Chapter 5

LangmuirLangmuir--Blodgett Blodgett Monolayer formation and Monolayer formation and

Solvent tuned Different Solvent tuned Different Aggregation Aggregation BehaviorBehavior of of RuRu(II)(II)--polypyridylpolypyridyl based based

MetallosurfactantsMetallosurfactants

Published in:

Chem. Commun. 2011, 47, 11074.Communicated

Chapter 5A

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5A. Interfacial and Film Formation Behaviour of Photoactive Ru(II)bipyridyl Based Metallosurfactants and a Monolayer Based Logic Gate Approach

5A.1. Introduction

Developing thin films and surface engineering are increasingly being acknowledged as

major research interests for the generation of new molecular electronic and photonic

materials.1 The fundamental understanding on molecular interactions and orientations

in such films or surfaces are essential for developing the device-quality mono and

multi-layers.2 Generally followed procedures for fabrication of thin-film devices are

vacuum deposition, spin coating, self-assembly and Langmuir-Blodgett (LB)

techniques. 3-7 Among these techniques, generation of mono- or multi-layers by LB film

technique has a special significance for developing ultra-thin films of appropriately

functionalized molecules with controlled thickness and well-defined molecular

orientation, as these have potential applications as biosensors, gas sensors and for

other high-end technological purposes. 8-11 The LB technique has been effectively used

to construct organized systems for efficient energy and electron transfer.12 Metal

containing soft materials like metallopolymers, metallomesogens, and

metallosurfactants, take advantage of the cooperativity between transition metals and

organic scaffolds to build up organized supramolecular architectures with unique

geometric, redox, and magnetic properties.13-15 For this, such metal-containing soft

materials are to be organized in highly ordered assemblies and transferred onto

surfaces. Among the metallosurfactants studied in LB films, ruthenium(II) bipyridyl

based amphiphilic complexes have received considerable attention, owing to their rich

photophysical and electrochemical properties, diversity of coordination forms, high

thermal, chemical and photophysical stability as well as their solubility in common

solvents.16-18 LB films of different ruthenium complexes have successfully been used in

making nonlinear optical (NLO) materials, light emitting diode (LED), and high-

Chapter 5A

204

performance sensors.19-21

Till date, enormous efforts have been devoted in the construction of molecular logic

gate which is an important device for information processing and computation at the

molecular level.22 At present there are several molecules reported that can perform

functions of various basic and complex logic operations; but most examples are limited

to the application in solution phase.22 For constructing solid-state molecular logic

devices, the immobilization of molecules onto solid substrates is essential. This has led

to an enormous opportunity for the development of hybrid system with molecules

fabricated on any solid surface, is capable of executing Boolean operations under the

influence of certain external stimulation(s).23 The Langmuir–Blodgett (LB) technique is

one of the most powerful techniques for building up nanoassemblies onto solid

supports.

In this chapter we report the synthesis of three new analogous complexes (1, 2 and 3

in Scheme 1) having varying alkyl chain lengths. We have also reported various

aggregated structures and films that were developed from complexes 1, 2, and 3 under

certain conditions. These metallosurfactants were found to form stable Langmuir

monolayer at the air-water interface. The morphology of the domains formed and real

time changes of the film during compression as well as the nature of the film after

collapse of the monolayer at the air-water interface were evaluated using Brewster

angle microscopy (BAM). Morphologies of these monolayer films on different solid

substrates were studied through Atomic force microscopy (AFM). Further spectral

responses on oxidation of the monolayer of complex 3 on glass plate could be used for

demonstrating a one input sequential logic gate. To the best of our knowledge this is

the first report on metallosurfactant based LB monolayer acting as a logic device.

Chapter 5A

205

5A.2. Experimental Section

5A.2.1. Materials

Analytical reagent grade solvents and compounds were used for studies unless

mentioned otherwise and were used as received. 4,4’-Dimethyl-2,2’-bipyridyl, 4-

methoxybenzaldehyde, 1-bromooctadecane, 1-bromotetradecane, 1-bromodecane,

butyllithium, diisopropylamine, ruthenium trichloride were procured from Sigma-Aldrich;

while all other chemicals were purchased from S. D. Fine chemicals (India). Nanopure

water having resistivity of 17.5 − 18 M.cm-1 was used for LB film studies. L1 and L2

were synthesized following previously reported procedures.24

5A.2.2. Instrumentation

Microanalyses (C, H, N) were performed using a Perkin-Elmer 4100 elemental

analyzer. Infrared spectra were recorded as a KBr pellets using Perkin Elmer Spectra

GX 2000 spectrometer. UV-Vis spectra of the complexes in solution were obtained by

using a Cary 500 Scan UV-Vis-NIR spectrometer. Uv-vis spectra of the LB films were

recorded on a Chemito UV 2600 spectrophotometer. Fluorescence spectra recorded

with HORIBA JOBIN YVON spectrophotometer. 1HNMR spectra were recorded on

Bruker 500 MHz FT NMR (model: Advance-DPX 500) spectrometer at room

temperature (25C). The chemical shifts () data and coupling constant (J) values are

given in ppm and Hz, respectively, throughout this chapter unless mentioned

otherwise. Tetramethylsilane (SiMe4) was used as an internal standard for all 1HNMR

studies. ESI-MS measurements were carried out on Waters QTof-Micro instrument.

5A.2.3. Compression Isotherms and Deposition of LB Films

Pressure area isotherm measurements were carried out by using computer controlled

KSV 5000 Langmuir double barrier Teflon trough. Surface pressure was measured with

platinum Wilhelmy plate microbalance. The compression rate (the barrier speed) used

was 5 mm/min at 25°C. Any impurities from the surface of freshly poured aqueous

Chapter 5A

206

subphase were removed by vacuum using a suction pump, after the compression of

the barriers. Spreading solutions for complexes 1, 2 and 3 having known concentration

(0.9-1.0 mg.mL-1) were prepared in HPLC grade chloroform. A known quantity (typically

80-90 µL) of one of these solutions was then allowed to spread on the clean aqueous

subphase with a Hamilton micro syringe. The system was allowed to equilibrate for ca.

20 min before monolayer compression. At least three independent measurements were

carried out for each experiment for ensuring reproducibility of the measurements. Using

the conventional vertical dipping method, LB films were transferred onto surfaces like

quartz and glass. The glass and quartz slides were kept overnight in chromic acid,

which were then rinsed thoroughly with Milli-Q water and immersed in ultrasonic bath of

chloroform for 10 min for appropriate cleaning. Then these slides were finally dried.

The transfer ratios were 0.93 in the upward stroke and 0.78 in the downward stroke.

5A.2.4. Brewster Angle Microscopy

The morphology of the Langmuir films at the air-water interface was observed by a

Brewster angle microscope (BAM). BAM images of the monolayer were recorded using

a nanofilm_ep3bam with polarized Nd:YAG laser (50 mW, 532.0 nm) and a CCD

camera (768 X 572 pixel) was used for recording images. The compression rate was 2

mm/min. The field of view was 487 X 387 microns and the lateral resolution was about

2 µm. The film was examined at different stages of the compression process. The

length scales of the images were corrected for the angle of incidence of the incident

laser beam. Images presented are typically 300 x 300 µm2 in area. Brewster angle (∼

53.1º) was maintained between the incident p-polarized light of 532 nm and the bare

air-water interface. At this stage, negligible amount of light reflected from the air-water

interface towards the CCD camera, so that the whole surface appeared black. Upon

spreading the amphiphilic material at the air-water interface the refractive index of the

interface was changed and a little portion (10-6 times) of the incident light was reflected,

Chapter 5A

207

which was captured by the CCD camera.25 Nature of the monolayer formed was

studied based on the reflected light from the interface.

5A.2.5. Atomic Force Microscopy

AFM studies were carried out under ambient conditions using Scanning probe

microscope NT-MDT (Model: Ntegra Aura; Moscow) in semi-contact mode using

rectangular cantilever of Si3N4.

5A.2.6. Synthesis and Characterization

Scheme 1. Synthetic methodology that was adopted for the synthesis of complexes 1, 2, and 3.

5A.2.6.1. Synthesis of Ligand L3

350 mg (1.22 mmol) of ligand L2 was dissolved in 60 ml dry DMF in a 100 ml 2 necked

round bottomed flask under N2 atmosphere. To this solution 251.9 mg (1.82 mmol, 1.5

mole equivalent) K2CO3 (hot and grinded) was added under vigorous stirring, when the

colour of the solution immediately changed from pale yellow to dark yellow. To this

solution 0.42 ml (1.22 mmol) of 1-bromooctadecane was added dropwise through a

syringe. Then to this solution 302.1 mg (1.82 mmol, 1.5 mole equivalent) of KI (hot)

was added. The whole mixture was heated around ~ 90C for 48 h. After that the

reaction mixture was cooled to room-temperature (25C) and filtered using Whatman-

Chapter 5A

208

42 filter paper. The residue was washed several times with CH2Cl2 and the filtrate was

evaporated to dryness through rotary evaporator to get the crude product (Crude yield

was 500 mg). Then the crude product was purified by column chromatography using

silica gel as stationary phase and CH2Cl2 as eluent.

Yield for pure L3 was evaluated based on the reactant used and was found to be 73%

(476 mg, 0.89 mmol). 1H NMR (500 MHz, CDCl3, SiMe4, J (Hz), (ppm)): 8.61 (1H, d, J

= 5, Hbpy6), 8.57 (1H, d, J = 4.5, Hbpy6), 8.48 (1H, s, Hbpy3), 8.25 (1H, s, Hbpy3), 7.49 (2H,

d, J = 8, Hphenyl3,5), 7.40 (1H, d, J = 16, Hvinyl), 7.34 (1H, d, J =4, Hbpy5), 7.15 (1H, d, J =

3.5, Hbpy5), 6.98 (1H, d, J = 16, Hvinyl), 6.91 (2H, d, J = 8, Hphenyl2,6), 3.98 (2H, t, HO-CH2),

2.45 (3H, s, Hbpy-CH3), 1.8-1.78 (2H, m, Hlongchain(-CH2)), 1.46-1.44 (2H, m, Hlongchain(-CH2)),

1.26 (28H, b, H(-CH2)14), 0.88 (3H, t, Hlongchain(-CH3)). ESI-MS (+ve mode): m/z 541.59

(100%) (M + H)+. Calc. for C37H52N2O = 540.82. Elemental Analysis data: Calc. C:

82.17, H: 9.69, N: 5.18; Found: C, 82.1; H, 9.64; N, 5.15.

5A.2.6.2. Synthesis of Ligand L4

Methodology used for the synthesis of L4 was similar to that for L3. L2 (500 mg, 1.74

mmol) was dissolved in ~ 60 ml of dry DMF in a two neck round bottom flask under the

positive pressure of N2 gas and to this finely ground pre-dried K2CO3 (365mg, 2.64

mmol) was added with rapid stirring. 1-bromotetradecane (0.52 ml, 1.74 mmol) was

added in a drop-wise manner using a syringe and the solution colour was found to

change to dark yellow. To this resulting reaction mixture, finely ground pre-dried KI

(439.25, 2.64 mmol in powder form) was added. Then the solution temperature was

raised to 90C and stirred for 48 h. After that, the reaction solution was allowed to

attain the room-temperature (25C) and filtered. Filtrate was collected and the solid

residue was washed thoroughly with CH2Cl2. This CH2Cl2 washing was added to the

filtrate and was evaporated to dryness through rotary evaporator to isolate the crude

product. Then the crude product was purified by gravity chromatography using silica

Chapter 5A

209

gel as stationary and CH2Cl2 as mobile phase.

Isolated yield of the purified compound (L4) (yield was calculated based on the starting

compounds) was found to be 65% (546 mg, 1.13 mmol). 1H NMR (500 MHz, CDCl3,

SiMe4, J (Hz), (ppm)): 8.61 (1H, d, J = 5.5, Hbpy6 ), 8.57 (1H, d, J = 5, Hbpy6), 8.48 (1H,

s, Hbpy3), 8.26 (1H, s, Hbpy3), 7.5 (2H, d, J = 8.5, Hphenyl3,5), 7.41 (1H, d, J = 16, Hvinyl),

7.34 (1H, dd, J1 = 5.5, J2 = 2, Hbpy5), 7.16 (1H, dd, J1 = 5, J2 = 1, Hbpy5), 6.98 (1H, d, J =

16, Hvinyl), 6.92 (2H, d, J = 9, Hphenyl2,6), 3.99 (2H, t, HO-CH2), 2.45 (3H, s, Hbpy-CH3), 1.82-

1.77 (2H, m, Hlongchain-CH2), 1.46-1.43 (2H, m, Hlongchain(-CH2)), 1.26 (20H, b, H(-CH2)10), 0.88

(3H, t, Hlongchain(-CH3)), ESI-MS (+ve mode): m/z 485.48 (100%) (M + H)+. Calc. for

C33H44N2O = 484.35. Elemental Analysis data: Calc. C, 81.77; H, 9.15; N, 5.78; Found:

C, 81.7; H, 9.11; N, 5.8.

5A.2.6.3. Synthesis of Ligand L5

Synthesis and purification procedures adopted for L5 was similar to that mentioned for

L4 with necessary change in one of the reactants, namely n-alkyl bromide. 1-

bromodecane (0.32 ml, 1.736 mmol) was used for this reaction instead of 1-

bromotetradecane.

Isolated yield of the compound L5 (yield was calculated based on the starting

compounds) was evaluated as 63% (421 mg, 0.98 mmol). 1H NMR (500 MHz, CDCl3,

SiMe4, J (Hz), (ppm)) : 8.61 (1H, d, J = 5.5, Hbpy6), 8.57 (1H, d, J = 5, Hbpy6), 8.48 (1H,

s, Hbpy3), 8.26 (1H, s, Hbpy3), 7.5 (2H, d, J = 8.5, Hphenyl3,5), 7.41 (1H, d, J = 16.5, Hvinyl),

7.35 (1H, d, J =5, Hbpy5), 7.16 (1H, d, J = 4.5, Hbpy5), 6.98 (1H, d, J = 16, Hvinyl), 6.92

(2H, d, J = 8.5, Hphenyl2,6), 3.99 (2H, t, HO-CH2), 2.46 (3H, s, Hbpy-CH3), 1.82-1.77 (2H, m,

Hlongchain-CH2), 1.48-1.43 (2H, m, Hlongchain-CH2), 1.28 (12H, b, H(-CH2)6), 0.88 (3H, t, Hlongchain-

CH3), ESI-MS (+ve mode): m/z 429.33 (100%) (M + H)+. Calc. for C29H36N2O = 428.61.

Elemental Analysis data: Calc. C, 81.27; H, 8.47; N, 6.54; Found. C, 81.2; H, 8.43; N,

6.5.

Chapter 5A

210

5A.2.6.4. Synthesis of Complex 1

Ligand L3 (170 mg, 0.315 mmol), RuCl3.xH2O (21.77 mg, 0.105 mmol) were added to

60 ml of ethanol-dioxan mixed solvent medium (1:1, v/v). The mixture was refluxed for

24 h under N2 atm with continuous stirring. After that it was cooled to room temperature

and evaporated to dryness. The crude product was chromatographed on alumina

grade-III using acetonitrile as an eluent.

Isolated yield of the complex 1 (yield was calculated based on the starting compounds)

25 % (141 mg, 0.078 mmol). 1H NMR (500 MHz, CD2Cl2, SiMe4, J (Hz), (ppm)): 9.13-

9.09 (6H, m, Hbpy6,6), 7.99 (3H, d, J = 17, Hvinyl), 7.65 (6H, d, J = 8, Hphenyl3,5), 7.59-7.5

(6H, m, Hbpy3,3), 7.38-7.37 (3H, m, Hbpy5), 7.22-7.20 (3H, m, Hbpy5), 7.09 (3H, d, J =

16.5, Hvinyl), 6.92 (6H, d, J = 8.5, Hphenyl2,6), 3.98 (6H, t, HO-CH2), 2.65 (9H, s, Hbpy-CH3),

1.78 (6H, b, Hlongchain-CH2), 1.46-1.42 (6H, m, Hlongchain-CH2), 1.26 (84H, b, Hlongchain-(CH2)14),

0.88 (9H, t, Hlongchain-CH3). ESI-MS (+ve mode): m/2z 861.65 (15%) (M2+/2). Calc. for

C111H156N6O3Ru = 1723.53. Elemental Analysis data: Calc. C, 74.3; H, 8.76; N, 4.68;

Found: C, 74.16; H, 8.81; N, 4.59.

5A.2.6.5. Synthesis of Complex 2

This complex was prepared by following a procedure that was adopted for 1. L4 (220

mg, 0.454 mmol) was dissolved in 25 ml ethanol-dioxan (1:1, v/v) mixed solvent

medium. To this RuCl3.xH2O (39.46 mg, 0.151 mmol) was added under an inert

atmosphere and refluxed for 24 h with continuous stirring. Then the reaction mixture

was cooled to room temperature and evaporated to dryness under reduced pressure.

The crude solid was chromatographed on alumina grade-III using acetonitrile as an

eluent. Major fraction was isolated and solvent was removed to isolate the desired

complex in pure form.

Isolated yield of the complex 2 (yield was calculated based on the starting compounds)

in pure form was 23% (170 mg, 0.104 mmol). 1H NMR (500 MHz, CD2Cl2, SiMe4, J

Chapter 5A

211

(Hz), (ppm)): 9.0 (3H, b, Hbpy6), 8.9 (3H, b, Hbpy6), 7.83 (3H, d, J = 16, Hvinyl), 7.62 (6H,

d, J = 6.5, Hphenyl3,5), 7.57-7.54 (6H, m, Hbpy3,3), 7.50 (3H, b, Hbpy5), 7.32 (3H, b, Hbpy5),

7.16 (3H, d, J = 17, Hvinyl), 6.96 (6H, d, J = 7.5, Hphenyl2,6), 4.0 (6H, t, HO-CH2), 2.61 (9H, s,

Hbpy-CH3), 1.79-1.75 (6H, m, Hlongchain-CH2), 1.46-1.42 (6H, m, Hlongchain-CH2), 1.25 (60H, b,

Hlongchain-(CH2)10), 0.87 (9H, t, Hlongchain-CH3). ESI-MS (+ve mode): m/2z 777.88 (100%)

(M2+/2), M2+ Calc. for C99H132N6O3Ru was 1553.23. Elemental Analysis data: Calc. C,

73.12; H, 8.18; N, 5.17; Found: C, 73.0, H, 8.2; N, 5.13.

5A.2.6.6. Synthesis of Complex 3

This complex was prepared following a procedure that was adopted for complex 2. L5

(200 mg, 0.467 mmol) was used instead of L4 for synthesis of this complex. The crude

product was purified by gravity chromatography using a Al2O3 grade-III column using

acetonitrile-chloroform (8:2, v/v) solvent mixture as eluent.

Yield was calculated based on the starting compounds used for the reaction. Analytical

data: 1H NMR (500 MHz, CD2Cl2, SiMe4, J (Hz), (ppm)): 9.06 (6H, b, Hbpy6,6), 7.96

(3H, d, J = 16, Hvinyl ), 7.64 (6H, d, J = 8, Hphenyl3,5), 7.6-7.51 (6H, m, Hbpy3,3), 7.38-7.37

(3H, m, Hbpy5), 7.21 (3H, b, Hbpy5), 7.09 (3H, d, J = 16, Hvinyl), 6.92 (6H, d, J = 8.5,

Hphenyl2,6), 3.99 (6H, t, HO-CH2), 2.65 (9H, s, Hbpy-CH3), 1.79-1.76 (6H, m, Hlongchain-CH2),

1.45-1.42 (6H, m, Hlongchain-CH2), 1.29 (36H, br, Hlongchain-(CH2)6), 0.88 (9H, t, Hlongchain-CH3).

ESI-MS (+ve mode): m/2z 693.86 (30%) (M2+/2). Calc. for C87H108N6O3Ru = 1386.90,

Elemental Analysis data: Calc. C, 71.68; H, 7.47; N, 5.76; Found: C, 71.53; H, 7.5; N,

5.73.

5A.3. Results and Discussion

5A.3.1. Amphiphilic Properties

Tris 2,2’-bipyridyl ruthenium(II) complexes are known to be soluble in polar solvent like

water when used as the dichloride salt. This led us to synthesize Ru[L]3Cl2 (Complexes

1, 2 and 3; L being L3 or L4 or L5) having the cationic Ru(II)-centre as the hydrophilic

Chapter 5A

212

head group and the n-alkyl chain substituted in Lx (X is 3, 4 or 5) as the hydrophobic

tail group. Chain length on the n-alkyl substituent was varied systematically in L3 or L4

or L5 and thus in respective complex 1, 2 and 3 from C18 to C10 with an aim to tune the

amphiphilicity of these photoactive metallosurfactants. The aggregation behaviour and

Langmuir monolayer formation of these complexes (1, 2 and 3) at the air-water

interface were studied by surface pressure vs. area (π vs. A) isotherms and Brewster

angle microscopy (BAM). The metallosurfactant was initially dissolved in a volatile

organic solvent like chloroform. Chloroform is immiscible with water and spreads on

water surfaces. Thus, chloroform solution of respective Ru(II)-polypyridyl complex was

allowed to spread on the water surface in a Langmuir trough. As the barriers of the

trough were compressed, the surface tension () at the air-water interface in the

presence of the amphiphilic species was expected to decrease as compared to that of

the bare air-water interface (0 = 72 mNm-1 at 25ºC), resulting in an increase in π (= 0 -

). Compression isotherms plot surface pressure (π, mNm-1) vs. mean molecular area

(A, expressed in Å2) provide fundamental information concerning the two dimensional

molecular organization of the monolayer at the air-water interface, collapse pressures

(πc), limiting areas per molecule (Alim), and average area of the molecule at the

collapse of the monolayer (Ac). Simultaneously Brewster angle microscopy evaluates

film homogeneity, domain and agglomerate formation upon passing vertically polarized

light through media possessing different refractive indexes.

Fig. 1 shows the π-A isotherms for complexes 1, 2 and 3 and suggests that these three

complexes are surface active. The π-A isotherm for metallosurfactant 1 reveals that

interactions between the molecules at the air-water interface start at 200 Å2. Further,

two distinctly different slopes are observed in the isotherm. For the initial region, till

surface pressure reaches the value of ~ 15 mNm-1; the increase in surface pressure (π)

with the change in mean moleculer area (A) is not sharp. This state is called the liquid

expanded state. However, after surface pressure reaches the value of ~ 20 mNm-1, a

Chapter 5A

213

0 75 150 2250

15

30

45

mMA (Å2)

π (m

Nm

-1)

steep rise in the π-A isotherm is observed (Fig. 1), which could be ascribed to the

formation of the liquid compressed state.26 At this situation, π rises steeply without

remarkable change in A (Å2). Fig. 1 further reveals that the isotherm of complex 1

shows a sudden drop in surface pressure after 51 mNm-1; with an average area per

molecule at collapse of 125 Å2 (Ac). This is termed as collapse area and could be

determined by extrapolating the steepest slope of the π versus A curve (before the

collapse) to zero pressure (i.e. π = 0).11 This collapse shows the signature of a constant

pressure collapse.27

Fig. 1. Surface pressure-area (π-A) isotherms of 1, 2 and 3.

Fig. 1 suggests that individual molecules of complex 2 start interacting at the air-water

interface at average molecular area of 196 Å2 (Fig. 1). The nature of the isotherm of

this complex is almost similar to complex 1, the only difference being the lower slope of

the π-A isotherm curve. The isotherm shows a high collapse pressure of 48 mNm-1 with

a constant pressure collapse similar to that of complex 1 and with a mean area at

collapse of 106 Å2 (Ac). Higher collapse pressures for complexes 1 and 2 indicate the

formation of a condensed and stable monolayer at the air-water interface. For complex

3 mean molecular area of 193 Å2 is evident (Fig. 1); while the surface pressure is found

to increase sharply until an inflection point is reached ca. at around 32 mNm-1. This

indicates a mesophasic change from liquid expanded state to the liquid compressed

state and the monolayer collapses at 48 mNm-1. Nearly analogous curves were

obtained when the compression-expansion cycle was repeated. No irreversible change

Chapter 5A

214

occurred during the second compression process. All three complexes showed

constant-pressure collapse and followed the Ries mechanism of folding, bending, and

breaking into multilayers.28 The large mean molecular areas (ca. 200 Å2 per molecule)

of these three metallosurfactants suggest a parallel orientation of these complexes with

respect to the air-water interface with alkyl chains pointing upward. These alkyl chains

serve as the hydrophobic part and the central Ru(bpy’)3 (bpy’ = 4-methyl, 4’-[2-(4-

alkoxy-phenyl)-vinyl]-[2,2’]bipyridinyl) moiety as the hydrophilic part of the amphiphilic

species in the resulting film.8d

Table 1. Mean molecular area and collapse pressures obtained from Langmuir-Blodgett (LB) isotherms for complexes 1-3.

Complex Mean moleculer area (Ǻ2) Collapse pressure mN/m-1

1 200 51 2 196 48 3 193 48

5A.3.2. Brewster Angle Microscopic Studies

Brewster Angle Microscopy (BAM) is a technique that allows the in situ study of thin

films at the gas-liquid or solid-gas interfaces. BAM is based on the study of the

reflected light coming from an interface illuminated by a p-polarized laser beam at the

Brewster angle.29 When the angle of incidence of this beam is at the Brewster angle,

the reflected intensity is a minimum for an air-water interface, which has a transition

region where the refractive index changes smoothly from one value to another. The

reflected intensity at this angle is strongly dependent on the interfacial properties,

mostly when a monolayer is involved in the interface. Using BAM, the homogeneity of

the film, agglomerates and domains in films at the air-water interface could be

recognized.30 Reflected light is a function of the orientation of the molecules in

monolayer domains. BAM images were recorded at different surface pressures during

the compression experiments for assessing the changes and domain formation at the

air-water interface. Selected BAM images of the air-water interface during compression

Chapter 5A

215

~ 30mN m-1

50 µm

~30 mN m-1

50 µm

(a) (b) (c)

50 µM

50 µm

~ 20mN m-1 ~ 20mN m-1 ~ 20 mN m-1

50 µm50 µm

~ 41 mN m-1~50 mN m-1 ~50 mN m-1

50 µm50 µm50 µm

50 µm

~ 6 mN m-1

50 µm

~ 6 mN m-1

50 µm

~ 6 mN m-1

~ 1.5 mN m-1 ~ 1.5 mN m-1

50 µm

~ 1.5 mN m-1

50 µm

50 µm

~ 0 mN m-1 ~ 0 mN m-1~ 0 mN m-1

50 µm 50 µm

50 µm

~ 30 mN m-1

50 µm

experiments after spreading chloroform solution of complexes 1, 2, and 3 on pure

water are shown in Fig. 2.

Fig. 2. Selected Brewster angle micrographs for complexes (a) 1, (b) 2 and (c) 3.

Initially the blank water surface was focused in such a way, that almost no reflection of

p-polarized light came from the bare air-water interface and appeared as black. After

spreading any of these three complexes on the air-water interface; non-uniform

Chapter 5A

216

domains with various morphology were formed, which were brighter compared to the

black water background. Initially at zero surface-pressure (π = 0 mNm-1) for complex 1,

the surface showed brighter domains of irregular shapes indicating scattered Langmuir

monolayer formation on water (dark background) (Fig. 2a). Initially with the increase in

surface pressure, movement of the domains were found to increase and it was difficult

to focus all the domains available in the CCD camera at a time; however, more uniform

monolayer film was formed at π = ~ 1.5 mNm-1 as brighter domains came closer to

each other. With further compression, ca. π 6 mNm-1, homogeneous surface with

random multiple ring-shaped bright circles were observed.31 These rings are called

Newton circles and are essentially multilayer granule that are formed by ejection of

matter due to the localized oscillation and reflect the thermodynamic instability of the

film (Fig. 2a).31 With further compression, the movement of these bright Newton circles

became faster which raised difficulty during focusing all the circles available for camera

at a time. With further lateral compression, more number of Newton circles having

lesser area was evident. For π ≥ 20 mNm-1, the density of these rings was even higher

and these rings combined to form a brighter film (Fig. 2a). After further compression

with π ≥ at 30 mNm-1, the homogeneous film became more compact and the

brightness of the film was enhanced (Fig. 2a). However after the collapse pressure, the

monolayer film collapsed and a rough and non-homogeneous film was observed (Fig.

2a).

For complex 2, similar changes at the air-water interface were observed (Fig. 2b) as

lateral compression was on. At π 1.5 mNm-1, almost a continuous monolayer was

formed and further compression lead to the formation of sporadic Newton rings.

However the size of these Newton rings were larger compared to complex 1. At π 20

mNm-1, the rings were more abundant and were more densely packed (Fig. 2b). At

even higher surface pressure (~ 50 mNm-1), rough film originated with dark (monolayer)

and bright (multilayer) patches, which was attributed to the collapse of the monolayer

Chapter 5A

217

and the formation of multilayer. Thus, on further increase in lateral compression with π

> 20 mNM-1, the interspatial distance between the resulting domains were found to be

narrower and eventually led to a collapse of the monolayer at π = 50 mNm-1 with the

resulting change in morphologies. According to Vollhardt and Wiedemann the growth

of these structures occurs due to crowding of the domains from supersaturation on the

surface of the surrounding phase.32 Again, it can also be argued that Ru(L)3Cl2 (L is L1,

L2 or L3) may remains in a dynamic equilibrium with its mono-halogenated and

completely solvated analogues, namely [Ru(L)3(H2O)Cl]+ and [Ru(L)3(H2O)2]2+, at the

air/water interface. Each of these species should exhibit distinct dipole moments which

increase molecular motion. Thus, the film stabilizes itself by the formation of domains

leading to a decrease in mobility.33

Complex 3 having the shortest chain length among three complexes was also studied.

Fig. 2c reveals that this complex forms almost uniform monolayer throughout the

compression with a very little Newton rings at higher pressure. At the higher lateral

compression, brighter film formation is evident (Fig. 2c), which signifies a more

compact film formation with increase in surface pressure.

5A.3.3. Atomic Force Microscopic Studies

Monolayer films of three amphiphilic complexes (1, 2 and 3) were deposited on

hydrophilic surface (glass) at different surface pressures. The morphology of these

films were analysed by Atomic force microscopy (AFM). Fig. 3A and 3B show the AFM

images of the monolayer of complex 1 deposited onto a glass surface at surface

pressures 10 and 30 mNm-1, respectively. Formation of a more compact monolayer film

is observed at higher surface pressure (ca. π 30 mNm-1). This is in accordance with

the π-A isotherm of complex 1 (Fig. 1); the π-A isotherm of complex 1 is less steeper at

π = 10 mNm-1 compared to than at π = 30 mNm-1. Thus, results obtained from both

these techniques were in good agreement. Fig. 3C and 3D showed the AFM images of

monolayers of complex 2 that were transferred at 10 and 30 mNm-1 surface pressures,

Chapter 5A

218

A B C D E

Fig. 3. AFM images of the transferred film of complex 1 that was deposited at surface pressure of (A) 10 mNm-1 (one monolayer) and (B) of 30 mNm-1 (one monolayer) on glass; AFM images of the transferred film of complex 2 that was deposited at surface pressure of (C) 10 mNm-1 and (D) 30 mNm-1 (one monolayer) on glass; (E) AFM images of the transferred film of complex 3 deposited at surface pressure 30 mNm-1 (one monolayer) on glass.

respectively. AFM image of the film, transferred at π = 10 mNm-1, showed monolayer

film formation with many bright spots (Fig. 3C); whereas the film that was transferred at

π = 30 mNm-1, appeared more compact and contained more number of bright spots

(Fig. 3D). Fig. 3E showed the AFM image of monolayer of complex 3 transferred on to

the hydrophilic glass surface at π = 30 mNm-1. Smooth homogeneous surface indicated

the formation of a compact homogeneous monolayer. These AFM results of the

transferred monolayer on the solid surfaces could be correlated well with the BAM

images (Fig. 2) at the corresponding surface pressure, which represented the

morphology of the monolayer formed at the air-water interface. BAM images (Fig. 2)

revealed that bright Newton rings were formed for complexes 1 and 2 at lower surface

pressure (Fig. 2a and 2b). The appearance of these Newton rings were found to

increase with the increase in the surface pressure (π) and after a certain pressure the

Newton rings were so close that they formed homogeneous film (Fig. 2a and 2b). AFM

image of complex 1 showed homogeneous film at 30 mNm-1, which was also observed

in the BAM images. The homogeneous nature of the film could be attributed to the

narrower interspatial distances between more numbers of Newton rings formed at that

high surface pressure. AFM image of the transferred film of complex 1 revealed more

number of bright spots for the film transferred at surface pressure of 30 mNm-1, as

compared to the one transferred at 10 mNm-1, which agreed well with the morphology

observed in BAM images for the corresponding surface pressures (Fig. 2a). Similar

increase in the density of bright spots in the AFM image of the transferred film of

Chapter 5A

219

300 400 500 6000.00

0.05

0.10

0.15

0.20

0.25

Film (11 layer)

Soln

Abs

orba

nce

Wavelength (nm)

complex 2 with increase in surface pressure was also evident in Fig. 3 and was in good

agreement with the higher numbers of Newton rings observed in the BAM images at 30

mNm-1 than that at 10 mNm-1 (Fig. 2b). At the surface pressure of 30 mNm-1, AFM

image of the transferred film of 3 showed homogeneous compact morphology, which

was also evident in the corresponding BAM image at comparable surface pressure

(Fig. 2c).

5A.3.4. Photophysical Studies of the Transferred Films

The LB films of these three complexes (complexes 1, 2 and 3), deposited on quartz

surfaces, were further studied by absorption and fluorescence spectroscopy. Multilayer

Y-type LB films of complexes 1, 2 and 3 were transferred on to hydrophilic quartz

plates in order to record the absorption and emission spectra. Absorption spectra of 1

in chloroform solution showed three peaks at 297, 366 and 474 nms (Fig. 4).

Fig. 4. Absorption spectra (normalized) of 1 in solution and in LB film that was deposited on a quartz substrate.

The high energy band at 297 nm was predominantly due to a ligand centered -

transition, whereas the absorption at 366 nm was predominantly due to intraligand

charge transfer transition. The absorption band at 474 nm was primarily arises due to a

metal to ligand (Rud L) charge transfer transition (1MLCT).34 In case of LB film,

respective absorption band appeared at 299, 366 and 495 nms (Fig. 4), respectively.

This indicated that max for the high energy transition bands remained almost

unchanged; while an appreciable red shift (~ 21 nm) was observed for the 1MLCT

Chapter 5A

220

0 2 4 6 8 10 120.00

0.05

0.10

0.15

0.20

486 nm

299 nm

Abs

orba

nce

No of layers300 400 500 600

0.00

0.05

0.10

0.15

0.20

Wavelength (nm)

11 layer9 layer

7 layer5 layer3 layer

Abs

orba

nce

A B

band. Further, this band was found to be broader for the LB film than that in solution

(Fig. 4). Similar red-shift for the 1MLCT band was also observed for complexes 2 and 3

(Table 2). This red shift could be ascribed to the aggregate nature of the molecules in

the solid LB film.35 Such red shift in the electronic spectrum was reported earlier for

Ru(II)-polypyridyl complexes36 as well as other organic molecules.37

Table 2. Absorption maxima for the 1MLCT band for complexes 1, 2, 3 in both chloroform solution as well as in LB-film.

Complex Absorbance maxima (nm) of 1MLCT band

Chloroform solution LB-film 1 474 495 2 471 487 3 470 487

The absorption spectra of the quartz plates with deposition of different multilayers (3 to

11) of complex 1 were recorded (Fig. 5A) and respective absorbance for each

multilayer at 299 (Ligand centred - band) and 486 nm (1MLCT band) were plotted

(Fig. 5B).

Fig. 5. (A) Absorption spectra recorded with 3 (bottom), 5, 7, 9, 11 (top) layers of deposition of complex 1 on a quartz substrate; (B) Plot of absorbances at 299 nm and at 486 nm for varying number of deposition on quartz surfaces.

Fig. 5B revealed that the absorbances at these two wavelengths followed the linear

relationship with the layer number. This linear dependence indicated that in all cases

the Langmuir monolayer was transferred in a regular and reproducible manner with

each dipping cycle.38 Similar phenomena were also observed for complexes 2 and 3.

From the linear plot of absorbance vs. no of layers of these metallosurfactants, the

Chapter 5A

221

surface concentration of the LB film could be derived using modified expression (Eq. 1)

of the Lambert-Beer law for two-dimensional concentration.38,39

= D/1000 (1)

Where , D and represented the surface concentration (mol cm-2), absorbance per

layer (AU/layer) and molar extinction coefficient (dm3 mol-1 cm-1), respectively at a

particular wavelength. D could be calculated from the slope of a linear plot of

absorbance vs. the number of layers for a given complex. The surface concentration at

the air-water interface could also be obtained from the limiting surface area (Ao)

determined by the extrapolation of the linear part to zero surface pressure in the π-A

isotherm following Eq. 2.39

= 1016/AoN (2)

Where , Ao and N represented the surface concentration (mol cm-2), limiting surface

area (Å2) and Avogadro’s number, respectively. The multilayer LB film forming

properties of the complexes are summarized in Table 3.

Table 3. Multilayer LB film-forming properties for complexes 1-3.

Complex Absorbance/layer Surface Concentration calculated by Lambert-Beer law/mol cm-2

calculated from A0/mol cm-2

1 8.9 x 10-3 1.31 x 10-10 0.83 x 10-10 2 8.0 x 10-3 1.26 x 10-10 0 .79 x 10-10 3 8.74 x 10-3 1.3 x 10-10 0.8 x 10-10

The surface concentrations in a LB layer as well as at the air-water interface calculated

following both methods (Equation 1 and 2) are consistent with each other. This further

validate that the monolayers of the LB film of the respective complex are regularly

transferred from the air-water interface onto the quartz substrates.

Emission spectra of these complexes in solution and in transferred films were recorded

and emission maxima are summarized in Table 4. In general, the emission spectra of

the complexes in chloroform solution and LB film showed broad emission band in the

Chapter 5A

222

600 650 700 750 8000

2

4

6

8

Soln.Film

Inte

nsity

(CPS

) x 1

0-6

Wavelength (nm) Fig. 6. Emission spectra of 1 in CHCl3 solution as well as the 9 monolayer of LB film of complex 1, deposited on quartz using ext = 490 nm as the excitation wavelength.

Table 4. Emission spectral data of complexes 1, 2 and 3 in solution the 9 monolayer of LB film of complex 1, deposited on quartz using ext = 490 nm as the excitation wavelength.

Complex Emission maxima (nm) in CHCl3 solution

Emission maxima (nm) of LB- film (9 monolayers)

1 680 659 2 680 666 3 678 665

region 659-680 nm. This emission band was attributed to the 3MLCT-based excited

state.40 The emission maxima of LB film on quartz surface was found to be blue shifted

compared to those in solution and were attributed to the phenomenon of luminescence

rigidochromism.41 In fluid state, 3MLCT state of the chromophore was stabilized by

reorganization of the solvent dipoles around the chromophore dipole, whereas in film

no such solvent stabilization was possible. The net result was the destabilization of the

3MLCT excited state in the rigid environment of the film and a shift of the emission

band to higher energies.

5A.3.5. Logic Gate Behaviour of Complex 3 Based Monolayer

Recenly, van der Boom and his coworkers have reported that a silica surface,

fuctionalized with a Os(II)-polypyridyl complex, could demonstrate the function of an

one input sequential logic gate.23a However, using LB film deposition technique we

could form monolayer of the complex 3 over glass surface with much ease and could

be used for executing a chemical input based solid state logic gate operation.

Absorption spectra was recorded for the monolayer of complex 3, deposited on glass

Chapter 5A

223

surface. This showed absorption maxima at 487 nm. On treatment with aqueous

solution of Ce4+ (ammonium ceric nitrate) partial bleaching in the intensity for this

absorption spectral band was observed due to the partial oxidation of the Ru(II) to

Ru(III) (Fig. 7A). This is anticipated as Ce4+ is known to oxidize Ru(II)-centre of the

Ru(II)-polypyridyl complex to Ru(III) and this is expected to lead to an overall decrease

in the Ru(II) bpy*-based MLCT transition. Fig. 7 reveals only a partial bleaching of

the band at 487 nm on treatment with Ce4+. This suggests that only a partial oxidation

of the Ru(II) based monolayer by Ce4+ could be achieved possibly due to an effective

shielding of the Ru(bpy)32+ moiety by hydrophobic long chains. The presence or

absence of an arbitrary chemical input was defined as a logical 1 or 0, respectively.

Entry Input (Ce4+)

Currentstate

Next State

OutputA487 nm

1 0 1 1 12 1 1 0 03 0 0 0 04 1 0 0 0

400 5000

4

8

0

Abs

orba

nce

x10-2

Wavelength (nm)

487 nm

1

A CB

Ce4+

Output

Feed back loop

Fig. 7. (A) Absorption spectra of Ru2+/Ru3+-based LB monolayer of complex 3. a) Ru2+, red line, b) Ru3+, blue line, c) baseline, black line. The absorption intensities at 487 nm was used as output (0 or 1). (B) Truth table with one chemical input (Ce4+) with different current states (Ru2+/Ru3+). (C) Sequential logic circuit based on the optical output with four combinations of one input.

The output was dependent on the formal oxidation state of the system, which was

monitored by UV/Vis spectroscopy. The threshold value was set at 5.0 x 10-2; output

values above and below of this threshold value were defined as 1 and 0, respectively

(Fig. 7A). Thus, a one-input sequential system was designed with Ce4+ ions in an

aqueous solution the input.23a The four possible combinations were demonstrated with

the same monolayer (Fig. 7B and 7C). In absence of any Ce4+ ion the monolayer was

in state 1 (Ru2+-complex existed), which was then changed to state 0 (predominant

Ru3+-complex; Fig. 7B). Since the current state was variable, the output became

dependent on the previous input of the logic gate. Thus based on the optical responses

of the monolayer of complex 3 sequential logic circuit could be constructed.

Chapter 5A

224

5A.4. Conclusion

Herein we have reported synthesis of a series of symmetrical Ru(bpy’)3 –based

metallosurfactants with three different chain-lengths, namely –C18H37 (1), –C14H29 (2)

and –C10H21 (3). All the three complexes formed stable monolayer at the air water

interface, which was evident from the surface-pressure area (π-A) isotherm

measurements as well as from the Brewster angle microscopy. From the π-A isotherm

the average area per molecule was obtained. No significant dependence of the

average area per molecule with variation in the alkyl chain-length of the

metallosurfactants was observed. This indicated that the hydrophobic long chain tails

are pointed upwards from the Ru(bpy’)32+ head-group with respect to the air-water

interface. The change in slope in the π-A isotherm of these complexes point to the

mesophasic changes from liquid expanded state to the liquid compressed state during

the compression of the monolayer. Again high collapse pressures of these isotherms

indicated the formation of homogeneous and compact monolayer film formation for the

complexes. From Brewster angle microscopy various domains formed by these

metallosurfactants at low surface pressure as well as the formation of compact

monolayer with gradual increase in surface pressure were observed. These LB

monolayers could be transferred successfully on the solid surfaces like glass and

quartz by vertical dipping method at different surface pressure. The morphology of the

transferred films was characterized by AFM and correlated with the surface

morphology at the air-water interface studied by BAM. Absorption and emission spectra

of the transferred films on the quartz surface are recorded, which support the uniform

and regular multilayer formation on quartz surfaces. The surface concentration of the

transferred film as well as at the air-water interface were calculated (Lambert-Beer law

modified for two-dimensional concentration (equation 1) and limiting surface area

(equation 2) and both data compared well with each other. More importantly, we had

shown that these amphiphilic Ru(II)-complexes could be used for developing LB film

Chapter 5A

225

that was capable of demonstrating the novel concept of optical one input sequential

logic gate operation. These results further revealed that Ru(bpy’)3 based monolayer in

presence of suitable input(s) could speak the complex language of information

technology.

Chapter 5A

226

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19. (a) Boubekeur-Lecaque, L.; Coe, B. J.; Harris, J. A.; Helliwell, M.; Asselberghs, I.; Clays, K.; Foerier, S.; Verbiest, T. Inorg. Chem. 2011, 50, 12886. (b) Verbiest, T.; Houbrechts, S.; Kauranen, M. Clays, K. Persoons, A. J. Mater. Chem. 1997, 7, 2175. (c) Chu, B. W.-K.; Yam, V. W.-W. Inorg. Chem. 2001, 40, 3324. (d) Nakano, T.; Yamada, Y.; Matsuo, T.; Yamada, S. J. Phys. Chem. B 1998, 102, 8569.

20. (a) Handy, E. S.; Pal, A. J.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 3525. (b) Rudmann, H.; Rubner, M. F. J. Appl. Phys. 2001, 90, 4338. (c) Gao, F. G.; Bard, A. J. J. Am. Chem. Soc. 2000, 122, 7426.

21. (a) Michinobu, T.; Shinoda, S.; Nakanishi, T.; Hill, J. P.; Fujii, K.; Player, T. N.; Tsukube, H. Ariga, K. J. Am. Chem. Soc. 2006, 128,14478. (b) Ferreira, M.; Dinelli, L. R.; Wohnrath, K.; Batista, A. A.; Oliveira, O. N. Thin Solid Films 2004, 446, 301. (c) Ferreira, M.; Riul, A.; Wohnrath, K.; Fonseca, F. J.; Oliveira, O. N.; Mattoso, L. H. C. Anal. Chem. 2003, 75, 953. (d) Borato, C. E.; Riul, A.; Ferreira, M.; Oliveira, O. N.; Mattoso, L. H. C. Instrum. Sci. Technol. 2004, 32, 21.

22. (a) de Silva, A. P.; Gunaratne, H. Q. N.; McCoy, C. P. J. Am. Chem. Soc. 1997, 119, 7891. (b) de Silva, A. P.; Dexon, I. M.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Maxwell, P. R. S.; Rice, T. E. J. Am. Chem. Soc. 1999, 121, 1393. (c) Suresh, M.; Jose, D. A.; Das, A. Org. Lett. 2007, 9, 441. (d) Suresh, M.; Ghosh, A.; Das, A. Tetrahedron Lett. 2007, 48, 8205. (e) Suresh, M.; Ghosh, A.; Das, A. Chem. Commun. 2008, 3906. (f) Suresh, M.; Kar, P.; Das, A. Inorg. Chim. Act. 2010, 363, 2881. (g) Kumar, M.; Kumar, R.; Bhalla, V. Org. Lett. 2011, 13, 366. (h) Kumar, M.; Dhir, A.; Bhalla, V. Org. Lett. 2009, 11, 2567. (i) Bhalla, V.; Vij, V.; Kumar, M.; Sharma, P. R.; Kaur, T. Org. Lett. 2012, 14, 1012.

23. (a) de Ruiter, G.; Tartakovsky, E.; Oded, N.; van der Boom, M. E. Angew. Chem., Int. Ed. 2010, 49, 169. (b) Gupta, T.; van der Boom, M. E. Angew. Chem., Int. Ed. 2008, 47, 5322. (c) Matsui, J.; Mitsuishi, M.; Aoki, A.; Miyashita, T. Angew. Chem., Int. Ed. 2003, 42, 2272.

24. Hayes, M. A.; Meckel, C.; Schatz, E.; Ward, M. D. J. Chem. Soc. Dalton. Trans. 1992, 703.

25. Hoenig, D.; Moebius, D. J. Phys. Chem. 1991, 95, 4590.

26. Oliveira Jr., O. N. Braz. J. Phys. 1992, 22, 60.

27. Kundu, S.; Datta, A.; Hazra, S. Langmuir 2005, 21, 5894.

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28. Ries, H. E. Nature 1979, 281, 287.

29. Henon, S.; Meunier, J. Rev. Sci. Instrum.1991, 62, 936.

30. Gericke, A.; Michailov, A. V.; Huehnerfuss, H. Vib. Spectrosc. 1993, 4, 335.

31. Galvan-Miyoshi, J.; Ramos, S.; Ruiz-Garcia, J.; Castillo, R. J. Chem. Phys. 2001, 115, 8178.

32. Weidemann, G.; Vollhardt, D. Thin Solid Films 1995, 264, 94.

33. Driscoll, J. A.; Allard, M. M.; Wu, L.; Heeg, M. J.; da Rocha, S. R. P.; Verani, C. N. Chem. Eur. J. 2008, 14, 9665.

34. Jose, D. A.; Kar, P.; Koley, D.; Ganguly, B.; Thiel, W.; Ghosh, H. N.; Das, A. Inorg. Chem. 2007, 46, 5576.

35. Vergeer, F. W.; Chen, X.; Lafolet, F.; De Cola, L.; Fuchs, H.; Chi, L. Adv. Funct. Mater. 2006, 16, 625.

36. Chen, Y.; Xu, W-C.; Kou, J-F.; Yu, B.-L.; Wei, X-H.; Chao, H.; Ji, L-N. Inorg. Chem. Commun. 2010, 13, 1140.

37. (a) An, B-K.; Kwon, S-K.; Jung, S –D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (b) Verma, S.; Ghosh, A.; Das, A.; Ghosh, H. N. J. Phys Chem B. 2010, 114, 8327.

38. Taniguchi, T.; Fukasawa, Y.; Miyashita, T. J. Phys. Chem. B 1999, 103, 1920.

39. Chu, B. W.-K.; Yam, V. W.-W. Inorg. Chem. 2001, 40, 3324.

40. Verma, S.; Kar, P.; Das, A.; Ghosh, H. N. Chem. Eur. J., 2011, 17, 1561.

41. (a) Yam, V. W.-W.; Li, B.; Yang, Y.; Chu, B. W.-K.; Wong, K. M.-C.; Cheung, K.-K. Eur. J. Inorg. Chem. 2003, 4035. (b) Kotch, T. G.; Lees, A. J. Inorg. Chem. 1993, 32, 2570. (c) Moya, S. A.; Schmidt, R.; Muller, U.; Frenzen, G. Organometallics 1996, 15, 3463.

Chapter 5B

230

5B. Solvent-Dependent Aggregation Behaviour of New Ru(II)-polypyridyl Based Metallosurfactants

5B.1. Introduction

Amphiphilic molecules self-assemble to generate various interesting structures that

differ in nature, size and shape. Nature of these self-assembled structures varies

between miceller/reverse miceller structures, lamellae, ellipsoids, disks, cylinders and

vesicles/reverse vesicles depending on the solvent environment, molecular architecture

and concentration.1,2 Among these various forms, vesicles composed of a spherical

bilayer of amphiphiles containing aqueous core, have drawn much attention from a

fundamental structural perspective, as well as for their potential applications in

modeling cell membranes, drug and gene delivery, as nanoreactors, templating

nanomaterials, gelation and molecular recognition.2a,3-8 Reverse vesicles are composed

of a reverse bilayer shell that encapsulate solvent of low polarity, where hydrophobic

portions of the precursor amphiphiles are exposed to the low polar solvent both in the

core and in the exterior.9 These are of great fundamental interest because they have

many impending applications like normal vesicles, e.g., encapsulation and controlled

delivery of hydrophobic solutes.10 To fully realize the promising aspects mentioned

above, a detailed understanding of the structural features of normal vesicles and

reverse vesicles is necessary. However, studies in which metallo-aggregates have

been used for vesicle formation are scarce in the literature and more importantly these

aggregates have been studied mainly in aqueous solution.11 Thus, studies with reverse

vesicle formation in non-polar organic solvents are even scarcer.12 Complexes

containing the Ru(bpy)32+ (where bpy = 2,2’-bipyridine) unit as building block are of

wide interest due to their distinct coordination geometry, stability in the ground and in

the photoexcited states, and rich redox/photophysical properties.13 Though, reports on

the aggregation behavior of amphiphilic complexes containing Ru(bpy)32+ as the polar

head group and aliphatic long chain as lipophilic tail are there, examples on the use of

Chapter 5B

231

such metallosurfactants for the formation of vesicles or reverse vesicles are

uncommon.14 Such structures are of immense importance for their application potential

as photodynamic therapeutic reagents. In the previous section, the Langmuir-Blodgett

film formation behavior and related studies for a series of Ru(II)-polypyridyl based

metallosurfactants (1, 2 and 3) have been described. These complexes were found to

form normal vesicle in water and reverse vesicle in non polar solvents like toluene and

cyclohexane—an observation that was unique in the contemporary literature. These

vesicles and reverse vesicles were characterized by Dynamic light scattering (DLS),

Static light scattering (SLS), Atomic force microscopy (AFM), Transmission electron

microscopy (TEM) and photophysical studies. Further, dye encapsulation ability of

these vesicles and reverse vesicles were also checked.

Scheme 1: Molecular structure of compound 1, 2 and 3 and cartoon representation of their aggregates formed in different solvents.

5B.2. Experimental Section

5B.2.1. Materials

Uranyl nitrate and Rhodamine B were purchased from Sigma-Aldrich and were used as

received. Solvents used for reactions and various other studies were of HPLC grade

(Merck, India) and were used as received. Details of all other chemicals used in this

study were described in details in the previous section of this chapter.

A B

Chapter 5B

232

5B.2.2. Instrumentation

UV-Vis spectra of the compounds in solution were obtained by using Cary 500 Scan

UV-Vis-NIR spectrometer. Fluorescence spectra recorded with HORIBA JOBIN YVON

spectrophotometer. AFM studies were carried out under ambient conditions using

Scanning probe microscope NT-MDT (Model: Ntegra Aura; Moscow) in semi-contact

mode using rectangular cantilever of Si3N4. The HRTEM images were recorded using

Jeol JEM-2100 Transmission Electron Microscope. DLS was performed on Malvern

Zetasizer ZS90 (UK). Ten repeat measurements were performed for each sample and

the average data was considered for analysis. Data evaluation of the DLS

measurements was performed using the CONTIN algorithm. SLS studies were

performed using NABITEC spectrosize TM 300 (Germany) Intensity of the scattered

light was measured at varying angle from 30 – 150º. For TEM study lacey carbon

formvar coated copper grid was used and for some experiment the sample was stained

with 1% uranyl nitrate solution. For AFM measurement freshly cleaved mica was used.

5B.2.3. Preparation of Sample Solution

The vesicle solution was prepared by dissolving 2 mg of the respective complex (1, 2

or 3) in 200 µL of methanol. To that solution 1.8 ml of water was added in a dropwise

manner and the solution was stirred for 24 hrs. The suspension was then centrifuged at

3500 rpm for 10 min. The clear solution was then filtered through 0.45 µm pore sized

micron filter and used for several studies.

The reverse vesicle solutions were prepared by sonication using 2 mg of the respective

complex (1, 2 or 3) in 2 ml of toluene/cyclohexane and then centrifuged at 3500 rpm for

10 min. The clear solution was then filtered through 0.45 µm pore sized micron filter

and used for several studies.

Chapter 5B

233

5B.2.4. Preparation of Sample for the AFM and TEM Studies

For AFM studies, one small drop of the complex solution (0.5 – 1 µM) was kept over

freshly cleaved mica sheet through a glass capillary and then dried overnight (with

appropriate cover to avoid any dust particle deposition) without any disturbance.

For TEM studies, similar procedure for sample solution was adopted. Sample solutions

were deposited over lacey carbon formvar coated Cu grid, which was hanged on a

clean forcep, after about 2 min the excess liquid was soaked gently through tissue

paper. For staining studies 1 drop of 1% of uranyl nitrate solution was deposited over

the sample loaded grid and after about 2 min the excess liquid was again soaked

gently through tissue paper and kept overnight hanging for drying.

5B.2.5. Determination of Radius of Gyration from Guinier Law

In static light scattering (SLS), the excess light scattering intensity of the aggregates in

different solvents were measured as a function of the angle of scattering. SLS permits

one to estimate the radius of gyration (Rg) of these aggregates. For low scattering

vectors, i.e. qRg 1, the scattering intensity follows the well known Guinier law,15

I(q) = I(0) exp(– q2Rg2/3)

where q = (4/) sin(/2), = refractive index and = wavelength of the light and =

angle of incident light.

ln(I) was plotted against q2 at different scattering angle and from the slope of the plot Rg

was calculated.

5B.2.6. Synthesis and Characterization

Synthesis and characterisation of ligands L3, L4 and L5 and complexes 1, 2 and 3 have

been described in chapter 5A.

5B.3. Results and Discussion

Complex 1, 2 and 3 (Ru(L3)3Cl2) were synthesized by reacting L3, L4 and L5,

respectively with RuCl3.3H2O in appropriate mole ratio in a mixed solvent (ethanol–

Chapter 5B

234

0 500 1000 15000

3

6

9

12

Inte

nsity

(%)

Diameter(nm)0 500 1000

0

3

6

9

12

Inte

nsity

(%)

Diameter (nm)0 250 500 750 1000

0

5

10

15

20

Inte

nsity

(%)

Diameter (nm)

A B C

dioxan, 1:1, v/v) medium at 90C and desired purity was achieved by column

chromatography. These complexes were expected to exhibit aggregation behaviour in

polar, as well as in nonpolar solvents due to their amphiphilic nature. It was found to

form aggregates on dropwise addition of water to the methanol solution of the complex

(1, 2 or 3), and the final solvent combination was adjusted to CH3OH:H2O = 1:9, (v/v) (~

10-6 M). DLS studies with the methanol-water solution of complex 1 showed

aggregates having distribution of varying hydrodynamic diameters, with mean diameter

(Dh) of 230 nm (PDI = 0.18) (Fig. 1A).

Fig. 1: Size distribution of (A) 1, (B) 2 and (C) 3 in methanol-water (1:9; v/v) medium.

The larger hydrodynamic diameter revealed that these aggregates were vesicles not

micelles, as for micelles diameter should be in the order of 3 – 7 nm.16 Again the mean

hydrodynamic diameters obtained from DLS, of the complexes 2 and 3 were 197 (PDI

= 0.2) and 175 nm (PDI = 0.23), respectively (Fig. 1). Static light scattering (SLS)

measurements were also performed in methanol-water medium (1:9, v/v) with varying

angles from 30 – 150 and radius of gyration (Rg) was evaluated from the slope of the

Guinier plot (Fig. 2).15

0.0000 0.0002 0.0004 0.000614

15

16

q2 (nm-2)

ln(I)

0.0000 0.0002 0.0004 0.0006

14

15

16

q2 (nm-2)

ln(I)

0.0000 0.0002 0.0004 0.000614

15

16

ln(I)

q2 (nm-2)

A B C

Fig. 2: Guinier plot obtained from SLS studies of (A) 1, (B) 2 and (C) 3 in methanol-water (1:9; v/v) medium.

Chapter 5B

235

0

1

2

0.5

1.5

2.5

µm

0 1.51 2 2.50.5 µm 0

100

200

250

nm

150

50

0.4 0.6 0.800

80

160

nm

nm10.2 1.2

The average Rg value obtained for the aggregates of complex 1 in methanol-water

medium was 116.6 nm and the Rg/Rh ratio (where Rh = 1/2 Dh) is 1.01. Whereas the Rg

value for the complexes 2 and 3 in CH3OH-H2O medium, obtained from SLS

experiments were 94 nm and 89.5 nm respectively (Table 1). Thus the Rg/Rh values for

these complexes were 0.96 and 1.02 respectively (Table 1). These values were also

very close to unity, indicating the spherical vesicle formations.17

Table 1: Dynamic and static light scattering results of complexes 1, 2 and 3, in methanol-water (1:9; v/v) medium.

Complex Dh (nm) Rh (nm) Rg (nm) = Rg/Rh 1 230 115 116.6 1.01 2 197 98.5 94 0.96 3 175 87.5 89.5 1.02

Thus comparison of the DLS and SLS results showed that all the complexes 1, 2 and 3

form vesicles in polar methanol-water (1:9; v/v) medium and radius of the vesicles

decreases with the decrease in chain length of these amphiphilic complexes. Vesicle

formation was also confirmed by AFM studies. For AFM experiment, cast film of

complex 1 was prepared over freshly cleaved mica on air drying of a small drop of

methanol-water (1:9; v/v) solution.

Fig. 3: AFM height contrast image of 1 on mica upon evaporation of methanol-water (1:9, v/v) as mixed solvent. Inset: Cross section of the selected portion of the image.

The morphology of these self-assembled vesicles (Fig. 3) enabled us to calculate the

diameter of the collapse dry aggregates. AFM images further revealed that the

aggregates were ellipsoid instead of spherical. The deposited droplet over the mica

surface did not undergo instant drying; instead there was a moving drying front which

allowed these vesicles to orient along its direction and accounted for the ellipsoid

Chapter 5B

236

shape. AFM cross-sectional analysis also revealed that the length, width, aspect ratio

and diameter to height ratios were in the order of 174 – 465 nm, 108 – 347 nm, 1.25 –

1.6 and 3.9 – 5.7, respectively. The diameters of these vesicles are thus larger than the

heights of the vesicles, which indicate flattening upon being transferred from solution to

the mica surface. Removal of solvent molecules from the hollow spheres during drying

process and the high local force applied by the AFM tip could have also contributed to

this. Thus these structures are robust, yet soft enough to deform upon drying.

Table 2: Average vesicle dimensions as measured by AFM of the complexes 1, 2 and 3, in methanol-water (1:9 v/v) medium.

Complex Length (nm) Width (nm) Aspect ratio Diameter to height ratio

1 174 – 465 108 – 347 1.25 – 1.6 3.9 – 5.7 2 155 – 435 95 – 315 1.3 – 1.63 4.1 – 6.1 3 132 – 398 78 – 274 1.27 – 1.56 3.7 – 5.9

AFM study also done for the cast films of aqueous-methanol solution of complexes 2

and 3 over mica sheet and the corresponding results were summarized in Table 2.

These results again confirmed that complexes 2 and 3 also form vesicle like complex 1,

which were stable even after drying.

Spherical nature of these vesicles was further confirmed by TEM studies. A small drop

of the methanol-water (1:9; v/v) solution was deposited on a carbon coated copper grid

and air dried. Fig. 4 shows the 2D projection of the polydispersed spherical vesicles of

the complexes 1, 2 and 3. These vesicles were in the 80 – 400 nm diameter range and

belong to medium size vesicles.2a

20 nm

6.8 nm6.8nm

7 nm

B C100 nm B6.8 nm

0.2 µm0.2 µm A200 nm

7 nm

Fig. 4: TEM images of the cast films of (A) 1, (B) 2 and (C) 3 in methanol-water (1:9; v/v) solution and arrow showing the cell wall thickness.

Chapter 5B

237

These images, on staining with 1% uranyl nitrate solution, showed darker cell wall with

a thickness of ~ 7 nm (Fig. 4A). The length of the fully extended individual molecule of

these complexes was ~ 3.5 nm and this suggested that the vesicle wall was a bilayer

and these vesicles were unilamelar and hollow in nature. Again fusion of two nearby

spheres and fission of the large spheres to form smaller spheres as observed from the

TEM images also supported the vesicular nature of these spherical structures (Fig. 4).

Complexes 1, 2 and 3 with Ru(bpy’)32+-core as the polar head group and three

symmetrical hydrophobic tail groups were expected to adopt a cone shape

geometry.17b Such cone shaped complexes were expected to form inverted micelles or

vesicles in nonpolar medium. We checked such a possibility in toluene and

cyclohexane. The solubility of these complexes in toluene was found to be higher than

in cyclohexane and complex 3 was even insoluble in cyclohexane, which could be

ascribed to the higher polarity of toluene. In the midst of the apolar environment the

apolar aliphatic chains were expected to be exposed to the solvent medium with the

hydrophilic head group situated at the core of the bilayer that faces each other

(Scheme 1). This was further supported by the fact that Ru(bpy)32+ derivatives are not

soluble in apolar solvents like toluene and cyclohexane.

The DLS study of freshly prepared solution of 1 (~ 10-6 M) in toluene and cyclohexane

revealed that mean Dh for these molecular aggregates were 308 nm (PDI = 0.35) and

229 nm (PDI = 0.19), respectively (Fig. 5). Rg values obtained from SLS experiments

for these aggregates were 166.3 and 121.4 nm (Inset, Fig. 5 and table 3). Thus the

Rg/Rh values were 1.08 and 1.06 in toluene and cyclohexane, respectively. These

values are very close to unity and corroborated the formation of reverse vesicles.18 The

DLS and SLS experiments for the complex 2 were also done in toluene and

cyclohexane mediums and for complex 3 in toluene medium and results were

summarized in table 3. These also indicated that complex 2 and 3 also form spherical

reverse vesicles like complex 1 in apolar medium. The size of the reverse vesicles

Chapter 5B

238

decreased in the order 1 > 2 > 3 and the alkyl chain lengths of these complexes

followed the similar order.

Fig. 5: Size distribution of 1 in (A) toluene and (B) cyclohexane medium. Insets: Guinier plot obtained from SLS studies in respective solvent medium.

Table 3: Dynamic and static light scattering results of complexes 1, 2 and 3, in Toluene and cyclohexane medium.

Complex Toluene Cyclohexane Rh (nm) Rg (nm) = Rg/Rh Rh (nm) Rg (nm) = Rg/Rh

1 154 166.3 1.08 114.5 121.4 1.06 2 114 111 0.97 99 103.2 1.04 3 79 81.7 1.02 --- --- ---

Smaller size of the reverse vesicles in cyclohexane could be accounted if one

considers that in media of lower polarity, polar head set to tighter inverted vesicles.

While toluene, being more polar among these, was expected to penetrate the vesicular

membrane better and caused the enhancement of the vesicle size.

The mean hydrodynamic diameter of complex 1 in toluene and in toluene-methanol

mixture of varying proportion (e.g. 25:1 and 25:4 (v/v)) were found to be 308, 61 and 39

nm respectively. This further confirmed the proposition that with the increase in polarity

of the medium the interaction of the amphiphiles became more flexible and thus the

rigidity of the inverted vesicles decreased, and finally they became soluble in the

mixture of the solvents.

Results of the AFM experiments with cast films on mica confirmed that complex 1

formed aggregates in toluene and cyclohexane solutions. The size of the reverse

vesicles obtained from the toluene solution was found to vary between 270 and 530 nm

in length and 170 and 320 nm in width; while the aspect ratio varied from 1.35 – 1.56

(Fig. 6A). Cross-section analysis of the typical structures revealed large ratios of

0 500 1000 15000

3

6

9

12In

tens

ity (%

)

Diameter (nm)0 250 500 750 1000

0

4

8

12

16

Inte

nsity

(%)

Diameter (nm)

0.0 3.0x10-4 6.0x10-4

9

12

15

In[I]

q2(nm)-20.0 3.0x10-4 6.0x10-412.0

13.5

15.0

In[I]

q2 (nm)-2

A B

Chapter 5B

239

diameter to height (4.4 – 6.8). In case of cyclohexane solution, length, width, aspect

ratio and diameter to height ratios were in the order of 134 – 333 nm, 117 – 280 nm,

1.2 – 1.5 and 3.7 – 6.7, respectively (Fig. 6B). Thus AFM results confirmed that the

reverse vesicles formed in both toluene and cyclohexane had flattened shape with

rounded edges and the hollow feature of the original inverted vesicle. The reverse

vesicles appeared with an ellipsoidal instead of a spherical shape, as during the drying

process of the deposited droplet, solvents were not dried instantly and there was a

moving drying front that allowed these vesicles to orient along its direction and

accounted for the ellipsoid shape. These vesicles were found to be stable even in the

dry state.

Fig. 6: AFM height contrast image of 1 on mica upon evaporation of (A) toluene and (B) cyclohexane solutions. Bottom: Cross section of the selected portion of the image.

Similar type of elliptical shaped aggregates was also found in AFM studies for complex

2 in toluene and cyclohexane solutions. Large sizes of these aggregates, high diameter

to height ratios (Table 4) indicated that these aggregates were reverse vesicles and

hollow in nature. AFM data for the complex 3 in toluene medium (Table 4) also

suggested that the complex form reverse vesicles which were found stable even after

drying. The size of the dry structures also followed the same ascending order as

obtained from the DLS and SLS studies.

More convincing information about the reverse vesicles was obtained from TEM

images. Cast films of complex 1 on carbon coated copper grid were performed through

0

1

2

0.5

1.5

2.5

µm

0 1.51 2 2.50.5 µm

0nm

600400200 800 1000060nm

120

0

100

300

400nm

200

B0

1

2

0.5

1.5

2.5

µm

0 1.51 2 2.50.5 µm

0nm

0.60.2 1.410

100nm200

0

100

200

250

nm

150

50A

Chapter 5B

240

evaporation of a small drop of toluene/cyclohexane solution in air. The mean diameters

of the aggregates obtained from TEM were 250 and 209 nm, respectively, for toluene

Table 4: Length (nm), width (nm), aspect ratio, diameter to height ratio obtained from AFM studies of complexes 1, 2 and 3, in Toluene and cyclohexane medium.

Com

plex

Toluene Cyclohexane Length(nm)

Width (nm)

Aspect ratio

Diameter to height ratio

Length(nm)

Width (nm)

Aspect ratio

Diameter to height ratio

1 270 –530

170 – 320

1.35 – 1.56

4.4 – 6.8 134 –333

117 –280

1.2 – 1.5

3.7 – 6.7

2 230 –450

140 –300

0.97 4.1 – 6.5 121 –312

105 –255

1.04 4.0 – 6.6

3 180 –350

122 –250

1.02 3.9 – 6.2 --- --- --- ---

and cyclohexane solutions (Fig. 7A & 7B). Again the aggregates formed from complex

2 in toluene and cyclohexane solution showed average diameter of 175 and 155 nm,

respectively (Fig. 7C & 7D). Whereas the diameter of the spherical structures obtained

after drying the toluene solution of complex 3 was 125 nm (Fig. 7E). These values are

lower than that of Dh valued obtained from DLS method. This is understandable if we

consider that DLS determines the hydrodynamic diameter, while TEM determines

radius of the collapsed dry aggregates.

100 nm50 nm

7 nm 7 nm

A B 50 nm20 nm20 nm

100 nm

7 nm

C D E

6.8 nm6.9 nm

20 nm

Fig. 7: TEM images of the cast films of complex 1 (A) in toluene, (B) in cyclohexane; complex 2 (C) in toluene, (D) in cyclohexane; and (E) complex 3 in toluene solution. Arrow showing the cell wall thickness.

The wall thickness of the reverse vesicle was found to be ~ 7 nm for all instances (Fig.

7A-E), which indicated that the reverse vesicles formed by the complexes 1, 2 and 3

either in toluene or in cyclohexane were of bilayer thick and these structures are

unilamelar. In order to check the dye encapsulation ability of these vesicles and

reverse vesicles, these aggregates of complex 1 were generated in presence of

Chapter 5B

241

rhodamine B (~ 106 M). Emission property of rhodamine B in vesicle/reverse vesicle

solution was compared with that of a pure rhodamine B solution having comparable

absorbance value at = 551 nm. Emission intensity recorded for rhodamin B with

vesicles in methanol-water medium and with reverse vesicle in toluene/cyclohexane

solution was found to be appreciably quenched as compared to that free rhodamine B

in water solution (Fig. 8).

600 650 7000.0

2.0x105

4.0x105

6.0x105

Inte

nsity

(cps

)

Wavelength (nm)600 650 700

0.0

5.0x106

1.0x107

1.5x107

2.0x107In

tens

ity (c

ps)

Wavelength (nm)600 650 700

0.0

5.0x106

1.0x107

1.5x107

Inte

nsity

(cps

)

Wavelength (nm)

Rhodamine B

Rhodamine BEncapsulatedin vesicle

Rhodamine B

Rhodamine BIn reversevesicle

Rhodamine BRhodamine BIn reversevesicle

A B C

Fig. 8: Emission spectra of absorption matched rhodamine B (A) in methanol-water (1:9, v/v) solution and in presence of vesicle; (B) in water and in presence of reverse vesicle in toluene solution; (C) in water and in presence of reverse vesicle in cyclohexane solution of complex 1.

This luminescence quenching was typically the result of the encapsulation of the dye

molecules inside these aggregates, which lead to an increase in local concentration

and thus the self quenching process (Fig. 8).19 Whereas, in reverse vesicles the effect

of quenching of the dye was much higher compared to the vesicles. Presumably, in

reverse vesicle the polar dye molecules were trapped in the reverse bilayer and more

organised upon interaction with the polar head groups, whereas in vesicles the dye

molecules were in the interior and were less organized.19 Similar dye encapsulation

behaviour and related photophysical studies also performed with complex 2 and 3 and

exactly similar results as complex 1 were obtained.

5B.4. Conclusion

In summary, a series of novel Ru(bpy)32+ based symmetrical amphiphilic complexes 1,

2, and 3 containing three C18, C14 and C10 chains, respectively, at equilateral positions

formed vesicles in polar aqueous-methanol (9:1; v/v) medium and reverse vesicles in

nonpolar medium like toluene and cyclohexane. These vesicles and reverse vesicles

Chapter 5B

242

were characterised by DLS, SLS, AFM and TEM studies. AFM and TEM confirmed the

spherical and hollow nature of these aggregates. Aggregate formation was also

confirmed by dye encapsulation and photophysical studies in respective solutions.

Chapter 5B

243

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