Synthetic Strategies for Hybrid Materials to Improve Properties for Optoelectronic Applications

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DOI: 10.1002/adfm.200701395 P
APER

Synthetic Strategies for Hybrid Materials to Improve Properties forOptoelectronic Applications**

By Olga Garcıa, Leoncio Garrido, Roberto Sastre, Angel Costela, and Inmaculada Garcıa-Moreno*

We report, for the first time to the best of our knowledge, a systematic study to relate the laser action from BODIPY dyes, doped

into monolithic hybrid matrices, with the synthetic protocols of the final materials prepared via sol-gel. To this aim, the influence

of both the hydrolysis time, increased in a controlled way, and the nature of the neutralization agent (pyridine, 3-amino-

propyltriethoxy-silane (APS), N-[3-(trimethoxysilyl)propyl]-ethylene diamine (TSPDA), and N1-[3-(trimethoxysilyl)propyl]-

diethylene triamine (TSPTA) on the laser action of PM567, incorporated into hybrid matrices based on copolymers of methyl

methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA), with methyltriethoxysilane (TRIEOS) as inorganic precursor,

was analyzed. The presence of the amine-modified silane TSPDA as neutralization agent, which is able at the same time to be

anchored to the inorganic network enhancing the inorganic-organic compatibility through the matrix interphase, and utilization

of hydrolysis times lower than 10 minutes, increased significantly the lasing efficiency and photostability of dye. The extension of

this study to the laser behavior of BODIPY dyes embedded in other different hybrid materials based on hydrolyzed-condensed

copolymers of MMA with 3-(trimethoxysilyl)propyl methacrylate (TMSPMA) in a 1/1 volumetric proportion, validates the

generalization of the above conclusions, which provide guides for the optimization of the synthesis of organic-inorganic hybrid

materials with optoelectronic innovative applications independently of their composition.

1. Introduction

The sol-gel chemistry has represented a real breakthrough in

the use of molecular chemistry for the design of new materials.

It allows the synthesis and development of organic-inorganic

hybrid matrices based on Si–O–Si networks, which are very

attractive materials for optical and optoelectronic applications,

mainly due to the relative high stability of the Si–C bond which,

in general, is not cleaved under sol-gel conditions and keeps the

organic group ‘‘alive’’.[1,2] Moreover, the quality of the Si–O–Si

network is excellent in terms of transparency and thermal and

chemical stability, resulting in stable materials with chemical

and physical properties well defined, which nevertheless can be

modified by a judicious choice of the organic and inorganic

components.[3–16]

One of the synthesis routes of these hybrid materials is based

on the simultaneous and ‘‘in situ’’ sol-gel process of the

inorganic part, catalyzed in an acid media, and the free radical

polymerization of the organic monomer part. These hybrid

materials exhibit attractive features and unique properties to

[*] Dr. I. Garcıa-Moreno, Dr. A. CostelaInstituto de Quımica-Fısica ‘‘Rocasolano’’, C.S.I.C.Serrano 119, 28006 Madrid (Spain)E-mail: [email protected]

Dr. O. Garcıa, Dr. L. Garrido, Prof. R. SastreInstituto de Ciencia y Tecnologıa de Polımeros, C.S.I.C.Juan de la Cierva 3, 28006 Madrid (Spain)

[**] This work was supported by Projects No MAT2004-04643-C03-01 andMAT2007-65778-C02-01 of the Spanish CICYT as well as by the ProjectNo 200660M025 of the Comunidad Autonoma de Madrid.

Adv. Funct. Mater. 2008, 18, 2017–2025 � 2008 WILEY-VCH Verlag

be used as hosts for organic dyes acting as active media in

solid-state dye lasers,[8–21] since they combine the excellent

thermal properties of inorganic glasses[22–25] with the excellent

optical properties and dye compatibility of polymeric materi-

als.[25–30]

Confirming the above specifications, when the laser dye

Rhodamine 6G (Rh6G) was incorporated into hybrid matrices

based on copolymers of methyl methacrylate (MMA) and 2-

hydroxyethyl-methacrylate (HEMA) as organic component,

and hydrolyzed-condensated methyltriethoxysilane (TRIEOS)

as inorganic counterpart, highly photostable laser action was

obtained, with no sign of degradation in the initial laser output

after 100,000 pump pulses, in the same position of the sample,

and at 10 Hz repetition rate.[15,16] This photostability was, to

the best of our knowledge, the highest achieved to this

date for organic, inorganic, and hybrid matrices doped with

this dye.

In the light of these promising results and in order to

improve the laser action of other organic dyes incorporated

into solid-state materials, over the past few years we have tried

the same approach with BODIPY dipyrromethene BF2

dyes.[12] However, these dyes lose their lasing ability under

the catalytic acid conditions employed in the sol-gel process,

what makes necessary the neutralization of the reaction media

before the addition of the monomer mixture with the dye

solution to the initial alkoxide precursor.

It is a well-established fact that control of the structure and

properties of the hybrid material is achieved by modifying the

composition, monomer functionality, chemical nature of the

organic and inorganic phases, size and morphology of these

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O. Garcıa, et al. / Synthetic Strategies for Hybrid Materials to . . .

2018

domains, as well as the nature of the interphase interactions.

However, attention has not been paid to the influence of the

synthesis conditions in relation to structure, properties and

applications of the so obtained final materials.

In this paper, we report for the first time a systematic study

on the influence that both the neutralization step and the time

of the hydrolysis reaction have on the laser behavior of two

BODIPY dyes incorporated into several hybrid organic-

inorganic materials, in order to define a general approach

that could be applicable to the optimization of the synthesis

routes of hybrid matrices with independence of compositions.

To this aim we tested the influence of different neutraliza-

tion agents such as pyridine, 3-amino-propyltriethoxy-silane

(APS), N-[3-(trimethoxysilyl)propyl-ethylene diamine (TSPDA),

and N1-[3-(trimethoxysilyl)propyl]-diethylene triamine (TSPTA),

(molecular structures shown in Fig. 1) on the final properties of the

photosensitized materials. Considering that the neutralization

Figure 1. Molecular structures of PM567 and PM597 dyes, monomers MMand TMSPMA, inorganic alkoxide TRIEOS and neutralization agents pyridTSPDA, and TSPTA.

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agent remains inside of the final structure of the material, the

last three amine-modified silanes were selected attending to

their character of ‘‘hybrid molecules’’ which allows their

anchorage to the inorganic network through their trimethox-

isilyl groups, thus enhancing the inorganic-organic compat-

ibility through the matrix interphase. In addition, the structure

of the three-dimensional inorganic network was modulated by

increasing, in a systematic and controlled way, the time of the

hydrolysis reaction in the sol-gel synthesis route.

2. Results and Discussion

2.1. Influence of the Neutralization Step

We first analyze the theoretical valoration acid/basic curves

to determine the optimum volume of pyridine, APS, TSPDA,

A, HEMAine, APS,

& Co. KGaA,

and TSPTA (133, 66, 30, and 15mL, respectively)

required to neutralize 25mL of HCl-37 vol % in

6 mL of H2O, which was the volume of acid added to

the total reaction volume in the synthesis route of

hybrid matrices that we had previously followed.[16]

Taking into account the complex and lengthy

synthesis process of hybrid materials, the dependence

on the neutralization agent of the PM567 emission

under UV light excitation was first evaluated in liquid

phase. To this aim the corresponding amount of each

amine was added to an acidulated ethanolic solution

of PM567 in a concentration of 1.5� 10�3M, and the

emission of the dye was analyzed. A solution of the

dye without HCl was used as reference.

As can be seen in the bottom of Figure 2, the

solutions neutralized with pyridine and TSPDA

keep the emission of the dye at the level of the

control solution, while the emission is lost in acid

medium or when the neutralization was carried out

with APS and TSPTA. To gain more insight, the

temporal evolution of the absorption and fluores-

cence spectra of each of the above solutions were

registered. The spectra of the dye solutions

neutralized with pyridine and TSPDA remained

unmodified after up to 6 days while the presence of

APS or TSPTA modified the absorption spectra of

the dye inducing, at the same time, a drastic

reduction of the fluorescence emission just after

48 hours (see Fig. 3).

Although no single cause is expected to fully

describe the mechanism at work, the above behavior

can be understood on the basis of the following

arguments. In the case of the amine-modified silane

APS; the analysis of its valoration curve (Fig. 4A),

theoretically determined, reveals that this amine

works as a buffer within the range corresponding to

the highest gradient in the dependence of the pH on

the amount of HCl added to the solution. Then,

minute variations, lower than 1mL, in the added

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Figure 2. Influence of the neutralization agent on the emission of dyePM567 (1.5� 10�3

M) in acidulated ethanolic solutions under Visible light(top) and UV light excitation (botton): A) only ethanol (pH¼ 6); B)ethanolþHCl (pH¼ 1); C) ethanolþHClþ 133mL pyridine (pH¼ 6);D) ethanolþHClþ 66mL of APS (pH¼ 6); E) ethanolþHClþ 30mLTSPDA (pH¼ 6), and F) ethanolþHClþ 15mL TSPTA (pH¼ 6).

igure 3. UV-VIS absorption (a,c) and fluorescence (b,d) spectra of PM567 (1.5� 10�3M in 6 mL of

Figure 4. Theorical valoration of pH curves of APS (A) and pyridine (B).

F
ethanolþ 25 mL of HCl) using as neutralization agents APS (a,b) and pyridine (c,d).
Adv. Funct. Mater. 2008, 18, 2017–2025 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

volume of acid induce strong mod-

ifications on the pH of the solution

(from 3 to 9), affecting drastically the

dye emission. On the other hand, the

valoration curve of pyridine (Fig. 4B)

proves that the volume added into

the dye solution and required to

reach the adequate values of pH is

within its regulation range. The

valoration curves of the amines

TSPDA and TSPTA could not be

theoretically evaluated because their

pKb values are unknown since, under

acid conditions, their trimethoxysilyl

groups can also undergo hydrolysis

and polycondensation processes,

inducing a decrease in the nominal

concentration of the base in the

medium. As the sol-gel process goes

forward, the presence of three amine

groups in each molecule of TSPTA

could induce a high local pH which

leads to a rapid polycondensation[31]

of the reactive silanol groups and

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Figure 5. Dependence of the lasing efficiency (as an average of the first 16pump pulses at 1 Hz) formulations under study and photostability of dyePM567 on both the weight proportion of TRIEOS added to the copolymerMMA/HEMA 1/1, and the neutralization agent: pyridine (solid bars) andTSPDA (dashed bars). Pump energy and repetition rate: 5.5mJ/pulse and10Hz, respectively.

0 20000 40000 60000 80000 1000000

20

40

60

80

100

Lase

r ou

tput

(%

)

Number of Pulses

A

B

Figure 6. Normalized laser output as a function of the number of pumppulses in the same position of the sample for dye PM567 incorporated intohybrid materials based on MMA/HEMA 1/1 v/v proportion with a 10wt %proportion of TRIEOS using as neutralizing agent A) TSPDA and B)pyridine.

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their subsequent precipitation, leaving an acid medium that

degrades the dye. For all the four neutralizing agents, pyridine,

APS, TSPDA and TSPTA, the local pH, determined one hour

after adding the reactives, was 6, 4, 7, and 4, respectively,

confirming the above asserts.

In the light of the above results, our next step was to analyze

the laser emission of the BODIPY PM567 dye, incorporated in

concentration of 1.5� 10�3M into hybrid matrices based on

copolymers of MMA/HEMA in a 1/1 volumetric proportion, as

organic component, and TRIEOS, as inorganic precursor, syn-

thesized using pyridine and TSPDA as neutralization agents.

In the synthesis of these hybrid matrices with increasing

amounts of TRIEOS, when adding pyridine to neutralize the

acidity of the medium, proportions of this alkoxide higher than

10 wt % resulted in both chemical degradation of the dye and

opacity of the final material. The strong volume contraction

induced by further increases of the inorganic content together

with the inability of pyridine to anchor to the inorganic net-

work could enhance the subsequent photo-oxidation and

exudation processes of this amine, reducing its neutralization

effectiveness and leaving a medium acid enough to impair

lasing emission and stability. On the other hand, the amine-

modified silane TSPDA allowed the synthesis of monoliths

with high inorganic content, up to 20 wt % proportion of

TRIEOS, keeping the adequate optical and mechanical

properties for this application.

Broadband laser emission with a pulse duration of �13 ns

FWHM and beam divergence of �5 mrad was obtained from

all the monolith materials under study. Laser emission maxima

were in the range of 560–564 nm, depending on the hybrid

composition, with oscillation bandwidths of 7–9 nm, typical of a

laser with many modes running simultaneously, due to the lack

of wavelength-selected elements in the cavity.

The lasing efficiencies of PM567 in the different hybrid

formulations under study, defined as the ratio between the

energy of the dye laser output and the energy of the pump laser

incident on the sample surface, are shown in Figure 5. For each

neutralization agent, the lasing efficiency does not seem to be

very dependent, within the experimental error, on the TRIEOS

content of the final matrix. For each weight proportion of

TRIEOS added to the monomeric mixture, the presence of

TSPDA enhances the lasing efficiencies of PM567 with respect

to those recorded in presence of pyridine. In addition the

efficiencies obtained in the presence of TSPDA are the highest

ones registered up to now for this dye incorporated into both

hybrid materials and silicon-containing organic matrices under

otherwise identical experimental conditions.[12,32]

The stability of the laser output as a function of the number

of pump pulses in the same position of the sample was studied

for the different materials at a repetition rate of 10 Hz and the

experimental data also collected in Figure 5. To clarify the

experimental behavior, the actual evolution of the laser output

with the number of pump pulses for two of the materials is

graphically represented in Figure 6. The lasing photostability

of PM567 dye resulted to be very dependent on the nature of

the neutralization agent. Once more, the amine-modified

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silane TSPDA enhances significantly the laser lifetime, since

the dye remains at up to 60% of its initial emission after 100,000

pump pulses in the same position of the sample and under the

demanding transversal pumping conditions used in this study.

The photostability exhibited by this system is one of the highest

achieved up to date for PM567 laser dye in inorganic and sol-

gel hybrid matrices without rotating or translating the medium

to distribute the thermal load over a large volume.[6,9,10,12,17,21]

The presence of pyridine reduces the laser lifetimes to values

even lower than those exhibited by PM567 embedded into pure

organic hosts, where the laser emission from the dye dissolved

in lineal and cross-linked matrices of MMA with different

co-monomers and pumped under identical experimental

conditions, remained up to 80% of the initial value after

45,000 pump pulses at 10 Hz repetition rate.[33]

These results confirm that the presence of TSPDA as neutra-

lization agent in the synthesis route of hybrid matrices based on

copolymers of MMA/HEMA 1/1 v/v proportion with increas-

ing amounts of TRIEOS optimizes the laser action of PM567.

The question that immediately arises is if this conclusion is

general and, consequently, holds in the synthesis of other

different hybrid materials and dyes for optical applications. To

assess this question, we have also analyzed the laser action of

other BODIPY dye, PM597, incorporated at 6� 10�4M, into

new hydrolyzed-condensed MMA/TMSPMA copolymers, and

synthesized using both pyridine and TSPDA as neutralizing

agents.

The behavior of these materials followed the pattern

previously observed: in the synthesis of the hybrids of MMA

with increasing amounts of TMSPMA as inorganic component,

if pyridine was added to neutralize the acidity of the medium,

volume proportions of TMSPMA monomer higher than 70%

could not be added without inducing chemical degradation of

the dye and opacity of the final material. On the other hand, the

amine TSPDA allowed synthesize monoliths of TMSPMA

homopolymer doped with PM597, with the adequate optical

and mechanical properties for this application.

A summary of the laser action of PM597 dye incorporated

into the different MMA/TMSPMA matrices and pumped

under identical experimental conditions as the samples

containing PM567 is presented in Table 1. Once again, the

Table 1. Laser parameters[a] of PM597 dye incorporated into differenthydrolyzed-condensed copolymers of MMA/TMSPMA with increasingvolumetric proportions using as neutralization agents pyridine and theamine-modified silane TSPDA.

Neutralization Agent Matrix material lmax [nm] Eff [%] I100,000 [%]

Pyridine MMA/TMSPMA 5/5 578 21 35

MMA/TMSPMA 4/6 577 18 45

MMA/TMSPMA 3/7 577 12 48

TSPDA MMA/TMSPMA 5/5 576 37 92

MMA/TMSPMA 3/7 577 34 95

MMA/TMSPMA 0/10 577 29 60

[a] lmax: peak of the laser emission; Eff: energy conversion efficiency; I100,000 (%):

intensity of the dye-laser output after 100,000 pump pulses in the same position of the

sample, referred to the initial intensity, I0.

Adv. Funct. Mater. 2008, 18, 2017–2025 � 2008 WILEY-VCH Verl

use of the amine-modified silane TSPDA as neutralization

agent increases significantly both the laser efficiency and

photostability of this dye.

The analysis by ESEM of the irradiated and non-irradiated

regions of the solid samples (Fig. 7) reveals that, for both dyes,

the pumping laser does not induce significant morphological

modifications in the surfaces. Consequently, the observed

decreases of the laser output as a function of the number of

pump pulses have to be directly related neither to the

photobleaching of the chromophores nor to structural damages

of the hybrid matrices caused by the incoming pump radiation.

The particles appearing randomly distributed over the sample

surfaces can be assigned to grinding and polishing abrasive

powders. They are not inorganic domains trapped inside the

polymer matrix since they disappeared from the ESEM images

of fresh fracture surfaces prepared by breaking the monolithic

samples immediately prior to the analysis. These last images

suggest very homogeneous, smooth, random, uniform, and

featureless surfaces, in agreement with the optical transpar-

ence shown by the samples.

2.2. Influence of the Time of the Hydrolysis Reaction

Different photosensitized hybrid materials were synthetized

based on PM567 incorporated into silicon-containing organic

matrices obtained by ‘‘in situ’’ and simultaneous hydrolysis-

condensation of methoxy groups of TMSPMA during their free

radical bulk polymerization with MMA, in 1/1 volumetric

proportions. Since, once again, the sol-gel process of the

alkoxide TMSPMA was catalyzed in an acid medium where

hydrolysis reactions are favored, in order to decrease the Hþ

concentration to pH> 6, 133mL of pyridine or 30mL TSPDA

were added to the solutions before the addition of PM567

dissolved in MMA. Taking into account that a basic catalysis in

a sol-gel process leads to polycondensation reactions between

the hydrolyzed groups, the neutralization step finished the

hydrolysis reaction. The time duration of this reaction was

increased, in a systematic and controlled way, from 1 minute,

minimum time required to overcome the initial cloud caused by

the addition of the acid to the reaction medium, to 120 minutes.

A summary of the laser action of PM567 dye incorporated

into the different hybrid matrices pumping under otherwise

identical experimental conditions are presented in Table 2. The

experimental results reveal a clear dependence of the laser

behavior of PM567 on the duration of the hydrolysis reaction.

This dependence was especially noticeable in the dye

photostability behavior, the most relevant parameter in the

development of solid-state dye lasers.

Once again, for all and each one of the hydrolysis time, the

presence of TSPDA as neutralizating agent optimizes the laser

action of the dye with respect to that induced in matrices

synthetized using pyridine. In each series, the laser photo-

stability first increases with the duration of the hydrolysis

reaction, reaching a maximum for relatively short times of

hydrolysis, within 5 and 10 minutes, for pyridine and TSPDA,

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Figure 7. ESEM photographs of the morphology of: (a) a polished surface of irradiated at 532 nm of alaser sample and (b) a fresh fracture of non-irradiated areas surface prepared by breaking a monolithiclaser sample based on PM567 incorporated into matrices of MMA/HEMA 1/1 v/v containing 5wt %proportion of TRIEOS.

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respectively. The highest photostability was obtained with the

matrix incorporating TSPDA, where the laser emission

remains at up to 85% of its initial value after 100,000 pump

pulses. Longer times of hydrolysis reaction result in a drastic

reduction of the lasing photostability: dye doped matrices

synthesized with 120 minutes as hydrolysis time stop emitting

laser radiation after only 35,000 pump pulses when pyridine

was the neutralization agent or drops to 36% of its initial value

after 100,000 pulses when the neutralization agent was

TSPDA.

In order to establish a possible relationship between

structure of the hybrid materials and their laser behavior as

a function of the hydrolysis time in the synthesis route, the

Table 2. Laser parameters[a] of PM567 dye incorporated into differenthydrolyzed-condensed copolymers of MMA/TMSPMA 1/1 synthesizedwith increasing hydrolysis times using as neutralization agents pyridineand the amine-modified silane TSPDA.

Neutralization agent Hydrolysis Time [min] lmax [nm] Eff [%] I60,000 [%] I100,000 [%]

Pyridine 1 564 30 22

5 564 31 33

10 564 31 16

20 563 32 11

30 564 31 15

60 563 32 6

120 563 31 0

TSPDA 1 562 45 44

5 561 46 54

10 562 45 86

20 565 46 56

30 568 48 64

60 567 46 44

120 567 41 36

[a] lmax: peak of the laser emission; Eff: energy conversion efficiency; In (%): intensity

of the dye-laser output after n pump pulses in the same position of the sample,

referred to the initial intensity, I0.

Figure 8. 29Si CP-MAS highcorresponding to the COP(Mthe presence of TSPDA as nranging from 1 to 120 minuaverages, a 5 s recycle delaresonances at �42, �50, �50, 1, 2, and 3 siloxane bonds

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structure of the materials was ana-

lyzed by NMR in solid state. The 29Si

NMR corresponding spectra are shown

in Figure 8. Four signals at �42, �50,

�59, and �67 ppm were observed for

each hybrid matrix assayed and their

assignment is based on previous studies

on silica gel.[7] Using the symbol Tn to

design the number of silicon atoms

with n siloxane bonds (n¼ 0, 1, 2, or 3),

the peakT0 at�42 ppm corresponds to

non-condensated silicon atoms repre-

sented as –CH2Si(OH)3 while the peak

T1 at �50 ppm corresponds to a silicon

atom bonded to two –OH groups and

one siloxane group, represented by

–CH2Si(OH)2O–. The peak T2 at

�59 ppm is related to the presence

of a two siloxane groups bonded to

the inorganic structure of type

–CH2SiOHO2–, and finally, the T3 peak at �67 ppm is associated

with silicon bonded to three siloxane groups, such as –CH2SiO3–.

Figure 9 shows the quantitative relationship between the

signals, comparable under the same experimental conditions.

On the basis of this relationship, it can be concluded that, for a

given hydrolysis time the crosslinking degree in inorganic

networks reached with TSPDA as neutralization agent is

higher than that determined by the presence of pyridine. In

fact, in the hybrid materials obtained with pyridine, the content

of T3 species decreases with the hydrolysis time from ca. 3% to

-power proton decoupling NMR spectrumMA/TMSPMA 1/1) hybrids synthesized in

eutralization agent and with hydrolysis timetes. The spectra were obtained with 6,000y, and a line broadening of 80 Hz. The9, and �67 ppm correspond to silicon withto the silicon of interest.

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Figure 9. Quantitative relationship of 29Si-CP-MAS signals, comparableunder the same experimental conditions, for the species T0, T1, T2, and T3presents in COP (MMA/TMSPMA 1/1) hybrids samples synthesized in thepresence of TSPDA (a) and pyridine (b) as a function of the hydrolysis time,where t0 is theminimum time required to overcome the initial cloud causedby the addition of the acid to the reaction medium and was estimated in 1minute.

0.5%, values that are at least one order of magnitude lower

than those determined in the presence of TSPDA. In addition,

in the pyridine series the relative amounts of non-croslinking

species such as T1 are much higher than the corresponding

percentages in the presence of TSPDA. Furthermore, for each

one of these neutralization agents the increase of the hydrolysis

time leads to a slight but progressive decrease of inorganic

crosslinking since the relative amount of (T0þT1) groups

grows up to be ca. 70% and 83% of inorganic network, for

TSPDA and pyridine, respectively.

The above structural results indicate clearly the complexity

of the mechanisms involved in the laser action of dyes

incorporated into hybrid materials and the specificity of the

dye/matrix combination that optimizes the laser operation.

From these results, a pattern arises allowing to establish a

direct relationship between the structure of the hybrids and

their laser behavior: inorganic networks dominated for cross-

linking degrees, defined as (ToþT1)/(T2þT3), near 57/43

represent the best compromise between the enhancement of

thermal dissipation in the matrix and the decrease of the

photochemical degradation of the dye in matrices containing

TSPDA. Variations in this proportion of the substituted sili-

cates impair the laser performance, reflecting that the rigidity

of the inorganic network is of the uppermost importance in


optimizing the laser action of a given dye. The presence of

pyridine increases, for any of the hydrolysis times, this struc-

tural proportion to at least ca. 70/30, and worsens drastically

the laser action of the dye.

Trying to better assess the influence of the synthetic protocol

on the laser photostability of PM567 and taking into account

that the chemical structure of the matrix material surrounding

dye molecules is modified during the synthesis reaction, ana-

lysis to evaluate variations on the concentration of –OH groups

as the hydrolysis time increase, were carried out. It would be

expected that higher hydrolysis times, before neutralization

with TSPDA, would render more hydroxyl groups in the

medium.

To confirm this hypothesis, the hydrolysis reaction was

followed in real time by conventional liquid nuclear magnetic

resonance (NMR). To this aim, the same monomer mixture

selected in the synthesis of solid samples (MMA/TMSPMA

1/1) was placed into a capillary contained D2O as internal

reference. In a next step, water-catalyst mixture was added in

amount equivalent to that of TMSPMA, preserving the experi-

mental conditions selected in the synthesis processes, and the1H-NMR spectra were registered at different hydrolysis times.

The hydroxyl group proportion was quantified referring to the

integrated signal corresponding to a monomer sample without

water-catalyst but with the theoretical proportions of MeOH,

assuming total hydrolysis of the methoxyl groups of TMSPMA

monomer. Taking this signal as reference value of 100%, the

integrated signals at the different hydrolysis times can be

correlated. The proportion of the hydroxyl groups increases

with the hydrolysis time but grows below than it was expected

since no total conversion was reached even after two hours of

reaction. In fact, the –OH proportion varies from just a 36%,

corresponding to the minimum time of hydrolysis, to a 57%, for

120 minutes.

3. Conclusions

All the results obtained in the systematic study presented in

this work confirm the initial hypothesis about the relevance of

the synthetic protocol on the design and development of

organic-inorganic hybrid materials with potential innovative

applications, especially in the optoelectronic fields. The control

of both the neutralization reaction and the hydrolysis time

results to be a determinant factor in order to enhance the laser

action of dyes incorporated to hybrid matrices.

The presence of amine-modified silanes such as TSPDA

allows their anchorage to the inorganic network enhancing the

inorganic-organic compatibility through the matrix interphase

and, consequently, the laser action of BODIPY dyes

incorporated into these hybrid materials with independence

on their compositions. However, pyridine, neutralization agent

common in the synthesis routes of hybrid materials, impairs the

lasing behavior leading to values of efficiencies and stabilities

lower than those registered when the dyes were incorporated

in pure organic matrices. In this case, the thermal benefits


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provided by the presence of silicon atoms incorporated into the

material are mitigated by the composition and the less cross-

linked structures of the inorganic networks induced by pyridine

compared to those determined by the amine-modified silanes.

The duration of the hydrolysis reaction determines, to a

significant extent, the properties of the final hybrid materials,

contrary to assertions found in the literature based on experi-

ments where this parameter was selected without systematic

criteria. Furthermore, the hydrolysis time that optimizes the

laser action is exclusive to the nature of the neutralization

agent although it seems to be always in a short range, lower

than 10 minutes. The variation of the hydrolysis time has a

higher influence on the structure of the inorganic network of

the hybrid materials than on its composition. In fact, the

proportion of the hydroxyl groups generated by increasing the

hydrolysis time from 1 to 120 minutes varies from ca. 36% to

57%, contrary to the general belief that one hour was time enough

to induce the total hydrolysis of the corresponding alcoxide.

The influence that the synthetic strategies has on the laser

action of different dyes incorporated into several hybrid

matrices indicates that the conclusions of the present work are

of general application in the synthesis of organic-inorganic

hybrid materials with optoelectronic innovative applications

independently of both the structure of the dye and the

composition of the matrix. Thus, the results obtained in the

present work, taking into account the quasi-infinity possibilities

of polymer-silica materials than can be prepared via sol-gel

process, provide a basis to redesign their synthesis.

4. Experimental

The BODIPY dyes commercially named PM567 and PM597(laser grade from Exciton) were used as received. The organicmonomers MMA, HEMA and 3-(trimethoxysilyl)propyl methacrylate(TMSPMA) (Aldrich, 99% purity) were distilled under reducedpressure before use. TRIEOS, from ABCR, was used as received. Thecorresponding molecular structures are shown in Figure 1. All theamines were supplied by Aldrich (97% purity) and used as received,except pyridine that was distilled under reduced pressure and kept dryover molecular sieves of 4 A.

The syntheses of new organic-inorganic hybrid matrices as well asthe silicon-modified organic materials were carried out following theprocedures previously described [15,16,34].

The solid monolith laser samples were cast in a cylindrical shape,forming rods of 10 mm diameter and 10 mm length. A cut wasmade parallel to the axis of cylinder to obtain a lateral flat surface of�6� 10 mm. These surfaces as well as the ends of the laser rods wereprepared for lasing experiments by using a grinding and polishingmachine (Phoenix Beta 4000, Buehler) until optical-grade finished. Theplanar grinding stage was carried out with a Texmet 1000 sand paper(Buehler) using a diamond polishing compound of 6mm as an abrasivein mineral oil as a lubricant. The final polishing stage was realized witha G-Tuch Microcloth (Buehler), using a cloth disk Mastertex (Buehler)with diamond of 1mm in mineral oil as an abrasive type.

The hybrid rods were transversely pumped at 532 nm with aNd:YAG laser (Monocrom OPL-10) at a repetition rate of 10 Hz.Details of the experimental set-up can be found elsewhere [16].

UV/VIS absorption and fluorescence spectra were recorded on aPerkin-Elmer Lambda-35 spectrophotometer and on Perkin-ElmerLS-50B spectrofluorimeter, respectively.

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH

29Si NMR experiments were performed in a Bruker AvanceTM 400spectrometer (Bruker Analytik GmbH, Karlsruhe, Germany),equipped with a Bruker UltraShieldTM 9.4-T (29Si resonance frequencyof 79.49 MHz), 8.9-cm vertical-bore superconducting magnet. CP-MASNMR spectra were acquired at ambient temperature by using astandard Bruker broad-band MAS probe. Representative sampleswere grounded and packed in 4-mm zirconia rotors, sealed withKel-FTM caps and spun at 5 kHz. The 908 pulse width was 3.5–4.5 msand, in all cases, high-power proton decoupling was used. Allfree-induction decays were subjected to standard Fourier transforma-tion and phasing. The chemical shifts were externally referenced toTMS.

The 29Si CP-MAS NMR spectra were obtained with 4-ms CPcontact time, 5-s recycle delay, 6000 averages and 75 Hz linebroadening. The spectra were deconvoluted by using Gaussian/Lorentzian fits, in terms of Ti where i¼ 1, 2, 3, corresponding to thenumber of siloxane bridges bonded to the silicon atom of interest. TheNMR spectra were evaluated with the software package XWIN NMRprovided by Bruker.

The morphology of the surfaces was examined by environmentalscanning electron microscopy (ESEM) with a Philips model XL 30microscope without any previous preparation of the samples.

Received: November 29, 2007Published online: June 25, 2008

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Synthetic Strategies for Hybrid Materials to Improve Properties for Optoelectronic Applications