Chemical Physics Letters 390 (2004) 427–432

www.elsevier.com/locate/cplett

Time resolved resonance Raman studies on triplet excited stateof 2-methoxy-naphthalene by photo-sensitization

Himansu Mohapatra a,1, Siva Umapathy a,*

a Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

Received 10 April 2003; in final form 9 March 2004

Available online 6 May 2004

Abstract

We report on the resonance Raman spectrum of the triplet excited state of 2-methoxy-naphthalene (3ROMe) generated by

benzophenone (BP) triplet sensitization. A comparison of the time resolved resonance Raman (TR3) spectra of 3ROMe obtained by

energy transfer with that of the spectrum obtained in the absence of BP reveals no change in vibrational frequencies due to weak

charge transfer interaction, as expected for a triplet exciplex. It is observed from our computational studies and the experimental

data that the unpaired electron in the * orbital of triplet state is more localized on the aromatic ring attached to the methoxy group.

� 2004 Published by Elsevier B.V.

1. Introduction

Photochemistry and photophysics of naphthalene

and its derivatives have been the subject of extensive

investigation by many scientists [1–11]. In particular,

transient absorption studies on the triplet excited state

of substituted naphthalenes, produced by triplet sensi-tization of carbonyl compounds, have revealed mecha-

nistic details of the photoreduction processes [1–4]. The

nature of the substitutent has been found to influence

the excited state properties, such as, S1–S0 transition

dipole movement, radiative lifetime and electronic state

mixing. In recent years, number of studies has been re-

ported on methoxy naphthalene in order to gain insight

into the effect of methoxy substituent on the intramo-lecular energetics and dynamics, and on the vibronic

structure of S1 $ S0 electronic transition [5–10]. How-

ever, little is known on the molecular structural details

of the excited state or the intermediates involved during

the photosensitization process.

* Corresponding author. Fax: +91-80-23601552/3600803.

E-mail address: umapathy@ipc.iisc.ernet.in (S. Umapathy).1 Swarnajayanti Fellow.

0009-2614/$ - see front matter � 2004 Published by Elsevier B.V.

doi:10.1016/j.cplett.2004.03.027

In the present study, we report on the photosensiti-

zation of methoxy naphthalene by benzophenone in-

vestigated by time resolved resonance Raman (TR3)

spectroscopy and density functional theoretical (DFT)

computation. In particular, we have analyzed the results

with a view to understanding the effect of substituent on

the structure and TR3 vibrational spectra of 2-methoxy-napthalene (ROMe) in the lowest triplet excited state in

comparison to the well known naphthalene triplet. It is

interesting to note that the oxygen atom of the substit-

utent methoxy group forms a sigma bond to the

aromatic ring, which exerts an inductive electron with-

drawing effect. On the other hand, the oxygen atom has

a nonbonding valence electron pair, thus, electron den-

sity is expected to flow into the aromatic ring by p–pconjugation (resonance). Such effects, imparted by the

substituent are expected to influence the structure in the

triplet excited state.

Transient absorption studies carried out by Shizuka

and co-workers show that 3ROMe is quenched by

ground state BP, through an intermidiate proposed to

be the triplet exciplex. In acidic medium the proton in-

duces electron transfer reaction in the exciplex to givecation radical of 2-methoxy-naphthalene and ketyl

radical of benzophenone [1]. In this Letter, (a) the effect

of weak charge transfer interaction in the exciplex has

428 H. Mohapatra, S. Umapathy / Chemical Physics Letters 390 (2004) 427–432

been investigated in the absence of proton and (b) the

effect of substituent on the structure of triplet naph-

thalene and also on TR3 spectra have been analyzed

from density functional calculation.

2. Experimental

The experimental apparatus and procedures used for

the TR3 spectroscopy have been described in detail

previously [23]. Briefly the third (355 nm) and fourth

harmonic (266 nm) output of an Nd-YAG laser (DCR-

11) was used as the photoexcitation pump source. Theprobe wavelength at 416 nm was obtained from a home

made H2 Raman shifter. The laser pulses were about

8–10 ns in temporal width and energies of about 1.0 and

0.5 mJ, respectively, for the pump and probe. The delay

between the laser pulses was provided by the standard

DG535 delay generator. A SPEX 1404 double mono-

chromator with two 600 groves grating was used to

disperse the scattered light. A liquid nitrogen cooledCCD (Princeton instrument) with 576� 376 pixels was

used as the multichannel detector. Benzophenone and

2-methoxy-napthalene were purchased from Aldrich.

The sample solution was circulated through a quartz

capillary using a micropump at a rate of 10 ml/s. In

order to avoid possible accumulation of photoproducts,

samples were replaced regularly. The solvent used was of

analytical grade and distilled before use. The recordedRaman spectra were calibrated using known solvent

bands as reference and the spectral resolution is esti-

mated as �5 cm�1.

3. Computational methods

DFT calculations were performed with GAUSSIANAUSSIAN

94/DFT program on IBM RS 6000 system employing

the standard 6-31G(d) basis set. The calculations for the

triplet state of ROMe were carried out using open shell

wavefunctions. The exchange functional used in the

DFT calculations is the gradient corrected functional of

Becke (B) [12] and the correlation functional is of Lee,

Yang and Paar (LYP) [13,14]. The three hybrid HF/DF

method used is B3LYP [15,16]. A complete geometryoptimization was carried out in all the cases employing

Bernys optimization algorithm. The vibrational fre-

quencies and the corresponding normal modes were

evaluated for the optimized geometries using analytical

differentiation algorithm contained within the program.

4. Results and discussion

It has been reported from transient absorption

studies that 3ROMe can be generated efficiently

(/ET ¼ 0:74) by triplet energy transfer from 3BP to

ROMe [1]. In the first instance, we have investigated

such an energy transfer process in acetonitrile using

TR3 spectroscopy. The TR3 spectra, at various time

delays for ROMe–BP system in acetonitrile usingpump and probe wavelengths of 355 and 416 nm,

respectively, are shown in Fig. 1. The pump laser

generates the 3BP and the probe laser wavelength

corresponds to the transient absorption maximum of3ROMe [1]. The solvent and the ground state bands

have been subtracted from the pump + probe spectra

and the spectra at various time delays have been

normalized with respect to acetonitrile band at 1375cm�1. The TR3 spectra are akin to the triplet state

spectra of parent naphthalene [11,17] and 2-bromo-

naphthalene [18]. It consists of one strong unsym-

metrical band at 1345 cm�1 and a weak band at 1570

cm�1. The 1345 cm�1 band has been resolved into two

bands, one at 1325 cm�1 and the other at 1345 cm�1,

by reducing the slit width of the spectrometer, which

is shown as inset in Fig. 1. The same experiment,which was carried out independently, each for ROMe

and BP in acetonitrile under the same conditions, does

not show Raman bands from any transient species.

This clearly suggests that the TR3 bands observed for

the acetonitrile solution containing mixture of ROMe

and BP are from 3ROMe and it originates due to

triplet energy transfer from 3BP to ROMe. The TR3

spectra of (a) ROMe–BP in acetonitrile; (b) BP inacetonitrile; (c) ROMe in acetonitrile have been

compared in Fig. 2. The solvent and ground state

bands have been designated by ‘*’ and excited state

band are given as ‘E’.

The results of transient absorption studies have

shown that the 3ROMe formed by triplet-triplet en-

ergy transfer is quenched by BP due to exciplex for-

mation and the equilibrium constant for the formationof exciplex has been reported to be 4.0 M�1 [1]. Thus,

it is plausible that the TR3 bands in Fig. 1 may also

originate from both the 3ROMe vibrations of the

triplet exciplex and/or the free 3ROMe, in different

proportions. Further, it is reasonable to expect that

structural changes in 3ROMe induced by weak charge

transfer in the exciplex may be reflected in the ob-

served TR3 spectra. That is, the TR3 spectra will bedifferent from the pure triplet state spectrum of the

triplet ROMe in terms of its position and width of the

band. Such a possibility can be clarified by comparing

the TR3 spectrum of free 3ROMe (generated by direct

photo-excitation) with that obtained by triplet energy

transfer from BP. The TR3 spectra of free 3ROMe in

the absence of BP can be recorded by changing the

pump wavelength to the ground state absorptionmaximum of ROMe. Fig. 3 shows the (pump+probe–

pump only–probe only) TR3 spectra of ROMe in

acetonitrile using 266 nm pump and 416 nm probe

1200 1300 1400 1500 1600 1700

(g)

(f)

(e)

(d)

(c)

(b)

(a)

Inte

nsity

in a

rb.u

nits

Raman shift in cm-1

1100 1200 1300 1400 1500 1600 1700 1800

1570 cm-1

1325 cm-1

1345 cm-1

(b)

(a)

Inte

nsity

in a

rb.u

nits

Raman shift in cm-1

Fig. 1. TR3 spectra of ROMe (3� 10�2 M)–BP (6.7� 10�2 M) system

in acetonitrile obtained at various delay time. Pump only and probe

only spectra are subtracted from each of the spectra (pump laser 355

nm probe laser 416 nm): (a) 15 ns; (b) 35 ns; (c) 55 ns; (d) 75 ns; (e) 105

ns; (f) 135 ns; (g) 175 ns. Inset: TR3 spectra of ROMe–BP system in

acetonitrile at a reduced slit width of the spectrometer. (a) 10 ns; (b)

110 ns.

1200 1300 1400 1500 1600 1700

(c)

(b)

(a)

Inte

nsity

in a

rb.u

nits

Raman shift in cm-1

E

*

**

*

*

*

*

*

*

*

E

Fig. 2. TR3 spectra at 40 ns (pump laser 355 nm probe laser 416 nm):

(a) ROMe (3� 10�2 M)–BP (6.7� 10�2 M) in acetonitrile; (b) BP

(6.7� 10�2 M) in acetonitrile; (c) ROMe (3� 10�2 M) in acetonitrile.

H. Mohapatra, S. Umapathy / Chemical Physics Letters 390 (2004) 427–432 429

wavelengths. We have observed fluorescence at the

pump wavelength, which masks the Raman bands in

the pump+probe spectrum. Because of this, the effi-

ciency of inter system crossing to the triplet state

decreases and the TR3 bands of 3ROMe are notably

weak. Close examination of these TR3 bands indicates

no observable change in their spectral band positions

or width with respect to the TR3 bands of 3ROMe

obtained by triplet energy transfer from BP. So weconclude that the extent of charge transfer from

ROMe to BP in the triplet exciplex might be very

weak or exciplex is not an intermediate. However,

considering the results observed here and from the

conclusions of the transient absorption studies, the

former seems more plausible.

Table 1

Structural parameters of the ground state is calculated using B3LYP/

6-31G(d) and triplet state of 2-methoxy-naphthalene (3ROMe) have

been calculated using UB3LYP/6-31G(d) methods

Parameter Ground statea Ground state Triplet state

Bondlength (�A)

C1–C2 1.374 1.378 1.431

C7–C8 1.378 1.380 1.455

C9–C10 1.364 1.371 1.433

C3–C4 1.375 1.378 1.438

C2–C3 1.414 1.415 1.368

C8–C9 1.422 1.423 1.369

C6–C7 1.427 1.423 1.408

C4–C5 1.419 1.419 1.400

C6–C1 1.424 1.421 1.410

C5–C10 1.426 1.423 1.421

C5–C6 1.423 1.432 1.451

C8–O17 1.375 1.365 1.358

aX-ray diffraction value [22].

Fig. 4. (a) Numbering of the atoms of 2-methoxy-naphthalene (ROMe)

and (b) highest occupied (MO) of 3ROMe.

1200 1300 1400 1500 1600 1700 1800

(c)

(b)

(a)

Inte

nsity

in a

rb.u

nits

Raman shift in cm-1

Fig. 3. TR3 spectra of ROMe (3� 10�2 M) in acetonitrile obtained at

various delay (pump laser 266 nm, probe laser 416 nm): (a) 20 ns; (b)

30 ns; (c) 50 ns; (d) 120 ns; (e) 220 ns.

430 H. Mohapatra, S. Umapathy / Chemical Physics Letters 390 (2004) 427–432

5. Structure and vibrational assignment

The optimized geometrical parameters corresponding

to Cs symmetry of ROMe ground state and triplet ex-

cited state as obtained by B3LYP/6-31G(d) are given in

Table 1. The numbering of atoms is as shown in Fig. 4

(a). The ground state of ROMe is known to exist in two

conformers. These conformers have been tentativelydifferentiated into cis and trans configuration of the

methoxy group relative to the long axis of the naph-

thalene subunit [19]. It is known from ab-initio calcu-

lations that the cis structure for the ground state

corresponds to the global minimum in the whole po-

tential energy surface [20]. Hence in our calculations, the

geometry optimizations for the ground state of ROMe

Table 2

DFT vibrational frequencies (cm�1) of 2-methoxy-naphthalene triplet

state as obtained by (UB3LYP/6-31G(d))

Symmetry Calc. Expt. PED %

a0 1022 CC (28), OC (32), CH(29)

1046 CC (35), dCH (41), dCCC (14)

1101 CC (36), dCH (56)

1150 CC (15), dCH (53)

1154 dCH (78)

1176 CH (23), dmethylCH (44)

1234 CC (20), CO (25), dCH (26)

1245 CC (15), dCH (60), dCCC (15)

1306 CC (31), dCH (58)

1337 1325 CC (35), dCH (57)

1367 1345 CC (46), dCH (45)

1388 CC (23), dCH (71)

1419 CC (36), dCH (52)

1427 CC (21), dCH (54), dmethylCH(18)

1439 CC (23), dCH (52), dmethylCH (18)

1451 CC (59), dCH (31)

1471 dmethylCH (31)

1510 CC (43), dCH (39)

1583 1570 CC (55), dCH (29)

UB3LYP/6-31G(d) frequencies scaled by 0.96.

H. Mohapatra, S. Umapathy / Chemical Physics Letters 390 (2004) 427–432 431

have been performed at the cis configuration. Interest-

ingly, in the case of the triplet excited state, the opti-

mization has been performed both for cis and trans

geometries and the potential energy for the trans con-

figuration is found to be 1180 cm�1 less than the cis

configuration. Hence, for the triplet excited state of

ROMe, we have employed the optimized geometry

corresponding to the trans configuration for frequency

calculations.

The computed optimized geometrical parameters

for the ground state of ROMe are similar to those

obtained from X-ray diffraction data [22]. In the

triplet excited state, the trend in the C–C bond lengthchanges with respect to the ground state show that,

the unpaired electron occupying the p� orbital is more

localized on the aromatic ring attached to the meth-

oxy group, which may be attributed to the effect of

substitutent. But the overall bond length changes be-

tween ground and the triplet state are consistent with

the changes induced by singly occupied antibonding p�

electron in the triplet state of naphthalene [21]. The pelectron density distribution of the highest occupied

M.O (shown in Fig. 4 (b)) in 3ROMe is also consis-

tent with these changes.

The vibrational frequency evaluated at Cs optimized

geometry for 3ROMe consists of 60 vibrational modes

out of which 40 are of a0 (totally symmetric modes) and

20 are of a00 (nontotally symmetric modes). The experi-

mental TR3 spectral bands are compared with the cal-culated vibrational frequencies of the totally symmetric

modes, since these are expected to be intense in the

resonance Raman spectrum. The calculated vibrational

frequencies (scaled by 0.96 [11]) and the potential energy

distribution along with the observed experimental fre-

quencies of 3ROMe are given in Table 2. The TR3

spectra of 3ROMe have been observed to be similar to

the triplet state of naphthalene and further, the struc-tural parameters obtained from calculation have shown

that the structure of naphthalene moiety of 3ROMe is

very close to that of the parent triplet state of naph-

thalene. Hence we presume that the most enhanced TR3

band at 1345 cm�1 is characteristic of the totally sym-

metric vibrations of the parent naphthalene molecule.

Based on the results obtained by Nakata et al. [11], the

m5a mode of the triplet state of naphthalene at 1354 cm�1

has been assigned to the intense band observed at 1336

cm�1 which differs from the calculated value by 18 cm�1.

Further, we have assigned the most intense band of3ROMe at 1345 cm�1 to the calculated one at 1367

cm�1, which differs by almost the same value. Similar

assignments have also been reported for the intense

resonance Raman band of triplet state of 2-bromo-

naphthalene. The bands at 1570 and 1325 cm�1 havebeen tentatively assigned to the calculated vibrational

frequencies at 1583 and 1337 cm�1, respectively. The

perturbation imposed by the methoxy group on the

structure and nature of TR3 spectra is quite weak. It has

also been observed from the structural parameters ob-

tained from DFT calculation, confirming similar nature

of the triplet state of ROMe to that of naphthalene.

6. Conclusion

TR3 spectroscopy combined with density functional

theory have been used to study the structure and vi-

brational spectra of 3ROMe generated by triplet sensi-

tization of BP. Comparison of TR3 spectra of 3ROMe

in presence of BP with those obtained in the absence

have shown that the interaction between ROMe and BP

in the triplet exciplex is very weak. The structural pa-

rameters obtained from DFT calculation have shownthat the effect of methoxy group results in localization

of the antibonding p� electron density on the ring

connected to the methoxy group. The experimental

TR3 bands of ROMe have been assigned to the calcu-

lated vibrational frequency obtained from DFT com-

putation. The TR3 spectra and structure of ROMe have

shown that the overall structure and nature of TR3

spectra are very similar to that of the triplet state ofnaphthalene.

Acknowledgements

The authors thank the Department of Science and

Technology (DST) and the Council of Scientific and

Industrial Research (CSIR) for financial support.

432 H. Mohapatra, S. Umapathy / Chemical Physics Letters 390 (2004) 427–432

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