TWO-PHOTON EXCITATION, HYPER-RAMAN SCATTERING, AND EXCITED STATE DECAY PROCESSES FOR POLYENES IN FREE JET EXPANSIONS

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TWO-PHOTON EXCITATION, HYPER-RAMAN

SCATTERING, AND EXCITED STATE DECAY

PROCESSES FOR POLYENES IN FREE JET

EXPANSIONS

J. Horwitz, B. Kohler, T. Spiglanin

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C7, suppldment au nO1O, Tome 46, octobre 1985 _{page C7-381}

TWO-PHOTON EXCITATION, HYPER-RAMAN SCATTERING, AND EXCITED STATE DECAY PROCESSES FOR POLYENES IN FREE JET EXPANSIONS

J.S. Horwitz, B.E. ~ohler' and T.A. Spiglanin

Chemistry Department, WesZeyan University, Middletown, CT 06457, U.S.A.

Abstract

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High resolution emission spectra, I-photon and 2- photon excitation spectra, vibronic level decay times and satu- ration behavior have been measured for the polyenes diphenylbutadiene and diphenylhexatriene seeded in supersonic helium expansions. For isolated diphenylbutadiene hyper-Raman scattering enhanced by 2-photon resonances has also been observed. Taken together these data provide a detailed picture of the nature of the most probable intramolecular excited state decay processes as a function of vibronic enerqy for these flexible molecules.

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INTRODUCTION

The electronic structure of conjugated linear polyenes is fundamental to understanding more complicated systems ranging from the visual pig- ment rhodopsin to conjugated polymers such as polyacetylene and the polydiacetylenes. It is reasonable to expect that the condensed phase photophysical and photochemical behavior of these molecules may be described in terms of intrinsic molecular potentials and, perhaps, the modification of these potentials by intermolecular interaction. How- ever, while much is known about the relevant potential energy surfaces of many polyenes in the condensed phase [ I ] , the role of intermolecular interactions in determining these potentials has been hard to eva- luate because of the lack of detailed information on the isolated molecules. Accordingly, we have started to measure optical spectra for

linear polyenes seeded in free jet expansions [2-81.

This paper summarizes some of the data that has been obtained for the symmetrically substituted polyenes diphenylbutadiene [3,6,8] and diphenylhexatriene [4,7] seeded in supersonic expansions of helium. Under our experimental conditions, highly structured spectra are observed. From the fluorescence spectra, I-photon and 2-photon fluorescence excitation spectra, and the time dependence of the total emission it is clear that: 1) 2 1 ~ g is the lowest energy excited singlet state for both molecules. 2) With increasing vibrational excitation in the excited electronic state there is increasingly rapid +at present : Chemistry Department, University of California, Riverside, CA 92521-0403, U.S.A.

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C7-382 JOURNAL DE PHYSIQUE

dephasing from the state initially prepared by the photon. 3) In diphenylbutadiene an yxtremely efficient radiationless decay channel appears at 1300 cm- above the 2 l ~ 0-0. No such channel is seen in diphenylhexatriene up to 5200 ~ m - ~ ~ a b o v e the 0-0. 4) In the case of 2-photon excitation (so far only done for diphenylbutadiene), in addition to fluorescence, incoherent scattering at twice the laser frequency minus a vibrational Stokes shift is observed when the exciting laser is tuned to I ~ A = - ~ I A , 2-photon resonances. We ascribe this to 2-photon resonance enhanced hyper-Raman scattering.

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EXPERIMENTAL

Diphenylbutadiene and diphenylhexatriene were obtained from Aldrich Chemical Company and further purified by vacuum sublimation. HPLC using an alumina column and absorbance detection showed that both com- pounds were free from impurities that absorbed in the region of our measured excitation spectra.

Gram quantities of the diphenylpolyenes were packed into a heated stainless steel tube that was part of the helium supply line for the nozzle. The source was heated to obtain diphenylpolyene pressures of less than 1 torr (140 C for diphenylbutadiene, 172 C for diphenylhexatriene), seeded into helium gas at 8-12 atm and then expanded through a 200 micron diameter nozzle (heated to 190 C to prevent clogging) in- to a chamber that maintained pressure at .35 torr. The jet was inter- sected at a right angle to the expansion direction approximately 3 mm from the nozzle by the laser beam (N2 or Nd-YAG pumped dye laser) and a f/l mirror whose axis was perpendicular to both the laser beam and the jet flow direction focussed the interaction region into the detection system. The detection system consisted of filters to isolate the wavelength region of interest followed by a cooled photomultiplier tube. For recording the emission spectra, a monochromator was inserted between the filters and the photomultiplier. Photocurrent versus time from this photomultiplier was either captured directly by a Tektronix 7912AD transient digitizer for determining the decay profiles or inte- grated by an electrometer set to Coulomb mode for the excitation and emission spectra. Data were collected and processed by a HP 9835A microcomputer. Further experimental details are to be found in pre- vious papers 12-81.

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RESULTS AND DISCUSSION

1. Diphenylhexatriene

The absorption spectrum for diphenylhexatriene vapor at room tempera- ture is shown in Figure 1.

Fig. 1

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Absorption spectrum

of diphenylhexatriene vapor at 441 K in a 5 cm path length cell. Full scale corresponds to an optical density of approximately 1.5.

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At this level of resolution, only the symmetry allowed SQ -> S2 (1 'A

to 1 'B ) transition is seen (0-0 at 29,157 cm-l). When d ~ ~ h e n ~ l h e x a - ~ trieneUis seeded into the supersonic expansion, there is a dramatic

increase in the information content of the observed spectra as seen in Figure 2.

Fig. 2

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Fluorescence excitation spectrum of diphenylhexatriene seeded in a supers nic helium expansion. The lowest energy intense band at 29,157 cn-' is the I1Ag-1 '8 0-0. The weak sharp features going to lower energy come from f ib?onically induced 1 1~ -2 lA ab-

sorption 4 9

In this excitation spectrum the symmetry allowed llAg to l l ~ U transition sharpens considerably allowing a number of vibronic features to be unambiguously assigned. Additionally, a set of weak sharp features can be seen going to lower energy from the llA to l1BU 0-0. These weak excitation bands belong to the symmetry £&bidden So ->S1 (1 ' A ~ to 2lA ) transition and derive intensity through vibronic coupling.

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From a detailed vibrational analysis [4] it is clear that the lowest energy I-photon excitation feature at 29,742 cm-l is not the 0-0 band but comes from absorption from the zero point level of the ground state to a level in the excited state where 1 quantum of a 27 cm-l odd-symmetry vibration is excited. This confirms that this weak series of absorption bands belong to a symmetry forbidden transition and establishes that the e uilibrium nuclear geometry is centrosymmetric

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in both the llAg and 2 A states.

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JOURNAL DE PHYSIQUE L i f e t i m e

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v e r s u s E n e r g y above Ag 0-0 0 0 1000 2000 3 0 0 0 4000 5000 E n e r g y , c m - l F i g . 3

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F l u o r e s c e n c e decay l i f e t i m e s a s a f u n c t i o n o f e x c i t a t i o n e n e r g y r e l a t i v e t o t h e l l A - 2 ' ~ 0-0 f o r d i p h e n y l h e x a t r i e n e seeded i n a s u p e r s o n i c helium expans$!on. $he 4 p o i n t s a t h i g h e s t e n e r g y ( r i g h t hand s i d e of t h e f i g u r e ) c o r r e s p o n d t o e x c i t a t i o n i n t o t h e o r i g i n and h i g h e r l y i n g v i b r o n i c f e a t u r e s o f t h e l l A - l l B t r a n s i t i o n . The l i n e t h a t c o n n e c t s t h e p o i n t s i s m e r e l y a v i s u z l a i 8 .

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Even though the molecules are isolated and, hence, energy is conserved, the emission spectrum observed for llBU excitation bears no relation to the mirror image of the llAg to l1BU absorption spectrum. It does, however, have considerable overlap with the mirror image of the llAq to 21Ag absorption spectrum showing that, following excitation into-a vibronlc level of the llB, state, there is rapid relaxation into the dense bath of isoenergetic 2lA vibronic levels which, by the Franck- Condon principle, emit to highYy excited vibronic levels of the I1Ag state.

2. Diphenylbutadiene

As in the case of diphenylhexatriene, the fluorescence excitation spectrum of diphenylbutadiene sharpens considerably when these molecules are seeded into a supersonic helium expansion (Figure 5 ) .

Ni

1 photon

F r e q u e n c y ( c r n - l )

Fig. 5

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Fluorescence excitation spectra for diphenylbutadiene seeded in a supersonic helium expansion. In the case of 2-photon excitation

(upper spectrum) the spectrum has been plotted against 2 times the laser energy.

However, in sharp contrast to the situation for diphenylhexatriene it is not possible to measure fluorescence excitation spectra beyond the

1 I A to 1 lBp 0-0 at approximately 30,800 cm-I due to the onset of a £as? radiat~onless channel. In the case of I-photon excitation, the fluorescence decays are again well fit by the convolution of a single exponential decay with the measured laser pulse profile. The appear- ance of the radiationless decay channel at approximately 1300 cm-I above the 1 1 ~-> 21A 0-0 is evident in the rapid decrease of fluorescence life?ime wit2 vibrational excitation as is shown in Figure 6.

When fluorescence is excited by 2-photon absorption, additional information on the 2 . l ~ excited state is obtained (Figure 5 ) . First of all, the I-photon an3 2-photon excitation spectra are complementary in the sense that they have no lines in common. This confirms the symmetry forbidden nature of the transition and establishes that, in their equilibrium configurations, the llAg and 2 ' states are strict- ~ ~ ly centro-symmetric. Second, fluorescence decay tlmes are approximately twice as long for 2-photon excitation within 150 cm-I of the 0-0

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JOURNAL DE PHYSIQUE L i f e t i m e v e r s u s E n e r g y i n Ag 1 2 0 0 0 5 0 0 1 0 0 0 1 5 0 0 E n e r g y , cm-1 F i g . 6

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F l u o r e s c e n c e decay l i f e t i m e s a s a f u n c t i o n o f e x c i t a t i o n e n e r g y r e l a t i v e t o t h e 1 l ~ ~ - 2 l ~ 0-0 f o r d i p h e n y l b u t a d i e n e s e e d e d i n a s u p e r s o n i c helium expansion. ~i!e p o i n t s p l o t t e d a s + I s a r e f o r 2-

photon e x c i t a t i o n and t h o s e p l o t t e d a s 1 ' s a r e f o r I-photon e x c i t a t i o n . The l i n e s c o n n e c t i n g t h e p o i n t s a r e v i s u a l a i d s o n l y .

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The coalescence of the fluorescence decay times for I-photon and 2- photon excitation at higher vibrational energies reflects the onset of a dephasing process that scrambles g and u symmetry labels on the timescale of the fluorescence decay. It is reasonable to postulate that this is due to some sort of larqe amplitude motion such as inter- nal rotation. The onset of this dephasing is also evident in the fluorescence spectra which change abruptly as the degree of vibrational excitation is increased beyond 150 cm-I.

Third, and perhaps most striking, the fluorescence decay profiles for 2-photon excitation show two components (Figure 81.

1

0 4 0 8 0 120 160 2 0 0

T i m e ( n s t c )

Fig. 8

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Time profiles at different excitation energies for light at

2 times the excitation laser frequency minus a Stokes shift of

1000 cm-I to 9000 cm-1 for diphenylbutadiene seeded in a supersonic

helium expansion. The excitation frequencies for curves a, b and c are labeled in Figure 5.

One component (normal fluorescence as discussed above) decays expo- nentially with a measurable lifetime. The other component exactly tracks the time profile of the exciting laser within our experimental error of approximately 0.2 nsec. The spectrum for two photon excitation at 15,430 cm-I (0.8 prompt emission and 0.2 normal fluorescence) is shown in Figure 9.

The decay time and yield for the normal fluorescence decrease with increasing vibrational excitation reflecting the increasing rate of radiationless decay. The persistance of the prompt emission for 2- photon excitation energies well above the threshold for radiationless decay for either g or u vibrational levels together with the geometry of the experiment argue that the prompt emission must be explained in terms of an incoherent scattering process.

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JOURNAL DE PHYSIQUE

S p e c t r u m o f S c a t t e r e d L i g h t

S h i f t f r o m 2 x L a s e r

Fig. 9

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Spectrum o f s c a t t e r e d l i g h t f o r 2-photon e x c i t a t i o n o f d i - p h e n y l b u t a d i e n e seeded i n a s u p e r s o n i c helium e x p a n s i o n . The e x c i t a - t i o n l a s e r was s e t a t 15,430 cm-'.

Hyper-Raman s c a t t e r i n g i s enhanced by 2-photon r e s o n a n c e s i n an ana- logous way t o t h e way i n which normal Raman s c a t t e r i n g i s enhanced by I-photon r e s o n a n c e s . F u r t h e r , f o r a s t r o n g l y allowed 2-photon t r a n s i - t i o n it c a n be shown t h a t t h e i n t e n s i t y o f hyper-Raman s c a t t e r i n g s h o u l d be comparable t o t h e i n t e n s i t y o f f l u o r e s c e n c e e x c i t e d by 2- photon a b s o r p t i o n [ 6 ] . Thus, t h e r e i s c l o s e agreement between a l l pro- p e r t i e s a s s o c i a t e d w i t h t h e prompt e m i s s i o n ( t h e s p e c t r a l d i s t r i b u - t i o n o f t h e s c a t t e r e d l i g h t , t h e t e m p o r a l p r o f i l e , t h e e x c i t a t i o n pro- f i l e and t h e i n t e n s i t y r e l a t i v e t o 2-photon e x c i t e d f l u o r e s c e n c e ) and t h e p r o p e r t i e s e x p e c t e d f o r hyper-Raman s c a t t e r i n g . T h i s c o r r e s p o n - dence t o g e t h e r w i t h t h e f a i l u r e of k i n e t i c schemes p r o c e e d i n g o u t o f e x c i t e d 2lA- l e v e l s ( w i t h o r w i t h o u t d e p h a s i n g ) t o a c c o u n t c o n s i s t e n t - l y f o r % %f t h e o b s e r v a t i o n s i s t h e b a s i s f o r a t t r i b u t i n g t h e

prompt p r o c e s s t o hyper-Raman s c a t t e r i n g enhanced by 2-photon r e s o - nance. I V

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CONCLUSIONS From t h e s e p r e l i m i n a r y s t u d i e s it i s c l e a r t h a t : 1 ) Highly r e s o l v e d e l e c t r o n i c s p e c t r a can be o b t a i n e d f o r i s o l a t e d f l e x i b l e molecules l i k e t h e l i n e a r p o l y e n e s u s i n g f r e e - j e t e x p a n s i o n t e c h n i q u e s . 2 ) These s p e c t r a show t h a t t h e l o w e s t e n e r g y e x c i t e d s i n g l e t s t a t e i n i s o l a t e d d i p h e n y l b u t a d i e n e and h e x a t r i e n e i s 21Ag and t h a t t h e d i f f e r e n c e i n e n e r g y between 21Ag and i n c r e a s e s w i t h i n c r e a s i n g c h a i n l e n g t h . 3) From t h e vibrational development o f t h e l l A t o 21Ag s p e c t r a ( i n p a r t i c u l a r , t h e I -photon f o r b i d d e n 2-photon algowed c h a r a c t e r o f t h e 0-0) it i s c l e a r t h a t i n t h e i s o l a t e d molecules t h e r e l a x e d e q u i l i - brium geometry i s c e n t r o s y m m e t r i c i n b o t h l l A and 2lA 4 ) A s t h e de-

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t h a t o b t a i n i n f r e e - j e t e x p a n s i o n s i t s h o u l d be p o s s i b l e t o combine l i n e a r and n o n - l i n e a r s p e c t r o s c o p i e s t o o b t a i n a d e t a i l e d p i c t u r e o f i n t r a m o l e c u l a r r e l a x a t i o n p r o c e s s e s i n t h e s e systems.

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REFERENCES

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[ 4 ] K o h l e r , B.E. and S p i g l a n i n , T.A., J . Chem. Phys.

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[ 6 ] Horwitz, J . S . , Kohler, B.E. and S p i g l a n i n , T . A . , J. Phys. Chem.

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[ 7 ] Kohler, B.E. and S p i g l a n i n , T.A., J. Chem. Phys. &2 (1985) 2939. [ 8 ] Horwitz, J . S . , K o h l e r , B.E. and S p i q l a n i n , T.A., J . Chem. Phys.,