Picosecond time-resolved CARS spectroscopy of a mixed excited singlet state of diphenylhexatriene

24 April 1998

Chemical Physics Letters 287 Ž1998. 8–16

Picosecond time-resolved CARS spectroscopy of a mixed excited singlet state of diphenylhexatriene S. Hogiu a , W. Werncke a , M. Pfeiffer a , A. Lau a , T. Steinke a

b

Max-Born-Institut fur ¨ Nichtlineare Optik und Kurzzeitspektroskopie, Rudower Chaussee 6, D-12489 Berlin, Germany b Konrad-Zuse-Zentrum fur ¨ Informationstechnik Berlin, Takustraße 7, D-14195 Berlin, Germany Received 5 December 1997; in final form 26 January 1998

Abstract Time-resolved coherent antistokes Raman spectroscopy ŽCARS. spectra from an excited singlet state of diphenylhexatriene ŽDPH. are reported. As determined from the CARS measurements, the excited-state population increases on a sub-picosecond time scale after excitation in the 1A g –1B u absorption band. We observe two strongly upshifted and anomalously broadened resonances in the double-bond stretching region. According to our semi-empirical calculations of vibrational frequencies, they can be assigned to polyenic p-chain modes of the 1B u and 2A g states, respectively. Frequency shifts as well as broadenings are extremely solvent-dependent, indicating a strong influence on the excited-state conformations. Our time-resolved measurements support a mixed excited singlet state with drastically different geometries of the local minima. q 1998 Elsevier Science B.V. All rights reserved.

1. Introduction A, v-diphenylpolyenes have attracted a great deal of attention as they can serve as models for vitamin A and visual pigments. Furthermore, the first three members in the homologous series, trans-stilbene ŽTS., diphenylbutadiene ŽDPB. and diphenylhexatriene ŽDPH., show an extremely high third-order nonlinearity, in contrast to the nonlinearity of diphenyloctatriene ŽDPO. which is only moderate w1–3 x. An additional remarkable feature of diphenylpolyenes is a reversal of the 1 1 B u and 2 1A g levels as lowest excited singlet state with increasing chain lengths and an increasing gap between the two states for longer polyenes w4,5x. In TS, the primarily excited 1B u state also represents the lowest excited singlet state. In DPH, however, the 2A g state is located slightly below the 1B u state w6x with a nar-

row gap depending mainly on the solvent polarizability w6,7x. In DPO, the gap amounts to approximately 2000 cmy1 w6,8x. In DPH, as a third interesting finding, tct- and ctt-photoisomers have been isolated as primary photoproducts w9x and it has been reported that the yield for terminal isomerization increases in polar solvents w10x. As a consequence, the excitedstate geometries determining the pathways of photoisomerization should be affected significantly by solvent polarity. The excited singlets of DPH populated after electronic excitation have been mainly investigated by time-resolved fluorescence w11,12x and transient absorption spectroscopy w13–15x. However, both the assignments of the transient absorption bands as well as the kinetics towards an excited-state equilibrium mixture are still the subject of controversy. Unfortunately, hardly any structural information can be ob-

0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 1 1 9 - 5

S. Hogiu et al.r Chemical Physics Letters 287 (1998) 8–16

tained from the broad and structureless bands of the excited-state absorption of DPH. Vibrational spectroscopy is suitable for determining the structure of the probed state. By applying coherent antistokes Raman spectroscopy ŽCARS. under electronic resonance conditions, the specific CARS line shapes are related to the spectral position of the relevant electronic transition w16,17x. We use time-resolved resonance CARS of the excited electronic state Ži. to assign the excited-state absorption by the signature of vibrational spectra which appear due to resonance with the transient excited-state absorption, Žii. to determine the time duration until equilibrium between the excited-electronic states has been established and Žiii. to get some structural information about the excited states in different environments in combining our spectroscopic results with a normal coordinate analysis on a semi-empirical ŽPM3. level. The main unexpected result of our study is the observation of two anomalously upshifted and broadened bands belonging to an C-C double-bond mode which are strongly solvent dependent. Our calculations show that the vibrational spectrum, probed in resonance with the 650 nm excited-state transition, should be assigned to an excited singlet of both 1B u and 2A g character which has been produced via the primarily populated 1B u Franck–Condon state on a sub-picosecond time scale. Both contributing states show geometrical differences of their bond lengths along the p-chain, which result in drastic differences of the respective local potentials from which the bands originate.

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recorded in rather high concentrated tetracarbon chloride solutions Ž10y2 molrl. to get a sufficient signal-to-noise ratio. The scheme of UV excitation and resonance CARS probing of DPH is depicted in Fig. 1. The spectra of the electronically excited DPH were recorded by a picosecond time-resolved multiplex CARS spectrometer as shown in Fig. 2. Briefly, pulses of 8 ps duration of a home-made dye laser pumped synchronously by the second-harmonic of a mode locked Nd:YAG laser ŽQuantronix 4216. were generated. The nearly transform limited pulses were tuned by a three-plate Lyot filter within the range of 750–705 nm using pyridin 2 as a laser dye. Adding 1,1X-diethyl-2,2-dicarbocyanine iodide ŽDDI. as saturable absorber to the pyridin 2 solution and replacing the three-plate Lyot by a two-plate Lyot filter pulse lengths were reduced to 2 ps at a wavelength of 736 nm. The pulses were amplified by a homemade three-stage dye amplifier pumped by the frequency-doubled radiation of a regenerative amplifier ŽContinuum RGA-50. at a repetition rate of 50 Hz resulting in an energy per amplified pulse of 100–150 mJ. Frequency doubling of these pulses was performed by a BBO crystal of 2 mm thickness with an efficiency of 20–25%. The excited states of DPH were either populated by the frequency-doubled radi-

2. Experimental Diphenylhexatriene ŽDPH. as commercially available ŽAldrich. was used without further purification. Samples were dissolved in cyclohexane and in methanol ŽMerck, spectroscopic grade., in concentrations ranging from 2 = 10y4 to 2 = 10y3 molrl. UVrVIS spectra of the solutions of DPH were recorded by a UVrVIS spectrometer ŽPerkin Elmer, Lambda 2S.. For spontaneous Raman spectroscopy we used a Dilor XY Raman spectrometer. The Raman spectra were excited by the 488 nm line of an argon Laser ŽCoherent model Innova 90, USA. and

Fig. 1. Scheme of photoexcitation of DPH. CARS probing at l1 ; 710 nm is close to the transient absorption at 650 nm w13–15x. Excitation is applied at lUV ; 355 nm. v 1,2 are the frequencies of the two laser radiations used for CARS generation at the frequency v 3 . n 0 and n 1 represent vibrational levels of the excited electronic state. ), Changed electronic energies due to solute–solvent interaction. Insert: molecular structure of DPH.

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broadband dye laser directly pumped by a part Ž20%. of the second harmonic of the regenerative amplifier. Weakening of the dye laser radiation l1 was essential to avoid bleaching of the excited states of DPH. We used rotating sample cells Ž d s 0.5 mm. to minimize photodegradation and thermal lensing of the material. The cell windows were made from thin microscopic plates Ž d s 0.15 mm. to reduce their four-wave mixing contribution. The CARS signals were spectrally analyzed by a polychromator ŽSOPRA SpectraPro-275, 1200 lrmm. and accumulated by a liquid nitrogen-cooled CCD Camera Ž S p ectro sco p y an d Im ag in g , L N r C C D 1100PBrUVAR CCD, back illuminated.. In addition to the spectra of the solutions of DPH, spectra of a 0.5 mm thick glass plate were recorded and served as a reference for dividing the spectra of the solutions through the spectrum of the glass plate. Fig. 2. Picosecond time-resolved CARS spectrometer. DA, threestage dye amplifier; FR, Fresnel romb; P, polarizer; F, filter.

3. Experimental results ation of the dye laser or by the third harmonics Ž355 nm, 80 ps, 30 mJ. of the Nd:YAG regenerative amplifier, respectively. Broadband radiation in a wavelength region l2 necessary for CARS generation was delivered by a

An excited-state CARS spectrum of DPH dissolved in cyclohexane Ž1 = 10y3 molrl. in the frequency range of 1000–1900 cmy1 obtained at l1 s 710 nm is shown in Fig. 3. The spectrum ŽI. was

Fig. 3. Excited-state resonance CARS spectrum ŽI. of DPH dissolved in cyclohexane Ž10y3 molrl. recorded in the frequency range of 1050–2000 cmy1 . High-frequency region of the excited-state resonance CARS spectrum ŽII. of DPH dissolved in methanol Ž10y3 molrl.. CARS spectra have been recorded at l1 s 710 nm with a 20 ps delay after excitation at lUV s 355 nm. x, Vibrational resonances; w, solvent lines. Raman spectrum ŽIII. of the electronic ground state of DPH.

S. Hogiu et al.r Chemical Physics Letters 287 (1998) 8–16

recorded with a pulse width of 8 ps and with a time delay between excitation at 355 nm and CARS probing of 20 ps. The spectrum of the excited state ŽI. is shown together with the spontaneous Raman spectrum ŽIII. of DPH in the electronic ground state for comparison. The vibrational resonances of the excited DPH Žspectrum I. in Fig. 3 as indicated by arrows in Fig. 3 are pronounced dips with an additional slight dispersion like contribution of the shape occurring within the electronic four-wave mixing background. In contrast to the DPH ground state spectrum, where only a few intense Raman lines are located near 1200 and 1600 cmy1 , respectively, we observe a much greater variety of vibrational resonances which are of comparable intensities. These resonances cover the whole frequency region. It should be noted that

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the solvent CARS line shapes of the solution are also affected by UV pumping, occurring on a strongly enhanced four-wave mixing background. The most surprising experimental result however, is the observation of two "high-frequency modes" at 1620 and 1780 cmy1 . These lines are extremely frequency broadened. Vibrational frequencies, intensities and line widths of the excited-state spectrum were obtained by a fitting procedure w18x. Briefly, the fit was carried out in three steps: Ži. First we fitted the CARS spectrum of the DPH solution without UV excitation. Žii. From a fit of rather isolated solvent CARS lines, which change their shapes and their CARS linerbackground intensity ratio after UV excitation, we determined the excited-state electronic hyperpolarizability of DPH.

Table 1 Comparison of Raman spectra of DPH dissolved in cyclohexane Želectronic ground state. with time-resolved resonance CARS spectra originating from the first excited singlet state CARS, excited state

n Žcmy1 .

< g R
970

13

1080 1131 1143

30 73 57

1160 1185 1240

57 60 57

1322

55

1410

45

1490 1542

1620 1780

Raman, ground state

n Žcmy1 .

Assignment related to Raman spectra of hexatriene Žhx. w20x and benzene Žbz. w21x

IRamanrG R Žau.

880

1.9

bz.a_A 1g

999

11.5

hx.n11

1144 1157

31.5 22

hx.n10 bz.b_E 2g

1178

20.8

bz.b_E 2g

1253 1295

21.6 3

hx.ng ,n 9 bz.t_B 2u

1331

3.8

bz.t_B 2u

1446 1496

15.2 5.7

bz.t_E 1u bz.t_E 1u

1570 1588 1595 1607

19.3 77 100 38

bz.t_E 2g hx.n6 hx.n5 bz.t_E 2g

100 98

60 17

Spectra were recorded in the frequency range: 950–2000 cmy1 . < g R
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S. Hogiu et al.r Chemical Physics Letters 287 (1998) 8–16

Žiii. Taking into account the electronic hyperpolarizability of DPH, as determined before, the CARS line shapes of excited-state DPH were fitted to get their frequencies and intensities. Fitting of the broad CARS resonances above 1600 cmy1 contains a considerable uncertainty. It is difficult to fit the broad resonances because of drifts of the spectral distribution of the four-wave mixing background after excitation with respect to the reference spectrum resulting in artificial changes of the shapes of the broad dips. Fitted CARS frequencies and relative cross-sections determined in cyclohexane solutions in the frequency range of 950–2000 cmy1 are summarized in Table 1. The corresponding spontaneous Raman data of the electronic ground state of DPH recorded in a highly concentrated carbon tetrachloride solution Ž10y2 molrl. of DPH are given for comparison. Additionally we show in Fig. 3 CARS spectrum of DPH dissolved in methanol ŽII. accumulated after ˚ . along the p-chain. Fig. 5. Calculated bond lengths of DPH Žin A ŽA. Electronic ground state; ŽB. local minima of the 2 1A g and 1 1 B u excited electronic states. j, Mixing of the states.

Fig. 4. Excited-state resonance CARS spectrum of DPH dissolved in cyclohexane Ž10y3 molrl. recorded after different delays between UV excitation and CARS probe. CARS spectra have been recorded at l1 s 726 nm with excitation at lUV s 363 nm. w, Solvent line.

UV excitation. It is obvious that the two highfrequency vibrations in the methanol solvent are further upshifted and are even more frequency broadened than in the cyclohexane solution. Dissolving in methanol instead of in cyclohexane we observed upshifts and broadenings of the two vibrations from 1620 Ž60. to 1670 cmy1 Ž80 cmy1 . and from 1780 Ž90. to 1880 cmy1 Ž120 cmy1 .. ŽThe half Raman line width G R is given in parentheses.. Fast kinetic measurements are shown in Fig. 4. The CARS spectra of DPH dissolved in cyclohexane were obtained with different time delays between pump and probe. Here we used pulses of 2 ps duration. The spectra shown in Fig. 5 are not divided through the reference glass spectrum because of time-dependent drifts of the background as has been mentioned before. It can be clearly seen that the CARS dips immediately grow after excitation, i.e. limited by our instrumental response. No differences between the time behaviour of different vibrations could be established. ŽShifts of the central frequencies seem to occur but have to be confirmed after a reliable fit, demanding improved spectra..

S. Hogiu et al.r Chemical Physics Letters 287 (1998) 8–16

4. Assignment of the vibrational spectra for the ground and excited states of DPH Structural chemical considerations suggest a plane molecular geometry in the S 0 state due to p-electron delocalization w19x. MNDO-CI calculations confirm this planarity at nearly C 2h symmetry also for the excited singlets. By this symmetry intense Raman lines are expected to occur only for modes belonging to the A g species. In the spectral range of interest Ž850–1650 cmy1 . these are in-plane stretching vibrations of the C—C skeleton Žt-modes., in-plane deformations of the CH bonds Žb-modes. and modes related to the breathing vibration Ž a-A 1g mode. of the benzene ring Žat 991 cmy1 in the isolated benzene.. The strongest Raman lines in the ground-state spectrum of DPH nearly coincide with frequencies of unsubstituted hexatriene assigned by Langkilde et al. w20x for the unsubstituted polyene. Analysis of the potential energy distribution ŽPED. derived from normal mode calculation for DPH shows that most of the modes between 880 and 1607 cmy1 are highly localized in either the chain- or benzene subsystems. Only three frequencies of the A g species between 1200 and 1350 cmy1 exhibit a mixture between chain- and ring-stretching modes. The benzene modes characterized by their species under D6h symmetry are degenerate modes which are split according to the attachment of benzene to the polyenic chain w21x. The modes related to the hexatriene segment are named by numbering of the A g modes for this molecule. The assignment given in the right column of Table 1 for the ground-state vibrations is based on our calculations. Semi-empirical calculations applying the MOPAC93 package w22x to the excited electronic states needs the inclusion of double excitations in the CI calculation in order to reproduce the correct term ordering of the 2 1A g and 1 1 B u states. This calculation nearly gives the gap known from experiments w6,7x. Applying the PM3-Hamiltonian and the CI option OPENŽ4,4. according to our calculation the 1 1 B u state is about 2000 cmy1 above the 2 1A g state. There are strong changes of bond lengths in the p-chain for the two states as shown in comparison to the ground state condition in Fig. 5. As typical for a polyenic structure the ground state configuration of DPH exhibits a pronounced alteration of C—C bond

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lengths of the chain. Both excited states show decreased bond orders of double bonds. However, in contrast to the 1 1 B u state where the bonds tend to equalize, in the 2 1A g state only two bonds remain as pronounced double bonds, with a reversal of the former double- and single-bond positions. According to our calculations, we have to expect upshifts of some tens of wavenumbers of the vibrational frequencies of the normal modes for the chain containing a high degree of single-bond chain C—C stretching motions Žf 1150–1300y1 cm in the electronic ground state., while those vibrations which are strongly localized in the phenyl rings exhibit only small shifts of some wave numbers. In the C-C double-bond stretching region of the chain Žf 1600 cmy1 in the electronic ground state. considerable down shifts of some tens of wavenumbers are calculated. These trends hold for the 2 1A g and 1 1 B u states as well. Quantitatively, the calculated down shifts in the double-bond region are stronger for the 2 1A g than for the 1 1 B u state. The exception from those "well behaving modes" which shift down if the bond order of the double bonds is decreased Žsee Fig. 5. is one totally symmetric double-bond stretching vibration. For this vibration we calculated upshifts of 211 and 84 cmy1 for the 2 1A g and 1 1 B u states, respectively.

5. Discussion of experimental and theoretical results We first briefly introduce the expression describing the shapes of vibrational resonances of a CARS spectrum. Generally the CARS spectrum generated at the frequencies v 3 due to excitation at v 1 and v 2 is composed of a Žnearly. frequency-independent contribution Ž A. caused by the electronic third-order hyperpolarizabilities g of solvent and solute and by the manifold of vibrational resonances at the frequencies v R w23x: I Ž yv 3 , v 1 , v 1 ,y v 2 . A A q Ý R

BR d R q C R

d R2 q GR2

Here d R s Ž v R y v 1 q v 2 . are the detunings from the vibrational resonances. BR and CR are line-shape parameters leading to dispersive and Lorentzian line

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shapes in the spectrum. Under off-resonance excitation conditions with respect to electronic transitions of the molecules the quantities BR , CR are always positive signed, resulting in peaks with more or less pronounced Lorentzian or dispersive contributions to their shapes. In contrast, under conditions of electronic resonance Žresonance Raman effect., BR and CR can change their signs w24x. If CR is negative the Raman resonances occur as dips in the four-wave mixing background. Now let us discuss the origin of the excited-state CARS spectra. Under off-resonance excitation conditions and for concentrations below 10y3 molrl CARS lines from DPH molecules in the electronic ground state can hardly be observed. The even lower concentrations of molecules in the excited states can only be detected by CARS spectroscopy due to signal enhancement by a transient electronic resonance of the excited state. The appearance of CARS lines of the excited DPH as dips further confirm resonance enhancement due to the absorption band of the excited electronic state near 650 nm. If we follow the assignment of the 650 nm transient absorption as a pure 2 1A g –n 1 B u electronic transition w13–15x, then the CARS spectra should originate from the 2 1A g state. However, this assumption is not in agreement with our experimental results. The observed two CARS resonances upshifted to 1620 and 1760 cmy1 in the excited cyclohexane solutions nearly coincide with the calculations predicting just one upshifted totally symmetric C-C double-bond vibration for the 1 1 B u and for the 2 1A g electronic state, respectively. Furthermore vibrational resonances detected in the fluorescence excitation spectra of long-chain polyenes observed in the range of 1750–1800 cm are due to the 2 1A g electronic state w24x. It is known that this strong upshift occurs because of 1 1A g –2 1A g vibronic coupling w24,25x. The excited-state CARS spectra of DPO shows only one rather sharp vibrational resonance at 1755 cmy1 in that region w26x, which nearly coincides with one excited-state vibration of DPH at 1780 cmy1 . The excited-state Raman spectrum of DPB has been attributed to the 1 1 B u Žor to a mixed. electronic state w27x. In this molecule one very broad line at 1650 cmy1 has been observed corresponding to a frequency at 1620 cmy1 calculated for the 1 1 B u state w28x. This resonance coincides with the other up-

shifted vibration of the excited DPH. Thus we conclude that in DPH we observe vibrations belonging to the 1 1 B u and to the 2 1A g electronic states. Following this conclusion we have to distinguish between a 1 1 B ur2 1A g thermalized mixture w6x and an excited singlet of mixed 1 1 B u and 2 1A g character. Assuming two separate excited electronic states after primary population of the 1 1 B u state, fast energy transfer to the 2 1A g Žor to the mixed. state is expected. Transfer may be faster than our time resolution. However, transfer from the hot but thermalized excited electronic states to the solvent cage should occur on a 10 ps time scale w29,30x. Assuming a gap of 1540 cmy1 of the excited states in the cyclohexane solution Žas has been determined from the shifts between absorption and fluorescence wavelengths., a thermal population of the 1 1 B u state at room temperature below 0.1% of that of the 2 1A g population should result w15x. Consequently the initially populated 1 1 B u state should dramatically drop to a value far below our detection limit for the vibration belonging to the 1 1 B u state after about 10 ps. In contrast, our time-resolved vibrational spectra show that the relative strength of the vibrational resonances attributed to the A g and B u electronic states are independent on the delay and that only one excited state is populated. The growth in the population of this electronic state is finished at least after 1–2 ps. Furthermore, in changing the solvent and hence the gap a strong alteration of the relative strength of vibrations belonging to different electronic states according to a changed Boltzmann equilibrium has to be expected. In contrast, the main effect for the methanol solvent are strong shifts and further broadening of both vibrational frequencies. Thus, we conclude that the excited electronic state which we observe is a vibronically mixed state and is populated on a sub-picosecond time scale Žwhich is in accordance with resent results of Atom Yee et al. w31x and unpublished time-resolved measurements of Rulliere ` ŽRulliere, ` private communication, 1997... The mixed state can be the result of vibronic coupling of the adiabatic 1B u and 2A g electronic states, mediated by a vibration belonging to the 1b u species because of acting symmetry. Alternatively vibronic coupling could be caused by an C-C double-bond vibration w32x, if the symmetries of the electronic states are distorted, e.g. due to excited-state ro-

S. Hogiu et al.r Chemical Physics Letters 287 (1998) 8–16

tamers. Let us now discuss the extreme frequency broadening of the upshifted vibrations. We note that an extreme broadening occurs in the case of nearly level crossing of the electronic states in DPB and DPH but not in DPO where the gap between the excited electronic states is considerably larger. The extreme broadening may be related to a mixing of the excited electronic states that opens a fast dephasing channel for the two vibrations by coupling the local potential minima in the two states. The coupling strengths and hence the shifts depend on the energy gap between the coupled states and therefore on the solvent. As reorientations of the chains relative to the solvent cage strongly influence the gap a distribution of excited-state conformers w11x should cause additional inhomogeneous broadening. In methanol solutions, additional shifts of the high-frequency chain modes are observed, indicating changes of bond orders in the chain. These changes should also determine the altered yields for the competing diradicaloid and zwitterionic pathways of photoisomerization of DPH detected in the solvents of different polarity w10x. In conclusion, we have observed mixing of two excited electronic states near their level crossing. Mixing results in two anomalously upshifted and extremely frequency broadened vibrational resonances. The resonances are due to an C-C doublebond vibration which can be related to two local minima with strongly altered geometries in the excited electronic state potentials. The mixed excited singlet state is generated on a sub-picosecond time scale and is strongly influenced by solute–solvent interactions. More detailed investigations of these solvent effects are now in progress.

Acknowledgements We thank Professor C. Rulliere, ` University of Bordeaux, France, for informing us of his yet unpublished results.We are grateful to Mrs. K. Kirchner for technical support and Dr. K. Lenz for recording the spontaneous Raman spectra of DPH. We also thank Professor T. Elsasser for critically reading the ¨ manuscript. One of us ŽSH. thanks the Deutsche Akademische Austauschdienst for financial support. Support of the Deutsche Forschungsgemeinschaft

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ŽProject No.: We 1489r5-1. is greatfully acknowledged.

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