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J. Phys. Chem. 1996, 100, 3308-3311
Evolutions of Singlet Excited-State Absorption and Fluorescence of all-trans-1,6-Diphenyl-1,3,5-hexatriene in the Picosecond Time Domain Edwin F. Hilinski,* William M. McGowan, Donald F. Sears, Jr., and Jack Saltiel* Department of Chemistry, The Florida State UniVersity, Tallahassee, Florida 32306-3006 ReceiVed: October 9, 1995; In Final Form: January 4, 1996X
Temporal behaviors of singlet excited-state absorption and fluorescence of all-trans-1,6-diphenyl-1,3,5hexatriene in solution are reported in the picosecond time domain. Contrary to an earlier study, the absorbance ratio at the two λmax (∼460 and ∼650 nm) is time (∼10 ps resolution) and, except for small shifts, temperature (238-298 K) independent. Similarly, in contrast to an earlier report, identical time evolution of fluorescence intensity is found at short (405 ( 10 nm) and long (g500 nm) λ. We conclude that the initially formed 11Bu state completely relaxes to a 11Bu/21Ag equilibrium mixture within our time resolution (∼10 ps). Since the latter is favored overwhelmingly, all transient absorption is assigned to n1Bu r 21Ag transitions, consistent also with small red shifts in λmax with increased medium polarizability. We discern no spectral manifestations of phenyl-vinyl torsional motions, or any associated stabilization of the 11Bu state, at times longer than 10 ps.
Introduction The spectroscopy and excited-state dynamics of R,ω-diphenylpolyenes have attracted a great deal of attention because these molecules serve as models for vitamin A and the visual pigments.1 The first member of the R,ω-diphenylpolyene family whose spectroscopy mimics that of the longer polyenes is alltrans-1,6-diphenyl-1,3,5-hexatriene (DPH). In contrast to the
two lower members of the series for which the initially formed 11Bu state is the lowest excited singlet state in solution, DPH and higher polyenes have the 21Ag state as the lowest excited singlet state.1a,b This is evident in the fluorescence spectrum of DPH that corresponds in large part to the 21Ag f 11Ag electronic transition.1,2 Although strongly symmetry forbidden and not easily observable in the absorption spectrum, the transition becomes somewhat allowed due to vibronic coupling between the 21Ag and the nearby 11Bu state.3,4 The extent of this mixing is highly sensitive to the energy gap, ∆Eba, between these two states.1,3 The 11Bu state is present in thermal equilibrium with the lower 21Ag state and its fluorescence is observed at the onset of the DPH emission spectrum.2 11Bu/ 21Ag fluorescence is associated with excitation of the thermodynamically favored all-s-trans conformer of DPH.5 Excitation of an s-cis conformer, present as a minor component at ambient temperature, yields an additional fluorescence spectrum that is red-shifted relative to the other two.5 The dynamics associated with relaxation of the initially formed 11Bu state to a 11Bu a 21Ag equilibrium mixture has been examined in the picosecond time domain using timeresolved fluorescence and absorption measurements.6,7 Fluorescence was detected using the frequency conversion gating technique with zero time defined as coincidence between 10ps 354.7-nm excitation and probe pulses.5 The 370-450 nm portion of the fluorescence spectrum of DPH in n-hexane at ambient temperature was probed at -3 and 416 ps. A subtle X
Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-3308$12.00/0
increase in intensity was noted in the 380-400 nm region close to the known origin of the 11Bu f 11Ag transition. Decay of this excess fluorescence to about 70% of its maximum value was observed within ∼30 ps suggesting that the initially formed 11Bu state relaxes to the equilibrium 11Bu/21Ag mixture with a rate constant exceeding 3 × 1010 s-1.6 Complementing these observations is a picosecond transient absorption study of DPH in methylcyclohexane (MCH), 3-methylpentane (3MP), and ethanol solutions.7 Absorption was probed in the 430-800 nm region in 10-ps steps following pulsed excitation at 355 nm. Spectra obtained in MCH are similar to those reported earlier using nanosecond-pulsed excitation,8 but the relative intensity of the two absorption bands at ∼460 and ∼650 nm is reversed in the picosecond relative to the nanosecond experiments. The ratio, r, of absorbances A460/A650 was reported to decrease with time, reaching a constant value within about 100 ps. In addition the value of r at the plateau was observed to decrease with decreasing temperature. The time evolution of r led to assignment of the 460 and 650 nm bands to n1Ag r 11Bu and n1Bu r 21Ag transitions, respectively, and the temperature dependence of r yielded ∆Eba ) 0.51 kcal/mol (180 ( 30 cm-1) in MCH/3MP (1:1).7 This ∆Eba value is about 7 times smaller than the value based on DPH absorption and fluorescence spectra.3 It is also much smaller than an estimated value based on the temperature dependence of the dual fluorescence of DPH.2d Furthermore, in earlier work both bands in the transient absorption spectrum had been assigned to transitions originating from the 21Ag state.8 These discrepancies prompted the work described in this paper. Experimental Section DPH was purchased from Aldrich and recrystallized from ethanol. MCH (Aldrich, 99%) was successively washed with 20% fuming sulfuric acid, 5% sodium bicarbonate, and water, then passed through an activated alumina column, and distilled prior to use. Cyclohexane (CH, Fisher, HPLC grade) and benzene (Bz, Aldrich, spectroscopic grade) were used as received and stored under argon prior to use. Picosecond-resolved measurements were made with the modified Quantel/Continuum Nd:YAG laser system described previously.9 To improve the signal-to-noise ratio, each differ© 1996 American Chemical Society
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J. Phys. Chem., Vol. 100, No. 9, 1996 3309 Temperature variation was achieved by circulating methanol at a selected temperature through the sample reservoir jacket. The temperature of the methanol was controlled with a Neslab Cryocool (Model CC-100 II) immersion cooler coupled with an Exatrol digital controller and a Haake circulating pump. The sample temperature was measured with an Omega Model 199P1 digital thermometer equipped with a platinum syringe RTD probe (Model HYP-4) that was positioned at the inlet of the sample reservoir. A stream of dry nitrogen gas was passed across the front and back faces of the optical flow cell to prevent water condensation from the air. Each spectrum shown in Figure 3 is the result of splicing two spectra recorded in the 442-680 and 560-791 nm regions at 500 ps after excitation. Time-resolved fluorescence was measured with the use of the picosecond-pulsed Nd:YAG laser system and a Hamamatsu C979-01 temporal disperser coupled to a Princeton Applied Research (PAR) 1254 multichannel detector whose output is digitized with a PAR 1216 detector controller interfaced with a microcomputer. Long-wave pass or bandpass filters were used to isolate the spectral regions monitored. Results and Discussion
Figure 1. Transient absorption spectra recorded at selected times (a-k correspond to -10, 10, 30, 40, 50, 70, 150, 150 (Bz), 800, 1800, 9800 ps, respectively) after 355-nm excitation of an 80 µM solution of DPH in MCH at 21 °C. Spectrum h is for a 108 µM solution of DPH in Bz at 22 °C. Each spectrum is normalized to a 55 µJ excitation pulse. Ordinate displacement between successive spectra is indicated by parallel lines.
ence absorption spectrum is the result of averaging data from at least 100 excitation laser shots. The sample solution was bubbled with a stream of argon for at least 60 min prior to an experiment, maintained under argon during an experiment, and flowed sufficiently rapidly through a 2-mm or 5-mm path length fused silica flow cell to ensure that fresh sample volumes were excited by individual laser pulses passing through the sample at a rate of 10 pulses/s. Excitation energies of 50-60 µJ/pulse at 355 nm and an excitation beam diameter of ∼1.5 mm were used. Monophotonic ground-state excitation was established by showing that the signal of transient absorbance varies linearly with excitation energy for an energy range exceeding the values employed. In general, the sample emission, which appears as a negative feature in the 420-nm region of some of the uncorrected transient absorption spectra recorded for DPH, was removed by the use of background correction data obtained while the sample was exposed to excitation laser pulses. The criterion employed for this small correction was the constraint that absorbance in the 402-413 nm be greater than or equal to 0. However, the intense emission and low probe light levels along with the shot-to-shot fluctuations in the excitation pulse energies result in uncertainty associated with absorbances for λ < 440 nm. Each difference absorption spectrum in Figure 1 is the result of splicing two spectra recorded in the 402-643 and 560-791 nm regions at a selected time after excitation. Spectra in the 402-643 nm region were recorded at the following delay times (ps): -30, -20, -10, 0, 10, 20, 30, 40, 50, 70, 100, 150, 200, 300, 500, 800, 1800, 4800, 9800, 19800; spectra in the 560-791-nm region were recorded at the following delay times (ps): -30, -10, 0, 10, 30, 40, 50, 70, 100, 150, 200, 300, 500, 800, 1800, 4800, 9800, 19800. For DPH in Bz, spectra in the 560-791-nm region were also recorded at -20 and 20 ps after excitation. Transient absorption spectra at temperatures other than ambient were recorded for samples flowed through a jacketed sample reservoir by means of an insulated recirculating pump.
At longer delay times (t g 70 ps) the transient absorption spectra in Figure 1 are in qualitative agreement with earlier spectra observed in CH8 and in MCH.7 Two broad absorption bands centered at ∼460 and ∼650 nm and a shoulder at ∼700 nm are common features in all spectra. The two absorption bands in Figure 1 are somewhat better resolved than in the spectra obtained in the nanosecond-time scale8 and much better resolved than in the earlier picosecond-time scale spectra.7 However, the absorbance ratio at the two λmax’s, r ) (A460/ A650) ) 1.7 ( 0.2, in our spectra (absorbance ratios are based on average values for 5 nm windows centered at 460 and 650 nm, respectively), while in nearly quantitative agreement with that reported by Goldbeck et al.,8a differs sharply from that reported by Rullie`re and Decle´my in the picosecond-time scale.7 Two possible reasons for the discrepancy are (i) the strong DPH fluorescence in the 400-500 nm region which, if not correctly accounted for, may lead to severe distortion of transient absorption in this region and lead to apparent diminished absorbance for the 460 nm band, (ii) formation of DPH radical cation that absorbs at 635 nm in CH,10 caused by two-photon excitation of DPH, if care to remain in the single-photon excitation regime were not taken. Formation of DPH•+ by twophoton absorption has been observed in acetonitrile.11 Professor Rullie`re has informed us that his published observations were subject to the effects of multiphoton absorption but not to distortions due to fluorescence.12 A superficial examination of the raw spectra in Figure 1 appears to confirm the claim7 that the absorbance ratio, r, undergoes temporal evolution in the picosecond-time scale, decreasing with time as it reaches a plateau at t g 70 ps (Figure 2). However, the spectra in Figure 1 are not corrected for chirp, the effects of group velocity dispersion associated with the continuum probe pulse. This phenomenon affects spectra recorded while excitation and probe pulses overlap,13,14 during the earliest ∼40 ps under our conditions.15 Absorption is sampled earlier by shorter wavelengths than by longer wavelengths during this time, giving the erroneous impression that transient absorption in the blue region of each spectrum grows in faster than absorption in the red region. When r is corrected for the known displacement of the two λmax’s in time, the temporal evolution evident in the uncorrected spectra disappears (Figure 2).12 We conclude, therefore, that relaxation of the initially formed 11Bu excited state to the 11Bu a 21Ag equilib-
3310 J. Phys. Chem., Vol. 100, No. 9, 1996
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Figure 2. Ratio of A460/A650 vs time: A460(t)/A650(t) (b), A460(t)/A650(t + 20 ps) (4), the chirp correction is applied only to spectra within the first 40 ps.
rium mixture occurs much faster than previously reported,6,7 being complete within ∼10 ps. Analogous results were obtained in Bz. Neglecting entropy differences between 21Ag and 11Bu states, 1 1 Bu populations of 0.42% and 0.054% in Bz and in MCH, respectively, can be based on ∆Eba ) 1120 and 1540 cm-1 that follow readily from the absorption and fluorescence spectra of DPH in these two solvents at ∼21 °C. Accordingly, most, if not all, absorption in the spectra in Figure 1 is assigned to n1Bu r 21Ag transitions in agreement with Goldbeck et al.8a Weak absorption bands in the 11Ag spectrum of DPH with origins near 260 and 220 nm16 may correspond to excitation of the ground state to the same 1Bu states. The transient absorption spectra in Bz are very similar to those in MCH except that the 650-nm band undergoes a significant red shift, as expected for a transition to a more polarizable excited state, and the shoulder at ∼700 nm gains intensity and becomes less discernible. Spectrum h in Figure 1 is typical. The gain in intensity in the 700-nm region might be considered to reflect the 8-fold increase in the population of the 11Bu state and, if so, it would represent the only feature in our spectra that may correspond to n1Ag r 11Bu transition. It is more likely, however, that it is a consequence of the relatively larger red shift of the 650-nm band (cf., also, below). Application of principal-component analysis17 to a spectral matrix composed of the nine transient absorption spectra of DPH in MCH in the 70 ps to 4.8 ns time range (four of these spectra are shown in Figure 1) reveals unambiguously a singlecomponent system whose only significant eigenvector is shown as spectrum b in Figure 3. The temporal evolution of the earlier picosecond-time scale transient absorption spectra7 aside, assignment of the 460 nm band to an n1Ag r 11Bu transition was based on the temperature dependence of plateau values of r over the 230-320 K range.7 Our transient absorption spectra recorded at 500 ps postexcitation in MCH over the range 238-298 K (Figure 3) exhibit more subtle changes in r. On decreasing the temperature, we observe only small red shifts of the 460- and 650-nm bands while the shoulder near 700 nm is unshifted but exhibits a slightly increased absorbance. The slight decrease in r with decreasing temperature is due, at least in part, to the red shift in the 650 nm band that brings it into stronger overlap with the band responsible for the 700 nm shoulder. This is reflected in the observation that the ratio of absorbance areas for the 440550 and 550-790 nm regions is nearly unity (1.01 ( 0.02), independent of temperature within experimental uncertainty. The red shifts of the two major bands reflect the increased polarizability of MCH at lower temperatures, consistent with
Figure 3. Transient absorption spectra recorded at 500 ps after excitation of a 76 µM solution of DPH in MCH at different temperatures (a-e corresponds to 24.6, 21, -1.5, -15.4, and -35.1 °C, respectively), except for spectrum b which is derived from the spectra in Figure 1 (see text). Each spectrum is normalized to a 53 µJ excitation pulse.
Figure 4. Fluorescence growth at 405 ( 10 nm (•) and g 500 nm ()) from a 25 µM DPH solution in CH at 22 °C. The solid lines are leastsquares fits which incorporate the instrument response function and a rise time of 8 ps.
their assignment to n1Bu r 21Ag transitions. Lowering the temperature diminishes the equilibrium concentration of the 11Bu state,2d though this effect is moderated somewhat by the decrease in ∆Eba due to the concomitant increase in medium polarizability. The gain in intensity in the 700 nm shoulder as the temperature is decreased is, therefore, inconsistent with the assignment of n1Ag r 11Bu absorbance in that region. It may reflect the stronger overlap between the 650 and 700 nm bands with decreasing temperature. There remains the report by Topp and co-workers6 that 11Bu relaxation to the equilibrium 11Bu/21Ag mixture is manifested in fluorescence decay of DPH in hexane solution observed at 395 nm within 30 ps postexcitation to 70% of its maximum value. We observe no such rapid decay in fluorescence monitored at ∼405 nm following excitation of a 25 µM sample of DPH in CH (Figure 4). A likely explanation for this discrepancy is a reported Raman band due to hexane that appears in the earlier work at 395 nm.6 The decay of this Raman band with the time constant of the instrument response function was considered in interpreting time-resolved fluorescence spectra of all-trans-1,8-diphenyl-1,3,5,7-octatetraene but was apparently
Letters disregarded in interpreting the DPH data in ref 6 and could result in the apparent time dependence of fluorescence intensity at 395 nm for the latter. A much discussed issue concerning the trans f cis photoisomerization of stilbene and its higher vinylogues is the dimensionality of the reaction coordinate.18 Reversal of double and single bond order is a structural change associated with 11Bu r 11Ag transitions. Accordingly, light absorption is expected to result in stiffening of torsional motions about the phenyl-vinyl bonds and in loosening of torsional motions about those CC bonds that are formally double in the ground state. The latter motions are of necessity along reaction coordinates for trans f cis isomerization. The question that is often raised is the degree to which motion along phenyl-vinyl torsional coordinates accompanies photoisomerization. Also, Rullie`re, Decle´my, and co-workers have proposed that the 11Bu state formed immediately following excitation experiences strong solvent-assisted stabilization as it relaxes toward a more planar geometry along phenyl-vinyl torsional coordinates. Presumably, this leads to reversal in 11Bu/21Ag state energy order in trans,trans-1,4-diphenyl-1,3-butadiene19 (DPB) and diminished ∆Eba in DPH.7 The temporal evolutions of the singlet excitedstate absorptions of DPB19a and DPH7 in the picosecond-time scale were supposed to be spectral manifestations of the phenylvinyl torsional motions and the associated stabilization of the 11Bu states in these two molecules. Recent transient absorption measurements on DPB, employing femtosecond-time resolution, reveal no spectral changes at times greater than ∼120 fs providing no evidence for thermal population of one excited state at the expense of another or for vibrational relaxation.20 Similarly, the results presented in this paper lead to the conclusion that any spectral manifestations of phenyl-vinyl torsional relaxation in DPH, associated with increased planarity and equilibration and mixing between 11Bu and 21Ag states, occur faster than ∼10 ps. Since the processes leading to trans f cis photoisomerization in DPH are highly inefficient,21 occurring in the nanosecond time scale, there exists no evidence requiring the coupling of phenyl-vinyl and CC torsional motions along photoisomerization coordinates. The need to invoke more than one-dimensional reaction coordinates for stilbene and DPB photoisomerizations in the liquid state has also been questioned.18 Acknowledgment. This research was supported by NSF, most recently by Grant No. CHE 93-12918. References and Notes (1) For reviews see: (a) Hudson, B. S.; Kohler, B. E. Annu. ReV. Phys. Chem. 1974, 25, 437-460. (b) Hudson, B. S.; Kohler, B. E.; Schulten, K.
J. Phys. Chem., Vol. 100, No. 9, 1996 3311 In Excited States; Lim, E. C., Ed.; Academic Press: New York, 1982; Vol. 6, p 1. (c) Allen, M. T.; Whitten, D. G. Chem. ReV. 1989, 89, 1691-1702. (d) Whitten, D. G. Acc. Chem. Res. 1993, 26, 502-509. (e) Saltiel, J.; Sun, Y.-P. In Photochromism, Molecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 64. (f) Waldeck, D. H. Chem. ReV. 1991, 91, 415-436. (2) (a) Alford, P. C.; Palmer, T. F. Chem. Phys. Lett. 1982, 86, 248253. (b) Alford, P. C.; Palmer, T. F. J. Chem. Soc., Faraday Trans. 2 1983, 79, 433-447. (c) Jones, G. R.; Cundall, R. B. Chem. Phys. Lett. 1986, 126, 129-133. (d) Itoh, T.; Kohler, B. E. J. Phys. Chem. 1987, 91, 17601764. (3) Andrews, J. R.; Hudson, B. S. J. Chem. Phys. 1978, 68, 45874594. (4) Birks, J. B.; Tripathi, G. N. R.; Lumb, M. D. Chem. Phys. 1978, 33, 185-194. (5) Saltiel, J.; Sears, D. F., Jr.; Sun, Y.-P.; Choi, J.-O. J. Am. Chem. Soc. 1992, 114, 3607-3612. (6) Felder, T. C.; Choi, K.-J.; Topp, M. R. Chem. Phys. 1982, 64, 175182. (7) Rullie`re, C.; Decle´my, A. Chem. Phys. Lett. 1987, 135, 213-218. (8) (a) Goldbeck, R. A.; Twarowski, A. J.; Russell, E. L.; Rice, J. K.; Birge, R. R.; Switkes, E.; Kliger, D. S. J. Chem. Phys. 1982, 77, 33193328. (b) Chattopadhyay, S. K.; Das, P. K. Chem. Phys. Lett. 1982, 87, 145-150. (9) Schmidt, J. A.; Hilinski, E. F. ReV. Sci. Instrum. 1989, 60, 29022914. (10) Almgren, M.; Thomas, J. K. Photochem. Photobiol. 1980, 31, 329335. (11) (a) Wang, Z.; McGimpsey, W. G. J. Phys. Chem. 1993, 97, 33243327. (b) Wang, Z.; McGimpsey, W. G. J. Phys. Chem. 1993, 97, 50545057. (12) Rullie`re, C., private communication. Professor Rullie`re has also informed us that the observations in ref 7 were not subject to distortions due to chirp. (13) (a) Topp, M. R.; Orner, G. C. Opt. Commun. 1975, 13, 276-281. (b) Topp, M. R.; Orner, G. C. Chem. Phys. Lett. 1975, 32, 407-413. (14) Sala, K. L.; LeSage, R.; Yip, R. W. Appl. Spectrosc. 1984, 38, 87-89. (15) Schmidt, J. A. Ph.D. Dissertation, Florida State University, Tallahassee, FL, 1989. (16) Lunde, K.; Zechmeister, L. J. Am. Chem. Soc. 1954, 76, 23082313. (17) (a) Lawton, W. H.; Sylvestre, E. A. Technometrics 1971, 13, 617633. (b) Sun, Y.-P.; Sears, D. F., Jr.; Saltiel, J. Anal. Chem. 1987, 59, 25152519. (c) Saltiel, J.; Sears, D. F., Jr.; Choi, J.-O.; Sun, Y.-P.; Eaker, D. W. J. Phys. Chem. 1994, 98, 35-46. (18) For an excellent discussion and brief review see: Lee, M.; Haseltine, J. N.; Smith, A. B., III; Hochstrasser, R. M. J. Am. Chem. Soc. 1989, 111, 5044-5051. (19) (a) Rullie`re, C.; Decle´my, A.; Kottis, Ph. Laser Chem. 1985, 5, 185-208. (b) Rullie`re, C.; Decle´my, A.; Kottis, Ph.; Ducasse, L. Chem. Phys. Lett. 1985, 117, 583-588. (20) Wallace-Williams, S. E.; Schwartz, B. J.; Møller, S.; Goldbeck, R. A.; Yee, W. A.; El-Bayoumi, M. A.; Kliger, D. S. J. Phys. Chem. 1994, 98, 60-67. Cf, also: Gehrke, E.; Mohrschladt, R.; Schroeder, J.; Troe, J.; Vohringer, P. Chem. Phys. 1991, 152, 45-56. (21) (a) Saltiel, J.; Ko, D.-H.; Fleming, S. A. J. Am. Chem. Soc. 1994, 116, 4099-4100. (b) Saltiel, J.; Wang, S. J. Am. Chem. Soc. 1995, 117, 10761-10762.
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