J . Phys. Chem. 1991, 95, 10380-10385
10380
Femtosecond Laser Studies of the cis-Stilbene Photoisomerization Reactions. The cis-Stilbene to Dihydrophenanthrene Reaction Stephen T. Repinec, Roseanne J.. Sension, Arpad Z. Szarka, and Robin M. Hochstrasser* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (Received: March 22, 1991; In Final Form: May 29, 1991) After optical excitation of cis-stilbene,ground-state dihydrophenanthrene (DHP) is formed in 1.7 ps (hexadeane) via kinetically required intermediate(s) present during the first 300 f 200 fs. The DHP is formed hot, and its cooling takes place over the time scale of 35 ps. The DHP spectrum in the region around 450 nm is present throughout the time scale studied (200 fs to 50 ms). The angle between the cis and resulting DHP transition dipoles is 38’ at the earliest times measured, which significantly deviates from the alignment expected from a one-dimensional reaction coordinate involving rotations of the phenyl rings. It is proposed that the ethylene bond axis rotates significantly during the isomerization to DHP, perhaps as a result of rotation of the whole molecule in the laboratory frame. Experiments with DHP indicate ultrafast (C500fs) ring opening leading toward DHP and cis-stilbene ground-state products. effects on the partitioning between the various reaction pathways could be investigated. Although the idea of an isomerization coordinate for stilbene involving a simple rotation about the ethylene double bond has been around for years, there is no direct evidence for it. In fact, evidence from resonance Raman scattering measurements,” the analysis of absorption and emission spectra of substituted stilbenes in argon clusters,’ and time-resolved anisotropy measurements12 all indicate more complex nuclear coordinate dynamics. All of the previous time-resolved spectroscopic studies of the photoreactions of stilbene have concentrated on the cis to trans or trans to cis reactions. In order to develop a more complete understanding of the excited-state potential energy surface in the cis configuration region, it is necessary to investigate both the cis to trans and the cis to DHP reactions. In this paper we present results of time-resolved absorption experiments investigating the photoinitiated cis to DHP/DHP to cis reactions, concentrating primarily on the formation of the DHP product. These experiments are the first in which subpicosecond time resolution has been used to study this particular reaction pathway in the stilbene system. The region of the spectrum exhibiting DHP product absorption was not investigated in previous studies of ~is-stilbene.~ The information obtained in these studies along with that obtained in the studies of cis* dynamic^^,^ and studies of the cis to trans r e a ~ t i o n ~ , ~ Jwill * - ’ help ~ to elucidate the excited-state potential energy surface and to lay the groundwork for more detailed studies of environmental effects on the potential energy surface and the partitioning between the various reaction pathways.
Introductien The photoisomerization reactions of cis- and tram-stilbene have been studied extensively as model systems for understanding the dynamics of intramolecular rearrangements in solution. Many of the processes crucial to the isomerism occur very rapidly and therefore require ultrafast laser methods for their study. Electronic excitation of trans-stilbene yields an excited-state population that is able to flow over a small barrier corresponding to rotation around the ethylene double bond and return to the ground electronic state surface forming cis- and trans-stilbene in approximately equal amounts. This process is well characterized in gases and liquids, the barrier crossing being friction dependent in a manner predicted approximately by Kramers’ theory.’ By contrast, electronically excited cis-stilbene molecules (cis*) escape from the cis configuration region in about 1 ps even in relatively high friction solvents at room suggesting either unrestricted motion of the structure on the excited-state surface or motion over a very small barrier. In addition, the cis population is partitioned between at least two reaction pathways, one involving rotation around the ethylene double bond leading to tram-stilbene and the other involving electronic rearrangement to form 4a,4b-dihydrophenanthrene(DHP). Steady-state measurements indicate that cis-stilbene excited at 3 13 nm forms trans-stilbene and DHP with quantum yields of 0.35 and 0.10, respectively.6 Our experiments indicate that the remaining cis* molecules return to the ground potential surface on the same time scale as trans and DHP formation. Excitation of DHP at 436 nm forms cis-stilbene with a quantum yield of 0.66. At 313 nm the yield is 0.30.6 The remaining DHP* molecules return to ground-state DHP. It has recently been suggested that 70% of the cis* molecules follow the cis to trans channel and 30% follow the cis to DHP channel.’ The various processes and their yields are shown schematically in Figure 1. A number of questions remain to be investigated in the stilbene system. Ideally, one would wish to have enough detailed information about the excited-state potential energy surface to use it to investigate statistical mechanical models of population dynamics. In particular, if the surface near the cis configuration was more clearly understood, the very interesting problem of environmental ( I ) For reviews see: (a) Hochstrasser, R. M. Pure Appl. Chem. 1980, 52, 2683-2691. (b) Saltiel, J.; Sun, Y.-P. In Photochromism Molecules and Sysfems; Durr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; pp 64-192. (c) Waldeck, D. H. Chem. Reu. 1991, 91, 415-436. (2) Greene, B. 1.; Farrow, R. C. J. Chem. Phys. 1983, 78, 3336-3338. (3) Doany, F. E.; Hochstrasser, R. M.; Greene, B. I.; Millard, R. R. Chem. Phys. Lett. 1985, 118, 1-5. (4) Abrash, S . ; Repinec, S.; Hochstrasser, R. M. J. Chem. Phys. 1990, 93,
1041-1053. (5) Todd, D. C.; Jean, J. M.; Rosenthal, S. J.; Ruggerio, A. J.; Yang, D.; Fleming, G. R. J. Chem. Phys. 1990, 93, 8658-8668. (6) Muszkat, K. A.; Fischer, E. J. Chem. SOC.B 1967, 662-678. (7) Petek, H.; Yoshihara, K.; Fujiwara, Y.; Lin, 2.;Penn, J. H.; Frederick, J. H. J . Phys. Chem. 1990, 94, 7539-7543.
Experimental Section The femtosecond studies were performed with a system based on a colliding pulse model-locked (CPM) dye laser.16 The experimental arrangement for transient absorption with 3 12-nm excitation was described previously! Briefly, an output beam of the CPM laser was amplified in a four-stage amplifier pumped by the second harmonic of a Nd:YAG laser. The amplified pulses were split into two parts to make the excitation pulse and the probe pulse. The excitation pulse was frequency doubled to obtain the 3 12-nm pump wavelength. A quartz half-wave plate was used
’
(8) Wismonski-Knittel, T.; Fischer, G.; Fischer, E. J . Chem. Soc., Perkin Trans. 2 1974, 1930-1940. (9) Sundstrom, V.; Gillbro, T. Chem. Phys. Lett. 1984, 109, 538-543. (10) Sumitani, M.; Yoshihara, K. Bull. Chem. Soc. Jpn. 1982, 55, 85-89. (11) Myers, A. B.; Mathies, R. A. J. Chem. Phvs. 1984.81, 1552-1558. (12) Sinsion, R. J.; Repinec, S. T.; Hochstrasser, R. M. J . Phys. Chem. 2946-2948. (13) Petek, H.; Yoshihara, K.; Fujiwara, Y.; Frey, J. J . Opr. SOC.Am. E 1990, 7, 1540-1544. (14) Sension, R. J.; Repinec, S. T.; Hochstrasser, R. M. J. Chem. Phys. 1990, 93, 9185-9188. (1 5) Sension, R. J.; Repinec, S. T ; Szarka, A. Z.; Hochstrasser, R. M. Manuscript in preparation. (16) Fork, R. L.; Greene, B. I.; Shank, C V. Appl. Phys. Leu. 1981,38, 671-672. 1991, 95,
0022-3654191 12095-10380%02.50/0 , 0 1991 American Chemical Society I
,
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10381
cis-Stilbene to Dihydrophenanthrene Reaction
Figure 1. Schematic drawing of the potential encrgy surfaces for the S, photochemical reactions of stilbene. Approximate quantum yields and branching ratios for the important processes are indicated. &, +cT, and $TC at 313 nm are from rcf 8. 4TCat 305 and 266 nm can be found in refs 9 and IO, respectively. GCD at 31 3 nm and bOcat 436 nm are from ref 6. In this figure the ground- and excited-state barrier heights are drawn to scale representing t h e best available values, as are the relative energies of the ground states of cis, trans, and DHP.
to rotate the 312-nm light to obtain the parallel and perpendicular pump-probe polarization geometries used in anisotropy measurements. Probe wavelengths in the visible region were obtained by continuum generation in a cell of HzO. Narrow band-passes of the continuum were selected with a set of interference filters. This visible probe could be amplified and frequency doubled to obtain probe wavelengths in the ultraviolet region. The sample was flowed in a 1-mm quartz sample cell. In order to obtain pump wavelengths throughout the visible and deeper into the ultraviolet region, a second Nd:YAG laser was added to the laser system. The two Nd:YAG lasers (both Quanta Ray DCR-2A) operated as master and slave with the slave laser having its flash lamps triggered by the oscillator sync output of the master laser. Both laser Q-switches were triggered by the same output pulse from a Quanta Ray SM-1 timing module, triggered by the CPM laser. The second and third harmonics from the slave laser (532 and 355 nm) were used to pump auxiliary amplifiers for the continuum. Pump wavelengths in the region of 250 nm were generated for transient absorption measurements by amplifying and then frequency doubling a portion of the continuum at 500 nm. The pulse at 500 nm was amplified in a twestage setup pumped by the third harmonic from the slave laser at 355 nm. The amplifiers were 1-cm quartz flow cells (Coumarin 500 at ca. 5 X 104 M). The cells were transversely pumped by approximately 4 and 40 mJ of the 355-nm light in the first and second stages, respectively. The 355-nm light was focused by cylindrical lenses to overlap the continuum beam. A pinhole of 0.5 mm separated the two stages in order to discriminate against amplified spontaneous emission. Pulses of approximately 18 pJ were generated from this two-stage amplifier. The amplified 500-nm pulses were then focused into a 1-mm crystal of BBO. The remaining 5Wnm light was removed by a filter, and the pulses a t 250 nm having approximately 2-pJ energy were used to excite the sample. The probe pulses were generated in the same manner as in the 312-nm pump experiments. The 532-nm light from the slave Nd:YAG laser was available to amplify selected portions of the continuum which could be frequency doubled to yield ultraviolet probe pulses. A Soleil-Babinet compensator in the probe arm rotated the polarization for anisotropy experiments. The signals Z,,(t) and ZL(t), corresponding to pump and probe beams with polarizations parallel and perpendicular, were separately collected. The pump and probe. beams could also be set at the ‘magic angle” so that (1/3)[Z,, 21,] was directly recordable. Pump pulses at 450 nm and probe pulses at 460 nm were generated in order to perform an experiment aimed at bleaching
+
the DHP absorption. The 450-nm pump pulses were obtained by amplifying a slice of continuum in a single-stage amplifier pumped with the Nd:YAG third harmonic. The amplifier cell contained Coumarin 450 at approximately 3 X M. The single-stage amplification resulted in a 2-3-pJ pump pulse. The 460-nm probe pulse was selected from the continuum by means of an interference filter. In all of the femtosecond experiments the time delay was achieved by a computer-controlled translation stage in one arm of the experiment. At each delay time 20-50 laser shots were averaged for a single scan, and many scans were averaged until the signal to noise was adequate. Spectra of the DHP product at long (many nanoseconds) delay times were taken with a nanosecond transient absorption instrument which has been described in detail elsewhere.” Briefly, the fourth harmonic (266 nm) of a IO-Hz Q-switched Nd:YAG laser focused into a sample cuvette provided the excitation pulse and the probe light, from a short-arc Xe continuum lamp, was split in a bifurcated quartz fiber to give two equal intensity probe beams. These probe beams were directed through a 1-cm sample cuvette perpendicular to the excitation pulse with only one probe overlapping the pump beam. The transmitted probes were collected with two quartz fibers and imaged onto the slit of a Spex 1681 spectrograph. The probe beams were detected simultaneously at the focal plane of the spectrograph by a dual diode array (Princeton Instruments Model DIDA-5 12). The reference (unpumped) and signal (pumped) were ratioed and converted to absorbance data. A spectrum of DHP at a delay time of 200 ps was obtained by using 312-nm pump pulses of the amplified and frequencydoubled CPM laser to excite cis-stilbene in hexadecane solution in a 2-mm flow cell. A colored glass filter with a 300-nm band-pass centered around 450 nm was used to select a large portion of the white light continuum which was used as the probe. The continuum beam was split in two with half being focused into the sample and overlapped with the pump beam; the other half was used as the reference. The reference and signal probe beams were guided with the quartz fibers and detected with the spectrograph and diode array system used in the nanosecond experiments. Fifty separate experiments of 1000 laser shots each were averaged together in order to obtain sufficient signal to noise. Sample Preparation. Samples of cis-stilbene (Aldrich) used in this study were purified by column chromatography so that they contained less than 0.1 % trans-stilbene when measured with HPLC. The samples were usually prepared as 0.15 mL of purified cis-stilbene in 250 mL of the appropriate solvent. For the experiments requiring DHP, a low-pressure mercury lamp was used to irradiate a sample of cis-stilbene to create a significant amount of DHP for study. The irradiated sample was in a circulating system which was sealed and constantly purged with nitrogen to prevent the oxidation of DHP to phenanthrene. The cis-stilbene in the circulator was exposed to the mercury lamp through two 1-cm quartz flow cells protected by Schott UG5 filters to block the visible light in the 450-nm region where DHP absorbs. The DHP solution then flowed through a 2-mm path length quartz cell used as the sample cell. The cis-stilbene solution (2% in hexane) was irradiated for several hours before the start of an experiment in order to achieve a suitable photostationary concentration of DHP. The sample reservoir was kept in an ice bath to minimize thermal isomerization back to cis-stilbene. With this arrangement optical densities of up to 0.1 at 450 nm were obtained for DHP in the 2-mm flow cell. Results Transient Absorption Dynamics and DHP Product Spectrum.
Transient absorption decay curves in the region of the DHP absorption, between 360 and 520 nm, exhibit the fast ca. 1-ps decay of excited cis-stilbene in addition to a persistent absorption ~
~
~
_
_
(17) Papp, S.; Vanderkooi, J. M.; Owen, C . S.; Holtom, G. R.; Phillips, C.M. Biophys. J . 1990, 58, 1 17-186. (18) Saltiel, J.; Waller, A,; Sun, Y.-P.; Sears, D. F. J . Am. Chem. SOC. 1990, 112, 4580-4581.
_
_
~
10382 The Journal of Physical Chemistry, Vol. 95, No. 25, 1991
Repinec et al. 0.010
0.020
480nm probelHexadecmne
-2
0.016
.-a
s
n
.-
+
0.012
0" -m
0.008
0
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0"
'
48Onm probelMethanol
I
' i
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'\
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--
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j
I
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6
3
0
12
9
15
Time (pa)
0.50
I
0.30
1
1
0.06 0.00
3
0
0.00
0
3
6
12
9
15
18
21
6
9
12
15
Time (ps)
Figure 4. Magic angle transient absorption decay and anisotropy decay
Time (pa)
Figure 2. (top) Magic angle transient absorption decay of cis-stilbene in hexadecane pumped at 312 nm and probed at 480 nm. (bottom) Anisotropy decay of above transient absorption. The solid line is a calculated curve (discussed in the text) with a rotational reorientational time of T~~~= 35 f 10 ps for the DHP product.
for cis-stilbene in methanol pumped at 312 nm and probed at 480 nm. The rotational reorientation time in this experiment is T ~ =~ 14, f 3 ps.
0.01s
480nm probelHexane 0,012
0.000
-0.004
'
-3
I
0
6
3
9
12
15
340
Time (ps) nI .
0.20 "'-"
-0.08
380
420
460
500
540
580
Wavelength (nm)
I
Figure 5. Spectra of vibrationally hot (ca. 5 ps, data points taken with 'magic angle" polarization) and cold (100 ns, solid line) dihydrophenanthrene formed from excitation of cis-stilbene in hexadecane.
I 0
I 3
6
9
12
15
Time (pa)
Figure 3. Magic angle transient absorption decay and anisotropy decay for cis-stilbene in hexane pumped at 312 nm and probed at 480 nm. The rotational reorientation time is T~,,, = 7 f 2 ps for the DHP product in this experiment.
at longer times. Typical "magic angle" decay curves for 3 12-nm pump and 480-nm probe experiments for cis-stilbene in hexadecane, hexane, and methanol are shown in Figures 2, 3, and 4, respectively. With probe wavelengths near the long wavelength edge of the DHP absorption band, the absorption coefficient drops with a time constant of ca. 35 ps before reaching a constant absorption level. This initial decay is attributed to cooling of the DHP photoproduct. By collecting decay curves at several
wavelengths in the 420-580-nm region, the spectra of what appear to be hot and cold DHP product are obtained. Figure 5 shows the early time ( 5 ps) spectrum of DHP formed from cis-stilbene overlaid on the spectrum taken at 100 ns. There is no significant change in the spectral shape out to the longest delays employed (a. 50 ms). This is shown in Figure 6 which displays the spectrum of DHP taken at delay times of 200 ps, 100 ns, and 50 ms. The kinetics of the appearance of DHP from cis-stilbene in hexadecane photolyzed by a 250-nm pump and probed at 460 nm are found to be similar to those obtained with the 312-nm pump. This result justifies the comparison of femtosecond experiments, which normally use 312 nm as the excitation wavelength, with nanosecond experiments which excite at 266 nm. The "magic angle" decay curves were tit with various functional forms. In the first procedure t h e exponential decay constant of cis* ( T ~ Jand the rise of the DHP product ( T ~ were ~ ~treated ) as free parameters using a functional form of the response given by the following I ( t ) = A I ex~(--t/~cis)+ A2(1 - ~xP(-~/TDHP)) (1) where A , is the amplitude of the cis* absorption signal and A2
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10383
&Stilbene to Dihydrophenanthrene Reaction
TABLE I growth solvent hexadecane
A, nm 540 520 500 480 465 450 420
hexane
7ciw
Ps
PS
~DHPI
1.3 1.3 1.3 1.6 1.5 1.4 1.3 av 1.4 f 0.1 1.15 0.50
480 480
methanol
intermediate 1.5 1.5 2.0 1.6 1.6 1.8 1.6 1.7 f 0.2 1.2 0.6
7ciw
PS
71,
1.4 1.3 1.3 1.5 1.5 1.5 1.3 1.4 f 0.1 1.1 0.51
fs
300 350 450 200 200 300 700’ 300 f 200“ 100 195
~DHP(O) 0.19 0.13 0.15 0.17 0.15 0.22 0.16 0.17 f 0.03 0.16 f 0.04 0.24 f 0.03
“The average value of 300 fs does not include the 420-nm data value which had poorer signal to noise. experiment and show no further dynamics out to 50 ps. In the case of cis-stilbene in methanol solvent probed at 480 nm a small long time growth (50 ps) is present which may be due to the absorption growing in as DHP cools. Anisotropy Measurements. The measured anisotropy for a transient absorption experiment is given as
=
[Ill(t)
- ZL(t)l/[qt)
+ 21,(t)l
(6)
Anisotropy decays for cis-stilbene in hexadecane, hexane, and methanol probed with 480-nm light are shown in Figures 2, 3, and 4. The solid line are anisotropies calculated from the relation
340
300
420
460
500
540
580
Wavelength (nm) Figure 6. Spectra of DHP formed from cis-stilbene in hexadecane at delay times of 200 ps (solid line), 100 ns (dashed line), and 50 ms (open circles). The 2 W p s spectrum includes a transient absorption feature due to trans-stilbene impurity which results in the peak at 580 nm.
is the amplitude of the DHP persistent absorption with a growth time of T D H ~ . The second procedure corresponds to a model in which the cis* population passes through an undetected intermediate state, I, on the way to ground-state DHP. The changes of the populations of cis*, the intermediate I, and DHP ground state are acis(t) = -klncis(t) (2) a ~ ( t= )
klncis(t)
- kzn~(t)
(3)
~DHP= ( ~k) d t ) (4) where kl = l/rcis and k2 = 1 / q . The growth of DHP product is then given as ( k , # k,) ~DHP= ( ~ncis(o)[(k2/(k~ ) - k,))X (1 - exp(-klt)) + (1 - (k2/(k2 - k J ) ) ( l - exp(-kd)l ( 5 ) where ncis(0)is the cis* population which ultimately isomerizes to DHP. In both procedures an additional exponential decay was added to account for the small ca. 35-ps decay of the DHP absorption coefficient assumed due to vibrational cooling. A summary of the results from the two fitting procedures for different probe wavelengths in hexadecane and 480-nm probe in hexane and methanol is given in Table I. The quality of the fits to eqs 1 and 5 are comparable as long as T D H in ~ eq 1 is allowed to vary freely. The value given in Table 1 of 0.5 ps for the decay of cis-stilbene in methanol is consistent with a value of T~~~= 0.47 f 0.07 ps obtained when probing absorption at 650 nm where the cis-stilbene excited state is the only absorbing species. The other 7,is decay times given in Table I are consistent with those obtained by probing at 650 nm, recorded previ~usly.~ They are also consistent with the cis* fluorescence decay times recorded in ref 5. The data obtained using a 250-nm pump was too noisy to be fit accurately and so is not included in Table I. However, these data exhibit the same relative ratio for the cis* and DHP signals as the 312-nm
r o d t ) = al(t) ras(t) + a2(t) rDHP(t) (7) where CY,and a2are the fractional absorption signals for the cis excited state and D H P ground state, respectively (an(t)= Z,,(t)/Z,oml(t) where Z,,(t) is obtained from the “magic angle” fit of the data). For example, in hexadecane solvent the initial anisotropy of cis* is rds(0) = 0.28 i 0.03. Upon disappearance of the cis* population, r0&) reduces to the anisotropy of the formed DHP product of rDHp(0) = 0.17 f 0.03. The “initial” anisotropy of the D H P product, rDHp(O), is defined as the anisotropy of the DHP formed at the earliest time obtainable from the extrapolation of experimental data by means of deconvolution of the instrument function from the assumed kinetic response. The long time anisotropy decay component, modeled as a single exponential, results from rotational diffusion of the DHP product molecules which display a rotational reorientation time of ca. 35 f 10 ps in hexadecane. The initial anisotropy of cis* in the region around 450 nm varies significantly with probe wavelength due to the presence of a gain signal from the cis-stilbene excited-state fluorescence.I8 The contributions of cis* gain and absorption to the anisotropy at early times can be separated, but due to the rapid decay of the cis excited-state population the measured anisotropy of formed DHP is hardly affected. The initial anisotropies of the DHP product (Le., rDHP(0))probed a t several wavelengths in hexadecane and for all three solvents probed at 480 nm are included in Table I. The wide variation in the anisotropy values reported in this table signifies the poor signal-to-noise ratio at most of the probe wavelengths studied. However, more extensive measurements were performed at 480 nm from which a best value of rDHp(0) = 0.17 f 0.03 in hexadecane was derived. This value is consistent with the average over all wavelengths studied. Dihydrophenanthrene. When pumped with a pulse a t 450 nm and probed at 460 nm, the DHP absorption is bleached and its recovery can be investigated. The results in hexane solvent are shown in Figure 7. The top panel is taken with the pump and probe polarizations set at the “magic angle”. The bottom panel is the Z,,(t) signal which contains orientational information. The small amount of signal prohibited the measurement of the perpendicular curve directly. The Zll(t) signal decays with a single exponential to a persistent bleach, giving a rotational reorientation time of 5 f 1 ps. Neither the Il,(t) nor “magic angle” data show any further dynamics out to the longest time measured of 50 ps. An instrument-limited positive absorption feature is present in the bleaching experiment when the pump and probe beams overlap, which appears to be due to two-photon absorption by the large
10384
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991
1
DHP BlmohlMag~C Angle
-00000
~
I
-0 00 1 5
-
-5
I 0
5
10
15
20
Time (PSI 0001,
I DHP BlsachIParallel Polarization
r
-0.00s
-5
0
5
10
15
20
Time (ps)
Figure 7. Decay curves for the 450-nm pump, 460-nm probe dihydrophenanthrene in hexane experiment: (top) magic angle bleach of DHP; (bottom) parallel polarization experiment which yields a rotational reorientation time of rrOt= 5 f 1 ps.
amount of cis-stilbene in the sample solution. The data have been plotted so that the data points that trace the two-photon absorption are off scale, but it can be seen in the calculated curves (solid lines). The two-photon absorption nature of this signal is confirmed by a number of measurements: it has an anisotropy of 0.4, it does not diminish when the frequency separation of the pump and probe beams is further increased (440-nm pump, 480-nm probe), and it grows in intensity as the DHP concentration drops. No recovery of the DHP bleach is observed. Any recovery with time constant T 5 500 fs would be obscured by the 800-fs instrument function and by the strong two-photon absorption. The DHP population recovery was not explored beyond 50 ps. Discussion Dihydrophenanthrene Formation. The data presented in the previous section indicate that vibrationally hot ground-state dihydrophenanthrene is formed very rapidly from electronically excited cis-stilbene (certainly within 300 fs of the decay of cis*). The vibrationally hot DHP molecules cool with a time constant of about 35 ps. The spectral change due to cooling of DHP is not as pronounced as that seen for the trans-stilbene product,14 presumably because of the Franck-Condon factors and because there is much less excess energy available. The presence of a signal from the small trans-stilbene (impurity) excited-state absorption on the long wavelength edge of the DHP absorption makes it difficult to make more quantitative statements regarding the DHP cooling at present. After the initial cooling of ground-state DHP, the data from 200 ps through 50 ms shows that the DHP spectrum remains essentially constant, exhibiting a peak around 450 nm with approximately the same width and shape as the steady-state spectrum (see Figure 6). Attempts to fit the experimental data to eq 1 with T , , ~and T~~~ as free parameters showed that the best fits were always obtained for T~~~ not equal to T,,,. The best fits consistently contain a delayed formation of the DHP signal. This is in contrast to the transient absorption results for the formation of trans-stilbene from cis excited at 312 nm. The trans-stilbene ground-state absorption has been found to rise with a time constant equal to that of the cis* d e ~ a y . ~ . ' ~ The formation of the DHP signal a few hundred femtoseconds after the disappearance of cis* is made even clearer by the probe
Repinec et al. experiments at wavelengths across the absorption band of DHP which show slightly different delays in the rise of the product absorption. In the model described by eq 5 it is assumed that there exists a kinetically significant but unobserved intermediate possessing a distinct lifetime. The most likely possibility is that the intermediate is a structure or distribution of structures located on the excited-state surface somewhere between the cis and DHP configuration regions as diagrammed in Figure 1. The other possibility, that DHP is formed in its electronically excited state which is perhaps stable for as long as a few hundred femtoseconds before undergoing internal conversion to the ground state, is deemed unlikely on energetic grounds. It must also be considered that the evolution of the DHP population as measured by absorption is not exponential as assumed. The intermediate having a lifetime of 300 f 200 fs is not observed directly but is demanded by fitting the kinetic response (eq 5) to the data. Unfortunately, no absorption spectral signature of an intermediate could be observed in the region 320-900 nm. Since the cis to DHP cyclization is understood to be a conrotatory process, allowed in a Woodward-Hoffman sense, there is no requirement for a conventional structural intermediate. So the observations more likely manifest motion on the cis to DHP potential surface. The distinction between the exponential response function (eq 5) and a nonexponential evolution of DHP will be difficult to make with experiments at the present time resolution. Clearly, investigations are required on the sub- 100-fs time scale. Statistical mechanical models for the appearance of DHP similar to those considered earlier4 for the trans-stilbene pathway can exhibit nonexponential kinetics. Thus, the present observations could result from motion toward DHP on a barrierless potential surface. Current work involves fitting the observed kinetics to these and otherI9 models of barrierless dynamics. Molecular Alignment and Rotational Relaxation. The measured anisotropies reported in Table I can be related to the angle between transition dipoles of the reactant cis* molecules and the product DHP molecules by the correlation function rcD(0) = 0.4(P2[cos &D]). is the angle between the cis and DHP transition dipoles in a molecule-fixed frame of reference, P2is the second Legendre polynomial (P2(X) = ' / 2 [ 3 x Z- l]), and ( ) indicates an ensemble average over the probability distribution of angles BcD in the molecule-fixed frame of reference. The So SI transition dipole of dihydrophenanthrene is directed along the longest axis of the molecule; this result is predicted by a simple exiton mode120and by recent QCFF/PI calculations which indicate a 2O displacement in the direction of greatest extension of the a-electron system.2' The So S, transition moment vector of cis-stilbene has been calculated by Dick22and is predicted to lie approximately along the ethylene bond with a 7 O displacement in the direction of greatest extension of the a-electron system. These dipole directions are imposed by the *-electron nodal patterns and are unlikely to have significant error. With these transition dipole directions and a simple reaction coordinate involving only phenyl ring rotation, the expected angle OcD should be near zero (ca. 5') so that the initial anisotropy of the DHP product should be close to 0.4. However, experimentally the initial anisotropy of the product DHP molecules in hexadecane is r(0) = 0.17 0.03, which is much smaller than expected. Assuming a narrow distribution of dipoles, the angle between the cis reactant and the DHP product transition dipoles is 6 c D = 38
-
-
*
=k
3O.
Two contributions to the large misalignment of the cis reactant and DHP product dipoles have been discussed previously.12 In the first of these the nature of the reaction coordinate results in the misalignment. In the second, the forced motion of the whole (19) Bagchi, B.: Fleming, G. R.: Oxtoby, D. W. J . Chem. Phys. 1983, 78, 1315-1 385. (20) Muszkat, K. A . Top. Curr. Chem. 1980, 88, 89-143. (21) Myers, A . B. Personal communication. (22) Dick, 8. Personal communication. (23) Lee, M.-Y.: Haseltine, J. N.; Smith, A. 9.; Hochstrasser, R. M. J . A m . Chem. Sac. 1989, I l l , 5044-5052.
cis-Stilbene to Dihydrophenanthrene Reaction
TABLE I1 T,",,
solvent hexane hexadecane methanol
PS
trans
DHP
14 f 3 82 f 7' 16f3
5f1,"7f2b 35 f lob 14 f 3 b
"From DHP bleach experiment shown in Figure 6. bFrom transient absorption at 480 nm shown in Figures 2-4. CReference23. molecule in the laboratory frame is considered. Both of these possibilities are consistent with but not explicitly brought out in previous attempts to define the reaction ~oordinate.~," The low DHP anisotropy may be partially attributed to the nature of the reaction coordinate. A displacement of the ethylene bond axis of cis-stilbene will result if the central carbon atoms rehybridize to tetrahedral geometry during the isomerization.I2 The cis-stilbene excited state has a high initial anisotropy (ca. 0.36) when probed in the region around 650 nm? indicating the transition dipoles of cis* and ground-state cis-stilbene transitions are near parallel. Therefore, the movement of the ethylene bond axis must occur during the isomerization reaction following the decay of cis*. The similarity in misalignments for both transstilbene and DHP products might evidence that these two isomerizing pathways have some important common features. However, at some point the population flow must partition between cis, trans, and DHP with trans-stilbene being formed promptly and the population going to DHP being delayed, as discussed earlier, for about 300 fs. Secondly, the coupling of internal motions of the isomerizing molecule to the overall rotation in the laboratory frame can also contribute to the loss of anisotropy.I2 Any rotation of the phenyl rings (or other reaction trajectory that generates angular momentum) will force the whole molecule to reorient in the laboratory frame and account for an initial DHP anisotropy that is lower than expected. In the simplest picture only the rotation of the phenyl rings need be common between the trajectories toward trans and DHP. Most of the alignment experiments have been carried out in hexadecane because of the slower overall rotation of cis-stilbene and DHP in this viscous solvent. However, we have also performed precise anisotropy measurements for the cis to DHP reactions in hexane and methanol probed at 480 nm. Table I1 lists the rotational reorientation times of DHP and trans-stilbene in the solvents used in this study. Even though the reorientation time of DHP in hexane is much faster than in hexadecane, the initial anisotropy of rDHp(0) = 0.16 f 0.04 indicates a similar initial alignment in these two alkane solvents. This result shows that the reaction coordinate is not strongly influenced by the solvent viscosity (7 = 3.34 and 0.326 CP at 20 O C for hexadecane and hexane, respectively; 7 = 0.597 CPat 20 O C for methanolz4). The initial anisotropy for DHP in methanol, rDHP(0)= 0.24 f 0.03, indicates a somewhat smaller angle (8CD = 31 f 3') between the reactant and product transition dipoles than in alkanes. This difference may arise from a solvent-induced modification of the potential surface surface such as occurs with trans-stilbene in alcohols.25 It is also interesting that the overall reorientation times of DHP and trans-stilbene are the same in methanol, but DHP reorientation is 2-3 times faster than that of trans-stilbene in alkanes. The much faster rotation time of DHP in alkanes is presumably a result of its more compact shape providing a significantly smaller (24) CRC Handbook of Chemistry and Physics, 62nd ed.; Weast R. C., Ed.; Chemical Rubber Co: Boca Raton, FL, 1981. ( 2 5 ) (a) Sundstrom, V.; Gillbro, T. Chem. Phys. Lett. 1984,109,538-543. (b) Hicks, J. M.; Vandersall, M. T.; Sitzmann, E. V.; Eisenthal, K. B. Chem. Phys. Lett. 1987, 135, 413-420. (c) Kim, S. K.; Courtney, S. H.; Fleming, G . R. Chem. Phys. Lett. 1989, 159, 543-548.
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10385 slip friction coefficient than does trans-stilbene.26 Formation of &-Stilbene from DHP. Steady-state measurements have shown that upon excitation of dihydrophenanthrene at 436 nm about 66% of the excited molecules isomerize to cisstilbene with the remaining 34% returning to DHP ground states6 Given these yields, the bleach of the DHP absorption at 450 nm should show a partial recovery. The absence of such a recovery in the magic angle bleach experiment shown in Figure 6 indicates a very rapid recovery, occurring within the period during which the two-photon absorption feature is dominant. This means that the lifetime of this electronically excited state of DHP is less than 500 fs. A recovery on a time scale much longer than the measured 50 ps could also account for the results. This would require a long-lived DHP excited state, unlikely because there is no significant transient absorption or fluorescence that can be attributed DHP. The conclusion regarding the ultrafast ring opening of DHP* is consistent with the potential energy surface in Figure 1 with no barrier on the DHP excited-state surface at the Franck-Condon region. The "intermediate" for cis to DHP and DHP to cis may be common, with the same lifetime of T I300 fs.
Conclusions In this paper the results of a series of time-resolved transient absorption and transient bleach studies of the photoinduced cis to DHP and DHP to cis reactions have been presented. These experiments represent the first time that picosecond and subpicosecond time resolutions have been used to study this reaction pathway in the stilbene system. The ground electronic state dihydrophenanthrene product is formed very quickly (q,HP = 1.7 f 0.2 ps) from cis-stilbene excited at 312 nm. When fit to a model including an intermediate state of finite lifetime (not observed directly), the lifetime of the intermediate is found to be 300 f 200 fs. There is no evidence for any intermediate state having a lifetime of more than a few hundred femtoseconds in the cis to DHP reaction. Given the current knowledge of the cis to DHP reaction pathway, the observed kinetics may indicate the occurrence of reactive motion on the potential energy surfaces in the crossing region, giving rise to a nonexponential population evolution. For example, the system may undergo some low-frequency oscillations near the surface intersection. The DHP appears to be formed vibrationally hot and cools on a time scale of approximately 35 ps. Afkr the initial vibrational cooling, the DHP spectrum remains essentially constant from 200 ps to 50 ms. Anisotropy measurements of the cis to DHP reaction indicate large angles (OCD= 38 f 3' for alkanes and 8CD = 31 f 3' for methanol) between the reactant cis-stilbene and initially formed product DHP transition dipoles. The unexpectedly low initial anisotropy is a result of reorientational motion of the relevant transition dipoles occurring during the isomerization process. The anisotropy results also allow a comparison of orientational relaxation times of trans-stilbene and DHP. In alkane solvents the reorientation time of DHP is 2-3 times faster than trans, but in methanol they are comparable. Transient bleach experiments performed on DHP show no recovery of the bleached ground-state absorption. This indicates that the excited state of DHP has a lifetime of less than 500 fs due to the ring-opening reaction. Acknowledgment. We thank Dr. C. M. Phillips for his help in obtaining the nanosecond spectra of DHP and Dr. A. R. McGhie for the use of his mercury lamp. This research was supported by the NSF and NIH. Registry No. DHP, 13020-78-5; cis-stilbene, 645-49-8; methanol, 67-56-1; hexane, 110-54-3; hexadecane, 544-76-3. (26) Hu, C.-M.; Zwanzig, R. J . Chem. Phys. 1974, 60, 4354-4357.
