J. Phys. Chem. 1992,96,6559-6563
6559
Picosecond TimeResolved Absorption Spectrum Measurements of the Higher Excited Singlet State of Dlphenylacetylene in the Solution Phase' Yoshinori Hirata,* Tadashi Okada, Noboru Mataga, Department of Chemistry, Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560, Japan
and Tateo Nomoto Department of Chemistry, Faculty of Education, Mie University, Tsu. Mie 51 4, Japan (Received: March 30, 1992)
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The photophysical properties of diphenylacetylene(DPA) in various solvents have been investigated by picosecond time-resolved absorption spectrum measurements. The absorption bands at 435 and 700 nm, assigned to the S, SItransition of DPA, decayed with a lifetime of about 200 ps, while the short-lived band at 500 nm was ascribed to the absorption from a second excited singlet state, S2 The fluorescence state of DPA was not the lowest excited singlet state but the short-lived S2 state. The change of the vibronic interaction between the excited singlet states from the intermediate case coupling to the statistical limit was deduced by the measurements of the excitation wavelength dependence of the Szyield. The lifetime of the 500-nm band followed Arrhenius type temperaturedependence, of which the activation energy was about 890 cm-I in n-hexane, while the lifetime of SIstate did not depend on temperature significantly. The large temperature effect of the S2 state lifetime coupling. might be due to the turnover of the Sl-S2
Introduction The lifetime of large organic molecules in the higher excited singlet states is usually much shorter than that of the SIstate. Therefore, except for a few molecules such as azulene and xanthione,2J no fluorescence from the higher excited states is detected in solution. Aromatic molecules with a Sz-SI energy gap of a few thousand wavenumbers should belong to the statistical limit for the S2 SIinternal conversion, while such separation seems to be so small that the interstate coupling is large, leading to the result that the lifetime of S2 falls in the subpicosecond region. Although time-resolved absorption studies will be able to provide information on such nonfluorescent excited states, there have been very few absorption studies with a picosecond time resolution! Diphenylacetylene(DPA) seems to be a unique molecule since it was reported to show a dual fluorescence in the gas phase in spite of a small S2-Sl ~eparation.~ DPA might be characterized by the fact that the lowest three excited singlet states are in close proximity i.e., within a few hundred wavenumbers. Only one of them leads to an allowed one-photon transition from the ground state. Although Okuyama et aL5investigated DPA in a supersonic free jet from a spectroscopic interest in large-amplitude torsional motion, details of its radiationless transitions in the excited states is not clear yet. According to the MO calculation,6 it is predicted that three excited singlet states are contained in the first absorption band of DPA. They are BzU,BIU,and A,, under D2hsymmetry. Only the transition B, A,, (So) is allowed. The B1, -A,, transition A,, transition is parity is orbitally forbidden, while the A,, forbidden. These facts are confirmed by fluorescence excitation and two-photon resonant four-photon ionization studies in the supersonic free jete5The Bzuand AI, states are located at 35 248 and 34 960 cm-I, respectively. Since only vibronically induced bands are observed for BIU,the origin lies at lower than 35 051 cm-'. In the solution phase, the energy levels should shift and it is difficult to get the information about either the molecular structure or the energy of these states. However, it has been confinned that the fmt UV-absorption band is polarized parallel to the long axis of the molecule? Therefore, the excitation around 290 nm should lead mainly to the vibronic manifold in the Bzustate, which i s the S3state in the gas phase. We have measured picosecond transient absorption spectra of DPA in various solvents, and the short-lived absorption band which should be attributed to the higher excited singlet state has been
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observed around 500 nm. The state with a lifetime of about 8 ps is a fluorescence state and a precursor of the dark SIstate. The triplet state was formed via the intersystem crossing from SIwith a time constant of about 200 ps. The rather long lifetime of the higher excited singlet state could be the result of a sparse level density of the accepting mode for the Sz SIinternal conversion. The excess energy dependence of the Sz state lifetime strongly suggests a change in the interaction between the excited singlet states from intermediate case coupling to the statistical limit with increasing excitation energy. A large temperature effect on the S2 state lifetime also suggests this hypothesis.
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Experimental Section Picosecond transient absorption spectra were measured by using a dye laser photolysis system pumped by the second harmonics of a mode-locked Nd3+:YAGlaser (Quantel, Picochrome). The details of this system were described &where.' The optical delay line which adjusts the delay time between the excitation and interrogating pulses moves stepwise with 5 pm/step and 50 step/s. Since the laser system was operated at 10 Hz, the transient absorption spectrum was obtained at every 0.167 ps. Some measurements were done with the scan rate of 100 step/s, and therefore the spectrum was measured every 0.33 ps. The time dependence of the transient absorbance was calculated and averaged for the given width of wavelength, while the spectra corrected for the wavelength dependent arrival time of the probe pulse were obtained by accumulating typically 25 data points, which correspond to a time window of 4.2 ps. Absorption spectra at long delay times, not corrected for the time-dependent arrival time of the probe pulse, were measured at a fined delay and were accumulated for typically 30 shots. The fluorescence lifetime was measured by using timwrrelated single-photon counting. The sample was excited with the synchronously pumped rhodamine 6G dye laser fitted with a cavity dumper operated at 4 MHz. For the measurements of the temperature effect, a sample cell was put into the quartz dewar with flat windows. The temperature was monitored by the thermocouple in contact with the cell holder, and the temperature was controlled by changing the flow rate of cold N2 gas evaporated from liquid Nz. DPA (Aldrich,, 99%) was purified by repeated recrystallization from ethanol. GR-grade isopentane, methylcyclohexane, and tram-decalin were purified by repeatedly passing them through
0022-3654/92/2096-6559303.00/00 1992 American Chemical Society
6568 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 r
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Hirata et al.
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w 1 p I
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500 600 700 800 9 0 0 WAVELENGTH I nn I Figure 1. Picosecond time-rtsolvcd absorption spcctra of DPA in nhexane at room temperature. Delay tima after the laser pulse excitation arc indicated in the figure.
400
a column of activated alumina and silica gel. Spectmgmde ethanol was dried by contacting with activated molecular sieves and then was distilled after decantation. Spectrograde n-hexane, cyclohexane, and acetonitrile were used without further purification. The samples were prepared in quartz cells of 1-cm optical path length. Samples were degassed by several freeze-pumpthaw cycles. Some samples were deaerated by using a flushing N2 stream. Convolution curves of the transient absorband were calculated by using ACOS-2000 of the Computation Center at Osaka University. Rdts (i) Picosecond Transfer Absorption Measurements. The picosecond time-resolved absorption spectra and time dependence of the transient absorbance of DPA in n-hexane at room temperature (2% IC) excited with a 295-nm dye laser pulse are shown in Figures 1 and 2, respectively. At delay times longer than 500 ps, the spectrum was chartsterized by a sharp and intense band around 415 nm. The band, which did not show si@icant decay in the measured delay time range, was assigned to the T,, TI transition of DPA because of the similarity of the peak position and the spectral shape to those of the reported spectrum of the triplet band of DPA.* A drastic change of the transient absorption spectrum was observed before the triplet-state formation. Immediately after excitation, the short-lived band appeared around 500 nm, while the bands around 435 and 700 nm were dominant in the 30-100-p range. At415nmrapidgrowthofthe~entabeorbancelimired by the response time of our apparatus was followed by additional slowergrowthsasshownin F i 2 . Althoughthemiddlegrowth was difficult to separate from the response-limited one at room temperature, the slowest one was ascribed to the triplet formation and was characterizcd by an exponential rise with a time constant of 190 f 15 ps. The decay monitored at 700 nm was a single exponential with a lifetime of 208 f 10 ps. The time dependence of the transient absorbance measured at 436 nm can be analyzed as a biexponential decay. The long-lived component was almost constant in the time range of our measurements, while the lifetime of the short-lived one was 205 f 23 p. These results clearly show that the 435- and 700-nm bands should be due to the same transient species that was assigned to a p~ecursorof the triplet state of DPA. The long-lived component at 435 nm should be due to the triplet band.
delay time [ p s ] Figure 2. Time dependence of the transient absorbance of DPA in nhexane. Probing wavelengths are indicated in the faure. logarithmic plota ( c l d symbols) are also given at 415 (rising component), 436 (fast decaying component), and 700 nm. r
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TIME
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Figure 3. Time dependence of the transient absorbance of DPA in nhexane in the short delay time region.
The time dependencies of the transient absorbance in the shorter delay times are shown in Figure 3. The smooth lines in the figure represent the simulation curves calculated with the assumption of the doublc-expondal decay kinetics. A"ing the excitation and interrogation pulse width of 9 and 18 pa, respectively, the rise time at 700 nm was determined to be abowt 8 ps. The decay time of the short-lived component at 500 nm was in agreement with the rise time at 700 nm. Similar time dependences of the transient absorption spectra were observed in other solvents, and the determined rise and decay times at various wavelengths are listed in Table I. No significant solvent effect on the decay times was
Higher Excited Singlet State of Diphenylacetylene
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6561
TABLE I: Rise and Decay T i m (piccweeoads) of the Transient Absorb.necObtained at Several Warelength at 2% K solvent isopentane n- hexane cyclohexane methylcyclohexane ferf-decalinb ethanol acetonitrile
415-nm rise 176 f 14 190 i 15 203 i 12 201 16 185 i 17 205 f 16 203 17
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436-nm decay 181 17 205 23 206 18 207 f 21 198 24 211 23 208 20
500-nm decay" 8 8 9 8 9
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700-nm d a y 195 & 13 208 IO 204 16 210 14 195 f 15 206 12 201 16
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Figure 5. Time dependence of the transient absorbance at 500 nm of DPA in isopentane at several temperatures.
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500 600 700 BOO 900 WAVELENGTH I n m 1 Figure 4. Picosecond time-resolved absorption spectra of DPA in isopentane at 123 K. Delay times after laser pulse excitation are indicated in the figure. 400
observed. These results suggest that the deactivation process of DPA excited with a near ultraviolet light pulse is characterized by the scheme So 2 X
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TI
where X and Y show the transient states responsible for the 500and 700- (435-) nm bands, respectively. (ii) Temperature Effect. A drastic temperature effect was observed for these systems between room temperature and the freezing point of the solvent. Figure 4 shows the time-resolved absorption spectra in isopentane at 123 K, and the decay curves of 5Wnm band at several temperatures are displayed in Figure 5. The lifetime of the 500-nm band increased with decreasing temperature. At 700 nm the rise time also increased with decreasing temperature, while the decay time did not change significantly at the higher temperatures. The intensity of the 7Wnm band decreased at low temperatures, and the band was hardly observed at lower than 180 K. These results indicate that the lifetime of X increases with decreasing temperature, while the lifetime of Y remains unchanged. In the low-temperature region the formation rate of Y was slower than its depopulation rate, and thus the concentration of Y decreased. If we could analyze the time dependence of the transient absorbance at 700 nm at low temperatures, the decay time should be the same as that of the 500-nm band. The rather poor dynamic range of our measurements did not allow such analysis. Figure 6 shows an Arrhenius plot for the decay rate measured at 5 0 0 and 700 nm and the rising rate at 415 nm in several solvents. The Arrhenius plot for the decay rate of the 500-nm band showed a good linear relation except for the temperature
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Figure 6. Arrhenius plot for the S2(open symbols) and SI(closed symbols) lifetimes of DPA in various solvents, isopentane (0),methylcyclohexane (A),and trans-decalin (0). Fluorescence lifetime (X) in isopentane is also shown in the figure. TABLE II: Activation Energies (cm-I) a d Frequency Factors in Several Solvents
methyl-
(1Ol* 9-l)
tert-
isowntane n-hexane cvclohexane dccalin ethanol activation energy 843 891 940 1100 925 frequency factor 4.2 6.3 1.8 18.2 7.1 activation energy 520 590 600 1160 lo00 of diffusion
region lower than 160 K. The deviation from a straight line in the lower temperature region should be due to the radiative process and the S2 TIintersystem crossing. The activation energies and the frequency factors deduced from the straight lines in the figure are listed in Table 11. In the case of trans-decalin solvent,
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6562 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 1 .c
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Hirata et al. was almost independent of the solvent. (iii) The ratio (R) of the initial population of X to Y showed an excitation energy dependence, while the lifetime of X did not show a significant excess energy dependence in the excitation wavelength region between 280 and 310 nm. R seemed to decrease with decreasing excess energy except for the region of the 0-0 band. The observations clearly show that the dynamic behavior of DPA in the excited state was different from that of the usual aromatic molecules. (i) Assignment. The fluorescence state was the short-lived X, although we cannot determine yet whether it was 'B2,, or IBIU. We do not know the assignment of the precursor of the TI state, Y, either. There are some possibilities of the assignments as shown in the following. Group 1 is concerned with the configurational isomerization, while Groups 2 and 3 involve the electronically higher excited state. (1.1) X is the SI state of DPA and Y is the configurationally different excited singlet state. In this case the SImight be assigned to the optically allowed Blu state, which is the S3state in the gas phase, because of a small Stokes shift and a rather large fluorescence yield of (1.2) X is SI (B2J and Y is the triplet state of which the configuration is different from that of the stable T I state. (2.1) X is the higher excited singlet state and Y is the SIstate. In this case X should be B2,, and Y should be the state, AI, or
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("m) Figure 7. UV absorption, fluorescence, and its excitation spectra of DPA in n-hexane and excitation wavelength dependence of the ratio of the initial yield of S2and SIstates, R (@), normalized at 290 nm. Relative fluorescence yield (0)is also shown in the figure. Smooth lines are just to guide of eye. both the activation energy and frequency factor obtained may be overestimated since the measured temperature region was not as wide as for other solvents. The plot may not be linear in the higher temperature region because the lifetime and (vibrational) cooling time may be comparable. The cooling time of many aromatic hydrocarbons in solution is a few tens of picoseconds, and in this time region the internal temperature of the excited molecules is higher than the envir~nment.~*'~ (iii) Fluorescence Measurements. Figure 7 shows the UV absorption, fluorescence and fluorescence excitation spectra of DPA in n-hexane at room temperature. The absorption spectrum was similar to that in the gas phase and showed a progression of the C W bond stretching mode (2150 cm-I). The relative fluorescence yield, which decreased drastically with increasing excitation energy, is also shown in the figure. A drastic decrease of the fluorescence yield was reported also in the supersonic beam.5 Fluorescence decay curves excited at 296 nm and monitored at 325 nm can be analyzed as a singleexponential decay, and the temperature dependence of the fluorescence lifetime is shown in Figure 6. The fluorescence lifetimes were in agreement with the decay time of the 5Wnm band. Therefore, the fluorescence state should be X, while the triplet state was formed from the dark state, Y. (iv) Excitation Wavelengtb Effect of the Initial Population of X and Y. Extrapolating the decay curves measured at 500 and 700 nm to t = 0, we can estimate the relative yields of X and Y. The excitation wavelength dependence of the ratio of initial intensity, R = 10(S00)/10(700) @(X)/(@(X)+ @(Y)),is also shown in Figure 7 . In the first absorption band, R was larger around 290 nm than in the shorter wavelengths, whereas the decay time at 500 nm did not change significantly in this wavelength region.
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Discussion The experimental observations can be summarized as follows: (i) In the solution phase, DPA did not show dual fluorescence and the fluorescence state (X)was difference from the precursor of the T I state. (ii) The lifetime of X showed a large temperature effect, while Y was affected little by changing temperature. The activation energy of the lifetime of X was about 900 cm-', which
BIU.
(2.2) X is the thermally populated higher excited singlet state with a large oscillator strength for the transition to the ground state and Y is the optically forbidden SI state. This is similar to the possibility 2.1, although the lifetime of X should be determined by the cooling time of the excited molecule. (3.1) X is SI and Y is higher triplet state (T2). The configurational changes (1.1,2) in the excited state may be ruled out because only a very small solvent viscosity effect was observed for the lifetimes of X and Y. The temperature effect should provide stronger evidence to distinguish among the possibilities. As listed in Table 11, the activation energy of the depopulation process of X was almost independent of solvent, although the activation energy of the diffusion (deduced from the viscosity of solvent) increases almost two times by changing solvent from isopentane to trans-decalin or ethanol. On the other hand, the decay kinetics of Y did not show significant temperature effects, and the activation energy should be much smaller than that of the diffusion. In addition to this, the observed lifetime of Y, i.e., about 200 ps, seems to be too long for the configurationalchange along the torsional motion of phenyl ring or the CsC-C bending motion. We can safely conclude that the possibility 1.2 is not plausible. The fluorescence excitation spectrum measurements in a supersonic free jet also provide evidence to eliminate the possibility of configurational change. The observed mutual exclusion principle between the one-photon and two-photon excitation spectra indicates that DPA has an inversion symmetry in both the ground and excited states and that the possible structure should be planar in the gas phase. The torsional mode of the phenyl ring has a higher frequency in Blu and B2,, states than that in the ground state, which indicates that the n-conjugation is stronger in the excited states. Therefore the configuration change from the planar to twisted form in the excited state is not plausible. Possibility 3.1 may be rejected since the lifetime of Y was too long for the electronically higher excited state (T2). We have other evidence to deny this possibility. As we have reported, both X and Y are formed by the photodissociation of diphenylcyclopropenone within 200 fs." It is reasonable to eliminate the possibility of the triplet state because of a quite rapid formation of Y. Possibility 2.2 should be related to the phenomena concerned with the dissipation of the vibrational excess energy into the solvent. Following the intramolecular vibrational redistribution, the local temperature of the excited molecule becomes high, and the excess energy is transferred to solvent molecules in the surroundings. Such a process is known to occur in the picosecond
Higher Excited Singlet State of Diphenylacetylene
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6563
to tens of picosar>ndstime scale both for the electronically excited9 and ground state.I0 If this is the case, the lifetime of X should be determined by the excess energy of excitation. Since the lifetime of X did not show significant excitation energy dependence (observation iii) and even by the 0-0 band (296 nm) or the hot band (305 nm) excitation the 500-nm band was observed, possibility 2.2 can be ruled out. Another evidence to reject such possibility is a extremely long lifetimes of X at low temperatures. Possibility 2.1 appears to be the most plausible case, and the explanation for this is as follows. The three lowest excited singlet states of DPA are located very close in energy, and only the transition B2,, AI, is allowed in one-photon absorption. Therefore the excitation at 295 nm should lead mainly to the B2,, state. Because of the small energy 8ap of less than 300 cm-I between the Bzuand SIstate,s the interstate coupling may be classified not to the statistical limit but to the intermediate case. If the S2-Sl energy separation is sufficiently large, the coupling should be the statistical limit and may result in a quite short lifetime of the S2 state. For example, phenanthrene provides an example of such a case, in which the S2-SIseparation is about 6000 cm-l; measuring the bandwidth of the S2 Soabsorption in a supersonic beam, the lifetime of S2 was estimated to be 0.5 ps.I2 On the contrary, S2 SIinternal conversion of DPA is much slower and should belong to the intermediatecase, where the level density of the final state is too low to ensure ultrafast internal conversion. By the way, the triplet state was formed by the SI T I intersystem crossing with a rise time of about 200 ps, which seems to be too fast for the usual aromatic hydrocarbons. Although the fluorescence lifetime of trans-stilbene in n-hexane is about 70 ps at room temperature, it is not determined by the intersystem crossing but essentially by the trans cis isomerization. We can expect rapid intersystem crossing for acetylene derivatives because of the presence of two *-orbitals, rIand r2.They are perpendicular to each other, and one conjugates strongly to phenyl rings, while the other localizes essentially on the acetylene bond. Presumably, the excited state due to the 7r1-r2 configuration will contribute to enhancing the spin-orbit coupling responsible for the intersystem crossing. The SIstate of diphenylbutadiyne decays with a lifetime of about 25 ps, producing the triplet state.I3 The faster intersystem crossing in diphenylbutadiyne should be due to the larger spin-orbit coupling caused by the smaller energy gap between the r1and r2 orbitals. (iichrnge ) from the Intermediate Case to the Statistical Limit. The observed large temperature effect on the S2 lifetime may suggest that the rate of the S2 SIinternal conversion increases rapidly with increasing excess energy. The frequency factor of lOI3 s-' appeared to be large enough for the usual S2 SIinternal
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conversion of aromatic molecules. These results indicate that the S2-Sl interaction changes from the intermediate case coupling to the statistid limit with increasing excess energy. The activation energy of about 900 cm-' may be the excess vibrational energy where the turnover from intermediate case coupling to statistical limit of the S2 SIinternal conversion occurs. The presence of such excess energy dependent S2 SIinternal conversion should result in the decrease of the fluorescence yield and R with increasing excess energy. According to Okuyama et al., the yields of fluorescence and one-photon resonant two-photon ionization decreased drastically with increasing excitation energy, and the excitation spectra in a supersonic jet were observed only in the longest wavelength part of the gas-phase absorption ~pectrum.~ They proposed that the quite rapid relaxation similar to the third channel in benzene occurred in the frequency region higher than 36000 cm-I. Nonradiative decay channel observed in the present investigation should not be the third channel like relaxation. Although it is difficult to compare the triplet yield at different temperatures because of the sharpening of the spectrum at low temperatures, the triplet yield appeared to decrease slightly with decreasing temperature. The decrease of the triplet yield may compensate the increase of the fluorescence yield at low temperature. We could not see any evidence of decrease in triplet yield at high temperatures, which clearly indicates that the rapid decay process of S2leads not to the direct formation of the ground state or isomer like the third channel in benzene but to the formation of the triplet state via the SIstate. +
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References and Notes (1) Part of this paper was presented at the VIIth International Symposium on Ultrafast Processes in Spectroscopy, Bayreuth, Germany, 1991. (2) Mahaney, M.; Huber, J. R. Chem. Phys. 1975,9,371. Huber, J. R.; Mahanev. M. Chem. Phvs. Lett. 1975.30.410. (3) Anderson Jr., R. W.; Hochstrakr, R. M.; Pownall, H. J. Chem. Phys. Lett. 1976, 43, 224. (4) Elsaesser, T.; Laermer, F.; Kaiser, W.; Dick, B.; Niemeyer, M.; Luttke, W. Chem. Phvs. 1988. 126.405. (5) Okuyama, K.; Hasegawa, T.; Ito, M.; Mikami, N . J . Phys. Chem. 1984.88. 1711. ( 6 ) Tanizaki, Y . ; Inoue, H.; Hoshi, T.; Shiraishi, J. Z . Phys. Chem. (Munich) 1971, 74, 45. (7) Hirata, Y.;Mataaa, N. J . Phvs. Chem. 1991. 95. 1640. (8) Ota, K.; Murofuihi, K.; Inoue, H. Tetrahedron Lett. 1974, 1431. (9) Hirata, Y.; Okada, T. Chem. Phys. Lett. 1991, 187, 203. Miyasaka, H.; Hagihara, M.; Okada, T.; Mataga, N. Chem. Phys. Letr.1992,188,259. (IO) Seilmeier, A.; Kaiser, W. Ultrashort laser pulses and applications; Kaiser, W., Ed.; Springer: Berlin, 1988, and references therein. ( 1 1 ) Hirata, Y . ;Mataga, N. Chem. Phys. Lett., in press. (12) Amirav, A.; Sonnenshein, M.; Jortner, J. J . Phys. Chem. 1984,88, 5593. (13) Hirata, Y.; Mataga, N., unpublished results,
