Photophysical properties of diphenylacetylene derivatives in solution


Department of Chemistry, Faculty of Education, Mie University, Tsu, Mie 514, Japan. Received: May 24, 19939. By using picosecond transient absorption ...
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J. Phys. Chem. 1993,97, 9677-9681

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Photophysical Properties of Diphenylacetylene Derivatives in Solution Phase III:' Thermal Repopulation of the S2 State of (Aminopheny1)phenylacetylene Yoshinori Hirata' and Tadashi Okada Department of Chemistry, Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560, Japan

Tateo Nomoto Department of Chemistry, Faculty of Education, Mie University, Tsu, Mie 514, Japan Received: May 24, 1993'

By using picosecond transient absorption spectrum and fluorescence lifetime measurement techniques, we have investigated photophysical properties of (aminopheny1)phenylacetylene in n-hexane solution. Besidesthe strongly temperature dependent S2- S1internal conversion which was reported for several diphenylacetylenederivatives previously, thermal repopulation of S2 from the longer lived S1was observed. From the temperature dependence of the S2 fluorescence decay, the energy gap between S1and S2has been determined to be about 1270 cm-l. The frequency factor of the S2 S1 internal conversion is more than 1 order of magnitude smaller than that of the reverse process, which suggests that SIhas a more restricted structure than S2. The large frequency factor of the reverse process is responsible for the high yield of the delayed S2 SOfluorescence and is characteristic to (aminopheny1)phenylacetylene.

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Introduction In previous papers, we have investigated the photophysical properties of diphenylacetylene (DPA)2 and various monosubstituted DPAs in the solution phase.' Our results of picosecond transient absorption and fluorescencedecay measurements clearly show that the fluorescence state of DPA is the lBlu state, for which the lifetime is about 8 ps at room temperature, and is not the lowest excited singlet state in many solvents. The lowest excited singlet state in the condensed phase seems to be the nonfluorescent 21ABstate. According to the results of supersonic beam experiments on DPA, the lBlu, lBzU,and 21ABstates are observed in the first absorption band region. The one-photon transition state allowed from the ground state is the lBlu state (35248 cm-1) which is located 288 cm-1 and more than 197 cm-l above the 21A,and lBzustates,re~pectively.~ Although theenergy gap between these states may not be the same in solution, we can expect that the three lowest excited singlet states are located close to each other even in the condensed phase. The level ordering of the excited singlet states of DPA derivatives is affected by thes~bstituents.~ The lBluis the lowest excited singlet state in cyano-DPA and CH3COO-DPAfor which the fluorescence shows a single exponentialdecay with a lifetime of several hundred picoseconds. On the other hand, DPA derivatives such as hydroxy-, methoxy-, and chloro-DPA keep the same ordering as DPA.3 The S2 SIinternal conversion of the latter group of DPA derivatives is much slower than that of a large aromatic molecule in solution and requires the excitation of the promotingvibrational mode in S2.' From the temperature dependence of the S2 lifetime, the frequency of the promoting mode was estimated to be 800-900 cm-1. The anomalous photophysical properties of the higher excited singlet state of DPA, which have been clearly demonstrated by our research, were explained by the sparse vibronic level density of S1 isoenergetic to the origin of S2caused by the small S a 1 energy gap2 In this paper, by using picosecond transient absorption spectrum and fluorescence lifetime measurement techniques, we have investigated photophysical properties of (aminopheny1)pheny-

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Abstract published in Advance ACS Abstracts, September 1, 1993.

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lacetylene in n-hexane solution. In this system, like DPA, the lBlu state is S2 and the strongly temperature dependent S2 SI internal conversion is observed. Not only the prompt S2 SO fluorescence but also the delayed S2 SOfluorescence due to the thermal repopulation from the longer lived SIstate are detected. No fluorescence from SIis observed. From the analysis of the fluorescence decay data we have evaluated the S f i l gap as 1270 cm-1. The frequency factor of the S2 SIinternal conversion is found to be about 35 times smaller than that of the reverse process, which leads to the observation of the delayed S2 fluorescence of this molecule. In the previous paper,2following Tanizaki et we used the coordinate system in which the x-axis is directed along the molecular long axis, the z-axis is directed perpendicular to the molecular plane, and the y-axis is perpendicular to both the xand z-axes. In this paper, the coordinate system recommended by I U P A P is used. Assuming the plane structure of DPA in D a symmetry, the old irreducible representations of Blu, Bzu, and Al, correspond to Bzu, Blurand AB,respectively in the IUPAP system.

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Experimental Section Picosecond transient absorption spectra were measured by using a dye laser photolysis system pumped by a second harmonic of a mode-lockedNd3+:YAGlaser(Quantel,PicochromeYG-503C/ PTL-10). The details of this system were described elsewhere.2 The sample was excited with a second harmonic of rhodamine6G (295 nm) or rhodamine-640 (313 nm), and the transient absorption spectrum between 380 and 980 nm was measured by using picosecond white light generated in a HzO/D20 mixture. Typically the signal was averaged for 30 shots. Fluorescence lifetimes were measured by using time-correlated single photon counting technique. The fluorescence was excited with a third harmonic of Ti:Sapphire laser (Spectra Physics, Tsunami; 820 nm) at a repetition rate of 82 MHz and was collected at right angles through a monochromator. The fluorescence decay data were fit to a sum of exponentials by iteratively convoluting trial decay curves with the instrumental response function and using the library program of Statistical Analysis with Least Squares fitting, SALS: on an ACOS-2OOO (NEC) of the Computation Center, Osaka University.

0022-3654/93/2097-9677%04.00/0 0 1993 American Chemical Society

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F i p e 1. Picosecond time-resolved absorption spectra of amino-DPA in n-hexane at (a) 295, (b) 225, and (c) 183 K. Transient absorption spectra of DPA in n-hexane at room temperature are shown by dotted lines. Delay times after laser excitation are indicated in the figure.

TABLE I: Decay Kinetic Parameters of the Transient Absorbance and the Fluorescence of Amino-DPA in &Hexane at Various Temperatures transient absorbance fluorescence 760 nm temp 500 nm rise decay 460 nm 71 TI R (K) decay (PS) (PSI (PSI rise (ps) (PS) (PSI

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.!,

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i “ .., Figure 2. Time dependence of the transient absorbance of amino-DPA at (a) 295, (b) 225, and (c) 183 K in the shorter delay time region.

Probing wavelengths are indicated in the figure. For the measurementsof the temperature dependence, a sample cell was put into a quartz dewar with flat windows. Temperature was monitored by a thermocouple contacted with the cell holder made from brass and was controlled by changing a flow rate of cold NZ gas evaporated from liquid Nz. For the fluorescence measurementsthe thermocouplecontacted the sample cell directly. The measurements were performed in the temperature range 180-298 K, where the solvent is liquid. Amino-DPA was synthesized.* Spectrograde n-hexane was used without further purification. All the samples prepared in quartz cells of 1-cm optical path length were deaerated by flushing in an Ar or NZ stream.

Results and Discussion I. Assignment of the Transient Absorption Spectra. Figure 1 shows picosecond time-resolvedabsofption spectra of amino-DPA in n-hexane at several temperatures. Transient absorptionspectra of DPA in n-hexane at 295 K are also shown in the figure. The spectra of DPA are attributed as follows:z The 500-nm band clearly seen immediately after excitation is assigned to the S, SZ(IBzu) transition, while the 410-nm band at 2 ns is due to the T, Ti transition. At 40 ps, not only the triplet band but the S, SI band is observed at 435 and 700 nm. Immediately after the laser pulse excitation of amino-DPA, the band around 500 nm, which is similar to theS,+ SZabsorption of DPA, was observed. The spectrum in the blue region is gradually replaced by the broader band peaking around 480 nm with increasing delay time. In the same time the 760-nm band

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is building up. Although the peak positions are shifted to the red, the spectral shape of the newly appeared bands is rather similar to that of the S, S1 (lAg) absorption of DPA. Spectra at the delay times longer than several hundred picoseconds show an absorption maximum around 460 nm. Figures 2 and 3 show the time dependence of the transient absorbance at several probing wavelengths. The time profile and the spectral shape seem to be independent of the excitation wavelength. The transient absorbance measured at 500 nmshows a rapid decay with a lifetime of about 7 ps followed by a slow decay at room temperature or a slight rise at low temperatures with a longer time constant. At 760 nm both a rapid rise and a slow decay were observed. The rise time of the 760-nm band is similar to the shorter decay time measured at 500 nm. As listed in Table I, the decay time at 500 nm and the rise time of the 760-nm band increase significantly with decreasing temperature. Comparing the spectral shape, the time dependenceof the transient absorbance, and the temperature dependenceof aminoDPA with those of DPA, we can safely assign the 500- and 760nm bands to the S, SZand S, Si transitions of amino-DPA, respectively. Because of the overlap of the transient absorption spectra, the time profile measured at 460 nm is complex at the short delay times. The rise time of the 460-nm band which does not show any decay up to 6 ns is similar to the decay time of the 760-nm band. It should be reasonable to attribute the 460-nm band to the T, Ti transition of amino-DPA. The T, T I band of amino-DPA shows a red shift compared with that of DPA. The temperature dependence of the rise time of the T, T1 band is not as significant as that of the decay time of SZ.Our results of

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The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9679

Delay Time [ ps 3 Figure 3. Time dependenceof the transient absorbance of amino-DPA in n-hexane at (a) 295, (b) 225, and (c) 183 K. Logarithmic plots of the rising (460 nm) and decaying (760 nm) parts are shown by filled circles. Probing wavelengths are indicated in the figure.

the transient absorption measurements show that the photophysical process of amino-DPA is similar to that of DPA: The lBlu state which is one-photon allowed from the ground state is S2,and the S2 S1internal conversion is exceptionally slow and is strongly temperature dependent. 11. Biexponential Decay of the Fluorescence. Fluorescence decay curves of amino-DPA in n-hexane observed at 330 nm are shown in Figure 4. An acceptable fit is obtained using two exponential decaying components,

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Zf(t) = A, exp(-t/.r,)

+ A, exp(-t/T,)

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k12

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and indicates that both the decay constants and the zero-time amplitudes of the short- and long-lived components depend on the temperature. Decay constants and the ratio of the zero-time amplitudes, R = Al/A,, obtained from the fitting are listed in Table I. The lifetime of the short-lived component, T,, shows a good agreement with the decay time of the transient absorbance measured at 500 nm, while the long-lived component, TI, has a similar lifetime to the decay time of the 760-nm band and to the rise time of the T, T1 absorption. The biexponential decay of the fluorescence is observed only for amino-DPA among the DPA derivatives we have studied, and it should be a key to elucidate the photophysical process of amino-DPA. Only the short-lived component is observed for DPA and neither the SISOfluorescence nor the delayed S2 SOfluorescence is detected. The fluorescencedecay profile does not depend on the monitor wavelength, and R is almost constant in the whole range of the fluorescence band between 320 and 400 nm. The lower the temperature, the smaller the value of R that was obtained and the Arrhenius plot of R, as displayed in Figure 5, shows a good linear relation. These results suggest that the longer-lived component of the fluorescence is not the SI SOfluorescence but the delayed S2 SOfluorescence. From the following reasons, we cannot expect that the S I S Iannihilation is responsible for the delayed S2 SOfluorescence: (1) The decay of SIis single exponential. (2) The lifetime of SIis independentof the excitation intensity. (3) The concentration of S1 is too low for interaction

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SCHEME I

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within a few hundred picoseconds. The delayed Sz So fluorescenceshould be due to the thermal excitation of the longerlived SI.It is consistent that we cannot observe the 500-nm band clearly in the delay time region longer than a few tens of picoseconds at room temperature because of the small R and of the overlap of the S, S2 and S, SIabsorption bands. III. Activation Energy of the S, SIInternal Conversion and the S& Energy Gap. A reaction scheme that is consistent with our data and conclusions is presented in Scheme I. k l and ~ k2~ are the rate constants of the intersystem crossing to the triplet state from S1and S2,while k2f is the radiative decay constant of S2. Since no SI SOfluorescence was observed, the radiative decay from SIis ignored. The rate constants of the S2 S1 internal conversion and the thermal repopulation of S2 from SI are represented by k21 and k12. The internal conversion should be the thermally activated process, and as we reported for DPA derivatives,3 it occurs through the vibronic level in S2 with the excited promoting vibrational mode indicated by the broken line in the scheme. The internal conversion rates from the excited singlet states to the ground state are klo and k20. Since we are

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Sz energy of amino-DPA in n-hexane is estimated to be 30 960 cm-1 which is about 2600 cm-1 lower than that of DPA in the same solvent. If we assume that the sample is excited by a &function light pulse and that at r = 0, the populations of S2 and SI are [SZ]Oand [Silo, respectively, the fluorescence intensity at time t, Z&), is given by

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(Y- X1)e-x~'] k12[Sl]o(e-X1'- e-X2t)] (2) where and Two rate constants, X1 and X2, are

The subscript 1 refers to the negative sign and the subscript 2 to the positive. A1 represents the rate of decay of delayed fluorescence X2 the rate of prompt fluorescence. If kZ1is much larger than klz, klT klo, and k2T k2f + k20, eq 2 can be reduced to

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kl,([S,IO + [SI],) e x P w 1 - r + k10)tH ( 6 ) From the lifetime of the shorter-lived component we can obtain l/(k21 klz), while l/(klT + klo) is obtained from the decay time of the longer-lived component. R is found to be

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Tlmo [ channol

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Figure 4. Fluorescence decay curves of amino-DPA in n-hexane at 298, 250,220, and 180 K. Dots show the observed data. Fitted curves and the response function of the apparatus are also shown in the figure. The time scale is 3.1 ps/channel.

Since the Arrhenius plot of R given in Figure 5 shows a good linear relation, the second term in eq 7 should be negligible. This implies that [SJOis much smaller than [SZ]Oand/or k12/k21 is neglegibly small. The former is consistent with the results of the transient absorption measurements of not observing the S, SI band clearly at short delay times. Thus we can obtain the S r S l energy gap, A E 2 1 = E2 - El, and the ratio of the frequency factors of the SZ SI and Sz SIinternal conversions, A12/A21 = exp(A&l/kB), from theslopeand intercept of the Arrheniusplot. and kB represent the difference of entropies of the first and second excited singlet states and the Boltzmann constant, respectively. The values of A& and A21/A12 so obtained are 1270 cm-1 and 35, respectively. The activation energy, E,, and the frequency factor A21 of the S2 S1 internal conversion are also determined from the Arrhenius plot of T,(= l/k21) shown in Figure 6. The value so obtained is about 600 cm-1 which is slightly smaller than those of DPA, chloro-DPA, hydroxy-DPA, and methoxy-DPA. A21 and A12are evaluated to be 1.5 X 10l2 s and 5.3 X 1013 s, respectively. Assumptions used to derive eq 6 seem to be satisfied because of (1) k12 = Rkzl< 0.1 kzl, (2) klT + klo = I / T I < 0.2k21 (183 K) and < 0.025k21 (298 K), (3) a low S2 fluorescenceyield and not observing the T.+ T I absorption at the short delay times. The linear relation of the Arrhenius plot of T* also suggests that kf k 2 + ~ kzo is much smaller than k21. We can detect the delayed SZ SOfluorescence for aminoDPA but not for other DPA derivatives. Although the S241 energy gap should not be the same for these compounds, the large AZ1/AI2we obtained should be responsible for the rather high yield of the delayed fluorescence. If the characters of Sz and SI, including the geometrical structure as well as the degree of solvation, are similar, the entropy of these two states should be similar. Since the experiments were performed in the nonpolar

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l / k B T [10-3/cm-1] Figure 5. Arrhenius plot of R.

not measuring the triplet yield, we cannot distinguishthe internal conversion to SOand the intersystem crossing to T1. From the fluorescence and absorption spectra in the near-UV region, the

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The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9681

Diphenylacetylene Derivatives in Solution Phase I11

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almost the same. The reported frequency factors of the S1 T1 intersystem crossing of eosin are 5 X lo8and 4 X IO7 s-1 and of the inverse process are 4 X lo7 and 1 X 107 s-1 in glycerol and ethanol, respectively.1° Although the difference in the values in the two solvents is significant, the frequency factors of both direction were thought to be almost the same within the experimental error and uncertainty caused by the assumptions made in analysis. On the contrary, introducing the amino group to DPA, we observed a significant difference in the frequency factors of the S2 S1 internal conversion and its reverse process. Among the substituted groups we have investigated, the amino group gives the largest red shift of the absorption spectrum in the near-UV region. We can expect the strong interaction between the amino group and the *-system of DPA, which may result in the state-dependent conformation. In order to clarify this point, time-resolved resonance Raman measurementsshould be helpful.

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Acknowledgment. The present work was partly supported by a Grant in Aid for Scientific Research (04640448) to Y.H. from the Ministry of Education, Science and Culture of Japan.

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References and Notes

Figure 6. Arrhtnius plot of the decay time of the transient absorbance at 500 nm ( 0 )and the short-lived component of the fluorescence (a). solvent, n-hexane, solvation may not play an important role. The large &/A12 value indicates that SItakes a more restricted geometrical structure than S2. Sincewe do not know the other case where the frequency factors of the S2 S1 internal conversion and its reverse process are determined, it should be better to compare our results to the SI T1 and inverse intersystem crossings. In the case of mronene in plastics? the delayed SI-SO fluorescence due to the thermal excitation of Tl is observed. The frequency factors of the S1 T1intersystem crossing and its reverse process are found to be

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(1) Our reports on the related subject were published in refs 2 (I) and 3 (11). (2) Hirata, Y.; Okada, T.; Mataga, N.; Nomoto,T. J. Phys. Chem. 1992, 96, 6559. (3) Hirata, Y.;Okada,T.;Nomoto,T. Chem.Phys. Lett. 1993,209,397. (4) Okuyama, K.; Hasegawa, T.; Ito, M.; Mikami, N. J. Phys. Chem. 1984,88, 1711. ( 5 ) Tanizaki, Y.; Inoue, H.; Hoshi, T.; Shiraishi, J. 2.Phys. Chem. NF 1971, 74, 45. (6) J . Chem. Phys. 1955,23, 1997. (7) Nakagawa, T.; Oyanagi, Y. In Recent Developments in Statistical inference and Data Analysis; Matusita, K., Ed.; North Holland Publishing: Amsterdam, 1980; pp 221-225. (8) Nomoto, T.; Tanaka, F.; Suzuki, N. Chem. Express 1991,6, 319. (9) Kropp, J. L.; Dawson, W. R.J. Phys. Chem. 1%7, 71,4499. (10) Parker, C. A,;Hatchard, C. G. Trans. Faraday Soc. 1%1,57,1894. Parker, C. A,; Hatchard, C. G. J. Phys. Chem. 1%2,66, 2506.