J. Phys. Chem. 1996, 100, 4689-4692
4689
Femtosecond Dynamics of a Thiophene Oligomer with a Photoswitch by Transient Absorption Spectroscopy Naoto Tamai* Light and Material Group, PRESTO, JRDC, Department of Chemistry, School of Science, Kwansei Gakuin UniVersity, 1-1-155 Uegahara, Nishinomiya 662, Japan
Tetsuyuki Saika* Light and Material Group, PRESTO, JRDC, Research Laboratories DAISO Co. Ltd., 9 Otakasu, Amagasaki 660, Japan
Takeo Shimidzu DiVision of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Kyoto 606-1, Japan
Masahiro Irie* Institute of AdVanced Material Study, Kyushu UniVersity, Kasugakoen 6-1, Kasuga, Fukuoka 816, Japan ReceiVed: October 30, 1995; In Final Form: January 26, 1996X
Femtosecond dynamics of the photochromic reaction of a thiophene oligomer with a diarylethene structure in solution was analyzed by transient absorption spectroscopy. It was found that the intermediate species with an absorption maximum at ∼515 nm was formed within 100 fs and that the closed-ring form of the thiophene oligomer was formed mainly from the intermediate with a time constant of ∼1.1 ps.
Introduction Recently, much attention has been given to molecular electronic devices from both theoretical and experimental points of view.1-3 One of the approaches to the molecular electronic devices is to integrate various functions into a molecule. A concept of “multimode chemical transducers” with multiple transformations has been proposed, and systematic studies of various property changes by external stimulations such as light, electric, and magnetic fields and chemicals have been done.4-6 A highly integrated transformation mode such as a molecular switch connected to a conductive polymer is one of such future molecular electronic devices. Polythiophene and thiophene oligomers having a photochromic moiety as an optical switch unit have been proposed as a new class of multimode chemical transducers, in which photochromic diarylethene derivatives have been used as the switch unit.7,8 One of the remarkable features of diarylethenes is their high degree of tolerance to photolysis in comparison to other photochromic compounds, which shows almost no degradation after thousands of photochromic cycles.9,10 Thermal stability is an another important feature of diarylethenes.9,10 Endocapped thiophene oligomers with a photochromic diarylethene structure were synthesized as model compounds.7 The photochromic reaction of a model compound is illustrated in Scheme 1. Upon UV irradiation, the open-ring form converts to the closed-ring one, in which π-conjugation extends throughout the molecule. The long π-conjugation is broken when the closedring form returns to the initial open-ring one on visible irradiation. As a result, the conductivity of the polymer (proconductivity of the oligomer) is expected to be optically switched by the photochromic reaction. In general, photochromic reactions in the singlet manifold occur on a picosecond time scale.11-16 We have recently X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-4689$12.00/0
analyzed the mechanism of a spirooxazine photochromic reaction by femtosecond transient absorption spectroscopy and elucidated the rate constants of C-O bond cleavage and the relaxation from the transition state to the metastable merocyanine to be (700 fs)-1 and (470 fs)-1, respectively.14 For a diarylethene compound, 1,2-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride, the rates of ring-closure and ring-opening processes were estimated to be faster than 10 ps by picosecond transient absorption spectroscopy.16 However, very little is known about the dynamics of photochromic diarylethenes. The photochromic reaction of diarylethenes belongs to the Woodward-Hoffmann type electrocyclic reaction.17-21 In the ring-closure reaction of a 6π-electron system, a concerted onestep mechanism predicts the stereochemistry proceeding through conrotatory pathways. The other possibility is that the photochromic reaction may proceed by a two-step mechanism involving a diradical intermediate. Recently, Pedersen et al. reported a femtosecond study on the role of diradicals in a cyclobutane ring-opening reaction by the photoinduced elimination of CO from cyclobutanone and related precursors.22 Therefore, direct femtosecond measurement is indispensable in analyzing the mechanism of the photochromic reaction. In the present study, we have investigated the primary processes of the photochromic reaction of a thiophene oligomer with a diarylethene structure in solution by femtosecond transient absorption spectroscopy. The rate constant and mechanism of the ring-closure reaction have been analyzed with femtosecond temporal resolution. Experimental Section A thiophene oligomer with a diarylethene structure (Scheme 1) was synthesized, the details of which are described in the literature.7 The sample was dissolved in 1,2-dichloroethane (WAKO, spectroscopic grade) and flowed through a 1-mm flow cell during the measurements to avoid the photoexcitation of © 1996 American Chemical Society
4690 J. Phys. Chem., Vol. 100, No. 12, 1996
Letters
SCHEME 1
the closed-ring form of the thiophene oligomer. In addition, the closed-ring form generated by laser excitation (≈360 nm) was converted to the open-ring form by irradiating the reservoir solution with visible light (532 nm) which was obtained by splitting a part of the output of the second harmonics of a cw mode-locked Nd:YAG laser (Coherent Antares 76S). Transient absorption spectra were measured by a femtosecond dye laser (Coherent Satori 774, 180 fs at ≈720 nm) amplified with a regenerative amplifier (Continuum RGA60 and PTA60, 10 Hz).14 The output energy of the dye laser was ≈400 µJ/ pulse. The pump pulse (≈360 nm) was obtained by frequency doubling the amplified dye laser output with a 1-mm BBO crystal. A white-light continuum was generated by focusing the residual of the fundamental laser pulse into a 1-cm H2O cell, which was split into probe and reference beams. The probe and reference spectra were averaged over 150 pulses and analyzed by a microcomputer-controlled ICCD detector (Princeton Instruments, ICCD-576-G) at each optical delay using a translation stage (Sigma Koki, STM-500X). The system response was estimated to be ≈200 fs (fwhm). The temporal dispersion of the white-light continuum for transient absorption spectra was corrected. The temperature of the laboratory was kept at 298 ( 2 K. Analyses of the rise and decay curves of transient absorption were carried out by a nonlinear least-squares iterative convolution method.23
completely converted to the open-ring form by irradiation at 532 nm (∼200 mW; SHG of mode-locked Nd:YAG laser). The quantum yield of the ring-closure reaction was determined to be 0.47 by the method described previously.10a Transient absorption spectra of the thiophene oligomer are shown in Figure 2a. Just after the excitation, the spectrum has a peak at around 460 nm. In addition, an another absorption band at ≈515 nm clearly appears at a delay time of 0.2-0.4 ps, indicating the delayed formation of this band compared with the spectrum at ≈460 nm. As time progresses, the absorbance at ≈515 nm decays rapidly and is replaced by a broad absorption band in the wavelength range from 590 to 660 nm. After 1.2 ps, the absorption maximum is positioned at approximately 620 nm. Because this absorption spectrum is very similar to that of the closed-ring form obtained by a steady experiment (Figure 1), it is clear that the closed-ring form is formed on a picosecond time scale. The spectral shape of the 620-nm absorption becomes narrower with time. For assignment of the transient species at ≈460 nm, the transient absorption spectrum of bithiophene should be noted
Results and Discussion Figure 1 illustrates the absorption spectra of the thiophene oligomer in 1,2-dichloroethane before and after UV irradiation. The spectrum of the open-ring form has a peak at ≈326 nm. Upon UV irradiation, the spectrum dramatically changes and has a sharp peak at 352 nm and broad absorption band at ≈620 nm, corresponding to the closed-ring form. The closed-ring form is very stable at room temperature in the dark and can be
Figure 1. Absorption spectra of an endo-capped thiophene oligomer with a photochromic diarylethene structure in 1,2-dichloroethane before and after UV irradiation.
Figure 2. (a, top) Transient absorption spectra of the thiophene oligomer excited at ∼360 nm. The delay times after excitation are written in the figure. (b, bottom) Contour plot of the transient absorption spectra illustrated in Figure 2a for a time scale up to 4 ps.
Letters
J. Phys. Chem., Vol. 100, No. 12, 1996 4691
Figure 4. Rise and decay curves of transient absorption (points) of the thiophene oligomer recorded at 515 and 450 nm. Simulation curves (smooth line) are also shown in the figure.
Figure 3. Rise and decay curves of transient absorption (points) of the thiophene oligomer recorded at 515 and 650 nm and their simulation curves (smooth line) with the sum of exponentials.
as a reference. It is reported that the absorption spectrum of bithiophene shows a time-dependent spectral shift in dioxane and has a peak at 460 nm at 0 ps and 495 nm at 1.3 ps.24 This suggests that the absorption at ≈460 nm is due to the Sn r S1 absorption of the open-ring form of the thiophene oligomer, although no spectral shift was observed in our experiments. It is worth noting that the absorbance at ≈515 nm decays faster than the peak at ≈460 nm. At a delay time of 30 ps, the spectrum at ≈515 nm completely disappears, although the spectrum at ≈460 nm still remains, in which the absorption spectrum is composed of 460 and 620 nm bands. This result indicates that the relaxation from the S1 state is not the main process of the ring-closure reaction. For further understanding of the mechanism, a contour plot of the transient absorption spectra is illustrated in Figure 2b. As clearly shown in the Figure, the decay at ≈460 nm is much slower than the decay at 515 nm. From inspection of the 620nm absorbance, the formation of the closed-ring form is almost finished within 1.5 ps. To estimate the time constants of the ring-closure reaction of the thiophene oligomer, we analyzed the rise and decay curves of the transient absorption. Figure 3 illustrates the time profiles of absorbance at 515 and 650 nm. The rise curve at 650 nm can be well fitted by a three-exponential function with two rises and a very long decay component (>5 ns). The fast and slow rise components were 1.1 (53%) and 9.5 ps (18%), respectively. The decay curve recorded at 515 nm was analyzed by the sum of two- or three-exponential functions. The fitting to the experimental data with a three-exponential function was always better than a two-exponential function. The fast decay component with a time constant of 1.1 ps (59%), corresponding to the fast rise time at 650 nm, and a very fast rise time of 70110 fs (73%), were obtained in addition to the long decay component. To clarify the time constant of the fast rise component at 515 nm, the initial part of the transient absorbance was analyzed by the sum of exponentials as illustrated in Figure 4. The rise time recorded at 515 nm was approximately 100 fs, whereas no rise component was observed at 450 nm. The analysis of
the absorbance at 450 nm revealed a fast decay constant of ∼2.5 ps which did not correspond to the rise time at 650 nm. From these results, it is concluded that the closed-ring form is mainly formed from the species with an absorbance maximum at ≈515 nm with a time constant of ∼1.1 ps. An ultrafast reaction of the order of 100 fs may be included in the formation of a transient species at ≈515 nm. The transient species at ≈460 nm assigned to the S1 state is probably not a main precursor of the closed-ring form. It is worth noting that the Sn r S1 absorption of 1,2-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride decays much slower than the rise time of the closed-ring form;16 the ring-closure process occurs within 10 ps, whereas the fluorescent state of the open-ring form decays with a time constant of 360 ps, which corresponds to the fluorescence lifetime in n-hexane (340 ps (98%), 1.1 ns (2%)).25 This observation is similar to our result; namely the decay time of the Sn r S1 absorption at ≈460 nm does not coincide with the rise time of the closed-ring form. Their result was interpreted as follows: During the relaxation process in the S1 state, some portions of the excited open-ring form undergo a cyclization reaction, and others relax to the fluorescent state in which the conformation is unfavorable for the cyclization reaction.25 A similar interpretation may exist for the current experimental results. It is documented that the diarylethene derivatives have parallel and antiparallel conformers and the photochromic ringclosure reactions proceed only from the antiparallel conformers.10b Therefore, upon excitation of the open-ring form, some intermediate with a peak at 515 nm is formed from the antiparallel conformer prior to the relaxation to the vibrationally relaxed S1 state. The closed-ring form is mainly formed from the intermediate species with a time constant of 1.1 ps. One possible interpretation of the species is a diradical-like intermediate. Unfortunately, there are no spectral data on the absorption spectrum of the diradical. The theoretical analysis predicts that the substituent causes substantial deviations from the synchronous concerted mechanism in electrocyclic reactions.26 The slow rise component in the formation process of the closed-ring form is very similar to that observed for a spirooxazine photochromic reaction, which includes the simultaneous narrowing of the spectral bandwidth.14 This phenomenon is interpreted in terms of thermal relaxation of a vibrationally hot closed-ring form of the thiophene oligomer in the ground state. The ground-state conformation of the open-ring form plays an important role in the photochromic reaction as stated above.
4692 J. Phys. Chem., Vol. 100, No. 12, 1996 In the current system, it was calculated that two thiophene oligomers have tilt angles of 74-86° with respect to the plane of perfluorocyclopentene, for the antiparallel conformer.27 After the cyclization, two thiophene rings are almost parallel with a tilt angle of 7.4-7.7° to the plane. On the average, the thiophene ring rotates with an angular velocity of ≈6°/100 fs during the ring-closure reaction in the photoexcited state. In conclusion, we have analyzed the photochromic ringclosure reaction of a thiophene oligomer with a diarylethene structure as a new class of multimode chemical transducers. It is clearly shown here for the first time that the closed-ring form is formed mainly from an intermediate with an absorption maximum at ≈515 nm with a time constant of 1.1 ps. This result suggests deviation from a synchronous concerted reaction. References and Notes (1) Molecular Electronic DeVices; Carter, F. L., Ed.; Marcel Dekker: New York, 1982. (2) Introduction to Molecular Electronics; Petty, M. C., Bryce, M. R., Bloor, D., Eds.; Edward Arnold: London, 1995. (3) Mirkin, C. A.; Ratner, M. A. Annu. ReV. Phys. Chem. 1992, 43, 719. (4) Iyoda, T.; Saika, T; Honda, K; Shimidzu, T. Tetrahedron Lett. 1989, 30, 5429. (5) Saika, T.; Iyoda, T.; Honda, K.; Shimidzu, T. J. Chem. Soc., Chem. Commun. 1992, 591. (6) Saika, T.; Iyoda, T.; Honda, K.; Shimidzu, T. J. Chem. Soc., Perkin Trans. 2 1993, 1181. (7) Saika, T.; Irie, M.; Shimidzu, T. J. Chem. Soc., Chem. Commun. 1994, 2123. (8) (a) Kawai, S. H.; Gilat, S. L.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1994, 1011. (b) Kawai, S. H.; Gilat, S. L.; Ponsinet, R.; Lehn, J.-M. Chem. Eur. J. 1995, 1, 285.
Letters (9) Irie, M. In Photo-reactiVe Materials for Ultrahigh Density Optical Memory; Irie, M., Ed.; Elsevier: Amsterdam, 1994; p 1. (10) (a) Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803. (b) Uchida, K.; Nakayama, Y.; Irie, M. Bull. Chem. Soc. Jpn. 1990, 63, 1311. (c) Hanazawa, M.; Sumiya, R.; Horikawa, Y.; Irie, M. J. Chem. Soc., Chem. Commun. 1992, 206. (11) Krysanov, S. A.; Alfimov, M. V. Chem. Phys. Lett. 1982, 91, 77. (12) Schneider, S.; Mindl, A.; Elfinger, G.; Melzig, M. Ber. BunsenGes. Phys. Chem. 1987, 91, 1222. (13) Ernsting, N. P. Chem. Phys. Lett. 1989, 159, 526. Ernsting, N. P.; Dick, B.; Arthen-Engeland, T. Pure Appl. Chem. 1990, 62, 1483. Ernsting, N. P.; Arthen-Engeland, T. J. Phys. Chem. 1991, 95, 5502. (14) Tamai, N.; Masuhara, H. Chem. Phys. Lett. 1992, 191, 189. (15) Kurita, S.; Kashiwagi, A.; Kurita, Y.; Miyasaka, H.; Mataga, N. Chem. Phys. Lett. 1990, 171, 553. (16) Miyasaka, H.; Araki, S.; Tabata, A.; Nobuto, T.; Mataga, N.; Irie, M. Chem. Phys. Lett. 1994, 230, 249. (17) Woodward, R. B.; Hoffmann, R. The ConserVation of Orbital Symmetry; Academic Press: New York, 1970. (18) Gill, G. B.; Willis, M. R. Perycyclic Reactions; Chapman and Hall: London, 1974. (19) Houk, K. N.; Li, Y.; Evanseck, J. D. Angew. Chem. 1992, 104, 711. (20) Houk, K. N.; Gonzalez, J.; Li, Y. Acc. Chem. Res. 1995, 28, 81. (21) Nakamura, S.; Irie, M. J. Org. Chem. 1988, 53, 6136. (22) S. Pedersen, S.; Herek, J. L.; Zewail, A. H. Science 1994, 266, 1359. (23) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969. (24) Lap, D. V.; Grebner, D.; Rentsch, S.; Naarmann, H. Chem. Phys. Lett. 1993, 211, 135. (25) Irie, M.; Sayo, K. J. Phys. Chem. 1992, 96, 7671. (26) Houk, K. N.; Gonzalez, J.; Li, Y. Acc. Chem. Res. 1995, 28, 81 and references therein. (27) Saika, T., manuscripts in preparation.
JP953181D
