Femtosecond Time-Resolved UV-Visible Absorption Spectroscopy of trans-Azobenzene in Solution Igor K. Lednev, † Tian-Qing Ye, Ronald E. Hester, and John N. Moore* Department of Chemistry, The UniVersity of York, Heslington, York YO1 5DD, UK ReceiVed: April 3, 1996; In Final Form: June 7, 1996 X Femtosecond time-resolved UV-visible absorption spectroscopy has been used to study the UV photochemistry of trans-azobenzene (t-AB) in solution at 30 °C. Photolysis of t-AB at 303 nm results in transient absorption at 370-450 nm, the decay of which can be fitted by a sum of two exponential components. The shorter- lived component has a lifetime of 0.9 ( 0.2 ps in hexane, cyclohexane, and hexadecane and 1.2 ( 0.2 ps in acetonitrile; this is attributed to the S 2 (ππ*) excited state of t-AB. The longer-lived component has a lifetime which is similar to the recovery time of the ground-state absorption of t-AB at 303 nm, found to be 13 ( 1 ps in hexane, cyclohexane, and hexadecane and 16 ( 1 ps in acetonitrile. This longer-time-scale process is attributed to the internal conversion of an intermediate excited state, S † , into ground state t-AB, and this intermediate is tentatively assigned as a twisted conformer of excited t-AB on the S 2 or S 1 potential energy surface. The vibrational relaxation of hot t-AB molecules in the ground state, formed by internal conversion from S † , may also contribute to this longer-time-scale process. Introduction Azobenzene (AB) and many of its derivatives exhibit reversible photochromism both in fluid solutions and in solids, arising from their photoisomerization reactions; 1 a strong interest in these compounds has developed recently because of their potential for application in optical switching and image-storage devices. 2-4 AB has been studied extensively by steady-state spectroscopic and photochemical methods including UV-visible absorption, 5-8 Raman, 9,10 and NMR 11 and by theoretical modeling. 12-16 The ground-state UV-visible absorption spec- trum of t-AB comprises a weak feature at 447 nm, arising from excitation to the S 1 (nπ*) state, and a strong feature at 316 nm which arises from excitation to the S 2 (ππ*) state. Excitation to either of these states results in photoisomerization to cis- azobenzene (c-AB), with excitation to the S 2 state giving the lower quantum yield of photoisomerization. There has been extensive discussion of the isomerization and relaxation mech- anisms, with an inversion mechanism having been proposed for isomerization via the S 1 state and an additional rotational mechanism proposed for isomerization via the S 2 state. 1 Relatively few time-resolved studies of AB photochemistry have been reported, 17,18 in contrast with the isosteric stilbene mol- ecule; 19 consequently, the isomerization mechanisms and the structures and dynamics of the intermediates are not yet firmly established. In the present study, femtosecond time-resolved UV-visible absorption spectroscopy has been used for the first time to study the photochemistry of t-AB on excitation to the S 2 state. Experimental Section t-AB, n-hexane, and hexadecane (Aldrich), acetonitrile and cyclohexane (Aldrich, HPLC Grade), were used as received. Solutions of t-AB (ca. 4 × 10 -4 mol dm -3 , 50 cm 3 reservoir volume) under air were circulated through a 1-mm path length quartz cell using a gear pump-driven flow system comprising Teflon, glass, and stainless steel components. A linear flow rate of >100 cm s -1 was used to ensure that each pair of laser pulses encountered fresh sample, and all measurements were made on solutions maintained at 30 ( 2 °C. UV-visible absorption spectra of the solutions were measured before and after the laser experiments to check the integrity of the sample; small differences in absorbance (<3%) were observed after several hours, and were consistent with changes arising from partial trans-to-cis isomerization. The ultrafast apparatus used for this study is described in detail elsewhere. 20,21 Briefly, an amplified dye laser system provided pulses (606 nm, 50 μJ, 200 fs) at a repetition rate of 1050 Hz. A portion of this output was frequency doubled to generate photolysis pulses (303 nm, 1.4-1.8 μJ), while the remainder was directed round a variable optical delay line and used to generate a white light continuum as the probe. This continuum probe was split into two beams of similar intensity and both were focused to a diameter of ca. 200 μm at two different positions in the sample cell. One probe beam was coincident with the photolysis beam, which was focused to a diameter of ca. 250 μm using near-collinear geometry. The emerging beams were analyzed using either a spectrograph and CCD detector or a 10-nm bandpass interference filter and photodiodes/lock-in amplifiers. In some experiments, a probe beam at 303 nm (attenuated to ca. 1 nJ/pulse) was generated by frequency doubling the delayed 606-nm beam instead of generating a white light continuum. In all cases, the relative polarization of the pump and probe beams was set at the “magic angle” of 54.7°. The spectral data obtained using the CCD detector were corrected for the dispersion of the white light at the sample by obtaining two spectra at appropriate delay times and using the measured dispersion and the fitted kinetics to calculate the actual spectrum. Results The 303-nm photolysis of t-AB in solution resulted in transient absorption at 370-450 nm and transient bleaching at 303 nm, within the ground-state absorption band. The kinetics of these transient features were measured for t-AB in hexane, cyclohexane, hexadecane, and acetonitrile. Figure 1 shows † Also affiliated with the Institute of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region 142432, Russia. X Abstract published in AdVance ACS Abstracts, July 15, 1996. 13338 J. Phys. Chem. 1996, 100, 13338-13341 S0022-3654(96)01006-4 CCC: $12.00 © 1996 American Chemical Society