Investigations of ultrafast photoisomerization of photochromic molecular switches by fs-time-resolved transient absorption spectroscopy
The photoinduced isomerization of trans- and cis- azobenzene and the cis-trans photoisomerization of the furylfulgide were studied with a pump-probe setup. The detection of the ultrafast transient absorption was realized using supercontinuum (SC) broadband probe pulses generated in sapphire or CaF2. A study of the cross-phase modulation showed that the SC could be described as linearly chirped pulses. Stimulated Raman amplification contribution was used to find the best setting for the pulse compression for the NOPA pump pulses. The photoinduced isomerization of azobenzene in CCl4 and ethanol at room temperature was studied after excitation at 466 nm and 387 nm. In the first case (466 nm), two wavelength-independent decay components of tau1 = 0.52(2) ps and tau2 = 3.22(2) ps were found, in agreement with previous work. The shortest time constant was interpreted as very fast motion of the excited state wavepacket out of the Franck-Condon region and ultrafast conversion to the S0 state via a conical intersection (CI), the second component was attributed to molecules that do not take the direct path. A much weaker third component of tau3 = 18 ps only found at lambda = 510 nm was assigned to the relaxation of vibrationally hot molecules in the S0 state. For excitation at 387 nm leading to S2 state, three decay components with tau1 = 0.08(5) ps, tau2 = 0.4(3) ps and tau3 = 13(3) ps were found. The fastest component (tau1) could be related to the conversion from the S2 to the S1 state. The second decay time (tau2) was attributed to motion in the S1 state towards the S1/S0-CI and the slowest component was assigned to cooling of vibrationally hot molecules in S0 state. It is not clear why, in contrast to literature, no tau ~ 3 ps component was observed. The photoisomerization of the furylfulgide Z-ACR 540 was studied after excitation at lambda = 387 nm in toluene and ethanol. In toluene, a short-lived component only present within the first 0.3 ps and a long lived second component were found. The latter had a rise time of tau = 16 ps and did not decay within 136 ps. That component was also observed in pure toluene and was assigned to the photoinduced formation of toluene excimers. In ethanol only one single fast decay component of tau = 248(2) fs was found. The time scale of the process could be interpreted as motion from the S1 state to the S0 state via a CI.
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