Excited-State Dynamics of trans- and cis-Azobenzene after UV Excitation in the ππ* Band

Abstract

Transient absorption measurements on cis-and trans-azobenzene after UV excitation in the ππ* band are presented and compared to data obtained after VIS excitation in the nπ* band. The data show a two-step process, where after a fast motion on the S 2 potential energy surface the molecule relaxes to the S 1 state and in both cases (trans f cis and cis f trans) the isomerization reaction involves large-amplitude motion on the S 1 potential energy surface.

Femtosecond fluorescence up-conversion spectroscopy of a rotation-restricted azobenzene after excitation to the S1state

“…Both processes are assumed to take place through CIs between the S 1 and S 0 potential energy surfaces (PES) of the molecules near the mid-points of the transcis isomerization reaction coordinates. 11,[14][15][16][17][18][19][20][21]26,27 As has been shown, S 1 fluorescence and excited state (S n ' S 1 ) absorption decay on the same time scale and this decay is accompanied by the appearance of ''hot'' ground state absorption as signature of the S 1 -S 0 internal conversion. 11,12,15,28 Thus, nonradiative electronic relaxation and isomerization of AB are closely connected.…”

“…14,15,21,26,27 The striking similarities then strongly suggest that photoisomerization of the AB derivatives 1 and 2 is governed by the same molecular mechanisms. Whether or to which extent this also applies for the unsubstituted parent AB 3 remains a question which we defer to the end of this discussion.…”

“…Here, possible differences have to be taken into account between the dynamics of unsubstituted AB 15,21,27 and the AB derivatives. We shall discuss common features first and specific differences afterwards.…”

“…Considering the unsubstituted parent trans-or cis-AB, the ''inversion'' mechanism has received support from Na¨gele et al and Satzger et al, 11,15,27 who compared time-resolved absorption results on trans-and cis-AB after S 1 (np*) and after S 2 (pp*) excitation. Transient signals of trans-AB in ethanol or DMSO in the case of S 1 excitation were found to exhibit decay times of t 1 E 0.3 and t 2 E 2-3 ps; those of cis-AB could be described with decay times of t 1 E 0.1-0.17 and t 2 E 0.9-1.0 ps, in excellent agreement with the fluorescence up-conversion data on the S 1 state obtained in our laboratory 16 and by Lu et al 14 Apart from an additional very fast decay time (t0.1 ps) attributed to internal conversion from the S 2 to the S 1 state, virtually the same decay times were found following S 2 excitation, in agreement in this case with fluorescence up-conversion data by Fujino et al 37 These results were interpreted by Satzger et al in terms of a two step model, whereby an ultrafast radiationless transition (t0.1 ps) takes the molecules first from S 2 to S 1 .…”

Observation of Isomerization of an Azobenzene Derivative in Gold Nanoparticle Aggregates Using SERS

“…4,5 So far, many experimental and theoretical studies have been performed on the dynamics of photoisomerization of azobenzene. [6][7][8][9] Novel metal nanoparticles and their aggregates provide unique environments for studying photochemistry. 10 They act as an electron source for photo-induced charge transfer which opens a new reaction pathway in addition to the regular photochemistry of adsorbates on their surfaces.…”