Ab initio molecular dynamics simulation of photoisomerization in azobenzene in the nπ∗ state

Abstract

Photoisomerization mechanism of azobenzene in the lowest excited state S 1 ͑n ‫ء‬ ͒ is investigated by ab initio molecular dynamics ͑AIMD͒ simulation with the RATTLE algorithm, based on the state-averaged complete active space self-consistent field method. AIMD simulations show that cis to trans isomerization occurs via two-step rotation mechanism, accompanying rotations of the central NN part and two phenyl rings, and this process can be classified into two types, namely, clockwise and counterclockwise rotation pathways. On the other hand, trans to cis isomerization occurs via conventional rotation pathway where two phenyl rings rotate around the NN bond. The quantum yields are calculated to be 0.45 and 0.28Ϯ 0.14 for cis to trans and trans to cis photoisomerizations, respectively, which are in very good agreement with the corresponding experimental results.

A multireference perturbation study of the NN stretching frequency of trans-azobenzene in nπ* excitation and an implication for the photoisomerization mechanism

“…This finding can explain, why in dynamics simulations the isomerization occurs through a rotation of the -N=N-fragment rather than a true torsion along the N-N bond. 47,55 Finally, we summarize significant results from the present calculations. First, we should mention that we employ the CASPT2 method in determinations of geometrical structures, vibrational frequencies, and reaction pathways.…”

“…27,28,30,31,42,58 On-the-fly dynamics simulations were also performed for the photoisomerization of azobenzene on the basis of semiempirical molecular orbital calculations with the surface hopping method 29,33,55,59,61 and with the multiple spawning method. 32 Recently, ab initio molecular dynamics (AIMD) simulations at the CASSCF level 26,47,54,57 and Car-Parrinello molecular dynamics simulations 34,45,49,50 were also performed for the photoisomerization of azobenzene in nπ * excitation. In our surface hopping AIMD simulation 47 at the state-averaged CASSCF (SA-CASSCF) level, it was shown that cis to trans isomerization in nπ * excitation occurs via two-step rotation mechanism, accompanying rotations of the central NN part and two phenyl rings, and this process can be classified into two types with respect to the orientation of the rotation, namely, clockwise and counterclockwise rotation pathways; the calculated quantum yields and lifetime in the excited states are in very good agreement with the corresponding experimental results.…”

“…32 Recently, ab initio molecular dynamics (AIMD) simulations at the CASSCF level 26,47,54,57 and Car-Parrinello molecular dynamics simulations 34,45,49,50 were also performed for the photoisomerization of azobenzene in nπ * excitation. In our surface hopping AIMD simulation 47 at the state-averaged CASSCF (SA-CASSCF) level, it was shown that cis to trans isomerization in nπ * excitation occurs via two-step rotation mechanism, accompanying rotations of the central NN part and two phenyl rings, and this process can be classified into two types with respect to the orientation of the rotation, namely, clockwise and counterclockwise rotation pathways; the calculated quantum yields and lifetime in the excited states are in very good agreement with the corresponding experimental results. 47 The similar reaction mechanism was also reported in the dynamics simulations by Doltsinis et al 45,49,50 and Thiel et al 55 As described above, the transient Raman spectra indicate that the NN stretching frequency of trans-azobenzene is almost unchanged (decreases by only 12 cm −1 ) in the S 1 (nπ * ) state.…”

“…In our surface hopping AIMD simulation 47 at the state-averaged CASSCF (SA-CASSCF) level, it was shown that cis to trans isomerization in nπ * excitation occurs via two-step rotation mechanism, accompanying rotations of the central NN part and two phenyl rings, and this process can be classified into two types with respect to the orientation of the rotation, namely, clockwise and counterclockwise rotation pathways; the calculated quantum yields and lifetime in the excited states are in very good agreement with the corresponding experimental results. 47 The similar reaction mechanism was also reported in the dynamics simulations by Doltsinis et al 45,49,50 and Thiel et al 55 As described above, the transient Raman spectra indicate that the NN stretching frequency of trans-azobenzene is almost unchanged (decreases by only 12 cm −1 ) in the S 1 (nπ * ) state. 12 There are several theoretical reports on vibrational frequencies for trans-azobenzene in the ground state at the MP2, density functional theory (DFT), and CASSCF levels, 20,21,23,25,30 while, to our knowledge, there is only one report on frequencies of trans-azobenzene in the S 1 (nπ * ) state, which employed the CASSCF method.…”

Communication: Time-resolved fluorescence of highly single crystalline molecular wires of azobenzene

“…From the viewpoint of fundamental photochemistry, particularly, trans-cis photoisomerization mechanism and excited state decay dynamics of azobenzene are still a long standing issue to be much debated between theoretical calculations and experiments because of very fast isomerization process. [5][6][7][8][9][10][11] Recent ultrafast time-resolved spectroscopic results from probing short-lived azobenzene excited states propose that the isomerization pathways are rather complicated processes involving multiple relaxation channels. 12 In principle, azobenzene undergoes trans-cis photoisomerization by the excitation of the S 1 (n → π*) and the S 2 (π → π*) states in the UV and visible region.…”