On the Mechanism of the cis−trans Isomerization in the Lowest Electronic States of Azobenzene:  S₀, S₁, and T₁

Abstract

Abstract:In this paper, we identify the most efficient decay and isomerization route of the S1,T 1 ,and S0 states of azobenzene. By use of quantum chemical methods, we have searched for the transition states (TS) on the S1 potential energy surface and for the S0/S1 conical intersections (CIs) that are closer to the minimum energy path on the S1. We found only one TS, at 60°of CNNC torsion from the E isomer, which requires an activation energy of only 2 kcal/mol. The lowest energy CIs, lying also 2 kcal/mol above the S1 minimum, were found on the torsion pathway for CNNC angles in the range 95-90°. The lowest CI along the inversion path was found ca. 25 kcal/mol higher than the S1 minimum and was characterized by a highly asymmetric molecular structure with one NNC angle of 174°. These results indicate that the S1 state decay involves mainly the torsion route and that the inversion mechanism may play a role only if the molecule is excited with an excess energy of at least 25 kcal/mol with respect to the S1 minimum of the E isomer. We have calculated the spin-orbit couplings between S0 and T1 at several geometries along the CNNC torsion coordinate. These spin-orbit couplings were about 20-30 cm -1 for all the geometries considered. Since the potential energy curves of S0 and T1 cross in the region of twisted CNNC angle, these couplings are large enough to ensure that the T1 lifetime is very short (∼10 ps) and that thermal isomerization can proceed via the nonadiabatic torsion route involving the S0-T1-S0 crossing with preexponential factor and activation energy in agreement with the values obtained from kinetic measures.

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.…”

“…In particular, calculations on the semi-linear inversion mechanism in the S 1 state predicted a potential energy barrier for that pathway. 14,17,18 In contrast, torsional motion has been suggested to lead to an easily accessible conical intersection (CI) between the S 1 and S 0 states that can mediate an ultrafast radiationless relaxation to the ground state near the 901 point of the CNNC out-of-plane rotation pathway. [17][18][19][20] In addition, Diau recently proposed a ''concerted inversion'' channel.…”

“…14,17,18 In contrast, torsional motion has been suggested to lead to an easily accessible conical intersection (CI) between the S 1 and S 0 states that can mediate an ultrafast radiationless relaxation to the ground state near the 901 point of the CNNC out-of-plane rotation pathway. [17][18][19][20] In addition, Diau recently proposed a ''concerted inversion'' channel. 17 Chang et al reported experimental evidence supporting the rotational isomerization route in low viscosity solvents (hexane) and the concerted inversion route in high viscosity solvents (ethylene glycol).…”

Calculation of non-adiabatic coupling vectors in a local-orbital basis set

“…These photoisomerization processes take place in the picosecond scale and the detailed mechanisms for these transitions have been under debate. 50,51 In this third example, we have used Eq. (18) to calculate the NACV, d α ij , between the HOMO and LUMO KS states for the azobenzene molecule along a pre-selected path between the cis-and trans-conformations.…”

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

“…After Ishikawa's report, 24 all theoretical studies at the CASSCF, multireference complete-active-space second-order perturbation theory (CASPT2), second-order approximated coupledcluster model with the resolution-of-the-identity approximation (RI-CC2), and time-dependent density functional theory (TDDFT) levels indicate that the rotation is the preferred pathway for nπ * excitation in the gas phase. 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.…”