Nonadiabatic simulation study of photoisomerization of azobenzene: Detailed mechanism and load-resisting capacity

Abstract

Non-adiabatic dynamical simulations were carried out to study cis-to-trans isomerization of azobenzene under laser irradiation and/or external mechanical loads. We used a semiclassical electron-radiation-ion dynamics method that is able to describe the co-evolution of the structural dynamics and the underlying electronic dynamics in a real-time manner. It is found that azobenzene photoisomerization occurs predominantly by an out-of-plane rotation mechanism even under a non trivial resisting force of several tens pN. We have repeated the simulations systematically for a broad range of parameters for laser pulses, but could not find any photoisomerization event by a previously suggested in-plane inversion mechanism. The simulations found that the photoisomerization process can be held back by an external resisting force of 90-200 picoNewtons depending on the frequency and intensity of the lasers. This study also found that a pure mechanical isomerization is possible from the cis to trans state if the azobenzene molecule is stretched by an external force of ~ 1250 -1650 pN. Remarkably, the mechanical isomerization first proceeds through a mechanically activated inversion, and then is diverted to an ultrafast, downhill rotation that accomplishes the isomerization. Implications of these findings to azobenzene-based nanomechanical devices are discussed.

The Kinetics of the Cis-to-Trans Thermal Isomerization of 4-Anilino-4’- Nitroazobenzene are Highly Influenced by Solvent Polarity

“…Upon photo-excitation of the trans form, an electron is excited from its ground-state (S 0 ) orbital to its first singlet excited state (S 1 ) or second singlet excited state (S 2 ) in which the electron retains its spin under an n-π* excitation condition or a π-π* excitation condition, respectively [1,2]. Azo groups are reported to photo-isomerize via two distinct mechanisms: the π-π* transition with an out-of-plane rotation mechanism in which the nitrogen-nitrogen π bond is ruptured heterolytically and a dipolar transition state is involved (Scheme 1), or the n-π* electronic transition with an inversion of one sp 2 hybridized nitrogen atom through an sp hybridized linear transition state in which the double bond is retained (Scheme 2) [2][3][4][5][6]. The rate of isomerization for the inversion mechanism is relatively rapid and mostly independent of the polarity of the medium or the electronic nature of substituents on the azobenzene, but the rate for the rotation mechanism increases rapidly with increasing solvent polarity [3].…”

“…Azobenzenes are common organic dyes that have been extensively studied both experimentally and theoretically owing to their potential applications in material science, medicinal chemistry, molecular switches and other devices [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. They are photo-reactive molecules that undergo reversible photo-isomerization from the more stable trans-isomer to the less stable cis-isomer.…”

Nonadiabatic ab initio molecular dynamics of photoisomerization in bridged azobenzene

“…Minimum-energy CIs between adiabatic S 0 and S 1 states were optimized at the two-state-averaged CASSCF (10,8) level with the 6-31G* basis set, which was used in CI optimizations by Liu et al 21 Furthermore, higher-level electronic calculations (CASSCF(14,12)/6-31G*) were performed to confirm the stationary points and CIs we located. Additionally, at the twostate-averaged CASSCF(10,8)/6-31G* level, PESs of S 0 and S 1 states were scanned as functions of the C1-N-N-C2 dihedral angle and N-N-C1 bond angle with all other geometrical parameters relaxed to explain the complicated photoisomerization mechanisms.…”

“…The photoisomerization mechanisms of azobenzene and its derivatives have been widely investigated [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] since the molecule plays a particularly important role in optical memory, logic devices, molecular motors, etc. As a derivative of azobenzene, bridged azobenzene has been very recently recognized to be a molecular photoswitch by Siewertsen et al 16,17 and turns out to have superior photochromic properties compared to azobenzene.…”

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

“…Photoisomerization of azobenzene has been extensively studied both experimentally [10][11][12][13][14][15][16][17][18][19][20][21] and theoretically [22][23][24][25][26][27][28][29][30][31][32][33][34][35] because of its significance as photochromic materials. The trans form of azobenzene has a planar structure, whereas in the cis form, phenyl rings are twisted around the C-N bonds to minimize steric repulsion, and thus the cis form is less stable than the trans form.…”