Influence of Laser Pulse Parameters on Dynamical Processes during Azobenzene Photoisomerization

Abstract

When a molecule is subjected to a short intense laser pulse, the ensuing dynamical processes depend qualitatively on the pulse parameters, including duration, frequency, and fluence. Here we report studies of cis to trans photoisomerization of azobenzene following femtosecond-scale laser pulses which are relatively short (10 fs) or long (100 fs) and which have a central frequency matched to either the first excited state (S1, or HOMO to LUMO in a molecular orbital picture) or the second (S2, or HOMO-1 to LUMO). The results presented here demonstrate that photoisomerization involves a rather intricate sequence of connected steps, with the nuclear and electronic degrees of freedom inextricably coupled. One important feature is the de-excitation required for the molecule to achieve its new ground-state after isomerization. If the primary excitation is to S1, then we find that only a single HOMO/LUMO avoided crossing is required and that this crossing occurs halfway along a rotational pathway involving the central CNNC dihedral angle. If the primary excitation is to S2, then the same HOMO/LUMO avoided crossing is observed, but it must be preceded by another avoided crossing that permits transfer of holes from the HOMO-1 to the HOMO, so that the HOMO is then able to accept electrons from the LUMO. We find that this earlier crossing can occur in either of two geometries, one near the cis configuration and the other near the trans. The fact that S2 (pi pi*) isomerization requires two steps may be related to the fact that isomerization yields are smaller for this (UV) excitation than for the S1 (n pi*, visible-light) excitation.

Photoisomerization in different classes of azobenzene

“…123 Dynamic simulations predict a multidimensional isomerization mechanism with rotation dominating the isomerization process. 71,111,[116][117][118] Multiple decay channels following S 1 ' S 0 excitation are predicted by some simulations. 71 Other simulations predict isomerization is caused by pedal-like motion of the azo nitrogens.…”

“…Theoretical calculations predict that AB quickly relaxes to the S 1 state after S 2 ' S 0 excitation, followed by isomerization by concerted inversion. 57,81,100,[108][109][110] Dynamic simulations 59,71,[111][112][113][114][115][116][117][118] and other ab initio calculations 119,120 predict a different relaxation pathway following S 2 ' S 0 excitation of t-AB. The S 2 state may relax directly to the S 1 state or to an alternative intermediate state, 59,119 which then relaxes to the S 1 state.…”

Ultrafast CSpiro–O Dissociation via a Conical Intersection Drives Spiropyran to Merocyanine Photoswitching

“…For applying a force, a fast reaction with a large geometric change is favorable, which is true for photoswitches undergoing photoisomerizations or pericyclic reactions. The most prominent photoswitch of that kind is azobenzene, whose central N–N double bond is isomerized upon excitation by light. − Recently, the idea of exploiting the exerted force to fold or unfold macromolecules has received considerable attention. , …”

Theoretical Study of Solvent Effects on the Ground and Low-Lying Excited Free Energy Surfaces of a Push–Pull Substituted Azobenzene

“…Despite the enormous theoretical effort devoted to the study of the thermal and photochemical isomerization in substituted and unsubstituted azobenzenes in gas and condensed phase, ,− theoretical studies on the 4-amino-4′-cyanoazobenzene are rather scarce. To the best of our knowledge, there exist only three works: the above-mentioned calculations by Dokic et al, the work by Makita et al, who made use of semiempirical calculations to explain their experimental measurements, and, more recently, that of Wang and Wang, who compared AB, ACAB and ANAB (4-amino-4′-nitroazobenzene).…”