Role of Ultrafast Internal Conversion and Intersystem Crossing in the Nonadiabatic Relaxation Dynamics of ortho-Nitrobenzaldehyde

ortho-Nitrobenzaldehyde (oNBA)is a well-known photoactivated acid and a prototypical photolabilenitro-aromatic compound. Despite extensive investigations, the ultrafastrelaxation dynamics of oNBA is still not properlyunderstood, especially concerning the role of the triplet states.In this work, we provide an in-depth picture of this dynamics by combiningsingle- and multireference electronic structure methods with potentialenergy surface exploration and nonadiabatic dynamics simulations usingthe Surface Hopping including ARbitary Couplings (SHARC) approach.Our results reveal that the initial decay from the bright & pi;& pi;*state to the S-1 minimum is barrierless. It involves threechanges in electronic structure from & pi;& pi;* (ring) to n & pi;* (nitro group), to n & pi;* (aldehydegroup), and then to another n & pi;* (nitro group).The decay of the & pi;& pi;* takes 60-80 fs and can betracked with time-resolved luminescence spectroscopy, where we predictfor the first time a short-lived coherence of the luminescence energywith a 25 fs period. Intersystem crossing can occur already duringthe S-4 & RARR; S-1 deactivation cascade butalso from S-1, with a time constant of about 2.4 ps andsuch that first a triplet & pi;& pi;* state localized on the nitrogroup is populated. The triplet population first evolves into an n & pi;* and then quickly undergoes hydrogen transfer toform a biradical intermediate, from where the ketene is eventuallyproduced. The majority of the excited population decays from S-1 through two conical intersections of equal utilization, apreviously unreported one involving a scissoring motion of the nitrogroup that leads back to the oNBA ground state andthe one involving hydrogen transfer that leads to the ketene intermediate.

Automated summary

In this study, the ultrafast relaxation dynamics of ortho-Nitrobenzaldehyde (oNBA) were investigated using single- and multireference electronic structure methods, potential energy surface exploration, and nonadiabatic dynamics simulations. The results showed that the initial decay from the bright ππ* state to the S-1 minimum is barrierless and involves three changes in electronic structure. The majority of the excited population decayed from S-1 through two conical intersections, leading back to the oNBA ground state or to the ketene intermediate.