We investigated the atmospheric sunlight-promoted photodegradation of the atmospheric pollutants 9-nitrophenanthrene (9NPh) and 8-nitrofluoranthene (8NFlu) based on quantum chemical calculations using CASSCF/CASPT2. We find that both 9NPh and 8NFlu possess a small energy gap between the Franck-Condon singlet excited and the receiver triplet states (Tr). The obtained kinetic constants for the intersystem crossing (ISC) S1 → Tr show that in both 9NPh and 8NFlu ISC can efficiently occur, although the kISC rate of 8NFlu is lower compared to 9NPh. Both molecules possess a lower-lying triplet π to π* excited state below the lowest singlet excited state. The reactive region in this process is the ISC point (S0/T1)ISC at which both 9NPh and 8NFlu adopt an epoxide-like geometry. There is an energy difference in the barrier between these two molecules the system must surmount, from the lowest-lying triplet state to the reactive region of the intersystem crossing with an epoxide-like geometry. 9NPh possesses a lower energy barrier of 0.39 eV in comparison to 1.71 eV of 8NFlu. After reaching the (S0/T1)ISC crossing point, both molecules can proceed toward the ground state characterized by the epoxide ring geometry. We show that these two molecular systems can degrade into nitrogen(II) oxide and the corresponding aryloxy radical through a thermally activated process, and in a polar environment such as dimethylsulfoxide the height of the barrier for the NO˙ release is significantly reduced for both systems.
Ostojić et al. (Thu,) studied this question.
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