Theoretical Studies On Photophysical And Photochemical Processes Of Nitro Polycyclic Aromatic Hydrocarbons And Their Derivatives

Nitro polycyclic aromatic hydrocarbons（NPAHs）are a class of atmospheric pollutants that are widespread in nature.Though NPAHs are one to two orders of magnitude lower than polycyclic aromatic hydrocarbons（PAHs）in the atmosphere,they are 10,000 to 100,000 times more mutagenic and carcinogenic than PAHs.NPAHs can be generated by incomplete combustion of petrol and diesel or photochemical reactions of PAHs in the atmosphere.When exposed to UV radiation,most of NPAHs can be photodegraded into other species.Thus,the photodegradation reactions and the possible products of NPAHs（such as the Ar–O·and NO·free radicals）have become an increasingly urgent concern to researchers.In addition,NPAHs possess some conspicuous photophysical properties among the organic compounds,i.e.,the ultrafast ISC pathways.However,the observation of the rapid singlet to triplet transitions and the exploration of the inherent driving forces for the ultrafast ISC processes are still being a great challenge to scientists.Herein,the high-level QM（MS-CASPT2//CASSCF）/MM method was adopted to study the photophysical and photochemical reaction mechanisms of2-nitrofluorene（2-NF）and p-Nitrophenylphenol（HNPB）.1.The photophysical and photochemical reaction mechanisms of 2-NF in the gas phase and acetonitrile solution have been studied theoretically.Under the irradiation of～330 nm,the 2-NF can be excited to the first bright state（¹ππ*）firstly,the 2-NF system can decay to triplet excited states via rapid intersystem crossing（ISC）processes through different surface crossing points or to the ground state via an ultrafast internal conversion（IC）process through the S₁/S₀ conical intersection.The ¹nπ*dark state will serve as a bridge when the system leaves the Franck–Condon（FC）region and approaches to the S₁minimum.The molecule maintains a planar geometry during the excited-state relaxation processes.The differences on excitation properties such as electronic configurations and spin–orbit coupling（SOC）interactions between those in the gas phase and acetonitrile solution cannot be neglected,indicating possible changes on the efficiency of the related ISC processes for the 2-NF system in solution.Once arrived at the T₁ state,it would further decay to the S₀ state or photodegrade into the Ar–O·and NO·free radicals.During the intramolecular rearrangement process,the twisting of the nitro group out of the aromatic-ring plane is regarded as a critical structural variation for the photodegradation of the 2-NF system.The free radicals finally form through oxaziridine-type intermediate and transition state structures.The present work provides important mechanistic insights to the photochemistry of nitro-substituted polyaromatic compounds.2.The mechanistic photophysics of HNBP have also been studied with the high-level QM（MS-CASPT2//CASSCF）method.On the ground of the electronic structure calculations,we found that the S₁ state is a spectroscopically bright state,while the S₂ state is a dark state at the Franck-Condon region.HNBP would rapidly decays to the excited triplet state via the ultrafast intersystem crossing（ISC）processes at crossing points,or relax to the ground state via the ultrafast internal conversion（IC）processes through the S₁/S₀ conical intersection.In the solution phase,p-Nitrophenylphenol has many stable triplet intermediate structures,but no similar structures exist in the gas phase.