Insights into the Complex Photophysics and Photochemistry of the Simplest Nitroaromatic Compound: A CASPT2//CASSCF Study on Nitrobenzene

Nitrobenzene is the simplest nitroaromatic compound and yet is characterized by a challenging and rich photophysics and photochemistry. In the present contribution, the main decay paths undertaken by the system after UV absorption from both the brightest (1)(L-a pi pi*) and the lowest (l)(n(A)pi*) singlet excited states have been characterized by means of CASPT2//CASSCF computations. The obtained results match with the main photophysical properties experimentally reported: the lack of fluorescence and phosphorescence emission is justified by the presence of accessible conical intersections and intersystem crossing regions between, respectively, the (l)(n(A)pi*) and (3)(n(A)pi*) states and the ground state, while the high triplet quantum yield is attributable to the strong coupling between the (1)(n(A)pi*) and (3)(pi(O)pi*) states along the main decay path of the former. Two not previously reported singlet triplet crossing regions, termed (T1/S0)(stc-NO) and (T1/S0)(stc-ep), have been here documented, from which the ground state can decay toward NO and phenoxy radical production and toward the formation of an epoxide ring structure, respectively. A possible mechanism leading to the photoisomerization of the nitro into the nitrite group, believed to be a key step in the photodegradation of nitrobenzene, has been proposed, based on the geometrical deformation recorded along the decay path leading from the (1)(n(A)pi*) state back to the original ground state through a conical intersection characterized by a significant shortening of the carbon-nitrogen bond.