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

Abstract

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(La) and the lowest 1(nA) 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 1(nA) and 3(nA) states and the ground state, while the high triplet quantum yield is attributable to the strong coupling between the 1(nA) and 3(O) 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(nA) state back to the original ground state through a conical intersection characterized by a significant shortening of the carbon-nitrogen bond.