Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

Abstract

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field//configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T(2) electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T(1)→T(3) and T(1)→T(5) transitions, supporting that the intermediate triplet state (T(2)) decays by internal conversion to T(1).

On the Primary Reaction Pathways in the Photochemistry of Nitro-Polycyclic Aromatic Hydrocarbons

“…An important aspect in the development of the kinetic model discussed above for the nitronaphthalene derivatives [29,38,39] is the fact that internal conversion from the lowest-energy excited singlet state to the ground state is negligible, as time-resolved anisotropy measurements for 1-nitronaphthalene indicated [24]. This is further supported by recent CASPT2//CASSCF(12/12) calculations in which conical intersections between the S 0 and excited singlet states were not found [37]. These nitronaphthalene derivatives also exhibit negligible fluorescence quantum yields of ca.…”

“…Importantly, NPAHs are frequently more toxic than their parent PAHs [8,20], and photochemical degradation of a number of NPAHs leads, in turn, to photoproducts, some of which are more toxic than their parent compounds [8,20,21]. However, the light-induced transformation mechanisms of NPAHs are still under debate and knowledge of their fates in the environment is of current interest [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. Fundamental gaps remain in our knowledge of the elementary steps that lead to product formation with regard to the competition among available reaction pathways, the intermediate species involved in the photochemical transformations, and the degradation rates as a function of compound structure, added co-solutes, and micro-environment [26,34,35,[40][41][42][43].…”

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

“…This is characterized by the same main configuration describing the 3 (π O π*) state, is energetically close (around 0.15 eV energy gap) and displays a SOC value of the order of 70 cm -1 . 16,24 These features are the same as those describing the 1 (n A π*) and 3 (π O π*) states in NB along the 1 (n A π*) MEP from the 1 (gs) min structure. Since the experimental data on NB and 1-nitronaphthalene are recorded in solution (ethanol and methanol, respectively) the interaction with the surrounding environment may lead to different effects on the two molecules.…”

“…Such a strategy was previously proved successful for different systems. 16,24,25 To evaluate the photoreactivity of the system a larger active space of 16 electrons in 13 orbitals has been used (see Photodegradation section). Within the CASPT2 calculations, an imaginary level-shift correction of 0.2 au has been used to minimize the effects of possible intruder states.…”

Mechanism of Ultrafast Intersystem Crossing in 2‐Nitronaphthalene

“…However, reaction rates calculated in ref. 30 using these SOCs and Fermi's golden rule predicted ISC to occur only on a ps time scale ( k ISC =1.8–5.2×10 11  s −1 , i.e., 1.9–5.2 ps) 30…”