Solvent effects on Stokes shifts, and NLO response of thieno[3,4-b]pyrazine: A comprehensive QM/MM investigation

The analysis of solvent effects on the fluorescence and nonlinear optical (NLO) response of thieno[3,4-b]pyrazine was performed using the sequential Monte Carlo/Quantum Mechanics (s-MC/QM) approach. For this, the theoretical analysis was based on time-dependent density functional methods considering several implicit and explicit solvent models. Explicit solvent models include either electrostatic or non-electrostatic intermolecular forces like Keeson, London, and Debye interactions. The first absorption transition occurs with a charge transfer from the pyrazine to the thieno ring leading to a decrease in the molecular dipole moment by changing the average number of hydrogen-bonds on nitrogen and sulfur atoms. Including the solvent effects, this leads to a strong Stokes shift (~110 nm), showing that the title compound and its derivatives are promising for solar cells or even as a biological marker. Regarding the NLO response of the chromophore, the results indicate that the electronic-correlation (EC) plays a fundamental role for the first hyperpolarizability (β) but the solvent effect is negligible. The second-order Møller-Plesset perturbation theory and the long-range and dispersion corrected DFT-based methods show that EC corrections can improve the description of β by ca. 150% when compared with the Hartree–Fock approximation.