Insights On The Crossing Of The Two Lowest N-Pi* And Pi-Pi* Absorption Lines Of Thieno[3,4-B]Pyrazine In An Aqueous Environment

Different degrees of quantum mechanics and solvation based on a polarizable continuum model and a sequential-QM/MM methodology is applied to investigate the two lowest singlet excited states in the UV-vis spectra of thieno[3,4-b]-pyrazine (TPy). The results show that the solvent affects the solute's photophysics severely, reversing the order of the lowest n-pi* and pi-pi* lines compared to vacuum conditions. The best description of the crossing-effect between n-pi* and pi-pi* lines must reconcile explicit solute-solvent hydrogen-bonds (HB) coupled to electrostatic interactions from the bulk. Long-range interactions play a fundamental role in TPy's photophysics.