To get a notion of photochemical properties of the PrdPyr-MeOH system, before calculating the potential energy profiles (PEPs), we performed unconstrained structural optimization of all complexes, firstly in the S 2 electronic state and, subsequently, in the S 1 state, with the latter optimization starting from the respective S 2 minimum. All resulting structures and their energies are shown in Figure . In the course of these optimizations, a local S 2 minimum positioned in the Franck-Condon region has been found for each complex. In all cases, the obtained geometries resembled closely the starting S 0 structures, with almost unchanged OH and NH interatomic distances. Following reoptimization of thus-obtained structures in the S 1 electronic state resulted in two other minima, obtained respectively for PPMet-1 and PPMet-6. In both cases, the identified structures were similar to their S 2 counterparts, differing by slight opening of the CNC angle in the pyridine ring, up to 128 • /124 • for PPMet-1 and PPMet-6, respectively. On the contrary, the unconstrained S 1 optimization of PPMet-4 resulted in a barrierless PCET reaction leading to an S 1 /S 0 crossing (final optimization convergence was not achieved due to numerical instability of the single-reference ADC(2) method). In the course of the optimization process, initially the O-N 6 distance decreased from 3.56 Å to ca. 2.15 Å, resulting in reduction of the S 1 /S 0 energy gap to about 0.8 eV. Subsequently, the hydrogen atom from the OH group was transferred to the N 4 center of the pyrazine ring, which was eventually followed by dissociation of the methoxy radical, upon which the SCF convergence failed.