Regulation mechanism of spin-orbit coupling in charge-transfer-induced luminescence of imidazopyrazinone derivatives

The spin transition in the reactions of the derivatives of imidazo[1,2-a]pyrazin-3(7H)-one (1H) with a triplet molecular oxygen (O-3(2)) has been investigated by the geometry optimization at the B3LYP/6-31+G(d) level and the evaluation of the electronic matrix elements for spin-orbit coupling (SOC) using the full Pauli-Breit SOC operator. The reductive activation for the O-3(2) reaction is affected by the proton activity and solvent polarity of a surrounding reaction field. In a polar aprotic solvent, a base-prompted anionic substrate may react with O-3(2) in a stepwise manner through complete electron transfer from the substrate anion to O-3(2), while the irreversible concerted O-3(2) addition via intersystem crossing may become complete in a less polar solvent. SOC in the thermal decomposition of a resulting peroxide adduct can be controlled by the protonation state of the substrate. There exists an optimal protonation state for the suppression of SOC in the charge-transfer-induced luminescence (CTIL) of the peroxide, which is closely related with the ability of a substituent to donate an electron. This will constitute a necessary condition for the high efficiency of chemi- and bioluminescence.