The hydrogen bond in 3 HQ shortens from 1.64 to 1.00 Å during donor relaxation, while the hydrogen bond involved in the hydrogen transfer in 3Dan displays a smaller change from 1.58 to 0.99 Å, indicating a reduced reorganization penalty compared to that of 3 HQ. Two explanations can be offered to unveil the role of the additional hydroxyl group at the R1 position of Dan. In the context of intramolecular resonance-assisted hydrogen bonding, the extra hydroxyl functionality can serve as a π-electron-donating group to strengthen the hydrogen bond motif and, subsequently, shortens it. As another aspect, the additional hydrogen bond donor can withdraw the electron density from the interacting hydroxyl group and lower its pKa facilitating the hydrogen transfer process back to the singlet ground state structure. We reoptimized the triplet structure of 3 Dan with the supplementary hydroxyl group to face the opposite direction and observed a slight elongation of the hydrogen bond length to 1.61 Å, which validates the dual effect of the hydroxyl group at R1 in Group B, as described in Figure .