Excited-State H-Aggregation Enables Proton Abstraction from Water in Weak Photobases

Photobasic molecules can abstract a proton from water in the picosecond range, enabling efficient photochemical reactions, especially photocatalytic water splitting. However, such molecules are rarely reported due to the challenge of achieving an excited-state pK_a (pK_a*) close to the pK_a of water ( 15.7). Here, we demonstrate that excited-state H-aggregation can substantially enhance proton-attracting capability by raising the energy of unprotonated species. According to the Förster cycle, the thermodynamic driving force for proton attraction is determined by the energy difference between the protonated and unprotonated species. For the model weak photobasic molecule citrazinic acid, excited-state H-aggregates are formed simply by adjusting the concentration. The energy of the unprotonated species increases by 0.38 eV upon aggregation, while that of the protonated species remains unchanged. The increased energy difference significantly increases the pK_a* from a ground-state value of 2.5 to 15.4. Consequently, the weak photobasic aggregates are enabled to abstract a proton from water, a capability absent in the isolated form. Meanwhile, the lower-energy state in H-aggregates functions as a rate-limiting intermediate state, delaying the proton transfer dynamics, but the delay can be modulated by the excitation wavelength. This work provides fundamental insights into H-aggregation-induced photobasicity, opening new avenues for modulating photochemical reactions simply through concentration and light.