Intermolecular interactions in crystals modulate intramolecular excited state proton transfer reactions

Proton transfer is a fundamental process underlying chemical and biological phenomena, and its dynamics are significantly influenced by the surrounding environment. This paper studies the excited state intramolecular proton transfer (ESIPT) process, which is crucial to the photostability of hydroxyanthraquinone-based pigments through efficient energy dissipation, by investigating how crystalline packing influences photoinduced proton transfer dynamics in single crystals of dihydroxyanthraquinone (DHAQ) constitutional isomers. Comparing the proton transfer dynamics in crystalline and solution phases, we show substantial differences due to the crystalline environment, particularly in the 1,4- and 1,5-DHAQ isomers. These isomers show intermolecular hydrogen bonding within the crystal lattice, resulting in larger excitonic couplings, which significantly alters their reaction pathways compared to their behaviour in solution. We show that 1,8-DHAQ, which does not form intermolecular hydrogen bonds in the crystal, shows minimal changes in dynamics between the phases. In contrast, in the case of 1,4-DHAQ, intermolecular interactions within the molecular crystal phase open up an ESIPT relaxation channel, which is not observed in the solution phase. These findings highlight the critical role of crystal packing in modulating proton transfer dynamics and offer insights into how molecular packing can be strategically manipulated to control and optimize reaction pathways in solid-state environments.