Theoretical Insights Into the Effects of Water Solvent and Intermolecular Hydrogen Bond on Excited State Intramolecular Proton Transfer ( <scp>ESIPT</scp> ) of 2‐Butyl‐4‐Hydroxyisoindoline‐1,3‐Dione

ABSTRACT The fluorophore 2‐butyl‐4‐hydroxyisoindoline‐1,3‐dione (BHD) has been obtained experimentally ( RSC Adv ., 2015, 5, 18177–18182). However, the influence of protic solvent and intermolecular hydrogen bond (InterHB) between BHD and solvent has not been considered. On this foundation, we systematically explored the effect of protic water on the ESIPT mechanism of BHD adopting density functional theory (DFT) and time‐dependent DFT (TD‐DFT). From a series of data, it can be ensured that the intramolecular hydrogen bonds (IntraHBs) and InterHBs in BHD and its hydrogen‐bonded complexes (BHDH 2 O, BHD(H 2 O) cyc ) are enhanced upon photoexcitation. For BHD, BHDH 2 O and BHD(H 2 O) cyc , the proton transfer process can only occur in the excited state. The potential barriers of ESIPT in BHD and BHDH 2 O are 2.6362 and 3.8611 kcal/mol, respectively. Clearly, the extra InterHB that does not involve in the proton transfer reaction hinders the ESIPT process in BHDH 2 O. For the cyclic hydrogen‐bonded complex BHD(H 2 O) cyc , excited state double proton transfer (ESDPT) process occurs in a concerted but asynchronous protolysis pattern, requiring only conquering a potential barrier of 0.0356 kcal/mol, as shown in the potential energy curve. Compared with BHD, the two InterHBs in BHD(H 2 O) cyc obviously reduce the potential barrier and effectively accelerate the ESPT process.