Theoretical study on an intriguing excited-state proton transfer process induced by weakened intramolecular hydrogen bonds.

In the present work, we have systematically investigated the dual hydrogen-bonded system 2,2'-3,3'-(4,4'-methylenebis(4,1-phenylene)bis(azanediyl)bis(1,3-diphenylprop-2-en-1-one)) (abbreviated as L) utilizing quantum chemistry methods, in which the excited-state intramolecular proton transfer (ESIPT) does not conform to the usual stereotype but proceeds along the weakened intramolecular hydrogen bonds (IHBs). Two primary configurations were confirmed to coexist in the ground state (, -L and -L) by calculating the Boltzmann distribution in three different solvents. Based on the cardinal geometrical parameters involved in IHBs and the interaction region indicator (IRI) isosurface, it can be revealed that the dual IHBs of L were both weakened upon photoexcitation, not least the N-H⋯O IHB was utterly destroyed in the excited state. The proton-transfer process of and in three solvents with different polarities has been analyzed by constructing S- and S-state potential energy surfaces (PESs). It can be concluded that only the single proton transfer behavior along N-H⋯O occurs in the S state, and the corresponding energy barrier is gradually enlarged with increasing solvent polarity. To further expound the weakened IHB-induced ESIPT mechanism, the scanned PESs connecting the transition state (TS) structures and the initial forms indicate that the ESIPT process is infeasible without the appropriate structural torsion. Our work not only unveils the extraordinary ESIPT process of L, but also complements the results obtained from previous experiments.