Excited-state hydrogen bond strengthening drives ultrafast electron transfer in coumarin 153/dimethylaniline complex: A TDDFT study

Abstract

The role of intermolecular hydrogen bonds in photoinduced electron transfer (PET) between coumarin 153 (C153) and N,N-dimethylaniline (DMA) was investigated using DFT and TDDFT. Calculations revealed that two intermolecular hydrogen bonds significantly strengthen in the excited state (S₂), with hydrogen-bond binding energy increasing from 8.24 kJ/mol to 157.35 kJ/mol, and dipole moment rising from 7.25 D to 23.43 D. Frontier molecular orbital analysis confirmed that the S₁ state is an intermolecular charge transfer (CT) state, facilitated by enhanced hydrogen bond coupling. Spectral shifts in C–H stretching vibrations validated hydrogen bond dynamics. These findings demonstrate that the reinforcement of hydrogen bonds in the excited state accelerates PET, offering valuable insights for the design of efficient photoactive materials.

Graphical Abstract

This study shows that the S excited state of the C153/DMA complex is a locally excited state on C153 with partial intramolecular charge transfer (CT) from N to the C=O group. The S₁ state, in contrast, is an intermolecular CT state involving photoinduced electron transfer (PET) from DMA to C153 and has low oscillator strength. Hydrogen bonds between molecules are stronger in the S₂ state, supporting this electron transfer.