Excited state proton transfer coupled with twisted intermolecular charge transfer for N,N-dimethylanilino-1,3-diketone in high polar acetonitrile solvent

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In the present work, N,N-dimethylanilino-1,3-diketone (DMADK), a new chromophore of the unsymmetrically substituted 1,3-diketone [R. Ghosh and D. K. Palit, Photochem. Photobiol. Sci. 12 (2013) 987–995], has been investigated about the excited state proton transfer (ESPT) based on the time-dependent density functional theory (TDDFT) method. The experimental UV/Vis and emission spectra are well reproduced by the calculated vertical excitation energies in the S0 and S1 states. For the optimized Enol-B* structure, the twisted intermolecular charge transfer (TICT) process can be confirmed in the S1 state. Hydrogen bond strengthening has been testified in the S1 state based on comparing primary bond lengths and bond angles involved in the intramolecular hydrogen bond between the S0 state and the S1 state. Furthermore, infrared spectra (IR) at the OH stretching vibrational region and Molecular electrostatic potential surface (MEPS) based on our calculation also declare the phenomenon of hydrogen bond strengthening. The frontier molecular orbitals (MOs) analysis, MEPS, Mulliken's charge distribution analysis, Hirshfeld charge distribution analysis and Natural bond orbital (NBO) analysis methods manifest the intramolecular charge transfer, which reveals the tendency of excited state intramolecular proton transfer (ESIPT) process. The constructed PESs of both S0 and S1 states demonstrate that ground state intramolecular proton transfer (GSIPT) as well as reversed GSIPT processes exists in the S0 state, and the ESIPT coupled with the TICT process can occur in the S1 state rather than sequential ESIPT and TICT processes.