Theoretical investigation on excited state intramolecular proton transfer of 1-aryl-2-(furan-2-yl) butane-1, 3-diones substitutions

The effect of different substituents on the excited state intramolecular proton transfer (ESIPT) of 1-aryl-2-(furan-2-yl)butane-1,3-diones (AYFBD), namely OFBD, TFBD, MFBD and FFBD are studied by employing the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. By comparing the geometric configurations and infrared vibrational spectra of the first excited state (S1) with those of the ground state (S0), it can be concluded that the intramolecular hydrogen bonds are strengthened in S1 state. The calculated peaks of absorption and fluorescence spectra of AYFBD derivatives in keto form are consistent with experimental results reported previously (Journal of Organic Chemistry, 2017, 82, 12097). We can also confirm that the intensities of the intramolecular hydrogen bonds in the S1 state are enhanced by analyzing the frontier molecular orbitals and the infrared spectra. To further investigate the ESIPT mechanism of AYFBD derivatives, the potential energy curves (PECs) of S0 and S1 states are scanned by varying O1 H2 distance, which demonstrates that ESIPT occurs in the S1 state of all the AYFBD derivatives and follows the order: FFBD > MFBD > TFBD > OFBD.