Ground- and excited-state proton transfer and rotamerism in 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole and its O/"NH or S"-substituted derivatives.

The intramolecular proton-transfer process, rotational process, and optical properties of 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-oxadiazole (HOXD) and its O/"NH"- and O/"S"-substituted derivatives, 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-triazole (HOT) and 2-(2-hydroxyphenyl)-5-phenyl-1,3,4-thiadiazole (HOTD), respectively, have been studied. DFT (B3LYP/6-31+G**) single-point energy calculations were performed using HF- and DFT-optimized geometries in the ground state (S0). TD-B3LYP/6-31+G** calculations using CIS-optimized geometries were carried out to investigate the properties of the first singlet excited state (S1) and first triplet excited state (T1). The computational results revealed that a high-energy barrier inhibits the proton transfer from cis-enol (Ec) to keto (K) form in S0, whereas the proton transfer in S1 can take place through a very-low-energy barrier. The rotation between Ec and trans-enol (Et) can occur in S0 through a low-energy barrier, whereas it is prohibited because of the high-energy barrier in S1 for each of the three molecules. The vertical excitation energies were calculated using the TD-B3LYP/6-31+G** method based on the HF- and CIS-optimized geometries. Absorption and fluorescence wavelengths of HOT show a hypsochromic shift (6-15 nm) relative to HOXD, while those of HOTD show a bathochromic shift (21-29 nm). The phosphorescence wavelength of HOTD shows a significant bathochromic shift relative to that of HOXD.