Theoretical study of the excited-state intramolecular proton transfer and rotamerism in 2,5-bis(2-hydroxyphenyl)-1,3,4-oxadiazole

Abstract

The intramolecular proton transfer process, rotational process, and optical properties of 2,5-bis(2-hydroxyphenyl)-1,3,4-oxadiazole have been studied. The effects of solvents were considered using the polarized continuum mode. The calculated results revealed that the high energy barriers inhibit the proton transfers for bis-enol (BE) forms (BE1 and BE2) in S0 states. The single proton transfers for BE forms in S1 states can take place through low energy barriers and exothermicity, while the corresponding processes are not feasible in T1 states due to endothermicity. The proton transfers for enol–keto (EK) forms (EK1 and EK2) are difficult to occur in S0, S1, and T1 states because of the high energy barriers and large endothermicity. The rotational processes of both BE1 → BE2 and EK1 → EK3 are feasible in S0 states, while difficult in S1 and T1 states because of the high energy barriers. The rotational process of both EK1 → EK2 and BK1 → BK2 are difficult to occur in S0, S1, and T1 states. Furthermore, 1EK1* in S1 state can undergo ISC to the T1 state PES of 3EK1*, which may be the reason that the excited state intramolecular proton transfers can facilitate ISC to T1 states and subsequently enhance the electroluminescent efficiency. The normal small Stokes shift emission can be assigned to 1BE1* form. The large Stokes shift emission can be assigned to the 1EK1* form, which was formed by the single proton transfer in S1 state. In T1 states, the 3BE1* form shows phosphorescence emissions both in gas phase as well as in solutions. Our results give good supplementary explanations for experimental results at the molecular level. The intramolecular proton transfer and rotational processes in the S0, S1, and T1 states for the different tautomers and rotamers of 2,5-bis(2-hydroxyphenyl)-1,3,4-oxadiazole have been theoretically investigated. Their optical properties in the different environments have been studied with the aim to get an in-depth explanation of the experimental results.