Exploring the effect of nitrile substituent position on fluorescence quantum yield of ESIPT-based oxazoline derivatives: A TDDFT investigation

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Vol. 272; p. 120953
• Main Authors: Li, Qi, Wan, Yu, Zhou, Qiao, Li, You, Li, Bo, Zhu, Lixia, Wan, Yongfeng, Yin, Hang, Shi, Ying
• Format: Journal Article
• Language: English
• Published: England Elsevier B.V 05.05.2022
• Subjects: Energy curve; ESIPT; Quantum yield; Spin-orbit coupling; Spin-orbit coupling; ESIPT; Quantum yield; Energy curve
• ISSN: 1386-1425; 1873-3557
• DOI: 10.1016/j.saa.2022.120953

Intersystem crossing of 1-CN is promoted greatly compared with 3-CN, which results in the quantum yield of 1-CN is lower than 3-CN. [Display omitted] •TheESIPTreactionsfor1-CNand3-CNareidentified.•The energy level difference is small enough to ensure the occurrence of ISC for two molecules.•Quantum yield difference is ascribed to promoted intersystem crossing of 1-CN compared with 3-CN. We explore the mechanism specifically on quantum yields difference of 2-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxybenzonitrile (1-CN) and 4-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxybenzonitrile (3-CN) by density functional theory and time-dependent density functional theory within the Tamm-Dancoff approximation. The structures optimization and the potential energy curves scanning of singlet excited state directly prove that the excited state intramolecular proton transfer (ESIPT) can take place in 1-CN and 3-CN molecules. The calculated spectra show that the fluorescence peaks of two molecules come from the emission of keto* configuration. The non-covalent interaction and the atomic dipole moment corrected Hirshfeld charge are also analyzed. Through the comparison of emission oscillator strength between 1-CN and 3-CN molecules suggests that the radiative transition process is not the main reason for the difference on quantum yields. Internal conversion process is also excluded on account of the large energy gap between S0 and S1. Considering the interaction between singlet and triplet states, both molecules can undergo intersystem crossing. The prominent difference is that, compared with 3-CN, the larger spin-orbit coupling constant and smaller energy level difference promote the intersystem crossing process of 1-CN. This provides direct evidence for the fluorescence quantum yield of 1-CN is lower than that of 3-CN. We envision that the present work can provide help for the synthesis and application of ESIPT compounds with high quantum yields.