Study on the Photoinduced Isomerization Mechanism of Hydrazone Derivatives Molecular Switch

Improving quantum yield is an important characteristic for enhancing the operational efficiency of light-driven molecular motors. Building upon Cigan et al.'s pioneering work on CH₃ substitution for H (RSC Adv., 2015, 5, 62449), we have developed a structural modification strategy for hydrazone-based molecular switches through the replacement of a single oxygen atom with two hydrogen atoms, resulting in a remarkable enhancement of the quantum yield. We systematically investigate the photoinduced isomerization mechanism of the hydrazone derivatives molecular switch using the Tully's surface hopping method on the semiempirical OM2/MRCI level. The results show that the calculated quantum yield for the E-to-Z photoisomerization of this molecular rotary motor is approximately (55 ± 3)% (16.01% for original (Pang, X.-J.; Zhao, K.-Y.; He, H.-Y.; Zhang, N.-B.; Jiang, C.-W. Photoinduced isomerization mechanism of isatin N²-diphenylhydrazones molecular switch. Acta Phys. Sin. 2024, 73 (17).) with an average lifetime of the excited state of 122 fs. Additionally, we calculate the time-dependent fluorescence emission spectra and observe a redshift in wavelength accompanied by fluorescence emission quenching, which shows a blue shift compared to the original isatin N²-diphenylhydrazone spectrum. Furthermore, we propose that this molecular switch may not have a "dark state".