The effect of resonance-assisted hydrogen bond on the second-order nonlinear optical properties of pyridine hydrazone photoswitches: a quantum chemistry investigation

International audience ; The effect of hydrogen bonds on the NLO properties was not considered as essential, in particular inpyridine hydrazone systems. Yet, we show in the present study that a control of these photoswitchesdepends on the strength of hydrogen bonds. In this study, we investigate a selection of 18 E/Z pyridinehydrazone photoswitch molecules to explore the impact of resonance-assisted hydrogen bond (RAHB)on the NLO properties in the E/Z isomers. Using quantum calculations at the oB97XD/6-311+g(d) levelof theory, we determine various electronic parameters, reactivity descriptors, bond length alternation(BLA) values, nuclear independent chemical shift (NICS) aromaticity indices, QTAIM topology, energy ofhydrogen bond (EHB), RAHB, and linear and nonlinear optical properties for these molecules. Theagreement between the quantum calculations and experimental spectra is illustrated through TD-DFTcalculations, showing small deviations. Contrary to conventional expectations, our findingsdemonstrated that the delocalization strength of the electrons and NLO properties of the Z isomers aresignificantly enhanced by the presence of a resonance-assisted hydrogen bond. The Z-isomer exhibiteda lower excited state energy, weaker energy gap, smaller BLA value, larger dipole moment variations forthe first excited state, higher FE-Z, and electron delocalization at the quasi-cycle closed (RAHB)compared to the E-isomer. Furthermore, we find that the hyperpolarizability value of the titlephotoswitches increases as the wavelength of the incident light decreases, i.e., b(695) 4 b(1064) 4b(1340) 4 b(N), and the dispersion has less effect at l = 1064 and 1340 nm. Additionally, we observe astrong relation between the photoisomerization quantum yield (FE$Z) and static hyperpolarizability (b) ofthe first and second isomer, where FE$Z is proportional to b of the second isomer and inverselyproportional to b of the first isomer. This inverse trend between static hyperpolarizability andphotoisomerization quantum yields is ...