Exploring the ESIPT dynamical processes of two novel chromophores: symmetrical structure CHC and asymmetric structure CHN

Abstract

The detailed excited-state intramolecular proton transfer (ESIPT) dynamical processes of two novel chromophores, 8,8′-((1E,1E′)-hydrazine-1,2-diylidenebis(methanylylidene))bis-(7-hydroxy-4-methyl-2H-chromen-2-one) (CHC) (symmetrical structure) and 7-hydroxy-8-((E)-((E)-((2-hydroxynaphthalen-1-yl)-methylene)hydrazono)methyl)-4-methyl-2H-chromen-2-one (CHN) (asymmetric structure), which were synthesized in a previous study (Xiao et al., New. J. Chem., 2014, 38, 2386), were investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Analysis of the bond lengths, angles and IR vibrational spectra confirmed that the intramolecular hydrogen bonds (HBs) of the CHC and CHN molecules were strengthened in the S₁ state, which could facilitate ESIPT reactions. In addition, intramolecular charge transfer based on frontier molecular orbitals (MOs) and maps of the electron density difference between the S₀ and S₁ states indicated the possibility of ESIPT reactions for these two molecules. Moreover, to explore the detailed ESIPT dynamical processes of the CHC and CHN molecules, the potential energy surfaces (PESs) in the S₀ and S₁ states were constructed. The low excited-state potential barriers illustrated that both stepwise and simultaneous double ESIPT processes could occur for these two molecules. For the symmetrical structure CHC, two pathways of ESIPT processes existed as pathway I (stepwise double PT) and pathway II (simultaneous double PT). The relationship of the potential barriers was pathway II (4.71 kcal mol⁻¹) < pathway I (6.15 and 7.62 kcal mol⁻¹), which manifested that pathway II was more prone to double ESIPT for the CHC molecule. For the asymmetric structure CHN, three pathways of ESIPT processes existed as pathway III (stepwise double PT: firstly H₈ from O₇ to N₉, and secondly H₁₁ from O₁₀ to N₁₂), pathway IV (other stepwise double PT: firstly H₁₁ from O₁₀ to N₁₂, and secondly H₈ from O₇ to N₉) and pathway V (double PT). The relationship of the potential barriers was pathway III (2.67 and 6.75 kcal mol⁻¹) < pathway IV (3.04 and 7.24 kcal mol⁻¹) < pathway V (7.69 kcal mol⁻¹), which indicated that pathway III was more susceptible to double ESIPT for the CHN molecule. Obviously, the ESIPT processes of the asymmetric structure CHN were more complicated than those of the symmetrical structure CHC.