Theoretical studies on the photophysical properties and ESIPT behavior of HPQ derivatives

In this work, DFT and TDDFT methods using both B3LYP and CAM-B3LYP functionals combined with 6-31G(d) and 6-31++G(d,p) basis set were used to study the ground state, excited state geometries, the electronic properties, vertical excitation and emission energies, as well as the ESIPT potential energy curves of the experimental synthesized HPQ derivatives (HPQ2-6) and the newly designed molecules DA1-9 and the extended conjugation structures. The effect of dispersion was also considered. We found that the substituents and solvent have significant effects on ESIPT behavior and its photophysical properties. The electron donating/withdrawing ability of substituents affects the electronic properties of excited molecules, which in turn regulates the emission wavelength. In addition, we found that the larger the polarity of the solvent, the more redshifted the emission wavelength and the smaller the energy barrier for ESIPT. Among the newly designed molecules DA1-9, DA8 with BBTD group substituted at the R₁ site, has potential application in near-infrared probes with emission wavelength of 972 nm at the TD-B3LYP-D3/6-31++G(d,p) level. In addition, extending conjugation not only facilitates the redshift of emission wavelength but also enhances the fluorescence intensity. This work provides important insights into the regulation of ESIPT dynamics and emission wavelengths of HPQ derivatives, which is important for designing novel NIR HPQ derivatives.