Fine-tuning the photophysical properties of the five quinolin based nanophotoswitches in the gas phase, polar and nonpolar solvents: A TD-DFT approach

Abstract

Fluorophores exhibiting excited state intramolecular proton transfer (ESIPT) have received increasing attention during the last decades. The swiftness of ESIPT process often avoids its inspection with only experimental methods, and theoretical explorations are often necessary to achieve a full justification of the observations and fine-tuning the ESIPT fluorescence properties. In this work, photo-switching in five novel red light emitting molecules 11hydroxy-indeno[1,2-b]quinolin-4ol (11HIQO), 4hydroxy-11H-indeno[1,2-b]quinolin-11one (4HIQO), 10H-indolo[3,2-b]quinolin-6-ol (10HIQO), benzofuro[3,2-b]quinolin-6-ol (BFQO) and benzothieno[3,2-b]quinolin-6-ol (BTQO) was comprehensively investigated using a robust protocol based on Time-Dependent Density Functional Theory (TD-DFT) at PBE0/6-311++G(2d,2p) level of theory. The interplay between π-conjugation and aromaticity was observed in all five photo-switching molecules in which are able to render both normal and keto fluorescence emissions with wavelengths longer than 610 nm and large Stokes shifts in gas phase (242–472 nm) and solvent media (187–385 nm). The normal and tautomer emissions were assigned to the relaxation of the initially excited Franck-Condon (FC) state to normal (S1-E) and tautomer (S1-K) states, respectively. The full photo-switching cycle and potential energy surfaces in the ground and excited states and structural parameters, H-bonding energy, absorption and emission wavelengths, vertical excitation and emission energies, oscillator strength, fluorescence rate constant, dipole moment, atomic charges and electron density at critical points for five new photochromic ESIPT switches were explored. The potential of these molecules as emissive switches in designing and generation of new displays and light sources in the molecular photo-devices were predicted based on their high red Stokes shifts.