Reverse solvatochromism in a family of probes having 2,6–di–tert–butylphenolate as electron–donor and 4–nitrophenyl as electron–acceptor groups

Abstract

Dyes presenting electron–accepting nitrophenyl and electron–donating phenolate groups have been used in the recent years as solvatochromic probes to investigate the polarity of solvents. Although several substituents in the phenolate have been used in studies involving these dyes, the use of tert-butyl groups has not yet been explored for these compounds. Bulky tert–butyl groups in the structure of these probes could bring interesting results because they make the dyes more lipophilic and hinder the formation of hydrogen bonds with hydrogen bond donating solvents. In this study, eight 2,6–di–tert–butylphenols were synthesized and characterized. They were then deprotonated to generate the corresponding phenolate dyes, which exhibited reverse solvatochromism. TD/DFT calculations were performed, being shown that the longest wavelength absorption bands of the dyes are due to π−π* transitions, with an intramolecular charge transfer from the 2,6–di–tert–butylphenolate to the 4–nitrophenyl group. Data indicated that the overall nature of the excitation is the same in all cases, and the differences registered between experimental and theoretical data suggest that a synergetic effect in the solvation of the probes involving many solvent molecules may take place, which can widen the gap between HOMO and LUMO. The tert–butyl groups play an important role in the nature of dye–solvent interactions. Molecular dynamics simulations demonstrated the effect of the tert–butyl groups in hindering hydrogen bonding between the phenolate group and methanol and water solvent molecules, with a more structured solvation shell being observed on the nitro substituent of the probes. In conclusion, the investigation carried out showed that the presence of tert–butyl groups in the electron–donor phenolate groups tunes the types of interactions between the probe and the solvent. This can occur through changes in the molecular and electronic structure of the probe or be a result of steric hindrance, which makes the interaction of the solvent with the oxygen of the donor phenolate group more difficult.