Fine-tuning photophysical properties in pyrene derivatives: Unravelling the impact of substitution patterns with incorporation of a strong electron-donating group

Abstract

To investigate the impact of substitution patterns on the photophysical properties of pyrene derivatives, six novel pyrene derivatives A1-A6 were synthesised by directly coupling anthracene with pyrene units. The objective was to explore the influence of strong electron-donating groups and substitution patterns on their photophysical behaviour. Experimental and theoretical studies were conducted to analyse the photophysical properties of these compounds, revealing a clear dependence on the position of substitution within the pyrene core. Analysis of molecular orbitals revealed significant contributions from both pyrene and anthracene moieties, with deviations from typical electronic distributions attributed to the presence of electron-donating groups. The energy gap values remained consistent across derivatives, contrary to previous findings involving acceptor motif substitutions. Structural analyses indicated deviations from planarity with significant changes in angles upon excitation. The emission spectra of mono- and disubstituted pyrene derivatives predominantly exhibited a single broad maximum with slight shoulders in each solvent. However, the emission spectra of the tetrasubstituted compound (A6) displayed a vibrational structure with three maxima in all solvents, reminiscent of pyrene itself but red-shifted by approximately 30 nm. Furthermore, the central maximum for compound A6 was around 20 nm blue-shifted compared to the other compounds, although its shoulder at around 440 nm corresponded to a maximum observed in other pyrene derivatives. Typically, 1,3,6,8-tetrasubstituted pyrene derivatives, compared to their disubstituted analogues, exhibited red-shifted emission maxima associated with increased π-conjugation due to additional units on the pyrene core. A comprehensive analysis was performed on the absorption and emission spectra of the pyrene derivatives in solvents of various polarities and in the solid state. The fluorescence intensity was influenced by solvent polarity, with A6 showing limited sensitivity (similar quantum efficiency of approximately 45 % in all solvents). Moreover, the quantum yield decreases with the increasing number of anthracene substituents, being the lowest for tetrasubstituted A6. Bi-exponential decay and shorter lifetimes were observed for A1-A6 compared to known compounds, with quantum yields decreasing as solvent polarity increased. In the solid state, emission peaks shifted towards longer wavelengths, with A1 exhibiting the highest quantum yield attributed to steric hindrance. At low temperatures (77 K), emission spectra displayed redshifts, and longer lifetimes were observed. Spectrometric tests in THF/water mixtures indicated luminescence quenching at higher water fractions.