Tuning Charge Transfer and Singlet Oxygen Generation in Donor–Substituted Benzophenones by Structural and Solvent Effects

Abstract

Donor–acceptor-substituted benzophenone (Bzp) derivatives are useful model compounds for studying intramolecular charge transfer (ICT) due to the presence of electron-donating and electron-withdrawing substituents. Connecting a Bzp core to various para-substituted donor groups gives rise to structures with distinct photophysical behaviour. This study presents the electronic, spectral, and photophysical properties of nine unsymmetrical Bzp-D derivatives in various solvents. The rational design of these compounds features a common Bzp donor and different acceptor groups with varying torsion angles of the donor moiety. These structural factors, together with solvent polarity, strongly influence their emission properties. A comprehensive study was conducted using steady-state and time-resolved emission measurements of Bzp-D derivatives in solvents spanning a wide polarity range, to understand their charge transfer behaviour. The photoluminescence (PL) spectra were effectively tuned from blue to orange based on the ICT effect. The emission behaviour can be adjusted through chemical modification to control the geometry of the ICT state. The fluorescence decay times of the locally excited (LE) and charge transfer (CT) states were found to depend on the basicity of the solvent, which emphasises the influence of hydrogen bonding on these states. Quantum chemical calculations (DFT and TDDFT) explained the observed spectral characteristics. The data indicate that, in both ground and excited states, the torsion angle and the nature of the substituent in the donor moiety significantly influence emissive properties. These findings suggest that extending the conjugation of a Bzp acceptor linearly can markedly enhance ICT interactions. High singlet oxygen sensitisation quantum yields (ϕ_Δ) were recorded for all derivatives in 2-methyltetrahydrofuran, with values ranging from 0.47 to 1.