Alkyne-Linked Spirobifluorene D–π–A–π–D Emitters: Synthesis, Solution-State ICT Photophysics, and Comparative Acceptor Effects

Abstract

The construction of sterically demanding donor-acceptor architectures remains a significant challenge in organic materials chemistry. Herein, two D-π-A-π-D type compounds, BTD-DPA and QXN-DPA, incorporating a 9,9′-spirobifluorene core with benzothiadiazole and quinoxaline acceptor units, were synthesized and investigated in solution. Conventional cross-coupling approaches, including Stille and Negishi reactions, were unsuccessful because of severe steric congestion between donor-bridge fragments and acceptor units. Introduction of alkyne spacers through Sonogashira coupling successfully enabled access to the target structures. Both compounds exhibited good stability in aerated chloroform solution, with no observable degradation after 30 days as monitored by 1H NMR spectroscopy. Photophysical studies revealed absorption maxima at 352 nm for BTD-DPA and 310 nm for QXN-DPA, with corresponding emission maxima at 400 nm and 450 nm, respectively. Fluorescence quantum yields of 16.4% for BTD-DPA and 14.3% for QXN-DPA were measured in toluene. Solvent-dependent fluorescence behavior indicated the presence of intramolecular charge-transfer excited states. Computational analysis further supported the mixed localized π-π* and charge-transfer character of the electronic excitations. Comparative analysis demonstrated that acceptor identity significantly influences the absorption energy, emission wavelength, and fluorescence efficiency of these spirobifluorene-based systems. These findings provide insight into structure-property relationships in sterically hindered donor–acceptor luminophores and establish an effective synthetic strategy for their preparation.