V-shaped D–π–A–π–D molecules based on benzothiophene-S,S-dioxide: tuning of excited states via donor strength to engineer photoactive materials

Abstract

Controlling locally excited (LE) and charge-transfer (CT) states and triplet-related pathways is crucial for the design of photoactive materials. Here, it is reported a family of V-shaped D–π–A–π–D small molecules (V-CBZ₂, V-POZ₂, and V-PTZ₂) that share a benzothiophene-S,S-dioxide acceptor and 1,4-dimethylphenylene bridges, while incorporating donors of increasing strength: carbazole, phenoxazine, and phenothiazine. The enforced V-shaped geometry limits the electronic coupling between the acceptor and donor units and prevents tight π–π stacking. Photophysical characterizations and TD-DFT calculations reveal that the donor strength dictates the nature of the excited states: from hybrid LE-CT character in V-CBZ₂ to predominantly CT in V-POZ₂ and V-PTZ₂. In solution, V-CBZ₂ behaves as a bright fluorophore that predominantly decays radiatively (PLQY up to 100%), whereas V-POZ₂ and V-PTZ₂ act as efficient singlet-oxygen (¹O₂) photosensitizers (Φ_Δ up to 89%). Increasing solvent polarity stabilizes their CT states, further modulating emission lifetimes, PLQYs and ¹O₂ generation. All compounds exhibit a reversible electrochromic response between transparent and coloured states, demonstrating that systematic donor variation provides straightforward control over the colour hue. In the solid state, all molecules show broad and featureless emission, with comparable PLQYs in doped polymer matrices and neat films, indicating that the twisted architecture effectively mitigates aggregation-induced quenching. Time-resolved studies reveal purely prompt fluorescence for V-CBZ₂, whereas V-POZ₂ and V-PTZ₂ display additional microsecond delayed components attributed to thermally activated delayed fluorescence (TADF). Finally, a solution-processed OLED based on a neat V-CBZ₂ layer delivers broad white electroluminescence, highlighting that this V-shaped D–π–A–π–D architecture can function as a versatile platform for matrix-free solution-processable organic light-emitting materials.