Spectroscopic investigation of phenazine-based donor–acceptor dyads with thermally activated delayed fluorescence: heavy halogen effects on photosensitizing properties

Abstract

The demand for metal-free, water-compatible photosensitizers for photocatalysis, bioimaging, and photodynamic therapy has inspired the exploration of purely organic systems with efficient visible light absorption and triplet harvesting. We report on the photophysical properties of a dicyanodibenzo[a,c]phenazine-based dye (H-TPA-A) and its dibrominated (Br-TPA-A) and diiodinated (I-TPA-A) derivatives, designed as broad range visible-light-absorbing photosensitizers with thermally activated delayed fluorescence (TADF) for aqueous environments. Partial conjugation of the large π-system of the acceptor with the triphenylamine donor enables absorption in the 350–650 nm range, while maintaining a small energy gap between the S₁ and T₁ states. Incorporation of heavy halogens enhances intersystem crossing (ISC) via the internal heavy atom effect, confirmed by increased spin–orbit coupling (SOC) from 0.05 cm⁻¹ (H-TPA-A) to 0.14 cm⁻¹ (Br-TPA-A) and 0.53 cm⁻¹ (I-TPA-A) between S₁ and T₁ states. Stable micellar dispersions in SDS enable (sub)microsecond storage of the excitation energy in water: the delayed fluorescence (DF) lifetimes reach 257 µs (H-TPA-A) and 176 µs (Br-TPA-A) in dilute environments. In neat micelles, aggregation not only reduces oscillator strength (∼3-fold) and photoluminescence quantum yield (PLQY <10%) but also promotes ISC through the external heavy atom effect, particularly in halogenated derivatives. Br-TPA-A exhibits the best balance of low (non)radiative deactivation and efficient triplet formation, making it the most promising candidate for photosensitization. Nonetheless, aggregation-induced quenching in aggregates limits the energy storage efficiency, indicating the need for structural modifications to suppress aggregation without compromising ISC enhancement.