BF2-Azadipyrromethenes: Probing the Excited-State Dynamics of a NIR Fluorophore and Photodynamic Therapy Agent

BF2-Azadipyrromethene dyes are a promising class of NIR emitter (nonhalogenated) and photosensitizer (halogenated). Spectroscopic studies on a benchmark example of each type, including absorption (one and two photon), time-resolved transient absorption (ps–ms) and fluorescence, are reported. Fast photodynamics reveal that intense nanosecond NIR fluorescence is quenched in a brominated analog, giving rise to a persistent (21 ?s) transient absorption signature. Kinetics for these changes are determined and ascribed to the efficient population of a triplet state (72%), which can efficiently sensitize singlet oxygen formation (ca. 74%), directly observed by 1?g luminescence. Photostability measurements reveal extremely high stability, notably for the nonhalogenated variant, which is at least 103-times more stable (?photodeg. = < 10–8) than some representative BODIPY and fluorescein dyes.