Synthesis and triplet sensitization of bis(arylselanyl)BOPHYs; potential application in triplet–triplet annihilation upconversion

Abstract

Owing to the growing interest in the development of novel triplet sensitizers, the introduction of arylselanyl (ArSe) groups at the 2 and 7 positions of bis(difluoroboron)-1,2-bis((1H-pyrrol-2-yl)methylene) hydrazine (BOPHY) scaffold was investigated for producing dyes 1 and 2. Direct Se–C bond formation was effective in facilitating the intersystem crossing (ISC) channel to the corresponding triplet state, endowing them with triplet sensitization. The quantum yield (Φ_Δ) of the singlet oxygen (¹O₂) generation was determined using diphenylisobenzofuran (DPBF), resulting in preferable Φ_Δ values of 73% and 74% for 1 and 2, respectively, which were significantly higher than those of halogenated BOPHY. This was ascribed to the selenium-based heavy-atom effect on the photophysical properties of BOPHY dye, which was rationalized by estimating the spin–orbit coupling (SOC) values of the S₁ →T_n transitions. The favorable ability to generate ¹O₂ under photoirradiation motivated us to apply the dye as a sensitizer to triplet–triplet annihilation upconversion (TTA-UC). Photoexcitation of a toluene solution of 1 and 9,10-diphenylanthracene (DPA) as the triplet energy acceptor with a 524 nm-wavelength laser allowed to detect upconverted emission with λ_em values of 412 nm and 430 nm, which were attributed to the DPA fluorescence. A significant anti-Stokes shift of approximately 94 nm was observed in the UC process with a of 3.9%. This study suggests that a judicious introduction of ArSe units into chromophores provides a facile but effective strategy for designing triplet sensitizers.