3,5-Anthryl–Bodipy dyad/triad: Preparation, effect of F–B–F induced conformation restriction on the photophysical properties, and application in triplet–triplet-annihilation upconversion

We prepared a series of compact Bodipy–anthryl electron donor/acceptor triads and dyads by attaching anthryl moieties at the 3-,5-positions of the Bodipy core, with a novel conformation restriction approach, to study the spin–orbit charge transfer intersystem crossing (SOCT-ISC). The conformation restrictions are imposed by the BF2 unit of Bodipy without invoking the previously reported method with 1,7-dimethyl or 1,3-dimethyl groups. Our new approach shows a few advantages, including the stronger electron accepting ability of the methyl-free Bodipy core (reduction potential anodically shifted by +0.3 V vs the methylated Bodipy), red-shifted absorption (by 21 nm), and longer triplet state lifetime (372 µs vs 126 µs). The effects of the different mutual orientations of the electron donor and acceptor on ultraviolet–visible absorption, fluorescence, triplet state quantum yields, and lifetimes were studied. Triads with orthogonal geometries show higher singlet oxygen quantum yields (ΦΔ = 37%) than those with more coplanar geometries. Since the non-radiative decay for the S1 state is significant in the parent Bodipy chromophore (ΦF = 6.0%), we propose that in dyads/triads, the charge separation and recombination-induced ISC outcompete the non-radiative decay to the ground state, which is new in the study of SOCT-ISC. Density functional theory computation indicated a shallow torsion potential energy curve as compared to the meso-anthryl–Bodipy dyad analog, which may contribute a low triplet state quantum yield of the new dyads/triads. Triplet–triplet annihilation upconversion was performed with the electron donor/acceptor dyads as the triplet photosensitizer, with an upconversion quantum yield of 12.3%.