Color-Tunable Delayed Fluorescence and Efficient Spin–Orbit Charge Transfer Intersystem Crossing in Compact Carbazole-Anthracene-Bodipy Triads Employing the Sequential Electron Transfer Approach

Spin–orbit charge transfer intersystem crossing (SOCT-ISC) is a promising approach to develop heavy-atom-free triplet photosensitizers. However, designing a strong visible-light harvesting heavy-atom-free triplet photosensitizer with efficient ISC ability in various solvents is still challenging. Most of the SOCT-ISC triplet photosensitizers exhibit efficient ISC only in solvent of particular polarity. To address this challenge, herein, two triads (BDP-AN-C-CZ and BDP-AN-N-CZ), composed of carbazole (CZ), anthracene (AN), and bodipy (BDP) moieties, were devised. In these triads, the distance, relative orientation, and position of CZ with respect to the AN moiety were varied to study the effect on photophysical properties, especially on SOCT-ISC efficiency. Electrochemical studies, steady-state, and time-resolved spectroscopies confirmed a sequential photoinduced electron transfer (PET) process in the triads. The fluorescence of the BDP moiety is quenched and a red-shifted CT emission band is observed in the triads, due to the enhanced PET effect, compared to the reference BDP-AN dyad. We observed that the SOCT-ISC yield can be enhanced taking advantage of sequential electron transfer. The triad BDP-AN-C-CZ, in which the CZ moiety was directly linked to the AN moiety, shows an efficient ISC ability both in low-polarity and high-polarity solvents, and unity triplet quantum yield (Φ_T) was observed in dichloromethane. Femtosecond transient absorption spectroscopy confirmed the fast charge separation process (1.8 ps) in BDP-AN-C-CZ as compared to the other triad BDP-AN-N-CZ (4.8 ps) and the reference BDP-AN dyad (7.7 ps). The triads were used as triplet photosensitizers for triplet–triplet annihilation (TTA) upconversion, and high upconversion quantum yield (Φ_UC = 18%) was observed. Interestingly, long-lived (τ_DF = 118 μs) and solvent-dependent color-tunable TTA delayed fluorescence was observed in the case of BDP-AN-C-CZ.