Truxene-bridged Bodipy fullerene tetrads without precious metals: study of the energy transfer and application in triplet–triplet annihilation upconversion

Abstract

Fullerenes can generate a triplet state efficiently without invoking the heavy-atom effect. However, their weak absorption in the visible spectral range makes fullerenes not an ideal triplet photosensitizer (PS). Herein, novel truxene-bridged fullerene and Bodipy tetrads were prepared by attaching Bodipy or Zn–porphyrin units (T-2B-C₆₀ and T-B-C₆₀-P) which show a stronger absorption band in the visible spectral range, and the maximum molar absorption coefficient is up to 5.63 × 10⁵ M⁻¹ cm⁻¹ at 428 nm for T-B-C₆₀-P. These tetrads show efficient singlet energy transfer from the Bodipy moiety to the fullerene/Zn–porphyrin moiety, the energy transfer rate constant is 2.72 × 10⁹ s⁻¹ and the efficiency is up to 90% for T-2B-C₆₀. Electrochemical and computational studies show that T-B-C₆₀-P exhibits photo electron transfer from fullerene to Zn–porphyrin and a T-B-C₆₀⁻˙-P⁺˙ charge-separated state is formed. Nanosecond time-resolved transient difference absorption spectra show that the triplet excited state of T-B-C₆₀-P is distributed on both the fullerene moiety and the Zn–porphyrin moiety, which can be used as the triplet PS for two-color excitable triplet–triplet annihilation upconversion, with a large anti-Stokes shift of 4074 cm⁻¹.