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