Truxene-Linked Zinc Porphyrin–Bis(Coumarin) Triad: Synthesis, Photophysical Properties and its Application in Triplet−Triplet Annihilation Upconversion

Abstract

An triplet photosensitizer (PS) featuring a long triplet lifetime and high yield of triplet quantum holds significant importance for triplet–triplet annihilation–upconversion (TTA-UC). Herein, a truxene based Zn porphyrin-dicoumarin triad (T-2C-P) was synthesized and employed as a novel heavy-atoms-free PS for TTA-UC. Steady-state UV-Vis absorption and fluorescence spectroscopy, and nanosecond time-resolved transient absorption spectroscopy were employed to investigate the photophysical properties. The triad T-2C-P exhibited a strong absorption peak, with the maximum molar absorption coefficient(e) reaching as high as 4.32 × 105 M−1 cm−1. The intramolecular singlet–singlet energy transfer from the coumarin moieties to the Zn-porphyrin units takes place with an efficiency of 77.6% under the photoexcitation of the coumarin.  In toluene, the triplet lifetime of T - 2C - P is 82.5 µs. Based on nanosecond transient absorption analysis, the triplet excited states are located on the Zn porphyrin moiety. The triad T-2C-P can be applied as triplet photosensitizer for two-color TTA-UC and the upconversion quantum yields (ΦUC) was determined to be 1.67% and 1.47 % with Py and PBI as triplet acceptor, respectively. Moreover, when Py acts as the triplet acceptor, T - 2C - P exhibits a substantial anti - Stokes shift of 5244 cm⁻¹.

Keywords: Triplet photosensitizer, Coumarin, upconversion, zinc porphyrin, triplet-triplet annihilation-upconversion