Bodipy Dimer for Enhancing Triplet-Triplet Annihilation Upconversion Performance

Abstract

Triplet-triplet annihilation upconversion (TTA-UC) has considerable potential for emerging applications in bioimaging, optogenetics, photoredox catalysis, solar energy harvesting, etc. Fluoroboron dipyrrole (Bodipy) dyes are an essential type of annihilator in TTA-UC. However, conventional Bodipy dyes generally have large molar extinction coefficients and small Stokes shifts (<20 nm), subjecting them to severe internal filtration effects at high concentrations, and resulting in low upconversion quantum efficiency of TTA-UC systems using Bodipy dyes as annihilators. In this study, a Bodipy dimer (B-2) with large Stokes shifts was synthesized using the strategy of dimerization of an already reported Bodipy annihilator (B-1). Photophysical characterization and theoretical chemical analysis showed that both B-1 and B-2 can couple with the red light-activated photosensitizer PdTPBP to fulfill TTA-UC; however, the higher fluorescence quantum yield of B-2 resulted in a higher upconversion efficiency (η_UC) for PdTPBP/B-2 (10.7%) than for PdTPBP/B-1 (4.0%). This study proposes a new strategy to expand Bodipy Stokes shifts and improve TTA-UC performance, which can facilitate the application of TTA-UC in photonics and biophotonics.

Keywords: Stokes shift; boron-dipyrromethene; fluorescence quantum yield; triplet-triplet annihilation upconversion; upconversion quantum efficiency.

Figure 1

(a) Schematic illustration of triplet-triplet annihilation upconversion mechanism; (b) Molecular structures of the photosensitizer (PdTPBP) and annihilators (B-1, B-2) in this study.

Scheme 1

The preparation process of B-2.

Figure 2

(a) UV–vis absorption and fluorescence spectra of B-1 and B-2, λ_ex = 470 nm, 10 μM; (b) UV–vis absorption and phosphorescence spectra of PdTPBP, λ_ex = 635 nm, 10 μM.

Figure 3

Theoretical chemical calculation results. (a) Calculated S₀, S₁, and T₁ configurations of B-2 in toluene, highlighting the dihedral angles between two Bodipy moieties, top panel is the top view, bottom panel is the side view. (b) Selected frontier molecular orbitals of B-2 including HOMO, LUMO, HOMO−1, and LUMO+1. (c) Triplet state spin density surfaces of B-1 and B-2, respectively, in toluene at the optimized triplet state molecular geometric configurations. Calculated with Gaussian 09 based on the DFT-B3LYP/6-31G.level.

Figure 4

Upconversion properties of the annihilators. (a) Upconversion emission spectra of B-1 and B-2 in toluene, λ_ex = 635 nm (1267.5 mW/cm²); (b) CIE diagram showing the adjustable upconversion emission colors; (c) Upconversion pictures of B-1 and B-2 with PdTPBP; (d) Power-dependence of TTA-UC for PdTPBP/B-2, a slope of 1.83 (black, quadratic) and a slope of 1.09 (red, linear), I_th is 53.7 mW/cm²; (e) Upconversion lifetime decay trace of PdTPBP/B-2 at 600 nm, in deaerated toluene, PdTPBP (10 µM); (f) Stern-Volmer plots of PdTPBP in response to B-2 addition in toluene.