a-PET and Weakened Triplet-Triplet Annihilation Self-Quenching Effects in Benzo-21-Crown-7-Functionalized Diiodo-BODIPY

Abstract

Weakening the triplet-triplet annihilation (TTA) self-quenching effect induced by sensitizers remains a tremendous challenge due to the very few investigations carried out on them. Herein, benzo-21-crown-7 (B21C7)-functionalized 2,6-diiodo-1,3,5,7-tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (DIBDP) was synthesized to investigate the influences of huge bulks and electron-rich cavities of B21C7 moieties on the fluorescence emission and triplet-state lifetimes of DIBDP moieties. Density functional theory (DFT)/time-dependent DFT (TDDFT) computable results preliminarily predicted that B21C7 moieties had influences on the fluorescence emissions of DIBDP moieties but not on their localization of triplet states of B21C7-functionalized DIBDP (B21C7-DIBDP). The UV-vis absorption spectra, fluorescence emission spectra, and cyclic voltammograms verified that there was an electron-transfer process from the B21C7 moiety to the DIBDP moiety in B21C7-DIBDP. However, the calculated results of ΔG _CS and E _CS values and nanosecond time-resolved transient absorption spectra demonstrated that the electron-transfer process from the B21C7 moiety to the DIBDP moiety in B21C7-DIBDP had direct influences on the fluorescence emission of DIBDP moieties but not on the triplet states of DIBDP moieties. The experimental values of triplet-state lifetimes of B21C7-DIBDP were obviously longer than those of DIBDP at a high concentration (1.0 × 10^-5 M); however, the fitted values of intrinsic triplet-state lifetimes of B21C7-DIBDP were slightly greater than those of DIBDP in the same solvent. These results demonstrated that the steric hindrance of B21C7 moieties could weaken the TTA self-quenching effect of DIBDP moieties at a high concentration and the a-PET effect induced a proportion of the produced singlet states of DIBDP moieties and could not emit fluorescence in the form of radiation transition but they could be transformed into triplet states through intersystem crossing (ISC) processes due to the iodine atoms in the DIBDP moiety. The stronger a-PET effects in polar solvents induced smaller fluorescence quantum yields so that more singlet states of DIBDP moieties were transformed into triplet states to weaken the TTA self-quenching effects.

Scheme 1. Synthetic Routes of B21C7-DIBDP

The molecular structure of the reference compound DIBDP is also shown in this scheme.

Figure 1

Selected frontier molecular orbitals of B21C7-DIBDP. The calculations are based on the optimized ground-state geometry (S₀ state) without any solvent at the B3LYP/6-31G(d) level using Gaussian 09W.

Figure 2

Isosurfaces of the spin densities of DIBDP and B21C7-DIBDP based on the optimized triplet-state geometries. The calculations were performed at the B3LYP/6-31G(d) level using Gaussian 09W.

Figure 3

UV–vis absorption spectra of DIBDP and B21C7-DIBDP (1.0 × 10^–5 M, 25 °C) in toluene (a), CHCl₃ (b), acetone (c), and CH₃CN (d).

Figure 4

Frontier molecular orbitals of fluorescecet molecules with PET processes. (a) a-PET process: photoinduced electron-transfer process from the electron donor to the fluorophore, which is an electron acceptor; (b) d-PET process: photoinduced electron-transfer process from the fluorophore (donor) to the electron acceptor.

Figure 5

Fluorescence emission spectra of DIBDP and B21C7-DIBDP (1.0 × 10^–5 M, 25 °C) in toluene (a), CHCl₃ (b), acetone (c), and CH₃CN (d).

Figure 6

Cyclic voltammograms of B21C7, DIBDP, and B21C7-DIBDP in deaerated CH₂Cl₂ containing 1.0 mM compound to be measured and ferrocene (Fc), 0.10 M Bu₄NPF₆ as the supporting electrolyte, and Ag/AgNO₃ as the reference electrode; scan rate was 0.1 V/s, 25 °C. Fc was used as the internal reference [E_1/2 = +0.64 V (Fc⁺/Fc) vs the standard hydrogen electrode].

Figure 7

Optimized ground-state geometries of DIBDP and B21C7-DIBDP at the DFT//B3LYP/6-31G(d) level using Gaussian 09W.