Thioetherification Inducing Efficient Excited Triplet State and Singlet Oxygen Generation: Heavy Atom-Free BODIPY Photosensitizer Based on the S₁(n,π*) State

Abstract

We have examined the ability of the newly synthesized thioalkyl and thiophenyl (SR) BODIPYs to generate the excited triplet state (T 1 ) using the laser flash photolysis technique. Compared to the unsubstituted BODIPY parent, SR BODIPYs are much more efficient in generating T 1 . We have also determined the capability of the SR BODIPYs to yield singlet oxygen O 2 ( 1 Δ g ) using two methods: (1) measuring the steady state and time-resolved NIR phosphorescence that peaked at 1270 nm; (2) recording DPBF decomposition caused by the reaction with singlet oxygen. The results show that both thioalkyl and thiophenyl very remarkably increase the quantum yield of singlet oxygen formation up to 0.83 from 0.10. To understand the mechanism, we have studied the related photophysics using steady state and time-resolved fluorescence and UV−vis absorption, as well as quantum chemical calculations based on Density Functional Theory. Both the thioalkyl and thiophenyl remarkably decrease the quantum yield of fluorescence and shorten the fluorescence lifetime of BODIPYs, they also cause the occurrence of a new S 1 (n,π*) with a slightly lower energy than S 1 (π,π*). Since intersystem crossing (ISC) from S 1 (n,π*) to T 1 (π,π*) is symmetrically allowed, while ISC from S 1 (π,π*) to T 1 (π,π*) is symmetrically forbidden, this explains why SR presence renders BODIPY efficient photosensitizer for excited triplet state and singlet oxygen generation. SR induced efficient T 1 formation is different from the cases of carbonyl (CO) or thiocarbonyl (CS). This finding provides a new method to get efficient heavy atom free photosensitizers, which is important in developing new generation photosensitizers for photodynamic therapy, photochemical up conversion of near IR sunlight in solar cell, and photoredox catalysis.

Modulating Room-Temperature Phosphorescence through the Synergistic Effect of Heavy-Atom Effect and Halogen Bonding

“…To achieve high-efficiency RTP emission, two key points need to be addressed: (1) sufficient generated triplet excitons through the ISC process and (2) stable radiative transition of triplet excitons. − Based on the above issues, some design principles have been developed, such as n−π* transition, , incorporation of heavy atoms or aromatic carbonyl , for promoting the ISC process, as well as crystallization, − H-aggregation, , host–guest doping, − and organic/inorganic frameworks , for stabilizing triplet excitons. Especially, the introduction of heavy atoms can achieve high-efficiency RTP due to heavy-atom effect (HAE). − However, excessive HAE can also be harmful to the phosphorescence lifetime to some extent because of the intrinsic competition between phosphorescence lifetime (τ p ) and quantum yields (Φ p ). − Thus, it still remains a big challenge to develop RTP materials with both high Φ p and long τ p .…”

“…To achieve high-efficiency RTP emission, two key points need to be addressed: (1) sufficient generated triplet excitons through the ISC process and (2) stable radiative transition of triplet excitons. 18−20 Based on the above issues, some design principles have been developed, such as n−π* transition, 21,22 incorporation of heavy atoms or aromatic carbonyl 23,24 for promoting the ISC process, as well as crystallization, 25−27 Haggregation, 28,29 host−guest doping, 30−32 and organic/inorganic frameworks 33,34 for stabilizing triplet excitons. Especially, the introduction of heavy atoms can achieve high-efficiency RTP due to heavy-atom effect (HAE).…”

Synthesis, Two-photon Fluorescence Imaging and Photodynamic Therapy of Near Infrared Thienyl-BODIPY Photosensitizer

“…光敏剂在光动力治疗过程中起着重要作用, 良 好的光敏剂应当具备较高的光稳定性、高单线态氧产 率、较好的生物相容性, 以及低暗毒性、高光毒性等特 点 [3] . 目前, 多种类型的光敏剂已被开发出来, 包括花 菁类光敏剂 [4][5] 、氟硼二吡咯类光敏剂 [6][7][8][9][10][11][12] 、罗丹明类光 敏剂 [13] 及荧光蛋白类光敏剂 [14][15] 等. 在诸多类型的光敏 剂中, 氟硼二吡咯(BODIPY)由于具有较高的摩尔消光 系数、明亮的荧光、优异的光稳定性及较好的生物相容 性而被认为是一类理想的光敏染料 [8,[16][17][18][19][20][21] .…”

Halogen-free photosensitizers based on meso-enamine-BODIPYs for bioimaging and photodynamic therapy

“…Some examples of halogen-free BODIPYs, in which different electron-donor groups have been incorporated into the chromophore core, 39,45,47,48,53,63–67 have been already reported as potential theragnostic agents. In most cases, the electron-donating unit is added to the meso position, considering the most sensitive one in terms of the photophysical impact because of the marked change in the electronic density that occurs upon excitation (Fig.…”