The internal heavy-atom effect on 3-phenylselanyl and 3-phenyltellanyl BODIPY derivatives studied by transient absorption spectroscopy

Abstract

Three monosubstituted 3-phenylselanyl and 3-phenyltellanyl BODIPY derivatives were synthesized and their spectroscopic properties were characterized and compared to those of iodine and chlorine-atoms containing analogues as well as an unsubstituted BODIPY derivative. The fluorescence quantum yields were found to decrease, whereas the intersystem crossing quantum yields (ΦISC), determined by transient spectroscopy, increased in the order of the H → Cl → Se/I → Te substitution. The maximum ΦISC, found for the 3-phenyltellanyl derivative, was 59%. The results are interpreted in terms of the internal heavy-atom effect of the substituents.

Construction of the carbon–chalcogen (S, Se, Te) bond at the 2,6-positions of BODIPY via Stille cross-coupling reaction

“…On the other hand, the chalcogen-substituted BODIPYs displayed broad emission bands and large Stokes shifts (Dñ = 3692-5128 cm À1 ; Fig. S28-S35, ESI †), fluorescence lifetimes (t f E 3-6 ns) in the same order as those of unsubstituted BODIPYs, 20 and surprisingly high F f values, especially in the case of ditelluro-analogue 7d (18%). BODIPY derivatives possessing large Stokes shifts based on geometrical changes upon excitation, 39 intramolecular charge transfer (ICT), 40 or extension of the chromophore 41 have already been reported.…”

“…We recently reported on the synthesis of several 3-chalcogen substituted BODIPYs, which exhibit intersystem crossing (ISC) quantum yields comparable to those of iodo-analogues. 20 Due to their high photostability and low toxicity, chalcogen-substituted BODIPYs are promising triplet sensitizers 21 which can be used in photodynamic therapy 22 or triplet-triplet annihilation upconversion. 4,23 Ortiz and coworkers have recently demonstrated that heavyatom substituents at the 2,6-positions have the highest impact on ISC quantum yields.…”

“…36 Non-substituted BODIPYs usually exhibit fluorescence with a lifetime of B5 ns and negligible ISC quantum yields, and their emission originates from the locally excited (LE) state. 20,37,38 Lower F f values were anticipated especially for the Se-, Te-and I-derivatives as a result of spin-orbit coupling between singlet and triplet states (the heavy-atom effect, HAE). 11,20,21 Indeed, the fluorescence lifetime of iodo-BODIPYs 1 and 6 was shortened by one order of magnitude (t f E 0.5 ns).…”

“…20,37,38 Lower F f values were anticipated especially for the Se-, Te-and I-derivatives as a result of spin-orbit coupling between singlet and triplet states (the heavy-atom effect, HAE). 11,20,21 Indeed, the fluorescence lifetime of iodo-BODIPYs 1 and 6 was shortened by one order of magnitude (t f E 0.5 ns). These compounds exhibited sharp emission bands and small Stokes shifts (Dñ = 600-1000 cm À1 ; Fig.…”

“…To evaluate the effect of chalcogen substituents as heavy atoms, the quantum yields of ISC (F ISC ) were obtained by ns transient absorption spectroscopy 20,49 for all prepared derivatives and compared with the quantum yields of singlet oxygen generation (F D ; Table 1), a common tool for determination of ISC efficiency in BODIPY derivatives. 50 All triplet-triplet absorption spectra (Fig.…”