Accessing the Long-Lived Triplet Excited States in Bodipy-Conjugated 2-(2-Hydroxyphenyl) Benzothiazole/Benzoxazoles and Applications as Organic Triplet Photosensitizers for Photooxidations

摘要

Bodipy derivatives containing excited statenintramolecular proton transfer (ESIPT) chromophores 2-(2-nhydroxyphenyl) benzothiazole and benzoxazole (HBT andnHBO) subunits were prepared (7−10). The compounds shownred-shifted UV−vis absorption (530−580 nm; ε up to 50000nM−1 cm−1) and emission compared to both HBT/HBO andnBodipy. The new chromophores show small Stokes shift (45nnm) and high fluorescence quantum yields (ΦF up to 36%),nwhich are in stark contrast to HBT and HBO (Stokes shift upnto 180 nm and ΦF as low as 0.6%). On the basis of steady statenand time-resolved absorption spectroscopy, as well as DFT/TDDFT calculations, we propose that 7−9 do not undergo ESIPTnupon photoexcitation. Interestingly, nanosecond time-resolved transient absorption spectroscopy demonstrated that Bodipylocalizedntriplet excited states were populated for 7−10 upon photoexcitation; the lifetimes of the triplet excited states (τT) are upnto 195 μs. DFT calculations confirm the transient absorptions are due to the triplet state. Different from the previous report, wendemonstrated that population of the triplet excited states is not the result of ESIPT. The compounds were used as organic tripletnphotosensitizers for photooxidation of 1,5-dihydroxylnaphthalene. One of the compounds is more efficient than the conventionaln[Ir(ppy)2(phen)][PF6] triplet photosensitizer. Our result will be useful for design of new Bodipy derivatives, ESIPT compounds,nand organic triplet photosensitizers, as well as for applications of these compounds in photovoltaics, photocatalysis andnluminescent materials, etc.