Novel group 14 element-bridged bithiophene dimers appended with terminal electron-withdrawing groups: Red-to-near infrared fluorescence and efficient photosensitized singlet oxygen generation

Abstract A series of group 14 element bridged bithiophene dimers bearing dicyanovinyl (DC) or thiobarbiturylmethylene (TB) electron-withdrawing groups at the molecular termini (series E-1a and series E-1b; E = C, Si, and Ge) were developed as red absorbing singlet oxygen (1O2) photosensitizing dyes. These dyes exhibited broad and strong absorption in the red-to-deep red regions and efficient red-to-NIR fluorescence (photoluminescence quantum yield ΦPL; 0.32–0.45 in dichloromethane). The terminal TB groups led to more red-shifted electronic absorption and fluorescence emission than the DC groups, owing to more intense intramolecular charge transfer. The 1O2 generation photosensitization abilities of series E-1a and series E-1b were evaluated by the 1,3-diphenylisobenzofuran method, and large 1O2 generation quantum yields (ΦΔ) were obtained upon irradiation of 630 nm light, ranging from 0.35 to 0.68 in dichloromethane. Therefore, these dyes show fluorescence properties as well as 1O2 generation photosensitization ability. Introduction of bromo groups at the 3,3′-positions in Si-1a and Si-1b afforded the new potential photosensitizers Si-2a and Si-2b that accompanied blue-shifted electronic absorption as well as blue-shifted fluorescence emission with the low ΦPL. Unfortunately, as Si-2a exhibited quite low absorptivity in the red region, 1O2 generation photosensitization with 630 nm light was not evaluated for Si-2a. On the other hand, Si-2b showed significantly efficient 1O2 generation photosensitization upon irradiation of 630 nm light (ΦΔ; 0.88). Considering the result of theoretical calculations showing that the bromo groups effectively contribute to the frontier orbitals, these bromo substituents should facilitate the intersystem crossing from the excited singlet state to the triplet excited state through the strong spin‒orbit coupling.