Donor-acceptor-acceptor-type near-infrared fluorophores that contain dithienophosphole oxide and boryl groups: effect of the boryl group on the nonradiative decay†

The use of donor-pi-acceptor (D-pi-A) skeletons is an effective strategy for the design of fluorophores with red-shifted emission. In particular, the use of amino and boryl moieties as the electron-donating and -accepting groups, respectively, can produce dyes that exhibit high fluorescence and solvatochromism. Herein, we introduce a dithienophosphole P-oxide scaffold as an acceptor-spacer to produce a boryl- and amino-substituted donor-acceptor-acceptor (D-A-A) pi-system. The thus obtained fluorophores exhibit emission in the near-infrared (NIR) region, while maintaining high fluorescence quantum yields even in polar solvents (e.g. lambda(em) = 704 nm and phi(F) = 0.69 in CH3CN). A comparison of these compounds with their formyl- or cyano-substituted counterparts demonstrated the importance of the boryl group for generating intense emission. The differences among these electron-accepting substituents were examined in detail using theoretical calculations, which revealed the crucial role of the boryl group in lowering the nonradiative decay rate constant by decreasing the non-adiabatic coupling in the internal conversion process. The D-A-A framework was further fine-tuned to improve the photostability. One of these D-A-A dyes was successfully used in bioimaging to visualize the blood vessels of Japanese medaka larvae and mouse brain.

Automated summary

The use of donor-pi-acceptor (D-pi-A) skeletons with amino and boryl moieties as the electron-donating and -accepting groups can produce fluorophores with red-shifted emissions. The introduction of a dithienophosphole P-oxide scaffold as an acceptor-spacer in the D-A-A pi-system results in near-infrared emission fluorophores with high fluorescence quantum yields. The boryl group plays a crucial role in generating intense emission by lowering the nonradiative decay rate constant through reducing non-adiabatic coupling in the internal conversion process.