Theoretical exploitation of acceptors based on benzobis(thiadiazole) and derivatives for organic NIR-II fluorophores

Small-molecule dyes with fluorescence emission in the second near-infrared (NIR-II) region (1000-1700 nm) have attracted considerable attention in the biomedical and bioimaging fields due to their greater imaging depths, better spatial resolution, and higher signal-to-background ratios. However, currently reported organic NIR-II fluorophores are still limited and there is great demand to develop other novel NIR-II fluorophores besides benzobisthiadiazole (BBT)-based fluorophores. More importantly, there is a lack of an appropriate level of theory capable of providing both efficient and accurate predictions of the electronic structures of organic NIR-II fluorophores. In this work, successful application of time-dependent density functional theory (TDDFT) using optimally-tuned range-separated functionals for calculations of both absorption and fluorescence spectral properties has been demonstrated, compared with the available experimental data. A series of thiadiazole-based acceptors (A) and derivatives based on the D-A-D skeleton are designed coupled with the triphenylamine donor (D). The structure-property relationships for these fluorophores are thus revealed by analyzing their ground (S0) and excited (S1) state geometries, frontier molecular orbitals (HOMO and LUMO), HOMO-LUMO energy gaps, oscillator strengths, hole-electron distributions and fluorescence wavelengths. It is suggested that the existence of a hypervalent structure leading to a much lower LUMO level and accompanying significant hole-electron separation plays a key role in the red-shift of fluorescence emission in the NIR-II region. In addition, the substitution of BBT oligomers and analogues as acceptor cores is an efficient way to achieve both red-shifted fluorescence wavelengths and enhanced oscillator strengths. The present work provides a reliable and efficient theoretical tool for predicting the related electronic and spectral properties of organic fluorophores and future screening out of potential candidates for excellent NIR-II molecular fluorophores.