Positional Tuning of Ether-Substituted NIR Donor–Acceptor–Donor Fluorophores

Recent advancements in near-infrared (NIR) donor-acceptor-donor (D-A-D) fluorophore design have unveiled the potential of variations in its architecture for attaching solubilizing and/or shielding groups to enhance dye solubility and mitigate fluorescence quenching caused by aggregation and interactions with solvent. However, a significant gap in research exists regarding how these alterations in the molecular structure influence the photophysical properties of the fluorophore itself. In this study, we present a comprehensive analysis of the effect of the position of the donor substituent group on the stability and optical properties of a series of D-A-D NIR fluorophores. We conjugated thienothiadiazole (TTD) with six different methoxyphenyl or dimethoxyphenyl donors to form a library of D-A-D NIR fluorophores with methoxy substituents positioned <i>ortho</i>, <i>meta</i>, and/or <i>para</i> with respect to the acceptor. Our experimental findings, supported by density functional theory (DFT) calculations, reveal that the position of the methoxy donor is strongly linked to the emission wavelength, brightness, and stability. The <i>para</i>-positioned donor, for instance, yields a longer emission wavelength (907 nm) than its <i>meta</i>-positioned counterpart (888 nm), while the di-<i>ortho</i> substitution significantly compromises the photostability of the dye, likely due to steric strain. Overall, the <i>para</i>-positioned donors exhibit longer emission wavelengths (902-907 nm), while the <i>meta</i>-positioned donors offer quantum yields (0.88-0.90%) that are higher than those of their <i>ortho</i>- and <i>para</i>-positioned counterparts (0.31-0.47%), making them promising candidates for potential applications in bioimaging.