Engineering the Photophysics of Cyanines by Chain C1´ Substituents

Cyanine dyes are widely used in bioimaging, sensing, optoelectronic, or even medicinal applications due to their tunable photophysical properties. However, controlling their electronic structures and photophysical properties remains a challenge. Here we report a novel synthetic route to pentamethine and heptamethine cyanines bearing C1´ chain substituents that allow unprecedented control of their electronic and photophysical properties. By varying the terminal heterocycle (symmetric or asymmetric cyanines) and introducing various substituents at the 1´-position, such as methyl, n-propyl, i-propyl, t-butyl, and cyano groups, we investigated the role of symmetry breaking and its impact on bond length alternation (BLA) and out-of-plane rotation (OPR). Our analysis shows that OPR, coupled with BLA, suppresses or hypsochromically shifts the first absorption band, thereby significantly altering the absorption properties of the studied dyes. This effect is particularly pronounced in structures with different heterocyclic end groups and bulky substituents at the 1´-position, which induce strong electronic and geometric distortions. Through quantum chemical calculations and spectroscopic analyses, we demonstrate how these modifications can be utilized to fine-tune their absorption and emission properties, paving the way for their further customization. Our study not only presents the synthesis of previously unex-plored novel basic cyanine derivatives, but additionally offers a versatile approach to tailor their photophysics through targeted structural modifications at the 1´-position.