Engineering the Photophysics of Cyanines by Chain C1' Substituents.

Cyanine dyes are widely used in bioimaging, sensing, optoelectronic, and medicinal applications due to their tunable photophysical properties. However, controlling their electronic structures and photophysical properties remains a challenge. Here we report a general synthetic route to pentamethine and heptamethine cyanines bearing C1' chain substituents that allow substantial control of their electronic, photophysical, and photochemical properties. By varying the terminal heterocycle and introducing various substituents at the 1'-position, 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 or electron deficient substituents at the 1'-position. Through quantum chemical calculations and spectroscopic analyses, we demonstrate how these modifications can be used to tune optical properties of these dyes across the visible region, paving the way for their further customization.