Photoluminescent Properties and Mechanism of Novel Cyanine-Borondifluoride Curcuminoid Hybrids as Red-to-Near Infrared and Endoplasmic Reticulum-Targeting Dyes

Abstract

Synthesis-oriented design led us to the discovery of a series of novel cyanine-borondifluoride curcuminoid hybrids called Nanchang Red (NCR) dyes that overcome the intrinsic low synthetic yields of symmetrical cyanine-difluoroboronate (BF₂ )-hybridized NIR dyes. The hybridization endows NCR dyes with high molar extinction coefficients, efficient red-to-NIR emission, and enlarged Stokes shifts. Quantum chemical calculations revealed that the asymmetrical layout of the three key electron-withdrawing and electron-donating fragments results in a special pattern of partial charge separation and inconsistent degrees of charge delocalization on their π-conjugated backbones. While the nature of the hemicyanine fragment exerts significant influence on the excitation modes of NCR dyes, the borondifluoride hemicurcuminoid fragment is the major contributor to the enlarged Stokes shifts. Cell imaging experiments illustrated that a subtle change in the N-heterocycle of the hemicyanine fragment has a remarkable effect on the subcellular localization of NCR dyes. Unlike other previously reported cyanine-BF₂ hybridized dyes, which mainly target mitochondria, the benzothiazole and indole-based NCR dyes accumulate in both the endoplasmic reticulum (ER) and lipid droplets of HeLa cells, whereas the benzoxazole and quinoline-based NCR dyes stain the ER specifically.

Keywords: borondifluoride curcuminoids; cyanines; endoplasmic reticulum-targeting; near infrared dyes; photoluminescent mechanism.