Synthetic Approach to Polymethine-Modified Trimethine Cyanines Gives Access to Viscosity-Sensitive Probes

Polymethine functionalization
of trimethine cyanine (Cy3) fluorophores
remains challenging due to limited synthetic strategies. Here, we
report a versatile synthetic strategy for accessing both symmetric
and unsymmetric meso-substituted Cy3 fluorophores
by using activated carboxyl intermediates. This three-step, high-yielding
approach enables polymethine-modified Cy3. Through systematic variation
of acyl chloride precursors and indolenine derivatives, we generated
a structurally diverse library of Cy3 fluorophores containing aliphatic,
aromatic, and electron-withdrawing meso-substituents.
The approach is compatible with a broad range of indolenines. The
Cy3 fluorophores have absorption and emission maxima between 550 and
590 nm and between 560 and 650 nm, respectively. Unsymmetric Cy3 derivatives
have a larger Stokes shift (up to 100 nm) as compared to symmetrical
Cy3. We show that meso-substituted benzothiazole-containing
analogues exhibit up to 20-fold enhancement in brightness in high-viscosity
environments and, as such, could be used as molecular rotors for viscosity
sensing. Time-dependent density functional theory (TD-DFT) calculations
further support the experimental observations by reproducing the substituent-dependent
trends in absorption, emission, and Stokes shift across the series.
The computed absorption wavelengths and Stokes shifts are consistent
with the experimental results. This work establishes a synthetic
scheme for meso-functionalized Cy3 to fine-tune photophysical
properties for imaging, sensing, and bioconjugation applications.