Bright NIR-Emitting Styryl Pyridinium Dyes with Large Stokes' Shift for Sensing Applications.

Two NIR-emitting donor-π-acceptor (D-π-A) type regioisomeric styryl pyridinium dyes ( 1a - 1b ) were synthesized and studied for their photophysical performance and environment sensitivity. The two regioisomers, 1a and 1b , exhibited interesting photophysical properties including, longer wavelength excitation (λ ex ≈ 530-560 nm), bright near-infrared emission (λ em ≈ 690-720 nm), high-fluorescence quantum yields (ϕ fl ≈ 0.24-0.72) large Stokes' shift (∆λ ≈ 150-240 nm) and high-environmental sensitivity. Probe's photophysical properties were studied in different environmental conditions such as polarity, viscosity, temperature, and concentration. Probes ( 1a - 1b ) exhibited noticeable changes in absorbance, emission and Stokes' shift while responding to the changes in physical environment. Probe 1b exhibited a significant bathochromic shift in optical spectra (∆λ ≈ 20-40 nm) compared to its isomer 1a , due to the regio-effect. Probes ( 1a - 1b ) exhibited an excellent ability to visualize bacteria ( Bacillus megaterium , Escherichia coli ), and yeast ( Saccharomyces cerevisiae ) via fluorescence microscopy.