Design of 2-Pyridone Fluorophores for Brighter Emissions at Longer Wavelengths

The recent discovery of blue fluorophores with high quantum yields based on pyridone structures inspired the development of new low-molecular-weight fluorophores with bright emissions at tunable wavelengths, which are highly attractive for various applications. In this study, we propose a rational design strategy for 2-pyridone-based fluorophores with bright emissions at long wavelengths. With a detailed understanding of the positional substitution effects on each carbon atom of the 2-pyridone core, we developed a bright blue fluorophore (λ_abs =377 nm; λ_em =433 nm; ϵ=13,200 M^-1  cm^-1 ; ϕ_F =88 %) through C₃ -aryl and C₄ -ester substitutions followed by cyclization. Furthermore, by applying the intramolecular charge transfer (ICT) principle, we invented a bright green fluorophore through C₃ - and C₄ -diester and C₆ -aryl substitutions. The ICT fluorophore based on the pyridone structure shows large molar absorptivity (ϵ=20,100 M^-1  cm^-1 ), longer emission wavelength (λ_em =539 nm), high emission quantum yield (ϕ_F =74 %), and large Stokes shift (Δv=5720 cm^-1 ), which are comparable to those of practical fluorescent probes.