Controlling the excited-state relaxation for tunable single-molecule dual fluorescence in both the solution and film states

Realizing single-molecule dual fluorescence with facile tunability in both the solution and film states through manipulation of the excited-state relaxation pathways is highly desirable but challenging. Herein, three synthesized fluorophores emit dual fluorescence originating from two excited-state intramolecular proton transfer tautomers. Their emission behaviors in the solution and film states are controlled by the excited-state kinetics and thermodynamics, and adjusted by the targeted chemical modification. Notably, the dual fluorescence of the methoxyl substituted molecule can be regulated via controlling the excited-state relaxation with the different excitation wavelengths. We found that the resulting twisted configuration in the S-2 state providing an additional pathway to yield the phototautomer is responsible for the modulating mechanism in the solution, while the proton transfer occurs in the S-2 state through suppression of S-2 -> S-1 internal conversion for that in the film state.