Ultrafast Photophysics of the Environment-Sensitive 4'-Methoxy-3-hydroxyflavone Fluorescent Dye.

The excited state intramolecular proton transfer (ESIPT) of 3-hydroxyflavone derivatives results in a fluorescence spectrum composed of two emission bands, the relative intensity of which is strongly influenced by the interaction with the local environment. We use time-resolved fluorescence and ultrafast transient absorption spectroscopies to investigate the photophysics of 4'-methoxy-3-hydroxyflavone in different solvents characterized by various polarities and hydrogen (H) bonding capabilities. We evidence that in this compound, the ESIPT reaction rate varies by more than 3 orders of magnitude, depending on the H-bonding capability of its local environment. This remarkable property is attributed to the moderate electron-donating strength of the 4'-methoxy substituent, and turns this fluorescent dye into a very promising fluorescent probe of biomolecular structures and interactions, where local structural heterogeneity may possibly be revealed by resolving a distribution of ESIPT reaction rates.