In particular, this kind of dual emission behavior is clearly observed in ESIPT molecules possessing five-membered ring hydrogen-bonding structures such as 3-hydroxyflavone (3HF), due to their rather weak intramolecular H-bond between the 3-hydroxyl and 4-carbonyl groups (Scheme2).17 As reported by Sengupta and Kasha,36 3HF with its π–π* absorption band at 335 nm exhibited large Stokes' shifted green K* emission at 520 nm in non-polar solvent such as 2-methylbutane. In methanol solution, however, 3HF was found to exhibit both the violet E* -solv fluorescence at 405 nm and the green K* fluorescence at 528 nm, as previously observed by Frolov et al.37 A more striking feature of this dual emission is a dramatic variation of the relative intensities of E* and K* emission bands according to the solvent polarity. It was actually shown that the ratio of them ranged from unity up to the complete disappearance of one of the forms. Such a unique dual emission characteristic of 3HF derivatives has already been written for the prototype biomolecular sensors which can probe polarity,28a,b electrostatic fields,28c and the structural variations in micelles28d and biomembrane models.