Tuning excited state intramolecular proton transfer in 3-hydroxyflavone derivative by reaction of its isothiocyanate group with an amine

In the present work, we developed an isothiocyanate derivative of 3-hydroxyflavone, 7-isothiocyanato-4′-diethylamino-3-hydroxyflavone. This dye exhibits dual fluorescence due to an excited state intramolecular proton transfer (ESIPT). On reaction with an amine, this dye shows a dramatic change in its dual emission as well as shifts in the absorption and emission maxima. The observed phenomenon is due to the conversion of the electron acceptor isothiocyanate group into an electron donor thiourea based group. This increase in the donor properties at 7-position of the 3-hydroxyflavone probably shifts the reversible ESIPT reaction towards the ESIPT product. Moreover, we show that the conjugate of the reactive dye with an amine exhibits a two-band emission sensitive to the environment, which makes it an attractive solvatochromic label of amino groups of biomolecules.