Photochemical behaviour of a new 1,2,3,4-tetrahydroxanthylium fluorescent dye with “rhodamine-like” structure in liquid media and adsorbed onto a TiO2 photo-responsive substrate

• A new synthesized fluorescent dye with a rhodamine-like structure was studied.
• The photochemical properties of the dye were investigated in three solvents.
• The behaviour of the dye adsorbed onto TiO2 solid substrate and cellulose was studied.
• Interesting results were found for the formation of the dye radical cation.

A new “rhodamine-like” fluorescent NIR dye based on the 9-(2′-carboxyphenyl)-6-(N,N-diethylamino)-1,2,3,4-tetrahydroxanthylium system was synthesized. Its spectral-luminescent properties were investigated both in solution and adsorbed onto a TiO2 photoactive solid substrate. Regarding liquid samples, the highest fluorescent quantum yield and fluorescence lifetime were obtained in dichloromethane (øF = 0.28, τF = 2.67 ns) while in ethanol and acetonitrile we observed the largest Stokes Shift, indicating a twisted intramolecular charge transfer mechanism. Additionally, the behaviour of the dye adsorbed onto TiO2 revealed some interesting features regarding the formation of the radical derived from the dye cation, following the electron injection in the conduction band of the semiconductor. Laser flash photolysis and ground state diffuse reflectance were used to evaluate and discriminate the mechanism of photoexcitation presented, as well as to characterize the photostability of the molecule. In this case, two different scenarios could be recognized: the self-photosensitization pathway and the photocatalytic mechanism.

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