Spectroscopic Studies of Fluorescent Dye N, N′- bis(2-hydroxy-5-methyl-benzylidene)-1,2-ethanediamine and its DNA Complex in Solution

Absorption and fluorescence emission properties of an N-salicylideneamine fluorescent dye molecule N, N′-bis(2-hydroxy-5-methylbenzylidene)-1,2-ethanediamine (1) have been studied in three typical solvents—2-methylbutane, ethanol, dimethyl sulphoxide (DMSO), and its DNA complex in methanol/H2O mixed solvent. The normal absorption band of 1 is observed in both aprotic and protic solvents and has been assigned to the l → aπ transition in the enol form of 1. The long-wavelength absorption band of 1, which is caused by the formation of a cis-keto species in the ground state, is absent in aprotic solvents, but is observable in protic ones. Normal fluorescence emission from the excited enol state of 1 is obtained only when the normal absorption band is excited, while the excited-state intramolecular proton transfer (ESIPT) emissions from both cis- and trans-keto species are recorded in all cases, being acceptable for the variation of the relative emission intensities. A preliminary spectroscopic study of the 1–DNA complex indicates an intercalation-binding mode, the convincing supporting evidence being the enhanced ESIPT fluorescence intensity of 1 when complexed with DNA. Finally, a universal energy-state diagram is given to interpret the experimental results.