Triazinylaniline derivatives as fluorescence probes. Part 1. Absorption and fluorescence in organic solvents and in aqueous media in relation to twisted intramolecular charge-transfer state formation, hydrogen bonding, and protic equilibria

Abstract

The UV absorption and fluorescence characteristics of new optical probes based on a 1,3,5-triazinylaniline chromophore and of general structure p-Et₂NC₆H₄C₃N₃(Cl)NR¹R² are reported. Increase in alkanol solvent polarity strongly quenches the fluorescence by a process attributed to TlCT (twisted intramolecular charge-transfer) state formation with co-operative solvent relaxation (log k_NR/s^–1 8.5–10.5). In aqueous media no changes in fluorescence band position or intensity are observed in the pH range 3.5 to 11 but below pH 3.5 protonation of the anilino nitrogen (pK_aca. 2.4) eliminates both the λ= 360 nm absorption band and the weak λ= 460 nm fluorescence band. Free and hydrogen-bonded species, with respect to the anilino nitrogen centre, are present in aqueous dioxane solutions.

The triazinylaniline derivatives are prototypes of promising sensitive probes for organized media and as models of proteins labelled with the triazinylaniline fluorophore.