2,6,10-Tris(dialkylamino)trioxatriangulenium salts: a new promising fluorophore. Ion-pair formation and aggregation in non-polar solvents

Abstract

The D_3h symmetric tris(dialkylamino)trioxatriangulenium ((R₂N)₃TOTA⁺) ions are structurally related to classical stains and fluorophores such as triphenylmethane dyes and rhodamines. New derivatives of (R₂N)₃TOTA⁺, in which long flexible alkyl chains surround the planar and rigid aromatic core, have been synthesized and isolated as hexafluorophosphate salts. In contrast to short-chain derivatives of triphenylmethane dyes, the new compounds described here are soluble in hydrocarbon solvents. The photophysical properties of these carbenium salts are investigated. Characteristic alterations of the photophysical properties as a function of solvent polarity, concentration and temperature are observed. A model to rationalize the phenomena is presented comprising three states of aggregation, i.e. freely solvated ions, tight ion-pairs, and dimers of ion-pairs. Reduced symmetry in the tight ion-pairs is held responsible for the observed splitting of the long wavelength absorption band. Exciton coupling in the ion-pair dimers leads to reduced oscillator strength of the S₀–S₁ transition. The model is supported by semi-empirical calculations of the structures and electronic transitions of the free cation and its ion-pair(s).