Synthesis and optical properties of trioxatriangulenium dyes with one and two peripheral amino substituents

Several derivatives of two new dye systems, with one or two dialkylamino donor groups attached to resonant positions at the periphery of a trioxatriangulenium ion, were synthesized. The mono- and bis-dialkylamino trioxatriangulenium salts (A(1)-TOTA(+) and A(2)-TOTA(+)) were prepared from methoxy-substituted triphenylmethylium (TPM) compounds by aromatic nucleophilic substitution with secondary amines and subsequent intramolecular ring closure. The optical properties of the new triangulenium dyes and their TPM precursors were investigated and compared to those of known TPM and xanthenium dyes. The optical properties were found to be dependent on symmetry and charge localization in the conjugated framework. The trioxatriangulenium dye with two amino groups (A(2)-TOTA(+)) was found to be a strong fluorophore with properties as a blue-shifted rhodamine B. The mono-substituted compound (A(1)-TOTA(+)) was found to be only weakly fluorescent. We assign the weak fluorescence of A(1)-TOTA(+) to an efficient but reversible formation of a nonfluorescent conformation in the excited state, favored by the large degree of charge localization in this dye with only one donor group.