Direct probing of ion pair formation using a symmetric triangulenium dye

Abstract

The 2,6,10-tris(dialkylamino)trioxatriangulenium dyes (ATOTA⁺) are highly stabilised cationic chromophores with D_3h symmetry. The symmetry gives rise to a degeneracy of the main electronic transition. In low polarity solvents significant splitting of this degenerate transition is observed and assigned to ion pair formation. Ion pairing of the 2,6,10-tris(dioctylamino)trioxatriangulenium ion with Cl⁻, BF₄⁻, PF₆⁻ and TRISPHAT anions was studied using absorption spectroscopy. A clear correlation is found between the size of the anion and the splitting of the ATOTA⁺ transitions. In benzene the Cl⁻ salt displays a splitting of 1955 cm⁻¹, while the salt of the much larger TRISPHAT ion has a splitting of 1543 cm⁻¹. TD-DFT calculations confirm the splitting of the states and provide a detailed insight into the electronic structure of the ion pairs. The different degree of splitting in different ion pairs is found to correlate with the magnitude of the electric field generated in each ion pair, thus leading to the conclusion that the effect seen is an internal Stark effect. By insertion of an amphiphilic derivative of the ATOTA⁺ chromophore in an oriented lamellar liquid crystal, it was possible to resolve the two bands of the double peak spectrum and show their perpendicular orientation in the molecular framework, as predicted by the calculations.