Characterization of Rose Bengal-N,N′-dimethyl-4,4′-bipyridinium complexes and their separation in aqueous SiO₂ colloids: Photophysical properties of Rose Bengal in the microheterogeneous system

Rose Bengal, Rb^2-, forms crystalline 1:1 and 2:1:1 complexes with N,N′-dimethyl-4,4′-bipyridinium, MV²⁺. In the 1:1 complex the two components are coplanar with charge neutralization within the complex structure. In the 2:1:1 complex, one MV²⁺ unit is intercalated between two Rb^2- units and the second MV²⁺ unit is positioned externally to this sandwich structure and acts in charge neutralization. In both complexes, electrostatic interactions, charge-transfer interactions, and π-interactions stabilize the intermolecular assemblies. In solution, Rb^2- and MV²⁺ form initially the 1:1 intermolecular complex [Rb^2-⋯MV²⁺]; K_a(298K) = 9000 ± 500, ΔG° = -22.56 ± 1.1 KJ·mol^-1, ΔH° = -36.62 ± 2 KJ·mol^-1, and ΔS = -0.048 ± 0.0025 KJ·mol^-1·K^-1. In the presence of added silica colloid, the [Rb^2-⋯MV²⁺] complex is separated through the selective association of MV²⁺ to the negatively charged colloid interface; K_a(298K) = 140 000 ± 20 000 M^-1. Flash photolysis experiments reveal that only free MV²⁺ is operative in the quenching of ³Rb^2-; k_q = 4.7 ± 1 × 10⁹ M^-1·s^-1.