Non-radiative decay in rhodamines: Role of 1:1 and 1:2 molecular complexation with β-cyclodextrin

The stoichiometry of complexes formed between Rhodamine 3B and Hexyl-Rhodamine B with β-cyclodextrin was investigated by electronic absorption, fluorescence spectroscopy and photokinetics and well supported by thermochromic studies. Complex 1:1 is formed at low β-cyclodextrin concentrations with Rhodamine 3B and Hexyl-Rhodamine B with estimated equilibrium constants K1 = 8.4 × 103 M−1 and K ′ 1 = 2.1 × 10 3   M − 1 , fluorescence quantum yields Φf = 0.12 and Φ ′ f = 0.15 and lifetimes τf = 610 and τ ′ f = 750   ps , respectively. At higher β-cyclodextrin concentrations 1:2 (cyclodextrin) complexes are formed with estimated equilibria constants of K2 = 2.8 M−1 and K ′ 2 = 7.5   M − 1 , fluorescence quantum yields Φf = 0.67 and Φ ′ f = 0.68 and lifetimes τf = 3360 and τ ′ f = 3390   ps , respectively for Rhodamine 3B and Hexyl-Rhodamine B. diationless rate constants in both 1:1 complexes with either rhodamine reflect essentially the activated charge transfers which are enhanced to kA = (1.1–1.4) × 109 s−1 as compared to 1:2 complexes of kA = (7.5–7.8) × 107 s−1. The difference in one order of magnitude is assigned to the specificity of the binding sites and geometry constraints in each case.