Complexation of alkali metal and alkaline earth ions by anthracene based fluorophores with one and two appended monoaza coronand receptors

Abstract

The complexation of alkali metal ions by 13-[2-(10-ethyl-9-anthryl)ethyl]-1,4,7,10-tetraoxa-13-azacyclopentadecane, 1, and 13-(2-{10-[2-(1,4,7,10-tetraoxa-13-azacyclopentadecanyl)ethyl]-9-anthryl}ethyl)- 1,4,7,10-tetraoxa-13-azacyclopentadecane, 2, to form fluorescent complexes in acetonitrile is reported. The fluorescence quantum yields, φ, are 0.25 and 0.03 for 1 and 2, respectively. At 298.2 K and I = 0.05 mol dm-3 (NEt4ClO4) the [M1]+ complexes are characterised by complexation constants K1 = (1.28 ± 0.01) × 105 (φ1 = 0.71), (9.27 ± 0.04) × 104 (φ1 = 0.64), (1.73 ± 0.02) × 104 (φ1 = 0.60), (3.08 ± 0.05) × 103 (φ1 = 0.53) and (2.17 ± 0.04) × 103 (φ1 = 0.34) dm3 mol-1, respectively, as M+ changes from Li+ to Cs+. Fluorophore 2 forms weakly fluorescent [M2]+ and possibly the "sandwich" complex, [M2′]+, which are jointly characterised by complexation constants K1 = (7.1 ± 0.03) × 105, (5.2 ± 0.3) × 105, (1.00 ± 0.03) × 104 and (1.8 ± 0.2) × 104 dm3 mol-1 for Li+, Na+, K+ and Rb+ and [M22]2+ characterised by K2 = (6.41 ± 0.01) × 104 (φ2 = 0.73), (4.84 ± 0.01) × 104 (φ2 = 0.53), (1.59 ± 0.06) × 103 (φ2 = 0.39) and (6.8 ± 0.1) × 102 dm3 mol-1 (φ2 = 0.15). (Cs+ induced insufficient fluorescence in 2 for quantitative study.) The alkaline earths form more stable complexes with 1 and 2 characterised by K1 and K2 ≥ 107 dm3 mol-1. The factors governing fluorescence and complex stability are discussed and the syntheses of 1 and 2 are described.