The formation of fluorescent alkaline earth complexes by 4-{2-[10-(2-morpholinoethyl)-9-anthryl]methyl}morpholine and its -ethyl}morpholine and -propyl}morpholine analogues in acetonitrile

Abstract

The formation of fluorescent alkaline earth complexes of 4-{2-[10-(2-morpholinoethyl)9-anthryl]methyl}morpholine and its new -ethyl}morpholine and -propyl}morpholine analogues, L = (1)–(3), in acetonitrile is reported. Each L has a ‘fluorophore–spacer–receptor’ structure in the sequence ‘anthracene–(methylene)n–morpholine’ in which quenching of the anthracene fluorophore becomes less effective as the receptor tertiary nitrogen becomes more distant with an increase in n from 1 to 3. Complexation by the alkaline earths (M2+) modulates photoinduced electron transfer (PET) and enhances the fluorescence of (1)–(3). The two alkyl morpholine receptors of (1)–(3) may either complex M2+ singly to form [ML]2+ and [M2L]4+ or cooperatively to form a [ML′]2+ ‘sandwich’ complex depending on the length of the alkyl spacer. Thus, (1) dominantly forms [ML]2+ and [ML′]2+ while (2) and (3) form all three complexes as exemplified by [Mg(2)]2+ and [Mg(2)′]2+ characterized by the overall complexation constant K1 = 9.19 × 104 dm3 mol−1 and [Mg2(2)]4+ characterized by the stepwise complexation constant K2 = 1.19 × 103 dm3 mol−1 at 298.2 K and I = 0.05 mol dm−3 (NEt4ClO4). The most stable and fluorescent complexes are formed by Mg2+ and Ca2+, consistent with M2+ size being an important factor affecting complex characteristics. Eighteen complexation constants and quantum yields are reported for the (1)–(3) complexes together with those for (1)–(3) alone. In 40 : 60 (v/v) 1,4-dioxan/water, protonation modulates PET to increase the fluorescence of (1)H22+–(3)H22+. (The pKa values for (2)H22+ are 6.38 and 5.62, and for (3)H22+ are 7.00 and 6.25.) The syntheses of (2) and (3) are reported.