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 pK_a 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.