Synthesis and photophysical properties of fluorescent anthracenophanes incorporating two polyoxadioxoalkane chains

Abstract

Anthracene macrolides I_n(n= 1–4) display dual fluorescence (‘monomer’/excimer) sensitive to the number of links of the chain n and temperature. The excimer fluorescence quantum yield is particularly high (0.24–0.39 in toluene) in comparison with those of related anthracenophanes or bichromophores. A thermodynamic study of the fluorescence emission has enabled us to evaluate the activation energy of excimer formation (25.1–27.2 kJ mol^–1 for n= 1, 3, 4, and 15.0 kJ mol^–1 for n= 2), its enthalpy of stabilization (–14.2 to –21.7 kJ mol^–1), and the repulsive potential in the ground state between two anthracenes after radiative deactivation of the intramolecular excited complex (38.5–53.1 kJ mol^–1). These values are discussed as a function of n and compared with literature data.

The spectroscopic properties of I_n were found to be weakly cation dependent except for I₄ and Sr²⁺ or Ba²⁺ where moderate effects were detected in UV absorption or fluorescence emission spectra. The failure to encapsulate the alkali and alkaline-earth cations is ascribable to the rigidity of the system which presumably hinders the formation of a stable co-ordinating sphere around the cation.

The fluorescence emission of the single crystals is described and correlated with the molecular packing mode. The excimer-type emitting crystals (n= 2 and 4) contain pairs or stacks of aromatic rings whereas ‘monomer’-type emitting crystals do not show any important overlap between anthracenes.