Spectroscopic properties and gelling ability of a set of rod-like 2,3-disubstituted anthracenes

Abstract

The UV absorption spectra of 2,3-didecyloxyanthracene (DDOA) recorded in methanol, ethanol, 1-propanol, acetonitrile and methylcyclohexane reveal interesting features: they show a striking contrast between the isotropic solution and the gel state where, at ambient temperature, a fine structure appears with an apparent bathochromic shift (Δν ≈ −700 cm⁻¹), as observed in the solid state. Such an effect was mimicked by 2,3-dioxydi- (and -tri-) methyleneanthracenes in which the conformational mobility of the two juxtanuclear oxygen atoms is reduced in a manner similar to that assumed in the gel state. The fluorescence emission of DDOA (10⁻⁵ M) at very low temperature exhibits a loss of fine structure and a bathochromic shift, for the gel state, in agreement with the presence of aggregates; the excitation spectra were found to be superimposable upon the absorption spectra of the isotropic and gel phases, respectively. Solvent screening for DDOA gelling ability has shown that the most efficient solvents are CH₃OH and CH₃CN. From the phase transition diagram (temperatures of gel setting, T_gel, and gel melting, T_m, versus concentration), thermodynamic parameters were derived: ΔH⁰_gel/kJ mol⁻¹ =  −70 (CH₃OH), −66 (CH₃CN); ΔS⁰_gel/(J K⁻¹ mol⁻¹ = −147 (CH₃OH), −140 (CH₃CN) and ΔG⁰_gel/kJ mol⁻¹ (at 300 K) ≈ −26 (CH₃OH), −24 (CH₃CN). These parameters attest to the good stability of these gel systems. Finally, the influence of the chain length (n = C₇H₁₅ to C₁₂H₂₅) on the efficiency of gel formation (or melting) was investigated in methanol, ethanol, acetonitrile and heptane. It emerges that, to form gels, n-decyl and n-undecyl were found to be the most suitable chains and methanol and ethanol the most efficient solvents. It should be noted that the ability to form gels in methanol at a concentration of 0.6 mM at ambient temperature qualifies DDOA as a supergelator.