Structure, Phase Diagram and Fluorescence Spectra of 2,3-Dimethylnaphthalene (Anthracene) Mixed Crystals

Abstract

An X-ray crystal structure determination for 2,3-dimethylnaphthalene (2,3-DMN) at 300 K was carried out. The two noncentrosymmetric molecules in the unit cell, space group P2₁/a, (a = 7.916(10) Å, b = 6.052(8) Å, c = 10.017(8) Å, β = 105.43(1)°) are statistically dipolarly disordered. Perfect cleavage along (001) is examined in terms of the disordered structure. There are two phase transitions, one of higher order from monoclinic to triclinic, occuring at ∼ 210K, and another one at ∼ 100K. The phase diagram of 2,3-DMN/anthracene exhibits a peritectic at 377.4K (104.2°C) with continuous mixed crystal formation from 0 to ∼ 23% anthracene (confirmed by X-ray Guinier technique). From normal freezing experiments a concentration-dependent distribution coefficient k of anthracene (A) in 2,3-DMN is derived with k(c_A → 0) = 2.16. Packing of the mixed crystals is analyzed in terms of lattice geometry and packing coefficients. Liq. He fluorescence and absorption spectra of anthracene in 2,3-DMN display a fairly large residual line width, Δ ∼ 300 cm⁻¹ which is attributed to the statistical disorder of the host lattice. Spectral changes of the anthracene guest fluorescence emission observed with increasing anthracene concentration are taken to be indicative of guest-guest interactions.