Effect of configurational isomerism and polymorphism on chalcone fluorescent properties

Abstract

Three chalcones, 3-(9-anthryl)-1-(4-methoxyphenyl)prop-2-en-1-one (I), 3-(9-anthryl)-1-phenyl-prop-2-en-1-one (II) and 3-(1-pyrenyl)-1-(4-methoxyphenyl)prop-2-en-1-one (III), were synthesized and crystallized in different solvents to provide different forms such as two forms of I (I_a and I_b), four forms of II (II_a, II_b, II_c, and II_d) and four forms of III (III_a, III_b, III_c, and III_d). Single crystal X-ray diffraction was conducted on the samples except for II_a, III_a, and III_c, and powder X-ray diffraction, thermal behavior, solid-state absorption and fluorescence spectroscopy were investigated for all the crystals. It was found that these forms possess different configurational isomers and conformational polymorphs. Among them I_a, II_d, III_b and III_d possess the trans configuration, whereas I_b, II_b, and II_c show the cis configuration with a higher melting point than their trans isomers. These forms with trans configuration are fluorescent and III_b has a new red shifted fluorescent peak relative to III_d, which can be explained by the strong slipped face-to-face π-stacked arrangement of the pyrene chromophores in III_b. II_d shows a new red shifted fluorescent peak relative to I_a, which can be explained by the overwhelming C(anthracene)–H⋯π(anthracene ring) interactions with an edge-to-face slipped π-stacked arrangement in II_d. Although we were unable to obtain a single crystal of II_a, it can still be speculated that II_a possesses the trans conformation and has stronger interactions of anthracene fluorophores than II_d by combining the two facts that II_a can be formed from II_d by heating, which was tested by DSC and HSM and it has only one new red shifted fluorescent peak relative to I_a or II_d.