Series of charge transfer complexes obtained as crystals in a confined environment

Abstract

A series of charge transfer complexes (CTCs) were successfully formed by solvent free processing techniques, using the 1,2,4,5-tetracyano benzene (TCNB) as π_A molecule and a series of p-dihydroquinones (H₂Qs) as π_D counterparts. Additionally to the classical co-evaporation techniques, we obtained CTCs in less than an hour, in a very simple confined environment, between two 100 μm – spaced glass plates. A systematical study by Raman spectroscopy on crystals highlighted the CTCs formation. Moreover, three new crystalline structures were obtained, namely TCNB-H₂Q that crystallizes in columns connected to each other by H-bonds, while with the methoxy- and dimethoxy-H₂Qs the CTC forms crystals with the stoichiometry 1 : 2, TCNB-(H₂Q_OMe)₂ and TCNB-(H₂Q_OMe2)₂. In TCNB-(H₂Q_OMe)₂ layers are formed due to intermolecular hydrogen bonds, while in TCNB-(H₂Q_OMe2)₂ molecules arrange in triads, with π_A–π_D interactions. In all cases, strong π_A–π_D interactions exist with intermolecular distances lower than the van der Waals distances, which results in strong absorption bands in the visible range.