The Role of Secondary Interactions in Centrosymmetry of Charge Transfer Complexes with Nitrated Acceptors

Abstract

Charge transfer (CT) cocrystals lacking an inversion center (noncentrosymmetric) have the propensity for nonlinear optical and ferroelectric behavior. Although noncentrosymmetry is rare, phase transitions can lead to symmetry breaking. Some cocrystals with weak CT undergo structural changes, resulting in an alteration of the degree of CT and the emergence of new non-CT interactions. In this paper, the influence of symmetry on secondary interactions of CT cocrystals with the nitrated acceptor 2-(2,7-dinitro-9H-fluoren-9-ylidene) (DDF) are investigated. Of the four cocrystals discovered, the presence of nitro group interactions correlated with the observation of centrosymmetry. An analysis of two polymorphs of DDF with the donor 5,10-dihydro-5,10-dimethylphenazine (DHDMP) reveals that the noncentrosymmetric (DDF:DHDMP I) form showed no participation of nitro groups, whereas the centrosymmetric form (DDF:DHDMP II) had close interchain C–H···O interactions. These nitro group interactions are accompanied by increased donor planarity, which is correlated to the increase in degree of charge transfer from ρ = 0.24 e (DDF:DHDMP I) to ρ = 0.41 e (DDF:DHDMP II), thereby demonstrating the influence of crystal packing on electronic properties.