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

Charge transfer (CT) cocrystals lacking an inversion center (noncentrosymmetric) have the propensity for nonlinear optical and ferroelectric behavior. Although non-centrosymmetry 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 N I, tric 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 center dot center dot center dot O interactions. These nitro group interactions are accompanied by increased donor planarity, which is correlated to the increase in degree of charge transfer from rho = 0.24 e (DDF:DHDMP I) to rho = 0.41 e (DDF:DHDMP II), thereby demonstrating the influence of crystal packing on electronic properties.