Comparing the Structures and Photophysical Properties of Two Charge Transfer Co-crystals

In the search for molecular materials for next-generation optoelectronic devices, organic co-crystals have emerged as a promising class of semiconductors for their unique photophysical properties. This paper presents a joint experimental-theoretical study of ground and excited state charge transfer (CT) interactions in two CT co-crystals. Reported herein is a novel CT co-crystal Npe:TCNQ, formed from 4-(1-naphthylvinyl) pyridine (Npe) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules via molecular self-assembly. The electronic structure and photophysical properties of Npe:TCNQ are compared with those of the co-crystal composed of Npe and 1,2,4,5-tetracyanobenzene (TCNB) molecules, Npe:TCNB, reported here with a higher-symmetry (monoclinic) crystal structure than previously published. Npe:TCNB and Npe:TCNQ dimer clusters are used as theoretical model systems for the co-crystals and their electronic structure is compared to that of the extended solids via periodic boundary conditions density functional theory (PBC DFT). UV-Vis absorption spectra of the dimers are computed with time-dependent density functional theory (TD-DFT) and compared to experimental UV-Vis diffuse reflectance spectra. Both Npe:TCNB and Npe:TCNQ are found to exhibit neutral character in the S0 state and ionic character in the S1 state. The degree of CT in the S0 state of Npe:TCNB is found to be slightly smaller than that of Npe:TCNQ, as predicted from differences in electron affinities of the acceptors. Furthermore, the degree of CT in the S1 state of Npe:TCNB is found to be slightly higher than that of Npe:TCNQ, aligning with predictions employing a recently developed orbital similarity metric.