Cocrystallization of Aromatic (Di)amines with Nitrobenzene Acceptors Leading to the Charge-Transfer Assemblies with Extended Near-Infrared Absorption

Molecular materials exhibiting absorption in the near-infrared region are of significant interest for the development of optoelectronic devices. A series of solid-state donor–acceptor associations were prepared and examined via a combination of X-ray structural and UV–vis-NIR measurements. Spectral and electrochemical studies, as well as computational analysis of the corresponding complexes and reactants, were performed in solution and in silico. In particular, cocrystallization of N,N,N’,N’-tetramethyl-p-phenylenediamine and related aromatic amines with trinitrobenzene or bromodinitrobenzene produced π-stacks of alternating donors and acceptors. The intermolecular C···C and C···N distances in these stacks were shorter than the van der Waals separations (except in the complexes of dodecahydro-3a,9a-diazaperylene, where propylene linkers prevent close contact between donors and acceptors). These cocrystals exhibit strong absorptions in the near-infrared (NIR) range, with optical gaps varying from approximately 0.8 to 1.7 eV. These values are about 1 eV lower than the energies of the absorption bands of the corresponding 1:1 donor–acceptor complexes in solution. The spectral characteristics of the solid-state and solution-phase associations, as well as the binding energies determined through computational analysis, correlate with the differences in the oxidation potentials of the donors and the reduction potentials of the acceptors. These findings confirmed the charge-transfer nature of these interactions. Energy decomposition analysis revealed, however, that orbital (charge-transfer) interactions contribute only about 20% of the attractive energy in all of these complexes, while the remainder is provided by (roughly equal in magnitude) dispersion and electrostatic interactions.