A long-wavelength photoabsorption of organic molecules has been noticed because of the potential as materials. In addition to the extension of π conjugation, molecular aggregation has been utilized to realize the elongation of absorption wavelength. We report strong near-infrared absorptions of trioxotriangulene neutral radicals in the crystalline state and large-scale theoretical calculations of the radical assemblies interpreting the mechanism of optical properties. Polarized absorption spectra and X-ray diffraction of the crystals clarified that an unusual π-stacking column consisting of π-dimers is key for this absorption. Quantum chemical calculations based on time-dependent density functional theory revealed that the π-dimer shows an electronic transition between frontier orbitals generated by strong coupling of the delocalized singly occupied orbitals of monomers. The interdimer interaction of transition dipole moments, which are parallel to the column, elucidated the increase of absorption wavelength. The divide-and-conquer Green function method enabled the large-scale time-dependent density functional theory calculation up to a 60mer, where the maximum number of atoms is 4380, reproducing the near-infrared absorptions of trioxotriangulene crystals. The present method to investigate the mechanism of the long-wavelength photoabsorption is useful for developing organic materials consisting of stable neutral radicals.

Our recent studies for the development of conducting charge-transfer complexes with well-defined assembled structures based on the cooperation of hydrogen-bond and charge-transfer interactions are described. We have designed and synthesized hydrogen-bonded charge-transfer complexes based on (1) tetrathiafulvalene derivatives having nucleobases, aminobenzo and imidazole moieties, and (2) tetracyanoquinodimethane radical anion salts with hydrogen-bond-functionalized phenalenyl and protonated oligo(imidazole) cations. The directional and site-selective hydrogen-bonds formed various supramolecular network structures, constructing segregated columns as the conduction path. Furthermore, the high polarity of hydrogen-bonds caused strong donor–acceptor or cation–anion electrostatic interactions modulating the ionicity of component molecules and the electronic structure of conduction columns. Such electronic modulation effects in addition to controlling the molecular arrangement realized the first purely organic molecular metal based on a hydrogen-bonded charge-transfer complex in the p-chloranil complex of an imidazole-functionalized tetrathiafulvalene derivative. The proton-donating/accepting ability of the hydrogen-bonding sites afforded the simultaneous proton- and charge-transfer complexes, demonstrating an important prompt for the realization of cooperative proton–electron-transfer systems.