Near-infrared absorption of π-stacking columns composed of trioxotriangulene neutral radicals

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. Organic electronics: strong near-infrared light absorption with radical columns Near-infrared light absorption is enhanced by trioxotriangulene radicals that self-assemble into one-dimensional columns. Organic materials are attractive candidates for solar cells as they are inexpensive and have strong light absorption at visible wavelengths. This usually does not extend into the near-infrared region, however, restricting their use in this technologically-important wavelength range. A team of researchers in Japan led by Yasushi Morita from Aichi Institute of Technology and Hiromi Nakai from Waseda University show that crystals of trioxotriangulene can have unusually strong near-infrared absorption. With the aid of large-scale quantum chemistry calculations, they show that the key to this property is the tendency of their trioxotriangulene derivatives to stack into one-dimensional columns, which led to an overlap of electronic orbitals from different molecules.