Organic Nonlinear Optical Materials: The Mechanism of Intermolecular Covalent Bonding Interactions of Kekule Hydrocarbons with Significant Singlet Biradical Character

The ground- and excited-state properties of benzene-linked bisphenalenyl (B-LBP), naphthaline-linked bisphenalenyl (N-LBP), and anthracene-linked bisphenalenyl (A-LBP) Kekule molecules and their respective one-dimensional (1D) stacks are investigated using time-dependent density functional theory (TD-DFT) and a range of extensive multidimensional visualization techniques. The results reveal a covalent pi-pi bonding interaction between overlapping phenalenyl radicals whose bond length is shorter than the van der Waals distance between carbon atoms. Increasing the linker length and/or number of molecules involved in the 1D stack decreases the HOMO-LUMO energy gap and increases the wavelength of the systems. The charge-transfer mechanism and electron coherence both differ with changes in the linker length and/or number of molecules involved in the 1D stack.