Third-Order Electric Field Response of Infinite Linear Chains Composed of Phenalenyl Radicals

Open shell singlet di- and multiradicals are of great importance in the framework of developing organic materials with enhanced nonlinear optical properties. We present here Hartree–Fock and DFT calculations of the structure and static second hyperpolarizabilty (γ) of the infinite periodic polymer containing stacked planar phenalenyl radicals. At the equilibrium geometry the oligomer structures are closed shell singlets with a significant alternation of the monomer–monomer distances in the stacking direction, thereby forming dimer units. However, the open shell singlet with reduced alternation becomes more favorable at chain lengths greater than about 40 dimer units. In the infinite polymer limit the alternation of stacking distance is essentially zero and the open shell singlet has intermediate diradical character, which is conducive to producing a high value of γ. DFT functionals, such as B3LYP and PBE0, largely overestimate the value of this property. However, a good estimate for the polymer γ = (1.0 ± 0.3) × 106 au per dimer is obtained by a hybrid approach wherein CCSD calculations for the dimer are combined with HF and LC-BLYP results for the polymer/dimer ratio. This value of γ is comparable to that found for the −(CH═CH)– unit of polyacetylene. An infinite polymer of stacked coronene molecules was also investigated, showing a considerably smaller enhancement of γ with respect to the corresponding dimer.