Abstract
Using the spin-unrestricted density functional theory method, we have investigated a novel class of open-shell singlet fused-ring molecules with helical scaffolds toward the development of highly efficient third-order nonlinear optical (NLO) systems. It turns out that the helical molecules possess pancake bonding with intermediate diradical character and thereby exhibit large enhancement of orientationally-averaged second hyperpolarizability γs values (third-order NLO properties at the molecular scale) as compared to their closed-shell analogues. It is also found that the asymmetric charge distribution between the cofacial radical sites located on both the terminal rings of the helical scaffold leads to further enhancement of the γ value in the intermediate open-shell character region as compared to the open-shell analogues without asymmetric charge distributions. These results indicate that the helical systems with intermediate diradical character are potential candidates for the building block of highly...
Highlights
Helicene-like molecules, that is, polycyclic aromatic compounds with nonplanar screw-shaped scaffolds, have been receiving increasing attention due to their unique structure characterized by helicity and their unusual physicochemical properties[1] like second-order nonlinear optical (NLO) properties,[2] two-photon circular dichroism,[3,4] circularly polarized luminescence,[5] and so on
We choose three helical model systems with intermediate diradical characters (8CTR, 7AZ, and 13C2NHTCN), which are investigated by the previous theoretical and experimental studies.[53−56] As references, we have investigated closed-shell analogues (10helicene, 7helicene, and 13C2NH), which have the same number of rings as those in the intermediate diradical counterparts (8CTR, 7AZ, and 13C2NHTCN), respectively. 8CTR indicates [8]cethrene, which substitutes two phenalenyl rings for two terminal benzene rings of [8]helicene-like [5]cethrene. 56 7AZ is an azulene-like [7]helicene analogue terminated by five- and seven-membered rings. 13C2NHTCN and 13C2NH indicate [13]-C2NH helicenes with and without tetracyano groups, respectively, and resemble C2S helicenes composed of thiophenes.[57]
Rings, D (at the (U)M05-2X/6-31G(d) level of approximation) and diradical characters y (at the projected unrestricted HF (PUHF)/6-31+G(d) level of approximation). 8CTR is found to have 0.09 Å shorter D than the closed-shell reference 10helicene, which has the same number of rings as 8CTR, due to the covalent-like interaction between the cofacial phenalenyl radical units. 13C2NHTCN is found to have 0.26 Å shorter D than the closed-shell reference 13C2NH, where the change in D is more significant than that between 8CTR and 10helicene because
Summary
Helicene-like molecules, that is, polycyclic aromatic compounds with nonplanar screw-shaped scaffolds, have been receiving increasing attention due to their unique structure characterized by helicity and their unusual physicochemical properties[1] like second-order nonlinear optical (NLO) properties,[2] two-photon circular dichroism,[3,4] circularly polarized luminescence,[5] and so on. A new design concept, open-shell singlet character-based design principle, has recently emerged for developing highly efficient quantum functional materials since the tuning of the open-shell character tends to cause unique electronic structures and physicochemical properties.[13,14] since this character is based on the effective bond nature characterized by bonding and antibonding frontier molecular orbitals in the ground-state molecular systems and is closely related to conventional chemical concepts like resonance structure and aromaticity, the investigations of practical chemical guidelines for tuning the open-shell character as well as of design realistic open-shell singlet systems have attracted much attention from theoretical and experimental researchers.[14−19] In previous studies, we have theoretically proposed a novel class of highly efficient NLO systems based on open-shell singlet molecules.[13,20,21] We introduce the diradical character y, which is a quantumchemically well-defined index and takes a value between 0. The present results are expected to contribute to constructing novel design guidelines for highly efficient openshell singlet NLO materials composed of helical pancakebonded systems
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