Details

Autor(en) / Beteiligte

• Fukuda, Kotaro
• Nakano, Masayoshi

Titel

Intramolecular Charge Transfer Effects on the Diradical Character and Second Hyperpolarizabilities of Open-Shell Singlet X−π–X (X = Donor/Acceptor) Systems

Ist Teil von

• The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 2014-05, Vol.118 (19), p.3463-3471

Ort / Verlag

United States: American Chemical Society

Erscheinungsjahr

2014

Quelle

MEDLINE

Beschreibungen/Notizen

• We investigate the effect of the quadrupole-type intramolecular charge transfer (ICT) in open-shell singlet donor−π–donor (D−π–D) molecules on the singlet open-shell (diradical) character and the longitudinal second hyperpolarizabilities γ (the third-order nonlinear optical (NLO) properties at the molecular scale). For this investigation we used the para-quinodimethane (PQM) with point charges (pc’s) model calculated with the unrestricted coupled cluster method including single and double excitations with a perturbative treatment of the triple excitations (UCCSD(T)). In this model, the diradical character y and the amount of the ICT, that is, the D−π–D nature, can be varied primarily by changing the exocyclic carbon–carbon bond (C–C) lengths and the external pc's Q, respectively. It turns out that the increase in the D−π–D nature decreases the y values, moves the y values (y max) giving the maximum γ (γmax) to the large y region, and enhances the γmax values, for example, the γmax of the singlet diradical PQM with Q = −2.8 au reaches twice that of the singlet diradical PQM without any pc's. This result indicates that open-shell singlet D−π–D systems with ICT are promising candidates for a new class of third-order NLO molecules, whose γ values are more enhanced than those of conventional closed-shell D−π–D systems and of symmetric open-shell singlet systems without the ICT. To confirm this tendency, we examine the boron-disubstituted PQM dianion model, which is found to exhibit further enhancement of γ as compared to the PQM model with intermediate diradical character due to the synergy effects of the intermediate open-shell singlet nature and the strong field-induced ICT nature in the dianionic state of the D−π–D system. Further investigation of the acceptor−π–acceptor (A−π–A) type ICT effect in the PQM-pc model shows that both D−π–D and A−π–A type symmetric ICTs give similar effects on the relationship between y and γ, though there are some differences originating in the orbital contraction and extension induced by the pc's. The present results contribute to understanding the third-order NLO properties of open-shell symmetric ICT systems and thus to constructing new design guidelines for highly efficient third-order NLO systems.