Large off-diagonal contribution to the second-order optical nonlinearities of Λ-shaped molecules

Mingli Yang, Benoît Champagne

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The diagonal and off-diagonal tensor components of the first hyperpolarizability of a series of Λ-shaped molecules are compared by means of ab initio and semiempirical molecular orbital calculations. The calculated results are rationalized using expressions derived from a simple valence-bond charge-transfer model in which one ground and two excited state are described as a combination of one valence-bond (VB) and two charge-transfer (CT) states. In addition to the CT character, the angle (2θ) between the two donor (D)/acceptor (A) branches is a key parameter to determine the relative magnitudes between the βxxz and βzzz components. In the VB-2CT model, βxxz presents a maximum value for 2θ = 109.47°, whereas βxxz and μz have the same sign. On the other hand, βzzz decreases monotonically with θ, and its variations with the CT character follow a curve with two extrema like for one-dimensional D/A chromophores. The ab initio and semiempirical calculations as well as the VB-2CT model show that Λ-shaped molecules with large first hyperpolarizability and specific η = βxxzzzz can be designed in order to build phase-matchable second-harmonic generation crystals.

langue originaleAnglais
Pages (de - à)3942-3951
Nombre de pages10
journalJournal of physical chemistry A
Numéro de publication19
Les DOIs
Etat de la publicationPublié - 15 mai 2003

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