Abstract
Using quantum chemical calculations and exciton dynamics simulations, we investigate the static second hyperpolarizability γ [the third-order nonlinear optical (NLO) property at the molecular scale] of slip-stacked pentacene dimer models in the correlated-triplet-pair [1(TT)] state created from the singlet excited state in the singlet fission (SF) process. It is found that the SF induces significant (∼20 times at maximum) enhancement of γ/monomer in the 1(TT) state as compared to that in the singlet ground state. The origin of the remarkable enhancement of γ/monomer is revealed by analyzing the γ density distribution and the intermolecular orbital interaction. Furthermore, we clarify molecular packings suitable for highly efficient SF and largely enhanced γ values of a novel class of SF-induced NLO systems, which have promising potential to surpass the conventional NLO systems.
Highlights
Third-order nonlinear optical (NLO) phenomena have attracted a great deal of attention in a variety of fields including physics, chemistry, biology, and materials science due to their wide applications including extremely large data storage,[1,2] ultrafast optical switching,[3] ultrahigh sensitivity spectroscopy,[4] and nanofabrication.[5,6] Because the materials with both the large third-order NLO property and ultrafast response time are indispensable for realizing such applications, many experimental and theoretical studies have been conducted to explore the mechanism of NLO properties in molecular systems as well as to construct design guidelines for highly active NLO materials
Nakano and co-workers have clarified spinmultiplicity dependences of γ in molecules with different diradical characters in the singlet state, that is, the relationship between spin-multiplicity, diradical character, and γ: the closed-shell or open-shell molecules with weak diradical character exhibit the enhancement of γ with the increasing spin-multiplicity, whereas the molecules with intermediate and large diradical characters exhibit the reduction of γ with the increasing spin-multiplicity.[25]. This finding suggests the possibility of another novel class of open-shell NLO materials using high spin states, which are far superior in magnitude to the conventional closed-shell NLO systems and even openshell singlet NLO systems with intermediate diradical character
We investigate the effects of singlet fission (SF) on the third-order NLO properties (γ) of slip-stacked pentacene dimer models with various intermonomer configurations to clarify the guiding principle for controlling the γ in the correlated-triplet-pair [1(TT)] state created by SF
Summary
Third-order nonlinear optical (NLO) phenomena have attracted a great deal of attention in a variety of fields including physics, chemistry, biology, and materials science due to their wide applications including extremely large data storage,[1,2] ultrafast optical switching,[3] ultrahigh sensitivity spectroscopy,[4] and nanofabrication.[5,6] Because the materials with both the large third-order NLO property and ultrafast response time are indispensable for realizing such applications, many experimental and theoretical studies have been conducted to explore the mechanism of NLO properties in molecular systems as well as to construct design guidelines for highly active NLO materials. Several structure−NLO property relationships and design guidelines have been revealed; for example, introduction of donor/acceptor groups,[4,7] extension of the π-conjugation length,[8] and tuning the charge states[9] have been applied to the closed-shell molecular systems to enhance the second hyperpolarizability γ, which is the third-order NLO property at the molecular scale. This finding suggests the possibility of another novel class of open-shell NLO materials using high spin states, which are far superior in magnitude to the conventional closed-shell NLO systems and even openshell singlet NLO systems with intermediate diradical character
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