Spin Multiplicity Dependence of Nonlinear Optical Properties

Spin‐restricted time‐dependent DFT for open‐shell systems is used to study the spin multiplicity dependence of nonlinear optical coefficients. Calculations are performed for various spin states of the pyrrole radical (see figure). The results indicate a substantial dependence on spin multiplicity of the NLO properties, which are sensitive to the basis set compared to closed shells at the DFT level of theory.Open‐shell spin‐restricted time‐dependent density functional theory is applied to explore the spin multiplicity dependence of linear and nonlinear optical properties. An open‐shell neutral conjugated system, the C 4 H 4 N radical in the doublet X 2 A 2 , quartet X 4 A 2 , and sextet X 6 A 1 states, is chosen as a model system to illustrate various aspects of the theory. It is found that irrespective of the exchange‐correlation functional employed, the components of the polarizability α (− ω,ω ) and first hyperpolarizability β (−2  ω,ω,ω ) show very different dependency with respect to the multiplicity, with an increasing trend for higher spin states. This is rationalized by the decrease in conjugation and stability of the system with increasing multiplicity, and by the way the interaction between unpaired electrons and the external field is shielded by remaining electrons of the molecule. The study suggests the applicability of open‐shell systems for frequency‐dependent nonlinear optical properties and for the possibility of spin control for such properties.