Spin multiplicity dependence of nonlinear optical properties

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 ₄H₄N radical in the doublet X²A₂, quartet X⁴A₂, and sextet X⁶A₁ 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,omega;) and first hyperpolarizability β(-2ω,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.