Ab initio diradical/zwitterionic polarizabilities and hyperpolarizabilities in twisted double bonds

Area of Science:

• Computational chemistry
• Materials science
• Quantum chemistry

Background:

• Twisted pi-electron systems are promising for nonlinear optical (NLO) applications.
• Previous studies suggested a zwitterionic character for tictoid analogues.

Purpose of the Study:

• Investigate the ground state nature and static response properties (dipole moment, polarizability, hyperpolarizability) of twisted pi-electron system NLO chromophores.
• Analyze substituted twisted ethylene and tictoid analogues.
• Compare theoretical predictions across different computational levels.

Main Methods:

• High-level quantum chemical calculations.
• Hartree-Fock (HF) method.
• Density functional theory (B3LYP).
• Møller-Plesset perturbation theory (MP2).
• Complete active space self-consistent field (CASSCF) method.

Main Results:

• CASSCF calculations predict a ground-state diradical character for tictoid analogues, contradicting prior zwitterionic assignments.
• Calculated static hyperpolarizability (beta) values at HF, B3LYP, MP2, and CASSCF levels are significantly lower than semiempirical predictions.
• Consistent predictions across multiple computational methods for the diradical nature.

Conclusions:

• The ground state of these NLO chromophores is diradical, not zwitterionic.
• Theoretical beta values are orders of magnitude smaller than previously estimated.
• Accurate computational methods are crucial for predicting NLO properties of twisted pi-electron systems.