Second Harmonic Generation Responses of Ion Pairs Forming Dimeric Aggregates

Area of Science:

• Computational Chemistry
• Materials Science
• Nonlinear Optics

Background:

• Understanding the nonlinear optical (NLO) properties of molecular aggregates is crucial for developing advanced optical materials.
• Anion-cation (AC) pairs are building blocks for NLO materials, but their aggregation behavior and impact on NLO responses require detailed investigation.

Purpose of the Study:

• To investigate the second harmonic generation (SHG) responses of stilbazolium-based AC pairs forming dimeric aggregates.
• To elucidate the relationship between the structural configurations of AC dimers and their NLO properties.
• To explain experimentally observed concentration-dependent EFISHG responses.

Main Methods:

• A sequential approach combining molecular dynamics (MD) and density functional theory (DFT) calculations.
• MD simulations were used to study the aggregation behavior of AC pairs under high-concentration conditions.
• DFT calculations were employed to determine the NLO responses (second- and third-order) of the formed dimeric aggregates.

Main Results:

• AC pairs form stable dimeric aggregates ((AC)₂) with various structural shapes (stacked, Λ, head-to-head).
• Dimeric structures exhibit different symmetries that modulate the second- and third-order NLO responses.
• The second-order contribution (μβ//) dominates the EFISHG response, and its magnitude in dimers is reduced by approximately half compared to monomers.
• Third-order contributions (γ//) are reduced by about 10% in dimers.
• Strong amino donor groups enhance both μβ// and γ// responses.
• Dimeric aggregation affects the hyper-Rayleigh scattering (HRS) response similarly to μβ//, reducing its one-dimensional character.

Conclusions:

• The study successfully models the NLO responses of AC aggregates in solution, providing insights into experimentally observed phenomena.
• Dimer formation, particularly Λ and head-to-head configurations, increases centrosymmetric character, leading to reduced second-order NLO responses.
• The findings offer a step forward in understanding and predicting the NLO properties of molecular aggregates.