Tunable Chiral Second-Order Nonlinear Optical Chromophores Based on Helquat Dications

Area of Science:

• Materials Science
• Organic Chemistry
• Nonlinear Optics

Background:

• Development of novel organic materials with significant nonlinear optical (NLO) properties is crucial for advanced photonic applications.
• Helquat (Hq) acceptors offer a promising scaffold for designing NLO chromophores due to their electronic characteristics.

Purpose of the Study:

• To synthesize and characterize new dipolar cations incorporating helquat acceptors with methoxy or tertiary amino electron donor groups.
• To investigate the structure-property relationships governing the nonlinear optical responses of these novel helquat derivatives.

Main Methods:

• Synthesis of fourteen new helquat-based dipolar cations as TfO- salts.
• Characterization using NMR, electronic absorption spectroscopy (UV-vis), and single-crystal X-ray diffraction.
• Determination of molecular quadratic NLO responses via hyper-Rayleigh scattering (HRS) and Stark spectroscopy.
• Computational analysis using Density Functional Theory (DFT) and time-dependent DFT (TD-DFT).

Main Results:

• Synthesized Hq derivatives exhibit intense, low-energy intramolecular charge-transfer (ICT) absorptions (λmax ≈ 400-600 nm).
• Static first hyperpolarizabilities (β0) range from moderate to large (9-580 × 10⁻³⁰ esu), with tertiary amino donors yielding higher values.
• DFT calculations confirm ICT character and predict large βtot values (472-1443 × 10⁻³⁰ esu).
• Optimal NLO response is achieved with para-conjugation between Hq and donor fragments.

Conclusions:

• The synthesized helquat derivatives are promising candidates for quadratic nonlinear optical applications.
• Tertiary amino donors and extended π-conjugation significantly enhance NLO properties.
• Para-conjugation is key for maximizing molecular hyperpolarizability in these systems.