Carbamate-Functionalized NLOphores via a Formal [2+2] Cycloaddition-Retroelectrocyclization Strategy

Area of Science:

• Organic Chemistry
• Materials Science
• Photochemistry

Background:

• Click-type [2+2] cycloaddition-retroelectrocyclizations typically have a limited scope.
• Developing new donor groups is crucial for expanding synthetic methodologies.
• Chromophores with tunable optical properties are essential for advanced materials applications.

Purpose of the Study:

• To introduce a novel donor group for activating click-type [2+2] cycloaddition-retroelectrocyclizations.
• To synthesize and characterize new chromophores with tunable optical and nonlinear optical (NLO) properties.
• To investigate the electronic and photophysical properties of the synthesized chromophores.

Main Methods:

• Isocyanate-free urethane synthesis for chromophore preparation.
• Spectroscopic techniques (UV-Vis, solvatochromism, protonation) to study optical properties.
• Thermogravimetric analysis (TGA) for thermal stability assessment.
• Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations for electronic structure and transitions.
• Experimental Electric Field Induced Second Harmonic Generation (EFISHG) measurements for NLO properties.

Main Results:

• A new donor group was successfully synthesized and utilized to activate the cycloaddition-retroelectrocyclization reactions.
• Chromophores with tunable optical properties were synthesized, exhibiting charge transfer (CT) bands with λmax values from 363 to 692 nm.
• Positive solvatochromism and support from protonation experiments confirmed the CT nature of the observed bands.
• Synthesized compounds demonstrated good thermal stability and promising nonlinear optical (NLO) properties, with experimental μβ values up to 5400×10⁻⁴⁸ esu.
• Theoretical calculations (DFT, TD-DFT) aligned well with experimental findings, elucidating HOMO-LUMO transitions and NLO responses.

Conclusions:

• The developed donor group significantly broadens the applicability of click-type [2+2] cycloaddition-retroelectrocyclizations.
• The new synthetic strategy allows for fine-tuning of chromophore optical properties for specific applications.
• The synthesized chromophores are promising candidates for nonlinear optical (NLO) materials due to their tunable electronic structures and significant NLO responses.