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

This study introduces a new donor group capable of activating click-type [2+2] cycloaddition-retroelectrocyclizations, generally known for their limited scope. Target chromophores were synthesized using isocyanate-free urethane synthesis. The developed synthetic method allows for the tuning of the optical properties of the chromophores by modifying the donor groups, the acceptor units, and the side chains. The charge transfer (CT) bands of the chromophores exhibit λ_max values ranging from 363 to 692 nm. The CT bands observed have been supported by solvatochromism and protonation experiments. The synthesized compounds exhibit positive solvatochromism. Due to their potential as NLOphore candidates, the stability of the synthesized compounds have been investigated both experimentally through TGA and theoretically by calculating parameters such as frontier orbital energy differences, electronegativity, and global hardness/softness. TD-DFT calculations were used to elucidate the nature of the electronic transitions, revealing that the bands correspond to CT arising from HOMO-to-LUMO excitations. The NLO properties of the chromophores were investigated theoretically by DFT methods and experimentally by the EFISHG technique. Both results are shown to be in agreement with HOMO-LUMO energy differences. The experimental μβ values of the selected molecules range from 470×10^-48 to 5400×10^-48 esu.