Obtaining enhanced circular dichroism in [4]heterohelicenium analogues

Area of Science:

• Computational Chemistry
• Spectroscopy
• Organic Chemistry

Background:

• Helicenium analogues are known for their unique electronic and chiroptical properties.
• Understanding their electronic circular dichroism (ECD) is crucial for developing new molecular materials.
• Previous studies have focused on specific derivatives, but a systematic investigation across various substituents is lacking.

Purpose of the Study:

• To investigate the electronic circular dichroism (ECD) of three helicenium analogues: dimethoxyquinacridinium (DMQA(+)), dimethoxychromenoacridinium (DMCA(+)), and dimethoxychromenoxanthenium (DMCX(+)).
• To identify optimal substituent patterns that enhance chiroptical response.
• To explore potential applications of these optimized systems as chromophores.

Main Methods:

• Time-dependent density functional theory (TD-DFT) calculations were employed.
• The CAM-B3LYP functional with the 6-311++G** basis set was used.
• Fifty-six derivatives with varying electron-donating and -withdrawing substituents and different net charges were systematically examined.

Main Results:

• A superior substitution pattern was identified, independent of bridging atoms, yielding high rotational strengths (approx. 90 × 10(-40) esu(2) cm(2)) for DMQA, DMCA, and DMCX.
• The optimal systems exhibit a highly allowed primary electronic transition.
• The angle between electronic and magnetic transition dipole moments was found to be approximately 50°, indicating a strong chiroptical response.

Conclusions:

• The identified substitution pattern significantly enhances the chiroptical properties of helicenium analogues.
• These optimized systems demonstrate potential as advanced chromophores due to their strong and tunable electronic transitions.
• The chiroptical response is primarily limited by the magnetic transition, offering avenues for further molecular design.