Comparative rotatory power of bent and twisted polyynes

Area of Science:

• Computational chemistry
• Theoretical chemistry
• Spectroscopy

Background:

• Linear polyynes are π-conjugated systems with high symmetry.
• Optical activity is typically associated with chirality, but its origins in π-conjugated systems are complex.
• Understanding how molecular geometry influences optical properties is crucial for designing new materials.

Purpose of the Study:

• To investigate the impact of bending and twisting on the optical activity of linear polyynes.
• To computationally explore the relationship between molecular structure, symmetry, and optical activity.
• To challenge the conventional association of optical activity solely with chirality.

Main Methods:

• In silico modeling of linear polyynes (C18H2) with varying degrees of bending and twisting.
• Computation of gyration tensors using linear response methods.
• Analysis of transition electric dipole-magnetic dipole and electric dipole-electric quadrupole polarizabilities.

Main Results:

• Bending polyynes significantly induces optical activity, even in achiral configurations.
• Twisting bent polyynes reduces their overall optical activity by promoting linearization.
• Optical activity generation is direction-dependent in oriented, non-chiral bent structures.

Conclusions:

• Molecular bending is a potent mechanism for generating optical activity, independent of chirality.
• Twisting counteracts the optical activity induced by bending in these π-conjugated systems.
• The study highlights the importance of considering molecular orientation and specific polarizabilities when analyzing optical activity in non-chiral systems.