Prashanna Poudel | Searcharxiv

arXiv2026

Helically Enhanced Chiroptical Response and Symmetry Breaking in Conjugated Polymers

Chiral $π$-conjugated polymers are an attractive material platform for spin polarized carrier-transport and spectroscopy, but fundamental considerations for how torsional disorder influences the response properties of the material have not been considered. Here we combine atomistic electronic structure modeling with with experimental spectroscopic measurements to examine symmetry breaking in the prototypical $π$-conjugated polymer polyacetylene, (CH)$_x$. Chiral (CH)$_x$ oligomers are generated in distinct conformations which differ in their out-of-plane tonsorial ordering. We find that a \textit{helical }conformation introduces orders of magnitude enhanced chiroptical activity due to a solenoid effect. This effect is visualized by the Transition Chiral Tensor analysis which shows signatures of domain ordering which eliminates destructive interference between electric and magnetic contributions. These findings highlight the capability to develop a hierarchical interpretation relating local, fragment symmetry breaking to global, nonlocal interactions governing chiroptical response in emerging chiral materials.

cond-mat.mtrl-sci↗

arXiv2024

The influence of dark excitons on the electroabsorption spectrum of polyacetylene

This study revisits the electroabsorption (EA) spectrum of polyacetylene, as functions of the electric field strength, isomerization degree, and light polarization states. The EA spectrum of $cis$-$(CH)_x$ reveals an oscillatory feature that follows the Stark shift-related first derivative of the materials absorption spectrum that contains v(0-1) and v(0-2) sidebands of the excited $C=C$ stretching vibration that agrees well with the Raman spectrum. EA spectrum of $trans $-$(CH)_x$ does not match the first derivative of the materials absorption spectrum, and the phonon sideband frequency does not agree with the RS spectrum. EA spectrum of $trans $-$(CH)_x$ reveals a band below the first allowed $1B_u$ exciton. We interpret this feature as due to the electric field activated even-parity dark (forbidden) exciton, namely $mA_g$ ($m >1$), showing that the nonluminescent $trans $-$(CH)_x$ is due to the reverse order of the excited states, where a dark $mA_g$ exciton lies below the allowed $1B_u$ exciton. This agrees with the unusual phonon sideband in $trans $-$(CH)_x$ absorption, since the excited state attenuation caused by the fast internal conversion from $1B_u$ to $mA_g$ influences the apparent frequency that determines the phonon sideband. Consequently, from the EA and RS spectra we estimate the $1B_u$ lifetime in $trans $-$(CH)_x$ to be $\sim 30$ fs. Integrated EA spectrum of $trans $-$(CH)_x$ shows a traditional Huang-Rhys type series with a relaxation parameter, $S \sim 0.5$. This indicates that the EA spectrum of the $trans $ isomer is also determined by a Stark shift related to the first derivative of the absorption spectrum, but preferentially for the longest chains in the films chain lengths distribution. This is due to the $N^3$ response of the non-linear susceptibility, $χ^{(3)}$ ($\sim$EA), dependence on the chain length having $N$ monomers.

cond-mat.other↗