A recent article published in Polymers demonstrated density functional theory (DFT)-based first-principle calculations to analyze the influence of inter-oligomer interactions on photoluminescence (PL) in cis-polyacetylene (cis-PA), a semiconducting conjugated polymer (CP).
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Background
CPs such as cis-PA have recently gained significant attention due to their PL properties, which arise from their alternating single and double bonds. These properties make CPs useful in various applications, including organic photovoltaics, thermo-electrics, supercapacitors, biosensing, photocatalysis, optoelectronic devices, transistors, and solar cells.
The structure of CPs exhibits short-range order without long-range regularity, complicating their solubility and resulting in polydispersity. This makes it challenging for computational models to represent these polymers accurately. Consequently, characteristics of longer polymeric chains are often inferred from those of shorter oligomeric chains. However, this extrapolation does not provide accurate electronic properties of polymers.
This study examined the excited state dynamics of cis-PA oligomer ensembles and compared the results to those of individual oligomers. The application of DFT helped comprehend the complex behavior of charge carriers in both individual and ensemble oligomers.
Computational Methods
The computations involved two components: DFT-based assessment of the ground state electronic structures and examination of excited state dynamic processes by calculating nonadiabatic couplings (NACs) based on Redfield theory.
Ground-state calculations were based on the fictitious one-electron Kohn-Sham framework. Firstly, an atomic model was defined through the initial ionic positions. Subsequently, the electronic structure was determined by solving the self-consistent field equations of DFT. Dissipative transitions were calculated using the adiabatic molecular dynamic (MD) trajectories framework.
DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP). The Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional accounted for the exchange-correlation effects in these calculations.
Two classes of cis-PA are considered in this study: a single oligomer with a molecular formula of C32H36 and an ensemble of six oligomers with a net molecular formula of C184H208. The latter depicted the collective behavior and interactions of the oligomers within a condensed phase.
Before conducting dynamic analyses, all models were relaxed to optimal geometries in the ground state. These were preheated for dynamic processes and maintained using a thermostat, followed by MD simulations at 300 Kelvin.
All simulations were performed at the gamma point. Both electronic and nuclear degrees of freedom were integrated through adiabatic MD simulations, and NACs among adiabatic states were computed via an on-the-fly technique.
For the ensemble model, oligomers were positioned to balance the pressure between unit cells and obtain a realistic density. Periodic boundary conditions were applied in both models.
Results and Discussion
The models with shorter-length oligomers exhibited a decreased computed conductivity, attributed to increased band gaps and sub-gaps due to quantum confinement. This opposed the generally observed mild redshift of transition energies due to increased chain length.
The CP oligomers formed finite-size bound excitons, primarily due to electron-hole interactions, underscoring the interaction between exciton polarons and interchain vibrational modes. This confinement effect was more pronounced in shorter oligomers, significantly affecting their electronic properties.
Alternatively, longer oligomers exhibited stable transition energies and were defined mainly by the size of the exciton and polaron instead of further increases in length. No significant increase in exciton size was observed beyond the 10 to 40 Å range. Thus, the researchers utilized shorter oligomers for further investigation.
Consistent charge carrier dynamics and PL behavior were observed across all molecular weights investigated. Other major effects observed include dispersion forces and orbital hybridizations between chains.
Terminal methyl groups incorporated in the oligomer configurations enhanced molecular stability through steric protection, reducing vulnerability to oxidative degradation and ensuring structural integrity during experimental analyses.
Electrons relaxed faster than holes in both individual and ensemble oligomers due to smaller sub-gaps and more prominent phonon interactions for electrons. The ensemble demonstrated faster relaxation than the individual oligomer, attributed to the higher density of states and more relaxation pathways.
Conclusion
The researchers successfully obtained the ground electronic structures and excited state dynamics of two different models of cis-PA (individual and ensemble) using DFT. Nonradiative relaxation of charge carriers and PL were computed using the Redfield theory.
PL spectra of both models demonstrated interband and intraband emissions. Additionally, the ensemble of oligomers exhibited broader line widths, red-shifted transition energies, and lower intensities than the individual oligomer.
The comparative analysis revealed that dispersion forces and orbital hybridizations between chains were mainly responsible for the different PL in the two models. Thus, the side-chain functionalization of CPs can mitigate inter-oligomer interactions.
The detailed analysis of the optoelectronic properties of cis-PA presented in this study can help improve nanostructured semiconductors for photovoltaic and film-forming CP processing.
Journal Reference
Keya, K. N., Han, Y., Xia, W., Kilin, D. (2024). Inter-Oligomer Interaction Influence on Photoluminescence in Cis-Polyacetylene Semiconductor Materials. Polymers. DOI: 10.3390/polym1613189
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