Bridging Molecular and Periodic Models: A Theoretical Study of the Spectroscopic Properties of Polythiophene †

Polymeric organic semiconductors play a pivotal role in the development of new organic thermoelectric materials, sensing, and organic optoelectronics. Understanding the relation between their structure and properties is essential for designing the next generation of materials with improved features. Theoretical modelling based on first-principles calculations is a powerful aid for interpreting experimental structural and spectroscopic data of π-conjugated polymers, both in their pristine and doped states. Two approaches are currently available, i.e., oligomeric models and one-dimensional (1D) periodic crystal, each of one with its advantages and disadvantages. The combination of both allows to overcome the limitations of each. However, to do this, it is fundamental to systematically address the differences between these two approaches (molecular models vs. crystals). Here, we present a first systematic comparison between oligomeric and (1D) crystal models for describing the structural and spectroscopic properties of polyconjugated polymers. Using polythiophene (PT) and poly(3-hexylthiophene) (P3HT) as prototypical benchmark systems, we examine the relationship between finite-size oligomers and the corresponding infinite polymer treated as a one-dimensional (1D) crystal under periodic boundary conditions. Density functional theory (DFT) calculations are performed to examine the convergence of geometrical parameters and vibrational features (IR and Raman) as the oligomer length increases. The 1D crystal model is then evaluated as the limiting case of the oligomeric series. Its advantages in simplifying vibrational mode analysis are illustrated. This study establishes a methodological framework connecting molecular and periodic models, with direct relevance to the interpretation of experimental vibrational spectra of conducting polymers.