Polaron Formation in Conducting Polymers: A Novel Approach to Designing Materials with a Larger NLO Response

Substantial efforts have been made to design and investigate new approaches for high-performance nonlinear optical (NLO) materials. Herein, we report polaron formation in conducting polymers as a new approach to designing materials with a large NLO response. A comparative study of polypyrrole and polypyrrole-based polaron (nPy⁺ where n = 1, 3, 5, 7, and 9) is carried out for optoelectronic and NLO properties. The studied polarons (PPy⁺) show excellent electronic properties and have reduced ionization potential (IP) as compared to neutral PPy, and a monotonic decrease is observed with increased chain lengths (1Py to 9Py). Interesting trends of global reactivity descriptors can be seen; the softness (S) increases with an increase in the chain length of PPy, while the hardness (η) decreases in the same fashion. The E_H-L gaps for the PPy⁺ polaronic state are significantly lower than their corresponding neutral PPy. In the polaronic model (PPy⁺), radicals decisively reduce the crucial excitation energy, reminiscent of excess electrons (alkali metals). The performed TDOS spectral analysis further justifies the better conductive and electronic properties of polarons (PPy⁺) with increased chain lengths (conjugation). The static hyperpolarizability response (β_o) is recorded up to 1.3 × 10² au for 9Py, while for polaron 9Py⁺, it has increased up to 3.2 × 10⁴ au. The static hyperpolarizability of the 9Py⁺ polaronic state is 246 times higher than that of the corresponding neutral analogue, 9Py. It is observed that the values of β obtained at the CAM-B3LYP/6-311+G(d,p) level of theory are comparable to those obtained at the LC-BLYP and ωB97XD functionals. The β values show a strong correlation with the total hyperpolarizability (β). Furthermore, the calculated second harmonic generation (SHG) values are up to 4.0 × 10⁶ au at 532 nm, whereas electro-optic Pockel's effect (EOPE) is much more pronounced at the smaller dispersion frequency (1064 nm). The TD-DFT study reveal the red-shifted absorption maxima (λ_max) with an increased length of PPy⁺. A significant reduction in excitation energy (ΔE) is observed with increased length of PPy and PPy⁺, which also favors the improved NLO response. Hence, the studied thermally conducting polypyrrole-based polarons (PPy⁺) are new entries into NLO materials with better electrical and optical features.