Polaron Formation in Conducting Polymers: A Novel Approach to Designing Materials with a Larger NLO Response
Atazaz Ahsin, Iqra Ejaz, Sehrish Sarfaraz, Khurshid Ayub, Haitao Ma
Abstract
High Resolution Image Download MS PowerPoint Slide 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 × 10 2 au for 9Py, while for polaron 9Py +, it has increased up to 3.2 × 10 4 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 o 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 vec values show a strong correlation with the total hyperpolarizability ( β o ) . Furthermore, the calculated second harmonic generation (SHG) values are up to 4.0 × 10 6 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 (Δ E ) 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.