The Importance of Excellent π−π Interactions in Poly(thiophene)s To Reach a High Third-Order Nonlinear Optical Response

Poly(thiophene)s have an inherently large third-order nonlinear optical (TONO) response, but applications are not straightforward due to unoptimized materials. Therefore, several structure−property relationships (molar mass, branching, regioregularity) are investigated to unravel which structural modifications give the highest TONO response. Poly(3- hexylthiophene) with different molar masses, poly[3-(2-ethylhexyl)- thiophene] with different molar masses, and random copolymers with different degrees of regioregularity are synthesized and measured by UV−vis spectroscopy and the third harmonic scattering technique. Every structural modification that leads to an increase in π−π interactions in poly(thiophene)s leads to an increase in the TONO response of the material. Therefore, a material with a high molar mass, an unbranched alkyl side chain, and a high regioregularity degree is preferably tested as a promising TONO material.