Third‐harmonic generation from regio‐regular and regio‐irregular poly(3‐dodecylthiophenes). Dependence of χ⁽³⁾ on conjugation length

Abstract

Conjugated polymers such as polyacetylenes and polythiophenes have spurred considerable research activity for several years because of their potential applications in, for example, photonic devices''"] and highly conducting organic thin films.14] In order to optimize the electrical and optical properties of such materials, control of both the microscopic and the macroscopic solid-state structures is important, since they collectively define the electronic structure. A clear demonstration of the relationship between structural regularity and improved electrical and optical properties is found in Naarmann's defect-free polya~ety1ene.l'~ Recently, enhanced electrical conductivity in regio-regular poly(3-alkylthiophenes) has also been demon~trated["~I ( Fig. 1).In the present communication the effect of regio-regularity in polythiophenes on the electronic contribution to the third order nonlinear optical susceptibility, ~(~) (-3w;w,w,o), is addressed. The experimental strategy has been to measure the nonlinear susceptibility x (~) of regio-regular poly(3-dodecylthiophene) (RR-PDDT) thin films by third harmonic generation (THG) and compare it with similar THG measurements on regio-irregular (RIR-PDDT i.e.

Ultrafast coherent vibronic oscillations in regioregular poly(3‐alkylthiophene)

“…One thus expects that conjugated polymers, with quite a few double bonds, exhibit a large value of χ (3) . Hence, conjugated polymers have attracted a great interest as one of promising organic nonlinear optical materials [2].…”

“…1 Introduction The magnitude of the third-order nonlinear susceptibility χ (3) of π-conjugated molecules is known to increase with the number of double bonds or the chain length [1]. One thus expects that conjugated polymers, with quite a few double bonds, exhibit a large value of χ (3) .…”

“…Among some classes of conjugated polymers, poly(3-alkylthiophene) (P3AT) is one of the most studied polymers owing to its potential applicability as organic semiconducting materials and its good solubility to several familiar solvents that enables the film fabrication by wet processes. Head-to-tail coupled, regioregular (RR) P3AT is particularly noticeable, because it has been demonstrated that the magnitude of χ (3) in P3AT is enhanced by arranging its alkyl sidechains in a head-to-tailed way [3][4][5].…”

“…In this article, the very early process of χ (3) (ω; ω, ω, −ω) signals in RR type poly(3-hexylthiophene) (P3HT) (Fig. 1) is addressed by DFWM experiments using sub-20 fs pulses at the resonance condition.…”

“…Among some characters of χ (3) reported on RR-P3AT, we focus on χ (3) (ω; ω, ω, −ω) determined using a degenerate four-wave mixing (DFWM) method. Previously, a large value of χ (3) (ω; ω, ω, −ω) whose magnitude reaches as much as 10 7 esu at the resonance condition was reported for the Langmuir-Blodgett (LB) film of RR-P3AT [6].…”