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Synthesis and Photovoltaic Properties of Regioregular Head-to-Head Substituted Thiophene Hexadecamers

We describe the expedient synthesis of regioregular thiophene hexadecamers head-to-head (hh) substituted with hexyl and hexylthio grous. The synthesis was carried out by means of a sequence of ultrasound-assisted selective monobrominations and microwave-assisted Suzuki reactions using 4,4,5,5-tetramethyl-1,3,2-dioxaborolane in THF:water. The hexadecamers, which are very soluble in organic solvents, were investigated in solution and thin film by a variety of techniques (UV, PL, CV, X-ray diffraction, FET charge mobility, SKFM) with the aim of elucidating the effect of the sulfur spacer on morphology and functional properties. We show that the sulfur spacer compensates for the decrease in π–π conjugation caused by the hh regiochemistry and that the λmax value and redox potentials of the S-alkyl-substituted hexadecamer are similar to those of head-to-tail substituted poly(3-hexylthiophene). Measurements in field effect transistor devices showed that the alkylthio-substituted hexadecamer is a p-type semicondu...