The isotope effect on charge transport had been proposed to judge the transport mechanism in organic semiconductors. By using quantum nuclear tunneling model, we found that isotopic substitution could reduce mobility. For deeply understanding the impacts of the isotopic substitution position, substitution number and even molecular structure on the isotope effect, we take 2,2′-bithiophene and its dihexyl substitutions as examples to study the deuteration effect on hole transport. For deuterated–bithiophene, the isotope effect is linearly increasing with deuteration number. However, when the number is identical, deuteration on 5(5′)-position of thiophene will lead to stronger isotope effect than 3(3′)- or 4(4′)-position, since the reorganization energy contributed by 5-position hydrogen atoms is larger. For di(n-hexyl)-bithiophene isomers, 5,5′-dihexyl substitution also exhibits the strongest isotope effect after hexyl-deuteration or all-deuteration, due to the larger reorganization energy contributed by hexyl group in 5(5′)-position rather than 3(3′)- and 4(4′)-positions. Our calculation indicates that for identical system, the isotope effect is closely related to the number and position of isotopic atoms, while for isomers, the isotope effect is also related to the molecular configuration, such as the position of side chain substitution.
Theoretical Chemistry Accounts – Springer Journals
Published: Feb 21, 2018
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