Alkyl Substituent Effects on Molecular Packing and Optoelectronic Properties of 2,3-Dialkyltetracenes

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A series of 2,3-dialkyltetracenes (alkyl = propyl to hexyl) were readily prepared from 2,3-dialkyl-5,12-tetracenequinone by a one-pot sequence involving hydride reduction with sodium borohydride and subsequent addition of tin(II) chloride in concd hydrochloric acid. X-ray diffraction patterns of their recrystallized powder samples demonstrated their crystalline nature. X-ray crystallographic analysis of the butyl derivative revealed that there were two disordered sets of butyl groups with 50% occupancies, and that in the molecular structure, one butyl group was coplanar with the tetracene core, while the other butyl group was perpendicular to the tetracene plane. Further, the crystal structure adopted a repeating bilayer structure of herringbone-packed tetracene cores and disordered alkyl groups. The conformational analysis demonstrated that there were a number of stable conformers, which would account for the occurrence of alkyl disorders. Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements of 2,3-dialkyltetracenes exhibited an increase in the product of charge carrier mobility (3.0 × 10−5 to 3.5 × 10−4 cm2 V−1 s−1) and its generation efficiency with increase in the length of the alkyl side chains, suggesting that a longer alkyl side chain plays an important role in the degree of transfer integral and in the suppression of charge recombination or exciton quenching.
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