Ultrafast Singlet Fission and Efficient Carrier Transport in a Lamellar Assembly of Bis[(trialkoxyphenyl)ethynyl]pentacene


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Pentacene derivatives carrying 3,4,5-trialkoxyphenylethynyl groups with different alkyl chain lengths (OCnH2n+1; n = 6, 12, 16) at the 6- and 13-positions of pentacene (denoted as PcCn) were newly synthesized to examine chain-length effects on self-assembly, optical, and electronic properties. Upon fast injection of a cyclohexane solution of PcCn into isopropyl alcohol, PcC6 and PcC12 underwent controlled self-assembly to give rise to spherical and fibrous nano-objects, respectively, while this self-assembly procedure could not be applied to PcC16 because of its poor solubility. Powder X-ray diffraction (PXRD) analysis showed that the nanofiber of PcC12 consists of a lamellar structure, where an effective overlap between the pentacene chromophores occur, resulting in significantly broadened and red-shifted absorption spectral features, compared with those observed for molecularly dispersed PcC12 in cyclohexane. In contrast, based on PXRD analysis, the fundamental assembly pattern of the nanoparticle of PcC6 was characterized by a two-dimensional hexagonal structure. Although a shorter core-to-core separation of 3.35 angstrom was observed for the nanoparticle, judging from the absorption properties, the pentacene chromophores involved in the PcC6 assembly do not effectively interact with each other. Interestingly, femtosecond transient absorption spectroscopy indicated that PcC6 in the spherical assembly tends to undergo deactivation of the singlet excited state, while PcC12 in the fibrous assembly shows ultrafast intermolecular singlet fission (apparent rate constant k(SF) >= 1012 s(-1)). A field-effect transistor (FET) fabricated using the nanofiber of PcC12 and an ion gel as the active layer and gate materials, respectively, exhibited a p-type behavior with a carrier mobility as high as 2.4 x 10(-4) cm(2) V-1 s(-1), despite the fact that fibrous materials, where nanoscale conducting domains discontinuously aggregate as an entity, are not favorable for FET applications. Through this study, we demonstrate that a proper design of the side-chain length enables a pentacene chromophore to assemble into an arrangement beneficial for exhibiting optical and electronic properties.
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