Naphthyl and Thionaphthyl End‐Capped Oligothiophenes as Organic Semiconductors: Effect of Chain Length and End‐Capping Groups

Abstract

Two series of oligothiophenes (OThs), NaTn and TNTn (n = 2–6 represents the number of thiophene rings), end‐capped with naphthyl and thionaphthyl units have been synthesized by means of Stille coupling. Their thermal properties, optical properties, single crystal structures, and organic field‐effect transistor performance have been characterized. All oligomers display great thermal stability and crystallinity. The crystallographic structures of NaT2, NaT3, TNT2, and TNT3 have been determined. The crystals of NaT2 and NaT3 are monoclinic with space group P21/C, while those of TNT2 and TNT3 are triclinic and orthorhombic with space groups P and P212121, respectively. All oligomers adopt the well‐known herringbone packing‐mode in crystals with packing parameters dependent on the structure of the end‐capping units and the number of thiophene rings. The shorter intermolecular distance in NaT3 compared to NaT2 indicates that the intermolecular interaction principally increases with increasing molecular length. X‐ray diffraction and atomic force microscopy (AFM) characterization indicate that the NaTn oligomers can form films with better morphology and high molecular order than TNTn oligomers with the same number of thiophene rings. The NaTn oligomers exhibit mobilities that are much higher than those for TNTn oligomers (0.028–0.39 cm2 V–1 s–1 versus 0.010–0.055 cm2 V–1 s–1, respectively). In particular, the NaTn oligomers with n = 4–6 all show a mobility higher than 0.1 cm2 V–1 s–1. This device performance is among the best in aryl end‐capped OThs, and indicates that naphthyl is an effectual building block for designing high‐performance organic semiconducting materials.

Structural stability of naphthyl end-capped oligothiophenes in organic field-effect transistors measured by grazing-incidence X-ray diffraction in operando

“…For the remainder of the 10 h period mobility values varies less than 4% (including one standard deviation) from the average. These hole mobilities are more than an order of magnitude lower than the values reported by Geng and coworkers [21]. However, our films were deposited with the substrate at room temperature, while these authors used a substrate heated up to 80…”

“…4 shows selected layer intensity profiles deduced from the 2D GIXRD patterns. Also shown are the calculated peak positions and scattering intensities (vertical bars) for various Miller hk indices using the single crystal NaT2 structure reported in the CSD database (644331.cif from Ref [21]). In the grazing incidence geometry the detector does not observe the central region of the h00 reflections and so only the calculated intensities are shown.…”

“…Previously reported lattice parameters for single crystal NaT2 were used as an starting point for this procedure [21]. The fitting error is calculated from the diagonal of the estimated variance-covariance matrix and given as ± 1 standard deviation.…”

“…Geng and co-workers introduced naphthyl end-capped oligothiophenes with good stability and crystallinity as well as relatively high mobilities [21]. The end-capping helped to prevent side reaction, which hinders polymerization and oxidation and the high polarizability, amongst other beneficial properties for OFETs.…”

“…Among these materials, the crystal growth of 5,5'-bis(naphth-2-yl)-2,2'-bithiophene (NaT2) shows also a peculiar property as the molecules may form extremely high aspect ratio fibers, 20 nm thick and several micrometers long, when grown on muscovite mica, with potential in phototransistors [22] and waveguides [23]. Structures of these materials were characterized in the single crystal bulk but not in OFETs [21].…”