Substrate steered crystallization of naphthyl end-capped oligothiophenes into nanofibers: the influence of methoxy-functionalization

Naphthyl end-capped oligothiophenes are a class of materials well suited for high-performance organics based devices. The formation of nanofibers on muscovite mica from 2,5-bis(naphth-2-yl) thiophene (NaT), 5,5 '-bis(naphth-2-yl)-2,2 '-bithiophene (NaT2), and 5,5"-bis(naphth-2-yl)-2,2 ': 5 ',2"-terthiophene (NaT3) as well as of the methoxy-functionalized variants MONaT, MONaT2, and MONaT3 is investigated via atomic force microscopy, X-ray diffraction, polarized fluorescence microscopy, and fluorescence spectroscopy. From polarized fluorescence microscopy spatially resolved molecular orientations are deduced revealing a profound anisotropy. Fibers from lying molecules grow along distinct substrate directions. Methoxy-functionalization substantially increases the crystallization into aligned fibers. In air Ostwald ripening is observed. The morphological variations of the aggregates result in specific optical signatures, disclosed by temperature dependent and spatially resolved fluorescence spectra.