Millimeter-long organic fiber arrays of intramolecular charge transfer merocyanine HB194 dye were prepared by evaporation-induced self-assembled method. X-ray diffraction spectroscopy indicated individual fibers are millimeter-sized HB194 single crystals. The elimination of defects and structural disorder enabled photoluminescence microscopy studies that revealed the intramolecular charge transfer (CT), bandgap excitonic state is long-lived and remains largely localized in the absence of π-orbital stacking in the crystalline structure. These nanosecond lifetimes explain the observation of a photoconductivity response upon irradiation with a 633 nm laser due to dissociation of the delocalized CT exciton to free carriers. At the same time, the photo response was increased 4.5 times by coating of the HB194 fiber array with polyvinyl alcohol. This increase is attributed to the larger dielectric field around the fibers that further facilitates the band-gap (CT) exciton dissociation.
Intermolecular interactions in small molecule crystals play an important role in the exciton delocalization process and can lead to robust exciton coherence. The spatial extension of this delocalization, and the exciton coherence length, significantly influence practical applications that rely on excitonic radiative recombination, dissociation or electron transport. The porphyrins and phthalocyanines derivatives family represent the ideal system for a comprehensive study of excitonic coherence because they represent an iconic example of a one-dimensional J -like aggregate system. At the University of Vermont, the Furis group focuses on exploring excitonic states in solution-processed phthalocyanine thin films using a dual Linear Dichroism/Photoluminescence Laser Scanning Microscope developed in -house. These studies are among the first to directly probe the supramolecular structure-property relationships in these electronic materials. The experiments reported here investigate the effect of alkyl substitutions and dynamic disorder (i.e. exciton-phonon coupling) on exciton coherence in solution-processed crystalline thin films of porphyrin and phthalocyanine derivatives. Specifically, the largest exciton coherence length (approx.15 nm) was measured for an octabutoxy derivative, where the saddle shape of the molecules and the crystalline packing result in weaker coupling to the acoustic phonons (low energy) modes. Enhancing the size of the macrocycle ring to a naphthalocyanine also results in long coherence lengths despite larger intermolecular nearest-neighbor (NN) distances. Most importantly, this study brings evidence that excitonic coherence can engineered in a systematic manner through chemical and physical routes. For example, applying uniaxial strain to the same thin films deposited on Kapton substrates results in continuous bandgap energy tuning, in analogy to inorganic semiconductors.
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