Tailoring 1D Exciton Coherence in Crystalline Small Molecules Thin Films

At the University of Oklahoma, the Furis group focuses on exploring excitonic states in solution-processed phthalocyanine thin films using Linear Dichroism/ Time-Resolved Photoluminescence Laser Scanning Microscopy and Transient Absorption spectroscopy. 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 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.