We report on polarization-resolved absorption and photoluminescence spectroscopy experiments that investigate the spatial orientation of excitonic transition dipoles at temperatures ranging from 4 K to room temperature in crystalline phthalocyanine thin films prepared with solution-based deposition methods. Octabutoxy phthalocyanines are quasi-1D systems with highly directional intermolecular interactions along a preferred crystalline axis. Experiments reveal the existence of redshifted delocalized bulk band gap exciton states at temperatures below 175 K. These states are the result of the strong π−π short-range coupling and long-range Coulomb coupling between nearest-neighbor molecules along the stacking axis. They are characterized by linearly polarized, nondegenerate dipoles that largely obey the Davydov selection rules. Photoluminescence studies reveal that these excitons couple to lattice and molecular vibrations, forming delocalized exciton-polarons at temperature below 175 K. Finally, by changing the incident light wave vector orientation, we find additional circularly and elliptically polarized states that most likely result from the mixing of HOMO-n (n > 1) orbitals with aza-nitrogen orbitals.
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.
The formation of delocalized excitonic states in organic semiconductors is highly desirable because it leads to efficient energy transport in devices. We investigate the potential of uniaxial strain as a "tuning dial" for delocalized excitons (i.e., exciton− polarons) in crystalline thin films of soluble octabutoxy phthalocyanine. Absorption and photoluminescence spectra confirm the formation of delocalized excitonic states in the presence of tensile strain, accompanied by a red shift of low-frequency vibration modes (<100 cm −1 ) in Raman spectroscopy, which are likely responsible for the delocalized exciton formation. Remarkably, an 80% enhancement in photoluminescence intensity and a 30 nm red shift in peak wavelength are observed for a tensile strain of 4.9%, which is equivalent to a temperature reduction approximately by 100 K below room temperature. These results show promise that strain engineering can efficiently modify the exciton−phonon coupling in octabutoxy phthalocyanine crystalline thin films toward enabling delocalization at room temperature.
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