We present spatially-, temporally- and polarization-resolved dual photoluminescence/linear dichroism microscopy experiments that investigate the correlation between long-range order and the nature of exciton states in solution-processed phthalocyanine thin films. The influence of grain boundaries and disorder is absent in these films because typical grain sizes are 3 orders of magnitude larger than focused excitation beam diameters. These experiments reveal the existence of a delocalized singlet exciton, polarized along the high mobility axis in this quasi-1D electronic system. The strong delocalized π orbitals overlap, controlled by the molecular stacking along the high mobility axis, is responsible for breaking the radiative recombination selection rules. Using our linear dichroism scanning microscopy setup, we further established that a rotation of molecules (i.e., a structural phase transition) that occurs above 100 K prevents the observation of this exciton at room temperature.

Exploration of optical properties of organic crystalline semiconductors thin films is challenging due to submicron grain sizes and the presence of numerous structural defects, disorder and grain boundaries. Here we report on the results of combined linear dichroism (LD)/ polarization-resolved photoluminescence (PL) scanning microscopy experiments that simultaneously probe the excitonic radiative recombination and the molecular ordering in solution-processed metal-free phthalocyanine crystalline thin films with macroscopic grain sizes. LD/PL images reveal the relative orientation of the singlet exciton transition dipoles at the grain boundaries and the presence of a localized electronic state that acts like a barrier for exciton diffusion across the grain boundary. We also show how this energy barrier can be entirely eliminated through the optimization of deposition parameters that results in films with large grain sizes and small-angle boundaries. These studies open an avenue for exploring the influence of long-range order on exciton diffusion and carrier transport.

Novel solution-processing deposition techniques in tandem with chemical synthesis design of small-molecule soluble derivatives represent a viable avenue for exploring organic analogues of semiconductor alloyed systems, where excitonic properties are tunable through alloy concentration. Here these properties are explored using absorption, grazing incidence X-ray diffraction (GIXRD), and temperature-dependent/time-resolved photoluminescence spectroscopy (TRPL) in a series of crystalline thin film alloys of metal-free (H2OBPc) and metal (MOBPc) octabutoxy-phthalocyanine, H2OBPc1–x MOBPc x (M = Co, Cu, Ni, or Mn) where 0.5 ≥ x ≥ 0.001. Films are fabricated using a solution-processed, novel hollow pen-writing technique that results in millimeter-sized crystalline grains with long-range macroscopic order for all concentrations. The spectroscopy experiments produce two important results that offer great insight into the fundamental quantum mechanics of delocalized excitons in small-molecule semiconductors. First, they indicate that the delocalization of bandgap excitons previously observed in pure H2OBPc films extends over approximately ten molecules, and second they reveal that the presence of the MOBPc molecule inhibits the formation of this delocalized exciton for x > 0.09. Furthermore, the MOBPc molecule introduces a highly localized state with a strong photoluminescence signature.

The origins of spin exchange in crystalline thin films of Copper Octabutoxy Phthalocyanine (Cu-OBPc) are investigated using Magnetic Circular Dichroism (MCD) spectroscopy. These studies are made possible by a solution deposition technique which produces highly ordered films with macroscopic grain sizes suitable for optical studies. For temperatures lower than 2 K, the contribution of a specific state in the valence band manifold originating from the hybridized lone pair in nitrogen orbitals of the Phthalocyanine ring, bears the Brillouin-like signature of an exchange interaction with the localized d-shell Cu spins. A comprehensive MCD spectral analysis coupled with a molecular field model of a σπ − d exchange analogous to sp-d interactions in Diluted Magnetic Semiconductors (DMS) renders an enhanced Zeeman splitting and a modified g-factor of −4 for the electrons that mediate the interaction. These studies define an experimental tool for identifying electronic states involved in spin-dependent exchange interactions in organic materials.

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