Exciton Delocalization in H₂OBPc_1–xMOBPc_x (M = Co, Cu, Ni, Mn) Crystalline Thin-Film Organic Alloys

Abstract

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.

Transition Dipole Moments of One-Dimensional Excitons in Soluble Phthalocyanine Thin Films

“…Figure c shows that the PL main peak intensity increases with the decreasing temperature. Similar results could be found in our previous research, which involves H 2 -OBPc, its “cousin” derivative H 2 -PcOC 8 , , and organic alloy compounds . As discussed earlier, no obvious transition dipole rotation is observed with the increasing temperature.…”

“…Our preliminary confocal Raman study on the PW thin films indicate that all these vibrational modes, including the low-energy phonons, are clearly present in these films (see Figure S8). At low temperature (below 175 K), the exciton coupling with localized molecular vibrations is suppressed, while the low-energy phonon coupling assists the delocalized exciton-polaron formation and results in the spreading of the wavefunction over the π-stacked molecular chain. ,,, Manning et al estimate the spatial extent of the delocalized exciton along the stacking axis in H 2 -OBPc system to be at least 10 lattice units. Such delocalization of the lowest-energy exciton could also be found in pentacene systems .…”

“…An ex situ polarized microscopic image of the same film (Figure b) shows clear color contrast between nearby grains. This is the result of different orientations of molecular stacking axes in adjacent grains, where H 2 -OBPc molecules are standing “edge-on” the substrate to form quasi-one-dimension chains (Figure d; H 2 -OBPc molecular structure is shown in Figure e). ,,, More images from pen-written films, deposited under different conditions along with more information about the pen-writing technique, may be found in ref . A scanning electron microscopy (SEM) image (Figure c) shows that the grains consist of straight and flat stripes oriented along the writing direction.…”

“…Phthalocyanine (Pc) and its derivatives are widely utilized in different applications because of their easy synthesis, high stability, and tunable π–π* transition in the visible/near-IR region. , Octabutoxy phthalocyanine (H 2 -OBPc) is a soluble derivative of the Pc that has eight butoxy electron-donating groups at the nonperipheral benzo positions. It features highly directional intermolecular interactions in the crystalline phase due to the nearest-neighbor π - orbital overlap and thus has self-assembly properties that lead to long-range molecular ordering and a one-dimensional character for carrier or exciton transport. , Previous experiments report that H 2 -OBPc crystalline thin films with grain sizes in the millimeter scale are obtained with solution-based deposition methods. − These ordered thin films with large grain sizes uniquely offer the opportunity to spatially explore optical and excitonic properties within a single crystalline grain free from the influence of grain boundaries or static disorder, enabling the usage of a variety of diffraction-limited optical techniques at low temperatures in a variety of experimental geometries. Theoretical studies , indicate that crystalline phthalocyanines are characterized by strong short-range intermolecular exchange interactions comparable to the reorganization energy for exciton energy transfer.…”

Nature of Optical Excitations in Porphyrin Crystals: A Joint Experimental and Theoretical Study

“…As mentioned above, in addition to the four a bands, these excitons also involve other states associated with bands in the b–e blocks, which show particularly strong dispersion along the π-stacking direction. − Other theoretical studies ,, have reported that in some organic molecular crystals, charge-transfer excitons (with charge delocalized on the first neighboring molecules) can be present and can be involved in optical and transport processes; nonetheless, those crystals are made of very different, rodlike molecules such as oligoacenes and display a completely different crystal packing. More recently, highly delocalized excitons are demonstrated by experimental observations in phthalocyanines, a class of molecules similar to porphyrins, where delocalization over >10 molecules along the π-stacking direction is observed in highly crystalline samples with a low degree of compositional disorder, like ours, thus lending support to the results discussed here.…”