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J. Phys. Chem. B 1997, 101, 11004-11006
Self-Organization in Thin Films of Liquid Crystalline Perylene Diimides Russell A. Cormier†,‡ and Brian A. Gregg*,† National Renewable Energy Laboratory, 1617 Cole BouleVard, Golden, Colorado 80401-3393, and Department of Chemistry, Metropolitan State College of DenVer, DenVer, Colorado 80204 ReceiVed: October 1, 1997; In Final Form: October 28, 1997X
A family of liquid crystalline organic semiconductors based on perylene-3,4,9,10-tetracarboxyldiimide is introduced. The thermal transitions, self-organizing behavior, and change in photophysical properties upon self-organization of one member of the family are described. Red, polycrystalline thin films of spin-coated N,N′-bis[3-[1,3-bis[2-(2-methoxyethoxy)ethoxy]-2-propoxy]propyl] perylene-3,4,9,10-tetracarboxyldiimide spontaneously form a highly crystalline black phase after about 24 h. The quantum yield for fluorescence from the black phase is enhanced 7-fold, and the width (fwhm) of the emission band is decreased by more than a factor of 2, with respect to the red phase. The self-organization process appears to decrease spontaneously both the energetic disorder and the density of exciton quenching sites in the film.
Introduction Liquid crystals (LCs) possess the remarkable and useful ability to self-organize spontaneously into highly ordered structures. At present, the technologically important liquid crystals are electrically insulating molecules used primarily for flat panel displays. However, the recent advent of liquid crystalline semiconductors1-4 significantly expands the realm of potential uses for this class of materials. Electronic properties are often highly correlated with the crystallinity of a material, but single crystals are generally difficult to prepare, especially in the form of thin films on an electrode, one of the most desirable structures.5 Liquid crystals, however, are ideally suited for preparing thin, crystalline films. The tendency of LCs to form ordered structures spontaneously also means that they tend to eliminate structural (and electronic) defects spontaneously. This self-ordering/self-healing property is a central feature of biological systems and may ultimately become a common feature of man-made structures. The use of LCs as active components in electroluminescent devices,6 molecular wires and fibers7,8 and photorefractive materials9,10 has been studied in the past few years. Liquid crystals have become important model systems for the study of charge carrier dynamics11-14 and energy transfer processes.15,16 The highest charge carrier mobility yet observed in an organic material other than a single crystal was measured in an LC triphenylene derivative.17 The photovoltaic properties of liquid crystal porphyrins have been investigated,18,19 and the same materials have been proposed as a high-information-density optical storage medium.20,21 Perylene diimides, along with the parent compound, perylene3,4,9,10-tetracarboxylic dianhydride, represent one of the most widely studied classes of organic semiconductors with possible applications in electrophotography, electroluminescent displays and photovoltaic cells.22-29 Perylene diimides are inexpensive and robust materials (the parent anhydride is a common automobile paint pigment), and they are highly fluorescent and exhibit singlet energy transfer over unusually long distances.30 Several LC derivatives of the basic perylene ring system were recently reported,31,32 but, to our knowledge, no reports of LC derivatives of the more technologically useful and synthetically tractable perylene diimide core have appeared. We report here the first examples of liquid crystalline perylene diimides and * Corresponding author. email:
[email protected] † NREL. ‡ Metropolitan State College. X Abstract published in AdVance ACS Abstracts, December 1, 1997.
S1089-5647(97)03206-9 CCC: $14.00
describe some of the thermal, photophysical, and self-organizing properties of one member of the family. Results and Discussion The structures of seven recently synthesized perylene diimides are presented in Scheme 1. Compounds 1a-1c are based on the well-known perylenebis(phenethylimide) structure (PPEI, R ) H), while compounds 2a-2d have purely aliphatic side chains that are linear (2a and 2b) or branched (2c and 2d). All compounds shown in Scheme 1 exhibit liquid crystalline phases as evidenced by differential scanning calorimetry (DSC) as well as by observation of the optical textures in a polarizing microscope equipped with a hot stage. The detailed synthesis and characterization of these materials will be reported separately.33 As one example of the characteristics of this new family of materials, we discuss here some of the properties of compound 2d. A liquid crystal at room temperature, 2d is the least viscous and lowest melting of the LC perylene diimides reported here. Scheme 2 shows the complete structure of 2d. Figure 1 shows the DSC traces of 2d on the first and second heating scans. Observations of films between crossed polarizers showed unambiguously that 2d is a liquid crystal below its clearing point (LC f isotropic liquid) of ∼55 °C. Around 45 °C, 2d is a relatively low-viscosity, ordered fluid; its viscosity increases rapidly on cooling. Although the transition temperatures seen in the DSC do not change substantially, the enthalpies of transition (areas under the curves) and the shapes of the transitions change markedly from the first heating to the second. No further changes were observed on subsequent scans. Similar behavior, though less pronounced, was observed in all the compounds shown in Scheme 1. The change between first and subsequent scans may be explained as follows. The first heating results in melting of the structures (crystals) formed by recrystallization from a solvent during the final purification step. In contrast, the second and subsequent heatings result in melting the structures formed by cooling a viscous, solvent-free neat liquid. Since the structures of these phases may not be identical (polymorphism is very common in molecular crystals24), the character of the thermal transitions may change after the first heating. Furthermore, even if the structure derived from the initial solvent recrystallization represents the global energy minimum, the viscosity of the molten neat phase may be so high on cooling that the molecules cannot reach this equilibrium configuration during the time scale of a DSC experiment. The slow (∼24 h) crystallization of spin-coated thin films of 2d © 1997 American Chemical Society
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J. Phys. Chem. B, Vol. 101, No. 51, 1997 11005
SCHEME 1: Liquid Crystalline Perylene Diimides
Figure 1. Differential scanning calorimeter traces of the first and second heatings of compound 2d. All subsequent scans closely matched the second scan. An approximate base line for each scan is included. Transition temperatures (enthalpies) of the two transitions on first heating were -43 °C (0.4 kcal/mol) and 55° (15.4); on subsequent heatings: -38° (2.0) and 52°-59° (3.4). Scan rate was 10 °C/min.
SCHEME 2: N,N′-Bis[3-[1,3-bis[2-(2-methoxyethoxy)ethoxy]-2-propoxy]propyl]perylene-3,4,9,10-tetracarboxyldiimide (PPMEEM, 2d)
described below lends credence to this explanation, as does the observation that when 2d is cooled rapidly on the microscope hot stage, it freezes into a visually isotropic glassy state that crystallizes slowly over a period of days. Figure 2 shows a photomicrograph taken between crossed polarizers of an approximately 200 nm thick film of 2d spincoated from THF solution onto an untreated glass microscope slide. Upon spin-coating, the film originally consisted of small (5-20 µm), randomly oriented crystallites. The photomicro-
Figure 2. Photomicrograph of a film of 2d viewed between crossed polarizers. The lower portion shows a highly crystalline (black) region spontaneously growing into the less ordered (red) region initially formed upon spin-coating. The contrast derives from the birefringence, not the color. Area of the micrograph is 0.8 × 1.2 mm2.
graph was taken 24 h after the initial film preparation and shows the spontaneous growth of a highly ordered structure of much larger ribbonlike crystals (bottom) growing into the more disordered phase initially formed (top). Two days after initial preparation, the entire film consisted of the more highly crystalline structure. The (unpolarized) absorption and emission spectra of the two parts of the film shown in Figure 2 are presented in Figure 3. The more disordered part of the film (top, Figure 2) appeared red, and its absorption spectrum had a maximum at 468 nm. As the film spontaneously crystallized into the more highly ordered structure (bottom, Figure 2), it turned black, with absorption maxima at 462 and 602 nm. The absorption spectrum of the black phase is similar to that of highly ordered films of perylenebis(phenethylimide), PPEI.25,28 The quantum yield for fluorescence is dramatically different for the two parts of the film (Figure 3). When illuminated at 518 nm, where the two crystal forms have equal absorbance, the integrated fluorescence of the more highly ordered part of the film (bottom, Figure 2) was more that seven times greater
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Letters disorder in the film and a decrease in the density of exciton quenching sites. The tendency toward spontaneous improvement in the photophysical properties of these new liquid crystalline semiconductors makes them promising compounds for future studies. Acknowledgment. We are grateful to the U.S. Department of Energy, Office of Energy Research, Division of Basic Energy Sciences, Chemical Sciences Division, for supporting this research. References and Notes
Figure 3. Absorption and emission spectra of the spin-coated film of 2d displayed in Figure 2 showing the difference between the more disordered part of the film (dashed lines, top of Figure 2) and the selforganized, highly crystalline part of the film (solid lines, bottom of Figure 2). For the emission spectra, excitation was at the isosbestic point, 518 nm.
than that of the less ordered structure (top, Figure 2). Furthermore, the full width at half-maximum (fwhm) of the emission band, which is a measure of the distribution of electronic energy levels in the film, decreased by over a factor of 2 (344 to 154 meV) during the transition from the red to the black phase. Figures 2 and 3 show that the formation of large, partially oriented crystallites occurs spontaneously in films of 2d and suggest that this transformation is accompanied by a decrease in energetic disorder in the film (decrease in width of the emission band) and a decrease in the density of exciton quenching sites (increase in the fluorescence intensity). The spontaneous improvement in the structural and photophysical properties of the LC perylene film indicates that the highly ordered, highly emissive state is more thermodynamically stable than the originally formed state and that there is a relatively low activation energy pathway leading to this preferred state. Similar structural and spectroscopic changes occur in thermally evaporated films of PPEI but require exposure to a solvent vapor such as methylene chloride to effect the transformation.30,34 The solvent vapor apparently provides the molecules with enough conformational mobility to rearrange into a low-energy configuration (the black phase) and, eventually, provides enough translational mobility to facilitate the growth of micrometer-size crystallites. Films of 2d spontaneously crystallize into structures hundreds of times larger than those occurring in PPEI films without the necessity of solvent vapor treatment. Perhaps more importantly, films of 2d appear to maintain an approximately uniform physical contact to the substrate, in contrast to PPEI films that lose much of their physical and electrical contact to the substrates upon solvent vapor annealing.30,34 A more complete description of the photophysical and photoelectrical properties of these new liquid crystalline semiconductors will be provided in future publications. Conclusions A family of seven poly(oxyethylene) derivatives of perylene diimides has been synthesized and found to have liquid crystalline phases. The characteristics of one of the compounds, 2d, were described here. It is a liquid crystal at room temperature and melts to an isotropic liquid at 59 °C. Spincoated films of 2d on untreated glass slides spontaneously selforganize into long, ribbonlike crystals. Both absorption and fluorescence measurements indicate that this spontaneous transformation is accompanied by a decrease in the energetic
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