J. Phys. Chem. B 2002, 106, 767-772
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Electronic Structure of Perylene Pigments as Viewed from the Crystal Structure and Excitonic Interactions J. Mizuguchi* and K. Tojo Department of Applied Physics, Graduate School of Engineering, Yokohama National UniVersity, 79-5 Tokiwadai, Hodogaya-ku, 240-8501 Yokohama, Japan ReceiVed: July 26, 2001; In Final Form: October 30, 2001
Perylene derivatives are industrially important pigments used not only as colorants, but also as materials for optical disks and photoconductors for electrophotographic photoreceptors. These pigments cover a variety of shades in the solid state from vivid red, via maroon to black, although their molecular absorption spectra are quite similar in solution. The color generation mechanism has therefore been investigated in three representative pigments (PR149, PR179, and PB31) with special attention to the crystal structure and intermolecular interactions. The color in the solid state is mostly determined by two absorption bands in the visible region: one is of molecular character and is due to nonresonance interactions, whereas the other is caused by interactions between transition dipoles. The red color (PR149) appears as a result of insignificant resonance interactions, whereas the colors of maroon (PR179) and black (PB31) are characterized by medium and strong interactions, respectively.
1. Introduction Perylene derivatives are well-known organic pigments on the market,1 and have attracted attention as materials useful for electrophotographic photoreceptors,2 photovoltaic elements3 as well as optical disks.4 Perylene pigments cover a variety of shades in the solid-state from vivid red (PR149), via maroon (PR179) to black (PB31) as shown in Figure 1,1 although no significant difference is recognized in solution spectra. These facts clearly indicate that the substituents in perylene pigments are not involved in the choromophore. Instead, the intermolecular interactions are directly responsible for the various shades in the solid state. Because the electronic characterization is essential to a proper understanding of the color in the solid state as well as to photonic applications, an attempt was made to clarify the electronic structure of three representative perylenepigments from the standpoint of the crystal structure and intermolecular interactions. Our characterization is based on an exciton model developed for the interpretation of the black color of PB31, as described below.4 A number of investigations have been carried out on the mechanism of various colors in perylene pigments.4-10 Graser and Ha¨dicke and their co-workers considered that the degree of “area overlap” of successive perylenes determines simply the shift of the absorption maximum.5-9 Kazmaier and Hoffmann proposed a new concept of the quantum interference effect in order to explain the spectral shift.10 Alternatively, we have pointed out the importance of resonance interactions between transition dipoles and interpreted the black color of PB31 in terms of these interactions.4 The color generation mechanism can be summarized as follows. The solution spectrum and solid-state spectra in evaporated films are shown in Figure 2(a). In solution, there is only one electronic transition coupled with vibrational transitions, giving a progression of absorption bands as designated by 0-0, 0-1, 0-2, and 0-3. To our surprise, the color is not black but vivid red as evaporated. However, the color changes from red to black when the evaporated film is heated at 100 °C for several seconds
Figure 1. Molecular structure of perylene derivatives.
to minutes as shown in Figure 2(b). The absorption spectrum in the red phase is quite similar to the one in solution, and oneto-one correspondence in absorption bands is possible. On the other hand, during the heat treatment, an additional absorption shoulder appears around 610 nm in 30-60 s that grows up to an absorption band to give a black color. This additional band is clearly attributed to the band due to intermolecular interactions because this band is not explicable by the absorption bands in solution (Figure 2(a)). On the basis of the crystal structure and intermolecular interactions, we have elucidated the black color as arising from two absorption bands: the shorter-wavelength band is of molecular character (i.e., due to nonresonance interactions) and the longer-wavelength-band is caused by resonance interactions between transition dipoles.4 2. Experimental Section 2.1. Materials. PR149 and PR179 were obtained from BASF and purified by vacuum sublimation using a two-zone furnace.11 The single crystals were grown from the vapor phase using the same sublimation equipment. PR149 was sublimed at about 740 K. After 60 h of vapor growth, a number of prism crystals were obtained. In a similar way, single crystals of PR179 were
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768 J. Phys. Chem. B, Vol. 106, No. 4, 2002
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Figure 2. UV-Vis absorption spectra of PB31: (a) solution spectrum and solid-state spectrum as evaporated and (b) spectral changes in evaporated films as a function of heat-treatment time at 100 °C. The film thickness is about 1000 Å.
TABLE 1: Crystallographic Parameters for PR149 and PR179 molecular formula formula weight molecular symmetry crystal system space group Z a (Å) b (Å) c (Å) β (°) volume
PR14913
PR17914
C40H26N2O4 598.66 Ci monoclinic P21/c 2 17.0313(5) 4.869(3) 17.096(5) 93.40(2) 1413(1)
C26H14N2O4 418.39 Ci monoclinic P21/c 2 3.8648(4) 15.5654(17) 14.6603(16) 97.414(2) 874.55(16)
obtained at a sublimation temperature of around 620 K. Both single crystals were used for structure analysis as well as for measurements of polarized reflection spectra. Evaporated thin films of PR149 and PR 179 were prepared onto plain glass slides (film thickness: about 1000 Å) using conventional vacuum equipment (Tokyo Vacuum Co. Ltd.: model EG240). 2.2. Measurements. UV-Vis spectra were recorded on a UV-2400PC spectrophotometer (Shimadzu). The temperature dependence of the absorption spectra in evaporated films were measured in the range between 20 and 300 K on a UV-2400PC spectrophotometer in combination with a cryostat from Iwatani Gas Co. Ltd. (model: CRT-105-OP). Diffuse reflectance spectra for powdered pigments were measured on a UV-2400PC spectrophotometer together with an integrating sphere attachment (ISR-240A from Shimadzu). Measurements for polarized reflection spectra were made on single crystals by means of a UMSP80 microscope-spectrophotometer (Carl Zeiss). An Epiplan Pol (×8) objective was used together with a Nicol-type polarizer. Reflectivities were corrected relative to the reflection standard of silicon carbide. 2.3. Molecular Orbital (MO) Calculation. The INDO/S program used for spectroscopic calculations is part of the ZINDO program package.12 Optical absorption bands for the molecule in crystal were calculated on the basis of the X-ray x, y, and z coordinate sets using the INDO/S Hamiltonian.
Figure 3. Molecular conformation: (a) PR149 and (b) PR179.
3. Results and Discussion 3.1. X-ray Structure Analysis. The structure of PR149 has been analyzed in the present investigation13 while the structure of PR179 was identified as the same phase as reported by Graser and Ha¨dicke.14 Table 1 details the crystallographic parameters for PR149 and PR179. In both structures, the molecular symmetry, crystal system, and space group are Ci, monoclinic and P21/c, respectively. Figure 3 shows the molecular conformation for PR149 and PR179. In both compounds, the perylene skeleton is entirely planar while the two 3,5-xylyl groups of PR149 are symmetrically twisted by 95.76° with respect to the perylene skeleton (top view and side view in Figure 3(a)). The molecules are stacked, as shown in Figure 4, along the b-axis in PR149 and along the a-axis in PR179. 3.2. Solution and Solid-State Spectra. Figure 5 (a) shows the solution spectrum, where the molecules are far enough apart that the intermolecular interactions can be neglected; whereas the solid-state spectra are shown in Figure 5 (b) in evaporated films for PR149, PR179, and PB31, respectively. No noticeable difference is recognized in solution spectra of these compounds, and the absorption spectrum of each compound exhibits a progression of absorption bands as designated by 0-0, 0-1, 0-2, and 0-3 transitions. This indicates that the substituents in these compounds are not involved in the chromophore. On the other hand, the absorption spectra in the solid state are vastly different as characterized by red, maroon and black colors for PR149, PR179, and PB31, respectively. The present difference in shades is obviously attributed to the molecular arrangement that exerts a profound influence on the interaction between transition dipoles. To elucidate the present phenomena, we focused our attention on the similarity in spectral changes
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Figure 4. Molecular arrangement along the stacking axis: (a) along the b-axis in PR149 and (b) along the a-axis in PR179. The molecules in the middle column in PR179 are located in the foreground by a half lattice.
Figure 5. (a) Solution spectra and (b) solid-state spectra in evaporated films for PR149, PR179, and PB31. The film thickness is about 1000 Å.
between band B in Figure 5(b) (as designated by a curved, long arrow) and the longer-wavelength band in PB31 in Figure 2(b). That is, the absorption shoulder around 570 nm in PR149 grows up to an absorption band in PR179 (Figure 5(b)) and the band is further displaced toward longer wavelengths in PB31, accompanied by intensity enhancement. This spectral change is striking similar to that in the heat-treatment process in PB31 as shown in Figure 2(b). The present similarity prompted us to
assume that the resonance interaction of band B is intensified in the order of PR149 (red), PR179 (maroon) and PB31 (black). On the basis of the present assumption, we proceed with the assignment that the band around 540 nm in PR149 corresponds to the (0-0) transition in solution as judged from the comparison between the solution and solid-state spectra and the shoulder around 570 nm is caused by interactions between transition dipoles. According to the present assignment, the corresponding (0-0) band for PR179 is also supposed to appear around 540 nm. In fact, it will appear or be recognized at low temperatures as shown later. 3.3. Polarized Reflection Spectra and Resonance Interactions between Transition Dipoles. Figures 6 and 7show the polarized reflection spectra of PR149 measured on the (012) and (100) planes together with their corresponding projections, respectively. Likewise, Figure 8 shows the polarized reflection spectra measured on the (102) plane together with its projection for PR179. The direction of the transition dipole (µ), as determined by MO calculations, is also shown on the projection in dotted lines. This transition is due to the HOMO/LUMO π-π* transition in both compounds. In PR149, polarized light was introduced parallel or perpendicular to the long-molecular axis of the molecule, i.e., along the calculated transition moment. Two prominent reflection bands appear around 475-525 and 575 nm for polarization parallel to the long-molecular axis. On the other hand, these intense bands are completely quenched by polarized light perpendicular to the molecular axis. These results evidently indicate that the transition dipole points along the long-molecular axis as predicted by MO calculations, and that all reflection bands in the visible region are assigned to the same electronic transition. The present polarized reflection spectrum bears
770 J. Phys. Chem. B, Vol. 106, No. 4, 2002
Figure 6. (a) Polarized reflection spectra measured on the (012) plane for PR149 and (b) projection of the crystal structure onto the (012) plane where the H-atoms are omitted for clarity.
Figure 7. (a) Polarized reflection spectra measured on the (100) plane for PR149 and projection of the crystal structure onto the (100) plane where only the perylene skeletons are depicted for clarity.
resemblance to the absorption spectrum in evaporated films (Figure 5(b)), although the two bands around 540 and 570 nm in Figure 5(b) appear superimposed with enhanced intensity in the polarized reflection spectrum (Figure 6a)). Because the molar extinction coefficient of perylene pigments is quite large over 50 000, the resonance interaction between transition diploes15-18 is most likely to occur in the solid state. The bathochromic or hypsochromic shift will result between translationally equivalent molecules, depending on the slip angle between two transition dipoles where the critical angle is 54.7°. On the other hand, the Davydov splitting appears due to an oblique arrangement of the transition dipoles in translationally inequivalent molecules. The Davydov splitting can easily be confirmed in polarization experiments based on single crystals and serves as evidence of short-range coherence of exciton packets. Furthermore, the existence of Davydov splitting acquires an extended significance that the excitonic interaction is also occurring between translationally equivalent molecules. For this reason, measurements were made on the (100) plane
Mizuguchi and Tojo
Figure 8. (a) Polarized reflection spectra measured on the (102) plane for PR179 and (b) simplified projection of the crystal structure onto the (102) plane where only the perylene skeletons are depicted for clarity.
of PR149 (Figure 7) on which the molecules are arranged in an oblique fashion as specified by the fractional coordinates at (1/ 2,0,0) and (1/2,1/2,1/2) (intermolecular distance ) 8.9 Å). The polarized spectra shown in Figure 7(a) clearly support the Davydov splitting as shown by the difference in reflection maxima around 550-560 nm for different polarizations, parallel or perpendicular to the b-axis. That is, the reflection bands are displaced toward longer wavelengths for polarization parallel to the b-axis as compared with those for polarization perpendicular to the b-axis. The split energy is about 474 cm-1. The existence of the Davydov splitting provides further evidence that the resonance interaction is also occurring between translationally equivalent molecules at (1/2,0,0) and (1/2,1,0) (intermolecular distance ) 4.9 Å; angle between transition dipoles ) 83.7 °) in Figure 7(b), or (1/2,0,0) and (1/2,0,1) (distance ) 17.1 Å; angle ) 45.9°). Eventually, the above two kinds of resonance interactions are superimposed in complicated ways to give rise to the present polarized reflection spectra in Figure 7(a). It should also be noted that a small reflection band appears around 660 nm for polarization parallel to the b-axis (Figure 7(a)). This band is not assignable at the moment, but it also appears in diffuse reflectance spectrum as shown later. PR179 exhibits similar spectroscopic behavior to that of PR149 as shown in Figure 8(a). Because the molecules are arranged in a zigzag fashion, polarization parallel to the b-axis is more effective for excitation of the molecules rather than that perpendicular to the b-axis. In addition, a small Davydov splitting is also observed around 580 nm (about 300 cm-1) as a result of interactions between translationally inequivalent molecules (i.e., (0,0,1/2) and (0,1/2,0); intermolecular distance ) 10.7 Å) just as in the case of PR149. PR 179 also includes two prominent translationally equivalent molecule-pairs which greatly contribute to the bathochromic shift: (0,1/2,1) and (0,1/2,0) (distance ) 15.6 Å and angle ) 18.2°), and (0,1/2,0) and (-1,3/2,0) (distance ) 16.0 Å and angle ) 12.3°). Another important factor that involves the optical absorption in the solid state is the effect called “crystal shift” on going from solution to the solid state. This is related to the “van der Waals” stabilization energy in both the ground state and the excited state.15-18 It is, however, an intractable task at the
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Figure 10. Temperature dependence of lattice parameters measured on single crystals: (a) PR149 and (b) PR179.
Figure 9. Temperature dependence of absorption spectra in evaporated films: (a) PR149, (b) PR179, and (c) PB31. The film thickness is about 1000 Å.
moment to analytically estimate the crystal shift as well as spectral displacements or band splitting due to resonance interactions between transition dipoles. 3.4. Temperature Dependence of Absorption Spectra and Lattice Contraction. On the basis of the preceding polarization experiments, band B in Figure 5(b) can be interpreted as being due to interactions between transition dipoles that depend on the molecular arrangement. If this is the case, band B should exhibit considerable temperature dependence because of enhanced intermolecular interactions due to lattice contraction at low temperatures. Figure 9(a), (b), and (c) show the temperature dependence of absorption spectra in evaporated films for PR149, PR179, and PB31, respectively, measured in the temperature range between 20 and 300 K. Significant temperature dependence is clearly recognized in band B in PR179 and PB31 while the temperature effect is quite insignificant in PR149. The present results clearly indicate that band B is due to interactions between transition dipoles and the present interaction increases in the order of PB31, PR179 and PR149, respectively. On the other hand, the shorter-wavelength bands (band A) in PR149, PR179, and PB31 are quite insensitive to temperature variations because band A is of molecular character and is due to nonresonance interactions. It is also to be noted that the second longest-wavelength band in PR179 [around 550 nm as denoted by an arrow (Figure 9(b))] was not observed at room temperature as described in section 3.2. However, it clearly appears as a small band at 20 K. This enables us to recognize one-to-one correspondence in absorption bands between PR149 and PR179.
Figure 11. Diffuse reflectance spectra of commercial powders: (a) PR149 and (b) PR179.
Figure 10 shows the temperature dependence of lattice parameters in single crystals for PR149 and PR179 measured in the temperature range between 150 and 300 K. In both cases, crystal lattice is predominantly contracted on the (a,c) plane. The present lattice contraction induces a significant temperature dependence of absorption spectra as shown in Figure 9. 3.5. Diffuse Reflectance Spectra. Figure 11(a) and (b) shows the diffuse reflectance spectra measured on commercial powdered products for PR149 and PR179, respectively. The spectrum for PR149 is in agreement with that of the evaporated film (Figure 5(b)) and also of the polarized reflection spectra (Figure 6(a)), although the band intensity at the longestwavelength component (550-580 nm) is different. In addition, a very small broad band around 660 nm is slightly recognized in Figure 9(a). This corresponds to the band around 660 nm in Figure 7(a) for polarization parallel to the b-axis. On the other hand, the diffuse reflectance spectrum for PR179 is in complete
772 J. Phys. Chem. B, Vol. 106, No. 4, 2002 agreement with that of the evaporated film (Figure 2(b)) as well as of the polarized reflection spectrum (Figure 8(a)). The reason the longest-wavelength component in PR149 is less enhanced in evaporated films and powders can presumably be attributed to the difference in crystallinity between the states of single crystals, evaporated films and powders. The crystallinity is generally better in single crystals than in evaporated films and powders. As a consequence, the effect of resonance interactions is more enhanced in single crystals than in evaporated films and powders. Further explanation about the difference in spectral shape is that both evaporated films and powders provide us with averaged information of the absorption spectrum; whereas the reflection spectra measured on single crystals are variously different, depending on the crystal plane and polarization direction of incident light etc. 4. Conclusions The mechanism of various shades in perylene pigments has been investigated from the standpoint of the crystal structure and intermolecular interactions. The solid-state color of perylene pigments is mainly determined by two characteristic absorption bands in the visible region: the shorter-wavelength band is of molecular character (i.e., due to nonresonance interactions) that appear in common in all perylene derivatives, whereas the longer-wavelength band is caused by interactions between transition dipoles. Therefore, the color in the solid state can variously change, depending on the extent of resonance interac-
Mizuguchi and Tojo tions between transition dipoles based on the molecular arrangement. The red color appears when the interaction is quite small, whereas the maroon and black colors are characterized by medium and strong interactions, respectively. References and Notes (1) Herbst, M.; Hunger, K. Industrial Organic Pigments; VCH: New York, 1993. (2) Borsenberger, P. M.; Regan, M. T.; Staudenmayer, W. J. US Pat.4 578 334, 1984. (3) Gregg, B. A. J. Phys. Chem. 1996, 100, 852. (4) Mizuguchi, J. J. Appl. Phys. 1998, 84, 4479. (5) Ha¨dicke, E.; Graser, F. Acta Crystallogr. 1986, C42, 189. (6) Graser, F.; Ha¨dicke, E. Liebigs Ann. Chem. 1980, 1994. (7) Graser, F.; Ha¨dicke, E. Liebigs Ann. Chem. 1984, 483. (8) Ha¨dicke, E.; Graser, F. Acta Crystallogr. 1986, C42, 195. (9) Klebe, G.; Graser, F.; Ha¨dicke, E.; Berndt, J. Acta Crystallogr. 1989, B45, 69. (10) Kazmaier, P. M.; Hoffmann, R. J. Am. Chem. Soc. 1994, 116, 9684. (11) Mizuguchi, J. Krist. Tech. 1981, 16, 695. (12) Zerner, M. C. ZINDO, A General Semiempirical Program Package. Department of Chemistry, University of Florida, Gainesville, FL. (13) Mizuguchi, J.; Tojo, K. Z. Kristallogr. NCS 2001, 3, 375. (14) Ha¨dicke, E.; Graser, F. Acta Crystallogr. 1986, C42, 189. (15) Kasha, M. Spectroscopy of the Excited State; Plenum Press: New York, 1976, p 337. (16) Craig, D. P.; Walmsley, S. H. Excitons in Molecular Crystals; W. A. Benjamin, Inc.: 1968. (17) Hochstrasser, R. M. Molecular Aspects of Symmetry; W. A. Benjamin, Inc.: 1966. (18) Davydov, A. S. Theory of Molecular Excitons; McGraw-Hill Book Company, Inc.: New York, 1962.
