J. Phys. Chem. B 1998, 102, 5985-5990
5985
Crystal Structures and Photocarrier Generation of Thioindigo Derivatives Kotaro Fukushima,* Kazumi Nakatsu,† Ryuichi Takahashi, Hiroshi Yamamoto, Keigo Gohda,‡ and Seiji Homma Pigment DiVision, Ciba Specialty Chemicals K. K., Takarazuka 665-8666, Japan ReceiVed: March 11, 1998; In Final Form: May 1, 1998
The X-ray crystal structures of thioindigo derivatives, 4,4′7,7′-tetrachlorothioindigo and 4,4′,5,5′,7,7′hexachlorothioindigo, have been determined to clarify the photocarrier generation process in these compounds. The crystals belong to the same space group (P21/a), but the stacking manner of the molecules differs with respect to the slip angles of the molecular stacking columns. In tetrachlorothioindigo, the stacking molecules are arranged in such a way that all of the atoms of the indigoid part effectively overlap, so that the superior overlap between the HOMOs and between the LUMOs of the nearest stacking neighbors are realized to those in hexachlorothioindigo and thioindigo. These overlaps and the resultant intermolecular interactions in the tetrachlorothioindigo crystal are also revealed by differences between the bond lengths in the indigoid part of the isolated molecule and those of the solid-state molecule. In hexachlorothioindigo, there are no such differences, indicating that neither significant overlap of molecular orbitals nor strong intermolecular interactions exists. As a result, in the tetrachlorothioindigo crystal, intermolecular charge transfer and subsequent electronhole pair dissociation occur more efficiently than in the others. This leads to a higher photocarrier generation efficiency of tetrachlorothioindigo expressed experimentally by a large thermalization length of 6 nm.
1. Introduction Recently, organic pigments and dyestuffs have been extensively studied for applications for various optoelectronic devices. Some of them have been successfully employed as charge generation materials (CGMs) of the layered-type organic photoreceptor in electrophotographic devices, where their superior photoconductive properties are utilized. For studies of CGMs, the geminate recombination theory derived by Onsager1,2 is usually exploited to characterize their photocarrier generation properties. In this theory, it is assumed that the absorption of a photon produces a bound electron-hole (geminate) pair that either recombines or dissociates into free carriers under the effects of the Coulombic attraction and the applied electric field. This photocarrier generation process is characterized by two parameters (i.e., the initial dissociation efficiency η0 and the electron-hole separation distance (thermalization length:r0)). The latter parameter of CGM with high photocarrier generation efficiency can be as large as several nanometers,3,4 beyond their unit cell size. This means that the electron-hole pair spreads over several molecules or that intermolecular charge transfer (CT) occurs. Therefore, the photocarrier generation process should be strongly influenced by the crystal structure, which reflects various types of intermolecular interactions. For this reason, many studies have been made to investigate the effects of intermolecular interactions on crystal structures in materials with polymorphs such as phthalocyanines derivatives, a family of the most promising CGMs for laser beam printers.5,6 We also investigated the relationship between absorption spectra and the polymorphs of 1,4-dithioketo-3,6-diphenyl-pyrrolo-[3,4c]-pyrrole (DTPP)7 and further reported the orientational † Emeritus Professor, School of Science, Kwansei Gakuin University, Nishinomiya 662-8501, Japan. ‡ Research Department, Novartis Phama K. K., Takarazuka 665-8666, Japan.
Figure 1. Molecular conformation of (a) TCTI and (b) HCTI. H atoms are not drawn in (b).
behaviors of the molecules in a vacuum-deposited films on various crystalline substrates aiming at the superior property as CGM.8 Some of the indigo and its derivatives, which are widely used as coloring materials, demonstrated potential as CGMs of organic photoreceptors,9 especially 4,4′,7,7′-tetrachlorothioindigo (TCTI) with ORTEP drawing10 in Figure 1a. However, the
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TABLE 1: Crystallographic Data in the Structure Determination of TCTI and HCTI formula Mr space group z a (Å) b (Å) c (Å) β (°) V (Å3) Dx (g/cm3) Dm (g/cm3) R Rw
TCTI
HCTI
C16H4O2S2Cl4 434.14 P21/a 2 13.090(4) 15.368(5) 3.799(1) 91.173 764.2(4) 1.887 1.879 0.033 0.049
C16H2O2S2Cl6 503.03 P21/a 2 12.011(9) 15.323(6) 4.878(2) 101.61(3) 879.4(5) 1.900 1.860 0.047 0.051
correlations between the photocarrier generation processes and their crystal structures have not yet been clarified. In this paper, we present the X-ray crystal structures of TCTI and 4,4′,5,5′,7,7′-hexachlorothioindigo (HCTI) (Figure 1b). The crystal structure of thioindigo (TI)11,12 is compared with the structures of TCTI and HCTI. The intermolecular interactions in these crystals are directly evaluated by investigating the difference in the geometrical conformations of the molecules in the solid state and in the isolated condition with the aid of ab initio molecular orbital (MO) calculations. Their photocarrier generation processes are discussed, paying special attention to the difference in the stacking manners of the molecules and the degree of overlap between molecular orbitals. Orientational disorder has been found to exist in the TCTI crystal, and its effect on the photocarrier generation is also discussed. 2. Experimental Section 2.1. Structure Analyses. Phenyl-1-thioglycollic acid was cyclized in chlorosulfonic acid and thionyl chloride, and the obtained material was oxidatively dimerized to thioindigo (TI). In the same manner, TCTI and HCTI were synthesized from 2,5-dichloropheny-1-thioglycollic acid and 2,4,5-trichlorophenyl1-thioglycollic acid, respectively, as starting materials.13,14 The compounds obtained were repeatedly purified by sublimation method in an argon atmosphere. The single crystals of TCTI and HCTI were grown at about 543 and 533 K, respectively, in quartz tubes with a temperature gradient. The dimensions of the single crystals used for the X-ray diffraction were 0.9 × 0.15 × 0.15 mm3 for TCTI and 0.35 × 0.1 × 0.1 mm3 for HCTI. The data collection of crystal structures was carried out by a Mo KR monochromatized four-circle X-ray diffractometer (MAC Science Co., Ltd.). Unit-cell dimensions were determined from 22 reflections for TCTI (12 < 2θ < 20°) and HCTI (16 < 2θ < 26°). Intensities of the independent 1507 and 1504 reflections with |Fobs| > 3σ(Fobs) over the range of 3 < 2θ < 55° were collected for the structure determinations of TCTI and HCTI, respectively. Corrections for absorption and secondary extinction were not applied. The densities of the single crystals were measured by flotation in an aqueous solution (1 wt %) of sodium dodecyl sulfate. The structures were solved by the direct methods using MULTAN 78.15 The structure refinements were conducted by full-matrix least-squares with anisotropic atomic displacement parameters. The positions of the hydrogen atoms were determined from difference-electron density maps. The results of the refinements are shown in Table 1 with the crystallographic data, and the final atomic positions and the equivalent isotropic
TABLE 2: Positional and Isotropic Displacement Parameters with the Estimated Standard Deviations in Parentheses for TCTI atom
x
y
z
B(iso)a
occupancy
Cl1 Cl2 S1 S2 O1 O2 C1 C2 C3 C4 C5 C6 C7 C8 C9 H1 H2
0.42195(6) 0.12840(5) 0.34515(7) 0.4937(1) 0.5394(2) 0.3343(4) 0.4631(2) 0.4666(3) 0.3710(2) 0.3413(2) 0.2459(2) 0.1805(2) 0.2092(2) 0.3050(2) 0.3602(6) 0.224(2) 0.117(2)
0.16644(5) 0.45853(5) 0.50879(7) 0.3579(1) 0.3424(2) 0.5540(3) 0.4712(2) 0.3777(3) 0.3343(2) 0.2476(2) 0.2264(2) 0.2912(2) 0.3776(2) 0.3992(2) 0.4812(5) 0.163(2) 0.277(2)
0.5013(2) -0.0453(2) 0.2696(3) 0.5428(5) 0.6345(9) 0.228(2) 0.4572(7) 0.501(1) 0.3695(7) 0.3506(7) 0.2114(8) 0.0900(8) 0.1077(7) 0.2468(7) 0.310(2) 0.201(8) 0.012(8)
3.68(2) 3.62(2) 2.46(2) 2.38(4) 3.44(8) 3.3(1) 2.33(6) 2.7(1) 2.37(6) 2.61(6) 3.08(7) 3.10(7) 2.57(6) 2.37(6) 2.8(2) 4.2(8) 4.0(7)
1 1 0.65 0.35 0.65 0.35 1 0.65 1 1 1 1 1 1 0.35 1 1
a B(iso) ) 4/ [a2B(1,1) + b2B(2,2) + c2B(3,3) + ab(cos γ)B(1,2) + 2 ac(cos β)B(1,3) + bc(cos R)B(2,3)].
TABLE 3: Positional and Isotropic Displacement Parameters with the Estimated Standard Deviations in Parentheses for HCTI atom
x
y
z
B(iso)
Cl1 Cl2 Cl3 S1 C1 C2 C3 C4 C5 C6 C7 C8 O1 H1
0.8168(1) 0.6373(1) 0.9148(1) 0.53151(8) 0.5329(3) 0.6154(3) 0.6781(3) 0.7655(4) 0.8087(3) 0.7679(4) 0.6846(4) 0.6395(3) 0.6240(3) 0.812(4)
0.49766(9) 0.16986(7) 0.31235(9) 0.35904(6) 0.4701(2) 0.4937(3) 0.4144(3) 0.4078(3) 0.3251(3) 0.2514(3) 0.2590(3) 0.3409(3) 0.5672(2) 0.197(3)
-0.0693(3) 0.4623(3) -0.1525(3) 0.5281(2) 0.4547(8) 0.2750(9) 0.2202(8) 0.0712(9) 0.0362(9) 0.146(1) 0.3042(9) 0.3380(8) 0.1904(7) 0.16(1)
4.45(4) 4.15(3) 4.45(4) 2.73(2) 2.48(9) 2.8(1) 2.51(9) 3.1(1) 3.1(1) 3.4(1) 3.0(1) 2.7(1) 3.67(9) 5(1)
thermal parameters for TCTI and HCTI are shown in Tables 2 and 3, respectively. For the structure determination of the TCTI crystal, two kinds of molecular orientations (orientational disorder) were taken into consideration to illustrate the diffusing of the electron density on S1, C2, and O1 in the Fourier map. One is shown in Figure 1a, which will be designated as MaO. The other was obtained by rotating the left half, for example, of the structure in Figure 1a by 180° around the axis of the C1dC1′ double bond, which will be designated as MiO. The existing ratio of MaO and MiO was optimized so that the R factor was minimized. The R factor thus obtained was 0.033 when the ratio, MaO:MiO, was 65:35. Ab initio MO calculations for geometry optimizations of the isolated molecules were also conducted for TCTI and HCTI in order to compare the bond lengths in the isolated molecules with those in the solid state. The optimizations were carried out, using 3-21G(*) basis-sets, by SPARTAN (Wave function Co.) on an INDIGO2 workstation (Silicon Graphics, Inc.). 2.2. Evaluations of Photocarrier Generation Properties. Dual-layered structures were prepared for the characterization of photocarrier generation properties. The charge generation layer (CGL) of 0.4 µm in thickness, including 90 wt % of a thioindigo compound and 10 wt % of a polyvinyl butyral binder (BMS from Sekisui Chem. Co., Ltd.), was coated on an aluminum electrode by means of a wire bar. A charge transport layer (CTL) of 25 µm in thickness was formed on the CGL in
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TABLE 4: Bond Lengths (in angstroms) with Estimated Standard Deviations in Parentheses TCTI Cl1-C4 Cl2-C7 Cl3-C5 S1-C1 S1-C8 C1-C1′ C1-C2 C1S2 C1-C9 C2-C3 C2O1 C3-C4 C3-C8 C3-S2 C4-C5 C5-C6 C6-C7 C7-C8 C8-C9 C9-O2
1.725(3) 1.727(3) 1.784(3)a 1.765(3) 1.346(2) 1.447(5) 1.815(3)b 1.457(2)b 1.494(5) 1.200(5) 1.390(4) 1.394(4) 1.761(2)b 1.385(4) 1.387(4) 1.380(4) 1.391(4) 1.469(3)b 1.2080(4)b
HCTI 1.712(5) 1.716(5) 1.725(5) 1.742(5) 1.765(5) 1.341(6) 1.495(6) 1.483(6) 1.211(6) 1.393(6) 1.383(6) 1.394(7) 1.381(7) 1.387(7) 1.392(7)
a Significant differences in bond lengths between TCTI and HCTI are designated in bold. b The bond lengths in the MiO molecule of TCTI.
the same manner. The CTL used contained in a weight ratio of 1:1 N,N′-diphenyl-N,N′-bis(2,4-dimethylphenyl)-1,1′-biphenyl-4,4′-diamine (4MeTPD) and polycarbonate (Z-200 from Mitsubishi Gas Chemistry Co., Ltd.). The electrophotographic sensitivity was evaluated by photoinduced discharge technique. The surface of the CTL was negatively charged up by corona charging. The initial surface potential was -700 V and its reduction by monochromatic irradiation (5 µW/cm2) was then measured. The reciprocal of the energy, E1/2, which was required to reduce the surface potential to a half of its original value, was taken as the electrophotographic sensitivity. The measurements were done at room temperature over the visible spectral range (400-700 nm). To further clarify the photocarrier generation process of TCTI, the quantum efficiency of the photocarrier generation, η, was evaluated by measuring the rate of potential discharge at the onset of visible light exposure with the following equation,2
η)
C dV eI dt
(1)
where C is the geometrical capacitance of the layered structure, dV/dt the initial discharging rate of the surface potential, I the incident light intensity in photons/s, and e the electronic charge. The dielectric constant of the CGMs was taken as three to evaluate the capacitance. Exposures of 550 nm were used to irradiate the CGL through the optically transparent CTL, where the maximum absorption of the CGLs occurs. The intensity was 5 µW/cm2. The measurements were made at various initial voltages (-87 ∼ -1617 V). The ionization potentials (Ip) of TCTI and 4MeTPD were measured with a surface analyzer (AC1, Riken Keiki) for discussion of the carrier injection from TCTI to 4MeTPD. 3. Results 3.1. Molecular Conformations and Arrangements in the Crystals. As listed in Table 4, the S1-C1MaO (C1-S2MiO) and C1-C2MaO (C1-C9MiO) distances in TCTI were 1.784(3) Å (1.815(3) Å) and 1.447(5) Å (1.457(2) Å), respectively. The
Figure 2. (a) Projections of a-c planes in TCTI (major orientation) and HCTI. Column axes are denoted as dotted lines. Columns A and B are at different height along the projection to each other. (b) Projections of a-b planes in TCTI (major orientation) and HCTI. Molecules A and B are in the stacking columns represented in Figure 2a.
subscripts indicate the origins of the oriented molecule (i.e., MaO and MiO) as mentioned in the experimental section. In HCTI, the S1-C1 and C1-C2 distances were 1.742(5) and 1.495(6) Å, respectively. The other bond lengths show no significant differences between TCTI and HCTI. The planarity of these molecules in the crystals was evaluated using the deviations of the atoms from the least-squares planes of the cyclic frameworks of thioindigo structures. The maximal deviations were less than 0.03 Å for TCTI and 0.08 Å for HCTI. This means that these two molecules are entirely planar in the crystalline state. Figure 2a shows the stacking configuration of TCTI and HCTI on an a-c plane. In the TCTI crystal, the molecules form stacking columns along the c-axis. The molecules in columns A and B are arranged in such a way that their long molecular axes are almost perpendicular to each other (Figure 2b). They do not exhibit close contacts between the columns. The slip angle (θs) is about 70° in both columns. In the HCTI crystal, the stacking manner of the molecule is similar to that in TCTI as shown in Figure 2. However, the slip angle (≈51°) is steeper than that of TCTI. Figure 3 shows the projections normal to the molecular planes for TCTI and HCTI. The projection of the TI crystal is also
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Figure 5. Electric field dependence of quantum efficiency of carrier generation using the TCTI as a CGM. Dots are experimental results and they fit to the line with r0 ) 6.0 nm and η0 ) 28% based on Onsager’s theory.
TABLE 5: Comparisons of Bond Lengths (in angstroms) between Experimental and Calculated Data TCTI Figure 3. Molecular overlapping manners of (a) TCTI (major orientation), (b) HCTI, and (c) TI. Remarkable overlaps of indigoid parts are denoted as shaded circles with bold lines.
Figure 4. Comparison of the reciprocals of E1/2 among TCTI, HCTI, and TI.
shown for comparison in Figure 3.11 In the TCTI crystal (Figure 3a), the upper molecule is slipped almost along the carbonyl chain from the lower molecule in this projection. As a result, all of the atoms of the indigoid parts are well overlapped between the two molecules; the carbonyl carbon atoms in the upper molecule are almost coincident with the carbonyl oxygen in the lower molecule and the double-bonded carbon atom with the sulfur atom. Their interatomic distances are 0.349 and 0.353 nm, respectively. In the case of HCTI (Figure 3b), the carbonyl groups and the double bonded carbon atoms of the indigoid parts almost overlap, whose interatomic distances are around 0.35 nm. However, because the molecular center of HCTI is shifted much more than that of TCTI, significant overlap of sulfur atoms could not be obtained between the stacking neighbors. The TI molecules in the crystal have the smallest shift of the molecule center, and it is along the direction of the carbon double bond. However, the indigoid part is poorly overlapped as shown in Figure 3c. The nearest interatomic distance of double-bonded carbons is 0.36 nm. In all cases, the molecules in the crystals seem to be slipped more or less to prevent the repulsion forces between the stacking neighbors. Furthermore, it was found that the stacking manner of TCTI molecules shows better overlap of the indigoid part than in the others (i.e., HCTI and TI). 3.2. Photocarrier Generation Characteristics. Figure 4 shows the reciprocal of the E1/2 of layered photoreceptors prepared using TCTI, HCTI, and TI as CGM. It is clear that the 1/E1/2 values of TCTI are much greater than those of HCTI and TI, indicating that the electrophotographic sensitivity of
MO calculation Cl1-C4 Cl2-C7 Cl3-C5 S1-C1(S2-C1) S1-C8(S2-C3) C1-C1′ C1-C2(C1-C9) C2-C3(C8-C9) C2-O1(C9-O2) C3-C4 C3-C8 C4-C5 C5-C6 C6-C7 C7-C8
crystal structure
1.733 1.739
1.725 1.727
1.756 1.767 1.326 1.494 1.473 1.212 1.381 1.392 1.382 1.381 1.380 1.376
1.784 (1.815) 1.765 (1.761) 1.346 1.447 (1.457) 1.494 (1.469) 1.200 (1.2080) 1.390 1.394 1.385 1.387 1.380 1.391
HCTI MO crystal calculation structure 1.724 1.736 1.734 1.754 1.765 1.326 1.494 1.477 1.211 1.385 1.388 1.385 1.382 1.376 1.377
1.712 1.716 1.725 1.742 1.765 1.341 1.495 1.483 1.211 1.393 1.383 1.394 1.381 1.387 1.392
TCTI is greater than that of the others, although the skeleton of these molecules is identical. The quantum efficiency of photocarrier generation measured with TCTI as CGL is shown in Figure 5 as a function of the electric field. The solid curve is calculated from Pai’s2 expression; which is derived from the Onsager theory.1 The calculated results fit the experimental values, especially in the higher electric field region, when the thermalization length, r0, and the efficiency at zero field, η0, are determined as 6.0 nm and 28%, respectively. This r0 value is the highest of those reported for other organic CGMs,4 excluding the Y-form of oxotithanium phthalocyanine.16 This indicates the efficient generation of CT states in the TCTI crystal. 4. Discussion 4.1. Effect of Intermolecular Interaction on Molecular Geometry. As described in the results, the X-ray crystal structures show that the molecules in the solid state are almost as planar as the isolated molecules, despite intermolecular interactions existing in the solid state. This may be due to the rigid structure of the thioindigo molecule. However, a detailed examination reveals that, for TCTI, the bond lengths in the crystal are different from those predicted for the isolated molecule, especially in the indigoid part. Table 5 shows the bond lengths of TCTI and HCTI molecules in the crystal, which were experimentally determined, and the lengths of isolated TCTI and HCTI molecules, which were predicted by the ab initio calculations. The S1-C1MaO or S2-C1MiO bond in the TCTI crystal shows the single-bond character, compared with that of the isolated molecule, and the C1-C2MaO or the C1-
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J. Phys. Chem. B, Vol. 102, No. 31, 1998 5989
Figure 7. Overlapping manner of TCTI molecules between major and minor orientations.
Figure 6. Schematic representations of electronic states of TCTI. (a) HOMO and (b) LUMO. White and black circles indicate their signs, and their sizes represent the value of the LCAO coefficients.
C9MiO bond exhibits double-bond character. These significant differences in bond lengths between the two states seem to reflect the differences in the intermolecular interactions. On the other hand, for HCTI, the bond lengths in the crystal did not show significant differences from those in the isolated molecule. This clearly indicates that the TCTI molecule, especially its indigoid part, the crystalline lattice experiences much stronger intermolecular interaction than the HCTI molecule in the solid state. The surface characterization of the CGM crystal is also important in evaluating the photocarrier generation process, and molecular conformations and their arrangements must reflect intermolecular interactions on the surface. Recently, we have observed the surface structure of the TCTI crystal by atomic force microscopy at the molecular level. It was found that the cell dimensions are almost the same as those estimated by crystal structure, and TCTI molecules formed arrayed structures along the stacking direction. This proved the strong intermolecular π-π interaction between stacking neighbors also on the surface.17 4.2. Generation of Intermolecular CT State. MO calculations have been already employed to investigate the electronic structure of thioindigo derivatives.18,19 The effect of various substituents on the structure has been studied to reveal that, although the coefficients in the linear combination of atomic orbital (LCAO) depend on the position and the type of substituents, the substantial contribution of indigoid parts in the HOMO and the LUMO is maintained. Schematic representations of the HOMO and the LUMO in the TCTI molecule are shown in Figure 6. Dominant contributions to the HOMO and the LUMO come from the atoms in the indigoid part, such as centered double-bonded carbons, carbonyl groups, and sulfurs. Therefore, the most important requirement for intermolecular CT and the occurrence of the subsequent carrier generation in the TI derivatives is the geometrical overlaps of atoms in the indigoid part. As shown in Figure 3a, the HOMOs and the LUMOs were effectively overlapped between the stacking neighbors in the TCTI crystal as a result of the well-overlapped manner of the indigoid parts. Therefore, an electron in the LUMO and/or a hole in the HOMO of the photoexcited molecule can easily migrate along the stacking columns. For the thermalization length to be large enough for an electron-hole pair to be easily dissociated, the molecular packing of TCTI must be preserved in the procedures of the layered-structure preparation. Otherwise, effective overlap of the molecular orbitals will be destroyed and carrier migration will be hindered, as usually observed in disordered molecular solids. The long thermalization length observed in TCTI-based structures indicates that the molecular packing of TCTI might be perfectly preserved in the
procedures of the layered-structure preparation due to its substantial insolubility against organic solvents. On the other hand, the poor overlap of the indigoid part, especially the sulfur atoms, in HCTI crystals leads to inhibition of carrier migration in the crystal. This is believed to be the reason for lower photocarrier generation efficiency of HCTI relative to that of TCTI. Due to the poor overlaps between the stacking neighbors, the migration of electrons or holes in TI does not seem to occur. Attention should be paid to the fact that the photocarrier efficiency measured for layered structures depends on the efficiency of photocarrier generation in a CGM and the efficiency of carrier injection from the CGM to a CTL. Popovic et al. proposed that for efficient carrier injection at the CGMCTL interface the ionization potential (IP) of the CTL should be equal to or smaller than that of the CGM so that holes crossing the interface do not experience an energy barrier.20 The IP values measured for TCTI and 4MeTPD were both 5.46 eV, indicating that there is no energy barrier for hole injection and that the experimental results of r0 can be attributed to the photocarrier generation properties of the CGM crystal. The layered structure prepared using HCTI showed the lower photocarrier generation efficiency compared with the structure with the TCTI layer. We cannot conclude whether the efficiency of photocarrier generation in HCTI or the efficiency of carrier injection from HCTI to the CTL is responsible to the low overall efficiency, because of the lack of the IP information of HCTI. However, it can be said that, comparing the molecular stacking manner of TCTI and HCTI, the low overall efficiency of the HCTI-based structure is likely due to the low photocarrier generation efficiency in the HCTI crystal. 4.3. Effect of Disorder on Photocarrier Generation. The distribution of the MiO molecules in the TCTI crystal may play a crucial role in the photocarrier generation process. Two extreme cases are considered here: One is that the MaO molecules form a domain structure without MiO molecules, and MiO molecules also form domains without MaO molecules, whereby they form one single crystal together. Provided that the sizes of the domain structures are larger than the thermalization length of TCTI, the photocarrier generation process is basically explained by the degree of overlapping of the indigoid part as in Figure 3a. The other case is that the MiO molecules are randomly distributed in the crystal lattice of TCTI. In this case, the overlapping between the MaO and MiO molecules must be considered. As shown in Figure 7, a good overlap of the indigoid part is still maintained, or it is at least better than that of the other thioindigo derivatives. To date, it is not clear which postulation is dominant in the TCTI crystal. In addition, the exact role of orientational disorder is not clear for the entire process in the photoreceptor. An MiO molecule may play a negative role in the photocarrier generation by acting as an exciton trap in the TCTI crystal, which shortens the exciton diffusion length in the crystal. It may play a positive role by generating the local electrical field around the disorder and thereby providing an electron-hole pair with a site for dissociation.21
5990 J. Phys. Chem. B, Vol. 102, No. 31, 1998 5. Conclusions The photocarrier generation process of the crystalline thioindigo derivatives was investigated from the viewpoint of their crystal structures. The X-ray crystal structures showed that the effective overlapping of the indigoid part between the nearest stacking neighbors was realized in TCTI crystals, as compared with HCTI and TI. This overlapping and the resultant intermolecular interaction were also revealed in TCTI by the difference in the bond lengths in the indigoid part between the isolated and the solid-state molecule. The large thermalization length observed for TCTI could be explained by this effective overlapping, which enables the intermolecular charge transfer and subsequent electron-hole pair dissociation to occur, even though the structure includes orientational disorder. Acknowledgment. The authors would like to express their sincere thanks to Mr. H. Yoshioka for his technical support and to Dr. K. Kodama for the critical reading of the manuscript. Mr. T. Deno is acknowledged for fruitful discussions and Dr. T. Yamamura for continuous encouragement. References and Notes (1) Onsager, L. Phys. ReV. 1938, 54, 554. (2) Pai, D. M.; Enck, R. C. Phys. ReV. 1975, B11, 5163. (3) Borsenberger, P. M.; Contois, L. E.; Hoesterey, D. C. J. Chem. Phys. 1978, 68, 637.
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