1. Introduction
Phthalocyanines (Pc) and their derivatives exhibit a variety of useful applications depending on the chemical constitution of the molecule and the crystal structure, as well as on the structural polymorphism when the same chemical constitution is present [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10]. An excellent tool for the determination of the molecular and crystal structures of these compounds is X-ray single-crystal studies, which reveal the arrangements and interactions that are needed within the materials for applications as active materials for organic electronics, xerography, sensors, photovoltaic cells, electrochromic devices, nonlinear optics, and recordable DVDs.
Structural determination using X-ray powder patterns of randomly distributed crystallites that all exhibit the same structure is difficult and can only be successful with an additional conformation and packing analysis for the Pcs with an almost firm conformation of the molecules to substitute the few available X-ray data. But this method can serve as an approximation only. Nevertheless, single-crystal data are still needed as prerequisites for such an investigation to supplement the missing data as the molecular conformation or as bond lengths and angles. Such a structural determination by an X-ray powder experiment was carried out to refine the structure of HOGaPc V [
2], starting with the isomorphic unit cell of TiOPc II or VOPc II and a fixed conformation of the HOGaPc molecule. The conformation was obtained by single-crystal analysis for these two compounds [
11]. The replacement of the central atoms, Ti or V, by Ga in the fixed conformation of TiOPc or VOPc slightly changed the packing arrangement of the HOGaPc V structure due to small differences in the size of the unit cells considered. Since the size of the unit cells is established by the interactions of all atoms, the few interactions with the central atoms, Ga, O, Ti or V, in the packing refinement can be neglected compared with the many interactions of all the other atoms of the Pc molecule, which all contribute to the total energy. This method of neglecting small contributions to the total energy was introduced in the structural calculations of monosaccharides by neglecting hydrogen bonds [
12]. A packing analysis with few variables then leads to an excellent approximation of the of HOGaPc V structure. The central atoms can then be determined with the few reflections available by an X-ray analysis of the powder pattern.
The relationship between crystal structure and photosensitivity of Pc polymorphs is described by Oka and Okada [
1]. The photosensitivity is caused by a certain overlap in the stacking of adjacent molecules, alongside the separation distance of the two molecules, allowing π- interactions, which was found to be optimal for HOGaPc V and high photosensitivity is the result.
X-ray powder diagrams without any relation to structural features are used to characterize properties of phthalocyanines, especially in the patent literature, without a detailed knowledge of the interactions of the molecules responsible for the proposed properties. It is sometimes rather difficult to obtain an accurate structure with powder X-ray data, or at least a reliable unit cell as prerequisite for a crystal structure determination. An interpretation of powder X-ray data is, in many cases, not trustworthy. Therefore, a single-crystal structure evaluation by X-ray analysis, as carried out for PcGaOGaPc and ClGaPc, is still an elegant way to obtain the required structures for a discussion of properties. However, this method requires special crystallization procedures to obtain the needed crystals. Single-crystal analysis can also provide the required geometrical data and interaction parameters for phthalocyanines to compare with results obtained by other methods, i.e., by spectroscopy, such as X-ray absorption spectroscopy (EXAFS [
9]), or by theoretical quantum chemical studies [
10].
3. Results and Discussions
The positional and averaged thermal parameters for PcGaOGaPc and ClGaPc are collected in
Table A1,
Table A2 and
Table A3 in the
Appendix A, the respective conformations, thermal ellipsoids with a probability of 50%, and labeling of the atoms are shown in
Figure 1 and
Figure 2 [
18]. In
Table 2 and
Table 3, some selected bond lengths are listed for a comparison of Pc structures. The two Ga-Pc structures investigated are isomorphous with the ones with a central Al atom studied and discussed by Wynne [
19,
20], space group P-1, with unit cell parameters for PcAlOAlPc of a = 7.694 (2) Å, b = 12.520 (3) Å, c = 12.705 (3) Å, α = 91.03 (2)°, β = 94.54 (2)°, γ = 90.37 (2)°, and V = 1219.8 Å
3, and for ClAlPc, a = 13.776 (6) Å, b = 13.775 (4) Å, c = 14.059 (5) Å, α = 98.36 (3)°, β = 108.60 (3)°, γ = 90.16 (3)°; V= 2498.2 Å
3. The two unit cells by Wynne agree almost perfectly with the ones listed in
Table 1. The packing for the PcGaOGaPc and PcAlOAlPc molecules agrees well (
Table A1 and COD 3000042 -file) and can serve as proof that the unit cells are determined by the total interactions of the atoms in the molecules neglecting the interactions of the central atoms. Wynne had difficulties in solving the structure of ClAlPc and was only able to present one complete molecule of the two in the asymmetric unit. He described the second one as two halves with uncertainties, with a high R factor of 0.137 for 2165 reflections and I > 3σ(I) of the 9454 unique reflections in a θ range of 2.8–140° (CuKa) and concluded that limited observed data were obtained as result of disorder in this structure. The highest peak in his difference Fourier amounted to 1.01 e/Å
3. Due to the perfect agreement of the unit cell with ClGaPc of
Table 1, the same arrangement as for ClGaPc can be assumed as listed in
Table A2. A structure for ClAlPc can be proposed with the coordinates of ClGaPc without the central atoms. These can be introduced by a refinement with few X-ray data. As suggested, the data of the two GaPc structures may also serve as models for conformations needed for X ray investigations of powder patterns with few reflections.
The asymmetric unit of PcGaOGaPc consists of one Pc molecule, which is linked by the P-1 symmetry of the space group through O to the symmetry-related Pc molecule, and both Pc molecules form a dimer, as depicted in
Figure 3. The Ga-O bond length established in the dimeric molecule amounts to 1.7336(6) Å (for Al-O, 1.679 (2) Å) and both bond angles Ga-O-Ga and Al-O-Al amount to 180°. The staggering angle α of the two bisphthalocyanine planes is 0°. In his review on phthalocyanines, Engel [
4,
5] lists many bisphthalocyanine compounds of various conformations and cites that the angle α will be 0° if the distance of the two planes in the dimer is bigger than the van der Waals distance. The distance of the adjacent intermolecular planes is 3.47 Å for GaPc and 3.43 Å for AlPc (c.f.
Table 4). The Ga atom lies 0.492 Å (Al 0.459 Å) above the isoindole nitrogen N1–N4 plane. The Pc rings of adjacent planes in the dimer are in an eclipsed conformation with an intraplanar spacing of 4.24 Å for GaPc and 4.13 Å for AlPc (c.f.
Table 4). The data of
Table 4 confirm the isomorphic structure between PcGaOGaPc and PcAlOAlPc, with close values except for the bond lengths Ga-O and Al-O, of which the interactions with the stiff ring can be neglected.
In contrast, the second modification of ClGaPc, for which two solutions have been considered, contains two molecules in the asymmetric unit and shows a completely different arrangement in the unit cell of the P-1 space group (
Figure 4). A projection down the a-axis reveals parallel planes formed by the four molecules present in the unit cell. The Ga-Cl bond length is 2.20 Å in both independent molecules of the asymmetric unit and Ga lies 0.439 Å above the isoindole nitrogen plane. These distances are collected in
Table 4 with some further parameters and are compared with related Pc structures.
The investigations of the two solutions for ClGaPc exhibit different disorder, as represented in
Table A2 and
Table A3, but show quite similar conformations. They differ in the equivalent isotropic displacement parameters, Ueq, as well as in the anisotropic displacement parameters, Uij matrix (c.f. CIF-files COD 300483, 300484), which could be a sign of distinct distortions of the first and second kind. In lattices affected by distortions of the first kind, the long-range periodicity of the atoms is preserved and included in the thermal matrix. In a lattice distortion of the second kind (paracrystallinity), the long-range order is lost, and each lattice point varies in position only in relation to its nearest neighbors and results in diminution of intensity and an increase in reflection breath with increasing reflection angle, θ, and needs a special additional description [
21]. The consideration of such a disorder may provide the possibility of describing the differences obtained for the two modifications, a and b, of ClGaPc with a high Fourier peak of 1.21 e/Å
3 when the reflections at larger reflection angles of sample b were included. In general, this disorder will cause changes. The maximum reflection angle, θ, of solution a extends to 20° that of solution b to 23°. The number of unique reflections differs by 2210 comparing solution a to b and, therefore, a big influence of the reflections at higher angles is expected.
The best approach for the disordered structure of ClGaPc seems to be solution a, where the temperature or disorder factor still represents a good approximation, and the coordinates describe the structure well with an R factor of 0.044 and the highest peak in difference Fourier reaching 0.54 e/Å
3. The number of observed reflections with I > 2σ(I) for both solutions surmounts, by far, the ones by Wynne [
20] (c.f.
Table 1).
The coordination of the metal atom M of all structures of
Table 4 is square pyramidal with four coordinated nitrogen atoms at the basal apices and Cl or O occupying the remaining apex. The Pc molecules considered exhibit a plano-convex shape with a planar and a convex side which leads to different interactions of adjacent Pc planes of either planar—planar or convex—convex side representations.
Wynne [
20] also studied a single-crystal structure of another polymorph of ClGaPc called ClGaPc 1, space group P-1, with only one Pc molecule in the asymmetric unit. In this structure Ga is centered 0.44 Å above the plane N1–N4. This structure compares well with all the data of the solutions a and b of ClGaPc listed in
Table 4. GaO
0.5 Pc has a little longer distance, d
1, with 0.49 Å. The lengths of the Ga-N bonds are the same for solution a, b as listed in
Table 3 and a small amount longer for GaO
0.5 Pc (
Table 2). The Ga-Cl distance for solutions a and b is ca. 2.200 Å and compares well with the bond length of 2.217 Å for the Pc modification ClGaPc 1 [
20]. The different interactions of the molecules in the two modifications cause differences in packing but has little effect on the conformation of the ClGaPc molecule.
Selected distances describing the conformation and packing of GaO
0.5Pc and GaClPc are compared in
Table 4 with the highly photosensitive TiOPc II [
8] and HOGaPc V [
2]. In contrast, both ClGaPc polymorphs are scarcely photosensitive.
Table 4 also provides similar data for AlO
0.5Pc compared with GaO
0.5Pc, except the distance, d
3 (M-O). These central atoms M and O atoms do not influence the packing of the Pc molecules in the unit cell. The data for HOGaPc V of Inami et al. [
2] and a refinement resulting in an excellent approximation for a powder X-ray study have also been added in
Table 4 for a comparison with the highly photosensitive TiOPc II.
The Ga-O bond and further bond lengths have been provided by an EXAFS determined structural study for PcGaOGaPc [
9] and are also listed in
Table 4. They show some differences compared with the geometrical data of this work. In particular, the Ga-O bond length, at 1.87 (2) Å [
9], differs significantly with the geometrical value of 1.7336 (6) Å and to the theoretical quantum calculated value of 1.776 Å [
10].
Table 4.
Selected distances describing the conformation and packing of GaO0.5Pc and GaClPc as compared with related phthalocyanines determined by single-crystal analysis, except HOGaPcV, determined by a powder pattern.
Table 4.
Selected distances describing the conformation and packing of GaO0.5Pc and GaClPc as compared with related phthalocyanines determined by single-crystal analysis, except HOGaPcV, determined by a powder pattern.
Compound | d/Å | d*/Å | d1/Å | d2/Å | d3/Å | | d4/Å | | |
---|
| | | | | | N1 | N2 | N3 | N4 |
TiOPc II [8] | 3.040 | 3.551 | 0.638 | 0.951 | 1.628 | 2.05 | 2.08 | 2.06 | 2.09 |
AlO0.5Pc + [19] | 3.431 | 4.127 | 0.459 | 0.384 | 1.680 | 1.99 | 1.98 | 1.97 | 1.97 |
GaO0.5Pc + [22] | 3.470 | 4.244 | 0.492 | 0.389 | 1.734 | 2.00 | 2.00 | 2.01 | 2.01 |
| | | | | | N5 | N6 | N7 | N8 |
| | | | | | 3.39 | 3.40 | 3.39 | 3.41 |
HOGaPcV [11] | 3.040 | 3.707 | 0.428 | 0.741 | 1.938 | 2.00 | 2.03 | 2.00 | 2.03 |
HOGaPcV [2] | 3.060 | 3.452 | 0.531 | 0.892 | 1.843 | 1.92 | 1.91 | 1.92 | 1.92 |
GaClPc 1 [20] | 3.248 | 3.331 | 0.440 | 0.519 | 2.218 | 1.98 | 1.98 | 1.98 | 1.99 |
GaClPc solution a [22] | | | | | | | | | |
Molecule 1 | 3.250 | 1→2 | 0.439 | 0.533 | 2.200 | 1.99 | 1.98 | 1.99 | 1.99 |
Molecule 2 | 3.250 | ≈3.377 & | 0.435 | 0.535 | 2.197 | 1.99 | 1.99 | 1.99 | 1.99 |
GaClPc solution b [22] | | | | | | | | | |
Molecule 1 | 3.247 | 1→2 | 0.426 | 0.518 | 2.218 | 1.99 | 1.98 | 1.98 | 1.99 |
Molecule 2 | 3.252 | ≈3.377 & | 0.432 | 0.520 | 2.216 | 1.99 | 2.00 | 1.98 | 1.98 |
EXAFS[PcGa]2O [9] | | | 0.45(9) | | 1.87(2) | 2.02(2) | Naza | 3.33(4) | |
(PcGa)2O [10] (theory, mixed basis set, D4h symmetry) | | | | | 1.776 | 2.03 | | | |
As demonstrated in
Figure 3, the planar sides of the phthalocyanine rings of GaO
0.5 Pc are arranged parallel in this dimeric structure, with a distance d = 3.47 Å, compared with 3.25 Å for the two modifications, ClGaPc 1 and ClGaPc a and b (
Table 4). Yet, a different overlap of the rings occurs for the two ClGaPc polymorphs when two adjacent rings are projected onto each other facing their planar sides.
Figure 4 represents the stacking of the two independent molecules of the second modification for ClGaPc in the crystal structure with a distance d = 3.25 Å between adjacent parallel and symmetry-related molecules 1 and 1″, which also extends to molecule 2 and 2′. The overlap resembles the stacking type observed for β-H
2Pc [
1] and provides low photosensitivity. The convex planes formed between adjacent molecules 1 and 2 deviate slightly from a parallel arrangement with d* ≈ 3.38 Å and an overlap of these two molecules 1 and 2 is not observed. The crystal structures listed in
Table 4 form stiff planes below and above the van der Waals distance, which, along with the π-π interactions of the atoms, is believed a necessary condition for photosensitivity.
Possible profiles of representing adjacent overlaps of Pc polymorphs are illustrated in
Figure 5. The stacking of neighboring planes of ClGaPc 1 is shown in
Figure 5a. This overlap is comparable to that of the metal-free X-H
2Pc phthalocyanine polymorph [
1] or TiOPc II [
8], both of which show high photosensitivity but a long separation distance d is found. A similar pattern shows up for PcGaOGaPc (
Figure 5b), with longer separation distances, d and d*, for possible effective π-interactions.
Figure 5c shows the overlap of molecule 1 of the asymmetric unit with a symmetry-related molecule 1 for ClGaPc and
Figure 5d shows the same type of overlap by molecule 2 with a symmetry-related molecule 2. The two representations look identical. These overlaps of the two independent ClGaPc molecules resemble that of the stacking type observed for β-H
2Pc [
1] and provide low photosensitivity. An overlap of two adjacent independent molecules 1 and 2 of the asymmetric unit of ClGaPc is not observed (c.f.
Figure 4).
The packing of GaPc molecules in the crystallites provides a different picture of the interactions of molecular planes and is represented in
Figure 6. GaOHPc V and also TiO II are assembled in staggered piles (
Figure 6a). The molecular planes in the piles possess alternate distances, d and d*, and adjacent molecules along the piles are arranged to exhibit the necessary overlap for excellent π-interactions. PcGaOGaPc also forms staggered piles with the neighboring pile, having the same form but lying behind the front stack in
Figure 6b. The overlap of neighboring molecules of planar- or convex-facing sides is shown in
Figure 3. The packing of ClGaPc 1 (
Figure 6c) can best be described by the arrangement of parallel planes perpendicular to the extension of the planes forming molecules in which the single molecule 1 of the asymmetric unit is present as well as the symmetry related one 1′. The distance between the planes alternates from d and d*, with planar-facing sheets between molecule 1 and inversion symmetry-related ones 1′ and convex-facing sheets between molecule 1 and 1′. Adjacent molecules with planar-facing sides show the overlap represented in
Figure 5a, adjacent convex molecules are not overlapped. The two independent molecules a and b of ClGaPc (
Figure 6d) are also placed in sheets as shown for ClGaPc 1 in
Figure 6c but rotated within the pile and show an overlap appearance represented in
Figure 5c,d. The same distance d = 3.25 Å occurs between molecule 1 and molecule 1 as well as between molecule 2 and 2. Despite the short distance between the adjacent molecules, little photosensitivity is observed due to the almost absent π-interaction. The distances, d and d*, of the represented planes in
Figure 6d alternate according to the facing of the planar or convex sides between sheets formed by the adjacent molecules 1 or 2. No overlap is observed between neighboring molecules 1 and 2 of distance d*, as demonstrated also in
Figure 6c.