The Position of Methyl and Nitrogen on Axial Aryloxy Substituents

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The Position of Methyl and Nitrogen on Axial Aryloxy Substituents Determines the Crystal Structure of Silicon Phthalocyanines Hasan Raboui, Alan J. Lough, Trevor G Plint, and Timothy P Bender Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00298 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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Crystal Growth & Design

The Position of Methyl and Nitrogen on Axial Aryloxy Substituents Determines the Crystal Structure of Silicon Phthalocyanines Hasan Rabouia, Alan J. Loughb, Trevor Plinta and Timothy P. Bendera,b,c*

a

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200

College St, Toronto, Ontario, M5S 3E5, Canada b

Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, M5S

3H6, Canada c

Department of Materials Science and Engineering, University of Toronto, 180 College St,

Toronto, Ontario, M5S 3H6, Canada *

Correspondence email: [email protected]

Abstract Phthalocyanines are a class of organometallic compounds that have been investigated extensively for emerging applications such as organic photovoltaics, organic light-emitting diodes, and thin-film transistors. For these applications, the understanding of molecular structures and intermolecular interactions is extremely crucial and is one of the most promising pathways for designing a new generation of functional materials. Here, the crystal structures of six axially substituted silicon phthalocyanines (SiPcs) with aryloxy groups are reported. Three bis(methylphenoxy) SiPcs with varying methyl position and two bis(pyridoxy) derivatives with varying nitrogen position are compared to the bare bis(phenoxy) SiPc. Quantitative analysis is conducted to describe the impact of the position and nature of the functional group/atom on the orientation of the axial aryloxy subtituents as well as on the aromaticity and planarity of the SiPc macrocycle. Quantitative analysis of changes in intermolecular interactions, in particular π–π interactions, and visual representation using Hirshfeld surfaces are also presented. In all cases, the axial aryloxy substituent determined the

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solid-state arrangement except for the bis(4-pyridoxy) SiPc which is isostructural to bis(phenoxy) SiPc. Keywords: crystal; structure; engineering; relationship; organic;

electronics; silicon;

phthalocyanine; phenoxylate.

Introduction Phthalocyanines (Pcs) are a family of compounds that have been known in the literature for almost a century. They have been produced at an industrial scale and applied as dyes and pigments. Recently, the interest in Pcs has increased due to their interesting optical and electrical properties. They have been investigated in a variety of research areas such as thinfilm transistors (TFTs), 1, 2 organic light-emitting diodes (OLEDs),3, 4 and organic photovoltaics (OPVs).5-8 For these types of applications, the solid-state arrangement of materials used has a great impact on device performance. For example, Jian et al recently correlated the electron mobility and hole mobility of copper and zinc phthalocyanine derivatives in TFTs to their intermolecular stacking distances.9 Zinc and copper phthalocyanine (CuPc and ZnPc respectively) are typical divalent Pcs with a distinctive overall planar structure. Less systematic studies have been conducted on trivalent and tetravalent Pcs with axial substituents such as silicon phthalocyanines (SiPcs), whereby the axial substituent yields a molecular shape significantly different than CuPc and ZnPc. The Cambridge Crystallographic Data Centre (CCDC) database was examined producing seventy crystal structures of SiPcs. The majority of structures have axial substituents with chains of sp3 hybridized atoms. The role of these flexible chains is to boost degrees of freedom of the molecule and, therefore, solubility. Alkyl groups are used to increase the solubility in non-polar organic solvents for applications such as electronic devices fabrication.10-12 On the other hand, ether chains13,

14

and amine groups15 are preferred for

water/polar solvent processability for biological applications. However, there are only a few publications comprising more than a single crystal structure and systematically investigated the effect of molecular structure on solid-state packing; rare examples of systematic studies include Yang et al.16 producing crystal structures of oligomeric SiPcs with varying number of repeating units and Marks et al.17, 18 performing a study on the effect of counterions in doped SiPc films. Sosa-Sánchez et al., also, investigated the effect of the size of the alkyl tail on solubility and Page 2 of 27 ACS Paragon Plus Environment

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solid-state arrangement of carboxylate SiPcs.19 All of the aforementioned studies are of large molecules with bulky axial substituents and lack π–π interactions, which is detrimental for electronic functional materials. In fact, only a handful of the published structures have axial substituents with less than ten non-hydrogen atoms allowing for appreciable π–π interactions but there is no systematic study on the effect of the axial group on solid-state arrangement. Our group has a growing interest in SiPcs as functional electronic materials. Axial functionalization of SiPc with trialkylsiloxy groups was first used to increase the solubility of SiPcs to enable solution processability for usage as a ternary additive in bulk-heterojunction OPVs.11 We also found that by substituting the axial chloride atoms in the dichloro SiPc derivative with non-bulky phenoxy groups, the packing in the solid-state could be engineered to increase the desired π–π interactions; SiPc derivative with a fully fluorinated phenoxy group was first synthesized and the performance of OPV devices increased eight folds.8 By altering the amount of fluorination on the axial fluorophenoxy group, device performance was affected again with minimal effects on the photophysical and electrochemical properties of these compounds.20 Based on the derivatives that were synthesized, drawing a correlation between device performance, the nature of substituents, the positioning of atoms on the axial fluorophenoxy group remains a challenge. In this paper, the crystal structures of six bis(aryloxy) SiPc compounds were determined and quantitively analysed in terms of their molecular structures and intermolecular π–π interactions; amongst are two sets of structural isomers, bis(methylphenoxy) and bis(pyridoxy) derivatives (Figure 1). Starting with the bare bis(phenoxy) SiPc compound ((PhO)2-SiPc) as a baseline, a methyl group was placed first in the ortho ((2MP)2-SiPc), meta ((3MP)2-SiPc), and para ((4MP)2-SiPc) positions. Then, the phenoxy group was substituted with pyridoxy groups with the nitrogen atom alternating between the meta ((3Pyr)2-SiPc) and para ((4Pyr)2SiPc) positions. The synthesis and purification of the derivative with the nitrogen atom in the ortho position was deemed challenging and, therefore, no crystal structure of the molecule was able to be determined.

Experimental Materials All solvents used are ACS grades and were obtained from Caledon Laboratories Ltd. Phenol, m-cresol, o-hydroxypyridine, m-hydroxypyridine, and p-hydroxypyridine were purchased from Page 3 of 27 ACS Paragon Plus Environment


Sigma Aldrich. o-cresol was purchased from Fisher Scientific and p-cresol was purchased from TCI America.

Dichloro silicon phthalocyanine (Cl2-SiPc) was synthesized according to

literature.21 All chemicals were used as received unless specified otherwise. Synthesis General Synthetic procedure. All the compounds were synthesized by reacting the dichloro silicon phthalocyanine (Cl2-SiPc) with 10 molar excess of the respective phenol in refluxing chloroaromatic solvent for 24 hours as published previously.8 Chlorobenzene was used as a solvent for all compounds except for (4Pyr)2-SiPc where o-dichlorobenzene was used instead. The reaction mixture is then allowed to cool down before precipitation in isopropyl alcohol. The precipitates were stored in the fridge overnight. The next day, the solvent was removed using vacuum filtration and the cake was washed with toluene, acetone, and methanol sequentially. After the cake dried, blue powder was collected and was further dried in a vacuum oven. The powders were then purified using train sublimation22 to produce blue crystals. Synthesis of Bis(phenoxy) silicon phthalocyanine ((PhO)2-SiPc). This compound was synthesized according to the aforementioned general procedure starting with 0.5 gram of Cl2SiPc. The reaction yielded 0.460 g (74.5%) of blue powder. The powder was further purified twice using train sublimation with 81% and 89% yields of the first and second sublimation respectively.TOF-EI MS calc’d for C44H26N8O2Si 726.1948, found 726.1903. EA calc’d for C44H26N8O2Si: C, 72.7; H, 3.6 N, 15.4. Found: C, 72.0; H, 3.4; N, 15.3. UV-Vis in dichloromethane λmax = 680 nm. Synthesis of Bis(3-pyridoxy) silicon phthalocyanine ((3Pyr)2-SiPc). This compound was synthesized according to the aforementioned general procedure starting with one gram of Cl2SiPc. The reaction yielded 0.941 g (79%) of blue powder. The powder was further purified twice using train sublimation with 45% and 82% yields of the first and second sublimation respectively. TOF-EI MS calc’d for C42H24N10O2Si 728.1853, found 728.1832. EA calc’d for C42H24N10O2Si: C, 69.2; H, 3.3 N, 19.2. Found: C, 68.9; H, 3.15; N, 19.1. UV-Vis in dichloromethane λmax = 682 nm. Synthesis of Bis(4-pyridoxy) silicon phthalocyanine ((4Pyr)2-SiPc). This compound was synthesized according to the aforementioned general procedure starting with one gram of Cl2SiPc. The reaction yielded 1.078 g (90%) of blue powder. The powder was further purified using train sublimation with 20% yield. TOF-EI MS calc’d for C42H24N10O2Si 728.1853, found Page 4 of 27 ACS Paragon Plus Environment

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728.1835. EA calc’d for C42H24N10O2Si: C, 69.2; H, 3.3 N, 19.2. Found: C, 68.6; H, 3.3; N, 18.95. UV-Vis in dichloromethane λmax = 685 nm. Synthesis of Bis(2-methylphenoxy) silicon phthalocyanine ((2MP)2-SiPc). This compound was synthesized according to the aforementioned general procedure starting with 0.5 gram of Cl2-SiPc. The reaction yielded 0.535 g (87%) of blue powder. The powder was further purified using train sublimation with 21% yield. TOF-EI MS calc’d for C46H30N8O2Si 754.2261, found 754.2257. EA calc’d for C46H30N8O2Si: C, 73.2; H, 4.0 N, 14.8. Found: C, 73.2; H, 3.9; N, 14.6. UV-Vis in dichloromethane λmax = 681 nm. Synthesis of Bis(3-methylphenoxy) silicon phthalocyanine ((3MP)2-SiPc). This compound was synthesized according to the aforementioned general procedure starting with 0.5 gram of Cl2-SiPc. The reaction yielded 0.411 g (46%) of blue powder. The powder was further purified using train sublimation with 20% yield. TOF-EI MS calc’d for C46H30N8O2Si 754.2261, found 754.2261. EA calc’d for C46H30N8O2Si: C, 73.2; H, 4.0 N, 14.8. Found: C, 72.7; H, 4.0; N, 14.8. UV-Vis in dichloromethane λmax = 681 nm. Synthesis of Bis(4-methylphenoxy) silicon phthalocyanine ((4MP)2-SiPc). This compound was synthesized according to the aforementioned general procedure starting with 0.5 gram of Cl2-SiPc. The reaction yielded 0.460 g (75%) of blue powder. The powder was further purified using train sublimation with 14% yield. TOF-EI MS calc’d for C46H30N8O2Si 754.2261, found 754.2235. EA calc’d for C46H30N8O2Si: C, 73.2; H, 4.0 N, 14.8. Found: C, 73.0; H, 3.9; N, 14.7. UV-Vis in dichloromethane λmax = 680 nm. Crystallization Crystallization of the compounds was achieved through sublimation, slow vapor diffusion, or solvent evaporation (Table 1). Table 1. Crystallization methods for the silicon phthalocyanine compounds under investigation. Compound (PhO)2-SiPc

Vapor diffusion of cyclohexane into chloroform

(2MP)2-SiPc

Sublimation

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(3MP)2-SiPc

Vapor diffusion of heptane into THF and of pentane into benzene. Evaporation of chloroform and of THF

(4MP)2-SiPc

Sublimation

(3Pyr)2-SiPc

Vapor diffusion of heptane into THF. Evaporation of THF

(4Pyr)2-SiPc

Vapor diffusion of heptane into chloroform. Sublimation.

Refinement The crystal of (2MP)2-SiPc was a twin with a twin law: -0.11

0.87

0.00

1.14 0.12

0.01

-0.57

-0.57 -1.00

and a BASF parameter which refined to 0.499(2). (4MP)2-SiPc, (3Pyr)2-SiPc, and (4Pyr)2SiPc lied across inversion centers. Hirshfeld analysis The Hirshfield surfaces were generated using Crystal Explorer. 23

Results and Discussion Crystal Preparation All compounds were synthesized using a past established procedure8 by reacting the precursor dichloro silicon phthalocyanine (Cl2-SiPc) with excess of a methyl-phenol derivative or a hydroxypyridine to yield the respective bis(aryloxy) SiPcs (Figure 1). Each crude product was initially purified by washing with organic solvents (see experimental section). After drying, each bis(aryloxy) SiPc was further purified using train sublimation.22 Some, upon train sublimation, yielded single crystals of diffractable size and quality (Table 1). For others, solution methods (e.g. vapor diffusion) were used for crystal growth. Note, for these materials, the crystals structures obtained via sublimation are extremely valuable; they are likely to represent the structure of the material when operating in an organic electronic device because these materials are usually deposited/integrated using thermal vacuum sublimation techniques. The other bis(aryloxy) SiPcs whereby crystals where grown Page 6 of 27 ACS Paragon Plus Environment

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by more than one method and yet yielded the same structure indicates that the reported structures are probably thermodynamic minima. Therefore, these materials, too, are likely to form the same solid state structure when used in an organic electronic device. The structures of (4Pyr)2-SiPc24 and (3MP)2-SiPc12 are published in the literature and are confirmed in this study to have the same unit cell dimensions. Molecular Structures Previously published reports of bis(aryloxy) SiPc derivatives lack the quantitative analysis of the molecular structure and have focused on intermolecular interactions. Although there is no common thread of differences between the bis(methylphenoxy) and bis(pyridoxy) SiPc derivatives in terms of molecular structure, we are aiming in this section to provide a quantitative approach to describe the molecular structure of any bis(aryloxy) SiPc derivative. The main focus in this section was on the positioning of the axial molecular fragments and the effects they have on the aromaticity and planarity of the Pc macrocycle. Three angles were used to define and quantify the position of the aryloxy groups situated on the top of the SiPc macrocycle (Figure 2). The first angle is the oxygen bending angle (θO). The second angle, is the angle between the plane containing the axial aryl group and the axial plane containing the two axial bonds Si–O and O–C (θArAx). The third angle is the angle between the isoindole cross-sectional plane and the axial plane (θIndAx). In general, the differences of θO and θArAx between the structures of interest is minimal while θIndAx shows the greatest variation.


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Figure 1. Chemical structures and ORTEP figures showing thermal ellipsoids at 50% probability level. Colors: white – carbon, red – oxygen, blue – nitrogen, and yellow – silicon. Dashed bonds in (2MP)2-SiPc denote disorder in the crystal. Hydrogen atoms were omitted for clarity. CDCC deposition numbers: (PhO)2-SiPc (1824908); (3Pyr)2-SiPc (1824909); (4Pyr)2-SiPc (1824910); (2MP)2-SiPc (1824911); (2MP)2-SiPc (1824912); (4MP)2-SiPc (1824913).


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θO is measured between the silicon, oxygen, and carbon atom (e.g. Si1–O1–C17 in (PhO)2-SiPc). It is lowest for the meta substituted derivatives (i.e. (3Pyr)2-SiPc and (3MP)2SiPc) and averaging around 133°. These values are slightly higher than other freely-rotating and oxygen-containing organic compounds indicating a slight stress on the axial oxygen in SiPcs; ethyl acetate for example has an oxygen bending angle of 116°.25When compared to other published SiPc derivatives bonded to axial oxygens, these values are similar to bis(methoxy) SiPc (130°)26 and smaller than the bis(trialkylsiloxy) SiPc derivatives (156°).27 The second angle, θArAx, describes the torsion of the axial group around the axis containing the oxygen-carbon bond and was measured through the Si–O–C–C bonds (e.g. Si1– O1–C17–C18 in (PhO)2-SiPc). θArAx of most of the compounds is close to 90° except for (2MP)2-SiPc. θArAx of (2MP)2-SiPc is much smaller (66°) because of the steric due to the methyl group in the ortho position on the axial phenoxy group. The most variation in the axial group positioning between the molecules is observed with the θIndAx. It is the measure of the torsion between the SiPc macrocycle and the axial aryloxy group along the Si–O bond. The angle is a measure of the alignment between the axial aryloxy groups and the isoindole groups within the SiPc macrocycle. The torsion angle was measured between the closest isoindole nitrogen to the axial group, the silicon atom, an oxygen atom, and the carbon atom attached to that oxygen atom (e.g. N1, Si1, O1, and C17 in (PhO)2SiPc). A zero-degree angle denote a total eclipse between the axial group and an isoindole group while a 45° denote maximum staggering. The axial groups in (3Pyr)2-SiPc and (2MP)2SiPc eclipse with their respective isoindole fragments while they stagger in (3MP)2-SiPc. The remaining compounds exhibit angles close to 20° that is between the staggered and eclipsed positions.



Figure 2. Angles defining the position of the axial group over the Pc macrocycle. Values for (2MP)2-SiPc are for the non-disordered side of the molecule. Then, the impact of the axial ligand on the aromaticity and planarity of the π-conjugated SiPc macrocycle, which includes 32 carbon atoms, 16 hydrogen atoms, 8 nitrogen atoms, and a silicon atom was investigated. There are two theoretical studies in literature on quantifying the aromaticity of Pcs using different metrics of theoretically optimized Pc geometries.28, 29 Amongst those metrices is the Harmonic Oscillator Model of Aromaticity (HOMA) which convey extra significance when the crystal structure, and therefore bond lengths, are known. HOMA is a molecule- or ring-averaged measure of the deviation in bond lengths from an absolutely aromatic compound (e.g. benzene). HOMA is equal to unity when the molecule is aromatic and approaches zero when aromaticity is decreased. HOMA is defined as:


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1

1

(1)

where n is the number of bonds involved in the calculation, α is a normalization constant, ROpt is the bond length of an absolutely aromatic compound, and Ri is the length of the bonds of the molecule. α and ROpt are set to 257.7 and 1.388 Å for C–C bonds and 93.52 and 1.334 Å for C–N bonds, respectively.30 The calculations involved all of C–C and C–N bonds within the macrocycle; the calculation excluded the axial aryloxy groups and bonds formed with silicon due to the lack of α and ROpt values (Figure 3). Overall, the deviation between the derivatives is subtle. The fact that the axial substituents have a minimal effect on the aromaticity of the macrocycle, which where chromphoric properties arise from, is in-line with our previous hypothesis that control of the packing of Pcs can be achieved by adjusting the axial position without disturbing the photophysical properties of Pcs significantly. 31 When planarity of the macrocycle is considered, there are many approaches that can be adopted to quantify the impact of the axial ligand. Here, three metrics were selected (Figure 3). The first one (θBnPy) quantifies the planarity within the isoindole fragments by measuring the angle between the benzene ring and the pyrrole ring in the same isoindole unit. The second two metrics measure the torsion between the isoindole units. This was done by measuring the torsion between to two inner C–C bonds in two different benzene rings (e.g. C3–C8 and C16– C11 in (PhO)2-SiPc) (inner benzene torsion) and the torsion between to two outer C–C bonds in two different benzene rings (e.g. C5–C6 and C13–C14 in (PhO)2-SiPc) (outer benzene torsion). The values of each of these metrics is variable depending which rings/bonds are considered in each molecule; only the highest values observed are presented in Figure 3. In contrast to aromaticity, the axial ligand affected the planarity of the macrocycles regardless of which quantitative approach was taken. The methyl group, especially in the meta position, seem to cause the most deformation of the SiPc macrocycle with outer benzene torsion as large as 11.4°. (PhO)2-SiPc and (Pyr)2-SiPc which are isostructural, as will be discussed in the following section, exhibit the lowest level of deformation.



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Figure 3. Impact of the axial substituent on the aromaticity (HOMA) and the planarity (θBnPy, inner benzene torsion, and outer benzene torsion) of the SiPc macrocycle. For the planarity parameters, only the largest observed value for each molecule is presented. Because these materials are likely to be deposited using thermal evaporation techniques, thermal stability of these materials is of great interest. A structural parameter that is relevant to thermal stability is the silicon–oxygen bond strength because it is the most likely source for thermal degradation. Bond strength usually correlates well to bond length for a given bond. The (4Pyr)2-SiPc has the shortest Si–O bonds and, therefore, is likely to be the most stable compound (Table 2). (3Pyr)2-SiPc, on the other hand, which differs only with the position of the nitrogen atom on the axial group and (3MP)2-SiPc have the longest Si–O bonds and are likely to be the least thermally stable compounds in the list. Table 2. Si–O bond length as an indicator of thermal stability.

Si–O Bond (Å)

(PhO)2-SiPc

(3Pyr)2-SiPc

(4Pyr)2-SiPc

1.7165(13)

1.7302(9)

1.7143(19)

(2MP)2-SiPc

(3MP)2-SiPc

1.7201(19),

1.735(4),

1.721(2)

1.720(4)


(4MP)2-SiPc 1.7166(10)

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Intermolecular π–π Interactions The long-range arrangement of these molecules is greatly dictated by the π–π interactions between the macrocycles. Because π–π interactions are of great significance in the field of organic electronics, modes and motifs of π–π interactions were carefully examined. Stacking modes will be used from hereafter to denote the type of π–π interaction two SiPc molecules display regardless of the long-range arrangement. Stacking motifs denote how the molecules arrange in the long range beyond two molecules. To characterize an interaction between two aromatic groups as a π–π interaction, the distance between the rings should not exceed 4 Å and the dihedral angle between the two aromatic planes should be less than 5°. Although none of the aromatic axial groups participated in any π–π interactions, the effects of these groups on the stacking are substantial in comparison to the baseline, (PhO)2-SiPc. The only exception is (4Pyr)2-SiPc which is isostructural to (PhO)2-SiPc. π–π Interaction Modes Previous reports on SiPcs12, 20 have recognized two modes of π–π interactions between two SiPc molecules (Figure 4). The first mode is the dual benzene stacking mode; in this mode, two benzene rings from each molecule participate in the π–π stacking. The second mode is the isoindole stacking mode where only one isoindole unit participate in the stacking from each molecule. Since the isoindole consists of two aromatic groups (benzene and pyrrole), isoindole stacking is characterized by two distances: the first distance is between the pyrrole group in one molecule and the benzene ring on the other molecule (B1); the second distance is the distance between the two benzene rings (B2). Here, two new π–π interaction modes of SiPcs are introduced. The first new mode is the dual isoindole–benzene stacking mode (Figure 4). In this mode, a benzene ring and an isoindole group from each molecule participate in the π–π stacking. Again, since isoindole units consist of two aromatic group, the π–π stacking is characterized by two sets of distances. C1 is the pyrrole–benzene distance and C2 is the benzene–benzene distance. The last mode of interaction is the single benzene mode (Figure 4). In this mode, only a single benzene from each molecule participates in the stacking. While previously reported SiPc structures contained only one of the aforementioned stacking modes. Here, we report crystal structures where more than one mode exists in the same crystal. We can divide the SiPcs under investigation into four categories based on the π– π modes they adopt. The first category includes (PhO)2-SiPc and (4Pyr)2-SiPc. These two Page 13 of 27 ACS Paragon Plus Environment


compounds interact through two modes of stacking simultaneously, the dual benzene stacking and the single isoindole stacking modes. (3Pyr)2-SiPc is in a category by itself exhibiting only a dual isoindole–benzene stacking. (2MP)2-SiPc is the only compound that assume three types of π–π interactions simultaneously; it exhibits the single isoindole stacking mode, the dual isoindole–benzene stacking mode, and the single benzene stacking mode. (3MP)2-SiPc and (4MP)2-SiPc interact only through the single isoindole stacking mode. This array of structures allows for investigating the direct effect of π–π interaction distances on organic electronic devices.


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Figure 4. π–π interaction modes of SiPcs and their respective distances. Compounds that assume more than one mode of stacking have the capability for 2D π– π stacking. These compounds are (PhO)2-SiPc, (4Pyr)2-SiPc, and (2MP)2-SiPc (Figure 5). The advantage of possessing two modes of π–π interactions is the formation of a 2D network of π Page 15 of 27 ACS Paragon Plus Environment


electrons in close proximity. This structure is potentially useful organic electronics facilitating for free-charge hopping in more than one dimension. The 2D structures of (PhO)2-SiPc and (4Pyr)2-SiPc are symmetric because only two modes of π–π stacking are observed in these structures. Within these crystals, the 2D stacking is composed of two 1D π–π stacks; in each of these stacks, only one mode of π–π interaction takes place. The 2D stacking in (2MP)2-SiPc, conversely, is asymmetric around a given molecule, but still symmetric in the long-range; each dimension comprises two modes of stacking in an alternating fashion. In one dimension, the dual isoindole–benzene stacking mode alternates with the single benzene stacking mode to form a continuous stack. The other dimension is formed by alternating between the single isoindole stacking mode and the single benzene stacking mode.


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Figure 5. Multidimensional π–π stacking of SiPcs. Red molecules are interacting through π– π stacking while grey molecules are not. Long-Range Motifs



Now that the types of interactions between molecules are well-defined, long-range motifs can be described. The two main stacking motifs are the herringbone motif and the lamellar motif (Figure 6). Both motifs can be described by the herringbone angle, which is measured between the planes of two SiPc macrocycles. A perfect herringbone motif assumes 90° herringbone angle between the SiPc macrocycles while a perfect lamellar motif assumes a zero-degree angle. (PhO)2-SiPc and (4Pyr)2-SiPc are the closest compounds to a herringbone motif with herringbone angles of 85.71° and 85.48°, respectively. (2MP)2-SiPc and (4MP)2SiPc exhibit a perfect lamellar motif. (3Pyr)3-SiPc and (3MP)2-SiPc arrange in intermediate motifs between the herringbone and lamellar motifs with herringbones angles of 22.14° and 68.37°, respectively. (PhO)2-SiPc and (4Pyr)2-SiPc adopt 2D stacking in a single plane, the crystal lattice consists of molecules inhabiting two planes perpendicular to each with the absence of π–π interaction between the planes. (4MP)2-SiPc conform to a lamellar motif but does not form a 3D network because the π–π stacking mode it assumes is only in one dimension. Because (2MP)2-SiPc adopt a lamellar motif and exhibit 2D π–π stacking structure within a single plane, it is the only molecule that forms a 3D network of π–π interacting molecules. Consequently, we predict that (2MP)2-SiPc will display the most superior charge transport properties. Electron and hole mobility measurements of these compounds coupled with the detailed structural analysis presented in this report will help solidify our understanding of the structure–property– device performance of these materials; to minimize the influence of variables beside the crystal structure (e.g. degree of crystallinity of the sample) on charge transport, these measurements are better conducted on single crystals in a similar fashion to that was demonstrated by Jiang et al.32


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Figure 6. Long range motifs and the herringbone angles measured of all SiPc molecules. Hirshfeld Surface Analysis Hirshfeld surface (HS) analysis is a powerful tool that can facilitate visualizing the interactions within a given crystal.33, 34Shape index and curvedness are properties of HS and can be used to identify π–π interactions (Figure 7). In the shape index map, red regions represent a concave region on a HS while the blue regions represent a convex region. Areas that have the same shape but are only different in color represent a complementary interacting pair. Curvedness is a similar property that measures the flatness of the surface. Green regions are relatively flat while dark blue lines represent large curvature. Flat regions on the curvedness surface are potential indicators of π–π stacking. Shape index and curvedness were calculated for SiPcs. Since the HS of most of these molecules are symmetric, only one face of the surfaces is shown with the exception of the disordered molecule (2MP)2-SiPc. Matching triangles, or “bowties”, in the shape index are characteristic of π–π stacking. The flatness on the curvedness maps is used to confirm that the interactions are planar π–π interactions rather than an atom interacting with an aromatic ring.



All aforementioned modes of π–π stacking were identified and are highlighted on the shape indices in Figure 7. Moreover, dnorm was used to identify the non-planar interactions. Regions on the dnorm map are shaded from red (for contacts shorted than the Van der Waals radii) to blue (for contacts longer than the Van der Waals radii). Red regions on the dnorm map on the face of an aromatic group do not necessarily indicate the presence of a π–π interaction. An example is the map of (3Pyr)2-SiPc; there are two red dots appearing on two of the outer benzene units of the SiPc macrocycle. These dots arise from C5C15 interaction. While the distance between the benzene rings containing C5 and C15 is close to 4 Å (4.147 Å), the dihedral angle between the rings are quite large (17.33°). Another example is the red dot on the face of the axial group of (3Pyr)2-SiPc, this dot arises from C–Hπ interaction with Hπ distance of 2.71 Å.

Figure 7. Hirshfeld analysis of SiPcs. The letters on the shape index map denote the different modes of π–π interactions. A – dual benzene stacking; B – single isoindole stacking; C – dual isoindole-benzene stacking; D – single benzene stacking.

Conclusion Crystals of six bis(aryloxy) SiPcs were grown and their structure were determined. The six compounds contain two sets of structural isomers, bis(methylphenoxy) and bis(pyridoxy) derivatives. While the (4Pyr)2-SiPc derivative is isostructural to the baseline, (PhO)2-SiPc, all of the remaining derivatives had unique structures. A quantitative analysis on the position of the axial aromatic groups and its impact on the aromaticity and planarity of the SiPc macrocycle Page 20 of 27 ACS Paragon Plus Environment

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was developed. Two new π–π stacking modes were identified and some derivatives experienced more than one mode in the same structure. (PhO)2-SiPc and (Pyr)2-SiPc exhibit a symmetric 2D network of molecules interacting through π–π stacking. (2MP)2-SiPc assumes a 3D network of π–π stacking molecules. This study proves that minuscule changes in the chemical structure of SiPcs can have drastic impacts on the structure in the solid-state. Understanding the crystal structures of these compounds can provide insight into the structure– property–device performance relationships with the goal of establishing design rules for new functional materials for organic electronic devices. Acknowledgements The authors would like to acknowledge ﬁnancial support from Saudi Basic Industries (SABIC). We would like to acknowledge the Natural Sciences and Engineering Research Council (NSERC) of Canada for their support through the Discovery Grant program. Supporting Information Thermal ellipsoid plots figures of individual molecules and detailed crystallographic data (bond lengths, atomic coordinates, etc.).

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For Table of Contents Use Only

The Position of Methyl and Nitrogen on Axial Aryloxy Substituents Determines the Crystal Structure of Silicon Phthalocyanines Hasan Rabouia, Alan J. Loughb, Trevor Plinta and Timothy P. Bendera,b,c*

a

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200

College St, Toronto, Ontario, M5S 3E5, Canada b

Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, M5S

3H6, Canada c

Department of Materials Science and Engineering, University of Toronto, 180 College St,

Toronto, Ontario, M5S 3H6, Canada

*

Correspondence email: [email protected]

Snopsis: Small changes in the chemical structure of aryloxy silicon phthalocyanine derivatives determine the molecular structures and intermolecular interactions (e.g. π-π interactions between two phthalocyanine macrocycles).



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Figure 1 66x24mm (300 x 300 DPI)





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The Position of Methyl and Nitrogen on Axial Aryloxy Substituents

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