One Well-Placed Methyl Group Increases the Solubility of Phenoxy

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One Well-Placed Methyl Group Increases the Solubility of Phenoxy Boronsubphthalocyanine Two Orders of Magnitude Andrew S. Paton,† Alan J. Lough,‡ and Timothy P. Bender*,†,‡ †

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada, M5S 3E5 ‡ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6 S Supporting Information *

ABSTRACT: Boronsubphthalocyanine (BsubPc) derivatives are materials that have recently seen an increase in attention, particularily for application as colorants and functional materials in organic electronic devices. However, the solubilities of most BsubPc derivatives are too low to utilize solution-based processing techniques. Where there are derivatives that are soluble, they rely on the addition of large alkyl chains for solubilization the result of which is an increase in the molecular weight and a decrease of the specific absorptivity of the BsubPc. In this paper we disclose the identity of two mass-efficient, highly soluble BsubPc derivatives: 3-methylphenoxy-BsubPc and 3,4-dimethylphenoxy-BsubPc. We compare their solubilities and crystal packing structures to previously known BsubPc derivatives. We put forward a symmetry-based argument for the reason for their surprisingly high solubility wherein we treat the molecules as two separate fragments: the phenoxy group and the BsubPc group. The symmetry-based argument suggests that by ignoring the BsubPc fragment common to each derivative a better estimate of the relative solubilities can be estimated through previously established empirical formulas.

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solid state.17−20 Similarly, others have used 2-allylphenoxyBsubPc as a soluble derivative amenable to solution processing.7 Recently, we have shown that this approach can be used to acquire very soluble PhO-BsubPc derivatives. [We will use the acronym PhO-BsubPc to denote a generic phenoxylated BsubPc. We will use the name phenoxy-BsubPc to denote the specific derivative made from Cl-BsubPc and phenol.] Our approach utilized the incorporation of the 3pentadecylphenoxy molecular fragment into a variety of positions of the PhO-BsubPc molecule.21 When the 3pentadecylphenoxy fragment is placed in either the axial or peripheral position of the BsubPc moiety, the resulting derivatives were very soluble in common organic solvents. This included the derivative made by the simple reaction of ClBsubPc with 3-pentadecylphenol.21 Given the solubility of this simple derivative and the considerable size of the pentadecyl chain, we were interested to know the smallest possible hydrocarbon chain which could be placed in the 3-position of the phenoxy molecular fragment of a PhO-BsubPc and still impart high solubility to the derivative. Stated another way, we were interested to understand the most mass efficient way to impart high solubility to a PhO-BsubPc derivative. While this would have no effect on the molar extinction coefficient (ε), high mass efficiency would have the net result of improving the specific absorptivity (absorption per unit mass, a value more commonly used in the colorant industry). We initially started with a methyl group and found the resulting derivative 3-methyl-

INTRODUCTION Boronsubphthalocyanine (BsubPc), the sole contracted member of the well-known phthalocyanine (Pc) family of compounds,1 has recently become of interest as a functional material for organic electronic devices.2−11 Due to its aromatic yet nonplanar π-electron system, BsubPc possesses desirable optical and electronic properties and has been studied for use in organic light-emitting diodes,2−6 organic photovoltaics,7−9 and organic thin film transistors.10 We have recently highlighted that BsubPc derivatives are relatively insoluble in common organic solvents.11 Solution-based fabrication techniques are often cited as necessary for organic electronic materials to achieve cost-effectiveness over their inorganic counterparts.12,13 The insolubility observed for BsubPc derivatives could therefore be a barrier to their application where solutionbased deposition techniques are desirable. A common method for increasing the solubility of normal phthalocyanines (nPcs) has been to use 3-t-butylphthalonitrile as a precursor material. The placement of t-butyl substituents peripherally on the nPc ring results in the formation of multiple constitutional isomers differing in the relative orientation of the t-butyl groups around the nPc structure. The formation of isomers combined with the size of the t-butyl group contributes to increasing the solubility. This method has also been used to form soluble derivatives of BsubPcs,14 and similar methods have been used to create water-soluble BsubPcs by using a phthalonitrile with ionically charged groups.15,16 Beyond cases related to nPcs and BsubPcs another common method for solubilizing small molecules for solution processing (without the formation of structural isomers) utilizes the incorporation of long-chain and/or branched alkyl substituents into the molecular structure to increase association with particular solvents or to physically inhibit aggregation in the © 2012 American Chemical Society

Received: Revised: Accepted: Published: 6290

December March 29, March 31, March 31,

21, 2011 2012 2012 2012

dx.doi.org/10.1021/ie202998f | Ind. Eng. Chem. Res. 2012, 51, 6290−6296

Industrial & Engineering Chemistry Research

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methylphenol (0.878 g, 0.00812 mol) in toluene (10 mL) in a cylindrical vessel fitted with a reflux condenser and argon inlet. The mixture was stirred and heated at reflux under argon until reaction was determined complete via HPLC analysis (approximately 72 h). The excess 3-methylphenol was removed by dissolving the product in toluene (300 mL) and extracting with 3.0 M KOH in distilled water (3 × 300 mL). The aqueous phase was discarded, and the solvent was removed under rotary evaporation. The crude product was purified on a Kauffman column using standard basic alumina (300 mesh) as the adsorbent and dichloromethane as the eluent. The product eluted from the Kauffman column, while the excess phenol remained adsorbed. The dichloromethane was then removed under reduced pressure yielding a dark pink/magenta powder with gold sheen. Yield 0.656 g (0.00131 mol, 84.6%). Tm = 280 °C 1H NMR 400 MHz (CDCl3 ref to TMS): δ = 1.94 (s, 3H), 5.13 (dd, 1H), 5.25 (s, 1H), 6.42 (d, 1H), 6.63 (t, 1H), 7.877.92 (m, 6H), 8.82−8.87 (m, 6H); UV−vis (CHCl3) λmax = 563 nm; HRMS (EI) Calcd. for [C31H19BN6O] ([M]+): m/z 502.1712, found 502.1713. 3,4-Dimethylphenoxyboronsubphthalocyanine (2b). ClBsubPc (0.493 g, 0.00114 mol) was mixed with 3,4dimethylphenol (0.723 g, 0.00592 mol) in toluene (10 mL). The reaction and purification proceeded as for 2a, except that reflux was maintained for 100 h. Yield 0.290 g (0.000562 mol, 49.1%). Tm = 280 °C; 1H NMR 400 MHz (CDCl3 ref to TMS): δ = 1.83 (s, 3H), 1.92 (s, 3H), 5.09 (d, 1H), 5.24 (s, 1H), 6.48 (d, 1H), 7.85- 7.89 (m, 6H), 8.81−8.85 (m, 6H); UV−vis (CHCl3) λmax = 563 nm; HRMS (EI) Calcd. for [C32H21BN6O] ([M]+): m/z 516.1868, found 516.1870.

phenoxy-BsubPc (compound 2a, Scheme 1) to be surprisingly soluble in common organic solvents. For example in chloroScheme 1. Synthesis of BsubPc Derivatives (2a−b) from ClBsubPc (1)a

a

Conditions: (i) 5 equiv, toluene, reflux, -HCl.

form it was ∼31 times more soluble than its counterpart with the methyl group in the 4-position (Table S1). In this article we suggest one factor in why we believe there is such a dramatic difference in solubility between these two BsubPc derivatives. Key to the discussion is a consideration of the symmetry of each molecular fragment of the PhO-BsubPcs.

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EXPERIMENTAL SECTION Materials. 3,4-Dimethylphenol was purchased from TCI America (Portland, Oregon). Toluene, dichloromethane, chloroform, acetone, chlorobenzene, THF, KOH (pellets), and standard basic alumina were all purchased from Caledon Laboratories (Caledon, Ontario). Propyl acetate, 3-methylphenol, and n-propanol were purchased from Sigma-Aldrich. All materials were used as received. General Methods. All nuclear magnetic resonance (NMR) spectra were acquired on a Bruker 400 MHz system in deuterated chloroform (CDCl3) purchased from Cambridge Isotope Laboratories which was used as received. All 1H NMR spectra where referenced to an internal standard of 0.05% TMS. All crystal structures were collected using computer-controlled KappaCCD system and an Oxford Cryostream variable temperature apparatus. All ultraviolet−visible (UV−vis) spectroscopy was performed using PerkinElmer Lambda 1050 with a 10 mm path length in a quartz cuvette. High pressure liquid chromatography (HPLC) analysis was conducted using a Waters 2695 separation module with a Waters 2998 photodiode array and a Waters 4.6 mm ×100 mm SunFireTM C18 3.5 μm column. HPLC grade acetonitrile and DMF were eluted at 0.6 mL/min during operation at a constant composition of 80:20, respectively. Mass spectrometry was performed on a Waters GC Time-of-Flight mass spectrometer with an electron ionization probe and accurate mass determination. X-ray diffraction data were collected on a Nonius Kappa-CCD diffractometer using monochromated Mo−Kα radiation and were measured using a combination of φ scans and ω scans with κ offsets, to fill the Ewald sphere. The data were processed using the Denzo-SMN package.22 Absorption corrections were carried out using SORTAV.23 The structure was solved and refined using SHELXTL V6.124 for full-matrix least-squares refinement that was based on F2. All H atoms were included in calculated positions and allowed to refine in riding-motion approximation with U∼iso∼ tied to the carrier atom. X-ray diffraction results were analyzed using PLATON 40 M25 for bond angles and lengths, and crystal packing images were generated using Mercury version 3.0.26 Methods. 3-Methylphenoxyboronsubphthalocyanine (2a). Cl-BsubPc (0.673 g, 0.00156 mol) was mixed with 3-

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RESULTS AND DISCUSSION In accordance with our statements above and with the goal of minimizing molecular weight, we initially targeted the smallest phenol derivative containing a hydrocarbon in the 3-position for reaction with Cl-BsubPc. On reaction of Cl-BsubPc with 3methylphenol (m-cresol) we formed 3-methylphenoxy-BsubPc (compound 2a, Scheme 1) using previously established procedures.11,27,28 We also synthesized 3,4-dimethylphenoxyBsubPc (compound 2b) as a point of comparison (by reaction of Cl-BsubPc with 3,4-dimethylphenol). After the reaction was complete, the excess phenol was partitioned into aqueous base. Upon removal of the organic phase the residue was purified via Kauffman column chromatography which on evaporation of the eluting solvent resulted in a powder with a gold luster. Identity, purity, and spectroscopic characteristics were established using standard techniques (see the Supporting Information). The solubilities of compounds 2a and 2b were measured, using our previously published method11 in which small amounts of the compounds were added to a variety of solvents until undissolved material remained after stirring at room temperature for 24 h. The results of the solubility measurements are summarized in Table S1. According to the definitions for dye and pigment we have previously proposed, a solubility of at least 3 mg/mL was chosen to represent a soluble derivative of BsubPc.11 Both compounds 2a and 2b were found to exceed the threshold to be classified as a dye for five of the seven solvents measured. Chloroform, chlorobenzene, propyl acetate, tetrahydrofuran (THF), and toluene dissolved >3 mg/ mL of both compounds. Solubilities below this limit were found for acetone and n-propanol, two of the more polar solvents in the series. Solubilities of compounds 2a−b in chloroform were found to be 103 mg/mL for compound 2a and 90 mg/mL for 6291



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Figure 1. Single crystal X-ray diffraction determined solid state arrangements. (a) Compound 2a grown from benzene/heptane with incorporated benzene: (i) front and (ii) a perspective approximately 45° from the front. (b) Compound 2b grown from benzene/heptane with incorporated benzene: (i) front and (ii) a perspective approximately 45° from the front. Colored for clarity as follows: BsubPc carbons − gray; benzene carbons − black; phenoxy molecular fragments − alternating orange and magenta; boron − yellow; oxygen − red.

compound 2b. For comparison, the solubilities of phenoxyBsubPc and 4-methylphenoxy-BsubPc in chloroform were previously found to be 2.9 and 3.2 mg/mL, respectively, ∼30 times less soluble despite only differing by the presence or location of a single methyl group.11 In an effort to explain the dramatic solubility difference we obtained single crystals of compounds 2a and 2b by two different vapor diffusion methods. Single crystals obtained from benzene/heptane (good solvent/diffusing solvent, respectively) were found to contain a benzene solvate (Figure 1), whereas crystallizations from acetone/heptane formed solvent-free single crystals (Figure 2) for both 2a and 2b. Crystallographic details including all bond lengths and angels are provided in the Supporting Information. The benzene solvates of compounds

2a and 2b (Figure 1) are in contrast to crystals of phenoxyBsubPc and 4-methylphenoxy-BsubPc which when grown from the same solvent system do not form solvated crystals.28 That 2a and 2b form a solvate with benzene under these conditions may be construed as evidence of their tendency to associate with aromatic solvents, leading to their high solubility. However, such a direct conclusion would be ill advised as we have observed benzene solvation for other phenoxy-BsubPc derivatives.28 Furthermore, it is not easily used as a design criterion for obtaining future soluble BsubPc derivatives. Each benzene solvate showed a similar solid-state arrangement. However, the distance between one BsubPc unit and its nearest neighbor (the d1 distance defined as the boron−boron distance between two nearest neighboring molecules28) is 6292



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Figure 2. Single crystal X-ray diffraction determined solid state arrangements. (a) Compound 2a grown from acetone/heptane with no incorporated solvent: (i) front and (ii) side. (b) Compound 2b grown from acetone/heptane with no incorporated solvent: (i) front and (ii) side. Associated dimer pairs of BsubPcs are shown in either cyan, yellow, or magenta for clarity.

increased to 12.26 Å for 2b compared with 11.32 Å for 2a due to two benzene molecules associated within the concave sides of the BsubPc fragment. The benzene solvent molecule was refined as disordered over two sets of sites with equal occupancy. Each component was refined as a ’variable metric’ group whereby the shape is maintained, but it may expand or contract during the refinement. The components of some of the anisotropic displacement parameters were subject to ’rigid bond’ restraints. It is also interesting to note that despite the potential for variation in the orientation of the 3-methylphenoxy and 3,4-dimethylphenoxy groups (solid-state rotational isomerism), there is no observed disorder in the position of the methyl groups as exemplified in the solid state arrangement of the solvated crystal of 2a (Figure 1a). The methyl group of the 3-methylphenoxy fragment alternates facing ‘left’ and ‘right’ in neighboring molecules along the crystallographic axis. Disorder within the solid would lead to higher solubility; however, since none is observed, we can rule this out as a reason for our observation regarding the high solubility of 2a and 2b. Comparison of the solvent-free crystal structures of 2a, 2b, phenoxy-BsubPc, and 4-methylphenoxy-BsubPc again highlights the similarities between this selection of compounds. Each structure shows the same close packed pairs of molecules (dimers), which we have previously noted as a common

packing motif for phenoxylated BsubPcs, including phenoxyBsubPc and 4-methylphenoxy-BsubPc.28 According to the metrics we have previously established28 a direct quantitative comparison of this packing structure shows that the spacing of the molecules in all of the solvent-free crystal structures varies only slightly (Table 1). The molecules making up the close packed pairs are the same distance apart (as indicated by the B−B d1 and Cg to Cg distances) in each arrangement, with the boron−boron (d1) distance between the closest molecules in different pairs ranging between 5.576(3) Å and 7.011(4) Å for phenoxy-BsubPc and compound 2a, respectively. Accompanying this change is an increase in the B−O−C angle and a corresponding small decrease in the crystal density (∼1.36 g/ cm−3 for 2a and 2b versus ∼1.42 g/cm−3 for the reference compounds). This comparison is summarized in Table 1. Due to the similarities between the solvent free crystal structures of compounds 2a and 2b and the reference compounds, we must conclude that it is not the solid state arrangement which leads to the noted increase in solubility for each compound. Ruling out crystal packing structure and solid-state disorder as causes for the high solubility of 2a and 2b, the only difference between these structures and phenoxy-BsubPc and 4-methylphenoxy-BsubPc is that 2a and 2b have a metapositioned methyl group on the phenoxy substituent. This difference means that phenoxy-BsubPc and 4-methylphenoxy6293



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Table 1. Selected Measurements Extracted from the Solid State Arrangement of BsubPc Derivatives 2a and 2b within Single Crystalsa

a See: aOrientation of the closest aromatic ring to ring center of gravity separation as calculated by PLATON.25 bconc = BsubPc concave face; conv = BsubPc convex face.

τ = nSP3 + 0.5nSP2 + 0.5nRING − 1

BsubPc possess C1V symmetry and while the molecular weights, molecular shapes, and solid state arrangement of the compounds are essentially the same, the presence of the meta methyl group removes the vertical reflection plane, giving compounds 2a and 2b a reduced symmetry of C1. We must then conclude that it is the loss of the vertical reflection plane or, in other words, the presence of rotational isomerism in the phenoxy molecular fragment that results in the large increase in solubility. While this is not seen in the respective solid state arrangement, it is surely present in solution. This conclusion may seem intuitive as it is well-known that molecular symmetry has an effect on both the melting temperature and solubility of organic crystals.29,30 Over the past two decades, empirical formulas have been developed based on thermodynamic arguments which allow reasonably accurate estimations of the ideal solubility from knowledge of just the molecular structure and the melting temperature of a small molecule organic compound.30−34 It would be of interest to see if these empirical frameworks can be used to support our observations and move from the realm of intuition to prediction. According to Yalkowsky and Wu,31 to estimate the ideal solubility, or crystal−liquid fugacity ratio (CLFR), the equation log(CLFR) = − (ΔSm /5700)(Tm(°C) − 25)

where nSP3 and nSP2 are the number of nonring, nonterminal sp3 and sp2 atoms in the molecule (respectively) and nRING is the number of fused ring systems present.31 It is a representation of the probability that any flexible part of a molecule will be in the correct orientation for incorporation into a crystal or solid. The application of the above variables and equations has been shown to provide accurate estimates of CLFR based on comparisons to measured entropies of melting for organic molecules.31 Common ranges for log(CLFR) are −0.1 to −4.0. If we consider the entire molecule (2a−b or one of the reference compounds) and following the definitions of σ and τ, the closely related derivatives all possess a σ of 1. Consideration of the BsubPc molecular fragment as a single fused ring system and the phenoxy as another each separated by a single sp3 oxygen atom yields a τ of 1 for each derivative. It then follows that application of eq 3 and eq 2 will yield the same value for ΔSm. Thus the only difference would come about from their difference in Tm and the application of eq 1 using the melting point of 4-methylphenoxy-BsubPc (307 °C). Considering only the difference in Tm yields values for log(CLFR) of −2.56 and −2.80 for compound 2a (3-methylphenoxy-BsubPc; Tm = 280 °C) and 4-methylphenoxy-BsubPc, respectively. Considering the same but instead treating the BsubPc fragment as 7 fused rings yields values for log(CLFR) of −3.39 and −3.75, respectively. These values are in the correct order of magnitude, but their difference is small and thus cannot adequately explain the large differences in solubilities we have observed. Although not the intention of Yalkowsky et al., who considers a molecule as a whole, if we consider the general phenoxy-BsubPc molecule as a two-component system (two molecular fragments) connected by a rigid rotor at the B−O−C bond this may allow for a comparison of the molecules by the symmetry of their substituent groups. Since the B−O−C bond allows the rotation of the phenoxy group, the orientation of

(1)

can be used, where Tm is the melting point (°C) and ΔSm is the entropy of melting (J·mol−1·K−1) given by ΔSm = 50 − (19.1)log(σ ) + 7.4τ

(3)

(2)

where σ is the molecular rotational symmetry number and τ is the molecular flexibility number. The value of σ is defined as the number of ways the molecule can be superimposed on itself through rigid rotation, and conceptually is proportional to the probability that a molecule will be properly oriented for incorporation in a crystal or solid.31 The flexibility number τ is defined as 6294



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difference between 4-methylphenoxy-BsubPc and 3-pentadecylphenoxy-BsubPc. While the use of the framework of Pinal allows us to move beyond intuition, there is clearly a shortfall to the application of this framework to the BsubPc system.

that group can be in any direction with respect to the BsubPc and this is the same for both 2a and 4-methyl-phenoxy-BsubPc. Because of this rotation, discussion of the symmetry of the molecule is the same whether or not the molecules are treated as a whole or in parts. It is simpler, in our opinion, to treat the BsubPc and phenoxy molecular fragments as separate moieties, as it eliminates the common BsubPc unit from the discussion. The lack of variation in the B−O−C angle for these and similar structures implies that this bond does not possess a great deal of flexibility unless deformed by other factors in the solid state.28 Thus, the linkage to the phenoxy moiety is more of a rigid rotor than a point of flexibility in the molecule, and it can also be eliminated from the discussion. Therefore taking the B− O−C bond as rigid (nSP3 = 0) and considering only one fused ring system (nRING = 1) yields a τ = 0.5. The reflection plane of the 4-methyl-phenoxy fragment sets σ = 2, whereas σ = 1 for the 3-methyl-phenoxy fragment. This gives a different value for ΔSm in eq 1 and after consideration of the difference in Tm yields a log(CLFR) of −2.40 and −2.34 for compound 2a (3methylphenoxy-BsubPc) and 4-methylphenoxy-BsubPc, respectively. Again, only a small difference which cannot help to explain the large difference in observed solubility. In a discussion which is complementary to that of Yalkowsky et al., Pinal considers the case of two isomers of identical chemical constituents that differ only in their symmetry.33 Since the chemical constituents of the two isomers are identical, flexibility is removed from the argument as it assumed each isomer is equally flexible. The author uses the similarity of the two molecules to compare their ideal solubilities with respect to their symmetry numbers. Because their symmetries differ, they will have different melting points, with that of compound 1 (Tm,1) being lower than compound 2 (Tm,2), but they will have similar thermodynamic properties otherwise due to their similarity in structure. The ideal solubility is related to melting point, enthalpy and entropy of melting, symmetry number, and difference in heat capacity in the solid and liquid. Taking the difference between the ideal solubility expressions of the two isomers and simplifying provides an equation relating the ratio of the ideal solubilities (CLFR) at 298 K to their symmetry numbers (σ) and the melting point of the more symmetric compound (Tm,2)

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CONCLUSIONS In conclusion, we have observed a large solubility difference between 3-methylphenoxy-BsubPc and 3,4-dimethylphenoxyBsubPc each containing a methyl group in the 3-position relative to their counterparts which do not. The result is a soluble BsubPc derivative which is also highly mass efficient (or has a high specific absorptivity) relative to other BsubPc derivatives. The single methyl group in this case increases the molar mass of the derivative by less than 3%. This simple addition of a methyl group in a particular position is in contrast to the more common method of solubilization of organic compounds which involves the addition of large ‘fatty’ hydrocarbon chains such as pentadecyl which results in considerable molar mass increase of the molecule. While that approach does result in soluble BsubPc, it is markedly less mass efficient than the presented method. We have attempted to find a justification which could lead to rational design criteria for further soluble BsubPc derivatives. After consideration of two empirical frameworks, we found the framework of Pinal33 to best fit our observed solubility increase but only after consideration of the PhO-BsubPc molecule as two individual molecular fragments. Overall, treatment of these large molecules as two smaller pieces produced a better estimation of their relative solubilities than treatment of the BsubPc molecules as a whole. We also hope that the identification of these highly soluble BsubPc derivatives will lead to the wider utilization of BsubPc derivatives in applications where high solubility for solution-based deposition techniques is desirable.

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ASSOCIATED CONTENT

S Supporting Information *

Displacement ellipsoid plots, bond distances, complete crystallographic data, and crystallographic information files (CIF) for compounds 2a and 2b. This material is available free of charge via the Internet at http://pubs.acs.org.

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ln(CLFR1/CLFR 2) = (2Tm,2(K) − 298)/298 × ln(σ2/σ1)

AUTHOR INFORMATION

Corresponding Author

(4)

*E-mail: [email protected].

The melting point of the less symmetric compound (Tm,1) was eliminated from eq 4 during its derivation. By considering only the phenoxy molecular fragment (refer to discussion above) and substituting the symmetry numbers σ2 = 2 and σ1 =1 (for 4-methyl-phenoxy and 3-methyl-phenoxy respectively) and Tm,2 of 4-methylphenoxy-BsubPc into eq 4 gives a CLFR1/ CLFR2 ratio of 7.32; meaning compound 2a (3-methylphenoxy-BsubPc) is predicted to have a solubility greater than that of 4-methylphenoxy-BsubPc by a factor of ∼7: a value which is more in line with the observed solubility difference. The application of this conceptual framework to a class of molecules such as phenoxy-BsubPc is untested and not considered by Pinal, but we can conclude that it better describes our observations. Unfortunately, it lacks a method of prediction as it is based on the measured Tm of a known compound. It would also fail to describe the difference in solubility between 2a (3methylphenoxy-BsubPc) and our previously reported 3pentadecylphenoxy-BsubPc. Perhaps more importantly it would also predict the same factor of 7 for a solubility

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Hatch Limited (Mississauga, Ontario, Canada) for their endowed scholarship to Andrew S. Paton. We also thank the Ontario Centres of Excellence (Ontario, Canada), the 65 Canadian Foundation for Innovation (CFI), and the Natural Sciences and Engineering Research Council (NSERC) of Canada for their financial support.

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REFERENCES

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