Experimentally Validated Model for the Prediction of the HOMO and

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Experimentally Validated Model for the Prediction of the HOMO and LUMO Energy Levels of Boronsubphthalocyanines Graham E. Morse,† Michael G. Helander,‡ Jason Stanwick,† Jennifer M. Sauks,† Andrew S. Paton,† Zheng-Hong Lu,‡ and Timothy P. Bender*,†,‡,§ †

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada ‡ Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario M5S 3E4, Canada § Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada W Web-Enhanced bS Supporting Information b

ABSTRACT: We report the rapid screening of phenoxyboronsubphthalocyanine (PhO-BsubPc) derivatives for sensitivity of their HOMO and LUMO energy levels (EH and EL) to substitution with electron-donating and electron-withdrawing functional groups using semiempirical methods. Subsequently, we have synthesized a selection of seven PhO-BsubPc derivatives, further modeled the seven derivatives using DFT methods, and measured or determined their respective EH and EL. We have used a combination of ultraviolet photoelectron spectroscopy (UPS), cyclic voltammetry (CV), and ultraviolet
visible (UV
vis) spectroscopy to correlate the computational predictions with experimental data. From these experimental and computational results, we have shown that EH and EL of PhO-BsubPcs are more sensitive to peripheral substitutions than to substitution on the axial phenoxylate. The frontier molecular orbitals, calculated using DFT methods, were found to be exclusively located on the boronsubphthalocyanine ligand among the seven PhO-BsubPc derivatives. A mathematical model correlating computational results with experimental data was determined which can be subsequently used to rapidly predict how structural factors influence the EH and EL to direct synthetic and engineering efforts.

’ INTRODUCTION The molecular structure of a phthalocyanine (Pc) comprises a metal atom bound within the internal cavity of a heterocyclic ligand which contains four repeating fragments.1 In general, Pcs are synthesized by polycondensation of either an anhydride (in the presence of urea), a diiminoisoindoline, or a 1,2-dinitrile arene precursor at high temperatures in the presence of a templating metal salt or metal alkoxide. An exception to this generalization occurs when the condensation of a 1,2-dinitrile is templated with boron trihalides. The result is the formation of a unique phthalocyanine having three repeat units which is known as boronsubphthalocyanine (BsubPc). BsubPc has a bowlshaped aromatic core with C3v symmetry (Figure 1) and a strong absorption in the visible spectrum making it a particularly interesting candidate for optoelectronic devices. While Pcs have been shown to have a robust chemistry that is easily adaptable and tolerant to structural changes, the chemistry of BsubPc (and resulting changes in physical and electrical properties) is less established but has progressed rapidly since the turn of the millennium.2 Current reports of its application as a functional material include its use as a chemical sensor for selective detection of fluoride and cyanide ions3 and as an electroactive r 2011 American Chemical Society

material for organic solar cells,4 organic light emitting diodes (OLED),5 and organic field-effect transistors (OFETs).6 In all of these applications, an understanding of the energy level of the HOMO (EH) and LUMO (EL) is critical to the successful application of the BsubPc derivative. Prior to any synthetic or engineering campaign targeting a BsubPc derivative for use in an optoelectronic device (an OLED for example), it would be advantageous to have a computational method to rapidly prescreen candidate derivatives for either EH, EL, or the gap between them (EG) or the sensitivity of the energy levels to changes in the chemical structure (i.e., magnitude of the change of EH, EL, and EG) with respect to the nature and placement of functional groups. The net effect of the computational study would be a targeted synthetic or engineering effort with a decrease in the amount of wet laboratory work or general experimentation needed. While some molecular modeling data and methodologies exist for BsubPcs,7 the studies are of limited scope and in some cases Received: September 27, 2010 Revised: April 15, 2011 Published: May 24, 2011 11709

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Figure 1. Model of C6H5O
BsubPc (compound 1): left, edge on view; right, top view. Carbon, black; hydrogen, white; nitrogen, blue; boron, yellow; and oxygen, red.

involve lengthy high level calculations. Furthermore, no comprehensive systematic study of readily accessible phenoxyBsubPc (PhO-BsubPc) derivatives has been reported. Herein, we report such a systematic study. We begin by rapidly screening 106 PhO-BsubPc derivatives computationally to examine the sensitivity of the EH, EL, and EG to substitution at both the axial and peripheral positions. We then take a selection of representative PhO-BsubPcs, synthesize them, and correlate ultraviolet photoelectron spectroscopy (UPS), cyclic voltammetry (CV), and ultraviolet
visible (UV
vis) spectroscopy determinations of their EH, EL, and EG with the models calculated at the semiempirical level (PM38 and RM19) as well as the density functional theory (DFT) level (6-31G* B3LYP10).

’ EXPERIMENTAL SECTION Methods and Materials. All solvents were purchased from Caledon Laboratories Ltd. (Caledon, Ontario, Canada) and used as received unless stated otherwise. Chromatography was performed using 50
100 μm silica gel or 50
200 μm standard basic alumina which was purchased from Caledon Laboratories Ltd. (Caledon, Ontario, Canada) and used as received. Thin film chromatography (TLC) was performed using Alugram 40  80 mm 0.2 mm thick silica gel on an aluminum substrate with fluorescence indicator or Alugram 5  20 cm 0.2 mm thick alumina gel on an aluminum substrate with a fluorescence indicator supplied by Macherey-Nagel through Caledon Laboratories Ltd. Soxhlet extractions were performed using Whatman single thickness cellulose extraction thimbles (33  118 mm). Each nuclear magnetic resonance (NMR) spectrum was acquired on a Varian Mercury 400 MHz system in deuterated chloroform (CDCl3) purchased from Cambridge Isotope Laboratories which was used as received. All 1H NMR spectra were referenced to an internal standard of 0.05% tetramethylsilane (Me4Si). All 19 F NMR spectra were referenced to an external standard (present as a concentric tube) of BF3 3 O(C2H5)2. All ultraviolet
visible (UV
vis) spectroscopy was performed using PerkinElmer Lambda 25 in a PerkinElmer quartz cuvette with a 10 mm path length for solution-phase samples. 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 SunFire C18 3.5 μm column. HPLC grade acetonitrile (ACN) purchased from Caledon Laboratories Ltd. (Caledon, Ontario, Canada) was eluted with an isocratic flow of 80/20 acetonitrile/dimethylformamide at 0.6 mL/min during operation. Cyclic voltammetry was performed with a Bioanalytical Systems C3 electrochemical workstation. The working electrode was a 1 mm platinum disk with a

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platinum wire used as a counter electrode. The reference electrode was Ag/AgCl in a saturated salt solution. All electrochemistry was done in spec-grade dichloromethane (DCM) (Caledon Laboratories, Caledon, Ontario, Canada) with decamethylferrocene as an internal reference. The solvent was purged with Argon at room temperature prior to use. Three cycles from þ1.6 to
1.6 V were measured for each compound at a scan rate of 100 mV/s. Schematics and instructions for use for both the Kauffman column chromatography and train sublimation apparatus have been previously disclosed (ref 5a). Carbon hydrogen nitrogen elemental analysis was performed using a Perkin-Elmer 2400 Series II CHNS Analyzer. Synthetic Methods. Chloro-boronsubphthalocyanine (ClBsubPc) was synthesized as previously reported.11 Chloro-hexachloroboronsubphthalocyanine (Cl-Cl6BsubPc) was synthesized by adaptation of a previously reported method.11 1,2-Dichlorobenzene (220 mL) was added to dichlorophthalonitrile (8.18 g) in a 500 mL three-neck round-bottom flask. Then, BCl3 (100 mL, 1.0 M solution of in heptane) was added in one portion. The mixture was then heated, and the heptane was removed by distillation under a moderate flow of nitrogen using a short-path condenser. After distillation, the short path condenser was removed, the gas changed to argon, and the flask heated at reflux (180.5 °C) for 20 h and then cooled. A positive pressure of argon was maintained throughout the experiment. The crude products were isolated by removal of the residual 1,2-dichlorobenzene by rotary evaporation. The resulting dry solid was placed in a cellulose thimble and extracted with acetonitrile in a Soxhlet extraction apparatus for 2 h. The solid was then placed in a vacuum oven at 50 °C overnight. Yield 9.661 g (purity 95%, by 1 H NMR). 1H NMR (400 MHz, CDCl3, Me4Si): δ 8.95 (6 H, s). Compounds 54, 72, and 1. Each of these compounds was synthesized following previously reported methods with slight modifications.5a One equivalent of Cl-BsubPc (for compound 54: 0.601 g, 0.0014 mol; for compound 72: 0.705 g, 0.0016 mol; for compound 1: 1.0 g, 0.0023 mol) was combined with five equivalents of phenol (for compound 54: 4-methoxyphenol, 0.925 g, 0.0085 mol; for compound 72: 3,4-dimethylphenol, 1.0 g, 0.0082 mol; for compound 1: phenol, 1.09 g, 0.0116 mol) in chlorobenzene (5 mL for 72 and 1) or toluene (10 mL for 54) in a 25 mL rounded-bottom flask fitted with a condenser and held under a constant pressure of argon gas. The reactants were heated at reflux for 24 h (for compounds 72 and 1) or 48 h (for compound 54) at which time the reaction was confirmed to be complete by HPLC analysis (absence of Cl-BsubPc) and was subsequently cooled to room temperature. The solvent was removed by rotary evaporation yielding crude product. Compound 54 was first purified by liquid
liquid extraction by dissolving the product in toluene (300 mL) and extracting with 3.0 M KOH solution in distilled water (3  300 mL). The still crude product was isolated by rotary evaporation of the toluene phase. The crude product 72 and 1 as well as the product after liquid
liquid extraction of 54 was purified using Kauffman column chromatography with basic alumina and dichloromethane as the eluent. Removal of the dichloromethane by rotary evaporation yielded a crude product (54: 0.475 g, 9.2  10
4 mol, 66% yield; 72: 0.968 g; 1: 0.922 g). Subsequently, 72 and 1 were further purified twice using train sublimation. The apparatus was operated under vacuum with a controlled flow of nitrogen gas generating an internal pressure of 5  10
2 Torr. The temperature was increased from room temperature to 300 °C over a period of 4 h and held constant for 2 h and then allowed to cool back to room temperature. 11710

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The Journal of Physical Chemistry C The slowest running band was confirmed as the PhO-BsubPc derivative and had the appearance of a metallic gold film (1: 0.667 g, 1.4  10
3 mol, 72% yield; from mass placed in the train sublimation apparatus, >99.9% purity; 72: 0.437 g, 8.5  10
4 mol, 44% yield). Compound 1, C6H5O-BsubPc: mp 307
308 °C. 1H NMR (400 MHz, CDCl3, Me4Si): δ 5.38
5.40 (2 H, d), 6.60
6.63 (1 H, t), 6.73
6.77 (2 H, t), 7.90
7.92 (6 H, m), 8.84
8.86 (6 H, m). Anal. Calcd for C30H17BN6O: C, 73.79; H, 3.51; N, 17.21. Found: C, 72.60; H, 3.43; N, 16.89. Compound 54, 4-MeO-PhO-BsubPc: mp 294.5 °C. 1H NMR (400 MHz, CDCl3, Me4Si): δ 3.56 (3 H, s), 5.31
5.36 (2 H, d), 6.28
6.30 (2 H, d), 7.89
7.93 (6H, m), 8.83
8.87 (6 H, m). Anal. Calcd for C31H19BN6O2: C, 71.83; H, 3.69; N, 16.21. Found: C, 71.86; H, 3.76; N, 14.71. Compound 72, 3,4-diMePhO-BsubPc: mp 303
304 °C. 1H NMR (400 MHz, CDCl3, Me4Si): δ 1.83 (3 H, s), 1.92 (3 H, s), 5.07
5.09 (1 H, d), 5.22
5.23 (1 H, dd), 6.46
6.48 (1 H, d), 7.89
7.92 (6 H, m), 8.84
8.86 (6 H, m). Anal. Calcd for C32H21BN6O: C, 74.43; H, 4.10; N, 16.03. Found: C, 74.22; H, 3.83; N, 16.03. Compound 107, F5-Cl6BsubPc: One equivalent of Cl-Cl6BsubPc (2.0 g, 0.0032 mol) was combined with five equivalents of pentafluorophenol (2.892 g, 0.0157 mol) in chlorobenzene (20 mL) in a 25 mL rounded-bottom flask fitted with a condenser and held under a constant pressure of argon gas. The reactants were heated at reflux for 21 h at which time the reaction was confirmed to be complete by HPLC analysis (absence of Cl-Cl6BsubPc) and was subsequently cooled to room temperature. The solvent was removed by rotary evaporation yielding crude product. The crude product was purified using Kauffman column chromatography with silica gel and dichloromethane as the eluent. Removal of the dichloromethane by rotary evaporation yielded a mass (2.0 g, 74% yield; >99.9% purity by HPLC maxplot, 1H NMR, and 19F NMR). Mp 288.5 °C. 1H NMR (400 MHz, CDCl3, Me4Si): δ 8.93 (6 H, s). 19F NMR (400 MHz, CDCl3, BF3 3 O(C2H5)2): δ
10.55 to
10.42 (1F, t),
9.97 to
9.84 (2F, t),
5.64 to
5.59 (1F, t). Anal. Calcd for C30H6BCl6F6N6O: C, 45.90; H, 0.77; N, 10.71. Found: C, 45.96; H, 1.02; N, 10.54. Compounds 100, 2, and 108: were synthesized as previously reported.5a Computational Methods. Consideration of Isomers. The use of monosubstituted phthalonitriles during the synthesis of PhOBsubPcs would result in the production of a mixture of isomers. For example, the condensation with 3-fluorophthalonitrile with BCl3 would result in a mixture of isomers, some of which are enantiomers of one another. Not surprisingly, we found that the phenoxy group of the PhO-BsubPc is orientated in space above and between two of the aromatic rings of the subPc ligand. This leads to the further possibility of conformational isomers when monosubstituted phthalonitriles are used. Therefore, when we were considering PhOBsubPcs derived from monosubstituted phthalonitriles we modeled all possible configurational and conformational isomers. The value given for the HOMO and LUMO is an average of all the possible isomers. Furthermore, we found the variation in energy levels between isomers was negligible. Semi-Empirical Methods. Each compound was constructed in HyperChem ver 8.0 for Windows OS using version 8.0. All models were optimized first using the MMþ molecular mechanics force field and then further refined using the appropriate semiempirical method to ensure each structure had an unbiased starting point. Geometric optimization for both the molecular mechanics and semiempirical methods was performed using the

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Polak
Ribiere conjugated gradient method with a 0.02 kcal/Å mol root-mean-squared convergence limit. Semiempirical calculations were performed using either the RM1 or PM3 method as adopted in HyperChem. It is known that the RM1 parameters, available in HyperChem and are incorrect and updated parameter files, which are freely available on the Internet,12 were substituted for the preinstalled files and used for the presented calculations. It is also known that RM1 is not yet parametrized for boron, and as such AM1 values are substituted into the parameter files in their absence. The result is therefore a mixed-RM1 model. We have also found that the PM3 parameters for boron are not consistent with those published by Stewart.13 Thus, we updated the PM3 parameter file accordingly. We have included these updated files (WEO 1, 2, 3, 4, 5, 6, and 7 as web enhanced files attached to the HTML version of this article. To use any of these files simply copy them to the folder C:\Hyper80 \Runfiles and delete the .txt extension. The spin-restricted Hartree
Fock approximation was assumed for each of the calculations. Optimization was achieved with a self-consistent-field convergence limit of 0.0001 and an iteration limit of 50. HOMO and LUMO energy levels were estimated using Koopmans’ theorem from their respective ionization potential and electron affinity estimates as implemented in HyperChem. Density Functional Theory Methods. Each compound was constructed, and all calculations were performed in SPARTAN ‘06 for windows. The Becke-Lee
Yang
Parr (B3LYP) exchange-correlation functional10 and the 6-31G* basis set were used for the geometric optimization of each compound (in the gas phase). All orbital energy estimates for EH and EL were performed on the optimized structures. Photoelectron Spectroscopy Characterization. UPS measurements were performed using a PHI 5500 Multi-Technique system attached to a Kurt J. Lesker multiaccess chamber ultra high vacuum (UHV) cluster tool (base pressure of ∼10
10). The spectrometer (hemispherical analyzer) was calibrated using XPS with monochromatic Al KR (hν = 1486.7 eV) as per ISO 15472. The zero of the binding energy scale for all measurements was referenced to the Fermi level of an Arþ sputter cleaned Au thin film in electrical contact with the samples (i.e., on the same sample holder). The energy resolution for UPS measurements was ∼135 meV determined from the width of the Fermi edge of Arþ sputter cleaned Au thin film. All UPS measurements were performed at a photoelectron takeoff angle of 90° and with a
15 V bias applied to the sample. The various organic molecules were deposited in a dedicated organic chamber from an alumina crucibles transfer arm evaporator (TAE) cell onto freshly cleaved highly ordered pyrolytic graphite (HOPG) substrates. HOMO energy levels for each molecule were determined from 3 nm thick films as measured by a calibrated quartz crystal microbalance.

’ RESULTS AND DISCUSSION (1). Rapid Screening for EH, EL, and EG Sensitivity. Compound Selection. We are interested in PhO-BsubPc derivatives specifically

because phenoxylation is a facile route to incorporate chemical functionality into the BsubPc molecular structure to affect the physical properties of the resulting derivatives including (but not limited to) their solubility,11 sublimation point, melting point, and crystal structure5a—which can in turn affect their performance in optoelectronic devices. We have recently shown that over a limited set of comparable fluorinated PhO-BsubPc derivatives (pentafluorophenoxy-boronsubphthalocyanine/F5BsubPc/compound 100, phenoxy-dodecafluoroboronsubphthalocyanine/F12BsubPc/compond 11711

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Scheme 1. Retrosynthetic Analysis of Phenoxy-boronsubphthalocyanines (PhO-BsubPcs)

2, and pentafluorophenoxy-dodecafluoroboronsubphthalocyanine/ F17BsubPc/compound 108) the EH and EL can be varied over a range of 5.86
6.65 eV and 3.71
4.08 eV, respectively.5a The most pronounced energy level variation occurred due to the presence of fluorine around the periphery of the BsubPc, an observation which was in line with electrochemical characterization of similar BsubPc derivatives.14 However, given the limited data set available from our lab and within the literature, it was unclear the ultimate sensitivity of the EH, EL, and EG to chemical substitution through the PhOBsubPc molecular structure. To answer this question we have elected to use a combination of computational and experimental methods. To begin we must first consider the general synthetic pathway to PhO-BsubPc derivatives. The synthesis of PhO-BsubPc derivatives is a two-step process. In the first step, condensation of phthalonitrile with either BCl3 or BBr3 results in the formation of Cl-BsubPc or Br-BsubPc, respectively. Further derivatization of the labile B-halogen bond with phenol and its derivatives yields a PhO-BsubPc derivative. A retrosynthetic analysis is shown for a generalized PhO-BsubPc (Scheme 1) starting from substituted phthalonitriles and illustrates the potential places on the molecular scaffold where functional groups can be incorporated. Two placements are possible: around the periphery of the BsubPc ligand, resulting from the use of substituted phthalonitriles as starting materials and axial to the boron atom resulting from the use of a substituted phenol. Each position lends to the possibility of introducing variation in the chemical structure (and as a consequence variance in the electronic structure and physical properties). Any functional groups (Rn) present in the phthalonitrile would end up placed around the periphery of the BsubPc heterocyclic ring system. Common phthalonitriles available commercially or synthetically are monosubstituted at the 3 or 4 position, disubstituted at the 4,5 positions, or tetrasubstituted at the 3,4,5,6 positions. We have limited the choice of functional groups (R) present on the phthalonitriles for this study to fluoro, chloro, bromo, iodo, methyl, trifluoromethyl, and methoxy. We have done this to span a range of electrondonating (methoxy) and electron-withdrawing (trifluoromethyl) substituents. Second, functional groups (R) present within the generalized phenol would result in their placement in a position axial to the boron atom. Here we chose fluoro, chloro, bromo, iodo, methyl, phenyl, trifluoromethyl, and methoxy substituents present in a variety of positions (ortho, meta, and para) and frequencies (mono-, di-, tri-, and penta-substitutions).

Figure 2. Calculated EH (red b), EL (blue 9), and EG (green 2) in eV for PhO-BsubPcs 1
106 at the PM3 semiempirical level.

EH and EL by Semiempirical Methods. We have previously shown a good correlation of calculated HOMO energy levels with observed electrochemical potentials using the PM3 semiempirical method15a across a series of electron-donating materials. Therefore, we initially chose to use the PM3 method for screening of PhO-BsubPc derivatives (Table S1, Supporting Information). Using this method, we were able to rapidly screen the 200þ models (including isomers) in less than one month using a standard workstation class computer. The same set of PhO-BsubPc derivatives was also modeled using the RM1 method for comparison (Table S2, Supporting Information). All models were optimized first using the MMþ molecular mechanics force field and then further refined using the appropriate semiempirical method (each using a rms gradient of 0.02 kcal Å
1 mol
1 and the Polak
Ribiere conjugate gradient optimization algorithm). From each of these methods, the BsubPc ligand adopts a bowl-like conformation (Figure 1) consistent with the reported X-ray determined structures of BsubPc derivatives.5a,16 Complete tables of results for PM3 models (Table S1) and RM1 models (Table S2) are available in the Supporting Information accompanying this article. The results of our PM3 and RM1 study can be summarized rather simply: both the EH and EL of PhO-BsubPcs are five times more sensitive to substitution around the periphery of the BsubPc ligand than they are to substitution on the phenol bound in the axial position to the boron atom. This is illustrated in Figure 2 for the PM3 models and in Figure S1 (Supporting Information) for the 11712

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Scheme 2. Synthesis of Compounds 1, 2, 54, 72, 100, 107, and 108a

a

Conditions: 5 mol equiv of appropriate phenol, toluene, or chlorobenzene, reflux, 24
50 h.

RM1 models, which show the energy levels (both HOMO and LUMO) of PhO-BsubPc bearing substituents around the periphery to the left and those bearing substituents on the phenoxylate to the right. With reference to the PM3 models, it is clear that energy level variations can be achieved from
9.91 to
7.41 eV (2.50 eV difference) in the HOMO and
4.23 to
2.00 eV (2.23 eV difference) in the LUMO for peripheral substituted PhO-BsubPcs. When substitution was limited to positions on the phenoxylate of BsubPc, energy levels varied from
8.13 to
7.70 eV (0.44 eV difference) in the HOMO and
2.68 to
2.13 eV (0.54 eV difference) in the LUMO. Where variation was observed in the positions of the HOMO and LUMO, the gap between the HOMO and LUMO remained constant at approximately 5.5 eV. (2). Representative PhO-BsubPc Synthesis and Characterization. Synthesizing a Series of PhO-BsubPc Derivatives. From this large data set (200þ compounds including isomers), it would be advantageous to derive a model to correlate computational EH and EL with their experimental values to validate the model and as a consequence have the ability to more accurately screen the electronic properties of targeted PhO-BsubPc compounds computationally. To achieve this, we targeted a series of PhO-BsubPc derivatives selected to experimentally probe a wide range in EH and EL by ultraviolet photoelectron spectroscopy (UPS), cyclic voltammetry (CV), and ultraviolet
visible absorption (UV
vis) spectroscopy. The PhO-BsubPc compounds were available from previous work in our laboratory5a,11 or were specifically made for this study (Scheme 2). Each of these compounds spanned a variety of peripheral and axial substitutions, of which three were hydrogenated BsubPcs: phenoxy-boronsubphthalocyanine (C6H5O-BsubPc, compound 1), 4-methoxyphenoxy-boronsubphthalocyanine (4-MeO-PhOBsubPc, compound 54), and 3,4-dimethylphenoxy-boronsubphthalocyanine (3,4-diMe-PhO-BsubPc, compound 72). One was a

chlorinated derivative: pentafluorophenoxy-hexachloroboronsubphthalocyanine (F5-Cl6BsubPc, compound 107). Finally, three were fluorinated PhO-BsubPcs available from our previous study:5a pentafluorophenoxy-boronsubphthalocyanine (F5BsubPc/compound 100), phenoxy-dodecafluoroboronsubphthalocyanine (F12BsubPc/compound 2), and pentafluorophenoxy-dodecafluoroboronsubphthalocyanine (F17BsubPc/ compound 108). The three hydrogenated PhO-BsubPcs were synthesized by reaction of Cl-BsubPc with 4-methoxyphenol, 3,4-dimethylphenol, and phenol in chlorobenzene or toluene at reflux to produce compound 54, 72, and 1, respectively. In each synthesis, the phenol was used in excess by five molar equivalents. Complete conversion of these compounds required 24
48 h. Purification of these compounds by Kauffman column chromatography on standard basic alumina with dichloromethane as eluent resulted in purities >99% as measured by HPLC, 1H NMR, and elemental analysis. To achieve the synthesis of F5-Cl6BsubPc (compound 107), a precursor Cl-Cl6BsubPc was synthesized using the analogous procedure for the synthesis Cl-BsubPc albeit with some modifications. Specifically, the reaction time was increased to 20 h, and purification was achieved by Soxhlet extraction of the crude solids with acetonitrile for 2 h. The resulting Cl-Cl6BsubPc was used without further purification (purity 96%) and was then reacted with an excess of pentafluorophenol to produce F5Cl6BsubPc. Complete conversion, taking a total of 21 h, was monitored by HPLC. Crude F5-Cl6BsubPc was purified by Kauffman column chromatography on standard basic alumina with dichloromethane as eluent. Its purity was established using HPLC, 19F NMR, 1H NMR, and elemental analysis. Probing E1/2,red, E1/2,ox, and EH by CV. Solution cyclic voltammetry (CV) of each PhO-BsubPc was performed to measure their oxidation and reduction half-wave potentials which has 11713

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Table 1. EH and EL for a Series of PhO-BsubPcs Experimentally Measured Using CV, UV
Vis, and UPSa HOMO E1/2,ox (V)

EH (eV), calc.

1, C6H5O-BsubPc

1.09


5.57

54, 4-MeO-PhO-BsubPc

Ir.

PhO-BsubPc

a

LUMO EH (eV), UPS


5.54

E1/2,red (V)

gap

EL (eV), UV
vis

EG (eV), UV
vis


5.46


1.04


3.32

2.14


5.42


1.05


3.28

2.14 2.14

72, 3,4-diMe-PhO-BsubPc

1.07


5.42


1.05


3.28

100, F5BsubPc

Ir.


5.86


0.88


3.72

2.14

107, F5-Cl6BsubPc

Ir.


6.18

Ir.


4.06

2.12

2, F12BsubPc

Ir.


6.61

Ir.


4.49

2.12

108, F17BsubPc

Ir.


6.65

Ir.


4.54

2.11

Ir., irreversible.

been previously shown to correlate to EH and EL.17 Each CV measurement was taken in dichloromethane (DCM) as the solvent containing tetrabutylammonium perchlorate (Bu4NClO4) as an electrolyte. Decamethylferrocene was used as an internal standard, and the results were corrected to its half-wave reduction potential (E1/2,red) which was previously established to be
0.012 V vs Ag/ AgCl.15 Each sample was scanned between 1.6 and
1.6 V, and the results are summarized in Table 1. Compounds 72 and 1 each showed a reversible oxidation with half-wave potentials (E1/2,ox) at 1.07 and 1.09 V, respectively; however, compounds 54 and 107 were found to undergo irreversible oxidation with peak potentials at 1.14 and 1.41 V, respectively. On the reduction side, reversible reductions were observed for compounds 54, 72, and 1 at
1.05,
1.05, and
1.04 V, whereas irreversible reductions were observed for compound 107 at a peak potential of
0.78 V. Therefore, within this series of compounds, modifying the axial phenol resulted in only a minor change in half-wave reduction and oxidation potentials. In comparison, the E1/2,red for compound 100 (F5BsubPc) is
0.88 V, showed a more substantial change in halfwave reduction potential, reflecting the strong electron-withdrawing nature of the five fluorine atoms substituting the phenol. Peripheral hexachlorination and perfluorination resulted in irreversible oxidation and reduction events in the PhO-BsubPcs tested. Despite its irreversible nature, a substantial change in peak reduction potential was observed for the series of compounds 100, 107, and 108 resulting from peripheral hexachlorination and perfluorination (each being axially substituted with pentafluorophenol). The peak potentials measured were
0.89,
0.78, and
0.52 V, respectively, for compounds 100, 107, and 108: reflecting a 0.11 V change from perhydrogenated to hexachlorinated and a 0.26 V change from hexachlorinated to perfluorinated. Probing EH by UPS. Due to the lack of reversible oxidations seen in our cyclic voltammetry studies, we opted to use ultraviolet photoelectron spectroscopy (UPS) to determine the HOMO energy levels of the PhO-BsubPc compounds in the solid state (as thin films). The molecules were deposited by sublimation on freshly cleaved highly ordered pyrolytic graphite, and the results were aligned to its vacuum level work function at 4.45 eV. In the valence band spectra of each compound (Figure 3), the peak at lowest binding energy represents the density of states corresponding to EH. The measured EH for 1, 2, 54, 72, 100, 107, and 108 are summarized in Table 1. In this series, a lowering of EH was observed for the series as follows: 54 > 72 > 1 > 100 > 107 > 2 > 108 (a trend identical to that observed from CV). Within the series, compounds 1, 54, 72, and 100 derived from Cl-BsubPc showed a

Figure 3. UPS spectra of four PhO-BsubPcs (compounds 1, 54, 72, and 107).

variation in EH from
5.42 to
5.86 eV with a difference of 0.44 eV, whereas compounds derivated from Br-F12BsubPc (2, 108) showed a variation in EH from
6.61 to
6.65 eV with a difference of 0.04 eV. The diminishing trend suggests that the sensitivity of axial derivation of BsubPc is reduced as the degree of electron withdrawing around periphery is increased. Similarly, trends in peripheral modification were observed by focusing attention to the pair of compounds made from phenol (compound 1 and 2) and to the corresponding compounds made from pentafluorophenol (compounds 100, 107, and 108). Compounds 1 and 2 had EH values from
5.46 to
6.61 eV spanning 1.15 eV, while compounds 100, 107, and 108 had EH values of
5.86,
6.18, and
6.65 eV, respectively, spanning 0.79 eV. Therefore, we can conclude the position of the EH of PhO-BsubPc is between 2 and 29 times more sensitive to peripheral derivation than axial derivation: an experimental result in agreement with our computational results. For compounds 1 and 72 we were able to observe and measure reversible oxidation events by cyclic voltametry. When the EH was calculated from the electrochemical data,17 the result was a moderately good fit to the UPS values for the two data points (Table 1). Probing EG and EL by UV
vis Spectroscopy. The EH
EL energy gap (EG) was probed using UV
vis spectroscopy. A solution of each PhO-BsubPc was prepared in DCM. The EG was taken as the absorption onset of on the red-most absorption band.4c,6a Each PhO-BsubPc derived from Cl-BsubPc (compounds 1, 54, 72, and 1) had an identical EG of 2.14 eV, while compounds 107, 100, and 108 (derived from Cl-Cl6BsubPc or Br-F12BsubPc) had an EG of 11714

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The Journal of Physical Chemistry C

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Table 2. EH and EL for a Series of PhO-BsubPcs Calculated Using PM3, RM1, and B3YLP Methods HOMO, EH (eV) PhO-BsubPc

LUMO, EL (eV)

PM3

RM1

B3YLP

PM3

RM1

B3YLP

1


7.763


7.256


5.166


2.212


1.822


2.431

54


8.732


8.287


5.141


2.245


1.864


2.406

72 100


7.788
7.744


7.302
7.235


5.139
5.312


2.190
2.441


1.801
2.028


2.404
2.587

107


7.933


7.452


5.931


2.779


2.686


3.246

2


8.222


8.058


5.868


3.288


3.047


3.254

108


8.896


8.470


6.000


3.495


3.231


3.399

Figure 5. Calculated EL (eV) using PM3 (green 9), RM1 (red b), and B3LYP (blue [) for compounds 1, 2, 54, 72, 100, 107, and 108 versus their EL (eV) measured experimentally by UPS.

values of 0.951, 0.961, and 0.901 with standard deviations of 0.132, 0.117, and 0.188 eV for the PM3, RM1, and B3LYP methods, respectively: the semiempirical methods therefore correlating best to the measured UPS data. From the linear regressions, the following equations (eqs 1
3) have been generated to estimate the EH of PhO-BsubPcs using any of the three computational methods

Figure 4. Calculated EH (eV) using PM3 (green 9), RM1 (red b), and B3LYP (blue [) for compounds 1, 2, 54, 72, 100, 107, and 108 versus their EH (eV) measured experimentally by UPS.

2.12, 2.12, and 2.11 eV, respectively. Thus, the EG remained relatively unaffected upon axial and peripheral derivation: a result which is consistent with our computational results above. The EG measured in this way was used to estimate EL for each compound using the EH measured by UPS (Table 1). Given the similarity in EG, the trend in EL was obviously similar to the trends in EH seen in UPS, specifically that 54 > 72 > 1 > 100 > 107 > 2 > 108 with values ranging between
3.24 and
4.54 eV (Table 1). (3). Correlating Computational Predictions with Experimental Data. We have extracted PM3 and RM1 values for both EH and EL for the synthesized compounds 1, 54, 72, 100, 107, 2, and 108 from Table S1 (Supporting Information) and summarized them in Table 2. In addition, we modeled this smaller subset of compounds using density functional theory (DFT, using the Becke
Lee
Yang
Parr (B3LYP) exchange-correlation functional10 and the 6-31G* basis set as implemented in Spartan ‘06) to compare its efficacy at predicting experimental data with the two semiempirical methods given the obvious and expected differences between our semiempirical results and the experimentally determined values. With regard to EH, it is immediately evident that the B3LYP method gave estimates closest to the experimentally measured value, followed by the RM1 method and then the PM3 method (Figure 4). However, when plotting the computational estimates from each method against the experimental data, the results from B3LYP showed the worst correlation to the experimental data. A linear regression of each data set produced goodness of fit (r2)

EH ¼ 1:105  ðEPM3 H Þ þ 3:067 ( 0:132

ð1Þ

EH ¼ 1:007  ðERM1 H Þ þ 1:834 ( 0:117

ð2Þ

EH ¼ 1:284  EB3LYP þ 1:132 ( 0:188 H

ð3Þ

Similar plots were generated for EL from the combined UPS and UV
vis data, and similarly the semiempirical methods produced a better correlation between experimental and computational data (Figure 5). For EL a linear regression of each data set produced r2 values of 0.967, 0.968, and 0.929 with standard deviations of 0.112, 0.109, and 0.163 eV for PM3, RM1, and B3LYP, respectively. The following equations (eqs 4
6) represent the best linear fit EL ¼ 1:012  ðEPM3 L Þ
1:118 ( 0:112

ð4Þ

EL ¼ 0:888  ðERM1 L Þ
1:723 ( 0:109

ð5Þ

EL ¼ 1:176  ðEB3LYP Þ
0:499 ( 0:163 L

ð6Þ

Having determined the mathematical relationship between experimental and modeled values for EH, EL, and EG we could then go back and re-examine the original library of 106þ compounds thereby allowing for the estimation of the ultimate variation of EH, EL, and EG which one could expect from synthetically accessible PhO-BsubPcs (Tables S1 and S2, Supporting Information). Considering first the PM3 data (Table S1, Supporting Information) in combination with eq 1 and eq 4, a range in EH of 2.758 eV (
7.881 to
5.123 eV) for peripheral modification and 0.485 eV (
5.922 to
5.436 eV) for axial modification can be predicted. Similarly for EL, peripheral modification results in a range of 2.256 eV (
5.396 to
3.140 eV), while axial modification 11715

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The Journal of Physical Chemistry C Table 3. Plots of the HOMO and LUMO Orbital Distribution for the Series of PhO-BsubPcs (1, 54, 72, 100, 107, 2, and 108) Calculated Using DFT Methods (B3LYP 6-31G*)

results in a range of 0.548 eV (
3.825 to
3.277 eV). Over this range, the EG predicted by the difference in EL and EH showed a

ARTICLE

maximum variation of 0.5 eV, reflecting the persistent absorption spectra characteristic of BsubPc derivatives.18 Likewise, using the RM1 data (Table S2, Supporting Information) and eq 2 and eq 5, a range in EH of 2.673 eV (
7.760 to
5.087 eV) for peripheral modification and 0.499 eV (
5.896 to
5.396 eV) for axial modification can be predicted. Lastly, for EL a range of 2.197 eV (
5.313 to
3.116 eV) for peripheral modification and 0.497 eV (
3.757 to
3.260 eV) for axial modification can be predicted. As an additional validation of the presented model, we can consider other nonphenoxylated BsubPc derivatives. For example, the absolute EH of Cl-BsubPc has been previously measured by UPS as
5.6 eV5c or
5.7 eV.4a The frontier orbitals of Cl-BsubPc were calculated using the RM1 method, and using eq 2, and an EH of
5.648 eV can be predicted, closely matching the experimental value. We have measured the EG to be 2.13 eV in DCM solution using UV
vis spectroscopy as described above. Using eq 5 we can estimate an EG of 2.140 eV: again closely matching the experimental value. The close fit of experimentally determined values of Cl-BsubPc with the presented model, which was constructed from PhO-BsubPc data alone, may indicate its relevance for other nonphenoxlated BsubPc derivatives. (4). Molecular Orbital Distribution. Images of the molecular orbital distributions for PhO-BsubPcs 1, 54, 72, 100, 107, 2, and 108 are shown in Table 3 (calculated using DFT). Among the compounds, both the HOMO and LUMO are situated entirely on the BsubPc ligand with no contribution from axial phenoxylate. The HOMO is evenly distributed on the carbon atoms of the BsubPc ligand. However, for the LUMO, the orbital density encompasses both the nitrogen and carbon atoms comprising the 14-π electron system, leaving one 6-carbon aromatic ring (pointing forward in each image, Table 3) with significantly less orbital density. In the case where halogen atoms appear around the periphery of the BsubPc ligand, small contributions from the halogens are made to the HOMO and LUMO. It should also be noted that there is an absence of contribution to the HOMO or LUMO from the boron atom. This observation coupled with the lack of orbital density on the phenoxylate visually confirms why there are no significant differences in EL or EH observed by changing substituents on the phenoxylate and why substitution around the periphery does; the orbital density is entirely located on the BsubPc ligand.

’ CONCLUSION We have shown the rapid screening of PhO-BsubPcs for sensitivity of their EH and EL to substitution with electrondonating and electron-withdrawing functional groups using semiempirical methods. We synthesized seven PhO-BsubPc derivatives to characterize their EH, EL, and EG by CV, UPS, and UV
vis spectroscopy to generate a mathematical correlation between the computational predictions and experimental values. Mathematical correlations were determined between the PM3 and RM1 semiempirical method as well as the B3LYP density functional theory method. From these experimental and computational results, we repeatedly established that the EL and EH of PhO-BsubPcs are more sensitive to peripheral substitutions than to substitution on the axial phenoxylate, both EH and EL being five times more sensitive measured computationally using the PM3 and RM1 methods and each being 2 to 29 times more sensitive measured experimentally, whereas that the gap between the HOMO and LUMO (EG) remains relatively 11716

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The Journal of Physical Chemistry C constant across different molecular compositions. We have compared the PM3 method with the newer RM1 method (albeit a mixed method due to the lack of RM1 parameters for boron) and the density functional theory method B3LYP and found that while the B3LYP method gave closer absolute estimates the semiempirical methods (PM3 and RM1) produced more accurate correlations between experimental and computed EH and EL data (eqs 1
6). We would therefore choose to use the PM3 or RM1 method in combination with eqs 1 and 4 or eqs 2 and 5 for further studies of BsubPc phenoxylates. Through these correlations (eqs 1
6) we have developed a methodology to rapidly predict structural factors which influence the HOMO and LUMO energy levels where such considerations are needed and warranted, thereby directing synthetic efforts. The spatial distribution of the frontier molecular orbitals, calculated using DFT, was exclusively located on the BsubPc ligand among the seven PhO-BsubPcs 1, 54, 72, 100, 107, 2, and 108: the HOMO being distributed among the aromatic carbon atoms and the LUMO being distributed among the 14-π electron nitrogen and carbon atoms. Peripheral modification by halogenations was seen to effect the distribution of the molecular orbitals by allocating orbital density on the halogen, while substitution on the axial phenoxylate showed no effect on the distribution of the orbitals. Finally, we can provide an estimation of computational time required to complete this study on standard workstation class computers of this time. Using semiempirical methods we were able to screen 200þ models in less than one month, whereas using the DFT method it took 3 days for 7 models (approximately 70 models per month).

’ ASSOCIATED CONTENT

bS

Supporting Information. EH, EL, and EG of compounds 1
108 modeled using the PM3 and RM1 method with eqs 1, 2, 4, and 5 and a plot of the calculated EH, EL, and EG for PhOBsubPcs 1
106 at the RM1 semiempirical level. This material is available free of charge via the Internet at http://pubs.acs.org. W Web Enhanced Feature. Updated PM3 parameter files b are available in the HTML version of the paper.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We wish to acknowledge the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding this research in the form of a Discovery Grant (TPB, ZHL), a Vanier Scholarship (M.G.H.), a Canada Graduate Scholarship (G.E.M.), and a Post Graduate Scholarship (A.S.P.). ’ REFERENCES (1) Lever, A. B. P. Phthalocyanines: properties and applications; VCH: New York, 1989; Vols. 1
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