Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2018, 122, 1091−1102
Ability To Fine-Tune the Electronic Properties and Open-Circuit Voltage of Phenoxy-Boron Subphthalocyanines through MetaFluorination of the Axial Substituent Kathleen L. Sampson,† David S. Josey,† Yiying Li,‡ Jessica D. Virdo,† 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 S Supporting Information *
ABSTRACT: Six different boron subphthalocyanines with fluorophenoxy axial substituents were synthesized to explore and fine-tune the HOMO and LUMO energy levels based on past observations of the unique levels of pentafluorophenoxyboron subphthalocyanine (F5BsubPc). Electrochemistry reduction potentials, ionization energies from ultraviolet photoelectron spectroscopy (UPS) measurements, and energy levels calculated using semiempirical methods reveal finely variable values between phenoxy-BsubPc (PhO-BsubPc), without any fluorine atoms, and F5BsubPc, with all five fluorines. There is no trend between the number of fluorines on the phenoxy group and the electronic properties, but there is an influence of meta fluorine(s) altering the redox potentials and ionization energy, leading us to categorize the fluorophenoxy-BsubPcs into two “buckets”: with and without meta fluorines. We then applied the fluorophenoxy-BsubPcs as electron acceptors in planar heterojunction organic photovoltaic devices. The categorization of these “buckets” was confirmed by the fine change in open-circuit voltage between 1.15 and 1.21 V, which is an exceptionally high value. This level of fine-tuning of properties and device metrics is a unique handle enabled by the phenoxy axial substituent of BsubPcs.
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INTRODUCTION The energy level alignment of the organic semiconductor materials has an important role in converting light into electrical current within organic photovoltaic (OPV) devices.1 The dissociation of excitons (electron−hole pairs) generated within the active layer of an OPV is dependent on the energetic spacing of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the electron-donating and electron-accepting materials, respectively.2,3 Therefore, the ability to fine-tune these energy levels is a potential handle to engineer efficient charge transfer and improve device performance. However, the ability to have a molecular fragment handle to fine-tune these properties is rare in organic electronic materials, especially for typical materials such as fullerene-based electron acceptors.1,4 A member of a broader class of materials called phthalocyanines (Pcs), boron subphthalocyanine (BsubPc) is a small-molecule, semiconducting material of interest for OPV applications. BsubPcs contain a boron center surrounded by three nitrogen-linked isoindoline groups, which form a unique bowl shape. We have proven experimentally that the physical © 2017 American Chemical Society
properties of BsubPcs, such as maximum absorption wavelength,5 sublimation temperature,6 solubility,7,8 melting point,9 crystal structure,10 extinction coefficient,5 and electronic characteristics,11 can be altered through the functionalization of the axial position or substitution of the hydrogens in the periphery.12−14 This array of modifications is highly desirable compared with the inability to alter the electronic and optical properties of typical fullerene electron acceptors, which are limited to an open-circuit voltage (VOC) of 99% purity. 1H NMR (400 MHz, CDCl3, Me4Si): δ 4.87−4.97 (2H, m), 6.07 (1H, tt), 7.88−7.98 (6H, m), 8.82−8.92 (6H, m) (Figure S2). MS (EI) exact mass calculated for C30H15BF2N6O: m/z 524.1368, found 524.1368. 246F3PhO-BsubPc. Amount of 2,4,6-trifluorophenol added to reaction vessel: 4.680 g, 31.6 mmol. Crude product after Kauffman column: 1.634 g, 3.01 mmol, 92% purity. Sublimation yield: 0.305 g, 5.62 mmol, 66.9% yield, > 99.9% purity. 1H NMR (400 MHz, CDCl3, Me4Si): δ 6.09−6.20 (2H, m), 7.87− 7.97 (6H, m), 8.81−8.91 (6H, m) (Figure S3). MS (EI) exact mass calculated for C30H14BF3N6O: m/z 542.1274, found 542.1277. 345F3PhO-BsubPc. Amount of 3,4,5-trifluorophenol added to reaction vessel: 4.692 g, 31.7 mmol. Crude product after Kauffman column: 1.583 g, 2.92 mmol, 95% purity. Sublimation yield: 0.225 g, 4.15 mmol, 58.6% yield, > 99% purity. 1H NMR (400 MHz, CDCl3, Me4Si): δ 5.02 (2H, ddt), 7.98−7.88 (6H, m), 8.92−8.82 (6H, m) (Figure S4). MS (EI) exact mass calculated for C30H14BF3N6O: m/z 542.1274, found 542.1274. 2356F4PhO-BsubPc. Amount of 2,3,5,6-tetrafluorophenol added to reaction vessel: 5.375 g, 32.4 mmol. Crude product after Kauffman column: 1.963g, 3.50 mmol, 99% purity. Sublimation yield: 0.338 g, 6.03 mmol, 76.8% yield, > 99.9% yield. 1H NMR (400 MHz, CDCl3, Me4Si): δ 6.35 (1H, tt), 7.88−7.98 (6H, m), 8.82−8.92 (6H, m) (Figure S5). MS (EI) exact mass calculated for C30H13BF4N6O: m/z 560.1180, found 5560.1183. F5BsubPc. Previously synthesized according to method reported by Morse et al.16 Semiempirical Methods. All compounds were modeled in HyperChem version 8.0 for Windows OS. Optimization using the MM+ molecular mechanics force field was performed first, followed by further refining using either RM1 or PM3 semiempirical methods. Molecular mechanics and semiempirical methods geometric optimization was performed using the Polak−Ribiere conjugated gradient algorithm with a 0.02 kcal/ Å·mol−1 root-mean-squared convergence limit. The semiempirical RM1 and PM3 files were modified as per our previously published experimentally verified BsubPc model by Morse et al.12 Device Fabrication. The OPV fabrication and characterization in this study followed previously described meth-
resonance (NMR) spectra were acquired from a Bruker Avance III 400 MHz system using deuterated chloroform (CDCl3) and referenced to an internal standard of 0.05% TMS. Photoluminescence spectra were measured from a PerkinElmer LS 55. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed using a Bioanalytical Systems C3 electrochemical workstation. The working electrode was a 1 mm platinum disk, the counter electrode was a platinum wire, and the reference electrode was Ag/AgCl in 3 M sodium chloride salt solution. Electrochemistry was done with 0.1 M tetrabutylammonium perchlorate (Sigma-Aldrich) electrolyte in anhydrous dichloromethane (Caledon Laboratories), a 30 min nitrogen purge prior to experimentation, and decamethylferrocene (Sigma-Alrich) as an internal reference. The redox potential of decamethylferrocene has been previously determined to be −0.012 V for CV versus Ag/AgCl, and all potentials were corrected to this value.19 For DPV, all potentials were corrected to the maximum potential of decemethylferrocene, which was calculated to be −0.037 V versus Ag/AgCl using the following equation: ΔE Emax = E1/2 − 2 , where Emax is the maximum potential, E1/2 is the known half-wave potential, and ΔE is the pulse amplitude.20 Samples were analyzed over a range of +1.6 to −1.6 V for CV at a scan rate of 100 mV/s for three cycles. For DPV, scan windows of +0.25 to −1.6 V and −1.6 to −0.25 V were used with a pulse amplitude of 0.05 V. Bromo-boronsubphthalocyanine (Br-BsubPc) Synthesis. Br-BsubPc was prepared based on the method previously reported21 by modifying a published procedure by Potz et al.22 Phenoxy-BsubPc (PhO-BsubPc) Synthesis. A similar procedure was followed as described by Morse et al.12 To a preheated 100 mL round-bottom flask fitted with a condenser and under pressure of argon, Br-BsubPc (3.0 g, 6.31 mmol) was dissolved in 65 mL of chlorobenzene. After 5 min of stirring, five equivalents of phenol (3.0 g, 31.6 mmol) were added to the flask. The reaction was heated to 131 °C at reflux and left overnight. In the morning, the heating was stopped and allowed to cool to room temperature before the solvent was removed by rotary evaporation. To purify the crude product, Kauffman chromatography was used with standard basic alumina and dichloromethane as the mobile solvent phase. The column yield was 1.972 g (48.0% yield) with 96.70% purity determined by HPLC. The PhO-BsubPc was further purified by train sublimation, resulting in >99.9% purity and 66.7% yield. Florinated-Phenoxy-BsubPc Synthesis. The synthetic procedure for all five fluorophenoxy-BsubPc’s was adapted from a previously reported process for pentafluorophenoxy boron subphthalocyanine (F5BsubPc).16 In a 100 mL round-bottomed flask fitted with a condenser, Br-BsubPc (3.0 g, 6.314 mmol) was dissolved in toluene (30 mL) under constant pressure of argon gas. To the vessel, three equivalents of the fluorinatedphenol were added and the mixture was refluxed at 111 °C for 4 h while monitoring the completion of the reaction (absence of Br-BsubPc) by HPLC analysis. The reaction was then cooled to room temperature and the toluene was removed by rotary evaporation. The crude product was subsequently purified using Kauffman column chromatography with standard basic alumina and dichloromethane as the mobile phase to remove the excess fluorophenol. The solvent was removed by rotary evaporation and yielded the crude product with purity determined by HPLC analysis. Further purification of the product was carried out by train sublimation. The apparatus 1093
DOI: 10.1021/acs.jpcc.7b11157 J. Phys. Chem. C 2018, 122, 1091−1102
Article
The Journal of Physical Chemistry C ods.23−25 In brief, the devices were fabricated on prepatterned, ITO-coated glass substrates (Figure S6). The substrates were cleaned using sonication in a detergent solution and solvents, followed by an atmospheric plasma treatment. After cleaning, the substrates were spin-coated with PEDOT:PSS and baked on a hot plate at 110 °C for at least 10 min. The substrates were then removed from atmospheric conditions for the remainder of fabrication and characterization by transferring to a nitrogen atmosphere glovebox attached to a custom vacuum deposition chamber. All subsequent organic layers were deposited at ∼1.0 Å/s and a working pressure of ∼1 × 10−7 Torr. Silver electrodes were deposited at 1.0 Å/s and a working pressure of ∼1 × 10−6 Torr through a shadow mask, defining an active area of 0.2 cm2.
and 564 nm (within experimental error, Table 1). The spectra also have distinct shoulders at 547 nm. The varying position Table 1. Maximum UV−vis Absorption, Fluorescence, Stokes Shift, and Quantum Yield Measurements for the Array of Fluorophenoxy-BsubPc compounds
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RESULTS AND DISCUSSION Synthesis of Fluorophenoxy-BsubPc Compounds. For this study, five fluorophenoxy-BsubPcs (FnPhO-BsubPcs) were synthesized in addition to phenoxy-BsubPc (nonsubstituted PhO-BsubPc) and pentafluorophenoxy-BsubPc (pentasubstituted, F5BsubPc): 3-fluorophenoxy-BsubPc (monosubstituted, 3F1PhO-BsubPc); 3,5-difluorophenoxy-BsubPc (disubstituted, 35F2PhO-BsubPc); 2,4,6-trifluorophenoxy-BsubPc (trisubstituted, 246F3PhO-BsubPc); 3,4,5-trifluorophenoxy-BsubPc (trisubstituted, 345F3PhO-BsubPc); and 2,3,5,6-tetrafluorophenoxy-BsubPc (tetrasubstituted, 2356F4PhO-BsubPc). The synthesis of all fluorophenoxy-BsubPc compounds was achieved by reacting Br-BsubPc with three molar equivalents of the respective fluorophenol in toluene under reflux (Scheme 1). The mechanism of this reaction has been established.26 We have previously shown that Br-BsubPc has a high phenoxylation reaction rate, achieving completion in only a few hours, and is therefore an ideal precursor for efficient synthesis of phenoxyBsubPc derivatives.9 All FnPhO-BsubPcs were purified by train sublimation prior to characterization and device fabrication. Spectroscopy Analysis. The UV−vis absorption and photoluminescence spectra of each fluorophenoxy-BsubPc compound were measured in toluene solution (Figure 1). All fluorophenoxy-BsubPc compounds were found to have very similar absorption spectra, which is typical of BsubPcs and directly comparable to F5BsubPc. This confirmed that the axial phenoxy substituent has no influence on the band gap nor basic photophysics of the BsubPc chromophore. The maximum absorptions for the fluorophenoxy-BsubPcs occur between 562
fluorophenoxy-BsubPc
maximum absorption wavelength (nm)
maximum fluorescence wavelength (nm)
Stokes shifts (nm)
quantum yield
PhO-BsubPc 3F1PhO-BsubPc 35F2PhO-BsubPc 246F3PhO-BsubPc 345F3PhO-BsubPc 2356F4PhO-BsubPc F5BsubPc
562 563 564 563 564 563 564
570 569 573 572 570 570 573
8 6 9 9 6 7 9
0.47 0.42 0.38 0.46 0.41 0.44 0.42
and quantity of fluorines on the axial phenoxy molecular fragment does, however, affect the UV region, where differences in extinction coefficients can be seen. Photoluminescence spectra were also measured in toluene solution by excitation at 540 nm, slightly lower in wavelength than the maximum absorbance. The fluorophenoxy-BsubPc compounds, again, had emission spectra of similar size and shape to that of other BsubPcs and F5BsubPc, in particular (Figure 1). The maximum photoluminescence peak emissions had minimal variations, ranging from 569 to 573 nm (Table 1). Each compound also had an emission shoulder at ∼615 nm, mirroring the absorption spectrum. Much like the UV−vis absorption spectra, the different fluorination has little effect on the emission spectra, which is consistent with previous observations.27 Stokes shifts are also similar and only range from 6 to 9 nm (Table 1). Fluorescence quantum yield measurements for each fluorophenoxy-BsubPc were measured at room temperature in toluene solutions relative to the previously determined standard F5BsubPc (0.42).16 The quantum yields of each material are similar to F5BsubPc (Table 1) and range from 0.38 (35F2PhO-BsubPc) to 0.47 (PhO-BsubPc). Although the quantum yields are very similar, it is notable that the BsubPcs with the lowest measured quantum yield have fluorines only in the meta position(s) on the phenoxy group, while the two highest measured quantum yield values correspond to the BsubPcs with fluorines only in the para and ortho position(s). Solid-State Arrangement Analysis. How the molecules arrange in the solid state is relevant to the formation of thin films and transfer of charge within an organic electronic device. Therefore, the crystal structure of each fluorophenoxy-BsubPc was examined by growing crystals suitable for X-ray diffraction. For the fluorophenoxy-BsubPcs (3F1PhO-BsubPc, 35F2PhOBsubPc, 246F3PhO-BsubPc, 345F3PhO-BsubPc, 2356F4PhOBsubPc), we focused mainly on crystals formed by train sublimation, as this would likely be the most representative arrangement of the vacuum-deposited films in devices. (A description of crystal structures from other methods is provided in the Supporting Information and Figure S7.) PhO-BsubPc crystals grown by vapor diffusion and F5BsubPc crystals grown by vapor diffusion (α-F5BsubPc) and sublimation (β-F5BsubPc) have been previously reported by Paton et al., Morse et al., and Castrucci et al.16,28,29 There are two general descriptions of BsubPc solid-state interactions: One details the interactions of the bowl shapes
Figure 1. Overlaid UV−vis absorption (solid line) and photoluminescence spectra (dotted line) of PhO-BsubPc (gray), 3F1PhOBsubPc (red), 35F2PhO-BsubPc (blue), 246F3PhO-BsubPc (green), 345F3PhO-BsubPc (purple), 2356F4PhO-BsubPc (orange), and F5BsubPc (pink). 1094
DOI: 10.1021/acs.jpcc.7b11157 J. Phys. Chem. C 2018, 122, 1091−1102
Article
The Journal of Physical Chemistry C
Figure 2. Three general solid-state interactions of adjacent fluorophenoxy-BsubPcs in terms of the relative arrangement of the bowls (concave− concave, convex−convex, and concave-ligand) and the position of the isoidoline groups (highlighted in yellow: head-to-head, convex−convex, and concave-to-ligand). The short π−π centroid contacts of each packing type are labeled with A through H.
der Waals radii (
