A Comprehensive Scope of Peripheral and Axial Substituent Effect on

Article Cite This: J. Phys. Chem. A 2018, 122, 4414−4424

pubs.acs.org/JPCA

A Comprehensive Scope of Peripheral and Axial Substituent Effect on the Spectroelectrochemistry of Boron Subphthalocyanines Kathleen L. Sampson,†,¶ Xiaoqin Jiang,‡,¶ Esmeralda Bukuroshi,† Aleksa Dovijarski,† Hasan Raboui,† Timothy P. Bender,*,†,§,∥ and Karl M. Kasdish*,‡

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†

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E4 ‡ Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States § Department of Chemistry, University of Toronto, 80 St George Street, Toronto, Ontario, Canada M5S 3H6 ∥ Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada, M5S 3E4 S Supporting Information *

ABSTRACT: An extensive study of the electrochemical and spectroelectrochemical properties of 14 boron subphthalocyanine (BsubPc) derivatives with various axial and peripheral substituents was performed in 1,2-dichloromethane (CH2Cl2) containing 0.1 M tetra-n-butyl-ammonium perchlorate (TBAP) as the supporting electrolyte. From the cyclic voltammetry results, all compounds exhibit one oxidation and at least two reduction processes within the solvent potential window of +1.6 to −1.8 V vs SCE. It was found that the reversibility of the redox reactions depends on the axial and peripheral substituents and the dipole moment of the boron-to-axial substituent. In general, UV−vis absorption spectra of the singly reduced BsubPc derivatives exhibit three equal intensity peaks in the 450 to 650 nm region that are derived from the maximum BsubPc absorbance peak upon reduction. Axial substituents affect the intensity of the three peaks upon reduction, while peripheral substituents shift the position of the peaks to higher wavelengths. Upon oxidation, the UV−vis absorption profile flattens considerably with only a single broad (∼300 nm) band apparent. Understanding the effect of substituents on the stability of the redox processes of BsubPcs will aid in further development of these materials for applications in organic electronic devices.

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materials, respectively.5,12,13 Upon dissociation of the excitons, the active material becomes either a radical cation (electron removal) or a radical anion (electron addition). Therefore, an understanding of the radical cation and radical anion stability is desirable for a material. The ability to determine and alter the HOMO/LUMO energy levels is also desirable and essential to the engineering of efficient charge transfer and OPV device performance. Several handles are available to alter the energy levels of BsubPcs, two of which are the ability to change the axial ligands on the boron center atom or the substituents on the peripheral positions of the subPc macrocycle.14−17 Morse et al. have shown that the redox potentials of BsubPcs can be altered more significantly by changing the peripheral rather than axial substituents.14 Thus, the ability to simultaneously characterize the redox behavior and the stability of BsubPcs is of particular importance to confirm the alignment of the energy

INTRODUCTION Boron subphthalocyanines (BsubPcs) are semiconducting organic small molecules that are a variant from the widely studied family of phthalocyanine (Pc) macrocycles. BsubPcs are uniquely templated and formed by cyclotrimerization of phthalonitrile in the presence of boron trihalides (BCl3 and BBr3). Because of their unique photophysical properties, BsubPcs are applicable within organic electronic devices, such as organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs).1−5 Within OPVs, BsubPcs have been proven to have multiple roles including facilitating exciton formation through strong photonic absorption6,7 and exciton dissociation through both hole-8,9 and electron-transport.10,11 The energy level alignment of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of organic semiconductor materials has an intrinsically important role in converting (sun)light into electrical current in OPV devices. The dissociation of excitons (electron−hole pairs) generated within the active layer of an OPV depends on the spacing between the HOMO and the LUMO of the electron-donating and electron-accepting © 2018 American Chemical Society

Received: February 28, 2018 Revised: April 17, 2018 Published: April 19, 2018 4414

DOI: 10.1021/acs.jpca.8b02023 J. Phys. Chem. A 2018, 122, 4414−4424

Article

The Journal of Physical Chemistry A

Figure 1. Structures of the investigated BsubPcs.

in our study an extensive set of phenoxylated-BsubPcs: 3methylphenoxy-BsubPc (3-MePhO-BsubPc, 8), phenoxyBsubPc (PhO-BsubPc, 1), pentafluorophenoxy-BsubPc (F5BsubPc, 7), and five additional fluorophenoxy-BsubPc compounds. These fluorophenoxy-BsubPcs include 3-fluorophenoxy-BsubPc (3F1PhO-BsubPc, 2), 3,5-difluorophenoxyBsubPc (35F2PhO-BsubPc, 3), 2,4,6-trifluorophenoxy-BsubPc (246F 3 PhO-BsubPc, 4), 3,4,5-trifluorophenoxy-BsubPc (345F3PhO-BsubPc, 5), and 2,3,5,6-tetrafluorophenoxy-BsubPc (2356-F4PhO-BsubPc, 6). In order to elucidate the influence of peripheral substituents, chloro-hexachloro-BsubPc (ClCl6BsubPc, 14) and phenoxy-hexachloro-BsubPc (PhOCl6BsubPc, 9) were chosen as part of this study due to their known functionality in OPVs.34−38 Finally, phenoxy-dodecachloro-BsubPc (PhO-Cl12BsubPc, 10) was also included based on previous observations that chloro-dodecachloro-BsubPc (ClCl12BsubPc) harvested triplets from pentacene in OPVs.37 ClCl12BsubPc has low solubility, but its phenoxylated version was easily characterized by spectroelectrochemistry.

levels, to efficiently extract charge, and to obtain a radical anion or a radical cation over long periods of time. The electrochemical, spectroscopic, and spectroelectrochemical properties of Pcs have previously been measured to determine the effect of both the macrocycle and its substituents on the electrochemical behavior of the compounds.18−22 Both the number and potential of redox reactions of Pcs can be affected by the substituents on the Pc macrocycle.23 While electrochemical techniques, such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV), have been used extensively to study the electrochemical properties of BsubPcs,14,24 only a few comprehensive studies have compared a wide range of BsubPc derivatives under the same solution and experimental conditions (solvent, supporting electrolyte, type of electrode material, and type of electrochemical experiment). Moreover, most of these studies were on BsubPc-ferrocene dyads and not on the individual monomeric units.25−28 The first oxidation of most BsubPcs is typically irreversible and the product is less stable than that of the first reduction, which is often a reversible process.14,24−27,29 Scanning to more positive or more negative potentials beyond the first oxidation or first reduction often reveals several additional redox reactions.26,27,30 These are not necessarily related to starting compounds, but may instead be due to redox reactions involving new products generated from the singly oxidized or singly reduced BsubPcs. Understanding the stability and reversibility of the redox species and identification of side reactions is, therefore, essential for evaluating the potential application of these materials in organic electronics. Spectroelectrochemistry is a useful technique to simultaneously determine the redox stability and to spectrally characterize both intermediates and final products of the redox reactions, which is important for organic electronic device performance. However, to date, it has been used only once to study BsubPcs.28 In this study, a series of 14 BsubPc derivatives with different axial and peripheral substituents were characterized using cyclic voltammetry and UV−vis spectroelectrochemistry (Figure 1). The BubPcs with differing axial substituents include the prototypical chloro-BsubPc (Cl-BsubPc, 13) and the less explored fluoro-BsubPc (F-BsubPc, 12).31−33 We also included

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EXPERIMENTAL SECTION Spectroelectrochemistry. All chemicals and solvents were of the highest grade available and were used without further purification. Dichloromethane (CH2Cl2) and tetra-n-butylammonium perchlorate (TBAP), used as the supporting electrolyte, were purchased from Sigma-Aldrich Co. UV−vis spectra were recorded with a Hewlett-Packard Model 8453 diode array spectrophotometer. Cyclic voltammetry was carried out at 298 K using an EG&G Princeton Applied Research 173 potentiostat/galvanostat. A three-electrode system was used for cyclic voltammetry measurements and consisted of a glassy carbon working electrode, a platinum counter electrode, and a saturated calomel reference electrode (SCE). The SCE was separated from the bulk of the solution by a fritted glass bridge of low porosity which contained the solvent/supporting electrolyte mixture. Thin-layer UV−vis spectroelectrochemical experiments were performed with a home-built thin-layer cell, which had a light transparent platinum-net working electrode.39,40 Potentials were applied and monitored with an EG&G PAR Model 173 potentiostat. High purity nitrogen 4415


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flask equipped with a condenser and gas inlet, dichlorophthalonitrile (9.654 g, 49 mmol) was added to bromobenzene (14.5 mL) and toluene (40.5 mL) under argon gas pressure. Boron tribromide (1.5 mL, 15.8 mmol) was added slowly through one neck of the reaction flask. The reaction was stirred overnight. In the morning, the reaction was turned off and left for 1 h. The mixture was filtered by gravity filtration and immediately washed with ∼75 mL of acetonitrile. The solid was dried with nitrogen blowing over the top for a few hours. Once sufficiently dry, the product was placed in the vacuum oven overnight. Yield 7.74 g, 70.4%. HPLC purity 89%, MS (ESI) m/ z: 681.8 [M + H]+. One equivalent of Br-Cl6BsubPc (0.8 g, 1.2 mmol) was combined with 5 equiv of phenol (0.55 g, 5.9 mmol) in chlorobenzene (15 mL) in a 50 mL round-bottom flask fitted with a condenser and held under a constant pressure of argon gas. The reactants were heated at reflux for 4 h, at which time the reaction was confirmed to be complete by HPLC analysis (absence of Br-Cl6BsubPc) and was subsequently cooled to room temperature. The solvent was removed by rotary evaporation, yielding the crude product. The crude product was purified using Kauffman column chromatography with silica gel and dichloromethane as the eluent. Removal of dichloromethane by rotary evaporation yielded the product, which was further purified by train sublimation. HPLC purity >99.9%. 1H NMR (400 MHz, CDCl3, Me4Si) δ 5.34−5.36 (2H, m), 6.65−6.75 (1H, t), 6.76−6.79 (2H, m), 8.90 (6H, s). MS (ESI) m/z: 693.9 [M + H]+. Phenoxy-Dodecachloro-Boron Subphthalocyanine (PhOCl12BsubPc, 10). A method adapted from Morse and Bender was used to synthesize PhO-Cl12BsubPc, the main difference being a new activating agent, tin(IV) chloride.46 Cl-Cl12BsubPc (1.026 g, 1.22 mmol, 1 equiv) was added to a round-bottom flask containing 1,2-dichlorobenzene (51 mL). To the stirring solution was added tin chloride (0.213 mL, 1.82 mmol, 1.5 equiv). The resulting mixture was heated at 60 °C under argon for 25 min. After 25 min, phenol (0.572 g, 6.08 mmol, 5 equiv) was added and the reaction mixture was left under heating at 60 °C for 27 h. The mixture was quenched with pyridine (10 mL). After cooling to room temperature, the solvent and other volatiles were removed by a rotary evaporator. The crude product was purified on a Kauffman column using standard silica gel as the adsorbent and dichloromethane as the eluent. The product eluted from the Kauffman column, while most of the excess phenol remained adsorbed. Dichloromethane was then removed by a rotary evaporator and the product was further purified by train sublimation with a yield of 0.064 g (24%). HPLC purity >99%. 1H NMR (400 MHz, CDCl3, Me4Si): δ 5.35−5.37 (2H, d), 6.70−6.77 (1H, t), 6.81−6.78 (2H, t). MS (EI) m/z: 901.7 [M]+.

from Trigas was used to deoxygenate the solution and a stream of nitrogen was kept over the solution during each electrochemical and spectroelectrochemical experiment. Synthesis of Boron Subphthalocyanines. All solvents used in the synthetic procedures and the 50−200 μm standard basic alumina for Kauffman chromatography were purchased from Caledon Laboratories (Caledon, Ontario, Canada) and used as received. The procedures and setup of the Kauffman column and train sublimation apparatus have been previously described.41 3-Methylphenol and phenol for the synthesis of 3MePhO-BsubPc (8) and PhO-Cl12BsubPc (10) were purchased from Sigma-Aldrich (Oakville, Ontario, Canada). High performance liquid chromatography (HPLC) was performed by 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. The HPLC mobile phase was composed of 80/ 20 acetonitrile/dimethylformamide solvent mixture (HPLC solvents purchased from Caledon Laboratories, used as received) and eluted at a rate of 0.6 mL·min−1. Extinction coefficients were measured using our previously described method and are summarized in Table S1 of the Supporting Information.7,31 The synthetic procedures for PhO-BsubPc (1), 3F1PhOBsubPc (2), 35F2PhO-BsubPc (3), 246F3PhO-BsubPc (4), 345F3PhO-BsubPc (5), 2356F4PhO-BsubPc (6),42 F5BsubPc (7),41 Ph-BsubPc (11),43 F-BsubPc (12),31 Cl-BsubPc (13),7 and Cl-Cl6BsubPc (14)14 have been previously described. 3-Methylphenoxy-Boron Subphthalocyanine (3-MePhOBsubPc, 8). Br-BsubPc was prepared based on the method previously reported44 by modifying a published procedure by Potz et al.32 A similar procedure to the one previously described by Paton et al. was followed.17 Br-BsubPc was used as a starting material instead of Cl-BsubPc to decrease the reaction time. It has been shown in our lab that Br-BsubPc has a higher rate of substitution with phenol than Cl-BsubPc.31 To a predried round-bottom flask with a condenser and under argon gas pressure, Br-BsubPc (5.0 g, 10.52 mmol) was dissolved in 75 mL of toluene. After 5 min of stirring, 2 equiv of 3methylphenol (2.28 g, 21.05 mmol) was added to the reaction flask and the mixture was refluxed at 111 °C overnight. The next day, the heating was stopped and the mixture was allowed to cool to room temperature. The crude product was purified by first dissolving the mixture in 160 mL of toluene and washing three times with 300 mL of 3 M KOH in a separatory funnel, keeping the organic layer. Toluene was removed by rotary evaporation and further purified, first by Kauffman chromatography using standard basic alumina and dichloromethane as the solvent and then by train sublimation. Reaction Yield 3.02 g, 47.7%. Sublimation Yield 1.47 g, 48.5%. HPLC purity 98%, 1H NMR (400 MHz, CDCl3, Me4Si): δ 1.94 (3H, s), 5.12−5.14 (1H, m), 5.25 (1H, s), 6.42−6.43 (1H, d), 6.63− 6.65 (1H, t), 7.88−7.93 (6H, m), 8.83−8.88 (6H, m). MS (ESI) m/z: 502.2 [M + H]+. Phenoxy-Hexachloro-Boron Subphthalocyanine (PhOCl6BsubPc, 9). A more time-efficient synthetic procedure for the BsubPc acceptor used in this study (PhO-Cl6BsubPc) was adapted from Ebenhoch et al., based on our synthetic methodology.31,34,45 Rather than refluxing phenol with Cl− Cl6BsubPc in toluene for 7 days, we have previously found that bromine in the axial position of BsubPc is more labile than chlorine and, thus, undergoes a faster reaction.31 Therefore, bromo-hexachloro-boron-subphthalocyanine (Br-Cl6BsubPc) was first synthesized. To an oven-dried 3-neck round-bottom

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RESULTS AND DISCUSSION The main goal of this study was to survey the spectroelectrochemistry of boron subphthalocyanines (BsubPcs), a previously unexplored analysis technique applied broadly to these compounds. To start, we first performed cyclic voltammetry (CV) measurements, on the series of 14 BsubPc derivatives with various axial and peripheral substituents (Figure 1) in order to measure the redox potentials and evaluate reversibility. This was followed by spectroelectrochemical measurements at applied potentials which were at least 100 mV beyond the peak or half-wave potential of the investigated redox processes. In this study, it was assumed that the solubility 4416


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F5BsubPc, 7), and a m-methyl-phenoxy-BsubPc (3-MePhOBsubPc, 8). Representative cyclic voltammograms are depicted in Figure 2 and a comprehensive set of the voltammograms are

does not significantly change between the neutral, reduced, and oxidized species of each compound during the time of the cyclic voltammetric or spectroelectrochemical measurements. Our discussion of the electrochemical and spectroelectrochemical data is divided into two main sections based on the effect of the axial and peripheral substituents on the redox behavior. Table 1 Table 1. Peak and Half-Wave Potentials (Ep and E1/2, V vs SCE) of Investigated BsubPcs in CH2Cl2, 0.1 M TBAP oxidation compound PhO-BsubPc (1) 3F1PhO-BsubPc (2) 35F2PhOBsubPc (3) 246F3PhOBsubPc (4) 345F3PhOBsubPc (5) 2356F4PhOBsubPc (6) F5BsubPc (7) 3-MePhOBsubPc (8) PhO-Cl6BsubPc (9) PhO-Cl12BsubPc (10) Ph-BsubPc (11) F-BsubPc (12) Cl-BsubPc (13) Cl−Cl6BsubPc (14) a

reduction

Ep (ox 1)

E1/2 (ox 1)

Ep (red 1)

E1/2 (red 1)

Ep (red 2)

E1/2 (red 2)

1.09 1.10

-

-

−1.06 −1.04

−1.57 −1.57

-

1.13

-

-

−1.03

−1.57

-

1.12

-

-

−1.04

−1.58

-

1.15

-

-

−1.02

−1. 56

-

1.14

-

-

−1.01

−1.55

-

1.15 1.12

-

-

−1.01 −1.06

−1.58 −1.62

-

1.31

-

-

−0.77

-

−1.31

-

1.44

-

−0.54

-

−1.09

1.10 1.14 1.37

1.05 -

−0.99 −0.72

−1.11 −1.00 -

−1.48 −1.22

−1.60a −1.53a -

Figure 2. Cyclic voltammograms of axially substituted BsubPcs in CH2Cl2 containing 0.1 M TBAP (PhO-BsubPc, 1; Ph-BsubPc, 11; FBsubPc, 12; and Cl-BsubPc, 13).

Quasi-reversible redox reaction.

depicted in Figure S1. We also examined phenyl-BsubPc (PhBsubPc, 11) and two halogenated-BsubPcs (F-BsubPc, 12, and Cl-BsubPc, 13). Each BsubPc derivative undergoes one oxidation and at least two reductions within the potential limit of the electrochemical solvent (+1.6 to −1.8 V vs SCE), a result which is similar to other electrochemistry results reported in the literature (Figure 2).26,27,29,30,43 A comparison of voltammograms for the eight investigated phenoxy-substituted BsubPcs (Figure S1) shows only a very slight variation in redox potentials. This can be explained from previous reports that explored modeling of the molecular

summarizes the peak or half-wave oxidation and reduction potentials for each compound and Table 2 lists the maximum absorption wavelengths for major peaks of the neutral, anionic, and cationic forms of the BsubPcs. Effect of Axial Substituents. Electrochemistry. In terms of axial substituents, we analyzed a series of axially phenoxyBsubPcs, including PhO-BsubPc (1), a range of fluorophenoxyBsubPcs (3F1PhO-BsubPc, 2; 35F2PhO-BsubPc, 3; 246F3PhOBsubPc, 4; 345F3PhO-BsubPc, 5; 2356F4PhO-BsubPc, 6; and

Table 2. UV−vis Spectral Data of Neutral Species, Anion Radicals, and Cation Radicals of Some of the Investigated BsubPcs in CH2Cl2 Containing 0.1 M TBAPa λmax (nm) neutral

compound PhO-BsubPc (1) F5BsubPc (7) 3-MePhO-BsubPc (8) PhO-Cl6BsubPc (9) PhO-Cl12BsubPc (10) Ph-BsubPc (11) F-BsubPc (12) Cl-BsubPc (13) Cl-Cl6BsubPc (14)

307 307 304 318 328 308 306 302 314

513sh 515sh 518sh 520sh 539sh 521sh 513sh 502sh 525sh

reduced 561 561 562 568 586 566 560 563 570

301 301 300 309 326 307 300 -

483 483 488 489 505 487 484 502 552

513 515 518 534 539 521 512 -

oxidized 566 567 565 606 613 584 566 -

600−800br 600−800br 670 720 703 690 657 657 705

321 321 325 350 348 317 366 322 322

350 365 364 400 522 -

sh = shoulder peak, br = broad peak. Absorption peaks for fluorophenoxy-BsubPcs (2−6) from spectroelectrochemical measurements can be found in Table S2 of the Supporting Information. a

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V as shown in Figure 2. The peak at −1.70 V is coupled to a reoxidation peak on the return scan at −1.48 V. It is difficult to determine the nature of these peaks and whether they are separate reduction and reoxidation processes of Cl-BsubPc (13) or if a new species is formed after reduction. This unclear cyclic voltammetry curve could be due to the known lower solubility of Cl-BsubPc (13) compared to other axially substituted BsubPcs.7,17,31 A previous theoretical study by Ferro et al. using semiempirical, density functional theory (DFT), and Hartree−Fock (HF) molecular modeling methods, found that the boron-toaxial ligand bond length varies with the axial substituent type and obeys the following sequence: F-BsubPc < PhO-BsubPc < Ph-BsubPc < Cl-BsubPc.47 This order of boron-to-ligand bond length is the same order determined from X-ray diffraction measurements in the literature.17,31,43,51 However, this order does not correlate well with the reversibility of redox potentials, with Ph-BsubPc having the second longest boron-to-axial ligand bond, but the most reversible redox processes and the most stable products of the electron transfer. The reversibility of the redox potentials seems to coincide more closely with the dipole moment of the axial ligand as measured with the same DFT calculations performed by Ferro et al., which results in the following order: Ph-BsubPc (3.1 D) < F-BsubPc (4.4 D) ∼ PhO-BsubPc (4.5 D) < Cl-BsubPc (5.5 D).47 The more electron withdrawing the axial substituent and the longer the boron-to-axial ligand bond are, the higher the dipole moment and the more difficult it is to reversibly oxidize or reduce the compound. For example, Ph-BsubPc (11) has the lowest dipole moment and the only studied compound with a reversible oxidation, a reversible first reduction, and a quasireversible second reduction. F-BsubPc (12) and PhO-BsubPc (1) have similar dipole moments and comparable electrochemistry with an irreversible oxidation, reversible first reduction, and an irreversible or quasi-reversible second reduction. Finally, Cl-BsubPc (13) has the bulkiest and most electronegative axial substituent, resulting in the largest dipole moment and all irreversible redox events. From the DFT study on the redox processes of Cl-BsubPc (13), Ferro et al. found that the boron-to-axial substituent dipole moment increases with oxidation and the boron-to-axial substituent bond length changes more with the oxidized species than the reduced species.48 This may contribute to the reversibility of the redox processes. Further studies on the change in boron-to-axial substituent bond length and dipole moment with reduction and oxidation of other axially substituted BsubPcs is needed in order to further understand this mechanism and the trends in stability. Spectroelectrochemistry. UV−vis spectra were recorded during controlled-potential reduction or oxidation of the axially substituted BsubPc derivatives. As predicted from the similar electrochemistry of the fluorophenoxy-BsubPc derivatives (1− 7) described above, Figure S2 depicts the comparable neutral UV−vis spectra, which are characterized by a major absorption band between 561−562 nm, shoulder peak in the range of 513−517 nm, and a second peak at 307 nm of lower intensity and extinction coefficient. During the one-electron addition to these fluorophenoxy-BsubPcs (1−7), the major absorption band between 561 and 562 nm decreases in intensity and is replaced by three major absorption bands of fairly equal intensity at 483−488, 513−517, and 563−569 nm. The peak at 307 nm also decreases in intensity and shifts to 301−305 nm as a new broad band in the range of 600−800 nm forms. Similar

orbitals for a series of BsubPcs and found that the first oneelectron oxidation and first one-electron reduction occur at the π-ring system of the conjugated subphthalocyanine macrocycle.47−50 Thus, axial substituents do not have much effect on the HOMO and LUMO values of the BsubPc. In our previous study of the fluorophenoxy-BsubPcs (1−7), we found a trend in this small variation which was due to the large electronic influence of fluorine in the meta position of the fluorophenoxy axial substituent.42 However, the cyclic votammograms in this study were done without an internal standard and the redox potentials, therefore, have an error of 10−25 mV. Thus, the same trends cannot be observed from the electrochemical data in which the redox potentials vary slightly from compound to compound. Our previous study used multiple other techniques, such as differential pulse voltammetry, ultraviolet photoelectron spectroscopy, semiempirical modeling, and OPV device characteristics with the fluorophenoxy-BsubPcs as electron acceptors, in order to firmly establish the influence of a fluorine in the meta position of the phenoxy axial substituent and the resulting effect of this substituted placement on OPV device performance.42 Redox potentials of the other axially substituted BsubPcs (Ph-BsubPc, 11; F-BsubPc, 12; and Cl-BsubPc, 13) are also fairly similar to the axial-phenoxy-substituted BsubPcs. However, the potentials of Ph-BsubPc (11) are slightly more negatively shifted than the other axially substituted BsubPcs, outside of the ∼25 mV error of these measurements. This could be due to the small dipole moment of the boron to axial substituent bond, which is described in more detail on the following pages. In this study, we also scanned to more negative potentials in order to observe the presence of a second reduction process. The second reduction involves a one electron transfer and was seen for all of the axially substituted BsubPcs. This reduction is also coupled to a new irreversible oxidation peak between −0.35 and −0.48 V on the return scan back toward 0 V (an exception is Cl-BsubPc, 13). The new oxidation peak may involve a new chemical species generated from the doubly reduced BsubPc, but further studies of compound identification of this product were not carried out and are of secondary interest to this study. While redox potentials of the axially substituted BsubPcs are comparable to each other, the reversibility of these processes depends upon the type of axial substituent. For the phenoxyBsubPcs (1−8), the first reduction is reversible, while the second reduction and first oxidation processes are both irreversible. This result is consistent with what was previously reported for these phenoxy-BsubPc derivatives.14,42 F-BsubPc (12) exhibits a reversible one electron reduction and an irreversible one electron oxidation at similar potentials, which are similar to both PhO-BsubPc (1) and the fluorophenoxyBsubPcs (2−7). However, the first reduction is less reversible for F-BsubPc (12), a compound with a fluorine axial ligand, than for the compounds with phenoxy axial ligands. In addition, the second reduction becomes quasi-reversible for F-BsubPc (12). When the BsubPc is axially coordinated with a phenyl group (Ph-BsubPc, 11), the first oxidation becomes reversible, a rare result for BsubPcs, and the second reduction becomes quasi-reversible. An exception to this trend of two reductions with the first being reversible is for Cl-BsubPc (13), where all of the redox reactions are irreversible and there are more than two reduction processes. The irreversible first reduction at −0.99 V is followed by two other irreversible peaks at −1.48 and −1.70 4418


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Figure 3. UV−vis spectral changes during controlled-potential first reduction (left spectra) and oxidation (right spectra) for PhO-BsubPc (1), PhBsubPc (11), F-BsubPc (12), and Cl-BsubPc (13) in CH2Cl2 containing 0.1 M TBAP as the electrolyte.

considerably more altered compared to the changes in spectra for the reduced species. This may indicate that the oxidized compound is less stable than the reduced compound. However, further studies of the spectroelectrochemical reversibility are needed to confirm this. Changing the axial ligand on Ph-BsubPc (11), F-BsubPc (12), and Cl-BsubPc (13) has very little effect on the spectra of the reduced or oxidized compounds (Figure 3). For the neutral species, these BsubPcs exhibit two major absorption bands: a major peak between 560−566 nm with a shoulder between 502−521 nm and a second lower intensity band in the range of 302−308 nm; these are typical bands of BsubPcs.7,14,29,31,43 However, the presence of the phenyl and chlorine axial ligands also leads to a moderate red-shift in the absorption band, which are located at 566 and 564 nm, respectively, as compared to 561 nm for PhO-BsubPc (1) and 560 nm for F-BsubPc (12).

changes are seen for 3-MePhO-BsubPc (8), which has bands before reduction at 304, 518, and 562 nm as compared to 300, 488, 518, 565, and 670 nm after the addition of one electron (Figure S3). The UV−vis spectra are also similar to that of the eight singly oxidized axial phenoxy-substituted BsubPcs. Examples of the spectral changes are given in Figure S3, which depicts the first reduction and oxidation spectra of F5BsubPc (7) and 3MePhO-BsubPc (8), compounds representative of the axial phenoxy-substituted BsubPcs. In general, the major peak at ∼560 nm flattens considerably to the baseline upon the oneelectron abstraction. For 3-MePhO-BsubPc (8), a peak at ∼600 nm appears upon oxidation but flattens to the baseline with further scans. The peak at ∼305 nm decreases in intensity and broadens with two ill-defined peaks at ∼320−325 nm and ∼365 nm. The absorption profile of the oxidized species is 4419


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Figure 4. Cyclic voltammetry of PhO-BsubPc (1), PhO-Cl6BsubPc (9), PhO-Cl12BsubPc (10), Cl-BsubPc (13), and Cl-Cl6BsubPc (14), in CH2Cl2 containing 0.1 M TBAP as the electrolyte.

this and to observe the reversibility of the reduction and oxidation for the currently investigated compounds, the BsubPc macrocycle was altered by the stepwise addition of six peripheral chlorine atoms to both PhO-BsubPc (1) and ClBsubPc (13), forming PhO-Cl6BsubPc (9), PhO-Cl12BsubPc (10), and Cl-Cl6BsubPc (14), respectively. The substantial effect of the peripheral electron-withdrawing chlorine substituents is depicted in the cyclic voltammograms of Figure 4. The addition of six chlorine groups to the macrocycle (PhO-BsubPc, 1, to PhO-Cl6BsubPc, 9) shifts the oxidation peak potential, the first reduction half-wave potential, and the second reduction peak potential by 220, 290, and 200 mV, respectively. These peak or half-wave potentials are further shifted positively by 170, 230, and 220 mV, respectively, when six more chlorine atoms are added to the macrocycle (comparing PhO-Cl6BsubPc, 9, and PhO-Cl12BsubPc, 10). Moreover, the second reduction becomes reversible for PhOCl6BsubPc (9) and the first oxidation and the two reductions are reversible for PhO-Cl12BsubPc (10). Thus, the electronwithdrawing chlorine peripheral substituents have two effects: they shift the redox potentials in a positive direction, making the two reductions easier and single oxidation harder, and they also cause the redox reactions to become more reversible. With peripheral substituents, the HOMO and LUMO energy levels of the BsubPcs can be deepened to be used as electron accepting materials in OPVs.3,5,34,36 The increased redox reversibility is likely related to how the electron-withdrawing peripheral substituents have been theoretically shown to shorten the boron-to-axial substituent bond distance as well as significantly lower the dipole moment, thus possibly leading to more electrochemically reversible species.50 The newly formed irreversible oxidation peak at ∼ −0.3 to −0.5 V that appears after the second reduction process for the axially substituted BsubPcs is not observed for PhO-Cl6BsubPc (9) or PhO-Cl12BsubPc (10). This is another indication of an improved stability of the compound or the anionic species generated with peripheral substitution. The result of peripheral substituents having a greater effect on the redox processes also

Similar UV−vis spectra are also observed for the anion radicals of Ph-BsubPc (11) and PhO-BsubPc (1), each compound being characterized by three major bands of similar intensity between 400 and 650 nm and a newly formed broad band between 600 and 700 nm upon reduction. The three absorption bands for the anionic form of Ph-BsubPc (11) are located at 487, 521, and 584 nm, slightly red-shifted from the bands of PhO-BsubPc (1) at 483, 513, and 566 nm. In the case of the fluoride and chloride derivatives, the bands in the 400− 650 nm region are flattened to two nonobvious bands at 485 and 512 nm for F-BsubPc (12) and further leveled out to one broad band at around 500 nm for Cl-BsubPc (13) after reduction. In addition, the broad band in the 600−700 nm region is more pronounced with these compounds and there is a peak at around 657 nm (Figure 3). This result is fairly consistent with the electrochemical data in Figure 2, in which the first reduction is less reversible for F-BsubPc (12) and irreversible for Cl-BsubPc (13). Again, the reason for this is attributed to the large dipole moment of the boron-to-halogen axial substituent and the resulting stretching of this bond during the reduction processes. As seen in Figure 3, the major 560−566 nm peak of the neutral compounds significantly flattens to the baseline upon oxidation. The peak at ∼300 nm also decreases in intensity and broadens, showing a maximum between 317−366 nm for PhBsubPc (11), F-BsubPc (12), and Cl-BsubPc (13). As the ∼560 nm peak of these three axially substituted compounds decreases, a small new absorption is initially observed at ∼600 nm, but with further scans, this new peak also reduces in intensity. Future work in analyzing the reversibility of the first reduction and oxidation processes by spectroelectrochemistry would be useful in establishing the stability of the anion and cation species. Further investigations of the spectra and reversibility during the second reduction would also contribute to our understanding of the overall stability of BsubPcs. Effect of Peripheral Substituents. Electrochemistry. As reported previously, the redox behavior of BsubPcs can be altered much more significantly by changing the peripheral substituents than by changing the axial ligands.14 To confirm 4420


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Figure 5. UV−vis spectral changes during the first reduction of PhO-BsubPc (1), PhO-Cl6BsubPc (9), PhO-Cl12BsubPc (10), Cl-BsubPc (13), and Cl-Cl6BsubPc (14), in CH2Cl2 containing 0.1 M TBAP as the electrolyte.

the anion radicals also follow a similar trend. The anion radicals of these three compounds have UV−vis absorption spectra consisting of three major, equal intensity bands, which are redshifted with increasing number of chlorine peripheral substituents. The formation of three equal intensity bands is consistent with what is seen for most of the axially substituted BsubPcs described above. The absorption bands shift from 483, 513, and 566 nm for PhO-BsubPc (1) to 490, 534, and 606 nm for PhO-Cl6BsubPc (9) and then to 505, 540, and 613 nm for PhO-Cl12BsubPc (10). A broad band in the region of 650−800 nm is also formed for the reduced species. In a similar fashion, the absorption profile shifts to higher wavelengths upon going from Cl-BsubPc (13) to Cl-Cl6BsubPc (14), with the main peaks moving from 302, 502, and 564 nm to 314, 525, and 570 nm, respectively, for the two neutral species. However, upon reduction, the maximum BsubPc absorbance peak flattens out and is characterized by a single broad band at 500 or 552 nm as opposed to the three equal intensity bands typical of other derivatives. The broad band in the 600−800 nm region also becomes more pronounced with peaks at 657 and 705 nm for Cl-BsubPc (13) and Cl-Cl6BsubPc (14), respectively. Again, the flattening of the major peaks upon reduction correlates with the irreversible and unclear reduction processes in the electrochemical measurements. This is likely

correlates with the theoretically shown location of the macrocyclic HOMO orbital.47,49 As seen in Figure 4, similar redox behavior is observed for ClBsubPc (13) and Cl-Cl6BsubPc (14) as the potentials for the first oxidation and reduction peaks are shifted more positively for Cl-Cl6BsubPc (14) by 230 and 270 mV, respectively. The cyclic voltammograms of both BsubPcs have ill-defined peaks and Cl-Cl6BsubPc (14) also has an irreversible reduction at −0.72 V, followed by three additional irreversible peaks of smaller current at −0.87, −1.22, and −1.76 V. As stated previously, the unclear and inconclusive peaks of the chlorine axially substituted BsubPcs could be due to their known lower solubility.7,31 However, none of the redox processes of ClBsubPc (13) or Cl-Cl6BsubPc (14) are reversible, which could be due to the high dipole moment associated with the chlorine axial substituent. Spectroelectrochemistry. The peripheral chlorine substituents on compounds PhO-Cl6BsubPc (9), PhO-Cl12BsubPc (10), and Cl-Cl6BsubPc (14) also affect the UV−vis spectra for the neutral, reduced, and oxidized species. As depicted in Figure 5, the successive addition of six chlorine atoms to the periphery of the BsubPc macrocycle shifts the main absorption bands from 307, 513, and 561 nm in the case of PhO-BsubPc (1) to 318, 520, and 568 nm for PhO-Cl6BsubPc (9) and 328, 540, and 586 nm for PhO-Cl12BsubPc (10). The UV−vis spectra of 4421


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Figure 6. UV−vis spectral changes during the first oxidation of PhO-BsubPc (1), PhO-Cl6BsubPc (9), PhO-Cl12BsubPc (10), Cl-BsubPc (13), and Cl-Cl6BsubPc (14), in CH2Cl2 containing 0.1 M TBAP as the electrolyte.

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due to the lower solubility and the large dipole moment of the boron-to-chlorine axial substituent.7,31,47 Figure 6 depicts the oxidation processes for these five compounds and reveals how the spectra of the generated oxidized products are flattened just like the axially substituted BsubPcs. For the phenoxy-axial substituted BsubPcs with peripheral chlorination, the ∼300 nm band broadens with peaks at 350 and 400 nm for PhO-Cl6BsubPc (9) and PhOCl12BsubPc (10). However, the major peak between 561 and 586 nm completely flattens to zero absorbance. The same disappearance of the major 564−570 nm band occurs with ClBsubPc (13) and Cl-Cl6BsubPc (14), but the ∼300 nm band decreases in intensity, shifts to 322 nm, and does not broaden as significantly as for the other peripherally chlorinated BsubPcs. Future exploration of the reversibility of the reduction and oxidation processes via spectroelectrochemical analysis will give a better idea of radical cation stability and whether the typical BsubPc absorbance profile can be regenerated from the flattened absorbance spectra of the oxidized species.

CONCLUSION

In summary, a series of 14 BsubPcs with different axial and/or peripheral substituents were synthesized and examined by electrochemistry and UV−vis spectroelectrochemistry in CH2Cl2 containing 0.1 M TBAP. Each compound undergoes two or more one-electron reductions and a single one-electron oxidation within the potential limit of the electrochemical solvent (+1.6 to −1.8 V vs SCE). In general, the peripheral substituents shift all of the redox potentials more positively, while the reversibility of these processes seems to depend upon solubility and upon both the peripheral and axial substituents, with peripheral chlorines and phenoxy axial ligand resulting in the most reversible processes. The reversibility of the redox reactions appears to correlate with theoretically calculated dipole moments of the boron-to-axial ligand bond and the lengthening and shortening of this bond upon oxidation or reduction, respectively, as indicated by the fact that the phenyl axial substituent has the smallest dipole moment and most reversible electrochemistry. 4422



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Spectroscopic properties of the singly reduced and singly oxidized BsubPcs can be tuned with changes in axial and peripheral substituents. For the BsubPcs without chlorine or fluorine on the axial ligand, the UV−vis spectra of the anion radical are characterized by three equal intensity peaks in the 450 to 650 nm region, a reduced intensity peak at ∼300 nm, and a broad band in the region of 600 and 800 nm. The absorption profiles of the cationic species are considerably flattened, compared to the anionic species. This is true for all of the BsubPcs which show only a broad peak between 300 and 400 nm. These trends are important in the design of BsubPc derivatives for use in high performance, organic electronic devices. Spectroelectrochemistry provides simultaneous characterization of photochemical and electronic properties, similar to the function of an OPV. Future work on the spectroelectrochemical reversibility for the redox processes of these BsubPcs, both in solution and in the solid state, will further enhance our understanding of their material and device stability and knowledge as to their use as better electron acceptor (stable anion) or electron donor (stable cation) materials in organic electronics.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b02023. Extinction coefficients of investigated BsubPcs, spectroelectrochemical data of neutral and anion radicals of the fluorophenoxy-BsubPcs, cyclic voltammograms of the axial phenoxy-substituted BsubPcs, UV−vis spectra of neutral and reduced fluorophenoxy-BsubPcs, and UV− vis spectral changes of F5BsubPc and 3-MePhO-BsubPc during first reduction and first oxidation processes (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Timothy P. Bender: 0000-0002-6086-7445 Karl M. Kasdish: 0000-0003-4586-6732 Author Contributions ¶

These two authors contributed equally to the preparation of this manuscript and the outlined experimentation.

Notes

The authors declare no competing financial interest.

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REFERENCES

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S Supporting Information *

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ACKNOWLEDGMENTS

This work was supported by a Natural Sciences and Engineering Research Council (NSERC) Alexander Graham Bell Canada Graduate Scholarship (Doctoral Level) and Ontario government Queen Elizabeth II Graduate Scholarship in Science and Technology (QEII-GSST) to K.L.S., an NSERC Alexander Graham Bell Canada Graduate Scholarship (Masters Level) to K.L.S, and the Robert A. Welch Foundation (K. M. K. Grant E-680). Support was also received through the NSERC via a Discovery Grant to T.P.B. 4423


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A Comprehensive Scope of Peripheral and Axial Substituent Effect on

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