Versatile Synthesis of Siloxy Silicon ... - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Versatile Synthesis of Siloxy Silicon Tetrabenzotriazacorroles and Insight into the Mode of Macrocycle Formation Hasan Raboui,† Alan J. Lough,‡ Anjuli M. Szawiola,‡ and Timothy P. Bender*,†,‡,§ †

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5 ‡ Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6 § Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4 S Supporting Information *

ABSTRACT: Tetrabenzotriazacorroles (Tbcs) are a family of molecules related to phthalocyanines but have the unique ability to intensely absorb both blue and red light. Here, we report the synthesis of four novel silicon tetrabenzotriazacorrole derivatives (SiTbcs) with varying sized axial ligands. SiTbcs are formed starting from bis(hydroxy) silicon phthalocyanine ((OH)2-SiPc) via a simple in situ axial functionalization and reductive chemical process using magnesium metal and the respective chlorosilane in pyridine. Systematic probing of the reaction conditions revealed that the reaction is acid-promoted and that the formation of the Tbc macrocycle occurs at temperatures as low as 40 °C. Results imply this chemistry can be extended to SiTbcs with any axial ligands using pyridine hydrochloride as an acid source. Single crystals of all compounds were grown and showed significant π−π interactions between the macrocycles in the solid state. Optical, electrochemical, and thermal characterization of these materials is also described. The SiTbcs exhibit interesting highly oxidative electrochemistry as well as high thermal stability and tunable phase transition behavior.

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INTRODUCTION Phthalocyanines (Pcs) have been known for almost a century, and structural variants have been produced as dyes and pigments at an industrial scale for some time.1 The intense color of Pcs due to their strong absorbency of photons in the visible portion of the electromagnetic spectrum has incentivized researchers to leverage their properties in emerging applications such as organic photovoltaics,2−8 photodynamic therapy,9−15 and photocatalysis.16−20 However, control over the optical properties of Pcs through structural variations, with the exception of π-conjugate extension, has been limited to small shifts to the Q-band, which lies in the red to near-infrared region of the electromagnetic spectrum. Tetrabenzotriazacorroles (Tbcs) are a family of photoactive compounds that are structurally related to Pcs but have been relatively unexplored.21 Symmetry in the chemical structure of Tbcs is reduced relative to Pcs due to the omission of one of the imine bridging nitrogen atoms in the macrocyclic structure (Scheme 1). The reduced symmetry of Tbcs results in a large red-shift in the absorbance of the Soret-band from the ultraviolet region of the electromagnetic spectrum (characteristic of Pcs) to the blue region while maintaining a strong absorbance in the red region, analogous to the absorbance of Pcs. Both bands of Tbc compounds exhibit high extinction coefficients and on the same order of magnitude (∼105 L mol−1 cm−1).22,23 Thus, Tbcs have the unique ability to absorb light at high intensities in two regions of the visible spectrum, both the © XXXX American Chemical Society

blue and red regions. In contrast, the absorbance of corroles, a similar family of well-studied asymmetric compounds, is skewed having Soret-bands with extinction coefficients an order of magnitude higher than the Q-bands.24 Triazacorroles, on the other hand, have relatively matching absorbance intensities between their Soret- and Q-bands but with lower extinction coefficients relative to Tbcs.23 The Tbc macrocycle holds a 3− formal charge balanced with a positively charged metal/metalloid central atom, compared to the 2− charge of a Pc macrocycle. Tbcs have been shown to have the capacity to host different central atoms such as phosphorus, aluminum, silicon, gallium, and germanium.25,26 Varying the central atom of Tbcs allows for additional control over the axial position and, therefore in principle, their properties. However, the synthesis and understanding of structure−property relationships of Tbcs remain very limited within the literature. Recently, a comprehensive review on Tbcs was published highlighting the scarcity of synthetic reports and the inconsistency in the characterization of these compounds.21 This is in contrast to Pcs where almost all atoms in the Periodic Table have been incorporated into the Pc macrocycle, and a sizable amount of literature has been established on the structure−property relationships. For example, tuning the Received: January 25, 2018

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DOI: 10.1021/acs.inorgchem.8b00181 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of SiTbc from (OH)2-SiPc Enabled by Magnesium and Chlorosilane

Table 1. Investigating the Impact of Different Reagents and Reaction Conditions on the Formation of SiTbcs from SiPcs variable investigated R1a R1b R1c R1d R2 R3 R4 R5a R5b R5c R5d R6 R7

size of the silane

in air absence of the silane absence of magnesium solvent

solubility acidity

SiPc

Silane

Mg

Solvent

atmosphere

conversion to SiTbc

(OH)2-SiPc (OH)2-SiPc (OH)2-SiPc (OH)2-SiPc (OH)2-SiPc (OH)2-SiPc (OH)2-SiPc (OH)2-SiPc (OH)2-SiPc (OH)2-SiPc (OH)2-SiPc (nHx3SiO)2-SiPc (nHx3SiO)2-SiPc

Hx3SiCl Bu3SiCl Pr3SiCl Ph3SiCl Hx3SiCl None Hx3SiCl Hx3SiCl Hx3SiCl Hx3SiCl Hx3SiCl none none

yes yes yes yes yes yes none yes yes yes yes yes yes

pyridine pyridine pyridine pyridine pyridine pyridine pyridine toluene dimethylaniline dimethyl sulfoxide 3-picoline pyridine pyridine + pyridine·HCl

N2 N2 N2 N2 Air N2 N2 N2 N2 N2 N2 N2 N2

yes yes yes yes no no formed a soluble SiPc no no slow slow no yes

later replicated by Zhang et al.22 The process proceeds by reacting dichloro SiPc (Cl2-SiPc) and benzyl alcohol in the presence of sodium borohydride (as a reducing agent) and anisole (as a co-solvent). The process yields a SiTbc derivative with a benzyl alkoxy moiety on the axial position. However, the applicability of the process to produce other derivatives is unclear due to the reactivity of sodium borohydride with alcohols, two main reagents in this reaction. Another process, which was reported by Myakov et al., proceeds by reacting bis(hydroxy) SiPc ((OH)2-SiPc) with an excess of trimethyl chlorosilane (Me3SiCl) in pyridine in the presence of magnesium metal as a reducing agent.33 The reported process was limited in two distinct ways: (1) the process was done at a scale only enabling UV−vis spectroscopic analysis and lacked a workup procedure, a yield, and additional physical characterization; (2) the versatility of the process was not demonstrated, and only one derivative was considered, trimethylsiloxy SiTbc (Me3SiO-SiTbc). Therefore, the report had no indication that it could produce considerable mass yields to enable the use of the material in any meaningful application nor an indication of the versatility of the process. In this paper, the synthesis of four novel SiTbcs with variable sized axial siloxy groups are reported by verifying the base chemistry outlined by Myakov et al., developing a workup procedure, and proving its versatility. Reaction parameters are also probed in order to provide further insight into the mechanism of this unknown reaction. Detailed characterization of the new SiTbc derivatives was also performed.

solubility of silicon Pcs (SiPcs) has been demonstrated by our group and others via axial functionalization of SiPcs with trialkyl siloxy groups.27−31 Such derivatives have been applied as ternary additives27−30 and recently as electron acceptors/charge transporters31 in organic photovoltaics (OPV). Our group has shown that control over the morphology of the deposited layers in OPVs can be achieved by changing the size of the alkyl groups on the axial siloxy moiety without affecting the basic photophysical properties of the SiPc chromophore.31 We have also recently demonstrated for the first time the functionality of Tbcs in OPV devices using oxy phosphorus tetrabenzotriazacorrole (POTbc) as a representative compound.32 POTbc was applied both as an electron-donating and as electron-accepting material, which indicates that POTbc can transport both positive and negative electrical charges. However, POTbc performed better as an electron donor which is attributable to the 3− formal charge of the Tbc macrocycle. We also verified the controversial chemical structure of POTbc using time-of-flight electron ionization mass spectrometry (TOF-EI MS), elemental analysis, and X-ray photoelectron spectroscopy (XPS). Now that the functionality of Tbcs is proven, the next sensible step is to produce Tbcs with flexible chemical and synthetic handles that can enable control over their properties to improve their functionality in the different applications. The goal would be to use a high yielding and versatile chemical process. Moving away from phosphorus, silicon typically has a 4+ oxidation state and, therefore, has an axial substituent remaining when complexing with the trianionic Tbc ligand. Tunability of the properties of Tbcs can then be achieved by controlling the axial substituent using the vast range of known silicon chemistries. The only known method to produce silicon Tbcs (SiTbcs) in a quantitative yield is the process reported by Fujiki et al.25 and

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RESULTS AND DISCUSSION Chemical Process Probing and Workup Development. In order to better understand the process introduced by Myakov et al.,33 a systematic approach was conducted whereby one reagent or reaction condition was changed at a time (see B


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Inorganic Chemistry Table 1 which summarizes this section). This approach was taken since there is no understanding of the Tbc formation mechanism. To start, we evaluated the versatility of the process by targeting other SiTbc derivatives by replacing Me3SiCl with tri-n-hexyl chlorosilane (Hx3SiCl), tri-n-butyl chlorosilane (Bu3SiCl), tri-i-propyl chlorosilane (Pr3SiCl), and triphenyl chlorosilane (Ph3SiCl) to produce tri-n-hexylsiloxy SiTbc (nHx3SiO-SiTbc), tri-n-butylsiloxy SiTbc (nBu3SiO-SiTbc), tri-i-propylsiloxy SiTbc (iPr3SiO-SiTbc), and triphenylsiloxy SiTbc (Ph3SiO-SiTbc) derivatives respectively (Scheme 1). The conversion of Pc to Tbc is easily identifiable by a change in color of the reaction mixture from deep blue to deep green. This change can be quantified by monitoring the evolution of the Soret-band of Tbc around 440 nm. The formation of axial bonds to the freely rotating siloxy groups is also accompanied by an increase in solubility. The reaction was conducted under inert atmosphere and was found to be inhibited when run in air. We also considered alternative solvents to pyridine. No conversion was detected when toluene and dimethylaniline were used as solvents, and the solution remained insoluble, which is indicative of either the insolubility or reactivity of silanes within the process in/with these solvents. When dimethyl sulfoxide and 3-picoline were used, conversion occurred but at rates much slower compared to when pyridine was used. The temperature of SiTbc formation was probed by increasing the reaction temperature by 10 °C every hour starting at 30 °C (Figure 1). Conversion from (OH)2-SiPc to

Next, we probed the role of magnesium and the R3SiCl reagents in facilitating the formation of SiTbc. A reaction mixture in the absence of R3SiCl remained blue and the (OH)2SiPc remained very insoluble. When the reaction was carried out without magnesium, the reaction mixture quickly became soluble but remained blue and UV−vis measurements indicated the lack of SiTbc formation. The noticeable change in solubility is an obvious indication of the formation of bis or mono (tri-nhexyl siloxy) SiPc which then could be intermediates in the formation of SiTbcs. Next, the reaction was performed starting from bis(tri-n-hexyl siloxy) SiPc ((nHx3SiO)2-SiPc) directly with magnesium in pyridine. However, no SiTbc formation was observed. This result led us to hypothesize that a byproduct of the reaction of (OH)2-SiPc and R3SiCl, likely hydrogen chloride (HCl), could be playing an important role in the formation of SiTbcs. This hypothesis was tested by reacting (nHx3SiO)2-SiPc with magnesium in the presence of pyridine hydrochloride (pyridine·HCl) at five molar equivalencies (same equivalency was used with silanes). The reaction changed color quickly, and the formation of SiTbc was confirmed by UV−vis measurements. This result means that the ring contraction of SiPc to SiTbc, and possibly Pcs to Tbcs in general, is acidpromoted. It is also well-known that the imine nitrogen, the nitrogen that is lost on Tbc formation, is Lewis basic. Honda et al.36,37 has shown this for peripherally phenylated Pc derivatives. Our group38 and Decréau et al.39 have shown the same for boron subphthalocyanines. Therefore, the protonation and nitrogen atom loss are likely to be intimately associated. Honda et al.36,37 monitored changes in the UV−vis absorption spectrum to confirm protonation. However, (nHx3SiO)2-SiPc did not show appreciable changes in the absorbance when run in the presence of excess pyridine·HCl and trifluoroacetic acid (analogous conditions to those of Honda et al.,36,37 Figure S1). Given the difference between reaction and spectroscopy conditions (temperature, concentration), this does not negate the conclusion that protic acid promotes the formation of Tbc. While the in situ process of axial functionalization and Tbc formation is more convenient for the siloxy SiTbc derivatives, this result can potentially enhance the versatility of this reaction to SiTbcs functionalized with groups other than siloxy moieties; other SiTbc derivatives can be accessed using this magnesiumenabled chemistry either by reacting Cl2−SiPc with hydrogencontaining nucleophiles such as alcohols or phenols (which will also generate HCl) or ultimately by reacting any SiPc with pyridine·HCl. Potentially, this chemistry can be extended to other Tbcs with central atoms other than silicon. Finally, a workup and separation procedure was developed for the in situ process (R1a-d) to separate the products from residual magnesium turnings and any unidentified byproducts. Attempts to purify the products using conventional and Kauffman column40 chromatography failed due to the modest solubility of the SiTbc derivatives. Considering the scale of the reaction (0.5 g of (OH)2-SiPc) and lack of high solubility, large volumes of solvent would be required for standard column chromatography. Therefore, a precipitation workup method was developed instead. The reaction had to be intensified by increasing the (OH)2-SiPc concentration in the reaction mixture from approximately 1 wt % (as reported by Myakov et al.) to 10 wt % in order to reduce the volume of the nonsolvent required for precipitation, isopropyl alcohol in this case. We also had to reduce the amount of magnesium and chlorosilane reagents to five molar equivalence each with respect to (OH)2-SiPc. As a result of concentrating the reaction

Figure 1. Monitoring the temperature of SiTbc formation.

SiTbc was first observed at 40 °C but did not increase significantly until the temperature reached 70 °C where conversion spiked almost to completion. The emergence of a significant Tbc absorbance peak at 40 °C is an indication that the activation temperature of the Pc-to-Tbc reaction step must be between 30 and 40 °C. The noise in the spectrum of the sample collected at 30 °C is due to the lack of solubility of the (OH)2-SiPc precursor preceding axial functionalization with the solubilizing siloxy group. Consequently, decoupling the kinetics of the axial functionalization and the ring-contraction reactions was unfeasible. The stagnation of conversion below 70 °C could be another sign that solubility is a limiting factor similar to the slower kinetics when the concentration was increased. For reference, the temperatures to form Pcs are often well over 180 °C.34,35 C


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Inorganic Chemistry

Figure 2. Crystal structures of SiTbcs. (A−D) ORTEP plots showing thermal ellipsoids at 50% probability. The dashed lines indicate a disorder in the structure. (E−H) Unit cells showing intermolecular interactions in dimeric forms. Hydrogens are omitted for clarity. CCDC deposition numbers: nHx3SiO-SiTbc (1817930); nBu3SiO-SiTbc (1817929); iPr3SiO-SiTbc (1817931); Ph3SiO-SiTbc (1817928).

Crystals were grown for all siloxy SiTbc derivatives and structures were resolved and confirmed using single-crystal Xray diffraction techniques (Figure 2). Crystal structures were also used to analyze the solid-state arrangement of the SiTbcs, which is critical to consider for many of the targeted applications for these molecules. In literature, there are only two reported crystal structures of Tbcs. The first structure is of a triethylsiloxy SiTbc (Et3SiO-SiTbc) crystal that was grown serendipitously and was resolved to a structure with disordered isoindole units equally over two sites.42 The other crystal structure, generated by the same group, was of a siloxy phosphorus Tbc.43 Phosphorus, unlike silicon, has a 5+ oxidation state and, therefore, forms bonds to two axial substituents of the Tbc macrocycles, triphenyl siloxy groups in this case. The compound is also decorated with eight phenoxy groups around the periphery. The bulky phenyl groups in both the axial and peripheral directions were obviously hindering any π−π interactions between the Tbc macrocycles. The size of the siloxy ligands impacted the structures of our SiTbc compounds but reflected minimal changes in the basic photophysical and electrochemical properties of the molecules (see later discussion). The Tbc macrocycles are planar with the central silicon atoms positioned slightly off-plane due to the asymmetry in the axial direction. The size of the Tbc cavity was measured in order to predict the limitation on which elements can be inserted and complexed with Tbcs. The size of the Tbc cavity was measured as the shortest distance between opposing isoindoline nitrogen atoms. The distance is approximately 3.5 Å for SiTbcs, which is slightly smaller than their SiPc counterparts (∼3.8 Å).31 This value is smaller than the covalent radii, a measure of the size of an atom when forming a covalent bond, of all elements across the Periodic Table except for some of alkali-metal elements. The long-range solid-state arrangement was greatly affected by the type of axial siloxy fragment which consequently influenced the solid material properties such as the melting and

mixture and reducing the equivalencies, time to full conversion increased from under 1 h (in the case of 1 wt %) to about 24 h (in the case of 10 wt %). This could be due to solubility limitations which was supported by examining the effects of temperature on kinetics as will be discussed later. Because most of the magnesium remained in its granular form, it was easily removed by pipetting the reaction solution out of the reaction vessel into the precipitation vessel. The participants were then filtered under vacuum and washed with more isopropyl alcohol. The cake was left to dry under air for a few hours before it was moved and kept in a vacuum-oven overnight. This separation sequence removed the bulk of the magnesium metal and residual silanes yielding between 70−82% of solid blue powders. Any remaining impurities were removed using train sublimation, a standard purification process in our laboratory.41 Structural Confirmation and Analysis. Structural confirmation of the SiTbcs was carried out using 1H NMR, TOFEI MS, and single-crystal X-ray diffraction. 1H NMR analysis was not entirely surprising (Supporting Information); due to the reduction in symmetry relative to SiPcs, the two multiplet peaks in the aromatic region arising from Hα and Hβ (Scheme 1) common to SiPcs were expected to split into eight different peaks. However, only five resonances were observed and were assigned to H1, H2, H3, H4, and H5 according to their relative concentrations. Protons at the bay positions (H1, H2, and H3) occurred at higher chemical shifts, while the protons at the terminal positions (H4 and H5) occurred at lower shifts. The change of the axial substituent had a small effect on the chemical shifts mostly noticeable with H1 and H2. Broadening of the peaks was observed at higher concentrations likely due to the tendency of these molecules to aggregate. In TOF-EI MS, m/z peaks corresponding to the entire molecules were observed, and all derivatives exhibited communal fragmentation patterns (Supporting Information); typical fragmentation observed was the loss of one R group, two R groups, three R groups, and the entire siloxy group. D


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Inorganic Chemistry

these extinction coefficients are slightly lower than that of SiPcs but are within the same order of magnitude. When the extinction coefficients of SiTbcs were considered on mass bases (εmass), SiTbcs become more superior because of the lower dilution of the SiTbcs chromophore (Table 2), a result of having only one of the optically passive alkyl chains on the axial position compared to two in SiPcs. The use of εmass rather than εmole is of practical importance for targeted applications including dyes and pigments or organic electronics. The optical energy gaps (Eg,Opt) of SiTbcs were calculated from the onset of the absorbance spectrum, and again all are consistent across the SiTbc derivatives and are identical to that of SiPcs (Eg,Opt = 1.82 eV). Photoluminescence was also measured. Excitation at the Soret-band (440 nm) resulted in photoluminescence at 667− 674 nm. This shift is approximately 1 eV drop in the energy of photons, and it is unclear if this relaxation is luminescent or rather vibrational. The drop in energy corresponds to a wavelength of 1300 nm, which is beyond our detector limit and is not visible to the eye. Electrochemical characterization of the new SiTbcs was conducted using cyclic voltammetry (CV) and differential-pulse voltammetry (DPV) (Figure 4). There were no significant differences in the redox potentials between the SiTbc derivatives (Table 2). As a result, the electrochemical energy gaps (Eg,ElCh) also remained unchanged and were measured to be slightly lower than the Eg,Opt. Within the scanning window, SiTbcs experienced three oxidation events and a single reduction event. While the reduction peaks are reversible in all compounds, iPr3SiO-SiTbc exhibited the most reversible behavior on the oxidation side. The lower reversibility of nHx3SiO-SiTbc and nBu3SiO-SiTbc is perhaps due to chargetransfer limitations caused by the bulky and electrically inert alkyl chains. When compared to SiPcs, SiTbcs exhibit lower oxidation potentials (EOX) and higher reduction potentials (ERED) than SiPcs. These results are expected considering that the Tbc chromophores hold a 3− formal charge compared to 2− of a SiPcs. These results are in-line with our previous work with POTbc showing favorable electron-donating properties of Tbcs when applied in OPV devices, which is indicative of a higher tendency to oxidation than reduction.32 Also, the three oxidation events experienced by all of the SiTbc derivative render these compounds strong candidates for energy storage applications. Thermal stability and phase transition behavior were assessed using thermal gravimetric analysis (TGA) and differentialscanning calorimetry (DSC) (Figure 5). SiTbcs are extremely stable and exhibit degradation temperatures slightly higher than SiPcs (Td > 390 °C). Thermal stability allows these materials for both vacuum processing as well as solution processing enabled by their moderate solubility; both methods are common for fabrication of organic electronic devices. Phase transition behavior is significantly affected by the axial substituents similar to the observation with SiPcs.31 The DSC experiment was run on a cyclic basis between 35 and 350 °C at a rate of 5 °C/min. Melting temperatures (Tm) and crystallization temperatures (Tc) exhibited resilience to thermal fatigue which sometimes occurs upon repetitive heating and cooling. The second cooling cycle was conducted at a much faster rate (100 °C/min) to imitate the uncontrolled cooling rates (flash cooling) typically experienced by materials used in relevant applications (e.g., OPVs). The flash crystallization temperatures (Tcf) are very close to Tc, indicating a negligible dependence of Tc on cooling rates. The difference in Tm and Tc

crystallization temperatures, as will be discussed in the next section. All of the four SiTbc compounds exhibited significant π−π interactions in dimeric forms (Figure 2). This is in contrast to their SiPc31 and phosphorus Tbc43 analogues which lack any π−π interactions because of the two axial groups, compared to one in the case of SiTbcs. nHx3SiO-SiTbc and nBu3SiO-SiTbc exhibited the same packing pattern where the linear alkyl groups on the axial position packed closely in addition to the stacking caused by π−π interactions. Conversely, the isopropyl and phenyl groups did not contribute greatly to the solid-state arrangement of iPr3SiO-SiTbc and Ph3SiO-SiTbc. The strength of π−π interactions between two molecules can be best characterized by the combination of the shortest distance between interacting aromatic groups (dAr−Ar) and the distance between the central silicon atoms (dSi−Si). nBu3SiO-SiTbc exhibits the strongest π−π interaction (dAr−Ar = 3.505 Å and dSi−Si = 4.839 Å), while Ph3SiO-SiTbc exhibits the weakest interactions (dAr−Ar = 3.673 Å and dSi−Si = 6.247 Å). It is noteworthy to mention that the crystals of nBu3SiOSiTbc and iPr3SiO-SiTbc are both disordered at ∼14−19% occupancy of the minor components. In nBu3SiO-SiTbc, the entire SiTbc macrocycle and the axial oxygen atom are rotated approximately 180°, while the axial tributyl silyl fragment remained unchanged. iPr3SiO-SiTbc is disordered in two modes. Two of the axial isopropyl units are disordered over two sites. This alkyl disorder pattern was previously observed with similar molecules such as bis(tri-n-hexylsiloxy) germanium Pc.30 In the same crystal, one of the isoindole fragments within the SiTbc macrocycle is disordered in a fashion similar to the disorder reported in the literature with Et3SiO-SiTbc.42 Optical, Electrochemical, and Thermal Characterization. Optical, electrochemical, and thermal properties were measured and compared against (nHx3SiO)2-SiPc.31 (nHx3SiO)2-SiPc was selected as a reference to allow for a direct comparison between SiPcs and SiTbcs with minimal influence from the axial moiety. Solution-based properties (optical and electrochemical) of Ph3SiO-SiTbc were not measured because of the extreme low solubility of this derivative caused by the lack of a flexible alkyl chain. Molar extinction coefficients (εmole) measured for the series of SiTbcs are almost constant (∼250 000 L mol−1 cm−1 and 150 000 L mol−1 cm−1 of the Soret and Q bands respectively) (Figure 3);

Figure 3. Molar extinction coefficient (solid) and normalized photoluminescence (dashed) of SiTbcs in chloroform. Excitation wavelength of the photoluminescence spectra is 440 nm. E


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Inorganic Chemistry Table 2. Optical and Electrochemical Characterization of SiTbcsa optical

electrochemical

Soret-band

nHx3SiOSiTbc nBu3SiOSiTbc iPr3SiOSiTbc SiPc a

Eg,Opt

Q-band

EOX

λmax, nm

εmole, 10 L mol−1 cm−1

εmass, L g−1 cm−1

λmax, nm

εmole, 10 L mol−1 cm−1

εmass, L g−1 cm−1

eV

εSoret/εQ

440

2.50

302

663

1.5

184

1.82

1.65

438

2.73

365

660

1.6

221

1.82

1.67

440

2.25

321

663

1.3

192

1.82

1.74

353

0.74

66

669

3.4

302

1.82

0.22

5

5

V 0.52, 0.76, 1.20 0.54, 0.74, 1.19 0.54, 0.75, 1.20 1.04

ERED

Eg,ElCh

V

eV

−1.21

1.73

−1.24

1.78

−1.21

1.74

−0.85

1.89

Optical measurements are run in chloroform. Half-wave redox potentials are measured using cyclic voltammetry (CV) in 1,2-dichloroethane.

Figure 4. Cyclic voltammetry (A) and differential-pulse voltammetry (B) of SiTbcs in 1,2-dichloroethane with decamethylferrocene as an internal reference (E1/2 = 0.007 V).

Figure 5. Thermal gravimetric analysis (A) and differential scanning calorimetry (B) of SiTbcs.

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between nHx3SiO-SiTbc and (nHx3SiO)2-SiPc is quite small (7 and 20 °C respectively), indicating a negligible impact of the macrocycle on phase transition. Phase transition temperatures increased going from hexyl to butyl groups perhaps due to the decrease in weight percentage of the axial ligand relative to the macrocycle (57% in nHx3SiO-SiTbc and 41% in nBu3O-SiTbc). iPr3SiO-Tbc has a low Tm of 60 °C, which renders this compound a strong candidate for applications requiring solution processing. Ph3SiO-SiTbc did not show any signs of phase transitions which is potentially attributable to the lack of flexible alkyl chains.

CONCLUSIONS

In summary, four SiTbc derivatives were synthesized at high yields with varying sized siloxy groups in the axial position using in situ axial functionalization and magnesium-enabled reductive chemistry starting from (OH)2-SiPc. The high yields permit the assessment of the properties of these materials as well as their incorporation in various applications. The reaction conditions enabling this conversion were probed by systematically investigating the roles of magnesium, chlorosilane, hydrogen chloride, solvent, and reaction temperature. This systematic study revealed that the SiTbc formation reaction is F


Article

Inorganic Chemistry

multilayer optics and were measured using a combination of ϕ scans and ω scans. The data were processed using APEX2 and SAINT.44 Absorption corrections were carried out using SADABS.44 The structures were solved and refined using SHELXT & SHELXL (Sheldrick, 2015a,b) for full-matrix least-squares refinement that was based on F2.45,46 Description of the disorder refinement of nBu3SiOSiTbc: all atoms apart from those of the Si(n-Bu)3 group are disordered over two sets of sites with refined occupancies 0.814(4) and 0.186(4). The disorder corresponds approximately to a rotation of 180 deg of the ring system and bonded Si−O group. Description of the disorder refinement of iPr3SiO-SiTbc. One of the isoindole fragments of the ring system is disordered over two sets of sites with refined occupancies 0.856(3) and 0.144(3). In addition, two of the isopropyl groups are disordered over two sets of sites with refined occupancies 0.844(11) and 0.156(11). Materials. All reactions were performed under an atmosphere of nitrogen gas, unless specified otherwise, using oven-dried glassware. All ACS grade solvents were purchased from Caledon Laboratories (Caledon, Ontario, Canada) and used without further purification. 1,3Diaminoisoindoline (DI3) was purchased from Xerox Corporation (Mississauga, ON) and used as received. Anhydrous pyridine, pyridine hydrochloride, magnesium turnings, silicon tetrachloride (SiCl4), and cesium hydroxide (50 wt % in water) were purchased from SigmaAldrich and used as received. All chlorosilanes were all purchased from Gelest, Inc. (Morrisville, PA, USA) and used as received. (OH)2-SiPc and (Hx3SiO)2-SiPc were synthesized according to literature procedures.30 Chemical Synthesis. General Synthesis of SiTbcs. The synthesis is a modified process from Myakov et al. with the addition of a workup procedure.33 (OH)2-SiPc (0.5 g, 0.87 mmol) was mixed with magnesium turnings (0.106 g, 5 molar equivalence) in 5 mL of anhydrous pyridine under nitrogen atmosphere. The reaction mixture was then heated to 100 °C. The respective chlorosilane (5 molar equivalence) was then added to the mixture using a syringe with the exception of triphenyl chloro silane which was dissolved in 3 mL of pyridine prior to injecting into a 2 mL of pyridine reaction mixture. Complete conversion was confirmed using UV−vis spectroscopy after 24 h. The mixture was cooled down to room temperature before precipitation in 50 mL of isopropyl alcohol and stirring for a few hours. The solids were vacuum-filtered and washed with isopropyl alcohol. The resulting deep-blue powders were left to air-dry for a few hours before leaving in the vacuum oven overnight. The products were further purified using train sublimation using nitrogen as a carrier gas in order to remove any residual silanes. Tri-n-hexylsiloxy Silicon Tetrabenzotriazacorrole (nHx3SiO-SiTbc). It was synthesized using the aforementioned general synthetic route of SiTbcs. The process yielded 0.527 g (73%) of product before sublimation. The sublimation yield was 47%. 1H NMR (500 MHz, C6D6): δ 9.88 to 9.84 (4H, dd), 9.70 to 9.69 (2H, d), 9.12 to 9.11 (2H, d), 8.01 to 7.96 (4H, m), 7.89 to 7.83 (4H, m), 0.90 to 0.83 (6H, m), 0.75 to 0.72(9H, m), 0.42 to 0.37 (6H, m), 0.11 to 0.06 (6H, m), −0.92 to −0.98 (6H, m), −1.89 to −1.93 (6H, m). TOF-EI MS calc’d for C50H55N7Si2O 825.4, found 825.4. EA calc’d for C50H55N7Si2O: C, 72.7; H, 6.7; N, 11.9. Found: C, 72.5; H, 6.7; N, 11.6. UV−vis in chloroform λmax (nm, L mol−1 cm−1) = (441, 2.5 × 105) and (662, 1.5 × 105). Crystals were grown by solvent techniques and diffracted using X-ray techniques. Tri-n-butylsiloxy Silicon Tetrabenzotriazacorrole (nBu3SiO-SiTbc). It was synthesized using the aforementioned general synthetic route of SiTbcs. The process yielded 0.451 g (70%) of product before sublimation. The sublimation yield was 46%. 1H NMR (500 MHz, C6D6): δ 9.80 (4H, s broad), 9.56 (2H, s broad), 8.90 (2H, s broad), 7.97 (4H, s broad), 7.77 (4H, s broad), 0.10 to 0.07 (9H, m), 0.03 to −0.04 (6H, m), −1.06 to −1.12 (6H, m), −2.03 to −2.06 (6H, m). TOF-EI MS calc’d for C44H43N7Si2O 741.3, found 741.3. EA calc’d for C44H43N7Si2O: C, 71.2; H, 5.8; N, 13.2. Found: C, 70.3; H, 6.7; N, 13.0. UV−vis in chloroform λmax(nm, L mol−1 cm−1) = (438, 2.7 × 105) and (660, 1.6 × 105). Crystals were grown by solvent techniques and were diffracted using X-ray techniques.

Table 3. Thermal Stability and Phase Transition Temperatures of SiTbcs Measured Using Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC), Respectively thermal stability nHx3SiO-SiTbc nBu3SiO-SiTbc iPr3SiO-SiTbc Ph3SiO-SiTbc SiPc

phase transition

Td (°C)

Tm (°C)

Tc (°C)

Tcf (°C)

411 395 403 462 342

180 247 60, 327

90 207 55, 282

196 51

173

110

103

acid-promoted. The reaction of (nHx 3 SiO)2 -SiPc with magnesium only formed a SiTbc when hydrogen chloride was intentionally added to the reaction. This can enhance the versatility of this reaction and extend its applicability to form SiTbcs with any axial substituent and perhaps to Tbcs with central atoms other than silicon. SiTbc formation was also shown to occur at temperatures as low as 40 °C. Strong π−π coupling is seen in the crystal structures, a desirable property for applications that require high charge-carrier mobility such as OPVs. Optical, electrochemical, and thermal characterization of the compounds was conducted. The dually absorbing SiTbcs exhibit extinction coefficients slightly lower than their SiPc counterparts on a molar basis but higher ones on a mass basis. For any desired application, capturing the same amount of light by utilizing less material is quite desirable. These compounds also exhibit attractive electrochemical behavior by undergoing three oxidation events at moderate potentials rendering the materials strong candidates for energy storage applications. Finally, thermal characterization of the molecules indicated high thermal stability as well as tunable melting and crystallization temperatures. Potential future investigation pathways include examining axial chemistries on SiTbcs beyond siloxy groups, Tbc compounds with different central elements beyond silicon, and the incorporation of SiTbcs into optical and electronic applications.

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EXPERIMENTAL SECTION

General Methods. Low-resolution mass spectra (LR MS) and high-resolution mass spectra (HR MS) were determined using Waters GC Premier using time-of-flight spectrometer with electron ionization source (TOF-EI MS). Proton nuclear magnetic resonance (NMR) spectra were recorded on a Agilent DD2 500 spectrometer at 23 °C in C6D6, operating at 500 MHz. Chemical shifts (δ) are reported in parts per million (ppm) referenced to benzene shift (7.16 ppm). UV−vis absorbance measurements were acquired on a PerkinElmer Lambda 25 UV/vis spectrometer using a PerkinElmer quartz cuvette with a 10 mm path length. Photoluminescence spectra were collected using a PerkinElmer LS 55. Thermogravimetric analysis was run using TA Instrument Q50 under an atmosphere of nitrogen and a heating rate of 5 °C/min. Differential scanning calorimetry (DSC) was conducted using TA Instrument Q1000. The DSC was performed under nitrogen using the following profile: (1) heating to 350 °C at a rate of 5 °C min−1, (2) cooling to 0 °C at 5 °C min−1, (3) heating again to 350 °C min−1, (4) flash cooling to 0 at 100 °C min−1, (5) third heating to 350 °C min−1, and finally, (6) cooling to 0 °C at 5 °C min−1. Electrochemical analysis was performed on a BASI C3 cell stand using an Ag/AgCl as the reference electrode, platinum counterelectrode, and a glassy carbon working electrode at a of 100 mV/s scanning rate. Samples were dissolved in a 0.1 M tetrabutyl ammonium perchlorate solution in 1,2-dichloroethane. Single-crystal X-ray diffraction data were collected on a Bruker Kappa APEX-DUO diffractometer using a Copper ImuS (microsource) tube with G


Article

Inorganic Chemistry Tri-i-propylsiloxy Silicon Tetrabenzotriazacorrole (iPr3SiO-SiTbc). It was synthesized using the aforementioned general synthetic route of SiTbcs. The process yielded 0.499 g (82%) of product before sublimation. The sublimation yield was 66%. 1H NMR (500 MHz, C6D6): δ 9.80 to 9.76 (4H, dd), 9.56 to 9.54 (2H, d), 8.88 to 8.87 (2H, d), 7.99 to 7.93 (4H, m), 7.79 to7.72 (4H, m), −1.21 to −1.23 (18H, m), −1.68 to −1.78 (3H, m). TOF-EI MS calc’d for C41H37N7Si2O 699.2598, found 699.2592. EA calc’d for C41H37N7Si2O: C, 70.35; H, 5.3 N, 14.0. Found: C, 68.6; H, 5.2; N, 13.8. UV−vis in chloroform λmax(nm, L mol−1 cm−1) = (441, 2.2 × 105) and (662, 1.3 × 105). Crystals were grown by solvent techniques and were diffracted using X-ray techniques. Triphenylsiloxy Silicon Tetrabenzotriazacorrole (Ph3SiO-SiTbc). It was synthesized using the aforementioned general synthetic route of SiTbcs. The process yielded 0.935 g of green product which is indicative of a significant silane residual. The silanes were removed using sublimation at 38% yield. TOF-EI MS calc’d for C50H31N7Si2O 801.2, found 801.2. EA calc’d for C50H31N7Si2O: C, 74.9; H, 3.9 N, 12.2. Found: C, 74.45; H, 3.8; N, 12.0. UV−vis in dichloromethane λmax = 441 and 665 nm. Crystals were grown by sublimation and diffracted using X-ray techniques.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00181. UV−vis absorbance measurements under acid conditions, 1H NMR spectra, TOF-EI MS spectra, and singlecrystal crystallographic data and figures (PDF) Accession Codes

CCDC 1817928−1817931 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Timothy P. Bender: 0000-0002-6086-7445 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge financial support from Saudi Basic Industries (SABIC). We would like to acknowledge the Natural Sciences and Engineering Research Council (NSERC) of Canada for their support through the Discovery Grant program. The authors also thank Datong Song for insightful comments with respect to the crystal structure refinement of nBu3SiO-SiTbc.

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REFERENCES

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