**Nathan J. Yutronkie 1, Trevor M. Grant 2, Owen A. Melville 2, Benoît H. Lessard 2,\* and Jaclyn L. Brusso 1,\***


Received: 10 April 2019; Accepted: 20 April 2019; Published: 24 April 2019

**Abstract:** Efficient synthesis of silicon phthalocyanines (SiPc) eliminating the strenuous reaction conditions and hazardous reagents required by classical methods is described. Implementation into organic thin-film transistors (OTFTs) affords average electron field-effect mobility of 3.1 <sup>×</sup> 10−<sup>3</sup> cm<sup>2</sup> V−<sup>1</sup> s−<sup>1</sup> and threshold voltage of 25.6 V for all synthetic routes. These results demonstrate that our novel chemistry can lead to high performing SiPc-based n-type OTFTs.

**Keywords:** silicon phthalocyanines; n-type organic semiconductors; organic thin-film transistors

## **1. Introduction**

In recent years, research focused on the design of various organic architectures that enable electron transport in organic semiconductors (OSCs) has led to significant advances, not only in terms of OSC design but also the requirements for integration into organic electronics. In regard to the latter, to be viable for organic thin-film transistors (OTFTs), OSCs must ultimately possess high field-effect mobilities, robust environmental stability and ease of processability. While these efforts have led to impressive progress, the development of n-type OSCs still lags behind their p-type counterparts in terms of carrier mobility and air stability [1,2]. This may be attributed to the challenges associated with designing new n-type OSCs, in which the number of electron-deficient π-building blocks available is limited, and even fewer have been demonstrated to lead to high-mobilities [1]. Nonetheless, by employing motifs such as imides, carbonyls, *N*-heterocycles, cyanoethylene, and fullerenes, a library of both molecular and polymeric n-type OSCs has been achieved.

Phthalocyanines (Pcs; Figure 1), which are conjugated molecules composed of a tetramer of nitrogen-linked isoindole units, are ideal candidates for organic electronics. This is due in large part to their unique photophysical signatures and electrophysical properties, which are readily tunable through structural modification. Furthermore, they are known for their chemical robustness, high thermal resistance and low photodegradation [3,4]. Pcs often chelate a metal or metalloid through four M–N bonds, resulting in highly stable materials that have been explored for a variety of applications including their employment as active materials in organic electronics, predominantly as p-type materials [3,5–10]. With respect to their potential as n-type OSCs, only a handful of metal phthalocyanines (MPcs) have been shown to possess air stable n-type mobilities when implemented into OTFTs, with values on the order of 10−<sup>1</sup> cm2V−1s−<sup>1</sup> [9,11–13].

**Figure 1.** Divalent metal phthalocyanine (MPc) and tetravalent silicon phthalocyanine (SiPc) frameworks.

Replacing the central metal ion in MPcs with silicon affords tetravalent silicon phthalocyanines (SiPc), a subclass of phthalocyanines that have recently experienced an increase in interest with respect to integration into organic light emitting diodes [14–16], organic solar cells [17–21] and OTFTs [22], positioning SiPcs as excellent candidates as n-type OSCs. In addition to their inherent n-type mobility, a key advantage to employing SiPcs over divalent MPcs is the ability to modify the axial substituents (e.g., R3), thereby providing a facile avenue for functionalization. Considering that the two key factors that determine the performance of OSCs in organic electronics are the frontier molecular orbitals (in terms of both energy levels and electronic distribution), and molecular packing in the solid state, axial substitution in SiPcs enables modification, and ideally control, of the film morphology. To that end, preliminary studies on SiPcs axially functionalized with aromatic substituents revealed electron mobilities up to 10−<sup>2</sup> cm2V−1s−1, demonstrating the influence of film morphology on device performance [22]. While axial substitution impacts molecular packing in the solid state, it has little effect on the energy levels of the frontier molecular orbitals. To that end, modifications to the R1 and R2 substituents about the Pc framework will not only influence the packing but also the energetics. This would therefore require appropriately substituted starting materials.

While MPcs are often prepared through the reaction of phthalonitrile with various metal precursors (e.g., MX2, where X = halide), this route is not effective for silicon-based derivatives as very low yields (i.e., less than 1%) [23] of SiPcCl2 are achieved upon the treatment of phthalonitrile with silicon tetrachloride (SiCl4) [24]. Other reported attempts to prepare SiPcCl2 involved the reaction of the disodium salt of phthalocyanine with SiCl4; however, no product was obtained through this methodology [25]. Alternatively, starting with diiminoisoindoline or o-cyanobenzamide in place of phthalonitrile affords enhanced yields of 71% and 35%, respectively [26]. Although this improved the yield of SiPcCl2, the preparation of such reagents proves problematic, especially of diiminoisoindoline (DIII). For example, the reported synthesis involves a constant flow of ammonia gas bubbling though a refluxing solution of phthalonitrile and sodium methoxide [26]. In addition to the harsh reaction conditions, the isolation of DIII of high purity is difficult to achieve, which may be attributed to many factors including its high solubility in a number of solvents, its tendency to cyclize upon dissolution and its sensitivity towards heat. This therefore limits the methods available for purification. As a result, DIII is often used in its crude form in most cases, or used directly in a one-pot synthesis of SiPcCl2 [27]. In order to tap into the potential of SiPcs, we sought to explore and develop new synthetic protocols to facilitate advancements within this exciting class of n-type OSCs. To that end, we herein present effective and alternative approaches to the classical preparative route for SiPcCl2, eliminating the undesired expenses attributed to the strenuous reaction conditions required and the hazardous reagents employed. Furthermore, integration of the resulting SiPc derivatives into high performing n-type OTFTs is presented, exhibiting the highest electron field-effect mobilities (μe) reported for silicon phthalocyanine-based OTFTs.

#### **2. Results and Discussion**

As mentioned, SiPcs can be isolated following Route A (Scheme 1); however, the conditions required are not appealing for large scale production due to the safety precautions necessary when dealing with corrosive gases and highly reducing sodium metal. To achieve a more versatile and less hazardous preparative method, we sought to eliminate the necessity of both sodium metal and ammonia gas in the preparation of DIII. To that end, utilization of lithium bis(trimethylsilyl)amide etherate (LiN(TMS)2·Et2O) offers several advantages over the reagents required for Route A. First and foremost, LiN(TMS)2·Et2O is a solid tolerant to atmospheric conditions, rendering it easier to handle and quantify than gaseous reagents such as ammonia. Not only does the use of LiN(TMS)2·Et2O eliminate the need for ammonia gas, but it also affords the necessary basicity without the use of sodium metal, thereby affording a preparative route that removes the use of toxic gases (ammonia) and highly reactive reagents (sodium metal). As outlined in Scheme 1, treatment of phthalonitrile with LiN(TMS)2·Et2O affords the anionic intermediate 1. Initial attempts to quench with either water or methanol led to various intractable by-products, likely due to decomposition of the desired product. Alternatively, quenching the anionic intermediate with gaseous hydrochloric acid afforded the dihydrogen chloride salt [H2DIII]Cl2 (Route B), which precipitates out of solution as a powdery yellow solid. Attempts to deprotonate [H2DIII]Cl2 to afford DIII were unsuccessful; however, preparation of SiPcCl2 could be achieved by reacting [H2DIII]Cl2 directly with SiCl4 in refluxing quinoline.

**Scheme 1.** Synthetic routes in the preparation of SiPcCl2. Reagents and conditions: (**a**) (i) Na, MeOH, (ii) NH3(g), Δ; (**b**) SiCl4, Quinoline, Δ; (**c**) LiN(TMS)2·Et2O, Toluene; (**d**) HCl(g); (**e**) (i) TMSCl, (ii) silica gel.

Although SiPcCl2 can be synthesized via Route B in 21% yield, employing gaseous hydrochloric acid to isolate [H2DIII]Cl2 does not eliminate the use of corrosive gases. Alternatively, quenching 1 with chlorotrimethylsilane (TMSCl) affords the silylated DIII intermediate, while the LiCl by-product precipitates out of solution (Route C). Protonation and purification can be achieved upon passing the resulting brown solution through silica gel, affording DIII as a pale orange solid. Subsequent treatment of the DIII with SiCl4 in refluxing quinoline affords SiPcCl2 as a purplish blue matte solid in 51% yield. Via Route C, we have therefore established a safer alternative to that previously published, as we have not only eliminated the necessity for highly reactive reagents and corrosive gases, but we have also ascertained a method in which quantifiable amounts of each reagent necessary can be used. Overall, Route C offers the best alternative in terms of yield and purity of the final product and, through the use of LiN(TMS)2·Et2O in place of sodium metal and ammonia gas, paves the way for the development of additional derivatives that would otherwise not be possible with the previous synthetic routes.

To test the efficacy of the various routes to prepare SiPcCl2, and to ensure the quality of the materials is maintained, samples from routes A–C were functionalized at the axial position with 3,4,5-trifluorophenoxy substituents and implemented into OTFTs. In that regard, bis(3,4,5-trifluorophenoxy) silicon phthalocyanine ((345F)2-SiPc) was synthesized from SiPcCl2 according to the literature [28], employing SiPcCl2 from Routes A, B and C. In all cases, the (345F)2-SiPc was purified by train sublimation and the products were obtained in similar yields with similar purities. UV–Vis absorption spectroscopy revealed no difference between all the (345F)2-SiPc derivatives in this study (Figure S1) and those reported in the literature [28]. (345F)2-SiPc has previously been reported to have a good thermal stability (decomposition temperature of >400 ◦C), significant pi–pi interaction by single crystal X-ray diffraction and a highest occupied molecular orbital (HOMO) level of −5.90 eV and a respective lowest unoccupied molecular orbital energy level of −4.05 eV (Figure 2c) [28]. Organic thin-film transistors (OTFTs) were therefore fabricated using (345F)2-SiPc (from Routes A, B and C) by thermal evaporation in a bottom-contact, bottom-gate configuration. Regardless of the synthetic route for the preparation of (345F)2-SiPc, an average electron field-effect mobility (μe) of 3.1 <sup>×</sup> 10−<sup>3</sup> cm2 V−<sup>1</sup> s−<sup>1</sup> and threshold voltage (VT) of 25.6 V were obtained. Although modest, this performance is superior to previously reported silicon-based phthalocyanine OTFT devices fabricated under similar conditions using bis(benzoate) silicon phthalocyanine as the semiconductor, where an average <sup>μ</sup><sup>e</sup> of 4.9 <sup>×</sup> <sup>10</sup>−<sup>4</sup> cm<sup>2</sup> <sup>V</sup>−<sup>1</sup> <sup>s</sup>−<sup>1</sup> and VT of 25.9 V were obtained [22]. For (345F)2-SiPc, performance varied over 75 devices with an average electron field-effect mobility of 3.1 <sup>×</sup> 10−<sup>3</sup> cm<sup>2</sup> V−<sup>1</sup> s−<sup>1</sup> and threshold voltage of 25.6 V, an on/off current ratio (Ion/off) ranging between 10<sup>3</sup> and 10<sup>5</sup> and a maximum <sup>μ</sup><sup>e</sup> of 1.64 <sup>×</sup> 10−<sup>2</sup> cm<sup>2</sup> V−<sup>1</sup> s−<sup>1</sup> for all synthetic routes. Characteristic output and transfer curves for (345F)2-SiPc devices are shown in Figure 2. We have previously shown that when integrated into bottom-gate, bottom-contact OTFTs, (345F)2-SiPc and other SiPc derivatives experienced significant variations in performance as a function of channel lengths due to substantial contact resistance (as apparent in Figure 2a) [29]. Current efforts involve device engineering, including electrode material selection as a way to reduce contact resistance and amplify n-type performance.

**Figure 2.** Characteristic output curve (**a**) and transfer curve (**b**) for bis(3,4,5-trifluorophenoxy) silicon phthalocyanine ((345F)2-SiPc) bottom-gate, bottom-contact (BGBC) organic thin-film devices with L = 5 um. (**c**) is the chemical structure of ((345F)2-SiPc), the respective highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of (345F)2-SiPc and a diagram of the BGBC organic thin-film transistor (OTFT) structure (from left to right) [28].

#### **3. Conclusions**

In closing, we have demonstrated an efficient and effective method for the preparation of SiPcCl2 and, upon axial substitution, have confirmed their n-type mobilities, which are the highest reported to date for SiPc-based OTFTs. Having developed the synthetic methodology to isolate SiPcCl2 using less toxic, less corrosive, non-gaseous reagents, we can now begin to explore other derivatives with modified molecular frameworks. This therefore provides an avenue to control the energy levels of the frontier MOs, enabling the development of air stable n-type OSCs, in synergy with the combined morphological control through axial substitution. Such studies are currently underway.
