Position of Methyl and Nitrogen on Axial Aryloxy Substituents Determines the Crystal Structure of Silicon Phthalocyanines

Abstract

Phthalocyanines are a class of organometallic compounds that have been investigated extensively for emerging applications such as organic photovoltaics, organic light-emitting diodes, and thin-film transistors. For these applications, the understanding of molecular structures and intermolecular interactions is extremely crucial and is one of the most promising pathways for designing a new generation of functional materials. Here, the crystal structures of six axially substituted silicon phthalocyanines (SiPcs) with aryloxy groups are reported. Three bis(methylphenoxy) SiPcs with varying methyl positions and two bis(pyridoxy) derivatives with varying nitrogen positions are compared to the bare bis(phenoxy) SiPc. Quantitative analysis is conducted to describe the impact of the position and nature of the functional group/atom on the orientation of the axial aryloxy subtituents as well as on the aromaticity and planarity of the SiPc macrocycle. Quantitative analysis of changes in intermolecular interactions, in particular, π−π interactions, and visual representation using Hirshfeld surfaces are also presented. In all cases, the axial aryloxy substituent determined the solid-state arrangement except for the bis(4-pyridoxy) SiPc, which is isostructural to bis(phenoxy) SiPc.

Cyanophenoxy-Substituted Silicon Phthalocyanines for Low Threshold Voltage n-Type Organic Thin-Film Transistors

“…16,48 Intermolecular interactions can be further enhanced to improve charge transport mobility, however these interactions are not necessarily related to electron donating or withdrawing character of functional groups. 10,13,15 While μ e of materials 3 and 4 are relatively low compared to state-of-the-art MPc and R 2 -SiPc materials, these observations confirm that designing R 2 -SiPc materials with stronger electron-withdrawing groups result in OTFTs with lower V T . Confirming that this trend continues to hold with stronger electron-withdrawing pendants will enable the design of next-generation R 2 -SiPcs with a combination of high μ e and low V T by further tuning pendant groups to optimize intermolecular interactions and electronic coupling 16,49 while incorporating electron-withdrawing moieties to reduce V T .…”

“…In this previous work, we observed a range of isotropic and anisotropic thin-film morphologies and found that anisotropic grain structures across the substrate enabled by favorable film growth was a greater driver of μ e than axial group selection. Additionally, DFT calculations have demonstrated that intermolecular electronic coupling has a strong impact on theoretical field-effect mobilities, making it a good predictor for high-mobility R 2 -SiPc materials as polycrystalline thin-films in devices. , Intermolecular interactions can be further enhanced to improve charge transport mobility, however these interactions are not necessarily related to electron donating or withdrawing character of functional groups. ,, While μ e of materials 3 and 4 are relatively low compared to state-of-the-art MPc and R 2 -SiPc materials, these observations confirm that designing R 2 -SiPc materials with stronger electron-withdrawing groups result in OTFTs with lower V T . Confirming that this trend continues to hold with stronger electron-withdrawing pendants will enable the design of next-generation R 2 -SiPcs with a combination of high μ e and low V T by further tuning pendant groups to optimize intermolecular interactions and electronic coupling , while incorporating electron-withdrawing moieties to reduce V T .…”

“…EA: expected wt %: C (67.97%), H (2.73%) and N (17.23%) − analysis wt % C (68.23%), H (2.66%), and N (17.43%). Given the low solubility of this compound, 13 C NMR spectroscopic analysis was not possible. Nonetheless, the identity of this compound was confirmed by 1 H NMR spectroscopy and elemental analysis.…”

“…SiPcs have recently shown great potential for applications as n-type or ambipolar (both n- and p-type) organic semiconductors, with various derivatives incorporated into OLEDs, OPVs, and OTFTs . Previous studies have demonstrated that the choice of axial substituent can significantly impact the solid-state arrangement of the SiPc macrocycle, including through variations in the π–π stacking distances, herringbone angle, and degree of molecular overlap, yielding a dramatically different device performance. − Vapor-deposited SiPcs with a phenoxy (PhO) axial substituent are of particular interest due to their relatively good performance in OTFTs and as donor and acceptor materials in solar cells . The highest reported electron mobility ( μ e ) of ∼0.5 cm 2 V –1 s –1 was achieved for pentafluoro-phenoxy silicon phthalocyanine (F 10 -SiPc), suggesting that properly designed SiPcs are capable of exceeding other MPcs in n-type OTFTs and can possibly rival current state-of-the-art n-type organic semiconductors.…”

Silicon Phthalocyanines for n-Type Organic Thin-Film Transistors: Development of Structure–Property Relationships

“…The highest reported electron mobility (μ e ) for n-type SiPcs is ∼0.5 cm 2 V –1 s –1 from F 10 -SiPc ( 3 , Figure ), which is comparable or exceeds that of MPcs in n-type OTFTs, implying the potential of this class of MPcs to rival other state-of-the-art n-type organic semiconductors. , Despite this excellent potential, only a few structure–property relationships have been established to determine the impact of the axial substituents on SiPc OTFT performance. Several studies indicate that the choice of the axial group affects SiPc packing in the single crystal, often changing the π–π stacking distance, herringbone angle, and degree of molecular overlap. , To this end, Gali et al demonstrated that density functional theory (DFT) calculations in combination with kinetic Monte Carlo simulations could be used to screen the potential of several compounds by estimating mobility and its directionality and identified compound 9 (3I-SiPc), which has never been previously incorporated into OTFTs, as possessing better 1D single-crystal charge transport and compound 3 (F 10 -SiPc) as the best for 2D single-crystal charge transport . Melville et al found that increasing the size of the molecular pendant decreased mobility for carboxyl-functionalized SiPcs 10 (PhCOO-SiPc) and 11 (NpCOO-SiPc) and their anthracene-substituted analogues in OTFTs .…”