The effect of dendronisation of arylamine centred chromophores on field effect transistor performance

Abstract

We report the design and synthesis of new dendronised hole transport materials and their first use in organic field effect transistors (OFETs) as solution processed amorphous channels. The dendrimers were comprised of a triphenylamine centre, core chromophores containing one or two thiophene units per arm, and first generation biphenyl dendrons with 2-ethylhexyloxy surface groups attached. The dendronised materials were found to be more easily processed than their non-dendronised equivalents. Top contact OFETS were fabricated and found to have hole mobilities in the saturated regime of 1.7 × 10⁻⁶ cm² V⁻¹ s⁻¹ and 1.1 × 10⁻⁵ cm² V⁻¹ s⁻¹ for dendrimers with one and two thiophene units in each arm of the chromophore, respectively. The devices had threshold voltages of around 10 V and ON/OFF ratios in the range 10² to 10³. The OFET results demonstrate that for amorphous films it is important that the chromophore is as large as possible to allow for maximal intermolecular interactions.

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Introduction

Conjugated dendrimers have had a significant impact on organic optoelectronic devices such as light-emitting diodes^1–5 and more recently photovoltaic cells.^6–10 A key strength of dendritic materials is that they are macromolecular compounds but because of their branching can be considered as being comprised of discrete molecular chromophores. The former property means that dendrimers can be solution processed in a similar manner to conjugated polymers while the latter allows the controlled design of individual chromophores akin to small molecule semiconductors. Conjugated dendrimers are composed of three main components: the core to which one or more dendrons (the branched units) are attached, and surface groups. In conjugated dendrimers the surface groups provide solubility and hence play an important role in the processing of the materials. The dendrimers are defined by their generation, which is the number of levels of branching in the dendrons. The dendrons themselves can be either insulating, playing a purely structural role,¹¹ or electroactive.^12,13 In many of the studies reported to date the core has been the optoelectronic chromophore but it is important to note that it also provides the basic dimensionality of the dendrimer. For example, bi(fluorene) cores lead to more planar dendrimers at low generation¹⁴ while a fac-tris(2-phenylpyridyl)iridium(III) core forms the basis of a three dimensional dendrimer.⁴ These structural components give rise to a number of important materials properties. For example, dendrimers with the same surface groups can be processed under essentially the same conditions. In addition, incorporation of poorly soluble small molecules into a dendritic architecture can enable them to be solution processed. These characteristics have been illustrated extensively for organic light-emitting diodes (OLEDs)¹⁵ and to a lesser extent photovoltaic cells but have not been explored in any detail for organic field effect transistors (OFETs).

The main efforts on field effect transistors have focused on small molecules with devices fabricated by evaporation, and solution processable conjugated polymers. However, more recently there have been a number of reports of OFETs that have solution processable small molecules with very good device performance.¹⁶ In contrast we are aware of only a few reports of dendrimers being used in an active layer of an OFET.^17–19 For example, in ref. 17 the active channel layer was formed from a second-generation PAMAM dendronised C₆₀ deposited using the Langmuir–Blodgett technique and the OFETs had mobilities of the order 2.7 × 10⁻³ cm² V⁻¹ s⁻¹. There have also been a number of reports of OFETs containing star shaped materials with three or more arms emanating from the central unit [see for example ref. 20], with mobilities generally in the range 10⁻⁵–10⁻² cm² V⁻¹ s⁻¹ for solution processed devices. In our development of light-emitting materials we have utilised time-of-flight methods to measure charge mobility and found that dendrimer films formed from simple solution processing can have good hole mobilities. For example, a first generation dendrimer with a bi(9,9-di-n-hexylfluorene) core, biphenyl dendrons and 2-ethyhexyloxy surface groups had a hole mobility of around 2 × 10⁻⁴ cm² V⁻¹ s⁻¹.²¹ Also, a highly dendronised iridium(III) complex cored dendrimer with carbazole based dendrons and fluorenyl surface groups had a hole mobility of ∼1 × 10⁻³ cm² V⁻¹ s⁻¹ at room temperature.¹³ In both cases the films were not annealed and hence these results suggest that dendrimers could be interesting materials for the active channel layers in OFETs.

In this article we describe the preparation of a family of materials (Fig. 1) that contain triarylamines at their centre and their use as the active layer in OFETs. Triarylamine based compounds have been used extensively as hole-transport materials in applications such as photocopiers, OLEDs and more recently solar cells. We discovered that such materials could also be used as the light-emitting layer in an OLED.²² These previous results of triarylamine based compounds have led to their investigation for the use as the active layers in OFETs.²³ We report the synthesis of a new family of OFET materials that are comprised of a triphenylamine centre linked to either first generation dendrons or a simple phenyl ring via one or two thiophene rings. We reveal the effect of the number of thiophene units between the triphenylamine at the centre and the distal unit (the alkoxyphenyl or dendron), and the role the dendron plays on charge transport. The triphenylamine centre was chosen as it is almost planar and in previous work we have shown that the ‘active chromophore’ is extended through the nitrogen atom thus giving a large electroactive moiety with the greatest possibility of intermolecular chromophore interactions.^24,25 A large electroactive chromophore should improve intermolecular interactions in the solid-state, and hence charge transport. The thiophene units were incorporated as a structural motif as sulfur–sulfur interactions have been shown to be important for good charge mobility in organic semi- and superconductors.

Fig. 1 Structures of the materials used in the active channel of the OFETs.

Results and discussion

Synthesis and physical properties

The synthetic pathway to the materials is shown in Scheme 1. In each case a convergent route was followed with the three extended thiophene containing units added in the last step. For the formation of the star compounds 1 and 2 (Fig. 1) the first step was the formation of the boronate ester 6 (Scheme 1) from 4-(2-ethylhexyloxy)phenylbromide 5, which was achieved by formation of the Grignard followed by reaction with trimethylborate, hydrolysis of the dimethylborate with aqueous acid, and finally esterification of the formed boronic acid with ethylene glycol. Under these conditions 6 was isolated in a yield of 65%. We prefer to use the boronate esters rather than the boronic acids as it avoids the formation of dimers and trimers and the material is easier to handle. The advantage of using the glycolate ester compared to the pinacolate esters we have traditionally used is that they are much less expensive although they are a little less stable and a small amount of hydrolysis is observed during chromatography over silica. The boronate ester 6 was then reacted with 5-bromo-2,2′-bithiophene²⁶ under Suzuki conditions to give 9 in a 50% yield. The thiophene containing materials 7²⁷ and 9 were brominated with N-bromosuccinimide to give 8 and 10 in yields of 98% and 97% respectively. The final step in the formation of 1 and 2 was the Suzuki coupling of 8 and 10 with tris[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]amine²⁸ with 1 being formed in a 60% yield and 2 being isolated in a 58% yield.

Scheme 1 Reagents and conditions: (i) Mg turnings, THF, reflux, 1 h; then B(OMe)₃, 0 °C to rt 12 h; then 3 M HCl_(aq), rt, 2 h; then ethylene glycol, toluene, reflux Dean–Stark conditions, 12 h; (ii) n-butyllithium, THF, −78 °C, Ar_(g); then B(OMe)₃, −78 °C to rt 12 h; and then 3 M HCl_(aq), rt, 2 h; then ethylene glycol, toluene, reflux Dean–Stark conditions, 8 h; (iii) N-bromosuccinimide, THF, 0 °C, 20 min; (iv) Pd(PPh₃)₄, 2 M Na₂CO_3(aq), EtOH, toluene, reflux, Ar_(g), 24 h.

The equivalent first generation dendrimers, 3 and 4, were formed by a similar process to that of 1 and 2. Compound 11 was reacted with n-butyllithium to form the anion, which was then treated with trimethylborate. Subsequent hydrolysis with dilute hydrochloric acid followed by reaction with ethylene glycol under Dean–Stark conditions gave the corresponding boronate ester focused dendron 12 in a 92% yield. To add a single thiophene ring 12 was reacted with 2,5-dibromothiophene under Suzuki conditions and this gave a mixture of the desired 13 and the debrominated material. The debrominated compound was then treated with N-bromosuccinimide to selectively brominate the α-position of the thiophene to give an overall yield of 13 of 45% over the two steps. Compound 12 was then reacted with 5,5′-dibromo-2,2′-bithiophene in a similar two step procedure to form 14 in an isolated yield of 49%. To complete the synthesis of the two dendrimers 13 and 14 were reacted with tris[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]amine under Suzuki conditions to give 3 and 4 in the good yields of 72% and 62% respectively. All four materials were thermally stable with decomposition temperatures above 400 °C (determined by thermal gravimetric analysis).

Electronic properties and transistor performance

It is important for good OFET performance to have the minimum offset in energy between the source and drain electrodes and the energy levels of the organic semiconductor in the active channel. The materials of this work lend themselves to p-type OFETs and hence we used cyclic voltammetry to determine the oxidation potentials. The E_1/2 for the first oxidation of 1 was found to be 0.2 V and that for 2, 3 and 4 was 0.3 V against the ferrocenium/ferrocene couple. The E_1/2s correspond to ionization potentials of 5.0 eV and 5.1 eV respectively given that the ionization potential of ferrocene has been reported to be 4.8 eV.²⁹ Ionisation potentials around 5 eV mean that gold is a suitable contact for the source and drain electrodes in p-type channel operation.

A top contact-bottom gate OFET (see Experimental section) architecture was used to test the new semiconducting materials. Fig. 2–5 show the electrical characteristics of devices containing semiconducting channels of 1–4 respectively. In all cases the top panes correspond to plots of I_DSversusV_DS for a range of gate voltages. From these plots we can see consistent p-type transistor behavior, low leakage currents and clear saturation. From the I_DSversusV_DS plots the transfer curves [V_GversusI_DS and (I_DS)^1/2, bottom panels in Fig. 2–5] for specific source-drain voltages have been derived. Using the I_DSversusV_DS plots, the transfer curves and eqn (1) and (2) (in the Experimental section) we calculate the key FET performance parameters including carrier mobilities from the linear and saturated regime (µ^lin and µ^sat respectively), the threshold voltage for transistor behavior (V_T) and the ON/OFF ratio, and these are summarized in Table 1. It is important to note that the values reported in Table 1 represent the averages from multiple devices fabricated with each material and that the plots in Fig. 2–5 all correspond to devices with the same W/L (channel width to length ratio) for direct comparison.

Fig. 2 Electrical characteristics of a field effect transistor with compound 1 (small molecule thiophene) as the semiconducting channel with L = 60 µm, W = 46 mm, W/L = 767: (top) I_DSvs.V_DS and (bottom) transfer curve for V_DS = −80 V.

Fig. 3 Electrical characteristics of a field effect transistor with compound 2 (small molecule bithiophene) as the semiconducting channel with L = 60 µm, W = 46 mm, W/L = 767: (top) I_DSvs.V_DS and (bottom) transfer curve for V_DS = −80 V.

Fig. 4 Electrical characteristics of a field effect transistor with compound 3 (thiophene dendrimer) as the semiconducting channel with L = 60 µm, W = 46 mm, W/L = 767: (top) I_DSvs.V_DS and (bottom) transfer curve for V_DS = −80 V.

Fig. 5 Electrical characteristics of a field effect transistor with compound 4 (bithiophene dendrimer) as the semiconducting channel with L = 60 µm, W = 46 mm, W/L = 767: (top) I_DSvs.V_DS and (bottom) transfer curve for V_DS = −80 V.

Table 1 Averaged threshold voltages, saturation and linear mobilities for FETs made using P3HT, small molecules 1 and 2 and dendrimers 3 and 4. Each data value were calculated using between 8 and 25 transistors

Material	µ ^sat (standard deviation)/cm^2 V^−1 s^−1	µ ^sat,max/cm^2 V^−1 s^−1	µ ^lin (standard deviation)/cm^2 V^−1 s^−1	µ ^lin,max/cm^2 V^−1 s^−1	V T/V	ON/OFF
1	1.0 × 10^−5 (0.2 × 10^−5)	2 × 10^−5	1.3 × 10^−5 (0.4 × 10^−5)	2 × 10^−5	−8.6	1.6 × 10^3
2	3.7 × 10^−5 (0.7 × 10^−5)	6.4 × 10^−5	4.5 × 10^−5 (0.76 × 10^−5)	7 × 10^−5	−7	5.6 × 10^3
3	1.75 × 10^−6 (0.9 × 10^−6)	2.9 × 10^−6	2.8 × 10^−6 (1.1 × 10^−6)	4.5 × 10^−6	−8	5.3 × 10^2
4	1.3 × 10^−5 (0.3 × 10^−5)	1.65 × 10^−5	1.5 × 10^−5 (0.2 × 10^−5)	1.9 × 10^−5	−10	1.9 × 10^3
P3HT	0.0147 (2.7 × 10^−3)	0.017	0.0142 (2 × 10^−3)	0.017	−0.5	1 × 10^5

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To calibrate our device architecture and fabrication procedures we used poly(3-n-hexylthiophene) (P3HT) as a model p-type channel material. The P3HT devices had mobilities of the order 10⁻² cm² V⁻¹ s⁻¹ and ON/OFF ratios of up to 1 × 10⁵. The mobilities of materials 1 to 4 were smaller than the P3HT but respectable for amorphous spin-coated films, and were in the range 10⁻⁶–10⁻⁵ cm² V⁻¹ s⁻¹. The threshold voltages were around 10 V and the ON/OFF ratios were between 10² and 10³. The mobility results show two general trends: first, increasing the number of thiophene units in the core increases the charge mobility for both the dendronised and non-dendronised materials; and second, the attachment of the dendron only reduces the measured mobility by a small amount (Table 1). For all four materials the electroactive chromophore is comprised of all three arms due to the delocalization of the π-system across the central nitrogen atom.²⁵ The improvement in charge mobility on adding the extra thiophene into 2 and 4 relative to 1 and 3 is therefore due to the improved overlap of the arms of adjacent dendrimers. This is particularly important for amorphous films. The slight decrease in mobility on addition of the first generation biphenyl-based dendrons is due to the fact that the insulating dendrons hold the chromophores on neighbouring dendrimers on average further apart and hence increases the hopping distance. However, the poorer charge transport is more than offset by the improvement in processing ability for the dendrimers which we qualitatively compared by measuring relative processability for 20 mg ml⁻¹ solutions of each molecule in toluene mixed at room temperature. It was also more difficult to form films of the non-dendronised materials 1 and 2, and particularly 2 reliably. In contrast the dendronised compounds 3 and 4 allowed facile formation of good quality thin films.

Conclusion

In summary we have designed and synthesized a family of amine centred materials that differ in the number of thiophene units between the nitrogen and the groups at the surface. We found that when the electroactive chromophore was dendronised it was more easily processed. The materials were used as the semiconductor in OFETs demonstrating for the first time that conjugated dendrimers can be used as the active layers in such a device. The study also illustrates important design criteria for charge transport in amorphous materials, namely that it is important to design the chromophore to have maximal overlap between adjacent molecules in the channel.

Experimental

General methods

Column chromatography was performed using the flash chromatography technique over silica. Solvent mixtures are stated as volume/volume proportions. Tetrahydrofuran was distilled from sodium and benzophenone under a nitrogen atmosphere before use. Where used light petroleum was 40–60 °C fraction.¹H and ¹³C NMR spectra were recorded using a 400 MHz Oxford Instruments or a 500 MHz Bruker spectrometer in deuterated chloroform; ThH = thienyl H, spH = surface phenyl H, G1-bpH = dendron branching phenyl H. Coupling constants are quoted to the nearest 0.5 Hz. Microanalyses were carried out in Microanalysis Laboratory of the School of Chemistry and Molecular Biosciences, The University of Queensland or at the Microanalysis Laboratory of the School of Human Sciences, London Metropolitan University. UV-Visible absorption measurements were recorded with a Cary Varian 5000 UV-Vis-NIR spectrophotometer. Melting points were measured in a glass capillary on a BUCHI Melting Point B-545 and uncorrected. Thermalgravimetric analysis (TGA) was carried out on a Mettler-Toledo TGA/DSC 1 STARe System at the Australian National Fabrication Facility (Queensland Node). Decomposition temperatures (T_d) are reported for a 10% decrease in mass. Mass spectra were recorded on either an Applied Biosystems Voyager matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) from 2-[(2-E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) in positive reflection mode or on a Finnigan MAT 900XL-TRAP (EI 70 eV) electron impact ionisation.

Electrochemistry was performed using a BAS Epsilon electrochemistry station with a glassy carbon working, Ag/AgCl reference, and platinum counter electrodes. All measurements were made at room temperature on samples at a 1 mM concentration in dichloromethane (HPLC grade), with 0.1 M tetra-n-butylammonium tetrafluoroborate as the electrolyte. The solutions were deoxygenated with argon and the ferricenium/ferrocene couple was used as standard.³⁰ The scan rate was 100 mV s⁻¹ and in all cases several scans were carried out to confirm the chemical reversibility of the redox processes.

Bottom-gate, top contact OFETs using 1–4 as the channel semiconductors were fabricated and tested in a glove box (MBRAUN) under a nitrogen environment at 23 °C with oxygen and water levels between 0.1 and 0.3 ppm respectively. For direct comparison and validation of the fabrication process, a series of transistors were also made using standard poly-3-n-hexylthiophene (95% RR, Merck). The general fabrication process was as follows: a heavily doped n-silicon wafer (University Wafer, P(100) 1–10 ohm cm⁻¹ SSP 500um Prime Grade) coated with chromium (5 nm) and gold (40 nm) was used as the gate electrode. The evaporation thicknesses were monitored with a calibrated quartz crystal monitor. Thermally grown SiO₂ (350 nm), whose thickness was confirmed using a NanoSPEC 210 thickness measurement system, was used as the gate dielectric. The SiO₂ layer was cleaned in a Class 1000 clean room by sonication in acetone (>99.8% by GC) then 2-propanol (>99.9% by GC) for 20 minutes at room temperature, dried with a stream of nitrogen, and then exposed to UV/ozone for 30 minutes (MBRAUN with OP 73). The SiO₂ surface was modified with hexamethyldisilazane (HMDS) by covering the substrate surface with 300 µL of neat HMDS and leaving it to stand for 60 seconds before spinning at 800 rpm for 60 seconds. This modification serves to cap potential Si–OH trap sites on the silica surface. The substrate was then baked at 110 °C for 20 minutes on a hot plate to remove excess HMDS. The semiconductor layer was formed by spin-coating filtered (0.25 µm PTFE filter) dendrimer solutions (20 mg mL⁻¹, 9.1 × 10⁻⁴ to 18.1 × 10⁻⁴ M) in toluene (>99.7%, anhydrous) at 1500 rpm for 60 seconds. The film thicknesses were measured using a Dektak 150 profilometer and were found to be approximately 55 nm. Finally, interdigitated gold source/drain electrode pairs were deposited by vacuum evaporation through an in-house prepared shadow mask, creating a series of OFETs with channel length L = 60 µm and width W = 46 mm. The materials used were annealed before testing at the following temperatures 1 = 40 °C, 2 = 83 °C, 3 = 46 °C, and 4 = 59 °C. These temperatures were found to produce the optimal OFET performances.

The OFETs were characterized using an Agilent B1500A Semiconductor Device Analyser and an SA-6 Semi-Auto Prober. The mobility in the saturation regime was extracted in the usual manner from eqn (1),³¹ where W is the channel width, L is the channel length, I_DS is the drain current, C_i is the capacitance per unit area of the dielectric layer, and V_G and V_T are the gate voltage and threshold voltage. V_T was determined from the relationship between the square root of I_DS at the saturation regime and V_G of the device by extrapolating the measured data to I_DS = 0. The mobility in the linear regime was extracted from eqn (2), where the ON/OFF ratios were calculated from the transfer curves at a V_D of −80 V and V_G of 0 to −100 V.

Synthesis

4-(2-Ethylhexyloxy)phenyl-1,3,2-dioxaborolane 6. A solution of 1-bromo-4-(2-ethylhexyloxy)benzene 5 (19.1 g, 67.0 mmol) in anhydrous tetrahydrofuran (20 cm³) was added drop-wise to magnesium turnings (1.67 g, 69 mmol) in tetrahydrofuran (5 cm³) under argon. After complete addition the reaction was stirred at reflux for 1 h. The reaction was cooled to 0 °C with an ice bath and tetrahydrofuran (20 cm³) was added followed by trimethylborate (12.7 mL, 134 mmol). The reaction was allowed to warm to room temperature and stirred for 12 h. Hydrochloric acid (3 M, 30 cm³) was added and the reaction was stirred for 2 h before being poured into water and filtered to remove any remaining magnesium. The filtrate was extracted with diethyl ether (4 × 80 cm³). The combined organic fractions were dried over magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using light petroleum ether : diethyl ether mixtures (1 : 0 then 0 : 1) as eluent to give 4-(2-ethylhexyloxy)phenylboronic acid as a viscous oil (12.2 g), which was used immediately in the next step. The 4-(2-ethylhexyloxy)phenylboronic acid and ethylene glycol (1.11 mL, 98.5 mmol) in toluene (250 cm³) were reacted for 12 h under Dean–Stark conditions. After cooling the ethylene glycol layer was removed and the toluene phase was washed with water (3 × 50 cm³) to give a viscous oil of 6 (12.2 g, 65%). Found: C, 69.9%; H, 9.4%. C₃₆H₄₉BO₄ requires C, 69.6%; H, 9.1%. λ_max (THF)/nm 238 [log (ε/dm³ mol⁻¹ cm⁻¹) (4.52)]; ¹H NMR (400 MHz, CDCl₃) δ: 0.88–0.94 (6H, m, CH₃), 1.29–1.58 (8H, m, CH₂), 1.68–1.78 (1H, m, CH), 3.87 (2H, m, ArOCH₂), 4.35 (4H, s, OCH₂), 6.90 and 7.74 (4H, AA′BB′, spH); ¹³C NMR (100 MHz, CDCl₃) δ: 11.1, 14.1, 23.0, 23.8, 29.1, 30.5, 39.3, 65.9, 70.3, 114.0, 136.5, 162.2; m/z (EI) found: 275.3 (26%), 276.3 (100%), 277.3 (21%), 278.3 (9%); C₁₆H₂₅BO₃ requires 275.2 (24%), 276.2 (100%), 277.2 (18%), 279.2 (2%).

2-Bromo-5-[4-(2-ethylhexyloxy)phenyl]thiophene 8. N-Bromosuccinimide (0.40 g, 2.19 mmol) was added to a stirred solution of 2-[4-(2-ethylhexyloxy)phenyl]thiophene 7²⁷ (0.59 g, 2.04 mmol) in tetrahydrofuran (20 cm³) which was cooled to 0 °C with an ice bath and protected from light. The reaction was stirred for 20 min at 0 °C before being poured into water (30 cm³). The mixture was extracted with chloroform (8 × 20 cm³). The organic phases were combined, dried over magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using light petroleum as eluent to give a white solid of 8 (0.73 g, 98%), mp = 88.8–89.1 °C; found: C, 58.8%; H, 6.2%; C₁₈H₂₃BrOS requires C, 58.85%; H, 6.3%; λ_max (THF)/nm 302 [log (ε/dm³ mol⁻¹ cm⁻¹) (4.36)], 315sh (4.29), 332sh (3.83); ¹H NMR (400 MHz, CDCl₃) δ: 0.89–0.95 (6H, m, CH₃), 1.30–1.56 (8H, m, CH₂), 1.68–1.77 (1H, m, CH), 3.86 (2H, m, OCH₂), 6.89 and 7.42 (4H, AA′BB′, spH), 6.92 (1H, d, J = 4, ThH), 6.99 (1H, d, J = 4, ThH); ¹³C NMR (100 MHz, CDCl₃) δ: 11.1, 14.1, 23.0, 23.8, 29.1, 30.5, 39.3, 70.6, 110.0, 115.0, 122.1, 126.2, 126.9, 130.7, 146.0, 159.3; m/z (EI) 366.2 (95%), 367.2 (21%), 368.2 (100%), 369.2 (21%), 370.2 (6%), 371.2 (2%); C₁₈H₂₃BrOS requires 366.1 (96%), 367.1 (20%), 368.1 (100%), 369.1 (20%), 370.1 (6%), 371.1 (1%).

Tris[4-(5-{4-[2-ethylhexyloxy]phenyl}thiophen-2-yl)phenyl]amine 1. A mixture of 8 (60 mg, 0.17 mmol), tris[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]amine²⁸ (18 mg, 28 µmol), toluene (6 cm³), ethanol (0.85 cm³), and aqueous sodium carbonate (2 M, 0.85 cm³) was deoxygenated by placement under high vacuum and backfilling with argon 5 times. Tetrakis(triphenylphosphine)palladium(0) (3.3 mg, 0.003 mmol) was added and the reaction mixture was deoxygenated again 5 times before being stirred at reflux for 24 h. The reaction was cooled to room temperature and water (10 cm³) was added. The mixture was extracted with chloroform (6 × 20 cm³). The combined organic layers were dried over magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using a petroleum ether : dichloromethane mixture (4 : 1 to 3 : 2) as eluent to give a light green solid of 1 (19 mg, 60%): mp = 61.2–63.8 °C; T_d = 438 °C; found: C, 77.9%; H, 7.4%; N, 1.3%; S, 8.9%; C₇₂H₈₁NO₃S₃ requires C, 78.3%; H, 7.4%; N, 1.3%; S, 8.7%; λ_max (THF)/nm 321 [log (ε/dm³ mol⁻¹ cm⁻¹) (4.99)], 395 (5.36); λ_max (film)/nm 327, 404, 427sh; ¹H NMR (500 MHz, CDCl₃) δ: 0.90–0.95 (18H, m, CH₃), 1.31–1.58 (24H, m, CH₂), 1.70–1.78 (3H, m, CH), 3.87 (6H, m, OCH₂), 6.92 (6H, 1/2AABB′, spH), 7.14–7.16 (9H, m, core PhH and ThH), 7.20 (3H, d, J = 3.5, ThH), 7.52–7.55 (12H, m, core PhH and spH); ¹³C NMR (125 MHz, CDCl₃) δ: 11.1, 14.1, 23.0, 23.8, 29.1, 30.5, 39.4, 70.6, 114.9, 122.8, 123.3, 124.4, 126.4, 126.8, 126.9, 129.3, 142.1, 143.1, 146.3, 159.0; m/z (MALDI) 1103.8 (100%), 1104.8 (85%), 1105.8 (51%), 1106.8 (18%), 1107.8 (8%); C₇₂H₈₁NO₃S₃ requires 1103.5 (100%), 1104.5 (84%), 1105.5 (48%), 1106.5 (20%), 1107.5 (7%), 1108.5 (2%).

5-[4-(2-Ethylhexyloxy)phenyl]-2,2′-bithiophene 9. A mixture of 6 (1.1 g, 4.0 mmol), 5-bromo-2,2′-bithiophene²⁶ (1.2 g, 4.8 mmol), toluene (15 cm³), ethanol (8 cm³), and aqueous sodium carbonate (2 M, 8 cm³) was deoxygenated by placement under high vacuum and backfilling with argon 5 times. Tetrakis(triphenylphosphine)palladium(0) (23 mg, 0.20 mmol) was added and the reaction was deoxygenated again 10 times before being heated at reflux for 24 h with stirring. The reaction was cooled to room temperature and water (20 cm³) was added. The mixture was extracted with chloroform (10 × 20 cm³). The combined organic layers were dried over magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using petroleum ether as eluent to give a cream solid of 9 (0.74 g, 50%); mp = 63.0–63.5 °C; found: C, 71.2%; H, 7.3%; S, 17.1%; C₂₂H₂₆OS₂ requires C, 71.3%; H, 7.1%; S, 17.3%; λ_max (THF)/nm 221sh [log (ε/dm³ mol⁻¹ cm⁻¹) (3.60)], 254sh (3.47), 277sh (3.34), 347 (3.47), 381sh (3.08); ¹H NMR (400 MHz, CDCl₃) δ: 0.89–0.96 (6H, m, CH₃), 1.30–1.58 (8H, m, CH₂), 1.70–1.78 (1H, m, CH), 3.87 (2H, m, OCH₂), 6.91 and 7.51 (4H, AA′BB′, spH), 7.02 (1H, dd, J = 3.5, J = 5, ThH), 7.09 (1H, d, J = 3.5, ThH), 7.11 (1H, d, J = 3.5, ThH), 7.17 (1H, dd, J = 3.5, J = 1, ThH), 7.20 (1H, dd, J = 5, J = 1, ThH); ¹³C NMR (100 MHz, CDCl₃) δ: 11.1, 14.1, 23.0, 23.8, 29.1, 30.5, 39.4, 70.6, 114.9, 122.5, 123.3, 124.1, 124.6, 126.6, 126.8, 127.8, 136.4, 143.3, 159.2; m/z (MALDI) found: 370.1 (100%), 371.1 (33%), 372.1 (18%), 373.1 (3%); C₂₂H₂₆OS₂ requires 370.1 (100%), 371.1 (26%), 372.1 (12%), 373.1 (3%).

5-Bromo-5′-[4-(2-ethylhexyloxy)phenyl]-2,2′-bithiophene 10. N-Bromosuccinimide (0.39 g, 2.19 mmol) was added to a solution of 9 (0.74 g, 1.99 mmol) in tetrahydrofuran (20 cm³), which was cooled to 0 °C with an ice bath and was protected from light. The reaction mixture was stirred for 20 min at 0 °C and then poured into water (30 cm³). The mixture was extracted with chloroform (10 × 20 cm³). The organic layers were combined, dried over anhydrous magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using light petroleum as eluent to give a white solid of 10 (0.87 g, 97%): mp = 131.3–133.6 °C; found: C, 58.5%; H, 5.6%; S, 14.4%; C₂₂H₂₅BrOS₂ requires C, 58.8%; H, 5.6%; S, 14.3%; λ_max (THF)/nm 226 [(log (ε/dm³ mol⁻¹ cm⁻¹) (3.97)], 257sh (3.59), 355 (4.15); ¹H NMR (500 MHz, CDCl₃) δ: 0.89–0.95 (6H, m, CH₃), 1.31–1.58 (8H, m, CH₂), 1.70–1.77 (1H, m, CH), 3.87 (2H, m, OCH₂), 6.90–6.92 (3H, m, spH and ThH), 6.96 (1H, d, J = 3, ThH), 7.04 (1H, d, J = 3.5, ThH), 7.08 (1H, d, J = 3.5, ThH), 7.49 (2H, 1/2AA′BB′, spH); m/z (EI) 448.2 (95%), 449.2 (23%), 450.2 (100%), 451.2 (24%), 452.2 (12%), 453.2 (1%) [M⁺]; C₂₂H₂₅BrOS₂ requires 448.05 (91%), 449.05 (24%), 450.05 (100%), 451.05 (25%), 452.04 (11%), 453.05 (2%).

Tris[4-(5′-{4-(2-ethylhexyloxy)phenyl}-2,2′-bithiophen-5-yl)phenyl]amine 2. A mixture of 10 (100 mg, 0.22 mmol), tris[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]amine (23 mg, 0.04 mmol), toluene (6 cm³), ethanol (1.11 cm³), and aqueous sodium carbonate (2 M, 1.11 cm³) was deoxygenated by placement under high vacuum and backfilling with argon 5 times. Tetrakis(triphenylphosphine)palladium(0) (3.8 mg, 3.3 µmol) was added and the reaction was deoxygenated again 5 times before being stirred at reflux for 24 h. The reaction was cooled to room temperature and water (15 cm³) was added. The organic aqueous mixture was extracted with chloroform (8 × 20 cm³). The combined organic layers were dried over anhydrous magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using a light petroleum ether : dichloromethane mixture (4 : 1 to 3 : 2) as eluent to give a yellow/orange solid of 2 (28.8 mg, 58%): mp = 209.8–211.3 °C; T_d = 452 °C; found: C, 74.6%; H, 6.4%; N, 1.0%; C₈₄H₈₇NO₃S₆ requires C, 74.7%; H, 6.5%; N, 1.0%; λ_max (THF)/nm 242sh [(log (ε/dm³ mol⁻¹ cm⁻¹) (5.10)], 284sh (4.47), 346sh (4.77), 422 (5.23); λ_max (film)/nm 240, 358sh, 440, 471sh; ¹H NMR (400 MHz, CDCl₃) δ: 0.89–0.96 (18H, m, CH₃), 1.31–1.58 (24H, m, CH₂), 1.70–1.77 (3H, m, CH), 3.87 (6H, m, OCH₂), 6.92 (6H, 1/2AA′BB′, spH), 7.11 (3H, d, J = 3.5, ThH), 7.13–7.18 (15H, m, ThH and core PhH), 7.51–7.53 (12H, m, core PhH and spH); m/z (MALDI) 1349.8 (100%), 1350.8 (96%), 1351.8 (67%), 1352.8 (41%), 1353.8 (15%), 1354.8 (8%), 1355.8 (1%) [M⁺]; C₈₄H₈₇NO₃S₆ requires 1349.5 (100%), 1350.5 (99.5%), 1351.5 (76%), 1352.5 (43%), 1353.5 (20%), 1354.5 (8%), 1355.5 (3%).

3,5-Bis[4-(2-ethylhexyloxy)phenyl]phenyl-1,3,2-dioxaborolane 12. A solution of 1-bromo-3,5-bis[4-(2-ethylhexyloxy)phenyl]benzene 11³² (2.3 g, 4.1 mmol) in anhydrous tetrahydrofuran (9 cm³) was deoxygenated by placement under high vacuum and backfilling with argon 3 times. The solution was then cooled to −78 °C and then n-butyllithium (1.6 M solution in hexane, 3.05 cm³, 4.9 mmol) was added drop-wise to the reaction. The reaction was stirred at −78 °C for 1 h and then anhydrous tetrahydrofuran (20 cm³) was added. Trimethylborate (0.78 cm³, 8.12 mmol) was then added to the reaction mixture and it was allowed to warm to room temperature and stirred for 12 h. The reaction was quenched by the addition of hydrochloric acid (3 M, 20 cm³). The mixture was extracted with diethyl ether (4 × 25 cm³). The combined organic layers were washed with (2 × 25 cm³) water, then saturated brine solution (1 × 25 cm³) before being dried over anhydrous magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using a petroleum ether : dichloromethane mixture (4 : 1) and then ethyl acetate as eluents to give a viscous oil of 3,5-bis[4-(2-ethylhexyloxy)phenyl]phenylboronic acid (1.88 g), which was used immediately. A mixture of 3,5-bis[4-(2-ethylhexyloxy)phenyl]phenylboronic acid (1.9 g), ethylene glycol (4.9 mL, 71 mmol), and toluene (150 cm³) was heated at reflux under Dean–Stark conditions for 8 h. The ethylene glycol layer was removed and the organic layer was washed with water (3 × 10 cm³), dried over anhydrous magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using dichloromethane as eluent to give a cream gum of 12 (2.10 g, 92%): found: C, 77.6%; H, 9.0%; C₃₆H₄₉BO₄ requires C, 77.7%; H, 8.9%; λ_max (THF)/nm 264 [(log (ε/dm³ mol⁻¹ cm⁻¹) (4.73)]. ¹H NMR (400 MHz, CDCl₃) δ: 0.90–0.97 (12H, m, CH₃), 1.32–1.59 (16H, m, CH₂), 1.71–1.80 (2H, m, CH), 3.89 (4H, m, ArOCH₂), 4.43 (4H, s, OCH₂), 6.98 and 7.59 (8H, AA′BB′, spH), 7.84 (1H, dd, J = 2, J = 2, G1-bpH), 7.94 (2H, d, J = 2, G1-bpH); m/z (MALDI) 555.6 (26%), 556.6 (100%), 557.6 (41%), 558.6 (6%), 559.6 (2%); C₃₆H₄₉BO₄ requires 555.4 (23%), 556.4 (100%), 557.4 (39%), 558.4 (8%), 559.4 (1%).

2-Bromo-5-[3,5-bis(4-{2-ethylhexyloxy}phenyl)phenyl]thiophene 13. A mixture of 3,5-bis[4-(2-ethylhexyloxy)phenyl]phenyl-1,3,2-dioxaborolane (215 mg, 0.38 mmol) and 2,5-dibromothiophene (110 mg, 0.45 mmol), toluene (10 cm³), ethanol (1.52 cm³), and aqueous sodium carbonate (2 M, 1.52 cm³) was deoxygenated by placement under high vacuum and backfilling with argon 3 times. Tetrakis(triphenylphosphine)palladium(0) (4.4 mg, 3.8 µmol) was added and the reaction was deoxygenated again 5 times before being stirred at reflux for 12 h. The reaction was cooled to room temperature and water (15 cm³) was added. The organic aqueous mixture was extracted with chloroform (4 × 15 cm³). The combined organic layers were dried over anhydrous magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using petroleum ether : dichloromethane mixtures (9 : 1 to 4 : 1) as eluent to give an opaque viscous oil of 13 (112 mg, 45%); found: C, 70.5%; H, 7.3%; S, 4.6%; C₃₈H₄₇BrO₂S requires C, 70.5%; H, 7.3%; S, 4.9%; λ_max (THF)/nm 239sh [log (ε/dm³ mol⁻¹ cm⁻¹) (4.67)], 280 (4.62); ¹H NMR (400 MHz, CDCl₃) δ: 0.91–0.98 (12H, m, CH₃), 1.33–1.60 (16H, m, CH₂), 1.73–1.82 (2H, m, CH), 3.91 (4H, m, OCH₂), 7.01 and 7.58 (8H, AA′BB′, spH), 7.06 (1H, d, J = 4, ThH), 7.15 (1H, d, J = 4, ThH), 7.60 (2H, d, J = 1.5, G1-bpH), 7.64 (1H, dd, J = 1.5, G1-bpH); m/z (EI) 646.2 (97%), 647.2 (36%), 648.3 (100%), 649.3 (33%), 650.3 (20%), 651.3 (5%), 652.3 (2%); C₃₈H₄₇BrO₂S requires 646.2 (90%), 647.3 (39%), 648.3 (100%), 649.2 (41%), 650.2 (12%), 651.2 (3%).

Tris[4-(5-{3,5-bis[4-(2-ethylhexyloxy)phenyl]phenyl}thiophen-2-yl)phenyl]amine 3. A mixture of 13 (50 mg, 0.08 mmol), tris[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]amine (8 mg, 0.01 mmol), toluene (6 cm³), ethanol (0.3 cm³), and aqueous sodium carbonate (2 M, 0.3 cm³) was deoxygenated by placement under high vacuum and backfilling with argon 3 times. Tetrakis(triphenylphosphine)palladium(0) (0.7 mg, 6 µmol) was added and the reaction was deoxygenated 5 times before being stirred at reflux for 24 h. The reaction was cooled to room temperature and water (10 cm³) was added. The mixture was extracted with chloroform (4 × 20 cm³). The combined organic layers were dried over anhydrous magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using a petroleum ether : dichloromethane mixture (4 : 1) as eluent to give a yellow solid of 3 (18 mg, 72%): mp = 76.8–78.2 °C; T_d = 439 °C; T_g = 46 °C; found: C, 81.3%; H, 8.1%; N, 0.71%; S, 4.9%; C₁₃₂H₁₅₂NO₆S₃ requires C, 81.5%; H, 7.9%; N, 0.72%; S, 4.9%; λ_max (THF)/nm 253sh [log (ε/dm³ mol⁻¹ cm⁻¹) (5.34)], 286sh (5.15), 321sh (4.70), 399 (5.11); λ_max (film)/nm 260sh, 282, 411; ¹H NMR (500 MHz, CDCl₃) δ: 0.90–0.97 (36H, m, CH₃), 1.33–1.58 (48H, m, CH₂), 1.73–1.80 (6H, m, CH), 3.91 (12H, m, OCH₂), 7.01 (12H, 1/2AA′BB′, spH), 7.18 and 7.58 (12H, AA′BB′, core PhH), 7.28 (3H, d, J = 3.5, ThH), 7.39 (3H, d, J = 3.5, ThH), 7.59–7.63 (15H, m, G1-bpH and spH), 7.73 (6H, d, J = 1.5, G1-bpH); m/z (MALDI) 1944.7 (82%), 1945.7 (100%), 1946.7 (97%), 1947.7 (74%), 1948.6 (29%), 1949.7 (9%). C₁₃₂H₁₅₃NO₆S₃ requires 1944.1 (66%), 1945.1 (100%), 1946.1 (85%), 1947.1 (51%) 1948.1 (25%), 1949.1 (10%), 1950.1 (4%), 1951.1 (1%).

5-Bromo-5′-[3,5-bis(4-{2-ethylhexyloxy}phenyl)phenyl]2,2′-bithiophene 14. A mixture of 12 (350 mg, 0.63 mmol) and 5,5′-dibromo-2,2′-bithiophene²⁶ (303 mg, 0.95 mmol), toluene (10 cm³), ethanol (2.5 cm³), and aqueous sodium carbonate (2 M, 2.5 cm³) was deoxygenated by placement under high vacuum and backfilling with argon 3 times. Tetrakis(triphenylphosphine)palladium(0) (7.2 mg, 6.3 µmol) was added and the reaction was deoxygenated 5 times before being stirred at reflux for 24 h. The reaction was cooled to room temperature and water (20 cm³) was added. The mixture was extracted with chloroform (5 × 10 cm³). The combined organic layers were dried over anhydrous magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using petroleum ether : dichloromethane mixtures (9 : 1 to 4 : 1) as eluent to give a mixture of the brominated and debrominated materials (488 mg). N-Bromosuccinimide (73 mg, 0.41 mmol) was added to a solution of the mixture (488 mg) in tetrahydrofuran (5 cm³) and the reaction mixture was stirred for 20 min in the dark. The reaction mixture was poured into water (10 cm³) and extracted with chloroform (5 × 10 cm³). The organic layers were combined, dried over anhydrous magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using a petroleum ether : dichloromethane mixture (4 : 1) as eluent to give a viscous yellow oil of 14 (493 mg, 49%). Found: C, 68.8%; H, 6.6%; S, 8.6%; C₄₂H₄₉BrO₂S₂ requires C, 69.1%; H, 6.8%; S, 8.8%; λ_max (THF)/nm 263 [log (ε/dm³ mol⁻¹ cm⁻¹) (5.14)], 286sh (5.11), 352 (5.07), 387sh (4.74); ¹H NMR (400 MHz, CDCl₃) δ: 0.91–0.98 (12H, m, CH₃), 1.30–1.60 (16H, m, CH₂), 1.73–1.82 (2H, m, CH), 3.92 (4H, m, ArOCH₂), 6.96 (1H, d, J = 4, ThH), 6.98–7.02 (5H, m, ThH and AA′BB′ spH), 7.11 (1H, d, J = 4 Hz, ThH), 7.30 (1H, d, J = 4, ThH), 7.60 (4H, AA′BB′ spH), 7.64 (1H, dd, J = 2, G1-bpH), 7.68 (2H, d, J = 2, G1-bpH); m/z (EI) 728.0 (84%), 729.0 (40%), 730.0 (100%), 731.0 (43%), 732.0 (17%), 733.0 (4%), 734.0 (1%) [M⁺]; C₄₂H₄₉BrO₂S₂ requires 728.2 (84%), 728.2 (41%), 730.2 (100%), 731.2 (45%), 732.2 (18%), 733.2 (5%), 734.2 (1%).

Tris[4-(5′-{3,5-bis[4-(2-ethylhexyloxy)phenyl]phenyl}-2,2′-bithiophen-5-yl)phenyl]amine 4. A mixture of 14 (58 mg, 0.08 mmol), tris[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]amine (8.25 mg, 0.013 mmol), toluene (6 cm³), ethanol (0.4 cm³), and aqueous sodium carbonate (2 M, 0.4 cm³) was deoxygenated by placement under high vacuum and backfilling with argon 3 times. Tetrakis(triphenylphosphine)palladium(0) (2 mg, 1.7 µmol) was added and the reaction was deoxygenated 5 times before being stirred at reflux for 24 h. The reaction was cooled to room temperature and water (10 cm³) was added. The mixture was extracted with chloroform (5 × 20 cm³). The combined organic layers were dried over anhydrous magnesium sulfate, filtered, and the solvent removed. The residue was purified by column chromatography over silica using petroleum ether : dichloromethane mixtures (4 : 1 to 3 : 2) as eluent to give a yellow/orange solid of 4 (17.8 mg, 62%), mp = 87.6–90.2 °C; T_d = 445 °C; found: C, 78.6%; H, 7.3%; N, 0.77%; S, 8.9%; C₁₄₄H₁₅₉NO₆S₆ requires C, 78.9%; H, 7.3%; N, 0.64%; S, 8.8%; λ_max (THF)/nm 236sh [log (ε/dm³ mol⁻¹ cm⁻¹) (5.49)], 260sh (5.36), 290sh (5.08), 349sh (4.78) 424 (5.23); λ_max (film)/nm 240, 360sh, 440, 468sh; ¹H NMR (400 MHz, CDCl₃) δ: 0.90–0.97 (36H, m, CH₃), 1.33–1.58 (48H, m, CH₂), 1.72–1.81 (6H, m, CH), 3.91 (12H, m, OCH₂), 7.02 and 7.60 (24H, AA′BB′, spH), 7.17 and 7.54 (12H, AA′BB′, core PhH), 7.19–7.21 (9H, m, ThH), 7.34 (3H, d, J = 3.5, ThH), 7.63 (3H, dd, J = 1.5, J = 1.5, G1-bpH), 7.70 (6H, d, J = 1.5, G1-bpH); m/z (MALDI) 2190.5 (58%), 2191.5 (82%), 2192.5 (100%), 2193.5 (76%), 2194.5 (46%), 2195.5 (18%), 2196.5 (10%), 2197.5 (4%), 2198.5 (1%). C₁₄₄H₁₅₉NO₆S₆ requires 2190.0 (60%), 2191.1 (100%), 2192.1 (100%), 2193.1 (73%), 2194.1 (44%), 2195.1 (22%), 2196.1 (10%), 2197.1 (4%), 2198.1 (1%).

Acknowledgements

This work was funded by the Australian Research Council (PLB Federation Fellowship), the Queensland State Government (PM Smart State Senior Fellowship), and the University of Queensland (Strategic Initiative—Centre for Organic Photonics and Electronics). GV is an ARC Australian Research Fellow. We also acknowledge use of equipment of the Australian Nanofabrication Facility Queensland Node (ANFF-Q) funded by the Australian Federal Government National Collaborative Infrastructure Strategy (NCRIS).

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