Reductive Coupling Synthesis of a Soluble Poly(9,10-anthrylene ethynylene)

Key words

conjugated polymers - poly(arylene ethynylene)s - reductive coupling - fluorescence anisotropy

Introduction

Attractive optical and electronic properties of conjugated polymers, in combination with a superior processability, predestine them for many applications, such as in organic field-effect transistors,[1] organic light emitting diodes (OLEDs),[2] [3] [4] photovoltaic devices,[5] or thin film sensors,[6] as well as for applications in biology.[7] [8] In this context, poly(arylene ethynylene)s (PAEs) that are composed of alternating arylene and ethynylene units represent one well-established and promising class of conjugated polymers.[3] Many PAEs show interesting photoluminescence (PL) properties, in particular stimuli-responsive PL features, together with a high PL efficiency.[9] [10] [11] [12] [13] The two main synthetic routes towards linear PAEs are Sonogashira–Hagihara-type cross-coupling protocols of difunctional monomers containing two ethynyl and halo functions (AA/BB- or AB-type monomers), or alkyne metathesis schemes starting from acyclic, dialkynylarylene monomers.[14]

Anthracene is a promising building block for conjugated polymers. Due to its unique features, such as a blueish PL of high quantum yield, it seems well suited for applications in luminescent devices.[15] [16] [17] [18] OLEDs based on anthracene-derived active materials exhibited promising blue electroluminescence properties.[9] [19] [20] [21] However, the photooxidation of anthracene under ambient conditions poses a challenge to its usability. Photooxidation can be limited by lowering the reactivity through electron-accepting substituents.[22] Therefore, the incorporation of anthracene building blocks into PAEs is particularly interesting.[9] [10] [17] [23] An intense PL can be expected if stacking of the anthracenes in such a polymer can be prevented. Nevertheless, soluble poly(9,10-anthrylene ethynylene)s have not been reported previously. The known anthracene-containing PAEs also contain non-anthracene arylene units.[9] Only insoluble, on-surface synthesized poly(anthrylene ethynylene)s were reported recently.[23]

Another interesting property of anthracene single crystals is the high degree of linear polarization of light emission, documented by the fluorescence polarization anisotropy.[24] The transition dipole moment is oriented along the short molecular axis of anthracene.[25] [26] [27] Emitters with a macroscopically polarized emission have been investigated for their potential to increase the luminescence efficiency of optical devices. For example, brightness losses caused by polarizers in displays could be reduced, potentially lowering the energy consumption.[28] Furthermore, it has been shown that a defined orientation of the emitting dipoles in OLEDs increases the outcoupling efficiency of light out of the device plane, limiting waveguide losses.[29] [30] [31]

Here, we report a novel synthesis scheme of a soluble poly(9,10-anthrylene ethynylene), as a prototypical, directed example for the synthesis of related polymers. Poly[2,6-(2-octyldecyl)-9,10-anthrylene ethynylene] (PAAE) was made in the course of the PhD project of one of the co-authors.[32] To prevent steric hindrance and distortion of the polymer backbones, and to possibly also suppress the interchain π-stacking of anthrylene units, branched and bulky alkyl side chains were introduced to the outer benzene rings of the anthracenes at the 2,6-positions. Our monomer synthesis is based on published anthracene substitution chemistry.[9] The optical properties of the PAAE polymer obtained were related to commercially available 9,10-bis(phenylethynyl)anthracene (BPEA) as a monomeric model compound. The chemical structures of the targets are depicted in [Figure 1].

Figure 1 Structure of poly[2,6-(2-octyldecyl)-9,10-anthrylene ethynylene] PAAE and the commercially available model compound BPEA.

Results and Discussion

Our new PAAE synthesis is a reductive, dehalogenative homocoupling of 9,10-bis(dibromomethylene)-2,6-bis(2-octyldecyl)-9,10-dihydroanthracene (6) as a single monomer. First, the three-step monomer synthesis is depicted in [Scheme 1].

Scheme 1 Synthesis of monomer 6: a) t-BuNO₂, CuBr₂, ACN, 85 °C, 4 h; b) NaH, Ph₃PMeBr, DMSO, rt, 20 h; c) 1. 9–BBN, THF, rt, 23 h, 2. Pd(PPh₃)₄, K₂CO₃, 75 °C, 23 h; d) CBr₄, Ph₃P, toluene, 80 °C, 24 h.

Commercially available 2,6-diaminoanthraquinone (1) was dibrominated to 2 (88% yield) in a Sandmeyer procedure, as described by Seidel et al.[33] The alkyl side chains were introduced by following a method reported by Müllen's group.[9] First, 9-heptadecanone (3) was converted to 9-methyleneheptadecane (4) in a Wittig reaction in 93% yield. The obtained olefin 4 was borylated with 9-borabicyclo[3.3.1]nonane (9-BBN). In a one-pot reaction, the obtained boronic ester was not isolated and immediately coupled with the 2,6-dibromoanthraquinone 2, leading to the 2,6-dialkylated anthraquinone 5 (28% overall yield). In the last step, a twofold Corey–Fuchs reaction was carried out, as described by Pola et al. [34] Under conversion of the keto into dibromomethylene functions, 5 was converted to the desired monomer 6 (86% yield).

Our target poly(anthrylene ethynylene) PAAE was synthesized as depicted in [Scheme 2]. This route is inspired by the on-surface coupling procedure of related non-alkylated monomers as reported by Sánchez-Grande et al.[23] For the resulting polymer, both possible isomeric electron structures are depicted, a cumulene-type and a PAE-type structure, which is expected to be more stable. Both electronic structures, of course, differ in their end groups. It is anticipated that the initially formed olefinic dibromomethylene >=CBr₂ end groups (corresponding to the cumulene-type structure) are converted into single-bonded end groups (present for the PAE structure) in an interplay of hydrolysis and water/methanol addition, under formation of carbonyl-containing end groups (e.g. −COOH, −COOMe, −CHO). In the IR spectrum of PAAE, as expected, carbonyl-related signatures at frequencies of 1727 and 1675 cm⁻¹ are observed (see [Figure S3]) that can generally be attributed to carbonyl end groups without the possibility of a clear structural assignment. The Raman spectrum of PAAE displays the expected carbon–carbon triple bond stretching vibration at 2161 cm⁻¹ ([Figure S4]).

Scheme 2 Synthesis of poly(9,10-anthrylene ethynylene) PAAE.

Different potentially promising reducing reagents have been reported in the literature.[35] [36] [37] [38] [39] [40] The results in terms of the PAAE synthesis are summarized in [Table 1]. Only one of these agents, the combination of n-butyllithium (n-BuLi) and copper (I) cyanide (CuCN), produced the desired polymer in acceptable yields. The crude products were purified and fractionated by repeated Soxhlet extractions (see the Experimental Section). The molecular weights of the ethyl acetate and chloroform fractions were determined by gel permeation chromatography (GPC, polystyrene calibration). Our method, using n-BuLi/CuCN as reagents, yielded 53% PAAE with M _n = 7100 g/mol and M _w = 16600 g/mol (PDI: 2.33, DP: ca. 10) in the ethyl acetate fraction accompanied by a low amount of higher M _n polymer in the chloroform fraction (see [Table 1]). The good solubility in ethyl acetate is caused by the long-chained, branched alkyl substituents.

Table 1 Reagent screening for the synthesis of PAAE

Reagent	Ethyl acetate fraction		Chloroform fraction
	Yield [%]	Molecular weights M n/M w [g/mol]	Yield [%]	Molecular weights M n/M w [g/mol]
Co2(CO)8	0.4	5500/7300	0.3	5300/8000
Cr2(OAc)4, Ni(COD)2, Ni(PPh3)(CO)2, Cu, Zn, Zn/CuCl	–		–
n-BuLi, CuCN	53	7100/16600	1	13000/44400

A thermogravimetric analysis (TGA) of PAAE was carried out in the temperature range from 35 °C to 950 °C with a heating rate of 10 K/min. PAAE shows a good thermal stability with a 5% weight loss point at 279 °C. No phase transitions were observed in a differential scanning calorimetry (DSC) analysis in the temperature range from −20 °C to 150 °C (heating rate: 10 K/min).

The absorption and emission spectra of PAAE and of the model compound BPEA in chloroform solution are displayed in [Figure 2]. The absorption spectrum of PAAE shows a broad absorption band with a peak at 502 nm, arising from the π–π* transitions; a similar peak position appears in the thin film at 506 nm. The fact that the optical spectra in solution and the solid state are very similar indicates the absence of interchain interactions. The absorption spectra are surprisingly broad, most probably because of distortion and coiling of the backbone. Indeed, the distortion of neighbouring 9,10-anthrylene units in a related dianthrylacetylene dimer has previously been reported,[41] as has the occurrence of broadened, unstructured absorption bands in related trimers.[42] [43] The bulky alkyl chains at the 2,6-positions of the 9,10-anthrylene units may cause additional disorder. These conclusions are supported by the large Stokes loss associated with the orange/red emission of PAAE (peaking at 611 nm in solution, and at 687 nm in the film).

Figure 2 Absorption and emission spectra of poly(9,10-anthrylene ethynylene) PAAE and the model compound BPEA in chloroform solution.

In contrast, the solution absorption spectrum of the monomeric BPEA shows the expected narrowed absorption band peaking at 459 nm (film, BPEA embedded in a PS matrix: λ_max 470 nm). The emission, on the other hand, shifts from 475 nm (solution, shoulders at 508 nm and 536 nm) to a weakly structured, broad band peaking at 568 nm in the film, probably caused by intermolecular interactions in the solid state. Comparing the monomer BPEA and the polymer PAAE, the overall absorption redshift upon going from the monomer to the polymer is only moderate, suggesting that the overall backbone conjugation is also rather modest. However, polymer formation is also accompanied by a pronounced spectral broadening of the absorption band, presumably as a result of a high degree of geometrical disorder within the polymer backbone. The measured PL quantum yields of the polymer PAAE are rather low: 0.072 in solution and 0.038 in films; for comparison, the values for BPEA are 0.93 in solution[40] and 0.19 in films.

By photoemission spectroscopy (see the Experimental Section), the HOMO energy level of PAAE was measured as E _HOMO = −5.49 eV. Together with an optical bandgap of E _g ^opt = 2.24 eV (calculated from the absorption onset of the film[44]), the energetic position of the LUMO was estimated to be E _LUMO = −3.25 eV.

A particularly interesting aspect regarding the optical properties of the PAAE polymer relates to the question of whether signatures of the two transition dipole moments of the molecule can be resolved. In a simplistic picture, one would expect one axis of polarization to be associated with transitions vertical to the backbone direction, along the axis of the anthracene moieties, and a second polarization component along the arylene ethynylene backbone. Bending of the polymer chain would be expected to limit the overall polarization anisotropy. It is particularly insightful to perform measurements of the loss in polarization memory as a function of time so as to resolve structural dynamics, rotational diffusion, and intramolecular electronic relaxation such as energy transfer from one part of the molecule to another.[45] Since the excited-state lifetime is dominated by non-radiative relaxation in the polymer, the PL lifetime is smaller than 1 ns. Using a conventional streak-camera system for picosecond polarization-resolved fluorimetry, we define the polarization memory as , where I indicates the fluorescence intensity, either parallel or orthogonal to the polarization plane of the excitation laser, and the factor G accounts for the polarization sensitivity of the grating used in the fluorimeter. The measurement setup is sketched in [Figure 3]. The anisotropy r can be measured either as a function of time, r(t), or as a function of emission wavelength, r(λ_em). A perfect dipole such as a dye molecule, randomly distributed in three dimensions, will generally yield an initial anisotropy value of r(t = 0) = 0.4,[25] which decreases with time because of rotational diffusion. The model monomer structure BPEA indeed behaves as such a near-perfect dipole, with r(t) dropping to zero within 250 ps after excitation by a femtosecond laser pulse. As would be expected, we find no linear dichroism in the luminescence, i.e. r(t) is the same over the entire emission spectrum. The situation is very different, and rather unusual, for the polymer PAAE as summarized in [Figure 3]. We begin by discussing r(t) in panel (a). Since the absorption spectrum shown in [Figure 2] is so broad, and presumably consists of contributions from both localized anthracene and delocalized PAAE chromophores, it is meaningful to alter the excitation wavelength used in the depolarization measurements while detecting the fluorescence in the spectral region of 500–550 nm. When exciting at 490 nm (red curve), r(t) begins at a substantial value of 0.3, dropping down to 0.15 within the measurement window of 2000 ps of approximately three times the fluorescence lifetime. Under this condition, a significant memory of the polarization plane of the incident laser is retained in the fluorescence and we propose that this case corresponds to direct excitation of the polymer backbone. The drop in r with time may arise from structural dynamics such as rotational diffusion, but could also relate to intrachain excited-state energy transfer in the bent polymer structure. As the excitation wavelength λ_ex is lowered from 490 nm to 400 nm, a dramatic change takes place, even though the PL spectra remain identical: the initial polarization anisotropy drops continuously with decreasing excitation wavelength, reaching zero within approximately the excited-state lifetime for λ_ex = 400 nm. Such a complete loss of polarization memory is unusual, even for the most distorted of polymer chains.[46] We propose that this complete depolarization arises from excited-state relaxation from an excited state of the polymer associated with an off-axis transition-dipole moment along the anthracene unit to the excited state associated with the PAAE backbone of the polymer. However, such intramolecular excited-state relaxation would be expected to be very fast and should occur within a few picoseconds. In contrast, a finite but small initial anisotropy of 0.05 is observed, which decays further over time, presumably due to rotation of the molecule in solution. The fact that such rotational diffusion can be observed, even for excitation of the anthracene unit, is consistent with the rather short chain length inferred for the polymer.

Figure 3 Sketch of the setup to measure the polarization anisotropy in the fluorescence of the polymer PAAE in toluene solution using vertically oriented polarization of the excitation laser and detecting either parallel or perpendicular emission polarization components. a) Temporal evolution of the polarization memory for different excitation wavelengths. The fluorescence was detected in a spectral window of 500–550 nm. b) Spectral dependence of the fluorescence depolarization, detected in a time window of 300 ps after photoexcitation.

It is insightful to examine the spectral dependence of the polarization anisotropy r(λ_em) as a function of λ_ex, shown in [Figure 3b]. As noted, for a monodisperse system such as a dye molecule, r(λ_em) shows a constant value over the entire PL spectra which decreases as r(t). However, if different molecular geometries are present in the sample and contribute to the spectral broadening both in the absorption and the emission, signatures of anomalous linear dichroism may arise. Indeed, for all λ_ex, we find that r(λ_em), recorded in the first 300 ps after photoexcitation, decreases with increasing λ_em. This unusual observation provides a clear indication of the heterogeneity of the polymer sample, presumably due to the high degree of polydispersity. Emission at longer wavelengths corresponds to longer conjugated segments, which in turn offer more opportunity for the polarization memory to be lost, for example due to distortions in the chain.

The measurement of r(λ_em) therefore offers a spectroscopic route of mapping out the origin of strong spectral broadening of the polymer spectra compared to the monomer. We note that, with a long-pass filter cutting off the luminescence above the maximum around 560 nm, perfectly unpolarized luminescence can be achieved for excitation at 400 nm. While this was not the original objective of our study, this observation appears to be quite unique in the context of the photophysics of conjugated polymers.

Conclusions

A soluble poly(9,10-anthrylene ethynylene), poly[2,6-(2-octyldecyl)-9,10-anthrylene ethynylene] PAAE, was made accessible with moderately low degrees of polymerization P_n of ca. 10 in a reductive, dehalogenative homocoupling scheme starting from a 2,6-dialkylated 9,10-bis(dibromomethylene)-9,10-dihydroanthracene monomer. By testing different potential coupling agents, it was found that only the n-BuLi/CuCN system allowed for PAAE synthesis. PAAE shows very broad, unstructured optical spectra, both in absorption and PL, with peak maxima at 506 and 611 nm, respectively, for chloroform solutions. The absorption tails into the 600–700 nm spectral region. Together with the large Stokes loss, these findings indicate a high degree of geometrical disorder of the chromophores that are incorporated into a highly distorted polymer backbone. This conclusion was confirmed by detailed time-resolved studies of the polarization anisotropy in fluorescence. The polarization memory was found to be strongly dependent on excitation wavelength, decreasing with decreasing excitation wavelength, thus providing an indication of orthogonal transition dipole moments within the polymer. In addition, the polarization memory decreases with increasing detection wavelength, which presumably probes molecules of greater extension and thus of greater structural disorder.

Experimental Section

All commercially available chemicals and solvents including dried solvents were obtained from the suppliers Fisher Scientific, Sigma-Aldrich Co., ABCR GmbH, VWR International GmbH, TCI Deutschland GmbH, chemPUR GmbH, Carl Roth GmbH + Co. KG, Santa Cruz Biotechnology Inc. and Merck KGaA, and were used without further purification. Acetonitrile was dried over calcium hydride and stored over a molecular sieve (3 Å). Reactions under argon atmosphere were carried out using standard Schlenk techniques and flame-dried glassware. Reactions were monitored by thin-layer chromatography using ALUGRAM^® SIL G/UV 254 silica gel plates from Macherey-Nagel with a thickness of 0.2 mm, the TLC plates visualized by UV light at 254 or 365 nm or by common staining reagents. Flash column chromatography was carried out on a Biotage (Isolera One) system with pre-packed silica gel columns from Büchi Labortechnik GmbH. ¹H- and ¹³C{H}-NMR spectra were recorded on a Bruker AVANCE 400 MHz- or AVANCE III 600 MHz-NMR spectrometers in deuterated solvents. All spectra were referenced to the residual solvent signal. Spin multiplicities were given as follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), dd (doublet of doublets). FD (field desorption) mass spectra were obtained from a JEOL AccuTOF-GCX spectrometer. GPC measurements were carried out on a PSS/Agilent SECurity GPC system equipped with a diode array detector (G1362A) and a refractive index detector (G1362A). Separation was carried out on a set of two PSS SDV Linear S columns (8 × 300 mm, particle size 5 µm) and a PSS SDV precolumn (8 × 300 mm, particle size 5 µm) at room temperature, using chloroform or THF as the eluents (flow rate: 1 mL/min) and polystyrene standards. UV/Vis absorption spectra were recorded on a Jasco V-670 spectrometer. The IR spectrum was prepared with a Jasco FT/IR 4700 spectrometer using an ATR unit. The Raman spectrum of a thin film of the polymer was measured with a confocal Raman microscope (MonoVista CRS+ from S&I), with 633 nm as the excitation wavelength. Thin films (thickness: 20 nm) were prepared by spin coating a polymer solution in chloroform (4 mg/mL) on a silicon substrate. To increase the Raman signal compared to the strong PL of the polymer, approximately 1 nm of silver was deposited by thermal evaporation on top of the polymer film. As a reference the same silver layer was also deposited on the pristine Si substrate. PL spectra were measured on a Horiba Scientific FluoroMax-4 spectrometer at room temperature. Quantum yields were obtained with an integrating sphere accessory (QuantaPhi). Spin-coated polymer films where obtained from 7 mg/mL solutions on quartz glass using a Süss MicroTec spin-coater (rotational speed: 1000 rpm). HOMO energy levels were determined by atmospheric pressure photoelectron spectroscopy on a Riken Keiki (AC-2) spectrometer. TGA and DSC were carried out with a Mettler Toledo TGA/DSC1 STAR-System under an argon stream of 50 mL/min and a heating rate of 10 K/min.

Picosecond fluorescence depolarization was measured using a frequency-doubled tunable femtosecond Ti:sapphire laser (Coherent, Chameleon Ultra II) for excitation and a streak camera (Hamamatsu c5680 series) coupled to a spectrometer (Brucker, 250is c68878) for detection. The analyte was dissolved in toluene solution and further diluted to rule out reabsorption effects during the measurements.

2,6-Dibromoanthraquinone 2

To a refluxing mixture of tert-butyl nitrite (8.4 mL, 63.6 mmol, 3.0 equiv), copper (II) bromide (11.91 g, 53.3 mmol, 2.5 equiv) and dry acetonitrile (350 mL) under an argon atmosphere, 2,6-diaminoanthraquinone 1 (5.0 g, 21.0 mmol, 1.0 equiv) was slowly added. After 4 hours the mixture was cooled to room temperature and poured into a 1 M aqueous hydrochloric acid solution. The crude, precipitated 2,6-dibromobromo-9,10-anthraquinone 2 was filtered off and the product purified by recrystallization from chloroform. The product was obtained as a light-yellow solid (6.74 g, 18.43 mmol, 88%).

¹H-NMR (600 MHz, CDCl₃, 323 K): δ [ppm] = 8.44 (2 H, d, J = 2.0 Hz), 8.17 (2 H, d, J = 8.3 Hz)_, 7.94 (2 H, dd, J = 8.3, 2.0 Hz). ¹³C-NMR (151 MHz, CDCl₃, 323 K): δ [ppm] = 181.4, 137.5, 134.7, 132.2, 130.6, 130.4, 129.3. MS (FD): m/z = 363.8816, calculated for [C₁₄H₆O₂Br₂]⁺ = 363.8735.

9-Methyleneheptadecane 4

60% Sodium hydride in mineral oil (0.79 g, 19.7 mmol, 1.0 equiv) was added to dry dimethyl sulfoxide (8 mL) under an argon atmosphere and heated to 75 °C for 20 minutes. Then, the mixture was cooled to 0 °C and triphenylmethylphosphonium bromide (7.02 g, 19.7 mmol, 1.0 equiv) in warm dimethyl sulfoxide (20 mL) was added. The resulting solution was stirred for 10 minutes at room temperature. Subsequently, 9-heptadecanone 3 (5.00 g, 19.7 mmol, 1.0 equiv) was added and the mixture stirred for 20 hours. Then, saturated aqueous ammonium chloride solution was added under cooling. The solution was extracted with pentane, dried over magnesium sulphate and evaporated. The crude product was purified by column chromatography (hexane) to yield product 4 as a colourless liquid (4.60 g, 18.2 mmol, 93%).

¹H-NMR (400 MHz, CDCl₃, 300 K): δ [ppm] = 4.69 (2 H, s), 2.00 (4 H, t, J = 7.6 Hz), 1.48–1.20 (24 H, m), 0.89 (6 H, t, J = 6.9 Hz). ¹³C-NMR (101 MHz, CDCl₃, 300 K): δ [ppm] = 150.6, 108.5, 36.3, 32.1, 29.7, 29.6, 29.5, 28.0, 22.9, 14.3. MS (FD): m/z = 252.2931, calculated for [C₁₈H₃₆]⁺ = 252.2817.

2,6-Bis(2-octyldecyl)anthraquinone 5

A 0.5 M 9-borabicyclo[3.3.1]nonane solution in THF (62 mL, 31.0 mmol, 2.7 equiv) was slowly added to 9-methyleneheptadecane 4 (7.13 g, 28.2 mmol, 2.5 equiv) under an argon atmosphere at room temperature. The mixture was stirred for 23 hours. Then, a 2.4 M aqueous potassium carbonate solution (10 mL, 23.5 mmol, 2.1 equiv), 2,6-dibromoanthraquinone 2 (4.14 g, 11.3 mmol, 1.0 equiv) and tetrakis(triphenylphosphino)palladium (0) (0.52 g, 0.45 mmol, 0.04 equiv) were added. The mixture was heated to 75 °C for a further 23 hours. Then, the reaction mixture was cooled down to room temperature, extracted with dichloromethane, dried over magnesium sulphate, and evaporated to dryness. The crude product was purified by column chromatography (eluent: hexane/CH₂Cl₂ 1/0 to 3/1). The product 5 was obtained as yellow oil (2.24 g, 3.14 mmol, 28%).

¹H-NMR (600 MHz, CDCl₃, 300 K): δ [ppm] = 8.21 (2 H, d, J = 7.9 Hz), 8.07 (2 H, d, J = 1.5 Hz)_, 7.55 (2 H, dd, J = 7.9, 1.7 Hz), 2.70 (4 H, d, J = 7.1 Hz), 1.72 (2 H, s), 1.37–1.15 (58H, m), 0.86 (12 H, t, J = 7.0 Hz). ¹³C-NMR (151 MHz, CDCl₃, 300 K): δ [ppm] = 183.6, 149.6, 135.0, 133.6 131.7, 127.7, 127.4, 41.1, 39.8, 33.4, 32.0, 30.1, 29.7, 29.5, 26.7, 22.8, 14.2. MS (FD): m/z = 712.6186, calculated for [C₅₀H₈₀O₂]⁺ = 712.6158.

9,10-Bis(dibromomethylene)-2,6-bis(2-otyldecyl)-9,10-dihydroanthracene 6

Dry toluene (5 mL) was added to triphenylphosphine (2.31 g, 8.79 mmol, 8.0 equiv) and tetrabromomethane (1.64 g, 4.95 mmol, 4.5 equiv) under an argon atmosphere. The mixture was stirred for 20 minutes at room temperature. Then, 2,6-bis(2-octyldecyl)-9,10-anthraquinone 6 (0.78 g, 1.10 mmol, 1.0 equiv) in toluene (5 mL) was added and the mixture was heated to 80°C for 24 hours. The suspension was cooled down to room temperature and filtered. The isolated solid was washed with toluene and the filtrate evaporated to dryness. The crude products were purified by column chromatography (eluent: hexane). Monomer 6 was obtained as colourless oil (0.97, 0.95 mmol, 86%).

¹H-NMR (600 MHz, CDCl₃, 300 K): δ [ppm] = 7.73 (2 H, d, J = 8.0 Hz), 7.60 (2 H, d, J = 1.5 Hz)_, 7.04 (2 H, dd, J = 8.0, 1.7 Hz), 2.52 (4 H, d, J = 6.8), 1.63 (2 H, m), 1.34–1.17 (58H, m), 0.88 (12H, t, J = 7.1 Hz). ¹³C-NMR (151 MHz, CDCl₃, 300 K): δ [ppm] = 141.1, 139.9, 135.9, 133.3, 128.4, 127.9, 127.4, 89.4, 40.6, 39.8, 33.5, 33.2, 32.1, 30.2, 30.1, 29.8, 29.7, 29.5, 29.4, 27.1, 26.9, 26.6, 22.8, 14.3. MS (FD): m/z = 1020.2935, calculated for [C₅₀H₈₀Br₄]⁺ = 1020.2994.

Poly[2,6-(2-octyldecyl)-9,10-anthrylene ethynylene] PAAE

A solution of the monomer 9,10-bis(dibromomethylene)-2,6-bis(2-otyldecyl)-9,10-dihydroanthracene 6 (1.34 g, 1.31 mmol, 1.0 equiv) in dry tetrahydrofuran (10 mL) was cooled to temperatures below −90 °C. Then, 1.6 M n-BuLi solution in hexane (1.64 mL, 2.62 mmol, 2.0 equiv) was slowly added. The solution was stirred for 1 hour. Subsequently, copper(I) cyanide (0.12 g, 1.31 mmol, 1.0 equiv) was added and the mixture allowed to slowly reach room temperature, over a period of c. 4 hours. The mixture was further stirred at room temperature for 18 hours. Then, the mixture was diluted with chloroform and washed twice with 25% aqueous ammonia solution. In the organic phase, most of the solvent was evaporated. The polymer was subsequently precipitated into acidified (2 M HCl) cold methanol, isolated by filtration, and washed with methanol. The crude product was purified by subsequent Soxhlet extractions (MeOH, acetone, EtOAc, CHCl₃). PAAE was obtained as a dark red, viscous mass (ethyl acetate fraction: 0.49 g, 0.70 mmol, 53%; chloroform fraction: 0.01 g, 0.01 mmol, 1%).

¹H-NMR (400 MHz, C₂D₂Cl₄, 353 K): δ [ppm] = 9.21–6.48 (m), 3.15–2.22 (m), 2.14–1.78 (m), 1.78–1.00 (m), 1.00–0.17 (m). GPC (THF) ethyl acetate fraction: M _n = 7100, M _w = 16600 g/mol; chloroform fraction: M _n = 13000, M _w = 44400 g/mol. UV/Vis CHCl₃ solution: λ_max. [nm] = 261, 502; film: λ_max. [nm] = 263, 506. PL CHCl₃ solution (λ_exc. [nm] = 500 nm): λ_max. [nm] = 611, PL quantum yield [PLQY] = 7.2%; film (λ_exc. [nm] = 500 nm): λ_max [nm] = 687, PLQY = 3.8%. HOMO/LUMO energies: E _HOMO [eV] = −5.49; E _LUMO [eV] = −3.25; E _g [eV] = 2.24.

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