Synthesis and Optical Characterization of Hybrid Organic–Inorganic

Article pubs.acs.org/Macromolecules

Synthesis and Optical Characterization of Hybrid Organic−Inorganic Heterofluorene Polymers Valentin H. K. Fell,† Annabel Mikosch,† Ann-Kathrin Steppert,† Wojciech Ogieglo,† Erdem Senol,§ Damien Canneson,‡ Manfred Bayer,‡ Franziska Schoenebeck,§ Alex Greilich,‡ and Alexander J. C. Kuehne*,† †

DWI − Leibniz Institute for Interactive Materials, RWTH Aachen University, Forckenbeckstraße 50, 52076 Aachen, Germany Experimentelle Physik 2, Technische Universität Dortmund, 44221 Dortmund, Germany § Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany ‡

S Supporting Information *

ABSTRACT: We synthesize heterofluorene monomers with Si, Ge, N, As, Se, and Te occupying the 9-position of the fluorene motif, which are then polymerized by Suzuki coupling. The optical properties of the obtained polymers are investigated in their solid state. We compare and elucidate effects in the materials absorption, emission, quantum yield (Φ), and fluorescence lifetime. Moreover, we determine the refractive indices n and absorption coefficient k by variable angle spectroscopic ellipsometry (VASE). We show that in addition to already known C, Si, and N containing polyfluorenes also Ge and As containing polymers exhibit amplified spontaneous emission.

P

inorganic and organic materials represent heterojunctions. Charge transport across the inorganic/organic interface is often impeded by the topological, morphological, and energetic mismatch of the materials.34 Self-assembled monolayers can be applied to minimize the morphological constraints at the interface and direct crystal growth of the inorganic component.35 To achieve a more gradual transition between the interfaces of an organic and an inorganic semiconductor, conjugated polymers with incorporated heteroatoms could be applied to reduce interfacial energy. The heteroatoms should represent elements and metalloids commonly applied in main group IV, III−V, and IV−VI inorganic semiconductor materials. Polysilafluorenes (PSiF) and polycarbazoles (PCz) are relatively well studied and characterized; however, polygermafluorenes (PGeF) have been prepared but are hardly characterized.14,15 Whereas polyselenafluorene (PSeF) has been theoretically described and characterized using DFT theory,36 polyarsafluorenes (PAsF), polystannafluorenes (PSnF), and polytellurafluorene (PTeF) have never been synthesized or investigated with computational chemistry. Here we synthesize a variety of heterofluorenes, which are copolymerized with fluorene using Suzuki coupling. We characterize the optical properties of the hybrid inorganic/ organic polymers with respect to their absorption, spectral refractive index, photoluminescence, quantum yield, and photoluminescence lifetime. We also investigate their ability

olyfluorene homo- and copolymers are a powerful class of polymers for optoelectronic applications such as lasers,1 solar cells,2,3 transistors,4 and OLEDs.5 Polyfluorenes exhibit good quantum yield,6 large optical gain,7 and high chargecarrier mobility.8 The polymer solubility is mediated through alkylation of the C9 position, where incomplete alkylation promotes oxidation,10 leading to a shift in fluorescence to the green spectrum8 and a decrease in quantum yield.11 To prevent oxidation and improve the solid state quantum yield, the 9position can be replaced with silicon. Such polysilafluorenes are thermally stable and resistant toward oxidation.9,12 Other fluorene polymers where the carbon atom in the 9-position has been replaced for nitrogen,13 germanium,14,15 phosphorus,16 oxygen,17 and sulfur18 have been reported. Nitrogen containing polycarbazoles are well-known hole-transport (p-type) materials.19,20 Polycarbazoles are applied in solar cells,21 PLEDs,22 and transistors.23 Because of the nitrogen lone pair at the fluorene 9-position, the carbazole moiety is fully aromatic.20 For metalloids, DFT studies predict an interaction between the σ* antibonding orbital of the metalloid to carbon bond and the π* antibonding orbital of the butadiene fragment.24,25 This interaction extends the π-conjugation26 and improves the electron affinity and conductivity of the heterofluorene polymer.12 The higher the atomic number of the atom at the C9 position, the less effective is this σ*−π* conjugation.24 There exist several examples of organic−inorganic semiconductor hybrid materials, where Si27 and Ge28 nanoparticles and GaAs,29 GaN,30,31 CdSe,32 and PbTe33 nanocrystals have been compounded in conjugated polymers for photonic and electronic applications and devices.34 Interfaces between © XXXX American Chemical Society

Received: December 7, 2016 Revised: February 26, 2017

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DOI: 10.1021/acs.macromol.6b02611 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Synthesis of heterofluorene monomers: (a) Cu, DMF, 125 °C, 3.5 h; (b) Sn, EtOH/HCl, 100 °C, 2.5 h; (c) HCl/MeCN/H2O, NaNO2, −10 °C, 1 h, then KI, −15 °C, then 80 °C, 67 h; (d) n-BuLi, THF, −78 °C, 2 h; (e) Ph2SiCl2, THF, −78 °C, then RT, overnight; (f) Ph2GeCl2, THF, −78 °C, then RT, overnight; (g) CdI2, Et2O, −78 °C, 1 h; (h) AsCl3, Et2O, −78 °C, 1 h, then reflux, 4 h; (i) PhLi, Et2O, −78 °C, 0.5 h, then RT, overnight; (j) H3PO4, 180 °C, 46 h; (k) Cu, K2CO3, PhI, DMF, 140 °C, 8.5 h; (l) CuI, Na2Se, NMP, then addition of 2, 190 °C, 5.5 h; (m) CuI, Na2Te, NMP, then addition of 2, 190 °C, 17 h.

Figure 2. Synthesis of heterofluorene copolymers: Aliquat 336, 2 M aqueous K2CO3, Pd(PPh3)4, toluene, 100 °C, 2 days. Overview over the synthesized polymers. PspiroF: X = C, R = spiro; PSiF: X = Si, R = diphenyl; PGeF: X = Ge, R = diphenyl; PCzF: X = N, R = Ph; PAsF: X = As, R = Ph; PSeF: X = Se, R = O; PTeF: X = Te, R = O.

Table 1. Molecular and Optical Properties of the Heterofluorene Monomers and Polymers polymer

θDFT (deg)

λmax abs (nm)

λmax em (nm)

nmax

PspiroF PSiF PGeF PCzF PAsF PSeF PTeF

101.44 91.48 89.02 108.46 85.54 87.03 81.83

379 367 383 402 387 381 376

423 417 429 450 458 424

1.96 1.97 2.30 2.05 2.21 2.22

Φ (%) 17.18 18.74 21.98 7.13 10.89 11.37

± ± ± ± ± ±

0.07 0.08 0.05 0.08 0.04 0.04

Mw (Da)

Đ

S0−1 τ1 (ns)

S0−1 τ2 (ns)

× × × × × × ×

3.2 2.7 2.8 4.1 3.4 1.7 1.5

0.79 0.77 0.65 0.86 0.78 0.80

1.69 1.83 1.61 1.75 4.79 1.62

1.5 6.4 5.6 3.8 1.6 6.3 1.5

104 103 104 103 104 103 103

amount of naphthalene).42 After conversion of these slurries with copper(I) iodide in NMP, compound 2 is added and while heating to 190 °C. 2,7-Dibromo-9,9-selenafluorene (SeF) and 2,7-dibromo-9,9-tellurafluorene (TeF) are obtained respectively (see Figure 1).43,44 To obtain conjugated polymers from these heterofluorene monomers, we apply Suzuki coupling with 9,9dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester as comonomer (see Figure 2). As a control polymer we polymerize a carbon equivalent using 2,7-dibromospirofluorene (spiroF). The obtained polymers are purified by Soxhlet extraction, followed by reprecipitation in methanol. The molecular weight and conversion of the polymers are monitored using size exclusion chromatography (SEC) and 1 H NMR (see Supporting Information). SnF cannot be polymerized into a soluble polymer, probably because the stannyl moiety takes part in a competing Stille-type coupling reaction producing a cross-linked product, which is insoluble.45 All other polymers can be obtained as soluble powders with medium to high molecular weights ranging between 4 and 16 kDa. Only PTeF is obtained in lower molecular weight (see Table 1).

to emit laser light. The materials have acceptable molecular weights, can be processed from solution, and are stable in air.

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RESULTS AND DISCUSSION We start by synthesizing the heterofluorene monomers. Most of the heterofluorenes are accessible through lithiation of 4,4′dibromo-2,2′-diiodobiphenyl (2) using n-butyllithium (see Figure 1).11 2,7-Dibromo-9,9-diphenylsilafluorene (SiF), 2,7dibromo-9,9-diphenylgermafluorene (GeF), and 2,7-dibromo9,9-di-n-butylstannafluorene (SnF) are prepared by adding the respective organyl metalloid dichloride to 3 (see Figure 1).37 2,7-Dibromo-9-phenyl-9H-carbazole (Cz) is synthesized by stirring 4,4′-dibromo-2,2′-diamine (1) in hot phosphoric acid to induce ring closure,38 followed by Ullmann coupling with iodobenzene (see Figure 1).39,40 Since compound 2 is too reactive toward arsenic trichloride, it is first converted into a cadmium fluorene before addition of AsCl3.41 Phenyllithium is added to afford the 2,7-dibromo-9phenylarsafluorene (AsF) (see Figure 1). The chalcogen fluorenes are prepared from disodium selenide and disodium telluride, which are derived by stirring elementary sodium with elementary selenide or telluride in dry THF (with a small B

DOI: 10.1021/acs.macromol.6b02611 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. Absorption (open circles), emission (full circles), refractive index, and absorption coefficient of the synthesized polymers PSpiroF (black), PSiF (dark blue), PGeF (blue), PCzF (green), PAsF (orange), PSeF (red), and PTeF (dark red). The absorption and emission of the main group V are red-shifted in comparison to the main group IV polymers. The emission behavior of the main group VI polymers is similar to the main group IV polymers.

of an interacting lone pair, the absorptions of the main group VI heterofluorene polymers (PSeF and PTeF) resemble that of the group IV polyheterofluorenes. Whereas PSeF also shows a similar fluorescence profile as the group IV polyheterofluorenes, PTeF does not exhibit fluorescence (see Figure 3). This result is somewhat surprising; however, it is in agreement with previously reported undetectably low photoluminescence in polytellurophenes.50−52 One can speculate that tellurium incorporated in an organic backbone quenches the fluorescence. To investigate whether the heteroatoms have an impact on the fluorescence lifetime, we probe fluorescence lifetime on thin films of our material. The 0 → 0 transition usually has considerable overlap with the absorption profile in fluorene derived polymers, which is why we examine the 0 → 1 transition instead. The fluorescence lifetime in our heterofluorene polymers is governed by biexponential decay with time constants τ1 and τ2 (see Figure S3 in the Supporting Information). For the τ1 decay time in the singlet state the trend is analogous to the Φ. In group IV the decay times τ1 are below 1 ns and decrease with increasing atomic number. This is also valid for group V heterofluorenes. No trend is observable for the τ2 decay times, which describes the leveling of the fluorescence decay. To determine the influence of the heteroatom in the organic polymer on its refractive index n, we perform variable angle spectroscopic ellipsometry (VASE) of the different heterofluorene polymers on a silicon wafer. VASE measurements reveal that the maximum refractive index of the polymers is around two or higher. For the main group IV and V polymers, the refractive index increases with the main atomic number of the heteroatom, as can be seen in Table 1. The determined absorption coefficients k exhibit the same maxima as the UV− vis absorption spectra of the thin films (see Figure 3). The agreement between the VASE and UV−vis absorption measurements reflects the good quality of the polymer films in terms of homogeneity, roughness, and depolarization effects as well as consistency of the optical model applied to fit the VASE data (see Figure 3). Altogether PGeF and PAsF exhibit the highest refractive indices at their respective maximum fluorescence wavelength, the highest quantum yields and lowest

To characterize the new heterofluorene monomers, we perform DFT studies46 to examine the ground state geometries (see Figure S1 and the applied computational methods in the Supporting Information). With increasing atomic number the carbon−heteroatom−carbon bond angles θDFT decrease. This is due to the increased atomic radius and the resulting separation from the pentacycle leading to a more acute angle (see Figure S1 and Table 1). To investigate the spectral influence of the heteroatoms, we perform absorption and fluorescence spectroscopy on thin films spin-coated from toluene on quartz. The main group IV polymers (PspiroF, PSiF, PGeF) exhibit very similar absorption spectra and vibronically resolved fluorescence spectra (see Figure 3). The fluorescence quantum yield (Φ) increases with rising atomic number of the heteroatom in the 9-position of the fluorene moiety (see Table 1). This could originate from different polymer packing in the solid state. The larger heteroatom radius may leads to an increased intermolecular distance similar to what has been observed in (benzofurano)tetrels.47 Smaller intermolecular distances entail increased nonradiative decay.47 PCzF and PAsF do not show vibronic structure in their photoluminescence spectra, and the absorption and emission of the main group V polymers are redshifted in comparison to the main group IV polymers (see Figure 3).48 We hypothesize that these properties originate from the heteroatom featuring a lone pair. To test this hypothesis, we perform DFT calculations to investigate the HOMO and LUMO frontier molecular orbitals of heterofluorene trimers (fluorene−heterofluorene−fluorene). We find that the HOMOs are delocalized over the trimer, whereas the LUMOs are primarily localized at the heterofluorene moiety (see Figure S2). In case of the group V heterofluorene trimers, the LUMOs comprise the heteroatoms, which therefore contribute to π-conjugation. The bathochromic shift in absorption of PCzF is larger than for PAcF, suggesting a weaker interaction of the more diffuse 4p lone pair orbital of arsenic with the π-electron system than the 2p lone pair orbital of nitrogen. This assumption is substantiated by the DFT results (see Figure S2). The Stokes shift of PAsF is greater than for PCzF, which results in reduced self-absorption leading to an increased Φ.49 In general, the Φ of the main group V polymers is lower than for main group IV polymers. Because of the lack C

DOI: 10.1021/acs.macromol.6b02611 Macromolecules XXXX, XXX, XXX−XXX

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fluorescence lifetimes decay rates in their respective main groups (see Table 1). When pumped with nanosecond laser pulses, films of PGeF and PAsF exhibit amplified spontaneous emission (ASE), where the fluorescence spectrum of the conjugated polymer films collapses into narrow line emission (see Figure 4). The threshold for this behavior is as low as 20 J/

ACKNOWLEDGMENTS This work was performed in part at the Center for Chemical Polymer Technology CPT, which was supported by the EU and the federal state of North Rhine-Westphalia (Grant EFRE 30 00 883 02).

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cm2. As such, PGeF and PAsF represent potentially powerful new gain materials for organic−inorganic hybrid photonics and laser devices.

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CONCLUSION We investigate a series of polyheterofluorene-9,9-dioctylfluorene copolymers for their optical properties. While amplified spontaneous and laser emission have previously been observed in polyfluorenes,53 polycarbazoles,54 and polysilafluorenes,55 we have identified two new heterofluorene polymers, namely PGeF and PAsF, with superior optical properties and show that they exhibit ASE. In the future, these polymers could present important materials for interfacing inorganic semiconductors such as elemental Ge and III−V arsenides (GaAs, AlAs, InAs, etc.) with organic materials. This will pave the way for hybrid optoelectronic devices with improved properties, merging the best qualities of both the inorganic and the organic worlds. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02611. Experimental details and fluorescence lifetime plots (PDF)

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Figure 4. ASE of PGeF (blue) and PAsF (orange). For PGeF, the laser emission arises from the 0−1 vibronic transition.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.J.C.K.). ORCID

Franziska Schoenebeck: 0000-0003-0047-0929 Alexander J. C. Kuehne: 0000-0003-0142-8001 Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.macromol.6b02611 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b02611 Macromolecules XXXX, XXX, XXX−XXX