Spiro‐Bridged Ladder‐Type Oligo(para‐phenylene)s: Fine ... · Stefan Kowarik, Sylke Blumstengel, and Stefan Hecht* A set of ladder-type quaterphenyls with an incremental number

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Spiro-Bridged Ladder-Type Oligo(para-phenylene)s: Fine Tuning Solid State Structure and Optical Properties

Björn Kobin, Jutta Schwarz, Beatrice Braun-Cula, Moritz Eyer, Anton Zykov, Stefan Kowarik, Sylke Blumstengel, and Stefan Hecht*

A set of ladder-type quaterphenyls with an incremental number of spiro-biflu-orene units in the bridge positions as well as an in-plane bent quaterphenyl carrying all bridges on one and the same side of the ribbon are synthesized and characterized. While spiro-bifluorene substituents lead to bathochromi-cally shifted maxima in the UV–vis absorption spectra, this effect can be com-pensated by in-plane bending. The influence of different deposition techniques on the solid state structure is analyzed by X-ray diffraction of single crystals obtained by crystallization from solution as well as sublimation. An increasing number of spiro-bifluorene substituents are found to aid thin-film formation.

DOI: 10.1002/adfm.201704077

Dr. B. Kobin, J. Schwarz, Dr. B. Braun-Cula, Prof. S. HechtDepartment of Chemistry & IRIS AdlershofHumboldt-Universität zu BerlinBrook-Taylor-Str. 2, 12489 Berlin, GermanyE-mail: [email protected]. Eyer, Dr. A. Zykov, Prof. S. Kowarik, Dr. S. BlumstengelInstitut für PhysikHumboldt-Universität zu BerlinNewtonstr. 15, 12489 Berlin, Germany

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201704077.

separating charge transport from light absorption/emission processes by con-fining them to the better performing com-ponent, thus allowing for higher overall efficiencies.[15] The S0 → S1 transition of all-methylated L4P (R1 = R2 = CH3 in Scheme 1) matches the emission of ZnO quantum wells, and thus is suited for the production of resonant devices.[16–18] However, in practice, the photochemical inertness and the processability of L4P still remained an issue. Similarly to poly-fluorenes,[19] LPPPs suffer from parasitic green defect emission, which is mainly

related to oxidative degradation in general and especially at syn-thetic defect sites accompanied by distortion of the backbone planarity.[20–25] One way to lower the sensitivity toward degrada-tion is the substitution pattern. It was shown for polyfluorenes as well as for LPPPs that aromatic bridge substitution gives the most inert structures,[26–33] which we could recently evaluate in a quantitative fashion.[34] In the case of LPPPs, the most common motif for aromatic bridge substituents is the spiro-bifluorene. It was incorporated into linear polymers[35,36] and oligomers,[37,38] carrying the bridging moieties on alternating sides of the ribbon, as well as in bent trimers, which deviate from this “zig-zag” substitution pattern.[39–41] The influence of the spiro-bifluorene substitution on the solid state structure is rather pronounced, as the steric demand of the molecules has a tremendous impact on their supramolecular interactions. These effects may influence the ability to form extended high-quality thin films as well as the solid-state optical properties. The latter are determined by the position of the optical transi-tion dipoles with respect to each other and thus by the molec-ular packing.[42–44]

In this contribution, the synthetic routes to subsequently exchanged symmetry equivalent methyl groups by spiro-biflu-orene units in the linear L4P (Scheme 1a; cyclopenta[2,1-b:3, 4-b′]difluorene core, referred to as L4P in the following) are pre-sented. Related structures have been reported in refs. [37] and [38]; however, we strictly omitted solubilizing groups to guarantee vacuum processability, and formal methyl to spiro-bifluorene substitution was carried out in a very systematic way. In line with this, we discuss the influence of the spiro-substitution on the molecular packing in solid state, as judged by X-ray crystal structures, as well as the influence on the optical properties and thin-film formation. Furthermore, the potential of band gap engineering by synthesis of the bent isomer of LOPPs (Scheme 1b; cyclopenta[1,2-a:4,3-a′]difluorene core, referred

Chromophores

1. Introduction

In the field of organic electronics oligo- and poly(para-phe-nylene)s have attracted great interest for more than two dec-ades now. Because of their relatively wide band gap and their strong optical transitions, these materials have been employed as singlet emitters for blue organic light-emitting diodes early on.[1–5] Within this class of materials, Scherf and co-workers introduced the ladder-type poly(para-phenylene)s (LPPPs) by bridging each phenylene unit with a five-membered ring to overcome the limited processability of unsubstituted poly(para-phenylene). In contrast to polyfluorenes, these polymers pos-sess a very planar conjugated backbone.[6–8] This molecular design leads to a unique electronic structure and particularly attractive optical properties.[9–14]

Due to their narrow and strong optical transitions, LPPPs and the corresponding oligomers (LOPPs) are favorable building blocks for hybrid resonant opto-electronic devices, exploiting dipolar coupling to excitons in inorganic semiconductors. In such devices, it would, in principle, be possible to harness the unique features of both inorganic and organic materials by

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to as iso-L4P in the following) will be discussed. While corre-sponding indenofluorene trimers have been described in the literature,[39,40] a strategy for the on-purpose synthesis has not been described for the tetramer thus far. Finally, these deriva-tives need to be evaluated toward their applicability in reso-nant energy-transfer (light-emitting) devices.[15–17] Beyond a matching absorption wavelength and high emission quantum yield in solution, it is of utmost importance to preserve these advantageous properties in the solid state with a high density of chromophores in a pure, vacuum-deposited, smooth thin film.

2. Results and Discussion

2.1. Synthesis of Spiro-LOPPs

The synthesis of ladder-type oligo- or poly(para-phenylene)s involves two major steps: buildup of the para-phenylene back-bone by cross-coupling reactions and subsequent bridging by Friedel–Crafts type reactions. In case of the linear tetramer (Scheme 1a), it is suitable to couple two terminal phenyl units to a central fluorene unit. The functional units for the intra-molecular bridging reaction may either be located at the central fluorene or at the phenyl groups, the latter route being simpler but not applicable in every case.[18,45] Ideally, the bridging is the last step of the synthesis, since the rigid products typically pos-sess low solubility. However, postbridging functionalization is carried out here, as well. To obtain the spiro compounds, 2-lith-iobiphenyl is added to the corresponding ketones, and the inter-mediate carbinols are reacted in acetic acid/HCl as described already very early for 9,9′-spirobifluorene.[46]

The synthesis of L4P has already been described.[18,25] In a first attempt (Scheme 2) of synthesizing L4P-sp, 2,7-dibromospiro-bif-luorene 1 was used as central building block and coupled to boronic ester 2. The product was oxidized to the ester 3 and reacted with methyl lithium. The resulting alcohol was converted to the fully bridged structure using BF3 as its tetrahydrofuran (THF) complex. But after initial purification of the product, it turned out that unwanted isomers were also formed. Although the low regioselectivity in the bridging reaction is unusual, it is not unknown.[40]

However, in this particular case, the sub-stitution pattern favors very low selectivity. The isomeric mixture contained only 55% of the target isomer L4P-sp. Since quantitative purification was not possible with reasonable

effort, another synthetic strategy had to be employed. Once the isomerically pure ketone L4P-O was available (Scheme 3a),[25] the syn-thesis of L4P-sp became relatively facile and good yields were found for the introduction of the spiro group. L4P-sp2 was synthesized in the same manner from the diketone L4P-O2 (Scheme 3b). Initially, the synthesis of L4P-sp3 was also planned via a bridged

diketone, but due to solubility issues in the keto stage and the low regioselectivity in the case of L4P-sp, it was decided to follow a route that does not need further functionalization after the bridging step. This was achieved by using methyloxymethyl protected (9-(2-bromophenyl)-9H-fluoren-9-yl)methanol 6 as terminal building block[47] and 2,7-dibromospirobifluorene 1 as the central one (Scheme 3c). Cross-coupling and final intra-molecular Friedel–Crafts alkylation were carried out without purification of intermediates in relatively low yields. Although the purification is demanding, L4P-sp3 can be synthesized on a gram scale.

As it turned out, the introduction of the spirobifluorene units shifts their S0 → S1 transition off-resonance with respect to the narrow excitonic features of ZnO and hence the need for slightly blue-shifted derivatives emerged. For indenofluorene derivatives, it was already shown that the relative position of the bridges shifts the absorption spectra.[39] Furthermore, the UV–vis absorption traces of analytical liquid chromatography (LC) runs of the isomeric mixture 4 showed a blue-shift for the unwanted isomer of L4P-sp. For that reason, an on-purpose syn-thesis for derivatives with the cyclopenta[1,2-a:4,3-a′]difluorene (iso-L4P) core was developed (Scheme 4). The desired regioi-somer may either be synthesized by attaching the bridging functionalities at the central fluorene unit in 1- and 8-positions or by protecting the usual 3- and 6-positions of the fluorene and using terminal phenyl units with the bridging function-ality. Here, it was decided to use the latter one and bromine was employed as protecting group. 3,6-dibromo-9,9-dimethyl fluorene 7 was diiodinated in 2- and 7-positions and subsequent cross-coupling with the phenylboronic ester prepared from 9

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Scheme 1. Structures of target molecules.

Scheme 2. Attempted synthesis of L4P-sp.



employing Pd(PPh3)4 as catalyst in THF and aqueous Na2CO3 gave a high selectivity for iodine. Diether 10 was converted to the dik-etone 12 by common methods of subsequent oxidation, saponification, and Friedel–Crafts acylation. Direct reaction of the dibromodik-etone 12 with 2-lithium biphenyl followed by a Friedel–Crafts alkylation gave 13 only in very low yields. However, bromine can be removed in a three-step sequence of reduc-tion of the ketones to the corresponding alcohol, halogen metal exchange, quenching with a proton source, and re-oxidation of the hydroxyl groups to the diketone iso-L4P-O2, which can finally be converted to iso-L4P-sp2 in 60% yield.

2.2. X-Ray Structures and Molecular Packing

In the solid state, intermolecular interactions have significant impact on the (opto-electronic) performance of a mate-rial. For the optical properties, mainly

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Scheme 3. Synthesis of linear tetramers: a) L4P-sp, b) L4P-sp2, and c) L4P-sp3.

Scheme 4. Synthesis of bent tetramer iso-L4P-sp2.



the distance, displacement and angle between the transi-tion dipole moments are of importance, which are expected to be parallel to the molecular long axis of the spiro-L4Ps. For simple crystal structures (usually Z = 2), stacks of rod-like organic π-conjugated molecules can be characterized by pitching (displacements along the long molecular axis) and rolling (displacement along the short molecular axis).[44,48] In π-stacked and herringbone arrangements with zero or low pitch, formation of H-aggregates may occur, accompanied by a hypsochromic shift in absorption and a bathochromic shift in emission as well as reduced radiative rates. Upon increasing the pitch angle, the interaction changes to J-aggregates, in which the lower energy transition is allowed. This leads to a bathochromic shift in absorption and emission as well as to increased radiative rates.[44] In case of hybrid devices as described above, the type of excitonic coupling is of great importance, as light emitting devices can have enhanced per-formance due to the formation of J-aggregates. However, any strong detuning of the absorption is supposed to reduce the energy-transfer efficiency.

For the three keto intermediates as well as for nearly all final products (except iso-L4P-sp2), single crystals of sufficient quality to conduct X-ray crystal structure analysis were obtained by different techniques. In the case of iso-L4P-sp2 only the bro-minated derivative 13 could be crystallized. In some cases, even for the same molecule, different molecular packing arrange-ments, i.e., different space groups, were obtained by different crystallization techniques. In the following, the structures of the different derivatives are discussed, including the molecular packing for some examples.

2.2.1. Ketone Intermediates

The π-system of the three ketones (Figure 1) is very planar in the crystal, except for L4P-O, which displays a slight twist along the long axis of the π-system. L4P-O was crystallized from chlo-robenzene at elevated temperatures with a space group of P-1 (Z = 2) in a layer structure. It forms stacks of molecules with the keto group pointing in alternating directions (see Figure 1b). The π-systems are not exactly on top of each other with respect to the short axis and the stacks have a pitch distance of roughly one-fourth of the molecular length. L4P-O2 was crystallized by sublimation in the space group Pna21 (Z = 4). The molecules are also forming stacks with an antiparallel orientation of the keto groups but with a pitch distance of half a molecular length. Stacks of opposite pitch are forming a herringbone structure with an angle of about 42° between the planes. iso-L4P-O2 was crystallized from chlorobenzene in P-1 (Z = 2). The molecular packing is similar to L4P-O, but with a larger pitch. While the CO-bonds in L4P-O2 are nearly parallel, in iso-L4P-O2, they are tilted by 36.7° with respect to each other.

2.2.2. Molecular Structures of Spiro-LOPPs

The molecular geometry of the final products in the crystal (Figure 2) deviates in most cases from the ideal structure con-cerning the planarity of the molecular backbone as well as the angle between the spiro-bifluorene units (defined by the planes through the 3, 6, and 9-carbon of the fluorenyl group) in the 12- and 15-positions in L4P-sp2 and L4P-sp3.

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Figure 1. Molecular structures (based on single crystal X-ray diffraction). Asymmetric units (left) and arrangement of molecules in the crystal (right) of a,b) L4P-O; c,d) L4P-O2; and e,f) iso-L4P-O2. The thermal ellipsoids are drawn at 50% probability level. Cell edges are marked in the following colors: a: red, b: green, and c: blue.



While in L4P (Figure 2a), the backbone is relatively planar, L4P-sp (Figure 2b) displays an s-shaped deformation over the whole length of the molecule. L4P-sp2 has a relatively planar backbone showing only small twist and bending, no matter if crystallized from chloroform/ethanol solution (Figure 2c) or by sublimation (Figure 2d). In the first case, the angle between the spiro groups amounts to 6.8° and in the latter one it is 13.6°. In crystals of L4P-sp3 grown by sublimation (Figure 2f), the back-bone shows strong distortions, especially the outer methylene carbons moved out of plane giving rise to a twist throughout the entire backbone. The angle between the outer spiro groups amounts to 17.5°. When crystals of L4P-sp3 were grown from chloroform/ethanol, the backbone is not twisted, only showing s-shaped bending and the outer spiro groups are nearly parallel (Figure 2e). Finally, compound 13 shows a distorted backbone as well (Figure 2g). Mainly the outer bridging methylene car-bons are twisted versus the central one. The angle between

the spiro groups amounts to 17.9°, which is only half of the expected value, compared to the angle between the carbonyl bonds in iso-L4P-O2. This deformation might be explained by repulsion between the methyl groups and the spiro groups (the distance between the fluorenyl plane and the methyl carbon is 3.26 Å).

2.2.3. Arrangement of the Chromophores

L4P was found to crystallize from THF in the space group Pbca with Z = 8, as already described by Kobin et al.[45] The molecular packing is rather complex. An evaluation of excitonic coupling in the solid state has recently been published. It was shown that nearest neighbor interactions only give very weak H-aggregates.[49] L4P-sp crystallizes from dioxane in P-1 (Z = 2). The molecular backbones are more or less parallel but the

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Figure 2. Molecular structures (based on single crystal X-ray diffraction). Asymmetric units, view along the short axis of the π-system of a) L4P (from THF), b) L4P-sp (from dioxane), c) L4P-sp2 (from CHCl3/ethanol), d) L4P-sp2 (from sublimation), e) L4P-sp3 (from CHCl3/ethanol), f) L4P-sp3 (from sublimation), and g) 13 (from CHCl3/ethanol). Thermal ellipsoids are drawn at the 50% probability level. Solvent molecules are omitted.



packing is not particular dense and π,π-distances are typically larger than 4 Å.

Luckily, for the two derivatives, L4P-sp2 and L4P-sp3, single crystals were not only obtained by crystallization from solution, but also by sublimation (with only little temperature gradient), which is of greater relevance in terms of application in hybrid devices fabricated by vacuum deposition. For L4P-sp2, solution-based crystallization was achieved from chloroform/ethanol. In this case, L4P-sp2 crystallizes in P-1, as well. All molecular backbones are oriented parallel without π-stacking. By subli-mation L4P-sp2 crystallizes in P21/c (Z = 4). In this structure, herringbone-like interactions can be found similar to L4P. The angle between the backbone planes of adjacent molecules is 88.5° and the long axis is tilted by 19.1° with respect to each other.

L4P-sp3 was also crystallized from chloroform/ethanol. In this case, the space group of C 2/c was found. Also for this compound, the arrangement of the chromophores is quite similar: all the backbones are parallel to each other and there is no direct π-stacking (Figure 3a,b). Upon sublimation L4P-sp3 crystallized in P21/c (Z = 4). The packing may be interpreted as a layer structure (Figure 3c,d). In each layer the backbones are rotated out of plane around the long axis of the molecule by 27°. After every second layer, there is a rotation of about

67° between the layers, and thus between the long axes of the molecules (Figure 3d). This structure is quite advantageous in terms of the above-mentioned application in a hybrid device, because it can be expected that there is some transition dipole moment in every direction. Furthermore, there is no π-stacking or herringbone structure. Thus, the optical properties in the crystalline state do not change apart from a small red-shift (see Figure S2 in the Supporting Information).

2.3. Optical Properties in Solution

Before discussing the optical properties of the spiro-L4Ps, it is worth having a look at the keto intermediates. It turns out that iso-L4P-O2 exhibits an unusual behavior. The absorption and emission spectra of iso-L4P-O2 (Figure 4) are compared to L4P-O2, which was already discussed elsewhere.[26] The linear L4P-O2 shows a vibronically structured L4P-like absorption starting around 3.4 eV (π → π*) and a very weak low energy band between 2.5 and 3.3 eV (n → π*) as well as a low energy emission with less than 1% quantum yield. In strong contrast, iso-L4P-O2 does not possess an L4P-like absorption feature around 3.5 eV. The π → π* absorption is hypsochromically shifted by about 0.2 eV compared to the linear isomer. The only

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Figure 3. Molecular arrangement of L4P-sp3 in the crystal grown from CHCl3 solution a) view along stacks and b) along short molecular axis; and by sublimation c) view perpendicular to layers and d) parallel to layers. Substituents and solvent molecules are omitted for clarity. Unit cell edges are marked in the following colors: a: red, b: green, and c: blue. Thermal ellipsoids are drawn at the 50% probability level.


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feature that is similar is the absorption band above 4 eV, which might be assigned to some rather localized excitation of the carbonyl moieties. Especially, the low energy transition shows unusual behavior. On the one hand, the absorption becomes relatively strong (ε ≈ 6000 L mol−1 cm−1 between 2.6 and 2.7 eV). On the other hand, the emission centered at 2.2 eV has a photo-luminescence (PL) quantum yield of ≈22%. Both findings indicate that the normally symmetry forbidden n → π* transi-tion becomes partly allowed due to distortion of the molecular geometry by the introduction of the 36.7° tilt angle between the carbonyl groups, while the backbone maintains its planarity.

In general, it can be stated, that the influence of the intro-duction of the spiro-bifluorene substituents on the optical prop-erties of the linear compounds is rather small. The spectra of L4P, L4P-sp, L4P-sp2, and L4P-sp3 (Figure 5a) exhibit nearly

the same shape with two main maxima due to vibronic pro-gression in the absorption and emission (see also Table 1). The Stokes shift is small (around 40 meV) and the fluorescence quantum yield ΦPL of all only-hydrocarbon derivatives is close to unity. However, with every additional spiro unit, absorption and emission are shifted bathochromically by about 2 nm. This bathochromic shift potentially leads to detuning with respect to the emission of ZnO quantum wells, demanding slightly blue-shifted products.[50–52] Accordingly, bent iso-L4P-sp2 was synthe-sized compensating for the spiro-induced red-shift as described above and indeed the absorption maximum of iso-L4P-sp2 is blue-shifted by 4 nm with respect to L4P (Figure 5b, Table 1). The Stokes shift is still small and the fluorescence quantum yield is also about unity. The absorption coefficient of the first maximum decreases to 73 000 L mol−1 cm−1 and the second (vibronic) maximum has higher intensity. Consequently, this set of new chromophores enables a very precise matching of the resonance wavelength in resonant hybrid devices.

2.4. Thin-Film Growth

L4P, L4P-sp2, and L4P-sp3 were grown on ZnO and Al2O3 sur-faces by organic molecular beam deposition under ultra-high vacuum (UHV) conditions and the growth was monitored by atomic force microscopy (AFM) as well as X-ray reflectivity (XRR) using Cu Kα radiation. Deposition of L4P on ZnO (and other oxide surfaces) results in the formation of 3D nano-aggregates (Figure 6a), independent of the substrate tempera-ture, which was varied between 90 and 300 K. High surface diffusivity and a strong tendency of L4P to crystallize favor the formation of large clusters with an average height of ≈250 nm. The deposited amount of material corresponds to a nominal film thickness of 6 nm only. Such heterogeneous films are obvi-ously not suited for the use in opto-electronic devices. Spiro-annulation of L4P changes the growth mode significantly. Both L4P-sp2 and L4P-sp3 display 2D Frank–van der Merwe growth[53] when deposited at 300 K (Figure 6b,c). The presence of the organic layer was evidenced by performing an AFM scan in contact mode in a selected area applying sufficient force to break up the closed film (see page S3 in the Supporting Infor-mation). When stored under inert conditions (N2 or vacuum), no postgrowth restructuring of the film morphology is observed even for very thin films (3 nm). However, when exposed to air, thin L4P-sp2 layers break up rapidly and form holes (Figure 6d), which increase in diameter over time. By attaching a third

Figure 4. Absorption (solid lines) and normalized photoluminescence (PL, dotted) spectra of L4P-O2 (black, λex = 330 nm) and iso-L4P-O2 (red, λex = 337 nm) in CH2Cl2 (10−6–10−5 mol L−1). The PL spectrum of L4P-O2 is only shown below 2.6 eV.

Figure 5. Absorption (solid lines) and normalized PL (dotted) spectra of final products 10−6–10−5 mol L−1 in CH2Cl2. a) L4P (black, λex = 352 nm), L4P-sp (blue, λex = 352 nm), L4P-sp2 (green, λex = 352 nm), and L4P-sp3 (red, λex = 360 nm). b) iso-L4P-sp2 (blue, λex = 320 nm), L4P (black, λex = 352 nm), and L4P-sp3 (red, λex = 360 nm).

Table 1. Absorption and photoluminescence data of the spiro-LOPPs.

λmaxa)

[nm]εmax

b) [103 M−1 cm−1] λ [nm]

λemc)

[nm]λex

d) [nm]

L4P 369, 351 104 (369) 373, 394 352

L4P-sp 370, 352 91 (370) 375, 397 352

L4P-sp2 372, 353 104 (372) 375, 398 352

L4P-sp3 374, 355 98 (374) 377, 400 360

iso-L4P-sp2 365, 346 73 (365) 370, 391 320

a)λmax: Absorption maxima; b)εmax (λ): Absorption coefficient and corresponding wavelength; c)λem: Emission maxima; d)λex: Corresponding excitation wavelength.



spiro group, the dewetting process can significantly be slowed down. We did not detect changes in the film morphology of 3 nm thick layers of L4P-sp3 when kept under ambient condi-tions for up to one day and thicker films are stable even over the course of several days, indicating that L4P-sp3 is especially suited for thin-film applications. The absence of Bragg reflec-tions in the XRR measurements performed on a ≈20 nm thick film of L4P-sp3 indicates that the layer is amorphous (Figure 7).

The large number of pronounced Kiessig oscillations[54] points toward a smooth film morphology also for thicker films.

3. Conclusion

Synthetic routes have been developed that allow us to subse-quently replace every pair of symmetry-equivalent alkyl groups in ladder-type quaterphenyl by a spiro-bifluorene group. With an increasing number of spiro groups, the optical gap for absorption and emission slightly decreases, which is disad-vantageous with respect to resonant energy transfer with ZnO. Thus, a synthetic route to a para-linked ladder-type quater-phenyl carrying all bridging units on one side of the ribbon (iso-L4P) was developed, which results in an in-plane bending of the para-phenylene. Indeed, the optical gap increased com-pared to the linear molecule, however, the absorption coefficient slightly decreased. Nearly all derivatives have been analyzed by single crystal X-ray structure analysis. In the cases of L4P-sp2 and L4P-sp3, it could even be shown that sublimation and crys-tallization from solution result in different crystal structures, of which the ones from sublimation are obviously more relevant in view of the typically employed vacuum deposition and might be advantageous in terms of application in light-emitting devices. Additionally, it was demonstrated that the introduction of spiro-bifluorene groups is advantageous for the formation and the stability of thin films on oxide surfaces, such that the optical properties could be preserved in pure, nondiluted thin films. Finally, promising spiro-L4P derivatives (mainly L4P-sp3) have

Figure 6. AFM topology (height) images of different LOPPs. a) L4P on ZnO. The nominal thickness is 6 nm. b) L4P-sp2 (3 nm) on ZnO measured in situ 2 h after deposition and storage in UHV. c) L4P-sp3 (3 nm) on ZnO recorded after exposure to air for 3 h. d) L4P-sp2 (3 nm) on ZnO measured after exposure to air for 40 min. All films were deposited at 300 K.

Figure 7. X-ray reflectivity scan of a ≈20 nm thick L4P-sp3 layer deposited on ZnO.



been employed in energy-transfer devices, for which highly effi-cient energy transfer from an inorganic quantum well to the organic layer followed by efficient light emission could success-fully be demonstrated.[51,52]

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsGenerous support by the German Research Foundation (DFG via SFB 951) and the Helmholtz Association (via Helmholtz Energy Alliance) is gratefully acknowledged.

Conflict of InterestThe authors declare no conflict of interest.

Keywordschromophores, hybrid materials, ladder oligomers/polymers, resonant energy transfer, small molecule emitters

Received: July 20, 2017Revised: August 18, 2017

Published online: October 16, 2017

[1] Organic Light Emitting Devices (Eds: K. Müllen, U. Scherf), Wiley-VCH, Weinheim, Germany 2006.

[2] J. Huber, K. Müllen, J. Salbeck, H. Schenk, U. Scherf, T. Stehlin, R. Stern, Acta Polym. 1994, 45, 244.

[3] G. Leising, S. Tasch, C. Brandstätter, W. Graupner, S. Hampel, E. J. W. List, F. Meghdadi, C. Zenz, P. Schlichting, U. Rohr, Y. Geerts, U. Scherf, K. Müllen, Synth. Met. 1997, 91, 41.

[4] R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. D. Santos, J. L. Brédas, M. Logdlund, W. R. Salaneck, Nature 1999, 397, 121.

[5] U. Mitschke, P. Bäuerle, J. Mater. Chem. 2000, 10, 1471.[6] U. Scherf, K. Müllen, Makromol. Chem., Rapid Commun. 1991, 12,

489.[7] J. Grimme, U. Scherf, Macromol. Chem. Phys. 1996, 197, 2297.[8] A. C. Grimsdale, K. Müllen, Macromol. Rapid Commun. 2007, 28,

1676.[9] J. Grimme, M. Kreyenschmidt, F. Uckert, K. Müllen, U. Scherf, Adv.

Mater. 1995, 7, 292.[10] T. Pauck, H. Bässler, J. Grimme, U. Scherf, K. Müllen, Chem. Phys.

1996, 210, 219.[11] E. J. W. List, J. Partee, J. Shinar, U. Scherf, K. Müllen, E. Zojer,

W. Graupner, G. Leising, Synth. Met. 2000, 111, 509.[12] J. Rissler, H. Bässler, F. Gebhard, P. Schwerdtfeger, Phys. Rev. B

2001, 64, 045122.[13] F. Schindler, J. Jacob, A. C. Grimsdale, U. Scherf, K. Müllen,

J. M. Lupton, J. Feldmann, Angew. Chem. Int. Ed. 2005, 44, 1520.[14] Q. Zheng, S. K. Gupta, G. S. He, L.-S. Tan, P. N. Prasad, Adv. Funct.

Mater. 2008, 18, 2770.

[15] V. M. Agranovich, Y. N. Gartstein, M. Litinskaya, Chem. Rev. 2011, 111, 5179.

[16] S. Blumstengel, S. Sadofev, C. Xu, J. Puls, F. Henneberger, Phys. Rev. Lett. 2006, 97, 237401.

[17] S. Blumstengel, S. Sadofev, C. Xu, J. Puls, R. L. Johnson, H. Glowatzki, N. Koch, F. Henneberger, Phys. Rev. B 2008, 77, 085323.

[18] B. Kobin, L. Grubert, S. Blumstengel, F. Henneberger, S. Hecht, J. Mater. Chem. 2012, 22, 4383.

[19] S. Gamerith, C. Gadermaier, U. Scherf, E. J. W. List, Phys. Status Solidi 2004, 201, 1132.

[20] W. Graupner, M. Sacher, M. Graupner, C. Zenz, G. Grampp, A. Hermetter, G. Leising, in Electrical, Optical, and Magnetic Prop-erties of Organic Solid-State Materials IV (Eds: J. R. Reynolds, A. K.-Y. Jen, M. F. Rubner, L. Y. Chiang, L. R. Dalton), Materials Research Society, Warrendale, PA, USA 1998, p. 789.

[21] E. J. W. List, J. Partee, J. Shinar, U. Scherf, K. Müllen, E. Zojer, K. Petritsch, G. Leising, W. Graupner, Phys. Rev. B 2000, 61, 10807.

[22] L. Romaner, G. Heimel, H. Wiesenhofer, P. Scandiucci de Freitas, U. Scherf, J.-L. Brédas, E. Zojer, E. J. W. List, Chem. Mater. 2004, 16, 4667.

[23] L. Liu, S. Qiu, B. Wang, W. Zhang, P. Lu, Z. Xie, M. Hanif, Y. Ma, J. Shen, J. Phys. Chem. B 2005, 109, 23366.

[24] L. Liu, S. Qiu, B. Wang, H. Wang, Z. Xie, Y. Ma, J. Phys. Chem. C 2009, 113, 5799.

[25] B. Kobin, F. Bianchi, S. Halm, J. Leistner, S. Blumstengel, F. Henneberger, S. Hecht, Adv. Funct. Mater. 2014, 24, 7717.

[26] W.-L. Yu, J. Pei, W. Huang, A. J. Heeger, Adv. Mater. 2000, 12, 828.[27] S. Setayesh, A. C. Grimsdale, T. Weil, V. Enkelmann, K. Müllen,

F. Meghdadi, E. J. W. List, G. Leising, J. Am. Chem. Soc. 2001, 123, 946

[28] J.-H. Lee, D.-H. Hwang, Chem. Commun. 2003, 2836.[29] D. Vak, C. Chun, C. L. Lee, J.-J. Kim, D.-Y. Kim, J. Mater. Chem. 2004,

14, 1342.[30] Y. Wu, J. Li, Y. Fu, Z. Bo, Org. Lett. 2004, 6, 3485.[31] D.-H. Hwang, M.-J. Park, J.-H. Lee, Mater. Sci. Eng. C 2004, 24, 201.[32] J. Jacob, S. Sax, M. Gaal, E. J. W. List, A. C. Grimsdale, K. Müllen,

Macromolecules 2005, 38, 9933.[33] L. Liu, S. Tang, M. Liu, Z. Xie, W. Zhang, P. Lu, M. Hanif, Y. Ma,

J. Phys. Chem. B 2006, 110, 13734.[34] B. Kobin, S. Behren, B. Braun-Cula, S. Hecht, J. Phys. Chem. A 2016,

120, 5474.[35] Y. Wu, J. Zhang, Z. Fei, Z. Bo, J. Am. Chem. Soc. 2008, 130, 7192.[36] B. Wang, M. Forster, E. Preis, H. Wang, Y. Ma, U. Scherf, J. Polym.

Sci., Part A: Pol. Chem. 2009, 47, 5137.[37] Y. Wu, J. Zhang, Z. Bo, Org. Lett. 2007, 9, 4435.[38] N. Cocherel, C. Poriel, J. Rault-Berthelot, F. Barrière, N. Audebrand,

A. M. Z. Slawin, L. Vignau, Chem. – Eur. J. 2008, 14, 11328.[39] D. Thirion, C. Poriel, J. Rault-Berthelot, F. Barrière, O. Jeannin,

Chem. – Eur. J. 2010, 16, 13646.[40] D. Thirion, M. Romain, J. Rault-Berthelot, C. Poriel, J. Mater. Chem.

2012, 22, 7149.[41] M. Romain, D. Tondelier, J.-C. Vanel, B. Geffroy, O. Jeannin,

J. Rault-Berthelot, R. Métivier, C. Poriel, Angew. Chem. Int. Ed. 2013, 52, 14147.

[42] J. Gierschner, Y.-S. Huang, B. van Averbeke, J. Cornil, R. H. Friend, D. Beljonne, J. Chem. Phys. 2009, 130, 044105.

[43] F. C. Spano, Acc. Chem. Res. 2010, 43, 429.[44] J. Gierschner, S. Y. Park, J. Mater. Chem. C 2013, 1, 5818.[45] B. Kobin, L. Grubert, S. Mebs, B. Braun, S. Hecht, Isr. J. Chem.

2014, 54, 789.[46] R. G. Clarkson, M. Gomberg, J. Am. Chem. Soc. 1930, 52, 2881.[47] T. Kowada, T. Kuwabara, K. Ohe, J. Org. Chem. 2010, 75, 906.[48] M. D. Curtis, J. Cao, J. W. Kampf, J. Am. Chem. Soc. 2004, 126, 4318.



[49] B. Dänekamp, B. Kobin, S. Bhattacharyya, S. Hecht, B. Milián-Medina, J. Gierschner, Phys. Chem. Chem. Phys. 2016, 18, 16501.

[50] M. Höfner, B. Kobin, S. Hecht, F. Henneberger, ChemPhysChem 2014, 15, 3805.

[51] F. Bianchi, S. Sadofev, R. Schlesinger, B. Kobin, S. Hecht, N. Koch, F. Henneberger, S. Blumstengel, Appl. Phys. Lett. 2014, 105, 233301.

[52] R. Schlesinger, F. Bianchi, S. Blumstengel, C. Christodoulou, R. Ovsyannikov, B. Kobin, K. Moudgil, S. Barlow, S. Hecht, S. R. Marder, F. Henneberger, N. Koch, Nat. Commun. 2015, 6, 6754.

[53] F. C. Frank, J. H. van der Merwe, Proc. R. Soc. London, Ser. A 1949, 198, 1053.

[54] H. Kiessig, Ann. Phys. 1931, 402, 769.

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