Beyond p‐Hexaphenylenes: Synthesis of Unsubstituted p‐Nonaphenylene by a Precursor Protocol

Abstract The synthesis of unsubstituted oligo‐para‐phenylenes (OPP) exceeding para‐hexaphenylene—in the literature often referred to as p‐sexiphenyl—has long remained elusive due to their insolubility. We report the first preparation of unsubstituted para‐nonaphenylenes (9PPs) by extending our precursor route to poly‐para‐phenylenes (PPP) to a discrete oligomer. Two geometric isomers of methoxylated syn‐ and anti‐cyclohexadienylenes were synthesized, from which 9PP was obtained via thermal aromatization in thin films. 9PP was characterized via optical, infrared and solid‐state 13C NMR spectroscopy as well as atomic force microscopy and mass spectrometry, and compared to polymeric analogues. Due to the lack of substitution, para‐nonaphenylene, irrespective of the precursor isomer employed, displays pronounced aggregation in the solid state. Intermolecular excitonic coupling leads to formation of H‐type aggregates, red‐shifting emission of the films to greenish. 9PP allows to study the structure–property relationship of para‐phenylene oligomers and polymers, especially since the optical properties of PPP depend on the molecular shape of the precursor.


General Remarks
The 1 H NMR and 13 C NMR spectra were recorded on a Bruker AVANCE 300 and Bruker AVANCE 500. For calibration the residual solvent peak was referenced. [1] Solid-state NMR CP/MAS experiments were performed with a Bruker AVANCE III console operating at 700 MHz 1H Lamor frequency using a commercial double resonance MAS probe supporting zirconia rotors with 2.5 mm outer diameter. The High-power frequency -swept TPPI hetero-nuclear dipolar decoupling of 100 kHz rf nutation frequency was applied during 13 C acquisition. Thermogravimetric analysis and differential scanning calorimetry -analyses were done using a Mettler Toledo TGA/DSC1 device. Thin-films were made via spin-coating with a Spin Coater SCV-10 on quartz glass or Si-wafers. Layer-thicknesses of thin-films were determined using a profilometer (DektakXT, Bruker). Solid-state UV-Vis absorption and emission spectra were recorded at room temperature on a Jasco V660 and FP6500 spectrophotometer of thin-films on glass substrates. Microscope images were taken with a Nikon Eclipse LV100POL polarization microscope. AFM images were taken on a Bruker Nanoscope MultiMode VIII in the ScanAsyst PeakForce mode. The resulting AFM measurements were treated with the software Gwyddion for row alignment and scar correction. Melting points were determined on a Büchi B-545 hot stage apparatus. Precursor solutions were spun-cast at 1000 rpm for 50 s in a nitrogen glove box with both water content and oxygen concentration below 3 ppm. Film thicknesses were determined via Vis ellipsometry and a Bruker DekTak XT profilometer. Films for mass analysis of 9PP were prepared using drop coating and measured on a SynaptG2-Si using 7,7,8,8-tetracyanoquinodimethane (TCNQ) as matrix. The matrix was evaporated on the films using a heat plate under air. To ensure full aromatization of the thicker films S2 the films were heated at 300 °C for 5 h. Sample preparation: For all IR spectroscopic measurements, silicon wafers (1.5 x 1.5 cm 2 , intrinsic, σ > 5000 Ω -1 cm -1 ) with a native oxide layer were used as substrate. The substrates were cleaned prior to fabrication via sonication in acetone and isopropanol and dried using a dry nitrogen stream.
Subsequently, the backside of the substrate was cleaned with CHCl3. Thermal aromatization was achieved on standard thermal hotplates in nitrogen atmosphere inside the glovebox.
Infrared (IR)-Spectroscopy: All samples for IR transmission measurements were measured at room temperature using a Fourier-transform IR spectrometer (Vertex 80v) from Bruker. The spectrometer was evacuated to 3 mbar to prevent absorption from ambient air (water and CO2). Spectra were recorded at near normal incidence (7 °) oras specifiedat 70° incident light. All spectra were recorded with an MCT detector, a resolution of 4 cm -1 and averaged over 200 scans.
Time resolved photoluminescence was determined on a Horiba FluoroCube-01-NL lifetime spectrofluorometer with emission monochromator.
The heating of the thin films was done stepwise in between measurements on a hotplate. After the specified time the sample was removed from the hotplate and put on a piece of aluminum at room temperature to prevent further heating and falsifying the time scale. Heating up and cooling down to under 50 °C took below 30 s, for 300 °C below 1 min.
Density functional theory (DFT) calculations were carried out with Gaussian 09 [2] under use of the B3LYP [3][4] functional and the basis set 6-311G(d,p). To speed up this process a single reduced molecule (p-pentaphenylene) was calculated in gas phase.
Simulations were carried out with SCOUT. [5] Details of the crystal data and a summary of the intensity data collection parameters are listed at the end of this chapter. Suitable crystals were measured with Bruker Smart CCD diffractometer. Graphite-monochromated Mo Kα radiation was used. The structures were solved by direct methods with SIR-97 and refined by the full-matrix least-squares techniques against F2 (SHELXL-97). The intensities were corrected for Lorentz and polarization effects. The nonhydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. The crystals structures were visualized using Mercury.

Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC)
The annealing process was studied by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the bulk precursor oligomers. Figure S1: DSC and TGA-trace of 9PPsyn,pre (left) and 9PPanti,pre (right) with a heating rate of 10 K/min. The TGA trace shows a loss in mass of the precursor oligomer at about 228-288 °C for the syn-isomer and 228-271 °C for the anti-isomer. This mass loss is attributed to demethoxylation to yield the corresponding phenylene units. Due to possible encapsulation of bulk material and the hindrance of conformational change from oligomer coils to rod-like oligomer the mass loss is slightly less than expected for both isomers (for syn-isomer: calc: 21.3%, observed: 18.2%; for anti-isomer: calc: 21.3%, observed: 20.0%;). However, the deviation of the expected vs. the observed mass loss for its polymeric analogues is much higher (calc: 21.38%, observed: 12.55%). Nevertheless, thin spin-coated thin films of the precursor materials (oligomer and polymer) show quantitative aromatization as evidenced by IR spectroscopy.

S4
2.2 Dewetting Process of 9PPsyn,pre Figure S2: Illustration of the dewetting process as a result of the different shapes of the rotamers (syn-vs anti-isomer) and their impact on the packing. Heating below the aromatization temperature leads either to sole rearrangements (anti-isomer) or to dewetting (hole formation in the films as starting point for dewetting precess). [6][7][8][9][10] Figure S3: Polarization micrographs of in situ-heating 9PPanti,pre. Before heating (a) and after gradually heating to 300 °C (b) the film appear identical. Only a drift of the whole sample is visible due to the hardware of the experiment. Figure S4: Polarization micrographs while heating 9PPsyn, pre. Before heating (a) the film shows only small holes and a scratch used for focusing. At 150 °C (b) the dewetting has already started. At 175 °C (c) it is already quite severe and at 300 °C (d) only bubbles remain.

Infrared Spectroscopy
While heating to 300 °C 9PPanti,pre did keep a closed film wheras 9PPsyn,pre started dewetting at temperatures as low as 120 °C, see Figure S3 and Figure S4. Despite the dewetting of the films IR measurements were still possible because our IR beam is about 3 mm in diameter averaging the signal over islands and holes. This led to a wavy baseline and further artefacts in the spectra ( Figure S12), which could still be used to roughly compare them to their 9PPanti,pre counterparts and to sketch the evolution over time ( Figure S13). Table S1. Band assignment of 9PPanti, pre. δ and ν correspond to bending and stretching vibrations, respectively. The experimental wavelength νexp is compared with the predicted wavelength νtheo from DFT calculations. From the latter the vibration can be assigned. Index "end" in the vibrational mode indicates that the vibration is mostly from the outermost phenyl ring, index "me" refers to vibrations only present in the methoxylated precursor and not in the converted 9PPanti. The orientation is given as inplane (ip) or out-of-plane (oop) if assignment was possible.
Number Figure S9 Table S2.  Figure S14: Time-resolved infrared spectroscopy of 9PPsyn, pre annealed at 250 °C (top) and 300 °C (bottom). As for 9PPanti, pre precursor specific absorption bands gradually decrease. Due to dewetting IR-measurement was stopped after 3 min.  Figure S15: Degree of conversion from 9PPsyn, pre to 9PPsyn at different temperatures. Dewetting leads to distortion especially at the beginning.

Mass analysis of thin films
Drop-coated films (2 mg/mL, chloroform) were prepared using 9PPanti,pre as precursor.
The films were thermally annealed at 300 °C and coated with a thin layer of TCNQ (7,7,8,8-tetracyanoquinodimethane) as a MALDI matrix. Due to aggregation effects of 9PP molecules, high laser intensities were needed for desorption during mass analysis, also resulting in partial fragmentation during the measurement, the ratio of which was highly dependent on the exact spot used for desorption. Under the same conditions, analysis of 9PPanti,pre films was unsuccessful -measurements resulted in fragmentation, some of the observed m/z ratios being identical to peaks observed for analysis of the 9PPanti films ,but no precursor molecular peak was found. Thus, we conclude that 9PP is indeed formed with our thermal aromatization procedure.  Lifetimes were fitted with a tris-exponential function. While the emission maxima at 485 nm, 520 nm and 560 nm show nearly the same lifetimes and fractions (both 9PPanti and 9PPsyn), the lifetimes for the weak emission at 430 nm are significantly shorter. In accordance with literature, [11,12] emission bands at 485, 520, 560 nm can be assigned to aggregates and the one at 430 nm to another species, most likely resulting from single-strand emission, as in thin-films more than one emitting species is likely to be present. The same trend was previously found for para-hexaphenylen. [13,14] Figure S19: Lifetime measurements of 9PPanti/syn in thin film.

Solid-State NMR Spectra
Solid-State 13 C NMR spectra of 9PPsyn/anti,pre and 9PPsyn,anti, obtained from thermal aromatization in the bulk at 300 °C for 5 h. The 13 C spectra of 9PPsyn,anti shows good agreement with other reported para-phenylenes. [15] Fitting of the 13 C signals in the product spectra confirms the chain length for both, 9PPsyn and 9PPanti, of 9 phenylene units.