Sequence-definition in stiff conjugated oligomers

The concept of sequence-definition in the sense of polymer chemistry is introduced to conjugated, rod-like oligo(phenylene ethynylene)s via an iterative synthesis procedure. Specifically, monodisperse sequence-defined trimers and pentamers were prepared via iterative Sonogashira cross-coupling and deprotection. The reaction procedure was extended to tetra- and pentamers for the first time yielding a monodisperse pentamer with 18% and a sequence-defined pentamer with 3.2% overall yield. Furthermore, three novel trimers with a 9H-fluorene building block at predefined positions within the phenylene ethynylene chain were synthesised in 23–52% overall yields. Hence, it was confirmed that a functionality of interest can be incorporated selectively at a pre-defined position of these monodisperse oligomers. All respective intermediate structures were fully characterised by proton and carbon NMR, mass spectrometry, size-exclusion chromatography, and IR spectroscopy. Additionally, thermal and optical transitions are reported for the different oligomers.


Table of Contents
Cyclohexane and ethyl acetate in technical grade were distilled before use.
Tetrahydrofuran in HPLC grade (≥99.7%, VWR Chemicals) was dried over sodium and subsequently distilled under argon. Benzophenone was used to indicate the abstinence of water and oxygen.

Characterisation
NMR spectra were recorded on a Bruker AVANCE DPX spectrometer operating at 300 MHz for 1  Thin layer chromatography (TLC) was performed on silica gel coated aluminum foil (silica gel 60 F254, Sigma-Aldrich). Compounds were visualized by UV at 256 and 365 nm.
Thermal properties were recorded via differential scanning calorimetry (DSC) with a Mettler Toledo DSC star e system operating under nitrogen atmosphere using approximately 3 mg of the respective sample.
Following method was used: heating from 25 to 200 °C with a heating rate of 20 K/min, cooling from 200 to 0 °C with a cooling rate of 20 K/min, an isothermic segment at 0 °C for 10 min and a further heating cycle from 0 to 200 °C with a heating rate of 20 K/min.
Optical attenuation was recorded on a UV/Vis spectrophotometer (LAMBD A1050, PerkinElmer), which was additionally equipped with an integrating sphere.
Steady-state photoluminescence spectra were taken from a 100 µMol concentrated solution at an excitation wavelength of 355 nm and with an excitation power of 300 µW. The Photoluminescence was spectrally dissolved by a spectrometer (Acton SpectraPro SP-2300, Princeton Instruments) and detected by a CCD-camera (PI-MAX4, Princeton Instruments).
Time-resolved photoluminescence spectra were recorded from the same solutions with a streak camera (C10910-02, Hamamatsu) operated in photon counting mode. For excitation, we used the frequencydoubled laser pulses at a wavelength of 380 nm from an 80 MHz titanium:sapphire laser (Chameleon UIltra II, Coherent) . Excitation power was 3 mW. The photoluminescence transients shown in this work are taken from the spectral regions of 420 to 620 nm.
Photoluminescence quantum yield (PLQY) values were measured in a home-built setup. The excitation laser (405 nm, DL5146-101S, Thorlabs Inc.) was directed in an integrating sphere (Labsphere) with a diameter of 15 cm. The luminescence was detected with a CCD spectrometer (AvaSpec-2048x64-TEC, Avantes). The measurement system (integrating sphere -optical fiber -spectrometer) was irradiance calibrated using a calibrating lamp (HL-3P, Ocean Optics). PLQY was measured according to a 3measurement procedure published elsewhere. [1,2] Measurement and calculation routine was automated using a Labview program. It should be pointed out that for the measured compounds 2-measurement and 3-measurement PLQY values were coinciding with 99% accuracy.

Synthesis of the Building Blocks
Synthesis of 1,4-dipropoxybenzene The Williamson Ether syntheses are based on a procedure published by H. Meier et al. [3] Hydroquinone (  Synthesis of 1,4-diiodo-2,5-dipropoxybenzene The iodinations are based on a procedure published by Park et al. [4] Periodic acid (3.20 g, 14.0 mmol, 0.636 eq.) was dissolved in 25 mL methanol and stirred for 10 minutes.