Photocatalytically active ladder polymers

Post-polymerization ladderization is explored as a promising technique to boost the photo-catalytic activity of conjugated polymers.


Experimental Section
Materials and methods: All reagents and dry solvents were obtained from Sigma-Aldrich or from TCI and used as received. 1,4-Dibromo-2,5-bis(methylsulfinyl)benzene (1) was synthesized according to a previously published procedure. 1 Water for the hydrogen evolution experiments was purified using an ELGA LabWater system with a Purelab Option S filtration and ion exchange column without pH level adjustment. Reactions were carried out under nitrogen atmosphere using standard Schlenk techniques. CHN Analysis was performed on a Thermo EA1112 Flash CHNS-O Analyzer using standard microanalytical procedures. Palladium content was determined on a Perkin Elmer ICP-MS NexION 2000 using a Perkin Elmer Microwave Titan for digestion of powdered samples in conc. nitric acid. Transmission FT-IR spectra were recorded on a Bruker Alpha at room temperature using an ATR diamond sample tip. Thermogravimetric analysis was performed on a Q500 TGA by heating (20°C min −1 ) samples under air (25 mL min −1 ) in open platinum pans up to 1000 °C. The UV-visible absorption spectra were recorded on a Shimadzu UV-2550 UV-vis spectrometer as powders in the solid-state. The fluorescence spectra of the polymer powders were measured with a Shimadzu RF-5301PC fluorescence spectrometer at room temperature. Imaging of the polymer morphology was performed on a Hitachi S4800 Cold Field Emission SEM, with secondary electron, backscatter and transmission detectors. EDX Measurements were performed on an Oxford Instruments INCA ENERGY 250 M/X. PXRD Measurements were performed on a PANalytical X'Pert PRO MPD, with a Cu X-ray source, used in high throughput transmission mode with Kα focusing mirror and PIXCEL 1D detector. Time-correlated single photon counting experiments (TCSPC) were performed on an Edinburgh Instruments LS980-D2S2-STM spectrometer equipped with picosecond pulsed LED excitation sources and a R928 detector, with a stop count rate below 3%. A 371.5 nm laser diode (instrument response 100 ps fwhm) was used. Suspensions were prepared by ultrasonicating the polymers in water (concentration is the same as used in the corresponding TA experiments). The instrument response was measured with colloidal silica (LUDOX® HS-40, Sigma-Aldrich) at the excitation wavelength and decay times were fitted in Fluoracle software, based on suggested lifetime estimates and pre-exponential factors.

Hydrogen evolution experiments:
A screw-top vial was charged with the polymer powder (ca. 1 mg mL -1 ) and a mixture of water/triethylamine/MeOH (1:1:1; 10 mL) and ultrasonicated until the photocatalyst was dispersed (30 min). The suspension was transferred into a quartz cuvette (2 × 4 × 1 cm, w × h × d), sealed with a septum and degassed by N 2 bubbling for 30 min. The reaction mixture was illuminated with a 300 W Newport Xe light-source (Model: 6258, Ozone free) for the time specified using appropriate filters. Gas samples were taken with a gas-tight syringe, and run on a Bruker 450-GC gas chromatograph equipped with a Molecular Sieve 13X 60-80 mesh 1.5 m × ⅛" × 2 mm ss column at 50°C with an argon flow of 40.0 mL min −1 . Hydrogen was detected with a thermal conductivity detector referencing against standard gas with a known concentration of hydrogen. Hydrogen dissolved in the reaction mixture was not measured and the pressure increase generated by the evolved hydrogen was neglected in the calculations. The rates were determined from a linear regression fit (see Section 8 for fits) and the error is given as the standard deviation of the amount of hydrogen evolved. No hydrogen evolution was observed for a mixture of water/methanol/trimethylamine under λ >295 nm illumination in absence of a photocatalyst. [4] External quantum efficiency measurements: The external quantum efficiencies of the polymer photocatalysts for hydrogen evolution were estimated using monochromatic light from LEDs controlled by an IsoTech IPS303DD power supply. The output of the LEDs was measured with a ThorLabs S120VC photodiode power sensor controlled by a ThorLabs PM100D Power and Energy Meter Console to be 1.80 mW for the λ = 375 nm LED, 3.18 mW for the λ = 420 nm LED, 1.95 mW for the λ = 515 nm LED, and 5.05 mW for the λ = 595 nm LED. For the experiments polymer (8 mg) was suspended in water, triethylamine, methanol (1:1:1 volume mixture) and an area of 8 cm 2 was illuminated (path length = 1 cm) and the amount of hydrogen was measured as described above. For cLaP1@Pt the sample was prepared, H 2 PtCl 6 solution was added and illuminated for 1 hour under λ > 295 nm illumination (300 W Xe light source). The sample was then degassed again and illuminated using a λ = 420 nm LED. The external quantum efficiencies were estimated using the equation below: Oligomer cLiP1 (400 mg) was added to cold trifluoromethanesulfonic acid (40 mL) in a roundbottom flask (100 mL) and stirred at room temperature for 22 h. The mixture was added to cold water (500 mL) with vigorous stirring to precipitate the product. The precipitate was filtered and washed with water several times. The methylated oligomer cLaP1 + -Me (336 mg) was obtained as a light brown powder, after drying in a vacuum overnight at room temperature. [5] The methylated oligomer cLaP1 + -Me (336 mg) was dissolved in a mixture of acetone/acetonitrile (80 mL, 1:1) at room temperature. Tetraethylammonium bromide (1.30 g) dissolved in an acetonitrile (10 mL) and water (2 mL) mixture was added to the oligomer cLaP1 + -Me solution and stirred vigorously at room temperature for 18 h. The yellow solid oligomer cLaP1 + -Me (223 mg), which precipitated as the result of demethylation, was collected by filtration, washed with water, and dried overnight at room temperature.
Oligomer cLaP1 + -Me (223 mg) was added to cold trifluoromethanesulfonic acid (15 mL) in a round-bottom flask and the reaction mixture was stirred at room temperature for 21 h. The reaction solution was poured into cold water (300 mL) with vigorous stirring to precipitate the product. The precipitate was filtered and washed with water several times. Oligomer cLaP1 was obtained as a brown powder (222 mg, 65%), after drying in a vacuum overnight at room temperature. Under inert conditions, a flask was charged with 3,7-dibromodibenzo[b,d]thiophene (1 g, 2.9 mmol, 1 eq), B 2 Pin 2 (1.6 g, 6.4 mmol, 2.2 eq.) and [Pd(Cl) 2 (dppf) 2 ] (64 mg, 3 mol%). Anhydrous DMSO (40 ml) and KOAc (1.7 g, 6 eq.) were added. The reaction mixture was heated to 80 °C for 28 hours. After cooling to room temperature, the mixture was poured onto ice and the precipitate was collected via filtration. The crude product was re-dissolved in ethyl acetate (120 ml) and filtered through a celite pad. The solvent was removed under reduced pressure. The beige product (965.2 mg, 75%.) was dried in vacuo at 40 °C for 48 hours. [8]   [12] [13] 4 Powder X-Ray Diffraction [14] [16] 6 Scanning Electron Microscope/Energy-dispersive X-ray spectroscopy cLiP1 cLaP1 cLaP2 Figure S22. SEM images for cLiP1, cLaP1 and cLaP2.   Figure S30. Hydrogen evolution of P60@Pt from a triethylamine/water/methanol mixture under λ >295 nm (left) and λ >420 nm (right) irradiation. [21] 9 External Quantum Efficiencies for cLaP1  Irradiation conditions and sample treatment run #1 8.3 mg, H2O/TEA/MeOH (1:1:1; 10 mL), degassed by bubbling N 2 , >295 nm cut-off filter, 300 W Xe lamp; Sample was irradiated for 7 hours, samples of the gas phase were taken hourly (black circles) and the hydrogen evolution rate was estimated via a linear fit function (red line). Sample was left irradiated overnight. A final sample of the gas phase was taken the next morning (blue square) to ensure that hydrogen evolution had continued overnight. Slight deviation from the linear fit function is expected due to increase in pressure and possible leakage through the punctured septum. Estimated HER: 1307 26 μmol h −1 g −1

±
Between run #1 and run #2, septum was exchanged and the sample was degassed.
run #2 sample from run #1, >295 nm cut-off filter, 300 W Xe lamp; Sample was irradiated for 8 hours, samples of the gas phase were taken hourly (black circles) and the hydrogen evolution rate was estimated via a linear fit function (red line). Sample was left irradiated overnight. A final sample of the gas phase was taken the next morning (blue square) to ensure that hydrogen evolution had continued overnight. Slight deviation from the linear fit function is expected due to increase in pressure and possible leakage through the punctured septum. Estimated HER: 1348 Between run #2 and #3, septum was exchanged and the sample was degassed.
run #3 sample from run #2, >295 nm cut-off filter, 300 W Xe lamp; Sample was irradiated for 6 hours, samples of the gas phase were taken hourly (black circles) and the hydrogen evolution rate was estimated via a linear fit function (red line). Sample was left irradiated overnight. A final sample of the gas phase was taken the next morning (blue square) to ensure that hydrogen evolution had continued overnight. Slight deviation from the linear fit function is expected due to increase in [23] pressure and possible leakage through the punctured septum. A lower hydrogen evolution rate was estimated. We attribute this to an exhaustion of the sacrificial donor mixture and possible "poisoning" due to accumulation of byproducts.  [25]    [27] [28] 13 (GFN/IPEA/sTDA)-xTB Calculations All calculations were performed using the semi-empirical density functional tight-binding approach, xTB, 8 recently developed for the rapid calculation of geometries and optoelectronic properties of large molecular systems. The structural optimisation method (GFN-xTB 8 ) is used to obtain optimized geometries from which ionisation potentials, electron affinities (obtained using IPEA-xTB 9 ) and excitation energies (obtained with sTDA-xTB 10 ) may be calculated. This latter method uses energy eigenvalues and wave functions obtained via xTB to calculate excited state properties. We have previously shown 11 that this semi-empirical approach, when a simple linear calibration procedure is performed using previously-obtained parameters, 11 produces absolute values of IP, EA and optical gap in excellent agreement with density functional theory.
For each polymer species (cLaP1, cLaP2 & cLiP1), we calculate IP, EA and optical gaps for varying lengths of oligomer chains (defined by the number of aromatic rings along the polymer backbone). This is done to ensure that converged values are obtained across both ladder and nonladder polymer species.