Supplementary Information Light Upconversion by Triplet-Triplet Annihilation in Diphenylanthracene-Based Copolymers

Low-power light upconversion by triplet–triplet annihilation (TTA-UC) was only recently achieved in glassy materials. Here, a new strategy based on covalent tethering of diphenylanthracene (DPA) emitters to a polymeric backbone is reported. The design aims to optimize the efficiency of this photophysical process in glassy polymeric materials by increasing the emitter content. To that end, DPA molecules were covalently attached to a methacrylate-type monomer and further copolymerized with methylmethacrylate (MMA). Green-to-blue (543 to 440 nm) upconversion was observed at power densities as low as 32 mW cm−2 in films prepared by solution casting and compression molding (co)polymers containing 8–72 wt% of DPA and palladium octaethyl porphyrin (PdOEP) as a sensitizer (0.03–0.7 wt%). The upconversion intensity was studied as a function of DPA and PdOEP contents and the results suggest that upconversion is optimal for DPA and PdOEP weight fractions of 34 and 0.05 wt% respectively.

Refractive Index Detector using GPC grade tetrahydrofuran (THF) as an eluent with 250 ppm BHT as inhibitor at a flow rate of 1 mL/min.Scheme S1 Synthesis of the DPA-containing monomer DPAMA.In a microwave tube, 9bromo-10-phenylanthracene (1.25 g, 3.7 mmol, 1 eq.), 4-(3-hydroxypropyl) benzeneboronic acid (0.74 g, 4 mmol, 1.1 eq.) and K 2 CO 3 (3 g, 21.7 mmol, 6 eq.) were suspended in a mixture of Electronic Supplementary Material (ESI) for Polymer Chemistry This journal is © The Royal Society of Chemistry 2014 S3 benzene (10 mL) and H 2 O (5 mL).The slurry was purged with argon for 15 min and Pd(PPh 3 ) 4 (21 mg, 0.017 mmol, 0.005 eq.) was then added to the mixture.The vessel was sealed and placed in the microwave synthesizer.After 2 min of pre-stirring, the content was heated to 150 °C at a pressure of 3 bar for 15 min, and cooled down to 50 °C.The suspension was then filtered through a fritted glass filter and the grey precipitate was washed with dichloromethane (DCM, 5 mL).DCM (10 mL) was added to the filtrate and the organic phase was extracted with brine (2 x 25 mL) and water (2 x 25 mL).The organic layers were collected and concentrated in vacuo.The crude solid was then purified by FCC (hexane:DCM = 3:1, R f = 0.35).A light yellowish solid was obtained in a yield of 65 % (930 mg); 1

3-[4-(10-Phenylanthracen-9-yl)phenyl]propyl methacrylate (DPAMA).
In a 250 mL twonecked round-bottom flask under argon, DPA(CH 2 ) 3 OH (0.9 g, 2.3 mmol, 1 eq.) and triethylamine (0.48 mL, 3.47 mmol, 1.5 eq.) were dissolved in dry THF (70 mL) and cooled to 0 °C.Methacryoyl chloride (0.34 mL, 3.47 mmol, 1.5 eq.) was added dropwise to the reaction mixture.The solution was allowed to warm up to r.t. and stirred for 1 day.After evaporation of the solvent, the off-white powder obtained was purified by FCC (hexane:ethylacetate = 5:1 elution, R f = 0.65) in a yield of 60 % (630 mg) 1  General procedure for the synthesis of poly(DPAMA-co-MMA) by free radical polymerization.Example for polymer 2 In a 5 mL Schlenk tube under argon, MMA(296.04 mg, 2.96 mmol), DPAMA(450 mg, 0.99 mmol), and recrystallized AIBN(1.08 mg, 0.0066 mmol) were dissolved in dry toluene (2 mL).After degassing under argon for 30 min in an ice bath, the mixture was placed in a pre-heated oil bath at 70 °C and the reaction was allowed to proceed at this temperature for 5 h.The content of the tube was subsequently precipitated dropwise into 200 mL of r.t.methanol.A solid polymer was collected after centrifugation at 3500 rpm for 15 min with a yield of 55 %.The composition of copolymers was determined by 1 H NMR spectra by comparing the integrations of the benzylic protons of DPAMA at δ = 4.0 ppm with the ester protons of both DPAMA and MMA at δ = 3.5 ppm (Fig. S6-S10).
Thermal Studies.Thermogravimetric analyses (TGA) were performed on a Mettler-Toledo TGA STAR instrument by heating the (co)polymer from 25 °C to 500 °C under nitrogen (N 2 ) at 10 °C/min (Fig. S17).Differential scanning calorimetry (DSC) experiments were performed on Mettler-Toledo STAR DSC instrument by heating the samples from 25 °C to 220 °C, cooling from 220 °C to 0 °C, and heating again to 250 °C under N 2 atmosphere.All experiments were conducted with heating or cooling rates of 10 °C/min (Fig. S18-S23).The glass transition temperatures (T g ) of the (co)polymers measured by DSC on second heating cycle with heating rate of 10 °C/min were compared to the T g calculated by the Fox and Flory equation (Equation 1, Fig. S24). 2 Materials processing.Method A: Solution casting.Poly(DPAMA-co-MMA) (30 mg) and PdOEP (0.05 wt%) were dissolved in ~1 mL of toluene.The solution mixture was drop-cast onto Electronic Supplementary Material (ESI) for Polymer Chemistry This journal is © The Royal Society of Chemistry 2014 S5 a glass slide and the solvent was evaporated at 50 °C in a vacuum oven (0.05 bar) for 2 days.The resulting films were used directly for subsequent photoluminescence analysis.The thickness of the resulting films, measured using a digital caliper (Millimess Inductive Digital comparator extramess 200, Mahr) was between 80-193 µm (Table S1).
Method B: Compression molding.Poly(DPAMA-co-MMA) (30 mg) and PdOEP (0.03 %, 0.05 % and 0.18 wt%) were dissolved in ~1 mL of toluene.The solution mixture was drop-cast onto a glass slide, and the solvent was evaporated at 105 °C by placing the glass slide onto a hot plate.
The resulting film was covered with a second glass slide and placed in a Carver® press at 160 °C applying light pressure.After compression-molding for 5 min, the films were delaminated from the glass slides and their thickness, measured using the aforementioned caliper, was between 110 and 245 µm.(Table S1).
Method C: Spin-coating.Poly(DPAMA-co-MMA) (10 mg) dissolved in DCM (0.5 mL) was drop-cast and spin-coated onto a glass slide at 2500 rpm for 3 min.The thicknesses of the thin films were determined by scratching the samples with a razor blade and measuring the depth using a JPK Nanowizard II atomic force microscope (AFM) with a SPM Controller III in air and tapping mode (AC mode) with a silicon nitrate tip cantilever (Nominal spring constant of 40 N/m and resonance frequency of 300 kHz) and the recorded thicknesses varied between 500-600 nm (Fig. S25).
Optical Experiments.The UV-Vis absorption and conventional photoluminescence spectra of the (co)polymers were determined on films that were prepared by spin-coating (Method C).The optical absorption spectra of the resulting thin films were recorded with a Shimadzu UV-2401PC UV-vis spectrophotometer.Their fluorescence spectra were measured by excitation at 370 nm Electronic Supplementary Material (ESI) for Polymer Chemistry This journal is © The Royal Society of Chemistry 2014 S6 using a solid sample geometry in which the emission was detected (at an angle of 90 ° from the incident light) with a spectrofluorometer from Photon Technology International.
Upconverted fluorescence emission was measured in air by exciting the upconverting films mounted on glass slides prepared by solution-casting (Method A) and compression-molding (Method B) at 543 nm with a green HeNe laser (2.0 mW, 320 mW/cm 2 ) from Thorlabs, and recording emission spectra.A green HeNe line filter at 543 nm from Thorlabs was used to ensure monochromatic excitation at 543 nm.The upconverting films were placed in a solid state sample holder at a 40 ° angle from the laser excitation source and detection was recorded at 90 ° from the incident light.The power dependence of the upconverted fluorescence versus excitation intensity was explored by varying the excitation intensity between 0.2 and 1.26 mW (measured with a power meter PM 100USB from Thorlabs) using a series of neutral density filters to attenuate the transmission from the HeNe laser.

Small-angle X-ray scattering (SAXS).
The spectrum was obtained by using a NanoMax-IQ camera (Rigaku Innovative Technologies, Auburn Hills, MI USA).The sample was kept at room temperature in vacuum during the measurements.Raw data were processed according to standard procedures, and the isotropic scattering spectrum is presented as a function of the momentum transfer q = 4π•λ -1 •sin (θ/2), where θ is the scattering angle and λ = 0.1524 nm is the photon wavelength.

S9
The DPAMA/MMA ratio in the copolymers (2-6) was determined by 1       Fig.S29 Small-angle X-ray scattering (SAXS) spectra of films of copolymers 3 (containing 34 wt% DPA, red curve) and 6 (8 wt% DPA, black), both films are doped with 0.05 wt% of PdOEP and prepared by Method B. The scattering curves do not exhibit any feature that could be related to a well-defined phase segregation.Instead, an apparent power like regime is present up to q = 2 nm -1 , strongly indicating rather disordered structural heterogeneities, and on the corresponding length scale there is not sign of aggregation.The intensity upturn beyond q = 2 nm -1 is related to the so-called amorphous halo.Fig. S32 Emission spectra of compression-molded films comprising (co)polymers, 1-6 and 0.05 wt% of PdOEP upon irradiation with a laser at 543 nm.Delayed upconverted fluorescence was observed between 400-520 nm for all films.Phosphorescence was observed exclusively in case of the film of copolymer 6.       S2.The data was converted into a 3D matrix on Origin 8.5 using xyz Random Gaussian template which is based on Random (thin plate spline) gridding method with 50x50 grids and extrapolation.The resulting matrix was plotted in 3D color map surface without smoothing, and coordinating wt% DPA on x-axis, wt% PdEOP on y-axis and upconversion intensity normalized to thickness on z-axis.For all the wavelengths monitored a significant decrease in fluorescence to ca. 1/3 intensity was found for the sample containing 0.33 wt% PdOEP, even when irradiating at 340 nm, efficiently absorbed by DPA only.Strong decrease of DPA-fluorescence regardless of the irradiated species (DPA or DPA and PdOEP) indicates formation of exciplexes or other non-radiative relaxation pathways induced by the presence of the sensitizer, e.g.energy transfer from the emitter to the sensitizer.
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Fig. S25 S22Fig. S28 S24
Fig. S25 Atomic force microscopy (AFM) image of a longitudinal section of the scratched spincoated DPAMA homopolymer film.The average of its thickness is 600 nm.

Fig. S30
Fig. S30 Optical micrographs of films of copolymers 3 (containing 34 wt% DPA) (a-b) and 6 (containing 8 wt% DPA) (c-d).Both films were prepared by Method B and are doped with 0.05 wt% of PdOEP.Pictures (a) and (c) were taken under parallel polarizers, while pictures (b) and (d) were taken under crossed polarizers.The scale bar corresponds to 20 μm (a-b) and 10 μm (cd).

Fig. S33
Fig.S33(a) Upconverted emission spectra of solution-cast (co)polymer films excited with HeNe laser at 543 nm with variation of the excitation power density from 80 mW/cm 2 to 320 mW/cm 2 .(b) Double logarithmic plots of the data shown in (a), and a least square fit of the data (slope = 1.7).

Fig. S34
Fig. S34 Upconversion intensity normalized to the thickness as function of wt% of PdOEP and DPA based on TableS2.The data was converted into a 3D matrix on Origin 8.5 using xyz Random Gaussian template which is based on Random (thin plate spline) gridding method with 50x50 grids and extrapolation.The resulting matrix was plotted in 3D color map surface without smoothing, and coordinating wt% DPA on x-axis, wt% PdEOP on y-axis and upconversion intensity normalized to thickness on z-axis.

ElectronicS31Fig. S36
Fig. S35Normalized upconverted emission intensity of films of copolymer 3 and 0.05 wt% PdOEP upon continuous wave excitation at 543 nm (320 mW/cm 2 ), monitored at 445 nm.The sample was prepared with Method B and a stable upconversion intensity under ambient conditions found when measuring over the course of 3.5h.

Table S2
Maximum upconversion intensity in counts/s as a function of PdOEP and DPA content in wt%.