The polymerization of the crosslinkable polymers and the non-crosslinkable reference materials was conducted by using Suzuki polycondensation. A generic reaction scheme is depicted in Scheme 4. The biphasic reaction system of toluene and aqueous Na2CO3 is thoroughly degassed before the catalyst Pd(PPh3)4 is added. To eliminate boronic acid ester and bromine terminal groups, the polymer is endcapped by bromobenzene and phenylboronic acid. Removal of boronic acid and bromine end groups is important to achieve a good low bandgap polymer. Residual boronic acid groups are prone to condensation. This might cause issues concerning the solubility of the polymer and molar mass determination.[138] Bromo endgroups severely influence the electronic properties of a conjugated polymer.
Scheme 4. Suzuki polycondensation for the synthesis of PFDTBT based low bandgap polymers. Conditions:
toluene:water, Pd(PPh3)4, Na2CO3, aliquat 336, RF, 4 d, endcapping with bromobenzene and phenylboronic acid.
In the following section the characterization of a series of fluorene based low-bandgap polymers is described. An important aspect of polymer characterization in this work is the comparison of the crosslinkable PFDTBTOx and the reference material PFDTBT in order to find out if the incorporation of oxetane groups has any detrimental effects on the chemical, thermal, and optical properties of the polymers. Figure 28 compares the SEC traces, TGA, and the absorption and emission spectra for the crosslinkable PFDTBTOx and the reference polymer PFDTBT. The data of both polymers are summarized in Table 5.
Figure 28. Characterization of the reference polymer PFDTBT and the crosslinkable low bandgap polymer PFDTBTOx. a) Chemical structures of PFDTBT and PFDTBTOx; b) SEC traces, eluent: THF, polystyrene calibration;
c) thermogravimetric analysis, 10 K min-1, N2; d) absorption and fluorescence spectra from THF solutions (c = 10-3 g mol-1).
SEC measurements (Figure 28b) of the raw polymers before Soxhlet extraction show comparable molecular weights, with number average molecular weights (𝑀𝑛) in the range of 15,000 g mol-1. This corresponds to a degree of polymerization of about 15. From the SEC there is no hint for any influence on the Suzuki polycondensation caused by the oxetane groups. Also, the steric demands of the dodecyl and the hexyloxy-oxetane chains are comparable, as both consist of twelve carbon atoms. Both materials show more than sufficient thermal stability. In nitrogen atmosphere degradation starts above 400 °C.
Oxetane causes no thermal degradation at lower temperatures (Figure 28b). Also the optical spectra look very similar (Figure 28c). Both polymers possess absorption maxima at about 360 nm and 510 nm. The optical bandgaps Eg are estimated to be 2.1 eV from the onset of the absorption edges of both polymers. Photoluminescence behavior is also very similar, with only a slight red-shift of the maximum of PFDTBTOx (639 nm, PFDTBT 635 nm).
Comparing both materials, a crosslinkable low bandgap polymer and its non-crosslinkable equivalent were successfully synthesized. Both materials possess similar molecular weights, thermal, and optical properties. This is important for the following crosslinking experiments and the use of these materials in organic solar cells.
PFDTBTOx
100 200 300 400 500 600 700
40
300 400 500 600 700 800 900
0.0
In the case of PFDTBTOx the concentration of crosslinkable groups is very high, with two oxetane groups being present in each repeat unit. To gain polymers with varying amounts of crosslinkable groups, a series of polymers was synthesized by copolymerization of the crosslinkable (4) and non-crosslinkable fluorene monomer (6) and dithienylbenzothiadiazole (11). The synthesis of these polymers is shown in Scheme 5.
Scheme 5. Synthesis of polymers with varying amounts of crosslinkable groups. Conditions: toluene:water, Pd(PPh3)4, Na2CO3, aliquat 336, RF, 4 d, endcapping with bromobenzene and phenylboronic acid.
The copolymers PFDTBTOx0.75, PFDTBTOx0.50, PFDTBTOx0.25, PFDTBTOx0.10, and PFDTBTOx0.05 were synthesized as described previously for PFDTBT and PFDTBTOx by Suzuki polycondensation. Table 4 lists the monomer feed ratios for the polymers of this series.
From 1H NMR spectra the amount of crosslinkable repeat units in the polymers is calculated. Figure 29 shows a generic structure of the crosslinkable (co-)polymers from this series and the NMR spectra. The relative amount of crosslinkable repeat units is determined by the ratio of the integrals of the singlet caused by the -O-CH2-oxetane protons at 3.40 ppm (marked in red in Figure 29) and the multiplet of the aromatic protons between 7.40 and 8.10 ppm (indicated in blue in Figure 29). In Table 4 these values are summarized. The amount of oxetane containing fluorene units found in the copolymers is in all cases very close to the number expected from the feed ratios.
Table 4. Series of low bandgap polymers with varying amounts of crosslinkable oxetane groups. The index x represents the crosslinkable fluorene monomer, the index y represents the non-crosslinkable didodecylfluorene monomer.
feed ratio found in polymera molecular weightb
x y x y 𝑀𝑛 𝑀𝑤
PFDTBTOx 1 0 1 0 14,800 37,900
PFDTBTOx0.75 0.75 0.25 0.76 0.24 14,200 33,400 PFDTBTOx0.50 0.50 0.50 0.52 0.48 6,200 13,200 PFDTBTOx0.25 0.25 0.75 0.25 0.75 12,500 22,500 PFDTBTOx0.10 0.10 0.90 0.10 0.90 11,800 24,000 PFDTBTOx0.05 0.05 0.95 0.04 0.96 11,000 21,100
PFDTBT 0 1 0 1 11,500 24,400
a The ratio of x/y was determined from 1H NMR spectra based on the integrals of the singlet at 3.40 ppm and the multiplet between 7.40 and 8.10 ppm.
b Determined from SEC, eluent: THF, 𝑀𝑛and 𝑀𝑤 were calculated from polystyrene calibration.
Apart from PFDTBTOx0.50 the molecular weights of all polymers from this series are in the same range.
Figure 29. Determination of the amount of oxetane groups in the series of crosslinkable polymers by 1H NMR
In addition to the series shown in Table 4, a PFDTBT with one oxetane group per repeat unit is synthesized by a different strategy. Instead of using equimolar amounts of the crosslinkable and non-crosslinkable fluorene monomers, 50% of crosslinkable repeat units were achieved by incorporating one heptyloxy-oxetane and one dodecyl chain to the fluorene monomer. The synthetic route is illustrated in Scheme 6.
Scheme 6. Synthetic steps towards PFDTBTOx0.50-alt.. Conditions: i: DMSO, 50% NaOH, phase-transfer catalysts benzyltriethylammonium chloride and tetrabutylammonium chloride, 100 °C; ii: THF, n-butyllithium, isopropoxyboronic acid pinacol ester, -78 °C; iii: toluene:water, Pd(PPh3)4, Na2CO3, aliquat 336, RF, 4 d, endcapping with bromobenzene and phenylboronic acid.
In the first step 2,7-dibromofluorene is alkylated in 9-position. Here, an equimolar mixture of 1-bromododecane and 3-(7-bromoheptyloxymethyl)-3-ethyloxetane is used. This results in a statistic mixture of the two symmetrically substituted dibromofluorenes, bearing either two dodecyl or two hexyloxy-oxetane chains, and the desired 2,7-dibromo-9-dodecyl-9-(heptyl-7,1-diyl-oxymethyl-3-ethyloxetane)-fluorene (13). Due to their differences in polarity, these components can be separated and purified by column chromatography. The yield of (13) is 42%. Afterwards the boronic acid pinacol ester groups are introduced. The monomer (14) was obtained in high purity after column chromatography in 27% yield. Polymerization was conducted as described above, yielding a PFDTBTOx with one oxetane group in each repeating unit. This should result in a comparable density of crosslinkable groups as expected in a copolymer of 50%
crosslinkable and 50% non-crosslinkable fluorene monomers as it was described previously. The amount of oxetane groups was checked by 1H NMR and found to be exactly 1.0 per repeat unit. From SEC the molecular weight of the polymer was determined to 𝑀𝑛 11,500 g mol-1 and 𝑀𝑤 23,900 g mol-1. This is within the molecular weight range of the other polymers from this series. In the following crosslinking experiments the polymer obtained from this alternative strategy, named PFDTBTOx0.50-alt., was used as the material with 1.0 oxetane groups per repeat unit. The experimental data of all polymers from this series are summarized in Table 5.
Table 5. Data of the crosslinkable polymers and the reference polymer PFDTBT.
SECa TGAb UV/Visc PLd
𝑀𝑛 𝑀𝑤 Td λmax λonset Eopt λmax
PFDTBTOx 14,800 37,900 410 °C 362, 510 nm 590 nm 2.1 eV 639 nm PFDTBTOx0.75 14,200 33,400 395 °C 365, 510 nm 584 nm 2.1 eV 634 nm PFDTBTOx0.50-alt. 11,500 23,900 402 °C 360, 510 nm 590 nm 2.1 eV 640 nm PFDTBTOx0.25 12,500 22,500 395 °C 364, 509 nm 584 nm 2.1 eV 633 nm PFDTBTOx0.10 11,800 24,000 419 °C 361, 505 nm 584 nm 2.1 eV 633 nm PFDTBTOx0.05 11,000 21,100 256 °C 363, 509 nm 586 nm 2.1 eV 634 nm PFDTBT 11,500 24,400 420 °C 365, 511 nm 587 nm 2.1 eV 635 nm
a 𝑀𝑛 and 𝑀𝑤 were calculated from polystyrene calibration.
b Measured under N2 at 10 K min-1. Td: decomposition temperature, here: temperature of 5% weight loss.
c Spectra from THF solutions (c = 10 -3 mg ml-1), λonset: determined from linear fit of absorption edge. Eopt = h×c/λonset.
d Spectra from THF solutions (c = 10 -3 mg ml-1), λexcitation= 360 nm.