3. Synthesis of Conjugated Polymers by Controlled Suzuki-Miyaura Coupling
4.3 Summary and Conclusion
precipitating the nanocrystals. The polymer might be stripped off from the nanoparticles by the addition of methanol, which is added to quench the polymerization and to precipitate the nanoparticles. The removal of carboxylic acids from PbSe nanocrystals by short chain alcohols has been described in literature.164 To prevent this, the PbS nanocrystals could be precipitated by the addition of an aprotic solvent e.g. acetonitrile. Alternatively, the ligand is displaced from the PbS nanoparticles at some point during initiator formation or polymerization, in which case stronger binding ligands are necessary.165
In conclusion, it was possible to extend the potential of surface initiated Suzuki-Miyaura coupling polymerization to PbS nanocrystals. By using benzothiadiazole compounds as ligands featuring an increased reactivity towards oxidative addition, a higher conversion of the surface-bound ligand was achieved under milder conditions. However, stronger binding ligands or different protocols for the isolation of the hybrid particles are necessary, as functionalized polymer was only obtained from the supernatant after removal of the nanoparticles by centrifugation.
4.3 Summary and Conclusion
Besides their potential application in optoelectronics, hybrid particles based on quantum dots and conjugated polymers are of fundamental interest as single photon sources, as they allow for ultrafast quantum optics on the few-photon level.
In previous work, a strategy for the synthesis of CdSe/polyfluorene hybrid particles was developed.98 CdSe nanocrystals were synthesized in the presence of aniline- or phenylphosphonic acid functionalized polyfluorene. This strategy was extended in this work to the synthesis of hybrid particles based on CdSe/CdS core-shell nanocrystals and functionalized polyfluorenes were introduced directly during the synthesis of the CdS shell.
For this purpose, the synthesis of CdSe cores and CdSe/CdS quantum dots was studied in more detail regarding the optimization of optical properties, influence of ligands during nanocrystal synthesis and upscaling. The findings were implemented subsequently in the synthesis of CdSe/CdS/polyfluorene hybrid particles. The use of functionalized polyfluorene as a ligand during
Figure 49. TEM images of CdSe/CdS-core shell nanocrystals generated with different ligands present during the shell synthesis. Left: 16000 equiv. of oleyl amine per CdSe QD. Center: 16000 equiv. of oleyl amine and 100 equiv. of aniline
functionalized polyfluorene per CdSe QD. Right: 100 equiv. of aniline functionalized polyfluorene per CdSe QD.
The addition of 100 equiv. of aniline functionalized polyfluorene additionally to 16000 equiv. of oleyl amine into the shell-growth reaction resulted in a narrowing of the particle size distribution and QDs with a uniform shape (Figure 49, center), compared to the QDs synthesized under standard conditions (16000 equiv. of oleyl amine). The replacement of oleyl amine by 100 equiv. of aniline functionalized polyfluorene also resulted in the formation of hybrid particles with a narrower size distribution (Figure 49, right). These experiments underline the significant interaction between the polymeric ligand and the growing nanocrystals.
Analytical ultracentrifugation experiments with core-shell quantum dots stabilized by aniline- or phenylphosphonic acid functionalized polyfluorene ligands, respectively, revealed that the aniline functionalized polymer is almost completely displaced from the nanocrystal surface in high dilution (< 5% of bound polymer), while approx. 65% of the phenylphosphonic acid functionalized polyfluorene is bound to the surface. These measurements were substantiated by studying ensemble photoluminescence spectra of these hybrids in different dilution. Dilution of the dispersions results in an increase in polyfluorene emission, due to the displacement of polymer ligands from the nanoparticle surface. This displacement is more pronounced for aniline functionalized polymer than for phenylphosphonic acid functionalized polyfluorene (Figure 50).
4.3 Summary and Conclusion
Figure 50. Polyfluorene emission intensity divided by the nanocrystal emission intensity vs. dilution for CdSe/CdS/aniline functionalized polyfluorene hybrid particles (green squares) and for CdSe/CdS/phenylphosphonic
acid functionalized polyfluorene hybrid particles (blue triangles).
Additionally, the increase in polymer emission alludes to an efficient energy transfer between the polymer and the nanocrystal in the hybrid particles, resulting in a quenching of nanocrystal-bound polyfluorene.
A limitation for this hybrid particle synthesis is the limited solubility of the polymer in the reaction mixture during the synthesis of the CdS shell around the CdSe cores.
As an alternative approach to conjugated polymer/quantum dot hybrid particles, the quenching of a Suzuki-Miyaura coupling polymerization with 4-mercaptophenylboronic acid functionalized CdSe/CdS quantum dots was studied (Scheme 18).
Scheme 18. Synthesis of CdSe/CdS/polyfluorene hybrid particles by the quenching of Suzuki-Miyaura coupling polymerizations with functionalized CdSe/CdS nanocrystals.
formation of 4-mercaptophenyl functionalized polyfluorene, the latter not being the main species however, indicating the quenching to be inefficient.
For this reason, a surface initiated polymerization method was developed. Quantum dots can be synthesized under optimized conditions and functionalized in a second step by a small ligand that allows for an efficient exchange. After the synthesis of a surface bound initiator complex, polymer can be grafted from the QD surface, resulting in conjugated quantum dot/polymer hybrid particles (Scheme 19).
Scheme 19. CdSe/CdS/conjugated polymer hybrid particle synthesis by surface confined Suzuki-Miyaura coupling polymerization.
The polymerization from quantum dots functionalized with 4-bromophenylposphonic acid and 4-iodophenylphosphonic acid was studied. The bromo-aryl based system turned out to be superior due to a higher stability of the surface-bound initiator complex and less formation of Pd-black.
Successful grafting of polyfluorene and poly(p-phenylene) from nanocrystals could be demonstrated (Figure 51, bottom), and additionally, precise end-capping of growing chains was accomplished, confirming the chain-growth nature of the reaction (Figure 51, top). Solution-initiated polyfluorene could be separated off from the hybrid particles by a precipitation and centrifugation step.
4.3 Summary and Conclusion
Figure 51. MALDI-TOF mass spectra of polyfluorene isolated after a surface-initiated Suzuki-Miyaura coupling polymerization from (4-bromophenyl)phosphonic acid functionalized CdSe/CdS quantum dots. The polymerizations were quenched by the addition of methanol (bottom) and by the addition of
2-[3,5-bis(trifluoromethyl)phenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (top).
The grafting of polythiophene from nanoparticles failed, as the thiophene monomer features a considerably higher reactivity, as demonstrated by NMR-experiments, resulting in predominantly solution-initiated polymerization by the reaction of residual [Pd(PtBu3)2] with monomer. This underlines the necessity of matching the monomers’ and surface bound initiators’ reactivity. In preliminary studies towards this identified need, benzothiadiazole derivatives were classified to be extremely reactive in an oxidative addition reaction with [Pd(PtBu3)2] and have therefore the potential as ligands in surface initiated Suzuki-Miyaura coupling polymerizations. A successful polymerization of polyfluorene from PbS nanocrystals functionalized with 7-bromobenzo[c][2,1,3]thiadiazole-4-carboxylic acid was demonstrated and hardly any non-functionalized polymer was observed.