Organic/Inorganic Semiconductor Hybrid Nanoparticles

consumption of the monosubstituted bromoaryl was observed, additionally underlining an association of the Pd-center to the growing chain during polymerization. Besides the absence of chain transfer reactions, an additional requirement for a controlled polymerization is the absence of chain termination, which is prevented by carefully excluding oxygen from the polymerization mixture and by deploying highly pure monomers, as monosubstituted arylboronic acid esters would terminate growing chains. Another possibility for chain termination is dehalogenation by water and base, both present in the polymerization mixture.⁵⁰ Interestingly, polymerization experiments with and without water performed by Kosaka et al. indicated that the presence of water suppresses intermolecular transfer of the Pd-moiety and that water is necessary for a controlled polymerization.⁵⁷

The relationship between monomer conversion and molecular weight (Mn) of the obtained polymer is linear, in agreement with a chain-growth polymerization and therefore allows for the precise adjustment of the molecular weight by the molar monomer to initiator ratio.⁴⁸

1.3 Organic/Inorganic Semiconductor Hybrid Nanoparticles

A synthetic access to the targeted hybrid particles (Figure 1) is challenging. The simple mixing of separately prepared conjugated polymers and inorganic nanocrystals normally results in phase separation and furthermore, the ligands introduced as stabilizers during quantum dot synthesis end up as insulators at the interface between the two semiconducting materials.^58-60 Therefore, it is beneficial to use nanocrystals with conjugated polymers directly bound to their surface to enable an optimal integration into optoelectronic devices. Such hybrid particles can additionally exhibit different photo physics relative to bulk blends.^61,62

There are several strategies for the synthesis of hybrid particles with the polymer directly bound to the pre-synthesized nanocrystal, namely the grafting onto, grafting through and grafting from approach.⁶³ Additionally, hybrid particles can be synthesized in-situ by performing the synthesis of the inorganic core in the presence of functionalized polymer ligands.⁶⁴ The grafting methods (depicted in Scheme 4) will be discussed in more detail in the following.

Scheme 4. Grafting methods for the synthesis of hybrid particles, adapted from Bousquet et al.⁶³ Copyright 2014, Elsevier.

1.3.1 Synthesis of Organic/Inorganic Semiconductor Hybrid Nanoparticles by the Grafting Onto Approach

Most reported procedures for organic/inorganic semiconductor hybrid nanoparticle synthesis are based on the grafting onto approach. The quantum dot ligands originating from the hot injection synthesis are directly replaced by a conjugated polymer featuring an appropriate functional group that can bind to the QD surface.^65-70 The attachment of a polymer by an end-group can allow for polymer chains to be oriented perpendicular to the particle surface, while multiple binding of polymer side-chains leads to a more dense and flat structure.⁶³ An advantage of the grafting onto approach is that the polymer and the nanocrystal are synthesized in separate steps and can be purified and analyzed thoroughly. Modern controlled coupling polymerizations allow for the synthesis of precisely functionalized polymers.46-48,56,64,71 The major drawback of the grafting onto approach is that it does not allow for high grafting densities. The already bound chains hinder further polymer chains from diffusing to the surface due to the bulkiness of the polymer ligand. Consequently, brush-like structures are difficult to achieve. In the case of grafting end-functionalized polymer directly onto fluorescent nanocrystals, a decrease in quantum yield of the inorganic emitter due to the generation of free dangling bonds on the nanocrystal surface as a result of an incomplete exchange reaction is often observed.⁷²

1.3.2 Synthesis of Organic/Inorganic Semiconductor Hybrid Nanoparticles by the Grafting Through Approach

In the grafting through approach, the nanocrystals are functionalized with a polymerizable group.

The polymerization is initiated in solution and the functionalized nanocrystals are co-monomers and

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1.3 Organic/Inorganic Semiconductor Hybrid Nanoparticles

through approach.⁷³ The CdSe nanocrystals were functionalized with a phosphine oxide ligand featuring a phenyl-bromide moiety and were deployed as co-monomer in a Pd-catalyzed Heck coupling. However, due to the step growth nature of the reaction, only tri- and tetramers were formed. The grafting through approach features a poor control over the grafting density and mostly results in grafted polymer with a low molecular weight.⁶³ Furthermore, formation of non-bound polymer in solution is observed.

1.3.3 Synthesis of Organic/Inorganic Semiconductor Hybrid Nanoparticles by the Grafting From Approach

Grafting from is the method of choice for polymer-functionalization of surfaces and for the synthesis of organic/inorganic hybrid particles, as it allows for control over molecular weight and grafting density. The grafting of non-conjugated polymers by controlled radical polymerization methods from nanoparticles is well established and polymer brushes can be grown from many different particles e.g. polymer-, silica-, metal-, metal oxide- or semiconductor nanoparticles.⁷⁴ However, the free-radical methods are not applicable to conjugated polymers as the organic part.

Conjugated polymers are normally formed by a step growth mechanism, which is incompatible with a surface-confined polymerization. The introduction of solution chain-growth polymerization methods for polythiophene^46,47, polyfluorene and poly(p-phenylene)⁴⁸ by Suzuki-Miyaura or Kumada coupling, represents a major advance towards grafting from of these conjugated polymers.

To this end, the nanocrystals need to be functionalized with a small molecule that acts as an initiation site. The functionalized nanocrystals are then reacted with a metal-precursor complex, forming the surface-bound initiator complex, and are finally mixed with the monomer solution to initiate the polymerization. Ideally, only surface bound polymer is formed, the grafting density can be adjusted by the precursor amounts and the molecular weight can be adjusted by the initiator to monomer ratio.

Only few examples for the grafting from of conjugated polymers have been reported to date, and they are mostly restricted to macroscopic flat substrates. The grafting of polythiophene^75-77,

spectroscopy, electron energy loss spectroscopy) or by scanning electron microscopy. However, with these methods, it is difficult to differentiate between adsorbed and grafted polymer. In none of these reports, the grafted polymer was isolated from the substrate/particles and analyzed separately. The first example for a surface initiated polymerization from nanocrystals including polymer analysis was published by Islam et al. Hydride terminated silicon nanoparticles with a diameter of 11 nm were reacted with 5-chloro-magnesio-2-bromo-3-hexylthiophene to yield 2-bromo-3-hexyl-5-thienyl functionalized nanocrystals, from which polythiophene was grafted by Kumada coupling polymerization.⁸⁹ They detached the polymer from the surface for MALDI-TOF MS analysis by destroying the inorganic core, however detached and solution-initiated polymer feature the same end-group after quenching of the polymerization (hydrogen).

To determine the organic initiator precursor/polymer density at the surface, thermogravimetric analysis is a common method, however rather inaccurate when the ligand composition is not exactly known.^90-92 Alternatively, cyclic voltammetry can be used as demonstrated by Sontag et al.⁷⁷ A gold substrate in the form of the working electrode was functionalized with a thiophene compound to yield a surface-bound redox couple and the ligand density was estimated to be approximately six molecules/nm² by integration of the oxidation wave in the cyclic voltammogram. The initiator density was estimated by analysis of a surface that was functionalized with 8-(5-bromothiophene-2-yl)octane-1-thiol, reacted with a Ni(0) precursor yielding a surface bound Ni(II) complex after oxidative addition, followed by the addition of a Grignard compound containing a ferrocene moiety. Cyclic voltammetry measurements allowed for the estimation of the yield of the reaction of the aryl-bromide with the Ni(0) precursor and the subsequent coupling with the ferrocene moiety, which was approximately 10%, translating to an initiator density of 0.6 per nm². However, such measurements are only possible with a substrate that can be used as electrode in the measurement setup. Thus, cyclic voltammetry is not suited to determine the ligand density at nanoparticle surfaces.

So far, there are no examples for the surface initiated polymerization of conjugated polymers from semiconducting nanocrystals by Suzuki-Miyaura coupling polymerization. The Suzuki-Miyaura protocol should be advantageous compared to Kumada coupling for the following reasons: The Grignard-type monomers used for Kumada coupling are very sensitive which strongly restricts the applicability of this method. This is not the case for the insensitive and storable compounds used for Suzuki-Miyaura coupling polymerization. Furthermore, no magnesium salts are left behind after polymerization, which seems beneficial for the optical properties of the hybrid particles.⁶³ Finally, the Pd-initiators that are applied in Suzuki-Miyaura coupling are less prone to disproportionation compared to the Ni-based initiators that are mostly used for Kumada coupling polymerization (Scheme 5). This can be of particular relevance when high grafting densities are desired.⁷⁷