Ligand Chemistry - Semiconductor Quantum Dots: Organometallic and Inorganic Synthesis - page 185

groups and two carboxylic acid groups, has been reported. In this case, the
ligand bound through the carboxylic acid groups leaving the thiol groups
pendant in solution and available for bioconjugation. 168
Despite being initially designed as ligands for bioimaging, the inherent
stability and ease of manipulation of thiol- and dithiol-based ligands has
resulted in their application in other areas. The treatment of QD surfaces with
thiols usually (as mentioned above) leads to at least some degree of emission
quenching, which is generally considered detrimental. The use of most
surfactants during synthesis enhances emission as a result of e

d n 1 y 4 n g | 6

ective steric
passivation, and also because most surfactants have a wide bandgap and are
electronically insulating. Thiols, however, have band structures that overlap
with QD bandgaps, and, although this is usually problematic in cellular
labelling, in applications such as photovoltaics the transfer of charge by such
means is essential. Therefore, QDs passivated by thiols, which allow hole
transfer from excited semiconductor QDs to ligands, may be of immense
bene

t. 169 For example, CdSe nanorods linked to an oligothiophene via
a pendant thiol group on the polymer quenched the polymer emission; this
was attributed to an energy transfer mechanism. 170 Similarly, TOPO-capped
CdSe rods have been passivated with tert -butyl N -(2-mercaptoethyl)carbamate,
which, in the presence of a photoacid generator and under UV irradiation, was
modi



ed to give shorter ligands 171 which were used in the manufacture of
polymer photovoltaic composites. 172 Nanoparticulate CdSe has also been
linked to thymine, a DNA base, via an alkyl thiol linkage, and this has been
coordinated to a polythiophene via an aminopyridine side group, 173 an
interesting mix of molecular recognition based on DNA base interactions and
potential photovoltaic applications. Similarly, a bifunctional carbodithioic
acid molecule has been used to replace TOPO on CdSe nanoparticles, linking
to the particle surface by the dithiol units leaving a pendant carboxylic acid
group which was then further reacted with an aniline tetramer, giving an
overall structure with possible applications in photovoltaics (Figure 6.5). 174
Attachment of the carbodithioic molecule resulted in enhanced photostability
for the nanoparticle, although all emission was quenched, and the attached
ligand retained its electrochemical activity. 175
Other applications have also been explored since attaching reactive groups
via a thiol became routine: for example, linking a photoactive spiropyran
molecule to CdSe/ZnS nanoparticles has been achieved, where the QD
emission can be switched o



.

by UV photoinduced ring-opening of the

functional group and reversibly ' switched back on ' by heat or irradiation by

visible light. 176
The phase transfer of QDs and nanoparticles is not always organic

/
aqueous. In applications where materials require processing in a water-free
environment, an aqueous

organic phase transfer is required, and this can
be achieved by using a ligand such as 1-dodecanethiol, 177 where the long
simple alkyl chain made nanoparticles passivated with such a ligand
hydrophobic. In this case, acetone was found to be an important constituent
to reduce the interface boundaries and induce e

/

cient interactions.