emission of CdSe/ZnS QDs was completely quenched by unspecific complex
formation with a diazaperylene dye not bearing a suitable binding functionality
[42]. Suitable anchor groups for CdSe or ZnS QDs are provided by amines, oxides
and thiols, which have different affinities for the QD surface and alter the fluorescence properties to different degrees. Amines often lead to an increase in the
fluorescence but bind only weakly to the QDs. The opposite is true for thiols,
which bind more strongly but typically reduce the fluorescence of the particles. To
overcome the weak binding of amines, Potapova et al. [43] used a dye-labeled
polymer ligand with multiple amine anchor groups, which formed stable complexes
with CdSe/ZnS QDs. Other approaches used functionalized peptides and proteins to
attach dyes to QDs [44–46]. As discussed below, carboxylic groups enable stable
binding of dye molecules to QDs. In most of the conjugates the QD (dye) acts as an
energy donor (acceptor) and EET signals complex formation. In a few cases the
opposite arrangement has also been considered, one example of which is given in
Sect. 2.3. The EET from QD to dye in several instances was satisfactorily modeled
in terms of the Fo ¨rster mechanism [47, 48], although deviations have also been
reported [49].
2.2.1 Semiconductor Quantum Dots
Several types of QDs have been employed in the assembly of QD/dye complexes.
In early attempts [42, 43] CdSe/ZnS core–shell QDs have been used, which were
synthesized following established protocols [50, 51]. Later, a synthetic route for
multishell QDs was developed [52] in which the shell composition of the CdSe core
was gradually changed from CdS to ZnS in the radial direction. The resulting
particles had a high crystallinity, large fluorescence quantum yields of up to 80%
and increased photochemical and colloidal stability. Moreover, the presence of the
passivating shell mediated the impact of ligand exchange on the fluorescence
properties of the multishell QDs. The particles discussed so far represent type-I
core–shell QDs in which electron and hole are confined in the core after light
absorption. In addition, type-II core–shell QDs [53–55] of composition CdTe/
CdSe/ZnS were provided for complex formation with light-harvesting complexes
as described in Sect. 2.3. In these particles, the hole is confined to the core while the
electron largely resides in the shell.
2.2.2 Functionalized Organic Dye Molecules
Furnishing organic dyes with carboxylic groups yields stable complexes with QDs
[56]. Thus, we designed a family of rylene dyes 36–38 bearing β-glutamic acid
groups at the imide positions for stable complexation with QDs. In the case of 36
(Scheme 9) [57], the starting perylene dye 39 was partially saponified under basic
conditions to give monoanhydride 40. Imidization of 40 with β-glutamic acid in
N-methyl-2-pyrrolidone (NMP) afforded 36. On the other hand, using the open form
Optical Properties of Assemblies of Molecules and Nanoparticles
75
formation with a diazaperylene dye not bearing a suitable binding functionality
[42]. Suitable anchor groups for CdSe or ZnS QDs are provided by amines, oxides
and thiols, which have different affinities for the QD surface and alter the fluorescence properties to different degrees. Amines often lead to an increase in the
fluorescence but bind only weakly to the QDs. The opposite is true for thiols,
which bind more strongly but typically reduce the fluorescence of the particles. To
overcome the weak binding of amines, Potapova et al. [43] used a dye-labeled
polymer ligand with multiple amine anchor groups, which formed stable complexes
with CdSe/ZnS QDs. Other approaches used functionalized peptides and proteins to
attach dyes to QDs [44–46]. As discussed below, carboxylic groups enable stable
binding of dye molecules to QDs. In most of the conjugates the QD (dye) acts as an
energy donor (acceptor) and EET signals complex formation. In a few cases the
opposite arrangement has also been considered, one example of which is given in
Sect. 2.3. The EET from QD to dye in several instances was satisfactorily modeled
in terms of the Fo ¨rster mechanism [47, 48], although deviations have also been
reported [49].
2.2.1 Semiconductor Quantum Dots
Several types of QDs have been employed in the assembly of QD/dye complexes.
In early attempts [42, 43] CdSe/ZnS core–shell QDs have been used, which were
synthesized following established protocols [50, 51]. Later, a synthetic route for
multishell QDs was developed [52] in which the shell composition of the CdSe core
was gradually changed from CdS to ZnS in the radial direction. The resulting
particles had a high crystallinity, large fluorescence quantum yields of up to 80%
and increased photochemical and colloidal stability. Moreover, the presence of the
passivating shell mediated the impact of ligand exchange on the fluorescence
properties of the multishell QDs. The particles discussed so far represent type-I
core–shell QDs in which electron and hole are confined in the core after light
absorption. In addition, type-II core–shell QDs [53–55] of composition CdTe/
CdSe/ZnS were provided for complex formation with light-harvesting complexes
as described in Sect. 2.3. In these particles, the hole is confined to the core while the
electron largely resides in the shell.
2.2.2 Functionalized Organic Dye Molecules
Furnishing organic dyes with carboxylic groups yields stable complexes with QDs
[56]. Thus, we designed a family of rylene dyes 36–38 bearing β-glutamic acid
groups at the imide positions for stable complexation with QDs. In the case of 36
(Scheme 9) [57], the starting perylene dye 39 was partially saponified under basic
conditions to give monoanhydride 40. Imidization of 40 with β-glutamic acid in
N-methyl-2-pyrrolidone (NMP) afforded 36. On the other hand, using the open form
Optical Properties of Assemblies of Molecules and Nanoparticles
75