used to conduct light-induced asymmetric reactions [12–14] or to control chiral
morphologies in nanostructures [15, 16]. Organic systems present significant advantages when it comes the modulation of key physical and chemical properties through
workable structural modifications which are available via the rich variety of chemical transformations offered by modern organic chemistry [1, 2]. Furthermore,
organic systems offer additional advantages in other fundamental aspects related to
the development of specific materials, for example, in materials processing or in
gaining biocompatibility, facilitating the preparation of ultra-fine materials or biomaterials, respectively. Additionally, the characteristic broadband emission of the
organic luminescent systems allows the selection of multiple wavelengths from the
same photonic material, which is interesting for the development of applications
requiring such a tunability [1, 2].
Among the CPL-enabling organic systems (molecules, polymers, supramolecular
aggregates, etc.), those based on simple and non-aggregated small organic molecules
(SOMs) have aroused great interest in recent years. The interest in CPL-enabling
SOMs (CPL-SOMs) is due to (1) their high potential for the development of specific
CPL materials due to properties associated with low molecular weight (e.g. in the
development of certain biomaterials beyond biosensors) and organic-solvent solubility (e.g. in the development of CPL-active dye-doped inclusion materials);
(2) their capability to achieve high emission quantum yields (ϕ); and (3) the ease
in which emission signatures can be modulated by accessible structural modification
of the chromophoric scaffold [1, 2]. However, examples of CPL-SOMs are scarce,
are based on a small set of chromophoric molecular scaffolds (biaryls, helicenes,
perylenes, BODIPYs) and usually exhibit low levels of circular polarization, as
measured by the luminescent dissymmetry factor (|g lum | typically in the 10
À5
–10
À2
range) [1, 2]. The development of chiral SOMs that able to exhibit CPL, with both a
large luminescent dissymmetry factor and a high emission quantum yield, is thus an
exciting challenge in the field of molecular photonics, due to the difficulty of
combining both key properties in the same SOM.
Boron dipyrromethenes (4-bora-3a,4a-diaza-s-indacenes or BODIPYs; see
Fig. 6.1) are valuable fluorescent dyes, which have been successfully used in the
development of many photonic tools [17–23]. BODIPYs are normally defined as
SOMs, despite the presence of a chelated boron atom in the core structure. This is
due to the metalloid nature of boron allowing properties closer to those of organic
molecules than to inorganic metal complexes. On the other hand, the BODIPY
chromophore itself is purely organic, as it is located in the aromatic-like
π-conjugated system of its organic dipyrromethene ligand (dipyrrin; highlighted in
N
N
B
F F
N
N B
F
F
1
2
3
4
5
6
7
8
meso
Fig. 6.1 Boron dipyrromethene scaffold for the simplest F-BODIPY, including both commonly
used numbering schemes. Dipyrrin chromophore highlighted in blue
118
M. J. Hall and S. de la Moya
morphologies in nanostructures [15, 16]. Organic systems present significant advantages when it comes the modulation of key physical and chemical properties through
workable structural modifications which are available via the rich variety of chemical transformations offered by modern organic chemistry [1, 2]. Furthermore,
organic systems offer additional advantages in other fundamental aspects related to
the development of specific materials, for example, in materials processing or in
gaining biocompatibility, facilitating the preparation of ultra-fine materials or biomaterials, respectively. Additionally, the characteristic broadband emission of the
organic luminescent systems allows the selection of multiple wavelengths from the
same photonic material, which is interesting for the development of applications
requiring such a tunability [1, 2].
Among the CPL-enabling organic systems (molecules, polymers, supramolecular
aggregates, etc.), those based on simple and non-aggregated small organic molecules
(SOMs) have aroused great interest in recent years. The interest in CPL-enabling
SOMs (CPL-SOMs) is due to (1) their high potential for the development of specific
CPL materials due to properties associated with low molecular weight (e.g. in the
development of certain biomaterials beyond biosensors) and organic-solvent solubility (e.g. in the development of CPL-active dye-doped inclusion materials);
(2) their capability to achieve high emission quantum yields (ϕ); and (3) the ease
in which emission signatures can be modulated by accessible structural modification
of the chromophoric scaffold [1, 2]. However, examples of CPL-SOMs are scarce,
are based on a small set of chromophoric molecular scaffolds (biaryls, helicenes,
perylenes, BODIPYs) and usually exhibit low levels of circular polarization, as
measured by the luminescent dissymmetry factor (|g lum | typically in the 10
À5
–10
À2
range) [1, 2]. The development of chiral SOMs that able to exhibit CPL, with both a
large luminescent dissymmetry factor and a high emission quantum yield, is thus an
exciting challenge in the field of molecular photonics, due to the difficulty of
combining both key properties in the same SOM.
Boron dipyrromethenes (4-bora-3a,4a-diaza-s-indacenes or BODIPYs; see
Fig. 6.1) are valuable fluorescent dyes, which have been successfully used in the
development of many photonic tools [17–23]. BODIPYs are normally defined as
SOMs, despite the presence of a chelated boron atom in the core structure. This is
due to the metalloid nature of boron allowing properties closer to those of organic
molecules than to inorganic metal complexes. On the other hand, the BODIPY
chromophore itself is purely organic, as it is located in the aromatic-like
π-conjugated system of its organic dipyrromethene ligand (dipyrrin; highlighted in
N
N
B
F F
N
N B
F
F
1
2
3
4
5
6
7
8
meso
Fig. 6.1 Boron dipyrromethene scaffold for the simplest F-BODIPY, including both commonly
used numbering schemes. Dipyrrin chromophore highlighted in blue
118
M. J. Hall and S. de la Moya