blue in Fig. 6.1), the boron acting as a mere linking atom maintaining chromophoric
planarity and rigidity, thus minimizing the rate of non-radiative relaxation of the
excited state and favouring radiative emission (i.e. fluorescence) [24].
Among the different organic chromophoric scaffolds, BODIPYs, and mainly
F-BODIPYs (4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes; see Fig. 6.1), present
significant advantages for the development of CPL-SOMs. This is due to their
outstanding physical, photophysical and chemical properties [25–30], including:
• High solubility in a wide range of solvents.
• Low aggregation capability.
• High chemical robustness.
• High light-absorption coefficients (ε max ).
• High fluorescent quantum yields (ϕ).
• High photostability.
• Fluorescence signatures which are mostly independent of solvent polarity.
• Ample structural diversity.
• Potential for modulation of physical, chemical, and biological properties through
molecular modification.
• Potential for modulation of photophysical properties, including CPL signatures
(e.g. g lum ) and CPL efficiency.
In other words, high fluorescence efficiency combined with a rich, well-known,
and workable synthetic chemistry, the latter allowing synthetic access and fine
modulation of key properties, makes the BODIPYs one of the most interesting
chromophoric scaffolds with which to develop novel photonic materials, including
CPL-SOMs [31].
Although the π-conjugated BODIPY chromophore is inherently achiral, it can be
chirally perturbed as to make it efficiently exhibit chiroptical properties, including
CPL [1, 2, 31]. This perturbation is generally achieved by embedding the BODIPY
chromophore within a chirally resolved molecular design, which is crucial to gain
chiroptical efficiency (i.e. to maximize |g lum | or ideally to maximize the product of
|g lum | and ϕ) [1, 2]. On the other hand, such a design should be as simple as possible
to maximize synthetically accessibility, thus allowing easy access to low-cost
chiroptical materials, including molecular CPL materials [1, 2].
To date, a number of preferred chiral designs have been reported as successful
approaches with which to endow the BODIPY chromophore with CPL activity.
These designs which can be initially separated into those based on (a) monomeric
chiral BODIPYs (mono(BODIPY)s) and (b) those involving two or more BODIPY
units (poly(BODIPY)s). Mono(BODIPY)-based CPL-SOMs can be further
subdivided into those in which (1) chiral moieties (one or more) are covalently
linked to the BODIPY core to obtain a chiral C 2 -symmetric mono(BODIPY) or
(2) the BODIPY core is embedded into a molecular architecture displaying helical
chirality. Whilst poly(BODIPY)-based CPL-SOMs can also be subdivided into
those in which (1) two (or more) BODIPY cores are directly linked via a covalent
bond to obtain an axially-chiral atropoisomeric bis(BODIPY) structure, (2) two
BODIPY moieties are covalently linked through a flexible chiral bridge to obtain a
6 BODIPY Based Emitters of Circularly Polarized Luminescence
119
planarity and rigidity, thus minimizing the rate of non-radiative relaxation of the
excited state and favouring radiative emission (i.e. fluorescence) [24].
Among the different organic chromophoric scaffolds, BODIPYs, and mainly
F-BODIPYs (4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes; see Fig. 6.1), present
significant advantages for the development of CPL-SOMs. This is due to their
outstanding physical, photophysical and chemical properties [25–30], including:
• High solubility in a wide range of solvents.
• Low aggregation capability.
• High chemical robustness.
• High light-absorption coefficients (ε max ).
• High fluorescent quantum yields (ϕ).
• High photostability.
• Fluorescence signatures which are mostly independent of solvent polarity.
• Ample structural diversity.
• Potential for modulation of physical, chemical, and biological properties through
molecular modification.
• Potential for modulation of photophysical properties, including CPL signatures
(e.g. g lum ) and CPL efficiency.
In other words, high fluorescence efficiency combined with a rich, well-known,
and workable synthetic chemistry, the latter allowing synthetic access and fine
modulation of key properties, makes the BODIPYs one of the most interesting
chromophoric scaffolds with which to develop novel photonic materials, including
CPL-SOMs [31].
Although the π-conjugated BODIPY chromophore is inherently achiral, it can be
chirally perturbed as to make it efficiently exhibit chiroptical properties, including
CPL [1, 2, 31]. This perturbation is generally achieved by embedding the BODIPY
chromophore within a chirally resolved molecular design, which is crucial to gain
chiroptical efficiency (i.e. to maximize |g lum | or ideally to maximize the product of
|g lum | and ϕ) [1, 2]. On the other hand, such a design should be as simple as possible
to maximize synthetically accessibility, thus allowing easy access to low-cost
chiroptical materials, including molecular CPL materials [1, 2].
To date, a number of preferred chiral designs have been reported as successful
approaches with which to endow the BODIPY chromophore with CPL activity.
These designs which can be initially separated into those based on (a) monomeric
chiral BODIPYs (mono(BODIPY)s) and (b) those involving two or more BODIPY
units (poly(BODIPY)s). Mono(BODIPY)-based CPL-SOMs can be further
subdivided into those in which (1) chiral moieties (one or more) are covalently
linked to the BODIPY core to obtain a chiral C 2 -symmetric mono(BODIPY) or
(2) the BODIPY core is embedded into a molecular architecture displaying helical
chirality. Whilst poly(BODIPY)-based CPL-SOMs can also be subdivided into
those in which (1) two (or more) BODIPY cores are directly linked via a covalent
bond to obtain an axially-chiral atropoisomeric bis(BODIPY) structure, (2) two
BODIPY moieties are covalently linked through a flexible chiral bridge to obtain a
6 BODIPY Based Emitters of Circularly Polarized Luminescence
119