5 BODIPY Dyes and Their Analogues
135
Fig. 5.19 B,O-Chelation of BODIPY dyes. a N,N,O,O-Boron-chelated BODIPY. b N,N,O,OBoron-chelated BODIPY showing CPL property. c N,O-Boron-chelated BODIPY
Helically chiral N,N,O,O-boron-chelated BODIPY shows circularly polarized luminescence (CPL) (Fig. 5.19b) (Alnoman et al. 2016). Similarly, N,O-boron-chelated
BODIPY (λ max = 580 nm) shows redshifted absorption compared to the non-chelated
derivative (λ max : 519 nm) (Fig. 5.19c) (Liu et al. 2018).
5.2.3.5 Relationship Between Fluorescence Quantum Yield
and Structure of BODIPY
The type and position of substituents also have an effect on the fluorescence quantum
yield of BODIPY chromophores. In the case of meso-aryl substituted BODIPY dyes,
BODIPY dye with two methyl substituents at the C1 and C7 positions (Φ f = 0.60)
showed a higher Φ f value compared to the corresponding derivative without the two
methyl substituents (Φ f = 0.29) because of the prevention of the free rotation of the
meso-tolyl group (Chen et al. 2011) (Fig. 5.20a). In the X-ray crystallographic structure, the dihedral angle of methyl derivative (81.14°) between the BODIPY core and
meso-tolyl moiety was larger than that of nonsubstituted derivative (64.69°), which
results in the prevention of free rotation and the slight blueshift in λ max (Me: λ max =
500 nm, H: λ max = 510 nm). Similarly, the Φ f value of meso-mesityl BODIPY (Φ f =
0.84) was higher compared to that of the corresponding phenyl derivative (Φ f = 0.03)
(Yu et al. 2011) (Fig. 5.20b). On the other hand, while meso-methyl BODIPY showed
a high Φ f (Φ f = 1.00) value, the bulkier meso-tert-butyl BODIPY shows a low Φ f
(Φ f = 0.04) value (Jiao et al. 2015) (Fig. 5.20c). Quantum chemical calculations
indicate that meso-tert-butyl BODIPY has a distorted nonplanar geometry in the S 1
excited state, which is reflected in the fast non-radiative deactivation (k nr = 8.76 ×
10
8 s
−1 ) and large Stokes shift (47 nm). Similar fluorescence quenching arising from
the distorted nonplanar geometry was observed in the meso-CF 3 derivative (Φ f =
0.008) with methyl groups at the β
positions (Fig. 5.20d) (Kim et al. 2015).
Although variance of the substituents on the boron atom of BODIPY dyes has a
small effect on the absorption and fluorescence maxima, it does affect the fluorescence quantum yield, solid-state fluorescence properties, HOMO and LUMO energy
levels, stability, and solubility. In BODIPY dyes, the value of k f tends to be less
135
Fig. 5.19 B,O-Chelation of BODIPY dyes. a N,N,O,O-Boron-chelated BODIPY. b N,N,O,OBoron-chelated BODIPY showing CPL property. c N,O-Boron-chelated BODIPY
Helically chiral N,N,O,O-boron-chelated BODIPY shows circularly polarized luminescence (CPL) (Fig. 5.19b) (Alnoman et al. 2016). Similarly, N,O-boron-chelated
BODIPY (λ max = 580 nm) shows redshifted absorption compared to the non-chelated
derivative (λ max : 519 nm) (Fig. 5.19c) (Liu et al. 2018).
5.2.3.5 Relationship Between Fluorescence Quantum Yield
and Structure of BODIPY
The type and position of substituents also have an effect on the fluorescence quantum
yield of BODIPY chromophores. In the case of meso-aryl substituted BODIPY dyes,
BODIPY dye with two methyl substituents at the C1 and C7 positions (Φ f = 0.60)
showed a higher Φ f value compared to the corresponding derivative without the two
methyl substituents (Φ f = 0.29) because of the prevention of the free rotation of the
meso-tolyl group (Chen et al. 2011) (Fig. 5.20a). In the X-ray crystallographic structure, the dihedral angle of methyl derivative (81.14°) between the BODIPY core and
meso-tolyl moiety was larger than that of nonsubstituted derivative (64.69°), which
results in the prevention of free rotation and the slight blueshift in λ max (Me: λ max =
500 nm, H: λ max = 510 nm). Similarly, the Φ f value of meso-mesityl BODIPY (Φ f =
0.84) was higher compared to that of the corresponding phenyl derivative (Φ f = 0.03)
(Yu et al. 2011) (Fig. 5.20b). On the other hand, while meso-methyl BODIPY showed
a high Φ f (Φ f = 1.00) value, the bulkier meso-tert-butyl BODIPY shows a low Φ f
(Φ f = 0.04) value (Jiao et al. 2015) (Fig. 5.20c). Quantum chemical calculations
indicate that meso-tert-butyl BODIPY has a distorted nonplanar geometry in the S 1
excited state, which is reflected in the fast non-radiative deactivation (k nr = 8.76 ×
10
8 s
−1 ) and large Stokes shift (47 nm). Similar fluorescence quenching arising from
the distorted nonplanar geometry was observed in the meso-CF 3 derivative (Φ f =
0.008) with methyl groups at the β
positions (Fig. 5.20d) (Kim et al. 2015).
Although variance of the substituents on the boron atom of BODIPY dyes has a
small effect on the absorption and fluorescence maxima, it does affect the fluorescence quantum yield, solid-state fluorescence properties, HOMO and LUMO energy
levels, stability, and solubility. In BODIPY dyes, the value of k f tends to be less