showed a number of short-wavelength transitions corresponding to those arising
from the binaphthalene moiety, along with longer wavelength absorptions
corresponding to those of the BODIPY fluorophores. Binaphthalene-substituted
mono(BODIPY) 3 showed a positive long-wavelength transition (λ ¼ 495 nm,
Δε max ¼ +10 Lmol
À1 cm
À1 ) related to the S 0 –S 1 absorption band (λ ¼ 507 nm) of
the BODIPY fluorophore. More interestingly binaphthalene-substituted bis
(BODIPY) 4 showed a positive, strong asymmetric couplet (λ ¼ 501 nm,
Δε max ¼ +53 Lmol
À1 cm
À1 ; λ ¼ 485 nm, Δε max ¼ À5 Lmol
À1 cm
À1 ). Quantumchemical calculations of the excited states of both mono(BODIPY) 3 and bis
(BODIPY) 4 suggest that the observed long-wavelength CD transition of
binaphthalene-substituted bis(BODIPY) 4 corresponds to an exciton-coupled transition arising from the interaction between the two BODIPY fluorophores (Fig. 6.3).
Due to a combination of long-wavelength CD transitions and the redox behaviour of the BODIPY fluorophores, binaphthalene-substituted bis(BODIPY) 4 was
subsequently shown to act as an electrochemically controlled chiroptical switch.
Thus CD-spectroelectrochemical measurements showed a decrease in the intensity
of the Cotton effect observed at 501 nm when reductive potential was increased,
attributed to the reversible reduction of BODIPY 4 to the corresponding dianion
(E 1/2 ¼ À1510 mV) in which one electron is transferred to each chromophore [33].
In related system by Beer, Rurack, and Daub, an enantiopure binaphthalenesubstituted bis(BODIPY) 5 was constructed containing free hydroxyl groups on the
chiral binaphthalene ((R)-BINOL) moiety, amine-mediated deprotonation of which
led to static quenching of the BODIPY fluorescence [34]. Interestingly in the
presence of optically active bases (i.e. (R)-(+)- or (S)-(À)-1-phenylethylamine),
N B
N
CO 2 Et
EtO 2 C
F F
H
OMe
CF 3
N B
N
Ph
F F
H
OMe
CF 3
H
MeO
F 3 C
2
1
Fig. 6.2 Mono(BODIPY)s
bearing chiral-phenyl
substituents at the meso- (1)
or α-positions (2)
O
O
O
O
N
B N
N
B N
N B
N
F
F
F
F
F
F
3
4
60
40
20
0
470
Lmol –1 cm –1
Δe /
λ / nm
480
490
500
510
520
Fig. 6.3 1,1
0 -Binaphthalene-based mono(BODIPY) 3 and bis(BODIPY) 4, including CD spectra
(3 (broken line) and 4 (solid line)) [adapted with permission from [33], copyright 2000 WILEYVCH Verlag GmbH]
6 BODIPY Based Emitters of Circularly Polarized Luminescence
121
from the binaphthalene moiety, along with longer wavelength absorptions
corresponding to those of the BODIPY fluorophores. Binaphthalene-substituted
mono(BODIPY) 3 showed a positive long-wavelength transition (λ ¼ 495 nm,
Δε max ¼ +10 Lmol
À1 cm
À1 ) related to the S 0 –S 1 absorption band (λ ¼ 507 nm) of
the BODIPY fluorophore. More interestingly binaphthalene-substituted bis
(BODIPY) 4 showed a positive, strong asymmetric couplet (λ ¼ 501 nm,
Δε max ¼ +53 Lmol
À1 cm
À1 ; λ ¼ 485 nm, Δε max ¼ À5 Lmol
À1 cm
À1 ). Quantumchemical calculations of the excited states of both mono(BODIPY) 3 and bis
(BODIPY) 4 suggest that the observed long-wavelength CD transition of
binaphthalene-substituted bis(BODIPY) 4 corresponds to an exciton-coupled transition arising from the interaction between the two BODIPY fluorophores (Fig. 6.3).
Due to a combination of long-wavelength CD transitions and the redox behaviour of the BODIPY fluorophores, binaphthalene-substituted bis(BODIPY) 4 was
subsequently shown to act as an electrochemically controlled chiroptical switch.
Thus CD-spectroelectrochemical measurements showed a decrease in the intensity
of the Cotton effect observed at 501 nm when reductive potential was increased,
attributed to the reversible reduction of BODIPY 4 to the corresponding dianion
(E 1/2 ¼ À1510 mV) in which one electron is transferred to each chromophore [33].
In related system by Beer, Rurack, and Daub, an enantiopure binaphthalenesubstituted bis(BODIPY) 5 was constructed containing free hydroxyl groups on the
chiral binaphthalene ((R)-BINOL) moiety, amine-mediated deprotonation of which
led to static quenching of the BODIPY fluorescence [34]. Interestingly in the
presence of optically active bases (i.e. (R)-(+)- or (S)-(À)-1-phenylethylamine),
N B
N
CO 2 Et
EtO 2 C
F F
H
OMe
CF 3
N B
N
Ph
F F
H
OMe
CF 3
H
MeO
F 3 C
2
1
Fig. 6.2 Mono(BODIPY)s
bearing chiral-phenyl
substituents at the meso- (1)
or α-positions (2)
O
O
O
O
N
B N
N
B N
N B
N
F
F
F
F
F
F
3
4
60
40
20
0
470
Lmol –1 cm –1
Δe /
λ / nm
480
490
500
510
520
Fig. 6.3 1,1
0 -Binaphthalene-based mono(BODIPY) 3 and bis(BODIPY) 4, including CD spectra
(3 (broken line) and 4 (solid line)) [adapted with permission from [33], copyright 2000 WILEYVCH Verlag GmbH]
6 BODIPY Based Emitters of Circularly Polarized Luminescence
121