5 BODIPY Dyes and Their Analogues
165
both the formazanate backbone and the dimethylanimophenyl group. Although 91
does not show electrochemiluminescence (ECL) in the absence of a coreactant, ECL
is induced upon the addition of tripropylamine, which reaches its maximum intensity at 0.54 V relative to the Fc/Fc
+ redox couple. The wavelength of maximum ECL
intensity is centred at 910 nm with an efficiency of 17.5% relative to the benchmark
[Ru(bpy) 3 ][PF 6 ] 2 /tripropylamine system.
5.2.4.4
5 NˆN and
7 NˆN Monoboron Complexes
A few examples (Li et al. 2013a; Frath et al. 2014) of five-membered ring NˆN
(
5 NˆN) type and seven-membered ring NˆN (
7 NˆN) monoboron complexes are
shown in Fig. 5.47. For the
5 NˆN type monoboron complexes, in 1969, Hohaus
and Umland reported a 8-aminoquinoline biphenylboron (BPh 2 aq) dye (Fig. 5.47a)
(Hohaus and Umland 1969). Although 92 has a small ε (4,100), it shows a large
Stokes shift (103 nm) (Nagata and Chujo 2008). Boron-complexation of 2,5-bis(4dimethylaminophenylazo)-1H-pyrrole (λ max = 575 nm, ε = 59,000) gives NIR
absorbing azopyrrole BF 2 complex 93 (λ max = 754 nm, ε = 60,000) (Fig. 5.47b)
(Li et al. 2009). Triazaborolopyridinium dyes show a relatively large Stokes shift
(94: 89 nm) and general insensitivity to solvent or pH; triazaborolopyridinium
dyes are membrane permeable and are suitable candidates of fluorescent probes
for intracellular targets (Fig. 5.47c) (Hapuarachchige et al. 2011).
Thermodynamically stable trans Az-BF 2 complex (λ max = 530 nm) isomerizes
to the cis isomer (λ max = 480 nm) upon irradiation at 570 nm; the irradiation of
the trans Az-BF 2 complex at 570 nm yields 97% of the cis isomer at the photostationary state (Fig. 5.47d) (Yang et al. 2012). Irradiation at 450 nm or storing the
cis isomer in the dark converts it back to the trans isomer; irradiation at 450 nm
produces 80% of the trans isomer at the photostationary state. The well separated π–
π* transitions between the trans and cis isomers enable efficient visible light-induced
isomerization. Phenanthro[9,10-d]imidazole-quinoline boron difluorides (PQBDs)
show a large Stokes shift (up to 115 nm) because of the efficient charge transfer from
the phenanthro[9,10-d]imidazole unit to the quinoline moiety and red fluorescence
in the solid-state (Φ f : up to 0.184) (Fig. 5.47e) (Li et al. 2013b). Despite 95 being
planar, no π–π stacking interactions in the crystal are observed, which accounts for
the relatively strong fluorescence in the solid-state.
The optical properties of BOIMPY are profoundly affected by the N-aryl
substituent group (R
1 and R
2 ) (Fig. 5.47f) (Lee et al. 2016). Although BOIMPY
96 has nearly co-planar arrangements of the iminopyrrolide fragment and the Naryl ring with a small torsion angle (17.17° and 11.70° for two crystallographically
independent molecules), 98 adopts an essentially orthogonal geometry with a large
torsion angle (89.48˚). 96 has a higher contribution to the quinoid-type resonance
structure, whereas 98 has a higher contribution to the benzoid resonance structure.
Thus, 96 shows a redshifted λ max compared with that of 98. Although 97 (Φ f = 0.15)
shows relatively high Φ f , 96 (0.02) and 98 (<0.001) hardly show fluorescence. Protonation of 96 and 98 by HBF 4 ·OEt 2 leads to a large blueshift of F max (96: 484 nm, 98:
165
both the formazanate backbone and the dimethylanimophenyl group. Although 91
does not show electrochemiluminescence (ECL) in the absence of a coreactant, ECL
is induced upon the addition of tripropylamine, which reaches its maximum intensity at 0.54 V relative to the Fc/Fc
+ redox couple. The wavelength of maximum ECL
intensity is centred at 910 nm with an efficiency of 17.5% relative to the benchmark
[Ru(bpy) 3 ][PF 6 ] 2 /tripropylamine system.
5.2.4.4
5 NˆN and
7 NˆN Monoboron Complexes
A few examples (Li et al. 2013a; Frath et al. 2014) of five-membered ring NˆN
(
5 NˆN) type and seven-membered ring NˆN (
7 NˆN) monoboron complexes are
shown in Fig. 5.47. For the
5 NˆN type monoboron complexes, in 1969, Hohaus
and Umland reported a 8-aminoquinoline biphenylboron (BPh 2 aq) dye (Fig. 5.47a)
(Hohaus and Umland 1969). Although 92 has a small ε (4,100), it shows a large
Stokes shift (103 nm) (Nagata and Chujo 2008). Boron-complexation of 2,5-bis(4dimethylaminophenylazo)-1H-pyrrole (λ max = 575 nm, ε = 59,000) gives NIR
absorbing azopyrrole BF 2 complex 93 (λ max = 754 nm, ε = 60,000) (Fig. 5.47b)
(Li et al. 2009). Triazaborolopyridinium dyes show a relatively large Stokes shift
(94: 89 nm) and general insensitivity to solvent or pH; triazaborolopyridinium
dyes are membrane permeable and are suitable candidates of fluorescent probes
for intracellular targets (Fig. 5.47c) (Hapuarachchige et al. 2011).
Thermodynamically stable trans Az-BF 2 complex (λ max = 530 nm) isomerizes
to the cis isomer (λ max = 480 nm) upon irradiation at 570 nm; the irradiation of
the trans Az-BF 2 complex at 570 nm yields 97% of the cis isomer at the photostationary state (Fig. 5.47d) (Yang et al. 2012). Irradiation at 450 nm or storing the
cis isomer in the dark converts it back to the trans isomer; irradiation at 450 nm
produces 80% of the trans isomer at the photostationary state. The well separated π–
π* transitions between the trans and cis isomers enable efficient visible light-induced
isomerization. Phenanthro[9,10-d]imidazole-quinoline boron difluorides (PQBDs)
show a large Stokes shift (up to 115 nm) because of the efficient charge transfer from
the phenanthro[9,10-d]imidazole unit to the quinoline moiety and red fluorescence
in the solid-state (Φ f : up to 0.184) (Fig. 5.47e) (Li et al. 2013b). Despite 95 being
planar, no π–π stacking interactions in the crystal are observed, which accounts for
the relatively strong fluorescence in the solid-state.
The optical properties of BOIMPY are profoundly affected by the N-aryl
substituent group (R
1 and R
2 ) (Fig. 5.47f) (Lee et al. 2016). Although BOIMPY
96 has nearly co-planar arrangements of the iminopyrrolide fragment and the Naryl ring with a small torsion angle (17.17° and 11.70° for two crystallographically
independent molecules), 98 adopts an essentially orthogonal geometry with a large
torsion angle (89.48˚). 96 has a higher contribution to the quinoid-type resonance
structure, whereas 98 has a higher contribution to the benzoid resonance structure.
Thus, 96 shows a redshifted λ max compared with that of 98. Although 97 (Φ f = 0.15)
shows relatively high Φ f , 96 (0.02) and 98 (<0.001) hardly show fluorescence. Protonation of 96 and 98 by HBF 4 ·OEt 2 leads to a large blueshift of F max (96: 484 nm, 98: