5 BODIPY Dyes and Their Analogues
153
for vacuum processable organic solar cells; bulk heterojunction organic solar cells
(BHJSCs) comprising KFL-4 as the electron donor and C 60 as the electron acceptor
exhibit a PCE of 6.1% (Li et al. 2017b).
Benzofuran and naphtho[1,2-b]furan-fused BODIPY dyes exhibit a further
redshifted λ max (33 (Li et al. 2018b): λ max = 676 nm, 34 (Li et al. 2018b): λ max
= 694 nm) (Fig. 5.36b, c). Single crystal X-ray analysis suggests that the slope angle
and the distance between layers are 21.08° and 21.68° and 3.5 Å and 3.9 Å for 33
and 34, respectively, which represents typical J-aggregation. The F max values of 33
(from 686 to 757 nm) and 34 (from 707 to 777 nm) are redshifted, and the Φ f values
become lower with increasing concentration in solution. The λ max of benzofuro[3,2b]pyrrole-based BODIPY 35 (Chen et al. 2000) (637 nm) is more redshifted than
that of benzofuro[2,3-b]pyrrole-based BODIPY 36 (Belmonte-Vázquez et al. 2019)
(586 nm) (Fig. 5.36d, e). BODIPY 36 is applied to a redemitting laser dye; the laser
efficiency (Eff) and laser peak wavelength (λ peak ) are 44% and 647.2 nm, respectively.
The absorption and fluorescence properties of thiphene-[b]-fused BODIPY dyes
are shown in Fig. 5.37. In 2011, You et al. reported the first synthesis of thiphene-[b]fused BODIPY dyes (Awuah et al. 2011). Thiphene-fused BODIPY dye SBDPiR731
(Awuah et al. 2011) (λ max = 731 nm, F max = 754 nm, Φ f = 0.37) shows slightly
redshifted λ max and F max values and lower Φ f compared with those of the corresponding furan-fused derivative KFL-4 (λ max = 723 nm, F max = 738 nm, Φ f =
0.56) (Figs. 5.36a and 5.37a). Ji et al. reported that the incorporation of sulfur atoms
into the π-conjugated skeleton of BODIPY is an efficient strategy to attain high
quantum yields for triplet-state formation (Fig. 5.37b) (Ji et al. 2015). The triplet
formation of 37 (triplet-state formation quantum yield Φ T = 0.637) is more efficient compared with that of the non-fused derivative 38 (Φ T = 0.061). Theoretical
calculations demonstrate that the increased intersystem crossing (ISC) mechanism
of 37 compared with that of 38 is caused by the participation of the sulfur atom in
the lowest-lying excited states, which leads to moderate spin-orbit coupling (SOC)
and the small singlet-triplet energy gap in 37.
While thiphene-fused BODIPY SBDPiR731 does not show singlet oxygen
generation, the brominated derivatives show efficient singlet oxygen generation
(Fig. 5.37a). Inclusion of heavy atoms tends to induce cytotoxicity in the absence of
light. Despite the absence of heavy atom substituents, nonsubstituted (SBDPiR690
(Awuah et al. 2013)) and trifluoromethyl-substituted (SBDPiR688 (Watley et al.
2015)) derivatives generate singlet oxygen atoms; the singlet oxygen quantum yields
(Φ ) of SBDPiR690 and SBDPiR688 are 0.42 and 0.47, respectively. Additionally,
SBDPiR690 (Φ f = 0.22) and SBDPiR688 (Φ f = 0.39) exhibit fluorescence. Such
dual functional photosensitizers are rare and can be applied to not only photodynamic
therapy (PDT), but also in fluorescence tumour detection (image-guided surgery).
SBDPiR690 and especially SBDPiR688 have a remarkable dual functionality of
brightness (BT = ε × Φ f : SBDPiR690: BT = 26,400 M
−1 cm
−1 , SBDPiR688:
BT = 82,290 M
−1 cm
−1 ) and phototoxic power (PP = ε × Φ : SBDPiR690: PP
= 50,400 M
−1 cm
−1 , SBDPiR688: PP = 211,000 M
−1 cm
−1 ) owing to the high ε
and balanced decay (Φ f and Φ ). The dual functionality of SBDPiR690 has been
successfully applied to preclinical optical imaging and PDT (Watley et al. 2015).
153
for vacuum processable organic solar cells; bulk heterojunction organic solar cells
(BHJSCs) comprising KFL-4 as the electron donor and C 60 as the electron acceptor
exhibit a PCE of 6.1% (Li et al. 2017b).
Benzofuran and naphtho[1,2-b]furan-fused BODIPY dyes exhibit a further
redshifted λ max (33 (Li et al. 2018b): λ max = 676 nm, 34 (Li et al. 2018b): λ max
= 694 nm) (Fig. 5.36b, c). Single crystal X-ray analysis suggests that the slope angle
and the distance between layers are 21.08° and 21.68° and 3.5 Å and 3.9 Å for 33
and 34, respectively, which represents typical J-aggregation. The F max values of 33
(from 686 to 757 nm) and 34 (from 707 to 777 nm) are redshifted, and the Φ f values
become lower with increasing concentration in solution. The λ max of benzofuro[3,2b]pyrrole-based BODIPY 35 (Chen et al. 2000) (637 nm) is more redshifted than
that of benzofuro[2,3-b]pyrrole-based BODIPY 36 (Belmonte-Vázquez et al. 2019)
(586 nm) (Fig. 5.36d, e). BODIPY 36 is applied to a redemitting laser dye; the laser
efficiency (Eff) and laser peak wavelength (λ peak ) are 44% and 647.2 nm, respectively.
The absorption and fluorescence properties of thiphene-[b]-fused BODIPY dyes
are shown in Fig. 5.37. In 2011, You et al. reported the first synthesis of thiphene-[b]fused BODIPY dyes (Awuah et al. 2011). Thiphene-fused BODIPY dye SBDPiR731
(Awuah et al. 2011) (λ max = 731 nm, F max = 754 nm, Φ f = 0.37) shows slightly
redshifted λ max and F max values and lower Φ f compared with those of the corresponding furan-fused derivative KFL-4 (λ max = 723 nm, F max = 738 nm, Φ f =
0.56) (Figs. 5.36a and 5.37a). Ji et al. reported that the incorporation of sulfur atoms
into the π-conjugated skeleton of BODIPY is an efficient strategy to attain high
quantum yields for triplet-state formation (Fig. 5.37b) (Ji et al. 2015). The triplet
formation of 37 (triplet-state formation quantum yield Φ T = 0.637) is more efficient compared with that of the non-fused derivative 38 (Φ T = 0.061). Theoretical
calculations demonstrate that the increased intersystem crossing (ISC) mechanism
of 37 compared with that of 38 is caused by the participation of the sulfur atom in
the lowest-lying excited states, which leads to moderate spin-orbit coupling (SOC)
and the small singlet-triplet energy gap in 37.
While thiphene-fused BODIPY SBDPiR731 does not show singlet oxygen
generation, the brominated derivatives show efficient singlet oxygen generation
(Fig. 5.37a). Inclusion of heavy atoms tends to induce cytotoxicity in the absence of
light. Despite the absence of heavy atom substituents, nonsubstituted (SBDPiR690
(Awuah et al. 2013)) and trifluoromethyl-substituted (SBDPiR688 (Watley et al.
2015)) derivatives generate singlet oxygen atoms; the singlet oxygen quantum yields
(Φ ) of SBDPiR690 and SBDPiR688 are 0.42 and 0.47, respectively. Additionally,
SBDPiR690 (Φ f = 0.22) and SBDPiR688 (Φ f = 0.39) exhibit fluorescence. Such
dual functional photosensitizers are rare and can be applied to not only photodynamic
therapy (PDT), but also in fluorescence tumour detection (image-guided surgery).
SBDPiR690 and especially SBDPiR688 have a remarkable dual functionality of
brightness (BT = ε × Φ f : SBDPiR690: BT = 26,400 M
−1 cm
−1 , SBDPiR688:
BT = 82,290 M
−1 cm
−1 ) and phototoxic power (PP = ε × Φ : SBDPiR690: PP
= 50,400 M
−1 cm
−1 , SBDPiR688: PP = 211,000 M
−1 cm
−1 ) owing to the high ε
and balanced decay (Φ f and Φ ). The dual functionality of SBDPiR690 has been
successfully applied to preclinical optical imaging and PDT (Watley et al. 2015).