456
K. Imato and Y. Ooyama
exhibit no
1 O 2 generation. In contrast, T-BODIPY-P2 (R = CF 3 ) with electron withdrawing groups shows high ε (211,000 M
–1 cm
–1 ) and balanced Φ fl (0.39) and Φ
(0.47) values and thereby, has excellent BT (82,290 M
–1 cm
–1 ) and PP (99,170 M
–1
cm
–1 ). In the case of T-BODIPY-P derivatives, a HOMO–LUMO energy gap of
more than 1.5 eV is necessary for them to exhibit high
1 O 2 generation characteristics
(Φ > 0.2).
Aza-BODIPY dyes exhibit strong photoabsorption and moderate fluorescence
properties in the phototherapeutic window. Their photoabsorption spectra are
bathochromically shifted by ca. 100 nm compared to those of BODIPY dyes, and
their photoabsorption maximum wavelengths are observed around 650–700 nm with
high ε (−10
5 M
–1 cm
–1 ) in the spectra. In addition, aza-BODIPY dyes show remarkable heavy atom effects similar to BODIPY dyes; the introduction of heavy atoms
such as bromine and iodine atoms onto the aza-BODIPY skeleton reduces Φ fl values
and dramatically improve Φ T and Φ values (Fig. 13.5c). For example, in the case
of Aza-BODIPY-1–6, iodinated Aza-BODIPY-6 has a photoabsorption maximum
wavelength around 680 nm (ε = ca. 50,000 M
–1 cm
–1 ) and high Φ T (0.86) and Φ
(0.80) values (Adarsh et al. 2012).
As described above, BODIPY dyes promise to construct a new group of photosensitizing dyes for PDT because of their high ε, Φ fl , and Φ values, and excellent
photostability. However, most of the previously developed BODIPY dyes absorb
light mainly below 500–600 nm, which should be bathochromically shifted to exhibit
excellent photosensitizing properties in the phototherapeutic window.
13.6 Xanthene and Phenothiazinium Photosensitizing Dyes
Xanthene dyes including rose bengal (RB), erythrosine B, eosin blue, fluorescein, and
rhodamine (Rhod), and phenothiazinium dyes including methylene blue (MB) show
photoabsorption maximum wavelengths at ca. 550–650 nm (ε = −10
5 M
–1 cm
–1 ).
These xanthene and phenothiazinium dyes have excited triplet energy levels (energy
difference between their ground states,
1 Dye, and excited triplet states,
3 Dye*) close
to the energy difference between
1 O 2 (
1
Δ g ) and
3 O 2 (
3
Σ g
– ) (94.1 kJ mol
–1
≈ 1270 nm
(
1 O 2 phosphorescence wavelength) ≈ 0.98 eV, Fig. 13.1c), and thereby, show high
Φ values due to the efficient energy transfer between
3 Dye* and
1 O 2 (Fig. 13.6a)
(DeRosa and Crutchley 2002; Ronzani et al. 2013). The Φ values of RB and MB are
ca. 0.7–0.8 and 0.5–0.6, respectively, and they are used as standard photosensitizing
dyes for xanthene and phenothiazinium dyes.
Many studies on the photochemical and electrochemical properties of xanthene
and phenothiazinium dyes have revealed that the introduction of bromine and
iodine atoms into the xanthene skeleton bathochromically shifts the photoabsorption
maximum wavelength, and the heavy atom effect of halogen atoms improves the Φ T
and Φ values. Therefore, tetraiodide-substituted RB and erythrosine B are efficient
photosensitizing dyes (Fig. 13.6a) (Pal et al. 1996). Detty et al. synthesized TMR-E,
analogues of rhodamine/rosamine dyes in which the bridge atoms are displaced by
K. Imato and Y. Ooyama
exhibit no
1 O 2 generation. In contrast, T-BODIPY-P2 (R = CF 3 ) with electron withdrawing groups shows high ε (211,000 M
–1 cm
–1 ) and balanced Φ fl (0.39) and Φ
(0.47) values and thereby, has excellent BT (82,290 M
–1 cm
–1 ) and PP (99,170 M
–1
cm
–1 ). In the case of T-BODIPY-P derivatives, a HOMO–LUMO energy gap of
more than 1.5 eV is necessary for them to exhibit high
1 O 2 generation characteristics
(Φ > 0.2).
Aza-BODIPY dyes exhibit strong photoabsorption and moderate fluorescence
properties in the phototherapeutic window. Their photoabsorption spectra are
bathochromically shifted by ca. 100 nm compared to those of BODIPY dyes, and
their photoabsorption maximum wavelengths are observed around 650–700 nm with
high ε (−10
5 M
–1 cm
–1 ) in the spectra. In addition, aza-BODIPY dyes show remarkable heavy atom effects similar to BODIPY dyes; the introduction of heavy atoms
such as bromine and iodine atoms onto the aza-BODIPY skeleton reduces Φ fl values
and dramatically improve Φ T and Φ values (Fig. 13.5c). For example, in the case
of Aza-BODIPY-1–6, iodinated Aza-BODIPY-6 has a photoabsorption maximum
wavelength around 680 nm (ε = ca. 50,000 M
–1 cm
–1 ) and high Φ T (0.86) and Φ
(0.80) values (Adarsh et al. 2012).
As described above, BODIPY dyes promise to construct a new group of photosensitizing dyes for PDT because of their high ε, Φ fl , and Φ values, and excellent
photostability. However, most of the previously developed BODIPY dyes absorb
light mainly below 500–600 nm, which should be bathochromically shifted to exhibit
excellent photosensitizing properties in the phototherapeutic window.
13.6 Xanthene and Phenothiazinium Photosensitizing Dyes
Xanthene dyes including rose bengal (RB), erythrosine B, eosin blue, fluorescein, and
rhodamine (Rhod), and phenothiazinium dyes including methylene blue (MB) show
photoabsorption maximum wavelengths at ca. 550–650 nm (ε = −10
5 M
–1 cm
–1 ).
These xanthene and phenothiazinium dyes have excited triplet energy levels (energy
difference between their ground states,
1 Dye, and excited triplet states,
3 Dye*) close
to the energy difference between
1 O 2 (
1
Δ g ) and
3 O 2 (
3
Σ g
– ) (94.1 kJ mol
–1
≈ 1270 nm
(
1 O 2 phosphorescence wavelength) ≈ 0.98 eV, Fig. 13.1c), and thereby, show high
Φ values due to the efficient energy transfer between
3 Dye* and
1 O 2 (Fig. 13.6a)
(DeRosa and Crutchley 2002; Ronzani et al. 2013). The Φ values of RB and MB are
ca. 0.7–0.8 and 0.5–0.6, respectively, and they are used as standard photosensitizing
dyes for xanthene and phenothiazinium dyes.
Many studies on the photochemical and electrochemical properties of xanthene
and phenothiazinium dyes have revealed that the introduction of bromine and
iodine atoms into the xanthene skeleton bathochromically shifts the photoabsorption
maximum wavelength, and the heavy atom effect of halogen atoms improves the Φ T
and Φ values. Therefore, tetraiodide-substituted RB and erythrosine B are efficient
photosensitizing dyes (Fig. 13.6a) (Pal et al. 1996). Detty et al. synthesized TMR-E,
analogues of rhodamine/rosamine dyes in which the bridge atoms are displaced by