458
K. Imato and Y. Ooyama
stability. Although there are few studies on the
1 O 2 generation characteristics of PDI,
the effects of substituents on the PDI skeleton on
1 O 2 generation efficiency have
been investigated (Fig. 13.7a). PDI-Br2 and PDI-Br4, in which the 1,7-positions
or 2,5,8,11-positions of the PDI skeleton are substituted with 2 or 4 bromine atoms,
show high Φ values (0.23 and 0.85, respectively) due to the heavy atom effect of
bromine atoms (Yang et al. 2016). The substitution with phenylethenyl groups at the
1,7-positions or 2,5,8,11-positions of the PDI skeleton (PDI-PhE2 and PDI-PhE4)
also enhances the Φ values. In particular, the Φ value of PDI-PhE4 modified with
four phenylethenyl groups reaches ca. 0.6. Ph-PDI (λ
abs
max = ca. 530 nm) modified
with para-substituted aryl groups at the 2,5,8,11-positions shows higher Φ T and Φ
values than unsubstituted PDI (λ
abs
max = 525 nm) (Yu et al. 2016). The Φ value
of MeSPh-PDI having methylthioxyphenyl groups is extremely high (0.8). The Φ T
values increase in the order of Ph-PDI (0.08) < MeOPh-PDI (0.54) < MeSPh-PDI
(0.86), which is attributed to the increased spin–orbit interaction.
A series of naphthalene diimides (NDIs) substituted with alkylamino groups
containing a quaternary ammonium salt at one or both of the 2,6-positions have high
water solubility and show photoabsorption bands at 500–600 nm (Fig. 13.7a, NDIn2N3Br, NDI-n3N3Br, NDI-n2N4, NDI-n3N4). NDI-n2N3Br and NDI-n3N3Br
exhibit high Φ values (ca. 0.4–0.6) due to the heavy atom effect of a bromine atom.
NDI-n3N4, on the other hand, has no bromine atoms but exhibits a moderate Φ
value (0.3) (Doria et al. 2013).
In aromatic carbonyl compounds such as benzophenone, highly efficient ISC
(from S 1 to T 1 ) from
1 Dye* to
3 Dye* is observed because of the low energy levels
of n-π* excited states. The reasons for the highly efficient ISC are that (1) the
transition from
1 (n-π)* to
3 (π-π)* is allowed although that from
1 (π-π)* to
3 (π-π)*
is forbidden and (2) the small electron exchange energy for the n-π* transition due
to the spatial separation of the n and π orbitals renders the energy gaps between
S 1 and T 1 of aromatic carbonyl compounds small (Fig. 13.1b). Zhao et al. reported
that a series of ketocoumarin dyes (KCOU-1–5) have photoabsorptions in a shortwavelength region (λ
abs
max = ca. 450 nm, ε = ca. 10,000 M
–1 cm
–1 , Fig. 13.7b),
but their triplet states are long-lived, which contributes to moderate
1 O 2 generation
characteristics (Φ = 0.28–0.48) (Huang et al. 2013).
Water-soluble bisarylidenecycloalkanone dyes (AC-1 and AC-2) have photoabsorption maxima in a short-wavelength region around 480 nm (ε = ca. 60,000 M
–1
cm
–1 ) but show two-photon absorption characteristics (AC-1, ca. 900 GM @λ
2PA
=
820 nm; AC-2, ca. 1100 GM @λ
2PA
= 820 nm) and moderate Φ values (AC-1,
0.26; AC-2, 0.14; Fig. 13.7c) (Zou et al. 2015).
As described above, heteropolycyclic photosensitizing dyes have relatively good
1 O 2 generation characteristics. However, most of them show photoabsorption bands
in short-wavelength regions below the phototherapeutic window (650–900 nm). To
be used as photosensitizing dyes for PDT, the photoabsorption of heteropolycyclic
photosensitizing dyes should be improved in long-wavelength regions.
K. Imato and Y. Ooyama
stability. Although there are few studies on the
1 O 2 generation characteristics of PDI,
the effects of substituents on the PDI skeleton on
1 O 2 generation efficiency have
been investigated (Fig. 13.7a). PDI-Br2 and PDI-Br4, in which the 1,7-positions
or 2,5,8,11-positions of the PDI skeleton are substituted with 2 or 4 bromine atoms,
show high Φ values (0.23 and 0.85, respectively) due to the heavy atom effect of
bromine atoms (Yang et al. 2016). The substitution with phenylethenyl groups at the
1,7-positions or 2,5,8,11-positions of the PDI skeleton (PDI-PhE2 and PDI-PhE4)
also enhances the Φ values. In particular, the Φ value of PDI-PhE4 modified with
four phenylethenyl groups reaches ca. 0.6. Ph-PDI (λ
abs
max = ca. 530 nm) modified
with para-substituted aryl groups at the 2,5,8,11-positions shows higher Φ T and Φ
values than unsubstituted PDI (λ
abs
max = 525 nm) (Yu et al. 2016). The Φ value
of MeSPh-PDI having methylthioxyphenyl groups is extremely high (0.8). The Φ T
values increase in the order of Ph-PDI (0.08) < MeOPh-PDI (0.54) < MeSPh-PDI
(0.86), which is attributed to the increased spin–orbit interaction.
A series of naphthalene diimides (NDIs) substituted with alkylamino groups
containing a quaternary ammonium salt at one or both of the 2,6-positions have high
water solubility and show photoabsorption bands at 500–600 nm (Fig. 13.7a, NDIn2N3Br, NDI-n3N3Br, NDI-n2N4, NDI-n3N4). NDI-n2N3Br and NDI-n3N3Br
exhibit high Φ values (ca. 0.4–0.6) due to the heavy atom effect of a bromine atom.
NDI-n3N4, on the other hand, has no bromine atoms but exhibits a moderate Φ
value (0.3) (Doria et al. 2013).
In aromatic carbonyl compounds such as benzophenone, highly efficient ISC
(from S 1 to T 1 ) from
1 Dye* to
3 Dye* is observed because of the low energy levels
of n-π* excited states. The reasons for the highly efficient ISC are that (1) the
transition from
1 (n-π)* to
3 (π-π)* is allowed although that from
1 (π-π)* to
3 (π-π)*
is forbidden and (2) the small electron exchange energy for the n-π* transition due
to the spatial separation of the n and π orbitals renders the energy gaps between
S 1 and T 1 of aromatic carbonyl compounds small (Fig. 13.1b). Zhao et al. reported
that a series of ketocoumarin dyes (KCOU-1–5) have photoabsorptions in a shortwavelength region (λ
abs
max = ca. 450 nm, ε = ca. 10,000 M
–1 cm
–1 , Fig. 13.7b),
but their triplet states are long-lived, which contributes to moderate
1 O 2 generation
characteristics (Φ = 0.28–0.48) (Huang et al. 2013).
Water-soluble bisarylidenecycloalkanone dyes (AC-1 and AC-2) have photoabsorption maxima in a short-wavelength region around 480 nm (ε = ca. 60,000 M
–1
cm
–1 ) but show two-photon absorption characteristics (AC-1, ca. 900 GM @λ
2PA
=
820 nm; AC-2, ca. 1100 GM @λ
2PA
= 820 nm) and moderate Φ values (AC-1,
0.26; AC-2, 0.14; Fig. 13.7c) (Zou et al. 2015).
As described above, heteropolycyclic photosensitizing dyes have relatively good
1 O 2 generation characteristics. However, most of them show photoabsorption bands
in short-wavelength regions below the phototherapeutic window (650–900 nm). To
be used as photosensitizing dyes for PDT, the photoabsorption of heteropolycyclic
photosensitizing dyes should be improved in long-wavelength regions.