4 Phthalocyanine and Related Analogues
107
4.4.2 Contracted and Expanded Pc Analogues with Unique
Properties
Contraction and expansion of the π-conjugated system of Pc by abstraction and
addition of isoindole rings and bridging imino nitrogen atoms cause blue-shift and
red-shift of the Q bands, respectively. These kinds of analogues are named contracted
Pc and expanded Pc according to the nomenclature of porphyrin analogues (Sessler
et al. 2017). In contrast to various expanded porphyrins (Tanaka and Osuka 2017),
there are only a few expanded Pcs known to date because of their synthetic difficulties.
As described in Sect. 4.2, Pc is formed by cyclotetramerization of phthalonitrile
or its derivatives. Because reactions are normally conducted at high temperatures, it
is difficult to isolate oligoisoindole intermediates, which can be used for a stepwise
synthesis. Considering that the availability of oligopyrrole precursors has enabled
the stepwise synthesis of expanded porphyrins, this is one of the reasons why the
synthesis of expanded Pcs is significantly difficult.
Superphthalocyanine (SuperPc) is a rare example of expanded Pcs. The first
synthesis of SuperPc traces back more than 50 years ago. When Bloor et al. investigated the synthesis of uranyl complex of Pc, they characterized the obtained product
as their target compound despite its significantly red-shifted Q band in the NIR region
ranging from 800 to 1000 nm (Fig. 4.19) (Bloor et al. 1964). The reason for the false
characterization of this compound is probably because H 2 Pc was obtained after the
removal of the central uranyl ion. Nowadays, it is known that ring contraction of
SuperPc to H 2 Pc occurs upon removal of the central uranyl ion. Although SuperPc
is intriguing not only as an expanded Pc but also as a NIR chromophore, only the
synthesis of unsubstituted and β-alkyl-substituted SuperPcs was reported (Day et al.
1975).
As Marks et al. reported, the reaction conditions are rather critical. The reaction
should be conducted under dry conditions because the presence of water in reagents
promotes the formation of H 2 Pc as a byproduct. They also found the use of UO 2 Cl 2
and N,N-dimethylformamide (DMF) is indispensable. A standard reaction scheme of
SuperPc is shown in Fig. 4.19. Owing to their pioneering work, SuperPc is characterized as a Pc-like aromatic compound, but with an expanded 22π-electron conjugation
(Day et al. 1975).
Recently, Furuyama and Kobayashi reported a benzo-ring-abstracted analogue
of SuperPc called superporphyrazine (SuperPz) (Furuyama et al. 2012). SuperPz
was synthesized from a similar uranyl template synthesis of 3,4-diarylpyrroline-2,5diimine (Scheme 4.11). Despite the loss of the benzo-rings, SuperPz also exhibits
broad absorption in the NIR region (Fig. 4.20). In an analogy to the theoretical
description of the absorption properties of Pc based on Gouterman’s four orbital
theory, this broad NIR absorption can be assigned as a Q-like band arising from
the 22π-electron aromatic conjugation. Faraday A terms were observed in both the
Soret and Q band regions, being indicative of the degenerate excited states. In the
same study, they also reported a low-symmetry SuperPz, which possesses one isoindole ring instead of a pyrrole ring. Due to the perturbation by the benzo-fusion,
107
4.4.2 Contracted and Expanded Pc Analogues with Unique
Properties
Contraction and expansion of the π-conjugated system of Pc by abstraction and
addition of isoindole rings and bridging imino nitrogen atoms cause blue-shift and
red-shift of the Q bands, respectively. These kinds of analogues are named contracted
Pc and expanded Pc according to the nomenclature of porphyrin analogues (Sessler
et al. 2017). In contrast to various expanded porphyrins (Tanaka and Osuka 2017),
there are only a few expanded Pcs known to date because of their synthetic difficulties.
As described in Sect. 4.2, Pc is formed by cyclotetramerization of phthalonitrile
or its derivatives. Because reactions are normally conducted at high temperatures, it
is difficult to isolate oligoisoindole intermediates, which can be used for a stepwise
synthesis. Considering that the availability of oligopyrrole precursors has enabled
the stepwise synthesis of expanded porphyrins, this is one of the reasons why the
synthesis of expanded Pcs is significantly difficult.
Superphthalocyanine (SuperPc) is a rare example of expanded Pcs. The first
synthesis of SuperPc traces back more than 50 years ago. When Bloor et al. investigated the synthesis of uranyl complex of Pc, they characterized the obtained product
as their target compound despite its significantly red-shifted Q band in the NIR region
ranging from 800 to 1000 nm (Fig. 4.19) (Bloor et al. 1964). The reason for the false
characterization of this compound is probably because H 2 Pc was obtained after the
removal of the central uranyl ion. Nowadays, it is known that ring contraction of
SuperPc to H 2 Pc occurs upon removal of the central uranyl ion. Although SuperPc
is intriguing not only as an expanded Pc but also as a NIR chromophore, only the
synthesis of unsubstituted and β-alkyl-substituted SuperPcs was reported (Day et al.
1975).
As Marks et al. reported, the reaction conditions are rather critical. The reaction
should be conducted under dry conditions because the presence of water in reagents
promotes the formation of H 2 Pc as a byproduct. They also found the use of UO 2 Cl 2
and N,N-dimethylformamide (DMF) is indispensable. A standard reaction scheme of
SuperPc is shown in Fig. 4.19. Owing to their pioneering work, SuperPc is characterized as a Pc-like aromatic compound, but with an expanded 22π-electron conjugation
(Day et al. 1975).
Recently, Furuyama and Kobayashi reported a benzo-ring-abstracted analogue
of SuperPc called superporphyrazine (SuperPz) (Furuyama et al. 2012). SuperPz
was synthesized from a similar uranyl template synthesis of 3,4-diarylpyrroline-2,5diimine (Scheme 4.11). Despite the loss of the benzo-rings, SuperPz also exhibits
broad absorption in the NIR region (Fig. 4.20). In an analogy to the theoretical
description of the absorption properties of Pc based on Gouterman’s four orbital
theory, this broad NIR absorption can be assigned as a Q-like band arising from
the 22π-electron aromatic conjugation. Faraday A terms were observed in both the
Soret and Q band regions, being indicative of the degenerate excited states. In the
same study, they also reported a low-symmetry SuperPz, which possesses one isoindole ring instead of a pyrrole ring. Due to the perturbation by the benzo-fusion,