62
J. Setsune
the cyclization of the linear ethyne-linked porphyrin octamer 36 by Sonogashira
coupling to give a 1:2 complex, cyclo-36•(T’ 4 ) 2 , in 32% yield. The fluorescence
band of cyclo-35•T’ 6 at 1073 nm is red-shifted relative to that of the linear hexamer,
linear-35, at 909 nm and the fluorescence quantum yield of cyclo-35•T’ 6 is smaller
by the factor of 1000, which is indicative of the highly delocalized singlet excited
state of the cyclohexamer.
Anderson, Rebane, and coworkers studied two-photon absorption efficiency of
their π-conjugated oligomers (Drobizhev et al. 2006). The 2PA cross section values
(σ) of the linear Zn porphyrin oligomers increased by 450 times upon going from
the monomer 32(SiR 3 ) 2 to the dimer 33(SiR 3 ) 2 , but only by 2.4 times from the
dimer 33(SiR 3 ) 2 to the tetramer 34(SiR 3 ) 2 . The σ value increased gradually from
22,000 GM to 37,000 GM when the porphyrin units increased from 4 to 8 in a
series of butadiyne-linked linear oligomers end-capped with trialkylsilyl groups. The
2PA cross section of cyclo-(32) 6 •T 6 determined by femto-second 2P fluorescence
intensity with laser pulses at 1000 nm was 151,000 GM that is much greater than
23,000 GM for the linear hexameric porphyrin oligomer of 32 end-capped with
trialkylsilyl groups (Mikhaylov et al. 2016). This great σ value is responsible for the
effective π-conjugation over the whole ring that is promoted by the complexation of
the template.
3.3 Porphyrins Substituted at Pyrrole β-Positions
3.3.1 Barton–Zard Pyrrole Synthesis
A number of porphyrins with substituents at the pyrrole β-positions are derived from
β-substituted pyrroles. Among many synthetic methods for pyrrole ring such as
classical Knorr pyrrole synthesis, Barton–Zard synthesis is frequently used because
of generality, applicability, and simple procedure. Nitro olefins are generated from
nitroaldol products (acetoxynitroalkanes 37) in situ under basic reaction conditions
of Barton–Zard synthesis and then undergo [3+2]-cycloaddition with isocyanoacetate to give 3,4-disubstituted pyrrole-2-carboxylate 38 (Fig. 3.15) (Barton and Zard
1985). The ester function at one of the pyrrole α-positions is regarded as a protecting
group, and it can be easily removed or utilized as a meso-carbon of porphyrin structure leading to versatile application. A related methodology, van Leusen pyrrole
synthesis, using tosylmethyl isocyanide (TosMIC) and Michael acceptor is useful in
the synthesis of 3,4-disubstituted pyrroles 39 having electron-withdrawing groups
(EWG) at the β-position (van Leusen et al. 1992). 2-Stannylpyrrole 40 was also
obtainable in good yields by one-pot procedure, and it is useful in the Stille coupling
reactions leading to various building blocks for porphyrin analogues (Dijkstra et al.
1998).
Naturally occurring porphyrins have alkyl substituents at all the pyrrole
β-positions. These 2,3,7,8,12,13,17,18-octaalkylporphyrins are different from
J. Setsune
the cyclization of the linear ethyne-linked porphyrin octamer 36 by Sonogashira
coupling to give a 1:2 complex, cyclo-36•(T’ 4 ) 2 , in 32% yield. The fluorescence
band of cyclo-35•T’ 6 at 1073 nm is red-shifted relative to that of the linear hexamer,
linear-35, at 909 nm and the fluorescence quantum yield of cyclo-35•T’ 6 is smaller
by the factor of 1000, which is indicative of the highly delocalized singlet excited
state of the cyclohexamer.
Anderson, Rebane, and coworkers studied two-photon absorption efficiency of
their π-conjugated oligomers (Drobizhev et al. 2006). The 2PA cross section values
(σ) of the linear Zn porphyrin oligomers increased by 450 times upon going from
the monomer 32(SiR 3 ) 2 to the dimer 33(SiR 3 ) 2 , but only by 2.4 times from the
dimer 33(SiR 3 ) 2 to the tetramer 34(SiR 3 ) 2 . The σ value increased gradually from
22,000 GM to 37,000 GM when the porphyrin units increased from 4 to 8 in a
series of butadiyne-linked linear oligomers end-capped with trialkylsilyl groups. The
2PA cross section of cyclo-(32) 6 •T 6 determined by femto-second 2P fluorescence
intensity with laser pulses at 1000 nm was 151,000 GM that is much greater than
23,000 GM for the linear hexameric porphyrin oligomer of 32 end-capped with
trialkylsilyl groups (Mikhaylov et al. 2016). This great σ value is responsible for the
effective π-conjugation over the whole ring that is promoted by the complexation of
the template.
3.3 Porphyrins Substituted at Pyrrole β-Positions
3.3.1 Barton–Zard Pyrrole Synthesis
A number of porphyrins with substituents at the pyrrole β-positions are derived from
β-substituted pyrroles. Among many synthetic methods for pyrrole ring such as
classical Knorr pyrrole synthesis, Barton–Zard synthesis is frequently used because
of generality, applicability, and simple procedure. Nitro olefins are generated from
nitroaldol products (acetoxynitroalkanes 37) in situ under basic reaction conditions
of Barton–Zard synthesis and then undergo [3+2]-cycloaddition with isocyanoacetate to give 3,4-disubstituted pyrrole-2-carboxylate 38 (Fig. 3.15) (Barton and Zard
1985). The ester function at one of the pyrrole α-positions is regarded as a protecting
group, and it can be easily removed or utilized as a meso-carbon of porphyrin structure leading to versatile application. A related methodology, van Leusen pyrrole
synthesis, using tosylmethyl isocyanide (TosMIC) and Michael acceptor is useful in
the synthesis of 3,4-disubstituted pyrroles 39 having electron-withdrawing groups
(EWG) at the β-position (van Leusen et al. 1992). 2-Stannylpyrrole 40 was also
obtainable in good yields by one-pot procedure, and it is useful in the Stille coupling
reactions leading to various building blocks for porphyrin analogues (Dijkstra et al.
1998).
Naturally occurring porphyrins have alkyl substituents at all the pyrrole
β-positions. These 2,3,7,8,12,13,17,18-octaalkylporphyrins are different from