4 Phthalocyanine and Related Analogues
93
4.3 UV/vis Absorption Properties of Pc and SubPc
4.3.1 Gouterman’s Four-Orbital Model and Theoretical
Description of Absorption Properties of Pc and SubPc
A theoretical framework for the description of absorption properties of porphyrin
was initially constructed to explain the energy and intensity differences between the
Soret and Q bands of porphyrin. Then the established framework, which is known
to date as “Gouterman’s four-orbital model,” was used to describe the absorption
properties of Pc (Gouterman 1959; Fukuda and Kobayashi 2010).
In the middle of the twentieth century, Simpson attempted to explain the absorption spectra of porphyrins using 18-membered cyclic polyene having 18π-electrons
(Simpson 1949). Although this model successfully described the energy differences
between the Soret and Q bands, completely forbidden Q bands in this model failed
to explain the fact that the Q bands of porphyrin can be observed as weak, but
distinct bands. Meanwhile, on the basis of π-molecular orbitals (MOs) of porphyrin
obtained by the LCAO technique, Platt et al. tried to describe the absorption properties
(Longuet-Higgins et al. 1950). The energy difference between the Soret and Q bands
were reproduced by their model because the degeneracy of the HOMO in Simpson’s
model is lifted by lowering the symmetry from the cyclic polyene to the D 4h metalloporphyrin skeleton. However, it was unable to explain the observed differences
in intensities between the Soret and Q bands. In 1959, Gouterman demonstrated
how Simpson’s model is related to Platt’s LCAO MOs by applying configurational
interactions to one-electron excited states consisting of transitions from the HOMO
(a 2u ) and HOMO–1 (a 1u ) to degenerate LUMO (e g ) (Fig. 4.3) (Gouterman 1959) The
degree of configuration largely depends on the relative energy differences between
the two one-electron excited states. In the case of porphyrin, the HOMO and HOMO–
1 (or the next HOMO of porphyrin π-skeleton when metal- or substituent-centered
MOs are inserted in between) is nearly degenerate. As a result, two excited states
strongly interact with each other to produce the Soret and Q bands as sum and difference of two configurations, respectively. Therefore, the Soret band becomes allowed
and more intense compared with the forbidden Q band.
In the case of Pc, the a 2u orbital is stabilized compared with the a 1u orbital by
more electronegative nitrogen than carbon because the a 2u orbital has large MO
coefficients (electron density) on the meso-positions (Fig. 4.3). The lifted degeneracy
of the HOMO causes smaller configurational interactions, resulting in more intense
Q bands of Pc than those of porphyrin. The major contribution of the HOMO–
LUMO transitions to the Q bands in the case of Pc also indicates that the intensity
and energy of the Q bands are significantly perturbed by molecular symmetry and
peripheral substituents. Considering that the Q band of Pc generally appears around
650 nm, Pc is one of the potential structures to create NIR chromophores.
The absorption properties of SubPc can be explained in a similar manner to Pc
because SubPc holds degenerate LUMO due to the three-fold molecular symmetry.
Therefore, SubPc exhibits an intense Q band around 550 nm, which is blue-shifted
93
4.3 UV/vis Absorption Properties of Pc and SubPc
4.3.1 Gouterman’s Four-Orbital Model and Theoretical
Description of Absorption Properties of Pc and SubPc
A theoretical framework for the description of absorption properties of porphyrin
was initially constructed to explain the energy and intensity differences between the
Soret and Q bands of porphyrin. Then the established framework, which is known
to date as “Gouterman’s four-orbital model,” was used to describe the absorption
properties of Pc (Gouterman 1959; Fukuda and Kobayashi 2010).
In the middle of the twentieth century, Simpson attempted to explain the absorption spectra of porphyrins using 18-membered cyclic polyene having 18π-electrons
(Simpson 1949). Although this model successfully described the energy differences
between the Soret and Q bands, completely forbidden Q bands in this model failed
to explain the fact that the Q bands of porphyrin can be observed as weak, but
distinct bands. Meanwhile, on the basis of π-molecular orbitals (MOs) of porphyrin
obtained by the LCAO technique, Platt et al. tried to describe the absorption properties
(Longuet-Higgins et al. 1950). The energy difference between the Soret and Q bands
were reproduced by their model because the degeneracy of the HOMO in Simpson’s
model is lifted by lowering the symmetry from the cyclic polyene to the D 4h metalloporphyrin skeleton. However, it was unable to explain the observed differences
in intensities between the Soret and Q bands. In 1959, Gouterman demonstrated
how Simpson’s model is related to Platt’s LCAO MOs by applying configurational
interactions to one-electron excited states consisting of transitions from the HOMO
(a 2u ) and HOMO–1 (a 1u ) to degenerate LUMO (e g ) (Fig. 4.3) (Gouterman 1959) The
degree of configuration largely depends on the relative energy differences between
the two one-electron excited states. In the case of porphyrin, the HOMO and HOMO–
1 (or the next HOMO of porphyrin π-skeleton when metal- or substituent-centered
MOs are inserted in between) is nearly degenerate. As a result, two excited states
strongly interact with each other to produce the Soret and Q bands as sum and difference of two configurations, respectively. Therefore, the Soret band becomes allowed
and more intense compared with the forbidden Q band.
In the case of Pc, the a 2u orbital is stabilized compared with the a 1u orbital by
more electronegative nitrogen than carbon because the a 2u orbital has large MO
coefficients (electron density) on the meso-positions (Fig. 4.3). The lifted degeneracy
of the HOMO causes smaller configurational interactions, resulting in more intense
Q bands of Pc than those of porphyrin. The major contribution of the HOMO–
LUMO transitions to the Q bands in the case of Pc also indicates that the intensity
and energy of the Q bands are significantly perturbed by molecular symmetry and
peripheral substituents. Considering that the Q band of Pc generally appears around
650 nm, Pc is one of the potential structures to create NIR chromophores.
The absorption properties of SubPc can be explained in a similar manner to Pc
because SubPc holds degenerate LUMO due to the three-fold molecular symmetry.
Therefore, SubPc exhibits an intense Q band around 550 nm, which is blue-shifted