4 Phthalocyanine and Related Analogues
105
using UV/vis absorption, CD, and MCD spectra, Shimizu and Kobayashi experimentally estimated the oscillator and rotational strengths and ascribed the observed
changes in the CD intensities to smaller magnetic dipole moments of the C 1 isomer
due to the oppositely arranged naphthalene ring. Since SubPc and its derivatives
exhibit fluorescence, the inherent chirality of low-symmetry SubPcs can be used to
generate circularly polarized luminescent materials.
4.4 Recent Examples of Pc Analogues with Unique Optical
Properties
In this section, recent examples of Pc analogues with unique properties are introduced, focusing on NIR absorption and antiaromaticity. Sections 4.4.1 and 4.4.2
describe the NIR absorbing phosphorous(v) Pc and contracted and expanded Pc
analogues, respectively.
4.4.1 Phosphorous(v) Pc as an NIR Chromophore
As described in the previous sections, the Q band of Pc in the far-red region (ca.
650 nm) is affected by several perturbations, such as molecular symmetry and
substituents. It is, therefore, rational to target the NIR absorption using Pc as a
fundamental chromophore skeleton. Synthetic investigation toward this goal has been
intensively conducted from the early stage of Pc chemistry. In most cases, peripheral
extension of the conjugated systems by fusing aromatic ring units or by oligomerization via peripheral benzene rings was attempted. Although the red-shifts of the Q band
were achieved based on this strategy, such NIR-absorbing Pc analogues were found
to be rather unstable due to the inevitably highly lying HOMO. Push-pull substitution also destabilizes the HOMO energy level because of the predominant push
effect compared with the pull effect. Therefore, to create stable NIR chromophores,
it is important to attain a narrow HOMO–LUMO gap by effectively controlling the
HOMO and LUMO energy levels.
Recently, Furuyama and Kobayashi reported a rational molecular design to
achieve such systems (Furuyama and Kobayashi 2017). As shown in Fig. 4.3., the
HOMO (a 1u orbital) has large electron density on the α-positions of the benzo-rings,
whereas large MO coefficients can be found on the central nitrogen atoms in the
LUMO (e g orbitals). They focused on these MO density distribution patterns and
introduced electron-donating chalcogen substituents on the α-positions and high
valent phosphorous(v) in the core, which destabilize the HOMO and stabilizes the
LUMO, respectively (Fig. 4.17).
α-Arylchalcogen substituted Pcs were synthesized by the lithium method, and a
phosphorous(v) was inserted by a reaction with excess POBr 3 in pyridine. After
105
using UV/vis absorption, CD, and MCD spectra, Shimizu and Kobayashi experimentally estimated the oscillator and rotational strengths and ascribed the observed
changes in the CD intensities to smaller magnetic dipole moments of the C 1 isomer
due to the oppositely arranged naphthalene ring. Since SubPc and its derivatives
exhibit fluorescence, the inherent chirality of low-symmetry SubPcs can be used to
generate circularly polarized luminescent materials.
4.4 Recent Examples of Pc Analogues with Unique Optical
Properties
In this section, recent examples of Pc analogues with unique properties are introduced, focusing on NIR absorption and antiaromaticity. Sections 4.4.1 and 4.4.2
describe the NIR absorbing phosphorous(v) Pc and contracted and expanded Pc
analogues, respectively.
4.4.1 Phosphorous(v) Pc as an NIR Chromophore
As described in the previous sections, the Q band of Pc in the far-red region (ca.
650 nm) is affected by several perturbations, such as molecular symmetry and
substituents. It is, therefore, rational to target the NIR absorption using Pc as a
fundamental chromophore skeleton. Synthetic investigation toward this goal has been
intensively conducted from the early stage of Pc chemistry. In most cases, peripheral
extension of the conjugated systems by fusing aromatic ring units or by oligomerization via peripheral benzene rings was attempted. Although the red-shifts of the Q band
were achieved based on this strategy, such NIR-absorbing Pc analogues were found
to be rather unstable due to the inevitably highly lying HOMO. Push-pull substitution also destabilizes the HOMO energy level because of the predominant push
effect compared with the pull effect. Therefore, to create stable NIR chromophores,
it is important to attain a narrow HOMO–LUMO gap by effectively controlling the
HOMO and LUMO energy levels.
Recently, Furuyama and Kobayashi reported a rational molecular design to
achieve such systems (Furuyama and Kobayashi 2017). As shown in Fig. 4.3., the
HOMO (a 1u orbital) has large electron density on the α-positions of the benzo-rings,
whereas large MO coefficients can be found on the central nitrogen atoms in the
LUMO (e g orbitals). They focused on these MO density distribution patterns and
introduced electron-donating chalcogen substituents on the α-positions and high
valent phosphorous(v) in the core, which destabilize the HOMO and stabilizes the
LUMO, respectively (Fig. 4.17).
α-Arylchalcogen substituted Pcs were synthesized by the lithium method, and a
phosphorous(v) was inserted by a reaction with excess POBr 3 in pyridine. After