Photochemical properties of phthalocyanines with transition metal ions

doi:10.1016/j.ccr.2022.214626

Coordination Chemistry Reviews

Volume 468, 1 October 2022, 214626

https://doi.org/10.1016/j.ccr.2022.214626 Get rights and content

Highlights

•
The Q absorption bands of phthalocyanine complexes show a blue or red shift with/without broadening owing to the interactions with transition metal ions.
•
The lowest excited triplet energies can be evaluated by observing the singlet–triplet absorption bands using the strong spin–orbit coupling of transition metal ions.
•
The second lowest excited triplet energies of low-symmetry phthalocyanine complexes can be determined by analyzing zero-field splitting owing to the spin–orbit coupling.
•
Numerous data on the excited-state dynamics are discussed in terms of spin-forbidden processes, such as phosphorescence decay rate and intersystem crossing.
•
Some attractive photofunctions of phthalocyanine complexes are summarized from the viewpoint of the properties of transition metal ions.

Abstract

This review focuses on phthalocyanine (Pc) complexes with transition metal ions, especially the second- and third-row transition metal ions, and summarizes their photochemistry in terms of spin-allowed singlet–singlet (S₀-S₁) absorption, spin-forbidden singlet–triplet (S₀-T_n) absorption, excited-state dynamics, and photofunctions. Characteristic photophysical and photochemical properties are identified from numerous data based on relevant theories. Firstly, it is demonstrated that the Q absorption bands show a blue or red shift with/without broadening owing to the interactions with transition metal ions, which is explained by the energy relationship among the d, π, and π* orbitals. Secondly, it is shown that the T_n energies can be determined based on the strong spin–orbit coupling of transition metal ions: the T₁ energy can be evaluated by observing spin-forbidden singlet–triplet (S₀-T_n) absorption, whereas the T₂ energy of low-symmetry Pc complexes can be determined by analyzing zero-field splitting. Thirdly, numerous data on the excited-state dynamics are summarized and discussed in terms of spin-forbidden processes, such as phosphorescence decay rate and intersystem crossing. Finally, some attractive photofunctions are introduced. To provide a systematic understanding of the roles of the transition metal ions in relation to the characteristic photophysical and photochemical properties of Pcs, brief analyses are provided. This review provides direction for designing various photofunctions based on metal Pc complexes.

Introduction

Phthalocyanines (Pcs) and related compounds have attracted interest due to their diverse electronic and optical properties [1]. They have considerable applications in pigments and dyes, luminescence imaging for diagnosis or sensors [2], nonlinear optics [3], singlet oxygen photosensitizers for photodynamic therapy [[4], [5], [6], [7]], dye-sensitized solar cells [[8], [9], [10]], red or near-infrared light absorbers in optical storage media, and organic photoconductors [11]. Fig. 1 shows the molecular structure of an unsubstituted Pc. Many compounds are derived from the parent Pc by exchanging the hydrogen atoms at the α and β positions of the four benzene rings to other functional groups. The tetrasubstituted Pcs which composed mixtures of four isomers show high solubility, and therefore, their photochemical studies are mainly introduced in this review.

In the case of transition metal Pc complexes, the selection of central metal ions is important for tuning their excited-state properties. Recently, in Pc complexes containing second- or third-row (d⁶ or d⁸) transition metal ions, that is, Ru(II) [[12], [13], [14], [15], [16], [17], [18], [19]], Pd(II) [[20], [21], [22], [23], [24], [25], [26]], Re(I) [[27], [28]], Ir(III) [29], and Pt(II) [[22], [30]], some interesting photochemical properties have been reported. The first is the changes in the UV–vis absorption spectra. The Q-band is known to exhibit a blue shift and/or broadening upon the insertion of transition metal ions. Recently, a red shift of the Q-band has been reported in RePc complexes. Thus, the excited-state energies, which are important for the photofunctions described above, can be altered on demand by changing the central transition metal ions. The second is the determination of the T_n energies, which are strongly correlated with the photochemical properties. Using strong spin–orbit coupling (SOC), the direct observation of spin-forbidden singlet–triplet (S₀-T_n) absorption has been recently reported in transition metal porphyrin complexes. Furthermore, the T₂ energies of low-symmetry Pcs can be determined through the analyses of the zero-field splitting (ZFS) dependent on the SOC. In addition, some attractive photofunctions owing to second- or third-row (d⁶ or d⁸) transition metal ions have been reported.

This review illustrates the photochemical properties of Pcs with transition metal ions, especially focusing on second- and third-row (d⁶ and d⁸) transition metal ions, by summarizing the characteristic photophysical and photochemical properties of transition metal Pcs. To functionalize the photophysical and photochemical properties of Pcs, it is essential to understand the roles of the transition metal ions in relation to the Pc rings systematically, based on not only quantum chemical approaches, but also excited-state dynamics. Thus, in order to extract the characteristic photophysical properties of Pcs owing to the second- and third-row (d⁶ and d⁸) transition metal ions, this article summarizes extensive data related to the excited-state properties obtained by luminescence and transient absorption measurements and illustrates the relevant theories. This review addresses four main topics, that is, spin-allowed singlet–singlet (S₀-S₁) absorption, spin-forbidden singlet–triplet (S₀-T_n) absorption, the excited-state dynamics of the S₁ and T₁ states, and photofunctions. Based on the extracted characteristic properties, it provides direction for the design of various photofunctions of metal Pc complexes.

Section snippets

General background of diamagnetic Pcs

Fig. 2 illustrates the photophysical processes. The commonly encountered photophysical processes are (1) spin-allowed singlet–singlet (S₀-S₁) absorption; (2) spin-forbidden singlet–triplet (S₀-T_n) absorption; (3) fluorescence from the S₁ to S₀ state; (4) phosphorescence from the T₁ to S₀ state; (5) intersystem crossing (ISC), spin-forbidden transitions from the S₁ to T₁ state or from the T₁ to S₀ state; and (6) internal conversion (IC), a spin-allowed transition from the S₁ to S₀ state. The

Sharp, intense Q-band dominated by ¹(π,π*) transition

Fig. 3 depicts the electronic absorption (UV–vis) and magnetic circular dichroism (MCD) spectra of zinc(II) tetra-tert-butyl-phthalocyanine (Zn(II)tbPc) [[27], [31], [32], [33]]. In the UV–vis spectrum, a sharp, intense absorption band, called the Q band, is observed at approximately 680 nm (full width at half-maximum (FWHM) of 450 cm⁻¹), which originates from the S₀ → S₁ transition. For the MCD spectrum of Zn(II)tbPc, a derivative-shaped A term is observed in the Q-band region, which implies

Theoretical background of SOC

When a Pc has a central heavy metal or axial ligands, the z-component of the SOC, either between the d_xz and d_yz orbitals of the metal atom or between the p_x and p_y orbitals of the axial ligands, becomes efficient [[38], [39], [40], [41], [42]]. The S₀-T_n absorption bands are observable even in the ³(π,π*) transition, when the e_g(π*, Pc) orbitals are admixed with the d_π(M) orbitals. Based on the z-component of the SOC between the d_xz and d_yz orbitals of the metal ion (Fig. 11), the ¹E_u_x (or ¹E_u_y

Excited-state dynamics of the S₁ and T₁ states

The excited-state dynamics from the S₁ and T₁ states can be evaluated using the excited-state lifetimes (τ_S and τ_T for singlet and triplet lifetimes, respectively), luminescence lifetimes (τ_F and τ_P for fluorescence and phosphorescence lifetimes, respectively), luminescence quantum yields (Φ_F and Φ_P for fluorescence and phosphorescence quantum yields, respectively), and triplet yield (Φ_T), as shown below.τ_S = τ_F = 1/(k_F + k_IC + k_ISC)τ_T = τ_P = 1/(k_P + k_ISC′)Φ_F = k_F/(k_F + k_IC + k_ISC)Φ_T = k_ISC/(k_F

Photo-induced CO-releasing molecules (photoCORMs)

The investigation of various CORMs has attracted significant attention, as a large amount of evidence has confirmed that CO plays a physiological role owing to its vasorelaxation effect, which is similar to that of nitric oxide (NO) [[64], [65], [66], [67], [68], [69], [70], [71], [72]]. The photodissociation of CO is a promising phenomenon because it can be exploited by activating CORMs with light of an appropriate wavelength, that is, photoCORMs. Metal carbonyl complexes are advantageous for

Summary

This review summarized the characteristic photophysical and photochemical properties of Pcs with transition metal ions, especially the second- and third-row (d⁶ and d⁸) transition metal ions, that is, Ru(II), Pd(II), Re(I), Ir(III), and Pt(II), from the perspective of the interactions among the d, π, and π* orbitals and strong SOC due to transition metal ions. The interactions among the d, π, and π* orbitals directly reflect the excited-state energies, which can be observed as a blue or red

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by JSPS KAKENHI (grant number JP17H06375).

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ReviewPhotochemical properties of phthalocyanines with transition metal ions

Highlights

Abstract

Introduction

Section snippets

General background of diamagnetic Pcs

Sharp, intense Q-band dominated by 1(π,π*) transition

Theoretical background of SOC

Excited-state dynamics of the S1 and T1 states

Photo-induced CO-releasing molecules (photoCORMs)

Summary

Declaration of Competing Interest

Acknowledgement

Coord. Chem. Rev.

J. Photochem. Photobiol. B Biol.

Organometallics

J. Inorg. Biochem.

Polyhedron

J. Catal.

Organic Electronics

Dyes Pigm.

J. Mol. Spectrosc.

Coord. Chem. Rev.

Chem. Phys. Lett.

Chem. Phys.

Inorg. Chim. Acta

Inorg. Chim. Acta

Coord. Chem. Rev.

Molecular orbituals and electronic spectra of phthalocyanine analogues

Nonlinear optical properties of phthalocyanines

Radiolabelled phthalocyanines for imaging and therapy

Porphyrins and phthalocyanines as photosensitizers and radiosensitizers

Photochemical and photophysical properties of metallophthalocyanines

Phthalocyanines and related macrocycles in organic photovoltaic junctions

On the significance of phthalocyanines in solar cells

Phthalocyanines as sensitizers in dye-sensitized solar cells

Phthalocyanines - Basic Physical Properties and Application to Functional Materials

Control of photobleaching in photodynamic therapy using the photodecarbonylation reaction of ruthenium phthalocyanine complexes via stepwise two-photon excitation

J. Phys. Chem. B

Unexpected rotamerism at the origin of A chessboard supramolecular assembly of ruthenium phthalocyanine

Chem. Eur. J.

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Chem. Eur. J.

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Chem Commun.

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Chem. Eur. J.

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Eur. J. Inorg. Chem

Direct observation of the S0→ T2 transition in phosphorescent platinum(II) octaethylporphyrin, evidenced by magnetic circular dichroism

J. Phys. Chem. Lett.

Molecular orbitals and electronic spectra of phthalocyanine analogues

A concept for controlling singlet oxygen (1Δg) yields using nitroxide radicals: phthalocyaninatosilicon covalently linked to nitroxide radicals

J. Am. Chem. Soc.

Relationship between symmetry of porphyrinic π-conjugated systems and singlet oxygen (1Δg) yields: low-symmetry tetraazaporphyrin derivatives

J. Phys. Chem. A

Experimental evidence for A selection rule of intersystem crossing to the excited quartet states: metallophthalocyanines coordinated by 4-amino-TEMPO

J. Phys. Chem. A

Time-resolved EPR, fluorescence, and transient absorption studies on phthalocyaninatosilicon covalently linked to one or two TEMPO radicals

J. Am. Chem. Soc.

Time-resolved electron spin resonance of gallium and germanium porphyrins in the excited triplet state

Inorg. Chem.

The lowest excited triplet states of lanthanide bisphthalocyanine complexes studied by time resolved electron paramagnetic resonance

Mol. Phys.

Electron spin polarizations of phthalocyaninatosilicon covalently linked to one TEMPO radical in the excited quartet and doublet ground states

J. Phys. Chem. A

Phosphorescence and Fluorescence of Phthalocyanines

J. Chem. Phys.

Electronic spectra and zeeman effect of ruthenium phthalocyanine in shpol'skii matrices

Mol. Phys.

Review
Photochemical properties of phthalocyanines with transition metal ions

Sharp, intense Q-band dominated by ¹(π,π*) transition

Excited-state dynamics of the S₁ and T₁ states

Near infra-red S₀↔T₁ transitions of Iridium(III) phthalocyanine: degeneracy of the T₁ state evidenced by magnetic circular dichroism

Direct observation of the S₀→ T₂ transition in phosphorescent platinum(II) octaethylporphyrin, evidenced by magnetic circular dichroism

A concept for controlling singlet oxygen (¹Δ_g) yields using nitroxide radicals: phthalocyaninatosilicon covalently linked to nitroxide radicals

Relationship between symmetry of porphyrinic π-conjugated systems and singlet oxygen (¹Δ_g) yields: low-symmetry tetraazaporphyrin derivatives