ReviewPhotochemical properties of phthalocyanines with transition metal ions
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 Tn energies, which are strongly correlated with the photochemical properties. Using strong spin–orbit coupling (SOC), the direct observation of spin-forbidden singlet–triplet (S0-Tn) 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-Tn) 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-Tn) 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 dxz and dyz orbitals of the metal atom or between the px and py orbitals of the axial ligands, becomes efficient [[38], [39], [40], [41], [42]]. The S0-Tn absorption bands are observable even in the 3(π,π*) transition, when the eg(π*, Pc) orbitals are admixed with the dπ(M) orbitals. Based on the z-component of the SOC between the dxz and dyz orbitals of the metal ion (Fig. 11), the 1Eux (or 1Euy
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 (τS and τT for singlet and triplet lifetimes, respectively), luminescence lifetimes (τF and τP for fluorescence and phosphorescence lifetimes, respectively), luminescence quantum yields (ΦF and ΦP for fluorescence and phosphorescence quantum yields, respectively), and triplet yield (ΦT), as shown below.τS = τF = 1/(kF + kIC + kISC)τT = τP = 1/(kP + kISC′)ΦF = kF/(kF + kIC + kISC)ΦT = kISC/(kF
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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