Tetrapyrrole complexes

Not only porphyrins, but tetrapyrrole ligands in general are interesting partners for the coordination of noble metals. This section will refer to the synthesis of some tetrapyrrole complexes other than porphyrins from time to time, derivatives of these ligands will be mentioned in the later sections on the reactions of noble metal porphyrins because most of these tetrapyrrole complexes behave similarly as regards their axial coordination chemistry. 

Phthalocyanines - Pthalocyanine complexes form a very large class among tetrapyrrole complexes. Their chemistry, physics, and applications for novel materials are being reviewed in a multi volume series [106]. Usually they are made by reductive tetramerization of phthalodinitrile with a metal salt (see below), but metallations of free phthalocyanine, H2(Pc), are also documented. Work in the last two decades has been concentrated on phthalocyanine complexes of Ru, Os, Rh, Pd, Pt, and Ag Berezin gives references on IrCl(Pc) and AuCl(Pc) [107]. 

Diazaporphyrins - Like the phthalocyanines, a 10,20-diaza-porphinoid palladium complex was composed by cyclooligomerization. The condensation of two suitable bispyrrole halves yielded the tetrapyrrole complex, cis [1,11-dimethoxy-2,2,3,3,7,7,8,8,12,12,13,13,17,17,18,18-hexadecamethyl- 10,20-diaza-decahydro-porphyrinato]palladium(II) [124]. 

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

Even a short glance at the chemical literature is sufficient to get an impression (and it is not only an impression but a reality) that during the two last decades, the photochemistry of tetrapyrrole complexes belongs to the fastest developing areas in the field of excited-state chemistry of coordination com-... 

Turning back to the definition of photochemistry and anticipating the classification of photochemical reactions of metallotetrapyrroles, it should be kept in mind that a true photochemical process is only that involving an electronically excited particle (in this review it means an excited tetrapyrrole complex). All subsequent reactions are spontaneous (in photochemistry they are familiarly called dark reactions). Exactly speaking, each classification of photochemical reactions should start with an answer to the question what is the nature of the primary photochemical step involving a complex in its photochemically reactive excited state It must be admitted that for the... 

Experimental data on the formation, thermodynamic and kinetic decay parameters, and the multiplicity of exciplexes and excimers involving tetrapyrrole complexes are summarized in Tables 1 and 2. Based on these data and on information on the spectral properties and chemical behavior, some conclusions... 

Table 1. Formation, thermodynamic and kinetic decay parameters (stability constant K, energy and entropy values, decay rate constant k, lifetime x, quantum yield < >) of exciplexes (A — Q) involving tetrapyrrole complexes A (energy values expressed in kJmol 1 entropy in Jmol"1 K-1 k and x in s 1 and s, respectively)... 

Generally, the differences in Gibbs energy, AG, of a solvated exciplex (A — Q) and its solvated constituents A and Q are not very large. The values of the formation constants Kf for the exciplexes involving a tetrapyrrole complex are of the order 103- 10s (for triplet exciplexes) and of some order lower for singlet exciplexes. 

The problem of the formation and existence of exciplexes is connected with their character (predominantly charge-transfer or covalent) and the polarity of solvent. Knowledge in this matter can be summarized as follows the higher the polarity of the solvent (its dielectric constant), the lower is the stability of charge-transfer exciplexes in it. Charge-transfer exciplexes are stabilized, therefore, in non-polar solvents. For the stability of non-polar exciplexes (a rare case of exciplexes containing a tetrapyrrole complex) the solvent polarity plays a negligible role. As it follows from the data in Table 1, there are some exceptions to the above rules. 

On the other hand, photosubstitutions of tetrapyrrole complexes are rare processes. The main reason for such distinctions between tetrapyrrole complexes and common inorganic compounds lies in the electronic structure of the two mentioned classes of compounds in their low-lying photoreactive excited states. 

Table 3. Photosubstitution, photoelimination, and photoaddition non-redox reactions of tetrapyrrole complexes... 

Based on given experimental results and their interpretation the following conclusions on photosubstitution reactions of tetrapyrrole complexes can be drawn ... 

In chemistry, however, the term isomers is generally used in connection with isolable compounds. Keeping this limitation in mind it has been found that, apart from chemistry and photochemistry of common inorganic complexes [1], isomerism is a very rare phenomenon in the chemistry of tetrapyrrole complexes. 

Table 4. Photoredox reactions of tetrapyrrole complexes involving central atoms... 

Discussion of photochemical properties of natural and artificial photosynthetic centers is beyond the scope of this chapter. This part of photochemistry of tetrapyrrole complexes involving systems seems to be the most elegant part in the field and one of the most exciting parts in photochemistry and photobiology as a whole. 

The subject of this Section is photochemical processes in which polynuclear tetrapyrrole complexes with metal-metal or metal-bridge-metal bonds are formed from or decomposed to mononuclear tetrapyrrole ligand containing... 

Table 6. Photochemical formation and decomposition of polynuclear tetrapyrrole complexes... 

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