**5.1 Complexation kinetics**

Rates of the complexation of porphyrins with metal ions are very much slower by several orders of magnitude than those of acyclic ligands (Funahashi, S. et al., 1984). Such very slow rates have been discussed in terms of the rigidity of the planar porphyrin framework. The electronic nature of porphyrins, and also the steric accessibility of the bound metal center,

The Use of Spectrophotometry UV-Vis for the Study of Porphyrins 99

Bain-Ackerman & Lavallee, 1979; Funahashi et al., 1984; Funahashi et al., 1986;Lavallee et al.,

The formation of metalloporphyrins can be accelerated substantially by the use of an auxiliary complexing agent. For example, the rate of complexation of TMPyP with Cu(II) and Mg(II) was accelerated by L-cysteine (Watanabe & Ohmori 1981) and 8-quinolinol (Makino & Itoh, 1981), respectively. The organic ligands with an extended -electron structure, such as imidazole or bipyridine (Giovannetti et al., 1995; Kawamura et al., 1988; Ishi & Tsuchiai 1987; Tabata & Kajhara, 1989) and L-tryptophan (Tabata & Tanaka, 1988) also show a tendency to accelerate the formation of metalloporphyrins because form intermediate molecular complexes with metal ions and porphyrin reagents. In the presence of tryptophan, the rate of incorporation of Zn(II) to TPPS4 is about 100 times greater than in

Hence, larger metal ions such as Pb 2+, Hg2+, or Cd2+ can catalyse the formation of regular metalloporphyrins via generation of SAT complexes as intermediates. This because in the SAT complexes, the distortion caused by the out-of-plane location of the larger metal center, makes two diagonal pyrrolic nitrogens more accessible to another metal ion, even with smaller ionic radius, on the other side of the porphyrin ligand that can easily coordinate to them (Stinson & Hambright, 1977; Inamo et al 2001; Wittmer & Holten, 1996; Tabata & Tanaka 1985; Tung & Chen, 2000; Tabata et al., 1995; Grant & Hambright 1969; .R. Robinson & Hambright,

Since the deformation of the porphyrin ring proved to be the main factor governing the acceleration of the metalloporphyrin formation (Lavallee, 1985; Tabata & Tanaka, 1991), this can also be achieved by substituents at the porphyrin core or at the peripheral ring. Thus, e.g., the peripheral or substituted octabromoporphyrins display a buckled structure due to steric hindrance between the substituents (Bhyrappa & Krishnan, 1991; Mandon et al., 1992; Henling et al., 1993; Brinbaum et al., 1995). Such a deformation profoundly enhanced the reactivity of the porphyrin even towards Hg2+ (Nahar & Tabata, 1998), the ionic radius of

An increasing interest in recent years is due to supramolecular assemblies of -conjugated systems for their potential applications in optoelectronic and photovoltaic devices (

Molecular aggregates of several dyes have been studied as organic photoconductors (Borsenberger et al.,1978), as markers for biological and artificial membrane systems (Waggoner, 1976), as materials with high non-linear optical properties suitable for optical devices ([Hanamura,1988; Sasaki & Kobayashi, 1993; Wang, 1986; Wang, 1991). Some properties of molecular structure of aggregates permit their use in superconductivity, and other processes (Kobayashi,1992; Schouten et al., 1991; Collman, 1986). Aggregation of small organic molecules to form large clusters is of large interest in chemistry, physics and biology. In nature, particularly in living systems, self-association of molecules plays a very important role; an example is given by molecular aggregates of chlorophyll that have been found to mediate the primary light harvesting and charge-transfer processes in photosynthetic complexes (Creightonet al., 1988; Kuhlbrandt, 1995). In fact, light-harvesting

1992; C. Stinson & Hambright, 1977; Barkigia et al., 1990; Giovannetti et al., 1998).

1986; Bartczak et al., 1990; Shimizu et al., 1992).

its absence (Tabata & Tanaka, 1988).

which is rather large anyway.

Schenning & Meijer, 2005).

**6. Aggregation of porphyrins** 

can be varied by using electron-donating or with drawing substituents at the meso carbon or in the pyrrolic positions. While such substituent-based changes have been seen to influence the extent of apical ligand binding, as well as the stability of the metal complexes, there is a relatively small effect on the ability to insert cations into the nitrogen core (Lin & Lash, 1995).

Fig. 13. Uv-Vis adsorption spectra of CPI, [MnIIICPI(H2O)2] [MnIIICPI(OH)2], [MnIV(O)CPI(OH)], [MnV(O)CPI(OH)], [MnIICPI(OH)] with in insert, the experimental conditions for the preparation of all complexes obtained with several reagents.


Table 1 Spectral characteristics of Mn–CPI complexes.

In the case of N-substituted porphyrins, in which one of the two hydrogen atoms bound to the pyrrole nitrogen atoms is substituted by an alkyl or aryl group, displacement of the substituent from the porphyrin plane due to its bulkiness causes tilting distortion of the pyrrole rings of the porphyrin as shown by X-ray crystallography (Lavallee & Anderson, 1982; Aizawa et al., 1993). The investigation of the kinetics of the complexation of Nsubstituted porphyrins with several metal ions, showing that N-alkylporphyrins form metal complexes much faster than corresponding non-N-alkylated porphyrins (Aizawa et al., 1993; Shah et al., 1971; Shah et al., 1971; Anderson & Lavallee, 1977; Lavalle et al., 1978; Anderson et al., 1980; Kuila & Lavallee, 1984; Schauer et al., 1987; Balch et al., 1990; McLaughlin, 1974;

can be varied by using electron-donating or with drawing substituents at the meso carbon or in the pyrrolic positions. While such substituent-based changes have been seen to influence the extent of apical ligand binding, as well as the stability of the metal complexes, there is a relatively small effect on the ability to insert cations into the nitrogen core (Lin & Lash, 1995).

Fig. 13. Uv-Vis adsorption spectra of CPI, [MnIIICPI(H2O)2] [MnIIICPI(OH)2],

[MnIICPIOH]\_ 414 544, 574 [MnIIICPI(H2O)2]+ 366, 458 542, 572 [MnIIICPI(OH)2] 348, 458 556 [MnIVOCPI(OH)]\_ 400 502, 610 [MnVOCPI(OH)] 384 526, 562

Table 1 Spectral characteristics of Mn–CPI complexes.

A

conditions for the preparation of all complexes obtained with several reagents.

[MnIV(O)CPI(OH)], [MnV(O)CPI(OH)], [MnIICPI(OH)] with in insert, the experimental

Wavelength (nm)

**Complex Soret band λ (nm) Q band (, ) λ (nm)**

In the case of N-substituted porphyrins, in which one of the two hydrogen atoms bound to the pyrrole nitrogen atoms is substituted by an alkyl or aryl group, displacement of the substituent from the porphyrin plane due to its bulkiness causes tilting distortion of the pyrrole rings of the porphyrin as shown by X-ray crystallography (Lavallee & Anderson, 1982; Aizawa et al., 1993). The investigation of the kinetics of the complexation of Nsubstituted porphyrins with several metal ions, showing that N-alkylporphyrins form metal complexes much faster than corresponding non-N-alkylated porphyrins (Aizawa et al., 1993; Shah et al., 1971; Shah et al., 1971; Anderson & Lavallee, 1977; Lavalle et al., 1978; Anderson et al., 1980; Kuila & Lavallee, 1984; Schauer et al., 1987; Balch et al., 1990; McLaughlin, 1974; Bain-Ackerman & Lavallee, 1979; Funahashi et al., 1984; Funahashi et al., 1986;Lavallee et al., 1986; Bartczak et al., 1990; Shimizu et al., 1992).

The formation of metalloporphyrins can be accelerated substantially by the use of an auxiliary complexing agent. For example, the rate of complexation of TMPyP with Cu(II) and Mg(II) was accelerated by L-cysteine (Watanabe & Ohmori 1981) and 8-quinolinol (Makino & Itoh, 1981), respectively. The organic ligands with an extended -electron structure, such as imidazole or bipyridine (Giovannetti et al., 1995; Kawamura et al., 1988; Ishi & Tsuchiai 1987; Tabata & Kajhara, 1989) and L-tryptophan (Tabata & Tanaka, 1988) also show a tendency to accelerate the formation of metalloporphyrins because form intermediate molecular complexes with metal ions and porphyrin reagents. In the presence of tryptophan, the rate of incorporation of Zn(II) to TPPS4 is about 100 times greater than in its absence (Tabata & Tanaka, 1988).

Hence, larger metal ions such as Pb 2+, Hg2+, or Cd2+ can catalyse the formation of regular metalloporphyrins via generation of SAT complexes as intermediates. This because in the SAT complexes, the distortion caused by the out-of-plane location of the larger metal center, makes two diagonal pyrrolic nitrogens more accessible to another metal ion, even with smaller ionic radius, on the other side of the porphyrin ligand that can easily coordinate to them (Stinson & Hambright, 1977; Inamo et al 2001; Wittmer & Holten, 1996; Tabata & Tanaka 1985; Tung & Chen, 2000; Tabata et al., 1995; Grant & Hambright 1969; .R. Robinson & Hambright, 1992; C. Stinson & Hambright, 1977; Barkigia et al., 1990; Giovannetti et al., 1998).

Since the deformation of the porphyrin ring proved to be the main factor governing the acceleration of the metalloporphyrin formation (Lavallee, 1985; Tabata & Tanaka, 1991), this can also be achieved by substituents at the porphyrin core or at the peripheral ring. Thus, e.g., the peripheral or substituted octabromoporphyrins display a buckled structure due to steric hindrance between the substituents (Bhyrappa & Krishnan, 1991; Mandon et al., 1992; Henling et al., 1993; Brinbaum et al., 1995). Such a deformation profoundly enhanced the reactivity of the porphyrin even towards Hg2+ (Nahar & Tabata, 1998), the ionic radius of which is rather large anyway.
