**6. Aggregation of porphyrins**

An increasing interest in recent years is due to supramolecular assemblies of -conjugated systems for their potential applications in optoelectronic and photovoltaic devices ( Schenning & Meijer, 2005).

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

The Use of Spectrophotometry UV-Vis for the Study of Porphyrins 101

1970; Nuesch et al., 1995). The H-aggregates are not known to have sharp spectra like the Jaggregates; nevertheless, there are many examples where the spectroscopic blue shift,

H-aggregate

J-aggregate

These aggregated are of particular interest because the highly ordered molecular arrangement present unique electronic and spectroscopic properties that can be predicted ( Fidder et al.,

H and J aggregates can be obtained under specific conditions (Misawa & Kobayashi, 1999; Maiti et al., 1995; Kanojk & Kobayashi, 2002; Pasternack et al., 1994; Akins et al., 1994; Ohno et al., 1993; Luca et al., 2005; Luca et al., 2006; Luca et al., 2006). Due to the distinct optical properties, also the control of the formation of H- and J-aggregated states of dyes has attracted much research interest (Maiti et al., 1998; Shirakawa et al., 2003; De Luca et al., 2006; Egawa et al., 2007; Yagai et al., 2008; Gadde et al., 2008; Ghosh et al., 2008; Zhao et al.,

As a result, a wide variety of self-assembled porphyrin structures are highly desirable for practical use, which can be applied to nonlinear optical materials (Collini et al., 2006; Liu et al., 2006; Matsuzaki et al., 2006; Terazima et al., 1997), organic solar cells (Hasobe et al., 2003)

In general, the aggregate formation of porphyrins have been studied in solution, and their physicochemical properties can be affected by the ionic strength, nature of the titrating acid,

1991; West & Carroll,1966; Furuki et al., 1989; Spano & Mukamel, 1989; Bohn, 1993).

evident for formation of H-aggregates, was observed.

Fig. 14. Schematic representation of H and J aggregates.

and sensor devices ( Fujii et al., 2005; Lucaet al., 2007).

2008; Delbosc et al., 2010).

monomer

and the primary charge-separation steps in photosynthesis are facilitated by aggregated species, i.e., chlorophylls.

Self-assembly of molecules, driven by not-covalent intermolecular interactions, is a convenient route for manufacturing of new functional materials (Lidzey at al., 2000; Van der Boom et al., 2002; Fudickar et al. 2002; Lagoudakis et al., 2004; Li et al., 2003).

Recently, porphyrin assembly has been used for light-driven energy transduction systems, copying the photophysical processes of photosynthetic organisms (Choi et al.,2004); Choi et al., 2003; Choi et al., 2002; Choi et al., 2001; Luo et al., 2005).

The aggregation and dimerization of porphyrins and metalloporphyrins in aqueous solution have been widely investigated (Borissevitch & Gandini, 1998; Pasternack et al, 1985) and it has been deduced that it is dependent strictly on physical-chemical characteristics, such as, ionic strength, pH and solvent composition; the combination of these factors can facilitate the aggregation processes (Kubat et al., 2003; Giovannetti et al., 2010).

The aggregation of porphyrins, changing their spectral and energetic characteristics, influences their efficacy in several applications thus, it is very important take on detailed informations about the formation dynamic and on the typology of aggregates. In the metal complexation of porphyrins the efficiency reaction is affected by their aggregation (Yusmanov et al, 1996). Several authors have observed that in the photogeneration of H2O2 by porphyrins, the efficiency of production was highly dependent on their aggregation state (Komagoe et al, 2006).

The diverse chemical and photophysical properties of porphyrins are in many cases due to their different aggregation mode and, as a result of interchromophoric interactions, perturbations in the electronic absorption spectra of dyes occur. Deviations from Beer's law are often used to investigate the porphyrin aggregation in solution.

Because the aggregates of porphyrins show peculiar spectroscopic properties, the molecular associations of porphyrins were generally investigated using UV–vis absorption and fluorescence spectroscopy (Ohmo, 1993).

The characteristic of porphyrin molecule with 22 -electrons causes a strong – interaction (Van de Craats, & Warman, 2001), facilitating the formation of two structure types: "H-type" with bathochromic shift of B and Q bands and "J-type" with blue shift of B band and redshift of Q band, with respect to those of monomer.

The J-type aggregates (side-by-side) were formed for transitions polarized parallel to the long axis of the aggregate, while H-type (face-to-face) for transitions polarized perpendicular to it (Fig. 14).

 *J-aggregates* are formed with the monomeric molecules arranged in one dimension such that the transition moment of the monomers are parallel and the angle between the transition moment and the line joining the molecular centers is zero (Bohn, 1993). The strong coupling of monomers results in a coherent excitation with a red-shift relative to the monomer band. *H-aggregates* are again a one-dimensional arrangement of strongly coupled monomers, but the transition moments of the monomers are perpendicular (ideal case) to the line of centers. On the contrary of J-aggregates, the arrangement in H-aggregates is face-to-face. The dipolar coupling between monomers leads to a blue shift of the absorption band (Czikklely et al.,

and the primary charge-separation steps in photosynthesis are facilitated by aggregated

Self-assembly of molecules, driven by not-covalent intermolecular interactions, is a convenient route for manufacturing of new functional materials (Lidzey at al., 2000; Van der

Recently, porphyrin assembly has been used for light-driven energy transduction systems, copying the photophysical processes of photosynthetic organisms (Choi et al.,2004); Choi et

The aggregation and dimerization of porphyrins and metalloporphyrins in aqueous solution have been widely investigated (Borissevitch & Gandini, 1998; Pasternack et al, 1985) and it has been deduced that it is dependent strictly on physical-chemical characteristics, such as, ionic strength, pH and solvent composition; the combination of these factors can facilitate

The aggregation of porphyrins, changing their spectral and energetic characteristics, influences their efficacy in several applications thus, it is very important take on detailed informations about the formation dynamic and on the typology of aggregates. In the metal complexation of porphyrins the efficiency reaction is affected by their aggregation (Yusmanov et al, 1996). Several authors have observed that in the photogeneration of H2O2 by porphyrins, the efficiency of production was highly dependent on their aggregation state

The diverse chemical and photophysical properties of porphyrins are in many cases due to their different aggregation mode and, as a result of interchromophoric interactions, perturbations in the electronic absorption spectra of dyes occur. Deviations from Beer's law

Because the aggregates of porphyrins show peculiar spectroscopic properties, the molecular associations of porphyrins were generally investigated using UV–vis absorption and

The characteristic of porphyrin molecule with 22 -electrons causes a strong – interaction (Van de Craats, & Warman, 2001), facilitating the formation of two structure types: "H-type" with bathochromic shift of B and Q bands and "J-type" with blue shift of B band and red-

The J-type aggregates (side-by-side) were formed for transitions polarized parallel to the long axis of the aggregate, while H-type (face-to-face) for transitions polarized

 *J-aggregates* are formed with the monomeric molecules arranged in one dimension such that the transition moment of the monomers are parallel and the angle between the transition moment and the line joining the molecular centers is zero (Bohn, 1993). The strong coupling of monomers results in a coherent excitation with a red-shift relative to the monomer band. *H-aggregates* are again a one-dimensional arrangement of strongly coupled monomers, but the transition moments of the monomers are perpendicular (ideal case) to the line of centers. On the contrary of J-aggregates, the arrangement in H-aggregates is face-to-face. The dipolar coupling between monomers leads to a blue shift of the absorption band (Czikklely et al.,

Boom et al., 2002; Fudickar et al. 2002; Lagoudakis et al., 2004; Li et al., 2003).

al., 2003; Choi et al., 2002; Choi et al., 2001; Luo et al., 2005).

the aggregation processes (Kubat et al., 2003; Giovannetti et al., 2010).

are often used to investigate the porphyrin aggregation in solution.

species, i.e., chlorophylls.

(Komagoe et al, 2006).

fluorescence spectroscopy (Ohmo, 1993).

perpendicular to it (Fig. 14).

shift of Q band, with respect to those of monomer.

1970; Nuesch et al., 1995). The H-aggregates are not known to have sharp spectra like the Jaggregates; nevertheless, there are many examples where the spectroscopic blue shift, evident for formation of H-aggregates, was observed.

Fig. 14. Schematic representation of H and J aggregates.

These aggregated are of particular interest because the highly ordered molecular arrangement present unique electronic and spectroscopic properties that can be predicted ( Fidder et al., 1991; West & Carroll,1966; Furuki et al., 1989; Spano & Mukamel, 1989; Bohn, 1993).

H and J aggregates can be obtained under specific conditions (Misawa & Kobayashi, 1999; Maiti et al., 1995; Kanojk & Kobayashi, 2002; Pasternack et al., 1994; Akins et al., 1994; Ohno et al., 1993; Luca et al., 2005; Luca et al., 2006; Luca et al., 2006). Due to the distinct optical properties, also the control of the formation of H- and J-aggregated states of dyes has attracted much research interest (Maiti et al., 1998; Shirakawa et al., 2003; De Luca et al., 2006; Egawa et al., 2007; Yagai et al., 2008; Gadde et al., 2008; Ghosh et al., 2008; Zhao et al., 2008; Delbosc et al., 2010).

As a result, a wide variety of self-assembled porphyrin structures are highly desirable for practical use, which can be applied to nonlinear optical materials (Collini et al., 2006; Liu et al., 2006; Matsuzaki et al., 2006; Terazima et al., 1997), organic solar cells (Hasobe et al., 2003) and sensor devices ( Fujii et al., 2005; Lucaet al., 2007).

In general, the aggregate formation of porphyrins have been studied in solution, and their physicochemical properties can be affected by the ionic strength, nature of the titrating acid,

The Use of Spectrophotometry UV-Vis for the Study of Porphyrins 103

The porphyrins represent a fascinating world of molecules with sensational properties. Many results have been obtained by careful observation and with detailed studies of their chemical and physical properties due to the use of UV-Vis spectrophotometry between the

In this contest, the light absorbing power of porphyrins and related compounds should be used in the near future for other many applications and much more can still be studied in

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**7. Conclusions** 

the future.

**8. References** 

interpretation of Soret and Q band transformations.

Wiley & Sons: New York.

*Chem.* 32, 476.

temperature, pH, peripheral substitution and presence of surfactants of ions (Choi et al., 2003; Ohno et al., 1993; Napoli et al., 2004; Kubat et al., 2003; Siskova et al., 2005).

H- and J-aggregates was formed by simply mixing aqueous solutions of two kinds of porphyrins with opposite charges (Xiangqing et al., 2007).

Moreover, self-assembly can be mediated by templates that allows for obtaining aggregates with additional properties, e.g., chiral templates (Koti & Periasamy, 2003; Mammana et al., 2007). In these systems, coupling of strong transition dipoles can result in a perturbations to the electronic absorption spectra of monomer with hyposochromic and bathochromic shift of the monomer Soret band, for H and J aggregates formation respectively ( Gourterman et al., 1977; Zimmermann et al., 2003; Scherz & Parson, 1984). The produced splitting is proportional to the magnitude of transition dipole coupling between adjacent molecules.

Although much has been studied about the spectroscopic features and excitonic interactions in molecular aggregates, the detailed information of geometrical structure, especially the molecular orientation, are still the subjects of continuing interests (Nikiforov et al., 2008; Jeukens et al., 2004).

The aggregates of porphyrins have been formed in solutions in the form of fibers, ribbons and tubules by the self-association or aggregation method (Rotomskis, 2004; Fuhrhop, 1993; Giovannetti et al., 2010).

Some H- or J-type aggregates of porphyrins play a role as light harvesting assemblies to gather and transfer energy to the assembled devices, and to obtain a higher incident photonto-photocurrent generation efficiency ( Kamat et al.,2000; Sudeep et al., 2002).

The structural, kinetic, and spectroscopic studies on J- and H-aggregates provide useful information for understanding molecular interactions in aggregation processes.

The kinetics of the formation of the porphyrin aggregate and its structure are sensitive of experimental conditions (Giovannetti et al., 2010). The monomer – aggregated species is a system of multiple equilibria. Spectrophotometric monitoring in the time of the Uv- Vis absorbance permit to obtain information of intermediate species, of type of the aggregate, and of their transformation. For this, for evaluated the polymerization kinetic constants, the concentrations of monomeric [M] and dimeric form [D], can be calculated from the relative absorption maxima at each time. If *kpol* is the polymerisation kinetic constant, CM and CD denoted the initial monomer and dimer concentrations, the reaction rate can be expressed as in equation (2):

$$k\_{pol}t = \frac{1}{C\_M - C\_D} \ln \frac{C\_D[M]}{C\_M[D]} \tag{2}$$

The plot of the right term of this equation versus t gave good straight lines the slopes of which represented the values of *kpol*.

The information derived from such studies can help in achieving appropriate design of photoactive aggregates for mimicking light-harvesting natural photosynthetic pigments, photodynamic therapeutic use, and advanced nonlinear optical materials.
