**7. Conclusions**

102 Macro to Nano Spectroscopy

temperature, pH, peripheral substitution and presence of surfactants of ions (Choi et al.,

H- and J-aggregates was formed by simply mixing aqueous solutions of two kinds of

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

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;

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;

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 photon-

The structural, kinetic, and spectroscopic studies on J- and H-aggregates provide useful

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

][

*pol* (2)

*M D*

][ ln <sup>1</sup> *DC MC*

*CC*

*DM*

The plot of the right term of this equation versus t gave good straight lines the slopes of

The information derived from such studies can help in achieving appropriate design of photoactive aggregates for mimicking light-harvesting natural photosynthetic pigments,

*tk*

photodynamic therapeutic use, and advanced nonlinear optical materials.

to-photocurrent generation efficiency ( Kamat et al.,2000; Sudeep et al., 2002).

information for understanding molecular interactions in aggregation processes.

2003; Ohno et al., 1993; Napoli et al., 2004; Kubat et al., 2003; Siskova et al., 2005).

porphyrins with opposite charges (Xiangqing et al., 2007).

between adjacent molecules.

Jeukens et al., 2004).

Giovannetti et al., 2010).

in equation (2):

which represented the values of *kpol*.

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 interpretation of Soret and Q band transformations.

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 the future.
