Dear Sir. Concerning your issue about the signs of porphyrin aggregation interpreting absorption spectrum. An increasing interest in recent years is due to supramolecular assemblies of pi-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 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 pi-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. For more details, I think the following below links may help you in your analysis:

http://cdn.intechopen.com/pdfs/37656.pdf

Thanks

Thank you Mr. Elgailani for your valuable answer and for providing this publication.