Physica A 249 (1998) 501–510 Fractal aggregation in aqueous solutions of porphyrins N. Micali a , L. Monsu’ Scolaro b , A. Romeo b , F. Mallamace c; ∗ a Istituto di Tecniche Spettroscopiche del CNR, Salita Sperone 31, 98166 Messina, Italy b Dipartimento di Chimica Inorganica, Analitica e Chimica Fisica, Universita’ di Messina and ICTPN-CNR, Messina, Italy c Dipartimento di Fisica, Universita’ di Messina, Vill. S.Agata C.P. 55, 98166 Messina, Italy Abstract We have studied the aggregation of the porphyrin trans-bis(N-methylpyridinium-4-yl)-diphenyl- porphyrine (t −H 2Pagg ) in aqueous solutions by means of light scattering (elastic and quasielastic) and UV-visible absorption measurements. Elastic and dynamic light scattering indicate that the aggregation produces large monodisperse clusters having a fractal structure driven by diusion limited aggregation (DLA). UV-visible absorption measurements have been applied to study the corresponding kinetics, obtaining the time dependence of the monomer concentration and of the mean cluster size. The obtained ndings are in agreement with the picture proposed by current theoretical models and molecular dynamics (MD) simulations on DLA. c 1998 Elsevier Science B.V. All rights reserved. Keywords: Complex uids; Aggregation; Fractal; Porphyrins 1. Introduction Porphyrins are an important class of compounds, as their properties are of large scientic and technological interest in many elds (physics, chemistry, material science, electronics, photonics, engineering and biology). They nd applications in xerography, coating of semiconductor microdevices, sensors, photonic devices, catalysts, molecular electronics, drug delivery, pharmaco kinetics, photodynamic therapy, etc. The dynamical and structural properties of these systems are inuenced by the occurrence of – stacking interactions, due to their extended aromatic regions. Such interactions lead to two dierent spatial arrangements [1]: (i) an oset −stacked, in which two molecules overlap each other with a lateral oset, and (ii) a T -shaped geometry, in which one molecule is rotated by 90 ◦ with respect to the other one. * Corresponding author. 0378-4371/98/$19.00 Copyright c 1998 Elsevier Science B.V. All rights reserved PII S0378-4371(97)00509-8