Control over the optical and morphological properties of UV-deposited porphyrin structures

UV irradiation of porphyrin solutions in halogenated solvents leads to various extents of protonation, aggregation, and formation of solid deposits depending on the nature of the specific porphyrin, the light source, the initial solution concentration, the halogenated solvent, and the nature of the supporting substrate. In order to evaluate the role of various factors in controlling the main characteristics of the deposited samples, we investigated the behavior of a series of meso-substituted aryl porphyrins bearing protonable pyridyl peripheral groups [i.e., tetrakis(pyridyl) porphyrin isomers (ortho TpyP(2), meta TpyP(3), and para TpyP(4))] or hydroxyl peripheral groups [i.e., tetrakis(4-hydroxyphenyl) porphyrin (THPP) and tetrakis(3,5-dihydroxyphenyl) porphyrin (OHPP) together with some metal derivatives (Zn(II) TpyP(4) and Mn(III) TpyP(4))]. In pulsed irradiation experiments, the size and morphology of the resulting porphyrin particles strongly depend on the single pulse duration, the total irradiation time and the delay between pulses. Size and dispersity of the samples can be controlled through a careful choice of the light source (pulsed deuterium lamp or He-Cd laser). The photogenerated HX halogen acid plays an important role in modulating the optical properties (UV/vis absorption and fluorescence emission) of the porphyrin particles. The protonation and ion-pairing (for TpyPs porphyrins) and the hydrogen-bonding properties (for THPP and OHPP) together with electrostatics contribute to stabilize the growing particles at the interface between the solution and the supporting surface. The possibility of controlling the size of the deposited particles in the nanoscopic range (100-200 nm) has been demonstrated.