Effects of Molecular Organization on Photophysical Behavior. 1. Steady-State Fluorescence and Fluorescence Quantum Yield Studies of Langmuir−Blodgett Monolayers of Some Surfactant Porphyrins

The surface chemistry of two new groups of surfactant carboxyporphyrins is described. These materials exhibit well-behaved monomolecular films at an air−water interface. In compressed monolayers, the porphyrin ring appears to be oriented so that the plane of the ring is perpendicular to the surface. The surface-pressure−molecular area isotherms change with the length and nature of the side chains in a manner suggesting that a long side chain allows the porphyrin rings to acquire the most ordered packing. A rigid chain structure was found to restrict the orientational flexibility of the porphyrin rings and hence prevented them from acquiring a well-ordered monolayer structure. The solid-state photophysical properties of Langmuir−Blodgett monolayers of these materials are strongly dependent on their degree of order. The absorption, fluorescence, and fluorescence excitation spectra of single monolayer films transferred onto quartz and SnO 2 slides using the Langmuir−Blodgett technique indicate that increasing the order in porphyrin monolayers leads to an increased red-shift of the Soret band and to decreased fluorescence quantum yields. On a SnO 2 semiconductor surface, the monolayers exhibit enhanced fluorescence quenching. This is interpreted as evidence for isoenergetic electron transfer from the porphyrin to the semiconductor. An approximate interfacial electron-transfer rate ( k et ) was estimated on the basis of the fluorescence yields on quartz and SnO 2 surfaces, respectively.