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 carboxyporphyrin s is described. These materials exhibit well-behaved monomolecular fil ms at an air-water interface. In compressed monolayers, the porphyrin ring appears to be oriented so that the plane of the ring is perpendic ular to the surface. The surface-pressure-molecular area isotherms cha nge with the length and nature of the side chains in a manner suggesti ng that a long side chain allows the porphyrin rings to acquire the mo st ordered packing. A rigid chain structure was found to restrict the orientational flexibility of the porphyrin rings and hence prevented t hem from acquiring a well-ordered monolayer structure. The solid-state photophysical properties of Langmuir-Blodgett monolayers of these mat erials are strongly dependent on their degree of order. The absorption , fluorescence, and fluorescence excitation spectra of single monolaye r films transferred onto quartz and SnO2 slides using the Langmuir-Blo dgett technique indicate that-increasing the order in porphyrin monola yers leads to an increased red-shift of the Soret band and to decrease d fluorescence quantum yields. On a SnO2 semiconductor surface, the mo nolayers exhibit enhanced fluorescence quenching. This is interpreted as evidence for isoenergetic electron transfer from the porphyrin to t he semiconductor. An approximate interfacial electron-transfer rate (k (et)) was estimated on the basis of the fluorescence yields on quartz and SnO2 surfaces, respectively.