DYNAMICS OF SPIN-POLARIZED RADICAL PAIRS AT THE SOLID SOLUTION INTERFACE/

Time resolved electron paramagnetic resonance (TREPR) spectroscopy has been used to investigate the magnetic and kinetic properties of monor adicals covalently bound to silicon oxide surfaces. Norrish I alpha-cl eavage of aliphatic ketones and photoreduction of aromatic ketone n,pi triplet states were used to produce the radicals. The ketones were a nchored to the silica surface via a chlorosilane-terminated alkane cha in. The aliphatic ketones were analogues of di-tert-butyl ketone, and the aromatic ketones were benzophenones connected at the ortho, meta, and para positions. A detailed description of the synthesis, attachmen t, and characterization procedures is given for all surface-anchored k etones. Comparison to TREPR spectra of similar structures in free solu tion shows that both radical pair (RPM) and triplet (TM) spin polariza tion mechanisms are affected by anchoring the molecules to the surface . Stronger TM polarization is observed in all cases, and increases in the line width are observed when the ''tether'' chain length is less t han five carbon atoms. Longer tethers show line widths similar to thos e observed in free solution. Polar solvents at the interface also affe ct the ratio of RPM to TM polarization. The T-1 of the surface-bound r adicals increases in some cases. These changes are discussed with resp ect to the rotational correlation time of the radicals and the ''effec tive viscosity'' at the interface. The ortho-alkylated benzophenone ex hibits a long-lived EPR signal which may be due to a ''photo-enol'' ty pe biradical, rather than monoradicals from the photoreduction process . Signals from the para-alkylated benzophenone show evidence for a spi n-correlated radical pair (SCRP) at very early delay times.