Steady-state and Transient Optical Spectroscopy: Chemically Modified Surfaces

The impact of restricted diffusion on the kinetics and mechanisms of free radical and ionic reactions during the pyrolysis of organic molecules covalently immobilized on the surfaces of nonporous fumed silica nanoparticles has been a focus area for our group in recent years. Fundamental interactions between adsorbed / chemically attached molecules and heterogeneous surfaces can influence the course of many chemical reactions and processes in environmental chemistry, surface catalysis, and solid-state synthesis. In heterogeneous systems such as silica, the nature of the network structure and the changes that the surface imposes on the physisorbed/chemically-attached molecules depends largely on the surface termination groups (e.g. silanols) and/or modifier groups. Steady state and time-resolved transient spectroscopy in the diffusely reflected mode offers enormous potential for probing such interactions on solid surfaces¹. The rotational and translational dynamics of physisorbed and chemically attached surface probes can be resolved using time-resolved fluorescence anisotropy. By studying the re-orientation dynamics of the molecules under a variety of conditions, it is possible to determine how the relaxation dynamics respond to various properties of the surface under investigation. The method allows us to distinguish between in-plane and out-of-plane rotations exhibited by the probe molecule on the surface. Thus, molecular motions can be interrogated and the rate constants for various rotations of the surface probe obtained using this technique. Pyrene has been chosen as the surface probe in these investigations. We have determined the diffusion rates for in-plane and out-of-plane rotational rate constants for chemically attached and pysisorbed pyrene on silica^2,3. The effect of tether chain length for surface-attached pyrene and the presence of co-attached spacer molecules on these rates have also been investigated².

1. Dabestani, R.; Higgin, J.; Stephenson, D.; Ivanov, I. N.; Sigman, M. E. J. Phys. Chem. B 2000, 104, 10235-10241.
2. Ivanov, I. N.; Dabestani, R.; Buchanan, III, A. C.; Sigman, M. E. J. Phys. Chem. B 2001, 105,
10308-10315.
3. Sigman, M. E.; Read, S.; Barbas, J. T.; Ivanov, I.; Hagaman, E. W.; Buchanan, III, A. C.; Dabestani, R.; Kidder, M. K.; Britt, P. F. J. Phys. Chem. B. 2003, in press.

Time-Resolved Transient Absorption/Emission Set-up