J. Phys. Chem. 1996, 100, 15551-15554
15551
Photochemistry of trans-Stilbene Adsorbed on Al2O3(0001) R. M. Slayton, N. R. Franklin, and N. J. Tro* Department of Chemistry, Westmont College, Santa Barbara, California 93108 ReceiVed: April 18, 1996; In Final Form: June 19, 1996X
Temperature-programmed desorption (TPD) was used to study the desorption and photochemistry of transstilbene on Al2O3(0001) in ultrahigh vacuum. At low coverages, the TPD traces of cis-stilbene and transstilbene both show only one peak, attributed to monolayer desorption, at 275 and 315 K, respectively. As the coverage is increased, a second, multilayer peak is seen at 230 K for cis-stilbene and at 260 K for transstilbene. The relative intensities of the monolayer and multilayer peaks were observed to fit a statistical adlayer sticking model similar to those of other hydrocarbons and described in previous work. When transstilbene is exposed to light at 313 nm, TPD traces show a new peak at 380 K which is attributed to the formation of the stilbene dimer, 1,2,3,4-tetraphenylcyclobutane (or isodistilbene). The photodimerization is coverage and temperature dependent with little or no photochemistry occurring at coverages below 0.10 ML or temperatures below 100 K. The data are consistent with the dimerization occurring through an excimer intermediate. Little or no photoisomerization was observed at any of the coverages studied.
Introduction The photochemistry of surface-adsorbed molecules is significant in many processes, such as photolithography, optical data storage, stratospheric ozone depletion, solar cell sensitization, and catalysis.1-5 Surface photochemistry has received a great deal of attention recently with several excellent review articles in the literature.6,7 Surfaces have been observed to affect photochemical pathways in a number of ways. In some cases, the surface, especially if it is a metal surface, quenches excited states and therefore reduces photochemical yields relative to the gas or solution phase.6,8 In other cases, the surface has been found to enhance chemical yields by absorbing energy from light and then transferring the energy to the adsorbate.9 In addition, the photochemistry of surface adsorbates may be completely different than gas or condensed phases because of the unique structure of molecular adlayers. In this paper, we report on the photochemistry of transstilbene adsorbed on Al2O3(0001). We are particularly interested in how a dielectric surface affects the photochemical pathways relative to the gas and solution phases. The photochemistry of trans-stilbene has been extensively studied in both gas and condensed phases.10-16 In the gas phase, in dilute solutions, and in the solid crystal, the primary photochemical pathway for trans-stilbene upon excitation at 313 nm is photoisomerization to cis-stilbene. The primary mechanism of isomerization is through the excited singlet state.17-19 In this state, trans-stilbene can overcome a small activation barrier and twist about its central bond to form a twisted intermediate. This intermediate then decays with equal probability to either ground state cis-stilbene or ground state trans-stilbene. trans-stilbene can also under photodimerization in solution at high concentrations. However, dimerization has only been observed in small percentages (
