J. Phys. Chem. 1995, 99, 6689-6636
6689
Primary Photochemical Processes of Anthracene Adsorbed on Silica Gel F. Wilkinson,* D. R. Worrall, and S. L. Williams Department of Chemistry, University of Technology, Loughborough, Leicestershire LE1 1 3TU, U.K. Received: December 20, 1994; In Final Form: February 17, 1995@
Time resolved and steady state absorption and emission characteristics of anthracene adsorbed at surface coverages of less than 5% of a monolayer on silica gel have been investigated as a function of sample loading. At low loadings, steady state absorption and emission spectra show considerable similarities with those from dilute methanolic solutions. As loading is increased, analysis of the spectra using the Kubelka-Munk theory reveals an apparent decrease in molar absorption coefficient which is attributed to the formation of aggregates. Studies of steady state emission spectra as a function of sample loading reveal a decrease in emission intensity at higher loadings, indicative of nonemitting aggregates. Transient absorption due to the triplet state of anthracene, the rate of decay of which increases with increased loading, has been studied using diffuse reflectance laser flash photolysis. At low loadings, the decay of delayed fluorescence due to triplet-triplet annihilation can be described using a fractal dimensional rate constant, but with increasing loading the kinetics conform more closely to a two-dimensional model. Transient absorption studies show that, in addition to the triplet state, the anthracene cation radical is formed by a two-photon or multiphoton process with a yield which depends on the concentration of monomeric anthracene present on the surface, as does the triplet yield. No transient absorption attributable to excited states of aggregated species has been observed. Time resolved emission studies reveal, in addition to the prompt fluorescence and delayed fluorescence originating from triplet-triplet annihilation, two further emissions; one centered at 450 nm, assigned as arising from the anthracene excimer, and one at 530 nm, assigned as arising from a reaction product present on the surface prior to irradiation.
Introduction Primary photochemical processes of molecules adsorbed on surfaces have been the subject of much recent Their understanding is of great importance in a number of technological areas, including photocatalysis, semiconductors, and dyed fabrics. We and others have made considerable progress in revealing some of the properties of importance in the photochemistry of molecules on surfaces, although a complete picture is still not available for many With this in mind, we have conducted a detailed study of the primary photochemistry of anthracene, the properties of which are well characterized in solution, when it is adsorbed at less than a monolayer coverage on microporous silica gel. The photodimerization of anthracene was one of the first photochemical reactions to be studied, being first observed by Fritszche in 1867.25 It was not until many years later that the correct structure of the photoproduct, dianthracene, was elucidated to involve bond formation between the 9-9' and 10-10' positions of the reacting anthracenes. Following on from this early research, anthracene and other polynuclear aromatic hydrocarbons have received considerable attention in many photochemical studies of s o l ~ t i o n s , ~of~ ~- ~r y~s t a l s , ~ 'and, -~~ to a lesser extent, when adsorbed on surface^.^-^^ The technique of fluorescencequenching has been used to investigate diffusion of polynuclear aromatic hydrocarbons when adsorbed on surfaces?0.21while diffuse reflectance laser photolysisz2allows direct observation of triplet-triplet and radical absorptions of adsorbed molecules. Pyrene has been studied extensively on both s i l i ~ a ' ~ ,and ' ~ ,a~l ~~ m i n a *surfaces, ~ , ~ ~ where triplet states and cation radicals are readily observed. Pyrene, in common with anthracene, is adsorbed on the surface via x-cloud interactions with silanol functionalities, which are present on @
Abstract published in Advance ACS Abstracts, April 1, 1995.
the surface unless pretreatment temperatures exceed 170 "C in vacuo34 which leads to their removal.
Experimental Section Samples of anthracene on silica gel were prepared as follows. Silica gel (David grade 635, 60-100 mesh, 6-nm pore size, surface area 480 m2 g-l; Aldrich) was dried under vacuum at 5x mbar and 125 "C for 6 h and maintained under vacuum at room temperature for a further 12 h. Anthracene (scintillation grade; Aldrich) was dissolved in n-hexane (spectrophotometric grade; Aldrich) previously dried by reflux over calcium hydride. The anthracene solution was added to the silica under dry nitrogen and the mixture periodically agitated for 3 h. The solvent was then decanted off and the sample finally dried under vacuum to 5 x mbar and sealed into a 22 mm diameter x 10 mm path length cuvette. Sample loadings in the range 0.1-7 mg g-l (corresponding to
