J . Phys. Chem. 1993,97, 5981-5994
5987
Photophysics of Polycyclic Aromatic Hydrocarbons Adsorbed on Silica Gel Surfaces. 2. Lifetime Distribution and Symmetry? Yuan S. Liu, Paul de Mayo, and William R. Ware’ Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 587 Received: December 28, 1992; In Final Form: March 12, I993
In a lifetime distribution analysis of six polycyclic aromatic hydrocarbons (PAHs) adsorbed on silica gel surfaces, a dramatic photophysical effect on the surface can be seen in the PAHs where the lowest excited singlet state is of ‘Lbtype. This suggests that this effect is related to the symmetry of the lowest excited singlet state of the particular molecule. In addition, a comparison of the photophysical behavior of these PAHs adsorbed on a highly dehydroxylated and on a hydroxylated silica gel surface suggests that the natures of these two types of adsorption may be different. These results show that the regularized lifetime distribution analysis is a useful tool in the surface photophysical studies.
Introductioo In the past decade, many interesting observations of the photophysical behavior of adsorbed aromatic hydrocarbons have been reported. One such report is that the lifetimes of pyrene and naphthalene, when adsorbed on a silica gel surface, are dramatically shortened on going from a hydroxylated to a highly dehydroxylated silica gel surface.’ A number of mechanisms could be suggested to interpret this phenomenon, but a detailed investigation was lacking. A difficulty associated with such a study was the fact that there was no satisfactory way to analyze and to represent the lifetimes of adsorbed molecules. Although the authors’ realized that the lifetimesof these adsorbed molecules were most likely represented by distributions, they were forced to present their results in terms of two discrete lifetimes, while, however, mentioning that the two lifetimes had no physical meaning. The difficulties involved in lifetime distribution analysis have been discussed in detail elsewhere.2 Using pyrene adsorbed on silica gel as a model system, we concluded that the use of a “smoothing”or “regularization”technique was essentialin dealing with surface decay data and that a bimodal distribution appeared to be an acceptable representation of the photophysical behavior of this system. In this paper, we report a comparative study of six polycyclic aromatic hydrocarbons (PAHs) adsorbed on hydroxylated and highly dehydroxylated silica gel surfaces, using the regularized lifetime distribution analysis technique. The six PAHs studied in this work were phenanthrene, chrysene, pyrene, perylene, 1,12-benzoperylene, and coronene. These compounds are all highly fluorescent and possess large a-electron systems. This selection was made to determine whether there was any obvious correlation between the size and shape of the a system of the PAH molecule and the magnitude of the “surface effect”.
Experimental Section Pyrene (Aldrich) was purified by recrystallizationfrom alcohol solution and then by vacuum sublimation. Phenanthrene was zone-refined. Chrysene (Princeton Organics), perylene (Litton Chemicals), 1,12-benzoperylene (Aldrich)and coronene (Aldrich) were used as received. The silica gel used was Kieselgel-60 (3570 mesh); its specific surface area, as specified by the manufacturer, was 640 m2/g.
’Publication No. 488 from the Photochemistry Unit, University of Western ~
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0022-3654193/2097-5987$04.O0/0
The pretreatment of the silica gel and the loading procedure of the PAHs are critical factors in the photophysical behavior of the adsorbate. Special attention must be given to highly dehydroxylated silica gel samples, because the surface of such silica gel is covered with isolated hydroxyl (silanol) groups and strained Si-oSi (siloxane) bonds and is extremely susceptible to moisture. Although a complete rehydration takes a long time, to completely destroy the “dry surface effect” can be just a matter of seconds or minutes. It has been repeatedly observed in this work that simply dumping the high-temperature treated silica gel from one apparatus to another may partly or even completely eliminate the dry surface effect. This is probably because the remaining isolated hydroxyl groups on a heated silica gel surface, whenever exposed to the atmosphere, tend to rapidly pick up water molecules and to become instantly hydrogen bonded (with water). Consequently, all the effects caused by these isolated hydroxyl groups disappear before the rehydration of Si-oSi bonds actually takes place. The manner of the transfer of PAH molecules to silica gel is also important. In the early stage of this work, the aromatic molecules were adsorbed onto the silica gel surface from a cyclohexane solution. However, it was too difficult to prevent the solution from absorbing traces of water, and such dissolved water may considerably reduce the dry surface effect. Therefore, the transfer was effected from the gas phase under a high-vacuum condition. Such “dry” loading techniques have been described by Oelkrug and co-~orkers.~ The apparatus used is shown in Figure la. Part A is a quartz tube or a cuvette that contains the prepared sample and is used for later measurements. Part B is a quartz bulb in which the silica gel is subject to preheating. C and D are vacuum valves, and D is attached to a joint used for connecting the apparatus to a vacuum line. In dehydroxylation experiments, the desired amount of PAH solution was introduced into A. The solvent was then evaporated through thevacuum line, leaving the PAH deposit in A. Then valve C was closed, so that A was maintained under a high vacuum (
