J. Phys. Chem. B 1999, 103, 8097-8103
8097
Intrazeolite Photochemistry. Photochemistry of 1-Azaxanthone in Zeolites in the Presence of Hydrogen Donors, Electron Donors, and Energy Acceptors S. Corrent, L. J. Martı´nez, and J. C. Scaiano* Department of Chemistry, UniVersity of Ottawa, Ottawa K1N 6N5, Canada
Hermenegildo Garcı´a and Vicente Forne´ s Instituto Tecnologı´a Quı´mica, Apartado 22012, 46071-Valencia, Spain ReceiVed: January 28, 1999; In Final Form: May 20, 1999
The photochemistry and photophysics of 1-azaxanthone included in faujasite zeolites have been examined. In organic solvents, 1-azaxanthone has been found to be a better H-abstractor than benzophenone, which led to the idea of its use as a probe for radical reactions in heterogeneous systems. However, in the highly polar zeolite interior, mixing of n,π* and π,π* states of 1-azaxanthone occurs, resulting in reduced probe reactivity. The reactions of excited 1-azaxanthone included in NaY in the presence of other organic guests in the supercages were also monitored. Co-inclusion of small amounts of basic molecules such as pyridine and triethylamine led to a substantial increase in the triplet lifetime. Larger amounts of triethylamine resulted in ketyl radical anion formation, and addition of 2-propanol, a good hydrogen donor, resulted in some ketyl radical formation along with another transient. This second transient is ascribed to the cyclohexadienyl radical of 1-azaxanthone. Inclusion complexes of benzophenone in NaY were also prepared and the reactivity was compared to the 1-azaxanthone inclusion complexes.
Introduction The photochemical and photophysical properties of 1-azaxanthone have been studied in organic solvents.1 From this work, it was postulated that this molecule may be a good choice for use as a probe in radical-radical reactions. At least in organic solvents, 1-azaxanthone has been found to undergo hydrogen abstraction reactions very efficiently, even more so than benzophenone. A state inversion from n,π* to π,π* as solvent polarity increases is less pronounced for 1-azaxanthone than for xanthone, but does occur in solvents of > 60, notably water.2,3 That is, the less reactive π,π* state is the populated state in aqueous media. The singlet-triplet energy gap for this molecule is also quite small, as is indicated by the extensive overlap of the fluorescence and phosphorescence spectra in aqueous solution.3
As part of our studies of the photochemical and photophysical properties of 1-azaxanthone in different media,1,3,4 we report on the reactivity of this molecule in the faujasite zeolites NaY and HY. In these zeolite studies, co-inclusions of additional organic molecules such as triethylamine, which is a very good electron donor, 2-propanol, a good hydrogen atom donor, and pyridine, which has been shown to influence the triplet lifetime of xanthone included in NaY,5 were performed. When possible, the results are compared to those obtained with benzophenone. Triplet energy transfer was also studied in zeolite NaY using 1-azaxanthone as donor and 1-methylnaphthalene as the acceptor. The loadings of 1-methylnaphthalene were varied, keeping
the 1-azaxanthone loading constant and monitoring the changes in the transient absorption spectra. Simple statistical analysis was performed, giving an indication of the distance requirements for this “semistatic” energy transfer process to occur in faujasite zeolites. These results are compared with those reported earlier using xanthone as the triplet energy donor.6 Experimental Section Zeolite NaY (Linde molecular sieves LY-52), triethylamine, and pyridine were obtained from Aldrich and used as received. 2-Propanol (HPLC grade) was obtained from VWR Scientific. HY was prepared according to reported procedures.7 1-Azaxanthone was purchased from Lancaster and recrystallized twice from ethanol prior to use. Benzophenone was purchased from Aldrich and was also recrystallized from ethanol prior to use. Zeolite inclusion compounds were prepared by activating the zeolite at 550 °C for at least 12 h, then adding it to a solution of a known amount of 1-azaxanthone or benzophenone in CH2Cl2. The resulting mixture was stirred for 3 h at which point it was filtered and the solid washed with fresh CH2Cl2 to ensure that none of the ketone had remained on the zeolite surface. The solid was then dried under reduced pressure (∼10 mTorr) overnight. The CH2Cl2 washings were analyzed by GC, and no peaks due to the probe molecule (1-azaxanthone or benzophenone) were detected. This verified that all of the probe molecule was included in the zeolite. The occupancy number, 〈S〉 , which indicates the number of molecules per supercage, could then be determined. This is calculated from the ratio of moles of guest to the moles of supercages. Silica gel samples were prepared by heating 1 g of silica gel at 80 °C overnight, and then adding it to 5 mg of 1-azaxanthone in 35 mL of CH2Cl2. The mixture was stirred for 2 h and the solvent removed by rotary evaporation.
10.1021/jp9903357 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/03/1999
8098 J. Phys. Chem. B, Vol. 103, No. 38, 1999 Pyridine, triethylamine, and 2-propanol inclusion into the zeolite containing the ketone was accomplished by placing the dry zeolite complex into a closed container containing a small amount of the respective liquid and allowing the free diffusion into the supercages. The amount of pyridine, 2-propanol, or triethylamine included was determined by the resulting change in weight of the inclusion compound. From the amount of guest included, the occupancy number 〈S〉 , was determined as described above. UV-visible absorption spectra were recorded on a Cary 1E spectrometer. Steady-state emission spectra were recorded on a Perkin-Elmer LS-50 luminescence spectrometer. IR spectra were obtained at room temperature using greaseless quartz cells fitted with CaF2 windows in a Nicolet model 710 FTIR spectrophotometer controlled by a work station. Wafers (10 mg) were pressed into disks and typically outgassed under 10-2 Pa at 200 °C for ∼1 h before recording the spectra. Laser flash photolysis experiments were carried out using a time-resolved diffuse-reflectance setup similar to that developed by Wilkinson and co-workers.8-10 Laser excitation at 355 nm was from a Nd:YAG Continuum Surelite laser and a Lumonics EX-530 excimer laser was used for 308 nm excitation. All pulse durations were
