J . Phys. Chem. 1989, 93, 5493-5500
5493
Singlet Molecular Oxygen ('Ago,)Formation upon Irradiation of an Oxygen (3Z,0,)-Organic Molecule Charge-Transfer Absorption Band Rodger D. Scurlock and Peter R. Ogilby* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 871 31 (Received: December 5, 1988)
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Singlet molecular oxygen ('$0,) phosphorescence (%io2 1$02:1270 nm) has been observed in a time-resolved experiment subsequent to pulsed UV laser irradiation of the oxygen (32i02)-organic molecule charge-transfer bands of liquid aromatic hydrocarbons (mesitylene,p-xylene, o-xylene, toluene, benzene), ethers (tetrahydrofuran, 1,4-dioxane, glyme, diglyme, triglyme), alcohols (methanol, propanol), and aliphatic hydrocarbons (cyclohexane, cyclooctane, decahydronaphthalene). Although 'Ago2could originate from a variety of different processes in these oxygenated solvent systems, we have used the results of several independent experiments to indicate that an oxygen-solvent charge-transfer (CT) state is the 'Ago2 precursor. Other transient species have also been observed in time-resolved absorption experiments subsequent to pulsed UV irradiation of the oxygen-solvent CT bands. Some of these molecular transients, or species derived from these intermediates, may be responsible for an observed increase in the rate of i A K 0 2decay under certain conditions.
Introduction It has long been recognized that a distinct red shift occurs in the absorption spectrum of a liquid organic solvent when it is exposed to molecular This structureless absorption band is found with a variety of different organic molecules, many of which are used as common solvents, and has been assigned to a transition from a ground-state oxygen (3Z-02)-solvent contact complex to an oxygen-solvent charge-transfer (CT) state.' The CT state is often represented as an excited-state complex with substantial solvent radical cation and oxygen radical anion character (~ol'+O~'-).'*~9~ The C T absorption band is reversibly removed when the solvent system is purged of oxygen. Our interest in this C T state comes as part of an experimental program to elucidate the nature of excited-state interactions between molecular oxygen and organic molecules. Specifically, we have been interested in (a) the process of ground-state oxygen (32;02)induced intersystem crossing in excited-state organic molecules (Le., Ti SI and So T,) which can result in energy transfer to form singlet molecular oxygen ('Ag0,)637 and (b) the ground-state organic molecule induced deactivation of 'A,02 to 32;02.8Our efforts in this field necessarily require a better understanding of the coupling between the various electronic states of an oxygen-organic molecule complex. Theoretical studies of the deactivation of organic molecule excited electronic states (M*) by 32-02indicate that there is a significant interaction between the &T state and other excited electronic states of the oxygen-M ~ o m p l e x , ' and * ~ ~it~thus appears that the C T state may be of considerable importance in the energy-transfer process from M* to 32;02to yield 1AK02.9 This is diagrammatically shown in Scheme I for the specific case of a triplet-state organic molecule (3M1)in which i*3(M'+O{-) states mix with i*3*5(3M1-.32,02) states." The importance of the C T state has been recognized in a variety of experimental studies.l2-I4 In one of these studies,13 evidence was presented to support the claim that oxygen quenching of an organic molecule excited state can result, under certain circumstances, in the formation of the In a second study, oxygen quenching of superoxide ion (02'-). an organic molecule excited state in a polar solvent was accompanied by the formation of the organic molecule radical cation.12 Also, there is evidence that electrochemically generated radical cations (M'+) and other oxidizing agents may interact with 0,'to generate IAK02.15J6 These experiments suggest that the C T state can indeed couple to other states of the oxygen-organic molecule complex that result in the formation of ]Ago2. However, the role of the C T state in the organic molecule induced transition to 3Z;02 is less ~ e r t a i n . ~ . ' ~ from 'Agoz If Scheme I is an accurate, albeit simplified, representation of our photophysical system, it should be possible to gain access to
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*To whom correspondence should be addressed.
the oxygen-organic molecule excited-state potential surfaces, and ultimately form ,Ago2, by selective irradiation of the CT absorption band (huCT). It has been shown that broad-band irradiation18of oxygenated hydrocarbons in the spectral region of the C T band, both in the solution phase and at cryogenic temperatures, gives rise to oxygenated products which implicate the intermediacy of either a caged radical ion pair or the free superoxide ion.ie23 was proposed to be the active In several cases, however, 'Ago2 oxygenating species.23 We recently reported that 'Ago2 can be spectroscopically detected subsequent to the pulsed UV laser photolysis of an oxygensolvent cooperative absorption band.24 We suggested, at the time, that a combination of two different channels may have been involved in IA,O2 formation: (a) excitation to the oxygensolvent
(1) Tsubomura, H.; Mulliken, R. S. J . Am. Chem. SOC. 1960, 82, 5966-5974. (2) Evans, D. F. J. Chem. SOC.1953, 345-347. (3) Munck, A. U.; Scott, J. F. Nature 1956, 177, 587. (4) (a) Kawaoka, K.; Khan, A. U.; Kearns, D. R. J . Chem. Phys. 1967, 46, 1842-1853. (b) Kawaoka, K.; Khan, A. U.; Kearns, D. R. Ibid. 1967, 47, 1883-1884. ( 5 ) Khan, A. U.; Kearns, D. R. J . Chem. Phys. 1968, 48, 3272-3275. (6) Iu, K.-K.; Ogilby, P. R. J . Phys. Chem. 1987, 91, 1611-1617. (7) Iu, K.-K.; Ogilby, P. R. J . Phys. Chem. 1988, 92, 4662-4666. (8) Scurlock, R. D.; Ogilby, P. R. J . Phys. Chem. 1987, 91, 4599-4602. (9) It is entirelypssible that this energy-transfer step may also result in the formation of 1Zg02.4v5 However, in liquid solutions, the transition ]Ago2 1Z+02is extremely rapid ( e 1 ns).'O (16) Arnold, S. J.; Kubo, M.; Ogryzlo, E. A. A h . Chem. Ser. 1968, 77, 133-142. (1 1) We have interchangeably used the symbols TI and to represent the lowest triplet excited state and So and IMo to represent the ground singlet state of an organic (solvent) molecule. S I refers to the lowest singlet excited state. (12) Potashnik, R.; Goldschmidt, C. R.; Ottolenghi, M. Chem. Phys. Lett. 1971. -. - ,9. - ,424-425. (13) Cox, G.S.; Whitten, D. G.; Giannotti, C. Chem. Phys. Lett. 1979, 67, 511-515. (14) Koizumi, M.; Usui, Y. Mol. Photochem. 1972, 4, 57-92. (15) Mayeda, E. A.; Bard, A. .I. J. Am. Chem. Soc. 1973,95,6223-6226. (16) Nanni, E. J.; Birge, R. R.; Hubbard, L. M.; Morrison, M.M.; Sawyer, D. T. Inorg. Chem. 1981, 20, 737-741. (17) Ogilby, P. R.; Foote, C. S. J. Am. Chem. Soc. 1983,105,3423-3430. (18) In each of these studies, a nonselective, polychromatic arc lamp was used as the photolysis source. Therefore, the specific molecular transition(s) excited may be uncertain.1e23 (19) Chien, J. C. W. J . Phys. Chem. 1965,69, 4317-4325. ( 2 0 ) Hashimoto, S.; Akimoto, H. J . Phys. Chem. 1986, 90, 529-532. (21) Kojima, M.; Sakuragi, H.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1987, 60, 3331-3336. (22) Stenberg, V. I.; Olson, R. D.; Wang, C. T.; Kulevsky, N. J . Org. Chem. 1967, 32, 3227-3229. (23) Onodera, K.; Furusawa, G.; Kojima, M.; Tsuchiya, M.;Aihara, S.; Akaba, R.; Sakuragi, H.; Tokumaru, K. Tetrahedron 1985,41,2215-2220. (24) Scurlock, R. D.; Ogilby, P. R. J. Am. Chem. Soc. 1988,110,640-641.
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0022-3654/89/2093-5493$01.50/00 1989 American Chemical Society
5494
The Journal of Physical Chemistry, Vol. 93, No. 14, I989
Scurlock and Ogilby
SCHEME I
h0+ 3&02
3(1~00*-3&02)
C T state followed by dissociation to yield '$02and (b) an 31;;02 enhanced solvent singlet (So) to triplet (T,) transitionZ5J6followed by the sensitized formation of '$O2 by the resulting solvent triplet state. In the current paper, we specifically indicate that excitation to an oxygen-solvent CT state results in the formation of 'Ago2. By using the time-resolved 1270-nm phosphorescence of 'Agoz as an experimental probe, we show that this is a general phenomenon common to a wide variety of organic solvents.
TABLE I: Pulsed Laser Excitation Waveleneths Nd:YAG harmonic wavelength. nm
S R S wavelength." nm ~~
355 309 (first anti-Stokes) 266 239 (first anti-Stokes) 299 (first Stokes) 341 (second Stokes)
energy per oulse.6 mJ ~
55c 1.5 40' 1.5 15 10
Experimental Section "Laser lines generated by stimulated Raman scattering (SRS) in Hz Absorption Spectra. Absorption spectra were recorded on a gas. Au = 4155 cm-'. *Typical values which define an upper limit to Beckman DU-40 UV-vis spectrophotometer using a 1-, 5-, or the laser energy range used in these experiments. In several cases, IO-cm path length cell. The solvents were oxygen-saturated by SRS lines substantially higher in energy were obtained. With the exbubbling with oxygen gas for 10 min. To remove the oxygen, the ception of several experiments (e.g., data in Figure 6 and the solvent solvents were bubbled with nitrogen gas for the same length of fluorescence study; see text), however, the laser energies used did not time. Both gases were passed through a 1-ft-long, in-line Drierite exceed -2.5 mJ/pulse.
