J . Phys. Chem. 1993,97, 1589-1595
1589
Electron Transfer Reactions of Triplet 9-Arylxanthenium and 9-Arylthioxanthenium Cations' L. J. Johnston' and D. F. Wong Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Ontario, Canada KIA OR6 Received: September 17, 1992; In Final Form: December 1, 1992
Triplet excited states of 9-arylxanthenium and 9-arylthioxanthenium cations have been characterized using laser flash photolysis. The triplet cations absorb at -300 nm and are long-lived ( T > 5-10 ps) in the absence of electron donors. They undergo efficient electron transfer reaction with a variety of aromatic donors, including biphenyl, naphthalene, substituted benzenes, and the precursor alcohols, to give the corresponding xanthenyl or thioxanthenyl radicals and the radical cation of the aromatic donor. Both the radical and radical cations are readily detectable by transient techniques. The rate constants for electron transfer quenching of the triplet cations decrease with increasing oxidation potential of the donor, as expected from the Rehm-Weller equation. The triplet cations are somewhat poorer oxidizing agents than the singlet excited states of the same cations due to their lower excitation energy. However, the triplet cations are much more useful sensitizers in that they give substantially higher yields of cage escape from the initial geminate pair than the corresponding singlets, based on a lower limit of 25% for biphenyl radical cation formation from the triplet 9-phenylxanthenium cation.
Introduction Carbocationsarekey intermediates in a widevariety of reactions and their ground-state chemistry has been widely studied.2 Until quite recently, most of the available data on the reactivity of these species was derived from competitive product studies for cations generated by SNI solvolyses. Structural data are also available from spectroscopic studies of stable species in strong acid media at low temperature. However, the recent development of efficient photochemical routes for cation generation has enabled the application of laser flash photolysis techniques to the direct measurement of the reactivity of aryl-substituted cations in reactive solvents. As a result, a considerable volume of data on absolute rate constants for cation reactivities toward various nucleophiles is now a~ailable.~ Much less information is currently available on the photochemistry of these important intermediate^.^^ A recent review of the area by Childs and Shaw notes that both isomerizationand electron transfer reactions appear to be of general importance in the photochemistry of carbocation~.~For example, electron transfer chemistry of a triplet excited state has been suggested as a possible mechanism for the photorearrangement of the triphenylmethylc a t i ~ n . The ~ . ~ photochemistryof this species is complex and depends strongly on the concentrationsof acid used to produce the initial cation. The involvement of electron transfer chemistry was also postulated to rationalize the products obtained by photolysis of triphenylcyclopropenium ions.I0 More recently, excitation of dianisylethyl cations in the presence of excess dianisylethanol has been shown to involve electron transfer reactions that lead to reduction of the cation." However, none of these studies reported detailed mechanistic work or characterization of the excited states involved. In fact, except for scattered reports on the emission from stable cations,9J2-I7little was known until quite recently about the excited-state properties or intermolecular reactivity of excited cations. During the last two years detailed studies of the fluorescence behavior and intermolecular reactivity of di- and triarylmethyl cationsI8 and of various 9-substituted xanthenium cations'8-20 have been undertaken. The reactivity of the first excited singlet state of the 9-phenylxanthenium cation toward a variety of substituted aromatic donors has been measured and shown to involve electron transfer to give 9-phenylxanthenylradical plus the radical cation of the aromatic donor.18-20 Rapid back electron transfer provides the major decay pathway for thesinglet geminate 0022-3654/93/2097-1589$04.00/0
radical/radical ion pair. However, results for reaction of excited singlet dibenzosuberenyl cation with benzyltrimethylsilane indicate that such electron transfer reactions can lead to efficient chemistry in cases where rapid chemical decay pathways of the radical cation can compete with back electron transfer.I8 In two related examples, picosecond transient absorption methods have been used to detect intermediate radical/radical cation pairs formed by excitation of charge transfer complexes between tropylium cations and aromatic donorsz1and covalent compounds composed of stable anions and triphenylcyclopropenyl cations.2* Electron transfer reduction of diarylmethyl cations has also been sensitized by triplets such as metho~ynaphthalene,~~ We have recently reported the characterization of the triplet excited state of the 9-phenylxantheniumcation using laser flash photolysis and phosphorescence technique^.^^ We present herein the results of a detailed examination of intermolecular electron transfer reactions of the triplet excited statesof severalxanthenium and thioxanthenium cations using laser flash photolysistechniques.
Results The 9-arylxanthenium and thioxanthenium cations, 1 and 2, can be readily generated in 2,2,2-trifluoroethanol(TFE) solvent by protonation and subsequent dehydrationof the parent alcohols, 3 and 4, using moderate concentrations (95% of the excited singlet cations yields no observable transients on a nanosecond time scale; (3) phosphorescence emission is observed from la. Quenching of the triplet cation by electron transfer from ground-state precursor alcohol present in the solution then accounts for the observed formation of radical Sa (Scheme I). The 9-(p-fluorophenyl)xantheniumcation, lb, behaves similarly to la. Spectra of the triplet cation produced by 355-nm excitationin TFE/TFA and the corresponding radical generated by reaction of the triplet with the parent alcohol are shown in Figure 2. The lifetimes for triplets la and l b are on the order of 4-5 ps at low concentrationsof the precursor alcohols and are limited by quenching by residual alcohol precursor and/or by traces of remaining oxygen. In the case of the p-methoxysubstituted 9-phenylxantheniumcation, IC,no transient species could be detectedwithin the time resolution of our flash photolysis setup. The transient behavior for thioxanthenium cations 2n and 2b is similar to that described above for the oxygen analogues. Figure 3 (top) shows the transient absorption spectrum obtained 1 ps after 355-nm excitation of 2a generated from 1 X 10-4 M C in TFE/TFA. In this case essentially all of thealcohol was converted to cation in order to have sufficient absorbance at the laser wavelength since the second absorption band for this species is shifted to slightly longer wavelength (385 nm). The transient that we assign to the triplet cation has X, at 300 nm as well as a very weak absorption in the 500-600 nm region. Under conditions where all of the precursor alcohol is converted to cation,
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1591
Electron Transfer Reactions of Triplet Cations
TABLE I: Spectroscopic Data and Redox Potentials for Cations la-c and 2a,b in TFE at Room Temperature Es, ET, cation kcal/mol @F T F , ns kcal/mol &d,' V la 57 0.80f0.04 36b 48 0.002
0.06
0.04
1R
0.02
-
-3 0.00
-
d 0
-0.02
lb IC
2s 2b
-
t
250
57 5 2' 52 52
0.55 f 0.02
*
0.002 0.020 f 0.002 0.031
47 0.14d
