The quenching rate constant of the triplet state by molecular oxygen and the efficiency of singlet oxygen generation have been measured for 12 amines in cyclohexane and benzene. For the best electron donors, the average rate constant for quenching by energy transfer is 6.0 × 10 9 M -1 s -1 . For the same compounds, the rate constant of quenching via enchancement of internal conversion is 1.2 × 10 10 M -1 s -1 . The energy transfer component of the total quenching rate constant is almost twice as fast as the maximum from the standard Porter model. The mechanisms of quenching via quintet, singlet, and triplet channels are discussed for amines and aromatic hydrocarbons, and intersystem crossing out of the quintet manifold is proposed.

The quenching rate constants of singlet oxygen O2(1Δg) luminescence at 1.27 μm by 29 amines and aromatic hydrocarbons were measured in acetonitrile and benzene. Quenching occurs via reversible charge transfer with the formation of an exciplex with a partial (δ ≈ 0.2 esu) electron transfer. An estimate of the intersystem-crossing rate constant for the exciplex of ca. 1010 s-1 is obtained, and a modest decrease with increasing exciplex energy is noted. The data predict and it is confirmed that charge transfer plays a significant role in the nonradiative decay of singlet oxygen in solvents as difficult to oxidize as toluene-d 8 and mesitylene. It is also confirmed that N,N,N‘,N‘-tetramethyl-p-phenylene diamine quenches O2(1Δg) nearly exclusively by full electron transfer in D2O, but electron transfer is not observed in other solvents.

Absorption spectra and extinction coefficients of phenylsulfinyl and phenylsulfenyl (thiyl) radicals are determined by nanosecond laser photolysis in various solvents. Direct observation and characterization of arylsulfinyl radicals from the photolysis of several aromatic sulfoxides provides the strongest evidence to date for R-cleavage as the predominant primary photochemical process for these compounds. The absorption spectrum of phenylsulfinyl, with λ max ) 300 and 450 nm and ) 1.1 × 10 4 and 1.3 × 10 3 M -1 cm -1 , is practically independent of solvent. Quantum yields of free sulfinyl radicals range from 0.09 to 0.18 in various solvents. Recombination rate constants very near diffusion control indicate that there is a large spin-orbital coupling in the radical pair. Rate constants for the reactions of arylsulfinyl radicals with stable nitroxide radicals are among the fastest known, but reactivity with O 2 is very modest. Computations indicate that the singly occupied molecular orbital is a π* orbital largely localized on the sulfur and oxygen atoms.

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3723this correlation. The data points, plotted as k!s/k:,, are compared with the predicted behavior of eq 7 (solid line). Also shown in this figure is the hard-sphere rate constant, represented by the dashed line of zero slope and intercept one. This plot illustrates graphically our observation made earlier that the rotational relaxation proceeds more efficiently than the hard-sphere rate constant since all data p i n t s lie below the dashed line. It is also shown in Figure 9 that the measured rate constants for most of the ga.w significantly exceed the Lennard-Jones collision rate since they lie below the predicted LJ line. This suggests that the interaction occurs over larger distances than allowed for in the LJ potential; i.e., the attractive part of the potential is more long range. ConclusionsIn this work we have measured collision-induced rotational relaxation rates corresponding to destruction of an initial, narrow rotational distribution within the 6l vibrational level of SI benzene vapor by six collision partners. The collision partners studied were H2, He, N2, Ar, Cd-IH,(So), and cyclohexane. All experiments were performed at ambient temperature (293 K).The relaxation rates exceed the hard-sphere rate for all collision partners and for most also exceed the Lennard-Jones collision rate.Presumably, the intermolecular potential has a longer range attractive force than is provided by the Lennard-Jones potential. The rotational relaxation rate constants determined represent state-to-field values, as individual relaxation pathways could not be seen in this study.Rotational relaxation by So benzene was found to be less than the value measured by Coveleskie and Parmenter for the 00 level.We suggest that this is a consequence of a reduced contribution from electronic energy transfer in our experiments on 6I than were present in Coveleskie and Parmenter's work on 00. We thus believe that our value gives an improved estimate of rotational relaxation due to So benzene collisions.There are comparatively few studies that report rotational relaxation in excited electronic states for polyatomic molecules. This study significantly extends the range of collision partners measured for a large polyatomic in an excited electronic state.The quenching rate constants of triplets by molecular oxygen (k 3 and the efficiency of singlet oxygen generation per quenching encounter (SA) were measured for various r,r* and n,r* tridet sensitizers containing heavy atoms using nanosecond laser photolysis and time-resolved 02('Ag) phosphorescence. For r,r* triplets (tetrahalo-p-benzoquinones and haloanthracenes), introduction of the heavy atom has practically no influence on SA. For n,r* triplets (haloacetophenones), SA values decrease sharply with increasing number of heavy atoms; for example, for 4-bromo-and 2,4'-dibromoacetophenone in benzene, SA = 0.10 and 0.032, respectively. An analysis of the mechanism of triplet quenching by oxygen was carried out, taking into account intersystem crossing between singlet and triplet states of the ...

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The efficiency of singlet oxygen generation (SA) was measured for acetophenone, p-aminobenzophenone, and Michler's ketone by the time-resolved 02(*Ag) phosphorescence method in various solvents. The SA value for acetophenone is 0.3-0.4 in benzene, cyclohexane, acetonitrile, and alcohols (practically independent of solvent); the lowest triplet state of acetophenone is therefore probably pure n,r* in all solvents. The values for Michler's ketone and p-aminobenzophenone are 0.24-0.4 in all solvents investigated (except in ethanol for Michler's ketone) and are also typical for ketones with n,?r* triplet states. For Michler's ketone in ethanol, SA = 0.65; the triplet state therefore probably has substantial C T character. Amino-substituted benzophenones do not undergo photoreduction in alcohols, which is explained partly by a low quantum yield of triplet and mainly by the considerable negative charge at the carbonyl oxygen because of intramolecular charge transfer, especially in alcohols. As a result, the oxygen atom in the 3n,r* state is considerably less electrophilic than in benzophenone.Internal conversion is the main deactivation channel for the excited singlet state of Michler's ketone and p-aminobenzophenone and probably occurs by fast back electron transfer with deactivation to the ground state; the rate of this process increases strongly with decreasing energy of the CT state.In tr od u c ti 0 nThe efficiency of singlet oxygen generation (SA) is very sensitive to the electron density distribution in the lowest state of ketones.For T,T* triplet states, SA -0.8-1.0, while for n,r* triplets SA -0.3-0.4.1-12 Intramolecular triplet charge-transfer states are similar in character to 3r,r* statesI3J4 and would therefore also be expected to have SA values near 1. For example, Chattophadhyay et al. showed that that the efficiency of 02(lAs) generation from oxygen quenching of the metal-to-ligand CT triplet state of a tris(bipyridine)ruthenium(II) complex is approximatelyThese results suggest that it should be possible to define the nature of the lowest triplet state of ketones by measuring the S A values. On the other hand, introduction of different substituents or changing the medium can lead toinversion of the ordering of the n,r* and r,r* or CT triplet states in ketones (see ref 13 for a review). For acetophenone, analysis of spectra, phosphorescence lifetimes,lS and triplet-triplet absorption spectra in various solventsI6 led to the conclusion that the 3n,?r* and 3~,~* states have very similar energies and that, in nonpolar hydrocarbons, the 3n,T* is the lowest state, in cyclohexane they are isoenergetic, but in acetonitrile or hydrogen-bonding solvents such as alcohols, the lowest state is %r,r*. Porter and Suppan (and many later investigators) have shown that 3n,?r* ketones characteristically abstract hydrogen atoms from donors very efficiently and have phosphorescence spectra with characteristic vibrational structure, shortphosphorescence lifetimes, and a small SI*-T ~p l i t t i n g .~~J~ Moreover, the dipole ...