Is Singlet Oxygen Formation Dominant in Triplet Quenching Processes? Measurement of the Quantum Yield of Singlet Oxygen Formation by the Time‐resolved Thermal Lens Method
Abstract:The quantum yields of singlet oxygen formation (ØΔ) by the quenching of triplet states of organic sensitizers are measured at various concentrations of the sensitizers by using the time‐resolved thermal lens method. Above a certain concentration, ØΔ is independent of the sensitizer concentration. Below the threshold, ØΔ gradually decreases as the concentration of the sensitizer decreases. The extrapolation of ØΔ to zero concentration indicates that singlet oxygen formation is not necessarily dominant in the qu… Show more
“…Recently, the surprising result that the 3' values for anthracene, 9,10-dibromoanthracene (DBA), and quinoxaline decrease very sharply at low concentrations of sensitizers was obtained by time-resolved thermal methods. 27 Decreasing the concentration of anthracene from 1.5 X 10"4 M to 1.5 X 10"5 M and of DBA from 1.7 X 10"4 M to 2 X 10"5 M was reported in ref 27 to decrease "1 by 20 and 32%, respectively. We checked these data by the time-resolved phosphorescence method.…”
6317absorption cross-section. We anticipate that the triplet population can be enhanced by optical excitation of an equivalent fluence but longer duration compared to the measurements reported here.Under such conditions there is more opportunity for the molecules to undergo intersystem crossing, since molecules that relax back to the ground state can be reexcited during the excitation pulse. This results in an increased population in the triplet. With the singlet parameters known from the present picosecond measurements, time-resolved studies of the triplet absorption for different optical excitation pulse widths should allow direct measurements of the triplet system parameters u34, N3, and ~1 3 . Such measurements should help to clarify the current discrepancy between the nanosecond and picosecond responses of the King's complex.The quenching of triplet fluoranil (FA) by molecular oxygen was studied by IR laser fluorimetry and nanosecond laser photolysis in various solvents. In acetonitrile, CCl,, chloroform, and cyclohexane, the charge-transfer (CT) interaction between triplet fluoranil and solvent (Sol.) is insignificant and the quantum yield of triplet formation and the probability of O2(IA ) generation from triplet fluoranil are close to unity. In aromatic solvents, a substantial decrease in the quantum yield (aA-? of 02(1$) formation (limiting value at [O,]a) was observed; for example, in p-xylene the estimated value is 110-2. In the weakly electron-donating solvents, benzene and chlorobenzene, triplet exciplexes with partial charge transfer [3( FAb-.Sol.6+)] and lifetime of -150 ns were observed. The absorption spectra of the triplet exciplexes are slightly broadened compared with the triplet-triplet spectrum of free fluoranil and have broad structureless CT bands in the near IR region. The values of aAm are -0.4-0.5, and the rate constants for quenching by oxygen [(6-8) X IO* M-' s-l ] o f the triplet exciplexes are lowerthan for triplet fluoranil. Molecular oxygen, a strong electron acceptor, probably interacts mainly with the FAb fragment of the triplet exciplex where excitation is localized, leading to efficient 0 2 ( ' $ , production. Fluoranil forms weak CT complexes with aromatic solvents in the ground state, and their direct excitation produces contact ion-radical pairs (CIRP). In benzene and chlorobenzene, the main deactivation channel for the CIRP is intersystem crossing, leading to triplet exciplex formation.
“…Recently, the surprising result that the 3' values for anthracene, 9,10-dibromoanthracene (DBA), and quinoxaline decrease very sharply at low concentrations of sensitizers was obtained by time-resolved thermal methods. 27 Decreasing the concentration of anthracene from 1.5 X 10"4 M to 1.5 X 10"5 M and of DBA from 1.7 X 10"4 M to 2 X 10"5 M was reported in ref 27 to decrease "1 by 20 and 32%, respectively. We checked these data by the time-resolved phosphorescence method.…”
6317absorption cross-section. We anticipate that the triplet population can be enhanced by optical excitation of an equivalent fluence but longer duration compared to the measurements reported here.Under such conditions there is more opportunity for the molecules to undergo intersystem crossing, since molecules that relax back to the ground state can be reexcited during the excitation pulse. This results in an increased population in the triplet. With the singlet parameters known from the present picosecond measurements, time-resolved studies of the triplet absorption for different optical excitation pulse widths should allow direct measurements of the triplet system parameters u34, N3, and ~1 3 . Such measurements should help to clarify the current discrepancy between the nanosecond and picosecond responses of the King's complex.The quenching of triplet fluoranil (FA) by molecular oxygen was studied by IR laser fluorimetry and nanosecond laser photolysis in various solvents. In acetonitrile, CCl,, chloroform, and cyclohexane, the charge-transfer (CT) interaction between triplet fluoranil and solvent (Sol.) is insignificant and the quantum yield of triplet formation and the probability of O2(IA ) generation from triplet fluoranil are close to unity. In aromatic solvents, a substantial decrease in the quantum yield (aA-? of 02(1$) formation (limiting value at [O,]a) was observed; for example, in p-xylene the estimated value is 110-2. In the weakly electron-donating solvents, benzene and chlorobenzene, triplet exciplexes with partial charge transfer [3( FAb-.Sol.6+)] and lifetime of -150 ns were observed. The absorption spectra of the triplet exciplexes are slightly broadened compared with the triplet-triplet spectrum of free fluoranil and have broad structureless CT bands in the near IR region. The values of aAm are -0.4-0.5, and the rate constants for quenching by oxygen [(6-8) X IO* M-' s-l ] o f the triplet exciplexes are lowerthan for triplet fluoranil. Molecular oxygen, a strong electron acceptor, probably interacts mainly with the FAb fragment of the triplet exciplex where excitation is localized, leading to efficient 0 2 ( ' $ , production. Fluoranil forms weak CT complexes with aromatic solvents in the ground state, and their direct excitation produces contact ion-radical pairs (CIRP). In benzene and chlorobenzene, the main deactivation channel for the CIRP is intersystem crossing, leading to triplet exciplex formation.
“…Many studies and arguments have been aimed to explain the differences in S Δ values between π,π* and n,π* sensitizers, since most examined π,π* sensitizers have shown high values (0.8−1.0) 1,4,11,15 while n,π* aromatic ketones displayed low efficiencies (0.3−0.4). , There are now several examples of n,π*-excited states available for which high S Δ values have also been substantiated, in particular the bicyclic azoalkanes 1 and 2 16 and indanetrione 3 , all of which possess a pure n,π* configuration and are shown below. The S Δ values of the aliphatic ketones (Table ) and several aromatic n,π*-excited ketones in protic solvents are also quite high (0.5−0.7). 5a, These examples, along with many examples of π,π* states for which low efficiencies for singlet oxygen production have been observed, , demonstrate that a general correlation between efficiency for singlet oxygen production and the sensitizer electronic configuration does not exist. …”
Section: Discussionmentioning
confidence: 98%
“…Since process 2a leads to the formation of singlet oxygen, O 2 ( 1 Δ g ), by energy transfer, many studies have been aimed to measure the efficiency for singlet oxygen production from triplet states ( S Δ ), which provides a measure for the relative contribution of process 2a. , Although it is expected that the S Δ values of a sensitizer increase with decreasing energy of the excited state 6a and decreasing charge-transfer (CT) character of the encounter complex or exciplex, these dependencies were not found to be universal. Despite earlier suggestions 5,6,15 that a π,π* triplet is required for highly efficient singlet oxygen formation, recent work demonstrates that the n,π* triplet states of bicyclic azoalkanes are very efficient sensitizers, while some π,π* states show low efficiencies. ,6a,, …”
Section: Introductionmentioning
confidence: 94%
“…Isotope exchange experiments ( 16 O/ 18 O) have been employed to evaluate the contributions of the mechanism in eq 3 of Scheme for an aromatic ketone ( N -methylacridone), and no isotopic exchange was observed 17a. To our knowledge such experiments have not been reported for aliphatic ketones.…”
The oxygen quenching rate constants of aliphatic ketones (acetone,
2-pentanone, 2-hexanone, 4-heptanone)
and diketones (biacetyl and camphorquinone) have been determined for
the singlet- and triplet-excited state
by time-resolved fluorescence and triplet transient absorption
spectroscopy, and the quantum yields for singlet
oxygen formation were measured. It was observed that the quenching
rate constants for the singlet state of
aliphatic ketones are considerably smaller than for their triplet
states. The efficiencies for singlet oxygen
production of the aliphatic ketones were 0.2−0.3, while those of most
diketones fall in the 0.4−0.6 range.
The changes in the efficiencies for singlet oxygen production and
of the oxygen-quenching rate constants for
singlet- and triplet-excited n,π* states, ketones and azoalkanes are
discussed in terms of variations in excited
state energies and charge-transfer interactions with
oxygen.
“…It is now well-known that, for ketones with , * triplets, the probability of 02( 8) generation on collisonal quenching of the triplet (Sa) in benzene lies in the range 0.3-0.4, while for ketones with , * triplets, SA ~0.8-1.0. [1][2][3][4][5][6][7][8][9][10][11] The physical reasons for this effect probably have to do with the fact that excitation is localized on the carbonyl group for ketones with , * triplets. In ref 11, it was shown that a heavy atom in sensitizers with , * triplets slightly decreases SA values, whereas in bromo-substituted acetophenones, a sharp decrease in SA values was observed.…”
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