11360
J. Phys. Chem. 1996, 100, 11360-11367
Oxygen Quenching of Excited Aliphatic Ketones and Diketones Werner M. Nau and J. C. Scaiano* Department of Chemistry, UniVersity of Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada K1N 6N5 ReceiVed: March 28, 1996X
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.
Introduction
SCHEME 1
Oxygen quenching of the excited states of organic molecules has been extensively examined,1 and the principal modes of interaction are shown in Scheme 1. Spin statistics allows diffusion-controlled quenching of singlet-excited sensitizers (sens) with rate constants (kq) of ca. 30 × 109 M-1 s-1 in solvents like benzene or acetonitrile (process 1).2 The encounter complex in process 1 may lead to the formation of singlet oxygen O2(1∆g), to oxygen-enhanced intersystem crossing (ISC), or to radiationless deactivation (processes 1a-c). In contrast, quenching of triplet states (process 2) should be limited to 4/9 of the diffusion-controlled rate, i.e., 3kq < 13 × 109 M-1 s-1, since the encounter of two triplets leads to a quintet encounter complex with a probability of 5/9; the latter cannot proceed to products but can only redissociate (process 2c).3 Although most experimental data seem to support the spin-statistical arguments for oxygen quenching of singlet4 and triplet5,6 states according to Scheme 1, exceptions are also known.4,7-9 ISC between the encounter complexes with different spin multiplicities for process 2, first suggested by Garner and Wilkinson,7b is held responsible in some cases (eq 3).7,8 Clearly, although spinstatistical arguments impose limitations on the highest expected rate constants, other factors might reduce the quenching efficiency and result in lower values than those expected from spin statistics. In particular, it has been theoretically predicted10,11 and experimentally confirmed for triplet π,π* states3a,5b,6a,12 that the quenching rate constants decrease with increasing excited state energy and also with decreasing chargetransfer (CT) contributions in the encounter complex or exciplex, i.e., partial electron-donation to oxygen.8,13,14 Oxygen quenching of triplet states has been most extensively examined owing to their longer lifetimes and their efficient population by ISC.1 Since process 2a leads to the formation of singlet oxygen, O2(1∆g), by energy transfer, many studies have been aimed to measure the efficiency for singlet oxygen production from triplet states (S∆), which provides a measure for the relative contribution of process 2a.5,6 Although it is expected that the S∆ values of a sensitizer increase with decreasing energy of the excited state6a and decreasing chargetransfer (CT) character of the encounter complex or exciplex,11 these dependencies were not found to be universal. Despite earlier suggestions5,6,15 that a π,π* triplet is required for highly X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(96)00932-X CCC: $12.00
1sens*
+ 3O2
3/3
3[sens–O
2]
3sens*
+ O2(1∆g)
(1a)
3sens*
+ 3O2
(1b)
sens + 3sens* 3sens* 3sens*
+ 3O2 + 3O2 + 3O2
5[sens–O
2]
1/9 3/9 5/9 ISC
3O
2
(1c)
1[sens–O
sens + O2(1∆g)
(2a)
3[sens–O
sens + 3O2
(2b)
2] 2]
5[sens–O
(2c)
2]
3[sens–O ] 2
ISC
1[sens–O
2]
(3)
efficient singlet oxygen formation, recent work demonstrates that the n,π* triplet states of bicyclic azoalkanes are very efficient sensitizers,16 while some π,π* states show low efficiencies.1,6a,11,17 Although much knowledge has been gained about the oxygen quenching of aromatic molecules, including triplet aromatic ketones,1 there is surprisingly little information available for the primary interactions of simple aliphatic ketones or diketones with oxygen.18-20 For this reason we are presently reporting our results on oxygen quenching and singlet oxygen formation for acetone, 2-pentanone, 2-hexanone, 4-heptanone, biacetyl, and camphorquinone.
This study also served to obtain data on the oxygen quenching of singlet-excited ketones and diketones. Experimental Section Acetone, acetonitrile, benzene, cyclohexane, ethanol, methanol, and 1,1,2-trichloro-1,2,2-trifluoroethane (Freon-113) were Omnisolv grade (BDH). Decafluorobenzophenone, 4-heptanone, (1S)-(+)-camphorquinone (Aldrich), and phenazine © 1996 American Chemical Society
O2 Quenching of Ketones and Diketones
J. Phys. Chem., Vol. 100, No. 27, 1996 11361
TABLE 1: Oxygen-Quenching Parameters of Selected Aromatic and Aliphatic Ketones and Diketones ketone acetophenonea benzophenonea acetonee 2-pentanonee 2-hexanonee 4-heptanonee biacetylt camphorquinonet benzilt,V
1τ (ns) ∼0.1 0.005b 1.95f,g 1.95f,g 0.75f,g 1.95f,g 9.2f 15.9x 2
3τ (µs) 3.0 2.5d 50h 0.240p 0.018r 0.230j 500u 300x 30
ΦT 1.0 1.0 1.0i 0.9i 0.6i [0.9]r 1.0u ∼1x ∼1
1 kq (109 M-1 s-1)
2.3 ( 0.5f,g 1.7 ( 0.6f,g s 2.7 ( 0.4f,g 0.7 ( 0.2f 0.3 ( 0.1y
