Photooxidation of a conjugated diene by an exciplex mechanism:: Amplification via radical chain reactions in the perylene diimide-photosensitized oxidation of α-terpinene

被引:18
作者
Chen, LH
Lucia, LA
Gaillard, ER
Whitten, DG
Icil, H
Icli, S
机构
[1] Univ Rochester, Dept Chem, Rochester, NY 14627 USA
[2] Univ Rochester, NSF, Ctr Photoinduced Charge Transfer, Rochester, NY 14627 USA
[3] Eastern Mediterranean Univ, Dept Chem, Mersin 10, N Cyprus, Turkey
[4] Ege Univ, Fac Sci, Dept Chem, TR-35100 Bornova, Izmir, Turkey
关键词
D O I
10.1021/jp983346t
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
Irradiation of the perylene diimide (1) or 9,10-dicyanoanthracene (DCA) in the presence of alpha-terpinene (2-HH) in the presence of molecular oxygen leads to moderately efficient oxidation of 2-HH to p-cymene (2). Although 1 might be expected to photosensitize oxidations by the conventional "singlet oxygen" pathways, spectroscopic studies indicate that while oxygen can quench the fluorescent singlet of 1, no singlet oxygen is produced. 2-HH is also an efficient quencher of the fluorescent singlets of 1 and DCA and, for nonpolar solvents such as methylene chloride, in each case the quenching results in formation of an exciplex or contact radical ion pair. Under conditions where quenching by 2-HH to form the exciplex is complete, maximum quantum yields of 2 are obtained, thus indicating that the exciplex is the precursor to its formation. Nonproductive decay of the exciplex to starting materials is its major fate, thus the moderately high quantum efficiencies for formation of 2 require a mechanism involving amplification. Spin-trapping experiments suggest the role of hydroperoxy radicals and amplification by a radical chain mechanism involving these radicals and the intermediate 2-H-. is proposed. Possible paths for reaching these radicals from the exciplex are considered; either oxygen quenching of the exciplex or proton transfer within the exciplex followed by oxygen interception of the semireduced perylene diimide appear viable. For the reaction of DCA with 2-HH and oxygen, it is found that the much longer-lived exciplex undergoes quenching by oxygen.
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收藏
页码:9095 / 9098
页数:4
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