PHOTOCHEMISTRY OF CARBOCYANINE ALKYLTRIPHENYLBORATE SALTS - INTRA-ION-PAIR ELECTRON-TRANSFER AND THE CHEMISTRY OF BORANYL RADICALS

被引:218
作者
CHATTERJEE, S
DAVIS, PD
GOTTSCHALK, P
KURZ, ME
SAUERWEIN, B
YANG, XQ
SCHUSTER, GB
机构
[1] UNIV ILLINOIS,ROGER ADAMS LAB,DEPT CHEM,URBANA,IL 61801
[2] MEAD IMAGING INC,MIAMISBURG,OH 45342
[3] ILLINOIS STATE UNIV,DEPT CHEM,NORMAL,IL 61761
关键词
D O I
10.1021/ja00173a022
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
Cyanine borate [R(Ph)3B−; R = alkyl] salts dissolved in nonpolar solvents exist predominantly as ion pairs. Irradiation of the ion pair with visible light absorbed by the cyanine leads to formation of a locally excited singlet state of the dye. The singlet excited cyanine in the ion pair is capable of oxidizing the borate anion to the boranyl radical [R(Ph)3B⋆]. The rate constant for this electron-transfer reactions (ket) depends on the free energy change (ΔGet). This latter value was estimated for a series of borates and the relationship between ket and ΔGet appears to give a maximum value consistent with the quadratic law predicted from Marcus theory. The boranyl radical undergoes carbon-boron bond cleavage to generate a free alkyl radical. The rate of bond cleavage depends directly on the stability of the alkyl radical formed. When stabilized alkyl radicals are formed, carbon-boron bond cleavage is faster than the back-electron-transfer reaction that regenerates the cyanine borate ion pair. Carbon-boron bond cleavage of the boranyl radical in the systems examined is irreversible and the bond dissociation energy of the parent hydrocarbon is a good predictor of the yield of radicals. The free alkyl radicals formed by the irradiation-electron-transfer-bond-cleavage sequence may be used in meaningful chemical processes such as the initiation of polymerization. © 1990, American Chemical Society. All rights reserved.
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页码:6329 / 6338
页数:10
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