INTRAMOLECULAR QUENCHING OF EXCITED SINGLET-STATES BY STABLE NITROXYL RADICALS
INTRAMOLECULAR QUENCHING OF EXCITED SINGLET-STATES BY STABLE NITROXYL RADICALS
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DOI：
10.1021/ja00176a038
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发表时间：
1990-09-26
影响因子：
15
通讯作者：
BLOUGH, NV
中科院分区：
文献类型：
--
作者：
GREEN, SA;SIMPSON, DJ;BLOUGH, NV
采用吸光度、稳态荧光和时间分辨荧光测量研究了一系列氮氧自由基-荧光团加合物中氮氧自由基对激发单重态荧光猝灭的机理。研究结果表明：（1）荧光团的吸收和发射能量不受氮氧取代基的影响，并且从加合物中观察到的残余发射来自荧光团的局部激发单重态，而不是电荷复合;（2）氮氧自由基分子内猝灭的速率常数（kq）较高（108-1010 s-1）并且随着氮氧自由基与荧光团距离的增加而显著降低-然而，较高的淬火速率（> 108 s-1）在12 Å的距离上观察到;（3）由于光谱重叠积分值较低，福斯特能量转移对猝灭没有显着贡献;（4）kq不随Dexter重叠积分的溶剂依赖性增加而成比例地增加，表明Dexter机制的能量转移不是猝灭的原因;与光诱导电子转移自由能的计算值没有明显的相关性，表明光诱导电子转移的猝灭途径也不重要;（6）对于血卟啉-氮氧自由基加合物，其荧光团的单重态能量低于氮氧自由基的第一激发态能量（因此排除能量转移），仍然观察到显著的猝灭速率;（7）对于具有类似的氮氧自由基-荧光团距离的化合物，在顺磁性化合物的kq与抗磁性参考化合物的非辐射速率常数之间观察到近似线性相关，这表明氮氧部分催化荧光团中预先存在的非辐射途径。这些结果表明，猝灭是通过电子交换发生的，这导致荧光团的（局部）单重态弛豫到三重态和/或基态。
Absorbance and steady-state and time-resolved fluorescence measurements were employed to examine the mechanism (s) of excited singlet state quenching by nitroxides in a series of nitroxide-fluorophore adducts. This work establishes the following:(1) the absorption and emission energies of the fluorophores are unaffected by the presence of the nitroxide substituent (s), and the residual emission that is observed from the adducts arises from the locally excited singlet of the fluorophore, not from charge recombination;(2) rate constants for intramolecular quenching by the nitroxides (kq) are high (108-1010 s'1) and decrease significantly with increasing nitroxide to fluorophore distance—however, relatively high rates of quenching (> 108 s" 1) are observed over distances as great as 12 Á;(3) Forster energy transfer does not contributesignificantly to the quenching due to the low values for the spectral overlap integrals;(4) the kq’s do not increase proportionally to the solvent-dependent increases in the Dexter overlap integral, indicating that energy transfer by the Dexter mechanism is not responsible for the quenching;(5) the values of k. show no obvious correlationwith the calculated free energies for photoinduced electron transfer, suggesting that this quenching pathway is also unimportant;(6) for hematoporphyrin-nitroxide adducts, which contain a fluorophore whose singlet energy is below that of the first excitedstate energy of the nitroxide (thus precluding energy transfer), significant rates of quenching are still observed;(7) for compounds with similar nitroxide-fluorophore distance, an approximately linear correlation is observed between the kq’s of the paramagnetic compounds and the nonradiative rate constants of the diamagnetic reference compounds, suggesting that the nitroxide moiety catalyses a preexisting nonradiative pathway in the fluorophore. These results indicate that the quenching arises through electron exchange which causes relaxation of the (local) singlet state to the triplet and/or ground state of the fluorophore.