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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Comment on the “Magnetic Field Effects on Exciplex Luminescence in Water-Tetrahydrofuran and Water-Dioxane Mixtures”
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Comment on the “Magnetic Field Effects on Exciplex Luminescence in Water-Tetrahydrofuran and Water-Dioxane Mixtures”

机译：评论“磁场对水-四氢呋喃和水-二恶烷混合物中复杂发光的影响”

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Cage recombination of radical ion pairs (RIP) produced during the course of photoinduced electron transfer in polar solutions of exciplex systems can be affected by a weak external magnetic field B ~ 100 G (for review see ref 1). Because of the law of spin conservation, the multiplicity of the product formed by geminate recombination is the same as that of its radical-ion precursor. The RIP, which is created, say, as a pure singlet state, undergoes spin evolution during its lifetime (~ 10 ns) to form a triplet state via hyperfine interaction (HFI) of its unpaired electrons. Applied magnetic fields affect the spin mixing, removing the degeneracy of the triplet sublevels of RIP, T↓(0), and T↓(±1). When the energy separation between them exceeds the size of HFI, only T↓(0) can mix with the RIP singlet state S. Thus, external magnetic fields diminish the probability of intersystem crossing and, therefore, change the concentration of both the singlet and triplet states of the RIP. Magnetic fields affect the spin evolution of the geminate RIP rather than that of pairs generated via random bulk recombination. A convenient method for detecting the magnetic field effect (MFE) on cage recombination of RIP is based on detecting exciplex fluorescence; the transition from the singlet state of RIP to exciplex is possible even in moderately polar solvents. Studying the MFE on exciplex fluorescence of pyrene/N,N-dimethylaniline solution in binary solvents, components of which were considerably different in terms of dielectric permit-tivity, we have found↑(2) that values of a relative enhancement of exciplex fluorescence caused by magnetic fields may be as large as 18% at B = 300 G in a benzene/dimethyl sulfoxide (DMSO) liquid mixture when the volume fraction of the polar component is ca. 0.26. This value of the MFE is considerably larger than those obtained in intermolecular exciplexes in neat solvents (typically 3-6%) and is the same order of magnitude as the MFE detected by Staerk et al.↑(3) in polar solutions of polymeth-ylene-linked pyrene/N,N-dimethylaniline systems. A characteristic features of these intramolecular systems is that dissociation of RIP is completely eliminated. Therefore, the bulk (homogeneous) recombination of free radical ions, which also results in exciplex fluorescence but unaffected by magnetic fields, is actually absent. This provides favorable conditions for detecting large values of the MFE (up to 47% depending on the length of polymethylene chain↑(3)). By analogy with intramolecular exciplex systems, we have assumed that in the binary liquid solvents, owing to preferential solvation, micro-domains of polar-component molecules are created around radical ion pairs, keeping radical ions within the cage.↑(2) The physical meaning of such a cage effect enhancement is as follows. When ions have the separated solvation shells (see Figure 1), this interaction is determined by #↓(m). If the solvation shells of partner ions intersect each others (Figure la), Cou-lombic forces are strongly diminished by a factor of ca. #↓(1)/#↓(m) Here #↓(1) is the dielectric constant of the polar component (ca. 50 for DMSO) and #↓(m) is that of the mixture. (For a benzene/ DMSO (25 vol %) mixture, #↓(m) is ca. 11 as measured by the capacity method.) So that the common solvation shell plays the role of a trap that increases the yield of cage recombination of radical ion pairs. Fluctuations of the polar component concentration is supposed to be a physically meaningful source of producing relatively large polar clusters. It is worth noting that concentration fluctuations in nonideal binary mixtures can considerably exceed a stochastic level, and the concept of microheterogeneous structure of binary mixtures has been involved in explaining experimental results on light scattering in binary mixtures.↑(4) Recently, Chowdhury et al.↑(5) have questioned the maximum value of MFE we had found earlier for the benzene/DMSO mixture. (These authors have
机译：在激基复合物体系的极性溶液中，在光致电子转移过程中产生的自由基离子对（RIP）的笼重组，可能受到弱的外部磁场B〜100 G的影响（有关综述，请参见参考资料1）。由于自旋守恒定律，通过双键重组形成的产物的多重性与其自由基离子前体的多重性相同。 RIP被创建为纯单线态，在其寿命（〜10 ns）内经历自旋演化，通过其未配对电子的超精细相互作用（HFI）形成三线态。施加的磁场会影响自旋混合，从而消除了RIP，T↓（0）和T↓（±1）的三重态子级的简并性。当它们之间的能量间隔超过HFI的大小时，只有T↓（0）可以与RIP单重态S混合。因此，外部磁场会减小系统间交叉的可能性，并因此改变单重态和RIP的三重态。磁场影响双峰RIP的自旋演化，而不是通过随机本体重组产生的成对的自旋演化。一种检测RIP笼重组的磁场效应（MFE）的简便方法是基于检测激基复合物荧光。即使在中等极性的溶剂中，也可能从RIP的单重态过渡到激基复合物。研究binary在二元溶剂中components / N，N-二甲基苯胺溶液的激基复合荧光的MFE，该溶剂的成分在介电常数方面有很大差异，我们发现↑（2）引起激基复合荧光的相对增强值当极性组分的体积分数为时，在苯/二甲亚砜（DMSO）混合液中，B = 300 G时，磁场引起的磁场强度可能高达18％。 0.26。 MFE的这个值比在纯溶剂中的分子间激基复合物所获得的值（通常为3-6％）大得多，并且与Staerk等人检测到的MFE数量级相同（↑3）。乙烯连接的// N，N-二甲基苯胺系统。这些分子内系统的特征是RIP的解离被完全消除。因此，实际上不存在自由基离子的整体（均质）重组，这也导致激基复合物荧光，但不受磁场的影响。这为检测较大的MFE（高达47％，取决于聚亚甲基链的长度↑（3））提供了有利条件。通过与分子内激基复合物系统的类比，我们假设在二元液体溶剂中，由于优先溶剂化，极性组分分子的微区在自由基离子对周围形成，从而使自由基离子保持在笼中。↑（2）这样的笼效应增强的含义如下。当离子具有分离的溶剂化壳时（见图1），这种相互作用由＃↓（m）确定。如果伙伴离子的溶剂化壳相互交叉（图1a），则库仑力将大大减小。 ＃↓（1）/＃↓（m）此处＃↓（1）是极性成分的介电常数（对于DMSO约为50），而＃↓（m）是混合物的介电常数。 （对于苯/ DMSO（25体积％）混合物，按容量法测得，＃↓（m）约为11。）因此，普通的溶剂化壳起着捕集阱的作用，从而提高了笼形重组的收率。自由基离子对。极性组分浓度的波动被认为是产生相对较大的极性团簇的物理上有意义的来源。值得注意的是，非理想二元混合物中的浓度波动可能会大大超过随机水平，并且二元混合物的微异质结构概念已被用于解释二元混合物中光散射的实验结果。↑（4）最近，Chowdhury等人。↑（5）质疑了我们早先发现的苯/ DMSO混合物的MFE最大值。 （这些作者有

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