Merkel Paul B, Dinnocenzo Joseph P
Department of Chemistry and the Center for Photoinduced Charge Transfer, University of Rochester, Rochester, New York 14627-0216, USA.
J Phys Chem A. 2008 Oct 30;112(43):10790-800. doi: 10.1021/jp802164d. Epub 2008 Oct 4.
With the judicious selection of triplet energy donor (D) and acceptor (A) pairs, a laser flash photolysis procedure has provided a sensitive method for the study of triplet energy transfer in rigid polymer films. By monitoring changes in triplet-triplet (T-T) absorptions the kinetics of triplet energy transfer were evaluated at short time scales, and overall energy-transfer quantum yields were also obtained. Combinations of xanthone- or thioxanthone-type donors and polyphenyl acceptors were particularly suited to these measurements because the former have high intersystem-crossing quantum yields and the latter have very high extinction coefficients for T-T absorption. For exothermic transfer most of the energy transfer that occurred within the lifetime of triplet D ( (3)D) took place in less than a few microseconds after (3)D formation in poly(methyl methacrylate), and triplet A yields were limited largely by the number of A molecules in near contact with (3)D. The kinetics of triplet energy transfer were modeled using a modified Perrin-type statistical arrangement of D/A separations with allowance for excluded volume in combination with a Dexter-type formula for the distance-dependent exchange energy-transfer rate constant. Experimental observations were best explained by constraining D/A separations to reflect the dimensions of intervening molecules of the medium. Rate constants, k 0, for exothermic energy transfer from (3)D to A molecules in physical contact are approximately 10 (11) s (-1) and very similar to triplet energy-transfer rate constants determined from solution encounters. Energy-transfer rate constants, k( r), fall off as approximately exp(-2 r/ 0.85), where r is the separation distance between D and A centers in angstroms. Exchange energy transfer is not restricted to (3)D and A in physical contact, but at </=0.4 M A at least 85% of the energy transfer arises from interaction of (3)D with a single nearest-neighbor A molecule. The modified Perrin model was also applied to quantum yields of quenching in rigid media. Comparison to the simple Perrin model for quenching shows that the latter may be adequate as long as molecular volumes are accommodated in the Perrin expression. Under these conditions the critical radius, r c, corresponds to the (3)D/A separation at which the effective rate constant for energy transfer equals the inverse of the (3)D lifetime.
通过对三重态能量供体（D）和受体（A）对进行明智的选择，激光闪光光解程序为研究刚性聚合物薄膜中的三重态能量转移提供了一种灵敏的方法。通过监测三重态 - 三重态（T - T）吸收的变化，在短时间尺度上评估了三重态能量转移的动力学，并且还获得了整体能量转移量子产率。吨吨酮或噻吨酮型供体与多苯基受体的组合特别适合这些测量，因为前者具有高的系间窜越量子产率，而后者对于T - T吸收具有非常高的消光系数。对于放热转移，在聚甲基丙烯酸甲酯中三重态D（³D）寿命内发生的大部分能量转移在³D形成后不到几微秒内就发生了，并且三重态A的产率在很大程度上受与³D近接触的A分子数量的限制。使用D / A间距的修正佩林型统计排列对三重态能量转移的动力学进行建模，该排列考虑了排除体积，并结合了用于距离依赖性交换能量转移速率常数的德克斯特型公式。通过限制D / A间距以反映介质中间分子的尺寸，能够最好地解释实验观察结果。从³D到物理接触的A分子的放热能量转移的速率常数k₀约为10¹¹ s⁻¹，并且与从溶液相遇确定的三重态能量转移速率常数非常相似。能量转移速率常数k(r) 随着大约exp(-2r / 0.85)下降，其中r是以埃为单位的D和A中心之间的分离距离。交换能量转移不限于物理接触的³D和A，但在A浓度≤0.4 M时，至少85％的能量转移来自³D与单个最近邻A分子的相互作用。修正的佩林模型也应用于刚性介质中的猝灭量子产率。与简单的猝灭佩林模型相比表明，只要在佩林表达式中考虑分子体积，后者可能就足够了。在这些条件下，临界半径r c对应于能量转移的有效速率常数等于³D寿命倒数时的³D / A间距。
