Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States.
J Phys Chem A. 2012 Mar 8;116(9):2184-91. doi: 10.1021/jp2125735. Epub 2012 Feb 27.
The impact of donor-acceptor electronic coupling and bridge energetics on the preference for hole or electron transfer leading to charge recombination in a series of donor-bridge-acceptor (D-B-A) molecules was examined. In these systems, the donor is 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) and acceptor is naphthalene-1,8:4,5-bis(dicarboximide) (NI), while the bridges are either oligo(p-phenyleneethynylene) (PE(n)P, where n = 1-3) 1-3 or oligo(2,7-fluorenone) (FN(n), where n = 1-3) 4-6. Photoexcitation of 1-3 and 4-6 produces DMJ(+•)-An-PE(n)P-NI(-•) and DMJ(+•)-An-FN(n)-NI(-•), respectively, which undergo radical pair intersystem crossing followed by charge recombination to yield both (3*)An and (3*)NI, which are observed by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. (3*)NI is produced by hole transfer from DMJ(+•) to NI(-•), while (3*)An is produced by electron transfer from NI(-•) to DMJ(+•), using the agency of the bridge HOMOs and LUMOs, respectively. By monitoring the initial population of (3*)NI and (3*)An in 1-6, the data show that charge recombination occurs preferentially by selective hole transfer when the bridge is PE(n)P, while it occurs by preferential electron transfer when the bridge is FN(n). Over time, the initial population of (3*)NI decreases, while that of (3*)An increases, indicating that triplet-triplet energy transfer (TEnT) occurs. The observed distance dependence of TEnT from (3*)NI to An is weakly exponential with a decay parameter β = 0.08 Å(-1) for the PE(n)P series and β = 0.03 Å(-1) for the FN(n) series. In the PE(n)P series, this weak distance dependence is attributed to a transition from the superexchange regime to hopping transport as the energy gap for triplet energy injection onto the bridge becomes significantly smaller as n increases, while in the FN(n) series the corresponding energy gap is small for all n resulting in triplet energy transport by the hopping mechanism.
研究了给体-受体电子耦合和桥键能对一系列给体-桥-受体（D-B-A）分子中导致电荷复合的空穴或电子转移偏好的影响。在这些体系中，给体是 3,5-二甲基-4-(9-蒽基)-乔利定（DMJ-An），受体是萘-1,8:4,5-双（二羧酸二酰亚胺）（NI），而桥则是聚（对亚苯基乙炔）（PE(n)P，其中 n = 1-3）1-3 或聚（2,7-芴酮）（FN(n)，其中 n = 1-3）4-6。1-3 和 4-6 的光激发分别产生 DMJ(+•)-An-PE(n)P-NI(-•)和 DMJ(+•)-An-FN(n)-NI(-•)，它们经历自由基对系间交叉，随后通过电荷复合生成均被时间分辨电子顺磁共振（TREPR）光谱观察到的（3*）An 和（3*）NI。（3*）NI 是由 DMJ(+•)向 NI(-•)的空穴转移产生的，而（3*）An 是由 NI(-•)向 DMJ(+•)的电子转移产生的，分别利用桥的 HOMO 和 LUMO。通过监测 1-6 中（3*）NI 和（3*）An 的初始种群，数据表明当桥为 PE(n)P 时，电荷复合优先通过选择性空穴转移发生，而当桥为 FN(n)时，优先通过电子转移发生。随着时间的推移，（3*）NI 的初始种群减少，而（3*）An 的初始种群增加，表明发生了三重态-三重态能量转移（TEnT）。从（3*）NI 到 An 的观察到的 TEnT 距离依赖性呈弱指数关系，PE(n)P 系列的衰减参数β=0.08 Å(-1)，FN(n)系列的β=0.03 Å(-1)。在 PE(n)P 系列中，这种弱的距离依赖性归因于从超交换区域到跳跃输运的转变，因为随着 n 的增加，三重态能量注入到桥的能隙显著减小，而在 FN(n)系列中，对于所有 n，相应的能隙都很小，导致通过跳跃机制进行三重态能量输运。
