Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
J Phys Chem B. 2013 Feb 21;117(7):2195-204. doi: 10.1021/jp311067q. Epub 2013 Feb 7.
Two X-shaped, cruciform electron donor(2)-acceptor-acceptor'(2) (D(2)-A-A'(2)) molecules, 1 and 2, in which D = zinc 5-phenyl-10,15,20-tripentylporphyrin (ZnTPnP) or zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP), respectively, A = pyromellitimide (PI), and A' = perylene-3,4:9,10-bis(dicarboximide) (PDI), were prepared to study self-assembly motifs that promote photoinitiated charge separation followed by electron and hole transport through π-stacked donors and acceptors. PDI secondary electron acceptors were chosen because of their propensity to form self-ordered, π-stacked assemblies in solution, while the ZnTPnP and ZnTPP donors were selected to test the effect of peripheral substituent steric interactions on the π-stacking characteristics of the cruciforms. Small- and wide-angle X-ray scattering measurements in toluene solution reveal that 1 assembles into a π-stacked structure having an average of 5 ± 1 molecules, when [1] =/~ 10(-5) M, while 2 remains monomeric. Photoexcitation of the π-stacked structure of 1 results in formation of ZnTPnP(•+)-PI-PDI(•-) in τ(CS1) = 0.3 ps, which is nearly 100-fold faster than the formation of ZnTPnP(•+)-PI(•-) in a model system lacking the PDI acceptor. The data are consistent with a self-assembled structure for 1 in which the majority of the intermolecular interactions have the ZnTPnP donor of one monomer cofacially π-stacked with the PDI acceptor of a neighboring monomer in a crisscrossed fashion. In contrast, 2 remains monomeric in toluene, so that photoexcitation of ZnTPP results in the charge separation reaction sequence: (1*)ZnTPP-PI-PDI → ZnTPP(•+)-PI(•-)-PDI → ZnTPP(•+)-PI-PDI(•-), where τ(CS1) = 33 ps and τ(CS2) = 239 ps. The perpendicular orientation of ZnTPnP and ZnTPP relative to PDI in 1 and 2 is designed to decrease the porphyrin-PDI distance without greatly decreasing the overall number of bonds linking them. This serves to decrease the Coulomb energy penalty required to produce D(•+)-PI-PDI(•-) relative to the corresponding linear D-PI-PDI array, while retaining the weak electronic coupling necessary to achieve long-lived charge separation, as evidenced by τ(CR) = 24 ns for ZnTPP(•+)-PI-PDI(•-).
两个 X 形的、十字形的电子给体（2）-受体-受体'（2）（D（2）-A-A'（2））分子，1 和 2，其中 D = 锌 5-苯基-10,15,20-三戊基卟啉（ZnTPnP）或锌 5,10,15,20-四苯基卟啉（ZnTPP），A = 均苯四甲酸二酰亚胺（PI），A' = 苝-3,4:9,10-双（二羧酸酰亚胺）（PDI），分别被制备以研究促进光引发的电荷分离，然后通过π堆积的供体和受体进行电子和空穴传输的自组装基序。选择 PDI 作为二级电子受体是因为它们有形成自有序、π堆积组装的倾向在溶液中，而 ZnTPnP 和 ZnTPP 供体则被选择来测试外围取代基的空间相互作用对十字形的π堆积特性的影响。在甲苯溶液中的小角和广角 X 射线散射测量表明，当 [1] =/~ 10(-5) M 时，1 组装成具有平均 5 ± 1 个分子的π堆积结构，而 2 保持单体状态。1 的π堆积结构的光激发导致 ZnTPnP(•+)-PI-PDI(•-)的形成，τ(CS1) = 0.3 ps，这比缺乏 PDI 受体的模型系统中 ZnTPnP(•+)-PI(•-)的形成快近 100 倍。数据与 1 的自组装结构一致，其中大多数分子间相互作用使一个单体的 ZnTPnP 供体与相邻单体的 PDI 受体以交叉方式共面π堆积。相比之下，2 在甲苯中仍保持单体状态，因此 ZnTPP 的光激发导致电荷分离反应序列：（1*）ZnTPP-PI-PDI → ZnTPP(•+)-PI(•-)-PDI → ZnTPP(•+)-PI-PDI(•-)，其中 τ(CS1) = 33 ps 和 τ(CS2) = 239 ps。1 和 2 中 ZnTPnP 和 ZnTPP 相对于 PDI 的垂直取向旨在减小卟啉-PDI 距离，而不会大大减小连接它们的键的总数。这有助于降低产生 D(•+)-PI-PDI(•-)所需的库仑能罚分相对于相应的线性 D-PI-PDI 阵列，同时保留实现长寿命电荷分离所需的弱电子耦合，这由 ZnTPP(•+)-PI-PDI(•-)的 τ(CR) = 24 ns 证明。
