Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
J Phys Chem A. 2010 Mar 18;114(10):3384-90. doi: 10.1021/jp908767x.
We report direct measurements of the excess polarizability volumes of butadiyne-bridged zinc porphyrin dimers at singly beta-to-beta (1Zn) and doubly beta-to-beta (2Zn) positions using the transient dc photoconductivity (TDCP) technique. The excess polarizability volumes of the singlet exciton for 1Zn and 2Zn are 110 and 270 A(3), respectively, while those of the triplet exciton are approximately 100 A(3) for both dimers. Our measurements suggest that the singlet exciton is mainly localized on one porphyrin subunit for 1Zn, similar to the case for the porphyrin monomer. While the exciton is fully delocalized on two porphyrin subunits in the case of meso-to-meso linked dimer (3Zn), the extent of exciton localization/delocalization for doubly beta-to-beta linked dimer lies between those of singly beta-to-beta and meso-to-meso linked dimers. Electronic structure calculations show that the dramatically different extents of exciton localization/delocalization are the results of frontier orbital coefficients being small at beta positions but large at meso positions. Two butadiyne linkages between the porphyrins at beta positions (2Zn) clearly facilitate electronic communication between the two porphyrin subunits by virtue of stabilization of cumulenic charge resonance structures through enforced planarity.
我们使用瞬态直流光电导（TDCP）技术直接测量了位于单β-β（1Zn）和双β-β（2Zn）位置的丁二炔桥联锌卟啉二聚体的过剩极化率体积。1Zn 和 2Zn 的单重激子的过剩极化率体积分别为 110 和 270 A(3)，而两者的三重激子的过剩极化率体积约为 100 A(3)。我们的测量结果表明，1Zn 中的单重激子主要定域在一个卟啉亚基上，与卟啉单体的情况类似。而在 meso-to-meso 连接的二聚体（3Zn）中，激子完全定域在两个卟啉亚基上，而在双β-β 连接的二聚体中，激子的定域/离域程度则介于单β-β 和 meso-to-meso 连接的二聚体之间。电子结构计算表明，激子定域/离域程度的显著差异是由于β位的前沿轨道系数较小，但 meso 位的系数较大所致。β位的两个丁二炔连接（2Zn）通过强制平面化稳定了Cumulenic 电荷共振结构，从而明显促进了两个卟啉亚基之间的电子通讯。
