Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States.
J Phys Chem B. 2011 May 12;115(18):5157-67. doi: 10.1021/jp105115n. Epub 2010 Aug 31.
Exciton sizes influence transport processes and spectroscopic phenomena in molecular aggregates and crystals. Thermally driven nuclear motion generally localizes electronic states in equilibrium systems. Exciton sizes also undergo dynamic changes caused by nonequilibrium relaxation in the lattice structure local to the photoexcitations (i.e., self-trapping). The α-phase of crystalline perylene is particularly well-suited for fundamental studies of exciton self-trapping mechanisms. It is generally agreed that a subpicosecond self-trapping process in α-perylene localizes photoexcited excitons onto pairs of closely spaced molecules (i.e., dimers), which then relax through excimer emission. Here, electronic relaxation dynamics in α-perylene single crystals are investigated using a variety of nonlinear optical spectroscopies in conjunction with a Frenkel exciton model. Linear absorption and photon echo spectroscopies suggest that excitons are delocalized over less than four unit cells (16 molecules) at 78 K prior to self-trapping. Stimulated Raman spectroscopies conducted on and off electronic resonance reveal significant vibronic coupling in a mode at 104 cm(-1), which corresponds to the displacement between perylene molecules comprising a dimer. Strong vibronic coupling in this mode suggests that motion along the interdimer axis is instrumental in driving the self-trapping process. The results are discussed in the context of our recent study of tetracene and rubrene single crystals in which similar experiments and models were employed.
激子尺寸会影响分子聚集体和晶体中的输运过程和光谱现象。热驱动的核运动通常会使电子态在平衡系统中局部化。激子尺寸也会发生动态变化，这是由光激发局域晶格结构的非平衡弛豫引起的（即自陷）。结晶苝的α 相特别适合于激子自陷机制的基础研究。人们普遍认为，α-苝中的亚皮秒自陷过程会将光激发的激子局域到紧密间隔的分子对（即二聚体）上，然后通过激子复合物发射来松弛。在这里，使用各种非线性光学光谱学并结合弗仑克尔激子模型研究了α-苝单晶中的电子弛豫动力学。线性吸收和光子回波光谱表明，在自陷之前，激子在 78 K 下的局域化程度小于四个单元（16 个分子）。在电子共振上和电子共振外进行的受激拉曼光谱显示，在 104 cm(-1) 处的模式中有显著的振子耦合，这与构成二聚体的苝分子之间的位移相对应。该模式中的强振子耦合表明，沿二聚体轴的运动是自陷过程的关键。这些结果是在我们最近对并五苯和荧蒽单晶的研究中讨论的，在该研究中采用了类似的实验和模型。
