Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA.
J Chem Phys. 2011 May 28;134(20):204703. doi: 10.1063/1.3590871.
A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32 cm(-1), far smaller than the measured value of 631 cm(-1) and of the wrong sign-a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601 cm(-1) and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport.
本文提出了一个用于晶状低聚物中偏振吸收的理论，该理论包括弗伦克尔激子耦合、弗伦克尔和电荷转移（CT）激子之间的耦合，以及所有中性和离子激发态与主要的环呼吸振动模式的耦合。对于并五苯，使用五个最低能量分子单重态之间的所有弗伦克尔耦合计算得到的最低能量（0-0）振子带的 Davydov 分裂（DS）仅为-32 cm（-1），远小于实验测量值 631 cm（-1），而且符号错误-负号表示下和上 Davydov 分量的极化与实验相反。包含弗伦克尔-CT 耦合极大地提高了与实验的一致性，得到了 0-0 DS 为 601 cm（-1），并且几乎定量地再现了 0-n 振子分量的相对光谱强度。我们的分析还表明，CT 混合随低聚物的大小而增加。我们讨论了这些结果对激子离解和输运的影响。
