低聚芴的激发态吸收光谱带形分析
Analysis of the excited-state absorption spectral bandshape of oligofluorenes
摘要：
本文报道了两种低聚芴衍生物在稀溶液中的超快瞬态吸收光谱。这些光谱显示清晰的电子振动结构，我们分析严格使用依赖于时间的吸收形式主义提取的主要激发态振动正常模式的频率，耦合到电子跃迁，较高的说谎激发态的构型位移，和重组能量。我们可以使用频率为450 cm（二聚体）或400 cm $^{-1}$（三聚体）和1666 cm $^{-1}$的两个完全对称的振动模式来模拟激发态吸收光谱。基态吸收的重组能量是相当不敏感的低聚物长度在230毫电子伏。然而，低聚芴二聚体和三聚体之间的激发态吸收从58到166 meV。基于先前的理论工作[Shukla等人，\textbf{67}，245203（2003）]，我们将吸收光谱归属于从$1B_u$激发态到更高的$mA_g$态的跃迁，并发现激发态跃迁能量相对于基态跃迁能量与这里研究的两种低聚物的理论预测非常一致。这些结果和分析允许在$π$共轭聚合物，这是作为在固态的有机半导体的普遍感兴趣的主题的激发态吸收的性质的深刻理解。
英文摘要：
We present ultrafast transient absorption spectra of two oligofluorene derivatives in dilute solution. These spectra display clear vibronic structure, which we analyze rigorously using a time-dependent formalism of absorption to extract the principal excited-state vibrational normal-mode frequencies that couple to the electronic transition, the configurational displacement of the higher-lying excited state, and the reorganization energies. We can model the excited-state absorption spectrum using two totally symmetric vibrational modes with frequencies 450 (dimer) or 400 cm$^{-1}$ (trimer), and 1666 cm$^{-1}$. The reorganization energy of the ground-state absorption is rather insensitive to the oligomer length at 230 meV. However, that of the excited-state absorption evolves from 58 to 166 meV between the oligofluorene dimer and trimer. Based on previous theoretical work [Shukla et al., Phys. Rev. B \textbf{67}, 245203 (2003)], we assign the absorption spectra to a transition from the $1B_u$ excited state to a higher-lying $mA_g$ state, and find that the energy of the excited-state transition with respect to the ground-state transition energy is in excellent agreement with the theoretical predictions for both oligomers studied here. These results and analysis permit profound understanding of the nature of excited-state absorption in $π$-conjugated polymers, which are the subject of general interest as organic semiconductors in the solid state.
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