Understanding strong two-photon absorption in π-conjugated porphyrin dimers via double-resonance enhancement in a three-level model
Understanding strong two-photon absorption in π-conjugated porphyrin dimers via double-resonance enhancement in a three-level model
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DOI：
10.1021/ja0445847
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发表时间：
2004-12-01
影响因子：
15
通讯作者：
Anderson, HL
中科院分区：
文献类型：
--
作者：
Drobizhev, M;Stepanenko, Y;Anderson, HL
本文研究了一系列共轭卟啉二聚体的双光子吸收（2PA）光谱，结果表明它们具有极大的飞秒本征峰2PA截面，在近红外区域达到σ2= 1 × 104GM；这是所有有机分子中测量到的最高值。此外，我们证明了二阶摄动理论应用于一个简单的三能级模型，给出了观测到的2PA截面的完美定量描述。通过比较二聚体的三能级模型与相应单体（σ2= 20 GM）的三能级模型的所有因素，我们解释了σ2的协同增强约500倍，并发现最重要的因素是激发态跃迁的强度。共轭卟啉二聚体的偶极矩矩阵元达到30 ~ 40 D的巨大值，这可以解释在该状态下的大离域半径（大电子-空穴分离）。我们还证明了这些卟啉二聚体在单光子和双光子激发下有效地产生单线态氧，这可以用于双光子启动的癌症光动力治疗。
We present the two-photon absorption (2PA) spectra of a series of conjugated porphyrin dimers and show that they possess extremely large intrinsic (femtosecond) peak 2PA cross sections, up to σ2= 1 × 104GM in the near-IR region; these are among the highest values measured for any organic molecule. Moreover, we demonstrate that the second-order perturbation theory applied to a simple three-level model gives a perfect quantitative description of the observed 2PA cross section. By comparing all the factors of the three-level model for dimers with those of corresponding monomer (for which σ2= 20 GM), we explain an ∼500-fold cooperative enhancement in σ2and find that the most important factor is the strength of excited-state transition. The matrix element of dipole moment of this transition amounts gigantic values of 30−40 D for conjugated porphyrin dimers, which can be accounted for a large delocalization radius (large electron−hole separation) in this state. We also demonstrate efficient generation of singlet oxygen upon one- and two-photon excitation of these porphyrin dimers, which can be useful for two-photon initiated photodynamic therapy of cancer.