Allen Taylor G, Benis Sepehr, Munera Natalia, Zhang Junxiang, Dai Shuixing, Li Tengfei, Jia Boyu, Wang Wei, Barlow Stephen, Hagan David J, Van Stryland Eric W, Zhan Xiaowei, Perry Joseph W, Marder Seth R
School of Chemistry and Biochemistry, Center for Organic Photonics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States.
CREOL, The College of Optics & Photonics, University of Central Florida, Orlando, Florida 32816-2700, United States.
J Phys Chem A. 2020 Jun 4;124(22):4367-4378. doi: 10.1021/acs.jpca.0c02572. Epub 2020 May 20.
The two-photon absorption (2PA) properties are investigated for two series of organic, π-conjugated, fused-ring, quadrupolar A-π-D-π-A chromophores of the type originally developed as nonfullerene acceptors for organic photovoltaics. These molecules are found to exhibit large nondegenerate two-photon absorption (ND2PA) cross-sections (ca. 6-27 × 10 GM) in the near-infrared (NIR). In the first series, involving molecules of varying core size, ND2PA spectra and cross-sections characterized by femtosecond ND2PA spectroscopy in chloroform solutions reveal that increases in core size, and thus conjugation length, leads to substantially red-shifted and enhanced 2PA. In a second series, variation of the strength of the terminal acceptor (A) with constant core size (seven rings, indacene-based) led to less dramatic variation in the 2PA properties. Among the two core types studied, compounds in which the donor has a thieno[3,2-]thiophene center demonstrate larger 2PA cross-sections than their indacene-centered counterparts, due to the greater electron-richness of their cores amplifying intramolecular charge transfer. Excited-state absorption (ESA) contributions to nonlinear absorption measured by open-aperture Z-scans are deduced for some of the compounds by analyzing the spectral overlap between 2PA bands and NIR ESA transitions obtained by ND2PA and transient absorption measurements, respectively. ESA cross-sections extracted from transient absorption and irradiance-dependent open-aperture Z-scans are in reasonable agreement, and their moderate magnitudes (ca. 10 m) suggest that, although ESA contributions are non-negligible, the effective response is predominantly instantaneous 2PA.
对两类有机π共轭稠环四极A-π-D-π-A发色团的双光子吸收（2PA）特性进行了研究，这类发色团最初是作为有机光伏的非富勒烯受体而开发的。发现这些分子在近红外（NIR）区域表现出较大的非简并双光子吸收（ND2PA）截面（约6 - 27×10 GM）。在第一类中，涉及不同核心尺寸的分子，通过飞秒ND2PA光谱在氯仿溶液中表征的ND2PA光谱和截面表明，核心尺寸的增加以及共轭长度的增加会导致2PA显著红移并增强。在第二类中，在核心尺寸恒定（七个环，基于茚并芴）的情况下改变末端受体（A）的强度，导致2PA特性的变化较小。在所研究的两种核心类型中，供体具有噻吩并[3,2 - ]噻吩中心的化合物比以茚并芴为中心的同类化合物表现出更大的2PA截面，这是由于其核心更高的富电子性增强了分子内电荷转移。通过分别分析由ND2PA和瞬态吸收测量获得的2PA带与近红外ESA跃迁之间的光谱重叠，推导了一些化合物在开孔Z扫描测量的非线性吸收中激发态吸收（ESA）的贡献。从瞬态吸收和辐照度依赖的开孔Z扫描中提取的ESA截面具有合理的一致性，并且它们的适度大小（约10 m）表明，尽管ESA贡献不可忽略，但有效响应主要是瞬时2PA。
