Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, United States.
Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, United States.
J Phys Chem A. 2021 May 13;125(18):3863-3873. doi: 10.1021/acs.jpca.1c01691. Epub 2021 May 4.
This paper reports a photophysical investigation of a series of phenylene ethynylene oligomers (OPE) that are end-substituted with a 1,8-naphthalene imide (NI) acceptor. The NI acceptor is attached to the terminus of the OPEs an ethynylene (-C≡C-) unit that is linked at the 4-position of the NI unit. A series of three oligomers is investigated, OPE1-NI, OPE3-NI, and OPE5-NI, which contain 1, 3, and 5 phenylene ethynylene repeat units, respectively. The properties of the OPEn-NI series are compared to a corresponding set of unsubstituted OPEs, OPE3 and OPE5, which contain 3 and 5 phenylene ethynylene repeats, respectively. The photophysics of all the compounds are interrogated using a variety of techniques including steady-state absorption, steady-state fluorescence, two-photon absorption, time-resolved fluorescence, and transient absorption spectroscopy on femtosecond-to-microsecond time scales. The effect of solvent polarity on the properties of the oligomers is examined. The results show that the NI-substituted oligomers feature a lowest charge transfer (CT) excited state, where the OPE segment acts as the donor and the NI moiety is the acceptor (OPE-NI). The absorption spectra in one-photon and two-photon exhibit a clear manifold of absorption features that can be attributed to direct CT absorption. In moderately polar solvents, the emission is dominated by a broad, solvatochromic band that is due to radiative decay from the CT excited state. Ultrafast transient absorption provides evidence for initial population of a locally excited state (LE) which in moderately polar solvents rapidly (∼1 ps) evolves into the CT excited state. The structure, spectroscopy, and dynamics of the CT state are qualitatively similar for OPE3-NI and OPE5-NI, suggesting that delocalization in the OPE segment does not have much effect on the structure or energetics of the CT excited state.
本文报道了一系列苯乙炔低聚物（OPE）的光物理研究，这些低聚物的末端被 1,8-萘二酰亚胺（NI）受体取代。NI 受体连接在 OPEs 的炔烃（-C≡C-）单元的 4 位上。研究了一系列三个低聚物，OPE1-NI、OPE3-NI 和 OPE5-NI，它们分别含有 1、3 和 5 个苯乙炔重复单元。将 OPEn-NI 系列的性质与相应的一组未取代的 OPE 进行了比较，OPE3 和 OPE5 分别含有 3 和 5 个苯乙炔重复单元。使用各种技术研究了所有化合物的光物理性质，包括稳态吸收、稳态荧光、双光子吸收、时间分辨荧光和飞秒至微秒时间尺度的瞬态吸收光谱。研究了溶剂极性对低聚物性质的影响。结果表明，NI 取代的低聚物具有最低的电荷转移（CT）激发态，其中 OPE 段作为供体，NI 部分作为受体（OPE-NI）。单光子和双光子吸收光谱显示出明确的吸收特征，可归因于直接 CT 吸收。在中等极性溶剂中，发射主要由宽的溶剂化带主导，这是由于 CT 激发态的辐射衰减引起的。超快瞬态吸收提供了初始局域激发态（LE）种群的证据，在中等极性溶剂中，LE 迅速（约 1 ps）演变为 CT 激发态。CT 态的结构、光谱和动力学对于 OPE3-NI 和 OPE5-NI 是定性相似的，这表明 OPE 段的离域化对 CT 激发态的结构或能级没有太大影响。
