Stewart David J, Dalton Matthew J, Swiger Rachel N, Fore Jennifer L, Walker Mark A, Cooper Thomas M, Haley Joy E, Tan Loon-Seng
Air Force Research Laboratory, Materials and Manufacturing Directorate, Functional Materials Division, Wright-Patterson AFB , Dayton, Ohio 45433-7750, United States.
J Phys Chem A. 2014 Jul 17;118(28):5228-37. doi: 10.1021/jp504319u. Epub 2014 Jul 7.
Three two-photon absorption (2PA) dyes (donor-π-donor (DPA2F), donor-π-acceptor (AF240), and acceptor-π-acceptor (BT2F); specifically, D is Ph2N-, A is 2-benzothiazoyl, and the π-linker is 9,9-diethylfluorene) are examined in a variety of aprotic solvents. Because the 2PA cross section is sensitive to the polarity of the local environment, this report examines the solvent-dependent linear photophysics of the dyes, which are important to understand before probing more complex solid-state systems. The symmetrical dyes show little solvent dependence; however, AF240 has significant solvatochromism observed in the fluorescence spectra and lifetimes and also the transient absorption spectra. A 114 nm bathochromic shift is observed in the fluorescence maximum when going from n-hexane to acetonitrile, whereas the lifetimes increase from 1.25 to 3.12 ns. The excited-state dipole moment for AF240 is found to be 20.1 D using the Lippert equation, with smaller values observed for the symmetrical dyes. Additionally, the femtosecond transient absorption (TA) spectra at time zero show little solvent dependence for DPA2F or BT2F, but AF240 shows a 52 nm hypsochromic shift from n-hexane to acetonitrile. Coupled with the solvatochromism in the fluorescence and large excited-state dipole moment, this is attributed to formation of an intramolecular charge-transfer (ICT) state in polar solvents. By 10 ps in AF240, the maximum TA in acetonitrile has shifted 30 nm, providing direct evidence of a solvent-stabilized ICT state, whose formation occurs in 0.85-2.71 ps, depending on solvent. However, AF240 in nonpolar solvents and the symmetrical dyes in all solvents show essentially no shifts due to a predominantly locally excited (LE) state. Preliminary temperature-dependent fluorescence using frozen glass media supports significant solvent reorganization around the AF240 excited state in polar solvents, and may also support a twisted intramolecular charge-transfer (TICT)-state contribution to the stabilization. Finally, time-dependent density functional theory calculations support ICT in AF240 in polar media and also allow prediction of the 2PA cross sections in the 0-0 band, which are much larger for AF240 than the symmetrical dyes.
研究了三种双光子吸收（2PA）染料（供体-π-供体（DPA2F）、供体-π-受体（AF240）和受体-π-受体（BT2F）；具体而言，D为Ph2N-，A为2-苯并噻唑基，π连接体为9,9-二乙基芴）在多种非质子溶剂中的情况。由于2PA截面对局部环境的极性敏感，本报告研究了染料的溶剂依赖性线性光物理性质，这在探究更复杂的固态体系之前是很重要的。对称染料显示出几乎没有溶剂依赖性；然而，AF240在荧光光谱、寿命以及瞬态吸收光谱中都表现出显著的溶剂化显色现象。从正己烷到乙腈，荧光最大值出现了114 nm的红移，而寿命从1.25 ns增加到3.12 ns。使用Lippert方程得出AF240的激发态偶极矩为20.1 D，对称染料的值较小。此外，在时间零点的飞秒瞬态吸收（TA）光谱显示，DPA2F或BT2F几乎没有溶剂依赖性，但AF240从正己烷到乙腈出现了52 nm的蓝移。结合荧光中的溶剂化显色现象和较大的激发态偶极矩，这归因于在极性溶剂中形成了分子内电荷转移（ICT）态。在AF240中，到10 ps时，乙腈中的最大TA已经移动了30 nm，这为溶剂稳定的ICT态提供了直接证据，其形成时间在0.85 - 2.71 ps之间，取决于溶剂。然而，非极性溶剂中的AF240以及所有溶剂中的对称染料由于主要处于局域激发（LE）态，基本上没有位移。使用冷冻玻璃介质进行的初步温度依赖性荧光研究支持了极性溶剂中围绕AF240激发态的显著溶剂重排，也可能支持扭曲分子内电荷转移（TICT）态对稳定性的贡献。最后，含时密度泛函理论计算支持了极性介质中AF240的ICT现象，并且还能预测0-0带中的2PA截面，AF240的这些截面比对称染料大得多。
