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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Photoinduced Intramolecular Charge Transfer in a Series of Differently Twisted Donor-Acceptor Biphenyls As Revealed by Fluorescence
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Photoinduced Intramolecular Charge Transfer in a Series of Differently Twisted Donor-Acceptor Biphenyls As Revealed by Fluorescence

机译：荧光显示的一系列不同扭曲的供体-受体联苯中的光诱导分子内电荷转移。

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This photophysical study addresses the general question of how electron transfer in bichromophoric molecules influences the conformational relaxation, which can be toward either more or less #pi#-conjugation. The effects of photoinduced intramolecular charge transfer on the electronic and molecular properties of a series of differently twisted 4-N, N-dimethylamion-4' -cyanobiphenyls are investigated by steady-state and time-resolved fluorescence. The dipole moments, radiative rates, and torsional relaxations in the excited state are analyzed by comparison with the absorption spectra and interannular twist angle (#phi#)-dependent CNDO/S calculations. Independent of the twist angle #phi# and solvent polarity, the first excited singlet state of these donor-acceptor (D-A) biphenyls (I-III) is an emissive intramolecular ~1CT state of the ~1L_a-type transferring charge from the dimethylaminobenzene (D) to the cyanobenzene (A) subunit. Similar to the planar restricted D-A fluorene I, the flexible D-A biphenyl II shows only a weak dependence of the fluorescence radiative rate constants k_f(0.4-0.6 ns~(-1)) on the solvent polarity, consistent with a planarization in the excited state of II. In contrast, the strongly pretwisted biphenyl III behaves similarly to I and II only in nonpolar solvents ()=0.3 ns~(-1), indicating partal excited-state relaxation toward planarity), whereas with increasing polarity the mean radiative rate decreases down to 0.03 ns~(-1). A fast equilibrium between a more planar and a more twisted rotamer distribution in the ~1CT state of III explains the appearance for III of additional photophysical effects such as (a) strong decrease of the radiative rates with increasing polarity, (b) two long (>200 ps) fluorescence lifetimes with precursor-successor relation, and (c) excited-state quenching by protic solvents.
机译：这项光物理研究解决了一个一般问题，即双发色分子中的电子转移如何影响构象弛豫，该构象弛豫可能会或多或少地与＃pi＃共轭有关。通过稳态和时间分辨荧光研究了光诱导的分子内电荷转移对一系列不同扭曲的4-N，N-二甲基氨基-4'-氰基联苯的电子和分子性质的影响。通过与吸收光谱和依赖于环间扭转角（＃phi＃）的CNDO / S计算进行比较，分析了激发态下的偶极矩，辐射速率和扭转弛豫。这些供体-受体（DA）联苯（I-III）的第一个激发单重态与扭转角＃phi＃和溶剂极性无关，是〜1L_a-型转移电荷从二甲基氨基苯（ D）为氰基苯（A）亚基。与平面受限DA芴I相似，柔性DA联苯II仅显示出荧光辐射速率常数k_f（0.4-0.6 ns〜（-1））对溶剂极性的弱依赖性，这与激发态下的平坦化相一致II。相反，仅在非极性溶剂中（）= 0.3 ns〜（-1），强加捻的联苯III的行为与I和II相似，表明部分激发态向平面性弛豫），而随着极性的增加，平均辐射速率降低至0.03 ns〜（-1）。 III的〜1CT状态下较平坦的和更扭曲的旋转异构体分布之间的快速平衡解释了III出现的其他光物理效应，例如（a）随着极性的增加辐射速率大大降低，（b）两个较长的（ > 200 ps）的荧光寿命与前体-后代关系，以及（c）质子溶剂激发态猝灭。

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