Dias Fernando B, Pollock Sam, Hedley Gordon, Pålsson Lars-Olof, Monkman Andy, Perepichka Irene I, Perepichka Igor F, Tavasli Mustafa, Bryce Martin R
OEM Research Group, Department of Physics, University of Durham, Durham DH1 3LE, UK.
J Phys Chem B. 2006 Oct 5;110(39):19329-39. doi: 10.1021/jp0643653.
The strong solvatochromism observed for two fluorene-dibenzothiophene-S,S-dioxide oligomers in polar solvents has been investigated using steady-state and time-resolved fluorescence techniques. A low-energy absorption band, attributed to a charge-transfer (CT) state, is identified by its red shift with increasing solvent polarity. In nonpolar solvents, the emission of these conjugated luminescent oligomers shows narrow and well-resolved features, suggesting that the emission comes from a local excited state (LE), by analogy to their conjugated fluorene-based polymer counterparts. However, in polar solvents, only a featureless broad emission is observed at longer wavelengths (CT emission). A linear correlation between the energy maximum of the fluorescence emission and the solvent orientation polarizability factor Deltaf (Lippert-Mataga equation) is observed through a large range of solvents. In ethanol, below 230 K, the emission spectra of both oligomers show dual fluorescence (LE-like and CT) with the observation of a red-edge excitation effect. The stabilization of the CT emissive state by solvent polarity is accompanied/followed by structural changes to adapt the molecular structure to the new electronic density distribution. In ethanol, above 220 K, the solvent reorganization occurs on a faster time scale (less than 10 ps at 290 K), and the structural relaxation of the molecule (CT(unrelaxed) --> CT(Relaxed)) can be followed independently. The magnitude of the forward rate constant, k(1)(20 degrees C) approximately 20 x 10(9) s(-1), and the reaction energy barrier, E(a) approximately 3.9 kcal mol(-1), close to the energy barrier for viscous flow in ethanol (3.54 kcal mol(-1)), show that large-amplitude molecular motions are present in the stabilization of the CT state.
利用稳态和时间分辨荧光技术，研究了两种芴 - 二苯并噻吩 - S,S - 二氧化物低聚物在极性溶剂中观察到的强溶剂化显色现象。一个归因于电荷转移（CT）态的低能量吸收带，通过其随溶剂极性增加而发生的红移得以识别。在非极性溶剂中，这些共轭发光低聚物的发射显示出窄且分辨良好的特征，这表明发射源自局域激发态（LE），类似于它们基于共轭芴的聚合物对应物。然而，在极性溶剂中，仅在较长波长处观察到无特征的宽发射（CT发射）。通过大范围的溶剂观察到荧光发射的最大能量与溶剂取向极化率因子Δf（Lippert - Mataga方程）之间存在线性相关性。在乙醇中，低于230 K时，两种低聚物的发射光谱均显示出双重荧光（类似LE和CT），并观察到红边激发效应。溶剂极性对CT发射态的稳定伴随着/随后是结构变化，以使分子结构适应新的电子密度分布。在乙醇中，高于220 K时，溶剂重组发生在更快的时间尺度上（在290 K时小于10 ps），并且分子的结构弛豫（CT（未弛豫）→CT（弛豫））可以独立跟踪。正向速率常数的大小，k(1)(20℃)约为20×10^9 s^(-1)，以及反应能垒，E(a)约为3.9 kcal mol^(-1)，接近乙醇中粘性流动的能垒（3.54 kcal mol^(-1)），表明在CT态的稳定过程中存在大幅度的分子运动。
