Structure and exciton dynamics in J-aggregates studied by polarization-dependent near-field scanning optical microscopy
Structure and exciton dynamics in J-aggregates studied by polarization-dependent near-field scanning optical microscopy
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DOI：
10.1021/jp9518217
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发表时间：
1996-01-25
影响因子：
--
通讯作者：
Barbara, PF
中科院分区：
文献类型：
--
作者：
Higgins, DA;Reid, PJ;Barbara, PF
用近场扫描光学显微镜（NSOM）研究了赝异氰（PIC）J聚集体薄膜，揭示了这些纳米结构薄膜中激子迁移的局部分子取向、分子有序性和距离尺度.聚（硫酸乙烯酯）膜中生长的J-聚集体的偏振相关的图像记录通过检测从激子状态的聚集体的荧光，并提出了三个不同的激发波长对应于激发的激子带的下部（570 nm）和上部（514.5和488 nm）区域。图像表明荧光沿聚集体的长轴沿着高度偏振。总吸收（即，荧光激发）在570 nm处沿着聚集体的长轴高度偏振，在514.5 nm处独立于偏振，在488 nm处垂直于长轴偏振。极化相关的结果被用来确定本地激子跃迁偶极取向，并确定聚集体内的取向顺序的程度。PIC单体在聚集体中的局部取向随后从偶极取向确定，并且基于这些结果提出了用于单体过渡偶极的组织的人字形结构模型。最后，观察到聚集体的荧光通过弯曲和相交的聚集体结构高度偏振;在这些区域中缺乏荧光去偏振证实了我们先前的结论，即激子迁移限于小于或等于50 nm。
Thin films of pseudoisocyanine (PIC) J-aggregates have been investigated by near-field scanning optical microscopy (NSOM), revealing the local molecular orientation, the molecular order, and the distance scale for exciton migration in these nanostructured films. Polarization-dependent images of J-aggregates grown in poly(vinyl sulfate) films were recorded by detecting the fluorescence from the excitonic state of the aggregates and are presented for three different excitation wavelengths corresponding to excitation to the lower (570 nm) and upper (514.5 and 488 nm) regions of the excitonic band. The images demonstrate that the fluorescence is highly polarized along the long axis of the aggregates. Aggregate absorption (i.e., fluorescence excitation) in the near field is highly polarized along the long axis of the aggregates at 570 nm, independent of polarization at 514.5 nm, and polarized perpendicular to the long axis at 488 nm. The polarization-dependent results are employed to determine the local excitonic transition dipole orientations and to determine the extent of orientational order within the aggregates. The local orientation of the PIC monomers in the aggregates is subsequently determined from the dipole orientations, and a herringbone-like structural model for the organization of the monomeric transition dipoles is proposed, based on these results. Finally, the fluorescence from the aggregates is observed to be highly polarized through curved and intersecting aggregate structures; the lack of fluorescence depolarization in these regions confirms our previous conclusion that exciton migration is limited to less than or equal to 50 nm.