Relaxation dynamics and structural characterization of organic nanoparticles with enhanced emission
Relaxation dynamics and structural characterization of organic nanoparticles with enhanced emission
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DOI：
10.1021/jp0502297
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发表时间：
2005-07-21
影响因子：
3.3
通讯作者：
Hsu, CS
中科院分区：
文献类型：
--
作者：
Bhongale, CJ;Chang, CW;Hsu, CS
采用再沉淀法制备了1，4-二[（E）-2-苯基-1-丙烯基]-苯（PPB）荧光纳米粒子，该纳米粒子在溶液中发射较弱，但聚集态时吸收蓝移，发射较强。这些PPB纳米粒子的皮秒荧光瞬态表现出双指数衰减，描述了一个连续的动力学模型，涉及两个发射状态。PPB纳米晶体的X射线衍射图案表明有两种类型的长程堆积结构，一种与单晶相同，另一种尚未确定。PPB分子在晶体中呈现人字形排列，PPB单元中的三个苯环几乎是平面的，单元中的两个甲基沿着同一方向指向，与孤立的PPB分子的扭曲结构形成对比。飞秒尺度上的PPB的荧光瞬变表明了一个有效的异构化通道，该通道在溶液中被激活用于游离PPB，但在PPB形成纳米颗粒中被抑制，这表明了影响激发态弛豫动力学的分子几何形状和扭转运动的重要性。这项工作中结合的多功能技术提供了强有力的证据，以提高我们对有机纳米粒子的光学性质依赖于大小的理解。
With a reprecipitation method, we prepared fluorescent organic nanoparticles of 1,4-di[(E)-2-phenyl-1-propenyl]-benzene (PPB) that feature weak emission in solution but exhibit blue-shifted absorption and strong emission as aggregates. Picosecond fluorescent transients of these PPB nanoparticles showed biexponential decay, described with a consecutive kinetic model involving two emissive states. X-ray diffraction patterns of PPB nanocrystals indicate long-range packing structures of two types, one the same as in a single crystal and the other not yet determined. PPB molecules in a crystal show an arrangement of a herringbone type with three benzene rings in a PPB unit being nearly planar and two methyl groups of the unit pointing along the same direction, in contrast to the twisted structure of an isolated PPB molecule. Fluorescence transients of PPB on a femtosecond scale indicate an efficient channel for isomerization that is activated for free PPB in solution but inhibited in PPB forming nanoparticles, demonstrating the significance of molecular geometry and twisting motions that affect the relaxation dynamics in the excited state. The versatile techniques combined in this work provide strong evidence to improve our understanding of optical properties in organic nanoparticles dependent on size.