Cycloparaphenylenes represent the smallest possible fragments of armchair carbon nanotubes. Due to their cyclic and curved conjugation, these nanohoops own unique photophysical properties. Herein, the internal conversion processes of cycloparaphenylenes of sizes 9 through 16 are simulated using Non-Adiabatic Excited States Molecular Dynamics. In order to analyze effects of increased conformational disorder, simulations are done at both low temperature (10 K) and room temperature (300 K). We found the photoexcitation and subsequent electronic energy relaxation and redistribution lead to different structural and electronic signatures such as planarization of the chain, electron–phonon couplings, wavefunction localization, and intra-ring migration of excitons. During excited state dynamics on a picosecond time-scale, an electronic excitation becomes partially localized on a portion of the ring (about 3–5 phenyl rings), which is not a mere static contraction of the wavefunction. In a process of non-radiative relaxation involving non-adiabatic transitions, the latter exhibits significant dynamical mobility by sampling uniformly the entire molecular structure. Such randomized migration involving all phenyl rings, occurs in a wave-like fashion coupled to vibrational degrees of freedom. These results can be connected to unpolarized emission observed in single-molecule fluorescence experiments. Observed intra-ring energy transfer is subdued for lower temperatures and adiabatic dynamics involving low-energy photoexcitation to the first excited state. Overall our analysis provides a detailed description of photo excited dynamics in molecular systems with circular geometry, outlines size-dependent trends and connotes specific spectroscopic signatures appearing in time-resolved experimental probes.

中文翻译：

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环对亚苯基中的光诱导动力学：平面化，电子-声子耦合，电子激发的局域化和环内迁移

环对亚苯基代表扶手椅碳纳米管的最小可能片段。由于它们的循环和弯曲共轭，这些纳米环具有独特的光物理特性。在本文中，使用非绝热激发态分子动力学模拟了大小为9至16的环对亚苯基的内部转化过程。为了分析构象障碍增加的影响，在低温（10 K）和室温（300 K）下均进行了仿真。我们发现光激发以及随后的电子能量弛豫和重新分布导致了不同的结构和电子签名，例如链的平面化，电子-声子耦合，波函数定位以及激子在环内迁移。在皮秒级的激发态动力学过程中，电子激发变得部分局限在环的一部分（约3-5个苯环）上，这不仅仅是波函数的静态收缩。在涉及非绝热转变的非辐射弛豫过程中，后者通过均匀采样整个分子结构而表现出显着的动力学迁移率。涉及所有苯环的这种随机迁移以与振动自由度耦合的波状方式发生。这些结果可以与单分子荧光实验中观察到的非偏振发射有关。对于较低的温度和涉及低能光激发到第一激发态的绝热动力学，观察到的环内能量传递受到抑制。