五苯荑分子剎車系統之分子運動模式與激發態鬆弛行為之探討

Abstract

螢光分子的發光效率由其激發態進行非輻射鬆弛而去活化的速率決定，而這些鬆弛過程已知可藉由分子內的運動來加速進行，這個行為被稱作「自由轉子效應」。藉由分子間的堆疊或者是環境的黏滯度來限制分子運動來壓抑此效應往往可以導致螢光量子產率的提升，以及螢光生命期的增長。若能對此效應的理論基礎有更深入的認知，對於高效發光材料的開發將會有很大的助益。 本論文針對過去曾研究過的五苯荑分子剎車系統o-Pp-Ss進行更深入的探討，以變溫NMR搭配DFT理論計算分析其分子內運動模式，並與其在不同黏度下的螢光生命期變化做連結。另外，另一個具有相似結構的p-Pp-Ss分子也被加入討論。經由本工作，我們對o-Pp-Ss系統的光物理行為有了更深入的認知，並據此提出了分子運動影響與非輻射鬆弛過程的理論模型。

The photoluminescence efficiency of a fluorescent molecule is governed by the rates of its non-radiative excited-state relaxation processes, which are known to be accelerated by certain intramolecular motions. This phenomenon is known as the “free-rotor effect” or “loose-bolt effect”. Suppressing this effect by restriction of intramolecular motion by intermolecular aggregation or environmental viscosities often lead to the enhancement of fluorescence quantum yields as well as the increase of fluorescence lifetimes. A deeper understanding of this behavior would provide guidelines for the design of efficient photoluminescent molecules. In this work, we perform a more detailed analysis of a previously studied pentiptycene-derived molecular braking system o-Pp-Ss. The molecular motions are investigated by VT-NMR experiments and DFT-calculations, and the correlations between the modes of the molecular motion and the non-radiative deacivation are discussed based on the viscosity-dependent fluorescence lifetimes. In addition, a model compound p-Pp-Ss has also been studied. From this study, we have gained more detailed insight into the photophysical behaviors of o-Pp-Ss, and a fundamental model for the motion-induced excited-state relaxation process is proposed.