Department of Electrical Engineering , The Pennsylvania State University , University Park , Pennsylvania 16802 , United States.
Department of Physics , The Pennsylvania State University , University Park , Pennsylvania 16802 , United States.
Nano Lett. 2018 Mar 14;18(3):1693-1698. doi: 10.1021/acs.nanolett.7b04738. Epub 2018 Feb 28.
Upper-excited state emission is not usually observed from molecules owing to competition with much faster nonradiative relaxation pathways; however, it can be made more efficient by modifying the photonic density of states to enhance the radiative decay rate. Here, we show that embedding the small molecule zinc tetraphenylporphyrin (ZnTPP) in a hyperbolic metamaterial enables an ∼18-fold increase in fluorescence intensity from the second singlet excited state ( S) relative to that from the lowest singlet excited state ( S). By varying the number of periods in the HMM stack, we are able to systematically tune the ZnTPP fluorescence spectrum from red (dominated by emission from S) to blue (dominated by emission from S) with an instrument-limited decay lifetime <10 ps. Our results are consistent with a broadband Purcell enhancement in the radiative rate of both transitions predicted via transfer matrix modeling and point to a general opportunity to harness upper-excited states for spectrally tunable, ultrafast fluorescence via radiative decay engineering.
上激发态发射通常不会在分子中观察到，因为它与更快的非辐射弛豫途径竞争；然而，通过修饰光子态密度以提高辐射衰减率，可以使上激发态发射更加高效。在这里，我们表明，将小分子锌四苯基卟啉（ZnTPP）嵌入双曲超材料中，可以使第二单重激发态（S）的荧光强度相对于最低单重激发态（S）提高约 18 倍。通过改变 HMM 堆叠的周期数，我们能够系统地调节 ZnTPP 的荧光光谱，从红色（主要由 S 态发射）到蓝色（主要由 S 态发射），仪器限制的衰减寿命<10 ps。我们的结果与通过转移矩阵建模预测的两种跃迁的宽带普塞尔增强一致，并指出了一种通过辐射衰减工程利用上激发态实现光谱可调、超快荧光的一般机会。
