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Optical Properties and Ultrafast Dynamics in Metal Cluster Hybrids with Diamondoids, and Conjugated pi-Systems

Subject Area Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2010 to 2017
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 120401550
 
Final Report Year 2017

Final Report Abstract

In this project, we have performed theoretical investigations along three lines: (1) We have studied the spectroscopic properties of nonmetallic clusters such as diamondoids and silicon clusters. In collaboration with the experimental project G we have elucidated the photoabsorption and –emission spectra of pristine diamondoids, revealing that their apparently simple vibrational structure emerges from a complicated interplay of many simultaneously excited vibronic transitions. In addition, in some diamondoids several low-lying electronic states are symmetryforbidden, leading to the observation that higher energy absorption is accompanied by emission out of energetically lower states which are only scarcely visible in absorption. These findings provide a basis for further investigations of the photodynamical properties of diamondoids and their hybrids. (2) In order to pave the way for ongoing research on hybrid systems consisting of metal clusters and diamondoids or other organic compounds, we have investigated the structural and optical properties of metal clusters interacting with organic ligands such as phenyl or thiolate. We could show that in such systems, characteristical traits of the silver cluster absorption remain intact, while the system itself becomes stabilized against environmental influences. In this way, silver clusters may be employed as absorption chromophores under ambient conditions due to the protecting organic groups around them. Beyond their stabilization, the precise geometric orientation of clusters on a template is desirable in the context of materials design. Therefore, we have studied the absorption properties of extended silver cluster arrays on polymeric porphyrin sheets. Such 1D and 2D structures are experimentally known, and the porphyrin subunits provide well-defined binding sites for metal clusters. As we have shown, silver clusters ordered in different arrangements on such porphyrin structures maintain characteristic features of the intense single-cluster absorption. However, due to the excitonic coupling of individual clusters, the specific spectral shape depends crucially on the geometry of the system, thereby providing a means to tailor the optical properties to the desired needs. (3) Beyond static calculations, we investigated the coupled electron nuclear photodynamics of several building blocks of the aforementioned systems. Specifically, we studied the nonradiative relaxation of diamantane, in which the low-lying excited states are symmetry forbidden in the Franck-Condon region. We showed that after photoexcitation into higher-lying optically bright states, nonradiative decay rapidly leads to the lowest excited state, from which photoemission finally takes place. Since ultimately, the hybridization of diamondoids with metal clusters is envisioned, we also investigated the nonradiative relaxation of metal clusters. On the example of small gold clusters, we could show, in collaboration with subproject F, that, depending on the excitation energy, both molecular-like dynamics involving large-amplitude nuclear motion as well as bulk-like electronic relaxation in a dense excited state manifold can be observed. This could be exploited for the development of cluster-based hybrid materials with a tailored optical response. Finally, we have extended our theoretical methodology for the simulation of field-induced photodynamics to also account for multiphoton transitions in the strongfield regime. This is especially relevant for the porphyrin systems to be used as support for ordered arrays of clusters as discussed in part (2), since they exhibit several energetically low-lying transitions which are dark in one-photon, but bright in two-photon absorption. Varying the field intensity and photon energy thus will allow to switch between different regimes of excitation in a precisely defined way, thereby providing a toehold for new strategies of optimal control.

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