How fast is optically induced electron transfer in organic mixed valence systems?

Abstract

The rate of thermally induced electron transfer in organic mixed valence compounds has thoroughly been investigated by e.g. temperature dependent ESR spectroscopy. However, almost nothing is known about the dynamics of optically induced electron transfer processes in such systems. Therefore, we investigated these processes in mixed valence compounds based on triphenylamine redox centres bridged by conjugated spacers by NIR transient absorption spectroscopy with fs-time resolution. These experiments revealed an internal conversion (IC) process to be on the order of 50-200 fs which is equivalent to the back electron transfer after optical excitation into the intervalence charge transfer band. This IC is followed by ultrafast cooling to the ground state within 1 ps. Thus, in the systems investigated optically induced electron transfer is about 3-4 orders of magnitude faster than thermally induced ET.

Control of Electron Transfer Processes in Multidimensional Arylamine-Based Mixed-Valence Compounds by Molecular Backbone Design

“…To this purpose, it is indeed crucial to (i) devise molecules allowing an intramolecular electron or hole transfer between at least two redox sites linked by a bridge and (ii) establish rules for the prediction and control of the electron propagation through this bridge. , Arylamine fragments are the most used redox centers in the construction of promising HTMs. − They exhibit a well-defined N •+/0 process in a relatively low potential region, good stability of radical species, and the ability to transport positive charge efficiently. In the last years, different studies have considered arylamine-based linear MVs as versatile molecular models for investigating the basic electron- and/or charge-transfer (ET/CT) phenomena and the strength of the intramolecular electron coupling between two redox centers. − On the contrary, charge-transfer phenomena in arylamine-based multidimensional MVs have not been much studied, − although a large number of these molecular structures have been synthesized and used as HTMs in various optoelectronic devices. ,, …”

Inversion of donor–acceptor roles in photoinduced intervalence charge transfers

“…ESMV is a more general concept, including also those systems where mixed valence interactions are already present in the ground state (GSMV). ESMV systems can result, for example, from photoexcitation on the GSIVCT band of a GSMV moiety, [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] or from photoinduced energy transfer to GSMV-donor-or GSMV-acceptor-centered excited states. 27,36,37 In contrast, the term PIMV is reserved only for those systems where GSMV interactions are absent.…”

Wave-Function Symmetry Control of Electron-Transfer Pathways within a Charge-Transfer Chromophore

“…The positive feature at ∼20 000 cm −1 is rather distinctive, and it is present in several IVCT states of related supramolecular bimetallic {Ru−Os} systems. 31−34 The IVCT state decays to the GS within 3 ps, matching those lifetimes observed for directly excited IVCT states in cyanide-bridged 35−44 or alkynylbridged 45,46 mixed-valent bimetallic complexes. The 1.2 ps process that leads from hot-3 MLCT to IVCT is an ultrafast electron transfer, enabled thanks to the large metal−metal electronic communication promoted by the cyanide bridge.…”

“…In this state, a negative charge density has been transferred from the ruthenium to the osmium acceptor ion. The positive feature at ∼20 000 cm –1 is rather distinctive, and it is present in several IVCT states of related supramolecular bimetallic {Ru–Os} systems. − The IVCT state decays to the GS within 3 ps, matching those lifetimes observed for directly excited IVCT states in cyanide-bridged − or alkynyl-bridged , mixed-valent bimetallic complexes. The 1.2 ps process that leads from hot- 3 MLCT to IVCT is an ultrafast electron transfer, enabled thanks to the large metal–metal electronic communication promoted by the cyanide bridge. , Additionally, the metallic dπ orbitals involved in both states extend along the z -axis and have matching (“parallel”) wave-function symmetries, which ensures strong excited-state coupling.…”