Vibrational Coherence Due to Promoting Mode Activity in the Relaxation Dynamics of the Class III Mixed-Valence Molecule [Ru₂TIEDCl₄]⁺

Abstract

We present the first measurements of the excited-state relaxation dynamics of a bimetallic class III mixedvalence molecule. The 800 nm absorption of [Ru 2 TIEDCl 4 ] + (TIED ) tetraiminoethylenedimacrocycle) relaxes in 250 and 1000 fs to at least two different intermediate states that can be followed with transient absorption spectroscopy. These states decay in 1.3 and 11.5 ps, and the absorption of the 1.3 ps intermediate displays a large amplitude, very low frequency, highly damped vibrational coherence that completely modulates the absorption. The coherence frequency is 20 ( 5 cm -1 , and the dephasing times range from 360 to 730 fs over the wavelength range of the absorption band. The occurrence of a low-frequency coherence at room temperature, the nearly 100% modulation amplitude, and the phase properties as a function of wavelength are consistent with a nonradiative rate modulation rather than the typical impulsive mechanism that creates a coherent Franck-Condon modulation of the absorption. A nonradiative rate modulation can occur from a vibronic coupling mechanism that is created by breakdown of the Born-Oppenheimer approximation. This type of electronic state coupling likely occurs via nontotally symmetric vibrations, and this is the first time domain measure of a vibronic coupling frequency for inorganic complexes. The resonance Raman activity of the ground-state absorption is consistent with very small mode displacements for the optically connected ground and excited states, as expected for a class III molecule. Since similar nonradiative rates are measured for both the optically excited-state and intermediate-state decays, they both require similar energy gaps in the range of 5000-7000 cm -1 . With these energy gaps, we infer that vibronic coupling matrix elements from 4500 to 11 200 cm -1 can explain the observed nonradiative decay time of 250 fs. These experiments show that class III molecules, and probably many other inorganic complexes, can have fast nonradiative decay channels from vibronic coupling when electronic states are available at lower energies. Therefore, applications with such molecules require careful molecular design to compete with or reduce rates of return to the ground state.

Unconventional Mixed‐Valent Complexes of Ruthenium and Osmium

“…Conventional molecular bridges contain donor atoms for metal binding in a bis-monodentate (1), [10][11][12] bis-bidentate (11), [53][54][55] bis-tridentate (12), [56][57][58][59] or bis-tetradentate [60] fashion (13), connected to a "spacer" which typically ensures p conjugation and determines the metal-metal distance.…”

“…[27] However, the use of increasingly variegated bridging ligands, especially with chelate functions of variable denticity (bis-bidentate, bistridentate, or bis-tetradentate ligands) [55][56][57][58][59][60] has shown that these two manifestations of metal-metal interaction may not run in parallel but can be very different in magnitude. [98] In comparison to the Creutz-Taube ion as a standard, Table 5 illustrates cases with small K c values and intense IVCT bands (bis-tridentate acceptor bridges) [56][57][58] as well as examples with very large K c values but weak or non-observable IVCT features, typically exhibited by diruthenium(III,II) compounds with tris-or bis-bidentate acceptor bridges.…”

Quantum entanglement between electronic and vibrational degrees of freedom in molecules

“…Such a system was first introduced by Horiuti and Polanyi in 1935 to describe proton and hydrogen transfer reactions 1 and was subsequently extended by Hush to oxidation-reduction processes 2 in 1953, now forming the basis of modern electron-transfer theory [3][4][5][6] describing for example exciton and charge transport through molecules, organic conductors and organic photovoltaics as well as electron-transfer reactions in biochemistry. [7][8][9][10][11][12][13][14][15][16][17][18] It also describes general racemization processes 19,20 and has been widely used in spectroscopic analyses, [21][22][23] forming the core of Herzberg-Teller theory. [24][25][26] Indeed, Bersuker has suggested that all chemical processes can be described in this way as a pseudo Jahn-Teller effect.…”

Final Report: Vibrational Dynamics in Photoinduced Electron Transfer

“…12 This molecule is an example of how many inorganic molecules delocalize to accommodate mixed valence oxidation states and behave according to molecular orbital states. In particular, this molecule was able to have two channels of relaxation back to the ground state with no fluorescence, and the fastest channel has a strong vibrational coherence.…”