Vibrational Coherence Controls Molecular Fragmentation: Ultrafast Photodynamics of the [Ag₂Cl]⁺ Scaffold

Abstract

The recently introduced pump-probe fragmentation action spectroscopy reveals a unique observation of excited state vibrational coherence (430-460 fs) in the isolated metal complex [Ag(Cl)(dcpm))] (dcpm = bis(dicyclohexylphosphino)methane) containing the [AgCl] scaffold. After photoexcitation by an XMCT transition (260 nm) in an ion trap, an unexpected correlation between specific fragment ions (loss of HCl/Cl vs loss of dcpm) and the phase of the wave packet is probed (1150 nm). Based on ab initio calculations, we assign the primary electronically excited state and ascribe the observed coherences (72-78 cm) to contain predominantly Ag-Ag stretch character. We propose specific probe absorption and vibronic coupling at the classical turning points to switch remarkably early on between the different fragmentation pathways. The overall excited state dynamics are fitted to a multiexponential decay with time constants: 0.2-0.4/3-4/19-26/104-161 ps. These findings open new perspectives for further dynamics investigations and possible applications in photocatalysis.

Excited state vibrational coherence in a binuclear metal adduct: wave packet phase dependant molecular fragmentation under variation of ligand size

“…Since the largest structural changes upon photoexcitation correspond to the decrease of metal distance (d(Ag-Ag) =-3.6%) and the bending of P-Ag-P angle (∢(P-Ag-P) = +3.3%)), this observation strengthens our previous assignment for the vibrational coherence to contain mainly Ag-Ag stretching mode character [1].…”

“…The early decay dynamics of 1 + with time constants 0.2-0.5 / 4-6 ps (Fig. 2) is very similar compared to that of 2 + with 0.2-0.4 / 3-4 ps [1]. Interestingly, a correlation between specific fragment ions (Cl loss vs loss of L Me + AgCl) and the phase of the wave packet is observed, as evident by strongly damped (damping constant  0.1-0.5 ps) oscillatory components in their τ-PD signals (Fig.…”

“…The experimental setup and methodology is described elsewhere [1][2][3]. The chloride adducts [Ag 2 (Cl)(L Me ) 2 ] + (1 + ) and [Ag 2 (Cl)(L Cy ) 2 ] + (2 + ) where generated by electrospray ionization from acetonitrile sample solutions, isolated and identified by mass analysis via their characteristic isotope patterns (m/z = 521 and 1065, resp.…”

“…The static photo-induced dissociation (PD) spectra show a band at λ max ~ 280 nm and a plateau-like form centered at ~ 260 nm for 1 + and 2 + , resp. Based on calculations (PBE0-D3(BJ)/evGW-BSE/def2-SVPD) [4], using 1 + as model system, we assign these excitations to HOMO-1  LUMO ( 1 XMCT) transitions with chloride-to-metal-metal charge transfer character [1]. Hereby a contraction of the Ag-Ag distance by ~ 0.1 Å is predicted (CC2/def2-TZVPP), indicating a strengthening of Ag-Ag interaction in the electronically excited state.…”

“…Our gas phase approach is based on time-resolved photodissociation (τ-PD) action spectroscopy [1][2][3] of mass-selected, ion-trapped molecular systems, probing their electronic dynamics by way of transient fragmentation patterns as a function of pump-probe delay. Besides information on excited state kinetics, we focus on the question of how a subtle quantum phenomenon such as vibrational coherence is influenced by structural and electronic parameters, modulating cooperative metal-metal interaction.…”