Equilibrium Excited State Dynamics of a Photoactivated Catalyst Measured with Ultrafast Transient 2DIR

Abstract

A detailed understanding of photocatalyzed reaction dynamics requires a sensitive means of investigating the transient catalytically active species. Ideally, the method should be able to compare the electronically excited photocatalyst directly to the ground state species. We use equilibrium and transient two-dimensional infrared (2DIR and t-2DIR) spectroscopy to study the ground and excited state spectral dynamics of [Re(CO)3(bpy)Cl] in tetrahydrofuran (THF). We leverage the long-lived triplet excited state of the molecule to re-establish an equilibrated state relative to intersystem crossing dynamics and external solvent fluctuations, allowing access to the dynamics experienced by the excited state photocatalyst. The decay of frequency correlations within the excited triplet state species differs significantly from the ground state (slower by a factor of 3), indicating that the electronic excitation and subsequent metal-to-ligand charge transfer and associated structural changes are sufficient to perturb the spectral dynamics as sensed by the carbonyl ligands. In addition, we observe a 2-fold slowdown in ground state spectral dynamics around the in-phase symmetric vibrational mode compared to the two lower frequency, out-of-phase symmetric and asymmetric modes. Following electronic absorption and metal-to-ligand charge transfer the symmetry of the vibrational modes are disrupted, and all vibrational modes experience inhomogeneous broadening and spectral diffusion. The qualitative change in broadening mechanisms arises from the charge redistribution, indicating that direct comparisons of vibrational spectral dynamics on different electronic states-reported here for the first time-can be highly sensitive indicators of changes in electronic structure and in the concomitant solvation dynamics that underlie the microscopic details of charge transfer reactions.

Dye aggregation identified by vibrational coupling using 2D IR spectroscopy

“…Coherent oscillations have been previously observed in systems similar to ours as strong evidence of energy transfer. 35,36,43,44 Shown in Fig. 8 are plots of the intensity of the cross peak regions above and below the diagonal (referred to as upper c.p.…”

“…Re-based dyes have been extensively studied over the last 10 years as model systems for charge injection into TiO 2 and other semiconductors. 12,[32][33][34][35][36] From a comparison to density functional theory (DFT) calculations of the frequencies, we concluded that the dyes attached to the TiO 2 as either monomers or aggregates of two or three molecules, because the coupling between metal carbonyl groups is large enough to excitonically delocalize the vibrations across multiple carbonyl groups on adjacent molecules. 31 An illustration of FIG.…”

Anharmonicity Effects in IR Spectra of [Re(X)(CO)3(α-diimine)] (α-diimine = 2,2′-bipyridine or pyridylimidazo[1,5-a]pyridine; X = Cl or NCS) Complexes in Ground and Excited Electronic States

“…The corresponding IR feature becomes more intense in the excited state and has to be taken into account when photoinduced dynamic IR shifts and 2D-TRIR spectra are interpreted. 4,[19][20][21][22]77 Furthermore, the full dimensionality anharmonic procedure overcomes the limits of a harmonic analysis in the description of the fine structure of the FTIR spectra in the 1100−1700 cm −1 region.…”

Von ultraschnellen Strukturbestimmungen bis zum Steuern von Reaktionen: mehrdimensionale gemischte IR/nicht‐IR‐Schwingungsspektroskopie

“…Eine vergleichbare Beschleunigung wurde außerdem für [Re(CO) 3 (Bipyridin)Cl] beobachtet. [69] Darüber hinaus wurde auch der Einfluss von Photodissoziation auf die Schwingungsrelaxation untersucht. [70] Ein ähnliches Experiment wurde genutzt, um die Schwingungsdynamik nach Elektronenrücktransfer in einem Cyano-verbrückten gemischtvalenten Übergangsmetallkomplex [(NC) 5 Fe II -CN-Pt IV (NH 3 ) 4 -NC-Fe II (CN) 5 ] 4À nach Anregung im sichtbaren Bereich zu zeigen.…”

“…Kubarych und Mitarbeiter wendeten dies auf einen Photokatalysator im elektronischen Grundzustand und im angeregten Zustand an. [69] Dabei wurde eine Verlangsamung der spektralen Diffusion nach elektronischer Anregung um den Faktor 3b eobachtet, was der veränderten Elektrostatik zugeschrieben wurde.E ine weit verbreitete Methode zur Untersuchung der Dynamik von Lçsungsmitteln basiert auf der Beobachtung der dynamischen Fluoreszenz-Stokes-Verschiebung.Während bei Messungen der Stokes-Verschiebung die Dynamik im Grundzustand und im angeregten Zustand vermischt wird, da die Übergangsenergie abgetastet wird, kann bei T2D-IR-Spektroskopie jeder dieser Zustände einzeln betrachtet werden. [69] Zudem lässt sich lokale Information gewinnen, da sich betrachtete Oszillatoren an verschiedenen Punkten des Moleküls befinden kçnnen.…”