Ultrafast Vibrational Dynamics of Biomimetic Catalysts

Ultrafast two-dimensional infrared (2D-IR) spectroscopy is used in this work to study the vibrational dynamics of a series of biomimetic catalysts. We set out to investigate the vibrational dynamics of catalytic compounds in systems directly relevant to molecular reactivity, specifically reactive oxidation states, catalytically relevant self-isomerizations, dendritically-induced nano-confinement, and excitonic coherence transfer. For most of the work performed for this thesis we used diiron hexacarbonyl small-molecule mimics of the [FeFe]-hydrogenase enzyme’s active site. The vibrational dynamics of [(1,1’-bis(diphenylphosphino)ferrocene)chromium-(CO)4] (DPPFCr) in its neutral, closed-shell state were compared to the vibrational dynamics of DPPFCr as a cation radical. This comparison is possible because molecular oxidation does not significantly change the vibrational displacements of the carbonyl modes, which are studied here. Molecular oxidation induces an acceleration of the vibrational relaxation of the carbonyl modes but does not significantly affect the spectral diffusion dynamics of the carbonyl groups. We attribute this to an idiosyncrasy of the non-interacting solvent used for the experiment, CH2Cl2, which was chosen specifically for the weak nature of its solvent-solute interactions. Unexpectedly pronounced and slow spectral diffusion in the carbonyl modes of (µ-pdt)[Fe(CO)3]2 (pdt = 1,3-propanedithiolate) was observed in alkane solvents. The contribution of solvent-solute interactions in alkane solvents to spectral diffusion is expected to be minimal, and we related the spectral diffusion to fluctuations of the carbonyl potential induced by a catalytically-relevant mode of molecular fluxionality in (µ-pdt)[Fe(CO)3]2. Comparison with a different diiron hexacarbonyl compound, (µ-edt)[Fe(CO)3]2 (edt = 1,2-ethanedithiolate), effectively ruled out isomerization of the bridging organic disulfide group, and a Boltzmann distribution of states derived from electronic structure calculations supported our hypothesis by suggesting that a significant distribution of molecular conformations were present in at room temperature. Other fluxional organometallic complexes M3(CO)12 (M=Ru, Os) displayed similar spectral diffusion. This is the first use of spectral diffusion to study molecular conformational flexibility. We also observed an unexpected dependence of the rate of intracarbonyl IVR on the chain length of the alkane solvent. Nano-confinement has been reported on several occasions to favorably modulate the reactivity of diiron hexacarbonyl compounds, and dendritic assemblies with diiron hexacarbonyl cores were synthesized and the vibrational dynamics of the carbonyl groups were compared to the vibrational dynamics of carbonyls on similar diiron hexacarbonyl compounds without dendritic groups. Slower IVR and an additional timescale of spectral diffusion were observed in dendritic assemblies, which are hypothesized to reflect nano-modulation of the carbonyl group’s first solvation shell by the dendritic groups. Three diiron hexacarbonyl compounds with differing bridging disulfide groups (edt, pdt, and o-xylyldithiolate) are found to display unusual modulations of cross peak intensity which have previously been identified as spectral signatures of vibrational coherence transfer. Specific modulations of cross peak amplitude are observed in all three compounds, suggesting that certain coherence transfer events are common in diiron hexacarbonyl compounds, and an oscillatory frequency resulting from coherence transfer between bright and dark vibrational modes is identified.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145788/1/zpeckert_1.pd

Using Wave-Packet Interferometry to Monitor the External Vibrational Control of Electronic Excitation Transfer

We investigate the control of electronic energy transfer in molecular dimers through the preparation of specific vibrational coherences prior to electronic excitation, and its observation by nonlinear wave-packet interferometry. Laser-driven coherent nuclear motion can affect the instantaneous resonance between site-excited electronic states and thereby influence short-time electronic excitation transfer (EET). We first illustrate this control mechanism with calculations on a dimer whose constituent monomers undergo harmonic vibrations. We then consider the use of nonlinear wave-packet interferometry (nl-WPI) experiments to monitor the nuclear dynamics accompanying EET in general dimer complexes following impulsive vibrational excitation by a sub-resonant control pulse (or control pulse sequence). In measurements of this kind, two pairs of polarized phase-related femtosecond pulses following the control pulse generate superpositions of coherent nuclear wave packets in optically accessible electronic states. Interference contributions to the time- and frequency-integrated fluorescence signal due to overlaps among the superposed wave packets provide amplitude-level information on the nuclear and electronic dynamics. We derive the basic expression for a control-pulse-dependent nl-WPI signal. The electronic transition moments of the constituent monomers are assumed to have a fixed relative orientation, while the overall orientation of the complex is distributed isotropically. We include the limiting case of coincident arrival by pulses within each phase-related pair in which control-influenced nl-WPI reduces to a fluorescence-detected pump-probe difference experiment. Numerical calculations of pump-probe signals based on these theoretical expressions are presented in the following paper

Beyond Vibrationally Mediated Electron Transfer: Coherent Phenomena Induced by Ultrafast Charge Separation

Wave packet propagation succeeding electron transfer (ET) from alizarin dye molecules into the nanocrystalline TiO2 semiconductor has been studied by ultrafast transient absorption spectroscopy. Due to the ultrafast time scale of the ET reaction of about 6 fs the system shows substantial differences to molecular ET systems. We show that the ET process is not mediated by molecular vibrations and therefore classical ET theories lose their applicability. Here the ET reaction itself prepares a vibrational wave packet and not the electromagnetic excitation by the laser pulse. Furthermore, the generation of phonons during polaron formation in the TiO2 lattice is observed in real time for this system. The presented investigations enable an unambiguous assignment of the involved photoinduced mechanisms and can contribute to a corresponding extension of molecular ET theories to ultrafast ET systems like alizarin/TiO2.Comment: This work was supported by the German Research Foundation (DFG) (Hu 1006/6-1, WA 1850/6-1) and European Union projects FDML-Raman (FP7 ERC StG, contract no. 259158) and ENCOMOLE-2i (Horizon 2020, ERC CoG no. 646669

Transfer of Vibrational Coherence Through Incoherent Energy Transfer Process in F\"{o}rster Limi

We study transfer of coherent nuclear oscillations between an excitation energy donor and an acceptor in a simple dimeric electronic system coupled to an unstructured thermodynamic bath and some pronounced vibrational intramolecular mode. Our focus is on the non-linear optical response of such a system, i.e. we study both excited state energy transfer and the compensation of the so-called ground state bleach signal. The response function formalism enables us to investigate a heterodimer with monomers coupled strongly to the bath and by a weak resonance coupling to each other (F\"{o}rster rate limit). Our work is motivated by recent observation of various vibrational signatures in 2D coherent spectra of energy transferring systems including large structures with a fast energy diffusion. We find that the vibrational coherence can be transferred from donor to acceptor molecules provided the transfer rate is sufficiently fast. The ground state bleach signal of the acceptor molecules does not show any oscillatory signatures, and oscillations in ground state bleaching signal of the donor prevail with the amplitude which is not decreasing with the relaxation rate.Comment: 11 pages, 9 figure

Vibronic mixing enables ultrafast energy flow in light-harvesting complex II.

Since the discovery of quantum beats in the two-dimensional electronic spectra of photosynthetic pigment-protein complexes over a decade ago, the origin and mechanistic function of these beats in photosynthetic light-harvesting has been extensively debated. The current consensus is that these long-lived oscillatory features likely result from electronic-vibrational mixing, however, it remains uncertain if such mixing significantly influences energy transport. Here, we examine the interplay between the electronic and nuclear degrees of freedom (DoF) during the excitation energy transfer (EET) dynamics of light-harvesting complex II (LHCII) with two-dimensional electronic-vibrational spectroscopy. Particularly, we show the involvement of the nuclear DoF during EET through the participation of higher-lying vibronic chlorophyll states and assign observed oscillatory features to specific EET pathways, demonstrating a significant step in mapping evolution from energy to physical space. These frequencies correspond to known vibrational modes of chlorophyll, suggesting that electronic-vibrational mixing facilitates rapid EET over moderately size energy gaps

Ultrafast mid-infrared spectroscopy by chirped pulse upconversion in 1800-1000cm(-1) region

Broadband femtosecond mid-infrared pulses can be converted into the visible spectral region by chirped pulse upconversion. We report here the upconversion of pump probe transient signals in the frequency region below 1800c