Daniel G. Kuroda

Mapping Excited-State Dynamics by Coherent Control of a Dendrimer’s Photoemission Efficiency

Phasing-In Emission In keeping with quantum mechanics, the dynamics that ensue when molecules absorb light depend not only on the lights frequency but also its phase, which can manipulate excited state trajectories through interference. Recently, an impressive degree of control has been achieved over complex light absorbers, including proteins, by iteratively adapting the phase of a laser excitation pulse. However, the optimal pulses used are often too complicated to reveal straightforward insights into the systems being manipulated. Kuroda et al. (p. 263; see the Perspective by Batista) applied iterative phase manipulation toward optimizing the photoemission efficiency of a macromolecular donor-acceptor system, followed by statistical analysis to isolate a fairly simple phase function underlying much of the control mechanism. Careful follow-up experiments revealed an intuitive picture of the excited state behavior. A simple manipulation of the phase of a laser pulse optimizes photoemission efficiency in a complex molecule. Adaptive laser pulse shaping has enabled impressive control over photophysical processes in complex molecules. However, the optimal pulse shape that emerges rarely offers straightforward insight into the excited-state properties being manipulated. We have shown that the emission quantum yield of a donor-acceptor macromolecule (a phenylene ethynylene dendrimer tethered to perylene) can be enhanced by 15% through iterative phase modulation of the excitation pulse. Furthermore, by analyzing the pulse optimization process and optimal pulse features, we successfully isolated the dominant elements underlying the control mechanism. We demonstrated that a step function in the spectral phase directs the postexcitation dynamics of the donor moiety, thus characterizing the coherent nature of the donor excited state. An accompanying pump-probe experiment implicates a 2+1 photon control pathway, in which the optimal pulse promotes a delayed excitation to a second excited state through favorable quantum interference.

Snapshot of the equilibrium dynamics of a drug bound to HIV-1 reverse transcriptase

The anti-AIDS drug rilpivirine undergoes conformational changes to bind HIV-1 reverse transcriptase and retain potency against drug-resistance mutations. Our discovery that water molecules play an essential role in the drug binding is reported. Femtosecond experiments and theory expose molecular level dynamics of rilpivirine bound to HIV-1 reverse transcriptase. The two nitrile substituents (-CN), one on each arm of the drug, have vibrational spectra consistent with their protein environments being similar in crystals and in solutions. Two-dimensional vibrational-echo spectroscopy reveals a dry environment for one nitrile while unexpectedly the other is hydrogen-bonded to a mobile water molecule, not identified in earlier X-ray structures. Ultrafast nitrile-water dynamics are confirmed by simulations. A higher (1.51 Å) resolution X-ray structure indeed reveals a water-drug interaction network. Maintenance of a crucial anchoring hydrogen bond, despite the enlargement and structural variation of the binding pocket, may help retain the potency of rilpivirine against the pocket mutations.

Water-Induced Relaxation of a Degenerate Vibration of Guanidinium Using 2D IR Echo Spectroscopy

The nearly degenerate asymmetric stretch vibrations near 1600 cm(-1) of the guanidinium cation in D-glycerol/D(2)O mixtures having different viscosity were studied by 2D IR photon echo spectroscopy. The polarization-dependent photon echo signal shows two separate frequency distributions in the 2D spectrum in D(2)O, even though only one band is evident from inspection of the linear FTIR spectrum. The split components are more clearly seen at higher viscosity where the distortion of the molecule from 3-fold symmetry is even more evident. The interactions with solvent induce energy transfer between the degenerate component modes on the time scale of 0.5 ps. The energy transfer between modes is directly observed in 2D IR and distinguished by the waiting time dependence of the cross peaks from the transfers between configurations of the distorted ion and solvent. The 2D IR analysis carried out for various polarization conditions gave frequency-frequency auto- and cross-correlation functions for the degenerate components which derive from the solvent induced wagging of the -ND(2) groups of the guanidinium ion.

Ultrafast Vibrational Spectroscopy of a Degenerate Mode of Guanidinium Chloride

Nearly degenerate asymmetric stretches with perpendicular transition dipole moments of the deuterated guanidinium cation (DGdm(+)) in D(2)O and D-glycerol/D(2)O mixtures at 1600 cm(-1) were investigated by linear FTIR spectroscopy and polarization dependent femtosecond pump-probe spectroscopy. The vibrational coupling of the asymmetric stretches of guanidinium occurs within 0.5 ps and leads to fast decay of the anisotropy to a level of 0.1. A systematic study of the influence of the coherence transfer on pump-probe signals is given. Following this decay, the anisotropy decays with a time constant of 4.1 ps in D(2)O by rotational diffusion about an axis perpendicular to the DGdm(+) mean plane. The presence of aggregation was demonstrated for concentrations higher than 0.2 M.

Two-dimensional infrared spectral signature and hydration of the oxalate dianion.

Ultrafast vibrational spectra of the aqueous oxalate ion in the region of its carboxylate asymmetric stretch modes show novel relaxation processes. Two-dimensional infrared vibrational echo spectra and the vibrational dynamics obtained from them along with measurements of the anisotropy decay provide a picture in which the localization of the oxalate vibrational excitation onto the carboxylate groups occurs in ~450 fs. Molecular dynamics simulations are used to characterize the vibrational dynamics in terms of dihedral angle motion between the two carboxylate planes and solvation dynamics. The localization of the oxalate vibrational excitation onto the carboxylates is induced by the fluctuations in the carboxylate vibrational frequencies which are shown by theory and experiment to have a similar correlation time as the anisotropy decay.

Ultrafast relaxation and 2D IR of the aqueous trifluorocarboxylate ion

The asymmetric stretching vibration of the amphiphilic trifluoroacetate ion and its (13)C=(16)O isotopologue in D(2)O were investigated with infrared spectroscopy (FTIR), ultrafast infrared pump probe, and two dimensional vibrational photon echo techniques and simulations. Trifluoroacetate ions have a nonexponential depopulation of the first vibrational excited state, which is well described by a kinetic mechanism involving a temperature dependent solvent assisted relaxation to the symmetric stretch mode. The vibrational spectrum of the asymmetric stretch of the (13)C=(16)O isotopologue presents an unusual spectral shape. The frequency-frequency autocorrelation function shows a static term not present in the (13)C=(16)O form, which is caused by an accidental degeneracy with a combinational mode. A newly developed frequency map for carboxylate is used to characterize the processes and dynamics observed in the frequency fluctuations of the carboxylate asymmetric stretch mode in aqueous solution. An assignment of the molecular processes that govern the frequency fluctuations is suggested from an analysis of the solvation shell configurations obtained from molecular dynamics simulations.

Dynamics of Energy Transfer in a Conjugated Dendrimer Driven by Ultrafast Localization of Excitations

Solar energy conversion starts with the harvest of light, and its efficacy depends on the spatial transfer of the light energy to where it can be transduced into other forms of energy. Harnessing solar power as a clean energy source requires the continuous development of new synthetic materials that can harvest photon energy and transport it without significant losses. With chemically-controlled branched architectures, dendrimers are ideally suited for these initial steps, since they consist of arrays of chromophores with relative positioning and orientations to create energy gradients and to spatially focus excitation energies. The spatial localization of the energy delimits its efficacy and has been a point of intense research for synthetic light harvesters. We present the results of a combined theoretical experimental study elucidating ultrafast, unidirectional, electronic energy transfer on a complex molecule designed to spatially focus the initial excitation onto an energy sink. The study explores the complex interplay between atomic motions, excited-state populations, and localization/delocalization of excitations. Our findings show that the electronic energy-transfer mechanism involves the ultrafast collapse of the photoexcited wave function due to nonadiabatic electronic transitions. The localization of the wave function is driven by the efficient coupling to high-frequency vibrational modes leading to ultrafast excited-state dynamics and unidirectional efficient energy funneling. This work provides a long-awaited consistent experiment-theoretical description of excited-state dynamics in organic conjugated dendrimers with atomistic resolution, a phenomenon expected to universally appear in a variety of synthetic conjugated materials.

An Ion’s Perspective on the Molecular Motions of Nanoconfined Water: A Two-Dimensional Infrared Spectroscopy Study

The vibrational population relaxation and hydration shell dynamics of the symmetric tricyanomethanide (TCM) anion are investigated in a sodium bis(2-ethylhexyl)sulfosuccinate reverse micelle as a function of the water pool radius. Two-dimensional infrared (IR) spectroscopy in combination with linear absorption and ultrafast IR pump-probe spectroscopy is utilized in this study. Spectroscopic measurements show that the anion has two bands in the 2160-2175 cm(-1) region, each with its own spectroscopic signatures. Analysis of the vibrational dynamics shows that the two vibrational bands are consistent with the anion located either at the interface or in the water pool. The sensitivity of the TCM anion to the environment allows us to unequivocally monitor the vibrational and hydration dynamics of the anion in those two different environments. A TCM anion located at the interface does not show any significant variation of the vibrational dynamics with the water pool size. On the contrary, the TCM anion inside the water pool exhibits a large and nonlinear variation of the vibrational lifetime and the frequency-frequency correlation time with the pool radius. Moreover, for the solvated anion in water pools of 49 Å in radius (W0 = 30), the vibrational lifetime reaches the values observed for the anion in bulk water while the frequency-frequency correlation time shows a characteristic time higher than that observed in the bulk. In addition, for the first time a model is developed and used to explain the observed nonlinear variation of the spectroscopic observables with the pool size. This model attributes the changes in the vibrational dynamics of the TCM anion in the water pool to the slow and radius-dependent water dynamics present in the confined environment of a reverse micelle.

Vibrational dynamics of a non-degenerate ultrafast rotor: The (C12,C13)-oxalate ion

Molecular ions undergoing ultrafast conformational changes on the same time scale of water motions are of significant importance in condensed phase dynamics. However, the characterization of systems with fast molecular motions has proven to be both experimentally and theoretically challenging. Here, we report the vibrational dynamics of the non-degenerate (C12,C13)-oxalate anion, an ultrafast rotor, in aqueous solution. The infrared absorption spectrum of the (C12,C13)-oxalate ion in solution reveals two vibrational transitions separated by approximately 40 cm(-1) in the 1500-1600 cm(-1) region. These two transitions are assigned to vibrational modes mainly localized in each of the carboxylate asymmetric stretch of the ion. Two-dimensional infrared spectra reveal the presence and growth of cross-peaks between these two transitions which are indicative of coupling and population transfer, respectively. A characteristic time of sub-picosecond cross-peaks growth is observed. Ultrafast pump-probe anisotropy studies reveal essentially the same characteristic time for the dipole reorientation. All the experimental data are well modeled in terms of a system undergoing ultrafast population transfer between localized states. Comparison of the experimental observations with simulations reveal a reasonable agreement, although a mechanism including only the fluctuations of the coupling caused by the changes in the dihedral angle of the rotor, is not sufficient to explain the observed ultrafast population transfer.

Hydration and vibrational dynamics of betaine (N,N,N-trimethylglycine)

Zwitterions are naturally occurring molecules that have a positive and a negative charge group in its structure and are of great importance in many areas of science. Here, the vibrational and hydration dynamics of the zwitterionic system betaine (N,N,N-trimethylglycine) is reported. The linear infrared spectrum of aqueous betaine exhibits an asymmetric band in the 1550-1700 cm(-1) region of the spectrum. This band is attributed to the carboxylate asymmetric stretch of betaine. The potential of mean force computed from ab initio molecular dynamic simulations confirms that the two observed transitions of the linear spectrum are related to two different betaine conformers present in solution. A model of the experimental data using non-linear response theory agrees very well with a vibrational model comprising of two vibrational transitions. In addition, our modeling shows that spectral parameters such as the slope of the zeroth contour plot and central line slope are both sensitive to the presence of overlapping transitions. The vibrational dynamics of the system reveals an ultrafast decay of the vibrational population relaxation as well as the correlation of frequency-frequency correlation function (FFCF). A decay of ∼0.5 ps is observed for the FFCF correlation time and is attributed to the frequency fluctuations caused by the motions of water molecules in the solvation shell. The comparison of the experimental observations with simulations of the FFCF from ab initio molecular dynamics and a density functional theory frequency map shows a very good agreement corroborating the correct characterization and assignment of the derived parameters.