Impact of Vibrational Coherence on the Quantum Yield at a Conical Intersection

We study the vibrationally coherent quantum dynamics of an electronic wave packet in the vicinity of a conical intersection within a three-state two-mode model. By transforming the coherent tuning and coupling modes into the bath, the underdamped dynamics of the resulting effective three-state model is solved efficiently by the numerically exact hierarchy equation of motion approach. The transient excited-state absorption and two-dimensional spectra reveal the impact of vibrational coherence on the relaxation pathways of the wave packet. We find that both the quantum yield and the isomerization rate are crucially influenced by the vibrational coherence of the wave packet. A less coherent wave packet can traverse the conical intersection more rapidly, while the resulting quantum yield is smaller. Finally, we show that repeated passages of the wave packet through the conical intersection can lead to measurable interference effects in the form of Stueckelberg oscillations

Theoretical Study of the Photophysics of 8‑Vinylguanine, an Isomorphic Fluorescent Analogue of Guanine

Paving the way for the application of the algebraic-diagrammatic construction scheme of second-order (ADC(2)) to systems based on the guanine chromophore, we demonstrate the this excited-state electronic structure method provides a realistic description of the photochemistry of 9<i>H</i>-guanine, in close agreement with the benchmark provided by the CASPT2 method. We then proceed to apply the ADC(2) method to the photochemistry of 8-vinylguanine (8vG), a minimally modified analogue of guanine which, unlike the naturally occurring nucleobase, displays intense fluorescence, indicative of a much longer-lived excited electronic state. The emissive electronic state of 8vG is identified as an <i>ππ</i>*-type intramolecular charge transfer (ICT) state, in which a charge of roughly −0.2 <i>e</i> is transferred from the guanine moiety onto the vinyl substituent. The main radiationless deactivation pathway competing with fluorescence is predicted to involve the molecule leaving the minimum on the ICT <i>ππ</i>* state, and reaching a region of the S₁ adiabatic state where it resembles the L_a <i>ππ</i>* state of unmodified 9<i>H</i>-guanine. The topology of the L_a <i>ππ</i>* region of the S₁ state favors subsequent internal conversion at a crossing seam with the ground electronic state. The sensitivity of this process to environment polarity may explain the experimentally observed fluorescence quenching of 8vG upon incorporation in single- and double-stranded DNA

Capturing Chemistry in Action with Electrons: Realization of Atomically Resolved Reaction Dynamics

One of the grand challenges in chemistry has been to directly observe atomic motions during chemical processes. The depiction of the nuclear configurations in space-time to understand barrier crossing events has served as a unifying intellectual theme connecting the different disciplines of chemistry. This challenge has been cast as an imaging problem in which the technical issues reduce to achieving not only sufficient simultaneous space-time resolution but also brightness for sufficient image contrast to capture the atomic motions. This objective has been met with electrons as the imaging source. The review chronicles the first use of electron structural probes to study reactive intermediates, to the development of high bunch charge electron pulses with sufficient combined spatial-temporal resolution and intensity to literally light up atomic motions, as well as the means to characterize the electron pulses in terms of temporal brightness and image reconstruction. The use of femtosecond Rydberg spectroscopy as a novel means to use internal electron scattering within the molecular reference frame to obtain similar information on reaction dynamics is also discussed. The focus is on atomically resolved chemical reaction dynamics with pertinent references to work in other areas and forms of spectroscopy that provide additional information. Effectively, we can now directly observe the far-from-equilibrium atomic motions involved in barrier crossing and categorize chemistry in terms of a power spectrum of a few dominant reaction modes. It is this reduction in dimensionality that makes chemical reaction mechanisms transferrable to seemingly arbitrarily complex (large N) systems, up to molecules as large as biological macromolecules (<i>N</i> > 1000 atoms). We now have a new way to reformulate reaction mechanisms using an experimentally determined dynamic mode basis that in combination with recent theoretical advances has the potential to lead to a new conceptual basis for chemistry that forms a natural link between structure and dynamics

Nanofluidic Cells with Controlled Pathlength and Liquid Flow for Rapid, High-Resolution In Situ Imaging with Electrons

The use of electron probes for in situ imaging of solution phase systems has been a long held objective, largely driven by the prospect of atomic resolution of molecular structural dynamics relevant to chemistry and biology. Here, we present a nanofluidic sample cell with active feedback to maintain stable flow conditions for pathlengths varying from 45 nm to several 100 nm, over a useable viewing area of 50 × 50 μm. Using this concept, we demonstrate nanometer resolution for imaging weakly scattering polymer and highly scattering nanoparticles side by side with a conventional transmission microscope. The ability to flow liquids allows control over sample content and on-the-fly sample exchange, opening up the field of high-throughput electron microscopy. The nanofluidic cell design is distinguished by straightforward, reliable, operation with external liquid specimen control for imaging in (scanning) transmission mode and holds great promise for reciprocal space imaging in femtosecond electron diffraction studies of solution phase reaction dynamics

Soft Picosecond Infrared Laser Extraction of Highly Charged Proteins and Peptides from Bulk Liquid Water for Mass Spectrometry

We report the soft laser extraction and production of highly charged peptide and protein ions for mass spectrometry directly from bulk liquid water at atmospheric pressure and room temperature, using picosecond infrared laser ablation. Stable ion signal from singly charged small molecules, as well as highly charged biomolecular ions, from aqueous solutions at low laser pulse fluence (∼0.3 J cm^–2) is demonstrated. Sampling via single picosecond laser pulses is shown to extract less than 27 pL of volume from the sample, producing highly charged peptide and protein ions for mass spectrometry detection. The ablation and ion generation is demonstrated to be soft in nature, producing natively folded proteins ions under sample conditions described for native mass spectrometry. The method provides laser-based sampling flexibility, precision and control with highly charged ion production directly from water at low and near neutral pH. This approach does not require an additional ionization device or high voltage applied directly to the sample

Early Events in the Nonadiabatic Relaxation Dynamics of 4‑(<i>N</i>,<i>N</i>‑Dimethylamino)benzonitrile

4-(<i>N</i>,<i>N</i>-Dimethylamino)­benzonitrile (DMABN) is the archetypal system for dual fluorescence. Several past studies, both experimental and theoretical, have examined the mechanism of its relaxation in the gas phase following photoexcitation to the S₂ state, without converging to a single description. In this contribution, we report first-principles simulations of the early events involved in this process performed using the nonadiabatic trajectory surface hopping (TSH) approach in combination with the ADC(2) electronic structure method. ADC(2) is verified to reproduce the ground- and excited-state structures of DMABN in reasonably close agreement with previous theoretical benchmarks. The TSH simulations predict that internal conversion from the S₂ state to the S₁ takes place as early as 8.5 fs, on average, after the initial photoexcitation, and with no significant torsion of the dimethylamino group relative to the aromatic ring. As evidenced by supporting EOM-CCSD calculations, the population transfer from S₂ to S₁ can be attributed to the skeletal deformation modes of the aromatic ring and the stretching of the ring-dimethylamino nitrogen bond. The non- or slightly twisted locally excited structure is the predominant product of the internal conversion, and the twisted intramolecular charge transfer structure is formed through equilibration with the locally excited structure with no change of adiabatic state. These findings point toward a new interpretation of data from previous time-resolved experiments

Direct Observation of Ultrafast Exciton Dissociation in Lead Iodide Perovskite by 2D Electronic Spectroscopy

The unprecedented success of hybrid organic–inorganic lead halide perovskites in photovoltaics motivates fundamental research to unravel the underlying microscopic mechanism for photoinduced charge generation. Recent studies suggest that most photoexcitations in perovskites are free charge carriers, although the contribution of the electron–hole pairs (i.e., excitons) at room temperature has been a matter of debate. We have employed ultrafast two-dimensional (2D) electronic spectroscopy to directly probe the elementary optical excitation of CH₃NH₃PbI₃ thin films with ∼16 fs temporal resolution. We distinctly capture the ultrafast dissociation of excitons to the charge carriers at room temperature and at 180 K. Interestingly, we also observe that the coherent oscillations of the off-diagonal signals in the 2D electronic spectra live for ∼50 fs at room temperature. The entropy-driven dissociation of excitons to charge carriers happens within the electronic dephasing time scale and is favored by the low exciton binding energy, which we determine to be ∼12 meV at room temperature. This ultrafast dissociation of excitons to charge carriers can be one of the important contributions to the high efficiency of perovskite-based photovoltaics

Visualization of Multimerization and Self-Assembly of DNA-Functionalized Gold Nanoparticles Using In-Liquid Transmission Electron Microscopy

Base-pairing stability in DNA-gold nanoparticle (DNA-AuNP) multimers along with their dynamics under different electron beam intensities was investigated with in-liquid transmission electron microscopy (in-liquid TEM). Multimer formation was triggered by hybridization of DNA oligonucleotides to another DNA strand (Hyb-DNA) related to the concept of DNA origami. We analyzed the degree of multimer formation for a number of samples and a series of control samples to determine the specificity of the multimerization during the TEM imaging. DNA-AuNPs with Hyb-DNA showed an interactive motion and assembly into 1D structures once the electron beam intensity exceeds a threshold value. This behavior was in contrast with control studies with noncomplementary DNA linkers where statistically significantly reduced multimerization was observed and for suspensions of citrate-stabilized AuNPs without DNA, where we did not observe any significant motion or aggregation. These findings indicate that DNA base-pairing interactions are the driving force for multimerization and suggest a high stability of the DNA base pairing even under electron exposure

Ring-Closing Reaction in Diarylethene Captured by Femtosecond Electron Crystallography

The photoinduced ring-closing reaction in diarylethene, which serves as a model system for understanding reactive crossings through conical intersections, was directly observed with atomic resolution using femtosecond electron diffraction. Complementary ab initio calculations were also performed. Immediately following photoexcitation, subpicosecond structural changes associated with the formation of an open-ring excited-state intermediate were resolved. The key motion is the rotation of the thiophene rings, which significantly decreases the distance between the reactive carbon atoms prior to ring closing. Subsequently, on the few picosecond time scale, localized torsional motions of the carbon atoms lead to the formation of the closed-ring photoproduct. These direct observations of the molecular motions driving an organic chemical reaction were only made possible through the development of an ultrabright electron source to capture the atomic motions within the limited number of sampling frames and the low data acquisition rate dictated by the intrinsically poor thermal conductivity and limited photoreversibility of organic materials

Ambient Mass Spectrometry Imaging with Picosecond Infrared Laser Ablation Electrospray Ionization (PIR-LAESI)

A picosecond infrared laser (PIRL) is capable of cutting through biological tissues in the absence of significant thermal damage. As such, PIRL is a standalone surgical scalpel with the added bonus of minimal postoperative scar tissue formation. In this work, a tandem of PIRL ablation with electrospray ionization (PIR-LAESI) mass spectrometry is demonstrated and characterized for tissue molecular imaging, with a limit of detection in the range of 100 nM for reserpine or better than 5 nM for verapamil in aqueous solution. We characterized PIRL crater size using agar films containing Rhodamine. PIR-LAESI offers a 20–30 μm vertical resolution (∼3 μm removal per pulse) and a lateral resolution of ∼100 μm. We were able to detect 25 fmol of Rhodamine in agar ablation experiments. PIR-LAESI was used to map the distribution of endogenous methoxykaempferol glucoronide in zebra plant (<i>Aphelandra squarrosa</i>) leaves producing a localization map that is corroborated by the literature. PIR-LAESI was further used to image the distribution inside mouse kidneys of gadoteridol, an exogenous magnetic resonance contrast agent intravenously injected. Parallel mass spectrometry imaging (MSI) using desorption electrospray ionization (DESI) and matrix assisted laser desorption ionization (MALDI) were performed to corroborate PIR-LAESI images of the exogenous agent. We further show that PIR-LAESI is capable of desorption ionization of proteins as well as phospholipids. This comparative study illustrates that PIR-LAESI is an ion source for ambient mass spectrometry applications. As such, a future PIRL scalpel combined with secondary ionization such as ESI and mass spectrometry has the potential to provide molecular feedback to guide PIRL surgery