1,720,986 research outputs found
Understanding Charge Dynamics in Dense Electronic Manifolds in Complex Environments
No full text[Image: see text] Photoinduced charge transfer (CT) excited states and their relaxation mechanisms can be highly interdependent on the environment effects and the consequent changes in the electronic density. Providing a molecular interpretation of the ultrafast (subpicosecond) interplay between initial photoexcited states in such dense electronic manifolds in condensed phase is crucial for improving and understanding such phenomena. Real-time time-dependent density functional theory is here the method of choice to observe the charge density, explicitly propagated in an ultrafast time domain, along with all time-dependent properties that can be easily extracted from it. A designed protocol of analysis for real-time electronic dynamics to be applied to time evolving electronic density related properties to characterize both in time and in space CT dynamics in complex systems is here introduced and validated, proposing easy to be read cross-correlation maps. As case studies to test such tools, we present the photoinduced charge-transfer electronic dynamics of 5-benzyluracil, a mimic of nucleic acid/protein interactions, and the metal-to-ligand charge-transfer electronic dynamics in water solution of [Ru(dcbpy)(2)(NCS)(2)](4–), dcbpy = (4,4′-dicarboxy-2,2′-bipyridine), or “N3(4–)”, a dye sensitizer for solar cells. Electrostatic and explicit ab initio treatment of solvent molecules have been compared in the latter case, revealing the importance of the accurate modeling of mutual solute–solvent polarization on CT kinetics. We observed that explicit quantum mechanical treatment of solvent slowed down the charge carriers mobilities with respect to the gas-phase. When all water molecules were modeled instead as simpler embedded point charges, the electronic dynamics appeared enhanced, with a reduced hole–electron distance and higher mean velocities due to the close fixed charges and an artificially increased polarization effect. Such analysis tools and the presented case studies can help to unveil the influence of the electronic manifold, as well as of the finite temperature-induced structural distortions and the environment on the ultrafast charge motions
From Charge-Transfer to Charge-Separated State: A Perspective from the Real-Time TDDFT Excitonic Dynamics
No full textTetracyanoethylene / radical anion coordination sites unveiled via electronic and vibrational fingerprints
No full textTetracyanoethylene (TCNE) is a very effective electron acceptor and can form stable complexes with various chemical species and is known for its interesting electronic and optical properties. The formation of the monoanion gives TCNE unique electronic and chemical properties, making it a material of interest for several technological applications including solar cells, organic battery electrodes, sodium/potassium-ion batteries and information storage devices. The chemico-physical characterization of Na+ and K+ coordination of TCNE in solution is not available so far, although it would be valuable for understanding the molecular mechanisms of charging and discharging processes in innovative battery devices. The present theoretical-computational study characterizes the vibrational and electronic properties of ion-coordinated TCNE in solution. We analyzed the ground electronic state through equilibrium structures, focusing on structural features, stability, optical properties and Raman vibrational modes. The Na+ and K+ cations, coordinated at two distinct sites, significantly alter the geometry, and modulate electronic and vibrational properties. Using ab initio molecular dynamics simulations, we computed vibrational spectra, finding that TCNE complexes serve as effective probes for detecting environmental variations, with key vibrational modes on CC and CN moieties being sensitive to cation interactions. Additionally, a correlation between C=C bond distances and vibrational frequencies was observed. In conclusion, this study provides further insight into the role of TCNE in optoelectronic applications and nonaqueous battery systems
Tuning ultrafast time-evolution of photo-induced charge-transfer states: A real-time electronic dynamics study in substituted indenotetracene derivatives
Get PDFPhoto-induced charge transfer (CT) states are pivotal in many technological and biological processes. A deeper knowledge of such states is mandatory for modeling the charge migration dynamics. Real-time time-dependent density functional theory (RT-TD-DFT) electronic dynamics simulations are employed to explicitly observe the electronic density time-evolution upon photo-excitation. Asymmetrically substituted indenotetracene molecules, given their potential application as n-type semiconductors in organic photovoltaic materials, are here investigated. Effects of substituents with different electron-donating characters are analyzed in terms of the overall electronic energy spacing and resulting ultrafast CT dynamics through linear response (LR-)TD-DFT and RT-TD-DFT based approaches. The combination of the computational techniques here employed provided direct access to the electronic density reorganization in time and to its spatial and rational representation in terms of molecular orbital occupation time evolution. Such results can be exploited to design peculiar directional charge dynamics, crucial when photoactive materials are used for light-harvesting applications
Understanding THz and IR signals beneath time-resolved fluorescence from excited state ab-initio dynamics
No full textThe detailed interpretation of time-resolved spectroscopic signals in terms of the molecular rearrangement during a photoreaction or a photophysical event is one of the most important challenges of both experimental and theoretical chemistry. Here we simulate a time resolved fluorescence spectrum of a dye in aqueous solution, the N-methyl-6-oxyquinolinium betaine, and analyze it in terms of far IR and THz frequency contributions, providing a direct connection to specific molecular motions. To obtain this result, we build up an innovative and general approach based on excited state ab-initio molecular dynamics and a wavelet-based time dependent frequency analysis of non-stationary signals. We obtain a nice agreement with key parameters of the solvent dynamics, such as the total Stokes shift and the Stokes shift relaxation times. As important finding, we observe a strong change of specific solute-solvent interactions upon the electronic excitation, with the migration of about 1.5 water molecules from the first solvation shell toward the bulk. In spite of this event, the Stokes shift dynamics is ruled by collective solvent motions in the THz and far IR, which guide and modulate the strong rearrangement of the dye microsolvation. By the relaxation of THz and IR contributions to the emission signal, we can follow and understand in detail the molecularity of the process. The protocol presented here is in principle transferable to other time-resolved spectroscopic techniques
Evidence of Excited-State Vibrational Mode Governing the Photorelaxation of a Charge-Transfer Complex
Get PDF: Modern, nonlinear, time-resolved spectroscopic techniques have opened new doors for investigating the intriguing but complex world of photoinduced ultrafast out-of-equilibrium phenomena and charge dynamics. The interaction between light and matter introduces an additional dimension, where the complex interplay between electronic and vibrational dynamics needs the most advanced theoretical-computational protocols to be fully understood on the molecular scale. In this study, we showcase the capabilities of ab initio molecular dynamics simulation integrated with a multiresolution wavelet protocol to carefully investigate the excited-state relaxation dynamics in a noncovalent complex involving tetramethylbenzene (TMB) and tetracyanoquinodimethane (TCNQ) undergoing charge transfer (CT) upon photoexcitation. Our protocol provides an accurate description that facilitates a direct comparison between transient vibrational analysis and time-resolved spectroscopic signals. This molecular level perspective enhances our understanding of photorelaxation processes confined in the adiabatic regime and offers an improved interpretation of vibrational spectra. Furthermore, it enables the quantification of anharmonic vibrational couplings between high- and low-frequency modes, specifically the TCNQ "rocking" and "bending" modes. Additionally, it identifies the primary vibrational mode that governs the adiabaticity between the ground state and the CT state. This comprehensive understanding of photorelaxation processes holds significant importance in the rational design and precise control of more efficient photovoltaic and sensor devices
Watching the Interplay between Photoinduced Ultrafast Charge Dynamics and Nuclear Vibrations
No full text: Here is presented the ultrafast hole-electron dynamics of photoinduced metal to ligand charge-transfer (MLCT) states in a Ru(II) complex, [Ru(dcbpy)2(NCS)2]4- (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine), a photoactive molecule employed in dye sensitized solar cells. Via cutting-edge computational techniques, a tailored computational protocol is here presented and developed to provide a detailed analysis of the electronic manifold coupled with nuclear vibrations to better understand the nonradiative pathways and the resulting overall dye performances in light-harvesting processes (electron injection). Thus, the effects of different vibrational modes were investigated on both the electronic levels and charge transfer dynamics through a theoretical-computational approach. First, the linear response time-dependent density functional (LR-TDDFT) formalism was employed to characterize excitation energies and spacing among electronic levels (the electronic layouts). Then, to understand the ultrafast (femtosecond) charge dynamics on the molecular scale, we relied on the nonperturbative mean-field quantum electronic dynamics via real-time (RT-) TDDFT. Three vibrational modes were selected, representative for collective nuclear movements that can have a significant influence on the electronic structure: two involving NCS- ligands and one involving dcbpy ligands. As main results, we observed that such MLCT states, under vibrational distortions, are strongly affected and a faster interligand electron transfer mechanism is observed along with an increasing MLCT character of the adiabatic electronic states approaching closer in energy due to the vibrations. Such findings can help both in providing a molecular picture of multidimensional vibro-electronic spectroscopic techniques, used to characterize ultrafast coherent and noncoherent dynamics of complex systems, and to improve dye performances with particular attention to the study of energy or charge transport processes and vibronic couplings
On the optical absorption of the anionic GFP chromophore in vacuum, solution, and protein
No full textIn spite of the large number of experimental and theoretical studies, the optical absorption trend of the green fluorescent protein (GFP) chromophore in several environments has not been fully understood. We calculated at the same level of time dependent density functional theory the vertical excitation energy of the anionic GFP chromophore in the protein and in ethanol, dioxane, methanol and water solutions. As result, we reproduced for the first time the experimental trend of the absorption peaks with 0.015 eV as the standard deviation of the accuracy. This systematic error allowed us to analyze with confidence the relative weight of several solvation effects on the vertical excitation energy. Experimental trends not correlated with the solvent polarity were therefore explained with a fine balance of different steric and electronic effects on the photophysics of the chromophore. As an indirect and remarkable result, the present analysis confirms that the optical absorption of the chromophore in the gas phase is close to the value of 2.84 eV extrapolated by Dong et al. (J. Am. Chem. Soc., 2006, 128, 12038), and, as a consequence, that the protein environment induces a red shift of 0.23 eV
Vibrational analysis of trans N-methyl-acetamide in aqueous solution from ab-initio molecular dynamics
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