Excited-state Energy Surfaces Research Articles

Fast and accurate excited states predictions: machine learning and diabatization.

The efficiency of machine learning algorithms for electronically excited states is far behind ground-state applications. One of the underlying problems is the insufficient smoothness of the fitted potential energy surfaces and other properties in the vicinity of state crossings and conical intersections, which is a prerequisite for an efficient regression. Smooth surfaces can be obtained by switching to the diabatic basis. However, diabatization itself is still an outstanding problem. We overcome these limitations by solving both problems at once. We use a machine learning approach combining clustering and regression techniques to correct for the deficiencies of property-based diabatization which, in return, provides us with smooth surfaces that can be easily fitted. Our approach extends the applicability of property-based diabatization to multidimensional systems. We utilize the proposed diabatization scheme to achieve higher prediction accuracy for adiabatic states and we show its performance by reconstructing global potential energy surfaces of excited states of nitrosyl fluoride and formaldehyde. While the proposed methodology is independent of the specific property-based diabatization and regression algorithm, we show its performance for kernel ridge regression and a very simple diabatization based on transition multipoles. Compared to most other algorithms based on machine learning, our approach needs only a small amount of training data.

Prototropically Controlled Dynamics of Cytosine Photodecay

The effect of the existence of several prototropic tautomers of cytosine on its UV/vis spectra and the excited state decay dynamics is studied by spectral and nonadiabatic molecular dynamics (NAMD) simulations in connection with the mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) method. Simulated UV/vis spectra provide a strong indication that the H3N keto-amino cytosine tautomer (the least anticipated species) may be present under experimental conditions. The NAMD simulations yield a wide range of excited state decay constants for various tautomers of cytosine, ranging from ∼1.3 ps for the biologically relevant H1N keto-amino tautomer to ∼0.1 ps for the keto-imino tautomer. The slowness of the H1N decay dynamics follows from the presence of a barrier on the excited state energy surface separating the Franck-Condon structure from the major decay funnel, the conical intersection seam. It is suggested that the experimentally observed photodecay dynamics may result from a combination of the decay processes of various tautomers (H3N in particular) present simultaneously under the experimental conditions.

Molecular Basis for the Difference in Singlet Oxygen Quantum Yield Between the First Genetically Encoded Photosensitizer, KillerRed, and its Monomeric Counterpart, SuperNova

AbstractThe application of protein‐based photosensitizers in diverse fields has been proposed due to their characteristic to generate reactive oxygen species (ROS) upon light irradiation. However, the first of its kind, KillerRed, has a limitation in its use because of its dimeric nature, so its monomeric counterpart, SuperNova, was developed subsequently to overcome this limitation. What is very intriguing is that, although KillerRed and SuperNova share highly similar structural and optical properties, the singlet oxygen quantum yield of SuperNova is shown to be almost three times larger than that of KillerRed, but the mechanistic details for this difference have not been addressed yet. In the present work, a hybrid quantum mechanics / molecular mechanics (QM/MM) approach with time dependent density functional theory (TDDFT) is employed to examine this issue. Excited state energy surfaces leading to intersystem crossing (ISC) for both KillerRed and SuperNova are explored, and the comparison of those results and additional electrostatic potential (ESP) calculations reveal that the lower singlet oxygen quantum yield of KillerRed is attributed to a high energy 3nπ* minimum due to a hydrogen bond between the chromophore and neighboring Asn145.

A computational scheme for evaluating the phosphorescence quantum efficiency: applied to blue-emitting tetradentate Pt(II) complexes.

Phosphorescent organic light-emitting diodes (PhOLEDs) are leading candidates for displays or lighting technologies. Recently, blue phosphorescent tetradentate Pt(II) complexes have been attracting extensive attention due to their high phosphorescence quantum efficiency and numerous chemical structures on account of flexible ligand frames and modifications. Using quantum chemistry coupled with our thermal vibration correlation function (TVCF) formalism, we investigated the triplet excited state energy surface and the decay processes involving both direct vibrational relaxation and minimum energy crossing point (MECP) via the transition state (3TS) to the ground state (S0) for 16 recently experimentally reported blue-emitting tetradentate Pt(II) emitters containing fused 5/6/6 metallocycles. We found that (i) in most cases, the direct vibrational relaxation deactivations dominated the triplet non-radiative decay because either the 3TS is too high or the MECP is not reachable. Hence, results from the TVCF formalism agreed well with the experiments for the phosphorescence quantum efficiency; (ii) only when both 3TS and MECP are low, for instance, for PtON1-oMe, deactivations via MECP dominated the triplet non-radiative decay.

Excited-State Energy Surfaces in Molecules Revealed by Impulsive Stimulated Raman Excitation Profiles.

Photophysical and photochemical processes are ruled by the interplay between transient vibrational and electronic degrees of freedom, which are ultimately determined by the multidimensional potential energy surfaces (PESs). Differences between ground and excited PESs are encoded in the relative intensities of resonant Raman bands, but they are experimentally challenging to access, requiring measurements at multiple wavelengths under identical conditions. Herein, we perform a two-color impulsive vibrational scattering experiment to launch nuclear wavepacket motions by an impulsive pump and record their coupling with a targeted excited-state potential by resonant Raman processes with a delayed probe, generating in a single measurement background-free vibrational spectra across the entire sample absorption. Building on the interference between the multiple pathways resonant with the excited-state manifold that generate the Raman signal, we show how to experimentally tune their relative phase by varying the probe chirp, decoding nuclear displacements along different normal modes and revealing the multidimensional PESs. Our results are validated against time-dependent density functional theory.

Quantum-Classical Simulation of Molecular Motors Driven Only by Light.

Molecular motors that exhibit controlled unidirectional rotation provide great prospects for many types of applications, including nanorobotics. Existing rotational motors have two key components: photoisomerization around a π-bond followed by a thermally activated helical inversion, the latter being the rate-determining step. We propose an alternative molecular system in which the rotation is caused by the electric coupling of chromophores. This is used to engineer the excited state energy surface and achieve unidirectional rotation using light as the only input and avoid the slow thermally activated step, potentially leading to much faster operational speeds. To test the working principle, we employ quantum-classical calculations to study the dynamics of such a system. We estimate that motors built on this principle should be able to work on a subnanosecond time scale for such a full rotation. We explore the parameter space of our model to guide the design of a molecule that can act as such a motor.

Theoretical characterization of the photochemical reaction CO2 + O(3P) → CO + O2 related to experiments in solid krypton

Formation and decomposition of the complex of carbon dioxide and atomic oxygen are characterized by quantum chemistry methods aiming to rationalize experimental studies in solid krypton. The observed FTIR spectra reflected the temporal evolution of the system after irradiation showing the bands of reactants, intermediates and products. Advanced quantum chemistry calculations show that the T-shape complex CO2…O(3P) can be formed in the matrix. Its excitation by the 193 nm light results in the charge-transfer state CO2+…O−, which evolves to the reaction intermediate CO3. The latter species decomposes to CO + O2 following pathways on the excited state energy surfaces.

Optimized effective potentials to increase the accuracy of approximate proton transfer energy calculations in the excited state

Many fundamental chemical reactions are triggered by electronic excitations. Here, we propose and benchmark a novel approximate first-principles molecular dynamics simulation idea for increasing the computational efficiency of density functional theory-based calculations of the excited states. We focus on obtaining proton transfer energy at the S1 excited state through actual density functional theory calculations at the T1 state with additional optimized effective potentials. The potentials are optimized as such to reproduce the excited-state energy surface obtained using time-dependent density functional theory, but can be generalized to other more accurate quantum chemical methods. We believe that the presented method is not only suitable for studies on excited-state proton transfer and ion mobility in general systems but can also be extended to investigate more involved processes, such as photo-induced isomerization.

Influence of Triplet Surface Properties on Excited-State Deactivation of Expanded Cage Bis(tridentate)Ruthenium(II) Complexes.

Calculations of excited-state potential energy surfaces (PESs) are useful to predict key properties relating to the deactivation cascade of transition-metal complexes. Here, we first perform full free optimizations of the relevant excited-state minima, followed by extensive two-dimensional PES calculations based on the minima of interest. Maps of the lowest triplet excited-state surfaces of two bistridentate RuII-complexes, [Ru(DQP)2]2+ and [Ru(DQzP)2]2+, are used to explain recent experimental findings including an unexpected order of magnitude difference in an excited-state lifetime. The calculations reveal significant differences in the fundamental shapes and spin transitions of the lowest triplet excited-state energy surfaces of the two complexes and, in particular, show that the metal-to-ligand charge transfer (MLCT) surface region of [Ru(DQzP)2]2+ with a shorter excited-state lifetime is significantly smaller than that of [Ru(DQP)2]2+. This leads to a minimum energy crossing between the triplet and singlet surfaces near the MLCT for [Ru(DQzP)2]2+ or near the MC for [Ru(DQP)2]2+. These results indicate that the experimentally observed difference in the excited-state lifetime is closely related to the set of energetically accessible 3MLCT conformations.

Observation of Vibronic-Coupling-Mediated Energy Transfer in Light-Harvesting Nanotubes Stabilized in a Solid-State Matrix.

Ultrafast vibrational spectroscopy is employed to obtain real-time structural information on energy transport in double-walled light-harvesting nanotubes at room temperature, stabilized in a host matrix to mimic the rigid scaffolds of natural light-harvesting systems. We observe evidence of a low-frequency vibrational mode at 315 cm-1, which transfers excitons from the outer wall of the nanotubes to a crossing point through which energy transfer to the inner wall can occur. This mode is furthermore absent in solution phase. Importantly, the coherence of this mode is not transferred to the inner wall upon energy transfer and is only present on the outer wall's excited-state energy surface, highlighting that complete energy transfer between the outer and inner walls does not take place. Isolation of the individual walls of the nanotubes provides evidence that this mode corresponds to a supramolecular motion of the nanotubes. Our results emphasize the importance of the solid-state environment in modulating vibronic coupling and directing energy transfer in molecular light-harvesting systems.

Mechanism of Charge Separation and Frontier Orbital Structure in Graphitic Carbon Nitride and Graphene Quantum Dots.

Graphene quantum dots (GQDs) and carbon nitride quantum dots (CNQDs), the latest addition to the carbon material family, are promising materials for numerous novel applications in optical sensing, photocatalysis, biosensing, and photovoltaics. However, understanding the photocatalytic capability of CNQDs compared to GQDs requires investigations of the charge behavior on the excited state energy surface. In this work, through time-dependent density functional tight binding (TD-DFTB) calculations, we show that CNQDs exhibit superior ground state frontier orbitals (FOs) localization. Strong localization of the FOs and excited state charge separation observed in the first excited state are caused by the relaxation of the structure. Excited energy surface investigations reveal spatial confinement of FOs to the stretched C-N bonds due to excited state structural relaxation. On the other hand, no such localized FOs structure was found for GQDs, presumably caused by its strong π-conjugated configuration not allowing large structural changes upon excitation. The optical absorption and emission of CNQDs is sensitive to size and does not show large variations with the shape of the QD. Our approach provides an explanation for the origin of the enhanced photocatalytic performance of CNQDs over GQDs and their characteristic FOs localization.

A quantitative analysis of light-driven charge transfer processes using voronoi partitioning of time dependent DFT-derived electron densities.

An analytical method is presented that provides quantitative insight into light‐driven electron density rearrangement using the output of standard time‐dependent density functional theory (TD‐DFT) computations on molecular compounds. Using final and initial electron densities for photochemical processes, the subtraction of summed electron density in each atom‐centered Voronoi polyhedron yields the electronic charge difference, Q VECD. This subtractive method can also be used with Bader, Mulliken and Hirshfeld charges. A validation study shows Q VECD to have the most consistent performance across basis sets and good conservation of charge between electronic states. Besides vertical transitions, relaxation processes can be investigated as well. Significant electron transfer is computed for isomerization on the excited state energy surface of azobenzene. A number of linear anilinepyridinium donor‐bridge‐acceptor chromophores was examined using Q VECD to unravel the influence of its pi‐conjugated bridge on charge separation. Finally, the usefulness of the presented method as a tool in optimizing charge transfer is shown for a homologous series of organometallic pigments. The presented work allows facile calculation of a novel, relevant quantity describing charge transfer processes at the atomic level. © 2017 The Authors Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

Vibrational spectroscopy of the double complex salt Pd(NH3)4(ReO4)2, a bimetallic catalyst precursor

Tetraamminepalladium(II) perrhenate, a double complex salt, has significant utility in PdRe catalyst preparation; however, the vibrational spectra of this readily prepared compound have not been described in the literature. Herein, we present the infrared (IR) and Raman spectra of tetraamminepalladium(II) perrhenate and several related compounds. The experimental spectra are complemented by an analysis of normal vibrational modes that compares the experimentally obtained spectra with spectra calculated using DFT (DMol3). The spectra are dominated by features due to the ammine groups and the ReO stretch in Td ReO4−; lattice vibrations due to the D4h Pd(NH3)42+ are also observed in the Raman spectrum. Generally, we observe good agreement between ab initio calculations and experimental spectra. The calculated IR spectrum closely matches experimental results for peak positions and their relative intensities. The methods for calculating resonance Raman intensities are implemented using the time correlator formalism using two methods to obtain the excited state displacements and electron-vibration coupling constants, which are the needed inputs in addition to the normal mode wave numbers. Calculated excited state energy surfaces of Raman-active modes correctly predict relative intensities of the peaks and Franck-Condon activity; however, the position of Raman bands are predicted at lower frequencies than observed. Factor group splitting of Raman peaks observed in spectra of pure compounds is not predicted by DFT.

Excitation energies from time-dependent generalized valence bond method

A generalized valence bond perfect-pairing (GVB-PP) wavefunction has been extensively used in computational chemistry methods due to its multiconfigurational character, which captures in an inexpensive way static electron correlation. GVB-PP has been mostly applied to ground states and not much is known about its performance in predicting electronic spectra of molecules. Here, we present the formalism based on the time-dependent linear response theory which provides excitation energies from the GVB-PP ground-state wavefunction. The accuracy of the excitation energy dissociation curves parallels that of the TD-HF method around ground-state equilibrium geometries of the investigated molecules. For stretched-bond molecules, when TD-HF breaks down, the proposed TD-GVB method remains reliable. TD-GVB may therefore serve as a useful and inexpensive tool for exploring potential energy surfaces of excited states of molecules.

Proton Transfer Dynamics of 4'-N,N-Dimethylamino-3-hydroxyflavone Observed in Hydrogen-Bonding Solvents and Aqueous Micelles.

Photophysical studies on the 4'-N,N-dimethylamino-3-hydroxyflavone fluorophore were performed in hydrogen-bonding solvents. Both in hydrogen-bonding acids and bases, clear evidence of excited state intramolecular proton transfer (ESIPT) emerged from steady-state and time-resolved spectroscopies. The same was also observed for the fluorophores residing in the hydrophilic shell region of aqueous micelles, where they come into close contact with water molecules at the micelle-water interface. Slow ∼100 ps ESIPT time-constants were determined in these systems that correlated well with solvation dynamics. The slow ESIPT time-constants are attributed to activated barrier crossing from the solvent-relaxed enol form to tautomer form in the excited state energy surface of the flavone. In contrast to the barrier-less ESIPT occurring in early (<1 ps) time-scales, this activated proton-transfer event necessarily requires extensive reorganization of flavone···solvent intermolecular hydrogen bonds, a process heavily modulated by the relatively slower dynamics of solvent relaxation around the excited fluorophore.

Linear energy relationships in ground state proton transfer and excited state proton-coupled electron transfer.

Proton-coupled electron transfer (PCET) processes are elementary chemical reactions involved in a broad range of radical and redox reactions. Elucidating fundamental PCET reaction mechanisms are thus of central importance for chemical and biochemical research. Here we use quantum chemical density functional theory (DFT), time-dependent density functional theory (TDDFT), and the algebraic diagrammatic-construction through second-order (ADC(2)) to study the mechanism, thermodynamic driving force effects, and reaction barriers of both ground state proton transfer (pT) and photoinduced proton-coupled electron transfer (PCET) between nitrosylated phenyl-phenol compounds and hydrogen-bonded t-butylamine as an external base. We show that the obtained reaction barriers for the ground state pT reactions depend linearly on the thermodynamic driving force, with a Brønsted slope of 1 or 0. Photoexcitation leads to a PCET reaction, for which we find that the excited state reaction barrier depends on the thermodynamic driving force with a Brønsted slope of 1/2. To support the mechanistic picture arising from the static potential energy surfaces, we perform additional molecular dynamics simulations on the excited state energy surface, in which we observe a spontaneous PCET between the donor and the acceptor groups. Our findings suggest that a Brønsted analysis may distinguish the ground state pT and excited state PCET processes.

Excited state localisation cascades in inorganic semiconductor nanoparticles

Excited state relaxation in zinc sulfide (ZnS) nanoparticles is studied as a model for the fate of the excited state in inorganic nanoparticles in general. A series of time-dependent density functional theory optimisations on the S1 and T1 excited states predict the existence of not merely isolated minima, as found before, but rather a connected cascade of excited state minima ending up in a conical intersection between the excited state energy surface and the ground state. The localisation of the excited state in the different minima increases down the cascade, while the barriers separating these minima, studied here for the first time for nanoparticles, are predicted to be in some cases electronic (strongly avoided crossing) in origin. The cartoon picture of excited state relaxation in inorganic nanoparticles that involves relaxation to the bottom of only one approximately harmonic well followed by photoluminescence appears for the ZnS nanoparticles studied here to be at best rather simplistic. The localisation cascade is finally found to strongly affect the excited state properties of nanoparticles and predicted to lead to the formation of defected nanoparticles after de-excitation in selected cases.

Spectral tuning in photoactive yellow protein by modulation of the shape of the excited state energy surface

Protein-chromophore interactions in photoreceptors often shift the chromophore absorbance maximum to a biologically relevant spectral region. A fundamental question regarding such spectral tuning effects is how the electronic ground state S(0) and excited state S(1) are modified by the protein. It is widely assumed that changes in energy gap between S(0) and S(1) are the main factor in biological spectral tuning. We report a generally applicable approach to determine if a specific residue modulates the energy gap, or if it alters the equilibrium nuclear geometry or width of the energy surfaces. This approach uses the effects that changes in these three parameters have on the absorbance and fluorescence emission spectra of mutants. We apply this strategy to a set of mutants of photoactive yellow protein (PYP) containing all 20 side chains at active site residue 46. While the mutants exhibit significant variation in both the position and width of their absorbance spectra, the fluorescence emission spectra are largely unchanged. This provides strong evidence against a major role for changes in energy gap in the spectral tuning of these mutants and reveals a change in the width of the S(1) energy surface. We determined the excited state lifetime of selected mutants and the observed correlation between the fluorescence quantum yield and lifetime shows that the fluorescence spectra are representative of the energy surfaces of the mutants. These results reveal that residue 46 tunes the absorbance spectrum of PYP largely by modulating the width of the S(1) energy surface.

Theoretical Study of Structure and Electronic Absorption Spectra of Some Schiff Bases and Their Zinc Complexes

Tetradentate Schiff bases (H(2)L(i)), derivatives of salicylic aldehyde (H(2)L(1), H(2)L(2)) or o-vanillin (H(2)L(3), H(2)L(4)) with ethylenediamine or o-phenylenediamine as a bridge, and their zinc complexes were studied experimentally and theoretically in view of their possible application as emitters in organic light emitting diodes (OLEDs). The composition of thin films of the complexes was analyzed using a combination of different experimental and molecular modeling techniques taking into account changes in the Gibbs free energy of dehydration and dimerization reactions. The absorption spectra of the initial Schiff bases were investigated in methanol solutions, while the absorption spectra of their zinc complexes were investigated in thin films. Experimental results of elemental analysis, IR spectroscopy, laser desorption/ionization mass spectrometry (LDI MS), and X-ray diffraction as well as theoretical analysis of electronic absorption spectra by the quantum-chemical TD DFT method demonstrate that thin films of the zinc complexes contain binuclear anhydrous molecules. This conclusion should be taken into account when considering both transport and luminescence properties of these complexes in OLED heterostructures. A comparison of the results of CIS, TD DFT/PBE, and TD DFT/PBE0 calculations reveals the crucial importance of the inclusion of the exact exchange in the E(XC) functional for the further correct description of potential energy surfaces of excited states for the systems studied.

A program for the calculation of ground state and excited state energy surfaces

A program for the calculation of ground state and excited state energy surfaces