Excited-state Phenomena Research Articles

Visualizing and Quantifying Interactions in the Excited State.

Determining the location and nature of the electron pairs within a molecule provides an intuitive representation of electronic structures. Yet, most of the available theoretical representations are not suitable for describing excited state phenomena. The density overlap region indicator (DORI) scalar field, which depends only on the density and its derivatives, overcomes previous limitations, while keeping the intuitiveness of popular scalar fields. Here, its usefulness is demonstrated by pinpointing visual and numerical DORI signatures for both intra- and intermolecular excited state situations.

Multidisciplinary Science at the Molecular Foundry

Multidisciplinary Science at the Molecular Foundry

Mechanistic studies of photoinduced spin crossover and electron transfer in inorganic complexes.

Electronic excited-state phenomena provide a compelling intersection of fundamental and applied research interests in the chemical sciences. This holds true for coordination chemistry, where harnessing the strong optical absorption and photocatalytic activity of compounds depends on our ability to control fundamental physical and chemical phenomena associated with the nonadiabatic dynamics of electronic excited states. The central events of excited-state chemistry can critically influence the dynamics of electronic excited states, including internal conversion (transitions between distinct electronic states) and intersystem crossing (transitions between electronic states with different spin multiplicities), events governed by nonadiabatic interactions between electronic states in close proximity to conical intersections, as well as solvation and electron transfer. The diversity of electronic and nuclear dynamics also makes the robust interpretation of experimental measurements challenging. Developments in theory, simulation, and experiment can all help address the interpretation and understanding of chemical dynamics in organometallic and coordination chemistry. Synthesis presents the opportunity to chemically engineer the strength and symmetry of the metal-ligand interactions. This chemical control can be exploited to understand the influence of electronic ground state properties on electronic excited-state dynamics. New time-resolved experimental methods and the insightful exploitation of established methods have an important role in understanding, and ideally controlling, the photophysics and photochemistry of transition metal complexes. Techniques that can disentangle the coupled motion of electrons and nuclear dynamics warrant emphasis. We present a review of electron localization dynamics in charge transfer excited states and the dynamics of photoinitiated spin crossover dynamics. Both electron localization and spin crossover have been investigated by numerous research groups with femtosecond resolution spectroscopy, but challenges in experimental interpretation have left significant uncertainty about the molecular properties that control these phenomena. Our Account will emphasize how tailoring the experimental probe, femtosecond resolution vibrational anisotropy for electron localization, and femtosecond resolution hard X-ray fluorescence for spin crossover can make a significant impact on the interpretability of experimental measurements. The emphasis on thorough and robust interpretation has also led to an emphasis on simpler molecular systems. This enables iteration between experiment and theory, a requirement for the development of a more predictive understanding of electronic excited-state phenomena and an essential step to the development of design rules for solar materials.

Excited state dynamics in the matrix-assisted laser desorption/ionization matrix 2,4,6-trihydroxyacetophenone: evidence for triplet pooling charge separation reactions.

Excited state pooling reactions are a central part of some models of ultraviolet matrix-assisted laser desorption/ionization (MALDI) mechanisms. Evidence has been found for pooling in several matrix materials, but a recent report of pure exponential fluorescence decay at MALDI-relevant laser fluences suggested that 2,4,6-trihydroxy-acetophenone (THAP) may be an example of a matrix in which pooling does not occur (Lin et al., Rapid Commun. Mass Spectrom. 2014, 28, 77). However, those data were instrumentally limited in dynamic range and signal/noise ratio, and the conclusion does not take into account several aspects of THAP excited state dynamics. Using time-correlated single photon counting, and absorption and emission spectroscopies, the excited state dynamics of THAP are reexamined. Like many other aromatic ketones and acetophenone, isolated THAP molecules undergo very efficient intersystem crossing. No fluorescence is observed in dilute solution. In the solid state, efficient fluorescence reappears, but is non-exponential even at very low excitation intensity. The solvent used for sample preparation was found to have a large effect on the spectra and decay curves. Needle-like crystals seem to be correlated with reduced intersystem crossing. THAP solid state fluorescence is entirely due to intermolecular interactions. Activation of fluorescence, instead of quenching, is a clear indicator of delocalized excited state phenomena in THAP. Contrary to the conclusions of Lin et al., the greatly increased singlet lifetime in the solid state substantially increases the probability that pooling-type reactions are indeed involved in ionization processes. The sensitivity of fluorescence and phosphorescence on sample morphology appears to reflect changes in intermolecular interactions due to crystal packing. Pooling charge separation pathways based on known triplet-triplet ionization reactions of aromatic ketones are proposed.

Ab initio non-adiabatic molecular dynamics

Adiabatic nuclear potential energy surfaces (PESs) defined via the Born-Oppenheimer (BO) approximation are a fundamental concept underlying chemical reactivity theory. For a wide range of excited-state phenomena such as radiationless decay, energy and charge transfer, and photochemical reactions, the BO approximation breaks down due to strong couplings between two or more BO PESs. Non-adiabatic molecular dynamics (NAMD) is the method of choice to model these processes. We review new developments in quantum-classical dynamics, analytical derivative methods, and time-dependent density functional theory (TDDFT) which have lead to a dramatic expansion of the scope of ab initio NAMD simulations for molecular systems in recent years. We focus on atom-centered Gaussian basis sets allowing highly efficient simulations for molecules and clusters, especially in conjunction with hybrid density functionals. Using analytical derivative techniques, forces and derivative couplings can be obtained with machine precision in a given basis set, which is crucial for accurate and stable dynamics. We illustrate the performance of surface-hopping TDDFT for photochemical reactions of the lowest singlet excited states of cyclohexadiene, several vitamin D derivatives, and a bicyclic cyclobutene. With few exceptions, the calculated quantum yields and excited state lifetimes agree qualitatively with experiment. For systems with ∼50 atoms, the present Turbomole implementation allows NAMD simulations with 0.2-0.4 ns total simulation time using hybrid density functionals and polarized double zeta valence basis sets on medium-size compute clusters. We close by discussing open problems and future directions.

Pressure-induced Co2+photoluminescence quenching in MgAl2O4

This work investigates the electronic structure and photoluminescence (PL) of Co${}^{2+}$-doped MgAl${}_{2}$O${}_{4}$ and their pressure dependence by time-resolved spectroscopy. The variations of the visible absorption band and its associated emission at 663 nm (\ensuremath{\tau} = 130 ns at ambient conditions) with pressure/temperature can be explained on the basis of a configurational energy model. It provides an interpretation for both the electronic structure and the excited-state phenomena yielding photoluminescence emission and the subsequent quenching. We show that there is an excited-state crossover (ESCO) ${[}^{4}{T}_{1}(P)\ensuremath{\leftrightarrow}{}^{2}\phantom{\rule{-0.16em}{0ex}}E(G)]$ at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 K and 100 K to a narrow-line emission at lower temperatures. Contrary to expectations from the Tanabe-Sugano diagram, instead of enhancing ESCO phenomena, pressure reduces PL and even suppresses it (PL quenching) above 6 GPa. We explain such variations in terms of pressure-induced nonradiative relaxation to lower excited states: ${}^{2}\phantom{\rule{-0.16em}{0ex}}E(G)\ensuremath{\rightarrow}{}^{4}$${T}_{1}$($F$). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation.

PERI-CC2: A Polarizable Embedded RI-CC2 Method.

We present a combination of the polarizable embedding (PE) method with the resolution-of-the-identity implementation of the approximate coupled-cluster singles and doubles method CC2. The new approach, termed PERI-CC2, allows one to study excited state phenomena of large solvated molecular systems with an accurate correlated wave function method. Central to the PE approach is the advanced description of the environmental electrostatic potential and inclusion of polarization, and the quintessence of RI-CC2 is efficient access to excited state properties while retaining the accuracy associated with CC theory. To maintain efficiency, an approximate truncated CC2 density is introduced to calculate the PE contributions. Explicitly, we derive the central equations and outline an implementation of polarizable embedding for the RI-CC2 approach. The new method is tested against previous PE-CC2 and PE-CCSD results for solvatochromic shifts, demonstrating how the important effects of polarization are incorporated well with PERI-CC2 but with a dramatically reduced overall computational cost. A follow-up investigation of the solvatochromic shift of uracil in aqueous solution further illustrates the potential of PERI-CC2. We discuss the need to explicitly incorporate several water molecules into the region treated by quantum mechanics in order to obtain a reliable and accurate description of the physical effects when specific solute/solvent interactions as, e.g., hydrogen-bonds are involved.

Observation of tunable two-photon induced excited-state and three-photon absorption phenomena by structure in oligomerfluorene derivatives

The nonlinear optical properties, including two-, three-photon and excited-state absorption phenomena, were observed in three oligomerfluorene derivatives (abbreviated as OF3, OF5 and OF7) by femtosecond (450 fs) laser pulses. Their two-, three-photon and, excited-state absorption properties were studied by open Z-scan and pump-probe measurements. Interestingly, when the numbers of fluorene units increased from 3 to 5, and to 7, the corresponding absorption phenomena changed from three-photon to two-photon induced excited-state phenomena, showed strong structure dependence in these derivatives. The results offer a method to design and synthesize organic molecules for multiphoton materials. It was noted that the three-photon cross-section value (δ 3) of OF3 was up to 1.36×10−76 cm6 s2. Correspondingly, the two-photon (δ 2) and excited-state cross-section (δ e ) values of OF5 and OF7 also were obtained, and they were 1230 GM and 0.91×10−18 cm2 for OF5, and 1780 GM and 1.18×10−18 cm2 for OF7, respectively.

Magnetic Field Effect Corroborated with Docking Study to Explore Photoinduced Electron Transfer in Drug−Protein Interaction

Conventional spectroscopic tools such as absorption, fluorescence, and circular dichroism spectroscopy used in the study of photoinduced drug-protein interactions can yield useful information about ground-state and excited-state phenomena. However, photoinduced electron transfer (PET) may be a possible phenomenon in the drug-protein interaction, which may go unnoticed if only conventional spectroscopic observations are taken into account. Laser flash photolysis coupled with an external magnetic field can be utilized to confirm the occurrence of PET and authenticate the spin states of the radicals/radical ions formed. In the study of interaction of the model protein human serum albumin (HSA) with acridine derivatives, acridine yellow (AY) and proflavin (PF(+)), conventional spectroscopic tools along with docking study have been used to decipher the binding mechanism, and laser flash photolysis technique with an associated magnetic field (MF) has been used to explore PET. The results of fluorescence study indicate that fluorescence resonance energy transfer takes place from the protein to the acridine-based drugs. Docking study unveils the crucial role of Ser 232 residue of HSA in explaining the differential behavior of the two drugs towards the model protein. Laser flash photolysis experiments help to identify the radicals/radical ions formed in the due course of PET (PF(•), AY(•-), TrpH(•+), Trp(•)), and the application of an external MF has been used to characterize their initial spin-state. Owing to its distance dependence, MF effect gives an idea about the proximity of the radicals/radical ions during interaction in the system and also helps to elucidate the reaction mechanisms. A prominent MF effect is observed in homogeneous buffer medium owing to the pseudoconfinement of the radicals/radical ions provided by the complex structure of the protein.

First-principles simulation of photoreactions in biological systems

First-principles simulations start to be applicable to the photochemistry and photophysics in biological systems. In this review the prerequisites for investigating such excited state phenomena in large systems are outlined. Generally, a quantum mechanical description of the electronic structure is combined with molecular dynamics simulations, which allows to describe the motion of the atoms in the field produced by the quantum-mechanical potential. Like this, bonds can be formed and broken, that is, chemical reactions can be simulated. The review focuses on applications of first-principles molecular dynamics to photoactive proteins.

Photoluminescence of MgAl2O4:Co2+ through time-resolved spectroscopy under pressure

This work investigates the Co2+(Td) electronic structure and photoluminescence (PL) in MgAl2O4:Co2+ and their variation with pressure by time-resolved spectroscopy. The absorption spectra and the pressure and temperature dependences of the emission band at 663 nm (τ=120 ns, at ambient conditions) can be explained on the basis of a configurational energy model, which provides an interpretation of the electronic structure and the excited-state phenomena. We show that there is an excited-state crossover [4T 1(P)↔2E (G)] at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 and 100 K to a narrow-peak emission at 6 K. The pressure, instead of enhancing excited-state crossover phenomena, reduces PL and suppresses it (PL quenching) above 6 GPa. We explain such variations in terms of pressure-induced non-radiative crossover relaxation to lower excited states: 2E (G)→4T 1(F). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation.

Excited-State Proton-Transfer Dynamics of 7-Hydroxyquinoline in Room Temperature Ionic Liquids

Excited-state proton transfer (ESPT) reaction of 7-hydroxyquinoline (7-HQ) mediated by methanol molecules has been studied in two room temperature ionic liquids (RTILs) using steady-state and time-resolved fluorescence measurements. While no ESPT is observable in neat RTILs, characteristic tautomer fluorescence of 7-HQ could be observed in the presence of small quantity of methanol (0.5-4.1 M). The observation of a rise time (350 ps-1.4 ns) associated with the tautomer fluorescence suggests that proton transfer in 7-HQ is indeed an excited-state phenomenon that requires considerable solvent reorganization prior to the relay of proton from the hydroxyl group to the distant ring nitrogen atom through suitably organized dimeric chain of methanol molecules. The rise time of the tautomer fluorescence, which has been found to decrease with increasing methanol concentration, is attributed to the change of viscosity of the medium upon methanol addition. While the influence of viscosity on the ESPT kinetics is evident from the data, lack of any definite correlation between the bulk viscosity and the rise time has been interpreted in terms of the microheterogeneous nature of the media that does not allow assessment of the microviscosity around 7-HQ from the bulk viscosity.

Stimulated emission three-pulse photo-echo peakshift: a mixed pump-probe and photon-echo technique for studying excited-state dynamics.

A novel four-pulse photon-echo technique for exploring condensed phase dynamics at different parts of the excited-state potential energy surface is presented. In contrast to traditional three-pulse photon-echo signals, the introduction of a fourth pump pulse allows the use of photon-echo techniques to probe excited-state phenomena. Here, a "proof of principle" experiment is presented where the excited-state solvent dynamics of the coumarin 153 chromophore dissolved in methanol is explored. The fluctuations of the stimulated emission transition is probed, in contrast to the ground-state absorption transition explored in traditional echo measurements. Distinctly different excited-state dynamics, in contrast to ground-state signals, is observed and discussed.

Theory of long-range forces in AFM and current versus distance curves in STM on Au(1 1 1)

Tunnelling currents on Au(1 1 1) are evaluated within a theory which regards the tunnelling as an excited state phenomenon. The competition between two effects determines the variations of the tunnelling current with tip–sample distance and leads to a local minimum in the current vs. distance dependence. The theory provides the explanation of various experimental observations in AFM and STM on Au(1 1 1): special features in the current vs. distance curve, the long-range attractive interactions between tip and sample and the anomalously high corrugation amplitudes in STM and AFM.

Photoluminescence enhancement of Ca1−xSrxF2:Mn2+ under pressure

This work investigates the surprising disappearance of the Mn2+ green photoluminescence (PL) observed at room temperature on passing from CaF2 to SrF2 along the Ca1−xSrxF2:Mn2+ fluorite series. The aim is to understand the microscopic origin of the excited-state relaxation phenomena leading to radiationless processes in these crystals. High-pressure experiments performed on Ca0.25Sr0.75F2:Mn2+ show that PL can gradually recover by application of pressure. The increase of intensity and lifetime with pressure is explained by a reduction of the fluorite lattice parameter, a. The variation of Mn2+ PL lifetime with pressure and x is described through the same equation by renormalising these parameters to a.

Influence of different environments on the excited-state proton transfer and dual fluorescence of fisetin

The influence of different protic and aprotic solvent environments on the excited-state intramolecular proton transfer (ESIPT) leading to a dual fluorescence behaviour of a biologically important, naturally occurring, polyhydroxyflavone, fisetin (3,3′,4′,7-tetrahydroxyflavone), has been investigated. The normal fluorescence band, in particular, is extremely sensitive to solvent polarity with ν max shifting from 24 510 cm −1 in dioxane ( E T(30)=36.0) to 20 790 cm −1 in methanol ( E T(30)=55.5). This is rationalized in terms of solvent dipolar relaxation process, which also accounts for the red edge excitation shifts (REES) observed in viscous environments such as glycerol at low temperatures. Significant solvent dependence of the tautomer fluorescence properties ( ν max, yield and decay kinetics) reveals the influence of external hydrogen bonding perturbation on the internal hydrogen bond of the molecule. These excited-state relaxation phenomena and their relevant parameters have been used to probe the microenvironment of fisetin in a membrane mimetic system, namely AOT reverse micelles in n-heptane at different water/surfactant molar ratio ( w 0).

Transient Raman Observations of Heme Electronic and Vibrational Photodynamics in Deoxyhemoglobin

Transient resonance Raman spectroscopy has been used to probe the vibrational dynamics of the heme active site of deoxyhemoglobin during photoexcitation. Near UV pulses of approximately 35 ps in duration were used to both excite the sample and generate resonance Raman spectra of the heme during its rapid electronic and vibrational relaxation. The behavior of the Stokes and anti-Stokes transitions as a function of incident laser flux directly reflects net heme vibrational populations and permits the isolation and characterization of ground and excited electronic state phenomena. Scattering from excited electronic states significantly influences the spectra only at the highest excitation fluxes used in this study. A simple model that accounts for the flux-dependent manifestations of the electronic and vibrational contributions to the heme transient resonance Raman spectra is discussed. In addition, the data presented here clearly show mode selectivity in the vibrational energy distribution associated with t...

Excited state phenomena

Recent progress in theoretical simulations of processes induced by electronic excitation in insulating solids has contributed to our understanding of some of the basic steps of the exciton self-trapping, recombination luminescence, radiation defect creation, photo-desorption and interaction between defects in various large bandgap materials. It is hoped that these examples will stimulate further progress in other systems, many of which have potential practical applications.

Excited State Phenomena in Solid State Fullerene

Photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high intensity illumination on the properties of Fullerenes. A highly nonlinear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This nonlinear increase is associated with a dramatic shift to the red of the emission maximum. The photoconductive response of the fullerenes is also seen to increase nonlinearly with input intensity. Temperature dependent measurements indicate that the nonlinear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies

Excited-state phenomena associated with solvation site heterogeneity: a large edge-excitation red-shift in a merocyanine dye-ethanol solution

The electronic spectra of a merocyanine dye in a polar solvent have been studied at different temperatures. Absence of excitation wavelength dependence at room temprature in a fluid medium is due to orientational and translational relaxation of the “solvent-cage”. For the rigid solution, lifetimes of various “solute-solvent conformations” are larger than the excited-state lifetime; hence excitation-wavelength dependent fluorescence occurs.