Frequency-resolved Fluorescence Research Articles

On-the-fly simulation of time-resolved fluorescence spectra and anisotropy.

We combine on-the-fly trajectory surface hopping simulations and the doorway-window representation of nonlinear optical response functions to create an efficient protocol for the evaluation of time- and frequency-resolved fluorescence (TFRF) spectra and anisotropies of the realistic polyatomic systems. This approach gives the effective description of the proper (e.g., experimental) pulse envelopes, laser field polarizations, and the proper orientational averaging of TFRF signals directly from the well-established on-the-fly nonadiabatic dynamic simulations without extra computational cost. To discuss the implementation details of the developed protocol, we chose cis-azobenzene as a prototype to simulate the time evolution of the TFRF spectra governed by its nonadiabatic dynamics. The results show that the TFRF is determined by the interplay of several key factors, i.e., decays of excited-state populations, evolution of the transition dipole moments along with the dynamic propagation, and scaling factor of the TFRF signals associated with the cube of emission frequency. This work not only provides an efficient and effective approach to simulate the TFRF and anisotropies of realistic polyatomic systems but also discusses the important relationship between the TFRF signals and the underlining nonadiabatic dynamics.

Exciton Dynamics and Time-Resolved Fluorescence in Nanocavity-Integrated Monolayers of Transition-Metal Dichalcogenides

We have developed an ab initio-based, fully quantum, numerically accurate methodology for the simulation of the exciton dynamics and time- and frequency-resolved fluorescence spectra of the cavity-controlled two-dimensional materials at finite temperatures and applied this methodology to the single-layer WSe2 system. Specifically, the multiple Davydov D2 Ansatz has been employed in combination with the method of thermofield dynamics for the finite-temperature extension of accurate time-dependent variation. This allowed us to establish dynamical and spectroscopic signatures of the polaronic and polaritonic effects as well as uncover their characteristic time scales in the relevant range of temperatures. Our study reveals the pivotal role of multidimensional conical intersections in controlling the many-body dynamics of highly intertwined excitonic, phononic, and photonic modes.

Monitoring the impact of ionizing radiation on CdSe/ZnS semiconductor quantum dot photoluminescence

The impact of 60Co gamma irradiation on the photoluminescent properties of surface passivated CdSe/ZnS semiconductor quantum dots (QDs) in toluene are investigated. Two sets of triplicate samples of the quantum dots in toluene were subjected to increasing absorbed doses of gamma radiation from a 60Co source. One set of samples was prepared with zero (0% volume) headspace. The other set was prepared with appreciable (90% volume) headspace to investigate the impact of air on the QD-gamma ray interaction. Samples were exposed to radiation for periods of 1, 5, 15, 30, or 45 min, producing radiation doses of 0.03, 0.15, 0.45, 1.02, or 1.55 kilo-gray (kGy). Monitoring the impact of gamma irradiation on CdSe/ZnS QDs using wavelength- and frequency-resolved fluorescence decay measurements allows construction of reaction progress Fourier-transform emission-decay matrices (RP-FT-EDMs), which can be analyzed numerically for the evolution of the spectra, decays, and relative intensities of the photoluminescent QD components during gamma irradiation. Analysis of the photoluminescence data shows that the QD emission is composed of three components that change with increasing gamma ray exposure: two quantum dot components that degrade with 0.45 kGy exposure dose; and one component that is associated with particle aggregation and is promoted by exposure in the zero headspace samples. The results also show that QD degradation is accelerated in the samples that have appreciable headspace. Dynamic light scattering measurements corroborated the disruption of the smaller QDs and formation of larger particles observed in the fluorescence measurements after gamma radiation, however, it did not reveal the QD heterogeneity observed in the fluorescence of unirradiated samples.

Simulation of Time- and Frequency-Resolved Four-Wave-Mixing Signals at Finite Temperatures: A Thermo-Field Dynamics Approach

We propose a new approach to simulate four-wave-mixing signals of molecular systems at finite temperatures by combining the multiconfigurational Ehrenfest method with the thermo-field dynamics theory. In our approach, the four-time correlation functions at finite temperatures are mapped onto those at zero temperature in an enlarged Hilbert space with twice the vibrational degrees of freedom. As an illustration, we have simulated three multidimensional spectroscopic signals, time- and frequency-resolved fluorescence spectra, transient-absorption pump-probe spectra, and electronic two-dimensional (2D) spectra at finite temperatures, for a conical intersection-mediated singlet fission model of a rubrene crystal. It is shown that a detailed dynamical picture of the singlet fission process can be extracted from the three spectroscopic signals. An increasing temperature leads to lower intensities of the signals and broadened vibrational peaks, which can be attributed to faster singlet-triplet population transfer and stronger bath-induced electronic dephasing at higher temperatures.

Simulation of Nonlinear Femtosecond Signals at Finite Temperature via a Thermo Field Dynamics-Tensor Train Method: General Theory and Application to Time- and Frequency-Resolved Fluorescence of the Fenna-Matthews-Olson Complex.

Addressing needs of contemporary nonlinear femtosecond optical spectroscopy, we have developed a fully quantum, numerically accurate wave function-based approach for the calculation of third-order spectroscopic signals of polyatomic molecules and molecular aggregates at finite temperature. The systems are described by multimode nonadiabatic vibronic-coupling Hamiltonians, in which diagonal terms are treated in harmonic approximation, while off-diagonal interstate couplings are assumed to be coordinate independent. The approach is based on the Thermo Field Dynamics (TFD) representation of quantum mechanics and tensor-train (TT) machinery for efficient numerical simulation of quantum evolution of systems with many degrees of freedom. The developed TFD-TT approach is applied to the calculation of time- and frequency-resolved fluorescence spectra of the Fenna-Matthews-Olson (FMO) antenna complex at room temperature taking into account finite time-frequency resolution in fluorescence detection, orientational averaging, and static disorder.

Efficient simulation of time- and frequency-resolved four-wave-mixing signals with a multiconfigurational Ehrenfest approach.

We have extended the multiconfigurational Ehrenfest approach to the simulation of four-wave-mixing signals of systems involving multiple electronic and vibrational degrees of freedom. As an illustration, we calculate signals of three widely used spectroscopic techniques, time- and frequency-resolved fluorescence spectroscopy, transient absorption spectroscopy, and two-dimensional (2D) electronic spectroscopy, for a two-electronic-state, twenty-four vibrational-mode conical intersection model. It has been shown that all these three spectroscopic signals characterize fast population transfer from the higher excited electronic state to the lower excited electronic state. While the time- and frequency-resolved spectrum maps the wave packet propagation exclusively on the electronically excited states, the transient absorption and 2D electronic spectra reflect the wave packet dynamics on both electronically excited states and the electronic ground state. Combining trajectory-guided Gaussian basis functions and the nonlinear response function formalism, the present approach provides a promising general technique for the applications of various Gaussian basis methods to the calculations of four-wave-mixing spectra of polyatomic molecules.

Multi-faceted spectroscopic mapping of ultrafast nonadiabatic dynamics near conical intersections: A computational study.

We studied spectroscopic signatures of the nonadiabatic dynamics at conical intersections formed by the lowest excited singlet states in pyrazine. We considered two ab initio models of conical intersections in the excited states of pyrazine developed by Sala et al. [Phys. Chem. Chem. Phys. 16, 15957 (2014)]: a two-state (B2u and B3u), five-mode model and a three-state (B2u, B3u, and Au), nine-mode model. We simulated the signals of three widely used techniques: time- and frequency-resolved fluorescence spectroscopy, transient absorption pump-probe spectroscopy, and electronic two-dimensional spectroscopy. The signals were calculated through third-order response functions, which, in turn, were evaluated with the numerically accurate multiple Davydov ansatz. We establish spectroscopic signatures of the optically dark Au state and demonstrate that the key features of the photoinduced dynamics, such as electronic/nuclear populations, electronic/nuclear coherences, and electronic/nuclear energy transfer processes, are imprinted in the spectroscopic signals. We show that a fairly complete picture of the nonadiabatic dynamics at conical intersections can be obtained when several spectroscopic techniques are combined. Provided that the time resolution is sufficient, time- and frequency-resolved fluorescence may provide the best visualization of the nonadiabatic dynamics near conical intersections.

Mapping of Wave Packet Dynamics at Conical Intersections by Time- and Frequency-Resolved Fluorescence Spectroscopy: A Computational Study.

Monitoring of wave packet dynamics at conical intersections by time- and frequency-resolved fluorescence spectroscopy has been investigated theoretically for a three-state two-mode model of a conical intersection coupled to a dissipative environment. The ideal and the actually measurable time- and frequency-gated fluorescence spectra are accurately and efficiently simulated by combining the hierarchy equations-of-motion method for dissipative quantum dynamics with the methodology of the equation-of-motion phase-matching approach for the calculation of spectroscopic signals. It is shown that time- and frequency-resolved fluorescence spectra reveal essential aspects of the wave packet dynamics at conical intersections and the effects of environment-induced dissipation. The results of the present work indicate that fluorescence up-conversion spectroscopy with femtosecond time resolution is an efficient tool for the characterization of ultrafast dynamics at conical intersections.

Time- and frequency-resolved fluorescence with a single TCSPC detector via a Fourier-transform approach.

We introduce a broadband single-pixel spectro-temporal fluorescence detector, combining time-correlated single photon counting (TCSPC) with Fourier transform (FT) spectroscopy. A birefringent common-path interferometer (CPI) generates two time-delayed replicas of the sample's fluorescence. Via FT of their interference signal at the detector, we obtain a two-dimensional map of the fluorescence as a function of detection wavelength and emission time, with high temporal and spectral resolution. Our instrument is remarkably simple, as it only requires the addition of a CPI to a standard single-pixel TCSPC system, and it shows a readily adjustable spectral resolution with inherently broad bandwidth coverage.

Inversion of two-dimensional potentials from frequency-resolved spectroscopic data

We report the first successful reconstruction of two-dimensional potential energy surfaces (PES) using the magnitudes and positions of a set of frequency-resolved fluorescence (or absorption) lines. The inversion proceeds by first extracting the phases of the transition-dipole matrix elements, yielding, together with the (ground) PES to (from) which emission (absorption) occurs, a point by point reconstruction of the two-dimensional excited state PES. The inversion procedure is highly accurate even for PES with multiple minima and many missing lines, with typical RMS errors <0.002 cm(-1) in the classically allowed region and <0.018 cm(-1) in the classically forbidden region.

Signatures of carrier multiplication in the frequency resolved fluorescence spectra from polaritons

Using the micro-cavity coupled to weakly confined excitons in quantum dots we investigated the signatures of carrier multiplication in the single and two-photon emitted frequency resolved fluorescence. Those are compared to the spectra provided by the conventional multi-photon induced fluorescence. The two processes are distinguished by the statistics of the initial polariton distribution. We argue that exciton emission spectra turned out to be CM insensitive, while the polaritons emission demonstrates strong spectral signatures of the CM. When the polaritonic effect dominates the confinement-boosted excitons' nonlinearities, increasing quantum yield leads to consecutive appearance of Rabi multiplets. The correlation spectra reveal the formation of more multiplets as compared to the single photon emission. When the confinement increases the multiplets start to overlap and their classification requires transient measurements of the spectral snapshots. These are based on the discrete Auger lifetimes of the multi-excitons.

Two-photon-coincidence fluorescence spectra of cavity multipolaritons: novel signatures of multiexciton generation.

In a simulation study, we show that correlated two-dimensional frequency-resolved fluorescence spectra of a quantum dot in a microcavity provide a sensitive probe for the distribution of multiexcitons. Polariton couplings lead to a fine structure of Rabi multiplets that allow us to resolve otherwise overlapping features of the different multiexcitons. These may be used to probe multiexciton generation.

Time- and frequency-resolved fluorescence spectra of nonadiabatic dissipative systems: What photons can tell us

The monitoring of the excited-state dynamics by time- and frequency-resolved spontaneous emission spectroscopy has been studied in detail for a model exhibiting an excited-state curve crossing. The model represents characteristic aspects of the photoinduced ultrafast dynamics in large molecules in the gas or condensed phases and accounts for strong nonadiabatic and electron-vibrational coupling effects, as well as for vibrational relaxation and optical dephasing. A comprehensive overview of the dependence of spontaneous emission spectra on the characteristics of the excitation and detection processes (such as carrier frequencies, pump/gate pulse durations, as well as optical dephasing) is presented. A systematic comparison of ideal spectra, which provide simultaneously perfect time and frequency resolution and thus contain maximal information on the system dynamics, with actually measurable time- and frequency-gated spectra has been carried out. The calculations of real time- and frequency-gated spectra demonstrate that complementary information on the excited-state dynamics can be extracted when the duration of the gate pulse is varied.

Fast Excited-State Intramolecular Proton Transfer and Subnanosecond Dynamic Stokes Shift of Time-Resolved Fluorescence Spectra of the 5-Methoxysalicylic Acid/Diethyl ether Complex

Excited-state intramolecular proton transfer (ESIPT) occurring in the salicylic acid (SA) derivative 5-methoxysalicylic acid (5-MeOSA) in an apolar solvent (cyclohexane) and in the presence of the hydrogen bond accepting agent diethyl ether (DEE) is investigated. Analysis of the directly measured subnanosecond time-resolved emission spectra (TRES) together with conventional steady-state fluorescence and time-correlated single-photon-counting (TCSPC) decays indicates that ESIPT in this system occurs much faster than fluorescence, and that the equilibrium between normal and tautomeric excited states is established before the emission from both states takes place. However, changes in time- and frequency-resolved fluorescence of the 5-MeOSA/DEE complex are observed due to structural relaxation within the complex, which is reflected in the dynamic Stokes shift of the tautomeric fluorescence band. The normal fluorescence band of 5-MeOSA/DEE does not exhibit marked changes within the investigated time range. A single-exponential relaxation time of 460 ps was determined for the dynamic Stokes shift of the tautomeric band, and it is attributed to a geometric change within the 5-MeOSA/DEE complex upon excitation. Since both tautomeric and normal emission bands are well resolved and exhibit different time-dependent behaviors, a double-well potential appears to be adequate to describe the excited state of the system studied.

Ultrafast Energy Transport in a First-Generation Coumarin−Tetraphenylporphyrin Dendrimer

Energy transfer in a newly synthesized coumarin−tetraphenylporphyrin donor−acceptor system was studied by time- and frequency-resolved fluorescence spectroscopy. The energy transfer kinetics was shown to be fast (transfer time ca. 500 fs) and efficient (quantum yield ca. 97%). The influence of interactions between excitations on the energy transfer dynamics was studied by intensity-dependent experiments. Although annihilation of excitations occurs for high irradiation doses, this does not affect the observed fluorescence transients. A kinetic model was constructed to explain these findings. Both the difference between the rate constants of energy transfer to singly and doubly excited acceptor states and the rate of radiationless decay from such doubly excited states were shown to be key parameters in the explanation of the intensity-dependent effects.

Femtosecond secondary emission arising from the nonadiabatic photoisomerization in rhodopsin

A microscopic quantum-mechanical model of the femtosecond photodynamics and the associated secondary emission of rhodopsin is presented. The formulation consists of a two-state two-mode model describing the nonadiabatic photoisomerization of retinal, a harmonic multi-mode ansatz accounting for the remaining Raman-active modes, and a low-frequency bath accounting for the coupling of retinal to the protein environment. The interaction between the various subsystems of the model is described in a mean-field approximation. Explicit simulations of absorption, resonance Raman and fluorescence spectra of rhodopsin are presented and compared to available experimental data. The model assumptions and the validity of the approximations involved are discussed in some detail. Furthermore, it is studied to what extent the secondary emission spectra reflect the photochemical reaction of the molecular system. It is shown that standard continuous-wave techniques such as absorption, resonance Raman and fluorescence spectra may yield only little direct information on the photoreaction. Considering the time- and frequency-resolved fluorescence spectrum, on the other hand, the time evolution of the excited-state wave function can be monitored, thereby providing a real-time measurement of the nonadiabatic photoreaction. Moreover, the proposed model of rhodopsin reveals recurrences of time-resolved emission which are shown to reflect coherent vibrational motion on coupled potential-energy surfaces.

X˜ 2 A′/Ã 2 A′ conical intersection effects on the fluorescence of NO 2

We have analyzed the effect of the X˜2A′/A2A′ conical intersection between the first two electronic surfaces of NO2 on the fluorescence of the molecule. Our computations are based on diabatic potentials obtained by a transformation of ab initio adiabatic potentials. We have computed time- and frequency-resolved fluorescence spectra from stationary and nonstationary states, showing the effects of the conical intersection, which slows down the fluorescence, shifts the emitted frequencies, redistributes the intensities, and increases the density of the lines. Furthermore, we have explained the low fluorescence yield observed by Dulcey et al. after optical excitation.

Solvent Reorganization in Long-Range Electron Transfer: Density Matrix Approach

The dynamics of charge transfer from a photoexcited donor to an acceptor coupled through a bridge is investigated by using a correlation-function approach in Liouville space that takes into account solvent dynamics with an arbitrary distribution of time scales. The time- and frequency-resolved fluorescence spectrum from the acceptor is used to probe the scaling of the ET rate with bridge size. The crossover between the coherent tunneling (transfer) and the incoherent sequential (transport) regimes and its implications on the nature of ET processes in DNA are discussed.

Structural Effects on the Isomerization Dynamics of trans-Stilbenes: IVR, Microcanonical Reaction Rates, and the Nature of the Transition State

Picosecond time-resolved fluorescence spectroscopy has been used for studying the intramolecular dynamics of trans-stilbene (TS) derivatives. Nonradiative decay rate constants (knr) of jet-cooled 4-methoxy-trans-stilbene (MS), 4,4‘-dimethoxy-trans-stilbene (DMS), 2-phenylindene (PI), 4,4‘-dihydroxy-trans-stilbene (DHS), and 1-(trans-β-styryl)cyclohexene (SCH) have been obtained from time-resolved fluorescence measurements with wide-band detection from photoselected vibrational states in the S1 manifold. Time- and frequency-resolved fluorescence have been measured to characterize intramolecular vibrational energy redistribution (IVR) in the first three of these molecules. Statistical nonadiabatic RRKM theory is applied to model the energy dependence of knr and extract the reaction barriers. Dispersed fluorescence is measured and analyzed in order to determine the substitution effect on vibrational frequencies most significant to the RRKM calculations. While knr is almost invariant to para substitution of a methoxy group, with similar isomerization thresholds for MS and TS (E0MS ∼ E0TS), a dramatic slowing of the rate is observed in both disubstituted derivatives, and reaction barriers of ∼1800 and 1700 cm-1 are derived for DMS and DHS, respectively. The altered conjugation of the system in SCH produces a large blue shift of the and results in a lowering of the isomerization barrier (E0SCH < 740 cm-1). The twisting around the ethylenic double bond (Ce−Ce) is prevented upon intramolecular bridging in PI, but another nonradiative channel, attributed to twisting around the Ce−Cφ single bond, is modeled by RRKM theory with E0PI ∼ 1770 cm-1. From the electronic structure effects on both E0 and the shift of the S1 transition, we elucidate the nature of the transition state, as zwitterionic, and we correlate substitution effects with the HOMO/LUMO (frontier orbital) description. The results are relevant to studies in polar solutions and to the dimensionality of the reaction coordinate in the dynamics of barrier crossing.

Accurate determination of excited state potentials from frequency-resolved fluorescence data and knowledge of the ground state potential

A new closed-form quantum mechanical formula for the inversion of spectroscopic data to yield excited state potentials is developed. The method uses fluorescence frequencies, line strength data and the ground state potential. Contrary to the RKR procedure, the method is not limited to diatomic molecules and is capable of yielding potentials at energies above the dissociation thresholds. Highly accurate determination of some of the excited states of the Nat molecule is demonstrated.