Vibronic Origin Research Articles

Unraveling the Impact of Flexibility and Solvent Effects on the UV-Vis Absorption Spectrum of Subphthalocyanine in Liquid Chloroform within the Born-Oppenheimer Molecular Dynamics Approach.

A study based on Born-Oppenheimer molecular dynamics (BOMD) of the subphthalocyanine (SubPc) with a chloride attached to the central boron atom was carried out. The BOMD simulation is used to access the dynamic evolution of the SubPc in liquid chloroform, and the electronic absorption spectrum is calculated using the Time-Dependent Density Functional Theory (TDDFT) considering explicit solvent models. We show that the conformational changes and solvent effects produce a red shift of the Q-band, where the largest contribution is due to the geometry changes of the symmetric structure of SubPc. A large splitting (0.2 eV) of the first electronic transition is also described, and it originates as a shoulder in the Q-band, which according to previous experimental studies is attributed to a vibronic origin. The red shift is obtained in agreement with experiment within less than 0.1 eV. The splitting is a consequence of the symmetry breaking in the SubPc central ring structure occurring during the molecular dynamics, with a significant contribution to the large red shift and the broadening of the spectrum.

Theoretical analysis of lactone and carboxylate forms of camptothecin in aqueous solution: Electronic states, absorption spectra, and hydration

The vibronic absorption spectra of the camptothecin drug in an aqueous solution have been calculated for the first time. The calculations were carried out by the TD-DFT method. It was shown that both for lactone and carboxylate forms an excellent agreement with the experimental spectrum gives the BMK/6-31++G(d,p)/IEFPCM theory level. It was demonstrated that absorption spectra of both forms in the 300–400 nm wavelength range are due to single HOMO → LUMO (S0 → S1) electronic transition, and the short-wavelength smaller peak and shoulder are of vibronic origin. This result is at odds with the previous conclusions of other authors that this peak has a dimeric nature or is caused by interaction with an aqueous medium, and the shoulder is due to a separate electronic transition. According to my calculations, strong hydrogen bonds of the lactone and carboxylate forms with water molecules do not affect their absorption spectra. This is because some of these H-bonds were enhanced by photoexcitation of alkaloid molecules, while others were weakened. The opening of the lactone ring does not affect the position of the vibronic absorption bands but only leads to their significant broadening. Therefore, two separate maxima of the lactone form of the alkaloid merge into one broad peak. The absence of the influence of the lactone E ring on the absorption of the alkaloid is since the frontier orbitals are localized on other rings of the chromophore. Calculations have shown that the dipole moment of both forms of the alkaloid decreases significantly upon excitation. In combination with the practically zero contribution of specific interactions with an aqueous environment, this result indicates the n → π* type of the considered electronic transition. It agrees with both the experimental and theoretical results of some authors. Various characteristics of excited states of both drug forms (IR spectra, atomic charges, electron density, and electrostatic potential distributions) were obtained.

Excited states of six oxazine 1 conformers in aqueous solution: TD-DFT/DFT study

The paper analyzes the electronic-vibrational states of six possible conformers of the dye oxazine 1 (also known as oxazine 725) in an aqueous solution using the SMD solvent model and the 6-31++G(d,p) basis set. Vibronic absorption spectra are calculated using a wide range of hybrid functionals. The X3LYP functional gives the best agreement with the experiment for the positions of the main maximum and shoulder. According to the calculations, the shoulder in the experimental spectrum is of vibronic origin. Various characteristics of the ground and excited states (Duschinsky matrices, IR spectra, atomic charges, dipole moments, maps of the distribution of the electrostatic potential) were obtained. All six investigated conformers have similar vibronic absorption spectra and frontier molecular orbitals. Significant differences between them are observed only for the set of vibronic transitions and relaxation from nonequilibrium to an equilibrium excited state. The effect of two strongly bound water molecules on the excitation of the dye was analyzed.

Resolving Jahn-Teller induced vibronic fine structure of silicon vacancy quantum emission in silicon carbide

Point defects in semiconductors are promising single-photon emitters (SPEs) for quantum computing, communication, and sensing applications. However, factors such as emission brightness, purity. and indistinguishability are limited by interactions between localized defect states and the surrounding environment. Therefore, it is important to map the full emission spectrum from each SPE, to understand the complex interplay between the different defect configurations, their surroundings, and external perturbations. Herein, we investigate a family of regularly spaced sharp luminescence peaks appearing in the near-infrared portion of photoluminescence (PL) spectra from $n$-type 4H-SiC samples after irradiation. This periodic emitter family, labeled the L lines, is only observed when the zero-phonon line signatures of the negatively charged Si vacancy (so-called V lines) are present. The L lines appear with $1.45\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ and $1.59\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ energy spacing after H and He irradiation and increase linearly in intensity with fluence---reminiscent of the intrinsic defect trend. Furthermore, we monitor the dependence of the L-line emission energy and intensity on heat treatments, electric field strength, and PL collection temperature, discussing these data in the context of the L lines. Based on the strong similarity between the irradiation, electric field, and thermal responses of the L and V lines, the L lines are attributed to the Si vacancy in 4H-SiC. The regular and periodic appearance of the L lines provides strong arguments for a vibronic origin explaining the oscillatory multipeak spectrum. To account for the small energy separation of the L lines, we propose a model based on rotations of distortion surrounding the Si vacancy driven by a dynamic Jahn-Teller effect.

The vibronic absorption spectrum and electronic properties of Azure B in aqueous solution: TD-DFT/DFT study

The vibronic absorption spectrum of Azure B (AB) in an aqueous solution is calculated using the time-dependent density functional theory (TD-DFT). The results of calculations are analyzed using all hybrid functionals supported by Gaussian16, the 6-31++G(d,p) basis set, and the IEFPCM and SMD solvent models. The solvent model IEFPCM gave significantly underestimated values of λmax in comparison with the experiment. This is a manifestation of the TD-DFT “cyanine failure”. However, the SMD model made it possible to obtain good agreement between the calculation results and experimental data. The best fit was achieved using the X3LYP functional. According to our calculations, the shoulder in the visible absorption spectrum of AB has a vibronic origin. However, the calculated shoulder is weaker than the experimental one. Explicit assignment of two water molecules, which form strong hydrogen bonds with a dye molecule, leads to a shift of the calculated absorption spectrum to longer wavelengths by approximately 17 nm but does not lead to an improvement in its shape. Comparative analysis of the calculated vibronic absorption spectra of Azure B with those obtained earlier for Azure A and methylene blue showed that the presence and intensity of the short-wavelength shoulder are determined by the location of the bands of higher vibronic transitions relative to the band of the 00 → 00 main transitions. Photoexcitation leads to an increase in the dipole moment of the dye molecule. An insignificant photoinduced electron transfer was found in the central ring of the chromophore of the dye molecule.

TD-DFT/DFT study of thionine in aqueous solution: Vibronic absorption spectrum and electronic properties

The vibronic absorption spectrum of thionine (TH) in an aqueous solution is calculated using the time-dependent density functional theory (TD-DFT). The calculations are performed using all hybrid functionals supported by Gaussian16, the 6-31++G(d,p) basis set with IEFPCM and SMD solvent models. IEFPCM gave significantly underestimated values of λmax in comparison with the experiment. However, SMD made it possible to obtain good agreement between the calculation results and experimental data. The best fit was achieved using the X3LYP functional. The shoulder in the visible absorption spectrum of TH has a vibronic origin. Photoexcitation leads to an increase in the dipole moment of the dye molecule. An insignificant photoinduced electron transfer was found in the central ring of the chromophore of the dye molecule. The absorption peak of the TH solution in the visible region of the spectrum is due to the π→π* type electronic transition. Vibronic coupling plays a significant role in the absorption spectra of TH.

Vibronic absorption spectrum and electronic properties of methylene blue in aqueous solution: TD-DFT study

The vibronic absorption spectrum of methylene blue (MB) in an aqueous solution is calculated using the time-dependent density functional theory (TD-DFT). The results of calculations are analyzed using all hybrid functionals supported by Gaussian16, the 6–31++G(d,p) basis set and the IEFPCM and SMD solvent models. The solvent model IEFPCM gave significantly underestimated values of λmax in comparison with the experiment. This is a manifestation of the TD-DFT “cyanine failure”. However, the SMD model made it possible to obtain good agreement between the calculation results and experimental data. The best fit was achieved using the X3LYP functional. According to our calculations, the shoulder in the visible absorption spectrum of MB has a vibronic origin. Explicit assignment of a water molecule strongly bound to the N10 endocyclic nitrogen atom does not change the shape of the absorption band, but shifts its maximum by 7 nm to the long-wavelength region of the spectrum. The dipole moments and atomic charges of the ground and excited states of the MB molecule have been calculated. Photoexcitation leads to an increase in the dipole moment of the dye molecule. An insignificant photoinduced electron transfer was found in the central ring of the chromophore of the dye molecule. In accordance with our results, the absorption peak of the MB solution in the visible region of the spectrum is due to the π → π* type electronic transition. Vibronic coupling plays a significant role in the absorption spectra of MB.

Engineering couplings for exciton transport using synthetic DNA scaffolds

Summary Control over excitons enables electronic energy to be harnessed and transported for light harvesting and molecular electronics. Such control requires nanoscale precision over the molecular components. Natural light-harvesting systems achieve this precision through sophisticated protein machinery, which is challenging to replicate synthetically. Here, we introduce a DNA-based platform that spatially organizes cyanine chromophores to construct tunable excitonic systems. We synthesized DNA-chromophore nanostructures and characterized them with ensemble ultrafast and single-molecule spectroscopy and structure-based modeling. This synthetic approach facilitated independent control over the coupling among the chromophores and between the chromophores and the environment. We demonstrated that the coupling between the chromophores and the environment could enhance exciton transport efficiency, highlighting the key role of the environment in driving exciton dynamics. Control over excitons, as reported here, offers a path toward the development of designer nanophotonic devices.

Monolayers of MoS2 on Ag(111) as decoupling layers for organic molecules: resolution of electronic and vibronic states of TCNQ.

The electronic structure of molecules on metal surfaces is largely determined by hybridization and screening by the substrate electrons. As a result, the energy levels are significantly broadened and molecular properties, such as vibrations are hidden within the spectral line shapes. Insertion of thin decoupling layers reduces the line widths and may give access to the resolution of electronic and vibronic states of an almost isolated molecule. Here, we use scanning tunneling microscopy and spectroscopy to show that a single layer of MoS2 on Ag(111) exhibits a semiconducting bandgap, which may prevent molecular states from strong interactions with the metal substrate. We show that the lowest unoccupied molecular orbital (LUMO) of tetracyanoquinodimethane (TCNQ) molecules is significantly narrower than on the bare substrate and that it is accompanied by a characteristic satellite structure. Employing simple calculations within the Franck–Condon model, we reveal their vibronic origin and identify the modes with strong electron–phonon coupling.

Pair-delocalization in trigonal mixed-valence clusters: new insight into the vibronic origin of broken-symmetry ground states.

A new vibronic mechanism for the stabilization of pair-delocalized electronic states in trigonal trimeric mixed valence complexes (such as iron-sulfur [Fe3S4]0 proteins) is proposed. Unlike the previously reported mechanism based on the Piepho-Krausz-Schatz model dealing with the local "breathing" vibrations, the present mechanism involves the multicenter vibrations of the triangular complex, which change the metal-metal distances. The former mechanism of the vibronic coupling in combination with the double exchange and Heisenberg type exchange (superexchange) interactions accounts for the existence of the pair-delocalized states of many-electron mixed valence clusters in the intermediate ground spin states. In contrast to the previous model, within which the conditions for the occurrence of pair delocalized states are spin-dependent, the pair delocalization phenomenon in the proposed model is exclusively based on a clear physical mechanism of the interplay between the inter-center vibronic coupling and electron transfer. Therefore, the proposed mechanism demonstrates that the pair delocalization is a more common feature of mixed valence complexes and can appear not only in multi-electron trimers as a result of the interplay between several parameters, but even in one-electron mixed valence trimers in which neither double exchange nor superexchange is operative. The conditions for the pair delocalization in the proposed model are shown to be dependent on the value and sign of the electron transfer parameter as well as on the ratio of the frequencies of the fully symmetric (a1) and doubly degenerate (e) vibrations of a triatomic molecule.

Coherent phenomena in photosynthetic light harvesting: part two-observations in biological systems.

Considerable debate surrounds the question of whether or not quantum mechanics plays a significant, non-trivial role in photosynthetic light harvesting. Many have proposed that quantum superpositions and/or quantum transport phenomena may be responsible for the efficiency and robustness of energy transportpresent in biological systems. The critical experimental observations comprise the observation of coherent oscillations or "quantum beats" via femtosecond laser spectroscopy, which have been observed in many different light harvesting systems. Part Two of this review aims to provide an overview of experimental observations of energy transfer in the most studied light harvesting systems. Length scales, derived from crystallographic studies, are combined with energy and time scales of the beats observed via spectroscopy. A consensus is emerging that most long-lived (hundreds of femtoseconds) coherent phenomena are of vibrational or vibronic origin, where the latter may result in coherent excitation transport within a protein complex. In contrast, energy transport between proteins is likely to be incoherent in nature. The question of whether evolution has selected for these non-trivial quantum phenomena may be an unanswerable question, as dense packings of chromophores will lead to strong coupling and hence non-trivial quantum phenomena. As such, one cannot discern whether evolution has optimised light harvesting systems for high chromophore density orfor the ensuing quantum effects as these are inextricably linked and cannot be switched off.

High Magnetic Field Detunes Vibronic Resonances in Photosynthetic Light Harvesting.

The origin and role of oscillatory features detected in recent femtosecond spectroscopy experiments of photosynthetic complexes remain elusive. A key hypothesis underneath of these observations relies on electronic-vibrational resonance, where vibrational levels of an acceptor chromophore match the donor-acceptor electronic gap, accelerating the downhill energy transfer. Here we identify and detune such vibronic resonances using a high magnetic field that exclusively shifts molecular exciton states. We implemented ultrafast pump-probe spectroscopy into a specialized 25 T magnetic field facility and studied the light-harvesting complex PC645 from a cryptophyte algae where strongly coupled chromophores form molecular exciton states. We detected a change in high-frequency coherent oscillations when the field was engaged. Quantum chemical calculations coupled with a vibronic model explain the experiment as a magnetic field-induced shift of the exciton states, which in turn affects the electronic-vibrational resonance between pigments within the protein. Our results demonstrate the delicate sensitivity of interpigment coherent oscillations of vibronic origin to electronic-vibrational resonance interactions in light-harvesting complexes.

Vibronic absorption spectra of the angular fused bisindolo- and biscarbazoloanthracene blue fluorophores for OLED applications

An in-depth analysis of the vibronic absorption spectra for the recently synthesized blue-fluorescent OLED emitters bis[(1,2)(5,6)]indoloanthracene and biscarbazolo[3,4-a:3′,4′-h]anthracene has been carried out computationally at the density functional theory level within the Franck-Condon approximation. These molecules are characterized by extended and rich electronic absorption spectra with most absorption bands being of vibronic origin. The first excited singlet state of bis[(1,2)(5,6)]indoloanthracene compound demonstrates a clear observable double-peak vibronic progression for two different active modes in the absorption spectrum, while the S2 state is vibronically inactive. In contrast, for the larger biscarbazolo[3,4-a:3′,4′-h]anthracene compound the S0 → S2 transition demonstrates well-resolved intense vibronic bands which overlap the less intense progressions of few modes in the S0 → S1 transition. We have also found, that even the higher-lying and very intense S0 → S4 and S0 → S5 transitions for bis[(1,2)(5,6)]indoloanthracene and biscarbazolo[3,4-a:3′,4′-h]anthracene, respectively, are characterized by clear vibronic progressions in excellent agreement with experimental spectra.

Franck–Condon simulation for unraveling vibronic origin in solvent enhanced absorption and fluorescence spectra of rubrene

The damped FC method shows how and which local modes are responsible to solvent enhance vibronic spectra.

Vibronic origin of long-lived coherence in an artificial molecular light harvester.

Natural and artificial light-harvesting processes have recently gained new interest. Signatures of long-lasting coherence in spectroscopic signals of biological systems have been repeatedly observed, albeit their origin is a matter of ongoing debate, as it is unclear how the loss of coherence due to interaction with the noisy environments in such systems is averted. Here we report experimental and theoretical verification of coherent exciton–vibrational (vibronic) coupling as the origin of long-lasting coherence in an artificial light harvester, a molecular J-aggregate. In this macroscopically aligned tubular system, polarization-controlled 2D spectroscopy delivers an uncongested and specific optical response as an ideal foundation for an in-depth theoretical description. We derive analytical expressions that show under which general conditions vibronic coupling leads to prolonged excited-state coherence.

Photolysis of nitric acid at 308 nm in the absence and in the presence of water vapor.

We have re-examined the NOx channels from the 308 nm gas-phase photolysis of nitric acid (HNO3) by using excimer laser photolysis combined with cavity ring-down spectroscopy. The photolysis products were monitored in the 552-560 and 640-648 nm regions. Direct comparison of the photolysis product spectrum in the 640-648 nm region with literature vibronic band origins and line intensities in electronically excited NO2 (NO2*) suggests that NO2* is not formed from HNO3 photolysis at 308 nm. A comparison of the photolysis product spectrum in the 552-560 nm region with a standard NO2 spectrum indicates that ground-state NO2 is a photolysis product. We have determined the NO2 quantum yield from the 308 nm HNO3 photolysis. We also investigated HNO3 photolysis in the presence of water vapor. For equilibrated HNO3/H2O mixtures, we did not observe significant variation of product absorption around 552 nm with delay times between the firing of the photolysis and the probe lasers. Transient product absorption measurements at 342.0 and 343.5 nm (respective wavelengths where the peak and valley of HONO absorptions are located) are consistent with ground-state NO2 being the predominant NOx product from the 308 nm photolysis of a HNO3/H2O mixture. Atmospheric implications are also discussed.

Theory of femtosecond coherent double-pump single-molecule spectroscopy: Application to light harvesting complexes.

We develop a first principles theoretical description of femtosecond double-pump single-molecule signals of molecular aggregates. We incorporate all singly excited electronic states and vibrational modes with significant exciton-phonon coupling into a system Hamiltonian and treat the ensuing system dynamics within the Davydov D1 Ansatz. The remaining intra- and inter-molecular vibrational modes are treated as a heat bath and their effect is accounted for through lineshape functions. We apply our theory to simulate single-molecule signals of the light harvesting complex II. The calculated signals exhibit pronounced oscillations of mixed electron-vibrational (vibronic) origin. Their periods decrease with decreasing exciton-phonon coupling.

Monte-Carlo simulations of spin-crossover phenomena based on a vibronic Ising-like model with realistic parameters.

Materials with spin-crossover (SCO) properties hold great potential in information storage and therefore have received a lot of concerns in recent decades. The hysteresis phenomena accompanying SCO are attributed to the intermolecular cooperativity whose underlying mechanism may have a vibronic origin. In this work, a new vibronic Ising-like model in which the elastic coupling between SCO centers is included by considering harmonic stretching and bending (SAB) interactions is proposed and solved by Monte Carlo (MC) simulations. The key parameters in the new model, k1 and k2, corresponding to the elastic constant of the stretching and bending mode, respectively, can be directly related to the macroscopic bulk and shear modulus of the material of study, which can be readily estimated either based on experimental measurements or first-principles calculations. Using realistic parameters estimated based on density-functional theory calculations of a specific polymeric coordination SCO compound, [Fe(pz)Pt(CN)4]·2H2O (pz = pyrazine), temperature-induced hysteresis and pressure effects on SCO phenomena are simulated successfully. Our MC simulations shed light on the role of the vibronic couplings in the thermal hysteresis of SCO systems, and also point out the limitations of highly simplified Ising-like models for quantitative description of real SCO systems, which will be of great value for the development of more realistic SCO models.

Vibronic origin of sulfur mass-independent isotope effect in photoexcitation of SO2and the implications to the early earth’s atmosphere

Signatures of mass-independent isotope fractionation (MIF) are found in the oxygen ((16)O,(17)O,(18)O) and sulfur ((32)S, (33)S, (34)S, (36)S) isotope systems and serve as important tracers of past and present atmospheric processes. These unique isotope signatures signify the breakdown of the traditional theory of isotope fractionation, but the physical chemistry of these isotope effects remains poorly understood. We report the production of large sulfur isotope MIF, with Δ(33)S up to 78‰ and Δ(36)S up to 110‰, from the broadband excitation of SO2 in the 250-350-nm absorption region. Acetylene is used to selectively trap the triplet-state SO2 ( (3)B1), which results from intersystem crossing from the excited singlet ( (1)A2/ (1)B1) states. The observed MIF signature differs considerably from that predicted by isotopologue-specific absorption cross-sections of SO2 and is insensitive to the wavelength region of excitation (above or below 300 nm), suggesting that the MIF originates not from the initial excitation of SO2 to the singlet states but from an isotope selective spin-orbit interaction between the singlet ( (1)A2/ (1)B1) and triplet ( (3)B1) manifolds. Calculations based on high-level potential energy surfaces of the multiple excited states show a considerable lifetime anomaly for (33)SO2 and (36)SO2 for the low vibrational levels of the (1)A2 state. These results demonstrate that the isotope selectivity of accidental near-resonance interactions between states is of critical importance in understanding the origin of MIF in photochemical systems.

Excess Dynamic Stokes Shift of Molecular Probes in Solution

The solvation dynamics of molecular probes is studied by broad-band fluorescence upconversion. The time-dependent position of the S1 → S0 emission band or of a vibronic line shape is measured with ~80 fs, 10 cm(-1) resolution. Polar solutes in acetonitrile and acetone, when excited into S1 with excess vibrational energy, show a dynamic Stokes shift which extends to the red beyond the quasistationary state. Equilibrium is then reached by a slower blue shift on a 10 ps time scale. In methanol, excess vibrational energy as large as ~14,000 cm(-1) shows no such effect. Nonpolar solutes exhibit an excess red shift of the emission band in both polar and nonpolar solvents even upon excitation near the vibronic origin. The observed dynamics are discussed in terms of transient heating of the excited chromophore, conformational change, and changes of the molecular cavity size. For solvation studies the optical excitation should be chosen close to the band origin.