Franck-Condon Research Articles

Modeling radiative and non-radiative pathways at both the Franck-Condon and Herzberg-Teller approximation level.

Here, we present a concise model that can predict the photoluminescent properties of a given compound from first principles, both within and beyond the Franck-Condon approximation. The formalism required to compute fluorescence, Internal Conversion (IC), and Inter-System Crossing (ISC) is discussed. The IC mechanism, in particular, is a difficult pathway to compute due to difficulties associated with the computation of required bosonic configurations and non-adiabatic coupling elements. Here, we offer a discussion and breakdown on how to model these pathways at the Density Functional Theory (DFT) level with respect to its computational implementation, strengths, and current limitations. The model is then used to compute the photoluminescent quantum yield (PLQY) of a number of small but important compounds: anthracene, tetracene, pentacene, diketo-pyrrolo-pyrrole (DPP), and Perylene Diimide (PDI) within a polarizable continuum model. Rate constants for fluorescence, IC, and ISC compare well for the most part with respect to experiment, despite triplet energies being overestimated to a degree. The resulting PLQYs are promising with respect to the level of theory being DFT. While we obtained a positive result for PDI within the Franck-Condon limit, the other systems require a second order correction. Recomputing quantum yields with Herzberg-Teller terms yields PLQYs of 0.19, 0.08, 0.04, 0.70, and 0.99 for anthracene, tetracene, pentacene, DPP, and PDI, respectively. Based on these results, we are confident that the presented methodology is sound with respect to the level of quantum chemistry and presents an important stepping stone in the search for a tool to predict the properties of larger coupled systems.

The vacuum ultraviolet absorption spectrum of norbornadiene: Vibrational analysis of the singlet and triplet valence states of norbornadiene by configuration interaction and density functional calculations.

A synchrotron-based vacuum ultraviolet (VUV) absorption spectrum of norbornadiene (NBD) is reported, and the extensive vibrational structure obtained has been analyzed. The previously known 5b13s-Rydberg state has been reinterpreted by comparison with our recent high-resolution photoelectron spectral analysis of the X2B1 ionic state. Additional vibrational details in the region of this Rydberg state are observed in its VUV spectrum when compared with the photoelectron 2B1 ionic state; this is attributed to the underlying valence state structure in the VUV. Valence and Rydberg state energies have been obtained by configuration interaction and time-dependent density functional theoretical methods. Several low-lying singlet valence states, especially those that arise from ππ* excitations, conventionally termed NV1 to NV4, have been examined in detail. Their Franck-Condon (FC) and Herzberg-Teller (HT) profiles have been investigated and fitted to the VUV spectrum. Estimates of the experimental 00 band positions have been made from these fits. The anomaly of the observed UV absorption by the 1A2 state of NBD is attributed to HT effects. Generally, the HT components are less than 10% of the FC terms. The calculated 5b13s lowest Rydberg state also shows a low level of HT components. The observed electron impact spectra of NBD have been analyzed in detail in terms of triplet states.

Electronic spectroscopy of the A1̃2A′′/A2̃2A′−X̃2A′ transitions of jet-cooled calcium ethoxide radicals: Vibronic structure of alkaline earth monoalkoxide radicals of C s symmetry

Laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down spectra of the A1̃2A''/A2̃2A'-X̃2A' electronic transition of the calcium ethoxide (CaOC2H5) radical have been obtained under jet-cooled conditions. An essentially constant Ã2-Ã1 energy separation for different vibronic levels is observed in the LIF spectrum, which is attributed to both the spin-orbit (SO) interaction and non-relativistic effects. Electronic transition energies, vibrational frequencies, and spin-vibrational eigenfunctions calculated using the coupled-cluster method, along with results from previous complete active space self-consistent field calculations, have been used to predict the vibronic energy level structure and simulate the recorded LIF/DF spectra. Although the vibrational frequencies and Franck-Condon (FC) factors calculated under the Born-Oppenheimer approximation and the harmonic oscillator approximation reproduce the dominant spectral features well, the inclusion of the pseudo-Jahn-Teller (pJT) and SO interactions, especially those between the A1̃2A″/A2̃2A' and the B̃2A' states, induces additional vibronic transitions and significantly improves the accuracy of the spectral simulations. Notably, the spin-vibronic interactions couple vibronic levels and alter transition intensities. The calculated FC matrix for the A1̃2A''/A2̃2A'-X̃2A' transition contains a number of off-diagonal matrix elements that connect the vibrational ground levels to the levels of the ν8 (CO stretch), ν11 (OCC bending), ν12 (CaO stretch), ν13 (in-plane CaOC bending), and ν21 (out-of-plane CaOC bending) modes, which are used for vibrational assignments. Transitions to the ν21(a″) levels are allowed due to the pJT effect. Furthermore, when LIF transitions to the Ã-state levels of the CaOC-bending modes, ν13 and ν21, are pumped, A1̃2A''/A2̃2A'→X̃2A' transitions to the combination levels of these two modes with the ν8, ν11, and ν12 modes are also observed in the DF spectra due to the Duschinsky mixing. Implications of the present spectroscopic investigation to laser cooling of asymmetric-top molecules are discussed.

Tracking the early nonadiabatic events of ESIPT process in 2-acetylindan-1,3-dione by quantum wavepacket dynamics

Spectroscopy and dynamics associated with the three low-lying singlet excited electronic states of 2-acetylindan-1,3-dione (AID) are studied theoretically to interpret the early events of the excited-state intramolecular proton transfer process (ESIPT). Upon populating “bright” S3, AID decays to “dark” lower electronic states (S2 and S1) on a timescale of about 100 fs. Nuclear densities associated with the OH vibrations obtained from wavepacket calculations reveal their participation in the S3→ S2/S1 internal conversion. We show that AID requires very minimal distortions along these vibrations to access the conical intersections located near to the Franck–Condon (FC) point. Our findings demonstrate that the ultrafast nonadiabatic dynamics taking place within the FC zone make the assignment of ESIPT pathways and associated timescales in AID a challenging task.

(Invited) Why Do Dipole Effects on Charge Transfer Matter?

Charge transfer (CT) and charge transport (CTr) are by far among the most important processes for sustaining life on Earth and for making our modern ways of living possible (Phys. Chem. Chem. Phys. 2020, 22, 21583-21629). In natural and manmade sensing systems, for example, well synchronized CT processes drive the conversion of mechanical, optical, chemical and thermal signals into electric outputs, which is essential for interfacing with processing modules. Concurrently, electric dipoles are everywhere and therefore, understanding how they affect CT and CTr is of principal importance for attaining and controlling the functionality of a wide range of materials and devices. Discussions of the idea about dipole effects on CT date back to the mid 20th century and reported experimental evidence from the 1990s and 2000s demonstrated their key importance. The dipole-generated localized electric fields induce Stark effects and modulate the electronic properties of the CT moieties in their vicinity. The notion for such effects focusses on dipole-induced changes in the reduction potentials of the donor and the acceptor, affecting the CT driving forces and thus, the Franck-Condon (FC) contributions to the CT kinetics. We recently demonstrated that to harness such effects, which are inherently enormous, (1) the dipoles should be placed as closed as possible to the electron donor and acceptor, and (2) the media polarity should be lowered (Angew. Chem. 2018, 57, 12365-12369). Polar media, indeed, stabilize charged states and in general enhances the rates of CT. Polar media, however, screen the field permeation and damps the dipole effect on the electron donor and acceptor. Using hydrocarbons as a medium, results in electron transfer rates along the dipole that are six times larger than the rates for the same system when in polar solvents, such as acetonitrile. The same localized field effects in non-polar medium completely shut down the electron transfer against the dipole. While in the vicinity of dipoles, the electric fields can reach GV/m, they fall of rapidly with distance. In our work, we incorporate the dipoles in the electron donors. This geometry provides the closest possible placement of the dipole to the CT moieties. Most importantly, it has also allowed us to discover for the first time dipole effects on donor-acceptor electronic coupling that dominate and oppose the established notion about effects on the FC contributions to the CT kinetics. These discoveries change the ways of thinking about dipole effects on CT and CTr, and set unprecedented paradigms for the development of electronic materials and devices.

Symmetry-Breaking Enhanced Herzberg-Teller Effect with Brominated Polyacenes.

Molecular symmetry is vital to the selection rule of vibrationally resolved electronic transition, particularly when the nuclear dependence of electronic wave function is explicitly treated by including Franck-Condon (FC) factor, Franck-Condon/Herzberg-Teller (FC/HT) interference, and Herzberg-Teller (HT) coupling. Our present study investigated the light absorption spectra of highly symmetric tetracene, pentacene, and hexacene molecules of point-group D2h, as well as their monobrominated derivatives with a lower Cs symmetry. It was found that the symmetry-breaking monobromination allows more vibrational normal modes and their pairs to contribute to FC/HT interference and HT coupling, respectively. Through a projection of a molecule's vibrational normal modes to its irreducible representations, a linear relationship between the FC/HT intensity to the polyacene's size was deduced alongside a quadratic dependence of the HT intensity. Both theoretically derived correlations were well justified by our numerical simulations, which also demonstrated an approximately 20% improvement on the agreement with experimental line shape if the HT theory is adopted to replace the FC approximation. Moreover, for these low-symmetry monobrominated polyacenes, the FC intensity was even weaker than its FC/HT and HT counterparts at some excitation energies, making the HT theory imperative to decipher vibronic coupling, a fundamental driving force behind numerous chemical, biological, and photophysical processes.

Origin of vibrational features in the excitation energy transfer dynamics of perylene bisimide J-aggregates.

We investigate the role of intramolecular normal mode vibrations in the excitation energy transfer (EET) dynamics of perylene bisimide J-aggregates composed of 2 or 25 units using numerically exact methods. The calculations employ a Frenkel exciton Hamiltonian where the ground and excited electronic states of each molecular unit are coupled to 28 intramolecular normal mode vibrations at various temperatures. The electronic populations exhibit strong damping effects, a lengthening of the EET time scale, and complex dynamical patterns, which depend on aggregate length, temperature, as well as electronic and vibrational initial conditions and which are not additive. The early evolution is dominated by high-frequency vibrational modes, but all modes are responsible for the observed dynamics after the initial 25 fs. Overall, we observe significant changes in the electronic populations upon varying the temperature between 0 and 600 K. With a Franck-Condon (FC) initial excitation, a strongly coupled vibrational mode introduces new peaks to the dimer populations, which show very weak temperature sensitivity. The first of these peaks is also seen in the long aggregate, but subsequent recurrences appear strongly quenched and merged. These structures are drastically altered if a non-FC initial condition is assumed. Additional insights are obtained from the diagonal elements of the dimer electronic-vibrational reduced density matrix. We find that the vibronic peaks result from depletion of the crossing region during the early coherent evolution of the vibrational density away from the crossing point, which allows the premature back-transfer of excitation to the initially excited unit.

Quantitative assessing of crystal field, nephelauxetic, and Stokes shift effects on the blue luminescence of Eu2+ ions incorporated in ZnS films

A quantitative study of the crystal field, nephelauxetic, and Stokes shift effects on the 5d → 4 f levels of Eu2+ ions incorporated in ZnS films is presented. The magnitude of the crystal effect was calculated using the simplest point charge model to express the crystal field splitting of Eu2+ 5d orbitals surrounded by four S2− ligands in tetrahedral coordination symmetry. The shift of the centroid of the 5d levels by nephelauxetic effect was analyzed using a model that involves the polarizability of the anion ligands, similar to that used to investigate this effect for Ce3+ in many inorganic compounds. The Stokes shift was evaluated according to the Frank-Condon principle, under the idealized situation where the Stokes shift and the emission bandwidth can be expressed in terms of the Huang-Rhys parameter. We found that the redshift of the 5d → 4 f transition for the free Eu2+ ion, due to crystal field splitting was half of the redshift due to the nephelauxetic effect. This result is consistent with the large size, large polarizability, and low electronegativity of the sulfide ions. We also found a relatively large Stokes shift which can be attributed to the autoionization of the 5d electrons to the conduction band levels. The total redshift of the 5d and 4 f energy levels with respect to the 5d and 4 f energy gap for a free Eu2+ in vacuum fits quite well with the blue luminescence peaked at 2.73 eV for Eu2+ ions incorporated in the ZnS films. Based on the experimental results and the calculations for the crystal field splitting, nephelauxetic, and Stokes shift effects, we also propose a simplified energy band and levels diagram which helps to explain the excitation-emission mechanisms of the blue luminescence of the Eu2+ ions incorporated in the ZnS films.

Structural Integration of Carbazole and Tetraphenylethylene: Ultrafast Excited‐State Relaxation Dynamics and Efficient Electroluminescence

The synthesis, characterization, and spectroscopic investigations of a new solution‐processable N‐styrylcarbazole‐linked tetraphenylethylene (TPE) derivative (TPE‐BCzS) are reported, exhibiting green–orange emission. While the material displays weak photoluminescence (PL) in solution with a low PL quantum yield (PLQY of 4 ± 0.2%), significant PL enhancement in neat‐film PLQY (55 ± 8%) is observed. Studies using steady‐state spectroscopy, and femtosecond and nanosecond transient absorption spectroscopy reveal details of the excited‐state dynamics, consisting of the Franck–Condon (FC) state, nonradiative conformational relaxation, and formation and decay of the triplet excited state generated via intersystem crossing (ISC) upon ultrafast photoexcitation. Solution‐processed organic light‐emitting diodes (OLEDs) based on TPE‐BCzS display maximum external quantum efficiencies of 1.8% and 1.7% for neat and blend films (20 wt% in a 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl host), approaching the theoretical efficiency limit for the determined PLQYs of the films. While TPE materials are typically associated with aggregation‐induced emission, it is reported that the enhanced PLQYs in the solid state are due to restriction of structural relaxations in the solid state and not due to the commonly misunderstood aggregation effect.

Excited state structure of isolated 2-cyanoindole and the binary 2-cyanoindole-(H2O)1 cluster from a combined Franck-Condon and rotational constants fit

Laser induced fluorescence and fluorescence emission spectra of 2-cyanoindole and the binary 2-cyanoindole-(H2O)1 cluster in a molecular beam were analyzed using a combination of Franck-Condon (FC) fits of the emission spectra and ab initio calculations. The structural changes upon electronic excitation to the lowest excited singlet state have been elucidated by a combined rotational constants/vibronic intensities fit. The experimentally determined geometry changes and the ab initio calculations point to an Lb-like excited state for the monomer. FC fits of emission spectra of the binary water cluster and quantum chemical calculations show that the emission of the water cluster also takes place from an Lb-like excited state structure. However, for 2-cyanoindole-(H2O)1 the second highest occupied molecular orbital mixes strongly with the highest one. Therefore, the difference between La and Lb characteristics are smaller than for other substituted indoles, including the other positional conformers of the n-cyanoindoles.

Photophysical investigation of maleimide units substituted [5]carbohelicene derivatives: Absorption, emission, ECD and CPL spectral-shapes

[5]Carbohelicene derivatives are generally unable to show chirality due to the easily occurred racemization. While stable chirality could be achieved by introducing bulky substituents. To understand the substituent effect on the optical properties, we present a systematic study on the chiroptical properties of two [5]carbohelicene derivatives (HeliIm and MeO-HeliIm) from pure electronic level to vibrational analysis. Electronic calculations show that the introduction of methoxy (MeO-HeliIm) promotes the electron transfer and causes narrower Molecular Orbital (MO) gaps of key orbital pairs with respect to HeliIm. This leads to the relative shift of the spectra between HeliIm and MeO-HeliIm. The vibrationally resolved spectra nicely captured the vibronic features and also correctly reproduced the different signs of chiroptical spectra, in nice agreement with the experiment. They show that Franck-Condon (FC) contribution is of vital importance in the reproduction of the experimental spectral shapes. However, Herzberg-Teller (HT) moderately improves the performance of the vibrationally resolved spectra. What is more, HT modes play a unique role in the analysis of the different optical properties. Finally, it is highlighted that the typical vibration of out-of-plane-deformation of methyl in MeO is responsible for causing the different transition processes.

Quantum calculations of the photoelectron spectra of the OH-·NH3 anion: implications for OH + NH3→ H2O + NH2 reaction dynamics.

We present the results of quantum dynamics calculations for analyzing the experimentally measured photoelectron spectra of the OH-·NH3 anion complex. Detachment of an excess electron of OH-·NH3 initially produces a molecular arrangement, which is close to the transition-state structure of the neutral OH + NH3→ H2O + NH2 hydrogen abstraction reaction due to the Franck-Condon principle, and thus finally leads to the OH + NH3 or H2O + NH2 asymptotic channel. We used both the path integral method and the reduced-dimensionality quantum wave packet method to simulate the photoelectron spectra of the OH-·NH3 anion. The calculated spectra were found to be in qualitative agreement with the experimental spectra. It was found that the photodetached complex mainly dissociates into the OH + NH3 channel; however, we found that the hydrogen exchange process also contributes to the photodetachment spectra.

Observation of a Symmetry-Forbidden Excited Quadrupole-Bound State.

We report the observation of a symmetry-forbidden excited quadrupole-bound state (QBS) in the tetracyanobenzene anion (TCNB-) using both photoelectron and photodetachment spectroscopies of cryogenically-cooled anions. The electron affinity of TCNB is accurately measured as 2.4695 eV. Photodetachment spectroscopy of TCNB- reveals selected symmetry-allowed vibronic transitions to the QBS, but the ground vibrational state was not observed because the transition from the ground state of TCNB- (Au symmetry) to the QBS (Ag symmetry) is triply forbidden by the electric and magnetic dipoles and the electric quadrupole. The binding energy of the QBS is found to be 0.2206 eV, which is unusually large due to strong correlation and polarization effects. A centrifugal barrier is observed for near-threshold autodetachment, as well as relaxations from the QBS vibronic levels to the ground and a valence excited state of TCNB-. The current study shows a rare example where symmetry selection rules, rather than the Franck-Condon principle, govern vibronic transitions to a nonvalence state in an anion.

Dynamic Symmetry in Dozy-Chaos Mechanics

All kinds of dynamic symmetries in dozy-chaos (quantum-classical) mechanics (Egorov, V.V. Challenges 2020, 11, 16; Egorov, V.V. Heliyon Physics 2019, 5, e02579), which takes into account the chaotic dynamics of the joint electron-nuclear motion in the transient state of molecular “quantum” transitions, are discussed. The reason for the emergence of chaotic dynamics is associated with a certain new property of electrons, consisting in the provocation of chaos (dozy chaos) in a transient state, which appears in them as a result of the binding of atoms by electrons into molecules and condensed matter and which provides the possibility of reorganizing a very heavy nuclear subsystem as a result of transitions of light electrons. Formally, dozy chaos is introduced into the theory of molecular “quantum” transitions to eliminate the significant singularity in the transition rates, which is present in the theory when it goes beyond the Born–Oppenheimer adiabatic approximation and the Franck–Condon principle. Dozy chaos is introduced by replacing the infinitesimal imaginary addition in the energy denominator of the full Green’s function of the electron-nuclear system with a finite value, which is called the dozy-chaos energy γ. The result for the transition-rate constant does not change when the sign of γ is changed. Other dynamic symmetries appearing in theory are associated with the emergence of dynamic organization in electronic-vibrational transitions, in particular with the emergence of an electron-nuclear-reorganization resonance (the so-called Egorov resonance) and its antisymmetric (chaotic) “twin”, with direct and reverse transitions, as well as with different values of the electron–phonon interaction in the initial and final states of the system. All these dynamic symmetries are investigated using the simplest example of quantum-classical mechanics, namely, the example of quantum-classical mechanics of elementary electron-charge transfers in condensed media.

Theory of molecular emission power spectra. I. Macroscopic quantum electrodynamics formalism.

We study the emission power spectrum of a molecular emitter with multiple vibrational modes in the framework of macroscopic quantum electrodynamics. The theory we present is general for a molecular spontaneous emission spectrum in the presence of arbitrary inhomogeneous, dispersive, and absorbing media. Moreover, the theory shows that the molecular emission power spectra can be decomposed into the electromagnetic environment factor and lineshape function. In order to demonstrate the validity of the theory, we investigate the lineshape function in two limits. In the incoherent limit (single molecules in a vacuum), the lineshape function exactly corresponds to the Franck-Condon principle. In the coherent limit (single molecules strongly coupled with single polaritons or photons) together with the condition of high vibrational frequency, the lineshape function exhibits a Rabi splitting, the spacing of which is exactly the same as the magnitude of exciton-photon coupling estimated by our previous theory [S. Wang et al., J. Chem. Phys. 151, 014105 (2019)]. Finally, we explore the influence of exciton-photon and electron-phonon interactions on the lineshape function of a single molecule in a cavity. The theory shows that the vibronic structure of the lineshape function does not always disappear as the exciton-photon coupling increases, and it is related to the loss of a dielectric environment.

Doubly resonant SFG and DFG spectroscopies: An analytic model for data analysis including distorted and rotated vibronic levels. II. Applications.

The influence of vibration mode distortion and mode mixing in the vibronic structure of molecules on the doubly resonant sum (SFG) and difference frequency generation spectroscopies is systematically studied in several examples. These phenomena modify the spectral overlap function at the heart of the modeling of the doubly resonant processes. When the visible beam is tuned, under the Franck-Condon approximation, each vibration mode generates two main peaks corresponding to the resonance of the molecular electronic transition with the visible and SFG energies, together with higher order ones driven by the amplitudes of the vibronic displacements. For a single mode system, mode distortion modifies the positions of the main peaks and the excitations of the higher order ones. For a two-mode system, mode mixing induces in addition a deep change in the balance between the intensities of the mixed modes. It also changes the phases of the vibrations, which makes curve fitting difficult. For multimode systems, mode mixing may greatly enhance intensity of a poorly active mode. For all these reasons, it seems mandatory to take mode distortion and mode mixing into account for an accurate analysis of second-order nonlinear spectroscopic experimental data.

Herzberg-Teller Effect on the Vibrationally Resolved Absorption Spectra of Single-Crystalline Pentacene at Finite Temperatures.

The line shape of an electronic spectrum conveys the coupling between electronic and vibrational degrees of freedom. In the present study, the light absorption spectra of single-crystalline pentacene were measured by polarized UV-vis microscopy at 77, 185, and 293 K. The vibronic coupling encoded in each spectrum was resolved by the Herzberg-Teller theory that considers the contributions from the Franck-Condon (FC) factor, Franck-Condo/Herzberg-Teller (FC/HT) interference, and Herzberg-Teller (HT) coupling. Specifically, excitation energies, electronic transition dipole moments, and their nuclear gradients were evaluated by the GW method to ensure numerical accuracy, while the computationally efficient density function theory was employed to determine the optimized structures and vibrational normal modes. For every pair of electronic transition and normal mode that gives rise to a strong vibronic transition intensity, we examined their spatial characteristics by projecting them onto the three crystal axes. It was found that all normal modes strongly coupled to the lowest-lying a-polarized electronic transitions oscillate along axis a, whereas none of their counterparts for the lowest-lying b-polarized electronic transitions is predominantly along axis b. This notable difference on the alignment between the electronic transition and molecular vibration could help the directional control of charge dissociation and/or spin separation. Moreover, a significant variance of the destructive FC/HT interference was discovered with increasing temperatures that can well explain the a-polarized fading tableland near 650 nm. Finally, the importance of HT coupling was corroborated by comparing its intensity with those of FC factor and FC/HT interference. Taken all together, the vibrational dependence of the electronic wave function is critical to resolve the light absorption spectra of single-crystalline pentacene and its temperature effects, facilitating the systematic design of functional optical materials based on pentacene and its derivatives.

Multiphonon Relaxation of the 1s(T2) States of a Singly Ionized Selenium Donor in Silicon

The rate of the multiphonon relaxation of the 1s(T2) level in Se+ donors in silicon is estimated. The calculation comprises the initial approach to the problem, in which the most simplified form of wave functions is used. To evaluate the transition probability, the Passler expression known from publications is used [R. Passler, Czech. J. Phys. B 24, 322 (1974)]. This expression was derived in terms of the so-called static approximation. The deformation potentials of optical and acoustic phonons were determined by the fitting procedure using published data on the luminescence spectrum of Se+ donors at the 1s(T2)–1s(A1) transition and Franck–Condon principle. Evaluation for the relaxation rate of 103 s–1 turned out to be five orders of magnitude smaller than the rate corresponding to the experimentally measured lifetime. The cause of disagreement with the experiment is the excessively simplified model, which does not taking into account several factors, the main one being the presence of quasi-local oscillatory modes. Analysis of the luminescence spectrum based on the mentioned transition leads to the conclusion that the energies of such oscillatory modes lie in the range from 26 to 61 meV. To attain satisfactory agreement with the experiment, the model should be made more complex taking into account the interaction with these modes.

Optical spectroscopy of cryogenic metalated flavins: The O2(+) isomers of M+lumiflavin (M=Li–Cs)

Flavin complexes are used by nature in many photobiological processes, because their photochemical response in the visible range can be strongly modulated by their environment. Herein, we report optical spectra of mass-selected metal complexes of lumiflavin (LF) with alkali metal ions (M=Li–Cs) recorded in the visible range by photodissociation (VISPD) at cryogenic temperatures (T<20 K). VISPD spectra are measured in a tandem mass spectrometer coupled to a cryogenic ion trap and an electrospray ionization source. The vibrationally-resolved VISPD spectra obtained in the 420–450 nm range are assigned to the S0←S1 (ππ*) transition of the O2(+) isomers of M+LF by comparison to time-dependent density functional theory calculations (PBE0/cc-pVDZ) coupled to multidimensional Franck–Condon (FC) simulations. The preferred binding motif and interaction strength strongly depend on the size of M+. The spectra of M+LF with the smaller M+ ions (M=Li and Na) are attributed to the most stable O2+ isomers characterized by a N1–M–O2 chelate binding motif in which M+ can interact with the lone pairs of both N1 and O2 of LF. Because of steric interaction with the CH3 group at N10, the tricyclic aromatic ring is slightly bent and the VISPD spectra feature low-frequency out-of-plane bending modes of LF. Such an O2+ structure is sterically repulsive for the larger M+ ions (M=Rb and Cs), which instead form planar O2 minima with nearly linear C2–O2–M bonds. Their VISPD spectra are characterized by low-frequency in-plane bending modes (β) describing the M+…LF interaction. The VISPD spectrum of K+LF with the intermediate-size K+ ion is more complex and features very low-frequency modes resulting from a very shallow potential along the O2↔O2+ isomerization coordinate, which cannot be described well by harmonic FC simulations. Electronic S1 excitation slightly weakens the M+…LF interaction (6–9%), as deduced from the small ΔS1 blueshifts upon metalation at the O2(+) binding site, consistent with the charge reorganization deduced from the molecular orbitals involved in ππ* excitation. Overall, the effects of O2(+) complexation of LF are drastically different from those of the previously studied O4+ isomers, which are characterized by large ΔS1 redshifts based on the strong increase in the M+…LF interaction (up to 20%) upon the same ππ* excitation of the LF chromophor. The drastic site-specific variation in the photophysical response of the O2(+) and O4+ isomers arises mainly from the different effect of electronic excitation on the M+…LF bond strength rather than from the metal-induced change of the electronic structure and molecular orbitals of LF responsible for ππ* excitation.

On the effective temperature of AlC molecular lines in sunspot umbral spectra

In the present work, a significant rotational lines of the $B^{4}\Sigma ^{-} \mbox{--} X^{4}\Sigma ^{ -} $ ( $0,0$ ) band system of the AlC molecule were identified in high resolution FTS sunspot umbral spectra in the region from 22,450 to 22,600 cm−1. Among the well-resolved identified lines, the rotational temperatures have been estimated to be 4018 K for photosphere and 3722 K for a hot umbra. Hence, the estimated effective rotational temperatures provide evidence for the possible presence of the AlC molecule in a sunspot umbra. The radiative transition parameters for $A^{4}\Pi ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ and $B^{4}\Sigma ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ systems of the AlC molecule have been estimated for experimentally known vibrational levels using the Rydberg–Klein–Rees (RKR) potential. The Franck-Condon (FC) factor of $B^{4}\Sigma ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ ( $0,0$ ) band system is most intense ( $q_{v'v''} = 0.741$ ). For the $B^{4}\Sigma ^{ -}$ state, the radiative lifetime of $v'=0$ level is found as 103.93 ns. The effective vibrational temperature of the $A^{4}\Pi ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ and $B^{4}\Sigma ^{ -} \mbox{--} X^{4}\Sigma ^{ -} $ band systems of the AlC molecule is 3981 K and 2706 K, respectively. Therefore, the effective vibrational temperature and radiative transition parameters help us to ascertain the possible presence of the AlC molecule in the solar atmosphere.