Intersection Seam Research Articles

Excited state dynamics of cis,cis-1,3-cyclooctadiene: Non-adiabatic trajectory surface hopping.

We have performed trajectory surface hopping dynamics for cis,cis-1,3-cyclooctadiene to investigate the photochemical pathways involved after being excited to the S1 state. Our calculations reveal ultrafast decay to the ground state, facilitated by conical intersections involving distortions around the double bonds. The main distortions are localized on one double bond, involving twisting and pyramidalization of one of the carbons of that double bond (similar to ethylene), while a limited number of trajectories decay via delocalized (non-local) twisting of both double bonds. The interplay between local and non-local distortions is important in our understanding of photoisomerization in conjugated systems. The calculations show that a broad range of the conical intersection seam space is accessed during the non-adiabatic events. Several products formed on the ground state have also been observed.

Multidimensional characterization of the conical intersection seam in the normal mode space.

Multidimensional conical intersection seam has been characterized by utilizing the dynamic resonances in the nonadiabatic transition probability experimentally observed in the predissociation of thioanisole isotopomers. The nonadiabatic bifurcation behavior of the reactive flux into either the Herzberg type-I (electronic) or type-II (vibrational) predissociation pathway is found to be strongly dependent on the quantum nature of the S1/S2 vibronic eigenstate, providing the essential information about structure and dynamic character of the conical intersection seam projected onto the normal mode space. By modifying the nature of the normal mode space through partial or full H/D substitution of the molecule, multiple aspects of the conical intersection seam could be characterized from different viewpoints set by the adjusted normal mode space. Theoretical calculations of potential energy curves along selected normal mode displacements support the experiment.

On the nonadiabatic collisional quenching of OH(A) by H2: a four coupled quasi-diabatic state description.

A four-state diabatic potential energy matrix (DPEM), Hd, for the description of the nonadiabatic quenching of OH(A2Σ+) by collisions with H2 is reported. The DPEM is constructed as a fit to adiabatic energies, energy gradients, and derivative couplings obtained exclusively from multireference configuration interaction wave functions. A four-adiabatic-electronic-state representation is used in order to describe all energetically accessible regions of the nuclear coordinate space. Partial permutation-inversion symmetry is incorporated into the representation. The fit is based on electronic structure data at 42 882 points, described by over 1.6 million least squares equations with a root mean square (mean unsigned) error of 178(83) cm-1. Comparison of ab initio and Hd determined minima, saddle points, and energy minimized points on C2v, Cs, C∞v, and C1 (noncoplanar) portions of two conical intersection seams are used to establish the accuracy of the Hd.

Vibronic structure and predissociation dynamics of 2-methoxythiophenol (S1): The effect of intramolecular hydrogen bonding on nonadiabatic dynamics.

Vibronic spectroscopy and the S-H bond predissociation dynamics of 2-methoxythiophenol (2-MTP) in the S1 (ππ*) state have been investigated for the first time. Resonant two-photon ionization and slow-electron velocity map imaging (SEVI) spectroscopies have revealed that the S1-S0 transition of 2-MTP is accompanied with the planar to the pseudoplanar structural change along the out-of-plane ring distortion and the tilt of the methoxy moiety. The S1 vibronic bands up to their internal energy of ∼1000 cm-1 are assigned from the SEVI spectra taken via various S1 vibronic intermediate states with the aid of ab initio calculations. Intriguingly, Fermi resonances have been identified for some vibronic bands. The S-H bond breakage of 2-MTP occurs via tunneling through an adiabatic barrier under the S1/S2 conical intersection seam, and it is followed by the bifurcation into either the adiabatic or nonadiabatic channel at the S0/S2 conical intersection where the diabatic S2 state (πσ*) is unbound with respect to the S-H bond elongation coordinate, giving the excited (Ã) or ground (X̃) state of the 2-methoxythiophenoxy radical, respectively. Surprisingly, the nonadiabatic transition probability at the S0/S2 conical intersection, estimated from the velocity map ion images of the nascent D fragment from 2-MTP-d1 (2-CH3O-C6H4SD) at the S1 zero-point energy level, is found to be exceptionally high to give the X̃/Ã product branching ratio of 2.03 ± 0.20, which is much higher than the value of ∼0.8 estimated for the bare thiophenol at the S1 origin. It even increases to 2.33 ± 0.17 at the ν45 2 mode (101 cm-1) before it rapidly decays to 0.69 ± 0.05 at the S1 internal energy of about 2200 cm-1. This suggests that the strong intramolecular hydrogen bonding of S⋯D⋯OCH3 in 2-MTP at least in the low S1 internal energy region should play a significant role in localizing the reactive flux onto the conical intersection seam. The minimum energy pathway calculations (second-order coupled-cluster resolution of the identity or time-dependent-density functional theory) of the adiabatic S1 state suggest that the intimate dynamic interplay between the S-H bond cleavage and intramolecular hydrogen bonding could be crucial in the nonadiabatic surface hopping dynamics taking place at the conical intersection.

On the Impact of Singularities in the Two-State Adiabatic to Diabatic State Transformation: A Global Treatment.

Diabatizations based on molecular properties can remove the singularity in the derivative coupling at a conical intersection in the diabatic representation. Yet, they can also create new singularities in the derivative couplings because of the defining equations of the adiabatic to diabatic state transformation. In the iconic two-state case, these singularities occur at points termed diabolical singular points and form a seam of dimension Nint-2, where Nint is the number of internal degrees of freedom. This seam is of the same dimension as the conical intersection seam, but is distinct. Here, the global topography of the diabolical singularity seam of 1,21A states of ammonia is reported using a Boys localization (BL) dipole-based diabatization and juxtaposed with a previously reported global representation of the coupled electronic state potential energy surfaces. The principal finding is that the seam of BL diabatization-induced singularities passes very close to the key saddle point on the 21A potential energy surface which connects the 21A equilibrium structure with the NH2(X̃,Ã) + H channel. The construction and detailed study of the reported diabolical singularity seam is made possible by a recently constructed analytic representation of the dipole and transition dipole moment surfaces.

Mechanochromic luminescent materials with aggregation-induced emission: Mechanism study and application for pressure measuring and mechanical printing

In this work, we report the synthesis and emission behavior of two analogue compounds, 2-((E)-((9H-fluoren-2-yl)methylene)amino)-3-aminomaleonitrile (C1) and 2-amino-3-((E)-(4-(diphenylamino)benzylidene)amino)maleonitrile (C2). The two compounds exert both aggregation-induced emission (AIE) nature and intramolecular charge transfer (ICT) character. They show mechanochromic luminescence (MCL) in different ways: The C1 solid powder has strong green emission but would be quenched without color change after grinding. While C2 shows strong green emission in powder form but turns orange-red with a 30% loss in the quantum efficiency after grinding. The force-induced emission of both C1 and C2 can be changed back to the starting state by a simple immersion treatment in organic solvents, suggesting high fatigue resistance. Their MCL mechanisms were studied base on spectroscopic analysis combined with quantum chemical calculation. Powder x-ray diffraction (PXRD) diffractograms suggest a decrease in crystallinity rather than a full amorphization, excluding the phase transition mechanism that has been widely accepted for most of the MCL materials in prior reports. The photophysical process has been modeled according to the quantum chemical calculation using DFT-PBE0/6-311g (d,p), two stable isomers of C1 were detected, which exhibit structural relaxation with a conical intersection seam. The conical intersection of C1 can be accessed by mechanical stimuli but is unable to reach in the starting crystalline phase due to conformation rigidity. In contrast, such structural relaxation is less possible with C2 due to high emission in solution state. And the MCL mechanism mainly involves conformation planarizartion, which leads to bathochromic-shift with its emission. The new working mechanism with C1 is superior to other counterparts on account of its sensitivity and practicability. A new type of pressure sensor based on C1 film was developed, showing remarkable quenching effect with the pressure. A linear relation between pressure and emission intensity was obtained, offering a low detection limit down to 22.92 Mpa. This mechanochromic system succeeds in mechanical printing using MCL: Pressure up to 30 Mpa clearly imprints the cartoon patterns in detail with high contrast using the MCL materials, demonstrating a feasible prototype of printing technology.

Theoretical Study of Ground- and Excited-State Charge Transfer in Fulvene-Based Donor-Acceptor Systems.

Donor-acceptor systems based on fulvene as the electron-accepting moiety are typified by exotic, strongly polar electronic structures. In this contribution, ab initio calculations have been performed to explore the ground- and excited-state properties of an archetypal compound of this class, which incorporates the exocyclic carbon atom of fulvene into a tetramethylimidazoline-like five-membered ring. In the electronic ground state, the compound under study has a pronounced zwitterionic character and is best described as consisting of a negatively charged cyclopentadienyl ring linked covalently to a positively charged tetramethylimidazolium ring. Both of these rings can be considered as aromatic. The excess negative charge localized on the cyclopentadienyl ring is highly labile in the photochemical sense: the low-lying valence excited states exhibit varying degrees of reverse charge transfer, whereby electron density is transferred from the cyclopentadienyl ring back onto the tetramethylimidazolium ring. The topographies of the excited-state potential energy surfaces favor rapid and efficient internal conversion at an extended, fulvene-like S1/S0 conical intersection seam. As a consequence, the excited-state lifetime of this compound is predicted to be on the order of 100 fs.

Potential‐Energy Surface and Dynamics Simulation of THBDBA: An Annulated Tetraphenylethene Derivative Combining Aggregation‐Induced Emission and Switch Behavior

AbstractElectronic structure calculations and nonadiabatic dynamics simulations were carried out on the excited state decay mechanism of a typical aggregation‐induced emission (AIE)‐active molecule, 10,10′,11,11′‐tetrahydro‐5,5′‐bidibenzo[a,d][7]annulenylidene (THBDBA), which was experimentally found to be almost non‐emissive in THF solution but highly luminescent as an aggregate. At the MS‐CASPT2//CASSCF level, the decay path from S1 leads to a conical intersection (CI) seam with the ground state (S0) via a cyclization process of two neighboring phenyl groups. Nonadiabatic dynamics simulations of 367 surface hopping trajectories show that all the structures of hopping points resemble those of the S1/S0 CI seam with cyclized geometry. The final structures of trajectories hopping to S0 belong to either Franck‐Condon (FC) like structures (66.8 %) or cyclized conformers (33.2 %), which agrees well with the experimental result of 23 % cyclization product (Y. Cai et al. Chem. Sci., 2011, 2, 2029–2034). Our results also indicate that long‐time irradiation leads to a photostationary state where the open and cyclized forms interconvert photochemically. The preference for photocyclization over double bond isomerization is due to the rigidity of the dihydrofulvene rings. From the point of view of AIE, the calculations suggest that THBDBA follows our restricted access to a CI (RACI) model because the photocyclization requires large displacements of the phenyl rings. These displacements must be blocked in the aggregate phase, inhibiting the decay and leading to luminescence. Our study contributes to understand the AIE and switch behavior of THBDBA and will be useful to support the design of new molecules with these features.

Ultrafast Photoinduced Dynamics of 1,3-Cyclohexadiene Using XMS-CASPT2 Surface Hopping.

A full-dimensional simulation of the photodissociation of 1,3-cyclohexadiene in the manifold of three electronic states was performed via nonadiabatic surface hopping dynamics using extended multistate complete active space second-order perturbation (XMS-CASPT2) electronic structure theory with fully analytic nonadiabatic couplings. With the 47 ± 8% product quantum yield calculated from the 136 trajectories, generally 400 fs-long, and an estimated excited lifetime of 89 ± 9 fs, our calculations provide a detailed description of the nonadiabatic deactivation mechanism, showing the existence of an extended conical intersection seam along the reaction coordinate. The nature of the preferred reaction pathways on the ground state is discussed and extensive comparison to the previously published full dimensional dynamics calculations is provided.

Site-Selective Isomerization of Cyano-Substituted Butadienes: Chemical Control of Nonadiabatic Dynamics.

The photochemistry of organic chromophores generally involves the co-evolution of the electronic and nuclear degrees of freedom. To obtain a specific and predetermined photochemical reaction outcome, chemical substitution can be used to selectively alter the underlying electronic potential energy surfaces to favor a particular reaction pathway. We show using ab initio simulation that the substitution of s- trans-1,3-butadiene with a cyano group can effectively "direct" a molecular wavepacket to particular regions of the seam of conical intersection and either favor or inhibit the photoinitiated cis-trans isomerization. The substituent is able to effect this control due to the formation of transient charge-separated electronic structures that arise during the nonadiabatic dynamical process. The atomic site at which this charge develops can be selectively stabilized (or destabilized) depending on the location of the cyano substituent and gives rise to a single dominant decay pathway. This work aims to demonstrate how the application of known electron density effects to ultrafast dynamics may be used to obtain desired photochemical reactions and properties.

MetaFALCON: A Program Package for Automatic Sampling of Conical Intersection Seams Using Multistate Metadynamics.

The multistate metadynamics for automatic exploration of conical intersection seams and systematic location of minimum energy crossing points in molecular systems and its implementation into the software package metaFALCON is presented. Based on a locally modified energy gap between two Born-Oppenheimer electronic states as a collective variable, multistate metadynamics trajectories are driven toward an intersection point starting from an arbitrary ground state geometry and are subsequently forced to explore the conical intersection seam landscape. For this purpose, an additional collective variable capable of distinguishing structures within the seam needs to be defined and an additional bias is introduced into the off-diagonal elements of an extended (multistate) electronic Hamiltonian. We demonstrate the performance of the algorithm on the examples of the 1,3-butadiene, benzene, and 9H-adenine molecules, where multiple minimum energy crossing points could be systematically located using the Wiener number or Cremer-Pople parameters as collective variables. Finally, with the example of 9H-adenine, we show that the multistate metadynamics potential can be used to obtain a global picture of a conical intersection seam. Our method can be straightforwardly connected with any ab initio or semiempirical electronic structure theory that provides energies and gradients of the respective electronic states and can serve for systematic elucidation of the role of conical intersections in the photophysics and photochemistry of complex molecular systems, thus complementing nonadiabatic dynamics simulations.

Nonadiabatic Dynamics in Photodissociation of Hydroxymethyl in the 32A(3p x) Rydberg State: A Nine-Dimensional Quantum Study.

The nonadiabatic predissociation dynamics of the hydroxymethyl radical (CH2OH) in its 32A(3p x) state is investigated using a nine-dimensional quantum mechanical model based on an ab initio three coupled diabatic state potential energy matrix. The calculated absorption spectrum, which is dominated by predissociative resonances, is in excellent agreement with experiment. The predissociation is facilitated by two conical intersection seams formed between the 32A(3p x) and 22A(3s) states near the Franck-Condon region. The h and g vectors of energy minimized points on these seams are analyzed using the normal modes of the 32A equilibrium structure. The low-lying predissociative resonances have been assigned and their lifetimes are less than 100 fs and moderately mode specific. The absorption spectrum is dominated by a CO vibrational progression, due apparently to the promotion of an electron from the ground state antibonding πCO* orbital to the carbon Rydberg orbital, which effectively increases the C-O bond order.

Six-dimensional potential energy surface for NaK-NaK collisions: Gaussian process representation with correct asymptotic form.

Constructing accurate global potential energy surfaces (PESs) describing chemically reactive molecule-molecule collisions of alkali metal dimers presents a major challenge. To be suitable for quantum scattering calculations, such PESs must represent accurately three- and four-body interactions, describe conical intersections, and have a proper asymptotic form at the long range. Here, we demonstrate that such global potentials can be obtained by Gaussian Process (GP) regression merged with the analytic asymptotic expansions at the long range. We propose an efficient sampling technique, which allows us to construct an accurate global PES accounting for different chemical arrangements with <2500 ab initio calculations. We apply this method to (NaK)2 and obtain the first global PES for a system of four alkali metal atoms. The resulting surface exhibits a complex landscape including a pair and a quartet of symmetrically equivalent local minima and a seam of conical intersections. The dissociation energy found from our ab initio calculations is 4534 cm-1. This result is reproduced by the GP models with an error of less than 3%. The GP models of the PES allow us to analyze the features of the global PES, representative of general alkali metal four-atom interactions. Understanding these interactions is of key importance in the field of ultracold chemistry.

Up to a Sign. The Insidious Effects of Energetically Inaccessible Conical Intersections on Unimolecular Reactions.

It is now well established that conical intersections play an essential role in nonadiabatic radiationless decay where their double-cone topography causes them to act as efficient funnels channeling wave packets from the upper to the lower adiabatic state. Until recently, little attention was paid to the effect of conical intersections on dynamics on the lower state, particularly when the total energy involved is significantly below that of the conical intersection seam. This energetic deficiency is routinely used as a sufficient condition to exclude consideration of excited states in ground state dynamics. In this account, we show that, this energy criterion notwithstanding, energy inaccessible conical intersections can and do exert significant influence on lower state dynamics. The origin of this influence is the geometric phase, a signature property of conical intersections, which is the fact that the real-valued electronic wave function changes sign when transported along a loop containing a conical intersection, making the wave function double-valued. This geometric phase is permitted by an often neglected property of the real-valued adiabatic electronic wave function; namely, it is determined only up to an overall sign. Noting that in order to change sign a normalized, continuous function must go through zero, for loops of ever decreasing radii, demonstrating the need for an electronic degeneracy (intersection) to accompany the geometric phase. Since the total wave function must be single-valued a compensating geometry dependent phase needs to be included in the total electronic-nuclear wave function. This Account focuses on how this consequence of the geometric phase can modify nuclear dynamics energetically restricted to the lower state, including tunneling dynamics, in directly measurable ways, including significantly altering tunneling lifetimes, thus confounding the relation between measured lifetimes and barrier heights and widths, and/or completely changing product rotational distributions. Some progress has been made in understanding the origin of this effect. It has emerged that for a system where the lower adiabatic potential energy surface exhibits a topography comprised of two saddle points separated by a high energy conical intersection, the effect of the geometric phase can be quite significant. In this case topologically distinct paths through the two adiabatic saddle points may lead to interference. This was pointed out by Mead and Truhlar almost 50 years ago and denoted the Molecular Aharonov-Bohm effect. Still, the difficulty in anticipating a significant geometric phase effect in tunneling dynamics due to energetically inaccessible conical intersections leads to the attribute insidious that appears in the title of this Account. Since any theory is only as relevant as the prevalence of the systems it describes, we include in this Account examples of real systems where these effects can be observed. The accuracy of the reviewed calculations is high since we use fully quantum mechanical dynamics and construct the geometric phase using an accurate diabatic state fit of high quality ab initio data, energies, energy gradients, and interstate couplings. It remains for future work to establish the prevalence of this phenomenon and its deleterious effects on the conventional wisdom discussed in this work.

Intersystem crossing in the exit channel.

Intersystem crossing plays an important role in photochemistry. It is understood to be efficient when heavy atoms are present due to strong spin-orbit coupling, or when strongly bound long-lived complexes are formed that increase the chance of finding the singlet-triplet intersection seam. Here we present evidence for a different intersystem crossing mechanism in the bimolecular reaction of O(3P) with alkylamines. In crossed-beam experiments, product velocity-flux maps are measured for aminoalkyl radicals produced from H abstraction from the methyl group, which also gives OH radicals as co-fragments. The low translational-energy release and isotropic angular distributions of the products indicate that such reactions undergo the formation of a complex before OH and aminoalkyl are produced. However, there is no well on the triplet potential energy surface that could support such a complex. Multi-reference ab initio calculations suggest, instead, that intersystem crossing occurs in the exit-channel region due to the long-range dipole-dipole interaction between the nascent radical product pair coupled with the vanishing singlet-triplet splitting at long range. Intersystem crossing then leads to a deep hydroxylamine well before OH elimination.

Comparative study of the photodynamics of malonaldehyde and acetylacetone

In this computational study we explore the photodynamics of malonaldehyde and compare it to that of the homologous acetylacetone. Upon photoexcitation to the bright S2(ππ∗) state malonaldehyde relaxes to the S1(nπ∗) state on an ultrashort time scale. Once in the S1(nπ∗) state the population of malonaldehyde splits. Part of the population undergoes internal conversion to the ground state by deplanarization of the H-chelate ring, while the other part crosses to the triplet manifold. The comparison with acetylacetone reveals a substantial increase of the internal conversion pathways, from less than 20% in acetylacetone to 40% in malonaldehyde. We show that the substitution of the aliphatic hydrogens by the methyl groups reduces the accessibility of the S1/S0 conical intersection seam in acetylacetone. The system then crosses to the triplet manifold and homolytic C–CH3 bond cleavage takes place.

Resolving the excited state relaxation dynamics of guanosine monomers and hydrogen-bonded homodimers in chloroform solution

The relaxation pathways of silyl-modified guanosine nucleoside monomers (G) and double-hydrogen-bonded homodimers (GG1) are compared in chloroform solution after 260-nm ultraviolet excitation. Transient absorption spectra support two previously reported relaxation pathways for the monomer with time constants of 210 ± 20 fs and 2.6 ± 0.1 ps. These pathways are associated with bifurcated approach to a seam of conical intersections between the excited 1ππ* 1La state and the ground electronic state. In the homodimer, an increase in the larger time constant to 18 ± 2 ps is attributed to slower passage through the minimum energy region of the 1ππ* state. A further time constant of 70 ± 10 fs indicates wavepacket evolution out of the 1ππ* state Franck-Condon region. A slow component of recovery of ground-state GG1 is proposed to result either from relaxation of the product of inter-base electron-driven proton transfer, or from the lowest triplet state (3ππ* , T1).

Multistate metadynamics for automatic exploration of conical intersections

We introduce multistate metadynamics for automatic exploration of conical intersection seams between adiabatic Born-Oppenheimer potential energy surfaces in molecular systems. By choosing the energy gap between the electronic states as a collective variable the metadynamics drives the system from an arbitrary ground-state configuration toward the intersection seam. Upon reaching the seam, the multistate electronic Hamiltonian is extended by introducing biasing potentials into the off-diagonal elements, and the molecular dynamics is continued on a modified potential energy surface obtained by diagonalization of the latter. The off-diagonal bias serves to locally open the energy gap and push the system to the next intersection point. In this way, the conical intersection energy landscape can be explored, identifying minimum energy crossing points and the barriers separating them. We illustrate the method on the example of furan, a prototype organic molecule exhibiting rich photophysics. The multistate metadynamics reveals plateaus on the conical intersection energy landscape from which the minimum energy crossing points with characteristic geometries can be extracted. The method can be combined with the broad spectrum of electronic structure methods and represents a generally applicable tool for the exploration of photophysics and photochemistry in complex molecules and materials.

Energy-switching potential energy surface for ground-state [formula omitted

The multiple energy switching scheme [J. Chem. Phys. 119 (2003) 2596] has been used to improve the double many-body expansion (DMBE II) potential energy surface of C3 near its linear global minima by morphing it with an accurate Taylor-series expansion [J. Chem. Phys. 144 (2016) 044307]. The final ES form attains the accuracy of the local form in reproducing the rovibrational spectrum of C3 while keeping unaltered all key attributes of the original DMBE II, namely conical intersection seams and dissociative channels. The ES form is therefore commended for adiabatic spectroscopic and reaction dynamics studies.

Ultrafast internal conversion in 4-aminobenzonitrile occurs sequentially along the seam

Upon UV light absorption, 4-aminobenzonitrile undergoes an ultrafast radiationless decay process from a charge-transfer state to a locally-excited state. This pathway proceeds through an extended seam of conical intersections between the second- and first-excited singlet electronic states. Quantum dynamics simulations show that planar geometries dominate the earlier times (<20 fs), after which the whole seam becomes explored, as the wavepacket spreads and breathes along both sides of the unstable ridge with respect to the amino-bending coordinate.