Minimum Energy Conical Intersections Research Articles

Ab Initio Molecular Dynamics Study of the Photoreaction of 1,1′-Dimethylstilbene upon S0 → S1 Excitation

Ab initio molecular dynamics (AIMD) simulations were carried out for ππ*-excited 1,1'-dimethylstilbene (dmSB) at the spin-flip time-dependent density functional theory (SF-TDDFT) level with the TSF-index technique, to get insights into the substitution effects on the photoisomerization dynamics of stilbene (SB). It is found that the reaction path from the Franck-Condon structure of cis-dmSB is oriented toward the 4,4-dihydrophenanthrene (DHP) side from the beginning, which is in contrast to the case of SB where the pathway is oriented toward the twist side in the initial stage. The optimized geometries of minima and the minimum-energy conical intersection (MECI) suggested that molecules in the DHP region could easily decay to the ground state. On the other hand, S1/S0-MECI and S1-minimum in the twist region have a relatively different geometry from each other, which is consistent with the experimental observation of the long lifetime of the perpendicular structure. AIMD simulations showed that more trajectories enter the well of the DHP side than the well of the twist side and that all of the trajectories going to the DHP side reached the S1/S0-CI region within ∼0.2 ps on average, while very few trajectories reached S1/S0-CI even after 1 ps in the twist region. Decrease in the S1 population in the cis and twist regions qualitatively reproduced the temporal profiles of the transient absorption bands of dmSB observed in the visible and ultraviolet regions, respectively.

New Insight into the Photoisomerization Process of the Salicylidene Methylamine under Vacuum.

The deactivation process of salicylidene methylamine in the gas phase has been explored using static calculations (CASSCF, CASPT2, and CC2) and on-the-fly surface hopping dynamics simulations (CASSCF). Five minimum energy conical intersections (MECIs) were located upon the geometry optimization calculations. One corresponds to the excited state intramolecular proton transfer (ESIPT) process, and the remaining four arise from CN bond rotational motion. Our calculation results found that the molecule prefers to decay to the ground state through the four rotational motion related MECIs rather than the ESIPT related one. This mechanistic scenario is verified by the energy profiles connecting the Franck-Condon point and the MECIs at CASSCF, CASPT2, and CC2 levels. Our proposed new decay mechanism can explain the previous experimental findings of femtosecond pump-probe photoionization spectroscopy and can provide additional guidance to the rational design of photochemically switchable molecules.

Nonadiabatic Pathways of Furan and Dibenzofuran: What Makes Dibenzofuran Fluorescent?

Automated searches for minimum energy conical intersection structures between the lowest two singlet electronic states (S0/S1-MECIs) of furan and dibenzofuran were made. In total, eight and sixty S0/S1-MECIs were found for furan and dibenzofuran, respectively, around their Franck–Condon (FC) regions. The difference in their experimental fluorescence quantum yields was discussed based on the barrier heights to reach S0/S1-MECIs from the FC regions. It was found that the stability of the FC region is the major factor giving the difference in their fluorescence quantum yields.

Semiempirical Quantum-Chemical Orthogonalization-Corrected Methods: Benchmarks of Electronically Excited States.

The semiempirical orthogonalization-corrected OMx methods have recently been shown to perform well in extensive ground-state benchmarks. They can also be applied to the computation of electronically excited states when combined with a suitable multireference configuration interaction (MRCI) treatment. We report on a comprehensive evaluation of the performance of the OMx/MRCI methods for electronically excited states. The present benchmarks cover vertical excitation energies, excited-state equilibrium geometries (including an analysis of significant changes between ground- and excited-state geometries), minimum-energy conical intersections, ground- and excited-state zero-point vibrational energies, and 0-0 transition energies for a total of 520 molecular structures and 412 excited states. For comparison, we evaluate the TDDFT/B3LYP method for all benchmark sets, and the CC2, MRCISD, and CASPT2 methods for some of them. We find that the current OMx/MRCI methods perform reasonably well for many of the excited-state properties. However, in comparison to the first-principles methods, there are also a number of shortcomings that should be addressed in future developments.

Non-adiabatic dynamics investigation of the radiationless decay mechanism of trans-urocanic acid in the S2 state

The trans-urocanic acid, a UV chromophore in the epidermis of human skin, was found to exhibit a wavelength dependent isomerization property. The isomerization quantum yield to cis-urocanic is greatest when being excited to the S1 state, whereas exciting the molecule to the S2 state causes almost no isomerization. The comparative photochemical behavior of the trans-urocanic on the S1 and S2 states continues to be the subject of intense research effort. This study is concerned with the unique photo-behavior of this interesting molecule on the S2 state. Combining the on-the-fly surface hopping dynamics simulations and static electronic structure calculations, three decay channels were observed following excitation to the S2 state. An overwhelming majority of the molecules decay to the S1 state through a planar or pucker characterized minimum energy conical intersection (MECI), and then decay to the ground state along a relaxation coordinate driven by a pucker deformation of the ring. A very small fraction of molecules decay to the S1 state by a MECI characterized by a twisting motion around the CC double bond, which continues to drive the molecule to deactivate to the ground state. The latter channel is related with the photoisomerization process, whereas the former one will only generate the original trans-form products. The present work provides a novel S2 state decay mechanism of this molecule, which offers useful information to explain the wavelength dependent isomerization behavior.

Highly Twisted N,N-Dialkylamines as a Design Strategy to Tune Simple Aromatic Hydrocarbons as Steric Environment-Sensitive Fluorophores.

The steric-environment sensitivity of fluorescence of 9,10-bis(N,N-dialkylamino)anthracenes (BDAAs) was studied experimentally and theoretically. A new design strategy to tune simple aromatic hydrocarbons as efficient aggregation-induced emission (AIE) luminogens and molecular rotors is proposed. For a variety of BDAAs, prominent Stokes shifts and efficient solid-state fluorescence were observed. Calculations on BDAA-methyl suggested that in the ground state (S0) conformations, the pyramidal amine groups are highly twisted, so that their lone-pair orbitals cannot conjugate with the anthracene π orbitals. Fluorescence takes place from the S1 minima, in which one or both amine groups are planarized. The stability of the S1 excited state minima as well as destabilization of the S0 state is the origin of large Stokes shift. Experimental measurement of the nonadiabatic transition rate suggests that para disubstitution by dialkylamino (or strongly electron-donating) groups is a key for fast internal conversion. Minimum energy conical intersection (MECI) between S1 and S0 states was found to have a Dewar-benzene like structure. Although this can be reached efficiently in liquid phase for fast internal conversion, a large amplitude motion is required to reach this MECI, which is prohibited in the solid state and caused efficient AIE. This strategy is used to find experimentally that naphthalene analogues are also efficient AIE luminogens. The flexibility of alkyl chains on amino groups is also found to be important for allowed charge-transfer transition. Thus, three points [(1) highly twisted N,N-dialkylamines, (2) substitution at the para positions, (3) with flexible alkyl groups] were proposed for activation of small aromatic hydrocarbons.

Exploring the Mechanism of Ultrafast Intersystem Crossing in Rhenium(I) Carbonyl Bipyridine Halide Complexes: Key Vibrational Modes and Spin-Vibronic Quantum Dynamics.

The mechanism of ultrafast intersystem crossing in rhenium(I) carbonyl bipyridine halide complexes Re(X)(CO)3(bpy) (X = Cl, Br, I) is studied by exploring the structural deformations when going from Franck-Condon (FC) to critical geometries in the low-lying singlet and triplet excited states and by selecting the key vibrational modes. The luminescent decay observed in [Re(Br)(CO)3(bpy)] is investigated by means of wavepacket propagations based on the multiconfiguration time-dependent Hartree (MCTDH) method. The dominant coordinates underlying the nonradiative decay process are extracted from minima, minimum energy seam of crossing (MESX) and minimum energy conical intersection (MECI) geometries obtained by the seam model function (SMF)/single-component artificial force induced reaction (SC-AFIR) approach. By choosing the normal modes used in MCTDH from the MECI and MESX geometries, not only the degenerate energy points but also the low-energy-gap regions are included. For this purpose a careful vibrational analysis is performed at each critical geometry and analyzed under the light of the pertinent nonadiabatic coupling terms obtained from the linear vibronic coupling (LVC) model augmented by spin-orbit coupling (SOC) in the electronic diabatic representation.

Dynamical Correlation Effects on Photoisomerization: Ab Initio Multiple Spawning Dynamics with MS-CASPT2 for a Model trans-Protonated Schiff Base.

We investigate the photoisomerization of a model retinal protonated Schiff base (trans-PSB3) using ab initio multiple spawning (AIMS) based on multistate second order perturbation theory (MSPT2). Discrepancies between the photodynamical mechanism computed with three-root state-averaged complete active space self-consistent field (SA-3-CASSCF, which does not include dynamic electron correlation effects) and MSPT2 show that dynamic correlation is critical in this photoisomerization reaction. Furthermore, we show that the photodynamics of trans-PSB3 is not well-described by predictions based on minimum energy conical intersections (MECIs) or minimum energy conical intersection (CI) seam paths. Instead, most of the CIs involved in the photoisomerization are far from MECIs and minimum energy CI seam paths. Thus, both dynamical nuclear effects and dynamic electron correlation are critical to understanding the photochemical mechanism.

Rapid intersystem crossings in anti bimanes.

We have carried out first principles electronic structure calculations on the ground and excited valence states of syn and anti bimanes. While syn bimanes fluoresce strongly after photoexcitation to the first excited singlet state (S1) and are commonly used as fluorophores in biological labeling studies, anti bimanes largely phosphoresce at low temperatures. We show that this is due to subtle differences in the energetic ordering of excited singlet and triplet states within the isomers. In particular, T2 in anti bimanes is characterized by a π→π* transition and large exchange interactions with the singlet counterpart cause it to lie below and energetically close to S1 at the Franck-Condon region. This opens up a pathway for very fast intersystem crossing (ca. 10(11) s(-1)) from the optically bright S1 state to the triplet manifold, which effectively quenches fluorescence. On the other hand, T2 is energetically inaccessible to S1 in syn bimanes and intersystem crossing via S1→ T1 cannot compete effectively with fluorescence to S0. We have also located minimum energy conical intersections between S0 and S1 in bimanes. However, these structures are significantly distorted from their equilibrium geometries as well as energetically much higher than S1 at the Franck-Condon region. They are therefore not expected to play a part in the photophysics of bimanes after excitation to S1.

Accidental degeneracy in the spiropyran radical cation: charge transfer between two orthogonal rings inducing ultra-efficient reactivity.

Photochromism of the spiropyran radical cation to the corresponding merocyanine form is investigated by a combination of electrochemical oxidation, UV/vis absorption spectroscopy, spectroelectrochemistry and first-principles calculations (TD-DFT, CAS-SCF and CAS-PT2). First, we demonstrate that the ring-opening of mono-spiropyrans occurs upon one-electron oxidation and that it can be driven photochemically as well as thermally, with trapping of the merocyanine by protonation. Second, in order to explain this experimentally observed spectroelectrochemical behaviour we suggest a theoretical mechanism based on the reactivity of the two lowest electronic excited-states, which promotes effective electron transfer from the indoline (nitrogen-ring) to the pyran (oxygen-ring) moieties (and vice versa) through a conical intersection seam of degeneracy. Characterisation of the minimum energy conical intersection on this crossing revealed that it presents a rare diabatic trapping topology. The excited state molecule cannot escape from crossing the intersection seam due to the presence of only one degeneracy-lifting coordinate that efficiently channels into the formation of the merocyanine photoproduct, so giving rise to a "kitchen sink" funnel-like effect. Therefore, assuming rapid relaxation after vertical excitation to a higher electronic state, photoconversion cannot be avoided in the D1 electronic state, which rationalises the remarkably efficient visible light driven excited-state reactivity observed experimentally.

Computational Design of a Family of Light-Driven Rotary Molecular Motors with Improved Quantum Efficiency.

Two new light-driven molecular rotary motors based on the N-alkylated indanylidene benzopyrrole frameworks are proposed and studied using quantum chemical calculations and nonadiabatic molecular dynamics simulations. These new motors perform pure axial rotation, and the photochemical steps of the rotary cycle are dominated by the fast bond-length-alternation motion that enables ultrafast access to the S1/S0 intersection. The new motors are predicted to display a quantum efficiency higher than that of the currently available synthetic all-hydrocarbon motors. Remarkably, the quantum efficiency is not governed by the topography (peaked versus sloped) of the minimum-energy conical intersection, whereas the S1 decay time depends on the topography as well as on the energy of the intersection relative to the S1 minimum. It is the axial chirality (helicity), rather than the point chirality, that controls the sense of rotation of the motor.

Exploration of Quenching Pathways of Multiluminescent Acenes Using the GRRM Method with the SF-TDDFT Method.

The quenching pathways were investigated for three types of multiluminescent acene derivatives, which show environment-dependent fluorescence. Spin-flip time dependent density functional theory (SF-TDDFT) combined with the Global Reaction Route mapping (GRRM) strategy is employed to locate minimum-energy conical intersections (MECIs). The energies and geometries of the MECIs relative to the Franck-Condon (FC) state control the difference in fluorescence behavior among the three derivatives. For the molecule with a phenyamide moiety, a MECI with energy lower than the FC state with large geometrical change from V-type to flat structure provides an efficient internal conversion (quenching) pathway in solution. For the same molecule, in a solid, this large geometrical change is inhibited, and the second MECI, with an energy lower than FC but higher than the first MECI requiring only a small geometry change of CH out-of-plane bending, contributes to the quenching. The molecule with the napthaleneimide moiety has only one low-energy MECI that requires large geometrical change from the V-type to flat structure. Although this MECI provides the quenching pathway in solution, in the solid, this large motion is inhibited, and the molecule will stay in the excited state and emit. The molecule with an anthraceneimide moiety has no conical intersection lower than the FC state, and no quenching pathway is available in solution or solid. In addition, in this molecule, at the local minimum of the excited state, the dipole transition to the ground state is allowed, and this molecule prefers emission rather than internal conversion.

Assessment of Approximate Coupled-Cluster and Algebraic-Diagrammatic-Construction Methods for Ground- and Excited-State Reaction Paths and the Conical-Intersection Seam of a Retinal-Chromophore Model.

As a minimal model of the chromophore of rhodopsin proteins, the penta-2,4-dieniminium cation (PSB3) poses a challenging test system for the assessment of electronic-structure methods for the exploration of ground- and excited-state potential-energy surfaces, the topography of conical intersections, and the dimensionality (topology) of the branching space. Herein, we report on the performance of the approximate linear-response coupled-cluster method of second order (CC2) and the algebraic-diagrammatic-construction scheme of the polarization propagator of second and third orders (ADC(2) and ADC(3)). For the ADC(2) method, we considered both the strict and extended variants (ADC(2)-s and ADC(2)-x). For both CC2 and ADC methods, we also tested the spin-component-scaled (SCS) and spin-opposite-scaled (SOS) variants. We have explored several ground- and excited-state reaction paths, a circular path centered around the S1/S0 surface crossing, and a 2D scan of the potential-energy surfaces along the branching space. We find that the CC2 and ADC methods yield a different dimensionality of the intersection space. While the ADC methods yield a linear intersection topology, we find a conical intersection topology for the CC2 method. We present computational evidence showing that the linear-response CC2 method yields a surface crossing between the reference state and the first response state featuring characteristics that are expected for a true conical intersection. Finally, we test the performance of these methods for the approximate geometry optimization of the S1/S0 minimum-energy conical intersection and compare the geometries with available data from multireference methods. The present study provides new insight into the performance of linear-response CC2 and polarization-propagator ADC methods for molecular electronic spectroscopy and applications in computational photochemistry.

An atomic orbital-based formulation of analytical gradients and nonadiabatic coupling vector elements for the state-averaged complete active space self-consistent field method on graphical processing units.

We recently presented an algorithm for state-averaged complete active space self-consistent field (SA-CASSCF) orbital optimization that capitalizes on sparsity in the atomic orbital basis set to reduce the scaling of computational effort with respect to molecular size. Here, we extend those algorithms to calculate the analytic gradient and nonadiabatic coupling vectors for SA-CASSCF. Combining the low computational scaling with acceleration from graphical processing units allows us to perform SA-CASSCF geometry optimizations for molecules with more than 1000 atoms. The new approach will make minimal energy conical intersection searches and nonadiabatic dynamics routine for molecular systems with O(10(2)) atoms.

Analytic first derivatives of floating occupation molecular orbital-complete active space configuration interaction on graphical processing units

The floating occupation molecular orbital-complete active space configuration interaction (FOMO-CASCI) method is a promising alternative to the state-averaged complete active space self-consistent field (SA-CASSCF) method. We have formulated the analytic first derivative of FOMO-CASCI in a manner that is well-suited for a highly efficient implementation using graphical processing units (GPUs). Using this implementation, we demonstrate that FOMO-CASCI gradients are of similar computational expense to configuration interaction singles (CIS) or time-dependent density functional theory (TDDFT). In contrast to CIS and TDDFT, FOMO-CASCI can describe multireference character of the electronic wavefunction. We show that FOMO-CASCI compares very favorably to SA-CASSCF in its ability to describe molecular geometries and potential energy surfaces around minimum energy conical intersections. Finally, we apply FOMO-CASCI to the excited state hydrogen transfer reaction in methyl salicylate.

Excited State Relaxation of Neutral and Basic 8-Oxoguanine.

8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dGuo) is one of the most common forms of DNA oxidative damage. Recent studies have shown that 8-oxo-dGuo can repair cyclobutane pyrimidine dimers in double-stranded DNA when photoexcited, making its excited state dynamics of particular interest. The excited state lifetimes of 8-oxo-dGuo and its anion have been previously probed using transient absorption spectroscopy; however, more information is required to understand the decay mechanisms. In this work, excited state potential energy surfaces for the neutral and deprotonated forms of the free base, 8-oxoguanine (8-oxo-G), are explored theoretically using multireference methods while the nucleoside is experimentally studied using steady-state fluorescence spectroscopy. It is determined that the neutral species exhibits ultrafast radiationless decay via easy access to conical intersections. The relatively long lifetime for the anion can be explained by the existence of sizable barriers between the Franck-Condon region and two S1/S0 minimum energy conical intersections. A Strickler-Berg analysis of the experimentally measured fluorescence quantum yields and lifetimes is consistent with emission from ππ* excited states in line with theoretical predictions.

Photochemical ring opening and closing of three isomers of diarylethene: spin-flip time-dependent density functional study.

The reaction mechanism of photochemical ring opening and closing transformation was investigated for diarylethene (DAE), which works as a molecular switch and photodevice. Spin-flip time-dependent density functional theory is employed to map the potential energy surfaces and to elucidate the photochemical mechanism of three isomers (normal, inverse, and mixed types) of 1,2-dithienylethene, a model DAE. The potential energy characteristics including the minimum-energy conical intersection reveals the origin of different product preferences of the three isomers. For the normal type, the excited state from either closed or open form reaches the same conical intersection that gives preferentially the closed product. The inverse type preferentially gives the closed product. The mixed type has two pathways that are easily convertible, and both open and closed reactants give both open and closed products.

Conical Intersection Optimization Using Composed Steps Inside the ONIOM(QM:MM) Scheme: CASSCF:UFF Implementation with Microiterations.

Three algorithms for optimization of minimum energy conical intersections (MECI) are implemented inside an ONIOM(QM:MM) scheme combined with microiterations. The algorithms follow the composed gradient (CG), composed gradient-composed steps (CG-CS), and double Newton-Raphson-composed step (DNR-CS) schemes developed previously for purely QM optimizations. The CASSCF and UFF methods are employed for the QM and MM calculations, respectively. Conical intersections are essential to describe excited state processes in chemistry, including biological systems or functional molecules, and our approach is suitable for large molecules or systems where the excitation is well localized on a fragment that can be treated at the CASSCF level. The algorithms are tested on a set of 14 large hydrocarbons composed of a medium-sized chromophore (fulvene, benzene, butadiene, and hexatriene) derivatized with alkyl substituents. Thanks to the microiteration technique, the number of steps required to optimize the MECI of the large molecules is similar to the one needed to optimize the unsubstituted chromophores at the QM level. The three tested algorithms have a similar performance, although the CG-CS implementation is the most efficient one on average. The implementation can be straightforwardly applied to ONIOM(QM:QM) schemes, and its potential is further demonstrated locating the MECI of diphenyl dibenzofulvene (DPDBF) in its crystal, which is relevant for the aggregation induced emission (AIE) of this molecule. A cluster of 12 molecules (528 atoms) is relaxed during the MECI optimization, with one molecule treated at the QM level. Our results confirm the mechanistic picture that AIE in DPDBF is due to the packing of the molecules in the crystal. Even when the molecules surrounding the excited molecule are allowed to relax, the rotation of the bulky substituents is hindered, and the conical intersection responsible for radiationless decay in solution is not accessible energetically.

Exploration of minimum energy conical intersection structures of small polycyclic aromatic hydrocarbons: toward an understanding of the size dependence of fluorescence quantum yields.

Minimum energy conical intersection (MECI) geometries were searched for polycyclic aromatic hydrocarbons (PAHs) consisting of up to 26 atoms. The energy barriers to the obtained MECIs showed a correlation with their fluorescence quantum yields. This provides a theoretical rationale for the size dependence of the fluorescence quantum yields seen in these PAHs.

Computational determination of the dominant triplet population mechanism in photoexcited benzophenone.

In benzophenone, intersystem crossing occurs efficiently between the S1(nπ*) state and the T1 state of dominant nπ* character, leading to excited triplet states after photoexcitation. The transition mechanism between S1(nπ*) and T1 is still a matter of debate, despite several experimental studies. Quantum mechanical calculations have been performed in order to assess the relative efficiencies of previously proposed mechanisms, in particular, the direct S1→ T1 and indirect S1→ T2(ππ*) → T1 ones. Multiconfigurational wave function based methods are used to discuss the nature of the relevant states and also to determine minimum energy paths and conical intersections. It is found that the T1 state has a mixed nπ*/ππ* character and that the T2(ππ*) state acts as an intermediate state between the S1 and T1 states. This result is in line with recent experiments, which suggested a two-step kinetic model to populate the phosphorescent state after photoexcitation [Aloïse et al., J. Phys. Chem. A, 2008, 112, 224-231].