Simulation of the non-adiabatic dynamics of an enone-Lewis acid complex in an explicit solvent

Unlocking the full potential of Lewis acid catalysis for photochemical transformations requires a comprehensive understanding of the ultrafast dynamics of substrate-Lewis acid complexes. In a previous article [Peschel et al., Angew. Chem. Int. Ed., 2021, 60, 10155], time-resolved spectroscopy supported by static calculations revealed that the Lewis acid remains attached during the relaxation of the model complex cyclohexenone-BF3. In contrast to the experimental observation, surface-hopping dynamics in the gas phase predicted ultrafast heterolytic dissociation. We attributed the discrepancy to missing solvent interactions. Thus, in this work, we present an interface between the SHARC and FermiONs++ program packages, which enables us to investigate the ultrafast dynamics of cyclohexenone-BF3 in an explicit solvent environment. Our simulations demonstrate that the solvent prevents the dissociation of the complex, leading to an intriguing dissociation-reassociation mechanism. Comparing the dynamics with and without triplet states highlights their role in the relaxation process and shows that the Lewis acid inhibits intersystem crossing. These findings provide a clear picture of the relaxation process, which may aid in designing future Lewis acid catalysts for photochemical applications. They underscore that an explicit solvent model is required to describe relaxation processes in weakly bound states, as energy transfer to the solvent is crucial for the system to reach its minimum geometries.

Molecular phosphorescence enhancement by the plasmon field of metal nanoparticles

A theoretical model is proposed that allows the estimation of the quantum yield of phosphorescence of dye molecules in the vicinity of plasmonic nanoparticles. For this purpose, the rate constants of the radiative and nonradiative intramolecular transitions for rhodamine 123 (Rh123) and brominated rhodamine (Rh123-2Br) dyes have been calculated. The plasmon effect of Ag nanoparticles on various types of luminescence processes has been studied both theoretically and experimentally. We show that in the presence of a plasmonic nanoparticle, the efficiency of the immediate and delayed fluorescence increases significantly. The phosphorescence rate of the rhodamine dyes also increases near plasmonic nanoparticles. The long-lived luminescence i.e., delayed fluorescence and phosphorescence is more enhanced for Rh123-2Br than for Rh123. The largest phosphorescence quantum yield is obtained when the dye molecule is at a distance of 4-6 nm from the nanoparticle surface. Our results can be used in the design of plasmon-enhancing nanostructures for light-emitting media, organic light-emitting diodes, photovoltaic devices, and catalysts for activation of molecular oxygen.

Tropospheric Photochemistry of 2-Butenedial: Role of the Triplet States, CO and Acrolein Formation, and the Experimentally Unidentified Carbonyl Compound-Theoretical Study

Solar irradiation of 2-butenedial in the lower troposphere mainly produces isomeric ketene-enol (a key intermediate product), furanones, and maleic anhydride, the formation pathways of which were investigated in a previous study. The other main products were carbon monoxide and an experimentally unidentified carbonyl compound. This was the subject of the present study. The oxidative reaction mechanisms were studied using DFT calculations. Water intervention is found essential. Its addition and subsequent water-assisted isomerizations (an ene-gem-diol/enol and a carboxylic acid/enol form), followed by cyclization, lead to an interesting cyclic carbonyl compound, but this pathway appears to be rather energy demanding. An alternative implies water cooperation in a ketene-enol + carboxylic acid/enol addition that gives the relevant anhydride. The anhydride is proposed as a candidate for the experimentally unidentified carbonyl product. Regarding CO and acrolein formation, the role of the triplet states, as defined by the probability of intersystem crossing from the excited singlet state S1 to T2 and T1, is discussed. The T1 photolysis pathway connecting butenedial to propenal + CO was then defined.

Channel Competition and Control of Relaxation Pathways in S1 State of Acrolein: Role of Conical Intersection and Surface Crossing

Channel competition and further photochemical control of relaxation pathways in excited molecules are of primary importance in photochemistry and related areas. Acrolein, as the simplest and most typical α,β-enone, is suitable to provide a model for understanding the photochemistry and photophysics of α,β-enones. Here, the ultrafast dynamics in acrolein following S1(nπ*) excitation has been studied by time-resolved photoelectron imaging (TRPEI) and mass spectroscopy. The competition between intersystem crossing (ISC) and internal conversion (IC) is investigated. The key factor influencing the decay pathways and the relative contributions are revealed to be the position of the excitation relative to the energy of the S1/S0 conical intersection (CI), which is obtained to be 3.65-3.76 eV experimentally. If the excitation is above the CI, IC is superior to ISC and most excited molecules go back to the ground. Otherwise, ISC will dominate the relaxation and lead the triplet products formation. These results show the potential of affecting the dynamics and governing the fate of excited molecules by adjusting the excitation conditions from the point of view of chemical control.

Deciphering Methylation Effects on S2(ππ*) Internal Conversion in the Simplest Linear α,β-Unsaturated Carbonyl

Chemical substituents can influence photodynamics by altering the location of critical points and the topography of the potential energy surfaces (electronic effect) and by selectively modifying the inertia of specific nuclear modes (inertial effects). Using nonadiabatic dynamics simulations, we investigate the impact of methylation on S₂(ππ*) internal conversion in acrolein, the simplest linear α,β-unsaturated carbonyl. Consistent with time constants reported in a previous time-resolved photoelectron spectroscopy study, S₂ → S₁ deactivation occurs on an ultrafast time scale (∼50 fs). However, our simulations do not corroborate the sequential decay model used to fit the experiment. Instead, upon reaching the S₁ state, the wavepacket bifurcates: a portion undergoes ballistic S₁ → S₀ deactivation (∼90 fs) mediated by fast bond-length alternation motion, while the remaining decays on the picosecond time scale. Our analysis reveals that methyl substitution, generally assumed to mainly exert inertial influence, is also manifested in important electronic effects due to its weak electron-donating ability. While methylation at the β C atom gives rise to effects principally of an inertial nature, such as retarding the twisting motion of the terminal -CHCH₃ group and increasing its coupling with pyramidalization, methylation at the α or carbonyl C atom modifies the potential energy surfaces in a way that also contributes to altering the late S₁-decay behavior. Specifically, our results suggest that the observed slowing of the picosecond component upon α-methylation is a consequence of a tighter surface and reduced amplitude along the central pyramidalization, effectively restricting the access to the S₁/S₀-intersection seam. Our work offers new insight into the S₂(ππ*) internal conversion mechanisms in acrolein and its methylated derivatives and highlights site-selective methylation as a tuning knob to manipulate photochemical reactions.

Spin-Orbit Coupling Calculation Combined with the Reference Interaction Site Model Self-Consistent Field Explicitly Including Constrained Spatial Electron Density Distribution

Studying the radiative and non-radiative decay processes of molecules in a solution is an important issue in the design of organic and functional molecules. Theoretical approaches have great potential for revealing this decay process through computation of various parameters, such as the energy surfaces at the excited state and spin-orbit coupling (SOC). The development of quantum chemical programs has enabled the calculation of SOC values to become popular for the gas phase. However, SOC calculations in solution have some difficulties that need to be overcome. In the present study, the authors combined the SOC calculations with the reference interaction site model self-consistent field explicitly including constrained spatial electron density distribution. To validate the reliability of our method, the decay process of dimethylaminobenzonitrile in cyclohexane and acetonitrile was studied. By computing the SOC values in both solution systems, the authors were able to investigate the decay process at the atomistic level. Furthermore, a natural transition orbital analysis and the measurement of the decomposed SOC values were found to provide a clear understanding of intersystem crossing.

Photochemical Ring Contraction of 5,5-Dialkylcyclopent-2-enones and in situ Trapping by Primary Amines

If substituted in the 5,5-position, cyclopent-2-enones undergo a smooth photochemical rearrangement to ketenes. A concomitant cyclopropane formation occurs due to a 1,3-shift of the C5 carbon atom from the carbonyl carbon atom (C1) to carbon atom C3. In this study, the cyclopropyl-substituted ketene intermediates were trapped in situ by primary amines providing an efficient entry into 2,2-disubstituted cyclopropaneacetic amides (24 examples, 49-95% yield). A remarkable feature of the reaction is the fact that the photochemical rearrangement can occur from either the first excited singlet (S₁) or the respective triplet state (T₁). In line with experimental results (triplet quenching, sensitization), XMS-CASPT2 calculations support the existence of efficient reaction pathways to the intermediate ketene both on the singlet and on the triplet hypersurface.

Generalized Ab Initio Nonadiabatic Dynamics Simulation Methods from Molecular to Extended Systems

Nonadiabatic dynamics simulation has become a powerful tool to describe nonadiabatic effects involved in photophysical processes and photochemical reactions. In the past decade, our group has developed generalized trajectory-based ab initio surface-hopping (GTSH) dynamics simulation methods, which can be used to describe a series of nonadiabatic processes, such as internal conversion, intersystem crossing, excitation energy transfer and charge transfer of molecular systems, and photoinduced nonadiabatic carrier dynamics of extended systems with and without spin-orbit couplings. In this contribution, we will first give a brief introduction to our recently developed methods and related numerical implementations at different computational levels. Later, we will present some of our latest applications in realistic systems, which cover organic molecules, biological proteins, organometallic compounds, periodic organic and inorganic materials, etc. Final discussion is given to challenges and outlooks of ab initio nonadiabatic dynamics simulations.

Identification of individual conformers in C4H6O isomers using conformer-specific vibrational spectroscopy

We measured the conformer-specific vibrational spectra of C4H6O isomers in neutral and cationic states using IR resonant vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy for the first time. Notably, the measured IR dip and hole-burn VUV-MATI spectra for each isomer represent the identifiable vibrational spectra of individual conformers in both states. Furthermore, we estimated the relative populations of individual conformers in crotonaldehyde (CA) and methyl vinyl ketone (MVK) isomers using the IR dip intensity, the corresponding Franck-Condon factor, and the IR absorption cross section. Our analysis revealed that the compositional ratio of s-trans to s-cis conformers in the CA isomer remained at 95.8 : 4.2 even under supersonic expansion, whereas that in the MVK isomer was determined as 90.6 : 9.4, which is consistent with previous research. These findings reveal that the conformational stability of each isomer depends on the position of the methyl group relative to the carbonyl group.

Activation of 2-Cyclohexenone by BF3 Coordination: Mechanistic Insights from Theory and Experiment

Lewis acids have recently been recognized as catalysts enabling enantioselective photochemical transformations. Mechanistic studies on these systems are however rare, either due to their absorption at wavelengths shorter than 260 nm, or due to the limitations of theoretical dynamic studies for larger complexes. In this work, we overcome these challenges and employ sub-30-fs transient absorption in the UV, in combination with a highly accurate theoretical treatment on the XMS-CASPT2 level. We investigate 2-cyclohexenone and its complex to boron trifluoride and analyze the observed dynamics based on trajectory calculations including non-adiabatic coupling and intersystem crossing. This approach explains all ultrafast decay pathways observed in the complex. We show that the Lewis acid remains attached to the substrate in the triplet state, which in turn explains why chiral boron-based Lewis acids induce a high enantioselectivity in photocycloaddition reactions.

Probing the Photoionization Dynamics of 2-Cyclopenten-1-one via High-Resolution VUV-MATI Spectroscopy

2-Cyclopenten-1-one (2CP), which is a cyclic enone, has been considered an important precursor because of its versatile functionality in the synthesis of natural products and materials for biofuels. Here, we report the adiabatic ionization energy (AIE) and cationic structure of 2CP in the ionic transition between the neutral S₀ and the cationic D₀ states probed by high-resolution vacuum ultraviolet mass-analyzed threshold ionization (VUV-MATI) spectroscopy. From the 0-0 band position in the VUV-MATI spectrum supported by the VUV-photoionization efficiency curve, the AIE of 2CP was determined to be 9.3477 ± 0.0004 eV (75,395 ± 3 cm^-1), which is in good agreement with the reference value but much more accurate. The measured MATI spectrum combined with the Franck-Condon fitting at the B3LYP/cc-pVTZ level revealed that the cationic structure of 2CP is twisted with the C₁ symmetry, whereas the neutral 2CP has the C_S symmetry. The results indicate that geometrical changes induced by ionization are mainly attributed to the electron removal from the highest occupied molecular orbital, which consists of nonbonding orbitals on the oxygen atom in the carbonyl group interacting with the σ orbitals in the molecular plane of 2CP. Consequently, lowering the C₁ symmetry for cationic 2CP led to the promotions of the ring-bending and ring-twisting modes in the MATI spectrum, which correspond to the ring puckering and C═C twisting in the S₀ state, respectively.

Calculating rate constants for intersystem crossing and internal conversion in the Franck–Condon and Herzberg–Teller approximations

Effective and fast algorithms for calculating rate constants for internal conversion (IC) and intersystem crossing (ISC) in the Franck–Condon and Herzberg–Teller approximations have been developed and implemented.

First-principles method for calculating the rate constants of internal-conversion and intersystem-crossing transitions

A method for calculating the rate constants for internal-conversion (k_IC) and intersystem-crossing (k_ISC) processes within the adiabatic and Franck–Condon (FC) approximations is proposed.

Brønsted Acid Catalysis in Visible-Light-Induced [2+2] Photocycloaddition Reactions of Enone Dithianes

1,3-Dithiane-protected enones (enone dithianes) were found to undergo an intramolecular [2+2] photocycloaddition under visible-light irradiation (λ=405 nm) in the presence of a Brønsted acid (7.5-10 mol %). Key to the success of the reaction is presumably the formation of colored thionium ions, which are intermediates of the catalytic cycle. Cyclobutanes were thus obtained in very good yields (78-90 %). It is also shown that the dithiane moiety can be reductively or oxidatively removed without affecting the photochemically constructed ring skeleton.

Cavity Ringdown Spectrum of 2-Cyclohexen-1-one in the CO/Alkenyl CC Stretch Region of the S1(n, π*)-S0 Vibronic Band System

The 2-cyclohexen-1-one (2CHO) molecule serves as a prototype for understanding the photochemical properties of conjugated enones. We have recorded the cavity ringdown (CRD) absorption spectrum of 2CHO vapor at room temperature over the 360-380 nm range. This portion of the spectrum encompasses the S₁(n,π*) ← S₀ vibronic band system in the region of the C═C and C═O stretch fundamentals. We have assigned about 40 vibronically resolved features in the spectrum, affording fundamental frequencies for 7 different vibrational modes in the S₁(n,π*) state, including the C═C (1554 cm^-1) and OC-CH (1449 cm^-1) stretch modes. The C═O stretch character is spread over at least four different vibrational modes in the S₁(n,π*) state, with fundamentals spanning the 1340-1430 cm^-1 interval. This finding stems from a significant reduction in C═O bond order upon excitation, which leads to near-coincidence of the C═O stretch and several CH₂ wag frequencies. Such complexities make 2CHO an ideal candidate for testing excited-state computational methods. We have used the present spectroscopic results to test EOM-EE-CCSD harmonic-frequency predictions for the S₁(n,π*) state. We have also benchmarked the performance of less costly computational methods, including CIS(D) and TDDFT. For certain density functionals (e.g., B3LYP and PBE0), we find that the accuracy of TDDFT frequency predictions can approach but not meet that of EOM-EE-CCSD.

Unravelling Photochemical Relationships Among Natural Products from Aplysia dactylomela

Aplydactone (1) is a brominated ladderane sesquiterpenoid that was isolated from the sea hare Aplysia dactylomela together with the chamigranes dactylone (2) and 10-epi-dactylone (3). Given the habitat of A. dactylomela, it seems likely that 1 is formed from 2 through a photochemical [2 + 2] cycloaddition. Here, we disclose a concise synthesis of 1, 2, and 3 that was guided by excited state theory and relied on several highly stereoselective transformations. Our experiments and calculations confirm the photochemical origin of 1 and explain why it is formed as the sole isomer. Irradiation of 3 with long wavelength UV light resulted in a [2 + 2] cycloaddition that proceeded with opposite regioselectivity. On the basis of this finding, it seems likely that the resulting regioisomer, termed "8-epi-isoaplydactone", could also be found in A. dactylomela.