Determining Excited-State Absorption Properties of a Quinoid Flavin by Polarization-Resolved Transient Spectroscopy.

As important naturally occurring chromophores, photophysical/chemical properties of quinoid flavins have been extensively studied both experimentally and theoretically. However, little is known about the transition dipole moment (TDM) orientation of excited-state absorption transitions of these important compounds. This aspect is of high interest in the fields of photocatalysis and quantum control studies. In this work, we employ polarization-associated spectra (PAS) to study the excited-state absorption transitions and the underlying TDM directions of a standard quinoid flavin compound. As compared to transient absorption anisotropy (TAA), an analysis based on PAS not only avoids diverging signals but also retrieves the relative angle for ESA transitions with respect to known TDM directions. Quantum chemical calculations of excited-state properties lead to good agreement with TA signals measured in magic angle configuration. Only when comparing experiment and theory for TAA spectra and PAS, do we find deviations when and only when the S0 → S1 of flavin is used as a reference. We attribute this to the vibronic coupling of this transition to a dark state. This effect is only observed in the employed polarization-controlled spectroscopy and would have gone unnoticed in conventional TA.

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.

Sub-optical-cycle light-matter energy transfer in molecular vibrational spectroscopy.

The evolution of ultrafast-laser technology has steadily advanced the level of detail in studies of light-matter interactions. Here, we employ electric-field-resolved spectroscopy and quantum-chemical modelling to precisely measure and describe the complete coherent energy transfer between octave-spanning mid-infrared waveforms and vibrating molecules in aqueous solution. The sub-optical-cycle temporal resolution of our technique reveals alternating absorption and (stimulated) emission on a few-femtosecond time scale. This behaviour can only be captured when effects beyond the rotating wave approximation are considered. At a femtosecond-to-picosecond timescale, optical-phase-dependent coherent transients and the dephasing of the vibrations of resonantly excited methylsulfonylmethane (DMSO2) are observed. Ab initio modelling using density functional theory traces these dynamics back to molecular-scale sample properties, in particular vibrational frequencies and transition dipoles, as well as their fluctuation due to the motion of DMSO2 through varying solvent environments. Future extension of our study to nonlinear interrogation of higher-order susceptibilities is fathomable with state-of-the-art lasers.

Photoinduced B-Cl Bond Fission in Aldehyde-BCl3 Complexes as a Mechanistic Scenario for C-H Bond Activation.

In concert with carbonyl compounds, Lewis acids have been identified as a versatile class of photocatalysts. Thus far, research has focused on activation of the substrate, either by changing its photophysical properties or by modifying its photochemistry. In this work, we expand the established mode of action by demonstrating that UV photoexcitation of a Lewis acid-base complex can lead to homolytic cleavage of a covalent bond in the Lewis acid. In a study on the complex of benzaldehyde and the Lewis acid BCl3, we found evidence for homolytic B-Cl bond cleavage leading to formation of a borylated ketyl radical and a free chlorine atom only hundreds of femtoseconds after excitation. Both time-dependent density functional theory and transient absorption experiments identify a benzaldehyde-BCl2 cation as the dominant species formed on the nanosecond time scale. The experimentally validated B-Cl bond homolysis was synthetically exploited for a BCl3-mediated hydroalkylation reaction of aromatic aldehydes (19 examples, 42-76% yield). It was found that hydrocarbons undergo addition to the C═O double bond via a radical pathway. The photogenerated chlorine radical abstracts a hydrogen atom from the alkane, and the resulting carbon-centered radical either recombines with the borylated ketyl radical or adds to the ground-state aldehyde-BCl3 complex, releasing a chlorine atom. The existence of a radical chain was corroborated by quantum yield measurements and by theory. The photolytic mechanism described here is based on electron transfer between a bound chlorine and an aromatic π-system on the substrate. Thereby, it avoids the use of redox-active transition metals.

Diels-Alder Reaction of Photochemically Generated (E)-Cyclohept-2-enones: Diene Scope, Reaction Pathway, and Synthetic Application.

Upon irradiation at λ = 350 nm, cyclohept-2-enone undergoes an isomerization to the strained (E)-isomer. The process was studied by XMS-CASPT2 calculations and found to proceed by two competitive reaction channels on either the singlet or the triplet hypersurface. (E)-Cyclohept-2-enone is a reactive dienophile in thermal [4 + 2] cycloaddition reactions with various dienes. Ten different dienes were probed, most of which─except for 1,3-cyclohexadiene─underwent a clean Diels-Alder reaction and gave the respective trans-fused six-membered rings in good yields (68-98%). The reactions with furan were studied in detail, both experimentally and by DLPNO-CCSD(T) calculations. Two diastereoisomers were formed in a ratio of 63/35 with the exo-product prevailing, and the configuration of both diastereoisomers was corroborated by single crystal X-ray crystallography. The outcome of the photoinduced Diels-Alder reaction matched both qualitatively and quantitatively the calculated reaction pathway. Apart from cyclohept-2-enone, five additional cyclic hept-2-enones and cyclooct-2-enone were employed in their (E)-form as dienophiles in the Diels-Alder reaction with 1,3-cyclopentadiene (80-98% yield). The method was eventually applied to a concise total synthesis of racemic trans-α-himachalene (four steps, 14% overall yield).

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.

Ultrafast Reactive Quantum Dynamics Coupled to Classical Solvent Dynamics Using an Ehrenfest Approach.

The inclusion of solvent effects in the theoretical analysis of molecular processes becomes increasingly important. Currently, it is not feasible to directly include the solvent on the quantum level. We use an Ehrenfest approach to study the coupled time evolution of quantum dynamically treated solutes and classical solvents system. The classical dynamics of the solvent is coupled to the wavepacket dynamics of the solute and rotational and translational degrees of freedom of the solute are included classically. This allows quantum dynamics simulations for ultrafast processes that are decided by environment interactions without explicit separation of time scales. We show the application to the dissociation of ICN in liquid Ar as a proof of principal system and to the more applied example of uracil in water.