A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photogeneration and Interconversion through Electron Transfer

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.

How Protic Solvents Determine the Reaction Mechanisms of Diphenylcarbene in Solution

We investigate the effects of small admixtures of protic solvent molecules, such as water and alcohols, on the ultrafast dynamics of diphenylcarbene in acetonitrile at room temperature. Broadband transient absorption measurements and quantum mechanics/molecular mechanics molecular dynamics simulations allow elucidating the dominant reaction mechanism of an intermediate hydrogen-bonded complex between singlet diphenylcarbene and a protic solvent molecule, thus competing with intersystem crossing. Analysis of the data indicates that complex formation is a diffusion-controlled process with orientational requirements. The reaction path involving a benzhydryl cation is less likely in neat bulkier alcohols, as it requires the interaction of the carbene with a protic solvent molecule being part of a hydrogen-bonded network. The simulations indicate a further reaction path toward O-H insertion and two side reactions depending on the involved protic solvent species. Thus, we established that not only the number but also the chemical nature of the protic solvent molecule determine which reaction path is pursued.

Ultrafast non-adiabatic dynamics of excited diphenylmethyl bromide elucidated by quantum dynamics and semi-classical on-the-fly dynamics

Carbocations and carboradicals are key intermediates in organic chemistry. Typically UV laser excitation is used to induce homolytical or heterolytical bond cleavage in suitable precursor molecules. Of special interest hereby are diphenylmethyl compounds (Ph2CH-X) with X = Cl, Br as a leaving group as they form diphenylmethyl radicals (Ph2CH˙) and cations (Ph2CH+) within a femtosecond time scale in polar solvents. In this work, we build on our methodology developed for the chlorine case and investigate the photodissociation reaction of Ph2CH-Br by state-of-the-art theoretical methods. On the one hand, we employ specially adapted reactive coordinates for a grid-based wave packet dynamics in reduced dimensionality using the Wilson G-matrix ansatz for the kinetic part of the Hamiltonian. On the other hand, we use full-dimensional semiclassical on-the-fly dynamics with Tully's fewest switches surface hopping routine for comparison. We apply both methods to explain remarkable differences in experimental transient absorption measurements for Cl or Br as the leaving group. The wave packet motion, visible only for the bromine leaving group, can be related to the crucial role of the central carbon atom, which undergoes rehybridization from sp3 to sp2 during the photoinduced bond cleavage. Comparable features are the two consecutive conical intersections near the Franck-Condon region controlling the product splitting to Ph2CH˙/Br˙ and Ph2CH+/Br- as well as the difference in delay time for the respective product formation.

Photometrics of ultrafast and fast broadband electronic transient absorption spectroscopy: State of the art

The physical limits of the photometric resolution in broadband electronic transient absorption spectroscopy are discussed together with solutions for how to reach these limits in practice. In the first part, quantitative expressions for the noise contributions to the transient absorption signal are derived and experimentally tested. Experimental approaches described in the literature are discussed and compared on this basis. Guide-lines for designing a setup are established. In the second part, a method for obtaining nearly shot-noise limited kinetics with photometric resolution of the order of 100 μOD in overall measurement times of a few minutes from femtosecond to microsecond time scale is presented. The results are discussed in view of other experiments of step-scan type which are subject to a background or to correlated noise. Finally, detailed information is provided on how to obtain transient absorption spectra where counting statistics are the sole source of noise. A method for how to suppress outliers without introducing bias is discussed. An application example is given to demonstrate the achievable signal-to-noise level and the fast acquisition time.

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.

Taking the plunge: chemical reaction dynamics in liquids

The dynamics of chemical reactions in liquid solutions are now amenable to direct study using ultrafast laser spectroscopy techniques and advances in computer simulation methods. The surrounding solvent affects the chemical reaction dynamics in numerous ways, which include: (i) formation of complexes between reactants and solvent molecules; (ii) modifications to transition state energies and structures relative to the reactants and products; (iii) coupling between the motions of the reacting molecules and the solvent modes, and exchange of energy; (iv) solvent caging of reactants and products; and (v) structural changes to the solvation shells in response to the changing chemical identity of the solutes, on timescales which may be slower than the reactive events. This article reviews progress in the study of bimolecular chemical reaction dynamics in solution, concentrating on reactions which occur on ground electronic states. It illustrates this progress with reference to recent experimental and computational studies, and considers how the various ways in which a solvent affects the chemical reaction dynamics can be unravelled. Implications are considered for research in fields such as mechanistic synthetic chemistry.

Optical transient absorption experiments reveal the failure of formal kinetics in diffusion assisted electron transfer reactions

The charge separation yield is shown to be strongly influenced by the distance dependence of the reactivity, viscosity and concentration and cannot be disentangled from the preceding events.

Competitive solvent-molecule interactions govern primary processes of diphenylcarbene in solvent mixtures

Photochemical reactions in solution often proceed via competing reaction pathways comprising intermediates that capture a solvent molecule. A disclosure of the underlying reaction mechanisms is challenging due to the rapid nature of these processes and the intricate identification of how many solvent molecules are involved. Here combining broadband femtosecond transient absorption and quantum mechanics/molecular mechanics simulations, we show for one of the most reactive species, diphenylcarbene, that the decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding dynamics determine which reaction channels are accessible in binary solvent mixtures at room temperature. In-depth analysis of the amount of nascent intermediates corroborates the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking analogy to complexes found at cryogenic temperatures. Our results show that adjacent solvent molecules take the role of key abettors rather than bystanders for the fate of the reactive intermediate.

Photochemistry in solution often involves coexisting reaction channels that may comprise intermediates capturing a solvent molecule. Here, the authors show for one of the most reactive species, diphenylcarbene, that the decision-maker is not the nearest solvent molecule but its neighbour.

Molecular features in complex environment: Cooperative team players during excited state bond cleavage

Photoinduced bond cleavage is often employed for the generation of highly reactive carbocations in solution and to study their reactivity. Diphenylmethyl derivatives are prominent precursors in polar and moderately polar solvents like acetonitrile or dichloromethane. Depending on the leaving group, the photoinduced bond cleavage occurs on a femtosecond to picosecond time scale and typically leads to two distinguishable products, the desired diphenylmethyl cations (Ph2CH(+)) and as competing by-product the diphenylmethyl radicals ([Formula: see text]). Conical intersections are the chief suspects for such ultrafast branching processes. We show for two typical examples, the neutral diphenylmethylchloride (Ph2CH-Cl) and the charged diphenylmethyltriphenylphosphonium ions ([Formula: see text]) that the role of the conical intersections depends not only on the molecular features but also on the interplay with the environment. It turns out to differ significantly for both precursors. Our analysis is based on quantum chemical and quantum dynamical calculations. For comparison, we use ultrafast transient absorption measurements. In case of Ph2CH-Cl, we can directly connect the observed signals to two early three-state and two-state conical intersections, both close to the Franck-Condon region. In case of the [Formula: see text], dynamic solvent effects are needed to activate a two-state conical intersection at larger distances along the reaction coordinate.

Quantum Dynamics of a Photochemical Bond Cleavage Influenced by the Solvent Environment: A Dynamic Continuum Approach

In every day chemistry, solvents are used to influence the outcome of chemical synthesis. Electrostatic effects stabilize polar configurations during the reaction and in addition dynamic solvent effects can emerge. How the dynamic effects intervene on the ultrafast time scale is in the focus of this theoretical study. We selected the photoinduced bond cleavage of Ph2CH-PPh3(+) for which the electrostatic interactions are negligible. Elaborate ultrafast pump-probe studies already exist and serve as a reference. We compared quantum dynamical simulations with and without environment and noticed the necessity to model the influence of the solvent cage on the reactive motions of the solute. The frictional force induced by the dynamic viscosity of the solvent is implemented in the quantum mechanical formalism with a newly developed approach called the dynamic continuum ansatz. Only when the environment is included are the experimentally observed products reproduced on the subpicosecond time scale.

The highly reactive benzhydryl cation isolated and stabilized in water ice

Diphenylcarbene (DPC) shows a triplet ground-state lying approximately 3 kcal/mol below the lowest singlet state. Under the conditions of matrix isolation at 25 K, DPC reacts with single water molecules embedded in solid argon and switches its ground state from triplet to singlet by forming a strong hydrogen bond. The complex between DPC and water is only metastable, and even at 3 K the carbene center slowly inserts into the OH bond of water to form benzhydryl alcohol via quantum chemical tunneling. Surprisingly, if DPC is generated in amorphous water ice at 3 K, it is protonated instantaneously to give the benzhydryl cation. Under these conditions, the benzhydryl cation is stable, and warming to temperatures above 50 K is required to produce benzhydryl alcohol. Thus, for the first time, a highly electrophilic and extremely reactive secondary carbenium ion can be isolated in a neutral, nucleophilic environment avoiding superacidic conditions.

Probing the Conical Intersection Dynamics of the RNA Base Uracil by UV-Pump Stimulated-Raman-Probe Signals; Ab Initio Simulations

Nonadiabatic electron and nuclear dynamics of photoexcited molecules involving conical intersections is of fundamental importance in many reactions such as the self-protection mechanism of DNA and RNA bases against UV irradiation. Nonlinear vibrational spectroscopy can provide an ultrafast sensitive probe for these processes. We employ a simulation protocol that combines nonadiabatic on-the-fly molecular dynamics with a mode-tracking algorithm for the simulation of femtosecond stimulated Raman spectroscopy (SRS) signals of the high frequency C–H- and N–H-stretch vibrations of the photoexcited RNA base uracil. The simulations rely on a microscopically derived expression that takes into account the path integral of the excited state evolution and the pulse shapes. Analysis of the joint time/frequency resolution of the technique reveals a matter chirp contribution that limits the inherent temporal resolution. Characteristic signatures of relaxation dynamics mediated in the vicinity of conical intersection are predicted. The C–H and N–H spectator modes provide high sensitivity to their local environment and act as local probes with submolecular and high temporal resolution.

Bimolecular photoinduced electron transfer beyond the diffusion limit: the Rehm-Weller experiment revisited with femtosecond time resolution

To access the intrinsic, diffusion free, rate constant of bimolecular photoinduced electron transfer reactions, fluorescence quenching experiments have been performed with 14 donor/acceptor pairs, covering a driving-force range going from 0.6 to 2.4 eV, using steady-state and femtosecond time-resolved emission, and applying a diffusion-reaction model that accounts for the static and transient stages of the quenching for the analysis. The intrinsic electron transfer rate constants are up to 2 orders of magnitude larger than the diffusion rate constant in acetonitrile. Above ∼1.5 eV, a slight decrease of the rate constant is observed, pointing to a much weaker Marcus inverted region than those reported for other types of electron transfer reactions, such as charge recombination. Despite this, the driving force dependence can be rationalized in terms of Marcus theory.

Photogeneration and reactions of benzhydryl cations and radicals: A complex sequence of mechanisms from femtoseconds to microseconds

Benzhydryl radicals and cations are reactive intermediates central to the understanding of organic reactivity. They can be generated from benzhydryl halides by UV irradiation. We performed transient absorption (TA) measurements over the range from femtoseconds to microseconds to unravel the complete reaction scheme. The 290–720-nm probe range allows the unambiguous monitoring of all fragments. The appearance of the radical is delayed to the optical excitation, the onset of the cation signal is found even later. Ab initio calculations show that this non-rate behavior in the 100 fs range is due to wavepacket motion from the Franck–Condon region to two distinct conical intersections. The rise of the optical signal with a quasi-exponential time of 300 fs is assigned to the planarization and solvation of the photoproducts. The bond cleavage predominantly generates radical pairs. A subsequent electron transfer (ET) transforms radical pairs into ion pairs. Due to the broad interradical distance distribution and the distance dependence, the ET is strongly non-exponential. Part of the ion pairs recombine geminately. The ET and the recombination are terminated by the depletion of close pairs and diffusional separation. The remaining free radicals and cations undergo further reactions in the nanosecond to microsecond regime.