Infrared Laser Excitation Controlled Reaction Acceleration in the

Perspective Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Infrared Laser Excitation Controlled Reaction Acceleration in the Electronic Ground State Karsten Heyne*,† and Oliver Kühn‡ †

Department of Experimental Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany Institute of Physics, University of Rostock, Albert Einstein-Strasse 23-24, 18059 Rostock, Germany

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‡

Activation of a thermally driven reaction is associated with overcoming the potential barrier along the RC as depicted in Figure 1a. Upon direct excitation of the RC or by energy relaxation into the RC with vibrational energy equal to or above the activation energy the chemical reaction is promoted. The photon energy of the IR light is transferred to vibrational energy of the RC thus accelerating a thermally driven reaction. The excess energy, i.e., the energy difference between

ABSTRACT: Propelling a ground state reaction by mode-specific vibrational excitation via infrared (IR) light offers a novel route to carry out ground state chemistry. Here, we describe the acceleration of a bimolecular alcoholysis reaction as a paradigm for IR light-driven ground state reactions. Instead of resorting to coherent control, IR light is used for direct or indirect vibrational excitation of the reaction coordinate (RC) overcoming the activation energy and promoting the ground state reaction with negligible heating of the sample. Thus, knowledge of the RC is crucial to pick the reaction accelerating vibrations. Alternatively, upon mapping the reaction accelerating vibrations an image of the RC can be reconstructed. We discuss the concept of RCs and examine strategies to use vibrational energy relaxation pathways to single out vibrations belonging to the RC. The influence of the solvent interaction and limitations due to conformational heterogeneity are considered. We provide an application example generating microstructures of polymers and address the use for chemical synthesis in general.

1. INTRODUCTION: A NEW TOOL TO MANIPULATE GROUND STATE REACTIONS The overwhelming majority of chemical reactions in solution phase takes place in the electronic ground state. Reaction properties, e.g., reaction efficiency, can be affected by concentration, temperature, pH, solvent properties, pressure, ultrasound, catalysts, and microenvironment (such as cages).1−6 Most of these influences are global and cannot target molecular subgroups or specific aspects of a reaction. In particular, temperature impacts all molecules in the same manner, populating vibrations according to the Boltzmann distribution. Temperature provides the stimulus for thermally driven reactions, but there are very limited options to accelerate a specific thermally driven reaction and leave others unaffected in the same sample. Here, we present a tool to accelerate a specific thermally driven reaction by infrared (IR) light excitation of those vibrations participating in the reaction coordinate (RC). In contrast to temperature, IR light absorption is mode specific and deposits vibrational energy in a single vibration before vibrational energy relaxation (VER) effects set in. Energy relaxation pathways strongly depend on the vibration excited and are governed by anharmonic couplings.7−10 Thus, in the same molecule different excited vibrations can result in disjoined energy relaxation pathways. © XXXX American Chemical Society

Figure 1. (a) Schematic view of the IR-laser-driven evolution along a RC from reactants A (phenylisocyanate, PHI) and B (cyclo-hexanol, CH-ol) via a transition state to product C (cyclohexyl-carbanilate, CC). (b) Schematic view of the IR-laser-driven evolution along two different RCs with different potential barriers and transition states. Starting from reactants A (2,2,2-trichloroethane-1,1-diol, TCD) and B (toluene-2,4-diisocyanate, TDI) the product C (urethane) is formed along RC 1; starting from reactants C 2,2,2-trichloro-1-hydroxy-ethylN-(3-isocyanato-4-methylphenyl)carbamate and A (TCD) the product E (bis(2,2,2-trichloro-1-hydroxy-ethyl)-N,N′-(4-methyl-1,3phenylene)dicarbamate) is formed along RC 2. Received: March 8, 2019 Published: June 28, 2019 A

DOI: 10.1021/jacs.9b02600 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society excitation energy and activation energy, dissipates into heat. This process is sketched in Figure 1a for a one-step chemical reaction (A+B → C) with a single transition state and a fast reaction process on a picosecond time scale or faster. In the example discussed below, A is phenylisocyanate (PHI), B is cyclohexanol (CH-ol), and C is cyclohexyl-carbanilate (CC). The process of a multiple-step ground state reaction (A+B → C+D → E) is sketched in Figure 1b (for the case A = D) with multiple potential barriers, transition states, and a slow overall process on the nanosecond time-scale or longer. The RC may change from the first to the second step (see Figure 1b), and between these steps additional processes, such as diffusion, molecular rearrangement, etc. could occur. In Figure 1b, A (=D) is 2,2,2-trichloroethane-1,1-diol (TCD), B is toluene-2,4diisocyanate (TDI), C is a 2,2,2-trichloro-1-hydroxy-ethyl-N(3-isocyanato-4-methylphenyl)carbamate, and E is bis(2,2,2trichloro-1-hydroxy-ethyl)-N,N′-(4-methyl-1,3-phenylene)dicarbamate, the first step to polyurethane formation. Here, we focus on single-step reactions with a well-defined RC consisting of vibrations of the participating molecules. Most of these vibrations are IR active and can be excited by light absorption. The properties of light, such as polarization direction, superposition of light waves, and contactless triggering, should facilitate new applications. With the knowledge of the RC this tool enables, in principle, acceleration of all kinds of thermally driven ground state reactions, from decomposition to synthesis including shifting of equilibria to the desired species. The first example of an accelerated ground state synthesis reaction we present here, was performed by femtosecond IR laser pulses. Applications could be local coating of sensitive devices by polymers or simply increasing the reaction yield by illuminating the reagents. A multitude of new applications can be envisioned for this chemical tool. We give a short introduction into the concept of a RC, and describe ways to characterize the RC by theoretical methods, followed by the description of vibrational energy relaxation processes, before we present our recent experiments to accelerate a thermally driven ground state reaction. We propose new experiments to identify the RC of ground state reactions, and discuss industrial applications and limitations of the new tool.

Figure 2. Two-dimensional reaction surface for H atom transfer in tropolone where the coordinates are characterized by displacement vectors given at the axes. The RC (cf. MEP X(s) (solid line)) is dominated by H atom displacement within the molecular plane as well as H-bond compression. The two geometries in the lower left correspond to the reactant (black) and transition state (gray) (adapted from ref 11).

models resemble the situation typically assumed when using the system-bath approach of dissipative dynamics and hence they can be extended to include environmental effects, e.g., due to a solvent.14 Viewing molecular dynamics in terms of normal modes of vibration, their importance for a reaction follows from the projections of the normal mode displacements onto the RC. This poses the question whether reactions can be influenced by excitation of particular vibrational modes, e.g., by means of an applied laser field. Note that this strategy needs to be distinguished from studies where the vibrational density of states has been changed, e.g., by chemical modification of the reactants. The latter situation is often described by RRKM theory, which assumes equipartition of vibrational energy due to rapid VER. But even this notion has been challenged and extensions such as localized RRKM had to be introduced to accommodate the fact that not all available vibrations have a projection onto the RC.15,16 IR laser control of unimolecular reaction dynamics has attracted particular attention from theory (for a recent account, see also ref 18). Isomerization reactions, such as the Cope rearrangement in a semibullvalene model19 or various H atom transfer reactions, were studied as exemplary cases within the development of general control strategies.20 A common assumption has been that it is possible to excite the RC directly, i.e., without much influence of anharmonic couplings to other coordinates leading to broad absorption bands or competing VER processes. In this case, reaction control can be achieved by means of the so-called pump−dump scheme shown in Figure 3. Experimental realization was hampered so far by the complications due to anharmonic couplings mentioned above. Notably, there has been some experimental IR control of the cis−trans isomerization in matrix-isolated HONO at 30 K, which features a simple absorption spectrum.21 Using laser excitation to modify bimolecular reaction rates and even control branching ratios has been successful, in particular, for gas phase reactions. In his 1999 review, Fleming

2. THE REACTION COORDINATE Our understanding of chemical reaction dynamics is based on the concept of the RC. A RC usually describes the changes in geometry for the transformation of reactants into products. A unique definition is provided by following the minimum energy path (MEP) connecting reactants and products via a first-order saddle point (the transition state, TS) on the multidimensional potential energy hypersurface (PES). Thus, motion along the RC usually involves collective displacements of potentially many nuclei (Figure 2). However, it is not only the MEP but also the valley around it which needs to be considered for accurate modeling of reaction rates. The interplay between the topography of the PES and the energy partitioning during a reaction was already realized early on and is known as Polanyi rules for bimolecular reactions of type A+BC.12 To more accurately account for this valley, reaction surface approaches have been developed to combine large amplitude motions along a few coordinates with a harmonic description of the majority of degrees of freedom such as to span a full-dimensional PES.11,13 In fact, such B


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Journal of the American Chemical Society

intermolecular in cases where, e.g., hydrogen bonds are formed), and Coulomb couplings (e.g., between dipoles associated with bond vibrations); for a systematic study, see ref 29. For monitoring reactions in solution, time-resolved spectroscopy has been established during the last decades.30 In a pioneering work, Hochstrasser and co-workers studied the hydrogen abstraction from cyclohexane due to the bimolecular reaction with Cl, formed by UV photolysis of Cl2. The UV excitation pulse serves as a trigger for the reaction and also sets the time zero. Transient changes of the absorption of a delayed IR probe pulse are used to quantify the reaction progress and to deduce a reaction rate.31 Using transient absorption spectroscopy to unravel complex reaction dynamics in solution has been taken to the extreme in the recent study of the reaction of 1,2-di(quinolin-2yl)disulfide with methyl methacrylate spanning 7 orders of magnitude in time (femto- to milliseconds) by Koyama et al.32 In order to guide the interpretation of experiment, Landau−Teller theory provides a standard tool for calculating relaxation rates. In any case, interaction with visible or UV light introduces a high amount of excess energy, heating the sample unspecifically in the illuminated volume, in contrast to specific IR excitation. However, note that IR excitation can also be used to control UV/vis-triggered excited state reactions. Recently, this has been demonstrated by the group of Weinstein, who influenced electron transfer pathways in donor−bridge−acceptor complexes by IR excitation of a CC bridge vibration being the RC for this reaction.33,34

Figure 3. Pump−dump control of an isomerization reaction. Left: Double-well potential along a one-dimensional RC. A pump pulse promotes the ground state Ψ0 to an intermediate state above the reaction barrier Ψi. This intermediate state is deactivated to the final product state Ψf by a dump pulse. Right: Laser field as well as state populations for a pump−dump control process (adapted from ref 17).

Crim stated that “Laser-driven, bond-selective bimolecular chemistry is reality”.22 In practice, this is realized by providing single-collision conditions in low-concentration supersonic expansion molecular beams. Examples include the reaction of photolytically generated Cl with H2O to form HCl and OH, which is accelerated by excitation of the third overtone of the OH stretching vibration. Branching ratios have been controlled, e.g., for the H+HOD reaction which can form either H2+OD or HD+OH depending on whether the OH or OD vibration is excited, respectively. However, one should keep in mind that laser excitation prepares bright states, which are usually not eigenstates but so-called zero order states. As a consequence, there is anharmonic coupling, e.g., to dark states which causes intramolecular vibrational relaxation (IVR), thus limiting reaction control.23 A more recent review also covering surface reactions can be found in ref 24. Moving from gas to solution phase changes the situation completely.25,26 At common liquid densities, collision times range in the few hundreds of femtoseconds regime. In the simplest picture this can be seen as giving rise to a frictional force exerted on the RC, described, e.g., by means of a (generalized) Langevin equation (GLE). The latter can be rigorously derived assuming the so-called Caldeira−Leggett model, where RC and solvent coordinates are bilinearly coupled.14 In transition states theory (TST) the reaction rate is calculated from the properties of the PES (or free energy surface) at the TS or more precisely at the surface dividing reactants and products.1 Assuming an inverted parabola shape of the barrier region, the respective inverse frequency gives an estimate for the time it takes to cross the barrier. This time scale has to compete with the collision time, which can give rise to a regime where the basic TST assumption of no barrier recrossing breaks down. To accommodate this situation, Grote and Hynes extended TST in the spirit of a GLE approach.27 The situation sketched so far applies to cases where the reacting system interacts only modestly with its environment. In general, the interaction can cause a dramatic modification of the shape of the PES, beyond just merely scaling the height of the reaction barrier; often-quoted examples are S N 2 nucleophilic substitution reactions in aqueous solution.28 The modification of the PES due to the solute−solvent interaction is reflected in the energy flow and partitioning during the reaction. Here, one has to distinguish between energy transfer due to collisions (vibration−vibration or vibration−translation), anharmonic couplings (intra- and

3. EXPERIMENTAL ASSESSMENT OF ENERGY RELAXATION In recent years, numerous investigations of vibrational relaxation dynamics in the electronic ground state were performed using femtosecond IR pump−IR probe experiments, and IR pump−Raman probe experiments.8,35−41 Moreover, two-dimensional IR spectroscopy has developed into a powerful tool for mapping couplings between different vibrations connected to chemical groups.42−44 Applications to solution phase reactions include, e.g., the hydrogen bond exchange between CH3OH and the CN group of CNCH345 or the recent study of DNA dehybridization kinetics in ref 46 to mention just two extreme examples. These spectroscopic methods allow for tracking vibrational dynamics, energy flow, and vibrational energy relaxation in molecules in real-time. In general, VER processes in solution phase are separated into (i) IVR, the vibrational energy is transferred to other vibrations within the same molecule; (ii) intermolecular vibrational energy transfer (IET), the vibrational energy is transferred to another solute molecule; and (iii) external vibrational relaxation (EVR), the vibrational energy dissipates into solvent vibrations. The IVR and IET processes can be solvent-assisted, in which the vibrational energy couples from one vibrational mode of the solute via an anharmonic interaction involving a solvent mode to another solute vibration (external IVR, and external IET).26 In contrast to the macroscopic temperature, IR light excitation deposits photon energy into electronic ground state vibrations and specific vibrations of a molecule can be addressed. This transfers the molecule from an equilibrated to a highly nonequilibrium state within femtoseconds. Depending on the type of vibration this can be a rather local process or delocalized over the whole molecule. C


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Journal of the American Chemical Society Upon IR excitation of a high-energy vibration the energy relaxes either via VER processes on a femtosecond to picosecond time scale to lower and low-frequency modes, or it populates a RC driving a chemical reaction followed by VER for the remaining excess energy. Those vibrations that are coupled most strongly to the excited ones receive the major part of the vibrational energy. The energy relaxation process can be described by a hierarchy of coupling strengths from strong to weak, and by distances from small to long governing the energy relaxation from the femtosecond to nanosecond time-scale until an equilibrium state is reached with slightly elevated temperature. Therefore, IVR processes are typically the dominating and fastest relaxation processes in molecules. Since many efforts to drive chemical reactions competing with IVR failed, it was claimed that one cannot beat IVR. However, unimolecular bond breaking was demonstrated by using sequential multiexcitation steps via vibrational ladder climbing in the gas phase.47,48

4. ACCELERATION OF GROUND STATE REACTIONS BY IR LIGHT ABSORPTION In contrast to reactions in the gas-phase47,48 solute−solvent interactions potentially limit the prospects of laser-driven bond selective chemistry and reaction rate control by means of vibrational excitation. Hence only recently, the concept of reaction rate acceleration successfully used in gas phase bimolecular reactions has been implemented in solution.49 There has been evidence that atomic Br obtained via photolysis of Br2 reacted with the solvent (dimethyl sulfoxide (DMSO) or methanol) in a vibrationally driven hydrogen abstraction to form HBr. Further, efforts were performed to use vibrational excitation to manipulate and initiate chemical reactions successfully.49,50 In contrast to other attempts, we demonstrated the acceleration of a thermally driven reaction, namely the alcoholysis of urethane.50 The bimolecular synthesis reaction between CH-ol and PHI produces cyclohexyl-carbanilate in tetrahydrofuran (THF) solution (see Figure 1a), with an activation energy of ∼2340 cm−1.51 We excited the OH stretching vibration of the CH-ol at 3500 cm−1 clearly above the activation energy, and tracked bleaching of CH-ol and PHI vibrations, and the formation of the CO stretching vibration of CC in real-time. To separate the individual VER channels of each educt from the overall VER, we investigated selected vibrational dynamics of each educt. The first CH-ol educt in THF was excited at the OH stretching vibration at 3500 cm−1, resulting in a bleaching band around 3470 cm−1 (blue area in Figure 4a), and a red-shifted excited state absorption of the OH stretching vibration around 3220 cm−1. As depicted in Figure 4a,b the vibrationally excited state and the bleaching signal decay with a time constant of about 3 ps, followed by formation of a hot ground state of the OH stretching with vibrational energy in other low-frequency modes within tens of picoseconds around 3550 cm−1. Excitation of the OH stretching vibration is transferred to PHI without bleach recovery, resulting in a red-shift of the OH bleaching signal on a picosecond time scale in Figure 4b. Around 100 ps in Figure 4a a small contribution from hot ground state absorption is visible, decaying on a longer time scale, as well as the OH bleaching around 3450 cm−1 mainly reflecting the consumption of CH-ol. The second educt PHI in THF was excited at the NCO stretching vibration at 2270 cm−1 to track the dynamics of the v(NCO) stretching vibration. As displayed in Figure 4c the vibrational excited

Figure 4. (a) Absorbance change of CH-ol in THF upon OHstretching excitation at 3500 cm−1. Negative signals (blue) around 3470 cm−1 indicate the bleaching signal of CH-ol. Positive signals (red) at delay times of several picoseconds at about 3600 cm−1 indicate hot ground state absorption. Instantaneous positive signals around 3250 cm−1 reflect excited state absorption. (b) Same data set as in panel a, but presented as difference absorption spectra for specific delay times. (c) Absorbance change of PHI in THF upon excitation of the NCO stretching vibration at 2270 cm−1. Positive values in the absorption difference spectra indicates excited state absorption and hot ground state absorption; negative features display bleaching and stimulated emission signals.

state of v(NCO) absorbs around 2250 cm−1, and the bleaching band at 2270 cm−1. Both signals decay with a time constant of about 4 ps. With tens of picoseconds the red-shifted hot ground state of the v(NCO) stretching vibration decays to zero in Figure 4c.52 These features are not visible upon v(OH) excitation of CH-ol (see Figure 5). In contrast, upon v(OH) excitation of CH-ol the positive signal around 2220 cm−1 rises within tens of picoseconds. We assign this signal to a hot ground state of the v(NCO) vibration on the nonreacting molecule. D


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the δ(NH) of CC product. The most prominent vibration of the CC product is the v(CO) vibration at 1730 cm−1. In Figure 5c the rise of this product band with a time constant of (10 ± 3) ps is presented. To observe only the CC formation due to femtosecond IR pulse excitation, we subtracted two identical measurements with a CH-ol and PHI mixture and a CH-ol and CC mixture, presented in Figure 5c. The resulting dynamics are free from heating effects of the CC. The data demonstrate formation of the CC product with a time constant of 10 ps, and an accelerated bimolecular reaction upon IR excitation of a specific vibration, the OH stretching vibration of CH-ol. Comparing the calculated number of absorbed photons in the OH stretching band and its absorption strengths with the v(CO) vibrational signal in Figure 5c and its absorption strength, we estimate the bimolecular reaction quantum yield to ∼0.3%. Although, specific excitation of the OH stretching vibration accelerates the bimolecular reaction between CH-ol and PHI, we concluded that the v(OH) is not an important part of the RC, but instead relaxes to vibrations being part of the RC. In the simplest model of a one-step reaction the product formation should be finished with the decay of the RC. Since the vibrational energy of the v(OH) decays with about 3 ps, and the CC product is formed by 10 ps, we expect the vibrational energy of the v(OH) to relax into vibrations being part of the RC. Moreover, our understanding of a bimolecular reaction between CH-ol and PHI is that both the OH group of CH-ol, and the NCO group of PHI have to be activated to initiate the alcoholysis reaction.

5. IDENTIFICATION OF THE RC A potential route to the experimental identification of the RC for known activation energies is to excite vibrations with energy equal to or above the activation energy by IR light and probe the outcome of the reaction, e.g., by vibrational spectroscopy. This results in IR excitation−acceleration spectra, where the acceleration per excitation photon is plotted versus the excitation energy. First, a single vibration of the educts and solvent (CH-ol, PHI, and THF) is excited to map the acceleration of the (alcoholysis) reaction as a function of the excited vibrations. If excitation of a THF solvent vibration speeds up the reaction, we have EVR or external IET. Subsequently, the coupled vibrations in CH-ol and PHI have to be identified. If excitation of a single vibration of CH-ol or PHI accelerates the reaction and shows an excited state decay time matching the accelerated product formation time, this vibration is most likely the RC. Since in the present case of alcoholysis the v(OH) vibration of CH-ol and the v(NCO) vibration of PHI decay faster than the product formation time of 10 ps, we expect none of these vibrations having a large projection onto the RC. Thus, we expect at least two vibrations one in CH-ol and one in PHI with vibrational energy lower than the activation energy contributing to the RC. As a consequence, identification of the RC of the alcoholysis reaction needs to excite two different vibrations with a summed energy equal to or above the activation energy. In the present case of a bimolecular reaction, we expect the RC being formed from at least one vibration of CH-ol, and one of PHI. Since v(OH) excitation of CH-ol accelerates the bimolecular reaction, vibrations belonging to the energy relaxation pathway of the OH-stretching vibration of CH-ol are promising candidates for the RC. For an intramolecular hydrogenbonded system it was reported that vibrational energy upon

Figure 5. Absorbance change for different IR pump−IR probe delay times upon excitation of the OH stretching vibration of CH-ol at 3500 cm−1 at different spectral regions. (a) NCO-stretching vibration of PHI, the bleaching signal increases with (10 ± 3) ps; scaled and inverted absorption spectrum of PHI (gray curve); (b) NCO stretching and CH-bending vibration at 1512 cm−1 of PHI rising with (10 ± 3) ps, and NH-bending vibration of cyclohexyl-carbanilate at 1505 cm−1, scaled and inverted linear absorption spectrum of the sample (gray line); (c) CO stretching vibration of cyclohexylcarbanilate increasing with (10 ± 3) ps, linear absorption of the product spectrum (gray line).

Mixing both educts CH-ol and PHI in THF enables the alcoholysis reaction. Vibrational dynamics of the bimolecular reaction between CH-ol and PHI should show distinct spectral and transient features compared to the dynamics of a single educt. We investigated high concentrated (0.375 M to 1.5M) 1:1 mixtures of CH-ol and PHI in THF upon IR excitation of the OH stretching vibration of CH-ol at 3500 cm−1. Upon OH stretching v(OH) excitation the NCO stretching v(NCO) bleaches in Figure 5a with increasing amplitude of a time constant of (10 ± 3) ps. No positive feature of the v(NCO) excited state was observed (compare Figure 4c). Transient and spectral features are distinct from direct excitation of v(NCO) presented in Figure 4c. This clearly demonstrates the consumption of PHI due to bimolecular reaction. A similar impact on the PHI vibrations upon v(OH) excitation of CH-ol is depicted in Figure 5b. The combination of NCO stretching and CH bending vibration v(NCO) and δ(CH) of PHI at 1512 cm−1 shows bleaching with increasing amplitude of a time constant of (10 ± 3) ps, accompanied by a rise of a positive band at 1505 cm−1. This positive band was assigned to E


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50 is caused by the low probability for adopting the proper reaction geometry of CH-ol and PHI. Thus, we expect molecular structures, e.g., cages,61 stabilizing the reaction geometry would significantly enhance the quantum yield of the IR activated ground state reactions. The significant effect of the solvent molecules on the reaction geometry and the activation energy was shown for the alcoholysis reaction in ref 50.

v(OH) excitation relaxes predominantly via the in-plane OH bending vibration, followed by the out-of-plane OH bending vibration, and low-frequency modes.8 Thus, OH bending vibration of CH-ol and NCO bending vibration of PHI could be promising candidates for the RC in the present alcoholysis reaction. If no pair of vibrations is found that accelerates the reaction upon excitation, and exhibits time constants similar to the product formation, the number of excited vibrations should be expanded. Since excited state lifetimes typically increase with decreasing frequency, the probability of finding matching time constants for product formation times of several picoseconds increases.8,35,37,52,53 An alternative way to comprehensive femtosecond experiments for every single reaction is to compare energy relaxation maps for similar molecular groups. Such energy relaxation maps were created for methanol and similar molecules by the group of D. D. Dlott,9,53−55 and vibrational energy transport through different polyatomic molecules was measured and calculated by the group of I. V. Rubtsov.56−60 These and other pioneering investigations35,37 should be extended to create energy relaxation maps describing the complete energy relaxation pathways with energy proportions and time scales. If these maps were available, IR excitation−acceleration spectra could be measured by using broadband quantum cascade lasers (QCLs) with powers of ∼0.5 W to excite individual vibrations and probe the reaction outcome by other methods, e.g., FTIR spectroscopy. If several vibrations are identified accelerating the reaction, every vibration will relax into vibrations belonging to the RC. The energy relaxation maps can be used to sort out the proper RC or at least to narrow the search. For example, the multistep reaction presented in Figure 1b has an activation energy of about 980 cm−1.51 Upon IR-laserpulse excitation of the OH-stretching excitation at 3490 cm−1, and upon v(NCO)excitation at 2270 cm−1 the reaction was accelerated by (10 ± 5)% and (22 ± 8)%, respectively. In contrast, no acceleration was observed upon CH stretching excitation at 3070 cm−1. The impact of NCO-stretching excitation on the reaction is stronger compared to the OHstretching excitation. The energy of both vibrations is much higher than the activation energy, indicating vibrations belonging to the RC that are populated by VER of the OHstretching and NCO-stretching vibration.

7. APPLICATION IN CHEMICAL SYNTHESIS Femtosecond time-resolved vibrational spectroscopy is an ideal tool to track IR light induced product formation of ground state reactions in real-time. Comparison of product rise time with vibrational energy relaxation time constants of individual vibrations provides information on possible energy relaxation pathways. Nevertheless, femtosecond set-ups are expensive and cheaper solutions for identifying the RC (see above) and accelerating chemical reactions should be found. If the RC is known and a single or several vibrational frequencies are identified to accelerate a reaction, single wavelengths IRQCLs can be used to illuminate the sample. Nowadays, IR QCLs exhibit lifetimes of more than 40 000 h at a power of >10 mW that equals 3.6 × 1025 photons at about 5 μm in the complete lifetime. This corresponds to a maximal product generation in case of 100% quantum yield of about 60 mol per QCL lifetime. For such low quantities IR accelerated reactions are only economically viable for high-priced products, such as antibiotics or natural products. Products with competing thermally driven reaction pathways or low stability at higher temperatures are best suited for this method. Moreover, potential applications, in the field of micromanufacturing, e.g., microstructuring of polymers, are promising. In Figure 6, we

6. CONFORMATIONAL INFLUENCE IR excitation assisted reaction acceleration in solution is dominated by statistics. The actual conformation of a single molecule or several molecules at a given instant is crucial for the reaction. In the case of the alcoholysis reaction, the IR excited OH group of CH-ol should be very close to a NCO group of PHI with a proper orientation.18 Otherwise, the vibrational excitation will decay to nonactivating vibrations before a proper reaction geometry between the two reacting groups is adopted. Thus, very short vibrational lifetimes reduce the possibility to sample the configurational space to reach a proper reaction geometry. This implies that strongly interacting vibrations, e.g., v(OH) with hydrogen bonds and Fermi resonances,52 are unfavorable candidates for reaction acceleration. Vibrational lifetimes in the picosecond range are well suited for IR excited reaction acceleration together with molecules preferably adopting configurations close to the reaction geometry. In fact, we assume the low quantum yield of 0.3% per photon measured for the alcoholysis reaction in ref

Figure 6. Two different polymer squares produced by illuminating a 1:1 mixture of TCD and TDI with IR pulses at 2270 cm−1. The polymer is not visible in solution, but on a dry CaF2 window. Here, we used a focal beam diameter of about 0.08 mm. The polymerization follows the illuminated region of the sample.

demonstrate the formation of a polymer structure in 1:1 solution of 2,2,2-trichloroethane-1,1-diol (TCD) and toluene2,4-diisocyanate (TDI) by IR light illumination. The sample was moved along a ∼3 × 3 mm square while the IR light pulses were focused into the sample. Here, the properties of the accelerating light field are used to set the width of the polymer displayed in Figure 6. The limits F


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Journal of the American Chemical Society of miniaturization have to be investigated, in particular the possibilities of subdiffraction structuring. It is to be expected that the ongoing improvement of IR diode manufacturing will reduce the price at higher output powers in the future, making other applications feasible.

Knowledge of the RC enables precise IR excitation for accelerating chemical reactions with optimal reduction of excess energy. Using this tool, in a complex synthesis a specific thermally driven reaction could be accelerated, while other competing reactions were not. Moreover, a conformational equilibrium could be shifted upon IR excitation, increasing preferred conformations for chemical reactions. Using the properties of IR light, microstructuring of reaction products was demonstrated. Polarization properties could induce ordered or oriented product structures, and circular polarized IR light could be used to increase the abundance of a specific enantiomer. In combination with temperature dependent set-ups the temperature can be reduced to almost halt the chemical reaction and purely induce the reaction by IR light. In this case it should be possible to use the polarization of the IR light beam to select IR transitions predominantly parallel to the IR polarization, thereby generating products with specific orientations. Thus, for step-growth polymerization a specific orientation could be picked, and the property of the polymer could be modified on a scale of the IR light focus. Finally, it should be emphasized that although we used pulsed IR light to accelerate ground state reactions in our studies, multiphoton effects for a single reaction step could be excluded. Thus, in principle continuous wave IR light from common laser systems should also facilitate acceleration of thermally driven reactions.

8. CONCLUSIONS AND OUTLOOK We demonstrated IR light excitation and vibrational spectroscopy to be the perfect tools to initiate and follow ground state chemical reactions, with interpretation being guided by quantum chemical calculations.50 The present approach, although providing laser control of a chemical reaction, does not rely on complex pulse shapes exploiting coherent molecular wave packet evolution, which are typically obtained using optimal control theory18 or pulse shaping algorithms.62,63 Further, these advanced control approaches have been successful in manipulating reactions in electronically excited states or via the electronically excited states only. For instance, Zeidler and co-workers employed coherent anti-Stokes Raman scattering to control the ground state vibrational dynamics of a polymer using a feedback learning loop.64 We are not aware of practical applications of all-IR coherent control to ground state reactions in solution phase. Successful demonstrations of IRdriven reactions have employed simple pulse shapes and were restricted to essentially isolated small molecules, e.g., the isomerization of HONO in a low-temperature Krypton matrix21 or the dissociation of CH2N2 in the gas phase.47,48 In contrast to photoreactions in the electronic excited state,65 in the ground state excess energies are much smaller and dynamics are typically slower, involving a multitude of coordinates. Unravelling these processes still poses considerable challenges to experiment and theory. Gas phase reaction dynamics of small molecules can be described at the quantum mechanical level with high accuracy, e.g., by combining ab initio potential energy surfaces with wave packet propagation methods.66 Such a level of rigor will not be possible for solution phase reactions although substantial progress is foreseeable. Currently, the problem of generating proper interaction potentials for trajectory-based simulations witnesses a major breakthrough due to the implementation of neural network concepts. On the other hand, methods for incorporating quantum effects such as zero energy and tunneling on a trajectory level are being developed.67 Even though practical implementation will still require one to distinguish between the reactive system and its environment, treated at different levels of approximation, the combination of both approaches will pave the way to a molecular level understanding of the key factors enabling laser-driven acceleration of chemical reactions. The added value of the experimental tool, IR excitation to accelerate chemical reactions, is the possibility to identify the RC of a reaction. Up to now, it is only possible to accelerate thermally driven reactions, thus the described methods to identify the RC are limited to this class of reaction in the electronic ground state. Nevertheless, an overwhelming majority of chemical reactions in the solution phase are in fact thermally driven reactions in the electronic ground state. In terms of practical realization, another potentially limiting factor exists for reactions taking place in aqueous solution. The broad and strong water absorption as well as the often considerable solute−solvent interaction might provide a serious challenge.

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Corresponding Author

*[email protected] ORCID

Karsten Heyne: 0000-0002-3243-9160 Oliver Kühn: 0000-0002-5132-2961 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Till Stensitzki, Jens Beckmann, Ashour Ahmed, and Alejandro Ramos for valuable discussion. The work was supported by the Deutsche Forschungsgemeinschaft (grant nos Ku952/6 and He5206/3, CRC 1078 ‘Protonation Dynamics in Protein Function’, Project B07, CRC 1114 ‘Scaling Cascades in Complex Systems’, Project B05).

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