Controlling photochemistry via isotopomers and IR preexcitation

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Controlling photochemistry via isotopomers and IR preexcitation Daniela Kern-Michler, Carsten Neumann, Nicole Mielke, Luuk J. G. W. van Wilderen, Matiss Reinfelds, Jan von Cosel, Fabrizio Santoro, Alexander Heckel, Irene Burghardt, and Jens Bredenbeck J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b08723 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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Journal of the American Chemical Society

Controlling photochemistry via isotopomers and IR preexcitation Daniela Kern-Michler‡a, Carsten Neumann‡a, Nicole Mielkea, Luuk J.G.W. van Wilderena, Matiss Reinfeldsb, Jan von Coselc, Fabrizio Santorod, Alexander Heckelb, Irene Burghardtc, Jens Bredenbecka,* a

Goethe-University Frankfurt, Institute of Biophysics, Max-von-Laue-Str. 1, 60438, Frankfurt am Main, Germany, b Institute of Organic Chemistry and Chemical Biology, Max-von-Laue-Str. 7, 60438, Frankfurt am Main, Germany, c d Institute of Physical and Theoretical Chemistry, Max-von-Laue Str. 7, 60438 Frankfurt am Main, Germany, Consiglio Nazionale delle Ricerche – CNR, Istituto di Chimica dei Composti Organo Metallici (ICCOM-CNR), UOS di Pisa, Via G. Moruzzi 1, I-56124 Pisa, Italy.

Supporting Information Placeholder ABSTRACT: It is a photochemist’s dream to be able to photo-induce a reaction of a specific molecular species in an ensemble of similar but not identical ones. The problem is that similar molecules often exhibit nearly identical UV-Vis absorption spectra, making them difficult or impossible to distinguish or to select spectroscopically. The ultrafast VIPER (VIbrationally Promoted Electronic Resonance) pulse sequence allows to pick a single species for electronic excitation based on its infrared spectrum. The latter usually shows more features that allow to discriminate between species than the UV-Vis spectrum. Here, we show that it is possible to induce and monitor species-selective photochemistry even for molecules with virtually identical UV-Vis spectra, which is the case for isotopomers. Next to isotope-selective photochemistry in solution, applications to orthogonal photouncaging and species-selective spectroscopy and photochemistry in mixtures are within reach.

approach for an extreme case of near-identical and nonseparable molecules in the form of two photoreactive mole13 cules, differing only in the position of a C atom (i.e. isotopomers). This is not to be confused with selectivity achieved 1 via a kinetic isotope effect. In the gas phase or in a weakly interacting or homogeneous environment (such as a cryogenic liquid or a crystal), electronic transitions have been found to be sufficiently narrow in some cases to allow preferential electronic excitation of one specific isotopomer

Introduction Many chemical and biological systems consist of mixtures of very similar molecules, e.g. ensembles of molecules involved in hydrogen bonding with varying strength, different protonation states or different types of isomers. The individual species in a mixture often have strongly overlapping UV-Vis absorption spectra. In case of photoreactive molecules this can make it impossible to induce and monitor photochemistry in only one selected species. Gaining control by light over one individual species in the presence of similar others will ultimately allow studying and controlling more complex chemical and biological systems. This goal could be achieved by combining the high selectivity of infrared (IR) excitation with the ability of UV or Vis excitation to induce photochemistry. Here, we demonstrate this

Figure 1. Scheme for spectroscopic investigation of photochemistry in a mixture of two molecules having similar UVVis (symbolized by the orange-colored molecules), but different IR absorption spectra (green vs. blue circles). (A) Unselective resonant UV-Vis excitation (filled pulse) monitored by UV-Vis-pump IR-probe spectroscopy. (B) Speciesselective excitation via VIPER spectroscopy. In the VIPER pulse sequence the IR-pump pulse excites only one of the (green or blue) species. The subsequent off-resonant UV-Vispump (orange-striped; by itself not providing sufficient energy to directly excite the molecule electronically) promotes the pre-selected molecules (colored in orange, while the

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other species stays white) to the electronically excited state leading to photochemistry. The VIPER spectra show only

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signals specific for the photoreaction of the selected species.

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Figure 2. Steady-state spectroscopy of DEACM-N3 isotopomers. (A) Chemical structures with C at position 2 (2 C; left mole13 cule) and at position 4 (4 C; right). (B) Normalized UV-Vis spectra of both isotopomers in acetonitrile. Orange arrows indicate the wavelength of the Vis-pump pulses (solid: resonant in Vis-pump IR-probe, dashed: off-resonant in VIPER). (C) Steady state 13 13 IR spectra of the single isotopomers and of a mixture of 40% 2 C and 60% 4 C in acetonitrile, normalized to the azide band (at label 6). The labels 1-3 denote νC=C ring modes, 4 and 5 νC=O modes. The colored arrows indicate the center wavenumbers of -1 IR excitations used in the VIPER measurement (shown in Figure 4C): black is not resonant (at 1571 cm ), blue is predominantly 13 -1 13 -1 -1 resonant with a 4 C ring mode (1584 cm ) and green with a 2 C ring mode (1609 cm ). Note the axis break between 1780 cm -1 and 2035 cm . 2–4

or isotopologue. In solution at room temperature, however, UV-Vis spectra of isotopomers are virtually identical and spectroscopic selection of isotopomers for electronic excitation appears to be impossible. Nevertheless, we show that addressing different isotopomers for electronic excitation in 5,6 a mixture can in fact be achieved by VIPER excitation. This paves the way for flexible isotope-selective photochemistry in solution. VIPER spectroscopy. The VIPER pulse sequence uses a combination of a resonant IR photon and an off-resonant UV-Vis photon for excitation. It was initially introduced to circumvent the limitation of the observation time window posed by the vibrational lifetime in 2D-IR exchange spectros5,6 copy. IR pre-excitation leads to a red shift of the Vis spectrum of the molecule picked by the narrow band IR pulse 7 due to vibronic coupling. It becomes resonant with the previously off-resonant UV-Vis pulse. Thus, if the offresonant UV-Vis pulse arrives during the lifetime of the vibrational excitation, the molecules are electronically excited. The population in the electronically excited state, and therefore the VIPER 2D-IR signal was shown to decay with the electronic lifetime, which allowed to measure 2D exchange processes on much longer timescales than typically 5 given by the vibrational lifetime. A very important parameter for VIPER signal generation is the coupling of the preexcited vibrational modes to the electronic transition, since this determines to which extend the visible absorption is shifted by the vibrational pre-excitation. The Huang-Rhys 8 factor provides a suitable measure of this vibronic coupling; i.e., modes along which the molecule exhibits a large structural deformation upon photoexcitation have large HuangRhys factors. This is confirmed by more detailed computational predictions for the shift caused by excitation of differ7 ent vibrational modes . Indeed our recent computational 7 studies are found to be in excellent agreement with experiment in that they predict the most efficient mode for VIPER excitation. Experimental principle: selecting isotopomers by VIPER. Figure 1 illustrates the VIPER principle for selecting

among two molecular species with very similar UV-Vis spectra but different IR spectra (illustrated by orange ellipses with green or blue circles in panel A). If a sample containing both species (in the present case isotopomers) is irradiated by UV-Vis light of suitable wavelength, both species will undergo photochemistry and no selection is achieved. If such a mixture is studied by conventional UV-Vis-pump IR-probe spectroscopy (see Figure 1A), both species will be electronically excited by the resonant UV-Vis pulse (orange-filled pulse). The resulting IR-probe spectrum is a superposition of the spectra of the two species, as illustrated by the presence of two ground state bleach signals (negative signals) and their accompanying excited state absorptions (ESA; positive signals). In contrast, the VIPER pulse sequence (Figure 1B) can be used to select a molecular species via a narrow-band IRpump pulse which is resonant with only one of the two species (depicted by its correspondingly colored pulse). The photoreaction is initiated by a subsequent off-resonant UVVis-pump pulse (orange-striped pulse). Its wavelength is tuned to be off-resonant with the molecules in their vibrational ground state but resonant with the vibrationally excited ones. Therefore, the molecules which have been selected by the IR-pump pulse become resonant with the Vis pulse (indicated by orange color as opposed to white for the not pre-excited species) are electronically excited and undergo photochemistry. The selection is illustrated by the presence of only one set of corresponding bleach and excited state absorption signals in the two schematic IR-probe spectra in Figure 1B. In this fashion, the VIPER experiment combines the ability of UV-Vis excitation to trigger photochemistry with the high selectivity of IR excitation to control which species undergoes the photoreaction. In contrast to analytical tools like triggered-exchange 2D-IR 9–11 spectroscopy, employing an (unselective) resonant visible pulse, not all molecules but just the vibrationally pre-excited molecules undergo photoreaction in the VIPER experiment. For analytical purposes also other nonlinear multidimensional pulse sequences are available. One example is the


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DOVE or EVV 2D-IR pulse sequence. It has been shown to be capable of quantifying and identifying peptides and 13,15 proteins. Multidimensional methods that can work at 16 micromolar concentrations are also available, while many other experiments including VIPER usually require millimolar concentrations. A different approach is to use transient 2D-IR (T2D-IR) which has been successfully applied to identify different transient isomers or to correlate product and 10,17 reactant during a photoreaction. For more details on these and many other multidimensional spectroscopic tools the 18–26,6 reader is referred to recent reviews. Here we apply the VIPER pulse sequence not as an analytical tool but to select between isotopomers of a photo-cage (Figure 2A), opening 27–29 up a novel approach to orthogonal photouncaging. Photocages are molecules which release a leaving group upon light-induced bond cleavage. Instead of attempting to design different photocages with minimized overlap of their UV-Vis 30 spectra to achieve orthogonality, a set of isotopomers of a single optimal cage can be used.

Results and Discussion DEACM-N3 isotopomers. For our study we used two isotopomers of DEACM, carrying azide as a leaving group 31 ([7-(diethylamino)coumarin-4-yl]methyl-azide; see Figure 2A for its structure and the SI for its synthesis and sample preparation details. After photoexcitation, molecules of the (coumarin-4-yl)methyl family release their leaving 32,33,31 group. The isotopomer on the left side of Figure 2A, 13 13 named 2 C in short, has a C label incorporated at position 2 of the coumarin ring system, the isotopomer on the right at 13 position 4 (i.e. 4 C). Steady state spectroscopy. The UV-Vis absorption spectra in solution do not reveal any differences (see Figure 2B) between the isotopomers, showing that a selection between the isotopomers (i.e. which species reacts) by UV-Vis excitation only is impossible. The IR absorption spectra, in contrast, show pronounced differences between the isotopomers (Figure 2C). Each isotopomer features an azide stretching vibration (band 6) whose central wavenumber is not affected 13 by the C label. The carbonyl stretching vibrations differ 13 considerably, as positioning of C in the carbonyl group 13 leads to a distinct down shift (band 5 of 4 C is shifted with 13 regard to band 4 of 2 C). Each isotopomer features two ring modes in the given wavenumber range (see Figure S2 for a larger version of the spectrum and a comparison to the unlabelled compound). The position of the high wavenumber ring mode (band 3) is largely unaffected by the position of 13 13 C. The low wavenumber ring mode (band 2 of 2 C) is shift13 ing to lower wavenumbers (band 1 of 4 C). The 40:60 mixture (orange curve in Figure 2C; see SI for sample details) shows the corresponding features of both isotopomers. Characterization by Vis-pump IR-probe spectroscopy. Before applying the VIPER pulse sequence to the mixture each individual isotopomer was studied separately by Vispump IR-probe measurements. These measurements were carried out to identify the released azide photoproducts and to compare the spectra of the pure DEACM-N3 isotopomers. Figure 3 shows selected Vis-pump IR-probe spectra of pure 13 13 2 C (green) and pure 4 C (blue) in acetonitrile at 2.5 ps (Figure 3A) and 3160 ps (Figure 3B) delay between the pump

and probe pulses. Only the spectral windows of the carbonyl and azide vibration are shown. At 2.5 ps the negative signals

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Figure 3. Vis-pump IR-probe spectra of pure 2 C (green) and 13 pure 4 C (blue) in acetonitrile after resonant Vis excitation at 400 nm (solid orange arrow in Fig. 2B) at delays of 2.5 ps (A) and 3160 ps (B). The * denotes the excited state absorption of each corresponding bleach. The roman numerals are the photoproducts: I N3 and II HN3 for both isotopomers and III and IV are carbonyl signals assigned to dimer formation. -1 -1 Note the axis break between 1820 cm and 1990 cm . labelled 4 and 5 are the bleaches of the correspondingly 13 labelled (see Figure 2C) carbonyl stretch modes of 2 C and 13 4 C in the S0 state. Band 5* in Figure 3 is the corresponding 13 13 ESA of the carbonyl stretch band 5 for 4 C in S1. For 2 C this ESA is outside the spectral window of Figure 3 and cancels with the bleach of the high wavenumber ring mode (band 3). Bands 6* and 6 represent the azide ESA and bleach contributions, respectively (identical for both isotopomers). At 3.16 ns these ESA contributions have decayed, but features assigned to the photoproducts (denoted by roman numerals) and the ground state bleaches 4-6 can be observed. The permanent bleaches of both isotopomers now represent the fraction of the excited molecules that underwent photochemistry. In the photoreaction the azide moiety is cleaved from the DEACM molecule and N3 (I) and HN3 (II) are formed. Additional bands (III and IV) have been assigned to photo-induced 31,34 coumarin dimers. Comparing the light-minus-dark difference spectrum after continuous illumination in reference 31 with our nanosecond spectrum we conclude that contributions of sample heating in the Vis-pump IR-probe data are not significant. A detailed analysis of the photochemistry of 31 DEACM-N3 is given in references. and is consistent with the data for the isotopomers. The bleach and the ESA features in the carbonyl wavenumber region show prominent differences for both isotopomers. Therefore this region is used to distinguish which isotopomer in a mixture is electronically excited to undergo photochemistry. Selecting isotopomers out of the mixture by VIPER. For the VIPER experiment we now mix both isotopomers in the same solution, and show that it is possible to preselect one species in the presence of the other isotopomer to undergo electronic excitation leading to photochemistry. The results of the VIPER experiment are shown in Figure 4C. For easier comparison the steady state IR (Figure 4A) and Vis-pump IRprobe (Figure 4B) spectra of the carbonyl region are also shown. As expected, the Vis-pump IR-probe spectrum of the 40:60 mixture exhibits carbonyl bleaches 4 and 5 of both



isotopomers (orange curve in Figure 4B), as well as the car13 bonyl ESA of 4 C (5*; see also Figure 3A). Clearly, the Vispump IR-probe data demonstrate the non-selective nature of the experiment by using only a resonant (for both isotopomers equally) Vis pulse for electronic excitation. In contrast, the VIPER spectra collected on the same mixture reveal selective electronic excitation induced by the IR preexcitation (Figure 4C). In order to demonstrate the method, the narrow-band IR-pump pulse in the VIPER sequence is chosen to be centered at two different ring mode vibrations

Figure 4. (A) Steady state IR spectra of the carbonyl region of 13 13 13 pure 2 C (green), pure 4 C (blue) and of a 40 % 2 C and 60 13 % 4 C mixture (orange) in acetonitrile after subtraction of the solvent. (B) Vis-pump IR-probe spectra of the carbonyl 13 13 region of pure 2 C (green), pure 4 C (blue) and of the mentioned isotopomer mixture (orange) in acetonitrile after 2.5 ps Vis-pump delay time. (C) VIPER spectra of the carbonyl region of the isotopomer mixture in acetonitrile. The data were collected with the IR-pump pulse arriving 1.5 ps before the Vis-pump pulse and the Vis-pump pulse (at 437 nm) 2.5 ps before the probe pulse. and one off-resonant wavenumber as indicated by the colored arrows in Figure 2C. Similar to previous VIPER experi5 ments on the laser dye coumarin 6, the lower frequency ring mode generates a stronger VIPER signal than the carbonyl mode. This experimental finding is corroborated by computations of the vibronic spectra without and with preexcitation of different modes shown in Figure 5. Therefore the ring mode and not the carbonyl mode is chosen for preexcitation, despite the large separation of the carbonyl modes which would improve the contrast. The Vis pulse is tuned to be off-resonant at 437 nm, see the striped orange arrow in Figure 2B. Experimentally, we determined the delay between the IR-pump pulse and the Vis-pump pulse t1=1.5 ps, i.e. just after the narrowband IR-pump pulse, and the delay between the Vis-pump pulse and the IR-probe pulse t2=2.5 ps to deliver the largest VIPER signal. In contrast to ion-dip or 35 ion-gain spectroscopy in molecular beams using nanosecond pulses, ultrashort pulses and delays are required to out-

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pace vibrational relaxation and vibrational energy dissipation to the solvent in the condensed phase. Application of VIPER to the isotopomer mixture is expected to reveal selective electronic excitation. If the IR-pump pulse is tuned to be (predominantly) resonant with the low fre13 quency ring mode band 1 of 4 C (see blue arrow in Fig. 2C), the blue spectrum predominantly shows the carbonyl band 5 13 bleach of 4 C (see Figure 4C). Tuning to the low frequency 13 ring mode band 2 of 2 C (green arrow in Fig. 2C), the green spectrum now shows a single bleach of the carbonyl band 4 13 of 2 C. As pointed out above, the observed bleaches in the carbonyl region can be used as a reporter of the identity of the species that undergoes uncaging. The VIPER sequence has therefore succeeded to induce photochemistry of individual species in the mixture. Off-resonant IR excitation (black arrow in Fig. 2C) results in the featureless black spectrum and confirms that resonant IR excitation is essential for selecting the isotopomer for photochemistry.

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Figure 5. Calculated electronic absorption spectra of 2 C in acetonitrile for different vibrational pre-excitations. The spectrum is shown for the vibrational ground state (black curve) and after IR pre-excitations of the lower frequency 13 ring mode (light blue; corresponding with band 2 of 2 C which is also denoted in brackets in the legend), the higher frequency ring mode (violet; corresponding with band 3 of 13 2 C) and the carbonyl mode (mint green; corresponding 13 with band 4 of 2 C). The wavenumber of the 0–0 transition is marked by the dashed black line. Spectra were calculated in the gas phase, and will be subject to significant broadening if solvent effects are accounted for. Computation of the influence of IR pre-excitation on the visible spectrum. Vibrationally-resolved electronic absorption spectra including the effect of vibrational preexcitation were computed to predict the vibrational modes that lead to the largest red shift of absorptivity of the elec13 13 tronic spectrum (see Fig. 5 for 2 C and Fig. S3 for 4 C). An 36 analytical time-domain approach was employed based on an harmonic approximation of the potential energy surface 7 (PES) including Duschinsky rotation effects . A summarized description of the computational approach can be found in the SI. As depicted in Fig. 5, pre-excitation of the lower fre-


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quency ring mode (the light blue spectrum, corresponding to band 2 in Fig. 2) has the largest impact on the long wavelength part of the Vis spectrum. Therefore, excitation of this ring mode, with the largest Huang-Rhys factor is expected to lead to the largest VIPER signal among the three modes 7 compared here. This is confirmed experimentally . The IRinduced shift itself and its time dependence can in principle be obtained experimentally via two-dimensional vibrational 37 pump-electronic probe spectroscopy. Computations taking into account vibrational energy transfer are planned. Factors influencing the VIPER signal size and contrast between species. A number of factors influence the VIPER signal size of the different species and therefore the contrast between species. Whether the selection of a distinct isotopomer is possible depends foremost on the availability of species-unique IR bands (i.e. their relative intensities and spectral overlap). In an extreme case, however, if the vibronic coupling of a mode of one species is zero, this mode might overlap completely with the mode of interest of the second species, because the mode without vibronic coupling will not generate a VIPER signal. Spectral position and width of the IR-pump pulse can be optimized for the respective situation. For the data shown here the optimal IR-pump positions generating the highest contrast requires them to be slightly detuned from the absorption maxima of the selected modes. The spectral width -1 and shape of the pump pulse (here a 15 cm broad Lorentzian from a Fabry-Perot filter having slowly-decreasing tails) might also lead to minor contributions from the not selected species. However, different means to generate narrow band pulses can be applied in future. Differences in vibronic coupling between the selected vibrational modes of each isotopomer lead to a different extend of red shift of the respective Vis spectrum upon IR excitation and accordingly a different VIPER signal size. In the present case, however, the computations (see Fig. 5 and Fig. S3) suggest a similar shift of the Vis spectra of the two isotopomers upon IR excitation. Experimentally, an additional IR-pumpVis-probe experiment could be used to quantify the shift and determine optimal timing and wavelength of the Vis pulse. It is also possible that molecules are directly excited by the UVVis pulse without having received the IR pre-excitation. For selective spectroscopy measurements this does not pose a problem because the (non-selective) signal contribution of directly excited molecules is simply subtracted by using a 5 suitable chopping scheme (see the SI and ref. ). For other (e.g. preparative) applications, however, it might be desirable to optimize the ratio between selective VIPER excitation and unselective direct background excitation. Although the direct background signal for DEACM-N3 is larger than the VIPER signal, we have successfully obtained VIPER-tobackground ratios of up to 20 in coumarin 6 and rhodamine 6G (data not shown). We found this ratio to increase linearly with IR-pump power and to be currently limited by the comparably low available pulse energy of about 100 nJ. The generation of 400 times more intense pulses has already been 38 shown and applied to 2D-IR spectroscopy, and its application drastically increase the possible signal-to-background ratio. Another means to affect VIPER signal size and thus contrast is the polarization dependence. The polarization dependent

prefactor for two signals in a three pulse fifth order experiment such as the presented one on the isotopomer mixture 39 can differ by a factor up to 15, much more than in a two pulse third order experiment such as pump-probe spectroscopy. This factor depends on the three transition dipole angles which are formed by the IR-pumped vibration, by the Vis-pumped transition and the IR-probed vibration. In the present case, isotopic substitution changes the vibrational modes (as is already obvious from the FTIR spectra in figure 2C, where the isotope labelling changes absorption cross sections and wavenumbers), therefore, also the directions of the vibrational transition dipoles will be affected. This needs to be considered in a quantitative treatment. Vibrational relaxation determines the population of IR excited molecules at the time when the Vis pulse arrives. In an experiment aiming at the selection between species the population of vibrationally excited molecules of each of the two species might be different (even if initially the same number of molecules of each of the species has been excited), leading to a different amount of molecules being electronically excited and thus a different VIPER signal size. Vibrational energy transfer within the molecule might be exploited to increase the VIPER signal, if the modes to which the energy is transferred have a larger vibronic coupling than the initially excited one and the delay time between IR preexcitation and visible excitation is chosen appropriately. Despite intramolecular energy transfer the original selection of the molecule by the IR pulse is maintained, however, if the delay between IR pump and Vis pump is too long, vibrational energy dissipation into the solvent will prevent effective electronic excitation. The effects discussed here can have a large influence on VIPER excitation and signal size and provide versatile degrees of freedom to further optimize VIPER control of photochemistry in future studies.

Conclusion In conclusion, we have demonstrated the selection of isotopomers for photochemistry by combined femtosecond IR and UV-Vis excitation. A future application of isotope selective photochemistry could be orthogonal photouncaging by exploiting the species selectivity available in the infrared spectrum. While isotopomers are an extreme case of molecular species with virtually identical UV-Vis spectra, the selection of one species for electronic excitation via its vibrational modes could find more widespread use in other applications where two or more species show significantly overlapping UV-Vis spectra. The photochemist's dream to be able to pickand-study a molecular species within mixtures of multiple species with identical UV-Vis spectra, without further purification or isolation steps, has therefore turned reality by applying the VIPER pulse sequence.

ASSOCIATED CONTENT Supporting Information Sample preparation, details of experimental setup, data evaluation and computations of vibronic spectra with vibrational 5,31,40–42,36,43–49 pre-excitation are discussed. This material is



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AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank Andreas Münch and Matthias Brandl for measuring the HRMS and the Deutsche Forschungsgemeinschaft for funding via RTG 1986 "Complex Scenarios of Light Control”.

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Controlling photochemistry via isotopomers and IR preexcitation

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