Quantum Control Spectroscopy of Competing Reaction Pathways in a

Article pubs.acs.org/JPCA

Quantum Control Spectroscopy of Competing Reaction Pathways in a Molecular Switch Cristina Consani, Stefan Ruetzel, Patrick Nuernberger,† and Tobias Brixner* Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S Supporting Information *

ABSTRACT: Excitation with shaped femtosecond laser pulses is a logical extension of coherent two-dimensional (2D) spectroscopy. Here we combine quantum control and information from 2D spectroscopy to analyze the initial steps in three competing reaction pathways of an isomerizing merocyanine dye. Besides the achievement of control objectives, we show how excitation with tailored pulses can be used to retrieve photochemical information that is inaccessible or experimentally demanding to obtain with other approaches.

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INTRODUCTION The desire for a deeper understanding of photophysical and photochemical processes, which are typically initiated on ultrafast time scales, triggered the development of sophisticated time-resolved spectroscopic techniques. Among them, multidimensional spectroscopies, both in the IR and in the UV− visible domains, gained popularity as they allow visualizing vibronic couplings, while simultaneously correlating the precursor states with their products.1−9 The sensitivity of coherent two-dimensional (2D) spectroscopy in the IR domain to dipolar couplings between vibrational states has been exploited to study conformational changes accompanying photochemical reactions.6,9,10 Electronic 2D spectroscopy in the UV−visible region was mostly employed to track photophysical processes such as energy transport and electronic relaxation in multichromophore complexes.5,6,11 Recently we have shown that electronic 2D spectroscopy can also be applied to investigate photochemical reactions, due to its capability to separate reactant from product features in congested spectral regions.12 In parallel with the emergence of 2D spectroscopy, the field of quantum control developed. The initial motivation was to guide the evolution of a photoexcited quantum system using the coherence properties of light.13−23 For example, in chemical-reaction control the desired reaction pathways can be enhanced while minimizing undesired side products. Apart from steering quantum dynamics, quantum control can be used as a spectroscopic tool. The basic idea behind “quantum control spectroscopy”24 is that the optimal pulse bears information on both, the topology of the system’s potential energy surface (PES) and the mechanisms involved in the investigated reaction.23,25−30 Here we present an experimental approach to obtain ultrafast photochemical information from an interplay between quantum control and coherent 2D spectroscopy. These two methods were recently applied simultaneously31−34 and are strongly © XXXX American Chemical Society

related. Indeed excitation with shaped pulses can be seen as the natural extension of multipulse techniques. We show that a combination of the information gained from these two techniques, performed in the same transient absorption setup and on the same molecular system, is highly beneficial for elucidating the ultrafast dynamics of a photochemical reaction. We apply this approach to a molecular switch, the merocyanine form of 6-nitro-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′indoline] (6-nitro BIPS). Molecular switches are systems whose photochromic properties are exploited for applications in biology, sensors, data storage and molecular nanomachines,35−37 and in the past decade they have been subject of extensive studies aiming at clarifying the relevance of different electronic states and nuclear degrees of freedom in photochemical reactions. Through manipulation of the photoexcited wavepacket, quantum control techniques potentially provide information on the initial evolution of the photoexcited system, additionally accessing multiphoton processes. Open-loop control schemes, in which the excitation pulses are designed for the specific system, allow for exploring the details of the excited PES14,26,38−41 and for selective investigation of the role of distinct processes for the overall photoreaction, e.g., pump− dump processes, ionization or branching between different reaction channels.24,42−46 In our approach, the information inferred from third-order nonlinear spectroscopies and quantum control spectroscopy is combined to elucidate the initial steps in the photophysics of 6-nitro BIPS. In particular, by employing parametrized control fields, we are able to characterize the early dynamics of three competing reaction pathways. Received: September 16, 2014 Revised: November 5, 2014

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dx.doi.org/10.1021/jp509382m | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. Experimental setup used for the experiments, as described in the text. The following abbreviations are used: BS (beam splitter); SLM (spatial light modulator); ND (neutral density filter); SM (spherical mirror); HM (hot mirror); S (sample).

Figure 2. Static and transient absorption spectra of the merocyanine form of 6-nitro BIPS in acetonitrile solution. (a) Normalized steady-state spectra of the TTC (red, solid) and the TTT (blue, dashed-dotted) isomers. The (green) dotted line is a typical laser spectrum used for the control experiments. (b) Transient absorption signal recorded at 8 ps upon excitation of 6-nitro BIPS with transform-limited pulses; excited-state absorption (ESA), ground-state bleach (GSB) and stimulated emission (SE) contributions are indicated. Inset: Sscaled(λprobe) signal (eq 1) at low (10 nJ, black dashed) and high excitation power (260 nJ, green). (c) Scheme of the relevant states accessible upon photoexcitation of 6-nitro BIPS in solution with the pulses shown in part a. Relevant pathways are indicated with arrows. Given N initially photoexcited molecules, the number of molecules in each channel is indicated. (d) Estimated transient spectrum of the radical cation formed after excitation of Sn according to the model shown in part c.

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EXPERIMENTAL METHODS Commercially available 6-nitro BIPS (ABCR GmbH & Co. KG) is dissolved in acetonitrile (Sigma-Aldrich, spectroscopic grade) without further purification. Typical concentrations used in the experiments are 3.8 mmol/L. During the experiment, the sample is circulated in a 0.2 mm thick flow-cell to minimize heating and degradation effects. A constant concentration of the merocyanine form is achieved by continuous illumination of the sample reservoir with UV-LEDs (λ = 365 nm). The experimental setup was described extensively elsewhere47 and is depicted in Figure 1. Briefly, ultrafast pulses (2.5 mJ, 100 fs, 797 nm) are provided by an amplified 1-kHz system

(Solstice, Spectra Physics). About a third of the laser output is used to pump a noncollinear optical parametric amplifier (TOPAS White, Light Conversion), which delivers tunable sub30 fs pulses in the visible range. The visible excitation pulses are injected into a pulse shaper based on a 640-pixel, 2-layer spatial light modulator (SLM-640, CRI). The pulse shaper is used to further compress the pulses to (almost) their Fourier limit (∼14−19 fs in this work). The suitable phase mask is retrieved by pulse-shaper-assisted collinear frequency-resolved optical gating (c-FROG) measurements48,49 in a 10-μm BBO crystal, and is added to all other spectral phases for dispersion compensation. The pump beam is chopped at 500 Hz and B



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focused onto a ∼ 50−60 μm spot in the sample. Typical excitation energies for the control experiments are about 220− 260 nJ. This corresponds to exciting between 1/4 and 1/3 of the molecules. The transient signal in absence of ionization is measured in the region where the signal is linear with the excitation energy (≈ 10 nJ). The broadband probe pulses are obtained by focusing a small fraction ( 100 fs,

⎡ ⎛ τ ⎞⎤ f (τ ) = A ·⎢1 − exp⎜ − ⎟⎥ + B ⎢⎣ ⎝ τd ⎠⎥⎦

(5)

with amplitude A, departure time τd, and an offset B describing the yield of each channel at long interpulse separations. The departure time retrieved from three independent fits of the data in Figure 5c−e is τd ≈ 55 fs for all channels. Earlier, we had shown that, upon photoexcitation of the S1(TTC) transition, the wavepacket can either propagate along the isomerization reaction coordinate, leading to formation of the TTT isomer in S1, or relax to the TTC ground state.12,50 The results from quantum control show that, in proximity of the Franck−Condon region for S0 → S1 absorption, a third channel is opened and resonant absorption of a second photon leads to formation of a long-lived radical cation photoproduct. Our previous experiments allowed us to determine that isomerization takes place within 200 fs.12,50 The formation of the TTT photoproduct was monitored through the appearance of coherent oscillations of the wavepacket in S1(TTT). The experiments shown here allow us to extend this picture by monitoring the departure of the wavepacket from the Franck− Condon regions for S0 → S1 and S1 → Sn absorption, which are almost at the same point on S1. In these experiments, pump− dump processes and ionization are the keys to control. When accounting for the former, photoionization influences the population of both the isomerization and the relaxation channels. In all shown experiments, interactions exceeding a temporal window of ∼100 fs do not have a significant effect on the population of the three reaction channels. This means that by this time the wavepacket has moved away from the initial Franck−Condon region for the S0 → S1 and S1 → Sn transitions, and does not return to it at later times. In combination with results from 2D spectroscopy, we are able to restrict the time window in which isomerization takes place, being between 100 and 200 fs after photoexcitation. Additionally, ionization affects the isomerization and relaxation channels over an identical time scale. This implies that the resonant condition for S1 → Sn absorption is satisfied for the same time for a wavepacket reacting to form the TTT photoproduct (along the R2 coordinate in Figure 4d) and for a wavepacket remaining in the S1(TTC) configuration (along R1 in Figure 4d). If the branching between isomerization and F



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ionization QY, Figures S1−S4 showing uncorrelated model for the population of the three channels, relaxation quantum yield, comparison between the experimental ionization yield and the calculated integrated intensity square of the pulses, and quantum yields for other spectral phases, respectively and Table S1, giving departure time data. This material is available free of charge via the Internet at http://pubs.acs.org.

relaxation occurred upon the initial photoexcitation, our observations would imply that the energy difference between S1 and Sn for a wavepacket evolving along R1 is identical to the energy difference between S1 and Sn for a wavepacket evolving along R2, and this would be valid for the same time window of about 100 fs. In other words, in proximity of the Franck− Condon region for S0 → S1 absorption, the curvature difference between S1 and Sn would be the same along the two reaction coordinates R1 and R2. Although this scenario cannot be excluded without theoretical calculations, our data can be completely explained by a delayed branching between relaxation and isomerization, occurring on a time scale of 100 fs or longer. We already reported the presence of a barrier for the isomerization in the excited state of 6-nitro BIPS, which causes a lowering of the isomerization yield in absence of significant excess vibrational energy.50 Hence, it is likely that the branching between relaxation and isomerization occurs at this barrier.

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*(T.B.) E-mail: [email protected]. Present Address †

(P.N.) Physikalische Chemie II, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding from the DFG Research Unit “Light-induced Dynamics in Molecular Aggregates” (FOR 1809) and the Bavarian research network “Solar Technologies Go Hybrid”. P.N. further thanks the DFG for support within the Emmy-Noether program and the Cluster of Excellence RESOLV (EXC 1069).

CONCLUSIONS Summarizing, this study shows that quantum control spectroscopy is a powerful complement to coherent 2D spectroscopy for photophysical and photochemical studies. Time-resolved techniques have the unique capability to monitor the temporal evolution of a photoexcited wavepacket, provided a suitable spectroscopic signal is available. At very short times after excitation, photophysical and photochemical processes occur simultaneously with energy relaxation. This often leads to congested spectra, making it difficult to distinguish between different processes that occur on similar time scales or to retrieve their time constants. A possible approach is to combine the information obtained from several experimental techniques which are sensitive to different processes. Here we show that extensive information on the early photochemistry of a molecular switch can be retrieved by combining two techniques, 2D spectroscopy and quantum control spectroscopy. This strategy has the additional advantage of employing techniques that can be performed in an identical experimental configuration. In the example presented here, 2D spectroscopy and quantum control spectroscopy provided, respectively, a maximum and minimum limit for the time scale of TTC → TTT isomerization in 6-nitro BIPS. By comparing the molecular response to trains of transform-limited pulses and pulses possessing different amounts of chirp, the electronic evolution in proximity of the Franck−Condon region for S0 → S1 absorption can be observed. Our analysis allowed us to separate effects arising from pump−dump effects from multiphoton absorption to higher-lying molecular states. Additionally, we exploited these multiphoton processes to retrieve information on the topology of the PES, which would otherwise require the implementation of more sophisticated fifth-order techniques. Systematic combination of 2D spectroscopy with coherent control spectroscopy is a promising strategy, capable of better addressing the initial photochemistry of complex molecules, for which several competing pathways come into play.

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AUTHOR INFORMATION

Corresponding Author

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Description of data evaluation, description of the evaluation of the statistical errors and the confidence values arising from systematic uncertainties, discussion concerning possible effects of changes in the temporal intensity profile of the pulses on the G



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Quantum Control Spectroscopy of Competing Reaction Pathways in a

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