Molecular Model of the Ring-Opening and Ring-Closure Reaction of a

Article pubs.acs.org/JPCA

Molecular Model of the Ring-Opening and Ring-Closure Reaction of a Fluorinated Indolylfulgide Artur Nenov,† Wolfgang J. Schreier,‡ Florian O. Koller,‡ Markus Braun,‡,§ Regina de Vivie-Riedle,*,† Wolfgang Zinth,‡ and Igor Pugliesi*,‡ †

Department für Chemie, Ludwig-Maximilians-Universität München, Butenandt-Strasse 11, 81377 München, Germany Lehrstuhl für BioMolekulare Optik, Ludwig-Maximilians-Universität München, Oettingenstrasse 67, 80538 München, Germany § Institut für Physikalische und Theoretische Chemie, Goethe-Universität Frankfurt am Main, Max-von-Laue Strasse 7, D-60438 Frankfurt am Main, Germany ‡

S Supporting Information *

ABSTRACT: A combination of experimental and theoretical techniques is used to study the photoinduced ring-opening/closure of a trifluoromethyl-indolylfulgide. Time-resolved UV/vis pump and IR probe measurements are performed in the subpicosecond to 50 ps time range. Probing in the mid-IR between 1200 and 1900 cm−1 provides mode-specific dynamics and reveals photochemical reaction dynamics as well as the presence of a noncyclizable conformer. Ringopening occurs with about 3 ps. Experiments on the open isomer confirm that the ring-closure occurs on the subpicosecond time scale. They also show a 10 ps transient that can be assigned to internal conversion of a noncyclizable conformer. Quantum chemical calculations with multireference methods are used to explore the complex potential energy landscape in the excited electronic state showing paths for ring-opening and closure reactions as well as for competing side-reactions. The calculations reveal that photoexcitation induces a charge transfer from the indole to the anhydride. The same charge transfer drives the system toward a low-energy conical intersection seam spreading from the closed to the open ring side and is responsible for the ultrafast relaxation to the ground state. The ultrafast photoreactivity comes at the expense of selectivity.

1. INTRODUCTION Molecules with photochromic properties are promising candidates for technical applications as molecular switches.1 For practical use, such molecules have to possess several qualities, e.g. designable absorption spectra and solubility, very high environmental and photochemical fatigue resistance, thermal stability, and low production costs. Promising classes of molecules fulfilling most of these requirements are fulgides and diarylethene.2−5 The decisive photochromic properties of fulgide and diarylethene switches rely on the pericyclic ring-opening and ring-closure reaction based on the embedded cyclohexadiene/ hexatriene motif.6−8 The closed form absorbs in the visible spectral range, and the open form shows absorption exclusively in the UV and blue. Thus, the ring-opening and ring-closure reaction can be selectively initiated with the appropriate wavelength. The ultrafast kinetics of ring-opening and -closure have been investigated with UV/vis transient absorption experiments and time-resolved fluorescence studies.9−26 In the indolylfulgides and related indolylfulgimides, the ring-opening occurs with a reaction time in the range between 2 and 20 ps and a quantum efficiency between 0.1 and 50% depending on sample temperature.20,21,23,24,26 The ring-closure proceeds on a subpicosecond © 2012 American Chemical Society

time scale (0.2−0.5 ps) with quantum yields between 10 and 17% and is not influenced by thermal or optical excess energy.25,22 These experiments performed in the UV/vis range have given important information. However, questions on the detailed molecular reaction mechanism could not be answered. Timeresolved spectroscopy in the mid-IR as complementary probe for structural and vibrational dynamics can fill this gap. The first time-resolved IR-probe experiments on indolylfulgimides in the groundstates have been carried out by Braun and coworkers.27−29 In this work, we explore the dynamics of both the ring-opening and the ring-closure reaction of trifluoromethyl-substituted indolylfulgide, dissolved in tetrachloroethylene, by means of time-resolved UV/vis-pump IR-probe spectroscopy, characterizing the reaction in the excited and ground states. The sensitivity of this spectroscopic technique toward structural and vibrational dynamics will allow to investigate the interactions of the functional groups with the photochromic core. A comparison to the previously studied indolylfulgimides will aid the design of Received: July 27, 2012 Revised: September 26, 2012 Published: September 29, 2012 10518

dx.doi.org/10.1021/jp307437f | J. Phys. Chem. A 2012, 116, 10518−10528

The Journal of Physical Chemistry A

Article

spectral resolution. Absorbance changes of less than 50 μOD were resolved by averaging typically 5000 probe pulses. To enable polarization-dependent measurements, the polarization of the pump pulses was set at 45° with respect to the probe pulses. Polarizers were placed in front of the spectrometers to simultaneously record the transient signals for parallel and perpendicular polarization of pump and probe pulses. All pump− probe signals shown in the figures correspond to magic angle polarization conditions calculated from the parallel and perpendicular signals. Between two consecutive laser shots, the sample was completely exchanged by a peristaltic pump. For the experiments monitoring the ring-opening reaction, flow-cells with CaF2 windows with a geometrical film thickness of 160 μm were used. For the experiments monitoring the ring-closure reaction, a liquid jet42 was used instead of the flow cell because of unwanted light-induced deposition of debris at the CaF2 windows. To avoid evaporation of the solvent, the jet was kept in a closed compartment with small apertures for the incoming pump and probe pulses. The thickness of the jet was about 90 μm. In order to regenerate the starting isomers lost by the photochemistry, the sample reservoir was subject to continuous illumination at the appropriate wavelength during the time-resolved measurements. 2.3. Ab Initio Calculations. The compound with a demethylated nitrogen was used for the quantum chemical calculations. Geometry optimizations were performed with the software packages Molpro 201043 and Gaussian 09.44 The complete active space self-consistent field (CASSCF)45 and the time-dependent density functional theory (TDDFT)46,47 with the CAM-B3LYP functional48 were utilized. The complete π-spaces of the indole and cyclohexadiene/hexatriene subunits were included in a (14 electrons/13 orbitals) large active space (see Supporting Information, S1−S9). Averaging over the three lowest states for the chosen active space was applied. At each stationary point, the missing dynamic correlation was recovered using multistate CAS second order perturbation theory (MS-CASPT2,49−51 correlating the three CASSCF states as implemented in Molcas 7.4.52 Thereby, a default ionization potential-electronic affinity shift of 0.25 au, as well as a real shift of 0.1 au were used for the energy calculations. Transition dipole moments were obtained with the CASSCF State Interaction53 method as implemented in Molcas 7.4. Frequency calculations at the single-state (SS)-CASSCF level were performed for ground state equilibrium geometries, as well as for the excited state local minima in the vicinity of the Franck− Condon (FC) regions. Normal modes, frequencies, and IR intensities were calculated from analytical first derivatives and dipole moments obtained via a parallelized numerical procedure. To verify the CASSCF results, IR spectra were also calculated at the TDDFT level. For comparison with experimental spectra, the SS-CASSCF frequencies were scaled by a factor of 0.86, the CAM-B3LYP frequencies by a factor of 0.94. The standard Pople 6-31G* basis set54,55 was used for the atoms included in the π-system, whereas the 6-31G basis set54 was used for the methyl and trifluoromethyl groups. Ground state and excited state topologies around conical intersections (CoIn) have been obtained by performing an unrelaxed scan in the twodimensional plane defined by the gradient difference (GD) and derivative coupling (DC) vectors. They lift the degeneracy between electronic states in a first order approximation, i.e., the branching space.56 The scan was performed at equidistant displacements (scaling factors: 0.05, 0.10, 0.15) from the CoIn along the orthonormalized GD and DC vectors. Electrostatic

photochromic switches with tailored properties like high quantum yields.17,30,31 State-of-the-art ab inito quantum chemical calculations resolve the missing structural resolution at the atomic level and permit the interpretation of the observed time constants in terms of electronic and structural changes. The article is organized as follows. In the second section, we briefly introduce the experimental and theoretical methods and specify the compounds. In the third section, the observed ultrafast dynamics of the ring-opening and ring-closure reactions are described and analyzed. In the fourth section, the results of the ab initio calculations are presented and merged with the experimental results. This leads to a detailed reaction scheme, which is presented in the fifth section. It turns out that charge transfer is the main driving force for the ultrafast photochromic activity.

2. MATERIALS, METHODS, AND CONVENTIONS 2.1. Sample Preparation and Steady-State Characterization. Trifluoromethyl-substituted indolylfulgide was prepared as reported previously.32−34 Tetrachloroethylene (Sigma Aldrich, purity 99%+, used as delivered) served as solvent for all experiments presented here. The chosen concentration was about 8 mM. As previously reported,35 the investigated fulgide has a high resistance against photochemical fatigue. In the experiments presented here, no indications for any occurrence of photodamage was observed. For the steady-state characterization isomer pure solutions were generated according to Wolak et al.35 The steady-state spectroscopy was performed with a Perkin-Elmer Lambda19 in the UV/vis and with a FTIR spectrometer, model IFS66, from Bruker in the infrared. In the mid-IR, cuvettes with CaF2 windows (thickness 2 mm) with a geometric path length of 160 μm were used. A cold light source (for ring-opening) and a Hg(Xe)-short arc lamp (for ring-closure), filtered with color glass filters from ITOS (for ring-opening, KG2 and OG570 for the VIS; for ring-closure, GG385 and BG3 for the UV) were employed for photoswitching. The illumination time was chosen to reach photostationary states. 2.2. Time-Resolved IR Spectroscopy. For the transient absorption measurements of the ring-opening reaction, the sample was excited with pump pulses centered at 580 nm with a pulse energy of 1.3 μJ delivered by a single-stage NOPA coupled to a home-built 1 kHz Ti:sapphire oscillatorregenerative amplifier system.36 The pulses were focused down to a beam diameter of ∼150 μm (fwhm) at the sample location. The ring-closure reaction was initiated by the second harmonic of the Ti:sapphire laser-fundamental (2.0 μJ at 402 nm, spot size ≈ 150 μm). Tunable mid-IR probe pulses were generated by multistage nonlinear optical parametric processes (white-light seeded, 402 nm pumped NOPA,37,38 804 nm pumped OPA,39 and difference frequency generation in AgGaS240,41), with a spectral width of about 120 cm−1. To cover the range from 1200 to 1900 cm−1, 11 different spectral probe regions were combined. At the sample location, the probe pulses had energy of ∼50 nJ and a diameter of ∼90 μm. Depending on the mid-IR probe wavelength, the temporal resolution was better than 400 fs. After the sample, the probe beam was split into two beams and spectrally dispersed in two separate spectrometers. By chopping the excitation at half of the laser repetition rate of the laser system (500 Hz), transient spectra were recorded by identical 32-element MCT detectors (Infrared Associates) at 3 cm−1 10519



Article

Scheme 1. Isomers of the Trifluoromethyl-Indolylfulgide Addressed in This Worka

a Notation refers to the configuration of the cyclohexadiene/hexatriene central unit, C and Z denote closed- and open-ring configuration, respectively. In the open-ring structures c = cis and t = trans describe the orientation of the functional groups with respect to the single bonds C2−C3 in the hexatriene unit. Coordinates relevant for the subsequent discussion are denoted: the C1−C6 distance and the torsion angles around the C2−C3 and C5−C6 bonds.

For both the ring-opening (Figure 1, left) and the ring-closure (Figure 1, right) ground state bleach signals of the initial isomers are overlaid with excitation induced absorption bands. The most prominent features can be observed in the range of the CO stretching vibrations between 1700 and 1850 cm−1 (asymmetric and symmetric CO stretch). Excitation induced absorption bands appear to both sides of the CO stretch bleach signals and indicate that the photoinduced reaction is associated with a substantial change in bond character of the CO groups on the maleic anhydride. Although we observe a complex temporal evolution of the transient signals, relevant time-scales of the C → Z and Z → C reactions can be identified. For both, the ring-opening and the ring-closure reactions, transient spectra recorded at a delay time of 50 ps agree well with the scaled steady-state difference spectra of the isomerically pure C and Z isomers (Figure 1a,d) For delay times larger than 50 ps, the signals remain constant (data not shown). Thus, both the C → Z ring-opening and the Z → C ring-closure reactions are completed within 50 ps. At early delay times, before and around time-zero up to ∼0.4 ps, the dominant signal contributions are due to perturbed free induction decay and cross-phase modulation.58,59 Furthermore, the solvent tetrachloroethylene shows signal contributions up to 0.4 ps. Thus, the time range containing information exclusively on the dynamics of the ring-opening and ring-closure reaction lies between 0.4 and 50 ps. The observed absorption changes can be classified into three groups according to their spectral signature: (i) recovery and cooling of reactant ground states (blue dashed lines in Figure 1b,e), (ii) decay of singlet excited states (red dashed lines in Figure 1b,e), and (iii) formation and cooling of product states (black dashed lines in Figure 1b,e). (i) In the transient data, one finds instantaneous bleaching that recovers on a time scale of 20 ps. Bands of this category can be found at 1768 and 1851 cm−1 for the ringopening reaction and at 1829 cm−1 for the ring-closure reaction. The expected bleaching signal around 1781 cm−1 for the ring-closure reaction could not be recorded due to technical reasons. All bands correspond to the ground state absorption of the carbonyl stretching vibrations. The bleach recovery is accompanied by an induced absorption on the lower frequency side, which undergoes a blue-shift of several wavenumbers. This signature includes excited state relaxation and hot ground state formation.60,36 (ii) Positive, instantaneous absorption changes decay predominantly on the 0.5 to 10 ps time scale. Bands of this category can be found around 1708 and 1785 cm−1 for the ring-opening reaction and at 1723 and 1800 cm−1 for

potentials at selected geometries were generated and visualized with Molden.57 For convenience, we summarize all theoretical acronyms used in the article below: CASSCF SS-CASSCF MS-CASSCF CASPT2 TDDFT CoIn GD DC Min TS

complete active space self consistent field single-state CASSCF multistate CASSCF CAS second order perturbation theory time-dependent density functional theory conical interscetion gradient difference vector derivative coupling vector minimum transition state

2.4. Nomenclature and Conventions. Selected isomers of the closed- and open-ring trifluoromethyl-indolylfulgide are shown in Scheme 1. The closed-ring isomer is denoted as C-form, the open-ring isomer as Z-form (with respect to the orientation of the functional groups to double bond C3−C4). The Z-form exists in two stable conformers, denoted as cZc- and tZc-form, indicating the different orientation of the functional groups with respect to the single bond C2−C3 in the hexatriene subunit (c = cis, t = trans). Illumination with visible light induces the electrocyclic ring-opening of the C-form. The electrocyclic ring-closure is initiated by UV light irradiation. Only the cZcform is cyclizable. Further noncyclizable open-ring conformers (e.g., cEc, tEc, not shown) are accessible from the Z-form. The isomerization from and into the E-form is not addressed as the Z/E ratio in every photostationary state is less than 2%.35

3. PHOTOINDUCED DYNAMICS PROBED IN THE IR Time-resolved infrared difference spectra for the ring-opening and ring-closure reactions were recorded between 1200 and 1900 cm−1. For the interpretation, we focus on the region of the carbonyl modes between 1650 and 1875 cm−1 shown in Figure 1. Data covering the whole spectral range between 1200 and 1900 cm−1 can be found in the Supporting Information, Figures S10 and S11. The contour plots (Figure 1b,e) provide a twodimensional representation of the experimental data. Negative absorption changes are marked in blue; positive absorption changes are indicated in red. In support of the spectral interpretation, the IR steady-state absorption spectra of the isomerically pure samples of C and Z isomers are shown (gray area in Figure 1c,f) together with transient spectra at selected pump− probe delays. 10520



Article

Figure 1. Time-resolved IR data for the C to Z reaction (left) and the Z to C reaction (right). (a, d) Comparison of the difference spectra obtained in a steady-state experiment (solid black line) with the transient spectra after 50 ps (gray circles). (b,e) Contour plot representation of the data. Dashed lines

the ring-closure reaction. These bands are assigned to the asymmetric and symmetric carbonyl vibrations of the electronically excited state (see section 4.3 for more details). (iii) The characteristic changes for product formation following the decay of the excited state absorption bands (ii) can be clearly observed at 1785 and 1831 cm−1 for the ring-opening reaction and at 1764 and 1850 cm−1 for the ring-opening reaction. They show a delayed rise combined with a decay on the 10 to 20 ps time scale to match the offset value found in the steady-state difference spectra (solid line in Figure 1a,d). A global analysis of the transient data using exponential functions shows that both, the C → Z ring-opening and the Z → C ring-closure reactions, proceed with a complex temporal evolution in the picosecond range with deviations from simple exponential behavior (see the Supporting Information, Figures S12 and S13). These deviations are due to cooling of vibrationally hot molecules.60 The transient traces of different spectral positions representative for ground state recovery, excited state decay, and product formation are depicted in Figures 2 and 3. Fits on the data at single selected probing wavelengths using a sum of exponentials are given as solid black lines. The resulting time constants are summarized in Tables 1 and 2. 3.1. Ring-Opening (C → Z) Kinetics. Time transients are given in Figure 2. At the position of the ground state absorption at 1851 cm−1, instantaneous bleaching is observed (i). The recovery of the bleach signals can be described by two time constants of 2.9 and 17 ps (blue). At 1708 cm−1, the decay of the excited state can be followed (ii). The decay can be described by a single time constant of 2.9 ps (red). Product formation is observed at 1785 cm−1 together with the decay of the excited state absorption (iii). The kinetics are well reproduced by one time constant of 2.6 ps (black). From the experimental data, we obtain the following picture for the ring-opening reaction. The initially excited state decays with about 3 ps. With this time constant, recovery of the reactant ground state via internal conversion and product formation is

Figure 2. Transient data for the C → Z reaction at the indicated frequencies. Fits on the data are given as solid lines with the color matching the respective category of the underlying molecular process. The resulting time constants are summarized in Table 1.

observed. Cooling of the hot reactant molecules to the solvent surrounding occurs within 10−20 ps. These findings are in line with recent UV/vis transient absorption experiments in cyclohexane, where a time constant of 2.2 ps was found for the excited state decay and the cooling was associated with a 7 ps time constant.21 3.2. Ring-Closure (Z → C) Kinetics. Time traces are shown in Figure 3. The ground state bleach recovery at 1829 cm−1 (i) can be described by a fast 0.5 ps and a slower 24 ps time constant (blue). At around 1723 cm−1, one finds a spectrally broad induced absorption that can be assigned to the singlet excited state (ii). In contrast to the ring-opening reaction, the kinetics of the excited state decay cannot be described monoexponentially. A good representation of the decay can be obtained by using two time constants of about 1 and 8 ps (red). Similar time constants are observed for the time trace at 1850 cm−1, which represents product formation together with ground state recovery of the 10521



Article

vibrational cooling of hot reactant ground state molecules occurs within 30 ps. These findings are comparable to the ring-closure dynamics observed in the UV/vis transient absorption experiments of ref 25. The appearance of the product state, observable at 1764 and 1850 cm−1, cannot be clearly assigned to either the 0.5 or 10 ps time constant if only these bands are taken into account. To resolve this ambiguity, the product band at 1555 cm−1 (outside the spectral range of Figure 1e) can be used. This band does not overlap with the spectral signatures of the reactant and can be described by a fast 0.5 ps and a slower 24 ps time constant, confirming product formation with the 0.5 ps time constant and subsequent cooling of the hot product within 24 ps. The 10 ps time constant observed at other frequencies cannot be clearly assigned to either of these processes. It will be discussed in section 5.2. 3.3. Quantum Yields. The quantum yield η of the photoreaction was determined by comparing the maximum bleach amplitudes with the remaining absorption losses at delay times later than 50 ps. For the C → Z ring-opening reaction, it is estimated to η = 10%, while for the Z → C ring closure reaction, η = 20%, in good agreement with steady-state measurements in the visible spectral range.20,25

4. STRUCTURAL, ELECTRONIC, AND VIBRATIONAL ANALYSIS FROM AB INITIO CALCULATIONS AND VIBRATIONAL SPECTRA The complex temporal evolution of the transient spectral signatures of the two C → Z and Z → C together with the moderate quantum yields are a strong indication that, after excitation, the fulgide evolves along a complex excited state potential energy surface with different competing pathways. To obtain a mechanistic insight into the nature of the excited state and the structural deformations involved in the ultrafast dynamics, ab initio quantum chemical calculations have been carried out together with a vibrational analysis of the ground and first excited singlet states. 4.1. Ground State Geometries. Three stable ground state isomers are relevant for the subsequent discussion of the excited state dynamics, the closed-ring C-form and two open-ring forms tZc and cZc (see Scheme 1). The C-form is planar, with a completely delocalized π-system. The open-ring isomers are nearly isoenergetic and lie 0.26 eV higher than the C-form. Torsion around the C2−C3 single bond (63.5° and 120.5° for cZc and tZc, respectively, Table 3) weakens the conjugation in the π-system. As a consequence, the π-orbitals localize either at the indole or at the anhydride. We note that only the cZc-form is photochemically cyclizable. Because of the steric interactions of the methyl groups at C1 and C6, the C ↔ cZc electrocyclic reaction proceeds in the ground state via a conrotatory motion, contrary to the Woodward−Hoffmann rules.61 The resulting high activation barrier (C ↔ cZc, 1.95 eV; cZc ↔ C, 1.86 eV) makes thermal isomerization practically impossible at room temperature. As the cZc ↔ tZc isomerization proceeds along a single bond torsion (Scheme 1), the barrier associated therewith is only 0.33 eV. Using the Eyring formula k = (kBT/h) exp(−ΔG/RT) (kB, h, and R the Boltzmann, Planck, and ideal gas constants and ΔG the Gibbs free activation energy), we estimate an isomerization rate constant k = 107 s−1, corresponding to an isomerization time of several tens of nanoseconds. Thus, at room temperature, both open-ring forms exist in a thermal equilibrium. Their ratio is estimated using Boltzmann distribution (PcZc/PtZc) = exp(−ΔG/ kBT) to 60%(cZc)/40%(tZc). When the pump pulse is applied to

Figure 3. Transient data for the Z → C reaction at the indicated frequencies. Fits on the data are given as solid lines with the color matching the respective category of the underlying molecular process. The resulting time constants are summarized in Table 2.

Table 1. Time Constants of the Ring-Opening Reactiona ν (cm−1)

τ1 (ps)

τ2 (ps)

τ3 (ps)

1851 (i) 1785 (iii) 1708 (ii)

2.9 2.6 2.9

17

∞ ∞ ∞

a

The Roman numbers in brackets refer to the category assignment described in the text. Data have been analyzed by a sum of exponential functions convoluted with the instrumental response function. Typical errors for the determined time constants are 20% and 10% for τ1 and τ2, respectively.

Table 2. Time Constants of the Ring-Closure Reactiona ν (cm−1)

τ1 (ps)

τ2 (ps)

1850 (iii) 1829 (i) 1764 (iii) 1723 (ii) 1555

0.7 0.5 0.5b 0.8 0.5

11 3.9 7.8

τ3 (ps) 24 24 23

τ4 (ps) ∞ ∞ ∞ ∞ ∞

a

The Roman numbers in brackets refer to the category assignment described in the text. Data have been analyzed by a sum of exponential functions convoluted with the instrumental response function. Typical errors for the determined time constants are 25−30%, 20%, and 10% for τ1, τ2, and τ3, respectively. bTime constant fixed during iteration.

reactant (iii). The product band at 1764 cm−1 (iii) can be modeled using three time constants of 0.5, 4, and 24 ps (black). Here, the transient behavior is partly overlaid by signals due to cooling of neighboring ground state absorption bands. The data can be interpreted with the following reaction scheme. After photoexcitation, we observe a decay of the excited state on a subpicosecond and on a 10 ps time scale. Additionally, 10522



Article

Table 3. Relative Stabilization on the Excited State ΔES1 in eV, Vertical Energy Gap ΔES1−S0 between Ground and Excited State in eV, Relevant Coordinates, Excited State Permanent (μS1) and Transition (μS1−S0) Dipole Moments for All Geometries Discussed in This Work; the Energy of FCC Is Used As Reference for ΔES1

The rotational angles around the C2−C3 and C5−C6 bonds were determined by constructing the normals (e.g., N2 through C2), perpendicular to the first substituents of each center. The rotational angles can then be read off the dihedrals angle N2−C2−C3−N3 and N5−C5−C6−N6. a

(MincZc and MintZc) with more pronounced structural changes and a stabilization of 0.5−0.6 eV with respect to the associated FC points (ΔES1 in Table 3). Bond rearrangements in the π-system are now accompanied by torsion around the C2−C3 bond (Scheme 1). For cZc, the torsional angle increases by approximately 10° to 74°, while for tZc, it decreases by approximately 15° to 106°. The vertical energy gaps at MincZc and MintZc (ΔES1−S0 in Table 3) indicate that the fluorescence bands overlap. Remarkably, the excited state gradient accelerates the cyclizable cZc-form toward increasing torsion around the C2−C3 bond, thus toward further ring-opening, rather than along the ring closure coordinate. However, we found that this gradient is very sensitive to the angle between the indole and the anhydride. Starting from a slightly less twisted initial geometry, we encounter a barrierless pathway along the ring-closure coordinate toward a low-lying conical intersection CoInC/cZc facilitating fast radiationless decay to the ground state. Its geometry is shown in Figure 6, top. 4.3. Excited State Charge Transfer and Vibrational Spectra. Recently, it has been shown that the formation of energetically low-lying CoIns in the present system30 as well as in a structurally related pyrrolylfulgide62 is driven by charge transfer. In the present case, the first excited state has partial charge transfer character already in the FC-region. A comparison of the electrostatic potentials of the ground state equilibrium geometries to the potentials of the excited state intermediates (Figure 4) demonstrates this charge transfer from the indole to the anhydride. Remarkably, the asymmetric shortening of the C−C(O) and N−C bonds (Table 3), identified as an essential prerequisite for CoIn formation,30 is initiated simultaneously. Thus, the system feels an excited state gradient toward the CoIns already in the FC region. The calculated electrostatic potentials (Figure 4) indicate that the carbonyl groups are suitable markers to track charge fluctuation, as the IR-bands associated with CO stretching vibrations are well separated from the remaining signal. The increase of the charge density in the carbonyl bonds upon excitation will induce a bathochromic shift in the IR signal of the excited state intermediates, an indicator for the CT formation. To verify the CT

a thermal equilibrium of a cZc/tZc mixture, only 60% of all openring fulgides are in a cyclizable form. 4.2. Excited State Geometries. The lowest excited electronic singlet states of C, cZc, and tZc are of ππ*-character. The transition is spectroscopically allowed, with an oscillator strength of approximately 0.10. The theoretically predicted excitation energies (ΔES1−S0 in Table 3) are in a fair agreement with the experimentally determined absorption maxima of 2.20 eV (C-form) and 2.98 eV (Z-form).25 The overestimation of the blue shift by approximately 0.65 eV is attributed to three factors, the limited basis set, the restrictions to the active space, and the omission of solvent effects in our gas-phase calculations. The active space used throughout this work does not include the π/π*-orbitals of the carbonyl groups. Their low-lying π*-orbitals interact with the active virtual orbitals and lower the excitation energy. This effect is not completely discarded as the molecular orbital coefficients are optimized during the CASSCF routine. The excited state permanent dipole moments are larger by a factor of 2 compared to the ground state dipole moments, which are 7.80 D for C, 8.40 D for cZc, and 9.14 D for tZc. Therefore, we expect the solvent tetrachloroethylene to lower the excitation energy. As the first excited state is energetically well separated from higher lying states, there should be no risk of higher lying states to mix in along the reaction path. Thus, the qualitative photochemistry should be described correctly. The bathochromic shift of the closed-ring isomer absorption is attributed to the intact π-system. Both open-ring isomers absorb at the same wavelength, which renders them spectroscopically indistinguishable. Optimization on the spectroscopically accessible first excited state from the C-form converges to a local minimum (MinC) lying approximately 0.5 eV below the FC point (ΔES1 in Table 3). It is reachable solely by bond length rearrangements (partial double bond/single bond inversion) in the aromatic system. Noticeable is the asymmetric change of the C4/5−C(O) bond lengths in the anhydride, as well as the rehybridization of the nitrogen from sp3 to sp2 (Table 3). Optimization on the spectroscopically accessible first excited state from the open-ring cZc- and tZc-forms yields local minima 10523



Article

Figure 5. Comparison between experimentally obtained and calculated ground and excited state vibrational spectra for the C-form (a,c) and the cZc/tZc-forms (b,d). Experimental excited state spectra (c,d) are extracted from the transient absorption data at 0.5 ps delay time according to a procedure similar to that described in ref 63 (black traces). Theoretical spectra are obtained from a frequency analysis at the local minima in the excited state MinC, MincZc, and MintZc (color traces). Calculations were performed at the SS-CASSCF(14/13) level with a scaling factor of 0.86.

Figure 4. (a) Schematic representation of the electronic charge transfer from the indole to the anhydride induced upon photoexcitation and the change in the conjugation pattern associated with the charge transfer. (b−d) Electrostatic potentials (range [−0.05:0.15] a.u.) on a density isosurface with a value of 0.003 electron/bohr3 for the ground state (left column) and excited state (right column) equilibrium geometries of C, cZc, and tZc. Negative potentials with high electron density are given in red. Positive potentials with low electron density are given in blue.

CASSCF normal-mode analysis were carried out for the ground state and excited state minima of C, cZc, and tZc isomers, and the calculated intensities were convoluted with Lorentzians with a width of 5 cm−1. The spectra are shown in Figure 5 as colored traces. Normal modes in the region from 1150 to 1950 cm−1 are provided in the Supporting Information. We can clearly assign the peaks at 1708 and 1785 cm−1 to the symmetric and antisymmetric CO stretching modes of the closed-ring form in the excited state. The peaks at 1723 and 1800 cm−1 are the symmetric and antisymmetric CO stretching modes of the electronically excited open-ring form. Both the scaled calculated spectra and the experimental data show the bathochromic shift of the CO bands caused by the charge transfer. The shift is indicated by the colored areas in Figure 5. An unambiguous assignment of the remaining peaks is not possible due to the high density of modes in the spectral region between 1200 and 1400 cm−1. However, some general features can be identified. For example, stretching vibrations of bonds, having a double bond character in the ground state, are red-shifted from 1450 to 1600 cm−1 due to double bond/single bond rearrangement (for further details, see Table S.1 in the Supporting Information). Again the red shift is a consequence of the CT. Similar well separated vibrational bands are observed in the measured excited state vibrational spectra. 4.4. Conical Intersections. To obtain first information about potentially accessible products from the CoIn, a two-dimensional scan around CoInC/cZc in the branching plane was performed.64,65 Following the gradient in the ground state revealed that CoInC/cZc facilitates the electrocyclic isomerization in both directions. In fact, CoInC/cZc is a point on an extended low-energy excited state/ ground state degeneracy region, known as CoIn seam. The location and analysis of this seam is of vital importance for deciphering the

character in the FC-region of the closed-ring and open-ring isomers, excited state vibrational spectra, extracted from the transient data and calculated with quantum chemical methods, were compared against each other and to the steady-state spectra of the ground state. The extraction of excited state vibrational spectra from the transient absorption data follows a procedure similar to that described in ref 63. The analysis of the temporal evolution of the absorption changes (section 3) shows that the transient signal at early delay times (about 0.5 ps) consists predominantly of the negative ground state bleach and the induced excited state absorption of the reactant. Signal contributions from (hot) product molecules can be neglected. The sum of the transient absorption spectrum at 0.5 ps and the scaled steady-state groundstate absorption spectrum was calculated to remove the contributions from the ground state bleach and to reconstruct the IR spectrum. Furthermore, it was verified that the obtained absorption bands show a transient decay behavior with time constants of the excited state lifetimes (group (ii) signals, section 3). Seven absorption bands could be identified for the excited C-form (1785, 1708, 1440, 1390, 1340, 1265, and 1195 cm−1) and the cZc/tZcmixture (1800, 1723, 1515, 1470, 1385, 1245, and 1195 cm−1), respectively. The spectral positions and intensities were determined by a fit to a sum of Lorentzians with a width of 5 cm−1. The results are shown in Figure 5c,d as black traces. 10524



Article

from that seam, one in the closed-ring region (CoInC) and one in the open-ring region (CoIncZc/tZc). CoInC (Figure 6, middle) is characterized by a pronounced twist of the C2−C3 bond and an initiated cleavage of the C1−C6 σ-bond (1.79 Å, Table 3). Its geometry can be rationalized with the rules of thumb given in ref 30. It is an example of a strongly polar ethylene derivative (C2−C3 fragment) functionalized with an electron withdrawing (anhydride) and an electron donating (nitrogen lone pair) group. As for other strongly polar ethylene derivatives,66 degeneracy with the ground state is reached only for a significantly large torsional angle. The structure and gradients indicate that CoInC does not mediate the electrocyclic reaction between the C- and the cZc-form. Indeed, even though the C1−C6 σ-bond is elongated by 0.16 Å compared to the ground state closed-ring geometry, a scan in the branching plane reveals that CoInC permits a barrierless relaxation only to the C-form (Figure 6, middle). A similar constellation is found at the open-ring side, where CoIncZc/tZc could be located (Figure 6, bottom). It also exhibits a pronounced twist of the C2−C3 bond, this time accompanied by a complete cleavage of the C1−C6 σ-bond (3.01 Å, Table 3). Another structural feature of CoIncZc/tZc is the large twist of the C5−C6 bond. Again, this structure can be rationalized30 as a weakly polar ethylene derivative (C5−C6 fragment) with an electron withdrawing carbonyl group conjugated to C5. The nitrogen lone pair is structurally decoupled and not conjugated to the ethylene fragment. The insufficient excited state stabilization via functionalization requires additional geometrical deformations to reach a CoIn like the shortening of the C2−C6 distance. The geometrical deformations shift CoIncZc/tZc energetically above CoInC/cZc and CoInC. The topograhic analysis (Figure 6, bottom) reveals that CoIncZc/tZc facilitates a radiationless cZc ↔ tZc isomerization via a combined motion of torsion around the C2−C3 and the C5−C6 bonds and shortening of the C2−C6 distance, accompanied by bond rearrangements in the π-system. Ring-closure cannot be initiated from this CoIn. For comparison, the thermal cZc ↔ tZc isomerization is carried out solely by torsion around the C2−C3 bond. As a consequence, the thermal and photochemical isomerization (via TScZc/tZc and CoIncZc/tZc, Figure 7) result in different conformers in the ground state with respect to the substituents at C6. Thus, labeling one of the methyl groups can be utilized to confirm the passage through CoIncZc/tZc upon photoexcitation.

5. REACTION MECHANISM On the basis of the spectroscopical and theoretical findings, we propose a detailed mechanism for the ring-opening and closure dynamics, assign the extracted time constants, and explain the observed quantum yields (Figure 7). 5.1. Ring-Opening. Excitation at 560 nm of the C-form populates the bright ππ* first excited state. Instantaneous relaxation from the FC region, dominated by bond rearrangements in the π-system, occurs within the first 100 fs and drives the system into a local minimum MinC with an intact C1−C6 σ-bond. This fast initial process is below the time resolution of the experiment. However, evidence for the population of an intermediate in the excited state on a subpicosecond scale is provided by time-resolved UV/vis experiments.25 Two radiationless decay mechanisms from MinC exist, a photoreactive one and a photostabilizing one. The photoreactive mechanism involves cleavage of the C1−C6 σ-bond and passes through CoInC/cZc. Here, a rapid decay to the ground state with bifurcation toward either the C- or the cZc-form is possible. The photostabilizing

Figure 6. CoIn topologies of CoInC/cZc, CoInC, and CoIncZc/tZc with associated ground state products. GD = gradient difference vector; DC = derivative coupling vector.

ultrafast dynamics of fulgides. In a recent study on the related compound trifluoromethyl-pyrrolylfulgide, we reported on a lowlying CoIn seam,31 spreading from the closed-ring to the open-ring side. In this work, the indole was replaced by a pyrrole, but we could show that this substitution does not influence the topography of the seam.30 Correspondingly, we can use the results from the pyrrolylfulgide seam to locate critical geometries for the indole derivative. Next, the role of the seam is elucidated by focusing on the energetic and structural characteristics of two further CoIns 10525



Article

Figure 7. Schematic reaction scheme with relevant structures and excited state relaxation channels. The coordinate for the ring-closure/opening and the cZc/tZc ground state isomerization lies in the drawing plane. The coordinates for the internal conversion through CoInC and for the cZc/tZc photoisomerization through CoIncZc/tZc lie perpendicular to the drawing plane as indicated by the gray panels.

CoInC. Scans in the branching space of selected geometries along this seam segment indicate that bifurcation in the ground state occurs only in the vicinity of CoInC/cZc (green line in Figure 8), while at large parts of the seam, relaxation to the C-form is favored. The time constant of τ1 = 2.6−2.9 ps provides an effective description for the collective decay process from the excited state intermediate MinC through the CoIn seam. Dynamic simulations are indispensable for recovering the population of the individual channels. Without this information, we resort to the experimental quantum yields to estimate the distribution and reconstruct the time constants of the individual relaxation processes. Thereby, we make the following assumptions: (a) deactivation pathways through the reactive and the nonreactive seam segments are each described by one effective time constant giving the collective decay along the segment; (b) a 1:1 bifurcation in the ground state is assumed for the reactive seam segment around CoInC/cZc, i.e., the exact topology of the seam and effects coming from momentum conservation are neglected; (c) the region in the vicinity of CoInC is completely nonreactive, i.e., an insurmountable ground state barrier toward cZc is assumed; (d) deactivation through fluorescence is neglected. Using the quantum yield for the ringopening η = 10%, we estimate a population distribution of 20%/ 80% into the reactive and nonreactive channels. Consequently, a ratio between the time constants τC→cZc for the reactive and τdeact for the stabilizing photoprocesses of 4:1 is obtained. Using this relationship and an effective time constant τ1 of about 3.0 ps in the rate eq 1/τ1 = ∑i1/τi, the time constants τC→cZc and τdeact are calculated to be 15.0 and 3.8 ps, respectively. The presence of an intermediate MinC prevents the C-form from immediately reaching CoInC/cZc and provides the necessary time window for energy redistribution into modes, which drive

Figure 8. Ground state product accessibility from the CoIn seam, plotted in the coordinate space of the C1−C6 bond and the torsional angles of the substituents around C2−C3 and C5−C6. The region around CoInC facilitates a relaxation solely to the C-form (red dashed lines). The region around CoIncZc/tZc facilitates a relaxation solely to the cZcform (blue dashed lines). In the region around CoInC/cZc (green solid line), both channels can be accessed.

mechanism involves a torsion around the C2−C3 double bond and drives the system toward CoInC and finally to the C-form. In fact, effective radiationless decay occurs at all intermediate geometries along the low-lying seam connecting CoInC/cZc and 10526



Article

the system toward the nonreactive region on the seam. Analysis of experimental data from the temperature dependence of the ring-opening reaction point in the same direction.23,24 5.2. Ring-Closure. At room temperature, about 60% of the open-ring molecules exist in the cyclizable cZc-form, while 40% are in the tZc-form. The mixture is excited at 420 nm to the first bright excited state of ππ*-character. Two competing processes occur in the FC region of the cZc-isomer. A barrierless relaxation toward CoInC/cZc facilitates the ring-closure, the main reaction path, with a time constant τcZc→C. We assign the recorded short time constant τ1 = 0.5−0.8 ps to τcZc→C. Concurrent torsional motion around the C2−C3 bond drives the system to the local minimum MincZc. From the noncyclizable tZc-isomer, a corresponding local minimum MintZc can be reached. The longer time constant τ2 = 3.9−11.0 ps is attributed to the passage from the intermediate minima MincZc and MintZc to the ground state of both open forms passing through CoIncZc/tZc and the closeby seam segment connecting to CoInC/cZc. Photoisomerization between the cZc- and tZc-forms occurs exclusively by this process. In summary, we expect the ring-closure of the cZc-form to proceed on a subpicosecond time scale, thereby being an order of magnitude faster than the ring-opening. The ring-closure is very efficient as the system has no time to redistribute the energy and follows the gradient on the excited surface in a ballistic fashion toward CoInC/cZc. Using the ring-closure quantum yield of 20% and assuming a 1:1 product distribution from CoInC/cZc, we find that two of every three molecules in the cyclizable cZc-form follow the ring-closure coordinate. The moderate quantum yield is attributed to the high percentage of noncyclizable tZc molecules in the initial equilibrium mixture.

rearrangements and drives the system toward reactive and nonreactive regions on the energetically low-lying CoIn seam. Since the functionalization pattern of the fulgide may stabilize the charge transfer state in certain geometrical arrangements, substitution may be used to tune and optimize the photochemical properties. With the acquired knowledge, it should become possible to develop strategies for the selective stabilization of regions on the CoIn seam and for the suppression of undesired side-reactions.

6. CONCLUSIONS AND PERSPECTIVES From a joint experimental−theoretical study, important information is obtained on the photoinduced ring-opening/closure of trifluoromethyl-indolylfulgide. Time-resolved IR measurements provide mode-specific dynamics on the few picosecond time scale, which can be related to excited and ground state processes. Quantum chemical calculations reveal a complex potential energy landscape in the excited electronic state with pathways for ring-opening and -closure reactions as well as for competing side-reactions such as internal conversion to local minimia and the ground state. The existence and the spectral signatures of an additional time constant found during the ring-closure reaction and the molecular modeling convincingly shows that the ensemble of open-ring fulgides contains conformers (tZc) that cannot undergo cyclization and that react via internal conversion back to the ground state within about 10 ps. The cyclizable cZc conformers react much faster on the 500 fs time scale via the CoInC/cZc. The existence of two conformers also answers the long-standing question, whether ring-opening and ring-closure occur via a common CoIn. It has been argued that both reactions proceed via different CoIns as the quantum yields do not sum up to 100%.25,67 The present study indicates that both the ringclosure of the cyclizable conformer and the ring-opening proceed in the vicinity of the CoInC/cZc with high yield. The loss in reaction yield is due to the bifurcation at CoInC during ringopening and to the presence of the nonreactive conformers tZc upon ring-closure. Furthermore, this study sheds light on the electronic nature of the ultrafast photoreactivity of fulgides. Upon photoexcitation, a charge transfer from the indole to the anhydride is initiated already in the Franck−Condon region. This facilitates bond

Notes

■

ASSOCIATED CONTENT

* Supporting Information S

Normal modes, unscaled frequencies, absolute intensities for the ground state equilibrium geometries, and excited state local minima at CASSCF and DFT levels; orbitals from the active space of all structures discussed in the article; time-resolved IR difference spectra for the ring-opening and -closure in the spectral range 1200−1900 cm−1; decay associated difference spectra for the ring-opening and -closure; full refs 43, 44, and 52; an assignment of bands in the calculated excited state IR spectra to bands of the ground state IR spectra; Cartesian coordinates and absolute energies for all structures discussed in the article. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*(R.d.V.-R.) Tel: + 49 89 2180 77533. Fax: +49 89 2180 77133. E-mail: [email protected]. (I.P.) Tel: + 49 89 2180 9255. Fax: +49 89 2180 9202. E-mail: igor.pugliesi@ physik.uni-muenchen.de. The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We would like to thank Jessica A. DiGirolamo and Watson J. Lees for the synthesis of the fulgide compounds and Tobias E. Schrader and Simone Draxler for fruitful discussions. The work was supported by the DFG-Cluster of Excellence, MunichCentre for Advanced Photonics, and by the SFB 749 (Dynamics and Intermediates of Molecular Transformations). I.P. gratefully acknowledges Eberhard Riedle for continuous financial and intellectual support. The Leibniz-Rechenzentrum LRZ Munich is greatly acknowledged for the allocation of CPU time.

■

REFERENCES

(1) Feringa, B. L. Molecular Switches; Wiley-VCH: New York, 2001. (2) Yokoyama, Y. Chem. Rev. 2000, 100, 1717. (3) Chen, Y.; Wang, C. M.; Fan, M. G.; Yao, B. L.; Menke, N. Opt. Mater. 2004, 26, 75. (4) Zarwell, S.; Dietrich, S.; Schulz, C.; Dietrich, P.; Michalik, F.; RuckBraun, K. Eur. J. Org. Chem. 2009, 2088. (5) Irie, M. Chem. Rev. 2000, 100, 1685. (6) Celani, P.; Bernardi, F.; Robb, M. A.; Olivucci, M. J. Phys. Chem. 1996, 100, 19364. (7) Kosma, K.; Trushin, S. A.; Fuß, W.; Schmid, W. E. Phys. Chem. Chem. Phys. 2009, 11, 172. (8) Nenov, A.; Kölle, P.; Robb, M. A.; de Vivie-Riedle, R. J. Org. Chem. 2010, 75, 123. (9) Ramsteiner, I. B.; Hartschuh, A.; Port, H. Chem. Phys. Lett. 2001, 343, 83. (10) Handschuh, M.; Seibold, M.; Port, H.; Wolf, H. C. J. Phys. Chem. A 1997, 101, 502. 10527



Article

(44) Frisch, M. J.; et al. Gaussian 09, revision A.01; Gaussian Inc.: Wallingford, CT, 2009; see Supporting Information for full reference. (45) Roos, B. O. Advanced Chemistry and Physics: Ab Initio Methods in Quantum Chemistry II; Wiley: Chicester, U.K., 1987; p 399. (46) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108, 4439. (47) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218. (48) Yanai, T.; Tew, D.; Handy, N. Chem. Phys. Lett. 2004, 393, 51. (49) Andersson, K.; Malmqvist, P.-Å.; Roos, B. O.; Sadlej, A. J.; Wolinski, K. J. Chem. Phys. 1990, 94, 5483. (50) Andersson, K.; Malmqvist, P.-Å.; Roos, B. O. J. Chem. Phys. 1992, 96, 1218. (51) Finley, J.; Malmqvist, P.-Å.; Roos, B. O.; Serrano-Andrés, L. Chem. Phys. Lett. 1998, 288, 299. (52) Anderson, K.; et al. Molcas, revision 7.4; University of Lund: Lund, Sweden, 2010; see Supporting Information for full reference. (53) Malmqvist, P.-Å.; Roos, B. O. Chem. Phys. Lett. 1989, 155, 189. (54) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (55) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; DeFrees, D. J.; Pople, J. A.; Gordon, M. S. J. Chem. Phys. 1982, 77, 3654. (56) Domcke, W.; Yarkony, D.; Köppel, H. Advanced Series in Physical Chemistry; World Scientific Publishing: River Edge, NJ, 2004. (57) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000, 14, 123. (58) Hamm, P. Chem. Phys. 1995, 200, 415. (59) Kovalenko, S. A.; Dobryakov, A. L.; Ruthmann, J.; Ernsting, N. P. Phys. Rev. A 1999, 59, 2369. (60) Hamm, P.; Ohline, S. M.; Zinth, W. J. Chem. Phys. 1997, 106, 519. (61) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. 1969, 8, 781. (62) Tomasello, G.; Bearpark, M. J.; Robb, M. A.; Orlandi, G.; Garavelli, M. Angew. Chem., Int. Ed. 2010, 49, 2913. (63) Schreier, W. J.; Pugliesi, I.; Koller, F. O.; Schrader, T. E.; Zinth, W.; Braun, M.; Kacprzak, S.; Weber, S.; Römisch-Margl, W.; Bacher, A.; Illarionov, B.; Fischer, M. J. Phys. Chem. B 2011, 115, 3689. (64) Frutos, L. M.; Sancho, U.; Garavelli, M.; Olivucci, M.; Castaño, O. J. Phys. Chem. A 2007, 111 (15), 2830. (65) Sellner, B.; Barbatti, M.; Lischka, H. J. Chem. Phys. 2009, 131, 024312. (66) Bonačić-Koutecký, V.; Koutecký, J.; Michl, J. Angew. Chem., Int. Ed. 1987, 26, 170. (67) Draxler, S.; Brust, T.; Malkmus, S.; Di Girolamo, J.; Lees, W. J.; Zinth, W.; Braun, M. Phys. Chem. Chem. Phys. 2009, 11, 5010.

(11) Kurita, S.; Kashiwagi, A.; Kurita, Y.; Miyasaka, H.; Mataga, N. Chem. Phys. Lett. 1990, 171, 553. (12) Parthenopoulos, D. A.; Rentzepis, P. M. J. Mol. Struct. 1990, 224, 297. (13) Seibold, M.; Port, H. Chem. Phys. Lett. 1996, 252, 135. (14) Tamai, N.; Miyasaka, H. Chem. Rev. 2000, 100, 1875. (15) Port, H.; Gartner, P.; Hennrich, M.; Ramsteiner, I.; Schock, T. Mol. Cryst. Liq. Cryst. 2005, 430, 15. (16) Siewertsen, R.; Renth, F.; Temps, F.; Sönnichsen, F. Phys. Chem. Chem. Phys. 2009, 11, 5952. (17) Siewertsen, R.; Strübe, F.; Mattay, J.; Renth, F.; Temps, F. Phys. Chem. Chem. Phys. 2011, 13, 3800. (18) Siewertsen, R.; Strübe, F.; Mattay, J.; Renth, F.; Temps, F. Phys. Chem. Chem. Phys. 2011, 13, 15699. (19) Heinz, B.; Malkmus, S.; Laimgruber, S.; Dietrich, S.; Schulz, C.; Rück-Braun, K.; Braun, M.; Zinth, W.; Gilch, P. J. Am. Chem. Soc. 2007, 129, 8577. (20) Brust, T.; Draxler, S.; Malkmus, S.; Schulz, C.; Zastrowa, M.; Rück-Braun, K.; Zinth, W.; Braun, M. J. Mol. Liq. 2008, 141, 137. (21) Cordes, T.; Malkmus, S.; Di Girolamo, J.; Lees, W. J.; Nenov, A.; de Vivie-Riedle, R.; Braun, M.; Zinth, W. J. Phys. Chem. A 2008, 112, 13364. (22) Draxler, S.; Brust, T.; Malkmus, S.; Koller, F. O.; Heinz, B.; Laimgruber, S.; Schulz, C.; Dietrich, S.; Rück-Braun, K.; Zinth, W.; Braun, M. J. Mol. Liq. 2008, 141, 130. (23) Brust, T.; Draxler, S.; Popp, A.; Chen, X.; Lees, W. J.; Zinth, W.; Braun, M. Chem. Phys. Lett. 2009, 477, 298. (24) Brust, T.; Malkmus, S.; Draxler, S.; Ahmed, S. A.; Rück-Braun, K.; Zinth, W.; Braun, M. J. Photochem. Photobiol., A 2009, 207, 209. (25) Cordes, T.; Herzog, T. T.; Malkmus, S.; Draxler, S.; Brust, T.; DiGirolamo, J. A.; Lees, W. J.; Braun, M. Photochem. Photobiol. Sci. 2009, 8, 528. (26) Ishibashi, Y.; Katayama, T.; Ota, C.; Kobatake, S.; Irie, M.; Yokoyama, Y.; Miyasaka, H. New J. Chem. 2009, 33, 1409. (27) Koller, F. O.; Schreier, W. J.; Schrader, T. E.; Sieg, A.; Malkmus, S.; Schulz, C.; Dietrich, S.; Rück-Braun, K.; Zinth, W.; Braun, M. J. Phys. Chem. A 2006, 110, 12769. (28) Malkmus, S.; Koller, F. O.; Draxler, S.; Schrader, T. E.; Schreier, W. J.; Brust, T.; DiGirolamo, J. A.; Lees, W. J.; Zinth, W.; Braun, M. Adv. Funct. Mater. 2007, 3657. (29) Koller, F. O.; Schreier, W. J.; Schrader, T. E.; Malkmus, S.; Schulz, C.; Dietrich, S.; Rück-Braun, K.; Braun, M. J. Phys. Chem. A 2008, 112, 210. (30) Nenov, A.; de Vivie-Riedle, R. J. Chem. Phys. 2011, 135, 034304. (31) Nenov, A.; de Vivie-Riedle, R. Submitted for publication. (32) Liang, Y. C.; Dvornikov, A. S.; Rentzepis, P. M. J. Photochem. Photobiol., A 1999, 125, 79. (33) Wolak, M. A.; Gillespie, N. B.; Thomas, C. J.; Birge, R. R.; Lees, W. J. J. Photochem. Photobiol., A 2001, 144, 83. (34) Islamova, N. I.; Chen, X.; Garcia, S. P.; Guez, G.; Silva, Y.; Lees, W. J. J. Photochem. Photobiol., A 2008, 195, 228. (35) Wolak, M. A.; Thomas, C. J.; Gillespie, N. B.; Birge, R. R.; Lees, W. J. J. Org. Chem. 2003, 68, 319. (36) Schrader, T.; Sieg, A.; Koller, F. O.; Schreier, W.; An, Q.; Zinth, W.; Gilch, P. Chem. Phys. Lett. 2004, 392, 358. (37) Riedle, E.; Beutter, M.; Lochbrunner, S.; Piel, J.; Schenkl, S.; Sporlein, S.; Zinth, W. Appl. Phys. B: Lasers Opt. 2000, 71, 457. (38) Wilhelm, T.; Piel, J.; Riedle, E. Opt. Lett. 1997, 22, 1494. (39) Cerullo, G.; De Silvestri, S. Rev. Sci. Instrum. 2003, 74, 1. (40) Kaindl, R. A.; Wurm, M.; Reimann, K.; Hamm, P.; Weiner, A. M.; Woerner, M. J. Opt. Soc. Am. B 2000, 17, 2086. (41) Rotermund, F.; Petrov, V.; Noack, F. Opt. Commun. 2000, 185, 177. (42) Laimgruber, S.; H, S.; Schmidt, B.; Zinth, W.; Gilch, P. Appl. Phys. B:Lasers Opt. 2006, 85, 557. (43) Werner, H.-J.; et al. MOLPRO, version 2010.1, a package of ab initio programs; Cardiff University: Cardiff, U.K., 2010; see Supporting Information for full reference. 10528


Molecular Model of the Ring-Opening and Ring-Closure Reaction of a

Recommend Documents