Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 665−671
pubs.acs.org/JPCL
Photoswitching an Isolated Donor−Acceptor Stenhouse Adduct James N. Bull,† Eduardo Carrascosa,† Neil Mallo,‡ Michael S. Scholz,† Gabriel da Silva,¶ Jonathon E. Beves,‡ and Evan J. Bieske*,† †
School of Chemistry, University of Melbourne, Parkville, Victoria 3010, Australia School of Chemistry, UNSW Sydney, High Street, Kensington, New South Wales 2052, Australia ¶ Department of Chemical Engineering, University of Melbourne, Parkville, Victoria 3010, Australia ‡
S Supporting Information *
ABSTRACT: Donor−acceptor Stenhouse adducts (DASAs) are a new class of photoswitching molecules with excellent fatigue resistance and synthetic tunability. Here, tandem ion mobility mass spectrometry coupled with laser excitation is used to characterize the photocyclization reaction of isolated, charge-tagged DASA molecules over the 450−580 nm range. The experimental maximum response at 530 nm agrees with multireference perturbation theory calculations for the S1 ← S0 transition maximum at 533 nm. Photocyclization in the gas phase involves absorption of at least two photons; the first photon induces Z−E isomerization from the linear isomer to metastable intermediate isomers, while the second photon drives another E−Z isomerization and 4π-electrocyclization reaction. Cyclization is thermally reversible in the gas phase with collisional excitation.
D
into the photocyclization intermediates came from the observation of a red-shifted transient absorption band following irradiation of linear-DASA in solution with green light.17,18 Although the signal associated with the intermediate showed a strong dependence on temperature and light intensity, its exact origin was unclear. Recent time-resolved infrared spectroscopic studies of DASAs in chloroform at 298 K suggested that the first step of the photocyclization reaction involves formation of the EEE isomer on a 2 ps time scale, consistent with rapid isomerization through a conical intersection, with subsequent steps entailing thermally driven E−Z isomerization and conrotatory 4π-electrocyclization.19 Although the time-resolved measurements outlined above showed clear evidence for formation of the intermediate EEE isomer, the role of the solvent in the isomerization process is unclear. For this reason, we have studied the photoisomerization of a charge-tagged version of DASA in the gas phase. We use photoisomerization action (PISA) spectroscopy to investigate photoconversion of the charge-tagged linear-DASA shown in Figure 1, which is based on N-methyltaurine and Nmethylated barbituric acid. Briefly, PISA spectroscopy combines ion mobility mass spectrometry with laser spectroscopy, allowing isomer-specific action spectroscopy in the gas phase by exploiting differences in precursor and product isomer collision cross sections with a buffer gas.20−23 The technique is suited to studying photoisomerization reactions yielding intermediates and products that are stable on a >20 ms time
onor−acceptor Stenhouse adducts (DASAs; see Figure 1) are a new class of photoswitching molecules that can be synthesized in a two-step reaction from common starting materials.1−3 In solution, the colored, linear (extended) form photoisomerizes to a colorless, cyclic (compact) form with exposure to visible light. Compared with conventional spiropyran and diarylethene photoswitch molecules, DASAs offer excellent fatigue resistance, high solubility, and improved synthetic tunability.1−3 DASAs have been incorporated into applications that include light-triggered micelle collapse,1 control of polymer wettability and nanoparticle solubility,4−6 polymer dot chemosensors for metal ions in solution,7 chemosensors for amines in solution,8 targeted drug release,9,10 orthogonal multiphotochromic molecules and logic switches,11−13 polymeric sensors for nerve agents and projectile impacts,14,15 and wavelength-specific photopatterning of polymers.16 DASA molecules not only undergo facile photoisomerization but are also susceptible to thermal isomerization, with the propensity for interconversion between linear and cyclic forms depending on substituent groups and solvent;1,2,17,18 polar solvents tend to favor the cyclic form, whereas nonpolar solvents tend to favor the linear form. The cyclic form is presumably favored in polar solvents due to a zwitterionic tautomer.1,2,18 Photocyclization of DASAs is thought to involve photoisomerization from the linear isomer (EEZ isomer) to the EZE isomer via intermediate isomers, followed by thermal conrotatory 4π-electrocyclization (see Figure 1).19 The thermal, ground-state character of the 4π-electrocyclization step has been inferred from the stereochemistry of the alkylamine group in the cyclic product using crystallography.17 The first insights © XXXX American Chemical Society
Received: December 24, 2017 Accepted: January 22, 2018 Published: January 22, 2018 665
DOI: 10.1021/acs.jpclett.7b03402 J. Phys. Chem. Lett. 2018, 9, 665−671
Letter
The Journal of Physical Chemistry Letters
Figure 1. DASA photocyclization mechanism following absorption of 450−580 nm light (hν); one photon is required in solution at room temperature,19 whereas two photons are required in the gas phase. Δ denotes a thermal process. Gas-phase energies (ΔE) are given relative to the cyclic isomer. Ωc are calculated collision cross sections in pure N2 buffer gas (see the SI). The zwitterionic cyclic isomer is stabilized in polar solvents.1,2,18 Electron delocalization means the three bonds labeled in this figure (C5−C4, C4−C3, C3−C2) have similar bond orders and barriers to geometrical interconversion (see calculated barriers later in the paper), such that 180° rotation around each bond is associated with distinct E or Z geometric isomers.
scale and is capable of distinguishing isobaric reaction products.20−24 We show that the linear-DASA (EEZ) isomer photoisomerizes to intermediate isomers in the 450−580 nm range, with a maximum response at 530 nm. The intermediate isomers, which have absorption spectra nearly identical to that of the linear isomer, undergo cyclization following absorption of a second photon. Cyclization is found to be thermally reversible in the gas phase via energetic buffer gas collisions. Experiments were performed using a custom tandem ion mobility mass spectrometer (IMMS),20,21 which is described in detail in the SI. Briefly, the linear (EEZ) isomer of the chargetagged DASA was dissolved in acetonitrile (≈10−5 mol L−1) and introduced into the gas phase using electrospray ionization. Packets of electrosprayed ions were injected into the drift region of the IMMS where they were propelled by an electric field (44 V cm−1) through buffer gas at ∼6 Torr, eventually reaching a quadrupole mass filter and ion detector. In the drift region, the isomers were separated temporally and spatially according to their collision cross sections with buffer gas molecules and arrived at the detector at characteristic times.25 An arrival time distribution (ATD, ion signal plotted against transit time) exhibits peaks corresponding to different isomers. For the gas-phase isomerization measurements, an ion gate situated half way along the drift region was opened briefly (100 μs) at an appropriate delay to select the target ions. These mobility-selected ions were excited immediately after the gate with either a pulse of light from an EKSPLA NT342B optical parametric oscillator (OPO) or through energetic collisions with buffer gas molecules in a short 3 mm collision zone (slammer) where the electric field could be varied.26 The ions then passed through the remainder of the drift region, in which isomers formed by light or collisions were separated. The intermediate and cyclic isomer ions for the PISA and slammer measurements were produced by irradiating the linear isomer ions at the start of the first drift region with a pulse of 532 nm light.27 ATDs for the charge-tagged DASA with N2 or CO2 buffer gas and different RF drive voltages applied to the first ion funnel (IF1) are shown in Figure 2. Running the ion funnel with a high RF drive voltage (“IF1 high”, black ATDs) causes collisional heating of the ions before injection into the drift region, promoting isomerization to form the more stable gasphase isomers. Figure 2a shows ATDs for the charge-tagged DASA electrosprayed from a fresh acetonitrile solution and using N2 buffer gas. The “IF1 off” ATD, for which no RF drive voltage was applied to IF1, is dominated by a peak at 16.05 ms
Figure 2. ATDs for the charge-tagged DASA obtained using (a) N2 buffer gas, (b) N2 buffer gas seeded with ∼1% propan-2-ol, and (c) CO2 buffer gas. In each plot, gray and black ATDs correspond to no (“IF1 off”) and high (“IF1 high”) RF drive voltages to the first ion funnel, respectively. In (a), the 530 nm ATD was obtained by exposing the solution to green light before electrospray.28 Note that the isomer arrival times depend on the buffer gas. ATD peak resolutions (t/Δt) in (a) are 70−80, consistent with the instrument resolution for singly charged ions.20,21
(measured collision cross section, Ωm = 212 ± 5 Å2), associated with the linear isomer, whereas the IF1 high ATD is dominated by a faster peak at 14.75 ms (Ωm = 195 ± 5 Å2), assigned to the more compact, cyclic isomer. These ATD peak assignments are consistent with the isomers’ calculated relative energies and collision cross sections (Ωc) given in Figure 1 (see the SI for other isomers). Irradiating the DASA solution with 530 nm light prior to electrospray produced an ATD dominated by the 666
DOI: 10.1021/acs.jpclett.7b03402 J. Phys. Chem. Lett. 2018, 9, 665−671
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The Journal of Physical Chemistry Letters faster peak. Given that DASA photocyclizes in solution,1−3 this lends further support to assignment of the faster peak to the cyclic isomer. To better resolve the intermediate DASA isomers, ATDs were accumulated using N2 buffer gas seeded with ∼1% propan-2-ol (Figure 2b), a common mobility modifier, which, due to isomer-specific ion−molecule interactions, can increase ATD peak separations.29,30 The ATDs in Figure 2b show a new shoulder feature on the early side of the linear isomer peak, particularly apparent in the IF1 high ATD. Comparison of peak areas in the IF1 high ATDs suggests that the shoulder contributes to the linear isomer peak in the pure N2 buffer gas ATD. The shoulder peak is assigned to intermediate DASA isomers, including EEE, EZZ, and possibly ZEZ, based on calculated energies and ground-state isomerization barriers (see the potential energy surface in Figure 5). Interestingly, ATDs recorded with the DASA solution exposed to 530 nm light ∼10 s before electrospray under IF1 off conditions showed no evidence for the intermediate isomers, consistent with previous time-resolved experiments that suggest the intermediates are transitory in solution.17,18 ATDs were also accumulated using CO2 buffer gas (Figure 2c), which has proven useful for resolving isomers with large differences in dipole moments.31 The IF1 off ATD shows linear and cyclic isomer peaks similar to those in ATDs recorded using N2 buffer gas. However, the IF1 high ATD has an additional peak on the early side of the cyclic isomer peak. Comparing peak areas of the IF1 high ATD peaks recorded using CO2 and N2 buffer gases suggests that the additional peak is now resolved from the cyclic isomer peak. Exposing the linear DASA ions to a pulse of 532 nm light at the start of the drift region produced only the slower cyclic ATD peak in CO2 buffer gas, implying that the additional isomer is formed thermally rather than photochemically. While the additional peak is consistent with a second cyclic isomer, it is unclear whether it is associated with the zwitterion form shown in Figure 1, which has a calculated dipole moment ∼10 D larger than that of the cyclic isomer. One would expect that due to enhanced buffer gas interactions and a larger collision cross section, the zwitterion isomer should arrive later than the cyclic isomer. We now consider the photoisomerization responses for the linear and intermediate DASA isomers. The photoresponse of linear DASA is illustrated in Figure 3a, which shows a light-off ATD and photoaction ATDs (light-on−light-off) at 530 nm with low and high light fluence. Depletion of the peak associated with the linear isomer is matched by the appearance of peaks associated with the intermediate and cyclic isomers. Photoaction ATD measurements with varying light fluence at 530 nm (Figure 3b) show that relative production of the cyclic and intermediate isomers falls to zero for low light fluence, indicating that intermediate isomers are predominately formed following single-photon absorption and that absorption of two or more photons is required to drive photocyclization (see further details in the SI). For light fluences > 0.2 mJ cm−2 pulse−1, the depletion signal exceeds the total appearance signal due to multiphoton absorption leading to loss of the charge-tag group (m/z 108). To produce intermediate isomers for investigation, the ion packet was exposed to a pulse of 532 nm light immediately after its injection into the drift region. The target isomers were selected with an ion gate before being irradiated with tunable radiation (see the SI for further details). Under low light
Figure 3. PISA spectroscopy of charge-tagged DASA, recorded using N2 buffer gas seeded with ∼1% propan-2-ol. (a) Example light-off and photoaction ATDs (530 nm) for the linear isomer. Because the second light pulse intercepts mobility-selected ion packets halfway along the drift region, photoisomer peaks appear between peaks for parent and product isomers if they were separated through the entire drift region (Figure 2b). (b) Cyclic/intermediate product ratio plotted against light fluence at 530 nm. (c) PISA spectra for linear and intermediate isomers (light fluence < 0.2 mJ cm−2 pulse−1) along with an absorption spectrum of linear DASA in acetonitrile solution. Estimated errors in the photoisomerization yields in (c) are ±5%.
fluence conditions ( 350 nm. Furthermore, no photoreversion was observed for the cyclic isomer at the selected wavelength of 280 nm. The PISA spectrum for the linear isomer, obtained by plotting the intermediate photoisomer signal (normalized with respect to light pulse energy) against wavelength, features a band extending from 450 to 580 nm, peaking at 530 nm. The PISA spectrum for the intermediate isomers (plot of the normalized cyclic photoisomer signal against wavelength), also shown in Figure 3c, is very similar to the linear isomer PISA 667
DOI: 10.1021/acs.jpclett.7b03402 J. Phys. Chem. Lett. 2018, 9, 665−671
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The Journal of Physical Chemistry Letters
Figure 4. Transformations of the (a) linear, (b) intermediate, and (c) cyclic isomers of charge-tagged DASA with collisional activation. See the SI for further details. At a high slammer potential difference (>120 V), there is collision-induced (thermal) interconversion between the three forms.
spectrum. Both the linear and intermediate PISA spectra resemble the absorption spectrum for the linear DASA isomer dissolved in acetonitrile, aside from a ∼20 nm red shift in the solution band. The observed peak at 530 nm (Figure 3c) agrees well with theoretical predictions with the S1 ← S0 vertical transition wavelength for the linear isomer predicted at 553 and 533 nm from state-averaged and state-specific XMCQDPT2(12,12)/cc-pVDZ calculations, respectively. Earlier gas-phase calculations by Jacquemin and co-workers32,33 on a similar DASA based on barbituric acid gave vertical excitation wavelengths of 443 nm [TD-M06-2X/6-311++G(2df,2p)] and 597 nm [SOS-CIS(D)/6-311++G(2df,2p)], in poorer agreement with the PISA data, possibly due to inadequate description of the charge-transfer character of the excited state.34 Present calculations of the S1 ← S0 vertical transition wavelengths for possible EEE and EZZ intermediate isomers are 527 and 538 nm, respectively, consistent with the PISA spectrum for the intermediate isomers. The corresponding transition for the EZE isomer was calculated to lie at 609 nm, which is inconsistent with the PISA spectrum for the intermediate isomers, suggesting that this isomer is either not formed in substantial quantities or is transitory in the gas phase. The S2 ← S0 and S3 ← S0 transitions for the EEZ, EEZ, and EZE isomers are calculated to have vertical transition wavelengths of 300 nm (S2) and ∼250 nm (S3) and ∼0 oscillator strengths, consistent with them making no significant contribution to the PISA spectra over the 350−600 nm range. The photoisomerization measurements described above demonstrate that in the gas phase linear DASA molecules undergo a one-way sequence of photoisomerization reactions culminating in photocyclization. To explore thermal transformations between the linear, intermediate, and cyclic isomers, collisional activation experiments were performed in which mobility-selected isomers were collided with buffer gas under the influence of an adjustable potential difference, ΔV, in a short “slammer” region.26,35,36 In these measurements, increasing ΔV across the slammer electrodes subjected the ions to more energetic collisions, raising their internal energy and promoting thermal isomerization. Results of the slammer measurements are shown in Figure 4a−c, demonstrating interconversion between the isomers associated with the three ATD peaks for ΔV > 120 V. It is not possible to relate the appearance thresholds to isomerization barriers because a robust theory for the collision-induced isomerization process has not yet been developed and, for the present case, multiple isomers may contribute to each ATD peak. To further characterize the mechanism of the cyclization reaction, we computed the potential energy surface shown in Figure 5 that links the linear, intermediate, and cyclic isomers
Figure 5. Potential energy surface for cyclization of charge-tagged DASA. Energies relative to the cyclic isomer are given at the DLPNO− CCSD(T)/aug-cc-pVDZ level of theory in kJ mol−1. Key: black, EEE isomer pathway; purple in parentheses, EZZ isomer pathway; red, EEE isomer pathway assuming a methanol solvent model.
and modeled the rates of thermal isomerization using RRKM theory. For simplicity, Figure 5 includes only pathways involving EEE and EZZ isomer intermediates because recent time-resolved infrared studies on a similar DASA in solution suggest that the EEE isomer is the predominant intermediate19 and because any alternative pathway involving the ZEZ isomer requires isomerization about more than two bonds. In the first step of the reaction in the gas phase, the linear (EEZ) isomer absorbs a photon (λ1) to produce a combination of EEE and EZZ isomers. This isomerization step can proceed via two mechanisms in the gas phase: (i) prompt isomerization by passage of the photoexcited population through a S1/S0 conical intersection seam or (ii) recovery of vibrationally hot ground electronic state linear isomers that subsequently thermally isomerize. RRKM modeling suggests that mechanism (ii) has a rate coefficient of kf1 = 3.9 × 106 s−1, assuming a total internal energy of E = 291 kJ mol−1 (equivalent to the average thermal energy of the ions at 300 K plus the energy of a 530 nm photon), which is several orders of magnitude lower than the collision rate in the drift region of ∼109 s−1. Despite tens to hundreds of collisions being required to thermalize photoactivated ions, collisional cooling should outcompete statistical EEZ → EEE isomerization (or EEZ → EZZ isomerization), thus discounting mechanism (ii). Mechanism (i) is consistent with the infrared time-resolved measurements of a DASA in 668
DOI: 10.1021/acs.jpclett.7b03402 J. Phys. Chem. Lett. 2018, 9, 665−671
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The Journal of Physical Chemistry Letters solution,19 which demonstrated that EEZ → EEE photoisomerization is a nonstatistical process occurring on a 2 ps time scale. In the present gas-phase experiments, following formation of the EEE isomer, the majority of ions should become trapped in this geometry because statistical isomerization rates (kf2 = 5.1 × 107 s−1 and kr1 = 2.0 × 108 s−1; both values assume E = 271 kJ mol−1) are comparable with the expected rate for collisional energy quenching. In the next step of the gas-phase reaction, EEE isomers can absorb a second photon (λ2), causing some fraction to photoisomerize to the EZE form, again most probably by passage through a conical intersection, because statistical EEE → EZE isomerization of energized molecules in the S0 state should be slow. Once formed, the EZE isomer has the appropriate geometry to undergo a conrotatory 4π-electrocyclization reaction, which is rapid (kc = 2.54 × 1010 s−1 with E = 279 kJ mol−1) due to a low ground-state cyclization barrier of 7 kJ mol−1; see the illustration of the transition state in the SI. Comparable reaction rates and dynamics are expected for the cyclization pathway involving the EZZ intermediate due to a similar potential energy surface (Figure 5, parentheses). The discussion so far pertains to the photocyclization reaction and thermal reversion of a DASA in the gas phase, with the proposed pathway linking linear DASA and the cyclic isomer shown in Figure 5. This mechanism is essentially the same as the photocyclization mechanism proposed by Feringa and co-workers19 for similar DASA molecules in solution, with several notable exceptions: whereas the present gas-phase experiments show a one-way photocyclization reaction (an hν
hν
using tandem ion mobility mass spectrometry combined with laser excitation and collisional activation. The gas-phase PISA spectrum for the linear isomer spans the 450−580 nm range, with a maximum response at 530 nm. The photoisomerization mechanism involves prompt formation of intermediate isomers, which subsequently absorb a second photon leading to further isomerization and a 4π-electrocyclization reaction. The ion mobility approach was applied to unsolvated DASA molecules, where thermal reversion barriers are larger than those in methanol solution, allowing intermediate isomers to be separated and probed in isolation. The study demonstrates that a judiciously chosen mobility modifier (propan-2-ol) seeded into the buffer gas can help resolve intermediates in multistep photoisomerization reactions.
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b03402. Synthetic procedure; NMR spectra; UV−vis spectrophotometry measurement of thermal isomerization in acetonitrile and methanol; experimental methods; computational methods; illustration of noncyclic isomers; theoretical collision cross sections; product yields with laser power; photoaction ATD for the intermediate isomers; further details of slammer measurements; calculated transition states; and RRKM rate coefficients (PDF)
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Δ
EEZ→ EEE→ EZE→ cyclic process), Feringa’s solution study showed that EEE photoisomerizes back to EEZ rather than to EZE and that subsequent steps toward cyclization hν
Δ
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Δ
involve thermal isomerization (an EEZ⇌ EEE→ EZE→ cyclic process). Remembering that Feringa and co-workers considered different DASA molecules, the differences in photoisomerization dynamics are possibly linked to intrinsic and solvent-induced differences in the excited-state potential energy surfaces, including access to and topology of relevant conical intersection seams,37,38 local excitation vs charge-transfer character of the vertical transition,39 or solvent-induced that may inhibit certain isomerization pathways.40,41 While excited-state potential energy surface calculations that include a full, explicit treatment of solvent are beyond the scope of the present study, we have modeled the solvent-induced changes to the ground-state potential energy surface assuming one explicit methanol solvent molecule hydrogen-bonded to the hydroxyl group and embedded the system in a C-PCM solvent background (see red italicized energies in Figure 5). The importance of explicit solvation of the hydroxyl group in DASA molecules was outlined in a recent theoretical study.34 The present calculations indicate that methanol solvation stabilizes the noncyclic isomers, suggesting that the ratedetermining step for ground-state (thermal) cyclization is the initial EEZ−EEE isomerization; the EEE, EZZ, and EZE isomers should be shorter lived than the EEZ isomer in solution. It is clear from these calculations that solvent significantly influences relative energies of isomers and transition states. Solvent may also modify excited-state conical intersection seams, affecting the possibility for reversible E−Z photoisomerization. In summary, photocyclization and thermal reversion for a charge-tagged DASA have been investigated in the gas phase
ORCID
James N. Bull: 0000-0003-0953-1716 Eduardo Carrascosa: 0000-0003-4338-8669 Michael S. Scholz: 0000-0003-3290-2722 Gabriel da Silva: 0000-0003-4284-4474 Jonathon E. Beves: 0000-0002-5997-6580 Evan J. Bieske: 0000-0003-1848-507X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported under the Australian Research Council’s Discovery Project funding scheme (DP150101427 and DP160100474 to E.J.B. and DP160100870 to J.E.B.) and Future Fellowship (FT170100094 to J.E.B. and FT130101304 to G.d.S.). Computational resources were provided by the Australian National Computational Infrastructure through award of Early Career Allocation ya1 and a Microsoft Azure Research Award to J.N.B. E.C. acknowledges support by the Austrian Science Fund (FWF) through a Schrödinger Fellowship (Nr. J4013-N36). M.S.S. thanks the Australian government for an Australian Postgraduate Award scholarship. Prof. Javier Read da Alaniz, University of California Santa Barbara, is thanked for providing preliminary samples from the study reported in ref 1.
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REFERENCES
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DOI: 10.1021/acs.jpclett.7b03402 J. Phys. Chem. Lett. 2018, 9, 665−671
