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, VIC 3010, Australia ‡School of Chemistry, University of New South Wales, High St, Kensington, NSW 2052, Australia ¶Department of Chemical Engineering, University of Melbourne, Parkville, VIC 3010, Australia

E-mail: [email protected]

1 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 character-

ize the photocyclization reaction of isolated, charge-tagged DASA molecules over the

450 – 580 nm range. The experimental maximum response at 530 nm agrees with mul-

tireference perturbation theory calculations for the S1 ← S0 transition maximum at 533 nm. Photocyclization in the gas phase involves absorption of at least two pho-

tons; 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.

Graphical TOC Entry

2 Donor-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 coloured, linear (extended) form photoisomerizes to a colourless, 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 favour the cyclic form whereas non-polar solvents tend to favour the linear form. The cyclic form is presumably favoured in polar solvents due to a zwitterionic tautomer.1,2,18

Figure 1: DASA photocyclization mechanism following absorption of 580 – 450 nm light (hν) – one photon is required in solution at room temperature19 whereas two photons are required in the gas phase. ∆ denotes a thermal process. Gas-phase energies (∆E) are given relative to the cyclic isomer. Ωc are calculated collision cross-sections in pure N2 buﬀer gas (see SI). The zwitterionic cyclic isomer is stabilized in polar solvents.1,2,18 Electron delocalization means the three bonds labelled 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.

Photocyclization of DASAs is thought to involve photoisomerization from the linear isomer

(EEZ isomer) to the EZE isomer via intermediate isomers, followed by a thermal conrotatory 4π- electrocyclization (see Figure 1).19 The thermal, ground state character of the 4π-electrocyclization

3 step has been inferred from the stereochemistry of the alkyl-amine group in the cyclic product using crystallography.17 The ﬁrst insights 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 the ﬁrst step of the photocyclization reaction involves formation of the EEE isomer on a 2 ps timescale, 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 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 study- ing photoisomerization reactions yielding intermediates and products that are stable on a >20 ms timescale, 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 charge-tagged 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

4 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 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 gas-phase 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

2 by a peak at 16.05 ms (measured collision cross-section, Ωm=212±5 Å ), associated with the linear

2 isomer whereas the ‘IF1 high’ ATD is dominated by a faster peak at 14.75 ms( Ωm=195±5 Å ), 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 SI for other isomers). Irradiating the DASA solution with 530 nm light prior to electrospray produced an ATD dominated by the 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 the shoulder

5 Figure 2: Arrival time distributions (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, grey and black ATDs correspond to no (‘IF1 off’) and high (‘IF1 high’) RF drive voltage 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/∆t) in (a) are 70 – 80, consistent with the instrument resolution for singly-charged ions.20,21

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 potential energy surface below). 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 demonstrate the intermediates are transitory in solution.17,18

ATDs were also accumulated using CO2 buﬀer gas (Figure 2c), which has proven useful for

6 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 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 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 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 photo-action 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. Photo-action 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 IF2 before being irradiated with tunable radiation (see SI for further details). Under low light ﬂuence conditions (<0.2 mJ cm−2 pulse−1), the intermediate isomers are photo-converted to the cyclic isomer but not to the linear isomer, suggesting this process is ineﬃcient compared with the 4π-electrocyclization reaction (see photo-action ATD in the SI). No photoisomerization response was observed for either of the two cyclic isomers with visible or near-UV light, suggesting cyclization

7 is not photo-reversible over the 350 – 600 nm range. This result is consistent with the UV-Vis spectra reported in the SI (Figure S4), which show the cyclic form does not absorb at wavelengths >350 nm.

Furthermore, no photo-reversion 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 photo-isomer signal (normalized with respect to light pulse energy) against wavelength, features a band extending from 450 nm to 580 nm, peaking at 530 nm. The PISA spectrum for the intermediate isomers

(plot of the normalized cyclic photo-isomer signal against wavelength), also shown in Figure 3c, is very similar to the linear isomer PISA 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 nm 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

34 state. Present calculations of the S1← S0 vertical transition wavelengths for possible EEE and EZZ intermediate isomers are 527 nm 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 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 photoiomerization 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

8 Figure 3: Photoisomerization action (PISA) spectroscopy of charge-tagged DASA, recorded

using N2 buffer gas seeded with ≈1% propan-2-ol. (a) Example light-off and photo-action ATDs (530 nm) for the linear isomer. Because the second light pulse intercepts mobility- selected ion packets halfway along the drift region, photo-isomer 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%.

9 Figure 4: Transformations of the (a) linear, (b) intermediate and (c) cyclic isomers of charge- tagged DASA with collisional activation. See SI for further details. At high slammer potential difference (>120 V), there is collision-induced (thermal) interconversion between the three forms. with buffer gas under the influence of an adjustable potential difference, ∆V, in a short ‘slammer’ region.26,35,36 In these measurements, increasing ∆V 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 ∆V>120 V. It is not possible to relate the appearance thresholds to isomerization barriers because a robust theory for 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 cyclicic isomers, and modelled the rates of thermal isomerization using RRKM theory. For simplicity, Figure 5 includes only pathways involving EEE and EZZ isomer intermediates since recent time-resolved infrared studies on a similar DASA in solution suggest the EEE isomer is the predominant intermediate,19 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 photoexcited population through a S1/S0 conical intersection seam, or (ii) recovery of vibrationally-hot ground electronic

10 state linear isomers that subsequently thermally isomerize. RRKM modelling suggests mechanism

6 −1 −1 (ii) has a rate coeﬃcient of kf1=3.9×10 s , assuming a total internal energy of E = 291 kJ mol (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 solution,19 which demonstrated that EEZ →EEE photoisomerization is a non-statistical process occurring on a 2 ps timescale. In the present gas-phase experiments, following formation of the EEE isomer, the majority of ions should become trapped in this geometry because

7 −1 8 −1 statistical isomerization rates (kf2=5.1×10 s and kr1=2.0×10 s with both values assuming 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 conrota-

10 −1 −1 tory 4π-electrocyclization reaction, which is rapid (kc=2.54×10 s with E=279 kJ mol ) due to a low ground state cyclization barrier of 7 kJ mol−1 – see 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 excep- tions: whereas the present gas-phase experiments show a one-way photocyclization reaction (a

EEZ −→hν EEE−→hν EZE−→∆ cyclic process), Feringa’s solution study showed that EEE photoisomerizes back to EEZ rather than to EZE, and that subsequent steps towards cyclization involve thermal hν ∆ ∆ isomerization (a EEZ )−*−EEE−→EZE−→cyclic process). Remembering that Feringa and co-workers considered diﬀerent DASA molecules, the diﬀerences in photoisomerization dynamics are possibly

11 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. 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 and solvent-induced friction 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 modelled 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 non-cyclic isomers, suggesting that the rate determining 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.

12 In summary, photocyclization and thermal reversion for a charge-tagged DASA have been investigated in the gas phase using tandem ion mobility mass spectrometry combined with laser excitation and collisional activation. The gas-phase photoisomerization action 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 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 multi-step photoisomerization reactions.

Acknowledgement

This research was supported under the Australian Research Council’s Discovery Project funding scheme (DP150101427 and DP160100474 to EJB, and DP160100870 to JEB) and Future Fellowship

(FT170100094 to JEB and FT130101304 to GdS). Computational resources were provided by the

Australian National Computational Infrastructure through award of Early Career Allocation ya1 and a Microsoft Azure Research Award to JNB. EC acknowledges support by the Austrian Science Fund

(FWF) through a Schrödinger Fellowship (Nr. J4013-N36). MSS 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.

Supporting Information Available

Synthetic procedure; NMR spectra; UV-Vis spectrophotometry measurement of thermal isomerization in acetonitrile and methanol; experimental methods; computational methods; illustration of non-cyclic isomers; theoretical collision cross-sections; product yields with laser power; photo- action ATD for the intermediate isomers; further details of slammer measurements; calculated

13 transition states; RRKM rates. This material is available free of charge via the Internet at http://pubs.acs.org/.

References

(1) Helmy, S.; Leibfarth, F. A.; Oh, S.; Poelma, J. E.; Hawker, C. J.; Read de Alaniz, J. Pho-

toswitching Using Visible Light: A New Class of Organic Photochromic Molecules. J. Am.

Chem. Soc. 2014, 136, 8169–8172.

(2) Helmy, S.; Oh, S.; Leibfarth, F. A.; Hawker, C. J.; Read de Alaniz, J. Design and Synthesis of

Donor-Acceptor Stenhouse Adducts: A Visible Light Photoswitch Derived From Furfural. J.

Org. Chem. 2014, 79, 11316–11329.

(3) Hemmer, J. R.; Poelma, S. O.; Treat, N.; Page, Z. A.; Dolinski, N. D.; Diaz, Y. J.; Tomlin-

son, W.; Clark, K. D.; Hooper, J. P.; Hawker, C. et al. Tunable Visible and Near Infrared

Photoswitches. J. Am. Chem. Soc. 2016, 138, 13960–13966.

(4) Singh, S.; Friedel, K.; Himmerlich, M.; Lei, Y.; Schlingloﬀ, G.; Schober, A. Spatiotemporal

Photopatterning on Polycarbonate Surface Through Visible Light Responsive Polymer Bound

DASA Compounds. ACS Macro. Lett. 2015, 4, 1273–1277.

(5) Sinawang, G.; Wu, B.; Wang, J.; Li, S.; He, Y. Polystyrene Based Visible Light Responsive

Polymer with Donor-Acceptor Stenhouse Adduct Pendants. Macromol. Chem. Phys. 2016,

217, 2409–2414.

(6) Ahrens, J.; Bian, T.; Vexler, T.; Klajn, R. Irreversible Bleaching of Donor-Acceptor Stenhouse

Adducts on the Surfaces of Magnetite Nanoparticles. ChemPhotoChem 2017, 1, 230–236.

(7) Zhong, D.; Cao, Z.; Wu, B.; Zhang, Q.; Wang, G. Polymer Dots of DASA-Functionalized

Polyethyleneimine: Synthesis, Visible Light/pH Responsiveness, and Their Applications as

Chemosensors. Sens. Actuators B 2018, 254, 385–392.

14 (8) Diaz, Y. J.; Page, Z. A.; Knight, A. S.; Treat, N. J.; Hemmer, J. R.; Hawker, C. J.; de

Alaniz, J. R. A Versatile and Highly Selective Colorimetric Sensor for the Detection of Amines.

Chem. Eur. J. 2017, 23, 3562–3566.

(9) Poelma, S. O.; Oh, S. S.; Helmy, S.; Knight, A. S.; Burnett, G. L.; Soh, H. T.; Hawker, C. J.;

Read de Alaniz, J. Controlled Drug Release to Cancer Cells from Modular One-Photon Visible

Light-Responsive Micellar System. Chem. Commun. 2016, 52, 10525–10528.

(10) Jia, S.; Du, J. D.; Hawley, A.; Fong, W.-K.; Graham, B.; Boyd, B. J. Investigation of Donor-

Acceptor Stenhouse Adducts as New Visible Wavelength-Responsive Switching Elements for

Lipid-Based Liquid Crystalline Systems. Langmuir 2017, 33, 2215–2221.

(11) Lerch, M. M.; Hansen, M. J.; Velema, W. A.; Szymanski, W.; Feringa, B. L. Orthogonal

Photoswitching in a Multifunctional Molecular System. Nat. Comm. 2016, 7, 12054.

(12) Yang, S.; Liu, J.; Cao, Z.; Li, M.; Luo, Q.; Qu, D. Fluorescent Photochromic Donor-Acceptor

Strenhouse Adduct Controlled by Visible Light. Dyes Pigm. 2017, 148, 341–347.

(13) Tang, F.-Y.; Hou, J.-N.; Lin, K.-X.; Liu, Y.; Deng, L.; Liu, Y.-N. A facile Way to Achieve All-

Photonic Logic Functions and Photo-Printing Based on a Donor-Acceptor Stenhouse Adduct.

New J. Chem. 2017, 41, 6071–6075.

(14) Balamurugan, A.; Lee, H.-I. A Visible Light Responsive On-Oﬀ Polymeric Photoswitch for the

Colorimetric Detection of Nerve Agent Mimics in Solution and in the Vapor Phase. Macromol.

2016, 49, 2568–2574.

(15) Mason, B. P.; Whittaker, M.; Hemmer, J.; Arora, S.; Harper, A.; Alnemrat, S.; McEachen, A.;

Helmy, S.; de Alaniz, J. R.; Hooper, J. P. A Temperature-Mapping Molecular Sensor for

Polyurethane-Based Elastomers. Appl. Phys. Lett. 2016, 108, 041906.

(16) Ulrich, S.; Hemmer, J. R.; Page, Z. A.; Dolinski, N. D.; Rifaie-Graham, O.; Bruns, N.;

Hawker, C. J.; Boesel, L. F.; Read de Alaniz, J. Visible Light-Responsive DASA-Polymer

Conjugates. ACS Macro. Letters 2017, 6, 738–742.

15 (17) Lerch, M. M.; Wezenberg, S. J.; Szymanski, W.; Feringa, B. L. Unraveling the Photoswitching

Mechanism in Donor-Acceptor Stenhouse Adducts. J. Am. Chem. Soc. 2016, 138, 6344–6347.

(18) Mallo, N.; Brown, P. T.; Iranmanesh, H.; MacDonald, T. S. C.; Teusner, M. J.; Harper, J. B.;

Ball, G. E.; Beves, J. E. Photochromic Switching Behaviour of Donor-Acceptor Stenhouse

Adducts in Organic Solvents. Chem. Commun. 2016, 52, 13576–13579.

(19) Donato, M. D.; Lerch, M. M.; Lapini, A.; Laurent, A. D.; Lagatti, A.; Bussotti, L.; Ihrig, S. P.;

Medved, M.; Jacquemin, D.; Szymański, W. et al. Shedding Light on the Photo-Isomerization

Pathway of a Donor-Acceptor Stenhouse Adduct. J. Am. Chem. Soc. 2017, 139, 15596–15599.

(20) Adamson, B. D.; Coughlan, N. J. A.; Continetti, R. E.; Bieske, E. J. Changing the Shape

of Molecular Ions: Photoisomerization Action Spectroscopy in the Gas Phase. Phys. Chem.

Chem. Phys. 2013, 15, 9540–9548.

(21) Adamson, B. D.; Coughlan, N. J. A.; Markworth, P. B.; Continetti, R. E.; Bieske, E. J. An

Ion Mobility Mass Spectrometer for Investigating Photoisomerization and Photodissociation

of Molecular Ions. Rev. Sci. Instr. 2014, 85, 123109.

(22) Czerwinska, I.; Kulesza, A.; Choi, C.; Chirot, F.; Simon, A.-L.; Far, J.; Kune, C.; de Pauw, E.;

Dugourd, P. Supramolecular Inﬂuence on cis–trans Isomerization Probed by Ion Mobility

Spectrometry. Phys. Chem. Chem. Phys. 2016, 18, 32331–32336.

(23) Simon, A.-L.; Chirot, F.; Choi, C. M.; Clavier, C.; Barbaire, M.; Maurelli, J.; Dagany, X.;

MacAleese, L.; Dugourd, P. Tandem Ion Mobility Spectrometry Coupled to Laser Excitation.

Rev. Sci. Instrum. 2015, 86, 094101.

(24) Coughlan, N. J. A.; Adamson, B. D.; Gamon, L.; Catani, K.; Bieske, E. J. Retinal Shows its

True Colours: Photoisomerization Action Spectra of Mobility-Selected Isomers of the Retinal

Protonated Schiﬀ Base. Phys. Chem. Chem. Phys. 2015, 17, 22623–22631.

(25) Eiceman, G. A.; Karpas, Z.; Hill, H. H. Ion Mobility Spectrometry, 3rd ed.; CRC Press, 2013.

16 (26) Coughlan, N. J. A.; Scholz, M. S.; Hansen, C. S.; Trevitt, A. J.; Adamson, B. D.; Bieske, E. J.

Photo and Collision Induced Isomerization of a Cyclic Retinal Derivative: An Ion Mobility

Study. J. Am. Soc. Mass Spectrom. 2016, 27, 1–8.

(27) Bull, J. N.; Scholz, M. S.; Carrascosa, E.; Bieske, E. J. From E to Z and Back Again: Reversible

Photoisomerization of a Charge-Tagged Azobenzene. Phys. Chem. Chem. Phys. 2018, 20, 509–

513.

(28) Bull, J. N.; Scholz, M. S.; Coughlan, N. J. A.; Kawai, A.; Bieske, E. J. Monitoring Isomerization

of Molecules in Solution Using Ion Mobility Mass Spectrometry. Anal. Chem. 2016, 88, 11978–

11981.

(29) Rorrer III, L. C.; Yost, R. A. Solvent Vapor Eﬀects on Planar High-Field Asymmetric Wave-

form Ion Mobility Spectrometry. Int. J. Mass Spectrom. 2011, 300, 173–181.

(30) Waraksa, E.; Perycz, U.; Namiesńik, J.; Sillanpää, M.; Dymerski, T.; Wójtowicz, M.; Puton, J.

Dopants and Gas Modiﬁers in Ion Mobility Spectrometry. Trends Anal. Chem. 2016, 82, 237–

249.

(31) Bull, J. N.; Coughlan, N. J. A.; Bieske, E. J. Protomer-Speciﬁc Photochemistry Investigated

Using Ion Mobility Mass Spectrometry. J. Phys. Chem. A 2017, 121, 6021–6027.

(32) Laurent, A. D.; Medved,˘ M.; Jacquemin, D. Using Time-Dependent Density Functional Theory

to Probe the Nature of Donor-Acceptor Stenhouse Adduct Photochromes. ChemPhysChem

2016, 17, 1846–1851.

(33) Belhboub, A.; Boucher, F.; Jacquemin, D. An ab initio Investigation of Photoswitches Ad-

sorbed onto Metal Oxide Surfaces: The Case of Donor-Acceptor Stenhouse Adduct Pho-

tochromes on TiO2 Anatase. J. Mat. Chem. C 2017, 5, 1624–1631.

(34) García-Iripea, C.; Marazzi, M. Level of Theory and Solvent Eﬀects on DASA Absorption

Properties Prediction: Comparing TD-DFT, CASPT2 and NEVPT2. Materials 2017, 10,

1025.

17 (35) Pierson, N. A.; Chen, L.; Russell, D. H.; Clemmer, D. E. Cis-trans Isomerizations of Proline

Residues are Key to Bradykinin Conformations. J. Am. Chem. Soc. 2013, 135, 3186.

(36) Pierson, N. A.; Clemmer, D. E. An IMS Threshold Method for Semi-Quantitative Determi-

nation of Activation Barriers: Interconversion of Proline cis-trans Forms in Triply Protonated

Bradykinin. Int. J. Mass Spectrom. 2015, 377, 646–654.

(37) Burghardt, I.; Cederbaum, L. S.; Hynes, J. T. Environmental Eﬀects on a Conical Intersection:

A Model Study. Faraday Discuss. 2004, 127, 395–411.

(38) Levine, B. G.; Martínez, T. J. Isomerization Through Conical Intersections. Ann. Rev. Phys.

Chem. 2007, 58, 613–634.

(39) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural Changes Accompanying Intramolecu-

lar Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures.

Chem. Rev. 2003, 103, 3899–4032.

(40) Rodier, J. M.; Myers, A. B. cis-Stilbene Photochemistry: Solvent Dependence of the Initial

Dynamics and Quantum Yields. J. Am. Chem. Soc. 1993, 115, 10791–10795.

(41) Malhado, J. P.; Spezia, R.; Hynes, J. T. Dynamical Friction Eﬀects on the Photoisomerization

of a Model Protonated Schiﬀ Base in Solution. J. Phys. Chem. A 2011, 115, 3720–3735.