From: E to Z and back again: Reversible photoisomerisation of an isolated charge-tagged azobenzene

Article abstract of DOI:10.1039/c7cp07278c

Substituted azobenzenes serve as chromophores and actuators in a wide range of molecular photoswitches. Here, tandem ion mobility spectrometry coupled with laser excitation is used to investigate the photoisomerisation of selected E and Z isomers of the c

Full text of DOI:10.1039/c7cp07278c

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From E to Z and back again: reversible
photoisomerisation of an isolated charge-tagged
azobenzene
Cite this:DOI: 10.1039/c7cp07278c
James N. Bull,
Evan J. Bieske
Michael S. Scholz,
*
Eduardo Carrascosa
and
Substituted azobenzenes serve as chromophores and actuators in
a
wide range of molecular
photoswitches. Here, tandem ion mobility spectrometry coupled with laser excitation is used to investigate
the photoisomerisation of selected E and Z isomers of the charge-tagged azobenzene, methyl orange.
Both isomers display a weak S1(np*) photoisomerisation response in the blue part of the spectrum peaking
at 440 nm and a more intense S2(pp*) photoisomerisation response in the near-UV with maxima at 370
and 310 nm for the E and Z isomers, respectively. The 60 nm separation between the S2(pp*) photo-
response maxima for the two isomers allows them to be separately addressed in the gas phase and to be
reversibly photoisomerised using different colours of light. This is an essential characteristic of an ideal
photoswitch. The study demonstrates that a sequence of light pulses at different stages in an ion mobility
spectrometer can be deployed to generate and probe isomers that cannot be electrosprayed directly from
solution or produced through collisions in the ion source.
Received 27th October 2017,
Accepted 27th November 2017
DOI: 10.1039/c7cp07278c
rsc.li/pccp
the Z isomer in solution. Most condensed phase spectroscopic
and ultrafast studies rely on creating a photostationary state and
Introduction
Photoswitch molecules can be converted between two or more subtracting the contribution of the E isomer to yield the con-
distinct geometric forms using light. Natural photoswitches include tribution of the Z isomer.12 Ambiguities associated with this
the retinal chromophore in opsin proteins that are involved in the subtraction approach are exacerbated if the E and Z isomers have
visual transduction cycle and bacterial photosynthesis,1 and photo- similar properties or if other species in the sample generate
active proteins, such as the photoactive yellow protein, found in interfering signals.
certain bacteria.2 Over the last decade, photoswitches based on
Recently, our group has combined tandem ion mobility spectro-
azobenzenes have been developed and deployed for a range of metry (IMS) and laser excitation to probe the photoisomerisation of
applications, including as actuators in molecular machines,3 to protonated azobenzenes, azoheteroarenes and other cationic
control solution pH or ionic liquid properties with light,4,5 in azoheteroarenes in the gas phase or in solution.13–16 For the
optogenetics,6 to control peptide and lipid structure,7,8 and for gas-phase photoisomerisation studies, charged isomers drifting
targeted activation or release of drugs in photopharmacology.9 under the influence of an electric field through a buffer gas are
The appeal of azobenzenes as core photoswitching units stems separated according to their drift speeds, which depend on their
from their rapid photoisomerisation between E and Z isomers with collision cross-sections. Typically, the target isomer is selected in
a high quantum yield (0.2–0.3), and the possibility of reversible a primary IMS stage and then exposed to wavelength tunable
control using different colours of light.10,11 To realise the full light, with the photo-isomers separated in a secondary IMS
potential of azobenzene molecules and substituted derivatives as stage. Using this approach, the E and Z isomers are spatially
chromophores and actuators in photoswitching applications, it is and temporally separated allowing one to record isomer-specific
desirable to characterise the spectroscopic and photoisomerisation photoisomerisation action (PISA) spectra. In an ideal case, the
properties of the E and Z isomers for a series of benchmark photoisomerisation quantum yield is independent of wave-
molecules. For most azobenzenes and azoarenes, the Z isomer length, so the PISA spectrum corresponds to the absorption
is less stable than the E isomer, meaning Z–E thermal reversion spectrum. The technique provides a direct means to characterise
limits the ability to cleanly isolate and spectroscopically probe the photoresponse of selected isomer, avoiding complications
associated with overlapping spectra of coexisting isomers that
sometimes bedevil other spectroscopic techniques such as photo-
dissociation spectroscopy. In previous studies of protonated
School of Chemistry, University of Melbourne, Parkville, VIC 3010, Australia.
E-mail: evanjb@unimelb.edu.au
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Fig. 2 Schematic illustration of the IMS-IMS-QMF instrument. Key:
ESI – electrospray ionisation; IF1 and IF2 – ion funnels; IG1 and IG2 – ion
gates; IMS1 and IMS2 – ion mobility drift regions; OPO – light beam passing
through the photoisomerisation zone; oct – octupole ion guide; QMF –
quadrupole mass filter. Total drift region length (IMS1 + IMS2) is 0.9 m.
Further details of the instrument are given in Adamson et al.24,25 This figure is
adapted from ref. 14.
Fig. 1 E and Z isomers of the methyl orange anion.
azobenzene, we obtained PISA spectra for E - Z isomerisation but
were thwarted in attempts to observe the reverse process because
we were unable to obtain sufficient amounts of the Z isomer.16
Parallel investigations of protonated aminoazobenzene suffered
the added complexity of multiple possible gas-phase protomers,
which may have different absorption profiles and photochemical
behaviour.17 Spectroscopic measurements on gas-phase molecules
also allows straightforward connections with theory, providing
confidence regarding predictions for photoisomerisation properties
of similar molecules.
into a differentially pumped octupole ion guide and QMF that
mass-selected the ions before they reached the ion detector. The
detector was connected to a multichannel scaler that produced a
histogram of ion counts against arrival time, t, corresponding to
an arrival time distribution (ATD). In all presented ATDs, t = 0
corresponds to the opening of IG1. The mobility resolution, t/Dt,
for a singly-charged anion is typically 80–90.
Here, we investigate photoisomerisation of the E and Z
isomers of methyl orange anions (4-[4-(dimethylamino)phenyl]-
diazenylbenzene-1-sulfonate, see Fig. 1) using the PISA technique.
Methyl orange is a charge-tagged azobenzene that has been
incorporated into push–pull switching dendrimers that can
respond to both light and solution pH.18,19 Femtosecond
photoisomerisation measurements20 of methyl orange in aqueous
solution show that E–Z photoisomerisation occurs on an ultrafast
0.6–0.9 ps timescale, similar to E-azobenzene in either the gas
phase and in solution.11,21–23 The crucial issues we wish to address
for methyl orange anions are whether we can produce both E and Z
isomers in the gas phase, and whether they can be reversibly
photoswitched using different colours of light.
For the PISA spectroscopy measurements, packets of ions
with similar collision cross-sections were selected using a
Bradbury–Nielsen ion gate after IMS1 (IG2, E100 ms opening
time). Immediately after this, the mobility-selected ions were
excited with a pulse of light from an optical parametric oscillator
(OPO, EKSPLA NT342B). Any change in the ions’ collision cross-
section due to photoisomerisation was manifested as a shift in
arrival time following passage through a second IMS drift region
(IMS2). The OPO was operated at 20 Hz, half the rate of ion
injection, allowing accumulation of light-on and light-off ATDs.
The difference between the light-on and light-off ATDs reflected
the photoresponse (photo-action ATD). PISA spectra were
derived by integrating the photo-action ATD signal, and normal-
izing with respect to light pulse fluence and total laser-off signal
at each wavelength. The PISA measurements were performed
with a light pulse fluence of o1 mJ cmÀ2 to minimise multi-
photon absorption and dissociation producing SO3À. The Z
isomer was synthesised in the gas phase by irradiating electro-
sprayed ions with a pulse of 355 nm light (Quantel Big Sky
Nd:YAG, 40 Hz) immediately after the ions were injected into the
drift region of IMS1.
Experimental methods
The photoisomerisation of methyl orange anions was investi-
gated in a custom tandem ion mobility spectrometer (IMS-IMS)
coupled with a quadrupole mass filter (QMF).24,25 The IMS-IMS-
QMF apparatus is illustrated schematically in Fig. 2. Briefly,
methyl orange anions were produced through electrospray
ionisation of a E10 mmol LÀ1 solution of sodium methyl orange
(Sigma Aldrich) dissolved in methanol (voltage À3 kV, flow
rate E10 mL minÀ1). Electrosprayed ions were transferred via a
Theoretical methods
heated capillary into a RF ion funnel (IF1), which radially Supporting electronic structure calculations for the methyl orange
gathered and confined the ions. An ion gate (IG1) at the end isomers were performed using the Gaussian 16 and ORCA 4.0.0.2
of IF1 injected E100 ms packets of ions at 40 Hz into the first software packages.27,28 Geometrical optimisations, vibrational
IMS drift region (IMS1) where they were propelled by an electric frequencies and isomerisation transition state searches were
field (44 V cmÀ1) through N2 buffer gas at a pressure of E6 Torr. performed at the oB97X-D/aug-cc-pVDZ level of theory,29,30
The E and Z isomers were separated spatially and temporally followed by single-point energy calculations at the DLPNO-
because more extended ions (E isomers) have larger collision CCSD(T)/aug-cc-pVTZ level of theory.31 Vertical excited state
cross-sections with the buffer gas than more compact ions energies were computed at the oB97X-D/aug-cc-pVDZ level of
(Z isomers).26 After traversing the drift regions (IMS1 + IMS2), theory within the TD-DFT framework. Collision cross-sections
a second ion funnel (IF2) collected the ions and introduced them were calculated using MOBCAL with the trajectory method
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parametrised for N2 buffer gas.32,33 Input charge distributions
The photoisomerisation of E and Z isomers of methyl orange
were computed at the oB97X-D/aug-cc-pVDZ level of theory with anions in the gas phase was probed by selecting one of the
the Merz–Singh–Kollman scheme constrained to reproduce the isomers using an ion gate (IG2) following mobility-separation
electric dipole moment.34 Sufficient trajectories were computed in the first drift region and exposing the ion packet to a pulse of
to give standard deviations of Æ1 Å2 for the calculated values.
tunable wavelength light (see Fig. 2). Fig. 4a shows light-off,
light-on, and photo-action (light-on–light-off) ATDs obtained by
exposing the E isomer to 370 nm light. The E–Z PISA spectrum,
generated by plotting the Z photo-isomer signal against wave-
length and normalizing with respect to light pulse intensity and
total laser-off ion signal, is presented in Fig. 4b. The PISA
spectrum exhibits a weak S1(np*) band over the 410–530 nm
range, peaking at 440 nm, and an intense S2(pp*) band in the
280–410 nm range, peaking at 370 nm. The Z–E PISA spectrum,
shown in Fig. 4c, features a similar S1(np*) band in the 370–540 nm
range, peaking at 440 nm. However, the Z–E S2(pp*) PISA band
peaks at 310 nm, blue-shifted by 60 nm from the E–Z PISA band.
The clear separation of the S2(pp*) PISA bands for the two isomers
means that gas-phase methyl orange anions can be reversibly
photo-switched with light of different colours, as required for an
ideal photoswitch. Specifically, exposing methyl orange anions to
370 nm light (E–Z PISA maximum) drives E - Z photoisomerisa-
tion, whereas irradiation with 310 nm light (or 440 nm) drives net
Z - E photoisomerisation.
The transition wavelengths and relative intensities for the
S1(pp*) ’ S0 and S2(pp*) ’ S0 transitions of the E and Z
isomers agree reasonably well with the calculated vertical
excitation energies and oscillator strengths (vertical bars in
Fig. 4b and c). The absorption spectrum of methyl orange in
methanol, included in Fig. 4b, differs markedly from the gas-
phase E–Z PISA spectrum. Specifically, the main S2(pp*) absorp-
tion band in solution is strongly red-shifted, peaking at 420 nm,
with perhaps a small contribution from the S1(np*) band
appearing as a shoulder at 460 nm.11 The solution absorption
spectrum shows evidence for two further absorption bands at
315 and 280 nm, that are not evident in the E–Z PISA spectra,
but which may correspond to the S3(pp*) ’ S0 and S4(pp*) ’ S0
transitions, which, in the gas phase, are calculated to lie at 309
and 284 nm with oscillator strengths of f 4 0.01 and f 4 0.02,
respectively. The absence of these bands in the E–Z PISA
spectrum may be due to low isomerisation yields for the
S3(pp*) and S4(pp*) states (i.e. direct internal conversion to S0),
as is the case for neutral azobenzene.21
Results and discussion
The arrival time distribution (ATD) of electrosprayed methyl orange
anions exhibits a single peak (Fig. 3a), that can be assigned to the
E isomer based on calculations that suggest it is 21 kJ molÀ1 more
stable than the Z isomer. We attempted to generate the Z isomer by
several means. First, exposing the methyl orange solution to 532, 425
or 365 nm light before electrospray ionisation produced no evidence
for a second isomer in the ATD, indicating Z - E thermal reversion
in methanol occurs in less than a few seconds, consistent with
previous observations.20,35,36 Second, we tried synthesising the Z
isomer by subjecting the E isomer ions to energetic collisions in IF1
prior to their injection into the drift region, again without success.
Finally, we managed to generate Z isomer ions by irradiating the E
isomer ions with a pulse of 355 nm light immediately after their
injection into the drift region of the IMS. This resulted in an
additional, faster ion packet (Fig. 3b), that can be assigned to the
more compact Z isomer. The ATD peak assignments are consistent
with calculated collision cross-sections of 196 and 174 Å2 for the E
and Z isomers, respectively. Because the E and Z isomers are
associated with clear, baseline-resolved ATD peaks and can be
individually selected, the timescale for thermal reversion in the drift
region must be long relative to the drift time (E13 ms).37 This is
consistent with the present calculated Z - E thermal reversion
barrier of 179 kJ molÀ1
.
Returning to a comparison of the spectra of methyl orange
in solution and in the gas phase, we note that the S1(pp*) ’ S0
band of methyl orange in methanol is red shifted with respect
to the corresponding band in the PISA spectrum (peak at
420 nm versus 370 nm). Such a solvent induced red shift is in
line with observations for other azobenzenes. For example, the
absorption bands of substituted azobenÀzenes such as butter
yellow (methyl orange without the –SO3 charge tag) are red
shifted in polar solvents compared to non-polar solvents.38 It is
also possible that the PISA spectrum does not mirror the
absorption spectrum due to a wavelength dependent photo-
isomerisation yield. Ultimately, the absorption spectra of the E and
Z isomers of methyl orange in the gas phase should be revealed
Fig. 3 Arrival time distributions for methyl orange anions: (a) light-off and
(b) 355 nm irradiation of the gas-phase ion packet at the start of the drift
region. Peak resolutions (t/Dt E 90) are consistent with singly-charged ions. through spectroscopic measurements of mobility-selected ions
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Conclusions
In summary, we have investigated photoisomerisation of selected E
and Z isomers of methyl orange anions in the gas phase. Both
isomers have similar, weak S1(np*) PISA bands in the visible and
more intense S2(pp*) PISA bands in the near-UV. Notably, the
S2(pp*) PISA bands for the two isomers are separated by E60 nm
meaning that methyl orange can be reversibly switched between E
and Z forms using light of different colours, a requirement of an
ideal photoswitch. The present study illustrates that ions which
cannot normally be generated through electrospray ionisation due
to short lifetimes in solution can be produced at the start of the
drift region of an ion mobility spectrometer using an intense laser
pulse and probed downstream with a second tunable laser pulse.
Significantly, we have shown that it is possible to photoswitch a
population of methyl orange anions in the gas phase from E to Z
and back again, demonstrating unprecedented control over the
isomeric state of an ion population.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was funded through the Australian Research Council
Discovery Project scheme (DP150101427 and DP160100474). J. N. B.
acknowledges resources from the Australian National Computa-
tional Infrastructure (NCI) through Early Career Allocation ya1 and
a Microsoft Azure Research Award. M. S. S. thanks the Australian
government for an Australian Postgraduate Award scholarship.
E. C. acknowledges support by the Austrian Science Fund (FWF)
¨
through a Schrodinger Fellowship (No. J4013-N36).
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