Reversible Multicolor Photochromism of Dihydroazulene Crystals

Article abstract of DOI:10.1002/chem.201804677

The photochemical conversion of 1,8a-dihydroazulene-1,1-dicarbonitrile (DHA) to vinylheptafulvene (VHF) is a positive T-type photoswitch that is well understood in solution, but has not been explored in the solid state. Upon excitation with UV light, DHA

Full text of DOI:10.1002/chem.201804677

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Accepted Article
Title: Reversible Multicolor Photochromism of Dihydroazulene Crystals
Authors: Ieva Liepuoniute, Patrick Commins, Durga Prasad Karothu,
Stefan Schramm, Hideyuki Hara, and Pance Naumov
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To be cited as: Chem. Eur. J. 10.1002/chem.201804677
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Reversible Multicolor Photochromism of Dihydroazulene Crystals
Ieva Liepuoniutea, Patrick Comminsa, Durga Prasad Karothua, Stefan Schramma, Hideyuki Harab,
Panče Naumova,*
The photochemical conversion of 1,8a-dihydroazulene-1,1-
dicarbonitrile (DHA) to vinylheptafulvene (VHF) is a positive T-type
photoswitch that is well understood in solution, but has not been
explored in the solid state. Here we report that upon excitation with
UV light, DHA is converted into VHF in the solid state with a distinct
color change from yellow to deep red and retention of crystallinity.
The structure of the ring-opened product was assigned to syn-VHF
using variable-temperature infrared spectroscopy and determined by
X-ray photodiffraction in a crystal enriched with the product by two-
photon excitation. A radical pathway becomes an observable
photoreaction channel at low temperatures, and includes a strongly
colored, short-lived diradical intermediate.
Photochromic units are ubiquitous as switches in
photoresponsive materials, and could become a tool for solar
energy harvesting and storage.1,2 This line of thought has
generated a plethora of structural photoswitchable motifs3,4,5,
based on dithienylethenes,6,7 fulgides,8 spiropyrans,9‒14 and
Stenhouse adducts.15,16 A much less explored, yet promising
Figure 1. General reaction mechanism for conversion between DHA and VHF,
class of photochromic materials are the derivatives of 1,8a-
dihydroazulene-1,1-dicarbonitrile (DHA).17‒19 Upon UV irradiation
in solution these compounds undergo a ring-opening reaction to
their more strongly colored metastable form, vinylheptafulvene
(VHF), which can exist in syn and anti conformations (Fig. 1).18,19
Based on computational analysis, it was proposed that the
single crystals and their UV-Vis absorbance spectra. The crystal of syn‒VHF
shown here was obtained by irradiating a crystal of DHA with UV light, while
the crystal of pure anti‒VHF was obtained by irradiating DHA with UV light in
solution and subsequent crystallization of the product (VHF) below room
temperature.
photochromic reaction occurs via
a zwitterionic transition
the syn-VHF is possible, steric hindrances are prohibitive to the
anti-VHF in the solid state, and thus generation of this isomer is
highly improbable. With an 18-hour exposure to UV radiation,
the syn‒VHF IR band continued to increase, but the reaction
progressed very slowly. The conversion yield, based on fitting of
Gaussian functions of the area of the DHA band, was estimated
to 10% after 18 hours of irradiation (Fig. S4 and SI Table S1).
state.20‒22 This reaction is attractive due to the anticipated large
structural difference between DHA and anti-VHF and the
possibility for fine-tuning of the switch by chemical
substitution.18,23–29 While the photochromism of the DHA‒VHF
switch in solution has been studied extensively, the
photochromism in the solid state was noted29 but has not been
thoroughly explored, and the details remain poorly understood.
Given the interest in solid-state photochromic activity outside the
main photochromic families,3‒16 here we report direct evidence of
the transformation of DHA to its metastable form in a single
crystal, and we provide details of the multi-color photochromism.
Exposure of yellow crystals of DHA to broadband UV radiation
causes rapid change in color to orange-red (Fig. 1)28 and
evolution of absorption at 490 nm (Fig. 2a). The IR spectrum of
DHA conveniently contains a diagnostic nitrile stretching band at
2250 cm‒1 (Fig. 2b; Fig. S1). Upon irradiation with UV light, a
new band from the VHF nitrile emerges at 2211 cm‒1. The
product is assigned to syn‒VHF based on prohibitively close
packing for isomerization to anti-VHF, and the identity of
syn‒VHF was confirmed by comparison with anti‒VHF obtained
by irradiation of DHA solution with UV light and crystallization at
low temperature (Fig. 2b, Fig. S1). While syn‒VHF and
anti‒VHF have similar nitrile stretches at 2211 cm‒1 and 2210
cm‒1, respectively, the anti‒VHF bands at 1630, 1545, 1371 and
1210 cm‒1 are not found in syn‒VHF (Fig. S2). The assignment
of the intermediate as syn‒VHF was further supported by
analysis of the reaction cavity (Fig. 3).30 Even though the cavity
of DHA (55.328 Å3) is very similar to the cavity in anti‒VHF
(57.054 Å3), their shapes are very different; the cycloheptyl ring
plunges downwards and impinges upon the neighboring
molecules, and there is an insufficient amount of space for this
conformation (Fig. S3). On the contrary, the computed structure
of syn‒VHF overlaid within the cavity of DHA shows that the
molecule of the product fits reasonably well in the cavity of the
reactant (SI). This simple analysis supports that while reaction to
Figure 2. Photoconversion of DHA to syn‒VHF in the solid state at room
temperature. (a) Change of the Kubelka-Munk function of the solid-state
diffuse reflectance spectra with increasing UV irradiation. The absorption band
centered at 490 nm represents the transformation of DHA to VHF. (b) A
selected region of the IR spectrum showing the conversion of DHA to VHF at
room temperature with increasing duration of UV irradiation (the complete
spectra are available as Fig. S1 in the Supporting Information). The growth of
the peak at 2211cm‒1 is diagnostic for the product, syn‒VHF. The peak at
2250 cm‒1 is from DHA.
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absorption maxima may be due to optical effects). The crystal
readily recovered its original color within seconds upon warming
to room temperature. This color change was repeated five
additional times without any signs of crystal deterioration.
The formation of the hitherto unreported intermediate was
investigated using variable-temperature (VT) FTIR spectroscopy.
Unlike room temperature, where UV irradiation resulted in one
new nitrile stretching band (Fig. 2b), irradiation of solid DHA at
77 K resulted in two new bands, at 2205 cm‒1 and 2217 cm‒1
(Fig. 4a, Fig. S7). Initially these bands were attributed to two
crystallographically non-equivalent nitrile groups. However, UV
irradiation of DHA at 298 K produced syn‒VHF and subsequent
cooling of the resulting syn‒VHF to 77 K revealed only one band
at 2217 cm‒1 (Fig. 4a). As such, the second band, at 2205 cm‒1,
was assigned to the photoinduced intermediate. At 150 K, the
band gradually decreased and blended into a shoulder on the
2217 cm‒1 band from syn‒VHF (Fig. 4c).
Figure 3. Reaction cavities in DHA and anti‒VHF. (a) DHA and anti‒VHF
superimposed onto each other. Cycloheptyl ring in anti‒VHF protrudes outside
the cavity of DHA. (b) Predicted structure of syn‒VHF in the cavity of DHA.
Previous reports have suggested that the reaction of DHA to
VHF occurs via
a
closed-shell zwitterionic intermediate.18
Considering the fact that the purple-violet intermediate is
stabilized at low temperature, we postulated that it may be a
diradical obtained by homolytic bond dissociation. In the VT EPR
spectrum, UV irradiation at 100 K gave a single strong signal at
g = 2.0026, consistent with organic carbon-centered radicals (g
= 1.99‒2.01), thus confirming an open-shell intermediate.32 At
100 K, the signal was very stable and no significant decay in its
intensity was observed for at least 60 minutes (Fig. S8a). There
is a possibility that the radical generated is an open shell singlet,
but since the radical’s lifetime was found to exceed 1 hour (Fig.
S8), we have assigned the molecule as an open shell triplet and
performed calculation on the triplet. Additionally, the sample was
irradiated at temperatures from 100 to 298 K (Fig. 4b), and the
signal was detected at all temperatures. However, the intensity
of the signal was severely diminished above 220 K, indicating
that the radical species is less stable at higher temperatures,
which is reasonably in agreement with the observation of the
color change and the IR spectra (see above). The detection of
the EPR signal at all temperatures ranging from 77 K to 298 K
(Fig. S8b) suggests that the diradical is necessary for the
reaction to occur, and may be the primary pathway for
generation of VHF from DHA.
Figure 4. Evidence for a diradical intermediate generated by photoirradiation
of DHA. (a) VT IR measurements showing the presence of nitrile stretches
from syn‒VHF and the diradical at 2217 cm‒1 and 2205 cm‒1, respectively
(magenta-colored spectrum). When cooled from room temperature (green-
colored spectrum) to 77 K (red-colored spectrum), the peak at 2217 cm‒1 from
syn‒VHF does not split, indicating that the peak at 2205 cm‒1 is due to the
diradical. (b) EPR spectra of DHA recorded during UV irradiation at variable
temperatures. The diradical species is observed at varying intensities at 160,
180, 200 and 220 K with g = 2.0026. (c) Thermal decay after 20 min of UV
irradiation over a 35-hour period at 150 K. The diradical intermediate (2205
cm‒1) decays with rate constant k = 0.0166 s‒1 to DHA and k = 0.0002 s‒1 to
syn‒VHF, while syn‒VHF (2217 cm‒1) decays with k = 8.411010‒5 s‒1 to DHA.
(d) Single-crystal solid state UV-vis spectrum of DHA at 77 K (before and after
UV irradiation) shows a broad absorbance peak at 550 nm (shown in blue)
that includes components from both DHA and the diradical. The spectrum
confirms incomplete photoconversion of DHA to the diradical species.
To determine the rate of conversion of DHA to VHF in the crystal,
the fluorescence decay was recorded at 298 K and at ~77 K.
The excitation and fluorescence spectra were recorded by
excitation at 350 nm and at 480 nm, respectively (Fig. S5), and
the data acquired at 298 K was fitted.31 Initially a first order
exponential decay for the transformation of DHA to syn‒VHF
and a first order exponential growth for the formation of DHA
was assumed. This returned a poor fit (Fig. S6), and did not
appear to be in accord with the proposed mechanism of DHA
converting to syn‒VHF in a single step (Fig. 1).23 Qualitatively,
the fluorescence decay appeared to be faster at 77 K than at
room temperature, indicating the formation of an intermediate
species that suppressed the fluorescence. The hypothesis was
tested by exposing a single crystal to 405 nm irradiation at 77 K
whereby it changed color (SI Movie S1; note that the difference
in color in the movie and the color expected based on the
Figure 5. Energy profiles of the ground state and the first three excited states
calculated with unresticted (a) tiplet and (b) singlet model with the DFT method.
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Unrestricted DFT and time-dependent DFT calculations were
performed to validate the reaction mechanism by IRC
calculations. The hybrid ONIOM QM/MM method was applied by
constructing syn‒VHF within the experimentally determined
crystal structure of DHA (Fig. S9) with the assumption that at
small conversion yield, the crystal enriched with syn‒VHF
retains the DHA unit cell. As expected, the energy profile of the
first excited state in an open shell model is closer to the ground
state energy compared to the closed shell one (Fig. 5). In
support of the observed diradical, there is a local minimum along
the reaction path of the first triplet excited state between DHA
and the transition-state (Fig. 5b). At the lowest energy point, the
PES of the ground state is very close to the PES of the first
excited state, with energy gap of 75 kJ mol‒1. We suggest that
upon UV irradiation at low temperatures the reactant molecules
populate the first excited state and are trapped at the energy
minimum. The configuration
increments until the respective syn‒VHF and diradical bands at
2017 cm‒1 and 2205 cm‒1 dissipated (Fig. S12 and S13). The
decay rate constants for the thermal back reaction of VHF to
DHA were measured by irradiating the sample at 150 K, and
monitoring the sample by IR spectroscopy over a 35-hour period.
As shown in Scheme 1, the diradical can decay by two possible
pathways—either to DHA or to syn‒VHF, and this is reflected in
the peak fitting of the bands at 2205 cm‒1 and 2017 cm‒1 (Fig.
S13). The first order biexponential decay indicated that the band
at 2205 cm‒1 from the diradical decayed to DHA with rate
constants k = 0.0166 s‒1 and 0.0002 s‒1 (a single exponential fit
was not satisfactory). The thermal decay of the band at 2217
cm‒1 from syn‒VHF converting to DHA however is unidirectional
and it was fitted well using a single exponential decay to rate
constant k = 8.41110‒5 s‒1 (Fig S11). These results show that
the two IR stretches decay with different thermal decay
constants, which further supports the claim that they are
associated with two different chemical processes.
Scheme 1 Revised reaction mechanism for the photoinduced conversion of
DHA to syn‒VHF via an open-shell diradical intermediate.
Figure 6. Time-resolved fluorescence decay spectra recorded at 298 K and at
77 K. The kinetics was fitted with a functional form that accounts for the
formation of the diradical (solid line; the values of the rate constants are given
in the text). The scale bar corresponds to 500 m.
With the evidence of the photochemical change of DHA to the
diradical and the syn‒VHF at hand, the structural changes
during the photochromic process were determined with X-ray
diffraction. The structure determination of DHA from a single
crystal at room temperature confirmed the previously reported
triclinic structure (space group P ).27 Upon continuous UV
irradiation with broadband light for 16 hours, the yellow DHA
crystal turned dark red, however only very small (1%) changes
in the unit cell were detected. Powder X-ray diffraction
experiments did not indicate any changes in the structure after
irradiation with broadband UV light (Fig. S14), which could also
be a result of the small conversion yield.
corresponding to the lowest energy value on the reaction path
can be assigned to the diradical intermediate. This result is also
in agreement with the assumption that, since there are small
changes in the unit cell during the photochromic reaction, the
geometry of the intermediate should be similar to DHA. Such
energy minima are not observed in the closed-shell (singlet
state) calculations, which further disproves the suggested
zwitterionic structure of the intermediate.
Having the diradical established, the mechanism was
reevaluated (Scheme 1) by a second fitting of the fluorescence
decays with a new functional form (SI Eq. S2). The fluorescence
of DHA decayed with k1 = 0.00847 s‒1 at 77 K, but at a slower
rate with k3 = 0.00561 s‒1 at 298 K. Since it is counterintuitive to
observe the faster rate constant at reduced temperatures, the
rate constants represent different kinetic processes occurring at
the respective temperatures (Fig. 6 and Fig. S10). This result
indicates that at 77 K the fluorescence is primarily suppressed
by the diradical intermediate, while at ambient temperatures the
fluorescence is suppressed by the formation of syn‒VHF. At 77
K k1 is smaller than k3 at 298 K and, therefore, the activation
energy barrier for the diradical formation is lower than the
transition of the diradical to syn‒VHF. As a consequence, the
fluorescence kinetics at 77 K is faster than at 298 K due to
lowering the energy barrier and thereby producing faster rates.
To increase the conversion yield in a single crystal, we turned to
two-photon excitation with pulsed laser light.33–35 A single crystal
of DHA was irradiated 300 minutes ex situ with pulses of energy
70 mJ cm‒2 per pulse from an optical parametric oscillator with a
maximum at 730 nm (bandwidth at half maximum: 30 nm) at 200
K. The color of the crystal changed from yellow to dark/black red.
At this temperature the open-shell intermediate is not expected
to exist in significant amounts, and thus the molecules should be
converted to a single closed-shell product, VHF. X-ray diffraction
data of the irradiated crystal was collected at 100 K to increase
the lifetime of the product and to decrease the thermal atomic
vibrations (Fig. 7). The crystallographic details of the structure
before and after irradiation are provided in SI Table S2. The
difference electron density map of the irradiated crystal showed
new peaks around the cyano group that were not present in the
electron maps of the non-irradaited and UV-irradiated samples
(Fig. 7d; Fig. S15). This excess electron density was assigned to
one carbon atom and two cyano groups of the product, the open
form of VHF (Fig. 7b). After modeling of the disorder, the
population of the open form was refined to 3.1% (Fig. 7b).
Form syn‒VHF can convert back to DHA upon standing
overnight in the dark at room temperature, or by gentle heating.
The reversible change between the two states was repeated five
times without any visible signs of fatigue or disintegration of the
crystal (Figure S11). The conversion of VHF to DHA was further
analyzed by IR spectroscopy by exposing DHA to UV light at 77
K and monitoring the conversion of VHF back to DHA with
successive heating. The temperature was raised in 10 K
The distance between C2 and C7 in the five-membered ring of
non-irradiated DHA is 1.586(2) Å. In the two-photon irradiated
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New York University Abu Dhabi, P.O. Box 129188, Abu Dhab (United Arab
Emirates)
Email: pance.naumov@nyu.edu (P. N.)
[b] Dr. H. Hara
Bruker Biospin K.K., 3–9, Moriya, Kanagawa, Yokohama, Kanagawa 221-
0022 (Japan)
DHA crystal, the distance between C2′ and C7′ increased to
3.150(5) Å, and confirmed that the bond between the two atoms
was broken. The notable shift of atom C2 from its original
position is expected from the change of its geometry due to
change of hybridization (sp3 to sp2). This shift of the C2 is
related to the displacement of the two cyano groups that are
attached to this atom. Aside of this movement of the reactive
portion of the molecule, the overall molecular conformation is
retained. The torsional angle between the five-membered ring
and the phenyl ring in non-irradiated DHA is reduced only
slightly, from 23.5(2) to 20.9(5). The small overall structural
change is consistent with retention of the crystal integrity for
small conversion yields (note that further increase of the yield of
the open form in the crystal was not possible due to deterioration
after prolonged excitation). Considering that the radical
intermediate is not stable above 160 K, the product determined
by X-ray diffraction corresponds to the closed-shell form.
Acknowledgements
We thank Dr. Liang Li, New York University Abu Dhabi (NYUAD), for their help
with the XRD experiments. This research was partially carried out using the
Core Technology Platform resources at NYUAD. The computations were
carried out on the High Performance Computing resources (Dalma) at NYUAD.
Conflict of interest
The authors declare no conflicts of interest.
Keywords: photochromism • dihydroazulene • vinylheptafulvene
• solid state • crystallography
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In summary, we demonstrated that the DHA‒VHF photoswitch
undergoes a photochromic reaction from DHA to VHF in the
solid state. We provided spectroscopic and diffraction evidence
that the reaction results in the ring-opened form, syn‒VHF, and
it occurs with minimal atomic motion. In addition to the closed-
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photochromic switch to maintain its crystallinity and operate
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[a] Ms. I. Liepuoniute, Dr. P. Commins, Dr. D. P. Karothu, Dr. S. Schramm,
Prof. P. Naumov
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Entry for the Table of Contents
COMMUNICATION
Crystals of dihydroazulene can be
reversibly switched between closed
and open forms with a distinct color
change from yellow to deep red. In
addition to the closed-shell ring-
opening reaction, a radical pathway
becomes dominant at low
temperatures, and includes a
strongly colored, short-lived
diradical intermediate.
Ieva Liepuoniute, Patrick Commins,
Durga Prasad Karothu, Stefan
Schramm, Hideyuki Hara, Panče
Naumov,*
Page No. – Page No.
Reversible Multicolor
Photochromism of Dihydroazulene
Crystals
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