Controlling Dark Equilibria and Enhancing Donor-Acceptor Stenhouse Adduct Photoswitching Properties through Carbon Acid Design

Article abstract of DOI:10.1021/jacs.8b06067

A novel library of tunable negative photochromic compounds, donor-acceptor Stenhouse adducts (DASAs), is reported. Tailoring the electron deficient "acceptor" moiety yielded DASAs that can be activated with mild visible and far red light. The effect of ac

Full text of DOI:10.1021/jacs.8b06067

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Communication
Controlling Dark Equilibria and Enhancing DASA
Photoswitching Proper-ties Through Carbon Acid Design
James R. Hemmer, Zachariah A. Page, Kyle D Clark, Friedrich
Stricker, Neil D Dolinski, Craig J. Hawker, and Javier Read de Alaniz
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06067 • Publication Date (Web): 03 Aug 2018
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Controlling Dark Equilibria and Enhancing DASA Photoswitching Prop-
erties Through Carbon Acid Design
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a
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b
James R. Hemmer, Zachariah A. Page, Kyle D. Clark, Friedrich Stricker, Neil D. Dolinski, Craig, J.
a,b
,a
Hawker and Javier Read de Alaniz*
a
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California, 93106
Materials Department, University of California, Santa Barbara, California, 93106
b
Supporting Information Placeholder
9a, 4a, 9bꢀf
wavelength light.
to
This growth can be attributed
ABSTRACT: A novel library of tunable negative photoꢀ
chromic compounds, donor acceptor Stenhouse adducts
(DASAs), is reported. Tailoring the electron deficient
“acceptor” moiety yielded DASAs that can be activated
with mild visible and far red light. The effect of acceptor
composition on reactivity, absorption, equilibrium, and
cyclability is exploited for the design of high perforꢀ
mance photoswitches. The structural changes to the carꢀ
bon acid acceptor also provide access to new, more
structurally diverse DASA derivatives by facilitating the
ringꢀopening reaction with electron deficient amine doꢀ
nors.
The ability to utilize light as a stimulus to induce moꢀ
lecular transformations has drawn significant attention,
given that it can lead to unique property alterations in
targeted applications that require spatial control. In parꢀ
ticular photoswitches have shown impressive changes to
absorption, polarity, and geometry upon exposure to
light, leading to desirable applications, such as cargo
1
2
3
delivery, sensing, and actuation. Consequently, an
immense amount of research has gone into the developꢀ
ment and optimization of photoswitch performance in
order to harness their full potential in the medically releꢀ
4
vant longꢀwavelength region (λ = 650ꢀ1450 nm). Altꢀ
Figure 1. Donor acceptor Stenhouse adducts (DASAs) (a) Genꢀ
eral structures for open/colored triene and closed/colorless cycloꢀ
pentenone isomers. (b) Chemical structures for carbon acid accepꢀ
tors examined previously (top) and novel structures from this
report (bottom).
hough photoswitch performance can be assessed through
a number of parameters, such as addressability, thermal
stability, efficiency, and reliability, the ability to moduꢀ
late photoswitchable systems with longꢀwavelength light
is of central importance to all fields of life and material
science; ranging from the nascent fields of photopharꢀ
5
a deeper understanding of the factors that influence the
thermal halfꢀlife and the bathochromic shift of the exciꢀ
tation wavelength. More recently, indigoid photoswitchꢀ
es have shown great promise as long wavelength phoꢀ
toswitches, with facile synthesis and highly tunable abꢀ
6
7
macology to multiꢀphotoresponsive molecular systems.
Additionally, the photoswitch should display quantitaꢀ
tive conversion between its two isomers to maximize
photoswitching performance.
10
sorption profiles, switching kinetics, and equilibria.
Although the utility of these systems is unquestionable,
azobenzeneꢀ and indigoidꢀbased photoswitches transiꢀ
tion from one colored state to another, as opposed to
cycling between colored and colorless states, which is
Several decades of research have led to the developꢀ
ment of a range of visible to far red lightꢀdriven phoꢀ
toswitches. Of these, azobenzene is one of the fastest
8
growing classes of photoswitches that respond to longꢀ
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often desirable to maximize light penetration and actuaꢀ
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tion power by driving photoisomerization volume
changes throughout the depth of a material. Photoꢀ
chromic materials that lose absorption at the waveꢀ
length(s) of irradiation upon switching maximize light
penetration. Negative photochromic material, which are
capable of converting the thermodynamically favored
colored isomer into an optically transparent colorless
isomer upon exposure to light, are ideally positioned to
Scheme 1. General DASA synthesis. (i) water, or neat with proꢀ
line, 30ꢀ89% yield; (ii) THF or methanol, 13ꢀ80% yield.
11
achieve just that. Donor acceptor Stenhouse adducts
The six carbon acid acceptors utilized in this work
were pyrazolidinedione (3), methyl isoxazolone (4),
trfluoromethyl isoxazolone (5), indandione (6), trifluoꢀ
romethyl pyrazolone (7), and hydroxypyridone (8),
which were all synthesized in one pot from commercialꢀ
ly available starting materials (see SI for more details)
(Figure 1b). Knoevenagel condensation of the various
carbon acids with furfural on water or neat (+ catalytic
proline) provided the desired activated furan, which was
susceptible to ringꢀopening by an amine donor to yield
the corresponding DASA derivative (Scheme 1). The
facile twoꢀstep procedure provides the desired comꢀ
pounds as solids that can be simply filtered and trituratꢀ
ed for purification.
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(DASAs) are an emerging class of negative photoꢀ
chromic material (Figure 1a), containing a “donor”
amine and a carbon acid “acceptor” linked through a
12
trieneꢀenol backbone. Upon light irradiation, the triene
undergoes a Z–E isomerization followed by a thermal 4π
electrocyclization to yield a compact colorless cyclopenꢀ
tenone, which thermally reverts back to the extended
13
linear colored isomer. Combined, these features proꢀ
vide a means to enhance light penetration depth and acꢀ
tuation in bulk materials. Despite tremendous
12
advances, the development of DASAꢀbased phoꢀ
toswitch materials that simultaneously controls the
thermal equilibrum between the open and closed isomer
and redꢀshift the excitation wavelength remains elusive.
The first generation DASAs contained secondary alkyl
amine donors and either Meldrum’s (1) or barbituric (2)
14
acid acceptors. While these derivatives were favorably
in the colored/triene state (>95%) at thermal equilibria
in dichloromethane, they had a limited absorption winꢀ
dow (λ = 545 ꢀ 570 nm) and were only reversibly active
in nonꢀpolar media (e.g., toluene, hexanes, etc.). To adꢀ
dress these limitations, second generation DASAs were
developed by tuning the donor group using a range of
aniline derivatives, while continuing to employ accepꢀ
7a, 15
tors 1 and 2.
While these modifications imparted
improved color tunability and switchability in polar and
nonꢀpolar media, the thermodynamic equilibrium beꢀ
tween the open and closed form was compromised,
ranging from as low as 2% to 70% in the colored/triene
Scheme 2. The reactivity of pꢀOMeꢀdiphenylamine donor with (a)
Meldrum’s acid and (b) CF3 isoxazolone furan adducts.
1
6
state in the dark in dichloromethane. For numerous
applications, such as sensing and actuation, isomerizaꢀ
tion in the dark must be avoided in order to maximize
the lightꢀdriven property changes.
In all cases the reaction between the amine donor and
the activated furan proceeded as expected, but, impresꢀ
sively, the ringꢀopening reactions with the activated fuꢀ
ran adduct of 5 were notably more exothermic than
those observed with other furan adducts. When using
diethylamine as the donor the reaction with 5 was cooled
to –78°C to prevent side reactions, which suggested that
the furan adduct of acceptor 5 had exceptional reactivity,
likely due to the highly electron withdrawing nature of
the carbon acid. To test this hypothesis, a weakly nucleꢀ
ophilic donor, bis(4ꢀmethoxyphenyl)amine, was mixed
with the furan adduct of 1 (as a control) and 5 (Scheme
2). While no reaction was observed using the previously
studied activated furan of 1, the new furan adduct of 5
reacted at room temperature. The resulting DASA was
photochromic, with an absorption maximum of ~630 nm
and near quantitative reversible switching (see SI for
Herein we report the facile synthesis of third generaꢀ
tion DASAs where the carbon acid acceptor group is
tailored to provide switching with nearꢀinfrared (NIR)
light while simultaneously controlling the thermal equiꢀ
librium and thus enabling >95% conversion from the
colored linear form to the colorless compact form upon
exposure to light. Additionally, these new derivatives
have increased synthetic diversity, wide color tunability
(
(
λ = 585ꢀ735 nm), and enhanced switching kinetics
Figure 1b).
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more details). This highlights how structural change in
~11.6 and ~6.3 ppm, which correspond to the alcohol
proton and one of the triene protons, respectively, and
~7.8 and ~6.4 ppm, which correspond to protons on the
cyclopentenone ring, were monitored in the absence of
light until equilibrium was reached (Figure 3a). Comꢀ
pared to previously studied acceptors 1 and 2 (~45%
triene at equilibrium), all new acceptor groups, apart
from 6, favored the triene (colored form) relative to the
cyclopentenone (colorless form) at equilibrium in chloꢀ
roform (Figure 3b) (i.e., the new carbon acceptors shiftꢀ
ed the equilbria toward the colored form). In particular
DASAs with acceptors 3, 4, and 8 were found to have
triene equilibria ranging from ~75ꢀ85%, while 5 and 7
were >95% in the triene form. Interestingly, when comꢀ
paring methyl isoxazolone (4) to the more electron defiꢀ
cient trifluoromethyl isoxazolone (5) acceptor, there is a
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the carbon acid acceptors provide access to new, more
structurally diverse DASA derivatives, with the potential
for extended πꢀconjugation and bathochromic shift in
absorption.
To compare the properties of the DASAs synthesized
from these new carbon acid acceptors, diethylamine and
2
ꢀmethylindoline were primarily used as the donor comꢀ
pounds. Analogous to our previous results, the indoline
derivatives provide both a bathochromic shift in absorpꢀ
tion and enhanced photoswitching properties relative to
the diethylamine counterparts, and as such, will be the
main subject of discussion, with complete details on all
DASAs provided in the SI. The peak absorption ranged
from 585 to 680 nm in going from acceptor 1 to 8 (Fig-
ure 2). Thus, the acceptor group was clearly shown to be
a useful handle to tune the absorption of DASAs, with a
marked improvement on the previously reported 15 nm
difference observed between DASAs with acceptors 1
and 2. Furthermore, the peak absorption could be exꢀ
tended to 690 and 735 nm by introducing an electron
donating paraꢀmethoxy or ꢀdiheptylamino substituent on
the indoline moiety (Figure S4).
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0% shift in triene equilibrium, from 80 to >95%, reꢀ
spectively. Although it is not fully understood at this
time, it is speculated that factors such as the acidity and
orbital energy levels of the acceptors may play a role in
dictating the equilibrium position for these DASA librarꢀ
ies.
Figure 2. Absorption spectra for DASAs with a 2ꢀmethylindoline
donor and various acceptors. Inset shows the general chemical
structure and numbers refer to the acceptor group shown in Figure
1. See SI for comprehensive list of spectra.
Having the ability to switch fully from one isomer to
the other maximizes the change in properties, and as
such, it is important to characterize the thermodynamic
equilibrium for new photoswitches. For DASAs, it is
desirable for the thermal equilibrium to favor (>95%)
the triene isomer (colored form), with a complete transꢀ
formation to the photostationary cyclopentenone isomer
(colorless form) upon exposure to visible light. An atꢀ
tractive feature of negative photoswitches, like DASAs,
is the improved light penetration upon excitation, which
assists in driving the transformation to a >95% photostaꢀ
tionary state (<5% triene). To determine the effect of
acceptor composition on equilibrium position, the phoꢀ
Figure 3. Equilibrium position for DASAs with
a 2ꢀ
methylindoline donor and indicated acceptors. (a) Representative
1
DASA with acceptor 3 and corresponding H NMR spectrum (see
1
toswitches were examined using H nuclear magnetic
SI for full spectra); (b) Equilibrium position for DASAs with
resonance (NMR) spectroscopy. Specifically, peaks at
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Page 4 of 6
1
acceptors 1–8. Note: the H NMR of the closed form results in a
mixture of diastereomers.
resistance, which may be due to a reduced stability of
the cyclopentenone isomer in less polar media. Howevꢀ
er, further improvements in stability during switching
cycles is currently under examination. Figure 5 shows
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In addition to the importance of maximizing the
change in properties upon switching by tuning the equiꢀ
librium position, it is also essential to modulate the rate
at which the photoisomerization reaction occurs, with
rapid switching between the two states being desirable.
To this end, DASA with acceptor 7 was examined in
more detail, given its beneficial optical properties (λmax
1
0 cycles of ON/OFF NIR irradiation for DASA with
acceptor 8 in a 3:1 mixture of toluene:octane. The
DASA with acceptor 8 reverted back to the colored form
5
ꢀtimes and 16ꢀtimes faster than DASA with acceptors 1
and 2, respectively. Beyond the enhanced switching
properties, this is the first demonstration of a DASA
photoswitch operating with 700 nm light and represents
a rare example of a photoswitch that responds in the bioꢀ
≈
650 nm, >95% triene at equilibrium). When comparꢀ
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ing to previous DASAs, bearing either acceptor 1 or 2,
the switching upon exposure to visible light was found
to be similar (~5 seconds), yet the rate of thermal equiliꢀ
bration was impressively different. The DASA with acꢀ
ceptor 7 reverted back to the colored form within 60
seconds in the dark, while with acceptors 1 and 2 it took
9e, 17
logically important far red region.
In summary, electron deficient acceptors in donor ac-
ceptor Stenhouse adducts are excellent handles to tune
the properties of this emerging class of photoswitch.
Sixteen different third generation DASA derivatives
were efficiently synthesized and characterized for their
photochromic behavior. Improved reactivity, color tunaꢀ
bility, thermodynamic equilibria, switching kinetics, and
activity to far red light were observed when compared to
previous generations. These improvements will guide
the rational design of future DASA photoswitches, with
potential applications as actuators for renewable energy
and wavelengthꢀselective delivery vehicles for mediꢀ
~
8 and ~2.5 hours, respectively (Figures 4, S5 and S6).
Furthermore, DASA with 7 reverted back to >95% triꢀ
ene, while DASA with 2 equilibrates to a ratio containꢀ
ing 50% of the triene. This stark difference highlights
how the acceptor composition can be tailored to alter the
switching kinetics of DASAs.
18
cine.
Figure 4. Increased DASA switching rate. (a) Chemical structure
for DASA containing 2ꢀmethylindoline as the donor and acceptor
2 (top) and 7 (bottom); (b) Kinetics tracking the absorption maxꢀ
imum over time. Experiments performed under ambient condiꢀ
tions in CDCl3.
Given the less destructive and less harmful nature of
longꢀwavelength light toward polymeric materials and
biological cells, the ability to activate DASAs with 700
nm light was examined. The DASA with acceptor 8 was
chosen since it provided the most redꢀshifted absorption
Figure 5. Fatigue resistance study was conducted under ambient
conditions open to air using a 3:1 mixture of toluene:octane.
ASSOCIATED CONTENT
(
CDCl λ = 675 nm with tail out to λ = 720 nm), and
3 max
a far red light emitting diode (LED, λex ≈ 700 nm) was
used as the excitation source. In nonpolar solutions this
DASA derivative was shown to respond quickly to this
light (< 30 seconds), but in pure toluene had notable
fatigue (~70% / cycle; Figure S24). Introducing octane
as a coꢀsolvent was found to greatly improve fatigue
Supporting Information. Synthetic procedures, experiꢀ
mental details and photophysical properties. The Supportꢀ
ing Information is available free of charge on the ACS Pubꢀ
lications website.
AUTHOR INFORMATION
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Corresponding Author
javier@chem.ucsb.edu
Stefan, H., Angew. Chem. Int. Ed. 2015, 54, 11338ꢀ11349;
(d) Calbo, J.; Weston, C. E.; White, A. J. P.; Rzepa, H. S.;
ContrerasꢀGarcía, J.; Fuchter, M. J., J. Am. Chem. Soc.
1
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3
4
5
6
7
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*
2
017, 139, 1261ꢀ1274; (e) Danielle, W.; W., L. J.; R., B.
ORCID
N., ChemMedChem 2017, 12, 284ꢀ287; (f) van Leeuwen,
T.; Pol, J.; Roke, D.; Wezenberg, S. J.; Feringa, B. L., Org.
Lett. 2017, 19, 1402ꢀ1405.
For select examples, see: (a) Beharry, A. A.; Sadovski, O.;
Woolley, G. A., J. Am. Chem. Soc. 2011, 133, 19684ꢀ
Javier Read de Alaniz: 0000ꢀ0003ꢀ2770ꢀ9477
ACKNOWLEDGMENT
(
9)
C. J. H. and J. R. A. acknowledge that the research reported
here made use of shared facilities of the UCSB MRSEC
(NSF DMR 172056), a member of the Materials Reserarch
Facilities Network (http://www.mrfn.org). We thank Caliꢀ
fornia NanoSystems Institute (CNSI) Challenge Grant Proꢀ
gram for support and Dr. Alexander Mikhailovsky for his
help constructing the optical setup used for the switching
experiments.
1
9687; (b) Bléger, D.; Schwarz, J.; Brouwer, A. M.; Hecht,
S., J. Am. Chem. Soc. 2012, 134, 20597ꢀ20600; (c)
Samanta, S.; Babalhavaeji, A.; Dong, M.ꢀx.; Woolley, G.
A., Angew. Chem. Int. Ed. 2013, 52, 14127ꢀ14130; (d)
Knie, C.; Utecht, M.; Zhao, F.; Kulla, H.; Kovalenko, S.;
Brouwer, A. M.; Saalfrank, P.; Hecht, S.; Bléger, D.,
Chem. Eur. J. 2014, 20, 16492ꢀ16501; (e) Yang, Y.;
Hughes, R. P.; Aprahamian, I., J. Am. Chem. Soc. 2014,
136, 13190ꢀ13193; (f) Hammerich, M.; Schütt, C.; Stähler,
C.; Lentes, P.; Röhricht, F.; Höppner, R.; Herges, R., J.
Am. Chem. Soc. 2016, 138, 13111ꢀ13114.
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M.; Lee, C. W., Chem. Commun. 2014, 50, 4251ꢀ4254; (b)
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(
(
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2
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Macromolecules 2016, 49, 2568ꢀ2574; (d) Diaz, Y. J.;
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(a) Lerch, M. M.; Wezenberg, S. J.; Szymanski, W.;
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(
b) Di Donato, M.; Lerch, M. M.; Lapini, A.; Laurent, A.
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Jacquemin, D.; Szymanski, W.; Buma, W. J.; Foggi, P.;
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(3)
(
4)
(
c) Lerch, M. M.; Medved, M.; Lapini, A.; Laurent, A. D.;
Iagatti, A.; Bussotti, L.; Szymanski, W.; Buma, W. J.;
Foggi, P.; Di Donato, M.; Feringa, B. L., J. Phys. Chem. A
2
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8
Mallo, N.; Brown, P. T.; Iranmanesh, H.; MacDonald, T.
S.; Teusner, M. J.; Harper, J. B.; Ball, G. E.; Beves, J. E.,
Chem. Commun. 2016, 52, 13576ꢀ13579.
These ratios are for the dark equilibria (% open colored
form) in CD Cl with Meldrum's acid acceptor. Note, the
2 2
dark equilibria vary depending on solvent and donor and/or
acceptor group attached to DASA. See reference 12 for
examples in various conditions.
(a) Dong, M.; Babalhavaeji, A.; Collins, C. V.; Jarrah, K.;
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018, 122, 955ꢀ964.
2
014, 43, 148ꢀ184.
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(
(
5)
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van Dijken, D. J.; Kovaříček, P.; Ihrig, S. P.; Hecht, S., J.
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(
15)
(16)
(17)
(
7)
2
017, 139, 13483ꢀ13486; (b) Klaue, K.; Garmshausen, Y.;
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(
18)
(
8)
For select examples, see: (a) Patel, D. G.; Paquette, M. M.;
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2
018, 140, 8027ꢀ8036.
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