Acylhydrazones as Widely Tunable Photoswitches

Article abstract of DOI:10.1021/jacs.5b09519

Molecular photoswitches have attracted much attention in biological and materials contexts. Despite the fact that existing classes of these highly interesting functional molecules have been heavily investigated and optimized, distinct obstacles and inherent limitations remain. Considerable synthetic efforts and complex structure-property relationships render the development and exploitation of new photoswitch families difficult. Here, we focus our attention on acylhydrazones: a novel, yet underexploited class of photochromic molecules based on the imine structural motif. We optimized the synthesis of these potent photoswitches and prepared a library of over 40 compounds, bearing different substituents in all four crucial positions of the backbone fragment, and conducted a systematic study of their photochromic properties as a function of structural variation. This modular family of organic photoswitches offers a unique combination of properties and the compounds are easily prepared on large scales within hours, through an atom-economic synthesis, from commercially available starting materials. During our thorough spectroscopic investigations, we identified photoswitches covering a wide range of thermal half-lives of their (Z)-isomers, from short-lived T-type to thermally stable P-type derivatives. By proper substitution, excellent band separation between the absorbance maxima of (E)- and (Z)-isomers in the UV or visible region could be achieved. Our library furthermore includes notable examples of rare negative photochromic systems, and we show that acylhydrazones are highly fatigue resistant and exhibit good quantum yields.

Full text of DOI:10.1021/jacs.5b09519

Article
pubs.acs.org/JACS
Acylhydrazones as Widely Tunable Photoswitches
Derk Jan van Dijken,† Petr Kovarícek,†,‡ Svante P. Ihrig, and Stefan Hecht*
̌
̌
Department of Chemistry, Humboldt-Universitat zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany
̈
S
* Supporting Information
ABSTRACT: Molecular photoswitches have attracted much
attention in biological and materials contexts. Despite the fact
that existing classes of these highly interesting functional
molecules have been heavily investigated and optimized, distinct
obstacles and inherent limitations remain. Considerable synthetic
efforts and complex structure−property relationships render the
development and exploitation of new photoswitch families
difficult. Here, we focus our attention on acylhydrazones: a
novel, yet underexploited class of photochromic molecules based
on the imine structural motif. We optimized the synthesis of these
potent photoswitches and prepared a library of over 40
compounds, bearing different substituents in all four crucial
positions of the backbone fragment, and conducted a systematic
study of their photochromic properties as a function of structural
variation. This modular family of organic photoswitches offers a unique combination of properties and the compounds are easily
prepared on large scales within hours, through an atom-economic synthesis, from commercially available starting materials.
During our thorough spectroscopic investigations, we identified photoswitches covering a wide range of thermal half-lives of their
(Z)-isomers, from short-lived T-type to thermally stable P-type derivatives. By proper substitution, excellent band separation
between the absorbance maxima of (E)- and (Z)-isomers in the UV or visible region could be achieved. Our library furthermore
includes notable examples of rare negative photochromic systems, and we show that acylhydrazones are highly fatigue resistant
and exhibit good quantum yields.
Photoswitch performance can be assessed through the
following performance parameters: addressability, thermal
stability, efficiency, and reliability. The addressability relates to
the maximum absorption wavelength (λmax), the difference in
absorption wavelengths between the two different isomers
(Δλmax), and the molar absorptivity (extinction coefficient ε),
that is, how strong the photoswitch absorbs light of a given
wavelength. Efficiency depends on how many photons that are
absorbed by the photoswitch are used to induce isomerization
of the molecule (quantum yield; Φ) and how high the degree
of photoconversion is in the photostationary state (PSS).
Thermal stability dictates how long the unstable isomer lives
before reverting to the thermodynamically more stable isomer
INTRODUCTION
■
Molecular photoswitches are molecules that can reversibly be
converted from one state, or isomer, into another with light.
Therefore, the properties of these functional molecules, and
potentially the properties of the entire system they are
integrated in, can be changed at will. The spatial and temporal
precision that light offers is unmatched by any other external
stimulus and since it is additionally noninvasive and clean, light
and consequently photoswitches have attracted much attention
and seen much use over the last decades.1 Photochromic
compounds are most often based on light-induced isomer-
izations around double bonds or ring-opening and -closing
reactions. The ability to modulate a system’s intrinsic functions
at will, be it due to a change in conjugation, geometry, dipole
moment or another property, justifies the enormous amount of
effort that has been spent on the development of the field.
Consequently, photoswitches now provide an invaluable tool
for a large variety of applications in all fields of chemistry;
ranging from the interface with biology and biochemistry,2
supramolecular and organic chemistry,1,3 materials science,1,4 all
the way to the interface with physics.5 Over the last decades,
however, the assortment of photoswitches that are available has
not broadened significantly and the most widely used classes
remain stilbenes and azobenzenes,2a,6 diarylethenes,3b and
spiropyrans,7 each bearing intrinsic advantages and limitations.
at a given temperature and is measured in thermal half-life t1/2
,
which is the time it takes for half of the molecules to undergo
back-isomerization at a given temperature. Depending on the
thermal half-life of the less stable isomer, photoswitches are
typically classified into T-type or P-type, with the latter
exhibiting no (or extremely slow) thermal back isomerization.
Finally, reliability is given by the resistance of a molecule to
undergo deterioration through undesired side reactions (fatigue
resistance) and gives a measure for how often a photoswitch
can isomerize without significant degradation.
Received: September 9, 2015
© XXXX American Chemical Society
A
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On top of the aforementioned photoswitch-specific proper-
ties, more mundane features are additionally required for real-
world applications: easy, scalable, and modular synthesis,
chemical robustness, and solubility. Note that the rational
design of photoswitches is typically challenging and not
straightforward, resulting in lengthy syntheses of complicated
molecules whose desired properties can not be probed until the
final stage.
synthetically easily accessible from esters or acyl chlorides via
their hydrazide precursors.
The combination of remarkably facile preparation, chemical
robustness of precursors and product, as well as the modular
approach to the design of acylhydrazones, paves the way to a
novel platform of photoswitches with a plethora of tunable “on-
demand” properties (Figure 1). By systematically altering and
Inspired by the work of Lehn8 and Aprahamian,9 we focused
our attention on acylhydrazones. Although hydrazones are
structurally related to imines, they possess different features,
such as enhanced stability toward hydrolysis in aqueous media,
while still offering the possibility of constitutional adaptation via
exchange of dynamic covalent bonds.10 The hydrazone
structural motif is used in different fields of chemistry due to
its facile synthesis, high stability, and unique structural
properties.11 An interesting feature of this class of molecules
is the photochemical, or chemical, E−Z isomerization around
the imine-like CN double bond, changing the configuration
and consequently the steric and electronic relation between the
moieties on each side of the hydrazone core.8a The reverse
reaction can be both light-induced (P-type) or heat-induced
(T-type), following either an out-of-plane rotation or a nitrogen
inversion mechanism.8d,12 Acylhydrazones present a particularly
interesting scaffold, allowing more feasible photochemical
isomerization than hydrazones13 and acylhydrazone photo-
switches have proven invaluable in several intriguing
applications.8c,14 However, the systems reported in the
literature thus far provide only scattered indications of
photochromic properties of this class of compounds.11b
Therefore, we have undertaken a systematic structure−property
relationship study in order to identify the effects of substitution
of all four crucial positions of the key backbone fragment
(Scheme 1). A sizable library of a novel family of readily
Figure 1. Wide tunability of key properties of acylhydrazone
photoswitches through functionalization of the R1-R4-positions.
evaluating ample acylhydrazone photoswitches, we have
unravelled the connection between structure and switching
properties of this emerging family of photochromic compounds
and gained access to the rational design of on-demand feature-
specific photoswitches. As detailed in the following, we have
established guidelines for the custom design of a target
photoswitch, which is based on this potent class of photo-
chromic acylhydrazones and exhibits the specific properties
needed for the desired goal.
Scheme 1. General Structure and Isomerization of
Acylhydrazone Photoswitches
RESULTS AND DISCUSSION
■
Synthesis. Acylhydrazones can be readily prepared in
excellent yields by condensation of a given hydrazide and a
carbonyl compound under mild conditions. A wide variety of
hydrazides are commercially available at low cost, and the route
described in Scheme 2 provides access to an array of
acylhydrazone photoswitches in one or two steps, following a
fast and high yielding synthesis.
accessible acylhydrazone photoswitches was prepared and
crucial characteristics were studied for each of the compounds
as a function of systematic changes in their structure.
Scheme 2. Optimized Synthesis of Acylhydrazones
Isomerization of acylhydrazones induces a configurational
change around the CN double bond, similar to azobenzene-,
stilbene-, and (thio)indigo-derived photoswitches. Unlike in
azobenzenes,6a,15 however, the geometry of both (E)- and (Z)-
acylhydrazone isomers can be controlled, which can be a major
advantage and offers a higher degree of control over their
properties. While (thio)indigo-derivatives offer the same
advantage, they generally suffer from very poor solubility in
most solvents. For structurally related hydrazones, the
thermodynamically less-favored (Z)-isomer needs to be
kinetically stabilized, for example, by introducing an intra-
molecular H-bonding interaction that can occur in the (Z)-
isomer.11b In other words, to obtain appreciable amounts of
(Z)-hydrazone, some stabilization is required, imposing
substantial structural constraints on the design of these
molecules. Furthermore, in comparison, acylhydrazones are
more robust and resistant to oxidation, and in addition they are
If required, preparation of hydrazides from their correspond-
ing esters is straightforward and can be performed on a large
scale. Upon reaction with hydrazine monohydrate in ethanol,
esters can be converted into the corresponding pure hydrazides
after concentrating the reaction mixtures on a rotary
evaporator. For the condensation reaction between any given
hydrazide and carbonyl-containing partner of choice, the two
are mixed in a 1:1 ratio in ethanol. A catalytic amount of
trifluoroacetic acid is added to promote the reaction, and the
reaction mixture is heated (typically 10 min) until a clear
solution is obtained. When allowed to cool to room
B
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temperature, the acylhydrazone 1 precipitates from solution
and a simple filtration yields the highly pure photoswitch as the
(E)-isomer in excellent yields (>80%). The reactions are
typically performed on a gram scale, yielding several grams of
the desired photoswitch, and can easily be further scaled up.
Postmodification of the hydrazone backbone is readily
accomplished, and, for example, the amide N atom can be
methylated with methyl iodide in the presence of sodium
hydride. Acylhydrazones showed no sign of degradation after
storing them on the bench under ambient conditions for several
months, highlighting the robustness of this class of photo-
switches.
initial (E)-isomer, showing that both isomers can be addressed
orthogonally; that is, acylhydrazones have good addressability.
Clear isosbestic points were observed in both switching
directions: the most red-shifted (henceforth denoted as IP) at
336 nm (Figure 2d), suggesting clean conversion from the (E)-
to the (Z)-isomer upon irradiation.17 The thermodynamically
less stable isomer 2Z reverts to 2E over time and does not
exhibit perfect thermal stability and is therefore classified as a T-
type photoswitch with a relatively long half-life of the (Z)-
isomer of t1/2 = 145 min at 25 °C. Note that back-isomerization
from 2Z to 2E (and in fact any other T-type photoswitch
reported in this work) can by accelerated by irradiation with
light of the appropriate wavelength (Figure 2e,f).
Acylhydrazone Performance. We first examined the
spectral properties of the simplest acylhydrazone 2 (Figure
2a; R1 = R4 = phenyl, R2 = R3 = H) by UV/vis spectroscopy
Probing the efficiency of acylhydrazone photoswitches was
complicated by difficult chromatographic separation of isomers,
and quantitative evaluation of photostationary states and
consequently determination of quantum yields was only
possible for four acylhydrazones reported in this work. We
1
successfully employed H NMR measurements to determine
the PSS for a selection of photoswitches (Figure 3)17 and found
Figure 3. Example 1H NMR spectra for photostationary state
determination by integrating the amide N−H for 3 (R1 = R4 = 2-
pyridine). Top: 3E. Bottom: 3Z,PSS
.
Figure 2. UV/vis absorption spectra upon photoisomerization of
parent acylhydrazone photoswitch 2.17 (a) Chemical structure of 2.
(b) Absorbance followed at λmax(E) = 297 nm (black line) and λmax(Z)
= 388 nm (red line) over time. (c) Conversion of 2E (black line) in
MeCN (4.43 × 10−5 M) to 2Z,PSS (red line) followed over time under
constant irradiation (λirr = 297 nm, tirr = 11 min); IP at 336 nm. (d)
Expansion of (c) of λ = 325−425 nm. (e) Conversion of 2Z,PSS (black
line) to 2E,PSS in MeCN (green line, 2.57 × 10−5 M) followed in time
(λirr = 365 nm, tirr = 200 min). (f) Absorbance followed at λ = 297 nm
over time as in (e).
a moderate to good content (51−82%) of (Z)-isomer for the
investigated acylhydrazones: 3 (81% 3z), 7 (51% 7z), 9 (61%
9z), and 10 (82% 10z). The PSS is reached by irradiation at
λmax(E) for 5−120 min, depending on the photoswitch. Molar
absorptivities are in the order of ε = 15000−20000 M−1·cm−1 at
λ = 297 or 302 nm and quantum yields for E to Z isomerization
under UV-irradiation (λirr = 297 or 302 nm) of these
photoswitches were calculated to be in the range of ΦE→Z
=
0.3−0.4, as obtained from the initial slope of photoconversion
(see the Supporting Information for details), which is in good
agreement with literature values13 and places acylhydrazones
somewhere between azobenzenes and diarylethenes in terms of
efficiency.
(Figure 2). As expected, 2E displays an absorption band in the
UV region (λmax = 295 nm), which upon irradiation (λirr = 297
nm, tirr = 11 min) decreased while a new weak band at the blue
edge of the visible spectrum (λmax = 388 nm) developed
(Figure 2b−d), indicating the isomerization to 2Z. Much to our
delight, 2 displays a large (54 nm) difference in absorbance
maxima of the most bathochromically shifted band between the
(E)- and (Z)-isomers. Good band separation is an important
requirement for reversible activation of photoswitches;1,3e,16
however, it is not a trivial property to foresee or optimize.
Irradiation of the (Z)-isomer with light of a longer wavelength
(Figure 2e,f; λirr = 365 nm, tirr = 200 min) regenerated the
A further crucial performance parameter is reliability, which
describes the fatigue resistance of the photoswitch. While it is
generally high for azobenzenes, poor fatigue resistance can be a
problem for spiropyrans,18 while diarylethenes can undergo
irreversible oxidation or rearrangement side reactions that give
rise to undesired byproducts.19 We have recently reported
general strategies to overcome this important issue for
diarylethenes20 and were interested to study the intrinsic
fatigue resistance for acylhydrazone photoswitches (Figure 4).
Light was employed to induce the isomerization of T-type
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the wavelength of the most red-shifted absorbance maximum
(λmax) while maintaining or further improving the large
difference in absorbance maxima between the (E)- and (Z)-
isomers, defined as Δλmax = λmax(Z) − λmax(E). Note that
shifting the (Z)-isomer’s absorption bathochromically as
compared to the thermally more stable (E)-isomer leads to
so-called negative photochromism.22 At the same time, we
aimed to reliably influence their thermal stability, that is, the
absence or presence of a thermal back-reaction (P-type versus
T-type) and being able to obtain a variety of different thermal
half-lives for the T-type photoswitches.
Acyl Moiety (R1). The structure of the parent carboxylic
acid transfers to the acylhydrazone photoswitch as the R1-
substituent in the hydrazide half. Keeping the carbonyl half for
two subsets of switches constant (Scheme 3), several aromatic
Scheme 3. Acylhydrazones Discussed in Table 1
Figure 4. Fatigue resistance of T-type and P-type acylhydrazones. (a)
Chemical structure of 4 and 5. (b) UV/vis absorption spectra of 4 (T-
type) in MeCN (2.49 × 10−5 M) during repetitive switching cycles
consisting of alternating between λirr(E to Z) = 313 nm (tirr = 10 min)
and thermal back-isomerization (T = 25 °C, t = 40 min). (c)
Corresponding absorbance of 4E,PSS and 4Z,PSS over time as a function
of the number of switching cycles followed at λmax(E) = 310 nm. (d)
UV/vis absorption spectra of 5 (P-type) in MeCN (2.35 × 10−5 M)
during repetitive switching cycles consisting of alternating between
hydrazides were employed in the condensation reaction and the
photochromic products were investigated. The results for the
first subset, 3 and 6−10 (R2 = R3 = H, R4 = 2-pyridine), are
summarized in Table 1.
Thermal stabilities for 3 and 6−10 are very high,
λirr(E to Z) = 405 nm (tirr = 15 min) and λirr(Z to E) = 254 nm (tirr
=
independent of the R1-substituent and 3 and 6−10 are all P-
25 min). (e) Corresponding absorbance of 5E,PSS and 5Z,PSS over time
as a function of the number of switching cycles followed at λmax(E) =
390 nm. Black line = (E)-isomer, red line = (Z)-isomer.
Table 1. Photochromic Properties of Acylhydrazones
Derived from (R4=) 2-Pyridinecarboxaldehyde, as a
Function of Altering the Hydrazide R1-Moiety (R2 = R3 = H)
acylhydrazone 4E to 4Z (Figure 4b, black to red line; λirr = 313
nm, tirr = 10 min) after which thermal back-isomerization
(Figure 4b, red to black line; T = 25 °C, t = 40 min) fully
restored 4E. Repetition of these cycles (n = 12) while measuring
λmax(E) at 310 nm showed no detectable fatigue over time
(Figure 4c). Fatigue resistance was furthermore probed at 60
°C and no fatigue could be observed for over 300 cycles (see
the Supporting Information). For P-type 5, both E to Z and Z
to E isomerization was readily achieved by irradiation with light
of different wavelengths (Figure 4d; λirr(E to Z) = 405 nm, tirr =
15 min and λirr(Z to E) = 254 nm, tirr = 25 min). No fatigue was
observed for 5 during nine switching cycles (Figure 4e).
Similarly, irradiating samples for an extended duration of time
with a powerful 500 W Hg(Xe)-lamp (confirmed for up to 3 h
in MeCN) did not give rise to new products as evaluated by
UV/vis and 1H NMR spectroscopy, demonstrating that
acylhydrazones are highly reliable photoswitches, showing no
sign of side product formation upon irradiation.
After refinement of the highly efficient synthesis, we gained
access to a large collection of acylhydrazones and prepared over
40 derivatives (see the Supporting Information). The
hydrazone linkage divides the variable positions into two sets:
R1 and R2, stemming from the hydrazide moiety as well as R3
and R4, originating from the carbonyl unit (Scheme 1). In the
following, we discuss how derivatization of these photoswitches
and systematically introducing different substituents in the
various positions around the acylhydrazone core influences the
performance parameters and properties of this family of
photoswitches. We focused our attention on improving the
addressability of acylhydrazones by bathochromically shifting
a
b
c
For the (E)-isomer. Defined as Δλmax = λmax(Z) − λmax(E). At 25
°C; stable photoswitches were defined as those where no thermal Z−E
isomerization was observed for at least 30 min. Insufficient band
separation for back-isomerization from Z to E with light.
d
D
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type photoswitches (Table 1), showing no detectable thermal
back-reaction for at least 30 min at 25 °C. This can be readily
understood, as in the (Z)-isomer of these acylhydrazones, the
2-pyridine moiety is perfectly set up to engage in a six-
membered intramolecular H-bond with the amide N−H of the
hydrazide moiety.8a Interestingly, the addressability is barely
affected by introducing different R1-substituents as absorbance
maxima for the (E)-acylhydrazones are relatively unaffected,
except for strongly electron-donating 4-dimethylaminobenzene-
derived 8. Moreover, only a small difference between λmax(Z)
and λmax(E) is observed (13 nm between the two extremes: 7
and 10). The results of the second subset, 11−16 (Scheme 4;
R2 = R3 = H, R4 = 2-thiophene), where no intramolecular H-
bonding is possible, are summarized in Table 2.
pyridine analogues (Table 1; 3 and 6−10). More importantly,
the difference in average Δλmax is very large between the two
subsets, bearing identical hydrazide moieties, favoring the (R4=)
2-thiophene moiety. The effect of the R1-substituent within this
subset is, however, rather modest and the large difference in
addressability between the two subsets is due to the different
R4-moiety, which will be discussed hereafter.
Notably, band separation is not a trivial feature for
photoswitches that rely on isomerization around a double
bond for their switching and while the band separation for the
2-pyridine hydrazones is not satisfactory (Δλmax = 1−13 nm), it
is exceptionally large for the 2-thiophene hydrazones; Δλmax
=
76−91 nm (illustrative example shown in Figure 5). We
Scheme 4. Acylhydrazones Discussed in Table 2
Table 2. Photochromic Properties of Acylhydrazones
Derived from (R4=) 2-Thiophenecarboxaldehyde, as a
Function of Altering the Hydrazide R1-Moiety (R2 = R3 = H)
Figure 5. Example UV/vis absorption spectra of R4 = 2-thiophene
derived acylhydrazones. (a) Chemical structure of 14. (b) MeCN
solution of 14E,Z (4.06 × 10−5 M) as synthesized, before (black line)
and after (red line) irradiation (λirr = 313 nm, tirr = 10 min); IP at 351
nm. (c) Photochemical back-isomerization from 14Z,PSS (black line) to
14E,PSS (green line, λirr = 405 nm, tirr = 45 min), following (b).
hypothesize that both (E)- and (Z)-isomers of (R1 = ) 2-
pyridine-derived acylhydrazones are planar, causing a low
Δλmax, while for (R1=) 2-thiophene the (Z)-isomer is nonplanar
and hence Δλmax increases significantly. Furthermore, it was
observed that all thiophene-derived acylhydrazones show a
typical “fingerprint” triplet in the absorbance spectra of the (Z)-
isomer around λ = 350−425 nm. The presence of this
fingerprint is an indication of structural rigidity and originates
from vibronically split bands due to the thiophene C−C
vibrations.21
We observed that 14 is obtained from the condensation
reaction as a mixture of (E)- and (Z)-isomers (Figure 5b), but
can be completely converted to the (E)-isomer upon irradiation
(Figure 5b,c). A large difference between absorbance maxima is
a highly sought after feature for photoswitches and a critical
requirement for orthogonal isomerization of P-type photo-
switches with light.
a
b
c
Carbonyl Moiety (R4). The carbonyl moiety is introduced
in the acylhydrazone from the carbonyl condensation partner.
We investigated the use of various aldehydes, bearing different
aromatic moieties that transfer to the photoswitch as the R4-
substituent of the carbonyl half, while keeping the hydrazide
moiety constant (Scheme 5; R1 = 4-pyridine). This set of
For the (E)-isomer. Defined as Δλmax = λmax(Z) − λmax(E). At 25
°C; stable photoswitches were defined as those where no thermal Z−E
isomerization was observed for at least 30 min. Despite poor band
separation, the shoulder at λ = 265−281 nm allows for back-
isomerization from Z to E with light.
d
As for the previous subset (Table 1), independent of the R1-
Scheme 5. Acylhydrazones Discussed in Tables 3 and 4
substituent, 11−16 all switch upon irradiation and are either P-
type photoswitches, or T-type with a long-lived half-life (t1/2
>
5 h at 25 °C), displaying high thermal stability. However, the
addressability changes as the absorbance maxima for the (E)-
isomers are higher by ∼20 nm for the (R4=) 2-thiophene
derivatives (Table 2; 11−16) as compared to their (R4=) 2-
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compounds was divided into three subsets: the first having
increasingly large aromatic moieties (Table 3), the second
bearing substituents that influence the electronic properties,
and the third having different heteroaromatic substituents
(Table 4).
Table 4. Photochromic Properties of Acylhydrazones
Derived from (R1 = ) 4-Pyridinecarboxylic acid Hydrazide;
Scope of heteroaromatic carbonyl R4-moieties (R2 = R3 = H)
Table 3. Photochromic Properties of Acylhydrazones
Derived from (R1 = ) 4-Pyridinecarboxylic acid hydrazide, as
a Function of Increasing the Aromatic Moiety (R4) in the
Carbonyl Half (R2 = R3 = H)
a
b
c
For the (E)-isomer. Defined as Δλmax = λmax(Z) − λmax(E). At 25
°C; stable photoswitches were defined as those where no thermal Z−E
isomerization was observed for at least 30 min. Insufficient band
separation for back-isomerization from Z to E with light. The Z band
d
e
is weak, but sufficient for back-isomerization from Z to E with light.
f
Despite poor band separation, the shoulder at λ = 312−357 nm allows
for back-isomerization from Z to E with light.
a
b
c
For the (E)-isomer. Defined as Δλmax = λmax(Z) − λmax(E). At 25
°C; stable photoswitches were defined as those where no thermal Z−E
isomerization was observed for at least 30 min.
Aromatic moiety variation in the R4-position of the
acylhydrazone photoswitches has a tremendous effect on both
addressability and thermal stability as shown in Table 3.
Increasing the size of the aromatic moiety, that is, going from
phenyl to naphthyl to anthryl to pyrenyl, extends the
conjugated system and consequently increases λmax(E) sig-
nficantly, with a difference of over 100 nm between 17 (R4 =
phenyl) and 19 (R4 = pyrene). Following the same trend, Δλmax
changes notably as a function of the expanded aromatic system
and compound 17 (R4 = phenyl) displays large band separation
(Δλmax = 96 nm) between the (E)- and (Z)-isomer.
Interestingly, for 18, 5, and 19 (R4 = naphthalene, anthracene
and pyrene, respectively) the thermodynamically favored (E)-
isomer is bathochromically shifted compared to the (Z)-isomer
(Δλmax < 0), classifying 18, 5, and 19 as negative photochromic
systems (Figure 6).22
Anthracene is well-known to dimerize, but anthracene-
derived 5 shows the typical monomer absorbance pattern for
both isomers, indicating that switching of the acylhydrazone
occurs, rather than dimerization of the anthracene moieties.23
For 19 (R4 = pyrene), the trend continues as both 5 and 19
display negative photochromism with increasing band separa-
tions of up to -54 nm for 19. Moreover, the thermal half-life of
the (Z)-isomer also increases substantially, meaning that while
17 is a T-type photoswitch with one of the lowest thermal half-
lives (t1/2 = 4.10 min at 25 °C) in our present library, we could
not detect a thermal back-reaction for 5 (R4 = anthracene) and
19 (R4 = pyrene) at 25 °C. Compound 18 (R4 = naphthalene)
Figure 6. Example UV/vis absorption spectra of an acylhydrazone-
based negative photochromic system. (a) Chemical structure of 19.
(b) Solution of 19E in CHCl3 (2.92 × 10−5 M), before (black line) and
19Z,PSS after (red line) irradiation (λirr = 405 nm, tirr = 20 min); IP at
356 nm. (c) Photochemical back-isomerization from 19Z,PSS (black
line) to 19E,PSS (green line, λirr = 334 nm, tirr = 15 min), following (b).
shows intermediate properties, that is, λmax and t1/2 increase in
the following manner: phenyl < naphthyl < anthryl ≈ pyrenyl.
Furthermore, for Δλmax, we find phenyl > naphthyl > anthryl >
pyrenyl, demonstrating that switching characteristics can be
extrapolated from such structure−property relationships (with
due caution), further highlighting the value of our diversity-
oriented approach.
Electronic properties of azobenzenes have a large effect on
their photochromic behavior,6a,15,24 and hence, in analogy we
investigated the possibility to use substituents with different
electronic character as the carbonyl R4-moiety. For acyl-
hydrazones bearing either electron-withdrawing (20, R4 = 4-
nitrophenyl) or -donating (21 and 22, R4 = 4-dimethylamino-
F
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Journal of the American Chemical Society
Article
phenyl and 3,4,5-trimethoxyphenyl, respectively) moieties,
large band separations (Δλmax = 66−86 nm) between the
(E)- and (Z)-isomer were observed. Switching was not
inhibited in any way and 20−22 are all T-type photoswitches,
such as the parent (R4=) phenyl-compound 17, with t1/2 = 27−
47 min.
Heteroaromatic moieties are a common and sometimes even
crucial feature in molecular photoswitches1b,c and acyl-
hydrazones bearing different heteroaromatic rings in the
carbonyl R4-position were investigated next and the results
are summarized in Table 4. All investigated heteroaromatic R4-
substituents (R1 = 4-pyridine) show an absorbance maximum
in the UV region for the (E)-isomer (λmax = 292−330 nm).
Switching to the (Z)-isomer is possible and band separation
between both isomers is excellent, Δλmax = 80−109 nm for 14
(R4 = 2-thiophene), 27 (R4 = 2-furan) and 24 (R4 = 2-pyrrole).
However, for 23 (R4 = 4-Me-2-thiazole) and 9 (R4 = 2-
pyridine) no clear band separation was observed. Fortunately,
back-isomerization of 9Z is possible by irradiation at higher
wavelength (λirr = 334 nm), due to broadening of the
absorbance band at λ = 313−355 nm for the (Z)-isomer,
which holds true for many of the acylhydrazones that have
demonstrated poor absolute band separation.17 Acylhydrazones
23, 27, 24, and 9 show P-type photochromic behavior, while 14
Figure 7. Example UV/vis absorption spectra of R3 = CH3 derived
acylhydrazones, compared to R3 = H. (a) Chemical structure of 25 and
17. (b) Solution of 25E in MeCN (4.76 × 10−5 M), before (black line)
and 25Z,PSS after (red line) irradiation (λirr = 297 nm, tirr = 25 min). (c)
Solution of 17E in MeCN (7.02 × 10−5 M), before (black line) and
17Z,PSS after (red line) irradiation (λirr = 297 nm, tirr = 16 min); IP at
342 nm. (d) Difference absorption spectrum of 25Z,PSS-25E. (e)
Difference absorption spectrum of 17Z,PSS−17E; inset shows a new
band appearing for 17Z at λ = 340−420 nm.
exhibits T-type behavior with a relatively long half-life of t1/2
=
340 min at 25 °C. In general, introduction of a heteroaromatic
moiety in the R4-position affects the addressability, in particular
Δλmax, but has little effect on the thermal stability.
Aldehyde- or Ketone-Derived Acylhydrazones (R3).
Acylhydrazones can be readily prepared from aldehydes as well
as ketones. We expected that introducing a Me-substituent in
the R3-position would induce a change in the addressability of
acylhydrazones, due to the increased steric bulk around the
CN double bond. Using methyl ketones as the starting
materials in the condensation reaction under the exact same
conditions as for the aldehydes, compounds 25, 26, and 28 (R1
= 4-pyridine, R2 = H, R3 = CH3) were prepared in high to
excellent yields (60−89%) and their photochromic properties
were compared with their aldehyde counterparts (17, 27, and
14; R1 = 4-pyridine, R2 = R3 = H). The effects on addressability
and thermal stability follow the same trends and are examplified
by comparing ketone-derived 25 and aldehyde-derived 17
(Figure 7). A large decrease in Δλmax was observed for 25 (R4 =
phenyl) compared to 17, which is due to the appearance of a
new bathochromically shifted band for the (Z)-isomer of the
aldehyde-derived acylhydrazone 18 (Figure 7c,e) that is not
present for the (Z)-isomer of the sterically more hindered
ketone-derived analogue 25 (Figure 7b,d).17
Scheme 6. Acylhydrazones Discussed in Table 5
Comparing acylhydrazones with different hydrazide halves
(R1 = 4-pyridine, 2-thiophene, and 4-nitrobenzene, R2 = H),
while keeping the carbonyl half identical (R3 = CH3, R4 = 2-
pyridine), showed no significant effects on the photochromic
properties between ketone- (R3 = CH3) and aldehyde-derived
(R3 = H) acylhydrazones (Table 5) in any case, showing that
the R3-position can be functionalized without inherent loss of
photochemical properties and without affecting the changes
induced by R1, thus providing orthogonal handles for
optimization of the photoswitches.
N-Methylated Acylhydrazones and Steric Strain (R2
and R3). The ability to reversibly switch steric bulk between the
different isomers of molecular photoswitches is important in
supramolecular chemistry as well as the developing fields of
photoswitchable catalysis3b,f,4d and photopharmacology.1c,2c,d,25
Conceptually, many approaches followed in these fields exploit
such photoinduced reversible structural changes and hence
prompted us to investigate if the introduction of steric
interactions (1,3-allylic strain) between the R2- and R3-
positions would affect the switching capacity of acylhydrazones
(Scheme 7). Therefore, two series (R1 = phenyl or 2-
thiophene) of compounds were prepared, where (i) R2 = R3
= H (11 and 16), (ii) R2 = CH3, R3 = H (33 and 34), (iii) R2 =
H and R3 = CH3 (35 and 36), and (iv) R2 = R3 = CH3 (37 and
38). The results are summarized in Table 6.
Furthermore, addressability is generally affected to some
extent due to the fact that λmax(E) decreases slightly by
introduction of a Me-moiety in the R3-position. And while the
thermal stability of 25 (t1/2 = 16 min) and 17 (t1/2 = 4 min) is
in the same range, it is greatly affected for the other two
comparisons and t1/2 decreases by nearly an order of magnitude
for 28 (R3 = CH3, R4 = 2-thiophene) compared to 14 (R3 = H).
Similarly, while 26 (R3 = CH3, R4 = 2-furan) has a thermal half-
life of 129 min at 25 °C, 27 (R3 = H) is a P-type photoswitch
(see the Supporting Information).17
We subsequently kept the carbonyl half of the acylhydra-
zones constant (Scheme 6; R3 = CH3, R4 = 2-pyridine) and
evaluated the effect of R3 = CH3 compared to R3 = H for
different R1-moieties (Table 5, R2 = H).
Exchanging the (R2) N−H (11, 35, 16, and 36) functionality
for N−CH3 (33, 37, 34, and 38) causes broadening of all
G
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Table 5. Photochromic Properties of Acylhydrazones
Derived from 2-Acetylpyridine (R2 = H, R3 = CH3, R4 = 2-
pyridine) as a Function of Different (R1) Hydrazides,
Comparing R3 = CH3 (29−31) to R3 = H (9, 10, and 32)
Table 6. Photochromic Properties of Acylhydrazones
Derived from (R1=) Benzohydrazide (11, 33, 35, and 37) or
2-Thiophenecarboxylic Acid Hydrazide (16, 34, 36, and 38),
as a Function of Increasing Steric Bulk in the Hydrazide R2-
and Carbonyl R3-Positions
a
b
c
For the (E)-isomer. Defined as Δλmax = λmax(Z) − λmax(E). At 25
°C; stable photoswitches were defined as those where no thermal Z−E
isomerization was observed for at least 30 min. While an increase in
d
a
b
c
For the (E)-isomer. Defined as Δλmax = λmax(Z) − λmax(E). At 25
the band at λ = 268 nm is observed, the shoulder at λ = 315−360 nm
°C; stable photoswitches were defined as those where no thermal Z−E
isomerization was observed for at least 30 min. An increase at λ =
e
d
does not go back thermally. Insufficient band separation for back-
isomerization from Z to E with light.
226−287 nm allows for back-isomerization from Z to E with light (λirr
e
= 280 nm). The shoulder at λ = 276−304 nm allows for back-
Scheme 7. Acylhydrazones Discussed in Table 6
isomerization from Z to E with light (λirr = 297 nm).
that the long wavelength absorbance band is hypsochromically
shifted, leading to lower band separations. All acylhydrazones in
Table 6, bearing either one or two Me-moieties in the R2- and/
or R3-positions, can be reversibly switched between the (E)-
and (Z)-isomers and display clean photoreactions. This proves
that while both addressability and thermal stability may be
altered by introducing steric bulk or by removing the hydrazone
N−H functionality, switching is not inhibited in any case.
CONCLUSIONS
■
After discussing the correlations of all different substituents and
the photochemical properties of this new family of photo-
chromic molecules, and in order to visualize key performance
parameters, in particular λmax, t1/2, and Δλmax, a graphical,
multidimensional representation was chosen to encompass all
acylhydrazone photoswitches in the current library (Figure 8).
Spheres were colorized with a gradient from red to blue
according to their t1/2, and the absolute values of λmax (X-axis),
absorbance bands and increases the intensity of the band at
lower wavelength (λmax ≈ 260 nm), while the band at higher
wavelength (λmax ≈ 405 nm) decreases or disappears, leading to
a much lower Δλmax.
17 In addition, the thiophene fingerprint is
no longer observed, indicating a loss of rigidity.21 The thermal
half-life on the other hand, increases for R2 = CH3, compared to
R2 = H, and even changes T-type (35 and 36) photoswitches to
P-type (37 and 38). We attribute this effect to destabilization of
the planar (E)-isomer due to increased 1,3-allylic strain
resulting in a higher thermal barrier for the less affected
nonplanar (Z)-isomer. Introducing a Me-group in the R3-
position does not affect the spectral properties significantly
when R2 = H (35 and 36 compared to 11 and 16); however,
the thermal half-life decreases substantially. Comparing R3 = H
(33 and 34) to R3 = CH3 (37 and 38), while R2 = CH3, shows
t
1/2 (Y-axis), and Δλmax (Z-axis) are projected on the respective
axes in yellow, orange, and cyan colors. Figure 8 presents a
visualization of all acylhydrazones presented in this work and
shows that the current collection of acylhydrazone photo-
switches covers an extensive range of all parameters, displaying
a wide variation in band separation, thermal half-lives, and
absorbance maxima, thereby offering a novel, profoundly
tunable class of photoswitches.
H
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Journal of the American Chemical Society
Article
Present Address
‡P.K.: J. Heyrovsky Institute of Physical Chemistry, Academy of
Sciences of the Czech republic, Dolejskova 2155/3, 18223
̌
Prague 8, Czech republic.
Author Contributions
†D.J.v.D. and P.K. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. A. Goulet-Hanssens, M. Kathan, and S. Fredrich
for insightful comments and useful discussions. Generous
support by the European Research Council (ERC via ERC-
2012-STG_308117 “Light4Function”) is gratefully acknowl-
edged. BASF AG, Bayer Industry Services, and Sasol Germany
are thanked for generous donations of chemicals.
Figure 8. Graphical representation of the acylhydrazone library
presented in this work and their position in a 3D-plot with respect to
thermal half-life (t1/2 on X-axis, projection: yellow circles), absorption
maximum/excitation wavelength (λmax on Y-axis, projection: orange
circles), and band separation (Δλmax on Z-axis, projection: teal circles).
Spheres are colorized with a gradient according to their t1/2 (red to
blue).
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The Supporting Information is available free of charge on the
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Experimental details, 1H and 13C NMR spectra and UV/
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