Solution and Solid-State Emission Toggling of a Photochromic Hydrazone

Article abstract of DOI:10.1021/jacs.8b07108

The proliferation of light-activated switches in recent years has enabled their use in a broad range of applications encompassing an array of research fields and disciplines. All current systems, however, have limitations (e.g., from complicated synthesis

Full text of DOI:10.1021/jacs.8b07108

Communication
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Solution and Solid-State Emission Toggling of a Photochromic
Hydrazone
Baihao Shao, Massimo Baroncini,‡,§,# Hai Qian,
†
,#
†,#
Laura Bussotti,∥,⊥ Mariangela Di Donato,∥,⊥
,
‡,§
,†
Alberto Credi,*
and Ivan Aprahamian*
†
Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States
‡
Center for Light Activated Nanostructures (CLAN), Universita
01, 40129 Bologna, Italy
̀
di Bologna and Consiglio Nazionale delle Ricerche, via Gobetti
1
§
Dipartimento di Scienze e Tecnologie Agro-alimentari, Universita
̀
di Bologna, viale Fanin 50, 40127 Bologna, Italy
∥
LENS − European Laboratory for Non-linear Spectroscopy, via N. Carrara 1, 50019 Sesto Fiorentino (FI), Italy
⊥
INO − Istituto Nazionale di Ottica, Largo Enrico Fermi 6, 50125 Firenze, Italy
S Supporting Information
*
not all, these shortcomings is a sought-after goal. This necessity
explains the surge witnessed in the past decade in the
ABSTRACT: The proliferation of light-activated
switches in recent years has enabled their use in a broad
range of applications encompassing an array of research
fields and disciplines. All current systems, however, have
limitations (e.g., from complicated synthesis to incompat-
ibility in biologically relevant media and lack of switching
in the solid-state) that can stifle their real-life application.
Here we report on a system that packs most, if not all, the
desired, targeted and sought-after traits from photo-
chromic compounds (bistability, switching in various
media ranging from serum to solid-state, while exhibiting
ON/OFF fluorescence emission switching, and two-
photon assisted near-infrared light toggling) in an easily
accessible structure.
1
4
development of new photochromic compounds, with an
15
emphasis on visible-light activated ones.
As part of our effort to address the drawbacks associated
16
with photochromic compounds, we report here on an easy-
17
to-make, robust, bistable, hydrazone-based photochromic
compound (1, Figure 1a) whose completely new functions and
properties in organic solutions, which include fluorescence
ON/OFF switching under both 1-photon and 2-photon
excitation (i.e., near-infrared (NIR) light), are maintained in
serum, and are transferable to the solid-state. This compound
combines the most significant properties of widely used
photoswitches (i.e., the large geometrical changes of
1
hotochromic compounds play a key role in diverse areas
P
of science and technology as they enable the control over
2
(
bio)molecular processes, properties, assemblies, and func-
3
tions with the high temporal and spatial resolution of light.
These capabilities have allowed for their incorporation in
4
5
8
responsive materials and surfaces, catalysis and separation,
6
7
information storage, energy conversion, and drug design,
and have made them an integral part of the field of artificial
9
molecular switches and machines. Several prominent photo-
switches have been investigated throughout the years (e.g.,
1
0
11
12
13
azobenzenes, diarylethenes, spiropyrans, and fulgides )
and while great strides have been made in their development,
optimization, and use, a few drawbacks hinder their deploy-
ment in real-life applications. Depending on the system, these
shortcomings include complicated synthesis, low thermal and/
or photochemical stability, undesired spectral features (e.g.,
UV light activation), inefficient photoconversion, lack of solid-
state switching and/or fluorescence emission, small amplitude
geometrical changes, pH and/or oxygen sensitivity, reduction
by glutathione, and more commonly inadequate (if any)
switching in aqueous media.
Figure 1. (a) Light induced E/Z isomerization of hydrazone 1. (b)
−
5
Solid lines: UV−vis spectra (1.0 × 10 M) of 1-Z and 1-E isomers in
−6
toluene. Dashed lines: fluorescence emission spectra (5.0 × 10 M;
λex = 420 nm) of 1 in toluene, before (red) and after irradiation
(blue).
Considering the prevalent and prospective use of photo-
chromic compounds in a broad range of fields and disciplines,
the development of new systems that can overcome some, if
Received: July 10, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b07108
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
azobenzenes, the bistability and solid-state switching of
As part of our solvent screening studies (Figures S19, S38),
we found that 1 can undergo photoisomerization in fetal
bovine serum (FBS) buffer (Figure S19). A solution of 1 in
10% FBS buffer (PBS with 10% DMSO, pH = 7.4) has a λmax
at 395 nm, which shifts to λmax = 343 nm upon isomerization
18
diarylethenes, and the emission toggling of oxazines and
1
9
spiropyrans ) while circumventing their shortcomings (e.g.,
reduction with glutathione, complicated synthesis, low photo-
chemical stability, switching in water, etc.).
Hydrazone 1 was synthesized as described in Scheme S1 and
fully characterized using NMR spectroscopy, mass spectrom-
etry and X-ray crystallography (Figures S2, S3 and S47). An
equilibrated solution (toluene; under dark) of 1 shows an
absorption maximum (λmax) at 395 nm (Figure 1b and Table
with 442 nm light (PSS442 = 96% 1-E; Φ
= 18.9 ± 0.8%).
Z→E
These observations highlight the transformation of the initially
present 1-Z form into the E isomer. The back E → Z
isomerization process (PSS340 = 76% 1-Z; Φ
can be triggered by irradiating with 340 nm light, reverting the
system to its original state. More interestingly, the emission of
= 6.9 ± 0.3%)
E→Z
−
1
1
), with a molar absorption coefficient (ε) of 14 600 M ·
1
-Z in the serum buffer is maintained (τ = 0.59 ± 0.01 ns and
FL
Table 1. Photophysical and Photochemical Properties of 1
Φ
= 3.5 ± 0.4%; Figure S32) with a maximum emission band
at 525 nm (λ = 420 nm). This behavior is contrary to the
ex
Parameter
Toluene
395
343
Serum
395
343
Solid-state
diminished fluorescence intensity observed for 1-Z in polar
protic solvents and aqueous solutions (Figures S19 and S39),
most likely because 1-Z interacts with the protein components
λmax,abs, Z-isomer (nm)
λmax,abs, E-isomer (nm)
λmax,em, Z-isomer (nm)
395
336
560
24
525
525
of the FBS buffer (Figure S22). The switching in serum
prompted us to also test the sensitivity of 1 toward glutathione
reduction. We found that 1 and its switching process are not
affected by glutathione (Figure S23).
a
Φ , Z-isomer (%)
τem(ns)
0.7 ± 0.1
0.19 ± 0.01
>99:<1
82:18
32.0 ± 0.9
14.7 ± 1.0
3.5 ± 0.4
0.59 ± 0.01
96:4
27.5 ± 1.4
0.07 ± 0.01
>90:<10
70:30
FL
(
(
[Z]:[E])PSS (%)
[E]:[Z])PSS (%)
76:24
We were pleasantly surprised to find that 1 photoswitches in
drop-casted films as well, which is a rare property in molecular
b
ΦZ→E (%)
ΦE→Z (%)
18.9 ± 0.8
6.9 ± 0.3
−
−
b
11
25
switches in general, and configurational ones in particular.
c
d
b
τ1/2 (y)
75 ±
3
−
−
The absorption and emission spectra (λ = 380 nm) of 1-Z in
ex
a
b
c
Powder sample. Not determined (see the SI). Half life of the
thermal E → Z transformation. Cannot be measured accurately in
the solid film (Figure 2) are very similar to those recorded in
d
serum because of ester group hydrolysis at elevated temperatures.
−
1
cm . The absorption maximum of 1-Z is shifted bathochromi-
cally by 28 nm compared to a previously reported hydrazone
16c
lacking the NMe electron donating group (λ = 367 nm).
2
max
Irradiation of the solution of 1 with a 442 nm light source
induces a Z → E photoisomerization (photostationary state
(
PSS) of >99% E; Φ
= 32.0 ± 0.9%; Figure S14),
Z→E
accompanied by the appearance of a new absorption band at
λmax = 343 nm (ε = 13 800 M ·cm ), and a color change
from light yellow to colorless (Figure 1b). The reverse process
−
1
−1
(
(
E → Z) can be triggered by using a 340 nm light source
PSS340 = 82% 1-Z; Φ = 14.7 ± 1.0%; Figure S15) and the
E→Z
Figure 2. Absorption spectra (solid lines, left scale) and emission
system can be cycled between the two isomers by alternating
4
spectra (dashed lines, right scale; λ = 380 nm) of a drop-casted film
ex
20
42 and 340 nm irradiation (Figures S17 and S18). The
of 1. The red lines are of 1-Z, the blue lines correspond to PSS450
thermal E → Z back-isomerization half-life (τ1/2) at 298 K was
measured to be 75 ± 3 years (Figure S45).
(>90% 1-E), and the brown lines depict the bands at PSS360.
The introduction of the NMe group drastically enhances
2
the emission of the Z isomer compared to the parent system,
which is not emissive (Figure S36). Upon excitation with
solution (Figure 1b) indicating that the same isomerization
process is taking place. Irradiation of the film at 450 nm brings
about absorption spectral changes that are almost identical to
those observed in solution, indicating efficient Z → E
isomerization in the solid-state (PSS450 = >90% 1-E).
21
blue light (λ = 420 nm), the toluene solution of 1-Z exhibits
ex
an intense emission band (λ ) at 525 nm (Stokes shift of 130
em
nm; Figure 1b). The fluorescence lifetime (Figure S31) and
quantum yield of the process were determined to be 193 ± 2
ps and 0.7 ± 0.1%, respectively (Table 1). Though this
emission quantum yield is lower than in other switchable
Remarkably, the emission (τ = 0.07 ± 0.01 ns and Φ
=
FL
27.5 ± 1.4%; Figure S33) of the Z isomer is maintained in the
26
solid-state (i.e., there is no aggregation-caused quenching),
and so is the toggling between the ON/OFF states, which is
1
1,18,19
22
fluorophores,
it is still high enough for applications.
27
Switching 1-Z to 1-E using 442 nm light leads to quenching of
fluorescence. Irradiation of the E-dominant solution with 340
nm light leads to a Z-dominant solution, which turns the
emission ON again. Such an ON−OFF fluorescence response
in photochromic compounds, especially those that function
also highly uncommon. The characteristic emission of the Z
form is quenched at the photostationary state, confirming that
the 1-E form is nonemissive in the solid-state as well. We
hypothesize, based on an analysis of the single-crystal structure
of 1-Z (Figure S47) that the absence of π−π interactions is
responsible for the unusual emission observed in the solid-
state. Unfortunately, we could not grow suitable crystals of 1-E
for analysis.
2
3
upon configurational changes, is highly unusual. The
fluorescence modulation can be cycled 10 times with no
significant signs of photobleaching (Figure S34).
B
DOI: 10.1021/jacs.8b07108
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
No changes in the absorption spectrum of the 1-E film were
observed after one month at room temperature, proving that
the thermal E to Z back isomerization is exceedingly slow in
the solid-state. Exposing the 1-E enriched film to 360 nm light
causes spectral changes consistent with E → Z photo-
isomerization (PSS360 = 70% 1-Z; Figure 2). The initial
absorption and fluorescence spectra cannot be fully recovered
because of the extensive overlap between the absorption
spectra of the E and Z forms at 360 nm. Nevertheless, the
system displays high fatigue resistance, as its absorption and
emission characteristics can be reversibly switched multiple
times by alternate irradiation at 450 and 360 nm (Figure S25).
A decrease in the emission intensity is observed after the first
switching cycle, which stabilizes in subsequent cycles. This
effect might be attributed to an annealing effect (e.g.,
reorganization of the solid-state structure) that the film
undergoes after the first switching cycle.
We also investigated the two-photon absorption properties
of 1 in both solution and solid-state. Indeed, the relevant
excited-state manifestations of 1, namely luminescence and
isomerization, can be triggered by the absorption of two near-
infrared photons (i.e., 750 and 800 nm), similarly to what we
observed upon conventional one-photon excitation. This
property is also highly unusual, considering that most small
molecules have negligible 2-photon absorption, and that our
compound was not specifically designed for this purpose
(Figure 4, Figures S56−S59). The absorption cross-section of
To get a better insight into what is happening to the drop
casted film upon photoswitching, we imaged it with an
epifluorescence polarizing optical microscope. The film
exhibits a strong fluorescence emission when excited with
3
60 nm light (Figure 3a), and significant optical birefringence
Figure 4. Absorption spectral changes of a toluene solution of 1-Z
(
red line) upon irradiation at 800 nm for 3 h (purple line) and
successive rest in the dark for 3 d (blue line). Inset: emission spectra
of 1-Z in solution (red line) and in a solid film (green line) upon
excitation at 800 nm.
1
-Z in toluene at 800 nm was measured to be 13.9 GM, which
is comparable to other photoswitches and slightly lower than in
28
spiropyran.
Finally, to demonstrate how easily 1, and its solution and
solid-state properties, can be taken advantage of, and as a proof
of concept, we devised two simple experiments (Figures S49
and S54). In the first one, a blue laser pointer was used to
reversibly draw sustained structures in a toluene solution of 1
(Video S1). The other is a “sketch-and-etch” type application
where a laser pointer is used to write on a hydrazone covered
transparency slide (Video S3), while UV is used to erase the
writing.
Figure 3. Optical micrographs obtained with 360 nm excited
fluorescence (left), cross-polarized (central) and bright field (right)
light illumination. (a, b, c) The as-prepared film; (d, e, f) the film after
blue light irradiation (430−470 nm) in a central spot for 10 min; (g,
h, i) the film in (d, e, f) after near-UV light irradiation (330−380 nm)
in the same central spot for 10 min. Scale bar, 100 μm. The black
arrows in panels b, e and h represent the relative orientation of the
polarizer and analyzer.
In conclusion, we developed an easy-to-make photochromic
hydrazone that features configurational switching and fluo-
rescence toggling with one- or two-photon irradiation in
solution (organic and aqueous solutions including serum) and
solid-state; the toggled state is stable for years in the dark at rt.
All these properties are unprecedented for an individual
compound. The combination of the desirable characteristics of
several families of photochromic species in a single, easily
accessible system, will enable the development of new adaptive
functions and systems. For example, the simple molecular
structure of 1, which does not require additional structural
under cross-polarized light illumination (Figure 3b), confirm-
ing an ordered arrangement of the molecules in the solid that
gives rise to anisotropic crystals. Upon irradiation with high
intensity blue light in a central spot, the morphology of the film
changes (Figure 3) and both the optical birefringence (Figure
3
e) and fluorescence emission (Figure 3d) disappear in the
irradiated area. This observation suggests that isomerization
induces a phase transition leading to amorphization of the
material. The emission intensity and birefringence of the
material are partially restored upon irradiation with high
intensity near-UV light (Figure 3g-h). This result confirms the
reversible nature of the light-induced switching process and
rules out local thermal effects as the cause of the observed
phenomenon.
2
9
30
manipulation for it to be emissive or introduced to water,
31
can open the way for theranostics, where diagnostics (by
following the distribution and localization of the drug using
super-resolution fluorescence microscopy ) and therapy
3
2
33
(using photopharmacology ) are combined.
C
DOI: 10.1021/jacs.8b07108
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b07108.
■
*
S
Crystallographic data for 1-Z (CIF)
Crystallographic data for 2-Z (CIF)
Real-time writing in solution (AVI)
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AUTHOR INFORMATION
Corresponding Authors
ivan.aprahamian@dartmouth.edu
alberto.credi@unibo.it
ORCID
Hai Qian: 0000-0002-5304-132X
Mariangela Di Donato: 0000-0002-6596-7031
Alberto Credi: 0000-0003-2546-9801
Ivan Aprahamian: 0000-0003-2399-8208
Author Contributions
B.S., M.B. and H.Q. contributed equally.
Notes
■
(e) Hammerich, M.; Schu
Hoppner, R.; Herges, R. J. Am. Chem. Soc. 2016, 138, 13111−13114.
̈
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tt, C.; Stahler, C.; Lentes, P.; Rohricht, F.;
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*
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(g) Harris, J. D.; Moran, M. J.; Aprahamian, I. Proc. Natl. Acad. Sci. U.
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*
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H.; Pramanik, S.; Aprahamian, I. J. Am. Chem. Soc. 2017, 139, 9140−
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by the NSF DMR (DMR-1506170),
the European Research Council (grant agreement No.
̀
92981), and the Universita di Bologna. We gratefully
acknowledge Prof. Richard Staples (Michigan State University)
for the X-ray crystallography data.
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