Stepwise structural and fluorescent colour conversion in rhodamine analogues based on light and acid stimulations

Article abstract of DOI:10.1039/c9tc05054j

The research and development of multi-stimuli, multi-responsive molecules have attracted considerable attention in chemistry, biology, and material science. Herein, we propose a multi-stimuli-responsive multi-fluorescence system in a single molecule. This system is based on the isomerization that involves ring-opening/closing reactions of spirolactones of rhodamine analogues (ABPXs), developed by our group, which can be independently controlled by light and chemical stimuli. UV light irradiation opens one of the spirolactones to give thermally stable coloured isomer (Z) in solution. Detailed synthetic and theoretical investigations reveal that the ring-opening reaction of ABPXs proceeds via the formation of a photo-induced charge separated state, followed by the recombination of the biradicals. Furthermore, we explore the structure-kinetic relationships and demonstrate that the introduction of electron-donating substituents into the xanthene ring can tune the lifetime of the photo-generated isomer. Chemical stimulation by an acid further promotes the ring-opening reaction to give a red-shifted isomer (D). These light and chemical input signals can be converted into output signals of distinct colour and fluorescence. The multi-stimuli-responsive multi-colour fluorescence could be distinctively expressed through the combinatorial logic gate.

Full text of DOI:10.1039/c9tc05054j

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Materials Chemistry C
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PAPER
Stepwise structural and fluorescent colour
conversion in rhodamine analogues based
on light and acid stimulations†
Cite this: J. Mater. Chem. C, 2020,
8, 543
ab
Masaru Tanioka,
Daisuke Sawada
‡
*
Shinichiro Kamino, ‡*ab Natsumi Kogaa and
ab
The research and development of multi-stimuli, multi-responsive molecules have attracted considerable
attention in chemistry, biology, and material science. Herein, we propose a multi-stimuli-responsive multi-
fluorescence system in a single molecule. This system is based on the isomerization that involves ring-opening/
closing reactions of spirolactones of rhodamine analogues (ABPXs), developed by our group, which can be
independently controlled by light and chemical stimuli. UV light irradiation opens one of the spirolactones to
give thermally stable coloured isomer (Z) in solution. Detailed synthetic and theoretical investigations reveal that
the ring-opening reaction of ABPXs proceeds via the formation of a photo-induced charge separated state,
followed by the recombination of the biradicals. Furthermore, we explore the structure–kinetic relationships
and demonstrate that the introduction of electron-donating substituents into the xanthene ring can tune
the lifetime of the photo-generated isomer. Chemical stimulation by an acid further promotes the ring-
opening reaction to give a red-shifted isomer (D). These light and chemical input signals can be converted
into output signals of distinct colour and fluorescence. The multi-stimuli-responsive multi-colour
fluorescence could be distinctively expressed through the combinatorial logic gate.
Received 13th September 2019,
Accepted 27th November 2019
DOI: 10.1039/c9tc05054j
rsc.li/materials-c
be manipulated by more than two types of external stimuli in a
single molecule is very challenging. A representative work was
Introduction
Stimuli-responsive organic molecular systems play a significant pioneered by Feringa and co-workers in 2006 (Fig. 1a),25 and such
role in fundamental research and practical applications. Conven- ‘‘smart molecules’’ have witnessed rapid development over the past
tional stimuli-responsive molecular systems can be reversibly decades.26–34 A promising design strategy to construct this mole-
interconverted between two states by the action of a physical or cular system is the combination of different stimulus-responsive
chemical stimulus such as light,1–6 temperature,7 chemical,8–11 molecules connected through covalent linker groups or assembled
electric field,12,13 or mechanical force.14–18 These switching mole- within polymers.35 For example, Andreasson et al. developed multi-
´
cules exhibit different structures and physicochemical properties
in response to external stimuli.
Multi-stimuli-responsive molecular systems have potential
applications in future technologies including chemical logic
gates,19 molecular robotics,20 drug delivery,21,22 and pharmaco-
therapy.23,24 However, the development of organic molecules
existing in three or more thermodynamically stable states that can
a Graduate School of Medicine, Dentistry and Pharmaceutical Sciences,
Okayama University, Okayama-shi, Okayama 700-8530, Japan.
E-mail: skamino@okayama-u.ac.jp, dsawada@okayama-u.ac.jp
b Advanced Elements Chemistry Laboratory, RIKEN-CPR, Hirosawa Wako-shi,
Saitama 351-0198, Japan
† Electronic supplementary information (ESI) available. CCDC 1879130 and
1879131. For ESI and crystallographic data in CIF or other electronic format
Fig. 1 Schematic representation of multi-stimuli responsive molecular
systems. (a) Bis-tricyclic aromatic enylidenes (BEAs), (b) fulgimide (FG)-
dithienylethene (DTE).
see DOI: 10.1039/c9tc05054j
‡ Present address: Aichi Gakuin University, School of Pharmacy, Kusumoto-cho,
Chikusa-ku, Nagoya, 464-8650, Japan.
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Fig. 2 (a) Structural conversion of rhodamines in response to external stimuli
(top) and example of stimuli-responsive rhodamines in previous studies (bottom).
(b) Stepwise structural conversions of ABPXs in response to external stimuli.
Fig. 3 (a) Photo-switching and thermal reversion of spirolactone form (1)
to zwitterionic form (1Z). (b) Absorption and (c) fluorescence spectra 40 mM
of 1 in CHCl3 before (black) and after (red) UV irradiation. The excitation
responsive molecules using different stimuli responses of diary- wavelength is 305 nm. Insets show the photographs of 1 in CHCl3 before
lethene (DTE) and fulgimide (FG) (Fig. 1b).36 Despite the successful
and after UV irradiation.
use of this design concept, the selective manipulation of coexisting
molecular switch units using different types of external stimuli in a
properties of 1 in CHCl3 by UV/Vis absorption and fluorescence
spectroscopy. All measurements were performed at 293 K. The
colourless closed form of 1 showed an absorption band in the
UV region. Irradiation in the UV region (lirr = 305 nm, tirr = 60 min,
xenon lamp) caused a bathochromic shift of absorption band,
and a new absorption band appeared in the visible region
(labs = 510 nm), as shown in Fig. 3b. A clear isosbestic point
was observed at 338 nm. Upon UV light irradiation, the solution
of 1 gradually changed from colourless to orange, as evident to
the naked eye. The vibronic absorption spectrum of the orange-
coloured species was almost identical to that of the mono-
cationic species of 1 (see Fig. S1, ESI†). On the other hand, no
colour change was observed when 1 was dissolved in CHCl3
previously irradiated with UV light (Fig. S2, ESI†). Upon warm-
ing the solution of 1, the absorption at 500 nm decreased and a
half-life of 9.1 h (k = 0.076 hÀ1) at 298 K (Fig. S3 and kinetics,
ESI†) was obtained, indicating that the orange-coloured species
is reconverted to 1. In addition, 1 showed yellow fluorescence at
around 580 nm upon UV irradiation (Fig. 3c).
single molecule is still limited.
Rhodamines and rhodamine analogues are utilized in many
research fields because of their excellent photo-physical and
chemical properties.37–41 Until now, several rhodamine dyes that
respond to light,42–44 acid,45 redox,46 and temperature47 through a
spiro-ring opening/closing reaction have been developed and are
widely used in fluorescence imaging probe,48–54 chemical
sensor,55–57 molecular memory,58 and ink technology (Fig. 2a).
More recently, photo-chromic rhodamines have served as fluores-
cence markers for super-resolution microscopy (photoactivated
localization microscopy: PALM) that relies on single-molecule
photo-switching.59–61 It has been proposed that the spiro-ring
opening reaction occurs, which can lead to the formation of polar
nature of Z.44,62 However, the photo-isomerization process of
rhodamines has not been clearly elucidated. In this work, we
demonstrated the stepwise structural conversion in the rhodamine
analogues, namely, aminobenzopyranxanthenes (ABPXs), based
on light and chemical stimuli (Fig. 2b). This molecular system is
based on the isomerization that involves the ring-opening/closing
reaction of spirolactones of ABPXs.63,64 During our study on the
stimuli-responsive mechanisms of ABPXs (1), we found that a
physical stimulus, such as light irradiation, caused the selective
opening of one of the two spirolactones. The detailed photo-
isomerization process of ABPX was investigated by isolating the
photo-reaction intermediate. Upon the successive addition of an
acid and/or heating, reversible interconversion could be precisely
controlled (Fig. 2b). This multi-stimuli-responsive multi-
fluorescence is expressed in a combinatorial logic gate.
Photochemistry
These spectral and colour features indicate that 1 isomerizes to
a zwitterionic form (1Z) by cleavage of one of the C–O bonds in
the two spirolactones. Ma and co-workers suggested that 1Z was
produced by the photo-induced ring-opening reaction.65 How-
ever, the identification of 1Z was just investigated by the
changes of absorption/fluorescence spectra and IR spectra
accompanying UV light irradiation. Hence, there is insufficient
evidence for the identification of 1Z. Moreover, the elucidation
of the photo-chemical reaction mechanism is necessary for the
construction of a stimuli-responsive molecular system.
Results and discussion
Absorption and fluorescence spectra
To obtain a strong support for the formation of 1Z, we
attempted to trap and characterize the zwitterion adducts from
To elucidate the photochemistry of the ring-opening/closing the reaction of 1 with trimethylsilyl cyanide (TMSCN) in argon-
reaction of spirolactones, we initially examined the spectral purged CHCl3 under UV irradiation with reference to the literature.66
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geometry and electrostatic potential (ESP) distribution of 1 are
shown in Fig. 4. The two phthalide moieties of 1 were ortho-
gonally arranged relative to the xanthene moiety in the S0 state,
which is in good accordance with the results of previous X-ray
crystallographic analysis (Fig. S4 and S5, ESI†). The charge
separation (CS) between the phthalide moiety and the diethyl
aniline moiety in the S1 state was larger than that in the case of
the planar xanthene structure in the S0 state, which might be
due to the asymmetric distorted xanthene structure. This type
of photo-induced CS state is often observed in the spirolactone
form of triarylmethanes or rhodamines with nearly perpendi-
cular donor–acceptor orientation.70,71 These calculations indi-
cated that the charge-separated structure (1CS*) is formed in the
excited state.
Scheme 1 Trapping and characterization of (a) photo-generated product
(2) by TMSCN, and (b) X-ray crystal structure of 2. ORTEP view of 2 was
shown with thermal ellipsoids shown at the 50% probability level.
Mechanistic investigations were further performed for a
better understanding of the photo-chemical reaction of 1. The
1H NMR spectrum of 1 in argon-purged CDCl3 was recorded
As expected, the monocyano adduct (2) was obtained in 15% yield, after UV irradiation at room temperature. The 1H NMR spectrum
while a substantial amount of 1 was recovered (Scheme 1a). In showed severe line broadening with prolonged light irradiation
contrast to UV irradiation, trace amounts of 2 were trapped without and the solution of 1 gave no resonance signals corresponding
UV irradiation under dark conditions. These results indicated the to the xanthene protons (Hc–Hg) after 60 min of irradiation
photo-induced spirolactone-ring opening of 1, followed by isomer- (Fig. S6, ESI†).
ization to 1Z. The crystal structure of 2 was established by single-
From the 1H NMR spectral behaviour, it is assumed that the
crystal X-ray diffraction analysis. The xanthene moiety of 2 adopted a photoisomerization from 1 to 1Z proceeds via the formation of
nonplanar distorted conformation. A pair of phenyl rings of 2 was a radical intermediate. Based on the hypothesis, the EPR
in the trans configuration with respect to the xanthene moiety spectrum of 1 was measured in argon-purged CHCl3 after UV
(Scheme 1b). Thus, the CNÀ addition to 1 caused the inversion of light irradiation at room temperature, but no EPR signals were
a phenyl ring.
observed. This result indicated that the radical intermediate
The difference between the ground- and excited-state dipole is unstable at room temperature. Therefore, we isolated
moments of 1 was then estimated by using Lippert–Mataga and the isopropyl-cyano-radical-trapped adduct 3 from the photo-
Bilot–Kawski equations and Reichardt’s ENT solvent polarity chemical reaction of 1 with azobisisobutyronitrile (AIBN), a
parameter (Fig. S9–S11, and Table S1, ESI†).67–69 These calculated typical radical initiator, in argon-purged 1,2-dichloroethane
dipole moments in the excited state were significantly larger than (DCE) at room temperature (Scheme 2a). By contrast, any cyano
those of the ground state (Fig. 4). Density functional theory (DFT) isopropyl radical adducts were not obtained upon heating.
calculations (CAM-B3LYP/6-31G**) were then carried out to corro- Further structural characterization of 3 by X-ray crystallo-
borate the experimental results and examine the excited electronic graphic analysis revealed that the radical derived from the
structure. Indeed, the large difference in dipole moment between
the S0 and S1 states (Dm = 9.53 D) was in good agreement with the
experimental results (DmAVE = 10.82 D), implying that the excited
state is more polarized than the ground state. The optimized
Fig. 4 DFT calculations of 1 in the S0 and S1 states at the CAM-B3LYP/6-
31G** level. Left panels: Electrostatic potential (ESP) distribution. Middle
panels: Optimized geometry. Right panels: Ground- and excited-state
dipole moments.
Scheme 2 Trapping and characterization of (a) photo-generated product
(3) by AIBN, and (b) X-ray crystal structure of 3. ORTEP view of 3 was shown
with thermal ellipsoids shown at the 50% probability level. (c) Photo-
generated products (4 and 5) by styrene.
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Fig. 5 Plausible mechanism underlying the photo-chemical reaction of 1.
CS: charge separation.
C–O bond cleavage of the spirolactone in 1 attacks the isopropyl
cyano radical. It is noteworthy that UV absorption of 1 in
degassed styrene solution led to a radical reaction that gave
styrene-inserted compounds 4 and 5 (stereochemistries were
not determined), supporting the formation of a bi-radical
intermediate (Scheme 2b).
From the experimental and computational studies, a plausible
mechanism for the photo-chemical reaction of 1 is proposed, as
illustrated in Fig. 5. Upon the absorption of UV light, intra-
molecular charge transfer from the diethyl aniline moiety to the
phthalide moiety in 1 occurs to form 1CS* in the excited state.
Then, 1CS* is rapidly converted to bi-radical species 6 after C–O
bond cleavage in one of the spirolactone-rings. This selective
photo-chromic reaction (opening of one spirolactone) might be
triggered from the asymmetrically distorted structure of 1CS*.
Compound 6 subsequently undergoes recombination of the
bi-radicals to afford the coloured 1Z in the ground state. Alter-
nately, homolytic C–O bond cleavage is likely to occur, to give
bi-radical species 7, indicating that the photo-chemical reaction of
1 proceeds via several radical pathways.
Fig. 6 (a) Photo-switching and thermal reversion of ABPXs 8, 9, 11, and 14
with different electron-donating amino substituents. Hammett s+ constants
of electron-donating substituents proposed by Mayer and co-workers are
described.72 (b) Synthesis of 11 and 14. Reagents and conditions: (i) phthalic
anhydride, toluene, reflux, 24 h. (ii) resorcinol, CH3SO3H, 110 1C, 6 h.
Structure–kinetic relationship
We next synthesized ABPX derivatives 8, 9, 11, and 14 with
different electron-donating amino substituents, in agreement
with the Hammett s+ constants, as proposed by Mayr and
co-workers,72 and explored the structure–kinetic relationship of
the photo-generated zwitterionic isomer (Fig. 6a). Compounds 8
and 9 were synthesized according to the previous research
Fig. 7 Correlation between the first-order kinetic rate constant (k313K) and
Hammett constants s+ of different electron-donating amino substituents
for ABPXs. The coloured characters show the half-lives of the ABPXs.
methods.73 Compounds 11 and 14 could be synthesized from parameters obtained from the Eyring plots and the s+ value
the corresponding benzophenone derivatives (Fig. 6b). The closed (Table S4, ESI†). From these results, it is concluded that the
forms of these derivatives were isomerized into the open forms 8Z, introduction of an electron-donating amino substituent enhances
9Z, 11Z, and 14Z in CHCl3 under continuous UV light irradiation the stability of the xanthylium cation and tunes the lifetime of the
(Fig. S14, ESI†). The absorbance of the open forms decayed in ring-opened state. As opposed to other reported zwitterionic form
CHCl3 in the temperature range 298 to 313 K over 8 h (Fig. S15, of rhodamines, the ABPX family has the advantage of photo-
ESI†). The thermal bleaching of these isomers followed first-order chromic features with a long lifetime (t1/2 = 4.1–8.6 h).
kinetics (Fig. S16 and Table S3, ESI†). The reaction rate constant
(k) obtained from the first-order time plots was small, as s+ had a
Molecular logic gate
large negative value. An excellent linear relationship was obtained Based on the result of photo-reaction of ABPXs, the acid
in the single regression of k313K for the value of s+ (Fig. 7). activated switching was further studied by UV/Vis spectroscopy.
However, there was no correlation between the activation For this purpose, CHCl3/MeOH solvent mixtures were employed
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Fig. 8 (a) Schematic illustration and (b) fluorescence spectra of three states (1, 1Z and 1D). The excitation wavelength is 365 nm. (c) Truth table and
(d) logic gate diagram for multi-stimuli responsive multi-colour fluorescence system.
because it was found that the photo-isomerization from 1 to 1Z Chemical stimulation by an acid allows the photo-generated
was accelerated by the addition of small amounts of MeOH to form to be further isomerized to the thermally stable form. The
CHCl3 solution (Fig. S7, ESI†). Upon light irradiation, photo- solution changes from colourless to orange and then to purple.
switching of 1 was consistently observed in 1.0% MeOH/CHCl3 On the other hand, the photo-responsive and chemically
solution to generate the orange-coloured 1Z. To our delight, the responsive coloured species can be reverted to the closed
addition of trifluoroacetic acid (TFA) caused a further batho- isomer form by a thermal back-reaction. It is noteworthy that
chromic shift of absorption band, and an absorption band three states of the multi-stimuli responsive multi-colour fluores-
appeared at approximately 550 nm and 590 nm, which was cence switching could be expressed through the combinatorial
attributed to the dicationic species (1D). The solution drastically logic gate. We believe that the multi-stimuli responsive molecular
changed from orange to purple in chemical-stimulated inter- system developed in this study offers a new perspective for various
conversions (Fig. S8, ESI†).
technologies applying molecular switching.
We finally applied this unique multi-stimuli responsive
multi-colour fluorescence system to the combinational logic gates.
This molecular logic gate consists of two physical/chemical inputs Conflicts of interest
and three different fluorescent outputs (Output 1, 2 and 3 for
emissions at 515 nm, 580 nm, and 630 nm, respectively) and
There were no conflicts to declare.
functions without complicated arithmetic operations (Fig. 8). The
physical and chemical inputs were defined as UV light irradiation
(305 nm) and 1.0 eq. of TFA in 1.0% MeOH/CHCl3 solution.
Acknowledgements
Naked-eye observation of 1 showed that the initial state (0, 0) was
This work was partially supported by Grant-in-Aid for KAKENHI
blue-green in fluorescence colour (Output 1) via a NOR logic gate.
As it can be seen from the truth table, this blue-green fluorescent
solution was changed to yellow fluorescent solution (Output 2)
upon UV irradiation (1, 0). The photo-input followed by chemical-
input operations (1, 1) further caused the fluorescence colour
change from yellow to orange (Output 3).
C (17K08211), and KAKENHI S (17H06173), and JSPS Research
Fellow (16J06559) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of the Japan Society
for the Promotion of Science (JSPS). We also thank Dr Tomo-
nori Fukuchi of RIKEN-BDR for useful discussions.
Notes and references
Conclusions
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