Turn-off mode fluorescent norbornadiene-based photoswitches

Article abstract of DOI:10.1039/c8cp04329a

Single-molecule fluorescence emission of certain positive photochromic systems such as diarylethenes have been exploited for biological imaging and optical memory storage applications. However, there is a lack of understanding if negative photochromic sys

Full text of DOI:10.1039/c8cp04329a

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Turn-off mode fluorescent norbornadiene-based
photoswitches†
Cite this:DOI: 10.1039/c8cp04329a
Behabitu Ergette Tebikachew,
Fredrik Edhborg,
Nina Kann,
Bo Albinsson * and Kasper Moth-Poulsen
*
Single-molecule fluorescence emission of certain positive photochromic systems such as diarylethenes
have been exploited for biological imaging and optical memory storage applications. However, there is a
lack of understanding if negative photochromic systems can be used for such type of applications.
Hence, to explore the potential of negative photochromic molecules for possible optical memory
storage applications, we have here synthesized and studied a series of four norbornadiene–quadricy-
clane (NBD–QC) photoswitching molecules. These molecules feature either linearly conjugated or
cross-conjugated pi-electron systems. Upon photoisomerization, the UV-vis absorption spectra of the
molecules revealed a strong blue shift in the QC-form, with a photoisomerization quantum yield close
to 80% for the cross-conjugated systems. In contrast, a strong intrinsic emission (up to Ff = 49%) for
the linearly conjugated compounds in the NBD form was observed. Upon light-induced isomerization,
the emission was completely turned off in the QC-form in all the compounds studied. Further, the
robustness of the system was evaluated by performing several switching cycles. Under nitrogen,
the emission can be turned off and recovered with almost no loss of emission. We also show that the
QC-form can be photochemically triggered to convert back to the NBD-form using a low energy UV
light (340 nm), allowing an all optical conversion to both species. The demonstrated properties can
make the NBD–QC system attractive for potential applications such as optical memory storage devices.
Received 9th July 2018,
Accepted 15th August 2018
DOI: 10.1039/c8cp04329a
rsc.li/pccp
Some photochromic molecules are intrinsically fluorescent in
one form and non-fluorescent in the other isomer. Another way
Introduction
The emission of light from certain photoresponsive materials to impart fluorescence properties to a photochromic molecule is
upon photoexcitation has intrigued scientists in the past and by attaching a fluorophore.9–11 In the latter case, the absorption
led to a variety of applications.1,2 More recently, however, the of one of the isomers of the photochromic unit needs to be
ability to modulate the emission of the emitting species has matched to the emission of the fluorophore so that efficient
drawn significant attention, as it lends itself for applications quenching occurs. Upon light-triggered photoisomerization, the
ranging from biological imaging to ultra-high density optical fluorescence can be switched back on. This light induced
memories.4 In this context, photochromic molecules present modulation of the emissive properties of a photochromic molecule
an attractive opportunity since they can be tailored towards a between the fluorescent and the non-fluorescent form has been
specific application.
successfully used in high-resolution fluorescence imaging
Photochromic molecules are molecules that change their techniques12–15 as well as ultrahigh density optical memories.16–18
colour reversibly upon irradiation with light. This colour change For a fluorescent photochromic molecule to be used for such types
arises from a change in the structural and stereoelectronic of applications, certain requirements need to be met. Two of the
arrangement in the molecule. Besides the colour change, a most critical ones are fatigue resistance and high fluorescence
change in absorption,5 electrical conductance4,6 and dipole quantum yield.4,5
moment7,8 of the photochromic molecule can be manifested.
Various fluorescent photochromic molecules have been reported
in the literature,4,7,19–23 where the most prominent so far has been
the diarylethene (DAE)-based systems. Since most DAEs are not
intrinsically fluorescent, a fluorescent unit such as anthracene,24
organic dyes25 or even nanoparticles9 have been attached to the DAE
photochromic unit. Some DAE derivatives, e.g. benzothiophenes,
are fluorescent in themselves; however, they suffer from poor
fluorescence quantum yields. Recently, certain highly fluorescent
Department of Chemistry and Chemical Engineering, Chalmers University of
Technology, SE-41296 Gothenburg, Sweden. E-mail: balb@chalmers.se,
mkasper@chalmers.se
† Electronic supplementary information (ESI) available: Synthetic procedures,
1H, 13C and COSY-NMR spectra, UV-Vis absorption, kinetics and Arrhenius
analysis. Photoisomerization quantum yield, fluorescence lifetime measurements
and TDDFT calculations data. See DOI: 10.1039/c8cp04329a
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featuring a high fluorescence quantum yield in the NBD-form, a
virtually non-emissive QC-form and excellent fatigue resistance.
Furthermore, we demonstrate fluorescence modulation through
multiple emission switching cycles by photoisomerization from
NBD to QC and thermally activated back-conversion. Moreover,
we also show the NBD to QC and QC to NBD switching, using
relatively low energy UV (340 nm) sources, making this an
entirely photoswitching system.
Scheme 1 Examples of fluorescent DAE-based molecules.22
Experimental details
General procedures
DAE derivatives with a fluorescent quantum yield close to 90%
were prepared by oxidizing the sulphur on the thiophene unit of
the DAE to a sulphur-S,S-dioxide (Scheme 1).26 However, the
photostability of these dioxides is reported to be lower than the
unoxidized counterparts.23
All reagents and HPLC grade solvents were obtained from
commercial sources and used as received. Dry solvents (toluene
and tetrahydrofuran) were obtained from MBraun MB SPS-800
solvent purification system. All reactions were performed in
oven-dried flasks under positive nitrogen pressure unless stated
otherwise. Flash chromatography was conducted using a Biotage
Isolerat Spektra One flash chromatography system. Thin-layer
chromatography (TLC) was performed on pre-coated aluminum
plates with Merck Silica gel 60 F254 and the spots were visualized
by using 254 and 365 nm handheld UV lamp. Infrared measure-
ments were carried out using Perkin Elmer Frontier instruments
provided with an ATR module.
Another photochromic molecule that has seen a renewed
interest recently due to possible applications in solar energy
storage27–30 and data storage3 is the norbornadiene–quadricy-
clane (NBD–QC) system (Scheme 2). The NBD–QC pair is a
T-type5 negative photochrome, where the unsaturated diene,
i.e. the NBD-form, converts to the saturated QC-form upon
photoirradiation. When desired, the QC-form can be triggered
to relax back to the NBD-form thermally,27 electrochemically,30
by catalytic activation31 or even using light.32–34 This class of
photochromic molecules has seen a recent rise in interest,
since a relatively low energy photon is used to initiate the
photoisomerization of the parent molecule to its isomer compared
to positive photochromes.35 This is particularly attractive for
biological applications as it is less damaging to cells and tissues.
There are few reported examples of fluorescent NBD–QC
photochromic systems. Maafi et al. described one of the earliest
examples of the fluorescence properties of norbornadiene
derivatives containing an ester unit (Scheme 2).36 Interestingly,
upon photoisomerization to its QC-form, the emission intensity
increased. The lower emission in the NBD-form was attributed
to intramolecular self-quenching of the phenyl ester fluorophore
emission by the NBD unit. A similar behaviour was observed in a
polyphenylene polymer containing a pendant norbornadiene
moiety.37 Babudri et al. also reported a conjugated polymer
containing an NBD backbone structure with a fluorescent
quantum yield of 24%.38 However, long exposure to UV irradiation
resulted in a significant decrease in the fluorescence quantum yield.
In the synthesis of various 2,3-disubstituted norbornadiene–
quadricyclane (NBD–QC) photochromic systems, we have observed
fluorescence in certain NBD-derivatives.3 Here, we report unprece-
dented emission properties for this class of photochromic systems,
Proton (1H), carbon (13C) and correlation spectroscopy (COSY)
nuclear magnetic resonance (NMR) were recorded on an auto-
mated Agilent (Varian) MR 400 (400 MHz) spectrometer equipped
with a ‘‘One-probe’’. Chemical shifts are given in parts per
million (ppm) downfield from tetramethylsilane referring to
1
the residual proton signal (CHCl3: 7.26 ppm in H-NMR) and
carbon signal (CDCl3: 77.0 ppm in 13C-NMR) in the NMR
solvent. Data are presented as chemical shift, multiplicity (s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet),
coupling constants in Hertz (Hz) and integration. Absorption
spectroscopy and kinetics experiments were carried out using a
Cary 50, Cary 100 or Cary 4000 UV-vis spectrophotometer
provided with a water heating system. Fluorescence emission
spectra were recorded on a Spex Fluorolog 3 spectrofluorimeter
from JY Horiba. Photoswitching experiments were performed
using Thorlabs LED lamps 340 nm (M340L4), 365 nm (M365F1)
and 405 nm (M405LP1). Photoisomerization quantum yields
were measured according to a published literature method39
using a concentrated sample (absorbance 4 2) at room temperature.
The details are provided in Section V of the ESI.† High resolution
mass (HRMS) was obtained using Agilent 1290 Infinity LC system
tandem to an Agilent 6520 Accurate Mass Q-TOF LC/MS with an
APCI source in a positive mode. Elemental analyses were performed
at Mikrolab Kolbe.
The fluorescence quantum yield (FF) was measured using
the procedure described in literature40 with Coumarin 102 in
ethanol (FF = 0.80) as reference. The concentration of the
sample and reference sample was chosen so that both had
the same absorption at the excitation wavelength. Excited state
lifetime measurements were carried out using time correlated
single photo counting (TCSPC) with a 377 nm excitation source
Scheme 2 Examples of fluorescent NBD–QC systems.37
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from PicoQuant. An MCP-PMT detector was used, with 2048 The synthesis of 3 and 4 was achieved by first preparing 5 and
channels and 10 000 counts in the top channel. The decay 6 from the corresponding commercially available bromoiodo-
curves were fitted to mono- or bi-exponentials decays by decon- benzenes via a double Sonogashira reaction. Deprotection of the
volution with the instrument response function. Vertical trimethylsilyl groups furnished the dialkynylated compounds 7
excitation energies and oscillator strengths were calculated by and 8 in excellent yields (490%). Subsequent Sonogashira
employing time dependent DFT (TDDFT) on DFT//B3LYP/6- reaction of 7 and 8 with 11 led to the triisopropyl silyl (TIPS)
31G** optimized structures.
protected intermediates 9 and 10. Ultimately, deprotection of the
TIPS groups using tributylammonium fluoride (TBAF), followed by
a Sonogashira coupling with 4-iodophenyl thioacetate, yielded 3
General procedure for the synthesis of the NBD compounds
In an oven-dried Schlenk flask (25 mL) provided with magnetic and 4 in approximately 20 and 40% yield, respectively. It is worth
stirring bar, were added Pd(PPh3)4 (5 mol%), CuI (10 mol%), noting that the synthesis of 4 has been described previously.3 In
2,3-dibromonorbornadiene (1.2 mmol, 1 eq.) and toluene addition, all four compounds were characterized by elemental
(10 mL). The solution was purged with nitrogen for 15 minutes. analysis, HRMS, IR and NMR (see ESI†).
Then, diisopropylamine (1 mL) and the alkyne (2.6 mmol,
Steady state absorption spectra
2.1 eq.) were added, at which point the colour of the solution
turned from yellow to dark red. The mixture was reacted until
the starting materials were consumed as indicated by TLC. The
solution was then diluted with dichloromethane and filtered
through a short silica gel plug. The volatiles were then removed
using a rotary evaporator. The crude product obtained was
submitted to automated flash chromatography using an eluent
gradient of 0–5% dichloromethane in hexane. The product was
obtained after the solvent was removed in vacuo. (For the detailed
synthetic procedure and characterization, see the ESI†).
The UV-vis absorption spectra of 1–4 in the NBD-form and
QC-form were measured in toluene, see Fig. 1. For compounds
1-NBD and 4-NBD, that have the longest linearly conjugated
systems, redshifted absorption spectra were observed in comparison
to the respective cross-conjugated systems 2-NBD and 3-NBD. There
is approximately a 50 nm difference between the onset of absorption
(defined as log(e) = 2) of the most redshifted spectrum of 4-NBD
compared to that of 2-NBD, see Table 1. The absorption spectrum of
4-NBD is not only redshifted, but also has the highest molar
absorptivity of the four compounds studied (see ESI,† Fig. S13).
The molar absorption coefficient (e) (43 000 MÀ1 cmÀ1) of 4-NBD
at the first maximum (lmax1 = 391 nm) is about two times higher
than the other three compounds and comparable to some of the
strongest diarylethene absorbers reported in the literature.26
Results and discussion
Synthesis
The synthesis of the NBD-derivatives, symmetrically substituted
with a thiophenyl moiety as well as a thioacetate-terminated
bis(ethynylphenylene), are shown in Scheme 3. The synthesis
Kinetics and photoisomerization quantum yields (UNBD–QC
)
employed 2,3-dibromonorbornadiene (11) as the starting material, Irradiating compounds 1-NBD and 4-NBD with 405 nm light, and
which in turn, was synthesized from bicyclo[2.2.1]hepta-2,5- 2-NBD and 3-NBD with 365 nm light, converts the molecules 1–4
diene (2,5-norbornadiene) according to a literature procedure.3 to their respective QC-forms with near quantitative conversion, as
Compounds 1 and 2 were obtained in good yields (ca. 60%) via shown in Fig. 1. Moreover, no photostationary state was observed
a Sonogashira reaction of 2- or 3-ethynylthiophene with 11. in any of the compounds studied, due to the strong blueshift in
Scheme 3 Synthetic routes to the norbornadiene derivatives 1–4 starting from bicyclo[2.2.1]hepta-2,5-diene (11): (i) 2-ethynyl thiophene (for 1) or
3-ethynyl thiophene (for 2), Pd(PPh3)Cl2, CuI, DIPA, 35 1C; (ii) (a) triisopropylsilyl acetylene, Pd(PPh3)2Cl2, CuI, DIPA, r.t., (b) trimethylsilyl acetylene, 60 1C,
12 h; (iii) K2CO3, MeOH:DCM, r.t.; (iv) 11, Pd(PPh3)2Cl2, CuI, toluene, Et3N, 35 1C; (v) (a) TBAF, 0 1C to r.t., (b) 4-(iodophenyl)thioacetate, Pd(PPh3)4, CuI,
toluene, 50 1C.
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Fig. 1 Normalized steady state absorption spectra (NBD- and QC-forms) of 1, 2, 3 and 4 and the corresponding emission spectra (lex = 364, 362, 330
and 407 nm) in toluene.
Table 1 Summarized photophysical properties of 1–4 in toluene
Compound lmax1 (nm) (e (MÀ1 cmÀ1)); lonset (nm) t1/2,251C (min) Ea (kJ molÀ1
)
FNBD-QC
FF
tF (ns) kF (sÀ1
)
knr (sÀ1
)
i
a
b
c
d
e
f
g
h
1
2
3
381 (1.8 Â 104); 443
357 (1.8 Â 104); 415
364 (1.5 Â 104)
49
201
130
93.8
103.7
135.6
0.30
0.54
0.77
0.04
0.21
0.002 0.02
1.8 Â 108 4.6 Â 109
1.0 Â 108 5.0 Â 1010
4.3 Â 108 5.0 Â 1010
3.9 Â 108 3.9 Â 108
0.01
0.02
301 (5.0 Â 104); 424
391 (4.3 Â 104)
4
78.6
100.7
0.15
0.49
1.28
332 (8.3 Â 104); 462
a
b
The first absorption maximum of NBD-forms. Absorption onset of the NBD-form is defined as the wavelength at which the molar absorptivity is
c
d
log(e) E 2. Half-life of the thermal back reaction QC - NBD-forms at 25 1C. Activation energy for the thermal back reaction of QC - NBD-
forms. Quantum yield for the photoisomerization of NBD-form to QC-form. The values given are the average of two measurements (see ESI
e
f
Section IV). Fluorescence quantum yield of NBD-forms. The values given are the average of three measurements (see ESI Section V).
g
h
i
Fluorescence lifetime of NBD-forms. Radiative rate constant. Nonradiative rate constant.
Steady state fluorescence spectra, fluorescence quantum yields
and lifetimes
absorption onset of the QC-form relative to the NBD-form. Since
the NBD–QC pair is a thermal photoswitch system, the thermal
recovery of the QC-forms of 1–4 was also measured (see ESI,† The emission spectra of 1–4 were recorded in toluene and the
Section III). The corresponding Arrhenius parameters and half- spectra are shown in Fig. 1. All four compounds show emission
lives are presented in Table 1. All four compounds have relatively upon photoexcitation in the NBD-form. The fluorescence quantum
short half-lives (ca. 1 h to 3 h at 25 1C) as a result of the low yields of the four compounds in toluene were measured using
average activation energy (Ea) (ca. 100 kJ molÀ1). In general, the Coumarin 102 as reference (FF = 0.80) and the results are shown in
linearly conjugated compounds 1-QC and 4-QC have a lower Table 1. Out of the four compounds, 4-NBD has the highest
Ea than the cross-conjugated compounds 2-QC and 3-QC, hence fluorescence quantum yield of 49%, followed by 1-NBD of 4%.
they have shorter half-lives. The NBD–QC photoisomerization This can be compared to the much lower fluorescence quantum
quantum yields (FNBD-QC) of 1–4 were measured using a yield of 1% and 0.2% for the respective cross conjugated
Ferrioxalate actinometer41 (see ESI,† Section IV). The results compounds 3-NBD and 2-NBD.
are presented in Table 1. The cross-conjugated compounds
To fully understand the underlying cause for such a high
2-NBD and 3-NBD are found to have higher photoisomerization variation in the fluorescence quantum yield, the excited state
quantum yields in comparison to the corresponding linearly lifetimes of 1–4-NBD were measured using time-correlated
conjugated compounds 1-NBD and 4-NBD, respectively. In single photon counting (TCSPC). The fluorescence decay curves
particular, 3-NBD stands out with a photoisomerization quantum were fitted to a mono- or bi-exponential function, but only the
yield of around 80%.
major time constant with the highest amplitude was considered
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(for detailed information see ESI,† Section VI). The result of the radiative rate of fluorescence presented in Table 1. Generally,
measured lifetimes, tF, and calculated radiative rate constants, the results from emission measurements and TDDFT calculations
kF, are summarized in Table 1. Previously, it has been reported show that the difference in fluorescence quantum yields of the
that compounds with longer linear conjugation are considered to four compounds are due to the large differences in the non-
exhibit stronger fluorescence.42 As expected, the longest linearly radiative decay mechanisms (Table 1).
conjugated compound 4-NBD was found to have the highest
Upon photoisomerization to the QC-form, the emission
fluorescence as well as a longer-lived excited state (1.28 ns), which intensity for compounds 1–4 decreases to essentially zero,
is six times longer compared to compound 1-NBD, and more contrary to those reported by Maafi et al.36,37 A plausible
than 50 times longer than the excited state lifetime of compound explanations for this observed difference is that the emission,
2-NBD and 3-NBD. Moreover, since substituted NBDs have higher in the case of Maafi et al., is due to the aromatic esters, and the
propensity to undergo photoisomerization to the QC-form on the fact that NBDs are known quenchers.45 Nevertheless, upon
singlet surface upon direct excitation,43 it is worth noting that photoisomerization, NBDs are progressively converted to QCs,
the photoisomerization and fluorescence quantum yields of thus diminishing the quenching process. In our case, we did
compounds 1–4 are complementary, similar to Stilbenes,44 i.e. not observe an increase in emission. Hence, the fluorescence
higher FNBD-QC results in lower FF and vice versa.
can be inferred to arise from the entire conjugated molecule. In
The result of the measured lifetimes, tF, and calculated addition, during the photoisomerization from NBD to QC, the
radiative (kF) and non-radiative rate constants (knr) are presented conjugation becomes interrupted in compounds 1–4 at half-
in Table 1. The fluorescence rate constants, kF = FF/tF, of the way, since the double bond becomes a single bond. This process
related NBD-derivatives 1 and 2, and 3 and 4 are nearly similar. turns off the emission in the QC-form, even for the longer
However, the non-radiative rate constants (knr) differ by an order conjugated molecules 3-QC and 4-QC. In fact, no emission was
of magnitude between the linearly conjugated compounds 4-NBD measured in the QC-form from any of the four compounds
and 1-NBD. This difference is about two orders of magnitude studied.
between compound 4-NBD and the cross-conjugated compounds
Once the QC-form is formed, it has several possible mechanistic
2-NBD and 3-NBD. Time dependent density functional theory pathways to revert back to the NBD-form,30–32 where one of the most
(TDDFT) was used to calculate the excited state energies as well as common routes is by thermal activation.27 When the QC-form is
the oscillator strength of transition from ground state to the first heated, it relaxes back to the NBD-form. The thermal recovery of
singlet excited state for compounds 1–4 in the NBD form, see fluorescence, going from 4-QC to the corresponding 4-NBD, was
ESI† for details. The calculated singlet excited state energies recorded at 50 1C under nitrogen and the result is shown in Fig. 2a.
are consistent with the trend in the lower energy lmax for A quantitative recovery of the NBD-form emission was observed at
the fully conjugated compounds compared to the respective this temperature.
cross-conjugated compounds, as observed in the absorption
Once modulation of fluorescence was realized upon light-
measurements. Also, the calculated oscillator strength shows activated photoisomerization of the NBD-form form to the
that all four compounds have strong and allowed transition QC-form, the fluorescence fatigue resistance was tested by carrying
from ground state to the first singlet excited state. The oscillator out multiple photoisomerization-thermal recovery steps, using 4 as
strength is similar for compounds 1-NBD and 2-NBD, and a model compound. The fluorescence measurements for the
slightly higher for 3-NBD and 4-NBD, which is in line with the NBD-form and QC-form were carried out at 6 1C to slow down
Fig. 2 (a) Fluorescence recovery of 4 upon thermal relaxation of the QC-form to NBD-form at 50 1C in toluene, and (b) cyclability of the modulation of
emissive property of 4 in toluene at 6 1C. The fluorescence switching ratio between the NBD-form and QC-form is about 100.
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the thermal back-conversion of the QC-isomer since the half- the respective molecules was found to play a significant role,
life of 4-QC at 25 1C is less than 2 h (Table 1, entry 4). However, particularly for compounds 3-NBD and 4-NBD.
at 6 1C, it is well over 8 h, as obtained by extrapolating the data,
In contrast, none of the four compounds show any emission
which was optimal to significantly reduce residual 4-NBD from the QC isomer. Emission spectra recorded from the
emission for the 4-QC isomer. In between the measurements, samples after photoisomerization only shows a weak emission
the cuvette was kept for 30 minutes at 50 1C in a heating profile, which resembles the emission spectrum of the NBD-
chamber to thermally recover the NBD-form. About 7 cycles isomer. This emission is therefore assigned to residual NBD-
were measured with full recovery of fluorescence of 4-NBD form of the molecules that did not photoisomerize. From Fig. 1
(Fig. 2b). The emission recovery pattern is found to follow a it is clear that all four compounds have a wavelength region
similar trend as the cyclability of photoisomerization of 4.3 This where the NBD isomer has a high absorptivity. However, the
means that under nitrogen, compound 4-NBD has virtually absorptivity of the respective QC isomer is essentially zero at
no loss of emission. In air, some minor degradation (about
l
max1. This means that all four compounds 1–4 in principle
0.2% per cycle) was observed for compound 4.3
could be photoisomerized 100% from the NBD to the QC form
Furthermore, since heat-induced back-isomerization takes without any photostationary states. The only limiting factor for
a few hours at 25 1C, we investigated light-induced back complete photoisomerization is the thermally activated back-
isomerization of 4-QC to 4-NBD using a 340 nm LED lamp isomerization from QC to NBD. For compound 4-QC, the half-
(Fig. 3). Gratifyingly, the NBD-form was formed readily in less life for the thermal relaxation ranges from about 80 minutes at
than a minute. This is an important feature that can be used to 25 1C to less than 6 minutes at 50 1C. Hence, for short-term
interconvert the two isomers in short period of time. Although memory storage applications, compound 4 could be a promising
light-induced QC to NBD back-isomerization has been reported photoswitch and its fluorescence emission, which can also be
previously,32–34 our system utilizes a relatively low energy near-UV turned off to essentially zero emission by photoisomerization,
light. This makes our system attractive for possible biological can be used as a readout. Moreover, we have also demonstrated
applications.
that it is possible to back-isomerize the QC-form to the NBD-form
In summary, photoisomerization quantum yields of the linearly instantaneously using light, making it an entirely light-activated
conjugated 1-NBD and 4-NBD have relatively large values photoswitch. Thus, the remarkable stability and high fatigue
compared to the cross conjugated forms 2-NBD and 3-NBD. In resistance of compound 4 demonstrated through a cyclability
particular, the marked difference between 3-NBD (FNBD-QC
=
test (Fig. 2b), particularly under nitrogen, makes it an appealing
77%) and 4-NBD (FNBD-QC = 15%) is worth noting. Moreover, photochromic system for optical memory storage applications.
compounds 1–4 are fluorescent in the NBD-form, with quantum
yields ranging from o1% to 49% with 4-NBD having the highest
value. The photoisomerization and fluorescence quantum yields
Conclusions
are found to be complementary. Furthermore, the effect of
We have synthesized and studied four fatigue resistant NBD/
p-conjugation due to meta/para-arrangement of the bonds in
QC-based negative photochromic systems with conjugated and
cross-conjugated electronic arrangements, and investigated
their emission properties. The conjugated derivatives are found
to be more emissive compared to the cross conjugated analogues.
In particular, the linearly conjugated NBD-form of compound 4 is
found to show a remarkable fluorescence quantum yield (49%).
This is the highest value for an NBD-based photochromic system
that has been reported so far. In contrast, the cross-conjugated
analogues show a high photoisomerization quantum yield (about
80%). We have demonstrated multiple alternating photoactiva-
tion and thermal relaxations compound 4 to turn off and recover
its emission in the NBD-form. Moreover, in addition to heat, low
energy UV (340 nm) light was found to induce the back isomer-
ization of QC-form to the NBD-form in less than a minute,
making it a fast-responsive and all-light operated photoswitch
system. In particular, compound 4, with an all-light activated
switching, remarkable fatigue resistance, and high emission, can
potentially be used as a vital component in optical memory
storages and imaging applications with a fluorescence readout.
Fig. 3 Dual light switching of compound 4. Upon irradiation with a
405 nm LED lamp, the NBD form (green) converts to the QC form (orange)
in about 60 s. Similarly, the QC form converts to the NBD form readily
upon irradiation with a 340 nm LED lamp while change in absorbance was
Conflicts of interest
probed at 430 nm. The rate of degradation per cycle is faster than for the
previously reported photo-thermal cycle.3
There are no conflicts to declare.
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20 T. Fukaminato, T. Kawai, S. Kobatake and M. Irie, J. Phys.
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21 H. Qian, B. H. Shao and I. Aprahamian, Tetrahedron, 2017,
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Acknowledgements
BET and KMP acknowledge funding from European Research
Council (ERC-StG #37221, SIMONE). NK acknowledges funding
from the Swedish Research Council (grant no. 2015-05360). BA
and FE acknowledge funding from Swedish Energy Agency.
23 Y.-C. Jeong, S. I. Yang, E. Kim and K.-H. Ahn, Tetrahedron,
2006, 62, 5855–5861.
24 T. Kawai, T. Sasaki and M. Irie, Chem. Commun., 2001, 711–712.
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