Design, synthesis and photoswitching of broad-spectrum fluorescent hexaarylbiimidazoles

Article abstract of DOI:10.1039/c4ra10451j

A series of novel hexaarylbiimidazole (HABI)-containing fluorophores with broad-spectrum emissions ranging from blue, green and red emissions, have been designed and synthesized. Their photochromism and fluorescence switching properties are investigated by UV-Vis absorption and fluorescence spectra in solutions and PMMA films. By connecting carbazole (Cz), naphthalimide (NI) and perylene-mono-imide (PMI) to HABI with a spacer, three molecular photoswitches with broad-spectrum fluorescence varying from deep blue (Cz-O-HABI, λ_em = 390 nm, Φ_F = 0.27), green (NI-O-HABI, λ_em = 490 nm, Φ_F = 0.25) to far red (PMI-O-HABI, λ_em = 650 nm, Φ_F = 0.25) have been obtained. The "ON"/"OFF" ratios are calculated to be about 8.9, 8.0 and 3.7 (calculated from FL spectra) for Cz-O-HABI, NI-O-HABI and PMI-O-HABI, respectively. Such a series of photoswitchable fluorophores constitute prototypes of a photochromic molecular library in which fluorescence resonance energy transfer can be reversibly modulated by an external stimulus, namely, UV light.

Full text of DOI:10.1039/c4ra10451j

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Design, synthesis and photoswitching of broad-
spectrum fluorescent hexaarylbiimidazoles†
Cite this: RSC Adv., 2014, 4, 64371
*
Wen-Liang Gong, Zu-Jing Xiong, Chong Li and Ming-Qiang Zhu
A series of novel hexaarylbiimidazole (HABI)-containing fluorophores with broad-spectrum emissions
ranging from blue, green and red emissions, have been designed and synthesized. Their photochromism
and fluorescence switching properties are investigated by UV-Vis absorption and fluorescence spectra in
solutions and PMMA films. By connecting carbazole (Cz), naphthalimide (NI) and perylene-mono-imide
(PMI) to HABI with a spacer, three molecular photoswitches with broad-spectrum fluorescence varying
from deep blue (Cz–O–HABI, lem ¼ 390 nm, FF ¼ 0.27), green (NI–O–HABI, lem ¼ 490 nm, FF ¼ 0.25)
to far red (PMI–O–HABI, lem ¼ 650 nm, FF ¼ 0.25) have been obtained. The “ON”/“OFF” ratios are
calculated to be about 8.9, 8.0 and 3.7 (calculated from FL spectra) for Cz–O–HABI, NI–O–HABI and
PMI–O–HABI, respectively. Such a series of photoswitchable fluorophores constitute prototypes of a
photochromic molecular library in which fluorescence resonance energy transfer can be reversibly
modulated by an external stimulus, namely, UV light.
Received 15th September 2014
Accepted 19th November 2014
DOI: 10.1039/c4ra10451j
www.rsc.org/advances
has attracted much attention since it emerged at the end of the
20th and the beginning of the 21st century. Proper photo-
Introduction
switches could be realized by integrating a selected uo-
rophore to a photochromic component within a common
covalent skeleton.8–10 Accordingly, numerous examples of uo-
rophore–photochrome dyads have already been demon-
strated,11–15 and their operating principles for uorescence
modulation have recently been extended to nano-structured
constructs2,16–19 and extensively reviewed.15 Under these condi-
tions, the reversible interconversion of photochrome between
the colorless and colored states results in the modulation of the
uorescence intensity by electron or energy transfer. Photo-
switches were originally considered for optical data storage,20
and they have also found diverse applications in super-
resolution uorescence microscopy.21,22
The application in super-resolution imaging generally relies
on the reversible transformation between the uorescent “ON”
and “OFF” states upon photo excitation of the photoswitches.
We have demonstrated that both polymer nanoparticles
(SP–PCL) and small molecule (DTE–TPE) could be applied to
super-resolution uorescence imaging.23 And as a ground-
breaking attempt of HABI in super-resolution imaging, we have
also designed and synthesized tetraphenylethene–HABI conju-
gate (TPE–HABI).24 It has been found that TPE–HABI with
photoswitchable aggregation-induced emission (AIE) properties
is suitable for super-resolution imaging in condensed state.
Thus, designing uorescent HABIs with high “ON/OFF” ratio
and fast photobleaching rates might open the door for HABI in
super resolution imaging application. Abe and coworkers
have recently reported the investigation of a uorescent HABI-
based photoswitch using a conjugated [2.2]-paracyclophane
Photochromic materials are a well-known class of molecules
that change their color upon UV light irradiation. Of them,
azobenzene,1 spiropyran,2 and diarythene3 are the three mostly
investigated photochromic materials. Hexaarylbiimidazole
(HABI) was rst discovered in the 1960s by Hayashi and Maede
through oxidizing 2,4,5-triphenylimidazole in a potassium
hydroxide aqueous solution.4 2,4,5-triphenylimidazole has the
common name lophine, hexaarylbiimidazoles (HABIs) and
2,4,5-triarylimidazolyl radicals are sometimes called lophine
dimers and lophyl radicals, respectively. Lophine is also known
as the rst chemiluminescent materials discovered by Radzis-
zewski in 1877.5 Usually, HABIs are colorless in solution and
aer UV light irradiation the dimer homolytical cleavage to
form a pair of colored 2,4,5-triarylimidazolyl radicals. Interest-
ingly, HABI is also sensitive to pressure and thermal stimuli,
which makes it different from usual photochromic materials.4,6
The uorescence technique has been widely applied to
numerous elds like optical materials, analytical chemistry,
protein conformation studies to biological assays by taking
advantage of its exquisite sensitivity, cost-effectiveness, facile
operation, and super-spatial and temporal resolutions.7 And the
photo-switching emission phenomenon, which could control
uorescence upon irradiation of appropriate wavelength light,
Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic
Information, Huazhong University of Science and Technology, Wuhan, Hubei,
430074, China. E-mail: mqzhu@hust.edu.cn; Fax: +86-027-8779341
† Electronic supplementary information (ESI) available: Details on the materials
for synthesis, instrumentation for analysis and spectral characterizations. See
DOI: 10.1039/c4ra10451j
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HABI-uorescein molecule.25 And we also reported a uorescent this way, the full color: blue, green and red uorescent HABIs
HABI-based photochrome, in which a highly uorescent naph- are designed and synthesized.
thalimide (NI) derivative is connected to HABI in a non-
The synthetic routes of HABIs including C8H17–O–HABI, Cz–
conjugated fashion.26 However, both of the two systems are O–HABI and PMI–O–HABI are shown in Scheme 2. The
limited to green emissive HABIs. In order to achieve a broad- synthetic procedures and characterization information like
spectrum emission with blue, green and red wavelength, a H NMR, MS are presented in ESI† about 3 (NI–O–HABI) could be
molecular design strategy toward emissive HABIs with high found in our former work.26
quantum yield, high “ON/OFF” ratio and good fatigue resis-
tance is demonstrated through isolating the uorophore and
HABI with a spacer.
In this paper, we choose carbazole, naphthalimide and
HABI in benzene as the model compound of Cz–O–HABI, NI–O–
perylene-mono-imide as the blue, green and red uorophores
and connect them to HABI with a spacer, which are Cz–O–HABI,
NI–O–HABI and PMI–O–HABI. Their pronounced photochro-
mism properties are investigated by UV-Vis absorption and
uorescence spectrometer. The as-synthesized photoswitchable
Photochromism and uorescence switching
We rst investigate the photochromism properties of C8H17–O–
HABI and PMI–O–HABI, which is slightly different from HABI
because of the electron-donating nature of alkoxy group. As we
can see from Fig. 1, C8H17–O–HABI shows a peak around 290–
300 nm (this could be seen either in another solvent instead of
benzene). Aer 5 seconds of UV (302 nm, 0.85 mW cmꢀ2, we use
uorophores exhibits moderate uorescence quantum yield of
27%, 25% and 25% in benzene using anthracene or Rhodamine
B as standard. Moreover, they show reversible switching of
uorescence to various certain extents upon intermittent UV
irradiation.
Results and discussion
The molecular design and photochromism of the uorescent
HABI-based photochromes and their model uorophores are
illustrated in Scheme 1. Our major purpose here is to develop a
series of broad-spectrum photoswitchable uorophores
including blue (Cz–O–HABI), green (NI–O–HABI) and red
(PMI–O–HABI) emissions. C8H17–O–HABI has been synthesized
as a non-uorescent model molecule. Generally, 1 is colorless
and non-uorescent in solution but turns into blue color aer
UV irradiation. Carbazole (Cz), naphthalimide (NI) and
perylene-mono-imide (PMI) are used as blue (Cz–O–HABI),
green (NI–O–HABI) and red (PMI–O–HABI) uorophores. Upon
proper light excitation, these uorophores emit blue, green and
red uorescence and aer UV irradiation, the four HABIs
decompose into two radicals named as C8H17–O–TPIR, Cz–O–
TPIR, NI–O–TPIR and PMI–O–TPIR, respectively. The radicals
could be used as uorescence quencher for all-spectrum emis-
sion because of their activated excited state scavenger role. In
Scheme 2 The synthetic procedures of (A) C8H17–O–HABI, (B)Cz–O–
Scheme 1 The photochromism process of HABIs.
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HABI and (C) PMI–O–HABI.
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to blue. Meanwhile, the absorbance of the three peaks at 316
nm, 328 nm and 344 nm increases with more dened struc-
tures. These characters indicate the formation of Cz–TPIRs aer
UV irradiation from Cz–O–HABI dimer. The absorbance at 390
nm and 610 nm increases from 1.62 and 0.59 to 2.22 and 0.81,
respectively, aer 10 s of UV irradiation. These absorption peaks
steadily increase by further extending the irradiation time to 20
s. However, the spectrum does not change aer 30 s UV irra-
diation, which might be because the process from Cz–O–HABI
to Cz–O–TPIR becomes saturated (see Fig. 2A).
Fig. 1 (A) UV-Vis spectra of C8H17–O–HABI in benzene (5.0 ꢁ 10ꢀ5
mol Lꢀ1) with different UV light (302 nm) irradiation time from 0, 5, 10,
20,40 to 60 s; (B) photograph of C8H17–O–HABI in benzene and solid
state before and after UV (302 nm) irradiation.
the same UV source in this work except for special statements)
irradiation, two peaks at 390 nm (absorbance, A ¼ 2.69) and
610 nm (A ¼ 1.02) are observed with the solution color changing
from colorless to blue. The optical density increases to 3.12
(390 nm), 1.21 (610 nm) and 3.22 (390 nm), 1.26 (610 nm) for 10
s and 20 s with an isosbetic point at 326 nm. At 40 s and 60 s the
optical intensity slightly decreases which are 3.20 (390 nm), 1.26
(610 nm) and 3.17 (390 nm), 1.23 (610 nm). The optical intensity
decreases probably because the radicals recombine to form
C8H17–O–HABI aer it has been irradiated over saturated state.
These data are consistent with the book by Rolf Dessauer.27
C8H17–O–HABI is non-uorescent both in dimer and radi-
cals. Therefore we decorated HABI with emissive uorophores
allowing for the measurements and tracing by UV-vis absorp-
tion and uorescence spectroscopy. The photochromic prop-
erties of Cz–O–HABI, NI–O–HABI and PMI–O–HABI have been
investigated in benzene solutions.
The oxygen at the para-position of 2-phenyl ring of the
imidazole blocks the p-conjugation between HABI and the
corresponding uorophores, thus resulting in the distinct
UV-Vis spectra of conjugated HABIs.24,26 Before UV irradiation,
the solution of Cz–O–HABI (5 ꢁ 10ꢀ5 mol Lꢀ1) is colorless with
three peaks at 316 nm, 328 nm and 344 nm, which could be
attributed to the characteristic peak of carbazole. Aer 5
seconds of UV irradiation, a sharp peak at 390 nm (A ¼ 1.62)
and a broad peak between 500–700 nm with lAbs,max ¼ 610 nm
(A ¼ 0.59) are observed and the solution changes from colorless
Fig. 2 (A) UV-Vis spectra and (B) FL spectra of Cz–O–HABI (5 ꢁ 10ꢀ5
mol Lꢀ1) in benzene with different irradiation time from 0, 5, 10, 15, 20,
30 to 60 s, excitation wavelength lex ¼ 320 nm, excitation bandwidth
and emission bandwidth 1 and 1; (C) photograph of Cz–O–HABI in
benzene and solid before and after UV light (302 nm, 0.85 mW cmꢀ2
irradiation.
)
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The corresponding changes in FL spectra of Cz–O–HABI
upon UV irradiation have also been investigated. Before FL
measurement, the excitation spectrum of Cz–O–HABI is per-
formed by detecting the emission intensity at 390 nm (see
Fig. S1†). The following experiments are performed using 320
nm as excitation wavelength which is determined as the best
excitation wavelength from the excitation spectrum. It is note-
worthy that Cz–O–HABI shows slight photochromism upon 320
nm excitation, thus resulting in slice uorescence quenching.
However, the resulting emission loss is negligible compared
with the uorescence quenching from alien UV irradiation (see
Fig. S2(A)†).
The Cz–O–HABI solution emits strong deep blue uores-
cence, with a broad peak between 350–500 nm and a maximum
emission wavelength at 390 nm. The uorescence quantum
yield FF of Cz–O–HABI is calculated to 0.27 using anthracene as
standard (see Fig. S3†). Aer 5 s of UV irradiation, the FL
intensity at 390 nm decreases rapidly by almost 10 times. Two
new emission peaks at 366 nm and 422 nm are observed with
decreased intensity (see Fig. 2B). With 20 s of UV irradiation, the
emission intensity slightly decreases and hardly changes aer
30 s of UV irradiation, which is consistent with the UV spectra.
This indicates the photochromism in dilute solution of Cz–O–
HABI becomes saturated aer 30 s of 302 nm irradiation.
Interestingly, the emission spectra of Cz–O–HABI changes
from single peak (390 nm) to two peaks (366 nm and 422 nm)
with decreased intensity. The emission of Cz–O–HABI at 390 nm
is because of the uorogenic carbazole in Cz–O–HABI. Upon UV
irradiation, Cz–O–HABI homolytic dissociates into two Cz–O–
TPIRs. As we can see from C8H17–O–HABI, C8H17–O–TPIR
possesses a sharp maximum absorption wavelength at 390 nm,
which could act as a quencher for the emission of carbazole.
When carbazole has been excited, the emission at 390 nm could
be transfer to the TPIR part via uorescence resonance energy
transfer mechanism (see Fig. 3). As a result, most of the emis-
sion decreases centered at 390 nm affording twin emission
peaks at 366 nm and 422 nm (Fig. 2B).
Fig. 4 (A) UV-Vis spectra and (B) FL spectra of NI–O–HABI in benzene
(5 ꢁ 10ꢀ5 mol Lꢀ1) with different irradiation time from 0, 5, 10, 20, 30,
60, 90 to 120 s; (C) photograph of NI–O–HABI in benzene and solid
before and after UV light (302 nm, 0.85 mW cmꢀ2) irradiation.
The corresponding optical properties of NI–O–HABI are
presented in Fig. 4. The NI–O–HABI exhibits one UV-absorption
peak at around 390 nm. Upon UV irradiation, the resulting UV-
Vis spectra (Fig. 4A) shows the apparent increase of two bands
centered at 390 nm and 600 nm, in which the former band
overlaps with the absorption band of the naphthalimide moiety.
These two new peaks are due to homolytic cleavage of NI–O–
HABI and the formation of radical pairs, NI–O–TPIRs. This
transitions accompanied by a color change from light yellow-
green to deep blue (Fig. 4C).
The NI–O–HABI solution emits strong green uorescence,
with a broad peak between 450–600 nm and a maximum
emission wavelength at 490 nm. The uorescence quantum
yield FF of NI–O–HABI is calculated to 0.25.26 Aer 10 s of UV
irradiation, the FL intensity at 490 nm decreases to half and the
emission intensity remains nearly unchanged at 60 s. Similarly,
the emission at 490 nm could be attributed to the uorogenic
naphthalimide moiety in NI–O–HABI. Aer UV irradiation,
NI–O–HABI dissociated into two NI–O–TPIRs accompanied with
the color of the solution changing from light yellow-green to
deep blue and uorescence quenching (Fig. 4C).
The situation of PMI–O–TPIR looks different from those of
Cz–O–HABI and NI–O–HABI. Before UV irradiation, PMI–O–
HABI exhibits characteristic peak of PMI at 525 nm with another
two peaks at 353 nm and 376 nm. Aer UV irradiation, the peak
Fig. 3 The proposed photochromism and fluorescent quenching
process of Cz–O–HABI.
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at 525 nm red-shis to 535 nm and a sharp peak at 380 nm and
a broad peak around 600 nm are observed (see Fig. 5A). The
intensity of the peaks increases with extending the irradiation
time and reach maximum at 2 min. These characters are
different from those of C8H17–O–HABI, Cz–O–HABI and NI–O–
HABI. For both Cz–O–HABI and NI–O–HABI, we can see the two
distinguish peaks at 391 nm and 610 nm, which are from the
radicals (the peak 391 nm is overlapped between NI and the
radical spectra). To investigate this difference, we make a plot
between the optical density changes of PMI–O–HABI and UV
irradiation time (see Fig. 5B). In this way, the typical radical
spectra with two peaks at 391 nm and 610 nm are observed.
Naturally, this could be attributed to absorption spectra overlap
between PMI and radicals. The strong and sharp peak at 525 nm
covers with the peak at 610 nm and makes it “red-shi” to 535
nm as a result. Similarly, the peaks at 353 nm and 376 nm
overlap with the peak at 390 nm and “blue-shi” it to 380 nm.
The FL spectra of PMI–O–HABI by different UV irradiation
time are shown in Fig. 5C. PMI–O–HABI emits strong red
light (lem ¼ 650 nm) with moderate uorescent quantum yield
(FF ¼ 0.25, see Fig. S4†). Aer 30 s UV irradiation, the uores-
cent intensity at 650 nm decreases to 52% of the initial, and
continuously decreases to 37% (1 min of UV irradiation), 27%
(2 min UV irradiation) and keeps constant at 25% (3 min UV
irradiation). As further increasing the irradiation time does not
affect the UV-Vis and FL spectra, and we suppose the conversion
between PMI–O–HABI and PMI–O–TPIR becomes saturated. As
there is only slight overlap between the emission spectra of PMI
and red-edge shoulder of TPIR absorption band plus the two
groups in close proximity, we believe the incomplete uores-
cence quenching results from the uorescence resonance
energy transfer from the PMI to the TPIR.
Photochromism properties of HABIs in PMMA lms
The photochromism properties of the synthesized HABIs are
investigated in PMMA lms since the color changes in solid
state for photochromic materials are also very important. The
samples are prepared by dissolving 2 mg of the corresponding
samples into 1 mL of PMMA solution in chloroform. The PMMA
solution is made by dissolving 10 g PMMA (MW ¼ 35 000,
Sinopharm Chemical Reagent Co. Ltd.) into 50 mL distilled
chloroform. This solution was stirred overnight and then it was
used for the spin-coating. During the spin-coating process, the
speed is controlled to 1000 rad sꢀ1 for 1 min. In this way, thin
lms for UV-Vis measurements are prepared.
As we can see from Fig. 6A, a sharp peak at 390 nm and a
broad peak around 610 nm appeared aer 5 s UV light irradi-
ation. The peak around 280–320 nm decreased with an isosbetic
point at 335 nm. During the experiments, the color of the lm
changed from colorless to blue aer exposure to UV light. These
two peaks kept increasing as we extending the irradiation time
to 60 s and remaining unchanged at 90 s. This could be
explained in the same way as that in solutions. These two new
peaks are from the Cz–O–TPIRs formed from the photocleavage
of Cz–O–HABI. The amount of Cz–O–TPIR increases with the
irradiation time until it was saturated at 90 s. The peaks around
Fig. 5 (A) UV-Vis spectra and (B) DOD (optical density) and (C) FL spectra
of PMI–O–HABI in benzene (5 ꢁ 10ꢀ5 mol Lꢀ1) with different irradiation
time from 0, 0.5, 1, 2, 3 to 4 min; (D) photograph of PMI–O–HABI in solid
and benzene before and after UV light (302 nm, 0.85 mW cmꢀ2
)
irradiation.
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overlap between TPIR and naphthalimide as we have reported
before.26
This phenomenon was further observed in the case of PMI–
O–HABI since the broad peak around 610 nm is greatly over-
lapped with the absorption of perylene-mono-imide. As a result
no signicant spectra changes were witnessed for PMI–O–HABI
from the UV-Vis spectra compared with other synthesized
HABIs (Fig. 6D). When we use the “DOD” (changes of the optical
density) instead of “Abs”, two clear peaks: 390 nm and 610 nm
appeared (see inset of Fig. 6D). From the UV-Vis spectra of Cz–
O–HABI, NI–O–HABI and PMI–O–HABI as exhibited in
Fig. 6B–D, we observed similar spectra changes, which could be
attributed to the same reason of photochromism properties
aer UV irradiation. These data demonstrated that all the
designed uorescent HABIs exhibited great color changes in
both solutions and polymer lms.
Thermal recombination of HABI, C8H17–O–HABI, Cz–O–HABI,
NI–O–HABI and PMI–O–HABI
The back reactions from TPIRs to HABIs are investigated by UV-
Vis absorption spectroscopy. The experiments are performed by
irradiating the solutions and then detecting the optical
density changes at 610 nm. For comparison, the decay prole of
HABI is also performed in the same condition (the detecting
wavelength is 555 nm for HABI). As we can see from Fig. 7, in the
rst 200 s, the absorbance of HABI decreases rapidly with a half
lifetime s1/2 ¼ 132 s and then the curve becomes more and more
at, which exhibits a second order kinetic for the recombination
of the two TPIRs. The other HABIs behave in a similar manner
and the s1/2 of C8H17–O–HABI, Cz–O–HABI, NI–O–HABI and PMI–
O–HABI are calculated to be 848, 1250, 1365 and 1085 s.
According to the book wrote by Rolf, the dimerization
constant of alkoxy-substituted HABI (at para-position of
2-phenyl ring) is the smallest compared to others, which
supports the result that s1/2 (C8H17–O–HABI) is larger than that
of HABI. What's more, s1/2 (Cz–O–HABI), s1/2 (NI–O–HABI)
and s1/2 (PMI–O–HABI) are all even larger than that of s1/2
Fig. 6 UV-Vis spectra of (A) C8H17–O–HABI, (B)Cz–O–HABI, (C) NI–
O–HABI and (D) PMI–O–HABI in PMMA film under UV light (302 nm,
0.85 mW cmꢀ2) irradiation, t ¼ 0, 5, 10, 20, 30, 60 and 90 s.
Fig. 7 Decay profiles of the colored species generated from HABI,
C8H17–O–HABI, Cz–O–HABI, NI–O–HABI and PMI–O–HABI, moni-
tored at 555 nm for HABI and 610 nm for the others in degassed
benzene (5 ꢁ 10ꢀ5 mol Lꢀ1). The measurements were performed at
room temperature (25 ꢂC).
300–325 nm in Fig. 6B were generated from the characteristic
absorption of carbazole moiety. The peak at 390 nm increases in
both solutions (Fig. 4A) and lms (Fig. 6C) is contributed to the
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(C8H17–O–HABI), which indicates the thermal recombination
speeds of them are much slower than that of C8H17–O–HABI.
As we all know, when substituted HABIs are transformed to
TPIRs upon UV irradiation, the radicals would diffuse randomly
in the surrounding solvents. If we treat the recombination of
two TPIRs forming HABIs as chemical reaction processes,
naturally, the collision between two molecular TPIRs containing
bulk groups will become less effective compared to that of
C8H17–O–HABI and HABI, and as a result all of them exhibit a
larger s1/2 value. On the other hand, a bulk groups substituted
TPIR might be clamped much easier by another pair of TPIRs
during the recombination process, forming a “sandwich like”
structure making the solution remaining colored and delaying
the back reaction speed of TPIRs as a result.
Acknowledgements
This work was supported by the NSFC (21174045, 21474034),
National Basic Research Program of China (Grant no.
2013CB922104). We also thank the support from the Analytical
and Testing Center of Huazhong University of Science and
Technology.
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The decay proles of C8H17–O–HABI, Cz–O–HABI, NI–O–
HABI and PMI–O–HABI are also investigated in PMMA lm (see
Fig. S6†). All of them exhibit similar characters with negligible
differences.
Fatigue resistance properties of Cz–O–HABI, NI–O–HABI and
PMI–O–HABI in solutions
The fatigue resistance properties of Cz–O–HABI, NI–O–HABI
and PMI–O–HABI are performed by detecting the absorbance
and FL intensity changes in solutions using UV and darkness
treatments. The detail procedures for all the experiments are
presented in ESI, Fig. S5.† We can see that all of these three
uorescent HABIs: Cz–O–HABI, NI–O–HABI and PMI–O–HABI
exhibit efficient fatigue resistance in benzene solutions.
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Conclusions
In this paper, we design and synthesis a series of novel hex-
aarylbiimidazole (HABI)-containing uorophores with broad-
spectrum emissions ranging from blue, green to red emis-
sions. All of them show moderate uorescent quantum yield of
25–27% in benzene. Their photochromism properties are
proved by UV-Vis absorption and FL spectra in solution and
PMMA thin lm. We conclude that the photochromism prop-
erties of all the designed non-conjugated emissive HABIs
(Cz–O–HABI, NI–O–HABI and PMI–O–HABI) are determinate by
the HABI part by comparison with C8H17–O–HABI. Upon UV
irradiation, the uorescence quenches by a FRET mechanism
from the uorophores (Cz, NI, PMI) to the TPIRs and in this
way, blue (390 nm), green (490 nm) and red (650 nm) uores-
cent HABIs photoswitches have been established. We also
investigate their kinetic of the radical recombination (back
reaction) and fatigue resistance properties by UV-Vis and FL
spectra. Although, the back reaction speeds of them are slower
than that of HABI and C8H17–O–HABI, we believe this defect
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formerly proposed by Jiro Abe's group. Accompanying the former
green uorescent HABI (NI–O–HABI), we demonstrate that the full
color: blue, green and red uorescent HABIs can be achieved by
proper molecular design. Further chemistry work that designing
diffusion-inhibited HABIs are undertaken in our lab.
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