Photoactivatable aggregation-induced emission of triphenylmethanol

Article abstract of DOI:10.1039/c7cc04693f

Triphenylmethanol (TPOH) is a non-fluorescent compound. Interestingly, after UV irradiation at 254 nm, TPOH showed a strong blue fluorescence on a solid surface. Moreover, the fluorescence can be quenched by UV irradiation at 365 nm, making the photoactiv

Full text of DOI:10.1039/c7cc04693f

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Photoactivatable Aggregation-induced Emission of
Triphenylmethanol
Received 00th January 20xx,
Accepted 00th January 20xx
Yue Zheng, Xiaokun Zheng, Yu Xiang and Aijun Tong*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Triphenylmethanol (TPOH) is
a non-fluorescent compound. characteristics of these triphenylmethane dyes. The color
Interestingly, after UV irradiation at 254 nm, TPOH showed strong changes from leuco state to colored state because of the red
blue fluorescence on solid surface. Moreover, the fluorescence shift of absorption spectrum when triphenylmethane becomes
can be quenched by UV irradiation at 365 nm, making the tritylium.26, 27 Early studies demonstrated that the fluorescence
photoactivatable fluorescence reversible. TPOH is thus applied for of some tritylium molecules in common solvents at low
rewritable photopatterning tuned by UV light.
viscosity was very weak,28, 29 while stronger fluorescence was
observed in highly viscous media (or at low temperature).30-32
These findings indicated the tritylium molecules might have
AIE-like characteristics, thus we thought tritylium-generating
triphenylmethane derivatives, such as triphenylmethanol
(TPOH), could be ideal photoactivatable AIE flurophores. One
key feature for the photo-induced transformation between
tritylium cation (TPM+) and TPOH could be reversibility,
therefore the photoactivatable AIE fluorescence of TPOH, if
present, might be reversible as well.
Photoactivatable fluorophores1 undergo fluorescent switches
upon light irradiation, which are attracting materials for a
broad range of applications in chemistry,2, materials4 and
3
6
biosensing.5, Conventional fluorescent molecules, such as
8
10
rhodamine7, and fluorescein,9,
encounter aggregation-
caused quenching (ACQ), so that photoactivatable
fluorophores based on them are usually not suitable for use in
aggregate or solid state. To address this challenge,
fluorophores with aggregation-induced emission (AIE)
characteristics11-13 display weak fluorescence when dissolved
in solvent but are strongly fluorescent in aggregates, making
them highly efficient solid-state fluorophores.14-19 Taking
advantage of this property, photoactivatable AIE fluorophores
based on tetraphenyl ethylene (TPE)20 and salicylaldehyde
hydrazine (SA)21 derivatives have been developed by Tang’s
group and our group, respectively, which showed strong
fluorescence in aggregates and solids upon irradiation by UV
light. Nevertheless, because the mechanism of photoactivation
for these designs are based on photolabile groups,22 the
photoactivatable AIE fluorescence is not reversible, hindering
their potential application as solid materials with rewritable
fluorescence.
Triphenylmethane derivatives are members of traditional
dyes with bright colors.23-25 Their switch between the
triphenylmethane and tritylium forms upon UV irradiation at
254 nm leads to the well-known photochromism
Fig 1 (a) Photographic images of TPOH luminescence on silica surface under UV1 and
UV2 irradiation. Visualization of fluorescence was by a flash excitation at 365 nm within
10 seconds to avoid irradiation effect. (b)Photographic images of TPOH on silica surface
before and after UV1 irradiation. (c) Schematic diagram of the photoactivation reaction
on silica surface.
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more clear photos, see Figure S3. (c) fw% = 1 and 60 before and after UV1 irradiation.
See Figure S4 and S5 for excitation spectra and photoactivation kinetics for fw% = 60
after UV1 irradiation. (d) Dynamic lighter scattering (DLS) analysis of fw%=60,
suggesting average particle diameters of 202 nm. The concentration of TPOH is 1 mM.
In this work, we found that TPOH was nonfluorescent but
subsequently emitted strong fluorescence in aggregate state
or on solid surface after irradiation by UV light at 254 nm
(UV1). Interestingly, the photo-induced fluorescence of TPOH
was quenched when irradiated by UV light at a longer
wavelength of 365 nm (UV2), and further repeatable on and
off by UV1 and UV2, respectively. The mechanism of the
photoactivatable AIE fluorescence was then studied, and such
property of TPOH was also applied for rewritable
photopatterning.
A visible image of the photoactivatable fluorescence of
TPOH on solid surface of silica was shown in Figure 1a. TPOH
was initially nonfluorescent. After UV irradiation at 254 nm
(UV1) for 5 min, strong photoactivatable fluorescence was
observed. Indeed, the TPOH spot on silica also changed from
colorless to slightly yellow upon UV1 irradiation (Figure 1b).
The yellowish color was the characteristic color of tritylium
cation, suggesting the photo-induced generation of tritylium
might be responsible for the photoactivatable fluorescence.
Interestingly, when the fluorescent state of activated TPOH
was placed under UV irradiation at 365 nm (UV2) for a longer
time (30 min), the fluorescence was quenched. Subsequently,
the fluorescence could be recovered and quenched again by
UV1 and UV2 irradiation, respectively (Figure S1, Supporting
Information). The reversible activation and deactivation of
fluorescence at solid surface could be repeatable for at least 5
times (Figure S2).
Fig 3 Partial 1H NMR spectrum (1 mM solution in acetonitrile-d3 with 1% D2O) of
triphenylmathanol (TPOH), Triphenylcarbenium tetrafluoroborate (TPM+) and TPOH
after UV1 irradiate for 1.5 h.
60% water, but giving little fluorescence in 1% water (Figure
2c). Dynamic light scattering (DLS) result showed that insoluble
particles with an average diameter of around 202 nm were
formed in 60% water (Figure 2d). These results suggested the
presence on solid surface or aggregates in solution, instead of
freely dissolved, was critical for the photoactivatable
fluorescence of TPOH.
To investigate the origin of TPOH’s photoactivatable
fluorescence, we carried out the study in solvents. As shown in
Figure 2a and 2b, TPOH completely dissolved in acetonitrile
(1% water fraction) and showed very weak fluorescence even
after UV1 irradiation, in contrast to its fluorescence
enhancement on solid surface (Figure 1a). With increasing
amounts of water fraction, the photoactivatable fluorescence
enhanced to its maximum at 60% water. Further addition of
water to the solvent to 70% or above caused severe cloudy
and precipitation, making the apparent fluorescence intensity
decreased. Photoactivation was essential for the fluorescence
to occur in
We hypothesized that the reason for this phenomenon was
likely because of two factors: (1) the tritylium cation (TPM+)
formed after UV irradiation of TPOH was not very stable when
dissolved in solvent containing trace water or the leaving OH−,
so that the half-life of TPM+ was sufficiently long only when
the molecule was on the solid surface or in the aggregates of
TPOH suspended in poor solvent; (2) even if TPM+ could be
stable in good solvent, its fluorescence might still be quenched
by the intramolecular rotations of phenyl moieties connected
by C-C single bonds.
To support our hypothesis, first, we studied the 1H NMR
spectrum of TPOH before and after UV1 irradiation, as well as
a TPM+ salt (tritylium tetrafluoroborate) in acetonitrile-d3
containing 1% D2O. Three nearly identical spectra with
neglectable difference in chemical shifts and relative intensity
of the peaks were collected for the above three samples
(Figure 3), indicating TPM+, either formed by UV1 irradiation or
originated from salt, transformed into TPOH in good solvent
within the time scale for our NMR sample preparation (~15
min). Then, the fluorescence of TPM+ salt in anhydrous
acetonitrile with different amounts of glycerin were measured.
We assumed that TPM+ should be sufficiently stable in
anhydrous acetonitrile during our measurement. In this case,
with more glycerin in the solvent to increase the viscosity of
the solution, the fluorescence of “TPM+” was enhanced
accordingly (Figure S6). This is a characteristic dependence
Fig 2 (a) Fluorescent spectra of TPOH in acetonitrile with different water fractions
(fw%). (b) Fluorescent intensity at λ=381 nm with different water volume fractions. For
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between fluorescence and viscosity for typical AIE spectra of TPOH in methanol before and after UV1 irradiation.
fluorophores. As shown in Figure S7, for TPOH in methanol without UV1
To prove the formation of TPM+ from TPOH after UV1 irrad-
irradiation, we then measured the ESI-MS (positive ion mode)
Fig 4 (a) Rewritable photopatterning materials for printing the word “CHEMISTRY” (b) “Face changing” between UV1 and UV2 irradiation
iation, a sole peak of m/z = 243.1 (TPM+) was observed. This for 30min. The dark “CHEMISTRY” in a bright background was
was normal, because even for a TPOH solution without any generated. In addition, complicated patterns were also
TPM+, TPM+ signal was usually dominant in ESI due to the ease printed, erased and rewrote on this material with good
of OH− leaving under the ionization condition. However, for resolution, such as an interesting “face changing” process. As
TPOH in methanol after UV1 irradiation for 1.5 h, another peak illustrated in Figure 4b, two different Chinese paper-cut were
of TPOMe ([M+H]+ = 275.1) was present, indicating the used as photomasks. Upon sequential printing/erasing by UV1
exchange between TPOH and TPOMe in methanol occurred and UV2, respectively, the two face images were switched
only under UV light, which was very likely through the between each other on a silica plate.
intermediate TPM+ formed by UV irradiation.
In summary, we investigated the photoactivatable AIE
Such photoactivatable fluorescence was also found in some fluorescence of triphenylmethanol. The mechanism of this
derivatives of TPOH. Compound 1-4 (Table S2, Supporting phenomenon is most likely through photo-induced generation
Information) all showed photoactivatable fluorescent of tritylium cation that is stabilized and fluorescent on solid
properties both on surface and in aggregates similar to TPOH, surface or in aggregate states. The molecule was applied as a
despite of the dramatic difference in the fluorescence intensity rewritable material for photopatterning solid-state
(Figure S8 and S9). Compound 1 with Cl substituting the OH of fluorescence. By printing and erasing cycles through UV
TPOH, had strong fluorescence because of the good leaving irradiation at 254 and 365 nm, the reversible photopatterning
group of Cl−. In contrast, MeO− is a bad leaving group, so that of “CHEMISTRY” and Chinese “face-changing” images was
compound
2
exhibited much weaker emission. The successfully achieved.
We are thankful for the financial support from the National
fluorescence of compound 3 was the weakest because of the
quenching effect of three Cl substitutions on phenyl groups. Natural Science Foundation of China (Nos. 21375074,
Introducing an auxochrome MeO- onto one of the phenyl 21675097, 21405091, 21621003), National Key Scientific
groups in compound 4 did not increase the fluorescence, Instrument and Equipment Development Project of China
probably because the UV light efficiently absorbed was not (2012YQ030111), Tsinghua University Initiative Scientific
that efficiently used for photoactivation.
Research Program (20131089220), and the Recruitment
Inspired by the attractive fluorescence switches of TPOH Program of Global Youth Experts of China. We appreciate Ms.
between UV1 and UV2 irradiations reversibly on silica surface, Yiwen Ding for her kind help in the experiments during the
we further applied it as rewritable photopatterning materials. revision process of this work.
As shown in Figure 4a, the blue fluorescent "CHEMISTRY"
pattern was successfully printed on silica by 5 min UV1
Notes and references
irradiation under a photomask. The image was of high contrast
because of the efficient “off-on” photoswitch. The photomask
was then removed to “erase” the “CHEMISTRY” by UV1
irradiation for 5 min to light up the whole plate. After that, we
covered the photomask again and irradiated the plate by UV2
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