A turn-on type stimuli-responsive fluorescent dye with specific solvent effect: Implication for a new prototype of paper using water as the ink

Article abstract of DOI:10.1016/j.saa.2017.04.079

In this study, we reported the photoluminescence (PL) behaviour of a new intramolecular charge transfer (ICT) compound, ((E)-2-(((2-hydroxynaphthalen-1-yl)methylene)amino)benzoic acid, (HABA), which shows ICT solvent effect in aprotic solvents as confirme

Full text of DOI:10.1016/j.saa.2017.04.079

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 7–12
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A turn-on type stimuli-responsive fluorescent dye with specific solvent
effect: Implication for a new prototype of paper using water as the ink
a
b
a
a
a
a,
Xiaochen Hu , Yang Liu , Yuai Duan , Jingqi Han , Zhongfeng Li , Tianyu Han ⁎
a
Department of Chemistry, Capital Normal University, 100048 Beijing, P.R. China
Beijing Key Laboratory of Radiation Advanced Materials, Beijing Research Center for Radiation Application, 100015 Beijing, P.R. China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, we reported the photoluminescence (PL) behaviour of a new intramolecular charge transfer (ICT)
compound, ((E)-2-(((2-hydroxynaphthalen-1-yl)methylene)amino)benzoic acid, (HABA), which shows ICT sol-
vent effect in aprotic solvents as confirmed by absorption and emission spectra. While in protic solvents including
water and ethanol, the charge transfer (CT) band significantly reduces. Remarkable fluorescence enhancement in
the blue region was also observed for HABA in polar protic solvents. We described such phenomena as “specific
solvent effect”. It can be ascribed to the hydrogen bonding formation between HABA and protic solvents, which
not only causes significant reduction in the rate of internal conversion but also elevates the energy gap. Density
functional theory (DFT) calculations as well as the dynamics analysis were performed to further verify the exis-
tence of hydrogen bonding complexes. Stronger emission turn-on effect was observed on HABA solid film when
it is treated with water and base solution. The stimuli-responsive fluorescence of HABA enables a new green
printing technique that uses water/base as the ink, affording fluorescent handwritings highly distinct from the
background. Thermoanalysis of the dye suggests the nice thermostability, which is highly desired for real-
world printing in a wide temperature range.
Received 19 January 2017
Received in revised form 26 April 2017
Accepted 27 April 2017
Available online 29 April 2017
Keywords:
Water-based ink
Water-sensitive
Fluorescent dye
Solvent effect
Paper
©
2017 Published by Elsevier B.V.
1
. Introduction
fabricate the “paper”, where printing or handwriting can be performed
by using specific stimulants (commonly nontoxic) as the “ink” [9–12]. In
this manner, store, collection and recycle of the involved hazardous ma-
terials, if any, will become easier, as they are in solid state. The stimuli-
responsive property is the first necessity of such chromophores. Re-
search efforts have been made to identify new stimuli-responsive
chromophores that can be used in the manufacture of printing in
order to reduce dependency on traditional dyes and lessen the envi-
ronmental impact. Among such materials, the ones responding to
mechanical force [13–14], voltage [15], light [16], ions [17] and
acid/alkali [18–19], have been intensely studied, as they are more
green and easy to access.
Although we are currently in an age inundated with electronic infor-
mation, paper and ink still play an important role in our daily life. Ac-
cording to a study, the printing ink market is growing by 3% per year,
leading to an increasing demand for traditional as well as new printing
inks [1]. Inks are generally composed of four ingredients, including col-
orant, binder, solvent and additives. The colorant, mostly aromatic com-
pound with hyperchrome units, is obviously the key component.
Unfortunately, it is easy to be photobleached, resulting in color fading
in long-term storage [2]. Additionally, the toxicity of the colorant
poses a health risk and causes environmental pollution with daily use
[
3–4]. Nanoparticles fabricated by noble metals represent an efficient
In our previous work, we have reported an archetype of ink-free
rewritable paper based on a mechanochromic luminogen [20]. Letters
or complex patterns can be written repeatedly on such paper. Unfortu-
nately, it often requires extremely high pressure to planish the molecu-
lar conformation of the mechanochromic luminogens in order to change
their emission/absorption. A quantitative study reported by Tian et al.
states that a full color change can only be realized under a pressure as
high as 78,164 times greater than the standard atmospheric pressure
[21]. On the other hand, most of the stimuli-responsive materials con-
taining mechanochromic luminogens, however, function in a “turn-
off” mode: an initially emissive dye becomes non-emissive when it
gets stimulated. Correspondingly, “turn-on” type materials are more
way to bestow yellow to red coloration, making themselves good candi-
dates for new inks [5–8]. They enjoy much higher light fastness com-
pared with organic dyes or pigments. However, a strong limitation to
such type of inks stems from their high cost, even if efficient coloring
performance can be achieved with very low concentrations. So it is dif-
ficult for them to be commercialized.
It is an exceptionally elegant way to imprint chromophores on the
solid substrate (paper, polymer matrix and quartz plate, etc.) to
⁎
Corresponding author.
E-mail address: phanw@163.com (T. Han).
http://dx.doi.org/10.1016/j.saa.2017.04.079
386-1425/© 2017 Published by Elsevier B.V.
1

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X. Hu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 7–12
welcome as they are more sensitive and less likely to generate false-
positive signals [22–24]. Herein we report a turn-on type fluorescent
dye with water-sensitive nature. We studied the PL behaviour, the spe-
cific solvent effect and the related mechanism. The main goal of this
study is to design an archetype of paper that uses water as the ink.
2
. Experimental Section
2
.1. Materials and Methods
Chemicals involved were purchased from Aldrich and used as re-
ceived without further purification. Ethanol was distilled from magne-
sium under nitrogen prior to use. High resolution mass spectra
(
HRMS) were recorded on a GCT premier CAB048 mass spectrometer
1
13
operated in MALDI-TOF mode. H NMR and C NMR spectra were mea-
sured on a Varian VNMRS 600 MHz spectrometer (internal refer-
ence: tetramethylsilane). UV–vis spectra were recorded on a
SHIMADZU UV 2550 spectrophotometer. PL spectra were recorded
on a HITACHI F-7000 FL spectrophotometer. The ground-state geom-
etries were optimized using DFT with MPWB1K/aug-cc-pVTZ//
MPWB1K/6–31 + g* and B3LYP/6–31 + g*. Conductor-like polariz-
able continuum model (CPCM) was employed to estimate the
solvent-solute interaction. All the calculations were performed
using the Gaussian 05 package. Thermogravimetric analysis (TGA)
was carried on a TGA Q5000 under air at a heating rate of 10 °C/
min. Thermal transitions of the luminogen were investigated by dif-
ferential scanning calorimetry (DSC) using a TA DSC Q1000 under air
at a heating rate of 10 °C/min.
Fig. 1. UV–vis absorption spectra of HABA in different organic solvents. Concentration:
0 μM. Abbreviation: hexane (HA), ethyl acetate (EA), tetrahydrofuran (THF), N, N-
dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol (EtOH).
1
3. Results and Discussion
3.1. Solvent Effect
HMAB possesses electron donor (D, phenolic hydroxyl) and acceptor
(A, benzoic acid) groups, as well as the single bonds (C\\C and C\\N)
capable of triggering geometry transition that can affect the energy
level. The introduction of D-A pairs will thus endue a fluorogen with dis-
tinct solvatochromic effects. In most cases, a polar solvent brings the
luminogen from the locally excited (LE) state to the CT state and then
stabilizes the resulting CT state by solvation. Consequently, the energy
gap between the ground and excited state will be narrowed, giving
rise to red-shifted emission/absorption spectra. The good solubility of
HABA allows us to investigate its photophysical properties in the sol-
vents with varying polarities. The absorption peak of HABA appears at
around 316 nm in hexane, corresponding to the π-π* transition
(Fig. 1). The shoulder peak at 439 nm can be ascribed to CT transition.
Similar to the absorption spectrum in hexane, HABA shows the dual-
absorption band in other solvents, too. The hypochromic peaks repre-
sent the LE state absorption, closely related to the π-π* transition,
while the bathochromic peaks denote the CT band. The former remains
2
.2. Synthesis
As shown in Scheme 1, to a round-bottom flask containing 2-
hydroxy-1-naphthaldehyde (2 g, 11.60 mmol, 1 equiv) was added
anhydrous ethanol (200 ml), 2-aminobenzoic acid (1.60 g,
1
1.60 mmol, 1 equiv). Two drops of acetic acid (Scheme 1) was
added to the reaction mixture before it was heated to 60 °C. After
stirring for 8 h, the resulting precipitate was collected by vacuum fil-
tration using ethanol and hexane to rinse, yielding HABA ((E)-2-
(
((2-hydroxynaphthalen-1-yl) methylene)amino)benzoic acid) as
a yellow-orange solid powder (yield: 61%). Structural analysis in-
1
13
cluding H NMR, C NMR and ESI-Mass spectra are listed in the
supporting information (Fig. S1-S3), the corresponding data are rep-
resented as follows:
1
6
H NMR (600 MHz, DMSO-d ) δ: 13.40 (br s, 1H), 9.34 (s, 1H), 8.36–
8
7
.35 (d, 1H), 7.97–7.94 (q, 2H), 7.83–7.81 (d, 1H), 7.69–7.67 (t, 2H),
.48–7.45 (t, 1H), 7.34–7.26 (m, 2H), 6.78–6.77 (t, 1H) (abbreviation:
br = broad, s = singlet, d = doublet, t = triplet, q = quartet, m =
multiplet).
13C NMR (600 MHz, DMSO-d
) δ: 176.32, 167.81, 151.51, 143.30,
6
1
1
38.89, 134.30, 134.10, 131.59, 129.47, 128.69, 126.84, 125.63, 124.79,
23.99, 122.17, 120.65, 119.47, 109.21.
HRMS (MALDI-TOF): m/z 292.0974 [(M + H) , calcd 292.0974].
+
Fig. 2. PL spectra of HABA in different organic solvents. Concentration: 5 mM. Excitation
Scheme 1. Synthetic route to HABA.
wavelength: 410 nm.

X. Hu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 7–12
9
Fig. 3. Time-dependent absorption spectra of HABA in aqueous solution (A) and EtOH (C) (10 μM); Time-dependent PL spectra of HABA in aqueous solution (B) and EtOH (D) (5 mM).
Excitation wavelength: 410 nm. Insets for A–D: corresponding tendency plots.
nearly unchanged in various solvents, however, the latter is remarkably
enhanced as the solvent polarity gradually increases, and turns into the
dominant absorption band in polar solvents such as DMF and DMSO.
In protic solvents including water and ethanol, the CT band becomes
inconspicuous. Such type of solvent effect was occasionally observed
with chromophores containing hydrogen bonding sites such as aniline
Fig. 4. Molecular orbital amplitude plots of HOMO and LUMO energy levels of (A) HABA-DMSO (HABA in CPCM model with DMSO) (B) HABA-EtOH (HABA in CPCM with EtOH),
C) HABA…EtOH (HABA forming hydrogen bonding in EtOH in CPCM model), (D) HABA-H O (HABA in CPCM model with water), (E) HABA…
O (HABA forming hydrogen bonding in
water in CPCM model), calculated at the MPWB1K/aug-cc-pVTZ//MPWB1K/6–31 + g* level.
(
2
H
2

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X. Hu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 7–12
and carboxylic acid derivatives [25–27]. It has been reported that hydro-
gen bonding between solute-solvent often affects the photodynamic
properties of chromophores as well as their static spectral properties
[28]. There are at least three hydrogen bonding sites: carboxylic acid,
phenolic hydroxyl and imine), the latter two are less likely to bond
with solvents as enol-imine unit is normally a six-membered,
hydrogen-bonded pseudo ring [12,29–30]. As a hydrogen bond donor,
the hydroxyl group of water and ethanol is readily to form hydrogen
bonding with carbonyl acid, as the latter, not only a lone pair electrons
owner but also a hydrogen donor, will serve as a strong hydrogen bond-
ing site. The electronegativity on carbonyl group is thus reduced upon
bonding with hydrogen, resulting in a relatively weaker electronic with-
drawing ability. In this manner, D-A efficacy is suppressed, giving rise to
a larger energy gap intrinsic to the LE state. Based on the above discus-
sion, the reduction of CT band in water and ethanol probably rises from
hydrogen bonding complexes.
The fluorescence spectra of HABA were also measured in solvents
with different polarities and hydrogen bond donating abilities to deter-
mine the solvent effect (Fig. 2). Evidently, the light emission is red-
shifted in wavelength and enhanced in intensity with increasing solvent
polarity when comparing the spectrum of HABA in HA, THF, DMF and
DMSO. The red-shift is due to narrowed energy gap of CT transition
when the molecule is surrounded by polar solvent [31]. The weakened
fluorescence can be attributed to internal conversion, which is a non-
radiative process caused by vibronic interactions between close-lying
S1 and S2 states according to related studies [32–33]. As expected, re-
markable fluorescence enhancement in the blue region was observed
for HABA in polar protic solvents, as hydrogen bonding causes signifi-
cant reduction in the rate of internal conversion [33].
Fig. 5. Emission spectra of the pristine and water/alkali-treated HABA solid film. Excitation
wavelength: 410 nm. Photographs were taken under UV illumination.
group, where the conjugation are much weakened relative to the
former, are excluded from electron delocalization (Fig. 4A). The LUMO
orbital partly shifts to carboxylic acid owning to its electronic withdraw-
ing ability. Calculations in EtOH-CPCM model (without regard to
intermolecular hydrogen bonding) give a similar electronic cloud distri-
bution and energy gap (Fig. 4B). Apparently the results disagree with
the spectral behaviour shown in Figs. 1 and 2. But if the hydrogen bond-
ing between HABA and EtOH was taken into consideration in EtOH-
CPCM model, the energy gap will rises from 5.4970 to 5.5132 eV
(Fig. 4C), which matches well with the hypochromatic shift in both ab-
sorption and emission spectra (Figs. 1 and 2). As the most polar solvent,
water is supposed to activate the CT transition as well, leading to an
asymmetric electronic cloud distribution. As shown in Fig. 4D, the
HOMO and LUMO orbitals together with the energy gap (5.4950 eV) cal-
culated by using CPCM model likewise (without hydrogen bonding), are
very close to that in DMSO (Fig. 4A). It is quite clear that this single-
3
.2. Dynamic Analysis
The real-time change in absorption spectrum of HABA is shown in
Fig. 3A. HABA was dissolved in water and the absorption was monitored
over time. An absorption peak at 370 nm is observed with a freshly pre-
pared aqueous solution, demonstrating the CT transition triggered by
the solvent polarity. The CT absorption band gradually decreases with
time, and remains almost unchanged after 20 min. The emission spec-
trum in water shows time-dependent nature as well: CT emission pro-
gressively blue-shifts and LE emission at 448 nm arises (Fig. 3B).
Isotherm of the emission intensity at 448 nm plotted in the inset of
Fig. 3B shows that the growth trend levels off after 460 min. The dynam-
ics analysis of HABA indicates the formation of hydrogen bonding com-
plex. In the first stage, there's not enough time to form hydrogen bond
between HABA and water, hence CT transition plays a major role.
When HABA molecules turn into hydrogen bonding complexes, LE
state will predominate. The EtOH solution of HABA shows the similar
spectral response: drop of CT absorption band, increased LE state emis-
sion and blue-shifted wavelength in both absorption and emission spec-
tra (Fig. 3C–D). In addition, the absorption spectrum of HABA in EtOH
changes visibly slower than that observed with aqueous solution, as
the hydrophilicity of benzoic acid moiety makes it easier to bond with
water compared with EtOH. The time-dependent absorption and emis-
sion spectra in DMSO shown in Fig. S4, by contrast, exert no obvious
change, indicating the absence of solute-solvent hydrogen bond in this
aprotic solvent where CT solvation effect plays the dominant role.
molecule model is inappropriate, as disproved by the spectral responses
…
to water (Figs. 1 and 2). Therefore we calculated the HABA H
2
O hydro-
gen bonding complexes and found its energy gap (5.5031 eV) slightly
increases as shown in Fig. 4E, corresponding to the blue-shift of HABA
3
.3. DFT Calculations
It has been reported that the MPWB1K method is fully fit for analyz-
ing weak interactions, especially hydrogen bonding [34–35]. So we per-
formed such method in order to seek out more evidences to explain the
specific solvent effect in protic solvent. Highest occupied molecular or-
bital (HOMO) and (lowest unoccupied molecular orbital) LUMO are
plotted and displayed in Fig. 4. HOMO orbital of HABA in DMSO-CPCM
model mainly locates on aromatic rings, while the carboxylic acid
Fig. 6. Molecular orbital amplitude plots of HOMO and LUMO energy levels of the sodium
carboxylate form of HABA (HABA-Na) calculated using B3LYP/6-31G* basis set.

X. Hu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 7–12
11
2 3
Fig. 7. Representative images taken under UV illumination showing fluorescent handwritings on the dye-loaded papers using water (A–C) and Na CO (D–E) as the ink.
in water medium. The counter-examples shown in Fig. 4B and D, actu-
ally prove the existence of solvent-solute hydrogen bonding.
blue-shift (523–518 nm). On one hand, hydrogen bond formation sup-
presses nonradiative pathway including CT and internal conversion. On
the other hand, molecular conformation and arrangement should be
taken into consideration when discussing the emission at aggregation
state. Inserting water molecules into HABA may enhance the steric hin-
drance, preventing the molecules from π-π stacking. Besides, the hydro-
gen bonding further restricts the intramolecular rotation to block the
non-radiative pathway. In either case, the fluorescence will be boosted.
However, the spectral response is less conspicuous than that in solution
state as the solid-state reaction suffers from insufficient conversion due
to weak physical contact between reagents [13].
3
.4. Stimuli-Responsive Fluorescence at Solid State
Inspired by the specific solvent effect with HABA, we try to achieve
the water-sensitive property of this luminogen at solid state. A filter
paper was immersed into a THF solution of HABA (5 mM) before
dried in a vacuum oven, by which the dye was adsorbed onto a solid
platform for PL measurements. The as-prepared sample exhibits a
weak emission peaking at 523 nm (Fig. 5). The emission quenching
can be ascribe to the plenary aromatic ring that favors π-π stacking
and the following excimers, which is readily to consume the energy of
excited molecule via nonradiative pathway. After the sample was
dipped into water and dried, the fluorescence enhanced with a slight
After being dipped into an aqueous solution of Na
2 3
CO (0.1 M) and
dried, the dye-loaded paper exhibits as high as 47-fold greater emission
intensity enhancement relative to the starting sample (Fig. 5). The great
signal enhancement can be easily observed by naked eyes as shown in
Fig. 8. TGA (A) and DSC (B) thermogram of HABA recorded under nitrogen at a heating rate of 10 °C/min.

12
X. Hu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 184 (2017) 7–12
the inset of Fig. 5. A hypochromatic shift (523–516 nm) was recoded as
well. The spectral changes could be fit with that of water-treated sample
because they share similar mechanisms. It is worth noting that the car-
boxylic acid group incorporated into the luminogen is apt to react with
analysis. The stimuli-responsive can also be performed at solid state, in
which the turn-on effect toward water- and base-ink is clearly visible
under UV irradiation, enabling a new printing technique that has the ad-
vantages of green and convenience.
2 3
Na CO to yield a deprotonated form, in which the molecules would be
well dispersed owing to intermolecular electrostatic repulsion, offering
the possibility of both conformational change and destruction of face-
to-face packing. Therefore, the stronger emission induction observed
with Na CO -treated sample is due to higher reactivity of the HABA
2 3
with base solution than that with water.
Acknowledgments
Support is acknowledged from the Beijing Municipal Education Com-
mission Project (Grant No. KM201610028007 and KM201610028006)
and National Natural Science Foundation of China (Grant No. 81573832).
HOMO and LUMO orbitals of the carboxylate form (HABA-Na) were
plotted in Fig. 6. The energy levels of both HOMO and LUMO (especially
HOMO) are dramatically elevated on account of the additional electron
that will raise the interelectronic kinetic energy. HOMO electrons are
delocalized on the entire molecule while in the LUMO orbital, electrons
cease to migrate to the carboxylic moiety as the uncharged molecules
do, because it is no longer an electron withdrawing group due to elec-
trostatic repulsion. Destruction of D-A effect gives rise to a higher ener-
gy gap (3.9293 eV), avoiding the pathway including CT and internal
conversion. The above DFT calculations agree well with the fluorescence
changes, further providing theoretical support for the stimuli-
responsive nature.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.saa.2017.04.079.
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solvent effect: turn-on stimuli-responsive fluorescence toward protic
solvents. Owing to the presence of D-A pairs, polar solvents will bring
the luminogen from the LE state to the CT state. While protic solvents
will severely suppress the CT transition and simultaneously boost the
LE transition. The reduction of CT in protic solvents rises from hydrogen
bonding complexes as confirmed by DFT calculations as well as dynamic
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