(Z)-3-(Quinolin-2-ylmethylene)-3,4-dihydroquinoxalin-2(1H)-one derivatives: AIE-active compounds with pronounced effects of ESIPT and TICT

Article abstract of DOI:10.1039/c3nj01439h

A new family of (Z)-3-(quinolin-2-ylmethylene)-3,4-dihydroquinoxalin-2(1H)- one derivatives, which exhibit aggregation-induced emission (AIE) activity, is synthesized and characterized. The qualitative structure property analysis reveals that the emission behaviors of these compounds are closely related to the locations of substituents on the molecules. The density functional theory (DFT) calculations elucidate that excited state intramolecular proton transfer (ESIPT) can smoothly occur in these compounds and a following twisted intramolecular charge transfer (TICT) in the lowest excited singlet state should account for their fluorescence quenching in solution. The restriction of such a TICT process in the aggregated state is assumed as the mechanism for their AIE behaviors. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3nj01439h

NJC
View Article Online
View Journal | View Issue
PAPER
(Z)-3-(Quinolin-2-ylmethylene)-3,4-
dihydroquinoxalin-2(1H)-one derivatives:
AIE-active compounds with pronounced
effects of ESIPT and TICT†
Cite this: New J. Chem., 2014,
38, 2693
Qi-Chao Yao, Xiao-Lin Lu and Min Xia*
A new family of (Z)-3-(quinolin-2-ylmethylene)-3,4-dihydroquinoxalin-2(1H)-one derivatives, which exhibit
aggregation-induced emission (AIE) activity, is synthesized and characterized. The qualitative structure
property analysis reveals that the emission behaviors of these compounds are closely related to the
locations of substituents on the molecules. The density functional theory (DFT) calculations elucidate that
excited state intramolecular proton transfer (ESIPT) can smoothly occur in these compounds and a
following twisted intramolecular charge transfer (TICT) in the lowest excited singlet state should account
for their fluorescence quenching in solution. The restriction of such a TICT process in the aggregated
state is assumed as the mechanism for their AIE behaviors.
Received (in Montpellier, France)
19th November 2013,
Accepted 22nd January 2014
DOI: 10.1039/c3nj01439h
www.rsc.org/njc
dramatically diminished fluorescence quantum yields of these
systems in solution. The considerable constraint of such move-
1 Introduction
Organic small molecules with visible fluorescence are very impor- ments in the aggregated molecules should account for their
tant species which have been widely used in an ever expanding enhanced emission in the excited state. For example, tetraphenyl-
multidisciplinary arena. A common phenomenon called the ethene6b,d,e and polyphenylsilole6a,c,f are two typical cores for AIE
concentration-quenching effect in the aggregated state always systems, since the multiple phenyls act as propellers to decay the
leads to the reduced or quenched fluorescent intensity of a dye excited energy in solution by their free rotation. However, the rigid
by forming sandwich-shaped excimers or exciplexes between dye and twisted molecular structure in the aggregated state makes the
molecules in the excited and ground states.1 Due to such a p–p stacking impossible so that the excited state has to be relaxed
deleterious effect, the conventional fluorescent technology has via the radiative channel. In the case of BF2 complexes with
to work in dilute solutions with drawbacks such as poor sensitivity N-phenyl-3-[(phenylimino)methyl]-2H-chromen/thiochromen/3,4-
and rapid photo-bleaching.
dihydronaphthalen-4-amine ligands,7 the flipping of the aliphatic
However, some unique fluorophores, which are intensely rings is considered as the main reason for their faint fluorescence
emissive in the aggregated state but faintly or even not fluorescent in solution and the inhibition of such movement is responsible
in solution, bring about the aggregation-induced emission (AIE) for their AIE activity. Although the twisted intramolecular charge
effect.2 These AIE-active dyes are revolutionary for luminescent transfer (TICT) process is also widely approved as an important
materials, as they can be directly utilized in the solid state without pathway for the radiativeless deactivation of the excited states in
a reduction in the emission intensity. In recent years, AIE-active solutions,8 the reports on molecules whose AIE activity is caused
molecules have found use in a wide range of applications, such as by prohibiting the occurrence of a TICT process in the aggregated
organic electroluminescence devices,3 fluorescent sensors4 and state are quite scarce,9 especially those coupled with the excited
photodynamic therapy5 and so on.
state intramolecular proton transfer (ESIPT) reaction, just like
In the literature, the most common types of AIE-active systems in the 2-(20-hydroxyphenyl)benzothiazole-based series,10a–c salicyl-
involve molecules with at least a rotatable s-bond axis or a flexible aldehyde azine family10d and its analogue.10e
alicyclic component. Either the rotation around an s-bond axis or
ESIPT reactions have been of great scientific and technological
the flipping of an alicyclic moiety is mainly responsible for the interest in recent years, as one of the most intriguing phenomena
that can be utilized in the design of novel fluorescent dyes.11
When either the acidic or basic part of the same molecule
becomes a stronger acid or base in the excited state, proton
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018,
P. R. China. E-mail: xiamin@zstu.edu.cn
transfer takes place in the excited state to form a phototautomer.
In general, ESIPT is extremely fast occurring, within picoseconds,
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj01439h
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 2693--2700 | 2693

View Article Online
Paper
NJC
and upon excitation the molecule passes to the potential well of in the presence of catalytic p-toluene-sulfonic acid (Scheme 1). The
the tautomeric species almost instantaneously and then relaxes NMR analysis reveals that the distribution of the two tautomers in a
vibrationally.13 Such molecules often show dual emissions, one given product is closely related to the location of the substituents
corresponding to the normal emission from the locally excited on the molecule. When a hydrogen atom or dimethoxyls are
species and the other from the phototautomer with an abnormally imposed on the quinolinyl moiety, 1b or 1c are exclusively offered
large Stokes shift which can effectively eliminate self-absorption and with a trace amount of 2b or 2c that cannot be detected by NMR.
improve the resolution of the emitted light against the incident ray. The case is identical for the product where the dimethoxylphenyl
This merit is particularly significant for the accurate fluorescence is grafted on the N atom of the amide, in which only 1d is the
analysis in field of chemistry and biology. Herein, a novel family detectable isomer. However, when the dimethoxyls are introduced
of (Z)-3-(quinolin-2-yl-methylene)-3,4-dihydroquinoxalin-2(1H)-one on the 3,4-dihydroquinoxalinonyl moiety, a mixture of 1a and 2a in
derivatives with a remarkable ESIPT character is synthesized and a ratio of about 3.3 : 1 is afforded. Unfortunately, it is hard to isolate
characterized. Detailed measurements and theoretical calculations 1a from 2a, so they have to be used as a mixture. Unexpectedly, the
are carried out to establish the fundamental structure–property sole tautomer 2e is gained in good yield when o-phenylenediamine
relationship in this family. Several experimental and computational is reacted with ethyl 2-oxo-3-(quinolin-2-yl)-butanoate under the
approaches are used to explore what is the decisive factor for the AIE same reaction conditions.
activity in these compounds, and the restraint of an excited-state
Calculations based on the density functional theory (DFT) at
TICT process in the aggregated state is presumed to be responsible the B3LYP/6-31G (d,p) level with the polarizable continuum
for their AIE behaviors. Compared with the other common AIE- model (PCM) can predict the difference of the Gibbs free energy
active molecules caused by the suppression of s-bond rotation or between 1 and 2 at 25 1C and 1 atm in DMSO (Table S1, ESI†). It
alicycle flipping, our unique AIE system involving a TICT coupled is shown that 1b–d and 2e are the overwhelmingly major isomers
with an ESIPT process displays a strong long-wavelength emission but 1a coexists with 2a in a ratio of about 3 : 1, in the respective
and an extraordinarily large Stokes shift over 100 nm. Such advan- thermodynamic equilibrium, according to the estimations of the
tages can be readily achieved without the necessity of building a DG = ꢀRT ln K equation (where DG is the difference of the Gibbs
molecule with a complicated structure.
free energy between 1 and 2, R is the gas constant, T is the
tautomerism temperature (298 K) and K is the equilibrium
constant of the tautomerism). Clearly, the DFT-based prediction
for the distribution of 1 and 2 in a given product is in very good
agreement with the results of the experimental determination.
Actually, there are two other tautomers, 1N [(Z)-3-(quinolin-
2 Results and discussion
2.1 Synthesis
Theoretically, a product with two tautomers (the conjugated 2-ylmethylene)-3,4-dihydro-quinoxalin-2(1H)-one] and 1T [(Z)-3-
isomer 1 and the non-conjugated one 2) can be generated when (quinolin-2(1H)-ylidenemethyl)quinoxalin-2(1H)-one], in compound
o-phenylenediamine reacts with 2-oxo-3-(quinolin-2-yl)-propanoate 1 (Scheme 1). Although the DFT calculations indicate that 1N is
Scheme 1 The structure and ratios of tautomers in AIE-active compounds.
2694 | New J. Chem., 2014, 38, 2693--2700
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
more thermodynamically stable than 1T in the ground state for
1a–d (Table S1, ESI†), the equilibrium between them is so
kinetically ultrafast that the common NMR technology at room
temperature is unable to differentiate them for each other.
Hence, only one set of NMR signals can be obtained for them.
2.2 Photophysical properties in solution state
In organic solutions, 1a–d are not emissive to the naked eye but
2e gives off a discernible blue fluorescence. In THF solution,
the quantum yields are determined to be 0.28% for 1a, 0.82%
for 1b, 0.34% for 1c, 0.32% for 1d and 18% for 2e (Table 1).
Obviously, the visually observable blue fluorescence of the 1a
and 2a mixture dominantly comes from the emission of 2a. The
photos of 1a–d and 2e in THF solution under a hand-held
ultraviolet lamp at 365 nm are presented in Fig. S1 (ESI†).
Usually, an ESIPT process is much faster than the radiative
Fig. 1 The normalized absorption, excitation (lem at 600 nm) and emis-
sion (lex at 450 nm) spectra of 1c in THF solution (1 ꢁ 10ꢀ4 M), PMMA film
relaxation of an excited normal isomer (N*)18 so that an abnormally and solid state.
large Stokes shift can result from the absorption of a ground-state
N isomer and the emission of an excited tautomeric isomer (T*).
band in the high energy region. It is observed that the band
Certainly, if a rapidly nonradiative channel exists in the T* state, its
emission will be drastically diminished in solution, just like the
situation in our AIE-active system.
intensity of the T isomer on the absorption spectra is the largest
in 1a but the least in 1b. Such a qualitative intensity order is in
good agreement with the order of the T isomer population in the
corresponding tautomeric equilibrium calculated by the Gibbs free
energy difference (Table S1, ESI†). Additionally, the excitation
spectra of these compounds monitored at 600–615 nm in THF
was investigated, and it was found that both the band shape and
the maximum are very similar to those of their absorption spectra
(Fig. 1 and Fig. S2–S4, ESI†). This indicates that all high-level
excited states can only relax to the same lowest excited state. Due to
an ultrafast ESIPT from an N* to a T* state, the N* emission is
dramatically suppressed, hence, the major band in the low energy
region and the minor band in the high energy region of the
fluorescence spectrum can be assigned to the T* and N* emission,
respectively. Similarly, the intensity of the N* emission is also in the
order of 1a 4 1c B 1d 4 1b. The Stokes shifts between the major
absorption and emission bands are more than 100 nm for all these
compounds. Moreover, the emission maximum makes a consider-
ably bathochromical shift for the molecule with electron-donating
groups on either the dihydroquinoxalinonyl or the quinolinyl
moiety. Owning to the disconnected p-conjugation between the
above two moieties, the maxima in the absorption and emission
spectra undergo a dramatic blue shift in 2e (Fig. S5, ESI†).
Although a ground-state N or T isomer cannot be discrimi-
nated by NMR technology, they can be resolved by the absorption
spectra (Fig. 1 and Fig. S2–S4, ESI†). As the DFT calculations verify
that the N isomer has the more thermodynamic stability and
population than the T isomer in the ground state, the major band
in the high energy region and the minor band in the low energy
region are assigned to the absorption of the N and the T isomer,
respectively. For 1b–d, the two bands are separated and each of
them has a double-peaked fine vibration. For 1a, however, one of
the double peaks in the low energy region is combined into the
Table 1 Photophysical properties of compound 1a–d and 2e
Stokes
lem
(nm) (nm) (nm) Fe
a
b, j
c, j
d, j
labs (nm)/emax
lem
shiftk lem
I/
ITHF
Compd. (ꢁ104 Mꢀ1 cmꢀ1) (nm)
f
1a
1b
1c
1d
458, 481 (1.12)
426, 450 (3.42)
427, 452 (2.17)
433, 455 (2.33)
317 (0.93)
566,
610
553,
596
541,
581
545,
586
425
108
127
114
112
108
561, 620
599
553, 592
591
542, 588, 0.0034 40
579 611
0.0028 18
0.0082 50
547 580, 0.0032 36
607
h
Interestingly, a unique excitation wavelength dependence
of the T*/N* emission intensity ratio is observed for 1a, 1c
and 1d (Fig. S15–S17, ESI†). It is illustrated in Fig. S8 (ESI†) that
g
h
2e
—
—
0.18i
—
a
The absorption spectra are determined in 2.0 ꢁ 10ꢀ5 M THF solution
b
and characterized for the major absorption peaks. The emission spectra the ratio is linearly increased as the excitation wavelength is
(excited at the absorption maxima) are measured in 2.0 ꢁ 10ꢀ4 M THF
bathochromically shifted. However, the intensity ratio does
c
solution and characterized for the major emission peaks. The emission
spectra are detected by doping 0.5% (wt) of compound in thin film with
not respond to the change of the excitation wavelength for
5% (wt) poly(methyl methacrylate) and excited at the absorption maxima. 1b. Such a phenomenon in 1a, 1c and 1d indicates that
d
The emission spectra are measured by solid powder and excited at
the ESIPT rates in these compounds are very sensitive to
variations of the excitation energy. A similar excitation wave-
e
the absorption maxima. Measured by reference to fluorescein in 0.1 N
f
NaOH aqueous solution (Ff = 0.91). Ratio of emission intensity in 90%
H2O–THF mixture and in pure THF solution. g Not determined. No length dependence of the T*/N* emission intensity ratio is also
h
emission. i Measured by reference to quinine sulfate in 0.05 N H2SO4
observed for 40-N,N-diethylamino-3-hydroxyflavone in ionic
j
aqueous solution (Ff = 0.55). In italics is the peak with the larger
liquids,12 but the truth about the dependency in our system
is still under investigation.
emission intensity. k The Stokes shift is referred to as the difference
between the major absorption and emission peak.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 2693--2700 | 2695

View Article Online
Paper
NJC
of 3a, the structure of 3a was proved to be 6,7-dimethoxy-3-
(quinoline-2-carbonyl)quinoxalin-2(1H)-one. It was observed
that the absorption edge of 3a perfectly passes through the
isosbestic point at 400 nm. Although the photo -oxidation can
occur on both 1a and 2a, the rate is much faster for 1a than for
2a. With reference to 1a, the relative photo-oxidation rate is
34.73% for 1b, 14.15% for 1c and 13.88% for 1d.
2.3 Photophysical features in aggregated state
In the solid state, compounds 1b–d exhibit an intense orange to
red fluorescence. As a cousin of 2e, which is detected to be barely
emissive, 2a can be safely considered as dark in the solid state.
Hence, the visible fluorescence of the 1a/2a mixture should be
solely contributed by the emission of 1a. The photos of 1a–d and
2e coated on the inner wall of bottles under a hand-held
ultraviolet lamp at 365 nm are presented in Fig. S1 (ESI†).
The emission spectra of 1a–d obtained by doping 0.5% (wt)
of the corresponding compound onto a film with 5% (wt)
poly(methyl methacrylate) (PMMA) (Fig. 1 and Fig. S2–S4, ESI†)
demonstrates that the peak shape and the maxima of the T*
emission are almost identical to those in THF solution. This
indicates that the molecules are still in the isolated state instead
of in the aggregated state in the PMMA film. However, due to the
strong intermolecular interactions among the aggregated mole-
cules, a remarkably red-shifted maxima can be observed for both
the solid-state excitation and emission spectra with respect to
those in THF solution.
1c was selected as a model to check its AIE activity. A THF
solution of 1c was titrated with water and the change in the
emission intensity was monitored (Fig. 3). In pure THF solution,
1c had a negligible emission intensity, which was gently
increased as the water fraction ( fw) was increased to less than
70% [Fig. S6(a), ESI†]. During this period, the aqueous solution
is still transparent. Beyond this fw point, however, the emission
intensity starts to drastically increase as the water content rises
[Fig. S6(c), ESI†]. At this stage, the aqueous solution becomes
Fig. 2 Absorption spectra of the 1a/2a mixture in 1.5 ꢁ 10ꢀ4 M THF
solution under irradiation at 254 nm and 25 1C in air; normalized absorp-
tion spectrum of isolated 3a in THF (dash line).
The photostability of these novel compounds have also inves-
tigated and the absorption spectra of 1a/2a mixture in THF were
recorded periodically under irradiation at 254 nm and 25 1C in air
(Fig. 2). It is illustrated that the absorbance value of the mixture
gradually decreases while that of a new band centered at 367 nm
simultaneously increases as the irradiation goes on. It was found
that no new peak appears if the mixture in the degassed THF
solution is exposed to irradiation under a N2 atmosphere for a
long enough time. The case is the same when the solution stands
in air without any irradiation. Hence, the new absorption peak
should originate from the photo-oxidation product 3a. However,
the structure of 3a cannot be determined by NMR analysis, as its
solubility in common deuterated solvents is so low that a satis-
factory 13C NMR spectrum could be achieved, even by the
prolonged measurement. Despite our best efforts, a single crystal
of 3a could not be obtained either. Thus, it was treated with benzyl
bromide (Scheme 2) and the corresponding product 3a-Bn was
found to have a largely enhanced solubility. According to the 1D
and 2D NMR spectra of 3a-Bn along with the 1H NMR spectrum
Scheme 2 Photo-oxidation reaction of the 1a/2a mixture and the following benzylation.
2696 | New J. Chem., 2014, 38, 2693--2700
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
the pure THF solution through to the high water content
medium. The case is the same for the absorption spectra of
1c in THF–H2O (1 : 9, v/v) mixtures with different concentra-
tions (Fig. S22, ESI†). Therefore, a specific mechanism should
account for the AIE behaviors of our compounds.
2.4 Theoretical calculations
The simulated UV-vis and fluorescence spectra of these com-
pounds by the DFT and TD-DFT calculations in THF reproduce
the experimental results at the low-energy absorption and emis-
sion bands. The calculated lab maxima are 432 (427, 452 nm) for
1c, 433 (426, 450 nm) for 1b and 443 (458, 481 nm) for 1a, while
the calculated lem maxima are 560 (541, 581 nm) for 1c, 564 (553,
596 nm) for 1b and 572 (566, 610 nm) for 1a. The calculated values
are in good agreement with the experimental ones (in brackets).
For all the N-isomers in 1a–d, the S0 - S1 transition exclusively
has the HOMO - LUMO (p/p*) character (Fig. S9, ESI†). It was
shown that the electrons on 3,4-dimethoxylphenyl (in 1d) partici-
pate the p-conjugation with the other part of the molecule in
Fig. 3 Emission spectra (excited at 450 nm) of 1c in 1 ꢁ 10ꢀ4 M THF
solution (the insert is the fluorescence intensity of 1c depending on the
water fraction in the H2O–THF mixture).
turbid and intensely emissive due to the formation of many neither the HOMO nor the LUMO. Therefore, the photophysical
visible aggregates by the addition of water [Fig. S6(d), ESI†]. The property of 1d is very close to that of 1c due to the lack of the
UV-vis spectrum of 1c in the THF–water mixture [Fig. S6(b), ESI†] p-contribution by 3,4-dimethoxylphenyl.
also confirms that the fw around 70% is the boundary for a
To better understand the photophysical properties of these
transparent or turbid solution. Below this content, 1c is well AIE-active compounds, construction of the potential energy
dissolved in the THF–water mixture and the absorption curves curves (PECs) in the S0 and the S1 state was undertaken by
are almost identical to that in pure THF solution, except for a using TD-DFT calculations. The distance of the transferable
slight reduction of the absorbance values. Above this content, hydrogen from the N1 atom (wherefrom it is dissociated during
remarkable level-off tails in the long wavelength region along the IPT reaction) in an N-isomer is considered as the independent
with the disappearance of fine vibrations and a decrease of the reaction coordinate following the ‘‘distinguished coordinate’’
absorbance values, caused by the Mie effect of nanoparticles,14 approach.16 Fig. 4 depicts such PECs at the S0 and the S1 state
can be readily observed. It was observed that the emission in a vacuum and THF solution for 1c, respectively. The inclusion
intensities in the aqueous THF solution with a 90% water of the solvent stabilization energy does not change the picture
fraction is 18, 50, 40 and 36-fold higher than that in pure THF qualitatively, but it has a greater impact on the S1 than on the S0
solution for 1a, 1b, 1c and 1d, respectively (Table 1). The average state, which implies that the S1 state has a larger dipole
nanoparticle size was determined by DLS to be 222.3 nm for 1a, moment than the S0 state.
402.8 nm for 1b, 310.3 nm for 1c and 392.1 nm for 1d in the
THF–H2O mixture with a 90% water content (Fig. S7, ESI†).
For 1a–c, the simulated PECs generated double-well potentials
in the two electronic states (Fig. 4 and Fig. S10–S11, ESI†): the
Notably, an intramolecular planarity or a J-aggregation effect global minimum rests on an N-isomer in the S0 state while the
in the solid state is not presumed to be related to our AIE-active same corresponds to a T*-isomer in the S1 state. Quantitatively,
compounds. Although the single crystals of these compounds the N-isomer is more stable than the T-isomer by 0.85 kcal molꢀ1
could not be obtained even, despite our best efforts, the for 1a, 1.81 kcal molꢀ1 for 1b and 1.30 kcal molꢀ1 for 1c in the S0
optimized geometries of 1a–d predicted by the DFT calculations state, while the T*-isomer is more stable than the N*-isomer by
show that they are completely planar in solution, owning to the 3.96 kcal molꢀ1 for 1a, 4.30 kcal molꢀ1 for 1b and 4.10 kcal molꢀ1
considerable restraints of the intramolecular H-bonds. Hence, for 1c in the S1 state. Thus, the IPT reactions of these compounds
it seems that there is no driving force for them to adopt the appear thermodynamically unfavorable in the S0 state but plau-
intramolecular planarity strategy so as to become emissive in sible in the S1 state. Kinetically, the PECs also explain the
the solid state. Besides, it is well known that the formation of feasibility of the ESIPT reactions, as the activation energy in the
J-aggregates is an important pathway to improve the emission S0 state is 6.54 kcal molꢀ1 for 1a, 6.98 kcal molꢀ1 for 1b and
in the solid state, and this effect is characterized by a consider- 6.78 kcal molꢀ1 for 1c while the same is reduced appreciably to
able red shift of the original absorption bands with largely 2.94 kcal molꢀ1 for 1a, 2.80 kcal molꢀ1 for 1b and 2.72 kcal molꢀ1
reduced bandwidths or the appearance of a sharp new band in for 1c in the S1 state. Both the thermodynamic and the kinetic
the long-wavelength region with respect to those of the isolated factors support the ESIPT reactions, which result in an abnormally
chromophores.15 However, the absorption spectra of 1c in the large Stokes shift between the absorption of an N-isomer and the
THF–H2O mixtures with different water fractions [Fig. S6(b), emission of a corresponding T*-isomer.
ESI†] clearly illustrate that neither the bathochromic shift of
After an ESIPT process, either a radiative or nonradiative
the absorption band nor the appearance of a new one occurs in relaxation is the destination of a T* state. The rotation via a
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 2693--2700 | 2697

View Article Online
Paper
NJC
Fig. 5 Relative molecule total energy with varied torsion angle of
N2–C5–C4–C3 in 1aT*, 1bT* and 1cT* by TD-DFT calculations at B3LYP/
PCM/6-31G(d,p) level in THF.
However, the maxima and the quantum yield of the major
emission band in 1c are very slightly varied in THF (lem = 541,
585 nm and F = 0.0034) and in MeCN (lem = 543, 590 nm and
F = 0.0029). Hence, the major emission band should be assigned
to the radiative relaxation of the residual 1cT* with H-bonded
planar conformation which is not promptly converted into the
nonfluorescent TICT state.
Fig. 4 Relative molecule total energy with varied bond length of N1-H in
1cN by TD-DFT calculations at B3LYP/PCM/6-31G(d,p) level in THF and
gas phase.
Although the perpendicular conformations via the C4–C5
bond is one of its nonradiative pathways. The PECs of 1cT* bond torsion have the global energy minima for 1a–c, their
built by considering the torsion angle of N1–C3–C4–C5 or kinetic processes are quite different (Fig. 5). The planar con-
N2–C5–C4–C3 as the independent reaction coordinate respectively formation (01 torsion angle) rests on the energy summits of the
show that the rotation via the C3–C4 bond is unfavorable in the S1 PECs in 1a and 1c, but there is a shallow energy well for the
state (Fig. S12, ESI†), as the energy minimum rests on a com- planar conformation in 1b. Accordingly, an ultrafast and
pletely planar conformation. Hence, the nonradiative decay by barrierless torsion toward the TICT state can be easily accessed
rotation via the C3–C4 bond should be irresponsible for the low for 1a and 1c in the S1 state so that the radiative channel of the
quantum yields of the solution-state T* emission in this family.
planar conformations is effectively blocked. However, the TICT
By contrast, the PECs reveal that the rotation in a T*-isomer process is delayed by the energy well to some extent in 1b so
via the C4–C5 bond is permitted in the S1 state (Fig. S13, ESI†). that a few of the planar conformations have the chance to emit
In the 1cT* state, the PEC produces a remarkable energy well at their fluorescence in time. Hence, the quantum yield of 1b is
the orthogonal conformation (901) and double peaks at the two about 3-fold higher than that of 1a and 1c. Although there is a
planar ones (01 and 1801). It should be noted that the initially larger energy fall between the planar and orthogonal conformations
formed 1cT* would be near the H-bonded position at 01 occurring in 1c (4.35 kcal molꢀ1) than in 1a (3.13 kcal molꢀ1), the
because the ESIPT occurs on the H-bonded planar 1cN*. The quantum yields of 1a and 1c are approximate to each other. This
peak energy of the H-bonded planar conformation is signifi- suggests that the rotation of double methoxyls in 1a may contribute
cantly lowered over the other planar conformation without the to the nonradiative decay, because the energy fall on the PECs is
H-bond, strongly supporting the role of the H-bond stabilization generated by using the N2–C5–C4–C3 torsion angle alone as the
in the torsion via the C4–C5 bond. Moreover, the orthogonal independent coordinate without considering the other modes of
conformation exhibits the obvious TICT characters: (i) most of movement. In spite of this, it is still assumed that the deactivation of
the p-electrons are concentrated on the quinoxalinone part a T* state is dominantly controlled by torsion via the C4–C5 bond
(donator) in the HOMO while they are almost entirely distributed instead of the rotation of substituents, as in 1b, which also possesses
on the dihydroquinoline part (acceptor) in the LUMO (Fig. S14, the double methoxyls and has a higher quantum yield than 1a.
ESI†); (ii) the calculated dipole moment is raised from 4.25 D for
Apparently, a TICT process involving a largely volume-consuming
the planar conformation in the S0 state to 13.57 D for the perpendicular torsion is readily accessible in low-viscosity solutions
orthogonal one in the S1 state. It is known that an obviously as the friction induced by such movement is quite small. However,
red-shifted maximum and considerably reduced quantum yield this torsion is substantially restricted and the emission is gradually
are generally observed for a TICT emission in a polar solvent. increased as the viscosity of the medium goes up.17
2698 | New J. Chem., 2014, 38, 2693--2700
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
NJC
Paper
intensely red fluorescence, which is very helpful for the bio-
analysis of living specimens. The simple preparation procedure
and the ready availability of the starting substrates make our AIE-
active molecules quite easily accessible. The application of them
in bioanalysis by further modification is under investigation.
Acknowledgements
We are grateful for the financial support of the Zhejiang
Natural Science Foundation (Y14B020057) and the Zhejiang
Province Key Innovation Team (2012R10038-09).
References
1 (a) S. W. Thomas III, G. D. Joly and T. M. Swager, Chem. Rev.,
2007, 107, 1339–1386; (b) Photophysics of Aromatic Molecules,
ed. J. B. Birks, Wiley-Interscience, London, 1970; (c) J. Liu,
J. W. Y. Lam and B. Z. Tang, Chem. Rev., 2009, 109,
5799–5867; (d) P. L. Burn, S.-C. Lo and I. D. W. Samuel,
Adv. Mater., 2007, 19, 1675–1688; (e) Photophysical and
Photochemical Properties of Aromatic Compounds, ed.
J. Malkin, CRC, Boca Raton, 1992; ( f ) Modern Molecular
Photochemistry, ed. N. J. Turro, University Science Books,
Mill Valley, 1991.
2 (a) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev.,
2011, 40, 5361–5388; (b) Y. Hong, J. W. Y. Lam and
B. Z. Tang, Chem. Commun., 2009, 4332–4353.
3 (a) H. Li, Z. Chi, X. Zhang, B. Xu, S. Liu, Y. Zhang and J. Xu,
Chem. Commun., 2011, 11273–11275; (b) W. Z. Yuan,
S. Chen, J. W. Y. Lam, C. Deng, P. Lu, H. H.-Y. Sung,
I. D. Williams, H. S. Kwok, Y. Zhang and B. Z. Tang, Chem.
Commun., 2011, 47, 11216–11218; (c) X. Zhang, Z. Chi, B. Xu,
H. Li, Z. Yang, X. Li, S. Liu, Y. Zhang and J. Xu, Dyes Pigm.,
2011, 89, 56–62.
4 (a) T. Han, X. Feng, B. Tong, J. Shi, L. Chen, J. Zhi and
Y. Dong, Chem. Commun., 2012, 48, 416–418; (b) F. Sun,
G. Zhang, D. Zhang, L. Xue and H. Jiang, Org. Lett., 2011, 13,
6378–6381; (c) X. Xu, J. Huang, J. Li, J. Yan, J. Qin and J. Li,
Chem. Commun., 2011, 47, 12385–12387; (d) K. Shiraishi,
T. Sanji and M. Tanaka, Tetrahedron Lett., 2010, 51,
6331–6333; (e) M. Wang, G. Zhang, D. Zhang, D. Zhu and
B. Z. J. Tang, J. Mater. Chem., 2010, 20, 1858–1867;
( f ) S. Chen, J. Liu, Y. Liu, H. Su, Y. Hong, C. K. W. Jim,
R. T. K. Kwok, N. Zhao, W. Qin, J. W. Y. Lam, K. S. Wongc
and B. Z. Tang, Chem. Sci., 2012, 3, 1804–1809; (g) Y. Sun,
Z. Liu, X. Liang, J. Fan and Q. Han, Spectrochim. Acta, Part A,
2013, 108, 8–13.
Fig. 6 Emission spectra of 1c (25 mM) with varied glycerol fraction in
THF–glycerol mixture.
Hence, it can be expected that a TICT process is extremely
suppressed in the aggregated state owning to its great viscosity
(estimated to be at least 1011 cP19). The emission dependence of
the viscosity experiments clearly show that the fluorescence of
compound 1a–d is continuously enhanced in the THF–glycerol
mixture, as the glycerol fraction rises (Fig. 6 and Fig. S18–S20,
ESI†). Notably, a slight red- shift of the emission wavelength
occurs even in the medium with 95% glycerol with respect to
that in pure THF solution. This may well be caused by the good
dissolution of these compounds in glycerol so that the aggregates
are not formed in the THF–glycerol mixture. Instead, these
compounds are very poorly soluble in water. Thus, the red-
shifted emission caused by the intermolecular interaction
appears in a high water content medium due to the aggregate
formation in it. Therefore, the AIE mechanism of our com-
pounds can be expected to result from the restraint of the TICT
process in the aggregated molecules in the excited state.
Actually, the shallow energy well on the PEC in 1b is presumed
to be caused by the enlarged friction between the dihydroquinolinyl
fragment with dimethoxyls and the surrounding environment,
compared with the situation in 1a and 1c.
3 Conclusion
Although there is a great number of reports on various AIE
compounds, few have been presented with the ESIPT aspect.
Herein, we describe the synthesis, characterization and photo-
physical properties of a novel family with significant AIE
activity. Each of them can readily go through an ESIPT reaction
followed by TICT-caused fluorescence quenching in solvent.
However, it was verified that only the ESIPT reaction, rather than
the TICT process, can be smoothly carried out in the aggregated
state. The restriction of the torsion between the quinoxalinone
and the dihydroquinoline fragment via the C4–C5 bond in the S1
state should predominantly account for the AIE phenomenon in
this system. Compared with other common AIE-active molecules
with short-wavelength emissions, our compounds exhibit an
5 C.-C. Chang, M.-C. Hsieh, J.-C. Lin and T.-C. Chang,
Biomaterials, 2012, 33, 897–906.
6 (a) Z. Zhao, D. Liu, F. Mahtab, L. Xin, Z. Shen, Y. Yu,
C. Y. K. Chan, P. Lu, J. W. Y. Lam, H. H. Y. Sung,
D. r. I. D. Williams, B. Yang, Y. Ma and B. Z. Tang,
Chem.–Eur. J., 2011, 17, 5998–6008; (b) J. Shi, N. Chang,
C. Li, J. Mei, C. Deng, X. Luo, Z. Liu, Z. Bo, Y. Q. Dong and
B. Z. Tang, Chem. Commun., 2012, 48, 10675–10677;
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
New J. Chem., 2014, 38, 2693--2700 | 2699

View Article Online
Paper
(c) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
NJC
and D. T. Quang, Chem. Rev., 2007, 107, 3780–3799;
(e) Chemosensors: Principles, Strategies, and Applications,
ed. B.Wang and E. V.Anslyn, Wiley, New York, 2011,
pp. 253–273; ( f ) A. D. Roshal, A. V. Grigorovich,
A. O. Doroshenko and V. G. Pivovarenko, J. Phys. Chem. A,
1998, 102, 5907–5914.
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740–1741; (d) Z. Zhao, S. Chen,
J. W. Y. Lam, P. Lu, Y. Zhong, K. S. Wong, H. S. Kwoka
and B. Z. Tang, Chem. Commun., 2010, 46, 2221–2223;
(e) D. Jana and B. K. Ghorai, Tetrahedron Lett., 2012, 53,
6838–6842; ( f ) H. Aizawa, K. Hatano, H. Saeki, N. Honsho, 12 K. Suda, M. Terazima and Y. Kimura, Chem. Phys. Lett.,
T. Koyama, K. Matsuoka and D. Terunuma, Tetrahedron
Lett., 2010, 51, 1545–1549.
7 K. Perumal, J. A. Garg, O. Blacque, R. Saiganesh, S. Kabilan,
2012, 531, 70–74.
13 S. J. Formosinho and L. G. Arnaut, J. Photochem. Photobiol.,
A, 1993, 75, 21–48.
K. K. Balasubramanian and K. Venkatesan, Chem.–Asian J., 14 (a) Q. Zhao, J. Zhou, F. L. Zhang and D. Lippens, Mater.
2012, 7, 2670–2677.
Today, 2009, 12, 60–69; (b) Absorption and scattering of light
by small particles, ed. C. F. Bohren and D. R. Huffmann, New
York, Wiley-Interscience, 2010; (c) H. Xu, X. Chen,
S. Ouyang, T. Kako and J. Ye, J. Phys. Chem. C, 2012, 116,
3833–3839.
8 (a) Z. R. Grabowski and K. Rotkiewicz, Chem. Rev., 2003, 103,
3899–4031; (b) W. Rettig, Angew. Chem., Int. Ed. Engl., 1986,
25, 971–988; (c) J.-F. Utard, R. Lapouyade and W. Rettig,
J. Am. Chem. Soc., 1993, 115, 2441–2447; (d) J. Herbich and
B. Brutschy, in TICT Molecules: In Electron Transfer in 15 (a) W. I. Gruszecki, J. Biol. Phys., 1991, 18, 99–109;
Chemistry, ed. V.Balzani, Wiley-VCH, Weinheim, 2001.
9 (a) J. Hu, Z. He, Z. Wang, X. Li, J. You and G. Gao, Tetra-
hedron Lett., 2013, 54, 4167–4170; (b) Y. Sun, X. Liang, J. Fan
and Q. Han, J. Lumin., 2013, 141, 93–98; (c) R. Hu, E. Lager,
A. Aguilar-Aguilar, J. Liu, J. W. Y. Lam, H. H. Y. Sung,
(b) T. E. Kaiser, H. Wang, V. Stepanenko and F. Wu¨rthner,
Angew. Chem., Int. Ed., 2007, 46, 5541–5544; (c) H. von
¨
¨
Berlepsch, C. Bo1ttcher and L. Dalhne, J. Phys. Chem. B,
2000, 104, 8792–8799; (d) S. Choi, J. Bouffard and Y. Kim,
Chem. Sci., 2014, 5, 751–755.
˜
I. D. Williams, Y. Zhong, K. S. Wong, E. Pena-Cabrera and 16 (a) A. L. Sobolewski and W. Domcke, Chem. Phys., 1998, 232,
B. Z. Tang, J. Phys. Chem. C, 2009, 113, 15845–15853.
10 (a) Y. Qian, S. Y. Li, G. Q. Zhang, Q. Wang, S. Q. Wang,
H. J. Xu, C. Z. Li, Y. Li and G. Q. Yang, J. Phys. Chem. B, 2007,
111, 5861–5868; (b) R. Hu, S. Li, Y. Zeng, J. Chen, S. Wang, Y. Li
and G. Yang, Phys. Chem. Chem. Phys., 2011, 13, 2044–2051;
257–265; (b) S. Maheshwari, A. Chowdhury, N. Sathyamurthy,
H. Mishra, H. B. Tripathi, M. Panda and J. Chandrasekhar,
´
J. Phys. Chem. A, 1999, 103, 6257–6262; (c) J. Catalan, J. C. del
Valle, J. Palomar, C. Di’az and J. L. G. de Paz, J. Phys. Chem. A,
1999, 103, 10921–10934.
(c) M. Cai, Z. Gao, X. Zhou, X. Wang, S. Chen, Y. Zhao, Y. Qian, 17 R. Hu, E. Lager, A. Aguilar-Aguilar, J. Liu, J. W. Y. Lam,
N. Shi, B. Mi, L. Xie and W. Huang, Phys. Chem. Chem. Phys.,
2012, 14, 5289–5296; (d) W. X. Tang, Y. Xiang and A. J. Tong,
J. Org. Chem., 2009, 74, 2163–2166; (e) R. Wei, P. Song and
A. Tong, J. Phys. Chem. C, 2013, 117, 3467–3474.
H. H. Y. Sung, I. D. Williams, Y. Zhong, K. S. Wong,
˜
E. Pena- Cabrera and B. Z. Tang, J. Phys. Chem. C, 2009,
113, 15845–15853.
18 C.-L. Chen, C.-W. Lin, C. C. Hsieh, C.-H. Lai, G.-H. Lee,
C.-C. Wang and P.-T. Chou, J. Phys. Chem. A, 2009, 113,
205–214.
11 (a) J. E. Kwon and S. Y. Park, Adv. Mater., 2011, 23,
3615–3642; (b) C. Fang, N. N. Frontiera, R. Tran and R. A.
Mathies, Nature, 2009, 462, 200–204; (c) S. M. Ormson and 19 R. Edgeworth, B. J. Dalton and T. Parnell, Eur. J. Phys., 1984,
R. G. Brown, Prog. React. Kinet., 1994, 19, 45–91; (d) J. S. Kim 5, 198–200.
2700 | New J. Chem., 2014, 38, 2693--2700
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014