Hydrogen-Bond and Supramolecular-Contact Mediated Fluorescence Enhancement of Electrochromic Azomethines

Article abstract of DOI:10.1002/chem.201600859

An electronic push–pull fluorophore consisting of an intrinsically fluorescent central fluorene capped with two diaminophenyl groups was prepared. An aminothiophene was conjugated to the two flanking diphenylamines through a fluorescent quenching azomethi

Full text of DOI:10.1002/chem.201600859

DOI: 10.1002/chem.201600859
Full Paper
&
Fluorescence Enhancement
Hydrogen-Bond and Supramolecular-Contact Mediated
Fluorescence Enhancement of Electrochromic Azomethines
Monika Wałe˛sa-Chorab,[a, b] Marie-Hꢀlꢁne Tremblay,[a] and William G. Skene*[a]
Abstract: An electronic push–pull fluorophore consisting of
an intrinsically fluorescent central fluorene capped with two
diaminophenyl groups was prepared. An aminothiophene
was conjugated to the two flanking diphenylamines through
a fluorescent quenching azomethine bond. X-ray crystallo-
graphic analysis confirmed that the fluorophore formed mul-
tiple intermolecular supramolecular bonds. It formed two hy-
drogen bonds involving a terminal amine, resulting in an an-
tiparallel supramolecular dimer. Hydrogen bonding was also
confirmed by FTIR and NMR spectroscopic analyses, and fur-
ther validated theoretically by DFT calculations. Intrinsic fluo-
rescence quenching modes could be reduced by intermolec-
ular supramolecular contacts. These contacts could be en-
gaged at high concentrations and in thin films, resulting in
fluorescence enhancement. The fluorescence of the fluoro-
phore could also be restored to an intensity similar to its
azomethine-free counterpart with the addition of water in
>50% v/v in tetrahydrofuran (THF), dimethyl sulfoxide
(DMSO), and acetonitrile. The fluorophore also exhibited re-
versible oxidation and its color could be switched between
yellow and blue when oxidized. Reversible electrochemically
mediated fluorescence turn-off on turn-on was also possible.
Introduction
energy-dissipating excited-state modes involve structural
modification. These include metal coordination[5] and incorpo-
rating hydroxyl groups that sustain excited-state intramolecular
proton transfer.[6] Although these are efficient means for fluo-
rescence enhancement, they significantly alter the azome-
thines properties. It would therefore be beneficial to use other
strategies for fluorescence enhancement, especially those that
do not alter the intrinsic spectroscopic, electronic, or electro-
chemical properties.
Conjugated azomethines have attracted much attention in
part because of their straightforward preparation. This has
been further spurred by their properties, which are ideally
suited for use as functional materials in plastic electronics. For
example, conjugated derivatives have been successfully used
as active layers in organic photovoltaics[1] and electrochromic
devices.[1c,2] Although property tailoring is possible through the
judicious selection of aromatics used for their preparation,
a key azomethine property that remains elusive is high fluores-
cence quantum yields (Ffl). Their quenched fluorescence has
proven to be a major impediment for azomethine use in high-
performance emitting applications.
Typically, fluorophore fluorescence decreases with increasing
concentration. This is in part because of screening effects and
intermolecular quenching. In contrast, in aggregation-induced
emission (AIE), the fluorescence is enhanced with increasing
concentration.[7] This is a result of intermolecular interactions
that suppress intrinsic molecular motions that deactivate the
excited state.[8] AIE enhancement has recently been demon-
strated as a viable means of suppressing otherwise fluores-
cence deactivation modes, especially in thin films and crys-
tals.[9] This has been beneficial for enhancing the performance
and color emitted from organic light-emitting devices.[9b,10] AIE
enhancement has also been used for field-effect transistors,[11]
sensors,[7b] and memory devices.[12] AIE enhancement is there-
fore a viable process for restoring the fluorescence of other-
wise nonfluorescent azomethines.
The principal intrinsic deactivation pathways responsible for
conjugated azomethine fluorescence quenching are photoin-
duced electron transfer and photoisomerization between the
E- and Z-isomers.[3] Although no net photoproducts are formed
because the Z-isomer thermally isomerizes to the thermody-
namically E-isomer,[4] E!Z isomerization is nonetheless an effi-
cient quenching manifold. Current strategies to suppress these
[a] Dr. M. Wałe˛sa-Chorab, M.-H. Tremblay, Prof. Dr. W. G. Skene
Laboratoire de caractꢀrisation photophysique des matꢀriaux conjuguꢀs
Dꢀpartement de chimie, Universitꢀ de Montrꢀal
CP 6128, Centre-ville Montreal, QC (Canada)
AIE has proven useful for suppressing the inherent emission
deactivation modes of azomethines and for increasing their
fluorescence. The emission enhancement modes have princi-
pally been from aromatic stacking[13] or weak binding of an an-
alyte.[14] Although it was recently demonstrated that supra-
molecular contacts that drive crystallization in the solid state
can be used for AIE enhancement,[15] only a limited number of
examples have used such contacts for enhancing the fluores-
E-mail: w.skene@umontreal.ca
[b] Dr. M. Wałe˛sa-Chorab
Current address:
Faculty of Chemistry, Adam Mickiewicz University
Umultowska 89b, 61-614 Poznan (country/>Poland)
´
Supporting information for this article and ORCID are available on the
WWW under
http://dx.doi.org/10.1002/chem.201600859.
Chem. Eur. J. 2016, 22, 1 – 13
1
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
cence of azomethines. These have relied on the imine nitrogen
pected to be electroactive, with reversible oxidation and color
switching between the neutral and oxidized states. The collec-
tive fluorescent and electrochemical properties would also
make the fluorophore an ideal candidate for electrochemically
induced fluorescence switching. The crystallographic structure,
fluorescence, electrochemical, spectroelectrochemical, and flu-
oroelectrochemical properties of the azomethine fluorophores
1 and 2 (Scheme 1) are herein presented to demonstrate con-
centration-activated supramolecular contacts for mediating
fluorescence enhancement.
[16]
[17]
to promote intermolecular =N···HSꢀ and =N···HCꢀ bonds
[17]
in addition to intramolecular =N···HOꢀ supramolecular con-
tacts.
Terminal NH2 groups would be ideal scaffolds for mediating
hydrogen bonding and enhancing azomethine fluorescence.
More importantly, such supramolecular contacts would be ben-
eficial for promoting lateral fluorophore interactions for ulti-
mately generating smooth and homogenous fluorescent thin
films. While supramolecular assembly mediated by hydrogen
bonds is well-known,[18] it remains relatively unexplored for
promoting azomethine fluorescence. We were therefore moti-
vated to prepare conjugated azomethines that could promote
lateral intermolecular hydrogen bonds and other supramolec-
ular contacts for fluorescence enhancement. The fluorophore
shown in Figure 1 was targeted to demonstrate such an AIE
Results and Discussion
The fluorene–triphenylamine–dialdehyde precursors 5 and 6
were prepared in a multistep synthesis as per Scheme 1. The
corresponding 2,7-dibromofluorene was reacted with diphenyl-
amine in a Pd-catalyzed N-arylation reaction, followed by stan-
dard Vilsmeier–Haack formylation. Condensation of 5 and 6
with 2,5-diaminothiophene 7 in ethanol with catalytic trifluoro-
acetic acid resulted in the targeted azomethines 1 and 2, re-
spectively.
Although the structures of both 1 and 2 were confirmed by
conventional characterization methods, unequivocal evidence
was sought by X-ray crystallography. The added benefit of
crystallography is that both intra- and intermolecular supra-
molecular bonding can be confirmed. For this reason, crystals
of the electrochromes were pursued. Crystals suitable for X-ray
diffraction were obtained by the slow diffusion of diisopropyl
ether into a dichloromethane solution of 1. Crystals of 2 for X-
ray diffraction could not be obtained because of the high
degree of disorder from the alkyls groups of the fluorene core.
The resolved X-ray structure shown in Figure 2 (top panel)
Figure 1. Electrochromic azomethine prepared and examined for AIE en-
hancement driven by intermolecular supramolecular contacts.
enhancement. It is an interesting fluorophore for investigating
AIE, in part because azomethines derived from its end-capped
2,5-diaminothiophene are known to have quenched fluores-
cence.[19] It therefore should minimally fluoresce. Its fluorene
core serves as the intrinsic fluorophore, the quenched fluores-
cence of which should be enhanced upon reducing the azo-
methine deactivation modes. This should be possible by en-
gaging concentration-mediated supramolecular contacts, in-
volving lateral intermolecular hydrogen bonding through the
terminal amines. The two amine–azomethine–phenylamine
arms are further expected to form a cleft for additional inter-
molecular contacts to reinforce the AIE. Moreover, the terminal
amines are expected to contribute to the donor–acceptor–
donor electronic arrangement of the amine–azomethine–phe-
nylamine structure, resulting in interesting spectroscopic and
electrochemical properties. The conjugated arm is further ex-
Figure 2. Resolved X-ray structure of 1 (top). Crystal packing of 1 illustrating
the inter- and intramolecular hydrogen bonds drawn as blue lines (bottom).
The solvent molecules were removed from the figures for clarity.
clearly confirms the structure of 1. Four molecules of 1 were
found to occupy the unit cell in the P21/c space group. On the
molecular level, the crystal structure also confirms the E-config-
uration of both imines. Other salient features of the resolved
structure are the antiparallel arrangement of the triphenyla-
mines and the planes described by the central fluorene and its
adjacent phenylamines connected to the azomethines. These
Scheme 1. Preparation of the fluorenylazomethine. Reagents and conditions:
i) Pd(OAc)2, tBu3P, CsCO3, toluene, 1208C, 24 h; ii) POCl3, DMF, dichloroethane,
reflux, 15 h; iii) TFA, EtOH, 808C, 15 h.
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
2
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
were found to be twisted by 65.08 for one arm and 75.68 for
the other arm. In this configuration, the terminal amines are
oriented in up and down directions from the central fluorene
along the c-axis, leading to a zigzag arrangement of the arms.
Meanwhile, the two planes described by the terminal thio-
phenes were found to be twisted by 21(4)8 from each other.
The collective arrangements are ideal for extended hydrogen
bonding of the terminal amines of the different molecules in
the crystal structure, leading to linear and parallel supramolec-
ular ribbons (see below). The triarylamines were also found to
be planar, rather than the expected tetrahedral configuration.
The planar arrangement is supported by the sum of the three
amine angles being equal to 359.9(10)8. This is most likely due
to steric and electronic effects. Of importance is the coplanari-
ty of the aminothiophene–azomethine–phenylamine moieties
in the two arms of 1. In this configuration, the arms are ex-
pected to be highly conjugated and its donor–acceptor–donor
arrangement should be sensitive to solvent effects. This would
result in pronounced solvatochromic effects.
Although extensive supramolecular contacts were confirmed
in the solid state by X-ray crystallography, such intermolecular
hydrogen-bonding may not necessarily occur in solution. Con-
centration-dependent NMR studies between 0.8 and 50 mm
were therefore undertaken to probe whether intermolecular
hydrogen-bonding occurred in solution. The distinct chemical
shift of the NH2 protons did not overlap with other protons.
This made it an ideal signal for probing concentration-induced
intermolecular hydrogen-bonding. The NH2 signal, which was
assigned based on 2D HMBC experiments (Supporting Infor-
mation, Figure S13), was expected to shift downfield with in-
creasing concentration with hydrogen bonding. As seen in
Figure 3, the chemical shift of the amine protons is shifted
Both intra- and intermolecular supramolecular contacts
could be confirmed by X-ray crystallography. In fact, multiple
hydrogen bonds and other supramolecular contacts were
found for 1. Of importance are the hydrogen bonds. Intramo-
lecular hydrogen-bonding was found between the ester car-
bonyl and the amine of the terminal thiophenes. This is illus-
trated by the blue lines in the termini of 1 in Figure 2
(bottom). The terminal amines were also found to undergo in-
termolecular hydrogen-bonding. Similar to the intramolecular
bonding, intermolecular hydrogen-bonding occurred between
the thiophene amine and the ester carbonyl. Two such bonds
take place between one terminus of two different molecules of
1, as seen in Figure 2. The intramolecular donor–acceptor
bond distances were found to be similar (2.80(3) ꢃ). The result-
ing hydrogen-bonded dimer places the thiophenes in a copla-
nar and antiparallel arrangement, having ꢀx, ꢀy, 1ꢀz symme-
try with respect to each other. The intermolecular supramolec-
ular dimer donor–acceptor distances were found to be
2.96(2) ꢃ. This results in a supramolecular zigzag type ribbon il-
lustrated in the bottom panel of Figure 2. The other ester of
the terminal thiophene also undergoes intermolecular hydro-
gen-bonding. In this case, the intermolecular contact is per-
pendicular to the plane described by the thiophene–imine–
phenyl and involves the NH2 of a third molecule of 1 through
a 1ꢀx, ꢀ1/2+y, 3/2ꢀz symmetry. The NH···OC bond distance
was calculated to be 2.25(5) ꢃ, whereas the N···O distance was
3.09(2) ꢃ. Multiple supramolecular contacts also occurred be-
tween the first and third molecules of 1. For example, edge-on
Figure 3. Chemical shifts of the NH2 protons 2 as a function of concentration
in CDCl3 (0.8 to 50 mm) Inset: NMR spectra of the NH2 protons of 2 with in-
creasing concentration in CDCl3 from top to bottom.
with increasing concentration of 2 in anhydrous CDCl3. In fact,
a linear correlation (r2 =0.993) of chemical shift to higher fre-
quencies contingent on concentration was observed. The NMR
data provide sound evidence for the concentration-induced
hydrogen-bonding of the terminal amines.
Infrared spectroscopy was also used to confirm intermolecu-
lar hydrogen-bonding of the fluorophores. Similar to the NMR
studies, significant differences in the IR spectra were expected
between the fluorophore with and without hydrogen bonding.
DFT calculations were first performed. These were used to
identify both the stretching frequencies involved in hydrogen
bonding and to assign the experimentally measured frequen-
contact between the two fluorenes, S···thiophene, S···C=O, C= cies. The X-ray crystal structure of 1 was used as the starting
O···HC=N, C=O···HCphenyl, and phenyl···phenyl contacts were
found. In this arrangement, the two molecules are nearly per-
pendicular, with the planes described by the two fluorenes
found to be twisted by 76.38. The collective X-ray data confirm
that 1 has multiple intermolecular contacts. The resulting ex-
tended supramolecular structure is expected to suppress oth-
erwise fluorescence-deactivation modes, resulting in concen-
tration-induced fluorescence turn-on.
geometry for optimizing the DFT calculated geometry. Com-
pound 1 was chosen for the calculations rather than 2 because
of its reduced number of atoms and decreased degrees of
freedom, which allowed the computational cost to be reduced.
The resulting fully optimized gas-phase DFT ground-state ge-
ometry was in good agreement with the X-ray structure (Sup-
porting Information, Figure S15).
Hydrogen bonding was calculated to occur between the
electron-deficient hydrogen and a region of high electron-den-
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
3
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
sity. Multiple hydrogen bonds were found with 1. The calculat-
ed interactions were intramolecular between the ester carbon-
yl and the terminal amine, along with intermolecular bonding
between the same groups between two different molecules.
These were consistent with the X-ray crystal structure. Given
the intermolecular hydrogen bonds were expected to enhance
the fluorescence, these were calculated in more detail.
Hydrogen bonding was expected to increase the NꢀH and
the C=O bond lengths. The calculated intermolecular hydro-
gen-bond distance was 3.06 ꢃ, consistent with the value mea-
sured by X-ray crystallography. The NꢀH bonds were calculated
to extend by 23%, whereas the carbonyl was calculated to
extend by 32%. These significant bond stretches should be no-
ticeable in the IR spectrum. The infrared spectra of 1 both with
and without hydrogen bonds were therefore calculated, with
particular attention to the NꢀH vibrations. The asymmetric and
symmetric stretching bands were found to shift with intermo-
lecular hydrogen-bonding. The asymmetric NꢀH stretching at
3678 cmꢀ1 decreased by 55 cmꢀ1 with hydrogen bonding. The
symmetric NꢀH stretching at 3510 cmꢀ1 was also calculated to
shift to lower wavenumbers by 10 cmꢀ1, albeit less than the
asymmetric stretching upon hydrogen bonding (Figure 4A).
The spectral shifts were also accompanied with an increase in
the signal intensity. The stretching signals increased twofold,
with the greatest increase found for the symmetric stretching
(Supporting Information, Table S2).
The experimentally measured FTIR spectra of 2 are shown in
Figure 4B. The spectra were recorded both as a highly dilute
solution and as a power. These two extremes were chosen to
ensure both the absence of hydrogen bonds and the hydro-
gen bonded forms of 2 were recorded. CCl4 was selected as
solvent for solution measurements because of its spectroscopic
invisibility in the region of interest. Whereas the absolute fre-
quencies between the measured and calculated spectra were
different, their trends with hydrogen bonding were consistent.
The symmetric stretching peak at 3335 cmꢀ1 was reduced by
26 cmꢀ1 upon hydrogen bonding. Whereas the peak at
3430 cmꢀ1 was found not to significantly shift, its intensity in-
creased 2.7-fold, compared with the symmetric stretching peak
with hydrogen bonding. This is consistent with the theoretical
calculations. The collective FTIR and NMR concentration-de-
pendence studies confirm that concentration-mediated inter-
molecular hydrogen-bonding between fluorophores occurs.
The solvatochromism of both 1 and 2 and their correspond-
ing dialdehydes were investigated. This was done to gain in-
sight into their fluorescence properties and to better under-
stand their concentration-induced fluorescence properties.
Both fluorophores 1 and 2 and their respective dialdehyde pre-
cursors were expected to exhibit solvent-dependent fluores-
cence changes. The triarylamine serves both as a basic site and
electron donor. Solvent can therefore interact with the aryla-
mine through hydrogen bonding and other electrostatic inter-
actions. These perturb significantly the excited-state molecular
dipole, resulting in strong solvent-dependent spectral changes.
Similarly, the electron donor–acceptor–donor arrangement of
the aminothiophene–azomethine–phenylamine configuration
of 1 and 2 was further expected to be sensitive to solvent. En-
&
*
Figure 4. A) Calculated DFT IR spectra of 1 with ( ) and without ( ) hydro-
&
*
gen bonding. B) FTIR spectra of 96 mm of 2 in CCl4 ( ) and as a powder ( ).
hanced solvatochromic effects of 1 and 2 were therefore ex-
pected compared with their corresponding analogues 5 and 6.
As seen in Figure 5A, the fluorescence of 2 was significantly
affected by the nine solvents examined. This was also the case
for 1, 5, and 6. Notably, the absorption of 1, 2, 5, and 6 varied
by up to 5 nm in the different solvents (Supporting Informa-
tion, Figures S18, S30, S41, and S53). The fluorescence change
with solvent therefore reflects excited-state solvent–fluoro-
phore interactions. For 1, 2, 5, and 6, the fluorescence was red-
shifted with solvent polarity upwards of 200 nm (Table 1). The
fluorescence redshift with solvent polarity is consistent with
a greater stabilization of the excited state relative to the
ground state. The pronounced positive solvatochromism ob-
served arises from the donor–acceptor effect of the fluoro-
phores involving the triarylamine conjugated with the elec-
tron-accepting moiety; the carbonyl for 5 and 6, and the azo-
methine for 1 and 2. The solvatofluorochromic effects of 2 and
6 are qualitatively seen in the converging slopes in Figure 5B.
The Stokes shift of the fluorophores contingent on the solvent
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
4
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
ed state adopting many orientations, each being perturbed by
the solvent.
The fluorescence quantum yield (Ffl) of 1, 2, 5, and 6 was
also evaluated in the different solvents. This was to benchmark
the values for concentration studies and to confirm the fluores-
cence quenching of the azomethine. As expected, the Ffl of
1 and 2 were consistently low (<1%), irrespective of solvent
(Table 1). The fluorescence is quenched by the azomethine
bonds. In contrast, the Ffl of 5 and 6 varied markedly with sol-
vent; they fluoresced appreciably in toluene, diethyl ether, and
tetrahydrofuran (THF).
The fluorescence of the fluorophores contingent on concen-
tration in THF was also investigated to determine whether the
intermolecular interactions measured experimentally by X-ray
diffraction, IR, and NMR along with the theoretically calculated
hydrogen-bonding, enhanced the fluorescence as surmised. As
seen in Figure 6, there was a marked effect of concentration
on the spectroscopic properties. For 2 in THF, a red color was
observed when the concentration was increased seven orders
of magnitude, owing to an increased absorption at 540 nm.
The fluorescence was redshifted with increasing concentration.
The observed bathochromic shifts in both the absorption and
fluorescence were assumed to be a result of increased fluoro-
phore–fluorophore interactions. Correcting for changes in ab-
sorption, the fluorescence emission also increased with con-
&
Figure 5. A) Normalized emission spectra of 2 in toluene ( ), diethyl ether
~
&
&
^
^ *
(
), ethyl acetate ( ), THF ( ), ethanol ( ), DMSO ( ), acetone ( ), dichloro-
&
*
*
)
methane ( ) and acetonitrile (?) at 1.0 mm. B) Stokes shift of 2 ( ) and 6 (
as a function of solvent ET(30) parameter.
Table 1. Selected photophysical data.
Solvent
h[a]
1
2
5
6
lfl
Ffl lfl
Ffl lfl
Ffl
lfl
Ffl
[nm][b] [%][c] [nm][b] [%][c] [nm][b] [%][c] [nm][b] [%][c]
CH2Cl2
toluene
Et2O
EtOAc
EtOH
acetone
THF
acetonitrile 1.344 570
DMSO 1.479 545
1.424 585
1.497 495
1.352 495
1.371 515
1.361 545
1.359 550
1.407 525
0.22 613
0.09 495
0.06 495
0.11 505
0.12 560
0.11 580
0.11 531
0.08 600
0.21 548
0.25
0.11
0.08
0.11
403 1.44
518 36.7 463/523 39.0
515 26.4
545 5.69
403 1.65
520 31.0
536 9.34
0.13 426/547 0.12 426/549 0.19
0.12
0.19
0.09 426/600 0.05 426/575 0.08
0.22 426/574 0.22 426/570 0.44
608 0.19
545 9.63
595 0.35
536 16.0
[a] Refractive index taken from the literature.[22] [b] Emission maximum wave-
length. [c] Reference against naphthalene (Ffl =23% in cyclohexane).[23]
ET(30) parameter is somewhat linear. The less than linear trend
suggests specific solute–solvent interactions in addition to
nonspecific interactions. This notwithstanding, the fluores-
cence redshift of 1 and 2 was also complemented with
a broadening of the emission. This is consistent with the excit-
Figure 6. Top: photographs showing the fluorescence of 2 in sample vials of
THF with increasing concentration (from right to left) when irradiated with
a UV lamp (A) and under ambient light (B). Bottom: fluorescence spectra of
2 in deoxygenated and anhydrous THF measured at 3.3·10ꢀ4 ( ), 33.3 ( ),
&
^
ꢀ1
~
*
170 ( ), and 1000 ( ) mmolL
.
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
5
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
centration, as per Figure 6. In fact, the fluorescence increased
twofold with increasing concentration. This suggests that oth-
erwise intrinsic fluorescence deactivation modes are dimin-
ished with intermolecular supramolecular contacts between
the azomethines. The observed bathochromic shift with in-
creasing concentration along with the increased fluorescence
are further consistent with the formation of J-type aggre-
gates.[20] In contrast, H-type aggregates would result in hypso-
chromic shifts and decreased emission with increasing concen-
tration.[21]
Figure 7. Quantitatively, the fluorescence was redshifted up-
wards of 100 nm and its fluorescence emission increased 2.6-
fold. Owing to differences in thin-film thickness and limitations
of relative actinometry, only relative emission yields could be
obtained. Notably, the intrinsic fluorescence is typically
quenched when fluorophores are cast as films and when not
embedded in a matrix. Bearing this in mind, the enhanced
fluorescence observed with 2 in thin film provides further evi-
dence for intermolecular interactions that reduces the other-
wise efficient azomethine fluorescence quenching modes.
To examine whether the azomethines underwent the pro-
posed enhanced AIE when aggregated in solution, the fluores-
cence of 1 and 2 were measured in THF, acetonitrile, and di-
methyl sulfoxide (DMSO) with varying amounts of water. These
organic solvents were chosen because of their miscibility with
water. Given the poor hydrophilic character of the azome-
thines, especially 2 owing the fluorene alkyl groups, the addi-
tion of water should aggregate 1 and 2. Redshifted fluores-
cence concomitant with a fluorescence enhancement should
both be observed with aggregate formation upon the addition
of water. As seen in Figure 8A, the fluorescence behavior of 2
The concentration-dependent fluorescence of 2 was also ex-
amined in thin films. This was to evaluate the fluorescence be-
havior with varying degrees of intermolecular interactions in
the solid state. For this, 2 was cast on glass slides from a stock
solution in dichloromethane. The azomethine was also mixed
in varying weight% ratios with poly(methyl methacrylate)
(PMMA) in dichloromethane. This polymer was chosen because
it is miscible with 2 and it does not phase separate. It also
forms a uniform and homogenous film on glass. This property
is key for accurate fluorescent measurement. The dependence
of emission wavelength on the concentration of 2 in thin films
(Figure 7) was consistent with what was observed in solution.
&
*
Figure 7. Normalized emission spectra of thin films of 2 in 1:0 ( ), 1:10 ( ),
~
!
&
^
1:30 ( ), 1:50 ( ), 1:80 ( ), and 1:100 ( ) weight ratios in a PMMA matrix.
Inset: pictures showing the emission of 2 in thin films cast on glass sub-
strates when irradiated with a UV lamp in 1:0 (1), 1:10 (2), 1:30 (3), 1:50 (4),
1:80 (5), 1:100 (6) weight ratios in a PMMA matrix.
For example, the most blueshifted fluorescence was observed
when 2 was the most dilute in the PMMA matrix (1:100 wt%
PMMA). Under these conditions, the lowest number of intra-
molecular contacts occur. The measured fluorescence of 2 in
the 1:100 wt% PMMA film was redshifted by 75 nm compared
with what was measured at 3.3 ꢄ10ꢀ4 m. This implies that 2
forms intermolecular contacts even when dispersed in the
PMMA matrix. With increasing wt% of 2 in the PMMA matrix,
its fluorescence was redshifted, owing to the increased
number of intermolecular contacts. The same emission was ob-
served for both the as-cast thin film from a stock solution with-
out the PMMA matrix and in a 1000 mm solution. The effect of
concentration of 2 in thin films on the color emitted and emis-
sion intensity can qualitatively be seen in the photographs in
Figure 8. A) Fluorescence spectra of 2 in THF with different volume% of
~
&
&
*
^
*
water: 0% ( ), 10% ( ), 30% ( ), 50% ( ), 80% ( ), and 90% ( ). B) Relative
~
&
*
quantum yield of 2 in THF ( ), acetonitrile ( ), and DMSO ( ) with varying
volume% of water. Inset: photographs showing the fluorescence of 2 in
vials of DMSO when irradiated with a UV lamp with 0 (A), 10 (B), 30 (C), 50
(D), 80 (E), and 90 (F) volume% of water.
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
6
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
was markedly dependent on water being added to the envi-
ronment. The fluorescence intensity decreases somewhat with
increasing amounts of water, in part due to variations in the
sample volume. This aside, the fluorescence emission redshift-
ed by 130 nm once the water content was greater than 50%.
Subsequent addition of water increased the fluorescence in-
tensity. In fact, the fluorescence increased eightfold with water.
The fluorescence of 1 and 2 measured, once the water
volume% threshold was met, was similar to what was mea-
sured at high concentrations in homogenous solutions. For the
absorption, no appreciable shifts were observed for the princi-
ple absorption at 432 nm upon the addition of water. Only the
absorption at 458 nm was water sensitive: it decreased by up
to 16% relative to the peak at 432 nm for both 1 and 2 upon
the addition of water (Supporting Information, Figures S21,
S24, S27, S32, S33, and S36). Given the absorptions of 5 and 6
were centered at 380 nm and varied only by ꢁ5 nm with sol-
vent (Supporting Information, Figures S41 and S53), the de-
crease in the absorption at 458 nm of 1 and 2 with water is
unlikely to result from the azomethine bonds being hydro-
lyzed. The same behavior of fluorescence redshift and fluores-
cence enhancement with water was also observed for both
DMSO and acetonitrile. The stark shift in both fluorescence
wavelength and intensity of 2 in DMSO with water are evident
in the photographs in Figure 8B. The measured Ffl of 1 and 2
in aggregated acetonitrile and DMSO with water was consis-
tent for those measured for 5 and 6 in the corresponding neat
solvents (Table 1). Under similar aggregation conditions, the
fluorescence enhancement of 5 and 6 in THF increased only
1.5-fold. In contrast, their fluorescence in DMSO and acetoni-
trile water mixtures was enhanced between 2.3-fold and 5-
fold, respectively, more than their azomethine counterparts
under similar aggregation conditions. The emission of 5 and 6
were blueshifted by about 110 nm relative to 1 and 2 under
similar conditions with added water. The collective spectro-
scopic data provide evidence for the formation of intermolecu-
lar fluorophore interactions that are engaged upon water addi-
tion. Although the intrinsic fluorescence of the fluorene core
cannot be completely restored with the azomethine supra-
molecular contacts, the amount of fluorescence quenching can
nonetheless be decreased, resulting in fluorescence enhance-
ment. Restoring the inherent fluorescence of the fluorophore
would require complementary means to fully suppress the effi-
cient fluorescence quenching modes, which is believed to in-
volve photoinduced electron transfer. Suppressing this deacti-
vation mode would be possible by adjusting the energy levels
of the fluorophore and the azomethine to make the process
endergonic, as per the Rehm–Weller relationship.[24]
gen-bonded dimer of 1 were calculated. The corresponding
values for 1 without hydrogen bonding (monomer) were also
calculated as a benchmark. For all the calculated excited
states, the energy levels of the hydrogen bonded 1 were lower
than its corresponding monomer (Table 2). Whereas the great-
Table 2. Calculated electronic transition energies and oscillator strengths
of 1.
Excited state
Monomer
Dimer
E [eV] Oscillator strength E [eV] Oscillator strength
1
2
3
4
5
1.65
2.29
2.68
2.77
2.80
0.075
0.352
1.711
0.018
0.233
1.65
2.10
2.28
2.30
2.34
0.089
0.000
0.529
0.005
0.000
est stabilization occurred with the 3rd and 4th excited states,
the strongest oscillator strength was calculated for the 3rd ex-
cited state. The calculations confirm that hydrogen bonding
with 1 has a strengthening effect, resulting in spectral red-
shifts. This is consistent with the spectroscopic data and fur-
ther supports the formation of J-type aggregates. Moreover,
the multiple transitions calculated to participate in the spectro-
scopic properties imply the spectra of the hydrogen-bonded
fluorophore should be broad and not well-defined. This is con-
sistent with the spectra experimentally measured at high con-
centration.
An interesting feature of 1 and 2 is their triphenylamine
moiety. This is known to undergo reversible oxidation in addi-
tion to color changes between the neutral and oxidized states
that can readily be tracked by the user. The extended conjuga-
tion of 1 and 2 also makes them interesting electrochromic
candidates because it should enhance the color difference be-
tween their neutral and oxidized states. The anodic electro-
chemical properties of 1 and 2 were therefore investigated by
cyclic voltammetry in various solvents. The azomethines were
found to exhibit either one or two oxidation waves contingent
on solvent. The oxidation was reversible in acetonitrile, di-
chloromethane, and THF (Figure 9A). In dichloromethane, both
fluorophores exhibited two reversible one-electron oxidations.
The absorption changes and corresponding colors of 1, 2, 5,
and 6 with electrochemical oxidation were also examined by
spectroelectrochemistry. The measurements were performed in
dichloromethane because of the reversible anodic processes
observed in cyclic voltammetry. Both 1 and 2 exhibited a dis-
tinct color change when oxidized. Their color switched from
yellow to blue when oxidized. In the case of 2, its absorption
at 443 nm decreased with the concomitant appearance of two
new absorptions upon oxidation. A well-defined peak at
595 nm and broad absorption band in the near infrared region
(ca. 1375 nm) were also observed (Figure 9B). Similar spectral
shifts were also observed for 1, with absorptions at 584 and
1290 nm when it was oxidized (Figure S68). Notably, a well-de-
fined isosbestic point was observed at 483 nm with electro-
chemical switching between the neutral and oxidized states.
Time-dependent (TD) DFT theoretical calculations were per-
formed to validate the observed intermolecular contact medi-
ated spectral changes. The effect of hydrogen bonding on the
excited-state dynamics and its impact on the spectroscopic
properties were previously described by theoretical means.[25]
It was found that spectral redshifting resulted from strengthen-
ing of the hydrogen bonds, whereas weakening of the bond
caused blueshifts. The electronic transition energies and the
oscillator strengths of the first five excited states of the hydro-
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
7
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
This confirms the presence of only two states (neutral and oxi-
dized) and their interdependence. Both 5 and 6 also exhibited
color switching. In contrast to 1 and 2, 5, and 6 could be
doubly oxidized (Supporting Information, Figure S67). This
gives rise to two electrochemically induced color changes, one
for each of the two one-electron oxidations. The two colors
perceived by the user when oxidizing 5 and 6 were orange
and blue (Supporting Information, Figure S69 and S70). These
colors corresponded to new absorptions appearing at 690 and
1300 nm for 6. The absorptions observed for 5 were blueshift-
ed by 10 nm relative to 6, with the corresponding CIE color
space summarized in Table 3. The parameters in this conven-
tion quantitatively describe the colors perceived by the
common user, where the L*, a*, and b* parameters refer to the
darkness, red/green, and yellow/blue color components, re-
spectively.[26] Positive a* and b* values denote the dominance
of red and yellow in the perceived color, whereas, green and
blue are dominant for negative values of a* and b*, respective-
ly. The illuminates refer to the lighting conditions, with A, B, C
corresponding to tungsten, direct, and shady daylight, respec-
tively. As per Table 2, the L* value of the oxidized form of both
1 and 2 was lower than the neutral state. The perceived color
of the oxidized form is therefore darker than that of the neu-
tral state. The a* value varied little between the neutral and
oxidized forms. This confirms the conclusion that neither red
nor green contribute to the perceived color. In contrast, the
positive b* value for the neutral state of 1 and 2 was signifi-
cantly reduced with oxidation. The predominant yellow color
perceived for the neutral form is therefore replaced with blue
when 1 and 2 are oxidized, as per the inset of Figure 9B.
Given the observed fluorescence enhancement, combined
with the reversible oxidation of 1 and 2, their fluoroelectro-
chemistry was also investigated. This involved monitoring the
fluorescence change with increasing applied potential when
exciting the sample at its corresponding absorption maximum.
For 1 and 2, the excitation wavelength selected was 430 nm.
Although aggregation-induced fluorescence enhancement was
observed in DMSO, acetonitrile, and THF, the fluoroelectro-
chemistry measurements were performed in a 1:9 (v/v) mixture
of THF/water. This solvent mixture was chosen because it gave
the best electrochemical conductivity for the electrochemical
measurements. With increasing potentials from 0 to 1.2 V, the
emission at 640 nm decreased (Figure 9C). This is a result of
the absorption of the neutral state being replaced with a red-
shifted absorption of the oxidized state upon oxidation. There-
fore, fewer neutral species are excited, resulting in the ob-
served fluorescence decrease. Reducing the oxidized state re-
stored the absorbance of the neutral state, which, in turn, re-
stored the original fluorescence of the neutral state at 640 nm.
This was observed by applying a negative potential to the oxi-
dized state. The fluorescence can therefore be turned-off and
turned-on electrochemically once the supramolecular contacts
are activated. The same behavior was also observed for 5 and
6, with the emission at 525 nm being modulated with applied
potential.
Figure 9. A) Cyclic voltammograms of 2 measured in anhydrous and deoxy-
~
&
^
*
genated acetone ( ), THF ( ), acetonitrile ( ), and dichloromethane ( ) at
100 mVsꢀ1 with 0.1m TBAPF6 and ferrocene as an internal reference. B) Spec-
troelectrochemistry of 2 in anhydrous and deoxygenated dichloromethane
&
*
with TBAPF6 as an electrolyte with applied potentials of 0 ( ) 700 ( ), 800
+
~
(
&
^
^
), 900 ( ) 1000 ( ) 1100 ( ), and ꢀ500 (?) mV vs. Ag/Ag held for 30 s
per potential. Inset: photographs showing the mesh electrode of the neutral
(left) and oxidized (right) states of 2. C) Change in fluorescence of 2 in
a THF/water mixture (1:9 v/v) with 0.1m KCl as electrolyte when excited at
~
&
*
^
430 nm with applied potentials of 0 ( ), 900 ( ), 1100 ( ), 1200 ( ), and
&
1300 mV ( ) for 60 s followed by ꢀ1000 mV (?).
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
8
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
Table 3. CIE coordinates with A, B and C illuminants with the 28 standard observer angle of 1, 2, 5, and 6.[a]
Compound
State
CIE A
CIE B
CIE C
L*
a*
b*
92.3
ꢀ28.9
65.8
ꢀ1.3
4.9
16.5
0.1
5.7
L*
a*
b*
L*
a*
b*
1
2
5
neutral
oxidized
neutral
oxidized
99.0
41.5
99.3
78.5
98.8
96.5
87.3
99.4
95.5
88.1
ꢀ2.3
9.1
ꢀ1.1
ꢀ2.0
0.6
98.0
41.8
98.5
78.7
98.7
95.5
87.7
99.4
94.3
87.9
ꢀ13.9
14.0
ꢀ11.1
ꢀ2.1
ꢀ0.8
6.4
ꢀ7.2
ꢀ2.5
11.1
103.1
ꢀ30.2
71.5
ꢀ0.9
5.6
14.7
0.8
6.9
97.4
42.1
98.0
78.7
98.6
95.1
87.8
99.3
93.8
87.8
ꢀ22.6
18.2
ꢀ18.7
ꢀ2.0
ꢀ2.0
5.2
ꢀ6.5
ꢀ3.9
10.1
1.0
106.7
ꢀ30.3
73.2
ꢀ0.8
5.9
14.0
1.1
7.4
neutral
1st oxidation
2nd oxidation
neutral
6.8
ꢀ8.5
ꢀ0.6
10.7
ꢀ1.3
6
1st oxidation
2nd oxidation
15.9
4.1
13.4
3.4
12.4
3.1
0.5
[a] Calculated for the spectral range 360–830 nm.
Theoretical calculations
Conclusion
The ground-state geometry was first optimized by using the X-ray
crystallographic coordinates of 1 as the starting point. The geome-
tries were then fully optimized. For the hydrogen-bonding studies,
the geometries of both the monomer and hydrogen-bonded
dimer were optimized. The calculations were carried out with
Gaussian 09. The ground-state geometry optimization of the two
systems was performed with DFT with B3LYP as the hybrid func-
tional and 6–31G(d) as the basis set. The electronic transition ener-
gies of the monomer and dimer were calculated by using time-de-
pendent DFT with the same functional and basis set.
It was found that the fluorescence of an intrinsically fluores-
cent fluorophore consisting of two diphenylamines and a fluo-
rene core was quenched, in part, by its two azomethine
bonds. Courtesy of the terminal amines, the fluorophore
formed an antiparallel dimer that was connected by two hy-
drogen bonds. The supramolecular assembly was reinforced by
a least three other intermolecular contacts. Intermolecular hy-
drogen-bonding involving the terminal amine was confirmed
experimentally by FTIR, NMR, and X-ray diffraction, and com-
plemented by DFT calculations. Engaging these intermolecular
contacts at high concentrations in solution and thin films,
along with water-mediated aggregation, resulted in pro-
nounced fluorescence shifts and upwards of an eightfold fluo-
rescence enhancement. Intermolecular contacts can therefore
be used to reduce the fluorescence deactivation modes of azo-
methines and enhance their otherwise quenched fluorescence.
The concentration-dependent fluorescence turn-on behavior,
coupled with other fluorescence enhancement strategies,
would be ideal to completely suppress the intrinsic azome-
thine fluorescence quenching modes and make the otherwise
weakly fluorescent fluorophores extremely fluorescent. Apply-
ing these strategies to enhance the thin-film fluorescence of
azomethines further would ultimately be beneficial for improv-
ing the efficiency of emitting devices.
Electrochemistry
Electrochemical measurements were performed with a multi-chan-
nel BioLogic VSP potentiostat. The compounds were dissolved in
the corresponding anhydrous and degassed solvent at 10ꢀ4 m with
0.1m tetrabutylammonium hexafluorophosphate as the electrolyte.
A platinum electrode was used as the working electrode with
a platinum wire as the auxiliary electrode. The reference electrode
was a silver wire electrode. Ferrocene was added to the solution as
an internal reference and the voltammograms were calibrated
against its reversible Fc/Fc+ redox couple.[27]
Spectroelectrochemistry
Spectroelectrochemical measurements were obtained with a poten-
tiostat using a Pt mesh electrode that was placed in a thin-cell
spectrometer cell. The mesh, counter, and reference electrodes
were all placed in the UV cell that was inserted into a UV/Vis/NIR
spectrometer. The electrodes were connected to the potentiostat.
Samples were prepared by dissolving the compound of study in
the corresponding anhydrous and degassed solvent with 0.1m tet-
rabutylammonium hexafluorophosphate as the electrolyte. A plati-
num mesh working electrode, platinum wire auxiliary electrode,
and silver wire reference electrode were used.
Experimental Section
Spectroscopy
Absorption spectra were measured with Varian Cary 500 combined
UV/Vis/NIR spectrometer. Fluorescence measurements were ob-
tained with an Edinburgh Instruments FLSP-920 combined steady-
state time-resolved spectrometer. Solutions for fluorescence and
quantum yields measurements were prepared with an absorbance
ꢂ0.1 at the corresponding absorption maxima. Prior to measure-
ments, the samples were purged with nitrogen for at least 20 min.
Quantum yields were calculated against naphthalene in cyclohex-
ane (Ffl =0.23) and correcting for the difference in refractive index
of the solvent used for the actinometry and the compound of
study.[23]
Fluoroelectrochemistry
The fluorescence changes at different applied potentials were mea-
sured by using a combined steady-state time-resolved spectrome-
ter connected to a potentiostat. The sample was rotated by 458
relative to the excitation beam. The samples were prepared by dis-
solving the compounds in THF (0.2 mL) and distilled water (1.8 mL)
to which was added KCl (15 mg). The solutions were purged with
nitrogen for at least 20 min. A platinum mesh working electrode,
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
9
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
platinum wire auxiliary electrode, and silver wire reference elec-
trode were used.
123.3, 120.9, 119.6, 37.0 ppm. HRMS: m/z calcd: 556.2145 [M*]+;
found: 556.2153; m/z calcd 557.2224 [M+H]+; found: 557.2238.
Compound 6: Anhydrous dimethylformamide (1.20 mL) and POCl3
(1.5 mL) were added at 08C to a solution of 4 (0.40 g, 0.60 mmol)
dissolved in anhydrous 1,2-dichloroethane (10 mL). The mixture
was heated to reflux for 15 h under a nitrogen atmosphere and
then cooled to RT and neutralized with a saturated solution of
NaHCO3. The crude product was extracted with dichloromethane
and purified by SiO2 column chromatography (ethyl acetate/
hexane, 1:9). The titled compound (370 mg, 85%) was isolated as
Preparation of thin films and encapsulation
Thin films were prepared by drop-casting dichloromethane solu-
tions of the fluorophores onto glass microscope slides. The thick-
ness of the as-cast film of 2 was 40 nm. Encapsulation of the azo-
methine in a PMMA matrix (Mn =70 000 gmolꢀ1) was performed by
dissolving the fluorophore and PMMA in 1:10, 1:30, 1:50, 1:80, and
1:100 weight ratios in dichloromethane. The solutions were drop-
cast onto glass slides. The thin films deposited had thicknesses of
200 nm (1:10), 150 nm (1:30), 160 nm (1:50), 400 nm (1:80) and
230 nm (1:100).
1
a yellow solid. H NMR (400 MHz, [D6]acetone): d=9.84 (s, 2H), 7.82
(d, J=8.1 Hz, 2H), 7.77–7.72 (d, J=8.8 Hz, 4H), 7.45–7.38 (m, 4H),
7.34 (d, J=1.9 Hz, 2H), 7.25 (dt, J=13.9, 4.3 Hz, 6H), 7.16 (dd, J=
8.1, 2.0 Hz, 2H), 7.05 (d, J=8.7 Hz, 4H), 2.05–1.92 (m, 4H), 1.17–
1.07 (m, 12H), 0.81 (t, J=7.1 Hz, 6H), 0.74–0.72 ppm (m, 4H);
13C NMR (100 MHz, [D6]acetone): d=190.8, 154.4, 153.8, 147.5,
146.5, 139.1, 132.0, 130.9, 130.6, 127.1, 126.7, 126.2, 122.5, 122.0,
120.1, 56.4, 40.7, 32.4, 29.8, 24.8, 23.3, 14.5 ppm; HRMS: m/z calcd:
725.4102 [M+H]+; found: 725.4092.
Synthesis
Compound 3: Diphenylamine (1.30 g, 7.7 mmol), CsCO3 (1.54 g,
8.00 mmol), Pd(OAc)2 (70 mg, 0.31 mmol), and tBu3P (62 mg,
0.31 mmol) were added to a solution of 2,7-dibromo-9H-fluorene
(1.00 g, 3.08 mmol) in anhydrous toluene (20 mL). The mixture was
heated under a nitrogen atmosphere at 1208C for 24 h. After cool-
ing, the solvent was evaporated under reduced pressure and the
resulting residue was solubilized in CH2Cl2 and extracted with
water and brine. The organic layer was dried over MgSO4, filtered,
and then concentrated. The crude product was purified by column
chromatography on silica gel (hexane/CH2Cl2, 9:1) to afford the
title compound (1.22 g, 79%) as a pale-brown solid. 1H NMR
(400 MHz, CDCl3): d=7.55 (d, J=8.2 Hz, 2H), 7.26–7.21 (m, 10H),
7.11 (d, J=7.8 Hz, 10H), 6.99 (t, J=7.3 Hz, 4H), 3.72 ppm (s, 2H);
13C NMR (100 MHz, CDCl3): d=148.2, 146.4, 144.7, 136.9, 129.3,
124.0, 123.9, 122.6, 121.6, 120.1, 36.9 ppm. HRMS: m/z calcd:
500.2247 [M*]+; found: 500.2258; m/z calcd: 501.2325 [M+H]+;
found: 501.2337.
Compound 1: A catalytic amount of TFA that was diluted in EtOH
was added to a solution of 5 (100.7 mg, 0.18 mmol) and 7
(93.4 mg, 0.36 mmol) in anhydrous ethanol (10 mL). The reaction
mixture was then heated at 808C for 15 h, then the TFA was neu-
tralized by adding a small amount of Et3N and the solvent was
evaporated. The crude product was purified by SiO2 column chro-
matography (ethyl acetate/hexane, 4:6 with 1% Et3N). The title
1
compound (150 mg, 80%) was isolated as a dark-red solid. H NMR
(400 MHz, CDCl3): d=7.83 (s, 2H), 7.57 (d, J=8.2 Hz, 2H), 7.53 (d,
J=8.8 Hz, 4H), 7.26–7.20 (m, 6H), 7.11–7.02 (m, 8H), 6.97 (d, J=
8.8 Hz, 4H), 6.24 (s, 4H), 4.33 (q, J=7.1 Hz, 4H), 4.19 (q, J=7.1 Hz,
4H), 3.70 (s, 2H), 1.34 (t, J=7.1 Hz, 6H), 1.25 ppm (t, J=7.1 Hz,
6H); 13C NMR (100 MHz, CDCl3): d=164.0, 163.1, 159.5, 150.6,
149.5, 146.1, 144.6, 144.2, 136.8, 132.7, 128.6, 128.4, 128.3, 125.3,
124.4, 123.8, 123.2, 121.5, 120.0, 119.6, 100.5, 59.6, 58.6, 35.6, 12.9,
12.8 ppm; HRMS: m/z calcd: 1037.3361 [M+H]+; found:
1037.3337.
Compound 4: Diphenylamine (1.03 g, 6.1 mmol), CsCO3 (1.22 g,
6.32 mmol), Pd(OAc)2 (55 mg, 0.24 mmol), and tBu3P (50 mg,
0.25 mmol) were added to a solution of 2,7-dibromo-9,9-dihexyl-
9H-fluorene (1.20 g, 2.44 mmol) in anhydrous toluene (20 mL). The
mixture was heated under a nitrogen atmosphere at 1208C for
24 h. After cooling, the solvent was evaporated under reduced
pressure and the residue was solubilized in CH2Cl2 and extracted
with water and brine. The organic layer was dried over MgSO4, fil-
tered, and then concentrated. The crude product was purified by
column chromatography on silica gel (hexane/CH2Cl2, 10:1) and
the title compound (1.31 g, 80%) was isolated as a white solid.
1H NMR (400 MHz, [D6]acetone): d=7.64 (d, J=8.2 Hz, 2H), 7.31–
7.25 (m, 8H), 7.15 (d, J=1.9 Hz, 2H), 7.10–7.07 (m, 8H), 7.04–6.98
(m, 6H), 1.89–1.81 (m, 4H), 1.18–1.07 (m, 12H), 0.81 (t, J=7.2 Hz,
6H), 0.75–0.70 ppm (m, 4H); 13C NMR (100 MHz, [D6]acetone): d=
152.9, 148.9, 147.6, 137.4, 130.1, 124.6, 124.4, 123.4, 120.9, 120.4,
55.8, 40.8, 32.3, 24.6, 23.2, 14.3 ppm. HRMS: m/z calcd: 669.4203
[M+H]+; found 669.4206.
Compound 2: A catalytic amount of TFA that was previously dilut-
ed in EtOH was added to the solution of 6 (163.4 mg, 0.22 mmol)
and 7 (116.4 mg, 0.45 mmol) in anhydrous ethanol (10 mL). The re-
action mixture was heated at 808C for 15 h, then the TFA was neu-
tralized by adding a small amount of Et3N and the solvent was
evaporated. The crude product was purified by SiO2 column chro-
matography (ethyl acetate/hexane, 4:7 with 1% Et3N) to afford the
title compound (165 mg, 61%) as
a
dark-red solid. 1H NMR
(400 MHz, CDCl3): d=7.90 (s, 2H), 7.59 (d, J=8.8 Hz, 4H), 7.53 (d,
J=8.1 Hz, 2H), 7.33–7.27 (m, 4H), 7.15 (d, J=7.6 Hz, 4H), 7.11–7.02
(m, 10H), 6.26 (s, 4H), 4.38 (q, J=7.1 Hz, 4H), 4.25 (q, J=7.1 Hz,
4H), 1.83–1.73 (m, 4H), 1.40 (t, J=7.1 Hz, 6H), 1.31 (t, J=7.1 Hz,
6H), 1.18–1.01 (m, 12H), 0.81 (t, J=7.2 Hz, 6H), 0.67 ppm (m, 4H);
13C NMR (100 MHz, CDCl3): d=165.8, 164.7, 159.2, 152.7, 152.5,
150.6, 147.2, 145.8, 137.2, 135.4, 129.8, 129.6, 129.0, 128.3, 125.2,
124.8, 124.0, 121.5, 120.4, 120.3, 103.2, 61.5, 60.4, 55.3, 40.2, 31.7,
29.6, 23.9, 22.6, 14.5, 14.3, 14.2 ppm; HRMS: m/z calcd: 1205.5239
[M+H]+; found: 1205.5237.
Compound 5: Anhydrous dimethylformamide (2.3 mL) and POCl3
(2.9 mL) were added at 08C to a solution of 3 (0.6 g, 1.2 mmol) dis-
solved in anhydrous 1,2-dichloroethane (5 mL). The mixture was
heated to reflux for 15 h under a nitrogen atmosphere and then
cooled to RT and neutralized with a saturated solution of NaHCO3.
The crude product was extracted with dichloromethane and puri-
fied by SiO2 column chromatography (dichloromethane/hexane,
8:2). The title compound (530 mg, 81%) was isolated as a yellow
Single-crystal structure determination
X-ray diffraction data (Table 4) were collected with a Bruker APEX-II
CCD diffractometer with GaKa radiation (l=1.34139 ꢃ). The crystal
was kept at 100 K during data collection. Using Olex2,[28] the struc-
ture was solved with the XT[29] structure solution program using
direct methods and refined with the XL[30] refinement package
using least-squares minimization. CCDC 1454905 contains the sup-
1
solid. H NMR (400 MHz, CDCl3): d=9.82 (s, 2H), 7.72–7.66 (m, 6H),
7.38–7.32 (m, 6H), 7.23–7.15 (m, 8H), 7.03 (d, J=8.7 Hz, 4H),
3.82 ppm (s, 2H); 13C NMR (100 MHz, CDCl3): d=190.6, 153.60,
146.4, 145.2, 145.1, 138.5, 131.5, 129.9, 129.3, 126.3, 125.6, 125.2,
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
10
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
2010, 114, 13106–13112; e) N. Kawatsuki, H. Matsushita, T. Washio, J.
Kozuki, M. Kondo, T. Sasaki, H. Ono, Macromolecules 2013, 46, 2092;
f) D. Tsang, M. Bourgeaux, W. G. Skene, J. Photochem. Photobiol. A 2007,
192, 122–129.
Table 4. Details of crystal structure determination of 1.
CCDC
1454905
formula
C59H52N6O8S2
1037.18
Dark red
0.08ꢄ0.08ꢄ0.04
100; 1.170
monoclinic
P21/c
32.5784(7)
9.0681 (2)
´
[4] a) Y. Luo, M. Utecht, J. Dokic, S. Korchak, H.-M. Vieth, R. Haag, P. Saal-
Mw [gmolꢀ1]; F(000)
crystal color and form
crystal size [mm]
frank, ChemPhysChem 2011, 12, 2311–2321; b) Y. Luo, S. Korchak, H.-M.
Vieth, R. Haag, ChemPhysChem 2011, 12, 132–135; c) J. Mielke, F. Leyss-
ner, M. Koch, S. Meyer, Y. Luo, S. Selvanathan, R. Haag, P. Tegeder, L.
Grill, ACS Nano 2011, 5, 2090–2097; d) L. Greb, J.-M. Lehn, J. Am. Chem.
Soc. 2014, 136, 13114–13117; e) P. J. Coelho, M. C. R. Castro, M. M. M.
Raposo, J. Photochem. Photobiol. A 2013, 259, 59–65; f) C. Gahl, D.
Brete, F. Leyssner, M. Koch, E. R. McNellis, J. Mielke, R. Carley, L. Grill, K.
Reuter, P. Tegeder, M. Weinelt, J. Am. Chem. Soc. 2013, 135, 4273–4281.
[5] a) A. Singh, R. Singh, M. Shellaiah, E. C. Prakash, H.-C. Chang, P. Raghu-
nath, M.-C. Lin, H.-C. Lin, Sens. Actuators B 2015, 207, 338–345; b) M.
Yang, Y. Zhang, W. Zhu, H. Wang, J. Huang, L. Cheng, H. Zhou, J. Wu, Y.
Tian, J. Mater. Chem. C 2015, 3, 1994–2002; c) M. Shellaiah, T. Simon, V.
Srinivasadesikan, C.-M. Lin, K. W. Sun, F.-H. Ko, M.-C. Lin, H.-C. Lin, J.
Mater. Chem. C 2016, 4.
T [K]; dcalcd. [gcmꢀ3
crystal system
space group
unit cell: a [ꢁ]
b [ꢁ]
]
c [ꢁ]
a [8]
19.9889(4)
90
b [8]
94.1330(14)
90
5889.8(2); 4
4.732 to 121.406; 99.8%
106070/13529; Rint =0.0969,
g [8]
V [ꢃ3]; Z
q range [8]; completeness
reflections: collected/independ-
ent; Rint
m [mmꢀ1
[6] a) G. Yang, S. Li, S. Wang, Y. Li, C. R. Chim. 2011, 14, 789–798; b) V. S. Pa-
dalkar, S. Seki, Chem. Soc. Rev. 2016, 45, 169–202.
R
sigma =0.0582
]
0.824
[7] a) J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam, B. Z. Tang, Chem. Rev.
2015, 115, 11718–11940; b) W. Lingyun, Y. Lingling, C. Derong, Curr.
Org. Chem. 2014, 18, 1028–1049; c) L. Zhu, Y. Zhao, J. Mater. Chem. C
2013, 1, 1059–1065; d) S. S. Babu, K. K. Kartha, A. Ajayaghosh, J. Phys.
Chem. Lett. 2010, 1, 3413–3424; e) L. Wang, G. Fang, D. Cao, J. Macro-
mol. Sci. Pure Appl. Chem. 2014, 51, 668–681; f) J. Chen, B. Z. Tang, Re-
stricted Intramolecular Rotations: A Mechanism for Aggregation-Induced
Emission, in Aggregation-Induced Emission: Fundamentals and Applica-
tions, Volumes 1 and 2 (Eds.: A. Qin, B. Z. Tang), John Wiley and Sons
Ltd, 2013, pp. 307–322; g) Y. Q. Dong, C. Li, W. Zhao, Y. Dong, B. Z.
Tang, J. Molecul. Eng. Mater. 2013, 1, 1340010/1–1340010/13.
[8] M. Tsuchimoto, N. Yoshida, A. Sugimoto, N. Teramoto, K. Nakajima, J.
Mol. Struct. 2016, 1105, 152–158.
abs. Corr.
0.937–0.968
R1(F); wR(F2) [I> 2s(I)]
R1(F); wR(F2) (all data)
GoF(F2)
R1 =0.0761; wR2 =0.2096
R1 =0.1186; wR2 =0.2322
1.039
0.42/ꢀ0.30
max. residual eꢀ density
plementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre.
[9] a) Z. Chen, J. Liang, X. Han, J. Yin, G.-A. Yu, S. H. Liu, Dyes Pigm. 2015,
112, 59–66; b) S. J. Ananthakrishnan, E. Varathan, N. Somanathan, V.
Subramanian, A. B. Mandal, J. D. Sudha, R. Ramakrishnan, J. Mater.
Chem. C 2014, 2, 9035–9044.
Acknowledgements
The authors acknowledge both NSERC Canada and the Canada
Foundation for Innovation for operating and equipment
grants, respectively. M.-H.T. acknowledges the Fonds de recher-
che du Quꢀbec - Nature et technologies for a graduate schol-
arship. The assistance of Dr. M. Simard is greatly appreciated
for the X-ray crystal structure resolution. WestGrid (www.west-
grid.ca) and Compute Canada/Calcul Canada (www.compute-
canada.ca) are both acknowledged for access to software and
computational resources. The Center for Self-Assembled Chem-
ical Structures is also acknowledged.
[10] a) C.-W. Lin, C.-T. Chen, AIE or AIEE Materials for Electroluminescence Ap-
plications, in Aggregation-Induced Emission: Fundamentals and Applica-
tions, Volumes 1 and 2, John Wiley and Sons Ltd, 2013, pp. 1–41; b) Y.
Gong, Y. Zhang, W. Z. Yuan, J. Z. Sun, Y. Zhang, J. Phys. Chem. C 2014,
118, 10998–11005; c) S. Jin, Y. Tian, F. Liu, J. Chen, S. Deng, N. Xu, J.
Mater. Chem. C 2015, 3, 8066–8073; d) S. Kamino, A. Muranaka, M. Mur-
akami, A. Tatsumi, N. Nagaoka, Y. Shirasaki, K. Watanabe, K. Yoshida, J.
Horigome, S. Komeda, M. Uchiyama, S. Enomoto, Phys. Chem. Chem.
Phys. 2013, 15, 2131–2140; e) C. Coya, C. Ruiz, A. L. Alvarez, S. Alvarez-
Garcia, E. M. Garcia-Frutos, B. Gomez-Lor, A. de Andres, Org. Electron.
2012, 13, 2138–2148; f) S. J. Ananthakrishnan, E. Varathan, V. Subrama-
nian, N. Somanathan, A. B. Mandal, J. Phys. Chem. C 2014, 118, 28084–
28094.
[11] B.-K. An, J. Gierschner, S. Y. Park, Acc. Chem. Res. 2011, 44, 544–554.
[12] J. W. Chung, S.-J. Yoon, B.-K. An, S. Y. Park, J. Phys. Chem. 2013, 117,
11285–11291.
Keywords: conjugation · electrochemistry · fluorescence ·
solvatochromism · supramolecular chemistry
[13] a) W. Luo, Y. Zhu, J. Zhang, J. He, Z. Chi, P. W. Miller, L. Chen, C.-Y. Su,
Chem. Commun. 2014, 50, 11942–11945; b) Y. Cao, M. Yang, Y. Wang, H.
Zhou, J. Zheng, X. Zhang, J. Wu, Y. Tian, Z. Wu, J. Mater. Chem. C 2014,
2, 3686–3694.
[14] S. Chopra, N. Singh, P. Thangarasu, V. K. Bhardwaj, N. Kaur, Dyes Pigm.
2014, 106, 45–50.
[1] a) M. L. Petrus, F. S. F. Morgenstern, A. Sadhanala, R. H. Friend, N. C.
Greenham, T. J. Dingemans, Chem. Mater. 2015, 27, 2990–2997; b) M. L.
Petrus, T. Bein, T. J. Dingemans, P. Docampo, J. Mater. Chem. A 2015, 3,
12159–12162; c) X. Ma, H. Niu, H. Wen, S. Wang, Y. Lian, X. Jiang, C.
Wang, X. Bai, W. Wang, J. Mater. Chem. C 2015, 3, 3482–3493.
[15] C. Mallet, C. Moussallem, A. Faurie, M. Allain, F. Gohier, W. G. Skene, P.
Frꢁre, Chem. Eur. J. 2015, 21, 7944–7953.
[16] S. Pramanik, V. Bhalla, M. Kumar, ACS Appl. Mater. Interfaces 2015, 7,
22786–22795.
[17] L. Wang, Z. Zheng, Z. Yu, J. Zheng, M. Fang, J. Wu, Y. Tian, H. Zhou, J.
Mater. Chem. C 2013, 1, 6952–6959.
[18] a) A. Priimagi, J. Vapaavuori, F. J. Rodriguez, C. F. J. Faul, M. T. Heino, O.
Ikkala, M. Kauranen, M. Kaivola, Chem. Mater. 2008, 20, 6358–6363;
b) D. Gonzꢅlez-Rodrꢆguez, A. P. H. J. Schenning, Chem. Mater. 2011, 23,
310–325; c) S. Yagai, K. Iwai, M. Yamauchi, T. Karatsu, A. Kitamura, S.
Uemura, M. Morimoto, H. Wang, F. Wꢇrthner, Angew. Chem. Int. Ed.
[2] a) X. Ma, Y. Wu, H. Wen, H. Niu, C. Wang, C. Qin, X. Bai, L. Lei, W. Wang,
RSC Adv. 2016, 6, 4564–4575; b) A. Leliꢁge, S. Barik, W. G. Skene, ACS
Appl. Mater. Interfaces 2014, 6, 6920–6929; c) L. Sicard, D. Navarathne,
T. Skalski, W. G. Skene, Adv. Funct. Mater. 2013, 23, 3549–3559; d) L.
Hao, W. Wang, Y. Sun, H. Niu, J. Electroanal. Chem. 2015, 742, 74–83.
[3] a) M. Shyamal, P. Mazumdar, S. Maity, G. P. Sahoo, G. Salgado-Morꢅn, A.
Misra, J. Phys. Chem. A 2016, 120, 210–220; b) A. Kathiravan, K. Sundar-
avel, M. Jaccob, G. Dhinagaran, A. Rameshkumar, D. Arul Ananth, T. Siva-
sudha, J. Phys. Chem. B 2014, 118, 13573–13581; c) G. Boice, B. O. Pat-
rick, R. McDonald, C. Bohne, R. Hicks, J. Org. Chem. 2014, 79, 9196–
9205; d) S. Dufresne, I. U. Roche, T. Skalski, W. G. Skene, J. Phys. Chem. C
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
11
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
2014, 53, 2602–2606; Angew. Chem. 2014, 126, 2640–2644; d) S. Yagai,
T. Karatsu, A. Kitamura, Chem. Eur. J. 2005, 11, 4054–4063; e) N. H. Dam-
rauer, J. M. Hodgkiss, J. Rosenthal, D. G. Nocera, J. Phys. Chem. B 2004,
108, 6315–6321; f) A. P. H. J. Schenning, J. v. Herrikhuyzen, P. Jonkheijm,
Z. Chen, F. Wꢇrthner, E. W. Meijer, J. Am. Chem. Soc. 2002, 124, 10252–
10253; g) H. Kar, S. Ghosh, Chem. Commun. 2014, 50, 1064–1066; h) M.
Shellaiah, Y. C. Rajan, H.-C. Lin, J. Mater. Chem. 2012, 22, 8976–8987;
i) M. Shellaiah, M. V. Ramakrishnam Raju, A. Singh, H.-C. Lin, K.-H. Wei,
H.-C. Lin, J. Mater. Chem. A 2014, 2, 17463–17476; j) C. B. Aakerçy, A. M.
Beatty, B. A. Helfrich, Angew. Chem. Int. Ed. 2001, 40, 3240–3242;
Angew. Chem. 2001, 113, 3340–3342.
[23] A. M. Brouwer, Pure Appl. Chem. 2011, 83, 2213–2228.
[24] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer,
New York, 2006.
[25] G.-J. Zhao, K.-L. Han, ChemPhysChem 2008, 9, 1842–1846.
[26] CIE Technical Report on Colorimetry, Commission Internationale de L’ꢂ-
clairage 2004 CIE 15:2004 79, 72 pages.
[27] N. G. Connelly, W. E. Geiger, Chem. Rev. 1996, 96, 877–910.
[28] O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard, H. Pusch-
mann, J. Appl. Crystallogr. 2009, 42, 339–341.
[29] G. Sheldrick, Acta Crystallogr. Sect. A 2015, 71, 3–8.
[30] G. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112–122.
[19] A. Bolduc, C. Mallet, W. G. Skene, Sci. China Chem. 2013, 56, 3–23.
[20] J. Gierschner, L. Lꢇer, B. Miliꢅn-Medina, D. Oelkrug, H.-J. Egelhaaf, J.
Phys. Chem. Lett. 2013, 4, 2686–2697.
[21] F. Wꢇrthner, T. E. Kaiser, C. R. Saha-Mçller, Angew. Chem. Int. Ed. 2011,
50, 3376–3410; Angew. Chem. 2011, 123, 3436–3473.
[22] a) I. M. Smallwood, Handbook of Organic Solvent Properties, 1996;
b) J. A. Riddick, W. B. Bunger, T. K. Sakano, Organic Solvents: Physical
Properties and Methods of Purification, Wiley, New York, 1986.
Received: February 23, 2016
Revised: April 11, 2016
Published online on && &&, 0000
&
&
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
12
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Turn on the lights! The fluorescence of
an electrochromic azomethine,
&
Fluorescence Enhancement
M. Wałe˛sa-Chorab, M.-H. Tremblay,
W. G. Skene*
quenched by its conjugated imines, can
be enhanced by activating supramolec-
ular intra- and intermolecular contacts
(see figure). The enhanced fluorescence
can also be switched off and on electro-
chemically.
&& – &&
Hydrogen-Bond and Supramolecular-
Contact Mediated Fluorescence
Enhancement of Electrochromic
Azomethines
Chem. Eur. J. 2016, 22, 1 – 13
www.chemeurj.org
13
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ