Aggregation-induced bathochromic fluorescent enhancement for fluorenone dyes

Article abstract of DOI:10.1016/j.dyepig.2015.08.011

Organic solid-state luminescence materials with aggregation-induced emission enhancement have attracted considerable interest in recent years. In this work, we develop a class of 2,7-diphenylfluorenone derivatives that exhibit prominent aggregation-induce

Full text of DOI:10.1016/j.dyepig.2015.08.011

Dyes and Pigments 123 (2015) 355e362
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Aggregation-induced bathochromic fluorescent enhancement for
fluorenone dyes
,
Mao-Sen Yuan a b, Xianchao Du a, Fan Xu a, Dong-En Wang a, Wen-Ji Wang a, Tian-Bao Li a,
,
*
Qin Tu a, Yanrong Zhang a, Zhenting Du a, Jinyi Wang a
a College of Science, Northwest A&F University, Yangling 712100, PR China
b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Organic solid-state luminescence materials with aggregation-induced emission enhancement have
attracted considerable interest in recent years. In this work, we develop a class of 2,7-diphenylfluorenone
derivatives that exhibit prominent aggregation-induced emission properties with high solid-state
quantum yields. Their solid-state powders show a ~160 nm bathochromically shifted fluorescence and
longer lifetime compared to their emission in dilute THF solution. The X-ray single structures and
photophysical properties reveal that the mechanism of aggregation-induced emission in this class of
fluorenone compounds is due to the formation of static excimers through hydrogen bonds in the solid
state. The emission of their THF solution peaked at 380 nm and originates from the single molecule and
the emission of their solid-state powder peaked at 550 nm originates from the excimers. The
aggregation-induced emission properties and luminescence mechanism are further verified by the
design of 2,7-diphenylfluorenone-diboronic acid adduct, which exhibited weak luminescence in high pH
buffer and red-shifted strong luminescence in acidic buffer.
Received 2 July 2015
Received in revised form
6 August 2015
Accepted 7 August 2015
Available online 28 August 2015
Keywords:
Aggregation-induced emission
Solid-state fluorescence
Fluorenone
Crystal structure
Luminescence mechanism
Static excimer
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
referred to as aggregation induced emission (AIE) enhancement,
was first reported by the Tang group [5]. This phenomenon offers a
Organic luminescent materials have attracted numerous inter-
est not only in the fundamental research field of photochemistry
but also in the applied field of optoelectronic devices [1]. Having
sufficient high solid-state luminescent efficiency is a fundamental
issue for luminescent materials, because most organic devices are
only able to function in either a film or crystalline state [2]. How-
ever, general organic fluorophores are highly emissive only in dilute
solution and tend to show a decrease of luminescent efficiency in
the solid state due to the aggregation-induced quenching (AIQ)
effect [3].
Great effort has been exerted to enhance the solid-state effi-
ciency of organic luminescent materials by using chemical, phys-
ical, and engineering approaches [4]. However, these attempts have
achieved limited success. In 2001, an anti-AIQ phenomenon that a
non-emissive compound in organic solution that exhibits an obvi-
ously enhanced fluorescence in its aggregated state, which is
new path to obtain ideal organic solid-state luminescent materials.
Consequently, a number of AIE molecules with different structures
were developed by various research groups in the past decade
[1c,6e16]. However, current studies of the AIE materials are pri-
marily limited to several classes, such as silole, tetraphenylethene
(TPE), tetraphenylpyrazine, cyano-stilbene and difluoroboron avo-
benzone compounds. For discoveries of new organic AIE systems,
the design and synthesis of new AIE materials with high lumines-
cent efficiency still remain important.
In addition, deciphering the AIE working principle is also of
great significance. A correct mechanistic understanding of the AIE
process may help to obtain new photophysical insights and develop
new AIE luminogens. For the reported AIE processes, several
possible mechanisms have been proposed to cater to the respective
spectral phenomena [17], including restriction of intramolecular
rotation (RIR) [17c], intramolecular planarization [6], the formation
of J-aggregates [8], inhibition of photoisomerisation and the
blockage of non-radiative relaxation pathways of the excited spe-
cies [17d]. At present, the RIR mechanism has been widely explored
* Corresponding author. Tel./fax: þ86 29 87082520.
E-mail address: jywang@nwsuaf.edu.cn (J. Wang).
http://dx.doi.org/10.1016/j.dyepig.2015.08.011
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

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M.-S. Yuan et al. / Dyes and Pigments 123 (2015) 355e362
and becomes the well accepted one in some AIE systems [17a].
However, the other proposed mechanisms have not been
completely confirmed and approved until now, because of the lack
of the in-depth verified experimental data. Elaborately designing
experiments and strategies to verify the AIE mechanisms still
remain a challenge and are of academic interest.
2. Experimental section
2.1. Synthesis and characterizations of the subject compounds
Solvents for reactions and spectral measurements were dried
and distilled before use. The reagents used for reactions were
purchased from J&K Scientific Ltd. 1H NMR spectra were recorded
at 25 ꢀC on Bruker Avance 500 MHz spectrometer using CDCl3 as
solvent. 13C NMR spectra were recorded at 25 ꢀC on Bruker Avance
125 MHz spectrometer using CDCl3 as solvent. Element analyses (C,
H) were performed using a PE 2400 autoanalyser. Mass spectrom-
etry analyses were performed by a Bruker Biflex III matrix assisted
laser desorption/ionization time of flight (MALDI-TOF) mass
spectrometer.
With this concept in mind, we have embarked on the devel-
opment of new AIE compounds to enlarge the AIE library. Fluo-
rene, with perfect rigidity planarity and
p-conjugation, has been
widely used as an optoelectronic material due to its excellent
luminescent properties. However, in contrast to fluorene, fluo-
renone compounds have seldom been studied because they are
almost non-emissive in solution [18]. Their AIE property was
firstly found and reported by the Tao group [18a]. To carry out
more extensive investigations and ascertain the AIE mechanisms,
we synthesized two aryl substituted fluorenone derivatives: 2,7-
diphenyl-9H-fluoren-9-one (DPF) and 2,7-bis(2-methylphenyl)-
9H-fluoren-9-one (MDPF) in this paper. Their molecular structures
are shown in Scheme 1; both of these compounds exhibit typical
AIE properties and possess high solid-state fluorescence quantum
yields. In addition, they exhibit a considerably different spectral
character with those reported character to classic AIE compounds,
which generally keep an almost identical emission peak position
regardless of whether they are in the solid state or in solution.
However, the solid-state fluorescence of the two fluorenone
compounds shows a ~160 nm red-shift in comparison with their
dilute THF solution. The abnormal spectra characteristics indicate
their abnormal AIE procedure. To reveal the mechanism and have
an insight into the optical process, we cultured single crystals of
each compound and determined the X-ray crystal structures.
Furthermore, enlightened from the reference reported by the Tang
group [19], we designed and synthesized a DPF-diboronic acid
adduct, ((9H-fluorene-2,7-diyl)bis(4,1-phenylene))diboronic acid
(BDPF), through functionalising the terminal positions of DPF
with two boronic acid units. The AIE mechanism was further
verified by the spectral characteristics of BDPF in various pH
buffers.
Compound DPF was synthesized according to literature method
reported by us [17b].
2.1.1. Synthesis of 2,7-bis(2-methylphenyl)-9H-fluoren-9-one
(MDPF)
A mixture of 2,7-dibromo-9H-fluoren-9-one (0.60 g,1.77 mmol),
2-methylphenylboronic acid (0.60 g, 4.41 mmol), Pd(PPh3)4 (20 mg,
0.02 mmol), toluene (20 mL), ethanol (5 mL) and 2 M K2CO3
aqueous solution (2 mL) was heated to 80 ꢀC with stirring under an
argon atmosphere. The yellowish-green crystals directly precipi-
tated from the reaction system after the mixture reacted for
approximately 3 h at 80 ꢀC. The crude crystals were recrystallized
from tetrahydrofuran (THF)/ethanol (3:7, v/v) to yield compound
MDPF (0.49 g, 78%). Melting point (m.p.): 138 ꢀCe140 ꢀC. IR (KBr,
cmꢁ1): 3414, 3051, 2952, 1925, 1710, 1605, 1463, 1381, 1111, 840,
758,735. 1H NMR (CDCl3, 500 MHz, ppm):
(m, 8H), 7.45e7.47 (d, J ¼ 7.5 Hz, 2H), 7.58e7.60 (d, J ¼ 7.6 Hz, 2H)
and 7.65 (s, 2H). 13C NMR (CDCl3, 125 MHz, ppm):
193.9, 143.1,
142.9, 140.6, 135.6, 135.3, 134.5, 130.6, 129.5, 127.9, 126.1, 125.2,
120.2 and 20.5. TOF-MS-EI: m/z 360.1 [M]þ. Elemental anal. calcd.
for C27H20O: C, 89.97 and H, 5.59. Found: C, 89.68 and H, 5.72.
d 2.31 (s, 6H), 7.25e7.30
d
2.1.2. Synthesis of 2,7-bis(4-bromophenyl)-9H-fluoren-9-one (1)
A mixture of compound DPF (0.30 g, 0.90 mmol), Br2 (0.20 mL,
3.9 mmol) and H2O (10 mL) were heated to reflux and stirred for
10 h. Afterwards, the mixture was cooled to room temperature and
filtered off under suction, washed with water. The residue was
recrystallized from THF to yield 2,7-bis(4-bromophenyl)-9H-fluo-
ren-9-one 1 (0.25 g, 57%) as an orange crystal. Melting point (m.p.):
283 ꢀCe284 ꢀC. IR (KBr, cmꢁ1): 3414, 3048, 1714, 1605, 1582, 1458,
1296, 1177, 1070, 1003, 815, 782, 744. 1H NMR (CDCl3, 500 MHz,
ppm):
d 7.51e7.53 (m, 4H), 7.61e7.65 (m, 6H), 7.72e7.74 (d,
J ¼ 7.7 Hz, 2H) and 7.90 (s, 2H). 13C NMR (CDCl3, 125 MHz, ppm):
d
193.5, 143.3, 141.1, 138.7, 135.3, 133.2, 132.1, 128.4, 122.9, 122.3 and
121.0. TOF-MS-EI: m/z 488.3 [M]þ.
2.1.3. Synthesis of 2,7-bis(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-9H-fluoren-9-one (2)
A mixture of compound 1 (0.20 g, 0.40 mmol), bis(pinacolato)
diboron (0.26 g, 1.0 mmol), potassium acetate (0.20 g, 2.0 mmol)
and bis(triphenylphosphine)palladium(II) chloride (10.2 mg) in
dimethylsulfoxide (DMSO, 10 mL) was stirred under Argon at 80 ꢀC
for 20 h. After completion, solvent was removed in vacuo and the
crude product was purified by column chromatography on silica gel
and elution with dichloromethane-petroleum ether (1:1, v/v)
yielded compound 2 (0.20 g, 87%) as yellow crystals. Melting point
(m.p.): >300 ꢀC. IR (KBr, cmꢁ1): 3419, 2976, 1714, 1608, 1520, 1465,
1361, 1144, 1091, 857, 824, 656. 1H NMR (CDCl3, 500 MHz, ppm):
Scheme 1. Molecular structures of compounds DPF and MDPF, and the synthesis of
compound BDPF. (a) Bromine, H2O, reflux, 10 h; (b) Bis(pinacolato)diboron,
[(C6H5)3P]2PdCl2, potassium acetate, DMSO, 80 ꢀC, 20 h; (c) Sodium periodate, hy-
drochloric acid, room temperature, 24 h.
d
1.38 (s, 24H), 7.59e7.65 (q, J ¼ 7.6 Hz, 6H), 7.75e7.78 (d, J ¼ 7.7 Hz,
2H) and 7.90e7.95 (t, J ¼ 7.9 Hz, 6H). 13C NMR (CDCl3, 125 MHz,
ppm):
d 193.7, 143.3, 142.4, 142.1, 135.5, 135.2, 133.5, 126.1, 123.2,

M.-S. Yuan et al. / Dyes and Pigments 123 (2015) 355e362
357
120.9, 83.9 and 24.9. TOF-MS-EI: m/z 583.9 [M]þ. Elemental anal.
calcd. for C37H38B2O5: C, 76.05 and H, 6.55. Found: C, 75.88 and H,
6.71.
single-molecular ground-state geometry. And the geometry of the
planar DPF was obtained by making the two 2,7-linked benzene
rings coplanar with the fluorenone unit.
2.1.4. Synthesis of ((9-oxo-9H-fluorene-2,7-diyl)bis(4,1-
3. Results and discussion
phenylene))diboronic acid (BDPF)
Compound 2 (0.20 g, 0.34 mmol) and sodium periodate (0.45 g,
2.0 mmol) were stirred in 10 mL of a 4:1 mixture of THF and water
at room temperature for 2 h. Then aqueous hydrochloric acid (1 N,
0.25 mL) was added to the suspension and the resulting reaction
mixture was stirred for additional 24 h. After which, the reaction
mixture was diluted with water (10 mL) and extracted with eth-
ylacetate (3 ꢂ 15 mL). The combined organic layers were washed
with water (20 mL) and brine (20 mL), dried over MgSO4. The crude
product was purified by column chromatography on silica gel and
elution with dichloromethane-alcohol (10:1, v/v) yielded com-
pound BDPF (0.09 g, 63%) as an orange powder. Melting point
(m.p.): 290 ꢀCe292 ꢀC. IR (KBr, cmꢁ1): 3396, 2956, 1708, 1604, 1464,
1405, 1364, 1263, 1180, 1151, 824, 787. 1H NMR (DMSO-d6, 500 MHz,
3.1. Synthesis
The molecular structures of compounds DPF and MDPF and the
synthetic approach to compound BDPF are outlined in Scheme 1.
Compounds DPF and MDPF were readily synthesized through a
conventional Suzuki reaction by reacting 2,7-dibromo-9H-fluoren-
9-one with the corresponding aryl boric acid. The synthesis of the
diboronic acid compound BDPF was first processed through the
bromination of DPF at the 4,4'-positions of substituted two phenyl
groups of fluorenone to yield 1. Then the borylation was accom-
plished by using bis(pinacolato)diboron to obtain the arylboronic
acid ester 2. Ultimately, the ester 2 was hydrolysed in the presence
of sodium periodate to obtain BDPF. All the new compounds were
characterized with 1H NMR, 13C NMR, elemental analysis and
MALDI/TOF mass spectroscopy. Compounds DPF and MDPF have
been verified by their X-ray single-crystal structures.
ppm):
d
7.74e7.76 (d, J ¼ 7.7 Hz, 4H), 7.91e7.98 (m, 10H) and 8.16 (s,
4H). 13C NMR (CDCl3, 125 MHz, ppm):
d
193.2, 143.2, 141.8, 140.7,
135.4, 135.0, 134.1, 131.9, 129.2, 120.9, 126.0 and 122.4. TOF-MS-EI:
m/z 419.3 [Mꢁ1]þ. Elemental anal. calcd. for C25H18B2O5: C, 71.49
and H, 4.32. Found: C, 71.63 and H, 4.51.
3.2. AIE and photophysical properties
2.2. Single crystal X-ray diffraction
Fluorenone compounds, which contain keto groups, generally
exhibit very weak single-molecular emission because this class of
The single crystals of compounds DPF and MDPF were obtained
by the slow diffusion of their respective CH2Cl2/cyclohexane solu-
tions for several days at room temperature. Since the two crystals
are stable under ambient condition, the data collection was done
without any inert gas protection at room temperature on a Bruker
SMART APEX-II CCD area detector using graphite-monochromated
molecules possess a lower ne
p* state than the charge-transfer
pep* state, which will result in the radiative transition being
forbidden according to Kasha's rule [22]. Actually, as shown in
Fig. 1a, our synthesized fluorenones DPF and MDPF exhibit only
weak fluorescence (fluorescence quantum yield
F ~ 1%) in dilute
THF solution (20 M), and give very weak PL spectra with an
m
Mo K
a
radiation (
l
¼ 0.71073 Å). Data reduction and integration,
emission maximum (lem) of 380 nm (Fig. 1d). In contrast, their
respective powders emit a bright yellow emission under 365-nm
UV light with lem of 536 nm for DPF and 548 nm for MDPF
(Fig. 1a and f, respectively), with the solid-state fluorescence
quantum yield of 54% for DPF and 63% for MDPF.
To further explore the luminescence properties of DPF and
MDPF, their PL emission behaviour was determined in the dilute
THF/water solutions with different water fractions. Because the two
compounds are insoluble in water but soluble in THF, changing the
water fractions in the mixed solvents can effectively tune the extent
of solute aggregation. We have prepared the THF/water mixture
together with global unit cell refinements were done by the
INTEGRATE program of the APEX2 software. Semi-empirical ab-
sorption corrections were applied using the SCALE program for area
detector. The structures were solved by direct methods and refined
by the full matrix least-squares methods on F2 using SHELX.
2.3. Photophysical properties measurement
UVevis absorption spectra for the solutions and the films were
recorded with a Shimadzu UV-2550 spectrophotometer. Photo-
luminescence (PL) spectra were recorded using a Shimadzu RF-
solution of DPF and MDPF (20 mM) with different water fractions,
5301PC spectrofluorimeter. The fluorescence quantum yield (
F
) in
including 50%, 60%, 70%, 80% and 90%. With the increase in the
water fraction from 50% to 80%, the PL spectra of the solutions
exhibit an increased intensity (Fig. 2b and d), which can be
attributed to the AIE effect caused by the formation of molecular
aggregates. The absorption spectra also verify the aggregates for-
mation with the absorbance increasing in the long-wavelength
region (Fig. 2a and c). The 80% water fraction THF/water mixture
solution was determined using rhodamine B in ethanol as a refer-
ence according to a previously reported method [20]. Quantum
yield of the solid-state powder was determined with a PTI C-701
calibrated integrating sphere system [21]. Steady-state fluores-
cence spectra and decay curves were obtained using an Edinburgh
FLS920 fluorescence spectrometer equipped with a time-correlated
single photon counting (TCSPC) card. Reconvolution fits of the
decay profiles were performed with F900 analysis software to
obtain the lifetime values.
solutions exhibit the highest PL intensity with
F 47% and 58% for
DPF and MDPF, which are several tenfold higher in comparison
with those of their THF solution (Fig. 1d). These spectral phenom-
ena indicate that the two fluorenone compounds are really highly
AIE-active. The reason why the emission of the THF/water mixture
solutions did not show a decrease in intensity when the water
fraction was raised to >80% is that the higher water fraction made
the aggregation susceptible to spontaneous self-assembly and
produced some precipitate.
Besides the PL intensity increase, we also noted another
remarkable spectral change when adding water into the THF so-
lution of DPF and MDPF. The PL maxima (lem) position of the two
fluorenone compounds in THF/water solution (or in solid state)
2.4. Theoretical calculation
In order to understand the spectral behavior and elucidate the
influence of the configurational change to the spectra, the orbital
energy of the optimized compound DPF and the planar DPF were
respectively calculated by using the Gaussian 09 program at the
B3LYP Time-Dependent Density Functional Theory (TD-DFT). The
geometry of the optimized compound DPF was obtained by using
the B3LYP functional and the 6-31G* basis set to optimize the

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M.-S. Yuan et al. / Dyes and Pigments 123 (2015) 355e362
Fig. 1. (a) Photos of the THF solutions (20
mM) and as-prepared solid powders of compounds DPF and MDPF under natural light (NL) and 365-nm UV light (UV); (b) Fluorescence
lifetime profiles of DPF and MDPF in THF (20
m
M) and in the solid state respectively; (c) UVevisible absorption spectra of the THF solutions (20
mM) of DPF and MDPF; (d) PL spectra
of the THF solutions (20 M) and the THF/water solutions (20
m
mM, 80% water) of DPF and MDPF; (e) UVevisible absorption spectra of the as-prepared solid powders of DPF and
MDPF; (f) Normalized PL spectra of the as-prepared solid powders of DPF and MDPF.
showed a larger red-shift (~160 nm) relative to those in pure THF
(Fig. 1d). The spectral change is obviously different from the spec-
tral characteristics of the majority of previously reported AIE dyes,
whose enhanced luminescence is caused by restricted intra-
molecular rotation in their aggregated states [17a]. In those AIE
systems, their peak positions of PL maxima are not significantly
different in the two different states of organic solutions and
aggregated states. In addition, their dilute THF solutions (20 mM)
also show very different fluorescence lifetimes from their solid-
state powders. As shown in Fig. 1b, their THF solutions have a
3.3. Theoretical studies
To better understand the spectral behaviour and inspect
whether the red-shifted solid-state absorption and emission of DPF
and MDPF are derived from the effects of intramolecular planar-
isation, we respectively calculated the molecular orbital energy of
Optimized DPF (the single-molecular ground-state geometry was
optimized using the B3LYP functional and the 6-31G* basis set) and
Planar DPF (the geometry was planed through two 2,7-linked
benzene rings coplanar with the fluorenone unit) by using the
Gaussian 09 program and Time-Dependent Density Functional
Theory (TD-DFT). As shown in Fig. 3, in Optimized DPF, the dihedral
angle between the fluorenone and the 2,7-linked phenyl rings is
36.1ꢀ. The energy gap of Optimized DPF between the highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbitals (LUMO) is 3.58 eV (346 nm). When the two 2,7-
short lifetime (
the fluorescence lifetime of their solid-state powder (
DPF and 23.0 ns for MDPF) reached more than ten times those of
the THF solution, which may indicate that they undergo different
relaxation routes or pathways from the molecular excited state to
the ground state in the two different states, respectively.
t
¼ 1.0 ns for DPF and 1.2 ns for MDPF). However,
t
¼ 18.0 ns for

M.-S. Yuan et al. / Dyes and Pigments 123 (2015) 355e362
359
Fig. 2. UVevis absorption spectra changes of DPF (a) and MDPF (c) depending on the water fractions in THF. PL spectra changes of DPF (b) and MDPF (d) depending on the water
fractions in THF.
linked benzene rings are coplanar with the fluorenone unit, the
energy level of the HOMO of Planed DPF had a modest increase, and
the energy gap of Planed DPF between HOMO and LUMO reduced
by only 0.16 eV (17 nm) compared to that of Optimized DPF.
Apparently, the computed results are not in good agreement with
the absorption spectral characteristics of DPF in dilute THF solution
and in the solid-state: the absorption peak of DPF in solid-state is
located at about 440 nm (Fig. 1e), which is red-shifted up to 100 nm
Fig. 3. The LUMOs, HOMOs and molecular geometries of optimized compound DPF (the geometry was optimized using the B3LYP functional and the 6-31G* basis set) and planar
DPF (the geometry was planed through two 2,7-linked benzene rings coplanar with the fluorenone unit).

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M.-S. Yuan et al. / Dyes and Pigments 123 (2015) 355e362
Fig. 4. X-ray crystallographic packing of compound DPF. (a) Side view of DPF crystal packing; Top (b) and side (c) view and illustration of the hydrogen bond bound dimer in DPF.
in comparison with that of DPF in THF (340 nm, Fig. 1c). Therefore,
we believe that the bathochromic absorption and emission of DPF
and MDPF in the solid-state are impossibly derived from the effects
of intramolecular planarisation.
different from the staggered face-to-face fashion reported by the
Tao group [18a]. The molecular pairs are formed by hydrogen bonds
with bond lengths of 2.51 and 2.60 Å for DPF and MDPF, respec-
tively (Figs. 4b and 5b). As can be observed from the theoretical
calculation (Fig. 3), upon one molecule being excited, the electrons
of the LUMO are confined mostly in the fluorenone core, and
greater electron density is present on the oxygen atom of the
carbonyl group, which is inclined to bind the electropositive
hydrogen atoms of an adjacent ground-state molecule to form a
dimer. The crystal packing of DPF and MDPF are very convenient for
the formation of such dimers. Simultaneously, we also observed
that the molecules of both DPF and MDPF in the crystal packing
were not parallel to their top-and-bottom adjacent molecules
3.4. X-ray crystallography and AIE mechanism
Determination of X-ray single-crystal structure is the most
effective tool for acquiring the ground-state molecular structure to
reveal the molecular structureeproperty correlations. The single
crystals of DPF and MDPF have been obtained by the slow diffusion
of their CH2Cl2/cyclohexane (1:2, v/v) solution for several days at
room temperature [23]. The molecular packing of DPF and MDPF
are shown in Figs. 4 and 5. The fluorenone fragment and the 2,7-
linked benzene rings of both DPF and MDPF are not coplanar,
and the dihedral angle between fluorenone and benzene are
36.4(3)ꢀ and 53.7(3)ꢀ for DPF and MDPF, respectively, which
further illustrates that their AIE fluorescence is not caused by
intramolecular planarisation. In their crystal packing diagram, the
obvious characteristic molecular pairs can be observed, and they
each pack in a parallel but side-by-side fashion, which is obviously
(Figs. 4a and 5a). So, there are no
pep interactions in their
respective crystals. From the structural information, we believe that
the red-shifted solid-state luminescence of DPF and MDPF derives
from these dimers. Because of the special crystal packing, the
prearranged ground-state dimer bound by hydrogen bond can
readily form excimers, i.e., static excimers. When the excimers are
excited, they do not undergo any energy-consuming configura-
tional rearrangement. Therefore, the crystal powder of DPF and
Fig. 5. X-ray crystallographic packing of compound MDPF. (a) Side view of MDPF crystal packing; (b) Top view and illustration of the hydrogen bond bound dimer in MDPF; (c) Top
view and illustration of the up-and down adjacent molecules in MDPF.

M.-S. Yuan et al. / Dyes and Pigments 123 (2015) 355e362
361
Fig. 6. (a) UVevisible absorption spectra and (b) PL spectra of compound BDPF in BrittoneRobinson buffers at different pH values (10 mM).
MDPF exhibited longer-wavelength luminescence and higher
quantum yield in comparison with their THF solution.
BDPF. The compound exhibited weak luminescence in high pH
buffer because BDPF was ionized by the alkaline medium and
became soluble, and exhibited a red-shifted enhanced lumines-
cence in acidic buffer because BDPF was insoluble and formed
suspended dimers. This further verified the properties of AIE and
the luminescence mechanism of DPF and MDPF.
3.5. Verification of AIE mechanism
To further explore the AIE mechanism of the two fluorenone
compounds DPF and MDPF, DPF was functionalized with two
boronic acid units to obtain compound BDPF using the synthetic
route shown in Scheme 1. The photophysical properties of the DPF-
diboronic acid adduct BDPF were determined in BrittoneRobinson
buffers at different pH values ranging from 5.72 to 11.82. As shown
in Fig. 6b, in high pH buffer, BDPF emitted a weak blue-violet light
with the emission peak at about 380 nm, which is very similar to
the emission of DPF in THF. Since the pKa of phenylboronic acid is
~9, BDPF was ionized by the alkaline medium and became soluble
in the buffers with high pH (>9.15). Obviously, the luminescence
peak at 380 nm came from the single molecule of BDPF. With the
pH reducing, a new green luminescence peak at 550 nm progres-
sively developed. In acidic buffer, BDPF was immiscible with water,
and the single molecules aggregated and formed suspended dimers
to emit a strong green light. From the absorption spectra (Fig. 6a),
we reported the emergence of a wide absorption band that peaks at
approximately 490 nm in the acidic buffer (pH 5.72), which indi-
cated the formation of the dimers. BDPF exhibited a reversible
fluorescent response for pH changing.
Acknowledgment
This study was supported by the National Natural Science
Foundation of China (21202132, 21175107 and 21375106), the Fund
of Youth Science and Technology Stars by Shaanxi Province
(2015KJXX-15), the Ministry of Education of the People's Republic
of China (NCET-08-602 0464), the Fundamental Research Funds for
the Central Universities (Z109021106 and Z109021303).
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