Aggregation-induced emission and intermolecular charge transfer effect in triphenylamine fluorophores containing diphenylhydrazone structures

Article abstract of DOI:10.1039/c6cp04755f

Three new chromophores incorporating acceptor-π-donor-π-acceptor structural motifs and mono-, di- and tri-branched diphenylsulfone base linked to triphenylamine through a hydrazone π-bridge were synthesized, and the photoluminescence properties of the three chromophores were studied in solutions as well as in aggregated states. All the fluorophores emitted strong blue fluorescence in THF. Mono- and di-branched triphenylamine both exhibited increasing blue fluorescence and displayed an AIEE effect in the aggregated state. Tri-branched triphenylamine emitted green fluorescence and presented the AIE effect in the aggregated state. These interesting phenomena have been interpreted by a molecular stacking mode with molecular dynamics (MD) and DFT calculations. The unique propeller shaped molecular configuration of triphenylamine prevented face to face π-π stacking and induced the hindered rotation, which resulted in the AIEE or AIE effect in the aggregated state. The enlarged coplanarity of diphenylhydrazone chains increased the conjugation of tri-branched triphenylamine, which was beneficial to the formation of ICT and AIE and resulted in emitting green ICT fluorescence in the aggregated state. Fluorescent microscope imaging and the fluorescent pictures of the powder states certified the strong AIEE effect or AIE effect in the solid.

Full text of DOI:10.1039/c6cp04755f

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Aggregation-induced emission and intermolecular charge transfer
effect in triphenylamine fluorophores containing
diphenylhydrazone structures
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Wen Yanga, Chunchun Li a, Mengmeng Zhanga, WeiqunZhoua*, Renyu Xueb, Haili Liuc,
Youyong Lic
www.rsc.org/
Three newly chromophores incorporating acceptor-π-donor-π-acceptor structural motifs and mono-, di- and tri-branched
diphenylsulfone base linked to triphenylamine through
a hydrazone π-bridge were synthesized, and the
photoluminescence properties of three chromophores were studied in solutions as well as in aggregated states. All
fluorophores emitted strong blue fluorecence in THF. Mono-, di-branched triphenylamine exhibited increasing blue
fluorescence and displayed AIEE effect in aggregated state. And tri-branched triphenylamine emitted green fluorescence
and presented AIE effect in aggregated state. These interesting phenomenons have been interpreted by molecular
stacking mode with Molecular Dynamics (MD) and DFT calculations. The unique propeller shaped molecular configuration
of triphenylamine prevented face to face π-π stacking and induced the hindered rotation which resulted in AIEE or AIE
effect in aggregated state. The enlarged coplanarity of diphenylhydrazone chains increased the conjugation of tri-
branched triphenylamine, which was beneficial to the formation of ICT and AIE and came into emitting green ICT
fluorescence in aggregated state. Fluorescent microscope imaging and the fluorescent pictures of powder states
certificated
the
strong
AIEE
effect
or
AIE
effect
in
solid.
13] on account of insolubility in water. Because the luminescent
materials used as emitter layers in the light-emitting devices
must be fabricated in a solid film form, it is limited to the
fabrication of efficient organic light-emitting diodes (OLEDs) [14]
Development of luminescent materials which are strongly
emissive in the solid and aggregated states is still a challenging
issue due ACQ and other non-radiative pathways occurring in
the excited state.
1. Introduction
Generally, most of organic materials exhibited very strong
fluorescence in dilute solution but showed weakly or non-
emissive in the aggregated states, which was known as ACQ
(aggregation-caused quenching) materials in the solid states,
due to both strong intermolecular face to face π-π interactions
leading to energy transfer and other non-radiative
pathways[1,2], such as forming excimers or exciplexes. However,
traditional organic molecular structural design strategy has
been to increase the extent of π-conjugation by melding more
and more aromatic rings together resulting bigger discotic
plates. The discotic big molecules enjoyed stacking together in
the manner of face to face π-π interactions and caused ACQ.
The ACQ effect has severely obstructed the application for the
real-world [3-10]. For example, the ACQ materials can’t be used
as sensors to detect biological molecules in physiological
buffers and as probes to monitor ionic species in river water [11-
.
The appearance of AIE (aggregation-induced emission) or AIEE
(aggregation induced emission enhancement) fluorescence
materials termed by Prof. Tang in 2001[15] made it a possibility
to apply the organic photoimaging materials in real-world
applications. The AIE or AIEE fluorescence materials are a new
family of luminescent materials, which show quenching or
weakened emission in solution, but exhibit strong fluorescence
upon aggregation. Various mechanistic pathways such as
conformational planarization, J-aggregate formation, E/Z
isomerization, twisted intramolecular charge transfer (TICT),
excited state intramolecular proton transfer (ESIPT), restriction
of intramolecular rotation (RIR), restriction of intramolecular
vibration (RIV), and restriction of intramolecular motion (RIM)
have been explored to achieve AIE by hampering the
intermolecular π-π stacking interaction[16-19]. The covalent
bonds have also been used to fasten the aryl rotors to
a
School of Chemistry
University, 199 Ren’ai Road, Suzhou, People’s Republic of China, 215123
School of Biology and Basic Medical Sciences, Soochow University, 199 Ren’ai
& Chemical Engineering and Material Science, Soochow
b
Road, Suzhou, 215123, People’s Republic of China, 215123
c Functional Nano & Soft Materials Laboratory, Soochow University, 199 Ren’ai
Road, Suzhou, People’s Republic of China, 215123
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internally set off the RIR process at the molecular level. Some (100 mL) on quartz cell (12.5×12.5×45 mm). The fluorescence
typical examples of the AIE systems have been developed such microscope imaging of the nano-aggregations were obtained
as hexaphenylsilole (HPS) [20, 21], which has a propeller shaped by Leica DM2500M. All the experiments were carried out at
non-planar molecule.
room temperature. Film samples for the measurements of UV-
The triphenylamine (TPA) moiety has been employed generally vis and fluorescence spectroscopy were prepared by drop
as an activator to construction of AIE-active luminogens casting and the subsequent spin-coating (2000 rpm, 30 s) from
associated with its propeller-like structure and excellent THF solutions (100 mL) on quartz cell (12.5 ×12.5×45 mm).
electron-donating character [22-25]. Diphenylsulfone has been Scanning electron microscope (SEM) images of the
used as an electron-acceptor group to construct luminogens aggregations were obtained using a Hitachi S4800.
due to its V-shape structure character. The present
2.2 Computational methods
contribution reports a distinctive approach for multi-branched
The molecules were optimized with the Becke-3-Lee-Yang-Parr
(B3LYP) [31-33] by density functional theory (DFT) calculations at
Indeed, the multi-branched strategy constitutes a relevant tool
triphenylamine containing diphenylhydrazone structure.
the level of 6-311G (d) with Gaussian 09 program [34]. The
to modulate the aggregated states of AIE-active fluorophores
vibrational frequencies calculations were also performed to
without drastic impacts on the energy position of the
ensure the stability of the calculation result and avoid virtual
electronic transitions [26–30]. In this spectral range, the multi-
frequency. The MD (molecular dynamic) simulations were
branched constructions were covalently linked to
performed by Discover module within Materials Studio 7.0
triphenylamine centers to induce hindered rotation and the
with the COMPASS force field for the studied molecules with
desired emission enhancement. The molecular structure
the amorphous cell [35]. It was carried out with constant
constructed by the control of intermolecular interactions and
temperature and constant volume ensemble (NVT).
intramolecular charge transfer (ICT) effect considering the
Temperature was set 300K and kept constant using a nose
strong fluorescence, where the triphenylamine served as the
thermostat [36]. Pressure was set 1 atm. Electrostatic energies
central donor core, diphenylsulfone as the acceptor, two
were calculated using the vdW & Coulomb method with a 9.5-
hydrazinyldiphenyl conjugated chains served as the π-bridge to
Å non-bonded cutoff. The total simulation time of 10.0 ps was
extend the conjugation length at the molecular level have not
carried out with time step of 1.0 fs.
yet been reported in the literature. The design strategy is
purposed to have new AIE materials, which can be applied in
both solution and solid state.
3. Result and Discussion
3.1 Synthesis
2. Experimental section
The synthetic route was shown in Scheme 1.
2.1 Materials and instruments
3.1.1 4-hydraziondiphenyl sulfone (1)
Triphenylamine, 4-chlorodiphenylsulfone, hydrazine hydrate
(85%), POCl3, DMF, NaOH, CH2Cl2, THF, MgSO4, CH3COOH,
DMSO, acetone, CDCl3 and MTT were used as HPLC grade
purchased from J&K (CHINA). Melting points were determined
on a Kofler melting point apparatus and uncorrected. IR
spectra were obtained in KBr discs using a Nicolet 170SX FT-IR
Two to three drops of triethylamine were added to a suspension of
4-chorophenyl phenyl sulfone (5.0g, 0.02mol) and 50mL hydrazine
hydrate (85%) in the round flask (250mL). The mixture was reflux
for 24 h until the solid was completely dissolved. The resulting
solution was cooled and diluted with ice water; the precipitates was
removed by filtration, wash to neutrality with water, and dried over
KOH, The yield was 5.0g (90.0%). Found: M+, 248; calculation, 248.
spectrometer. Elemental analyses were performed on
a
Yannaco CHNSO Corder MT-3 analyzer. 1H NMR has been
recorded on INOVA 400 at 400.13MHz, with TMS as internal
standard using DMSO as deuterated solvents with chemical
shifts reported as ppm. Thermogravimetric analysis (TGA) of
the compounds was carried out simultaneously by employing a
NETZSCH thermal analyzer. The sample was heated at a rate of
3.1.2 4-(diphenylamino) benzaldehyde (2) and Bis (4-benzoyl)
aniline (3)
30mL dry DMF was ice-bathed and 29.5mL POCl3 was added
dropwise. After Stirred for 40 minutes under 0℃, the temperature
was raised to room temperature. Then 2.5g triphenylamine was
added and the reaction mixture was then heated at 45℃ and
stirred for 14 h. After cooling down to room temperature, the
reaction mixture was poured into ice-bath and neutralized with
NaOH, then extracted with chloroform. The combined organic layer
was dried with magnesium sulfate anhydrous, filtered.
Concentrated the organic layer, the white purified compound was
obtained by column chromatography from petroleum ether and
ethyl acetate mixed solvent (10:1). The compound 3 was prepared
according to the general procedure for compound 1 by using 15.5
10
fluorescence spectra were recorded on a CARY50 UV-VIS
spectrophotometer and an FLS920 fluorescence
℃/ min in a nitrogen atmosphere. The absorption and
spectrophotometer. Fluorescence quantum yields (Φf) in pure
THF and THF/water mixtures determined with anthracene as
the fluorescence reference in ethanol (Φ = 0.27). Film samples
for the measurements of UV-vis and fluorescence
spectroscopy were prepared by drop casting and the
subsequent spin-coating (2000 rpm, 30 s) from THF solutions
2 | J. Name., 2012, 00, 1-3
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mL dry DMF, and 19 mL POCl3, and 2.0 g triphenylamine. Compound 1151(SO2). 1H-NMR (400 MHz, DMSO-D6, Fig.S2): δ 10.95 (s, 3H),
2: Yield: 2.1g (85%).1H-NMR (DMSO, 400 MHz) δ (ppm): 9.76(s, 1 H), 7.88 (m, 8H), 7.78 (m, 8H), 7.58 (m, 15H), 7.17 (t, J=8.8 Hz, 4H), 7.09
7.71(d, 2 H), 7.41(t, 4 H), 7.21(m, 6 H), 6.88(d, 2 H). Compound 3: (d, J=8.8Hz, 4H), 6.83 (d, J=8.8Hz, 3H). 13C-NMR (100 MHz, DMSO,
Yield: 1.6g (76%). 1H-NMR (DMSO, 400 MHz) δ (ppm): 9.87(s, 2H), Fig.S3) δ: 124.38, 126.83, 127.07, 128.02, 129.54, 129.82, 129.96,
7.84(d, 4H), 7.47(t, 2 H), 7.31(s, 1 H), 7.21(d, 2H), 7.16(d, 4H).
132.97, 133.32, 140.30, 143.19, 143.80, 147.30. MS calc. for (M + H
+) +: 1019.2593, found: 1020.2762 (Fig.S4).
3.1.3 Tris (4-benzoyl-yl) amine (4)
The TGA curves of synthesized compounds are depicted in Fig.S5. In
the TGA curves of all compounds, the weight loss starts at around
200℃. The high thermal stability of these compounds enables them
to be used in AIEgens applications.
5.8mL dry DMF was ice-bathed and 7.6mL POCl3 was added
dropwise. After they were stirred for 1h under 0 ℃, the
temperature was raised to room temperature. 0.98g Bis (4-benzoyl-
yl) aniline was added and the reaction mixture was then heated at
95℃ and stirred for 4 h. After cooling down to room temperature,
the reaction mixture was poured into ice-bath and eutralized with
NaOH, and then extracted with chloroform. The combined organic
layer was dried with magnesium sulfate anhydrous, filtered.
Concentrated the organic layer, the white purified compound was
obtained by column chromatography from petroleum ether and
ethyl acetate mixed solvent (4:1). Yield: 0.55g (65%).1H-NMR
(DMSO, 400MHz) δ (ppm): 9.94(s, 3 H), 7.91(s, 6 H), 7.28(s, 6 H)
O
O
S
O
H2N NH2
S
NH
NH2
Cl
triethylamine
reflux
O
1
O
S
N
HN
O
CHO
S
NH
NH2
O
O
1
POCl3/DMF
N
N
Dioxane
N
2
5
POCl3/DMF
CHO
O
S
H
N
HN
N
NH
NH2
N
O
S
N
N
3
O
O
1
S
O
O
Dioxane
OHC
3.1.4 Compound 5, 6 and 7
6
POCl3/DMF
20 mmol 2, 3, 4 and 40 mmol N, N- dimethyl-p-phenylenediamine
were added to flask, then add 10 mL dioxane. The reaction mixtures
were then heated at 60℃and stirred for 2 h. After cooling down to
room temperature and concentrated the organic layer, the yellow
purified solid powder 5, 6, 7 were obtained by recrystallized from
ethanol. Yield: 81% (5), 82% (6) and 65% (7).
H
N
N
CHO
N
O
S
O
HN
N
O
O
S
O
S
NH
NH2
O
N
4
1
NH
N
Dioxane
OHC
CHO
O
S
O
7
5: m.p.:166℃. Anal. Calc. formula: C31H25N3O2S. (%): C, 73.93; H,
5.00; N, 8.34, Found: C31H25N3O2S.(%): C, 73.66; H, 4.98; N, 8.31. IR
(Fig.S1): 3257, 3184 (N-H), 2895, 2822 (C-H), 1593 (C=N), 1273,
1145 (SO2). 1H-NMR (400 MHz, DMSO-D6, Fig.S2) δ 10.92 (s, 1H),
7.93 (t, J=8.8Hz, 3H), 7.78 (d, J=9.2Hz, 2H), 7.68 (m, 1H), 7.62 (m,
Scheme 1 the synthetic route of compound 5, 6 and 7
3.2 Photo-physical properties
4H), 7.38 (t, J=8.8 Hz, 4H), 7.18(d, J=8.8Hz, 2H), 7.14(t, J=9.2Hz, 2H), Optical properties of 5, 6 and 7 were investigated by UV-visible
7.11(m, 4H), 7.01 (d, J=8.8Hz, 2H). 13C-NMR (100 MHz, DMSO, Fig.S3) and fluorescence spectroscopy. Three compounds had good
δ: 112.24, 122.50, 123.56, 124.94, 127.10, 127.51, 127.75, 129.06, solubility in common organic solvents such as THF, chloroform,
129.36, 129.70, 130.73, 132.49, 140.19, 142.87, 147.14, 148.45, toluene, acetonitrile, and DMSO, partial solubility in protic
148.96. MS calc. for (M + H +) +: 503.6141, found: 504.1565 (Fig.S4).
solvents (methanol and ethanol) and insoluble in water and
hexane. The absorption and emission spectra of them in THF
were depicted in Fig. 1(a). All compounds presented two
absorption bands in various solvents between 300 to 400 nm
in their UV spectra. The absorption maxima shifted red with
the increase of branch numbers. The maxima of high energy
absorption located at 301, 309 to 315 nm and the second
located at 378, 381 to 399 nm respectively for 5(single
branch), 6 (double branch) and 7(triple branch) (Table 1).
Similarly, the maxima emission wavelength (λem) (446, 453 to
476 nm for 5, 6 and 7 respectively) shifted red with the
addition of branch numbers. All of them are highly emissive in
different polarity solvents, and the fluorescence quantum
efficiencies are particular high (Φf: 42.8, 46.1 and 58.8% for 5,
6 and 7 respectively) in pure THF.
6: m.p.:176℃. Anal. Calc. formula: C44H35N5O4S2. (%): C, 69.36; H,
4.63; N, 9.19; Found: C44H35N5O4S2. (%): C, 69.28; H, 4.64; N, 9.21. IR:
(Fig.S1): 3336, 3276 (N-H), 3058, 2955(C-H), 1582(C=N), 1284,
1139(SO2). 1H-NMR (400 MHz, DMSO-D6, Fig.S2): δ 10.14 (s, 2H),
7.15 (s, 2H), 7.10 (d, J=9.2Hz, 4H), 6.97 (d, J=9.2Hz, 4H), 6.83 (m,
10H), 6.59 (t, J=8.8Hz,2H), 6.38 (d, J=8.8Hz, 5H), 6.33 (d, J=8.8Hz,
2H), 6.26 (d, J=8.8Hz, 4H). 13C-NMR (100 MHz, DMSO, Fig.S3) δ:
112.11, 123.70, 125.40, 127.06, 127.94, 129.18, 129.92, 129.96,
130.21, 133.31, 140.40, 141.30, 143.20, 146.71, 146.78, 147.78,
147.75, 149.63. MS calc. for (M + H +) +: 761.2130, found: 762.2235
(Fig.S4).
7: m.p.:199℃. Anal. Calc. formula: C57H45N7O6S3. (%): C, 67.11; H,
4.45; N, 9.61; Found: C57H45N7O6S3. (%): C, 67.18; H, 4.42; N, 9.59. IR
(Fig.S1): 3366, 3282, 3209 (N-H), 2988, 2822(C-H), 1666(C=N), 1267,
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Fig. 1 (a) UV- spectra and fluorescent emissions of 5, 6 and 7 in THF; (b) the absorption and fluorescence spectra of triple branched compound (7) in
various solvents, (c) Lippert–Mataga plot of 7(υa = absorption wavenumber, υf = emission wavenumber and Δƒ =orientation polarizability = (ɛ ‒ 1)/(2ɛ + 1)
‒ (n2 ‒ 1)/(2n2 + 1) , where ɛ = dielectric constant and n = refractive index).
Table 1 Maximum absorption wavelength, maximum emission wavelength and fluorescent quantum yield of 5, 6 and 7 in solutions, aggregated states and
film states
THF
λa2(nm)
THF-water
λem(nm)
Film
λa1 (nm)
λem(nm)
Φf(%)
Water (%)
Φf(%)
λem(nm)
Φf(%)
5
6
7
301
378
381
399
446
42.8
60
460
468
527
58.2
460
23.8
309
315
453
476
46.1
58.8
60
70
64.6
6.1
469
529
42.2
5.8
All compounds have the same donor and acceptor, the (Fig. 1(c)) gives an upward straight line with a moderate slope,
difference among them is only the number of branches. It is indicative of a moderate ICT feature [38]
.
noteworthy that the effects of solvent polarity of 7 on the
absorption and emission properties. UV-visible absorption
3.3 AIE Properties
spectra and emission spectra of 7 in different solvents were
A solvent-poor-solvent photoluminescence (PL) test, which was
commonly used for studying the AIE phenomenon [39-41], was
performed to investigate the luminescent behavior of the purpose
compounds. The dispersion system of nano-aggregated was
obtained by gradual addition of water into THF with different ratios.
AIE effect (I/I0) of three derivatives was drawn in Fig.2 (a-c). The
formation of aggregates can also be confirmed by the changes of
UV−vis, PL spectra and fluorescence microscope imaging in
THF/water mixtures with varying water volume fractions (fw).
Single-branched chromophore 5 exhibited the strong absorption
peaks at 378 nm and the weak absorption peaks at 301 nm in pure
THF solution. With an increase in fw from 0% to 90%, the strong
peaks decreased and the weak peaks increased (Fig.2 (a)). At the
same time, in the solvent mixtures, both peaks shifted red and the
maxima absorption located at 310 and 388 nm when the fw was
increased to 80% and the leveled-off tails appeared in the visible
region at fw equal to 60%. It indicated that the aggregates were
probably at nanoscale sizes. Chromophore 5 also emitted strong
blue fluorescence at 446 nm (Φf =42.8%) in dilute THF solution (5×
10-5 mol·L-1) (Table 1). The fluorescence intension increased when
the fw was increased to 40% and the maximum emission shifted red
presented in Fig. 1(b). The absorption band located 315 nm
could be assigned to π–π* electronic transition of the
respective TPA units [37], and 399 nm was due to the transition
of the conjugated TPA–diphenylsulfone backbone. Both peak
positions of the absorption bands were almost unchanged in
polar (acetonitrile and DMF), non-polar (toluene), and
moderately polar (THF and dichloromethane) solvents with
small contribution from tailing of the lower transition energies.
This indicates that there was little influence of solvent polarity
on the ground state and a negligible charge transfer process
between the TPA donor and diphenylsulfone acceptor in
ground state. Interestingly, the emission properties of 7 were
dependent on solvent polarity (Fig.1 (b)), (Φf: 72.0, 59.2, 58.8,
43.6, 40.8 % in n-Hexane, CH2Cl2, THF, CH3CN, and DMF) and
show positive sol-vatochromism effect, which indicated the
significant dipole moment change in excited state. For
example, it was emitted at 454 nm in toluene, while in DMF it
was at 485 nm, which indicated an intramolecular charge
transfer emission of 7 in solution. A Lippert–Mataga plot of
Stokes shift against the orientation polarizability of the solvent
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when the fw was at 50%. The maximum emission reached the the aggregation effect of double-branched chromophore 6. As
highest fluorescence quantum yield (Φf =58.2%) and the maximum shown in Fig.2 (b), when the fw increased to 50%, both absorption
emission shifted red to 460 nm when the fw was at 60%. The and emission spectra presented an increase of fluorescence and
inserted micro-imaging of Fig. 1 exhibiting bright blue fluorescence accompanied an obvious bathochromic-shift. When the fw was
supported it. Subsequently, the fw increased continuously, the increased to 60%, the maximum emission reached the highest Φf
emission intensity decreased gradually and invariableness finally (64.6%) and the maximum emission shifted red to 468 nm. The
with the fw increased.
more bright blue fluorescence micro-imaging was showed in
The influence of steric hindrance effects on the AIE properties was inserted picture of Fig.2 (b) due to the higher Φf than that of mono-
investigated by introduction of mutli-branch substituted into branched chromophore 5. Subsequently, the fw increased
triphenylamine central. The fluorescence enhancement was continuously, the emission intensity decreased gradually and
exhibited with the increase the branch numbers. Fig.2 (b) depicted invariableness finally with the fw increased.
Fig.2 (a-c) UV and emission spectra of 5, 6 and 7 respectively in THF–water mixtures (5×10-5 M) (the fluorescence micro imaging were inserted (λe=365nm)); (d) AIE
effect (I/I0) of 7 with the fw from 0 to 90% (left to right) under the UV lamp (365 nm).
(365 nm) with the fw from 0 to 90% (left to right). The blue
Comparing with the absorption spectra in pure THF, the absorption fluorescence changed to green with the increase of the fw. The
spectra of triple-branched chromophore 7 in THF/water mixture, fluorescence micro-imaging in inserted picture of Fig.2(c) exhibited
nothing seemed to have changed that much. The only difference faint green fluorescence due to its low Φf (6.1%). Subsequently, the
was that the leveled-off tails was found appeared in the visible fw increased continuously, the emission intensity is hardly changed
region at fw equal to 50% due to the aggregates at nanoscale sizes with the fw increased.
(SEM, Fig. S6). However, the PL spectra were different from those of
5 and 6, 7 in pure THF solution (5× 10-5 mol/L) emitted strong blue 3.4 Emission characteristic in solid states
fluorescence at 476 nm and the Φf reached to 58.8%. When the As shown in Fig.3 (a-c), both
5 and 6 emitted bright blue
water was added to THF, the fluorescence intension decreased. The fluorescence in powder states. However, the fluorescence of 7 was
lowest fluorescence intensity appeared at the fw equal to 40%. faint green under the UV lamp (365nm). In order to determining the
When the fw was at 50%, double peak appeared in the fluorescence Φf in solid states, the film samples were prepared by drop casting
spectra. The maxima emissions were located at 469 and 513nm and the subsequent spin-coating from THF solutions (100 mL) on
respectively. After that, the emissions at 469 nm decreased and the quartz cell. Fig.3 (d) showed the fluorescence emission in film and
emissions at 513 nm increased gradually and shifted red with the the Φf was listed in Table 1. The level-off tail stretched into the
water increased sequentially. When fw was equal to 70%, the longer wavelength region also proved the formation of aggregation
fluorescence increased to a maximum value and the maximum state. Calculations of full-width half maximum (fwhm) revealed that
emission was located at 527 nm. Connecting with the solvent effect the emission spectra in the film state are comparable or narrower
of 7 (Fig. 1(b)), the emission at 527 nm had ICT characteristic of D-π- as comparing with the emission spectra obtained in THF/water due
A-π-D motifs. Fig. 2(d) showed the fluorescence under the UV lamp to the formation of higher order aggregates in film.
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Fig.3 (a-c) the fluorescence image of 5, 6, 7 in powder state under the UV lamp (365nm) from left to right; (d) the fluorescence spectra of 5,6,7 in film states (λe=375nm)
3.5 Theoretical studies and AIE mechanism
branches. It led to the gradually bathochromis-shifts from single-,
To better understand the spectral behavior of these TPA double- and tri-branched triphenylamine in both absorption and
derivatives, the fully optimized structures of 5, 6 and 7 calculated at emission bands. Time-dependent (TD) combining a SCRF method
DFT level (B3LYP/6-31G (d)) are carried out. Molecular structures of (CPCM, by THF as solvent) at the same level were applied to
three derivatives exhibited quite similar structures as shown in Fig.4 indicate the energies of HOMO and LUMO. The longest wavelength
(top). The triphenyl moiety globally presented D3-like symmetry electronic transition corresponded to
a pure HOMO-LUMO
with twist angles around the bond connecting the central nitrogen transition, which indicated an electronic delocalization all along the
[42, 43]
atom to the phenylene rings for all molecules
. The compound structure (π–π* type) with a slight charge transfer from
diphenylhydrazone branches adopted a quasi-planar conformation, the amino group to the diphenylsulfone moieties (Fig.4 middle and
which indicated the increase conjugation with the increase of bottom).
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Fig.
4
Molecular structures (top), HOMO (middle) and LUMO orbital (bottom) from left to right for 5,
6
and
7
calculated by B3LYP/6-31G (d)
A fixed molecules can hindered the possibility of face to face π–π bonding (HB) interactions and strong molecular interactions. Fig.5
stacking, and the rigid structure can also suppress the motion or (a) showed the strong intermolecular hydrogen bonding and some
vibration of the phenyl rings in aggregation states and can be strong interactions in packing cell of 5 calculated by MD method. A
related to the restricted intramolecular rotation (RIR), which hydrogen bonding interactions of H-N-N···H-N-N existed between
[44-46]
provides an essential prerequisite of AIE phenomena
.
two hydrazone of the adjacent molecules. The distances between N
Hydrogen bonds and strong molecular interactions are important and H were 1.943Å respectively. The strong interaction N···N with
intermolecular interactions in supramolecular structures of the the distance 2.51 Å was also found. The interation O···O with the
solid state. The formation of intermolecular interactions can rigidify distance 2.65 Å forced two sulfone groups to approach each other.
molecular structures and activate their RIR processes. AIEE effect in All of these interactions compelled a cross-shaped packing mode
solid state of 5 should be attributed to intermolecular hydrogen (Fig. 5(b)) and caused the AIEE effect of 5.
Fig.5 hydrogen bonding and strong interactions (a) and packing mode (b) in packing cell of 5, calculated by MD
The packing manner of 6 (Fig.6) was different from that of 5 on respectively. The distance O···N are 2.577 and 2.687 Å respectively.
account of increasing diphenylhydrazone branches. There are four The strong interaction is O···N interaction with the distance of 2.623
atom-atom interactions among three molecules in packing cell of 6 and 2.876 Å respectively. Another molecular packing mode is
(Fig.6 (a)), which is constructed by an intermolecular hydrogen constructed by weak intermolecular HB interaction (O···H-C) and
bond (HB) interaction and a strong intermolecular interaction. The some weak interactions. These interactions gather sulfones of four
HB of O···H-N is formed by oxygen atom of sulfone and H-N of the molecules together (Fig.6 (b)). The hydrogen bonding interactions
hydrazone of two hydrazone branch in one molecule with H-N of and strong intermolecular interactions resulted in a rigid structure,
hydrazone and O of sulfone of one branch in other two molecules which induced the AIEE effect of 6.
respectively The distances of O···H are 1.866 and 1.883
Å
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Fig.6 three molecular (a) and four molecular (b) packing modes in the cell of 6, calculated by MD
The packing mode of 7 was exhibited in Fig.7. Four atom-atom molecule formed O···H-N HB. The six atoms interaction, N-
strong interactions (Fig.7 (b)) was constructed by a intermolecular H···O=S=O···O (Fig.7(c)) constructed between another branch of the
HB, O···H-N (1.840 Å) interaction, and a strong intermolecular above-mentioned molecule and another molecule with the distance
interaction, O···N (2.61 Å), which was similar to that of 6. Here, an H···O, 1.713 Å and O···O, 2.879 Å. Three atom interactions, O···O···O
oxygen atom of sulfone on one hydrazone branch in one molecule (Fig.7 (d)) existed in the adjacent two molecules, the distances O···O
as acceptor and H-N of hydrazone of another molecule as donor were 2.667 and 2.597 Å respectively.
with H-N of hydrazone and O of sulfone of a branch of the another
Fig. 7 (a) packing model, (b, c and d) the strong intermolecular interactions in the cell of 7, calculated by MD
Above all, all of molecules in cell are fixed effectively by the strong intermolecular interactions, such as S=O···O=S (5, 7) restrict
intermolecular interactions. The motion or vibration of the intramolecular rotation (RIR) and avoid the possibility of face to
diphenylhydrazone in aggregation states can be suppressed by HBs, face π–π stacking, which was considered as a basic requirement of
H–N···H-N(5) and S=O···H-N(6, 7) respectively. The strong AIE and AIEE phenomena.
Fig. 8 the dihedral angles (C-N-N-C) of hydrazone in molecule 5, 6 and 7(left to right), calculated by MD
From the results of fully optimized structures of 5, 6 and 7(Fig 4), it (donating unit) to the diphenylsulfone moieties (accepting units).
can be found that the charges of all three compounds are partial. On the other hand, the moderate Stokes shift 68, 72 and 77 nm in
And the charge transfer of all compounds from the amino group THF and 82, 87 and 128 nm in THF-water for 5, 6 and 7 respectively
8 | J. Name., 2012, 00, 1-3
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were corresponding to this weak ICT characteristic. And the imaging and the fluorescent pictures of powder states certificated
increase of Stokes shift with branch numbers increasing indicate the the AIEE and ICT effect in solid.
incremental emission of ICT characteristic, which can attributed to
the enlarged conjunction with the enhancement of coplanarity for
Acknowledgements
diphenylhydrazone chains.
This work was supported by the National Natural Science
The increase of ICT with branches increasing can be observed by the
Foundation of China (No. 51079094) and the Natural Science
transmission from HOMOs to LUMOs (Fig 4 middle and bottom).
Foundation of Jiangsu Province (No. BK2010215). The Project
The charge transfer from the amino group to the diphenylsulfone
was funded by the Priority Academic Program Development of
moieties was feeble for 5 and obvious for 7. The clear ICT
Jiangsu Higher Education Institutions.
characteristic could also be depicted by the solvent effect of PL
spectrum. As shown in Fig. 1(b), the UV spectrum of 7 had two
peaks at 315 nm and 399 nm in toluene, which changes little when
Notes and references
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