Synthesis of Fluorene-Bridged Arylene Vinylene Fluorophores: Effects of End-Capping Groups on the Optical Properties, Aggregation Induced Emission

Article abstract of DOI:10.1002/cjoc.201500240

We have synthesized a series of fluorene-based fluorophores, in which a central fluorene core has been modified with different peripheral arylene vinylene substituents that are able to activate aggregation-induced emission (AIE) characteristics. 9,9-Dioctylfluorene doubly end-capped at the 2,7-positions with triphenylethene groups, such as 4-(2,2-diphenylvinyl)phenyl (F1-(2,2)-HTPE) and 4-(1,2-diphenylvinyl)phenyl (F1-(1,2)-HTPE) were synthesized and compared to the tetraphenylethene analogue (F1-TPE). Both F1-(2,2)-HTPE and F1-(1,2)-HTPE glow with a deep blue fluorescence in THF solution with emission maxima (λ_em) of 426 and 403 nm, respectively. The λ_em slightly red-shifts in the solid-state to 458 nm for F1-(2,2)-HTPE and 437 nm for F1-(1,2)-HTPE. The fluorescence quantum yields (Φ;_F) of F1-(2,2)-HTPE (?_F=35.1 %) and F1-(1,2)-HTPE (?_F=26.2%) were found to be higher in solution compared to the near quenching of F1-TPE (?_F=0.1%). Consequently, this results in weaker AIE-stability of F1-(2,2)-HTPE (α_AIE=1.5) and F1-(1,2)-HTPE (α_AIE=1.9) compared to F1-TPE (α_AIE=125), suggesting that four phenyl groups are necessary for efficient AIE-activity of these fluorene bridged arylene vinylene type materials. In addition, decreasing the steric hindrance around the arylene vinylene moiety by removal of a phenyl ring is another method to decrease the AIE characteristics, in a similar manner to the commonly known ?phenyl-locking?. Non-polar triphenylethenes are poorer AIE materials than their tetraphenylethene analogues. Replacing the hydrogen atom of F1-(2,2)-HTPE with a cyano group affords fluorene end-capped with 2,3,3-triphenylacrylonitrile (F1-TPAN), which boosts the AIE-effect to α_AIE=90.5 and red-shifts the solid-state emission (λ_em=528 nm) with near quenching in THF solution (?_F=0.12%). X-ray crystallographic analysis of F1-TPAN indicates that the introduction of cyano groups can not only diminish the intramolecular steric hindrance in comparison of F1-TPE, but also improve the molecular cohesion ability via multiple C-H···N interactions. Four different arylene vinylene substituents attached to the fluorene were synthesized, and the disparities of AIE-effect of these compounds were discussed. The cyano-groups in F1-TPAN will not only reduce the steric congestion of the peripheral phenyl rings, but also appreciably improve the molecular cohesion ability via multiple C-H?N interactions.

Full text of DOI:10.1002/cjoc.201500240

FULL PAPER
DOI: 10.1002/cjoc.201500240
Synthesis of Fluorene-Bridged Arylene Vinylene Fluorophores:
Effects of End-Capping Groups on the Optical Properties,
Aggregation Induced Emission
Guo-Feng Zhang,a Tao Chen,a Ze-Qiang Chen,a Matthew P. Aldred,*,b
Xianggao Meng,*,c and Ming-Qiang Zhu*,a
a Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information,
Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
b Department of Chemistry, Durham University, South Road, Durham DH1 3LE, UK
c Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry,
Central China Normal University, Wuhan, Hubei 430079, China
We have synthesized a series of fluorene-based fluorophores, in which a central fluorene core has been modified
with different peripheral arylene vinylene substituents that are able to activate aggregation-induced emission (AIE)
characteristics. 9,9-Dioctylfluorene doubly end-capped at the 2,7-positions with triphenylethene groups, such as
4-(2,2-diphenylvinyl)phenyl (F1-(2,2)-HTPE) and 4-(1,2-diphenylvinyl)phenyl (F1-(1,2)-HTPE) were synthesized
and compared to the tetraphenylethene analogue (F1-TPE). Both F1-(2,2)-HTPE and F1-(1,2)-HTPE glow with a
deep blue fluorescence in THF solution with emission maxima (λem) of 426 and 403 nm, respectively. The λem
slightly red-shifts in the solid-state to 458 nm for F1-(2,2)-HTPE and 437 nm for F1-(1,2)-HTPE. The fluorescence
quantum yields (ΦF) of F1-(2,2)-HTPE (ΦF＝35.1 %) and F1-(1,2)-HTPE (ΦF＝26.2%) were found to be higher in
solution compared to the near quenching of F1-TPE (ΦF＝0.1%). Consequently, this results in weaker AIE-stability
of F1-(2,2)-HTPE (αAIE＝1.5) and F1-(1,2)-HTPE (αAIE＝1.9) compared to F1-TPE (αAIE＝125), suggesting that
four phenyl groups are necessary for efficient AIE-activity of these fluorene bridged arylene vinylene type materials.
In addition, decreasing the steric hindrance around the arylene vinylene moiety by removal of a phenyl ring is an-
other method to decrease the AIE characteristics, in a similar manner to the commonly known “phenyl-locking”.
Non-polar triphenylethenes are poorer AIE materials than their tetraphenylethene analogues. Replacing the hydro-
gen atom of F1-(2,2)-HTPE with a cyano group affords fluorene end-capped with 2,3,3-triphenylacrylonitrile
(F1-TPAN), which boosts the AIE-effect to αAIE＝90.5 and red-shifts the solid-state emission (λem＝528 nm) with
near quenching in THF solution (ΦF＝0.12%). X-ray crystallographic analysis of F1-TPAN indicates that the intro-
duction of cyano groups can not only diminish the intramolecular steric hindrance in comparison of F1-TPE, but
also improve the molecular cohesion ability via multiple C－H…N interactions.
Keywords aggregation-induced emission, fluorene, triphenylethene, tetraphenylethene, 2,3,3-triphenylacrylo-
nitrile
Introduction
and finally metal-catalysed cross-coupling reactions to
afford final materials. Polyfluorene is a class of
hairy-rod polymer that allows extensive core enlarge-
ment without significant loss in conjugation. Therefore,
structural modification at the 9-position does not sig-
nificantly affect planarity which loses at the 2- and
7-positions and shows excellent control of conjugation
length. Additionally, fluorene is an important core
component for the construction of light-emitting
nematic liquid crystals[11-14] and is also widely utilized
as fluorescent probes for sensors and bioimaging.[15]
Most conventional fluorophores suffer from aggre-
Fluorene is an excellent building block for the syn-
thesis of semiconducting materials for applications in
optoelectronics and electronics, i.e. organic light emit-
ting diodes (OLEDs), organic field-effect transistors
(OFETs) and organic photovoltaics (OPVs).[1-10] Fluo-
rene chemistry within the aforementioned research areas
usually involves functionalising the 9-position with
solubilising groups or other functionalities by nucleo-
philic substitution, derivatizing the 2- and 7-positions by
electrophilic substitution reactions, i.e., bromination,
*
E-mail: mqzhu@hust.edu.cn; mpaldred@hotmail.com; mengxianggao@mail.ccnu.edu.cn; Tel.: 0086-027-87793419
Received March 24, 2015; accepted April 17, 2015; published online June 2, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500240 or from the author.
Chin. J. Chem. 2015, 33, 939—947
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Zhang et al.
FULL PAPER
gation caused quenching (ACQ), in which the solid-
state fluorescence diminishes relative to its fluorescence
in dilute solution. The latter state represents isolated
molecules and does not suffer from detrimental aggre-
gation effects that can decrease the fluorescence quan-
tum yields. Due to the fact that many applications of
light-emitting materials are in the solid-state, i.e.
OLEDs, efficient ways to alleviate solid-state fluores-
cence quenching have been important research topics.
One excellent example of this is aggregation induced
emission (AIE),[16] in which the solution-state fluores-
cence diminishes relative to its solid-state fluorescence.
A phenomenon that is the exact opposite of ACQ was
unexpected and in recent years, the benefits from this
strategy in improving fluorescence properties in the
“aggregate” state have been exploited in several re-
search areas, including OLEDs, sensors and bioimag-
ing.[17-33]
C8H17
C8H17
H
H
H
F1-(2,2)-HTPE
H
C8H17
C8H17
F1-(1,2)-HTPE
C8H17
C8H17
One archetypal AIE-active material is tetraphenyl-
ethene (TPE) and has been extensively reported in the
literature due to its AIE properties, high solid-state
fluorescence quantum yields and ease of synthesis/
modification.[17-22] Recently, we have reported intrigu-
ing AIE-active adducts and investigated the optical and
photophysical properties. We have demonstrated re-
markable fluorescence quenching in solution with en-
hanced emission in the “aggregate” state of an analo-
gous series of fluorene oligomers end-capped with
TPE.[32] The mechanism of the enhanced emission in the
“aggregate” state compared to the near quenching in
solution was explained in terms of hindered internal
bond motions in the solid-state, compared to excessive
bond motions in solution, in which the latter opens up
non-radiative channels after excitation. We also con-
cluded that as the number of central fluorene units in-
creases in between the two end-capped TPE groups, the
AIE-effect of the associated material gradually de-
creases, indicating that the TPE units have a reduced
influence on the AIE-effect as the conjugation length
increases. On the basis of these interesting results, we
further developed these materials and here we report our
latest findings regarding the end-capping of a mono-
fluorene core with different arylene-vinylene groups
(Figure 1) with investigation of their optical, AIE prop-
erties.
F1-TPE
C8H17
C8H17
NC
CN
F1-TPAN
Figure 1 Structure of the fluorene derivatives end-capped with
different arylene-vinylene units.
instruments (FLSP920 spectrometers), respectively.
Mass Spectrometry was recorded on matrix-assisted
laser desorption/ionization time-of-flight mass spec-
trometry (MALDI-TOF) using cyano-4-hydroxy-
cinnamic acid (CHCA) as the matrix. The energy levels
of the four compounds were obtained from the simula-
tion based on Gaussian program. For X-ray crystallo-
graphic studies, diffraction data were collected at room
temperature using an Apex Duo X-ray CCD diffracto-
meter working with graphite-monochromatized Mo Kα
X-radiation (λ＝0.71073 Å). Empirical absorption cor-
rections were applied to the data using the SADABS
program. Structural refinements were performed by the
full-matrix least-squares method on F2 with SHELXTL-
97 program. Data collection, frame integration and data
reduction were performed using the multi-scan method,
and structure determination was carried out using
APEX2 attached software. Anisotropic displacement
parameters were refined for all non-hydrogen atoms.
Hydrogen bonds to carbon atoms were placed at calcu-
lated positions with the appropriate AFIX instructions
and refined using a riding model. Related parameters of
some short contacts in the crystal structures are meas-
ured using PLATON. The synthetic routes followed for
the synthesis of the fluorene derivatives F1-(2,2)-HTPE,
F1-(1,2)HTPE, F1-TPE and F1-TPAN are depicted in
Scheme 1. The compounds of (E,Z)-1-(4-bromophenyl)-
1,2-diphenyl-ethene [(1,2)-HTPE-Br], (E,Z)-(4-(1,2-
diphenylvinyl)-phenyl)-boronic acid ((1,2)-HTPE-BA),
Experimental
The commercially available starting materials, re-
agents and solvents were purchased from Sinopharm,
Aladdin Chemical Reagent Co. Ltd, and used as sup-
plied unless otherwise stated. THF and toluene were
dried using sodium wire and benzophenone as the indi-
cator. The NMR spectra relative to tetramethylsilane
(TMS) as an internal standard were recorded on Bruker
AV400 or AV600 spectrometer. UV-Vis and photolumi-
nescence spectra were recorded on Shimadzu UV-VIS-
NIR spectrophotometer (UV-3600) and Edin-burgh
940
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Chin. J. Chem. 2015, 33, 939—947

Synthesis of Fluorene-Bridged Arylene Vinylene Fluorophores
(4-(2,2-diphenylvinyl)-phenyl)boronic acid ((2,2)-
HTPE-BA),[34] (4-(1,2,2-triphenyl-vinyl)phenyl)boronic
acid (TPE-BA)[35] and 3,3-diphenyl-2-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxa-borolan-2-yl)-phenyl)acryl-
onitrile (TPAN-BE)[36-37] were synthesized following
previous reports.
(CDCl3, 100 MHz) δ: 151.80, 151.78, 151.74, 143.59,
142.32, 142.28, 142.20, 140.77, 140.42, 140.35, 140.22,
140.17, 139.53, 139.48, 139.26, 137.50, 137.39, 130.99,
130.43, 129.64, 129.62, 128.73, 128.42, 128.27, 128.09,
128.02, 127.82, 127.61, 127.51, 127.20, 126.94, 126.83,
126.72, 125.911, 121.39, 121.18, 120.07, 55.36, 55.32,
40.49, 40.46, 31.79, 30.05, 29.24, 29.22, 23.85, 22.61,
14.08. HR-MS (APCI): 898.4868.
2,7-Dibromo-9,9-dioctylfluorene A mixture of
2,7-dibromofluorene (1, 20.0 g, 61.7 mmol), KOH (20.8
g, 370.4 mmol), KI (1.02 g, 6.2 mol) in DMSO (250 mL)
was stirred in an ice bath for 10 min. Subsequently,
1-bromooctane (29.8 g, 154.3 mmol) was slowly added
to the above mixture over a period of 30 min. After stir-
ring overnight, the reaction mixture was poured into
ice-water (300 mL) and extracted with hexane (250 mL
×2). The organic layers were combined, washed with
brine (200 mL×3), water (100 mL×1), dried (MgSO4)
and filtered. The solution was concentrated under re-
duced pressure via rotary evaporator and the crude
product was purified by recrystallization using ethanol
9,9-Dioctyl-2,7-bis(4-(1,2,2-triphenylvinyl)phe-
1
nyl)-9H-fluorene (F1-TPE) Yield 62.8%; H NMR
(600 MHz, CDCl3) δ: 7.68 (d, J＝7.8 Hz, 2H), 7.50 (t,
J＝7.8 Hz, 4H), 7.42 (d, J＝8.0 Hz, 4H), 7.04－7.16 (m,
34H), 1.96－1.99 (m, 4H), 1.02－1.10 (m, 20H), 0.77 (t,
J＝6.0 Hz, 6H), 0.64－0.66 (m, 4H); 13C NMR (CDCl3,
100 MHz) δ: 151.60, 143.83, 142.63, 141.04, 140.57,
140.04, 139.40, 139.29, 131.82, 131.15, 131.40, 131.39,
127.83, 127.71, 127.65, 126.51, 126.49, 126.43, 126.20,
125.80, 121.02, 119.90, 55.24, 40.48, 31.77, 30.04,
29.23, 29.20, 23.77, 22.60, 14.09. MS m/z (APCI):
1051.9.
1
to yield a white crystalline solid (28.5 g, 84.1%). H
NMR (400 MHz, CDCl3) δ: 7.51 (d, J＝8.8 Hz, 2H),
7.44－7.46 (m, 4H), 1.89－1.93 (m, 4H), 1.05－1.19
(m, 20H), 0.83 (t, J＝6.4 Hz, 6H), 0.57 (q, J＝6.8 Hz,
4H). MS m/z (ACPI): 548.4 (M+).
9,9-Dioctyl-2,7-bis(4-(1-cyano-2,2-diphenylvinyl)-
phenyl)-9H-fluorene (F1-TPAN) Yield 60.4%; 1H
NMR (400 MHz, CDCl3) δ: 7.75 (d, J＝8.0 Hz, 2H),
7.52－7.57 (m, 8H), 7.46－7.50 (m, 4H), 7.41－7.44
(m, 6H), 7.35 (d, J＝8.4 Hz, 4H), 7.28－7.30 (m, 2H),
7.21－7.25 (m, 4H), 7.08 (d, J＝8.4 Hz, 4H), 1.99－
2.03 (m, 4H), 1.07－1.19 (m, 20H), 0.77 (t, J＝6.0 Hz,
6H), 0.68－0.70 (m, 4H); 13C NMR (CDCl3, 100 MHz)
δ: 157.49, 151.82, 141.40, 140.56, 140.40, 139.29,
138.00, 133.64, 130.79, 130.15, 129.96, 129.03, 128.47,
128.39, 127.03, 126.01, 121.24, 120.15, 120.05, 111.41,
55.37, 40.37, 31.73, 29.95, 29.13, 23.78, 22.55, 14.02.
MALDI-TOF MS: found 949.337.
General procedure for the synthesis of fluorene
derivatives via Suzuki coupling reaction To a
two-neck flask (100 mL), 2,7-dibromo-9,9-dioctyl-
fluorene (1 equiv.), boronic acid (3 equiv.), Pd(PPh3)4
(0.1 equiv.), tetrabutylammonium hydrogen sulfate (0.3
equiv.), K2CO3 (6 equiv.) in toluene (40 mL) and water
(20 mL) were added and heated to 90 ℃ with stirring
under nitrogen overnight. After cooling down to ambi-
ent temperature, the organic layer was separated and the
water layer was extracted with DCM. The combined
organic layers were dried (Na2SO4) for 2 h. After filtra-
tion, the resulting solution was concentrated by rotary
evaporator and the crude residue was purified by col-
umn chromatography (silica gel) using DCM-PE solvent
mixture as the eluent to afford the final products. The
1H NMR spectra of the final products are shown in the
supporting information (Figure S1 to Figure S4).
9,9-Dioctyl-2,7-bis(4-(2,2-diphenylvinyl)phenyl)-
9H-fluorene (F1-(2,2)-HTPE) Yield 58.3%; 1H NMR
(600 MHz, CDCl3) δ: 7.70 (d, J＝7.8 Hz, 2H), 7.45－
7.52 (m, 8H), 7.27－7.37 (m, 20H), 7.11 (d, J＝6.6 Hz,
4H), 7.02 (s, 2H), 1.97－1.98 (m, 4H), 0.85－1.16 (m,
20H), 0.78 (t, J＝6.6 Hz, 6H), 0.65－0.67 (m, 4H); 13C
NMR (CDCl3, 150 MHz) δ: 151.64, 143.43, 142.59,
140.53, 140.10, 139.78, 139.39, 136.30, 130.41, 130.02,
128.80, 128.25, 127.77, 127.60, 127.55, 127.52, 126.59,
125.76, 121.10, 119.96, 55.25, 40.49, 31.77, 30.03,
29.20, 23.77, 22.59, 14.09. HR-MS (APCI): 899.5535.
9,9-Dioctyl-2,7-bis(4-((E,Z)-1,2-diphenylvinyl)-
phenyl)-9H-fluorene (F1-(1,2)-HTPE) Yield 62.8%;
1H NMR (600 MHz, CDCl3) δ: 7.78－7.80 (m, 2H),
7.62－7.67 (m, 8H), 7.25－7.57 (m, 14H), 7.11－7.17
(m, 8H), 6.99－7.06 (m, 4H), 2.02－2.05 (m, 4H), 1.06
－1.19 (m, 20H), 0.73－0.79 (m, 10H); 13C NMR
Results and Discussion
The synthetic routes towards the fluorene derivatives
are depicted in Scheme 1. Triphenylethene-sub-
stituted boronic acids ((2,2)-HTPE-BA) and ((1,2)-
HTPE-BA) were synthesised by Horner-Wadsworth-
Emmons reaction using the benzyl phosphonate with
benzophenone ((2,2)-HTPE-Br) and 4-bromoben-
zophenone ((1,2)-HTPE-Br) followed by the usual
treatment with n-BuLi in dry THF, followed by subse-
quent borylation (B(Oi-Pr)3) and hydrolysis to afford
the respective boronic acid. With respect to (1,2)-
HTPE-Br, the crude product was a mixture of (Z)-
and (E)-stereoisomers and was not further purified. Ra-
thores method was adopted for the synthesis of TPE-Br,
followed by similar boronic acid formation to that men-
tioned above.[37] Knoevenagel condensation by nucleo-
philic addition of 4-bromobenzonitrile to the carbonyl
group of benzophenone gives TPAN-Br. The reactivity
of the cyano group to strong base prevented the usual
boronic acid formation, therefore, Pd(dppf)Cl2-catalysed
borylation using bis(pinacolato)diboron was employed
to yield the boronic ester TPAN-BE. A convergent ap-
proach using Pd(PPh3)4-catalysed Suzuki coupling
Chin. J. Chem. 2015, 33, 939—947
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Zhang et al.
FULL PAPER
Scheme 1 Synthetic routes towards the fluorene-bridged arylene vinylene derivatives
Br
Br
(ii)
(i)
(iii)
Br
B(OH)2
OEt
O
H
P
Br
(2,2)-HTPE-Br
Br
(2,2)-HTPE-BA
EtO
B(OH)2
(iii)
(i)
(iv)
OEt
O
H
P
Br
(1,2)-HTPE-Br
(1,2)-HTPE-BA
EtO
(v)
(vi)
(iii)
B(OH)2
Br
Br
OH
TPE-BA
TPE-Br
(viii)
Br
(vii)
O
O
B
Br
NC
NC
CN
TPAN-Br
TPAN-BE
C8H17
C8H17
H
H
H
F1-(2,2)-HTPE
H
C8H17
C8H17
C8H17
C8H17
C8H17
C8H17
(ix)
F1-(1,2)-HTPE
(x)
Br
Br
C8H17
C8H17
F1-TPE
C8H17
C8H17
NC
CN
F1-TPAN
Reagents and conditions: (i) P(OEt)3; (ii) benzophenone; KOAc, THF; (iii) BuLi; −78 ℃; B(OiPr)3; 2 mol/L HCl; THF, (iv) 4-bromobenzophenone; (v)
BuLi; benzophenone; (vi) PTSA, toluene; (vii) NaH, 4-bromophenylacetonitrile, benzene; (viii) bis(pinacolato)diboron, Pd(dppf)Cl2, KOAc, dioxane; (ix)
C8H17Br, KOH, KI, DMSO; (X) the relevant boronic acid or boronic acid ester, Pd(PPh3)4, K2CO3, toluene, water, 90 ℃.
942
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© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2015, 33, 939—947

Synthesis of Fluorene-Bridged Arylene Vinylene Fluorophores
reactions, involving 2,7-dibromo-9,9-dioctylfluorene and
the aforementioned boronic acid/ester, were adopted to
yield the final materials; F1-(2,2)-HTPE, F1-(1,2)-
HTPE, F1-TPE and F1-TPAN. Structural characteriza-
1
tion of the final materials was performed by H NMR,
13C NMR, MALDI-TOF and/or high-resolution MS
with satisfactory results. All four final materials exhibit
good solubility in common organic solvents, such as
THF, dichloromethane and chloroform.
Only one fluorene unit was used to bridge the two
end-capped aryl vinylene groups, due to its ease of syn-
thesis and strong AIE propensity compared to higher
fluorene oligomers. We have previously reported excel-
lent AIE-activity of F1-TPE, which exhibits nearly total
quenching in solution,[32] compared to enhanced emis-
sion in the “aggregate” state. Here, the “aggregate” state
is termed as a state that prevents excessive internal bond
motions around the vinylic groups. These hindered bond
motions occur in the nanoparticle state, solid-state
(crystals, powder or thin film), micelles and even upon
complexation with biomolecules/ions. Compared to the
solution state, intramolecular rotations are extremely
restricted, and as a result, enhanced emission of the
AIE-active fluorophores can be observed in the solid
state. Restriction of intramolecular rotations (RIR) as
the basical mechanism of AIE phenomenon is demon-
strated.[39-43]
As we know, the internal bond motions of the phenyl
rotors belonging to the TPE unit in F1-TPE are impor-
tant in understanding its quenching behaviour. When the
phenyl rings in the TPE unit of F1-TPE are locked by
replacing the TPE units with 9,10-diphenyl-
phenanthrene (DPP) units, the ΦF in THF solution in-
creases from 0.3% (F1-TPE) to 53% (F1-PP) with a
subsequent diminished “aggregate” fluorescence.[29]
Therefore, upon “locking” the phenyl rings together the
materials fluorescence properties become inverted and a
transition from AIE-active to ACQ-active occurs (see
Figure S5). Similarly, TPE alone when oxidised to
9,10-diphenyl-phenanthrene (DPP) displays a similar
AIE to ACQ transition, in which DPP glows brighter in
solution compared to TPE.[44] Phenyl-locking creates a
more rigid structure and, thus, minimises radiationless
deactivation that enhances solution fluorescence effi-
ciency but detriments “aggregate” fluorescence effi-
ciency.[45] Interestingly, diphenyldibenzofulvene
(DPDBF-open form) still exhibits AIE characteristics
despite having half of its phenyl rings locked in place.[46]
This could mean that hindering the internal bond mo-
tions of non-geminal phenyl rings is more important
than hindering those of geminal phenyl rings in disturb-
ing AIE characteristics.[47] To further probe the effects
of the phenyl rotors on the optical properties we have
synthesized other fluorene bridged materials with dif-
ferent numbers and locations of the peripheral phenyl
rings around the vinylic core. Herein, we wonder
whether decreasing the number of phenyl rings around
the vinylic group of the arylene vinylene units can
Figure
2
The normalised UV-vis absorption spectra of
F1-(2,2)-HTPE, F1-(1,2)-HTPE, F1-TPE and F1-TPAN (10−5
mol/L in THF solvent).
decrease the AIE-effect.
As depicted in Figure 2 and Table S1, the
maximumUV-Vis absorption peaks (λabs) of F1-(2,2)-
HTPE, F1-(1,2)-HTPE, F1-TPE and F1-TPAN are
located at 365, 356.5, 357 and 367 nm, respectively. The
maximum photoluminescence (PL) peaks (λem) for the
as-prepared solids of F1-(2,2)-HTPE, F1-(1,2)-HTPE,
F1-TPE and F1-TPAN are 458, 437, 492 and 528 nm,
respectively (Figure 3). The features of UV-Vis absorp-
tion spectra of F1-TPE and F1-(1,2)-HTPE are nearly
overlapped, the disparity of F1-TPE and F1-(2,2)-HTPE
in chemical structure is caused by the difference of
hydrogen atom and phenyl ring present in end-capped
arylene vinylene substituents resulting in the red-shift of
the λabs from 357 nm (F1-TPE) to 365 nm (F1-(2,2)-
HTPE). With comparison of the two extremes in λem of
F1-(2,2)-HTPE and F1-TPAN, the latter is 70 nm
red-shifted because of the electron withdrawing effect of
the cyano group. Due to the well-known morphology-
dependent emission of arylene-vinylene based materials,
this comparison is open for consideration because the
as-prepared sample of F1-(2,2)-HTPE is amorphous as
opposed to the crystalline nature of F1-TPAN.
Figure 3 The normalised PL spectra of F1-(2,2)-HTPE,
F1-(1,2)-HTPE, F1-TPE and F1-TPAN in the powder state.
The other apparent difference in the data from Table
S1 is the differences in ΦF of F1-(2,2)-HTPE and
F1-TPE in solution. F1-TPE exhibits considerable
fluorescence quenching in solution (ΦF ＝ 0.3%)
compared to relatively high fluorescence efficiency of
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F1-(2,2)-HTPE (ΦF ＝ 35.1%), which is similar to
F1-(1,2)-HTPE (ΦF ＝26.2%). The difference is the
replacement of a phenyl ring in F1-TPE with a
hydrogen atom in F1-(2,2)-HTPE. Recently, we have
shown that structural alterations at the geminal positions
of tetraarylethene-based materials cause significant
steric effects and subsequent changes in photophysics.[47]
Additionally, increasing the steric hindrance at the
geminal positions by incorporating bulky ortho-methyl
groups can induce a reduction in the AIE-effect and an
enhanced emission in the solution.[43] The steric hin-
drance of two geminal phenyl rings is minimal,[43]
therefore, we assume that by removal of one phenyl ring,
as in the case for F1-(2,2)-HTPE, no dominant steric-
optical property relationships are observed. We attribute
this reduction in the AIE-effect to the decreased
rotational motions of the arylene vinylene unit with
three phenyl rings, compared to four phenyl rings.
Similar phenomenon was observed for F1-(1,2)-HTPE
in comparison with F1-(2,2)-HTPE. This suggests that
the addition of one phenyl ring is essential in enhancing
internal bond motions in solution, which subsequently
leads to strong fluorescence quenching and a strong
AIE-effect. Interestingly, the incorporation of a cyano
group to the double bond of F1-(2,2)-HTPE to give
F1-TPAN decreases the solution fluorescence efficiency
and increases the AIE-effect.
calculations of molecular orbitals of F1-(2,2)-HTPE,
F1-(1,2)-HTPE, F1-TPE and F1-TPAN were carried out
using density functional theory (DFT) and the
B3LYP/6-31G(d) basis set. Three stereoisomers are
involved in F1-(1,2)-HTPE [i.e., 9,9-dioctyl-2,7-
bis(4-((Z)-1,2-diphenylvinyl)phenyl)-9H-fluorene, 9,9-
dioctyl-2,7-bis(4-((E)-1,2-diphenylvinyl)-phenyl)-9H-
fluorene and 9,9-dioctyl-7-(4-((E)-1,2-diphenylvinyl)-
phenyl)-2-(4-((Z)-1,2-diphenyl-vinyl)-ph-enyl)-9H-fluo-
rene], and the optimized HOMO and LUMO levels of
these three stereoisomers are almost identical to each
other (Figure S6). Based on this, electron densities of
the HOMO for 9,9-dioctyl-2,7-bis(4-((E)-1,2-diphenyl-
vinyl)phenyl)-9H-fluorene are selected and depicted as
follows. As shown in Figure 4, the electron densities for
the HOMO and LUMO levels of F1-(2,2)-HTPE,
F1-(1,2)-HTPE and F1-TPE are occupied over the entire
molecules. With regard to F1-TPAN, the electron
density for the HOMO of F1-TPAN is dominant on the
fluorene conterpart, and the electron density for the
LUMO of F1-TPAN is prevailing in the TPAN moity,
indicative of its electron-accepting attribute.
These characteristics are typical for a “push-pull”
type molecule in which the fluorene unit behaves as the
electron donor and the TPAN units behave as the
electron acceptor resulting in an A-D-A system. The
push-pull effect in the molecular structure of F1-TPAN
probably results in the further bathochromic shift of the
λabs and λem compared with F1-(2,2)-HTPE, F1-(1,2)-
HTPE and F1-TPE. Additionally, the HOMO and
LUMO levels of these analogues from DFT calculations
are shown in Table S1, and the energy band gap (Eg)
trends of F1-(2,2)-HTPE, F1-(1,2)-HTPE, F1-TPE and
F1-TPAN are consistent with the data estimated from
the onset wavelength (λonset) of their UV-vis absorption
spectra. The fluorescence lifetimes of F1-(2,2)-HTPE,
F1-(1,2)-HTPE, F1-TPE and F1-TPAN in the solid state
are 1.10, 0.89, 3.00 and 1.98 ns (Figure S7 and Table
S1), respectively.
TPE is a well-known AIE-active fluorophore, and
the majority of more complex fluorophores that inte-
grate TPE units also exhibit AIE feature. It is our inter-
est to systematically investigate the AIE characteristics
of the analogues depicted in Figure 1. To our surprise,
after end-capping fluorene at the 2,7-positions with
triphenylene units, the resulting compounds F1-(2,2)-
HTPE and F1-(1,2)-HTPE display weak AIE character-
istics (low AIE-effect, αAIE) (Figure 5a and 5b). As
shown in Figure 5a and Figure 5b, the PL intensities of
F1-(2,2)-HTPE formed in 80% water-THF solvent mix-
ture and F1-(1,2)-HTPE formed in 90% water-THF sol-
vent mixture are slightly increased compared to their
respective THF solutions. For F1-(2,2)-HTPE, the re-
duction of PL intensity F1-(2,2)-HTPE can be observed,
which is probably due to the formation of agglomerates
at high water content. With respects to F1-(1,2)-HTPE,
the respective ΦF of its aggregates in 90% water-THF
and THF solution are 51.5% and 26.2%, which is
Figure 4 HOMO and LUMO diagrams of F1-(2,2)-HTPE,
F1-(1,2)-HTPE, F1-TPE, and F1-TPAN.
Recently, using naphthalimide-TPE (NI-TPE),[31]
perylene diimide-TPE (PDI-TPE) and perylene diimide-
TPAN (PDI-TPAN)[27] as the models, we have
demonstrated that the TPE unit can serve as an
electron-donating unit, and oppositely, TPAN behaves
as an electron-accepting unit, due to the cyano group in
the molecular structure. To further understand the
different electron density distributions of the fluorene
derivatives caused by the end-capped groups, the
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Chin. J. Chem. 2015, 33, 939—947

Synthesis of Fluorene-Bridged Arylene Vinylene Fluorophores
Figure 5 The changes of PL intensity of (a) F1-(2,2)-HTPE, (b) F1-(1,2)-HTPE, (c) F1-TPE, (d) F1-TPAN in different THF-water
solvent mixtures. (e) The AIE-effect of the above four fluorophores.
similar to F1-(2,2)-HTPE.
the fluorene core; (ii) exhausting the energy of the excit-
ed state via intramolecular vibrational or rotational mo-
tions. The effective conjugation of the end-capped units
structurally corresponds to stilbene and may frequently
rotate in dilute solution, which will partly or completely
consume the energy of the excited state through non-
radiative processes manifesting in fluorescence
quenching. Oppositely, in the solid-state this non-radia-
tive process will be extremely impeded, inducing the
recovery of the fluorescence. The distinct AIE charac-
teristics amongst these four fluorene-bridged materials
probably stem from the disparities and the extent of
intramolecular rotation by the peripheral AIE moieties.
To further understand the molecular packing mode
of these four structures depicted in Figure 1, single
crystals of the associated compounds were tried to
obtain via slow solvent evaporation methodology using
DCM and methanol. However, we were unfortunate to
acquire the single crystals of F1-(2,2)-HTPE, F1-(1,2)-
In contrast, F1-TPE and F1-TPAN exhibit strong
AIE characteristics (high AIE-effect, αAIE) (Figure 5c
and Figure 5d). Under UV irradiation, the emission of
F1-TPE and F1-TPAN in dilute THF solution cannot be
observed by the naked eyes. Initially, when the water
content is progressively increased in the water-THF
solvent mixture, no obvious changes in PL intensity
occur. However, when the water fraction (fw) is further
increased nano-precipitation occurs and for fw＝70% to
90%, the emission intensity is extremely boosted
(Figure 5c and Figure 5d). The respective AIE-effects
(defined as αAIE＝Φaggre/Φsol, Φaggre＝resulting quantum
yield after gradual water addition, Φsol＝quantum yield
in 10−5 mol/L THF solution at 0% water content) of
F1-(2,2)-HTPE, F1-(1,2)-HTPE, F1-TPE and F1-TPAN
are 1.5, 1.9, 125.3, 90.5 (Figure 5e and Table S1).
The end-capped arylene vinylene units serve two
functions: (i) increasing the molecular conjugation with
Chin. J. Chem. 2015, 33, 939—947
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Zhang et al.
FULL PAPER
HTPE and F1-TPE via changing the other mixture
solvent systems, i.e. DCM and hexane, toluene,
chloroform and hexane, which is probably due to that
these three materials are inclined to be in amorphous
state (Figure 6).
and S7 (the disordered C66'-C71' atoms and their
relative hydrogens are omitted for clarity), in which the
end-capped TPAN units are quite distorted. The detailed
crystallographic data of F1-TPAN is shown in the Table
S2.
Calculated by PLATON,[48] the efficient intermolec-
ular interactions involving π…π and C－H…π are
absent in the crystals. As depicted in Figure 8, all the
F1-TPAN molecules are put into the same orientation, in
which the di-cyano groups are extended towards
different directions. The contiguous molecules are
packed via van der Waals' force and C — H … N
interactions, and the distances of C27—H27…N2 and
C32—H32…N1 are 2.69 and 2.66 Å, respectively.
Comparing F1-TPAN with F1-TPE, the replacement of
the phenyl rings with cyano-groups will probably
decrease the intramolecular steric congestion, which is
due to that the cyano-groups possess a smaller volume
than the phenyl rings in F1-TPE. In comparison of
F1-(2,2)-HTPE and F1-(1,2)-HTPE, the introduction of
cyano-groups will efficiently improve the molecular
cohesion ability via the formation of multiple C—H…N
interactions. Therefore, the introduction of cyano-
groups can not only diminish the intramolecular steric
hindrance, but also improve the intermolecular cohesion
via the introduction of C—H…N interactions, which is
further beneficial for the molecular packing.
Figure
6
XRD spectra of the as-prepared samples of
F1-(2,2)-HTPE, F1-(1,2)-HTPE, F1-TPE and F1-TPAN and the
simulation from F1-TPAN CIF.
From the XRD spectra (Figure 6) F1-(2,2)-HTPE,
F1-(1,2)-HTPE and F1-TPE show a broad amorphous
halo peak without sharp crystalline diffractions. On the
contrary, F1-TPAN shows sharp diffractions indicating
that the as-prepared sample of F1-TPAN is crystalline.
The ORTEP structure of F1-TPAN is shown in Figure 7
Figure 7 ORTEP structure of F1-TPAN. The disordered atoms are omitted for clarity.
Figure 8 Part of the molecular packing in the F1-TPAN crystals with C—H…N interactions shown as dotted lines
946
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Chin. J. Chem. 2015, 33, 939—947

Synthesis of Fluorene-Bridged Arylene Vinylene Fluorophores
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Doubly end-capping fluorene at the 2- and 7-posi-
tions with different arylene vinylene units has been
achieved by convergent synthesis with an ultimate Su-
zuki coupling reaction. The resultant fluorophores pos-
sess AIE characteristics with different degrees of AIE-
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motions of the end-capped units. In comparison with
F1-TPE, the introduction of cyano-groups in F1- TPAN
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Acknowledgement
This work was supported by the National Natural
Science Foundation of China (Nos. 21174045,
21474034), the National Basic Research Program of
China (Nos. 2015CB755602, 2013CB922104). M.P.A.
acknowledges the NSFC Research Fellowship for Inter-
national Young Scientists (No. 21150110141) and the
Key Fellowship of China Post-doctoral Science Founda-
tion. We also thank the Analytical and Testing Center of
Huazhong University of Science and Technology and
the Center of Micro-Fabrication and Characterization
(CMFC) of WNLO for use of their facilities.
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