Fluorescence response of pyridinyl- and/or dibutylaniline-Capped 2,6,9,10-Tetravinylanthracenes to metal ions

Article abstract of DOI:10.1016/j.dyepig.2013.09.037

The solution photophysical properties of a series of dibutylaniline- and/or pyridine-containing 2,6,9,10-tetrarylvinylanthracene cruciforms with and without metal ions have been determined. The results show that the dibutylaniline-containing cruciforms show stronger intermolecular charge transfer and obviously red-shifted emission spectra than the only pyridine-containing ones, and fluorescence responses of these cruciforms to select several metal ions are also different. The alkali K⁺, alkaline-earth Mg²⁺ and Ca²⁺ hardly affect the emission behavior, but the transition Cu²⁺ and Zn²⁺ could red-shift, blue-shift or quench the fluorescence emissions, depending on the nature of end groups. The fluorescence responsive behaviors of the different cruciforms to metal ions are explained qualitatively through their spatially separated frontier molecular orbital (FMO) features and coordination-induced FMO changes. These findings suggest that these anthracene-centered cruciforms could be promising functional scaffolds selectively for differential transition metal sensor arrays.

Full text of DOI:10.1016/j.dyepig.2013.09.037

Dyes and Pigments 101 (2014) 109e115
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Fluorescence response of pyridinyl- and/or dibutylaniline-capped
2,6,9,10-tetravinylanthracenes to metal ions
*
Meng Zheng, Mingxiao Sun, Deteng Zhang, Tongliang Liu, Shanfeng Xue, Wenjun Yang
Key Laboratory of Rubber-Plastics of Ministry of Education (QUST), School of Polymer Science & Engineering,
Qingdao University of Science & Technology, 53, Zhengzhou Road, Qingdao 266042, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The solution photophysical properties of a series of dibutylaniline- and/or pyridine-containing 2,6,9,10-
tetrarylvinylanthracene cruciforms with and without metal ions have been determined. The results show
that the dibutylaniline-containing cruciforms show stronger intermolecular charge transfer and obvi-
ously red-shifted emission spectra than the only pyridine-containing ones, and fluorescence responses of
these cruciforms to select several metal ions are also different. The alkali Kþ, alkaline-earth Mg2þ and
Ca2þ hardly affect the emission behavior, but the transition Cu2þ and Zn2þ could red-shift, blue-shift or
quench the fluorescence emissions, depending on the nature of end groups. The fluorescence responsive
behaviors of the different cruciforms to metal ions are explained qualitatively through their spatially
separated frontier molecular orbital (FMO) features and coordination-induced FMO changes. These
findings suggest that these anthracene-centered cruciforms could be promising functional scaffolds
selectively for differential transition metal sensor arrays.
Received 16 July 2013
Received in revised form
9 September 2013
Accepted 11 September 2013
Available online 6 October 2013
Keywords:
Anthracene-centered cruciform
Intramolecular charge transfer
Chemosensor
Transition metal ion
Frontier molecular orbital
Fluorescence responsive behaviors
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
We have been interested in anthracene-centered linear and
2-D fluorophore cruciforms because of their unique structures
Most organic chromophores exhibit spatially superimposed
HOMO and LUMO. However, when center-crossed two-dimen-
sional (2-D) conjugated molecules with an aromatic ring as the
and interesting optical properties that are different from benzene-
centered cruciforms For instance, 2,6,9,10-
[23ꢀ28].
tetrasubstituted anthracene-centered cruciforms are non-
centrosymmetric chromophores and show enhanced two-photon
absorption effect [24,25], however, the centrosymmetric ben-
zene- or pyrazine-based cruciforms do not exhibit the enhanced
two-photon absorption (2PA) cross sections [26ꢀ28]. Also, some
cruciforms have potential applications for the molecular electronics
[29ꢀ31] and the construction of functional scaffolds for differential
amine sensor arrays [15,16,32ꢀ34]. These results imply that
common
p-center (named cruciform) are added donor and/or
acceptor terminals, independent electronic shifts into opposite di-
rections in HOMO and LUMO could be attained [1ꢀ8]. Generally,
the HOMO is located on the electron-rich (or donor-containing)
branch, while the LUMO is situated on the electron-deficient (or
acceptor-incorporating) axis of molecule. As a result, the intra-
molecular charge transfer absorption and emission properties of
such cruciform are dependent on both their structures and chem-
ical environments [9ꢀ16], to render them unique and interesting
optical and optoelectronic properties. For example, 1,2,4,5-
tetrasubstituted benzene-centered cruciform with dibutylamine
and/or pyridine end-groups show dramatic blue and/or red shifts of
emission and absorption upon coordination of metal cations, which
have been regarded as valuable functional scaffolds for differential
metal sensor arrays [1,16ꢀ22].
different types of p-centers and molecular symmetry could endow
cruciforms with different optical properties and applications. In the
previous works, we have reported the one- and two-photon ab-
sorption and excitation fluorescence properties of several
anthracene-centered cruciforms and 2-D cross-conjugated poly-
mers [24,25,35]. Motivated to further understand of anthracene-
centered cruciforms’ optical properties and develop their applica-
tions, we have recently designed and synthesized a new series of
2,6,9,10-tetra(arylvinyl)anthracene cruciforms with dibutylaniline
and/or pyridine as the end groups, and their aggregation-enhanced
one- and two-photon excitation fluorescence properties have been
investigated [35]. It is known that pyridine and dibutylaniline
* Corresponding author. Tel.: þ86 532 84023670; fax: þ86 532 84023977.
E-mail address: ywjph2004@qust.edu.cn (W. Yang).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.09.037

110
M. Zheng et al. / Dyes and Pigments 101 (2014) 109e115
(C4H9)2N
N
N
N(C4H9)2
N
N
(C4H9)2N
3
2
1
N(C4H9)2
N
(C4H9)2N
N
N(C4H9)2
(C4H9)2N
6
5
4
N
N
N(C4H9)2
Fig. 1. Chemical structures of dibutylaniline and/or pyridine-capped 2,6,9,10-tetravinylanthracene cruciforms 1e6.
moieties have coordination potential with metal cations because of
the existence of lonely electronic pairs at nitrogen, and the donor/
acceptor-substituted anthracene-centered cruciforms have been
characteristic of spatially separated HOMO and LUMO. Therefore, it
is necessary to examine the fluorescence sensing properties of
these anthracene-centered cruciforms to metal cations. Here we
have prepared six permutations of the donor- and/or acceptor-
containing tetrakis(arylvinyl)anthracenes (Fig. 1), and several
different physiologically active metal cations (alkali Kþ, alkaline-
earth Mg2þ and Ca2þ, and transition Cu2þ and Zn2þ) are chosen
as examples [36ꢀ40], which should permit a more systematic
investigation on the effect of structural variations (nature and po-
sition of donor/acceptor moieties) and metal ions on the absorption
and emission properties. For the sake of comparison, the parent
pyridylaldehyde, triflates of Zn2þ, Cu2þ, Mg2þ, Ca2þ, and Kþ
were commercially available from Energy Chem. Co. Ltd., China.
Other solvents and reagents were analytical grade and used as
received without further purification. Cruciforms 1e5 and
PC, 2,6-bis(diethylphosphorylmethyl)-9,10-dibromoanthracene
and 4-dibutylaminobenzaldehyde were from the previous work
[35].
2.2. Synthesis of 2,6-bis(diethylphosphorylmethyl)-9,10-
distyrylanthracene
A pressuretubecontaininga mixtureofstyrene(0.37 g, 3.6mmol),
2,6-bis(diethylphosphorylmethyl)-9,10-dibromoanthracene (0.45 g,
0.71 mmol), Pd(OAc)2 (15 mg, 0.067 mmol), tris(o-tolyl)phos-
phine (0.13 g, 0.42 mmol), N(C2H5)3 (3 mL) and THF (3 mL) was
sealed under nitrogen and refluxed at 80 ꢁC for 24 h. The
mixture was separated by a column chromatography on silica
gel using ethyl acetate/hexane (3/1) as the eluent. Yield: 0.35 g
cruciform,
2,6-bis(p-tert-butylstyryl)-9,10-bis(styryl)anthracene
(PC, Scheme 1) is also included.
2. Experimental section
(73%). 1H NMR (500 MHz, CDCl3):
d 8.36 (d, 2H), 8.21 (s, 2H), 7.90
2.1. Materials
(d, 2H), 7.70 (d, 4H), 7.48 (m, 6H), 7.39 (m, 2H), 6.94 (d, 2H), 4.03
(m, 8H), 3.33 (d, 4H), 1.23 ppm (m, 12H). Anal. Calcd. for
Triethylamine (NEt3) over calcium hydride and tetrahydro-
furan over metallic sodium were distillated before use. 4-
C
40H44O6P2: C, 70.37; H, 6.50; O, 14.06; P, 9.07. Found: C, 70.46;
H, 6.47.
PO(OEt)
2
PO(OEt)
2
N(C4H9)
2
Pd
(
OAc
)
2/P
(o-tolyl)
3
/Et3N/
T
HF
B
r
Br
t
-
BuOK/
C4H9)
2
THF
N
(
6
(
C4H9)2N
(
EtO)2OP
(
EtO)2OP
CHO
Scheme 1. The synthesis and structure of cruciform 6.

M. Zheng et al. / Dyes and Pigments 101 (2014) 109e115
111
2.3. Synthesis of 2,6-bis(4-dibutylaminostyryl)-9,10-
distyrylanthracene (6)
3.2. Spectroscopic properties of the cruciforms 1e6 and PC in
dichloromethane
Potassiumt-butoxide(0.16g,1.43mmol)wasaddedtothemixture
of 2,6-bis(diethylphosphorylmethyl)-9,10-distyrylanthracene (0.13 g,
0.19 mmol) and 4-dibutylaminobenzaldehyde (0.13 g, 0.56 mmol) in
anhydrous THF (15 mL) at room temperature under N2. The
mixture was stirred overnight and then 100 mL of methanol was
added. The precipitate was collected and purified by a column
chromatography on silica gel using hexane/dichloromethane (2/
1) as the eluent. Yield: 91 mg (57%). 1H NMR (500 MHz, CDCl3)
The normalized absorption and emission spectra for cruciforms
1e6 and PC in dichloromethane (DCM) are displayed in Fig. 2, and
the peak wavelengths of lowest-energy absorption brand (lmax
)
and emission spectra (lem) are summarized in Table 1. The lmax and
lem of parent cruciform PC are 433 and 536 nm, respectively. When
phenyl units at 2,6- or 9,10-positions are replaced by pyridine unit,
the resulting cruciforms only show the slightly red-shifted lmax
(436 nm for 1, and 438 nm for 5), implying pyridine unit is a weak
electron-withdrawing moiety. However, when phenyl units at 2,6-
or 9,10-positions are changed into dibutylaniline units, significantly
bathochromic shifted lmax are observed (Fig. 2a and Table 1),
indicating that dibutylaniline unit is a strong electron-donating
moiety. Moreover, the lmax of 2, and 6 are longer than those 3,
and 4, implying the intramolecular charge transfer (ICT) effect
induced by dibutylamine unit at 2,6-positions is stronger than that
at 9,10-positions, which could be ascribed to their different
conjugation pathways and utility of resonance energy. Overall,
there is a significant ICT effect between 9,10- and 2,6-branches for
all the dibutylamine-containing cruciforms.
d
(ppm): 8.32 (d, 2H), 8.17 (s, 2H), 7.92 (d, 2H), 7.75 (m, 6H), 7.51
(m, 4H), 7.42 (m, 6H), 7.11 (m, 4H), 6.98 (d, 2H), 6.63 (d, 4H),
3.30 (t, 8H), 1.59 (m, 8H), 1.37 (m, 8H), 0.97 (m, 12H). 13C NMR
(125 MHz, CDCl3):
d (ppm) 147.9, 137.3, 134.9, 134.5, 132.2, 130.8,
129.3, 128.9, 128.2, 128.0, 127.9, 127.0, 126.7, 125.4, 124.4, 124.1,
122.6, 118.6, 111.7, 50.8, 29.5, 20.4, 14.1. Anal. Calcd. for
C
62H68N2: C, 88.52; H, 8.15; N, 3.33. Found: C, 88.41; H, 8.19; N,
3.37.
2.4. Measurements
The lem of PC is at 540 nm, and only pyridine-containing cru-
ciforms 1 and 5 show the moderately red-shifted lem (22, and 7 nm,
respectively) due to the weak ICT effect (Fig. 2b, and Table 1).
However, significantly red-shifted fluorescence emission (33e
73 nm) are observed for those dibutylamine-containing cruciforms
because of their stronger ICT effect that decreases energy gaps
between the ground and FranckeCondon states. Although cruci-
forms 1e6 and PC are all 9,10-di(arylvinyl)anthracene-containing
derivatives and have similar conjugation skeleton, dibutylamine-
containing cruciforms show very low fluorescence quantum
UVevis absorption spectra were recorded on a Hitachi U-4100
spectrophotometer. Fluorescence spectra were measured on a
Hitachi F-4600 spectrophotometer, and the peak wavelength of
the lowest energy absorption band was used as the excitation
wavelength. The fluorescence quantum yield was determined by
the literature method using rhodamine B as the reference [41].
The uncertainty for measured UV or PL is <2%. NMR spectra were
recorded on an AVANCE 500 (Bruker) spectrometer in CDCl3 with
tetramethylsilane (TMS) as an internal standard. The elemental
analysis was performed on PerkineElmer 2400. Metal ion coor-
dination to cruciforms 1e6 was performed in dichloromethane
yields (F) (Table 1). These low F could be ascribed to the lower
energy of the emitting states and/or the free intramolecular
torsional motion of severely distorted 9,10-diarylvinyl moieties
facilitated by the flexible dibutylamino moieties. It is observed that
the lem of donoreacceptor-substituted cruciforms are longer than
that of only donor-containing cruciforms, that is, 2 > 6, 4 > 3,
implying that the weak acceptor could still stabilize the emitting
states. Moreover, obviously red-shifted emissions for 9,10- over 2,6-
dibutylamino-contaning cruciforms are observed, i.e. 4 > 2, 3 > 6,
although their lmax are in the opposite order, 2 > 4, 6 > 3. This
indicates that dibutylamine linked at 9,10-positions could stabilize
the emitting states more than that at 2,6-positions. As a sequence,
the fluorescence Stokes shifts of cruciforms 3 and 4 are larger than
those of cruciforms 2 and 6 and others (Table 1).
(DCM, analytical grade). 5e60 m
L of triflate salt (10ꢀ2 M in DCM)
was respectively added to the solutions of 1e6 (5 mL, w10ꢀ5 M in
DCM) in a 10 mL of sample bottle via microsyringe to afford the
cruciform solution with different equiv of metal ions. The
resulting solutions were capped and shaken for 1 min and
transferred into the four-sided quartz spectrophotometry
cuvette. The fluorescence spectra were measured at room
temperature.
3. Results and discussion
3.1. Synthesis
Donor and/or acceptor-capped anthracene-centered cruciforms
3.3. Fluorescence responses of 1e6 to select metal cations in DCM
presented here are the
positions of a central anthracene ring are linked respectively with
two different arylvinyl arms. Since anthracene is non-
p-systems in which the 2,6- and 9,10-
Since pyridine- and/or dibutylaniline-capped benzene-centered
cruciforms have been used as functional scaffolds for differential
metal sensor arrays, it is interesting to examine the fluorescence
response of anthracene-centered cruciforms 1e6 to metal ions.
Here we have chosen several different physiologically-active metal
cations (triflates of alkali Kþ, alkaline-earth Mg2þ and Ca2þ, and
transition Cu2þ and Zn2þ) as examples to investigate the effect of
metal ions on emission properties of cruciforms 1e6 in DCM. Fig. 3
shows the fluorescence images of cruciforms 1e6 upon addition of
an excess of metal ions (6 equiv). It is observed that there are no
changes in fluorescence colors for these cruciforms upon addition
of Kþ, Mg2þ, and Ca2þ (Fig. 3a), indicating that these cruciforms
have no sensing ability to the selected alkali and alkaline-earth
metal ions, probably due to their weak coordination abilities each
other. However, three kinds of different fluorescence responses are
observed upon addition of Cu2þ or Zn2þ (Fig. 3b): (a) bathochromic
a
centrosymmetric aromatic ring, 2,6,9,10-tetraarylvinylanthracenes
have twice (six) as many permutations as benzene-centered ana-
logues (three). The syntheses of 1e5 and PC have been described
elsewhere [35], and the preparation of 6 is depicted in Scheme 1.
The general synthetic route to 1e6 and PC starts from 2,6-
bis(diethylphosphorylmethyl)-9,10-dibromoanthracene, which is
first subjected to a Heck coupling with arylvinylenes (styrene, 4-
dibutylaminostyrene, and 4-vinylpyridine), then followed by a
WittigeHorner reaction with 4-tert-butylbenzaldehyde, 4-
pyridylaldehyde, and 4-dibutylaminobenzaldehyde to produce
desirable cruciforms in good yields and purity after purification by
a column chromatography on silica gel. Their structures and com-
positions have been unambiguously characterized by 1H and 13C
NMR and element analysis.

112
M. Zheng et al. / Dyes and Pigments 101 (2014) 109e115
a
b
Cruciforms:
PC
1.0
1.0
0.8
0.6
0.4
0.2
0.0
Cruciforms:
PC
1
1
0.8
2
2
3
3
0.6
4
4
5
5
6
0.4
6
0.2
0.0
450
500
550
600
650
700
750
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 2. Absorption and emission spectra of cruciforms 1ꢀ6 and PC in DCM at 1.0 ꢂ 10ꢀ5 M.
shifted emissions for 1 and 5 with Zn2þ, (b) hypsochromic shifted
emissions for 2, 3, 4 and 6 with Zn2þ or for 3 and 6 with Cu2þ, and
(c) quenched fluorescence for 1, 2, 4 and 5 with Cu2þ. It is noted that
all the pyridine-containing cruciforms show quenched fluores-
cence upon the addition of Cu2þ. This implies that the interaction
between Cu2þ and pyridine moiety could lead to fast non-
radioactive decay of the excited states of these cruciforms, which
is probably related to the electronic structure of Cu2þ. However, the
real origin for this difference between Cu2þ and Zn2þ is unclear, and
further investigations are needed. Nevertheless, Cu2þ, and Zn2þ can
be distinguished from each other through these cruciforms. Their
features are that single member of such a family do not well-
recognize a specific analyse, but an ensemble of different ones
will work. These findings suggest that cruciforms 1e6 could be
regarded as a new family of differential sensor arrays selectively for
transition metal ions.
The emission spectra of cruciforms 1e6 upon addition of 6 equiv
of Cu2þ or Zn2þ are depicted in Fig. 4, and the relative spectroscopic
data are summarized in Table 2. It is shown that Cu2þ can quench
the fluorescence emissions of all the pyridine-containing cruci-
forms and blue-shift the emission spectra of only dibutylaniline-
containing cruciforms (3 and 6). In contrast, Zn2þ does not
quench the emission spectra of any cruciforms. Zn2þ can blue shift
monotonous spectral shift of cruciform 3 is understandable since it
contains only one coordination axis. However, cruciform 4 with
two different coordination axes exhibits still the monotonous
spectral shift upon titration of metal ions, which requires further
explanation since this phenomenon is different from that observed
in the benzene-centred analogues whose spectral response to
metal ions are generally two-stage (stepwise) [20,42]. Benzene ring
is a real centrosymmetric
p-center and its two ICT pathways are
less twisted and more straight, which makes the coordination
ability of two branches depend on predominately the nature of end
functional groups. To our cruciforms, anthracene ring is an asym-
metric
p-center and the 9,10- and 2,6-ICT pathways are very
different, one is zigzag (2,6-direction) and another is seriously
twisted (9,10-direction). This could reduce the difference of coor-
dination abilities between the two axes; moreover, the 2,6-zigzag
and 9,10-twisted ICT pathways should weaken their coordination
abilities to metal ions. This is possibly the reason why anthracene-
centered cruciforms have no sensing ability to the alkali and
alkaline-earth ions such as Kþ, Mg2þ, and Ca2þ. On considering that
the main difference between the benzene- and anthracene-
centered cruciforms is the
p-center, it could be concluded that
the different type of -center and molecular symmetry might
p
endow the cruciforms with different optical properties and prac-
tical applications. It could be expected that changing the terminal
recognition elements would lead to cruciforms with further
enhanced and differentiated response capabilities, which is un-
derway in our laboratory.
the emission spectra and enhanced the
but red-shift the emission spectra and decrease the
1 and 5. These changed emissions and could be explained in
F
of cruciforms 2, 3, 4 and 6,
F
of cruciforms
F
terms of their frontier molecular orbital (FMO) features and fluc-
tuations in energy levels upon coordination with metal ions (vide
infra).
3.4. FMO features and coordination-induced fluctuations
To gain further understanding of the impact of transition metal
ions on the fluorescence behavior, cruciforms 3 and 4 are used as
Organic chromophores with disjoint frontier molecular orbital
(FMO) features are promising materials in optical and optoelec-
tronic fields but still quite rare. Therefore, it is very interesting to
know the FMO features of functional anthracene-centerd cruci-
forms presented here and the changes in the energy level caused by
metal ions. The HOMO and LUMO features of cruciforms 1e6 and
PC calculated with B3LYP/6-31G* are depicted in Fig. 6. The LUMO
of PC is very similar to its HOMO (Fig. 6a). PC shows spatially su-
perimposable or congruent FMOs as a result of the lack of donor
and/or acceptor end groups. However, when the phenyl units are
replaced with dibutylaniline and/or pyridine moieties, the resulting
cruciforms 1e6 all exhibit the spatially separated or disjoint FMOs
with the central anthracene ring acting as an integral part of both
orbital, regardless of the nature and linking positions of donor/
acceptor units (Fig. 6b). Moreover, the HOMO density is always
located on the dibutylaniline-containing branch, and LUMO density
lies on invariably the pyridine-containing axis.
the representatives to be added different equiv of Cu2þ and Zn2þ
,
respectively. As shown in Fig. 5, their emission spectra are
monotonously blue shifted with the increase of metal ions and then
remain almost unchanged at high concentrations of metal ions. The
Table 1
Spectroscopic data of cruciforms 1e6 and PC in DCM at 1.0 ꢂ 10ꢀ5 M.
Cpd
PC
1
2
3
4
5
6
lmax/nma 435
439
562
123
484
591
107
466
587
121
474
613
139
440
547
107
479
573
94
l
em/nmb 540
Dn/nmc
105
0.61
Fd
0.50
0.037
0.045
0.049
0.42
0.15
a
Peak wavelength of the lowest energy absorption band.
Peak wavelength of emission spectra.
Fluorescence stokes shift.
b
c
d
Fluorescence quantum yield.

M. Zheng et al. / Dyes and Pigments 101 (2014) 109e115
113
Fig. 3. Fluorescence images of cruciforms 1e6 in DCM at 1.0 ꢂ 10ꢀ5 M upon adding 6 equiv of metal ions under illumination with a 365 nm UV lamp. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Emission spectra of cruciforms 1e6 in DCM at 1.0 ꢂ 10ꢀ5 M upon adding 6 equiv of Zn2þ and Cu2þ
.
It has been observed that cruciform 1 and 5 show bathochromic
shifts and 2 and 4 hypsochromic shifts upon coordination with
Zn2þ, and 3 and 6 show blue shifts upon coordination with both
Zn2þ and Cu2þ. These fluorescence responses to metal ions can be
rationalized by qualitatively examining relative energy levels of the
FMOs as the coordination happens (Fig. 7.). In cruciform, pyridine
and dibutylaniline act as an electron-withdrawing and an electron-
donating group, respectively. Upon coordination with metal ions,
the interaction between pyridine units and metal ions will enhance
the electron-withdrawing ability of pyridine, and the coordination
between dibutylaniline units and metal ions will weaken the
electron-donating ability of dibutylamino. Thus, the coordination
will lower the LUMO levels of only pyridine-containing cruciforms
1 and 5 (stabilizing the excited states), while the HOMO will not be
greatly affected because it is situated on the non-coordinating
distyrylanthracene branches of the cruciforms (Fig. 7a). The nar-
rowed band-gaps will red-shift the emission spectra and decrease
the
F due to the lower energy of the emitting states that facilitate
the non-radioactive pathways. In contrast, the coordination will
lower the HOMO levels of only dibutylaniline-containing cruci-
forms 3 and 6 (stabilizing the ground states), while the LUMO will
not be greatly affected because it is located on the dis-
tyrylanthracene arms without coordination ability (Fig. 7b). In this
case, the band-gaps were broadened, blue-shifted emission spectra
and increased
F values were observed. For cruciforms 2 and 4 with
donoreacceptor substituted motif, upon addition of Zn2þ, the co-
ordination will lower both the HOMO situated on dibutylaniline-
containing branch and the LUMO located on pyridine-containing
arms (Fig. 7c). Since the emission spectra are blue-shifted and
F
increased upon coordination, the lowering extent of HOMO should
Table 2
Peak emission wavelengths and fluorescence quantum yields of cruciforms 1e6 in DCM at 1.0 ꢂ 10ꢀ5 M upon adding 6 equiv of Zn2þ and Cu2þ
.
Cruciforms
1
2
3
4
5
6
lem (no ions)
562
591
587
613
547
573
F
(no ions)
0.50
620
0.045
Not recorded
Quenched
0.037
569
0.072
Not recorded
Quenched
0.045
548
0.26
543
0.37
0.049
568
0.11
Not recorded
Quenched
0.42
579
0.089
Not recorded
Quenched
0.15
526
0.21
518
0.11
lem (þZn2þ
)
F
(þZn2þ
)
lem (þCu2þ
)
F
(þCu2þ
)

114
M. Zheng et al. / Dyes and Pigments 101 (2014) 109e115
2+
15
2+
4+Zn
80
60
40
20
0
3+Cu
2+
2+
Zn equiv
0
Cu equiv
12
9
1.0
2.0
4.0
6.0
8.0
10.0
0
1.0
2.0
4.0
6.0
8.0
6
3
0
450
500
550
600
650
700
750
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
Fig. 5. Emission spectra of cruciforms 3 and 4 in DCM at 1.0 ꢂ 10ꢀ5 M upon titration of Cu2þ and Zn2þ, respectively.
Fig. 6. Frontier molecular orbital plots of cruciforms 1e6 and parent PC calculated with B3LYP/6-31G* using Spartan.
Fig. 7. Schematic diagrams of the changes in energy levels of FMOs for cruciforms 1e6 upon coordination with metal ions.

M. Zheng et al. / Dyes and Pigments 101 (2014) 109e115
115
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that the donor segment (dibutylaniline) is more basic than the
acceptor pyridine [42].
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4. Conclusions
We have demonstrated that 2,6,9,10-tetra(arylvinyl)anthracene
cruciforms end-capped with dibutylaniline and/or pyridine exhibit
spatially separated frontier molecular orbital features and strong
intermolecular charge transfer effect. These anthracene-centered
cruciforms have only sensing abilities to transition metal ions
such as Zn2þ and Cu2þ, but they do not respond to alkali and
alkaline-earth ions, such as Kþ and Mg2þ and Ca2þ. Moreover, the
emission shifts upon titration of transition metal ions are monot-
onous even for those anthracene-centered cruciforms with two
different coordination terminals. These fluorescence responsive
behaviors to metal ions have been rationalized by qualitatively
examining relative energy levels of the FMOs as the coordination
happens. Compared with the sensing properties of the benzene-
centered analogues to metal ions, it is concluded that the
drophobic interactions of
a series of pyrene end-labeled poly(ethylene
different type of p-centers and molecular symmetry might endow
the cruciforms with different optical properties and practical
applications.
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Acknowledgments
We thank the NSF of China (no. 51173092, 51073083), NSF of
Shandong Province (no. ZR2010EM023, ZR2012-EMQ003), and
Open Project of State Key Laboratory of Supramolecular Structure
and Materials of Jilin University (no. SKLSSM201310).
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