Understanding the unconventional effects of halogenation on the luminescent properties of oligo(phenylene vinylene) molecules

Article abstract of DOI:10.1002/asia.201300732

It is commonly known that halogenation tends to decrease the luminescence quantum yield of an organic dye, owing to the high electronegativity and heavy-atom effect of the halogen atom. However, based on an investigation of the effects of halogenation on the luminescence of the oligo(phenylene vinylene) (OPV) framework, we demonstrate that halogenation can have positive impact on the solid-state fluorescence and electrochemiluminescence (ECL) properties of OPV derivatives. The chlorinated OPV exhibits a very high solid-state fluorescence quantum yield (91 %), whilst the brominated analogue gives the highest ECL emission intensity. Time-dependent density functional theory calculations, natural bond orbital analysis, and natural transition orbital analysis were performed to assist the understanding of the origin of these positive halogenation effects, which provide insight into the rational design of highly luminescent halogenated organic materials for solid-state devices and ECL applications. Copyright

Full text of DOI:10.1002/asia.201300732

FULL PAPER
DOI: 10.1002/asia.201300732
Understanding the Unconventional Effects of Halogenation on the
Luminescent Properties of Oligo(Phenylene Vinylene) Molecules
Chun-Lin Sun,[a] Jun Li,[a] Hong-Wei Geng,[b] Hui Li,[a] Yong Ai,[a] Qiang Wang,[a]
Shan-Lin Pan,*[b] and Hao-Li Zhang*[a]
Abstract: It is commonly known that
halogenation tends to decrease the lu-
minescence quantum yield of an organ-
ic dye, owing to the high electronega-
tivity and heavy-atom effect of the hal-
ogen atom. However, based on an in-
vestigation of the effects of halogena-
tion on the luminescence of the
trochemiluminescence (ECL) proper-
ties of OPV derivatives. The chlorinat-
ed OPV exhibits a very high solid-state
fluorescence quantum yield (91%),
whilst the brominated analogue gives
the highest ECL emission intensity.
Time-dependent density functional
theory calculations, natural bond orbi-
tal analysis, and natural transition orbi-
tal analysis were performed to assist
the understanding of the origin of
these positive halogenation effects,
which provide insight into the rational
design of highly luminescent halogenat-
ed organic materials for solid-state de-
vices and ECL applications.
Keywords: density functional calcu-
lations · halogenation · lumines-
cence · molecular packing · oligo-
mers
oligo(phenylene
vinylene)
(OPV)
framework, we demonstrate that halo-
genation can have positive impact on
the solid-state fluorescence and elec-
Introduction
resulting in fluorescence quenching.[4] This effect works by
enhancing the spin–orbit coupling between the excited-state
electrons of a compound and the massive nucleus of the
heavy atom. Such an effect is proportional to Z4, where Z is
the atomic number. Therefore, heavy halogen atoms may
significantly decrease the fluorescence QY and excited-state
lifetime of a molecule. Although the HAE increases the
probability of intersystem crossing and results in more mole-
cules occupying the triplet excitation state, it is impractical
for enhancing the phosphorescence of organic molecules be-
cause the highly bonded nature of electrons in organic mate-
rials leaves them little freedom and less impetus to emit
from triplet states at room temperature. Moreover, the high
electronegativity of F, Cl, and Br atoms has been reported
to decrease the QY by withdrawing electron density from
the conjugated structure.[5] Therefore, the common conclu-
sion is that halogenation is a negative factor in determining
the luminescence properties of organic dyes.
Halogenation is a very important method for the design of
functional conjugated molecules and it can be used to tune
various molecular properties, such as the energy levels of
the molecular orbitals (MOs), bandgap, and solid-state
packing.[1] A large number of halogenated conjugate organic
molecules have been reported for potential applications in
the fields of organic electronic and optoelectronics, such as
organic field-effect transistors (OFETs)[1a,2] and photovolta-
ics.[3]
However, halogenation is generally avoided in the design
of luminescent materials with high quantum yields (QYs)
for several reasons. The heavy-atom effect (HAE) is one of
the most well-known halogenation effects on the lumines-
cent properties of organic molecules, which suggests that
halogen atoms promote singlet–triplet conversion, thereby
Recently, some investigations have shown that halogen
atoms affect the luminescence properties of organic materi-
als in a more-complicated manner than just through conven-
tional HAEs, for example the recent discovery of crystalliza-
tion-induced phosphorescence (CIP)[6] and the “directed”
heavy atom effect (DHAE).[7] These reports have shown
that, by designing chromospheres that contain triplet-pro-
ducing aromatic aldehydes and triplet-promoting bromine
atoms, crystal-state halogen-bonding interactions can pro-
duce highly efficient organic solid-state phosphorescence at
room temperature. Wꢀrthner and co-workers reported an
unexpected increase in the fluorescence QY of a series of
halogenated squaraines.[8] Duan and co-workers demonstrat-
[a] Dr. C.-L. Sun, Dr. J. Li, Dr. H. Li, Y. Ai, Dr. Q. Wang,
Prof. H.-L. Zhang
State Key Laboratory of Applied Organic Chemistry (SKLAOC)
College of Chemistry and Chemical Engineering
Lanzhou University, Lanzhou, 730000 (China)
Fax : (+86)931-8912365
E-mail: haoli.zhang@lzu.edu.cn
[b] Dr. H.-W. Geng, Prof. S.-L. Pan
Department of Chemistry
The University of Alabama
Tuscaloosa, Alabama 35487 (USA)
Fax : (+1)205-348-9104
E-mail: span1@bama.ua.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300732.
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Shan-Lin Pan, Hao-Li Zhang et al.
ed a tuning of the fluorescence emission of a single chromo-
Results and Discussion
phore molecule through different halogen- and hydrogen-
bonding interactions.[9] These recent works have indicated
that more investigations are needed to gain a deeper insight
into the effects of halogenation on the fluorescence proper-
ties of organic molecules and to identify possible positive ef-
fects that may benefit future materials design.
Herein, we report a combined experimental and theoreti-
cal investigation on a series of halogenated oligo(phenylene
vinylene) (OPV) molecules. The OPV backbone was select-
ed as a model structure because it is a highly fluorescence
chromophore that is widely used in various optoelectronic
applications.[10] Previously, we reported that an appropriately
modified OPV molecule may exhibit high solid-state fluo-
rescence and stimulated emissions.[11] Herein, four model
compounds (Scheme 1) were synthesized and their optical
Electronic Properties in Solution
The experimental absorption and emission spectra of the
four OPV molecules as dilute solutions in THF are shown in
Figure 1a; the key parameters are summarized in Table 1.
To assist the analysis, the absorption and emission properties
of the OPVs were also studied by TD-DFT simulations. The
simulated spectra (Figure 1b) are in good agreement with
the experimental ones. Notably, the absolute values of the
transition energies are systematically underestimated in the
theoretical calculations, consistent with previous reports.[12]
The linear fitting to the calculated and experimental transi-
tion energies shows good linearity, with a small systematic
error of 0.3 eV, thus indicating that the simulation is reliable.
The calculations indicate that the main electronic transition
for these four molecules is the HOMO!LUMO transition,
which can also be attributed to the p!p* transition. The
excited state in this transition is the first singlet excitation
state (S1). The highest-energy absorption peak is due to the
transition from the ground state to the second singlet excita-
tion state (S2), which mainly corresponds to the
HOMOꢀ1!LUMO electronic transition.
Scheme 1. Structures of the four OPV molecules that were used in this
study.
The absorption spectra of all of the OPV molecules are
very similar, with two prominent absorptions within the
properties, both in solution and
in the solid state, as well as
their electrochemiluminescence
(ECL), were studied and com-
pared. Aided by theoretical cal-
culations, detailed mechanisms
on how the different halogen
atoms affect the luminescence
properties of the OPV back-
bone are proposed.
This report is organized into
three sections. First, the spec-
troscopic properties of these
compounds in dilute solution
are discussed. Theoretical cal-
culations, including time-depen-
dent density functional theory
(TD-DFT), natural bond orbital
(NBO) analysis, and natural
transition orbital (NTO) analy-
sis are performed to investigate
the effects of halogenation on
the optical properties of these
Figure 1. Absorption and fluorescence spectra of OPV-H and OPV-X (X=F, Cl, Br) molecules a) in THF and
b) obtained from theoretical calculations. Inset shows a linear fitting to the calculated and experimental ener-
gies, including the energies of the two absorption peaks and the emission peak.
molecules. Second, the luminescence properties of these
OPV molecules in the solid state are studied and the corre-
lation between their fluorescence properties and molecular
packing in the crystal is discussed. Finally, we discuss the
ECL behavior of three model OPV-X molecules under elec-
trochemical conditions.
range 2.2–4.5 eV. The lowest electronic transition (S0!S1)[13]
gives a maximum absorption band at about 3.1 eV and the
S0!S2 transition gives a slightly weaker band at about
3.8 eV. For OPV-H, OPV-F, OPV-Cl, and OPV-Br, the S0!
S1 transition occurs at 3.15, 3.18, 3.12, and 3.11 eV, respec-
tively. Compared with OPV-H, OPV-F exhibits a higher
S0!S1 transition energy, whereas OPV-Cl and OPV-Br
show lower transition energies, thus indicating that the halo-
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Shan-Lin Pan, Hao-Li Zhang et al.
Table 1. Spectroscopic parameters of the four molecules as obtained from experimental observations in solution and theoretical calculations based on
TD-DFT methods.[a]
abs
exp
abs0
exp
em
exp
abs
theor
abs
theor
em
theor
E
E
E
E
E
E
mge
mge’
me*g
t
Fsol
kr
knr
OPV-H
OPV-F
OPV-Cl
OPV-Br
3.15
3.18
3.12
3.11
3.83
3.84
3.76
3.75
2.81
2.83
2.77
2.76
2.80
2.83
2.74
2.73
3.49
3.51
3.43
3.42
2.51
2.52
2.45
2.44
4.46
4.83
4.79
4.93
1.89
1.88
2.00
2.00
4.94
4.92
5.32
5.48
1.74
1.78
1.70
1.31
0.93
0.77
0.95
0.60
0.53
0.43
0.56
0.46
0.045
0.13
0.029
0.31
[a] Experimental (Eexp) and calculated vertical-transition energies (Etheor) of the lowest- (Eabs, in eV) and higher-energy allowed excited states (Eabs’, in
eV); energy of the maximum of the fluorescence band (Eem, in eV); transition dipole moments to the lowest- (mge) and higher-energy excited states (mge’
)
and transition dipole moments between the optimized excited state and the ground state (me*g), which defines the radiative decay rate; the fluorescence
lifetime (t, in ns); and the absolute fluorescence quantum yield (F). The latter two columns show the rate constant for radiative deactivation, S1!S0 (kr,
in nsꢀ1), and the rate constant for non-radiative deactivation (knr, in nsꢀ1).
ꢀ
gen atoms have different effects on the molecular orbitals of
the OPV backbone. The emission spectra of these four mol-
ecules show maximum emission energies at 2.81, 2.83, 2.77,
and 2.76 eV for OPV-H, OPV-F, OPV-Cl, and OPV-Br, re-
spectively. The order of the emission energies is consistent
with the order of their maximum absorption energies, thus
suggesting that the halogen atoms affect the electronic struc-
tures of the backbone in both the ground and excited states.
The influence of the halogen atoms on the electronic struc-
tures of both the ground and excited states was confirmed
by the calculated wavefunctions of the MOs (see the Sup-
porting Information, Figure 1). The electron density around
the halogen atom increases in the order OPV-F<OPV-Cl<
OPV-Br and all of the orbitals that are involved in these
transitions are affected by the halogen atoms.
Moreover, the calculations reveal that the optimized
structures of OPV-H, OPV-Cl, and OPV-Br adopt a planar
conformation, whereas OPV-F adopts a non-planar confor-
mation, with a dihedral angle of 168 between the terminal
and central benzene rings. Therefore, the higher S0!S1 tran-
sition energy for OPV-F compared with all of the other mol-
ecules is mainly due to its non-planar conformation.
around the C X bonds in the order OPV-F<OPV-Cl<
OPV-Br, similar to the results of the MO analysis. Owing to
the increased size of the bonding orbitals, that is, 2s2p, 3s3p,
ꢀ
and 4s4p for F, Cl, and Br, respectively, the size of C X or-
ꢀ
bitals increased in the same sequence. In the C X bond, the
natural electron configurations of the C atoms are sp3.46
,
sp3.34, and sp3.49 for OPV-F, OPV-Cl, and OPV-Br, respec-
tively, where the numbers represent the composition of the
p orbital in the hybridized orbital. Notably, the natural elec-
tron configuration as calculated from the NBO analysis is
different from the terms of hybridization in fundamental or-
ganic chemistry.[14] For comparison, in the terminal benzene
group of OPV-H, the natural electron configuration at the C
atom in the same position is sp2.31. The natural electron con-
figurations of OPV-X clearly have a larger p-orbital compo-
nent, which can be attributed to the polarization effect of
the halogen atoms. The configurations of the F, Cl, and Br
atoms are sp2.42, sp4.71, and sp6.30, respectively.
The contour maps in Figure 2b suggest that, with increas-
ing size of the halogen atom, the bonding orbitals extend
further towards the carbon-atom side of the molecule, that
ꢀ
is, the C C anti bond around the benzene rings, thus indicat-
ꢀ
ꢀ
The nature of the carbon halogen (C X) bonds in these
OPVs were investigated by using natural bond orbital
(NBO) analysis, which considered a multielectron molecular
wavefunction in terms of localized electron-pair “bonding”
units. The NBO diagrams show increased electron density
ing a greater contribution to the p system of the OPV back-
bone. The orbital extension can be evaluated by second-
order Fock matrix analysis. The interactions result in a loss
of occupancy from the localized NBO of the idealized Lewis
structures into an empty non-Lewis orbital.[15] The stabiliza-
ꢀ
Figure 2. a) NBO diagrams of the C X bonds in OPV-F, OPV-Cl, and OPV-Br at a density of 0.09 and b) the corresponding contour-line maps. c) Natu-
ꢀ
ral bonding configuration of the C X bonds.
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Shan-Lin Pan, Hao-Li Zhang et al.
tion energies (E2) that are associated with delocalization
reasonable to expect that OPV-Cl would show a somewhat
ꢀ
ꢀ
ꢀ
from the C X bond to the C C anti bond, are calculated to
be 3.2, 4.6, and 5.7 Kcalmolꢀ1 for OPV-F, OPV-Cl, and
OPV-Br, respectively. Such an increase in E2 can be attrib-
uted to four reasons: First, the orbital size increases, owing
to the increase in the size of the halogen atoms. Second, as
shown from the above configuration calculations, the p-orbi-
tal component increases with increasing halogen-atom size.
Third, a small part of the d orbitals participates in the bond-
lower QY than OPV-H. NBO analysis of the C Cl bond
(Figure 2) suggests that the d orbital of the Cl atom can
extend to the antibonding orbital of the C atom and in-
crease the molecular conjugation. Therefore, the high QY of
OPV-Cl suggests that the extension of the Cl d orbital has
a stronger positive impact on the fluorescence properties
than the negative influence factors, such as HAE, thereby
resulting in an overall high QY of OPV-Cl.
ꢀ
ꢀ
ing for C Cl and C Br and the d orbital can extend to the
Time-resolved photoluminescence (TRPL) measurements
were performed for the four molecules. The fluorescence
lifetime of OPV-H is 1.74 ns, close to reported vales for sim-
ilar molecules.[16] The lifetimes of OPV-F, OPV-Cl, and
OPV-Br are 1.78, 1.70, and 1.31 ns, respectively. The lifetime
provides information regarding the de-excitation process.
From the lifetime measurements and the QYs, the rate con-
stants for the radiative (kr) and non-radiative processes (knr)
were calculated (Table 1). The kr for OPV-H, OPV-F, OPV-
Cl, and OPV-Br are 0.53, 0.43, 0.56, and 0.46 nsꢀ1, respec-
tively.
For a molecule in dilute solution, the de-excitation pro-
cess can be described by the molecule exciton model. We
performed natural transition orbital (NTO) analysis to
obtain the “real” picture of the excited states, in which the
excited “particle” and the empty “hole” can be described
and analyzed.[17] The calculated NTOs of the brightest excit-
ed states are shown in the Supporting Information, Fig-
ure S2. The NTOs of these molecules are different from the
MOs as obtained from the TD-DFT calculations. The dis-
tinct difference between the NTOs and the MOs is at the
halogen atoms. In the MOs, there is more electron density
around the halogen atom in the HOMO than the LUMO,
owing to the electron-withdrawing nature of the halogen
atoms. However, the NTOs show less difference between
the “hole” and the “electron” around the halogen atoms.
Overlapping of the orbitals of the ground and excited states
is known to be a key factor in the probability of electronic
transition. The NTO results suggest that the lone-pair elec-
trons of the halogen atoms participate in the excited states.
The halogen atoms are increasingly involved in the exited
state in the order OPV-F<OPV-Cl<OPV-Br.
The excited-state-to-ground-state transition dipole
moment (me*g) was also obtained by NTO analysis. The rela-
tionship between the transition dipole moment and the radi-
ative-decay rate is non-trivial, based on the Strickler–Berg
equation.[18] However, considering the similar spectroscopic
properties in these four molecules, the enhanced transition
dipole moment for the emissive state should lead to faster
radiative-decay rates.[19] That is, a larger transition moment
generally results in a higher kr.[20] The transition moments as
calculated for OPV-H, OPV-F, and OPV-Cl are 4.94, 4.92,
and 5.32, respectively, which correspond to their experimen-
tal radiative rates. Also, the knr for OPV-F is larger than
those of OPV-H and OPV-Cl, which can be attributed to
the non-planar conformation and high electronegativity of
the F atom. However, OPV-Br exhibits the largest calculat-
ed transition dipole moment among the four compounds,
ꢀ
orbitals of the C C anti bond (Figure 2c). Fourth, the elec-
tronegativity decreases from F to Br and, hence, the elec-
tron-withdrawing effect decreases.
In the OPV chromophore, the double bonds are more sus-
ceptible to the influence of the substituents than the bene-
zene rings. Therefore, we studied the effects of halogenation
on the deviation angles of the double bonds. In the results
of the angular properties of the natural hybrid orbitals, the
deviation angle represents the angle between the bonding
orbital and the line of the bonded nuclei.[14] The ideal devia-
tion angle for a C=C double bond is 908, which means that
the two p orbitals are perpendicular to the line between two
carbon atoms. Deviation of the orientation of the p orbital
from the ideal angle causes a decrease in its strength. The p-
orbital-deviation angles for the double bonds in OPV-H, F,
Cl, and Br are 90, 88.3, 89.5, and 89.98, respectively. Fluori-
nation gives the largest change in the deviation angle, whilst
OPV-Br had an almost-ideal 908 deviation angle.
ꢀ
Briefly, the NBO analysis reveals that the C X bond
affect the molecular conjugation in two ways: 1) The elec-
tron cloud from the halogen atoms extends to the aromatic
rings; 2) the p-orbital-deviation angles of the C=C double
bonds are influenced by the halogen atoms. These effects
have rarely been discussed in previous investigations.
Halogenation has a significant impact on the fluorescence
properties of the OPVs. The fluorescence QY of the OPV-H
backbone in solution is very high (95%). However, OPV-F
shows a much lower QY (77%). The decrease in QY upon
halogenation is generally attributed to the high electronega-
tivity of the halogen atoms.[5] However, this NBO analysis
suggests that the lowest p-orbital-deviation angle of the C=
C double bond in OPV-F could be a major reason for the
decrease in QY.
The QY of OPV-Br is about 60%, even lower than that
of OPV-F. NBO analysis suggests that OPV-Br has an ideal
p-orbital-deviation angle of the C=C double bond, that is,
good conjugation. Moreover, the d orbital of the Br atom
extends over the molecule, which should be beneficial for
the fluorescence properties. However, the strong HAE from
the Br atoms overwhelms the above two positive factors and
affords a significant decrease in the QY.
In contrast to OPV-F and OPV-Br, OPV-Cl shows an un-
expectedly high QY of 95%, thus indicating that chlorina-
tion does not decrease the fluorescence properties of the
OPV-H backbone in this case. Given that the deviation
angles as calculated from the NBO analysis is slightly lower
than the ideal value and the HAE from the Cl atom, it is
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Shan-Lin Pan, Hao-Li Zhang et al.
albeit only with a moderate experimental kr value. This ex-
ception probably arises from the influence of ISC on the
rate, which is strongly promoted by Br but is not included in
our calculation model. Moreover, the lifetime of OPV-Br is
much shorter than those of the other three molecules, which
is expected to stem from two factors: First, the large transi-
tion moment of OPV-Br promotes a fast S1!S0 transition.
Second, the heavy-atom effect of Br promotes the ISC prog-
ress and, hence, augments the non-radiative rates.
properties and molecular conformations can be significantly
different from those in solution.[22]
The absorption spectra of these molecules are very simi-
lar, with a broad absorption band in the range 2.0–4.5 eV
and a peak at about 2.7 eV. The absorption spectra of the
OPV-X molecules have more structure in the lower-energy
peaks than that of OPV-H, thus suggesting weaker intermo-
lecular interactions in the microcrystals of OPV-X.
The emissions of the molecules are all about 2.4 eV. The
emission maxima of OPV-F, OPV-Cl, and OPV-Br are at
2.42, 2.39, and 2.29 eV, respectively. The emissions from the
solid samples are all bathochromically shifted compared to
that in solution, but the sequence is the same as that in solu-
tion.
Because of the aggregation-caused quenching (ACQ)
effect,[23] the fluorescent QY of solid organic materials is
typically much lower than that in solution. One example is
perylene, which exhibits a fluorescence quantum yield of
94% in solution, but gives no emission in the solid state. To
quantitatively evaluate the influence of halogen atoms on
solid-state fluorescence, the parameter of fluorescence-
quenching efficiency (h) is introduced, which is defined ac-
cording to Equation (1).
Optical Properties in the Solid State
OPV derivatives are potentially useful in solid-state devices,
such as OLEDs,[21] organic lasers,[10c–e,11] and photovola-
tics.[10g] Thus, we investigated the effects of halogenation on
the solid-state emission properties of these OPVs. Figure 3
shows the absorption and emission spectra of the OPVs in
their microcrystalline state; the key parameters are listed in
Table 2. In contrast to their solution-state behavior (see
above), the molecules in the crystalline state are subject to
many intermolecular interactions; therefore, their emission
h ¼ ðFsolꢀFsolidÞ=Fsol
ð1Þ
The parameter h helps to rule out the difference between
the intrinsic fluorescence properties of different molecules,
so that the effects of different halogen atoms on the quench-
ing behavior of the molecules in the solid state can be com-
pared. For OPV-H, OPV-F, OPV-Cl, and OPV-Br, h=50.5,
11.7, 4.2, and 26.7, respectively. Clearly, OPV-X have much-
smaller quenching efficiencies than OPV-H, thus suggesting
that halogenation could suppress the ACQ effect. Excitingly,
the fluorescent QY of OPV-Cl in the solid state is 0.91,
which is exceptionally high for solid organic materials. Solid
OPV-Cl almost retains its high fluorescence QY in dilute so-
lution, thus indicating an almost-negligible ACQ effect.
The time-resolved photoluminescence (TRPL) spectra of
the OPVs in the solid state were also measured (Figure 4).
Fitting to the decay curves indicates more than one expo-
nential decay in all of the samples, which may arise from
sample heterogeneity, non-exponential population decay,
surface effects,[24] etc. Assigning individual contributions by
decay-curve analysis is not possible at present. Therefore,
Figure 3. Absorption and emission spectra of the OPV molecules; the
numbers 0, 1, and 2 denote the vibrational energy levels.
Table 2. Experimental data of OPV-H, OPV-F, OPV-Cl, and OPV-Br in the solid state.[a]
abs
solid
em
solid
E
E
Fsolid
Fsol
h [%]
t
kr
knr
q
Type
d
Contact
none
OPV-H
OPV-F
OPV-Cl
OPV-Br
2.78
2.79
2.70
2.76
2.36
2.42
2.39
2.29
0.46
0.68
0.91
0.44
0.93
0.77
0.95
0.60
50.5
11.7
4.2
1.57
3.23
1.80
1.27
0.29
0.21
0.51
0.35
0.35
0.10
0.05
0.44
34.89/22.85
5.36
68.25/44.65
13.72
lamellar
3.69
ꢀ
herringbone
herringbone
herringbone
2.96
C H···F
4.89[b]
3.51
none
ꢀ
26.7
C H···p
[a] Experimental maximum absorption energy (Eabs, in eV) and maximum emission energy (Eem, in eV); the absolute fluorescence quantum yield in the
solid state (Fsolid) and in solution (Fsol); the intensity-weighted average lifetime (t, in ns); the quenching efficiency (h); the dihedral angle between the
central and terminal phenyl rings (q); and the distance between adjacent layers (d, in ꢂ). The latter two columns show the rate constant for radiative de-
activation, S1!S0 (kr, in nsꢀ1), and the rate constant for non-radiative deactivation (knr, in nsꢀ1). [b] Because of the non-planar conformation of OPV-Cl,
the distance was measured between the central phenyl rings.
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Shan-Lin Pan, Hao-Li Zhang et al.
Figure 5. Molecular packing of the OPVs, viewed along the bc plane.
CCDC 933885 (OPV-F), CCDC 933886 (OPV-Cl), and CCDC 933887
(OPV-Br) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Figure 4. Time-resolved fluorescence spectra of the OPVs in the micro-
crystals. The symbols represent the experimental fluorescence-decay
curves and the lines show the fitting.
the curves were simply fit to a double-exponential form to
obtain the average lifetime, which represents a characteristic
time constant.[24] The average lifetimes for OPV-H, F, Cl,
and Br are 1.57, 3.23, 1.80 and 1.27 ns, respectively. The
average lifetime of OPV-F is much longer in the solid state
than in solution (1.78 ns), whereas the average lifetimes of
OPV-Cl and OPV-Br are close to those in solution. The pro-
longed lifetime of OPV-F can be explained by two factors:
First, the vibration-relaxation process may be suppressed by
the rigid structure in the crystal state, that is, suppression of
the non-radiative deactivation of the excited state owing to
vibrational relaxation is responsible for enhanced fluores-
cence emission and the longer lifetime of OPV-F compared
with those in solution.[24] Second, the pitch angle of OPV-F
in the crystal is 73.58 and is assigned as H-aggregation-in-
duced packing, in which the lowest-energy state is only
weakly coupled to the ground state, thus dramatically en-
hancing the lifetime.[25] The existence of exciton coupling is
evident by the fact that the emission spectrum of OPV-F is
structure-less (Figure 3).
ent. All of the OPV-X molecules exhibit herringbone-type
packing, owing to the polarity that is caused by the introduc-
tion of a halogen atom. Although the three OPV-X mole-
cules have the same packing type, they pack differently in
detail, thereby resulting in different optical properties in the
solid state.
In the crystal of OPV-F, a strong F···H interaction is ob-
served between the terminal F atom and the H atom of the
methoxy group in the neighboring molecule, with a F···H
distance of 2.57 ꢂ. The distance between the two molecular
layers is 2.96 ꢂ, but the strong F···H interaction results in
slippage along the short axis and there is little overlap be-
tween the neighboring molecules. Moreover, the dihedral
angle between two adjacent columns is 81.358. The almost-
orthogonal intersection structure weakens the interactions
between the columns, so that the non-radiative deactivation
of the excited molecules is suppressed. As a result, the
OPV-F crystals show a smaller decrease in QY (h=11.7%)
compared to the strong quenching of OPV-H (h=50.5%).
Overall, the solid-state fluorescence QY of OPV-F (0.68) is
higher than that of OPV-H (0.46).
OPV-Cl exhibits the best solid-state fluorescence proper-
ties of all four OPVs, with a QY in the solid state as high as
0.91. Moreover, the knr value is almost as small in the solid
state as in solution, which suggests a very small ACQ effect.
This superior property is associated with its unique molecu-
lar packing in the crystal. There are two coexisting molecu-
lar conformations in the crystal (see the Supporting Infor-
mation, Figure 5), which have different dihedral angles be-
tween the terminal and the central benzene rings, that is,
68.25 and 44.658. The coexistence of two crystallographically
independent conformations is similar to the highly fluores-
cent crystals as reported by Ma and co-workers.[27] More-
over, it has been reported, including in recent studies on the
aggregation-induced emission (AIE) effect,[28] that the non-
planar conformation is very effective at suppressing the
ACQ effect. Firstly, the torsional conformation enhances the
steric hindrance between adjacent molecules. The distance
between the central benzene rings of molecules in the two
The single-crystal structure of OPV-H has been reported
previously.[26] Single crystals of the OPV-X molecules were
grown from solution and the crystal structures are shown in
Figure 5. In general, the main factors that may induce fluo-
rescence quenching in molecular crystals are various inter-
molecular interactions, which provide 3D deactivating chan-
ꢀ
nels, including p p interactions, dipole–dipole interactions,
Coulombic interactions, and other weak donor–acceptor in-
teractions, such as hydrogen bonding and halogen bonding
in the solid state. Moreover, potential exciton diffusion and/
or energy transfer within the aggregates may also quench
the fluorescence.
OPV-H shows 1D lamellar-type packing and the long axis
of the OPV-H molecules are almost parallel to one other in
the crystal. The lamellar-type packing of OPV-H is favora-
ble for dipole–dipole and charge-transfer interactions, there-
by resulting in strong quenching in the solid state. Com-
pared with the packing types of the backbone molecule
OPV-H, the packing types of OPV-X are dramatically differ-
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ꢀ
molecular layers is 4.89 ꢂ, which is much longer than the p
p distance. The interaction between transition dipole mo-
ments is also suppressed by the long distance and the non-
parallel dipole moments of two adjacent molecules. Second-
ly, the dihedral angle between any two nearby terminal ben-
produce the radical anion of OPV by scanning the same
working electrode potential towards the negative direction
[Eq. (3)]. The formed radical anions and cations are annihi-
lated to form the excited state of OPV [Eq. (4)], followed
by radiative decay [Eq. (5)] to give off light. As shown in
Figure 6, we obtained intense ECL of OPV-Cl at about
ꢀ
zene rings is 69.28, so that there is no p p packing between
ꢀ
the terminal benzene groups. Moreover, there are no CH p
interactions because the shortest distance between the side
CH unit and the nearby benzene ring is 3.16 ꢂ, which is
larger than the CH p distance (typically <3.0 ꢂ).[29] Thirdly,
ꢀ
the calculated energy levels (HOMO/LUMO) for the two
coexisting molecular conformations are different, that is,
ꢀ1.72/ꢀ5.40 eV and ꢀ2.07/ꢀ5.39 eV, respectively. It is
known from Marcus–Hush theory that the effective intermo-
lecular transfer integral is decreased if there is an energy
difference between the two adjacent molecules. Therefore,
the energy difference between the two different conforma-
tions prevents energy transfer between neighboring mole-
cules and suppresses the fluorescent quenching from inter-
molecular energy transfer.
The fluorescence QY of OPV-Br in the solid state is only
0.44, which is the lowest of the four OPVs. This low QY can
be attributed to two reasons: First, as discussed above, the
intrinsic QY of OPV-Br, as measured in solution, is the
lowest, owing to the heavy-atom effect. Second, the crystal
of OPV-Br exhibits the highest h value (26.7%) of the three
Figure 6. CV and ECL spectra of 0.24 mgmLꢀ1 OPV-Cl in MeCN with
0.1m tetrabutylammonium hexafluorophosphate (TBA·PF6). Scan rate:
25 mVsꢀ1
.
ꢀ
OPV-X molecules. The high h value is attributed to p p in-
teractions between the OPV-Br molecules. In the crystal,
the distance between the two molecular layers is only 3.5 ꢂ
and the benzene ring overlays with the double bond of the
ꢀ2.0 V (versus Ag QRE) after completing a linear potential
scan from 0.0 to 2.0 V. ECL generation requires both ener-
getic species OPVCꢀ and OPVC+ to have adequate energy for
producing the excited states of OPV; therefore, no ECL was
observed when the electrode potential was only scanned
within the range of 0.0 and ꢀ2.0 V or 0.0 and 2.0 V. Only
weak ECL was observed if OPV-Cl was first reduced and
then oxidized because the reduced species appeared to be ir-
reversibly converted into other species through a subsequent
homogenous reaction, as shown by cyclic voltammetry
(CV). The oxidized OPV-Cl was relatively stable for ECL
generation when the electrode potential was scanned at
a high scan rate.
Instead of using CV to probe the transient ECL character-
istics, the electrode potential was changed stepwise between
2.0 and ꢀ2.4 V (versus Ag QRE). As shown in Figure 7, the
current response follows a Cottrell decay. Pronounced ECL
can only be obtained when a negative potential step of
ꢀ2.4 V is applied. No ECL or only a weak ECL is observed
at the positive potential step, which is consistent with the
CV experiment, as shown in Figure 7. The ECL intensity at
the working electrode appears to be unstable and decreases
in the first ten potential-step periods before remaining
stable under a balanced supply of the oxidized and reduced
species. Electrode-surface passivation in the presence of
ECL by-products or their polymerized species might also ac-
count for the decreased ECL.
ꢀ
adjacent molecule, thus allowing p p interactions to occur.
Furthermore, in the crystal, the fluorescence of one OPV-Br
molecule is not only quenched by its own Br atoms, but also
by the Br atoms in the nearby molecules in a process that is
similar to the external heavy-atom effect in solution.[30]
Electrochemiluminescence Properties
The effects of the halogen atoms on the electrogenerated
chemiluminescence (ECL) have also been studied. Unlike
the photoluminescence method, ECL is electrochemically
generated and allows us to understand the light-emitting
characteristics of a chromosphere.[31] A general route for
ECL generation from the OPVs is proposed in Equa-
tions (2)–(5) to help understand the light-emitting mecha-
nism of the chromospheres in an electrochemical cell.
Cþ
OPVꢀeꢀ ! OPV
OPVþeꢀ ! OPV
ð2Þ
ð3Þ
ð4Þ
ð5Þ
Cꢀ
Cþ
Cꢀ
*
OPV þOPV ! OPV
*
OPV ! OPVþhn
First, the OPV molecule is oxidized to generate a radical
cation under a positive bias [Eq. (2)] and then reduced to
The insert in Figure 8b shows the shape and peak position
of the ECL spectra of OPV-X. The ECL spectrum of OPV-
7
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Shan-Lin Pan, Hao-Li Zhang et al.
Figure 4). All of the OPV-X molecules show ECL in the
negative potential range after a positive potential scan. All
of the OPVs are irreversibly reduced whilst ECL is generat-
ed. Unlike the fluorescence studies, OPV-Br was the most-
efficient OPV for ECL generation under the indicated con-
ditions as shown in Figure 8b; then, OPV-Cl is the next
most efficient for ECL generation and OPV-F is the weak-
est. This result is completely contrary to the conventional
HAE effect, as explained by the reversibility of the reduced
state of the three OPVs. OPV-Br shows relatively stable re-
duced states and ECL can be generated from about ꢀ1.5 V,
whilst OPV-F starts generating ECL above ꢀ2.0 V. The dif-
ference with the fluorescence data can also be attributed to
the complex nature of ECL generation, which relies on
redox reactions and mass transfer in a different electro-
chemical environment. The effect of the electrode surface
might also account for such differences. The complex mech-
anism of the ECL process makes HAE less important. As
a result, the ECL intensity in-
Figure 7. Current and ECL responses between 2.0 V and ꢀ2.4 V for
0.24 mgmLꢀ1 OPV-Cl in MeCN with 0.1m TBA·PF6.
creases in the sequence OPV-
F<OPV-Cl<OPV-Br, which is
contrary to the common per-
ception based on HAE.
Conclusions
Our investigation into the ef-
fects of halogenation on lumi-
nescence properties of organic
molecules under different con-
ditions indicates that the halo-
gen atoms affect their emission
in a complex way. In solution,
OPV-Br shows a low fluores-
cence QY, mainly owing to
a strong HAE. However, the
chlorinated molecule, OPV-Cl,
Figure 8. a) CV and b) the corresponding ECL spectra of 0.14 mgmLꢀ1 OPV-F, 0.24 mgmLꢀ1 OPV-Cl, and
0.12 mgmLꢀ1 OPV-Br in MeCN with 0.1m TBA·PF6. Scan rate: 25 mVsꢀ1. Inset shows the ECL spectra of
OPV-X (F, Cl, and Br) in MeCN with 0.1m TBA·PF6. ECL was recorded for 1 min between 2.0 V and ꢀ2.4 V
for six cycles.
Cl is quite consistent with the fluorescence spectrum. The
difference between the ECL and fluorescence spectra can
be attributed to the difference between their generation
mechanisms.[32] ECL is generated in the electrode-diffusion
range close to the electrode surface, where the redox reac-
tion is performed, whilst fluorescence is generated in the
bulk solution. Spectrum broadening for OPV-F and OPV-Br
is observed, which is attributed to the poor solubility of
these two halogen-substituted OPVs in MeCN.[33] The ECL
intensity appears to be much lower than the fluorescence in-
tensity because the energetic species are not stable and the
yield of excited-state generation depends on other effects,
such as mass transfer of these redox species. Trace amounts
of contaminates might well quench the excited state in
strong electrolyte.
exhibits very high fluorescence QY, mainly because the d or-
bital of the Cl atom positively contributes to the molecular
conjugation. The solid-state luminescence properties are
largely determined by the molecular conformation and
packing in the crystals. The microcrystals of OPV-Cl exhibit
an exceptionally high solid-state fluorescence quantum
yield, owing to its twisted conformation and unique molecu-
lar packing, which efficiently prohibit the ACQ process. In
the ECL experiment, the OPV-Br molecule, which has the
lowest fluorescence QY in solution, exhibits the highest
emission intensity, thus suggesting that the complex ECL
mechanism renders HAE less important. This work provides
more insight into the working mechanism of halogenation
effects on the luminescence properties of organic molecules
under different conditions beyond the conventional consid-
eration of HAE. This work should help the future design of
new organic fluorescence materials for various applications.
To determine the effect of halogen atoms on the ECL in-
tensity, the CVs and the ECL responses for all three OPV-X
molecules are shown in Figure 8 (for the current responses
of OPV-F and OPV-Br, see the Supporting Information,
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of Higher Education (SRFDP; 20110211130001), the Fundamental Re-
search Funds for the Central Universities, and the 111 Project.
Experimental Section
Materials and Methods
The four OPV molecules were synthesized according to
a
one-pot
method by using a classical Horner–Wadsworth–Emmons coupling reac-
tion,[34,35] similar to the method reported in our previous work[11a] (for de-
tails of the synthesis, see the Supporting Information).
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Steady-state absorption spectra were recorded on a T6 UV/Vis spectrom-
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Electrochemiluminescence Measurements
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This work was supported by the National Basic Research Program of
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Received: May 30, 2013
Published online: && &&, 0000
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Chem. Asian J. 2013, 00, 0 – 0
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

FULL PAPER
Halo jump: Typically, halogenation
decreases the luminescence quantum
yield of an organic dye, owing to
heavy-atom effects. Herein, appropri-
ate halogenation has a positive impact
on the solid-state fluorescence and
electrochemiluminescence (ECL)
properties of oligo(phenylene vinyl-
ene)s (OPVs). The chlorinated OPV
exhibits a very high fluorescence quan-
tum yield (91%), whilst the bromi-
nated OPV affords the highest ECL
emission intensity.
Halogenation
Chun-Lin Sun, Jun Li,
Hong-Wei Geng, Hui Li, Yong Ai,
Qiang Wang, Shan-Lin Pan,*
Hao-Li Zhang*
&&&& — &&&&
Understanding the Unconventional
Effects of Halogenation on the Lumi-
nescent Properties of Oligo(Phenylene
Vinylene) Molecules
11
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Chem. Asian J. 2013, 00, 0 – 0
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
These are not the final page numbers! ÞÞ