Synthesis and zinc(II) complexation modulated fluorescence emission properties of two pyrene-oligo(phenylene vinylene)-2,2′-bipyridine conjugated molecular rods

Article abstract of DOI:10.1039/b617497c

A series of conjugated rods in which the pyrene chromophore is connected to a 2,2′-bipyridine unit via an oligo(phenylene vinylene) bridge have been synthesized and their photophysical properties in solution investigated. They are shown to display intense

Full text of DOI:10.1039/b617497c

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Synthesis and zinc(II) complexation modulated fluorescence emission
properties of two pyrene-oligo(phenylene vinylene)-2,20-bipyridine
conjugated molecular rodsw
a
a
b
c
´ ´ ´ ´
Stephanie Leroy-Lhez,* Magali Allain, Jean Oberle and Frederic Fages*
Received (in Durham, UK) 30th November 2006, Accepted 21st March 2007
First published as an Advance Article on the web 19th April 2007
DOI: 10.1039/b617497c
A series of conjugated rods in which the pyrene chromophore is connected to a 2,20-bipyridine
unit via an oligo(phenylene vinylene) bridge have been synthesized and their photophysical
properties in solution investigated. They are shown to display intense visible electronic absorption
and fluorescence emission properties. The excited-states characteristics are compared to analogous
systems containing an oligo(phenylene ethynylene) spacer. The zinc complexes of the new
molecules possess very polar excited states, which lead to strong solvatochromic shifts. Semi
empirical calculations were performed which confirmed the experimental data. The X-ray analysis
of a pyrene-containing p-conjugated bipyridine ligand is described.
metal ions. A series of analogous systems based instead on the
Introduction
oligo(phenylene ethynylene) (OPE) backbone in which the
subunits are connected through acetylenic linkages were in-
vestigated and also displayed remarkable tunable fluorescence
and luminescence emission properties.23–25
Molecular rods represent a fascinating class of molecules that
are the subject of a tremendous interest because of their
applications in many areas.1,2 Among this broad class of
systems, oligo(phenylene vinylene) (OPV)-based materials
have received considerable attention. They have been em-
ployed as active layer in organic light-emitting diodes,3 or-
ganic field effect transistors,4 and in solar cells,5,6 liquid
crystals,7 or as building blocks in dendrimers,8 functional
nanostructures9,10 and hybrid materials.11 OPVs have been
long known to have versatile light-emission properties both in
solution and the solid state.12 As such they represent attractive
scaffolds on which to base the design of molecular systems and
materials with stimuli-sensitive fluorescence emission charac-
teristics.13–17
In that connection, the incorporation of the 2,20-bipyridine
unit into conjugated backbones has led to highly fluorescent
chromophores with interesting metal ion sensory properties.18
For example, 2,20-bipyridine-containing poly(phenylene viny-
lene) were reported as luminescent materials exhibiting highly
ionochromic effects as a result of the interplay between
photophysical and complexation properties.19–21
In this paper, we present the synthesis and a photophysical
study of compound 1 and a series of related compounds.
Especially, information on the influence of conjugation en-
hancement and terminal aromatic group on the excited state
properties are provided by compounds 2 and 3, respectively.
The results outline the occurrence of a polar singlet excited
state in the free ligands and their corresponding zinc(II)
complexes, which gives rise to highly solvatochromic shifts
of the fluorescence emission maximum. The experimental
results are strongly supported by theoretical calculations and
are discussed in the light of the data obtained for related
OPE-based molecular rods.
Recently,22 we showed that a properly designed conjugated
molecular rod, 1, in which the pyrene chromophore and the
2,20-bipyridine ligand unit are connected via an OPV bridge
could give rise to spectacular changes in optical responses
depending on the nature of the solvent and the presence of
a
´
CIMMA UMR 6200 CNRS, Universite d’Angers, UFR Sciences, 2
Bd Lavoisier, 49045 Angers Cedex, France. E-mail:
stephanie.lhez@univ-angers.fr
b CPMOH UMR 5798 CNRS, Universite´ Bordeaux 1, 352 Cours de
´
la Liberation, 33405 Talence Cedex, France
c
´
´
´
´
GCOM2 UMR 6114 CNRS, Universite de la Mediterranee, Faculte
des Sciences de Luminy, Case 901, 13288 Marseille Cedex 9, France.
E-mail: fages@luminy.univ-mrs.fr
w CCDC reference numbers 641181. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b617497c
ꢀc
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The 1H NMR spectra of free ligands were recorded at room
temperature in CDCl3 or CD2Cl2 as solvent (ca. 15 mM) (Fig.
1). The assignment of the proton resonances was performed by
comparison of the spectra obtained for 1, 2, 3 and 4 and using
literature data.27,28 In the case of compound 1, a COSY
experiment (Fig. 2) allowed full confirmation of the assign-
ment. Typical coupling constant values for trans-ethylenic
configuration (ca. 16–17 Hz) were found.
Scheme 1 Synthesis of zinc(II) complex 1–ZnCl2.
Single orange crystals of compound 1 were obtained by slow
evaporation of a mixture of dichloromethane–methanol and
analyzed by X-ray diffraction (Fig. 3). The 2,20-bipyridine unit
is in trans conformation with a small torsion angle of 14.4(1)1
between the two pyridine rings. The dihedral angles between
the two planes formed by pyrene or bipyridine and the central
benzene ring are respectively 175.4(1)1 and 36.5(1)1.
Results and discussion
Synthesis
The syntheses of ligands 1–3 and reference compound 4 are
based upon classical Wittig-type olefin bond formation meth-
odology followed by iodine-catalysed isomerization. The all-
trans configuration was assessed by 1H NMR spectroscopy for
all compounds and confirmed by X-ray crystallography
analysis in the case of 1.
Photophysical properties of the free ligands
Electronic absorption. Electronic absorption spectra of com-
pounds 1–4 were recorded in toluene and THF at room
temperature. They are given in Fig. 4 and the photophysical
data are collected in Table 1. All compounds show a broad
and intense low-energy absorption band in the visible region
except 4 which absorbs in the near-UV. For all pyrene-
containing compounds, the spectra are characterized by a
single absorption band, whereas for 3 and 4 two bands are
clearly observed. In those compounds, the high-energy band is
likely to arise from the small axis-polarized transition. When
the oligomer length increases, such as in 1 and 2, the intensity
of this band is known to decrease and the lowest-energy
transition, which is polarized along the long axis, becomes
predominant.29 The molar absorption coefficient of the lowest-
energy transition band is found to be 45 600 Mꢁ1 cmꢁ1 for 1 in
THF. This value is much higher than for pyrene and its alkyl-
substituted derivatives that are known to possess a forbidden
1Lb state as lowest-energy excited state.30 In the case of 1, the
The synthesis of both pyrene-2,20-bipyridine compounds 1
and 3, containing one phenylene-vinylene unit as a spacer,
followed a one-pot procedure involving two Wittig reactions
between the bis-phosphonium salt derivative of 1,4-bis(bro-
momethyl)-2,5-bis(octyloxy)benzene, 7,19 and a mixture of
2,20-bipyridine-5-carbaldehyde26 and benzaldehyde or pyr-
ene-1-carbaldehyde. This statistical method afforded inevita-
bly a mixture of symmetrical and dissymmetrical oligomers,
but all of them could be easily separated by column chroma-
tography owing to their markedly different polarity. Reference
compound 4 was also obtained by reacting 7 and benzalde-
hyde in a 1 : 2 molar ratio. Oligomers 3 (11% yield) and 4
(55% yield) were prepared using dichloromethane as solvent
and EtOLi as a base, while 1 was obtained (20% yield) in THF
in the presence of NaH. The mononuclear zinc(II) complex of
1, 1–Zn was obtained by mixing the free ligand and ZnCl2 in
dichloromethane (Scheme 1).27 The purity and the 1 : 1 metal/
ligand stoichiometry were confirmed by elemental analysis.
In the case of longest oligomer 2, containing three pheny-
lene-vinylene units, a stepwise synthetic route was chosen
(Scheme 2). It is based on the use of the dialdehyde 6 which
was obtained according to published procedure10 from dialde-
hyde19 8 and the bis-phosphonium salt 7. Dialdehyde 6 was
subjected to a first Wittig reaction using the phosphonium
salts of 1-bromomethylpyrene to produce the pyrene-contain-
ing mono-aldehyde 5. The latter was then reacted with the
phosphonium salt of 5-bromomethyl-2,20-bipyridine under the
same Wittig conditions to afford the target oligomer 2. Sub-
sequent iodine-catalyzed isomerization afforded 2 in 25% yield
from 6 (Scheme 2). Owing to the presence of the alkyl chains
on the phenyl rings, all compounds are soluble in common
organic solvents.
1
long-axis polarized La transition is substantially stabilized at
1
the expense of the Lb one as a result of p-conjugation. This
effect is even more pronounced in the case of 2 which possesses
a molar absorption coefficient of about 111 500 Mꢁ1 cmꢁ1 for
the lowest-energy transition band in THF.
The bathochromic shift observed when going from 1 to 2 is
consistent with the increase of conjugation with oligomer
length.31–33 Also consistent with an increase of electronic
delocalization is the red-shifted absorption of the pyrene-
terminated ligand 1 relative to its benzene-containing reference
compound 4. These features are in keeping with those pre-
viously reported for OPE-bridged oligomers.24 The fact that
the vinylene compounds 1–4 absorb at lower energy (ca.
+15–30 nm) with respect to the ethynylene systems24 is
indicative of a somewhat better conjugation, consistent
with previous observations.34,35 Moreover, the electronic
Scheme 2 Synthesis of compound 2. Reagents and conditions: (a) EtOLi, CH2Cl2, rt, 81%; (b) EtOLi, CH2Cl2, rt, 52%; (c) EtOLi, CH2Cl2, rt then
I2, CH2Cl2, rt, 47%.
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Fig. 1 Aromatic part of the 1H NMR spectra of ligands 1 (250 MHz, CDCl3, 298 K) and 2 (200 MHz, CD2Cl2, 298 K) including the assignment of
the protons of the ethylenic bonds between pyrene and phenyl rings (K), between phenyl and bipyridine rings (’), and between phenyl rings (X).
absorption spectra of compounds 1–4 were found to be
independent of solvent polarity, which is indicative of the
absence of significant charge transfer effect in the ground state,
in agreement with previous observations on related pyrene-
containing ligands.24,30
All compounds are found to be strongly emissive (room
temperature fluorescence quantum yields are between 0.5 and
0.8) in the visible range. The progression in the fluorescence
maxima from 1 to 4 is found identical to that recorded for the
corresponding absorption spectra. It is also noteworthy that
fluorescence emission spectra are red-shifted relative to those
of OPE-bridged pyrene-bipyridine derivatives,24 which is con-
sistent with the electronic absorption results. The fact that the
fluorescence spectra of 1, 2 and 4 display a clear vibrational
structure is also in keeping with the behavior of the acetylenic
compounds. The vibronics spacings are found to be ca. 1250
cmꢁ1 which is in agreement with the stretching modes of
aromatic nucleus. In the case of ligand 3, the vibrational
structure was observed only in less polar solvents such as
n-hexane. The vibronic spacing is then found to be the same as
that of the other compounds. The Stokes’ shift values in both
toluene and THF are in the 2800–3100 cmꢁ1 range, except
Fluorescence emission. Corrected fluorescence emission
spectra were recorded in toluene and THF and were found
to be independent of the excitation wavelength (Fig. 5).
Fluorescence excitation spectra matched the absorption profile
over the entire wavelength range.
Fig. 2 Part of the 400 MHz COSY 1H/1H spectrum of compound 1
(CDCl3, rt) showing the correlations of ethylenic protons between
pyrene and phenyl rings (K), and between phenyl and bipyridine rings
(’).
Fig. 3 ORTEP view of crystallographic structure of 1 with 50%
probability ellipsoids.
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Fig. 4 Normalized electronic absorption spectra of compounds 1–4
in THF (25 1C, 1–5 ꢃ 10ꢁ6 M).
Fig. 5 Normalized, corrected fluorescence emission spectra of com-
pounds 1–4 in THF (conc. o3 ꢃ 10ꢁ6 M, 20 1C, lexc = 400 nm).
Photophysical properties of the zinc(II) complexes
again for 3 for which it depends on the solvent polarity and is
hence greater in THF (ca. 3500 cmꢁ1). Clearly, these values are
between those obtained for bipyridine-pyrene dyads connected
by a phenylene-ethynylene bridge24 and those connected by a
single C–C bond.30 Moreover, the shape and position of
fluorescence spectra were not affected by solvent polarity,
which points to the absence of any significant charge transfer
character of the excited state in the free ligands.24 Only in the
case of 3, a weak solvatochromic shift was observed.
Electronic absorption. Addition of an excess of ZnCl2 or
Zn(BF4)2 to solutions of ligands 1–3 induced a red-shift of the
lowest energy transition band in the UV-visible spectrum
(Dl ca. +3 to +28 nm), the value of the bathochromic shift
being independent of the solvent nature (Table 2). The largest
effect is recorded, as in the case of OPE-bridged ligands,24 for
the phenyl-terminated ligand 3. So, even if the terminal
bipyridine becomes more electron affinic upon zinc(II) com-
plexation, the ground state does not present a significant
charge transfer character. The modification of absorption
spectrum of 1 in THF upon gradual addition of Zn(BF4)2
was used to monitor the metal complexation equilibrium
which indicated the formation of a 1 : 1 (metal–ligand)
complex with a stability constant log K of 5.5 ꢂ 0.2. The same
value was found with the analogous OPE-bridged ligand22 and
is of the same order of magnitude as that found for a related
terpyridine complex.37 The 1 : 1 stoichiometry was also
confirmed by the synthesis and elementary analysis of the
mononuclear zinc complex of 1 (see Synthesis).
The fluorescence lifetimes in THF are monoexponential in
the nanosecond regime. Ligand 2 displays a particularly short
fluorescence lifetime (0.61 ns in THF) compared to the other
compounds 1, 3 and 4. The radiative rate constants, kf, and
their reduced values, kf/n3n3,30 were calculated in THF (Table
1). Both values are close to that found for OPE-bridged
compounds.24 All together, the photophysical data obtained
for 1 and 2 are consistent with the occurrence of a highly
allowed pp* singlet excited state in which the excited state of
the pyrene chromophore exhibits a strong 1La character. Such
stabilization of the long-axis polarized transition seems to be
more effective for vinylic spacers than for acetylenic ones.
Finally, it has to be mentioned that no phosphorescence
emission from the ligands could be detected in low tempera-
ture THF matrix (77 K) containing 10% of iodobutane. This is
not surprising according to the particularly high radiative
decay rate constants obtained for these compounds.24,36
Fluorescence emission. The most apparent ionochromic
effects of ligands 1–3 upon complexation of zinc(II) are their
instant colour changes from fluorescent blue (3), green (1) or
yellow (2) to light orange (3–ZnCl2), orange (1–ZnCl2) or red
(2–ZnCl2) in THF solution. Indeed,
a
considerable
Table 1 Photophysical properties of compounds 1–4
Absorption
Emission
Toluene
THF
Toluene
THF
l
max/nm
(emax/Mꢁ1cmꢁ1
l
max/nm
(emax/Mꢁ1cmꢁ1
lmax
nm
/
Dna/
kf/108 sꢁ1
)
)
lmax/nm
Dna/cmꢁ1
Ffb
cmꢁ1
Ffb
t/ns
(kf/n3n3/10ꢁ5 ꢁ1cm3)
s
1
2
3
4
a
430 (57 500)
463 (99 900)
404 (47 600)
388 (27 140)
b
431 (45 600)
463 (111 500)
404 (62 130)
388 (75 400)
493
534
462
441
2947
2889
3131
3123
0.81
0.70
0.64
0.79
493
534
472
440
2910
2872
3566
3046
0.82
0.51
0.64
0.66
1.69
0.610
1.61
4.9 (2.1)
8.4 (4.6)
4.0 (1.5)
4.1 (1.3)
B1.6
Stokes’ shift. Fluorescence quantum yield.
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Table 2 Spectroscopic data for the zinc(II) complexes of compounds 1–3 in THF and in solvents of different polarity at room temperaturea,b
Absorptionb
Emission
lmax/nm
TOL
EE
AE
THF
DCE
BN
l
max/nm
e
max/Mꢁ1cmꢁ1
43 600
Dnc/cmꢁ1
rd/A
mexe/D
1
2
3
a
455 (+25)
466 (+3)
432 (+28)
558
590
544
591
ns
570
605
643
582
611 (+118)
655 (+121)
590 (+118)
628
670
613
c
638
q
633
5611
6192
6199
6.65
8.33
6.25
16
23
15
101 500
55 200
b
q: quenched emission; ns: non-soluble. For solvent abbreviations see experimental section. Values in THF; in parentheses, bathochromic
d
shifts relative to the free ligands. Onsager cavity radius, approximated according to the solute density method, an average value of 1 g cmꢁ1 being
e
taken for the density. Excited state dipole moment.
bathochromic shift of the fluorescence maximum (Dl ca.
+120 nm) was obtained upon zinc(II) complexation (Table
2). The fluorescence emission spectra are not dependent on the
excitation wavelength and the fluorescence excitation spectra
were observed to match the absorption profiles. Furthermore,
metal ion complexation is also accompanied by a broadening
of the fluorescence emission bands along with a loss of
vibronic structure. The half-height widths are observed to be
larger than those of the free ligands (Fig. 6). These results, as
well as the large Stokes’ shifts, are consistent with fluorescence
emission arising from a highly polar, charge transfer state.
Moreover, this change in the nature of the excited state of the
conjugated ligand upon zinc(II) chelation is intramolecular as
the fluorescence emission spectrum of 4 is not affected upon
addition of ZnCl2 under the same conditions. Consistently, a
strong solvatochromic effect is observed (Fig. 7 and Table 2),
which is the result of the bipyridine ligand moiety becoming a
strong withdrawing group upon zinc binding.24 From the
dependence of the emission maximum on the solvent polarity
factor, an estimated value of the dipole moment of the excited
state could be obtained for each ligand using the Lippert–
Mataga equation (Table 2), assuming the ground state as
weakly polar (mgsE 0).38 Indeed absorption maximum was
found independent on solvent polarity. The high values ob-
tained (mex ca. +15 to +23 D) are in the range of those
calculated for OPE-bridged ligands and thus with the forma-
tion of a giant dipole along the molecular axis.39 As in the case
of the OPE-bridged ligands,24 an increase of the dipole mo-
ment is observed with the increase in oligomer length. How-
ever, the dipole moment values calculated for the OPV ligands
are found to be 20% lower than those calculated for the OPE
corresponding ligands. This could be explained by a better
efficiency in charge transfer of triple bonds compared to
double bond, which is consistent with the results obtained
by Ziessel and Harriman on acetylenic bridges.35
Another effect of zinc(II) complexation is the decrease of
fluorescence quantum yield as well as the increase fluorescence
lifetimes, these observations being the most important in the
case of the longest oligomer 2 (Table 3). All fluorescence
decays recorded are still monoexponential in solvents such
as toluene and THF. Given the very short temporal resolution
(ca. 50 ps) of the time-resolved laser spectroscopy setup used in
this study, this confirmed the occurrence of a single fluorescent
complex species in solution in agreement with UV-visible
titration results. The consequence of these results are a de-
crease of the radiative constants in the zinc(II) complex and a
concomitant increase of the nonradiative rate, knr, in particu-
lar in the case of ligand 2, as expected in the case of
fluorescence emission from a low-lying charge transfer state
(Table 3).
Fig. 7 Corrected fluorescence emission spectra of zinc(II) complexes
of ligand 1 in solvents of increasing polarity (toluene, EE, THF, DCE).
Fig. 6 Normalized, corrected fluorescence emission spectra of com-
pounds 1–3 in the presence of an excess of ZnCl2 in THF (ligand con.
o3 ꢃ 10ꢁ6 M, metal conc. ca. 10ꢁ4 M, 20 1C, lexc = 400 nm).
(ligand conc. o3 ꢃ 10ꢁ6 M, metal conc. ca. 10ꢁ4 M, 20 1C, lexc
=
400 nm).
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Table 3 Photophysical properties of the zinc(II) complexes of com-
pounds 1–3 in THF at room temperature
Ff
t/ns
kf/108 sꢁ1 kf nꢁ3nꢁ3/10ꢁ5
s
ꢁ1cm3
knr/108 sꢁ1
1
2
3
0.28 2.05
0.04 0.317
0.43 2.80
1.4
1.3
1.5
1.1
1.3
1.1
3.51
30.3
2.04
Quantum chemical calculations
Quantum chemical calculations were performed in order to
validate the spectroscopic results obtained in absence and
presence of zinc(II) using the methylated model structures of
1–Me and 3–Me (Scheme 3), respectively.
Fig. 8 AM1 geometry optimisation of model ligand 1–Me and its
zinc(II) complex 1–Me–ZnCl2. The values of the dihedral angles Fp
(between pyrene and central benzene rings), b (between the central
aromatic ring and the first pyridine nucleus of the bipyridine unit) and
a (between the two pyridine rings in the 2,20-bipyridine) are given
(see text).
The geometry of both models have been optimized at the
Hartree–Fock semiempirical Austin model 1 (AM1) level. In
the energetically most favourable ground-state structure found
for free ligands, the non-substituted pyridine ring is twisted
with respect to the other one by an angle a of 401, consistent
with literature data.24,31 In the case of ligand 1–Me, the other
important dihedral angle values are reported in Fig. 8A. The
angle between pyrene and central benzene rings (Fp) is due to
the steric interaction with the peri hydrogen and is consistent
with the X-ray data (vide supra). Similarly, the value of the
dihedral angle between the central aromatic ring and the first
pyridine nucleus of the bipyridine unit (b) is found very close
to that observed in the crystal. In the case of the corresponding
zinc(II) complexes, the angle between the two pyridine rings
was kept to zero and the results show no significant conforma-
tional reorganization of the conjugated backbone as compared
to free ligand either for 1–Me or 3–Me (Fig. 8B for 1–Me).
As was previously found for analogous acetylenic com-
pounds,24 the calculation show that the S0–S1 transition is
always the most important one both for free ligand and zinc(II)
complexes, and involves molecular orbitals (MOs) that are of
p character. However, in the case of free ligands, only one
electronic transition (HOMO to LUMO) is necessary to
describe the transition from the ground to the first excited
state whereas for zinc(II) complexes, two electronic transitions
(HOMO to LUMO and HOMO to LUMO + 1) are needed.
Another interesting feature outlined by calculations, is the
dependence of localization of the MOs on both the nature of
the terminal aromatic nucleus and the presence of chelated
zinc (II). Indeed, on one hand, in the case of free ligand 1–Me
both HOMO and LUMO are localized only on the pyrene and
phenylene-vinylene bridge whereas in the case of 3–Me, the
HOMO orbital is localized from phenyl ring to the first
pyridine ring of the bipyridine unit and the LUMO extends
all over the molecular skeleton. This result confirmed the
occurrence of a more polar excited state in the case of 3 than
in the case of 1, as outlined by the solvatochromic effect
observed in the fluorescence study of 3. On the other hand,
upon chelation of zinc(II), an important effect on the localiza-
tion of MOs was noticed. Indeed, the electronic density upon
light excitation was shifted from the terminal aromatic nucleus
and the phenylene-vinylene bridge for both 1–Me and 3–Me to
the entire molecule in the case of 1–Me, and to the phenylene-
vinylene bridge and the bipyridine in the case of 3–Me. The
calculations thus confirm the increase of charge transfer
character of the first excited state upon metal chelation.
Conclusion
As observed in the case of OPE-based ligands, the OPV
counterparts described in this study are shown to exhibit
strong emission colour modulation effect when they are ex-
posed to zinc(II) ion. The emission colours were found to
depend on the number of phenylene vinylene units in the
conjugated backbone. As such they behave as an efficient class
of Zn2+-fluorogenic chemosensors or as tunable fluorophores
for optoelectronic applications.
Experimental
General
Anhydrous zinc(II) chloride (Aldrich Chemical Co., 99.999%)
was handled under an inert atmosphere using a glove box. All
solvents used for spectroscopic measurements were of spectro-
scopic grade and were used as commercially available. The
following are the abbreviations used for the solvents: toluene
(TOL), diethyl ether (EE), ethyl acetate (AE), tetrahydrofuran
(THF), dichloroethane (DCE), butyronitrile (BN), acetonitrile
(AN). Electronic absorption spectra were recorded with a
Hitachi U3300 spectrophotometer. Fluorescence spectra were
recorded in non deoxygenated solvents at 20 1C with either a
Hitachi F4500 or a Spex Fluorolog spectrofluorimeter. We did
Scheme 3 Structures of model ligands 1–Me and 3–Me as used in the
semi empirical calculations.
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not find any difference in emission properties with and without
degassing the solutions. Fluorescence quantum yields were
determined using quinine sulfate as a standard (Ff = 0.55 at
25 1C in 1 N H2SO4). The relative error in the quantum yields
is ꢂ5%. Low-temperature experiments were carried out using
a variable-temperature liquid nitrogen Optistat cryostat from
Oxford instruments. The laser setup used in these experiments
was as described elsewhere.24
orange solid. 1H NMR (250 MHz, CDCl3), d = 0.81–0.91 (m,
6H, CH3), 1.20–1.66 (m, 20H, CH2), 1.87 (m, 4H, OCH2CH2),
4.12–4.19 (m, 4H, OCH2), 7.21 (s, 1H, Hphenyl), 7.22 (d, 1H, J
= 16.5 Hz, CHQCH), 7.31 (s, 1H, Hphenyl), 7.31–7.38 (m, 1H,
H5’), 7.66 (d, 1H, J = 16.5 Hz, CHQCH), 7.69 (d, 1H, J =
16.2 Hz, CHQCH), 7.87 (td, 1H, J1 = 9.85 Hz, J2 = 2.15 Hz,
H4’), 8.00–8.38 (m, 10H, CHQCH, H4 and Hpyr), 8.45–8.49
(m, 2H, H3 and H3’), 8.53 (d, 1H, J = 9.5 Hz, H2pyr), 8.71–8.73
(m, 1H, H6’), 8.83 (s, 1H, H6). MS (EI), m/z: 740 (M+).
Analysis calc. (found) for C52H56N2O2: C, 84.28(83.88); H,
7.62(7.23); N, 3.78(3.93)%.
Syntheses
All chemicals were purchased from Aldrich Chemical Co. and
1
were used as received. Solvents were distilled prior to use. H
and 13C NMR spectra were recorded with either a Brucker
AC200 or a Brucker AC250 spectrometer. Mass spectra were
obtained using a VG Autospec-Q Micromass spectrometer
either in the EI or LSIM+ (NBA matrix) mode.
Ligand 2. To a solution of 5 (0.31 g, 0.22 mmol) and the
phosphonium salt of 5-bromomethyl-2,20-bipyridine (0.169 g,
0.33 mmol) in freshly distilled dichloromethane (75 mL) was
added dropwise a solution of lithium ethanolate (0.37 mL, 1 M
in ethanol) under argon atmosphere at room temperature. The
reaction mixture was stirred at room temperature for 4 h. The
organic layer was then washed with water and dried over
Na2SO4. The solvent was removed under vacuum and the
crude product was subjected to column chromatography
(silica gel) eluting with dichloromethane–methanol (99 : 1
v/v). The mixture of cis and trans isomers thus obtained was
stirred with iodine (0.5 g, 1.96 mmol) in dichloromethane
(60 mL) for 24 h at room temperature. The iodine was then
quenched with a 1 M aqueous solution of Na2S2O3. The
organic layer was washed with brine, dried over Na2SO4 and
concentrated under vacuum. The crude product was subjected
twice to column chromatography (silica gel) eluting with
dichloromethane and then crystallized from a mixture of
dichloromethane–methanol. Compound 2 was obtained as
the trans isomer (0.16 g, 50% yield) as an orange solid. 1H
NMR (200 MHz, CD2Cl2), d = 0.84–0.93 (m, 18H, CH3),
1.30–1.62 (m, 60H, CH2), 1.84–1.99 (m, 12H, OCH2CH2),
4.00–4.19 (m, 12H, OCH2), 7.21–7.35 (m, 8H, CHQCH and
Pyrene-containing mono-aldehyde 5. To a solution of dialde-
hyde 6 (0.6 g, 0.54 mmol) and the pyrene-containing phos-
phonium salt (0.308 g, 0.54 mmol) in freshly distilled
dichloromethane (100 mL) was added dropwise a solution of
sodium ethanolate (0.6 mL, 1 M in ethanol) under an argon
atmosphere at room temperature. The reaction mixture was
stirred at room temperature for 4 h. The organic layer was
then washed with water and dried over Na2SO4. The solvent
was removed under vacuum and the crude product was
subjected to column chromatography (silica gel) eluting with
petroleum ether–dichloromethane (50 : 50 v/v). Compound 5,
an orange solid, was obtained as a mixture of cis and trans
isomers which was used without further purification (0.36 g,
1
52% yield). H NMR (200 MHz, CDCl3), d = 0.80–0.91 (m,
18H, CH3), 1.20–1.60 (m, 60H, CH2), 1.80–2.00 (m, 12H,
OCH2CH2), 3.98–4.12 (m, 12H, OCH2), 7.00–8.36 (m, 15H,
Haryl and ethylene hydrogens), 10.43 (s, 0.55H, CHO), 10.45
(s, 0.45H, CHO).
H
phenyl), 7.24 (d, 1H, J = 17.2 Hz, CHQCH), 7.31 (m, 1H,
Ligand 1. To a suspension of 7 (4.2 g, 4 mmol) in freshly
distilled THF (60 mL), 95% NaH (0.65 g, 25.7 mmol) was
added portionwise at room temperature. After dropwise addi-
tion of a solution of pyrene-1-carbaldehyde (0.92 g, 4 mmol) in
THF (10 mL), the reaction mixture was heated at 30 1C and
then a solution of 2,20-bipyridine-5-carbaldehyde (0.8 g, 4.35
mmol) in THF (10 mL) was added dropwise. The reaction
mixture was heated at 30 1C for 40 h, cooled down to room
temperature and concentrated under vacuum. The crude
product obtained was dissolved in dichloromethane. The
organic layer was washed with water and dried over Na2SO4.
The solvent was removed under vacuum and the crude pro-
duct was subjected to column chromatography (silica gel)
eluting with dichloromethane. The mixture of cis and trans
isomers thus obtained was stirred with iodine (1.045 g, 4.11
mmol) in dichloromethane (125 mL) overnight at room tem-
perature. The iodine was then quenched with a 1 M aqueous
solution of Na2S2O3. The organic layer was washed with brine,
dried over Na2SO4 and concentrated under vacuum. The
crude product was subjected to column chromatography
(silica gel) eluting with dichloromethane and then crystallized
from a mixture of dichloromethane–methanol. Compound 1
was obtained as the trans isomer (0.592 g, 20% yield) as an
H5’), 7.55 (s, 1H, Hphenyl), 7.58 (s, 1H, Hphenyl), 7.66 (d, 1H, J
= 17.2 Hz, CHQCH), 7.77 (d, 1H, J = 16.5 Hz, CHQCH),
7.83 (td, 1H, J = 7.6 Hz, H4’), 8.37 (d, 1H, J = 16.5 Hz,
CHQCH), 7.98–8.46 (m, 11H, H3, H3’, H4 and Hpyr), 8.53 (d,
1H, J = 9.4 Hz, H2pyr), 8.65–8.68 (m, 1H, H6’), 8.79 (s, 1H,
H6). MS (LSIMS+), m/z: 1458 (M + 1). Analysis calc. (found)
for
C100H132N2O6: C, 82.41(81.34); H, 9.06(9.30); N,
1.92(1.98)%.
Ligand 3. To a solution of 7 (3.69 g, 3.5 mmol), 2,20-
bipyridine-5-carbaldehyde (0.65 g, 3.5 mmol) and benzalde-
hyde (0.36 mL, 3.54 mmol) in freshly distilled dichloro-
methane (150 mL) was added dropwise a solution of lithium
ethanolate (9.1 mL, 1 M in ethanol) under argon atmosphere
at room temperature. The reaction mixture was stirred at
room temperature for 0.5 h and then poured onto a diluted
aqueous chlorhydric acid solution. The organic layer was then
washed with water and dried over Na2SO4. The solvent was
removed under vacuum. The mixture of cis and trans isomers
thus obtained was stirred with iodine (1.7 g, 6.7 mmol) in
dichloromethane (150 mL) overnight at room temperature.
The iodine was then quenched with a 1 M aqueous solution of
Na2S2O3. The organic layer was washed with water, dried over
ꢀc
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New J. Chem., 2007, 31, 1013–1021 | 1019

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Na2SO4 and concentrated under vacuum. The crude product
was subjected twice to column chromatography: first on
deactivated alumina with petroleum ether–dichloromethane
(5 to 100%) mixture as eluant and second on silica gel with
dichloromethane–methanol (1 to 4%) mixture as eluant.
Compound 3 was obtained as the trans isomer (0.238 g,
twist angles between two adjacent rings was systematically
investigated. To be sure that the optimized geometry corre-
sponds to the lowest energy, vibrational frequencies were also
determined.41 Excited state energies were obtained for each
optimized ground-state geometry by single configuration in-
teraction method involving excitations from 10 occupied to 10
unoccupied molecular orbitals. As the number of possible
excitations was huge, a truncation of the configuration inter-
action active space was made, as the lowest-energy excited
states are of main interest. Care was taken to ensure that the
energies and oscillator strengths of the transitions of interest
were not significantly dependent on the number of configura-
tions used in the calculation.
1
11% yield) as an orange solid. H NMR (250 MHz, CDCl3),
d = 0.83–0.92 (m, 6H, CH3), 1.20–1.60 (m, 20H, CH2), 1.88
(m, 4H, OCH2CH2), 4.07 (t, 2H, J = 6.4 Hz, OCH2), 4.08 (t,
2H, J = 6.4 Hz, OCH2), 7.16 (d, 1H, J = 16.5 Hz, CHQCH),
0
7.14–7.56 (m, 9H, CHQCH, H5 , and Hphenyl), 7.43 (d, 1H, J
= 16.5 Hz, CHQCH), 7.61 (d, 1H, J = 16.5 Hz, CHQCH),
0
7.83 (td, 1H, J1 = 7.6 Hz, J2 = 1.8 Hz, H4 ), 7.99 (dd, 1H, J1
0
= 8.2 Hz, J2 = 2.1 Hz, H4), 8.38–8.42 (m, 2H, H3 and H3 ),
X-ray crystallography
0
8.69 (d, 1H, J = 4.6 Hz, H6 ), 8.79 (d, 1H, J = 2.1 Hz, H6).
MS (EI), m/z: 616 (M+). Analysis calc. (found) for
Data were collected at 293 K on a STOE-IPDS diffractometer
equipped with a graphite monochromator utilizing Mo Ka
radiation (l = 0.71073 A). The structure was solved and
refined on F2 by full matrix least-squares techniques using
SHELX-97 package. All non-H atoms were refined anisotro-
pically and the H atoms were included in the calculation
without refinement. Absorption was corrected by gaussian
technique.
C42H52N2O2: C, 81.78(81.68); H, 8.50(8.69); N, 4.54(4.46).
1,4-Bis(octyloxy)-2,5-distyrylbenzene 4. To a solution of 7
(1.85 g, 1.77 mmol), and benzaldehyde (0.36 mL, 3.54 mmol)
in freshly distilled dichloromethane (75 mL) was added drop-
wise a solution of lithium ethanolate (5 mL, 1 M in ethanol)
under argon atmosphere at room temperature. The reaction
mixture was stirred at room temperature overnight and then
poured onto a diluted aqueous chlorhydric acid solution. The
organic layer was then washed with water and dried over
Na2SO4. The solvent was removed under vacuum. The mix-
ture of cis and trans isomers thus obtained was stirred with
iodine (0.9 g, 3.54 mmol) in dichloromethane (80 mL) for 24 h
at room temperature. The iodine was then quenched with a
1 M aqueous solution of Na2S2O3. The organic layer was
washed with water, dried over Na2SO4 and concentrated
under vacuum. The crude product was subjected twice to
column chromatography: first on silica gel with petroleum
ether–dichloromethane (75 : 25 v/v) mixture as eluant and
second on deactivated alumina with petroleum ether–dichlor-
omethane (0 to 25%) mixture as eluant. Compound 4 was
obtained as the trans isomer (0.52 g, 55% yield) as a yellow
Crystal dataw for 1: C52H56N2O2, M = 740.99, triclinic, space
ꢀ
group P1, a = 9.0140(8) A, b = 14.873(2) A, c = 16.749(2) A,
a = 77.96(1)1, b = 83.05(1)1, g = 77.57(1)1, V = 2137.8(4) A3,
Z = 2, rcalc = 1.151 g cmꢁ3, m(Mo Ka) = 0.069 mmꢁ1, F(000)
= 796, ymin = 2.071, ymax = 25.981, 20 865 reflections collected,
7724 unique (Rint = 0.0545), restraints/parameters = 0/507,
R1 = 0.0471 and wR2 = 0.1123 using 3089 reflections with
I 4 2s(I), R1 = 0.1248 and wR2 = 0.1413 using all data,
GOF = 0.794, ꢁ0.135 o Dr o 0.339 e Aꢁ3
.
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