Propylamino-connected fluorescent terpyridine dimer and trimer: Syntheses, photophysical properties and formation of duplex-type complexes with Cd(ii)

Article abstract of DOI:10.1039/c2ob26609a

By using propylamine as a connector, a dimer (L2) and trimer (L3) of 2,2′:6′,2′′-terpyridine (tpy) are synthesized. They showed the lowest energy π-π* absorption at 354 nm and blue fluorescence at 420 nm with a moderate quantum yield in solution. Spectral titrations, electrospray ionization mass spectrometry (ESI-MS), ¹H-¹H rotating-frame nuclear Overhauser effect correlation spectroscopy (ROESY), and structure simulation (MOPAC/PM5) indicated that both L2 and L3 form duplex-type multi-component complexes with Cd(ii), [Cd ₂(L2)₂]⁴⁺ and [Cd₃(L3) ₂]⁶⁺, respectively. While the luminescence of these complexes was quite weak in solution (quantum yield Φ < 0.005), the complexes showed enhanced emission in the solid state with remarkable increase of the quantum yield (Φ = 0.13). This journal is

Full text of DOI:10.1039/c2ob26609a

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PAPER
Propylamino-connected fluorescent terpyridine dimer and trimer: syntheses,
photophysical properties and formation of duplex-type complexes with
Cd(II)†
Yuka Kamoya, Keisuke Kojima, Gentaro Tanaka, Ryo Tanaka, Toshiki Mutai and Koji Araki*
Received 19th June 2012, Accepted 14th September 2012
DOI: 10.1039/c2ob26609a
By using propylamine as a connector, a dimer (L2) and trimer (L3) of 2,2′:6′,2′′-terpyridine (tpy) are
synthesized. They showed the lowest energy π–π* absorption at 354 nm and blue fluorescence at 420 nm
with a moderate quantum yield in solution. Spectral titrations, electrospray ionization mass spectrometry
(ESI-MS), 1H–1H rotating-frame nuclear Overhauser effect correlation spectroscopy (ROESY), and
structure simulation (MOPAC/PM5) indicated that both L2 and L3 form duplex-type multi-component
complexes with Cd(II), [Cd2(L2)2]4+ and [Cd3(L3)2]6+, respectively. While the luminescence of these
complexes was quite weak in solution (quantum yield Φ < 0.005), the complexes showed enhanced
emission in the solid state with remarkable increase of the quantum yield (Φ = 0.13).
for functionalization and oligomerization of tpy units. Oligopyri-
Introduction
dine and its analogous compounds are being actively studied,10
2,2′:6′,2′′-Terpyridine (tpy) has high chemical and thermal
stability, and the suitable arrangement of its three ring nitrogens
contributes to its excellent chelating ability.1 A wide variety of
transition metal complexes with tpy ligands have been syn-
thesized and their unique photophysical properties, such as
excited-state energy and/or electron transfer and long-lived lumi-
nescence, have been studied extensively.2 As ligand-centred
excited states often play critical roles in the unique photophysical
properties of the tpy complexes, an absence of energy dissipation
pathways from the excited state within the ligand is desirable.
However, tpy and its derivatives are generally non-fluorescent,
owing to their nonradiative relaxation process; relatively few tpy
derivatives exhibit efficient fluorescence.3 Among them, 5,5′′-di-
(4-methoxy-2,6-dimethylphenyl-4′-phenyl)-tpy,4 4′-phenyl-tpy,5,6
6-amino-tpy7 and several other tpy derivatives have been
reported to show efficient fluorescence,3 and the luminescent
properties of their complexes have also been studied.8
and formation of metal complexes in helical or double helical
structures has been reported.11 Therefore, use of the amino-con-
nector provides a convenient method for preparation of oligo-tpy
derivatives. However, the properties of the amino-connected
oligo-tpys and the structures of their metal complexes have not
yet been clarified. In metallo-supramolecular chemistry, well-
defined supramolecular architectures composed of organic
ligands and metal cations are one of the recent topical subjects.12
Tpy has three nitrogen atoms suitably arranged to coordinate to
the metal cation in a convergent mode. Since adjacent tpy units
connected with the 6-amino group cannot coordinate to the same
metal centre, the metal complex of oligo-tpy should be multi-
component. Therefore, as a first step, it is essential to clarify the
structure and complexability of the amino-connected tpy dimer
and trimer as model compounds of oligo-tpys.
Here, we report the synthesis and photophysical properties of
the amino-connected tpy dimer (L2) and trimer (L3), their com-
plexability with Cd(II), and the structures of the complexes.
These polydentate ligands showed high complexability for
Cd(II), and formation of multi-component complexes with
duplex-type structures was indicated. Furthermore, the com-
plexes showed unique emission enhancement in the solid state.
We previously reported9 that the fluorescent properties of
6-amino tpy can be finely tuned by the introduction of alkyl or
phenyl groups into the amino nitrogen, without impairing the
highly fluorescent nature of 6-amino tpy. This confirms that the
amino-N-substitution does not cause rapid nonradiative energy
dissipation of the amino-tpy-centred excited state. As amino-N-
substitution with a variety of functional groups can be readily
achieved, the 6-amino group serves as a convenient connector
Results
Synthesis of amino-connected tpy dimer and trimer
Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba,
Meguro-ku, Tokyo 153-8505, Japan. E-mail: araki@iis.u-tokyo.ac.jp
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2ob26609a
To increase the solubility of tpy derivatives, we introduced a
4-tert-butylphenyl group at the 4′ position of tpy (BPtpy), and
used the resultant compound as a building block for the
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Scheme 1 Synthesis of L1, L2, and L3.
tpy-dimer and trimer. Syntheses of the monomer L1, dimer L2
and trimer L3 are illustrated in Scheme 1. The starting com-
(shoulder peak), respectively, presumably due to the π–π*
transition of the amino-tpy unit.10 In the case of L2 and L3,
a small additional peak at 313 nm was also observed.
pounds,
bromo-4′-(4-(tert-butyl)phenyl)-2,2′:6′,2′′-terpyridine
(BPtpy-1) and 6,6′′-dibromo-4′-(4-(tert-butyl)phenyl)-2,2′:6′,2′′-
terpyridine (BPtpy-2), were prepared by a similar procedure to
that reported previously.5 Subsequent Pd-catalyzed amination of
BPtpy-1 (Buchwald–Hartwig reaction)13 with dipropylamine in
the presence of 1,1′-bis(diphenylphosphino)ferrocene (dppf)
yielded monomeric L1. A similar reaction of BPtpy-1 with pro-
pylamine proceeded to yield dimeric L2 as a white microcrystal.
Use of a bulky ligand, ( )-2,2′-bis(diphenylphosphino)-1,1′-
binaphthyl (BINAP), instead of dppf in the reaction of BPtpy-1
with propylamine gave 6-propylamio-substituted BPtpy (BPtpy-3).
The subsequent Pd-catalyzed coupling of BPtpy-2 with BPtpy-3
in the presence of dppf yielded L3 (Scheme 1). The amino-
connected tpy dimer and trimer were soluble in benzene,
dichloromethane, acetone and acetonitrile.
Compounds L1, L2 and L3 exhibited blue fluorescence at
around 420–431 nm (λex = 285 nm) with moderate quantum
yields (Φ). The excitation spectra did not differ greatly from
their absorption spectra, confirming that N-substitution does not
impair the emissive properties of the 6-amino-tpy unit. The
emission spectra of L1, L2 and L3 are also shown in Fig. 1.
Compound L1 showed slightly red-shifted absorption and fluor-
escence bands when compared with L2, L3, and parent 6-amino-
tpy (λabs = 319 nm and λem = 386 nm in dichloromethane9).
These differences were ascribed to the high electron-donating
ability of the dipropylamino group in L1 compared to that of the
tpy- and propyl-substituted amino group, which presumably
decreases the HOMO–LUMO gap by increasing the HOMO
level of tpy.9 In acetonitrile, absorption and fluorescence spectra
were practically the same as those in dichloromethane, though
lower in fluorescence quantum yields (Table 1).
Photophysical properties of L1–L3
Complexation of L1, L2 and L3 with Cd2+ in acetonitrile
The electronic spectra of these compounds in dichloromethane
are shown in Fig. 1, and their photophysical properties are sum-
marized in Table 1. The lowest energy absorption bands of L1,
L2 and L3 appeared at 366, 354 (shoulder peak), and 354 nm
Addition of divalent cations, Fe2+, Cu2+, and Cd2+, to a L2
(2.0 × 10−5 mol dm−3) – acetonitrile solution caused a red shift
of the π–π* absorption band of the amino-tpy moiety (Fig. S1†),
indicating the formation of the complex. However, the spectral
changes were not monotonic and required detailed analysis.
Therefore, we selected Cd2+, which is suitable for studying struc-
tures of the complexes by a variety of methods, and conducted
the detailed spectroscopic analysis.
As the first step to study the complexability of the tpy dimer
and trimer, we performed spectral titration of L1, L2 and L3
(2.0 × 10−5 mol dm−3) by addition of Cd(ClO4)2 in acetonitrile
at 20 °C. By increasing the [Cd2+]/[L1] molar ratio to 0.5, a
spectral change of L1 with clear isosbestic points was induced,
but no further spectral change was observed in response to the
increase in the [Cd2+]/[L1] molar ratio (Fig. 2a). These results
clearly indicated one-step formation of the Cd-L1 complex with
[Cd2+]/[L1] = 0.5. Fluorescence spectral titration (Fig. 3a) by
excitation at 291.5 nm, which is one of the isosbestic points in
the absorption spectral titration (Fig. 2a), also revealed a spectral
change up to the [Cd2+]/[L1] molar ratio of 0.5. The blue
fluorescence of L1 based on the amino-tpy decreased
Fig. 1 Absorption and fluorescence spectra of L1 (dotted line),
L2 (solid line), and L3 (broken line) in dichloromethane (2.00 ×
10−5 mol dm−3).
8896 | Org. Biomol. Chem., 2012, 10, 8895–8902
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Table 1 Photophysical properties of the tpy derivatives L1–L3
Dichloromethane
Acetonitrile
Solid state
Compound
λabs a/nm (ε/104)
λem/nm (Φ)
λabs a/nm (ε/104)
λem/nm (Φ)
λabs a/nm
λem/nm (Φ)
L1
L2
L3
258 (3.71), 366 (0.57)
261 (6.19), 313 (3.03), 354s
262 (8.62), 314 (4.38), 354s
431 (0.18)
420 (0.19)
420 (0.16)
277 (3.14), 359 (0.61)
275 (6.54), 312s, 354s
262 (8.82), 313s, 356s
441 (0.07)
425 (0.11)
426 (0.09)
280, 364
284, 355s
278, 361s
425 (0.30)
410 (0.11)
420 (<0.01)
a s denotes a shoulder peak.
Fig. 2 Absorption spectral changes of (a) L1 ([Cd2+]/[L1] = 0–1.0
equiv.), (b) L2 ([Cd2+]/[L2] = 0–2.0 equiv.), and (c) L3 ([Cd2+]/[L3] =
0–2.0 equiv.) by addition of Cd(ClO4)2 in acetonitrile. Insets show plots
of the absorbance at (a) 310 nm, (b) 246 and 375 nm, and (c) 308.5 and
367.5 nm as a function of [Cd2+]/[L] molar ratio.
Fig. 3 Fluorescence spectral changes of (a) L1 ([Cd2+]/[L1] = 0–1.0
equiv.), (b) L2 ([Cd2+]/[L2] = 0–2.0 equiv.), and (c) L3 ([Cd2+]/[L3] =
0–2.0 equiv.) by addition of Cd(ClO4)2 in acetonitrile. Insets show emis-
sion intensities at (a) 443 nm, (b) 500 nm, and (c) 428 nm as a function
of [Cd2+]/[L] molar ratio.
considerably in response to the addition of Cd2+ and red-shifted
to a greenish yellow colour with low efficiency (Φ < 0.005). To
confirm the composition of the Cd-L1 complex, ESI-MS
measurement of an acetonitrile solution containing [Cd2+]/[L1]
= 0.5 was performed, and a set of isotope peaks corresponding
to [Cd(L1)2](ClO4)+ (m/z = 1141.0) was observed. Therefore,
the Cd-L1 complex was confirmed to be [Cd(L1)2](ClO4)2.
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In the case of L2, increasing the [Cd2+]/[L2] molar ratio to
0.5 induced an absorption spectral change similar to that of L1
with a clear set of isosbestic points. However, increasing the
molar ratio from 0.5 to 1.0 caused a further spectral change with
different isosbestic points. No further spectral change was
observed when the molar ratio was increased beyond 1.0. The
plots of absorbance at 375 and 246 nm, which are the isosbestic
points of the first and second spectral changes, respectively, are
shown in Fig. 2b as an inset. It is apparent that the spectral
change occurred in two steps, from 0 to 0.5 and from 0.5 to 1.0,
indicating the step-wise formation of Cd-L2 complexes having
the [Cd2+]/[L2] molar ratios of 0.5 and 1.0. The ESI-MS spec-
trum of an acetonitrile solution containing equimolar amounts of
Cd(ClO4)2 and L2 showed a peak (m/z = 2095.0) corresponding
KM2
ꢁꢁꢁ*
L
2
2þ
4þ
½Cdð L2Þ2ꢂ þ Cd2þ )ꢁꢁꢁ ½Cd2ð L2Þ2ꢂ
ð3Þ
KML
2
2þ
L3:Cd2þ þ 2ꢀL3 )ꢁꢁꢁ ½CdðL3Þ2ꢂ
ð4Þ
ð5Þ
ð6Þ
ꢁꢁꢁ*
KM2
L
2
2þ
4þ
6þ
½Cdð L3Þ2ꢂ þ Cd2þ )ꢁꢁꢁ ½Cd2ð L3Þ2ꢂ
ꢁꢁꢁ*
KM3
L
2
4þ
½Cd2ðL3Þ2ꢂ þ Cd2þ )ꢁꢁꢁ ½Cd3ðL3Þ2ꢂ
ꢁꢁꢁ*
Association constants in acetonitrile
Association constants (KM L ) of L1 and L2 with Cd2+ were
determined from the absorption spectral titration curves in aceto-
nitrile at 20 °C by a non-linear curve fitting method.15 As the
complex formation of L1 was almost quantitative, the third-order
+
to [Cd2(L2)2](ClO4)3 (Fig. S2†). Therefore, composition of the
i j
complex formed at the [Cd2+]/[L2] = 1.0 was not 1 : 1 but 2 : 2.
Thus, L2 underwent step-wise formation of [Cd(L2)2](ClO4)2
and [Cd2(L2)2](ClO4)4 as the [Cd2+]/[L2] molar ratio increased.
Fluorescence spectral titration of L2 with Cd2+ (λex = 291.5 nm)
revealed only the first step of the complexation. Increasing the
Cd2+ concentration induced marked quenching of the blue L2
fluorescence, which almost completely disappeared at [Cd2+]/
[L2] = 0.5 (Fig. 3b). The relative intensities of the emission at
500 nm are shown as an inset. A very weak greenish yellow
emission at around 480 nm was observed by a further increase of
the Cd2+ concentration, but the emission was too weak to be
analyzed.
association constant, KML , was too large to be determined
2
accurately by the curve fitting method and was estimated to be
1 × 1016 dm6 mol−2 from the absorbance at 310 nm (Fig. S3a†).
In the case of L2, the third-order association constant KML was
2
also large and was estimated to be approximately 9 × 1014 dm6
mol−2 from the absorbance at 246 nm, which was the isosbestic
point of the second spectral change (Fig. S3b†). The second-
order association constant KM L for the second step to form the
2
2
2 : 2 complex was successfully determined to be 7.8 × 106 dm3
mol−1 from the absorbance at 375 nm, the isosbestic point of the
first step (Fig. S3c†). However, as shown in Fig. 2c, the steps in
the L3 spectral change were not clear, and hence, the association
constants could not be determined.
Step-wise spectral changes were also observed during absorp-
tion spectral titration of L3 (Fig. 2c). Plots of the absorbance at
308.5 and 367.5 nm suggested the presence of three steps within
the molar ratio ([Cd2+]/[L3]) from 0 and 1.5; however, these
steps were not as clearly separated as those of the Cd2+-L2
system. Conversely, fluorescence spectral titration clearly showed
the first step of the complexation (Fig. 3c). The rapid decrease of
L3 fluorescence to [Cd2+]/[L3] = 0.5 was identical to that of L2,
indicating formation of the [Cd(L3)2]2+ complex. Furthermore,
signals corresponding to [Cd(L3)2](ClO4)+ (m/z = 2624.3) and
Structure of [Cd2(L2)2]4+ complex
Attempts to crystallize [Cd2(L2)2]4+ and [Cd3(L3)2]6+ using
various anions and solvents were unsuccessful and only yielded
micro-crystals which were not suitable for X-ray crystallographic
analysis. As the composition of the complex was confirmed by
spectral titration and ESI-MS, the structure of the complex was
analyzed by 1H and 13C NMR in acetone-d6. To accomplish this,
we selected [Cd2(L2)2]4+ because of its spectral simplicity. Since
only one set of 1H and 13C signals of the tpy moiety were
observed in the complex, the four tpy units of the two L2
molecules were in identical environments in the complex. Most
of the 13C-signals of the tpy units showed lower-field shifts upon
formation of the 2 : 2 complex, indicating coordination of all
pyridine nitrogen atoms to the metal centre.
+
[Cd2(L3)2](ClO4)3 (m/z = 2935.9) were observed in the
ESI-MS of an acetonitrile solution ([Cd2+] : [L3] = 3 : 2)
(Fig. S2†). No mass signal corresponding to the Cd3(L3)2
complex was observed, which was likely because the appropriate
concentration of the sample solution for ESI-MS measurement
was too low to form equilibrium species of the five components.
Based on the above results, we concluded that [Cd(L3)2]2+,
[Cd2(L3)2]4+ and [Cd3(L3)2]6+ were successively formed in
solution by increasing the concentration of Cd2+.
Thus, complexation of L1, L2, and L3 with Cd2+ in aceto-
nitrile is expressed by the following equations, where KM L rep-
Fig. 4 shows the 1H NMR spectra of the L2 and the
[Cd2(L2)2]4+ complex, for which the signals were assigned suc-
i
j
resents the association constant of formation of MiLj(2i)+
.
Quantitative formation of [Cd(L1)2]2+, [Cd3(L3)2]6+ and
[Cd2(L2)2]4+ was indicated under the experimental conditions,
indicating the high complexability of L1–L3 as ligands for
Cd2+.
cessfully by H–1H correlation spectroscopy (COSY; Fig. S4†).
1
The formation of the 2 : 2 complex induced a lower shift in the
signals of the phenyl moiety and the pyridine rings A (H3 ∼ H5)
and B (H3′ and H5′). However, the proton signals of the pyridine
ring C (H3′′ ∼ H6′′) and α-CH2 (Hα) of the propyl group shifted
to a higher magnetic field, and the extent of the upfield shift was
largest for H6′′ (Δδ = −0.96 ppm). As the 13C signals of the ring
C showed a downfield shift upon complexation, the observed
KML
2
ꢁꢁꢁ*
2þ
L1:Cd2þ þ 2ꢀL1 )ꢁꢁꢁ ½Cdð L1Þ2ꢂ
ð1Þ
ð2Þ
1
upfield shift of the H-signals is indicative of the ring-current
KML
2
2þ
L2:Cd2þ þ 2ꢀL2 )ꢁꢁꢁ ½CdðL2Þ2ꢂ
ꢁꢁꢁ*
effect14 of the other L2 molecule within the complex.
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Fig. 4 1H and 13C NMR spectra of (a) L2 and (b) [Cd2(L2)2](ClO4)4 in acetone-d6.
Fig. 6 Double-helical structures of (a) [Cd2(L2)2]4+ and (b)
[Cd3(L3)2] 6+ estimated by MOPAC/PM5 calculation.
rings, the tpy-dimer was conveniently synthesized by using the
amino group as the connector. In the 2 : 2 complex, the tertiary
amino group did not participate in the metal coordination.
Instead, the amino group served as a joint between the
Cd-BPtpy2 units, which allowed formation of the double helical
structure. These results suggest that the structure of the
[Cd3(L3)2]6+ complex also has a duplex-type structure, in which
three Cd-BPtpy2 units are connected with propylamino groups.
The estimated structure of the [Cd3(L3)2]6+ complex simulated
by MOPAC/PM5 is also shown in Fig. 6.
Fig. 5 1H–1H ROESY spectrum of [Cd2(L2)2](ClO4)4 in acetone-d6.
Cross peaks were marked with dotted circles.
To further clarify the unique arrangement of L2 in the
complex, a 1H–1H ROESY measurement16 was performed. In
the spectrum (Fig. 5), cross peaks were observed between H3
and H3′, H5′ and H3′′, and H5 and Hα. Because cross peaks arise
from protons that are close to each other in space, the presence
of the H3–H3′ and H5′–H3′′ cross peaks confirmed the synperi-
planar conformation of the adjacent pyridine rings in the tpy
units due to coordination of all of the pyridine nitrogens to Cd2+.
The H5–Hα cross peak indicated that these hydrogen atoms are
located close to one another in the complex.
Based on these observations and MOPAC/PM5 calculation,
the 2 : 2 complex is inferred to be the double helical structure
shown in Fig. 6. In this structure, the pyridine ring C is located
immediately above the pyridine ring of the other ligand mole-
cule, and the H5 and Hα atoms are situated close enough to each
other to yield the cross peak in the ROESY spectrum. A similar
double helical structure has been reported for M2+-oligopyridine
complexes. Instead of a direct connection between pyridine
Solid-state photophysical properties of L1–L3 and their Cd2+
complexes
Fluorescent organic compounds tend to lose their emissive pro-
perties in the solid state due to increased intermolecular inter-
actions inducing nonradiative deactivation. However, L1 and L2
retained their emissive properties in the solid state. Furthermore,
the powder samples of the [Cd(L1)2]2+, [Cd2(L2)2]4+ and
[Cd3(L3)2]6+ complexes displayed noticeable greenish yellow
luminescence (Table 2). Therefore, we studied the photophysical
properties of the 6-amino-tpy derivatives L1–L3 and their com-
plexes in the solid state (Fig. 7). The absorption and fluorescence
bands of the 6-amino-tpy derivatives in the solid state were
similar to those in the dilute solution (Table 1), indicating that
no dimer or excimer formation occurred in the solid state. In
the case of the complexes, the powder samples displayed
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Table 2 Luminescence of the tpy derivatives and their complexes in
the solid state
enhanced luminescence in the solid state or upon formation of
aggregates was recently reported and has attracted considerable
interest.17,18 In the case of the complexes, the enhanced emission
was observed not only for the powdery solid, but also for the
frozen dilute solution and in the amorphous polymer matrix.
Therefore, no specific intermolecular interaction or mode of
molecular packing contributed to their emission enhancement in
the powder state. It is likely that the suppression of ligand
exchange or any other molecular or atomic motion(s) upon soli-
dification prevents the nonradiative dissipation process.
λem/nm (Φ)
Complex
In acetonitrile
Powder
[Cd(L1)2] (ClO4)2
[Cd2(L2)2] (ClO4)4
[Cd3(L3)2] (ClO4)6
—a (<0.005)
534 (0.10)
489 (0.13)
505 (0.13)
440–480a (<0.005)
430–450a (<0.005)
a Maximum wavelength was not or only roughly determined due to a
noisy spectrum.
Conclusion
The presence of the 6-amino group enabled preparation of the
amino-connected fluorescent tpy-dimer L2 and trimer L3 as the
model compounds of oligo-tpys. These compounds served as
polydentate ligands and formed duplex-type multi-component
complexes with Cd2+. Although the complexes exhibited only
weak luminescence in solution, they showed a marked emission
enhancement in the solid state. These results suggest that the
duplex-type multi-component complex in the solid state has
sufficient rigidity to suppress a variety of nonradiative excitation-
energy dissipation pathways, coupled with atomic and molecular
motions, which is an attractive property for future photo-active
oligomeric tpy systems.
Experimental
Measurements
In solution, absorption spectra were recorded on a Shimadzu
UV-3100PC spectrophotometer and fluorescence spectra on a
Shimadzu RF-5300PC spectrofluorometer. The fluorescence
quantum yield in solution was calculated using 2-aminopyridine
(excitation 285 nm; Φ = 0.37; ethanol) as the standard. The
absorption spectra of the solids were measured by Kubelka–
Munk conversion19 of diffuse reflectance spectra, which were
measured on a JASCO FP-6600 spectrofluorometer equipped
with an integral sphere, into which a quartz cell containing
ground crystal (1 mg) diluted with sodium chloride powder (1 g)
was placed. The solid-state luminescence spectra were also
recorded using an ILF-533 integral sphere, and the quantum
yields were calculated using the system’s built-in software.20
Fast atom bombardment mass spectrometry (FAB-MS) spectra
(matrix: 3-nitrobenzyl alcohol) and ESI-MS spectra (solvent:
acetonitrile) were recorded on a JOEL JMS-600H mass spectro-
Fig. 7 Solid-state absorption and emission spectra of (a) L1 (dotted
line), L2 (solid line), and L3 (broken line), and (b) Cd2+ complexes,
[Cd(L1)2](ClO4)2 (dotted line), [Cd2(L2)2](ClO4)4 (solid line), and
[Cd3(L3)2](ClO4)6 (broken line).
appreciable greenish yellow luminescence with moderate
quantum yields (Table 2), even though the complexes were not,
or were only weakly, fluorescent in solution (Φ < 0.005). We
found that the very weak greenish yellow emission of the com-
plexes in a dilute acetonitrile solution was intensified when the
solution was frozen at 77 K. A poly(methyl methacrylate)
(PMMA) film prepared by casting an acetonitrile solution of
PMMA and [Cd2(L2)2]4+ complex (1 wt% to PMMA) also
exhibited greenish yellow fluorescence with moderate quantum
yield (477 nm, Φ = 0.12). Though there was a large difference in
the emission intensities, the emission spectra of the powder
samples appeared to be similar to those of the dilute solution
and the matrix-separated PMMA film (Fig. S5†). Therefore, the
solidified state of the complexes induced the emission enhance-
ment. A novel class of luminescent compounds that exhibit no,
or only weak, luminescence in solution, but exhibit greatly
1
meter. H and 13C NMR spectra were measured in chloroform-d
(CDCl3) or acetone-d6 using tetramethylsilane (Me4Si) as a
standard on JEOL JNM-ECS400 and JNM-ECP600 spectro-
meters. The association constant of the Cd(II) complexes (KM L
)
i
j
at each complexation step was evaluated by non-linear curve
fitting using the Newton–Raphson method21 performed on
Microsoft Excel 2007, according to the literature.22
Synthesis
6-Bromo-4′-(4-tert-butylphenyl)-2,2′:6′,2′′-terpyridine (BPtpy-1)
and 6,6′′-dibromo-4′-(4-(tert-butyl)phenyl)-2,2′:6′,2′′-terpyridine
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(BPtpy-2) were synthesized according to modified procedures
8.31 (2H, d, 3-H), 8.67–8.80 (8H, m, 3′,5′,3′′,6′′-H). δC
(100 MHz; CDCl3) 156.69, 156.51, 156.17, 155.76, 154.62,
152.25, 149.68, 149.11, 137.85, 136.84, 135.66, 126.83, 125.95,
123.71, 121.39, 118.63, 118.41, 114.73, 114.08, 50.34, 34.73,
31.32, 21.63, 11.92. FAB-MS: m/z 786.2 [M + H]+.
reported previously.5
4′-(4-tert-Butylphenyl)-6-(propylamino)-2,2′:6′,2′′-terpyridine
(BPtpy-3). To a mixture of BPtpy-1 (0.80 g, 1.80 mmol),
Pd(dba)2 (34.3 mg, 0.06 mmol), BINAP (53.6 mg,
0.087 mmol), and sodium tert-butoxide (2.16 g, 22.5 mmol) in
toluene (30 mL) was added propylamine (5 equiv. to BPtpy-1),
and the resulting mixture was maintained at 80 °C for 2 days.
The cooled mixture was diluted with dichloromethane, then
filtered through celite. The filtrate was evaporated and the
residual solid was applied to a column chromatography (alumi-
num oxide/benzene–hexane = 3 : 1, then benzene–acetonitrile =
5 : 1) yielded BPtpy-3 as a pale yellow powder (0.72 g, 96%).
Mp 147.0–148.0 °C. δH (400 MHz; CDCl3; Me4Si) 1.0 (3H, t,
J = 6.4 Hz,), 1.4 (9H, s), 1.7 (2H, m), 3.3 (2H, m), 4.7 (s, 1H),
6.4 (1H, d, J = 6.4 Hz), 7.3 (1H, m), 7.6 (3H, m), 7.8 (3H, m),
7.9 (1H, d, J = 8.0 Hz), 8.7 (4H, m). δC (100 MHz; CDCl3)
158.51, 156.55, 156.45, 155.59, 154.54, 152.02, 149.72, 149.01,
138.18, 136.78, 135.90, 126.94, 125.82, 123.60, 121.41, 118.60,
118.27, 110.35, 106.70, 44.22, 34.69, 31.31, 22.86, 11.68.
FAB-MS: m/z 423.4 [M + H]+.
6,6′′-Bis{N-[4′-(4-tert-butylphenyl)-2,2′:6′,2′′-terpyridin-6-yl]-
propylamino}-4′-(4-tert-butyl phenyl)-2,2′:6′,2′′-terpyridine (L3).
A suspension of BPtpy-2 (75 mg, 0.14 mmol), BPtpy-3
(120.6 mg, 0.285 mmol), Pd(dba)2 (18.9 mg, 0.030 mmol), dppf
(24.2 mg, 0.041 mmol), and sodium tert-butoxide (205 mg,
2.13 mmol) in toluene (6 mL) was maintained at 80 °C for
5 days. The cooled mixture was diluted with CH2Cl2, then
filtered through celite. The filtrate was evaporated to give a crude
product, which was then purified by column chromatography
(aluminum oxide/benzene) yielded L3 as a pale yellow powder
(55.1 mg, 32%). Mp 138.6–139.5 °C. δH (400 MHz; CDCl3;
Me4Si) 1.16 (6H, t, CH2CH3), 1.40 (27H, s, C(CH3)3), 2.05
(4H, m, CH2CH3), 4.49 (4H, t, NCH2), 7.35–7.38 (6H, m,
pyridyl m-H), 7.55–7.58 (6H, m, phenyl m-H), 7.77 (2H, t,
pyridyl p-H), 7.78 (2H, t, pyridyl p-H), 7.83–7.88 (6H, m,
phenyl o-H), 7.90 (2H, t, pyridyl p-H), 8.30 (2H, d, pyridyl
m-H), 8.33 (2H, d, pyridyl m-H), 8.71–8.78 (10H, m). δC
(150 MHz; CDCl3) 156.65, 156.62, 156.46, 156.16, 156.05,
155.72, 154.70, 154.55, 152.22, 152.21, 149.63, 149.07, 137.84,
137.80, 136.82, 135.82, 135.63, 126.82, 126.64, 126.03, 125.93,
123.69, 121.37, 118.60, 118.37, 118.32, 114.68, 114.65, 114.12,
114.00, 50.32, 34.71, 31.30, 21.60, 11.92. FAB-MS: m/z
1207.0 [M + H]+. Thin-layer chromatography (aluminum oxide):
Rf = 0.75 (benzene–pyridine 9 : 1), 0.17 (cyclohexane–pyridine
9 : 1).
4′-(4-tert-Butylphenyl)-6-(dipropylamino)-2,2′:6′,2′′-terpyridine
(L1). To a mixture of BPtpy-1 (0.51 g, 1.14 mmol), Pd(dba)2
(22.4 mg, 0.034 mmol), dppf (34.8 mg, 0.063 mmol), and
sodium tert-butoxide (1.36 g, 14.1 mmol) in toluene (18 mL)
was added dipropylamine (5 equiv. to BPtpy-1), and the result-
ing mixture was maintained at 80 °C for 2 days. The cooled
mixture was diluted with dichloromethane, then filtered through
celite. The filtrate was evaporated and the residual solid was
applied to a column chromatography (aluminum oxide/benzene–
hexane = 33 : 67 to 100 : 0) yielded L1 as a white microcrystal
(0.31 g, 60%). Mp 128.6–129.6 °C. δH (400 MHz; CDCl3;
Me4Si) 8.68–8.74 (4H, m, 3′,5′,3′′,6′′-H), 7.85 (2H, d, phenyl
o-H), 7.83–7.90 (2H, m, 3,4′′-H), 7.56 (1H, t, 4-H), 7.54 (2H, d,
phenyl m-H), 7.33 (1H, t, 5′′-H), 6.54 (1H, d, 5-H), 3.53 (4H, t,
NCH2), 1.75 (4H, m, CH2CH3), 1.40 (9H, s, C(CH3)3), 1.01
(6H, t, CH2CH3). δC (150 MHz; CDCl3) 157.32, 157.05,
156.70, 155.51, 153.96, 152.07, 149.34, 149.00, 137.77, 136.80,
135.97, 126.81, 125.89, 123.57, 121.46, 118.44, 117.97, 108.16,
105.93, 51.26, 34.72, 31.33, 20.95, 11.66. HRMS (FAB)
465.3014 [M + H]+. C31H37N4 requires 465.3018.
Acknowledgements
The study is partly supported by Grant-in-Aid for Scientific
Research (B) (No. 21350109) from the Japan Society for the
Promotion of Science (JSPS).
Notes and references
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amine (L2). To a mixture of BPtpy-1 (0.80 g, 1.80 mmol),
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