Strong π-delocalization and substitution effect on electronic properties of dithienylpyrrole-containing bipyridine ligands and corresponding ruthenium complexes

Article abstract of DOI:10.1039/c2dt12367c

The first dithienylpyrrole (DTP)-based bipyridine ligands has been prepared and coordinated with ruthenium to give the corresponding homoleptic complexes. Bipyridine was bound at pyrrole (DTP₁) or thiophene (DTP₂) ring. A strong bath

Full text of DOI:10.1039/c2dt12367c

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PAPER
Strong π-delocalization and substitution effect on electronic properties of
dithienylpyrrole-containing bipyridine ligands and corresponding ruthenium
complexes†
Sajida Noureen,a Stefano Caramori,b Antonio Monari,c Xavier Assfeld,c Roberto Argazzi,b
Carlo A. Bignozzi,b Marc Beleya and Philippe C. Gros*a
Received 7th December 2011, Accepted 24th January 2012
DOI: 10.1039/c2dt12367c
The first dithienylpyrrole (DTP)-based bipyridine ligands has been prepared and coordinated with
ruthenium to give the corresponding homoleptic complexes. Bipyridine was bound at pyrrole (DTP1) or
thiophene (DTP2) ring. A strong bathochromic effect was obtained by switching from pyrrole to
thiophene for ligands and complexes. Interestingly the DTP2 series offered a wide absorption window
from UV to visible domain with an almost constant absorbance. These effects are due to a larger extent of
delocalization as supported by DFT calculations and photophysical measurements.
solar cells.17–19 With the aim to further extend electron delocali-
zation in the compounds above, we decided to examine the
Introduction
Ruthenium–polypyridine complexes are highly important
coordination compounds due to their unique photophysical and
redox properties as well as electron transfer and light harvesting
ability.1–4 As a consequence of these features, ruthenium com-
plexes rapidly became ideal candidates for photosensitization in
several applications exploiting light energy such as dye-sensi-
tized solar cells,5–8 switches,9,10 or molecular engines.11–13 The
efficiency of these devices is highly dependent on the com-
plexes’ ability to strongly absorb the sunlight in a large domain,
ideally from the UV to NIR range. This is made possible by a
metal-to-ligand charge transfer (MLCT) occurring between
ruthenium and the ancillary ligands at low energy. The energetic
level of the MLCT can be tuned and drawn to lower values by
modifying the ligand structure or its substitution. Bipyridines
bearing π-delocalized moieties have been found to be efficient
for this purpose14–16 also allowing a concomitant increase of the
molar extinction coefficient. Our group has shown recently that
2,2′-bipyridines bearing pyrrole-based π-extended moieties
brought interesting properties to ruthenium complexes such as
wide absorption range and good performance in dye-sensitized
effect of mixed pyrrole-based oligomers moieties. Despite better
oxidation capability and enhanced luminescence properties,20,21
2,5-dithienylpyrrole (DTP) has been less studied22–25 and used
for organic electronic applications than oligothiophenes essen-
tially due to lack of efficient synthetic routes. In addition, in con-
trast with oligothiophenes26,27 and oligopyrroles,28,29 the DTP
moieties have not been bound to a bipyridine ligand yet and
their effect on the electronic properties of the corresponding
complexes remains underexplored. Herein we report the syn-
thesis of a range of new DTP-based bipyridine ligands and their
corresponding homoleptic ruthenium complexes (Fig. 1) as well
as their characterization. A dramatic substituent effect has been
observed for the DTP2 series (bound to bipyridine via its thio-
phen ring) and their corresponding complexes leading to a
marked bathochromic effect with strong increase of absorption in
the visible domain. This effect on ligands and complexes is dis-
cussed on the basis of transient spectroscopy as well as density
functional (DFT) and time dependent density functional
(TD-DFT) theory calculations.30–36
Results and discussion
aGroupe SOR, SRSMC, Université de Lorraine, CNRS, Faculté des
Sciences, Boulevard des Aiguillettes, F-54506 Vandoeuvre-Lès-Nancy,
France. E-mail: philippe.gros@srsmc.uhp-nancy.fr; Fax: +33(0)
383684785; Tel: +33(0)383684979
1. Ligands and complexes synthesis
The synthetic strategy to access the bipyridine ligands was based
on the Wadsworth–Emmons reaction between a bipyridine
diphosphonate and aldehydes to create the styryl link needed to
ensure efficient electronic communication between the DTP
moiety and bipyridine and metal center in the complex. Thus we
had to prepare various DTP-based compounds bearing carbox-
aldehyde functions at various positions i.e. on the thiophene and
the pyrrole ring (Scheme 1). The DTP moiety was built using
bChemistry Department, University of Ferrara, Via Luigi Borsari 46,
44100 Ferrara, Italy. E-mail: cte@unif.it; Fax: +39.0532.240709;
Tel: +39 0532 291163
cEquipe CBT, SRSMC, Université de Lorraine, CNRS, Faculté des
Sciences, Boulevard des Aiguillettes, F-54506 Vandoeuvre-Lès-Nancy,
France. E-mail: antonio.monari@cbt.uhp-nancy.fr; Fax: +33(0)
383684371; Tel: +33(0)383684380
†Electronic supplementary information (ESI) available. See
DOI: 10.1039/c2dt12367c
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Table 1 Preparation of ligands and ruthenium complexes
Aldehyde
DTP-X, yield (%)
Ru(DTP-X), yield (%)
9
6
7
8
10
11
12
DTP1-H, 68
DTP1-Br, 38
DTP1-F, 31
DTP1-Hex, 36
DTP2-F, 73
DTP2-Me, 46
DTP2-Hex, 40
Ru(DTP1-H), 76
Ru(DTP1-Br), 79
Ru(DTP1-F), 75
Ru(DTP1-Hex), 63
Ru(DTP2-F), 65
Ru(DTP2-Me), 40
Ru(DTP2-Hex), 66
yields. When 1 was reacted under the same conditions, an inse-
1
parable mixture containing aldehyde 9 as revealed by H NMR
of the crude product. A selective route to 9 was next found using
6 as precursor. The aldehyde was first protected as a dioxolane
and the bromine was removed by treatment with BuLi. After
deprotection under acidic conditions, 9 was obtained in 81%
yield.
Fig. 1 DTP-based ligands and complexes studied in this work.
Finally, we investigated the introduction of the aldehyde func-
tion on the thiophene ring in 3, 4 and 5 using a bromination–
lithiation sequence. The monobromination occurred mainly on
the thiophene ring using 1 equiv. of NBS at −50 °C in the pres-
ence of propionic acid. Further bromine lithium exchange was
performed using BuLi and the lithiated compound treated with
DMF leading to 10, 11 and 12 in 43, 55 and 43% yield, respect-
ively. With aldehydes 6–12 in hand, we turned to the preparation
of the DTP-based bipyridine ligands. The Wadsworth–Emmons
reaction between bipyridine diphosphonate and aldehydes was
investigated. The bipyridine diphosphonate was prepared in 3
steps from 4,4′-dimethyl-2,2′- bipyridine using the Fraser’s pro-
cedure.39,40 Reaction with 2 equiv. of aldehyde under basic con-
ditions gave the expected ligands (Table 1). The symmetrically
coordinated complexes Ru(DTP-X) were then prepared by react-
ing RuCl2(DMSO)4 with 3 equiv. of ligand under microwave
irradiation. The complexes were obtained in good yield after
10 min of stirring.
The new compounds were next characterized by UV-vis spec-
troscopy and electrochemistry in order to measure the effect of
DTP on electronic properties (Tables 2 and 3).
Scheme 1 Preparation of DTP-based aldehydes. Reagents and con-
ditions: (i) aniline (4 equiv.), AcOH–toluene, reflux (24 h). (ii) POCl3 (4
equiv.), DMF, r.t. (12 h) then 70 °C (1h). (iii) 1,2-ethanediol, APTS
(2.5%), toluene, reflux (12 h). (iv) (1) n-BuLi (1 equiv.), THF, −78 °C
(0.5 h); (2) H2O; (3) APTS (1.2 equiv.), acetone, 4-picoline (1 equiv.),
toluene reflux, (24 h). (v) NBS, (1 equiv.), CHCl3/EtCOOH (10 : 1),
−50 °C (1.5 h). (vi) (1) n-BuLi (1 equiv.), THF, −40 °C (1 h); (2) DMF
(1.5 equiv., −78 °C then r.t.
2. Characterization of ligands
As shown in Fig. 2, ligands of the DTP1-X series bound to the
styryl–bipyridine through the pyrrole ring exhibited absorption
spectra with a similar shape. An intense band was observed near
310 nm and another with a lower intensity in the 350–384 nm
range. A significant red-shift was observed with DTP1-Hex
bearing the electron-donating hexyl group (λabs = 384 nm
instead of 350–360 nm for the other ligands). The ε values were
found to be dependent on the substitution, the best absorption
was obtained for DTP1-Br and the lowest for DTP1-Hex. Sur-
prisingly, unsubstituted DTP1-H and DTP1-F bearing the most
the Paal–Knorr reaction between substituted anilines and 1,4-
dithienyl-butanedione.37 Among several conditions tried, the use
of acetic acid in refluxing toluene was retained, giving the
expected compounds 1–538 in 55–94% yield. The Vilsmeier–
Haack formylation was first chosen to next introduce the carbox-
aldehyde, which was introduced exclusively on the pyrrole ring
from 2, 3 and 5 leading to 6, 7 and 8, respectively, in acceptable
4834 | Dalton Trans., 2012, 41, 4833–4844
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electron-withdrawing fluorine group gave almost the same ε
values.
Such an ε increase could be assigned to an increase of the
molecule dipolar moment by the electron withdrawing fluorine
group. All the ligands were found emissive (see Table 2). λem for
DTP1-X were found in the 530–540 nm range while in agree-
ment with absorption, a notable red-shift was observed with
DTP2-X (λem = 580 nm for DTP2-Hex). Thus the way the DTP
moiety was attached to bipyridine dramatically affected the elec-
tronic properties of the corresponding ligand. This probably
resulted from differences in π delocalization extent in the
ligands. Indeed in the case of DTP2-X where DTP was bound
by the thiophene ring a more extended delocalization was
offered. In contrast, in DTP1-X the thiophene ring did not seem
to participate to the delocalization. In order to analyze the mol-
ecular structure and electron distribution into the ligands, ab
initio calculations were performed. All the ligand geometries
have thus been optimized at DFT level using B3LYP41 exchange
Absorption spectra of the DTP2-X series strongly contrasted
with those of the DTP1-X one since a strong bathochromic
effect toward the visible domain was observed (λmax = 433, 435
and 439 nm for DTP2-F, DTP2-Me and DTP2-Hex respect-
ively). Furthermore, the absorption of high intensity near
305 nm observed with the other ligands was dramatically
weakened.
Table 2 Photophysical and electrochemical properties of ligands
λabs-max (nm)a
b
(ε (103 L M−1 λem-max
Epac (L+/L) Epcd (L/L−)
Ligand
cm−1))
(λexc.) (nm) (V/SCE)
(V/SCE)
DTP1-H
DTP1-Br
DTP1-F
352 (42.1)
311 (53.9)
356 (42.4)
309 (61.0)
360 (39.0)
310 (52.5)
542 (352)
542 (311)
531 (356)
—
1.02 (irrev.) −1.90 (irrev.)
—
—
0.98 (irrev.) −1.90 (irrev.)
—
1.0 (irrev.)
—
—
−1.94 (irrev.)
—
—
540 (310)
540 (381)
540 (304)
575 (432)
575 (316)
580 (435)
—
DTP1-Hex 381 (26.0)
0.96 (irrev.) −1.92 (irrev.)
304 (41.6)
432 (59.5)
316 (24.9)
—
—
DTP2-F
0.84 (irrev.) −1.63 (irrev.)
—
—
DTP2-Me 435 (48.2)
318 (19.1)
0.80 (irrev.) −1.65 (irrev.)
—
—
DTP2-Hex 439 (56.1)
323 (27.5)
580 (438)
580 (322)
0.84 (irrev.) −1.65 (irrev.)
—
—
a Measured in DMSO at 25 °C. b Photomultiplier corrected emission
maxima for the complexes in DMSO in the absence of O2, A < 0.05.
c , d First oxidation and reduction potentials, respectively, standardized
with Fc+–Fc as internal standard and converted into SCE scale by
adding 0.47 V (E1/2 Fc+–Fc). Recorded in DMF using Bu4N+PF6 as
−
supporting electrolyte at 100 mV s−1
.
Fig. 2 Absorption spectra of ligands in DMSO.
Table 3 Photophysical and electrochemical properties of complexes
λabs-max a (nm)
λem-max b (ligand
based) or (MLCT)
τsinglet c (ns)
τtriplet (ns)
E
1/2 RuIII–RuII d
Epad (L+/L)
(V/SCE)
E1/2 e (L/L−)
(V/SCE)
Complex
(ε(103 L M−1 cm−1))
(V/SCE)
Ru(DTP1-H)
495 (50)
550 (ligand)
687 (MLCT)
—
541 (ligand)
691 (MLCT)
—
476 (ligand)
698 (MLCT)
—
541 (ligand)
692 (MLCT)
—
512 (ligand)
No MLCT emission
554 (ligand)
No MLCT emission
577 (ligand)
No MLCT emission
—
1.23
215
—
1.64
217
—
1.57
212
—
1.68
220
—
2.01
200
0.58
200
0.56
155
—
1.05 (ΔEp = 0.18)
—
—
0.98 (ΔEp = 0.20)
—
—
0.98 (ΔEp = 0.24)
—
—
0.96 (ΔEp = 0.20)
—
—
0.80 (ΔEp = 0.120)
—
0.77 (ΔEp = 0.120)
—
0.76 (ΔEp = 0.120)
—
—
1.17 (irrev.)
—
—
0.95 (irrev.)
—
—
1.07 (irrev.)
—
—
1.00 (irrev.)
—
—
0.86 (irrev.)
—
0.82 (irrev.)
—
0.82 (irrev.)
—
—
−1.16 (ΔEp = 0.08)
—
—
−1.17 (ΔEp = 0.08)
—
—
396 (62.2)
308 (132.8)
488 (33.1)
390 (46.6)
264 (112.5)
490 (20.3)
398 (28.2)
306 (64.4)
489 (48.3)
397 (63.1)
306 (48.3)
508 (72.7)
452 (72.3)
512 (67.9)
451 (62.5)
513 (78.3)
443 (75.9)
360 (62.7)
Ru(DTP1-Br)
Ru(DTP1-F)
−1.17 (ΔEp = 0.08)
—
—
Ru(DTP1-Hex)
n.d.f
—
—
−1.07 (ΔEp = 0.08)
—
−1.10 (ΔEp = 0.08)
—
−1.11 (ΔEp = 0.08)
—
—
Ru(DTP2-F)
Ru(DTP2-Me)
Ru(DTP2-Hex)
a Measured in CH3CN at 25 °C. b Photomultiplier corrected emission maxima for the complexes in DMF A < 0.05. c Ligand based singlet emission
lifetime measured by TCSPC and triplet absorption lifetime upon 532 nm nanosecond (FWHM 7 ns) laser excitation. All measurements performed in
deaerated DMF. d Oxidation potentials standardized with Fc+–Fc as internal standard and converted into SCE scale by adding 0.47 V (E1/2 Fc+–Fc).
Recorded in DMF using Bu4N+PF6− as supporting electrolyte at 100 mV s−1 e First reduction potential. f n.d. = not detected.
.
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Table 4 Ligands TDDFT computed principal excitation wavelengths
and oscillator strength
X
H
DTP1-X λa (nm), (f)
DTP2-X λa (nm), (f)
355.51 (2.06)
318.48 (1.56)
—
433.66 (3.52)
318.44 (0.07)
286.07 (0.56)
429.08 (3.51)
316.79 (0.06)
285.19 (0.55)
433.61 (3.52)
318.44 (0.07)
286.07 (0.56)
433.61 (3.52)
318.44 (0.07)
286.07 (0.56)
437.65 (3.55)
316.73 (3.13)
287.91 (0.48)
Br
351.76 (2.20)
342.40 (0.01)
315.43 (1.02)
352.08 (2.11)
342.72 (0.01)
315.52 (0.93)
355.77 (2.11)
346.33 (0.05)
318.56 (1.01)
355.79 (2.16)
346.34 (0.01)
318.60 (1.13)
F
Fig. 3 TDDFT optimized geometry of DTP1-Hex (top) and DTP2-
Hex (bottom).
Me
Hex
a Oscillator strength in parentheses.
Fig. 4 Calculated dihedral angles in DTP1-Hex (left) and DTP2-Hex
(right). Half the molecule is depicted.
correlation functional. A double zeta 6-31G42 basis was used
throughout. In order to assure a proper comparison we computed
all the possible systems obtained from DTP1 and DTP2 moieties,
making a total of 10 ligands. Optimized geometries for DTP1-
Hex and DTP2-Hex are given in Fig. 3.
Dihedral angles between relevant pyrrole and thiophene rings
were measured from the optimized structures (Fig. 4). In DTP1-
Hex, the bipyridine–styryl–pyrrole sequence was found to be
coplanar, while thiophene rings were distorted from the plane.
The styryl moiety appeared to induce a beneficial effect on the
dihedral angle with regard to planarity when thiophene was
bound at the ortho position (147° vs. 130° for the thiophene at
the meta position). Thus, one thiophene could be expected to
participate in the π-delocalization process. In DTP2-Hex, the
loss of planarity was minimized when a thiophene was bound to
both styryl and pyrrole (θ = 157°). This was in agreement with a
more extended delocalized system and favoured absorption at the
lower energy domain.
The 25 first excited states have been computed by using
TDDFT formalism for all the optimized ligands. CAM-B3LYP43
functional has been used in order to better account for the long
range corrections, a slightly larger augmented and polarized
double zeta basis (6-31+G(d,p)42) has been used. The solvato-
chromic effect of the solvent has been taken into account by
using the continuum PCM model.44 Note that the transoid struc-
ture presented in Fig. 3 appeared as the most stable ones in sol-
ution, anyway the computed cisoid structure spectra showed
only a very slight deviation from the previous ones. Computed
UV-vis spectra wavelengths and oscillator strength are reported
in Table 4. A fairly good agreement between experiment and
theory was observed (see Table 4).
Fig. 5 Computed spectra of DTP1-Hex and DTP2-Hex.
substituent as expected from its pseudo-orthogonality with
regard to the pyrrole ring (Fig. 3). Interestingly, the unsubstituted
DTP1 compounds do not show the small absorption at about
340 nm like the other members of that family; this is coherent
with the less pronounced shoulder in the experimental spectrum
evidenced in that region. A comparison of the computed spectra
for the DTP1-Hex and the DTP2-Hex is shown in Fig. 5, where
the spectrum has been obtained enveloping each transition with
a Gaussian function of fixed half length width of 0.06 eV. One
can see that besides some difference in relative intensities the
general structure of the spectrum is well reproduced in the higher
wavelength region.45 All the computed transitions for both
classes of compounds are of π–π* type as it is confirmed by an
excited state analysis. The frontier Kohn–Sham orbitals are
reported in Fig. 6 for DTP1-Hex and DTP2-Hex and although
somehow difficult to glance from a simple (delocalized) molecu-
lar orbital picture, the more extended nature of the conjugated π
system in case of DTP2 appears to be confirmed.
As shown in Table 2, an irreversible oxidation of the ligands
occurs at around 1 V (vs. SCE) for the DTP1 series and at 0.8 V
(vs. SCE) for the DTP2 series, respectively. For the same con-
centration, DTP1 series exhibited a current twice higher than in
the DTP2 series. Therefore, this irreversible oxidation can be
attributed to radical cation formation on the external thiophene.
This comparison confirms that the electronic interaction between
the DTP group and the bipyridine via the styryl moiety is higher
in the DTP2 series as described above. In the negative potential
The DTP2 structures confirmed the presence of a very impor-
tant red-shift of about 80 nm as compared to DTP1. On the other
hand, the different members of the same family give quite repro-
ducible spectra, confirming the small influence of the phenyl
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Fig. 6 DTP1-Hex (top) and DTP2-Hex (bottom) frontier orbitals iso-
density contour.
Fig. 7 Transient triplet absorption of DTP1-F (a) and DTP2-F (b) in
DMF (λexc = 355 nm, 0–838 ns interval). 0 delay corresponds to the
initial absorption. The negative signal due to singlet bleaching and to
laser induced emission is here excluded.
part, the reduction mechanisms of the ligands are also irrevers-
ible. They correspond to the addition of an electron in the
LUMO, which is centered on the bipyridine group as shown in
Fig. 6. In agreement with the electronic interaction, the LUMO
potentials are 0.3 V more negative in the DTP1 series. Due to
the pseudo-orthogonality of the dihedral angle between the
phenyl ring and the pyrrole group, the donor (methyl, hexyl) or
withdrawing (F, Br) effects of the substituent on the phenyl are
scarcely significant on the oxidation and reduction potential
values. These small effects were also difficult to detect due to
the irreversible nature of the electrochemical processes under
investigation. All the free ligands were found to be emitting in
fluid solution of DMSO (Table 2), DMF and THF (ESI, Tables
S1 and S2†) in the nanosecond time scale. However, while a
change in the solvent did not cause a notable energy shift of the
ground state absorption (ESI, Fig. S2†), it resulted in an evident
modification of the emission maxima, which underwent a
30–40 nm blue-shift passing from DMF to THF (ESI, Fig. S2
and S3†), probably due to excited state destabilization in the less
polar solvent, suggesting the presence of some degree of charge
transfer/separation in the excited state. The emission kinetics
were not trivial either, being biexponential for both the DTP1
and DTP2 series in DMF (ESI, Tables S1 and S2†). In the DTP2
series the emission decays became mono-exponential in THF
(ESI, Table S1†) and were accompanied by a threefold increase
in lifetime (from ca. 0.5 to 1.5 ns). In the case of the DTP1
series, the bi-exponentiality of the decay was maintained also in
THF, where two components weighing approximately 50% with
a respective lifetime of ca. 0.8 and 2 ns were observed. Any
attempt to fit the decay with a monoexponential function was, in
such cases, a failure, generating unacceptable χ2 > 10. The
DTP1 behavior was tentatively explained by the presence of two
energetically close excited states, both contributing to the broad
emission band. Indeed the excitation spectrum showed a depen-
dence upon the observation wavelength, showing two reasonably
well resolved bands (323 and 367 nm) (ESI, Fig. S4†) whose
relative intensity changed as a function of the observation wave-
length. In particular it was observed that the excitation spectrum
bore a closer resemblance to the ground state absorption spec-
trum when the emission was observed at 430 nm (in the blue
portion of the emission band), whereas the 367 nm band gradu-
ally gained intensity when the observation wavelength was
moved to the red. The emission lifetime changed accordingly,
undergoing, for example in the case of DTP1-F, a ≈ 20% short-
ening when measured at 430 nm (0.65 ns) with respect to the
value obtained in the band maximum (500 nm, 0.84 ns). On the
Fig. 8 Absorption spectra of complexes in acetonitrile.
other hand the excitation spectra of the DTP2 family were gener-
ally in good agreement with the ground state absorption (ESI,
Fig. S5†).
Upon 355 nm laser excitation the DTP1 family originated a
laser pulse limited negative signal due to ground state bleaching
and laser induced emission, followed by a relatively long lived
triplet absorption (monoexponential, τ ≈ 240 ns) with a distinct
470 nm maximum, followed by a 540 nm shoulder (Fig. 7(a)
showing the transient triplet absorption of DTP1-F). The DTP2
ligands were characterized by a relatively weak ground state
bleaching in the 400–500 nm interval, followed by an intense
and broad absorption (monoexponential τ ≈ 300 ns) with a
plateau between 550 and 650 nm (Fig. 7(b) showing the transient
triplet absorption of DTP2-F). The same features were generally
found in the transient spectra of the Zn2+–ligand adducts (ESI,
Fig. S7 and S8†).46
3. Characterization of complexes
The absorption spectra of ruthenium homoleptic complexes,
reported in Fig. 8 and Table 3, also reflected the influence of the
two ligand families. Ru(DTP1-X) complexes showed three
absorption bands: a first intense band in the UV region near
300 nm and two others in the visible region near 400 nm and
490 nm. Ru(DTP1-H) and Ru(DTP1-Hex) bearing respectively
a hydrogen and a hexyl group on the phenyl gave similar spectra
with comparable ε values. In contrast, when electron withdraw-
ing groups were introduced on the ligand, such as bromine (Ru
(DTP1-Br)) or fluorine (Ru(DTP1-F)), the absorbance was
decreased, especially with the latter complex for which half the ε
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value was obtained (20 300 L M−1 cm−1 (490 nm) compared to
50 000 L M−1 cm−1 (495 nm) for Ru(DTP1-H)).
the ligand centred excited state and the incomplete relaxation to
the MLCT state.
In contrast, the Ru(DTP2-X) series exhibited a wide absorp-
tion domain in the visible region with λmax at 443–451 and
512–513 nm. In comparison with the Ru(DTP1-X) complexes,
these bands in the visible region were found to be red-shifted
and were found to be of lower intensity in the UV region. The
high molar extinction coefficients were almost constant along the
visible domain. The electrochemical behavior of the ruthenium
complexes was studied by cyclic voltammetry. They show two
oxidation waves, the first one is semi-reversible and corresponds
to the RuII–RuIII couple, the second one is irreversible and is
attributed to the formation of radical cation on the thiophene as
described for the ligands. The first reduction wave corresponds
to the transfer of an electron in the bipyridine which is obviously
easier in the complexes that in the ligands. The comparison of
these potentials confirms the electronic behaviour previously
described for the ligands, i.e. a higher degree of conjugation in
the DTP2 compared to the DTP1 series.
Surprisingly, no MLCT type emission was observed within
the RuDTP2 series, the only emission being that of the LC type
in the 512–580 nm region, as confirmed by the similar energy,
lifetime and by the excitation spectra obtained in correspondence
of the emission maxima (ESI, Fig. S12†). Attempts to detect
MLCT emission upon excitation at 532 nm at low temperature
(77 K) for RuDTP2-F also failed indicating that the non-radia-
tive decay is dominant in the RuDTP2 series even in a frozen
matrix. Interestingly, compared to the parent free ligand, the
RuDTP2-F emission was substantially blue-shifted (512 vs.
570 nm) and its lifetime increased accordingly (from 0.4 to 2
ns), probably due to destabilization caused by strong interaction
with the dπ orbital of the metal. The TA spectra obtained follow-
ing 532 nm excitation (Fig. 10) were generally characterized by
The RuDTP1 series was emitting in fluid solution. When the
ligand manifold was excited, two distinct emission bands were
observed, one centered in the 480–550 nm region, depending on
the ligand, bearing a close similarity in both energy and lifetime
with the free ligand fluorescence, and one in the red part of the
spectrum (centered around 690 nm) originated by the typical
3MLCT radiative deactivation. Excitation of the lowest energy
absorption band (490–505 nm in DMF) resulted only in the low
energy emission (ESI, Fig. S10†), whose maximum varied very
little within the above mentioned series. The excitation spectrum
observed in correspondence of the low energy emission
(687–692 nm) was in excellent agreement with the absorption
spectrum of the complex, showing three distinct well resolved
bands (ESI, Fig. S11†), whereas the excitation spectrum ob-
served in correspondence of the ligand centred (LC) emission
revealed two UV bands at 300 and 362 nm, whose shape recalled
the absorption features of the Zn2+-DTP1 adducts (Fig. S6†).
Transient absorption (TA) spectra of the RuDTP1 complexes
in DMF by using a laser excitation at 532 nm (≈10 mJ per
pulse) (Fig. 9) exhibited similar characteristics consistent with a
long lived triplet MLCT excited state.
Fig. 9 Transient absorption spectra of RuDTP1 series in DMF (λexc
532 nm). RuDTP1-H (a), RuDTP1-Br (b), RuDTP1-F (c), RuDTP1-
Hex (d).
=
An intense absorption was observed in the blue region at
480 nm, followed by an equally intense bleaching of the MLCT
band with a minimum centered at 510 nm, followed by a strong
featureless triplet absorption in the red part of the visible
domain. The excited state lifetime was in the 200–220 ns range
for all complexes and the decay was monoexponential. The
excited/ground state isosbestic point was found at about 490 nm.
By comparison with the TA of the free ligand and of the Zn2+
adduct (ESI, Fig. S6 and S8†), the 480 nm band could be
assigned to ligand centred LUMO → LUMO + n absorption,
populated by excitation of the charge transfer band.
Upon 355 nm (mainly ligand absorption manifold) excitation
(ESI, Fig. S14†) similar spectra were obtained; however, com-
pared to that observed upon 532 nm excitation, the bleaching of
the MLCT band was about half of the intensity of the character-
istic 480 nm ligand centred (LC) absorption, the low energy (λ >
600 nm) absorption was flat and about half less intense, both
characteristics recalling the spectral features of the parent ligand
and of the relative Zn2+ adduct and indicating the persistence of
Fig. 10 Transient absorption spectra of RuDTP2 series in DMF (λexc
= 532 nm). RuDTP2-Hex (a), RuDTP2-Hex (b) (λexc = 355 nm),
RuDTP2-Me (c), RuDTP2-F (d).
4838 | Dalton Trans., 2012, 41, 4833–4844
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Table 5 Complexes
wavelengths and oscillator strength
TDDFT computed
principal
excitation
a monoexponetial decay, with lifetimes in the 150–200 ns range.
All TA spectra shared common features, summarized by the
ground state bleaching, which mirrored the two overlapping
bands of the ground state absorption, and by the strong triplet–
triplet absorption with a maximum at 700 nm. The isosbestic
point could be quite accurately individuated at 600 nm. The
strong absorption into the red part of the spectrum is evident in
the TA of the free ligand and of the Zn2+ adduct (ESI, Fig. S7
and S9†), although its maximum was blue-shifted by about
100 nm and probably originates from the LUMO → LUMO + n
absorption. In this sense, the 100 nm red-shift in the Ru(II) com-
plexes may not be surprising, given that the LUMO π* orbital
may be more strongly destabilized upon interaction with the
occupied dπ orbitals of the metal resulting in a decreased
LUMO–LUMO + n energy gap. In this case, the TA spectra col-
lected following 355 nm excitation (Fig. 10(b)) were almost
superimposable to those obtained with the 532 nm excitation,
suggesting a strong coupling between the metal and the ligand.
This fact and the lack of a distinct MLCT emission even upon
direct excitation of the lowest energy band (λ > 500 nm) may
suggest that the description of the excited state of the RuDTP2
complexes in terms of usual localized states (hole on the metal,
electron on the LUMO orbital of the ligand) may not be entirely
appropriate, and, as a result of the strong mixing of the HOMOs
of the metal and of the ligand, a photoexcited hole delocalization
would result in a favored deactivation of the lowest excited state
by internal conversion.
X
H
Ru(DTP1-X) λa (nm), (f)
Ru(DTP2-X) λa (nm), (f)
433.80 (0.99)
433.23 (1.02)
431.00 (0.70)
374.28 (2.42)
—
498.23 (1.94)
496.40 (1.81)
490.32 (3.30)
487.90 (2.47)
486.57 (1.92)
420.15 (0.50)
492.86 (2.01)
492.15 (1.90)
484.81 (3.54)
483.02 (2.03)
482.30 (1.78)
419.08 (0.56)
492.09 (1.86)
491.50 (2.01)
483.75 (3.72)
482.19 (1.76)
481.61 (1.89)
419.13 (0.58)
498.79 (1.81)
497.93 (1.98)
490.67 (3.86)
489.33 (1.82)
488.34 (1.98)
420.64 (0.49)
500.40 (1.91)
499.12 (1.94)
492.24 (3.58)
490.57 (1.98)
489.37 (2.05)
422.49 (0.49)
—
Br
433.03 (1.04)
432.39 (1.07)
430.62 (0.66)
371.89 (2.68)
—
—
F
432.32 (1.02)
431.97 (1.05)
429.98 (0.64)
371.58 (2.58)
—
—
Me
Hex
434.03 (0.95)
433.49 (1.01)
431.12 (0.73)
375.31 (1.77)
—
—
433.67 (0.85)
433.00 (0.90)
430.77 (0.78)
375.36 (2.51)
—
—
As for the ligands, the homoleptic complexes have been opti-
mized at DFT level with B3LYP functional. Excited states have
been computed at TD-DFT level with CAM-B3LYP correlation
exchange functional. In that case we used a LANL2DZ47 basis
allowing to treat Ru inner electrons with pseudopotentials. Due
to the very high computational cost excited states have been
computed using again the relatively small LANL2DZ basis. The
latter is certainly not sufficient to provide a qualitative agreement
with experimental data, but the main feature of the spectrum can
be inferred and the nature of the transition can be easily inter-
preted. The computed principal transition in the lower energy
region of the spectrum can be seen in Table 5 for the two
families. Consistent with experimental results the DTP2 family
is significantly red-shifted with respect to the DTP1 members,
the intensities also appear much higher. Note also that the low
lying spectrum of the DTP2 family is composed of a series of
transitions all having almost the same intensity, an occurrence
that can be related to the extended plateau observed in the exper-
imental spectrum, although in the computed result the low fre-
quency transition appears closer between them than in the
experimental one.
The last occurrence, as well as the general blue-shifting of the
spectrum can be related to the small basis set used during com-
putation due to the important size of the system. In order to
better analyse the excited state’s nature we considered natural
transition orbitals (NTO)48,49 representation of the electronic
transition. For the reader’s convenience we remind that NTOs
are obtained by a singular value decomposition (SVD) of the
transition density matrix, and they can be considered as the
optimal orbitals to represent an electronic transition in the
TDDFT formalism. In contrast with Kohn–Sham molecular orbi-
tals base, which require many occupied–virtual orbital couples
a Oscillator strength in parentheses.
in NTO base, only one or at maximum two couples entirely
describe all the physics underlining the transition. Therefore,
“occupied” NTO can be seen as the “hole” orbital, i.e. the orbital
from which electron is removed during transition, while “virtual”
NTO is the orbital in which electron is placed in the excited
state.
NTOs for one of the low lying transitions of the DTP1-F and
DTP2-F are shown in Fig. 11, (note that the other substituents
do not qualitatively alter the orbitals). It can be seen easily that
although in the two cases the transition are mainly of MLCT
nature a significant participation of the ligand in the occupied
orbital is observed (especially for DTP2-F). This effect can be
extremely important in the case of their use as DSSC sensitizers,
since such a transition will leave the “hole” far from the semi-
conductor surface, so diminishing recombination occurrence and
facilitating the access of the redox mediator. One can also see
that, as expected and consistent with the observed red-shift, the
DTP2-F shows a larger delocalization of the electronic density
in the virtual orbital, leading to a stabilization of the excited state
and hence to a red-shift.
Conclusions
A new family of ligands has been obtained by binding bipyridine
to dithienylpyrrole (DTP) moieties. To our knowledge, despite
interesting electronic properties of DTP, neither DTP-containing
bipyridine ligands nor their ruthenium complexes have been
reported yet. The electronic properties are deeply modified with
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Dalton Trans., 2012, 41, 4833–4844 | 4839

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tetramethylsilane (TMS) or signals from the residual protons of
the deuterated solvents, and are reported in parts per million
(ppm) from low- to high-field. Standard abbreviations indicating
multiplicity are used as follows: s = singlet; d = doublet; t =
triplet; m = multiplet; dd = doublet of doublets. All coupling
constants are reported in Hz. High-resolution mass spectrometry
(HRMS) data were obtained by using a Bruker micrOTOF-Q
spectrometer. UV-vis spectra were recorded in a 1 cm path
length quartz cell on a LAMBDA 1050 (Perkin Elmer), spectro-
photometer. Emission and excitation spectra were obtained on
optically diluted solutions by using a Fluoromax 2 (Jobin Yvon)
Spectrofluorometer.
Cyclic voltammetry was performed on a Radiometer PST006
potentiostat using a conventional three-electrode cell. The pot-
assium chloride saturated calomel electrode (SCE) was separated
from the test compartment using a bridge tube. The test solution
was DMF containing 0.1 M tetrabutylammonium hexafluoropho-
sphate as supporting electrolyte. The working electrode was a
10 mm Pt wire and the counter-electrode a 1 cm2 vitreous
carbon disc. The solutions were purged with argon before each
measurement.
Fig. 11 NTOs isodensity surface: DTP1-F at 432 nm (top) and DTP2-
F at 492 nm (bottom). See Table 5 for transitions.
A 0.5 mM solution of the studied compound was generally
used. All potentials were quoted versus SCE. In these conditions
the redox potential of the couple Fc+–Fc was found at 0.47 V. In
regard to the binding site of bipyridine to DTP. When bound to
the pyrrole ring (DTP1), the ligands absorbed the UV part of the
spectrum. In this series, substitution of DTP by a fluorine atom
induced a huge increase in the absorbance. When bipyridine was
bound to the thiophene (DTP2) a strong bathochromic effect was
observed leading to a strong absorption in the visible region.
The corresponding homoleptic ruthenium complexes exhibited
the same features. The complexes from the DTP2 series offered
a promising absorption range in the visible domain with a
notable and constant molar extinction coefficient all along this
domain. Calculations as well as transient spectroscopy were used
to explain such differences by evidencing a larger π-delocaliza-
tion extent in DTP2 series. In summary, DTP-containing ligands
and ruthenium complexes are promising as light harvesters and
work is in progress to involve them in photosensitization of
semiconductors in dye-sensitized solar cells.
all the experiments the scan rate was 100 mV s−1
.
Singlet emission lifetimes were acquired by using a Picoharp
300 time correlated single photon counting (TCSPC) apparatus
by using the 380 nm excitation generated by a nano-led with a
repetition rate of 10 MHz. The maximum reliable time resolution
of the apparatus was 300 ps. The average number of fluorescence
counts per seconds (cps) in optically diluted solutions (A380
≈
0.2) were in the range 103–104. The emission decay was decon-
volved and statistically elaborated by means of the Fluofit® dedi-
cated program. The fitting was deemed satisfactory when 0.99
<χ2 < 1.02 and the residues were homogeneously distributed
around 0 along the whole time interval under consideration (typi-
cally 20 ns). In the case of multiexponential decay, the amplitude
weighted average lifetime was considered.
Transient absorption spectroscopy experiments were carried
out by using a previously described nanosecond transient
absorption apparatus.50 If necessary, in the case of weak signals
(ligand based triplet absorption), to obtain a satisfactory S/N
ratio, oscillographic traces were averaged over 5–10 laser shots.
Experimental
Synthesis
All reactions were carried out under an argon atmosphere,
whereas workup procedures were done in air. THF and toluene
were distilled through MBraun solvent purification system (MB
SPS-800) prior to use. N,N-Dimethylformamide (DMF) was
purified by distillation under reduced pressure. Deuterated sol-
vents and commercially available reagents were used as received.
Compounds 1–5 38 and tetraethyl(4,4′-diphosphonate-2,2′-bipyri-
dine)39 were synthesized by the procedures already reported in
literature. Microwave synthesis was performed on CEM Discover
device fitted with infrared probe temperature control.
Computations
All quantum chemistry calculations have been performed using
GAUSSIAN 09 suite of codes.51 In the case of the ligands geo-
metry optimization has been done by using 6-31G basis set and
B3LYP exchange correlation functional. Subsequently UV-vis
spectrum has been simulated computing 25 excited states at
TD-DFT level using CAM-B3LYP functional and 6-31G+(d,p)
basis set. In the case of the complexes geometry optimization
was performed using LANL2DZ basis and B3LYP functional,
again 25 excited states have been computed using the same
LANL2DZ basis and CAM-B3LYP functional. Excited states
analysis in terms of NTOs have been performed using a local
produced and free downloadable code NancyEX (see http://
www.nancyex.sourceforge.net/).
Measurements
1H and 13C NMR spectra were performed on AC200, AC250, or
DRX400 Bruker spectrometers at ambient temperature. The
chemical shifts (δ), were calibrated by using either
4840 | Dalton Trans., 2012, 41, 4833–4844
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Synthesis of ligands
(0.79 mmol, 150.6 mg) were dissolved in 25 mL of acetone.
Then 0.06 mL of 4-picoline was added and the reaction mixture
was refluxed for 24 h. Then it was cooled and extracted with
CH2Cl2 and washed with saturated aqueous NaHCO3 and water,
dried over MgSO4, concentrated in vacuo and purified by
column chromatography on silica gel (eluent: cyclohexane : ethyl
General procedure for synthesis of aldehydes 6–8. In a round
bottomed flask, phosphorus oxychloride (6 mmol, 0.55 mL) was
added to DMF (12 mL) at 0 °C and the mixture was stirred for
15 minutes. 2, 3 or 4 (1.5 mmol) was dissolved in 5 mL of DMF
and added to the previous solution over a period of 30 minutes
at 0 °C. The resulting red coloured mixture was stirred overnight
and then heated at 70 °C for 1 h. The reaction mixture was then
cooled and poured into ice. The yellow precipitates were formed
that were filtered and dried. The crude product was purified by
column chromatography on silica gel.
1
acetate 4 : 1) to afford green powdered product, yield: 81%. H
NMR (250 MHz, CDCl3), δ (ppm): 9.90 (s, 1H), 7.39 (m, 3H),
7.34 (d, J = 3.3 Hz, 1H), 7.22 (d, J = 7.0 Hz, 2H), 7.14 (d, J =
5.0 Hz, 1H), 7.04 (s, 1H), 6.96 (d, J = 3.1 Hz, 2H), 6.85 (t, J =
3.8 Hz, 1H), 6.61 (d, J = 3.34 Hz, 1H). 13C NMR (62.5 MHz,
CDCl3), δ (ppm): 186.6, 137.4, 137.2, 133.4, 131.7, 131.0,
129.6, 129.2, 128.8, 127.2, 127.0, 126.2, 125.7, 125.2, 108.0.
6: Yellow solid, yield: 50%. Eluent: cyclohexane : ethyl
1
acetate (4 : 1). H NMR (250 MHz, CDCl3), δ (ppm): 9.90 (s,
1H), 7.51 (dd, J = 8.7 and 2.8 Hz, 2H), 7.38 (dd, J = 5.0 and 1.3
Hz, 1H), 7.19 (dd, J = 5.2 and 1.1 Hz, 1H), 7.09 (dd, J = 8.6
and 2.8 Hz, 2H), 7.02 (s, 1H), 6.97 (m, 2H), 6.89 (dd, J = 5.1
and 3.7 Hz, 1H), 6.64 (dd, J = 3.7 and 1.1 Hz, 1H). 13C NMR
(62.5 MHz, CDCl3), δ (ppm): 186.5, 132.9, 131.6, 131.4, 129.4,
127.7, 127.6, 126.9, 126.4, 125.5, 108.5.
General procedure for preparation of 10a, 11a, 12a. 3, 4 or 5
(6.14 mmol) was dissolved in a mixture of 90 mL chloroform
and 9 mL of propanoic acid at −50 °C. To this solution N-bro-
mosuccinimide (6.45 mmol, 1.15 mg) was added. The reaction
mixture was stirred at this temperature for 1.5 hours. After that,
the reaction was quenched by adding water. The reaction mixture
was washed with 1% aqueous NaOH solution and water and
then dried over anhydrous MgSO4. Solvent was evaporated and
the crude product was used as such for next step without any
purification.
7: Yellow solid, yield: 65%. Eluent: cyclohexane : ethyl
1
acetate (2 : 1). H NMR (250 MHz, CDCl3), δ (ppm): 9.91 (s,
1H), 7.36 (dd, J = 4.5 and 1.8 Hz, 1H), 7.21 (dd, J = 2.8 and 1.8
Hz, 1H), 7.18 (dd, J = 3.5 and 2.2 Hz, 2H), 7.08 (dd, J = 3.5
and 2.2 Hz, 2H), 7.05 (s, 1H), 6.98 (m, 2H), 6.88 (dd, J = 5.1
and 3.6 Hz, 1H), 6.65 (dd, J = 3.6 and 1.1 Hz, 1H). 13C NMR
(62.5 MHz, CDCl3), δ (ppm): 188.3, 166.1, 162.0, 138.3, 133.0,
131.6, 131.4, 131.3, 131.1, 129.2, 129.0, 127.3, 127.1, 126.4,
125.6, 126.0, 125.3.
General procedure for preparation of 10, 11, 12. 10a, 11a or
12a (2 mmol) was dissolved in 25 mL of THF. To this solution
n-BuLi (1.6 M solution in hexanes; 3 mmol, 1.87 mL) was
added dropwise at −40 °C. Reaction progress was monitored by
TLC, after completion of metallation (about 1.5 h) the tempera-
ture was decreased at −78 °C and a THF solution of DMF
(8 mmol, 0.56 mL) was added dropwise. Temperature was
slowly raised to room temperature. The reaction mixture was
stirred overnight and quenched by adding water. After extraction
with CH2Cl2, the separated organic layer was washed with water
and dried over MgSO4. The crude product was purified by
column chromatography on silica gel.
8: Yellow solid, yield: 60%. Eluent: cyclohexane : ethyl
1
acetate (2 : 1). H NMR (250 MHz, CDCl3), δ (ppm): 9.92 (s,
1H), 7.33 (dd, J = 3.5 and 0.7 Hz, 1H), 7.16 (m, 5H), 7.06 (s,
1H), 6.96 (m, 2H), 6.84 (dd, J = 5.0 and 1.2 Hz, 1H), 6.61 (dd,
J = 3.5 and 2.7 Hz, 1H), 2.69 (t, J = 7.6 Hz, 2H), 1.63 (m, 2H),
1.35 (m, 6H), 0.93 (m, 3H). 13C NMR (62.5 MHz, CDCl3), δ
(ppm): 186.8, 131.7, 131.0, 129.3, 128.7, 127.2, 127.0, 126.0,
125.6, 125.1, 107.7, 36.0, 32.1, 31.5, 29.2, 23.1, 14.6.
9a: 6 (1 mmol, 414 mg), ethylene glycol (6 mmol, 0.33 mL)
and p-toluenesulfonic acid (0.025 mmol, 4.75 mg) were dis-
solved in 30 mL of toluene. The reaction mixture was refluxed
for 16 h under Dean–Stark conditions and then washed three
times with 1% aqueous NaOH solution and water respectively.
The organic phase was dried over anhydrous MgSO4, filtered
and evaporated under vacuum to afford 9a, yield: 94%. 1H
NMR (250 MHz, CDCl3), δ (ppm): 7.44 (d, J = 8.6 Hz, 2H),
7.25 (dd, J = 4.1 and 1.9 Hz, 1H), 7.11 (dd, J = 5.9 and 1.0 Hz,
1H), 7.05 (d, J = 8.6 Hz, 2H), 6.93 (m, 2H), 6.85 (dd, J = 5.1
and 3.6 Hz, 1H), 6.71 (s, 1H), 6.57 (dd, J = 3.6 and 1.0, 1H),
5.78 (s, 1H), 4.21 (m, 2H), 4.01 (m, 2H).
9: 9a (0.80 mmol, 370 mg) was dissolved in 12 mL of THF.
To this solution n-BuLi (1.5 M solution in hexanes) (0.96 mmol,
0.64 mL) was added dropwise at −40 °C. Reaction progress was
monitored by TLC, after completion of metallation (about
2 hours) the reaction mixture was hydrolyzed by adding water
and extracted with CH2Cl2 and the separated organic layer was
washed with water, dried over MgSO4 and evaporated under
vacuum to afford a green product that was subjected to the next
step without purification. The above obtained product
(0.66 mmol, 250 mg), 10 mL water and p-toluenesulfonic acid
10: Yellow solid, yield: 45%. Eluent: ethyl acetate :
cyclohexane (1 : 4). H NMR (200 MHz, CDCl3), δ (ppm): 9.86
1
(s, 1H), 7.57 (d, J = 4.0 Hz, 1H), 7.40 (d, J = 8.6 Hz, 2H), 7.34
(d, J = 8.6 Hz, 2H), 7.21 (dd, J = 5.2 and 0.9 Hz, 1H), 6.94 (dd,
J = 4.8 and 3.7 Hz, 1H), 6.86 (d, J = 4.0 Hz, 1H), 6.78 (d, J =
4.0 Hz, 1H), 6.71 (dd, J = 2.5 and 1.0 Hz, 1H), 6.68 (d, J = 3.9
Hz, 1H). 13C NMR (62.5 MHz, CDCl3), δ (ppm): 182.4, 144.9,
140.9, 136.9, 133.9, 133.1, 132.0, 129.6, 127.4, 125.4, 123.3,
117.6, 117.0, 112.8, 111.1.
11: Dark orange oil, yield: 55%. Eluent: ethyl acetate :
1
cyclohexane (1 : 4). H NMR (250 MHz, CDCl3), δ (ppm): 9.69
(s, 1H), 7.44 (d, J = 4.1 Hz, 1H), 7.25 (m, 4H), 7.09 (dd, J = 5.1
and 1.1 Hz, 1H), 6.83 (dd, J = 5.1 and 3.7 Hz, 1H), 6.77 (d, J =
4.0 Hz, 1H), 6.66 (d, J = 4.1 Hz, 1H), 6.59 (m, 2H), 2.51 (s,
3H). 13C NMR (62.5 MHz, CDCl3), δ (ppm): 182.3, 145.7,
140.7, 137.3, 135.4, 133.2, 130.9, 130.1, 129.6, 127.4, 125.1,
124.2, 112.7, 110.8, 21.8.
12: Green oil, yield: 43%. Eluent: ethyl acetate : cyclohexane
1
1 : 4). H NMR (250 MHz, CDCl3), δ (ppm): 9.75 (s, 1H), 7.43
(d, J = 4.1 Hz, 1H), 7.26 (m, 4H), 7.07 (d, J = 5.1 Hz, 1H), 6.81
(dd, J = 5.0 and 3.7 Hz, 1H), 6.76 (d, J = 4.0 Hz, 1H), 6.60 (m,
3 H), 2.72 (t, J = 7.4 Hz, 2H), 1.69 (m, 2H), 1.34 (m, 6H), 0.92
This journal is © The Royal Society of Chemistry 2012
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(t, J = 6.7 Hz, 3H). 13C NMR (62.5 MHz, CDCl3), δ (ppm):
182.78, 145.78, 140.85, 137.26, 135.68, 134.79, 133.37, 130.37,
129.95, 127.72, 125.33, 124.38, 112.82, 110.95, 36.37, 32.35,
31.94, 29.46, 23.29, 14.84.
131.3, 130.9, 129.5, 129.1, 127.4, 127.2, 126.9, 126.2, 125.2,
124.8, 124.1, 122.3, 120.2, 118.7, 106.4, 35.9, 32.1, 31.51, 29.1,
23.0, 14.5. HRMS calcd for C62H58N4S4 [M + H]+: 987.3617.
Found: 987.3544. Anal. Calcd for C62H58N4S4: C, 75.41; H,
5.92; N, 5.67; S, 12.99%. Found: C, 75.14; H, 5.53; N, 5.59; S,
12.64%. UV-vis (DMSO), λmax (nm) (ε (103 M−1 cm−1)) = 304
(41.6) and 381 (26.0).
DTP2-F: Starting from 10, light brown solid, yield: 73%. Due
to poor solubility in organic solvents, NMR spectra could not be
obtained. HRMS calcd for C50H32F2N4S4 [M + H]+: 855.1478.
Found: 855.1551. Anal. Calcd for C50H32F2N4S4: C, 70.23; H,
3.77; N, 6.55; S, 15.00%. Found: C, 69.96; H, 3.74; N, 6.36; S,
14.94%. UV-vis (DMSO), λmax (nm) (ε (103 M−1 cm−1)) = 316
(24.9) and 433 (59.5).
DTP2-Me: Starting from 11, light brown solid, yield: 46%.
Due to poor solubility in organic solvents, NMR spectra could
not be obtained. HRMS calcd for C52H38N4S4 [M + H]+:
847.1979. Found: 847.2052. Anal. Calcd for C52H38N4S4: C,
73.72; H, 4.52; N, 6.61; S, 15.14%. Found: C, 73.34; H, 4.53;
N, 6.43; S, 15.01%. UV-vis (DMSO), λmax (nm) (ε (103 M−1
cm−1)) = 318 (19.1) and 435 (48.2).
DTP2-Hex: Starting from 12, light brown solid, yield: 40%.
1H NMR (400 MHz, CDCl3), δ (ppm): 8.59 (d, J = 5.1 Hz, 2H),
8.43 (s, 2H), 7.42 (d, J = 16.0 Hz, 2H), 7.27 (m, 12H), 7.05 (dd,
J = 5.0 and 0.7 Hz, 2H), 6.85 (d, J = 3.84 Hz, 2H), 6.82 (dd, J =
4.9 and 3.7 Hz, 2 H), 6.71 (d, J = 16.0 Hz, 2H), 6.62 (d, J = 3.8
Hz, 2H), 6.56 (dd, J = 6.6 and 3.8 Hz, 2H), 6.39 (d, J = 3.8 Hz,
2H), 2.75 (t, J = 7.5 Hz, 4H), 1.71 (m, 4H), 1.35 (m, 12H), 0.91
(t, J = 6.7 Hz, 6H). 13C NMR (100 MHz, CDCl3), δ (ppm):
156.89, 149.95, 146.11, 145.15, 139.96, 136.46, 136.21, 135.14,
131.40, 130.31, 129.95, 129.71, 128.71, 127.18, 126.57, 124.63,
124.42, 124.20, 120.86, 117.78, 110.42, 110.05, 36.14, 32.14,
31.67, 29.22, 23.08, 14.56. HRMS calcd for C62H58N4S4 [M +
H]+: 987.3617. Found: 987.3614. Anal. Calcd for C62H58N4S4:
C, 75.41; H, 5.92; N, 5.67; S, 12.99%. Found: C, 75.14; H,
5.62; N, 5.46; S, 12.61%. UV-vis (DMSO), λmax (nm) (ε (103
M−1 cm−1)) = 323 (27.5) and 439 (56.1).
General procedure for synthesis of ligands from aldehydes
6–12. The aldehyde (0.43 mmol, 145 mg) and tetraethyl(4,4′-
diphosphonate-2,2′-bipyridine) (0.216 mmol, 98.4 mg) were dis-
solved in 10 mL of deoxygenated anhydrous THF. Solid potass-
ium tert-butoxide (0.64 mmol, 71.8 mg) was added rapidly. The
resulting solution was stirred at room temperature overnight.
Afterwards, methanol was added to the reaction mixture, brown
precipitates were formed that were filtered and washed with an
excess of methanol.
1
DTP1-H: Starting from 9, yellow solid, yield: 68%. H NMR
(250 MHz, DMSO-d6), δ (ppm): 8.64 (d, J = 4.8 Hz, 2H), 8.43
(s, 2H), 7.63 (d, J = 4.5 Hz, 2H), 7.54 (m, 12H), 6.32 (m, 6H),
7.19 (s, 2H), 7.10 (t, J = 3.4 Hz, 2H), 6.97 (m, 4H), 6.74 (d, J =
2.7 Hz, 2H). 13C NMR (62.5 MHz, DMSO-d6), δ (ppm): 155.8,
149.8, 146.1, 137.3, 133.4, 130.8, 130.4, 129.9, 129.4, 128.5,
127.3, 127.0, 125.9, 125.4, 124.2, 121.9, 120.5, 117.1, 106.6.
HRMS calcd for C50H34N4S4 [M + H]+: 819.1666. Found:
819.1734. Anal. Calcd for C50H34N4S4: C, 73.32; H, 4.18; N,
6.84; S, 15.66%. Found: C, 72.95; H, 4.17; N, 6.54; S, 15.31%.
UV-vis (DMSO), λmax (nm) (ε (103 M−1 cm−1)) = 311 (53.9)
and 352 (42.1).
DTP1-Br: Starting from 6, yellow solid, yield: 38%. 1H NMR
(400 MHz, CDCl3), δ (ppm): 8.58 (d, J = 5.1 Hz, 2H), 8.35 (s,
2H), 7.46 (dd, J = 8.6 and 1.8 Hz, 4H), 7.42 (d, J = 16.1, 2H),
7.33 (m, 4H), 7.18 (dd, J = 5.1 and 1.0 Hz, 2H), 7.07 (dd, J =
8.6 and 1.9 Hz, 4H), 7.0 (m, 4H), 6.90 (dd, J = 5.1 and 1.4 Hz,
2H), 6.87 (s, 2H), 6.85 (dd, J = 3.5 and 1.0 Hz, 2H), 6.61 (dd, J
= 3.6 and 1.0 Hz, 2H). 13C NMR (100 MHz, CDCl3), 157.2,
149.7, 146.9, 137.4, 134.2, 132.3, 131.5, 131.2, 131.0, 130.7,
129.8, 129.4, 129.0, 127.8, 127.3, 127.1, 125.8, 125.3, 124.5,
122.9, 120.3, 118.7, 107.0. HRMS calcd for C50H32Br2N4S4 [M
+ H]+: 974.9873. Found: = 974.9933. Anal. Calcd for
C50H32Br2N4S4: C, 61.48; H, 3.30; N, 5.73; S, 13.13%. Found:
C, 61.16; H, 3.47; N, 5.42; S, 12.76%. UV-vis (DMSO), λmax
(nm) (ε (103 M−1 cm−1)) = 309 (61.0) and 356 (42.4).
Synthesis of complexes
1
DTP1-F: Starting from 7, yellow solid, yield: 31%. H NMR
3 equiv. of ligand and 1 equiv. of RuCl2(DMSO)4 were sus-
pended into 10 mL of ethylene glycol (see product details for
exact amounts). The mixture was irradiated in the microwave
oven (250 W, 200 °C, 10 min). On cooling, the red solution was
poured into a saturated aqueous solution of KPF6. A few drops
of acetone were also added and left at room temperature over-
night. Complex was obtained as red solid that was filtered and
washed with water and diethyl ether.
(250 MHz, CDCl3), δ (ppm): 8.60 (d, J = 5.3 Hz, 2H), 8.45 (s,
2H), 7.39 (d, J = 16.1 Hz, 2H), 7.33 (dd, J = 4.1 and 1.0 Hz,
2H), 7.19 (m, 8H), 7.03 (m, 10H), 6.89 (m, 4H), 6.64 (dd, J =
2.6 and 1.0 Hz, 2H). 13C NMR (62.5 MHz, CDCl3), δ (ppm):
165.0, 161.1, 149.0, 133.9, 133.8, 133.8, 131.4, 131.3, 131.0,
130.1, 129.9, 127.7, 127.2, 127.1, 125.7, 125.4, 124.0, 122.6,
120.5, 119.3, 116.4, 116.1, 106.8. HMRS calcd for
C50H32F2N4S4 [M + H]+: 855.1478. Found: 855.1551. Anal.
Calcd for C50H32F2N4S4: C, 70.23; H, 3.77; N, 6.55; S, 15.00%.
Found: C, 69.92; H, 3.96; N, 6.85; S, 15.06%. UV-vis (DMSO),
λmax (nm) (ε (103 M−1 cm−1)) = 309 (52.5) and 360 (39.0).
DTP1-Hex: Starting from 8, yellow solid, yield: 36%, 1H
NMR (400 MHz, CDCl3), δ (ppm): 8.60 (d, J = 5.0Hz, 2H),
8.38 (s, 2H), 7.35 (d, J = 16.1 Hz, 2H), 7.30 (d, J = 5.1 Hz, 2H),
7.15 (m, 12H), 6.98 (m, 4H), 6.93 (s, 2H), 6.86 (t, J = 4.1 Hz,
4H), 6.61 (dd, J = 2.6 and 0.9 Hz, 2H), 2.67 (t, J = 7.6 Hz, 4H),
1.63 (m, 4H), 1.31 (m, 12H), 0.90 (m, 6H). 13C NMR
(62.5 MHz, CDCl3), 157.1, 149.8, 147.1, 144.3, 135.8, 134.8,
Ru(DTP1-H): Dark red solid, yield: 76%. 1H NMR
(400 MHz, DMF-d7), δ (ppm): 9.03 (s, 6H), 7.92 (d, J = 5.9 Hz,
6H), 7.69 (m, 12H), 7.50 (m, 24H), 7.38 (d, J = 6.7 Hz 12H),
7.25 (d, J = 15.9 Hz, 6H), 7.12 (s, 6H), 7.03 (m, 12H), 6.97 (t, J
= 4.5 Hz, 6H), 6.75 (d, J = 2.8 Hz, 6H). HRMS (ESI): calcd for
C
150H102N12RuS12 m/z = 1278.2016 [M − 2PF6]2+. Found:
1278.1987. UV-vis (CH3CN), λmax (nm) (ε (103 M−1 cm−1)) =
308 (132.8), 396 (62.2) and 495 (50.0).
Ru(DTP1-Br): Dark red solid, yield: 79%. 1H NMR
(400 MHz, CD3CN), δ (ppm): 8.43 (s, 6H), 7.56 (m, 24H), 7.46
(m, 12H), 7.32 (m, 12H), 7.19 (d, J = 8.2 Hz, 12H), 7.10 (d, J =
4842 | Dalton Trans., 2012, 41, 4833–4844
This journal is © The Royal Society of Chemistry 2012

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16.5 Hz, 6H), 7.01 (s, 6H), 6.93 (m, 12H), 6.74 (d, J = 2.6 Hz,
6H). HRMS (ESI): calcd for C150H96Br6N12RuS12 m/z =
1515.9319 [M − 2PF6]2+. Found: 1515.9256. UV-vis (CH3CN),
λmax (nm) (ε (103 M−1 cm−1)) = 264 (112.5), 390 (46.6) and
488 (33.1).
helpful measurements. Funding from PRIN 2008 (UNIFE) is
also gratefully acknowledged.
Notes and references
Ru(DTP1-F): Dark red solid, yield: 75%. 1H NMR
(400 MHz, DMF-d7), δ (ppm): 9.06 (s, 6H), 7.93 (d, J = 6.0 Hz,
6H), 7.69 (m, 12H), 7.48 (m, 24H), 7.35 (t, J = 8.4 Hz, 12H),
7.20 (d, J = 16.0 Hz, 6H), 7.14 (s, 6H), 7.08 (m, 12H), 7.03 (dd,
J = 5.1 and 1.4 Hz, 6H), 6.85 (d, J = 3.6 Hz, 6H). HRMS (ESI):
calcd for C150H96F6N12RuS12 m/z = 1332.1751 [M − 2PF6]2+.
Found: 1332.1518. UV-vis (CH3CN), λmax (nm) (ε (103 M−1
cm−1)) = 306 (64.4), 398 (28.2) and 490 (20.3).
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Ru(DTP1-Hex): Dark red solid, yield: 63%. 1H NMR
(400 MHz, DMF-d7), δ (ppm): 9.20 (s, 6H), 8.09 (d, J = 6 Hz,
6H), 7.82 (m, 12H), 7.62 (d, J = 4.8 Hz, 6H), 7.48 (m, 36H),
7.28 (s, 6H), 7.20 (m, 12H), 7.13 (t, J = 4.8 Hz, 6H), 6.93 (d, J
= 3.1 Hz, 6H), 2.85 (t, J = 7.0 Hz, 12H), 1.78 (m, 12H), 1.46
(m, 36H), 1.04 (m, 18H). HRMS (ESI): calcd for
C
186H174N12RuS12 m/z = 1530.4833 [M − 2PF6]2+. Found:
1530.4938. UV-vis (CH3CN), λmax (nm) (ε (103 M−1 cm−1)) =
306 (48.3), 397 (63.1) and 489 (48.3).
Ru(DTP2-F): To remove unreacted ligand resultant product
was dissolved in acetone. Complex was soluble whereas ligand
remained insoluble. The solution was filtered and complex was
again precipitated with saturated aqueous solution of KPF6. Dark
red precipitate was obtained and was filtered and washed with
1
water and diethyl ether, yield: 65%. H NMR (400 MHz, DMF-
d7), δ (ppm): 9.25 (s, 6H), 8.15 (m, 12H), 7.82 (m, 12H), 7.65
(t, J = 7.4 Hz, 12H), 7.58 (d, J = 4.7 Hz, 12H), 7.38 (d, J = 2.9
Hz, 6H), 7.15 (m, 12H), 7.00 (dd, J = 8.0 and 3.7 Hz, 12H),
6.92 (dd, J = 8.1 and 3.6 Hz, 12H). HRMS (ESI): calcd for
C
150H96F6N12RuS12 m/z = 1332.1751 [M − 2PF6]2+. Found:
1332.1698. UV-vis (CH3CN), λmax (nm) (ε (103 M−1 cm−1)) =
452 (72.3) and 508 (72.7).
Ru(DTP2-Me): Unreacted ligand was removed by following
the same method as for Ru(DTP2-F). Dark red solid was
1
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obtained, yield: 40%. H NMR (400 MHz, DMF-d7), δ (ppm):
9.07 (s, 6H), 8.01 (m, 12H), 7.64 (m, 6H), 7.38 (m, 30H), 7.15
(d, J = 3.8 Hz, 6H), 6.92 (m, 6H), 6.86 (d, J = 15.5 Hz, 6H), 6.76
(m; 12H), 6.69 (d, J = 3.3 Hz, 6H), 6.63 (d, J = 3.3 Hz, 6H), 2.46
(s, 18H). HRMS (ESI): calcd for C156H114N12RuS12 m/z =
1320.2502 [M − 2PF6]2+. Found: 1320.2426. UV-vis (CH3CN),
λmax (nm) (ε (103 M−1 cm−1)) = 451 (62.5) and 512 (67.9).
Ru(DTP2-Hex): Dark red solid, yield: 66%. 1H NMR
(400 MHz, DMF-d7), δ (ppm): 9.12 (s, 6H), 7.95 (m, 12H), 7.66
(d, J = 5.9 Hz, 6H), 7.44 (m, 30H), 7.15 (d, J = 3.7 Hz, 6H),
6.90 (t, J = 4.7 Hz, 6H), 6.85 (d, J = 15.8 Hz, 6H), 6.80 (m,
12H), 6.73 (d, J = 3.7 Hz, 6H), 6.70 (d, J = 3.7 Hz, 6H), 2.75
(m, 12H), 1.71 (m, 12H), 1.27 (m, 36H), 0.77 (t, J = 6.4 Hz,
18H). HRMS (ESI): calcd for C186H174N12RuS12 m/z =
1530.4833 [M − 2PF6]2+. Found: 1530.4808. UV-vis (CH3CN),
λmax (nm) (ε (103 M−1 cm−1)) = 360 (62.7), 443 (75.9) and 513
(78.3).
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Acknowledgements
The authors would like to thank Brigitte Fernette (NMR), Sté-
phane Parant (UV-vis) and François Dupire (Mass) for their
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45 In order to get
wavelength region,
a
a
more detailed description of the shorter
much higher number of states should be
computed, thus leading to a significant increase in the computational
cost.
46 Zinc–polypyridine complexes are known to display strong ILCT tran-
sitions and no MLCT due to the high third ionization potential of zinc
and therefore are useful to identify ligand-based processesupon excitation
in ruthenium complexes. See for example: T. Renouard and H. Le Bozec,
Eur. J. Inorg. Chem., 2000, 229–239.
4844 | Dalton Trans., 2012, 41, 4833–4844
This journal is © The Royal Society of Chemistry 2012