Luminescent cyclometallated Ir(III) complexes of conjugatable carboxy-functionalized ligandsf

Article abstract of DOI:10.1039/b001670p

Novel bifunctional ligands based on a 6′-phenyl-2,2′-bipyridine moiety are synthesised and used to prepare iridium(m) complexes [Ir(ppy)₂(L)]⁺ (ppy = 2-phenylpyridlne anion; L = 1,2, and 4) which have the potential to serve as lumine

Full text of DOI:10.1039/b001670p

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Luminescent cyclometallated Ir(III) complexes of conjugatable
carboxy-functionalized ligands†
Francesco Neve,*a Alessandra Crispini,a Frédérique Loiseaub and Sebastiano Campagnab
a Dipartimento di Chimica, Università della Calabria, I-87030 Arcavacata di Rende (CS),
Italy. E-mail: f.neve@unical.it
b Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica,
Università di Messina, via Sperone 31, I-98166 Messina, Italy
Received 1st March 2000, Accepted 16th March 2000
Published on the Web 6th April 2000
Novel bifunctional ligands based on a 6Ј-phenyl-2,2Ј-
bipyridine moiety are synthesised and used to prepare
iridium(III) complexes [Ir(ppy)2(L)]؉ (ppy = 2-phenyl-
pyridine anion; L ؍ 1, 2, and 4) which have the potential to
serve as luminescent probes.
Since the pioneering work by Meares et al.,1 bifunctional
chelates have been designed to accomplish many different
functions. While the first component of the chelate is usually
able to coordinate a metal ion, the second (and often remote)
functional group must guarantee a potentially different per-
formance such as conjugation with biomolecules, grafting
on a solid support, or pH-dependent activity. Besides their
early use in the delivery of radiometals,1,2 bifunctional chelates
have been also primary tools for the construction of tethered
DNA intercalators,3 metallo-dendrimers,4 photosensitizers
for solar cells,5 pH sensors,6 and self-assembling organo-
metallic polymers7 amongst others. Nevertheless, the incorpor-
ation of a functional metal complex into a biomolecule for
analytical or therapeutic applications is still one of the major
targets.8
Within the latter approach, the search for luminescent metal
probes has already produced excellent examples of labeling via
transition metal complexes. While most of the candidates for
labeling experiments so far reported contained Ru() as the
photoactive center,9 we thought it would be of interest to
investigate whether functionalized Ir() cyclometallated com-
plexes could be a likely alternative.
The ligand 4Ј-(4-carboxyphenyl)-6Ј-phenyl-2,2Ј-bipyridine
(2) recently proved capable of chelating an “Ir(ppy)2” fragment
(ppy = 2-phenylpyridine anion) by using two of the potentially
available tridentate sites (i.e. the bipyridyl moiety) affording a
strongly luminescent chromophore (Φ = 0.017 in acetonitrile
solution at room temperature).10 Following this result, our next
target molecules were ligands still bearing a 6Ј-phenyl-2,2Ј-
bipyridine (hereafter pbpy) moiety on one side and a carboxyl
function on the opposite side. We have therefore prepared
two new bifunctional ligands where the COOH group is either
directly bound to the pbpy fragment in the 4Ј position (ligand 1)
or sitting at the end of a flexible spacer (ligand 4, Scheme 1). In
our opinion the series of ligands 1, 210 and 4 may then ensure
a modulation of the chemical reactivity of the conjugatable
function while keeping the coordinating abilility of the pbpy
fragment rather similar. We report here the synthesis of both
the ligands and the corresponding series of [Ir(ppy)2(L)]ϩ com-
plexes (L = 1, 2, and 4), and on the photophysical properties of
the Ir() derivatives.
N-(2-pyridacyl)pyridinium iodide11 in the presence of excess
ammonium acetate.12 Scheme 1 displays the synthetic procedure
for ligand 4.‡ Coupling of the protected 1-bromo-5-carb-
oxypentane with the chalcone I afforded the intermediate II,
which in turn was reacted with N-(2-pyridacyl)pyridinium
iodide under Kröhnke conditions.12 The final carboxylic acid 4
was obtained upon deprotection of the corresponding methyl
ester 3 by a standard method.
A room temperature reaction of 1 with [Ir(ppy)2Cl]2 13 (molar
ratio 2:1) in CH2Cl2–MeOH followed by counterion exchange
to PF6Ϫ gave the desired complex [Ir(ppy)2(1)]PF6 in high yield
(ESI).† Reflux conditions were however necessary for the
preparation of [Ir(ppy)2(4)]PF6.† For the latter species, it was
also observed that occasionally a partial esterification of the
carboxy group of 4 occurred (clearly shown by 1H NMR spec-
troscopy), which was promoted by the solvent. Since the
presence of methanol could not be avoided in the reaction
mixture, a pure [Ir(ppy)2(4)]PF6 species was easily recovered
upon subsequent hydrolysis under basic conditions. [Ir-
(ppy)2(1)]PF6 and [Ir(ppy)2(4)]PF6 are very soluble in chlorin-
ated solvents and acetonitrile. In fact, it should be noted that
[Ir(ppy)2(1)]PF6 is somewhat labile in acetonitrile solution,
leading to partial dissociation of ligand 1. Therefore this
solvent could not be used for spectroscopic studies. Both [Ir-
(ppy)2(1)]PF6 and [Ir(ppy)2(4)]PF6 were fully characterized by
analytical, spectrometric, and spectroscopic techniques.†
Ϫ
The absorption spectra of the PF6 salts of [Ir(ppy)2(1)]ϩ,
[Ir(ppy)2(2)]ϩ, and [Ir(ppy)2(4)]ϩ in dichloromethane solution
are characterized by intense spin-allowed ligand-centered (LC)
bands with maxima in the 260–280 nm range and by moder-
ately intense spin-allowed metal-to-ligand charge-transfer
(MLCT) bands at lower wavelengths (Table 1). Spin-forbidden
MLCT bands are also present in the visible region. Based on a
comparison with literature data,14 the lowest-energy MLCT
transition is always assigned to the coordinated pbpy fragment.
It is worth noting that the MLCT bands of [Ir(ppy)2(1)]ϩ are
at significantly lower energy than the corresponding bands of
[Ir(ppy)2(2)]ϩ and [Ir(ppy)2(4)]ϩ. This may be explained con-
sidering that the electron withdrawing COOH group has a more
significant effect on the electronic properties of the pbpy moiety
in [Ir(ppy)2(1)]ϩ where it is directly linked to the polypyridine
backbone, than in [Ir(ppy)2(2)]ϩ and [Ir(ppy)2(4)]ϩ, in which
In the preparation of ligand 1,‡ 3-benzoylacrylic acid
was converted directly into the final product by reaction with
† Electronic supplementry information (ESI) available: synthetic details
and further characterization data for [Ir(ppy)2(1)]PF6 and [Ir(ppy)2(4)]-
PF6. See http://www.rsc.org/suppdata/dt/b0/b001670p/
DOI: 10.1039/b001670p
J. Chem. Soc., Dalton Trans., 2000, 1399–1401
This journal is © The Royal Society of Chemistry 2000
1399

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Table 1 Spectroscopic and photophysical dataa
Luminescence, 298 Kb 77 K
Absorptionb
λmax/nm (ε/MϪ1 λmax
/
λmax/
nm
Compound
cmϪ1
)
nm
τ/ns
Φ
τ/µs
[Ir(ppy)2(1)]ϩ 267 (38500)
378 (6300)
735
392
9 × 10Ϫ4 585c 3.9c
402 (4600)
515 (400)
[Ir(ppy)2(2)]ϩ 270 (55500)
292 (44600)
640
606
249
345
0.044
0.074
565d 3.3d
537c 3.7c
388 (7300)
480 (800)
[Ir(ppy)2(4)]ϩ 273 (60200)
295 (46300)
385 (11200)
482 (800)
a Data for deoxygenated solutions. Absorption maxima (or shoulders)
of the spin-allowed LC and MLCT bands are given. b In CH2Cl2. c In
butyronitrile. d In MeOH–EtOH 4:1 (v/v).
Fig. 1. Room temperature luminescence spectra of: (a) [Ir(ppy)2(1)]ϩ,
(b) [Ir(ppy)2(2)]ϩ, and (c) [Ir(ppy)2(4)]ϩ in dichloromethane solution.
Scheme 1 Reagents and conditions: (i) MeOH (10 equiv.), H2SO4 (0.25
equiv.), reflux, 5 h, 92%. (ii) K2CO3 (5 equiv.), KI (0.1 equiv.), acetone,
reflux, 5 h, 65%. (iii) NH4OAc (10 equiv.), MeOH, reflux, 16 h, 63%.
(iv) KOH (4.5 equiv.), THF–water (2:1, v/v), reflux, 4 h; the reaction
mixture is then acidified with 2 N HCl (85%).
not strictly homogeneous as far as the deactivation processes
are concerned, and further investigation is needed to fully clarify
the photophysical data.
In summary, we have prepared two new functional Ir com-
plexes that, together with the already available [Ir(ppy)2(2)]ϩ
species, constitute a family of luminescent species bearing a
COOH functionality at varying distance from the photoactive
subunit. Modulation of the spectroscopic and photophysical
properties of the complexes was clearly obtained. Because of
the known reactivity of the carboxyl group, the reported species
look promising candidates to be interfaced with suitable
substrates for the generation of multicomponent systems
featuring sensing properties triggered by light.
interposed subunits are present. As a consequence, the acceptor
orbital of the MLCT transition is more stabilized in [Ir(ppy)2-
(1)]ϩ than in [Ir(ppy)2(2)]ϩ and [Ir(ppy)2(4)]ϩ, and the corre-
sponding absorption bands are displaced accordingly.
The luminescence spectra of [Ir(ppy)2(1)]ϩ, [Ir(ppy)2(2)]ϩ and
[Ir(ppy)2(4)]ϩ (Table 1, Fig. 1) can be safely assigned to triplet
Ir→pbpy CT excited states.14 Once again, the separation
between the photoactive site and the COOH functional group
is responsible for the differences among the complexes. The
luminescence spectra of [Ir(ppy)2(1)]ϩ (both in solution at room
temperature and in a rigid matrix at 77 K) are noticeably red
shifted with respect to the other two complexes, as a con-
sequence of the effect of the proximity between the acceptor
orbital of the MLCT excited state and the electron withdrawing
COOH group.
Acknowledgements
We acknowledge the financial support from the Italian
MURST, the CNR, and the European Community TMR
Program (Research Network on Nanometer Size Metal
Complexes). F. N. also thanks Dr A. De Nino for the mass
spectra.
Although smaller, a similar difference exists between the
emission spectra of [Ir(ppy)2(2)]ϩ and [Ir(ppy)2(4)]ϩ. This can
also be attributed to the electron withdrawing ability of the
substituents of the pbpy acceptor ligand in the two complexes.
Whereas the excited-state energy difference is fully rationalized,
the difference among the luminescence lifetimes and quantum
yields of the three complexes is not straightforward to explain.
The energy gap law,10,15 which states that the radiationless
deactivations become faster on decreasing excited state energy,
is not able to justify the data. Most likely the complexes are
Notes and references
‡ Synthetic details for the preparation of ligands 1 and 4 will be
reported elsewhere. Both ligands were characterized by 1H NMR
analysis, melting point and elemental analysis.
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