Photophysical properties of osmium(II) complexes with the novel 4'-p-phenylterpyridine-triarylpyridinium ligand

Article abstract of DOI:10.1039/a901237k

A new family of electron acceptor ligands complexed with osmium(II) and associated with appropriate electron donor subunits within heteroleptic compounds, results in multicomponent assemblies of potential interest for both synthetic chemistry and supramol

Full text of DOI:10.1039/a901237k

Photophysical properties of osmium(ii) complexes with the novel
4A-p-phenylterpyridine-triarylpyridinium ligand
Philippe Lainé* and Edmond Amouyal
Molecular Photonics Group, Laboratoire de Physico-Chimie des Rayonnements, CNRS UMR 8610, Université
Paris-Sud, Bâtiment 350, 91405 Orsay, France. E-mail: philippe.laine@lpcr.u-psud.fr
Received (in Basel, Switzerland) 14th February 1999, Accepted 11th April 1999
A new family of electron acceptor ligands complexed with
osmium(ii) and associated with appropriate electron donor
subunits within heteroleptic compounds, results in multi-
component assemblies of potential interest for both synthetic
chemistry and supramolecular photochemistry.
derivatives are known to undergo charge transfer reactions
when covalently linked to some donor groups as is the case for
solvatochromic Reichardt’s dyes like betaine 30,5,6 the pro-
moted electron being located on the pyridinium moiety. Our
proposal is thus to replace the latter donor by a light-triggered
electron donor, namely an inorganic chromophore (P) of the Ru/
Os-bis(terpyridyl) type.
As part of our studies devoted to photochemical conversion of
solar energy1 we are interested in producing long-lived
photoinduced charge separation (CS) states within inorganic
supramolecular species. Determining criteria that govern intra-
molecular electron transfers (ET) in multicomponent systems
(i.e. polyads) have already been addressed in previous work.2,3
A representative composition for a polyad system devised for
CS purposes consists of a linear disposition of peripheral
electron donating (D) and withdrawing (A) groups borne by a
central photosensitiser P in a trans configuration e.g. D–P–A
(triad).4 Spacers do play an important role in such constructions
by providing the rigidity of the architecture and mediating the
electronic coupling between the components. Regarding non-
porphyrinic inorganic polyads, widespread strategy is to
modulate the efficiency of A (and/or D) vs. intramolecular ET
by varying the length and nature of saturated spacers; our
approach hereby described is to preferably involve intrinsically
weak electron acceptors fairly strongly coupled to P rather than
the usual good acceptors almost insulated from P. The aim
remains the same in both cases i.e. favour the formation of the
CS state D+–P–A2 by retarding the charge recombination
(CR).
It is noteworthy that R1 R2TP+-pterpy (A) specifically fits to
2
polyad architectures where the electron-donating part (D) is
built from 4A-(4-N,N-disubstituted-aminophenyl)terpy (R2N-
pterpy, R = alkyl, aryl). In such a structure, both pyridinio and
amino nitrogen atoms are located at equal distance dMN from the
metal centre and connected to it with identical spacers (Scheme
1). From a topological viewpoint, the CS-state lifetime is
expected to be increased in this particular arrangement that
allows an optimal competition between the possible reductive
quenching reactions of oxidised photosensitiser (D–P+–A2
?
D+–P–A2 vs. D–P+–A2 ? D–P–A). Another advantage of
R1 R2TP+-pterpy ligands comes from the tuneable redox
2
properties of the pyridinium moiety5,7 the potential of which
can be varied systematically by altering substituents R1 and
R2.
These acceptor-modified terpyridyl ligands (A) are readily
synthesised following a general convergent approach8 consist-
ing in separately preparing the triarylpyrylium salt and the
amino derivative of the chelating polyimine which are subse-
quently reacted together to afford the target molecule in
reasonable yield (ca. 60% vs. amine). The chemical variability
Based on hitherto established structural and energetic
requirements, we have designed the new 4A-[4-N-(2,4,6-tri-
of R1 R2TP+-pterpy actually stems from pyrylium precursors,
2
the chemistry of which is very rich and also well documented
from the synthetic viewpoint.9 In addition, this route to
pyridinium derivatives can be successfully applied to other
oligopyridine molecules.10 Typically, the first element of the
ligand family (R1 = R2 = H), H3TP+-pterpy, is obtained by the
reaction of 4A-p-aminophenyl-terpy (Na2S2O4-reduced 4A-p-
arylpyridinio)phenyl]-2,2A:6A,2B-terpyridine ligand (R1 R2TP+-
2
pterpy) depicted in Scheme 1. Related triphenylpyridinium
R1
2
nitrophenyl-terpy) with a triphenylpyrylium cation (HSO4
salt, 1 equiv.) in refluxing EtOH for 6 h in the presence of
anhydrous NaOAc. Regarding the electron-donor component,
the model ligand used is 4A-p-dimethylaminophenyl-terpy
(Me2N-pterpy) synthesised following the classical method for
para-substituted phenyl terpy.11 Up to 12 RuII and OsII
complexes were then synthesised corresponding to the different
combinations between the following three ligands: H3TP+-
pterpy, Me2N-pterpy and 4A-p-tolyl-terpy (tterpy); after stan-
N
N
N
R2
+
N
R1
R12R2TP+-pterpy
2
dard work-up purification these compounds prepared as PF6
salts afforded microcrystalline powders which were fully
characterised;† experimental details will be given elsewhere.10b
Owing to their better emission properties at room temperature
than that of analogous RuII complexes, preliminary photo-
physical results are given for the OsII complexes only i.e.
[Os(tterpy)2]2+ 1, [Os(Me2N-pterpy)2]2+ 2, [Os(pterpy-
3+
R1
M = Ru2+, Os2+
N
N
N
N
R
N
R
N
M
N
N
+
R2
+
TPH3 )2]4+ 3, [(Me2N-pterpy)Os(tterpy)]2+ 4, [(tterpy)Os-
+
+
(pterpy-TPH3 )]3+ 5 and [(Me2N-pterpy)Os(pterpy-TPH3 )]3+
6. Properties of two H3TP+-pterpy complexes 5 and 6,
compared to that of 1 are more specifically discussed.
R1
As illustrated in Fig. 1 for this series, the luminescence is
shown to be gradually weakened when going from 1 to 6.
Compared to the reference chromophore 1 with an emission
dMN
dMN
Scheme 1
Chem. Commun., 1999, 935–936
935

elucidate whether the luminescence behaviour of this series
results from effective intra-molecular ET or an alteration of the
electronic properties of the complexes due to their inter-
component coupling.14
The chemical variability provided by R1 and R2 (Scheme 1)
allows the modulation of the electron withdrawing ability of the
acceptor as well as leading to the opportunity to expand the
structure along the C2-axis (via R2) in polyads such as D–P–A1–
A2 (redox cascade). Hence, R1 R2TP+-pterpy is a promising
2
candidate to perform sequential and vectorial long-range ET
over extended rod-like rigid systems engineered for CS.
Helpful discussions with Dr Valérie Marvaud were much
appreciated by P. L.
Notes and references
† All new compounds exhibited satisfactory elemental, spectroscopic (1H
and 13C NMR), spectral (UV–VIS–NIR) and mass (ES-MS for 2–6 and CI-
MS for H3TP+-pterpy) data.
Fig. 1 Emission spectra of OsII complexes in deaerated MeCN solutions at
room temperature, measured in identical experimental conditions (in
particular, same OD at lexc. = 600 nm).
Selected spectroscopic data for H3TP+-pterpy. dH (Bruker AC300, 300
MHz, CDCl3): 8.65 (d, J 4.8 Hz, 2 H), 8.57 (d, J 7.9 Hz, 2H), 8.46 (s, 2 H),
8.15 (s, 2 H), 7.94 (d, J 8.0 Hz, 2 H), 7.83 (dd, J 7.9, 7.6 Hz, 2 H), 7.75 (d,
J 8.7 Hz, 2 H), 7.62 (m, 4 H), 7.59 (d, J 8.7 Hz, 2 H), 7.54 (m, 3 H), 7.33
(dd, J 4.8 Hz, 2 H), 7.29 (m, 6 H). For 6: dH(300 MHz, CD3CN): 8.98 (s,
2 H), 8.81 (s, 2 H), 8.60 (d, J 8.0 Hz, 2 H), 8.57 (d, J 8.3 Hz, 2 H), 8.50 (s,
2 H), 8.17 (dd, J 7.8, 1.4 Hz, 2 H), 8.08 (d, J 8.9 Hz, 2 H), 7.98 (d, J 8.6 Hz,
2 H), 7.80 (m, 4 H), 7.73 (m, 3 H), 7.62 (d, J 8.6 Hz, 2 H), 7.54 (m, 4 H),
7.47 (m, 6 H), 7.34 (d, J 5.4 Hz, 2 H), 7.18 (d, J 5.0 Hz, 2 H), 7.12 (dd, J
7.3, 5.7 Hz, 2 H), 7.04 (d, J 8.8 Hz, 2 H), 7.03 (dd, J 5.9 Hz, 2 H), 3.15 (s,
6 H). For 6 ES-MS (modified NERMAG R10-10 quadrupole mass
Table 1 Luminescence propertiesa
lmaxb/nm
102Fc
Irelc,d(%)
te/ns
1
4
5
6
734
747
750
764
2.00
1.48
1.02
0.24
100
247
206
168
57
74.3
50.9
12.1
spectrometer, solvent: MeCN): m/z 1448 (190Os) and 1450 (192Os), [M 2
a Room temperature measurements; deaerated MeCN solutions. b Emission
2 3+
PF62]+; 652 [M 2 2PF62 2+; 386 [M 2 3PF6
] ] .
c
maxima for uncorrected spectra. Luminescence quantum yields were
Selected electrochemical data [differential pulse voltammetry, E1/2/V vs.
determined relative to [Os(bpy)3]2+ (Fem = 5 3 1023 in MeCN at 298 K).12
SSCE in freshly distilled MeCN/0.1 m TBAPF6 at 298 K]. For 6: E(OsIII/II
)
d
e
Irel
=
100 3 [Fem(species)/Fem(1)].
Determined by laser flash
= +1.03 (rev.), E(dma+/0) = +0.77 (rev.), E(TP+/0) = 20.93 (rev.),
E(TP0/21) = 21.01 (rev.), E(terpy0/21) = 21.24 (rev.).
photolysis, lexc = 308 nm.13
1 E. Amouyal, in Homogeneous Photocatalysis, ed. M. Chanon, John
Wiley, Chichester, 1997, ch. 8, 263; E. Amouyal, Sol. Energy Mater.
Sol. Cells, 1995, 38, 249.
2 V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis
Horwood, Chichester, 1991.
3 J.-P. Sauvage, J.-P. Collin, J.-C. Chambron, S. Guillerez, C. Coudret, V.
Balzani, F. Barigelletti, L. De Cola and L. Flamigni, Chem. Rev., 1994,
94, 993; V. Balzani, A. Juris, M. Venturi, S. Campagna and S. Serroni,
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4 E. C. Constable, Adv. Inorg. Chem. Radiochem., 1986, 30, 69; J.-P.
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9 A. T. Balaban, A. Dinculescu, G. N. Dorofeyenko, G. W. Fischer, A. V.
Koblik, V. V. Mezheritskii and W. Schroth, in Pyrylium Salts:
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Supplement 2.
quantum yield of 2.0 3 1022, 3MLCT emission of the D–P–A
triad 6 is ca. 90% quenched and lifetime shortened to 57 ns
(Table 1).
Note that the contribution of the donor fragment to the
quenching phenomenon should not be overestimated as re-
vealed by Irel values for 2 and 4: 80.5 and 74.3%, respectively.
A synergetic effect of electroactive subunits D and A upon
emission is, however, clearly identified in the case of 6 when
compared with luminescence properties of heteroleptic com-
pounds 4 and 5.
The rate constant kQ for the bimolecular quenching reaction
(1) of the excited state of the reference chromophore 1 (*P) by
the model-acceptor species N-phenyl-2,4,6-triphenylpyridin-
+
ium (p-TPH3 ) was experimentally determined (Stern–Volmer
plot). This kQ value of ca. 6 3 107 dm3 mol21 s21 (with tem of
1 = 247 ns) cannot account for the more efficient quenching
process observed within the dyads and triad; therefore, the
effective contribution of an inter-molecular ET to the quenching
mechanism is negligible.
10 (a) Work in progress; (b) manuscript in preparation.
11 W. Spahni and G. Calzaferri, Helv. Chim. Acta, 1984, 67, 450.
12 E. M. Kober, J. V. Caspar, R. S. Lumpkin and T. J. Meyer, J. Phys.
Chem., 1986, 90, 3722.
13 E. Amouyal, M. Mouallem-Bahout and G. Calzaferri, J. Phys. Chem.,
1991, 95, 7641.
+
•
*[Os(tterpy)2]2+ + p-TPH3 ? [Os(tterpy)2]3+ + p-TPH3 (1)
In addition, owing to photophysical properties of organic
colourless D and A electroactive fragments (UV-absorbing)
with respect to that of *P (NIR–VIS-emitting MLCT state at
ca. 740 nm), the above reported quenching effect cannot
originate from energy transfer.
Further photophysical investigations at low temperature and
picosecond time-scale are currently under way in order to
3
14 M. Maestri, N. Armaroli, V. Balzani, E. C. Constable and A. M. W.
Cargill Thompson, Inorg. Chem., 1995, 34, 2759.
Communication 9/01237K
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Chem. Commun., 1999, 935–936