Conformationally gated photoinduced processes within photosensitizer- acceptor dyads based on osmium(II) complexes with triarylpyridinio- functionalized terpyridyl ligands: Insights from experimental study

Article abstract of DOI:10.1021/ja058357w

[(ttpy)Os(tpy-ph-TPH₃⁺)]³⁺ (2), [(ttpy)Os(tpy-xy-TPH₃⁺)]³⁺ (3), [(ttpy)Os(tpy-ph-TPH₂(NO₂)⁺)]³⁺ (4), and [(ttpy)Os(tpy-xy-TPH₂(NO

Full text of DOI:10.1021/ja058357w

Published on Web 05/18/2006
Conformationally Gated Photoinduced Processes within
PhotosensitizersAcceptor Dyads Based on Osmium(II)
Complexes with Triarylpyridinio-Functionalized Terpyridyl
Ligands: Insights from Experimental Study
Philippe P. Laine´,*,† Fethi Bedioui,‡ Fre´de´rique Loiseau,§ Claudio Chiorboli,| and
Sebastiano Campagna*,§
Contribution from the Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques,
CNRS UMR-8601, UniVersite´ Rene´ Descartes, 45 rue des Saints Pe`res, F-75270, Paris Cedex
06, France; Laboratoire de Pharmacologie Chimique et Ge´ne´tique, CNRS UMR-8151 +
INSERM U-640, EÄ cole Nationale Supe´rieure de Chimie de Paris, 11 rue P. et M. Curie,
F-75231 Paris Cedex 05, France; UniVersita` di Messina, Dipartimento di Chimica Inorganica,
Chimica Analitica e Chimica Fisica, Via Sperone 31, I-98166 Messina, Italy, and ISOF-CNR,
Sezione di Ferrara, 44100 Ferrara, Italy
Received December 9, 2005; Revised Manuscript Received March 28, 2006; E-mail: philippe.laine@univ-paris5.fr; campagna@unime.it
Abstract: [(ttpy)Os(tpy-ph-TPH3+)]3+ (2), [(ttpy)Os(tpy-xy-TPH3+)]3+ (3), [(ttpy)Os(tpy-ph-TPH2(NO2)+)]3+
(4), and [(ttpy)Os(tpy-xy-TPH2(NO2)+)]3+ (5) are a series of dyads made of an Os(II) bis-tpy complex (tpy
) 2,2′:6′,2”-terpyridine) as the photosensitizer (P) and 2,4,6-triarylpyridinium group (TP+) as the electron
acceptor (A). These dyads were designed to form charge-separated states (CSS) upon light excitation.
Together with analogous Ru(II) complexes (7-10), they have been synthesized and fully characterized.
We describe herein how intramolecular photoinduced processes are affected when the electron-accepting
strength of A (by nitro-derivatization of TP+) and/or the steric hindrance about intercomponent linkage (by
replacing a phenyl spacer by a xylyl one) are changed. Electronic absorption and electrochemical behavior
revealed that (i) chemical substitution of TP+ (i.e., TP+-NO2) has no sizable influence on P-centered
electronic features, (ii) reduction processes located on TP+ depend on the intercomponent tilt angle.
Concerning excited-state properties, photophysical investigation evidenced that phosphorescence of P is
actually quenched in dyads 4 and 5 only. Ultrafast transient absorption (TA) experiments allowed attributing
3
the quenching in conformationally locked dyad 5 to oxidative electron transfer (ET) from the MLCT level
to the TP+-NO2 acceptor (kel ) 1.1 × 109 s-1). For 4, geometrically unlocked, the 3MLCT state was shown
to first rapidly equilibrate (reversible energy transfer; keq ≈ 2 × 109 s-1) with a ligand centered triplet state
before undergoing CSS formation. Thus, the pivotal role of conformation in driving excited-state decay
pathways is demonstrated. Also, inner P structural planarization as a relaxation mode of the 3MLCT states
has been inferred from TA experiments.
advancement of research fields like nanosciences6,7 (including
molecular electronics8 and photonic9) and bioinorganic photo-
chemistry,10 which are both relevant frameworks for the present
work devoted to the design of PMDs capable of handling
intramolecular directional photoinduced electron transfer (PET).11
1. Introduction
High level of organization is counted among basic features
of natural functional assemblies. This finding has contributed
to lead to the foundation of the conceptual corpus of supramo-
lecular chemistry,1a with the goal to create molecule(s)-based
artificial systems functioning as devices.1b When such well-
defined supermolecules are getting to interact with light in a
specific manner to become photochemical molecular devices
(PMDs), we are entering the domain of supramolecular photo-
chemistry.2 Deriving3 from “classical” inorganic photochemis-
try,4,5 supramolecular photochemistry has a share in the
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Scandola, F. Supramolecular Photochemistry; Ellis Horwood: Chichester,
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† Universite´ Rene´ Descartes.
‡ ENSCP.
§ Universita` di Messina.
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| ISOF-CNR.
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J. AM. CHEM. SOC. 2006, 128, 7510-7521
10.1021/ja058357w CCC: $33.50 © 2006 American Chemical Society

Conformationally Gated Processes within Dyads. 1.
A R T I C L E S
Chart 1. Salient Features of TP+-derivatized Tpy Ligands
Depending on whether light input is viewed as a trigger signal
(quantum of information) or as an energy source (to be conveyed
or converted), targeted applications are rather directed toward
information processing or artificial photosynthesis purposes,
respectively.12
When dealing with photoinduced processes, “reaction centers”
of natural photosynthetic organisms, that perform conversion
of solar energy into chemical fuels for their cellular machinery,
are benchmark functional assemblies.11,13-15 Beyond its com-
plexity, order within photosynthetic apparatus has been clearly
highlighted thanks to X-ray diffraction which has proved to be
a pertinent analytical method to investigate this type of
supramolecular assemblies of biological origin.16 The photo-
synthetic organization, in which involved photo- and/or redox-
active elements were found to be harmoniously set out with
utmost precision,17 presents a hierarchical association between
two functions.18 The first one is specialized in the collection of
photons and the funneling of light energy (antenna) while the
second one is in charge of the transduction of previously
conveyed excitation energy into usable electrochemical potential
(“redox energy”) by carrying out so-called charge separation
(CS). Brought together, structural issues and insights gained
from time-resolved spectroscopic experiments have allowed to
evidence another analytical grid of photosynthetic organizations,
which lies at the energetic level. Indeed, an arrangement of
active components according to energy gradients was revealed,
that could be superposed on overall spatial orderliness.19-21
Therefore, key-organization of operative functional assembliess
at least when photoinduced processes are concernedsis actually
dual, being attached to both spatial and energy layouts.
made computationally designed proteins27), liposome bilayers,28
thin films and lamellar solids29 or purely mineral materials, for
example zeolites.30 For an alternative approach relying on
genuine PMDs, components are held together by covalent
links,12-15,31,32 weak supramolecular-interactions7,33 or even
mechanical contacts.34 The counterpart at the molecular level
of dual organization of reference natural functional systems lies
in combined rigidity and chemical Variability.12-15 Rigidity
results in structurally well-defined architectures35,36 while
chemical Variability allows both tuning of electronic properties
of subunits and spatial expansion of the supermolecule so as to
generate energy gradients (e.g., “redox cascade” for CS). In
addition, functional integration is achieved by using a strongly
absorbing photosensitizer (P) which also behaves as a primary
intramolecular electron donor when photoexcited.
Design of artificial photochemical charge separation devices,
that is, functional models for directional ET more specifically
conceived to form charge separated states (CSS) upon light
excitation, goes through consideration of the above-identified
features.22-24 So far, two main strategies have been proposed
to obtain such hierarchical molecular assemblies integrating the
two functions of antenna and CS. One approach is relying on
the use of supports25 and matrices as rigid host (organizing
medium) to fix active components in a predetermined manner.
The matrices can either be proteins (using natural apo-proteins
according to semi-synthetic strategy26 or using synthetic tailor-
In an effort to exert more accurate control over both structural
and electronic features of supermolecules intended to perform
photoinduced CS, we have recently proposed to rely on
oligopyridyl ligands functionalized with the 2,4,6-triarylpyri-
dinium (TP+) electron-acceptor group, A (Chart 1).37 The overall
architecture of complexes with 4′-[p-(TP+-derivatized)-phenyl]-
2,2′:6′,2”-terpyridine native ligands (tpy-ph-TP+) is rigid despite
possible conformational fluctuations along main single-axis
symmetry that gives the rodlike shape to derived supermolecules.
Chemical variability results from the possible substitution on
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A R T I C L E S
Laine´ et al.
Chart 2. Label of the Molecules Studied. Counteranions Are BF4- for Organic Models and PF6- for Complexes
the TP+ unit (R1, R2), choice of d6 transition metal cation (M)
forming the [M(tpy)2] bis-tpy core of the photosensitizer (P)
and the choice of ancillary ligand which can bear an electron-
donating group. The structure is potentially extendable (via the
R2 position) to form a so-called “redox cascade” for long-range
PET and long-lived CSS. When the chemical variability
concerns the bridging element (linker) such as R0 substituents
of the spacer (Chart 1), the level of conformational restriction
is first tuned. There exists however a strong correlation between
structural and electronic properties as the TP+-derivatized
organic ligand is potentially fully conjugated. This relationship
is a supplementary reason for finely controlling the intramo-
lecular geometry. Indeed, when designing PMDs, one has to
ensure that electronic identity of various active components is
retained2,12 so as to perform vectorial ET by hopping from one
discrete state to another38,39 according to a predetermined energy
gradient referred to as “redox cascade”. To do so, we37 and
others39-46 have relied on a “geometrical decoupling” strategy
to minimize adverse intercomponent electronic coupling, at least
as far as π-π interactions, which have by far the largest
contribution to electronic communication, are concerned. Such
a geometrical decoupling consists in forcing connected sub-
units to adopt a perpendicular conformation by increasing steric
hindrance about the intercomponent linkage. Here, the structural
effect is mainly originating from the bulky phenyl substituents
ortho to the Npyridinio atom of TP+ (Chart 1) and has been shown
to be effective in the ground state.37,39
A recent theoretical analysis47 of the electronic behavior of
the reference photosensitizer complex [Os(ttpy)2]2+ (1; ttpy is
the 4′-tolyl-tpy, also used as ancillary ligand in dyads; Chart 2)
and nonoptimized native dyad [(ttpy)Os(tpy-ph-TPH3+)]3+ (2)
represented in Chart 2, has shown that a structural reorganization
is very likely to occur in the excited-state despite molecular
rigidity, as a consequence of (photoinduced) charge redistribu-
tion. Such geometrical changes have already been observed48-53
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Conformationally Gated Processes within Dyads. 1.
A R T I C L E S
or postulated54 for a few other inorganic compounds upon
charge-transfer (CT) excitation. In the present case, torsional
motions are anticipated to correlate with undesired fluctuation
of the determining intercomponent electronic coupling.47,55,56
Beyond ground state requirements, it is therefore of paramount
importance to take into account possible structural changes that
could occur in the excited state when engineering photoactive
supramolecular architectures intended to undergo large electronic
redistribution.
The present work is aimed at (i) taking advantage of appealing
potentialities of triarylpyridinio-functionalized tpy ligands (Chart
1) to improve the working mode of derived PMDs and (ii)
gaining new insights into the basic factors which control
photoinduced processes at the intramolecular level.35 In this
study, Ru(II) complexes (6 - 10; Chart 2) are mainly viewed
as reference compounds for the more interesting Os(II)-based
analogues (1 - 5; Chart 2). Thus, on one hand, a further increase
of conformational locking about the P-A linkage has been
achieved by replacing the phenyl spacer (ph) by a meta-xylyl
one (xy) in order to make sure that geometrical decoupling is
real especially in the excited state. On the other hand, to improve
thermodynamics of intramolecular PET between *P (the primary
electron donor) and A subunits, we show here that derivatization
of TP+ by an electron-withdrawing group such as nitro (to give
TPH2(NO2)+; Chart 2) indeed results in an increased efficiency
for CSS formation. These two parameters (that is xylylic
modification and nitro-substitution) have been varied separately
by synthesizing dyads [(ttpy)M(tpy-xy-TPH3+)]3+ (3, 8) and
[(ttpy)M(tpy-ph-TPH2(NO2)+)]3+ (4, 9), respectively. Effects of
these specific changes over photophysical behavior of dyads
were studied by comparison with the corresponding reference
[(ttpy)M(tpy-ph-TPH3+)]3+ (2, 7) native systems.37 Dyads
comprising both improvements, [(ttpy)M(tpy-xy-TPH2(NO2)+)]3+
(5, 10), were also investigated and demonstrated to behave in
a distinct manner from their parent complexes. In particular, a
comparative study of 4 and 5 has allowed to assess the role of
intramolecular conformation in gating intercomponent photo-
induced processes like PET and electronic energy transfer (EnT).
Figure 1. ORTEP drawing of the tpy-xy-TPH2(NO2)+ ligand with thermal
ellipsoids (50% probability). The BF4 counterion and the cocrystallized
solvent molecule are omitted for clarity.
-
Chart 3. Relevant Angular Parameters and Nomenclature. Ph1
Becomes Xy When R0 ) Me
to help in labilizing chloride ligands of (ttpy)OsCl3 chemically
inert precursor), the tpy-sp-TP+-NO2 ligand was involved in
a reductive degradation. To overcome this unexpected drawback,
the so-called “chemistry-on-the-complex” approach57-60 has
been adopted. Here, this strategy consists first in building the
inorganic core bearing an amino substituent at the right location
(typically the photosensitizing subunit (ttpy)M(tpy-sp-NH2)59)
so that, in a second step, one is allowed to perform in mild
conditions (that is T e 350 K) the reaction leading to the
formation of the targeted nitro-derivatized triarylpyridinio
fragment (Scheme SI-1b; Supporting Information). Identity of
all organic models, ligands and related complexes has been
confirmed by standard analytical techniques, including 1H NMR
spectroscopy, mass spectrometry and elemental analysis.
2.1.2. Structural Characterization. Ligand tpy-xy-TPH2-
(NO2)+ is readily crystallized as a tetrafluoroborate salt (Figure
1) by slow evaporation of acetonitrile solution.
2. Results and Discussion
2.1. Syntheses and Structural Characterization. 2.1.1.
Synthesis. Triarylpyridinio-derivatized organic ligands, tpy-sp-
TP+ (sp is the phenyl or xylyl spacer), are readily available in
good yields by reacting properly chosen amine precursors (tpy-
sp-NH2) and pyrylium salts.37b To obtain heteroleptic com-
pounds (i.e., dyads), the usual synthetic route12 consists in
reacting tpy-sp-TP+ with an inorganic precursor bearing a ttpy
The main structural feature of the ligand is the dihedral angle
(θ1; Chart 3) between the planes of the m-xylyl spacer and
pyridinium ring: 74.03°, to be compared with that of the closely
+
affiliated tpy-ph-TPH3 native ligand: 72.60°.37b Apparently,
increased steric hindrance about the intercomponent linkage does
not result in significantly larger geometrical decoupling although
passing from restrained conformation to a more constrained
one. This finding indicates that equilibrium conformation of the
native ligand in the ground state is virtually the same as that of
optimized ligand. This equilibrium geometry has been shown
to change slightly upon ligand complexation, the related twist
ancillary ligand ((ttpy)MCl3) to give [(ttpy)M(tpy-sp-TP+)]3+ 37b
.
This strategy could successfully be applied for the Ru(II)-based
species but not for the Os(II) analogues in the cases of the
ligands bearing nitro-substituted TP+ acceptors. Indeed, it
appeared that during the last step of complexation that requires
rather harsh conditions (T g 383 K and use of a reducing agent
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Verhoeven, J.; Mu¨llen, K.; Xie, X. S.; De Schryver, F. Proc. Natl. Acad.
Sci. U.S.A. 2004, 101, 14343-14348.
(56) Weiss, E. A.; Tauber, M. J.; Kelley, R. F.; Ahrens, M. J.; Ratner, M. A.;
Wasielewski, M. R. J. Am. Chem. Soc. 2005, 127, 11842-11850.
(60) Johansson, O.; Lotoski, J. A.; Tong, C. C.; Hanan, G. S. Chem. Commun.
2000, 819-820, and refs therein.
9
J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006 7513

A R T I C L E S
Laine´ et al.
Table 1. Electronic Absorption Data and Assignmentsa
λ
max [nm] (
ꢀ
[104 M-1 cm-1])
1MLCT
entry
LC
3MLCT
0
1
2
3
4
230 (4.70), 270 (5.13), 312 (8.10)
286 (7.64), 314 (8.64)
477 (1.74), 536sh (0.74)
490 (3.01)
492 (3.32)
491 (3.27)
492 (3.07)
632 (0.43), 658 (0.47)
645 (0.67), 668 (0.77)
642 (0.68), 668 (0.77)
642.9 (0.68), 669.2 (0.78)
643.2 (0.64), 669.7 (0.72)
643.2 (0.70), 670.1 (0.80)
289 (7.81), 315 (11.22)
290 (7.56), 315 (11.30)
290sh (8.34), 314 (9.86)
289sh (9.40), 314 (11.46)
285 (7.86), 310 (8.77)
5
6
491 (3.37)
490 (3.39)
7
8
9
10
288 (8.62), 312 (11.65)
288sh (7.91), 312.3 (10.96)
289sh (9.62), 309.3 (10.64)
289sh (9.28), 310.6 (11.19)
492 (3.63)
492 (3.39)
492 (3.34)
492 (3.48)
a Acetonitrile solutions; sh: shoulder.
Table 2. Electrochemical Data and Formal Assignment (See Text)
of Redox Processes for Examined Model Acceptors and
Complexes in Acetonitrile + 0.1 M TBABF4 at Pt Electrodea
/
II
0
/-
TP0
/-
P0
2-
MIII
TP+/
P-/
entry
E1
n
E1
/
n
E1
/
n
E1
/
n
E1
n
/
2
2
2
2
/
2
+b
ph-TPH3+
-0.93 1 -1.08
-0.98
-0.70d 1d -0.70d 1d
-0.66 1 -0.81
1
xy-TPH3
1
c
ph-TPH2(NO2)+
xy-TPH2(NO2)+
1
0
+0.95 1
+0.90 1
-1.165 1 -1.45
-1.20 1 -1.47
1
1b
2b
3
1
1
1
+0.93 1 -0.91 1e -1.00 1e -1.21 1 -1.47
+0.95 1 -0.93
1
c
-1.17 1 -1.46
4
5
+0.93 1 -0.65f 1 -0.74f 1 -1.21g nd -1.47g nd
+0.93 1 -0.63 1 -0.79 1 -1.17 1 -1.45
+1.24 1 -1.24 1 -1.47
+1.27 1 -0.90 1 -1.00 1 -1.25 1 -1.50
+1.27 1 -0.94 -1.22 1 -1.48
1
1
1
1
6b
7b
8
1
c
Figure 2. Electronic absorption spectra of the 2-5 series of Os-based
compounds in solution in MeCN.
9
10
+1.27 1 -0.66f 1 -0.73f 1 -1.24g nd -1.51g nd
+1.27 1 -0.63 1 -0.78 1 -1.20 1 -1.46
1
angle getting even more pronounced (θ1 ≈ 85.4° on average).61
Thus, rather than increasing geometrical decoupling, methyl
substituent of the meta-xylyl spacer are actually expected to
lock the tilted conformation so as to preVent from possible
planarization in the excited state, instead. In other words, R0
substituents are expected to play a structural role mostly in the
excited state.
2.2. Absorption Spectra. Ground-state electronic absorption
spectra of reference chromophores and related inorganic dyads
are collected in Table 1. Figure 2 shows the superimposed
absorption spectra of 2-5.
The electronic spectra of compounds 1-5 exhibit intense
absorption bands in the UV region, due to ligand-centered (LC)
transitions (ꢀ in the range 105-106 M-1 cm-1), and moderately
intense absorption bands in the visible region of the spectrum,
attributed to spin allowed singlet metal-to-ligand charge transfer
(1MLCT) transitions (ꢀ ≈ 105 M-1 cm-1). At lower energy,
spin-forbidden 3MLCT are also observed thanks to the presence
of osmium(II) heavy metal ion. From inspection of electronic
absorption properties (Table 1), it appears that the P1/M
chromophore photosensitizer (Chart 3), modeled by [Os(tpy-
ph-Me)2]2+ i.e., 1, is only very scarcely affected by TP+. Thus,
ground-state electronic absorption spectra of the new dyads 3-5
and 8-10 are almost the same as those of reference native dyads
2 and 7 already published,37b respectively. This is indicative of
a very weak electronic coupling between the P and A subunits
in the ground state.
a E1/2/V (vs SCE) is calculated as (Epa + Epc)/2 where Epa and Epc are
the anodic and cathodic peak potentials measured by cyclic voltammetry
at 0.2 V s-1 ; n is the number of electrons involved in the redox process
determined by hydrodynamic voltammetry by comparison with reference
components or with respect to the internal monoelectronic oxidation of the
metal center for complexes; nd: not determined. b ref 37a. c Out of the
investigated potential window (below -1.70 V). d Two unresolved mono-
electronic processes corresponding to TP+/0 and TP0/- observed as a
bielectronic peak. e Two monoelectronic partially resolved peaks: TP+/0
cathodic peak merges with TP0/- f ill-resolved peaks. g Reported value refers
.
to Epc and ill-defined reversible process due to electrode surface modification
upon potential scanning below -1.1 V.
2.3. Redox Behavior. The electrochemical properties of
model organic acceptors, reference complex-photosensitizers and
dyad systems are gathered in Table 2, where Ru(II) complexes
data are also included for comparison.
Assignment of redox processes within the various dyads
investigated has been made on the basis of issues obtained from
the electrochemical study of model parent species. The following
picture can be drawn out of these electrochemical data:
+
(i). In the ph-TPH3 model acceptor (Chart 2) and in the
corresponding metal complexes 2 and 7, the TP+ subunit is
doubly reduced at mild potentials (ca., -1 V; Table 2). The
difference in potential between the two TPH3+-based reduction
processes is relatively small (150 mV for the isolated parent
acceptor and equals 90 and 100 mV for the osmium and
ruthenium complexes, respectively), so indicating a large
delocalization of the redox-active orbital, which is probably
extended over many of the rings.
(ii). When comparing various TPH3+-based species (Chart
2, Table 2, Figure 3a), it appears that when θ1 is conforma-
(61) Laine´, P. P.; Ciofini, I.; Ochsenbein, P.; Amouyal, E.; Adamo, C.; Bedioui,
F. Chem. Eur. J. 2005, 11, 3711-3727.
9
7514 J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006

Conformationally Gated Processes within Dyads. 1.
A R T I C L E S
in the case of conformationally locked xy-TPH2(NO2)+ model
and related dyads 5 and 10. Thus, the main, chemical,
contribution to redox active orbital is apparently shifted from
the [Ph1/Xy-Py+] system toward the [Py+-Ph4-NO2] terminal
one. It remains nonetheless that the model acceptor, when on
the whole a monoanion species, is somewhat energetically less
favored in the case of [xy-TPH2(NO2)]- due to constrained
rotation about θ1 (E ) -0.81 V) than in [ph-TPH2(NO2)]- (E
≈ -0.70).53 This point, together with the fact that TPH2(NO2)+
could only be doubly reduced (like unlocked native acceptor
unit) but not three times reduced, as would be expected for a
nitrophenyl moiety decoupled from the pyridinium core, further
confirms the relevance of the delocalized description of the
pyridinium-centered A component comprising the N-pyridinio
aryl substituent (Ph1 or Xy) and the Ph4-NO2 group.
+
+
(iv). The comparison of ph-TPH3 to xy-TPH3 and of ph-
TPH2(NO2)+ to xy-TPH2(NO2)+ data definitively indicates that
the reduction pattern of the pyridinium core is not only sensitive
to the nature of attached N-pyridinio substituent, as shown in
previous works,37,61 but also to the tilt angle θ1. More precisely,
the TP+ fragment is found to be two-times reducible at the most
under favorable conditions including allowed planarization
about θ1 and/or deriVatization with conjugated and strongly
electron-withdrawing substituents such as NO2.
(v). The redox processes related to the photosensitizer unit
(metal- and terpyridyl ligands- centered) within the various new
dyads are almost unaffected by the modification of neither the
acceptor moiety nor the spacer (with respect to native dyads, 2
and 6 Chart 2; Figure 3b). Moreover, it is noteworthy that,
although bielectronically reduced, [TP]- does not influence a
lot the subsequent (ph-)tpy-centered reduction process referred
to as P0/- (Table 2). Taken together, these findings demonstrate
the weak intercomponent electrostatic interaction (and electronic
coupling) in the ground state.
2.4. Spectroelectrochemical Behavior. To facilitate the
interpretation of the transient absorption spectra and more
particularly to identify the formation of charge-separated states
where the photosensitizer is oxidized (metal-centered process),
while the acceptor unit is reduced, three spectroelectrochemical
experiments have been undertaken. On one hand, the gradual
reduction of the model acceptor xy-TPH2(NO2)+ has been
performed, up to add two electrons per molecule (i.e., successive
formation of the singly and doubly reduced species xy-TPH2-
(NO2)0 and xy-TPH2(NO2)-, respectively). On the other hand,
the controlled monoelectronic oxidation of the osmium- and
ruthenium-based optimized dyads (5 and 10, respectively) was
performed in order to get the spectroscopic signature of the
embedded P1/M photosensitizers when oxidized (Supporting
Information: Figure SI-2a-c).
+
Figure 3. (a) Cyclic voltammograms (V ) 0.2 V s-1) for xy-TPH3 (c )
5.5 × 10-4 M; dotted line), ph-TPH2(NO2)+ (c ) 3.1 × 10-4 M; dashed
line) and xy-TPH2(NO2)+ (c ) 3.3 × 10-4 M; solid line). (b) Cyclic
voltammograms for 6 (c ) 2.4 × 10-4 M, V ) 0.3 V s-1; dotted line)37a
and 10 (c ) 1.4 × 10-4 M, V ) 0.2 V s-1; solid line). The * indicates
redox processes related to the acceptor unit.
tionally constrained, as in the case of xy-TPH3+, the second
reduction of the acceptor is displaced to much more negative
potentials, out of the investigated (and accessible) potential
window. In the case of rings for which rotation around θ1 is
only restrained (e.g., ph-TPH3+), once the pyridinium has gained
one electron, the three-ring segment (comprising the N-phenyl
group (Ph1), the pyridinium ring (Py+) and terminal phenyl
moiety (Ph4); Chart 3), can adopt a more planar conformation
as a result of a quinoid-like electronic redistribution of the
reduced pyridinium. Such a propensity for planarization is
expected to get even more pronounced upon second reduction,
i.e., on going from radical to monoanion, as the second gained
electron is anticipated to be delocalized over the whole
segment38 instead of being preferably “localized” on the
heterocycle.62 Upon limiting (i.e., constraining and even locking)
torsional motion about θ1 (case of xy-TPH3+), the latter
supplementary planarization is hindered so that the second
reduction is found to be energetically unfeasible under standard
conditions.
(iii). Addition of a nitro substituent onto TP+ at the R2
position does result in a substantial increase of the electron-
withdrawing capability of this latter, as expected. Indeed, a
positive potential shift of ca. +0.25 V of the redox processes is
clearly observed (Table 2), which well compare with redox
behavior of a series of 4-[substituted-phenyl]-pyridinium species
reported in the literature.63 Interestingly, possibility of double
reduction of the acceptor is restored upon nitro-derivatization
The reduced model acceptor as well as the oxidized photo-
sensitizers within [5]+ and [10]+ are good chromophores as they
exhibit new absorption bands with extinction coefficient larger
than 10 000 M-1 cm-1. Unfortunately, the main spectroscopic
signatures of [A]- and [P]+ are found to be intermingled in the
of 400 - 500 nm spectral region, as already noticed for 2,37,47
making it difficult to interpret transient absorption difference
spectra of photoexcited dyads.47,64 Actually, the more diagnostic
(64) Noteworthy, the broad features situated at 400-500 nm also overlap
absorption of nitrobenzene radical anion if formed, which occurs at ca.
455 nm in DMF (Miertus, S.; Kysel, O.; Danciger, J. Collect. Czech. Chem.
Commun. 1980, 45, 360-368).
(62) Bock, H.; Herrmann, H.-F. HelV. Chim. Acta 1989, 72, 1171-1185.
(63) Hasegawa, E.; Kang, D.; Sakamoto, K.; Mitsumoto, A.; Nagano, T.;
Minakami, S.; Takeshige, K. Arch. Biochem. Biophys. 1997, 337, 69-74.
9
J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006 7515

A R T I C L E S
Laine´ et al.
Table 3. Absorption Maxima of the Electrochemically Reduced and Oxidized Key Speciesa
entry
λmax [nm] (ꢀ
[104 M-1 cm-1])
xy-TPH2(NO2)0 (i.e., Aref + 1e-)
440 (2.28)
474 (2.45)
404 (2.80)
375 sh (2.55)
471 (2.70)
526 (2.33)
517 (0.64)
830 sh(0.83)
>900 (>1.50)
xy-TPH2(NO2)- (i.e., Aref + 2e-)
[5]+
608 (0.37)
690 (0.35)
[10]+
a As the singly and doubly reduced model acceptor cannot be obtained in pure forms (due to the close potential values of the two successive redox
processes related to the reduction of the pyridinium) the given value for the respective extinction coefficients of their characteristic absorption bands are only
estimates.
Table 4. Photophysical Data for the Various Dyads and
Reference Photosensitizers
Table 5. Excited-State Properties of the Various Dyads and
Reference Photosensitizersa
1
1
293 Ka
102) Iemrel [%]c
77 Kb
max [nm]
entry
E(III/II*)[V]
E(II*/I)[V]
∆
GET[eV]
kr [s-
]
knr [s-
]
entry
λ
max [nm]
Φem
(
×
τ
[ns]
λ
τ
[
µ
s]
1
2
3
4
5
6
9
10
-0.820
-0.778
-0.753
-0.780
-0.771
-0.737
-0.652
-0.676
+0.520
+0.498
+0.533
+0.495
+0.531
+0.737
+0.682
+0.746
8.1 × 104
6.1 × 104
6.4 × 104
5.3 × 103
>3.2 × 104
5.5 × 104
1.6 × 104
9.0 × 104
4.0 × 106
5.9 × 106
5.2 × 106
5.7 × 106
>9.9 × 107
1.7 × 109
1.4 × 108
9.9 × 108
+0.132
+0.177
-0.125
-0.141
0
1d
2d
3
4
5
6
9
10
724
736.5
751
749
745
743.5
640
676
1.40
2.00
1.02
1.23
70
100
50.9
269
247
168
191
174
<10
689
3.9
3.1
2.7
3.2
3.1
3.2
721, 790sh
726, 790sh
728, 790sh
725, 790sh
729, 790sh
61.7
4.6
1.6
0.092
0.0318
0.0032
0.011
0.009
+0.0075
-0.046
0.58d 627, 684shd 11.7d
7
1
645, 696sh
637, 685sh
10.0
12.5
a Excited-state redox potentials E(III/II*) and E(II*/I) as well as radiative
(kr) and nonradiative (knr) rate constants were calculated according to refs
71 and 72.
674
a In acetonitrile. b In butyronitrile. sh: shoulder. Iemrel ) 100 ×
[Φem(species)/Φem(1)]. d ref 37a.
c
derivatized aryl-tpy ligand is made easier to reduce than the
nonsubstituted one, so decreasing the MLCT excited-state
energy.65 Other additional modifications concerning either the
photosensitizer itself (i.e., the methyl substituents of the xylyl
moiety, as in 3) or the acceptor group (i.e., NO2, as in 4) or
both (as in 5) are actually found to barely influence the emitting
level of the luminophore, which roughly lies at the same energy
throughout the 2-5 series. This fact is illustrated in normalized
rt and 77 K emission spectra of the whole series of complexes
(Figures SI-3a and b; Supporting Information). These findings
are not surprising for the following reasons. First, even if methyl
groups of the xylyl spacer (in 3 and 5) are weak electron-
donating substituents (+I effect), inductive contribution is
known to be a short-range effect. Second, although NO2
substituent exerts pronounced π-extending (-M mesomeric
effect) and inductive electron-withdrawing (-I effect) influences
over the terminal phenyl of TP+ and linked pyridinium ring
within 4 (and 5), these effects are anticipated to hardly propagate
to the metal center, over ca. 18 Å and through 16/17 covalent
bonds. Indeed, the π-system is not fully delocalized due to
cumulated structural distortions along the main molecular axis,
especially about the canted pyridinium (θ1) (Chart 3).
Figure 4. Emission spectra of the Os(II) series of complexes in deaerated
MeCN solutions at room temperature, measured in identical experimental
conditions (in particular, same OD at λexc. ) 600 nm).
region appears to be situated at λ > 710 nm and in particular
in the NIR domain (around 900 nm) where the mono-reduced
acceptor only (that is xy-TPH2(NO2)0) exhibits a significant
absorption (Table 3).
2.5. Photophysical Behavior. 2.5.1. Luminescence Proper-
ties. All the Os(II) species investigated here are luminescent,
both in acetonitrile fluid solution at room temperature (rt) and
in butyronitrile rigid matrix at low temperature (lt), i.e., 77 K
(see Table 4). In all cases, emission can be straightforwardly
attributed to the lowest lying 3MLCT state of the photosensitizer.
The luminescence data of 1-3 suggest that oxidative electron
transfer from the MLCT chromophore to the TP+ acceptor is
not effective. In fact, the driving force of the process is almost
negligible or even endoergonic (Table 5), making electron-
transfer inefficient. As far as species 2 and 3 are concerned,
When compared to complex 1, complexes 2-5 all exhibit
reduced luminescence quantum yields (Figure 4) and lifetimes
at room temperature. From the analysis of the photophysical
data, it appears that, as far as the emission energy is concerned,
(65) Noticed differential sensitivity of P1/Os electronic features to TP+
attachment depending on whether electronic absorption or emission
properties are considered is indicative of differences stemming from the
actual nature of both involved P/A partners and their electronic interplay.
Indeed, greater sensitivity of emission properties (Table 4) as compared to
that of absorption properties (Table 1) is most likely related to the (ML)-
CT nature of the intramolecular probe directly linked to the TP+ group,
which is [(ttpy)Os3+(tpy-ph)-] in the former case versus [(ttpy)Os2+(tpy-
ph)] in the latter one. In the present case, ground state picture derived from
methods like electronic absorption and electrochemistry cannot be straight-
forwardly extrapolated to excited state. This finding is a consequence of
existing nonnegligible intercomponent electronic coupling and is not
observed when electronic insulation is strictly attained, as in the case of
methylene linked assemblies; see ref 66.
3
the main perturbation of the luminescent MLCT state in this
series of complexes results from the covalent appending of the
positively charged triarylpyridinium (TP+) group to P1/Os
luminophore (by taking 1 and 6, for the analogous Ru(II)
species, as references). Indeed, emission of complexes 2-5 is
red-shifted compared to that of 1, most likely because the TP+-
9
7516 J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006

Conformationally Gated Processes within Dyads. 1.
A R T I C L E S
the above cited luminescence properties difference can now67
be essentially attributed to the energy gap law.68 The fact that
3 is slightly more emissive than 2 can be ascribed to increased
rigidity of the supermolecule architecture69 and is more specif-
ically indicative of the efficacy of R0 methyl groups in
constraining conformational fluctuations in the excited state,70
as expected. Thus, 3 and 2 are now worth considered as new
references (for P1/Os luminophore with locked and unlocked
chromophoric ligand, respectively) rather than 12a for forthcom-
ing photophysical study of photoinduced processes within dyads
5 and 4, respectively.
Energy gap law alone cannot justify the significant reduction
in luminescence quantum yields and lifetimes of 5 and -to a
smaller extent- 4 (Table 4 and Figure 4). Therefore, at least
another decay channel has to be active in these species. At 77
K, the luminescence properties of all the species are quite similar
one another (Table 4), suggesting that the supplementary decay
channel operating in 4 and 5 at room temperature is not active
at lt, in rigid matrix.
In the present case, such a contribution makes the slightly
exoergonic reactions at room temperature largely endoergonic
at 77 K. However, reduction in emission quantum yield at room
temperature is not accompanied by a reduction in phosphores-
cence lifetime in 4, suggesting that the hypothesis of a simple
ET quenching does not hold, at least for this latter species.
It should be noted that when dealing with excited-state energy
of the 3MLCT state, which is relevant for the calculation of the
electron-transfer driving force, the following particular point
should be considered. Although 4 and 5 (as well as 2 and 3)
adopt the same -more stable- axially distorted geometry resulting
from intraligand steric hindrance (especially about θ1; Chart 3)
in the ground state, the question arises of whether experimentally
derived Eem(0-0) values actually correspond to dyads in their
tilted (frozen ground state) conformation or in their planarized
geometry (see below: section 2.5.2A). In the former case,
emission spectra recorded at low temperature (in rigid matrix)
afford Eem(0-0) values related to the room temperature not
3
relaxed (NR) MLCT state of P1/Os, which is different from
When comparing the estimated excited-state reducing strength
of the metal chromophore in *[5] (E(III/II*) ) -0.77 V) to the
the room temperature -thermalized (i.e., relaxed, R)- planar
3MLCT emitting state. Emission maxima measured at 77 K are
therefore worth referred to as Eem(0-0)NR (where index “NR”
also identifies the “tilted conformation”) and Eem(0-0)R,
first reduction potential of the acceptor TP+ (i.e., E((NO2)H2TP+/0
)
) -0.63 V), it appears that an oxidative quenching of the
excited photosensitizer (primary light-triggered donor) by the
TP+ subunit is thermodynamically allowed by ca. ∆G ) -0.140
eV. Similar exoergonicity of ca. -0.125 eV is found for 4. One
can note that the driving force for an intramolecular PET is
only very slightly favorable in the case of the ruthenium
analogue of 5, that is 10. Therefore, the reduction in lumines-
cence lifetimes (for 5) and quantum yields (for both 4 and 5)
could in principle be attributed to the occurrence of a photo-
induced intramolecular ET (oxidative quenching of *P). Obvi-
ously, because of the small driving force, this process is expected
to be inefficient in rigid matrix at 77 K, and this would explain
the low-temperature emission properties. Indeed, due to pre-
cluded solvent repolarization on the time scale of the excited-
state lifetime, electronic redistribution that accompanies PET
process is estimated to require ca. +0.6 eV to take place.73,74
respectively. Indeed, it is anticipated that Eem(0-0)NR > Eem
-
(0-0)R due to extended electronic delocalization over the whole
planarized aryl-tpy ligand.75
Interestingly, it has been stated in the related case of
N-methyl-4,4′-bipyridinium (MQ+) acceptor that although the
planarization mode could be inhibited in the rigid glass,53 it
appears not to be in the complex [(4,4′-(NH2)2bpy)Re(CO)3-
(MQ+)]2+. In this complex, emission from the equilibrated dπ
f π* (MQ+) state is observed in the glassy state at low
temperature.76 Also, it has been estimated a rather large
thermodynamic driving force of ca. 7 kcal mol-1 (i.e., 29 kJ
mol-1 or 0.3 eV) for achieving the planarization of reduced
phenyl-bpy (that models the ligand when involved in the (ML)-
CT state of related complex).52 On the basis of the latter
energetic estimate for planarization, the assumption that ex-
perimentally measured value for Eem(0-0) is actually related
to Eem(0-0)NR rather than Eem(0-0)R would mean that PET
processes (i.e., CS formations) within 4 and 5 are not slightly
exoergonic (by ca. -0.125 eV and -0.141 eV for 4 and 5,
respectively) but slightly endoergonic (by ca. +0.175 eV and
+0.159 eV, respectively). Clearly, such an inference is not
consistent with experimental issues herein reported, therefore,
we would tentatively state that there is room for the phenyl
ring of the aryl-tpy ligand to rotate in the rigid glass,77
correlatively implying that experimentally derived value for Eem-
(0-0) actually corresponds to Eem(0-0)R.
(66) Collin, J.-P.; Guillerez, S.; Sauvage, J.-P.; Barigelletti, F.; De Cola, L.;
Flamigni, L.; Balzani, V. Inorg. Chem. 1992, 31, 4112-4117.
(67) Hitherto, the status of native dyad 2 with respect to CSS formation has
47
remained somewhat unclear.37, In particular, we could not get clear-cut
picture on the reason for the decrease of emission quantum yield of P1/Os
luminophore within 2: (i) Energy gap law, due to electronic perturbation
of P1/Os by appending of TP+, for which there is no simple equation to
quantify a priori the contribution (i.e., without available experimental data
for a series of affiliated compounds; Anderson, P. A.; Keene, F. R.; Meyer,
T. J.; Moss, J. A.; Strouse, G. F.; Treadway, J. A. J. Chem. Soc., Dalton
Trans. 2002, 3820-3831 and refs therein) and/or (ii) Poor efficiency of
the P+-A- state (CS state) population from the emitting 3MLCT level
(PET process) moreover leading to negligible spectral changes.47 This
complexity is further increased by potentially misleading effects of
intercomponent electronic coupling, as discussed in ref 47. Actually, in
borderline cases like 2, only comparison with closely affiliated species
unambiguously demonstrated to undergo PET process with CSS formation
can definitely help to decide. The present study provides such reference-
dyads (cf. cases of 4 and 5).
2.5.2. Ultrafast Transient Absorption Spectroscopy. To
better characterize and clarify the excited-state properties of the
new osmium complexes, we performed ultrafast (fs time range)
transient absorption (TA) spectroscopy in acetonitrile at room
temperature. For all the osmium compounds, differential absorp-
tion spectra after laser pulse (λexc. ) 400 nm; first observation
(68) Claude, J. P.; Meyer, T. J. J. Phys. Chem. 1995, 99, 51-54.
(69) Nijegorodov, N. I.; Downey, W. S. J. Phys. Chem. 1994, 98, 5639-5643
and references therein.
(70) (a) McFarland, S. A.; Finney, N. S. J. Am. Chem. Soc. 2002, 124, 1178-
1179. (b) McFarland, S. A.; Finney, N. S. J. Am. Chem. Soc. 2001, 123,
1260-1261.
(71) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.; Meyer,
T. J. J. Am. Chem. Soc. 1982, 104, 6620-6627.
(72) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277.
(75) Strouse, G. F.; Schoonover, J. R.; Duesing, R.; Boyde, S.; Jones, W. E.,
Jr.; Meyer, T. J. Inorg. Chem. 1995, 34, 473-487.
(76) Chen, P.; Danielson, E.; Meyer, T. J. J. Phys. Chem. 1988, 92, 3708-
3711.
(77) Barich, D. H.; Pugmire, R. J.; Grant, D. M.; Iuliucci, R. J. J. Phys. Chem.
A 2001, 105, 6780-6784.
(73) Amoroso, A J.; Das, A.; McCleverty, J. A.; Ward, M. D.; Barigelletti, F.;
Flamigni, L. Inorg. Chim. Acta 1994, 226, 171-177.
(74) Gaines, G. L.; III.; O’Neil, M. P.; Svec, W. A.; Niemczyk, M. P.;
Wasielewski, M. R. J. Am. Chem. Soc. 1991, 113, 719-721.
9
J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006 7517

A R T I C L E S
Laine´ et al.
Figure 5. Selected ultrafast differential transient absorption spectra of 3.
Table 6. Ultrafast Data with Attribution
Figure 6. Selected ultrafast differential transient absorption spectra of 5.
compared to that of [Os(tpy)2]2+ (i.e., 0, λmax ) 724 nm), which
implies an extended delocalization of the electron within the
ligand framework in the emitting 3MLCT state, once more
similar to that reported for the [Ru(dpb)2]2+ complex.50-52 To
further test our hypothesis, we also performed the same TA
experiment on [Ru(ttpy)2]2+ (6), analogous to 1, and we found
the same process occurring with the lifetime of 2 ps (cf. Figure
SI-5; Supporting Information), so definitely also showing that
this process does not directly involve metal orbitals.
electron
transfer
longer
excited-
state
aryl-tpy
planarization
process
(or
complexes
equilibration)
lifetime
1
2
3
4
5
6
2 ps
5 ps
2 ps
2 ps
2 ps
2 ps
>1ns
>1ns
>1ns
>1ns
870 ps
580 ps
400 ps
870 ps
Interestingly, Maestri et al., within the framework of an early
thorough analysis of substituents electronic effects upon the
photophysical properties of the *[Ru(tpy)2]2+ luminophore,81
have suspected some excited-state structural relaxation when
substituent is a phenyl (analogous of 6), even if planarization
is not explicitly stated. Of note, however, more recently
performed subnanosecond time scale photophysical investiga-
tions on [Ru(ttpy)2]2+ (6) and related dinuclear compounds did
not allow the authors to suspect the occurrence of such a
planarization although they studied the delocalization of MLCT
excited states.82 Actually, the differential transient absorption
spectrum they reported82 for 6 (taken at the end of a 35 ps pulse)
closely resembles to the one recorded here for similar delay,
but their temporal resolution was not sufficient to observe
ultrafast (2 ps) spectral evolution.
The constancy (and the similitude) of the profile of the
differential transient absorption spectra after 20 ps for complexes
1-3 confirms the conclusions inferred from luminescence
properties as far as the inefficiency of ET quenching by the
TP+ subunit is concerned.
B. Excited-State Behavior of Dyads 4 and 5 at Room
Temperature in Fluid Solution. At rt, differential transient
absorption spectra of complexes 4 and 5, after the fast ps-scale
processes, exhibit a different behavior compared to those of the
other osmium species (1-3). Again in this case, the more
diagnostic spectral region is that at λ > 700 nm. Compound 5
transient spectrum decays to the ground state, without any
significant modification in the spectral profile (apart from those
related to the above-mentioned planarization process), with a
single lifetime of about 900 ps (Table 6, Figure 6 and Figure
SI-6 in Supporting Information). For 4, the transient absorption
after 500 fs from pulse) show a strong bleach at about 490 nm
and a weaker bleach at 680 nm, which can be assigned to the
expected bleaching of the spin-allowed and spin-forbidden
MLCT absorption bands, respectively.12 Transient absorption
taking place in both the range of 550-630 nm and at
wavelengths longer than 700 nm, is mainly ascribed to absorp-
tion of the radical anion of ligand(s).12
A. Excited-State Behavior of Compounds 1-3 at Room
Temperature in Fluid Solution. The shape of the differential
absorption spectra clearly changes in the region around 740 nm
for all the species (1-5). In particular, there is a rise of the
absorption accompanied by a change in the band maximum, in
a time scale of a few picoseconds (Figure 5, Table 6). After
this process, the transient spectra of 1-3 remain roughly
constant (on the ps time scale), in agreement with the lumines-
cence lifetimes (Table 4). The change of the transient absorption
in the 740 nm region can be attributed to a modification in the
structure of the radical anion: a similar effect has been indeed
reported for [Ru(dpb)3]2+ (dpb ) 4,4′-diphenyl-2,2′-bipyridine)
and was attributed to the planarization of the two phenyl rings
with respect to the bpy framework (the phenyl rings are canted
with respect to bpy in the ground state and as a consequence in
the initially prepared Franck-Condon excited state).52,78 An
analogous change in structure is believed to occur upon
reduction of biphenyl79,80 and dimethyl-viologen.75 Also, such
a structural relaxation toward planarity, resulting from a CT
state, has been evidenced for the N-methyl-4,4′-bipyridinium
ligand (MQ+) within [Re(MQ+)(CO)3(4,4′-Me2bpy)]2+ 48,49,53
.
Time scale for the planarization of the phenyl rings in the Ru(II)
complex mentioned above was of a few ps, the same time scale
in which the fast process takes place in 1-3. On the basis of
literature data, we assign the picosecond process to planarization
of the aryl rings directly linked to the tpy ligands (θ0 and θ0A
f 0; Chart 3).
(79) Almenningen, A.: Bastiansen, O.; Fernholt, L.; Cyvin, B. N.; Cyvin, S. J.;
Samdal, S. J. Mol. Struct. 1985, 128, 59-76.
(80) Furuya, K.; Torii, H.; Furukawa, Y.; Tasumi, M. THEOCHEM 1998, 424,
225-235.
A further argument supporting our assignment is the red shift
of the room temperature emission energy of 1 (Table 4)
(81) Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Cargill Thompson,
A. M. W. Inorg. Chem. 1995, 34, 2759-2767.
(82) Hammarstro¨m, L.; Barigelletti, F.; Flamigni, L.; Indelli, M. T.; Armaroli,
N.; Calogero, G.; Guardigli, M.; Sour, A.; Collin, J.-P.; Sauvage, J.-P. J.
Phys. Chem. A 1997, 101, 9061-9069.
(78) McCusker, J. K. Acc. Chem. Res. 2003, 36, 876-887.
9
7518 J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006

Conformationally Gated Processes within Dyads. 1.
A R T I C L E S
Figure 7. Selected ultrafast differential transient absorption spectra of 4.
Figure 8. Schematized energy level diagram of 4 and 5. Dashed lines and
solid lines represent nonradiative and radiative processes, respectively. In
4, ken competes with kel and the rate constant of the equilibration process,
keq is (ken + k′en). In 5, kel . ken.
spectrum (Figure 7) also does not present significant differences
in shape, but its decay (Figure SI-7 in Supporting Information)
from the “planarized” CT state is biphasic, exhibiting an initial
lifetime of 400 ps. Then, the whole transient spectrum remains
approximately constant in the experimental time scale (1 ns),
in agreement with the relatively long lifetime (174 ns) of the
emitting state (Table 4). For more details, refer to the Experi-
mental Section and Supporting Information.
is evidencing that, for both 4 and 5, another triplet state is lying
3
close in energy to the lowest MLCT level. Such a state has
been computed as a ligand centered (LC) state largely involving
the nitro group. Supporting the theoretical issues, 77 K
millisecond phosphorescence of the two Aref organic acceptor
models, ph-TPH2(NO2)+ and xy-TPH2(NO2)+ (Chart 2), has
been found at about 725 nm, that is very close to the 77 K
3MLCT emission of 4 and 5 (Table 4).84 In both 4 and 5,
therefore, energy transfer (EnT) from the lowest-lying 3MLCT
state to such a LC (nitro-based) level85-90 can a priori compete
with the ET producing the CSS at room temperature, at least
as the primary deactivation process (see Figure 8). Whether ET
or EnT dominates is a matter of balance between thermodynam-
ics and electronic coupling of the active sites. Apparently, ET
is the dominant primary deactivation process for 5, while this
is not the case for 4. For this latter species, EnT is assumed to
dominate the decay from the 3MLCT state, and this can lead to
equilibration between the two triplet states, when suitable kinetic
and thermodynamic parameters are fitted.91 The equilibrated 3-
MLCT state level can then (i) undergo ET to form the targeted
CSS, which rapidly decays back to the ground state (CR), as
for 5 or (ii) directly deactivate to the ground state. Both of the
hypotheses justify the reduced quantum yield of 4 in comparison
with that of 1, as well as its relatively long luminescence lifetime
(174 ns). In both cases, we tentatively assign the 400 ps decay
obtained by transient spectroscopy to the equilibration time while
the luminescence lifetime can be ascribed (i) to the quenched
lifetime of the equilibrated state, whose unquenched intrinsic
To rationalize the behavior of 4 and 5, let us start from our
3
hypothesis of an oxidative ET from the MLCT state to the
TP+-NO2 unit. We indeed tentatively attribute the short lifetime
of the excited state of 5 (870 ps from transient absorption data,
< 10 ns from luminescence properties) to the occurrence of an
intercomponent ET process. Rate constant of this process (taking
as unquenched reference the luminescence lifetime of 3) would
be 1.1 × 109 s-1. The (very short-lived and less efficient)
emission would therefore be due to the quenched 3MLCT state.
However, formation of the charge-separated state (CSS) is not
directly visible in transient absorption spectroscopy, suggesting
that deactivation of the CSS to the ground state (i.e., the charge
recombination process, CR) is much faster than the forward
process. In other words, reduced acceptor does not accumulate,
as is the case for the Os(tpy)-based triad *[D+-P1-A-] (with
P1 ) Os(II)-bis-ttpy, A ) methyl viologen and D ) di-p-
anisylamine) reported by Sauvage et al.66 Possible contributions
to this fast deactivation process could come from radiationless
pathways involving the nitro group: indeed it is known that
the NO2 group induces very fast radiationless decay of MLCT
levels in metal polypyridine complexes (compare the lumines-
cence lifetimes at room temperature in aqueous solution of [Ru-
(bpy)2(5-nitro-1,10-phenanthroline)]2+ and [Ru(bpy)2(1,10-
phenanthroline)]2+, e7 ns and 684 ns, respectively),72 most
likely via solvent-assisted processes.83 Similar processes could
also accelerate CR in 5.
(84) Part 2: Laine´, P. P.; Loiseau, F.; Campagna, S.; Ciofini, I.; Adamo, C.
Inorg. Chem. 2006, in press.
(85) Maubert, B.; McClenaghan, N. D.; Indelli, M. T.; Campagna, S. J. Phys.
Chem. A 2003, 107, 447-455.
The situation appears more complicated for 4. For this species,
a priori and on thermodynamic basis, the transient decay of 400
ps (which translates into a quenching rate constant of 2.5 ×
109 s-1) could still be attributed to electron transfer, but this
decay process does not lead to the ground state, which is reached
on quite a longer time scale, in agreement with the luminescence
lifetime (Table 4).
To understand the properties of 4, it’s worth taking into
account the recently obtained results of a theoretical investiga-
tion based on Density Functional Theory (DFT).84 This study
(86) Ford, W. E.; Rodgers, M. A. J J. Phys. Chem. 1992, 96, 2917-2920.
(87) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak, P.;
Jin, X.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11012-11022.
(88) Tyson, D. T.; Castellano, F. N. J. Phys. Chem. A 1999, 103, 10955-10960.
(89) Passalacqua, R.; Loiseau, F.; Campagna, S.; Fang, Y.-Q.; Hanan, G. S.
Angew. Chem., Int. Ed. 2003, 42, 1608-1611.
(90) McClenaghan, N. D.; Leydet, Y.; Maubert, B.; Indelli, M. T.; Campagna,
S. Coord. Chem. ReV. 2005, 249, 1336-1350.
(91) It is known that when in multichromophoric species where luminescent
metal-based MLCT triplet states are close in energy with triplet states
involving organic chromophores, equilibration between the triplet states
can occur.85-90 In particular, when the rate constant of any other decay is
negligible compared to the equilibration time, the lifetime of the equilibrated
state is dictated by weighted linear combination of the intrinsic lifetimes
of the components according to Boltzmann distribution.
(83) Gabrielsson, A.; Matousek, P.; Towrie, M.; Hartl, F.; Zalis, S.; Vlcek, A.,
Jr. J. Phys. Chem. A 2005, 109, 6147-6153.
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J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006 7519

A R T I C L E S
Laine´ et al.
lifetime would be quite longer in this case,92 or (ii) to the lifetime
of the equilibrated state.93 We have no simple way to discrimi-
nate between the routes (i) and (ii), also considering that the
LC triplet is not expected to have significant transient absorption
features in comparison with the MLCT state, so the lack of
changes in the transient spectrum could be misleading. It should
be noted that case (i) would indicate a slow electron-transfer
rate (kel ≈ 6 × 106 s-1) for the oxidative PET from the
equilibrated state to the acceptor A subunit. However, such an
electron transfer from an equilibrated state could be a compli-
cated process, so we do not feel this is enough to rule out this
decay route. In any case, as a matter of fact the primary
deactivation process of the luminescent MLCT state in 4 and 5
appears to be different.
to TP+ slightly exoergonic, and indeed such a process is
effective at room temperature for the conformationally locked
compound 5, leading to targeted CSS formation. For the
conformationally unlocked compound 4, fast equilibration
between the MLCT and a NO2-based LC triplets takes place
(reversible EnT) and precedes ET decay with CSS formation.
This finding strongly points out that excited-state relaxation and
thermal structure fluctuations about restrained (i.e., weakly
restricted) or constrained (i.e., strongly restricted and even
locked) conformations can have pivotal roles in determining
both the rate constant of the decay processes and also the
dominant mechanism of the decay itself. In other words, we
have evidenced here a conformational gating of photoinduced
processes.
Beyond providing guidelines for the design of next generation
of TP+-based PMDs (toward “redox cascades”), the present
work affords issues of relevance for the field of supramolecular
photochemistry as aryl-substituted oligopyridine ligands are of
widespread use to build complexes photosensitizers within
photoactive functional arrays or supramolecular architec-
tures.12,96,97 Also, propensity for increasing compactness of
PMDs’ in view to multifunctional integration de facto correlates
with the rising importance of monitoring intercomponent
electronic coupling arranged by the interplay of active units with
the orbitals of intervening spacer elements. Last, possible
implications of conformational gating for the future regarding
switching effects and sensors relying on modulation of emission
lifetimes98 must be considered.
It remains to be commented the reason for the different
behavior of 4 and 5 with regard to the ET/EnT quenching
processes, in particular the fact that ET appears to dominate in
5 while EnT dominates in 4, at least at an early stage. From a
thermodynamic viewpoint, ET and EnT processes in 4 and 5
are in fact very similar. Also, these species virtually have the
same conformation (about θ1) on average. Basically, the dyads
are only differentiated by existing conformational locking within
5 ensured by R0 methyl substituents of the xylyl spacer (Charts
1 and 2), which reduce the amplitude of thermal structure
fluctuations as compared to 4. Thus, the minimized electronic
coupling between the MLCT chromophore (P1/Os) and the
electron and energy acceptor (A) unit is maintained in the
excited state. Noteworthy, A is in both cases the TP+-NO2
moiety. The importance of electronic coupling is larger for
electron exchange EnT, that is the mechanism considered to
drive EnT in our systems, than for ET. Thus, it can be foreseen
that EnT decay is slowed to a much larger extent than the ET
process on passing from 4 to 5. As a consequence, whereas
EnT (that is, the equilibration process) is competitive in 4, it is
much less an efficient decay route in 5. There are several
examples reported in the literature in which restricted spacer
conformations limit intercomponent coupling and therefore ET
and/or EnT processes.41,43,45,57,61,94,95
4. Experimental Section
4.1. Syntheses, Characterization and General Experimental
Details. Electronic absorption spectra were measured on a JASCO
V-570 spectrophotometer. 1H NMR spectra were recorded on a Bruker
ARX 250 spectrometer. Elemental analyses were performed at the
Institut de Chimie des Substances Naturelles, France. ESI(+) mass
spectra (solvent, acetonitrile) were recorded with a LCQ-advantage
(ThermoFinnigan) mass spectrometer.
4.2. Crystal Structure Determination. Suitable crystal of ligand
tpy-xy-TPH2(NO2)+ was mounted on a Bruker-Nonius KappaCCD
diffractometer. Orientation matrix and lattice parameters were obtained
by least-squares refinement of the reflections obtained by a θ-ø scan
(Dirax/lsq method). Data were collected at 293(2) K using graphite-
monochromated Mo-KR radiation (λ ) 0.710 73 Å). A summary of
the crystallographic data and structure refinement is given in Table
SI-1 (Supporting Information). The indexes of data collection were -13
e h e 11, -18 e k e 18, -17 e l e 20. Of the 8997 measured
independent reflections in the θ range 2.15-28.0°, 5494 have I g 2σ
(I). All the measured independent reflections were used in the analysis.
All calculations for data reduction, structure solution, and refinement
were done by standard procedures (WINGX).99 The structure was solved
by direct methods and refined with full-matrix least-squares technique
on F2 using the SHELXS-97100 and SHELXL-97101 programs. A careful
inspection of the difference maps and subsequent refinement of the
site occupation factors (sof) showed two statistical positions for the
Furthermore, we cannot exclude that EnT in 4 is favored by
a particular planar conformation on the occasion of thermal
structure fluctuations about θ1 mean angle,41b while such a
conformation is not possible for 5 whose intercomponent
geometrical decoupling is locked.
3. Summary
We synthesized and studied a series of osmium(II)- and
ruthenium(II)-based photosensitizer-electron acceptor dyads
containing triarylpyridinio-functionalized (TP+) terpyridyl ligands.
Our results indicate the following:
-Planarization is apparently an efficient way of structural
relaxation of MLCT states.
-A nitro substituent on the TP+ electron acceptor subunit
makes MLCT emission quenching by oxidative electron transfer
(94) Schlicke, B.; Belser, P.; De Cola, L.; Sabbioni, E.; Balzani, V. J. Am. Chem.
Soc. 1999, 121, 4207-4214.
(95) Scandola, F.; Chiorboli, C.; Indelli, M. T.; Rampi, M. A. in Electron
Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001;
Vol. 3, p 337.
(92) Within such an assumption, the emission lifetime of 4 would be adventi-
tiously falling in the time scale typical for P1/Os-based luminophores when
barely affected, as is the case for dyads 2 and 3, and could be of several
orders of magnitude longer in the absence of the PET process.
(93) The unquenched lifetime of the equilibrated state depends on the energy
gap between the involved states and on the intrinsic lifetime of the LC
(nitro-based) level at room temperature. Although we know the 77 K
lifetime of the free ligands containing the nitro group (which is in the ms
time scale)84 at the moment we have no information on the room-
temperature lifetime of the LC state in the complexes.
(96) (a) Hofmeier, H.; Schubert, U. S. Chem. Soc. ReV. 2004, 33, 373-399. (b)
Andres, P. R.; Schubert, U. S. AdV. Mater. 2004, 16, 1043-1068.
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1996, 96, 759-833.
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6669.
(99) Farrugia, L. J. WINGX, J. Appl. Crystallogr. 1999, 32, 837-838.
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7520 J. AM. CHEM. SOC. VOL. 128, NO. 23, 2006

Conformationally Gated Processes within Dyads. 1.
A R T I C L E S
nitrogen atom and one carbon atom of one acetonitrile molecule. The
atoms of the other acetonitrile molecule were refined anisotropically
with fixed occupancy factors which were established in a previous
refinement with isotropic thermal factors (sof is 0.5). All hydrogen
atoms were both set in calculated positions and isotropically refined.
The final Fourier-difference map showed maximum and minimum
height peaks of 0.627 and -0.491 e Å-3. The final geometrical
calculations and the graphical manipulations were carried out with
DIAMOND101 program.
4.3. Electrochemical and Spectroelectrochemical Measurements.
The electrochemical and UV-vis. spectroelectrochemical experimental
setup have been described in ref 37a. Cyclic voltammetric data were
used to estimate formal potentials E1/2 as (Epa + Epc)/2 where Epa and
Hamamatsu PLP 2 laser diode (59 ps pulse width at 408 nm) and the
nitrogen discharge (pulse width, 2 ns at 337 nm) were employed.
Picosecond time-resolved experiments were performed using a
previously described pump-probe setup,105 based on the Spectra-
Physics Hurricane Ti:sapphire laser source and an Ultrafast Systems
Helios spectrometer. The pump pulse was generated by a frequency
doubler (400 nm). The probe pulse was obtained by continuum
generation on a sapphire plate (useful spectral range, 450-800 nm).
The effective time resolution was ca. 300 fs, the temporal chirp over
the white-light 450-750 nm range was ca. 200 fs, and the temporal
window of the optical delay stage was 0-1000 ps. The decays reported
in the text are best fitting values related to an analysis of the transient
spectra decays performed at twenty probe wavelengths between 500
and 760 nm.
Epc are the anodic and cathodic peak potentials related to the considered
redox process. In some cases, E1/2 was evaluated directly on the
hydrodynamic voltammograms as being the half-wave potential value.
The number n of electrons involved in the redox process was determined
by (i) comparison with reference components, or (ii) with respect to
the internal monoelectronic oxidation of the metal center within the
examined complexes or (iii) from the value of Epc-Ep/2 which is equal
to 56.5/n mV at 25 °C for reversible redox processes (where Ep/2 is the
potential value at the half peak current intensity).102
Acknowledgment. We are indebted to Dr. Carine Guyard-
Duhayon, Dr. Mar´ıa Herna´ndez-Molina and Lise-Marie
Chamoreau for the X-ray structure determination of [tpy-xy-
TPH2(NO2)](BF4) as well as to Dr. Edmond Amouyal for the
determination of emission lifetimes of 3 and 5. The authors
thank Prof. Carlo Adamo and Dr. Ilaria Ciofini for stimulating
discussions. P.P.L. is grateful to the French ministry of research
for financial support (ACI project no. JC4123). S.C. and C.C.
also acknowledge MIUR (FIRB and PRIN projects) for funding.
4.4. Photophysical Properties. For 1-5, uncorrected emission
spectra were obtained with a Jobin Yvon Spex Fluorolog FL 111
spectrofluorimeter (λexc. ) 600 nm). For 0, 6, 9, and 10, luminescence
spectra were recorded with a Spex-Jobin Yvon Fluoromax-P spectro-
fluorimeter equipped with a Hamamatsu R3896 photomultiplier, and
were corrected for photomultiplier response using a program purchased
with the fluorimeter (λexc. ) 450 nm). Luminescence quantum yields
for argon-degassed acetonitrile solutions of the complexes have been
calculated by using the optically dilute method.103 These quantum yields
were determined relative to reference acetonitrile solutions of [Os-
Supporting Information Available: Experimental details
regarding synthesis and characterization of new compounds
1
including H NMR spectra of 3-5 and 8-10; a summary of
the crystallographic data and structure refinement for tpy-xy-
TPH2(NO2)+ ligand (1 Table); UV-vis. spectroelectrochemical
spectral changes for xy-TPH2(NO2)+ upon reduction as well as
for 5 and 10 upon oxidation; normalized rt and lt emission
spectra of the 1-5 series of complexes; ultrafast differential
transient absorption (TA) decays for 3, 4 and 5, as well as TA
spectra and decays for 6. (PDF). X-ray structural data for the
tpy-xy-TPH2(NO2)+ ligand (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
104
71
(bpy)3]2+ (Φem ) 5 × 10-3
)
or [Ru(bpy)3]2+ (Φem ) 6.2 × 10-2
)
at 298 K, for Os-based and Ru-based species, respectively.
Excited-state lifetimes of 1,37a 2,37a 3, and 5 were determined by
laser flash spectroscopy using a nanosecond setup previously described
(λexc. ) 308 nm).37a For the luminescence lifetimes of 0, 4, 6, 9, and
10, an Edinburgh OB 900 time-correlated single-photon-counting
spectrometer was used in the ns range. As excitation sources, a
JA058357W
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Applications, Second Edition; Wiley: New York, 2001, p 213.
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