Triplet energy transfers in electrostatic host-guest assemblies of unsaturated organometallic cluster cations and carboxylate-containing porphyrin pigments

Article abstract of DOI:10.1021/ic801006g

The unsaturated cyclic [M₃(dppm)₃(CO)]²⁺ clusters (M = Pt, Pd; dppm = Ph₂PCH₂PPh₂; such as PF₆^- salt) exhibit a cavity formed by the six dppm-phenyl groups placed like a picket fence above the unsaturated triangular M₃ dicationic center. Electrostatic interactions of the M ³⁺ units inside this cavity with the carboxylate anion RCO ₂^- [R = tetraphenylporphyrinatozinc(II), ZnTPP; p-phenyltritolylporphyrinatozinc(II), ZnTTPP; p-phenyltritolylporphyrinato- palladium(II), PdTTPP] form dyads for through-space triplet energy transfers. The binding constants are on the order of 20 000 M^-1 in all six cases (298 K). The energy diagram built upon absorption and emission spectra at 298 and 77 K places the [Pt₃(dppm)₃(CO)]²⁺ and [Pd₃(dppm)₃(CO)]²⁺ as triplet energy donors, respectively, with respect to the ZnTPPCO₂^-, ZnTTPPCO ₂^-, and PdTTPPCO₂^- pigments, which act as acceptors. Evidence for energy transfer is provided by the transient absorption spectra at 298 K, where triplet-triplet absorption bands of the metalloporphyrin chromophores are depicted at all time (at 298 K) with total absence of the charge-separated state in the nanosecond to microsecond time scale. Rates for energy transfer (ranging in the 10⁴ s^-1 time scale) are extracted from the emission lifetimes of the [Pt ₃(dppm)₃CO)]²⁺ donor in the free chromophore and the host-guest assemblies. The emission intensity of [Pd₃(dppm) ₃(CO)]²⁺ is too weak to measure its spectrum and emission lifetime in the presence of the strongly luminescent metalloporphyrin-containing materials. For the [Pd₃-(dppm)₃(CO)]²⁺... metalloporphyrin dyads, evidence for fluorescence and phosphorescence lifetime quenching of the porphyrin chromophore at 298 K is provided. These quenchings, exhibiting rates of 10⁴ (triplet) and 10⁸ s^-1 (singlet), are attributed to a photoinduced electron transfer from the metalloporphyrin to the cluster due to the low reduction potential.

Full text of DOI:10.1021/ic801006g

Inorg. Chem. 2008, 47, 9930-9940
Triplet Energy Transfers in Electrostatic Host-Guest Assemblies of
Unsaturated Organometallic Cluster Cations and Carboxylate-Containing
Porphyrin Pigments
Shawkat M. Aly,† Charfedinne Ayed,‡ Christine Stern,‡ Roger Guilard,*,‡ Alaa S. Abd-El-Aziz,§
and Pierre D. Harvey*,†
De´partement de Chimie, UniVersite´ de Sherbrooke, 2550 BouleVard de l’UniVersite´, Sherbrooke,
Quebec, Canada J1K 2R1, Institut de Chimie Mole´culaire de l’UniVersite´ de Bourgogne (ICMUB,
UMR 5260), UniVersite´ de Bourgogne, Dijon, France, and UniVersity of British Columbia at
Okanagan, 3333 UniVersity Way, Kelowna, British Columbia, Canada V1V 1V7
Received June 1, 2008
The unsaturated cyclic [M3(dppm)3(CO)]2+ clusters (M ) Pt, Pd; dppm ) Ph2PCH2PPh2; such as PF6- salt) exhibit
a cavity formed by the six dppm-phenyl groups placed like a picket fence above the unsaturated triangular M3
dicationic center. Electrostatic interactions of the M3+ units inside this cavity with the carboxylate anion RCO2- [R
) tetraphenylporphyrinatozinc(II), ZnTPP; p-phenyltritolylporphyrinatozinc(II), ZnTTPP; p-phenyltritolylporphyrinato-
palladium(II), PdTTPP] form dyads for through-space triplet energy transfers. The binding constants are on the
order of 20 000 M-1 in all six cases (298 K). The energy diagram built upon absorption and emission spectra at
298 and 77 K places the [Pt3(dppm)3(CO)]2+ and [Pd3(dppm)3(CO)]2+ as triplet energy donors, respectively, with
respect to the ZnTPPCO2-, ZnTTPPCO2-, and PdTTPPCO2- pigments, which act as acceptors. Evidence for
energy transfer is provided by the transient absorption spectra at 298 K, where triplet-triplet absorption bands of
the metalloporphyrin chromophores are depicted at all time (at 298 K) with total absence of the charge-separated
state in the nanosecond to microsecond time scale. Rates for energy transfer (ranging in the 104 s-1 time scale)
are extracted from the emission lifetimes of the [Pt3(dppm)3(CO)]2+ donor in the free chromophore and the host-guest
assemblies. The emission intensity of [Pd3(dppm)3(CO)]2+ is too weak to measure its spectrum and emission
lifetime in the presence of the strongly luminescent metalloporphyrin-containing materials. For the [Pd3-
(dppm)3(CO)]2+ · · · metalloporphyrin dyads, evidence for fluorescence and phosphorescence lifetime quenching of
the porphyrin chromophore at 298 K is provided. These quenchings, exhibiting rates of 104 (triplet) and 108 s-1
(singlet), are attributed to a photoinduced electron transfer from the metalloporphyrin to the cluster due to the low
reduction potential.
Introduction
bisporphyrin dyads (Chart 1), where the donor and acceptor
macrocycles are held together by a rigid spacer (except for
DPOx, which is flexible). The rigidity of the spacers allows
the bismacrocycles to adopt a slipped dimer geometry,
keeping the transition moment perfectly parallel. Hence, these
Through-space singlet energy transfers [ET(S1)] and
energy delocalization and migration (exciton) across the
photosynthetic antennas of photosynthetic bacteria, algea, and
plants are the two basic nonradiative processes that secure a
good constant excitation of the special pair in the reaction
center (RC).1,2 Recently, we investigated both the singlet
[ET(S1)] and triplet [ET(T1)] energy transfers in cofacial
(1) Light-HarVesting Antennas in Photosythesis (AdVances in Photosyn-
thesis and Respiration); Green, B. R., Parson, W. W., Eds.; Kluwer
Academic Publishers: Dordrecht, The Netherlands, 2003.
(2) Harvey, P. D.; Stern, C.; Gros, C. P.; Guilard, R. J. Inorg. Biochem.
2008, 102, 395–405.
(3) Gros, C. P.; Aly, Sh. M.; El Ojaimi, M.; Barbe, J.-M.; Brisach, F.;
Abd-El-Aziz, A. S.; Guilard, R.; Harvey, P. D. J. Porphyrins
Phthalocyanines 2007, 11, 244–257.
* To whom correspondence should be addressed. E-mail: Pierre.Harvey@
USherbrooke.ca. Tel. +1 (819) 821-7092. Fax: +1 (819) 821-8017.
†
Universite´ de Sherbrooke.
Universite´ de Bourgogne.
University of British Columbia at Okanagan.
‡
(4) Gros, C. P.; Brisach, F.; Meristoudi, A.; Espinosa, E.; Guilard, R.;
Harvey, P. D. Inorg. Chem. 2006, 46, 125–135.
§
9930 Inorganic Chemistry, Vol. 47, No. 21, 2008
10.1021/ic801006g CCC: $40.75  2008 American Chemical Society
Published on Web 10/10/2008

Energy Transfers in Electrostatic Host-Guest Assemblies
Chart 1
Chart 2
cofacial species can mimic the photophysical behavior of
one pair of bacteriochlorophylls a located inside the nine
apoproteins of the light-harvesting device II (LH II) of the
photosynthetic membrane in the purple photosynthetic
bacteria.1
Whereas ET(S1) operates via both Fo¨rster (most encoun-
tered) and Dexter mechanisms,6,9 ET(T1) usually proceeds
via Dexter’s mechanism. The latter involves a double electron
exchange [LUMO(donor) f LUMO(acceptor) and HO-
MO(acceptor) f HOMO(donor)], which requires close
proximity of the donor and acceptor for better orbital overlap.
One of the consequences is that the rate for ET(T1), kET(T1),
is slower than that for ET(S1). This was demonstrated in a
recent literature survey where both kET(S1) and kET(T1) were
compared.10 This paper also showed the relatively low
number of data available for kET(T1).
One of the questions that now arises is, to what extent
can through-space ET(T1) still operate in electrostatically held
dyads in the triplet state because there are no other ex-
amples.10 The separation between an energy donor and
acceptor may be variable from one system to another, and
so much is still to learn, such as the effect of the separation
versus the rate for energy transfer, particularly when the
triplet-state energy transfer is known to operate only via a
double electron exchange mechanism (i.e., no Fo¨rster
process). In line with the above comments, we now report a
study on through-space ET(T1) operating in cluster
(donor)2+ · · ·-O2C-C6H4 (acceptor) dyads across electrostas-
tic bridges composed of unsaturated organometallic clusters
of the type M3(dppm)3(CO)2+ (M ) Pt, Pd; donor) and
metallo(II)tetraarylporphyrin carboxylates (acceptor) (Chart
2 and Figure 1). These clusters were chosen because of their
structurally well-defined electrostatic ion-pair adducts (the
ligand enters the cavity with a 90° angle with respect to the
M3 plane because of steric reasons; Figure 1) and their
luminescence properties using these signals as probes for
triplet energy transfer (see the text below and references
indicated therein).
Experimental Section
Materials. The [M3(dppm)3(CO)]2+ clusters (such as PF6- salts)
were prepared according to literature procedures.11 The sodium
benzoate was available in our laboratory as (BDH) grade. The 5-(4-
carboxyphenyl)-10,15,20-triphenylporphyrinatozinc(II) and 5-(4-
carboxyphenyl)-10,15,20-tritolyporphyrinatopalladium(II) and -z-
inc(II) as well as tetraphenylporphyrinatopalladium(II) (PdTPP)
were synthesized according to the literature.12
The carboxylate salts [ZnTPPCO2Na (I), ZnTTPPCO2Na (II),
and PdTTPPCO2Na (III); Chart 2] were obtained by neutralization
of the above corresponding 4-carboxyphenylporphyrin acids. The
porphyrin-containing carboxylic acid (0.1 g) was reacted with a
large excess (3 g) of sodium carbonate in acetone for 48 h. The
reaction mixture was poured into distilled water to dissolve the
excess Na2CO3. The resulting sodium salts were filtered and washed
with a slightly alkaline solution (aqueous Na2CO3, pH ∼ 8). The
removal of the carboxylic proton was confirmed by 1H NMR. Data
1
for I. H NMR (DMSO-d6, ppm): δ 7.79-7.81 (m, 9H, phenyl-
H
m,p), 8.05 (d, 2H, COO-, COO--phenyl-Hm, J ) 7.93 Hz), 8.18
(d, 6H, phenyl-Ho, J ) 7.50 Hz),, 8.23 (d, 2H, COO-, COO--
phenyl-Hm, J ) 7.79 Hz), 8.76 (m, 8H, ꢀ-pyrrole H). Data for II.
1H NMR (DMSO-d6, ppm): δ 2.67 (s, 9H, tolyl-CH3), 7.62 (d, 6H,
phenyl-Hm, J ) 7.94 Hz), 8.08 (d, 2H, COO-, COO--phenyl-Ho,
J ) 7.93 Hz), 8.20 (d, 2H, COO-, COO--phenyl-Hm, J ) 7.80
Hz), 8.10 (d, 6H, phenyl-Ho, J ) 7.91 Hz), 8.80 (m, 8H, ꢀ-pyrrole
1
H). Data for III. H NMR (DMSO-d6, ppm): δ 2.66 (s, 9H, tolyl-
CH3), 7.62 (d, 6H, phenyl-Hm, J ) 7.94 Hz), 8.06 (d, 2H, COO-,
COO--phenyl-Ho, J ) 7.93 Hz), 8.34 (d, 2H, COO-, COO--phenyl-
Hm, J ) 7.93 Hz), 8.48 (d, 6H, phenyl-Ho, J ) 8.00 Hz), 8.78 (m,
8H, ꢀ-pyrrole H).
(5) Harvey, P. D.; Stern, C.; Gros, C. P.; Guilard, R. Coord. Chem. ReV.
2007, 251, 401–428.
(6) Faure, S.; Stern, C.; Guilard, R.; Harvey, P. D. J. Am. Chem. Soc.
2004, 126, 1253–1261.
(7) Poulin, J.; Stern, C.; Guilard, R.; Harvey, P. D. Photochem. Photobiol.
2006, 82, 171–176.
(8) Faure, S.; Stern, C.; Espinosa, E.; Guilard, R.; Harvey, P. D.
Chem.sEur. J. 2005, 11, 3469–3481.
(9) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings:
Menlo Park, CA, 1978; Chapter 9, pp 302-309.
(10) Harvey, P. D. Recent Advances in Free and Metalated Multi-Porphyrin
Assemblies and Arrays; A Photophysical Behavior and Energy
Transfer Perspective. In The Porphyrin Handbook; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2003;
Vol. 18, pp 63-250.
(11) (a) Ferguson, G.; Lloyd, B. R.; Puddephatt, R. Organometallics 1986,
344–348. (b) Manojlovic-Muir, L.; Muir, K. W.; Lloyd, B. R.;
Puddephatt, R. J. Chem. Soc., Chem. Commun. 1983, 1336–1337.
(12) (a) Kibbey, C. E.; Meyerhoff, M. E. Anal. Chem. 1993, 65, 2189–
2196. (b) Fungo, F.; Otero, L. A.; Sereno, L.; Silber, J. J.; Durantini,
E. N. J. Mater. Chem. 2000, 10, 645–650. (c) Fanti, C.; Monti, D.;
La Monica, L.; Ceccacci, F.; Mancini, G.; Paolesse, R. J. Porphyrins
Phthalocyanines 2003, 7, 112. (d) Scalise, I.; Durantini, E. N. J.
Photochem. Photobiol. A 2004, 162, 105–113.
Inorganic Chemistry, Vol. 47, No. 21, 2008 9931

Aly et al.
using a double-monochromator Fluorolog 2 instrument from Spex.
The emission lifetimes were measured on a TimeMaster model TM-
3/2003 apparatus from PTI. The source was a nitrogen laser with
a high-resolution dye laser (fwhm ∼ 1400 ps), and the fluorescence
lifetimes were obtained from deconvolution or distribution lifetime
analysis. The uncertainties were about 50-100 ps. The phospho-
rescence lifetimes were performed on a PTI LS-100 using a 1 µs
tungsten flash lamp (fwhm ∼ 1 µs). The flash photolysis spectra
and the transient lifetimes were measured with a Luzchem
spectrometer using the 355 nm line of a YAG laser from Continuum
(Serulite) and the 530 nm line from a OPO module pump by the
same laser (fwhm ) 13 ns).
Quantum Yields. Measurements were performed in 2MeTHF
at 77 K. Three different measurements (i.e., different solutions)
were prepared for each photophysical datum (quantum yields and
lifetimes). The sample and standard concentrations were adjusted
to obtain an absorbance of 0.05 or less. This absorbance was
adjusted to be the same as much as possible for the standard and
sample for a measurement. Each absorbance value was measured
10 times for better accuracy in the measurements of the quantum
yields. The reference used for quantum yield was TPP(Pd) (Φ )
0.14).14
Figure 1. Top: Space-filling model of [M3(dppm)3(CO)]2+ (M ) Pd, Pt)
showing the cavity above the unsaturated face of the M3 center. The variable
size of the cavity depends on the relative conformation of the dppm ligand.
Bottom: Investigated photophysical processes occurring when the
[M3(dppm)3(CO)]2+ energy donor is anchored with one of the acceptors
shown in Chart 2.
Results and Discussion
Absorption Data. The frontier molecular orbitals (MOs)
of the M3(dppm)3CO2+ clusters (M ) Pd, Pt) (C3V point
group) consist of a HOMO (a1) that is mostly composed of
M d orbitals with some pz and some CtO n and σ characters,
forming a weakly Pd-Pd bonding in the M3 plane and Pd-C
antibonding.15,16 The HOMO-1 consists of degenerate
orbitals (e), which are almost entirely composed of in-plane
M d and p orbitals and are formally M-M bonding. The
Spectroscopic Measurements. The spectroscopic and photo-
physical measurements were carried out in 2MeTHF, which was
distilled over calcium hydride under argon. For the [Pd3(dppm)3-
(CO)]2+ cluster, the absorption measurements associated with the
binding constant evaluation were carried out in methanol because
it was reported to give the largest difference in λmax between the
free cluster and host-guest assembly.13
2
2
LUMO (a2) is composed of M in-plane d orbitals (dxy, dx -y )
Methodology. The binding constants (K11) were measured
according to methodology outlined in ref 13. The typical method
used to measure the binding constant was as follows. Two different
solutions (A and B) were prepared in methanol or 2MeTHF.
with some minor px and py contributions and some P p
components (py, px), forming M-M and M-P antibonding
(σ*) MOs. The two lowest-energy electronic transitions are
the symmetry-forbidden a1 f a2 and the allowed and
polarized M3 in-plane e f a2 transitions. The latter leads to
an intense band, for both M ) Pd and Pt. The lowest-energy
excited states were assigned to 1,3[1(a2)1(a1)]*, which are
analogous to the typical dσdσ* ones for the dinuclear M-M
bonded systems.17
-
Solution A contained the “free cavity” cluster as the PF6 salt
([Pd3(dppm)3(CO)]2+ ) 6.70 × 10-5 M, [Pt3(dppm)3(CO)]2+ ) 5.85
× 10-5M). Solution B was prepared by mixing of the cluster with
exactly the same concentration as that used in solution A with an
exactly equal concentration of the porphyrin carboxylate salt in
order to achieve a 1:1 host-guest assembly. The spectroscopic
changes induced by solution A as a result of additions of constant
volume (0.1 mL) of solution B were monitored by measuring the
absorption spectra after each addition. The competitive binding
constants (K11) were measured by plotting -1/∆A vs 1/[substrate]
(Benesi-Hildebrand), where ∆A is the absorbance change upon an
increase in the substrate concentration. The substrate concentration
was corrected based on the change of the total volume at each
addition, and these adjusted values were used for the plots. The
ratio of intercept/slope in this plot gives K11. As counterchecks,
the K11 values were also evaluated by using the Scatchard and Scott
plots ∆A/[substrate] vs -∆A with K11 ) -slope (Scatchard) and
-[substrate]/∆A vs [substrate] with K11 ) slope/intercept (Scott)
and are all found to be the same within the experimental uncertain-
ties (10%. The uncertainties are based on multiple measurements.
Instruments. UV-visible spectra were recorded on a Varian
Cary 300 spectrophotometer. The emission spectra were obtained
The [Pd3(dppm)3(CO)]2+ cluster in methanol,15b,18 exhibits
a structureless absorption band at ∼494 nm (i.e., e f a2) at
298 K. In the presence of electron-donor guests, these anions
coordinatively add to the unsaturated sites of the
[Pd3(dppm)3(CO)]2+ cluster inside the cavity [a picture
showing the cavity formed by the dppm-phenyl groups is
given as Supporting Information (SI)]. This host-guest
process is accompanied by spectroscopic changes in the
UV-visible spectra with obvious isosbestic points.18 The
(14) Bolze, F.; Gros, C. P.; Harvey, P. D.; Guillard, R. J. Porphyrins
Phthalocyanines 2001, 5, 569–574.
(15) (a) Harvey, P. D.; Hubig, S. M.; Ziegler, T. Inorg. Chem. 1994, 33,
3700–3710. (b) Harvey, P. D.; Provencher, R. Inorg. Chem. 1993,
32, 61–65.
(16) Harvey, P. D.; Mugnier, Y.; Lucas, D.; Evard, D.; Lemaitre, F.; Vallat,
A. J. Cluster Sci. 2004, 15, 63–90.
(17) Harvey, P. D.; Murtaza, Z. Inorg. Chem. 1993, 32, 4721.
(18) Manojlovic-Muir, L.; Muir, K. W.; Lloyd, B.; Puddephatt, R. J.
J. Chem. Soc., Chem. Commun. 1985, 536–537.
(13) Provencher, R.; Aye, K. T.; Drouin, M.; Gagnon, J.; Boudreault, N.;
Harvey, P. D. Inorg. Chem. 1994, 33, 3689–3699.
9932 Inorganic Chemistry, Vol. 47, No. 21, 2008

Energy Transfers in Electrostatic Host-Guest Assemblies
Table 1. K11 Values for I-III with [Pd3(dppm)3(CO)]2+ in MeOH at
298 Ka
-1
K11 (M
)
substrate
Benesi-Hildebrand
Scott
Scatchard
-
ZnTPPCO2 (I)
22 500
18 900
20 100
19 300
17 100
18 500
19 000
20 300
20 200
-
ZnTTPPCO2 (II)
PdTTPPCO2 (III)
a The uncertainties are (10% based on multiple measurements.
-
Figure 2. UV-visible spectra for the addition of ZnTPPCO2Na (I) into a
methanol solution of [Pd3(dppm)3(CO)]2+. [[Pd3(dppm)3(CO)]2+] ) 6.70
× 10-5 M (curve a); [I] ) 0.42 × 10-5 M (curve b). Curves c-m were
obtained with successive additions of 0.67 × 10-5 M of I. (The arrows
indicate the direction of absorption change with the addition of guests.)
binding constant, K11, of a substrate inside the cavity is
related to the charge, electron-donating ability, size, and
hydrophobic properties of the substrates, which is consistent
with the bifunctional recognition properties of the cluster
cavity (cationic metal center and hydrophobic pocket).13 λmax
of absorption of this intense lowest-energy band (e f a2) of
the [Pd3(dppm)3(CO)]2+ cluster (such as the PF6- salt) blue-
shifts from 494 to about 466 nm (the exact position depends
on the substrate), in the absence and presence of a guest
molecule, respectively.15
The spectroscopic changes in the absorption spectra upon
successive additions of three carboxylate-containing por-
phyrin solutions at constant concentration (I-III, Chart 1)
to a [Pd3(dppm)3(CO)](PF6)2 solution (6.70 × 10-5 M) in
methanol at 298 K are now presented. The presence of
isosbectic points in the evolution of the UV-visible spectra
indicating a 1:1 equilibrium between the free cluster (empty
cavity) and host-guest assemblies is depicted (eq 1).
Figure 3. UV-visible spectra for the addition of ZnTPPCO2Na (I) into a
2MeTHF solution of [Pt3(dppm)3(CO)]2+. [[Pt3(dppm)3(CO)]2+] ) 5.85 ×
10-5 M (curve a); [I] ) 0.79 × 10-5 M (curve b). Curves c-l were obtained
with successive additions of 0.67 × 10-5 M of I. (The arrows indicate the
direction of absorption change with the addition of guests.)
same K11 constants are also found for all three complexes
(I · · · [Pd3(dppm)3(CO)]2+, II · · · [Pd3(dppm)3(CO)]2+, and
III · · · [Pd3(dppm)3(CO)]2+), which is consistent with the
nature of the electrostatic interactions (Pd32+ · · ·-O2C-R) as
well as the steric hindrance being the same for all three cases.
The observed K11 values are higher (by a factor of ∼2)
than that reported for the sodium benzoate · · · [Pd3-
(dppm)3(CO)]2+ complex (10 000 M-1).13 We tentatively
explain this difference by a contribution of the high electron
density of porphyrin inducing a high electron density at the
carboxylate center. This increase would result in stronger
interactions with a divalent cluster cation and hence increas-
ing binding constants. This hypothesis is based on our
previous studies on various RCO2- substrates where R was
varied (Me, Ph, etc.).13
[Pd3(dppm)3(CO)]2+
+
substrate ymethanolz [Pd3(dppm)3(CO)]2+···substrate (1)
where substrate ) I-III. The spectroscopic changes ac-
companying the successive addition of ZnTPPCO2Na (I) to
a solution of [Pd3(dppm)3(CO)](PF6)2 are shown in Figure
2 as an example. The addition of ZnTPPCO2Na (I), while
keeping the concentration of [Pd3(dppm)3(CO)]2+ constant,
induces a decrease of the absorption at λmax of 492 nm (free
cavity) and an increase of a new band at λmax of 460 nm
(filled cavity).
The [Pt3(dppm)3(CO)]2+ cluster behaves similarly. The
addition of solutions containing the porphyrin anions into
the cluster-containing solution resulted in the formation of
a 1:1 complex as evidenced by the spectral changes,
isosbestic points, and data analysis. Indeed, spectroscopic
changes in the UV-visible spectra of a [Pt3(dppm)3(CO)]2+
solution upon the addition of ZnTPPCO2Na (I) are noticed
(Figure 3). Despite the strong overlap between the a2 f e
band of the cluster and the Soret absorption of the porphyrin
chromophore, monitoring is possible. The decrease of the
shoulder absorption located at 445 nm (free [Pt3(dppm)3-
(CO)]2+) was monitored (Figure 3), hence extracting the
binding constant. The band associated with the host-guest
complex, which is expected to be blue-shifted in comparison
with the free cluster, is hidden under the strong Soret band
of ∼420 nm.
The formation of 1:1 host-guest complex (here
I · · · [Pd3(dppm)3(CO)]2+) was further demonstrated by ex-
amining the Benesi-Hildebrand, Scatchard, and Scott
plots.16,19 These plots exhibit straight lines with concentra-
tions of guests constants varying from 0.42 to 2.84 × 10-5
M (see the SI). The extracted K11 values (Table 1) using the
three methods, are almost the same, which is consistent with
the 1:1 complex formation.
-
Very similar results are obtained for ZnTTPPCO2 and
PdTTPPCO2 with [Pd3(dppm)3(CO)]2+ (see the SI). The
-
-
The K11 values for the ZnTPPCO2 (I) · · · [Pt3(dppm)3-
(CO)]2+ pair were also extracted from the Benesi-Hildebrand,
Scatchard, and Scott plots (Table 2). Again, the plots (see
(19) Connors, K. A. Binding Constants: The Measurements of Molecular
complex Stability; John Wiley & Sons: New York, 1987.
Inorganic Chemistry, Vol. 47, No. 21, 2008 9933

Aly et al.
Table 2. K11 Values for I-III with [Pt3(dppm)3(CO)]2+ in 2MeTHF at
298 Ka
-1
K11 (M
)
substrate
Benesi-Hildebrand
Scott
Scatchard
-
ZnTPPCO2 (I)
21 900
19 600
17 100
25 500
20 500
18 300
19 900
19 300
18 400
-
ZnTTPPCO2 (II)
PdTTPPCO2 (III)
-
a The uncertainties are (10% based on multiple measurements.
the SI) exhibit straight lines consistent with the formation
of a 1:1 complex formation.16,19 Moreover, the binding
constants were also almost the same in all three methods.
The K11 values for all three cases of metalloporphyrin-
containing substrates are very similar to those obtained for
the [Pd3(dppm)3(CO)]2+ pairs. All in all, the close similarity
in K11 values reinforced that the interactions are driven by
electrostatic interactions.
-
Figure 4. Emission of (a) [Pt3(dppm)3(CO)]2+ alone (as the PF6 salt),
λex ) 450 nm, (b) III, λex ) 515 nm, and (c) [Pd3(dppm)3(CO)]2+ (as the
-
PF6 salt), λex ) 450 nm, in 2MeTHF at 77 K. The emission intensities
were adjusted to fit in the frame. The emission band for [Pd3(dppm)3(CO)]2+
is, in fact. very weak (by at least 2 orders of magnitude). Smoothing was
applied in this case.
In order to secure this conclusion, control experiments
were performed. Sodium benzoate and PdTPP (Chart 2) were
placed in the presence of [Pt3(dppm)3(CO)]2+. The spectrum
recorded for the sodium benzoate-[Pt3(dppm)3(CO)]2+ sys-
tem (see Figures SI.12-SI.15 in the SI) exhibit isosbestic
points as expected, consistent with the formation of a
host-guest complex.18 The spectral changes are difficult to
observe, where a shift of only ∼2 nm is observed between
the free [Pt3(dppm)3(CO)]2+ (λmax ) 417 nm) and the
host-guest complex (λmax ) 415 nm) (see Figures SI.12 and
SI.13 in the SI). The calculated K11 value (10 100 M-1) using
the same methodologies (Benesi-Hildebrand, Scatchard, and
Scott) agrees with the value reported for the case of sodium
benzoate-[Pd3(dppm)3(CO)]2+, 10 000 M-1.13 Moreover, the
addition of PdTPP to [Pt3(dppm)3(CO)]2+ and [Pd3-
(dppm)3(CO)]2+ solutions did not lead to spectral changes
(i.e., blue-shift of the a2 f e band), except for linear
mathematical additions of the intensity of the added com-
ponent, or to the formation of any isosbestic points (see
Figures SI.14 and SI.15 in the SI). Examination of the
spectroscopic changes in the cases of both sodium benzoate
and PdTPP clearly demonstrates that the spectral changes
observed in the absorption spectra of [Pt3(dppm)3(CO)]2+ and
[Pd3(dppm)3(CO)]2+ are due to the interactions between the
negatively charged carboxylate anion and the positive charge
of the cluster. The size of the K11 data (∼20 000 M-1)
indicates that the interactions are strong but reversible.19
Photophysical Measurements. Previous work from our
group illustrated that the host-guest interactions are still
present in the excited states of the [M3(dppm)3(CO)]2+
clusters (M ) Pd, Pt).15,20 Interestingly, the cluster cavity
in the excited state is still hydrophobic but exhibits a larger
size. Indeed, the cavity size in the ground state has a diameter
of about 3-4 Å, whereas in the excited state, the antibonding
nature of the LUMO makes the M-M and M-P bond
distances longer than that of the ground state, which will
result in increased cavity sizes. The hydrophobic nature and
increased cavity size for the excited state would intuitively
achieve better substrate-cluster interactions.
Scheme 1
optically silent at 298 K. The photophysical measurements
on the 1:1 host-guest complexes described above between
ZnTPPCO2Na (I), ZnTTPPCO2Na (II), and PdTTPPCO2Na
(III) and the [M3(dppm)3(CO)]2+ clusters (M ) Pd, Pt) were
carried out in 2MeTHF at both 77 and 298 K because I-III
are emissive in both conditions.
The 0-0 phosphorescence peak for III in 2MeTHF (for
example) is observed at 690 nm at 77 K, whereas the 0-0
peaks for I and II are found at 780 nm (see Figures SI.16
and SI.17 in the SI). The exact position of the 0-0 peak of
the clusters is not known; one can approximate their position
by determining the position where the emission starts
appearing on the high-energy side (∼600 and ∼700 nm for
[Pt3(dppm)3(CO)]2+ and [Pd3(dppm)3(CO)]2+, respectively;
Figure 4). The energy gap between these two latter values
(2380 cm-1) appears reasonable in comparison with the
energy gap measured at the maximum of the absorption
bands of the two clusters (445 and 495 nm, respectively;
2270 cm-1). Using these values, an energy diagram can be
constructed (Scheme 1) and allows to predict that both
clusters and the metalloporphyrins should act as energy
donors and acceptors, respectively.
The only uncertainty is the relationship between the triplet
state of [Pd3(dppm)3(CO)]2+ and III (Figure 4). Experimental
evidence provided below demonstrates that the cluster is also
the donor in this pair. Moreover, in the presence of acetate
and benzoate, the emission maximum of [Pd3(dppm)3(CO)]2+
blue-shifts (modestly) by about 260 cm-1 (see the SI),
whereas the emission band for [Pt3(dppm)3(CO)]2+ remains
little affected in the presence of the same substrates. All in
all, the predicted role of the clusters as energy donors
The clusters exhibit emission bands at 755 and 655 nm
for M ) Pd and Pt, respectively, at 77 K (Figure 4) but are
(20) Harvey, P. D. J. Cluster Sci. 1993, 4, 377–402.
9934 Inorganic Chemistry, Vol. 47, No. 21, 2008

Energy Transfers in Electrostatic Host-Guest Assemblies
Figure 6. Time-resolved emission spectra for 1:1 III· · ·[Pt3(dppm)3(CO)]2+
assembly in 2MeTHF at 77 K excited at 450 nm. [PdTTPPCO2-] )
-5
[[Pt3(dppm)3(CO)]2+] ) 6 × 10
M. Time windows: 5- 50, 10- 50,
20-50, and 30-50 µs for a, b, c, and d, respectively.
is largely decreased in the 1:1 mixture where a weak shoulder
at ∼650 nm (i.e., emission of the [Pt3(dppm)3(CO)]2+ cluster)
is barely noticeable. The two other complexes (with I and
II) are provided in the SI (see Figures SI.16 and SI.17). The
corresponding spectra recorded at 298 K exhibit a similar
behavior, and the spectra are also given in the SI (see Figures
SI.18-SI.20). The excitation spectra of III · · · [Pt3-
(dppm)3(CO)]2+ at 77 K in 2MeTHF monitored at 820 nm,
where the cluster emission is no longer apparent but the
phosphorescence of the metalloporphyrin is still observable,
exhibit all of the absorption features belonging to both
chromophores (Figure 5, bottom). This result indicates that
the observation of metallophosphorescence arises from both
the macrocycle itself and the cluster, meaning that triplet
energy transfer from the cluster to the metalloporphyrin takes
place. The two other [Pt3(dppm)3(CO)]2+-containing as-
semblies (I and II) exhibit the same behavior (see Figures
SI.31 and SI.32 in the SI).
The time-resolved spectra in the microsecond time scale
for the 1:1 PdTTPPCO2-/[Pt3(dppm)3(CO)]2+ mixture (Figure
6) exhibit emissions from both the cluster and the porphyrin
chromophore. Both emissions are readily observed, but the
650 nm band (due to the cluster emission) relaxes more
quickly than that for the porphyrin macrocycle. In compari-
son with the free [Pt3(dppm)3(CO)]2+, the cluster emission
in the host-guest assembly decays faster, whereas the
metalloporphyrin chromophore exhibits the same rate of
decay. These observations support the presence of a triplet
energy transfer from [Pt3(dppm)3(CO)]2+ to the PdTTPPCO2-
unit in the assembly.
The two other assemblies (with I and II) behave the same
way (see Figures SI.21 and SI.22in the SI), where the 650
nm (cluster emission) signal decreases faster than of the 700
and 800 nm phosphorescence signals of the two other
metalloporphyrins.
Transient Absorption. The recorded transient absorption
spectra of PdTTPPCO2Na (i.e., triplet-triplet absorption) and
a 1:1 III/[Pt3(dppm)3(CO)]2+ mixture in 2MeTHF at 298 K
(Figure 7) exhibit a broad signal centered at ∼500 nm. The
two signals are essentially the same, but the transient absorption
undergoes some blue shift (∼5 nm) resembling the effect of
the host-guest behavior on the absorption spectra. The blue
shift is most likely due to host-guest interactions.
Figure 5. Top: Emission spectra of (a) [Pt3(dppm)3(CO)]2+, (b) III, (c)
1:1 III· · ·[Pt3(dppm)3(CO)]2+ in 2MeTHF at 77 K excited at 450 nm.
[PdTTPPCO2-] ) [[Pt3(dppm)3(CO)]2+] ) 6 × 10-5 M in all cases. Bottom:
Comparison of the absorption (black) and excitation (red) spectra of the
PdTTPPCO2 · · ·[Pt3(dppm)3(CO)]2+ assembly at 77 K in 2MeTHF, moni-
tored at 820 nm.
remains, as well as the uncertainty between [Pd3(dppm)3-
(CO)]2+ and III.
Host-Guest Interactions with [Pt3(dppm)3(CO)]2+
(Luminescence Spectra). A typical experiment where the
emission spectra of the cluster, the metalloporphyrin unit,
and the 1:1 host-guest assembly are compared is shown in
Figure 5. The other two anionic metalloporphyrins, I and
II, behave the same way, and the corresponding experiments
are provided in the SI. No significant spectroscopic changes
were observed for the emission bands of the free metal-
loporphyrin carboxylate anions compared to the 1:1 assembly
with [Pt3(dppm)3(CO)]2+. In all cases [Figures 5 (top) and
SI.16 and SI.17 in the SI], both the free base porphyrin and
the 1:1 assembly showed a weak band at ∼605 nm assigned
to a ππ* fluorescence of the metalloporphyrin chromophore.
The signal is expectedly weak because of the heavy-atom
effect induced by the Pd metal. Long-lived and intense
emission bands are observed in the range of 650-850 nm
and are assigned to be the ππ* phosphorescence of the
metalloporphyrin chromophore.14,21
No spectroscopic changes in the band shape and intensity
(keeping the concentrations the same) are observed for the
metalloporphyrin emission in the 1:1 complexes. On the other
hand, the 650 nm emission intensity of [Pt3(dppm)3(CO)]2+
(21) (a) Pekkarinen, L.; Linschitz, H. J. Am. Chem. Soc. 1960, 82, 2407–
2411. (b) Harriman, A. J. Chem. Soc., Faraday Trans. 2 1981, 77,
1281–1291. (c) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.;
Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem.
Soc. 1996, 118, 11771–11782. (d) Sessler, J. L.; Jayawickramarajah,
J.; Gouloumis, A.; Torres, T.; Guldi, D.; Maldonado, S.; Stevenson,
K. J. Chem. Commun. 2005, 1892–1894.
Inorganic Chemistry, Vol. 47, No. 21, 2008 9935

Aly et al.
Figure 7. Transient absorption spectra of (a) III and (b) 1:1 PdTTPPCO2- · · ·[Pt3(dppm)3(CO)]2+. [PdTTPPCO2-] ) [[Pt3(dppm)3(CO)]2+] ) 6 × 10
-5
M, in 2MeTHF at 298 K. (Spectra were measured at different delay times and λex ) 355 nm.)
Scheme 2
Table 3. Transient (Triplet-Triplet) Absorption Lifetimes of the
Metalloporphyrin Species in the Free Molecules and 1:1 Host-Guest
[Pt3(dppm)3 (CO)]2+ · · ·-O2CR Assemblies Measured in 2MeTHF at
a
298 K in the Presence and Absence of O2
τ (µs)
1:1 [Pt3(dppm)3(CO)]2+ · · ·
substrate alone
substrate
in the
in the
without presence
without
O2
presence
of O2 (air)
substrate
O2
of O2 (air)
-
ZnTPPCO2 (I)
145
137
220
5
4
2
148
141
227
6
5
3
-
ZnTTPPCO2 (II)
PdTTPPCO2 (III)
a The uncertainties are (7% based on multiple measurements.
-
This 500 nm band for the uncomplexed porphyrin is due
to triplet-triplet absorption. Because the band shape is the
same in the assembly and in the free metalloporphyrin, the
signal is confidently assigned to triplet-triplet absorption
as well.
Table 4. Emission Lifetimes of [Pt3(dppm)3(CO)]2+ and Its 1:1
Assemblies in 2MeTHF at 77 K (λem ) 640 nm)a
τe (µs)
kET (s-1
)
b
[Pt3(dppm)3CO]2+
12.63 ( 0.07
12.40 (0.05
The reported triplet-triplet absorption for zinc tetraphe-
nylporphyrin (ZnTPP) exhibits transient bands at 470 nm
and that for PdTPP at 450 nm, agreeing with the assign-
ment.21 On the other hand, a very broad transient absorption
band in the 600-1100 nm region is reported for the
corresponding radical cation.21 However, because of the very
large signal generally seen for the radical cation, it would
not be reasonable to totally preclude its existence. However,
because the fluorescence and phosphorescence lifetimes for
three metalloporphyrin remain the same in the uncomplexed
species and in the assemblies (see below), no quenching by
electron transfer of these species occurs. The recorded spectra
given in Figure 7 compare favorably to those previously
reported for metalloporphyrins while showing clear differ-
ences with the reported spectra of the porphyrin radical
cation. All in all, this means that there is no or a very slow
electron-transfer process occurring (knowing that the charge-
separated state would generate species absorbing at a
different place and with a different band shape). Scheme 2
shows the energy diagram for the [Pt3(dppm)3(CO)]2+ · · ·
-
[Pt3(dppm)3(CO)]2+ · · ·PhCO2
no transfer
(Na+ salt)
-
[Pt3(dppm)3(CO)]2+ · · ·ZnTPPCO2
9.57( 0.04
10.30 ( 0.03
11.80( 0.08
2.4 × 104
2.1 × 104
0.6 × 104
-
[Pt3(dppm)3(CO)]2+ · · ·ZnTTPPCO2
-
[Pt3(dppm)3(CO)]2+ · · ·PdTTPPCO2
a All of the emission decay traces are monoexponential. b ΦP for this
compound was found to be 0.073 ( 10%.
No triplet-triplet absorption spectra were observed for
[Pt3(dppm)3(CO)]2+ alone, presumably because its triplet
lifetime was too short at 298 K (i.e., <13 ns).
Emission Lifetime and Energy Transfers. The change
in emission lifetimes of the [Pt3(dppm)3(CO)]2+ cluster upon
the addition of porphyrin substrates as well as that of the
free metalloporphyrins and their corresponding 1:1 as-
semblies with [Pt3(dppm)3(CO)]2+ are listed in Tables 4 and
5.
The emission lifetimes for [Pt3(dppm)3(CO)]2+ and the
[Pt3(dppm)3(CO)]2+ · · ·-O2CPh assembly in 2MeTHF at 77
K are practically identical (Table 4), consistent with the
absence of triplet energy transfer. On the other hand, the
emission lifetimes of the [Pt3(dppm)3(CO)]2+ · · ·I, · · · ·II, and
· · · III assemblies are shorter than that for the cluster alone,
indicating obvious interactions from the Pt3 center to the
metalloporphyrin chromophore. The decrease in the emission
lifetime is due to triplet energy transfer, and the kET values
are extracted from eq 2:10
-
PdTTPPCO2 assembly (as an example) describing the
energy transfer. The transient absorption data are available
in Table 3.
The transient absorption was measured in the presence and
absence of O2, and the obtained spectra are given in the SI
(Figures SI.23-SI.27). The transient lifetime was measured
in the absence and presence of O2, and the obtained lifetimes
are listed in Table 3. The observed decrease in the lifetime
clearly supports the interpretation of the obtained signal to
the transient porphyrin triplet state.
1
1
kET
)
-
(2)
τe0
τ
(
)
e
where τe0 is the emission lifetime for the free donor (i.e.,
[Pt3(dppm)3(CO)]2+) where no energy transfer takes place
and τe is the emission lifetime of the donor in the dyad (the
9936 Inorganic Chemistry, Vol. 47, No. 21, 2008

Energy Transfers in Electrostatic Host-Guest Assemblies
Table 5. Photophysical Data for the Free Metalloporphyrins I-III and Their 1:1 Host-Guest Complexes with [Pt3(dppm)3(CO)]2+
substrate alone
1:1 [Pt3(dppm)3(CO)]2+ · · ·substrate
Φb (520 nm)c
ΦF ΦP τP (ms) (λmax)
Φb (520 nm)c
d
d
d
d
substrate in 2MeTHF at 77 K
ΦF
ΦP
τF (ns) (λmax
)
τP (ms) (λmax
)
τF (ns) (λmax
)
0.013 2.73 ( 0.02 (600)d 25.95 ( 0.28 (780)d 0.039
0.013 2.70 ( 0.02 (600)d 26.20 ( 2.72 (780)d
-
ZnTPPCO2 (I)
0.039
0.036
0.002 2.52 ( 0.24 (600)d 19.40 ( 0.40 (780)d 0.039
0.003 2.53 ( 0.08 (600)d 15.20 ( 1.20 (780)d
-
ZnTTPPCO2 (II)
PdTTPPCO2 (III)
>0.001 0.124 0.11 ( 0.03 (605)d 1.54 ( 0.05 (690)d >0.001 0.120 0.13 ( 0.02 (605)d 1.56 ( 0.05 (690)d
-
substrate alone
1:1 [Pt3(dppm)3(CO)]2+ · · ·substrate
d
d
d
d
substrate in 2MeTHF at 298 K
τF (ns) (λmax
)
τP (µs) (λmax
)
τF (ns) (λmax
)
τP (µs) (λmax)
ZnTPPCO2- (I)
ZnTTPPCO2- (II)
PdTTPPCO2- (III)
0.63 ( 0.03 (605)d
0.38 ( 0.04 (605)d
e
e
e
0.65 ( 0.03 (605)d
0.33 ( 0.03 (605)d
e
e
e
277 ( 14 (700)d
295 ( 1.85 (700)d
a All of the emission decay traces are monoexponential. b ΦF and ΦP are reported at (10% of uncertainties. c Excitation wavelength. d The values in
parentheses are the wavelengths where the lifetimes were measured. e The intensity is too weak to be measured.
Scheme 3
1:1 assemblies). The kET values are in the 104 s-1 range
(Table 4), which can be described as being on the slow side
in comparison with other assemblies of polyporphyrins.10
This is consistent with the through-space process where
orbital overlaps are more difficult. A comparison with other
cofacial systems, where through-space triplet energy transfer
also take place,7,8 is made in Scheme 3.
Figure 8. Top: Phosphorescence spectra of (a) III and (b) 1:1
III· · ·[Pd3(dppm)3(CO)]2+ in 2MeTHF at 77 K excited at 515 nm. At this
wavelength, both the cluster and the porphyrin chromophores are excited
(see Figure 1). [PdTTPPCO2-] ) [[Pd3(dppm)3(CO)]2+] ) 2 × 10
M.
-5
Insert: identical but this is the fluorescence. The spectra are adjusted so
that their intensities would match. There is no evidence for
[Pd3(dppm)3(CO)]2+ emission because it is weak in comparison with the
metalloporphyrin. Bottom: Comparison of the absorption (black) and
excitation (red) spectra of the PdTTPPCO2- · · ·[Pd3(dppm)3(CO)]2+ as-
sembly at 77 K in 2MeTHF, monitored at 685 nm.
The electrostatic assemblies reported here are about 1 order
of magnitude larger than the cofacial systems. The average
Pd · · · O distance between the related assemblies [Pd3-
(dppm)3(CO]2+ · · ·- O2CCF3 is ∼2.7 Å.13 Assuming that this
distance is transferable to the assemblies studied here
(because the Pd and Pt atoms exhibit about the same covalent
and van der Waals radii), the anticipated donor-acceptor
separations in these dyads are shorter than that found for
Cmeso · · · Cmeso of the diphenylene (3.80 Å) and the xanthene
spacer systems (4.32 Å),6 and so they predict slower rates
(as observed).10 The comparison is crude because the
[Pt3(dppm)3(CO)]2+ · · ·-O2CR assemblies are not cofacial,
but the observed rates are still considered logical with the
amplitude of the donor-acceptor separations. The through-
bond process is generally found to be faster than the through-
space one.10 Scheme 3 shows a related example containing
a cofacial dyad held by a Rh-Sn single bond (∼2.6 Å). The
observed rates range between 106 and 2.2 × 108 s-1,7 much
faster than others operating via a through-space mechanism.
The rates in the cluster assemblies make sense.
Interactions with [Pd3(dppm)3(CO)]2+ (Luminescence
Spectra). [Pd3(dppm)3(CO)]2+ is predicted to be a triplet
energy donor with respect to both I and II (Scheme 1). In
the [Pd3(dppm)3(CO)]2+ · · · PdTTPPCO2Na case, this is un-
certain from the Figure 4 only. Evidence that this cluster
also acts as a triplet energy donor is provided below.
The emission spectra of PdTTPPCO2Na (as an example)
and its corresponding 1:1 assembly with [Pd3(dppm)3(CO)]2+
in 2MeTHF at 77 K are shown in Figure 8. The other two
anionic porphyrins (I and II) behave the same way and are
placed in the SI (see Figures SI.24 and SI.25). Both the
uncomplexed metalloporphyrin and the 1:1 aggregates show
Inorganic Chemistry, Vol. 47, No. 21, 2008 9937

Aly et al.
Scheme 4
the expected band at ∼605 nm, associated with a weak ππ*
fluorescence for the PdTTPPCO2- chromophore and a long-
lived porphyrin-localized ππ* phosphorescence in the
650-850 nm range.14,21 No significant changes are observed
in the emissions band shape of the free porphyrin chro-
mophore compared to the 1:1 assembly, except that we find
that the emission intensity is lower in the assembly.
Figure 9. Time-resolved emission spectra of the 1:1 PdTTPP-
CO2- · · ·[Pd3(dppm)3(CO)]2+ assembly in 2MeTHF at 77 K excited at 450
nm. [PdTTPPCO2-] ) [[Pt3(dppm)3(CO)]2+] ) 6 × 10
M. Time
-5
windows: 5-50, 10-50, 20-50, and 30-50 µs for a, b, c, and d,
respectively.
unfruitful, suggesting that emission is far too weak and short-
lived to be found in comparison with the much stronger
phosphorescence signal of the metalloporphyrin chro-
mophores or is totally quenched.
The τF values of the metalloporphyrin chromophores at
77 K remain, at first glance, little affected in the
[Pd3(dppm)3(CO)]2+ · · ·-O2CR assemblies (Table 6), but in
absolute value (i.e., excluding the uncertainties), a small
decrease is noted, meaning quenching. Similarly, the phos-
phorescence lifetimes decrease by a factor of 2-3 in the
host-guest assemblies, contrasting with the [Pt3(dppm)3-
(CO)]2+ · · ·-O2CR systems (I-III).
One major difference is, however, noted for three possible
assemblies with the [Pd3(dppm)3(CO)]2+ cluster. The excita-
tion spectra do superpose the absorption ones perfectly,
particularly for the band due to the [Pd3(dppm)3(CO)]2+
cluster, which appears to be almost totally absent from the
excitation spectra. We used two different emission wave-
lengths at 650 and 820 nm. These wavelengths were selected
because they are situated on the high- and low-energy sides
of the [Pd3(dppm)3(CO)]2+ cluster emission, respectively, and
within the fluorescence and phosphorescence bands of the
metalloporphyrins and for comparison purposes. Because the
phosphorescence intensity of the Zn-containing chro-
mophores is weak, the measurements of the excitation spectra
monitored at 820 nm lead to lesser quality spectra even using
multiscans. Nonetheless, the absence (or the weakness) of
the excitation signal associated with the cluster component
indicates that excitation in the cluster band does not
contribute efficiently to the metalloporphyrin emission in the
dyad complexes. Hence, this result demonstrates the absence
of efficient triplet energy transfers. For a demonstration that
energy transfer is still operating, the [Pd3(dppm)3-
A comparison of the data in Tables 5 and 6 shows clearly
the trend. If we consider these changes in τP values as a
quenching process of the metalloporphyrin chromophore by
the cluster, then quenching rates (kQ) extracted the same way
as in eq 2 (kET is replaced by kQ) are found to be on the
order of 101-102 s-1 for all three cases. These values are
clearly slow. An element for the assignment for this slow
quenching, which is absent in the Pt3-containing cluster
assemblies, is found in the 298 K data. Indeed, the τP value
-
of PdTTPPCO2 in 2MeTHF at 298 K on going from the
-
-
(CO)]2- · · · PdTTPPCO2 assembly was investigated using
uncomplexed compound PdTTPPCO2 to the electrostatic
685 nm as monitoring wavelength where the phosphores-
cence is very intense (but not the cluster), hence improving
the quality of the signal. Indeed, two weak features between
450 and 500 nm attributable to the cluster are observed
(Figure 8) in the excitation spectra of this assembly. Again
the intensity is weak, meaning that the transfer is not efficient
but is clearly present (Scheme 4). So, the [Pd3(dppm)3(CO)]2+
cluster and PdTTPPCO2 · · · act as a triplet energy donor and
acceptor, respectively. On the basis of this conclusion, one
can also reasonably state that triplet energy transfer in the
two other [Pd3(dppm)3(CO)]2+ cluster assemblies must also
occur (Scheme 4).
The emission spectra at 298 K behave the same way as
those at 77 K and are placed in the SI (see Figures
SI.26-SI.28). The emission band located at ∼750 nm for
the [Pd3(dppm)3(CO)]2+ cluster is not observed because of
its relatively very weak intensity. In an attempt to discrimi-
nate this band from the assembly, time-resolved spectra for
the PdTTPPCO2- · · · [Pd3(dppm)3(CO)]2+ assembly were
studied (Figure 9). Despite several attempts, the search was
dyad [Pd3(dppm)3(CO)]2+ · · · -O2CPdTTPP also decreases,
but kQ is on the order of 0.70 × 104 s-1, which is slow but
faster than that measured at 77 K. This key information (i.e.,
kQ faster at 298 K with respect to kQ at 77 K) allows one to
suspect that an electron-transfer process going from the
metalloporphyrin center to the [Pd3(dppm)3(CO)]2+ cluster
takes place.
Transient Absorption. The transient spectra of the
metalloporphyrins exhibit the same transient triplet-triplet
absorption band (see III and its 1:1 host-guest complex with
[Pd3(dppm)3(CO)]2+ in 2MeTHF at 298 K in Figure 10),
meaning that despite quenching some residual species are
still present. The corresponding transient spectra for I and
II in the presence and absence of O2 are available in the SI
(see Figures SI.33-SI.37). The triplet-triplet spectra for the
studied porphyrins and their 1:1 host-guest assemblies with
[Pd3(dppm)3(CO)]2+ are almost the same and superpose that
reported for the other porphyrins21 and that presented in
Figure 7 for the [Pt3(dppm)3(CO)]2+ · · · I, · · · II, and · · ·III
assemblies. The presence of this triplet-triplet absorption
9938 Inorganic Chemistry, Vol. 47, No. 21, 2008

Energy Transfers in Electrostatic Host-Guest Assemblies
a
Table 6. Photophysical Data for the Porphyrins I-III and Their 1:1 Host-Guest Assemblies with [Pd3(dppm)3(CO)]2+
substrate alone
1:1 [Pd3(dppm)3(CO)]2+ · · ·substrate
Φb (520 nm)c
ΦF ΦP
Φb (520 nm)c
substrate in 2MeTHF
at 77 K
triplet
d
d
d
d
ΦF
ΦP
τF (ns) (λmax
)
τP (ms) (λmax
)
τF (ns) (λmax
)
τP (ms) (λmax
)
kET (s-1
)
0.013 2.73 ( 0.02 (600)d 25.95 ( 0.28 (780)d
0.037 0.011 2.69 ( 0.09 (600)d 10.40 ( 0.72 (780)d
71
96
65
-
ZnTPPCO2 (I)
0.039
0.036
0.002 2.48 ( 0.18 (600)d 19.40 ( 0.40 (780)d
0.022 0.001 2.30 ( 0.24 (600)d 6.94 ( 0.25 (780)d
-
ZnTTPPCO2 (II)
PdTTPPCO2 (III)
>0.001 0.124 0.11 ( 0.03 (605)d 1.54 ( 0.05 (690)d >0.001 0.103 0.15 ( 0.03 (605)d 1.40 ( 0.011 (690)d
-
substrate alone
1:1 [Pd3(dppm)3(CO)]2+ · · ·substrate
substrate in 2MeTHF
at 298 K
d
d
d
d
τF (ns) (λmax
)
τP (µs) (λmax
)
τF (ns) (λmax
)
τP (µs) (λmax
)
kQ (s-1
)
-
ZnTPPCO2 (I)
0.63 ( 0.03 (605)d
f
f
0.41 ( 0.04 (605)d
f
f
8.5 × 108 (S1)
5.9 × 108 (S1)
0.70 × 104 (T1)
0.38 ( 0.04 (605)d
0.31 ( 0.04 (605)d
-
ZnTTPPCO2 (II)
PdTTPPCO2 (III)
f
277 ( 15 (700)d
f
94 ( 1 (700)d
-
a All of the emission decay traces are monoexponential. b ΦF and ΦP are reported at (10%. c Excitation wavelength. d The values in brackets are the
wavelengths where the lifetimes were measured. e There is no quenching of the fluorescence within the uncertainty. This column presents only the triplet kET
.
f The intensity is too weak to be measured.
Figure 10. Transient absorption spectra of (a) III and (b) 1:1 PdTTPPCO2- · · ·[Pd3(dppm)3(CO)]2+, [PdTTPPCO2-] ) [[Pd3(dppm)3(CO)]2+] ) 2 × 10 -5
M, in 2MeTHF at 298 K. (Spectra were measured at different delay times and λex ) 355 nm; x ) an artifact signal.)
Table 7. Transient (Triplet-Triplet) Absorption Lifetimes for 1:1 Host-Guest [Pd3(dppm)3(CO)]2+ · · ·-O2CR Assemblies Measured in 2MeTHF at 298
a
K in the Presence and Absence of O2
τT1 (µs)
substrate alone
in the presence
1:1 [Pd3(dppm)3(CO)]2+ · · ·substrate
in the presence
substrate
without O2
of O2 (air)
without O2
of O2 (air)
kQ (s-1
)
-
ZnTPPCO2 (I)
145
137
220
5
4
2
67
55
93
3
3
2
0.80 × 104
1.09 × 104
0.62 × 104
-
ZnTTPPCO2 (II)
PdTTPPCO2 (III)
a The uncertainties are (5% based on multiple measurements.
band does not preclude electron transfer. This transfer can
simply turn out to be slow. However, the intensity of this
band as well as the ZnTPPCO2Na (I) system is quenched
(see the SI), which makes no sense in an energy-transfer
model, as suggested in Scheme 1. Because I and II are not
phosphorescent at 298 K (Table 6), the triplet-state lifetimes
are measured from the transient spectra (Table 7).
The kQ values determined from the emission and transient
lifetimes for the PdTTPPCO2- · · · [Pd3(dppm)3(CO)]2+ as-
sembly are the same (considering the uncertainties), and the
overall kQ values for the three assemblies range from 0.6 ×
104 to 1.1 × 104 s-1. Knowing that electron transfers are
slow at low temperatures because of the large solvent
reorganization energy and faster at warmer temperatures for
the same reason, we propose that a slow photoinduced
electron transfer from the metalloporphyrins to the clusters
occurs. Indeed, the [Pd3(dppm)3(CO)]2+ cluster in THF is
easily reduced at 0.40 V vs SCE.22 On the other hand, zinc
porphyrins in various solvents are oxidized with potentials
of E0/1+ > +0.6 V vs SCE.23 On the basis of Figure 1, the
0-0 peak of the Q absorption band would be around 590
nm (i.e., 17 200 cm-1; ∼2.1 eV). So, the driving force for
photoinduced electron transfer would be at least <1.5 V vs
SCE [1(metalloporphyrin)* f (metalloporphyrin)+], which
is large enough to reduce [Pd3(dppm)3(CO)]2+. As for the
reported energy transfer, the rate for electron transfer is
expected to be slow because of the through-space process.
Indeed, by taking the 298 K τF data in Table 6, rates on the
order 6 × 108-9 × 108 s-1 are calculated using eq 2, where
kET is replaced by kQ. These values lie on the slow side for
electron transfers but are right in the range for supramolecular
assemblies.10 The question is, why do we not see any
evidence for the charge-separated state in the transient
absorption spectra. By examination of the reported spectra
(22) (a) Gauthron, I.; Mugnier, Y.; Hierso, K.; Harvey, P. D. Can. J. Chem.
1997, 75, 1182–1187. (b) Brevet, D.; Lucas, D.; Cattey, H.; Lemaitre,
F.; Mugnier, Y.; Harvey, P. D. J. Am. Chem. Soc. 2001, 123, 4340–
4341. (c) Harvey, P. D.; Mugnier, Y.; Lucas, D.; Evrard, D.; Lemaˆıtre,
F.; Vallat, A. J. Cluster Sci. 2004, 15, 63–90.
(23) Kadish, K. M.; Van Caemelbecke, E. Electrochemistry of metallopor-
phyrins in nonaqueous media. In Encyclopedia of Electrochemistry;
Bard, A. J., Stratmann, M., Eds.; Wiley: Weinheim, Germany, 2002;
Chapter 9, pp 175-228.
Inorganic Chemistry, Vol. 47, No. 21, 2008 9939

Aly et al.
in ref 21, again we see that the transient spectra are spread
over 600-1100 nm. With the narrower triplet-triplet
absorption peak, it is possible that this feature is buried under
the former. No electrochemical data for the corresponding
[Pt3(dppm)3(CO)]2+ are reported so far. On the basis of the
lack of quenching of the reported metalloporphyrins, it is
reasonably anticipated that the reduction potential may be
higher than that of the [Pd3(dppm)3(CO)]2+ cation.
Acknowledgment. The Natural Sciences and Engineering
Research Council of Canada (NSERC; P.D.H.) and the
Centre National de Recherche Scientifique (CNRS, UMR
5633; R.G., C.S., and C.A.) are acknowledged for funding.
The Egyptian government and the Egyptian Cultural Bureau
in Montreal are also acknowledged for the scholarship
offered to S.M.A.
Supporting Information Available: UV-visible spectra for the
Final Comments. This work reports triplet energy trans-
fers in [M3(dppm)3(CO)]2+-containing assemblies held via
strong electrostatic interactions, here between an unsaturated
organometallic cluster cation and carboxylate-functionalized
metalloporphyrins. The rate of transfer is 104 s-1 for
[Pt3(dppm)3(CO)]2+. For [Pd3(dppm)3(CO)]2+, no rate could
be measured because of the very weak intensity of its
emission. Knowing that triplet energy transfer operates
according to a double electron-transfer mechanism (Dexter
mechanism), then a single photoinduced electron transfer
should also be possible. In this same sense, the [Pd3-
(dppm)3(CO)]2+ cluster exhibits two one-electron low-
potential reduction processes ([Pd3(dppm)3(CO)]2+ + e- a
-
-
electrostatic interactions of both ZnTTPPCO2 and PdTTPPCO2
with [M3(dppm)3(CO)]2+ (M ) Pd, Pt) and the corresponding plots
used for the K11 calculations (Benesi-Hildebrand, Scatchard, and
Scott), the emission spectra of the 1:1 assemblies of ZnTPPCO2
and ZnTTPPCO2 with [M3(dppm)3(CO)]2+ (M ) Pd, Pt) in
-
-
2MeTHF at 77 K as well as the emission spectra for all three 1:1
assemblies for ZnTPPCO2- · · · , ZnTTPPCO2- · · · , and PdT-
TPPCO2- · · ·[M3(dppm)3(CO)]2+ (M ) Pd, Pt) in 2MeTHF studied
at 298 K, time-resolved spectra of the 1:1 ZnTPPCO2- · · · and
ZnTTPPCO2- · · ·[Pt3(dppm)3(CO)]2+ assemblies, triplet-triplet ab-
sorption spectra for the 1:1 ZnTPPCO2- · · ·[Pt3(dppm)3(CO)]2+ and
· · · [Pd3(dppm)3(CO)]2+ assemblies, corrected emission spectra of
Pd3(dppm)3(CO)2+ in 2MeTHF at 77 K in the absence and presence
of acetate and benzoate, and comparison of the absorption and
excitation spectra of all of the assemblies at 77 K in 2MeTHF.
This material is available free of charge via the Internet at
http://pubs.acs.org.
[Pd3(dppm)3(CO)]+ and [Pd3(dppm)3(CO)]+ + e-
a
[Pd3(dppm)3(CO)]0),22 and the reported data in this work
strongly suggested that this was indeed the case. Future work
in this area is in progress.
IC801006G
9940 Inorganic Chemistry, Vol. 47, No. 21, 2008