Photophysics of Re(I) and Ru(II) diimine complexes covalently linked to pyrene: Contributions from intra-ligand charge transfer states

Article abstract of DOI:10.1021/ic010522v

The photophysical properties of Ru(II) and Re(I) polypyridyl complexes including a bis-bipyridyl pyrene ligand are presented. The complexes {[(bpy)₂Ru]₂bpb}⁴⁺ and [(CO)₃ReCl(bpb)] = 2,2′-bipyridine, bpb = 1,6-bis-(4(2,2′-bipyrid-yl)-pyrene) were designed with the intent of examining intramolecular energy migration between MLCT states localized on the metal complexes and pyrene-localized ³(π-π*) states. Absorption spectroscopy of both complexes containing the bpb ligand reveals that in addition to the MLCT and the pyrene-centered ¹(π-π*) transitions, a new absorption band is observed near 400 nm for both complexes. Absorption spectral data for the Re(I) complex strongly suggest the presence of a pyrene(π) to bpy(π*) intraligand charge transfer (ILCT) transition. Emission spectra at room temperature and at 77 K are almost identical for the Ru(II) and Re(I) complexes containing the bpb ligand. The ³MLCT emission of related bipyridyl compounds lacking the pyrene is observed at higher energy than for the pyrene-containing complexes, {[(bpy)₂Ru]₂bpb}⁴⁺ and [(CO₃ReCl(bpb)]. The Ru(II) complex emits at room temperature with a remarkably long lifetime (130 μs in degassed DMSO). This emission is also strongly sensitive to oxygen and is almost entirely quenched in an aerated solution. In addition, excited-state absorption spectra exhibit features not consistent with ³MLCT or ³(π-π*) states of the parent chromophores. The combined characteristics suggest the emission arises from either ³(π-π*) or ³ILCT states or a state with mixed parentage.

Full text of DOI:10.1021/ic010522v

Inorg. Chem. 2002, 41, 359−366
Photophysics of Re(I) and Ru(II) Diimine Complexes Covalently Linked
to Pyrene: Contributions from Intra-Ligand Charge Transfer States
,†
Andre´ Del Guerzo,† Ste´phanie Leroy,‡ Fre´de´ric Fages,‡ and Russell H. Schmehl*
Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118, and Laboratoire de
Chimie Organique et Organome´tallique, UMR 5802 CNRS, UniVersite´ Bordeaux 1, 351, Cours de
La Libe´ration, 33405 Talence Cedex, France
Received May 17, 2001
The photophysical properties of Ru(II) and Re(I) polypyridyl complexes including a bis-bipyridyl pyrene ligand are
presented. The complexes {[(bpy)2Ru]2bpb}4+ and [(CO)3ReCl(bpb)] (bpy ) 2,2′-bipyridine, bpb ) 1,6-bis-(4-
(2,2′-bipyrid-yl)-pyrene) were designed with the intent of examining intramolecular energy migration between MLCT
states localized on the metal complexes and pyrene-localized 3(π−π*) states. Absorption spectroscopy of both
complexes containing the bpb ligand reveals that in addition to the MLCT and the pyrene-centered 1(π−π*) transitions,
a new absorption band is observed near 400 nm for both complexes. Absorption spectral data for the Re(I) complex
strongly suggest the presence of a pyrene(π) to bpy(π*) intraligand charge transfer (ILCT) transition. Emission
spectra at room temperature and at 77 K are almost identical for the Ru(II) and Re(I) complexes containing the
bpb ligand. The 3MLCT emission of related bipyridyl compounds lacking the pyrene is observed at higher energy
than for the pyrene-containing complexes, {[(bpy)2Ru]2bpb}4+ and [(CO ReCl(bpb)]. The Ru(II) complex emits at
3
room temperature with a remarkably long lifetime (130 µs in degassed DMSO). This emission is also strongly
sensitive to oxygen and is almost entirely quenched in an aerated solution. In addition, excited-state absorption
spectra exhibit features not consistent with 3MLCT or 3(π−π*) states of the parent chromophores. The combined
characteristics suggest the emission arises from either 3(π−π*) or 3ILCT states or a state with mixed parentage.
Introduction
ground state, recent efforts of several groups have been
focused on making complexes with longer excited-state
lifetimes.4-11
One approach is to prepare diimine complexes having
ligand localized excited states that are lower in energy than
the 3MLCT state of the complex. These complexes generally
There are a large number of transition metal complexes
that have metal-to-ligand charge transfer (MLCT) excited
states involving diimine ligands.1 Within this group, the
photophysical behavior of complexes of Re(I), Ru(II), and
Os(II), have been investigated in considerable detail.1,2 This
is in part due to the fact that diimine complexes of these
metals serve as effective sensitizers for photochemical
reactions involving net electron transfer.3 Since nonradiative
relaxation in these complexes is dominated by a combination
of energy gap effects and a thermally activated population
of ligand-localized excited states that decay rapidly to the
(4) Giordano, P. J.; Fredericks, S. M.; Wrighton, M. S.; Morse, D. L. J.
Am. Chem. Soc. 1978, 100, 2257-9.
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N.; DeGraff, B. A. Inorg. Chem. 1990, 29, 4335-40. (b) Sacksteder,
L.; Lee, M.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. Soc. 1993,
115, 8230-8.
(6) Shaw, J. R.; Schmehl, R. H. J. Am. Chem. Soc. 1991, 113, 389-94.
(7) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1992, 96, 2917-20.
(8) (a) Wilson, G. J.; Launikonis, A.; Sasse, W. H. F.; Mau, A. W. H. J.
Phys. Chem., A 1997, 101, 4860-4866. (b) Wilson, G. J.; Sasse, W.
H. F.; Mau, A. W. H. Chem. Phys. Lett. 1996, 250, 583-588.
(9) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak,
P.; Jin, X. Q.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11012-
11022.
* To whom correspondence should be addressed. E-mail: russ@tulane.edu.
† Tulane University.
‡ Universite´ Bordeaux 1.
(1) (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
Von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277. (b)
Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159-244.
(2) Chen, P.; Meyer, T. J. Chem. ReV. 1998, 98, 1439-1477.
(3) (a) Balzani, V.; Bolletta, F.; Ciano, M.; Maestri, M. J. Chem. Educ.
1983, 60, 447-450. (b) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163-
170. (c) Graetzel, M. Coord. Chem. ReV. 1991, 111, 167-7.
(10) (a) Tyson, D. S.; Castellano, F. N. J. Phys. Chem. A 1999, 103, 10955-
10960. (b) Tyson, D. S.; Castellano, F. N. Chem. Commun. 2000,
2355-2356.
(11) Michalec, J. F.; Bejune, S. A.; McMillin, D. R. Inorg. Chem. 2000,
39, 2708-10.
10.1021/ic010522v CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/27/2001
Inorganic Chemistry, Vol. 41, No. 2, 2002 359

Del Guerzo et al.
Figure 1. Complex structures from refs 6 and 9.
Figure 2. Ligand bpb and geometric representation of the meso diaste-
reoisomer of the complex Ru2bpb.
exhibit no luminescence in solution at room temperature but
occasionally have structured luminescence in solid matrixes
and frozen solution arising from the ligand localized excited
state. In the mid-1970s Wrighton and co-workers attributed
the low-temperature emission of [Re(I)(bpy)(pyC(O)Ph)Cl]
to phosphorescence from the coordinated benzoylpyridine
n-π* excited state.4 More recently, Re(I) complexes having
low-temperature ligand-localized luminescence from diimine
π-π* excited states were observed by Demas and DeGraff.5
Room-temperature solution phosphorescence from a ligand
localized π-π* state was observed by our group in 1991
for the bimetallic Re(I) complex [Re(CO)3(NCCH3)(styb)]+
(Figure 1).6 In all of these cases, except for a few of the
complexes reported by Demas and DeGraff, a relatively large
presented convincing evidence that luminescence arises, at
least in part, from a pyrene to terpyridyl intraligand charge
transfer (ILCT) state.11 Observation of the transition in the
visible occurs because of the relatively low one-electron
oxidation potential of the pyrene. In addition, Albano et al.
attributed unique absorption and emission features of pro-
tonated and Zn2+ complexes of a terpyridyl-anthracene
derivative to anthracene to terpyridyl ILCT transitions.12
There are several interesting characteristics of such com-
plexes. First, the terpyridyl pyrene ligand only exhibits visible
ILCT absorption and emission when the terpyridyl is
protonated or coordinated to various metals; this occurs
because the one-electron reduction potential of the terpyridine
is decreased significantly (approximately 400 mV) upon
coordination, thus lowering the energy of the ILCT transition.
In addition, the complex reported by McMillin and co-
3
energy gap (>1000 cm-1) exists between the MLCT state
and the ligand localized excited state.
In the early 1990s Rodgers’ group reported a Ru(II)
diimine complex covalently linked to pyrene by an alkyl
bridge.7 In addition, Sasse and co-workers observed similar
behavior in a related complex with a shorter alkyl tether.8
The energy gap between the 3MLCT of the Ru(II) complex
and the 3(π-π*) state of the tethered pyrene in these
complexes is small, almost certainly under 500 cm-1. The
complexes have relatively long-lived (>5 µs) emission from
the MLCT state, resulting from reversible intramolecular
3
workers has a ILCT state with a lifetime long enough
(possibly due to spin-orbit coupling effects of the Pt) to
make it a viable sensitizer for photoredox reactions. The
excited state redox potentials of ILCT states will depend on
the energy of the excited state and the one-electron potentials
of the appropriate ligand components (in the McMillin case,
this is the pyrene oxidation and the first reduction of the
coordinated terpyridine). Last, it is in principle possible to
make complexes having both MLCT and ILCT transitions
in the visible, with one of the triplet CT states as the active
state for photoredox reactions. As such, this represents a new
approach to preparing chromophores that absorb throughout
the visible and have a long-lived charge transfer excited state
for use in photoredox reactions. In principle, the energies of
the MLCT and ILCT states can be independently controlled.
3
3
energy transfer between the MLCT state and the (π-π*)
state of pyrene. Photoexcitation of the MLCT transition of
the complex is followed by rapid relaxation to an equilibrium
mixture of the two states. Significantly, the transient absorp-
tion spectra reveal features that can be assigned to the
3
3MLCT state of the Ru(II) complex and the (π-π*) state
of pyrene. More recently, we reported similar behavior for
a complex having pyrene covalently attached to 2,2′-
bipyridine through a single C-C bond ([Ru(bpy)2(bpy-
pyr)]2+, Figure 1).9 The complex has a remarkably long
excited-state lifetime of 50 µs in deaerated CH3CN at room
temperature. In this case, it was concluded the pyrene
In an effort to explore the possible role of ILCT states in
the photophysics of Ru(II) and Re(I) complexes, we have
examined the behavior of two complexes of the pyrene-
containing ligand bpb (Figure 2): {[(bpy)2Ru]2(bpb)}(PF6)4
(Figure 2) and [(CO)3ClRe(bpb)]. The combined results of
luminescence, excited-state absorption, and solvent-depend-
ent behavior suggest that the emission arises from either a
3
localized triplet is not in equilibrium with the MLCT state
and the long excited-state lifetime is due to slow back energy
transfer from the π-π* state to the 3MLCT state. This work
was followed by reports of Castellano and co-workers
showing that further synthetic elaboration of pyrene-diimine
systems yields complexes with excited-state lifetimes in
excess of 100 µs.10
3
3ILCT state or a ILCT state with a significant amount of
3
3IL (pyrene (π-π*)) character.
Very recently, McMillin’s group examined the photo-
(12) Albano, G.; Balzani, V.; Constable, E. C.; Maestri, M.; Smith, D. R.
physical behavior of a Pt(II) terpyridyl-pyrene complex and
Inorg. Chim. Acta 1998, 227, 225-231.
360 Inorganic Chemistry, Vol. 41, No. 2, 2002

Photophysics of Re(I) and Ru(II) Diimine Complexes
The gating was adapted according to the phosphorescence lifetime,
and the acquisition was started after a delay of 0.1 ms. To compare
all the spectra obtained at 77 K, the phosphorescence spectrum of
protonated bpb (see Results, Figure 6c, setup 3) has been corrected,
using the ratio of the spectra of Ru2bpb obtained with the two
different setups (1 and 3).
Luminescence lifetimes were acquired using laser excitation (N2
pumped coumarin 460 at 460 nm, or Quantel Brilliant Nd:YAG at
532 nm), by filtering the emitted light through a Heath-McPherson
Model EU-700 monochromator and detecting the emitted light with
a Hamamatsu R928 PMT. Decays typically represented the average
of 50-500 pulses and were collected on a Lecroy 9370 digital
oscilloscope. The same system served to measure transient absorp-
tion spectra, using as an excitation source the 355 nm output of a
Coherent Infinity 40-100 Nd:YAG laser or the output of a Quantel
Brilliant Nd:YAG at 532 nm. A 150 W Xe arc lamp (Osram) was
used as the light source for the analytical beam.
Radiative Rate Constants. In the case of a monoexponential
emission decay, the radiative constant can simply be estimated by
ηisckr ) Φem/τem. For Ru2bpb, two components contribute to the
emission, and several cases can be distinguished (see Results). (i)
At 665 nm, the emission intensity of Ru2bpb is 18% relative to
that of the reference [Ru(bpy)3]2+ and has an amplitude-weighted
average lifetime of 26 µs. If both emission components have the
same radiative constant, the product of the intersystem crossing
efficiency and radiative decay rate constant, ηisckr, can be estimated
from
Experimental Section
Synthesis. The synthesis of the bpb ligand has been described
elsewhere.13 [[Ru(bpy)2]2(bpb)]4+ (Ru2bpb) was prepared from a
mixture of bpb (52 mg, 0.1 mmol) and an excess of Ru(bpy)2Cl2
14
(145 mg, 0.3 mmol) in chloroform:ethanol 7:3 by refluxing under
argon for several hours. The complex was extracted from the dried
reaction mixture with pure ethanol; the unreacted ligand remained
undissolved. The complex was then further purified by several
reprecipitations in ether and finally by flash chromatography on
silica (eluent, H2O:KNO3 (1 M):MeCN 4:1:5 + 1-5% trimethy-
lamine). The nitrate salt was then converted into a PF6 salt by adding
an excess of KPF6 to an aqueous solution of the complex and by
subsequently filtering the precipitated complex. The red solid was
washed several times with water, dissolved in ethanol, and dried
on a watch glass (analyses: C, 45.1; H, 2.84; N, 8.01; Ru2C76H54N12-
P4F24‚C2H6O·0.65KPF6: C, 45.0; H, 2.90; N, 8.07). A fraction of
this complex was then converted into its chloride salt by using an
anion exchange column (Sephadex A25).
[Re(CO)3Cl(bpb)] (Rebpb) was prepared by refluxing bpb (52
mg, 0.1 mmol) and an excess of Re(CO)5Cl (0.3 mmol) in toluene
under argon for several hours. The Re(I) complex readily precipi-
tates from the solution and is subsequently filtered, washed with
toluene, dissolved in MeCN, and reprecipitated several times from
ether. The complex was then dissolved in CH2Cl2 (a fraction of
the solid does not dissolve), and the solution remaining after
filtration is evaporated on a watch glass to yield Rebpb. The
complex was characterized by cyclic voltammetry, UV-vis spec-
troscopy, and MALDI-TOF mass spectrometry (using R-cyano-4-
hydroxycinnamic acid as a matrix). The MALDI spectrum showed
a prominent peak at 782.6, corresponding to M - Cl (calculated
780.8). Further evidence that Rebpb is a monometallic complex is
provided by the appearance in acetonitrile of a new absorption band
at 365 nm with the addition of Zn(acetate), arising from the
sample
ref
I665
Fabs
τref
(ηisckr)sample ) (ηiisckr)ref
)
ref
sample
abs
(
)(F
)(τ
)
I665
sample
6 × 102 s-1
At 665 nm, 98% of the emission intensity is due to the component
with the long lifetime of 100 µs. If the two components have a
different radiative constant and we assume that the short component
has the same radiative constant as [Ru(bpy)3]2+, 99% of the excited
states contribute to the long component, and
chelation of the free bipyridine by Zn2+
.
[Re(CO)3Cl(mpb)] (Rempb). The same procedure as that used
for Rebpb was used to synthesize the monometallic complex Rempb
(mpb ) 4-(4-methylphenyl)-2,2′-bipyridine). The complex was
characterized by UV-vis spectroscopy and MALDI-TOF mass
spectrometry, exhibiting a prominent peak at 518.6, which corre-
sponds to M - Cl (calculated 516.5).
Electrochemistry. Cyclic voltammograms and differential pulse
polarograms were recorded using an EG&G Versastat according
to procedures described earlier.9 The complexes were dissolved in
freshly distilled and nitrogen-purged acetonitrile with 0.1 M
triethylammonium perchlorate as supporting electrolyte. SCE was
used as reference electrode and ferrocene was added when necessary
as an internal reference.
ηisckr(long) ) 105(0.18 × 0.98)em(0.99)abs(86 × 10-8)/
(100 × 10-6
)
τlong ) 2 × 102 s-1
Results
Syntheses. The bimetallic complex [[Ru(bpy)2]2(bpb)]4+
(Ru2bpb, Figure 2) was prepared by reacting an excess of
[Ru(bpy)2Cl2] with bpb and was purified by column chro-
matography on silica. The bimetallic complex, Ru2bpb, exists
as a mixture of diastereomers, since each metal center is
chiral. Keene and co-workers have recently succeeded in
separating the diastereomeric forms of a variety of bimetallic
Ru(II) diimine complexes, and unique redox and spectro-
scopic behavior is only observed in rare cases.15 This work
reports the redox and photophysical behavior of the mixed
diastereomers of Ru2bpb. [Re(CO)3Cl(bpb)] (Rebpb) and
[Re(CO)3Cl(mpb)] (Rempb) were prepared by reacting an
excess of [Re(CO)5Cl] with the modified bipyridine and were
purified by repeated reprecipitation and washing to remove
unreacted ligand.
Spectroscopic Methods. UV-vis absorption spectra were
obtained using a Varian Cary 100 scanning spectrometer. Steady-
state luminescence spectra were obtained using a SPEX fluorolog
equipped with a 450 W Xe arc lamp. Emission was detected using
either a liquid nitrogen cooled CCD (setup 1) or a Hamamatsu R928
PMT in a thermoelectrically cooled housing (setup 2). Reported
spectra are not corrected for detector response. Phosphorescence
spectra and lifetimes at 77 K were obtained on a Varian Eclipse
spectrometer (setup 3). The excitation source is a pulsed Xe arc
lamp, and detection is achieved using a Hamamatsu R928 PMT.
(13) Soujanya, T.; Philippon, A.; Leroy, S.; Vallier, M.; Fages, F. J. Phys.
Chem. A 2000, 104, 9408-9414.
(14) Sprintschnik, G.; Sprintschnik, H. W.; Kirsch, P. P.; Whitten, D. G.
J. Am. Chem. Soc. 1977, 99, 4947-54. Lay, P. A.; Sargeson, A. M.;
Taube, H. Inorg. Synth. 1986, 24, 291-9.
(15) (a) Keene, F. Richard. Coord. Chem. ReV. 1997, 166, 121-159. (b)
Yeomans, Brett D.; Kelso, Laurence S.; Tregloan, Peter A.; Keene,
F. Richard Eur. J. Inorg. Chem. 2001, 239-246.
Inorganic Chemistry, Vol. 41, No. 2, 2002 361

Del Guerzo et al.
Table 1. Absorption and Emission Properties of Mono- and Bimetallic Complexes in Various Solventsa
Ru2bpb Rebpb
Rubpy
Rempb
solvent
λmaxabs, nm (log ꢀ)
λmaxem, nm
φem
λmaxabs, nm
λmaxem, nm
λmaxem, nm
λmaxem, nm
H2O
380 (4.45), 394 (4.44), 459 (4.50)
381 (4.43), 397 (4.43), 459 (4.50)
381 (4.43), 402 (4.44), 458 (4.50)
385 (4.42), 403 (4.43), 462 (4.50)
384 (4.44), 403 (4.74), 460 (4.50)
385 (4.50), 405 (4.39), 461 (4.50)
669
670
677
679
678
682
686
0.0083
0.0086
0.0035
0.013
MeCN
EtOH
DMSO
THF
CH2Cl2
toluene
402
407
407
674
680
620
593
0.0065
605
584
a Ru2bpb ) {[(bpy)2Ru]2(bpb)}4+; Rebpb ) [(CO)3ClRe(bpb)]; Rubpy ) [Ru(bpy)3]2+; Rempb ) [(CO)3ClRe(mpb)].
Figure 4. Electronic absorption spectra of (a) Ru2bpb, (b) Rebpb, (c)
Rempb, and (d) bpb in acetonitrile. Spectra have been scaled in order
to simplify their comparison. (Extinction coefficients are reported in
Table 1).
Figure 3. Cyclic voltammogram of Ru2bpb in nitrogen-purged acetonitrile.
The supporting electrolyte is tetraethylammonium perchlorate (0.1 M) and
the potentials are reported versus SCE, which was used as a reference
electrode.
reduction is observed at -1.55 V. The reoxidation wave,
shown in Figure 3, appears as a sharp anodic peak. This
behavior is characteristic of adsorption of reduced complexes
followed by anodic desorption from the electrode surface
(Pt or glassy carbon).18
The redox potentials of Rebpb have been estimated by
cyclic voltammetry and differential pulse polarography;
limitations in solubility prevent a definitive determination
of the reversibility of the redox processes. The first oxidation
of the complex is observed at a potential of +0.96 V and,
by comparison with other Re(I) complexes, is assigned as a
Re(II/I) wave. The pyrene oxidation is observed at +1.4 V,
similar to the pyrene oxidation in Ru2bpb. The reduction of
the coordinated bpb ligand occurs at -1.32 V.
Electronic Absorption Spectroscopy. Absorption spectra
of Ru2bpb were measured in a variety of solvents, and
absorption maxima are reported in Table 1. By analogy with
the parent compound [Ru(bpy)3]2+, the lowest energy
absorption band of the complex can be attributed to a metal-
to-ligand charge transfer (MLCT) transition (λmax ) 459 nm
in MeCN, Figure 4a). The energy of the transition is similar
to that of the related monometallic complex [(bpy)2Ru(bpy-
pyr)]2+,9 but it is slightly lower than that of [Ru(bpy)3]2+;
the results are in qualitative agreement with correlations of
redox data and absorption maxima for charge-transfer
transitions.19,20 The extinction coefficient of the MLCT
Electrochemistry. The redox behavior of Ru2bpb was
examined by cyclic voltammetry (Figure 3) and differential
pulse polarography in acetonitrile. A reversible Ru(III)/(II)
wave is observed at +1.17 V vs SCE (∆Ep ) 78 mV) and
oxidation of the pyrene is observed as an irreversible wave
with a peak of +1.40 V. This Ru(III/II) potential is thus
slightly less positive than that of [Ru(bpy)3]2+ (Eox ) +1.25
V).16 The oxidative voltammetry differs from that of the
closely related complex [(bpy)2Ru(bpy-pyr)]2+, which has
only a single bipyridine linked to pyrene;9 this complex
exhibits a single quasireversible oxidation with an anodic
peak potential of +1.5 V vs SCE. Another feature of the
oxidative portion of the cyclic voltammogram of Ru2bpb is
that only a single, presumably two-electron, Ru(III/II) wave
is observed. In addition, differential pulse polarograms of
the Ru(III/II) wave are not broadened. This suggests minimal
electronic interaction of the two Ru centers of the complex.
This behavior is consistent with that observed for other
bimetallic Ru(II) diimine complexes having aromatic hy-
drocarbon bridges between the two metal centers.17
The first reversible reduction of Ru2bpb occurs at -1.38
V, more negative than the first reduction of [Ru(bpy)3]2+
(-1.3 V). The reduction of the bpb coordinated to Ru(II)
might be expected to be at a slightly more positive potential,
based on the potentials observed for other 4-aryl-substituted
bipyridine complexes.1 The voltammetric results for Ru2bpb
make determination of localization of the electron on a
particular bipyridine unclear. The cathodic peak of the second
(18) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.;
Wiley: New York, 2001.
(19) On the basis of the correlation of ∆Ep, the difference between the E°
values for the Ru(III/II) and bpy(0/-1) couples, with the energy of
the MLCT absorption maximum. This correlation has been made for
several series of Ru(II) diimine complexes.20
(20) (a) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444-53. (b)
Kober, E. M.; Marshall, J. L.; Dressick, W. J.; Sullivan, B. P.; Caspar,
J. V.; Meyer, T. J. Inorg. Chem. 1985, 24, 2755-63.
(16) (a) McDevitt, M. R.; Addison, A. W. Inorg. Chim. Acta 1993, 204,
141-6. (b) Wacholtz, W. F.; Auerbach, R. A.; Schmehl, R. H. Inorg.
Chem. 1986, 25, 227-34.
(17) (a) Liang, Y. Y.; Baba, A. I.; Kim, W. Y.; Atherton, S. J.; Schmehl,
R. H. J. Phys. Chem. 1996, 100, 18408-18414. (b) Baba, A. I.; Ensley,
H. E.; Schmehl, R. H. Inorg. Chem. 1995, 34, 1198-207.
362 Inorganic Chemistry, Vol. 41, No. 2, 2002

Photophysics of Re(I) and Ru(II) Diimine Complexes
than that of [Ru(bpy)3]2+,22 with a quantum yield in degassed
MeCN of 0.0086. The emission is nearly completely
quenched (>97%) in aerated solutions; thus, quenching is
much more efficient than oxygen quenching of [Ru(bpy)3]2+.
The emission of Ru2bpb also exhibits a subtle shoulder at
shorter wavelength. In aerated solutions, the emission is
extremely weak and the emission maximum is somewhat
blue-shifted (655 nm). This suggests that two different states
contribute to the emission, and the states are very likely not
in equilibrium, since they are quenched by oxygen with
vastly different efficiencies.23 It is also possible that the weak
emission component at shorter wavelength arises from an
impurity; we have not been able to exclude impurity
emission, although the weak component persists even fol-
lowing a variety of micropreparative TLC separations
immediately prior to spectroscopic analysis.24
A blue shift of the emission maximum of deaerated
solutions of Ru2bpb occurs upon increasing the solvent
polarity (e.g., λmax ) 670 nm in MeCN and 682 nm in CH2-
Cl2). This effect is opposite to that observed for [Ru(bpy)3]2+
(λmax ) 620 nm in MeCN and 605 nm in CH2Cl2), where
the emission has clearly been assigned as arising from a
3MLCT state. While assignment of the emission transition
based on solvent shifts is complicated by effects due to
mixing of transitions and a variety of other factors,25 the fact
that the qualitative trend differs from that of the parent
chromophore ([Ru(bpy)3]2+) indicates that the emission
transition is very likely not MLCT in origin. It should also
be noted that no definite trend is observed for the emission
quantum yields as a function of the polarity of the solvent.
Emission spectra of Rebpb in various environments are
very similar to those of Ru2bpb. As shown in Figure 5, the
emission spectra of Rebpb and Ru2bpb have nearly identical
emission maxima in MeCN, but the emission yield for the
Re(I) complex is much lower than that of Ru2bpb. The
emission spectrum of Rebpb in aerated solution also differs
from that in degassed solution. An extremely weak, higher
energy emission component is observed at shorter wave-
length; however, the emission is so weak that a definitive
determination of the maximum is not possible. When the
solvent is changed from acetonitrile to DMSO, a red shift
of the emission occurs, as is the case for Ru2bpb. A
comparison with the less polar solvents dichloromethane and
THF is prohibited by the limited solubility of the complex.
The emission of Rebpb differs markedly from the parent
complex, Rempb, which has a weak emission with a
Figure 5. Normalized emission spectra of Ru2bpb (a, dashed line), Rebpb
(b), Rempb (c, full lines) in degassed acetonitrile at room temperature. The
excitation wavelength is 459 nm for the Ru(II) complex (8 µM) and 402
nm for the Re(I) complexes (20 µM).
transition in Ru2bpb (ꢀ459 ) 3.2 × 104 M-1 s-1) is consistent
with that of other bimetallic Ru-diimine complexes. The
magnitude is essentially equal to twice that of [Ru(bpy)3]2+,
indicating the two Ru diimine centers covalently linked
through the pyrene are very weakly coupled to the pyrene
substituent (MLCT dipole does not extend onto the pyrene).
Two distinct absorption bands are observed at lower wave-
lengths (λmax ) 382 and 397 nm in MeCN). The two
transitions have a lower energy than the π-π* transition of
the free bpb ligand (broad absorption at 361 nm in MeCN,
Figure 4d). A large bathochromic shift of the absorption of
bpb is also observed upon protonation or chelation by Zn-
(II), with the Zn(II) complex having a maximum of 380 nm.
The superposition of different transitions in Ru2bpb prevents
a more thorough examination of the solvent effect.
The absorption spectra of the Re(I) complexes Rebpb and
its parent compound Rempb are very different. The mpb
complex has only a long wavelength tail from a higher
energy absorption in the 350-450 nm wavelength region
(Figure 4c). For closely related Re(I) diimine complexes,
the lowest energy absorption occurs in this wavelength range
and has been assigned as a MLCT transition.5 In contrast,
Rebpb has two distinct absorption maxima at 382 and 402
nm in MeCN (Figure 4b). The unique longer wavelength
transition of the Re(I) complex of bpb at 402 nm is not
MLCT and is similar to the unique band observed at 400
nm in the spectrum of Ru2bpb. It was also found that the
long wavelength absorption transition of Rebpb exhibits some
degree of solvatochromism; a 5 nm red shift is observed
when the solvent is changed from acetonitrile to the less polar
solvent dichloromethane.
Luminescence Spectroscopy. Emission spectra and quan-
tum yields were recorded at room temperature in various
solvents, and the observed maxima are displayed in Table
1. The λmax of Ru2bpb emission in CH3CN is red-shifted as
compared to that of the parent compound [Ru(bpy)3]2+ and
(22) Kawanishi, Y.; Kitamura, N.; Kim T.; Tazuke, S. Riken. Q. 1984, 78,
212-219.
(23) The state emitting at higher energy is quenched by oxygen with a
much smaller efficiency, due to its shorter lifetime (see below). If the
state had been in equilibrium with the longer lived, lower energy state,
the higher energy emission would have been quenched with an
efficiency paralleling that of the long-lived state.
(24) In an attempt to separate possible impurities in Ru2bpb and the Re
complexes, samples were subjected to micropreparative TLC im-
mediately prior to spectroscopic examination. Following TLC plate
development using a variety of eluents, the band containing the
complex was scraped from the plate and washed from the support
(silica or alumina) into the cuvette with an appropriate solvent.
(25) For a review of solvent effects on charge-transfer transitions, see ref
2.
the closely related complex [(bpy)2Ru(bpy-pyr)]2+ (λmax
)
670, 620, and 640 nm, respectively). The emission quantum
yield exhibits a complex concentration dependence due to a
self-quenching process; yields were therefore determined at
complex concentrations of 3-8 µM.21 In degassed MeCN
(Figure 5), the emission of Ru2bpb is 10 times less intense
(21) In MeCN, the absorption at 458 and 397 nm increases linearly with
concentration up to 10 µM.
Inorganic Chemistry, Vol. 41, No. 2, 2002 363

Del Guerzo et al.
Figure 6. Emission spectra of Ru2bpb (a, ---, λexc ) 459 nm) and Rebpb
(b, s, λexc ) 402 nm) in EtOH:MeOH (4:1) glass at 77 K. Phosphorescence
of Hbpb+ (c, [, λexc ) 450 nm) in EtOH:MeOH (4:1) plus 10% of ethyl
iodide and bpb (d, 0, λexc ) 360 nm) in butyronitrile (+10% EtI).
Figure 7. Emission decay at 660 nm after pulsed excitation (λexc ) 532
nm) of Ru2bpb (5 µM) in acetonitrile at room temperature. The solid line
shows the exponential fitting of the data (τlong ) 100 µs). The inset shows
a measurement taken at a shorter time scale (τshort ) 1.8 µs).
maximum 80 nm to the blue of the Rebpb emission
maximum. In addition, the solvent effect on the emission
maximum is the opposite that of Rebpb. Since it is known
3
that the emissive excited state of Rempb is a MLCT state,
the emission of Rebpb is very likely not of MLCT origin
given the observed behavior.
In EtOH/MeOH (4:1) glasses at 77 K, all the complexes
exhibit structured luminescence. Spectra of Ru2bpb, Rebpb,
and the ligand are shown in Figure 6. All four exhibit
structured emission with a maximum at approximately 650
nm and a shoulder at 710 nm. The emission spectrum of
Ru2bpb is similar in band shape, but significantly red-shifted
as compared to [Ru(bpy)3]2+ and [(bpy)2Ru(bpy-pyr)]2+.9 The
emission lifetime of Ru2bpb in the EtOH/MeOH frozen glass
is 510 µs. The spectrum of the protonated ligand, bpb,
obtained in matrixes containing ethyl iodide (which favors
intersystem crossing to the triplet state), has a maximum at
653 nm with a shoulder around 710 nm and a lifetime of
410 µs (Figure 6). However, the nonprotonated ligand, bpb,
when excited at 360 nm, exhibits phosphorescence with a
maximum of 638 nm. The Re complex of bpb has a
maximum at slightly higher energy than Ru2bpb and a
lifetime of nearly 2 ms. The parent analogues of the two
bpb complexes, Rempb and [Ru(bpy)3]2+, both emit from
3MLCT states with maxima that differ from one another and
from the bpb complexes, providing further evidence that the
lowest lying excited states of the bpb complexes are not
3MLCT states.
Room-Temperature Time-Resolved Absorption and
Emission Spectroscopy. Room-temperature luminescence
decays of the bpb complexes can all be fit as double
exponentials. The emission decay of Ru2bpb with two
exponential components in the microsecond time scale is
shown in Figure 7. In MeCN, analyses of the longer decay
gives a lifetime of 100 µs, which is much longer than that
of the parent compound [Ru(bpy)3]2+ (τ ) 0.8 µs).1 This
lifetime is solvent dependent and reaches 130 µs in DMSO.
The short component has a lifetime of 1.8 µs in MeCN, and
its contribution to the emission decreases with increasing
emission wavelength (23% at 600 nm vs 5% at 660 nm),
consistent with the shorter lived emission being associated
with the weaker emission of Ru2bpb. The self-quenching
process mentioned above affects the long lifetime, whereas
the shorter component does not change significantly. The
Figure 8. Transient absorption spectrum of Ru2bpb (10 µM) in acetonitrile
at room temperature 5 µs after pulsed excitation (λexc ) 355 nm). The
spectrum is build by measurement of the decays each 10 or 20 nm. The
same spectra are obtained by exciting at 532 nm, and with a complex
concentration of 50 µM. Decays are single exponential at each wavelength.
wavelength dependence of the relative amplitude of each
decay and the difference of the emission spectra under
aerated and deoxygenated conditions indicate the existence
of two different emitting excited states. The faster decaying
3
excited state, probably the MLCT state, emits at shorter
wavelength and is rapidly deactivated, either to the ground
state or to the longer lived emissive excited state. It is also
possible that the short lifetime emission component arises
from an impurity; however, samples obtained from different
micropreparative TLC purification routines give very similar
relative intensities and lifetimes for the two emission
components. The double exponential behavior observed for
Ru2bpb parallels that observed for [(bpy)2Ru(bpy-pyr)]2+.9
In an effort to further explore the nature of the emissive
excited state, nanosecond transient absorption spectra were
obtained for the complexes studied. The spectrum of Ru2-
bpb exhibits a bleaching of the ground state between 350
and 470 nm, with a minimum at 380 nm (Figure 8). At longer
wavelengths, a broad positive transient extends to the near-
IR detection limit of the system used, with apparent maxima
at 490 and 780 nm. At each wavelength, the transient decay
is monoexponential with a time constant identical within
364 Inorganic Chemistry, Vol. 41, No. 2, 2002

Photophysics of Re(I) and Ru(II) Diimine Complexes
experimental error to that of the long decay observed in the
emission. The transient absorption spectrum of Rebpb also
has characteristics similar of those of the Ru complex, with
a bleaching that occurs farther to the blue. The lifetime of
the Re complex in acetonitrile is shorter than the Ru(II)
complex (11 µs) but still much longer than the lifetime of
the parent complex (Rempb). These transient spectra differ
significantly from both the transient absorption spectra of
3
related complexes having MLCT excited states and the
absorption spectrum of the pyrene triplet. The triplet excited
states of pyrene and substituted pyrene derivatives have a
strong absorption between 450 and 500 nm and no significant
absorption in the red and near-infrared.26 In addition, transient
spectra of Ru(II) bipyridyl complexes covalently linked to
pyrene with an alkyl tether have maxima characteristic of
the pyrene triplet state (maximum between 400 and 500 nm)
and exhibit no absorption to the red of 600 nm.8 Thus, the
transient spectra observed for Ru2bpb and Rebpb suggest
that the long-lived excited states of these complexes are not
Figure 9. Schematic state diagram for the complexes Ru2bpb and Rebpb.
The absorption and emission are shown as solid lines. Waved arrows
represent intersystem crossing and nonradiative decays.
similar. This would not be expected if emission arose from
3
a MLCT state, since related Ru(II) and Re(I) complexes
have significantly different emission energies, especially in
low-temperature glasses. The solvent dependence of the
emission of Ru2bpb also serves as evidence that the emission
of the two complexes examined here is not of MLCT origin.
The emission of Ru2bpb exhibits a decrease in λmax with
decreasing solvent polarity, a trend opposite that observed
for [Ru(bpy)3]2+.28 The protonated bpb ligand also exhibits
phosphorescence at nearly the same energy as the Ru and
Re complexes; in an independent study, the lowest energy
absorption transition of protonated bpb was proposed to be
a pyrene (π) to bpy(H+) (π*) CT transition.13 The similarity
of the emission spectra suggests that the emitting state of
the three substances is related and may be of ILCT origin.29
However, the weak 77 K phosphorescence observed for the
unprotonated bpb ligand has maxima and vibronic structure
very similar to the protonated bpb, suggesting that the two
states cannot be distinguished by 77 K luminescence
spectroscopy. The high degree of oxygen quenching observed
for both Ru2bpb and Rebpb is characteristic of aromatic
hydrocarbon triplet states and might serve to suggest that
intramolecular energy transfer to a pyrene localized triplet
is important in these complexes, like other covalently linked
metal diimine-pyrene complexes.6-10 However, it is worth
noting that the pyrene to bipyridine ILCT state also proposed
here is similar to widely studied twisted intramolecular
charge transfer (TICT) states of purely organic chro-
mophores; in fact, triplet TICT states are known to be readily
quenched by oxygen in organic solvents.30
3
pyrene triplet (3IL) or MLCT in nature.
Discussion
The vast difference in the transient absorption features
observed for the Ru(II) and Re(I) complexes of bpb relative
to other covalently linked metal diimine/pyrene complexes7,8
clearly indicate that the lowest energy excited state is not a
MLCT state. The combined spectroscopic data are most
consistent with the emitting excited-state being either a triplet
state localized on the pyrene (3IL) or a pyrene (π) to
bipyridine (π*) charge transfer state (3ILCT) similar to that
observed by McMillin and co-workers for [ClPt(tpy-pyr)]+.11
The discussion below relates each piece of spectroscopic
evidence to this conclusion.
The electrochemical data indicate that the one-electron
oxidation potential of pyrene in bpb is close to that of the
Ru(III/II) and Re(II/I) potentials. The redox data of Ru2bpb
indicate that the potential for one-electron oxidation of the
Ru(II) is less than 150 mV more negative than the pyrene
oxidation. On the reductive side, it is known that protonated
and metalated bipyridine ligands are much more easily
reduced than free bipyridines.27 These combined redox data
indicate that it is possible for protonated bpb to have a
relatively low-energy pyrene (π) to bipyridine (π*) ILCT
transition and that it is reasonable that the pyrene (π) to bpy
(π*) ILCT and the Ru(II) (dπ) to bpy (π*) MLCT absorption
transitions may be relatively close in energy. In addition,
Re(I) complexes such as Rempb have Re(II/I) and bpy (0/-
1) potentials nearly the same as those of Rebpb, yet the
MLCT absorption of Rempb is at significantly higher energy
than the lowest energy absorption band observed for Rebpb.
The implication is that the absorption of Rebpb at 402 nm
arises from a pyrene (π) to bpy (π*) ILCT transition.
The luminescence spectra of Ru2bpb and Rebpb in solution
at room temperature and in low-temperature glasses are very
Further evidence for the emitting excited state being of
ILCT origin in these complexes is derived from the broad
and complex transient absorption spectra that are not
consistent with either MLCT or IL excited states. In other
systems exhibiting reversible energy transfer between pyrene
triplet and a MLCT excited state, transient absorption spectra
resemble some clear combination of the spectra of the pyrene
triplet and the MLCT excited state of the complex.7,8
(28) Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23,
2098-104.
(29) The protonated ligand differs from the chelated ligand in that the
rotation about the bond linking the two pyridines is inhibited in the
chelated ligand; the protonated ligand can adopt a variety of conformers
where electronic interaction of the two pyridines is much less than
that of the metal-chelated bipyridine.
(26) Bensasson, R.; Land, E. J. Trans. Faraday Soc. 1971, 67, 1904-15.
(27) (a) Krejcik, M.; Vlcek, A. A J. Electroanal. Chem. Interfacial
Electrochem. 1991, 313, 243-57. (b) Braterman, P. S.; Song, J. I. J.
Org. Chem. 1991, 56, 4678-82.
(30) Hashimoto, M.; Hamaguchi, H. J. Phys. Chem. 1995, 99, 7875-7.
Inorganic Chemistry, Vol. 41, No. 2, 2002 365

Del Guerzo et al.
Given these considerations, a state diagram for the
complexes studied is as shown in Figure 9. The spin-allowed
absorption involves overlapping MLCT and ILCT transitions
as the lower energy transitions. The Re(I) complex has an
ILCT that is distinctly lower in energy than the MLCT
transition. The initially formed singlet charge transfer state
intersystem crosses rapidly into the triplet manifold. The
lowest energy, long-lived, emissive triplet state appears to
be either the IL (pyrene π-π*) or ILCT state for Ru2bpb
and Rebpb. The 3MLCT state is at slightly higher energy in
each complex and could be at the origin of the weak shorter
lived emission.
Conclusion
The similarity between the emission properties and the
transient absorption spectra of Ru2bpb and Rebpb strongly
suggests that the long-lived excited state is very likely a
3ILCT state or a ILCT state with significant pyrene triplet
3
character. The presence of heavy atoms substantially in-
creases the spin-orbit coupling and allows observation of
luminescence from the triplet ILCT state in Ru2bpb, Rebpb,
and protonated bpb (in the presence of ethyl iodide). In each
case, the attribution of the triplet state to a MLCT transition
can be excluded, and transient absorption data argue against
the emission arising exclusively from a pyrene 3(π-π*) state.
The presence of ILCT states, in addition to MLCT states,
provides an additional approach for tuning excited-state
energies in metal diimine complexes.
3
3
It should be mentioned that the transient spectra of both
Ru2bpb and Rebpb are similar to those of [(bpy)2Ru(bpy-
pyr)]2+ previously reported by our group. The long-lived
excited state of [(bpy)2Ru(bpy-pyr)]2+ was assigned to
Acknowledgment. A.D.G. and R.H.S. would like to
thank the United States Department of Energy, Office of
Basic Energy Sciences (DE-FG-02-96ER14617) for support
of this work. S.L. and F.F. thank the Universite´ Bordeaux
1, the CNRS, and La Re´gion Aquitaine for financial support.
3
3
coexisting MLCT and (π-π*)pyr states;9 however, data
obtained for this complex are also consistent with a single
3ILCT state. In fact, the transient absorption data obtained
for [(bpy)2Ru(bpy-pyr)]2+ are more easily explained in terms
3
of the ILCT state model.
IC010522V
366 Inorganic Chemistry, Vol. 41, No. 2, 2002