Photophysics of π-conjugated metal-organic oligomers: Aryleneethynylenes that contain the (bpy)Re(CO)3Cl chromophore

Article abstract of DOI:10.1021/ja015813h

A comprehensive study of a series of four monodisperse, metal-organic π-conjugated oligomers of varying length is reported. The oligomers are based on the aryleneethynylene architecture, and they contain a 2,2′-bipyridine-5,5′-diyl (bpy) metal binding unit. The photophysical properties of the free oligomers and their complexes with the (L)Re^I(CO)₃X chromophore (where L = the bpy-oligomer and X = Cl or NCCH₃) were explored by a variety of methods including electrochemistry, UV-visible absorption, variable temperature photoluminescence (PL), transient absorption (TA), and time-resolved electron paramagnetic spectroscopy (TREPR). The absorption of the free oligomers and the metal complexes is dominated by the π,π* transitions of the π-conjugated oligomers. The free oligomers feature a strong blue fluorescence that is quenched entirely in the (L)Re^I(CO)₃X complexes. The metal-oligomers feature a weak, relatively long-lived red photoluminescence that is assigned to emission from both the ³π,π* manifold of the π-conjugated system and the dπ Re → π* bpy-oligomer metal-to-ligand charge transfer (³MLCT) state. On the basis of a detailed analysis of the PL, TA, and TREPR results an excited-state model is developed which indicates that the oligomer-based ³π,π* state and the ³MLCT states are in close energetic proximity. Consequently the photophysical properties reflect a composite of the properties of the two excited-state manifolds.

Full text of DOI:10.1021/ja015813h

J. Am. Chem. Soc. 2001, 123, 8329-8342
8329
Photophysics of π-Conjugated Metal-Organic Oligomers:
Aryleneethynylenes that Contain the (bpy)Re(CO)3Cl Chromophore
Keith A. Walters,† Kevin D. Ley,† Carla S. P. Cavalaheiro,† Scott E. Miller,§
David Gosztola,| Michael R. Wasielewski,§ Alejandro P. Bussandri,‡
Hans van Willigen,‡ and Kirk S. Schanze*,†
Contribution from the Department of Chemistry, UniVersity of Florida, PO Box 117200, GainesVille,
Florida 32611-7200, DiVision of Chemistry, Argonne National Laboratory, Argonne, Illinois 60439,
Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, and
Department of Chemistry, UniVersity of Massachusetts, Boston, Massachusetts 02125
ReceiVed March 13, 2001. ReVised Manuscript ReceiVed June 23, 2001
Abstract: A comprehensive study of a series of four monodisperse, metal-organic π-conjugated oligomers
of varying length is reported. The oligomers are based on the aryleneethynylene architecture, and they contain
a 2,2′-bipyridine-5,5′-diyl (bpy) metal binding unit. The photophysical properties of the free oligomers and
their complexes with the (L)ReI(CO)3X chromophore (where L ) the bpy-oligomer and X ) Cl or NCCH3)
were explored by a variety of methods including electrochemistry, UV-visible absorption, variable temperature
photoluminescence (PL), transient absorption (TA), and time-resolved electron paramagnetic spectroscopy
(TREPR). The absorption of the free oligomers and the metal complexes is dominated by the π,π* transitions
of the π-conjugated oligomers. The free oligomers feature a strong blue fluorescence that is quenched entirely
in the (L)ReI(CO)3X complexes. The metal-oligomers feature a weak, relatively long-lived red photolumi-
nescence that is assigned to emission from both the 3π,π* manifold of the π-conjugated system and the dπ Re
f π* bpy-oligomer metal-to-ligand charge transfer (3MLCT) state. On the basis of a detailed analysis of the
PL, TA, and TREPR results an excited-state model is developed which indicates that the oligomer-based 3π,π*
state and the 3MLCT states are in close energetic proximity. Consequently the photophysical properties reflect
a composite of the properties of the two excited-state manifolds.
Introduction
optical response.12 This work has led to the development of a
reasonably clear picture of how molecular-level structure
influences the optical and electronic properties of a material.
Although much of the work in this area has focused on medium-
to high-molecular weight polymers, many investigations have
explored the optical and electronic properties of low- to medium-
molecular weight monodisperse oligomers.13-27 The oligomer
π-Conjugated polymers and oligomers have been the focus
of considerable research during the past 2 decades. Much of
the work on π-conjugated materials has focused on exploring
the chemical and physical basis for their unique electronic and
optical properties.1,2 However, there also is a significant
possibility that π-conjugated materials may comprise the basis
for a new generation of organic material-based electronic and
photonic devices.3,4
Studies of organic π-conjugated materials have explored the
relationship between molecular structure and the properties of
the materials. Properties of interest include the band-gap and
absorption spectra,5 thermal- and photoconductivity,6-8 photo-
and electroluminescence,9,10 electrochromism,11 and nonlinear
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* To whom correspondence should be addressed, kschanze@chem.ufl.edu.
† University of Florida.
§ Northwestern University.
| Argonne National Laboratory.
‡ University of Massachusetts.
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10.1021/ja015813h CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/04/2001

8330 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Walters et al.
work is very important, since it allows scientists to understand
the important relationship between conjugation length and
electronic and optical properties of the materials.16,24
and co-workers,34-36 and others28-33,37-39,41,48 demonstrated that
introduction of Ru(II)-, Os(II)- or Re(I)-bipyridine chromophores
into the backbone of a PPV or PPE π-conjugated polymer has
a dramatic influence on the absorption, photoluminescence (PL),
photoconductivity, and photorefractivity of the materials. Al-
though these studies demonstrate that the metals strongly
influence the properties of the π-systems, due to the structural
complexity of the polymers it is difficult to obtain detailed
insight concerning the mechanisms by which the π-conjugated
systems are modified by the presence of the transition-metal
centers.
In view of the lack of detailed information concerning
structure-property relationships for π-conjugated metal-
organic systems, we initiated a study having the objective of
synthesis and photophysical characterization of a series of
structurally well-defined, high-molecular weight π-conjugated
oligomers that contain the 2,2′-bipyridine unit as a metal binding
site.31 We selected as targets a series of oligomeric arylene-
ethynylenes (OAEs) that contain a 2,2′-bipyridine-5,5′-diyl metal
chelating unit at their core. The choice of the OAE system was
based on several factors including synthetic accessibility31 and
the expectation that the 3π,π* manifold in the OAEs would be
comparatively high in energy, thereby minimizing complications
that would arise due to interaction of the triplet states with metal
complex-based charge-transfer states.
More recently, a number of groups have examined the optical
and electronic (or electrochemical) properties of π-conjugated
materials that contain transition metals that interact strongly with
the delocalized π-electron systems.28-47 These efforts have the
objective of using the widely variable optical, electronic and
magnetic properties of the metals as a tool to allow the design
of π-conjugated polymers having unique materials properties.
Although a number of advances have been made in this area,
the complex electronic structure of transition-metal complexes
hinders the detailed study of the effect of the metal on the
delocalized π-electron system. Consequently, the need has arisen
to examine well-defined oligomer model systems to provide a
basis for understanding the interactions between the π-electron
system and a transition-metal center.
One area that has received particular attention recently has
been the study of poly(phenylenevinylene) (PPV)- and poly-
(phenyleneethynylene) (PPE)-type polymers that are “doped”
with transition-metal bipyridine chromophores along the π-con-
jugated backbone.28-41,45-48 These polymers are of interest
because of the possibility that the transition metals will strongly
influence the optical and electronic properties of the materials
through the introduction of new accessible redox states and
charge-transfer based excitations. Indeed, recent studies by Yu
The present contribution provides a detailed report of the
photophysical properties of unmetalated OAEs 1-4 (Scheme
1) and the metal-OAE complexes wherein -ReI(CO)3X is
coordinated to the 2,2′-bipyridine-5,5′-diyl moiety (X ) Cl or
CH3CN, complexes Re-1-Re-4 and ReACN-2 and ReACN-
3, Scheme 1). The (diimine)ReI(CO)3X chromophore was
selected for these studies for several reasons: (1) Small molecule
complexes of this type have been the focus of many photo-
physical investigations, and consequently the properties of this
chromophore are well understood.49-51 (2) (Diimine)ReI(CO)3X
complexes feature a lowest excited-state based on dπ Re f π*
diimine metal-to-ligand charge transfer (MLCT), and the energy
of this state can be predicted based on the reduction potential
of the diimine acceptor ligand.50,51 The MLCT excited state is
typically long-lived and photoluminescent and gives rise to a
strong and characteristic transient absorption in the near-UV
and visible.50,51 These features make it relatively easy to detect
and characterize the Re f diimine MLCT state even in complex
supramolecular systems.
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Synthesis and Characterization of 1-4 and Re-1-Re-4. The
methodology used to synthesize oligomers 1-4 relied extensively upon
Sonogashira coupling of appropriately protected and functionalized aryl
iodides and terminal acetylenes.52 Details of the synthesis and complete
characterization of 1-4 and Re-1-Re-4 are presented elsewhere.31
Synthesis of ReACN-2 and ReACN-3. The preparation involved
Ag+OTf- promoted substitution of Cl for CH3CN. The same procedure
was followed for both complexes, and we provide the details for
ReACN-2. Re-2 (12 mg, 6.1 µmol) was dissolved in 20 mL of
methylene chloride, whereupon 5 mL of CH3CN and Ag(CF3SO3) (12
mg, excess) was added. The solution was stirred at room temperature
overnight. TLC of the reaction mixture (silica, 1:1 hexanes:CH2Cl2)
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π-Conjugated Metal-Organic Oligomer Photophysics
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8331
Scheme 1
showed two spots, Rf ) 1 (ReACN-2) and Rf ) 0.6 (Re-2). Stirring
for an additional 6 h allowed the reaction to go to completion. Solid
AgCl was removed from the sample solution by filtration through Celite
on a medium-porosity glass frit, and the solvent was removed under
the third harmonic of a Nd:YAG laser (355 nm, 10 ns fwhm, 5 mJ
pulse-1{TA}, 8-30 µJ pulse-1 {LIOAS}) as the excitation source.
Primary factor analysis followed by first order (A f B) least-squares
fits of the transient absorption data was accomplished with SPECFIT
global analysis software (Spectrum Software Associates). LIOAS data
(average of four data acquisitions) was obtained with both 1 and 5
MHz transducers, and deconvolution analysis of the LIOAS data was
performed with Sound Analysis software (Quantum Northwest, Inc.).
Picosecond transient absorption spectroscopy was carried out using
instrumentation that has been previously described.57-59
Electrochemical Measurements. Electrochemical measurements
were conducted on THF solutions with tetrabutylammonium hexafluo-
rophosphate (TBAH, Aldrich) as the supporting electrolyte. Cyclic
voltammetry was carried out on nitrogen bubble-degassed solutions
(TBAH ) 0.1 M) with a BAS CV-27 Voltammograph and MacLab
Echem software. A GCE disk working electrode, platinum wire
auxiliary electrode, and silver wire quasi-reference electrode were used,
-
reduced pressure to afford ReACN-2 as the CF3SO3 salt. Due to the
small amount of material available, further purification was impractical.
Characterization of ReACN-2: IR (THF soln, cm-1), 1966, 2101,
2135 (υCtO), 2278 (υCtN).53 ESI-MS calcd for C131H182N3O7Re: 2096;
found: 2053 (parent - CH3CN).
Characterization of ReACN-3: IR (THF soln, cm-1) 1967, 2097,
2133 (υCtO), 2280 (υCtN).53 ESI-MS calcd for C149H195N2O11Re: 2416;
found: 2414 (parent), 2372 (parent - CH3CN).
Photophysical Measurements. All photophysical studies were
conducted in 1 cm square quartz cuvettes unless otherwise noted. All
room-temperature studies were conducted on THF solutions that were
deoxygenated using argon bubbling, and all low-temperature studies
were conducted on 2-methyltetrahydrofuran (2-MTHF) solvent glasses
degassed by 4 freeze-pump-thaw cycles (ca. 10-4 Torr) unless
otherwise noted. Absorption spectra were recorded on a HP 8452A
diode-array spectrophotometer. Corrected steady-state emission mea-
surements were conducted on a SPEX F-112 fluorimeter. Emission
quantum yields were measured by relative actinometry, with 9,10-
dicyanoanthracene (Φem ) 0.89) and perylene (Φem ) 0.89) in ethanol
as actinometers.54 Time-resolved emission decays were obtained by
time-correlated single photon counting on an instrument that was
constructed in-house. Excitation was effected by using either a pulsed
LED source or a blue pulsed diode laser (IBH instruments, Edinburgh,
Scotland). The pulsed LED and diode laser sources have pulse widths
of ca. 1 ns. The time-resolved emission was collected using a red-
sensitive, photon-counting PMT (Hamamatsu R928), and the light was
filtered using 10 nm band-pass interference filters. Lifetimes were
determined from the observed decays by using fluorescence lifetime
deconvolution software. Low-temperature emission measurements were
conducted in 1 cm diameter glass tubes contained in an Oxford
Instruments DN1704 optical cryostat connected to an Omega CYC3200
automatic temperature controller. Nanosecond transient absorption
spectra55 and laser-induced optoacoustic spectroscopy (LIOAS) mea-
surements56 were obtained on previously described instrumentation, with
and potentials were corrected to values vs SCE via an internal standard
ox
(ferrocene, E ) +0.40 V vs SCE). A scan rate of 100 mV-sec-1 was
1/2
employed in all measurements. Spectroelectrochemistry measurements
were conducted in a custom cell on solutions degassed by four freeze-
pump-thaw cycles (ca. 10-4 Torr; TBAH ) 0.05 M). Platinum mesh
working and auxiliary electrodes and a silver wire quasi-reference
electrode were used. Absorption measurements on this cell were
conducted in an Ocean Optics in-line filter holder connected to an Ocean
Optics S2000 CCD-spectrometer by a UV-rated fiber optic cable (600
µm diameter) and a xenon arc lamp excitation source.
Time-resolved cw (continuous wave) EPR (TREPR) spectra were
recorded with a Varian E9 X-band spectrometer with variable temper-
ature accessory adapted for direct detection of EPR signals with a PAR
model 162 boxcar averager.60 Samples in a 1:1 toluene-chloroform
glass at ∼130 K were excited with the third harmonic (355 nm, 10
mJ/pulse, 10 Hz) of a Quanta Ray GCR12 Nd:YAG laser. The boxcar
detection window typically was set at 200-700 ns post flash.
(56) Walters, K. A.; Schanze, K. S. The Spectrum; Newsletter of the
Center for Photochemical Sciences; Bowling Green State University:
Bowling Green, OH; http://www.bgsu.edu/departments/photochem/
Summer1998Spectrum.pdf; 1998; Vol. 11, pp 1-4.
(57) Gosztola, D.; Yamada, H.; Wasielewski, M. R. J. Am. Chem. Soc.
1995, 117, 2041-2048.
(53) For comparison purposes, Re-2 and Re-3 (KCl pellet) exhibit
stretches at 1902, 1927, and 2025 cm-1. The shift of the νCO bands to higher
frequency in ReACN-2 and Re-ACN-3 is consistent with substitution of
Cl with η1-NCCH3.
(54) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry, 2nd ed.; Marcel Dekker: New York, 1993.
(58) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R.
J. Am. Chem. Soc. 1996, 118, 6767-6777.
(59) Lukas, A. S.; Miller, S. E.; Wasielewski, M. R. J. Phys. Chem. B
2000, 104, 931-940.
(60) van Willigen, H.; Vuolle, M.; Dinse, K. P. J. Phys. Chem. 1989,
93, 2441-2444.
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8332 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Walters et al.
Scheme 2
Results
Structures and Synthesis. This study examines the photo-
physical properties of the series of oligomeric aryleneethy-
nylenes (OAEs) 1-4, and the corresponding fac-(L)Re(CO)3Cl
complexes Re-1-Re-4 (Scheme 1). Each oligomer contains a
“core” consisting of a 5,5′-(2,2′-bipyridyl) unit. The primary
repeat unit of the π-conjugated OAE chain is shown in Scheme
2. Including the 2,2′-bipyridine-5,5′-diyl unit, oligomers 1-4,
respectively, contain approximately 1.5, 2.5, 3.5, and 5.5 “repeat
units” in the OAE chain.
The bipyridine containing oligomers were synthesized by an
iterative sequence in which the key steps involved Pd-mediated
cross coupling of a terminal acetylene and an aryl iodide
(Sonogashira coupling).52 The oligomers were characterized by
1H and 13C NMR and mass spectroscopy. The (L)Re(CO)3Cl
complexes Re-1-Re-4 were prepared by reaction of Re(CO)5Cl
with the oligomer “ligand”.51 The metal complexes were also
Figure 1. Absorption spectra for THF solutions. (a) 1: (___); 2: (- -
-); 3: (__‚__); 4: (__‚‚__). (b) Re-1: (___); Re-2: (- - -); Re-3: (__‚__);
Re-4: (__‚‚__).
1
characterized by H and 13C NMR and FTIR spectroscopy.
Details regarding the synthesis and spectral characterization of
1-4 and Re-1-Re-4 were reported in a recent communication.31
The(L)Re(CO)3(NCCH3)+complexesReACN-2andReACN-3
were prepared by reaction of the corresponding (L)Re(CO)3Cl
complexes with AgOTf in CH3CN solution. The Cl-to-CH3CN
ligand exchange reactions were carried out on a very small scale,
and consequently the ReACN-2 and ReACN-3 products were
not characterized by NMR spectroscopy. However, the products
were characterized by FTIR and mass spectroscopy. In addition,
TLC analysis indicated that Cl-to-CH3CN ligand exchange was
quantitative in both cases.
Photophysics of Oligomers 1-4. UV-visible absorption
spectra of 1-4 in THF solution are shown in Figure 1a, and
the absorption maxima and molar absorptivities are listed in
Table 1. The oligomers feature two absorption bands in the near-
UV and visible regions. The low-energy band is due to the long-
axis polarized π,π* transition, while the higher-energy band is
due to the short-axis π,π* transition. Both absorption bands shift
to significantly lower energy on going from 1 to 2; however,
the energy of the bands remains relatively constant for 2 to 4.
This trend indicates that the conjugation length is defined
relatively early in the series. It is noteworthy that the absorption
spectra of 3 and 4 are closely similar to those of structurally
related OAEs and PPE polymers that have been previously
reported.18,22,23,25,26,61-65 Table 1 also lists the approximate molar
absorptivity per repeat unit (see above for definition of repeat
unit) for the lowest-energy absorption band in each oligomer.
This parameter remains relatively constant across the series,
indicating that to a first-order approximation the oscillator
strength increases linearly with oligomer length.
a listing of the fluorescence quantum yields, lifetimes and
radiative decay rates (φf, τf, and kr, respectively). Comparison
of the fluorescence spectra of the oligomers reveals that the
spectral bandwidth decreases with increasing oligomer length.
In addition, a clear trend emerges for oligomers 2-4: the
intensity of the 0,1 band relative to the 0,0 band decreases along
the series 2 > 3 > 4. This trend signals that the electron-
vibration coupling in the 1π,π* state decreases with increasing
oligomer length. The trend observed in the fluorescence
bandwidth and vibronic structure for the oligomers implies that
the 1π,π* state becomes more delocalized as the oligomer length
increases.13,14
The fluorescence lifetimes of the oligomers range from 2.4
to 0.68 ns, and the general trend is that the lifetime decreases
with increasing oligomer length. By contrast, the fluorescence
quantum yield is highest for 1 and then is comparatively constant
for the longer oligomers. The radiative decay rate, kr, computed
from the ratio φf/τf, gradually increases with increasing oligomer
length. The experimentally observed kr’s are in reasonable
agreement with those computed using the Strickler-Berg
equation (krSB, Table 1),13 which confirms that the fluorescence
emanates from the same state that gives rise to the lowest
absorption band. In addition, the fact that the radiative decay
rates are relatively large and close to those calculated with the
Strickler-Berg relationship indicates that the fluorescence arises
from a strongly allowed radiative transition. We infer this as a
confirmation that in oligomers 1-4 the lowest excited state is
1
of Bu symmetry.13,14
Laser flash photolysis of 1-4 in THF solution at room
temperature affords long-lived transient absorptions. For each
oligomer the transients are very long-lived (τ > 150 µs, see
Table 1) and are quenched by low-energy triplet energy
acceptors (ET < 50 kcal-mol-1). On the basis of these observa-
All of the oligomers feature a strong blue fluorescence. Figure
2 illustrates the fluorescence spectra of 1-4 and Table 1 contains
(61) Swager, T. M. Acc. Chem. Res. 1998, 31, 201-207.
(62) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995,
99, 4886-4893.
(63) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; Studer-Martinez,
S. L.; Bunz, U. H. F. Macromolecules 1998, 31, 8655-8659.
(64) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605-1644.
(65) Palmans, A. R. A.; Smith, P.; Weder, C. Macromolecules 1999,
32, 4677-4685.
3
tions we assign the long-lived transients to the π,π* excited
states of the oligomers. The absorption difference spectra
(triplet-triplet) for the 3π,π* states of 1-4 are shown in Figure
3. Each spectrum is characterized by strong bleaching of the
ground-state absorption band (380-420 nm) and broad absorp-
tion throughout the visible region and continuing into the near-

π-Conjugated Metal-Organic Oligomer Photophysics
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8333
Table 1. Photophysical Properties of Oligomers 1-4a
absorption
fluorescence
triplet state
λmax
/
ꢀmax
103 M-1 cm-1 b
/
λmax
/
τf/
kr/
krSB
/
ET/
τT/
µsf
nmc
ns
φf
109 s-1
109 s-1d
0.65
kcal-mol-1
φT
e
compound
nm
1
330
370
38.5
54.0
(36.0)
69.0
98.0
(39.2)
95.0
135
(38.5)
152
455
(60.8)
2.4
0.89
0.37
0.76
0.61
1.2
52.3
50.6
50.6
50.0
0.05(1)
0.21(6)
0.26(3)
0.17(4)
160
450
340
410
2
3
4
320
400
454
(63.0)
0.95
1.1
0.72
0.68
0.79
0.87
1.2
340
404
453
(63.1)
336
406
452
(63.3)
0.68
2.3
251
(46.0)
a All data for argon-deoxygenated THF solutions. Errors: ꢀmax (5000 M-1 cm-1; τf (0.4 ns; φf (0.06; τT (50 µs. b Number in parentheses is
molar absorptivity per “repeat unit”, see text. c Number in parentheses is fluorescence energy in kcal-mol-1 d Calculated using Strickler-Berg
.
equation (ref 13). e Number in parentheses is the experimental error (in the last digit). f Freeze-pump-thaw degassed solution.
Figure 2. Fluorescence spectra of 1-4 in THF solution at 298 K.
Excitation wavelength: 350 nm.
IR. The triplet-triplet spectra of oligomers 2-4 are remarkably
similar to the triplet-triplet spectrum of a structurally related
PPE polymer.66
The energies of the triplet states (ET, Table 1) were
determined by carrying out a series of Stern-Volmer quenching
experiments on each oligomer using six triplet energy acceptors.
The triplet energies were determined by fitting the resulting
Sandros plots to a Marcus-type expression (see Supporting
Information for the Sandros Plots).66 Interestingly, the triplet
energy of 2-4 is the same within experimental error (∼50 kcal-
mol-1); however, ET for 1 is higher by a few kcal-mol-1. Note
also that ET for the bipyridine-containing oligomers is very
similar to the triplet energy of a structurally related PPE polymer
(51.2 kcal-mol-1).66
Time-resolved photoacoustic calorimetry was used to deter-
mine the triplet yields for oligomers 1-4, and the resulting
values are listed in Table 1.56 Within experimental error, the
Figure 3. Transient absorption difference spectra of 1-4 in argon-
triplet yields for 2-4 are approximately the same (∼20%).
Furthermore, the sum of the fluorescence and triplet quantum
yields (φf + φT) is close to unity, which indicates that
photoexcitation of the oligomers leads almost exclusively to
radiative decay or intersystem crossing (i.e., very little nonra-
diative decay occurs from the singlet excited state).
Absorption and Luminescence Properties of Re-1-Re-4.
The UV-visible absorption spectra of Re-1-Re-4 are shown
in Figure 1b, and band maxima and molar absorptivity values
are listed in Table 2. The spectrum of each metalated oligomer
features at least two bands, one in the visible and a second,
deoxygenated THF solution at 298 K following 355 nm pulsed
excitation. Spectra were acquired at 40 µs increments after laser
excitation, and arrows indicate direction of change of ∆A with
increasing time. (a) 1. (b) 2. (c) 3. (d) 4.
more intense band in the near UV. Interestingly, the wavelength
maximum and oscillator strength of the visible absorption band
is comparatively constant for the series Re-2-Re-4, while the
higher energy band red-shifts and increases in oscillator strength
across the series. This feature suggests that the low-energy band
is associated with a chromophore that remains “constant” for
Re-2-Re-4, while the higher energy bands arise from a segment
of the molecule that is “varying” across the series. Although
(66) Walters, K. A.; Ley, K. D.; Schanze, K. S. J. Chem. Soc., Chem.
Commun. 1998, 10, 1115-1116.

8334 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Walters et al.
Table 2. Photophysical and Electrochemical Properties of fac-(L)Re(CO)3Cl Complexesa
absorption
emission
λmax
/
ꢀmax
103 M-1 cm-1
/
λ
max/nm
298 K
τi/ns (Ri)b
λ
max/nm
80 K
τi/ns (Ri)b
complex
nm
298 K
80 K
τ
TA/nsc
20
E1/2red/Vd
-0.88
Re-1
Re-2
Re-3
Re-4
332
410
336
440
352
444
338
388
444
340
449
348
447
29.0
34.0
83.0
73.4
118
83.0
168
207
103
94.9
73.8
147
94.9
2.4
690
1.6 (0.93)
14 (0.07)
2.0 (0.99)
154 (0.01)
2.4 (0.99)
174 (0.01)
3.8 (0.99)
170 (0.01)
586
659
642
642
61 (0.53)
6240 (0.47)
2.4 (0.99)
2490 (0.01)
5.1 (0.99)
5110 (0.01)
7.7 (0.99)
7610 (0.01)
650
652
650
145
198
161
-0.87
-0.86
-0.91e
ReACN-2
ReACN-3
652
650
0.18 (0.998)
8.5 (0.002)
0.17 (0.997)
9.2 (0.003)
39h
589
625
629
0.36 (0.99)
5.7 (0.01)
0.32 (0.99)
5.7 (0.01)
3120
9710f
5010f
-
-
(bpy)Re(CO)3Clg
400h
360
642h
540
540
-
-
-
-1.35i
-1.26k
(bpy)Re(CO)3(ACN)+ f,j
3.0
1100
a Room-temperature data for THF solutions and 80 K data for 2-MTHF glasses, unless otherwise noted. Errors: ꢀmax (5000 M-1 cm-1; τem (10
ns; τTA (10 µs. b Lifetimes and normalized amplitudes (in parentheses) obtained from two-component fits of emission decays according to the
equation, I(t) ) R1 exp{-t/τ1} + R2 exp{-t/τ2}. c Decay lifetime of transient absorption. d Reduction potential of metal-organic oligomer in
THF/0.05 M TBAH solution, V vs SCE. e Irreversible reduction. f CH2Cl2 solution. g Data from ref 50. h 2-MTHF solution. i CH3CN solution. j Data
from ref 88. k DMF solution, data from ref 89.
we reserve a detailed discussion of the absorption spectra until
a later discussion, at this point we note that the intensity of the
low-energy absorption band for each oligomer is considerably
stronger than expected if the transition is MLCT.49 Therefore,
all of the clearly resolved bands present in the absorption spectra
of Re-1-Re-4 are believed to arise from π,π* “intraligand”
transitions localized on the OAE. We infer that the Re f bpy-
oligomer MLCT bands are “buried” under these considerably
more intense π,π* transitions.
The PL of Re-1-Re-4 was carefully examined over the
temperature range from 77 to 298 K with dilute (c ≈ 2-5 µM)
samples dissolved in 2-methyltetrahydrofuran (MTHF). First,
all of the complexes feature only very weak emission in the
region where fluorescence from the free oligomers is observed
(400-550 nm). In most cases emission in this spectral region
is more than 100-fold weaker compared to that of the free OAEs.
Although the observed fluorescence may emanate from the
metal-organic complexes, it is impossible to rule out the
possibility that the emission does not originate from a trace
amount of the unmetalated oligomers present in the samples.
In any event, it is safe to conclude that fluorescence from the
1π,π* manifold is efficiently quenched by the metal complex
chromophore in Re-1-Re-4.
By contrast, weak luminescence is observed for Re-1-Re-4
in the 500-800 nm region. Figure 4 shows the PL spectra of
each complex over the 77-298 K temperature range. For all of
the complexes the room-temperature emission is very weak (φ
, 0.005). The emission intensity increases substantially upon
cooling (a 20-fold increase on cooling from 298 to 77 K is
typical) so that at low temperature the emission is moderate in
intensity. Nonetheless, due to the low intensity of the room-
temperature luminescence, quantum yield measurements were
not attempted. At 298 K the emission of Re-1 is characterized
Figure 4. Temperature-dependent emission spectra of complexes Re-
1-Re-4 in a vacuum degassed 2-MTHF solution (glass). Temperatures
by a broad-structureless band with λmax ) 680 nm (see inset,
Figure 4a). By contrast, at 77 K the emission is significantly
are listed in order of decreasing emission intensity. (a) Re-1: T ) 80,
blue-shifted (λmax ) 586 nm) and is highly structured, showing
100, 122, 140, 160, 180, 200, 220, 240, 260 K. (Inset) 280 K, spectrum
a clear vibronic progression with ∆ν ≈ 1350 cm-1. The emission
normalized. (b) Re-2: T ) 81, 103, 121, 141, 182, 204, 222, 240,
spectra of Re-2-Re-4 are similar to one another but differ from
260, 280, 298 K. (c) Re-3: T ) 80, 110, 128, 140, 160, 196, 222, 249,
that of Re-1. First, at 77 K, the emission of the three complexes
298 K. (d) Re-4: T ) 81, 110, 142, 170, 204, 230, 260, 298 K.
is distinguished by three “peaks” that are reasonably well-
resolved in Re-2 and Re-3, but less so in Re-4. Close inspection
of the 77 K spectra of Re-2 and Re-3 suggests that these three
“peaks” arise from the superposition of a structureless emission
band with λmax between 580 and 600 nm, and a structured

π-Conjugated Metal-Organic Oligomer Photophysics
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8335
Figure 6. Picosecond transient absorption difference spectra for Re-4
in THF solution following 400 nm excitation (80 fs pulse width). (___
)
__
100 ps delay; (__ __) 500 ps delay, (__‚__) 2500 ps delay.
complexes is characterized by: (1) bleaching of the ground-
state absorption bands in the 350-450 nm region, (2) a strong
mid-visible absorption band in the 500-550 nm region, and
(3) a broad, featureless absorption in the red that continues into
the near-IR region. It is interesting to note that the dominant
feature in all of the spectra is the bleach/absorption feature in
the 400-550 nm region. This derivative-shaped feature suggests
that the position of the lowest-energy π,π* transition is red-
shifted in the excited state (relative to the ground state). The
lifetime of Re-1 is rather short (τ ≈ 20 ns); however, the
transients observed for the longer oligomers decay with
comparable lifetimes of 150-200 ns. Note that there is a strong
correlation between the lifetimes of the transients observed by
ns flash photolysis and the long-lived emission decay component
(compare lifetimes in Table 2). This correspondence suggests
that the TA arises from the emitting excited state, or alternatively
it is an excited state that is in equilibrium with the luminescent
state.67-73
Figure 5. Transient absorption difference spectra of Re-1-Re-4 in
argon-deoxygenated THF solution at 298 K following 355 nm pulsed
excitation. Spectra acquired at 4 ns increments for Re-1 and at 80 ns
increments for Re-2-Re-4. (a) Re-1. (b) Re-2. (c) Re-3. (d) Re-4.
A limited series of studies were carried out to characterize
the TA properties of the metal-organic oligomers on the
picosecond time scale. The observations made with Re-4 are
typical of those seen for the oligomers and therefore we only
present data for this complex herein. Figure 6 shows absorption
difference spectra obtained for Re-4 in THF solution at 100,
500 and 2500 ps following 400 nm pulsed excitation (80 fs
pulse width), and the inset shows the transient absorption decay
kinetics monitored at 650 nm. As can be seen from the data,
there is clearly a relaxation occurring on the ps time scale
following initial excitation of the complex. Analysis of the
kinetic data reveals the following: (1) an ultrafast decay (τ ≈
16 ps), (2) a “slower” relaxation process with τ ≈ 770 ps, (3)
a transient absorption that persists into the nanosecond time
domain. Inspection of the time-resolved difference spectra shows
that on the 100-2500 ps time scale there are only minor changes
emission with λmax ≈ 650 (the 0,0 band) nm and a vibronic
sideband (the 0,1 band) that appears to the red of the main band.
The emission intensity of Re-2-Re-4 decreases in intensity with
warming, but no significant shift in the position of the emission
band is observed. Indeed, for all three complexes λmax is virtually
the same at 77 and 298 K. At 298 K the PL, although weak, is
dominated by a structured band with λmax ≈ 650 nm.
The emission decay profiles of Re-1-Re-4 are multiexpo-
nential. Table 2 contains a listing of parameters recovered from
two-component fits of the emission decays for the complexes
at 80 and 298 K. At 298 K the decays are characterized by a
large amplitude, very short-lived component (τ ≈ 1 ns, R >
0.95), and a low-amplitude component with a considerably
longer lifetime (τ ≈ 20-200 ns) that varies somewhat with
oligomer length. At 80 K in a rigid solvent glass the decays are
qualitatively similar. In each case the lifetime of the short-lived
decay component remains almost the same; however the lifetime
of the long-lived component increases significantly.
(67) Shaw, J. R.; Webb, R. T.; Schmehl, R. H. J. Am. Chem. Soc. 1990,
112, 1117-1123.
(68) Shaw, J. R.; Schmehl, R. H. J. Am. Chem. Soc. 1991, 113, 389-
394.
Transient Absorption of Metal-Oligomers Re-1-Re-4.
Nanosecond laser flash studies were carried out on Re-1-Re-4
using the third harmonic of a Nd:YAG laser for excitation (λ
) 355 nm, 10 ns fwhm). In all cases, strongly absorbing
transients with lifetimes in the 10-200 ns time domain were
observed. Figure 5 illustrates time-resolved transient absorption
(TA) difference spectra for the four complexes. The TA decay
lifetimes (τTA), obtained from global kinetic analysis of the time-
resolved spectral data, are listed in Table 2. As can be seen in
Figure 5, the absorption difference spectrum for each of the
(69) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak,
P.; Jin, X.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11012-11022.
(70) Baba, A. I.; Shaw, J. R.; Simon, J. A.; Thummel, R. P.; Schmehl,
R. H. Coord. Chem. ReV. 1998, 171, 43-59.
(71) Schmehl, R. H. The Spectrum; Newsletter of the Center for
Photochemical Sciences; Bowling Green State University: Bowling Green,
OH; http://www.bgsu.edu/departments/photochem/; 2000; Vol. 13, pp 17-
21.
(72) Tyson, D. S.; Castellano, F. N. J. Phys. Chem. A 1999, 103, 10955-
10960.
(73) Tyson, D. S.; Bialecki, J.; Castellano, F. N. Chem. Commun. 2000,
2355-2356.

8336 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Walters et al.
in the spectral band shapesthe dominant process that occurs
on this time scale is associated with the loss of a significant
fraction of the initial TA amplitude. Nonetheless, there is a blue-
shift in the transient absorption band (610 nm f 530 nm)
occurring on this time scale. Note that the spectrum observed
for Re-4 at “long times” (i.e., 2500 ps delay) in the picosecond
pump-probe experiment corresponds well with the transient
absorption difference spectrum observed by nanosecond flash
photolysis (compare Figures 5d and 6). Finally, there is
reasonable correspondence between the lifetime of the inter-
mediate decay component seen in the picosecond pump-probe
experiment (τ ≈ 770 ps) and the fast component observed in
the emission decay experiments. This fact suggests that the sub-
ns time scale dynamics that are observed by PL and transient
absorption may arise from the same physical relaxation process.
Electrochemistry and Spectroelectrochemistry of Re-1-
Re-4. Cyclic voltammetry was carried out on the metal-organic
oligomers in an effort to determine the potentials for oxidation
and reduction of the complexes. Owing to the poor solubility
of the oligomers in polar solvents typically used for electro-
chemistry, these studies were limited to CH2Cl2 and THF
solution. After extensive trial-and-error, it was found that the
optimal conditions for CV were obtained in THF solution with
a glassy carbon working electrode.74 Under these conditions,
reversible one-electron reduction waves were observed for the
first reduction of Re-1-Re-3, while the first reduction of Re-4
was quasi-reversible (unsymmetrical anodic and cathodic waves).
Figure 7. Spectroelectrochemistry of (a) Re-3 and (b) Re-4. The
samples were dissolved in THF/0.05 M TBAH and vacuum degassed.
Spectra obtained at 2 min intervals after the working electrode was
stepped to -1.2 V (Re-3) or -1.7 V (Re-4) vs SCE. Arrows indicate
the direction of change with increasing electrolysis time.
For each of the complexes the reduction occurs with E1/2
≈
nm is hardly affected by the reduction, suggesting that the
chromophore involved in this optical transition is not affected
significantly by the electrochemical reduction. This observation
is consistent with the idea that the long-wavelength ground-
state absorption band arises from absorption in the “bpy” core
of the OAE oligomer and that the shorter wavelength bands
arise from the OAE “periphery” (further discussion of this point
is presented below).
-0.90 V vs SCE (see Table 2). Unfortunately, sweeps to positive
potentials were limited by the accessible potential window in
THF (anodic limit ≈ +1.4) and therefore it was not possible to
obtain potentials for oxidation of the complexes.
Quite clearly the first reduction of Re-1-Re-4 arises from
one-electron reduction of the metalated oligomer. Given that
the reduction potential does not shift significantly across the
series, we conclude that the anion radical is localized primarily
on the 2,2′-bipyridine “segment” of the conjugated oligomer
(i.e., the LUMO of the oligomer has the highest density on the
metal-bipyridine unit). Nonetheless, the first reduction poten-
tials of Re-1-Re-4 are considerably less than those for the
parent complex (bpy)Re(CO)3Cl (-1.35 V),50 which indicates
that the aryl-ethynyl moieties that are at the 5,5′ positions on
the bipyridine stabilize the reduced complex (i.e., the LUMO
energy is reduced by the π-conjugation). This further implies
that in the reduced complex there is some delocalization of
electron density into the appended aryl-ethynyl units.
Spectroelectrochemistry was carried out on solutions of Re-3
and Re-4 to obtain information concerning the absorption of
the reduced complexes. Figure 7 illustrates the spectra obtained
when vacuum degassed solutions of the complexes were reduced
at a potential negative of the first reduction wave observed by
CV. In each case the original spectrum could be reproduced
when the solution was reoxidized by switching the potential
back to 0 V, suggesting that the species observed in the
spectroelectrochemistry arises from one-electron reduction of
the complex. For both complexes, reduction results in bleaching
of the lowest energy absorption band (λ ≈ 450 nm) with a
concomitant increase in the absorption in the red. It is important
to notice that for Re-4 the strong absorption band at λ ) 395
Photophysics of ReACN Complexes. In an effort to provide
further information concerning the nature of the lowest excited
states in the metal-OAE complexes, the photophysics of
ReACN-2 and ReACN-3 were examined. These complexes are
analogues of Re-2 and Re-3 wherein the Cl ligand is replaced
by η1-NCCH3. The CH3CN complexes were prepared because
it is known from work on the “parent” bipyridine complexes
that substitution of Cl with NCCH3 strongly influences the
photophysics of the complex. Specifically, replacement of Cl
with CH3CN typically has the following effects:50,51 (1) the
energy of the MLCT state increases by 0.3-0.4 eV, and (2)
the emission quantum yield and lifetime increase by 1-2 orders
of magnitude. These effects are clearly illustrated by comparing
the photophysical parameters of (bpy)Re(CO)3Cl with those of
(bpy)Re(CO)3(NCCH3)+ (see Table 2). On the basis of these
“typical” effects, it was anticipated that Cl-to-CH3CN ligand
exchange in the metal-OAEs would afford materials that are
strongly luminescent and have longer excited-state lifetimes
compared to those of Re-1-Re-4. Moreover, observation of
the anticipated shift in the emission energy would provide
evidence that the luminescence of the metal-oligomers ema-
nates from the MLCT manifold.
Table 2 contains a listing of various photophysical parameters
for the two CH3CN complexes, ReACN-2 and ReACN-3, and
Figures 8 and 9 illustrate their temperature-dependent emission
spectra and room-temperature TA spectra. First, the absorption
spectra of ReACN-2 and ReACN-3 are nearly indistinguishable
from those of the corresponding Cl-substituted complexes (see
Table 2 to compare the absorption maxima). This is not
(74) In other solvents, the electrochemistry of the oligomers was found
to be irreversible, presumably due to precipitation and adsorption of the
reduced complex onto the electrode surface. Even in THF solution, it was
often observed that a good CV could be obtained only on the first sweep.
On successive scans the peak current would diminish, and the degree of
reversibility of the electrode process would decrease.

π-Conjugated Metal-Organic Oligomer Photophysics
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8337
modestly, and the spectra broaden significantly. The most
noticeable effect is that at low temperature the emission of the
two complexes appears as if it consists of two superimposed
emission bands, one with an energy that is similar to that
observed at room temperature (λmax ≈ 640 nm) and a second,
new band that appears at higher energy (λmax ≈ 575 nm).
Finally, consistent with the low emission intensities, emission
decay experiments indicate that the decay time of the lumines-
cence from ReACN-2 and ReACN-3 is very short at all
temperatures (Table 2). This contrasts sharply with the emission
decay kinetics of the Cl complexes, where a low-amplitude but
long-lived decay component is observed at ambient temperature.
TA spectra obtained on ReACN-2 and ReACN-3 are il-
lustrated in Figure 9. Note that the absorption difference spectra
of the two CH3CN complexes are Very similar in appearance to
those of the corresponding Cl complexes (compare with Figure
5). This strongly suggests that the nature of long-lived transients
detected by transient absorption is the same, regardless if the
ancillary ligand is Cl or CH3CN. However, the unusual feature
is that the lifetimes of the transients observed by transient
absorption for ReACN-2 and ReACN-3 are considerably longer
than those of Re-1-Re-4 (see Table 2). It is also noteworthy
that the transient absorption lifetimes for ReACN-2 and
ReACN-3 are several orders of magnitude longer than their
emission decay lifetimes. This feature makes it clear that the
emission emanates from an excited state that is unique from
that observed by TA.
Figure 8. Temperature-dependent emission spectra of complexes
ReACN2 and ReACN3 in a vacuum degassed 2-MTHF solution (glass).
Temperatures are listed in order of decreasing emission intensity. (a)
ReACN2: T ) 81, 110, 140, 170, 199, 230, 260, 298 K (b) ReACN3:
T ) 80, 113, 140, 169, 200, 232, 259, 298 K.
Time-ResolvedElectronParamagneticResonance(TREPR).
In an effort to gain further information on the nature of the
long-lived transients observed following excitation of the metal-
organic oligomers, TREPR experiments were carried out on
selected oligomers and metal-oligomers. In these experiments
the compounds were excited at 355 nm using a Nd:YAG laser,
and the TREPR spectra were detected ∼500 ns after laser
excitation. The objective of these experiments was to use EPR
to confirm that the long-lived transients observed by flash
photolysis are the 3π,π* states based on the π-conjugated
system. All measurements were performed on samples in frozen
solution at 130 K, because modulation of the dipole-dipole
interaction between the unpaired electrons in a triplet generally
broadens the EPR signals beyond detection in liquid solution.
The initial TREPR experiments were carried out using several
of the free oligomers (i.e., 2-4), however, in these measure-
ments no signals were detected. At first this was a surprise,
given that the ambient temperature photophysical studies on 2-4
indicate that the 3π,π* state is formed in moderate yield (Table
1). However, after discovering the EPR result, a transient
absorption study was carried out on OAE 3 at 100 K. This
3
experiment failed to detect the absorption of the π,π* state,
Figure 9. Transient absorption difference spectra of ReACN2 and
ReACN3 in argon-deoxygenated CH2Cl2 solution at 298 K following
355 nm pulsed excitation. Spectra acquired at 4 µs increments and
arrows indicate direction of change of ∆A with increasing time. (a)
ReACN2. (b) ReACN3.
indicating that this state is quenched in the low temperature
glass. Previously reported fluorescence studies of 3 and 4 show
that in dilute solution these compounds aggregate at T < 200
K.75 Thus, the quenching of the 3π,π* state in 3 at low-
temperature is believed to arise due to the aggregation. This
triplet quenching mechanism accounts for the fact that an EPR
signal is not detected for the free oligomers.
surprising, given that the absorption is dominated by π,π*
transitions localized on the oligomers. However, contrary to
expectations, the PL of ReACN-2 and ReACN-3 is very weak,
both at room temperature in CH2Cl2 solution and at 80 K in
2-MTHF (φem < 0.005, see Table 2 and Figure 8). In addition,
room-temperature emission maxima for the two CH3CN com-
plexes are very similar to their Cl counterparts. This latter feature
is contrary to the expected emission blue-shift, and suggests
that the room-temperature PL from ReACN-2 and ReACN-3
does not emanate from the MLCT manifold. Upon cooling to
80 K, the PL intensity from ReACN-2 and ReACN-3 increases
Further attempts to detect the TREPR spectrum of the 3π,π*
state in an OAE were carried out using oligomer 5 (see Figure
10 for structure). Compound 5 was selected for the low-
temperature FT-EPR experiment because laser flash photolysis
of the compound at 100 K revealed the characteristic transient
3
absorption of the π,π* state.66 As shown in Figure 10, the
(75) Walters, K. A.; Ley, K. D.; Schanze, K. S. Langmuir 1999, 15,
5676-5680.

8338 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Walters et al.
typically feature short-lived fluorescence with high quantum
yields (>0.50 is typical), which implies that the lowest singlet
state is also strongly allowed. Taken together, this information
1
implies that the lowest excited state in these systems is of Bu
character.
Most previous studies of PPE conjugated systems have
examined materials in which the repeat unit consists of either
2-alkyl-1,4-phenyleneethynylene or 2,5-bisalkoxy-1,4-phenyle-
neethynylene moieties. In these systems the conjugation length,
as assessed by the shift of the absorption or fluorescence
maximum, continues to increase for oligomers having >10
phenyleneethynylene repeats and levels off as the number tends
to 16.26,77 This result suggests that the conjugation length of
PPE materials that contain only 1,4-phenyleneethynylene repeats
is at least 10 repeat units. West and co-workers reported that
the absorption and fluorescence maxima of polymers with
structures consisting of alternating 2,5-bisalkoxy-1,4-phenyle-
neethynylene and 1,4-phenyleneethynylene repeat units were
approximately 415 and 450 nm, respectively.78
Figure 10. TREPR spectra of compound 5 (dotted line) and ReACN2
(solid line) in toluene-chloroform (1:1) at 130 K. Delay between laser
excitation and detection was ∼500 ns.
TREPR spectrum of 5 at 130 K reveals a broad resonance (g ≈
2, peak-to-peak width ≈ 1200 G) which is observed at delay
times <5 µs after the 355 nm excitation pulse. The signal is
spin polarized with the low field half in emission (E) and the
high field in absorption (A). The EPR signal is clearly due to
As noted above, the absorption and fluorescence properties
of oligomers 2-4 are very similar to those of other PPE
oligomers and polymers. This point indicates that the bipyridine
unit does not significantly perturb the properties of the lowest
1π,π* excited state in 2-4. However, an important feature with
respect to 2-4 is that the maximum of the lowest-energy
absorption band red-shifts only very slightly with increasing
oligomer length, which indicates that the conjugation length is
reached early in the series. This is surprising in view of the
results obtained by other groups on PPE oligomers that suggest
that the conjugation length is not established until n > 10. The
restricted conjugation length in 2-4 likely arises because the
1,4′-biphenylene unit effectively disrupts the conjugation, owing
to twisted inter-ring geometry (Scheme 3, top). The conclusion
that the biphenylene unit disrupts conjugation is supported by
the fact that the conjugation length in p-phenylene oligomers
is ∼40% shorter than in PPEs.27
3
the π,π* state, and the characteristic EA pattern arises from
spin-polarization that is generated by the spin-selective singlet-
to-triplet intersystem crossing step.76 Next, TREPR experiments
were carried out on ReACN-2 at 130 K. Interestingly, a
resonance with g ≈ 2 is also observed for this metal-organic
complex. However, in this case the resonance is less intense
and not as broad as that observed for model 5 (for ReACN-2
the peak-to-peak width is ∼700 G). More importantly, the signal
for ReACN-2 exhibits an AE polarization (low field absorption,
high field emission), which is opposite to the EA pattern
observed for model 5.
The TREPR resonance observed for ReACN-2 is also likely
due to an oligomer based 3π,π* state. This conclusion is based
on the fact that an MLCT state would have a large contribution
from Re-based d orbitals, and consequently the EPR resonance
would be expected to have a g value that is shifted significantly
from the free electron value. Moreover, an EPR signal would
probably not be observed at all for an MLCT state, since spin-
lattice relaxation would be exceedingly fast so that any spin
polarization would vanish prior to detection of the signal. The
fact that an AE pattern is observed for ReACN-2, while an EA
pattern is observed for 5, suggests that in the metalated oligomer
the 3π,π* state is produced by a route other than direct singlet-
to-triplet intersystem crossing. As discussed below, this pathway
Although the absorption spectra imply that the conjugation
length is similar in 2-4, there is an easily discernible change
in the fluorescence band shape which indicates that the relaxed
1π,π* excited-state experiences a greater degree of delocalization
with increasing oligomer length. Recent studies of intramolecular
energy transfer in ethynylene-linked porphyrins indicate that
inter-chromophore singlet energy transfer occurs on the 10-
100 ps time scale, even when the process is isothermal.79,80 On
the basis of these results, we anticipate that due to its long
1
lifetime (τ ≈ 1 ns), the π,π* excited state in 2-4 effectively
samples the entire oligomer. This rapid exciton diffusion likely
is responsible for the decreased electron-vibrational coupling
which gives rise to the change in fluorescence band shape with
increasing oligomer length.
3
may involve the intermediacy of a MLCT state.
The field range covered by the resonance peaks is a measure
of the strength of the dipole-dipole interaction between the
unpaired electrons in the triplets. The interaction is proportional
to R-3 where R is the average distance between the unpaired
electrons. The pronounced reduction in spectral width observed
upon going from 5 to ReACN-2 points to a contribution of the
bipyridine ligand to the orbital of the unpaired electrons.
The transient absorption and photoacoustic calorimetry studies
of 1-4 indicates that in each case direct excitation of the
oligomers leads to moderate yields of the 3π,π* state. Moreover,
the triplet energy is comparatively constant across the series,
consistent with the finding that the conjugation length is defined
early in the series. The singlet-triplet (S-T) splitting in 1-4
is ∼0.55 eV (4430 cm-1), which is significantly less than that
Discussion
(77) Pearson, D. L.; Tour, J. M. J. Org. Chem. 1997, 62, 1376-1387.
(78) Li, H.; Powell, D. R.; Hyashi, R. K.; West, R. Macromolecules 1998,
31, 52-58.
Photophysics of Oligomers 1-4. Several previous studies
have examined the absorption and fluorescence properties of
PPE-type oligomers and polymers.18,22,23,25,26,61-65 These studies
have demonstrated that the lowest-energy absorption is strongly
allowed and long-axis polarized. In addition, PPE materials
(79) Li, F. R.; Yang, S. I.; Ciringh, Y. Z.; Seth, J.; Martin, C. H.; Singh,
D. L.; Kim, D. H.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S.
J. Am. Chem. Soc. 1998, 120, 10001-10017.
(80) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Johnson, T. E.; Weghorn,
S. J.; Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem.
Soc. 1999, 121, 8604-8614.
(76) Sixl, H.; Schwoerer, M. Z. Z. Naturforsch. 1970, 25A, 1383.

π-Conjugated Metal-Organic Oligomer Photophysics
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8339
Figure 11. Jablonski Diagrams for: (a) free oligomers 1-4; (b) metal-oligomers Re-2-Re-4; (c) metal-oligomers ReACN-2 and ReACN-3.
Solid and dashed lines indicate radiative and nonradiative transitions, respectively. Processes are labeled using numbers for ease of reference in the
text.
Scheme 3
of the “parent chromophores” diphenyl acetylene (1.39 eV) and
biphenyl (1.30 eV),54 presumably because of the delocalized
nature of the singlet and triplet states in the π-conjugated
systems. However, it is surprising that the S-T splitting in the
OAEs is also considerably less than that of other well-studied
π-conjugated systems such as R-septithiophene (0.90 eV)14 and
MEH-PPV (0.72 eV).81 The reason that the S-T splitting is
lower in the OAEs compared to that in these other π-conjugated
systems is unclear at the present time.
lie at 2.75 and 2.21 eV, respectively. Excited-state decay is
dominated by fluorescence, with S1 f T1 intersystem crossing
1
being the second most efficient decay process from the π,π*
manifold. Phosphorescence is not observed from any of the
oligomers (77 K in 2-MTHF, 1 M ethyl iodide added). We
believe the lack of phosphorescence may be due to the
propensity of the oligomers to aggregate at low temperature,
1
which leads to quenching of the π,π* state (and therefore
reducing the triplet yield).
In previously published work we demonstrated that the
spectroscopy and photophysics of oligomers 3 and 4 are strongly
influenced by intrachain conformations (i.e., the phenylene-
ethynylene-phenylene “ring-ring” torsion angles) and intramo-
lecular aggregation.75 In particular, the fluorescence spectra and
polarization from dilute solutions of these oligomers display a
strong temperature dependence. At intermediate temperatures
(200-300 K), spectral changes are believed to arise from
changes in intrachain conformation, while at lower temperatures
(T < 200 K) the spectral (and polarization) changes are
dominated by the presence of intermolecular aggregates.
Although these effects have not been considered in the present
report, we point them out to make it clear that intrachain
conformation and interchain aggregation effects may play a role
in the temperature-dependent photophysics of the metal com-
plexes Re-1-Re-4.
Metal-Organic Oligomers: Absorption Spectra. As in-
dicated previously, the dominant feature in the UV-visible
absorption spectra of Re-2-Re-4 is the intense absorption band
with λmax ≈ 440 nm and ꢀ ≈ 70 000 M-1 cm-1 (a similar band
appears slightly blue-shifted and with a smaller ꢀ in Re-1). This
band is obviously associated with the -Re(CO)3Cl unit;
however, for the reasons outlined below it is clear that the band
arises from an OAE-based π,π* transition, not Re f bpy-
oligomer MLCT. First, although complexes of the type (diimine)-
Re(CO)3Cl typically feature an MLCT absorption band in the
400-450 nm region,50 the MLCT band is only weakly allowed
with ꢀ ≈ 2000-4000 M-1 cm-1 (cf. Table 2). Thus, the 445
nm absorption band in Re-2-Re-4 is too intense to be due to
Re f bpy-oligomer MLCT. Second, in previous studies of PPE,
PPV, and polythiophene polymers that contain 2,2′-bipyridine-
5,5′-diyl chromophore, complexation with a variety of metals
(including the closed shell Zn(II) ion) induces a 40-50 nm red-
shift in the lowest-energy π,π* transition of the conjugated
system.33-35,45 Recently published semi-empiricial studies sug-
gest that metal-complexation of the bpy unit decreases the
HOMO-LUMO gap of the π-system due to the following
factors.82 (1) Metal complexation forces the bipyridine rings
The Jablonski diagram in Figure 11a summarizes the pho-
tophysics of OAEs 2-4. The absorption, fluorescence and ns-
µs transient absorption spectroscopy of the oligomers are
dominated by the lowest 1π,π* and 3π,π* excited states which
(81) Scurlock, R. D.; Wang, B.; Ogilby, P. R.; Sheats, J. R.; Clough, R.
L. J. Am. Chem. Soc. 1995, 117, 10194-10202.

8340 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Walters et al.
into a syn, planar conformation, effectively increasing the inter-
ring π-conjugation. (2) Interaction between the dπ metal orbitals
and the bpy π-system lowers the energies of both the π and π*
levels; however, this effect is more pronounced for the π* orbital
which results in a decreased π-π* gap.82
with the OAE core. The energy of this state is ∼1.90 eV based
on the position of the weak phosphorescence band.
It is more problematic to pinpoint the energies of the 1MLCT
and 3MLCT states in Re-2 - Re-4, because the MLCT
absorption(s) are obscured by the OAE-based π,π* bands and
the luminescence spectra are complicated. Thus, in the absence
of direct spectroscopic evidence, the state energies are estimated
While the low-energy absorption band is relatively constant
in energy and intensity across the series Re-2-Re-4, all of the
complexes feature a second intense band in the near-UV that
red-shifts and increases in intensity significantly as the oligomer
length increases. This property suggests that Re-2-Re-4 es-
sentially feature two oligomer-based chromophores we refer to
as “core” and “periphery” (refer to Scheme 3). First, the low-
energy absorption band is associated with the core chromophore
consisting of the bipyridine moiety and four flanking phenyle-
neethynylene units (two on each side of the bipyridine).
Importantly, all of the phenylene rings in the core are able to
attain the coplanar conformation that facilitates π-delocalization.
This core is present in each of Re-2-Re-4, which explains why
the position and intensity of the long-wavelength absorption
band is the same in these complexes. Second, the higher-energy
π,π* transition is associated with the periphery chromophore
of the OAE oligomer system (Scheme 3). The energy and
intensity of the transition associated with the periphery chro-
mophore varies along the series Re-2-Re-4 because the length
of this chromophore varies substantially in this series.
1
on the basis of two facts. (1) The energies of the MLCT and
3MLCT states of the parent complex, (bpy)Re(CO)3Cl are ∼3.3
and 2.0 eV, respectively. (2) The energies of the MLCT
manifold in Re-2-Re-4 will scale with the difference in
reduction potentials between the oligomer complexes and (bpy)-
Re(CO)3Cl.50 The difference in these reduction potentials is ∼0.3
( 0.1 V83 On the basis of these facts, we estimate that the
1MLCT and 3MLCT states in Re-2 - Re-4 lie at approximately
3.0 and 1.7 eV, respectively. These energies are used in the
diagram shown in Figure 11b.
Finally, consider the excited-state diagram for the CH3CN
substituted complexes ReACN-2 and ReACN-3 (Figure 11c).
Since it is anticipated that substitution of Cl for CH3CN will
not affect the energies of the π,π* states for the OAE π-system,
these states are positioned the same in ReACN-2 and ReACN-3
1
3
as in Re-2-Re-4. However, the MLCT and MLCT states of
(bpy)Re(CO)3(NCCH3)+ are approximately 0.35 eV higher in
energy relative to the same states in (bpy)Re(CO)3Cl. On the
1
basis of this energy difference, it is expected that the MLCT
Although the metal-organic oligomers do not fluoresce,
and 3MLCT states will be increased a similar amount in
ReACN-2 and ReACN-3 relative to the energies of the states
in Re-2-Re-4. This places the states at 2.05 and 3.3 eV,
respectively.
based on the absorption shift that accompanies metalation (i.e.,
1
400 nm f 445 nm), we estimate that the relaxed π,π* state
for the core chromophore in Re-2-Re-4 is approximately 0.3
eV lower in energy compared to the relaxed (fluorescent) 1π,π*
state in the free oligomers. Moreover, assuming that the S-T
splitting is unaffected by metalation, we estimate that the energy
of the 3π,π* state of the core chromophore in Re-2-Re-4 will
also be lowered by ∼0.3 eV relative to that of the free oligomers.
This analysis raises several important issues regarding the
lowest excited states in the metal-OAEs. First, it is evident
that in all of the metal-organic oligomers the 3π,π*c and
3MLCT states are in close energetic proximity. This close
proximity makes it possible that both states will contribute to
the observed photophysics. Second, it is also clear that the state
ordering is switched in the two series: for the Cl-substituted
3
This estimate suggests that the lowest π,π* state in Re-2-
Re-4 should be approximately 1.90 eV (652 nm). Interestingly,
this estimate corresponds exactly to the observed emission
energy of the metalated oligomers, which strongly supports the
3
complexes, Re-2-Re-4, MLCT is lowest in energy, while in
3
the CH3CN-substituted complexes, ReACN-2 and ReACN-3,
3π,π*c is lowest. As discussed below, this switch in state
ordering is believed to be responsible for the significant
difference in excited-state dynamics in the two families of
complexes.
premise that the structured emission emanates from the π,π*
state of the core chromophore.
Excited-State Energetics. Before turning to an interpretative
discussion of the remaining photophysical data, it is necessary
to establish the energies for the various low-lying excited states
in the OAEs. This presentation centers on complexes Re-2-
Re-4, ReACN-2 and ReACN-3, because the energetics for these
systems are believed to be related. (The properties of Re-1 are
considered in a later section.) Figure 11 provides Jablonski
diagrams for the free OAEs and the two categories of metal-
OAE complexes. First, consider the situation for the free
oligomers (2-4, Figure 11a). Each of these molecules features
a 1π,π* state at 2.75 eV (based on the fluorescence) and a 3π,π*
state at 2.21 eV (based on triplet quenching).
Photophysics of the Metal-Organic Oligomers. The vari-
able temperature PL spectra of Re-2-Re-4, ReACN-2, and
ReACN-3 are qualitatively similar (Figures 4 and 8). Specifi-
cally, at T > 200 K, the PL is dominated by a structured band
with λmax ≈ 650 nm, while at lower temperatures a second broad
band appears at higher energy. The structured 650 nm emission
band is assigned to phosphorescence from the triplet excited
state of the “core” chromophore (3π,π*c) on the basis of the
following facts: (1) The emission energy corresponds exactly
to the energy that is predicted for this state (vide supra). (2)
The energy of the structured emission does not change
significantly when the solvent glass point is reached. (A blue-
shift is expected if the emission arises from an MLCT state
due to the rigidochromic effect.49)
Next consider the excited-state diagram for the Cl-substituted
complexes, Re-2 - Re-4 (Figure 11b). In this series, at least
three excited states associated with the OAE π-system are
identified. (1) The high-energy singlet, 1π,π*p, arises from the
periphery OAE units. The energy of this state lies in the region
of 2.7-3.3 eV (it varies with oligomer length). (2) The lowest-
The origin of the broad emission band that appears on the
blue edge of the 3π,π*c phosphorescence as the temperature is
lowered is less certain. An important consideration in the
assignment of this band is that its apparent λmax decreases with
1
energy singlet state, π,π*c, is associated with the OAE core.
The energy of this state is estimated as outlined in the previous
section, and it is not expected to vary significantly with oligomer
3
length. (3) The lowest triplet state, π,π*c, is also associated
(83) The large error is due to the fact that the electrochemical potentials
for the two systems have been determined in different solvent media and
with different working electrodes.
(82) Manas, E. S.; Chen, L. X. Chem. Phys. Lett. 2000, 331, 299-307.

π-Conjugated Metal-Organic Oligomer Photophysics
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8341
temperature. As noted above, a blue-shift with decreasing
temperature is characteristic for charge-transfer emission,49 and
on the basis of this point we suggest that the broad band
induced by the rhenium ion may increase the rate of direct triplet
relaxation via intersystem crossing (i.e., step 10 in Figure 11c).
Finally we consider the origin of the fast emission decay
components (τ ≈ 1 ns) and the large amplitude decay observed
by ps transient absorption (τ ≈ 700 ps). While it is tempting to
assign these processes to energy transfer and intersystem
crossing within the OAE chain, we believe that the decays
correspond to establishment of the equilibrium between the
3
emanates from the MLCT excited state.
The PL spectra of Re-2-Re-4, ReACN-2 and ReACN-3
indicate that both low-lying excited states, 3π,π*c and 3MLCT,
are populated to a significant extent at times >1 ns following
excitation. The dual emission implies that these two excited
states are in equilibrium.67,68 Further information concerning
this equilibrium comes from consideration of the TA and
emission decay kinetics (Table 2).
3
3MLCT and π,π*c excited states.84 Thus, we suggest that for
1
all of the complexes excitation populates the π,π* manifold
(steps 1 and 2, Figure 11, b and c). Subsequently, ultrafast
energy transfer and intersystem crossing ensues to afford a
nonequilibrium distribution of the 3MLCT and 3π,π*c states (i.e.,
paths 3, 4, and 5 in Figure 11, b and c occur with τ < 20 ps).
On the basis of experimental evidence that indicates that in
diimine transition-metal complexes the 3MLCT manifold is
populated very rapidly following excitation,85 we favor an
interpretation where the dominant ultrafast relaxation in the
For the Cl-substituted complexes (Re-2-Re-4) there is good
agreement between the TA decay lifetime and the lifetime of
the long-lived PL decay component. In addition, the excited-
state decay lifetimes are consistent with those expected for a
3MLCT state in a (diimine)ReI(CO)3Cl complex.50 Taken
together, these facts are supported by the excited-state model
in Figure 11b. Thus, for Re-2-Re-4, the lowest excited state
3
metal-organic complexes affords mainly MLCT (i.e., k5 >
3
3
is MLCT. However, because π,π*c is only slightly higher in
k4, where subscripts refer to processes in Figure 11, b and c). If
energy, it is populated via the equilibrium (step 6, Figure 11b).
this model is correct, then the fast PL and TA decay components
3
Since MLCT is lower in energy and has a relatively large
3
observed for Re-2-Re-4 correspond to the rate of MLCT f
nonradiative decay rate constant (step 9, Figure 11b, k9 ≈ 106-
107 s-1),50 decay via this state is the main deactivation mode,
and the rate of this process controls the overall lifetime of the
excited-state population.67-73
3π,π*c energy transfer. For ReACN-2 and ReACN-3 virtually
all of the PL decays in less than 1 ns. This is consistent with
3
the fact that the π,π* state is lower in energy compared to
3MLCT in these complexes, and because of this state ordering
3
3
Despite the fact that in Re-2-Re-4 MLCT is lowest in
the lifetime of MLCT is expected to be very short.
energy and is responsible for the relatively rapid decay of the
excited state, we believe that the TA spectra of Re-2-Re-4 are
dominated by the 3π,π*c state. This assignment is based
primarily on the fact that the difference spectra of the excited
complexes are nearly indistinguishable from those of ReACN-2
and ReACN-3, and in the latter complexes there is ample data
to support assignment of the long-lived transient absorption to
the 3π,π*c state (vide infra). It is likely that the difference molar
Photophysics of Re-1. The previous discussion centered on
the longer metal-OAEs, which all share the common core
chromophore. The situation for the shortest OAE complex (Re-
3
1) is different because in this system the energy of the π,π*
state is higher due to the shorter π-conjugated segment. On the
basis of the blue-shift of the lowest-energy absorption band (∼35
nm relative to the maxima of Re-2-Re-4) we estimate that in
3
Re-1 the π,π* state is at ∼2.1 eV. Thus, it is clear that in
3
Re-1 at ambient temperature 3MLCT is the lowest excited state,
absorptivity (∆ꢀ) for the π,π state is very large, and this also
3
may account for the fact that this state dominates the TA spectra,
since based on the electrochemistry the energy of MLCT is
3
that is, the absorption of the MLCT state is obscured by the
similar in all of the metal-OAE complexes (i.e., the reduction
potential for all of the complexes is ∼-0.9 V). The MLCT
assignment is supported by the broad emission band with Emax
≈ 1.8 eV (see inset, Figure 4a) and by the lifetime (20 ns),
which is comparable to that of the parent complex. Given the
MLCT assignment for the emission, then it follows that the TA
difference spectrum of Re-1 (Figure 5a) can be assigned to the
MLCT state. Although there are clear differences in the TA
spectra of Re-1 and the longer oligomers, it is interesting to
note that the excited state difference spectra exhibit qualitatively
similar features, despite the fact that they arise from states of
different orbital character (i.e., dπ,π* and π,π*).
stronger absorbance of 3π,π. (It is also possible that the
difference spectra of the 3π,π*c and MLCT states are similar.)
The striking feature for the CH3CN substituted complexes
ReACN-2 and ReACN-3 is that their TA decay lifetimes are
considerably longer than those for Re-2-Re-4 (Table 2). In
addition, the TA decay lifetimes for ReACN-2 and ReACN-3
are several orders of magnitude longer than their PL decay
lifetimes. These data are consistent with the excited-state model
presented in Figure 11c. Thus, for the CH3CN complexes, the
3
3π,π*c state is lowest in energy, with MLCT lying somewhat
higher in energy. Because of this state ordering the equilibrium
3
The PL of Re-1 develops a well-defined vibronic structure,
blue-shifts, and increases significantly in intensity at low
temperatures (Figure 4a). These features are characteristic of
MLCT emission,49 making it tempting to assign the low-
favors the π,π*c state, and at long times after excitation the
excited-state population resides mainly in this state. Conse-
quently, the TA difference spectra of these complexes is clearly
3
due to the π,π*c state. In addition, the EPR signature of this
3
temperature PL from Re-1 to the MLCT state. However, it is
state can be observed at low temperature by TREPR (Figure
10).
important to recognize that due to the increased energy of the
Although the transient absorption lifetimes of ReACN-2 and
ReACN-3 are considerably longer than typical for a MLCT
state, they are considerably shorter than expected for organic
(84) It is unclear why the amplitude of the fast emission decay component
is so large relative to that of the slow component in Re-2-Re-4. It is
possible that some of the fast decay component arises from the very short-
lived 1π,π* manifold (i.e., fluorescence). In this case, because of the large
radiative decay rate of the 1π,π* state, the amplitude of the fast component
would be expected to be large compared to that of the MLCT state, which
has a considerably lower radiative decay rate. In support of this hypothesis
it was observed that the amplitude of the fast component is larger when
the emission decay is obtained on the blue edge of the emission band.
(85) Damrauer, N. H.; Cerullo, G.; Yeh, A.; Boussie, T. R.; Shank, C.
V.; McCusker, J. K. Science 1997, 275, 54-57.
3
triplets. For example, the triplet lifetimes of the oligomers 1-4
3
are >100 µs (Table 1). The “reduced” lifetimes of the π,π*c
states in ReACN-2 and ReACN-3 likely arise because the
dominant excited-state decay channel is via thermally activated
population of the 3MLCT state (i.e., step 6, followed by step 9
in Figure 11c). It is also possible that spin-orbit coupling

8342 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Walters et al.
3MLCT state in the low-temperature glass, under these condi-
the nonradiative decay rate of an MLCT state due to decreased
electron-vibrational coupling.86,87 Although the excited-state
lifetimes of Re-2-Re-4 are enhanced by almost a factor of 10
compared to the parent complex (bpy)Re(CO)3Cl, due to the
3
3
tions the MLCT and π,π* states are close in energy. It is
therefore possible that the PL observed at low temperature arises
from both excited states, as is the case for the longer metal-
OAEs.
3
close energetic proximity of the OAE-based π,π*c state it is
unclear whether the enhanced lifetimes can be attributed solely
to a decreased nonradiative decay rate for the MLCT state or
whether there is some (configuration) interaction between the
states that influences the observed nonradiative decay rates. Such
conclusions must await further studies that will explore metal-
oligomer systems that are designed specifically to raise the
Conclusions
This report describes the first comprehensive examination of
the photophysics of a series of well-defined π-conjugated
oligomers that contain the d6 (diimine)ReI(CO)3Cl (MLCT)
chromophore. These compounds feature a rich and complicated
array of excited states that are based on both the π-conjugated
oligomer system and the MLCT chromophore. The oligomers
absorb strongly throughout the near-UV and into the visible
region and the absorption spectra of the metal-oligomers are
dominated by the π,π* transitions. Little direct evidence for
Re f bpy-oligomer MLCT absorption is observed. Although
the unmetalated oligomers are strongly fluorescent, the fluo-
rescence from the metalated oligomers is largely quenched. The
fluorescence is replaced by a weak, red photoluminescence that
3
energy of the π,π* states relative to the MLCT manifold.
Acknowledgment. This work was carried out with the
support of the National Science Foundation under Grant CHE-
9901862 (K.S.S., K.A.W., and K.D.L.). Work at the University
of Massachusetts, Northwestern University, and Argonne Na-
tional Laboratory was supported by the Division of Chemical
Sciences, Office of Basic Energy Sciences of the U.S. Depart-
ment of Energy under Grants DE-FG02-84ER-13242 (H.v.W.)
and DE-FG02-99ER-14999 (M.R.W.) and Contract W-31-109-
Eng-38 (D.G.).
3
is believed to emanate primarily from the π,π* manifold of
the OAE π-system. Transient absorption and TREPR spectros-
Supporting Information Available: A figure illustrating
Sandros Plots for triplet quenching of OAEs 1, 3, and 4. This
material is available free of charge via the Internet at http://
pubs.acs.org. See any current masthead page for ordering
information and Web access instructions.
copy provides additional evidence that the long-lived excited
3
state in the metal-oligomers is π,π* in nature.
3
Due to the involvement of the OAE-based π,π* states in
the photophysics of the metal-oligomers, it is not possible to
clearly assess the influence of the π-conjugated electronic system
on the properties of the Re f bpy-oligomer MLCT excited state.
For example, in previous work it has been demonstrated that
increased conjugation in the diimine “acceptor” ligand decreases
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