Rhenium-to-benzoylpyridine and rhenium-to-bipyridine MLCT excited states of fac-[Re(Cl)(4-benzoylpyridine)2(CO)3] and fac-[Re(4-benzoylpyridine)(CO)3(bpy)]+: A time-resolved spectroscopic and spectroelectrochemical study

Article abstract of DOI:10.1021/ic049659m

The lowest allowed electronic transition of fac-[Re(Cl)(CO) ₃(bopy)₂] (bopy = 4-benzoylpyridine) has a Re → bopy MLCT character, as revealed by UV-vis and stationary resonance Raman spectroscopy. Accordingly, the lowest-lying, long-lived, excited state is Re → bopy ³MLCT. Electronic depopulation of the Re(CO)₃ unit and population of a bopy π* orbital upon excitation are evident by the upward shift of v(C=O) vibrations and a downward shift of the ketone v(C=O) vibration, respectively, seen in picosecond time-resolved IR spectra. Moreover, reduction of a single bopy ligand in the ³MLCT excited state is indicated by time-resolved visible and resonance Raman (TR³) spectra that show features typical of bopT^.-. In contrast, the lowest allowed electronic transition and lowest-lying excited state of a new complex fac-[Re(bopy)(CO)₃(bpy)]⁺ (bpy = 2,2′-bipyridine) have been identified as Re → bpy MLCT with no involvement of the bopy ligand, despite the fact that the first reduction of this complex is bopy-localized, as was proven spectroelectrochemically. This is a rare case in which the localizations of the lowest MLCT excitation and the first reduction are different. ³MLCT excited states of both fac-[Re(Cl)(CO) ₃(bopy)₂] and fac-[Re(bopy)(CO)₃(bpy)] ⁺ are initially formed vibrationally hot. Their relaxation is manifested by picosecond dynamic shifts of v(C≡O) IR bands. The X-ray structure of fac-[Re(bopy)(CO)₃(bpy)]PF₆·CH ₃CN has been determined.

Full text of DOI:10.1021/ic049659m

Inorg. Chem. 2004, 43, 4523−4530
Rhenium-to-Benzoylpyridine and Rhenium-to-Bipyridine MLCT Excited
States of fac-[Re(Cl)(4-benzoylpyridine)2(CO)3] and
fac-[Re(4-benzoylpyridine)(CO)3(bpy)]+: A Time-Resolved Spectroscopic
and Spectroelectrochemical Study
Michael Busby,† Pavel Matousek,‡ Michael Towrie,‡ Ian P. Clark,‡ Majid Motevalli,†
,†
Frantisˇek Hartl,§ and Anton´ın Vlcˇek, Jr.*
Department of Chemistry and Centre for Materials Research, Queen Mary, UniVersity of London,
Mile End Road, London E1 4NS, United Kingdom, Central Laser Facility, CCLRC Rutherford
Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom, and Molecular
Photonic Materials, Van’t Hoff Institute for Molecular Sciences, UniVersity of Amsterdam,
Nieuwe Achtergracht 166, NL-1018 WV Amsterdam, The Netherlands
Received March 15, 2004
The lowest allowed electronic transition of fac-[Re(Cl)(CO)3(bopy)2] (bopy ) 4-benzoylpyridine) has a Re f bopy
MLCT character, as revealed by UV−vis and stationary resonance Raman spectroscopy. Accordingly, the lowest-
lying, long-lived, excited state is Re f bopy 3MLCT. Electronic depopulation of the Re(CO)3 unit and population
of a bopy π* orbital upon excitation are evident by the upward shift of ν(CtO) vibrations and a downward shift
of the ketone ν(CdO) vibration, respectively, seen in picosecond time-resolved IR spectra. Moreover, reduction of
3
a single bopy ligand in the 3MLCT excited state is indicated by time-resolved visible and resonance Raman (TR )
•-
spectra that show features typical of bopy . In contrast, the lowest allowed electronic transition and lowest-lying
excited state of a new complex fac-[Re(bopy)(CO)3(bpy)]+ (bpy ) 2,2′-bipyridine) have been identified as Re f
bpy MLCT with no involvement of the bopy ligand, despite the fact that the first reduction of this complex is
bopy-localized, as was proven spectroelectrochemically. This is a rare case in which the localizations of the lowest
3
MLCT excitation and the first reduction are different. MLCT excited states of both fac-[Re(Cl)(CO)3(bopy)2] and
fac-[Re(bopy)(CO)3(bpy)]+ are initially formed vibrationally hot. Their relaxation is manifested by picosecond dynamic
shifts of ν(CtO) IR bands. The X-ray structure of fac-[Re(bopy)(CO)3(bpy)]PF ‚CH CN has been determined.
6
3
Introduction
with specially designed L are used as sensors.23-28 Re f L
and Re f NN metal to ligand charge transfer (MLCT)
excited states in these complexes often lie in a close energetic
proximity to intraligand (IL) states localized either on NN
Rhenium(I) carbonyl complexes of the type fac-[Re(Cl)-
(CO)3(L)2] or fac-[Re(L)(CO)3(NN)]n+ (NN ) polypyridine
or R-diimine, L ) electron-accepting ligand) often exhibit
very interesting properties, such as strong emission from
long-lived excited states, inter- and intramolecular excited-
state electron transfer, or ligand isomerization.1-22 Complexes
(4) Wang, Y.; Hauser, B. T.; Rooney, M. M.; Burton, R. D.; Schanze, K.
S. J. Am. Chem. Soc. 1993, 115, 5675-5683.
(5) Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K. S.; Anthon,
D.; Neveux, P. E., Jr.; Meyer, T. J. Inorg. Chem. 1987, 26, 1116-
1126.
* Author to whom correspondence should be addressed. E-mail:
a.vlcek@qmul.ac.uk.
(6) Chen, P.; Duesing, R.; Graff, D. K.; Meyer, T. J. J. Phys. Chem. 1991,
95, 5850-5858.
† University of London.
(7) Trammell, S.; Goodson, P. A.; Sullivan, B. P. Inorg. Chem. 1996, 35,
1421-1422.
‡ CCLRC Rutherford Appleton Laboratory.
§ University of Amsterdam.
(1) Stufkens, D. J. Comments Inorg. Chem. 1992, 13, 359-385.
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10.1021/ic049659m CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/17/2004
Inorganic Chemistry, Vol. 43, No. 14, 2004 4523

Busby et al.
or L. Such a situation gives rise to a variety of photochemical
and photophysical deactivation mechanisms that may be
subtly tuned by structural variations of the NN and L ligands
and the medium.2
The complex29 fac-[Re(Cl)(CO)3(bopy)2] contains the
4-benzoylpyridine (bopy) ligand, which is an electron
3
acceptor. It possesses a relatively low-lying nπ* IL state,
in which the electron is excited from a nonbonding orbital
on the ketone oxygen atom. Although an interplay between
MLCT and IL states can be expected, this complex displays
only a 3MLCT emission both in a low-temperature glass and
fluid solution.29 The bopy ligand is actually very similar to
Figure 1. Schematic structures of fac-[Re(Cl)(bopy)2(CO)3] (left) and fac-
[Re(bopy)(CO)3(bpy)]+ (right). These complexes are hereinafter denoted
Re(bopy)2 and Re(bopy)(bpy), respectively. The prefix fac will be omitted.
3
benzophenone, whose long-lived nπ* excited state shows
a radical-like reactivity of the C-O• group, abstracting
hydrogen atoms from substrates such as 2-propanol. In inert
media, benzophenone acts as a sensitizer by triplet energy
transfer to suitable energy acceptors. Bopy, like benzophe-
none, can be reduced to the corresponding ketyl radical
anion.30 Electrochemical reduction of coordinated bopy
ligands in [Re(Cl)(CO)3(bopy)2] occurs in two closely spaced
one-electron steps at -1.53 and -1.64 V vs Fc/Fc+.30 The
small potential difference demonstrates that the redox orbitals
are almost entirely localized on single bopy ligands, with
any bopy-bopy electronic interaction being very small.
Reduction of one bopy ligand in fac-[Re(Cl)(CO)3(bopy)2]
has also been accomplished photochemically31 by reductive
quenching of the 3MLCT excited state with NEt3: the long-
(bopy•-)]- that subsequently reacts by a H-atom transfer
forming an alcohol complex.31 In [Re(bopy)(CO)3(bpy)]+,
optical excitation may populate the Re f bopy or Re f
bpy MLCT excited state, hereinafter called MLCT(bopy) and
MLCT(bpy), respectively. Furthermore, population of a bopy
1nπ* IL state is also a possibility. The nature of the lowest
excited-state cannot thus be a priori predicted. The MLCT-
(bpy) exited state can either be stable and long-lived, as usual
in this type of complexes, or undergo a bpy•- f bopy
interligand electron transfer, similar to that found8,9,11,16,17,19
for fac-[Re(N-methyl-4,4′-bipyridinium)(CO)3(bpy)]2+.
Aiming at detailed characterization of low-lying electronic
transitions and excited states of Re-bopy complexes, we
have investigated ground-state resonance Raman spectra and
time-resolved visible and IR absorption spectra of fac-[Re-
(Cl)(CO)3(bopy)2] and fac-[Re(bopy)(CO)3(bpy)]+, herein-
after called Re(bopy)2 and Re(bopy)(bpy); see Figure 1.
Through a combination of spectroscopic techniques, comple-
mented with spectroelectrochemical measurements, it is
shown that the lowest excited state of Re(bopy)2 and Re-
(bopy)(bpy) have Re f bopy 3MLCT and Re f bpy 3MLCT
character, respectively. The X-ray structure of the novel
compound Re(bopy)(bpy) is also presented.
3
lived MLCT excited state, which may be formally viewed
as [ReII(Cl)(CO)3(bopy)(bopy•-)], is reduced in a bimolecular
reaction with triethylamine to produce [ReI(Cl)(CO)3(bopy)-
(11) Liard, D. J.; Busby, M.; Farrell, I. R.; Matousek, P.; Towrie, M.; Vlcˇek,
A., Jr. J. Phys. Chem. A 2004, 108, 556-567.
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124.
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Phys. Chem. 1995, 99, 11801-11804.
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1961-1968.
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(16) Westmoreland, T. D.; Le Bozec, H.; Murray, R. W.; Meyer, T. J. J.
Am. Chem. Soc. 1983, 105, 5952-5954.
Experimental Section
(17) Chen, P.; Curry, M.; Meyer, T. J. Inorg. Chem. 1989, 28, 2271-
2280.
Materials. fac-[Re(Cl)(CO)3(bopy)2] was synthesized according
to the literature method.30 The novel complex fac-[Re(bopy)(CO)3-
(bpy)]PF6‚CH3CN was prepared using a modified standard proce-
(18) Liard, D. J.; Kleverlaan, C. J.; Vle`ek, A., Jr. Inorg. Chem. 2003, 42,
7995-8002.
-
dure20 for fac-[Re(L)(CO)3(NN)]+, in which the PF6 counterion
(19) Liard, D. J.; Vlcˇek, A., Jr. Inorg. Chem. 2000, 39, 485-490.
(20) Yam, V. W.-W.; Lau, V. C.-Y.; Wu, L.-X. J. Chem. Soc., Dalton
Trans. 1998, 1461-1468.
was used instead of ClO4-, through addition of an excess of NH4-
PF6 to fac-[Re(bopy)(CO)3(bpy)]OTf dissolved in methanol.
(21) Sun, S.-S.; Lees, A. J. Organometallics 2002, 21, 39-49.
(22) Lewis, J. D.; Perutz, R. N.; Moore, J. N. Chem. Commun. 2000, 1865-
1866.
Characterization of the Complexes. fac-[Re(Cl)(CO)3(bopy)2]:
89% yield;
(23) Sun, S.-S.; Lees, A. J.; Zavalij, P. Y. Inorg. Chem. 2003, 42, 3445-
FT-IR (acetonitrile) νCO ) 2028 (s), 1928 (s), 1893 (s), 1670
3453.
(m) cm-1
;
(24) Lo, K. K.-W.; Hui, W.-K.; Ng, D. C.-M.; Cheung, K.-K. Inorg. Chem.
2002, 41, 40-46.
1H NMR [(CD3)2CO; 270 MHz] 7.59 (4H, dd, J 7.4, aromatic
CH), 7.74 (2H, d, J 7.2, aromatic CH), 7.83 (4H, d, J 7.4, aromatic
CH), 7.87 (4H, d, J 7.8, aromatic CH), 9.09 (4H, d, J 7.1, aromatic
CH);
13C NMR [(CD3)2CO; 67.9 MHz] 125.15, 128.95, 130.18, 134.07,
135.40, 146.87, 154.69, 193.47 (CO).
(25) Yam, V. W.-W.; Lo, K. K.-W.; Cheung, K.-K.; Kong, R. Y.-C. J.
Chem. Soc., Chem. Commun. 1995, 1191-1193.
(26) Yam, V. W.-W.; Wong, K. M.-C.; Lee, V. W.-M.; Lo, K. K.-W.;
Cheung, K.-K. Organometallics 1995, 14, 4034.
(27) Guo, X.-Q.; Castellano, F. N.; Li, L.; Lakowicz, J. R. Anal. Chem.
1998, 70, 632-637.
(28) Oriskovich, T. A.; White, P. S.; Thorp, H. H. Inorg. Chem. 1995, 34,
1629-1631.
fac-[Re(bopy)(CO)3(bpy)]PF6‚CH3CN: 70% yield;
(29) Giordano, P. J.; Fredericks, S. M.; Wrighton, M. S.; Morse, D. L. J.
Am. Chem. Soc. 1978, 100, 2257-2259.
FT-IR (acetonitrile) νCO ) 2036 (s), 1933 (br), 1672 (s) cm-1
;
(30) Shu, C.-F.; Wrighton, M. S. Inorg. Chem. 1988, 27, 4326-4329.
(31) Fredericks, S. M.; Wrighton, M. S. J. Am. Chem. Soc. 1980, 102,
6166-6168.
1H NMR [(CD3)2CO 270 MHz] 7.52 (2H, dd, J 8.1, aromatic
CH), 7.70 (5H, m, aromatic CH), 8.01 (2H, dd, J 6.5, aromatic
4524 Inorganic Chemistry, Vol. 43, No. 14, 2004

MLCT of fac-[Re(Cl)(4-benzoylpyridine)2(CO)3]
CH), 8.47 (2H, dd, J 6.3, aromatic CH), 8.74 (2H, d, J 8.2, aromatic
CH), 8.80 (2H, d, J 6.8, aromatic CH), 9.49 (2H, d, J 6.9, aromatic
CH);
13C NMR [(CD3)2CO; 67.9 MHz] 125.07, 125.82, 128.92, 129.29,
130.03, 134.18, 135.02, 141.62, 147.46, 153.14, 154.17, 156.11,
191.65 (CO), 192.87 (CO), 195.55 (CO);
ES-MS m/z 610.2, [M+] - PF6.
Instrumentation. X-ray data were recorded on a CAD4 dif-
fractometer fitted with a low-temperature device operating in the
ω/2θ scan mode. The structures were solved by standard heavy
atom techniques32 and refined by least-squares fits32 on F2. The
hydrogen atoms were calculated geometrically and refined with a
riding model. The program33 ORTEP-3 was used for drawing the
molecules and WinGX34 was used to prepare material for publica-
tion. Refinement details as well as a full list of atomic coordinates
and bond lengths and angles are available in the Supporting
Information. CIF data has been deposited at the CCDC, under
deposition no. 230056.
Ground-state resonance Raman spectra were obtained using a
Coherent Innova 90-5 UV argon-ion laser, with a power output of
ca. 50 mW at 457.9 nm and 30 mW at 351.1 nm. The Raman
spectra were recorded using 2 mm quartz flow tubes, and the
scattered light was collected at 90° from the excitation beam passed
through a Spex Triplemate 187 Series monochromator and dispersed
across a Princeton Instruments LN/CCD-1024 CCD camera. Spectra
were calibrated and solvent bands subtracted using CSMA CCD
Spectrometric Multichannel Analysis Software, version 2.4a, Prin-
ceton Instruments Inc. program.
Time-resolved UV-vis absorption spectra were measured using
the experimental setup at the Institute of Molecular Chemistry,
University of Amsterdam.35 A ∼130 fs and 390 nm pump pulse
was generated by frequency doubling of the Ti:sapphire laser output.
White-light continuum probe pulses were generated by focusing
the 800 nm fundamental in a sapphire plate.
Time-resolved IR and Kerr-gate resonance Raman used the
equipment and procedures described in detail previously.11,36-41 In
short, the sample solution was excited (pumped) at 400 nm, using
frequency-doubled pulses from a Ti:sapphire laser of ∼200 fs
duration (fwhm) in the case of time-resolved visible and IR
absorption spectroscopy, while pulses of 1-2 ps duration were used
for Raman and emission studies. TRIR spectra were probed with
IR (∼200 fs) pulses obtained by difference-frequency generation.
The IR probe pulses cover spectral range ca. 200 cm-1 wide and
are tunable from 1000 to 5000 cm-1 (i.e., 2-10 µm). Kerr-gate
TR3 spectra were probed at 600 or 400 nm using an OPA or
Figure 2. ORTEP diagram of [Re(bopy)(CO)3(bpy)]PF6‚CH3CN, showing
40% probability ellipsoids. The solvent molecule is omitted.
frequency-doubled Ti:sapphire laser pulses, respectively. Kerr gate
was used to remove all long-lived emission from the Raman signal.
TR3 spectra were corrected for the Raman signal due to the solvent
and the solute ground state by subtracting the spectra obtained at
negative time delays (-50, -20 ps) and by subtracting any weak
residual emission that passed through the Kerr gate. The sample
solutions for picosecond TR3 and TRIR experiments were flowed
as a 0.5 mm open jet and through a 0.5 mm CaF2 raster-scanned
cell, respectively.
The spectral bands observed in TRIR spectra were fitted by
Lorentzian functions to determine accurately their center positions,
widths, and areas. All spectral fitting procedures were performed
using Microcal Origin 7 software.
The cyclic voltammogram of Re(bopy)(bpy) was recorded on a
model 270/250 potentiostat from EG&G Instruments Inc., Princeton
Applied Research. A home-built electrochemical cell was used with
a three-electrode system: working electrode (0.5 mm2 Pt); auxiliary
electrode (Pt coil); an Ag-coil pseudoreference electrode. All values
were obtained at a scan rate of 100 mV/s with 0.1 M Bu4NPF6
supporting electrolyte and 1-2 mM concentration of the complex.
Spectroelectrochemical experiments were performed in n-PrCN
using OTTLE cells whose design was published previously.42-44
Results
(32) Sheldrick, G. M. SHELX97, Program for crystal structure determi-
nation and refinement; University of Go¨ttingen: Go¨ttingen, Germany,
1998.
X-ray Crystallography. Crystals of [Re(bopy)(CO)3(bpy)]-
PF6‚CH3CN were grown from an acetonitrile solution by
slow solvent evaporation. The structure was determined by
X-ray diffraction. The ORTEP diagram is shown in Figure
2, and selected bond lengths and angles are summarized in
Table 1, while all the structural data are collected in the
Supporting Information. Bond distances and angles around
the Re(1) metal center are typical for [Re(L)(CO)3(NN)]+
complexes,45 with the N(1)-Re(1)-N(2) bite angle being
(33) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(34) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
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2002, 327, 126-133.
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Parker, A. W.; Grills, D. C.; George, M. W. J. Chem. Soc., Dalton
Trans. 2002, 701-712.
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Opt. Comm. 1996, 127, 307-312.
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Technol. 1998, 9, 816-823.
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1999, 53, 1485-1489.
(42) Krejc´ik, M.; Dan`ık, M.; Hartl, F. J. Electroanal. Chem., Interfacial
Electrochem. 1991, 317, 179-187.
(40) Matousek, P.; Towrie, M.; Ma, C.; Kwok, W. M.; Phillips, D.; Toner,
W. T.; Parker, A. W. J. Raman Spectrosc. 2001, 32, 983-988.
(41) Towrie, M.; Grills, D. C.; Dyer, J.; Weinstein, J. A.; Matousek, P.;
Barton, R.; Bailey, P. D.; Subramaniam, N.; Kwok, W. M.; Ma, C.
S.; Phillips, D.; Parker, A. W.; George, M. W. Appl. Spectrosc. 2003,
57, 367-380.
(43) Hartl, F.; Luyten, H.; Nieuwenhuis, H. A.; Schoemaker, G. C. Appl.
Spectrosc. 1994, 48, 1522-1528.
(44) Mahabiersing, T.; Luyten, H.; Nieuwendam, R. C.; Hartl, F. Collect.
Czech.. Chem. Commun. 2003, 68, 1687-1709.
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Chim. Acta 2004, 357, 167-176.
Inorganic Chemistry, Vol. 43, No. 14, 2004 4525

Busby et al.
Figure 3. UV-vis absorption spectra of [Re(Cl)(bopy)2(CO)3] (solid line)
and [Re(bopy)(CO)3(bpy)]+ (dashed line) measured in acetonitrile.
Table 1. Selected Bond Lengths and Angles of
fac-[Re(bopy)(CO)3(bpy)]PF6
Lengths/Å
Re(1)-C(1)
Re(1)-C(3)
Re(1)-C(2)
Re(1)-N(2)
Re(1)-N(1)
Re(1)-N(3)
1.910(5)
1.923(6)
1.931(5)
2.159(4)
2.166(4)
2.218(4)
O(1)-C(1)
O(2)-C(2)
O(3)-C(3)
O(4)-C(9)
C(6)-C(9)
C(9)-C(10)
1.158(7)
1.131(6)
1.152(7)
1.222(7)
1.516(7)
1.479(8)
Angles/deg
C(1)-Re(1)-C(2)
C(1)-Re(1)-C(3)
C(3)-Re(1)-C(2)
C(1)-Re(1)-N(1)
C(3)-Re(1)-N(1)
C(2)-Re(1)-N(1)
C(1)-Re(1)-N(2)
C(2)-Re(1)-N(2)
C(3)-Re(1)-N(2)
86.9(2)
89.4(2)
88.1(2)
96.84(18)
97.61(18)
173.18(18)
171.93(18)
101.20(19)
91.21(19)
C(1)-Re(1)-N(3)
C(2)-Re(1)-N(3)
C(3)-Re(1)-N(3)
N(1)-Re(1)-N(2)
N(1)-Re(1)-N(3)
N(2)-Re(1)-N(3)
O(4)-C(9)-C(6)
O(4)-C(9)-C(10)
C(10)-C(9)-C(6)
95.34(19)
90.61(19)
175.02(19)
75.10(16)
83.35(15)
84.30(15)
118.2(5)
Figure 4. Ground and excited-state resonance Raman spectra: (a) Kerr-
gate picosecond time-resolved resonance Raman spectrum (TR3) of [Re-
(Cl)(bopy)2(CO)3], measured using 600 nm probe at 8 ps after 400 nm laser
pulse excitation; (b) ground-state resonance Raman spectrum of [Re(Cl)-
(bopy)2(CO)3], measured using stationary 457.9 nm irradiation with emission
background subtracted; (c) ground-state resonance Raman spectrum of [Re-
(bopy)(CO)3(bpy)]+, measured upon stationary irradiation at 351.1 nm. All
spectra were measured in acetonitrile. Asterisks indicate regions of solvent
subtraction.
121.1(5)
120.6(5)
lying electronic transitions of both complexes, their (pre)-
resonance Raman spectra have been investigated.
characteristically small, 75.10°. The bopy C(9)-O(4) bond
distance is 1.22 Å. This is a typical value for aromatic
ketones such as benzophenone, for which an identical CdO
bond length was determined.46 The nonplanarity between the
two rings of the bopy ligand indicates that there is no
substantial π orbital overlap and conjugation that would
overcome the steric repulsion between the rings, at least in
the solid state. A similar twisted structure was found for
benzophenone.46
UV-Vis Absorption and Resonance Raman Spectros-
copy. UV-vis absorption spectra of both complexes (Figure
3) are very similar, exhibiting a broad absorption band
between ca. 330 and 410 nm, which is characteristic of an
MLCT transition, although IL(bopy) transitions may con-
tribute as well. In addition to the bands seen for Re(bopy)2,
the spectrum of Re(bopy)(bpy) shows a typical structured
absorption with sharp peaks at 320 and 307 nm due to bpy
intraligand (IL) transitions. The band seen for both complexes
at ca. 260 nm originates in an IL(bopy) ππ* transition(s).
To obtain more detailed information on the nature of low-
The preresonance Raman spectrum of Re(bopy)2 acquired
upon CW excitation at 457.9 nm, Figure 4b, shows an
enhanced band at 2028 cm-1 which is due to the A′(1)
ν(C≡O) vibration of the Re(CO)3 moiety and a group of
bands corresponding to bopy-localized vibrations, including
a band due to the bopy ν(CdO) vibration29,30 at 1671 cm-1.
The most intense band at 1616 cm-1 seems to contain at
least two unresolved vibrations, which correspond to ring
C-C stretching modes.47 Other bopy-localized vibrations are
present between 1000 and 1250 cm-1 and are assigned to
C-Ph/py stretching and C-H bending modes.47 The reso-
nance enhancement of Raman bands due to vibrations of both
the Re(CO)3 and the bopy ligand is indicative of the 457.9
nm excitation being in preresonance with the MLCT(bopy)
transition, although a small contribution from a nearby IL-
(bopy) transition cannot be entirely excluded.
Re(bopy)(bpy) shows a significantly different resonance
Raman spectrum, Figure 4c. Excitation at 351.1 nm was used
(46) Kutzke, H.; Klapper, H.; Hammond, R. B.; Roberts, K. J. Acta
Crystallogr., Sect. B 2000, 56, 486.
(47) Tahara, T.; Hamaguchi, H.; Tasumi, M. J. Phys. Chem. 1987, 91,
5875-5880.
4526 Inorganic Chemistry, Vol. 43, No. 14, 2004

MLCT of fac-[Re(Cl)(4-benzoylpyridine)2(CO)3]
Raman spectrum (TR3); see Figure 4a. The spectrum was
measured at 8 ps after 400 nm excitation using a probe
wavelength of 600 nm that was tuned into the strong TA
band. The TR3 spectrum is consistent with the reduction of
one bopy ligand upon excitation. The ground-state band at
1671 cm-1 due to the ketone ν(CdO) vibrations disappears.
This vibration seems to be manifested in the excited-state
TR3 spectrum by a ∼1610 cm-1 shoulder, as is indicated by
comparison with TRIR spectra; vide infra. The new bands
at 1586, 1470, 1330, 1187, and 996 cm-1 are attributed to
the to C-Ph/py stretching and C-H bending modes of the
bopy•- radical-anionic ligand.47
For Re(bopy)(bpy), the TA signal is much weaker and
comprises of a broad shoulder between 500 and 600 nm and
an absorbance sharply rising into the UV region; see Figure
5b. The TA spectrum of Re(bopy)(bpy) is very similar to
the spectra of typical 3MLCT excited states49,50 of [Re(Etpy)-
(CO)3(dmb)]+ (dmb ) 4,4′-dimethyl-2,2′-bipyridine) and
[Re(Cl)(CO)3(bpy)], warranting the same assignment of the
lowest excited state of Re(bopy)(bpy). The TA signal does
not decay during the time interval investigated, in agreement
Figure 5. Time-resolved visible absorption spectra of (a) [Re(Cl)(bopy)2-
(CO)3], measured at 2, 3, 10, and 75 ps, and (b) [Re(bopy)(CO)3(bpy)]+,
measured at 4, 40, and 1000 ps. Both were measured in acetonitrile, after
excitation at 393 nm. The spectra evolve in the directions of the arrows.
to avoid interfering emission. The spectrum shows a strongly
enhanced band pertaining to the A′(1) ν(C≡O) vibration at
2037 cm-1. Its relative intensity is much stronger than in
the case of Re(bopy)2. The band due to the bopy CdO
stretching vibration is extremely weak, probably not reso-
nantly enhanced. Bands between 1000 and 1600 cm-1 can
all be attributed9,48,49 to vibrations of the bpy ligand. Overall,
the resonance Raman spectrum of Re(bopy)(bpy) is similar
to those9,48 of [Re(Cl)(CO)3(bpy)] and [Re(4-Etpy)(CO)3-
(bpy)]+, perhaps with the exception of the 1615 cm-1 band,
whose much larger relative intensity could indicate a bopy
contribution. It can be concluded that the lowest absorption
band of Re(bopy)(bpy) originates predominantly in a MLCT-
(bpy) transition, with a possible IL(bopy) contribution.
Time-Resolved Visible Absorption (TA) and Resonance
Raman Spectra (TR3). TA spectra of the two complexes
measured using 393 nm excitation show very different
behavior; see Figure 5. For Re(bopy)2, a very broad absorp-
tion is seen, with a maximum at ca. 575 nm, which closely
resembles the UV-vis spectrum of the electrochemically
generated [Re(Cl)(CO)3(bopy)(bopy•-)]- radical anion.30 This
provides evidence that the 575 nm TA band originates in a
ππ* transition of a single reduced bopy•- ligand which is
formed upon MLCT(bopy) excitation. The TA spectrum is
3
with the expected long MLCT(bpy) excited-state lifetime.
The assignment of the lowest excited state of Re(bopy)-
3
(bpy) as MLCT(bpy) is further supported by its emission
spectrum which shows a broad, unstructured MLCT-type
band both in a glass and a fluid solution. The following
emission maxima were determined (corrected for the instru-
ment response): 515 nm (77 K, MeOH/EtOH (4/5 v/v)
glass), 578 nm (RT (room temperature), CH3CN), 580 nm
(RT, MeOH/EtOH (4/ 5 v/v)). The Re(bopy)(bpy) emission
in fluid CH3CN occurs very close to the typical MLCT(bpy)
emission of Re(4-Etpy)(CO)3(bpy)]+ (571 nm).51 The ob-
served rigidochromism is also characteristic of MLCT(bpy)
states.
Time-Resolved Infrared Spectroscopy, TRIR. TRIR
experiments were performed to investigate the behavior of
the CtO and CdO stretching modes of the Re(CO)3 and
bopy moieties, respectively, after 400 nm excitation.
For Re(bopy)2, excitation essentially preserves the ground-
state ν(CtO) IR pattern with all the bands being shifted to
higher wavenumbers, Figure 6. In particular, the A′(1)
ν(C≡O) vibration shifts by +54 cm-1 upon excitation in both
CH2Cl2 and acetonitrile, while the ketone ν(CdO) band30
undergoes a downward shift52 of -55 cm-1, from 1670 to
3
3
1615 cm-1, Figure 7. This behavior confirms the MLCT-
thus attributed to a MLCT(bopy) excited state, formulated
approximately as [ReII(Cl)(CO)3(bopy)(bopy•-)]. Interest-
ingly, the TA band is seen to narrow and blue-shift during
the first 10 ps, which is assigned to vibrational cooling of
the excited molecule. Once the cooling is complete, the
absorption decreases very slowly in intensity, indicating that
(bopy) character of the lowest excited state: Decrease of
the electron density at the Re atom upon MLCT excitation
diminishes the Re f CO π back-donation, which is
manifested by an upward shift of ν(CtO) frequencies.51,53-57
3
(50) Kalyanasundaram, K. J. Chem. Soc., Faraday Trans. 2 1986, 82,
2401-2415.
the MLCT(bopy) excited state lives into the nanoseconds
time domain.
(51) Dattelbaum, D. M.; Omberg, K. M.; Schoonover, J. R.; Martin, R.
L.; Meyer, T. J. Inorg. Chem. 2002, 41, 6071-6079.
(52) The assignment of the 1615 cm-1 band as ν(CdO) is based on its
high intensity and comparison with IR spectra62 of radical anions of
benzophenone derivatives bearing an electron-accepting substituent
on the phenyl rings, whose spectra were assigned by isotopic
substitution.
The assignment of the lowest excited state of Re(bopy)2
as 3MLCT is further supported by its time-resolved resonance
(48) Smothers, W. K.; Wrighton, M. S. J. Am. Chem. Soc. 1983, 105,
1067-1069.
(49) Liard, D. J.; Busby, M.; Matousek, P.; Towrie, M.; Vlcˇek, A., Jr. J.
Phys. Chem. A 2004, 108, 2363-2369.
(53) Glyn, P.; George, M. W.; Hodges, P. M.; Turner, J. J. J. Chem. Soc.,
Chem. Commun. 1989, 1655-1657.
Inorganic Chemistry, Vol. 43, No. 14, 2004 4527

Busby et al.
tively.58 (Note that the magnitude of the shift is related to
the charge separation upon MLCT excitation.) Downward
shifts of organic ν(CdO) vibrations were seen previously
upon MLCT excitation of complexes with carboxylated 2,2′-
bipyridine ligands [Ru(bpy)2(4-CO2Et)-4′-CH3-2,2′-bpy)]2+ 59
and [Re(4-Etpy)(CO)3(4,4′-CO2Et)-2,2′-bpy)]+ 60 and in [Pt-
(Cl)2(4,4′-CO2Et)-2,2′-bpy)].61 The ν(CdO) shifts in these
complexes fall between -26 and -46 cm-1. The -55 cm-1
ν(CdO) shift, found herein, is less than that accompanying
benzophenone reduction, -262 cm-1, but close to values
found for the reduction of benzophenone derivatives bearing
electron-accepting substituents.62 This can be rationalized by
the excited electron density being largely delocalized over
the bopy•- ligand, polarized toward the pyridine ring. The
N-coordinated ReII(CO)3 unit acts as an electron acceptor
toward the bopy•- ligand. It follows that the charge separation
in the excited state is not complete and the CdO group
acquires only partial ketyl, C•-O-, character. This conclusion
agrees with the relatively small upward shift of the ν(CtO)
vibrations of Re(CO)3.
Figure 6. Time-resolved infrared spectra in the region of Re(CO)3 ν(Ct
O) vibrations measured in acetonitrile after 400 nm excitation: (a) [Re-
(Cl)(bopy)2(CO)3], spectra measured at 2, 4, 8, 10, 100, and 1000 ps; (b)
[Re(bopy)(CO)3(bpy)]+, spectra measured at 2, 3, 4, 10, 15, and 1000 ps.
The IR features undergo small dynamical shifts in the directions of the
arrows. Experimental points are separated by 4-5 cm-1
.
The upward shift of ν(CtO) IR bands consists of two
components: one instantaneous that occurs within the
instrument time resolution; a dynamic one that follows on a
picosecond time scale and originates in vibrational relaxation
and solvation.63 In particular, the +54 cm-1 shift of the A′1
band consists of a ca. +45.6 cm-1 “instantaneous” shift and
a +8.4 ( 0.9 cm-1 dynamic shift that is biexponential with
time constants of 1.8 ( 0.2 ps (88%) and 17.1 ( 4.3 ps
(12%). The width of this band narrows by ca. 30% with a
time constant of 5.6 ( 0.9 ps. The bopy ν(CdO) band shows
a small dynamic blue-shift of about +1.5 cm-1 with a time
constant estimated as 14.5 ( 2.5 ps.
For Re(bopy)(bpy) in acetonitrile, the TRIR spectrum
displays an upward shift of the Re(CO)3 ν(C≡O) bands on
excitation, while the lower ground-state broad band, which
corresponds to merged A′′ + A′(2) vibrations, splits into two
bands, Figure 6b and Table 2. As observed also in TA, the
TRIR bands do not undergo any decay, in accordance with
the very long excited-state lifetime. The change of the
excited-state ν(C≡O) IR spectral pattern and the +42 cm-1
upward shift of the high-energy 2034 cm-1 A′(1) ν(C≡O)
band upon excitation are similar to those observed for [Re-
(Etpy)(CO)3(dmb)]+ (34 cm-1),11,49 providing good evidence
that the excited electron density is localized on the bpy
ligand, instead of bopy. This conclusion is further supported
by the lack of any changes in the region of ketone ν(CdO)
vibrations, between 1600 and 1700 cm-1. It can thus be
concluded that the lowest excited state of Re(bopy)(bpy) has
Figure 7. Time-resolved infrared spectra [Re(Cl)(bopy)2(CO)3] in the
region of the ketone ν(CdO) measured in acetonitrile at 2, 4, 40, 100, and
1000 ps after 400 nm excitation. The spectra evolve in the directions of the
arrows. Experimental points are separated by 4-5 cm-1
.
At the same time, the excited electron density populates a
π* orbital of a bopy ligand, weakening the ketone CdO
bond. This results in a decrease of the CdO stretching
frequency. The observed ν(CtO) shift is higher than that
measured for [Re(Cl)(CO)3(4,4′-bpy)2] (+30 cm-1),54,55
though less than that found for [Re(Cl)(CO)3(XQ+)2]2+,
where XQ+ is N-methyl-4,4′-bipyridinium (MQ+) and N-phen-
yl-4,4′-bipyridinium (PQ+), (+67 and +65 cm-1, respec-
3
a MLCT(bpy) character.
(58) Busby, M.; Matousek, P.; Towrie, M.; Vlcˇek, A., Jr. Manuscript in
preparation.
(54) George, M. W.; Johnson, F. P. A.; Westwell, J. R.; Hodges, P. M.;
Turner, J. J. J. Chem. Soc., Dalton Trans. 1993, 2977-2979.
(55) Gamelin, D. R.; George, M. W.; Glyn, P.; Grevels, F.-W.; Johnson,
F. P. A.; Klotzbucher, W.; Morrison, S. L.; Russell, G.; Schaffner,
K.; Turner, J. J. Inorg. Chem. 1994, 33, 3246-3250.
(56) Dattelbaum, D. M.; Meyer, T. J. J. Phys. Chem. A 2002, 106, 4519-
4524.
(59) Chen, P.; Omberg, K. M.; Kavaliunas, D. A.; Treadway, J. A.; Palmer,
R. A.; Meyer, T. J. Inorg. Chem. 1997, 36, 954-955.
(60) Chen, P.; Palmer, R. A.; Meyer, T. J. J. Phys. Phem. A 1998, 102,
3042-3047.
(61) Weinstein, J. A.; Grills, D. C.; Towrie, M.; Matousek, P.; Parker, A.
W.; George, M. W. Chem. Commun. 2002, 382-383.
(62) Juchnovski, I. N.; Kolev, T. M. Spectrosc. Lett. 1985, 18, 481-490.
(63) Asbury, J. B.; Wang, Y.; Lian, T. Bull. Chem. Soc. Jpn. 2002, 75,
973-983.
(57) Za´lisˇ, S.; Farrell, I. R.; Vlcˇek, A., Jr. J. Am. Chem. Soc. 2003, 125,
4580-4592.
4528 Inorganic Chemistry, Vol. 43, No. 14, 2004

MLCT of fac-[Re(Cl)(4-benzoylpyridine)2(CO)3]
Table 2. Ground- and Excited-State ν(CO) Wavenumbers of [Re(Cl)(bopy)2(CO)3], [Re(Cl)(4,4′-bpy)2(CO)3], [Re(bopy)(CO)3(bpy)]+, and
[Re(Etpy)(CO)3(dmb)]+, Measured in Acetonitrile, at 500 ps, Once Vibrational Relaxation Is Completed
ground state
excited state
∆ν
compd
Re(bopy)2
Re(4,4-bpy)2
CdO
A′′
A′(2)
A′(1)
CdO
A′′
A′(2 )
A′(1 )
CdO
-55
A′′
A′(2)
A′(1)
1670
1895
1891
1929b
1930b
1923
1927
1929
1930
2026
2025
2035
2034
1615
1976
1957
1968
1970
1999
1992
2012
2013
2080
2055
2076
2068
+81
+66
+39
+40
+76
+65
+83
+83
+54
+30
+41
+34
a 55
Re(bopy)(bpy)
1670
Re(Etpy)(dmb)11,49
a Measured in CH2Cl2. b Merged doublet.
Figure 8. Low-temperature IR spectroelectrochemical reduction of Re-
(bopy)(bpy) measured in butyronitrile in the presence of 10-1 M Bu4NPF6
at 223 K: b, Re(bopy)(bpy); O, Re(bopy•-)(bpy); 0, Re(bopy•-)(bpy•-);
9, [Re(PrCN)(CO)3(bpy2-)]-.
Figure 9. Room-temperature IR spectroelectromical reduction of Re-
(bopy)(bpy) measured in butyronitrile in the presence of 10-1 M Bu4NPF6
at 293 K: b, Re(bopy)(bpy); O, [Re(PrCN)(CO)3(bpy)]+.
The A′(1) ν(C≡O) band shift has an instantaneous
component of +21.7 ( 2 cm-1 followed by a biexponential
dynamic shift of +21.3 ( 2 cm-1, which occurs with 1.6 (
0.2 ps (85%) and 26.6 ( 3.6 ps (15%) time constants. The
band narrows by 30% with a 3.0 ( 0.2 ps time constant.
The middle excited-state ν(C≡O) band at 2012 cm-1 also
shows an upward shift and narrowing. These vibrational
dynamics are very similar to those found for [Re(Etpy)(CO)3-
(dmb)]+ in acetonitrile11,49 and [Re(Cl)(CO)3(bpy)] in di-
methylformamide.63
Electrochemistry and Spectroelectrochemistry of Re-
(bopy)(bpy). Re(bopy)(bpy) in a CH3CN solution is reduced
in two electrochemically and chemically reversible one-
electron steps at E1/2 ) -1.44 and -1.66 V (vs Fc/Fc+).
These values are close to the first reduction potentials of
Re(bopy)2 (-1.53 V)30 and [Re(4-Et-pyridine)(CO)3(bpy)]+
(-1.56 V),51 respectively. This makes it hard to decide
whether the first reduction is localized on the bopy or the
bpy ligand. Nevertheless, the bopy localization of the first
reduction step was confirmed by IR spectroelectrochemistry
measured at low temperature, 223 K; see Figure 8. In situ
reduction initially produces a species characterized by IR
bands at 2022, 1919, 1905, and 1624 cm-1. The disappear-
ance of the parent ketone ν(CdO) band at 1670 cm-1 and
its shift to 1624 cm-1 demonstrate52,62 that the first reduction
is indeed bopy-localized, producing Re(bopy•-)(bpy). This
conclusion is further supported by the ν(CO) wavenumbers,
which are very close to those measured for [Re(L)(CO)3-
(bpy)] complexes where L is an anionic ligand (e.g.: L )
HCOO-, 2019, 1916, 1894 cm-1; L ) Cl-, 2020, 1914, 1897
cm-1).64 A much larger downward shift of ν(CO) wavenum-
bers would be expected for a bpy-localized reduction.64,65
UV-vis spectroelectrochemistry studied at 223 K in n-PrCN
shows that reduction of Re(bopy)(bpy) is accompanied by
growth of a band at ca. 360 nm and of a broad band with
maxima at 500 and ∼570 nm, which tails toward ca. 700
nm. These features resemble the visible spectrum30 of
Re(bopy•-)(bopy), again supporting the formulation of the
first reduction product as Re(bopy•-)(bpy). Accordingly, the
typical66 bpy•- ππ* bands at 800 nm and longer wavelengths
are missing. The IR features of Re(bopy•-)(bpy) decrease
during continuing reduction, being replaced by IR bands at
1997 and 1884 cm-1, which are attributed to the second
reduction product Re(bopy•-)(bpy•-). This assignment is
based on comparison with the spectra64 of the complexes
[Re(Cl)(CO)3(bpy•-)]- (1998, 1885, 1868 cm-1) and [Re-
(HCOO)(CO)3(bpy•-)]- (1997, 1879 cm-1) which contain
bpy•- and an anionic axial ligand. Further reduction is
accompanied by bopy•- substitution, producing [Re(PrCN)-
(CO)3(bpy2-)]-, which is characterized by ν(CO) bands at
1980, 1863, and 1851 cm-1.64
IR spectroelectrochemistry at room temperature (Figure
9) shows completely different behavior, whereby the bopy
ligand undergoes an electrode-catalyzed substitution67,68 by
the solvent forming [Re(PrCN)(CO)3(bpy)]+ (2040, 1935 (br)
(64) Johnson, F. P. A.; George, M. W.; Hartl, F.; Turner, J. J. Organo-
metallics 1996, 15, 3374-3387.
(65) van Outersterp, J. W. M.; Hartl, F.; Stufkens, D. J. Organometallics
1995, 14, 3303-3310.
(66) Krejcˇ´ık, M.; Vlcˇek, A. A. J. Electroanal. Chem. 1991, 313, 243-
257.
(67) Miholova´, D.; Vlcˇek, A. A. J. Organomet. Chem. 1982, 240, 413-
419.
Inorganic Chemistry, Vol. 43, No. 14, 2004 4529

Busby et al.
cm-1).65 Free bopy, characterized by an undiminished 1670
cm-1 band, is also present in the solution.
a similar complex with two electron-accepting ligands, [Re-
(MQ+)(CO)3(dmb)]+ (MQ+ ) N-methyl-4,4′-bipyridine, dmb
) 4,4′-dimethyl-2,2′-bipyridine), whose MLCT(MQ) lies
below the MLCT(dmb) state. Nevertheless, optical excitation
of [Re(MQ+)(CO)3(dmb)]+ populates first MLCT(dmb) and
the MLCT(MQ) state is formed subsequently by a dmb•-
f MQ+ interligand electron transfer (ILET).8,11,16,17 Neither
bopy•- f bpy nor bpy•- f bopy ILET occurs upon
excitation of Re(bopy)(bpy) since the optically populated
MLCT(bpy) state is the lowest excited state. Finally, it should
be noted that no evidence was found for any delocalization
of the excited electron density from bpy•- to bopy, i.e., for
mixing between MLCT(bpy) and MLCT(bopy) excited
states.
Discussion
By a combination of steady-state and time-resolved
spectroscopic techniques, it has been demonstrated that the
lowest allowed electronic transition of Re(bopy)2 is Re f
bopy MLCT, which is closely followed in energy by a ππ*
bopy IL transition. Accordingly, the lowest-lying electronic
excited state is 3MLCT(bopy), in which the excited electron
density is predominantly localized on a single bopy ligand,
which acquires bopy•- character. This excited state is long-
lived, and a radical-like photochemical reactivity can thus
be expected from Re(bopy)2.
The observation of dynamic shifts of ν(CtO) IR bands
upon MLCT excitation shows that 3MLCT excited states of
both complexes are formed initially vibrationally hot. Since
no V ) 1 f 2 ν(CtO) bands are seen in the TRIR spectra,
we can conclude that the energy released during intersystem
crossing from the optically populated 1MLCT excited states
(ca. 6400 cm-1)49 is rapidly (<1 ps) redistributed into low-
frequency skeletal, solvent-solute, and first solvation shell
vibrations, whose relaxation occurs on a picosecond time
scale by energy dissipation into the bulk solvent. This
vibrational dynamics are typical for MLCT excited states of
[Re(L)(CO)3(diimine)]n+ complexes.11,49,63
In conclusion, the MLCT(bopy) and MLCT(bpy) were
found to be the lowest excited states of [Re(Cl)(CO)3(bopy)2]
and [Re(bopy)(CO)3(bpy)]+, respectively. It follows that the
bpy ligand introduces a MLCT(bpy) state at rather low
energies even if another, more strongly, electron-accepting
ligand (i.e. bopy) is present in the coordination sphere. The
different localizations of the lowest MLCT excitation and
the first reduction are caused by an extra stabilization of the
ReII(bpy•-) chelate ring in the MLCT(bpy) excited state. Both
excited states are long-lived, but rather different photochemi-
cal behavior can be expected for these complexes. This study
also demonstrates the great power of TRIR spectroscopy in
revealing the nature and dynamics of low-lying excited states
of organometallics, by following the spectral and temporal
changes of vibrations of both the metal-carbonyl unit and
functional groups of an organic ligand.
Re(bopy)(bpy) is an interesting case of a complex that
contains two electron-accepting ligands which have similar
reduction potentials. The lowest excited state of Re(bopy)-
(bpy) can be either of a MLCT(bopy) or MLCT(bpy)
character. As was demonstrated electrochemically and spec-
troelectrochemically, the bopy ligand is reduced first, 0.22
V less negatively than bpy. On the basis of electrochemical
data, MLCT(bopy) is expected to be the lowest excited state.
Nevertheless, resonance Raman, emission and time-resolved
visible, and IR absorption spectra clearly show that Re(bopy)-
(bpy) has a Re f bpy lowest-allowed electronic transition
and a 3MLCT(bpy) lowest-lying excited state, which can be
formulated as [ReII(bopy)(CO)3(bpy•-)]+. The complex Re-
(bopy)(bpy) thus represents a very unusual case where the
lowest unoccupied redox orbital, i.e., π*-bopy, is different
from the lowest unoccupied optical orbital, i.e., π*-bpy. To
understand the different localizations of the first electro-
chemical reduction and the lowest MLCT excitation, it has
to be considered that the electrochemical potentials and
excited-state energies are pertinent to species with different
Re formal oxidation states, that is, I and II, respectively. A
π delocalization of the excited electron over the ReII(bpy•-)
chelate ring will stabilize the MLCT(bpy) excited state
relative to the MLCT(bopy) state where the excited electron
is delocalized over the ketone group, farther away from the
ReII center. Moreover, the monodentate pyridine unit of the
bpy ligand provides weaker π interaction with Re than
chelating bpy. In an alternative, but conceptually equivalent,
view, we can express the extra stabilization of a MLCT by
the term -e2/reh, where e is the electron charge and reh is
the effective distance between the excited electron and hole.
This effective distance is much shorter when the electron is
excited to bpy than to bopy, stabilizing the MLCT(bpy) state
over MLCT(bopy). Obviously, these effects are not included
in the reduction potentials since neither stabilizing π inter-
action nor the electron-hole interaction occurs in the reduced
state. The Re(bopy)(bpy) complex behaves differently from
3
3
Acknowledgment. Mr. Michiel M. Groeneveld of the
Molecular Photonic Materials, van ’t Hoff Institute for
Molecular Sciences, University of Amsterdam, is thanked
for his help with measuring the time-resolved UV-vis, and
Dr. Anders Gabrielsson (QMUL) is thanked for measuring
the emission spectra. EPSRC and COST Action D14 are
acknowledged for funding this research.
Supporting Information Available: Additional tables. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(68) Hershberger, J. W.; Klingler, R. J.; Kochi, J. K. J. Am. Chem. Soc.
1983, 105, 61-73.
IC049659M
4530 Inorganic Chemistry, Vol. 43, No. 14, 2004