Experimental studies of light-induced charge transfer and charge redistribution in (X2-Bipyridine)Re1(CO)3Cl complexes

Article abstract of DOI:10.1021/ic011015g

Stark emission spectroscopy, transient DC photoconductivity (TDCP), and ground-state dipole moment measurements have been used to evaluate charge transfer (CT) within various (X₂-bipyridine)Re¹(CO)₃Cl complexes following ³MLCT excited-state formation. The Stark technique reports on vector differences between ground-state (μ_g) and excited-state (μ_u) dipole moments, while TDCP, when combined with independently obtained μ_g information, reports on scalar differences. For systems featuring collinear, same-signed ground- and excited-state dipole moments, the scalar and vector differences are equivalent. However, for the low symmetry systems studied here, they are distinctly different. The vector difference yields the effective adiabatic one-electron-transfer distance (R₁₂), while the combined vector and scalar data yield information about dipole rotation upon ground-state/excited-state interconversion. For the systems examined, charge transfer distances are substantially smaller than geometric electron-donor/electron-acceptor site separation distances. The measured distances are significantly affected by changes in acceptor ligand substituent composition. Electron-donating substituents decrease CT distances, while electron-withdrawing substituents increase CT distances, as do aromatic substituents that are capable of expanding the bipyridyl ligand (acceptor ligand) π system. The Stark measurements additionally indicate that the CT vector and the transition dipole moment are significantly orthogonal, a consequence of strong polarization of the Re-Cl bond (orthogonal to the metal/ acceptor-ligand plane) in the ground electronic state and relaxation of the polarization in the upper state. The ground-state Re-Cl bond polarization is sufficiently large that the overall ground-state scalar dipole moment exceeds the overall excited-state scalar dipole moment, despite transfer of an electron from the metal center to the diimine ligand. This finding provides an explanation for the otherwise puzzling negative solvatochromism exhibited in this family of compounds. Combining TDCP and Stark results, we find that the dipole moment can be rotated in some instances by more than 90° upon ³MLCT excited-state formation. The degree of rotation or reorientation can be modulated by changing the identity of the acceptor ligand substituents. Reorientational effects are smallest when the compounds feature aromatic substituents capable of spatially extending the π system of the acceptor ligand.

Full text of DOI:10.1021/ic011015g

Inorg. Chem. 2002, 41, 2909−2919
Experimental Studies of Light-Induced Charge Transfer and Charge
I
Redistribution in (X -Bipyridine)Re (CO)3Cl Complexes
2
Keith A. Walters, Young-Jin Kim, and Joseph T. Hupp*
Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois 60208-3113
Received September 28, 2001
Stark emission spectroscopy, transient DC photoconductivity (TDCP), and ground-state dipole moment measurements
I
have been used to evaluate charge transfer (CT) within various (X -bipyridine)Re(CO)3Cl complexes following
2
3MLCT excited-state formation. The Stark technique reports on vector differences between ground-state (µg) and
excited-state (µe) dipole moments, while TDCP, when combined with independently obtained µg information, reports
on scalar differences. For systems featuring collinear, same-signed ground- and excited-state dipole moments, the
scalar and vector differences are equivalent. However, for the low symmetry systems studied here, they are distinctly
different. The vector difference yields the effective adiabatic one-electron-transfer distance (R ), while the combined
12
vector and scalar data yield information about dipole rotation upon ground-state/excited-state interconversion. For
the systems examined, charge transfer distances are substantially smaller than geometric electron-donor/electron-
acceptor site separation distances. The measured distances are significantly affected by changes in acceptor ligand
substituent composition. Electron-donating substituents decrease CT distances, while electron-withdrawing substituents
increase CT distances, as do aromatic substituents that are capable of expanding the bipyridyl ligand (acceptor
ligand) π system. The Stark measurements additionally indicate that the CT vector and the transition dipole moment
are significantly orthogonal, a consequence of strong polarization of the Re−Cl bond (orthogonal to the metal/
acceptor-ligand plane) in the ground electronic state and relaxation of the polarization in the upper state. The
ground-state Re−Cl bond polarization is sufficiently large that the overall ground-state scalar dipole moment exceeds
the overall excited-state scalar dipole moment, despite transfer of an electron from the metal center to the diimine
ligand. This finding provides an explanation for the otherwise puzzling negative solvatochromism exhibited in this
family of compounds. Combining TDCP and Stark results, we find that the dipole moment can be rotated in some
instances by more than 90° upon 3MLCT excited-state formation. The degree of rotation or reorientation can be
modulated by changing the identity of the acceptor ligand substituents. Reorientational effects are smallest when
the compounds feature aromatic substituents capable of spatially extending the π system of the acceptor ligand.
Introduction
the CT distance can report on upper-state charge delocal-
ization (for example, within extended aromatic electron-
acceptor ligands). It can also determine, in large measure,
the degree to which solvent and other environmental factors
couple to an electronic transition. We6-8 and many others3,9-14
have taken advantage of electroabsorption spectroscopy
(Stark spectroscopy) to determine CT distances in inorganic
Long-lived, photochemically generated excited states of
second- and third-row transition metal complexes usually
feature significant triplet character and often are based on
internal charge transfer, for example, metal-to-ligand charge
transfer (MLCT).1 From a fundamental perspective, a key
element of any molecular charge-transfer process is the
adiabatic CT distance, R12.2-5 From a functional perspective,
(2) (a) Oh, D. H.; Boxer, S. G. J. Am. Chem. Soc. 1990, 112, 8161-
8162. (b) Oh, D. H.; Sano, M.; Boxer, S. G. J. Am. Chem. Soc. 1991,
113, 6880-6890.
* Corresponding author. E-mail: jthupp@chem.nwu.edu. Fax: (847) 491-
7713.
(1) (a) Newton, M. D. Chem. ReV. 1991, 91, 761-792. (b) Barbara, P.
F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100, 13148-
13168.
(3) Shin, Y. K.; Brunschwig, B. S.; Creutz, C.; Sutin, N. J. Am. Chem.
Soc. 1995, 117, 8668-8669.
(4) Reimers, J. R.; Hush, N. S. J. Phys. Chem. 1991, 95, 9773-9781.
(5) Cave, R.; Newton, M. D. Chem. Phys. Lett. 1996, 249, 15-19.
10.1021/ic011015g CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/19/2002
Inorganic Chemistry, Vol. 41, No. 11, 2002 2909

Walters et al.
Scheme 1
systems. Briefly, the second-derivative component of an
electroabsorption spectrum yields the excited-state/ground-
state dipole moment difference (vector difference), |∆µv|,
which in turns equals e‚R12, where e is the unit electronic
charge.
While the electroabsorption technique is indeed powerful,
there are circumstances where it is inapplicable. For example,
the desired electronic transition may be obscured by other
more intense transitions, or the excited state of interest may
be a dark state. A case in point is the prototypical inorganic
chromophore (bpy)ReI(CO)3Cl (bpy is 2,2′-bipyridine).15-18
The compound emits from an MLCT excited state (eq 1)
that is nominally triplet in character, making direct excitation
to this state effectively spin-forbidden.
(bpy•-)ReII(CO)3Cl f (bpy)ReI(CO)3Cl + hυem (1)
1
The corresponding MLCT transition is allowed and does
feature significant extinction. However, it overlaps strongly
1
with higher energy bipyridine-localized π-π* transitions,
severely complicating electroabsorption investigations. We
reasoned that CT distance information could more effectively
be obtained by instead examining the reverse process,
We have previously reported in a preliminary fashion on
the TDCP behavior of (bpy)ReI(CO)3Cl (1).8 Like Stark
spectroscopy, TDCP reports on excited-state/ground-state
dipole moment differences. The quantities obtained, however,
are the scalar, rather than Vector, dipole moment differences
(Scheme 1). TDCP further differs from Stark spectroscopy
in that it also yields the sign of the dipole moment difference.
A particularly striking outcome from the preliminary study
of 1 was the observation that the scalar dipole moment of
the “charge separated” excited state was substantially smaller
than the dipole moment of the ground state.8 This finding,
while clearly unusual, provided an explanation for some
otherwise puzzling behavior patterns for 1, including negative
solvatochromism.15,17,18
As described further in this work, by combining these two
techniques, one can move beyond simple CT distance
assessment and begin to examine light-induced charge
redistribution. We present here the results of combined Stark
emission and TDCP studies of an extended family of (bpy)-
ReI(CO)3Cl type complexes (1-7, Scheme 2) featuring a
range of chemically modified bipyridine ligands (electron-
accepting, -donating, or -delocalizing ligands). We were
particularly interested in understanding how expanding the
ligand π system would affect CT distances and charge
redistribution. We were similarly interested in understanding
whether and how ligand substituent properties would modu-
late CT distances and redistribution behavior. We find that
both kinds of perturbations are significant. More generally,
the new study offers insight into the phenomenon of and
consequences of photoexcitation of low symmetry charge-
transfer systems.
3
emission from the MLCT excited state, via electric-field-
effect spectroscopy (Stark emission spectroscopy),19 and by
additionally employing a conceptually different approach:
transient direct current photoconductivity (TDCP).20,21
(6) Karki, L.; Hupp, J. T. Inorg. Chem. 1997, 36, 3318-3321.
(7) (a) Karki, L.; Vance, F. W.; Hupp, J. T.; LeCours, S. M.; Therien, M.
J. J. Am. Chem. Soc. 1998, 120, 2606-2611. (b) Karki, L.; Williams,
R. D.; Hupp, J. T.; Allen, C. B.; Spreer, L. O. Inorg. Chem. 1998, 37,
2837-2840. (c) Vance, F. W.; Hupp, J. T. J. Am. Chem. Soc. 1999,
121, 4047-4053.
(8) Vanhelmont, F. W. M.; Hupp, J. T. Inorg. Chem. 2000, 39, 1817-
1819.
(9) For a recent review on Stark-effect spectroscopy, see: Bublitz, G.
U.; Boxer, S. G. Annu. ReV. Phys. Chem. 1997, 48, 213-242.
(10) For a review of Stark-effect spectroscopy of metal complexes, see:
Vance, F. W.; Williams, R. D.; Hupp, J. T. Int. ReV. Phys. Chem.
1998, 17, 307-329.
(11) (a) Locknar, S. A.; Peteanu, L. A. J. Phys. Chem. B 1998, 102, 4240-
4246. (b) Chowdury, A. C.; Locknar, S. A.; Premvardhan, L. L.;
Peteanu, L. A. J. Phys. Chem. A 1999, 103, 9614-9625. (c)
Premvardhan, L. L.; Peteanu, L. A. J. Phys. Chem. A 1999, 103, 7506-
7514.
(12) Walters, K. A.; Premvardhan, L. L.; Liu, Y.; Peteanu, L. A.; Schanze,
K. S. Chem. Phys. Lett. 2001, 339, 255-262.
(13) Shin, Y. K.; Brunschwig, B. S.; Creutz, C.; Sutin, N. J. Phys. Chem.
1996, 100, 8157-8169.
(14) Brunschwig, B. S.; Creutz, C.; Sutin, N. Coord. Chem. ReV. 1998,
177, 61-79.
(15) Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998-1003.
(16) Wrighton, M. S.; Morse, D. L.; Pdungsap, L. J. Am. Chem. Soc. 1975,
97, 2073-2079.
(17) Giordano, P. J.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101, 2888-
2897.
(18) Worl, L. A.; Duesing, R.; Chen, P.; Della Ciana, L.; Meyer, T. J. J.
Chem. Soc., Dalton Trans. 1991, 849-858.
(19) (a) Lockhart, D. J.; Hammes, S. L.; Franzen, S.; Boxer, S. G. J. Phys.
Chem. 1991, 95, 2217-2226. (b) Baumann, W.; Bischof, H. J. Mol.
Struct. 1985, 129, 125-136. (c) Baumann, W.; Spohr, E.; Bischof,
H.; Liptay, W. J. Lumin. 1987, 37, 227-233. (d) Lockhart, D. J.;
Boxer, S. G. Chem. Phys. Lett. 1988, 144, 243-250. (e) Lockhart, D.
J.; Goldstein, R. F.; Boxer, S. G. J. Chem. Phys. 1988, 89, 1408-
1415.
Experimental Section
Materials and Characterization. All reagents and materials
were used as received from Aldrich or Fluka. Tetrahydrofuran
(THF) was distilled from sodium metal/benzophenone. Triethyl-
(20) Smirnov, S. N.; Braun, C. L. J. Phys. Chem. 1994, 98, 1953-1961.
(21) Smirnov, S. N.; Braun, C. L. ReV. Sci. Instrum. 1998, 69, 2875-
2887.
2910 Inorganic Chemistry, Vol. 41, No. 11, 2002

(X2-Bipyridine)ReI(CO)3Cl Complexes
Scheme 2
amine and toluene were distilled prior to use. Re(CO)5Cl was
purchased from Aldrich. The ligands 2,2′-bipyridine (bpy), 4,4′-
dimethyl-2,2′-bipyridine (dMe-bpy), and 4,4′-diphenyl-2,2′-bipyr-
idine (dφ-bpy) were also purchased from Aldrich, along with the
substituted aryl halides 4-bromobiphenyl, 1-bromo-4-nitrobenzene,
and 4-iodoaniline. 4,4′-Dibromo-2,2′-bipyridine (dBr-bpy) was
prepared according to previously published procedures,22 along with
the rhenium complexes (bpy)Re(CO)3Cl (1), (dMe-bpy)Re(CO)3Cl
(2), (dBr-bpy)Re(CO)3Cl (3), and (dφ-bpy)Re(CO)3Cl (4).18 All
syntheses were performed under argon unless otherwise noted. 1H
and 13C NMR spectra were recorded on a Varian Mercury-400 MHz
spectrometer. Mass spectrometry (MS) measurements used standard
FAB conditions for ligands and metal complexes. All spectra agreed
with appropriate simulations. Elemental analyses were performed
by ORS, Oneida, NY.
Synthesis. para-Substituted Ethynylbenzene Compounds.
These compounds were prepared by modification of previously
published procedures.23 To a mixture of trimethylsilylacetylene (2.36
g, 24 mmol) and substituted aryl halide (20 mmol) in dry
triethylamine (80 mL) bis(Triphenylphosphine)palladium dichloride
(0.28 g, 0.4 mmol) and copper(I) iodide (0.02 g, 0.2 mmol) were
added. The reaction mixture was stirred at room temperature for 6
h under nitrogen followed by solvent evaporation. The residue was
dissolved in ethanol, aqueous potassium hydroxide (10 mL, 1.0
M) was added, and the mixture was stirred at room temperature
for 2 h. After solvent removal, the residue was extracted with
chloroform and washed with water. The chloroform was finally
removed by rotary evaporation. The crude products were recrystal-
lized from ethanol.
1H), 7.39 (d, 2H, J ) 7.2 Hz), 7.47 (d, 2H, J ) 8.4 Hz), 7.58 (m,
5H) ppm. MS (FAB): m/z ) 178 [M+].
para-nitroethynylbenzene. Substituted aryl halide: 1-bromo-
4-nitrobenzene. Yield: 2.3 g (78%). 1H NMR (CDCl3): δ ) 3.37
(s, 1H), 7.65 (d, 2H, J ) 12.0 Hz), 8.21 (d, 2H, J ) 12.0 Hz) ppm.
MS (FAB): m/z ) 147 [M+].
para-aminoethynylbenzene. Substituted aryl halide: 4-iodo-
aniline. Yield: 1.2 g (51%). 1H NMR (CDCl3): δ ) 2.97 (s, 1H),
6.61 (d, 2H, J ) 11.2 Hz), 7.31 (d, 2H, J ) 11.6 Hz) ppm. MS
(FAB): m/z ) 117 [M+].
4,4′-diarylethynyl-2,2′-bipyridine Compounds. A solution of
4,4′-dibromo-2,2′-bipyridine (1.2 g, 3.82 mmol), a para-substituted-
ethynylbenzene (8.0 mmol), bis(triphenylphosphine)palladium dichlo-
ride (0.28 g, 0.4 mmol), copper(I) iodide (0.02 g, 0.2 mmol), and
triethylamine (15 mL) in dry THF (40 mL), was refluxed for 16 h.
After this period, the reaction mixture was cooled and the solvent
removed by rotary evaporation. The residue was dissolved in
methylene chloride and washed with water. The organic extract
was dried over anhydrous sodium sulfate, and the solvent was
removed by rotary evaporation. The crude product was purified by
flash column chromatography (silica gel, eluent CH2Cl2/hexanes/
MeOH 6:3:1) and recrystallized from warm ethanol to yield the
final product.
4,4′-di(phenylethynylbenzene)-2,2′-bipyridine (DPE-bpy).
Yield: 0.95 g (49%). 1H NMR (CDCl3): δ ) 7.54 (d, 4H, J ) 4.8
Hz), 7.65 (d, 4H, J ) 4.8 Hz), 8.55 (d, 2H, J ) 5.6 Hz), 8.68 (s,
6H), 8.93 (d, 2H, J ) 5.6 Hz), 9.18 (d, 2H, J ) 4.8 Hz) ppm.
Anal. Calcd for C38H24N2: C, 89.12; H, 4.69; N, 5.47. Found: C,
89.15; H, 4.51; N, 5.42. MS (FAB): m/z ) 510 [M+].
4,4′-di(nitroethynylbenzene)-2,2′-bipyridine (DNE-bpy). Yield:
para-phenylethynylbenzene. Substituted aryl halide: 4-bromo-
1
1
biphenyl. Yield: 1.89 g (53%). H NMR (CDCl3): δ ) 3.14 (s,
0.97 g (57%). H NMR (CDCl3): δ ) 7.47 (s, 2H), 7.58 (d, 4H,
J ) 8.4 Hz), 7.67 (d, 2H, J ) 8.0 Hz), 7.74 (d, 2H, J ) 8.0 Hz),
8.28 (d, 2H, J ) 8.4 Hz) ppm. Anal. Calcd for C26H14N4O4: C,
69.97; H, 3.14; N, 12.56. Found: C, 69.82; H, 3.07; N, 12.52. MS
(FAB): m/z ) 445 [M+].
(22) Maerker, G.; Case, F. C. J. Am. Chem. Soc. 1958, 80, 2745-2748.
(23) (a) Allen, A. D.; Cook, C. D. Can. J. Chem. 1963, 41, 1084-1087.
(b) Lavastre, O.; Ollivier, L.; Dixneuf, P. H.; Sibandhit, S. Tetrahedron
1996, 52, 5495-5504.
Inorganic Chemistry, Vol. 41, No. 11, 2002 2911

Walters et al.
4,4′-di(aminoethynylbenzene)-2,2′-bipyridine (DAE-bpy).
Yield: 2.5 g (32%). 1H NMR (CDCl3): δ ) 6.68 (d, 4H, J ) 8.4
Hz), 7.43 (d, 4H, J ) 8.4 Hz), 7.68 (d, 2H, J ) 5.0 Hz), 8.33 (s,
2H), 8.92 (d, 2H, J ) 5.6 Hz) ppm. Anal. Calcd for C26H28N4: C,
78.79; H, 7.09; N, 14.14. Found: C, 78.62; H, 7.16; N, 14.11. MS
(FAB): m/z ) 387 [M+].
fac-(4,4′-X2-2,2′-bipyridine)Re(CO)3Cl Complexes. (4,4′-X2-
bipyridine)Re(CO)3Cl complexes were synthesized via a general
procedure modified from ref 18. A solution of Re(CO)5Cl (0.103
g, 0.285 mmol) and the bipyridine ligand (0.285 mmol) in 20 mL
of dry toluene was refluxed for 1.5 h. After the mixture cooled to
room temperature,24 the (diimine)Re(CO)3Cl precipitated, and the
yellow or orange product was collected by filtration on a sintered
glass filter and dried.
(DAE-bpy)Re(CO)3Cl (5). Yield: 0.16 g (87%). 1H NMR (CD3-
COCD3): δ ) 6.73 (d, 4H, J ) 7.6 Hz), 7.37 (d, 4H, J ) 8.4 Hz),
7.72 (d, 2H, J ) 5.2 Hz), 8.00 (s, 2H), 8.96 (d, 2H, J ) 5.2 Hz)
ppm. 13C NMR (CD3COCD3): δ ) 179, 172, 157, 154, 150, 134,
132, 129, 115, 108, 83, 72, 48 ppm. Anal. Calcd for C29H18N4O3-
ClRe: C, 50.33; H, 2.60; N, 8.10. Found: C, 49.86; H, 2.84; N,
8.03. MS (FAB): m/z ) 693.4 [M+], 658 [M+ - Cl].
solution through a 0.22 µm Teflon filter, and drop casting a film
in an aluminum dish that was dried overnight. Film thicknesses
were determined by measuring the spacing of interference peaks
in IR spectra of the films and were typically 180-200 µm.6 Prior
to measurement, a 2 × 2 cm square of the film was pressed between
two ITO-coated pieces of glass with spring clips and heated in a
drying oven (∼50 °C) for 20 min to evaporate any residual
dichloroethylene and ensure a good electrical contact. Samples were
placed in a home-built liquid nitrogen immersion dewar, and an
electric field was generated with a Joseph Rolfe Associates Model
1100 AC power supply (typical fields were 2 × 107 V m-1).
Samples were excited at their MLCT absorption maximum. Emitted
light was measured through a horizontal polarizer in the Fluorolog
spectrophotometer using front-face acquisition geometry. The
emission spectrum and change-in-emission spectrum were simul-
taneously recorded on a Stanford Research Systems SR 850 digital
lock-in amplifier at twice the AC field modulation frequency (200
Hz). Spectra were recorded at angles (ø) of 90° and 62.5° between
the light propagation vector and electric field.27
Analysis of the data obtained is done in a fashion closely
analogous to Stark absorption6 and is only briefly summarized here.
The Stark emission data are fit to a linear combination of the zeroth,
first, and second derivatives of the luminescence spectrum F(υ):
(DNE-bpy)Re(CO)3Cl (6). Yield: 0.12 g (84%). 1H NMR
(DMSO-d6): δ ) 7.03 (d, 4H, J ) 8.0 Hz), 7.09 (d, 2H, J ) 5.2
Hz), 7.43 (d, 4H, J ) 8.0 Hz), 8.20 (d, 2H, J ) 12.0 Hz), 8.28 (s,
2H) ppm. 13C NMR (DMSO-d6): δ ) 186, 175, 140, 136, 133,
128, 125, 124, 117, 87, 60, 48, 41 ppm. Anal. Calcd for C29H14N4O7-
ClRe: C, 46.32; H, 1.86; N, 7.45. Found: C, 46.11; H, 1.98; N,
7.36. MS (FAB): m/z ) 752.7 [M+], 717.1 [M+ - Cl].
x
2
2∆F(υ)
Fmax
)
υ3d[F(υ)/υ3]
dυ
υ3d2[F(υ)/υ3]
dυ2
Bø
Cø
30h2c2
2
AøF(υ) +
+
Eint (2)
{
}
15hc
(DPE-bpy)Re(CO)3Cl (7). Yield: 0.18 g (79%). 1H NMR
(DMSO-d6): δ ) 6.19 (d, 4H, J ) 5.0 Hz), 6.28 (d, 4H, J ) 5.0
Hz), 6.83 (d, 2H, J ) 8.0 Hz), 7.06 (s, 6H), 7.88 (s, 2H), 8.25 (d,
2H, J ) 8.0 Hz) ppm. 13C NMR (DMSO-d6): δ ) 174, 171, 164,
163, 156, 152, 150, 147, 143, 142, 140, 136, 134, 133, 129, 127,
125, 123, 116, 108 ppm. Anal. Calcd for C41H24N2O3ClRe: C, 60.5;
H, 2.95; N, 3.44. Found: C, 59.7; H, 2.43; N, 3.12. MS (FAB):
m/z ) 815.1 [M+], 779.2 [M+ - Cl].
Conventional Photophysical Measurements. UV-vis spectra
were measured for CH2Cl2 solutions with a HP 8452A diode array
spectrophotometer. IR data (KBr pellets) were collected on a Biorad
FTIR spectrophotometer. Steady-state fluorescence spectra were
obtained using an ISA Fluorolog Model FL3-11 spectrophotometer.
Luminescence lifetime data were obtained with a Photon Technolo-
gies International Timemaster stroboscopic detection instrument
with a gated nitrogen lamp (337 nm excitation) using a scatter
solution to profile the instrument response function. Lifetimes were
deconvoluted using an iterative reconvolution procedure. The
luminescence lifetime of (DAE-bpy)ReI(CO)3Cl was separately
obtained using a previously described setup.25 Transient absorption
lifetimes (where specified) were obtained using 355 nm excitation
and a setup profiled earlier.26 Single exponential decays were
obtained in all cases.
In eq 2, ∆F(υ) is the frequency-dependent emission change resulting
from the electric field modulation, Fmax is the intensity maximum
of F(υ), h is Planck’s constant, c is the speed of light in a vacuum,
υ is the frequency of the emitted light, and Eint is the internal electric
field experienced by the chromophore.28 The coefficients Aø, Bø,
and Cø provide information about the changes in the transition
dipole, polarizability, and dipole moment, respectively, and are
described as follows:
rm
3
Aø )
+ 1/30(3 cos2 ø - 1)[3 âm - 2 rm
]
(3)
Bø ) 5/2Tr∆R + (3 cos2 ø - 1)(3/2gˆ‚∆R‚gˆ - 1/2Tr ∆R) (4)
2
Cø ) |∆µv| [5 + (3 cos2 ê - 1)(3 cos2 ø - 1)]
(5)
In these equations, rm and âm are the scalar portions of the
transition moment polarizability and hyperpolarizability tensors,
Tr∆R is the trace of the polarizability change between the ground
and excited electronic states, gˆ‚∆R‚gˆ is the polarizability change
along the transition moment (gˆ is the unit vector), ∆µv is the vector
change in dipole moment, and ê is the angle between the transition
dipole moment and ∆µv vector.6 Three film samples were prepared
and measured for each complex, and the resulting calculated
parameters were averaged.
TDCP Measurements. All measurements were performed with
the same instrumentation as described earlier,8,25 with the exception
that a flow cell was used (∼8 mL min-1 flow rate) to minimize
sample photodegredation and reduce the potential for irreversible
electrochemistry to degrade the sample. A 1000 V potential was
Stark Emission Measurements. Stark emission measurements
were conducted on poly(methyl methacrylate) (PMMA) thin films
of the rhenium complexes. The films were prepared by dissolving
the analyte complex in a dichloroethylene solution containing
PMMA (Aldrich, Mw ≈ 996 kD, 0.75 g/15 mL), filtering the
(24) In the case of (DNE-bpy)Re(CO)3Cl, the reaction mixture was filtered
hot to remove any unreacted ligand. The product was subsequently
precipitated by slowly adding hexanes.
(25) Vanhelmont, F. W. M.; Johnson, R. C.; Hupp, J. T. Inorg. Chem.
2000, 39, 1814-1816.
(26) (a) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R.
J. Am. Chem. Soc. 1996, 118, 6767-6777. (b) Gaal, D. A.; Hupp, J.
T. J. Am. Chem. Soc. 2000, 122, 10956-10963.
(27) The second angle takes into account the refraction of the incident
radiation through the ITO cell; see ref 11b.
(28) Eint ) f‚Eexternal; f(PMMA) has been estimated as 1.11 by: Ponder,
M.; Mathies, R. J. Phys. Chem. 1983, 87, 5090-5098.
2912 Inorganic Chemistry, Vol. 41, No. 11, 2002

(X2-Bipyridine)ReI(CO)3Cl Complexes
Table 1. (X2-bpy)ReI(CO)3Cl Photophysical Characterizationa
applied over the 0.46 mm electrode gap during the experiments.
All complexes were studied in saturated solutions of 1:1 chloroform/
toluene that were prepared by dissolving the samples in chloroform,
filtering through a 0.22 µm Teflon filter, and diluting the filtrates
dropwise with equal quantities of toluene. All solutions were argon
bubble deoxygenated for 30 min prior to study and excited with
the 355 nm third harmonic of a Quantel Brilliant Nd:YAG laser
(10 Hz, 4 ns fwhm). Laser energies of ∼150-250 µJ pulse-1 were
typically used to irradiate the samples. Solution optical densities
were between 0.5 and 1.0 at the excitation wavelength.
λ
max/nm
λem/nm
(τem/ns)
X
(ꢀmax/103 M-1 cm-1
)
υ(CtO)/cm-1
1
2
3
4
5
6
7
H
CH3
Br
388 (2.1)
378 (3.4)
408 (2.7)
396 (9.9)
408 (18)
422 (8.4)
410 (7.1)
598 (39b)
589 (49b)
657 (8)
2023, 1917, 1900b
2022, 1915, 1897b
2019, 1935, 1871
2019, 1918, 1875b
2021, 1911, 1894
2021, 1916, 1897
2020, 1935, 1872
φ
610 (56b)
628 (54)
653 (70c)
640 (12c)
DAE
DNE
DPE
a All measurements are at room temperature in CH2Cl2 except for
The methodology of TDCP data analysis has been reported
previously,8,25 but the basic ideas are summarized here. The
υ(CtO), which is in KBr. Errors for τem are (10%. b Reference 18.
c Lifetime determined by transient absorption measurements.
mathematical theory of TDCP stems from the polarization of a
29
solution (mole mL-1), kb is Boltzmann’s constant (1.38066 × 10-16
erg K-1), and ꢀ0 ) 1/4π. Dielectric constant measurements were
conducted with a Dielectric Products Company (Watertown, MA)
Model 350G three-electrode liquid dielectric cell33 and a GenRad
GR1658 Digibridge digital impedance meter. Conductivity and
dispersion values of 1:1 chloroform/toluene solutions of the
complexes34 were determined using eqs 10 and 1135 with a 1 kHz
test frequency and averaged over 10 data acquisition cycles.
single analyte system in the probed solution, Psolute
:
µi2Ec
Psolute ) n
i3kbT
(6)
In eq 6, ni is the solute dipole concentration, µi is the solute dipole
moment, Ec is the internal electric field, kb is Boltzmann’s constant,
and T is temperature. If the temporal evolution of Psolute concomitant
with photoexcitation is considered, one obtains
Cp
ꢀ )
(10)
(11)
Ec
C0
2
∆Psolute(t) ) ne(t)(µe2 - µg )
(7)
3kbT
Cx
1 + D2
Cp )
In this equation, ne(t) is the time-dependent excited-state dipole
population. Note that the dipole moment difference is reflected as
a difference of squares, thus requiring independent determination
of µg to calculate charge transfer distances. The evolving transient
current, ν(t), can be related to ∆Psolute(t) by the following expression:
In the equations, C0 is the measured air capacitance of the cell (33
pF), Cx is the measured sample solution capacitance, and D is the
measured sample solution dispersion (typically ≈ 0). At least three
sample concentrations in the concentration range 2-5 × 10-4
M
2
æRV0µe2 - µg dne
d
were measured for each complex, producing a linear plot based on
dν(t)
dt
ν(t) + τRC
)
(8)
eq 9 that was used to extract µg,exp
.
3kbT dt
Calculations. A geometry optimization was performed on each
of the rhenium complexes using the PC SPARTAN Plus software
package. The final molecular coordinates from the optimization
were used for a single point ZINDO-1 CI calculation in Hyperchem
5.11. Parameters for manganese and chlorine were used as needed
in place of rhenium and bromine, respectively, because semiem-
pirical parameters were not available for these atoms.
In eq 8, τRC is the circuit RC time constant, æ is a solvent-dependent
correction parameter that varies between 1.3 and 1.9,20,30 d is the
distance between electrodes in the TDCP cell, R is the resistance
that the measured voltage is taken across (50 Ω), and V0 is the
applied DC voltage. Five measurements were recorded on each
sample solution, deconvoluted on the basis of eq 8 to obtain the
so-called “effective” scalar dipole moment change (∆µs,eff
)
Results
2
2
|µ -µ |), and averaged to obtain the reported value.
x
e
g
Synthesis and Photophysical Characterization. A series
of (X2-bpy)Re(CO)3Cl complexes 1-7 (Scheme 2) was
synthesized and characterized. Complexes 1, 2, and 4 were
previously investigated by Meyer and co-workers, and our
photophysical results duplicate their observations.18,36 Spec-
tral characterization data, including IR peaks in the CtO
stretch region, UV-vis absorption maxima, and lumines-
cence maxima and lifetimes, are listed in Table 1. All
absorption spectra show intense UV absorptions due to
Ground-State Dipole Measurements. Ground-state dipole mo-
ments were obtained from solution conductivity measurements via
eq 9:31
2
η2 - 1
4πNAµg C
ꢀ - 1
-
)
(9)
(
ꢀ + 2) (η2 + 2
)
9kbTꢀ0
where ꢀ is the sample solution dielectric constant, η is the sample
solution refractive index,32 NA is 6.02 × 1023 molecules mol-1, µg
is the analyte molecule ground-state dipole moment in esu‚cm (1
D ) 1 × 10-18 esu‚cm), C is the analyte concentration in the sample
(33) (a) D 150-87: Standard Test Methods for A-C Loss Characteristics
and Permittivity (Dielectric Constant) of Solid Electrical Insulating
Materials. American Society for Testing and Materials, 1987. (b) D
924-92: Standard Test Method for Dissipation Factor (or Power
Factor) and Relative Permittivity (Dielectric Constant) of Electrical
Insulating Liquids. American Society for Testing and Materials, 1992.
(34) Ground-state dipole moment measurements on complexes 2, 4, and 7
were performed in chloroform solutions because of the limited
solubilities of these complexes in 1:1 chloroform/toluene.
(29) Debeye, P. Polar Molecules; Dover: New York, 1929.
(30) Smirnov, S. N.; Braun, C. L.; Greenfield, S. R.; Svec, W. A.;
Wasielewski, M. R. J. Phys. Chem. 1996, 100, 12329-12336.
(31) (a) Janini, G. M.; Katrib, A. H. J. Chem. Educ. 1983, 60, 1087-
1088. (b) Chen, C. T.; Liao, S. Y.; Lin, K. J.; Chen, C. H.; Lin, T. Y.
J. Inorg. Chem. 1999, 38, 2734-2741. (c) Kott, K. L.; Whitaker, C.
M.; McMahon, R. J. Chem. Mater. 1995, 7, 426-439.
(32) In eq 6, the refractive index of the solvent without analyte is typically
used as η since the value does not significantly change after analyte
addition.
(35) Breitung, E. M.; Vaughan, W. E.; McMahon, R. J. ReV. Sci. Instrum.
2000, 71, 224-227.
(36) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952-957.
Inorganic Chemistry, Vol. 41, No. 11, 2002 2913

Walters et al.
bipyridine-centered 1π-π* transitions. In each case, a
separate band or shoulder at lower energy (the reported
maximum) is observed that is strongly solvatochromic.
Consistent with the behavior of other (diimine)ReI(CO)3Cl
and ReI(CO)3Cl(pyridine)2 complexes,15,17,18 negative solva-
tochromic effects are observed (i.e., absorption bands shift
to higher energy with increasing solvent polarity). These
observations lead to the assignment of the shoulders as
rhenium-to-bipyridine charge-transfer transitions. Three strong
stretches are observed in the CtO stretch region in each of
the IR spectra, consistent with a facial orientation of the
carbonyl ligands.18
Before proceeding further, a brief comment on the
electron-donating, -withdrawing, and/or -delocalizing nature
of each substituted bipyridine is in order. For the “simple”
bipyridines, the order of increasing electron-withdrawing
ability is 2 < 1 < 3. Relative to 1, the phenyl-substituted
bipyridine complex (4) exhibits both electron-withdrawing
and -delocalizing effects. The addition of ethynyl spacers is
anticipated to amplify both of these effects. Complex 6 is
clearly more strongly withdrawing than 5, but the strength
of 5 relative to 4 is not clear. The available IR data for 7,
especially for the middle-frequency CO ligand, suggest that
the substituents for this compound are strongly electron
withdrawing.18
The solution-phase MLCT absorption is highest in energy
when X is an electron-donating group (378 nm for 2) and
lowest when X is an electron-withdrawing group (408 nm
for 3). The same trend is observed in the emission band
maxima (589 and 657 nm, respectively, for the two com-
plexes). The maxima are blue-shifted by ∼20 nm in the rigid
low-temperature PMMA environment (see Table 1). The
spectra offer no evidence of chromophore aggregation.37 Like
the excited-state emission energies, excited-state lifetimes
(τ, Table 1), acquired at room temperature in solution, also
display a sensitivity to ligand substituent composition.
Figure 1. Stark emission data of 1 in a PMMA thin film (182.1 µm thick)
at 77 K. (a) Normalized emission spectrum. (b) Second derivative of
emission spectrum. (c) Stark emission signal at ø ) 90° (solid line, × 2)
and ø ) 62.5° (dashed line).
Substitution of aryl groups on the bipyridine ligand
decreases the MLCT absorption (e.g., 396 nm for 4 compared
to 388 nm for 1) and emission (610 and 598 nm, respectively)
energies. These variations likely reflect opposing effects: (1)
an energy decrease (wavelength increase) due to increased
intraligand electron delocalization in the aryl-containing
MLCT excited state and (2) a smaller energy decrease due
to the electron-withdrawing nature of the aryl substituents
(note, in particular, compound 7). The presence of electron-
donating and -withdrawing groups in 5 and 6, respectively,
further contributes to their photophysical characteristics via
combinations of the trends described previously.
shown in Figure 1. The top panel shows the normalized
emission spectrum, and its second derivative is shown in
the middle panel. The Stark emission signals for two values
of ø are shown in the bottom panel. Note the similar band
shapes in the second and third panels, implying a significant
Cø term, and consequently |∆µv|, contribution to the Stark
spectrum (eq 5). Further illustration of the contributions of
each derivative component of eq 2 is shown in Figure 2,
again indicating a significant second derivative component.
The values for |∆µv|, Tr∆R, gˆ‚∆R‚gˆ, and ê resulting from
fits to eqs 2-5 are listed in Table 2.
For the series of compounds, a few trends are clearly
evident. Electron-donating substituents on either the simple
bipyridine ligand (2) or the phenyl-ethynyl bipyridine ligand
(5) decrease |∆µv|, while electron-withdrawing substituents
(3 and 6) increase its value. The addition of phenyl rings
alone (4) does not result in a dramatic change in |∆µv|, but
further expansion of the diimine ligand in 7 does yield an
increase. The polarizability data exhibit a more complex
Stark Emission Measurements. Even at reduced tem-
3
peratures, (X2-bpy)ReI(CO)3Cl MLCT phosphorescence is
weak, resulting in comparatively noisy Stark emission
spectra. Consequently, only the portion of the spectrum
immediately around the phosphorescence band was employed
in data fits. A representative Stark emission study for 1 is
(37) Walters, K. A.; Ley, K. D.; Schanze, K. S. Langmuir 1999, 15, 5676-
5680.
2914 Inorganic Chemistry, Vol. 41, No. 11, 2002

(X2-Bipyridine)ReI(CO)3Cl Complexes
Figure 3. Representative TDCP signal (lines) and fit based on eq 8 (dots)
of complex 3. Data from 1:1 chloroform/toluene solution.
Table 3. (X2-bpy)ReI(CO)3Cl TDCP Dataa
b
c
d
g
i
∆µs,eff
D
/
µg,calcd
D
/
µg,exp
D
/
µe / ∆µsh/ |∆µv
/
Figure 2. Stark emission deconvolution components of 1 in a PMMA
thin film (182.1 µm thick, ø ) 62.5°) at 77 K. The solid line is experimental
data, and the deconvolution fit is shown as circles. The contributions of
the zeroth, first, and second derivatives to the fit (eq 2) are indicated by
the dashed, dot-dashed, and dot-dot-dashed lines, respectively.
X
D
D
D
θ
∆µj/°
1
2
3
4
5
6
7
8
bpy
-7.0
-8.8
-7.8
-8.0
-7.5
-4.5
-8.3
12
13
12
13
14
13
13
12
7.7
3.0 -4.7
4.0 -5.6
9.0
7.4
110
64
120
60
40
75
Me
Br
φ
9.7e
9.0
4.7 -4.4 12
11e
7.7 -3.3
13 -2.0
11 -0.9 14
3.7 -5.4 12
9.5
9.0
Table 2. (X2-bpy)ReI(CO)3Cl Stark Emission Dataa
DAE
DNE
DPE
Mn(bpy)
(15)f
(12)f
b
X
|∆µV| /D (e Å)
Tr∆Rc/Å3
gˆ‚∆R‚gˆd/Å3
êe/°
9.0e
130
6.9
1
2
3
4
5
6
7
H
CH3
Br
9 (1.9)
7.4 (1.6)
12 (2.6)
9.5 (2.0)
9 (1.9)
22
460
300
27
880
240
90
90
90
50
47
49
43
a All measurements performed in 1:1 chloroform/toluene unless otherwise
noted. b Effective scalar dipole moment difference (see footnote 39).
c Ground-state dipole moment determined by semiempirical calculations.
d Ground-state dipole moment determined by dielectric constant measure-
φ
400
430
DAE
DNE
DPE
-220
-140
-290
-1400
-840
-690
14 (3.0)
12 (2.5)
ments. e Chloroform solution. µg,exp value extrapolated from the linear
f
relationship of µg,exp to µg,calcd, see text. g Excited-state dipole moment.
h Scalar dipole moment difference. i Vector dipole moment difference.
j Angle between µg and µe.
a All measurements performed on PMMA thin films (∼180 µm thickness)
at 77 K. Uncertainties in |∆µv| varied between (8 and 11% (standard
deviations from measurements with multiple samples). b Vector dipole
moment difference. c Trace of the polarizability change. d Polarizability
change along the transition moment. e Angle between the transition dipole
moment and the ∆µv vector.
were negative at short and intermediate times after the laser
pulse, indicating smaller values for |µe| than |µg|. Fits of the
TDCP signals using eq 88,21 yielded values for µe2 - µg .
2
pattern. Positive values for both polarizability parameters are
found for complexes 1-4, but negative values are found for
5-7.
The square root of this quantity, ∆µs,eff, is listed in Table
3.39
Estimation of scalar dipole moment changes, ∆µs ) µe -
µg, from the TDCP response requires values for µg. These
values were determined experimentally (µg,exp) via dielectric
constant measurements on solutions with varying complex
concentrations (eqs 9-11). Values for six of the eight
rhenium complexes and one manganese analogue are listed
in Table 3. Notably, the ground-state dipole moment is
measurably smaller for (bpy)MnI(CO)3Cl than (bpy)ReI-
(CO)3Cl.
Unfortunately, because of poor solubility in the low
polarity solvents required for ground-state dipole moment
measurements, values for 5 and 6 could not be obtained via
dielectric constant measurements. Consequently, we resorted
to computational estimates (semiempirical electronic structure
calculations). Because parameters were unavailable for
One parameter that returned unusual results was ê, the
angle between the transition dipole, which in this molecule
runs from the rhenium center towards the bipyridine ligand,
and the ∆µ vector. For complexes 1-3 (i.e., those with
ligands lacking electron-delocalizing substituents), ê was 90°,
suggesting approximate orthogonality between these vectors.
However, complexes featuring electron-delocalizing substit-
uents (4-7) were characterized by significantly smaller ê
values.
TDCP and Ground-State Dipole Moment Measure-
ments. The desire to understand in greater detail the relative
geometries of ground- and excited-state dipole moments
prompted the application of the TDCP technique. Represen-
tative TDCP signals are shown in Figure 3 for complex 3.
Consistent with our earlier studies of 1,8,38 all TDCP signals
2
2
(39) Note that µe - µg is negative for all complexes studied here. The
square root of this difference would be an imaginary number, so the
absolute value of this difference is used to determine ∆µs,eff. The
negative sign is assigned to this value to illustrate the negative
difference in the squares of the dipole moments.
(38) In ref 8, we reported a ∆µs,eff of ∼9 D. This value is higher than the
current value, likely because of slight sample degradation during the
earlier TDCP experiment, an effect that has been mitigated here by
employing a flow cell.
Inorganic Chemistry, Vol. 41, No. 11, 2002 2915

Walters et al.
Figure 4. Experimentally determined ground-state dipole moments (µg,exp) for (X2-bpy)ReI(CO)3Cl complexes 1-7.
rhenium, calculations were performed on manganese ana-
logues of 1-7. The calculated values (µg,calcd) significantly
exceed the experimental values. Nevertheless, with the
exception of 7, a fair correlation exists between µg,calcd (Mn)
and µg,exp (Re). Advantage was taken of this correlation to
extrapolate estimates for µg,exp (Re) for 5 and 6, which are
included in Table 3.
Returning to the calculations, the single most important
structural factor in determining the relative magnitudes of
µg,calcd within the series of tricarbonyl chloro compounds
appeared to be the size of the diimine ligand rather than the
electron-withdrawing or -donating strength of its substituents
(see Figure 4). In other words, the total number of valence
electrons within the ligand and its substituents appears to
be more important than the electronegativities of the atoms
comprising the substituents, at least for the limited range of
substituents examined.
With estimates or measurements of µg in hand, values for
the excited-state dipole moment and the scalar difference in
dipole moment (∆µs) can be obtained from the available
TDCP parameters. These results are listed in Table 3. From
the table, it is clear that electron-donating substituents render
∆µs more negative, while the weak electron-withdrawing
substituents in compound 3 lead to a slightly more positive
value. Note that these substituents exert the opposite effect,
in terms of absolute magnitude, to those observed for vector
dipole moment changes, |∆µv|. Aryl and ethynyl-aryl sub-
stituents exert a more substantial influence upon ∆µs, yielding
less negative values than those recorded for the parent
bipyridine compound 1 and other compounds lacking sub-
stituents capable of extending the ligand π system. Again,
however, 7 deviates from the pattern.
The availability of both scalar (µg and µe) and vector
(|∆µv|) parameters permits the geometric relationship be-
tween ground- and excited-state dipole moments to be
elucidated. The numerical inequivalence of the scalar and
vector dipole moment changes is evidence for the noncol-
linearity of ground- and excited-state dipole moments. More
quantitatively, the angle between µg and µe (θ∆µ, the extent
3
of rotation of the molecular dipole moment upon MLCT
excited-state formation) is given by the following expression:
|∆µv| ) µg2 + µe2 - 2µgµe cos(θ∆µ)
(12)
2
Values for θ∆µ based on eq 12 are listed in Table 3.40 In all
cases, the dipole moments deviate substantially from col-
linearity (θ∆µ ) 0). Notably, however, electron-withdrawing
substitutents increase the dipole angle, while electron-
donating substituents diminish it. Electron-delocalizing sub-
stituents (with the exception of 7) also diminish the angle.
Discussion
(X2-bpy)ReI(CO)3Cl Energetics. Meyer and co-workers
investigated the photophysical properties of a large series
of (4,4′-X2-bipyridine)ReI(CO)3Cl complexes and uncovered
trends that are replicated in this study.18 They noted that, as
one would anticipate, electron-donating substituents increase
the energy of the ligand-based LUMO orbital energy, while
(40) Equation 8 assumes that dipole moments in solution are identical to
those in rigid, polymeric environments. The assumption is probably
least reasonable for compounds 4 and 7. These almost certainly feature
different µe values in the two environments, because of the ability of
pendant phenyl groups, in solution, but not in rigid environments, to
rotate and achieve coplanarity with the coordinated bipyridine radical
anion created by photoexcitation.
2916 Inorganic Chemistry, Vol. 41, No. 11, 2002

(X2-Bipyridine)ReI(CO)3Cl Complexes
Table 4. (X2-bpy)ReI(CO)3Cl µg,calcd Dataa
e‚Å. Values for R12 are listed in Table 2. Consistent with
several previous studies of coordination complexes, internal
charge transfer distances are much shorter than geometric
donor-acceptor separation distances.2,3,13,43 For example,
molecular modeling produces a distance from rhenium to
the center of the phenyl-substituted bipyridine ligand of 4
of 5.2 Å, more than double the effective CT distance returned
by the Stark measurement.
b
c
d
e
µg,calcd
D
/
µg,Cl-M-OC
/
µg,M-Cl
/
µg,M-bpy
/
X
D
D
D
θµg-bpyf/°
1
2
3
4
5
6
7
bpy
Me
Br
11.7
12.5
11.9
12.8
14.3
13.3
13.1
11.2
10.9
11.3
10.5
11.5
11.7
10.6
14.5
14.2
13.1
12.5
13.1
14.4
12.3
3.3
6.2
3.8
7.3
8.5
6.4
7.7
107
120
109
125
127
119
126
φ
DAE
DNE
DPE
One consequence, again noted in previous studies, is that
initial-state/final-state electronic coupling energies (Hab), as
calculated from MLCT oscillator strengths via the two-state
Hush model (eq 14),3,5 are larger than one would anticipate
on the basis of geometric CT distances.5,44 The values
obtained range from ∼4000 cm-1 for 1 to ∼7000 cm-1 for
7, with significant uncertainty attending the estimates because
a All calculations performed on geometry-optimized structures using
ZINDO-1 semiempirical parameters. b Ground-state dipole moment deter-
mined by semiempirical calculations. c Component of µg,calcd aligned with
the Cl-M-OC axis. d Component of µg,calcd associated with the M-Cl
bond. e Component of µg,calcd associated with the M-bpy plane. f Angle
between µg,calcd and the M-bpy plane. Angles over 90° indicate the dipole
is pointing away from the diimine ligand.
1
of overlap of MLCT transitions with other features in the
electron-withdrawing substituents have the opposite effect.
The energy of the rhenium dπ (HOMO) orbital is largely
unaffected by the ligand substituents. These effects account
for the observed sensitivity of 1MLCT absorption and 3MLCT
luminescence energies to substituent electron-donation and
-withdrawal properties. Again following Meyer and co-
workers,18 as well as McCusker et al.,41 the energy-lowering
effects of aryl and aryl-ethynyl substituents are ascribed to
intraligand electronic delocalization in the MLCT excited
state (i.e., expansion of the “π* box” occupied by the
transferred electron).
electronic absorption spectrum. The availability of the cou-
pling parameters permits diabatic charge transfer distances,
Rab, to be calculated from the adiabatic distances:44,45
1/2
ꢀmax∆υ1/2
P12 ) 2.06 × 10-2
(13)
(14)
(15)
[
]
υ
maxb
P12υmax
Hab
)
R12e
R12
Ground-State Dipole Moments. Measured (µg,exp) ground-
Rab
)
1 - 2(Hab2/υmax
)
state dipole moments significantly exceed calculated (µg,calcd
)
2
dipole moments (see Table 3). Comparison of 1 and 8
indicates that a portion of the discrepancy is due to
replacement of rhenium by manganese in the semiempirical
calculations, although substantial discrepancies remain. The
calculations do, however, appear to capture the trends in the
experimental data and thus should prove instructive in
understanding the experimental behavior. The calculations
further indicate that polarization of the metal-chloro bond,
which is approximately orthogonal to the plane of the
coordinated diimine ligand, accounts for a significant fraction
of the total ground-state dipole moment of each compound.
This finding is summarized in Table 4, where calculated
dipole moments have been separated into vector compo-
nents: (a) aligned with the carbonyl-metal-chlorine axis
(µg,Cl-M-OC), (b) associated specifically with the metal-
chloro bond (µg,M-Cl),42 and (c) aligned with the metal-
diimine plane (µg,M-bpy). Also included are calculated µg
angles with respect to the metal-diimine plane (θµg-bpy).
These parameters qualitatively corroborate the experimental
observation that µg increases as the diimine ligand size
increases (Figure 4), while also corroborating the expected
(secondary) influence of electron-withdrawing and electron-
donating properties of the diimine ligand substituents.
Charge Transfer Distance. As noted previously, the
vector dipole moment change can be equated with the
adiabatic charge-transfer distance (R12), where 1 D ) 0.21
In these equations, P12 is the transition dipole moment
(closely related to the oscillator strength), υmax is the band
maximum, ꢀmax is the molar absorptivity at υmax, ∆υ1/2 is
the absorption bandwidth, and b is the degeneracy term (2
in this case). We find in these complexes that the calculated
diabatic distances are marginally greater, ∼0.2 Å, than the
measured adiabatic distances. Following Cave and Newton,5
this difference can be interpreted as the amount that partial
metal charge delocalization onto the ligand in the ground
state, as well as ligand delocalization onto the metal in the
excited state, contributes to the diminution of the effective
CT distance. Clearly, the effect is small.46 It seems likely
that the balance of this effect comes from MLCT excited-
state self-polarization and related phenomena (i.e., Re(II)/
coordinated diimine radical anion Coulombic interactions).14
For the available compounds, the effective CT distance
(or equivalently, |∆µv|) increases with the addition of
electron-withdrawing groups and decreases when electron-
donating groups are incorporated. Substituent electronic
(43) (a) Karki, L.; Lu, H. P.; Hupp, J. T. J. Phys. Chem. 1996, 100, 15637-
15639. (b) Karki, L.; Hupp, J. T. J. Am. Chem. Soc. 1997, 119, 4070-
4073.
(44) Creutz, C.; Newton, M. D.; Sutin, N. J. Photochem. Photobiol. A 1994,
82, 47-59.
(45) Vance, F. W.; Slone, R. V.; Stern, C. L.; Hupp, J. T. Chem. Phys.
2000, 253, 313-322.
(46) The effect may be even smaller than indicated because Hab describes
ground-state mixing with the initially formed and predominantly singlet
MLCT excited state. Mixing with the predominantly triplet, emissive
MLCT state presumably is less, making Hab smaller and reducing the
difference between Rab and R12.
(41) Damrauer, N. H.; Boussie, T. R.; Devenney, M.; McCusker, J. K. J.
Am. Chem. Soc. 1997, 119, 8253-8268.
(42) µg,M-Cl was calculated by multiplying the difference in charges on
the two atoms by their separation distance.
Inorganic Chemistry, Vol. 41, No. 11, 2002 2917

Walters et al.
properties necessarily influence both ground and excited
states. The effects are greater, however, in the MLCT excited
state. The differential effect is manifest as a change in |∆µv|.
The signs of ∆R parameters have sometimes been interpreted
as qualitative indicators of the relative importance of
configuration interactions.
The effects of adding potentially electron-delocalizing
groups are more complex. For example, the added phenyl
rings of 4 do not dramatically increase the CT distance as
measured by Stark emission spectroscopy, presumably
because they lack coplanarity with the bipyridine portion of
the ligand, thus limiting delocalization of the transferred
charge. Indeed, McCusker and co-workers41 showed that the
The measurements presented here involve transitions from
the lowest electronic excited state to the ground state.
Consequently, if sign conventions are preserved, polariz-
ability changes measured by electroemission (∆Rg-e) should
be opposite in sign to those measured by electroabsorption
(∆Re-g). To verify this supposition, we examined the model
compound 4-(dimethylamino)-4′-nitrostilbene and obtained
oppositely signed ∆R parameters from the two techniques.
Returning to the polarizability changes for the rhenium
coordination complexes, one interpretation is that compounds
1-4 feature emissive MLCT excited states that are com-
paratively poorly mixed with upper excited states, while 5-7
feature emissive MLCT excited states that are more exten-
sively mixed. Another interpretation emphasizes a more
intuitive description: An excess electron in the extended π
system of a large ligand, such as those found in MLCT
excited states of 5-7, will be comparatively easy to polarize.
An electron confined to a d orbital on a single atom (Re) in
the electronic ground state should be much less polarizable.
The polarizability change upon excited-state to ground-state
conversion should therefore be negative, consistent with
experiment. The argument becomes less compelling as the
size of the chromophoric ligand decreases (compounds 1-4),
and less negative, or even positive, polarizability changes
are expected.
2+
phenyl rings in Ru(dφ-bpy)3 are canted between 40° and
50° with respect to the bipyridyl plane in the ground state.
While the phenyl groups of 4 likely rotate to a coplanar
conformation in the excited state in liquid environments,41,47
the combination of low temperature (77 K) and immobiliza-
tion of the sample in a polymer matrix precludes this motion
under the conditions of the Stark experiment. This restriction
reduces the ability of the rings to increase |∆µv|. Further
excited-state electronic delocalization likely occurs within
the even larger ligand associated with complex 7. Consistent
with that reasoning, a still larger |∆µv| value is observed.
However, the value is perhaps not as large as expected,
presumably because of the second set of phenyl rings being
partially orthogonal to the remainder of the ligand.
Scalar Dipole Moment Changes. As noted previously,
∆µs values measured by TDCP seemingly respond in
opposite fashion to |∆µv| when electron-withdrawing or
electron-donating substituents are introduced. These appar-
ently contradictory observations are actually self-consistent.
Briefly, µg exceeds µe when scalar quantities are considered
because of the large ground-state polarization of the Re-Cl
bond. The addition of electron-withdrawing substituents will
preferentially increase µe, leading to a smaller absolute scalar
difference between µe and µg. While the difference between
1 and 3 is essentially equivalent to the experimental error,
the proposed electron-withdrawing effect can more easily
be observed when 4 and 6 are compared. The opposite effect
occurs with electron-donating substituents, as observed in
compounds 1 and 2. When vector differences are considered,
the greater influence of electron-donating (or -withdrawing)
substituents upon the excited-state dipole moment decreases
(or increases, respectively) the vector difference. The absolute
value of this difference is the quantity measured by the Stark
emission measurement.
Dipole Moment Positioning. All of the TDCP signals
obtained from the rhenium complexes are negative, indicating
that µe is smaller in absolute magnitude than µg. Indeed,
experiments as well as ZINDO-1 calculations here and in
previous work8 confirm that µg is relatively large (g8 D)
and support the idea that µe < µg even though an electron is
promoted from the central metal to the bipyridine ligand.
This dipole moment reduction upon excited-state formation
provides an attractive explanation for the negative solvato-
chromism typically observed in these complexes.15,17,18
Previously, this behavior has been ascribed to an unusually
strong influence of solvent polarity on the internal molecular
structure.17
A remaining question is why µe is smaller than µg, even
in complexes with large, delocalized bipyridine ligands. The
answer lies in the orientations of the individual dipole
moments. Illustrations of the dipole positioning obtained from
semiemiprical calculations and experimental results for
complexes 1, 3, and 4 are shown in Scheme 3, where NkN
represents the substituted bipyridine ligand. Note that these
dipole moments extend to the center of mass in the molecule,
which is located between the rhenium center and the
bipyridine ligand and varies with bipyridine substitutent
composition.48 The ground-state dipole moment is largely
oriented in the direction of the Re-Cl bond because of its
Polarizability Changes. The observed large polarizability
changes, and especially the differences in the ∆R parameter
sign for 1-4 versus 5-7, are difficult to interpret in any
detail. Brunschwig and co-workers, in their Stark absorption
studies of ruthenium ammine complexes,3,13,14 noted that a
basic two-level electronic treatment yields an excited-state
polarizability that is equal, but opposite in sign, to the
necessarily positive polarizability of the ground state, making
∆Re-g a negative quantity. However, as additional excited
states are incorporated, ∆Re-g is expected to become positive.
(48) The illustration of the dipole moments extending to the center of mass
is a formalism of the Hyperchem calculation, but the calculated
components of µg associated with the varying molecular axes are
correct regardless of the spatial representation of the dipole moment.
(47) (a) Schoonover, J. R.; Chen, P.; Bates, W. D.; Dyer, R. B.; Meyer, T.
J. Inorg. Chem. 1994, 33, 793-797. (b) Chen, P.; Palmer, R. A.;
Meyer, T. J. J. Phys. Chem. A 1998, 102, 3042-3047.
2918 Inorganic Chemistry, Vol. 41, No. 11, 2002

(X2-Bipyridine)ReI(CO)3Cl Complexes
Scheme 3
explanation is that the significant electron-withdrawing nature
of ethynyl-biphenyl substitutents12,49 more than offsets the
θ
∆µ attenuation expected from enhanced excited-state delo-
calization.
Conclusions
A series of (X2-bipyridine)ReI(CO)3Cl complexes, where
X is an electron-donating, -withdrawing, or -delocalizing
substituent, have been synthesized, and their photophysical
properties, including charge transfer properties, have been
evaluated. A complementary pair of techniques, Stark
emission spectroscopy and TDCP, provide information on
both CT distances (vector dipole moment changes) and more
general charge redistribution effects concomitant with MLCT
excitation. Electron-donating substituents increase the MLCT
transition energy and decrease the effective electron transfer
distance, while electron-withdrawing and -delocalizing groups
produce the opposite effect. The noncollinearity of the
ground- and excited-state dipole moments presents an
interesting opportunity to understand in a more detailed way
the mechanics of charge transfer and redistribution in these
complexes through determination of the angle between µg
and µe. The angle is finite in all cases, indicating rotation of
the dipole moment upon 3MLCT state formation. Occurrence
of nonzero angles is indicated by experimentally different
absolute values for vector dipole moment changes (Stark
emission measurements) versus scalar dipole moment changes
(TDCP measurements). The angle between µg and µe is larger
for complexes featuring electron-withdrawing groups than
for complexes featuring either electron-donating groups or
electron-delocalizing groups. These low symmetry com-
pounds are additionally characterized by noncollinearity of
the transition dipole moment and ∆µ vectors.
polarization but is partially rotated from collinearity with
this bond because of the mass and electron density of the
bipyridine ligand. Upon MLCT excitation, the dipole swings
above the Re-Cl bond and closer to the bipyridine ligand
where the photoexcited electron lies (see Scheme 3 for
orientation). However, the resulting excited-state dipole
moment is the sum of polarizations resulting from both
electron promotion and residual Re-Cl bond polarization.
It is this sum of these two differing, noncollinear contribu-
tions that leads to µe being smaller than µg.
Examination of the ligand-composition dependence of θ∆µ
(Table 3) provides further insight into photoinduced dipole
moment reorientation within these complexes. When an
electron-withdrawing substituent is present on the bipyridine
ligand (3), the excited-state dipole experiences more of a
“pull” from the bipyridine, so it rotates closer to the plane
of the bipyridine ligand. Furthermore, on the basis of
calculations, µg remains essentially in the same orientation
with respect to the Re-Cl bond (see Table 4), leading to a
θ∆µ increase (Scheme 3). An electron-donating substituent
exhibits the opposite effect in the sense that its “pull” on
the dipole is decreased.
Introduction of electron-delocalizing substituents (4) causes
a sharp decrease in θ∆µ. This decrease stems from the
dramatic “pull” exhibited on both µg and µe by the signifi-
cantly more delocalizing diimine ligand coupled with a center
of mass shift toward the ligand. Calculations further show
that µg is influenced more than µe by this substitution, leading
to the observed angle decrease. The substitution of electron-
donating and -withdrawing substituents on the phenyl rings
yields the same (qualitative) angle decreases and increases,
respectively, for reasons analogous to those described
previously. In contrast to other arene-substituted diimine
complexes, 7 exhibits a large θ∆µ increase. One possible
Acknowledgment. We thank Dr. Frederik W. M. Van-
helmont for initial TDCP instrument implementation and
preliminary measurements, Prof. Sergei Smirnov (New
Mexico State University) and Prof. Linda Peteanu (Carnegie
Mellon University) for helpful discussions, Prof. Frederick
D. Lewis for the use of the luminescence lifetime instrument,
Dr. Rajdeep Kalgutkar for his help with the luminescence
lifetime measurements, and Dennis A. Gaal for the transient
absorption lifetime measurements. We gratefully acknowl-
edge financial support from the U.S. Department of Energy,
Office of Science (Grant DE-FG02-87ER13808).
IC011015G
(49) (a) Ley, K. D.; Schanze, K. S. Coord. Chem. ReV. 1998, 171, 287-
307. (b) Walters, K. A.; Ley, K. D.; Cavalaheiro, C. S. P.; Miller, S.
E.; Gosztola, D.; Wasielewski, M. R.; Bussandri, A. P.; Van Willigen,
H.; Schanze, K. S. J. Am. Chem. Soc. 2001, 123, 8329-8342.
Inorganic Chemistry, Vol. 41, No. 11, 2002 2919