Synthesis and solution phase characterization of strongly photooxidizing heteroleptic Cr(III) tris-dipyridyl complexes

Article abstract of DOI:10.1021/ic1009972

We report the preparation and characterization of Cr(III) coordination complexes featuring the dimethyl 2,2′-bipyridine-4,4′-dicarboxylate (4-dmcbpy) ligand: [(phen)₂Cr(4-dmcbpy)](OTf)₃ (1), [(Ph₂phen)₂Cr(4-dmcbpy)](OTf)₃ (4), [(Me ₂bpy)₂Cr(4-dmcbpy)](OTf)₃ (7), and [Cr(4-dmcbpy)₃](BF₄)₃ (8), where phen is 1,10-phenanthroline, Ph₂phen is 4,7-diphenyl-1,10-phenanthroline, and Me₂bpy is 4,4′-dimethyl-2,2′-bipyridine. Static and nanosecond time-resolved absorption and emission properties of these complexes dissolved in acidic aqueous (1 M HCl) solutions are reported. Emission spectra collected at 297 K show a narrow spectrum with an emission maximum ranging from 732 nm (1) to 742 nm (4). The emissive state is thermally activated and decays via first order kinetics at all temperatures explored (283 to 353 K). At 297 K the observed lifetime ranges from 7.7 μs (8) to 108 μs (4). The photophysical data suggest that in these acidic aqueous environments these complexes store ～1.7 eV for multiple microseconds at room temperature. Of the heteroleptic species, complex 4 shows the greatest absorption of visible wavelengths (ε = 1270 M^-1 cm^-1 at 491 nm), and homoleptic complex 8 has improved absorption at visible wavelengths over [Cr(bpy)₃]³⁺. The electrochemical properties of 1, 4, 7, and 8 were investigated by cyclic voltammetry. It is found that inclusion of 4-dmcbpy shifts the Cr^III/II E_1/2 by +0.22 V compared to those of homoleptic parent complexes, with the first reduction event occurring at -0.26 V versus Fc⁺/Fc for 8. The electrochemical and photophysical data allow for excited state potentials to be determined: for 8, Cr^III/II lies at +1.44 V versus ferrocenium/ferrocene (～+2 V vs NHE), placing it among the most powerful photooxidants reported.

Full text of DOI:10.1021/ic1009972

Inorg. Chem. 2010, 49, 7981–7991 7981
DOI: 10.1021/ic1009972
Synthesis and Solution Phase Characterization of Strongly Photooxidizing
Heteroleptic Cr(III) Tris-Dipyridyl Complexes
Ashley M. McDaniel,†,§ Huan-Wei Tseng,‡,§ Niels H. Damrauer,*,‡ and Matthew P. Shores*,†
†Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, and
‡Department of Chemistry and Biochemistry, University of Colorado, Boulder,
§
Colorado 80309-0215. These authors contributed equally to this work.
Received May 19, 2010
We report the preparation and characterization of Cr(III) coordination complexes featuring the dimethyl 2,20-bipyridine-
4,40-dicarboxylate (4-dmcbpy) ligand: [(phen)2Cr(4-dmcbpy)](OTf)3 (1), [(Ph2phen)2Cr(4-dmcbpy)](OTf)3 (4),
[(Me2bpy)2Cr(4-dmcbpy)](OTf)3 (7), and [Cr(4-dmcbpy)3](BF4)3 (8), where phen is 1,10-phenanthroline, Ph2phen
is 4,7-diphenyl-1,10-phenanthroline, and Me2bpy is 4,40-dimethyl-2,20-bipyridine. Static and nanosecond time-
resolved absorption and emission properties of these complexes dissolved in acidic aqueous (1 M HCl) solutions
are reported. Emission spectra collected at 297 K show a narrow spectrum with an emission maximum ranging from
732 nm (1) to 742 nm (4). The emissive state is thermally activated and decays via first order kinetics at all
temperatures explored (283 to 353 K). At 297 K the observed lifetime ranges from 7.7 μs (8) to 108 μs (4).
The photophysical data suggest that in these acidic aqueous environments these complexes store ∼1.7 eV for multiple
microseconds at room temperature. Of the heteroleptic species, complex 4 shows the greatest absorption of
visible wavelengths (ε = 1270 M-1 cm-1 at 491 nm), and homoleptic complex 8 has improved absorption at visible
wavelengths over [Cr(bpy)3]3þ. The electrochemical properties of 1, 4, 7, and 8 were investigated by cyclic
voltammetry. It is found that inclusion of 4-dmcbpy shifts the “CrIII/II” E1/2 by þ0.22 V compared to those of homoleptic
parent complexes, with the first reduction event occurring at -0.26 V versus Fcþ/Fc for 8. The electrochemical and
photophysical data allow for excited state potentials to be determined: for 8, CrIII*/II lies at þ1.44 V versus ferrocenium/
ferrocene (∼þ2 V vs NHE), placing it among the most powerful photooxidants reported.
Introduction
conversion, including initiation of catalytic oxidative reactions
critical for water splitting,8-17 photocathodic solar cells (where
A large body of research has clarified the physical and syn-
thetic prerequisites for achieving efficient light-to-electrical
energy conversion in dye-sensitized solar cells (DSSCs) wherein
excited states of inorganic chromophores can inject elec-
trons into wide band gap semiconductors.1-7 Early experi-
mental successes, promising economic factors, and the sheer
magnitude of the scientific issues involved have meant that
other paradigms for dye-sensitization of charge transport
remain relatively unexplored. One such opportunity involves
photoinduced interfacial hole transfer. Optimization of this
paradigm would expose numerous opportunities in solar energy
current runs in the direction opposite to Gratzel cells),18-21 and
€
(8) Gersten, S. W.; Samuels, G. J.; Meyer, T. J. J. Am. Chem. Soc. 1982,
104, 4029–4030.
(9) Lei, Y. B.; Hurst, J. K. Inorg. Chim. Acta 1994, 226, 179–185.
(10) Limburg, J.; Vrettos, J. S.; Liable-Sands, L. M.; Rheingold, A. L.;
Crabtree, R. H.; Brudvig, G. W. Science 1999, 283, 1524–1527.
(11) Wada, T.; Tsuge, K.; Tanaka, K. Angew. Chem., Int. Ed. 2000, 39,
1479–1482.
(12) Sens, C.; Romero, I.; Rodriguez, M.; Llobet, A.; Parella, T.; Benet-
Buchholz, J. J. Am. Chem. Soc. 2004, 126, 7798–7799.
(13) Zong, R.; Thummel, R. P. J. Am. Chem. Soc. 2005, 127, 12802–
12803.
(14) Tseng, H.-W.; Zong, R.; Muckerman, J. T.; Thummel, R. Inorg.
Chem. 2008, 47, 11763–11773.
(15) Kanan, M. W.; Nocera, D. G. Science 2008, 321, 1072–1075.
(16) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S. J. Am.
Chem. Soc. 2008, 130, 210–217.
*To whom correspondence should be addressed. E-mail: niels.damrauer@
colorado.edu (N.H.D.), matthew.shores@colostate.edu (M.P.S.).
€
(1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737–740.
€
(2) Nazeeruddin, M. K.; Gratzel, M. In Photofunctional Transition Metal
(17) Eisenberg, R.; Gray, H. B. Inorg. Chem. 2008, 47, 1697–1699, and
papers published in the forum.
Complexes; Yam, V. W. W., Ed.; Springer: Berlin, 2007; pp 113-175.
(3) Robertson, N. Angew. Chem., Int. Ed. 2006, 45, 2338–2345.
(4) Polo, A. S.; Itokazu, M. K.; Iha, N. Y. M. Coord. Chem. Rev. 2004,
248, 1343–1361.
€
(18) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarstrom, L. J.
Phys. Chem. C 2008, 112, 9530–9537.
(19) Qin, P.; Wiberg, J.; Gibson, E. A.; Linder, M.; Li, L.; Brinck, T.;
Hagfeldt, A.; Albinsson, B.; Sun, L. J. Phys. Chem. C 2010, 114, 4738–4748.
(20) Qin, P.; Linder, M.; Brinck, T.; Boschloo, G.; Hagfeldt, A.; Sun, L.
Adv. Mater. 2009, 21, 2993–2996.
€
(5) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269–277.
(6) Ardo, S.; Meyer, G. J. Chem. Soc. Rev. 2009, 38, 115–164.
(7) Martinson, A. B. F.; Hamann, T. W.; Pellin, M. J.; Hupp, J. T.
Chem.;Eur. J. 2008, 14, 4458–4467.
r
2010 American Chemical Society
Published on Web 08/09/2010
pubs.acs.org/IC

7982 Inorganic Chemistry, Vol. 49, No. 17, 2010
McDaniel et al.
tandem photovoltaic cells,22-24 where both electrodes are
Scheme 1. Target Structures of [(NN)2Cr(4-dmcbpy)]3þ Complexesa
photoactive.25
Despite these promises, relatively little is known about the
physio-chemical factors that must be controlled if photoin-
duced hole injection processes are to be exploited for solar
energy conversion. To our knowledge, there are only a few re-
ports in the literature where this initial photophysical mecha-
nism drives a photocathodic current in a DSSC device.21,28-31
There are only three systems reported where hole injection is
time-resolved and shown to be ultrafast18,32,33 and only three
disclosures where hole transfer participates in a dye-sensitized
heterojunction solar cell.34-36 Finally, only in three reports has
hole injection functioned as one-half of a tandem photovoltaic
cell.23,37,38 The latter of these is the current efficiency record
holder for p-type DSSCs (0.20% overall efficiency). Clearly,
whereas the paucity of results alludes to the significant chal-
lenges involved in this area, it also offers the freedom to explore
new materials and methods for controlling energetics and
carrier-transfer rates.
a The (NN) ligands impart electronic tunability, while 4-dmcbpy
makes possible covalent attachment to semiconductor surfaces.
Searching for molecular sensitizers capable of initiating
excited-state oxidation of wide band gap semiconductors, we
note tris-dipyridyl complexes of Cr(III) as one promising class
of compounds. Serpone and Hoffman studied homoleptic
analogues for solar energy conversion purposes about 25 years
ago.39-45 Parent complexes such as [Cr(bpy)3]3þ or [Cr-
(phen)3]3þ have excited state redox potentials sufficient to
oxidize water to dioxygen if 4e- oxidation could be achieved.
They also have long excited state lifetimes, which should
promote hole injection into an attached semiconductor surface.
Although they absorb visible light ∼50 times more weakly than
[Ru(bpy)3]2þ (at 450 nm),39,46 chromium is several orders of
magnitude more abundant than ruthenium,47 and ligand modi-
fications can improve absorption properties (vide infra).
Heteroleptic polypyridyl complexes of Cr(III) represent
potentially functional model systems, which to our knowledge
have not been studied as components of hybrid materials. Dipy-
ridyl ligands with carboxylate functional groups located at the 4
and 40 positions can serve to anchor the sensitizer to metal oxide
surfaces, as has been demonstrated extensively in Ru(II)-con-
taining analogues.1-7 As discussed in this paper, the electronic
properties of the Cr(III) center can be tuned by judicious choice
of the ancillary dipyridyl-type ligands (NN). Although structu-
rally homologous with Ru(II) complexes, the synthesis of hete-
roleptic Cr(III) dipyridyl complexes is not straightforward, as
efforts to activate the inert metal center often result in ligand
scrambling.48 Nevertheless, a recently disclosed methodology
employing [(NN)2Cr(OTf)2]þ complexes as synthons48-50
shows the way to a new class of molecular species with potential
for efficient hole injection into semiconductor substrates. Here-
in, we describe the preparations as well as electrochemical and
photophysical investigations of a family of structurally related
heteroleptic Cr(III) dipyridyl complexes (Scheme 1). The solu-
tion phase investigation of these compounds demonstrates their
(21) Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.; Sun, L.
J. Am. Chem. Soc. 2008, 130, 8570–8571.
(22) Nattestad, A.; Mozer, A. J.; Fischer, M. K. R.; Cheng, Y. B.; Mishra,
A.; Bauerle, P.; Bach, U. Nat. Mater. 2010, 9, 31–35.
€
(23) He, J.; Lindstrom, H.; Hagfeldt, A.; Lindquist, S.-E. Sol. Energy
Mater. Sol. Cells 2000, 62, 265–273.
(24) Mizoguchi, Y.; Fujihara, S. Electrochem. Solid-State Lett. 2008, 11,
K78–K80.
(25) Thetheoreticalefficiencylimitforthirdgeneration26 photovoltaic tandem
cells, where each of the chromophores absorbs a different portion of the solar
spectrum, is ∼45%.23,27 This compares favorably to the maximum ∼30%
€
efficiency achievable in Gratzel-type cells operating with one active electrode.
(26) Green, M. Third Generation Photovoltaics: Advanced Solar Energy
Conversion; Springer-Verlag: Berlin, 2003.
(27) Hanna, M. C.; Nozik, A. J. J. Appl. Phys. 2006, 100, 074510–074518.
(28) Nattestad, A.; Ferguson, M.; Kerr, R.; Cheng, Y.-B.; Bach, U.
Nanotechnology 2008, 19, 295304.
(29) Mori, S.; Fukuda, S.; Sumikura, S.; Takeda, Y.; Tamaki, Y.; Suzuki,
E.; Abe, T. J. Phys. Chem. C 2008, 112, 16134–16139.
€
(30) Borgstrom, M.; Blart, E.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A.;
€
Hammarstrom, L.; Odobel, F. J. Phys. Chem. B 2005, 109, 22928–22934.
(31) Odobel, F.; Le Pleux, L.; Pellegrin, Y.; Blart, E. Acc. Chem. Res. DOI:
10.1021/ar900275b.
(32) Morandeira, A.; Edvinsson, T.; Le Pleux, L.; Blart, E.; Boschloo, G.;
€
Hagfeldt, A.; Hammarstrom, L.; Odobel, F. J. Phys. Chem. C 2008, 112,
1721–1728.
€
(33) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarstrom, L.
J. Phys. Chem. B 2005, 109, 19403–19410.
(34) O’Regan, B.; Schwartz, D. T. J. Appl. Phys. 1996, 80, 4749–4754.
(35) O’Regan, B.; Lenzmann, F.; Muis, R.; Wienke, J. Chem. Mater.
2002, 14, 5023–5029.
(36) Rusop, M.; Soga, T.; Jimbo, T.; Umeno, M. Surf. Rev. Lett. 2004, 11,
577–583.
(37) Nakasa, A.; Usami, H.; Sumikura, S.; Hasegawa, S.; Koyama, T.;
Suzuki, E. Chem. Lett. 2005, 34, 500–501.
(38) Gibson, E. A.; Smeigh, A. L.; Le Pleux, L.; Fortage, J.; Boschloo, G.;
€
Blart, E.; Pellegrin, Y.; Odobel, F.; Hagfeldt, A.; Hammarstrom, L. Angew.
Chem., Int. Ed. 2009, 48, 4402–4405.
(46) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
Vonzelewsky, A. Coord. Chem. Rev. 1988, 84, 85–277.
(47) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;
Elsevier Butterworth-Heinemann: Oxford, 1997.
(48) Barker, K. D.; Barnett, K. A.; Connell, S. M.; Glaeser, J. W.;
Wallace, A. J.; Wildsmith, J.; Herbert, B. J.; Wheeler, J. F.; Kane-Maguire,
N. A. P. Inorg. Chim. Acta 2001, 316, 41–49.
(49) Isaacs, M.; Sykes, A.; Ronco, S. Inorg. Chim. Acta 2006, 359, 3847–
3854.
(50) Donnay, E. G.; Schaeper, J. P.; Brooksbank, R. D.; Fox, J. L.; Potts,
R. G.; Davidson, R. M.; Wheeler, J. F.; Kane-Maguire, N. A. P. Inorg. Chim.
Acta 2007, 360, 3272–3280.
(39) Serpone, N.; Jamieson, M. A.; Henry, M. S.; Hoffman, M. Z.;
Bolletta, F.; Maestri, M. J. Am. Chem. Soc. 1979, 101, 2907–2916.
(40) Serpone, N.; Jamieson, M. A.; Emmi, S. S.; Fuochi, P. G.; Mulazzani,
Q. G.; Hoffman, M. Z. J. Am. Chem. Soc. 1981, 103, 1091–1098.
(41) Serpone, N.; Jamieson, M. A.; Sriram, R.; Hoffman, M. Z. Inorg.
Chem. 1981, 20, 3983–3988.
(42) Hoffman, M. Z.; Serpone, N. Isr. J. Chem. 1982, 22, 91–97.
(43) Bolletta, F.; Maestri, M.; Moggi, L.; Jamieson, M. A.; Serpone, N.;
Henry, M. S.; Hoffman, M. Z. Inorg. Chem. 1983, 22, 2502–2509.
(44) Serpone, N.; Hoffman, M. Z. J. Chem. Educ. 1983, 60, 853–860.
(45) Jamieson, M. A.; Serpone, N.; Hoffman, M. Z. Coord. Chem. Rev.
1981, 39, 121–179.

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 7983
ability to act as strong photooxidants, and the electronic
flexibility afforded by ligand substitution allows us to explore
fundamental structure/function relationships in our search for
efficient hole transfer to semiconductor substrates.
afford 0.47 g (83%) of product. IR (KBr pellet): νCdN 1625 cm-1
.
ESþMS (CH3CN): m/z 1014.20 ([3 - OTf]þ). The compound was
used in the next synthetic step without further purification or
characterization.
[(Ph2phen)2Cr(4-dmcbpy)](OTf)3 (4). Solid 4-dmcbpy (0.140 g,
0.515 mmol) was added to a solution of 3 (0.209 g, 0.17 mmol) in 30
mL of dichloromethane and heated to reflux. Over 14 days, a yellow
precipitate formed. The solid was isolated by filtration, washed with
dichloromethane (3 ꢀ 10 mL) and collected to afford 0.07 g (27%)
of product. IR (KBr pellet): νCdO 1734 cm-1. μeff (295 K): 3.36 μB.
ESþMS (CH3CN): m/z 329.80 ([4 - 3OTf]3þ), 1285.60 ([4 -
OTf]þ). Anal. Calcd. for C65H44N6CrF9O13S3: C, 54.36; H, 3.09;
N, 5.85. Found: C, 54.12; H, 3.05; N, 5.75.
Experimental Section
Preparation of Compounds. Unless otherwise noted, the syn-
theses of heteroleptic tris-dipyridyl Cr(III) complexes were per-
formed in air with atmospheric moisture excluded by use of a
CaCO3-filled drying tube. For synthetic routes employing Cr(II)
starting materials and for the preparation of [Cr(NN)2(OTf)2]OTf
(OTf=trifluoromethanesulfonate), compound manipulations
were performed either inside a dinitrogen-filled glovebox (MBRA-
UN Labmaster 130) or via Schlenk techniques on an inert gas (N2)
manifold. The commercially obtained ligand 4,40-dimethyl-2,20-bi-
pyridine (Me2bpy) was recrystallized from ethyl acetate before use.
The ligand dimethyl 2,20-bipyridine-4,40-dicarboxylate (4-dmcbpy)
was synthesized according to the literature.51 The preparations of
[(phen)2Cr(OTf)2](OTf), [(bpy)2Cr(OTf)2](OTf), and [Cr(CH3-
CN)4(BF4)2] have been described elsewhere.52,53 The homoleptic
complexes [Cr(NN)3](OTF)3, where (NN) is phen, Ph2phen, or
Me2bpy, were prepared by refluxing [Cr(NN)2(OTf)2]OTf in CH2-
Cl2, with 5 equiv of the same (NN) ligand for 16 h and collecting the
precipitated yellow solids by filtration. The complex [Cr(bpy)3]-
(BF4)3 was prepared analogously to [Cr(4-dmcbpy)3](BF4)3 (8),
using bpy in place of 4-dmcbpy. Electronic absorption spectra,39
electrospray ionization mass spectrometry (ESI-MS), and clean
electrochemical traces confirmed the identity and purity of the
previously reported homoleptic complexes. Pentane was distilled
over sodium metal and subjected to three freeze-pump-thaw
cycles. Other solvents were sparged with dinitrogen, passed over
alumina, and degassed prior to use. All other reagents were ob-
tained from commercial sources and were used without further
purification.
[(Me2bpy)2CrCl2]Cl (5). Solid anhydrous CrCl3 (0.43 g, 2.71
mmol) was added to a solution of Me2bpy (1.00 g, 5.43 mmol) in
60 mL of absolute ethanol. A trace amount (<4 mg) of zinc dust was
added, and the mixture heated to reflux for 1 h, resulting in a green-
brown solution. A gray-green solid precipitated from the reaction
mixture upon cooling. It was collected by filtration, washed with
cold absolute ethanol (3 ꢀ 10 mL), and dried in vacuo to afford
1.12 g (78%) of product. IR (KBr pellet): νCdN 1616 cm-1. ESþMS
(CH3CN): m/z 490.00 ([5 - Cl]þ). The compound was used in the
next synthetic step without further purification or characterization.
[(Me2bpy)2Cr(OTf)2]OTf (6). Under a dinitrogen atmosphere,
trifluoromethanesulfonic acid (2 mL, 22.60 mmol) was added to
solid 5 (0.35 g, 0.67 mmol) to give a red-orange solution. Dinitro-
gen was bubbled through the stirring solution for 24 h, after which
the solution was cooled in an ice bath. Diethyl ether (250 mL) was
slowly added to form a pink precipitate. The solid was isolated
by filtration and rinsed with diethyl ether (3ꢀ50 mL) to afford
0.53 g (92%) of product. IR (KBr pellet): νCdN 1627 cm-1
.
ESþMS (CH3CN): m/z 718.13 ([6 - OTf]þ). The compound was
used in the next synthetic step without further purification or
characterization.
[(Me2bpy)2Cr(4-dmcbpy)](OTf)3 (7). Solid 4-dmcbpy (0.07 g,
0.26 mmol) was added to a solution of 6 (0.20 g, 0.23 mmol) in 30
mL of dichloromethane and heated to reflux. Over 9 days, a light
yellow precipitate was formed. The solid was isolated by filtra-
tion, washed with dichloromethane (3 ꢀ 10 mL) and dried in
vacuo to afford 0.13 g (48%) of product. IR (KBr pellet): νCdO
1739 cm-1. μeff (295 K): 4.01 μB. ESþMS (CH3CN): m/z 230.93
([7 - 3OTf]3þ), 989.60 ([7 - OTf]þ). Anal. Calcd. for C41H36N6-
CrF9O13S3: C, 43.20; H, 3.18; N, 7.37. Found: C, 42.94; H, 3.11;
N, 7.28.
[(phen)2Cr(4-dmcbpy)](OTf)3 (1). Solid 4-dmcbpy (0.71 g,
2.62 mmol) was added to a solution of [(phen)2Cr(OTf)2]OTf
(1.50 g, 1.75 mmol) in 125 mL of dichloromethane and heated to
reflux. Over 5 days, a bright yellow precipitate formed. The solid
was isolated by filtration, washed with dichloromethane (3 ꢀ 30
mL), and dried in vacuo to afford 1.86 g (94%) of product. IR
(KBr pellet): νCdO 1728 cm-1. μeff (295 K): 3.90 μB. ESþMS
(CH3CN): m/z 228.27 ([1 - 3OTf]3þ), 981.67 ([1 - OTf]þ). Anal.
Calcd. for C41H28N6CrF9O13S3: C, 43.51; H, 2.49; N, 7.42.
Found: C, 43.23; H, 2.33; N, 7.27. Crystals suitable for X-ray
analysis were obtained by slow diffusion of diethyl ether into an
acetonitrile solution of the compound.
[Cr(4-dmcbpy)3](BF4)3 (8). Under a dinitrogen atmosphere, a
solution of [Cr(CH3CN)4(BF4)2] (0.13 g, 0.74 mmol) in 4 mL of
acetonitrile was added to a suspension of 4-dmcbpy (0.33 g, 2.44
mmol) in 4 mL of acetonitrile to form a forest green solution.
The solvent was removed in vacuo to afford a forest green solid.
Addition of AgBF4 (0.04 g, 0.19 mmol) to a solution of the
isolated green solid (0.20 g, 0.19 mmol) in 5 mL of acetonitrile
resulted in a yellow solution along with a light gray solid. The
solution was filtered, and the filtrate was treated with 15 mL of
diethyl ether to precipitate a bright yellow solid. The solid was
recrystallized by diethyl ether diffusion into acetonitrile result-
ing in bright yellow crystals. The crystals were collected by
filtration, washed with dichloromethane (3 ꢀ 3 mL) followed by
diethyl ether (3 ꢀ 5 mL) and dried in vacuo to afford 0.10 g
(45%) of product. IR (mineral oil): νCdO 1737 cm-1. μeff (295
K): 4.15 μB. ESþMS (CH3CN): m/z 289.67 ([8 - 3BF4]3þ). Anal.
Calcd. for C42H36N6CrF12B3O12: C, 44.67; H, 3.21; N, 7.44.
Found: C, 44.46; H, 3.08; N, 7.28.
[(Ph2phen)2CrCl2]Cl (2). Solid anhydrous CrCl3 (0.10 g, 0.60
mmol) was added to a suspension of Ph2phen (0.40 g, 1.20
mmol) in 35 mL of absolute ethanol. A trace amount (<2 mg) of
zinc dust was added, and the mixture was heated to reflux for 1
h, resulting in a green-brown mixture. The mixture was filtered,
and an olive green solid was isolated from the filtrate by rotary
evaporation to afford 0.49 g (98%) of product. IR (KBr pellet):
ν
CdN 1620 cm-1. ESþMS (CH3CN): m/z 788.27 ([2 - Cl]þ). The
compound was used in the next synthetic step without further
purification or characterization.
[(Ph2phen)2Cr(OTf)2]OTf (3). Under a dinitrogen atmos-
phere, trifluoromethanesulfonic acid (2 mL, 22.60 mmol) was
slowly added to solid 2 (0.40 g, 0.49 mmol) to give a red-orange
solution. Dinitrogen was bubbled through the stirring solution for
24 h, after which the solution was cooled in an ice bath and 250 mL
of diethyl ether was added. After standing 4 h, a beige-peach
colored solid precipitated from solution. The solid was isolated
by vacuum filtration and rinsed with diethyl ether (3 ꢀ 30 mL) to
X-ray Structure Determination. A suitable crystal of 1
3
1.3CH3CN was coated with Paratone-N oil and supported on
a Cryoloop before being mounted on a Bruker Kappa Apex II
CCD diffractometer under a stream of dinitrogen. Data collec-
tion was performed at 110 K with Mo KR radiation and a
graphite monochromator. Crystallographic data and metric
parameters are presented in Table 1. Data were integrated and
(51) Garelli, N.; Vierling, P. J. Org. Chem. 1992, 16, 3046–3051.
(52) Ryu, C. K.; Endicott, J. F. Inorg. Chem. 1988, 27, 2203–2214.
(53) Henriques, R. T.; Herdtweck, E.; Kuhn, F. E.; Lopes, A. D.; Mink,
J.; Romao, C. C. J. Chem. Soc., Dalton Trans. 1998, 1293–1297.

7984 Inorganic Chemistry, Vol. 49, No. 17, 2010
McDaniel et al.
Table 1. Crystallographic Dataa for [(phen)2Cr(4-dmcbpy)](OTf)3 1.3CH3CN
lifetimes were measured every 10 °C from 10 to 80 °C, and the
lifetime measured at 23-24 °C was also included.
3
(1 1.3CH3CN)
3
For the normalized emission quantum yields reported in Table 3,
we compared the integrated emission of each complex relative to
that of the standard [Cr(phen)3]3þ in 1 M HCl(aq) according to the
eq 1.58 Both measurements are made back to back.
1 1.3CH3CN
3
formula
formula wt
color, habit
T, K
C43.60H31.90CrF9N7.30 O13S3 S3
1185.24
yellow plates
110(2)
P1 (triclinic)
2
12.6215(7)
13.9620(8)
17.2034(15)
99.574(4)
106.338(4)
115.479(2)
2477.6(3)
1.589
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Astd
Istd
ηunk
ηstd
space group
Z
Φunk ¼ Φstd
ð1Þ
Aunk
˚
a, A
˚
b, A
In this expression, Φunk and Φstd are the emission quantum yields of
the unknown and standard, respectively, under the conditions of
the measurement. To our knowledge Φstd (for [Cr(phen)3]3þ) has
not been previously measured and is set to one in this column of
Table 3 as has been done previously.39,57 The quantities Iunk and Istd
are the integrated emission intensities of the sample and the
standard, respectively, at 650-850 nm. The quantities Aunk and
Astd are the absorbances of the sample and the standard, respec-
tively, at the excitation wavelength (320 nm). Care was taken to
ensure that these are both close to 0.1. Finally ηunk and ηstd are the
indices of refraction of the sample and the standard solution,
respectively. Since the same solvent was used in both measure-
ments, thelasttermineq1canbeignored.Forthesemeasurements,
no differences were observed in absorption spectra collected before
and after the emission measurements.
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
dcalc, g/cm3
GOF
1.026
4.64 (10.17)
R1 (wR2)b, %
a Obtained with graphite-monochromated Mo KR (λ = 0.71073 A)
˚
P
P
P
P
2
radiation. b R1 = ||Fo| - |Fc||/ |Fo|, wR2 = { w(Fo - Fc2)2/
w(Fo2)2}1/2 for Fo >4σ(Fo).
corrected for Lorentz and polarization effects using SAINT,
and semiempirical absorption corrections were applied using
SADABS.54 The structure was solved by direct methods and
refined against F2 with the SHELXTL 6.14 software package.55
Unless otherwise noted, thermal parameters for all non-hydro-
gen atoms were refined anisotropically. Hydrogen atoms were
added at the ideal positions and were refined using a riding
model where the thermal parameters were set at 1.2 times those
of the attached carbon atom (1.5 for methyl protons). In the
To determine the emission quantum yields reported in Ta-
ble 3, we first measured Φunk for [Cr(phen)3](OTF)3 in 1 M
HCl(aq) relative to [Ru(bpy)3](PF6)2 in acetonitrile for which the
absolute quantum yield (Φstd) of 0.062 is known.59 The refrac-
tive indices of pure acetonitrile and 1 M HCl(aq) were used in the
calculation. The quantum yields for the rest of the seven com-
plexes were then calculated with respect to Φunk determined for
[Cr(phen)3]3þ. The error bars reported with these seven quan-
tum yields are determined by combining the percentage experi-
mental errors from the individual normalized emission quantum
yields (A%) with the percentage error in the measurement made
between [Cr(phen)3]3þ and [Ru(bpy)3]2þ (B%) according to the
equation Error% = (A2 þ B2)1/2%.
structure of 1 1.3CH3CN, two of the triflate anions show
3
positional disorder, and one solvent molecule is only partially
occupied. Complete experimental parameters and disorder
treatment are discussed in the Supporting Information.
Photophysical Measurements. All photophysical measure-
ments were undertaken with complexes dissolved in 1 M HCl(aq)
.
This alters the nucleophilicity of the solvent, thereby decreas-
ing the quantum yield of associative excited state reactions
(formation of seven-coordinate solvento species) which ulti-
mately would lead to polypyridyl ligand substitution by solvent
molecules.39,56,57 The ground-state absorption spectra were ob-
tained with a Hewlett-Packard 8453 spectrophotometer in quartz
cuvettes with 1 cm or 1 mm path lengths; experiments were
performed at room temperature. Quartz cuvettes (1 cm ꢀ1 cm)
with silicone septa seal screw caps were used for the following
measurements. Transient absorption spectra were obtained at
20-21 °C without deoxygenation (see Supporting Information
for details). Emission spectra, emission quantum yields, and
single-temperature emission lifetime experiments were measured
at 23-24 °C with deoxygenation on dilute samples (see Support-
ing Information for details). Teflon tubing was used to introduce
a stream of argon into the solution. Prior to measurements,
samples were purged with argon for 30 min to remove oxygen.
For the longer lifetime species such as [Cr(phen)3]3þ, which are
highly sensitive to 3O2 concentrations, this methodology proved
easier and far more effective at achieving reproducible results
than using freeze-pump-thaw cycles. Each measurement was
done with argon flowing on top of the solution. In these emission
experiments the temperature of the sample was controlled using a
water-jacket cuvette holder connected to a constant-temperature
circulator. For temperature-dependent experiments, emission
Further detailed information about instrumentation and
methods for photophysical measurements are included in the
Supporting Information.
Other Physical Methods. Infrared spectra were measured with
a Nicolet 380 FT-IR spectrometer. Mass spectrometric measure-
ments were performed in the positive ion mode on a Finnigan
LCQ Duo mass spectrometer, equipped with an analytical elec-
trospray ion source and a quadrupole ion trap mass analyzer.
Cyclic voltammetry experiments were carried out inside a dini-
trogen filled glovebox in 0.1 M solutions of (Bu4N)PF6 in
acetonitrile unless otherwise noted. The voltammograms were
recorded with either a CH Instruments 1230A or 660C potentio-
stat using a 0.25 mm Pt disk working electrode, Ag wire quasi-
reference electrode, and at a Pt mesh auxiliary electrode. All
voltammograms shownweremeasuredwitha scanrateof0.1 V/s.
Reported potentials are referenced to the ferrocenium/ferrocene
(Fcþ/Fc) redox couple and were determined by adding ferrocene
as an internal standard at the conclusion of each electrochemical
experiment. Solid state magnetic susceptibility measurements
were performed on finely ground samples prepared in air using
a Quantum Design model MPMS-XL SQUID magnetometer at
295 K. The data were corrected for the magnetization of the
sample holder by subtracting the susceptibility of an empty
container; diamagnetic corrections were applied using Pascal’s
constants.60 Elemental analyses were performed by Robertson
Microlit Laboratories, Inc. in Madison, NJ.
(54) APEX 2; Bruker Analytical X-Ray Systems, Inc.: Madison, WI, 2008.
(55) Sheldrick, G. M. SHELXTL, Version 6.14; Bruker Analytical X-Ray
Systems, Inc.: Madison, WI, 1999.
(56) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin
Complexes; Academic Press: London, 1992.
(57) Brunschwig, B.; Sutin, N. J. Am. Chem. Soc. 1978, 100, 7568–7577.
(58) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991–1024.
(59) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.;
Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620–6627.
(60) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532–536.

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 7985
Scheme 2
Scheme 3
Results and Discussion
Synthesis and Characterization of Heteroleptic Cr(III)
Complexes. Although there is literature precedent for
heteroleptic Cr(III) dipyridyl complexes, the preparation of
species that contain at least one carboxylate group (for even-
tual attachment to semiconductor surfaces) was not known
prior to our efforts. The preparative routes we have devel-
oped are outlined in Schemes 2-3. Typically, heteroleptic
dipyridyl complexes of Cr(III) are synthesized using [(NN)2-
Cr(OTf)2](OTf) precursors: the weakly coordinating triflate
anions can be facilely removed from otherwise inert Cr(III)
centers, and replaced by a third diimine species with minimal
ligand scrambling.48-50 Initial attempts to prepare complexes
using 2,20-bipyridine-4,40-dicarboxylic acid (dcbpy) or its
disodium salt do not afford pure products. Whereas mass
spectral analyses show peaks at anticipated m/zratios (consis-
tent with [(phen)2Cr(4-dcbpy)]þ), analysis of isotopic dis-
tribution patterns reveal 2þ charges for those ions. We
speculate that the products actually formed are dimeric
[(NN)2Cr(4-dcbpy)]22þ species, where carboxylates coordi-
nate in preference to the imines because of the oxophilic
nature of Cr(III), allowing the 4-dcbpy ligand to bridge bet-
ween two metal centers. Masking the carboxylates on dcbpy
by conversion to methyl ester groups (4-dmcbpy) avoids
undesirable metal coordination by the carboxylates. Where
(NN) is phen, Ph2phen, or Me2bpy, the ester-protected
ligand 4-dmcbpy is found to react cleanly, albeit sluggishly,
with[(NN)2Cr(OTf)2](OTf) via Scheme 2 to form the hetero-
leptic complexes 1, 4, and 7, respectively, as yellow solids.
The homoleptic complex salt [Cr(4-dmcbpy)3](OTf)3 (8)
cannot be prepared via Scheme 2, since deprotection of the
esters during triflate exchange with [Cr(4-dmcbpy)2Cl2]Cl
results in dimerization of the Cr complexes. Instead, we find
that a Cr(II) solvento complex, [Cr(CH3CN)4(BF4)2], can
serve as a suitable starting material.53 The labile Cr(II) ion is
easily ligated by 3 equiv of the 4-dmcbpy ligand. Oxidation
by Ag(I) affords the tetrafluoroborate salt of the homoleptic
Cr(III) complex in reasonable yield (Scheme 3).
Figure 1. Structure of the [(phen)2Cr(4-dmcbpy)]3þ complex cation, as
observed in 1 1.3CH3CN, rendered with 40% ellipsoids. H atoms are
omitted for clarity. The complex resides on a general position. Bond
distances and angles are available in the Supporting Information.
3
less rigid dipyridyl ligands offer greater opportunity for
ligand scrambling. Attempts to make heteroleptic bpy-
containing complexes via the more reactive Cr(II) synthon
(Scheme 3) have led thus far to intractable mixtures of
homo- and heteroleptic products.
Besides the usual methods employed for identification and
characterization of the complexes, the solid state structure of
the phen-containing complex 1 has been confirmed by X-ray
crystallography (Figure 1). All bond distances and angles are
as expected for a Cr(III) ion in a tris-chelate ligand environ-
ment. Relevant crystallographic data are shown in Table 1;
additional structural and geometric data are included in the
Supporting Information.
Exploration of Photophysics and Electrochemistry of
Cr(III) Systems in Solution. We have explored the basic
photophysical and electrochemical properties of these sys-
tems to better understand the nascent and long-lived excited-
states that will be called upon to drive hole-injection follow-
ing photoexcitation in later studies. Key questions to be
addressed here include: (1) how efficient are these complexes
as optical absorbers; (2) how much energy is stored in the
excited state; (3) for how long is it stored in unbound solution
phase systems; (4) with what driving force might we expect
hole transfer photochemistry; and (5) what are the transient
absorption features we might use in later femtosecond pump/
probe studies to determine hole-injection rates.
Electronic Absorption. Sensitization of hole transfer
photochemistry demands light absorption in the materi-
al-bound metal complex as an initial step. If Cr(III) poly-
pyridyl complexes are to serve in this role it is important
that ligand modifications render visible light absorption
more efficient than pure spin-allowed ligand-field ex-
citation (ε=50-100 M-1 cm-1). UV-visible absorption
spectra in 1 M HCl(aq) for complexes containing substi-
tuted bipyridine ligands are shown in Figure 2. Also inclu-
ded for purposes of comparison are spectra for [Cr(bpy)3]3þ
and [Cr(Me2bpy)3]3þ. Absorption data for all complexes
considered in this manuscript are presented in Table 2. Note
that spectra for previously reported homoleptic complexes39
were reacquired to allow for unambiguous quantitative com-
parisons to the new heteroleptic complexes.
In contrast to the phen-containing heteroleptic complex
1, the bpy-containing analogue [(bpy)2Cr(4-dmcbpy)]3þ
resists formation via Scheme 2, although the reasons for
this are not known at this time. To our knowledge, only two
successful syntheses of bpy-containing heteroleptic com-
48
plexes have been reported, [(bpy)2Cr(phen)](OTf)3 and
[(bpy)2Cr(DPPZ)](OTf)3 (where DPPZ is dipyridophena-
zine).61 We speculate that the ring-locked configuration and
higher basicity of the phenanthroline-type ligands minimize
opportunities for deleterious ligand exchange before re-
placement of the more labile triflate anions, whereas the
(61) Vandiver, M. S.; Bridges, E. P.; Koon, R. L.; Kinnaird, A. N.;
Glaeser, J. W.; Campbell, J. F.; Priedemann, C. J.; Rosenblatt, W. T.;
Herbert, B. J.; Wheeler, S. K.; Wheeler, J. F.; Kane-Maguire, N. A. P. Inorg.
Chem. 2010, 49, 839–848.

7986 Inorganic Chemistry, Vol. 49, No. 17, 2010
McDaniel et al.
Table 2. Room Temperature Electronic Absorptions for Cr(III) Tris-Dipyridyl Complexes
complex
λ )
max/nm (ε/M-1 cm-1
[Cr(phen)3](OTf)3
269 (52500); 285 (31800), sh; 323 (11200), sh; 342 (6810), sh;
358 (3710); 405 (821), sh; 435 (558), sh; 454 (285), sh
283 (103000); 307 (108000); 362 (35300), sh; 380 (26200), sh;
445 (2820), sh; 484 (1730), sh
[Cr(Ph2phen)3](OTf)3
[Cr(bpy)3](OTf)3
265 (15700); 275 (15500); 305 (21300), sh; 313 (23100); 346 (8100);
360 (5680), sh; 402 (950), sh; 428 (682), sh; 458 (291), sh
278 (23100); 307 (25800); 342 (8530); 354 (6590), sh; 394 (892), sh;
418 (602), sh; 446 (256), sh
[Cr(Me2bpy)3](OTf)3
[(phen)2Cr(4-dmcbpy)](OTf)3 (1)
[(Ph2phen)2Cr(4-dmcbpy)](OTf)3 (4)
[(Me2bpy)2Cr(4-dmcbpy)](OTf)3 (7)
[Cr(4-dmcbpy)3](BF4)3 (8)
269 (50600); 285 (29900), sh; 329 (14300); 368 (5020), sh;
418 (1130), sh; 450 (634), sh; 480 (163), sh
284 (63300); 311 (59300); 368 (21500), sh; 389 (11600), sh;
455 (1950), sh; 491 (1270), sh
279 (23000), sh; 308 (21500); 333 (13500); 351 (9570), sh;
369 (4410), sh; 398 (1140), sh; 422 (866), sh; 451 (429), sh
294 (14800), sh; 322 (20000), sh; 333 (22700); 360 (10500), sh;
372 (8210), sh; 420 (1280), sh; 448 (1020), sh; 480 (415), sh
Figure 2. Electronic absorption spectra for Cr(III) dipyridyl complexes
in 1 M HCl(aq) at room temperature.
Figure 3. Electronic absorption spectra for Cr(III) complexes contain-
,
ing phenanthroline-based ligands: [Cr(phen)3]3þ and 1 in 1 M HCl(aq)
[Cr(Ph2phen)3]3þ and 4 in 1 M HCl(aq) with MeOH (2% v/v). All spectra
were collected at room temperature.
As shown above, these complexes are strongly absorptive
in the UV owing to ligand-centered π f π* transitions; these
Here we accept that trigonal splitting is small (∼20 cm-1 64,65
)
are also observed in the free ligands. For [Cr(bpy)3]3þ
,
[Cr(Me2bpy)3]3þ, and 7, this includes the band in the vicinity
of 300 nm. Ligand-centered π f π* transitions due to the
presence of 4-dmcbpy are seen red-shifted by ∼10 nm in 7
and ∼30 nm in 8. For [Cr(bpy)3]3þ, [Cr(Me2bpy)3]3þ, and 7,
the band in the vicinity of 350 nm appears to be charge
transfer in nature based on its intensity and the fact that it is
absent in the free ligand. A similar band is observed for 8 as a
shoulder at ∼375 nm. This red shift is consistent with ex-
pectations for metal-to-ligand charge-transfer (MLCT) gi-
ven the presence of electron withdrawing substituents on the
4-dmcbpy ligands. However, one might also expect red
shifting for ligand-to-metal charge transfer (LMCT) if such
ligands serve to reduce electron-electron repulsion in the
metal-centered orbitals. It is noted that the ∼350 nm band in
[Cr(bpy)3]3þ is most often attributed to LMCT52,62,63 since
there is an energetic penalty for oxidizing Cr(III) to Cr(IV) as
would formally occur during MLCT.
and that these systems can be discussed with state designa-
tions normally reserved for systems with octahedral sym-
metry. The origin of this intensity enhancement is currently
uncertain, but it has been claimed that spin-spin coupling
between the (4A2) Cr(III) center and the triplet states of the
aromatic ligand is a possible source.52,66,67 Juban and
McCusker have argued that visible absorption enhance-
ment in [Cr(bpy)3]3þ is due to intensity borrowing from
ligand-centered transitions.68 This should be kept in mind as
a possible mechanism in future explorations of 4-dmcbpy
ligand-containing systems. It is noted here that for a Cr(III)
species, [Cr(4-dmcbpy)3]3þ (8) shows appreciable sensitiza-
tion of visible light, with a molar absorptivity of 940 M-1
cm-1 at 450 nm. This portends eventual control of this
critical property through judicious structural and electronic
modifications.
Figure 3 presents UV-visible absorption spectra for
homo- and heteroleptic complexes containing phen-based
ligands. Addition of 2% MeOH by volume to 1 M HCl(aq)
For each of the bpy-containing complexes, a broad and
modestly structured absorption feature is observed tailing into
the visible spectrum (Figure 2 inset). Even for the known tris-
homoleptic complexes [Cr(bpy)3]3þ and [Cr(Me2bpy)3]3þ
,
(64) Hauser, A.; Maeder, M.; Robinson, W. T.; Murugesan, R.; Ferguson,
J. Inorg. Chem. 1987, 26, 1331–1338.
(65) Schonherr, T.; Atanasov, M.; Hauser, A. Inorg. Chem. 1996, 35,
2077–2084.
(66) Ohno, T.; Kato, S.; Kaizaki, S.; Hanazaki, I. Chem. Phys. Lett. 1983,
102, 471–474.
the observed molar absorptivities are larger than would
be expected for pure spin-allowed ligand field transitions
(4A2 f 4T2), and have been discussed in the literature.52
(67) Ohno, T.; Kato, S.; Kaizaki, S.; Hanazaki, I. Inorg. Chem. 1986, 25,
3853–3858.
(62) Milder, S. J.; Gold, J. S.; Kliger, D. S. Inorg. Chem. 1990, 29, 2506–
(68) Juban, E. A. Ph.D. Thesis, University of California, Berkeley,
California, 2006.
2511.
€
(63) Konig, E.; Herzog, S. J. Inorg. Nucl. Chem. 1970, 32, 585–599.

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 7987
state) transition and the shoulder that occurs at ∼700 nm
2
is assigned to the Tf4A (ground state) transition.39 Inte-
restingly, and somewhat counter to the above discussion,
Figure 4 and Table 3 show that the two species containing
the Ph2phen ligand, [Cr(Ph2phen)3]3þ and [(Ph2phen)2Cr(4-
dmcbpy)]3þ (4), have an emission λmax that is shifted ∼14 nm
to the red. For [Cr(Ph2phen)3]3þ this has also been repor-
ted elsewhere, although no explanation has been offered.39
Electron delocalization via the larger π-system of the aryl-
substituted ligand would reduce electron-electron repulsion
in the t2g3 configuration of the 2E state. However, a complete
explanation of emission shifting must be more subtle as a
similar reduction of repulsion in the t2g3 configuration of the
4A ground state might be expected to cancel the effect in the
excited state thereby leading to no shift in the emission
maximum. One plausible explanation lies in the fact that
doublet configurations involving spin pairing within indi-
vidual metal orbitals contribute to the description of the
emissive states in all of the these molecules explored herein,
but for the aryl-substituted versions, the energetic perturba-
tion due to these configurations is smaller as intraligand
electronic delocalization takes effect.69 Ultimately it will be
important to determine whether there are connections be-
tween these emission shifting effects and the states playing
roles in the absorption intensity borrowing (vide supra).
The emission spectra shown in Figure 4 allow us to
determine the amount of energy stored in the long-lived
excited states of these systems. This is a critical piece of
information in assessing the oxidation potential available
following absorption of UV-visible light. The values
reported in Table 3 are for E00 (equivalent to the ΔG
stored in the excited state) where we have modified the
observed maximum of the 0-0 vibronic transition (E0)
with its width (Δv0,1/2) according to the expression
Figure 4. Emission spectra for Cr(III) polypyridyl complexes in deox-
ygenated 1 M HCl(aq) following excitation at 320 nm. On this intensity
scale, the value 1 corresponds to the peak height in emission collected for
[Cr(phen)3]3þ
.
was necessary to increase solubility for the Ph2phen-contain-
ing complexes.39
Like the bipyridine-containing complexes discussed in
Figure 2, intense ligand-centered π f π* transitions are
observed in the UV. For complexes with the unsubstituted
phen ligand, this is most prominently observed at ∼270 nm
with very little shifting relative to the free ligand (264 nm for
phen in CH2Cl2). Such transitions are significantly stronger
and red-shifted in complexes containing the Ph2phen ligand
as evidenced by intense absorption bands at ∼285 nm and
∼310 nm. This is likely due to the presence of a larger and
more delocalized π-system. For these two complexes, [Cr-
(Ph2phen)3]3þ and 4, new absorption features appear at
∼375 nm upon formation of the metal complex. It is also
noted that the intensity of the ∼310 nm band mentioned
above changes significantly relative to the free ligand where it
appears as a shoulder to the bluer π f π* band. We believe
these substantive changes upon complexation herald charge
transfer transitions (again, LMCT, MLCT, or both). For
[Cr(phen)3]3þ and 1, features absent in the free ligand spectra
are observed at ∼340 nm. The Ph2phen-containing species 4
shows even more promising absorption of the visible spec-
trum than [Cr(4-dmcbpy)3]3þ (8) discussed above: at 450 nm,
2
ð2Þ
E00 ¼ E0 þ ðΔv0,1=2Þ =16kBT ln 2
For emissive MLCT species (commonly, RuII, OsII, and ReI)
one generally uses a Franck-Condon analysis to determine
70-72
E0 and Δv0,1/2
.
Here, because emissive features from
the 2E f4A ground state are relatively narrow, we determine
these quantities directly from the energy and width of the
most intense band in the emission spectra.
ε=1960 M-1 cm-1, and at 480 nm, ε=1490 M-1 cm-1
.
Static Emission. Each of the bis-heteroleptic polypyri-
dyl Cr(III) complexes that we have synthesized (1, 4, 7, and
8) is emissive at room temperature following electronic
excitation. Emission spectra are shown in Figure 4 for dilute
samples of the four ester-containing species in thoroughly
deoxygenated 1 M HCl(aq) following excitation at 320 nm.
The peaks of these spectra have been scaled to reflect relative
intensity with respect to the nearly simultaneous measure-
ment of a standard ([Cr(phen)3]3þ in thoroughly deoxyge-
nated 1 M HCl(aq)). We note that the wavelength of maxi-
mum emission at ∼730 nm in 1, 7, and 8 is largely invariant
to any changes made to the polypyridyl ligands. As shown in
Table 3, the model homoleptic species [Cr(phen)3]3þ, [Cr-
(bpy)3]3þ, and [Cr(Me2bpy)3]3þ emit most strongly at this
approximate wavelength. Such emission λmax invariance is
common to Cr(III) polypyridyl species and indicates that the
lowest energy excited state is insensitive to the ligand field, as
would be the case for a 2E state with a t2g3 configuration in-
volving a spin flip.45 By analogy to a large number of known
emissive Cr(III) polypyridyl complexes, the main emission
We also report emission quantum yields in Table 3 for
1, 4, 7, 8, and our set of tris-homoleptic species relative to
[Cr(phen)3]3þ. The expression for the emission quantum
yield φem in terms of radiative (kr) and non-radiative (knr)
rate constants is shown in eq 3. Here kobs and τobs are
inversely related and refer to the observed rate constant
and lifetime, respectively.
P
P
X
kr
kr
P
P
φem
¼
¼
¼ τobs
kr
ð3Þ
kr þ
knr
kobs
If a comparison is drawn between [Cr(phen)3]3þ and
[(phen)2Cr(4-dmcbpy)]3þ (1), it is seen that introduction
of a single 4-dmcbpy ligand drops the quantum yield to
28% of its value in the homoleptic species. For the
(69) Kirk, A. D. Chem. Rev. 1999, 99, 1607–1640.
(70) Claude, J. P.; Meyer, T. J. J. Phys. Chem. 1995, 99, 51–54.
(71) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.
Chem. 1986, 90, 3722–3734.
2
band seen at ∼730 nm is assigned to the Ef4A (ground
(72) Chen, P. Y.; Meyer, T. J. Chem. Rev. 1998, 98, 1439–1477.

7988 Inorganic Chemistry, Vol. 49, No. 17, 2010
McDaniel et al.
Table 3. Photophysical Data for Cr(III) Polypyridyl Complexes in 1 M HCl(aq)
normalized
emission
emission
quantum
emissiona E00
/
emission
Ea
/
b
f
complex
[Cr(phen)3](OTf)3
[Cr(Ph2phen)3](OTf)3
[Cr(bpy)3](OTf)3
[Cr(Me2bpy)3](OTf)3
[(phen)2Cr(4-dmcbpy)](OTf)3 (1)
[(Ph2phen)2Cr(4-dmcbpy)](OTf)3 (4)
[(Me2bpy)2Cr(4-dmcbpy)](OTf)3 (7)
[Cr(4-dmcbpy)3](BF4)3 (8)
λmax /nm
eV
quantum yielda,c,d
yielda,e ꢀ 103 lifetimea,c /μs
ln(A)f
A /s-1
kJ mol-1
3
730
744
729
732
732
742
734
733
1.70
1.67
1.71
1.70
1.70
1.68
1.70
1.70
1
12 ( 1c
304 ( 4
425 ( 20
69 ( 2
196 ( 8
87 ( 3
108 ( 8
47 ( 1
7.7 ( 0.3
28.9 ( 0.5 3.5 ꢀ 1012
51 ( 1
45 ( 3
43 ( 1
49 ( 1
46 ( 1
45 ( 1
46 ( 1
37 ( 1
2.5 ( 0.4
30 ( 5g
26 ( 1
2.4 ꢀ 1011
0.21 ( 0.01
0.74 ( 0.03
0.28 ( 0.01
0.55 ( 0.02
0.15 ( 0.01
0.012 ( 0.002
2.5 ( 0.2g
8.9 ( 0.9g
3.4 ( 0.4g
6.6 ( 0.7g
1.8 ( 0.2g
0.14 ( 0.02g
27.0 ( 0.3 5.5 ꢀ 1011
28.5 ( 0.5 2.3 ꢀ 1012
27.8 ( 0.5 1.2 ꢀ 1012
27.3 ( 0.5 7.5 ꢀ 1011
28.6 ( 0.2 2.6 ꢀ 1012
26.7 ( 0.4 4.1 ꢀ 1011
a The measurements were done at 23-24 °C. b See text for details on the calculation of E00. c The errors represent 2σ (two times of standard deviation)
from nine measurements (three independent experiments for each sample, three measurements for each experiment.) d The normalized emission quantum
yields were determined with respect to [Cr(phen)3]3þ e Emission quantum yields reported are relative to the standard [Ru(bpy)3]2þ in acetonitrile with a
.
0.062 absolute emission quantum yield. See Experimental Section for details. f Errors reported reflect 2σ from the fitting of a single temperature-data
set to a linear Arrhenius model ln kobs = ln A - Ea/RT. g See Experimental Section for details on the calculation of these error bars.
comparisons between [Cr(Me2bpy)3]3þ and [(Me2bpy)2-
Cr(4-dmcbpy)]3þ (7) or between [Cr(Ph2phen)3]3þ and
[(Ph2phen)2Cr(4-dmcbpy)]3þ (4), these values are 21%
and 21.5%, respectively. In the comparison between
[Cr(bpy)3]3þ and [Cr(4-dmcbpy)3]3þ(8), introduction of
three 4-dmcbpy ligands in place of the three bpy ligands
drops the quantum yield to ∼6% of the value prior to the
substitution. The origin of these quantum yield changes is
discussed below.
Time-Resolved Emission. To understand these trends in
radiative quantum yields, we have measured the observed
lifetimes (τobs) of the emissive 2E states in the full set of com-
plexes and these are also reported in Table 3. For the new
complexes reported here, the lowest energy excited state life-
Figure 5. Temperature dependence of the observed rate constant kobs
1/τobs for [Cr(bpy)3]3þ and 8 in degassed 1 M HCl(aq)
=
times range from 108 μs (for 4) to 7.7 μs (for 8). This is
encouraging as it suggests there may be ample time in the 2E
excited-state of the respective complexes to engage in hole
transfer photochemistry. In the comparison between [Cr-
(phen)3]3þ and [(phen)2Cr(4-dmcbpy)]3þ (1), the observed
lifetime drops from 304 to 87 μs upon introduction of the
4-dmcbpy ligand. This latter value is 29% of that measured
for the tris-homoleptic species. Similar comparisons made
between [Cr(Me2bpy)3]3þ and [(Me2bpy)2Cr(4-dmcbpy)]3þ
(7) or between [Cr(Ph2phen)3]3þ and [(Ph2phen)2Cr(4-
dmcbpy)]3þ (4) yield the values 24% and 25%, respec-
tively. The comparison between [Cr(bpy)3]3þ and [Cr(4-
dmcbpy)3]3þ(8) shows that introduction of a full set of
ester-containing ligands drops the observed lifetime
(7.7 μs) to 11% of the value obtained for [Cr(bpy)3]3þ
(69 μs). The various percentage changes in τobs shown
here are similar to those seen above for φem. This corres-
pondence suggests (by eq 3) that introduction of the
.
rate constant relative to temperature-independent contribu-
tions to kr and knr.73 Table 3 includes a listing of the measured
Arrhenius pre-exponential (A) and activation energy (Ea) for
each system. With this information we can see general trends
emerge, especially if we draw comparisons, as before, bet-
ween pairs of compounds where ancillary ligands are the
same. For example in comparing [Cr(phen)3]3þ and 1 it is
seen that introduction of the ester-containing ligand serves to
drop the activation energy by 5 kJ/mol. Given that the pre-
exponential A drops modestly between [Cr(phen)3]3þ and 1,
the concomitant decrease in Ea is responsible for the short-
ening of the lifetime from 304 to 87 μs. A similar decrease in
Ea is also observed between [Cr(Me2bpy)3]3þ and 7 and the
system with shortest lifetime, 8, also has the smallest Ea which
is 6 kJ/mol less than that measured for [Cr(bpy)3]3þ. These
observations can be explained if introduction of the electron
withdrawing ester ligand weakens the ligand field via an in-
ductive effect, thereby decreasing the barrier to excited states
having a t2g2eg configuration. Such states would have signi-
ficant nuclear distortions (primarily metal-ligand bond dis-
tances) and through these displacements larger non-radiative
decay rates. Given the magnitude of the Arrhenius pre-
exponential A measured for these systems, it is unlikely that
these complexes are thermally activated directly through
4-dmcbpy ligand mainly affects the rate constants for
non-radiative relaxation pathways ( knr).
P
To better understand the origin of the lifetime drop for
8, we have explored the temperature dependence of
emission lifetimes for the complete series of complexes
over the range 283 K-353 K. A representative example is
shown in Figure 5 for 8 compared to [Cr(bpy)3]3þ as the
natural log of the observed rate constant (kobs) versus
1000/T. Other data sets for the remaining compounds are
shown in the Supporting Information, Figure S3.
2
4
back-intersystem-crossing from the E to the T manifold.
Rather one might need to invoke deactivation through
states with appropriate nuclear distortion but also with
doublet electron spin character. It is noted that differences
In all cases we observe temperature dependence. The
high-degree of linearity for all data sets suggests that in
this temperature range (throughout which we have a fluid
solution) the activated process dominates the observed
(73) Forster, L. S. Coord. Chem. Rev. 2002, 227, 59–92.

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 7989
containing a single 4-dmcbpy. For this species the presence
of three ligands carrying electron withdrawing groups is able
to move the “Cr-I/-II” and “Cr-II/-III” reductions to within
the acetonitrile window, leading to the observation of six
reversible waves. While each of these reductions is listed as a
change in the Cr formal oxidation state (matching the
literature precedent), these processes are more likely due
to ligand reduction,63,74,75 and it is reasonable to surmise
that the presence of electron withdrawing groups allow for
each 4-dmcbpy ligand to be reduced by two electrons.
The half wave reduction potential of the excited state
can be estimated using the ground state half wave reduc-
tion potential and the one electron potential correspond-
ing to the spectroscopic excited state:76
E1=2ðCrIIIꢁ=IIÞ ¼ E00ðCrIIIꢁ=IIIÞ þ E1=2ðCrIII=II
Þ
ð4Þ
The use of E00 as a potential (rather than energy) is acceptable
in this context because the excited state is only acting as a one-
electron acceptor. As discussed in the context of Table 3, E00
is largely invariant to ligand substitution patterns. Others
have noted that the 2E emission maximum in Cr(III) species
is solvent insensitive, and we have observed that E00 does not
change between aqueous (where our spectroscopic measure-
ments have been made) and acetonitrile (where our electro-
chemical measurements have been made) environments. On
the other hand, the ground state (first) reduction potentials
show ligand dependence (Table 4). Thus, the excited state
reduction potential is tunable. These values are compiled in
Table 4 for all the complexes studied. Equation 4 allows us to
predict a CrIII*/II couple of þ1.22 to þ1.27 V versus Fcþ/Fc
for the heteroleptic complexes. To compare the excited state
potentials of these Cr(III) complexes to other photoelectro-
chemically active species reported in the literature, the mea-
sured redox potentials have been converted to reference
Figure 6. Comparison of cyclic voltammograms for the 4-dmcbpy
containingcomplexes1, 4, 7, and 8 in 0.1M TBAPF6 acetonitrile solution.
Arrows indicate the starting point and direction for each voltammogram.
For 4, 7, and 8, the potential is referenced to ferrocene (from a voltammo-
gram which includes Fc collected immediately after the displayed
voltammogram).
in activation energy alone are not sufficient to explain the
significant lifetime variation between [(Ph2phen)2Cr(4-
dmcbpy)]3þ (4) where τobs = 108 μs, and [Cr(Ph2phen)3]3þ
where τobs = 425 μs. Here, differences in the Arrhenius pre-
exponential A are the primary origin of the observation. It
does not appear to be simply a consequence of the reduced
rigidity of the diester ligand and related entropic effects as we
do not see A increase between [Cr(phen)3]3þ and [(phen)2-
Cr(4-dmcbpy)]3þ (1). Detailed theoretical exploration is
needed to determine, for example, whether there are changes
in the density of electronic states at the activation energy of
∼45 kJ/mol in the comparison between [Cr(Ph2phen)3]3þ
and [(Ph2phen)2Cr(4-dmcbpy)]3þ (4) that might influence
the relative mechanisms for non-radiative decay.
Ground and Excited State Reduction Potentials. Along
with emission data, the second critical component for
finding excited state redox potentials is the determination
of ground state “CrIII/II” and “CrIV/III” couples. Each of
the heteroleptic dipyridyl complexes 1, 4, and 7 show 4
reversible 1e- reductions, and in the case of 8, six rever-
sible waves are seen. The “CrIV/III” couples for these
systems have not been observed as they are outside of
the acetonitrile solvent oxidation window. These data are
presented here in Figure 6 and Table 4.
As shown, the first reduction for complexes containing
the ester ligands is facile, with E1/2 occurring between -0.47
and -0.42 V versus Fcþ/Fc for all species containing a single
ester ligand. We find that the position of the first wave can
be tuned through the functional groups present on the
attached ligands. The presence of two strongly electron
withdrawing ester groups on the ligand 4-dmcbpy shifts
the initial “CrIII/II” process to more positive potentials
compared to those previously reported for hetero- and
homoleptic [Cr(NN)3]3þ complexes.49,57 This is shown in
Table 4: for 1, 4, and 7, the first “CrIII/II” reduction occurs at
a potential at least 0.22 V more positive than the homoleptic
species that lack an ester ligand. For [Cr(4-dmcbpy)3]3þ (8)
this first reduction is remarkably shifted an additional ∼0.2
V in the positive direction compared to the complexes
NHE (Fcþ/Fc is 0.40 V vs SCE in 0.1 M TBAPF6 and
77
SCE is 0.241 V vs NHE78). We note that this comparison is
approximate because of differences in solvent and supporting
electrolyte, but it is still useful.79 In the case of 8, the CrIII*/II
couple is calculated to be a remarkable þ2.08 V versus NHE
while 1, 4, and 7 are þ1.92 V, þ1.87 V, and þ1.87 V versus
NHE, respectively. As a point of reference, a similar ester-
functionalized [Ru(NN)3]2þ complex shows an excited state
reduction potential of þ1.26 V (vs NHE) for RuII*/I.80 Reece
and Nocera reported þ1.78 V (vs NHE) for a ReI*/0
complex,81 while Sullivan and co-workers reported þ2.71
V (vs NHE) for a ReII*/I complex.82 Reports of CrIII*/II
potentials from non-polypyridyl complexes are very rare in
the literature; however, an amine based CrIII*/II system has
been reported with an excited state reduction potential of
(74) Saito, Y.; Takemoto, J.; Hutchinson, B.; Nakamoto, K. Inorg. Chem.
1972, 11, 2003–2011.
(75) Creutz, C. Comments Inorg. Chem. 1982, 1, 293–311.
(76) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V.
J. Am. Chem. Soc. 1978, 100, 7219–7223.
(77) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–910.
(78) Bard, A. J.; Faulkner, L. R. Electrochemical methods fundamentals
and applications, 2nd ed.; Wiley: New York, 2001.
(79) Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298, 97–
102.
€
(80) Monserrat, K.; Foreman, T. K.; Gratzel, M.; Whitten, D. G. J. Am.
Chem. Soc. 1981, 103, 6667–6672.
(81) Reece, S. Y.; Nocera, D. G. J. Am. Chem. Soc. 2005, 127, 9448–9458.
(82) Del Negro, A. S.; Seliskar, C. J.; Heineman, W. R.; Hightower, S. E.;
Bryan, S. A.; Sullivan, B. P. J. Am. Chem. Soc. 2006, 128, 16494–16495.

7990 Inorganic Chemistry, Vol. 49, No. 17, 2010
McDaniel et al.
Table 4. Ground and Excited State Reduction Potentials for Cr(III) Polypyridyl Complexes
(E1/2 vs Fcþ/Fc, V)a
0/1-
b
3þ/2þ
2þ/1þ
1þ/0
1-/2-
2-/3-
*3þ/2þ
[Cr(phen)3](OTf)3
[Cr(Ph2phen)3](OTf)3
[Cr(bpy)3](OTf)3
[Cr(Me2bpy)3](OTf)3
[(phen)2Cr(4-dmcbpy)](OTf)3 (1)
[(Ph2phen)2Cr(4-dmcbpy)](OTf)3 (4)
[(Me2bpy)2Cr(4-dmcbpy)](OTf)3 (7)
[Cr(4-dmcbpy)3](BF4)3 (8)
-0.65
-0.67
-0.63
-0.79
-0.42
-0.45
-0.47
-0.26
-1.17
-1.11
-1.15
-1.29
-1.01
-0.99
-1.11
-0.68
-1.71
-1.63
-1.72
-1.82
-1.61
-1.54
-1.71
-1.21
-2.21
-2.05
-2.34
þ1.05
þ1.00
þ1.08
þ0.91
þ1.28
þ1.23
þ1.23
þ1.44
-1.90
-1.87
-1.92
-1.74
-1.93
-2.10
a Conditions for cyclic voltammetry of Cr complexes: electrolyte, 0.1 M TBAPF6 in CH3CN; WE, Pt; CE, Pt Wire; scan rate, 100 mV/s. b Calculated
from eq 4.
Figure 7. Transient absorption spectra on a microsecond time scale for 8 (left) and 4 (right) in 1 M HCl(aq). The spectra were determined from a single
exponential fit to transient absorption (or bleach) kinetics collected at each of the wavelengths for which there is a dot. The lines are included as guides to the
eye.
þ0.77 V (vs NHE).83 Indeed, the ester-containing Cr(III)
complexes 1,4,7,and8offer themselves as potentially power-
ful excited state oxidants.
with the solvent.68,69,85-91 TA spectra collected for the tris-
homoleptic compounds [Cr(phen)3]3þ, [Cr(bpy)3]3þ, [Cr-
(Me2bpy)3]3þ, and [Cr(Ph2phen)3]3þ are shown in Support-
ing Information, Figure S2, and these agree with previously
reported spectra collected under similar experimental con-
ditions.39 Spectra for the two additional bis-heteroleptic
complexes 1 and 7 are also shown in the Supporting Infor-
mation, Figure S2. In the case of 7, the spectrum is quite simi-
lar to that of the related tris-homoleptic species [Cr(Me2-
bpy)3]3þ. There are some qualitative differences between 1
and [Cr(phen)3]3þ in terms of relative band intensities, but
these are unremarkable and are not considered here further.
In Figure 7 are plotted transient absorption (TA) spectra for
8 (left) and 4 (right). As discussed above, these species have
the most promising molar absorptivities of visible wave-
lengths and are likely, therefore, to be used in future semi-
conductor sensitization experiments.
Transient Absorption. In future studies involving hole
transfer photochemistry between Cr(III) systems and wide
band gap semiconductors to which they are bound, it will be
important to be able to interrogate the time-dependent beha-
vior of the lowest energy excited-state manifold in these
complexes without relying on emission and on potentially
very short time scales. A valuable tool in this context is tran-
sient electronic absorption (TA) spectroscopy. With time
resolution from tens of femtoseconds to milliseconds, TA
spectroscopy has been central to unraveling the mechanism
in dye-sensitized heterojunction solar cell materials.6,7,84
Here we do not consider the earliest transient events but
rather explore absorption features once these molecules are
2
electronically relaxed in the E excited states. The spectro-
meter used here employs ∼10 ns excitation pulses centered at
355 nm and the transient features probed with white light
decay with single exponential behavior on microsecond time
scales. Within such time scales, it is well established that the
2E excited-state of these complexes in fluid solution at room
temperature is vibrationally cool and thermally equilibrated
Both difference spectra show a resolved bleach feature
centered at 360 nm, a strong absorption feature at ∼400 nm,
and a broad absorption further to the red. In the case of 4,
this redder absorbance is very broad and appears to peak at
∼550 nm. The magnitude of the bleach in both transient
spectra provides a useful metric for estimating excited-state
molar absorptivities at various wavelengths. In the ground
state optical spectra shown in Figure 2 and Figure 3, com-
plexes 8 and 4 exhibit molar extinction coefficients at 360 nm
(83) Ramasami, T.; Endicott, J. F.; Brubaker, G. R. J. Phys. Chem. 1983,
87, 5057–5059.
of ε360 nm=10500 M-1 cm-1 and ε360 nm=23400 M-1 cm-1
,
€
(84) Gratzel, M. Inorg. Chem. 2005, 44, 6841–6851.
(85) Serpone, N.; Jamieson, M. A.; Hoffman, M. Z. J. Chem. Soc., Chem.
respectively. In the limiting case that the -ΔA at 360 nm for
Commun. 1980, 1006–1007.
(86) Serpone, N.; Jamieson, M. A.; Sharma, D. K.; Danesh, R.; Bolletta,
F.; Hoffman, M. Z. Chem. Phys. Lett. 1984, 104, 87–92.
(87) Kirk, A. D.; Porter, G. B.; Sharma, D. K. Chem. Phys. Lett. 1986,
123, 548–550.
(89) Rojas, G. E.; Dupuy, C.; Sexton, D. A.; Magde, D. J. Phys. Chem.
1986, 90, 87–92.
(90) Rojas, G. E.; Magde, D. J. Phys. Chem. 1987, 91, 689–691.
(91) Serpone, N.; Hoffman, M. Z. J. Phys. Chem. 1987, 91, 1737–1743.
(88) Serpone, N.; Hoffman, M. Z. Chem. Phys. Lett. 1986, 123, 551–552.

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 7991
these two spectra is entirely due to loss of ground state
absorption, then it is possible to assign a lower limit to
excited-state molar absorptivities (ε*(λ)) for observed absorp-
tion features. In the case of 8, we measure ΔA=-0.014 at 360
nm and ΔA=0.020 at 400 nm such that ε*400 nm g 14400
called upon to drive hole-injection following photoexcitation
in later studies.
The electronic absorption studies show a combination of
ligand-centered, metal-centered ligand field, and charge trans-
fer transitions. Nevertheless, it is noted here that for a Cr(III)
species with Ph2phen ancillary ligands, [(Ph2phen)2Cr(4-
dmcbpy)]3þ (in 4), shows appreciable sensitization of visible
light, with a molar absorptivity of 1270 M-1 cm-1 at 491 nm.
Time-resolved emission studies show that the introduction of
the 4-dmcbpy ligand decreases the doublet excited state life-
times of the complexes with respect to their tris-homoleptic
analogues. Notwithstanding, excited state lifetimes are suffi-
ciently long that we can anticipate photoinduced hole transfer
to suitable semiconductor substrates. Additionally, preliminary
electrochemical studies show that the introduction of the
4-dmcbpy significantly shifts reduction potentials of the com-
plexes to more positive values. The combination of cyclic
voltammetry and static emission studies indicates that strongly
oxidizing excited states are possessed by this class of molecules.
The strongly oxidizing excited states found for the Cr(III)
dye complexes coupled with the reported long excited state
lifetimes lead us to believe these species will be capable of hole
injection into p-type semiconductors with suitably aligned
valence bands. Further, the excited-state absorption features
for these heteroleptic complexes should serve as handles for
studying the hybrid dye-sensitized materials. Efforts are
underway to incorporate these dye complexes with semicon-
ductors into hybrid materials.
M
-1 cm-1. A similar observation is made for 4 although the
absorptivities are even larger. In this case ε*400 nm g 23300
M
-1 cm-1 and ε*550 nm g16500 M-1 cm-1. In these spectra
(and very likely in the whole series of Cr(III) polypyridyl
complexes we have considered), the transient absorption
features with appreciable ΔA are clearly not assignable to
ligand-field absorption occurring from the t2g3 configuration
2
of the E excited state. The strength of the absorption
features suggest these are charge transfer in nature originat-
2
2
ing from the E and T excited states, although it is not
possible at this time to assign MLCT and/or LMCT to any
3
2
particular feature. The t2g configuration of the E excited
state is not expected to significantly perturb the ligand
π-system, so ligand-centered π f π* transitions are unlikely
to play a prominent role. It may be possible that the observed
transitions, especially those near 400 nm, borrow some
intensity from such excitations as Juban and McCusker68
have argued occurs in the ground state of [Cr(bpy)3]3þ. The
important point to stress here is that excited-state absorption
features in the visible spectrum for these complexes have
significant oscillator strength. In more complex environ-
ments such as when complexes are bound to heterogeneous
semiconductor surfaces, such features may be a useful and
easily observable diagnostic of 2E lifetime and related hole-
injection rate constants.
Acknowledgment. This research was supported by
Honda (2008 Honda Initiation Grant), the Center for
Revolutionary Solar Photoconversion, Colorado State
University, and the University of Colorado. We thank
Prof. C. M. Elliott and Prof. C. Koval for helpful discus-
sions and Dr. Md. K. Kabir for initial efforts on prepar-
ing [Cr(dcbpy)n]xþ complexes.
Conclusions and Outlook
The preparation of heteroleptic dipyridyl Cr(III) com-
plexes that contain at least one carboxylate group (for even-
tual attachment to semiconductor surfaces) was not known
prior to our efforts. We have synthesized four such com-
plexes, including three heteroleptic [(NN)2Cr(4-dmcbpy)]3þ
species. We have also explored the basic photophysical and
electrochemical properties of these systems to better under-
stand the nascent and long-lived excited-states that will be
Supporting Information Available: X-ray structural data (cif);
details of spectroscopic and electrochemical characterizations
(pdf). This material is available free of charge via the Internet at
http://pubs.acs.org.