Synthesis, crystal structure, and optical and photoelectrochemical properties of a N∩O- rhenium(I) complex

Article abstract of DOI:10.1021/om100311b

The neutral dimer complex Re₂(CO)₆(BIZ)₂ was directly obtained from a simple reaction between Re(CO)₅Cl and N∩OH-type ligand 2-(1-phenyl-1H-benzoimidazol-2-yl)phenol (HBIZ). The further reaction of Re₂(CO)₆(BIZ)₂ with pyridine (Py) led to the monomeric Re(CO)₃(BIZ)(Py), which was characterized by means of single-crystal X-ray diffraction analysis, ¹H NMR, elemental analysis, and UV-visible absorption, photoluminescence, and infrared spectroscopy as well as cyclic voltammetry. The photoelectrochemical properties of Re(CO)₃(BIZ)(Py) spin-coated film modified indium-tin oxide electrode were investigated, and a large cathodic photocurrent up to 0.55 μA/cm² at-0.3 V bias potential was observed. The LUMO and HOMO energy levels of Re(CO)₃(BIZ)(Py) were studied by density functional theory calculations. The possible mechanism of the photocurrent generation was discussed.

Full text of DOI:10.1021/om100311b

712 Organometallics 2011, 30, 712–716
DOI: 10.1021/om100311b
Synthesis, Crystal Structure, and Optical and Photoelectrochemical
Properties of a N∩O- Rhenium(I) Complex
Chuan-Chuan Ju, An-Guo Zhang, Hao-Ling Sun, and Ke-Zhi Wang*
College of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China
Wei-Li Jiang, Zu-Qiang Bian, and Chun-Hui Huang
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials
Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing,
100871, People’s Republic of China
Received April 18, 2010
The neutral dimer complex Re2(CO)6(BIZ)2 was directly obtained from a simple reaction between
Re(CO)5Cl and N∩OH-type ligand 2-(1-phenyl-1H-benzoimidazol-2-yl)phenol (HBIZ). The further
reaction of Re2(CO)6(BIZ)2 with pyridine (Py) led to the monomeric Re(CO)3(BIZ)(Py), which was
characterized by means of single-crystal X-ray diffraction analysis, 1H NMR, elemental analysis, and
UV-visible absorption, photoluminescence, and infrared spectroscopy as well as cyclic voltammetry.
The photoelectrochemical properties of Re(CO)3(BIZ)(Py) spin-coated film modified indium-tin oxide
electrode were investigated, and a large cathodic photocurrent up to 0.55 μA/cm2 at -0.3 V bias potential
was observed. The LUMO and HOMO energy levels of Re(CO)3(BIZ)(Py) were studied by density
functional theory calculations. The possible mechanism of the photocurrent generation was discussed.
þ
been heavily studied.11-14 In contrast, the Re(CO)3 com-
Introduction
plexes containing the anionic N∩C-, N∩S-, and N∩O-
bidentate ligands have received little attention; particularly,
their photoelectric properties have been ill explored.15-23 We
have previously reported electroluminescent properties of a
complex of this family.21 In order to continue our efforts on
Re(CO)3(N∩N)(X)-5 and Re(CO)3(N∩O-)(X)21-type Re(I)
complexes, we have synthesized, structurally characterized,
and photoelectrochemically studied a new Re(I) complex,
Re(CO)3(BIZ)(Py), with N∩O--type ligand BIZ {BIZ =
deprotonated 2-(1-phenyl-1H-benzoimidazol-2-yl)phenol}.
In recent decades, the Re(I) diimine carbonyl complexes have
attracted considerable attention due to their intriguing photo-
physical, photochemical, and electrochemical properties.1-3
They also have excellent potential applications in molecular
and supramolecular luminescence sensing and switching devices,1
probes of environmental changes,4 electroluminescence,5-8
photocatalysis,9 and therapeutic radiopharmaceutþicals.10
Among the Re(I) complexes reported, the Re(CO)3 com-
plexes of general formula Re(CO)3(N∩N)X (where X = Cl-,
Br-, I-, or a neutral ligand such as acetonitrile or pyridine)
with N∩N aromatic diamine bidentate neutral ligands have
Experimental Section
Materials and Apparatus. The synthesis of the target Re(I)
complex is given as follows, and the other materials and instru-
mentation details are given in the Supporting Information.
Synthesis of 2-(1-Phenyl-1H-benzoimidazol-2-yl)phenol, HBIZ.
A solution of N-phenylbenzene-1,2-diamine (0.92 g, 5 mmol)
and 2-hydroxybenzaldehyde (0.61 g, 5 mmol) in 2-(2-methox-
yethoxy)ethanol (10 mL) was microwave heated at 160 ꢀC for 3 h
*To whom correspondence should be addressed. E-mail: kzwang@
bnu.edu.cn.
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r
pubs.acs.org/Organometallics
Published on Web 01/11/2011
2011 American Chemical Society

Article
Organometallics, Vol. 30, No. 4, 2011 713
under N2.24 Upon cooling to room temperature, the reaction
mixture was poured into water (200 mL). The precipitation
formed was purified by column chromatography on silica gel
using dichloromethane as eluent. The crude product obtained was
recrystallized from hexane-dichloromethane (v/v = 1:1), afford-
ing 0.35 g of white crystals (25% yield).
Synthesis of Re2(CO)6(BIZ)2. A solutionof Re(CO)5Cl(0.10 g,
0.28 mmol) and HBIZ (0.08 g, 0.28 mmol) in toluene (10 mL) was
refluxed for 6 h under N2. The solid, precipitated upon cooling the
solution to room temperature, was filtered, washed with dichloro-
methane, and dried at 100 ꢀC in vacuo to obtain a white powder
(0.12 g, 78% yield), which was directly used for the following
synthesis without further purification due to solubility problems.
Anal. Calcd for C44H26N4O8Re2: C 47.56, H 2.36, N 5.04. Found:
C 48.20, H 2.40, N 4.64.
Synthesis of Re(CO)3(BIZ)(Py) (Py = pyridine). A suspension
of Re2(CO)6(BIZ)2 (0.12 g, 0.11 mmol) in pyridine (5 mL) was
refluxed at 120 ꢀC for 3 h under N2. The orange solid obtained
after the solvent was removed under reduced pressure was recrys-
tallized from hexane-dichloromethane (v/v = 1:1) to afford 0.11 g
Figure 1. Perspective drawing of Re(CO)3(BIZ)(Py) with atom-
ic numbering scheme. Thermal ellipsoids are shown at the 30%
probability level.
1
of yellow crystals (85% yield). H NMR (CDCl3, 500 MHz) δ
(ppm):8.74 (d, 2H, J = 5.1), 7.90 (d, 1H, J = 8.2), 7.69 (t, 1H, J =
7.6), 7.54 (m, 3H), 7.34 (t, 1H, J=7.7), 7.27(m, 3H), 7.18(m,3H),
7.12 (t, 1H, J = 7.0), 6.97 (d, 1H, J = 7.9), 6.55 (d, 1H, J = 7.9),
6.15 (t, 1H, J = 7.1). Anal. Calcd for C27H18N3O4Re: C 51.09, H
2.86, N 6.62. Found: C 51.17, H 2.94, N 6.64.
in Re(CO)3(BIZ)(Py) than in Re(CO)3(BIP)(Py), namely, Npyr
more strongly binds to Re in Re(CO)3(BIZ)(Py) than in Re-
(CO)3(BIP)(Py), as illustrated by the shorter bond length of
Results and Discussion
˚
˚
Re-Npyr (2.217 A) in Re(CO)3(BIZ)(Py) than that (2.230A) in
Re(CO)3(BIP)(Py). Selected bond angles of Re(CO)3(BIZ)(Py)
(see Table S3) clearly indicate that the CO ligands are linearly
coordinated; C22, C21, O1, N1, Re1 as well as C22, C20, O1,
N3, Re1 are coplanar. In other words, the pyridine ring
coordinated via its N atom to Re is nearly perpendicular to
the plane consisting of C22, C21, O1, N1, and Re1. The bond
angle of C(1)-O(1)-Re(1) is found to be 117.7ꢀ, characteristic
of an sp3 hybrid state of O(1). The bond angles between
adjacent CO {C(20)-Re(1)-C(21), C(22)-Re(1)-C(20), C-
(22)-Re(1)-C(21)} are 87.6-89.1ꢀ, which are close to 90ꢀ, but
the bond angle between the coordinated nitrogen and oxygen
atom in ligand BIZ, {O(1)-Re(1)-N(1)}, is 80.41ꢀ, which is
much less than 90ꢀ, due not only to the steric requirement of the
bidentate ligand BIZ but also to a twist of the imidazole ring
relative to the phenol plane. This twist is clearly verified by the
dihedral angle (∼32ꢀ) between the imidazole and the phenol rings.
The IR spectrum of Re(CO)3(BIZ)(Py) (see Figure S3,
Supporting Information) exhibited two distinct νCO bands in
the region of CO stretching vibration (1800-2100 cm-1),
which are different from three νCO bands observed for Re2-
(CO)6(BIZ)2 and previously reported pseudooctahedral Re(I)
complexes with three facial CO ligands.21,22,28 This could be
understood by the fact that two of three C-Re bond lengths
Re(CO)3(BIZ)(Py) was synthesized according to the route
described in Scheme S1 (Supporting Information) and was
1
characterized by elemental analysis, H NMR spectroscopy,
and single-crystal X-ray diffraction. As shown by thermogra-
vimetric and differential thermal curves (see Figures S1 and S2,
Supporting Information), Re2(CO)6(BIZ)2 and Re(CO)3(BIZ)-
(Py) did not melt before they started thermal decomposition at
418 and 218 ꢀC, respectively.
The crystals of Re(CO)3(BIZ)(Py) suitable for single-crystal
X-ray analysis were obtained by slow evaporation of a hexane-
dichloromethane (v/v = 1:1) solution. A perspective drawing
of the structure of Re(CO)3(BIZ)(Py) with atomic number-
ing scheme is shown in Figure 1, and the crystallographic
data are given in Table S1 in the Supporting Information.
Selected bond lengths and angles for Re(CO)3(BIZ)(Py) are
shown in Tables S2 and S3 (Supporting Information), respec-
tively. The bond length of Re-Nchel (from the N∩O- ligand,
˚
2.177 A) is shorter than that of Re-Npyr (from pyri-
˚
dine, 2.217 A), revealing that the Nchel atom binds to Re more
strongly than the Npyr atom. The bond length of Re-Ochel
˚
˚
(2.125 A) is shorter than that of Re-Nchel (2.177 A), indicating
stronger coordination ability of Ochel than that of Nchel and the
covalent character of the Re-Npyr and Re-Nchel bonds.25-27
˚
{1.901(3), 1.900(3), and 1.916(3) A} as well as two of three
˚
˚
The bond lengths of Re-Ochel (2.125 A) and C-Ochel (1.326 A)
support the ionic character of the Ochel atom, and accordingly
the enol character of the ligand. The bond length of Re-Ochel in
Re(CO)3(BIZ)(Py) is close to that previously reported in Re-
(CO)3(BIP)(Py)20 {BIP = deprotonated 2-(1-methyl-1H-ben-
zoimidazol-2-yl)phenol} within experimental error, while the
bond length of Re-Nchel (2.177 A) in Re(CO)3(BIZ)(Py) is
longer than that (2.155 A) in Re(CO)3(BIP)(Py). It can also be
seen that there is less steric hindrance for pyridine coordination
˚
C-O bond lengths {1.153(4), 1.148(3), and 1.158(3) A} in
Re(CO)3(BIZ)(Py) are almost the same within experimental
errors.
Re(CO)3(BIZ)(Py) is well soluble in a wide range of organic
solvents, polar and less polar as well. On the contrary, dimeric
Re2(CO)6(BIZ)2 is much less soluble in organic solvents than
the monomeric Re(CO)3(BIZ)(Py). Re2(CO)6(BIZ)2 is not
stable in strongly coordinating solvents with relatively high
values of the donor number DN, such as pyridine, nitriles, and
amides,29,30 dissociating into Re(CO)3(BIZ)(solv). However, it
˚
˚
(24) Lin, S. Y.; Isome, Y.; Stewart, E.; Liu, J. F.; Yohannes, D.; Yu,
L. B. Tetrahedron Lett. 2006, 47, 2883.
(25) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F. Inorg.
(28) Stor, G. J.; Hartl, F.; van Outersterp, J. W. M.; Stufkens, D. J.
Organometallics 1995, 14, 1115.
Chem. 1986, 25, 3858.
(26) Donges, D. J.; Nagle, K.; Yersin, H. J. Lumin. 1997, 72-74, 658.
(27) Donges, D. J.; Nagle, K.; Yersin, H. Inorg. Chem. 1997, 36, 3040.
(29) Reinchardt, C. Solvent Effects in Organic Chemistry; Verlag
Chemie: Weinheim, NY, 1979.

714 Organometallics, Vol. 30, No. 4, 2011
Ju et al.
for Re2(CO)6(8-quinolinato)2 in pyridine-dichloromethane
mixture,19 and is the same order of magnitude as ksolv values
of 3.4 ꢀ 10-3 and 4.5 ꢀ 10-3 M-1 min-1 previously reported
for Re2(CO)6(BOP)2 {BOP = deprotonated 2-benzoxazol-
2-ylphenol} in pyridine-dichloromethane mixture20 and
Re2(CO)6(TOB)2 in neat pyridine,21 respectively. However, it
is much greater than ksolv values of 3.3 ꢀ 10-5 and 1.1 ꢀ 10-4
M-1 min-1 previously reported for Re2(CO)6(BIP)2 and
Re2(CO)6(BTP)2 {BTP = deprotonated 2-benzothiazol-2-yl-
phenol} in neat pyridine, respectively.20 The intrinsic charac-
teristics of different complexes may be the cause of the
difference in the ksolv values. Re2(CO)6(BIZ)2 and Re2(CO)6-
(BIP)2 differ only in methyl and phenyl substitutes, but the ksolv
value of Re2(CO)6(BIZ)2 is 2 orders of magnitude greater than
that of Re2(CO)6(BIP)2. This may be because the planar
phenyl in Re2(CO)6(BIZ)2 has less steric hindrance to pyridine
attack than the tetrahedral methyl in Re2(CO)6(BIP)2.
The photoluminescence excitation and emission spectra of
Re(CO)3(BIZ)(Py) in dichloromethane solution are shown in
Figure S5 (Supporting Information). The excitation spectrum
(λem=510 nm) showed two distinct bands at 295 and 380 nm,
which are close to the UV-vis absorption maxima. Upon
excitation at both 295 and 380 nm, Re(CO)3(BIZ)(Py) exhib-
ited a broad emission band centered at 510 nm from metal-to-
ligand charge-transfer (3MLCT) excited state,19-21 which is
evidently different from two emission bands at 405 and 480 nm
observed for the free HBIZ in the same solvent (λex = 320 nm)
and is blue-shifted by ∼130 nm with respect to Re(CO)3-
(TOB)(Py),21 probably due to the better electron-donating
property of BIZ than TOB. The photoluminescence quantum
yield for Re(CO)3(BIZ)(Py) in air-saturated dichloromethane
solution was determined to be 0.14% (λex = 294 nm) and 0.2%
Figure 2. Evolution of UV-visible absorption spectra of 10 μM
Re2(CO)6(BIZ)2 in pyridine with time (from 0 to 250 min).
Arrow shows spectral changes upon increasing time.
is stable toward weakly coordinating solvents such as dichloro-
methane and toluene, similar to analogous N∩O- Re(I)
complexes.20 The UV-vis absorption spectra of Re2(CO)6-
(BIZ)2 and Re(CO)3(BIZ)(Py) in dichloromethane are com-
pared in Figure S4 (Supporting Information). Re2(CO)6(BIZ)2
exhibited a metal-to-ligand charge-transfer (MLCT) absorp-
tion band (shoulder) centered at around 330 nm and a N∩O-
ligand-based π-π* absorption band centered at 295 nm,
which was red-shifted by 10 nm and blue-shifted by 15 nm,
respectively, relative to Re2(CO)6(TOB)221 {TOB = deproto-
nated 2-(2H-benzo[d][1,2,3]triazol-2-yl)-4-(1,1,3,3-tetra-
methylbutyl)phenol} and Re2(CO)6(BIP)2 in the same
solvent.22 Re(CO)3(BIZ)(Py) in dichloromethane showed a
strong well-defined absorption band at 295 nm that is almost
the same as that observed for the dimer, and a low-energy band
centered at 380 nm that is close to that previously reported for
structurally similar Re(CO)3(BIP)(Py)22 and blue-shifted by
40 nm compared with Re(CO)3(TOB)(Py).21 The evident
difference in the UV-vis absorption spectra between the
monomeric and dimeric complexes can be utilized to monitor
their presence in solution. In particular the absorbance above
350 nm may serve as a fingerprint of the monomeric form. In
this region, Re2(CO)6(BIZ)2 is almost transparent, whereas
Re(CO)3(BIZ)(Py) shows intense absorption; thus we can moni-
tor the spectral evolution of Re2(CO)6(BIZ)2 with time in a
coordinating medium. The time-dependent spectral changes
of Re2(CO)6(BIZ)2 in neat pyridine are shown in Figure 2.
The data were analyzed according to pseudo-first-order rate
eq 1:19-21
(λex = 380 nm), and 0.16% (λex = 294 nm) and 0.23% (λex
=
380 nm) for N2-degassed dichloromethane solution, which are
much lower than 2.1-3.2% previously reported for mono-
meric Re(CO)3(L)(Py) (in which L = BIP, BOP, and BTP)22
and even lower than 0.36% for Re(CO)3(TOB)(Py)21 in the
same solvent. On the contrary, dimeric Re2(CO)6(BIZ)2 was
too weakly emissive to be detected, similar to previously
reported Re2(CO)6(BIP)2.20 The excited-state relaxation data
at 510 nm for Re(CO)3(BIZ)(Py) in air-saturated dichloro-
methane were well fit to a single-exponential emission decay
lifetime of 126.9 ( 1.9 ns (see Figure S6, Supporting In-
formation). Assuming an intersystem crossing yield of unity,31
radiative (kr) and nonradiative (knr) rate constants were
calculated to be kr=1.1ꢀ 104 s-1 and knr=7.9 ꢀ 106 s-1
.
Cyclic voltammograms (see Figure 3) of Re(CO)3(BIZ)(Py)
exhibited two irreversible BIZ-based reduction waves at
Epc = -2.18 and -2.68 V with an onset reduction potential
of -2.04 V and one irreversible oxidation wave at E1/2
=
þ1.10 V with an onset oxidation potential of þ0.96 V, which is
assigned to a metal-based ReII/ReI oxidation process and is
close to an E1/2 value of þ1.20 V we previously reported for
Re(CO)3(TOB)(Py).21 The ligand HBIZ exhibited two couples
of irreversible reduction waves at E1/2 = -1.27 and -1.72 V vs
SCE (see Figure S7, Supporting Information), which is less
negative than those observed for the BIZ-based reduction in
Re(CO)3(BIZ)(Py), indicating that BIZ in Re(CO)3(BIZ)(Py)
is more difficult to reduce than in the free HBIZ. Unfortu-
nately, the cyclic voltammetry of Re2(CO)6(BIZ)2 could not be
accomplished due to the solubility problem. From the onset
At=A¥ ¼ 1 - expð - ksolvCsolvtÞ
ð1Þ
whereksolv is the solvolysis rate constant, and At and A¥ are the
absorbance of the solution at time t and at the end of the
reaction, respectively. If a highly excessive coordinating agent
is applied, coordinating agent concentration Csolv at a given t
stays approximately constant. On the basis of the data shown
in Figure 2, a solvolysis rate constant ksolv is derived to be
(3.9 ( 0.1) ꢀ 10-3 M-1 min-1 for Re2(CO)6(BIZ)2 in pyridine,
is much lower than 3.8 ꢀ 10-1 M-1 min-1 previously reported
(30) Gutmann, V. The Donor-Acceptor Approach to Molecular
Interactions; Plenum Publ. Corp.: New York, 1978.
(31) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
Zelewsky, A. V. Coord. Chem. Rev. 1988, 84, 85.

Article
Organometallics, Vol. 30, No. 4, 2011 715
and HOMO energies of the complex were computed to
be -1.95 and -5.40 eV, respectively, giving a gap of 3.45 eV
between the these two energy levels, which is greater than those
(2.95-3.0 eV) obtained from the cyclic voltammetry and the
visible absorption spectrum. This difference between the
experimental and the calculated energy values is in good
agreement with the error reported as a constant for transition
metal complexes, which may be attributed to a systemic error
coming from theory calculations and environment factors,
e.g., molecular solvation, geometrical changes, and polariza-
tion in aqueous solution and so on.38,39 Moreover, such a
systemic error can be obviously reduced by using a standard
sample correlation method, e.g., [Ru(bpy)3]2þ for Ru(II)
complexes.40,41
Re(CO)3(BIZ)(Py) was also spin-coated from its dichloro-
methane solution onto ITO substrates for photoelectrochem-
ical studies. The concentrations of the dichloromethane
solutions were varied to prepare a Re(CO)3(BIZ)(Py)-mod-
ified ITO electrode of varied thickness. As shown in Figure S9
(Supporting Information), the absorbances at 380 nm or the
film thicknesses of the Re(CO)3(BIZ)(Py) films increased
linearly with increasing concentrations of the spin-coating
solutions. As seen from Figure S10 (Supporting Information),
the photocurrents attained saturation for a film prepared at a
spin-coating solution concentration of 4 mg/mL and then
decayed as the concentrations were further increased. This
photocurrent trend is independent of the applied potentials. A
4 mg/mL concentration of the spin-coating solution was thus
selected for making a spin-coating film for the photoelectro-
chemical studies described below. The film that was spin-
coated at this concentration showed relatively continuous
island-like clusters or aggregates that were closely packed
and randomly stacked, making the film rough and jagged at
the surface, as revealed by scanning electron microscopy (see
Figure S11, Supporting Information). The photoresponses of
the Re(CO)3(BIZ)(Py) film on the ITO electrode under white-
light irradiation (100 mW/cm2) at different bias potentials are
shown in Figure 4. As the negative bias potentials were applied
to the working electrode, stable and rapid cathodic photo-
currents were induced by switching on the irradiation to
a Re(CO)3(BIZ)(Py) film. A photocurrent density up to
0.55 μA/cm2 at -0.3 V bias potential was obtained. The
photocurrent generation is prompt and reproducible with
negligible decay under many on/off cycles of irradiation. As
shown in Figure S12 (Supporting Information), the photo-
currents increased as the applied bias potential becomes more
negative, indicating that the photocurrent polarity is cathodic.
In other words, the negative bias potential speeded up the
generation of photoinduced electron-hole pairs in transfer-
ring the conduction band electron of ITO to the HOMO of
Re(CO)3(BIZ)(Py) and then to the electron acceptor in the
supporting electrolyte, inhibiting the recombination of electron-
hole pairs. There is another possible reason that the bias
potential on the electrode would increase the energy of the
conduction band of ITO, making the electron transfer from the
conduction band of ITO to the HOMO of Re(CO)3(BIZ)(Py)
Figure 3. Dependence of the cyclic voltammograms of Re-
(CO)3(BIZ)(Py) in acetonitrile on the scan rates. The inset is
the dependence of anodic peak currents for the oxidation wave
at þ1.19 V on the square root of scan rates.
anodic peak potential for oxidation process [Epa(onset)]ox
and onset cathodic peak potential for reduction process
[Epc(onset)]red, the HOMO and LUMO energy levels of Re-
(CO)3(BIZ)(Py) were estimated by taking 4.74 eV for SCE
with respect to the vacuum level according to the formulas
EHOMO = -4.74 - [Epa(onset)]ox, ELUMO = -4.74 -
[Epc(onset)]red, and ELUMO - EHOMO in eV = 1240/λmax
,
where λmax is the wavelength in nm at which the material has
the lowest-energy onset absorption.5,21,32-34 These calcula-
tions give the HOMO and LUMO energy levels of -5.70
and -2.70 eV, respectively. The gap between the LUMO and
HOMO energy levels is thus derived to be 3.00 eV, which is
about 0.05 eV greater than the value of 2.95 eV obtained from
an absorption edge at 421 nm of visible absorption spectrum
for Re(CO)3(BIZ)(Py) in CH2Cl2 and is greater than a gap
value of 2.65 eV we previously reported for Re(CO)3(TOB)-
(Py).21 The inset of Figure 3 shows a linear dependence of the
anodic peak currents for the oxidation wave of Re(CO)3(BIZ)-
(Py) at þ1.19 V on the square root of scan rates, indicating
diffusion-controlled redox reactions.35-37
The energies of some frontier molecular orbitals of Re-
(CO)3(BIZ)(Py) were also computed using the density func-
tional theory (DFT) method, and the results are shown in
Figure S8 (Supporting Information). As shown in Table S3
(Supporting Information), the computed bond lengths and
angles for Re(CO)3(BIZ)(Py) are very close to those structu-
rally determined, indicating that the results of the geometry
optimization by the DFT method are reliable. It can be seen
that the highest occupied molecular orbital (HOMO) of Re-
(CO)3(BIZ)(Py) is localized over the center metal atom (Re)
and phenolic and carbonyl groups and the lowest unoccupied
molecular orbital (LUMO) on the pyridine, indicating that the
HOMO-LUMO excitation could move the electron distribu-
tion from the center Re atom to the pyridine unit. The LUMO
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716 Organometallics, Vol. 30, No. 4, 2011
Ju et al.
dissolved oxygen played an important role in the process of
photocurrent generation. Figure S14 (Supporting Information)
shows the possible mechanism of the photocurrent generation.
For highly n-doped semiconductors such as ITO, there are no
unique values for the energies of the conduction band (EC) and
valence band (EV) edges. However, from the electron affinity
it is possible to estimate the value of EC as approximately -4.5
eV.42,43 When Re(CO)3(BIZ)(Py) in its ground state was
excited by white-light irradiation (730 nm > λ > 325 nm),
the excited-state Re(CO)3(BIZ)(Py)* was formed, which was
then oxidized by donating an electron to the electron acceptor
dioxygen in the supporting electrolyte solution. The resulting
oxidized Re(CO)3(BIZ)(Py)þ was finally reduced by accepting
an electron from the conduction band of ITO, completing the
circuit.
In conclusion, a new N∩OH ligand-based monomeric Re(I)
complex, Re(CO)3(BIZ)(Py), has been synthesized and struc-
turally and spectroscopically characterized, which belongs to a
rarely reported Re(CO)3(N∩O-)(X) family of complexes. The
dimeric Re2(CO)6(BIZ)2 in neat pyridine was found to convert
into Re(CO)3(BIZ)(Py) at an intermediate rate constant of
(3.9 ( 0.1) ꢀ 10-3 M-1 min-1. Re(CO)3(BIZ)(Py) in dichlor-
omethane is weakly photoluminescent, with a quantum yield
of ∼0.2%. The modified electrode of Re(CO)3(BIZ)(Py) on
ITO exhibited a large cathodic photocurrent up to 0.55 μA/cm2
under white-light irradiation of 100 mW/cm2 at -0.3 V bias
potential. This study would provide fundamental experimental
data for potential applications of Re(CO)3(BIZ)(Py) in photo-
electric conversion devices.
Figure 4. Current changes induced by switching on and off the
white-light irradiation (100 mW/cm2) of the Re(CO)3(BIZ)(Py)
film (0.28 cm2) at different bias potentials vs SCE. Supporting
electrolyte: 0.1 M Na2SO4.
highly favorable energetically. The increase of the driving force
accelerates the electron-transfer rate, and accordingly
increases the photocurrent.42-45
In order to get insight into the mechanism of the photo-
current generation, the photoelectrochemical experiments
were carried out in both deoxygenated and air-equilibrated
supporting electrolyte solutions. As shown in Figure S13
(Supporting Information), the dissolved oxygen in the sup-
porting electrolyte strongly influenced the photocurrent gen-
eration. When the supporting electrolyte was deoxygenated
with bubbling N2 for 30 min, the photocurrent decreased by
75% (from 40 to 10 nA), 69% (from 77 to 24 nA), and 54%
(from 95 to 44 nA) as the applied potentials were set at -0.1,
-0.2, and -0.3 V, respectively. So we can conclude that the
Acknowledgment. The authors thank the National
Natural Science Foundation (20771016, 20971016, and
90922004), the Fundamental Research Funds for the
Central Universities, and Analytical and Measurements
Fund of Beijing Normal University for financial support.
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Supporting Information Available: Experimental details and
CIF files giving crystallographic data of Re(CO)3(BIZ)(Py).
This material is available free of charge via the Internet at
http://pubs.acs.org.