Diimine triscarbonyl re(I) of isomeric pyridyl-fulvene ligands: An electrochemical, spectroscopic, and computational investigation

Article abstract of DOI:10.1021/ic301559s

The synthesis and characterization of a novel family of positively charged fac-[Re(bpy)(CO)₃(L)]PF₆ (bpy = 2,2′-bipyridine) complexes are reported, where L is a pyridine functionalized in para or meta position with a fulvene moiety, namely, 4-fluoren-9-ylidenemethyl-pyridine (pFpy) and 3-fluoren-9-ylidenemethyl-pyridine (mFpy). The complexes were prepared in high yield (86%) by direct addition at room temperature of the corresponding pyridine to the tetrahydrofuran (THF) adduct fac-[Re(bpy)(CO) ₃(THF)][PF₆] precursor. Both ligand and complex structures were fully characterized by a variety of techniques including X-ray crystallography. The complexes did not exhibit the expected triplet mixed metal-ligand-to-ligand charge transfer (MLLCT) emission, because of its deactivation by the non-emissive triplet excited state of fulvene. The absorption profile shows that the MLLCT is overshadowed by the fulvene centered φ-φ* transition of higher molar absorptivity as shown by time dependent density functional theory (TD-DFT) calculations. The position of the fulvene on the pyridyl ring has a large effect on this transition, the para position displaying a much higher absorption coefficient (21.3 ×10 ³ M^-1 cm^-1) at lower energy (364 nm) than the meta position (331 nm, 16.0 ×10³ M^-1 cm ^-1)

Full text of DOI:10.1021/ic301559s

Article
pubs.acs.org/IC
Diimine Triscarbonyl Re(I) of Isomeric Pyridyl-fulvene Ligands: an
Electrochemical, Spectroscopic, and Computational Investigation
Daniel Chartrand, Carlos A. Castro Ruiz, and Garry S. Hanan*
́ ́ ́ ́
Department of Chemistry, Universite de Montreal, Montreal, Quebec H3T 1J4, Canada
S
* Supporting Information
ABSTRACT: The synthesis and characterization of a novel family of positively
charged fac-[Re(bpy)(CO)3(L)]PF6 (bpy = 2,2′-bipyridine) complexes are re-
ported, where L is a pyridine functionalized in para or meta position with a
fulvene moiety, namely, 4-fluoren-9-ylidenemethyl-pyridine (pFpy) and 3-fluoren-
9-ylidenemethyl-pyridine (mFpy). The complexes were prepared in high yield
(86%) by direct addition at room temperature of the corresponding pyridine
to the tetrahydrofuran (THF) adduct fac-[Re(bpy)(CO)3(THF)][PF6] pre-
cursor. Both ligand and complex structures were fully characterized by a variety
of techniques including X-ray crystallography. The complexes did not exhibit
the expected triplet mixed metal−ligand-to-ligand charge transfer (MLLCT)
emission, because of its deactivation by the non-emissive triplet excited state of
fulvene. The absorption profile shows that the MLLCT is overshadowed by the fulvene centered π−π* transition of higher molar
absorptivity as shown by time dependent density functional theory (TD-DFT) calculations. The position of the fulvene on the
pyridyl ring has a large effect on this transition, the para position displaying a much higher absorption coefficient (21.3 × 103 M−1
cm−1) at lower energy (364 nm) than the meta position (331 nm, 16.0 × 103 M−1 cm−1)
fluorescence to a longer-lived, higher-energy triplet state
centered on the rhenium bipyridine that will act as a reservoir
for the triplet state centered on the fulvene. This process is
well described for Ru(II) diimine complexes containing fused
polycyclic moieties.7
INTRODUCTION
■
Rhenium triscarbonyl diimine chromophores have been studied
extensively for the past 30 years; their unique characteristics (e.g.,
high stability, high energy excited states, capacity for reductive
and oxidative electron transfer) make them ideal for photo-
sensitizers.1 They are found as photoactive components in various
roles: chromophores supplying electrons to catalysts (e.g., hydrogen
evolution);2 photocatalysts reducing carbon dioxide to carbon
monoxide;3 photosensitizers permitting visible light photo-
isomerization;4 and as chromophoric building-blocks for supra-
molecular assemblies.5 One common variant involves a 2,2′-
bipyridine (bpy) ligand and a secondary neutral ligand L in
complexes of the type [Re(bpy)(CO)3L]+, with pyridine being
the motif of choice for further functionalization of the rhenium
chromophore through ligand L. To expand the utility of the
rhenium-bpy unit, a meta and para fulvene functionalized
pyridine was used as ligand L in this study, and the chemical
and photophysical properties of its rhenium complexes were
determined.
The properties of the fulvene ligands are similar to those of
stilbene molecules in that they can undergo isomerization through
their triplet excited state,6 albeit the irradiation energy necessary
for this isomerization is often found only in the UV region unless
highly conjugated molecules are used. In addition, the quantum
yield of isomerization is usually relatively low because of the triplet
state of the molecule competing with the permitted fluorescence
decay of the singlet excited-state. As such, grafting a rhenium onto
the fulvene core may enhance both the absorption, by causing
its red-shift with a concomitant increase in the coverage of a
broader spectrum of light, and the efficiency, by quenching the
Although energy transfer has been investigated using rhe-
nium complexes displaying intramolecular charge transfer more
than 25 years ago,8 recent research has examined the addition
of photoisomerizable ligands, containing stilbene analogues and
other moieties, that once coordinated quench the luminescence
of the rhenium in favor of their isomerization.4a−i Closer to our
design is the 4-styrylpyridine motif, which upon coordination to
rhenium was shown to undergo photoisomerization with lower
energy light and at higher efficiency by spectroscopic and high
level computational studies.4k−m In our case, no isomerization
is observable because of the symmetry of the ligand, but energy
transfer can still be observed because of the absence of emission
of the rhenium complex, suggesting that the non-radiative decay
of the excited state passes by the triplet state of the fulvene.
Another interesting aspect of this study is the delocalization of
the fulvene orbitals onto the rhenium core, which is seen in
only one of the pyridyl isomers.
EXPERIMENTAL SECTION
■
General Considerations. All of the organic reagents were ob-
tained from Sigma Aldrich, rhenium carbonyl from Pressure Chemical
Co. and solvents from Fischer and Anachemia and were used as received
Received: July 17, 2012
Published: November 15, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Article
without any further purification. IR spectra were recorded on solid
samples using a diamond ATR Perkin-Elmer Spectrum 100 FT-IR.
Nuclear magnetic resonance spectra were recorded using Bruker spec-
for C19H13N: C, 89.38; H, 5.13; N, 5.49. Found: C, 88.85; H, 5.11; N,
5.28.
4-((9H-fluoren-9-ylidene)methyl)pyridine (pFpy). Synthesized
as the previous example, purification by crystallization from ethyl acetate/
hexane (1:2 v:v) instead of column chromatography. Afforded pale
yellow crystalline product. Yield: 38−50%. 1H NMR (300 MHz,
CDCl3) δ ppm 8.71 (d, J = 6 Hz, 2He), 7.76 (d, J = 8 Hz, 1Hh), 7.70
(d, J = 8 Hz, 2Hmm′), 7.50 (s, 1Hi), 7.48 (d, J = 5 Hz, 2Hf), 7.44 (d,
J = 8 Hz, 1Hj′), 7.41 (dt, J = 7, 1 Hz, 1Hk), 7.34 (dt, J = 7, 1 Hz,
2Hll′), 7.06 (dt, J = 7, 1 Hz, 1Hk′). 13C{1H} NMR (75 MHz, CDCl3)
δ ppm 150.2, 141.6, 140.3, 139.5, 138.8, 138.7, 135.9, 129.3, 129.0,
127.2, 126.9, 124.5, 123.8, 123.2, 120.5, 120.0, 119.7. HRMS (ESI,
MeCN) (m/z): [M+H]+ (C19H14N) calcd 256.11207; found
256.11240. Anal. Calcd for C19H13N: C, 89.38; H, 5.13; N, 5.49.
Found: C, 89.35; H, 5.17; N, 5.50.
trometers (300 and 400 MHz) at room temperature, with 1H and 13
C
chemical shifts referenced to residual solvent resonances. Elemental
analyses were performed on the desolvated bulk samples by the uni-
versity departmental service. Room temperature Photophysical mea-
surements were done in air-equilibrated and degassed freshly distilled
dichloromethane (DCM), using a quartz cell. Low-temperature emis-
sion were recorded in degassed 1 MeOH: 4 EtOH (v:v) and 1 toluene:
1 DCM (v:v) glasses at 77 K in a borosilicate tube. Absorption and emis-
sion spectra were recorded using a Cary 500i UV−vis−NIR spectro-
photometer and a Cary Eclipse 300 fluorimeter, respectively. Oscillator
strengths and peak maxima are obtained from integrated fitted Gaussian
curves of the molar absorptivity spectrum in function of wavenumber,
following the following equation: f = 1.44 × 10−9∫ ε(ν) dν. The emission
spectra used the maximum of absorption of the lowest energy band of
the studied molecules as excitation wavelength. Electrochemical mea-
surements were carried out in argon-purged DCM at room temperature
with a BAS CV50W multipurpose equipment interfaced to a PC. The
working electrode was a Pt electrode, the counter electrode was a Pt wire,
and the pseudoreference electrode was a silver wire. The reference was
set using an internal 1.0 mM ferrocene/ferrocinium sample with its redox
couple adjusted to 460 mV vs SCE in dichloromethane.9 The con-
centration of the compounds was around one mM. Tetrabutylammo-
nium hexafluorophosphate (TBAP) was used as supporting electrolyte,
and its concentration was 0.10 M. Cyclic voltammograms (CVs) were
obtained at scan rates of 50 and 100 mV/s. For reversible processes,
half-wave potentials (vs SCE) were measured with square wave
voltammetry (SWV) experiments performed with a step rate of 4 mV,
a square wave amplitude of 25 mV, and a frequency of 15 Hz. For
irreversible oxidation processes, the cathodic peak was used as E, and
the anodic peak was used for irreversible reduction processes. The
criteria for reversibility were the separation of 60 mV between cathodic
and anodic peaks, the close to unity ratio of the intensities of the
cathodic and anodic currents, and the constancy of the peak potential
on changing scan rate. Experimental uncertainties are as follows: ab-
sorption maxima, 2 nm; molar absorption coefficient, 10%; emission
maxima, 5 nm; redox potentials, 10 mV.
3-((9H-fluoren-9-ylidene)methyl)pyridine-2,2-bipyridine-
tricarbonyl-rhenium(I) Hexafluorophosphate [Re(bpy)-
(CO)3(mFpy)][PF6]. (1) Under an inert atmosphere, fac-[Re(bpy)(CO)3-
(THF)][PF6] (79.3 mg, 123 μmol) and mFpy (32.2 mg, 126 μmol)
were mixed in THF (5 mL) and left to stir for 16 h at room tem-
perature. The product was then precipitated by addition of hexane
(50 mL), resulting in a yellow solid. The product was purified by dis-
solution in a minimum of dichloromethane (2 mL) with a few drops of
toluene and followed by slow evaporation of DCM in a closed, toluene-
containing jar, affording a pale yellow solid. Yield: 87.0 mg (105 μmol)
1
(86%). H NMR (400 MHz, (CD3)2CO) δ ppm 9.48 (ddd, J = 5, 2,
1 Hz, 2Ha), 8.76 (d, J = 8 Hz, 2Hd), 8.72 (dd, J = 2, 1 Hz, 1He), 8.66
(d, J = 5 Hz, 1Hf), 8.41 (dt, J = 8, 2 Hz, 2Hb), 8.22 (d, J = 8 Hz, 1Hh),
7.90 (ddd, J = 8, 5, 1 Hz, 2Hc), 7.83 (d, J = 8 Hz, 3Hjmm′), 7.66 (dd,
J = 8, 5 Hz, 1Hg), 7.58 (s, 1Hi), 7.45 (dt, J = 8, 1 Hz, 1Hk), 7.37 (ddd,
J = 13, 8, 1 Hz, 2Hll′), 6.97 (dt, J = 8, 1 Hz, 1Hk′), 6.77 (d, J = 8 Hz,
1Hj′). 13C{1H} NMR (75 MHz, (CD3)2CO) δ ppm 196.3, 156.8, 154.9,
152.8, 152.2, 142.4, 142.4, 141.2, 140.43, 140.35, 139.4, 137.3, 136.3,
130.43, 130.26, 129.96, 128.36, 128.23, 127.8, 125.8, 124.4, 121.84, 121.28,
121.19, 120.7. HRMS (ESI, MeCN) (m/z): [M]+ (C43H21N3O3187Re)
calcd 682.11349; found 682.11275. IR (ATR, cm−1) νCO: 2031s, 1944s,
1911s; νPF 837s. Anal. Calcd for C32H21F6N3O3PRe + 0.5 toluene: C,
48.85; H, 2.89; N, 4.81. Found: C, 48.53; H, 2.76; N, 4.81.
4-((9H-fluoren-9-ylidene)methyl)pyridine-2,2-bipyridine-
tricarbonyl-rhenium(I)hexafluorophosphate [Re(bpy)-
(CO)3(pFpy)][PF6]. (2) Complex 2 was synthesized in the same
manner as the previous example. The product was purified by dis-
solution in a minimum of chloroform (2 mL) with a few drops of ethyl
acetate and crystallized by slow diffusion of ethyl acetate in a closed jar.
Yield: 90.0 mg (108 μmol) (86%). 1H NMR (400 MHz, (CD3)2CO) δ
ppm 9.54 (ddd, J = 6, 2, 1 Hz, 2Ha), 8.79 (d, J = 8 Hz, 2Hd), 8.64 (dd,
J = 5, 2 Hz, 2He), 8.51 (dt, J = 8, 2 Hz, 2Hc), 8.04 (ddd, J = 8, 3, 1 Hz,
2Hb), 7.83−7.79 (m, 3Hjmm′), 7.68 (dd, J = 7, 1 Hz, 2Hf), 7.60 (s,
1Hi), 7.42 (dt, J = 8, 1 Hz, 1Hk), 7.38 (dt, J = 8, 1 Hz, 1Hl′), 7.33 (dt,
J = 8, 1.0 Hz, 1Hl), 7.25 (d, J = 8 Hz, 1Hj′), 7.01 (dt, J = 8, 1 Hz,
1Hk′). 13C{1H} NMR (75 MHz, (CD3)2CO) δ ppm 196.5, 156.8,
155.0, 153.1, 149.8, 142.88, 142.37, 141.2, 140.4, 139.5, 135.9, 130.97,
130.56, 130.07, 128.5, 127.89, 127.78, 125.76, 125.05, 122.62, 122.01,
121.2, 120.8. HRMS (ESI, MeCN) (m/z): [M]+ (C43H21N3O3187Re)
calcd 682.11349; found 682.11374. IR (ATR, cm−1) νCO: 2028s,
1925s, 1903s; νPF 832s. Anal. Calcd for C32H21F6N3O3PRe: C, 46.49;
H, 2.56; N, 5.08. Found: C, 46.56; H, 2.50; N, 4.99.
Computational Methods. All calculations were performed with
the Gaussian 03 software.17 All models used crystallographic structure
data as starting point for ground-state geometry optimization (singlet
and triplet). The geometry optimization was carried out with the den-
sity functional theory (DFT) method using the B3LYP functional in
the gas phase.18 The 6-31G** basis set was used for C, H, N, and O
while the relativistic LANL2DZ with effective core potentials and one
additional f-type polarization function was implemented for the Re
atom (αf = 0.890).19 Energy levels and frequencies for both singlet and
triplet optimized geometries were calculated using single-point energy
calculation with a polarized continuum model (PCM) using dichloro-
methane as solvent.20 The absorption spectra properties in DCM were
calculated by the time-dependent DFT (TD-DFT) approach
Synthetic Methods. The rhenium complexes Re(CO)5Br,10 fac-
[Re(bpy)(CO)3Br],11 fac-[Re(bpy)(CO)3(MeCN)][PF6],12 fac-[Re-
(bpy)(CO)3(pyridine)][PF6] (3), and fac-[Re(bpy)(CO)3(THF)]-
[PF6]13 were prepared following the literature procedures. The ligands
mFpy and pFpy were prepared by a modified literature method.14
They have been synthesized by a similar route before, but with only
basic characterization.15 Herein, their free base versions are fully char-
acterized. See Supporting Information, Chart S1 for the labeling of the
proton peaks in NMR. Note that the hydrochloride salts were char-
acterized more recently by proton NMR as well.16
3-((9H-fluoren-9-ylidene)methyl)pyridine (mFpy). Fluorene
(4.023 g, 24.2 mmol) and KOH (2.028 g, 36.1 mmol) were taken
in ethylene glycol dimethyl ether (35 mL) in a round bottomed flask
and stirred at reflux for 10 min. 3-Pyridinecarboxaldehyde (3.89 g,
36.3 mmol) was added dropwise, and the stirring was continued at the
same temperature for 4 h, during which the initial red mixture turned
dark green. After the mixture was cooled to room temperature, water
(200 mL) was added, and the mixture stirred at ambient temperature
for 45 min, resulting in a yellow-orange suspension. The suspension
was isolated by filtration, and the resulting yellow precipitate was
washed with hexane (4 × 40 mL) and purified by column chro-
matography (Al2O3, DCM/AcOEt (v:v) (1:1)) to afford a pale yellow
pure compound. Yield: 70%.1H NMR (CDCl3 300 MHz) δ ppm 8.85
(s, 1He), 8.65 (d, J = 4 Hz, 1Hf), 7.89 (d, J = 8 Hz, 1Hh), 7.78 (d, J =
7 Hz, 1Hj), 7.71 (d, J = 7 Hz, 2Hmm′), 7.57 (s, 1Hi), 7.45−7.37 (m,
3Hgj′k), 7.34 (ddt, J = 7, 3, 1 Hz, 2Hll′), 7.06 (dt, J = 8, 1 Hz, 1Hk′).
13C{1H} NMR (75 MHz, CDCl3) δ ppm 150.2, 149.0, 141.5, 139.4,
138.9, 138.5, 136.5, 136.1, 133.9, 129.1, 128.7, 127.2, 126.9, 124.2,
123.3, 122.6, 120.4, 119.9, 119.7. HRMS (ESI, MeCN) (m/z):
[M+H]+ (C19H14N) calcd 256.11207; found 256.11255. Anal. Calcd
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Table 1. Crystal Data and Structure Refinement Information for mFpy, pFpy, 1, and 2
mFpy
pFpy
1
2
formula
cryst. syst.
space grp
a (Å)
C19H13N
triclinic
C19H13N
monoclinic
P21/c
[C32H21N3O3Re][F6P]·0.5(C4H8O2)
orthorhombic
Pca21
[C32H21N3O3Re][F6P] ·CH2Cl2
triclinic
P1
̅
P1
̅
6.2909(4)
11.8465(7)
18.5314(11)
90.475(3)
96.308(3)
105.025(3)
1324.79(14)
4
9.2782(4)
21.8542(9)
6.9306(3)
90
29.1631(5)
12.2319(2)
18.7356(44)
90
8.1758(3)
14.2872(5)
14.8093(5)
90.429(1)
90.852(2)
102.281(2)
1689.99(10)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
104.675(2)
90
90
90
1359.46(10)
4
6683.4(2)
4
Z
R1; wR2
(I > 2σ(I))
R1; wR2
(all data)
GoF (F2)
Flack par.
0.0472;
0.0422;
0.1111
0.0466;
0.1142
1.035
0.0263;
0.0730
0.0258;
0.0735
1.065
0.0313;
0.1151
0.0832
0.0685;
0.0313;
0.1227
0.0833
0.985
1.089
n.a.
n.a.
−0.002(6)
n.a.
associated with the polarized continuum model (PCM).20
GaussSum 2.2 was employed to draw absorption spectra (simulated
with Gaussian distribution with a full-width at half-maximum (fwhm) set
to 3000 cm−1) and extract the atomic orbital population information
from calculated data.21
Chart 1. Synthesis of the Rhenium Chromophores with the
Labeling of the Species
Crystal Structure Determination. X-ray crystallographic data
were collected from a single crystal sample, which was mounted on a
loop fiber. For both ligands, data were collected with a Bruker Plat-
form diffractometer, equipped with a Bruker SMART 4K Charged-
Coupled Device (CCD) Area Detector and a Nonius FR591 rotat-
ing anode Cu Kα X-ray radiation source equipped with a Montel
200 optics at 200 (2) K. For the two complexes, data were collected
with a Bruker Microstar diffractometer equipped with a Platinum 135
CCD Detector at 150 (2) K. The data was integrated with APEX2
software and corrected for absorption using the SADABS package.22
Following analytical absorption corrections and solution by direct
methods, the structures were refined against F2 with full-matrix least-
squares using the program SHELXL-97.23 All H-atoms were added
at calculated positions and refined by use of riding models with
isotropic displacement parameters based on those of the parent atoms.
Anisotropic displacement parameters were employed throughout for
the non-hydrogen atoms. Images were generated using Ortep III and
Pov-Ray.24 X-ray quality crystals were obtained from slow evaporation
of a chloroform solution for the ligands pFpy and mFpy. Crystals of
1 were obtained by slow diffusion of ethyl acetate into a chloroform
solution of 1. Complex 2 crystallized following the given purification
procedure. Specific parameters of each measurement are located in
Table 1. Note that pFpy has been reported before but without any
complete data set deposited in the Cambridge structural database.15b
The IR spectra of both complexes are consistent with the facial
configuration of the carbonyl ligands as indicated by three
intense absorptions, which are observed in the 2031 to 1911
cm−1 region for 1 and the 2028 to 1903 region for 2. These
bands are similar to other rhenium complexes, for example,
[Re(bpy)(CO)3(pyridine)][CF3SO3] (3), that has bands at
2026, 1921, and 1907 cm−1.25 No bands around 1600 cm−1
could be clearly identified as being part of the fulvene vinyl
group, as they are masked by the other CC stretches of the
bipyridine and pyridine.
The proton chemical shifts remain essentially the same for
both complexes, the biggest change is the downfield shift of the
fluorene proton nearest to the vinyl proton of complex 1 (j′ see
Chart S1 for labeling in Supporting Information). The chemical
shift passes from 7.45 ppm to 6.77 ppm for 1 vs 7.25 ppm for 2.
This extra shielding comes from the Re(bpy)(CO)3 fragment
as it is nearer in the meta version because of the free rotation of
the pyridyl group along its single bond to the fulvene core. The
pyridine protons ortho to the nitrogen ligand shift to similar
amounts upon coordination to the metal center (8.66 and
8.72 ppm for 1 vs 8.64 ppm for 2), as expected.
Both ligand and complexes were fully characterized by X-ray
crystallography and have excellent structure quality, with dis-
order present for the complexes only on the PF6 counterion or co-
crystallized solvent. It is noteworthy that complex 1 and mFpy
crystallized as two independent molecules in the asymmetric unit
RESULTS AND DISCUSSION
■
The ligands were synthesized by a condensation reaction between
fluorene and pyridinecarboxaldehyde, following a slightly mod-
ified protocol for benzylidene-fluorene synthesis.14 Established
procedures for the synthesis of 9-pyridylidenefluorene, studied
mainly for their medicinal properties, are less accessible syn-
thetically.15a,16 Our first attempts to coordinate the ligands to
the rhenium ion, using the precursor [Re(bpy)(CO)3(acetonitrile)]-
[PF6] and 10 equiv of the ligand, following an established meth-
odology, produced only moderate yields.12 [Re(bpy)(CO)3(THF)]-
[PF6], first reported in 1985, was a better starting material as it
is easily synthesized and is much more reactive, as the THF adduct
exchanges neatly at room temperature with only 1 equiv of the
pyridyl ligand (Chart 1).13 Near quantitative yields were obtained,
and no purification was needed prior to crystallization.
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cell, which gives a good indication on the precision of the bond
lengths and inherent variability of the spatial arrangement of
the atoms. Full unit-cell views of all molecules are found in the
Supporting Information, Figures S1 to S4.
The fulvene ligands are similar in structure, with the pyridyl
ring being rotated out of the plane of the fluorene ring. The
angle of rotation is more dependent on the packing itself, as
there is a notable difference in the angle for the two crystallo-
graphically independent molecules of mFpy (50.4° and 41.6°),
while pFpy has an angle is 60.8°. In the complexes these angles
become 36.5° for complex 1 (both molecules have the same
angle) and 45.9° for 2.
The planarity of the fluorene is a good indication of its con-
jugation; it is measured by the angle of both planes formed by
each fused phenyl ring. Both ligands and complexes show a
similar planarity with no significant variations. The biggest
angle change is seen for the two crystallographically independent
molecules of 1 at 4.5° and 2.8°, giving an almost 2° shift as a
normal fluctuation because of crystal packing. The mFpy ligand
has distortion angles of 4.6° and 5.1°, the pFpy ligand is at 3.9°
(also reported at 4.1°)15b while in 2 it is at 4.4°. There is clearly
no effect on the distortion angle from the coordination of rhe-
nium to the ligand and no appreciable differences between the
meta and para ligands.
A look at the length of the double bond bridging the pyridyl
and the fluorene shows an average distance of 1.35 0.01 Å
(for both ligand and 1); in the case of 2, it is shorter at 1.318(8) Å.
This could imply a stronger double bond and can be rationalized
looking at the reduced torsion angle present in the vinyl bond of
2 (5 1° for 2 versus 10 2° for 1). The increased planarity of
the vinyl bond in 1 brings the twisted pyridine ring closer to the
fulvene plane.
The rhenium centers have standard coordination geometry.
The CO bonds are not significantly different, as can be shown
in the case of the two distinct molecules present for 1, where up
to 0.02 Å variations can be observed for otherwise identical car-
bonyl environments. As shown in Figure 1, there is an observable
coplanarity of the rhenium core (the plane formed by N3, N2,
C32, and C30) and the fluorene moiety in the case of complex 2;
the rhenium atoms are lying only 0.17 Å above the plane, and
there is an angle of only 2° between the fluorene and the rhenium
cores. In the case of 1 the geometry of the rhenium core is com-
pletely twisted off the fluorene plane. The coplanarity of the
rhenium core with the fulvene in 2 will be shown to have a role
in its electronic properties despite the 45° rotation of the pyridine
ring.
Electrochemistry. The electrochemistry reveals the energy
of the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) and can give important
information about the energy levels involved in electronic
transitions.
On the oxidation side, a rhenium pyridyl complex should
have a partially reversible oxidation around 1.85 V vs SCE
in dichloromethane.26 Also we can have oxidation of the
fulvene ligand, measured as an irreversible oxidation at 1.72 V vs
SCE (in CH2Cl2), very close to the reported 1.67 V for the closest
analogue, 9-benzylidenefluorene.27 As reported in Table 2 and
Supporting Information, Figure S5, both complexes 1 and 2 have
an oxidation at 1.84 V vs SCE in CH2Cl2. It is difficult to assign
this redox couple as either the expected Re(I)/Re(II) or as Fpy/
Fpy+· or both. A square-wave voltammetry suggests an electron
count of at least two, implying both redox couples are occurring in
the same region (Supporting Information, Figure S6).
Figure 1. ORTEP view of the X-ray crystal structures of 1 (top) and
2 (bottom) along the fulvene plane (50% probability displacement
ellipsoids; anion, solvent, and other chemically equivalent molecule
removed).
On the reduction side, two redox couples are reported to be
centered on the bipyridine ligand of complex 3.26 The first is
reversible at −1.2 V and the other is irreversible and reported
at −1.7 V. From the electrochemistry of the free Fpy ligand, we
expect a semi-reversible reduction centered as low as −1.55 V,
but because of the electron withdrawing effect of the rhenium,
this reduction could occur at less negative potential.
Complexes 1 and 2 have four reduction processes. In the
case of 1, only three reduction waves are seen; in this case the
first reduction wave is broad, at −1.10 V (160 mV width), and
suggests a ratio of two-to-one electrons as compared to the second
reduction wave observed at −1.45 V (Supporting Information,
Figure S6), suggesting that two unresolved reductions occur
next to each other. This suggestion is sustained by the CV of 2,
showing two distinct and reversible peaks located at −1.08 and
−1.23 V. The first reduction is without a doubt the bpy/bpy•−
redox couple, the second one has no choice to be centered on
the fulvene ligand, since in 3 the second reduction of the bpy is
at a much more negative potential (−1.7 V). Since a third wave
is seen for both complexes around −1.45 V, two redox couples
are centered on the fulvene ligand even if in the case of the free
molecule only one was observable. The first one, in the case 1,
occurs at slightly more negative potential than the bpy reduc-
tion and overlaps with it. While for 2, there is a clean separation
between the peaks, this can be partly due to the higher de-
localization of the fulvene, enabling it to interact more with the
singly reduced bpy and push its own reduction to a more nega-
tive potential; this delocalization will be further discussed with
the spectral data. The second reduction of the fulvene is at
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Table 2. Electrochemical Data of the Complexes in CH2Cl2, Bu4NPF6 (0.1 M) Electrolyte
ox
red
E1/2 (ΔE (mV))
E1/2 (V) (ΔE (mV))
ReII/ReI (Fluo+·/Fluo)
bpy/bpy−·
Fluo/Fluo−·
Fluo−·/Fluo−2
bpy−·/bpy−2
a
1
2
3
1.84 (250)
1.84 (120)
1.85 (irr)
−1.11 (160)
−1.08 (66)
−1.2 (60)
−1.45 (230)
−1.434 (230)
−1.71 (irr)
−1.74 (irr)
−1.7 (irr)
−1.23 (60)
b
c
Fpy
1.72 (irr)
−1.55 (260)
Unresolved two one-electron reductions. From literature, triflate salt with Bu4NPF6(0.2 M) electrolyte.26 mFpy and pFpy gave the same result.
a
b
c
Table 3. Measured and Calculated Electronic Absorption Properties of the Re Complexes and Ligand in DCM
λmax (nm); (ε (cm−1 M−1)); [f (a.u.)]
lowest energy transition
calc.
bpy transition
fluorene transition
calc.
calc.
expt.
Fp→Fpy
Re→bpy
expt.
bpy→LUMO
expt.
fluorene→LUMO
mFpy
330 (15000) [0.27]
327 (13600) [0.25]
331 (16000) [0.45]
364 (21300) [0.43]
359 (4800) [0.13]
347 [0.65]
347 [0.60]
367 [0.49]
397 [0.79]
260 (39000) [0.22]
260 (41100) [0.25]
261 (48300) [0.29]
263 (44000) [0.35]
259 [0.48]
261 [0.61]
pFpy
b
1
2
3
360 [0.02] 373 [0.13]
358 [0.04] 365 [0.03]
361 [0.01] 375 [0.10]
321 (22800) [0.009]
321 (18500) [0.010]
299 [0.16]
260 1 [0.53]
a
b
300 4 [0.16]
300 [0.14]
262 1 [0.44]
321 (13500) [0.015]
a
b
Total oscillator strength of all stated transitions in a 4 nm range. In a 1 nm range.
more or less the same potential of 1 at −1.45 V. Finally, the
second irreversible bpy reduction is seen in the expected range
for both complexes.
transitions originate from HOMO and HOMO-1 states, localized
on the diphenylfulvene and the pyridine (Supporting Informa-
tion, Figure S8). For both ligands there is a strong red-shift in the
position of the first calculated transition; however, all of the other
measured values are in good accordance with the spectra. This is
especially true of the relative oscillator strength between ligands;
they always follow the same trend as the spectra even if they are
far from the experimental absolute values (Table 3). The other
major transition at 260 nm only involves fluorene-based molec-
ular orbitals and matches the energy of the calculated transition.
It has a 2-fold increase in maximum absorbance vs fluorene
molecule (∼40000 vs 18000 M−1 cm−1).
The electrochemistry does not allow clear identification of
the individual processes occurring on the oxidation side, but it
is clear on the reduction side that the bpy is the first to reduce,
then the Fpy ligand follows. In term of energy gap, all of the
complexes have a HOMO LUMO gap around 2.94 ( 0.02) V,
suggesting again very little effect of the Fpy toward the rhenium
core and that the oxidation potential of Fpy is very closely
matched to that of its rhenium complex.
Photophysical Investigation and Comparison with
Theoretical Models. The absorption spectra of the ligands as
well as of the complexes were measured in dichloromethane
solutions and are compared to the TD-DFT calculations, with a
solvent dielectric potential to simulate solvation. It is note-
worthy that these calculations only take into account the singlet-
state transitions. In the case of rhenium complexes, there can be
some absorption from triplet-state transitions due to the spin−
orbit coupling induced by heavy atoms, but these effects are not
modeled using Gaussian03.28 Although the calculated elec-
tronic transitions do not correlate very well in terms of energy,
usually being under evaluated, the relative ratio of calculated
oscillator strength matches very well with the relative ratios
measured in experimental intensities between the meta and the
para variants of both complexes and ligands.
In the case of the ligands, both show a very similar absorp-
tion spectrum (Figure 3 and Supporting Information, Figure S7),
characterized by a broad band around 320 nm and a sharper
peak at 260 nm, very near fluorene’s maximum intensity peak of
264 nm. Smaller maxima are found near 300 and 290 nm cor-
responding to other fluorene transitions. The lowest energy
transition is seen at 323 nm, very comparable to the nearest
fulvene analogue, 9-benzylidenefluorene, with a band at 326 nm,
implying very little effect of the heterocyclic ring on the overall
electronic delocalization.29 This transition implies a delocalized
π−π* transition involving the pyridyl ring. This is confirmed by
the TD-DFT calculation, in this case the lower energy transi-
tion is at 347 nm (a shift of +26 nm), and in both cases the
The low energy absorption and the complete delocalization
of the HOMO and LUMO orbitals of both ligands suggest that
the fulvene moiety will have an effect on the properties of the
associated rhenium chromophores.
In the case of the rhenium complexes (Table 3), the wave-
length of the expected mixed singlet metal−ligand-to-ligand
charge transfer (MLLCT) transition for pyridine adducts is
found around 360 nm with a molar absorptivity of about
5000 M−1 cm−1.12 Note that the MLLCT term is used instead
of metal-to-ligand charge transfer (MLCT) term because the
implied molecular orbitals of the ground state possess a strong
contribution from the carbonyl ligands.30 Complex 1 only shows
a decreasing band at that specific wavelength, but it has twice the
molar absorptivity. As can be seen in both Table 3 and Figure 2,
the deconvoluted bands of 1 place the first peak maximum at
330 nm, which is the same position as the deconvoluted peak of
the ligand. Only the larger width of the band suggests a buried
MLLCT band. The MLLCT can be seen more clearly if we
compare the spectrum of 1 with a superimposition of the spectra
of 3 and mFpy (Figure 3). They are almost identical in peak posi-
tions, indicating that the tailing in complex 1 above 375 nm is due
to the MLLCT of the rhenium bipyridine chromophore.
In contrast, the spectrum of 2 does not superimpose well with
the addition of the spectra of the ligand pFpy and 3 (Figure 3).
There is a new band at 270 nm (43000 M−1 cm−1) of
comparable intensity with the band at 260 nm (44000 M−1
cm−1) and the expected band at 330 nm is shifted to 364 nm
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Figure 2. Absorption spectra in dichloromethane of the complexes (in blue; Top: 1; middle: 2; bottom: 3.); Gaussian fits and sum of fits (black; full
and dashed); Calculated curves from TD-DFT (dotted green); fitted oscillator strength (black circle); calculated from TD-DFT oscillator strength:
Rhenium centered (red disk), Fulvene/pyridine centered (green triangle), bipyridine centered (orange triangle).
(22000 M−1 cm−1). This molar absorptivity is four times greater
than the pyridyl adduct and suggests a buried MLLCT (∼5000
M−1 cm−1), hidden beneath a red-shifted ligand-based absorption
(∼13000 M−1 cm−1). A better comparison can be obtained by look-
ing at the oscillator strengths of the implied transitions (Table 3):
MLLCT in 3 has a strength of 0.12, the pFpy’s π−π* transition
has 0.25 oscillator strength, for a total of 0.37 versus 0.43 for 2,
a net gain of 0.05 in oscillator strength. This is similar to 1, for
which we have 0.27 for the ligand first transition and 0.45 for the
formed complex, again a gain of 0.06 in the oscillator strength.
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Figure 3. Comparison of the absorption spectra of 1 (blue line),
2 (red line), 3 (black line), mFpy (dotted blue line), and pFpy (dotted
red line) with the additive spectra of 3 and their respective ligand
mFpy and pFpy (dashed blue and red line).
The small gain in both cases can be attributed to an increase in
the absorbance cross section of the fulvene brought by the com-
plexation of the rhenium, increasing its polarity.
Another characteristic band of Re(bpy)(CO)3L+ is the sharp
peak at 321 nm; it is present in all of the pyridyl complexes,
with a constant oscillator strength, suggesting again that the ab-
sorption transitions of the rhenium core are not affected by the
fulvene moiety.
All theoretical transitions for the complexes are classified with
their implied ground state orbitals as being rhenium-carboxyl,
fulvene/pyridine or bipyridine based. This information is visualized
in Figure 2 and Supporting Information, Figure S6, and the implied
orbitals are visualized in Figure 4. The pyridyl complex (3) is
the simplest to analyze, looking only at the major calculated os-
cillator strengths, we can see clearly three features: the MLLCT
transition sitting around 375 nm (0.10), a bpy π−π* transi-
tion at 300 nm (0.14), and other MLLCT transitions at 275 nm
(totaling 0.35 oscillator strength). The first two were assigned
on the experimental spectra as the transition at 353 nm (0.13)
(−22 nm shift) and the transition at 321 nm (0.15) (+21 nm
shift). The last transition is not clearly assignable because of the
quantity of peaks present at that wavelength, but fits well with
the peak at 270 nm (−5 nm) and the measured oscillator strength,
0.68, is a very rough estimate because of the quantity of peaks in
the area and its proximity to the upper edge of the spectra.
Complex 1 shows the same transitions as the pyridyl complex:
MLLCT at 373 nm (0.13 strength), bipyridine π−π* at 299 nm
(0.16 strength) and MLLCT transitions centered at 275 nm
(0.20 strength). Additional transitions from the fulvene ligand
come into play. There is a group of fluorene-based transitions at
260 nm, matching very well with the measured 261 nm transition.
Lastly, it has a fulvene π−π* transition (HOMO-1 to LUMO+1)
at 367 nm (0.50 strength), a +20 nm shift from the calculated
ligand absorption. This result does not compare well to the mea-
sured maximum of mFpy and 1, 330 and 331 nm, where there is
no real shift of the fulvene π−π* transition, which suggests that
the coordination of the metal has less influence on the delocaliza-
tion of the fulvene orbital (HOMO-1) contrary to what the cal-
culation indicates. Lastly, it is to note that the lowest energy
transition, HOMO to LUMO, has near zero oscillator strength,
because of lack of spatial overlap of the ground state and excited
Figure 4. Topmost: ORTEP X-ray crystal structure of 1 (left) and
2 (right) used as starting point for the geometry optimization
(50% probability displacement ellipsoids, anion, solvent and other
chemically equivalent molecule removed); Top center: optimized
structure of 3 starting from crystallographic data.31 Middle: Frontier
molecular orbitals and orbital energies obtained from DFT (rb3lyp/
LanL2DZ(f)[Re]6-31G**[C,H,N,O]) CPCM(CH2Cl2).
state: the HOMO being localized only on the fluorene moiety is
disconnected from the bpy-centered LUMO (Figure 4).
The calculated transitions of 2 differ from the two previous
compounds: the lowest permitted transition is the HOMO-1 to
LUMO+1 (π−π* fulvene centered) at 397 nm (0.87 strength),
+50 nm shift from the free ligand. If we compare the measured
value, 364 and 327 nm for 1 and pFpy, respectively, we have a
shift of +34 nm. In this case, the calculation models the ex-
tended delocalization in an error of about 20 nm, comparable
to the deviation observed for 1. A closer look of HOMO-1 reveals
the origin of the lowering of the transition energy: the HOMO-1
delocalization now extends through the rhenium and the carbonyls;
this is not the case for 1. The extended delocalization also
affects the LUMO+1 orbital (LUMO is centered on the bpy in
all cases), where in 1 it is centered mainly on bpy and pyridine,
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and in 2 it is exclusively the delocalized antibonding orbital of
pFpy. This extended π−π* transition leads to a lower energy
transition, as observed.
The rest of the calculated transitions of 2 are also affected
by this interaction, as indicated by the splitting of the transi-
tions and a greater part of fulvene orbitals being present in the
various expected transitions of the Re core. The MLLCT splits
to 368 and 358 nm (0.07 total strength); the bpy transition is
mixed with a fulvene-based transition at 298 and 302 nm (0.08
total strength). Other noteworthy transitions are the metal-based
transition seen at 276 nm (0.07) and finally the fluorene transi-
tion, seen at its expected position at 263 nm (0.4 total strength).
Emission and Excited State Theoretical Studies.
Neither the ligands nor the complexes emit at room tempera-
ture or at 77 K. The closest relative to the pyridylfulvene ligands,
9-benzylidenefluorene, is also reported to have no fluorescence at
room temperature or at 77 K.32 From fluorescence and laser flash
photolysis studies, it was reported that the deactivation pathway
lies through the triplet state. The transient absorption signal, seen
at 450 nm, had a decay of approximately 100 ns. We propose that
both mFpy and pFpy follow this pathway, passing from 1π−π* to
3π−π* efficiently by intersystem crossing.
As previously mentioned neither complex 1 nor 2 emits, in
contrast to the pyridyl analogue 3, which has a very strong emis-
sion at 540 nm (500 nm at 77 K). Solutions of the complexes that
stood for several hours did have very weak emission, centered at
545 nm (512 nm at 77 K), but these were proven to be dissocia-
tion products attributable to trace Re(bpy)(CO)3(solvent)+, as
seen from the 77 K emission spectra. Note that this dissocia-
1
tion is very small and not perceptible by UV−vis or H NMR
Figure 5. Top: Optimized single vs triplet ground state structure
of 1 (left) and 2 (right); Top Center: optimized geometry of 3 triplet
ground state; Bottom: SOMO and SOMO−1 molecular orbitals and
orbital energies obtained from DFT (ub3lyp/LanL2DZ(f)[Re]6-
31G**[C,H,N,O]), CPCM(CH2Cl2).
spectroscopy.
The general absence of a strong emission at low temperature
suggests a rapid quenching mechanism of the normally emissive
3MLLCT, suggesting it is no longer the lowest excited energy
state. This can be readily rationalized by examining the HOMO
and LUMO orbital of the singlet ground state (1GS), the LUMO
being a good approximation of the singly occupied molecular orbital
(SOMO) in the singlet excited state (1ES) and by identifying
the SOMO orbital from the lowest energy triplet excited state
(3ES) of the molecule, obtained with a DFT optimization of the
triplet “ground state”. In the case of 3, the calculated LUMO
1.52 eV (814 nm) and 1.49 eV (835 nm) for 1 and 2, while 3 is
estimated at 2.76 eV (450 nm) (Supporting Information, Table S1).
Although these estimates are approximate (with 50 nm error from
the measured emission of 3 at 77 K), they nonetheless suggests an
important driving force for the observed quenching of the 3MLLCT
emission.
1
orbitals suggest a ES centered on the bpy (Figure 4, center).
The 1MLLCT state, centered on the bpy ligand, 1π−π*(fulvene)
state, and the whole Fpy ligand (including the pyridyl) are very
3
This excited state is very similar to the ES, assigned to the
3MLLCT transition state, where the SOMO is also centered on
the bpy (Figure 5, center). However, the DFT calculation ob-
tained for 1 and 2 shows a HOMO 1GS centered on the fluorene,
not the rhenium core (Figure 4, left and right), while the LUMO,
1
close in energy to each other, the MLLCT state being slightly
lower energy for 1 and of similar energy for 2. Furthermore, theses
two states are very close spatially, being separated only by the
rhenium ion. As such, it is expected that singlet−singlet Dexter
energy transfer can occur easily between these states. Both of these
excited singlet states can undergo intersystem crossing to their
respective triplet state, the MLLCT ISC is very fast, in the order of
200 fs as reported for similar rhenium complexes.33 A fast triplet−
1
representing the ES lies on the expected bipyridine moiety. The
Re based orbitals are located at lower levels: HOMO−2, −3, and
−5 for both complexes. The SOMO and SOMO−1 of the 3ES of
both complexes are not centered on the expected bpy, they are
instead on the fulvene and the optimized geometry showed a 90°
rotation of the fluorene around the vinyl double bond (Figure 5).
This twist in the 3ES is reminiscent of 4-styrylpyridine, bound to a
rhenium chromophore, which has been extensively studied by
high-level calculations.4k−m Fulvene’s ability to twist is also known
and has been studied in detail as well.6c−f The measured triplet
energy of unsubstituted pentafulvene is around 2.34 eV (530 nm),
and the dibenzofulvene is expected to be lower because of its
extended delocalization.6b By the difference of total energy
calculated from triplet and singlet ground states, the follow-
ing estimated triplet energy can be made for our complexes:
3
3
triplet Dexter energy transfer between MLLCT and the π−π*-
(fulvene) states would follow, as described for 4-styrylpyridine con-
taining rhenium complexes.33 The non-radiative 3π−π*-
(fulvene) state acts as the sink for the system, since it is of
3
lower energy than the MLLCT state. It thus only needs a
shorter lifetime than the 3MLLCT to completely quench the
emission coming from that state. From literature data, the
3
lifetime of the MLLCT of 3, 669 ns, is longer-lived than
the transient lifetime of the closest analogue to our ligand,
9-benzylidenefluorene, at around 100 ns. We propose that
the lifetime of the 3π−π*(fulvene) state of our Fpy ligands is
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similar and leads to the observed quenching.12,32 Chart 2 illustrate
the possible pathways for non-radiative relaxation.
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Chart 2. Possible Relaxation Pathways for Complexes 1 and 2
CONCLUSIONS
■
Incorporating a pyridyl-dibenzofulvene moiety in the coordi-
nation sphere of the rhenium(I) complex significantly increases
its absorptivity, especially in the case of 2 where the red-shifting
of the permitted 1π−π* transition of the fulvene ligand is at the
1
same level as the MLLCT of the rhenium chromophore. In
the case of 1, the rhenium core has low impact on the fulvene
ligand electronic state, as it only increases the absorption of
the ligand marginally. From the absence of any fluorescence or
phosphorescence of the complex, we can postulate that the
fulvene ligand has a non-emissive triplet state of lower energy
then the 3MLLCT of the rhenium core, and a fast energy transfer
occurs between these two states. This non-emissive triplet state of
the fulvene can lead to isomerization. Of course, in this case, no
net structural change has occurred, but a small modification on
either side of the fluorene would confirm if the implied fulvene
triplet non-radiative deactivation pathway occurs through the
expected photoisomerization. With the break in symmetry, elec-
tronic spectroscopy or NMR can monitor the isomerization. Further
studies of this quenching would prove interesting, since the addition
of the rhenium complex can lead to an increased isomerization
quantum yield to the fulvene class of photoisomers.
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ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, extra figures, and
computational data. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: garry.hanan@umontreal.ca. Tel: 1-514-340-5156. Fax:
1-514-343-7586.
■
Notes
(7) (a) Passalacqua, R.; Loiseau, F.; Campagna, S.; Fang, Y.-Q.;
Hanan, G. S. Angew. Chem., Int. Ed. 2003, 42, 1608. (b) Wang, J.;
Hanan, G. S.; Loiseau, F.; Campagna, S. Chem. Commun. 2004, 2068.
(c) Wang, J.; Fang, Y.-Q.; Bourget-Merle, L.; Polson, M. I. J.; Hanan,
G. S.; Juris, A.; Loiseau, F.; Campagna, S. Chem.Eur. J. 2006, 12,
8539. (d) Tyson, D. S.; Luman, C. R.; Zhou, X.; Castellano, F. N.
Inorg. Chem. 2001, 40, 4063. (e) Wang, J.; Fang, Y.-Q.; Hanan, G. S.;
Loiseau, F.; Campagna, S. Inorg. Chem. 2005, 44, 5. (f) Simon, J. A.;
Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak, P.; Jin, X.;
Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11012. (g) Ford, W. E.;
Rodgers, M. A. J. J. Phys. Chem. 1992, 96, 2917.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the Natural Sciences and Engineering Re-
■
́
search Council of Canada, the Minister
̀
e de l’Education du Queb
l for financial support and for the
l’s Altix super computer supported by le
́ ́
ois de Calculs de Haute Performances (RQCHP)
́
ec,
and the Universite
Universite de Montrea
Reseau Quebec
́ ́
de Montrea
́
́
́
for computer calculations.
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