Complexes with redox-active ligands: Synthesis, structure, and electrochemical and photophysical behavior of the Ru(II) complex with TTF-annulated phenanthroline

Article abstract of DOI:10.1021/ic4006949

Ru(II) complexes with chelating ligands, 4′,5′- ethylenedithiotetrathiafulvenyl[4,5-f][1,10]phenanthroline (L1), 1,3-dithiole-2-thiono[4,5-f][1,10]phenanthroline (L2), and 1,3-dithiole-2-ono[4, 5-f][1,10]phenanthroline (L3), have been prepared and their structural, electrochemical, and photophysical properties investigated. Density functional theory (DFT) calculations indicate that the highest occupied molecular orbital of [Ru(bpy)₂(L1)](PF₆)₂ (1) is located on the tetrathiafulvalene (TTF) subunit and appears ～0.6 eV above the three Ru-centered d orbitals. In agreement with this finding, 1 exhibits three reversible oxidations: the two at lower potentials take place on the TTF subunit, and the one at higher potential is due to the Ru³⁺/Ru ²⁺ redox couple. Complexes [Ru(bpy)₂(L2)](PF ₆)₂ (2) and [Ru(bpy)₂(L3)](PF₆) ₂ (3) exhibit only the Ru³⁺/Ru²⁺-related oxidation. The optical absorption spectra of all complexes reveal a characteristic metal-to-ligand charge transfer (MLCT) band centered around 450 nm. In addition, in the spectrum of 1 the MLCT band is augmented by a low-energy tail that extends beyond 500 nm and is attributed to the intraligand charge transfer (ILCT) transition of L1, according to time-dependent DFT calculations. The substantial decrease in the luminescence quantum yield of 1 compared to those of 2 and 3 is attributed to the reductive quenching of the emissive state via electron transfer from the TTF subunit to the Ru³⁺ center, thus allowing nonradiative relaxation to the ground state through the lower-lying ILCT state. In the presence of O₂, complex 1 undergoes a photoinduced oxidative cleavage of the central Ci=C bond of the TTF fragment, resulting in complete transformation to 3. This photodegradation process was studied with ¹³C NMR and optical absorption spectroscopy.

Full text of DOI:10.1021/ic4006949

Article
pubs.acs.org/IC
Complexes with Redox-Active Ligands: Synthesis, Structure, and
Electrochemical and Photophysical Behavior of the Ru(II) Complex
with TTF-Annulated Phenanthroline
Lawrence K. Keniley, Jr.,† Nathalie Dupont,‡ Lipika Ray,† Jie Ding,‡ Kirill Kovnir,†,§ Jordan M. Hoyt,†,∥
Andreas Hauser,*,‡ and Michael Shatruk*,†
†Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States
‡Department of Physical Chemistry, University of Geneva, 30 quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
S
* Supporting Information
ABSTRACT: Ru(II) complexes with chelating ligands, 4′,5′-
ethylenedithiotetrathiafulvenyl[4,5-f ][1,10]phenanthroline
(L1), 1,3-dithiole-2-thiono[4,5-f ][1,10]phenanthroline (L2),
and 1,3-dithiole-2-ono[4,5-f][1,10]phenanthroline (L3), have
been prepared and their structural, electrochemical, and
photophysical properties investigated. Density functional
theory (DFT) calculations indicate that the highest occupied
molecular orbital of [Ru(bpy)2(L1)](PF6)2 (1) is located on
the tetrathiafulvalene (TTF) subunit and appears ∼0.6 eV
above the three Ru-centered d orbitals. In agreement with this
finding, 1 exhibits three reversible oxidations: the two at lower
potentials take place on the TTF subunit, and the one at
higher potential is due to the Ru3+/Ru2+ redox couple.
Complexes [Ru(bpy)2(L2)](PF6)2 (2) and [Ru(bpy)2(L3)]-
(PF6)2 (3) exhibit only the Ru3+/Ru2+-related oxidation. The optical absorption spectra of all complexes reveal a characteristic
metal-to-ligand charge transfer (MLCT) band centered around 450 nm. In addition, in the spectrum of 1 the MLCT band is
augmented by a low-energy tail that extends beyond 500 nm and is attributed to the intraligand charge transfer (ILCT) transition
of L1, according to time-dependent DFT calculations. The substantial decrease in the luminescence quantum yield of 1
compared to those of 2 and 3 is attributed to the reductive quenching of the emissive state via electron transfer from the TTF
subunit to the Ru3+ center, thus allowing nonradiative relaxation to the ground state through the lower-lying ILCT state. In the
presence of O2, complex 1 undergoes a photoinduced oxidative cleavage of the central CC bond of the TTF fragment,
resulting in complete transformation to 3. This photodegradation process was studied with 13C NMR and optical absorption
spectroscopy.
geometrically predictable arrangement of the TTF subunit. In
particular, TTF or its derivatives have been connected to
chelating dithiolates, pyridines, phosphines, carboxylates, and N-
heterocyclic ligands such as pyrazine, 2,2′-bipyridine, or 1,10-
phenanthroline. Thiolates were the first ligands ever used to
attach TTF to a metal center, for which Rivera et al. reported the
preparation of a nickel(II) bis(TTF-dithiolene) complex that
exhibits an unusually high room temperature conductivity of 30 S
cm−1.3 Despite initial reports of a TTF-pyridine ligand in 1984,4
it was not until 2001 that Ouahab and co-workers reported the
first metal complex of TTF-pyridine in which coordination to
copper(II) led to a conducting paramagnetic complex.5 The first
metal complexes to incorporate TTF-containing phosphines or
carboxylates also appeared in the literature during the late 20th to
the early 21st century.6 Among the aforementioned TTF ligand
INTRODUCTION
■
Multifunctionality represents one of the central themes in
contemporary materials science. Imparting different functions to
the same material and achieving synergetic interaction between
them constitute an innovative line of attack for the development
of new technologies and devices.1 Coordination chemistry
provides an appealing approach to the design of multifunctional
materials via simple combination of properties characteristic of a
metal ion and a ligand that are bound together in the
coordination compound. In this vein, complexes with ligands
that contain a redox-active tetrathiafulvalene (TTF) unit have
received a lot of interest in recent years because of the prospect of
combining the conducting properties of the TTF substructure
with magnetic, optical, or electrochemical properties of the metal
ion.2
A number of studies have focused on the design of molecules
in which a TTF-containing fragment is connected to a chelating
ligand that affords robust binding to a metal ion with a
Received: March 20, 2013
© XXXX American Chemical Society
A
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Article
anhydrous commercial solvents used in anaerobic reactions was
achieved by passing them through a double-stage drying/purification
system (Glass Contour Inc.). Otherwise, ACS grade solvents were used.
5,6-Dibromo-1,10-phenanthroline (Br2phen). A 100 mL heavy-wall
pressure vessel was cooled in an ice bath and charged with 5.45 g (30.2
mmol) of 1,10-phenanthroline and 44 mL of fuming sulfuric acid; 1.55
mL of bromine (30.2 mmol) was added with a pipet, and the vessel was
closed tightly with a Teflon bushing equipped with a chemically resistant
O-ring. The vessel was immersed in an oil bath preheated to 120 °C.
After being allowed to react for 12 h at constant temperature, the
mixture was cooled to room temperature. The deep red solution
obtained was poured carefully into 1.5 L of chilled water with vigorous
stirring, and the pH was adjusted to 3 with sodium bicarbonate, resulting
in precipitation of a solid product. (It is essential that the pH remains
below 3.0 to avoid contamination of the desired product with 1,10-
phenanthroline-5,6-dione byproduct.) After filtration, the filter cake was
extracted with CH2Cl2 (3 × 130 mL). The solution was dried over
MgSO4 and evaporated to dryness. The product can be recrystallized
from ethanol to afford white needles, but the fluffy white powder (crude
product) proved to be of sufficient purity to be used in the subsequent
reactions. Yield: 62% (6.33 g). Anal. Calcd (found) for C12H6N2Br2, wt
%: C, 42.64 (43.04); H, 1.79 (1.89); Br, 47.28 (46.91); N, 8.29 (8.13).
1H NMR (CDCl3, 600 MHz) δ: 9.20 (dd, 2H, J = 1.5, 5.7 Hz), 8.76 (dd,
2H, J = 1.6, 10.0 Hz), 7.72 (dd, 2H, J = 4.3, 12.6 Hz). ESI-MS, m/z
(relative intensity): 339 ([M + 1]+, 100), 260 ([M − Br + 2H]+, 4).
1,3-Dithiole-2-thiono[4,5-f ][1,10]phenanthroline (L2). The com-
pound was prepared according to the literature method with a slight
modification of the workup procedure.11 The yellow solid obtained from
successive extractions with CHCl3 contained the desired product L2
contaminated with the starting material Br2phen. The latter was
removed by washing the mixture with hot ethanol. Yield: 96%. Anal.
Calcd (found) for C13H8N2OS3 (L2·H2O), wt %: C, 51.29 (51.29); H,
types, the N-heterocyclic TTF-containing ligands are of most
relevance to the current work. The first successful attachment of
TTF to 1,10-phenanthroline was achieved by Becher and co-
workers and used to prepare precatenate complexes of CuI and
AgI that act as redox responsive sensors for various metal ions.7
Recently, we have reported the synthesis of edt-TTF-phen
(L1), a chelating ligand in which ethylenedithiotetrathiafulvalene
(edt-TTF) is fused directly to 1,10-phenanthroline (phen), and
preliminary results on the preparation and crystal structure of its
Ru(II) complex.8 Herein, we report a detailed study of the
photophysical and electrochemical behavior of [Ru(bpy)2(L1)]-
(PF6)2 (1) and interpret its properties based on the electronic
structure calculated by density functional theory (DFT)
methods. We compare the behavior of 1 to that of related
complexes that include only half of the TTF unit. Finally, we
describe a study of an unexpected light-induced oxidative CC
bond cleavage of the TTF fragment of 1.
EXPERIMENTAL SECTION
Spectroscopic Measurements. H NMR spectra were measured
■
1
on either a Bruker 400 or Bruker 600 spectrometer operating at 400 or
600 MHz, respectively, with chemical shifts internally referenced to the
residual proton signals of the deuterated solvent CDCl3 (7.26 ppm) or
CD3CN (1.94 ppm).9 13C NMR spectra were measured on either a
Mercury 300 or Bruker 600 spectrometer operating at 75.5 or 151 MHz,
respectively, with chemical shifts internally referenced to the deuterated
solvent signals in CDCl3 (77.2 ppm) or CD3CN (118.3 ppm).9 Two-
dimensional correlation spectroscopy (COSY) and heteronuclear
single-quantum coherence (1H−13C HSQC) experiments were carried
out on a Bruker 600 spectrometer. Electrospray ionization (ESI) and
electron impact (EI) mass spectra were acquired on JEOL AccuTOF
JMS-T100LC and JEOL JMS600 mass spectrometers, respectively.
Infrared (IR) spectra were measured in the 600−4000 cm−1 range as
solid samples pressed on a ZnSe crystal of the universal attenuated total
reflectance (ATR) sampling accessory on a Perkin-Elmer Spectrum 100
FT-IR spectrometer. Elemental analyses were carried out by Atlantic
Microlab, Inc. (Norcross, GA).
1
2.65 (2.53); N, 9.20 (9.16); S, 31.60 (31.35). H NMR (CDCl3, 600
MHz) δ: 9.26 (dd, 2H, J = 1.6, 5.9 Hz), 8.08 (dd, 2H, J = 1.6, 9.8 Hz),
7.74 (dd, 2H, J = 4.3, 12.5 Hz). EI-MS, m/z (relative intensity): 286
([M]+, 100), 242 ([M − CS]+, 25), 210 ([M − CS2]+, 18). IR (ZnSe
ATR), ν, cm−1: 642 (m), 736 (vs), 804 (s), 918 (m), 1063 (s), 1093
(CS, vs), 1414 (s), 1477 (w), 1495 (m), 1569 (m), 1682 (s), 2954
(w).
1,3-Dithiole-2-ono[4,5-f ][1,10]phenanthroline (L3). To a clear
yellow solution of 123 mg (0.429 mmol) of L2 in 150 mL of CHCl3
were added sequentially 200 mL of glacial acetic acid and 342 mg (1.07
mmol) of Hg(OAc)2. The solution turned to a white cloudy suspension
within 5 min. The suspension was stirred for 3 h at room temperature.
The white precipitate was removed by filtration. The clear and colorless
organic phase was washed with an equal volume of H2O, three times
with an equal volume of a saturated aqueous NaHCO3 solution, and
finally with an equal volume of H2O. The resulting CHCl3 solution was
dried over MgSO4 and evaporated to dryness to afford L3 as a white
solid. Yield: 98% (113 mg). 1H NMR (CDCl3, 400 MHz) δ: 9.24 (dd,
2H, J = 1.6, 6.0 Hz), 8.11 (dd, 2H, J = 1.6, 9.8 Hz), 7.73 (dd, 2H, J = 4.3,
12.6 Hz). IR (ZnSe ATR), ν, cm−1: 732 (vs), 790 (vs), 915 (m), 1016
(m), 1259 (m), 1414 (s), 1472 (w), 1490 (m), 1640 (vs), 1701 (CO,
m), 2919 (w).
4′,5′-Ethylenedithiotetrathiafulvenyl[4,5-f ][1,10]phenanthroline
(L1). A mixture of 222 mg (0.775 mmol) of L2, 484 mg (2.33 mmol) of
4,5-ethylenedithio-1,3-dithiol-2-one, and 10 mL of P(OEt)3 was placed
in a Schlenk tube and heated at reflux under N2 for 12 h. After cooling to
room temperature, the obtained red precipitate was filtered off, washed
with cold methanol (3 × 20 mL), and dried under vacuum. The product
was loaded onto a 1 in. bed of silica gel and washed with CH2Cl2 to
remove byproducts. Then the eluent was gradually changed to CH2Cl2/
MeOH (2:1 v/v), and the collected fraction was filtered through Celite
and evaporated to dryness to afford pure L1 as an orange solid. Yield:
54% (187 mg). Recrystallization from CHCl3/hexanes resulted in
orange crystals of L1·CHCl3. Anal. Calcd (found) for C19H11N2S6Cl3
(L1·CHCl3), wt %: C, 40.32 (40.37); H, 1.96 (1.89); N, 4.95 (5.15); S,
33.99 (33.64). 1H NMR (CDCl3, 600 MHz) δ: 9.17 (dd, 2H, J = 1.6, 5.9
Hz), 8.01 (dd, 2H, J = 1.6, 9.8 Hz), 7.68 (dd, 2H, J = 4.3, 12.5 Hz), 3.34
(s, 4H, CH2). 13C NMR (CDCl3, 75.5 MHz) δ: 30.5, 80.6, 114.3, 123.9,
Electronic absorption (UV−vis) spectra were collected in the 200−
1000 nm range on a Perkin-Elmer Lambda 950 UV/vis/NIR or a Cary
50 Bio UV/vis spectrophotometer. Emission and excitation spectra were
measured on a Horiba JobinYvon FluoroMax-4 spectrofluorometer.
Luminescence lifetimes on the nanosecond time scale were recorded by
exciting the samples at 458 nm using the third harmonic of a pulsed
Nd:YAG laser (Quantel Brilliant, 7 ns pulse width) to pump an optical
parametric oscillator (Opotek Magic Prism). The system used for
detection consisted of a single monochromator (Spex 270M), a
photomultiplier (Hamamatsu R928), and a digital oscilloscope
(Tektronix TDS 540B) with a time resolution of 15 ns. The reported
values for the luminescence quantum yields of complexes 1−3 were
calculated using [Ru(bpy)3](PF6)2 as a standard with a quantum yield of
6.2% in CH3CN at room temperature.
Electrochemistry. Cyclic voltammetry (CV) was performed on a
CH Instruments 600D electrochemical analyzer at a sweep rate of 0.100
V s−1 with a 0.100 M (Bu4N)PF6 electrolyte solution, a Pt disk working
electrode, a Pt wire counter electrode, and a Ag+(0.01 M AgNO3)/Ag
nonaqueous reference electrode. All the potentials were referenced to
the standard Fc+/Fc couple (Fc = ferrocene). Fc was added as an
internal standard upon completion of each CV experiment.
Synthesis. All reactions were performed in an inert N2 atmosphere
using standard Schlenk techniques, unless noted otherwise. Commer-
cially available 1,10-phenanthroline (99%), fuming sulfuric acid (20%
SO3), bromine (99.5%), carbon disulfide (anhydrous, 99.9%),
potassium (poly)sulfide (>42% K2S basis), mercury(II) acetate (99%,
Aldrich), glacial acetic acid (EMD), sodium bicarbonate (99%,
Mallinckrodt), and 4,5-ethylenedithio-1,3-dithiol-2-thione (TCI) were
used as received. Triethylphosphite (98%, Aldrich) was distilled under
nitrogen prior to use. 4,5-Ethylenedithio-1,3-dithiol-2-one was prepared
according to the published procedure.10 Additional purification of
B
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Inorganic Chemistry
Article
Table 1. Data Collection and Structure Refinement Parameters for L1·CHCl3, L2, 1·3.6CH3CN, and 3·0.8CH3CN
C19H11N2S6Cl3
(L1·CHCl3)
C45.2H36.8N9.6Ru1S6P2F12
C34.6H24.4N6.8O1Ru1S2P2F12
C13H6N2S3 (L2)
(1·3.6CH3CN)
(3·0.8CH3CN)
CCDC number
718515
718514
751678
751912
space group
P212121
P21/c
P21/c
P1
̅
unit cell parameters (Å and deg)
a = 7.127(1)
b = 17.269(3)
c = 18.462(3)
a = 3.851(1)
b = 10.297(3)
c = 28.547(9)
β = 91.511(4)
a = 22.57(2)
b = 22.54(2)
c = 32.53(3)
β = 92.07(1)
a = 8.928(3)
b = 12.162(4)
c = 17.426(5)
α = 89.738(4)
β = 86.754(4)
γ = 83.664(4)
1878(1)
2
V (Å3)
2272.2(7)
4
1131.7(6)
4
16539(26)
12
Z
ρcalc (g cm−3
)
1.655
1.681
1.563
1.778
T (K)
173
173
173
173
λ (Mo Kα, Å)
μ (mm−1
0.71093
0.967
0.71093
0.632
0.71093
0.653
0.71093
)
0.716
2θmax (deg)
52.8
52.8
52.0
52.0
reflections collected
Rint
21876
0.138
7011
62897
19226
0.063
0.082
0.046
unique reflns
4652
2295
31363
7312
parameters/restraints
R1, wR2 [Fo > 4σFo]
goodness of fit
322/0
0.051, 0.075
0.952
163/0
0.042, 0.077
0.975
1938/6
0.073, 0.178
0.962
528/0
0.054, 0.138
1.019
diff. peak and hole (e/Å3)
0.35 and −0.39
0.35 and −0.34
1.26 and −0.80
1.20 and −0.54
124.2, 131.0, 133.3, 145.3, 150.2. ESI-MS, m/z (relative intensity): 916
([2M + Na]+, 14), 470 ([M + Na]+, 30), 447 ([M]+, 100), 419 ([M −
C2H4]+, 11). UV−vis (CH2Cl2), λmax, nm (log ε): 232 (4.48), 277
(4.37), 310 (4.24), 336 (4.08), 403 (3.51). IR (ZnSe ATR), ν, cm−1: 660
(w), 737 (vs), 769 (m), 790 (m), 917 (s), 1146 (w), 1400 (w), 1418 (s),
1479 (w), 1576 (m), 2934 (w).
(4.85), 372 (4.36), 385 (4.30), 445 (4.21). Emission (CH3CN, λexc
450 nm), λmax, nm (quantum yield, ΦF): 632 (0.066).
=
[Ru(bpy)2(L3)](PF6)2 (3). Yield: 78%. Anal. Calcd (found) for
RuC33H27N6O3.5S2P2F12 (3·2.5H2O), %: C, 38.91 (38.87); H, 2.67
(2.56); N, 8.25 (8.25); S, 6.30 (6.16). 1H NMR (CD3CN, 600 MHz) δ:
8.53 (d, 2H, J = 8.1 Hz), 8.49 (d, 2H, J = 8.2 Hz), 8.40 (dd, 2H, J = 1.1,
9.5 Hz), 8.13 (dd, 2H, J = 1.1, 6.4 Hz), 8.10 (td, 2H, J = 1.4, 17.3 Hz),
8.01 (td, 2H, J = 1.5, 17.3 Hz), 7.81 (d, 2H, J = 4.9 Hz), 7.77 (dd, 2H, J =
5.3, 13.7 Hz), 7.55 (d, 2H, J = 5.0 Hz), 7.44 (m, 2H), 7.24 (m, 2H). 13C
NMR (CD3CN, 151 MHz) δ: 187.9, 158.2, 158.0, 154.0, 153.1, 153.0,
147.8, 139.1, 139.0, 134.9, 130.5, 128.7, 128.5, 128.0, 127.4, 125.4, 125.3.
UV−vis (CH3CN), λmax, nm (log ε): 256 (4.61), 285 (4.95), 340 (3.94),
449 (4.16). Emission (CH3CN, λexc = 450 nm), λmax, nm (quantum
yield, ΦF): 628 (0.097).
[Ru(bpy)2(L1)](PF6)2 (1). A suspension of L1 (50 mg, 0.112 mmol) in
anhydrous ethanol (10 mL) was added to a suspension of [Ru-
(bpy)2Cl2]·2H2O (53 mg, 0.102 mmol) in anhydrous ethanol (15 mL).
The mixture was refluxed in the dark under N2 for 12 h, after which the
reaction mixture was cooled to room temperature and filtered. A
solution of NH4PF6 (58 mg, 0.356 mmol) in anhydrous ethanol (25
mL) was added dropwise to the filtrate. The solution was left
undisturbed overnight at −23 °C, resulting in precipitation of a brown
product. Yield: 76% (89 mg). X-ray quality single crystals were obtained
by vapor diffusion of diethyl ether into an acetonitrile solution of the
complex in the dark. Anal. Calcd (found) for RuC38H28N6O1S6P2F12
(1·1H2O), %: C, 39.07 (39.19); H, 2.42 (2.42); N, 7.19 (7.40); S, 16.47
(16.30). 1H NMR (CD3CN, 600 MHz) δ: 8.55 (d, 2H, J = 8.2 Hz), 8.52
(d, 2H, J = 8.1 Hz), 8.11 (td, 2H, J = 1.4, 17.4 Hz), 8.05 (m, 4H), 7.98
(dd, 2H, J = 0.9, 9.2 Hz), 7.84 (dd, 2H, J = 0.7, 6.2 Hz), 7.66 (m, 4H),
7.47 (m, 2H), 7.34 (m, 2H), 3.32 (d, 4H, J = 3.0 Hz). 13C NMR
(CD3CN, 151 MHz) δ: 158.7, 158.4, 153.8, 153.6, 153.4, 148.5, 139.5,
139.5, 134.7, 134.5, 129.1, 129.1, 128.3, 127.0, 125.8, 125.8, 115.1, 113.3,
66.7, 16.1. ESI-MS (CH3OH), m/z (M = [Ru(bpy)2(L1)]2+, relative
intensity): 1005 ([M + PF6]+, 100), 481 ([M − L1 + Cl + CH3OH]+,
52), 449 ([M − L1 + Cl]+, 30), 430 ([M]2+, 24), 416 ([M − C2H4]2+,
41). UV−vis (CH3CN), λmax, nm (log ε): 240 (4.73), 253 (4.66), 285
X-ray Crystallography. In a typical experiment, a selected single
crystal was suspended in Paratone-N oil (Hampton Research) and
mounted on a cryoloop, which was placed in an N2 cold stream and
cooled at 5 K/min to 173 K. The data sets were recorded as ω-scans at
0.3° step width and integrated with the Bruker SAINT software
package.12 In all the experiments, a multiscan adsorption correction was
applied based on fitting a function to the empirical transmission surface
as sampled by multiple equivalent measurements (SADABS).13
Determination of the space group, solution, and refinement of the
crystal structures was carried out using the SHELX suite of programs.14
The final refinement was performed with anisotropic atomic displace-
ment parameters for all but hydrogen atoms. The H atoms were placed
in calculated positions. A summary of pertinent information relating to
unit cell parameters, data collection, and refinements is provided in
Table 1.
Theoretical Calculations. DFT calculations were performed with
the Gaussian 03 package15 using the B3LYP hybrid functional16 and the
LanL2DZ basis set.17 Starting geometries for complexes 1−3 were taken
from the refined crystal structure parameters. All geometries were
optimized in the ground state without symmetry restraints. Single-point
time-dependent DFT (TD-DFT) calculations were carried out on the
optimized geometries using the conducting polarized continuum
medium (CPCM, CH3CN) solvation model to include solvent
polarization effects. The UV−vis spectra were calculated with the
SWizard program, revision 4.7,18 using the pseudo-Voigt model. The
half-bandwidths, Δ1/2, were set equal to 3000 cm−1. Molecular fragment
(4.87), 343 (4.06), 450 (4.18). Emission (CH3CN, λexc = 450 nm), λmax
nm (quantum yield, ΦF): 636 (0.012).
,
The complexes [Ru(bpy)2(L2)](PF6)2 (2) and [Ru(bpy)2(L3)]-
(PF6)2 (3) were prepared in a similar manner as described above for
complex 1.
[Ru(bpy)2(L2)](PF6)2 (2). Yield: 93%. Anal. Calcd (found) for
RuC33H23N6O0.5S3P2F12 (2·0.5H2O), %: C, 39.68 (39.75); H, 2.32
1
(2.33); N, 8.41 (8.73); S, 9.63 (9.44); F, 22.64 (22.75). H NMR
(CD3CN, 600 MHz) δ: 8.53 (d, 2H, J = 8.0 Hz), 8.50 (d, 2H, J = 8.0 Hz),
8.39 (dd, 2H, J = 1.1, 9.5 Hz), 8.15 (dd, 2H, J = 1.1, 6.4 Hz), 8.10 (td, 2H,
J = 1.4, 17.3 Hz), 8.02 (td, 2H, J = 1.4, 17.3 Hz), 7.81 (d, 2H, J = 5.6 Hz),
7.78 (dd, 2H, J = 5.3, 13.7 Hz), 7.56 (d, 2H, J = 5.0 Hz), 7.45 (m, 2H),
7.25 (m, 2H). UV−vis (CH3CN), λmax, nm (log ε): 255 (4.62), 287
C
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a
Scheme 1. Synthesis of L1−L3 and Their Ru(II) Complexes 1−3
a
(i) K2S, CS2, DMF, 60 °C, 36 h; (ii) Hg(O2CCH3)2, CH3CO2H, CHCl3, 3 h; (iii) 4,5-ethylenedithio-1,3-dithiole-2-one, P(OEt)3, reflux, 12 h.
contributions to frontier orbitals were calculated using the AOMix
program.18b,19
result in the formation of the desired complexes that can be
isolated as hexafluorophosphate salts, [Ru(bpy)2(Li)](PF6)2 (i =
1, 2, or 3) upon addition of an excess of NH4PF6. The crystals of
these complexes are obtained by slow diffusion of diethyl ether
into an acetonitrile solution of the complex.
RESULTS AND DISCUSSION
■
Synthesis. The synthetic pathway toward ligands L1−L3 is
outlined in Scheme 1. Ligand L2 can be prepared in several ways.
The approaches, reported by Almeida and co-workers,20
Hudhomme and co-workers,21 and our group,8 proceed via
substitution of benzylthiolates for the bromo substituents in
Br2phen. The benzyl groups are then cleaved with t-BuOK or
AlCl3, and the generated dithiolate can be converted to the 1,3-
dithiole-2-thione ring with thiophosgene or CS2 under basic
conditions. Despite relatively good yields (∼32% based on
Br2phen), these methods require multistep synthesis and the use
of hazardous benzylmercaptan. Recently, Qin et al. demonstrated
that L2 can be prepared in high yield by simply treating Br2phen
with K2CS3 in DMF.11 This route provides a simple and
inexpensive one-pot reaction for the preparation of L2.
Nevertheless, our attempts to repeat the published procedure
led consistently to a mixture of L2 and unreacted Br2phen, but
the latter can be easily removed because of its solubility in hot
ethanol. The conversion from L2 to L3 is achieved by a standard
synthetic protocol that includes treating the solution of L2 in
CHCl3 and glacial acetic acid with Hg(OAc)2.22
NMR Spectroscopy. All ligands and complexes were
1
unambiguously characterized by NMR spectroscopy. The H
NMR spectra of all ligands exhibit three distinct series of peaks in
the aromatic region that can be explicitly assigned to the protons
on the phenanthroline moiety (Figure 1). The 1H NMR
spectrum of L1 shows that the Ha proton of the phen moiety
resonates at lowest field (δ = 9.17 ppm), followed by the Hb
proton (δ = 8.01 ppm) and the more shielded Hc proton (δ =
7.68 ppm).
In addition to 1D NMR experiments, complexes 1−3 were
1
1
also examined by 2D H COSY and H−13C HSQC methods
that allowed unequivocal assignment of signals in the more
complicated aromatic region (Supporting Information, Figure
S1). This region contains signals that can be assigned to the
bipyridine and phenanthroline moieties. Upon coordination of
phenanthroline-based ligands to the Ru(II) center, the most
dramatic changes are observed for the α-protons, whose signals
are shifted upfield by ∼1.2 ppm. This shift can be explained by
shielding arising from the proximity of these protons to the π-
system of bipyridine ligands (Supporting Information, Figure
S2), which is a commonly observed effect in such complexes with
chelating polypyridyl ligands. The 1H NMR spectrum shows that
the two bipyridine ligands are magnetically equivalent and thus
produce eight signals that can be identified with four protons
from each pyridyl moiety. The bipyridine protons were assigned
in accordance with previously reported NMR assignments for
ruthenium polypyridyl complexes.24
The ligands L2 and L3 provide an entry into the synthetic
chemistry of TTF-annulated phenanthrolines, as demonstrated
by our preparation of the asymmetric ligand L1 via a
triethylphosphite-mediated cross-coupling reaction between L2
and 4,5-ethylenedithio-1,3-dithiole-2-one (Scheme 1).23 The
same result can be obtained by reacting L3 and 4,5-ethyl-
enedithio-1,3-dithiole-2-thione in triethylphosphite. A separa-
tion of L1 from the symmetric byproduct, bis(ethylenedithio)-
tetrathiafulvalene (BEDT-TTF), formed by self-coupling of 4,5-
ethylenedithio-1,3-dithiole-2-one, is achieved by using a short
column (2.5 cm) and washing out the BEDT-TTF impurity with
CH2Cl2, after which the eluent is switched to CH2Cl2/CH3OH
(2:1 v/v) to wash out the pure L1 fraction.
Because of the never-diminishing interest in ruthenium(II)
complexes that incorporate N-heterocyclic polypyridyl ligands,
we were incited to investigate the electrochemical and photo-
physical properties of such complexes prepared with ligands L2
and L3 and especially with the redox-active ligand L1. Reactions
between these ligands and [Ru(bpy)2Cl2] in refluxing ethanol
Crystal Structure. The ligand L1 crystallizes in the chiral
orthorhombic space group P212121. The chirality stems not from
the molecular structure but from the crystal packing. A careful
examination of the crystal structure did not reveal any additional
symmetry. The crystal structure refinement revealed that the
compound crystallizes as a racemic twin. The dark orange crystals
exhibit rod-shaped morphology and contain an interstitial
solvent molecule, resulting in the formula L1·CHCl3. In the
crystal structure, the molecule of L1 is almost planar (Figure 2a).
The rms deviation from a least-squares plane through all atoms is
0.035(6) Å, excluding the peripheral ethylenedithio (edt)
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P21/c. Similar to the structure of L1, the ligand remains nearly
planar in the structure of 1·3.6CH3CN, with a 0.12(5) Å rms
deviation from a least-squares plane fit through all atoms but the
carbon atoms of the edt fragment. The crystal packing features a
unique stacking motif of TTF moieties propagating along the b
axis (Figure 2c,d). An asymmetric unit includes three [Ru-
(bpy)2(L1)]2+ dications of the same chirality. An inversion
operation generates three dications of the opposite chirality,
resulting in a stacking of alternating “triplets” of Δ- and Λ-
isomers, with six dicationic species in a repeat unit of the stack.
Thus, the overall structure is indeed centrosymmetric. The
plane-to-plane separation between TTF units varies from 3.44 to
3.78 Å.
The crystals of 1·3.6CH3CN quickly lose crystallinity when
removed from mother liquor, which can be explained by the loss
of interstitial solvent. The crystals remain indefinitely stable
when kept under mother liquor in the dark, but when exposed to
light in the presence of air, these dark red crystals gradually
convert to bright orange crystals of 3·0.8CH3CN. This
conversion is described in detail in Photodegradation of the
TTF-Containing Complex 1.
Electrochemistry. The electrochemical properties of ligand
L1 and complexes 1−3 were investigated in a CH3CN/CH2Cl2
(3:2 v/v) solution (Table 2). Ligand L1 exhibits two reversible
one-electron redox processes that are associated with the
successive oxidations of the TTF subunit to the radical cation
TTF+• and the dication TTF2+ (Figure 3). The half-wave
potentials of L1 (0.17 and 0.52 V vs Fc+/Fc) are positively shifted
relative to those observed for unsubstituted TTF (−0.10 and
0.27 V),25 as expected from the π-accepting nature of the
phenanthroline moiety.
The corresponding Ru(II) complex 1 exhibits three reversible
oxidation processes (Figure 3), in contrast to complexes 2 and 3,
which exhibit only one reversible oxidation (Table 2). A
comparison of the redox behavior of 1 to that of 2, 3, and L1
indicates that the first two one-electron oxidations in 1 (at 0.26
and 0.58 V) are TTF-based. They are shifted to slightly more
positive potentials relative to those observed for L1 because of
the electrostatic inductive effect of the Ru(II) ion bound to L1.
The third oxidation for complex 1 at 1.04 V corresponds to the
Ru3+/2+ redox couple, as does the only oxidation observed for
either 2 or 3, at 0.97 and 0.96 V, respectively. This couple is also
the only oxidation observed for the reference compound
[Ru(bpy)2(phen)](PF6)2 at 0.89 V (Table 2).26 All three
oxidations in 1 are similar to those reported earlier for
[Ru(bpy)2(TTF-dppz)](PF6)2 (0.29, 0.61, and 0.99 V).27 Two
reversible one-electron reduction waves were observed for 1 in
the cathodic region (at −1.54 and −1.92 V). They are also
comparable to the reversible reductions observed for the
reference compounds, [Ru(bpy)2(phen)](PF6)2 (−1.71 and
−1.91 V) and [Ru(bpy)2(TTF-dppz)](PF6)2 (−1.35 and −1.79
V), and attributed to the reduction of the phenanthroline and
bipyridine moieties, respectively.
Electronic Structure. DFT and TD-DFT calculations have
been shown to be indispensible for the analysis of photophysical
behavior of Ru(II) polypyridyl complexes.28 Therefore, we
carried out such calculations to elucidate the electronic structure
of complexes 1−3 and aid in the interpretation of their
photophysical properties. The energy diagram of selected
frontier molecular orbitals (MOs) appears in Figure 4, while
the corresponding orbital energies for each complex are listed in
Table 3.
Figure 1. Selected aromatic and aliphatic regions of the 1H NMR spectra
of L1−L3 (top to bottom, respectively; CDCl3, room temperature). The
residual solvent signal is marked with an asterisk.
subunit, the carbon atoms of which are disordered over two
positions ∼0.4 Å above and below the molecular plane. The
molecules are stacked in columns parallel to the a axis in a head-
to-tail fashion (Supporting Information, Figure S3) and exhibit
face-to-face π−π contacts with an interplanar separation of 3.56
Å. Large channels are present along the a axis, which are filled
with disordered CHCl3 molecules.
The mononuclear complex [Ru(bpy)2(L1)](PF6)2 (1)
crystallizes as a dark red acetonitrile solvate, 1·3.6CH3CN
(Figure 2b), in the centrosymmetric monoclinic space group
E
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Figure 2. (a, b) Molecular structures of ligand L1 and its Ru(II) complex, [Ru(bpy)2(L1)]2+. Thermal ellipsoids are at the 50% probability level. (c, d)
Side and top views of the packing of [Ru(bpy)2(L1)]2+ cations in the crystal structure of 1·3.6CH3CN. In b−d, the H atoms have been omitted for
clarity. Color scheme: Ru, light blue; S, yellow; N, blue; C, gray; H, off-white.
Table 2. Redox Potentials (V vs Fc+/Fc) of Ligand L1 and
Complexes 1−3 in CH3CN/CH2Cl2 (3:2 v/v) and of
Reference Compounds TTF in CH2Cl2,25
[Ru(bpy)2(phen)](PF6)2 in CH3CN,26 and [Ru(bpy)2(TTF-
dppz)](PF6)2 in CH2Cl2/CH3CN (5:1)27
oxidation
reduction
(1)
(2)
(3)
(1)
(2)
compound
E1/2
E1/2
E1/2
E1/2
E1/2
L1
1
0.17
0.26
0.52
0.58
1.04
0.97
0.96
−1.54
−1.80
−1.84
−1.92
−2.03
−2.04
2
3
TTF
−0.10
0.37
0.61
[Ru(bpy)2(phen)](PF6)2
0.89
0.99
−1.71
−1.35
−1.91
−1.79
[Ru(bpy)2(TTF-dppz)]
(PF6)2
0.29
Figure 3. Cyclic voltammograms of L1 and 1 (dashed blue and solid red
lines, respectively) recorded in a 0.100 M solution of (Bu4N)PF6 in
CH3CN/CH2Cl2 (3:2 v/v) at 0.100 V/s. Potentials are given vs Fc/Fc+.
The most obvious difference between the three complexes
appears in the nature of the highest occupied MOs (HOMOs).
Complexes 2 and 3 are characterized by the presence of metal-
based HOMO, HOMO−1, and HOMO−2, corresponding to
the d orbitals of the Ru ion, as typical of many Ru polypyridyl
complexes. These three orbitals are preserved in complex 1, but
about 0.6 eV above them lies the new π-type HOMO centered
entirely on the TTF fragment. This finding is in agreement with
the observation of two TTF-centered oxidations that precede the
Ru3+/2+ redox couple in 1. The purely TTF-based HOMO and
essentially unchanged nature of the metal-centered orbitals in 1
as compared to that of 2 and 3 (Table 3) indicate that the
electronic coupling between the RuII ion and the TTF fragment
is negligible.
respectively, while in 1 the HOMO−4 is centered on the TTF-
phen.
The four lowest unoccupied MOs (LUMOs) are similar in all
three complexes, corresponding to π* orbitals of polypyridyl
ligands and spanning a range of only ∼0.2 eV. The LUMO and
LUMO+1 are centered almost entirely on the phenanthroline
fragment. The LUMO+2, LUMO+3, and LUMO+4 are largely
bipyridine-based, with the exception of LUMO+3 in 2, which is
centered on L2. It appears that the change from the CC and
CO bonds in 1 and 3, respectively, to the CS bond in 2
results in the lower energy of the L2-centered π* orbital, which
has an important ramification on the appearance of the optical
absorption spectrum of 2, as discussed below.
In all three complexes, below the three metal-based orbitals
appear ligand-centered π orbitals. In 2 and 3, the HOMO−3 is
localized on the dithiocarbonate-phenanthroline (DTC-phen)
and trithiocarbonate-phenanthroline (TTC-phen) fragments,
Optical Spectra. The optical absorption spectra of 1−3 are
similar, revealing three major bands centered around 35000,
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Figure 4. Frontier molecular orbitals of 1−3. H atoms have been omitted for clarity. All energies have been converted to the normal hydrogen electrode
(NHE) scale assuming that the NHE potential is −4.5 V vs vacuum level.29
a
b
Table 3. Energies (in eV) and Composition (in %) of Frontier Molecular Orbitals of 1−3
1
2
3
MOs
energy
Ru
L1
bpy
energy
Ru
L2
bpy
energy
Ru
L3
bpy
L+4
L+3
L+2
L+1
L
2.52
1.81
0.90
5.39
9.45
4.43
89.65
90.18
80.33
3.90
1.79
1.76
5.62
0.16
1.95
96.46
11.71
97.24
86.43
5.37
92.43
3.38
2.50
1.80
0.71
5.57
22.79
2.53
76.50
91.89
84.81
2.18
1.76
4.79
14.89
94.71
80.08
99.93
5.38
1.73
3.95
84.35
1.65
1.74
4.43
10.76
96.62
84.09
5.42
1.72
1.39
1.63
1.12
1.66
1.20
1.61
2.06
17.86
0.02
1.56
2.83
10.74
12.06
10.88
19.96
1.36
1.58
2.52
13.39
11.99
11.33
19.94
1.90
H
−1.16
−1.73
−1.87
−1.90
−2.15
0.05
−1.76
−1.91
−1.93
−2.32
−2.52
82.58
70.14
76.30
5.81
−1.75
−1.90
−1.92
−2.62
−3.05
82.60
72.32
76.30
4.30
H−1
H−2
H−3
H−4
82.68
69.43
76.28
5.68
11.94
10.87
19.97
1.12
18.98
3.74
16.36
3.75
19.70
3.76
92.83
93.80
93.20
0.03
99.95
0.02
0.45
b
0.68
98.86
All energies have been converted to the NHE scale assuming that the NHE potential is −4.5 V vs vacuum level.29 H = HOMO; L = LUMO.
a
29500, and 22500 cm−1. The absorption spectra simulated from
the results of TD-DFT calculations reproduce well the character
of the experimental spectra (Figure 5) and thus can be used to
assign the experimentally observed transitions (Tables 4−6).
In each case, the intense absorption band at higher energy
(around 35000 cm−1) is attributed to spin-allowed intraligand
π−π* transitions, with some contribution from ligand-to-ligand
charge transfer (LLCT) transitions. The lowest-energy band
around 22500 cm−1 is due to the metal-to-ligand charge transfer
(MLCT) transitions, as commonly seen in the optical spectra of
Ru polypyridyl complexes. In contrast to those of 2 and 3, the
MLCT band of 1 reveals a low-energy tail that extends below
20000 cm−1. This feature is attributed to the intraligand charge
transfer (ILCT) excitation from the TTF-centered HOMO to
the phenanthroline-centered LUMO+1.
The intermediate-energy band in the optical spectrum of 2
differs significantly from those found in the spectra of 1 and 3. In
the latter two complexes, this band appears as a broad shoulder at
∼29500 cm−1 on the tail of the intense higher-energy band. In
the spectrum of 2, this band is red-shifted to 26000 cm−1. Given
the chemical similarity between complexes 2 and 3, one would
have to attribute this bathochromic shift to the substantial change
in the bonding character when going from the CS bond in 2 to
the CO and CC bonds in 3 and 1, respectively. Indeed, an
examination of excitations that compose the intermediate-energy
band confirms this assumption. In the case of 1, this band
involves a mix of transitions of MLCT (Ru → phen, Ru → bpy),
LLCT (TTF → bpy), and ILCT (TTF → phen) character, which
correspond to excitations from the Ru-based HOMO−1,
HOMO−2, or HOMO−3, or from the TTF-based HOMO to
the polypyridyl-centered π* orbitals (Table 4). In both 2 and 3,
this band includes MLCT (Ru → bpy) and ILCT (TTC → phen
or DTC → phen, respectively) transitions (Tables 5 and 6), but
in 2, the band is dominated by a strong HOMO−3 to LUMO+3
excitation that corresponds to the π−π* transition of the TTC
fragment. Thus, the red shift of the intermediate-energy band in
2 relative to the corresponding bands in 1 and 3 stems from the
stabilization of one of the unoccupied π* orbitals (LUMO+3 in
2) upon introduction of the additional S atom in L2.
Photophysical Properties. Complexes 1−3 exhibit lumi-
nescence in a CH3CN solution at room temperature (Supporting
Information, Figure S4) when excited at 450 nm (22220 cm−1).
The emissions appear as broad and structureless bands centered
at ca. 600 nm (16670 cm−1), which is characteristic of 3MLCT
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emission in Ru polypyridine complexes.30 In comparison to
reference complexes [Ru(bpy)3](PF6)2 and [Ru(bpy)2(phen)]-
(PF6)2, the emission maximum (λem) of 1−3 is red-shifted by
∼400 cm−1. The emission of complex 1 is shifted to a lower
energy relative to those of 2 and 3, which agrees with the smaller
HOMO−LUMO gap in the former. Compiled in Table 7 are the
photophysical properties of complexes 1−3 and the reference
compounds. The emission lifetime (τobs) and quantum efficiency
(ΦF) of 1 in deoxygenated CH3CN at room temperature are
1.77(4) μs and 1.2%, respectively. For 2 and 3, these parameters
are 1.47 μs and 6.6% and 2.08(4) μs and 9.7%, respectively.
The radiative lifetimes (τr) of 2 and 3 are 22 and 21 μs,
respectively, as calculated from the respective quantum
efficiencies and observed lifetimes. These τr values are of the
order usually observed for the spin-forbidden transition from the
3MLCT state of Ru(II) polypyridyl complexes.31 For 1, the
observed lifetime (τobs) is comparable to those of 2 and 3, but the
overall luminescence quantum yield is significantly quenched.
On the basis of the absorption spectra of the three compounds,
the radiative lifetime of the 3MLCT state of 1 is expected to be
similar to those calculated for 2 and 3. Therefore, the reduced
luminescence quantum efficiency of 1 must be due to a
nonradiative decay to a state at lower or similar energy competing
with the intersystem crossing from the initially excited 1MLCT
states to the 3MLCT state.27
The luminescence quenching observed for complex 1 may be
associated with the different nature of frontier orbitals in 1 as
compared to those of 2 and 3 (see Figure 4 and the related
discussion above). In the latter complexes, the lowest-energy
excitations, Ru2+ → bpy and Ru2+ → phen, correspond to the
population of nearly degenerate 1MLCT states, bpy/bpy•−-Ru3+-
phen and (bpy)2-Ru3+-phen•−, respectively, that are known to
relax via intersystem crossing (ISC) and interligand electron
hopping to the lowest-energy emissive 3MLCT state.32,33 In the
case of 1, however, the ISC can be quenched by electron transfer
from the TTF donor moiety to the Ru3+ center, thus resulting in
the decreased luminescence quantum yield. The mechanism of
Figure 5. Experimental (red line) and simulated (blue line) optical
absorption spectra of 1−3 (top to bottom, respectively). The gray bars
indicate the energy and oscillator strength of each electronic excitation.
Table 4. Assignments of Optical Absorption Bands of 1 Based on TD-DFT Calculations
λ (nm)
E (cm−1
)
observed (ε × 10−4, M−1 cm−1
)
calcd (osc. strength)
515 (0.014)
excitation
assignment
19194
521 (0.24)
H → L+1 (66%)
TTF → phen
22321
448 (1.62)
447 (0.080)
439 (0.102)
H−2 → L (41%)
Ru → phen
Ru → phen
Ru → bpy
Ru → bpy
Ru → bpy
Ru → phen
Ru → bpy
H−3 → L+1 (38%)
H−2 → L+2 (37%)
H−3 → L+2 (52%)
H−2 → L+3 (38%)
H−3 → L+1 (55%)
H−3 → L+3 (38%)
428 (0.163)
422 (0.118)
29155
35088
343 (1.25)
333 (0.035)
328 (0.042)
327 (0.054)
318 (0.062)
305 (0.152)
H → L+8 (55%)
TTF → bpy
Ru → bpy
H−1 → L+8 (63%)
H → L+9 (57%)
TTF → phen
Ru → bpy
H−2 → L+6 (47%)
H−3 → L+7 (34%)
H−2 → L+9 (33%)
Ru → bpy
Ru → phen
285 (7.78)
286 (0.331)
284 (0.233)
281 (0.259)
H−9 → L+1 (33%)
H−4 → L+4 (54%)
H−7 → L+3 (41%)
phen → phen
TTF → bpy
bpy → bpy
H
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Table 5. Assignments of Optical Absorption Bands of 2 Based on TD-DFT Calculations
λ (nm)
E (cm−1
)
observed (ε × 10−4, M−1 cm−1
)
calcd (osc. strength)
excitation
assignment
22573
443 (1.74)
449 (0.140)
428 (0.142)
H−1 → L (50%)
Ru → phen
Ru → phen
Ru → bpy
Ru → phen
Ru → bpy
H−1 → L+1 (48%)
H−2 → L+2 (42%)
H−2 → L+1 (51%)
H−2 → L+4 (37%)
427 (0.140)
361 (0.049)
26882
25974
372 (2.41)
H−3 → L+1 (66%)
TTC → phen
385 (2.13) sh
332 (0.031)
330 (0.382)
327 (0.038)
317 (0.057)
H → L+6 (60%)
H−3 → L+3 (58%)
H → L+8 (61%)
H−1 → L+6 (45%)
Ru → bpy
TTC → TTC
Ru → bpy
Ru → bpy
34965
286 (8.09)
289 (0.056)
282 (0.114)
278 (0.798)
276 (0.050)
275 (0.065)
254 (0.053)
H−5 → L+2 (52%)
H−7 → L (57%)
H−5 → L+4 (41%)
H−3 → L+5 (59%)
H−7 → L+3 (47%)
H−2 → L+11 (62%)
bpy → bpy
phen → phen
bpy → bpy
TTC → bpy
phen → TTC
Ru → phen
Table 6. Assignments of Optical Absorption Bands of 3 Based on TD-DFT Calculations
λ (nm)
E, cm−1
observed (ε × 10−4, M−1 cm−1
)
calcd (osc. strength)
excitation
assignment
22573
443 (1.67)
448 (0.088)
427 (0.169)
H−1 → L (42%)
Ru → phen
Ru → bpy
Ru → phen
Ru → phen
Ru → bpy
H−2 → L+2 (52%)
H−1 → L+1 (32%)
H−2 → L+1 (53%)
H−2 → L+3 (38%)
426 (0.119)
29412
35211
340 (0.97)
332 (0.062)
317 (0.057)
H−3 → L+1 (63%)
H−1 → L+6 (45%)
DTC → phen
Ru → bpy
284 (10.26)
286 (0.065)
281 (0.330)
280 (0.263)
277 (0.825)
264 (0.258)
258 (0.109)
H−6 → L+1 (52%)
H−5 → L+3 (50%)
H−6 → L (37%)
H−6 → L (39%)
H−3 → L+4 (60%)
H−3 → L+5 (63%)
DTC → phen
bpy → bpy
DTC → phen
DTC → phen
DTC → bpy
DTC → DTC
a
Table 7. Photophysical Properties of Complexes 1−3 and Reference Compounds [Ru(bpy)3](PF6)2 and
26
[Ru(bpy)2(phen)](PF6)2
EMLCT (cm−1)/λMLCT (nm)
(ε × 10−4 (M−1 cm−1))
Eem (cm−1)/λem
νST
ΦF
τr
τnr
(μs)
complex
(nm)
(cm−1
)
(%)
τobs (μs) (μs)
kr (s−1
)
knr (s−1
)
1
2
3
22321/448 (1.62)
22573/443 (1.74)
22573/443 (1.67)
22172/451 (1.32)
22271/449 (1.57)
15723/636
15823/632
15898/629
16260/615
16340/612
6598
6750
6675
5912
5931
1.2
6.6
9.7
6.2
6.0
1.77(4)
1.47(4)
2.08(4)
0.86
22
21
14
13
1.58
2.31
0.92
0.85
4.5 × 104
4.7 × 104
7.2 × 104
7.5 × 104
6.4 × 105
4.3 × 105
1.1 × 106
1.2 × 106
[Ru(bpy)3](PF6)2
[Ru(bpy)2(phen)]
(PF6)2
0.80
a
EMLCT/λMLCT = energy/wavelength of maximum MLCT absorption; Eem/λem = energy/wavelength of maximum emission; νST = Stokes shift; ΦF =
luminescence relative quantum yield under 450 nm excitation, using [Ru(bpy)3]2+ in CH3CN as a standard with Φf = 0.062; τobs = measured
luminescence lifetime; τr = calculated radiative lifetime (ΦF = τobs/τr); τnr = calculated nonradiative lifetime (1/τr + 1/τnr = 1/τobs); kr = radiative rate
constant (kr = ΦF/τobs); knr = nonradiative rate constant (kr + knr = 1/τobs).
such quenching requires special attention. The initial excitation
because of the presence of the TTF moiety in the vicinity of
1
1
1
from the ground state of 1 also creates two MLCT states,
the excited electron in MLCT1 (Scheme 2). In the MCLT2
state, the electron can be transferred from TTF to Ru3+ to
produce, in a cascade, first a lower-energy, charge-separated
(bpy)2-Ru2+-phen•−-TTF+ (1MLCT1) and bpy/bpy•−-Ru3+-
phen-TTF (1MLCT2), which have quite different natures
I
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Scheme 2. Energy Level Scheme for the Charge Transfer
a
States Prevalent in 1 upon Excitation
Figure 6. Kinetic profile of the difference absorbance spectrum of 1 in
CH3CN at room temperature. λpump = 488 nm; λprobe = 458 nm.
Experimental data, red; single exponential fit, blue.
signal.35 In the case of 1, however, we observed only a very weak
signal at 370 nm, which decayed at a single-exponential rate but
with a high degree of uncertainty: 1.6(8) μs (Supporting
Information, Figure S5). This observation reinforces our
assumption that the luminescence of 1 is of the 3MLCT nature.
Photodegradation of the TTF-Containing Complex 1.
The solid form of 1 can be stored in air for a prolonged period of
time without any visible degradation, but when dark red crystals
of 1 were left under O2-saturated mother liquor (CH3CN/Et2O)
for about a week, a complete transformation of this compound to
a new orange crystalline solid was observed. The crystal structure
determination revealed that the TTF fragment of 1 had
undergone an oxidative cleavage of the central CC bond,
resulting in the complete transformation of 1 to 3.
a
GS = ground state; MLCT = metal-to-ligand charge transfer excited
states; LLCS = ligand-to-ligand charge-separated excited state; ILCT =
intraligand charge transfer excited state. The most probable radiative
and nonradiative relaxation pathways are depicted as solid and dashed
red arrows, respectively.
LLCT state, bpy/bpy•−-Ru2+-phen-TTF+, and finally a non-
emissive ILCT state, (bpy)2-Ru2+-phen•−-TTF+. For the (bpy)2-
Ru3+-phen•−-TTF (1MCLT1) state, the TTF → Ru3+ electron
transfer is expected to be slower because of the localization of the
excited-state electron on the phen moiety. This blockage of the
electron transfer results in more efficient ISC to the
corresponding 3MLCT1 state that undergoes the radiative
decay with the characteristic lifetime of 1.77 μs.
To investigate the cause of this unexpected transformation,
13C NMR spectroscopy was utilized to monitor CD3CN
solutions of 1 as well as control CH2Cl2 solutions of L1, kept
under ambient light and in the dark (Figure 7). The experiment
In principle, the TTF → Ru3+ electron transfer can also occur
in the corresponding longer-lived 3MLCT2 state, and it would be
of interest to investigate the possible competition of this process
with the radiative decay by combining advanced transient
spectroscopy and theoretical calculations. However, on the basis
of the spectroscopic and the electrochemical data, the driving
3
force for electron transfer in the MLCT2 state would be
significantly smaller than in the MLCT2 state. Therefore, this
1
process would be slower and would have to compete with the
electron localization in the lowest-energy 3MLCT state, namely
3
the MLCT1 state, via interligand electron hopping on the
picosecond time scale.33 This puts the electron transfer rate into
1
3
direct competition with the MLCT to MLCT ISC. Thus, we
believe that the luminescence quenching in complex 1 stems
primarily from the inability of the 1MLCT2 state to undergo ISC
to the corresponding 3MLCT2 state.
In an effort to better understand the nature of the
luminescence (and thus the quenching mechanism) of 1, time-
resolved transient absorption spectra were collected in a
deaerated CH3CN solution at room temperature.34 A 488 nm
(20492 cm−1) pulse into the tail of the mixed Ru → phen/Ru →
bpy 1MLCT band was used for excitation, and the bleach of this
band was monitored at 458 nm (21834 cm−1). There is no
indication of an intermediate state in the transient signal, and the
kinetic trace was fit to a single exponential (Figure 6) with a
lifetime of 1.75(7) μs, which coincides with the luminescence
lifetime of 1 (Table 7). Unfortunately, there is no indication of an
intermediate charge-separated state that would be sufficiently
long-lived on the scale of the detection limits of the instrument
(>10 ns). If such a state exists, experiments on a faster time scale
will be required to observe it.
Figure 7. 13C NMR spectra of 1 in CDCl3 before (top) and after
(bottom) being exposed to light and air for a period of 20 days.
monitored solutions kept in both air-saturated and air-free
environments. An analysis of the 13C NMR spectra showed that 1
remained intact in solutions kept in air-free or dark environments
for 3 weeks but converted to 3 in solutions exposed to air and
light. Furthermore, upon conversion, the portion of L1 cleaved
from the Ru(II) complex precipitated as a light orange solid. This
solid was filtered prior to obtaining the final NMR spectrum,
which explains the absence of peaks for the corresponding
carbons (labeled r, s, and t in Figure 7). There is also a substantial
change in the chemical shift of carbon q upon conversion, from
115 to 188 ppm, as expected for the formation of the carbonyl
We also probed a transient signal at 370 nm (27027 cm−1)
under 488 nm excitation. At this energy, the reference compound
[Ru(bpy)3](PF6)2 exhibits a significant transient absorption
J
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Article
carbon upon transformation. The free ligand L1 did not show
decomposition in any of the solutions tested (i.e., its 13C NMR
spectra remained unchanged after 3 weeks). These results
indicate the need for the presence of the photoactive RuII ion for
the transformation of L1 to L3.
Taking into account the difference in the photophysical
properties of 1 and 3 (Table 7), we recorded absorption and
emission spectra of a CH3CN solution of 1 before and after
irradiation for 4 h with a 458 nm pulsed laser (Figure 8). The
solution of 1 (Table 7). The continuous irradiation led to a
gradual decrease in the luminescence lifetime until it reached a
constant value of 0.353(3) μs after 4 h of irradiation. The
substantial decrease in the luminescence lifetime points toward
the incidental seepage of atmospheric oxygen into the cuvette,
which results in substantial luminescence quenching.36 Thus, the
luminescence decay of a freshly prepared and degassed CH3CN
solution of 1 was measured immediately after the solution was
exposed to air under visible light, resulting in τobs = 0.279(4) μs.
This value, however, is significantly shorter than the value of
0.353(3) μs obtained for the solution kept in a capped cuvette
under irradiation for 4 h. Because the presence of oxygen and
irradiation can also result in the photodegradation of 1 to 3, the
same measurement was performed on a degassed CH3CN
solution of 3 immediately after it was exposed to air. The
measured τobs = 0.358(8) μs is substantially shorter than the
2.08(4) μs value obtained for a deaerated solution of 3 (Table 7).
Nevertheless, it is in excellent agreement with the value of
0.353(3) μs observed after continuously irradiating the CH3CN
solution of 1 in a capped cuvette for 4 h. These results confirm
our initial assumption about the slow photodegradation of 1 to 3
due to the seepage of air into the cuvette. They also indicate that
the photodegradation process occurs quite fast once the solution
is exposed to air under visible light.
Figure 8. Absorption (solid lines) and emission (dashed lines) spectra
of 1 in a CH3CN solution before (red) and after (blue) pulsed-laser
irradiation at 458 nm for a period of 4 h.
CONCLUSIONS
■
A detailed study of Ru(II) complexes with redox-active TTF-
annulated phenanthroline (edt-TTF-phen, L1) and its analogues
that contain only half of the TTF unit (TTC-phen, L2, and DTC-
phen, L3) reveals that [Ru(bpy)2(L1)](PF6)2 (1) exhibits
behavior distinctly different from that observed for [Ru-
(bpy)2(L2)](PF6)2 (2) and [Ru(bpy)2(L3)](PF6)2 (3). This
difference stems from the presence of an additional redox-active
MO in 1 which is centered on the TTF unit, serving as the
HOMO, and which is located ∼0.6 eV above the Ru-based d
orbitals. Such a high-energy ligand-centered HOMO is absent in
2 and 3. Consequently, 1 exhibits three reversible oxidations, of
which the two occurring at lower potentials are TTF-based. A
low-energy tail of the MLCT absorption band, observed only for
1, is assigned to the ILCT within the L1 ligand, according to TD-
DFT calculations. Upon irradiation into the characteristic low-
energy MLCT band, complexes 1−3 exhibit emission with
luminescence lifetimes of ∼1−2 μs. The monoexponential rate of
luminescence decay indicates that the MLCT state is the only
one contributing to the observed emission. In contrast to 2 and 3,
complex 1 also experiences intramolecular reductive excited-
state electron transfer from the TTF moiety to the Ru center.
This process is evident from the substantially lower luminescence
quantum yield of 1 (ΦF = 0.012) when compared to those of 2
and 3 and the reference compounds [Ru(bpy)3](PF6)2 and
[Ru(bpy)2(phen)](PF6)2 (ΦF = 0.060−0.095). This finding is in
agreement with the well-established electron-donating proper-
ties of TTF. Contrary to expectations from the previous reports
on related Ru(II) complexes with TTF-containing ligands, the
presence of the low-energy ILCT state (TTF → phen) is not
accompanied by a long-lived charge-separated state (>10 ns) that
is detectable as a transient species when probing the excited-state
absorbance profile. Solutions of 1 exposed to air and visible light
exhibit photodegradation that leads to the cleavage of the central
CC bond of the TTF unit and quantitative generation of 3.
The interesting photophysical behavior of complex 1 and the
related previously reported complex, Ru(bpy)2(TTF-dppz)]-
(PF6)2,27 call for further studies of heteroleptic Ru(II) complexes
attenuation of both the low-energy tail of the MLCT band above
500 nm and the mixed ILCT/LLCT band (TTF → phen/bpy,
Table 4) that appears as a shoulder at 340 nm, along with the
hypsochromic shift of the emission maximum from 642 to 630
nm, is in accord with the conversion of 1 to 3. A control
experiment was also performed on a CH3CN solution of 1 that
was kept in a dark and O2-free environment. The absorption and
emission spectra of this solution remained essentially unchanged
after the 4 h period without irradiation (Supporting Information,
Figure S6), yet again confirming the necessity of irradiation for
the oxidative cleavage of the CC bond in the TTF unit of 1.
Given the difference in the luminescence lifetimes of 1 and 3
(1.77(4) and 2.08(4) μs, respectively), we monitored the change
in this parameter for a deaerated CH3CN solution of 1 under
continuous irradiation with the 458 nm pulsed laser for 4 h
(Figure 9). During this period, the sample was kept in a capped
cuvette. The initial measurement was performed immediately
after degassing the solution and prior to turning on the laser. It
resulted in τobs = 1.74(5) μs, as expected for the deaerated
Figure 9. Monitoring the luminescence decay at 640 nm (λexc = 458 nm)
of a 10 μM solution of 1 in CH3CN at 298 K. Sample cuvette was
exposed to pulsed irradiation for a period of 4 h.
K
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(9) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;
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with redox-active ligands. In particular, we are currently
investigating the nature of excited states in these and related
complexes by ultrafast spectroscopy and more thorough
theoretical calculations. The results of these studies will be
reported in due course.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 2D COSY NMR spectra of 1−3, molecular structure of
[Ru(bpy)2(L1)]2+, crystal packing of L1, emission spectra of
complexes 1−3, kinetic profile of the difference absorbance
spectrum of 1, and absorption and emission spectra of a control
CH3CN solution of 1. This material is available free of charge via
the Internet at http://pubs.acs.org.
(14) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008,
A64, 112−122.
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M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.;
Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
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Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: andreas.hauser@unige.ch (A.H.), shatruk@chem.fsu.
edu (M.S.).
Present Addresses
§K.K.: Department of Chemistry, University of California at
Davis, Davis, CA 95616.
∥J.M.H.: Department of Chemistry, Princeton University,
Princeton, NJ 08544.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(17) (a) Dunning, T. H., Jr.; Hay, P. J. Mod. Theor. Chem. 1977, 3, 1−
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This research was partially supported by the National Science
Foundation (award CHE-0911109 to M.S.).
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