Chromium(III) Bis-Arylterpyridyl Complexes with Enhanced Visible Absorption via Incorporation of Intraligand Charge-Transfer Transitions

Article abstract of DOI:10.1021/acs.inorgchem.7b00953

A series of chromium(III) bis-arylterpyridyl complexes containing intraligand charge-transfer (ILCT) excited states were prepared and characterized. These complexes show significant absorption in the visible region due to the ILCT bands. The ILCT bands are tunable across the UV and visible spectrum via incorporation of electron-withdrawing and electron-donating groups on the aryl ring. The absorption of Cr(4′-(4-methoxyphenyl)-2,2′:6′,2″-terpyridine)₂³⁺ (4) in particular is much stronger in the visible region (? = 11 900 M^-1 cm^-1 at 450 nm and ? = 5090 M^-1 cm^-1 at 500 nm) than that of the parent complex Cr(tpy)₂³⁺ (tpy = 2,2′:6′,2″-terpyridine; ? = 2160 M^-1 cm^-1 at 450 nm, and ? = 170 M^-1 cm^-1 at 500 nm). Emission experiments on this series reveal Cr(III)-based phosphorescence with lifetimes from 140 to 600 ns upon excitation into the ILCT bands, which indicates funneling of the excitation energy from ligand-localized excited states to Cr(III)-based excited states. Cyclic voltammograms exhibit at least three reversible ligand-based reductions. The first reduction shows shifts of up to -160 mV compared to Cr(tpy)₂³⁺. The excited-state reduction potential of these complexes ranges from +0.95 to +1.04 V vs the ferrocene/ferrocenium couple, making them potent photooxidants.

Full text of DOI:10.1021/acs.inorgchem.7b00953

Article
pubs.acs.org/IC
Chromium(III) Bis-Arylterpyridyl Complexes with Enhanced Visible
Absorption via Incorporation of Intraligand Charge-Transfer
Transitions
Johanna C. Barbour,†,‡ Amy J. I. Kim,†,‡ Elsemarie deVries,†,§ Sarah E. Shaner,§,∥
and Benjamin M. Lovaasen*,†
†Department of Chemistry, Wheaton College, 501 College Avenue, Wheaton, Illinois 60187, United States
∥Department of Chemistry, Benedictine University, 5700 College Road, Lisle, Illinois 60532, United States
S
* Supporting Information
ABSTRACT: A series of chromium(III) bis-arylterpyridyl
complexes containing intraligand charge-transfer (ILCT)
excited states were prepared and characterized. These
complexes show significant absorption in the visible region
due to the ILCT bands. The ILCT bands are tunable across
the UV and visible spectrum via incorporation of electron-
withdrawing and electron-donating groups on the aryl ring.
The absorption of Cr(4′-(4-methoxyphenyl)-2,2′:6′,2″-terpyr-
idine)23+ (4) in particular is much stronger in the visible region
(ε = 11 900 M−1 cm−1 at 450 nm and ε = 5090 M−1 cm−1 at
3+
500 nm) than that of the parent complex Cr(tpy)2 (tpy =
2,2′:6′,2″-terpyridine; ε = 2160 M−1 cm−1 at 450 nm, and ε =
170 M−1 cm−1 at 500 nm). Emission experiments on this series
reveal Cr(III)-based phosphorescence with lifetimes from 140 to 600 ns upon excitation into the ILCT bands, which indicates
funneling of the excitation energy from ligand-localized excited states to Cr(III)-based excited states. Cyclic voltammograms
exhibit at least three reversible ligand-based reductions. The first reduction shows shifts of up to −160 mV compared to
Cr(tpy)23+. The excited-state reduction potential of these complexes ranges from +0.95 to +1.04 V vs the ferrocene/ferrocenium
couple, making them potent photooxidants.
appended groups with excited states of their own has led to
chromophores with enhanced absorption properties.37−44
While Cr(III) polypyridyls generally absorb at λ < 450 nm,
they phosphoresce at λ > 700 nm. Therefore, ligand-localized
transitions that absorb visible light are expected to be
energetically competent to sensitize the long-lived doublet
excited states typical of Cr(III) polypyridyl complexes.
However, we know of no previous systematic attempts to
investigate the ability of visible-absorbing, ligand-localized
transitions to funnel excitation energy into Cr(III)-based
excited states.
Compared to Cr(III) tris-bipyridyl complexes, the electro-
chemical and photophysical properties of Cr(III) bis-terpyridyl
complexes have received little attention.11,14,15,45−57 Many of
those reports are aimed at understanding the relatively short
(<200 ns) excited-state lifetimes of this family of complexes.
Nonetheless, Cr(III) terpyridyl complexes present opportuni-
ties to address the challenges posed by the poor photostability
and weak visible absorption of Cr(III) tris-bipyridyls.
Addressing the first challenge, the increased denticity of the
INTRODUCTION
■
Coordination complexes of Cr(III) have long been known as
chromophores with rich photochemistry and redox chem-
istry.1−5 Recently, there has been renewed interest in the
chemistry of these complexes, especially Cr(III) polypyr-
idines.6−22 Most of the studies on Cr(III) polypyridines have
focused on Cr(III) tris(2,2′-bipyridyl) and analogues with
bidentate ligands. These complexes are potent photooxidants
with excited-state reduction potentials of ≥+1.0 V vs the
ferrocene/ferrocenium couple (Fc/Fc+) and excited-state
lifetimes in the hundreds of microseconds.4,6,23,24 Chromium
is an inexpensive, earth abundant metal. In light of these
properties, Cr(III) chromophores are promising targets for use
in schemes for solar energy conversion and photoredox
catalysis.25−33
Challenges to the widespread use of Cr(III) polypyridyl
chromophores in a variety of applications include weak
absorption bands in the visible portion of the solar spectrum
and photoinduced ligand substitution reactions.4 Ligand
variation has been a successful strategy for tuning the
absorption properties of transition metal chromophores,
especially chromophores with charge-transfer excited
states.34−36 Additionally, incorporation of ligands or ligand-
Received: April 12, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b00953
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Photophysical Measurements. Absorption spectra were ob-
tained using a Shimadzu UV-2450 spectrophotometer with samples in
1 cm quartz cuvettes at room temperature. Extinction coefficients were
determined from plots of absorbance versus concentration for five or
more samples prepared via dilution of a stock solution. Samples for
emission experiments were sealed in screw-top cuvettes with silicone
septa under a nitrogen atmosphere. This method produced sufficiently
low oxygen concentrations to yield repeatable emission lifetimes and
quantum yield measurements for these samples.
Emission spectroscopy was performed on a Horiba Jobin Yvon
Fluorolog-3 equipped with a red-sensitive Hamamatsu R928 PMT.
Steady-state emission experiments were performed with a 455 nm long
pass filter between the sample and the emission monochromator.
Relative emission yields were obtained from optically dilute (OD <
0.1) samples of a standard, Cr(tpy)2(PF6)3, and the unknown
measured in succession. The relative emission intensity was calculated
from eq 1:68
terpyridine ligand has the potential to stabilize the excited
species toward ligand substitution. While recent reports reveal
unexpected thermal lability for Cr(III) terpyridyl complexes,
there have been no reports on the photolability for these
complexes.14,15 Addressing the second challenge, terpyridines
with an aryl substituent in the 4-position on the central pyridine
ring are known to contain strongly absorbing charge-transfer
(CT) states that are tunable via variation of the groups on the
aryl substituent.58−60 Furthermore, complexes of arylterpyr-
idines and metal centers (including Cr, Zn, Ru, Ir, Pt) are
reported to exhibit intraligand charge-transfer (ILCT) bands in
the visible region.59,61−66 A singular report details the synthesis
of Cr(III) bis-arylterpyridyl complexes with identified ILCT
bands but does not report on the photophysical properties of
these complexes.15
Herein, we report the synthesis and photophysical character-
ization of a series of homoleptic Cr(III) complexes with 4′-aryl-
2,2′:6′,2″-terpyridines as ligands. These complexes exhibit
ILCT bands that greatly enhance the absorption properties of
Cr(III) terpyridyls; the ILCT bands are tunable across the UV
and visible region and absorb with ε of up to 56 000 M−1 cm−1.
Emission spectroscopy indicates that, in all but one case, the
excitation energy is funneled from the ILCT state into Cr(III)-
based doublet excited states, leading to phosphorescence with
lifetimes up to 600 ns. These complexes are potent photo-
oxidants with excited-state reduction potentials of up to +1.04
V vs Fc/Fc+.
⎛
⎜
⎝
⎞⎛
⎞
⎟
⎠
Iunk Astd
rel em =
⎟⎜
I
A
unk
std ⎠⎝
(1)
where I is the integrated emission intensity of the phosphorescence
band and A is the absorption of the sample at the excitation
wavelength. Emission spectra were corrected for PMT response, and a
solvent background was subtracted from each spectrum. The emission
spectra of 6 were further adjusted to yield a baseline at zero. The
emission spectra of 4 were each fit to a Gaussian function from 625 to
715 nm; a representative emission spectrum and Gaussian function are
shown in Figure S1. To determine the integrated doublet emission
intensity for 4, the area under this function was subtracted from the
area under the emission spectrum from 600 to 825 nm.
Time-resolved emission experiments were performed using a
Fluorolog-3, equipped with a time-correlated single photon counting
module using the same detector and emission channel as the steady-
state emission experiments. The excitation source was a 454 nm
Horiba NanoLED with a 1.2 ns pulse width. A 715 nm long pass filter
was placed between the sample and the emission monochromator for
lifetimes monitored at λ > 720 nm. Emission experiments were
performed on at least three samples for each complex, and the
reported lifetime is the average of the three samples. Lifetimes were
found by fitting the decay traces to a single exponential function over
5τ. In the case of 4 and 6 a fast feature decayed with the instrument
response function; time points that include the rapid feature were
excluded from the fit. Inspection of the residuals confirmed the
appropriateness of the fits. Absorption spectra of the samples showed
no change before and after the emission measurements.
EXPERIMENTAL SECTION
■
General Information. All manipulations involving CrCl2 were
performed using standard Schlenk techniques or in a MBRAUN
Labstar glovebox under N2 atmosphere, until the oxidation step. 4′-(4-
Bromophenyl)-2,2′:6′,2″-terpyridine (L2),67 4′-phenyl-2,2′:6′,2″-ter-
11
pyridine (L1),67 and Cr(tpy)2(PF6)3 were synthesized via reported
procedures. All other reagents were obtained from commercial
1
sources. H NMR spectra were obtained using a Bruker AVANCE
III 400Mz NMR. All NMR spectra were recorded between 293 and
296 K in CDCl3 and referenced to the residual solvent peak. IR spectra
were recorded on a PerkinElmer Spectrum Two with UATR accessory.
Elemental analyses were performed by Midwest Microlab, LLC, in
Indianapolis, IN. Mass spectrometry was performed in either positive
or negative ion mode on a Thermo Finnigan LCQ Deca quadrupole
mass spectrometer equipped with an electrospray ion source.
Tetrabutylammonium hexafluorophosphate (Sigma-Aldrich, 98%)
was used as an electrolyte for electrochemical experiments and was
recrystallized from 95% ethanol and dried under vacuum at 70 °C for
at least 24 h. Acetonitrile used for electrochemical experiments was
filtered through activated alumina and stored over molecular sieves
prior to use. Ferrocene (Sigma-Aldrich, 98%) was sublimed under
vacuum. Burdick and Jackson brand solvents were used for
spectroscopic measurements. Acetonitrile (Burdick and Jackson
brand) for steady-state and time-resolved emission experiments was
dried over 4 Å molecular sieves for at least 48 h, freeze−pump−thaw
degassed, and vacuum transferred before use.
Electrochemistry. All cyclic voltammetry experiments were
performed at room temperature with an Epsilon EClipse BASi
electrochemical analyzer and a C3 cell stand. A three-electrode setup
consisting of a glassy carbon working electrode (3 mm diameter), Pt-
wire auxiliary electrode, and a Ag-wire quasi-reference electrode was
used, and electrodes were polished with alumina prior to each
experiment. Samples consisted of the chromium complex analyte
dissolved in a nitrogen-purged 0.1 M nBu4NPF6 solution in
acetonitrile. All cyclic voltammograms were collected at a scan rate
of 100 mV/s. Ferrocene was added as an internal reference, and under
the experimental conditions, the Fc/Fc+ couple exhibited reversible
behavior.
Synthesis of Ligands and Chromium Complexes. 4′-(4-
Methylphenyl)-2,2′:6′,2″-terpyridine (L3). Toluylbenzaldehyde (4.72
mL, 0.0400 mol), 2-acetylpyridine (10.5 mL, 0.0936 mol), and NaOH
(s) (12.0 g, 0.300 mol) were combined using a mortar and pestle until
the sticky aggregate became a pale orange powder (ca. 30 min). The
powder was added to a suspension of ammonium acetate (21.6 g,
0.280 mol) in 120 mL of glacial acetic acid and was refluxed for 2 h.
Upon the addition of 650 mL of water, a brown precipitate formed,
was collected via filtration, and was washed with water. The brown
solid was >98% pure by 1H NMR and used without further
1
purification (10.045 g, 0.0311 mol, 77.7%). H NMR: δ = 8.74 (m,
4H), 8.67 (d, J = 8 Hz, 2H), 7.88 (td, J = 7.7 Hz, J = 1.7 Hz, 2H), δ =
7.83 (d, J = 8 Hz, 2H), δ = 7.35 (m, 2H), δ = 7.32 (d, J = 8 Hz, 2H), δ
1
= 2.43 (s, 3H). H NMR data is consistent with a previous report for
the same compound.58
4′-(4-Methoxyphenyl)-2,2′:6′,2″-terpyridine (L4). p-Anisaldehyde
(6.10 mL, 0.050 mol), 2-acetylpyridine (10.5 mL, 0.0936 mol), and
NaOH (s) (11.8 g, 0.295 mol) were combined using a mortar and
pestle until the sticky aggregate became an orange powder (ca. 1 h).
The powder was added to a suspension of ammonium acetate (21.6 g,
0.280 mol) in 150 mL of glacial acetic acid and was refluxed for 2 h.
Upon the addition of 650 mL of water, a black precipitate formed and
was collected via filtration. The resulting sticky, black solid was
triturated with ethanol, yielding a dark green solid. The green solid was
B
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Inorganic Chemistry
Article
1
used without further purification (3.061 g, 0.00902 mol, 18.0%). H
NMR: δ = 8.73 (m, 2H), 8.70 (s, 2H), 8.66 (d, J = 8 Hz, 2H), 7.89−
7.85 (m, 4H), 7.34 (m, 2H), 7.02 (d, J = 8.8 Hz, 2H), δ = 3.88 (s, 3H).
1H NMR data is consistent with a previous report for the same
compound.58
mmol) in 200 mL of methanol. The reaction mixture was stirred at
room temperature. After 3 days, air was bubbled through the mixture
for 20 min, the mixture was filtered, and excess sodium
hexafluorophosphate was added to the filtrate, resulting in a brown
precipitate (1.144 g; 34% yield of crude product). 0.595 g of the brown
solid was chromatographed on silica gel with ACN:H2O (7:1). After a
purple band eluted, ACN:H2O:sat. aq KNO3 (7:1:0.5) was used to
elute the product as a yellow band. The eluent was concentrated, and
NaPF6 (0.482 g, 2.87 mmol) in 5 mL of water was added to precipitate
a yellow-orange solid. The solid was collected via filtration and
recrystallized from acetonitrile via slow diffusion of diethyl ether,
yielding yellow crystals (0.144 g, 0.0127 mmol, 24.2% yield for
purification). ES−MS (CH3CN): m/z 1278.17 ([Cr(L3)2(PF6)4]−).
ES+MS (CH3CN): m/z 232.71 ([Cr(L3)2]3+), 358.56 ([Cr-
(L3)2F]2+), 735.88 ([Cr(L3)2F2]+). Anal. Calcd for C44H34N6CrP3F18:
C, 46.61; H, 3.02; N, 7.41. Calcd for 3·2.5CH3CN: C, 47.60; H, 3.38;
N, 9.63. Found: C, 47.85; H, 3.36; N; 9.46. IR (solid, ATR): νAr,Py
1618, 1601, 1570, 1548, 1480, 1435, 1407, 1367 cm−1.
4′-(4-Dimethylaminophenyl)-2,2′:6′,2″-terpyridine (L5). 4-Dime-
thylaminobenzaldehyde (5.97 g, 0.0400 mol), 2-acetylpyridine (10.5
mL, 0.0936 mol), and NaOH (s) (11.8 g, 0.295 mol) were combined
using a mortar and pestle until the sticky aggregate became an orange-
yellow powder (ca. 30 min). The powder was added to a suspension of
ammonium acetate (21.6 g, 0.280 mol) in 150 mL of glacial acetic acid
and was refluxed for 2 h. Upon the addition of 650 mL of water, a dark
precipitate formed, was collected via filtration, and was washed with
1
water. The dark brown solid was >98% pure by H NMR and used
1
without further purification (7.867 g, 0.0231 mol, 55.8% yield). H
NMR: δ = 8.73 (m, 2H), 8.69 (s, 2H), 8.65 (d, J = 8 Hz, 2H), 7.88−
7.84 (m, 4H), 7.34 (m, 2H), 6.81 (d, J = 8 Hz, 2H), δ = 3.04 (s, 6H).
1H NMR data is consistent with a previous report for the same
compound.58
[Cr(L4)2](PF6)3 (4). CrCl2 (0.201g, 1.64 mmol) was dissolved in 25
mL of water and added via cannula to a suspension of L4 (1.19 g, 3.49
mmol) in 50 mL of methanol. The brown-red mixture was stirred at
room temperature. After 21 h, air was bubbled through the mixture for
20 min, resulting in an orange-red mixture. The mixture was filtered,
and NaPF6 (0.964 g, 5.84 mmol) was added to the filtrate to
precipitate a red-orange solid (0.910 g, 44% yield of crude product).
The product was recrystallized three times by vapor diffusion of
diethyl ether into a concentrated acetonitrile solution. The resulting
red solid was triturated with methylene chloride (10 mL) and
recrystallized a fourth time to yield red crystals (0.0448 g, 0.0354
mmol, 2.2% yield). Crystallizations were performed until successive
solids gave identical UV−vis absorption spectra. ES−MS (CH3CN):
m/z 1310.18 ([Cr(L4)2(PF6)4]−). ES+MS (CH3CN): m/z 243.37
([Cr(L4)2]3+), 374.56 ([Cr(L4)2F]2+), 767.87 ([Cr(L4)2F2]+). Anal.
Calcd for C44H34N6O2CrP3F18: C, 45.33; H, 2.94; N, 7.21. Calcd for 4·
H2O·2CH3CN: C, 45.54; H, 3.34; N, 8.85. Found: C, 45.26; H, 3.12;
N, 8.61. IR (solid, ATR): νAr,Py 1608, 1590, 1574, 1545, 1525, 1479,
1440, 1419, 1365 cm−1.
[Cr(L5)2](PF6)3 (5). CrCl2 (0.144 g, 1.17 mmol) was dissolved in 25
mL of water and added via cannula to a suspension of L5 (0.966 g,
2.59 mmol) in 50 mL of methanol. The dark purple-red reaction
mixture was stirred at room temperature. After 20 h, air was bubbled
through the mixture for 30 min, and the mixture turned green-purple.
The mixture was filtered, and NaPF6 (0.806 g, 4.80 mmol) was added
to the filtrate to precipitate the dark product (0.4508 g, 31% yield of
crude product). The product was recrystallized twice by vapor
diffusion of diethyl ether into a concentrated acetonitrile solution to
give a green crystalline powder (0.0917 g, 0.0731 mmol, 6.2% yield).
ES−MS (CH3CN): m/z 1335.80 ([Cr(L5)2(PF6)4]−). Anal. Calcd for
C46H40N8CrP3F18: C, 46.36; H, 3.38; N, 9.40. Calcd for 5·3.5H2O: C,
44.03; H, 3.78; N, 8.93. Found: C, 43.70; H, 3.41; N, 8.87. IR (solid,
ATR): νAr,Py 1607, 1578, 1563, 1539, 1476, 1445, 1420, 1382, 1363
cm−1.
[Cr(L6)2](PF6)3 (6). CrCl2 (0.114 g, 0.928 mmol) was dissolved in 25
mL of water and added via cannula to a suspension of L6 (0.686 g,
1.95 mmol) in 50 mL of methanol. The brown mixture was stirred at
room temperature. After 17 h, air was bubbled through the mixture for
30 min with no significant color change. The mixture was filtered, and
excess NaPF6 (0.772 g, 4.60 mmol) was added to the filtrate to
precipitate a dark yellow solid (0.500 g, 45.2% crude yield). 0.434 g of
the dark yellow solid was recrystallized from acetonitrile/diethyl ether
to yield dark red crystals (0.371 g, 0.312 mmol, 85.5% yield for
purification steps). ES−MS (CH3CN): m/z 1334.26 ([Cr-
(L6)2(PF6)4]−). ES+MS (CH3CN): m/z 251.41 ([Cr(L6)2]3+),
386.61 ([Cr(L6)2F]2+), 791.93 ([Cr(L6)2F2]+). Anal. Calcd for
C48H42N6CrP3F18: C, 48.45; H, 3.56; N, 7.07. Found C, 48.49; H,
3.72; N, 7.34. IR (solid, ATR): νAr,Py 1607, 1570, 1540, 1480, 1449,
1425 cm−1.
4′-(2,4,6-Trimethylphenyl)-2,2′:6′,2″-terpyridine (L6). Mesitalde-
hyde (5.9 mL, 0.0400 mmol), 2-acetylpyridine (10.5 mL, 0.0936
mmol), and NaOH (s) (12.0 g, 0.300 mol) were combined using a
mortar and pestle until the sticky aggregate became a caramel-colored
powder (ca. 45 min). The powder was added to a suspension of
ammonium acetate (21.6 g, 0.280 mol) in 100 mL of glacial acetic acid
and was refluxed for 2.5 h. Upon the addition of 650 mL of water, a
green precipitate formed, was collected via filtration, and was washed
with water. The resulting sticky, green solid was triturated with
ethanol, yielding a pale solid, which was used without further
purification (5.149 g, 0.0147 mol, 34.2%). 1H NMR: δ = 8.68 (m, 4H),
8.31 (s, 2H), 7.88 (td, J = 7.6 Hz, J = 2.0 Hz, 2H), 7.35−7.31 (m, 2H),
1
6.95 (s, 2H), 2.34 (s, 3H), 2.07 (s, 6H) H NMR data is consistent
with a previous report for the same compound.62
[Cr(L1)2](PF6)3 (1). CrCl2 (0.1 g, 0.9 mmol) was dissolved in 25 mL
of water and added via cannula to a suspension of L1 (0.529 g, 1.71
mmol) in 50 mL of methanol. The dark purple reaction mixture was
stirred at room temperature. After 20 h, air was bubbled through the
mixture for 20 min, resulting in an orange-brown mixture. The mixture
was filtered, and excess NaPF6 was added to the filtrate to precipitate
an orange solid (0.2866 g, 31% crude yield). The orange solid (ca. 0.2
g) was chromatographed on silica gel with ACN:H2O (7:1). After a
purple band eluted, ACN:H2O:sat. aq KNO3 (7:1:0.5) was used to
elute the product as an orange band. The eluent was concentrated,
NaPF6 (0.203 g, 1.21 mmol) in 5 mL of water was added, and the
solution was cooled to 2 °C to precipitate an orange crystalline solid
(0.1011 g, 0.090 mmol, 50% yield for purification steps). ES−MS
(CH3CN): m/z 1250.14 ([Cr(L1)2(PF6)4]−). ES+MS (CH3CN): m/z
223.37 ([Cr(L1)2]3+), 344.57 ([Cr(L1)2F]2+), 707.84 ([Cr(L1)2F2]+).
Anal. Calcd for C42H30N6CrP3F18: C, 45.62; H, 2.74; N, 7.60. Calcd
for 1·0.5CH3CN: C, 45.86; H, 2.82; N, 8.09. Found: C, 45.67; H, 3.09;
N, 8.01. IR (solid, ATR): νAr,Py 1610, 1596, 1570, 1550, 1482, 1449,
1422, 1366 cm−1.
[Cr(L2)2](PF6)3 (2). CrCl2 (0.018 g, 0.15 mmol) was dissolved in 5
mL of water and added via cannula to a suspension of L2 (0.12 g, 0.31
mmol) in 50 mL of methanol. The dark purple mixture was stirred at
room temperature. After 20 h, air was bubbled through the mixture for
15 min, resulting in a yellow-brown mixture. The mixture was filtered,
and excess NaPF6 was added to the filtrate to precipitate a yellow-
orange solid. The crude product was washed with water, diethyl ether,
and acetone (0.120 g). The product was recrystallized three times by
slow diffusion of diethyl ether into a concentrated acetonitrile solution
to yield bright yellow crystals (0.049 g, 24% yield). ES−MS (CH3CN):
m/z 1407.91 ([Cr(L2)2(PF6)4]−). ES+MS (CH3CN): m/z 275.97
([Cr(L2)2]3+), 423.46 ([Cr(L2)2F]2+), 865.65 ([Cr(L2)2F2]+). Anal.
Calcd for C42H28N6CrP3F18Br2: C, 39.93; H, 2.23; N, 6.65. Calcd for
2·2CH3CN: C, 41.06; H, 2.55; N, 8.33. Found: C, 40.99; H, 2.69; N,
8.19. IR (solid, ATR): νAr,Py 1610, 1587, 1571, 1545, 1500, 1480, 1435,
1396, 1371 cm−1.
[Cr(L3)2](PF6)3 (3). CrCl2 (0.332 g, 2.70 mmol) was dissolved in 100
mL of water and added via cannula to a suspension of L3 (1.84 g, 5.68
C
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part to the extensive purification required to obtain samples
that are sufficiently pure for emission spectroscopy. In contrast
to the bright yellow [Cr(tpy)2](PF6)3, these complexes range
from orange-yellow, 1−3, to bright orange, 4, to red-orange, 6,
to dark green (vivid purple in solution), 5. The purity and
identity of these compounds are established via elemental
analysis, mass spectrometry, and IR spectroscopy.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Compounds. L1−
L6 are known compounds that are typically synthesized via one
of two methods: condensation of a substituted benzaldehyde
with 2-acetylpyridine in the presence of base in solvent,
followed by ring closure using an appropriate nitrogen source,
or Suzuki coupling of 4-bromoterpyridine with an appropriate
aryl boronic acid.59,60,69 We prepared these complexes via a
solventless condensation of 2-acetylpyridine and an appropriate
benzaldehyde followed by refluxing the resulting 1,5-diketone
with ammonium acetate in acetic acid. This method developed
by Raston extends the principles of green chemistry to
terpyridine synthesis. L1 and L2 have been prepared by this
method previously.67 For this study we extended this method
to prepare L3−L6 via a solventless condensation and
subsequent ring closing with ammonium acetate for the first
time. Ligands L1−L5 were selected to incorporate a range of
electron-donating groups that serve to modulate the energy of
the CT transitions in the target chromium complexes; L6 was
chosen to prevent planarization between the 4′-aryl ring and
the central pyridyl ring. A previous report gives a value of 67.5°
for the dihedral angle between the mesityl group and central
terpyridine ring in L6 in contrast to 11° for the dihedral angle
between the toluyl group and central terpyridine ring in L3.62,70
The lack of planarity between the aryl ring and the central
terpyridyl ring should serve to inhibit conjugation between
these two moieties in both the ground and excited states.
Cr(III) complexes 1−6 are synthesized via the complexation
of arylterpyridyl ligands to a Cr(II) salt in stoichiometric
amounts, followed by oxidation in air and a salt metathesis with
excess sodium hexafluorophosphate to yield the hexafluor-
ophosphate salts (Scheme 1). Complexes 1−3 have been
Photostability Studies. Cr(III) bis-terpyridyls have been
recently shown to undergo both thermal ligand substitution by
selected solvents over a period of several hours, and rapid
ligand substitution by OH− in aqueous solution at pH >7.14,15
We investigated the photostability of Cr(tpy)23+ to evaluate the
potential of Cr(III) bis-terpyridyls as a platform for the design
of photostable chromophores. Solutions of Cr(tpy)2(PF6)3 or
Cr(bpy)3(PF6)3 in 1 M HCl were irradiated with UV light (26
W CFL black light bulb, λmax = 370 nm) over a period of 24 h.
3+
The UV−visible absorption spectrum of Cr(tpy)2 remained
unchanged during 24 h of irradiation, while under the same
conditions, Cr(bpy)33+ was completely consumed (Figure 1). It
is worth noting that for Cr(bpy)33+ the Φ for photosubstitution
is reported to be 0.007 at this pH.71 The previously proposed
mechanism for the photoaquation of Cr(III) polypyridyls
involves an initial coordination of waterlikely water that is
preorganized in the intraligand pocketto the Cr center,
followed by deprotonation by OH− and subsequent water
coordination/loss of the polypyridine ligand. Even the short 2Eg
3+
(Oh) lifetime for Cr(tpy)2 (∼50 ns in 1 M HCl) should be
sufficient for the first step to occur efficiently; the short excited-
3+
3+
state lifetime of Cr(tpy)2 compared to Cr(bpy)3 does not
explain the discrepancy in photostability.49,71,72 These results
suggest that Cr(III) terpyridyls are a promising platform for
design of photostable Cr(III) polypyridyl complexes.
Absorption Spectroscopy. The absorption spectrum of
3+
Cr(tpy)2 has been reported previously to exhibit an intense
Scheme 1. Synthesis of Cr(III) Arylterpyridyl Complexes
absorption band in acetonitrile at 364 nm (ε = 23 000 M−1
cm−1), with vibrational structure on the blue edge of the band
(Figure 2); this band is assigned as a terpyridine-localized π →
π* transition, which is singlet with respect to the terpyridine
ligand and quartet with respect to the complex, 4[1(π → π*)].50
Additionally, in the blue region there is a structured feature
centered at 443 nm (ε = 2540 M−1 cm−1). The intensity of this
feature is greater than expected for chromium-localized ligand-
field transitions, even considering the lack of a center of
inversion in Cr(tpy)23+, and has been assigned to a terpyridine-
3
localized (π → π*) transition coupled with a spin flip in the
4A2g (Oh) ground state of Cr(III), resulting in a quartet excited
57
4 3
state, [ (π → π*)], though this assignment is uncertain.
Previous studies on unligated arylterpyridines assign the
1
lowest energy absorption bands to a (π → π*) transition
1
within the terpyridine π-system and a CT transition from the
aryl π-system to terpyridine π*-orbitals.58 The energy of the
1CT transition has been shown to be sensitive to the nature of
1
the groups on the aryl substituent, while the energy of the (π
→ π*) transition is nearly unperturbed by substitution of the
aryl ring. Density functional theory calculations on arylterpyr-
1
synthesized previously, but their photophysical properties have
not been fully characterized; 4−6 are novel complexes.15,48,56
Purification of these complexes is achieved via column
chromatography in the case of 1, column chromatography
followed by recrystallization from an acetonitrile−ether
solution in the case of 3, and recrystallization from
acetonitrile−ether solutions in the cases of 2 and 4−6. The
low yields (<40%) for these compounds can be attributed in
idines indicate that the donor orbitals involved in the CT
transitions are delocalized over both the aryl π-system and the
substituents on the aryl ring. For arylterpyridines with electron-
withdrawing groups or moderately electron-donating groups
(such as L1−L3), the lowest energy absorption band has been
1
assigned to the (π → π*) transition; for arylterpyridines with
strongly electron-donating groups (such as L4 and, L5), the
order of the 1(π → π*) and 1CT transitions is inverted, and the
D
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Inorganic Chemistry
Article
3+
3+
Figure 1. Absorption spectra of Cr(bpy)3 (a) and Cr(tpy)2 (b) in 1 M HCl after irradiation with a 26 W CFL bulb (λmax = 370 nm) for 0 h
(black), 8 h (red), and 24 h (blue).
3+
the analogous feature in Cr(tpy)2 (Table 1). In some cases
this band is observed primarily as a shoulder on the edge of an
ILCT band (Figure 3). Additionally, a structured 4[3(π → π*)]
Figure 2. Electronic absorption spectra in acetonitrile for Cr-
(tpy)2(PF6)3 (black), 5 (gray), and 6 (brown).
1
lowest energy absorption band has been assigned to the CT
transition.58,60 Additionally, coordination of arylterpyridines to
metals (e.g., Cr, Zn, Ru, Ir, Pt) has been reported to lead to
substantial red shifts of both the 1(π → π*) and 1CT
Figure 3. Electronic absorption spectra in acetonitrile for 1 (red), 2
bands.15,59,61−66 As described in this study, the electronic
(green), 3 (blue), 4 (purple), and 5 (gray). The arrows mark the
location of the ILCT band in each spectrum. Note the shared feature
near λ = 363 nm in all spectra.
absorption spectra of 1−6 exhibit features characteristic of both
3+
Cr(tpy)2 and ILCT transitions.
1−5 all display features that are attributed to terpyridine-
localized π → π* and ILCT transitions in acetonitrile. The
band is evident for 1−3 that is nearly unshifted compared with
Cr(tpy)23+. 4 and 5 exhibit intense bands in this region that
obscure the [ (π → π*)] band. 1−5 display an additional
4[1(π → π*)] band appears at λmax = 364
compounds studied; this band appears nearly unshifted from
4 nm for all
4 3
Table 1. Electron Absorption Data for Cr(III) Terpyridyl Complexes
λmax/nm (ε/M−1 cm−1
)
complex
4[1(π→π*)]
4[3(π→π*)]
ILCT
349 (44 200)
a
3+
Cr(tpy)2
364 (23 000), 350 (18 700), 326 (12 700), 314 (10 600)
421 (2320), 443 (2540), 473 (1560)
444 (3210), 471 (1760),
a
a
a
a
a
a
,
,
,
,
b
b
b
b
1
2
3
4
5
6
365 (35 500)
365 (50 200)
443 (4190), 473 (2080)
353 (51 900)
367 (51 800)
441 (5280), 469 (3210)
357 (47 900)
366 (35 200), 347 (27 600), 325 (20 400), 313 (20 900)
406 (43 900), 499 (5140)
532 (56 500), 691 (13 600)
360 (25 800)
362 (25 500), 348 (22 300), 325 (15 900), 313 (13 900)
367
438 (3990), 472 (2150)
444, 472
c
[Cr(L5-H+)2]5+
331
a
b
In acetonitrile. These spectra exhibit substantial overlap between (π → π*) and ILCT bands. Reported ε values should be understood to arise
c
from a combination of both bands. In 1 M HCl.
E
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Inorganic Chemistry
Article
intense band that is not present in [Cr(tpy)2]3+. The location of
this band is strongly dependent on the electron-donating nature
of the groups on the aryl substituent and exhibits a red shift as
the electron-donating ability of these groups increases. It is
found at 347 nm in 1 and 532 nm in 5, which is consistent with
an assignment of an ILCT transition from the aryl ring to a
terpyridyl-localized π*-orbital. This band is assigned as a
4[1ILCT] transition that is singlet with respect to the
terpyridine ligand and quartet with respect to the complex.
H+)2]5+, which contains the strongly electron-withdrawing
−NMe2H+ in place of the strongly electron-donating −NMe2.
4 1
The [ ILCT] band for [Cr(L5-H+)2]5+ in 1 M HCl shows a
large blue shift from 532 to 331 nm compared with 5 in water
4 1
These [ ILCT] bands are intense with ε = 40 000−56 000
4 1
4 1
M−1 cm−1, though the [ ILCT] and [ (π → π*)] bands
overlap substantially in 1−4 and the contribution to ε from the
4[1ILCT] band is lower than these values.
Complexes 1−5 all show enhanced visible absorption
compared to Cr(tpy)23+. In the case of 4 and 5 the visible
absorption is greatly enhanced. 4 absorbs visible light with ε =
11 900 M−1 cm−1 at 450 nm and ε = 5090 M−1 cm−1 at 500 nm,
compared to ε = 1960 M−1 cm−1 at 450 nm and ε ∼ 1000 M−1
cm−1 at 500 nm for the Cr(III) tris-bipyridyl complex that has
the strongest previously reported visible absorption profile.6 5 is
a remarkably strong absorber of visible light with ε > 7500 M−1
cm−1 for the entire visible region. However, unlike 4, there is no
evidence that the excitation energy in 5 is funneled to the
Figure 4. Electronic absorption spectra for 5 in water (gray) and 1 M
HCl (black).
(Figure 4). This band is higher in energy than any of the
4[1ILCT] bands in 1−4, consistent with the strongly electron-
withdrawing ability of −NMe2H+. This significant blue shift
upon protonation is consistent with the aryl-ring-to-terpyridyl-
π*-system character of the ILCT transition, with substantial
electron delocalization over the aryl π-system and the
dimethylamino substituent. [Cr(L5-H+)2]5+ also displays a
2
Cr(III) doublet state, Eg (Oh), vide infra. 1−5 represent a
series of Cr(III) arylterpyridyl complexes with intense ILCT
absorption bands that are tunable across the near UV to visible
region via judicious selection of the substituents on the aryl
ring.
The well-resolved absorption features in 5 provide an
opportunity to further investigate the assignment of the lowest
energy UV and visible bands (Figure 2). 5 displays an
absorption band in the UV at 360 nm (ε = 25 800 M−1
4 3
4[1(π → π*)] band at 367 nm and a structured [ (π → π*)]
3+
band in the blue region, similar to those observed in Cr(tpy)2
.
Upon addition of NaOH to [Cr(L5-H+)2]5+ in 1 M HCl, the
4[1ILCT] band at 331 nm decreases in intensity and the
4[1ILCT] band from 5 at 530 nm grows with increasing pH.
Basic pH results in the decomposition of 5, as reported
previously for Cr(III) terpyridyls (Figure S3).14 The significant
blue-shift of the 4[1ILCT] band for [Cr(L5-H+)2]5+
containing strongly electron-withdrawing groupsin 1 M
HCl compared to 5containing strongly electron-donating
4 1
cm−1) that is similar in location and intensity to the [ (π →
π*)] band in Cr(tpy)23+, though any vibrational structure on
the blue edge of the band is obscured by an intense feature that
we cannot definitively assign.73 Two broad bands at 532 nm (ε
= 56 500 M−1 cm−1) and 689 nm (ε = 13 700 M−1 cm−1)
dominate the visible region. The higher energy band is assigned
4 1
to a [ ILCT] transition. Possible assignments for the lowest
energy band include vibrational progression within the
4 1
groupsin water further confirms the [ ILCT] assignment.
4[1ILCT] transition and a triplet ILCT transition that is
The electronic absorption spectrum of 6 is striking in its
4 3
3+
coupled to a spin flip in the Cr(III) quartet state, [ ILCT],
similarity to that of Cr(tpy)2 (Figure 2). The absorption
analogous to the 4[3(π → π*)] band in Cr(tpy)23+. The
relatively large energy difference between these bands (4300
cm−1) suggests a 4[3ILCT] assignment over vibrational
progression. An analogous band is observed at 499 nm for 4,
and a similar feature may in fact be present in all the
arylterpyridyl chromium complexes studied, but the congested
nature of the absorption spectra of 1−3 prevents definitive
identification of this feature.
3+
features for Cr(tpy)2 and 6 at wavelengths longer than 300
nm occur with λmax within 2 nm of each other, and ε values are
10−20% higher for 6. 6 lacks a defined ILCT transition in this
region, suggesting that substantial mixing between the aryl and
the terpyridyl π-systems is necessary for low energy ILCT
transitions. However, the 4[1(π → π*)] transition is left
unaffected by this mixing. This provides additional insight into
strategies to design Cr(III) polypyridyl complexes with desired
absorption properties.
4 1
The [ ILCT] assignment for the feature at 532 nm in
acetonitrile in 5 can be further confirmed by investigating it
Emission Spectroscopy. The emission spectra of Cr(III)
4 1
both in other solvents and upon protonation. The [ ILCT]
tris-bipyridyl complexes are well-known and consist of a sharp
emission feature assigned to a 2Eg → 4A2g (Oh) transition and a
band in 5 shows a weak, negative solvatochromism, though
solvents with protic hydrogens are notably out of order,
presumably due to the effect of hydrogen bonding with the
amino group (Figure S2). Weak solvatochromism is consistent
with previous studies on ILCT bands in homoleptic
arylterpyridyl Ir(III) complexes.62 Due to the limited solubility
of 5 in the solvents used, it is difficult to make definitive
statements about the solvatochromism of the band near 690
nm. Protonation of the amino groups in 5 forms [Cr(L5-
2
broader, less intense feature to the blue assigned to a T1g
→
4A2g transition (Oh); these features are commonly referred to in
octahedral symmetry (Oh or O), though D3 is a more
appropriate descriptor of the symmetry of these
compounds.6,50 Many fewer examples of Cr(III) bis-terpyridyl
complexes exist. The reduced symmetry of Cr(III) bis-
terpyridyl complexes may be expected to significantly alter
the excited state manifold with respect to Cr(III) tris-bipyridyl
F
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Article
complexes. In D2d symmetry the 2Eg (Oh) state splits into a pair
state with emission from the other component obscured under
the Eg (Oh) emission.
2
2
2
of singly degenerate states, A1 (D2d) and B1 2(D2d), while the
2
2T1g (Oh) state splits into A2 (D2d) and E (D2d) states.
In addition to these sharp features, the emission spectrum of
4 exhibits a broad band centered at ∼725 nm. No other
complex studied exhibits a similar feature in this wavelength
range. The excitation spectrum for emission from 4 at 670 nm
reveals peaks at the same wavelengths as those found in the
Fluorescence line-narrowing spectroscopy has previously been
used to find a value of ∼23 cm−1 for the splitting between the
74
3+
2
2A1 (D2d) and B1 (D2d) excited states for Cr(tpy)2
.
The
2
3+
splitting of the T1g (Oh) state for Cr(tpy)2 in D2d symmetry
is expected to be greater than the ∼23 cm−1 splitting of the 2Eg
(Oh) state. In a previous study on trans-CrN4X2 complexes,
where N is an amine or pyridine, the 2T1g (Oh) state splits into
two states. The magnitude of this splitting is large enough when
X = F− that one of the doublet states of 2T1g (Oh) parentage is
in fact lower in energy than states of 2Eg (Oh) parentage.75 In all
4 1
absorption spectrum. However, the intensity of the [ (π →
π*)] band in the excitation spectrum is enhanced with respect
to those from ILCT transitions (Figure S4). The broad
emission profile of this band suggests that it arises from a
ligand-localized transition. Additionally, this feature exhibits an
emission lifetime of ≤250 ps, vide infra, which suggests
fluorescence versus phosphorescence. One possible assignment
for this emission is fluorescence from the 4[1ILCT] state. If this
assignment is correct, it represents a Stokes shift of ∼10 000
cm−1. The observed Stokes shift for L5 fluorescence has been
reported as 8300 cm−1 in CH2Cl2; Stokes shifts as large as
11 000 cm−1 have been previously reported for CT fluorescence
in unligated 4′-arylterpyridines.58,60
2
of the complexes studied here, a Eg (Oh) band is the lowest
energy feature observed in the emission spectra and there is at
2
most one discernible T1g (Oh) band, vide infra. Due to the
similarity to Cr(III) tris-bipyridyl complexes, state labels from
Oh symmetry are appropriate to approximate the doublet
3+
excited states for Cr(tpy)2 and 1−6.
Emission spectra were obtained upon excitation of
2
4 1
Two additional aspects of the Eg (Oh) phosphorescence in
acetonitrile solutions of 1−5 at λmax of the [ ILCT] band
and of 6 at λmax of the 4[1(π → π*)] band (see Table 1 for λmax
1−4 and 6 bear noting. First, the sole previous investigation of
3+
4 1
4 1
the room temperature steady-state emission of Cr(tpy)2
of [ ILCT] and [ (π → π*)] bands). Complexes 1−4 and 6
each display a sharp emission feature with λmax between 776
and 796 nm; 1−4 also display a weak shoulder on the blue edge
of this band at ∼750 nm (Figure 5).76 Complex 5 is
2
reports T1g (Oh) emission at 710 nm (in 1 M HCl) and a
2Eg−2T1g (Oh) energy gap of ∼1100 cm−1 compared to ∼600
cm−1 for Cr(III) bipyridyls.50 We do not observe 2T1g emission
from acetonitrile solutions of either Cr(tpy)23+ or 6 (Figure 5).
2
Curiously, we do observe T1g (Oh) emission for the Cr(III)
arylterpyridyl complexes 1−4, with an energy difference
2
2
between the Eg (Oh) and T1g (Oh) features of ∼600 cm−1.
3+
We do not currently have an explanation as to why Cr(tpy)2
and 6 do not display T1g (Oh) emission. Second, the
2
phosphorescence of 1−4 and 6 is red-shifted compared to
3+
Cr(tpy)2 and the magnitude of this red shift depends on the
substitution on the aryl ring. These red shifts of up to 26 nm
are large compared to the red shifts observed for aryl-
substitution on Cr(III) tris-bipyridyls. This is mildly surprising
considering the t2g3 electron configuration of both the 2Eg (Oh)
4
excited state and the A2g (Oh) ground state. One possible
2
explanation has been offered previously for the red-shifted Eg
emission in a Cr(III) complex with a phenyl-substituted
phenanthroline ligand. In that case, the authors suggested
that spin-pairing effects between electrons in individual metal
orbitals become less significant as the electron delocalization is
increased upon extension of the aromatic system.6 The
3+
emission in Cr(tpy)2 is already red-shifted 42 nm from that
of Cr(bpy)23+, which would imply greater t2g electron
50
3+
3+
delocalization in Cr(tpy)2 compared to Cr(bpy)3
.
Aryl
substitution in 1−4 leads to a significant red shift from
Cr(tpy)23+ emission. This red shift increases with an increase in
the electron-donating ability of the aryl substituents (Table 2).
It is curious that 1 and 2 emit at the same wavelength, though 2
contains electron-donating Br substituents. The origin of this
red shift upon increased conjugation is in need of further study.
In comparison, the emission from 6containing electronically
isolated mesityl groupsis red-shifted by only 6 nm from
Cr(tpy)23+. Therefore, the Cr(III) terpyridyl platform may be a
useful one for the tuning of primarily Cr(III)-based excited
states via ligand modification utilizing both steric and electronic
effects.
3+
Figure 5. (top) Relative emission spectra for Cr(tpy)2 (black), 1
(red), 2 (green), 3 (blue), and 6 (brown) in acetonitrile under N2 and
(bottom) emission spectrum for 4 (purple) in acetonitrile under N2.
λex for 1−4 is the absorbance maximum for the ILCT band (Table 1);
λex for 6 is 363 nm.
nonemissive. The shapes of emission bands and shoulders
found in 1−4 and 6 are similar to those found for Cr(III) tris-
bipyridyl complexes. The sharp, prominent features in 1−4 and
6 are assigned to the Eg → A2g (Oh) Cr(III) ligand-field
transitions, and the broader shoulders near 750 nm in 1−4 are
assigned to the 2T1g → 4A2g (Oh) ligand-field transitions. In the
case of 1−4, an alternative assignment for the shoulder near
750 nm is emission from one component of the split 2T1g (Oh)
2
4
The energy of the 2Eg (Oh) excited state, E00, is approximated
3
by λem. This approximation is valid due to the shared t2g
electronic configuration between the 2Eg (Oh) excited state and
G
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Table 2. Electrochemical and Emission Data for Cr(III) Complexes
a,b
emission data
electrochemical data
c
d
e
f
E1/2 *
/2+/V
3+
complex
λem/nm
E00/eV
rel em
τ/ns
E1/23+/2+/V
E1/22+/1+/V
E1/21+/0/V
3+
Cr(tpy)2
770
785
785
788
796
1.61
1.58
1.58
1.57
1.56
1.0
2.6
1.9
2.8
4.3
140
280
300
400
−0.53
−0.55
−0.54
−0.60
−0.61
−0.69
−0.59
−0.94
−0.92
−0.89
−0.95
−0.98
−1.03
−0.98
−1.44
−1.42
−1.42
−1.44
−1.45
−1.46
1.08
1.03
1.04
0.97
0.95
1
2
3
4
5
6
g
g
h
h
600
776
1.60
0.8
140
−1.48
1.01
a
b
c
Potentials reported vs Fc/Fc+. Measurements conducted at room temperature with 0.1 M [NBu4][PF6] in acetonitrile. E00 is estimated from the
d
emission energy. λex is ILCT absorption maximum for 1−4; λex is [1(π → π*)] absorption maximum for 6; relative emission intensity vs
4
e
f
g
Cr(tpy)2(PF6)3 in acetonitrile with matching λex. 5%. Calculated from eq 1. Limited solubility in the electrolyte solution for electrochemical
h
experiments resulted in larger than expected ΔEp values for the third reduction wave. This value represents the long-lived component. An additional
component with a τ < 250 ps is observed for these complexes.
4
2
the A2g (Oh) ground state, and the sharp profile of Eg (Oh)
emission.24,50 Previous calculations of E00, using the energy and
bandwidth of emission from Cr(III) bipyridyl complexes, yield
a E00 that differs at most by 0.01 eV from the value estimated
using the emission energy alone.6 For the complexes in this
study, E00 is found to be between 1.56 and 1.60 eV compared to
The emission lifetimes of complexes 1−4 are all longer than
the emission lifetime for Cr(tpy)23+, and they increase with an
increase in the electron-donating ability of aryl substituents,
ranging from 280 to 600 ns (Table 2). Curiously, this increase
in emission lifetime is accompanied by a decrease in emission
energy, which is contrary to what is expected from the energy
gap law. By contrast, the emission lifetime of 6 was found to be
the same as that of Cr(tpy)23+. One possible explanation is that
increased electron delocalization onto the arylterpyridyl ligands
in the 2Eg (Oh) excited state leads to elongation of the emission
lifetime. Particularly noteworthy is the 600 ns emission lifetime
of 4. While this lifetime is shorter than the ∼1−700 μs emission
lifetimes typical of Cr(III) bipyridyls, it is on the order of
lifetimes exhibited by Ru and Ir chromophores.4,6,24,34,35 As
such, 4 has an excited-state lifetime that is sufficiently long for
use as a photoredox sensitizer.
3+
1.61 eV in Cr(tpy)2 (Table 2).
3+
Previous reports reveal that Cr(tpy)2 is weakly emissive in
room temperature solution (ϕ ∼ 10−5 in 1 M HCl).77 We
report the relative phosphorescence intensity of complexes 1−4
3+
and 6 compared to Cr(tpy)2 (Table 2). To isolate the
intensity of the phosphorescence from emission of the broad
feature centered at ∼725 nm in 4, vide supra, we fit the blue
edge of the broad emission to a Gaussian function and
subtracted this function from the overall emission band (Figure
S1). The emission from 6 is slightly weaker than that from
Cr(tpy)23+. 1−4 are stronger emitters than Cr(tpy)23+. This
further indicates migration of the excitation energy from the
Electrochemistry. The cyclic voltammograms of 1−6 each
exhibit at least three reversible reduction waves within the
solvent window (Figures S6 and S7). These waves are assigned
to successive reductions into terpyridine-localized π*-orbitals,
4 1
arylterpyridine-localized [ ILCT] state to the emissive, Cr-
(III)-based 2E state. The excitation spectrum of 4 in acetonitrile
overlays the absorption spectrum at λ > 400 nm (Figure S4),
confirming that excitation into both ILCT features leads to
population of the 2Eg (Oh) excited state. Complexes 1−4
demonstrate that judicious modification of ligands in Cr(III)
complexes is capable of introducing strongly absorbing ILCT
states that funnel excitation energy into the Cr(III) localized
2Eg (Oh) state.
11
3+
as has been found previously for Cr(tpy)2
.
The cyclic
voltammogram of 5 exhibits an additional reversible oxidation
wave at +0.69 V vs Fc/Fc+ that is presumably due to oxidation
of the dimethylamino group, analogous to a similar feature
found previously for the diphenylamino analogue.15 No
oxidative features were observed for 1−4 or 6. The first
reduction shifts to negative potentials as the substituents on the
aryl ring become more electron-donating (Table 2). This shift
is less pronounced for the second reduction and is nearly
absent for the third reduction. The presence of o-methyls on
the mesityl ring in 6 has no notable effect on the first reduction
potential in comparison to 3, which contains a toluyl ring. This
suggests that the shift to negative potentials for the first
reduction is due more to inductive effects than to mixing
between the terpyridyl and aryl π systems. These results are
consistent with previous studies on Pt(4′-Ar-tpy)Cl+ and Ir(4′-
3+
Time-resolved emission measurements for Cr(tpy)2 and
1−3 (monitored at λem from Table 2) exhibit single exponential
decays with lifetimes in the hundreds of nanoseconds, while 4
and 6 (monitored at λem from Table 2) each exhibit a fast
component (at least as fast as the instrument response
function; τ ≤ 250 ps) in addition to a long-lived component
with a lifetime in the hundreds of nanoseconds (Figure S5).
The emission of 4 monitored at 675 nmwhere only the
broad feature emits, vide supraalso decays with the
instrument response function yielding an emission lifetime of
≤250 ps; the two-component decay of 4 upon monitoring at
796 nm is likely due to overlapping emission from both the
phosphorescence bands and the tail of a broad feature to the
blue of the phosphorescence. The sub-nanosecond lifetime for
3+
Ar-tpy)2 complexes, which show that aryl substitution has a
small effect on terpyridine-localized reduction potentials.61,78
3+
The excited-state reduction potential, E1/2
*
/2+, for Cr(III)
complexes can be calculated according to eq 2, using the first
reduction potential for the ground state, E1/23+/2+, and the
energy of the excited state, E00:79
2
3+*/2+
3+/2+
the fast component in 4 is shorter than expected for Eg (Oh)
phosphorescence and likely originates from a quenched
fluorescence pathway. The longer-lived component is con-
sistent with Eg (Oh) phosphorescence.
E1/2
= E00 + E1/2
and E00 both decrease as the substituents on the aryl
(2)
3+/2+
E1/2
2
ring increase in electron-donating ability. These two effects
H
DOI: 10.1021/acs.inorgchem.7b00953
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3+
Present Address
combine to lower E1/2
*
/2+; the effects are relatively small in
3+
the series studied, contributing to a decrease in E1/2
by 0.13 V compared to Cr(tpy)2
*
/2+ for 4
§E.d.V.: Department of Geological Sciences, The University of
North Carolina at Chapel Hill, 104 South Road, Chapel Hill,
NC 27599-3315, United States. S.E.S.: Department of
Chemistry, Southeast Missouri State University, Cape
Girardeau, MO 63701, United States.
3+
.
CONCLUSIONS
■
We have synthesized and characterized a series of Cr(III)
arylterpyridines with intensely absorbing ILCT transitions. The
location of the ILCT bands in the absorption spectra of these
complexes is tunable via judicious selection of the substituents
on the aryl ring. The ILCT bands are found in the visible region
when strongly electron-donating substituents are used; this is
most pronounced in 5, which absorbs strongly across the entire
visible region. The absorption properties of this series of
chromophores represent a marked improvement over pre-
viously reported Cr(III) polypyridines and validate the strategy
of incorporating visible-absorbing, ligand-localized excited
states to improve the absorption characteristics of Cr(III)
complexes.
Author Contributions
‡J.C.B. and A.J.I.K. contributed equally.
Funding
B.M.L. gratefully acknowledges Wheaton College and the
Wheaton College Alumni Association for funding. S.E.S.
gratefully acknowledges Benedictine University College of
Science for funding.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Most of the Cr(III) arylterpyridines studied exhibit Cr(III)-
localized Eg (Oh) phosphorescence upon excitation into the
■
2
We would like to thank Michael D. Hopkins and Peter K.
Walhout for helpful discussions regarding this manuscript.
terpyridine-localized ILCT bands. This demonstrates the ability
of Cr(III) bis-arylterpyridyl complexes to funnel excitation
energy from ligand-localized excited states into Cr(III)-based
excited states. These complexes emit with larger emission
quantum yields and longer excited-state lifetimes than Cr-
(tpy)23+. This is most pronounced in 4, which absorbs strongly
in the blue with significant visible absorption that extends to
560 nm. Excitation of 4 leads to 2Eg (Oh) phosphorescence with
a lifetime of 600 ns and an excited-state reduction potential of
+0.95 V vs Fc/Fc+. 4 is an example of the promise of using
ligand-localized absorptions in the visible region to generate
photooxidizing excited states in Cr(III) polypyridyl complexes.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00953.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: benjamin.lovaasen@wheaton.edu.
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ORCID
Benjamin M. Lovaasen: 0000-0003-4617-6231
I
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