Synthesis and electronic investigation of mono- and di-substituted 4-nitro- and 4-amino-pyrazol-1-yl bis(pyrazol-1-yl)pyridine-type ligands and luminescent Eu(III) derivatives

Article abstract of DOI:10.1039/c7dt01153a

Four new disubstituted and monosubstituted nitro- and amino- bis(pyrazol-1-yl)pyridine (bppy) ligands, substituted at the pyrazole 4-position (1, 2, 5, 6) have been synthesized, along with two luminescent Eu(iii) tris-β-diketonate derivatives of the amino

Full text of DOI:10.1039/c7dt01153a

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Chemical Science
ARTICLE
Synthesis and Electronic Investigation of Mono- and Di-
substituted 4-Nitro- and 4-Amino-pyrazol-1-yl Bis(pyrazol-1-
yl)pyridine-type Ligands and Luminescent Eu(III) Derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
DOI: 10.1039/x0xx00000x
D. J. Strohecker, V. M. Lynch, B. J. Holliday† and R. A. Jones †
www.rsc.org/
Four new disubstituted and monosubstituted nitro- and amino- bis(pyrazol-1-yl)pyridine (bppy) ligands, substituted at the
pyrazole 4-position (1, 2, 5, 6) have been synthesized, along with two luminescent Eu(III) tris-β-diketonate derivatives of
the amino substituted ligands (7, 8). The compounds have been studied using UV-Vis absorbance spectroscopy and cyclic
voltammetry which has allowed for characterization of the electronic environments of these ligands. The calculated
HOMO-LUMO gap values (1: 3.54 eV; 2: 3.53 eV; 5: 3.01 eV; 6: 3.66 eV) differ from that of bppy (3.86 eV) and the range is
indicative that tuning of the ligand electronic environment is possible. Additionally, fluorescence spectroscopy studies
1 1
were employed to determine ligand T energy levels of the amine-bearing ligands 2 and 6, yielding values of T of 25,381
-1
-1
cm and 26,201 cm , respectively. These ligands were employed in the synthesis of Eu(III) complexes 7 and 8, for which
the absolute and intrinsic quantum yields, lifetimes and ligand sensitization efficiencies were determined.
Introduction
the lesser basicity of pyrazole in addition to a higher π* energy
of the aromatic system, resulting in lessened ligand binding
5
Lanthanide complexes continue to be of significant interest strength. This deviation in electronic behavior in a ligand that
for potential applications in magnetism, optoelectronics and as is otherwise structurally similar to terpy, along with the
1
–4
probes in biological systems.
The useful photophysical synthetic ease with which bppy may be derivatized on the 4-
properties of certain lanthanide complexes include large position of pyridine, as well as the 3- and 5-positions of the
pseudo-Stokes shifts, discreet emission wavelengths and long pyrazole rings, has led to the increasing popularity of bppy as a
lifetimes. Due to their ease of synthesis and favourable complementary platform for research into new d- and f-block
emissive properties lanthanide complexes of multidentate metal complexes. Halcrow published an especially thorough
ligands containing pyridyl or pyrazolyl groups, or both, are now review of such ligands and complexes as they pertain to
well established. Two of the most important ligands of this catalysis, dye-sensitized solar cells, spin-crossover materials,
class are, 2,6-bis(pyrazol-1-yl)pyridine (bppy) and 2,6-bis(2- and emissive materials for LEDs and biomarkers, with a special
pyridyl)pyridine (terpy) (Figure 1). First reported in the interest taken on spin-crossover and luminescent lanthanide
6
pioneering work by Jameson et al. in 1989, bppy has been complexes.
shown to serve as a versatile analogue to the well-known
5
As Zoppellaro and Baumgarten reported in 2005, bppy
tridentate nitrogen-donor terpy. With respect to terpy, bppy derivatives substituted on the 4-position of the pyrazole ring
is a weaker σ-donor as well as a weaker π-acceptor, owing to are relatively rare, due to unwieldy synthetic routes involving
7
non-selective lithiations or protection/deprotection steps.
There are consequently few examples of this type of
substitution, being limited to a handful of (4-halopyrazol-1-
yl)pyridines, a modest number of carbon-coupling reaction
products of these compounds, and some carbonyl-derived
ligands, either stemming from the formylation of said (4-
halopyrazol-1-yl)pyridines or nucleophilic substitution of 2,6-
7
–15
Figure 1: depiction of tridentate N-donor ligands terpy and
dibromopyridine with ethyl pyrazole-4-carboxylate.
bppy.
Numerous heteroleptic lanthanide complexes consisting of
mixed hard O- and N-chelating ligands have been reported
which exhibit promising photoluminescent properties and
stability; as such these motifs are desireable targets for
1
6–21
luminescent materials.
and electronic properties of the 4,4” symmetrically substituted
di-nitro and di-amino derivatives (NO bppy ( ), (NH bppy)
Herein, we report the synthesis
2
)
2
1
2 2
)
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(
2
), the 4-mono-substituted NO
compounds as well as the synthesis and spectroscopic from Sigma-Aldrich in Sure/Seal bottles and used without
characterization of two Eu(III) derivatives of the amino further drying. Eu(tta) •2H O was synthesized via a literature
substituted ligands, Eu(tta) (NH bppy and procedure and the crystalline product was characterized by
Eu(tta) NH bppy ( ) (tta = 2-thenoylacetonate) (Scheme 1).
2
bppy (
5
) and NH
2
bppy (
6
)
used without further purification. Dry diglyme was obtained
TM
3
2
3
2
)
2
(7)
2
2
3
2
8
LRMS and melting-point determination. Dry acetonitrile
MeCN) for electrochemical measurements was obtained using
(
Experimental
an Innovative Technology PureSolv 400 solvent purifier with a
double purifying column and stored in Schlenk storage flasks
over 3 Å molecular sieves under nitrogen atmosphere.
Materials and Methods
All reagents were purchased from commercial sources and
Single-crystals for X-ray diffraction studies on 3 (see ESI), 5
a)
b)
c)
Scheme 1: a) Synthesis of 1 and 2. i. NaH, N , 120 °C, 7 d; yield = 93% ii. 20% PtO , 2:3 EtOH: ethyl acetate, H 3 d; yield = 81% b) Synthesis
2
2
2
of 5 and 6. i. NaH, N
2
2 2
, 70 °C, 2 d; yield = 50% ii. NaH, N , 120 °C 4 d; yield = 89% iii. 10% PtO2, 1:1 EtOH: ethyl acetate, 25 – 45°C H 2 d;
yield = 70% c) Synthesis of 7 and 8. i. refluxing acetone, 1 d; 7: R = R’= NH
2
, yield = 76%; 8: R = H, R’ = NH
2
, yield = 46%.
2
| J. Name., 2012, 00, 1-3
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and were grown by slow evaporation of saturated d under an N
Ple Da sae l tdo onn To rt aa nd sj ua sct t mi o an r sg ins
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DOI: 10.1039/C7DT01153A
ARTICLE
6
2
atmosphere using standard Schlenk techniques.
dichloromethane solutions (see supplemental information After cooling to room temperature, the reaction was quenched
SI)). Diffraction data for were collected on a Rigaku AFC12 with H O (10 mL) and solvent was removed under vacuum to
(
5
2
diffractometer with a Saturn 724+ CCD using a graphite give a light brown solid. The solid was washed with deionized
monochromator with MoKα radiation (λ = 0.71073Å) at 100 K ice water (three x 50 mL portions), vacuum-filtered and then
with the use of a Rigaku XStream low temperature device. subjected to silica column chromatography using
Diffraction data for
f
3 and 6 data were collected on an Agilent dichloromethane eluent. The product (R = 0.45) was isolated
Technologies SuperNova Dual Source diffractometer using a μ- as a lustrous white needle-like crystalline solid (yield: 1.768g,
1
focus Cu Kα radiation source (λ = 1.5418Å) with collimating 93%). H-NMR (500 MHz, d-DMSO, 130 °C): 9.99 (s, 2H), 8.506
1
3) and 173 K (6) with the (s, 2H), 8.316 (t, 1H J = 1.5) 8.058 (d, 2H J = 6.0). C{ H}-NMR
3
1
mirror monochromators at 100 K (
use of an Oxford Cryostream low temperature device. X-ray (100 MHz, d-DMSO, 130 °C): 148.0, 142.8, 137.1, 137.0, 127.6,
+
powder diffraction data of
Technologies instrument, but powder diffraction data of
were collected on a Rigaku R-Axis Spider diffractometer with (m, N – O), 1,405 (s, N – O). M.P.: 344.16 – 346.10 °C.
3
was obtained on the same Agilent 111.6. HR-MS (CI ): calcd for C11
H
7
N
7
O
4
m/z 301.0560, found
-1
5
and 301.0557. IR ( ꢀꢁ , cm ): 3,162 (s, C – H), 3,148 (m, C – H), 1,615
6
an image plate detector utilizing a graphite monochrometer, Elemental Analysis: calcd C 43.86% H 2.34% N 32.55%, found C
CuKα radiation (λ = 1.5418 Ǻ) and Rigaku RINT RAPID control 43.73% H 2.28% N 32.32%.
software for system automation. Further information
regarding crystal structure data including powder diffraction yellow crystalline solid by-product in yields up to 31%, in the
data, refinement details, and tables of select bond lengths and previous reaction and isolated as the R = 0.8 band with silica
angles is provided in the SI. column chromatography using dichloromethane eluent.
UV-Vis absorption spectrophotometry and fluorescence NMR (400 MHz, CDCl ): 9.24 (s,1H), 8.25 (s, 1H), 7.98 (d, 1H J =
spectroscopy excitation/emission measurements were 8.0), 7.73 (t, 1H J = 7.9), 7.52 (d, 1H J = 7.8). C{ H}-NMR (100
performed in Starna quartz fluorometer cells with 1.0 cm path MHz, CDCl ): 149.8, 141.3, 140.6, 137.74, 137.65, 127.8, 126.4,
lengths. All room-temperature solution absorption, excitation 111.6. HR-MS (ESI ): calcd for C
and emission spectra were obtained in dichloromethane 270.96490, found 270.96520. IR ( ꢀꢁ , cm ): 2,921 (m, C – H),
DCM) solvent. Luminescence measurements were recorded 1,566 (m, N – O), 1,501 (m, N – O), 1,038 m, C – Br). M.P. 136.1
2-bromo-6-(4-pyrazol-1-yl)pyridine (3) was formed as a
f
1
H
3
1
3
1
3
+
+
8 6 4 2
H BrN O (M + H) m/z
-1
(
on a Photon Technology International QuantaMaster 4 – 137.5 °C. Elemental Analysis: calcd C 35.71% H 1.87% N
spectrofluorometer equipped with a calibrated 6-in. diameter 20.82%, found C 34.89% H 1.73% N 20.13%.
K Sphere-B integrating sphere used for absolute quantum yield
2,6-bis(4-aminopyrazol-1-yl)pyridine
Eu) measurements. All excitation spectra were obtained by (0.0811 g, 20% catalyst loading) was suspended in a 2:3
monitoring the maximum emission wavelength of the mixture of EtOH: ethyl acetate (EtOAc) (500 mL). H gas was
molecule; all emission spectra were obtained by irradiation of sparged through this mixture for 1 h. (0.497 g, 1.65 mmol)
the sample with the maximum wavelength observed in the was then added to this mixture in one portion and the stirred
corresponding excitation spectra. Absolute quantum yields suspension was sparged with H gas for an additional hour
were calculated by dividing the area under the emission peak then sealed under a H atmosphere. The sealed reaction
of the complex by the difference between the area under the mixture was stirred an additional 3 d at room temperature
excitation peak of the sample and that of either a solvent with 1 h H sparge cycles every 24 h. The reaction was then
blank or solid BaSO sample. vacuum-filtered through Celite and concentrated under
Electrochemical experiments were carried out in a dry box vacuum to yield a brown solid. The solid was subjected to silica
under a nitrogen atmosphere using a GPES system from Eco. column chromatography using 5% v: methanol:
Chemie B. V. in 0.1 M solutions of tetrabutylammonium dichloromethane eluent. The last band (R = 0.15) was isolated
(2):
2 2
PtO •H O
L
(Φ
2
1
2
2
2
4
a
v
f
-
1
hexafluorophosphate in dry MeCN at a scan rate of 0.100 V
∙
The electrochemical cell consisted of a 1.6 mm-diameter Pt
s . and concentrated to give a yellow solid (yield: 0.320 g, 81%).
1
H-NMR (400 MHz, d-DMSO): 7.93 (d, 2H J = 1.0), 7.92 (t, 1H J =
button working electrode, Pt wire counter electrode and 8.0), 7.54 (d, 2H J = 8.0), 7.38 (d, 2H J = 0.98), 4.41 (s, 4H).
1
3
1
C{ H}-NMR (100 MHz, d-DMSO): 150.1, 141.5, 135.1, 134.1,
Ag/AgNO
3
reference electrode. All potentials were externally
+
referenced to the ferrocene/ferrocenium (Fc/Fc ) redox 111.09, 106.54. HR-MS (CI ): calcd for C11
+
H
11
N
7
m/z 241.1076,
-1
couple.
found 241.1075. IR ( ꢀꢁ , cm ): 3,310 (b, N – H), 2,962 (m, C – H),
,603 (s, N – H), 1,577 (s, N – H), 1,259 (s, C – N amine). M.P.
168.19 – 213.16 °C (decomp.). Elemental Analysis: calcd C
1
Synthesis
2,6-bis(4-nitropyrazol-1-yl)pyridine (1): In a dry box, 4- 54.76% H 4.60% N 40.64%, found C 54.93% H 4.41% N 38.24%.
nitropyrazole (1.798 g, 15.90 mmol) was dissolved in dry N analyses were consistently slightly low on this compound. At
diglyme (15 mL). To this solution NaH (0.520 g, 21.67 mmol) present we do not have a simple explanation for this.
was added in small portions and stirred until H
ceased. 2,6-dibromopyridine (1.502 g, 6.34 mmol) was then prepared following a slightly-modified literature procedure.
added in one portion with stirring, and additional dry diglyme In a dry box, 1-H-pyrazole (0.657 g, 9.75 mmol) was dissolved
50 mL) was added to the mixture. The resulting solution was in dry diglyme (15 mL). To this solution NaH (60 % in oil, 0.458
removed from the dry box and heated in a 120 °C oil bath for 7 g, 11.41 mmol) was added in small portions and stirred until H
2
(g) evolution
2-bromo-6-(pyrazole-1-yl)pyridine (4): This compound was
1
7
(
2
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(
g) evolution ceased. 2,6-dibromopyridine (2.457 g, 10.37 7.85 (t, 1H J = 8.0), 7.77-7.68 (m, 3H), 7.43 (d, 1H J = 1.0), 6.47
1
3
1
mmol) was then added in one portion with stirring, and (dd, 1H J = 1.6, 0.7), 3.12 (s, 2H). C{ H}-NMR (100 MHz,
additional dry diglyme (20 mL) was added to the mixture. The CDCl ): 150.1, 142.4, 141.2, 135.4, 131.4, 127.1, 114.0, 110.2,
resulting solution was removed from the dry box and heated in 108.7, 108.6, 108.0. HR-MS (ESI ): calcd for C11
3
+
+
H
10
N
6
([M+H] )
atmosphere using standard m/z 227.10400, found 227.10390. IR ( ꢀꢁ , cm ): 3,347 (m, CN –
Schlenk techniques. After cooling to room temperature, the H), 3,203 (m, N – H), 2,922 (m, C – H), 2852 (m, C – H) 1,602 (s,
reaction was quenched with H O and solvent was removed to N – H), 1,578 (s, N – H). M.P. 150.2 – 158.4 °C. Elemental
give a light brown solid. The solid was extracted with DCM, Analysis: calcd C 58.40% H 4.46% N 37.15%, found C 58.19% H
washed with then subjected to silica column 4.34% N 36.13%
chromatography using DCM eluent. The product (R = 0.65) Eu(tta) X (complexes 7 – 8): Both complexes were synthesized
was isolated as a white crystalline solid (1.17 g, 50% yield). H- following the same procedure; the synthesis of complex 7 is
NMR (400 MHz, CDCl ): 8.31 (d, 1H J = 2.6), 7.93 (d, 1H J = 8.0), described as a general example: Ligand 2 (0.025 g, 0.104 mmol) was
.73 (d, 1H, J = 1.68), 7.65 (td, 1H J = 7.8, 1.2), 7.35 (d, 1H J = combined with Eu(tta) •2H O (0.088 g, 0.104 mmol) and dissolved
.72, 0.8), 6.46 (m, 1H). in acetone (20 mL). The solution was refluxed for 30 min, stirred at
-(4-nitropyrazol-1-yl)-6-(pyrazol-1-yl)pyridine (5): In a dry room temperature overnight and then concentrated under vacuum
-1
a 70 °C oil bath for 2 d under an N
2
2
2
H O
f
3
1
3
7
7
3
2
2
box, 4-nitropyrazole (0.505 g, 4.46 mmol) was dissolved in dry to give a pale brown solid. The solid was washed with hexanes,
diglyme (15 mL). To this solution NaH (60% wt. in oil, 0.156 g, dried under vacuum and collected. We have, so far, been unable to
3
.90 mmol) was added in small portions and stirred until H
evolution ceased. (0.500 g, 2.23 mmol) was then added in purity was assessed by elemental analysis and confirmed by the
one portion with stirring, and additional dry diglyme (5 mL) absence of splitting in the 5D → 7F Eu(III) ion emission spectra
2
(g) obtain single crystals suitable for X-ray diffraction studies; bulk
4
0
0
2
3
was added to the mixture. The resulting solution was removed (indicative of one emissive species in the sample).
from the dry box and heated in a 120 °C oil bath for 4 d under
2 2
Eu(tta)(NH ) bppy (7): (83.4 mg, 76% yield) HR-MS
an N
cooling to room temperature, the reaction was quenched with
O (10 mL) and solvent was removed under vacuum to give a
canary-yellow solid. This solid was dissolved in DCM, washed
with H O then subjected to silica column chromatography
using DCM eluent. The product (R = 0.5) was isolated as a
white crystalline solid (0.509 g, 89% yield). H-NMR (400 MHz,
CDCl ): 9.25 (d, 1H J = 0.72), 8.58 (dd, 1H J = 2.6, 0.7), 8.29 (d,
H J = 0.5), 8.07 – 7.99 (m, 2H), 7.94 – 7.87 (m, 1H), 7.82 – 7.77
2
atmosphere using standard Schlenk techniques. After
+
+
(
6 7 4 2
MALDI ): calcd for C27H19EuF N O S ([M-tta] ) m/z 836.00509,
-1
found 836.0045. IR ( ꢀꢁ , cm ): 3,105 (b, N – H), 2,962 (m, C – H).
M.P. 104.1°C (decomp.). Elemental Analysis: calcd C 39.78% H
H
2
2
.19% N 9.28%, found C 40.19% H 2.37% N 8.71%.
Eu(tta)NH bppy (8): (32.2 mg, 46% yield) HR-MS (ESI ):
calcd for C35
064.97290. IR ( ꢀꢁ , cm ): 3,131 (b, N – H), 2,926 (w, C – H).
M.P. 99.2 – 145.4 °C (decomp.). Elemental Analysis: calcd C
0.35% H 2.13% N 8.07%, found C 40.59% H 2.22% N 7.01%.
2
+
2
f
+
1
9 6 6 3
H22EuF N O S ([M+Na] ) m/z 1064.97210, found
-1
1
3
1
1
3
1
4
(m, 1H), 6.55 (dd, 1H J = 2.6, 1.7). C{ H}-NMR (100 MHz,
CDCl ): 151.6, 143.1, 142.7, 137.6, 127.3, 125.9, 112.0, 110.0,
3
+
08.8. HR-MS (ESI ): calcd for C11
+
([M+Na] ) m/z
Results and Discussion
1
H
8
-1
N
6
O
2
279.0610, found 279.03090. IR ( ꢀꢁ , cm ): 3,175 (s, C – H), 3,130
Ligand Synthesis
(
m, C – H), 2,922 (m, C – H), 1,605 (m, N – O), 1,397 (s, N – O).
M.P. 167.0 °C (decomp.). Elemental Analysis: calcd C 51.56% H
.15% N 32.80%, found C 51.88% H 3.12% N 32.68%.
-(4-aminopyrazol-1-yl)-6-(pyrazol-1-yl)pyridine
•H O (0.0074 g, 5% catalyst loading) was suspended in a
:1 mixture of EtOH: EtOAc (100 mL). H gas was sparged
through this mixture for 20 min. (0.154 g, 0.599 mmol) was
then added to this mixture in one portion and the stirring
suspension was sparged with H gas for an additional hour
then sealed under a H atmosphere. The sealed reaction
The synthesis of the mono- and disubstituted nitro- ligands
1
and
substitution of 2,6-dibromopyridine affording satisfactory
yields. The synthesis of the di-substituted species was
5 was achieved via the established nucleophilic
3
2
(6):
1
PtO
2
2
achieved by reacting excess of the pyrazolate of 4-
nitropyrazole and 2,6-dibromopyridine, followed by
1
2
5
hydrogenation to yield
2. Although it has been noted that this
procedure may be expected to result in the decomposition of
2
2
4
the 4-nitropyrazolate rather than the desired substitution,
we have found that by limiting the reaction temperature to
20 °C and increasing the reaction time to 7 days in diglyme
yields in high yield (93%). Initial attempts to form using
2
mixture was stirred an additional 1 d at room temperature and
reaction progress was monitored via TLC (5% v/v MeOH/DCM
on silica). After 1 d an additional 0.007 g (5% cat. loading)
1
1
1
stoichiometric amounts of 4-nitropyrazole and reaction
temperatures above 130 °C gave low yields and formation of
PtO
2
•H
2
O was added to the stirring reaction accompanied by a
. The reaction was allowed to proceed
30 min sparge with H
2
the expected mono-substitution product,
approximately 30%. Reaction of with an additional
equivalent of 4-nitropyrazole can be employed to further
improve the yield of . Optimization of reaction conditions
3, in yields of
for an additional day in a 45 °C oil bath. The reaction was then
vacuum-filtered through Celite and concentrated to yield a
yellow oil. This oil was subjected to silica column
chromatography using a 2% v: v methanol: dichloromethane
3
1
such as introducing one extra equivalent of the pyrazole and
NaH each, as well as extending reaction times to 7 days
eluent. The last band (R
to give a brown crystalline solid (0.095 g, 70% yield). H-NMR
400MHz, CDCl ): 8.53 (d, 1H J = 2.64), 8.08 (d, 1H J = 0.96),
f
= 0.10) was isolated and concentrated
1
eventually yielded
1 in excellent yield with minimal mono-
(
3
4
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substitution by-products. The subsequent reduction to form
2
ring bearing the primary amine, with a dihedral angle of
was problematic due to the poor solubility of
1. Limited 12.40 ° between the two heterocycles. The primary amine N‒H
success was achieved by reducing with SnCl in basic media as bond distances are 0.963 Ǻ and 0.880 Ǻ. The difference in
2
2
5
per the literature, however higher catalytic loading of bond lengths arise from two distinct H-bonding interactions
Adams’s catalyst and careful adjustment of the volumetric involving the separate H atoms, one involving an adjacent
ratio of EtOH: EtOAc (2:3 was found to be optimal) enabled the primary amine N6 (2.312 Ǻ) and the other involving the
desired reduction to
range of 80%.
2
in consistent satisfactory yields in the pyrazole imine N5 atom of an adjacent 4-aminopyrazole ring
(2.143 Ǻ) (see ESI).
Although we did not attempt synthesis of
of 2,6-difluoropyridine derivatives with the pyrazolate Ligand Spectroscopy
nucleophile as has been reported previously, this may be a The results of UV-Vis absorption spectroscopy are
Nucleophilic aromatic summarized in Table 1. The absorption spectra of all ligands
substitution methods involving fluorinated pyridine starting are characterized by two ꢂ→ꢂπ* transitions in the UV region in
materials may be advantageous owing to the increased the range of 260 nm – 317 nm, with the exception of (Figure
reactivity of fluoride leaving groups, which may lessen the 3.a). The bathochromic shift of the low-energy transition seen
reaction time and temperature required. Additionally, in (327 nm) suggests a lower energy ꢂ→ ꢃ transition
ꢂ→ꢂπ*
1 by substitution
26–31
desirable synthetic route to pursue.
π
2
2
n
*
employing a 4-bromo-2,6-difluoropyridine intermediate as combining with or completely covering the expected
π
reported by Charbonnière et al. may provide a path to transition.
improving the solubility of
1
via reaction of the Br- group,
Fluorescence spectroscopy of these molecules reveals
excitation profiles that closely resemble the respective
32,33
which would be very advantageous.
Synthesis of the asymmetrically-substituted derivatives absorbance spectra, with excitation λmax being closely
was also performed with similar nucleophilic substitutions associated with the λmax of absorption in all cases. At room
followed by nitro- group reduction, but careful consideration temperature,
was required when choosing the order of pyrazolate 404 nm and 414 nm, respectively (Figure 3.b);
nucleophilic substitution. Attempts to synthesize by treating seen to emit very faintly in the UV region. Observation of
with pyrazole and NaH resulted in low yields, with large by- and in a 2:2:1:1 volumetric ratio of ethyl iodide: diethyl
and 4-nitropyrazole as well as bppy, ether: ethanol: toluene (EEET) solvent glass upon cooling to 77
2
and
6
exhibit broad emission peaks centered at
1
and can be
5
5
2, 5
3
6
product formation of
4
indicating competition between substitution of the 4- K permitted the phosphorescence of these molecules to be
nitropyrazole and bromine with pyrazolate. Step-wise observed; in the case of and , emission from the T manifold
formation of via nucleophilic substitution of 2-6- appears as an additional shoulder in the emission spectrum of
dibromopyridine with pyrazole, followed by substitution with centered at 486 nm and a resolved phosphorescence
-nitropyrazole proved to be effective in synthesizing the manifold in (Figure 3.c). Identification of the blue edges of
2
6
1
4
2
4
6
mono-substituted 5. Nucleophilic aromatic substitution using phosphorescence allowed for the determination of the T
-1
and 26,201 cm for
1
-
1
4
-aminopyrazole to directly produce
2
is undesirable, as the energy of 25,381 cm for
2
6
. This
amine would necessitate a longer synthetic route entailing information is of key importance in the consideration of this
protection and deprotection steps; additionally the cost of this molecule as a sensitizing ligand for lanthanide luminescence.
reactant is quite high.
The blue edge of phosphorescence from 2 was determined by
fitting the 77 K emission spectrum to a multi-peak Gaussian
distribution model and extrapolating the resultant
Ligand Crystallography
In order to unequivocally establish the identities of
5
and
6
phosphorescence curve (see SI). In the case of 5 there appears
single crystal X-ray structures were obtained (Figure 2). In the to be significant overlap between fluorescence and
solid state structure of , a slight distortion from co-planarity phosphorescence processes, making determination of the T1
6
with regard to the pyridine ring is apparent for the pyrazole energy level difficult; the shape and range of the 77 K emission
b)
a)
Figure 2: a) single crystal structure of 5. b) single crystal structure of 6. Displacement ellipsoids are scaled to the 50% probability level.
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Table 1: UV-Vis absorbances of ligands and complexes in DCM solution
1
2
5
6
7
8
λ (nm);
--
--
259; 12,499
276; 13,114
312; 14,503
248; 18,073
--
266; 49,402
--
---
-
ε (M cm )
1
-1
2
89; 19,489
17; 18,397
279; 5,638
327; 14,571
276; 27,902
340; 51,415
3
314; 22,017
338; 84,565
spectrum of
5 changes drastically depending on the chosen Cyclic Voltammetry
excitation wavelength, and attempts to determine emission
Cyclic voltammetry (CV) was performed to determine the
lifetimes for phosphorescence peak identification were not electrochemical behavior as well as energies of the HOMO
possible with our current instrumentation. Excitation with the (EHOMO), LUMO (ELUMO), and HOMO-LUMO gap (ΔE) of these
lowest-energy excitation wavelength that was discrete from species by identifying the onset potential of each initial
the 77 K emission profile of
5 is shown in Figure 3.c, as this reductive and oxidative event of the ligands. The resultant
wavelength should maximize the phosphorescence character voltammograms are shown with that of bppy under identical
of the emission. This emission appears as a well-resolved conditions in Figure 4; redox event peak potentials and frontier
manifold, though we hesitate to assign a ligand T
1
energy using orbital energy values are reported in Table 2. Electrochemical
the emission edge. Unfortunately no phosphorescence could energy levels were calculated utilizing onsets of oxidation and
3
4
be observed from 1.
reduction according to equations 1 and 2.
b)
a)
c)
d)
Figure 3: a) Absorption spectra of ligands and complexes in DCM solution b) Room-temperature excitation and emission spectra of ligands
in DCM solution c) 77 K excitation and emission spectra of 2, 5, and 6 in solid EEET solutions d) DCM solution (solid lines) and solid-state
(dashed lines) excitation and emission spectra of 7 and 8.
6
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Figure 4: Cyclic voltammograms of bppy and substituted bppy derivatives in ACN solution with t-butylammonium hexafluorophosphate
electrolyte.
potentials being required for the molecules bearing NO2-
ꢄꢅꢆꢇꢆ ꢈꢂꢉꢂꢊꢄꢋꢌꢍꢎꢏ,ꢋꢐ.ꢑꢍ.ꢒꢓ/ꢒꢓ
ꢔ
ꢔ
ꢕ 5.1ꢖꢗꢘꢙꢚ
ꢕ 5.1ꢖꢗꢘꢙꢚ
(1) groups and lesser potentials required for the NH -bearing
2
molecules.
(2) peak at positive potentials, and a reversible nitro-group redox
couple upon sweeping to negative potentials. and undergo
To the best of our knowledge, the electrochemical multiple irreversible oxidative events, and can be seen to
behavior of the parent bppy ligand has not been reported until undergo two irreversible reductive events. Performing
now. Electrochemistry of bppy reveals two irreversible negative potential sweeps of from 0 V to -2.5 V yields no
1 and 5 each exhibit only one irreversible oxidative
ꢄꢛꢜꢇꢆ ꢈꢂꢉꢂꢊꢄꢋꢌꢍꢎꢏ,ꢝꢎꢞ.ꢑꢍ.ꢒꢓ/ꢒꢓ
2
6
2
2
oxidative events at relatively high potentials; this suggests that reductive current, indicating coupling between the two
the irreversible oxidative events seen in all of these ligands reductive events and the oxidative events which correspond to
which take place at approximately 1.1 V and greater are due to neither bppy oxidation nor reduction of the oxidized residue;
the oxidation of the aromatic system, which is corroborated by we therefore attribute these signals to redox processes
the presence of similar irreversible oxidative signals in each of undergone by the amine functionalities.
the rest of the series. The electron-rich or deficient nature of
the amino- and nitro-bppy derivatives are reflected in the of
Since the reductive events evident in the voltammograms
are coupled to these initial oxidative peaks and no
or bppy, the ELUMO of
2
potentials at which the aromatic system is oxidized, with larger reductive events are discernible in
6
Table 2: Cyclic voltammetry redox potentials and HOMO/LUMO energy levels of tridentate N-donors in solution
Ligand
E
red (V)
E
ox (V)
E
1/2 (V)
E
HOMO (eV)
–6.18
E
LUMO (eV)
–2.35
ΔE (eV)
3.83
bppy
---
1.53, 2.06
–1.52, 2.11
---
1
2
5
6
–1.64
–1.12, –0.74
1.64
–1.58
---
–7.04
–3.64
3.54
0.51, 0.86, 1.34, 1.71
–1.54, 1.70
–5.35
–1.82
3.53
–1.59
---
–6.68
–3.59
3.01
---
0.48, 1.16, 1.72
–5.46
–1.80
3.66
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Figure 5: HOMO-LUMO gaps of the ligands described in this work.
these three ligands cannot be determined electrochemically. Luminescent lifetimes (ꢟꢋꢠꢍ) and absolute quantum yields
ꢛ
Therefore, the ELUMO and ΔE of these molecules were
(ꢡ ) were also measured. Attempts to model ꢟꢋꢠꢍ of 7 and 8
ꢢꢣ
calculated by combining optical measurement data with EHOMO reveals two components of luminescent relaxation in both
values obtained from CV studies. The energy of the red edge of solution and solid phases; in solution the longer-lived
the UV-Vis absorption spectra, corresponding to the 0 – 0 S
0
component appears to have a much greater contribution to
ꢂ→ꢂ electronic transition, was assigned the ΔE of these the luminescence decay, whereas in the solid state, the
S
1
ligands. The assignment of frontier orbital energies for these shorter-lived decay component exhibits an increased
ligands allows for empirical LUMO energy determination and involvement in the decay process (see Table S1). The greatly-
comparison across the series (Figure 5). As expected, the increased contribution of the shorter lifetime in the solid state
2
electron-deficient NO -containing ligands exhibit lowered is consistent with the decreased quantum yield. By combining
E
HOMO values and raised ELUMO values, whereas the electron- the weighted contributions of these components into an
3
5–37
rich NH
2
- ligands exhibit the opposite.
Interestingly, the averaged lifetime measurement (ꢟꢋꢠꢍ,ꢤꢑꢥ) (see Table S1) and
disparity in ΔE is greatest with the mono-substituted bppy calculating the radiative lifetime (ꢟꢝꢤꢞ), it is possible to obtain
ligands whereas the di-substituted ligands exhibit nearly an approximation of sensitization efficiency (ꢦꢍꢎꢌꢍ) and
ꢢꢣ
identical ΔE values but at different energies. In addition to intrinsic quantum yield (
3
serving as versatile synthetic intermediates, characterization of reported in Table 3.
ꢡ
) (Equations 3 – 5); these data are
ꢢꢣ
9,40
the electronics of these ligands indicate that they offer a
means of tuning energy levels and binding strengths of bppy
complexes.
1/ꢟꢝꢤꢞ ꢈꢂꢧꢇꢨ,ꢩꢪꢫꢗꢬꢏꢋꢏ/ꢬꢇꢨ
ꢚ
(3)
(4)
(5)
ꢢꢣ
ꢡ
ꢈ ꢟꢋꢠꢍ/ꢟꢝꢤꢞ
ꢢꢣ
Eu(III) Complex Synthesis and Spectroscopy
ꢦꢍꢎꢌꢍ ꢈꢂꢂꢡꢛ ⁄ ꢡ
ꢢꢣ
The new compounds were also investigated as ligands for
sensitizing Eu(III) luminescence, owing to the highly-
ꢢꢣ ꢢꢣ
3
8
luminescent nature of the parent bppy complex. Presently, The absence of any observable ligand luminescence in all of
all attempts at forming lanthanide complexes with and the luminescence emission spectra is indicative of optimal
have failed, presumably due to the exceedingly electron- intra-complex energy matching and minimal energy loss to
deficient nature of the ligands. However, metalation of and radiative processes (Figure 3.d). One trend that holds true for
upon refluxing with Eu(tta) •2H in acetone yielded both complexes is the decrease of , and ꢦꢍꢎꢌꢍin solid-state
complexes Eu(tta) (NH bppy ( ) and Eu(tta) NH bppy ( ) (tta measurements with respect to solution measurements. This
2-thenoylacetonate) (Scheme 1.c). The absorption profiles of may be due to stacking interactions which affect the ligand T
1
5
2
6
3
2
O
ꢟ, ꢡ
3
2
)
2
7
3
2
8
=
1
these complexes (Figure 3.a) are primarily dominated by energy levels, and therefore sensitization of the Eu(III) center.
absorption of the tta moieties, and feature greatly enhanced Alternatively, the solid samples may allow for closer packing of
absorptivity values with respect to the ligands themselves. the primary amine N‒H oscillators and consequently greater
Solution and solid-state excitation and emission spectra of non-radiative quenching of the chromophore excited states.
these complexes are shown in Figure 3.d. The excitation We also attribute the difference in emission efficiencies of
7
-
spectra indicate that population of the tta chromophore and
8
to quenching involving the amine groups, as the 820 cm
difference in T levels of the differing bppy ligands is unlikely
absorption feature of the Eu(III) to affect Eu(III) sensitization to a large extent; this is especially
ion can be seen in the excitation spectra of the solid samples. likely since the excitation spectra of all of these complexes are
1
excited states is responsible for sensitization of Eu(III)
1
5
7
0 0
luminescence; the D ← F
8
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Table 3: Photophysical characterization of Eu(III) emission of 7 and 8
ꢭꢮꢯꢰ
ꢭꢮꢯꢰ,ꢱꢲꢳ
ꢷ
ꢵꢶ
Complex
7
Solvent
DCM
solid
λ
ex (nm)
ꢴ
(%)
ꢭꢸꢱꢹ (μs)
1,150
615
ꢴ (%)
ꢵꢶ
ꢺꢰꢻꢼꢰ (%)
ꢵꢶ
(
μs)
6 ± 9;
80 ± 10
(μs)
6
350
339
132
424
252
24.2 ± 0.8
1.1 ± 0.2
30 ± 2
29
82
5
3
60 ± 2;
386
21
52
35
2
12 ± 2
1
60 ± 20;
90 ± 10
DCM
solid
353
821
58
29
4
8
41 ± 3;
386
10.2 ± 0.1
726
3
61 ± 6
suggestive of Eu(III) luminescence occurring at wavelengths
corresponding with light absorption of the tta moieties.
Current research is underway to improve the luminescent
efficiency of the amino-bppy complexes by synthetic
modifications to remove the amine N‒H oscillators in an effort
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pyrazolyl- hydroxylamines,
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The synthesis of nitro- and amino-bis(pyrazol-1-yl)pyridines was achieved, allowing for tuning
of frontier orbital energies and Eu(III) complex spectroscopic investigations.