Emissive Azobenzenes Delivered on a Silver Coordination Polymer

Article abstract of DOI:10.1021/acs.inorgchem.8b02845

Azobenzene has become a ubiquitous component of functional molecules and polymeric materials because of the light-induced trans → cis isomerization of the diazene group. In contrast, there are very few applications utilizing azobenzene luminescence, since the excitation energy typically dissipates via nonradiative pathways. Inspired by our earlier studies with 2,2′-bis[N,N′-(2-pyridyl)methyl]diaminoazobenzene (AzoAMoP) and related compounds, we investigated a series of five aminoazobenzene derivatives and their corresponding silver complexes. Four of the aminoazobenzene ligands, which exhibit no emission under ambient conditions, form silver coordination polymers that are luminescent at room temperature. AzoAEpP (2,2′-bis[N,N′-(4-pyridyl)ethyl]diaminoazobenzene) assembles into a three-dimensional coordination polymer (AgAAEpP) that undergoes a reversible loss of emission upon the addition of metal-coordinating analytes such as pyridine. The switching behavior is consistent with the disassembly and reassembly of the coordination polymer driven by displacement of the aminoazobenzene ligands by coordinating analytes.

Full text of DOI:10.1021/acs.inorgchem.8b02845

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Emissive Azobenzenes Delivered on a Silver Coordination Polymer
Jingjing Yan,† Liam Wilbraham,‡,§ Prem N. Basa,†,⊥ Mischa Schu
̈
ttel,† John C. MacDonald,†
Ilaria Ciofini,‡ Francois-Xavier Coudert,‡ and Shawn C. Burdette*,†
̧
†Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts
01609-2280, United States
‡Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris, 75005 Paris, France
S
* Supporting Information
ABSTRACT: Azobenzene has become a ubiquitous component of functional
molecules and polymeric materials because of the light-induced trans → cis
isomerization of the diazene group. In contrast, there are very few applications
utilizing azobenzene luminescence, since the excitation energy typically dissipates via
nonradiative pathways. Inspired by our earlier studies with 2,2′-bis[N,N′-(2-
pyridyl)methyl]diaminoazobenzene (AzoAMoP) and related compounds, we
investigated a series of five aminoazobenzene derivatives and their corresponding
silver complexes. Four of the aminoazobenzene ligands, which exhibit no emission
under ambient conditions, form silver coordination polymers that are luminescent at
room temperature. AzoAEpP (2,2′-bis[N,N′-(4-pyridyl)ethyl]diaminoazobenzene)
assembles into a three-dimensional coordination polymer (AgAAEpP) that undergoes
a reversible loss of emission upon the addition of metal-coordinating analytes such as
pyridine. The switching behavior is consistent with the disassembly and reassembly of the coordination polymer driven by
displacement of the aminoazobenzene ligands by coordinating analytes.
pyridyl)ethyl]diaminoazobenzene), which utilizes a pyridine
ligand and an ethylene spacer. With AzoAMoP and AzoAEoP
in hand, we reasoned that using a 3-pyridyl (meta) or 4-pyridyl
(para) ligand in the existing aminoazobenzene scaffold would
allow retention of the hydrogen-bonded anilino-hydrogen-
diazene core, while orienting the pyridine nitrogen atoms on a
trajectory favorable for coordinating metal ions. Since the low-
temperature emission of AzoAMoP was attributed to hydrogen
bonding and from being embedded in a frozen solvent glass,
we hypothesized that integrating the aAB chromophore into
coordination polymers would restrict nonradiative decay
pathways sufficiently to produce emissive systems at ambient
temperature.
INTRODUCTION
■
Azobenzene (AB) undergoes trans → cis isomerization on
irradiation and cis → trans isomerization on heating or upon
exposure to a different wavelength of light. Since light
absorption depends on the conjugated π-orbital system, ring
substituents change the photoisomerization process and the
optical properties of the AB chromophore. The S2 ← S0
transition of aminoazobenzene (aAB) shifts to longer wave-
lengths, which typically leads to an overlap with the S1 ← S0
transition.1−3 As we previously reported, AzoAMoP (1, 2,2′-
bis[N,N′-(2-pyridyl)methyl]diaminoazobenzene) exhibits
overlapping S2 ← S0 and S1 ← S0 transitions with a λmax
value at 490 nm and 30-fold higher emission in comparison to
AB at 77 K. Furthermore, AzoAMoP undergoes minimal trans
→ cis isomerization due to intramolecular hydrogen bonding
between the anilino protons and the pyridyl and diazene
nitrogen atoms.4 Hydrogen bonding imposes an energetic
barrier that prevents the aryl rings from adopting the
prerequisite collinear conformation necessary for isomerization
via the concerted inversion mechanism.
To further develop the photochemistry of these unique aAB
derivatives, we replaced the pyridine ligand of the amino-
methylpyridine groups with a series of hydrogen bond
acceptors and changed the linker between the amine and
pyridine to an ethylene.5 Investigations with the small library
of compounds suggested that reproducing the structure-
induced photophysical properties observed with AzoAMoP
would be difficult with a standalone AB chromophore. This
study included the report of AzoAEoP (2; 2,2′-bis[N,N′-(2-
EXPERIMENTAL SECTION
■
General Procedures. All reagents were purchased and used
without further purification. The 2,2′-diaminoazobenzene (DAAB)
synthon was prepared as previously described.6 Toluene, dichloro-
methane (CH2Cl2), dichloroethane (DCE), and diethyl ether (Et2O)
were sparged with argon and dried by passing through alumina-based
drying columns. All chromatography and thin-layer chromatography
(TLC) were performed on silica (200−400 mesh). TLCs were
developed by using CH2Cl2 or solvent mixtures containing CH2Cl2,
ethyl acetate (EtOAc), hexanes, or methanol (CH3OH). H and 13C
1
NMR spectra were recorded with a 500 MHz Bruker Biospin NMR
instrument. Chemical shifts are reported in ppm relative to
tetramethylsilane (TMS). FT-IR spectra were recorded using a
Received: October 8, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02845
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Bruker Optics FT-IR spectrometer equipped with a Vertex70
attenuated total reflection (ATR) accessory by collecting 1024
scans over a scan range from 4000 to 400 cm−1 at 4 cm−1 resolution.
Thermogravimetric analysis (TGA) measurements were carried out
on a TA Instruments Hi-Res TGA 2950 Thermogravimetric Analyzer
from room temperature to 800 °C under a nitrogen atmosphere at a
heating rate of 10 °C/min. LC/MS was carried on a Agilent
Technologies single quadrupole 1200 series LC system. High-
resolution mass spectra were obtained at the University of Notre
Dame mass spectrometry facility using a microTOF instrument
operating in positive ionization mode. Melting-point information was
obtained using a Mel-Temp Hydrothermal instrument.
2,2′-Bis[N,N′-(2-pyridyl)methyl]diaminoazobenzene (1,
AzoAMoP). AzoAMoP was synthesized as previously reported4
with minor modifications. DAAB (0.640 g, 3.02 mmol), 2-
pyridinecarboxaldehyde (0.600 mL, 6.31 mmol), and 3 Å molecular
sieves (0.940 g) were combined in CH2Cl2 (30 mL) and stirred for 24
h at room temperature. Sodium triacetoxyborohydride (NaBH-
(OAc)3, 1.34 g, 6.32 mmol) was added, and the mixture was stirred
at room temperature for 24 h. The reaction mixture was diluted with
water (20 mL), and the product was extracted into CH2Cl2 (3 × 40
mL). The combined organic layers were dried over sodium sulfate
(Na2SO4), and the solvent was removed. Flash chromatography on
silica (24/1 EtOAc/MeOH) yielded an orange-red powder (387 mg,
32.5%). Analytical data matched previously reported values.4
2,2′-Bis[N,N′-(2-pyridyl)ethyl]diaminoazobenzene (2,
AzoAEoP). AzoAEoP was synthesized as previously reported5 with
minor modifications. DAAB (0.640 g, 3.02 mmol) and 2-vinylpyridine
(316 μL, 2.93 mmol) were combined in CH3OH (3 mL) and acetic
acid (AcOH, 10 mL). The reaction mixture was stirred for 3 h at 45
°C before a second portion of 2-vinylpyridine (632 μL, 5.86 mmol)
was added. After it was stirred for 24 h at 45 °C, the reaction mixture
was cooled to room temperature and diluted with 10 mL of water and
the pH was adjusted to ∼8 with ammonium hydroxide (NH4OH).
The product was extracted into EtOAc (3 × 50 mL), the combined
organic materials were dried over Na2SO4, and the solvent was
removed. Flash chromatography on silica (CH2Cl2/CH3OH, 25/1)
gave AzoAEoP (448 mg, 35.1%) as a dark red solid. Analytical data
matched previously reported values.5
extracted into CH2Cl2 (3 × 40 mL). The combined organic layers
were dried over Na2SO4, and the solvent was removed. Flash
chromatography on silica using CH2Cl2/CH3OH (10/1) yielded
AzoAMpP as a red-orange solid (474 mg, 39.8%). Slow evaporation of
chloroform (CHCl3) provided orange-red needles suitable for X-ray
analysis. TLC: Rf = 0.20 (silica, CH2Cl2/CH3OH, 24:1). Mp: 160−
1
161 °C. H NMR (500 MHz, CDCl3): δ 8.58 (d, J = 5.7 Hz, 4 H),
8.55 (s, 2 H), 7.63 (dd, J = 7.9, 1.6 Hz, 2 H), 7.33 (d, J = 6.0 Hz, 4
H), 7.19 (t, J = 7.7 Hz, 2 H), 6.79 (t, J = 7.6 Hz, 2 H), 6.61 (d, J = 8.5
Hz, 2 H), 4.54 (s, 4 H). 13C NMR (125 MHz, CDCl3): δ 150.2,
148.2, 143.0, 136.6, 131.8, 127.3, 122.0, 116.9, 112.1, 46.1. FT-IR
(neat, cm−1): 3358.0, 3111.4, 2892.3, 2130.1, 1734.2, 1652.8, 1615.2,
1528.5, 1492.4, 1415.2, 1370.5, 1312.1, 1300.9, 1244.1, 1289.1,
1206.4, 1148.1, 1124.5, 1045.8, 1016.7, 987.5, 941.5, 883.6, 804.2,
740.7, 666.3, 620.0. HRMS (+ESI): calculated for MH+ 395.1979 and
observed 395.1985.
2,2′-Bis[N,N′-(4-pyridyl)ethyl]diaminoazobenzene (5,
AzoAEpP). DAAB (0.640 g, 3.02 mmol) and 4-vinylpyridine (316
μL, 2.96 mmol) were combined in CH3OH (3 mL) and acetic acid
(AcOH, 10 mL). The reaction mixture was stirred for 3 h at 45 °C
before a second portion of 4-vinylpyridine (632 μL, 5.91 mmol) was
added. After it was stirred for 24 h at 45 °C, the reaction mixture was
cooled to room temperature and diluted with 10 mL of water and the
pH was adjusted to ∼8 with NH4OH. The product was extracted into
EtOAc (3 × 50 mL), the combined organic materials were dried over
Na2SO4, and the solvent was removed. Flash chromatography on silica
using CH2Cl2/CH3OH (15/1) yielded AzoAEpP as a red-orange
solid (500 mg, yield 39.1%). Slow evaporation from toluene/ethanol
(1/1) provided orange-red needles suitable for X-ray analysis. TLC:
Rf = 0.35 (silica, DCM/CH3OH, 10/1). Mp: 149−150 °C. 1H NMR
(500 MHz, CDCl3): δ 8.51 (d, J = 4.4 Hz, 4 H), 8.06 (s, 2 H), 7.32
(dd, J = 8.0, 1.6 Hz, 2 H), 7.24 (t, J = 7.8 Hz, 2 H), 7.17 (d, J = 4.7
Hz, 4 H), 6.79 (d, J = 8.4 Hz, 2 H), 6.74 (t, J = 7.5 Hz, 2 H), 3.57 (q,
J = 6.4 Hz, 4 H), 2.97 (t, J = 6.9 Hz, 4 H). 13C NMR (125 MHz,
CDCl3): δ 150.0, 148.1, 143.0, 136.4, 131.6, 127.2, 124.1, 116.3,
111.7, 42.9, 34.8. FT-IR (neat, cm−1): 3205.4, 3046.8, 2917.4, 2851.4,
2171.9, 2330.4, 1601.0, 1565.1, 1509.1, 1458.6, 1321.6, 1204.0,
1147.8, 1072.3, 1042.8, 836.7, 785.9, 368.8. HRMS (+ESI): calculated
for MH+ 423.2292 and observed 423.2270.
{[Ag(AzoAMoP)](CF3SO3)}n (AgAAMoP). AzoAMoP (25.4 mg,
64.4 μmol) in toluene (1.8 mL) was added dropwise to a toluene
solution (1.8 mL) of silver trifluoromethanesulfonate (AgOTf, 16.6
mg, 64.6 μmol). When the reaction mixture was stirred for 30 min, an
orange-red solid slowly precipitated. CH3CN (1 mL) was added to
redissolve the precipitate, and the reaction mixture was stirred at
room temperature for 2 h and filtered. Slow evaporation provided
orange rectangular plates suitable for X-ray analysis. FT-IR (neat,
cm−1): 3361.6, 3069.5, 2904.1, 2324.2, 1981.4, 1597.2, 1581.2,
1493.6, 1466.0, 1436.7, 1371.0, 1322.3, 1285.2, 1240.9, 1219.3,
1159.1, 1107.3, 1075.2, 1052.2, 1028.3, 991.5, 888.3, 823.5, 755.2,
699.3, 633.5. Anal. Calcd for C27H25AgF3N7O3S (AgAAMoP·
CH3CN): C, 46.79; H, 3.61; N, 14.15. Found: C, 46.50; H, 3.90;
N, 14.53. TGA shows a 0.7% weight loss between 60 and 120 °C,
which may correspond to absorbed solvent. Decomposition occurs at
175 °C.
{[Ag(AzoAMmP)]CF3SO3}n (AgAAMmP). AzoAMmP (25.4 mg,
64.4 μmol) in toluene (1.8 mL) was added dropwise to a toluene
solution (1.8 mL) of AgOTf (16.6 mg, 64.6 μmol). When the reaction
mixture was stirred for 30 min, an orange-red solid slowly
precipitated. CH3CN (1 mL) was added to redissolve the precipitate,
and the reaction mixture was stirred at room temperature for 2 h and
filtered. Slow evaporation of the filtrate at room temperature provided
crystals as orange-red blocks suitable for X-ray analysis. FT-IR (neat,
cm−1): 3230.2, 3076.0, 2941.6, 2889.8, 2362.8, 2314.7, 2165.8,
1982.2, 1862.7, 1739//.6, 1603.8, 1578.9, 1507.0, 1437.0, 1431.2,
1370.4, 1297.1, 1265.5, 1265.3, 1254.3, 1148.0, 1024.0, 1051.3, 941.1,
741.7, 612.6, 612.3. Anal. Calcd for AgAAMmP, C25H24.5AgF3N6O4.5S
(AgAAMmP·1.5H2O): C, 44.29; H, 3.64; N, 12.40. Found: C, 44.01;
H, 3.31; N, 12.02. The TGA shows no weight loss before
decomposition at 177 °C.
2,2′-Bis[N,N′-(3-pyridyl)methyl]diaminoazobenzene (3,
AzoAMmP). DAAB (0.640 g, 3.02 mmol), 3-pyridinecarboxaldehyde
(600 μL, 6.39 mmol), and 3 Å molecular sieves (0.940 g) were
combined in DCE (30 mL) and stirred for 24 h at room temperature.
NaBH(OAc)3 (1.34 g, 6.32 mmol) was added, and the reaction
mixture was stirred at room temperature for 24 h. The reaction
mixture was diluted with water (20 mL), and the product was
extracted into CH2Cl2 (3 × 40 mL). The combined organic layers
were dried over Na2SO4, and the solvent was removed. Flash
chromatography on silica using a gradient elution (EtOAc/hexanes 2/
1 → EtOAc/CH3OH) yielded AzoAMmP as a dark red solid (532
mg, 44.7%). Diffusion of Et2O into an CH3CN solution of AzoAMmP
provided orange-red blocks suitable for X-ray analysis. TLC: Rf = 0.30
1
(silica, EtOAc/CH3OH, 49/1). Mp: 158−159 °C. H NMR (500
MHz, CDCl3): δ 8.65 (s, 2 H), 8.53 (d, J = 4.9 Hz, 2 H), 8.43 (s, 2
H), 7.67 (d, J = 7.8 Hz, 2 H), 7.55 (dd, J = 8.0, 1.6 Hz, 2 H), 7.25 (t, J
= 6.2 Hz, 2 H), 7.19 (t, J = 7.8 Hz, 2 H), 6.76 (t, J = 7.6 Hz, 2 H),
6.70 (d, J = 8.4 Hz, 2 H), 4.50, (s, 4 H). 13C NMR (125 MHz,
CDCl3): δ 149.1, 149.0, 143.0, 136.6, 134.8, 134.3, 131.7, 127.3,
123.6, 116.8, 112.0, 44.7. FT-IR (neat, cm−1): 3302.0, 3062.1, 3030.0,
2991.1, 2880.2, 2617.1, 2056.7, 1498.0, 1476.3, 1465.2, 1419.9,
1309.2, 1247.5, 1199.2, 1153.9, 1122.6, 1025.9, 1041.0, 906.9, 787.0,
748.0, 706.7, 601.8. HRMS (+ESI): calculated for MH+ 395.1979 and
observed 395.1980.
2,2′-Bis[N,N′-(4-pyridyl)methyl]diaminoazobenzene) (4,
AzoAMpP). DAAB (0.640 g, 3.02 mmol), 4-pyridinecarboxaldehyde
(600 μL, 6.37 mmol), and 3 Å molecular sieves (0.940 g) were
combined in DCE (30 mL) and stirred for 24 h at room temperature.
NaBH(OAc)3 (1.34 g, 6.32 mmol) was added, and the reaction
mixture was stirred at room temperature for 24 h. The reaction
mixture was diluted with water (20 mL), and the product was
B
DOI: 10.1021/acs.inorgchem.8b02845
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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{[Ag(AzoAMpP)]NO3}n (AgAAMpP). AzoAMpP (10.0 mg, 25.4
μmol) was dissolved in 2.5 mL of a CH3OH/CH3CN (1/4) mixture
with a few drops of DMF. The solution was added dropwise to a
CH3CN solution (0.5 mL) containing silver nitrate (AgNO3, 4.5 mg,
27 μmol) and tetrabutylammonium hexafluorophosphate (n-Bu4PF6,
11.0 mg, 0.0284 mmol). The reaction mixture was stirred for 30 min
to precipitate an orange-red solid. The solid was isolated by filtration,
dissolved in 2 mL of a 1/1 CH3OH/CH3CN mixture, and stirred at
room temperature for 2 h. The mixture was filtered, and slow
evaporation provided orange-red blocks suitable for X-ray analysis.
FT-IR (neat, cm−1): 3516.5, 3484.4, 3262.2, 2862.0, 2826.4, 1942.1,
1639.4, 1613.5, 1610.4, 1454.6, 1454.6, 1445.0, 1322.0, 1207.8,
1206.6, 1155.8, 1100.9, 1069.1, 1028.0, 963.3, 889.5, 765.7. Anal.
Calcd for C24H22N7O3Ag (AgAAMpP): C, 51.03; H, 3.90; N, 17.37.
Found: C, 50.92; H, 4.01; N, 17.43. TGA shows a 3.6% weight loss
between 60 and 120 °C, which may correspond to absorbed solvent.
Decomposition occurs at 154 °C.
[Ag(AzoAEoP)]NO3 (AgAAEoP). AzoAEoP (30.0 mg, 71.1 μmol)
in DCM (1 mL) was added dropwise into an CH3CN solution (1
mL) of AgNO3 (12.0 mg, 70.6 μmol) and n-Bu4PF6 (28.0 mg, 72.3
μmol). The reaction mixture was stirred for 30 min to precipitate an
orange solid. The solid was isolated by filtration, dissolved in 2 mL of
a 1/1 CH3OH/CH3CN mixture, and stirred at room temperature for
2 h. The mixture was filtered, and slow evaporation provided orange-
red blocks suitable for X-ray analysis. FT-IR (neat, cm−1): 3247.2,
3049.8, 2911.3, 2863.4, 2324.7, 2164.5, 2051.2, 1981.6, 1903.6,
1604.1, 1566.1, 1497.9, 1482.5, 1439.3, 1423.7, 1375.0, 1322.0,
1284.7, 1249.4, 1221.3, 1177.2, 1156.4, 1129.0, 1105.9, 1084.8,
1064.8, 1044.1, 1025.4, 1004.8, 959.6, 938.5, 882.3, 846.5, 825.4,
800.5, 762.5, 752.0, 738.4, 647.8, 616.5. Anal. Calcd for C26H27N7O3.5
Ag (AgAAEoP·0.5H2O): C, 51.93; H, 4.53; N, 16.30. Found: C,
51.84; H, 4.43; N, 16.13. The TGA shows no weight loss before
decomposition at 166 °C.
{[Ag(AzoAEpP)2]PF6}n (AgAAEpP). AzoAEpP (30.0 mg, 71.1
μmol) in DCM (1 mL) was added dropwise into an CH3CN solution
(1 mL) containing AgNO3 (12.0 mg, 70.6 μmol) and n-Bu4PF6 (28.0
mg, 72.3 μmol). The reaction mixture was stirred for 30 min to
precipitate a yellow solid. The solid was isolated by filtration,
dissolved in 2 mL of a 1/1 CH3OH/CH3CN mixture, and stirred at
room temperature for 2 h. The mixture was filtered, and slow
evaporation provided orange-red blocks suitable for X-ray analysis.
FT-IR (neat, cm−1): 3426.5, 3068.1, 2861.8, 2360.3, 2324.7, 2050.9,
1981.4, 1604.0, 1564.4, 1501.5, 1464.6, 1430.7, 1316.2, 1240.8,
1223.5, 1211.6, 1183.9, 1154.6, 1123.0, 1104.1, 1079.5, 1066.5,
1028.6, 939.4, 880.9, 847.3, 823.3, 800.0, 760.1, 740.8, 615.2. Anal.
Calcd for C52H52N12F6PAg (AgAAEpP): C, 56.89; H, 4.77; N, 15.31.
Found: C, 56.72; H, 4.48; N, 15.02. TGA shows a 0.7% weight loss
between 60 and 120 °C, which may correspond to absorbed solvent.
Decomposition occurs at 197 °C.
X-ray Crystallography. Structural analysis was carried out in the
X-ray Crystallographic Facility at Worcester Polytechnic Institute.
Crystals were glued on the tip of a glass fiber or were covered in
PARATONE oil on 100 μm MiTeGen polyimide micromounts and
were mounted on a Bruker-AXS APEX CCD diffractometer equipped
with an LT-II low-temperature device. Diffraction data were collected
at room temperature or at 100(2) K using graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å) using the ω-scan technique.
Empirical absorption corrections were applied using the SADABS
program.7 The unit cells and space groups were determined using the
SAINT+ program.8 The structures were solved by direct methods and
refined by full-matrix least squares using the SHELXTL program.9
Refinement was based on F2 using all reflections. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms on carbon atoms
were all located in the difference maps and subsequently placed at
idealized positions and given isotropic U values 1.2 times that of the
carbon atom to which they were bonded. Hydrogen atoms bonded to
oxygen atoms were located and refined with isotropic thermal
parameters. Mercury 3.1 software was used to examine the molecular
structure. Relevant crystallographic information is summarized in
Tables 1 and 2, and the 50% thermal ellipsoid plots are shown in
Figures 1−5.
Table 1. Selected Interatomic Distances (Å) and Angles
(deg) for the Five AzoAXxP Ligands
bond length
bond angle
AzoAMoP
N(3)−N(3) 1.2793(15)
N(3)−C(12) 1.4127(16)
C(12)−N(2) 1.3573(18)
N(2)−C(6) 1.4434(18)
N(3)−N(3)−C(12) 116.87(10)
N(3)−C(12)−C(7) 126.89(11)
N(2)−C(7)−C(12) 121.64(11)
C(6)−N(2)−C(7) 123.93(11)
N(2)−C(6)−C(5) 110.70(10)
N(1)−C(5)−C(6) 118.15(12)
AzoAMmP
N2−N2 1.268(3)
C2−N2 1.414(3)
C3−N1 1.373(3)
C8−N1 1.444(3)
N2−N2−C2 114.9(2)
N2−C2−C3 115.07(17)
N1−C3−C2 119.75(18)
C3−N1−C8 122.48(17)
N1−C8−C4 113.39(17)
AzoAMpP
N1−N1 1.265(3)
C7−N1 1.415(2)
C12−N2 1.373(2)
C1−N2 1.449(2)
N1−N1−C7 115.08(18)
N1−C7−C12 115.58(15)
N2−C12−C7 119.91(15)
C12−N2−C1 122.46(15)
N2−C1−C2 115.93(16)
AzoAEoP
N1−N1 1.267(3)
C1−N1 1.408(2)
C6−N2 1.354(2)
C7−N2 1.446(2)
C9−N3 1.340(2)
N1−N1−C1 115.48(18)
N1−C1−C6 115.84(15)
N2−C6−C1 120.30(16)
C6−N2−C7 124.91(17)
N2−C7−C8 112.68(15)
N3−C9−C8 117.21(17)
AzoAEpP
N2−N2 1.283(4)
N2−C7 1.408(3)
N1−C6 1.358(3)
N1−C5 1.450(3)
N2−N2−C7 117.2(3)
N2−C7−C6 127.1(2)
N1−C6−C7 121.0(2)
C6−N1−C5 125.0(2)
N1−C5−C4 109.4(2)
Powder X-ray Diffraction. PXRD data were collected on a
Bruker-AXS D8-Advance diffractometer using Cu Kα radiation with
X-rays generated at 40 kV and 40 mA. Bulk samples of crystals were
placed in a 20 cm × 16 cm × 1 mm well in a glass sample holder and
scanned at room temperature from 3° to 50° (2θ) in 0.05° steps at a
scan rate of 2°/min. Simulated PXRD patterns from single-crystal
data were compared to experimental PXRD patterns of five AzoAXxP
silver complexes, to confirm the uniformity of the crystalline samples.
General Spectroscopic Methods. Solution UV−vis absorption
spectra were acquired in 1.0 cm quartz cuvettes at room temperature
and recorded on a Thermo Scientific Evolution 300 UV−vis
spectrometer with inbuilt Cary winUV software. Steady-state diffuse
reflectance UV−vis spectra were obtained on the same instrument
with a Harrick Praying Mantis diffuse reflectance accessory (Harrick
Scientific Products) and referenced to magnesium sulfate. Solution
emission spectra were recorded on a Hitachi F-4500 spectropho-
tometer with excitation and emission slit widths of 5 nm. The
excitation source was a 150 W Xe arc lamp (Ushio Inc.) operating at a
current rate of 5 A and equipped with a photomultiplier tube with a
power of 400 V.
Emission and Quantum Yield Determination. Steady-state
emission spectra were recorded on a Hitachi F-4500 spectropho-
tometer with excitation and emission slit widths of 5 nm. The
excitation source was a 150 W Xe arc lamp (Ushio Inc.) operating at a
current rate of 5 A and equipped with a photomultiplier tube with a
power of 400 V. Quantum yields in the solid state were determined in
C
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Table 2. Photophysical Properties of AzoAXxP Ligands and Coordination Polymers
compound
H-bond distance NH···NN (Å)
ε (M−1 cm−1
)
abs λmax (nm)
em λmax (nm)
rel em intensity (room temp)
a
AzoAMoP
AzoAMmP
AzoAMpP
AzoAEoP
AzoAEpP
AgAAMoP
AgAAMmP
AgAAMpP
AgAAEpP
2.046(15)
12334
13729
13702
15874
15195
n.a.
n.a.
n.a.
n.a.
495
497
495
501
502
440
440
475
508
602
607
603
619
616
600
618
590
n.a.
n.a.
n.a.
n.a.
n.a.
0.035
0.067
0.069
1.0
a
2.33(3)
2.34(2)
2.36(2)
2.00(3)
2.43(3), 2.47(5)
2.12(9), 1.96(2)
2.02(3)
a
a
a
b
b
b
b
2.00(3), 2.05(1), 2.01(3), 2.02(2)
641
a
b
λex 490 nm. λex 523 nm.
a
Scheme 1. Synthetic Protocols for Preparing Five AzoAXxP Ligands
a
Reagents and conditions: (a) NaBH(OAc)3,CH2Cl2, room temperature; (b) AcOH, MeOH, 45 °C.
triplicate following published procedures using Na2SO4 as the
reference.10,11
was resuspended in 2 mL of toluene and stirred for 30 min before
remeasuring the emission. The emission loss and restoration steps
were repeated in triplicate with single analyte additions to reach 100
μM pyridine. When excess pyridine was added (500 μM), the
emission completely disappeared and could not be restored.
Measurements for N-methylmorpholine (NMM) were acquired
using analogous procedures, where each addition provided a final
concentration of 100 μM NMM. Measurements with imidazole were
acquired similarly, but emission loss was irreversible. Measurements
with dimethylamine (DMA) were acquired similarly except toluene
was replaced with THF, and the DMA concentration after each
switching cycle was 10 μM. After each DMA addition, the emission
Analyte Detection by Emission. A 100 μM suspension, with
respect to AzoAEpP units, of AgAAEpP in toluene (2 mL) was
prepared, and the emission spectra were recorded (λex = 523 nm).
Upon the addition of each aliquot of analyte, the mixture was
equilibrated by stirring for 30 min before recording the emission
spectra. The emission response to all analytes was measured by
integrating the emission band between 550 and 800 nm. Pyridine was
added from a 2 mM stock solution in toluene in three equal portions,
to obtain final concentrations of 33, 67, and 100 μM, and the emission
was measured. After the third addition, the solvent and pyridine were
removed by sparging with N2 gas, and the resulting crystalline material
D
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Figure 1. (A) Preparation of AgAAMoP showing the thermal ellipsoid representations of AzoAMoP and AgAAMoP at the 50% probability level and
selected atom labels. Hydrogen atoms except for those engaged in intramolecular hydrogen bonds are omitted for clarity. The triflate anion and a
noncoordinating CH3CN solvent molecule are omitted for clarity. AgAAMoP was prepared in toluene using AgOTf as the silver source using a slow
ligand exchange process with CH3CN. During the polymer formation, the azo N−C and anilino−methylene N−C bonds rotate by 180 and 81°,
respectively. (B) Thermal ellipsoid representation at the 50% probability level of the expanded helical polymer chain. Hydrogen atoms, triflate
anions, and CH3CN solvent molecules are omitted for clarity.
was restored by the addition of aliquots of 1.58 mM nitric acid (12.7
pyridine nitrogen atoms of AzoAMoP are 8.37 Å apart;
however, the pyridine lone pairs engage in hydrogen bonding
with the anilino hydrogen atom, which orients the potential
coordination sites inward. In preliminary screenings, we
observed that AzoAMoP did not appear to coordinate metal
ions such as Zn2+, presumably due to the thermodynamic
stability of the hydrogen bonds. We reasoned that a
thermodynamically stable, kinetically inert metal−ligand
bond could provide access to a metal complex with AzoAMoP
by shifting the equilibrium from hydrogen bonding to metal−
ligand bonding toward complex formation. Ag+ appeared to be
a good candidate for coordination polymer formation on the
basis of other successful investigations12−16 as well as the
stability of Ag−pyridine bonds.17 We further speculated that
the photoactivity of Ag+ might provide access to additional
functionality in the resulting AB materials.
The other four AzoAXxP derivatives do not exhibit
hydrogen bonding with the pyridine lone pairs, so ostensibly
the ligands could bind metal ions more like typical bridging
bipyridine derivatives. Two-coordinate metals with bridging
bipyridine ligands are rare18−20 except for Ag+.12,14,21,22 The
two AzoAXxP derivatives containing methylene linkers,
AzoAMmP and AzoAMpP, can rotate at the azobenzene−
anilino C−N bond, anilino−methylene N−C bond, and
methylene−pyridine C−C bond. The two AzoAXxPs with
ethylene linkers, AzoAEoP and AzoAEpP, can also rotate about
the C−C ethylene bond. The multiple degrees of freedom
suggest that our ligands might go through noticeable
reorientation during the formation of Ag+ complexes or
polymers.
μL, 20.1 nmol) to achieve a final H+ concentration of 20 μM.
RESULTS AND DISCUSSION
■
Synthesis and Structure. Using the synthetic protocols
for preparing AzoAMoP and AzoAEoP, we prepared the three
additional aAB-bipyridyl ligands AzoAMmP (3, 2,2′-bis[N,N′-
(3-pyridyl)methyl]diaminoazobenzene), AzoAMpP (4, 2,2′-
bis[N,N′-(4-pyridyl)methyl]diaminoazobenzene), and
AzoAEpP (5, 2,2′-bis[N,N′-(4-pyridyl)ethyl]-
diaminoazobenzene) from the common DAAB scaffold (2,2′-
diaminoazobenzene, 6) building block (Scheme 1). Collec-
tively, we refer to the aABs as AzoAXxP compounds, where
“X” represents the variable components. The m-pyridyl ligand
containing the ethylene linker (AzoAEmP) has not been
accessed yet owing to difficulty in obtaining a stable and
reactive synthon. AzoAEoP and AzoAEpP were prepared via a
Michael reaction with the corresponding vinylpyridine;
however, the vinyl group becomes a poor Michael acceptor
with the nitrogen atom in the meta position. The anilino
nitrogen atom also appears to be a weak nucleophile, as
reactions with 3-(2-bromoethyl)-pyridine or 2-(pyridin-3-
yl)ethyl-4-methylbenzenesulfonate failed to produce the
desired product. Attempts to prepare AzoAEmP by reductive
amination with 2-(pyridin-3-yl)acetaldehyde also proved
unsuccessful, owing to difficulty in isolating sufficient
quantities of the aldehyde precursor.
The five ligands can be viewed as a library of extended
bipyridine ligands where each pyridine ligand can coordinate a
different metal ion to form an extended network. Fortuitously,
all five AzoAXxP ligands are highly crystalline in the solid state
and could be subjected to crystallographic analysis. The two
Although not absolutely predictive, the geometric structure
of Ag+ metal organic polymer chains depends on the pyridine
E
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Figure 2. (A) Preparation of AgAAMmP showing the thermal ellipsoid representations of AzoAMmP and AgAAMmP at the 50% probability level
and selected atom labels. Hydrogen atoms except for those engaged in intramolecular hydrogen bonds are omitted for clarity. The triflate anion is
removed for clarity. AgAAMmP was prepared in toluene using AgOTf as the silver source using a slow ligand exchange process with CH3CN.
During the polymer formation, the azo N−C bonds rotate by 180°. (B) Thermal ellipsoid representation at the 50% probability level of the
expanded helical polymer chain. Hydrogen atoms and triflate anions are omitted for clarity.
substitution pattern, the directionality of the pyridine lone
pairs, and the conformational freedom of the ligand. Solvent
and anion interactions also can affect the extended macro-
molecular structure. In bipyridine ligands containing o-pyridyl
nitrogen atoms, the lone pairs tend to face inward in the apo
form but often reorient during the polymerization process to
minimize crowding between the side chain and the Ag+
binding site, resulting in zigzag silver chain structures.22,23 In
meta- and para-pyridine bipyridine ligands, the lone pairs tend
to face outward, and therefore do not necessarily need to
undergo structural rearrangement in two-coordinate Ag+
structures. Some meta-bipyridine derivatives exhibit minimal
reorientation,24 or asymmetric changes on one-half of the
molecule.22 In more rigid m- and p-bipyridine ligands, linear
geometries are more common because of partially or
completely restricted movement.25,26
On the basis of the structural trends with other bipy
derivatives,22,23 we hypothesized that the pyridine nitrogen
lone pairs in AzoAMoP would flip outward to accommodate
Ag+ coordination if the intramolecular hydrogen bonds were
disrupted. We prepared the AzoAMoP complex using protocols
designed to gradually deliver Ag+ through ligand exchange with
CH3CN.14 A [Ag(AzoAMoP)]+ complex precipitated imme-
diately from toluene solution but redissolved upon addition of
CH3CN. Slow evaporation of CH3CN led to nitrile−pyridine
ligand exchange and the formation of a AgAAMoP
coordination polymer that was isolated as well-defined orange
crystals. In comparison to the structure of the apo ligand, the
azo N−C and anilino−methylene N−C bonds in the Ag+
polymer with an AzoAMoP ligand are rotated by 180 and 81°,
respectively (Figure 1). This rearrangement orients the two
pyridine rings perpendicular to the azo core but retains inward-
facing pyridyl nitrogen lone pairs in the formation of a helical
structure where each Ag+ is coordinated by a pyridyl nitrogen
atoms from two different AzoAMoP ligands. Ag+ binding
increases the length of the hydrogen bonds between the anilino
hydrogen atoms and the diazene lone pairs of the DAAB core
by approximately 0.4 Å and introduces a slight asymmetry in
the halves of the molecule. The core NN bond decreases
slightly from 1.28 to 1.26 Å owing to the decrease in hydrogen
bonding (Table 1). Although the changes in hydrogen bonding
might suggest a less rigid and therefore less emissive
azobenzene chromophore, we expected that the metal
complexation and crystal-packing interactions would offset
the loss of intramolecular forces.
In comparison to AzoAMoP, AzoAMmP and AzoAMpP
exhibit more conformational freedom, since both lack
hydrogen bonding between the anilino hydrogen atoms and
pyridine nitrogen atoms. The point-to-point distances between
the two pyridyl nitrogen atoms are 15.96 and 16.52 Å,
respectively, but unlike a linear bridging analogue such as 4,4′-
bipyridine, the effective distance obtained by projecting one
pyridine nitrogen on a plane containing the other pyridine is
shorter, 14.76 and 14.70 Å, respectively. We calculate the
effective distance by combining the offset between the two
pyridine ligands in the y and z directions where the x axis is
defined by a line going through one pyridine nitrogen atom
through its para carbon atom and defining the xy plane by that
pyridine ring. The y and z offset distances are therefore the
displacements from a linear bipyridine ligand (e.g., 4,4′-
bipyridine) from the defined x axis. Therefore, for AzoAMmP
the pyridyl nitrogen atoms are offset from linearity by 5.41 Å
(y) and 2.12 Å (z) (Figure S33), and AzoAMpP has a greater
displacement along the y trajectory (6.70 Å) but a shorter
deviation in the z direction (1.79 Å) (Figure S34). In contrast
to AzoAMoP, the pyridine lone pairs in both AzoAMmP and
AzoAMpP point away from the diazene core to afford extended
bipyridine derivatives more reminiscent of the bridging ligands
F
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Figure 3. (A) Preparation of AgAAMpP showing the thermal ellipsoid representations of AzoAMpP and AgAAMpP at the 50% probability level
and selected atom labels. Hydrogen atoms except for those engaged in intramolecular hydrogen bonds are omitted for clarity. The PF6 anion is
removed for clarity. AgAAMpP was prepared in CH3OH/CH3CN (1/4) using AgNO3 as the silver source. Subsequent anion exchange with n-
Bu4PF6 in CH3CN provided X-ray-quality crystals. During the polymer formation, the azo N−C bonds rotate by 180°. (B) Thermal ellipsoid
representation at the 50% probability level of the expanded zigzag polymer chain. Hydrogen atoms and PF6 anions are omitted for clarity.
Figure 4. Preparation of AgAAEoP showing the thermal ellipsoid representations of AzoAEoP and AgAAEoP at the 50% probability level and
selected atom labels. Hydrogen atoms except for those engaged in intramolecular hydrogen bonds are omitted for clarity. The PF6 anion is removed
for clarity. AgAAEoP was prepared in CH3OH/CH3CN (1/4) using AgNO3 as the silver source with added n-Bu4PF6 to provide X-ray-quality
crystals. During the complex formation, one azo N−C bond rotates by 180° while the other remains stationary to form a 17-membered ring. The
two hydrogen bonds formed between two anilino hydrogen atoms and the same azo N atom are asymmetric with lengths of 2.07 and 2.38 Å,
respectively.
found in coordination polymers. Structural changes in m- and
p-bipy derivatives are typically minimal during Ag+ complex
formation due to limited or no orientational freedom.22,24−26
On the basis of these trends, we expected the AgAAMmP and
AgAAMpP polymers, where the pyridine nitrogen atoms face
outward in the apo ligands, would likewise undergo minimal
structural changes.
AgAAMmP was synthesized by procedures identical with
those used to prepare AgAAMoP. Due to the limited solubility
of AzoAMpP ligand in toluene, however, the solvent was
replaced with a 1/4 CH3OH/CH3CN mixture and a minimal
amount of DMF, and AgOTf was substituted with AgNO3. To
obtain X-ray-quality crystals, anion metathesis with n-Bu4PF6
−
was used to replace the NO3− anion with PF6 . In comparison
G
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Figure 5. (A) Preparation of AgAAEpP showing the thermal ellipsoid representations of AzoAEpP and AgAAEpP at the 50% probability level and
selected atom labels. Hydrogen atoms except for those engaged in intramolecular hydrogen bonds are omitted for clarity. The PF6 anion is removed
for clarity. AgAAEpP was prepared in DCM using AgNO3 as the silver source with added n-Bu4PF6. The resulting precipitate was dissolved in
CH3CN to yield X-ray-quality crystals from slow solvent evaporation. During the polymer formation, the ethylene C−C bonds rotate by 180° and
the ethylene−pyridine C−C bond rotates minimally. (B) Thermal ellipsoid representation at the 50% probability level of the three-dimensional
polymer chain. Hydrogen atoms and PF6 anions are omitted for clarity.
to the structure of the apo ligand, the azo N−C bonds in the
Ag+ polymer with AzoAMmP and AzoAMpP ligands are
rotated by 180° (Figures 2 and 3), which is identical with the
azo N−C bond rotation observed during the formation of
AgAAMoP. This reorients the two pyridine rings but retains
outward-facing pyridyl nitrogen lone pairs in the formation of a
linear or zigzag chain where each Ag+ is coordinated by a
pyridyl nitrogen atom from each of two different AzoAMmP or
AzoAMpP ligands.
The DAAB core in AzoAMmP and AzoAMpP exhibit nearly
identical hydrogen bond lengths, 2.33 and 2.34 Å, respectively;
however, Ag+ binding produces different changes in the three
derivatives containing a methylene spacer. Unlike AgAAMoP,
the hydrogen bond length between the anilino hydrogen atoms
and the diazene lone pairs decreases. While the hydrogen
bonds in AgAAMpP (2.02 Å) are symmetric, AgAAMmP
contains longer (2.12 Å) and shorter (1.96 Å) bonds that are
both contracted in comparison to the apo ligand. While
AgAAMoP exhibited some asymmetry in the hydrogen bond
length, the difference is much more pronounced in
AgAAMmP. As expected, the increased hydrogen-bonding
interaction results in the opposite effect on the NN bond
lengths, which increases slightly from 1.27 to 1.28 Å for both
coordination polymers.
of the ethylene linkers and ligand flexibility provide no a priori
reason to predict a reorientation would be required to
coordinate Ag+. Using a procedure identical with that used
to prepare AgAAMpP, a discrete monomeric complex was
obtained (Figure 4). The single rotation about the azo C−N
bond required to achieve this coordination geometry suggests
kinetic trapping of a 17-membered metallacycle instead of a
polymer. To the best of our knowledge, this is the largest Ag
metallacycle formed from a bipydyl derivative to date. Large
metallacycles with pyridyl donor groups are relatively
uncommon. The most closely related examples include 13-
membered-ring metallacycles of Pt and Pd27 and an 11-
membered-ring metallacycle of Ag.23
The intramolecular hydrogen bonds in AzoAEoP may be a
significant factor in the preference for metallacycle formation
over a polymeric structure. Two 2.36 Å hydrogen bonds
between anilino hydrogen atom and the azo nitrogen atoms
appear in the apo ligand. When the ligand rotates to close the
metallacycle, the hydrogen bonding between one of the anilino
hydrogens and azo nitrogen is unavailable, and a new 2.07 Å
hydrogen bond forms between the same proton and the other
azo nitrogen atom. The hydrogen bonding in the upper half-
molecule remains unchanged, but the distance increases
slightly from 2.36 to 2.38 Å. The shorter, stronger hydrogen
bond formed in AgAAEoP may provide a thermodynamic
driving force for metallacycle formation.
The pyridine nitrogen atoms of AzoAEoP are separated by
15.04 Å. While the pyridine lone pairs face inward, the length
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Of the five AzoAXxP ligands, AzoAEpP most closely
resembles a linear bridging ligand such as 4,4′-bipyridine.
The point-to-point distance between the pyridine nitrogen
atoms of 17.95 Å is the longest distance of all the AzoAXxP
ligands, but the trajectory of the two pyridine groups is linear
and the aromatic rings are coplanar in the solid state. Linear
bipyridines with long nitrogen−nitrogen distances form linear
Ag+ polymers26,28 and three-dimensional networks with four-
coordinate Ag+ sites.29,30 The preference for two-coordinate or
four-coordinate Ag+ appears to correlate with the reorientation
ability and rigidity of the ligands, as well as the distance
between pyridine nitrogen atoms.
Although the pyridine ligands in apo-AzoAEpP are coplanar,
rotation about the C−N bond in the ethylene spacers
decreases the point-to-point distance from 17.95 to 6.65 Å.
The effective distance is obtained similarly to that described
above to yield y and z offset distances of 2.22 and 9.09 Å,
respectively (Figure S35). Each Ag+ is four-coordinate, with
each pyridine coming from one of four unique AzoAEpP
ligands. We partially attribute the increased coordination to the
reduced steric requirements at the donor ligand site of the p-
pyridine isomer. Each AzoAEpP ligand also coordinates two
separate Ag cations to form a three-dimensional polymer with
distorted-tetrahedral Ag+ sites (Figure 5B). The lack of steric
hindrance at the coordination site of the p-pyridine ligands
appears to be an important prerequisite for three-dimensional
polymer formation with bipyridines. In previous Ag+ polymers
and three-dimensional networks with four-coordinate Ag+ sites,
most of the ligands undergo minimum or no reorienta-
tion.26,28,30 Unlike these linear bipyridine ligands, AzoAEpP
exhibits a high degree of freedom, which allows optimization of
steric and electrostatic factors in Ag+ complex formation.
Emission. AzoAMoP exhibits no measurable emission at
room temperature but emits when frozen in a solvent glass at
77 K. The four additional AB compounds behave similarly.
Although the wavelengths vary, all the five compounds emit
with a λmax value between 566 and 610 nm (Table 2). Similar
to AzoAMoP and AzoAEoP,4,5 none of the new AzoAXxP
derivatives exhibit significant evidence for photoisomerization
upon irradiation. While quantitative measurements of mini-
mally emissive complexes are imprecise, qualitatively, AzoAEoP
and AzoAEpP exhibit brighter emission in comparison to
AzoAMmP and AzoAMpP. AzoAMoP appears to emit more
weakly than the other four derivatives.
Figure 6. Solid-state emission spectrum of the five AzoAXxP silver
complexes (λex 523 nm) showing the relative intensities.
Figure 7. Solid-state diffuse reflectance (black) and solid-state
emission (λex 523 nm, pink) spectra of AgAAEpP. Inset: cuvettes
containing a suspension of the complex crystals in toluene (100 μM)
without irradiation (left) and on excitation with 365 nm light (right).
To assess the photochemistry of the Ag+ complexes, the
emissions of AgAAMoP, AgAAMmP, AgAAMpP, and
AgAAEpP were evaluated in toluene as suspensions of
powdered crystals. The four coordination polymers form
semihomogeneous dispersions in toluene that remain
suspended for several months. Similarly to the free ligands,
the emission λmax value of the Ag polymers occurs at around
600 nm for all of the complexes (Table 2). In the solid state,
the relative integrated emission intensities reveal a nearly 30-
fold brighter emission for the four-coordinate AgAAEpP
complex in comparison to the least emissive AgAAMpP
complex (Figure 6). Complex formation red-shifts the
absorbance maximum of AzoAEpP from 502 to 594 nm with
an emission peak centered at 641 nm (Figure 7). Similar to the
absorbance, the emission of AgAAEpP red-shifts 31 nm from
that of free AzoAEpP measured at 77 K, which is a greater
change than for the other three polymeric complexes. The
luminescence response appears to validate the initial
hypothesis that embedding the AzoAXxP ligand in a solid-
state material increases the degree of radiative decay of the AB
excited state.
The emission of AB in solution at room temperature is too
weak to be detected by conventional emission instruments
owing to efficient decay of the excited state via nonradiative
pathways.31 In many aromatic chromophores, the emission of a
photon accompanies a S1(ππ*) → S0 transition as the excited
molecule returns to the ground state. In AB, however, the
S2(ππ*) ← S0 absorbance has the largest extinction coefficient
but a weakly absorbing interstitial S1(nπ*) state is present.
Although the photophysical processes remain an active area of
investigation,32−34 a rapid intersystem crossing to a vibration-
ally excited S1(nπ*) state occurs following S2(ππ*) ← S0
absorbance. AB isomerization then occurs via the concerted
inversion pathway from the S1 state.35,36 A modest increase in
emission intensity can be observed in a frozen matrix when
relaxation pathways involving molecular motion are impaired.
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Radiative decay of AB can be increased by manipulating the
frontier orbitals of the chromophore as observed in 2-
borylazobenzenes.37−40 Engaging the diazene lone pair in a
dative bond with a boron atom lowers the energy of the lone
pair, or B−N bonding orbital, below the diazene π-bond. Thus,
forming the nπ* state would require promoting an electron
into the half-occupied, higher energy diazene π orbital. The
absence of the interstitial S1(nπ*) state removes the optically
forbidden S1(nπ*) → S0 transition and opens the S2(ππ*) →
S0 emission channel.
We hypothesized that introducing intramolecular hydrogen
bonds between the diazene lone pair and anilino hydrogen
atom might lead to a similar, albeit less drastic, change in the
frontier orbitals to those observed in 2-borylazobenzenes,
increasing the radiative decay of the excited state. This
hypothesis was supported by our previous observations that an
AzoAMoP derivative lacking intramolecular hydrogen bonds
had significantly weaker emission at 77 K, similar to the
luminescence of AB under the same conditions.4 To support
the increased emission hypothesis, we interrogated the ground-
state electronic structure of each ligand using density
functional theory (DFT). In order to capture the structural
effect of the solid-state environment on the electronic structure
for each ligand, unit cell vectors and atom positions were
determined using three-dimensional periodic boundary con-
ditions before extracting a single molecule for analyzing the
electronic structure, an approach we have previously applied to
luminescence metal−organic systems.41 The first 10 excited
states were computed vertically using time-dependent DFT
(TDDFT) for all of the ligands.
meta/para nitrogen atom within the pyridine unit or an
ethylene spacer increases the distance between the nitrogen
donor atom and the diazene core, rendering this interaction
negligible and yielding a net stabilization of the lone pair, as
seen in the other four derivatives. The lack of emission in
solution and subsequent increase in a frozen matrix is also
consistent with hydrogen bonding as a key contributor to the
emission behavior. The restricted motion at low temperature
would be expected to lead to a stabilization of the hydrogen
bonds and therefore the energy levels of the frontier orbitals,
which would not necessarily occur in solution.
The electron-donating pyridine moieties destabilize the π
(HOMO) orbital for all ligands with respect to the DAAB
control molecule. This destabilization exceeds the stabilization
of the diazene lone pair orbital (LUMO), which results from
hydrogen bonding between the diazene lone pair and the
anilino hydrogen atom (maximum 0.95 eV). AzoAMoP again
provides an exception, where the π* orbitals are stabilized
upon the addition of the pyridine substituents (0.52 eV). The
combination of these effects results in a general stabilization of
the ππ* excited state and destabilization of the nπ* state,
causing an inversion of the emissive ππ* and nonemissive nπ*
states with respect to both AB and DAAB. As expected, this
results in the removal of the lower-lying nπ* state as a
radiationless decay channel.40 Notably, the inversion of states
is the least pronounced in AzoAMoP. In an absolute sense, the
nπ* state still lies higher in energy than the ππ*, but the small
energy gap (0.04 eV) could permit internal conversion
between these two states, accounting for the lower emission
intensity of AzoAMoP.
The computational approach provides values that correlate
closely with experimentally measured absorption energies. As
expected, the gap between the n and π* orbitals decreases
significantly on going from AB to DAAB, but the order remains
unchanged (Figure S36). The red shift of the ππ* absorption
maxima between AB and DAAB was calculated to be 0.95 eV,
in reasonable agreement with the experimental value of 1.23
eV. Likewise, the calculated and experimental values for the
five aAB ligands showed reasonable agreement. The minimum
and maximum red shifts of the ππ* transition of the five aAB
ligands were measured as 0.139 and 0.142 eV with respect to
DAAB. The equivalent red shifts determined computationally
were 0.250 and 0.294 eV, a spread similar to that of the
experimental values shifted by approximately 0.1 eV.
Since our series of modified aAB ligands are significantly
more emissive in a frozen matrix in comparison to AB and
DAAB, we reasoned that the anilino substituent must induce
further changes in the frontier orbitals. In examining the
frontier orbital energies, we observed that the pyridine
moieties induce a stabilization of the diazene-centered n-type
orbital for the four aAB ligands (maximum 0.33 eV). This
observation is in line with the expectation that the inductive
effect of the pyridine substituent results in a slightly more
electron rich secondary anilino nitrogen atom, leading to a
slightly more stable π* orbital and making the diazene lone
pairs somewhat better hydrogen bond acceptors. AzoAMoP,
which exhibits a relative orbital destabilization of approx-
imately 0.44 eV, provides an exception to this trend. AzoAMoP
shows a reduced gap between the n and π* orbitals in
comparison to the other four derivatives. We attribute this
difference to the additional hydrogen bonding provided by the
pyridine substituent, reducing the interaction between the
diazene lone pair and the anilino hydrogen atom. Introducing a
To generate emission at room temperature, we hypothesized
that embedding the ligands in coordination polymers would
provide the requisite restrictions on molecular motion needed
to stabilize the diazene lone pair−anilino hydrogen atom
hydrogen bond. Our hypothesis was validated by the emission
measurements on the Ag+ systems. The 2.43 Å diazene lone
pair−anilino hydrogen atom bond length in [Ag(AzoAMoP)]n
is the longest of the four polymeric complexes and correlates
with the lowest relative integrated emission intensity (Table
2). As the hydrogen bond length shortens in [Ag-
(AzoAMmP)]n (2.12 Å) and [Ag(AzoAMpP)]n (2.03 Å), the
relative emission intensity increases. As our hypothesis
predicts, [Ag(AzoAEpP)2]n has the brightest emission and
the shortest hydrogen bonds. In order to understand the effects
of the structural reorganization imposed by Ag+ coordination
on the frontier orbitals and excited states, the same
computation procedures were applied to the polymeric
complexes. These calculations only capture the structural
effects of the Ag+ coordination since a neutral ligand is
extracted from the periodic solid-state calculations; therefore,
the model does not account for the electronic effects of Ag+
coordination.
While the trend between hydrogen bond length and
emission intensity correlates as expected, the computational
results predict a small energy gap between the n and π*
orbitals. Since the structural reorganization of the anilino
moieties after polymer formation causes only a slight
modulation in the lowest-lying excited states, the qualitative
ordering of the states remains unchanged with respect to the
results obtained for the crystalline ligands alone. Interestingly,
[Ag(AzoAEpP)2]n is over 1 order of magnitude more emissive
than the other three polymeric complexes, which indicates
other contributing factors to the photophysical behavior. The
J
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a
Scheme 2. Proposed Action of the On−Off Response of AgAAEpP
a
Addition of Ag coordinating ligands like pyridine leads to loss of the polymeric structure necessary to induce luminescence from the azobenzene
chromophore. Removal of pyridine leads to a reassembly of the coordination polymer.
Figure 8. (A) Normalized emission response of AgAAEpP to pyridine. AgAAEpP (100 μM) was suspended in toluene (black), treated with 100
μM pyridine (red), and redispersed in toluene after removing the solvent and analyte by sparging with N2 (blue). The response also shows stepwise
decreases when incremental amounts of pyridine were added to reach final concentrations of 33, 67, and 100 μM (inset). (B) Normalized emission
response of AgAAEpP to multiple cycles of pyridine addition and removal. AgAAEpP (100 μM) was exposed to 100 μM of pyridine followed by a
sparging and redispersal process a total of four times. (C) Normalized emission response of AgAAEpP to multiple cycles of DMA addition and
removal. AgAAEpP (100 μM) was exposed to 10 μM of DMA followed by an addition of nitric acid to reach a final concentration of 20 μM H+ a
total of four times.
K
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Inorganic Chemistry
Article
calculations based on extracted ligands only capture structural
effects imposed by coordination to Ag+. While we are able to
demonstrate a correlation between hydrogen bond length and
emission behavior across the series of materials, we cannot
draw any conclusions regarding the influence of Ag+ atoms or
the crystalline environment on orbital or excited state energies.
Together, these results show that the enhanced luminescence
observed in these azobenzene derivatives could be based on
the inversion of the aforementioned electronic excited states
relative to azobenzene, but no conclusions can be made
concerning the differences in emission intensity observed upon
the formation of silver complexes or the differences in
luminescence intensity observed between different silver-
based polymers.
through the formation of insoluble halide complexes leads to
an irreversible loss of emission and a UV spectrum consistent
with free AzoAEpP.
The ability to reversibly detect analytes through changes in
extended structure is not limited to removal by evaporation.
Addition of DMA leads to a loss of emission analogous to the
response to pyridine. The addition of dilute nitric acid causes
the AgAAEpP to reassemble. This process can also be repeated
for multiple cycles without significant loss of absolute emission
intensity (Figure 8C). The emission loss and restoration
process by adding and removing volatile analytes such as
pyridine and NMM demonstrates the viability of a disassembly
and reassembly process for emission detection. While physical/
electrostatic interactions are more common signal transduction
pathways,42−44 our system provides an alternative sensing
mechanism driven by coordination events but primarily
demonstrates that the coordination polymer is required for
the AzoAXxP ligands to be emissive at room temperature.
AgAAEpP exhibits the brightest emission of all the Ag+
coordination polymers with a quantum yield of 0.8%. The
other complexes have quantum yields of less than 0.1%, and
therefore the exact values are not reliable. We examined the
emission response to the Ag+-coordinating analytes pyridine,
N-methylmorpholine, DMA, and imidazole. When 5 equiv of
pyridine with respect to AzoAEpP units is added to a
suspension of AgAAEpP in toluene, no emission was detected;
however, the absorbance spectrum showed the presence of
AzoAEpP. When a substoichiometric amount of pyridine was
added, the coordination polymer emission decreased and the
absorbance spectrum showed evidence for the release of some
AzoAEpP (Scheme 2). We suspected that pyridine was
displacing AzoAEpP pyridine ligands bound to the Ag+ sites,
which would cause the loss of extended structure in the
coordination polymer. The release of AzoAEpP, which is not
emissive at room temperature in solution, would produce an
on/off switching behavior. To provide supporting evidence for
this signaling mechanism, the pyridine was removed by
sparging the solution with N2 gas. The recovery of emission
after the evaporation and redispersion of the material in
toluene suggests the reassembly of the original AgAAEpP
structure. The disassembly/reassembly process can also be
observed by PXRD. Upon the addition of pyridine, the
diffraction pattern indicates the formation of amorphous
material, and the reassembled material exhibits an spectrum
identical with that of the original coordination polymer.
The reversible structure disassembly process can be
monitored by emission spectroscopy. Initially, the emission
spectrum of AgAAEpP exhibits features with maxima at 620
nm (Figure 8A). Upon the addition of pyridine, the peak
intensity decreases gradually and correlates with expected
changes in absorbance from AzoAEpP. The pyridine removal
process restores the original spectrum characteristic of
AgAAEpP. This process is fairly robust and can be repeated
with minimal loss of emission. After five cycles, the material
retains 90% of the original emission intensity (Figure 8B), and
the slight erosion of the maximum response is likely due to
irreversible loss of structure or incomplete removal of pyridine.
NMM shows results similar to those for pyridine. The boiling
points of pyridine and NMM are 115.2 and 116 °C,
respectively; therefore, the sparging with N2 removes both
the analyte and solvent (toluene, bp 111 °C).
CONCLUSION
■
Although ABs are not typically emissive, embedding AB
chromophores in a rigid coordination polymer can enhance the
degree of radiative decay of the excited state. In the aAB
ligands investigated, both Ag+ coordination and intramolecular
hydrogen bonds contribute to the emission enhancement;
however, the hydrogen bonding accounts for most of the
changes in the frontier molecular orbitals. The stabilization of
the ππ* excited state below the interstitial nπ* usually involved
in the nonradiative decay of the excited state leads to
luminescence in both the solid state and crystalline samples
dispersed in solvent. Our initial investigations suggest three-
dimensional networks are more emissive than linear coordina-
tion polymers; however, additional examples will be required
to confirm this hypothesis. The coordination polymers can be
completely or partially disassembled by the addition of Ag+-
binding analytes such as amines and re-formed after analyte
removal to provide a sensor-like system. This signal trans-
duction mechanism differs from that of many polymeric
sensors and will be investigated as an alternative approach to
designing practical luminescent probes.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02845.
1H and 13C NMR spectra for new AzoAXxP compounds
synthesized, TGA and PXRD spectra for all new Ag+
coordination polymers and complexes reported, room
temperature and 77 K emission spectra of all AzoAXxP
compounds, diffuse reflectance and emission spectra of
all Ag+ coordination polymers, emission data for analyte
additions to AgAAEpP, diagrams of Ag+-AzoAXxP units
showing distance calculations based on the trajectory
and positions of the pyridine units, additional
computation details and calculated energies and bond
lengths, and complete X-ray data and fully labeled
thermal ellipsoid representations of all coordination
polymers and complexes (PDF)
In contrast, on exposure to imidazole, the emission loss is
not reversible. Although imidazole displaces AzoAEpP from
the coordination sphere of Ag+ in the coordination polymer,
the lack of volatility (bp 256 °C) makes removal nearly
impossible. A similar response occurs upon treatment of
AgAAEpP with potassium bromide. The removal of Ag+
Accession Codes
CCDC 1854329−1854336 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
L
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free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for S.C.B.: scburdette@wpi.edu.
ORCID
Ilaria Ciofini: 0000-0002-5391-4522
Franco̧ is-Xavier Coudert: 0000-0001-5318-3910
Shawn C. Burdette: 0000-0002-2176-0776
Present Addresses
§L.W.: Department of Chemistry, University College London,
20 Gordon Street, London WC1H 0AJ, U.K.
⊥P.N.B.: Department of Chemistry, Syracuse University,
Syracuse, New York 13244, United States
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Christopher Lambert for guidance with diffuse
reflectance and solid-state emission measurements. This work
was supported by the American Chemical Society Petroleum
Research Fund grant 53977-ND3 and Worcester Polytechnic
Institute.
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