Solid-state and solution photoluminescence of platinum(II) complexes with 4′-substituted terpyridine ligands-structural, spectroscopic and electrochemical studies

Article abstract of DOI:10.1039/c6ce00890a

Platinum(ii) complexes with the formula [PtCl(L)]CF₃O₃S (where L = 4′-(4-chlorophenyl)-2,2′:6′,2′′-terpyridine (L₁), 4′-(4-bromophenyl)-2,2′:6′,2′′-terpyridine (L₂), and 4′-(benzothiophene-2-yl)-2,2′:6′,2′′-terpyridine (L₃)) were synthesized from precursor [Pt(PhCN)₂Cl₂] and characterized by FT-IR, NMR, UV-vis spectroscopy, X-ray crystallography and electrochemical studies. The emission properties of the coordination compounds were studied in solution and in the solid state at ambient temperature and in methanol:ethanol frozen glass matrices at 77 K. The quantum yields of fluorescence, lifetimes and the nature of the excited states were described on the basis of the molecular properties of the compounds. To elucidate the structural, spectroscopic and bonding properties of the obtained compounds, calculations at the DFT level were undertaken. Coordination compounds are emissive in both solution and solid states. Emission energies and the character of excited states in solution are governed by the donor-acceptor properties of the 4′ substituent of the terpyridine core. The emission properties in the solid state are attributed to the aggregation induced phenomenon. Moreover, the compounds [PtCl(L₁)]CF₃O₃S and [PtCl(L₂)]CF₃O₃S display a bathochromic shifted emission corresponding to J-aggregation (head-to-head Pt?Pt interactions), while in the case of compound [PtCl(L₃)]CF₃O₃S a hypsochromic shifted emission compared to that in solution is characteristic of the H-aggregation process (head-to-tail Pt?π interaction). The coordination compounds display a strong rigidochromic effect with decay times changing from the ps/ns scale at ambient conditions to the μs domain in frozen glass matrices.

Full text of DOI:10.1039/c6ce00890a

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Solid-state and solution photoluminescence of
platinumIJII) complexes with 4′-substituted
terpyridine ligands – structural, spectroscopic and
electrochemical studies†
Cite this: DOI: 10.1039/c6ce00890a
a
A. Maroń, A. Szlapa,b K. Czerwińska,a J. G. Małecki,a S. Krompiecb and B. Machuraa
*
PlatinumIJII) complexes with the formula [PtClIJL)]CF3O3S (where
L = 4′-(4-chlorophenyl)-2,2′:6′,2″-
terpyridine (L1), 4′-(4-bromophenyl)-2,2′:6′,2″-terpyridine (L2), and 4′-(benzothiophene-2-yl)-2,2′:6′,2″-
terpyridine (L3)) were synthesized from precursor [Pt(PhCN)2Cl2] and characterized by FT-IR, NMR, UV-vis
spectroscopy, X-ray crystallography and electrochemical studies. The emission properties of the coordina-
tion compounds were studied in solution and in the solid state at ambient temperature and in methanol :
ethanol frozen glass matrices at 77 K. The quantum yields of fluorescence, lifetimes and the nature of the
excited states were described on the basis of the molecular properties of the compounds. To elucidate the
structural, spectroscopic and bonding properties of the obtained compounds, calculations at the DFT level
were undertaken. Coordination compounds are emissive in both solution and solid states. Emission ener-
gies and the character of excited states in solution are governed by the donor–acceptor properties of the
4′ substituent of the terpyridine core. The emission properties in the solid state are attributed to the aggre-
gation induced phenomenon. Moreover, the compounds [PtCl(L1)]CF3O3S and [PtCl(L2)]CF3O3S display a
bathochromic shifted emission corresponding to J-aggregation (head-to-head Pt⋯Pt interactions), while
in the case of compound [PtCl(L3)]CF3O3S a hypsochromic shifted emission compared to that in solution is
characteristic of the H-aggregation process (head-to-tail Pt⋯π interaction). The coordination compounds
display a strong rigidochromic effect with decay times changing from the ps/ns scale at ambient conditions
to the μs domain in frozen glass matrices.
Received 18th April 2016,
Accepted 28th May 2016
DOI: 10.1039/c6ce00890a
www.rsc.org/crystengcomm
optics,4–6 solar energy conversion7,8 and also luminescence
sensing materials.9 The preferred properties are usually
Introduction
PlatinumIJII) coordination compounds as luminophores are
connected with long-lived room-temperature emission from
triplet metal-to-ligand charge transfer (3MLCT) states ob-
served for platinumIJII) compounds. However, the terpyridine
platinumIJII) coordination compounds display weak photo-
luminescence properties in solution at room temperature due
to the thermally accessible triplet metal-centered (3MC)
state.10 Through electron-withdrawing or electron-donating
substituents on the 4′-position of the terpyridine core it is
possible to increase the energy barrier between the 3MLCT
and the 3MC states and prevent thermal deactivation.11
Electron-withdrawing substituents (such as cyano and methyl-
known for their possible applications in OLEDs,1–3 non-linear
a Department of Crystallography, Institute of Chemistry, University of Silesia, 9th
Szkolna St, 40-006 Katowice, Poland. E-mail: ank806@wp.pl
b Department of Inorganic, Organometallic Chemistry and Catalysis, Institute of
Chemistry, University of Silesia, 9th Szkolna St, 40-006 Katowice, Poland
† Electronic supplementary information (ESI) available: FT-IR and NMR spectra
of compounds, hydrogen bond in coordination compounds; stacking interac-
tions in coordination compounds; composition of selected LUMOs and HOMOs
of [PtClIJL1)]+ and [PtCl(L2)]+ cations; selected calculated transitions for coordina-
tion compounds; 2D fingerprint plots, coordination cations, Hirshfeld surfaces
and percentage contributions to the surface area for the various close inter-
molecular contacts for molecules of compounds; density of states diagrams cal-
culated for cationic forms of compounds; excitation and emission spectra in the
form normalized intensity vs. wavelength of compounds; excitation and emis-
sion spectra with decay curves for compounds; comparison of emission spectra
of compounds at different conditions. CIF files giving crystallographic data for
complexes (1)–(3) CCDC 1033725, 1033726 and 1039074. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c6ce00890a
3
sulfonyl groups) decrease the energy of the MLCT state; how-
ever, electron-donating groups (such as thioether or dimethyl-
amino substituents) cause the appearance of
a triplet
intraligand charge transfer (3ILCT) state similar in energy to
the 3MLCT state, thus the emissive excited state is mixed
3MLCT/3ILCT/3LC in character. However, relatively less known
are the coordination platinumIJII) compounds with
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terpyridines functionalized by substituted aryl groups or
heterocycles and need further careful studies especially on
the fields of impact of supramolecular architecture on solid-
phase emission.
spectra were recorded for powder samples with the use of a
1
60 mm Spectralon® integrating sphere. The H and 13C NMR
spectra were obtained at room temperature in CDCl3 using a
Bruker 500 MHz spectrometer. Elemental analyses (C, H, N)
were performed on a Perkin-Elmer CHN-2400 analyzer.
Coordination compounds of platinumIJII) usually display
enhanced solid state photoluminescence properties.12,13 This
phenomenon is connected with stacking intermolecular in-
teractions. The square-planar geometry of d8 PtIJII) com-
pounds facilitates aggregation through attractive Pt–Pt and li-
gand–ligand interactions, which allows bi-molecular states
(B-M) in the ground state or in the excited state.14 Thus, com-
pared to d6 octahedral coordination compounds of
iridiumIJIII) or rutheniumIJII), the platinumIJII) compounds show
an advantage due to the emission of broad visible light and
possible generation of white light with a single dopant in lu-
minescent materials. The arrangement of the stacking inter-
action in [PtIJ4′-R-terpy)Cl]+ additionally depends on the na-
ture of the counter ions, which determines the solid-state
colours and luminescence.9
Photoluminescence spectra were measured on an FLS-980
spectrophotometer. The room-temperature spectra were pre-
pared in acetonitrile solution and in the solid state (powder).
Low-temperature emission spectra were measured in an
EtOH : MeOH mixture (4 : 1) frozen-glass matrix at the temper-
ature of liquid nitrogen with a Dewar assembly. Quantum
yields of optically diluted (0.05 < OD < 0.1) solutions and
powders were measured with an integrating sphere using sol-
vent (for solution samples) or Spectralon® reflectance stan-
dard (for powdered samples) as blanks. Time-resolved mea-
surements were carried out at room temperature using the
time correlated single photon counting method (TCSPC) on
an FLS-980 spectrophotometer. The excitation wavelength
was obtained using a set of picosecond pulsed diodes
(EPLED-340 nm, EPL-375 nm) with a 500 ns pulse period as a
light source and a PMT+500 nm (Hamamatsu, R928P) in a
cooled housing was used as a detector. The system was
aligned at the emission wavelengths. Additionally for the
analysis of fluorescence decay, an instrument response func-
tion (IRF) was obtained. The IRF contains the information
about the time response of the overall optical and electronic
system. The IRF was designed using Ludox solution as a stan-
dard at excitation wavelengths.
In this report, we present a study on the synthesis and the
structural, photophysical and electrochemical properties of
platinumIJII) chloride compounds with the formula
[PtClIJL)]CF3O3S (where
L
=
4′-(4-chlorophenyl)-2,2′:6′,2″-
terpyridine (L1), 4′-(4-bromophenyl)-2,2′:6′,2″-terpyridine (L2),
and 4′-(benzothiophene-2-yl)-2,2′:6′,2″-terpyridine (L3)). The
electronic structure is discussed on the basis of DFT calcula-
tions and used to interpret the luminescence properties of
the compounds. The crystal and molecular structures were
determined by X-ray crystallography. The spectroscopic prop-
erties were obtained using IR, UV-vis and NMR spectroscopy.
The luminescence of compounds was studied at ambient
temperature in solution and in the solid state. The solid state
emission properties were correlated with the analysis of the
Pt⋯Pt, π⋯π, Pt⋯π, Cl⋯π stacking interactions. The
rigidochromic effect was examined to demonstrate the radia-
tionless deactivation via the 3MC state. The quantum yield
and lifetime measurements were recorded to discuss the
characteristics of the excited states.
Cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) measurements were carried out on an
Autolab potentiostat (Eco Chemie). A three-electrode one-
compartment cell was used to contain the solution of coordi-
nation compounds and supporting electrolyte in acetonitrile.
Deaeration of the solution was achieved by bubbling argon
through the solution for about 10 min before measurement.
The compound and supporting electrolyte (n-Bu4NPF6) con-
centrations were 5 × 10−6 mol dm−3 and 0.02 mol dm−3, re-
spectively. The scan rate was 0.1 V s−1. A glassy carbon disk
working electrode (3 mm diam.) and a Ag/Ag+ reference
electrode were used.
electrode was added as an internal standard (the original
data reported vs. Ag/Ag+ have been converted using E(Fe/Fe+)
A
ferrocene/ferrocenium (Fe/Fe+)
Experimental
Materials and methods
=
E(Ag/Ag+) − 0.5 V to obtain the potential measured against
Fe/Fe+). The energy of the HOMO and LUMO orbitals of the
compounds was obtained from cyclic voltammograms using
All reagents used for the syntheses of the coordination com-
pounds are commercially available and were used without
further purification. The [PtIJPhCN)2Cl2] starting compound
was obtained from Strem Chemicals Inc. The ligands, 4′-(4-
chlorophenyl)-2,2′:6′,2″-terpyridine (L1), 4′-(4-bromophenyl)-
2,2′:6′,2″-terpyridine (L2), and 4′-(benzothiophen-2-yl)-2,2′:6′,2″-
terpyridine (L3), were synthesized according to procedures de-
scribed previously in the literature.15–17
the following equations: EHOMO = −5.1 − Eox, ELUMO = −5.1 − Ered
.
All electrochemical experiments were carried out under ambi-
ent conditions.
Synthetic procedure
Infrared spectra were recorded on a Nicolet iS5 FT-IR
spectrophotometer in the range 4000–400 cm−1 using KBr pel-
lets. Electronic absorption spectra were measured on a Nico-
let iS50 UV-VIS-NIR spectrophotometer in the range of 900–
180 nm in acetonitrile solution. The solid state (reflectance)
A suspension of [PtIJPhCN)2Cl2] (0.10 g, 0.21 mmol) in aceto-
nitrile (10 mL) was treated with an equimolar amount of
Ag(CF3O3S) (0.054 g, 0.21 mmol) dissolved in acetonitrile (5
mL). The reaction mixture was heated under reflux for 16 h
in the dark, the AgCl precipitate was removed by filtration
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and one equivalent of ligand was added to the filtrate. The re-
action mixture was heated in a solvothermal reactor under at-
mospheric pressure for an additional 24 h and then gradually
cooled for another 24 h. Crystals suitable for X-ray analysis
were obtained directly from the reaction mixtures.
Crystallography
Crystals of the coordination compounds (1)–(3) were
mounted in turn on a Gemini A Ultra Oxford Diffraction au-
tomatic diffractometer equipped with a CCD detector and
used for data collection. X-ray intensity data were collected
with graphite monochromated MoKα radiation (λ = 0.71073
Å) at a temperature of 295(2) K with ω scan mode. Ewald
sphere reflections were collected up to 2θ = 50.10°. Details
concerning crystal data and refinement are gathered in
Table 1. The unit cell determination and data integration
were carried out using the CrysAlis package18 of Oxford Dif-
fraction. Lorentz, polarization and empirical absorption cor-
rections using spherical harmonics implemented in SCALE3
ABSPACK scaling algorithm were applied. The structures were
solved by the Patterson method and subsequently completed
by difference Fourier recycling. All the non-hydrogen atoms
were refined anisotropically using full-matrix least-squares
techniques. All hydrogen atoms were positioned in geometri-
cally idealized positions and were allowed to ride on their
parent atoms with Uiso(H) = 1.2Ueq. The Olex2,19 SHELXS, and
SHELXL20 programs were used for all the calculations.
Atomic scattering factors were incorporated into the com-
puter programs.
(1) [PtClIJL1)]CF3O3S: yield 70%. C21H14Cl2N3Pt, CF3O3S.
Anal. calc: C 36.53%, H 1.95%, N 5.81%. Anal. found: C
36.45%, H 1.98%, N 5.78%.
IR (KBr; cm−1): 3082 ν(ArH); 1610, 1554 ν(CN, CC); 1480,
1397 δ(C–CH out of the plane); 1265 ν(C–N); 1149, 1027 δ(C–CH in the
plane); 833, 779, 751, 717 δ(C–C out of the plane); 636 δ(C–C in the
.
plane)
1H NMR (400 MHz, DMSO) δ 8.92 (s, 2H), 8.88 (d, J = 5.3
Hz, 2H), 8.75 (t, J = 11.4 Hz, 2H), 8.54–8.43 (m, 2H), 8.18 (t, J
= 10.3 Hz, 2H), 7.96–7.87 (m, 2H), 7.75 (t, J = 9.0 Hz, 2H).
13C NMR (126 MHz, DMSO) δ 158.52, 154.92, 151.76,
143.02, 135.91, 133.65, 130.14, 130.00, 129.66, 126.45, 121.83.
UV-vis (ACN, nm (ε)): 404.5 (36 000), 385.0 (30 400), 352.4
(34 800), 333.6 (107 800), 307.3 (120 400), 281.8 (155 800),
260.1 (156 200); (solid, nm (Abs)): 512.6 (0.929), 426.4 (1.055),
346.6 (0.923), 252.5 (0.751).
(2) [PtClIJL2)]OTf: yield 70%. C21H14ClBrN3Pt, CF3O3S. Anal.
calc: C 34.41%, H 1.84%, N 5.47%. Anal. found: C 34.50%, H
1.80%, N 5.43%.
IR (KBr; cm−1): 3082 ν(ArH); 1611, 1587 ν(CN, CC); 1479,
1414 δ(C–CH out of the plane); 1247, 1224 ν(C–N); 1149, 1029 δ(C–CH Quantum calculations
in the plane); 829, 780, δ(C–C out of the plane); 636 δ(C–C in the plane)
.
The calculations were carried out using the Gaussian0921
program. Molecular geometries of the singlet ground state of
the cationic forms of the platinumIJII) compounds were opti-
mized in acetonitrile solutions with the Polarizable Contin-
uum Model (PCM) model22 at the B3LYP level of theory.23
For each compound a frequency calculation was carried out,
verifying that the optimized molecular structure obtained cor-
responds to a local energy minimum; thus only positive fre-
quencies were found. The Pt-mDZP basis set24 was used to
describe the platinum atom and the basis set used for the
lighter atoms (Cl, N, B, C, H) was 6-31G**. The TD-DFT
method25–27 was employed to calculate the electronic absorp-
tion spectra in the PCM22 using acetonitrile as a solvent.
1H NMR (400 MHz, DMSO) δ 8.79 (d, J = 9.4 Hz, 2H), 8.61
(dd, J = 28.4, 6.3 Hz, 4H), 8.39 (t, J = 7.7 Hz, 2H), 8.04 (d, J =
8.5 Hz, 2H), 7.87 (d, J = 8.3 Hz, 2H), 7.84–7.75 (m, 2H).
13C NMR (126 MHz, DMSO) δ 158.16, 154.62, 151.61,
151.50, 142.92, 133.87, 132.89, 130.20, 129.67, 126.48, 126.28,
122.49, 121.43, 119.83.
UV-vis (ACN, nm, (ε)): 404.2 (20 800), 384.6 (17 000), 331.9
(64 800), 308.6 (68 000), 281.6 (91 000), 260.2 (88 800); (solid,
nm (Abs)): 513.6 (0.825), 422.8 (1.018), 343.7 (0.889), 252.8
(0.701).
(3) [PtClIJL3)]OTf: yield 78%. C23H15ClN3PtS, CF3O3S. Anal.
calc: C 38.69%, H 2.03%, N 5.64%. Anal. found: C 38.75%, H
2.12%, N 5.58%.
IR (KBr; cm−1): 3059 ν(ArH); 1611, 1554 ν(CN, CC); 1478,
1429 δ(C–CH out of the plane); 1256, 1223 ν(C–N); 1157, 1029 δ(C–CH
Results and discussion
Crystal structure
in the plane); 782, 754 δ(C–C out of the plane); 637 δ(C–C in the plane)
.
1H NMR (400 MHz, DMSO) δ 8.63 (s, 2H), 8.43 (s, 2H),
8.35 (t, J = 7.5 Hz, 2H), 8.03 (d, J = 7.8 Hz, 2H), 7.90 (d, J = 7.6
Hz, 2H), 7.77 (s, 1H), 7.50 (dt, J = 14.7, 7.1 Hz, 4H).
13C NMR (126 MHz, DMSO) δ 157.87, 154.56, 151.48,
146.37, 142.71, 141.11, 139.76, 138.29, 129.65, 128.39, 127.81,
126.45, 126.18, 125.71, 123.33, 122.48, 120.07.
Syntheses of platinumIJII) coordination compounds were
conducted with [PtCl2(NCPh)2] as a precursor using a modifi-
cation of Dhara's method.28 The coordination compounds
crystallize in the monoclinic space groups P21/c and P21/n
and their structures, as ORTEP representations with label-
ling schemes, are shown in Fig. 1 while selected experimen-
tal and calculated bond distances and angles are given in
Table 2 (see also Tables S1 and S2 in the ESI†). The bond
lengths and angles calculated by DFT method for cationic
forms of the coordination compounds are in agreement
with experimental values as can be seen from the data col-
lected in Table 2.
UV-vis (ACN, nm, (ε)): 423.3 (48 000), 401.0 (34 200), 323.4
(53 600), 283.4 (81 800), 255.0 (65 600), 228.2 (67 400), 207.5
(88 000); (solid, nm (nm (Abs)): 592.1 (0.774), 546.6 (0.889),
473.8 (1.135), 444.1 (1.147), 419.2 (1.140), 356.3 (1.059), 272.8
(0.943).
FT-IR and NMR spectra for compounds (1)–(3) are col-
lected in the ESI.†
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Table 1 Crystallographic data for complexes (1)–(3)
(1)
(2)
(3)
Empirical formula
Formula weight
C21H14Cl2N3Pt, CF3O3S
723.41
C21H14ClBrN3Pt, CF3O3S
767.87
C23H15ClN3PtS, CF3O3S
745.05
Temperature [K]
295(2)
295(2)
295(2)
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/n
Unit cell dimensions
a [Å]
b [Å]
c [Å]
α [°]
7.3698(3)
7.4264(4)
16.2788IJ13)
7.0245(7)
21.716(2)
90
21.6379IJ12)
14.5584(7)
90
21.7744IJ14)
14.5258(8)
90
β [°]
102.509(4)
90
2266.47IJ19)
4
2.120
6.576
102.970(5)
90
2289.0(2)
98.966(7)
90
2452.9(4)
4
2.018
6.056
γ [°]
Volume [Å3]
Z
4
Calculated density [Mg m−3
]
2.228
8.140
1456
Absorption coefficient [mm–1
F (000)
]
1384
1432
Crystal dimensions [mm]
θ range for data collection [°]
Index ranges
0.31 × 0.13 × 0.05
3.44–25.05
−8 ≤ h ≤ 7
−25 ≤ k ≤ 25
−15 ≤ l ≤ 17
12 767
0.39 × 0.11×0.04
3.55–25.05
−8 ≤ h ≤ 7
−25 ≤ k ≤ 23
−17 ≤ l ≤ 16
11 339
0.12 × 0.10 × 0.04
3.36–25.05
−19 ≤ h ≤ 19
−8 ≤ k ≤ 8
−20 ≤ l ≤ 25
14 019
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness of fit on F2
4006ijRIJint) = 0.0328, Rσ = 0.0344]
4006/0/316
1.034
4039ijRIJint) = 0.0679, Rσ = 0.0682]
4039/0/316
1.039
4335ijRIJint) = 0.0367, Rσ = 0.0354]
4335/0/334
1.041
Final R indices [I > 2σ(I)]
R1 = 0.0272
wR2 = 0.0583
R1 = 0.0374
wR2 = 0.0614
0.760/−0.621
R1 = 0.0545
wR2 = 0.1342
R1 = 0.0713
wR2 = 0.1448
2.938/−1.901
R1 = 0.0288
wR2 = 0.0631
R1 = 0.0403
wR2 = 0.0673
0.689/−0.602
R indices (all data)
Largest diff. peak and hole
Fig. 1 ORTEP drawing of complexes (1)–(3) with 50% probability displacement ellipsoids.
Table 2 Selected experimental and calculated bond lengths [Å] and angles [°] for complexes (1)–(3)
(1)
(2)
(3)
Exp
Calc
2.07
Exp
Calc
2.07
Exp
Calc
2.07
Bond length [Å]
PtIJ1)–NIJ1)
2.016(4)
2.017(8)
2.015(4)
PtIJ1)–NIJ2)
PtIJ1)–NIJ3)
PtIJ1)–ClIJ1)
1.930(3)
2.025(4)
2.2994(11)
1.98
2.07
2.40
1.931(6)
2.019(8)
2.299(2)
1.98
2.07
2.40
1.931(3)
2.018(4)
2.2961(14)
1.98
2.07
2.40
Angle [°]
NIJ2)–PtIJ1)–NIJ1)
NIJ2)–PtIJ1)–NIJ3)
NIJ1)–PtIJ1)–NIJ3)
NIJ1)–PtIJ1)–ClIJ1)
NIJ2)–PtIJ1)–ClIJ1)
NIJ3)–PtIJ1)–ClIJ1)
80.88(15)
81.05(15)
161.92(14)
99.06(10)
179.61(12)
99.01(11)
80.2
80.9(3)
80.5(3)
161.4(3)
99.3(2)
179.7(3)
99.3(2)
80.2
81.11(15)
81.19(15)
162.25(15)
98.57(11)
179.12(12)
99.14(11)
80.1
80.2
160.4
99.8
180.0
99.8
80.2
160.4
99.8
180.0
99.8
80.1
160.3
99.9
180.0
99.9
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The platinumIJII) metal center in the coordination cations
adopts a distorted square-planar geometry due to the geomet-
ric constraints imposed by the terpyridyl ligand. The N–Pt–N
angles deviate from the ideal 90° and 180° by about 10° and
19° (see Table 2), respectively, due to the steric demand of
the terpyridine ligands. The geometries (bond lengths, an-
gles) of the coordination sphere around the platinum ion are
almost the same in each of the compounds: geometry indices
29,30
τ4 and τ’4
are respectively equal to 0.1309°, 0.0761° (1);
0.1340, 0.0773 (2) and 0.1321, 0.0798 (3). However, some dif-
ferences are visible within the geometries of the terpy li-
gands. The coordinated terpyridine moieties are almost pla-
nar, with the dihedral angles between the central pyridine
plane and the two lateral ones equal to 5.90° and 2.87° in (1),
5.87°, 3.11° in (2) and 4.22°, 1.27° in (3), whereas the 4′ sub-
stituent planes are inclined to the central pyridine rings at
10.50°, 9.40° and 1.17° in (1)–(3), respectively.
The crystal packings of (1) and (2) show the stacking inter-
actions between pairs of the coordination cations (Table S2†
and Fig. 2),31 with alternating intermolecular Pt⋯Pt dis-
tances of 3.57 Å. In the crystal structure of (3) the Pt⋯Pt
interaction does not exist; instead a head-to-tail stacking cre-
ates a Pt–π benzene ring interaction with a distance of 3.70 Å
(see Table S2C† and Fig. 2). Examination of the packing dia-
grams reveals also that the structures of the coordination
compounds exhibit π–π interactions between aromatic
terpyridine moiety rings; C–H⋯π and C–F⋯π ring32 interac-
tions occurred in compounds (1) and (2). Moreover, in the co-
ordination compounds (1) and (2) the Pt–Cl⋯π ring interac-
tions are visible with distances close to 3.8 Å. The various
intermolecular interactions further confirmed by Hirshfeld
surface analysis33–35 (see Fig. S1 in the ESI†) affect the lumi-
nescence properties of these compounds, especially the pres-
ence of the interactions between the platinum centers in the
structures of compounds (1) and (2) and in the case of com-
pound (3) the planarity of 4′-(benzothiophene-2-yl)-2,2′:6′,2″-
terpyridine.
Electronic absorption and emission spectra
Absorption spectra in solution and solid state. Solution
samples of the coordination compounds display well-shaped,
wide charge transfer bands with maxima at 404 nm for (1),
(2) and 423 nm for (3) on the absorption spectra. The energy
of these absorption bands depends mostly on the π-donating
ability of the substituent in the 4′ position of the terpyridine
moiety, increasing in the order L1 < L2 < L3, which is addi-
tionally confirmed by DFT calculations. The participation of
the substituent in the 4′ position of the terpyridine core in
the HOMO increases in the range (1) < (2) < (3) as one can
see from the data collected in Table S3 in the ESI,† where the
compositions of the selected LUMOs and HOMOs of the coor-
dination cations are presented and from the densities of
states (DOS) diagrams (cf. Fig. S2†) calculated using the
GaussSum program.36 The transitions in this range (according
to Table S4 in ESI†) can be assigned as H-1/H-2 → LUMO and
Fig. 2 Contact interactions in the crystal structures of (1)–(3).
can be characterized as mixed XLCT/MLCT in the case of (1)
and (2) or ILCT in the case of benzothiophene-2-yl substituted
compound (3).
Solid state absorption spectra are dominated by one or
two bands, which are bathochromic shifted compared to
solution-phase spectra. The ranges of these shifts are equal
to 1278 cm−1, 1056 cm−1, and 1118 cm−1 for compounds
(1)–(3), respectively. Visible broadening of the solid state ab-
sorption spectra compared to the solution-phase spectra is
characteristic of strong intermolecular coupling in the aggre-
gates.37 The experimental and calculated electronic
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absorption spectra in solution are graphically compared with
absorption spectra in the solid phase of the compounds in
Fig. 3.
Emission spectra in solution, solid state and rigid matrix.
The excitation and emission spectra are presented in Fig. 4.
The spectroscopic and electrochemical properties are listed
in Table 3 (see also Fig. S3 and S4 in the ESI†).
ples as a result of intermolecular interactions observed in
the crystal arrangement of compounds (see ‘Crystal struc-
ture’ section). It is well known that enhanced properties (Φ
and τ) of emission in the solid state are usually attributed
to the aggregation induced phenomenon.41 The solid-state
emission spectra of (1) and (2) at room temperature show
structureless bands with the maxima largely bathochromic
shifted with respect to the values reported for solutions
(5080 cm−1 (1), 4755 cm−1 (2)) due to the head-to-head
Pt⋯Pt interactions predisposed to the J-aggregation pro-
cess. However, in the case of compound (3) the structured
solid state emission band shows a hypsochromic shift com-
pared to emission maxima in solution (2398 cm−1), which
is associated with the head-to-tail Pt–π stackings that allow
the formation of H-aggregates (see Fig. 2 and S5 in the
ESI†).
Acetonitrile solutions of the compounds at ambient condi-
tions display excitation bands at 368 nm, 432 nm (1), 366 nm
(2), and 465 nm (3). Excitation of (1) and (2) in solution gives
rise to one structureless emission band with a maximum at
518 and 528 nm, respectively, while excited compound (3)
displays a structured emission band with maxima at 545 nm,
607 nm, and 668 nm (sh). Calculated transitions correspond-
ing to excitation wavelengths are HOMO → LUMO/L+1
according to Table S4† (italic lines). On this basis, it can be
concluded that the orbital parentages of excited states in the
case of (1) and (2) are mixed and the excitation has a partly
d–π*, π(Cl)–π*(core) and π(R)–π*(terpy) character. However, for
compound (3) the excited state has an ILCT character due to
the greater ability of the benzothiophene-2-yl substituent to
donate electrons to the terpy core than the 4-chloro/
bromophenyl substituents. Efficient decreasing of the energy
gap caused by the benzothiophene-2-yl substituent explains
the significant bathochromic shift of the emission maxima in
the case of (3) (see also ‘Electrochemical properties’ section).
Compound (2) shows weaker luminescence properties (Φ and
τ) than compound (1) in solution due to the internal heavy
atom effect observed when changing the chloro substituent
to a bromo substituent. Compared with known [Pt(4′-X-T)Cl]+
(where X = CN, SO2Me, H, SMe, NMe2), an increase in quan-
tum efficiency by an order of magnitude is observed.11 Such
an increase in the quantum efficiency is characteristic of plat-
inum terpyridine compounds with electron-donating substit-
uents38 or polycyclic substituents39 due to mixing of the
3MLCT excited state with 3ILCT and/or 3LC states. However,
the values of the decay times indicate thermal deactivation of
The rigidochromic effect was studied in methanol : etha-
nol frozen glass matrices at the temperature of liquid nitro-
gen to better understand the nature of the lowest emissive ex-
cited states. For all three compounds low-temperature
emission spectra reveal a vibronic structure with three to four
bands (see Table 3 and Fig. 4). It is well known that the
vibronic shape of the emission spectra in a rigid matrix is
typical of emission originating from both MLCT and ILCT
states and a distinction between these states can be raised by
the time-resolved spectra. Thus, the lifetime values at low
temperature compared to those at ambient conditions, in-
creasing from the nanosecond/picosecond scale in the case
of ambient temperature to the microsecond domain at 77 K,
indicate the elimination of the thermal deactivation process
observed at ambient conditions. Moreover, a very similar
range of emission, the decay times (∼20 μs) and the slight
hypsochromic shift of low-temperature spectra relative to the
emission band at ambient conditions (equal to 226 cm−1 and
592 cm−1) in the case of compounds (1) and (2) indicate the
3
emission from the MLCT excited state. However, the spectra
of compound (3) are significantly bathochromic shifted (1225
cm−1), which allows the interpretation that the emission of
(3) is connected with different excited states and should be
considered as phosphorescence of 3ILCT. The longest life-
time, about 60 μs, additionally is in accordance with this
study.
3
the emissive excited states by MC states, which was already
observed earlier.9,40
A major feature of the studied compounds is the emis-
sion in the solid phase. All three compounds display a rela-
tively long-lived emission (∼500–1000 ns) as powdered sam-
Fig. 3 Absorption spectra of complexes (1)–(3).
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Electrochemical properties
Cyclic voltammograms of the compounds are shown in
Fig. 5. The spectroscopic and electrochemical data for coordi-
nation compounds (1)–(3) are listed in Table 3.
The examined platinumIJII) coordination compounds ex-
hibit a two-step ligand based reduction. The first reduction
couple is quasi-reversible in nature as is consistent with the
peak potential separations generally larger than the ideal
Nernstian value of 59 mV for a one-electron transfer. The ΔE
values are 78.1 mV, 83.0 mV and 65.9 mV for compounds
(1)–(3), respectively. The half-wave potentials of ligand based
reduction are similar for compounds (1) and (2); however, for
compound (3), due to the π-acidity differences of substituents
in the 4′ position of the terpy core, they have significantly a
more positive value.
The oxidation peaks at 0.735 V (1), 0.817 V (2) and 1.061 V
(3) are irreversible. The pulses in this range can be attributed
to the oxidation of the metal center; however the increasing
sharing of oxidation within substituents should be taken into
account. The potentials of the oxidation process increased
with the increasing sharing of ligand orbitals in HOMOs
gradually from (1) to (3).
The energies of LUMO orbitals obtained from cyclic
voltammograms of the compounds differ between −3.36 eV
and −3.88 eV; however, the energy of HOMOs are in the range
from −5.84 eV to −6.16 eV. The decreasing tendency of the en-
ergy band gap characteristic of (3) is clearly visible from the
electrochemical data and generally explains the lowering of
the energy of emission observed in solutions of compounds
(3) compared to (1) and (2). The electrochemically deter-
mined band gaps are in good agreement with the optical
band gaps. Moreover, the electrochemical data indicate
rather the significant stabilization of the energy of LUMOs
than the destabilization of the energy of HOMOs, which prac-
tically means a greater impact of electron-accepting proper-
ties than the electron-donating effect of the substituent in
the terpy core especially in the case of compound (3). Taking
into account electrochemistry studies, the influences of the li-
gand substituents on the emission process of coordination
compounds should be considered as both electron-donating
and electron-withdrawing effects of the substituents in the 4′
position of the terpy core. In the case of compound (3) with
the benzothiophene-2-yl group present in the molecular
structure, both electron-donating and electron-withdrawing
effects have an impact on the emission properties and cause
the efficient shift of emission compared to compounds (1)
and (2).42
Conclusions
Summarizing, by modification of Dhara's method triflate
platinumIJII) coordination compounds with 4′-substituted de-
rivatives of 2,2′:6′,2″-terpyridine were synthesized. The com-
pounds were studied by NMR, FT-IR, UV-vis spectroscopy and
cyclic voltammetry. The molecular structures of the
platinumIJII) coordination compounds (1)–(3) have been
Fig. 4 Excitation and emission spectra of complexes (1)–(3).
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Table 3 Spectroscopic and electrochemical properties of compounds (1)–(3)
λexc [nm] (cm-1)
λem [nm] (cm-1)
τ [ns] (Φem [%])
Solid
ACN
77 K
Solid
ACN
77 K
Solid
ACN
77 K
(1) 372 (26882), 368 (27174), 286 (34965), 703 (14225)
518 (19305)
512 (19531),a
550 (18182),
593 (16863),
649 (15408) (sh)
496.32 (6.32)
3.01 (6.21) 23693.0
479 (20877)a 432 (23148)a 310 (32258),
335 (29851),
356 (28090),
385 (25874),
407 (24570)a
(2) 450 (22222), 366 (27322)
314 (31847), 705 (14184)
334 (29940),
528 (18939)
545 (18349),
512 (19531),a
549 (18215),
595 (16807),
647 (15456) (sh)
584 (17123),a
636 (15723),
821.80 (7.30)
2.02 (0.61) 23389.4
499 (20040)a
386 (25907),
407 (24570)a
(3) 328 (30488)
465 (21505)
307 (32573), 482 (20747),a
1191.04 (1.12) 4.94 (2.01) 62434.3
339 (29498), 505 (19802) (sh), 607 (16474),a
402 (24876), 613 (16313) (sh) 668 (14970) (sh) 699 (14306) (sh)
424 (23585)a
ΔE [cm−1
]
Egopt [eV]
CV
Solid
ACN
77 K
Solid
ACN
77 K
Eox [V]
Ered [V] (ΔE [mV])
EHOMO [eV]
ELUMO [eV]
EgCV [eV]
(1)
(2)
(3)
6652
5856
9741
3843
8383
5031
5039
5039
6462
1.76
1.76
2.57
2.40
2.35
2.04
2.42
2.42
2.12
0.735
0.817
1.060
−1.741 (78.1), −2.463
−1.718 (83.0), −2.167
−1.219 (65.9), −2.410
−5.84
−5.92
−6.16
−3.36
−3.38
−3.88
2.48
2.54
2.28
a
The most intensive band; sh – shoulder.
determined by X-ray diffraction studies on single crystals.
The coordination compounds display both solid-state and
solution-phase emission. The emission properties in the solid
phase are attributed to the aggregation induced phenome-
non. Solid-state emission of compounds (1) and (2) are signif-
icantly bathochromic shifted compared to solution-phase
emission due to the J-aggregation process connected with
head-to-head Pt⋯Pt interaction. However, the hypsochromic
shift of solid-state emission in the case of compound (3) is at-
tributed to the H-aggregation process via head-to-tail Pt–π
intermolecular interactions. The electrochemical properties
of coordination compounds are in agreement with the optical
properties in solution phase. On this basis, it can be con-
cluded that the lowering of emission energy in the case of
compound (3) compared to that in compounds (1) and (2)
can be attributed to the decrease in the energy band gap
caused by substituent effect. The strong rigidochromic effect
characteristic of coordination compounds indicates on the
one hand that the thermal deactivation process via the MC
state occurs at ambient conditions and on the other hand
proves the different character of the lowest emissive excited
state in the case of coordination compounds (1) and (2) com-
pared to coordination compound (3).
3
Acknowledgements
The research was co-financed by the National Research and
Development Center (NCBiR) under Grant ORGANOMET No:
PBS2/A5/40/2014. Calculations have been carried out in
Wroclaw Centre for Networking and Supercomputing (http://
www.wcss.wroc.pl) under grant number 18.
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