Neutral heteroleptic cyclometallated Platinum(II) complexes featuring 2-phenylbenzimidazole ligand as bright emitters in solid state and in solution

Article abstract of DOI:10.1016/j.dyepig.2018.11.029

A series of 8 neutral cyclometallated platinum(II) complexes featuring 2-phenylbenzimidazole chromophoric ligand and acetylacetonate as ancillary ligand has been synthesized and characterized. The introduction of a variety of substituent on the phenyl ring allows to finely tune the emission wavelength. The emission data reveal high quantum yield in deaerated solution and in bulk solid state (micro-crystalline powder), with values up to an impressive 51%. The emission for all of the complexes is attributed to a mixed ³LC-MLCT state. Both absorption and emission data have been correlated successfully with the redox potentials and the Hammett parameters.

Full text of DOI:10.1016/j.dyepig.2018.11.029

Accepted Manuscript
Neutral heteroleptic cyclometallated Platinum(II) complexes featuring 2-
phenylbenzimidazole ligand as bright emitters in solid state and in solution
Pierre-Henri Lanoë, Angelica Moreno-Betancourt, Laura Wilson, Christian Philouze,
Cyrille Monnereau, Hélène Jamet, Damien Jouvenot, Frédérique Loiseau
PII:
S0143-7208(18)31960-0
https://doi.org/10.1016/j.dyepig.2018.11.029
DYPI 7173
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 5 September 2018
Revised Date: 13 November 2018
Accepted Date: 14 November 2018
Please cite this article as: Lanoë P-H, Moreno-Betancourt A, Wilson L, Philouze C, Monnereau C, Jamet
Héè, Jouvenot D, Loiseau Fréé, Neutral heteroleptic cyclometallated Platinum(II) complexes featuring 2-
phenylbenzimidazole ligand as bright emitters in solid state and in solution, Dyes and Pigments (2018),
doi: https://doi.org/10.1016/j.dyepig.2018.11.029.
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ACCEPTED MANUSCRIPT

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Neutral
Heteroleptic
Cyclometallated
Platinum(II)
Complexes Featuring 2-Phenylbenzimidazole Ligand as
Bright Emitters in Solid State and in Solution.
a
a
a
Pierre-Henri Lanoë,*a Angelica Moreno-Betancourt, Laura Wilson, Christian Philouze, Cyrille
Monnereau, b Hélène Jamet, a Damien Jouvenot a and Frédérique Loiseau* a
a Univ. Grenoble Alpes, CNRS, DCM, F-38000 Grenoble, France
b
Univ Lyon, Ens de Lyon, CNRS UMR 5182, Université Lyon 1, Laboratoire de Chimie, F69342,
Lyon, France
Corresponding Authors
pierre-henri.lanoe@univ-grenoble-alpes.fr
frederique.loiseau@univ-grenoble-alpes.fr
Keywords: platinum • solid state emission • phosphorescence • organometallic • photophysics
Abstract:
A
series of
8
neutral cyclometallated platinum(II) complexes featuring 2-
phenylbenzimidazole chromophoric ligand and acetylacetonate as ancillary ligand has been synthesized
and characterized. The introduction of a variety of substituent on the phenyl ring allows to finely tune
the emission wavelength. The emission data reveal high quantum yield in deaerated solution and in bulk
solid state (micro-crystalline powder), with values up to an impressive 51%. The emission for all of the
3
complexes is attributed to a mixed LC-MLCT state. Both absorption and emission data have been
correlated successfully with the redox potentials and the Hammett parameters.
1. Introduction
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Phosphorescent organo-transition metal complexes have become a focus of intense research,
particularly due to their applicability as efficient emitters in optoelectronics,[1–4] molecular sensors,[5–
9] photosensitizer in light conversion systems,[10–12]… These numerous applications in optical
technologies are based on the very high luminescent quantum yields that those compounds can exhibit.
The luminescence stems from the radiative deactivation of the lowest triplet excited state to the singlet
ground state. Triplet to singlet transitions are formally forbidden, but they become partially allowed
with the strong spin-orbit coupling (SOC) induced by presence of a heavy metal center. In particular,
Platinum(II) and Iridium(III) complexes have demonstrated to have somewhat matchless luminescent
properties regarding their emission features that can span from blue[13–20] to red/near-infrared,[21–27]
and quantum yields that can be close to unity.[28–32]
Platinum(II) complexes have emerged as valuable candidates as light emitting materials in particular
when coordinated to strong field ligand, such as 2-phenylpyridines, 6-phenylbipyridines and 1,3-di(2-
pyridyl)benzene.[2,33–37] Indeed, numerous Pt(II) complexes with bipyridine or terpyridine ligands are
not or weakly emissive in solution at room temperature, often because their metal-centered states (MC,
d-d) are accessible.[38,39] The dx2-y2 orbital is strongly antibonding; its population leads to Pt-L
elongation and severe distortions leading to efficient nonradiative deactivation, providing a decay
pathway for the emissive state.[35] The luminescence of cyclometallated platinum complexes is often
based on the radiative deactivation of a low lying triplet excited state centered on the ligand (3LC) to the
ground state.[1,2,34,35,40,41] This emission is characterized by a low radiative constant, but mixing
with higher states (1,3MLCT) may lead to the increase of the radiative constant.[41] The excited state
decay time will determine greatly the field of application of the emitter. While complexes with long
emission decay times will be advantageous for sensors, other complexes with relatively short decays
will be devoted to lighting display.
Generally, chromophores that exhibit highly emissive luminescence in dilute solutions notoriously
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suffer from “aggregation-caused quenching” (ACQ) in concentrated solutions or in the solid state
because of the formation of excimers, exciplexes and, dealing with phosphorescent compounds, could
lead to triplet-triplet annihilation.[42,43] For example, [Ir(ppy3)] displays photoluminescence quantum
efficiency of 0.97[44] in dilute solution, whereas in neat film the value drops below 0.03[45] and as an
amorphous solid the value drops to 0.004[46] due to self-quenching through intermolecular π-stacking
interactions.[47,48] Platinum(II) complexes with square planar geometry are known to display weak
metal-metal interactions by an overlap of their dz2 orbitals. The end-on interaction can form weakly
bonding and antibonding dσ and dσ* molecular orbitals. As a consequence the nature of the lowest
excited state can switch from 3LC or 3MLCT (metal to ligand charge transfer) to metal-metal to ligand
charge transfer triplet excited state (3MMLCT), with generally weak emission quantum yield.[35] For
many applications, such as sensing or lighting devices, maintaining phosphorescence in solid state is a
sought-after property.[49] In addition, absorption properties, brilliance and photostability can be
increased by the high number of molecules contained in solid objects, rather than in dilute solutions.[50]
Among the emissive platinum(II) complexes explored, those based on the form [C^N]Pt(O^O), with
[C^N] cyclometallating ligand and (O^O) of the family of acetylacetonate, are very attractive especially
for their ease of synthesis.[4,35,51] Complexes featuring the 2-phenylpyridine have been largely
explored and demonstrate to have good to excellent emission properties.[13,34,51–65] On the other
hand, it appears that 2-phenylbenzimidazole (phbz) based cyclometallated platinum(II) complexes have
not been as intensely[66–68] explored yet. Nonetheless, the ease of synthesis and the commercial
availability of phbz compounds make this moiety attractive to use as ligands. In this contribution, we
describe the synthesis of neutral cyclometallated platinum complexes with acetylacetonate as ancillary
ligand. The cyclic voltammetry, absorption and emission spectroscopies, as well as theoretical
calculations reveal that (i) this family of complexes are efficient triplet emitters with quantum yield
around 40% for several compounds in solution under inert atmosphere, (ii) most of the complexes
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display an emission quantum efficiency in solid state (microcrystalline solid) that generally exceeds
10% and can reach an impressive 51% for one of the compounds, (iii) the fluorine atoms linked to the
arene have prevalent donating mesomeric effect, (iv) the ethyl group offers sufficient steric hindrance to
inhibit the interactions in the solid state.
2. Experimental Section
2.1.General
Commercially available reagents were purchased from Sigma-Aldrich, Alfa Aesar, Acros Organics,
TCI Chemical, Merck, Strem or Fluorochem and used as received unless otherwise specified.
Compounds 1a-d and 1f-h were purchased from the same supplier when available or synthesized from
known procedures.[69–71] Solvents were obtained from same commercial sources and used without
1
further purification. Glassware was oven-dried prior to use. H NMR spectra were recorded on a 300
MHz and on a 400 MHz in deuterated solvent (CDCl3, DMSO-d6 or CD2Cl2) and data are reported as
follows: chemical shift in ppm from tetramethylsilane with the solvent as an internal indicator (CDCl3
7.26 ppm, dmso-d6 2.50 ppm, CD2Cl2 5.32 ppm), multiplicity (s = singlet, d = doublet, t = triplet, q =
quartet, p = pentet, m = multiplet or overlap of non-equivalent resonances), integration. 13C{1H} NMR
spectra were recorded either on a 75.4 MHz or on a 101 MHz in suitable deuterated solvent and data are
reported as follows: chemical shift in ppm from tetramethylsilane with the solvent as an internal
indicator (CDCl3 77.16 ppm, DMSO-d6 39.52 ppm, CD2Cl2 53.84 ppm). 19F NMR spectra were
recorded on a 376 MHz in the suitable deuterated solvent.
All electrochemical measurements were performed in a standard three-compartment electrochemical
cell under an argon atmosphere. Silver/silver nitrate in CH CN (Ag/AgNO 10 mM/CH CN) and a
3
3
3
platinum coil were used as reference and counter electrodes, respectively. Modified carbon electrodes
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were used as working electrodes. Cyclic voltammograms were recorded with CHI 600 or 630
potentiostats (CH Instruments), allowing on-line ohmic drop compensation.
Absorption and emission spectra in solution and at 77 K (not absorption) were recorded on a Cary 300
UV−visible spectrophotometer (Varian) and a Fluoromax 4 (Horiba), respectively, using quartz cuvette
with 1 cm optical path. Lifetimes were measures using LP900 spectrometer with a Flashlamp pumped
Q-switched Nd:Yag laser operating 355 nm and with photomulitplie (PMT) detector, or with a
picosecond Nd:YAG laser operating at 400 nm and using a time-correlated single photon counting
detection (TCSPC, PicoHarp 300). Solid state fluorescence spectra were obtained on a Horiba-Jobin-
Yvon Fluorolog-3® spectrofluorimeter, equipped with a three-slit double-grating excitation and
emission monochromator with dispersions of 2.1 nm.mm-1 (1200 grooves.mm-1 and a R928
photodetector. Sample was introduced into an open capillary quartz tube, which was placed into the
cavity of a GMP G8 integration sphere. Solid state quantum yields were determined using the
previously published general methodology. In order to minimize differences between the peak
intensities of the lamp and emission profiles, and ensure that the collected signal remained in the linear
range of the detector, density filters (0.5%) were used to attenuate the intensity of the lamp profile.Solid
state lifetime measurements were obtained using the same integration sphere/ spectrophotometer setup,
using a NanoLED 390 source operating at 390 nm with 1.2 ns pulses, which was controlled by Horiba-
DataStation software. Repetition rate was set at 50 kHz The fluorescence decay signal was
reconstructed from time correlated single photon counting (TCSPC), and deconvoluted using the Decay
Analysis Software (DAS)
2.2.Computational part
All calculations were performed using the GAUSSIAN 09 package.[72] Geometry optimizations were
carried out for all systems using the hybrid exchange-correlation PBE0 functional accounting for
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CH2Cl2 solvation effects using the IEF-PCM continuum solvent model. Small-core relativistic energy-
consistent pseudopotentials for the 5d orbitals of Pt were employed with the VTZ basis set, and the 6-
31G* basis set was used for all of the other atoms.[73] Vibrational frequency calculations were
performed to ensure that each geometry optimization converged to a real minimum.The NTO formalism
was used to elucidate the nature of the transitions to the excited states.
The synthesis of the compounds 1e and 2a-h is depicted in the supplementary materials.
2.3.General procedure for the complexation:
In a mixture of glacial acetic acid and acetonitrile (2/1, v/v) or pure acetic acid was degassed by
bubbling Ar for 20 min, the ligand (1.6 to 2 eq.) was dissolved at room temperature, then K2PtCl4 (1
eq.) was added. The solution was refluxed overnight under Ar. At room temperature, water was added
and the precipitate was filtered off, washed with water and dried with a small amount of methanol. The
crude product was put in suspension in dry acetone or toluene and sodium acetylacetonate (4 eq.) was
added. The solution was degassed 20 min by bubbling Ar and refluxed. The reaction was monitored by
TLC (SiO2, CH2Cl2/C5H12, 8/2). After full conversion, solvent was evaporated, the crude product
dissolved in CH2Cl2 and the salts were filtered off. See the supplementary materials for detail synthesis.
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3. Results and Discussion
3.1.Synthesis.
The synthesis of the ligands and the complexes is depicted in Scheme 1. The starting 2-
phenylbenzimidazole derivatives have been obtained through known procedures[69,74] or bought from
commercial suppliers, with the exception of compound 1e. Compound 1e was obtained by mixing 3,4,5-
trifluorobenzaldehyde with o-phenylenediamine in the presence of sodium pyrosulfite in a mixture of
water/DMF under reflux. Simple precipitation and filtration gave analytically pure 1e in high yield
(94%). The N-alkylation of compounds 1a-h proceeded in moderate to high yields (37-90%) in
anhydrous DMF in the presence of sodium hydride and bromoethane, followed by precipitation upon
water addition. The choice of the ethyl group instead of a methyl one has been motivated in order to
enhance the solubility of both ligands and complexes. The latters have been obtained using a robust two
steps method: (i) an excess of ligand was dissolved in a suspension of K2PtCl4 in pure AcOH at 120 °C
or in a mixture of AcOH/CH3CN at reflux and gave the dichloro-bridged dimer [(N^C)Pt(µ-Cl)]2 ; (ii)
the latter was dissolved/suspended in toluene or acetone in the presence of sodium acetylacetonate and
the mixture was refluxed. The complexes have been obtained in low to moderate yields (14-53%) after
column chromatography (SiO2). Complexes were recrystallized in CH2Cl2/Et2O or CH2Cl2/C5H12 before
characterizations.
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Scheme 1. Synthesis of the precursor 1e, the ligands 2a-h, and the platinum(II) complexes 3a-h. (i)
Na2S2O5, DMF/H2O reflux; (ii) BrCH2CH3, NaH, DMF 40 °C; (iii) K2PtCl4 AcOH 120 °C or
AcOH/CH3CN reflux; (iv) sodium acetylacetonate acetone or toluene, reflux.
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Table 1. Selected bond distances and angles from single crystal X-Ray diffraction spectroscopy for
complexes 3a, 3e and 3g. In the case of complexes 3a and 3e, the data corresponding to two complexes
(A and B) of the asymmetric unit are presented.
Complex Pt-C1 (Å) Pt-N1 (Å)
Pt-O1 (Å)
Pt-O2 (Å)
C1-Pt-N1 (°) O1-Pt-O2 (°)
1.985 (5)
1.983 (6)
1.989 (4)
2.020 (4)
1.982 (4)
1.985 (5)
1.978 (5)
1.986 (4)
1.965 (4)
1.992 (2)
2.003 (4)
1.992 (3)
2.004 (3)
2.002 (3)
2.010 (2)
2.082 (3)
2.083 (4)
2.057 (3)
2.041 (3)
2.094 (3)
81.1 (2)
80.9 (2)
79.7 (2)
79.8 (2)
80.4 (1)
92.1 (2)
91.7 (1)
90.9 (1)
91.2 (1)
91.77 (9)
3a (A)
3a (B)
3e (A)
3e (B)
3g
3.2. NMR spectroscopy
19
All complexes were characterized by 1H and 13C NMR, and, when relevant, F NMR. The 1H NMR
spectra of the complexes feature the characteristic appearance of a singlet in the 5.52-5.68 ppm range
and two singlets in the 2.12-1.99 ppm range, corresponding to the enolic proton and the methyl groups
of the ancillary ligand, respectively. Furthermore, in the case of complexes 3c, 3d, 3f, 3g and 3h, the 1H
NMR spectra display doublets (3d and 3c) or singlets (3f, 3g and 3h) at 7.77 ppm, 7.78 ppm, 7.93 ppm,
7.92 ppm and 8.03 ppm respectively, showing weak and broad satellites. The presence of satellites is
due to the 3J195Pt-H coupling between the platinum(II) and the proton in α-position with respect to the C-
Pt bond, with a constant between 40 Hz and 50 Hz (see supporting information). These signals confirm
1
that cyclometallation took place. In the case of complex 3a, the H NMR displays a weak and broad
signal superimposed to a doublet at 7.44 ppm and to the residual solvent peak (CHCl3). Such feature
3
could be assigned to the J195
coupling, but we were unable to measure accurately the coupling
Pt-H
1
constant. For complex 3h, cyclometallation can obviously occur in two positions. The H NMR
spectrum displays two singlets at 8.03 ppm and 7.91 ppm, of which the first displays the two satellites
attributed to the 195Pt-H coupling (vide supra). The second singlet is assigned to the proton in β’-
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position with respect of the C-Pt bond. It is worth noting that, this proton is shifted up-field upon the
cyclometallation, with a chemical shift of 8.26 ppm for the ligand and 7.91 ppm for the complex. The
second isomer that could be formed with a cyclometallation in β’-position, has not been observed. The
3
19
19F NMR spectrum of complex 3b displays an interesting feature, being a J coupling between F and
195Pt with a constant of roughly 55 Hz.
3.3. Crystal structures
Figure 1. Top left, view of compound 3a and top right, 3g. Bottom, view of complex 3g highlighting
Pt…Pt interactions. All structures are drawn with 50% probability displacement ellipsoids and hydrogen
atoms omitted for clarity.
Suitable crystals for X-Ray diffraction spectroscopy were obtained for complexes 3a, 3e and 3g by
slow vapor diffusion of pentane or diethyl ether in a dichloromethane solution. The structures are
depicted in figure 1 and 2, and selected distances and angles are gathered in table 1.
The crystal structures of the three cycloplatinates reveal an expected square-planar geometry around
the PtII core, with minor distortion. In addition, for compounds 3a and 3e, two molecules are located in
one asymmetric unit with similar bond distances and angles respectively (see figure S1 and figure S2),
such features have been observed in previously reported cycloplatinates.[52,60,75] Pt-C1 and Pt-N1
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bond distances lie in the range 1.982(4) Å to 2.020(4) Å and 1.965(4) Å to 1.992(2) Å, respectively. It
can be noted that for complex 3e, the two molecules in the asymmetric unit display a difference of
roughly 0.03 Å for the Pt-C bond distance. Such variation has been observed previously between two
molecules located in one asymmetric unit.[34] Within the series, two of the complexes (3a and 3e)
display longer Pt-C bond than Pt-N bond, which differ from related complexes reported. However, a
similar behavior was observed in complexes integrating the dipivaloylmethane ligand.[52] Overall,
those values are comparable with the average values observed in similar reported structures, 1.97 Å and
1.99 Å bond distances for Pt-C and Pt-N respectively.[34,52,66,75–77] The Pt-O bond distances are
longer, in the range 1.992(3) Å to 2.094(3) Å and 2.010(2) Å to 2.083(4) Å, for Pt-O1 and for Pt-O2
respectively. It is worth noting that the Pt-O2 bonds display longer distances than the Pt-O1 and this is
likely due to the trans effect, with respect to the opposite strong σ-donating cyclometallating carbon
(C1). The observed bond distances fall in the average value (2.04 Å) reported for related [(N^C)PtO^O]
complexes.[34,52,66,75–77] The C1-Pt-N1 angles range from 79.7(2) ° to 81.1(2) ° and the O1-Pt-O2
angles, from 90.9(1) ° to 92.1(2) °. These angle values straddle the value reported in related
[(N^C)PtO^O] complexes, with average angles of 82 ° and 92 ° for C-Pt-N angle and O-Pt-O angle
respectively.[34,52,66,75–77]
The packing of compound 3g evidences intermolecular interactions that are made possible by the
square planar geometry of the platinum(II) complexes (figure 1). Indeed, the analysis of the crystal
structure reveals short Pt-Pt distance of 3.221(1) Å, which is smaller than the sum of the Pt Van Der
Waals radii (3.4 Å),[78] along with π-π stacking with distances of 3.386(5) Å. Molecules are organized
pairwise in a head to tail fashion, with an alternation of Pt…Pt interactions and π-π stacking (figure S3).
11

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Figure 2. Top left, view of compound 3e, with all hydrogen atoms omitted for the sake of clarity.
Bottom left, dimer view exhibiting a 106 ° shift. Right, crystal packing of compound 3e, intermolecular
interactions have been highlighted, blue π-π interactions and orange H…F bonds. All structures are
drawn with 50% probability displacement ellipsoids.
The crystal packing of complex 3e displays interesting features. Indeed, the molecules are organized
pairwise and in a shifted manner, with roughly 106 ° (torsion angle: C1A-Pt1A-Pt1B-C1B) between
two paired molecules (figure 2). In addition, the pairs are shifted roughly by 115 ° between each other.
The assembly shows the presence of both H-F bonding and π-π stacking with distances in the range 2.35
Å - 2.66 Å and 3.36(1) Å respectively, which are smaller than the sum of the Van Der Waals radii (2.67
Å and 3.4 Å, respectively). In addition, H-F bonds are also present between adjacent pairs within the
packing. Finally, no Pt…Pt interactions are observed in the supramolecular structure.
3.4. Electrochemistry
The electrochemical behavior of the complexes was studied by cyclic voltammetry in acetonitrile
solution with n-Bu4NPF6 as supporting electrolyte, glassy carbon electrode as working electrode and
Ag/AgNO3 (10—2 M) as reference electrode. The redox data are gathered in table 2. To the exception of
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complex 3h that was not soluble in the solvent, all complexes exhibit a quasi-reversible or irreversible
reduction peak between —2.63 V and —1.99 V. For cyclometallated platinum(II) complexes, the first
reduction peak corresponds to the addition of an electron in the LUMO, which is located on the
cyclometallating ligand (C^N) according to previous work on similar platinum(II) complexes and with
the theoretical calculation (vide infra).[2,24,34,79,80] It is commonly admitted that the introduction of
electron donating or electron withdrawing groups on the phenyl moiety should have a notable impact on
the reduction potential.[2,24,34,79,80]
Figure 3. HOMO and LUMO calculated for complex 3c.
The observed reduction potentials evolve as expected with respect to the unsubstituted compound 3c
that sets our reference. HOMO and LUMO for complexes 3a-c, 3e and 3g were computed and plotted in
table S1 and in figure 3. As it was previously described in the literature,[66] the LUMO is partially
located on the position 4 of the arene. Introduction of an electron-withdrawing group, such as a nitrile,
stabilizes this orbital at this position and thus the reduction occurs at a less negative potential.
Interestingly, the introduction of a fluorine atom in position 4 in complex 3b does not shift the reduction
potential toward less negative values. Indeed, the evolution of the reduction potential follows the same
trend than the reduction potential of complex 3a. Such behavior seems to indicate that fluorine atoms
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directly attached to the phenyl ring show a predominant electron donating-mesomeric effect. The
electron-donating behavior has been previously observed in related complexes, where fluorine atom was
in position 5 of the arene (in para of the metal carbon bond), and tempers the electron withdrawing
effect,[34,81,82] but within the present series the mesomeric effect seems to prevail, regardless of the
position. From the theoretical calculation (Table S1), the introduction of the methoxy group or the
fluorine atom adds an antibonding interaction to the LUMO, which destabilizes the orbital compared to
3c.
In contrast, the theoretical calculations show that no additional contribution appears on the
positioning of the LUMO for 3e compared to 3b, but given the positions of the extra fluorine atoms
(ortho and para from M-C bond), their electron-donating effect delocalizes up to the metal center
therefore destabilizing the HOMO more than the LUMO. The ligand is mainly subject to the fluorine
inductive withdrawing effect, explaining the value of the reduction potential of 3e (—2.26 V). These
combined effects decrease the HOMO-LUMO gap of 3e.
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Table 2. Reduction potentials vs Ag/AgNO3 in acetonitrile at r.t.
Complex Eredpeak (V)
-2.63
-2.55
-2.52
-2.14
-2.26
-2.25
-1.99
3a
3b
3c
3d
3e
3f
3h
In the anodic region, the complexes display an irreversible peak around +0.5 V, which is similar to
those of other platinum complexes,[34,83,84] and is assigned to the oxidation of Pt(II) to Pt(III). Again,
the small variation of the oxidation potential can be correlated to the fact that the position 4 has a
limited impact on the HOMO level. It is well documented that cyclometallated platinum(II) complexes
tend to display quasi-reversible or irreversible oxidation and reduction peaks, as their square planar
geometry makes Pt(III) and Pt(I) susceptible to nucleophilic attack by solvent molecules, much like a
free radical.[34,80,85]
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Figure 4. Plot of the lowest absorption band in energy (red) and plot of the first emission band (CH2Cl2
at r.t., blue) in energy vs Epred and linear least-squares fit to the data.
3.5. Photophysical properties
The absorption and emission spectra are displayed in figure 5 and the photophysical data are gathered
in table 3 (77 K) and table 4. For all complexes, the absorption spectra display intense structured bands
below 350 nm, over 104 L.mol—1cm—1, which arise from the spin-allowed ligand centered (LC) π-π*
transitions.[34,41] Between 350 nm and 450 nm, moderate absorption bands, with maxima around 104
L.mol-1cm-1, absent in the spectrum of the parent ligand are observed (Figure S4). In agreement with
previous studies and the TD-DFT calculations (vide infra),[34,41] they could be ascribed to the spin-
allowed metal-to-ligand charge transfer transitions from the platinum(II) to the cyclometallating ligand
(dπ(Pt) Ï π*[C^N], 1MLCT).
The substituents do affect the lowest absorption band and the absorption can be tuned with a shift of
2000 cm—1 going from 3a (OMe) to 3g (CN). The observed variations are consistent with the fact that
the substituent in position 4 has an influence mainly on the LUMO. Indeed, the evolution of the energy
of the lowest energy absorption band evolves linearly with the reduction potential (figure 4). The
fluorine atom and the methoxy group in position 4 affect the energy of the lowest absorption band
similarly ( 25 700 cm—1). The lowest absorption energy has been successfully correlated with the
Hammett parameter of the substituents on the ligands of different metal complexes.[86,87] In our case,
the plot of the lowest absorption energy vs the Hammett parameter σ is presented in figure 6 and gives
an excellent correlation (r2 = 0.99). This value is obtained taking out of consideration complexes 3b and
3e where the fluorine atoms are directly linked to the arene, as the positive σ value of the Hammett
parameter[88] does not fully reflect the mesomeric donating effect observed in the series. This is in
agreement with the electrochemical studies that show that the donating mesomeric effect prevails over
the electron withdrawing effect in the series (vide supra).
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Figure 5. Absorption (top) and normalized emission (bottom) spectra in dilute CH2Cl2 solutions.
Emission measurement have been performed in deaerated solution.
TD-DFT calculated transitions (347 nm for 3a and 3b, 355 nm for 3c, 354 for 3e and 385 nm for 3g)
confirm the trends observed experimentally. Natural transition orbital analysis provides a reasonable
assignment of the lowest absorption bands (Table S2) and illustrates the charge transfer process from
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the platinum(II) to the cyclometallating ligand. The shapes of the orbitals involved in the transitions are
close to those of the HOMO and LUMO of all complexes. This corroborates the observed variation of
the energy of the lowest energy absorption bands following the influence of the substituent in position 4
and the reduction potential (figure 4).
The emission properties of the complexes have been studied in dilute CH2Cl2 solution under Ar and
air equilibrated conditions (figure 5 and table 3). Low temperature measurements (77 K, figure S5) have
been performed in butyronitrile except compound 3h, which was not soluble in this solvent, or in other
solvents (2-methyltetrahydrofuran, diethylether/isopentane/ethanol mixture and EtOH/MeOH mixture).
Most of the complexes display intense emission under inert atmosphere in solution at room temperature
in the visible region of the spectrum, with quantum yield values over 0.1. The emission intensities
turned out to be very sensitive to the presence of oxygen. The bimolecular rate constants for quenching
by O2 were found to be in the range 6.6 x 108 M—1 s—1 to 18.0 x 108 M—1 s—1. Lifetimes under inert
atmosphere span between 400 ns and 10 µs, confirming that the emission can be assigned to
phosphorescence.
In addition, the emission spectra are structured both at r.t. and at low temperature (77 K). At low
temperature, vibronic progression of roughly 1400 cm-1 is observed for each complex, along with
lifetimes beyond 10 µs. Based on these results and on previous studies, the emission could be assigned
to the radiative deactivation of the ligand centered triplet excited state (3IL) to the ground
state.[34,35,41] Note that complex 3e has a relatively short lifetime (432 ns) at room temperature; this is
3
likely due to a greater contribution of the MLCT to the excited state. Complexes 3b and 3c
demonstrate to be very good phosphorescent emitters with quantum yields (QY) around 0.4. The
estimations of the rate constants kr and
knr are close (3b kr = 0.2 x 104 s—1 and
knr = 17.2 x 104 s—1) and the radiative deactivation pathway is therefore more competitive.
Σ
knr from the QY and τ explain this behavior. For each of these
complexes kr and
Σ
Σ
knr = 0.3 x 104 s—1; 3c; kr = 13.6 x 104 s—1
and
Σ
However, for the other complexes of the series, the rate constant values differ by at least one fold in
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favor of
Σ
knr constant, making the complexes less emissive. Finally, the energy of the first emission
band correlates well with the reduction potentials showing again the influence of the position 4 on the
LUMO. For instance compound 3a (OMe) has a blue-shifted emission along with a more negative
potential, with respect to complex 3c. The complex bearing a nitrile group (3g) in position 4 displays a
bathochromic shift in emission with a less negative reduction potential. Finally, the graph confirms that
the fluorine atoms directly attached to the arene influence the HOMO-LUMO energy gap by mesomeric
donating effect. Complex 3e with fluorine atoms in positions 3, 4 and 5 displays a red-shifted emission
and is more easily reduced, with respect to the complex 3c. Such behavior confirms that the fluorine
atoms destabilize both the HOMO and the LUMO by mesomeric electron-donating effect (vide supra).
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Figure 6. Plot of the lowest absorption band in energy (red) and plot of the first emission band (CH2Cl2
at r.t., blue) in energy vs the Hammett parameter σ and linear least-squares fit to the data. Data for 3b
and 3e were ignored for the linear least-square fit. σp was used for all the complexes, except for 3e,
where σ is the weighted sum of σp and σm.
20

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Table 3. Absorption and emission spectroscopy data.
Solution CH2Cl2
Kr[b]/10
τ
Φ[a] x 10-2
kQO2[c] /108
Abs / nm
Σ
Knr[b]/104
4
Emission
[Air]
Complex
/nm
(M-1.s-1)
(ε / mol.L-1.cm-1 x 103)
(s-1)
[Air]
(s-1)
5.7
(ns)
251 (34.8); 299 (24.3); 313 (30.5);
350 (15.1); 362 (6.4); 389 (7.8)
479; 513*;
553; 600
10.3
[0.9]
2688
[297]
31.5
0.3
13.6
15.3
3a
3b
285 (37.3); 296 (47.2); 309 (55.8);
352 (28.3); 370 (18.6); 389 (8.2)
478; 512*;
552; 601
39.9
[2.1]
1957
[297]
0.2
289 (9.6); 299 (15.2); 313 (15.1);
369 (6.4); 403 (2.0)
486*; 523;
559
44.1
[2.4]
3241
[259]
13.6
17.2
16.1
3c
291 (15.2); 302 (21.2); 316 (24.9);
363 (10.4); 378 (0.8); 399 (0.3)
493; 530*;
573; 625
31.8
[2.7]
3900
[400]
8.1
6.7
7.2
25.6
203
10.2
8.9
3d
3e
291 (12.7); 300 (18.7); 313 (21.3);
360 (10.5); 402 (1.1)
492; 528*;
560; 617
3.2
[1.5]
475
[245]
268 (19.4); 295 (19.8); 305 (26.0);
319 (25.1); 377 (12.1); 413 (0.2)
505; 543*;
580; 639
29.9
[4.6]
4160
[527]
24.04
7.53
3f
3g
252 (37.5); 303 (19.0); 315 (25.3);
330 (22.4) ; 392 (10.2); 430 (1.3)
530; 573*;
620; 681
11.1
[5.8]
3180
[564]
3.5
0.6
28.0
9.5
6,6
9.7
293 (32.8); 321 (23.1); 337 (27.6);
378 (8.2); 420 (2.5)
526; 570*;
620; 678
6.4
[0.3]
9890
[445]
3h
* Wavelength of the emission maximum. [a] Photoluminescence were measured in dilute solution with
irradiation at 390 nm except for 3e which was irradiated at 360 nm, Ru(bpy)3Cl2 was used as standard
with φ = 0.029 in aerated water except for 3e which QY was estimated with 9,10-diphenylanthracene as
standard with φ = 0.90 in cyclohexane. [b] kr and knr are estimated from the relations kr = φ/ τ and knr =
τ-1 – kr, φ and τ values taken in Ar condition. [c]Bimolecular rate constant for quenching by O2,
estimated from the luminescence lifetimes in degassed and air-equilibrated solutions and taking [O2] =
2.2 mM in CH2Cl2 at p = 1 atm air and T = 298 K.
At 77 K, the complexes display a more pronounced structured emission with a hypsochromic shift,
which is referred as rigidochromic effect.[89] This can be construed by the restriction imposed by the
solid matrix, that concomitantly decreases the average inter-nuclear distances in the excited state and
limits the solvent reorientation in response to the polarity change of the excited state.[90] Therefore, the
rigidochromic effect is indicative of the charge transfer character of the excited state. Thus, the lowest
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triplet excited state results in a mixture of 3LC and 3MLCT, as it is commonly observed for [C^N]Ptacac
complexes.[34,35,75]
Figure 7. Solid state emission spectra under air condition
Solid state emission measurements have been performed under atmospheric conditions at r.t. and
quantum yields were measured by the absolute method using an integrating sphere.[27,91] Samples
were microcrystalline powders obtained by liquid diffusion of a counter-solvent (Et2O or C5H12) in a
concentrated dichloromethane solution of a given complex at low temperature (around —10 °C). It is
noteworthy that some strategies are efficient to significantly limit the intermolecular interaction in bulk
solid state, like introducing bulky substituents[57,59,62,92–95] or “shielding” the metal center.[96,97]
Despite the small steric hindrance induced by the ethyl group, most of the complexes are brightly
luminescent. Complexes 3g and 3h are weakly emissive (Φ <0.01, figure S5), that can be attributed to
intermolecular interactions. Regarding 3g, such result could be explained by the tendency of this
complex to have intermolecular interactions in the solid state (vide supra). In the case of complex 3h,
the loss of structure of the emission spectrum suggests that π- π stacking and/or Pt…Pt interactions (see
figure S5) occur. In stark contrast, the other complexes display significant emission with quantum
22

ACCEPTED MANUSCRIPT
efficiencies over 0.10 (to the exception of 3e which displays a quantum efficiency of 0.05), along with
lifetimes in the µs regime (see table 4 and figure 7). Complex 3b displays an impressive
photoluminescence quantum efficiency of 0.51, ranking among the best quantum yields in bulk solid yet
reported for platinum complexes.[47,49,94,96–109] Moreover, the photoluminescence quantum
efficiency in solid state is higher than in solution (≈ 0.4). Such behavior could be explained by the fact
that the ethyl acts as a lariat in solution, offering flexibility, thus pathways to non-radiative deactivation.
The emission spectra of the complexes are slightly red-shifted in comparison with the emission spectra
in solution (r.t.), showing that there could be a small π-π interaction in the solid state.[94,110] Thus, the
emission is believed to originate from the same excited state as in solution.
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Table 4. Emission data in solid state and at 77 K in butyronitrile rigid matrix
Solid state
77 K
Emission /nm
Φ[a] x 10-2
τ (µs)
16.1
15.3
Complex
Emission/nm
478; 510*; 550;
600
τ (µs)
12.7
1.66
468*; 504; 544
3a
485, 522*, 551,
609
51.0
3.26
466*, 502, 542,
586
3b
3c
3d
1.06 [b]
510; 541*; 572;
642
13.0
23.0
5.0
1.24
1,37
476*; 513; 554
13.1
18.0
479*; 518; 560;
607
505, 542*, 568
508; 540*; 586
478, 517*, 553,
584
1.30
16.5
11.3
3e
3f
489*; 528; 572;
612
507, 550*, 589
15.0
3.88
0.33 [b]
514*, 555, 600,
3g
3h
546; 557*; 623
610
<1
<1
n.d.
n.d
9.3
n.d
656
n.d
Photoluminescence quantum yields were measured in air with irradiation at 390 nm except for 3e
[a]
which was irradiated at 360 nm. * Wavelength of the emission maximum. n.d. = not determined
Quantum yields were measured using an integrating sphere apparatus
[b]
Biexponetial decays were
observed for compounds 3b and 3f, the long-lived decays represent 75% and 86% of the signal,
respectively.
4. Conclusion
In summary, we have designed and prepared a series of neutral platinum(II) complexes featuring 2-
phenylbenzimidazole as chromophoric ligand and acetylacetonate as ancillary ligand. The spectroscopic
studies revealed that this family of complexes displays good photoluminescence quantum efficiencies in
3
solution, which is attributed to a mixed LC-MLCT state. Moreover, we showed that, in the series, the
electron donating ability by mesomeric effect of the fluorine atoms (complexes 3b and 3e) dominates
the inductive withdrawing effect. The cyclic voltammetry as well as the plots of the lowest energy
absorption and the first emission band energy values against the Hammett parameter confirm these
results. The complexes display bright phosphorescence in bulk solid state and under air with quantum
efficiencies up to 0.51 which is among the best reported to date for cyclometallated platinum(II)
complexes. Our efforts are currently devoted to extend this family of ligands to other transition metal
complexes and their integration in solid state lighting device.
24

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Acknowledgements
The project was support by the Université Grenoble Alpes (UGA) and the Centre National pour la
Recherche Scientifique (CNRS). The present work was also partially supported by the Labex Arcane
(ANR-11-LABX-0003-01) and the ANR program (ANR TROICAT ANR-15-CE07-0025-02). We are
grateful to Dr. Olivier Pérez for usefull discussions, advice and help concerning the anharmonic
refinements.
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