Proton sensitive charge-transfer excited states in bis-terdentate cyclometalated Ir(III) complexes: Spectroscopic and theoretical investigation

Article abstract of DOI:10.1016/j.ica.2017.10.018

Two bis-terdentate cyclometalated Ir(III) complexes with polypyridyl (N^N^N) and cyclometalated (C^N^C) ligands [Ir-Py]⁺ and [Ir-Py-Me]²⁺ have been synthesized and characterized. Their absorption and emission properties have been exa

Full text of DOI:10.1016/j.ica.2017.10.018

Accepted Manuscript
Proton Sensitive Charge-Transfer Excited States in bis-Terdentate Cyclometa-
lated Ir(III) Complexes: Spectroscopic and Theoretical Investigation
Alexandre Jacques, Thomas Auvray, Robin Bevernaegie, Frédérique Loiseau,
Mihaela Cibian, Garry S. Hanan, Andrée Kirsch-De Mesmaeker, Benjamin
Elias
PII:
S0020-1693(17)31384-1
https://doi.org/10.1016/j.ica.2017.10.018
ICA 17945
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Accepted Date:
6 September 2017
16 October 2017
Please cite this article as: A. Jacques, T. Auvray, R. Bevernaegie, F. Loiseau, M. Cibian, G.S. Hanan, A. Kirsch-
De Mesmaeker, B. Elias, Proton Sensitive Charge-Transfer Excited States in bis-Terdentate Cyclometalated Ir(III)
Complexes: Spectroscopic and Theoretical Investigation, Inorganica Chimica Acta (2017), doi: https://doi.org/
10.1016/j.ica.2017.10.018
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Proton Sensitive Charge-Transfer Excited States in
bis-Terdentate Cyclometalated Ir(III) Complexes :
Spectroscopic and Theoretical Investigation
Alexandre JACQUES a, Thomas AUVRAY b, Robin BEVERNAEGIE a, Frédérique LOISEAU c,
Mihaela CIBIAN b, Garry S. HANAN b, Andrée KIRSCH – De MESMAEKER d and Benjamin
ELIAS a*
a) Université catholique de Louvain (UCL), Institut de la Matière Condensée et des
Nanosciences (IMCN), Place Louis Pasteur 1, box L4.01.02, B-1348 Louvain-la-Neuve,
Belgium
b) Département de Chimie, Pavillon J.-A. Bombardier, 5155 Chemin de la Rampe, Université de
Montréal, Montréal, Québec, Canada, H3T 2B1
c) Département de Chimie Moléculaire, Université Grenoble-Alpes, CNRS UMR 5250,
BP53 38041 Grenoble, France
d) Chimie Organique et Photochimie, Université libre de Bruxelles, CP 160/08, 50 avenue F.D.
Roosevelt, B-1050 Bruxelles, Belgium.
Corresponding Author
* Prof. Benjamin ELIAS – benjamin.elias@uclouvain.be
1

ABSTRACT
Two bis-terdentate cyclometalated Ir(III) complexes with polypyridyl (N^N^N) and
cyclometalated (C^N^C) ligands [Ir-Py]+ and [Ir-Py-Me]2+ have been synthesized and
characterized. Their absorption and emission properties have been examined, more particularly
in MeCN as a function of the addition of different acids. Depending on the acid strength and
concentration, two distinct effects have been observed. For acetic acid at high concentration or
trifluoroacetic acid (TFA) at low concentration, the pyridine moiety of [Ir-Py]+ is protonated; its
spectroscopic behaviour is then similar to that of [Ir-Py-Me]2+. Moreover at higher
concentration in TFA, the methoxy group of both complexes is protonated. The spectroscopic
and electrochemical data as well as the DFT and TD-DFT calculations support contributions of
charge-transfer excited states in absorption and emission from metal-ligand/ligand based
HOMOs to ligand-based LUMOs, i.e. MLLCT.
KEYWORDS
Iridium – protonation – MLLCT – terdentate – photophysics
ABBREVIATIONS
[Ir-Py]+ = [Ir(4′-Py-tpy)(4′-MeO-tppy)]+
[Ir-Py-Me]2+ = [Ir(4′-Me-Py-tpy)(4′-MeO-tppy]2+
[Ir-Py-H]2+ = [Ir(4′-H-Py-tpy)(4′-MeO-tppy]2+
[Ir-Py-H-Me]3+ = [Ir(4′-Me-Py-tpy)(4′-MeOH-tppy]3+
[Ir-Py-H2]3+ = [Ir(4′-H-Py-tpy)(4′-MeOH-tppy]3+
2

1. INTRODUCTION
Transition metal complexes have attracted the interest of increasing numbers of researchers
these past decades due to their exceptional properties.[1] Among the available transition metal
ions, Ir(III) is becoming one of the most interesting elements for designing new complexes
because of its strong spin-orbit coupling, its high quantum yield of luminescence with organic
ligands and its fine-tunable properties by ligand modifications.[2] Therefore iridium (III)
complexes are useful for the development of supramolecular devices, with photo-induced
energy[3, 4] and/or electron transfer.[5-7] More recently, the incorporation of strong donor
ligands has been exploited to red-shift the absorption and emission bands in transition metal
complexes, such as Ru(II),[8-14] Re(I)[15] and Ir(III)[16] complexes. Generally greater effects
result by using anionic cyclometalating ligands, such as derivatives of 2-phenylpyridine, that
produce Ir(III) complexes like [Ir(C^N)2(N^N)]+ (C^N = cyclometalated ligand, N^N =
polypyridine). These complexes have attracted much attention because of their potential
applications in various fields such as photo-catalysis,[17-19] dye-sensitized solar cells,[20]
electroluminescence and electrochemiluminescence,[21, 22] DNA sensors[23] and cellular
imaging.[24, 25] Compared to cyclometalated Ir(III) tris-bidentate complexes, the bis-terdentate
equivalents have been less investigated mainly due to difficulties of synthesis and
purification.[26, 27] However, these compounds exhibit several important advantages. Ir(III)
terpyridyl complexes derivatized on the 4′ -position by suitable substituents circumvent the
occurrence of geometrical isomers as with tris-bidentate arrangements. This problem has been
well documented in Ru(II) chemistry of bidentate and tridendate ligands.[28]Moreoverthe Ir(III)
complexes of tridentate ligands also allow the design of systems for linear charge separation by
photo-induced intramolecular electron transfer. In the literature, cyclometalated Ir(III) complexes
involving two tridentate ligands, one N^N^N tpy derivative (tpy = 2:2′ ,6′ :2′ ′ -terpyridine) and
3

one C^N^C dppy derivative (dppy = 2,6-diphenylpyridine), have been first reported by Scandola
et al.[29] Based on computational results, the predominant emitting excited state has been
attributed to a Ligand-to-Ligand charge-transfer state (LLCT) from the cyclometalated iridium
fragment to the tpy ligand.[30] However, experimental proof of LLCT transitions are not always
easy to obtain. Because the photo-redox properties of this type of complexes and the
understanding of their origins are essential for potential applications, it has become of prime
importance to design and study other Ir(III) complexes constructed with different derivatives of
tridentate ligands. Such studies could allow gathering new data on the complicated photophysics
of this type of complexes.
Therefore in this work, we report the synthesis and characterization of two new Ir(III)
complexes obtained from the coordination of a terpyridyl ligand and a triphenylpyridyl ligand
i.e. [Ir(4′ -Py-tpy)(4′ -MeO-tppy)]+ (4′ -Py-tpy = 4′ -(4-pyridinyl)-2,2′ ;6′ ,2′ ′ terpyridine and 4′ -
MeO-tppy = 2,6-diphenyl-4-(4′ -methoxyphenyl)pyridine) and its N-methylated equivalent i.e.
[Ir(4′ -Me-Py-tpy)(4′ -MeO-tppy)]2+ (4′ -Me-Py-tpy
=
4′ -(4-methyl-pyridinio)-2,2′ ;6′ ,2′ ′ -
terpyridine) termed [Ir-Py]+ and [Ir-Py-Me]2+, respectively (Figure 1). The impact of acidity on
the spectroscopic properties of both complexes in MeCN is analyzed from an experimental and
theoretical approach and provides arguments in favor of contributions of Metal-Ligand/Ligand to
Ligand centered charge-transfer excited states.
2. EXPERIMENTAL SECTION
2.1. Synthetic procedures
4′ -((4-Pyridinyl)-2,2′ ;6′ ,2′ ′ terpyridine) (4′ -Py-tpy) was prepared following the previously
described methods from 2-acetyl-pyridine and 4-pyridinecarboxaldehyde.[31, 32] (4′ -(4-Methyl-
pyridinio)-2,2′ ;6′ ,2′ ′ -terpyridine) ([4′ -Me-Py-tpy]+) was synthetized by a selective methylation
of 4′ -Py-tpy as previously described.[33]2,6-Diphenyl-4-(4′ -methoxyphenyl)pyridine) (4′ -MeO-
4

tppy) was prepared by grinding two equivalents of acetophenone and NaOH in the presence of p-
anisaldehyde followed by cyclization with ammonium acetate.[34]
2.2. Materials and methods
1H NMR (300 and 500 MHz) experiments were performed in CD3CN on a Bruker AC-300
Avance II (300 MHz) or on a Bruker AM-500 (500 MHz). The chemical shifts (given in ppm)
were measured versus the residual peak of solvent as internal standard. HRMS were recorded on
a Q-Extractive orbitrap from ThermoFisher. The samples were ionized by ESI (capillary
temperature: 250 °C, vaporizer temperature: 250 °C, sheath gas flow rate: 20). UV-vis
absorption spectra were recorded on a Shimadzu UV-1700. Room temperature fluorescence
spectra were recorded on Varian Cary Eclipse. Complexes were excited at 520 nm.
Luminescence lifetimes were measured with a modified Applied Photophysics laser kinetic
spectrometer by exciting the samples at 355 nm with a Nd-YAG pulsed laser (Continuum NY
61-10). Emitted light as a function of time was detected with a R-928 Hamamatsu
photomultiplier tube the output of which was applied to a digital oscilloscope (Hewlett-Packard
HP 54200A) interfaced with a Dell Dimension DE051 computer. The signals were averaged over
at least 16 shots and the baseline was corrected. Igor 6.1 software was used for the decay
analysis. Transient absorption spectra were acquired using a LP920K system from Edinburgh
Instruments. Excitation was carried out from the third-harmonic (355 nm) of a Brilliant-Quantel
Nd:YAG Laser at 6 Hz. A Xe900 pulsed Xenon Lamp is used as probe source. The photons were
dispersed using monochromator, transcripted by a R928 (Hamamatsu) photomultiplicator and
recorded on a TDS3012C (Tektronix) oscilloscope. Cyclic voltammetry was carried out in a one-
compartment cell, using a glassy carbon disk working electrode (approximate area = 0.03 cm2), a
platinum wire as counter electrode and an Ag/AgCl electrode as reference. The potential of the
working electrode was controlled by an Autolab PGSTAT 100 potentiostat through a PC
5

interface. The cyclic voltammograms were recorded with a sweep rate of 100 or 400 mV s-1 in
dried acetonitrile (Acros, HPLC grade). Tetrabutylammonium perchlorate (0.1 M) was used as
supporting electrolyte and the samples were purged by argon before each measurement. DFT
calculations were performed using Gaussian 09.[35] The B3LYP method[36-38] was used to
carry the DFT[39-42] and TD-DFT[43-45] calculations. The 6-31G* basis set[46] was used for
H, C, N and O, and the VDZ (valence double ζ) with the SBKJC effective core potential basis
set[47-50] was used for Ir. Geometry optimizations were conducted without symmetry
constraints. Frequency calculations on each optimized structure confirmed that energy minima
had been reached in all cases. The energy, oscillator strength, and related MO contributions for
the 100 lowest singlet–singlet and 10 lowest singlet–triplet excitations were obtained from the
TD-DFT/singlets and the TD-DFT/triplets output files, respectively, for the S0-optimized
geometry. GaussView3.0.9[51], GaussSum 3.0[52] and Chemissian 4.33[53] software were used
for data analysis, visualization and surface plots. All calculations were performed with MeCN as
solvent by use of the polarized continuum (PCM) solvation model as implemented in Gaussian
09.[54, 55]
-
[Ir(4′-Py-tpy)(4′-MeO-tppy)]+.NO3 ([Ir-Py]+)
(NH4)3IrCl6 (100 mg, 0.21 mmol) was added to a solution of 4′ -Py-tpy (65 mg, 0.21 mmol) in
DMF (3 mL). The mixture was heated at 120°C for 6h. The dark red precipitate was isolated and
washed with water (twice), EtOH and CHCl3.The crude [(4′ -Py-tpy)-IrCl3] (100 mg, 0.16 mmol),
4′ -MeO-tppy (55 mg, 0.16 mmol) and AgNO3 (136 mg, 0.80 mmol) were refluxed in degassed
ethylene glycol (4 mL) in the dark and under Ar for 16h. On cooling, the mixture was filtered
through Celite to remove AgCl. The Celite plug was washed with MeOH, which was
subsequently evaporated. Water was added and the aqueous layer was extracted with CHCl3 (3
times). The organic solutions were combined and evaporated to dryness. The crude product was
6

purified by column chromatography on SiO2 (MeCN/saturated aqueous KNO3 (10/1)). The first
eluted orange band was recovered and the solvent evaporated. The red powder was washed with
water and dried under vacuum to afford [Ir-Py]+ (55 mg, 29 %). 1H NMR (CD3CN, 300 MHz) : δ
(ppm), 8.99 (s, 2H, H3′ and H5′ of tpy-C6H5N), 8.93 (d, 2H, Ha of tpy-C6H5N, JHa-Hb = 6.1 Hz), 8.66
(d, 2H, H3 of tpy-C6H5N, JH3-H4 = 7.6 Hz), 8.27 (s, 2H, H3′ and H5′ of tppy-OMe), 8.11-8.07 (m,
4H, Hb of tpy-C6H5N and H3 of tppy-OMe), 8.01-7.95 (m, 4H, Hb of tppy-OMe and H4 tpy-
C6H5N), 7.88 (d, 2H, H6 of tpy-C6H5N, JH6-H5 = 5.8 Hz), 7.30-7.25 (m, 2H, H5 of tpy-C6H5N), 7.24
(d, 2H, Ha of tppy-OMe, JHa-Hb = 8.8 Hz ), 6.99 (td, 2H, H4 of tppy-OMe, JH4-H5 (H3) = 7.3 Hz, JH4-H6
=
1.0 Hz), 6.75 (td, 2H, H5 of tppy-OMe, JH5-H4 (H6) = 7.3 Hz, JH5-H3 = 1.2 Hz), 6.23 (dd, 2H, H6 of
13
tppy-OMe, JH6-H5 = 7.3 Hz, JH6-H4 = 1.0 Hz), 3.95 (s, 3H, OMe). C NMR (CD3CN, 125 MHz) : δ
(ppm), 167.0, 165.2, 158.3, 153.6, 153.4, 151.7, 151.5, 148.3, 138.9, 134.9, 131.3, 130.7, 130.6,
128.8, 127.3, 126.4, 125.4, 123.6, 118.7, 116.0, 115.1, 56.7. HRMS (ESI): m/z calculated for
[C44H31N5O1193Ir – NO3]+, 838.2152; found: 838.2146. Elem. Anal. Calcd for [C44H31N5OIr]PF6: C,
53.77; H, 3.18; N, 7.12. Found: C, 53.94; H, 2.97; N, 7.30.
[Ir(4′-Me-Py-tpy)IrCl3]+.I-
[4′ -Me-Py-tpy]+.I- (50 mg, 0.11 mmol) and (NH4)3IrCl6 (53 mg, 0.11 mmol) were heated at
120°C for 6h in DMF (3 mL). The dark red precipitate was isolated and washed with water
(twice), MeCN and Et2O. Yield: 34 mg (42%). 1H NMR (DMSO-d6, 300 MHz) : δ(ppm), 9.51 (s,
2H, H3′ and H5′), 9.34 (d, 2H, Ha, JHa-Hb = 6.6 Hz), 9.23 (dd, 2H, H6, JH6-H5 = 5.6 Hz, JH6-H4 = 1.2 Hz),
9.01 (d, 2H, Hb, JHb-Ha = 6.6 Hz), 8.98 (d, 2H, H3, JH3-H4 = 7.9 Hz), 8.38 (td, 2H, H4, JH4-H3(H5)) = 8.0
Hz, JH4-H6 = 1.2 Hz), 8.03 (m, 2H, H5), 4.45 (s, 3H, Me). MS (ESI): m/z calculated for
[C21H17N4Cl3193Ir – I]+, 623.01; found: 622.92.
-
[Ir(4′-Me-Py-tpy)(4′-MeO-tppy]2+.2NO3 ([Ir-Py-Me]2+)
7

This complex was prepared using the same procedure as that used for complex ([Ir-Py]+)
except that [Ir(4′ -Me-Py-tpy)IrCl3]+.I- was used instead of [(4′ -Py-tpy)-IrCl3]. Yield: 10 mg
1
(25%). H NMR (CD3CN, 500 MHz) : δ(ppm), 9.16 (s, 2H, H3′ and H5′ of tpy-C6H5N-Me), 8.96
(d, 2H, Ha of tpy-C6H5N-Me, JHa-Hb = 6.7 Hz), 8.78 (d, 2H, Hb of tpy-C6H5N-Me, JHb-Ha = 6.8 Hz),
8.75 (d, 2H, H3 of tppy-OMe, JH3-H4 = 8.0 Hz), 8.29 (s, 2H, H3′ and H5′ of tppy-OMe), 8.10 (d, 2H,
Hb of tppy-OMe, JHb-Ha = 8.8 Hz), 8.04-8.00 (m, 4H, H4 and H3 tpy-C6H5N-Me), 7.91 (dd, 2H, H6
of tpy-C6H5N-Me, JH6-H5 = 5.8 Hz, JH6-H4 = 1.1 Hz), 7.31 (ddd, 2H, H5 of tpy-C6H5N-Me, JH5-H4 = 7.3
Hz, JH5-H6 = 5.8 Hz, JH5-H3 = 1.3 Hz ), 7.24 (d, 2H, Ha of tppy-OMe, JHa-Hb = 8.8 Hz ), 6.99 (td, 2H, H4
of tppy-OMe, JH4-H5 (H3) = 7.3 Hz, JH4-H6 = 1.1 Hz), 6.75 (td, 2H, H5 of tppy-OMe, JH5-H4 (H6) = 7.3 Hz,
JH5-H3 = 1.1 Hz), 6.21 (dd, 2H, H6 of tppy-OMe, JH6-H5 = 7.3 Hz, JH6-H4 = 0.8 Hz), 4.44 (s, 3H, Me)
13
3.95 (s, 3H, OMe). C NMR (CD3CN, 125 MHz) : δ(ppm), 166.5, 164.7, 162.7, 157.7, 153.5,
153.1, 151.2, 147.8, 147.3, 139.0, 134.7, 131.1, 130.3, 128.7, 127.1, 126.9, 126.4, 125.3, 124.0,
115.7, 114.9, 56.4, 49.2. HRMS (ESI): m/z calculated for [C45H34N6O4193Ir – NO3]+, 915.2265;
found: 915.2273, m/z calculated for [C45H34N5O193Ir – 2NO3]2+, 426.6191; found: 426.6192. Elem.
Anal. Calcd for [C45H34N5OIr]P2F12: C, 47.29; H, 3.00; N, 6.12. Found: C, 48.27; H, 3.05; N, 5.54.
3. RESULTS AND DISCUSSION
3.1. Synthesis and characterization
The synthesis of the two Ir(III) complexes (Figure 1) was achieved in two steps from the Ir(III)
chloride salt according to methodologies previously described;[29, 30] they were purified by
column chromatography (see experimental section). Both complexes were unambiguously
1
13
characterized by H and C NMR spectroscopy and high-resolution mass spectrometry (HRMS)
(see experimental section).
8

Figure 1. Structures of [Ir(4′ -Py-tpy)(4′ -MeO-tppy)]+ ([Ir-Py]+) and [Ir(4′ -Me-Py-tpy)(4′ -MeO-
tppy)]2+ ([Ir-Py-Me]2+).
The redox potentials of the two complexes have been determined from cyclic voltammetry in
order to obtain more information on the relative energy of their HOMO and LUMO levels. The
data are gathered in Table 1. Both complexes display one reversible oxidation wave at +
1.16/1.19 V vs. Ag/AgCl (Figures S1-S2). These similar oxidation potentials are in agreement
with a similar oxidation process, which indicates that the HOMOs have a similar energy level in
the two compounds. The potential values are also in a range similar to those of other
cyclometalated complexes[29, 30]. The oxidation process most likely involves the
cyclometalated iridium fragment (Ir-tppy) as also proposed in the literature for the non-
substituted carbometalated Ir compound.[30]
At negative potentials, there are two reduction waves for [Ir-Py-Me]2+, the first one at -0.71 V
vs. Ag/AgCl (Figure 2) is ascribed to the reduction of the N-methyl-pyridinium substituent
9

similar to the attribution of the first reduction wave of [Pt(4′ -Me-Py-tpy)(Cl)](ClO4)2.H2O (-0.45
vs. Ag/AgCl).[33] As the potential of this first wave can be related to the LUMO level of the
complex, this means that the LUMO of [Ir-Py-Me]2+ is much more stabilized than that of [Ir-
Py]+. The second reduction wave of [Ir-Py-Me]2+ at -1.14 V vs. Ag/AgCl is ascribed to the
reduction of the tpy motif, as for [Ir-Py]+, with only a weak effect caused by the substituent
(Table 1 and Figures 2 and S3).
Figure 2. Reduction voltammogram of [Ir-Py-Me]2+ (10-5 M) in MeCN (0.1 V/s) with Bu4NClO4
0.1 M as supporting electrolyte.
3.2. Absorption and emission spectroscopy
The absorption and emission spectra in MeCN are shown in Figure 3. The data are gathered in
Table 1 for MeCN. Intense absorption bands between 280-330 nm are ascribed to Ligand
Centered transitions (LC), based on previous studies of cyclometalated Ir(III) complexes[29]; as
shown in Table 1, they are not affected upon methylation of the pyridine substituent.
Table 1. Electrochemical and spectroscopic data for the complexes [Ir-Py]+ and [Ir-Py-Me]2+.
Complexes
[Ir-Py]+
[Ir-Py-Me]2+
a
Eox (V
1.16 (r, ΔEp = 1.19 (r, ΔEp =
70 mV) 62 mV)
vs.Ag/AgCl)
-1.16 (r, ΔEp = -0.71 (r, ΔEp
72 mV) = 73 mV) ;
Ereda (V
10

-1.14 (r, ΔEp
vs.Ag/AgCl)
= 71 mV)
514 (5.5), 445 540
(6.7),
(9.3),
λ
max abs,nmb (ε, x
10-3 M-1cm-1)
(9.1),
324 476
283 327 (31.8),
281 (42.2)
(41.6),
(53.6)
λ
max Em under air
c
705
673
0.22
-
at 298 K
λ
max Em at 77 K
720
τ (s) under air
at 298 K
c
-
τ (s) under
argon at 298 K
c
1.05
-
d
c
ф(Air)
0.0009
0.0041
3.9
-
d
c
ф(Ar)
-
kr (x 10-3 s-1)e
-
c
aElectrochemical data were recorded at room temperature in MeCN ([Complex] = 10-5 M) with
Bu4NClO4 0.1 M as supporting electrolyte; (r) = reversible, (ΔEp) (peak-to-peak difference).
c
d
bMeasured in MeCN. Non emissive. Quantum yield was measured under air and argon using
[Ru(bpy)3]2+ as reference ф = 0.018, ф = 0.094 in MeCN,[56] λ = 450 nm, 293 K. eRadiative
air
argon
Exc
rate constant under argon at 298 K (kr=ф/τ).
11

Figure 3. Absorption spectra of [Ir-Py]+ (green) and [Ir-Py-Me]2+ (red) in MeCN; emission of
[Ir-Py]+ (dotted green line) under air at room temperature and excitation at 520 nm. Inset:
emission spectra at 77 K of [Ir-Py]+ (green), [Ir-Py-Me]2+ (red) and [Ir-Py-H]2+ (blue) in a
mixture EtOH/MeOH (4/1)( λ = 520 nm).
exc
Absorption bands also appear in the visible. In contrast to the LC transitions in the UV, these
bands are affected by methylation of the pyridine substituent. Thus, a red shift of ca. 30 nm is
observed upon methylation of [Ir-Py]+ into [Ir-Py-Me]2+ (Figure 3). Regarding the luminescence
properties at 298 K, only the [Ir-Py]+ is emissive (Figure 3). A shoulder in the emission
spectrum might indicate the contribution of more than one excited state. At 77 K, the
luminescence of [Ir-Py]+ in MeOH/EtOH is shifted to the blue (Figure 3, inset). For [Ir-Py-
Me]2+ in MeOH/EtOH at 77K a weak emission with a maximum around 725 nm can be detected
(Figure 3, inset). Pulsed laser induced transient absorption spectra of [Ir-Py]+ have also been
recorded in MeCN under Ar (Figure S4). Transient absorption (TA) is observed around 700 nm,
concomitant with the bleaching of the ground state. The associated lifetime of the excited state
obtained from the TA measurements is 216 ns, in good agreement with that of the emission
lifetime (Table 1). Strikingly, the transient absorption is reminiscent of the reduced methyl
12

viologen absorption[57]. All these observations lead to different conclusions. (i) For [Ir-Py]+, the
unstructured room temperature emission, which is blue-shifted at 77 K, is characteristic of a CT
emission. (ii) The fact that a very weak emission for [Ir-Py-Me]2+ can be detected at 77 K
around 725 nm (Figure 3, inset) indicates also the participation of a luminescent excited state for
the methylated compound. This excited state is however affected by important radiationless
deactivation at 298 K, probably caused by an important stabilization of this state and thus
governed by the energy gap law[58], (iii) An emission lifetime of the order of 1 s with kr values
of ca. 104 s-1 for [Ir-Py]+ is compatible with CT excited state lifetimes.[59]
The spectroscopy in absorption and emission is thus compatible with the attribution of the
lowest energy transitions to CT transitions for both Ir complexes. Based on the electrochemistry,
the transient absorption results and data in the literature[29], the CT would take place from the
whole fragment C-N-C-Ir towards the tpy ligand with possibly participation of more than one CT
for [Ir-Py]+.
3.3. Acid-base titration studies
In order to check further the CT characteristics in absorption and emission of [Ir-Py]+, acid-
base titration experiments were performed with this complex. Indeed, if MLLCT/LLCT
transitions are predominant, the corresponding absorption and emission energy should be greatly
affected by protonation of the pyridine group of the tpy. In contrast no protonation effect should
be observed with [Ir-Py-Me]2+. Therefore acid-base titrations experiments were also carried out
with the methylated complex for comparison purposes. MeCN was used as solvent in both cases,
with two acids that have different pKa values, acetic acid (pKa = 23.51)[60] and trifluoroacetic
acid (TFA) (pKa = 12.65).[61]
3.4. Spectroscopic behaviour upon addition of acetic acid or TFA at low concentration
(i.e. in the mM range in TFA)
13

Modifications of absorption and emission bands are shown in Figure 4 for [Ir-Py]+ upon
addition of acetic acid in MeCN. For [Ir-Py-Me]2+ (Figure S5) in the same conditions, no change
is observed, as expected for the methylated compound. For [Ir-Py]+, there is a clear
bathochromic shift of the visible band upon addition of acetic acid. The presence of isosbestic
points (387 nm, 455 nm) indicates at least the existence of two species in equilibrium. No shift is
noticed for the higher energy absorption bands (283 nm and 324 nm) but the absorbance slightly
decreases. The emission of [Ir-Py]+ dramatically decreases with the addition of acetic acid.
These absorption and emission modifications are due to the gradual protonation of the pyridine
substituent of [Ir-Py]+, which once protonated, becomes a better acceptor in the CT transition
with a concomitant decrease in the LUMO energy. No further change by supplementary addition
of acetic acid can be observed with the exception of slight solvatochromic effects caused by a too
large amount of acid in MeCN.
Figure 4. Absorption and emission (inset) spectral changes for a 2.2 x 10-5 M solution of [Ir-
Py]+ in MeCN for increasing concentrations of acetic acid (0.5 M, 0.9 M, 1.4 M, 1.8 M , 2.2 M,
2.6 M, 2.9 M, 3.3 M, 3.6 M).
14

The addition of low concentrations of TFA (mM range) induces similar changes in the [Ir-Py]+
spectrum as those observed with acetic acid in absorption (with the same isosbestic points) and
decreases the emission of [Ir-Py]+ (Figures 4 and 5). Moreover, as in the case of acetic acid,
these low TFA concentrations have no effect on the absorption of [Ir-Py-Me]2+ (Figure S6) as
expected. The effect of a stronger acid in mM concentration range is thus the same as with the
acetic acid.
The spectroscopic properties of protonated [Ir-Py]+ (with acetic acid or TFA in the mM range)
look very much like those of the methylated complex, i.e. a bathochromic shift in absorption
(Figure S7) and disappearance of emission in MeCN at RT. At 77 K in EtOH/MeOH in strong
acidic conditions (100 % protonation with TFA) a slight emission (Figure 3, inset) exists with the
same emission maximum as [Ir-Py-Me]2+. These similar properties with a very poor emission
can be attributed to the important stabilization of the excited state and to a decrease of emission
due to the energy gap law effect[58].
From the absorption and emission changes of [Ir-Py]+ with the TFA addition in mM range of
concentration, an association constant Ka1 between TFA (Figure 5, insets) and the complex
(reaction 1) could be determined by the Benesi-Hildebrand method[62] (equation 1):
15

16

where A0 is the absorbance of the [Ir-Py]+ without acid, A is the absorbance at different TFA
concentrations at 525 nm, Ka1 is the association constant, and Δε [Ir-Py]+ is the difference between
the molar extinction coefficient of [Ir-Py]+ and its associated form with TFA. The adjustment to
a linear plot obtained by the Benesi-Hildebrand relation (Figure 5, insets) is in agreement with a
1:1 binding stoichiometry (reaction 1). The corresponding values of Ka1 calculated from the
absorption and emission data are 1400 ± 200 M-1 and 1530 ± 75 M-1, respectively. These similar
values indicate that the red-shift of the absorption and loss of emission correspond to the
appearance of the red-absorbing and non-luminescent protonated [Ir-Py]+ form, i.e. [Ir-Py-H]2+.
17

Figure 5. Absorption (A) and emission (B) changes for a 2.6 x 10-5 M solution of [Ir-Py]+ in
MeCN at low and increasing concentrations of trifluoroacetic acid (0.24 mM, 0.47 mM, 0.69
mM, 0.89 mM , 1.09 mM, 1.27 mM, 1.45 mM, 1.62 mM). Inset: Corresponding Benesi-
Hildebrand plot with CF3COOH when monitoring absorbance changes at 525 nm (A) and
emission changes at 704 nm (B).
3.5. Spectroscopic behaviour upon addition of TFA at higher concentrations
18

When higher concentrations of TFA (from 0.1 M range till 0.6 M) are used, both absorption
spectra, i.e. that of [Ir-Py]+ and [Ir-Py-Me]2+ (Figure 6), seem to be modified with the addition
of TFA. An absorption band appears at ca. 360 nm and there is a hypochromic effect in the
visible. The spectral modification of both complexes could be attributed to protonation of the
common fragment, i.e. the 4′ -MeO-tppy. At such high proton concentrations, the MeO group of
1
the ligand 4′ -MeO-tppy can indeed be protonated. We have checked by H NMR analysis that
for these high TFA concentrations, the complex does not decompose (see Supporting
Information). The modifications of absorption introduced by biprotonation are in agreement with
the attribution of the lowest energy transitions to a mixture of different CT transitions. Indeed,
the protonation of the MeO group induces a stabilization of the HOMO orbital centered on this
fragment and a clear hypsochromic shift of the visible band to form a new absorption band at ca.
360 nm. However, if the HOMO was exclusively centered on the ligand 4′ -MeO-tppy, the
complete protonation of the MeO group should induce a total disappearance of the absorption
bands in the 400-600 nm region. But, as shown on Figure 6B, some absorption remains in the
visible, i.e. at 430 nm (Figure 6B, inset). These transitions should be probably attributed to other
CT transitions from the fragment C-N-C-Ir without the participation of the MeO group towards
the tpy ligand.
DFT and TD-DFT calculations were performed with [Ir-Py]+ and [Ir-Py-Me]2+ in order to
better understand the nature of the different lowest energy transitions in the protonated and
unprotonated forms.
19

Figure 6. Absorption changes for a 2.6 x 10-5 M solution of [Ir-Py]+ (A) and a 3.6 x 10-5 M
solution of [Ir-Py-Me]2+ (B) in MeCN with increasing concentrations of TFA (0.10 M, 0.19 M,
0.28 M, 0.37 M, 0.45 M, 0.53 M, 0.60 M) . Inset for 6B: titration curve by monitoring the
absorbance at 430 nm.
3.6. Computational studies
The ground and vertical excited state electronic structures were investigated by the means of
DFT/TD-DFT calculations using the B3LYP method[36-38], the 6-31G* basis set[46] for H, C,
N and O and the VDZ (valence double ζ) with the SBKJC effective core potential basis set[47-
50] for Ir. Solvent (MeCN) was included by a polarizable continuum model. The computed
20

HOMO, HOMO-1, HOMO-2, LUMO and LUMO+1 for [Ir-Py]+, [Ir-Py-H]2+, [Ir-Py-Me]2+,
[Ir-Py-H2]3+ and [Ir-Py-H-Me]3+ are depicted in Table 2 while further details on electronic and
geometrical structure can be found in Tables S1-S15. Figure 7 compares the energy levels of
selected occupied and unoccupied molecular orbitals (MOs) of the S0 state. The contributions of
the Ir(III) as well as those of the two types of ligands are represented with different colors and
the percentages of these contributions can be found in Tables S1-S5. The calculated lowest
energy transitions follow the trend observed in the electrochemical measurements as well as in
the absorption spectra, with [Ir-Py]+ exhibiting a larger energy gap than [Ir-Py-Me]2+.
Methylation stabilizes the LUMO levels, as suggested by the higher reduction potential of [Ir-
Py-Me]2+, while the HOMO levels are almost unaffected.
As also summarized in Figures S8, S9 and Tables S6, S8, the TD-DFT results show that the
lowest energy transitions should have a charge transfer characteristic as they correspond to
transitions involving orbitals centered on the Ir-tppy fragment to an orbital centered on the tpy
ligand.
It is difficult to clearly define these CT transitions as pure Metal-Ligand to Ligand (MLLCT),
due to the participation of the Ir and carbometalated ligand, or as Ligand to Ligand (LLCT),
because the % of participation of the carbometalated ligand is higher than that of the Ir. These
cases are more aptly seen as being of mixed origins and are due to the fact that there are different
CT transitions of very close energy for [Ir-Py]+. Although the LUMOs are clearly tpy centered,
there is a slight difference between [Ir-Py]+ and the methylated derivative in the electron
accepting tpy. For [Ir-Py]+ the LUMO and LUMO+1 participating in the lowest energy
transition are mainly tpy type, whereas for [Ir-Py-Me]2+ and [Ir-Py-H]+, the participating
LUMO is localized not only on the tpy but also on the pyridinium group as suggested by the
presence of two reduction waves in the voltamogram of [Ir-Py-Me]2+. Moreover the
21

computational studies performed with the protonated [Ir-Py]+ complex show the same
characteristics in the results as with the methylated [Ir-Py]+ (see the comparisons in Figure 7 and
Tables 2 and S1-S15), i.e. a bathochromic shift of absorption as compared to [Ir-Py]+ and also a
more important participation of the pyridinium group in the LUMO (Table 2) of the tpy ligand in
the CT transition.
These conclusions from computational calculations agree very well with the electrochemical
data and spectroscopic behavior before and after the first protonation.
For further understanding of the system, the protonated forms of [Ir-Py-Me]2+ and [Ir-Py-
H]2+, namely [Ir-Py-H-Me]3+ and [Ir-Py-H2]3+ were also studied by DFT/ TD-DFT. The energy
levels of selected occupied and unoccupied molecular orbitals (MOs) of the S0 state are presented
on Figure 7. As a consequence of the protonation of the MeO-tppy ligand, the HOMO and
HOMO-1 levels are slightly stabilized. This stabilization leads to a small increase in the band
gap thus, as observed in TD-DFT studies (Figure 7, Table S1-15), a blue-shift of the low energy
MLLCT absorption band. The situation is however more dramatic for the HOMO-2 levels that
are clearly strongly stabilized (Figure 7 and Table 2). Thus, the transitions involving this level
are shifted to higher energy leading to the hypochromic effect and the appearance of a band in
near-UV at high concentrations of TFA. These observations from computational calculations
bring further evidence that absorption bands in the visible for [Ir-Py]+ and [Ir-Py-Me]2+ involve
a mixture of CT transitions from the cyclometalated iridium fragment (Ir-tppy) towards the tpy
ligand.
Table 2. Representation of the molecular orbitals from HOMO-2 through LUMO+1 for [Ir-Py]+,
[Ir-Py-H]2+, [Ir-Py-Me]2+, [Ir-Py-H2]3+ and [Ir-Py-Me-H]3+.
[Ir-Py]+
[Ir-Py-H]2+
[Ir-Py-Me]2+
[Ir-Py-H2]3+
[Ir-Py-H-Me]3+
22

LUMO+1 (-2.5 eV)
LUMO+1 (-2.7 eV) LUMO+1 (-2.68 eV) LUMO+1 (-2.68 eV) LUMO+1 (-2.74 eV)
LUMO (-2.63 eV)
LUMO (-3.33 eV)
LUMO (-3.27 eV)
HOMO (-5.89 eV)
LUMO (-3.38 eV)
LUMO (-3.33 eV)
HOMO (-5.78 eV)
HOMO (-5.9 eV)
HOMO (–6.01 eV)
HOMO (-6 eV)
HOMO-1 (-5.79 eV) HOMO-1 (-5.94 eV) HOMO-1 (-5.93 eV) HOMO-1 (-6.02 eV) HOMO-1 (-6.01 eV)
HOMO-2 (-6.06 eV) HOMO-2 (-6.11 eV)
HOMO-2 (-6.1 eV)
HOMO-2 (-6.62 eV) HOMO-2 (-6.61 eV)
23

Figure 7. Energy diagram for [Ir-Py]+, [Ir-Py-H]2+, [Ir-Py-Me]2+, [Ir-Py-H2]3+ and [Ir-Py-H-
Me]3+. The contribution of Ir (red), tppy (green) and tpy (blue) are presented for each orbital.
4. CONCLUSION
The electrochemical and spectroscopic data for [Ir-Py]+ and [Ir-Py-Me]2+ are in full
agreement with the computational results which indicate that the lowest transitions correspond to
CT transitions. Moreover the effect of mono- and bi-protonation observed on the absorption and
emission behavior of the two complexes also corroborates the theoretical data.
However it is difficult from the experimental and theoretical results to describe more
specifically the type of CT transition occurring in these complexes, i.e. a metal-ligand to ligand
(MLLCT) or ligand to ligand (LLCT) charge transfer transition. Based on the calculations, it
might be possible that for [Ir-Py]+, at least two transitions or more participate to the absorption
and emission processes i.e. (i) a CT from the HOMO and HOMO-1 (almost isoenergetic) to the
tpy ligand or the pyridine moiety of the tpy ligand and (ii) a CT involving the HOMO-2 level.
The situation should be different for the methylated complex in which the CT excitation would
correspond better to an electron transfer from the cyclometalated fragment to the pyridinium
moiety.
24

The first protonation of [Ir-Py]+ on the pyridine substituent stabilizes the LUMO level and this
stabilization is similar to the one due to the methylation of 4′ -Py-tpy in [Ir-Py-Me]2+. Therefore,
the absorption band of [Ir-Py-H]2+, shifted to the red with respect to [Ir-Py]+, becomes quite
similar to that of [Ir-Py-Me]2+. After the second protonation of [Ir-Py]+, or first protonation of
[Ir-Py-Me]2+, on the methoxy group of both complexes, the corresponding HOMO levels are
stabilized, resulting in a large hypochromic effect and hypsochromic shift of the lowest energy
absorption of both complexes. The experimental results of this work show also that the HOMO
of the two Ir complexes are not purely Ir centred as is the case, for example, for MLCT
transitions in Ru(II) complexes, but it is characteristic of the whole fragment (Ir-tppy-MeO) and
could thus be termed MLLCT (Metal-Ligand fragment to Ligand Charge Transfer) as referenced
by Williams et al. for other cyclometalated bis-terdentate Ir(III) complexes.[63, 64] The
attribution of MLLCT to the lowest energy absorption and emission (at 298 K and 77 K) for [Ir-
Py]+, protonated [Ir-Py-H]+, and [Ir-Py-Me]2+ in MeCN is also in perfect agreement with the
conclusions from theoretical calculations reported by Scandola et al. for other cyclometalated
Ir(III) complexes.[29]
ASSOCIATED CONTENT
1
Supporting Information. Cyclic voltammograms, H NMR spectra and HRMS data for
complexes [Ir-Py]+ and [Ir-Py-Me]2+. Absorption changes for [Ir-Py-Me]2+ in the presence of
acetic acid and low TFA concentrations. Comparison of absorption spectra of [Ir-Py]+ upon
addition of 0.1 M in TFA and [Ir-Py-Me]2+. Spectral absorption changes for [Ir-Py-Me]2+ upon
addition of TFA in H2O. Computational studies details: geometrical and electronic structure,
detailed TD-DFT results.
ACKNOWLEDGMENT
25

The authors gratefully acknowledge the Fonds National pour la Recherche Scientifique
(F.R.S.-FNRS), the Fonds pour la Formation à la Recherche dans l’Industrie et dans
l’Agriculture (F.R.I.A.), the Région Wallonne, the Université catholique de Louvain and the Prix
Pierre et Colette Bauchau for financial support. GSH, MC and TA thank the Natural Sciences
and Engineering Research Council (NSERC) of Canada and the Université de Montréal for
financial support, as well as Compute Canada / West Grid for the computational resources. BE
and GSH thank the Wallonie Bruxelles International program for travel grants.
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29