Cyclometalated mono and dinuclear rhodium(III) and iridium(III) complexes with imidazolyl phenanthrolines: Synthesis and, photophysical and electrochemical characterization

Article abstract of DOI:10.1016/j.poly.2015.04.011

Cyclometalated rhodium(III) and iridium(III) complexes were synthesized with imidazolyl modified phenanthroline ligands (1-2) of general formula [M(ppy)₂(1)]Cl [M = Rh/Ir] and ['(2)]Cl₂ [M = Rh/Ir] (3-9). All the compounds 1-9, have been characterized spectroscopically to reveal their dependence on the solvent polarity and proticity of the environment. The photophysical details show that the ligands as well as the cyclometalated complexes have spectrum of variations depending on the immediate solvent environment and pH of the medium. Electrochemical studies show variation in the electrochemical behaviour due to the change of the metal centres.

Full text of DOI:10.1016/j.poly.2015.04.011

Accepted Manuscript
Cyclometalated mono and dinuclear rhodium(III) and iridium(III) complexes
with imidazolyl phenanthrolines: Synthesis and, photophysical and electro-
chemical characterization
Sourav Kanti Seth, Soumik Mandal, Pradipta Purkayastha, Parna Gupta
PII:
S0277-5387(15)00200-4
DOI:
http://dx.doi.org/10.1016/j.poly.2015.04.011
POLY 11275
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
5 February 2015
9 April 2015
Please cite this article as: S.K. Seth, S. Mandal, P. Purkayastha, P. Gupta, Cyclometalated mono and dinuclear
rhodium(III) and iridium(III) complexes with imidazolyl phenanthrolines: Synthesis and, photophysical and
electrochemical characterization, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.04.011
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Cyclometalated mono and dinuclear rhodium(III) and iridium(III)
complexes with imidazolyl phenanthrolines: Synthesis and,
photophysical and electrochemical characterization
Sourav Kanti Seth, Soumik Mandal, Pradipta Purkayastha* and Parna Gupta*
Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER)
Kolkata, Mohanpur 741246, India. Fax: +91-33-25873020; Tel: +91-33-25873019; E-mail:
parna@iiserkol.ac.in, ppurkayastha@iiserkol.ac.in
Abstract: Cyclometalated rhodium(III) and iridium(III) complexes were synthesized with imidazolyl
modified phenanthroline ligands (1 - 2) of general formula [M(ppy)2(1)]Cl [M = Rh/Ir] and
[{M(ppy)2}2(2)]Cl2 [M = Rh/Ir] (3 - 9). All the compounds 1 - 9, have been characterized
spectroscopically to reveal their dependence on the solvent polarity and proticity of the environment.
The photophysical details show that the ligands as well as the cyclometalated complexes have
spectrum of variations depending on the immediate solvent environment and pH of the medium.
Electrochemical studies show variation in the electrochemical behaviour due to the change of the metal
centres.
1. Introduction
Cyclometalated iridium(III) complexes with polypyridyl based ancillary ligands exhibit rich
photophysical properties. They are well addressed for their application as sensors,1-5 in biological
imaging, 6 light-emitting electrochemical cells,7-11 organic light-emitting diodes,12-20 catalysts for water
-28
-35
splitting,21-23 dye-sensitized solar cells (DSSCs)24
and organic transformations.29
In contrast,
reports on photophysical properties of the cyclometalated rhodium(III) analogues are limited in
literature.4,29,30,36-47 Spectroscopic transitions in these systems are usually metal centered (MC) and
ligand centered (LC), or through charge transfer (either metal-to-ligand, MLCT, or ligand-to-metal,
LMCT) and corresponding oxidation and reduction processes are either metal or ligand centered.
Ligand-field stabilization in these complexes depends on the oxidation state, nature of the
cyclometalating ligand and d-orbitals. However, dissimilarity in photophysical behaviour between the
cyclometalated iridium(III) and rhodium(III) complexes originates from the large difference in spin-
orbit coupling (ξIr = 3909 cm–1 and ξRh = 1259 cm–1) between these two metals. As a result,
intersystem crossing from first excited singlet state to triplet state occurs efficiently in the iridium
complexes. Hence, cyclometalated iridium(III) complexes are generally characterized by their long-
lived triplet excited states. Iridium(III) complexes with strong-field cyclometalating ligands, such as
phenylpyridine, usually emit from the admixture of spin-allowed metal-to-ligand charge-transfer,
(1MLCT) and π(ppy) - π*(phen) ligand-to-ligand (1LLCT) processes, whereas, the emission of
rhodium(III) complexes are mainly ligand-based.8,29-30,36-51Cyclometalated rhodium complexes of
phenylpyridines were recently explored for their catalytic properties.52–55 Polynuclear transition metal
complexes bridged through polypyridyl ligands are always interesting for their through-bridge
1

electronic communication. Choice of the bridging ligand is crucial to design such systems. This is
important from the structural viewpoint as well as for electronic coupling between the separated
subunits.56-75 Transition metal complexes containing polypyridyls are largely employed as energy donor
or acceptor for fast and efficient energy transfer due to their rich photophysics and long-lived triplet
excited states. Bis(imidazo[4,5-f][1,10]phenanthroline)s are well suited bridging ligands for this
purpose.76 - 82 They are fully conjugated, have two N∩N-bidentate coordination sites, and possess protic
sites (N-H) that induce pH-dependence. In addition, the photophysical properties can be modulated by
incorporating two different cyclometalated subunits into the structure of this tetradentate system. The
photoactive characteristic of one subunit can be coupled with the redox active centre in the other.
Combined use of cyclometalated iridium(III) complexes and rhodium polypyridyl systems in
homogeneous and heterogeneous photocatalytic processes have already been reported.83 -85 The former
acts as photosensitizer, and the latter serves as catalyst for proton induced reduction. The photophysical
properties of a wide range of cyclometalated heteroleptic iridium(III) complexes with imidazolyl based
phenanthroline as ancillary ligand have already been explored.86 - 95
In the present work, we have synthesised mononuclear, homo and hetero-dinuclear coordination
complexes comprising of cyclometalated iridium and rhodium cores with polypyridyl ligands. In all the
complexes the imidazolyl and bis-imidazolyl phenanthrolines bind to cyclometalated iridium and
rhodium units as ancillary bidentate ligands. Photophysical characterization of the complexes was done
to understand the electronic communication between the metal centres in homodinuclear and
heterodinuclear complexes. Cyclic voltammetric measurements for the complexes were done to
understand the electrochemical behaviour of the complexes and to understand the potential of
heterodinuclear system in the catalytic processes.
2. Experimental
2.1. Chemicals and materials
The starting materials, RhCl3.3H2O, IrCl3.3H2O, 2-phenylpyridine, 8-aminoquinoline, 1-napthylamine
and 1-methyl-2-imidazole carboxaldehyde were purchased from Sigma-Aldrich and used without
further purification. All the solvents were dried by usual methods prior to use. The cyclometalated
iridium(III) and rhodium(III) chloro bridged dimer [Ir(ppy)2Cl]2 and [Rh(ppy)2Cl]2,97 1,10-
96
phenanthroline-5,6-dione,98 2-(4-formylphenyl)imidazo[4,5-f][1,10]phenanthroline(1),78 and 2,2-p-
Phenylene(imidazo[4,5-f][1,10]phenanthroline(2)78 were synthesized according to the literature
methods.
2.2. Synthesis of complexes
2.2.1 Synthesis of [Rh(ppy)2(1)]Cl (3)
A mixture of [Rh(ppy)2Cl]2 (0.1 mmol), 1 (0.235 mmol), methanol (15 mL), dichloromethane (15 mL)
and acetonitrile (15 mL) was refluxed for 5 h at 90̊ C that yielded a reddish orange solution. The
product was purified by column chromatography with 5 to 10% (v/v) methanol in dichloromethane as
eluent. A reddish orange compound was obtained after the eluent was dried. Yield: 72%. ESI-MS (m/z):
2

[M-Cl]+ : 735.137 (Calcd. 735.13). Elemental Analysis: Calcd. C: 65.42, H: 3.66, N: 10.90; Expt. C:
1
65.79, H: 3.87, N: 11.02; H-NMR [CDCl3]: δ 10.03 (s, 1H), 9.50 (d, 2H), 8.77 (d, 2H), 8.07 (d, 2H),
7.98 (d, 2H), 7.92 (d, 2H), 7.75 (m, 4H), 7.66 (q, 2H), 7.37 (d, 2H), 7.13 (t, 2H), 7.02 (t, 2H), 6.85 (t,
2H), 6.42 (d, 2H)(for more details see Figure S8). ῡmax/cm-1: 3423s (br), 2924m, 2360w, 1697s, 1605s,
1580s, 1480s, 1210m, 1118m, 758m, 732m.
2.2.2 Synthesis of [Ir(ppy)2(1)]Cl (4)
The synthetic procedure is similar to that of 3. A reddish orange compound was obtained on drying the
eluent . Yield: 69%. ESI-MS (m/z): [M-Cl]+ :825.173 (Calcd. 825.19). Elemental Analysis: Calcd. C:
58.63, H: 3.28, N: 9.77; Expt. C: 59.10, H: 3.45, N: 9.23 ;1H-NMR [CDCl3]: δ 10.06 (s, 1H), 9.59
(broad, 2H), 8.79 (d, 2H), 8.13 (d, 2H), 8.02 (d, 2H), 7.94 (d, 2H), 7.73 (m, 6H), 7.38 (d, 2H), 7.10 (t,
2H), 6.99 (t, 2H), 6.85 (t, 2H), 6.42 (d, 2H) (for more details see Figure S9). ῡmax/cm-1: 3430m, 3043m,
2920m, 2850m, 1698s, 1607vs, 1582s, 1478s, 1383m, 757m, 730m.
2.2.3 Synthesis of [Rh(ppy)2(2)]Cl (5)
A mixture of [Rh(ppy)2(1)]Cl (0.0581mmol), 1,10-phenanthroline-5,6-dione (0.0762 mmol), glacial
acetic acid (12 mL) and ammonium acetate (1.01 g, 13.12 mmol) was refluxed for 4 h at 110̊ C
followed by cooling down to room temperature and dilution with distilled water. A precipitate appeared
on neutralizing the solution with aqueous ammonia in a cooling bath. The resulting precipitate was
filtered, washed first with water for several times and then with dichloromethane producing a brown
coloured compound. Yield: 64%. ESI-MS (m/z): [M-Cl]+: 925.205 (Calcd. 925.20). Elemental Analysis:
Calcd. C: 67.47, H: 3.57, N: 14.57; Expt. C: 66.96, H: 3.13, N: 14.89; 1H-NMR [DMSO-d6]: δ 9.09 (d,
2H), 9.03 (d, 2H), 8.98 (d, 2H), 8.59 (d, 2H), 8.37 (d, 2H), 8.28 (d, 2H), 8.01 (m, 4H), 7.95 (t, 2H), 7.87
(m, 4H), 7.47 (d, 2H), 7.12 (t, 2H), 7.08 (t, 2H), 7.03 (t, 2H), 6.34 (d, 2H)(for more details see Figure
S10).. ῡmax/cm-1: 3403m (br), 2924m, 1605s, 1479m, 1449s, 1351s, 1121s, 810m, 757m, 740m.
2.2.4 Synthesis of [Ir(ppy)2(2)]Cl (6)
The synthetic procedure is similar to that of 5. A dark brown coloured compound was obtained as the
final product. Yield: 61%. ESI-MS (m/z): [M-Cl]+: 1015.23 (Calcd. 1015.26). Elemental Analysis:
Calcd. C: 61.73, H: 3.26, N: 13.33; Expt. C: 62.94, H: 2.87, N: 13.67; 1H-NMR [DMSO-d6]: δ 9.09 (d,
2H), 9.01 (d, 4H), 8.59 (d, 2H), 8.40 (d, 2H), 8.26 (d, 2H), 7.95 (d, 4H), 7.86 (m, 6H), 7.50 (d, 2H),
7.06 - 7.00 (m, 4H), 6.95 (t, 2H), 6.32 (d, 2H) (for more details see Figure S11). ῡmax/cm-1: 3386s(br),
3060s, 1606s, 1581s, 1563s, 1478s, 1449s, 1397s, 758m, 740s.
2.2.5 Synthesis of [{Rh(ppy)2}2(2)]Cl2 (7)
A mixture of 6 (0.047 mmol), [Rh(ppy)2Cl]2 (0.035 mmol) and acetonitrile/ dichloromethane/ methanol
(1:1:1, v/v, 30 mL) was refluxed at 90̊ C for about 3 h. The resulting yellowish orange coloured solution
was cooled down to room temperature. Subsequently, the compound was purified by thin layer
chromatography with 10% methanol in dichloromethane mixture. A dark yellow compound was
obtained on drying. Yield: 65%. ESI-MS (m/z): [M-2Cl]2+/2: 713.13 (Calcd. 713.14), [M-2Cl-H+]+ =
1425.25 (calcd. [M-2Cl]+ = 1426.29); Elemental Analysis: Calcd. C: 62.44, H: 3.45, N: 11.50; Expt. C:
3

62.76, H: 3.85, N: 11.20; 1H-NMR [CDCl3]: 9.74 (broad, 4H), 8.77 (s, 4H), 8.15 (d, 2H), 8.12 (d, 2H),
7.95 (d, 4H), 7.79 – 7.72 (m, 12H), 7.38 (t, 4H), 7.14 (t, 2H), 7.08 (t, 2H), 7.02 (t, 2H), 6.97 (t, 2H),
6.91 (t, 2H), 6.86 (t, 2H), 6.42 (d, 4H) (for more details see Figure S12)..ῡmax/cm-1: 3433s (br), 3043m,
2922m, 1606s, 1581m, 1479s, 1449m, 758m, 731m.
2.2.6 Synthesis of [{Ir(ppy)2(2)Rh(ppy)2}](PF6)2 (8)
8: was synthesised by refluxing [Rh(ppy)2Cl]2 (0.25 mmol) with
2
(0.25 mmol) in
EtOH/dichloromethane mixture (2:1 v/v, 30 mL) for 4 h. The mixture was heated until the volume was
reduced to 8 mL followed by addition of 10 mL water. The desired compound was precipitated by
adding an excess of aqueous NH4PF6 to the solution. Filtration yielded a yellow coloured compound.
+
Yield: 68%. ESI-MS (m/z): [M-2PF6]2+/2: 668.09 (Calcd. 668.12).[M-2PF6-H+]+ = 1335.22; (calcd.
[M-2PF6]+ = 1336.24); Elemental Analysis: Calcd. C: 61.59, H: 3.40, N: 11.34; Expt. C: 61.78, H: 3.56,
N: 12.02 ;1H-NMR [DMSO-d6]: δ 9.26 (d, 4H), 8.60 (s, 4H), 8.30 (d, 4H), 8.23 (d, 4H), 8.12 (t, 4H),
8.03 (d, 4H), 7.97 (t, 4H), 7.49 (d, 4H), 7.15 (t, 4H), 7.07 – 7.02 (m, 8H), 6.32 (d, 4H)(for more details
see Figure S13). ῡmax/cm-1: 3430s(br), 1606s, 1578s, 1480s, 1451m, 846vs, 757s, 733m.
2.2.7 Synthesis of [{Ir(ppy)2}2(2)]Cl2 (9)
A mixture of [Ir(ppy)2Cl]2 (0.1 mmol), 2 (0.1 mmol) and acetonitrile/dichloromethane (1:1 v/v, 30 mL)
was refluxed for 4 h at 90̊ C to obtain an orange coloured mixture. It was cooled down to room
temperature and the solvent was evaporated using a rotary evaporator. The residue was purified by
column chromatography (5% methanol in dichloromethane was used as eluent) yielding a brownish
orange compound. Yield: 70%. ESI-MS (m/z): [M-2Cl]2+/2: 758.12(calcd. 757.17).[M-2Cl-H+]+ =
1515.27 (Calcd. [M-2Cl]+ = 1516.35); Elemental Analysis: Calcd. C: 58.85, H: 3.25, N: 10.84; Expt. C:
59.23, H: 3.27, N: 10.29; 1H-NMR [CDCl3]: δ 9.69 (broad, 4H), 8.73 (s, 4H), 8.09 (d, 4H), 7.94 (d, 4H),
7.71 (m, 12H), 7.38 (d, 4H), 7.05 (t, 4H), 6.95 (t, 4H), 7.85 (t, 4H), 6.41 (d, 4H) (for more details see
Figure S14). ῡmax/cm-1: 3412s (br), 2924vs, 1605s, 1581s, 1477s, 1451s, 1377s, 758s, 729s.
2.3 Physical measurements
Elemental analyses were performed on a Perkin–Elmer 2400 series II CHN series. IR spectra were
obtained on a Perkin-Elmer Spectrum RXI spectrophotometer with samples prepared as KBr pellets.
1
Electronic spectra were recorded on a U-4100, HITACHI spectrometer. H NMR spectra were obtained
on a Bruker Avance III-500 NMR spectrometer using TMS as the internal standard. Electrochemical
measurements were done using a PAR model 273 potentiostat. A platinum disk working electrode, a
platinum wire auxiliary electrode and Ag/Ag+ (reference electrode) were used in a three electrode
configuration. Electrochemical measurements were made under a dinitrogen atmosphere. All
electrochemical data were collected at 298 K and are uncorrected for junction potentials. Fluorescence
spectra were taken on a HORIBA JOBINYVON spectrofluorometer Fluoromax 4. Quantum yield data
reported here were measured relative to quinine sulfate in 0.1 M H2SO4 (λex = 350 nm, Φ = 0.577). The
integration of the emission spectra was obtained from the Fluoromax-4 instrument directly. The
photoluminescence spectra were measured on a Horiba Jobin Yvon spectrometer equipped with a 150
4

W Xe lamp. Mass spectra were recorded on a Q-Tof Micromass spectrometer by positive-ion mode
electrospray ionization.
3. Results and Discussion
The Bis(imidazo[4,5-f][1,10]phenanthroline system as bridging ligand has been chosen judiciously as,
cyclometalated iridium(III) complexes with N∩N polypyridyl as ancillary ligands are luminescent in
nature.36-40, 99 - 100 The synthesis of complexes 3, 4 and 8 - 9 have been done by refluxing [M(ppy)2Cl]2
(M = Ir, Rh) with 1 - 2 or 5 - 6 in dichloromethane: acetonitrile (1:1) solution at 60oC for 4 h (Scheme
1).
O
O
O
N
H
N
O
NH4OAc, Reflux, 2h
N
N
O
Conc. H2SO4
Conc. HNO3,
KBr
Glacial Acetic Acid
N
N
N
N
N
O
1
O
O
N
H
N
O
NH4OAc, Reflux, 2h
N
N
N
N
Conc. H2SO4
Conc. HNO3,
KBr
Glacial Acetic Acid
N
N
N
H
N
N
2
+
N
N
N
H
N
Cl
Cl
N
N
N
O
N
C
C
C
C
C
C
1, CH2Cl2
M
N
M
N
M
N
=
MeOH,
Reflux, 4h
N
C
M=Rh, 3
M=Ir, 4
O
O
+
+
N
N
N
M
H
N
H
N
N
Glacial Acetic Acid
N
N
N
O
N
C
C
C
C
M
N
NH OAc, Reflux, 2h
4
N
N
N
H
N
N
N
N
M=Rh, 5
M=Ir, 6
N
Rh
N
N
Rh
N
Cl
Cl
2+
C
C
C
+
C
N
N
Rh
N
N
Ir
N
H
N
H
N
N
N
N
N
N
N
N
N
N
C
C
C
C
C
Ir
C
CH2Cl2, MeOH,
Reflux 4h
N
N
H
N
N
H
N
N
7
N
N
Cl
2+
C
C
C
M
N
M
N
N
C
N
H
N
H
N
Cl
N
N
N
N
N
N
C
N
C
C
M
N
M
N
C
CH2Cl2, MeOH,
Reflux4h
N
N
H
N
N
H
N
N
N
M=Rh, 8
M=Ir, 9
Scheme 1. Synthetic details of ligands 1 - 2 and complexes 3 - 9, with procedures starting from 1,10-
phenanthroline as ligand and [Ir(phenylpyridine)2Cl2]as metal precursor.
Complexes 5 and 6 has been synthesized by treating phen-5, 6-dione with 3, 4 in glacial acetic acid in
presence of ammonium chloride respectively. Complex 7 has been synthesized by refluxing
[Rh(ppy)2Cl]2 with 6 in dichloromethane: acetonitrile (1:1) solution. Except, complex 8 (precipitated as
5

PF6 salt), the complexes 3 – 7 and 9 have been obtained as chloride salt. The synthesized compounds
1
were purified by standard techniques and characterized by H NMR, mass spectrometry and infrared
1
(IR) spectroscopy. ESI-MS data depict the composition of the complex as shown in Scheme 1 and H
NMR spectra of the complexes (Figure S8 – S14) confirm the structural aspect of the complexes quite
well. Despite, several attempts to get good quality crystals, the quality of the crystals was very poor and
could not be solved.
3.1. Spectral features in the ground electronic state
Ligands 1 and 2 display absorption bands at around 375 nm and 284 nm in common organic solvents
(Figure 1). Bands at 375 nm get blue shifted with increase in solvent polarity for 1. Ligand 2 has an
insignificant blue shift but is more structured compared to 1. The higher energy absorption bands 1 and
2 show high extinction coefficient (ε ≈ 2.7 - 12 × 104 M-1cm-1L) around 290 nm and hence can be
assigned to the spin allowed ligand-centered transitions within the π-conjugated system (1π → π*). The
TDDFT of ligands 1 and 2 help us to correlate the spectral data and the electronic transition along with
the main orbital contributions of the transition of 1 and 2 are given in (Table 1) and the frontier orbital
surface diagrams are shown in Figure 2.
Fig. 1 Absorption spectra of ligands 1 and 2 in solvents of different polarities
6.250x104
5.000x104
1
2
Conc. 4x10-6M
Conc. 4x10-6M
5.000x104
3.750x104
THF
DCM
1-Prop
3.750x104
2.500x104
THF
DCM
2.500x104
1-Prop
Acetone
Methanol
Acetonitrile
Water
Acetone
Methanol
Acetonitrile
Water
1.250x104
1.250x104
0.000
0.000
300
400
500
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Table 1: The correlation of electronic spectral data with the theoretically calculated spectral assignment
Key transition (expt.)
Absorption (nm)
377, 357, 284
Calculated (nm,
force constant)
Compound
(Dichloromethane)
1
370.46 (f=0.4175)
362.64 (f=0.0156)
299.26 (f=0.0592)
HOMO -> LUMO (87%)
HOMO -1 -> LUMO (86%)
HOMO -> LUMO +2 (33% )
6

371, 354, 314, 287 374.59 (f=1.0825)
350.03 (f=0.0029)
HOMO -> LUMO (89%)
HOMO - 2 -> LUMO (68%)
HOMO -> LUMO+3 (37%)
HOMO-6 -> LUMO (54%)
2
311.50 (f=0.0089)
295.67 (f=0.2363)
Fig 2. The frontier orbital diagrams of HOMO and LUMO for the compounds 1 and 2
The absorption spectral studies show that the complexes 3 - 9 absorb at around 290 nm, 350 nm and 470
nm in common organic solvents (Figure 3). It is interesting to note that, ε of the high energy bands
increases sharply once the phenyl pyridine ligands are first introduced in 3. This can be attributed to πppy
- π*phen ligand-to-ligand (1LLCT) charge transfer.101 The absorption bands around 390 nm have ε ≈ 2.5 –
9 × 104 M-1cm-1L (Table S1). Increase in absorption at this wavelength (390 nm) is presumably due to
the contribution from the admixture of spin-allowed metal-to-ligand charge transfer.14 The weak band
around ~470 nm with low ε (≈ 0.2 - 10×103 M-1cm-1L, Table S1), can be assigned to spin-forbidden
3
3MLCT and LLCT transitions.102-104 Summarily, the basis of the above assignments are: (i) the high
extinction coefficients of the bands, which are consistent with π - π* transitions in polypyridine metal
complexes and (ii) similar ppy-centered bands in the protonated ppy and other rhodium(lII) and
iridium(III) cyclometalated compounds.29-30, 36 - 51 It is perceivable that influence of 5d and 4d metal ions
are quite similar on the absorption spectra of the complexes (3 - 9).
7

Fig. 3 Absorption spectra of 3-9 in solvents of different polarities.
6.250x104
7.500x104
4
3
Conc. 4x10-6M
Conc. 4x10-6M
5.000x104
3.750x104
2.500x104
1.250x104
0.000
6.250x104
5.000x104
3.750x104
2.500x104
THF
DCM
THF
DCM
1-Prop
Acetone
Methanol
Acetonitrile
Water
1-Prop
Acetone
Methanol
Acetonitrile
Water
1.250x104
0.000
300
400
500
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
1.25x105
1.00x105
7.50x104
5.00x104
8.750x104
7.500x104
6.250x104
5.000x104
3.750x104
2.500x104
5
6
Conc. 4x10-6M
Conc. 2x10-6M
THF
DCM
THF
DCM
1-Prop
Acetone
Methanol
Acetonitrile
Water
1-Prop
Acetone
Methanol
Acetonitrile
Water
2.50x104
0.00
1.250x104
0.000
300
400
500
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
1.25x105
1.25x105
1.00x105
7.50x104
5.00x104
8
7
Conc. 4x10-6M
Conc. 4x10-6M
1.00x105
7.50x104
THF
DCM
THF
DCM
1-Prop
Acetone
Methanol
Acetonitrile
Water
1-Prop
Acetone
Methanol
Acetonitrile
Water
5.00x104
2.50x104
0.00
2.50x104
0.00
300
400
500
600
300
400
500
600
Wavelength (nm)
1.25x105
9
Wavelength (nm)
Conc. 4x10-6M
1.00x105
7.50x104
5.00x104
2.50x104
0.00
THF
DCM
1-Prop
Acetone
Methanol
Acetonitrile
Water
300
400
500
600
Wavelength (nm)
8

3.2. Emission spectral characteristics
The fluorescence spectra of 1 – 9 and an image of the emission of the compounds (solvent:
dichloromethane, λex = 366 nm) are shown in Figure 4. Characteristic locally excited (LE) and charge
transfer (CT) state emissions were observed from 1 in solvents of different polarity. The benzaldehyde
moiety in ligand 1 attracts the electron cloud facilitating intramolecular charge transfer (ICT) in
solvents of higher polarity. LE emission occurs at lower wavelengths showing bands at at 401, 425 nm
and 445 nm. The reason behind appearance of a single structureless band at 425 nm in dichloromethane
is unknown. In protic solvents, such as propanol, methanol and water, 1 shows ICT emission at ~490
nm. However, acetonitrile, which is polar and aprotic in nature, does not show ICT. Hence, it is
apparent that for 1, ICT is promoted in polar protic solvents. The –CHO group in 1 can form hydrogen
bond with protic solvents that results into enhancement of its electron withdrawing ability. Ligand 2, on
the other hand, shows emission only from the LE state in all the solvents. Symmetric nature of 2 makes
it relatively insensitive to solvent polarity and hence no ICT band is observed in this case.
Complexes 3 and 4 were synthesized using the ligand 1 with cyclometalated rhodium and iridium
core respectively. Due to the difference in photophysical characteristics of iridium(III) and
rhodium(III),29-30,36-51 the CT (3MLCT) emission is prominent in complex 4. Complex 3 shows intense
ligand-based emission (3LC) peak at 453, 464 and 471 nm in THF, DCM and acetonitrile. However, the
emission maximum shifts towards 540 nm in propanol, methanol and water probably contribution of
3
MLCT increases in these solvent. The emission from MLCT in complex 4 dominates irrespective of
the nature of solvent because of the presence of 5d electrons in iridium(III) in contrast to 4d electrons in
rhodium(III) due to large shift in the spin-orbit coupling constant (ζ/cm-1: Rhodium: 1259; Iridium
3909). This phenomenon renders intersystem crossing much easier for the iridium(III) complex. Thus,
CT emission in 3 is aided by hydrogen bonding between the –CHO group in of the ligand and the protic
solvents. In 4, MLCT becomes more labile and hence ICT emission is observed readily in all the
solvents. Emissions from 5 and 6 are quite similar to 2, with a less intense MLCT based emission
observed only in the cyclometalated iridium complex 6 (λmax = 560 nm).
The complexes 7 to 9 constitute the cyclometalated dinuclear complexes with imidazolyl
phenanthroline bridging ligand. The emission spectra of 7, 8 and 9 are clearly MLCT based and emit in
the range 540 – 580 nm. In 8, although MLCT based emission is prominent but LLCT based emission is
also observed at around 400 – 415 nm. Similar spectral characteristic is observed for 7 and 9
presumably due to the presence of iridium(III). The corresponding fluorescence quantum yields for the
complexes in various solvents are given in Table 2. The image of the complexes shown in cuvette
irradiated at 366 nm, completely in agreement with the experimental observation.
9

Fig. 4. A. Fluorescence spectra of (A-I) 1-9 in solvents of different polarities. Excitation wavelength for
1 - 9 is 350 nm. B.
(C)
(A)
(B)
Complex 3
THF
DCM
Ligand 1
Ligand 2
Propanol
Methanol
Acetonitrile
Water
THF
DCM
Propanol
THF
DCM
Propanol
Methanol
Acetonitrile
Water
Methanol
Acetonitrile
Water
400
450
500
550
600
650
700
400
450
500
550
600
650
700
(E)
400
450
500
550
600
650
700
(D)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
(F)
Complex 4
Complex 6
Complex 5
THF
DCM
Propano
Methanol
Acetonitrile
Water
THF
DCM
THF
DCM
Propanol
Methanol
Propanol
Methanol
Acetonitrile
Water
Acetonitrile
Water
400
450
500
550
600
650
400
450
500
550
600
650
700
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
(H)
(I)
(G)
Complex 8
THF
DCM
Complex 9
Propanol
Methanol
Complex 7
THF
DCM
THF
DCM
Acetonitrile
Water
Propanol
Methanol
Acetonitrile
Water
Propanol
Methanol
Acetonitrile
Water
400
450
500
550
600
650
700
400
450
500
550
600
650
700
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
B. Visible emission of ligands 1-2 and complexes 3-9 in dichloromethane (λex = 366 nm)
10

Table 2. Fluorescence quantum yield of the complexes in solvents of different polarities measured with
respect to that of quinine hemisulfate in 0.1M H2SO4
Complex
THF
DCM
0.0171
0.0978
0.1480
0.0413
0.0679
0.0023
0.0956
Propanol
0.0131
0.0042
0.2583
0.0242
0.0025
0.0074
0.0020
Methanol
0.0080
0.0084
0.0356
0.0192
0.0088
0.0067
0.0072
Acetonitrile
0.0165
0.0108
0.1596
0.0237
0.0263
0.0012
0.0284
Water
0.0063
0.0067
0.0017
0.0053
0.0048
Negligible
0.0056
0.0103
0.0164
0.1471
0.0215
0.0142
0.0039
0.0200
3
4
5
6
7
8
9
3.3. Time resolved fluorescence measurements
Time resolved fluorescence decay data from the ligands and the metal complexes could be fitted by a
double exponential function indicating coexistence of two excited state species in all the solvents. The
compounds were excited at 340 nm. The 450 nm and 425 nm emissions were monitored for 1 and 2,
respectively. The nanoseconds component in the fit is attributed to the LE state and the picoseconds one
to the CT state (Table 3).
Table 3. Time-resolved fluorescence decay data for the ligands 1 and 2 in different solvents. The
samples were excited at 340 nm and the 450 nm and 425 nm emissions were monitored. Values in
parentheses are the percentage contributions to the fits and χ2 values indicate goodness of the fits.
Ligand
Solvent
THF
τ1 (ns)
τ2 (ps)
χ2
2.89 (87)
1.67 (89)
1.71 (90)
1.04 (13)
2.41 (31)
2.24 (71)
821 (14)
641 (11)
547 (10)
2010 (87)
570 (69)
425 (29)
1.28
1.11
1.18
1.14
1.18
1.07
1
DCM
Acetonitrile
Methanol
Water
THF
2
DCM
1.79 (74)
1.55 (91)
1.79 (94)
1.77 (95)
709 (26)
478 (9)
709 (6)
442 (5)
1.19
1.09
1.19
1.06
Acetonitrile
Methanol
Water
11

This is because percentage contribution of the slower component is greater than the faster one in low
polarity solvents, whereas, in water (higher polarity) the contribution gets reverted. It is known that
solvents with higher polarity promote CT which explains the result for 1. However, in 2 we found that
the LE state is the major contributor in all the solvents. This is probably due to the absence of a strong
electron withdrawing group at an appropriate position as that in 1. The data from the time-resolved
fluorescence experiment corroborate well with the findings from the steady state measurements for 1
and 2.
Cyclometalated dinuclear rhodium and iridium complexes are known to show slow decay because of
intersystem crossing (ISC) to the triplet states.76-79 Hence, we measured the phosphorescent lifetimes of
7, 8 and 9 as given in Table 4. Although the triplet state phenomena need further studies, but the data
indicate that ISC is better exhibited in 7 in low polarity solvents. In case of 8 the trend reverses and for
9 ISC equally less favourable in all sorts of solvents. This is again due to the presence of rhodium(III) in
the complexes 7 and 8, where ISC is not favourable like iridium.29-30, 36 – 51
Table 4. Phosphorescence lifetimes (µs) of 7, 8 and 9. The values in parentheses are the percentage fit
Complex THF(% Fit) DCM(% Fit)
ACN(% Fit) Propanol(% Fit)
Methanol(% Fit)
9.2 (94.9)
9.3 (99)
10.1 (99.9)
9.1 (86)
9.8 (99.8)
6.2 (38.6)
11.3 (99.6)
9.8 (99.9)
9.1 (73.8)
9.9 (99.8)
7.9 (90.9)
9.4 (99.5)
9.7 (99.7)
7
8
9
10.1 (99.8)
9.4 (99)
3.4. Effect of pH on the photophysics of 7-9
The absorption spectra of 7, 8 and 9 are shown in Figure 5 collected at different pH in aqueous
medium. The complexes were dissolved in water using dimethylsulfoxide (DMSO) as co-solvent to
acquire a final concentration of 6 µM. 7 and 8 show an intense absorption at 360 nm and a weak
shoulder at ~410 nm in low pH (acidic condition), whereas 9 shows absorption bands at 307 nm and
370 nm. Up to pH 6 we observed enhancement in the lower wavelength band along with a concomitant
decrease of the shoulder. This helped us to attribute the lower wavelength band to the neutral
benzimidazolyl species and the shoulder to its cationic form. The spectral features in all the three cases
show an abrupt shift in the absorption spectrum as the medium acquires pH > 6. The sudden change
observed is due to ionization of the imidazolyl group at around pH 5.5. Under alkaline condition, 7 and
8 show an intense peak at 370 nm and a weak shoulder at 480 nm. The peak at 370 nm is due to the
normal anionic form of the benzimidazolyl moiety and the shoulder at 480 nm is for its tautomer
(Scheme 2).105 In 9, all the three species coexist at all pH. Figure 6 presents the fluorescence spectral
changes for 7, 8 and 9 under different pH from acidic to alkaline conditions. All the complexes show
emission at three wavelengths. Emission in the lowest wavelength with very low fluorescence yield
could be due to the benzimidazolyl cationic species. In 7 the emission from this species decreases with
increase in solution pH.
12

Fig. 5 Absorption spectra of (A) 7, (B) 8 and (C) 9 at different pH.
1.0
1.0
0.8
0.6
0.4
0.2
0.0
0.5
(B)
(A)
(C)
0.8
0.6
0.4
0.2
0.0
0.4
0.3
0.2
0.1
0.0
pH 1
pH 2
pH 3
pH 4
pH 5
pH 1
pH 3
pH 5
pH 6
pH 7
pH 9
pH 1
pH 6.5
pH 9.3
pH 10
pH 2.8
pH 5
pH 6
pH 11.3
pH 12
pH 13.3
pH 8.6
pH 10
pH 12
pH 11
pH 12
300
350
400
450
500
550
600
650
350
400
450
500
550
600
650
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
H
N
H
N
Acidic
condition
N
N
N
N
N+
H
N
Cation
Neutral
Alkaline
condition
N-
N
N-
N
N
N
N
N
Tautomer
Anion
Scheme 2 Molecular structure of the chromophoric part of the ligands (1 and 2) under different pH
conditions.
Fig. 6 Fluorescence spectra of (A) 7, (B) 8 and (C) 9 at different pH of the medium. The samples were
excited at 370 nm.
(A)
(B)
(C)
pH1
pH 2
pH 1
pH 2
pH 3
pH 4
pH 5
pH 6
pH 7
pH 8
pH 9
pH 10.8
pH 12
pH 1
pH 3
pH 4
pH 5
pH 6
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
pH 2
pH 3
pH 4
pH 5
pH 6.5
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
500
550
600
650
700
500
550
600
650
700
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
The fluorescence spectra of 8 at different pH are quite different from the rest as we could see ligand-
centered emission from the cationic species up to pH 3 that decreases in intensity as the pH increases
from 1 to 3. There is a red shift in the emission maximum to 550 nm with further increase in pH of the
medium. The intensity of emission increases until pH 6 followed by a decrease. An intense band is
13

observed for 7 ( λmax =565 nm) at pH = 2 decreases with increase in pH. For 8 and 9 the bands appear
at 550 nm and 580 nm are considered to be due to the neutral form of the benzimidazolyl moiety. Over
pH 6 we observed a profuse reduction in intensity of the band with a bathochromic shift. We attribute
this band to the anionic form of the benzimidazolyl moiety. All the three species coexist for 9. The
findings match well with those observed from the absorption studies. The behavioural difference
between 8 with 7 and 9 could be due the presence of rhodium metal at both sides of the bridging ligand.
Rhodium being more redox active than iridium, can diffuse the electron density to phenylpyridine
moiety easily. In case of the iridium(III) complex, however, the bridging ligand bear the impact of the
change in pH more than in complex 8.
3.6. Cyclic voltammetry
The complexes 3, 4, 7, 8 and 9 were subjected to a scan from -2.00 to 2.00V to understand the redox
behaviour (Table 5). Due to extremely poor solubility we could not record the cyclic voltammograms
for 5 and 6. The rhodium complexes 3, 7 and 8 have similarity in their first oxidation potentials >1.42 V
(3 = 1.53V, 7 = 1.42V, 8 = 1.46V) attributed to the M(III) - M(IV) oxidation, whereas, purely
homonuclear iridium complexes 4 and 9 have their first oxidation around ~1.07V(Ir(III)-Ir(IV).106
Interestingly, complex 7, which contain both the metals shows peaks quite similar to the rhodium
complexes 3 and 8. The Ir(III)-Ir(IV) oxidation is not visible in complex 7 which may be due to the
current height of the rhodium oxidation peak. In addition to the metal centered peaks, the other
oxidation and reduction peaks are certainly ligand based.
Table 5. The redox potential of complexes 3 - 4 and 7 - 9.
Complex
Redox potential (V)
Eox
E red
0.67, 1.019, 1.53
1.07, 1.33 (90 mV)a, 1.84
0.66, 0.92, 1.42
0.63, 1.46
-1.048, -1.33, -1.75
-0.79, -1.24, -1.54 (80 mV)a
-0.99, -1.44
3
4
7
8
9
-0.80, -1.29, -1.46, 1.72 (100 mV)a
1.08, 1.35, 1.62
-0.78, 1.26, -1.63
a ∆Ep = Epa – Epc in mV
4. Conclusions
In summary, we have synthesized three homodinuclear and heterodinuclear cyclometalated
rhodium(III) and iridium(III) complexes. To have a better understanding on the photophysical and
electrochemical properties of the systems the mononuclear precursors were also synthesized and studied
in detail. The effect of ligand emission on the purely rhodium based complexes over MLCT based
14

emission and pronounced effect of MLCT-based emission for iridium based complexes can be
visualized very prominently on excitation at 366 nm. It is quite interesting to note that, the
photophysical property of the heterodinuclear complex behaves like pure iridium based complexes.
Conversely, the electrochemical property of the heterodinuclear complex is quite similar to the rhodium
analogue. We are working on the device fabrication with heterodinuclear system, where well
established electrochemical character of rhodium
1
5. Supplementary materials: [Figure S1-S7: ESI-MS of the complexes. Figure S8 - S14: H NMR of
the complexes. Table S1: Molar extinction coefficients of ligand 1-2 and complex 3-9
6. Acknowledgement
SKS and SM are thankful to the Council of Scientific and Industrial Research for Senior Research
Fellowship. This work is supported by Department of Science and Technology through research grants
ST/FT/CS-057/2009 (PG) and SR/S1/PC-35/2011 (PP).
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19

Cyclometalated mono and dinuclear rhodium(III) and iridium(III)
complexes with imidazolyl phenanthrolines: Synthesis, photophysical and
electrochemical characterization
Sourav Kanti Seth, Soumik Mandal, Pradipta Purkayastha* and Parna Gupta*
Homonuclear and heterobinuclear cyclometalated rhodium(III) and iridium(III) complexes 3 -
9 were synthesized with imidazolyl modified phenanthroline ligands. The detail photophysical
studies reveal that the emission behaviour of the rhodium complexes is greatly influenced by
ligand emission behaviour.
20