Bright orange/red-emitting rhodium(iii) and iridium(iii) complexes: Tridentate N^C^N-cyclometallating ligands lead to high luminescence efficiencies

Article abstract of DOI:10.1039/c3dt51211h

Rhodium(iii) complexes rarely display strong phosphorescence, in contrast to well-known iridium(iii) complexes with cyclometallating ligands. This study shows how 1,3-bis(1-isoquinolyl)benzene cyclometallates to Rh(iii) through N^C^N-coordination to give

Full text of DOI:10.1039/c3dt51211h

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Bright orange/red-emitting rhodium(III) and iridium(III)
complexes: tridentate N^C^N-cyclometallating ligands
lead to high luminescence efficiencies†
Cite this: DOI: 10.1039/c3dt51211h
Received 8th May 2013,
Accepted 5th June 2013
DOI: 10.1039/c3dt51211h
Louise F. Gildea, Andrei S. Batsanov and J. A. Gareth Williams*
www.rsc.org/dalton
3
Rhodium(III) complexes rarely display strong phosphorescence, in be predominantly π–π* in character and, since emission from
contrast to well-known iridium(III) complexes with cyclometallat- triplet states requires mixing with higher-lying 1MLCT states to
ing ligands. This study shows how 1,3-bis(1-isoquinolyl)benzene introduce the necessary SOC,6 an increase in the energy of the
cyclometallates to Rh(III) through N^C^N-coordination to give latter will serve to reduce kr. (iii) The weaker ligand-field split-
complexes with unprecedented phosphorescence quantum yields ting of 2nd versus corresponding 3rd row transition metals
at room temperature. Their highly emissive Ir(III) analogues are means that metal-centred d–d excited states – which are
also described.
strongly distorted relative to the ground state and hence poten-
tially highly deactivating for d6 complexes – are more likely to
be thermally accessible for Rh(III) than for Ir(III).7
The excited-state properties of cyclometallated iridium(III) com-
plexes have attracted a great deal of interest over the past 15
years, owing particularly to the high quantum yields of emis-
sion from triplet excited states that have been found for many
such compounds.1 These properties have rendered them excit-
ing molecular materials for application as emitters in organic
light-emitting devices2 and in biological stains and sensors
suitable for time-resolved detection procedures.3 In contrast,
rhodium(III) complexes have been much less explored for lumi-
nescence,4 their phosphorescence quantum yields being typi-
cally much lower. Thus, while bis-cyclometallated complexes
of the form [Ir(N^C)2(N^N)]+ are brightly luminescent,1a emis-
sion from analogous Rh(III) complexes can scarcely be detected
under such conditions5 (here N^C represents a ligand such as
orthometallated phenylpyridine and N^N a diimine such as
bipyridine).
The much weaker emission of rhodium complexes arises
from the combination of a number of factors: (i) The lower
spin–orbit coupling (SOC) constant of Rh compared to Ir will
result in smaller triplet radiative rate constants, kr. (ii) For a
given type of complex, the RhIII|RhIV couple will lie at higher
potential than the corresponding IrIII|IrIV couple, such that
metal-to-ligand charge-transfer (MLCT) states will be raised in
energy.5 Lowest-lying excited states are therefore more likely to
Amongst cyclometallated complexes, the brightest systems
hitherto reported appear to be those of the form [Rh-
(pba)2(N^N)]+, where pba is 4-(2-pyridyl)benzaldehyde and
N^N is a diimine ligand such as a 2,2′-bipyridine or 1,10-phe-
nanthroline. This family of complexes were reported to have
quantum yields of 1–3% and lifetimes of a few microseconds
in solution at 298 K, rationalised in terms of the influence of
the aldehyde group leading to a pba-localised state.8 Mean-
while, a family of rhodacyclopentadiene-based complexes
display intense fluorescence from the singlet state, reflecting an
unusually low rate of intersystem crossing.9
We have previously shown how the use of tridentate,
N^C^N-cyclometallating ligands can provide access to excep-
tionally highly luminescent Ir(III) and Pt(II) complexes, prob-
ably due to the enhanced rigidity and the short M–C bonds
imposed by the tridentate binding of the ligand.10,11 Here
we show how such a ligand, namely 1,3-bis(1-isoquinolyl)-
benzene, can be used to generate unusually brightly-emitting
rhodium complexes. We also describe the analogous Ir(III)
complexes, which rival the most efficient deep-red triplet emit-
ters reported.12
1,3-Bis(1-isoquinolyl)benzene (biqbH) was synthesised
readily by Suzuki cross-coupling of benzene-1,3-diboronic acid
with 2 equivalents of 1-chloroisoquinoline under standard
conditions (Scheme 1). This method is attractive compared to
the more commonly used Stille route to N^C^N-coordinating
ligands,10a as it avoids the use of the rather toxic heterocyclic
stannane reagents. Moreover, heterocyclic substrates in which
the carbon–halogen bond is ortho to the heteroatom are highly
activated, such that chloro derivatives can be used. Reaction of
Department of Chemistry, University of Durham, Durham, DH1 3LE, UK.
E-mail: j.a.g.williams@durham.ac.uk; Fax: +44 (0)191 384 4737
†Electronic supplementary information (ESI) available: Synthesis and character-
isation of all compounds; TD DFT; cyclic voltammograms; crystal structure data
in CIF format. CCDC 937441–937443. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3dt51211h
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Scheme 1 Synthesis of the proligand biqbH and of the rhodium and iridium complexes.
Fig. 1 Bidentate N^C4 coordination is the predominant binding mode dis-
played by dpyb (a) whereas tridentate N^C2^N coordination predominates for
biqb (b).
biqbH with MCl3·3H2O (M = Rh or Ir) under the conditions
typically used in the synthesis of [M(ppy)2(μ-Cl)]2 dimers
(ethoxyethanol–water at 80 °C) led to a precipitate, yellow for
Rh and orange for Ir. Owing to their very low solubility in all
common solvents, these products could not be characterised
readily at this stage but – on the basis of subsequent reactions
and our earlier work with substituted pyridyl analogues11
–
were deduced to be the chloro-bridged dimers [M(biqb)Cl-
(μ-Cl)]2 1 (Scheme 1). This contrasts with the behaviour of 1,3-
di(2-pyridyl)benzene (dpybH), where metallation with Ir(III) or
Rh(III) occurs at the 4-position to give bidentate N^C-coordi-
nation (Fig. 1a), rather than at the 2-position to give tridentate
N^C^N coordination11a,b – which can only be achieved by
introducing blocking substituents at the 4 and 6 positions
(e.g., CH3, CF3, F).11 Biqb apparently favours N^C2^N over N^C4
binding, without the need for such substituents (Fig. 1b).
Treatment of dimers 1 with 2-phenylpyridine (ppyH) gave
mononuclear complexes, M(biqb)(ppy)Cl, yellow for M = Rh
(2a)‡ and bright orange for M = Ir (2b). Single-crystal X-ray
diffraction study§ of 2a·3CDCl3 and 2b·5/3MeOH·2/3H2O
obtained by slow evaporation of chloroform and methanol
solutions respectively, showed very similar molecular geometry
Fig. 2 X-ray structures of 2a (top) and 2b (molecule B, mirror plane passing
through Ir(2), Cl(2), C(36), C(39) and the ppy ligand; molecule A has the same atom
numbering as 2a). Dotted lines show short H⋯H contacts. Thermal ellipsoids are
drawn at 50% probability level. See also ESI.†
As in the related Ir(III) complexes with 1,3-dipyridyl-
(Fig. 2, Table 1) with distorted octahedral coordination of the benzene11 and 1,3-bis(1-methylbenzimidazol-3-yl)-benzene,13
metal atom. The latter structure contains two symmetrically in 2 the two cyclometallating carbon atoms (of the ppy and
non-equivalent molecules of 2b, one of which (A) has no crys- biqb ligands) adopt a cis disposition, rather than trans which
tallographic symmetry, like molecule 2a, and the other (B) is disfavoured by their strong trans influence. The latter effect
has Cs symmetry.
is apparent instead in lengthening of the M–Cl bonds,
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monoanionic, bidentate ligand such as acac). Moreover, the
resulting structures also incorporate one terminal chloride
ligand, which can undergo metathesis with other ligands. As
an example, adducts 3 with a 3,5-bis(trifluoromethyl)phenyl-
acetylide ligand in place of the chloride were prepared under
Sonogashira-type conditions, by treating the parent complexes
with the corresponding alkyne in the presence of base and
copper(I) iodide (Scheme 1). In contrast to the field of plati-
num(II) chemistry, there are relatively few examples of acetylide
adducts of cyclometallated iridium(III) complexes.13a,15
The UV-visible absorption spectrum of 2b (Fig. 4, Table 2)
is typical of cyclometallated Ir(III) complexes, showing very
intense absorption bands at <330 nm attributable to intra-
ligand π–π* transitions, and a set of bands at longer wave-
lengths with ε = 4000–7000 M−1 cm−1, typical for spin-allowed
charge-transfer transitions involving the metal. Some weaker,
but nevertheless well-defined bands at longer wavelength (e.g.,
λ = 591 nm, ε = 440 M−1 cm−1) are likely to be due to corres-
ponding spin-forbidden transitions to triplet excited states,
promoted by the SOC associated with the metal ion. These
visible-region bands are significantly red-shifted compared to
complexes with simple arylpyridine ligands, e.g. Ir(ppy)3,
reflecting the more extended conjugation associated with the
isoquinoline groups. Indeed, the spectrum is very similar to
those of Ir(piq)2(acac) and Ir(piq)3, where piq = 1-phenyl-
isoquinoline.12b,c,e Time-dependent density functional theory cal-
culations (B3LYP, with 6-311G and LanL2DZ basis sets for
ligands and metal ion respectively) indicate that the lowest-
lying singlet and lowest-lying triplet transitions both have pri-
marily HOMO–LUMO character. The corresponding orbital
plots (Fig. 5) show that the LUMO is based on the tridentate
ligand, while the HOMO is delocalised over the metal, the
cyclometallating phenyl ring of the ppy, and the chloride
Table 1 Selected bond lengths (Å) and angles (°) in 2a and 2b
2a
2b, A
2b, B
M–Cl(1)
M–N(1)
M–N(2)
M–N(3)
M–C(2)
M–C(25)
N(1)–M–N(2)
Cl(1)–M–C(25)
C(2)–M–N(3)
N(1)–M–C(2)
N(2)–M–C(2)
2.479(1)
2.050(3)
2.049(3)
2.178(3)
1.907(4)
1.993(4)
160.5(1)
173.0(1)
174.3(2)
80.0(2)
2.462(7)
2.043(13)
2.064(17)
2.183(16)
1.945(17)
2.04(2)
159.7(6)
172.5(6)
172.9(8)
79.6(7)
Ir(2)–Cl(2)
Ir(2)–N(4)
2.461(9)
2.041(13)
Ir(2)–N(5)
Ir(2)–C(36)
Ir(2)–C(49)
N(4)–Ir(2)–N(4′)
Cl(2)–Ir(2)–C(49)
C(36)–Ir(2)–N(5)
N(4)–Ir(2)–C(36)
2.13(2)
1.95(3)
1.96(4)
160.5(8)
170.3(9)
174.8(11)
80.3(4)
80.5(2)
80.2(6)
cf. typical values of 2.31–2.38 Å in RhN3Cl3 and 2.34–2.39 Å in
IrN3Cl3 chromophores.14
Whilst the crystal structure study,†,§ of biqbH revealed
planar quinoline moieties inclined by 49° to each other and to
the central benzene ring, in 2 two fused metallacycles i form a
planar system and the resulting strain (short intra-biqb H⋯H
contacts, see Fig. 2) is partly relieved by warping of the ligand.
Thus, in 2a rings ii and iii tilt to opposite sides of plane i, by
11.5° and 11.4° (Fig. 3). In 2b these rings tilt in the same direc-
tion, by 4.0° and 14.1° in molecule A, and 14.6° in B, but ring
iv tilts in opposite direction (towards the Cl ligand) by 8.2° and
13.2°, respectively.
These systems are unusual in that they comprise two
different cyclometallating ligands, a feature difficult to attain in
complexes based on bidentate ligands, such as the well-known
Ir(N^C)3 or Ir(N^C)2(L^X) classes for example (L^X represents a
ligand. The excited state may thus be described as d/πN^C
π*N^C^N (mixed MLCT/L′LCT) in character.
→
The spectrum of 2a is similar to that of 2b, except for the
absence of the weak, low-energy bands, and the blue-shift of
the intense visible-region absorption bands by ca. 30 nm. The
former can be explained by the smaller spin–orbit coupling
Fig. 3 Comparison of the molecular conformation of biqb in the crystal struc-
tures of biqbH, 2a and two independent molecules of 2b. Intra-ligand H⋯H
contacts shown with arrows, measure 1.74–1.80 Å in 2a and 1.89–1.90 Å in 2b,
assuming C–H bond distances of 1.08 Å (= internuclear distance established by
neutron diffraction). Refinement of the direction (with fixed length) of these
C–H bonds showed them bend out of the corresponding aromatic ring planes,
so as to increase the H⋯H distance; however, the effect lies within the experi-
mental error.
Fig. 4 UV-visible absorption spectra of complexes 2 and 3 in CH2Cl2 at 298 K
(left scale), and the excitation spectrum of 2a (green, right scale).
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Table 2 Photophysical properties of the rhodium(III) and iridium(III) complexes in CH2Cl2 at 298 K except where indicated otherwisea
Emission at 77 Kb
Comp.
Absorption λmax/nm (ε/M−1 cm−1
)
Emission λmax/nm
τ/μs
Φlum
λmax/nm
τ/μs
2a
2b
252sh (24 400), 330 (6790), 345 (6960), 426 (6200), 447 (6200)
259sh (29 100), 320sh (12 400), 347sh (7810), 414 (4280),
455sh (5590), 477 (6490), 549sh (870), 591 (440)
575, 618, 678sh
611, 653
16 [0.91]
1.7 [0.49]
0.10
0.40
564, 611, 667, 735
593, 644, 703
53
2.8
3a
3b
242 (47 700), 286 (29 400), 345 (11 700), 428 (7210), 453 (9490)
250 (52 200), 307 (32 000), 346sh (16 400), 406 (6270),
462 (9250), 488 (8880), 538sh (2450), 584 (880)
574, 616, 675sh
603, 644
15.6 [0.95]
1.7 [0.38]
0.06
0.51
561, 608, 662, 729
585, 636, 698
49
3.2
a Experimental details are given in the ESI. b In diethyl ether–isopentane–ethanol (2 : 2 : 1 v/v).
Fig. 6 Cyclic voltammograms of M(biqb)(ppy)Cl, M = Rh (2a, blue) and Ir (2b,
red). Recorded in CH2Cl2 at 298 K, in the presence of Bu4NPF6 (0.1 M) as sup-
porting electrolyte; scale is relative to the Fc|Fc+ couple.
Fig. 5 Frontier orbital plots of the Rh(III) complex 2a (left) and of the Ir(III)
complex 2b (right).
constant of Rh compared to Ir, so that the spin selection rule
that inhibits the S0 → T1 transition will not be relaxed to the
same extent. The shift, observed earlier for cyclometallated Rh
complexes,5 can be understood in terms of the lower energy of
the Rh 4d compared to the Ir 5d orbitals, which will serve to
raise the energy of those excited states that feature significant
MLCT character. This interpretation is supported by electro-
chemical evidence: the cyclic voltammogram (versus Fc|Fc+, in
CH2Cl2 at 298 K, Fig. 6) of 2b displays a reversible oxidation at
+0.69 V, compared to +1.07 V for 2a.
Fig. 7 Normalised luminescence spectra of 2a (blue) and 2b (red) in CH2Cl2 at
298 K (solid lines) and in Et2O–isopentane–ethanol (2 : 2 : 1 v/v) at 77 K (dotted
lines).
The absorption spectra of the acetylide adducts 3 resemble
those of the parent complexes 2, but show higher extinction
coefficients throughout the spectra (Fig. 4, Table 2).
Upon irradiation into the visible or UV bands, 2a displays
intense orange-red luminescence in deoxygenated solution at decay is substantially reduced compared to Rh(III) complexes
room temperature (Fig. 7, Table 2). The quantum yield of phos- with bidentate cyclometallating ligands, an effect likely to be
phorescence of 0.10 is unprecedented for a cyclometallated linked – at least in part – to the high rigidity associated with
rhodium complex and, indeed, apparently for any type of tridentate binding.16 However, given that an analogous Rh(III)
rhodium(III) complex. Moreover, the room temperature lifetime complex of dpyb displays a quantum yield an order of magni-
of 16 μs is exceptionally long, and contrasts with values of <10 tude lower than 2a, other factors must also be involved.17 The
ns for most complexes based on the Rh(ppy)2 unit, for spectrum recorded at 77 K is slightly blue-shifted compared to
example (with the exception of those of ref. 8a). The long life- room temperature, and displays a well-resolved vibrational pro-
time and high quantum yield of 2a suggest that non-radiative gression, ν ∼ 1400 cm−1. Such a spectrum is typical of an
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emissive excited state that has significant ligand-centred char-
acter, as supported by the TD DFT calculations (Fig. 5).†
The acetylide adduct 3a displays similar luminescence pro-
perties to the parent chloro complex (Fig. 8); there is no aug-
mentation in efficiency of the type often observed in
complexes of metals such as Pt(II) when there are low-lying d–d
Notes and references
‡Synthesis and characterisation of 2a. A suspension of 1,3-bis(1-isoquinolyl)-
benzene (0.050 g, 0.150 mmol) and rhodium trichloride trihydrate (0.053 g,
0.150 mmol) in 2-ethoxyethanol (7 mL) and water (3 mL) was heated to 80 °C for
24 h. After cooling to room temperature, the precipitate was collected via cen-
trifugation and washed with water (3 × 5 mL), ethanol (3 × 5 mL) and diethyl
states present, which are displaced to higher energy on the ether (3 × 5 mL). Upon drying in vacuo, the crude product (1a) was obtained as a
introduction of a strong-field acetylide co-ligand.18
The Ir(III) complexes 2b and 3b both display very intense
red luminescence in solution (Fig. 7 and 8). Again, some
yellow solid; further product was obtained by repeating the procedure with the
remaining solutions (0.059 g, 66%). The crude product, silver triflate (0.056 g,
0.22 mmol) and 2-phenylpyridine (200 μL, 1.78 mmol) were placed in a Schlenk
tube, which was degassed three times via evacuating and backfilling with nitro-
vibrational structure is apparent at room temperature, well-
gen. The mixture was heated at 110 °C under nitrogen for 20 h then cooled to
resolved at 77 K, suggestive of more ligand-centred character room temperature and CH2Cl2 (35 mL) was added. Solid residue was removed by
filtration and the filtrate washed with HCl (1 M, 3 × 35 mL), dried over MgSO4
than Ir(ppy)3 for example. The emission maxima and spectral
profiles satisfy well the requirements for good red emitters for
displays; e.g. C.I.E. colour coordinates for 2b are (0.65, 0.34).
Moreover, the luminescence quantum yields of around 0.5
and the solvent removed under reduced pressure. Purification was carried out by
column chromatography (silica, CH2Cl2–methanol, gradient elution from 100 : 0
to 99 : 1) to give the product as a yellow solid (0.065 g, 35%). 1H NMR (CDCl3,
700 MHz) δ = 10.27 (1H, d, 3J = 5.0, H6-ppy), 9.12 (2H, d, 3J = 7.7, H3-NCN), 8.62
(Table 2) are particularly high for red-emitting complexes, (2H, d, 3J = 7.7, H4′-NCN), 8.14 (1H, d, 3J = 11.2, H3-ppy), 8.10 (1H, td, 3J = 8.4,
3J = 1.4, H4-ppy), 7.63–7.70 (8H, m, H4-NCN and H5-ppy and H3′-ppy and H5-NCN
where the combined effects of increased non-radiative decay
and H8-NCN, and H6-NCN or H7-NCN), 7.53–7.56 (3H, m, H5′-ppy, and H6-NCN
through vibrational deactivation and decreased radiative decay
or H7-NCN), 7.22 (2H, d, 3J = 4.5, H5-NCN or H8-NCN), 6.74 (1H, td, 3J = 6.0, 4J =
constants (which vary as ν3) tend to limit efficiencies compared
1.0, H4′-ppy), 6.51 (1H, td, 3J = 5.5, 4J = 1.0, H5′-ppy), 5.91 (1H, d, 3J = 8.4, H6′-
ppy). 13C NMR (CDCl3, 176 MHz) δ = 188.5, 167.9, 165.7, 157.9, 150.7, 143.8,
143.8, 142.7, 138.0, 136.8, 130.7, 129.8, 128.2, 127.7, 127.6, 125.9, 123.0, 122.6,
122.3, 121.9, 120.9, 119.0, 113.6, 110.2, 92.3, 92.1. MS (ASAP+) m/z 623.0 [M]+.
HRMS (ASAP+) Calcd for C35H2335ClN3108Rh: m/z 623.0636. Found: m/z 623.0643.
Mp > 250 °C.
to structurally similar green emitters. The values are compar-
able to the most efficient red phosphors reported.12
In summary, bis(isoquinolyl)benzene is shown to bind as a
tridentate N^C2^N ligand to Rh(III) or Ir(III). Complexes incor-
porating this ligand in combination with a second, bidentate,
cyclometallating ligand are highly luminescent in the orange
or red region respectively. 2a has higher phosphorescence
quantum yield than any rhodium complex hitherto reported,
whilst the Ir(III) complexes 2b and 3b are excellent red emitters.
Given the recent successful demonstration of complexes of
related dipyridylbenzene ligands as OLED emitters,11b,13b and
the ability to further fine-tune the emission wavelength
through the bidentate ligand, these new compounds may be
attractive in such applications, as well as in the area of time-
resolved bioimaging, where intense, long-lived emission in the
red region – to which biological tissue is most transparent – is
particularly desirable.3d
§CCDC 937441 (biqbH), 937442 (2a), 937443 (2b).
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