Fraternal twin iridium hemicage chelates

Article abstract of DOI:10.1039/c1dt11236h

The synthesis and complete photophysical characterization of rigidified neutral hemicage iridium complexes are presented. The hemicage ligands were obtained via a modular synthesis, which will facilitate the expansion of future hemicage syntheses. Slight

Full text of DOI:10.1039/c1dt11236h

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PAPER
Fraternal twin iridium hemicage chelates†
Gabriel St-Pierre, Se´bastien Ladouceur, Daniel Fortin and Eli Zysman-Colman*
Received 28th June 2011, Accepted 19th September 2011
DOI: 10.1039/c1dt11236h
The synthesis and complete photophysical characterization of rigidified neutral hemicage iridium
complexes are presented. The hemicage ligands were obtained via a modular synthesis, which will
facilitate the expansion of future hemicage syntheses. Slight variations in structure between the two
iridium hemicage podates reveal subtle differences in photophysical behavior, which will aid in the
design of functional materials. A parallel computational investigation corroborates the experimental
findings. The insight gleaned from this study will have an impact for the design of iridium-based
luminophores for OLED-type applications.
than their acyclic congeners;10b,10e nonradiative decay processes are
inhibited as a result of increased rigidity of the ligand framework
resulting in greater luminescence.8,10f
Herein we report the modular synthesis of two HC ligands, HC1
(Scheme 1) and HC2 (Scheme 2), and their complexation with
iridium(III), and compare the photophysical properties of these
podates to that of fac-Ir(ppy)3 (ppyH = 2-phenylpyridine) (1) and
the chiral iridium hemicage constructed by Von Zelewski.11
Introduction
Tripodal structures, which are nominally formed from a benzene
skeleton equipped with three pendant ligand arms flexibly attached
to the arene with meta dispositions with respect to each other,
serve as important scaffolds used within a variety of applications.
When a guest analyte is introduced, the C3-symmetric podand
host, or hemicage (HC), adopts an all syn conformation wherein
the three arms of the host act to complex the guest on the
same face. Hexasubstitutued analogs are particularly effective,
though not necessary, for sterically preorganizing the host. The
arms of the host are staggered thus preorienting the three ligand-
containing arms onto the same face of the central arene.1 The
hemicage architecture has been exploited towards the generation
of fluorescent chemosensors for heparin,2 as logic gates,3 as
siderophores,4 as selective hosts for both cations5 and anions6 and
as air-stable lanthanide catalysts for Diels–Alder reactions.7
In particular, metals such as In, Al and Ga have been complexed
with 8-hydroxyquinoline-derived HCs,8 while Fe,9 Ru9–10 and Zn10e
complexes have formed from bipyridine-derived HCs. Notably,
there is but one report of an Ir–HC complex,11 the ligand arms be-
˚
ing derived from phenylpyridine with the Ir situated ca. 10 A above
the plane of the arene. Recently, Velders and co-workers reported
the first IrIII caged complex, bearing tris(2-aminoethyl)amine
caps.12 Owing to their structure, metal complexes of HCs are
endowed with several advantages: they are more chemically inert
Scheme 1 Synthesis of HC1: (a) 1. 1.05 equiv. tBuLi/THF, -78 ◦C,
30 min, 2. 1.05 equiv. ZnCl2/THF, -78–0 ◦C, 150 min, 3. 0.95 equiv.
2,5-dibromopyridine, 5 mol% Pd(PPh3)4/THF, reflux, 18 h; (b) 2.4 equiv.
TMSA, 16 mol% CuI, 6 mol% Pd(PPh3)4/3 : 1 THF/i-Pr2NH, reflux, 16 h;
(c) 2.30 equiv. K2CO3/MeOH, RT, 30 min; (d) 0.25 equiv. 5, 3 mol%
CuI, 2.5 mol% Pd(PPh3)4/1 : 1 PhMe/NEt3, reflux, 18 h; (e) H2, 10 mol%
Pd/C/THF, RT, 3 d.
2500 Blvd de l’Universite´, Sherbrooke, J1K 2R, Canada. E-mail: eli.zysman-
colman@usherbrooke.ca; Fax: 819-821-8017; Tel: 819-821-7922
† Electronic supplementary information (ESI) available: Complete exper-
imental section and compound characterization including 1H assignment
for ligands and complexes. Summary of crystallographic parameters for
Ir.HC2. Methodologies used for photophysical characterization. Details of
computational methodology. Full computational output, including figures
of calculated absorption spectra, tables of 50 lowest energy transitions
calculated by TD-DFT and tables of calculated emission energies and
isodensity surface plots for selected orbitals of Ir.HC1 and Ir.HC2. CCDC
reference number 831090. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11236h
Results and discussion
The modular synthetic approach undertaken enabled a facile
and convergent construction of the desired podands. 2-
Phenyl-5-bromopyridine 2 was quickly accessed in excellent
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Scheme 2 Synthesis of HC2: (a) pyridine, RT, 10 min; (b) 1.0 equiv.
Methacrolein, 5 equiv. NH4OAc/MeOH, reflux, 18 h; (c) 1.5 equiv. TMSA,
8 mol% CuI, 10 mol% Pd(PPh3)4/1 : 1 PhMe/Et3N, reflux, 18 h; (d) 2.3
equiv. K2CO3/MeOH, RT, 30 min; (e) 0.25 equiv. 5, 2 mol% CuI, 2.5 mol%
Pd(PPh3)4/1 : 1 PhMe/NEt3, reflux, 18 h; (f) H2, 10 mol% Pd/C/THF, RT,
3 d.
yield via a Negishi13 reaction between bromobenzene and
2,5-dibromopyridine (Scheme 1). Sonogashira14 coupling with
trimethylsilylacetylene (TMSA) followed by base deprotec-
tion afforded 5-ethynyl-2-phenylpyridine 4 in excellent yield.
Three equivalents of 4 were efficiently coupled with 1,3,5-
tribromobenzene 5 to afford 6, which was subjected to hydro-
genation conditions over 3 days to cleanly and efficiently afford
HC1 (7).
HC2 was constructed in an analogous sequence in excellent
yield (Scheme 2). The methyl group was incorporated into 10 as
the yield for the Kro¨hnke15 condensation of 9 with methacrolein
was superior to that with acrolein. The methyl group should have
little impact upon the photophysical properties of metal complexes
of HC2.
Fig. 1 Complexation of Ir(acac)3 with (a) HC1 and (b) HC2. 1H
NMR of ligand in red and complex in blue in CD2Cl2, with peak
assignments. Computed structures are shown for the complexes. * indicates
non-deuterated DCM solvent.
Treatment of each of HC1 and HC2 with Ir(acac)3 in refluxing
ethylene glycol over 24 h produced the homoleptic complexes with
facial geometries Ir.HC1 and Ir.HC2 in 33% and 37%, respectively.
The 1H NMR spectrum of the hemicaged complexes Ir.HC1
and Ir.HC2 reveal a single set of phenylpyridine protons, thus
establishing the three-fold symmetry of the complexes (Fig. 1).
Exact structural assignment was determined through a series
of 1D- and 2D-NMR experiments (DQCOSY, COSY, HMBC
NOESY). Notably, protons H1 and H3¢ are significantly shifted
upfield (ca. 2 ppm) upon complexation. In fact, H1 in Ir.HC1 and
H3¢ in Ir.HC2 are oriented to reside in the anisotropic cone of two
Fig. 2 Crystal structure of Ir.HC2. (a) Crystal packing of dimeric pair
of complexes; (b) ORTEP perspective representation (ellipsoids at 50%
probability). A molecule of CHCl3 has been removed for clarity. The red
spheres represent calculated centroids of the aromatic rings.
2
aryl rings and within the dz orbital of the iridium metal, resulting
in extensive shielding of the proton; aromatic H3¢ in Ir.HC2 for
example is shifted to ca. 5.4 ppm.
The crystal structure of Ir.HC2 was obtained by slow evap-
oration of a chloroform solution and crystallizes as a distorted
octahedron within a centrosymmetric trigonal space group (R-3),
where each unit cell contains six molecules as three enantiomeric
˚
iridium metal resides 5.47 A above the centroid of the central
arene while there are C–H–p interactions between H3¢ and the two
proximal arenes (Fig. 2). There are no intermolecular interactions
between hemicages in the unit cell.
The photophysical properties for Ir.HC1 and Ir.HC2 are
summarized in Table 1 and Fig. 3 and are compared to archetypal
fac-Ir(ppy)3 (1).
dimeric pairs (Fig. 2a).16 Bond lengths and bond angles (Ir–Cppy
=
2.01 A; Ir–Nppy = 2.14 A; Nppy–Ir–Cppy = 80◦) are unremarkable
when compared to those in 117 or in Von Zelewski’s iridium
hemicage.11 Thus, the presence of short 2-carbon methylene arms
does not unduly impact complex formation nor significantly
distort the octahedral geometry about the iridium center. The
˚
˚
Absorption and emission profiles for Ir.HC1 are quite similar
to those of Ir.HC2, exhibiting only a slight bathochromic shift of
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the periphery of the hemicage. Based on this electronic analysis,
solvent molecules should stabilize less Ir.HC1 than Ir.HC2. A
quantification and corroboration of the assignment of dipoles is
discussed below. Emission spectra for the three complexes in this
study obtained at 77 K in the glass state in 1 : 1 EtOH/MeOH were
found to be blue shifted and displayed fine vibrational structure
compared to those taken at 298 K (see ESI†).
Quantum yields are highly solvent dependent. Ir.HC1 was
poorly soluble in most organic solvents and exhibited low
quantum efficiency in chloroform (U = 17%), which is consid-
erably lower than that obtained for 1 (U = 41% – standard:
[Ru(bpy)3](PF6)2; UPL = 9.5%).21 Ir.HC2 was even less luminescent
in CHCl3. Each, however were highly luminescent in polar
aprotic solvents (UBuCN(Ir.HC1) = 78%; UACN(Ir.HC2) = 79%),
exhibiting quantum yields comparable to that found for 1 (UACN
=
78%) but superior to that found for Von Zelewski’s hemicage
(UACN = 51%).11 By contrast, K-Ir(pppy)3 (pppyH = (8R,10R)-2-
(2¢-phenyl)-4,5-pinenopyridine) was found to be less luminescent
than the corresponding hemicage (UACN = 64%), perhaps due
to the flexibility inherent in the structure of the podand.11 As
first elucidated by Watts and co-workers,22 the lower quantum
yields and shorter lifetimes (see below) found in CHCl3 are most
probably due to the photoreactivity of the solvent that leads
to quenching of the emission of the complexes, similar to that
observed for the behavior of 1 in DCM. The relative quantum
yield for 1 in 2-MeTHF (U = 97%) was found to be much
greater than that first reported by Thompson and co-workers (U =
40%).23 More recently, the room temperature quantum efficiency
of 1 has been measured using the absolute method24 in solid
matrix hosts such as 4,4¢-N,N¢-dicarbazole-biphenyl (CBP) and
polymethylmethacrylate (PMMA) to be in excess of 90%,25 while
RT measurements in degassed 2-MeTHF and DCM were found
to be 97%26 and 90%,18a,25b respectively. The quantum yield for 1 in
deaerated toluene using the relative method with quinine sulphate
dehydrate as the reference compound was recently reported to be
73%.27 From our internally reproducible results we can conclude
that in nitrile solvents that Ir.HC1 and Ir.HC2 are just as brilliant
phosphors as 1.28 Quantum efficiencies in air-equilibrated solution
were comparable to that found by Von Zelewski and co-workers.
Lifetimes at 77 K in the glass state were on the microsecond
timescale while those at 298 K were on the submicrosecond time
scale. Whereas Ir.HC2 has a room temperature lifetime (t = 888
ns) in chloroform solution similar to that of 1, Ir.HC1 possesses
a lifetime that is half as long (t = 443 ns). Room temperature
lifetimes obtained in polar aprotic nitrile solvents reveal a different
behavior. In BuCN, Ir.HC1 exhibits a lifetime of 1482 ns, longer
than that found for either 1 or the hemicaged iridium complex
under investigation by Von Zelewski. The lifetime for Ir.HC2
in BuCN is similar (1445 ns) to that of Ir.HC1 but decreases
somewhat in ACN (887 ns), a similar value to that obtained in
chloroform. The increased lifetime observed in BuCN compared
to ACN is most likely due to the increased viscosity of the former,
which limits molecular movement that can lead to non-radiative
dissipation of energy.29 The lifetime of Ir.HC2 in ACN is noticeably
shorter than that measured for 1 (1322 ns).
Fig. 3 Photophysical properties of hemicage podates. (a) Photos of
N2-degassed solutions in CHCl3 and nitrile solvents of 1, Ir.HC1 and
Ir.HC2, irradiated at 325 nm; (b) Absorption (solid) and emission (dashed)
for 1, Ir.HC1 and Ir.HC2 at 298 K in ACN (the spectra for Ir.HC1 were
obtained in BuCN).
5 nm for lem in CHCl3. Intense bands found at wavelengths inferior
to 300 nm were assigned to ligand-centred (1LC) p–p* transitions
1
1
of the ppy ligand.18 The mixed LC and MLCT d–p* band at
ca. 380 nm is more intense for Ir.HC2 compared to either Ir.HC1
or 1 and this difference in intensity is more marked in CHCl3
than in nitrile solvents (see ESI†). The small absorption bands at
longer wavelengths have been assigned to spin-forbidden mixed
3LC and 3MLCT transitions and are of similar amplitudes for the
three complexes under investigation. The assignments for these
transitions are similar to those reported by Hofbeck and Yersin18a
for 1 and are corroborated by our computational investigation (see
below).
The phosphorescence emission spectrum is broad and essen-
tially featureless for both podates at 298 K and are not dissimilar
from that of 1, exhibiting a hypsochromic shift by ca. 12 nm in
CHCl3 compared to the spectrum of 1 (lem = 518 nm). The shape
of the emission spectrum implies that the origin of the emission
3
is in part characteristic of a MLCT transition. It is generally
accepted that emission from complexes such as 1 can be described
3
3
as an admixture of MLCT and LC transitions.19 Remarkably,
though there is a pronounced red shift of 21 nm for Ir.HC2 when
migrating from CHCl3 to the more polar nitrile solvents ACN and
BuCN,20 similar to that found for 1, there is little observed change
in lem for Ir.HC1 (there is also essentially no change in absorption
and emission energies between ACN and BuCN for Ir.HC2; see
Fig. S5†). For Ir.HC1, BuCN was necessary to solubilize the
complex. Based on the behavior of Ir.HC2 in nitrile solvents, the
surprising lack of solvatochromism inherent in Ir.HC1 is thus
not due to the identity of the alkyl group on the nitrile solvent.
One possible explanation for the lack of solvatochromism is based
on an analysis of the orientation and magnitude of the dipole
moments of each of the hemicages. The orientation of the dipole
should emanate from the iridium center and be projected towards
the three nitrogen atoms. Ir.HC1 would thus have a dipole directed
towards the central benzene while Ir.HC2’s dipole is projected to
Radiative (kr = UPL/t) and non-radiative (knr = kr(1 - UPL)/t
were determined for the three complexes under study. Of par-
ticular note are the large non-radiative (knr) decay constants in
CHCl3, which are particularly significant for the two hemicage
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podates being of an order of magnitude larger than their
corresponding kr.
A combined DFT and TD-DFT computational study was
performed without imposition of symmetry in order to elucidate
the photophysical behavior of each. These results are summarized
in Fig. 4 (see ESI for details†).
Emission energies were determined as the difference between
the total energy of the triplet state (T1) and the total energy of the
ground state (S0) for each of the complexes, with structures opti-
mized at their respective states.13b,32 Emission energy predictions
of 473 nm and 479 nm for Ir.HC1 and Ir.HC2 are nearly identical
and reflect quite well the experimental values obtained at 77 K of
491 nm and 494 nm, respectively. The emission energy prediction
for 1 is 480 nm, hypsochromically shifted by 16 nm compared that
measured in the EtOH/MeOH glass state.
The magnitude of the calculated ground state dipole moments
for 1, Ir.HC1 and Ir.HC2 were found to be 6.52, 13.51 and
10.29 Debye, respectively. The magnitude of the corresponding
calculated excited triplet state dipole moments for 1, Ir.HC1 and
Ir.HC2 were found to be 5.80, 11.82 and 8.79 Debye, respectively.
Inspection of the orientation of the dipole moments in each of
the hemicages reveals that they are aligned principally along the
x-axis of the molecule. Whereas both the ground and excited state
dipole moments for Ir.HC2 are oriented away from the central ring,
the orientation of the ground and excited state dipole moments
towards the central benzene ring of Ir.HC1 explains the absence
of solvatochromism for this complex compared to the other two
in the study. In this latter case, dipole–dipole interactions will be
weaker despite an overall larger magnitude for the dipole moments
in both the ground and the excited state.
Fig. 4 Calculated absorption (HOMO➞LUMO) and emission energies,
assigned transition type and isodensity surface plots for each of the
3HSOMO and 1HOMO for (a) Ir.HC1 and (b) Ir.HC2.
For each of the hemicage complexes, electron density in the
Conclusions
2
ground state (S0) is found principally on the iridium (dz ),
extending onto the phenyl fragment of the ppy-type ligand. In
the case of Ir.HC2, this results in the preponderance of the
electron density being sterically shielded by the central arene
whereas in the case of Ir.HC1, the electron density is mainly on
the periphery of the complex. This is due to the difference in
the placement of the pyridine ring in each of the complexes. For
Ir.HC1, the principle low energy absorption transition predicted
by TD-DFT (f = 0.011) is found to be at 415 nm (see ESI†)
and results mainly from a HOMO ➞ LUMO transition and
In summary, we have synthesized two hemicage podands via
a modular synthetic approach. The facial iridium(III) podates
subsequently formed exhibit similar photophysical characteristics
with two stark exceptions: their lifetime behavior and emission
maxima in disparate solvents. Whereas Ir.HC2 emits with a
lifetime of about 888 ns in each of CHCl3 and ACN and exhibits
a large bathochromic shift of 21 nm between the two solvents,
the lifetime for Ir.HC1 in BuCN was found to be four times
longer at 1.5 ms than in CHCl3 with lem being essentially solvent-
insensitive. The long lifetime and lack of solvatochromism in
BuCN for Ir.HC1 may be attributed to the increased steric bulk
of the butyl group, which impedes a significant stabilization of the
polar excited state and thus the observed red shift. In all other
respects, Ir.HC1 behaves similarly to 1. The large bathochromic
shift in emission maxima for Ir.HC2 is due to a combination of
its electronically shielded ground state and electronically exposed
excited state (Fig. 4). The polar ACN solvent can preferentially
stabilize the excited state more than the ground state leading to
the larger red shift. The similar lifetimes observed in CHCl3 and
ACN for Ir.HC2 seem to primarily be due to an increased knr/kr
ratio in chloroform (8.3 for Ir.HC2 vs. 5.0 for Ir.HC1). Upon
irradiation, reactive chlorinated species can more easily attack
1
can be characterized as mixed MLCT/1LC/1ILCT transition.
Analogously, for Ir.HC2, the HOMO ➞ LUMO transition at
420 nm is significant (f = 0.013) and can similarly be characterized
as mixed 1MLCT/1LC/1ILCT transition. By contrast for 1, nearly
degenerate transitions occurring at 401 nm from HOMO-1 ➞
LUMO (f = 0.022) and HOMO-2 ➞ LUMO (f = 0.022) are
dominant.
In the excited triplet state for each of the hemicage complexes
the electronic density of the 3HSOMO, (see ESI for other ground
and excited state MOs†) is found to be localized on a single arm
of the hemicage, distributed equally about the ppy-type moiety,
similar to that described by Koseki and co-workers.30 There is also
a small contribution from the metal in the form of a dyz orbital.
Inspection of the 3HSOMO and 3LUMO orbitals in the triplet state
for each of the complexes reveals that their superposition mirrors
the 1LUMO found for their corresponding S0 ground states. The
majority of the electron density found in the 3HSOMO is localized
on the ppy ligands whereas in the ground state HOMO, it is found
on both the metal and on the phenyl fragment of the ppy ligand.
Thus, the nature of the emission in 1 can be generally characterized
2
the exposed Ir(dz ) orbital (S0 state) in Ir.HC2 than in Ir.HC1.
The incorporation of these two complexes into OLED devices is
currently under investigation and results thereof will be reported
elsewhere.
Acknowledgements
3
3
as an admixture of MLCT and LC transitions, consistent with
experimental analyses (see above). This conclusion mirrors those
found in other theoretical investigations.31
Computational time on the Mammouth supercomputer at
the Universite´ de Sherbrooke was underwritten by the
RQCHP (re´seau que´be´cois de calculs de haute performance).
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EZ-C acknowledges CFI (Canadian Foundation for Innovation),
NSERC (the National Sciences and Engineering Research Council
of Canada), FQRNT (Le Fonds que´be´cois de la recherche sur la
nature et les technologies) and the Universite´ de Sherbrooke for
financial support. SL acknowledges the Universite´ de Sherbrooke
for an institutional scholarship.
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