Luminescence and theoretical studies of Cu(tripod)X [tripod = 1,1,1-tris(diphenylphosphanylmethyl)ethane; X- = halide, thiophenolate, phenylacetylide]

Article abstract of DOI:10.1002/ejic.200400959

Complexes of the type Cu^I(tripod)X [tripod = 1,1,1 -tris (diphenylphosphanylmethyl)ethane; X^- = Br^-, I^-, PhS^-, PhC≡C^-] are phosphorescent in solution and in the solid state (λ_max ≈ 465 nm). Calculations show that the emissive triplet is of mixed MLCT/LLCT character. The emission is facilitated by the rigid tetrahedral structure, which is imposed by the tripod ligand. Accordingly, a distortion towards a square-planar geometry, which should occur upon MLCT excitation, is prevented. On the other hand, in the triplet state the phenyl substituents of the phosphane ligands undergo a rotation which favours radiationless deactivation. As a result, the emission efficiency is relatively small in solution, but much higher in the solid state owing to the rigidity of the lattice. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200400959

FULL PAPER
Luminescence and Theoretical Studies of Cu(tripod)X [tripod = 1,1,1-
Tris(diphenylphosphanylmethyl)ethane; X– = Halide, Thiophenolate,
Phenylacetylide]
Valeri Pawlowski,[a][‡] Günther Knör,[a] Christian Lennartz,*[b] and Arnd Vogler*[a]
Keywords: Electronic spectra / Luminescence / Copper complexes / Phosphane ligands / Quantumchemical calculations
Complexes of the type CuI(tripod)X [tripod = 1,1,1-tris(di-
phenylphosphanylmethyl)ethane; X– = Br–, I–, PhS–, PhCϵC–] substituents of the phosphane ligands undergo a rotation
vented. On the other hand, in the triplet state the phenyl
are phosphorescent in solution and in the solid state (λmax
Ϸ
which favours radiationless deactivation. As a result, the
emission efficiency is relatively small in solution, but much
higher in the solid state owing to the rigidity of the lattice.
465 nm). Calculations show that the emissive triplet is of
mixed MLCT/LLCT character. The emission is facilitated by
the rigid tetrahedral structure, which is imposed by the tripod
ligand. Accordingly, a distortion towards a square-planar ge-
ometry, which should occur upon MLCT excitation, is pre-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
plexes of the type CuI(tripod)X [tripod = 1,1,1-tris(diphen-
ylphosphanylmethyl)ethane; X– = Br–, I–, PhS–, PhCϵC–]
were expected to serve this purpose (Scheme 1).
Introduction
The luminescence of transition metal complexes is a
rapidly expanding research field.[1,2] Potential applications
such as optical sensors or OLEDs (organic light-emitting
diodes) have stimulated many studies in this area.[3,4] The
appearance of a luminescence at room temperature is
largely restricted to complexes of metals which belong to
the second and third transition row, while complexes of
first-row transition metals are frequently not emissive under
ambient conditions. However, there is one notable excep-
tion. A variety of mononuclear and polynuclear CuI com-
plexes are luminescent at room temperature in solution or
in the solid state.[5–8] In particular, mononuclear complexes
with polypyridine ligands have attracted much interest.[8,9]
These compounds are characterized by emissive low-energy
MLCT (metal-to-ligand charge transfer) states. (Phos-
phane)CuI complexes constitute another group of lumines-
cent compounds[5] which, however, have not been investi-
gated to the same extent as (polypyridine)CuI complexes.
In particular, the nature of the emitting excited states of
(phosphane)CuI complexes is not clear at all. The present
study was undertaken to gain more insight into the excited-
state properties of (phosphane)CuI complexes and to define
structural requirements for potential applications. Com-
Scheme 1.
The rigid structure of these complexes,[10] which is im-
posed by the tripod ligand, should facilitate this work.
These complexes are kinetically stable, soluble in a variety
of organic solvents and volatile at higher temperatures with-
out decomposition. Moreover, the rigidity of the com-
pounds prevents extensive excited-state distortions. Accord-
ingly, calculations of the frontier orbitals may be sufficient
to characterize the lowest-energy excited state. In addition,
the lack of flexibility could block radiationless deactivations
and increase the luminescence efficiency. Finally, the rigid-
ity should reduce the Stokes shift. As a result, the emission
is expected to undergo a blue-shift. In this context, it should
be mentioned that for certain applications such as OLEDs
blue triplet emitters are requested, but are not yet available
to the same extent as green and red molecular phosphors.
[a] Institut für Anorganische Chemie, Universität Regensburg,
93040 Regensburg, Germany
E-mail: arnd.vogler@chemie.uni-regensburg.de
[b] BASF AG, Computational Chemistry and Biology, GVC/C-
A30,
Results and Discussion
67057 Ludwigshafen, Germany
Experimental Results
E-mail: christian.lennartz@basf-ag.de
[‡] Present address: Institute of Molecular and Atomic Physics,
National Academy of Science of Belarus, 220072 Minsk, Be-
larus
The neutral compounds Cu(tripod)X [X– = Br–, I–, PhS–,
PhCϵC–] were readily obtained as white or slightly yellow-
Eur. J. Inorg. Chem. 2005, 3167–3171
DOI: 10.1002/ejic.200400959
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3167

V. Pawlowski, G. Knör, C. Lennartz, A. Vogler
FULL PAPER
ish solids by reaction of the corresponding copper(i) precur- tylide complex 4 displays a conspicuos structuring with a
sors CuX with the tridentate phosphane ligand tripod rather sharp maximum at 465 nm and a clearly resolved
(Scheme 1). Their monomeric composition was confirmed shoulder in the region of 510 nm. The peak separation of
by elemental analysis, and in the case of the halide deriva- about 2000 cm–1 approximately matches the CϵC stretch-
tives 1 and 2 also by conductivity and molecular weight ing frequency of the ligand X. In solution, however, a broad
measurements.[11]
and nearly structureless band with λmax = 475 nm is present
While the electronic spectrum of the tripod ligand in in the organometallic derivative 4 as well. The spectroscopic
CH2Cl2 exhibits a featureless ultraviolet band at λmax data for all of the copper(i) complexes 1–4 in the solid state
=
251 nm, the absorption spectra of the corresponding copper and in dichloromethane solution are summarized in
complexes display their maxima in the 300 nm spectral re- Table 1.
gion (Figure 1). These bands are rather intense (ε =
13650 m–1 cm–1 at λmax = 298 nm for 2 in CH2Cl2) and sig-
nificantly red-shifted in the order of 4000–6500 cm–1 from
the absorption of free tripod. In the case of the thiophenol-
ate derivative 3, an additional splitting of the main absorp-
tion band occurs with two well-resolved peaks at 270 and
290 nm.
Figure 2. Electronic absorption (a) and emission (e) spectrum of
Cu(tripod)SPh (3) at room temperature. Absorption: CH2Cl2, 1-
cm cell. Emission: λexc = 300 nm, solid and in CH2Cl2 solution
(dashed curve), intensity in arbitrary units.
Table 1. Absorption and emission maxima of Cu(tripod)X at room
temperature.
X
λmax [nm]
(CH2Cl2)
λem [nm] (solid)
λem [nm]
(CH2Cl2)
Figure 1. Electronic absorption (a) and emission (e) spectra of Cu-
(tripod)I (2) at room temperature and at 77 K (dashed curve). Ab-
sorption: 7.36×10–5 m in CH2Cl2, 1-cm cell. Emission: λexc
=
Br
I
SPh
CϵCPh
296
298
270, 290
297
462
461
465
462
465
508
475
300 nm, solid and in toluene matrix, intensity in arbitrary units.
465, 510
All of the Cu(tripod)X complexes 1–4 exhibit an intense
blue emission with peaks around 465 nm when excited with
ultraviolet light in the solid state. This feature is shown in
Figure 1 for Cu(tripod)I (2) with λmax = 462 nm. At 77 K
in a low-temperature toluene matrix, the luminescence
maximum of 2 is blue-shifted to λmax = 453 nm and the
Discussion
The compounds Cu(tripod)X are monomeric and tetra-
spectrum is slightly narrowed, while no significant structur- hedral.[10,11] They are kinetically stable in distinction to
ing or larger change of shape of the emission band is ob- other CuI complexes with monodentate or bidentate phos-
served (Figure 1). In argon-saturated dichloromethane solu- phane ligands, which undergo dissociation and dimerization
tion, the luminescence of compound 2 shows a broad maxi- in solution.[12,13] The electronic spectra of various (aryl-
mum with λmax = 465 nm. A rather low quantum yield of phosphane)CuI complexes have been reported.[5] However,
φ = 8×10–4 is obtained in CH2Cl2 relative to quinine sulfate assignments for the lowest-energy excited states seem to be
as a standard, and the luminescence in solution is found to controversial. In most cases it has been suggested that these
be only moderately quenched by dioxygen. A quite similar states are of the phosphane IL (intraligand) type. This exci-
behaviour is observed for the corresponding bromide deriv- tation involves the promotion of an electron from the Cu–
ative 1.
P σ-bond into π*-orbitals, which are partially delocalized
The thiophenolate complex 3 also shows a blue emission over the aryl substituents. Since the lone-pair of the free
at λmax = 465 nm in the solid state, but in contrast to the phosphane is stabilized by coordination, a blue-shift of this
halide derivatives 1 and 2, a green luminescence with a sig- transition should take place. However, this has been fre-
nificantly shifted, structureless band at λmax = 508 nm is quently not observed. Generally, the lowest-energy transi-
dominant in dichloromethane solution (Figure 2). As a so- tions of such (arylphosphane)CuI complexes are very close
lid compound, the luminescence spectrum of the phenylace- to those of the free ligands, sometimes at somewhat higher
3168
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 3167–3171

Luminescence and Theoretical Studies of Cu(tripod)X
FULL PAPER
and sometimes at slightly lower energies. The longest-wave-
length absorptions of Cu(tripod)X (Table 1) are assigned to
spin-allowed transitions in agreement with their high inten-
sities.
Generally, the luminescence of CuI complexes is a phos-
phorescence, but occasionally a delayed fluorescence at
shorter wavelengths has also been observed.[14–17] In the
case of Cu(tripod)X complexes, the emission is certainly a
phosphorescence, since it does not overlap with the absorp-
tion spectrum. Moreover, at low temperatures Cu(tripod)X
complexes do not show a new long-wavelength emission,
although the appearance of a phosphorescence should be
favoured under these conditions.
Quantum chemical calculations were carried out for all
complexes to determine the electronic character of the emis-
sion. TD-DFT calculations of the T1-state at the T1-geome-
try based on the singlet orbitals showed that the electronic
character of the state is dominated by a one-electron transi-
tion from HOMO to LUMO (weight of the leading config-
urations Ͼ 0.95). Therefore the interpretation is based on
HOMO and LUMO taken from the singlet wave function
calculated at the T1-state geometry (Figure 3).
Figure 3. Frontier orbitals of Cu(tripod)I (2): HOMO (bottom)
and LUMO (top).
The character of the HOMO is dominated by the d-p-π-
antibonding interaction between Cu and X. Additionally,
there is an antibonding interaction between the phosphane
lone pair and the copper d-orbital. The LUMO is mainly
localized on the phenyl rings and displays π* character (e2u
benzene-type). Accordingly, the electronic character of the
transition is best described as MLCT(CuI Ǟ phosphane) +
LLCT(X– Ǟ phosphane). This is shown in Figure 4 for X =
I, but is qualitatively very similar for the other Cu(tripod)-
X complexes.
Comparison of the calculated geometries of the singlet
ground state and triplet state (Table 2) shows that the main
geometrical changes are due to rotations of the phenyl
groups in the case of X = Br, I. The maximum rotation
angles found among the phenyl groups amount to 50° and
35° for Br– and I–, respectively. The rms distances of all
atoms add up to 68 pm and 49 pm, respectively. The Cu–
Br/Cu–I bond length is slightly shortened (2.9/3.7 pm) in
the triplet state due to the depopulation of the Cu–Br/Cu–
Figure 4. Schematic representation of the electronic transition.
Only one of the six phenyl groups is shown.
I antibonding orbital. Distortions are substantially lower in The Cu–S bond is shortened by 4.7 pm in the triplet state.
the case of X = SPh. The maximum rotation is found on Nearly no distortions are found for X = CϵC–Ph (rms d =
the thiophenyl group and amounts to 12° (rms d = 26 pm). 11 pm; elongation of the Cu–C bond: 3.9 pm). In all cases
Table 2. Calculated geometries of the singlet ground states and triplet states of Cu(tripod)X.
Parameter
X = I
X = Br
singlet
X = SPh
singlet
X = CCPh
singlet
triplet
triplet
triplet
singlet
triplet
Cu–X[a]
2.584
2.307
2.303
2.304
1.416
2.547
2.310
2.322
2.372
1.425
2.380
2.301
2.301
2.229
1.416
2.351
2.358
2.298
2.344
1.416
2.283
2.323
2.305
2.342
1.414
2.236
2.306
2.346
2.370
1.420
1.909
2.319
2.322
2.324
1.413
1.870
2.317
2.325
2.372
1.415
Cu–P1[b]
Cu–P2[b]
Cu–P3[b]
max (CphCph
[c]
)
max (phtor)[d]
35
0.49
50
0.68
12[e]
0.26
2
0.11
rms d[f]
[a] Distance in Å. [b] Distance between Cu and P atoms of the tripod ligand in Å. [c] Maximum C–C distance in Å observed in the
phenyl rings. [d] Maximum changes in torsional angles [°] of phenyl groups found on comparison of singlet and triplet geometry. [e]
Thiophenyl group. [f] Root-mean-square distance variation in Å of all atoms in an alignment of singlet and triplet structure.
Eur. J. Inorg. Chem. 2005, 3167–3171
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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V. Pawlowski, G. Knör, C. Lennartz, A. Vogler
Table 3. Emission maxima, orbital energies and calculated dipole moments of Cu(tripod)X.
FULL PAPER
X
λem [eV] (exp.)
solid CH2Cl2
λem [eV] (calcd.)[a]
unshifted
Orbital energy [eV]
Dipole [D]
S0
shifted[b]
HOMO
LUMO
T1
Br
I
SPh
CϵCPh
2.68
2.69
2.67
2.67
2.68
2.67
2.44
2.61
1.53
1.62
1.31
2.00
2.68
2.78
2.46
3.16
–3.78
–3.88
–3.46
–3.91
–2.21
–2.22
–2.14
–1.90
7.30
8.50
8.70
8.03
6.50
6.96
2.50
5.83
[a] TD-DFT/TZVP. [b] Energies shifted by 1.16 eV to fit the emission energy of X = Br, since this method is known to yield systematical
red-shifts.
there is a slight increase of the P–Cu bond length in the ationless transitions are hampered when the flexibility in-
triplet state.
cluding excited-state distortions is restricted. Indeed, all
The calculated emission energies of Cu(tripod)X with X four Cu(tripod)X complexes are luminescent in solution
= I, Br and SPh were found to be systematically red-shifted. and at room temperature (Table 1). However, in solution at
Therefore, we added a constant shift of 1.16 eV to facilitate room temperature the efficiency is still rather low. We sug-
a comparison with the experimental values (Table 3). This gest that the rotations of the phenyl groups at the phos-
shifting behaviour may be understood by the fact that stan- phane ligand, which take place in the lowest triplet state,
dard approximate exchange-correlation functionals suffer favour radiationless deactivations. In the solid state the
from an artificial stabilization of CT states due to the wrong phosphorescence of Cu(tripod)X is much more intense and
asymptotic behaviour of the corresponding exchange-corre- shifts to shorter wavelengths. This is certainly a conse-
lation functionals.[18] We performed additional single-CI quence of a diminished flexibility in the lattice of the solid
calculations (TZVP basis set) in order to determine the compound. However, since the extent of structural changes
character of the lowest excited state for X = Br. Although in the solid state is not well defined, the calculations should
this method only gives qualitative results for energies, it be better compared with the emission spectrum in solution.
does not suffer from the artificial stabilization of charge- While the formal assignments of MLCT/LLCT transitions
transfer states. We found that the CT state actually is the are certainly appropriate, the actual amount of charge re-
lowest-lying state in Cu(tripod)Br. Therefore, the relative distribution seems to be rather small as indicated by the
TD-DFT energies should reflect the right trend. Indeed, the dipole moments (Table 3). Accordingly, a distinct solvent-
shifted TD-DFT energies nicely correlate with the observed dependent shift is neither expected nor observed.
phosphorescence energies of of Cu(tripod)X with X = I, Br
In conclusion, the complexes Cu(tripod)X are charac-
and SPh (Table 3). The red-shift observed for the thiophen- terized by a blue emission, which originates from a mixed
olate complex is caused by a greater destabilization of the MLCT (CuI Ǟ tripod)/LLCT (X– Ǟ tripod) triplet. Owing
HOMO which contains a rather large sulfur contribution.
to the rigid structure imposed by the tripod ligand, this
For the phenylacetylide complex, there is a distinct dis- phosphorescence appears not only in the solid state, but
crepancy between the calculated and the observed phospho- also at room temperature in solution.
rescence energies. While in comparison to other Cu(tripod)-
X complexes a blue-shift is calculated, a red-shift is indeed
observed. Probably, here the TD-DFT calculation fails to
describe the first excited state correctly due to the problems
Experimental Section
Materials: All solvents used for spectroscopic measurements were
already mentioned above. For phenylacetylide complexes,
such as (PR3)Au(CϵC–Ph),[19] an IL (phenylacetylide)
of spectrograde quality. CuBr, CuI, CuSPh, CuCϵCPh and the
tripodal ligand 1,1,1-tris(diphenylphosphanylmethyl)ethane were
phosphorescence occurs at energies which are comparable
commercially available (Aldrich and Strem) and used without fur-
to the calculated transition energies (Table 3). It is conceiv-
ther purification. The title compounds were obtained by the follow-
ing synthetic routes.
able that contrary to the calculation, the emission of Cu(tri-
pod)(CϵC–Ph) now originates from the phenylacetylide IL
Cu(tripod)Br (1): In a procedure different from the published
triplet, which in reality is located below the MLCT/LLCT
route,[11,27] a solution of CuBr (0.16 g, 1.1 mmol) and tripod
triplet. Therefore, the artificial stabilization of the CT states
(0.71 g, 1.1 mmol) in 40 mL of acetonitrile was refluxed for 20 min,
may result in a qualitatively wrong picture here.
and then cooled to room temperature. The resulting white precipi-
tate was collected by filtration, washed with acetonitrile and diethyl
The mixed MLCT/LLCT character of the lowest-energy
excited state of Cu(tripod)I does not only follow from the
ether, and dried yielding 0.77 g (88%). C41H39BrCuP3 (768.12):
present calculations; it is also supported by previous calcu-
lations on related systems.[20] Moreover, similar conclusions
have been drawn for the assignment of the lowest-energy
electronic transition in tetranuclear copper(i) clusters.[6,7] In
this context it should be kept in mind that the structures of
CuI complexes in their MLCT states are planar and not
tetrahedral as in their ground states.[21–26] The rigidity of
calcd. C 64.11, H 5.12, Br 10.40; found C 64.02, H 5.06, Br 9.61.
Cu(tripod)I (2): Similar to the bromide complex 1, a solution of
CuI (0.10 g, 0.5 mmol) and tripod (0.32 g, 0.5 mmol) in 20 mL of
acetonitrile was refluxed for 20 min, and then cooled to room tem-
perature. The resulting white precipitate was collected by filtration,
washed with acetonitrile and diethyl ether, and dried yielding 0.32 g
(73%). C41H39CuIP3 (815.12): calcd. C 60.41, H 4.82, I 15.57;
Cu(tripod)X is expected to facilitate the emission since radi- found C 60.23, H 4.57, I 15.23.
3170 © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjic.org
Eur. J. Inorg. Chem. 2005, 3167–3171

Luminescence and Theoretical Studies of Cu(tripod)X
FULL PAPER
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90 min. After filtration of the hot, yellowish mixture, the solution
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Acknowledgments
Financial support of the Regensburg group by BASF is gratefully
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Received: November 17, 2004
Published Online: June 22, 2005
Eur. J. Inorg. Chem. 2005, 3167–3171
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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