Simple Cu(I) complexes with unprecedented excited-state lifetimes

Article abstract of DOI:10.1021/ja012247h

This report describes new, readily accessible copper(I) complexes that can exhibit unusually long-lived, high quantum yield emissions in fluid solution. The complexes are of the form [Cu(NN)(POP)]⁺ where NN denotes 1,10-phenanthroline (phen), 2

Full text of DOI:10.1021/ja012247h

Published on Web 12/08/2001
Simple Cu(I) Complexes with Unprecedented Excited-State Lifetimes
Douglas G. Cuttell, Shan-Ming Kuang, Phillip E. Fanwick, David R. McMillin,* and Richard A. Walton*
Department of Chemistry, Purdue UniVersity, 1393 Brown Building,West Lafayette, Indiana 47907-1393
Received September 27, 2001
This communication describes new mixed-ligand, copper(I)
phenanthroline complexes that exhibit unusually efficient, long-
lived photoluminescence signals even in a coordinating solvent.
The development of practical components for chemical sensors,
display devices, and solar-energy conversion schemes has fueled
interest in complexes of polypyridine and phenanthroline ligands
with transition metals,1-5 especially heavy metal ions, e.g. ruthe-
nium(II) or rhenium(I).5,6 Because of the comparative cost advan-
tage, copper-based systems are beginning to receive more attention;
however, the emission signals from charge-transfer (CT) excited
states of copper(I) complexes are typically weak and short-lived7-9
because the lowest energy CT state of a d10 system involves
excitation from a metal-ligand dσ* orbital.7 Moreover, in donor
solvents the possibility of expanding the coordination number
renders the copper systems vulnerable to an unusual, but very
effective form of exciplex quenching.7,8 Mixed-ligand systems
involving triphenylphosphine initially looked promising because
they exhibit long lifetimes in the solid state and frozen solution.10,11
However, detailed studies of [Cu(dmp)(PPh3)2]+ (dmp ) 2,9-
dimethyl-1,10-phenanthroline) have shown (1) that exciplex quench-
ing is important in methanol despite the presence of the bulky
phosphines12 and (2) that the speciation is hard to control in the
noncoordinating solvent dichloromethane (DCM).13 An obvious
possibility was that incorporation of a chelating phosphine might
suppress ligand dissociation, and studies reported herein establish
that systems such as [Cu(dmp)(POP)]+ are, in fact, vastly superior
luminophores (POP ) bis[2-(diphenylphosphino)phenyl]ether).14
An unanticipated finding is that solvent-induced exciplex quenching
is relatively inefficient for the CT excited state of the POP complex,
even though the triphenylphosphine moieties in [Cu(dmp)(PPh3)2]+
have greater steric requirements.
Reaction of [Cu(NCCH3)4]BF4 with POP and a phenanthroline
ligand gives good yields of the copper(I) complexes [Cu(NN)(POP)]-
BF4, where NN ) 1,10-phenanthroline (phen), dmp, or 2,9-di-n-
butyl-1,10-phenanthroline (dbp).15 The ORTEP drawing of the dmp
derivative in Figure 1 reveals a distorted tetrahedral coordination
environment about Cu(I) with P-Cu-P and N-Cu-N bond angles
of 116.44(4)° and 80.88(13)°, respectively. The unbound ether
oxygen of the POP ligand lies 3.24 Å, the closest interligand contact,
from the C(21) methyl group of the dmp ligand. All three [Cu-
(NN)(POP)]+ complexes exhibit similar structures (Figure 1).16
However, the photophysical properties of [Cu(NN)(POP)]+
systems vary dramatically with the steric requirements of the NN
ligand. Whereas the emission from [Cu(phen)(POP)]+ is very
similar to that reported for the [Cu(phen)(PPh3)2]+ system in room-
temperature DCM solution,12 under the same conditions [Cu(dmp)-
(POP)]+ exhibits almost a 100-fold greater emission efficiency and
a much longer excited-state lifetime (Table 1). In each case the
emission is broad, unstructured, and characteristic of a CT state.7-9
Figure 1. ORTEP representation of the structure of the cation in [Cu-
(dmp)(POP)]BF4‚CH2Cl2. Thermal ellipsoids are drawn at the 50% prob-
ability level except for the phenyl carbon atoms of the Ph2P groups which
are circles of arbitrary radius. Selected bond distances (Å) and bond angles
(deg) are as follows: Cu-P(1) 2.2691(11), Cu-P(2) 2.2728(11), Cu-N(1)
2.104(3), Cu-N(12), 2.084(3), Cu‚‚‚O(12) 3.151; P(1)-Cu-P(2) 116.44-
(4), N(1)-Cu-N(12) 80.88(13). Bond distances (Å) and bond angles (deg)
in the structure of [Cu(phen)(POP)]BF4‚1.5Et2O‚CH3CN: Cu-P(1) 2.2314-
(8), Cu-P(2) 2.2614(9), Cu-N(1) 2.071(3), Cu-N(10), 2.064(3), Cu‚‚‚
O(12) 3.205; P(1)-Cu-P(2) 110.81(3), N(1)-Cu-N(10) 80.83(11). Bond
distances (Å) and bond angles (deg) for [Cu(dbp)(POP)]BF4‚CH3CN: Cu-
P(1) 2.2712(7), Cu-P(2) 2.2793(6), Cu-N(1) 2.097(2), Cu-N(12) 2.109-
(2), Cu‚‚‚O(12) 3.257; P(1)-Cu-P(2) 112.91(2), N(1)-Cu-N(10) 80.51(8).
Table 1. Photophysical Data for Mixed Ligand Cu(I) Complexes in
DCM at Room Temperature
corrected emission
complex
λ
max (Abs) nm
λmax, nm
φa
τb, µs
[Cu(phen)(POP)]+
[Cu(dmp)(POP)]+
[Cu(dbp)(POP)]+
[Cu(phen)(PPh3)2]+ c
[Cu(dmp)(PPh3)2]+ d
[Cu(dmp)(dppe)]+ e
391
383
378
370
365
400
700
570
560
680
560
630
0.0018
0.15
0.16
0.0007
0.0014
0.010
0.19
14.3
16.1
0.22
0.33
1.33
a Error (10%. b Error (5%. c Reference 12. d Reference 10; recorded
in methanol. e Cuttell, D. G., unpublished observations.
Energy differences between the absorption and emission maxima
reveal that the CT state of [Cu(phen)(POP)]+ suffers by far the
largest geometry change of the three [Cu(NN)(POP)]+ species due
to the lack of sterically active substituents on the NN moiety. In
contrast, the system with the bulkiest phenanthroline ligand, namely
[Cu(dbp)(POP)]+, has the highest energy emission and longest
excited-state lifetime (16.1 µs in fluid DCM) within the series. The
steric influence of the alkyl substituents is also evident in the
electrochemistry. Normally, electron-donating groups stabilize the
oxidized form of the complex, but cyclic voltammetry reveals that
the E0 of the Cu(II)/Cu(I) couple shifts from +1.23 V vs Ag/AgCl
for the phen complex to +1.38 V (+1.39 V) for the dmp (dbp)
system. The shift to higher potential occurs for complexes with a
bulkier phenanthroline because the ligand framework resists rear-
rangement to a more flattened structure that is appropriate for the
Cu(II) oxidation state.
* To whom all correspondence should be addressed. E-mail: mcmillin@
purdue.edu; rawalton@purdue.edu
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10.1021/ja012247h CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
At the same time, the structures reveal that the POP ligand
meshes with the dmp ligand better than a pair of PPh3 ligands
because the Cu-N and Cu-P distances are about 0.02 Å longer17
on average in [Cu(dmp)(PPh3)2]+ as compared with [Cu(dmp)-
(POP)]+. Another potentially important consequence of the ether
linkage is that the P-Cu-P angle decreases from 122.7° in the
[Cu(dmp)(PPh3)2]+ complex17 to 116.4° in [Cu(dmp)(POP)]+. In
view of the bulkiness of the PPh3 ligands, it is striking that a five-
coordinate form is more accessible in the case of the [Cu(NN)-
(PPh3)2]+ system. Thus, the lifetime of the CT state of [Cu(dmp)-
(PPh3)2]+ is only 330 ns in methanol due to solvent-induced
quenching,10,12 whereas methanol has little or no effect on the CT
state of [Cu(dmp)(POP)]+ (τ ) 2.4 µs) or [Cu(dbp)(POP)]+ (τ )
5.4 µs). Note that the presence of a bulky phenanthroline like dmp
or dbp is also essential for suppression of exciplex quenching
because, like [Cu(phen)(PPh3)2]+, [Cu(phen)(POP)]+ exhibits a
weak, short-lived emission signal in methanol. Emission from [Cu-
(dmp)(POP)]+ also persists in acetone (τ ) 3.8 µs) and acetonitrile
(τ ) 1.1 µs) but quenching is essentially complete in the high donor
number solvent dimethylformamide. Full details of the solvent
dependence will be reported in due course.
The [Cu(NN)(POP)]+ systems with bulky NN ligands are
unprecedented in that they exhibit CT states with ca. 15 µs lifetimes
in DCM solution and emission efficiencies approaching 20%.
Although past work has shown that increasing delocalization within
the π system of the phenanthroline ligand can enhance the lifetime,18
an even more effective strategy is to raise the energy of the excited
state so as to take advantage of the energy gap law.19 To minimize
structural relaxation in the CT excited state and retain as much of
the excitation energy as possible, most efforts involving [Cu(NN)2]+
systems have focused on incorporating bulky substituents in the
2,9 positions of the ligand.7,8 However, σ-antibonding interactions
with the lone pair orbitals of the phenanthrolines destabilize the
high-energy d orbitals of the D2d ground state and inevitably reduce
the energy of the CT excited state.7 In the case of a mixed-ligand
POP complex the CT state shifts to higher energy due to differences
in bite angles as well as donor type, and the lifetime becomes much
longer.
Establishing the basis of the profound influence the POP ligand
has on the ground and excited-state chemistry of the [Cu(dmp)-
(POP)]+ and [Cu(dbp)(POP)]+ systems will require systematic
studies involving ligand variations and theoretical work. As a first
step in that direction, a comparison with other data in Table 1
reveals that the excited state of the [Cu(dmp)(POP)]+ system is
also significantly longer lived than that of the dppe analogue (dppe
) 1,2-bis(diphenylphosphino)ethane). The lifetimes of the photo-
excited [Cu(NN)(POP)]+ complexes are the more intriguing because
the ether oxygen of the POP ligand is also capable of coordinating
to the central metal.20 One might have expected a hapticity increase
to occur in the excited state so as to foster internal exciplex
quenching, a process that has been found to be very efficient in
other copper(I) systems,21 but this does not occur. The persistence
of microsecond lifetimes for the photoexcited states of [Cu(dmp)-
(POP)]+ and [Cu(dbp)(POP)]+ in methanol is also remarkable
because solvent-induced exciplex quenching is normally a very
potent process in copper systems. Perhaps the single most unex-
pected result is that solvent-induced quenching is so much less
efficient for the two POP systems than [Cu(dmp)(PPh3)2]+, a
structurally similar complex with a bulkier complement of phos-
phines. Up to now, the unwritten rule has been that “bigger is better”
in designing ligand frameworks that suppress solvent-induced
quenching.7-9,17,21 With [Cu(NN)2]+ systems involving bulky
biquinoline ligands, Riesgo et al. have emphasized that interlocking
ligand-ligand interactions can also enforce rigidity and suppress
solvent-induced quenching,22 and a similar effect may play a role
in the [Cu(dmp)(POP)]+ and [Cu(dbp)(POP)]+ systems. What is
clear from our study is that there are easy routes to very simple
Cu(I) complexes with highly emissive excited states and obvious
potential for luminescence-based applications and devices.
Acknowledgment. The National Science Foundation provided
funding for this research through grant number CHE 01-08902 (to
D.R.M). One of us (R.A.W.) thanks the John A. Leighty Endow-
ment Fund for support.
Supporting Information Available: X-ray crystallographic files
(CIF); tables of experimental data (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
References
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(15) Synthesis of [Cu(NN)(POP)]BF4 complexes. A typical procedure is as
follows. A mixture of [Cu(NCCH3)4]BF4 (31 mg, 0.10 mmol) and bis-
[2-(diphenylphosphino)phenyl]ether14 (54 mg, 0.10 mmol) in CH2Cl2 (20
mL) was stirred at room temperature for 2 h and then treated with a
solution of 2,9-dimethyl-1,10-phenanthroline (21 mg, 0.10 mmol) in CH2-
Cl2 (5 mL). The reaction mixture was stirred for an additional 1 h and
filtered, and the clear yellow filtrate was concentrated to ca. 5 mL.
Acetonitrile (about 5 mL) was added and the vapor diffusion of diethyl
ether into the resulting solution gave yellow crystals of the complex; yield
57 mg (63%). Anal. Calcd for C50.5H41BClCuF4N2OP2 (i.e. [Cu(dmp)-
(POP)]BF4‚0.5CH2Cl2): C, 64.32; H, 4.43. Found: C, 64.32; H, 4.42.
(16) Crystal data for [Cu(phen)(POP)]BF4‚1.5Et2O‚CH3CN (150 K): space
group P1h (No. 2) with a ) 12.9326(3) Å, b ) 14.3210(3) Å, c ) 15.1443-
(4) Å, R ) 105.8830(13)°, â ) 99.3086(13)°, γ ) 106.9454(12)°, V )
2490.1(2) Å3, Z ) 2, dcalcd ) 1.362 g cm-3, µ(Mo KR) ) 0.561 mm-1
,
)
2
2
26584 reflections measured (11164 unique). A cutoff Fo > 2.0σ(Fo
2
was used for R-factor calculations to give R(Fo) ) 0.061, Rw(Fo ) ) 0.154,
and GOF ) 1.030. Crystal data for [Cu(dmp)(POP)]BF4‚CH2Cl2 (150
K): space group P21/c (No. 14) with a ) 10.7713(2) Å, b ) 14.7971(2)
Å, c ) 28.8529(5) Å, â ) 98.1590(7)°, V ) 4527.6(2) Å3, Z ) 4, dcalcd
) 1.441 g cm-3, µ(Mo KR) ) 0.727 mm-1, 23669 reflections measured
2
(10274 unique). A cutoff Fo2 > 2.0σ(Fo ) was used for R-factor calculation
to give R(Fo) ) 0.062, Rw(Fo2) ) 0.161, and GOF ) 1.050. Crystal data
for [Cu(dbp)(POP)]BF4‚CH3CN (150 K): space group P1h (No. 2) with a
) 11.6779(2) Å, b ) 14.0495(2) Å, c ) 17.6505(3) Å, R ) 77.3452(6)°,
â ) 71.4646(7)°, γ ) 66.7766(6)°, V ) 2508.57(10) Å3, Z ) 2, dcalcd
)
1.353 g cm-3, µ(Mo KR) ) 0.555 mm-1, 34562 reflections measured
2
(11413 unique). A cutoff Fo2 > 2.0σ(Fo ) was used for R-factor calculation
to give R(Fo) ) 0.049, Rw(Fo2) ) 0.125, and GOF ) 1.037.
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McKenzie, A. T.; Chen, S. Inorg. Chem. 1985, 24, 3928-3933.
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