A highly emissive heteroleptic copper(I) bis(phenanthroline) complex: [Cu(dbp)(dmp)]+ (dbp = 2,9-di-tert-butyl-1,10-phenanthroline; dmp = 2,9- dimethyl-1,10-phenanthroline)

Full text of DOI:10.1021/ja9901415

4292
J. Am. Chem. Soc. 1999, 121, 4292-4293
plexes, heteroleptic complexes are expected to scramble. The
beauty of the dbp ligand is that it allows the preparation of a
heteroleptic complex since the formation of [Cu(dbp)2]+ is
sterically impossible.14 Here it is demonstrated that [Cu(dbp)-
(dmp)]+ shows much larger improvements in τ and φ than
previously examined homoleptic complexes; these effects are
attributed to the size of the tert-butyl groups of dbp (vide infra).
Importantly, this report represents the first demonstration of how
significant photophysical effects can be achieved with heteroleptic
CuI complexes and opens the door to a family of molecules for
further investigation.
A Highly Emissive Heteroleptic Copper(I)
Bis(phenanthroline) Complex: [Cu(dbp)(dmp)]+ (dbp
) 2,9-Di-tert-butyl-1,10-phenanthroline; dmp )
2,9-Dimethyl-1,10-phenanthroline)
Mark T. Miller, Peter K. Gantzel, and Timothy B. Karpishin*
Department of Chemistry and Biochemistry
UniVersity of California, San Diego
La Jolla, California 92093-0358
ReceiVed January 15, 1999
The complex [Cu(dbp)(dmp)](PF6) (1) is prepared by first
stirring 1 equiv of dbp15 with 1 equiv of [Cu(CH3CN)4](PF6)16 in
CH2Cl2 under N2. One equivalent of dmp is then added, and the
solution immediately turns from yellow to deep orange. Recrys-
tallization (MeOH) is sufficient for purification from a small
amount of the side product [Cu(dmp)2](PF6) and yields the air-
stable, orange 1.17 The crystal structure of 1 demonstrates the
heteroleptic coordination about the copper (Figure 1).18 The
coordination geometry is distorted from a D2d pseudotetrahedral
geometry that might be expected for a d10 ion. The geometry is
best described as trigonal pyramidal with molecular Cs symmetry,
in which the dmp ligand is canted from D2d symmetry along a
mirror plane. Several structures of [Cu(dmp)2]+ have been shown
to adopt geometries distorted from D2d symmetry.19 In most cases,
the largest distortion is a flattening of the phenanthroline (phen)
ligands with respect to each other, attributed to crystal-packing
forces.20,21 In the structure of 1, however, the tert-butyl groups
of the dbp ligand prevent the flattening distortion and result in a
nearly orthogonal orientation of the two phen planes.22 In addition,
in the structure of 1, there are two independent molecules of [Cu-
(dbp)(dmp)]+ in the asymmetric unit. However, each of the
complex cations adopts identical geometries (rms error ) 0.022
Å for the CuN4 cores, see Supporting Information).
Complex 1 emits brightly at room temperature upon illumina-
tion with a hand-held UV lamp. Since many of the applications
of luminescent inorganic complexes require the complexes
attached to solid supports or embedded in solid matrices,23-25 the
properties of 1 in the solid state are of interest. The excited-state
lifetime and emission spectrum (Figure 2) of the complex were
recorded upon excitation into the MLCT band (λmaxabs ) 454 nm,
mineral oil mull).26 Complex 1 emits with a maximum at 595
nm, and the decay is best fit by multiple exponentials (Figure
2).28 The solid-state lifetime and emission spectrum of [Ru(bpy)3]-
Photoluminescent diimine complexes of RuII have been ex-
tensively investigated for a wide variety of applications including
solar-energy conversion and molecular sensing.1-4 Although much
less studied, certain CuI bis(diimine) complexes display useful
properties including strong visible absorption, long (>100 ns)
excited-state lifetimes, and excited-state redox potentials that
render them viable as photocatalysts.5-9 Because of the substantial
price differential between copper and ruthenium, copper-based
devices and sensors are economically attractive. A central problem
associated with the use of CuI systems has been the low quantum
yields (φ ≈ 0.1-0.4%)8,10 of the complexes. Here we report a
CuI complex that exhibits impressive photophysical properties in
solution and in the solid state. In the solid state, the quantum
yield is equivalent to the most widely studied RuII complex ([Ru-
(bpy)3]2+; bpy ) 2,2′-bipyridine), and the excited-state lifetime
is longer.
The ligand dmp has been known for more than fifty years,11
but it was not until 1980 that Blaskie and McMillin demonstrated
that [Cu(dmp)2]+ is emissive upon excitation into the visible
MLCT band.12 It is now known that 2 and 9 phenanthroline
substituents are necessary for [Cu(NN)2]+ complexes (NN ) a
1,10 phenanthroline) to be emissive.13 The 2 and 9 substituents
sterically inhibit molecular distortion that occurs in the vibra-
tionally relaxed excited state.5 This distortion results from the
tendency of the CuII ion to adopt a square-planar (flattened)
geometry, the MLCT state having considerable CuII character.
Recent work has shown that increasing the steric requirements
of the 2 and 9 substituents leads to improvements in the excited-
state lifetimes (τ) and quantum yields (φ) of [Cu(NN)2]+
complexes by further inhibiting excited-state distortion.10 There
is a limit however. Increasing the substituent size by too much
leads to the inability to form the [Cu(NN)2]+ complex. Utilizing
molecular models, we predicted that only one ligand in a [Cu-
(NN)2]+ complex needs to have bulky 2 and 9 substituents to
prevent the flattening distortion. Thus, a heteroleptic complex in
which one phenanthroline contains tert-butyl groups at the 2 and
9 positions (dbp) should lead to a complex with an extremely
rigid coordination sphere. Because of the lability of CuI com-
(14) This concept has been utilized with ligands that contain bulky 2,9-
diaryl substituents: Schmittel, M.; Lu¨ning, U.; Meder, M.; Ganz, A.; Michel,
C.; Herderich, M. Heterocyc. Commun. 1997, 3, 493-498.
(15) Pallenberg, A. J.; Koenig, K. S.; Barnhart, D. M. Inorg. Chem. 1995,
34, 2833-2840.
(16) Kubas, G. J. Inorg. Synth. 1979, 19, 90-91.
1
(17) The H NMR spectrum of 1 is found in the Supporting Information.
(18) Complex 1 crystallizes in space group P21 with a ) 15.489(7) Å, b
) 11.983(7) Å, c ) 18.102(13) Å, â ) 91.23(5)°, V ) 3359(4) Å3, and Z )
4. For 6359 unique data with F > 4.0σ(F), R ) 6.48%.
(1) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159-244.
(2) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277.
(3) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163-170.
(4) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Horwood:
Chichester, U.K., 1991.
(19) CSD codes for the structures of [Cu(dmp)2]+ are CABKEV, DAWKOB,
DMPNCU, DMPNCU01, DMPRCU, MPHCUN, MPHCUN01.
(20) Goodwin, K. V.; McMillin, D. R.; Robinson, W. R. Inorg. Chem.
1986, 25, 2033-2036.
(5) McMillin, D. R.; Kirchhoff, J. R.; Goodwin, K. V. Coord. Chem. ReV.
(21) Dobson, J. F.; Green, B. E.; Healy, P. C.; Kennard, C. H. L.;
Pakawatchai, C.; White, A. H. Aust. J. Chem. 1984, 37, 649-659.
(22) The dihedral angle between the ligands (θz)21 is 90.1° and 90.2° for
each of the independent molecules.
1985, 64, 83-92.
(6) Kutal, C. Coord. Chem. ReV. 1990, 99, 213-252.
(7) Horva´th, O. Coord. Chem. ReV. 1994, 135/136, 303-324.
(8) Ruthkosky, M.; Kelly, C. A.; Castellano, F. N.; Meyer, G. J. Coord.
Chem. ReV. 1998, 171, 309-322.
(23) Meyer, G. J. J. Chem. Educ. 1997, 74, 652-656.
(24) Yan, S. G.; Lyon, L. A.; Lemon, B. I.; Preiskorn, J. S.; Hupp, J. T. J.
Chem. Educ. 1997, 74, 657-662.
(9) McMillin, D. R.; McNett, K. M. Chem. ReV. 1998, 98, 1201-1219.
(10) Eggleston, M. K.; McMillin, D. R.; Koenig, K. S.; Pallenberg, A. J.
Inorg. Chem. 1997, 36, 172-176.
(25) Demas, J. N.; DeGraff, B. A. J. Chem. Educ. 1997, 74, 690-695.
(26) Solid-state samples were prepared by applying ground samples of 1
27
(11) dmp ) 2,9-dimethyl-1,10-phenanthroline: Case, F. H. J. Am. Chem.
Soc. 1948, 70, 3994-3996.
or [Ru(bpy)3](PF6)2 onto Whatman No. 50 filter paper.
(27) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583-5590.
(28) Photoluminescent molecules in the solid state typically exhibit multiple
excited-state lifetimes: Castellano, F. N.; Heimer, T. A.; Tandhasetti, M. T.;
Meyer, G. J. Chem. Mater. 1994, 6, 1041-1048; see also refs 29 and 30.
(12) Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980, 19, 3519-3522.
(13) Ichinaga, A. K.; Kirchhoff, J. R.; McMillin, D. R.; Dietrich-Buchecker,
C. O.; Marnot, P. A.; Sauvage, J.-P. Inorg. Chem. 1987, 26, 4290-4292.
10.1021/ja9901415 CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/14/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 17, 1999 4293
Figure 1. One of the independent [Cu(dbp)(dmp)]+ complex cations.
Hydrogens have been removed for clarity. Selected bond lengths (Å) and
angles (deg) for both independent cations: Cu(1)-N(1), 2.051(6); Cu-
(1)-N(2), 2.087(6); Cu(1)-N(2A), 2.088(6); Cu(1)-N(1A), 2.094(6);
Cu(2)-N(2B), 2.064(6); Cu(2)-N(1B), 2.080(7); Cu(2)-N(1C), 2.085-
(6); Cu(2)-N(2C), 2.096(6); N(1)-Cu(1)-N(2), 81.4(3); N(1)-Cu(1)-
N(2A), 131.5(2); N(2)-Cu(1)-N(2A), 113.3(2); N(1)-Cu(1)-N(1A),
133.6(2); N(2)-Cu(1)-N(1A), 114.6(2); N(2A)-Cu(1)-N(1A), 83.9-
(2); N(2B)-Cu(2)-N(1B), 81.2(3); N(2B)-Cu(2)-N(1C), 132.1(3);
N(1B)-Cu(2)-N(1C), 115.7(2); N(2B)-Cu(2)-N(2C), 131.0(2); N(1B)-
Cu(2)-N(2C), 115.0(3); N(1C)-Cu(2)-N(2C), 84.4(2).
Figure 3. Absorption spectrum and corrected emission spectrum (λex
450 nm) of 1 in CH2Cl2 at room temperature.
)
The absorption and emission spectra of 1 were recorded in CH2-
Cl2 (Figure 3). The absorption bands centered at 440 nm (ꢀ )
7000 Μ-1 cm-1) are assigned to MLCT transitions, analogous to
those of [Cu(dmp)2]+.35 Complex 1 emits in CH2Cl2 with a
maximum at 646 nm (Figure 3); the absolute quantum yield is
1.0%.36 The excited-state lifetime of 1 in degassed CH2Cl2 is 0.73
µs.38 Previous to this report, the longest τ and φ for a [Cu(NN)2]+
complex in solution were found for [Cu(dsbp)2]+ (dsbp ) 2,9-
di-sec-butyl-1,10-phenanthroline) τ ) 0.40 µs and φ ) 0.45%.10
Relative to [Cu(dsbp)2]+, 1 exhibits an 82% increase in τ and a
120% increase in φ. The higher φ and longer τ of 1 versus [Cu-
(dsbp)2]+ are primarily attributed to a substantial reduction
(-45%) in the nonradiative rate constant, knr.39 This likely results
from the maximal interligand steric interactions in 1 (Figure S6)
that prevent adoption of a flattened geometry in the vibrationally
relaxed excited state, leading to a higher energy emissive state,
which in turn reduces the vibrational overlap between the
emitting40 and ground states.
Complex 1 represents a landmark improvement in the photo-
physics of [Cu(NN)2]+ complexes.41 This study also illustrates
that inexpensive copper-based complexes can exhibit photophysi-
cal properties that are equivalent to, or better than, ruthenium-
based analogues.
Figure 2. Time-resolved, room-temperature photoluminescent spectra
of [Cu(dbp)(dmp)](PF6) (____) and [Ru(bpy)3](PF6)2 (- - -) at 675 nm; λex
) 455 nm. Since each decay is best-fit by multiple exponentials, an
effective half-life (τeff) was calculated from the time the total area under
the curves decreased by 50%: τeff, [Cu(dbp)(dmp)](PF6) ) 1.5 µs; τeff
,
Acknowledgment. Partial support was provided from a Hellman
Faculty Fellowship. We thank Professor D. Magde for the use of his
laser equipment and Professor Y. Tor for the use of his emission
spectrometer.
[Ru(bpy)3](PF6)2 ) 0.21 µs. Inset: corrected solid-state emission spectra
of [Cu(dbp)(dmp)](PF6) (____) and [Ru(bpy)3](PF6)2 (- - -); λex ) 450 nm.
(PF6)2 were also measured using identical methods (Figure 2).
The quantum yield of 1 was determined to be 1.19 ( 0.25 times
higher than [Ru(bpy)3](PF6)2 under ambient conditions.31,32 We
estimate the solid-state φ of 1 is at least 50 times higher than
that of one of the most emissive, previously examined [Cu(NN)2]+
complexes, [Cu(bfp)2](PF6).33,34 Further, the solid-state lifetime
of 1 is considerably longer than that found for [Ru(bpy)3](PF6)2
(Figure 2).
Supporting Information Available: Stereoviews, ORTEP diagrams,
positional parameters, bond lengths, bond angles, anisotropic thermal
parameters, H-atom coordinates, an NMR spectrum, and a space-filling
view of 1 (PDF). An X-ray crystallographic file, in CIF format for 1.
This material is available free of charge via the Internet at http://pubs.acs.org.
JA9901415
(35) Everly, R. M.; McMillin, D. R. J. Phys. Chem. 1991, 95, 9071-9075.
(36) The absolute quantum yield ((20%) of 1 was determined in degassed
CH2Cl2 using [Ru(bpy)3](PF6)2 as the standard: φ (H2O) ) 0.042.27,37
(37) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853-
4858.
(29) Hartmann, P.; Leiner, M. J. P.; Lippitsch, M. E. Anal. Chem. 1995,
67, 88-93.
(30) Kneas, K. A.; Xu, W.; Demas, J. N.; DeGraff, B. A. Appl. Spectrosc.
1997, 51, 1346-1351.
(38) Lifetime data were determined with excitation at 445 or 455 nm with
observation at 675 or 690 nm; the error in τ is estimated to be (5%.
(39) The kr and knr values are: 1, kr ) 1.37 × 104 s-1, knr ) 1.36 × 106
(31) Wrighton, M. S.; Ginley, D. S.; Morse, D. L. J. Phys. Chem. 1974,
78, 2229-2233.
(32) The absolute quantum yields in the solid state were not determined
because of a lack of suitable solid-state standards.
(33) bfp ) 2,9-bis(trifluoromethyl)-1,10-phenanthroline; Miller, M. T.;
Gantzel, P. K.; Karpishin, T. B. Angew. Chem., Int. Ed. 1998, 37, 1556-
1558.
s-1; [Cu(dsbp)2]+, kr ) 1.13 × 104 s-1, knr ) 2.49 × 106 s-1
.
(40) The emitting state is likely composed of at least two MLCT excited
states.35
(41) Although complex 1 is stable in CH2Cl2 and MeOH, upon dissolution
in CH3CN or DMSO, it readily dissociates to [Cu(dmp)2]+, [Cu(dbp)S2]+ (S
) solvent) and dbp. We believe that dissociation occurs in CH3CN and DMSO
because the stability of the complex is not high enough to offset the formation
of [Cu(dbp)S2]+.
(34) Although the solution-state quantum yield of [Cu(bfp)2](PF6) has been
measured (φ ) 0.33%),33 the solid-state emission spectrum of [Cu(bfp)2](PF6)
was very weak, only allowing an estimate of the value of φ relative to 1.