Efficient luminescence from easily prepared three-coordinate copper(i) arylamidophosphines

Article abstract of DOI:10.1039/c000818d

A series of brightly luminescent, three-coordinate copper(i) arylamidophosphine complexes have been prepared from readily available precursors in high yield. Emission maxima span 102 nm in the visible spectrum from 461 (blue) to 563 nm (yellow) while photoluminescence quantum yields range from 0.11 to 0.24 in fluid solution at room temperature.

Full text of DOI:10.1039/c000818d

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Efficient luminescence from easily prepared three-coordinate copper(I)
arylamidophosphinesw
Kenneth J. Lotitoz and Jonas C. Peters*
Received 13th January 2010, Accepted 30th March 2010
First published as an Advance Article on the web 14th April 2010
DOI: 10.1039/c000818d
A series of brightly luminescent, three-coordinate copper(I)
arylamidophosphine complexes have been prepared from readily
available precursors in high yield. Emission maxima span 102 nm
in the visible spectrum from 461 (blue) to 563 nm (yellow) while
photoluminescence quantum yields range from 0.11 to 0.24 in
fluid solution at room temperature.
Compounds 1–5, and 7 are prepared by treating a sus-
pension of CuBrꢀSMe2 in benzene with two equivalents of
triarylphosphine followed by salt metathesis with the corres-
ponding lithium amide. Compound 6 is prepared in an analogous
fashion employing a single equivalent of the chelating ligand
DPEphos (DPEphos = bis-[2-(diphenylphosphino)phenyl]ether).
1–6 are obtained as diamagnetic, bright yellow-green solids
while 7 is obtained as a diamagnetic pale yellow solid whose
luminescence is only visible upon ultraviolet irradiation. Analyti-
cally pure material of 1–7 is obtained in good to excellent yield
(64–91%) following filtration to remove lithium bromide and
recrystallization from a mixture of n-pentane and benzene
(Fig. 1). Alternatively, 1 can be prepared in substantially lower
yield (19%) by treating a benzene solution of mesitylcopper(I)
with triphenylphosphine and diphenylamine. In all cases,
crystals suitable for single crystal X-ray diffraction are obtained
by layering a benzene solution of the copper complex with
n-pentane.
Synthetic simplicity is a desirable property for luminescent
molecular systems as it can facilitate the rapid expansion and
tuning of photophysical properties to targeted functions including
biological imaging,1 photochemical catalysis,2 light-driven fuel
production,3 and electroluminescent devices.4 Indeed, their
comparative ease of synthesis has in part driven the wide-
spread success enjoyed by iridium cyclometalates in roles
ranging from dopants in OLED displays to sensitizers in
photochemical hydrogen production.5,6
Copper-based luminophores7 have been investigated as an
inexpensive alternative to the more ubiquitous noble metal
emitters and recently our group introduced copper(I) amido-
phosphine complexes as a class of highly luminescent compounds.
To date we have focused on binuclear8 and mononuclear9
copper(I) complexes that feature tridentate and bidentate aryl-
amidophosphine ligands, respectively. While these compounds
exhibit quantum efficiencies as high as 70% with microsecond
luminescence lifetimes, their synthetic versatility hinges on the
steps required for the synthesis of the chelating ligands them-
selves. These steps include (i) catalytic aryl amination to assemble
the ligand backbone and (ii) use of lithium reagents and/or strong
phosphide nucleophiles to install the phosphine unit.
All of the reported complexes are thermally stable but are
highly sensitive to adventitious oxygen and water and discolor
rapidly in aerated solution giving insoluble brown solids with
concomitant loss of luminescence. The complexes are relatively
robust in the solid-state, however, with powdered materials
retaining both their color and bright luminescence after several
weeks in the open atmosphere. Compounds 1–5 and 7, which
are prepared with monodentate phosphine ligands, exhibit
some decomposition in solution upon dilution to micromolar
concentrations as determined by UV-vis spectroscopy. Excita-
tion spectra, however, can be obtained for each compound and
provide an indication of their respective absorption profiles
(see ESIw). In contrast, 6, which is prepared with the chelating
phosphine DPEphos, is more robust at low concentrations,
suggesting that phosphine dissociation may be problematic for
1–5 and 7 at low concentrations.
We now report that highly emissive, mononuclear, three-
coordinate copper(I) arylamidophosphines can be quickly
assembled from common, commercially available reagents
such as diphenylamine and triphenylphosphine. While their
quantum efficiencies are not in general as high as those
complexes highlighted above,8,9 they are favorable when com-
pared to other classes of copper emitters.7 Moreover, their
ease of structural variation makes this class of complexes a
convenient family for further expansion and study.
The solid-state structures of 1–3 and 7 were determined by
single-crystal X-ray diffraction and revealed monomeric com-
plexes in which the amide ligand adopts a monodentate
coordination mode (Fig. 2). Well defined examples of mono-
meric copper complexes with terminal amide ligands are rare
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, MA 02139, USA
w Electronic supplementary information (ESI) available: Synthetic
procedures and characterization, absorption spectrum of 6, excitation
and emission spectra, electrochemical data, and solid state structures.
CCDC 766167 (1), 766168 (2), 766169 (3), 766170 (7). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c000818d
z Current address: Division of Chemistry and Chemical Engineering,
California Institute of Technology, 101-20 Pasadena, CA 91125, USA.
E-mail: jpeters@caltech.edu; Fax: +1 626 395 6948; Tel: +1 626 395 4036.
Fig. 1 Schematic representation of complexes 1–7.
ꢁc
This journal is The Royal Society of Chemistry 2010
3690 | Chem. Commun., 2010, 46, 3690–3692

View Article Online
Fig. 2 (A) Displacement ellipsoid representation of 1. (B) Displace-
ment ellipsoid representation of 7. Thermal ellipsoids at 50% proba-
bility. Hydrogen atoms, solvent, and the minor component of the
disorder omitted for clarity.
Fig. 3 Photoluminescence from compounds 1, 6, and 7 in MeCy
upon long-wave UV irradiation.
and were first reported in the past few years.10,11 Complex 7 is,
to the best of our knowledge, the first reported carbazolate
complex of copper.
Each structurally characterized complex exhibits a three-
coordinate metal center with a nearly ideal trigonal planar
geometry; 1–3 and 7 are rigorously planar as indicated by the
angles about copper which sum to 360.001 within experimental
error (see ESIw for structural data) and in all cases the P–Cu–P
and N–Cu–P bond angles are within 101 of the ideal value of
1201. The amido ligand exhibits a large C–N–Cu–P dihedral
angle suggesting little to no contribution of an N(p) - Cu(4p)
interaction to the copper–amide bonding. The Cu–N inter-
nuclear distances range from 1.9363(17) (2) to 1.9602(16) A
(1), which fall between those reported for other structurally
characterized examples.10 Variation of the substituent in the
amido ligand in the series 1–3 has no apparent structural
consequence.
Fig. 4 Photoluminescence spectra of 1–7 in MeCy at room temperature.
lex = 390 nm.
Complexes 1–7 exhibit pseudo-C2 symmetry in the solid-
state, with the two-fold axis coincident with the Cu–N vector.
C2-symmetry is preserved in benzene solution at room tempera-
ture according to 1H and 31P NMR measurements that show a
single phosphorous resonance and equivalent protons on the
amide ligands, where expected (see ESIw).
The electrochemical properties of each complex were investi-
gated by cyclic voltammetry. Compounds 1 and 3–7 exhibit
irreversible oxidation waves around +600 mV versus the
ferrocene/ferrocenium couple in THF. In contrast, complex
2 exhibits a fully reversible oxidation at +420 mV. (See ESIw
for detailed electrochemical results.)
the more electron-releasing tri-p-tolylphosphine (4) results in a
modest 12 nm blue shift in the emission while employing the
relatively electron withdrawing P(p-FPh)3 ligand (5) shifts the
emission wavelength 14 nm to the red. While these results
suggest that the room temperature emission maximum may be
rationally tuned by judicious choice of phosphine electronic
properties, substitution of triphenylphosphine for the electro-
nically similar DPEphos (6) results in a substantial red shift,
giving a broad, yellow emission centered at 563 nm.
Solution quantum yields were determined by comparison of
the integrated emission intensity to a standard sample of
perylene and excited-state lifetimes were determined by time-
resolved emission spectroscopy (Table 1). 1–7 are comparably
efficient emitters, with FPL varying over the relatively narrow
range 0.11–0.24. Each complex exhibits a monoexponential
luminescence decay profile in solution with lifetimes on the
microsecond timescale suggesting the involvement of a triplet
excited state. The lifetimes for the isostructural series 1–3 and
1, 4, and 5 show little alteration upon para-substitution of the
amide and phosphine ligands. Exchange of the diphenylamide
for the more rigid carbazolate ligand, however, results in a
four-fold enhancement of the lifetime. While not as bright as
the amidophosphine complexes supported by chelating ligands
that we have reported previously,8,9 complexes 1–7 do represent
a substantial improvement over copper bis-diimine com-
plexes (FPL E 0.01) and compare favorably to heteroleptic
Each copper complex is brightly luminescent in both the
solid-state and in solution at room temperature. The parent
diphenylamide complex (1) exhibits a featureless green emission
band centered at 521 nm in methylcyclohexane (MeCy) at
room temperature (Fig. 3 and 4). Substitution with fluorine at
the para position of the amide phenyl rings (3) results in a
negligible bathochromic shift in lem while functionalization
with methyl groups (2) shifts the emission by 25 nm to longer
wavelength. In contrast to the diphenylamido complexes,
which tend towards yellow-green emission, copper carbazolate
7 exhibits cyan emission centered at 461 nm and demonstrates
that the photophysical properties of this series of compounds
are highly dependent on the identity of the amide ligand.
Variation of the ancillary phosphine also has an impact on
the emission wavelength. Substituting triphenylphosphine for
ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 3690–3692 | 3691

View Article Online
Table 1 Photophysical parameters of copper complexes at room
temperature in MeCy
tunability, and ease of synthesis. The ease of synthetic modifi-
cation afforded by these complexes provides an opportunity to
expand the available emission colors and to facilitate a more
detailed study of the photophysical states contributing to their
luminescence properties.
a,b
Complex
lem/nm
FPL
t/msc
1
2
3
521
546
525
509
535
563
461
0.23
0.22
0.13
0.13
0.11
0.18
0.24
3.17(5)
3.1(2)
2.5(1)
This work was supported by an NSF Center for Chemical
Innovation (CHE-0802907). Luminescence measurements
were carried out in the laboratory of T. M. Swager with
assistance from T. L. Andrews. K. J. L. gratefully acknowledges
a fellowship from the MIT Energy Initiative.
4
2.5(1)
5d
6
2.71(2)
1.70(1)
11.7(6)
7
a
Uncertainty in quantum yield measurements is estimated to be ꢂ0.05.
b l = 390 nm. c
ex
d
lex = 337 nm. In C5H10
.
Notes and references
1 J. Riegler and T. Nann, Anal. Bioanal. Chem., 2004, 379, 913–919.
2 P. Ciesla, P. Mytych and Z. Stasicka, J. Mol. Catal. A: Chem, 2004,
227, 17–33.
3 G. M. Brown, B. S. Brunschwig, C. Creutz, J. F. Endicott and
N. Sutin, J. Am. Chem. Soc., 1979, 101, 1298–1300; N. Robertson,
ChemSusChem, 2008, 1, 977–979.
4 H. Yersin, Top. Curr. Chem., 2004, 241, 1–26; M. Noto, Y. Goto
and M. Era, Chem. Lett., 2003, 32–33.
5 M. A. Baldo, M. E. Thompson and S. R. Forrest, Nature, 2000,
403, 750–753; S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-
Razzaq, H.-E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and
M. E. Thompson, J. Am. Chem. Soc., 2002, 123, 4304–4312.
6 E. D. Cline, C. M. Kraml, N. Byrne, D. M. Ho, Q. Qin,
F. J. Coughlin, S. Bernhard and R. A. Pascal, Inorg. Chem.,
2008, 47, 10378–10388; J. I. Goldsmith, W. R. Hudson,
M. S. Lowry, T. H. Anderson and S. Bernhard, J. Am. Chem.
Soc., 2005, 127, 7502–7510.
Fig.
5 (A) HOMO of 2 as calculated by DFT. Energy =
ꢃ84.3 kcal molꢃ1. (B) LUMO of 2 as calculated by DFT. Energy =
ꢃ23.4 kcal molꢃ1. Isovalue = 0.03. Hydrogen atoms omitted for clarity.
7 N. Armaroli, G. Accorsi, F. Cardinali and A. Listorti, Top. Curr.
Chem., 2007, 280, 69–115; P. C. Ford, E. Cariati and J. Bourassa,
Chem. Rev., 1999, 99, 3625–3627; O. Horvath, Coord. Chem. Rev.,
1994, 135/136, 303–32; V. Balzani, A. Juris, M. Venturi,
S. Campagna and S. Serroni, Chem. Rev., 1996, 96, 759–833;
R. F. Ziolo, S. Lipton and Z. Dori, J. Chem. Soc. D, 1970,
1124–1125; C. Kutal, Coord. Chem. Rev., 1990, 99, 213–252;
M. W. Blaskie and D. R. McMillin, Inorg. Chem., 1980, 19,
3519–3522; D. G. Cuttell, S.-M. Kuang, P. E. Fanwick,
D. R. McMillin and R. A. Walton, J. Am. Chem. Soc., 2002,
124, 6–7; A. M. James, R. K. Laxman, F. R. Fronczek and
A. W. Maverick, Inorg. Chem., 1998, 37, 3785–3791.
phosphinodiimine complexes (FPL r 0.16) that have been the
most extensively studied monomeric copper emitters.7
The electronic structure of 2 was explored using Density
Functional Theory (DFT) calculations at the B3LYP/
6-31+G* level of theory. The computed highest occupied
molecular orbital (HOMO) is localized primarily on the
diphenylamide ligand with substantial nitrogen pp and C–C
p character (Fig. 5). There is a small contribution from a Cu
d-orbital that is p-antibonding with respect to the amide
p-orbital. The HOMO is nearly identical to those calculated
for a related class of complexes9 and indicates that tuning of
the ground state energy is possible through ortho- or para-
substitution of the amide phenyl rings. The calculated lowest
unoccupied molecular orbital (LUMO) is localized on the
triphenylphosphine ligands and has C–C p* character. While
these computational results imply a ligand-to-ligand-charge-
transfer (LLCT) excitation, computational and experimental
studies on related systems suggest that amide ligand-centered
(LC) excited states may also play an important role.10,12
Further studies aimed at understanding the photophysics
and electronic structure of these luminescent copper(I) amido-
phosphines are underway for comparison to a recent detailed
photophysical study on the dicopperphosphinoamide complex
[Cu(PNP-tBu)]2 (PNP-tBu = bis[(2-diisopropylphosphino-4-
tert-butyl)phenyl]amide).13
8 S. B. Harkins and J. C. Peters, J. Am. Chem. Soc., 2005, 127,
2030–2031.
9 A. J. M. Miller, J. M. Dempsey and J. C. Peters, Inorg. Chem.,
2007, 46, 7244–7246.
10 E. D. Blue, A. Davis, D. Conner, T. B. Gunnoe, P. D. Boyle and
P. S. White, J. Am. Chem. Soc., 2003, 125, 9435–9441; L. A. Goj,
E. D. Blue, C. Munro-Leighton, T. B. Gunnoe and J. L. Petersen,
Inorg. Chem., 2005, 44, 8647–8649; P. Reiß and D. Z. Fenske, Z.
Anorg. Allg. Chem., 2000, 626, 1317–1331; N. P. Mankad,
W. E. Antholine, R. K. Szilagyi and J. C. Peters, J. Am. Chem.
Soc., 2009, 131, 3878–3880; M. M. Melzer, S. Mossin, X. Dai,
A. M. Bartell, P. Kapoor, K. Meyer and T. H. Warren, Angew.
Chem., Int. Ed., 2010, 49, 904–907.
11 Yamamoto and co-workers had previously formulated a series of
Cu(I) bisphosphinoamidates as three-coordinate. No structural
data were presented. One of these complexes was observed to be
luminescent: T. Yamamoto, Y. Ehara, M. Kubota and
A. Yamamoto, Bull. Chem. Soc. Jpn., 1980, 53, 1299–1302.
12 S. B. Harkins, N. P. Mankad, A. J. Miller, R. K. Szilagyi and
J. C. Peters, J. Am. Chem. Soc., 2008, 130, 3478–3485.
13 J. C. Deaton, S. C. Switalski, D. Y. Kondakov, R. H. Young,
T. D. Pawlik, D. J. Giesen, S. B. Harkins, A. J. M. Miller,
S. F. Mickenberg and J. C. Peters, 2010, manuscript submitted
for publication.
In conclusion, we have described a series of three-coordinate
copper(I) diarylamidophosphine complexes that combine efficient
luminescence in solution at room temperature, spectroscopic
ꢁc
This journal is The Royal Society of Chemistry 2010
3692 | Chem. Commun., 2010, 46, 3690–3692