Solid-State Structure Dependence of the Molecular Distortion and Spectroscopic Properties of the Cu(I) Bis(2,9-dimethyl-1,10-phenanthroline) Ion

Article abstract of DOI:10.1021/ic0348805

The relation between the geometry and spectroscopic properties of a series of salts of the Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) ion, (Cu ^(I)(dmp)₂)⁺, is explored. The distortions from the idealized D_2d geometry, which include flattening, rocking of the dmp ligands, and displacement of the Cu atoms out of the dmp planes, show considerable variation, indicating the importance of packing forces in the crystalline environment. The change in the absorption spectra upon flattening of the complex, expressed as the variation of the angle between the dmp planes, which varies from 88° in the BF₄ and tosylate salts to 73° in the picrate, agrees qualitatively with parallel DFT calculations. No correlation is found between ground state geometry and luminescence lifetimes, recorded both at room temperature and at 16 K. The low temperature lifetimes vary by a factor of 8 among the (Cu^(I)(dmp)₂)⁺ salts examined, the longest lifetime (2.4 μs at 16 K) being observed for the tosylate salt.

Full text of DOI:10.1021/ic0348805

Inorg. Chem. 2003, 42, 8794−8802
Solid-State Structure Dependence of the Molecular Distortion
and Spectroscopic Properties of the
Cu(I) Bis(2,9-dimethyl-1,10-phenanthroline) Ion
Andrey Yu. Kovalevsky, Milan Gembicky, Irina V. Novozhilova, and Philip Coppens*
Chemistry Department, UniVersity at Buffalo, State UniVersity of New York,
Buffalo, New York 14260
Received July 25, 2003
The relation between the geometry and spectroscopic properties of a series of salts of the Cu(I) bis(2,9-dimethyl-
(I)
1,10-phenanthroline) ion, (Cu (dmp)2)+, is explored. The distortions from the idealized D geometry, which include
2d
flattening, rocking of the dmp ligands, and displacement of the Cu atoms out of the dmp planes, show considerable
variation, indicating the importance of packing forces in the crystalline environment. The change in the absorption
spectra upon flattening of the complex, expressed as the variation of the angle between the dmp planes, which
varies from 88° in the BF and tosylate salts to 73° in the picrate, agrees qualitatively with parallel DFT calculations.
4
No correlation is found between ground state geometry and luminescence lifetimes, recorded both at room temperature
(I)
and at 16 K. The low temperature lifetimes vary by a factor of 8 among the (Cu (dmp)2)+ salts examined, the
longest lifetime (2.4 µs at 16 K) being observed for the tosylate salt.
Introduction
with an identical Cu(I) cation but different molecular environ-
ment has been systematically explored by both crystal-
lographic and spectroscopic methods.
A fundamental understanding of photoinduced electron
transfer requires knowledge of the geometry changes ac-
companying the transfer process. As part of our time-resolved
diffraction studies on the geometry of molecular excited
states,1 we have, as a necessary first step, examined the
variation in the ground state structure and spectroscopic
behavior of Cu(I) bisphenanthroline complexes, which un-
dergo photoinduced metal-to-ligand charge transfer (MLCT).2,3
They absorb light in the visible spectral region and show
phosphorescence with nanosecond to microsecond scale
lifetimes.
Experimental Section
Preparation of the Copper(I) Complexes. Starting Materials.
CuBr, Cu(BF4)2‚6H2O, Cu(NO3)2‚6H2O, [Cu(NCCH3)4]PF6,
[Cu(CNCH3)4]BF4, copper metal powder, 1,10-dimethyl-2,9-
phenanthroline (dmp), tetramethylammonium tosylate (TMATos),
sodium 9,10-anthraquinone-2-sulfonate (NaAQSO3), and L-ascorbic
acid are commercially available from Aldrich and were used without
furtherpurification.Tetramethylammoniumcalix[4]arenate(TMACalix)
was synthesized by a method of Harrowfield et al.4 Tetramethyl-
ammonium picrate (TMAPic) was prepared with quantitative yield
by combining a saturated solution of picric acid (Aldrich, Inc.) in
water and an equimolar amount of tetramethylammonium hydroxide
(25% in water) (Aldrich, Inc.). The yellow crystalline precipitate
of TMAPic was filtered and dried overnight over molecular sieves.
[Cu(dmp)2]BF4 (1), [Cu(dmp)2]BF4‚0.5Acetone (2), and
[Cu(dmp)2]BF4‚0.5dmp (3). The [Cu(dmp)2]BF4 complex was
prepared by the method described previously.5 Crystals of 1 were
obtained by slow evaporation of an acetonitrile solution, while
crystals of 2 were prepared by diethyl ether vapor diffusion into
The current paper concerns the relation between the
structural variation and spectroscopic changes of a series of
salts of the Cu(I) bis(2,9-dimethyl-1,10 phenanthroline) ion,
(Cu(I)(dmp)2)+. This is the first time that a series of solids
* To whom correspondence should be addressed. E-mail: coppens@
buffalo.edu.
(1) (a) Kim, C. D.; Pillet, S.; Wu, G.; Fullagar, W. K.; Coppens, P. Acta
Crystallogr., Sect. A 2002, 58, 133-137. (b) Novozhilova, I.; Volkov,
A. V.; Coppens, P. J. Am. Chem. Soc. 2003, 125, 1079-1087. (c)
Coppens, P.; Novozhilova, I. Faraday Discuss. 2002, 122, 1-11. (d)
Coppens, P. Chem. Commun. 2003, 1317-1320. (e) Coppens, P.;
Graber, T.; Vorontsov, I.; Wu, G.; Kovalevsky, A. Yu.; Gembicky,
M.; Chen, Y.-S. To be published.
(2) Scaltrito, D. V.; Thompson, D. W.; O’Callaghan, J. A.; Meyer, G. J.
Coord. Chem. ReV. 2000, 208, 243-266.
(3) Armaroli, N. Chem. Soc. ReV. 2001, 30, 113-124.
(4) Harrowfield, J. M.; Ogden, M. I.; Richmond, W. R.; Skelton, B. W.;
White, A. H. J. Chem. Soc., Perkin Trans. 1993, 2, 2183-2190.
(5) McMillin, D. R.; Buckner, M. T.; Ahn, B. T. Inorg. Chem. 1977, 16,
943-945.
8794 Inorganic Chemistry, Vol. 42, No. 26, 2003
10.1021/ic0348805 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/26/2003

Cu(I) Bis(2,9-dimethyl-1,10-phenanthroline) Ion
an acetone solution of the copper complex. Product 3 in which
unligated dmp cocrystalizes with the complex was obtained by slow
evaporation of an acetonitrile/water (2:1 v/v) solution containing
the copper compound and an equimolar amount of free dmp ligand.
Every procedure described here and below gave crystals suitable
for X-ray diffraction analysis.
squares against F2. The displacement parameters of non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were located
in difference electron density maps and subsequently refined using
the riding model for aromatic and methylene H atoms with Uiso
)
1.2Ueq of the connected carbon, and in idealized positions for CH3,
OH, and H2O with Uiso ) 1.5Ueq.
[Cu(dmp)2]PF6 (4) and [Cu(dmp)2]PF6‚0.5CH2Cl2 (5).
[Cu(dmp)2]PF6 was synthesized by a known procedure.5 Crystals
of 4 were prepared by slow evaporation of an acetonitrile/water
(2:1 v/v) solution, while crystals of 5 were grown by diethyl ether
vapor diffusion into a solution of the complex in dichloromethane.
[Cu(dmp)2]NO3‚2H2O (6). Cu metal powder was added to a
green acetonitrile solution containing Cu(NO3)2‚6H2O and dmp (4-
fold excess) in order to reduce Cu(II) to Cu(I). The resulting mixture
was suspended in an ultrasonic bath for 15 min. After the insoluble
residue was removed by filtration, the clear red solution was slowly
evaporated to give a crystalline product (∼90% yield). Compound
6 can also be prepared by the same procedure but with L-ascorbic
acid replacing copper metal powder as a reducing agent.
[Cu(dmp)2]Calix (7). An acetonitrile solution of CuBr and a
stoichiometric amount of dmp were refluxed for 2 h. To the resulting
red clear solution was added TMACalix (10% excess in acetonitrile)
and the mixture left to slowly evaporate over a period of several
days. Well-formed crystals of the product were collected by
filtration and dried in air to give an 80% yield.
Acetone and dichloromethane solvent molecules in 2 and 5,
respectively, are disordered into two equally occupied positions
through inversion centers. In 3 the nonligated dmp molecule and
the acetonitrile solvent molecule included in the crystal occupy the
same crystallographic site with 50%/50% population. In addition,
two of the four fluorine atoms of the BF4- counterion are disordered
over two equally populated positions. The atoms of the disordered
molecules were refined isotropically. The picrate counterion in 11
is disordered by 120° rotations around the local pseudo-3-fold axis
perpendicular to its central benzene ring. The oxygen atom of the
picrate ion occupies three positions with 70%/20%/10% population
and was refined isotropically.
No phase transitions were observed for any of the crystals studied
upon cooling from room temperature to 90 K.
UV-Vis Absorption and Photoluminescence Spectroscopy.
UV-vis absorption experiments were performed on a Perkin-Elmer
Lambda 35 UV-vis spectrometer equipped with an integrating
sphere for reflectance spectroscopy. The spectra were collected in
the 300-1100 nm range at room temperature from freshly drybox
prepared KBr pellets. Photoluminescence measurements were
carried out on a home-assembled emission detection system.
Samples (∼0.5-1 mm-sized single crystals, except for compound
10, for which several small single crystals were used) were mounted
on a copper pin attached to a DISPLEX cryorefrigerator. A metallic
vacuum chamber with quartz windows is attached to the cryostat,
and the chamber is evacuated to approximately 10-7 bar with a
turbo-molecular pump. Samples can be cooled to 15-16 K. The
crystals were irradiated with 365 nm and/or 500 nm light from a
pulsed N2-dye laser. The emitted light was collected by an Oriel
77348 PMT device, positioned at 90° to the incident laser beam,
and processed by a DSO-2102S computer-based digital oscilloscope
with 100 MHz sampling rate.
[Cu(dmp)2]Tos (8). [Cu(NCCH3)4]BF4 and a stoichiometric
amount of dmp were dissolved in an acetonitrile/water mixture (2:1
v/v). TMATos (10% excess) was added to the resulting solution
which was refluxed for 2 h. Slow evaporation of the solution gave
good crystals of the product in 70% yield.
[Cu(dmp)2]AQSO3‚CH3CN (9) and [Cu(dmp)2]AQSO3‚0.5H2O
(10). Stoichiometric amounts of NaAQSO3 and Cu(BF4)2‚6H2O
were dissolved in water (50 mL) at 70 °C. To the clear solution
was added a stoichiometric amount of dmp dissolved in ethanol.
A greenish precipitate was formed immediately. Ascorbic acid was
added to the suspension to reduce Cu(II) to Cu(I). The color of the
precipitate changed to orange. The resulting suspension was stirred
for an additional 2 h at 70 °C and cooled to room temperature.
The orange solid was filtered, washed with diethyl ether, and dried
in air to give 86% of [Cu(dmp)2]AQSO3. Crystals of 9 (large
orange-red prisms) were grown by diethyl ether vapor diffusion
into an acetonitrile solution of the product. Crystals of 10 (orange
plates) were obtained by the same method using an acetone solution.
[Cu(dmp)2]Pic (11). Complex 11 was prepared in quantitative
yield in a similar way as 8, with the exception that TMATos was
replaced by TMAPic. The crystals were obtained by a slow
evaporation of the reaction mixture.
X-ray Crystallography. X-ray diffraction data on 1-11 were
collected at 90(1) K using a Bruker SMART1000 CCD diffracto-
meter installed at a rotating anode source (Mo KR radiation) and
equipped with an Oxford Cryosystems nitrogen gas-flow apparatus.
The data were collected by the rotation method with 0.3° frame
width (ω scan) and 20-40 s exposure time per frame. For each
complex, four data sets (600 frames in each set) were collected,
nominally covering half of reciprocal space. The data were
integrated, scaled, sorted, and averaged using the SMART software
package.6 The structures were solved by direct methods, using
SHELXTL NT version 5.10,7 and refined by full-matrix least-
Theoretical Calculations. Calculations were carried out with
the ADF2002 suite of programs.8 The VWN and B88LYP func-
tionals were used in the local density approximation (LDA) and
the general gradient approximation (GGA), respectively. The atomic
orbitals of copper were described by a triple-ú Slater-type basis
set (ADF database TZP), while for the carbon, nitrogen, and
hydrogen atoms a double-ú Slater-type basis sets with one polariza-
tion function (ADF database DZP) was used. The (1s2s2p)10 core
shell of Cu and (1s)2 core shell of C and N were treated by the
frozen-core approximation. Relativistic effects were taken into
account using the “zeroth order regular approximation” (ZORA).
Geometry optimization was performed in the D2 point group. The
geometry was converged to the threshold of 10-4 Hartree/Å on the
Cartesian gradients. The integrals were evaluated numerically with
an accuracy of 8 significant digits. Time-dependent density
functional theory (TD-DFT) calculations were carried out with the
same functional used in the geometry optimization.
(8) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca
Guerra, C.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput.
Chem. 2001, 22, 931-967.
(9) Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980, 19, 3519-3522.
(10) Scaltrito, D. V.; Thompson, D. W.; O’Callaghan, J. A.; Meyer, G. J.
Coord. Chem. ReV. 2000, 208, 243-266.
(6) SMART and SAINTPLUSsArea Detector Control and Integration
Software, version 6.01; Bruker AXS: Madison, WI, 1999.
(7) SHELXTLsAn Integrated System for SolVing, Refining and Displaying
Crystal Structures from Diffraction Data, version 5.10; Bruker AXS:
Madison, WI, 1997.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8795

Kovalevsky et al.
Results and Discussion
Type 1 occurs in compounds 6, 8, and 9 and is described
by slight distortions around the copper center, with θ angles
ranging from 87.1° to 89.7° for θx, 84.3° to 86° for θy, and
84.8° to 88.1° for θz.
Type 2 occurs in compounds 1, 2, 3, and 10 (the second
of the two independent molecules) and is manifested by the
rocking being more pronounced than the flattening distortion,
the θ angles being 78.2-89.8° for θx, 74.0-81.5° for θy,
and 81.5-88.1° for θz.
Type 3 occurs in compounds 4, 5, 7, 10 (the first
molecule), and 11, and is characterized by the flattening
being equal to or more pronounced than the rocking
distortion, the θ angles ranging from 80.2° to 88.8° for θx,
74.4° to 85.4° for θy, and 72.8° to 81.9° for θz.
While flattening-only lowers the molecular symmetry from
D2d to D2, and rocking-only leads to Cs symmetry, in all cases
both distortions occur to some extent, so that the actual
symmetry is lowered to C1.
+
Structures of the Cu(dmp)2 ion in Different Solids.
Crystallographic and selected structural data for complexes
1-11 are given in Tables 1 and 2, respectively. Final
positional, isotropic, and anisotropic displacement parameters
together with full list of bond lengths and angles are listed
in the Tables S1-S44 of the Supporting Information.
Since the first synthesis of Cu(dmp)2+ by McMillin et al.5
in 1977 and the discovery9 that it is photoluminescent at room
temperature in solution, a great amount of effort has been
invested in the exploration of the photochemistry and
photophysics of this and related systems.10 However, the
+
reported Cu(dmp)2 structures are limited to room temper-
ature determinations,11 and a detailed study of the effect of
the packing in the crystal on the cation’s geometry and
spectroscopic behavior is lacking.
In the solids studied, the copper cation adopts a variety
of conformations with various degrees of rocking and
flattening distortions. A recent calculation shows the fre-
quencies of the normal modes corresponding to these
distortions to be only about 20 cm-1; thus, they can easily
be induced by relatively weak intermolecular forces.12
+
The least distorted geometry of the Cu(dmp)2 cation is
found in the tosylate complex 8, which shows rocking and
flattening of only 4° and 2°, respectively. The highest degree
of rocking distortion takes place in the tetrafluoroborate
acetone semisolvate compound 2, with a θy value of 74.0°,
while the most flattened copper(I) cation is that of the picrate
salt, the dmp planes being rotated with respect to each other
by almost 17° with θz equaling 72.8° (Figure 2). Although
the nitrate dihydrate complex 6 has a very insignificantly
distorted Cu(I) cation, it is 22.6° flattened (θz ) 67.4°) in
the case of a similar nonhydrated compound reported by
Hamalainen and co-workers.11e
The rocking distorts the molecules from a close-to-
tetrahedral to a trigonal pyramidal geometry. This results in
one of the Cu-N bonds moving to the axial position of the
trigonal pyramid, and consequently becoming longer than
the other three bonds. The flattening, on the other hand,
distorts the molecules toward the square planar geometry.
Following White and co-workers,11b these distortions can be
described by the three angles θx, θy and θz, where the first
two θ’s describe the rocking distortions and the third θz
describes the flattening (Figure 1). The perfect tetrahedral
geometry is illustrated by the three angles being 90°. In the
case of the trigonal pyramidal geometry, θx or θy is 0°, while
θz remains 90°. When the Cu(I) system is square planar, θx
and θy values reach 90°, while θz decreases to 0°.
It is worth noting that the copper atom is displaced from
one or both of the phenanthroline planes in all the complexes
(Table 2), and that this displacement is correlated with the
extent of the rocking distortion: the larger the rocking, the
more the copper is displaced. In complex 2, which exhibits
the largest rocking distortion, the copper atom is displaced
by 0.442 Å from the plane of one of the coordinated dmp
ligands. Flattening on the other hand does not produce such
large copper displacements. An additional correlation exists
between the rocking distortion and the lengthening of one
of the Cu-N bond distances. As one of the Cu-N bonds
gets closer to the axial position of the trigonal pyramid, it
becomes longer than the other three bonds in pseudoequa-
torial positions.
The Cu(dmp)2+ cations form π-π dimers in the majority
of the crystals studied. The exceptions are 1, 7, and 9, in
which the packing is such that the dmp ligands do not interact
with each other. The π-π stacking modes are schematically
illustrated in Figure 3. In 3, the dimers are interconnected
by the cocrystallized free dmp molecules, forming stacks
interrupted randomly by the included acetonitrile solvent
molecules. The distances between the π-π interacting dmp
planes in these dimers are in the range 3.30-3.60 Å. The
closest contact occurs in a dimer of complex 11, the distance
being 3.30 Å, although the dmp ligands involved do not
overlap much, as shown in Figure 3. The largest plane-to-
The distortions of complexes 1-11 are compared in Table
2. In the complexes studied, the θ angles range from 78.2°
to 89.8° for θx, from 74.0° to 86° for θy, and from 72.8° to
88.1° for θz. Three distinct types of Cu(dmp)2 geometries
can be distinguished (Table 2).
+
(11) (a) Hoffman, S. K.; Corvan, P. J.; Singh, P.; Sethulekshmi, C. N.;
Metzger, R. M.; Hatfield, W. E. J. Am. Chem. Soc. 1983, 105, 4608-
4617. (b) 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. (c) Dessy, G.; Fares, V. Cryst. Struct. Commun. 1979, 8, 507-
510. (d) Kon, A. Yu.; Burshtein, I. F.; Proskina, N. N.; Ibragimov, B.
T. Koord. Khim. 1987, 13, 260-263. (e) Hamalainen, R.; Ahlgren,
M.; Turpeinen, U.; Raikas, T. Cryst. Struct. Commun. 1979, 8, 75-
80. (f) Hamalainen, R.; Turpeinen, U.; Ahlgren, M.; Raikas, T. Finn.
Chem. Lett. 1978, 199-202. (g) Blake, A. J.; Hill, S. J.; Hubberstey,
P.; Li, W.-S. J. Chem. Soc., Dalton Trans. 1998, 909-916.
(12) Zgierski, M. Z. J. Chem. Phys. 2003, 118, 4045-4051.
8796 Inorganic Chemistry, Vol. 42, No. 26, 2003

Cu(I) Bis(2,9-dimethyl-1,10-phenanthroline) Ion
Inorganic Chemistry, Vol. 42, No. 26, 2003 8797

Kovalevsky et al.
Figure 1. Schematic representation of the distortions around the copper
center in Cu(I)(dmp)2+ complexes. The θx and θy angles describe the rocking
distortion, and θz the flattening distortion. The coordinate system is chosen
so that triangle N1A-Cu-N1B lies in the XZ plane. Unit vector ê bisects
angle N2A-Cu-N2B, and unit vector η is perpendicular to triangle N2A-
Cu-N2B. θx is the angle between ê and X. θy is the angle between ê and
Y. θz is the angle between η and Y.
plane separation (3.60 Å) is within dimers in the PF6
complexes 4 and 5. In 4, but not in 5, the dmp ligands overlap
strongly.
In the crystal of 11 containing the flat picrate (Pic)
counterion, no π-π interactions are present between the dmp
ligands and the picrate anions. Instead, the Pic- anions form
infinite stacks, with plane-to-plane distances of 3.39 and 3.32
Å. A completely different packing is found in the crystals 9
and 10, in which the counterion is electron accepting 9,10-
anthraquinone-2-sulfonate (AQSO3) (Figure 4). In 9, one dmp
ligand and the anion form infinite alternating stacks (Figure
4a), with a minimum separation of 3.30 Å and almost parallel
dmp and AQSO3 planes. The Ndmp-OAQSO3 contact is 3.5
Å, and close interactions O2‚‚‚C4′ (x + 1, y, z) 3.31 Å and
O2‚‚‚C5′ 3.19 Å exist with the other dmp ligand. In 10,
which contains two independent molecules, AQSO3- anions
form a larger number of contacts with the dmp ligands,
although with less overlapping of the ring systems. The
closest contacts are O1‚‚‚N6 3.62 Å, C67‚‚‚C1′ (x + 2, y +
1, z) 3.31 Å (Figure 4b), and O6‚‚‚N2′′ (1 - x, y, z) 3.75 Å,
C80‚‚‚C31′′′ (x + 1, y + 2, z + 1) 3.43 Å, C81‚‚‚C30′′′
3.30 Å (Figure 4c). In addition, two of the dmp ligands
interact with a plane-to-plane separation of 3.50 Å.
UV-Vis Absorption. UV-vis absorption (reflectance)
and photoluminescence measurements were performed on
+
crystalline samples of all 11 Cu(dmp)2 complexes. The
reflectance spectra are similar in the visible region, although
there are differences in detail (Table 3).
+
The idealized ground state geometry of the Cu(dmp)2
complexes corresponds to a pseudotetrahedral arrangement
of phenanthroline ligands around the copper center with D2d
symmetry. According to Parker and Crosby13 and more recent
calculations,12,14 in D2d symmetry four low-energy singlet-
singlet transitions from the degenerate highest occupied
orbital of E symmetry to the lowest unoccupied orbitals (exz,yz
f edmp, with z along the principal 2-fold axis, x and y along
the other 2-fold axes) are possible. Only one of these
transitions has its dipole moment operator along the 2-fold
axis and is therefore symmetry-allowed in D2d. Thus, in
solution the absorption spectrum of Cu(dmp)2+ contains only
(13) Parker, W. L.; Crosby, G. A. J. Phys. Chem. 1989, 93, 5692-5696.
(14) Novozhilova, I.; Coppens, P. To be published.
8798 Inorganic Chemistry, Vol. 42, No. 26, 2003

Cu(I) Bis(2,9-dimethyl-1,10-phenanthroline) Ion
+
Figure 2. Geometries of Cu(dmp)2 cation in the complexes 2 (tetrafluoroborate acetone), 8 (tosylate), and 11 (picrate). The complexes are drawn in the
plane containing the N3-C-C-N4 atoms.
Figure 3. The schematic representation of the π-π stacking modes of the dmp ligands in the crystals of 2-6, 8, 10, and 11. The free cocrystallized dmp
molecule of complex 3 is in red.
one visible absorption band around 450 nm. However, in
the solid state, symmetry-lowering to point group C1 resulting
from the geometric distortions lifts the symmetry selection
rules. As a result, the MLCT transition in the solid state
spectra gives rise to two bands which occur around 450 and
550 nm.15
As reported by Karpishin et al.,16 small 5-15° deviations
of the θ angles are sufficient for the appearance of the red-
shifted second band. This is confirmed by the reflectance
spectra of complexes 1-10, which show two low-energy
absorption bands centered at 422-465 and 564-588 nm.
For the picrate salt 11 which displays the highest degree of
the ground-state flattening distortion (θz ) 72.8°, Table 2),
the second band is further red-shifted to 622 nm, while the
first band blue shifts to the extent that it becomes a shoulder
at the 425 nm band (Table 3). These shifts result in the color
of 11 being almost purple, compared to the orange-red color
of the other complexes. Similar shifts of the absorption bands
have been reported for other complexes with large flattening
distortion.17
The two MLCT absorption bands of type 1 complexes are
at 451-465 and 570-585 nm and are significantly red- and
(15) Felder, D. F.; Nierengarten, J.-F.; Barigelletti, F.; Ventura, B.;
Armaroli, N. J. Am. Chem. Soc. 2001, 123, 6291-6299.
(16) Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. Inorg. Chem. 1999,
38, 3414-3422.
(17) Cunningham, C. T.; Moore, J. J.; Cunningham, K. L. H.; Fanwick, P.
E.; McMillin, D. R. Inorg. Chem. 2000, 39, 3638-3644.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8799

Kovalevsky et al.
In summary, when the flattening distortion occurs without
any significant rocking, the lower-energy band red-shifts
considerably, corresponding to the HOMO-LUMO gap
being decreased, and the higher-energy band blue-shifts. Our
DFT calculations of the orbital energies of Cu(I)(dmp)2+ ion
at different levels of flattening (Figure 5) indicate increasing
splitting of the dxz, dyz HOMO upon distortion from D2d to
D2 symmetry (Figure 5a). The energies of the absorption
bands calculated with TD-DFT show the effect of the
increased splitting and are in agreement with the observed
shift upon flattening (Figure 5b).
The ∼450 nm band also moves to higher energy in the
complexes with large and virtually equal rocking and
flattening distortions; however, the lower-energy band does
not change, as in the compounds with very small distortions.
Interestingly, the rocking distortion alone or in combination
with small-to-moderate flattening distortion (4-8°) does not
change the absorption spectrum, which is practically the same
as for the undistorted systems. For the solids studied here,
the distortions (especially flattening) have to be more than
10° in order to have a pronounced effect on the UV-vis
absorption spectrum.
The absorption bands in the UV region observed at 330-
340 nm are assigned to the (π-π*) transitions centered on
the dmp ligands,18 a conclusion supported by our calculations.
The near-UV bands appearing for all the complexes, except
11, at 403-428 nm are due to the dxy f π*dmp transition,
which does not shift upon distortion of the cation (Figure
5b).
Emission Spectra and Lifetimes. In the crystalline state,
complexes 1-9 and 11 show weak red photoluminescence.
The emission is characterized by the broad featureless spectra
with the maxima positioned in the 660-720 and 690-760
nm range at room and low (16 K) temperatures, respectively
(Table 3).
It has been observed before that the luminescence intensity
decreases with lowering the temperature, but the excited state
lifetime increases. This behavior has been explained by
McMillin and co-workers19 on the assumption that the
emission originates from two different excited states, a singlet
and a triplet MLCT state, which are in thermal equilibrium.
This would result in a significantly weaker photolumines-
cence from the lower-energy state and a very low-emission
intensity at helium temperatures.20
Examination of the emission data from the 11 compounds
at room temperature and at 16 K and other considerations
do not support this explanation. First, the energy gap between
these states, reported as 1000-2000 cm-1,13,19 would be too
high compared with kT (∼210 cm-1 at room temperature)
to allow establishment of a thermal equilibrium. Second,
+
Figure 4. Stacking interactions between AQSO3 anions and Cu(dmp)2
-
cations in complexes 9 (a) and 10 (b, c). AQSO3 is sandwiched between
dmp ligands.
blue-shifted, respectively, relative to the similar bands of
complex 11. The positions of the absorption bands of type
2 complexes 1, 2, and 3 fall in an almost identical range of
455-460 and 564-588 nm. In the case of compound 10,
which has two independent and differently distorted mol-
ecules in its asymmetric unit, the spectrum is the same as
that measured for the type 1 and 2 systems, the bands being
centered at 454 and 585 nm. In contrast, the higher-energy
bands in greatly flattened type 3 complexes 4, 5, and 7 are
blue-shifted and appear at 422-445 nm, similar to the shift
observed for the picrate complex. However, the lower-energy
bands remain at 564-575 nm as in the other compounds.
(18) Basara, H. J. Lumin. 1983, 28, 73-86.
(19) (a) Kirchhoff, J. R.; Gamache, R. E.; Blaskie, M. W.; Del Paggio, A.
A.; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380-
2384. (b) Everly, R. M.; McMillin, D. R. J. Phys. Chem. 1991, 95,
9071-9075.
(20) (a) Dietrich-Buchecker, C. O.; Nierengarten, J. F.; Sauvage, J. P.;
Armaroli, N.; Balzani, V.; De Cola, L. J. Am. Chem. Soc. 1993, 115,
11237-11244. (b) Armaroli, N.; Balzani, V.; Barigelletti, F.; De Cola,
L.; Flamigni, L.; Sauvage, J. P.; Hemmert, C. J. Am. Chem. Soc. 1994,
116, 5211-5217.
8800 Inorganic Chemistry, Vol. 42, No. 26, 2003

Cu(I) Bis(2,9-dimethyl-1,10-phenanthroline) Ion
Table 3. Solid State Absorption and Emission Data
BF4
1
BF4‚0.5AC BF4‚0.5DMP
PF6
4
PF6‚CH2Cl2 NO3 ‚H2O calix[4]arene p-Tos R-AQS
â-AQS
10
picrate
11
2
3
5
6
7
8
9
Reflectance (nm)
330
420
460sh 464sh
330sh
415
340sh
426
455sh
564
340sh
410
440sh
564
330sh
404
422sh
564
330sh
432
465sh
585
330sh
403
445
330sh 335sh
418 428
458sh 451sh
330sh
420
454sh
585
340
425sh
588
570
575sh
570
580sh
622
Emission
τ, µs (RT)
0.32
0.76
0.34
0.80
0.56
1.3
0.34
0.50
0.44
0.95
0.25
0.11
0.44a
720
0.18
0.30
0.95
2.40
0.26
0.80
no emission 0.34
no emission 0.60
τ, µs (16 K)
λ
max, nm (RT)
max, nm (16 K) 720
700
690
710
660
705
705
710
690
720
710
750
660
690
695
730
no emission 715
no emission 760
λ
730
a Biexponential fit to the emission decay curve.
of Shinozaki and Kaizu are from multiphase samples, as the
copper(I) complexes can occur in several polymorphic
modifications, and/or cocrystallize with solvent molecules.
Third, the reported room temperature excited state lifetimes
are hundreds of nanoseconds, being longer than expected
for fluorescence emission (Table 3), and the depicted pseudo-
Stokes shifts are appreciable, as is typical for phosphores-
cence but not for fluorescence emission.
The lifetimes increase by a factor of 2-2.5 upon cooling
the samples. The emission maxima move to lower energies,
although the red-shifts are different for different compounds.
For instance, for the PF6- compound 4 the phosphorescence
shifts merely 5 nm to the red on cooling, but for complexes
9 and 11 the shifts are 45 nm. Such shifts may be due to the
contraction of the crystals, which will affect both the initial
ground state and excited state energy levels. The temperature
shift of the emission is just 100 cm-1 for complex 4, but it
increases to around 700 cm-1 for complexes 9 and 11, while
for the other compounds the values lie between these two
extremes. When irradiated with 365 nm light, the emission
intensity from crystals of the calixarenate (7) is so low that
it could not be detected in our experiments. Interestingly,
with 500 nm light illumination a very weak emission could
be detected at both room and low temperatures.
3
The pronounced variation of the MLCT excited state
lifetime with the counterion is striking. It varies from 0.44
µs at 16 K for nitrate complex 6, to 2.4 µs at 16 K for the
tosylate compound, which is an increase of more than five
times even though the ground state geometries of the
+
Cu(dmp)2 cations in these two solids are essentially
identical, the π-π stacking interactions between the dmp
ligands are weak in both compounds, and closest contacts
with the neighboring counterions are comparable (Table 2).
Thus, neither the ground state geometry nor the packing
interactions in the crystalline phase appear to correlate with
the photoluminescence properties of the complexes inves-
tigated. The extent of the cation’s geometrical distortion upon
photoexcitation to the triplet state remains as a possible
source of the differences observed.
Emission Quenching in Anthraquinone Sulfonate Salts.
Complexes 9 and 10 differ from the others studied here by
having the electron accepting AQSO3 as counterion, thus
allowing the possibility of interionic charge transfer as a
mechanism for phosphorescence quenching. Compounds 9
Figure 5. Top: Energy of the frontier orbitals of [Cu(I)(dmp)2]+ as a
function of the angle between the dmp planes. Bottom: Excitation energies
from TD-DFT calculations.
unlike the data obtained by Shinozaki and Kaizu,21 our
emission decay curves could all be fitted adequately with a
single-exponential decay, with the exception for compound
6, for which the 16 K emission decay curve was fitted
biexponentially. It is not impossible that the solid-state data
(21) Shinozaki, K.; Kaizu, Y. Bull. Chem. Soc. Jpn. 1994, 67, 2435-2439.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8801

Kovalevsky et al.
and 10 are the first such copper(I) systems of which the
structure and photoluminescent properties have been studied
in the solid state. In previous studies of solutions of
substituted bis-phenanthroline and bis(phosphine)phenan-
throline copper(I) compounds,5,22 it was shown that electron
accepting molecules, such as substituted benzenes, an-
thracenes, viologens, and even small inorganic Co and Cr
complexes, can easily quench the emission from a copper
complex by either energy or electron transfer. The mecha-
nism of the phosphorescence quenching was described as
dynamic in nature. After excitation of the Cu(I) complex into
its MLCT excited state, the excited molecule encounters the
acceptor molecule, and the electron or energy is transferred
during the lifetime of the encounter complex. The back
reaction again occurs through such an encounter complex,
without the copper molecule emitting light. However, in the
solid state all the molecules are immobilized in the crystal
matrix and subject only to the relatively small-amplitude
vibrations and/or librations allowed by the crystal packing.
Therefore, in the crystalline state the electron or energy
transfer can happen only through space to neighboring
molecules. For 10, which is a crystalline hydrate with two
independent Cu complexes in the asymmetric unit, no
emission could be detected either at room or low temperature,
indicating that the phosphorescence is largely quenched.
However, in 9 (the R-form), the emission can be detected
when the excitation wavelength is 500 nm (Table 3). A
similar behavior was observed in the case of the picrate
complex, which contains a weak acceptor ion. The fact that
the two polymorphs have considerably different emissive
properties points to the effect of the crystal packing on the
luminescence properties. In the R-form, the AQSO3- coun-
terion overlaps the nearest dmp ligand (Figure 4), though
few close contacts occur, and only one 4.43 Å contact exists
involving the copper atom. On the other hand, in the â-form
(10), the AQSO3- molecules form several contacts with both
neighboring dmp ligands and with copper atoms (Table 2).
In addition, each of the two copper atoms of the two
symmetrically independent molecules has two close contacts
of about 4.3-4.6 Å with oxygen atoms of the neighboring
anthraquinone-sulfonate anions. Thus, the observed lack of
luminescence of 10 may be due to the energy or electron
transfer to the anion.
Conclusions
The comparison of a series of different solids all containing
the Cu(I)(dmp2)+ ion shows a large variation in geometric
distortion upon variation of the crystalline environment and
a pronounced difference in spectroscopic properties. Red
shifts of the longest wavelength absorption band are observed
when the flattening exceeds 10° and the rocking distortion
is less than about 5°. The 16 K phosphorescence lifetimes
vary by a factor of about 5 between the different solids. In
the anthraquinone salts, the phosphorescence is largely
quenched, which is tentatively attributed to electron or energy
transfer to the counterion. The lack of an obvious correlation
between the ground state geometry differences among the
complexes and the luminescence properties indicates the need
for information on the excited state geometry as a function
of the molecular environment. Such information can be
obtained by time-resolved diffraction studies at atomic
resolution, which are now becoming a reality.1
Acknowledgment. Financial support by the National
Science Foundation (CHE9981864 and CHE0236317) and
the Department of Energy (DE-FG02-02ER15372) is grate-
fully acknowledged.
(22) (a) Sakaki, S.; Koga, G.; Ohkubo, K. Inorg. Chem. 1986, 25, 2330-
2333. (b) Sakaki, S.; Koga, G.; Hinokuma, S.; Hashimoto, S.; Ohkubo,
K. Inorg. Chem. 1987, 26, 1817-1819. (c) Crane, D. R.; Ford, P. C.
J. Am. Chem. Soc. 1991, 113, 8510-8516. (d) Sakaki, S.; Mizutani,
H.; Kase, Y.-I.; Arai, T.; Hamada, T. Inorg. Chim. Acta 1994, 225,
261-267. (e) Sakaki, S.; Mizutani, H.; Kase, Y.-I.; Inokuchi, K.-J.;
Arai, T.; Hamada, T. J. Chem. Soc., Dalton Trans. 1996, 1909-1914.
(f) Ruthkosky, M.; Castellano, F. N.; Meyer, G. J. Inorg. Chem. 1996,
35, 6406-6412. (g) Ruthkosky, M.; Kelly, C. A.; Castellano, F. N.;
Meyer, G. J. Coord. Chem. ReV. 1998, 171, 309-322.
Supporting Information Available: Tables of crystallographic
information. X-ray crystallographic files in CIF format for the
structure of 1-11. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC0348805
8802 Inorganic Chemistry, Vol. 42, No. 26, 2003