An illuminating framework: Understanding the photoluminescence of α- CuAlCl4

Article abstract of DOI:10.1021/ja9925204

CuAlCl₄ exemplifies a new class of phosphors based on metal halide analogues of aluminophosphates. The α- and β-CuAlCl₄ phases show brilliant blue to blue-green luminescence. The electronic structure of the CuAlCl₄ corner sharing tetrahedral frameworks has been explored by fluorimetric and diffuse reflectance measurements on the isomorphous series α-CuAlBr(x)Cl(4- x) (x = 0-4). The photoluminescence is demonstrated to be a bulk property of the electronically isolated CuCl(4/2) tetrahedra within the framework matrix. Remarkably, the structural framework is flexible such that at least 1 mol of small molecule gases per CuAlCl₄ formula unit can be reversibly sorbed. The sorption/desorption of small molecule gases further results in the reversible quenching of the photoluminescence.

Full text of DOI:10.1021/ja9925204

10092
J. Am. Chem. Soc. 1999, 121, 10092-10097
An Illuminating Framework: Understanding the Photoluminescence
of R-CuAlCl4
Roger M. Sullivan and James D. Martin*
Contribution from the Department of Chemistry, North Carolina State UniVersity,
Raleigh, North Carolina 27695-8204
ReceiVed July 16, 1999
Abstract: CuAlCl4 exemplifies a new class of phosphors based on metal halide analogues of aluminophosphates.
The R- and â-CuAlCl4 phases show brilliant blue to blue-green luminescence. The electronic structure of the
CuAlCl4 corner sharing tetrahedral frameworks has been explored by fluorimetric and diffuse reflectance
measurements on the isomorphous series R-CuAlBrxCl4-x (x ) 0-4). The photoluminescence is demonstrated
to be a bulk property of the electronically isolated CuCl4/2 tetrahedra within the framework matrix. Remarkably,
the structural framework is flexible such that at least 1 mol of small molecule gases per CuAlCl4 formula unit
can be reversibly sorbed. The sorption/desorption of small molecule gases further results in the reversible
quenching of the photoluminescence.
Introduction
stitution to form the isomorphous R-CuAlBr4 and R-CuGaCl4
structures2 also quenches the room temperature luminescence.
A diverse range of materials in which copper(I) serves as
the luminescent activator exhibit emissions across the visible
spectrum. When doped into silicate glasses6 or zinc chalcogenide
phosphors7 a blue to green luminescence is observed, whereas
a red to orange emission is observed for Cu4I4L4 clusters (L )
alkylamine).8 Similarly, models used to describe Cu(I) lumi-
nescence vary widely.9 Molecular orbital models are frequently
used to describe copper(I) doped alkali or alkaline metal
halides,10 ion-exchanged zeolites,11 and other oxides12 in which
cuprous ions reside in well-characterized coordination environ-
ments. By contrast, in zinc chalcogenides, copper(I) is described
as an extrinsic defect in an electronic band model.13 Emissions
originating from copper(I) cluster excited states are reported
both for molecular tetramers and clustered defects in extended
solids such as SrCl2:Cu+.14 In both halide and heavier chalco-
genides, significant mixing of the ligand p and metal d orbitals
has led some to emphasize the halide-to-metal charge transfer
(XMCT) character of the 3d10 f 3d94s absorption.15 Species
We recently described the use of CuCl4/2 and AlCl4/2
tetrahedral building units for the construction of open framework
solids.1,2 Like silicates, the assembly of corner sharing tetra-
hedral metal halide units can be exploited for the synthesis of
a variety of framework materials. In the metal-halide system,
ZnCl2 can be envisioned as a silicate-type parent phase. (Note
that R-ZnCl2 is isostructural to the high cristoballite phase of
SiO2.)3 Substituting certain of the Zn(II) sites with Cu(I) results
in the formation of anionic frameworks, [CunZnm-nCl2m
n-
] ,
analogous to zeolitic aluminosilicates.4 Similarly, structural
analogues to aluminophosphates, AlmPmO4m, can be synthesized
utilizing Cu(I) and Al(III) to yield CumAlmCl4m frameworks.
Two such materials, R- and â-CuAlCl4, represent condensed-
phase framework materials derived from closest packed anion
sublattices.2
An apparent flexibility of this framework allows for the
remarkable chemisorption of small molecule gases. The revers-
ible binding of small molecules such as CO, C2H4, and C6H6
to CuAlCl4 in solution has previously elicited significant interest
for both separations and catalysis applications.5 During our
investigation of the chemisorption into crystalline R- and
â-CuAlCl4, we observed a room temperature blue luminescence
under both UV and X-ray excitation. The photoluminescence
described here provides a useful probe of the interaction between
the framework and sorbed gases since the sorption of small
molecules quenches the blue luminescence. Heavy atom sub-
(6) Debnath, R.; Das, S. K. Chem. Phys. Lett. 1989, 52.
(7) Kroeger; Hellingman Trans. Faraday Soc. 1948, 156.
(8) Kyle, K. R.; Ryu, C. K.; DiBenedetto, J. A.; Ford, P. C. J. Am. Chem.
Soc. 1991, 113, 2954.
(9) Blasse, G. AdV. Inorg. Chem. 1990, 35, 319.
(10) (a) Chermette, H.; Pedrini, C. J. Chem. Phys. 1981, 75, 1869. (b)
Fussgaenger, K. Phys. Status Solidi 1969, 36, 645. (c) Baldin, G.; Jean, A.;
Spinolo, G. Phys. Status Solidi 1968, 25, 557. (d) Patil, R. R.; Moharil, S.
V. J. Lumin. 1995, 63, 339. (e) Patil, R. R.; Miharil, S. V. J. Lumin. 1995,
65, 321.
(11) Texter, J.; Strome, D. H.; Herman, R. G.; Kller, K. J. Phys. Chem.
1977, 81, 333.
(12) Barrie, J. D.; Dunn, B.; Stafsudd, O. M.; Nelson, P. J. Lumin. 1987,
37, 303.
(13) (a) Peka, P.; Schulz, H.-J. Solid State Commun. 1984, 89(3), 225.
(b) Bowers R.; Melamed N. T. Phys. ReV. 1955, 99, 1781. (c) Van Gool,
W. Thesis, University of Amsterdam, The Netherlands, 1961.
(14) (a) Yamashita, N.; Ebisumori, K.; Nakamura, K. J. Lumin. 1994,
62, 303-311.(b) Payne, S. A.; McClure, D. S. J. Phys. Chem. 1984, 88,
1379-1385.
(15) Jørgensen K. Chemical Bonding Inferred from Visible and Ultra-
violet Absorption Spectra. In Solid State Physics; AdVances in Research
and Applications; Seitz, F., Turnbull, D., Eds.; Academic: New York, 1962;
Vol. 13, p 430.
(1) Martin, J. D.; Dattelbaum, A. M.; Thornton, T. A.; Sullivan, R. M.;
Yang, J.; Peachey, M. T. Chem. Mater. 1998, 10, 2699.
(2) Martin, J. D.; Leafblad, B. R.; Sullivan, R. M.; Boyle, P. Inorg. Chem.
1998, 37, 1341.
(3) (a) Brehler, B. Naturwissenschaften 1959, 554. (b) Barth, T. F. W.
Am. J. Sci. 1932, 350.
(4) Martin, J. D.; Greenwood, K. B. Angew. Chem., Int. Ed. Engl. 1997,
36, 2072.
(5) (a) Walker, D. G. Prepr. Am. Chem. Soc., DiV. Pet. Chem. 1983, 28,
746. (b) Hirai, H.; Nakamura, M.; Komiyama, M. Bull. Chem. Soc. Jpn.
1983, 56, 2519. (c) Hirai, H.; Wada, K.; Komiyama, M. Bull. Chem. Soc.
Jpn. 1986, 59, 1043. (d) Hirai, H.; Hara, S.; Komiyama, M. Bull. Chem.
Soc. Jpn. 1986, 59, 1051.
10.1021/ja9925204 CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/03/1999

Understanding the Photoluminescence of R-CuAlCl4
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10093
Figure 1. The excitation and emission spectra of R-CuAlCl4 and
â-CuAlCl4.
with low-lying π* ligand orbitals, such as Cu4I4py4, exhibit
emissions arising from metal-to-ligand charge transfer (MLCT)
excited states.16 The unique structure of copper aluminum
chloride, in which the d10 copper centers are isolated within a
tetrachloroaluminate matrix, provides an excellent host to study
the luminescence of Cu(I). Furthermore, the ability to form a
chloride/bromide solid solution affords the opportunity to
examine the role of the anion in the luminescence mechanism.
Luminescence of the metastable â-phase and samples of the
R-phase prepared by rapid thermal quenching of the melt appear
to have an additional contribution from a red-shifted defect site
Cu(I) activator. While both R- and â-CuAlCl4 exhibit brilliant
room temperature photoluminescence, this manuscript focuses
on the luminescent mechanism for copper(I) on the unique
crystallographic site of the R-phase unit cell.
Figure 2. Reflectance data for the series CuAlBrxCl4-x, plotted as the
remission function, F(R∞) ) (1 - R∞)2/2R∞ (based on the Kubelka-
Munk theory of diffuse reflectance):17 (a) CuAlCl4, (b) CuAlBrCl3, (c)
CuAlBr2Cl2, (d) CuAlBr3Cl, and (e) CuAlBr4.
Results
Excitation/Emission. The excitation and emission spectra for
R- and â-CuAlCl4 are plotted in Figure 1. The brilliant blue
emission of the R-phase, (solid line) at 20.5 × 103 cm-1 (483
nm), is also readily observed upon illumination with a hand
held mercury lamp (254 nm excitation) under daylight condi-
tions. The ratio of incident to emissive intensity is approximately
2/3 that of a CaWO4 standard. Similar emission is observed
upon excitation with X-rays. (The emission at room temperature
due to X-ray excitation is brilliant when using synchrotron
radiation (16 MeV, 1 Å) and more faint when using radiation
from a sealed copper tube.) The emission maximum is slightly
red shifted for â-CuAlCl4, (dashed line) at 19.8 × 103 cm-1
(505 nm). Excitation maxima for both the R-phase (solid line)
and â-phase (dashed line) are observed at 37.0 × 103 cm-1 (275
nm). As previously reported, the â-phase is metastable with
respect to the R-phase, and is prepared by rapid cooling of the
molten salt in ice water.2 Samples of R-CuAlCl4 prepared from
a rapid thermal quench of the melt also show this red shift in
the emission maximum under 254 nm excitation. The energy
shifts between the excitation and emission maxima (16.1 × 103
cm-1 for R-CuAlCl4 and 17.2 × 103 cm-1 for â-CuAlCl4) are
notably large.
Diffuse Reflectance. The diffuse reflectance data for
R-CuAlCl4 are plotted as the remission spectrum in Figure 2a.
Remission spectra may be interpreted in the same manner as
absorption spectra.17 This spectrum is described by two broad
absorption features centered around 40.3 × 103 [248 nm] and
49.8 × 103 cm-1 [201 nm]. Addition of bromine to form the
CuAlBrxCl4-x solid solution has no significant influence on the
onset of absorption (i.e., the band edge falls at 35.4 ((0.5) ×
103 cm-1 [282 ((3) nm] for all R-CuAlBrxCl4-x (x ) 0-4)),
as shown in Figure 2. The resolution of two maxima upon
increasing bromide substitution suggests that in the all chloride
Figure 3. The normalized emission intensity of the solid solutions
R-CuAlBrxCl4-x, x ) 0-4 (plotted as solid circles). The solid line is a
plot of the probability of the four halide sites around each copper cation
being occupied by only chloride anions ([(4 - x)/4]4).
spectrum the dominant low-energy feature is the sum of multiple
features. By contrast the same anion substitution causes the
higher energy features to broaden and dramatically increase in
intensity.
Luminescence of the Mixed Halide Solid Solution. Sub-
stitution of either the chloride anions with bromide ions or the
aluminum cations with gallium cations results in the quenching
of the room temperature luminescence. In a liquid nitrogen bath
both R-CuAlBr4 and R-CuGaCl4 luminesce blue under 254 nm
excitation. Taking advantage of this room temperature quenching
of the luminescence upon anion substitution, we examined the
series of mixed chloride-bromide phases, R-CuAlBrxCl4-x (x
) 0-4). A nonlinear decrease in the luminescent intensity with
an increase in the bromide content, x, was observed (Figure 3),
in contrast to the Vegard’s law linear dependence of the unit
cell volume on the bromide substitution (Figure 4).18
Luminescence Lifetime. The luminescent lifetime of
R-CuAlCl4 was determined by frequency domain fluorometery.19
The modulation and phase curves extracted from the time
(16) Ford, P. C. Coord. Chem. ReV. 1994, 2196.
(17) Wendlandt, W. W.; Hecht, H. G. Reflectance Spectroscopy; Inter-
science: New York, 1966; Chapter 3.
(18) West, A. R. Solid State Chemistry and Its Applications; Wiley: New
York, 1984; p 51.

10094 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
SulliVan and Martin
Figure 4. Plot of the unit cell volume as a function of the solid solution.
Figure 6. Emission intensity (monitored at λ ) 483 nm) as a function
of the reversible sorption of ethylene into R-CuAlCl4.
accessible to our instrumentation (below 52.6 × 103 cm-1 [190
nm, 6.5 eV]). The contribution of aluminum orbitals to
R-CuAlX4 is expected to be only at high energy, and thus they
participate only in the high-energy features of the diffuse
reflection spectra. Extended Huckel calculations25 indicate that
both R-CuAlCl4 and CuCl exhibit a direct band gap in which
the valence band is a mixture of Cl 3p and Cu 3d orbitals and
the orbital character of the conduction band edge is dominated
by the Cu 4s contribution. The low-energy edge of the diffuse
reflectance spectrum in Figure 2a yields an estimated band gap
for R-CuAlCl4 of 34.9 × 103 cm-1 [287 nm, 4.3 eV].26 The
band gap of this ternary halide is much higher in energy than
the 25.8 × 103 cm-1 [388 nm, 3.2 eV] reported for CuCl.27
This disparity in band gaps is consistent with a localized versus
delocalized electronic structure for CuAlCl4 and CuCl, respec-
tively. In the structure of R-CuAlCl4 the CuCl4/2 tetrahedral units
are completely separated from each other by corner sharing
AlCl4/2 tetrahedra such that the nearest copper-copper contacts
are 5.43 Å. In the binary chloride the CuCl4/2 tetrahedra are
also corner sharing, but each tetrahedron is linked to four other
CuCl4/2 tetrahedra with copper-copper separations of 3.74 Å.
To further understand the relationship between structure and
optical properties we prepared the isomorphous series R-CuAlBrx-
Cl4-x (x ) 0-4). The series forms a true solid solution,
displaying a Vegard’s law linear dependence of unit cell volume
on bromide substitution (Figure 4) and no superstructure in the
powder X-ray diffraction. The diffuse reflectance spectra for
the series R-CuAlBrxCl4-x are displayed in Figure 2. Because
the higher energy absorption centered around 47 × 103 cm-1
broadens and increases in intensity upon increasing bromide
substitution, we have assigned this absorption to halide-to-metal
charge transfer (XMCT). An increased participation of the
bromide 4p orbitals is expected since they are higher in energy
than the chloride 3p. The low-energy feature centered about 40
× 103 cm-1 in the R-CuAlCl4 spectra (Figure 2a) resolves into
multiple peaks on increasing bromide substitution. In contrast
to the absorption edge of the respective binary halides (CuCl
Figure 5. Chemisorption as a function of relative pressure for the
sorption of ethylene into R-CuAlCl4.
dependent intensity data fit a model with a 95% contribution
by a single-exponential decay, τ ) 14.2 µs (ø2 ) 2.6). The
remaining 5% of the signal is attributed to scattered light from
a grating artifact at twice the excitation wavelength.
Luminescence Quenching by Gas Sorption. CuAlCl4 can
repeatedly and reversibly sorb and desorb, at least 1 equiv per
framework formula unit of certain small molecule gases (such
as ethylene, carbon monoxide, and benzene vapor).20 Data for
the room temperature equilibrium volumetric measurements for
the sorption of ethylene (shown in Figure 5) exhibit a type-V
isotherm, which is reminiscent of a cooperative binding of the
gas to the framework.21 Upon gas sorption there is a marked
change to the framework structure22 that further results in the
quenching of all photoluminescence. The sorbed gas is readily
removed under dynamic vacuum resulting in a return of the
luminescence as seen in Figure 6.
Discussion
Reflectance and Luminescence. The structures of R-CuAlCl4
and its binary precursors, CuCl and AlCl3, are related but the
ternary chloride exhibits remarkable luminescence not seen in
the binary precursors. Aluminum trichloride displays a cubic
closest packed (ccp) chloride sublattice in which aluminum
cations occupy octahedral interstices. Both CuCl and R-CuAlCl4
also display a cubic closest packing (ccp) of the chloride anions,
but with all cations distributed among corner sharing tetrahedral
interstices.23,24 No light is absorbed by AlCl3 at energies
(19) Lakowicz, J. R.; Gryczynski, I. Frequency-Domain Fluorescence
Spectroscopy. In Topics in Fluorescence Spectroscopy; Vol. 1. Techniques;
Lakowicz, J. R., Ed.; Plenum: New York, 1991; pp 293-336
(20) Sullivan, R. M.; Martin, J. D. Luminescent Materials (McKittrick,
J., Ed.) Proc. Mater. Res. Soc. 1999, 560, in press.
(21) Sing, K. S. W. Charachterization of Adsorbents. In Adsorption,
science and technology; Rodrigues, A. E., LeVan, M. D., Eds.; Kluwer
Academic Publishers: Boston, 1989; pp 3-14.
(22) (a) Liu, H.; Cirolo, M. F.; Grey, C. P.; Hanson, J.; Martin, J. D.;
Sullivan, R. M. BNL NSLS ActiVity Rep. 1997, B-72. (b) Sullivan, R. M.;
Martin, J. D.; Liu, H.; Grey, C. P.; Hanson, J. BNL NSLS ActiVity Rep.
1998, in press (www.nsls.bnl.goV/Pubs/ActiVR/AR-98/MA1218.pdf).
(23) Monier, J.-C.; Kern, R. C. R. Hebd. Seances Acad. Sci. 1955, 241,
69.
(24) Hildebrandkt, K.; Jones, P. G.; Schwarzmann, E.; Sheldrick, G. M.
Z. Naturforsch. B: Anorg. Chem. Org. Chem. 1982, 37, 1129.
(25) Atom coordinates were taken from the single-crystal structures.
Default atomic parameters were used as found in the CAESAR adaptation
of the Extended Hu¨ckel programs. Whangbo, M.-H. CAESAR; Department
of Chemistry, North Carolina State University: Raleigh, NC, 1998.
(26) Pankove, J. Optical Processes in Semiconductors; Dover: New
York, 1971; Chapter 3.
(27) (a) Garro, N.; Cantarero, A.; Cardona, M.; Ruf, T.; Go¨bel, A.; Lin,
C.; Reimann, K.; Ru¨benacke, S.; Stuebe, M. Solid State Commun. 1996,
98, 27. (b) Ferhat, M.; Zaoui, A.; Certier, M.; Khelifa, B. Phys. Lett. A
1996, 216, 187.

Understanding the Photoluminescence of R-CuAlCl4
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10095
) 24.9 × 103 cm-1 and CuBr ) 23.0 × 103 cm-1), the low-
energy features do not substantially shift in energy. Comparing
the diffuse reflectance spectra in Figure 2, it is observed that
the onset of absorption for the series R-CuAlBrxCl4-x falls at
nearly the same value, 35.4 ( 0.5 × 10-3 cm-1 (268 ( 3 nm).
The independence of the remission band edge from halide
substitution supports the assignment to a copper centered 3d10
f 3d94s absorption.
Scheme 1
The excitation maximum for the blue luminescence in
CuAlCl4, 37.0 × 103 cm-1 (275 nm), falls in the region of the
absorption edge and thus is assigned to a Cu 3d10 f 3d94s
transition. While the excitation energy of the luminescence
remains essentially the same for all values of the Br/Cl solid
solution, the intensity of the blue emission dies out with
increasing bromide substitution as shown in Figure 3. Remark-
ably, the nonlinear loss of emission intensity can be fit to an
equation that describes the probability of any copper cation being
surrounded by four chlorides. By assuming that in the solid
solution all chlorides and bromides are randomly distributed
among the halide crystallographic sites, the probability that any
copper is surrounded by four chlorides is equal to the probability
of any one site being occupied by a chloride, raised to the fourth
power, [(4 - x)/4]4. The super position of this probability curve
onto the normalized emission intensity (Figure 3) demonstrates
that the emission intensity is directly proportional to the decrease
in the number of all chloride (CuCl4/2) centers upon bromide
substitution. The quenching of luminescence by substitution with
a heavy element, such as bromine, is well-known. Typically
the increased spin-orbit coupling by the heavy element relaxes
the ∆S ) 0 selection rule such that a nonradiative pathway from
a triplet excited state to the singlet ground state is favored.
Emission from a triplet state of R-CuAlCl4 is confirmed by the
measured lifetime of 14.2 µs. The lifetime data are best fit to a
single exponential, indicating that the observed blue lumines-
cence arises from a single emissive center. We conclude from
this that the activator for the blue luminescence is the copper
tetrachloride tetrahedral building block, CuCl4/2, and that the
luminescence is a bulk property of R-CuAlCl4. We suspect that
the red shifting of the emission maximum observed for the
â-phase (Figure 1), prepared by rapid cooling from the melt,
and R-phase samples with a similar history of rapid cooling, is
a result of an increased defect population. A defect structure in
which interstitial coppers or coppers on aluminum sites form
clustered copper defects could result in such a decreased energy
gap. The sum of less intense, red-shifted, defect cluster emission
and the dominant blue emission of the isolated copper(I) centers
could red shift the observed emission maximum.
Scheme 2
above, the halide contribution is not sufficient to influence any
significant variation in the energy of the transitions upon
bromide for chloride substitution. The triplet states derived from
these orbitals are also given in Scheme 1. While these triplet
states do not correspond to allowed transitions from the singlet
ground state, the emission lifetime of 14.2 µs implies that a
triplet state is involved in the emissive relaxation.
Based on the above assignments, the Jablonski diagram in
Scheme 2 provides a summary of the observed optical behavior
of the R-CuAlCl4 system. Starting from the right, the diagram
shows the ground-state singlet and the three singlet excited states
belonging to the 3d10 f 3d94s orbital transition. The lowest
lying triplet state, which represents a formally forbidden
transition from the singlet manifold, is schematically shown at
the left. The overlay of the emission, excitation, and diffuse
reflectance spectra (Scheme 2b) leads us to conclude that the
maximum absorbance (abs) is into a high-lying excited state
of the singlet manifold. After rapid relaxation (R) to the
vibrational ground state of a high-lying singlet excited state,
the process of internal conversion (IC) allows for the nonra-
diative relaxation through the singlet manifold to the singlet
ground state. However, absorbance at the excitation maximum
(ex) is into the lowest lying singlet state (close to the absorption
edge). Again rapid relaxation to the vibrational ground state of
the lowest singlet excited state occurs, but now with few
accessible energy overlaps on the excited-state hypersurface
leading to the singlet ground state, intersystem crossing (ISC)
into the lowest lying triplet state becomes competitive with
internal conversion. Once formed the lowest lying triplet state
It is useful to treat the optical properties of this extended
solid in terms of a localized molecular orbital model, similar to
the case of calcium tungstate. A localized molecular orbital
model of an isolated, d0, WO42- tetrahedron has been very useful
for explaining its optical properties.28 By extension, we utilize
an isolated, d10, CuCl4 tetrahedron to interpret the optical
3-
properties of R-CuAlCl4. The copper(I) cations reside on a
crystallographic position with S4 site symmetry, which exhibits
a slight compression of the CuCl4/2 tetrahedra along the S4 axis.
The allowed transitions for a CuCl43- fragment of S4 symmetry
are described in Scheme 1. The set of three allowed 3d10
f
3d94s transitions and two 3d10 f 3d94p transitions, as well as
some possible transitions from the chloride lone pairs into the
copper 4s, gives rise to the absorption feature centered at 40.3
× 103 cm-1 [248 nm, 5 eV] (Figure 2a). However, as noted
(28) (a) Brasser, R.; Pitt, E.; Scharmann, A.; Zimmer, G. Phys. Status
Solidi . 1975, 69, 359. (b) Hameka, H. F.; Vlam, C. C. Physica 1953, 19,
943.

10096 J. Am. Chem. Soc., Vol. 121, No. 43, 1999
SulliVan and Martin
decays radiatively to a vibrationally excited singlet ground state
(em). The singlet-triplet energy difference is equal to twice
the exchange integral,29 which is theoretically large in a highly
localized system such as an isolated cation. The bulk of the
16.3 × 103 cm-1 energy shift between the excitation and
emission maxima is thus associated with the spin-forbidden
transition from the singlet to triplet excited states.30 In addition
a smaller, but significant, contribution to the energy shift is
expected from the vibronic coupling of the electronic transition
with a Jahn-Teller-type distortion of the 3d94s excited state.31
In addition to the excited-state Jahn-Teller-type distortion,
which compresses the tetrahedra along the S4 axis, vibronic
coupling with the totally symmetric stretch is expected, based
on the strong antibonding nature of the copper 4s orbital. The
full width at half-maximum for the emission peak is ∼4000
cm-1. This is consistent with a large difference in the equilib-
rium nuclear configurations of the excited-state triplet and the
singlet ground state.9
Luminescent Quenching by Gases. Variation in the room
temperature luminescence, on exposure to small molecule gases,
includes copper aluminum chloride in a group of inorganic
materials which can act as a gas sorption sensors.32,33 A plot of
gas sorbed by R-CuAlCl4 at room temperature as a function of
relative pressure, PS/P0, is shown in Figure 5. After reaching
the relative pressure for the onset of sorption, approximately
0.02 Torr, there is a rapid increase in the amount of gas sorbed
up to about 1 equiv of gas per formula unit. The shape of this
curve is suggestive of a cooperative binding process, where
coordination of a few sorbant molecules alters the structure of
the sorbate such that additional molecules sorb more readily.34
Upon exposure of copper aluminum chloride to ethylene an
abrupt drop in emission intensity is observed corresponding to
the gas sorption (Figure 6). Upon exposure to dynamic vacuum
(∼0.05 Torr) the sorbed gas is removed and the luminescence
returns. These 10 min gas on, 10 min gas off cycles, shown in
Figure 6, are repeated demonstrating the reversibility of the
process. A major structural reorganization is required to
accommodate the sorption of ethylene into the framework of
R-CuAlCl4, which we are currently studying by time-resolved
powder X-ray diffraction.22 Precise structural details are not yet
fully understood; however, the weak sorbant-sorbate interac-
tions favor a nonradiative relaxation of the copper-centered
excited state.
directly proportional to the statistical distribution of Br through-
out the lattice. The flexibility of the metal halide structure is
responsible for the reversible binding of small molecules by
R-CuAlCl4. The coupling of photoluminescence and gas sorptive
properties leads to a useful probe of the chemistry of the system.
Experimental Section
General Methods and Procedures. All reactions and products are
air and moisture sensitive and were handled under the inert atmosphere
of an N2-filled glovebox or using vacuum and Schlenk line techniques.
Aluminum trichloride (Fluka) was purified by sublimation from an Al/
NaCl flux.35 Aluminum bromide was prepared by reaction of the
elements36 (Aldrich) and purified by sublimation from an Al/KBr flux.
The cuprous halides were prepared by comproportionation of copper
metal (EM Science) and CuCl2 (Aldrich) or CuBr2 (Matheson) dissolved
in the appropriate mineral acid (HCl or HBr) and precipitated upon
dilution with deoxygenated water.37 The cuprous halides were then
purified by sublimation. Gallium chloride was used as received from
Aldrich. All powder X-ray diffraction measurements were obtained
using an Enraf-Nonius Guinier camera and were indexed with respect
to silicon as a standard.
Synthesis of r-CuAlX4 (X ) Cl, Br). As previously reported,1
R-CuAlCl4 (mp 236 °C) is prepared from the melt of the respective
binary halides. To prepare materials of the highest optical purity, it is
important to heat the reaction mixture to near the melting point of the
cuprous halide (430 °C for CuCl and 504 °C for CuBr) and to perform
the reaction with a slight excess (1.0 wt %) of the aluminum trihalide.
In a typical reaction, CuCl (4.250 g, 42.93 mmol) and AlCl3 (5.781 g,
43.36 mmol) were placed into a fused silica tube and sealed under
vacuum. The reaction vessel was then heated to 430 °C for 1 h and
cooled to room temperature at a rate of 10 deg per h. The resulting
product was characterized by powder X-ray diffraction and by diffuse
reflectance spectroscopy. The mixed halide materials R-CuAlBrxCl4-x
,
as well as R-CuGaCl4, were similarly prepared from stoicheometric
melts of the binary components. There is no evidence for a superstruc-
ture by powder diffraction in the R-CuAlBrxCl4-x phases and the mixed
halides show a linear dependence of the unit cell volume on bromide
substitution, x (cell volume ) (12.35x/4 + 286.77) Å3).
Spectroscopy. Diffuse reflectance measurements were carried out
on a Cary 3e UV-vis spectrophotometer equipped with an integrating
sphere. Spectra were measured with respect to a pressed polytetra-
fluoroethylene powder standard. Reflectance spectra were collected
as R∞ ) Rsample/Rrefrence, then plotted as the remission function,
F(R∞))(1 - R∞)2/2R∞ (based on the Kubelka-Munk theory of diffuse
reflectance).17
Steady-state fluorescence spectra were measured with an ISS PC-1
fluorometer equipped with a Xenon arc lamp using a front face detection
geometry to orient the sample face at an angle of 22.5° with respect to
the incident beam. A low band-pass UG-11 filter was placed after the
excitation monochromator. Excitation spectra were corrected by a
rhodamine-B quantum counter. Emission spectra were corrected with
respect to a standard quinine sulfate solution and the NIST spectrum.
Emission spectra were plotted using the relative emitted energy per
constant interval (ΦE ) Φλλ2/hc).38 Emission intensity measurements
were normalized using CaWO4 as an external standard. Scattered light
from a grating artifact at 346 nm was also found to be a valid internal
standard for normalizing emission intensities.
Conclusion
The unique choice of building blocks for the three-
dimensional framework of R-CuAlCl4 has led to its brilliant,
room temperature, blue luminescence. The CuCl4/2 corner-
sharing tetrahedral building units are well isolated by AlCl4/2
tetrahedra in the framework structure. These isolated copper(I)
centers serve as intrinsic activators giving rise to a brilliant blue
luminescence. The heavy ion substitution in the solid solution
CuAlBrxCl4-x results in the quenching of luminescence that is
Time-resolved phosphorescence measurements were performed on
an ISS K2-003 Fastscan Multi-frequency Phase and Modulation
Fluorometer. Data were collected for 10 frequencies in the range of 10
to 70 kHz. Standard deviation for the measurements was 0.2° for the
phase values and 0.004° for the modulation values.
(29) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy
of the Triplet State; Prentice Hall: Englewood Cliffs, NJ, 1969; Chapter 3.
(30) Forster, L. S. In Concepts of Inorganic Photochemistry; Adamson,
A. W., Fleischauer, P. D., Eds; Wiley: New York; p 28.
(31) (a) Ruthkosky, M.; Kelly, A.; Zaros, M. C.; Meyer, G. J. J. Am.
Chem. Soc. 1997, 119, 12004. (b) Balzani, V.; Carassiti, V. Photochemistry
of Coordination Compounds; Academic: New York, 1970; Chapter 6.
(32) Martin, J. D. U.S. Patent 5,876,637, 1999.
(35) Campbell, J. L. E.; Johnson, K. E. J. Am. Chem. Soc. 1989, 111,
525.
(33) Ko, M. C.; Meyer, G. J. Photoluminescence of Inorganic Semicon-
ductors for Chemical Sensor Applications. In Optoelectronic Properties of
Inorganic Compounds; Roundhill, D. M., Fackler, J. P., Jr., Eds.; Modern
Inorganic Chemsitry Series; Plenum: New York, 1999; p 269.
(34) Lehninger. A. L. Biochemistry, 2nd ed.; Worth: New York, 1975;
p 146.
(36) Nicholson, D. G.; Winter, P. K.; Fineberg, H. In Inorganic syntheses;
Audrieth, L. F., Ed.; McGraw-Hill: New York, 1950; Vol. III, p 30.
(37) Kauffman, G. B.; Fang, L. In Inorganic syntheses; Holt, S. L., Jr.,
Ed.; McGraw-Hill: New York, 1983; Vol. XXII, p 101.
(38) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer-
Verlag: Berlin, 1994; p A4.

Understanding the Photoluminescence of R-CuAlCl4
J. Am. Chem. Soc., Vol. 121, No. 43, 1999 10097
Gas Sorption. Measurement of the luminescence upon gas sorption
and desorption were carried out on samples prepared by coating 2 g of
ground fused silca particles (300-800 µm) with 0.06 g of R-CuAlCl4
(mp 236 °C) in a fused silica tube. The tube was heated to 250 °C and
cooled at a rate of 1 deg per min. The tube was then connected to a
gas/vacuum manifold and the intensity at the emission maximum
measured as a function of time, during which the sample was alternately
evacuated and exposed to gas. Data for C2H4 are presented in this work;
however, other small molecule gases including CO, NO, and benzene
have also been examined.20 All gases were used as received from
Aldrich or National Gas Products. Gunier powder X-ray diffraction
films were obtained for samples of R-CuAlCl4 in capillaries before
gas sorption and were found to match the same samples after desorption.
Gas adduct phases were also characterized by powder X-ray diffraction
films.
Samples of R-CuAlCl4 (∼30 mg) were placed in the sample chamber
at room temperature. A free volume measurement was performed using
He gas before the sample was dosed with calibrated aliquots of ethylene.
Using this system, it was possible to perform equilibrium measurements
up to a pressure of approximately 200 Torr.
Acknowledgment. The authors gratefully acknowledge Dr.
Martin van de Ven of ISS Fluorescence Instrumentation for the
frequency domain lifetime measurements, Michael Peachey for
the volumetric gas sorption measurements, and Coulter Corpo-
ration for the loan of the SA3100 BET surface area analyzer.
This work was supported by the National Science Foundation,
CAREER award (DMR-9501370). J. D. Martin is a Cottrell
Scholar of the Research Cooperation.
Equilibrium volumetric measurements for the sorption of ethylene
were made on modified Coulter SA3100 BET surface area analyzer.
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