Polymorph and isomer conversion of complexes based on CuI and PPh 3 easily observed via luminescence

Article abstract of DOI:10.1039/c1dt11462j

Reactions between copper(i) iodide and triphenylphosphine have been explored in solution and in the solid state and six luminescent coordination complexes have been obtained and characterized by X-ray diffraction and UV-vis spectroscopy and photophysics.

Full text of DOI:10.1039/c1dt11462j

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PAPER
Polymorph and isomer conversion of complexes based on CuI and PPh3 easily
observed via luminescence†
Lucia Maini,*a Dario Braga,a Paolo P. Mazzeoa and Barbara Ventura*b
Received 3rd August 2011, Accepted 15th September 2011
DOI: 10.1039/c1dt11462j
Reactions between copper(I) iodide and triphenylphosphine have been explored in solution and in the
solid state and six luminescent coordination complexes have been obtained and characterized by X-ray
diffraction and UV-vis spectroscopy and photophysics. Solid-state reactions of CuI with PPh3 in
different conditions (kneading, vapour digestion) and stoichiometries resulted in the formation of high
ratio ligand : metal compounds while tetrameric structures could be obtained only by solution
reactions. Crystal structures were determined by single crystal X-ray diffraction while purity of the bulk
product was checked by powder diffraction (XRPD). Three different tetrameric structures with 1 : 1
stoichiometry have been synthesized: two closed cubane-type polymorphs [CuI(PPh3)]4 (form 1a) and
[CuI(PPh3)]4 (form 1b) and an open step-like isomer [CuI(PPh3)]4 (form 2). The conversions between
the polymorphs and isomers have been studied and characterized by XRPD. The most stable form
[CuI(PPh3)]4 (form 1b) can convert into the open step-like isomer [CuI(PPh3)]4 (form 2) in a slurry
experiment with EtOH or CH2Cl2 or AcCN and converts back into [CuI(PPh3)]41b when exposed to
vapors of toluene. At room temperature all the tetrameric compounds exhibit luminescence in the solid
state and, notably, the two polymorphs show a dissimilar dual emission at low temperature. The
luminescence features in the solid state seem to be peculiarly related to the presence of the aromatic
phosphine ligand and depend on the Cu–Cu distance in the cluster.
Introduction
Copper(I) halide aggregates constitute a large family of com-
pounds studied for decades for their photochemical and pho-
tophysical properties.1–11 This class of compounds is attracting
renewed interest12–16 because of its potential applications in high-
efficiency OLEDs.17–19 Coordination systems based on copper
halides show a remarkable structural diversity,15 which arises from
the many possible combinations of coordination numbers (two,
three and four) available for copper(I) and for the geometries that
can be adopted by the halide ions (from terminal to m2- and up to
m8-bridging modes).
Organophosphine copper(I) halides have been extensively stud-
ied in the past20–22 and several species with different metal coor-
Fig. 1 Schematic representation of some examples of different geome-
tries and stoichiometries for CuI aggregates; a)[Cu2I2L2]; b)[Cu2I2L3];
c)[Cu2I2L4]; d)[Cu4I4L4] step-like; e) [Cu4I4L4]cubane-like; f)[CuIL]•
coordination polymer.
dination numbers and metal : ligand stoichiometries (Fig. 1) have
been already synthesized and the crystal structure determined,
while the luminescence properties have been explored only recently
without a clear assignment on the nature of the emission bands.
With respect to Cu(I) structures with halogens and amine-based
ligands,5,23,24 when the organic ligand is an aromatic phosphine,
further interactions such as back bonding from the metal to
the phosphorous system must be taken into account25,26 and
can play a role in the photophysics of the complexes. Since
the organophosphine copper(I) iodide complexes could find
applications in light emitting diode technology a full compre-
hension of the emission bands is essential to tune the emission
properties.
aDipartimento di chimica “G. Ciamician”-Universita` di Bologna, Via F. Selmi
2, Bologna, Italy. E-mail: l.maini@unibo.it; Tel: 39 051 2099536
bIstituto per la Sintesi Organica e la Fotoreattivita` (ISOF)-CNR, Via P.
Gobetti 101, 40129, Bologna, Italy. E-mail: bventura@isof.cnr.it
† Electronic supplementary information (ESI) available. CCDC reference
numbers 838119–838123. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11462j
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Fig. 2 a) [CuI(PPh3)3] (form a) and, b) [CuI(PPh3)3] (form b); hydrogen atoms are omitted for clarity.
The 1 : 1 : 1 Cu : halogen : ligand complexes CuX(PR3) have
tetrameric structures which may either be cubane-like (with all
copper atoms pseudo-tetrahedral and all X atoms serving as m3-
bridges) or step-like (with the tetramer containing two pseudo-
tetrahedral and two three-coordinated copper atoms and two m3-
and two m2- iodides). The 1 : 1 : 2 adducts CuX(PR3)2 may have
either monomeric three-coordinate structures or dimeric pseudo-
tetrahedral structures. The dimer compounds Cu2X2(PR3)y (y = 2–
4) nicely illustrate the propensity of the organophosphine copper(I)
systems to form different polynuclear complexes with similar
stabilities with different copper geometries of coordination.27 At
last the 1 : 1 : 3 adducts all seem to have monomeric pseudo-
tetrahedral coordination.28
In solution, different complexes are simultaneously present
due to the high constant of dissociation of the PPh3ligand.25
Despite the presence of a plethora of complexes in solution, pure
compounds can be obtained as precipitates: the stoichiometry
ratio determines the nuclearity of the compounds, while the
polymorph or isomer outcome can be influenced by the choice
of solvent or by the procedure used.
make these complexes interesting as sensor-like materials. The
photophysical characterization was carried out in the solid and in
solution and the results were interpreted on the basis of current
theories on the luminescence of Cu(I) cubane clusters4,37,38 with
new indications for the specific case of organophosphine based
complexes, whose properties have been scarcely investigated up to
now.
Results and discussions
Synthesis and structure of [CuI(PPh3)3] (form a) and [CuI(PPh3)3]
(form b)
Monomeric coordination compounds with a 1 : 3 molar ratio
stoichiometry of CuI salt and PPh3 were synthesized utilizing
the experimental instructions previously reported.29 The product
obtained was a mixture of two concomitant polymorphic forms:
[CuI(PPh3)3] (form a), which corresponds to a known structure
and [CuI(PPh3)3] (form b) whose structure was determined by us.
[CuI(PPh3)3] (form b) (Fig. 2b) crystallizes in the monoclinic P21/c
space group while [CuI(PPh3)3] (form a) (Fig. 2a) crystallizes in
a denser structure in the trigonal P3 space group (1.39 g cm-3
and 1.42 g cm-3 respectively) which suggests that [CuI(PPh3)3]
(form a) is the stable form at room temperature, according to the
Kitaigorodskii’s density rule.39 The stability of [CuI(PPh3)3] (form
a) was confirmed by an overnight slurry experiment in chloroform
where the polymorph b converted into form a. No further analyses
were performed on these two monomeric forms because of the
difficulty to obtain them as a pure powder.
In this work we report on the synthesis and characterization
(both structural and photophysical) of different Cu(I) coordina-
tion compounds with the PPh3 ligand. All compounds have been
obtained either by methods described in literature29–32 and/or via
solvent-free reactions such as kneading or vapor digestion.33,34 It
is well known that these approaches can be a valid alternative to
traditional synthetic methods, since they reduce the amount of
solvent required and are usually quantitative in yields. Sometimes,
non-solution methods also allow the synthesis of different com-
pounds which are not found via solvent synthesis.35 As previously
reported in the literature, the synthesis with a CuI : PPh3 ratio
of 1 : 3 yielded upon crystallization the monomer [CuI(PPh3)3]
(form a),29 but we observed also a new polymorph [CuI(PPh3)3]
(form b) which is a metastable form. A CuI : PPh3 ratio of 1 : 2
Synthesis and structure of [CuI(PPh3)1.5]2
Dimeric compound [CuI(PPh3)1.5]2 has been synthesized following
the experimental procedure previously reported in the literature.32
The [CuI(PPh3)1.5]2 structure consists of a 4-atom ring (Cu2I2),
in which the halogen atom acts as a bridge between copper
atoms. Different copper coordination numbers are observed in
this complex as shown in Fig. 3: one copper atom binds two P-
based ligands and it has a distorted tetrahedral geometry while the
other copper atom has a trigonal geometry. The distance between
32
yielded the dimer [CuI(PPh3)1.5]2 and, finally, a CuI : PPh3 ratio
of 1 : 1 yielded the crystallization of the cubane-like compounds
[CuI(PPh3)]4 (form 1a),30 [CuI(PPh3)]4 (form 1b)36 and the step-like
compound [CuI(PPh3)]4 (form 2).30 The conversions within the
isomers and polymorphs were studied in solution and via vapor
digestion.
˚
the two metal atoms is 3.044(5) A; luminescence features linked
We focused our attention on the luminescence properties of
the dimer and the tetramers, which exhibit remarkably different
emission features depending on the nature of the compound, which
to Cu–Cu distances are reported in the following section. We
obtained also the solvate species [CuI(PPh3)1.5]2·CH2Cl2 from the
crystallization process (see ESI†) which shows a Cu–Cu distance
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[CuI(PPh3)]4 (form 1a) is therefore the metastable form at RT,
as also suggested by the lower density compared to 1b (1.72 g cm-3
versus 1.77 g cm-3 at RT).39
Diffraction data for both polymorphs 1a and 1b were collected
at RT and 90 K and structures were found to be in agreement
with those previously reported.29,36 The cubane-like clusters of
[CuI(PPh3)]4 form 1a and 1b (Fig. 5 and Fig. 6) are distorted
(Table 1) with the shortest Cu–Cu distance equal to 2.839(3)
˚
˚
A and 2.945(4) A respectively at RT which are longer than the
43
˚
van der Waals distance (2.80 A) All Cu–Cu distances show a
dramatic decrease on lowering the temperature with minima Cu–
˚
˚
Cu distances equal to 2.7427(8) A and 2.828(2) A for 1a and 1b
at 90 K. On the basis of the copper distances, only 1a at low
temperature should shows features ascribable to cluster-centered
interactions while the luminescence data suggests the presence of
such interactions also at RT for both compounds.
Fig. 3 The [CuI(PPh3)1.5]2unit; hydrogen atoms are omitted for clarity.
˚
(3.0444(5) A) comparable to [CuI(PPh3)1.5]2 and it is isomorphic
to [CuCl(PPh3)1.5]2·CH2Cl2 reported in literature.40
Synthesis and structure of [CuI(PPh3)]4
Compounds with 1 : 1 : 1 ratio have been obtained only as in the
form of tetrameric units; however it was possible to isolate two
polymorphs of [CuI(PPh3)]4: a cubane-like cluster and a step-like
cluster, as shown in Fig. 1d–e.
The first synthesis was carried out by following the reported
procedure30 resulting in the expected [CuI(PPh3)]4 (form 1a);
however on repeating the synthesis, a second phase appeared which
we characterized as form 1b of [CuI(PPh3)]4.36,41
Interestingly, once form 1b had been obtained, a mixture of
1a and 1b was subsequently obtained on repeating the synthesis.
At each repetition step, the percentage of form 1b became
predominant and increasing at each reaction, until only form 1b
could be obtained (see Fig. 4).
Fig. 5 The [CuI(PPh3)]4cubane-like unit; hydrogen atoms are removed
for clarity and in shorter distances Cu atoms are bonded together.
Fig. 6 Overlap between Cu4I4 clusters in[CuI(PPh3)]41a (yellow) and
[CuI(PPh3)]4 1b (purple); phenyl rings are removed for clarity.
The step-like cluster [CuI(PPh3)]4 form 2 (Fig. 7) has been
obtained according to the reported synthesis.30 However no single
crystals could be obtained and X-ray powder diffraction data
was measured at 90 K to ascertain that the crystal structure was
retained. (Fig. S6, ESI† .)
Fig. 4 a) calculated X-ray powder pattern of [CuI(PPh3)]4 (form 1a);
b) X-ray powder diffractogram of compounds obtained from the first
synthesis corresponding to [CuI(PPh3)]4 (form 1a); c) X-ray powder
diffractogram of compounds obtained from the second synthesis where
both form 1a and 1b of phases [CuI(PPh3)]4 could be recognized; d) X-ray
powder diffractogram of compounds obtained in subsequent syntheses
corresponding to form 1b; e) calculated X-ray powder pattern of form 1b.
The metastable [CuI(PPh3)]4 (form 1a) product could no longer
be obtained as a pure form in our laboratory, and mixtures of
forms 1a and 1b were obtained when the synthesis was carried out
in a different laboratory and with a fast crystallization process.
This complex can be counted as a new example of disappearing
polymorphs.42
Fig. 7 The [CuI(PPh3)]4 form 2 unit; hydrogens are removed for clarity.
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˚
Table 1 Cu–Cu distances in cuprous organophosphine compounds. All distances are reported in A. Labels are in agreement with those reported in ESI†
[CuI(PPh3)1.5 [CuI(PPh3)]4 1a RT [CuI(PPh3)]4 1a 90 K [CuI(PPh3)]4 1b RT [CuI(PPh3)]4 1b 90 K [CuI(PPh3)]4 2
]
2
Cu1 Cu2 3.0444(5) Cu1
Cu2
Cu3
Cu4
Cu3
Cu4
Cu4
2.928(8)
2.896(6)
2.873(4)
2.839(3)
3.165(3)
3.109(4)
Cu1 Cu2 2.7431(4) Cu1 Cu1 2.945(3) Cu1 Cu1 2.8281(4) Cu1
Cu1 Cu3 3.0809(4) Cu1 Cu2 3.174(5) Cu1 Cu2 3.1051(4) Cu1
Cu1 Cu4 2.7971(8)
Cu2 Cu3 2.9985(7)
Cu2 Cu4 2.8085(8)
Cu1
Cu2
2.834(2)
3.404(3)
Cu1
Cu1
Cu2
Cu2
Cu3
Cu3 Cu4 2.8080(9)
The step-like cluster [CuI(PPh3)]4 form 2 is characterized by two
copper atoms pseudo-tetrahedral and two three-coordinate copper
atoms; although a quite short Cu–Cu distance is observed in the
When [CuI(PPh3)]4 (form 1a) is exposed to vapors of toluene
it rapidly converts into form 1b which subsequently transforms
into form 2. The conversion from 1b to 2 was observed in
other experimental conditions such as in kneading experiments
with acetonitrile or toluene or slurry experiments with ethanol,
dichloromethane or acetonitrile. The conversion is reversible since
it is possible to obtain 1b from a slurry of form 2 with seeds of
form 1b. All interconversions are summarized in Scheme 1.
˚
complex (2.834(2) A at RT) the different coordination around the
copper atoms seems to be responsible for the weak luminescence
properties of this compound (vide infra).
Solvent free reactions
The possibility of obtaining compounds based on CuI : PPh3 via
solvent free reactions with CuI : PPh3 ratios of 1 : 1, 1 : 2 and 1 : 3
was also explored. Kneading process (i.e grinding the physical
mixture with a drop of solvent)33 produced only [CuI(PPh3)]4 (form
2) irrespective of the ratio or solvent used.
The physical mixture between CuI and PPh3 with different
ratios was exposed to the vapor of organic solvents and the final
products depended on the starting solvents and ratios. Form 2 was
obtained with the CuI : PPh3 = 1 : 1 molar ratio exposed to vapor of
acetonitrile; monomeric [CuI(PPh3)3] (form a) was obtained with
the CuI : PPh3 = 1 : 2 molar ratio exposed to acetonitrile vapour
while in all the other cases only the monomer [CuI(PPh3)3] (form
b) was observed.
Scheme 1 Summary of all interconversions between polymorphic and
isomeric forms. (VD = vapour digestion, Kn = kneading).
Photophysical properties
Interconversions between isomers
The photophysical properties of compounds [CuI(PPh3)]4 1a, 1b,
2 and [CuI(PPh3)1.5]2 and of the free ligand PPh3 have been
investigated in solution and in the solid state, both at room
temperature and at 77 K. The main luminescence data are
summarized in Table 2.
Toluene has been used as a solvent for 1a, 1b and [CuI(PPh3)1.5]2,
to retain the main cluster features of the complexes. For the
same reason, acetonitrile was chosen for the solubilization of
[CuI(PPh3)]4 form 2, but this compound was found to be un-
stable in solution. It is worth recalling that previous studies on
Cu : Cl : PPh3 complexes in solution have highlighted the presence
of different equilibria between various complexes due to ease
of exchange of the PPh3 ligand.25 The absorption spectra of
compounds 1a, 1b and [CuI(PPh3)1.5]2 in toluene show absorption
features extending up to 350–400 nm, ascribable to CT transitions,
while the absorption of the free ligand PPh3 does not exceeds
320 nm (see Fig. S7, ESI†).31,44
The isomer conversion via exposure to solvent vapour was also
investigated. It has been observed that the two isomeric forms
[CuI(pic)]• and [CuI(pic)]4 interconvert in presence of organic
solvent.24 In our case form 1b, when exposed to saturated
atmosphere of acetonitrile, fully converts into form 2 as confirmed
by XRPD measurements (Fig. 8).
Forms 1a, 1b and [CuI(PPh3)1.5]2 are non emissive in air-
equilibrated solutions and display a weak emission centered
around 620 nm in air-free toluene at room temperature (fem = 2–3 ¥
10-3), with lifetimes on the order of 200–300 ns (Fig. S7, ESI† and
Table 2). The similarities between the emission features of the three
compounds point to a loss of the rigid cluster structure in solution
in favour of a comparable pseudo-tetrahedral geometry and the
observed luminescence can tentatively be assigned to MLCT
(metal-to-ligand charge transfer) excited states.45 In the dimeric
Fig. 8 a) [CuI(PPh3)]4 (form 1b) calculated; b) [CuI(PPh3)]4 (form 1b)
experimental; c) [CuI(PPh3)]4 (form 2) experimental after conversion; d)
[CuI(PPh3)]4 (form 2) calculated.
Since solid form 1b is characterized by a bright green emission
while form 2 shows only a very weak luminescence (vide infra),
the conversion between the two isomeric forms produces a switch-
on/switch-off signal which can be easily detected by naked eyes.
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Table 2 Luminescence parameters of the ligand and Cu(I) complexes in air–free toluene solutions and in the solid state, at room temperature and at
77 K
Emission at RT
Emission at 77 K
b
c
c
lmaxa/nm
468
437
628
544
628
518
620
482
fem
t /ms
lmaxa/nm
445
434
412
410, 591
412
434, 516
412
414
t /ms
PPh3
Solution
Solide
6.0 ¥ 10-3
2.9 ¥ 10-3
1.7 ¥ 10-3
1.9 ¥ 10-3
—
0.6 ¥ 10-3
0.8 ¥ 10-3
0.26
4.7
0.22
3.2
0.28
7.1
—
13.6 ¥ 103
2.4 ¥ 103
224.8
[CuI(PPh3)]4 1a
[CuI(PPh3)]4 1b
Solution
Solid
Solution
Solid
117.3, 26.0d
190.4
271.8, —
185.8
397.0
[CuI(PPh3)1.5
]
Solution
Solid
2
[CuI(PPh3)]4 2
Solution
Solid
—
—
—
527
3.4
515
11.7
a From corrected emission spectra. b Emission quantum yields in solution, determined by comparing corrected emission spectra, using quinine sulfate in
air-equilibrated 1 N H2SO4 (fem = 0.546)51 as a standard for PPh3 and [Ru(bpy)3]Cl2 in air-equilibrated water (fr = 0.028)51 as a standard for the complexes,
excitation at 300 nm for PPh3 and at 330 nm for the complexes. c Excitation at 278 or 300 nm for PPh3 and at 331 nm for the complexes. d A rise time of
7.3 ms is detected, see text. e Emission spectra of PPh3 in the solid state are shown in Fig. S10, ESI.†
compound [CuI(PPh3)1.5]2, Cu(I) experiences both a trigonal and
a tetrahedral geometry, the latter with two phosphine ligands,
and the tetrahedral part of the cluster is probably responsible for
the emission. Compound 2, where the tetrahedral coordination of
copper includes only one phosphine ligand is instead non stable
in solution, since only a weak emission from PPh3 was observed.
At 77 K in glassy toluene all the three emissive compounds
show a luminescence band centered around 415 nm, that is
largely hypsochromically shifted in comparison with the room
temperature emission, and is blue-shifted also with respect to
the low temperature emission of the free PPh3 ligand (Fig. S7,
ESI† and Table 2). The 77 K luminescence of PPh3, in fact, is
centered at 445 nm with a lifetime of 13.6 ms and is attributed
to phosphorescence.31 The spectral features and the lifetimes, on
the order of 200 ms, of the luminescence of the complexes at
77 K, suggest a ligand contribution to the emission, that can
be tentatively assigned to a mixed MLCT/LC (ligand-centered)
phosphorescence. A similar long-lived blue emission in solution
at 77 K has been recently reported for Ag(I)-bis(diphosphine)
complexes, and has been attributed to 3MLCT/3IL (intra-ligand)
states based on tetrahedral geometry.46
Reflectance spectra of solid samples of PPh3 and complexes 1a,
1b, [CuI(PPh3)1.5]2 and 2 (Fig. S8, ESI†), revealed absorption bands
below 350 nm for the ligand, with maxima at 212 and 264 nm,
ascribable to phenyl-centered and n→ p* phosphorous→phenyl
transitions, respectively,25,47 and a longer extension of the absorp-
tion up to 370 nm for the complexes, with the exception of 2 that
tails till 420 nm. A clear band at 300 nm is more pronounced for
the two solids 1a and 1b while solid [CuI(PPh3)]4 2 displays a more
pronounced absorption around 350 nm.
studied by Ford and co-workers, where a dual emission from CC
(cluster-centered) and XLCT (iodide to ligand charge transfer)
poorly coupled triplet excited states occur. The room temperature
emission of form 1a can thus be attributed to 3CC (low energy, LE,
emission), and the same band red-shifts at 591 nm at 77 K due to
the shortening of the Cu–Cu distance at low temperature.35,48 The
band at 410 nm that appears at 77 K, with a lifetime of 117 ms,
can then be ascribed to 3XLCT (high energy, HE, emission). This
˚
compound presents a Cu–Cu distance in the cluster of 2.839(3) A
˚
at RT, slightly longer than the sum of the orbital radii (2.80 A),
˚
which markedly shortens up to 2.7427(8) A at 90 K. Interestingly,
the LE band at 591 nm shows a rise of 7.3 ms, followed by a
decay of 26.0 ms (see Fig. S9, ESI†). Ford and co-workers also
describe LE rise-times considerably shorter than the HE emission
lifetime (ns vs. ms range) for Cu4I4L4 clusters with pyridinic ligands,
speculating on a third higher energy state that directly populates
the LE state.5 Excitation spectra monitored at 590 nm and 410
nm at low temperature and that measured at 544 nm at room
temperature (Fig. 9a), closely resemble those reported for 3XLCT
and 3CC bands of solid [CuI(py)]4.49 The excitation spectrum
measured on the LE band is more confined below 350 nm, even
if a clear differentiation is difficult due to the overlap of the two
emission bands.
Solid 1b behaves quite differently from its polymorphic form 1a,
showing at room temperature a green emission centered at 518 nm
with a lifetime of 3.2 ms, significantly blue-shifted with respect to
the room temperature emission of 1a. At low temperature, then, a
dual emission is also observed, but with one band centered at 434
nm and a less intense band at 516 nm, substantially unshifted with
respect to the room temperature case (Fig. 9b and Table 2). The
description of the dual luminescence of compound 1b in terms
of 3XLCT and 3CC is less straightforward, in consideration of the
fact that all the Cu–Cu distances estimated in the cluster are longer
With respect to the solution case, the solid state luminescence
differentiates to a larger extent the four examined complexes.
The two polymorphic forms 1a and 1b, show different features.
Form 1a displays a bright green–yellow emission at 544 nm at
room temperature whereas at 77 K a dual emission is clearly
observed, with maxima at 410 and 591 nm (see Fig. 9a and Table
2), that noticeably renders the resultant emission color nearly
white. These luminescence features closely resemble the behavior
of Cu(I) cubane structures with aromatic amine ligands,5–8,23 widely
˚
˚
than 2.80 A (2.945(4) A, see Table 1). It is however emerging from
recent examples in the literature that organophosphine copper(I)
˚
halides with Cu–Cu distances in the range 2.9–3.0 A show this
type of thermochromism, characterized by a dual emission at low
temperature with a LE band almost unshifted with respect to
the room temperature emission and a blue HE band.17,36,50 The
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Fig. 9 Normalized emission and excitation spectra of solids (a) [CuI(PPh3)]4 form 1a, (b) [CuI(PPh3)]4 form 1b,(c) [CuI(PPh3)1.5]2 and (d) [CuI(PPh3)]4
form 2, at room temperature (black lines) and at 77 K (red lines). For (a) and (b) the excitation spectra at 77 K measured on the HE band are reported as
green lines whereas those measured on the LE band are reported as blue lines.
excitation spectra measured at both temperature are similar to
those observed for [CuI(PPh3)]4 1a (Fig. 9b), indicating a similar
nature of the excited states and confirming the CC nature of
the LE band. The reduced shift in the position of the LE band
with temperature can be interpreted considering that at longer
Cu–Cu distances the distortion of the CC state with respect to
the ground state is reduced.49 The occurrence of a CC based
Compound [CuI(PPh3)1.5]2 emits at 482 nm at room temperature
and at 414 nm at 77 K with no evidence of dual emission (Fig.
9c). The Cu–Cu distances in this case are much longer than the
˚
sum of the orbital interaction radii (3.044(5) A) and this leads to
describe the luminescence at room temperature in terms of 3XLCT
phosphorescence; the excitation spectrum recorded at 482 nm is in
line with this analysis. The excitation spectrum measured at 414 nm
at low temperature, instead, is markedly blue-shifted, suggesting
a more pronounced LC nature of the emission, supported also by
the long lifetime of 397 ms.
˚
emission in clusters with Cu–Cu distances longer than 2.8 A,
however, seems to be correlated to the presence of the aromatic
phosphine ligand. It is known that the phosphorous–metal bond
of triphenylphosphine–transition metal complexes is not simply
describable by the charge donation from the phosphorous lone-
pair orbital to the metal, but interactions between the metal d
orbitals and the phosphorous–carbon s orbitals also participate
to the bond.26 We can thus speculate that the involvement of
3d electrons in a “back-bonding” toward the ligand renders the
d→s,p metal centered contribution to the CC state less effective:
the 3CC emission should be of predominant halide-to-metal charge
transfer (XMCT), thus possible also at distances slightly longer
than the sum of the van der Waals radii and less sensitive
to the contraction of the Cu–Cu cluster occurring when the
temperature is decreased. DFT calculations have been performed
on Cu4X4L4 structures with L = PH3 or NH3,47 indicating a larger
X contribution to the HOMO and a larger Cu contribution to the
LUMO when L = PH3, but further calculations are required to
investigate the nature of the transitions when the phosphine bear
aromatic groups.
It can be noticed that in all the three previously discussed
cases the 77 K solid state emission (considering the HE bands
for polymorphs 1a and 1b) is very similar for spectral features and
lifetime to that registered in solution at the same temperature and
this point to a similar nature of the emitting excited state that
involves the ligand orbitals.
[CuI(PPh3)]4 form 2, that is non-emissive in solution, shows
a very weak luminescence also in the solid state. The room
temperature emission, centered at 527 nm with a lifetime of
3.4 ms, slightly blue-shifts at 515 nm at low temperature, with
a lifetime of 11.7 ms (Fig. 9d and Table 2). The excitation
spectrum at room temperature show a sharp peak at 354 nm,
responsible of the observed emission (excitation below 300 nm
yielded a more broadened and red-shifted emission). In this
compound, the crystal data evidenced the presence of only one
˚
Cu–Cu distance in the unit cell of 2.83 A (Table 1), close to
the orbital interaction limit, so the observed weak emission can
536 | Dalton Trans., 2012, 41, 531–539
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3
be tentatively interpreted in terms of XLCT emission at both
temperatures.
On slow cooling and evaporation of the solvent, the products have
been separated as colourless crystal. A mixture of polymorphs a
and b was obtained.
A different synthetic strategy has been developed to make
[CuI(PPh3)3] b:
Conclusions
CuI salt (1 mmol; 0.190 g) was dissolved in dimethylchloride
under reflux then a solution of PPh3 (3 mmol; 0,786 g) in n-hexane
was added. After two days colourless crystals of [CuI(PPh3)3] b
appeared.
Polymorph b was also obtained exposing: a) a 1 : 1 molar ratio
physical mixture of CuI and PPh3 to vapour of toluene; b) a 1 : 2
molar ratio physical mixture of CuI and PPh3 to vapour of toluene,
acetonitrile and dichloromethane; c) a 1 : 3 molar ratio physical
mixture of CuI and PPh3 to vapour of toluene and acetonitrile.
Our study confirms the tendency of organophosphine copper(I)
iodide compounds to form polynuclear complexes of different
structural types but of similar stabilities. On the other hand the
observation of the existence of two polymorphs: [CuI(PPh3)3] a
and b and [CuI(PPh3)]41a and 1b for both species suggests also
the propensity of organophosphine copper(I) iodide compounds
to afford polymorphic modifications, probably because of the
flexibility of copper(I) in coordination systems. Coordinating
solvents seem to stabilize complexes with three coordinated copper
atoms, while the stoichiometry influences the nuclearity of the
complexes.
Synthesis of [CuI(PPh3)1.5]2
The extensive photophysical characterization in solution and
solid allowed to a deeper understanding on the nature of the
luminescence bands. Room temperature emission is postulated
The synthesis of [CuI(PPh3)1.5]2 was previously reported in the
literature.32 We report an alternative synthetic strategy. CuI
(1 mmol, 0.190 g) was dissolved in 20 mL of acetonitrile stirring
at 70 ◦C. Then, a solution of PPh3 (2 mmol, 0.524 g) in AcCN
(20 mL) was added to the clear solution. The solution was stirred
3
to be of CC nature also in compound [CuI(PPh3)]4 (form 1b)
˚
which has Cu–Cu distances longer than 2.80 A, a behavior not
observed in similar complexes with nitrogen based ligands. The
experimental evidence suggests that the 3CC excited state has
a predominant halide-to-metal charge transfer (XMCT) nature,
which is in agreement with the absence of bathochromic shift in
lowering the temperature. The contraction of the cubane cluster
[CuI(PPh3)]4 (form 1a) at low temperature reduces the distances
◦
at 70 C overnight, and crystals with prismoidal shape appeared
after two days.
Synthesis of [CuI(PPh3)]4 form 1a and [CuI(PPh3)]4 form 1b
The synthesis of compound [CuI(PPh3)]4 form 1a has been made
utilizing the experimental instruction as previous reported in the
literature.29 CuI (1 mmol, 0.190 g) and PPh3 (1 mmol, 0.262 g) have
been refluxed overnight in toluene. Very slow cooling (overnight)
of the resultant clear solution to room temperature resulted in well
formed crystals of the complex [CuI(PPh3)]4.
As described in discussion section, we had some difficulties to
re-prepare [CuI(PPh3)]4 form 1a; the second time we followed the
above described experimental procedure, we obtained a mixture
of polymorph 1a and 1b, and from the third time on, we obtained
always the most thermodynamically stable polymorph 1b.
Compound [CuI(PPh3)]4 form 1a could be re-prepared in
drastic kinetic conditions: copper(I) iodide (0.190 g, 1 mmol) was
dissolved in toluene under reflux and than a solution in toluene of
PPh3 (0.262 g, 1 mmol) was added. The transparent solution was
suddenly evaporated under reduced pressure. The predominant
compound was mainly 1a even if minor quantities of 1b have been
observed.
˚
among the Cu atoms till 2.7431(4) A, which is shorter than the sum
3
of the van der Waals radii. In this case the CC emission can be
considered of both d → s,p metal centered and XMCT nature, and
the red-shift of the LE band at low temperature is indicative of the
effect of the shortening of the Cu–Cu distance on the transition.
In solution the room temperature emission is described as 3MLCT
whereas the blue band registered at 77 K is attributed to mixed
3MLCT/3LC.
In summary, we have prepared and characterized a rather
unusual pair of polymorphs and isomers and studied their
interconversion and luminescence properties. Our results have
established that [CuI(PPh3)3] as form a is more stable than as form
b at RT, while in the case of the tetrameric compounds [CuI(PPh3)]4
form 1b is more stable than form 1a. The relative stability of the
isomers, on the other hand, depends on the nature of the solvent.
[CuI(PPh3)]4 as form 1b converts into form 2 in the presence
of solvents such as ethanol, acetonitrile and dichloromethane,
while the reverse reaction takes place only in slurry in toluene.
The conversion can be easily detected by naked eye because
forms 1b and 2 show a dramatic difference in the emission
output and the system could be used as a switch-on/switch-off
signal.
Synthesis of [CuI(PPh3)]4 form 2
The synthesis of compound [CuI(PPh3)]4 form 2 was made
utilizing the experimental instruction as previously reported in
the literature.20 Stoichiometric quantities of CuI (1 mmol, 0190 g)
and PPh3 (1 mmol, 0.262 g)were refluxed for 1 h in chloroform
and the hot, clear solution filtered. On slow cooling and solvent
evaporation, the product separate as colourless crystal
The same compound has been obtained by grinding a physical
mixture of CuI and triphenylphosphine in 1 : 1 molar ratio with
two drops of toluene or grinding a physical mixture of CuI and
triphenylphosphine in 1 : 2 and 1 : 3 molar ratio with two drops of
acetonitrile.
Experimental section
Synthesis of [CuI(PPh3)3] forms a and b
The synthesis of the two forms a and b of compound [CuI(PPh3)3]
was carried out by utilizing the experimental instruction as
previous reported in the literature.29 Stoichiometric quantities of
CuI (1 mmol, 0,190 g) and PPh3 (3 mmol, 0.786 g) were refluxed for
2–4 h in chloroform and the hot, clear solution has been filtered.
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Photophysics
added in calculated positions. Mercury54 was used for the graphical
representation of the results.
Solvents used for photophysical determinations were of spectro-
scopic grade (C. Erba). Dilute solutions were analyzed in 10 mm
path length square optical Suprasil Quartz (QS) cuvettes at room
temperature, solid samples were placed inside two quartz slides,
while capillary tubes immersed in liquid nitrogen in a home-made
quartz Dewar were used for measurements at 77 K of both solids
and solutions. The absorption spectra of solutions were obtained
with a Perkin-Elmer Lambda 950 UV/vis/NIR spectrophotome-
ter, whereas reflectance spectra of solid samples were acquired
on a Perkin-Elmer Lambda 9 UV/vis/NIR spectrophotometer
equipped with a 60 mm integrating sphere.
Emission and excitation spectra were measured in a SpexFlu-
orolog II spectrofluorimeter, equipped with a Hamamatsu R928
phototube, in right angle mode for solutions and front face mode
for solids. Corrected emission spectra are reported, the uncertainty
on the band maxima is 2 nm. Luminescence quantum yields
in solution were evaluated from the area of the luminescence
spectra, corrected for the photomultiplier response, with reference
to quinine sulfate in air-equilibrated 1 N H2SO4 (fem = 0.546)51
Acknowledgements
We thank MIUR, the University of Bologna and CNR (Project
PM-P04-010 MACOL) for financial support to this research. PPM
thanks the University of Bologna for a PhD grant. We thank Dr
Nicola Armaroli for helpful discussion.
Notes and references
1 F. G. Mann, D. Purdie and A. F. Wells, J. Chem. Soc., 1936, 447–460.
2 A. E. Arbusow, J. Russ. Phys. Chem. Soc., 1906, 38(ii), 293.
3 G. Tartarini, Gazz. Chim. Ital., 1933, 63, 597–600.
4 N. Armaroli, G. Accorsi, F. Cardinali and A. Listorti, Top. Curr. Chem.,
2007, 280, 69–115.
5 K. R. Kyle, C. K. Ryu, P. C. Ford and J. A. DiBenedetto, J. Am. Chem.
Soc., 1991, 113, 2954–2965.
6 P. C. Ford and A. Vogler, Acc. Chem. Res., 1993, 26, 220–226.
7 P. C. Ford, Coord. Chem. Rev., 1994, 132, 129–140.
8 H. D. Hardt and A. Pierre, Inorg. Chim. Acta, 1977, 25, L59–L60.
9 H. D. Hardt and A. Pierre, Naturwissenschaften, 1975, 62, 298.
10 H. D. Hardt, H. Gechnizdjani and A. Pierre, Naturwissenschaften,
1972, 59, 363.
for PPh3 and to [Ru(bpy)3]Cl2 in air-equilibrated water (fr
=
0.028)51 for the complexes. The samples with absorbances <0.1
were bubbled with Ar for approximately 15 min and sealed in
home made 10 mm optical cells. The luminescence tail of toluene
emission below 450 nm was previously subtracted from the spectra.
The estimated error on the quantum yields is 20%.
11 H. D. De Ahna and H. D. Hardt, Z. Anorg. Allg. Chem., 1972, 387,
61–71.
12 R. Peng, S. R. Deng, M. Li, D. Li and Z. Y. Li, CrystEngComm, 2008,
10, 590–597.
13 C. H. Arnby, S. Jagner and I. Dance, CrystEngComm, 2004, 6, 257–275.
14 S. Q. Bai, J. Y. Kwang, L. L. Koh, D. J. Young and T. S. A. Hor, Dalton
Trans., 2010, 39, 2631–2636.
15 R. Peng, M. Li and D. Li, Coord. Chem. Rev., 2010, 254, 1–18.
16 J. B. Liu, H. H. Li, Z. R. Chen, J. B. Li, X. B. Chen and C. C. Huang,
J. Cluster Sci., 2009, 20, 515–523.
17 S. Perruchas, X. F. L. Goff, S. Maron, I. Maurin, F. o. Guillen, A.
Garcia, T. Gacoin and J.-P. Boilot, J. Am. Chem. Soc., 2010, 132, 10967–
10969.
18 C. Tard, S. Perruchas, S. Maron, X. F. Le Goff, F. Guillen, A. Garcia,
J. Vigneron, A. Etcheberry, T. Gacoin and J. P. Boilot, Chem. Mater.,
2008, 20, 7010–7016.
19 Z. Liu, M. F. Qayyum, C. Wu, M. T. Whited, P. I. Djurovich, K. O.
Hodgson, B. Hedman, E. I. Solomon and M. E. Thompson, J. Am.
Chem. Soc., 2011, 133, 3700–3703.
Luminescence lifetimes in the range 0.5 ns–100 ms were mea-
sured with an IBH 5000 F time-correlated single-photon counting
device, by using pulsed NanoLED excitation sources at 278 or
331 nm (pulse width £ 0.6 ns). Longer lifetimes were measured
on the same IBH 5000 F TCSPC device, by using a pulsed
SpectraLED excitation source at 370 nm or on a Perkin-Elmer
LS-50B spectrofluorimeter equipped with a pulsed Xe lamp and
in a gated detection mode. Analysis of the luminescence decay
profiles against time was accomplished with the Decay Analysis
Software DAS6 or with the PHOSDecay Software provided by
the manufacturers, with an estimated error on the lifetimes of
10%. In some cases the low temperature luminescence of the solid
samples showed multiexponential decays; Table 2 reports lifetimes
corresponding to pre-exponential factors >80%.
20 M. R. Churchill, B. G. DeBoer and D. J. Donovan, Inorg. Chem., 1975,
14, 617–623.
21 M. R. Churchill and K. L. Kalra, Inorg. Chem., 1974, 13, 1065–1071.
22 B.-K. Teo and J. C. Calabrese, Inorg. Chem., 1976, 15, 2467–2474.
23 P. C. Ford, E. Cariati and J. Bourassa, Chem. Rev., 1999, 99, 3625–3648.
24 E. Cariati, X. H. Bu and P. C. Ford, Chem. Mater., 2000, 12, 3385–3391.
25 D. J. Fife, W. M. Moore and K. W. Morse, Inorg. Chem., 1984, 23,
1684–1691.
X-ray powder diffraction
26 C. Bourg, S. Gamblin and D. Urch, J. Electron Spectrosc. Relat.
X-ray powder diffractograms were collected on a Panalytical
X’Pert PRO automated diffractometer with Cu-Ka radiation and
X’Celerator detector without a monochromator equipped with a
Anton Paar TK 450 chamber to collect data at low temperature.
The program PowderCell52 was used for calculation of X-ray
powder patterns.
Phenom., 1995, 73, 163–172.
27 G. Costa, E. Reisenhofer and L. Stefani, J. Inorg. Nucl. Chem., 1965,
27, 2581–2584.
28 P. H. Davis, R. L. Belford and I. C. Paul, Inorg. Chem., 1973, 12,
213–218.
29 P. F. Barron, J. C. Dyason, P. C. Healy, L. M. Engelhardt, C.
Pakawatchai, V. A. Patrick and A. H. White, J. Chem. Soc., Dalton
Trans., 1987, 1099–1106.
30 J. C. Dyason, P. C. Healy, L. M. Engelhardt, C. Pakawatchai, V. A.
Patrick, C. L. Raston and A. H. White, J. Chem. Soc., Dalton Trans.,
1985, 831–838.
Crystal structure determination
Crystal data for 1a and 1b were collected on a Oxford Xcalibur S
with Mo-Ka radiation, l = 0.71073 A, monochromator graphite
31 D. J. Fife, W. M. Moore and K. W. Morse, Inorg. Chem., 1984, 23,
˚
1545–1549.
and equipped with a liquid nitrogen Oxford-Cryostream device.
Crystal data and details of measurements are summarized
in Table S1.† SHELX9753 was used for structure solution and
refinement based on F2. Non-hydrogen atoms were refined
anisotropically. Hydrogen atoms bound to carbon atoms were
32 P. G. Eller, G. J. Kubas and R. R. Ryan, Inorg. Chem., 1977, 16, 2454–
2462.
33 D. Braga, S. L. Giaffreda, K. Rubini, F. Grepioni, M. R. Chierotti and
R. Gobetto, CrystEngComm, 2007, 9, 39–45.
34 D. Braga, S. L. Giaffreda, F. Grepioni, M. R. Chierotti, R. Gobetto,
G. Palladino and M. Polito, CrystEngComm, 2007, 9, 879–881.
538 | Dalton Trans., 2012, 41, 531–539
This journal is
The Royal Society of Chemistry 2012
©

View Article Online
35 D. Braga, F. Grepioni, L. Maini, P. P. Mazzeo and B. Ventura, New J.
Chem., 2011, 35, 339–344.
36 H. Kitagawa, Y. Ozawa and K. Toriumi, Chem. Commun., 2010, 46,
6302–6304.
Armaroli, B. Delavaux-Nicot, A. Kaeser, M. Holler, J.-F. Nierengarten
and A. Degli Esposti, THEOCHEM, 2010, 962, 7–14.
46 K. Matsumoto, T. Shindo, N. Mukasa, T. Tsukuda and T. Tsubomura,
Inorg. Chem., 2010, 49, 805–814.
37 M. Vitale and P. C. Ford, Coord. Chem. Rev., 2001, 219, 3–16.
38 F. De Angelis, S. Fantacci, A. Sgamellotti, E. Cariati, R. Ugo and P. C.
Ford, Inorg. Chem., 2006, 45, 10576–10584.
39 A. I. Kitaigorodskii, Organic Chemical Crystallography, New York.
40 D. J. Darensbourg, M. W. Holtcamp, K. K. Klausmeyer and J. H.
Reibenspies, Z. Kristallogr., 1995, 210, 615–616.
41 Structure of 1b has been published by Kitagawa et al. during the prepa-
ration of the manuscript. The nomenclature of the two polymorphs is
in agreement with the Kitagawa articles.
47 P. A. Grutsch and C. Kutal, J. Am. Chem. Soc., 1979, 101, 4228–4233.
48 D. Braga, L. Maini, P. P. Mazzeo and B. Ventura, Chem.–Eur. J., 2010,
16, 1553–1559.
49 M. Vitale, C. K. Ryu, W. E. Palke and P. C. Ford, Inorg. Chem., 1994,
33, 561–566.
50 C. Tard, S. Perruchas, S. Maron, X. F. Le Goff, F. o. Guillen, A. Garcia,
J. Vigneron, A. Etcheberry, T. Gacoin and J.-P. Boilot, Chem. Mater.,
2008, 20, 7010–7016.
51 M. Montalti, A. Credi, L. Prodi and M. T. Gandolfi, in Handbook
of Photochemistry, Third Edition, CRC Press, Taylor & Francis: Boca
Raton, 2006.
42 J. D. Dunitz and J. Bernstein, Acc. Chem. Res., 1995, 28, 193–
200.
43 A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
44 H. H. Jaffe` and L. D. Freedman, J. Am. Chem. Soc., 1952, 74, 1069–
1071.
45 (a) O. Moudam, A. Kaeser, B. Delavaux-Nicot, C. Duhayon, M. Holler,
G. Accorsi, N. Armaroli, I. Seguy, J. Navarro, P. Destruel and J.-F.
Nierengarten, Chem. Commun., 2007, 3077–3079; (b) G. Accorsi, N.
52 W. Kraus and G. Nolze, Powder Cell, 1999.
53 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Go¨ttingen, Germany, 1997.
54 C. F. Macrae, P. R. Edgington, P. McCabe, E. Pidcock, G. P. Shields,
R. Taylor, M. Towler and J. van De Streek, J. Appl. Crystallogr., 2006,
39, 453–457.
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