New structural motifs, unusual quenching of the emission, and second harmonic generation of copper(I) iodide polymeric or oligomeric adducts with para-substituted pyridines or trans-stilbazoles

Article abstract of DOI:10.1021/ic050143s

The structural, emissive, and nonlinear optical properties of new Cul adducts with para-substituted trans-stilbazolic and pyridinic ligands are reported. Single-crystal X-ray diffraction results indicate that the para-substituent on the organic ligand greatly influences the structural motif by its steric (tert-butyl), electronic/steric (dimethylamino), or bridging-donor (cyano) properties so that two absolutely new structural motifs, polymeric and oligomeric, are found when trans-stilbazole and pyridine carry a dimethylamino group in the para-position. In addition, a surprising Photoemission behavior is observed, being the solid-state emission of [Cul(trans-4-stilbazole)] _n, [Cul(trans-4′-(dimemylamino)-4-stilbazole)]_n, and [Cul(4′-tert-butyl-4-stilbazole)]_n totally quenched. In the case of the noncentrosymmetric Cul adduct of trans-4′-(dimethylamino) stilbazole a discrete second harmonic generation (SHG) occurs. 2005 American Chemical Society.

Full text of DOI:10.1021/ic050143s

Inorg. Chem. 2005, 44, 4077−4085
New Structural Motifs, Unusual Quenching of the Emission, and Second
Harmonic Generation of Copper(I) Iodide Polymeric or Oligomeric
Adducts with Para-Substituted Pyridines or trans-Stilbazoles
Elena Cariati,*,† Dominique Roberto,† Renato Ugo,† Peter C. Ford,‡ Simona Galli,*,§ and Angelo Sironi|
Dipartimento di Chimica Inorganica Metallorganica e Analitica, UniVersita` degli Studi di Milano,
Centro di Eccellenza CIMAINA, UdR di Milano dell’INSTM and Istituto di Scienze e Tecnologie
Molecolari del CNR (ISTM-CNR), Via Venezian 21, I-20133 Milano, Italy, Department of
Chemistry, UniVersity of Santa Barbara, Santa Barbara, California 93106, Dipartimento di
Scienze Chimiche e Ambientali, UniVersita` degli Studi dell’Insubria, Via Valleggio 11, I-22100
Como, Italy, and Dipartimento di Chimica Strutturale e Stereochimica Inorganica, UniVersita`
degli Studi di Milano, Centro di Eccellenza CIMAINA, UdR di Milano dell’INSTM and Istituto di
Scienze e Tecnologie Molecolari del CNR (ISTM-CNR), Via Venezian 21, I-20133 Milano, Italy
Received January 28, 2005
The structural, emissive, and nonlinear optical properties of new CuI adducts with para-substituted trans-stilbazolic
and pyridinic ligands are reported. Single-crystal X-ray diffraction results indicate that the para-substituent on the
organic ligand greatly influences the structural motif by its steric (tert-butyl), electronic/steric (dimethylamino), or
bridging-donor (cyano) properties so that two absolutely new structural motifs, polymeric and oligomeric, are found
when trans-stilbazole and pyridine carry a dimethylamino group in the para-position. In addition, a surprising
photoemission behavior is observed, being the solid-state emission of [CuI(trans-4-stilbazole)]n, [CuI(trans-4
′-
(dimethylamino)-4-stilbazole)]n, and [CuI(4 -tert-butyl-4-stilbazole)]n totally quenched. In the case of the noncen-
′
trosymmetric CuI adduct of trans-4
′
-(dimethylamino)stilbazole a discrete second harmonic generation (SHG) occurs.
Introduction
with a pseudoaromatic donor nitrogen atom (L) are particu-
larly intriguing for both the variety of structural motifs and
During the past decade, interest for new materials exhibit-
ing worthwhile photoluminescence and nonlinear optical
(NLO) properties has been focused on crystalline inorganic-
organic hybrids, capable of merging the advantages of the
organic compounds (appreciable response speed and intensity
in a wide spectral range, straightforward synthetic approach)
to those of the inorganic ones (chemical, thermal, and
mechanical stabilities).1 Among the inorganic-organic hy-
brid materials, the adducts between group 11 metal halides
(MX, with X ) Cl, Br, or I) and monohapto Lewis bases
the related optical emission displayed.2
We have recently investigated the photoemission and the
second-order NLO properties of a series of known, noncen-
trosymmetric [CuX(L)]n single-stranded “chains” and double-
stranded “stairs” species, namely [CuCl(quinoline)]n, [CuCl-
(2-methylquinoline)]n, [CuBr(pyridine)]n, [CuBr(2,4,6-tri-
methylpyridine)]n, [CuBr(2-methylquinoline)]n, and [CuI-
(2,4,6-trimethylpyridine)]n.3 Solid-state emissive properties
of chains were explored for the first time, while those of
stairs were added to the few already studied in the literature
([CuI(pyridine)]n, [CuI(3-methylpyridine)]n, and [CuI(4-
methylpyridine)]n).2,4 For all compounds, the high-energy
(HE) emissions observed were retraced to a triplet halide-
to-ligand charge transfer [3XLCT*, pX f π*L] excited state
* Authors to whom correspondence should be addressed. E-mail:
simona.galli@uninsubria.it (S.G.).
† Dipartimento di Chimica Inorganica Metallorganica e Analitica,
Universita` degli Studi di Milano.
‡ Department of Chemistry, University of Santa Barbara.
§ Dipartimento di Scienze Chimiche e Ambientali, Universita` degli Studi
dell’Insubria.
(2) Ford, P. C.; Cariati, E.; Bourassa, J. Chem. ReV. 1999, 99, 3625 and
references therein.
(3) Cariati, E.; Roberto, D.; Ugo, R.; Ford, P. C.; Galli, S.; Sironi, A.
Chem. Mater. 2002, 14, 5116.
| Dipartimento di Chimica Strutturale e Stereochimica Inorganica,
Universita` degli Studi di Milano.
(1) See e.g.: (a) Di Bella, S. Chem. Soc. ReV. 2001, 30, 355. (b) Lacroix,
P. G. Chem. Mater. 2001, 13, 3495.
(4) Cariati, E.; Xianhui Bu; Ford, P. C. Chem. Mater. 2000, 12, 3385.
10.1021/ic050143s CCC: $30.25
Published on Web 05/05/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 11, 2005 4077

Cariati et al.
Chart 1
300 spectrometers) and elemental analyses carried out in the
Dipartimento di Chimica Inorganica Metallorganica ed Analitica
of the University of Milano.
For the numbering used in the attribution of the 1H NMR signals,
see Chart 1.
Synthesis and Characterization of Compounds 1-8. Synthesis
of [CuI(trans-4-stilbazole)]n (1). A 0.153 g (0.844 mmol) amount
of trans-4-stilbazole dissolved in 10 mL of CH2Cl2 was added under
stirring to a solution of 0.196 g (0.182 mmol) of [CuI(pyridine)]4
in 25 mL of CH2Cl2. A yellow solid separated, which was filtered
(ES), their energy being influenced by both the polarizability
of the halides and the electronic properties of the nitrogen
donor ligands. Moreover, we have tested their solid-state
second-order NLO activities, proving evidence for a cor-
relation between the latter and the structural motif, stairs
always showing a better response than chains.3
Among monohapto Lewis bases with a nitrogen donor in
a pseudoaromatic system, trans-4-stilbazoles para-substituted
with electron donor groups have been extensively investi-
gated for their significant second-order NLO activities, either
as metal complexes5 or as salts, these latter particularly in
the solid state.6
We have thus decided to pursue the investigation of solid-
state photoemissive and second-order NLO responses of new
CuI adducts with either trans-4-stilbazoles, having either a
sterically demanding (tert-butyl) or a strong electron-donor
(dimethylamino) group in position 4′, or the corresponding
para-substituted pyridines. The more extended π-conjugation
of trans-4-stilbazolic ligands with respect to pyridines allows
one to judge the importance of π-conjugation length on the
photophysical emission properties of these adducts.
In the following, we thus report on the structural, unusual
lack of emission, and, when available, second-order NLO
features of new CuI adducts with (see Chart 1) trans-4-
stilbazole (1), trans-4′-(dimethylamino)-4-stilbazole (2),
4-tert-butylpyridine (3), and 4-(dimethylamino)pyridine (4).
Other adducts of CuI with trans-4′-tert-butyl-4-stilbazole (5)
and trans-4-[4-(4-(dimethylamino)phenyl)buta-1,3-dienyl]-
pyridine (6), as well as of CuCl (7) and CuBr (8) with trans-
4′-(dimethylamino)-4-stilbazole, were investigated for their
emissive and second-order NLO properties only, since we
were unable to obtain suitable single crystals for an X-ray
structural characterization.
1
out and washed twice with 10 mL of CH2Cl2 (yield 82.7%). H
NMR (CD3CN, 25 °C): δ (ppm) 8.57 (AA′BB′, 2H, H2, and H6,
broad), 7.63 (2 H, H2′, and H6′), 7.50 (d, 1 H, H8, J ) 16.2 Hz),
7.50 (AA′BB′, 2 H, H3, and H5), 7.45 (1H, H4′), 7.40 (2 H, H3′,
and H5′), 7.21 (d, 1 H, H7, J ) 16.2 Hz). Anal. Calcd for C13H11-
NCuI: C, 42.00; H, 2.96; N, 3.77. Found: C, 41.34; H, 3.18; N,
3.73. Crystals of 1 suitable for X-ray diffraction were obtained by
dissolution in boiling CH3CN followed by slow cooling to room
temperature and then 0 °C.
Synthesis of [CuI(trans-4′-(dimethylamino)-4-stilbazole)]n (2).
A 0.195 g (0.872 mmol) amount of trans-4′-(dimethylamino)-4-
stilbazole dissolved in 20 mL of CH2Cl2 was added under stirring
to a solution of 0.202 g (0.187 mmol) of [CuI(pyridine)]4 dissolved
in 25 mL of CH2Cl2. A yellow solid separated, which was filtered
out and washed twice with 10 mL of CH2Cl2 (yield 76%). 1H NMR
(CD3CN, 25 °C): δ (ppm) 8.51 (AA′BB′, 2H, H2, and H6, broad),
7.51 (AA′BB′, 2 H, H2′, and H6′, J ) 8.9 Hz), 7.43 (AA′BB′, 2 H,
H3, and H5, J ) 5.4 Hz), 7.40 (d, 1 H, H8, J ) 16.2 Hz), 6.95 (d,
1 H, H7, J ) 16.1 Hz), 6.81 (AA′BB′, 2 H, H3′, and H5′, J ) 8.9
Hz), 3.00 (s, 6 H, NMe2). Anal. Calcd for C15H16N2CuI: C, 43.43;
H, 3.86; N, 6.76. Found: C, 43.40; H, 3.55; N, 6.53. Rather small
crystalline needles of 2, poorly suitable for X-ray diffraction, were
obtained by dissolution in hot CH3CN followed by slow cooling
to room temperature and then 0 °C.
Synthesis of [CuI(4-tert-butylpiridine)]4 (3). Compound 3 was
prepared by dissolving 0.314 g (1.65 mmol) of CuI in an aqueous
solution saturated with KI and adding, under stirring, 0.28 mL
(0.2582 g, 1.91 mmol) of 4-tert-butylpiridine. A white solid
separated, which was washed with 12 mL of an aqueous solution
saturated with KI, 20 mL of distilled water, 15 mL of methanol,
and 10 mL of n-hexane (yield 78.8%). The tetrameric nature of
compound 3 was suggested by its yellow-orange emission4 by
irradiation with a UV lamp emitting at 266 nm. By the addition of
20 mL of methanol to the reaction mother liquor, an additional
amount of white solid (yield 13.7%) was obtained, which, by
irradiation with the UV lamp, showed both a yellow-orange and a
blue emission, suggesting the presence of both a tetrameric and a
Experimental Section
General Comments. [CuI(pyridine)]4,7 trans-4′-tert-butyl-4-
stilbazole,8 and trans-4-[4-(4-(dimethylamino)phenyl)buta-1,3-di-
enyl]pyridine5 were prepared according to the literature. trans-4-
Stilbazole and trans-4′-(dimethylamino)-4-stilbazole were purchased
from Eastman Organic Chemicals and Sigma-Aldrich, respectively,
and used without further purification. All complexes were charac-
1
polymeric species,4 respectively. H NMR ((CD3)2CO, 25 °C): δ
(ppm) 8.90 (AA′BB′, 2H, H2, and H6, broad), 7.66 (AA′BB′, 2 H,
H3, and H5, broad), 1.38 (s, 9 H, t-Bu). 1H NMR ((CD3)2CO, -80
°C): δ (ppm) 8.80 (AA′BB′, 2H, H2, and H6, J ) 5.2), 7.72
(AA′BB′, 2 H, H3, and H5, J ) 5.2), 1.35 (s, 9 H, t-Bu). Anal.
Calcd for C9H13NCuI: C, 33.18; H, 3.99; N, 4.30. Found: C, 33.60;
H, 3.98; N, 4.24. Crystals of 3 suitable for X-ray diffraction were
obtained by addition of methanol to a CH2Cl2 solution of the
compound.
Synthesis of [CuI(4-(dimethylamino)pyridine)]6 (4). A 0.162
g (0.150 mmol) amount of [CuI(pyridine)]4 was dissolved in 20
mL of CH2Cl2; 0.085 g (0.696 mmol) of 4-(dimethylamino)pyridine,
dissolved in 3 mL of CH2Cl2, was then added. The solution was
concentrated under vacuum up to 5-6 mL, and then 30 mL of
n-pentane was added. The white-greenish precipitate (73.4% yield)
was filtered out, washed with n-pentane (50 mL), and dried under
1
terized by H NMR (Bruker AC-200 and a Bruker Avance DRX
(5) See e.g.: (a) Kanis, D. R.; Lacroix, P. G.; Ratner, M. A.; Marks, T. J.
J. Am. Chem. Soc. 1994, 116, 10089. (b) Roberto, D.; Ugo, R.; Bruni,
S.; Cariati, E.; Cariati, F.; Fantucci, P.; Invernizzi, I.; Quici, S.; Ledoux,
I.; Zyss, J. Organometallics 2000, 19, 1775.
(6) (a) Marder, S. R.; Perry, J. W.; Yakymyshyn, C. P. Chem. Mater.
1994, 6, 1137. (b) Coradin, T.; Cle´ment, R.; Lacroix, P. G.; Nakatani,
K. Chem. Mater. 1996, 8, 2153. (c) Be´nard, S.; Yu, P.; Audie`re, J. P.;
Rivie`re, E.; Cle´ment, R.; Guilhem, J.; Tchertanov, L.; Nakatani, K. J.
Am. Chem. Soc. 2000, 122, 9444.
(7) Malik, A. U. J. Inorg. Nucl. Chem 1967, 29, 2106.
(8) Lucenti, E.; Cariati, E.; Dragonetti, C.; Manassero, L.; Tessore, F.
Organometallics 2004, 23, 687.
4078 Inorganic Chemistry, Vol. 44, No. 11, 2005

Copper(I) Iodide Polymeric or Oligomeric Adducts
vacuum. The solid showed an orange emission by irradiation with
a UV lamp emitting at 266 nm. H NMR ((CD3)2CO, 25 °C): δ
solution of 0.164 g (0.159 mmol) of trans-4′-(dimethylamino)-4-
stilbazole dissolved in 20 mL of CH2Cl2 was added. A yellow solid
separated, which was filtered out under nitrogen (yield 83.5%). 1H
NMR (CD3CN, 25 °C): δ (ppm) 8.86 (AA′BB′, 2H, H2, and H6,
broad), 7.58 (AA′BB′, 2 H, H2′, and H6′, broad), 7.51 (AA′BB′, 2
H, H3, and H5, J ) 8.6 Hz), 7.42 (d, 1 H, H8, J ) 16.4 Hz), 6.94
(d, 1 H, H7, J ) 16.4 Hz), 6.81 (AA′BB′, 2 H, H3′, and H5′, J )
6.9 Hz), 3.01 (s, 6 H, NMe2). Anal. Calcd for C15H16N2CuBr: C,
48.98; H, 4.35; N, 7.62. Found: C, 48.70; H, 4.51; N, 7.99. The
compound can be recrystallized under nitrogen by dissolution under
reflux in CH3CN, followed by slow cooling to 0 °C. It must be
kept and manipulated under nitrogen to avoid easy oxidation by
air.
Photoluminescence Measurements. Solid state and solution
emission spectra were recorded using a Spex Florolog 2 spectro-
fluorometer equipped with a Hamamatsu R 928 A water-cooled
photomultiplier tube. The purity of the batches of compounds 1-4
used to record the spectra was assessed by checking the agreement
between their XRPD diffractograms and those calculated on the
basis of the single-crystal results.
1
(ppm) 8.14 (AA′BB′, 2H, H2, and H6, J ) 7.8 Hz), 7.16 (AA′BB′,
2 H, H3, and H5, J ) 7.8 Hz), 3.38 (s, 6 H, NMe2). Anal. Calcd for
C7H10N2CuI: C, 26.88; H, 3.20; N, 8.96. Found: C, 26.50; H, 3.15;
N, 9.05. Crystals of 4 suitable for X-ray diffraction were obtained
by slow diffusion of n-pentane in a CH2Cl2 solution of the
compound.
Synthesis of [CuI(trans-4′-tert-butyl-4-stilbazole)]n (5). A
0.099 g (0.42 mmol) amount of trans-4′-tert-butyl-4-stilbazole
dissolved in 20 mL of CH2Cl2 was added under stirring to a solution
of 0.095 g (0.088 mmol) of [CuI(pyridine)]4 dissolved in 20 mL
of CH2Cl2. A yellow solid separated from the deep yellow solution
by addition of an excess of n-pentane (yield 93%). 1H NMR (CD3-
CN, 25 °C): δ (ppm) 8.91 (AA′BB′, 2H, H2, and H6, broad), 7.52
(AA′BB′, 2 H, H2′, and H6′, J ) 8.4 Hz), 7.44 (2 H, H3′, and H5′,
J ) 8.4 Hz), 7.36 (d, 1 H, H8, J ) 16.3 Hz), 7.31 (AA′BB′, 2 H,
H3, and H5, J ) 8.2 Hz), 7.03 (d, 1 H, H7, J ) 16.3 Hz), 1.36 (s,
9H, t-Bu). Anal. Calcd for C17H19NCuI: C, 47.72; H, 4.44; N, 3.27.
Found: C, 47.70; H, 4.58; N, 3.36.
Second-Order NLO Kurtz-Perry9 Measurements. The 1064
nm wavelength of a Nd:YAG pulsed laser beam was directed on
sample-containing capillaries. The scattered radiation was collected
by an elliptical mirror, filtered to select only the second-order
contribution, and recollected with a Hamamatsu R 5108 photo-
multiplier tube. SHG efficiency was evaluated by taking as reference
the SHG signal of quartz.
Synthesis of [CuI(trans-4-[4-(4-(dimethylamino)phenyl)buta-
1,3-dienyl]pyridine)]n (6). A 0.087 g (0.35 mmol) amount of trans-
4-[4-(4-(dimethylamino)phenyl)buta-1,3-dienyl]pyridine dissolved
in 10 mL of CH2Cl2 was added under stirring to a solution of 0.094
g (0.087 mmol) of [CuI(pyridine)]4 in 20 mL of CH2Cl2. An orange
solid separated, which was filtered out and washed twice with 10
1
mL of CH2Cl2 (yield 71%). H NMR (CD3CN, 25 °C): δ (ppm)
X-ray Data Collection and Solution for Compounds 1-4.
Suitable crystals were mounted in air on the glass fiber tip of a
goniometer head. Data collections were performed at room tem-
perature with a Bruker AXS SMART CCD area-detector diffrac-
tometer using graphite-monochromatized Mo KR radiation
(λ ) 0.710 73 Å). A total of 2000 (1), 2400 (2 and 3), or 1800 (4)
frames was acquired by applying the ω-scan method, with ∆ω )
0.3°, t ) 30 (1, 3, and 4) or 45 (2) s/frame, and sample-detector
distance fixed at 3.93 (1), 5.52 (2), 4.98 (3), or 3.95 (4) cm. The
first 50 frames were recollected at the end of each measurement:
crystal decay was never observed. An empirical absorption cor-
rection was applied to the integrated reflections.10 The structures
were solved by direct methods11 and successfully refined with full-
matrix least squares calculations.12 Anisotropic temperature factors
were assigned to all non-disordered atoms but hydrogens, which
were made riding their parent atoms with a common isotropic
displacement parameter.
Since 2 is almost insoluble in the most common organic solvents,
many recrystallization attempts invariably yielded small needles
of rather modest quality. Lowering the temperature to 193 K
resulted in no significant improvements in data collection, so that
the room-temperature acquisition was definitely retained. Due to
the poor data quality, difficulties were encountered in assigning
the correct space group, a few orthorhombic choices being equally
probable. As both the E-statistics and (particularly) the SHG results
(see below) prompted a noncentrosymmetric crystal packing,
structure solution was carried on in Pna21. The conventional figures
of merit obtained from the subsequent refinement were not
completely satisfactory, so that reconsideration of the space group
8.5 (AA′BB′, 2H, H2, and H6, broad), 7.39 (4 H, H3, H5, H2′, and
H6′), 7.27 (1 H, H8), 6.87 (1 H, H9), 6.80 (3 H, H7, H3′, and H5′),
6.60 (d, 1 H, H10, J ) 15,3 Hz), 3.00 (s, 6 H, NMe2). Anal. Calcd
for C17H18N2CuI: C, 46.32; H, 4.09; N, 6.36. Found: C, 45.90; H,
4.20; N, 6.55.
Synthesis of [CuCl(trans-4′-(dimethylamino)-4-stilbazole)]n
(7). A 0.106 g (1.07 mmol) amount of CuCl was added under
nitrogen atmosphere to 50 mL of saturated KCl aqueous solution.
The suspension was kept under stirring up to the complete
dissolution of CuCl. A 0.2 mL (0.196 g, 2.478 mmol) volume of
pyridine was slowly added under a flux of nitrogen. [CuCl-
(pyridine)]4 separated as white solid. After the mixture was filtered,
the compound was washed with 10 mL of saturated KCl aqueous
solution, 10 mL of methanol, and 10 mL of n-hexane (yield 86%).
A 0.059 g (0.083 mmol) amount of this white solid was dissolved
under nitrogen in 20 mL of CH2Cl2. With working being done under
a flux of nitrogen, 0.088 g (0.395 mmol) of trans-4′-(dimeth-
ylamino)-4-stilbazole dissolved in 10 mL of CH2Cl2 was added. A
yellow solid separated, which was filtered out under nitrogen (yield
86.3%). The solid must be kept and manipulated under nitrogen to
avoid easy oxidation by air. Due to the compound instability to
1
oxidation to paramagnetic species of Cu(II), the H NMR charac-
terization could not be performed. Anal. Calcd for C15H16N2-
CuCl: C, 55.67; H, 4.95; N, 8.66. Found: C, 55.70; H, 5.05; N,
8.78.
Synthesis of [CuBr(trans-4′-(dimethylamino)-4-stilbazole)]n
(8). A 0.11 g (0.772 mmol) amount of CuBr was added under
nitrogen atmosphere to 50 mL of saturated KBr aqueous solution.
The suspension was kept under stirring up to the complete
dissolution of CuBr. Under a flux of nitrogen, 0.1 mL (0.098 g,
1.244 mmol) of pyridine was slowly added. [CuBr(pyridine)]4
separated as white solid. After the mixture was filtered under
nitrogen, the compound was washed with 10 mL of saturated KBr
aqueous solution, 10 mL of distilled water, and 10 mL of n-hexane
(yield 89.6%). A 0.096 g (0.108 mmol) amount of this white solid
was dissolved under nitrogen in 20 mL of CH2Cl2, and then a
(9) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798.
(10) Sheldrick, G. M. SADABS: program for empirical absorption cor-
rection; University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(11) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Gagliardi, A.; Moliterni,
A. G. G.; Burla, M. C.; Polidori, G.; Cavalli, M.; Spagna, R. SIR97:
package for structure solution by direct methods; 1997.
(12) Sheldrick, G. M. SHELX97: program for crystal structure refinement;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 44, No. 11, 2005 4079

Cariati et al.
Table 1. Crystallographic Data and Refinement Details for Compounds 1-4a
1
2
3
4
formula
fw
C13H11CuIN
371.67
C30H32Cu2I2N4
829.48
C36H52Cu4I4N4
1307.72
C21H30Cu3I3N6
937.83
cryst system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
monoclinic
P21/c
15.978(6)
4.148(2)
19.338(8)
90
103.34(1)
90
1247.1(9)
4
orthorhombic
Pna21
7.574(4)
11.662(6)
33.970(19)
90
90
90
3001(3)
4
orthorhombic
Pbcn
23.341(11)
20.409(9)
19.177(9)
90
90
90
9135(7)
8
monoclinic
P21/c
14.061(6)
11.917(5)
17.848(7)
90
105.58(1)
90
2881(2)
4
F(000)
712
1.980
4.20
300(2)
1616
1.836
3.50
293(2)
4992
1.894
4.57
299(2)
1776
2.162
5.43
297(2)
Dc (Mg m-3
µ(Mo KR) (mm-1
temp (K)
)
)
cryst morphology
cryst size (mm)
measd reflcns
unique reflcns
Rint
yellow platelet
0.40 × 0.14 × 0.04
14 404
3521
0.038
yellow needle
0.34 × 0.06 × 0.04
24 839
5394
0.053
colorless platelet
0.32 × 0.20 × 0.04
33 473
2764
0.035
colorless platelet
0.24 × 0.12 × 0.10
32 431
8140
0.048
Rσ
0.047
0.047
0.014
0.056
I > 2σ(I) reflcns
data/restr/params
GoF
2298
4351
2583
4479
3521/0/145
S(F2) ) 0.82
R(F) ) 0.024
wR(F2) ) 0.045
R(F) ) 0.051
wR(F2) ) 0.048
0.42, -0.85
5394/44/296
S(F2) ) 1.12
R(F) ) 0.068
wR(F2) ) 0.148
R(F) ) 0.086
wR(F2) ) 0.154
1.84, -3.06
2764/0/419
S(F2) ) 1.12
R(F) ) 0.041
wR(F2) ) 0.102
R(F) ) 0.045
wR(F2) ) 0.105
0.86, -0.62
8140/0/298
S(F2) ) 0.86
R(F) ) 0.031
wR(F2) ) 0.053
R(F) ) 0.086
wR(F2) ) 0.061
0.60, -0.66
FOM for I > 2σ(I)
FOM for all data
diff peak, hole (e Å-3
)
2
2
2
2
2
2
2
4
2
2
a Rint ) Σ|Fo - Fmean |/Σ|Fo |; Rσ ) Σ|σ(Fo )|/Σ|Fo |; R(F) ) Σ||Fo| - |Fc||/Σ|Fo|; wR(F2) ) [Σw(Fo - Fc )2/ΣwFo ]1/2; S(F2) ) [Σw(Fo - Fc )2/(n
2
2
2
- p)]1/2, with n number of reflections and p number of refined parameters; w ) 1/[σ2(Fo ) + (0.019P)2 + 1.88P], where P ) (Fo + 2Fc )/3.
seemed necessary. Being that the glide plane a unambiguously
confirmed by systematic absences, attempts were made to refine
the structure in the nonisomorphic subgroup Pa of Pna21. No
appreciable improvements were however obtained, so that Pna21
was definitely accepted. The disorder (46%) affecting one of its
two independent organic ligands was modeled by superimposing
two organic moieties almost related by a 180° rotation about their
longest inertial axis (ligands 2A and 2B in Figure 2). Coincidence
of chemically equivalent bond distances and angles was imposed.
All atoms were assigned identical, but refinable, isotropic thermal
parameters. No hydrogens were added.
The disorder affecting the tert-butylic groups of two organic
ligands in compound 3 (ligands 3 and 4 of Figure 4 with disorder
percentages of 18 and 25, respectively) was modeled by imposing
the contemporaneous presence of two tert-butyls reciprocally tilted
by 60°. Refinement was driven to completeness by assigning
identical but refinable isotropic thermal parameters and imposing
the coincidence of chemically equivalent bond distances. No
hydrogen atoms were added. Further refinement details and crystal
data are gathered in Table 1.
saturated aqueous solutions and the proper ligand or the
complex [CuI(pyridine)]4 by ligand exchange; compounds
7 and 8 were obtained by ligand exchange from [CuX-
(pyridine)]4 (X ) Cl, Br) (see Experimental Section).
For compounds 1-4, suitable single crystals for X-ray
diffraction were obtained. The structural motifs observed are
somehow influenced by the nature of the para-substituents
on the organic ligands.
In the trans-4-stilbazole adduct, [CuI(trans-4-stilbazole)]n
(1), arrangement of the asymmetric units defines the well-
known double-stranded “stair” motif (Figure 1). As already
3
reported for [CuI(4-acetylpyridine)]n and for known [CuI-
(L)]n stairs,13 the presence of a planar, pseudoaromatic
nitrogen-donor ligand bound to the metal implies a certain
distortion from an ideal stair, i.e. from orthogonal steps. The
stair translation axis is parallel to the monoclinic one: the
strands thus describe 21 degenerate helixes of pitch 4.15 Å;
i.e., they are intrinsically chiral. The latter asymmetry is yet
vanished by the overall centrosymmetric packing in the space
group P21/c. Any copper atom is tetracoordinated (to three
iodine atoms and the nitrogen of one ligand) in distorted
tetrahedral stereochemistry. The three Cu-I vectors and the
Cu-N one are comparable to those of known [CuI(L)]n stairs.
Table 2 collects Cu-I, Cu-N, Cu‚‚‚Cu, and I‚‚‚I distances
for both 1 and a pool of Cambridge Structural Database
(CSD) retrieved stairs.13 The Cu‚‚‚Cu#1 diagonal distance
(2.85 Å) is greater than the sum of the van der Waals radii
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion Nos. CCDC 261941, 261942, 261943, and 261944. Copies of
the data can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, U.K. (fax (+44)1223 336-
033; E-mail deposit@ccdc.cam.ac.uk).
Results and Discussion
Synthesis, X-ray Structure Determination, and Chemi-
cal Behavior. Compounds 1-6 were synthesized by straight-
forward methods, starting from either CuI dissolved in KI
(13) The reported values result from a statistical analysis on the [CuI(L)]n
stairs (L ) nitrogen-based pseudoaromatic ligand) retrieved from the
2003 edition of the Cambridge Structural Database.
4080 Inorganic Chemistry, Vol. 44, No. 11, 2005

Copper(I) Iodide Polymeric or Oligomeric Adducts
Table 2. Comparison between the Values of Cu-I, Cu-I#1, Cu-I#2, Cu-N, Cu‚‚‚Cu, and I‚‚‚I Distances in Compound [CuI(trans-4-stilbazole)]n, 1,
and the Average Values of the Same Distances in Known [CuI(L)]n “Stairs” Retrieved from the 2003 Version of the CSDS
compd
Cu-I (Å)
Cu-I#1 a (Å)
Cu-I#2 (Å)
Cu-N(1) (Å)
Cu‚‚‚Cu (Å)
I‚‚‚I (Å)
1
2.62
2.67
2.69
2.71
2.66
2.64
2.04
2.05
2.85
3.07
4.47
4.39
[CuI(L)]n stairs
a Consult Figure 3 for the labeling scheme.
Figure 1. Representation, with 50% probability Ortep, of the double-
stranded stair of compound [CuI(trans-4-stilbazole)]n, 1. Hydrogen atoms
have been omitted for clarity. The labeling scheme adopted throughout the
manuscript has been highlighted. The stair deviates from the ideality, i.e.
from perfectly orthogonal steps. The stairs translation axes are parallel to
b; i.e., the polymers evolve as degenerate 21 helices of pitch b (4.15 Å).
The organic moieties stack on both sides of the strand with an interplanar
distance of 4.15 Å (#1, -x, y + 1/2, -z + 1/2; #2, -x, y - 1/2, z + 1/2).
Figure 3. Representation, with 50% probability Ortep, of the structural
motif of compound [CuI(trans-4′-(dimethylamino)-4-stilbazole)]n, 2. The
absence of both hydrogen atoms and of the ligand 2A of the disordered
model is due to clarity purposes. The polymer translation axis is parallel to
a; the presence of Cu-I bonding interactions between consecutive dimers
along this same axis results in an unprecedented degenerate helicoidal motif.
The absence of stacking of consecutive ligands along both sides of the
polymeric skeleton can be appreciated.
Table 3. Comparison, on the Basis of the Metal Coordination Number,
between the Values of Geometrical Parameters Cu-I(1,2), Cu-N, and
I(1)-Cu-I(2) in Compound
[CuI(trans-4′-(dimethylamino)-4-stilbazole)]n, 2, and the Average Values
for the Same Parameters in Known [CuI(L)n]2 (n ) 1, 2) Dimers
Retrieved from the 2003 Version of the CSDS
Figure 2. Representation, with 50% probability Ortep, of the dimeric [CuI-
(trans-4′-(dimethylamino)-4-stilbazole)]2 asymmetric unit of compound
[CuI(trans-4′-(dimethylamino)-4-stilbazole)]n, 2. Hydrogen atoms have been
omitted for clarity. The labeling scheme adopted throughout the manuscript
has been highlighted. Each metal coordinates two bridging halides and the
nitrogen of a ligand. One of the independent ligands is affected by disorder
(ca. 46%), modeled by superimposing two moieties (2A and 2B) related
by a 180° rotation about their longest inertial axis.
Cu-I(1,2) (Å)
[CuI]2
Cu-N (Å) I(1)-Cu-I(2) (deg)
Cu coord no.
2
2
[CuI]2
2
[CuI]2
3
4
2.55, 2.58 2.55, 2.57 1.99 2.00
2.62, 2.75 2.63, 2.65 2.05 2.07
123.9
114.7
118.7
110.5
and I(2) are tri and dicoordinated, respectively (Figure 3).
Actually, both coordination number and stereochemistry
influence Cu(1) and Cu(2) geometrical parameters. On going
from the former to the latter, (i) Cu-I and Cu-N distances
decrease and (ii) I-Cu-I angles increase (Table 3). Cu-I
and Cu-N bond distances in 2 are comparable to the average
values of Cambridge Structural Database retrieved [CuI(L)x]2
dimeric species (x ) 1 or 2, Table 3).14 In 2, the
metal‚‚‚metal distance within a dimer (2.63 Å) is lower than
the sum of the van der Waals radii (2.80 Å). In known [CuI]2
dimers the latter distance undergoes a notable variety,
spanning the 2.53-3.45 Å range, with a mean value of 2.85
Å.14 As in most of the dimeric [CuI]2 moieties, even in the
present case the dimer is not planar: the angle between the
planes described by the I(1)-Cu(1)-I(2) and I(1)-Cu(2)-
I(2) triangles is about 14°. Both independent ligands ap-
(2.80 Å), as commonly (but not exclusively) happens in
known [CuI(L)]n stairs. The halide is tricoordinated in
distorted pyramidal geometry (diagonal I‚‚‚I#1 ) 4.47 Å vs
an average of 4.39 Å of known stairs). The organic moieties
approximately preserve their planarity and stack along both
sides of the CuI skeleton, the distance between their mean
planes being 4.15 Å.
Within the asymmetric unit of [CuI(trans-4′-(dimeth-
ylamino)-4-stilbazole)]n (2), each copper coordinates the
nitrogen donor atom of one organic ligand, while the iodine
atoms simply bridge two metal centers (Figure 2). The
copresence of a 21 screw axis parallel to a and of a Cu-I
bond between consecutive dimers creates a degenerate
helicoidal strand of pitch a ) 7.57 Å (Figure 3), a structural
motif previously unknown for adducts of CuI of this kind.
Along the polymeric unit, both copper and iodine experience
two different coordination numbers: Cu(1) is tetracoordi-
nated with a distorted tetrahedral geometry, while Cu(2) is
tricoordinated with a planar trigonal stereochemistry. I(1)
(14) The reported values result from a statistical analysis on the [CuI(L)n]2
dimers (L ) nitrogen-based pseudoaromatic ligand; n ) 1, 2) retrieved
from the 2003 edition of the Cambridge Structural Database.
Inorganic Chemistry, Vol. 44, No. 11, 2005 4081

Cariati et al.
Table 4. Comparison between the Average Values of Cu-I, Cu-N,
Cu‚‚‚Cu, and I‚‚‚I Distances in Compound [CuI(4-tert-butylpyridine)]4,
3, and in Known [CuI(L)]4 Tetramers Retrieved from the 2003 Version
of the Cambridge Structural Database
compd
Cu‚‚‚Cu (Å) I‚‚‚I (Å) Cu-I (Å) Cu-N (Å)
3
2.72
2.69
4.48
4.48
2.70
2.75
2.04
2.05
[CuI(L)]4 tetramers
Figure 4. Representation, with 30% probability Ortep, of the tetrameric
structural motif [CuI(4-tert-butylpyridine)]4, 3. Hydrogen atoms have been
omitted for clarity. The labeling scheme adopted throughout the manuscript
has been highlighted. A distorted tetrahedron of copper atoms is surrounded
by a distorted tetrahedron of halogens. Each copper is tetracoordinated to
three iodine atoms and the nitrogen of one organic ligand. The disorder
affecting the substituents on ligands 3 and 4 (ca. 18 and 25%, respectively)
has been modeled by imposing the contemporaneous presence of two tert-
butylic moieties each, reciprocally tilted by 60°.
proximately preserve their planarity. As pointed out in the
Experimental Section, one of them is affected by some
orientational disorder (46%). As already shown for trans-
stilbene and similar compounds,15 this feature could be
evidence of a local dynamic process comparable to a “pedal
motion”. The least-squares planes of the independent ligands
are reciprocally tilted with respect to the Cu-Cu vector, thus
forming an angle of ca. 66°. Interestingly, at variance with
the “classical” [CuI(L)]n chains and stairs, no stacking of
consecutive organic moieties is observed along the CuI
skeleton: actually, along both sides, two distinct piles of
ligands, having interplanar distance of 7.57 Å, are present
(Figure 3).
The significant change of structural motif on going from
1 to 2 could be originated by the different steric hindrance
of the ligands, which, in the latter compound, prevents their
stacking on both sides of the CuI skeleton. To further
strenghten this hypothesis, we synthesized the CuI adduct
with 4′-tert-butyl-4-stilbazole (5). Concomitantly, to inves-
tigate the role of the length of the π-delocalized “bridge”
on the structural motif, we prepared the CuI adduct with
trans-4-[4-(4-(dimethylamino)phenyl)buta-1,3-dienyl]pyri-
dine (6). Both derivatives were characterized by elemental
analysis and 1H NMR, but unfortunately we were unable to
obtain suitable single crystals for X-ray diffraction.
Figure 5. Representation, with 50% probability Ortep, of the hexameric
structural motif in [CuI(4-dimetilaminopyridine)]6, 4. Hydrogen atoms have
been omitted for clarity. The labeling scheme adopted throughout the
manuscript has been highlighted. The hexamer reminds the distorted portion
of a stair, the distortion implying absence of complete stacking of the ligands
on both sides of the CuI skeleton (angles between the average planes of
ligands 1/3 and 3/2#1 of 6 and 68°, respectively) (#1, -x, -y, -z).
of 2.75, 2.05, 2.69, and 4.48 Å, respectively; Table 4).16
Notably, the average Cu‚‚‚Cu distance of 3 is shorter than
the sum of the van der Waals radii (2.80 Å). As depicted by
the thermal ellipsoids of Figure 4, the aromatic rings undergo
a conspicuous thermal motion about the normal to their mean
planes. tert-Butyl moieties as well are characterized by a
high degree of motion. For ligands 3 and 4 (Figure 4) the
latter is so pronounced to require a disordered model,
according to which two tert-butyl groups, reciprocally tilted
by 60°, have been contemporarily refined (see Experimental
Section). Probably, the tert-butyl group in the para position
hinders the stabilization of the polymeric stair structure,
easily formed with the “naked” pyridine. However, by
emission studies, we had evidence of the presence, in the
reaction mother liquor, of a very minor amount of the
polymeric stair form of 3 (see Experimental Section).
By substitution of the tert-butyl group with a dimethyl-
amino group (4), characterized by significantly different
electronic properties and by a minor steric hindrance, the
structural motif is strongly affected. Structure solution of 4
revealed the first example of a hexameric [CuI(L)]6 oligomer
(L ) (dimethylamino)pyridine) (Figure 5), which could be
viewed as a “cut” portion of a stair. Among the consequences
of the cut, it is worth noting the loss of complete stacking
of the ligands: on both sides of the CuI skeleton, just ligands
1 and 3 (Figure 5) are approximately piled (acute angle
between their least-squares planes 6.3°); the average plane
When we shifted toward adducts of CuI with para-
substituted derivatives of pyridine, we found, with 4-tert-
butylpyridine, a structural motif unaltered (3) with respect
to [CuI(pyridine)]4: a distorted tetrahedron of copper atoms
is included in a larger I4 tetrahedron, whose vertexes cap
the triangular faces of the Cu4 polyhedron (Figure 4). The
fourth coordination site of each Cu is occupied by a 4-tert-
butylpyridine ligand. Average Cu-I, Cu-N, Cu‚‚‚Cu, and
I‚‚‚I distances are in good agreement with those of known
[CuI(L)]4 tetramers (2.70, 2.04, 2.72, and 4.48 Å vs averages
(16) The reported values result from a statistical analysis on the [CuI(L)]4
tetramers (L ) nitrogen-based pseudoaromatic ligand) retrieved from
the 2003 edition of the Cambridge Structural Database.
(15) Galli, S.; Mercandelli, P.; Sironi, A. J. Am. Chem. Soc. 1999, 121,
3767 and references therein.
4082 Inorganic Chemistry, Vol. 44, No. 11, 2005

Copper(I) Iodide Polymeric or Oligomeric Adducts
Table 5. Comparison between the Average Values of Cu-I, Cu-N,
Cu‚‚‚Cu, and I‚‚‚I Distances in Compound
[Cu3I3(4-dimethylaminopyridine)3]2, 4, and in Known [CuI(L)]n Stairs
Retrieved from the 2003 Version of the Cambridge Structural Database
compd
Cu-I (Å) Cu-N (Å) Cu‚‚‚Cu (Å) I‚‚‚I (Å)
4
2.65
2.67
2.01
2.05
2.84
3.07
4.46
4.35
[CuI(L)]n stairs
of ligand 2#1 is tilted by 68.0° with respect to that of 3. Mean
Cu‚‚‚Cu and I‚‚‚I diagonal interactions and mean Cu-I and
Cu-N bond distances have values of 2.84, 4.46, 2.65, and
2.01 Å, respectively. The quoted figures support the inter-
pretation of 4 as a distorted portion of a stair; actually, known
[CuI(L)]n stairs show average Cu‚‚‚Cu and I‚‚‚I diagonal
interactions of 3.07 and 4.35 Å and, particularly, Cu-I and
Cu-N bond distances of 2.67 and 2.05 Å, respectively (Table
5).13
These latter two structures, together with the CuI adduct
with para-cyanopyridine,17 clearly add further evidence that
even in the case of simple ligands such as pyridine, the para-
substituent on the organic ligand can influence the structural
motif by its steric (tert-butyl), electronic/steric (dimethyl-
amino), or bridging-donor (cyano) properties, with disruption
of the stair structural motif.
Probably due to their intrinsic instability toward oxidation,
we could not obtain suitable single crystals of 7 and 8 for
X-ray diffraction. However, X-ray powder diffraction showed
that they are not isomorphous to 2. Furthermore, the lack of
second harmonic generation under the Kurtz-Perry experi-
mental conditions (see below) prompts, differently from 2,
a centrosymmetric crystallographic arrangement. This seems
to suggest that even the nature of the bridging halides, not
just that of the organic moiety, may play a nonnegligible
role in the definition of the structural motif.
Figure 6. Solid-state emission spectra of compounds 3 and 4 (continuous
and dotted lines, respectively). In the case of 4 the spectrum has been
magnified (1.5×) for the sake of clarity.
Table 6. Positions of Solid-State Emission Maxima for Compounds
[CuI(trans-4-stilbazole)]n, 1,
[CuI(trans-4′-(dimethylamino)-4-stilbazole)]n, 2,
[CuI(4-tert-butylpyridine)]4, 3, [Cu3I3(4-(dimethylamino)pyridine)3]2, 4,
[CuI(pyridine)]n, [CuI(3-methylpyridine)]n, [CuI(4-methylpyridine)]n,
[CuI(4-acetylpyridine)2]n, and [CuI(4-acetylpyridine)]n
compd
λmaxem (nm)
ref
[CuI(trans-4-stilbazole)]n, 1
quenching
this work
[CuI(trans-4′-(dimethylamino)-4-stilbazole)]n, 2 quenching
this work
[CuI(4-tert-butylpyridine)]4, 3
[Cu3I3(4-(dimethylamino)pyridine)3]2, 4
[CuI(pyridine)]n
[CuI(3-methylpyridine)]n
[CuI(4-methylpyridine)]n
[CuI(4-acetylpyridine)2]n
[CuI(4-acetylpyridine)]n
452, 602
580
437
454
437
this work
this work
2
4
4
3
3
612
700
Photoluminescent Emission Studies and Solid-State
Second-Order NonLinear Optical Properties. Table 6
reports the emission maxima positions (λemmax) for com-
pounds 1-4. For comparison, λemmax’s of [CuI(pyridine)]n,
[CuI(3-methylpyridine)]n, [CuI(4-methylpyridine)]n, [CuI(4-
acetylpyridine)2]n, and [CuI(4-acetylpyridine)]n are reported
as well.
It is worth mentioning that, during the synthesis of 2, traces
of [CuI(cis-4′-(dimethylamino)-4-stilbazole)] (2′) formed, as
evidenced by 1H NMR spectroscopy. To check for a possible
influence of the cis:trans conformation of the ligand on the
structural motif, we tried to prepare pure 2′ from 2 irradiated
with a 125 W Hg/Xe lamp. In this regard, we observed that
although irradiation of 2 in CH3CN allows the formation of
good amounts of 2′, the latter is obtained in much better
yield (1H NMR evidence) by photoisomerization of trans-
4′-(dimethylamino)-4-stilbazole in CH2Cl2, followed by
exchange reaction with [CuI(pyridine)]4 in CH2Cl2. Unfor-
tunately, various attempts to prepare suitable single crystals
of 2′ invariably failed; however, the proof of its belonging
to a noncentrosymmetric space group came from a not nihil
SHG signal (see below). Interestingly, contrarily to 2, the
related compounds 1, 5, and 6, carrying a trans-stilbazolic
ligand as well, and the CuCl (7) and CuBr (8) adducts of
trans-4′-(dimethylamino)-4-stilbazole, much more easily
oxidized by air to Cu(II) species, did not present evidence
for a trans to cis isomerization of their ligands during their
preparation.
First we investigated the behavior of the CuI adducts 3
and 4, with the two para-substituted pyridines studied in this
work. The room-temperature solid-state photoluminescence
spectrum of 3 (see Figure 6) shows a broad, bright yellow,
structureless emission, centered at 602 nm.18 Solid-state
photoluminescent behavior of CuI tetramers have been
systematically analyzed.2 Their yellow emission (centered
around 580-625 nm) has been interpreted as a decay from
a triplet “cluster centered” (3CC*) excited state; the latter
involves both the I4 and Cu4 tetrahedra and has mixed halide-
to-metal charge transfer (3XMCT*) and “metal cluster
centered” [3MCC*, dCu f (s,p)Cu] characters. The emission
3
of 3 can be undoubtedly considered a decay from a CC*
3
excited state, the existence of a MCC* contribution being
supported by a Cu‚‚‚Cu nonbonding interaction of 2.72 Å.
Although the structural motif of 4 was neither reported nor
photophysically studied before, its emission, characterized
by a LE maximum at 580 nm (see Figure 6), can be
(18) It is worth mentioning that occasionally a very weak blue emission
centered at 452 nm is also observed. This weak high-energy HE
emission can be due to the presence of a minor amount of the
polymeric chain adduct, since the solid-state room-temperature emis-
sion of these polymers is reported to be centered at about 430-460
nm.2
(17) Graham, A. J.; Healy, P. C.; Kildea, J. D.; White, A. H. Aust. J. Chem.
1989, 42, 177.
Inorganic Chemistry, Vol. 44, No. 11, 2005 4083

Cariati et al.
tentatively attributed, as for the adducts with a chain or stair
actions present in the solid state,20 where the π systems stack
at a distance of 6.33 Å, below the critical value proposed by
Bredas.19 Noteworthy, in acetone solution, the emission
spectrum of 1 shows a maximum at 420 nm, not too different
from that of the free ligand. This could be due to lability in
solution of the stilbazolic ligands in this kind of Cu(I)
3
polymeric network,3 to a XLCT* excited state. Moreover,
the Cu‚‚‚Cu distance of 2.84 Å, higher than the sum of the
van der Waals radii (2.80 Å), allows discarding any
contribution from excited states involving metal-delocalized
orbitals. However, a 3MLCT* process cannot be excluded.3
Rather surprisingly, the room-temperature solid-state emis-
sion spectra of both 1 and 2 show no luminescence. The
structural motif of 2 was previously unknown in the literature,
so that no preexisting models are at hand to discuss its
emissive properties: the only available clue is its Cu‚‚‚Cu
diagonal distance (2.63 Å), shorter than the sum of the van
der Waals radii, which would anticipate the possible
participation of 3MCC* contributions to the emission process.
At variance, being 1 is a typical stair with a Cu‚‚‚Cu distance
(2.85 Å) higher than the sum of the van der Waals radii, a
1
complexes, as evidenced by the broad H NMR signals of
H2 and H6 (see Experimental Section).
In the case of 2, in the solid state, an emission quenching
process takes place even if the consecutive trans-stilbazolic
moieties stack at a distance of 7.57 Å, aboVe that for any
relevant interligand π interaction.19 On the contrary, trans-
4′-(dimethylamino)-4-stilbazole itself, whose moieties are
stacked at a distance similar to that in 2 (7.66 Å)21 shows,
in its solid-state emission spectrum, two peaks at 477 and
503 nm. Moreover, as for 1, any additional emission which
can be attributed to the CuI polymeric network is completely
lacking (on the basis of the rather short Cu‚‚‚Cu distance,
2.63 Å, a 3MCC* contribution to the emission involving the
Cu skeleton, could be expected2). The inadequate distance
for significant interligand π-π interactions to be at work,
supports, even for 2, the role, in the quenching process, of
orbitals belonging to the CuI skeleton. A nonemissive
3MLCT, e.g., cannot thus be discarded. As observed for 1,
also in the case of 2, an emission process is restored in
acetone solution, with a maximum centered at 468 nm and
a shoulder at 500 nm, analogous to the emission, in the same
solvent, of the free trans-stilbazolic ligand.
3
HE emission of the XLCT* type is expected.2 The stairs
for which a HE emission was reported ([CuI(pyridine)]n,
[CuI(3-methylpyridine)]n, [CuI(4-methylpyridine)]n, and [CuI-
(4-acetylpyridine)]n) are characterized by ligands of pyridinic
type. Significantly, even within this limited pool of ligands,
a nonnegligible effect of their electronic properties on the
emission maxima energy was already noticed.3 In light of
this, being the metal and the halide in 1 are identical with
those in the above pyridinic stairs, the presence, in 1, of
trans-stilbazole inevitably drives the attention on the role of
π-conjugation extension.
The more extended π-conjugation of trans-stilbazole may
be the key factor in making quenching of luminescence
effective. Intermolecular interactions of π-delocalized mol-
ecules, in the solid state, have been already invoked to
explain the process of luminescence quenching. Actually,
calculations carried out on cofacial stilbenic arrangements
have shown that, for intermolecular distances above 8 Å,
there is no exciton transfer between the two π electronic
systems, which behave as separate entities, as in solution.19
However, by a decrease of the intermolecular distance below
the critical value of 7 Å, the mixing of HOMO and LUMO
orbitals produces excited states characterized by a potentially
reduced efficiency of the emission process. Indeed, the stair
adduct of trans-stilbazole with CuI, 1, where the organic
moieties stack at a distance of only 4.15 Å, does not emit at
all. The quenching of the emission process could possibly
be due to an extensive HOMO and LUMO orbital mixing
of π systems closely interacting. Quite unexpectedly, there
is no evidence even for the emission typical of a CuI stair
network, attributed to a 3XLCT excited state.2 More probably,
then, the quenching of the emission involves a mixing of
the π systems of trans-stilbazoles and of orbitals delocalized
Similar is the behavior of 5, with total quenching of the
emission in the solid state and emission at 412 nm in acetone
solution. Since for 5 the structural motif is not available, we
cannot explain its solid-state quenching on a structural basis.
Due to the larger steric hindrance of the tert-butyl group
with respect to the dimethylamino one, we may argue that
the polymeric motif of the stair is unlikely. The above
evidence strongly suggests that, when a trans-stilbazolic
ligand is bound to a polymeric CuI skeleton, complete solid-
state quenching occurs of the expected emissions of both
the trans-stilbenic moiety and the CuI skeleton (the latter
involving either a XLCT* or a MCC* excited state). We
have evidence that the quenching process probably does not
take place only through space, between the adjacent π
delocalized systems of trans-stilbazoles, but involves a
mixing of the excited states of adjacent π-delocalized trans-
stilbazoles with excited states located mainly or partially on
the polymeric CuI skeleton.
As a final confirmation to this hypothesis, we have found
that in the case of compound 6, with an even more π
delocalized ligand and therefore π* excited states of lower
energy, the solid-state emission is not totally quenched. A
relatively weak emission is indeed observed at 580 nm, while
the free ligand shows, in the solid state, a strong emission
at 540 nm with a shoulder at 565 nm. The presence of a
weak, in 6, emission is in agreement with the reported
3
3
3
on the CuI skeleton: a nonemissive MLCT, for example,
could be at work. This hypothesis is confirmed by the solid-
state emission spectrum of trans-stilbazole itself, which
shows an intense emission in the blue spectral region plus a
broad and much weaker band in the 500-700 nm region.
The latter, which is absent in solution emission spectra of
stilbazole, is probably originated by intermolecular inter-
(20) Cariati, E.; Roberto, D.; Ugo, R.; Srdanov, V. I.; Galli, S.; Macchi,
P.; Sironi, A. New J. Chem. 2002, 26, 13.
(19) Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Bredas, J. L. J.
Am. Chem. Soc. 1998, 120, 1289.
(21) Lacroix, P. G.; Daran, J. C.; Nakatani, K. Chem. Mater. 1998, 10,
1109.
4084 Inorganic Chemistry, Vol. 44, No. 11, 2005

Copper(I) Iodide Polymeric or Oligomeric Adducts
decreased effect on the emission quenching by increasing
the length of π delocalized systems.19 As in the other cases,
in acetone solution, the emission spectrum of 6 shows an
emission band centered at about 550 nm quite similar to that
of the free ligand in the same solvent. Unfortunately, even
for 6 we have not been able to grow single crystals suitable
for X-ray structural characterization.
Testing the second harmonic generation (SHG) efficiencies
on unsieved powders of 1-8 via the Kurtz-Perry powder
method9 working with an incident wavelength of 1.064 nm,
resulted in lack of SHG not only for 1, 3, and 4 (as expected
because of their well-established centrosymmetric space
groups) but also for 5-8 thus suggesting at least a cen-
trosymmetric space group. Only 2 has shown a discrete SHG
with efficiency of 22.5 times that of quartz. A comparable
SHG efficiency has been observed also for a mixture of 2′and
2 with a ratio of 13:1.
As described by the “two-level model”,22 the molecular
second-order NLO response of organic and, to a lesser extent,
organometallic species, can be retraced to one prevailing
charge transfer (CT) process. In the solid state, this contribu-
tion is “tuned” by the reciprocal arrangement of the molecular
entities. This concept was successfully tackled by Zyss,23
who eventually proposed a set of geometrical relationships
linking the molecular quadratic hyperpolarizability along the
CT axis (âCT) to the crystalline nonlinearity/molecule (beff).
Thus, in the presence of such a performing chromophore
as trans-4′-(dimethylamino)-4-stilbazole, we tried to explain
the poor SHG efficience of species 2 by taking advantage
of the geometrical model developed by Zyss. The latter was
applied (i) adopting an “oriented gas” model, i.e. supposing
that the chromophores interact only weakly, (ii) assuming,
at first approximation, that SHG active moieties can be
described by a monodimensional system whose axis coin-
cides with the CT direction, and (iii) identifying the CT axis
with the longest inertial axis of the trans-4′-(dimethylamino)-
4-stilbazole chromophore, i.e. with the N‚‚‚N vector.
According to the crystallographic class of 2, mm2, the two
predicted by Zyss’ model, this allowing an explanation of
the modest SHG signal detected.
Conclusions
CuI adducts with a series of para-substituted trans-
stilbazoles and pyridines show a rich structural behavior. In
particular, the introduction of a dimethylamino group in the
para-position either of a trans-stilbazolic moiety or of a
pyridine ring produces two new structural motifs. The
polymeric helicoidal motif of compound [CuI(trans-4′-
(dimethylamino)-4-stilbazole)]n, totally new, arranges in a
noncentrosymmetric crystal structure and affords a discrete
SHG when irradiated at 1064 nm. Differently from pyri-
dines,2 to rationalize the solid-state photoemission behavior
of CuI adducts with para-substituted trans-stilbazoles, the
structural motif cannot be considered the only parameter to
be taken into consideration. In these latter adducts, any solid-
state photoemission originated either from the π system of
the trans-stilbazolic moiety or from the CuI skeleton is
unexpectedly quenched, both totally (as in the case of [CuI-
(trans-4-stilbazole)]n, [CuI(trans-4′-(dimethylamino)-4-stil-
bazole)]n, and [CuI(4′-tert-butyl-4-stilbazole)]n) or partially
(as in the case of [CuI(trans-4-[4-(4-(dimethylamino)phenyl)-
buta-1,3-dienyl]pyridine]). On the basis of the different
arrangement of the trans-stilbazolic moieties in the crystalline
structures of 1 and 2 (Figures 1 and 2), the quenching seems
not to be due only to a mixing of their π HOMO and LUMO
levels, but probably it involves mixing of the level of π*
excited states of the organic ligands and orbitals of the CuI
3
skeleton (3XLCT* or MCC*).
In conclusion, we have produced an unexpected evidence
for a surprising quenching of the photoemission process in
hybrid inorganic-organic materials such as adducts between
CuI and a series of trans-stilbazoles.
We have also evidenced a specific role of the dimethyl-
amino group, when located in para position to a pseudo-
aromatic (pyridine) or aromatic (the phenyl group of a trans-
stilbazolic moiety) ligand, in producing new and unusual
structural motifs of the CuI skeleton either polymeric or
oligomeric.
phase matchable components of beff have expression bZXX
)
sin2(φ) cos(θ) sin2(θ) âCT and bZYY ) cos2(φ) cos(θ) sin2(θ)
âCT, where θ and φ are the angles between the assumed CT
axis and c, and between the projection of the CT axis on ab
and a, respectively. The ratios bZXX/âCT and bZYY/âCT reach
their optimum value of 0.385 for θ ) 54.74° and φ ) 90°
or 0°, respectively. In 2, the actual values of θ and φ are
-14.89 and 75.12° or 15.15 and 74.86° for the two
independent ligands (L1 or L2), unfavorably directed toward
almost opposite directions with respect to c. An overall beff
) |(bZXX/âCT + bZYY/âCT)L1 - (bZXX/âCT + bZYY/âCT)L2| value
of |(0.060 + 0.004)L1 - (0.0061 + 0.004)L2| ) 0.001 results.
The SHG performance of 2 is thus about 4% of the optimum
Acknowledgment. This work was supported by the
Ministero dell’Istruzione, dell’Universita` e della Ricerca
(Programma di ricerca FISR 2001; research title “Nanotec-
nologie molecolari per l’immagazzinamento e la trasmissione
delle informazioni: Nanotecnologie per la comunicazione
ottica”). We deeply thank Dr. Francesca Tessore and Dr.
Maurizio Gallina for experimental help and Mr. Pasquale
Illiano for NMR measurements. We also acknowledge one
of the reviewers for his fruitful comments.
(22) (a) Oudar, J. L.; Chemla, D. S. J. Chem. Phys. 1977, 66, 446. (b)
Oudar, J. L. J. Chem. Phys. 1977, 67, 2664.
(23) (a) Zyss, J.; Oudar, J. L. Phys. ReV. A 1982, 26, 2028. (b) Zyss, J.;
Chemla, D. S. In Nonlinear Optical Properties of Organic Molecules
and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press Inc.:
Orlando, FL, 1987; Vol. 1, pp 23-187.
Supporting Information Available: X-ray crystallographic data
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
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