Polymorphic Forms of a Gold(I) Arylacetylide Complex with Contrasting Phosphorescent Characteristics

Article abstract of DOI:10.1021/ja0382415

The spectroscopic properties and crystal structures of the gold(I) arylacetylide complexes [(R₃P)-Au(C≡CAr)] (R = Cy, Ar = 4-nitrophenyl, 1; 4-trifluoromethylphenyl, 2; pentafluorophenyl, 3; R = Ph, Ar = 4-nitrophenyl, 4) have been examined. Th

Full text of DOI:10.1021/ja0382415

Published on Web 12/02/2003
Polymorphic Forms of a Gold(I) Arylacetylide Complex with
Contrasting Phosphorescent Characteristics
Wei Lu, Nianyong Zhu, and Chi-Ming Che*
Contribution from Department of Chemistry and HKU-CAS Joint Laboratory on New
Materials, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong SAR, China
Received August 31, 2003; E-mail: cmche@hku.hk
Abstract: The spectroscopic properties and crystal structures of the gold(I) arylacetylide complexes [(R3P)-
Au(CtCAr)] (R ) Cy, Ar ) 4-nitrophenyl, 1; 4-trifluoromethylphenyl, 2; pentafluorophenyl, 3; R ) Ph, Ar
) 4-nitrophenyl, 4) have been examined. The dipole-allowed and -forbidden transitions of 1 (4 in
parentheses) at λmax 340 (336) and ca. 485 (470) nm in CH2Cl2 solution at 298 K are assigned to the
singlet and triplet intraligand charge transfer (ILCT) transitions of the 4-nitrophenylacetylide moiety, whereas
2 (3 in parentheses) shows localized singlet and triplet acetylenic ππ* transitions at λmax 287 (276) and 426
nm, respectively. Two polymorphs of 1 with contrasting phosphorescent characteristics have been identified.
At 298 K, the emissive form of 1, as well as 2-4, are highly phosphorescent with peak maximum at 504,
425, 521, and 495 nm, respectively; the other polymorph of 1 is nonemissive at 298 K but emission is
detected at 77 K with peak maximum at 486 nm. Crystallographic studies reveal that the major differences
between the emissive and nonemissive forms of 1 are the orientations of the molecular dipoles and the
dihedral angles between neighboring 4-nitrophenyl moieties. Crystal 2 is isostructural to the nonemissive
form of 1, but does not display polymorphism. The molecular planes of two neighboring lumophores are
coplanar in the emissive form of 1, parallel in 4, and nearly perpendicular (78.6°) to each other in the
nonemissive form of 1. Both the nature of the excited state and the dihedral angle between adjacent [Au-
(CtCAr)] moieties determine the phosphorescent properties of these molecular crystals.
Introduction
the yellow form. The solid-state luminescent behavior of these
polymorphs are reported to be dependent on the nature and
The spontaneous aggregation of two-coordinate gold(I)
complexes in the solid state and in fluid and glassy solutions
has generated considerable interest in recent years.1 Numerous
studies have revealed that aurophilic interactions, which are
observed to be sensitive to the presence and nature of counte-
rions and solvent molecules in the crystal lattice, play a key
role in the structural and luminescent properties of gold(I) solids.
There are several reports in the literature on the polymorphism
(the occurrence of different crystalline forms with the same
chemical components) of luminescent gold(I) complexes with
or without close Au‚‚‚Au contacts (shorter than 3.6 Å). For the
two polymorphs of [(CyNC)2Au]PF6 (Cy ) cyclohexyl) isolated
by Balch and co-workers,2 one is colorless and emits at λmax
424 nm while the other is yellow and emits at λmax 480 nm.
The crystallographical findings revealed that the colorless form
contains linear chains of Au‚‚‚Au interactions (ca. 3.18 Å),
whereas shorter Au‚‚‚Au contacts (ca. 2.97 Å) are evident in
configuration of the infinite gold(I) chains arising from attractive
aurophilic interactions in the crystal lattices. Regarding the two
colorless polymorphic forms of crystalline [(Ph3As)AuCl],3
which are needle- and prism-shaped, both emit with peak
maxima at ca. 360 nm at ambient temperature, and a phenyl-
localized 3ππ* excited state has been assigned. Crystallographic
studies on these two crystal forms showed that they exist in the
same orthorhombic P212121 space group but differ slightly in
the intramolecular orientation of the phenyl groups and the
intermolecular crystal packing. Interestingly, appreciable Au‚‚‚Au
interactions were absent from these two polymorphs.
Excitonic coupling4 between nonconjugated organic chro-
mophores is normally encountered in the condensed phase,5 and
is cited to account for aggregation-induced fluorescence en-
hancement in organic lumophores.6 Such phenomenon, however,
has not been reported for phosphorescent metal-organic com-
pounds, which have been intensely studied as potential elec-
(1) (a) Schmidbaur, H., Ed.; Gold: Progress in Chemistry, Biochemistry and
Technology; John Wiley & Sons: Chichester, 1999. Schmidbaur, H. Chem.
Soc. ReV. 1995, 24, 391-400. (b) Pathaneni, S. S.; Desiraju, G. R. J. Chem.
Soc., Dalton Trans. 1993, 319-322. (c) Yam, V. W. W.; Li, C. K.; Chan,
C. L. Angew. Chem., Int. Ed. 1998, 37, 2857-2859. (d) Fu, W. F.; Chan,
K. C.; Miskowski, V. M.; Che, C. M. Angew. Chem., Int. Ed. 1999, 38,
2783-2785. (e) Vickery, J. C.; Olmstead, M. M.; Fung, E. Y.; Balch, A.
L. Angew. Chem., Int. Ed. Engl. 1997, 36, 1179-1181. (f) Lee, Y.;
McGarrah, J. E.; Lachicotte, R. J.; Eisenberg. R. J. Am. Chem. Soc. 2002,
124, 10 662-10 663.
(3) (a) Weissbart, B.; Larson, L. J.; Olmstead, M. M.; Nash, C. P.; Tinti, D.
S. Inorg. Chem. 1995, 34, 393-395. (b) Larson, L. J.; McClauley, E. M.;
Weissbart, B.; Tinti, D. S. J. Phys. Chem. 1995, 99, 7218-7226.
(4) (a) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721-722. (b) Kasha,
M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371-392.
(5) Recent examples: (a) Liang, K.; Farahat, M. S.; Perlstein, J.; Law, K. Y.;
Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 830-831. (b) Piet, J. J.;
Taylor, P. N.; Anderson, H. L.; Osuka, A.; Warman, J. M. J. Am. Chem.
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2003, 125, 1033-1040.
9
10.1021/ja0382415 CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 16081-16088
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A R T I C L E S
Lu et al.
Chart 1
trophosphorescent dopants in high-performance organic light-
emitting devices.7 Excitonic coupling may provide a radiative
pathway for interacting triplet emitters, thereby obviating the
diminished efficiencies typically observed in devices with high
emitter doping levels thought to arise from triplet-triplet
annihilation.7 Hence, the geometry and consequences of interac-
tions between phosphorescent emitters in molecular crystals
merit closer scrutiny, and the polymorphic forms of a metal-
based lumophore with contrasting phosphorescent characteristics
would be an ideal candidate for this purpose.
Gold(I) alkynyl compounds are of particular interest owing
to their stability, ease of preparation, novel solid-state structures,8
and interesting physical properties, such as nonlinear optical
response,9 liquid crystallinity,10 and photoluminescence.11,12
Very recently, several heterobimetallic complexes based on
Au-CtC ligation have been reported for application as
potential optoelectronic materials.13 Here, we report the intrigu-
ing solid-state emissive and structural properties of a phospho-
rescent polymorphic gold(I) complex, namely [(Cy3P)-
AuCtCC6H4-4-NO2] (Cy ) cyclohexyl) (1) (Chart 1), where
the heavy [(Cy3P)Au]+ moiety is employed to enhance spin-
orbit coupling and the electron-withdrawing nitro group is
Figure 1. Absorption spectra of complexes 1-4 in CH2Cl2 at 298 K.
introduced to increase the transition dipole moment of this
lumophore. To the best of our knowledge, this is the first report
of a polymorphic Au(I) complex without short Au‚‚‚Au contacts
in the crystal lattices that exhibits contrasting luminescent
properties. Furthermore, the present study offers a rare op-
portunity to study excitonic coupling in a phosphorescent metal-
organic compound. The derivatives 2-4 were studied for
comparison and to probe the effects of the substituents on the
arylacetylide and phosphine auxiliaries.
Results
Electronic Absorption Spectroscopy. The UV-vis absorp-
tion spectra of 1-4 in CH2Cl2 at 298 K are shown in Figure 1.
There is an intense, structureless absorption band at λmax 340
and 336 nm for 1 and 4, respectively, with ꢀ of ca. 2.5 × 104
dm3 mol-1 cm-1. This band shows positive solvatochromism,
thus the absorption maximum of 1 shifts from 336 to 340 nm
when the solvent changes from toluene to CH2Cl2. In addition
to this high-energy band, there is a weak absorption shoulder
at around 485 nm (ꢀ ≈ 5 dm3 mol-1 cm-1) for 1 and 470 nm
(ꢀ ≈ 3 dm3 mol-1 cm-1) for 4. We tentatively assign the 340
and 485 nm bands of 1 to the singlet and triplet intraligand
charge transfer (ILCT) transition, respectively. On the basis of
the absorption spectrum of 1, the transition dipole moment of
the lowest singlet excited state (P01) in CH2Cl2 solution at 298
K was calculated to be ca. 6.6 D according to eq 114
(6) (a) Zahn, S.; Swager, T. M. Angew. Chem., Int. Ed. 2002, 41, 4225-
4230. (b) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem.
Soc. 2002, 124, 14 410-14 415.
(7) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151-154.
(8) (a) Mu¨ller, T. E.; Mingos, D. M. P.; Williams, D. J. J. Chem. Soc., Chem.
Commun. 1994, 1787-1788. (b) Mingos, D. M. P.; Yau, J.; Menzer, S.;
Williams, D. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 1894-1895. (c)
Vicente, J.; Chicote, M.-T.; Abrisqueta, M.-D.; Guerrero, R.; Jones, P. G.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1203-1204. (d) McArdle, C. P.;
Irwin, M. J.; Jennings, M. C.; Puddephatt, R. J. Angew. Chem., Int. Ed.
1999, 38, 3376-3378. (e) McArdle, C. P.; Vittal, J. J.; Puddephatt, R. J.
Angew. Chem., Int. Ed. 2000, 39, 3819-3822.
(9) Donor-acceptor Au(I) acetylides have been intensely studied by Humphrey
and co-workers as NLO materials, for example: (a) Whittall, I. R.;
Humphrey, M. G.; Samoc, M.; Luther-Davies, B. Angew. Chem., Int. Ed.
Engl. 1997, 36, 370-372. (b) Whittall, I. R.; Humphrey, M. G.; Samoc,
M.; Luther-Davies, B.; Hockless, D. C. R. J. Organomet. Chem. 1997,
544, 189-196.
P012 ) (9.166 × 10-3) ꢀ(ν)dν/ν
(1)
∫
m
(10) Kaharu, T.; Ishii, R.; Adachi, T.; Yoshida, T.; Takahashi, S. J. Mater. Chem.
1995, 5, 687-692.
where ꢀ(ν) is the molar extinction coefficient at wavenumber
ν, and νm is the position of absorption maximum. For 2 and 3,
there is a structured absorption band with λ0-0 at 287 and 276
(with 285 nm shoulder) nm, and ꢀ of ca. 2.3 × 104 and 3.0 ×
104 dm3 mol-1 cm-1, respectively. This band shows negative
solvatochromism; thus the absorption maximum shifts from 289
to 287 and 283 nm for 2 and from 277 to 276 and 289 nm for
3, when the solvent changes from toluene to CH2Cl2 and EtOH.
The salient vibrational spacings of these bands are 1830 (for 2)
and 1930 (for 3) cm-1, which fall in the range expected for
CtC stretching in the excited state, and hence these bands are
(11) (a) Jia, G.; Puddephatt, R. J.; Scott, J. D.; Vittal, J. J. Organometallics
1993, 12, 3565-3574. (b) Yam, V. W. W.; Choi, S. W. K. J. Chem. Soc.,
Dalton Trans. 1996, 4227-4232. Irwin, M. J.; Vittal, J. J.; Puddephatt, R.
J. Organometallics 1997, 16, 3541-3547. (c) Vicente, J.; Chicote, M. T.;
Abrisqueta, M. D.; Jones, P. G. Organometallics 1997, 16, 5628-5636.
(d) Wang, H. M. J.; Chen, C. Y. L.; Lin, I. J. B. Organometallics 1999,
18, 1216-1223.
(12) (a) Li, D.; Che, C. M.; Lo, W. C.; Peng, S. M. J. Chem. Soc., Dalton
Trans. 1993, 2929-2932. (b) Xiao, H.; Cheung, K. K.; Guo, C. X.; Che,
C. M. J. Chem. Soc., Dalton Trans. 1994, 1867-1871. (c) Xiao, H.; Weng,
Y. X.; Peng, S. M.; Che, C. M. J. Chem. Soc., Dalton Trans. 1996, 3155-
3157. (d) Che, C. M.; Chao, H. Y.; Miskowski, V. M.; Li, Y.; Cheung, K.
K. J. Am. Chem. Soc. 2001, 123, 4985-4991. (e) Lu, W.; Xiang, H. F.;
Zhu, N.; Che, C. M. Organometallics 2002, 21, 2343-2346. (f) Chao, H.
Y.; Lu, W.; Li, Y.; Chan, M. C. W.; Che, C. M.; Cheung, K. K.; Zhu, N.
J. Am. Chem. Soc. 2002, 124, 14 696-14 706. (g) Lu, W.; Zhu, N.; Che,
C. M. J. Organomet. Chem. 2003, 670, 11-16.
(14) (a) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/
Cummings Publishing Co., Inc.: Menlo Park, CA, 1978; Chapter 5. (b)
Levitsky, I. A.; Kishikawa, K.; Eichhorn, S. H.; Swager, T. M. J. Am.
Chem. Soc. 2000, 122, 2474-2479.
(13) (a) Back, S.; Gossage, R. A.; Lang, H.; van Koten, G. Eur. J. Inorg. Chem.
2000, 1457-1464. (b) Shiotsuka, M.; Yamamoto, Y.; Okuno, S.; Kitou,
M.; Nozaki, K.; Onaka, S. Chem. Commun. 2002, 590-591.
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Polymorphic Forms of a Gold(I) Arylacetylide Complex
A R T I C L E S
Figure 2. Solid-state emission spectra (λex ) 330 nm) of the E- and
N-forms of 1 at 298 K.
assigned as predominantly 1(ππ*) acetylenic transitions. In
addition to this high-energy absorption band, 2 exhibits a weak
absorption beyond 350 nm with λ0-0 at 426 nm (ꢀ ≈ 2 dm3
mol-1 cm-1), which is assigned to the localized 3(ππ*)
acetylenic transition.
Emission and Excitation Spectroscopy. Complex 1 is
weakly emissive in fluid solutions at 298 K and the emission
energy is dependent upon the solvent polarity; the emission
maximum shifts from 492 to 524 and 545 nm from toluene to
CH2Cl2 and EtOH, respectively. The positive solvatochromism
(i.e., transition energy decreases with greater solvent polarity)
indicates that the ground state is less polar than the excited state.
This complex is strongly emissive in 77 K glassy toluene
solution with λ0-0 at 491 nm. Two crystal forms of 1 were
obtained upon slow diffusion of Et2O into a CH2Cl2 solution.
Both are yellow in color but one is rodlike and the other is
platelike in shape. Under UV light illumination, the rodlike
crystals (denoted as E-form) are strongly luminescent but the
platelike ones (denoted as N-form) are virtually nonemissive.
The E/N-form weight ratio after recrystallization is estimated
to be 1/9. The contrasting emission properties between these
two crystalline forms facilitated their separation by hand under
UV illumination. The solid-state emission spectra of the E- and
N-forms of 1 at 298 K are shown in Figure 2. The E-form emits
intense greenish light with peak maxima at 504 and 538 nm
(shoulder at ∼560 nm); a vibronic progression of 1250 cm-1 is
apparent, while the long lifetime (23 µs) suggests phosphores-
cence. We suggest that the emission originates from the triplet
intraligand excited state (the absorption maximum of which
occurs at around 485 nm in CH2Cl2 at 298 K). In contrast, the
emission from N-form is exceedingly weak, with peak maxima
at ca. 480 and 520 nm.
Figure 3. Solid-state emission and excitation spectra of the E- (top, λex
330 nm, λem ) 547 nm) and N- (bottom, λex ) 350 nm, λem ) 526 nm)
forms of 1 at 77 K.
)
(Figure 3, bottom). This emission is also long-lived (193 µs)
and can be similarly assigned to a triplet intraligand excited
state like for the E-form. Furthermore, the excitation spectrum
of the N-form exhibits a λ0-0 line at 482 nm, plus fine structures
in the 438-473 nm range that are comparable in pattern to the
excitation spectrum of the E-form. The blue-shifts (ca. 730
cm-1) of peak maxima from the E- to the N-form, both for the
emission and excitation spectra, together with the stark contrast
between the luminescent properties of these two crystalline
forms of 1 at 298 K, are unusual.
The emission and excitation spectra of complexes 2-4 are
given in the Supporting Information. Complex 2 is highly
emissive in fluid and alcoholic (methanol/ethanol ) 1/4, v/v)
glassy solutions (λ0-0 426 nm) and in the solid state (λ0-0 425
nm at 298 K). The emission maximum of 2 in toluene, CH2Cl2
and EtOH are virtually identical with λmax at 426 nm, and a
localized 3ππ excited state is assigned. Complex 3 is nonemis-
sive in fluid solutions but a crystalline sample emits at λmax
521 nm at 298 K and 490 nm at 77 K. In 77 K alcoholic glassy
solution, 3 emits at λ0-0 485 nm. The solvent-dependent
emissive behavior of 4 in fluid solutions is similar to that of 1;
the emission maximum shifts from 486 to 514 and 534 nm from
At 77 K, the E-form exhibits long-lived (335 µs) and highly
structured emission, and two vibronic progressions, namely 1250
(major) and 2110 cm-1, are apparent (Figure 3, top). Although
the peak maximum appears at 512 nm, a distinct shoulder at
504 nm is observed, which matches the emission maximum
found at 298 K (Figure 2). The excitation spectrum of the
E-form gives a well-defined λ0-0 line at 499 nm together with
vibronically structured bands in the 453-489 nm range. The
N-form emits intensely at 77 K and shows a less-resolved
emission spectrum with the λ0-0 line appearing at 486 nm
9
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A R T I C L E S
Lu et al.
Table 1. Crystal Data
1 (E-form)
1 (N-form)
2
3
4
formula
fw
C26H37AuNO2P
623.50
C26H37AuNO2P
623.50
C27H37AuF3P
646.50
C26H33AuF5P
668.46
C26H19AuNO2P
605.36
T, K
301(2)
301(2)
253(2)
253(2)
253(2)
color
yellow
yellow
colorless
colorless
0.2 × 0.2 × 0.1
monoclinic
P21
11.573(2)
8.533(2)
yellow
crystal size
crystal system
space group
a, Å
b, Å
c, Å
0.5 × 0.3 × 0.2
triclinic
P-1
0.3 × 0.2 × 0.1
monoclinic
P21/c
12.258(3)
16.590(3)
12.550(3)
0.4 × 0.3 × 0.2
monoclinic
P21/c
12.316(3)
16.415(3)
12.889(3)
0.3 × 0.15 × 0.07
monoclinic
P21/c
8.611(2)
18.466(4)
14.184(3)
9.381(2)
10.029(2)
14.374(3)
108.53(3)
93.02(3)
98.31(3)
1261.7(4)
2
14.077(3)
R, deg
â, deg
96.68(3)
98.54(3)
106.87(3)
91.28(3)
γ, deg
V, Å3
2534.8(10)
4
1.634
5.889
2576.8(10)
4
1.666
5.805
1330.3(5)
2
1.669
5.636
2254.8(9)
4
1.783
6.618
Z
Dc, g cm-3
µ, mm-1
F(000)
1.641
5.915
620
1240
1280
656
1168
2θmax, deg
no. reflections
no. independent
reflections
no. variables
GOF on F2
51.3
5915
51.3
15265
4479 [R(int) ) 0.043]
51.2
14096
4800 [R(int) ) 0.048]
50.7
8566
51.2
10341
3980 [R(int) ) 0.044]
3724 [R(int) ) 0.030]
4561 [R(int) ) 0.045]
280
0.93
0.029
0.066
280
1.09
0.027
0.079
289
1.11
0.031
0.093
298
1.03
0.038
0.102
280
0.99
0.028
0.077
a
R1
wR2
b
residual F, e Å-3
+0.488, -1.161
+0.548, -1.540
+0.659, -1.687
+1.555, -1.776
+0.626, -1.058
2
2
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
toluene to CH2Cl2 and EtOH, respectively. A crystalline sample
of 4 emits with λ0-0 at 495 nm at 298 K and 493 nm at 77 K.
The excitation spectrum of this sample exhibits a λ0-0 line at
489 nm plus fine structures in the 437-480 nm range, and is
similar in pattern to the excitation spectra of the E- and N-forms
of 1. Hence, polymorphism and opposing solid-state emissions
such as that observed for 1 were not evident for complexes 2-4.
Crystal Structures and Intermolecular Contacts. All
crystals were obtained from CH2Cl2/Et2O solutions. The crystal
data are shown in Table 1. The bond distances and angles for
these crystal structures are normal for gold(I) acetylide com-
plexes,15 which have been extensively documented in the
literature, so we focus here on their crystal packing arrangements
and short intermolecular contacts. The sum of van der Waals
radii taken from Bondi’s data16 is 2.90 Å for H‚‚‚C and 2.67 Å
for H‚‚‚F, hence intermolecular distances that are shorter than
these values may be considered as weak interactions.
The crystal structures of the E- and N-forms of 1 are shown
in Figures 4 and 5, respectively. No solvent molecules are
present in their crystal lattices, hence the E- and N-forms of 1
are exact polymorphs. The E-form crystallizes in the triclinic
P-1 space group. The nitro group is coplanar with the phenyl-
acetylide moiety. Neighboring molecules are arranged in pairs
with molecular dipoles in opposite directions. There are weak
C-H‚‚‚π(CtC) contacts between H8 of the phenyl ring and
C1 of the acetylide moiety (H‚‚‚C(CtC) 2.791 Å; C-H‚‚‚
C(CtC) 158°). The two phenyl rings are coplanar and these
discrete dimers are packed into parallel sheets in the crystal
lattice (Figure 4, bottom). In contrast, the N-form crystallizes
in the monoclinic P21/c space group and shows an infinite
slanted chainlike structure along the c-axis. Adjacent molecules
Figure 4. Crystal structure and packing diagram of the E-form of 1 with
dashed lines indicating intermolecular contacts that are shorter than the sum
of van der Waals radii.
are joined by C-H‚‚‚π(CtC) contacts between H7 of the
phenyl ring and C1 of the acetylide group (H‚‚‚C(CtC) 2.810
Å; C-H‚‚‚C(CtC) 143°). The dihedral angle between adjacent
phenyl rings is 79°, and the molecular dipoles are oriented in
the same direction. Thus, the major structural differences
between the E- and N-form are the orientations of the molecular
dipoles and the dihedral angles between neighboring 4-nitro-
phenyl moieties.
(15) Transition metal acetylides: Manna, J.; John, K. D.; Hopkins, M. D. AdV.
Organomet. Chem. 1995, 38, 79-154, and references therein.
(16) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
9
16084 J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003

Polymorphic Forms of a Gold(I) Arylacetylide Complex
A R T I C L E S
Figure 5. Crystal structures of the N-form of 1 (Cy groups are omitted for clarity) with dashed lines indicating intermolecular contacts that are shorter than
the sum of van der Waals radii.
Figure 6. Crystal structures of 2 (Cy groups are omitted for clarity) with dashed lines indicating intermolecular contacts that are shorter than the sum of
van der Waals radii.
Figure 8. Unit cell of crystal 3 (only selected C-H‚‚‚F-C contacts are
shown for clarity).
of the asymmetric units of crystals 2 and 1 (N-form)]. Values
of Π close to zero and Ii(n) approaching 100 implies isostruc-
turality. The calculated value of Π and Ii(21), for crystals 2
and 1 (N-form) is 0.004 and 97, respectively, suggesting
excellent isostructurality. Furthermore, the molecular arrange-
ment in the crystal lattice of 2 are identical to that of 1 (N-
Figure 7. Unit cell of crystal 4 with dashed lines indicating the distances
shorter than the sum of van der Waals radii.
form). Thus neighboring molecules are joined by C-H‚‚‚π(Ct
C) contacts between H5 of the phenyl ring and C1 of the
acetylide group (H‚‚‚C(CtC) 2.801 Å; C-H‚‚‚C(CtC) 147°),
and the dihedral angle between adjacent phenyl rings is 89°.
Crystal 2 (Figure 6) exists in the monoclinic P21/c space
group and is isostructural to the N-form of 1. The extent of
isostructurality for these two crystals was calculated by the unit
cell similarity index,17 Π ) |1 - (a + b + c)/(a′ + b′ + c′)|
[where a, b, c, a′, b′ and c′ are the orthogonalized lattice
parameters of crystals 2 and 1 (N-form)], and the isostructurality
index,17 Ii(n) ) |1 - [Σ(∆Ri)2/n]1/2| × 100 [where n is the
number of distance differences (∆Ri) between the crystal
coordinates of identical non-H atoms within the same section
The crystal structure of complex 4 was previously reported
by Humphrey and co-workers18 but the crystal packing was not
mentioned, so in this work, this structure was re-determined.
Crystal 4 (Figure 7) exists in the monoclinic P21/c space group
but the packing is different from those of 1 (N-form) and 2. In
(18) Whittall, I. R.; Humphrey, M. G.; Houbrechts, S.; Persoons, A.; Hockless,
(17) Ka´lma´n, A.; Pa´rka´nyi, L.; Argay; G. Acta Cryst. 1993, B49, 1039-1049.
D. C. R. Organometallics 1996, 15, 5738-5745.
9
J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003 16085

A R T I C L E S
Lu et al.
Figure 9. Schematic representations of geometry of adjacent lumophores in crystals 1 (E- and N-forms) and 4. Bold squares represent the P-Au-Ct
CC6H4-4-R (R ) NO2 or CF3) moieties with the blue side indicating phosphine and the red side indicating R; plain squares represent the molecular planes
of the arylacetylide ligands and dashed lines indicate C-H‚‚‚π(CtC) contacts.
the crystal lattice of 4, one molecule is connected to two
neighboring molecules by C-H‚‚‚π(CtC) contacts between
H11 and H14 of the Ph3P phenyl rings and C1 and C2 of the
acetylide group (H‚‚‚C(CtC) 2.74-2.82 Å; C-H‚‚‚C(CtC)
139-140°). Adjacent 4-nitrophenyl moieties are nearly parallel,
as indicated by the dihedral angle of 8°.
Intermolecular C-H‚‚‚π(CtC) contacts are not observed in
the crystal lattice of 3 (Figure 8), which contains no protons on
the perfluorinated phenylacetylide ligand for interaction with
the CtC moiety. Instead, there are extensive C-H‚‚‚F-C
contacts19 between protons of the Cy3P cyclohexyl groups and
F atoms of the C6H5 moiety (H‚‚‚F 2.56-2.62 Å; C-H‚‚‚F
135-147°). The dihedral angle between neighboring pentafluo-
rophenyl planes is 53°.
conjugation is also signified by the crystal structures of both
the E- and N-forms of 1, where the nitro group is exactly
coplanar with the phenylacetylide moiety. On the basis of this
evidence, the magnitude of the transition dipole moment in 1
should be significantly larger than that in 2.
Extrinsic Crystal-Packing Effects. The relationships be-
tween two neighboring molecules in crystals 1 (E- and N-forms)
and 4 are schematically depicted in Figure 9. The [(Cy3P)AuCt
CAr] molecules adopt a linear configuration and these rodlike
entities in crystals 1 (E- and N-forms) and 4 are virtually parallel
to each other; the distance between the long-axial of the closest
neighboring molecules is ca. 5.4, 5.2, and 4.9 Å for 1 (E-form),
1 (N-form) and 4, respectively. Assuming that the nearest
interacting lumophore acts as an energy trap for the molecular
crystal upon UV excitation, the geometry of adjacent lumo-
phores in the crystal lattice deserve close scrutiny. A correlation
between the solid-state emission properties and crystal packing
is tentatively proposed below.
Discussion
Intrinsic Electronic Effects. [(Cy3P)Au]+ is isolobal to
H+,1a,12d and the former has recently been shown to induce
phosphorescence in π-conjugated carbon-rich compounds through
Au-CtC ligation.12d-g In the discrete monomeric state of 1,
the excited 4-nitrophenylacetylide chromophore is expected to
cross from the singlet to triplet state through efficient spin-
orbit coupling that is enhanced by the [(Cy3P)Au]+ moiety.
Indeed, the lowest S0 f T1 transition of 1 has been observed at
ca. 485 nm with an ꢀ value of ca. 5 dm-3 mol-1 cm-1 in CH2-
Cl2 solution; this is substantially larger than those reported for
spin-forbidden triplet absorptions of pure organic chromophores.
Long-lived phosphorescence has been observed from this excited
state. We adopt the λ0-0 of 491 nm recorded in toluene glass at
77 K as the S0-T1 energy gap for monomeric 1.
The molecular dipoles of the closest neighboring lumophores
are arranged in a head-to-tail fashion in 1 (E-form) but in a
slipped manner in 1 (N-form) and 4. On the other hand, the
molecular planes of the two lumophores are coplanar in 1 (E-
form) and nearly parallel in 4, but almost perpendicularly tilted
(end-on) in 1 (N-form). In view of the fact that 1 (E-form) and
4 are highly emissive, whereas 1 (N-form) is virtually non-
emissive at 298 K, we infer that the angle between the two
molecular planes of the interacting lumophores (θ in Figure 9)
is linked to the emissive properties of these solids; i.e., when θ
deviates significantly from 0 or 180°, the emission becomes
diminished.
Comparison of the emission properties between 1 and 2, both
of which contain an electron-withdrawing group at the 4-position
of the phenylacetylide ligand, provide information regarding the
effect of electronic factors upon the contrasting solid-state
emissions exhibited by the polymorphs of 1. The crystal packing
and intermolecular interactions of the N-form of 1 are identical
to those in crystal 2, but their solid-state luminescent properties
are distinctly different. The likelihood that lowest excited states
in 1 (ILCT) and 2 (acetylenic ππ*) are different is indicated
by the following: (1) the low-energy dipole-allowed transition
band in the absorption spectrum is structureless for 1 but highly
structured with acetylenic vibronic progression for 2; and (2)
the emission energy for 1 in fluid solution is solvent-sensitive
but this is not the case for 2. The extension of π-conjugation
from the phenylacetylide moiety to the NO2 unit in 1 but not
the CF3 group in 2 may account for this disparity. This
Interestingly, the above-mentioned angle θ in these crystals
is related to the C-H‚‚‚π(CtC) contacts between neighboring
molecules. As originally reported by Mingo and co-workers,8a
this type of weak interaction is ubiquitous in gold(I) acetylide
complexes and is constructed from an acidic proton and the
electron-rich Au-CtC moiety. There are two identical
C-H‚‚‚π(CtC) contacts between adjacent lumophores in 1 (E-
form), whereas only one such interaction is observed in 1 (N-
form). When the substituents of the phosphine ligand is changed
from Cy3P to Ph3P, or the arylacetylide from 4-nitro- to
pentafluorophenylacetylide, such C-H‚‚‚π(CtC) contacts are
absent. Thus, the crystal packing arrangements derived from
these weak C-H‚‚‚π(CtC) contacts are crucial for the different
emission properties of the two polymorphs of 1.
Exciton-Coupling Interactions? In the crystal lattice of 1,
the arylacetylide lumophores are packed into dimers or infinite
chains through weak C-H‚‚‚π(CtC) contacts, hence excitonic
interactions become possible. It is apparent that both electronic
(19) Thalladi, V. R.; Weiss, H. C.; Bla¨ser, D.; Boese, R.; Nangia, A.; Desiraju,
G. R. J. Am. Chem. Soc. 1998, 120, 8702-8710.
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Polymorphic Forms of a Gold(I) Arylacetylide Complex
A R T I C L E S
and crystal packing factors cooperatively affect the excitonic
interactions of arylacetylide units in the solid state. Because
the triplet emissive excited state of 1 is intraligand charge-
transfer in nature, we can assume that the polarization of the
singlet transition dipole moment occurs in the molecular plane
while the polarization of the triplet one is perpendicular to the
molecular plane.14a Thus, the dihedral angle between the triplet
transition dipole moment vectors of the two 4-nitrophenylacetyl-
ide moieties correlates to the angle θ in Figure 9. As described
in previous studies on excitonic coupling-induced split-type
Cotton effects in circular dichroic spectra,20 the amplitude of
excitonic coupling between two neighboring chromophores is
dependent on the dihedral angle between the electronic transition
dipole moment vectors of the two chromophores; if the angle
between the two coupled transition dipoles is approximately 70°,
exciton splitting is maximized; on the other hand, if the two
transition dipoles are parallel (θ ) 0 or 180°), no exciton
splitting is observed. Bearing this in mind, it is notable that
there could be significant excitonic interaction(s) between
neighboring 4-nitrophenylacetylide moieties in the N-form of
1 (θ ) 79°) but negligible excitonic coupling in the E-form of
1 (θ ≈ 180°) and in 4 (θ ≈ 0°). We suggest that the excitonic
coupling in the N-form of 1 results in the transfer of excitation
energy to nonradiative energy traps that are presumably located
at the lattice flaws. These processes are apparently suppressed
at low temperatures, as indicated by the substantial enhancement
of the emission intensity of 1(N-form) at 77 K. In view of the
efficient spin-orbit coupling conferred by the [(Cy3P)Au]+
moiety (as evidenced by the ꢀ value for the S0 f T1 absorption
of 1 in CH2Cl2 solution), the triplet excitonic splitting energy
for the 0-0 transition in the present study could be substantially
larger than that for the anthracene (17 cm-1) and 1,4-dibro-
monaphthalene (30 cm-1) systems.21 At this stage, it is
nevertheless difficult to assign a tangible meaning to the value
of 730 cm-1 by which the emission energy of the N-form of 1
is blue-shifted from the E-form at 77 K.
crucial roles in possible excitonic coupling interactions in this
system. These findings not only enrich the diverse luminescent
nature of gold(I) complexes, but also provide valuable informa-
tion for crystal engineering studies of phosphorescent metal-
organic compounds.
Experimental Section
General Procedures and Materials. All starting materials were
purchased from commercial sources and used as received unless stated
otherwise. The solvents used for synthesis were of analytical grade.
Details of solvent treatment for photophysical studies have been
described earlier.23 [(Cy3P)AuCl],12d 4-nitrophenylacetylene,24a 4-trifluoro-
methylphenylacetylene,24a and pentafluorophenylacetylene24b were pre-
1
pared according to literature methods. H and 13C NMR spectra were
recorded on a Bruker Avance 400 or 300 DRX FT-NMR spectrometer
(referenced to residual solvent) at 298 K. 19F and 31P NMR spectra
were recorded on a Bruker Avance 400 at 298 K. Mass spectra (FAB)
were obtained on a Finnigan MAT 95 mass spectrometer. Elemental
analyses were performed by Beijing Institute of Chemistry, Chinese
Academy of Sciences. UV-vis spectra were recorded on a Perkin-
Elmer Lambda 19 UV/vis spectrophotometer. Emission spectra were
obtained on a SPEX Fluorolog-2 Model F11 fluorescence spectropho-
tometer. Emission lifetime measurements were performed with a Quanta
Ray DCR-3 pulsed Nd:YAG laser system (pulse output 355 nm, 8 ns).
Synthesis. Complexes 1-4 were prepared by reacting [(Cy3P)AuCl]
or [(Ph3P)AuCl] with 4-R-C6H4CtCH (R ) NO2, CF3) or C6F5Ct
CH in the presence of NaOMe in CH2Cl2/MeOH (1/1, v/v) and purified
with flash chromatography (neutral alumina, CH2Cl2 as eluent).
[(Cy3P)AuCtCC6H4-4-NO2] (1): Anal. Calcd. for C26H37-
NO2PAu: C, 50.16; H, 5.83; Found: C, 50.11; H, 6.10%. FAB MS:
m/z 624 [M+]. 1H NMR (CDCl3): δ ) 8.10 (d, 2H, 3J ) 8.7 Hz), 7.58
(d, 2H, 3J ) 8.7 Hz), 2.10-1.18 (m, 33H, Cy). 31P{1H} NMR
(CDCl3): δ ) 56.4.
[(Cy3P)AuCtCC6H4-4-CF3] (2): Anal. Calcd. for C27H37F3PAu:
C, 50.16; H, 5.77; Found: C, 50.07; H, 6.09%. FAB MS: m/z 647
[M+]. 1H NMR (CDCl3): δ ) 7.58 (d, 2H, 3J ) 8.1 Hz), 7.48 (d, 2H,
3J ) 8.3 Hz), 2.08-1.24 (m, 33H, Cy). 19F{1H} NMR (CDCl3): δ )
-62.5. 31P{1H} NMR (CDCl3): δ ) 56.4.
[(Cy3P)AuCtCC6F5] (3): Anal. Calcd. for C26H33F5PAu: C, 46.72;
1
H, 4.98; Found: C, 47.01; H, 4.75%. FAB MS: m/z 669 [M+]. H
Concluding Remarks
NMR (CDCl3): δ ) 2.05-1.25 (m, 33H, Cy). 19F{1H} NMR
(CDCl3): δ ) -141.7 (d, 2F, J ) 22.4 Hz), -162.1 (t, 1F, J ) 22.4
Hz), -166.3 (t, 2F, J ) 22.4 Hz). 31P{1H} NMR (CDCl3): δ ) 56.5.
[(Ph3P)AuCtCC6H4-4-NO2] (4): Anal. Calcd. for C26H19NO2-
PAu: C, 51.58; H, 3.16; Found: C, 51.75; H, 3.49%. FAB MS: m/z
It is apparent that polymorphism can occur in two-coordinate
gold(I) complexes supported by phosphine auxiliaries, and
aurophilic interactions are not a prerequisite for this phenom-
enon.22 In the present study, we have been able to prepare and
isolate two distinct polymorphs of the neutral gold(I) 4-nitro-
phenylacetylide complex with the ancillary Cy3P ligand, which
exhibit highly contrasting phosphorescent characteristics. By
comparing the X-ray crystal structures and emission properties
of these polymorphic solids and the related complexes 2-4, it
is evident that both electronic and crystal packing factors play
1
3
606 [M+]. H NMR (CDCl3): δ ) 8.13 (d, 2H, J ) 8.4 Hz), 7.60-
7.49 (m, 17H). 31P{1H} NMR (CDCl3): δ ) 56.4.
X-ray Crystallography. All single crystals were obtained by slow
diffusion of Et2O vapor into a CH2Cl2 solution. Data were collected
on a MAR diffractometer with a 300 mm image plate detector using
monochromatized Mo KR radiation (λ ) 0.71071 Å). Data collection
was made with 3° oscillation step of æ, 300 s exposure time and scanner
distance at 120 mm. The images were interpreted and intensities
(20) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy: Exciton
Coupling in Organic Stereochemistry; University Science Books: Mill
Valley and Oxford University Press: Oxford, 1983.
(23) Chan, S. C.; Chan, M. C. W.; Che, C. M.; Wang, Y.; Cheung, K. K.; Zhu,
N. Chem. Eur. J. 2001, 7, 4180-4190.
(21) (a) Avakian, P.; Ern, V.; Merrifield, R. E.; Suna, A. Phys. ReV. 1968, 165,
974-980. (b) Hochstrasser, R. M.; Whiteman, J. D. J. Chem. Phys. 1972,
56, 5945-5958. (c) Pope, M.; Swenberg, C. E. Electronic Processes in
Organic Crystals and Polymers; Oxford University Press: New York, 1999.
(22) (a) Lock, C. J. L.; Turner, M. A. Acta Crystallogr. 1987, C43, 2096-
2099. (b) Houlton, A.; Mingos, D. M. P.; Murphy, D. M.; Williams, D. J.;
Phang, L. T.; Hor, T. S. A. J. Chem. Soc., Dalton Trans. 1993, 3629-
3630. (c) Fackler, J. P., Jr.; Staples, R. J.; Khan, M. N. I.; Winpenny, R.
E. P. Acta Crystallogr. 1994, C50, 1020-1023. (d) Bowmaker, G. A.;
Brown, C. L.; Hart, R. D.; Healy, P. C.; Rickard, C. E. F.; White, A. H. J.
Chem. Soc., Dalton Trans. 1999, 881-889. (e) Leznoff, D. B.; Rancurel,
C.; Sutter, J.-P.; Rettig, S. J.; Pink, M.; Paulsen, C.; Kahn, O. J. Chem.
Soc., Dalton Trans. 1999, 3593-3599. (f) Smyth, D. R.; Vincent, B. R.;
Tiekink, E. R. T. Crystal Growth & Design 2001, 1, 113-117.
(24) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara N. Synthesis
1980, 627-630. (b) Zhang, Y. D.; Wen, J. X. Synthesis 1990, 727-728.
(25) DENZO. In The HKL Manual-A Description of Programs DENZO,
XDISPLAYF and SCALEPACK, written by Gewirth, D. with the cooperation
of the program authors Otwinowski, Z. and Minor, W. Yale University:
New Haven, USA, 1995.
(26) SIR-97: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
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Crystallogr. 1998, 32, 115.
(27) SHELXL-97: Sheldrick, G. M. SHELX97, Programs for Crystal Structure
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Analysis (Release 97-2), University of Goetingen, Germany, 1997.
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A R T I C L E S
Lu et al.
integrated using program DENZO.25 The structure was solved by direct
methods employing SIR-9726 except for 4 (SHELXS-9727). The Au, P
and many non-H atoms were located according to direct methods and
successive least-squares Fourier cycles. Positions of other non-H atoms
were found after successful refinement by full-matrix least-squares using
SHELXL-9728 program. In the final stage of least-squares refinement,
all non-H atoms were refined anisotropically. The positions of H atoms
were calculated based on riding mode with thermal parameters equal
to 1.2 times that of the associated C atoms and participated in the
calculation of final R-indices. One crystallographic asymmetric unit
consists of one formula unit for all crystal structures in the present
study.
Acknowledgment. We are grateful for financial support from
The University of Hong Kong and the Research Grants Council
of Hong Kong SAR, China [HKU 7039/03P]. W.L. acknowl-
edges the receipt of studentship from The University of Hong
Kong.
Supporting Information Available: Additional crystallo-
graphic plots and CIFs; supplementary absorption and emission
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0382415
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16088 J. AM. CHEM. SOC. VOL. 125, NO. 51, 2003