External heavy-atom effect of gold in a supramolecular acid-base π stack

Article abstract of DOI:10.1039/b506319a

The nucleophilic trinuclear Au(I) ring complex Au₃(p-tolN=COEt) ₃,1, forms a sandwich adduct with the organic Lewis acid octafluoronaphthalene, C₁₀F₈. The 1.C₁₀F ₈ adduct has a supramolecular structure consisting of columnar interleaved 1 : 1 stacks in which the Au₃(p-tolN=COEt)₃ π-base molecules alternate with the octafluoronaphthalene π-acid molecules with distances between the centroid of octafluoronaphthalene to the centroid of 1 of 3.458 and 3.509 A. The stacking with octafluoronaphthalene completely quenches the blue photoluminescence of Au₃(p-tolN=COEt) ₃, which is related to inter-ring Au-Au bonding, and leads to the appearance of a bright yellow emission band observed at room temperature. The structured profile, the energy, and the lifetime indicate that the yellow emission of the 1·C₁₀F₈ adduct is due to monomer phosphorescence of the octafluoronaphthalene. The 3.5 ms lifetime of the yellow emission of 1·C₁₀F₈ is two orders of magnitude shorter than the lifetime of the octafluoronaphthalene phosphorescence, thus indicating a strong gold heavy-atom effect. The diffuse-reflectance spectrum of the solid adduct shows new absorptions that are red-shifted from the absorptions of the monomeric organic and inorganic components alone, indicating charge transfer. Luminescence excitation spectra suggest that these new absorptions represent the major excitation route that leads to the yellow luminescence of 1·C₁₀F₈, which is different from the conventional heavy-atom effect in which the phosphorescence route entails simply the enhancement of the S₁-T₁, intersystem crossing of the organic compound. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506319a

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F U L L P A P E R
External heavy-atom effect of gold in a supramolecular acid–base
p stack
Ahmed A. Mohamed,a Manal A. Rawashdeh-Omary,b Mohammad A. Omary*b and
John P. Fackler, Jr.*a
a Department of Chemistry, Texas A & M University, College Station, Texas, 77843, USA.
E-mail: fackler@mail.chem.tamu.edu
b Department of Chemistry, University of North Texas, Denton, TX, 76203, USA.
E-mail: omary@unt.edu
Received 5th May 2005, Accepted 20th June 2005
First published as an Advance Article on the web 4th July 2005
=
The nucleophilic trinuclear Au(I) ring complex Au3(p-tolN COEt)3, 1, forms a sandwich adduct with the organic
Lewis acid octafluoronaphthalene, C10F8. The 1·C10F8 adduct has a supramolecular structure consisting of columnar
=
interleaved 1 : 1 stacks in which the Au3(p-tolN COEt)3 p-base molecules alternate with the octafluoronaphthalene
p-acid molecules with distances between the centroid of octafluoronaphthalene to the centroid of 1 of 3.458 and
˚
3.509 A. The stacking with octafluoronaphthalene completely quenches the blue photoluminescence of
=
Au3(p-tolN COEt)3, which is related to inter-ring Au–Au bonding, and leads to the appearance of a bright yellow
emission band observed at room temperature. The structured profile, the energy, and the lifetime indicate that the
yellow emission of the 1·C10F8 adduct is due to monomer phosphorescence of the octafluoronaphthalene. The 3.5 ms
lifetime of the yellow emission of 1·C10F8 is two orders of magnitude shorter than the lifetime of the
octafluoronaphthalene phosphorescence, thus indicating a strong gold heavy-atom effect. The diffuse-reflectance
spectrum of the solid adduct shows new absorptions that are red-shifted from the absorptions of the monomeric
organic and inorganic components alone, indicating charge transfer. Luminescence excitation spectra suggest that
these new absorptions represent the major excitation route that leads to the yellow luminescence of 1·C10F8, which is
different from the conventional heavy-atom effect in which the phosphorescence route entails simply the
enhancement of the S1–T1 intersystem crossing of the organic compound.
charge-transfer complexes with nitro-9-fluorenones12b and some
Introduction
form hourglass figures on standing or when placed in an acid.12c
Extended linear chain compounds that contain metal atoms
exhibit important aspects in chemical bonding as well as
fascinating chemical and physical properties.1–3 Many classes
of coordination compounds that exhibit such structures have
been reported, which has given rise to advances in several
fundamental and applied research aspects in areas that include
supramolecular architecture, acid–base chemistry, metallophilic
bonding, luminescent materials, optical sensing, and various
optoelectronic applications. Among the many recent develop-
ments in this area are reports about a vapochromic light-
Gabba¨ı and co-workers have reported that a trinuclear Hg(II)
complex forms 1 : 1 adducts with aromatic hydrocarbons, which
become brightly phosphorescent at room temperature due to a
mercury heavy-atom effect.13,14 Recently, Omary and Dias have
shown that trinuclear Cu(I) pyrazolato complexes exhibit bright
emissions that can be tuned to multiple luminescence colors
across the visible region by varying the temperature, solvent, or
concentration.15
We have been studying trinuclear Au(I) compounds with
aromatic-substituted imidazolate and carbeniate bridging lig-
ands. These compounds are colorless and do not form extended-
chain structures. However, it has been shown that they can form
sandwich adducts with a variety of electrophiles to produce
supramolecular chains (Chart 1) that exhibit interesting bonding
and optoelectronic properties that are related to the chain
structure.16–21 While most adducts show 2 : 1 assemblies, we
recently communicated a notable exception for the 1·C6F6
adduct, which assembled in a 1 : 1 ratio and is non-luminescent.
Here, we report a detailed structural and photophysical study
about the formation of a sandwich adduct of 1 with octafluo-
ronaphthalene (C10F8). The new 1·C10F8 adduct forms a 1 : 1
supramolecular stack and exhibits phosphorescence charac-
teristic of the organic molecule due to a gold heavy-atom
effect. The photophysical data illustrate a different excitation
route in 1·C10F8 from the conventional heavy-atom effect in
organic molecules and show signature bands for the secondary
p interactions in the absorption and not the emission process
of the adduct. A discussion of the puzzling differences in the
luminescence and structural properties of the various 1 : 1
and 1 : 2 donor–acceptor stacks shown in Chart 1 is also
presented.
emitting diode from linear chain Pt(II)/Pd(II) complexes,4
a
luminescent switch consisting of an Au(I) dithiocarbamate
complex that possesses a luminescent linear chain form only
in the presence of vapors of organic solvents,5 a vapochromic
complex, {Tl[Au(C6Cl5)2]}n, that shows a quantum dot effect
and reversible color changes upon binding to volatile organics,6 a
modified form of Magnus’ green salt, [(NH3)4Pt][PtCl4], that ex-
hibits semiconducting properties,7 a variety of d8 complexes that
interact with inorganic8 and organic9 molecules to form donor–
acceptor extended-chain adducts with interesting conducting
and/or magnetic properties, and new interesting complexes
that exhibit strong heterobimetallic bonding between different
closed-shell metal ions.10,11 A specially intriguing class of linear-
chain species involves trinuclear d10 complexes, which have
garnered considerable interest in recent years owing in large
part to their fascinating luminescence properties. For example,
Balch and co-workers reported that a trinuclear carbeniate
Au(I) complex exhibits “solvoluminescence”, i.e. it produces
spontaneous orange emission upon contact with solvent in
samples that had been irradiated with long-wavelength UV
light.12a The same and related complexes have been found to form
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 2 5 9 7 – 2 6 0 2
2 5 9 7
©

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tronic absorption and diffuse-reflectance spectra were acquired
using a Perkin-Elmer Lambda 900 double-beam UV/VIS/NIR
spectrophotometer. Absorption measurements were carried out
for freshly-prepared solutions in HPLC-grade CH3CN using
matching 1-cm or 1-mm Suprasil quartz cuvettes obtained
from Wilmad for the sample and the blank simultaneously
in the double-beam setup. The solid-state diffuse-reflectance
spectra were measured using a 150-mm integrating sphere (by
Labsphere) accessory to the Lambda 900 spectrophotometer.
This was done by packing crystals of the compound in a 0.1 mm
Suprasil quartz cuvette (Wilmad).
Melting points were measured on a Unimelt capillary melting
point apparatus and are reported uncorrected. 19F NMR spectra
were obtained on a Varian XL-200 spectrometer, or a Varian
UnityPlus-300 spectrometer. 19F NMR data are expressed in
parts per million (ppm) and referenced externally to CFCl3.
Structure determination
Data were collected using a Siemens (Bruker) SMART CCD
(charge coupled device) based diffractometer equipped with a
LT-2 low-temperature apparatus operating at 110 K. A suitable
crystal was chosen and mounted on a glass fiber using cryogenic
grease. Data were measured using omega scans of 0.3◦ per
frame for 60 s, such that a hemisphere was collected. The first
50 frames were recollected at the end of data collection as a
monitor for decay. No decay was detected. Cell parameters were
retrieved using SMART software and refined using SAINT on
all observed reflections.23 Data reductions were performed using
SAINT software.24 Absorption corrections were applied using
SADABS.25 The structure was solved by direct methods using
SHELXS-97 and refined by least squares on F2, with SHELXL-
97 incorporated in SHELXTL-PC V 5.03.26,27 The structure
was determined in the noncentric space group P212121 (Flack
parameter = 0.000(14)) by analysis of systematic absences. Hy-
drogen atom positions were calculated by geometrical methods
and refined as a riding model.
Chart 1 Structures of the trinuclear AuI compounds studied and their
adducts with various electrophiles.
Experimental
General
Octafluoronaphthalene (96%) was obtained from Matrix Sci-
entific and used as received. Synthesis of 1 was performed
according to a published procedure.22 Dichloromethane was
distilled over P4O10.
Synthesis
The adduct 1·C10F8 was synthesized by dissolving 100 mg
(0.09 mmol) of 1 in 2 mL of dichloromethane and adding to
it 25 mg (0.09 mmol) of octafluoronaphthalene. The solution
was stirred for 1 h and diethyl ether added to form a white
product. In another preparation, an excess of octafluoronaph-
thalene was added to 1 dissolved in dichloromethane and the
solvent evaporated. The excess octafluoronaphthalene sublimed
at room temperature to leave a white product. Colorless single
crystals used in the crystallographic and luminescence studies
were grown by slow evaporation from dichloromethane. Yield
(107 m◦g, 87%), mp 198–200 ◦C (mp of octafluoronaphthalene is
89–90 C and of 1 is 180 ◦C). 19F NMR: 20 (s) and 28 (s) ppm.
Anal. Calc. (found) for C40H36Au3F8N3O3: C, 35.57 (35.14); H,
2.68 (2.41); N, 3.11 (3.08); F, 11.26 (11.97)%.
Crystal data. C40H36Au3F8N3O3, Mr = 1349.62, unit-cell
dimensions: a = 7.292(2), b = 21.289(6), c = 25.589(7), V =
3
˚
3972.7(19) A , Z = 4, T = 110 K, l = 11.131, number of
reflections measured: 6601, Rint = 0.0531, R1 = 0.0386, wR2 =
0.0968 (all data). The 342 restraints were developed by using
ISOR values of 0.005 on C8 and C18 with 49 omitted reflections.
CCDC reference number 270181.
See http://dx.doi.org/10.1039/b506319a for crystallographic
data in CIF or other electronic format.
Physical measurements
Results and discussion
The luminescence and diffuse-reflectance measurements were
carried out at the University of North Texas for crystalline
material carefully examined by optical microscopy. Steady-state
luminescence spectra were acquired with a PTI QuantaMaster
Model QM-4 scanning spectrofluorometer equipped with a 75-
watt xenon lamp, emission and excitation monochromators, ex-
citation correction unit, and a PMT detector. The emission and
excitation spectra were corrected for the wavelength-dependent
detector response and lamp output, respectively. Lifetime data
were acquired using a nitrogen laser interfaced with a tunable
dye laser and a frequency doubler, as part of fluorescence and
phosphorescence sub-system add-ons to the PTI instrument.
The 337.1 nm line of the N2 laser was used directly to generate
the time-resolved data. Luminescence measurements at 77 K
were acquired by placing the crystalline samples in a Suprasil
quartz capillary tube inserted into a liquid-nitrogen Dewar flask
with a Suprasil quartz cold finger, which was positioned and
aligned in the sample compartment of the fluorometer. Elec-
Synthesis of the 1·C10F8 adduct
The reaction of 1 with octafluoronaphthalene in dichloro-
methane forms the sandwich complex described here. The
white product shows a strong yellow luminescence at room
temperature under UV light. The 19F NMR spectra for nearly
saturated solutions of 1·C10F8 in CDCl3 show two singlet
peaks at the same chemical shift (20 and 28 ppm) as those
observed for C10F8 alone in CDCl3 indicative of dissociation
in solution. The C10F8 moiety melts at ∼90 ◦C and the title
complex melts at ∼200 ◦C. Heating the 1·C10F8 adduct at
65 ◦C results in a loss of the yellow luminescence within
3 min and, instead, a blue luminescent material is produced
characteristic of the trinuclear starting material, 1.20 Cooling the
solid sample to room temperature followed by adding drops of
C10F8 in dichloromethane regenerates the yellow luminescence.
The process was repeated three times without loss of the yellow
luminescence.
2 5 9 8
D a l t o n T r a n s . , 2 0 0 5 , 2 5 9 7 – 2 6 0 2

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◦
˚
˚
Table 1 Selected bond distances (A) and angles ( ) for 1·C10F8
The interactions between Au and F (3.458–3.509 A) are prob-
ably augmented by electrostatic forces between the trinuclear
entities and the C10F8 rings. The eight F atoms participate in
intrastack Htolyl · · · F contacts that range between 2.478 and
Au(2)–C(8)
Au(2)–C(18)
Au(2)–Au(1)
Au(3)–N(3)
Au(1)–C(28)
C(39)–C(40)
1.925(11)
1.975(11)
3.2796(10)
2.058(9)
1.960(12)
1.421(17)
Au(2)–N(2)
Au(2)–Au(3)
Au(3)–C(18)
Au(3)–Au(1)
Au(1)–N(1)
2.055(9)
3.2790(9)
1.975(11)
3.3368(9)
2.055(10)
˚
2.9993 A. The F · · · H interactions appear to be an important
factor in forming a short-range supramolecular structure. The
octafluoronaphthalene molecules of one stack lie closer to the
trinuclear molecules of adjacent stacks, but not exactly opposite
to them. The various interactions induce Au–C–N–Au torsion
angles of 6.55–11.71◦ in the trinuclear complex.
C(8)–Au(2)–N(2)
C(18)–Au(3)–N(3)
C(28)–Au(1)–N(1)
177.7(4)
178.4(4)
178.7(4)
Au(3)–Au(2)–Au(1)
Au(2)–Au(3)–Au(1)
Au(2)–Au(1)–Au(3)
61.164(12)
59.43(2)
59.408(19)
The distance between the centroid of C10F8 to the centroid of
˚
˚
1 is 3.509 A compared to the slightly longer distance of 3.565 A
in 1·C6F6. Perfluoroaromatic molecules are known to behave as
electron acceptors and able to form compounds by interaction
with electron rich trinuclear gold complexes. The fact that no
significant change in the distances in the fluorinated naphthalene
rings after intercalation is augmented by a recent report by
Schafer and Cotton.28 They showed that the neutral and anion
C10F8 possess D2h and the electron affinity of the neutral species is
1.01 eV compared to 0.69 eV for hexafluorobenzene. The larger
electron affinity of C10F8 than that of C6F6 explains the shorter
interplanar separation in 1·C10F8 compared to 1·C6F6.
Structural properties
The adduct 1·C10F8 crystallizes in the noncentric orthorhombic
space group P212121. It is noted that 1·C10F8 satisfies an
important requirement for second-order NLO properties to
appear with photoluminescent materials. Noncenteric space
groups are relatively uncommon in inorganic compounds.
Selected bond distances and angles are presented in Table 1.
The molecular structure of 1·C10F8 is shown in Fig. 1 and the
stacking pattern is shown in Fig. 2. The Au · · · Au intramolecular
˚
interactions in 1 (3.224, 3.288, 3.299 A) become longer upon
The metallocycles are puckered and irregular in both the
dimeric 1 and the 1·C10F8 adduct. The three angles in the Au3 unit
are nearly equal, ∼60◦. The complex packs in the slipped stacks
face-to-edge. The naphthalene rings are slipped such that one
of the fused rings sits atop the trinuclear center and the other
ring below another trinuclear center. The slip angle between the
naphthalene rings is ∼17◦.
˚
adduct formation in 1·C10F8 (3.279, 3.280, 3.337 A). The longest
˚
Au · · · Au interaction of 3.337 A is parallel to the plane passing
through C(39)–C(40) in C10F8. A weak interaction between
˚
the fluorine and gold atoms of ∼3.5 A exists, which probably
plays a role in the adduct stabilization. The change in Au–C
and Au–N distances on going from 1 to 1·C10F8 is negligible.
There is essentially no change in the bond distances of aromatic
C–C bonds in the naphthalene ring, consistent with Gabbai’s
observations for [Hg3(C6F4)3]·C10H10.13,14
Photophysical properties
The structured blue emission of 1 is related to intermolecular
Au–Au interactions in its dimeric crystallographic form.19,20
Hence, the luminescence is quenched in 1 : 1 adducts of 1 with
organic Lewis acids. In the case of the 1·hexafluorobenzene
adduct reported earlier, the adduct is not luminescent and
exposure of the Au3 solid compound to vapors of C6F6 quenches
its luminescence within minutes.20 In the case of the 1·C10F8
adduct described here, a bright yellow structured emission
band is observed even at ambient temperature and the vibronic
structure becomes more resolved upon cooling to 77 K. Fig. 3
shows that the profile of the emission spectrum of crystals
of 1·C10F8 at 77 K is essentially the same as that of crystals
of uncomplexed octafluoronaphthalene but with a slight red
shift. At ambient temperature, the yellow organic-centered
luminescence is very bright for 1·C10F8 but undetectable for
solid octafluoronaphthalene. Lifetime measurements show that
the yellow luminescence of 1·C10F8 is phosphorescence. Fig. 4
shows that the decay curve at 550 nm shows a single exponential
Fig. 1 ORTEP (50%) of 1·C10F8.
fit with s = 3.57
0.07 ms. The magnitude of the lifetime
does not change significantly with variations in either the laser
excitation wavelength or the monitored emission wavelength.
The luminescence intensity and lifetime both increase upon
cooling. This is as expected because of the corresponding
decrease in the rate constant of the non-radiative decay.
The structured profile, energy, and lifetime of the yellow
emission of the 1·C10F8 adduct indicate that the emission
is monomer phosphorescence of the octafluoronaphthalene,
for which the energy of the T1 → S0 radiative transition is
red-shifted only slightly. The 3.57 ms lifetime of the yellow
emission of 1·C10F8 is two orders of magnitude shorter than the
lifetime of the octafluoronaphthalene phosphorescence, which
was reported by different authors to be in the range of 0.25–
0.38 seconds in frozen glasses.29–31 The brightness of the yellow
phosphorescence at room temperature for the 1·C10F8 solid
adduct along with the great reduction in the triplet lifetime
compared to free octafluoronaphthalene are indicative of a
strong gold heavy-atom effect. The slight red shift in the emission
energy of 1·C10F8 is also a known consequence of the heavy-atom
Fig. 2 Packing diagram of 1·C10F8 showing the Au · · · F interaction.
D a l t o n T r a n s . , 2 0 0 5 , 2 5 9 7 – 2 6 0 2
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1·C10F8 to ms levels due to an external heavy-atom effect of
gold is much more significant than the common reduction in sP
due to external heavy-atom effects and is more comparable to the
reduction in sP due to internal heavy-atom effects. The spin–orbit
coupling “f” parameter for the 5d orbital of Au(I) is 5100 cm−1,39
comparable to the f values for the Br and I atoms of 2460 and
5700 cm−1, respectively.35,40 The internal heavy-atom effect in a-
iodonaphthalene leads to sP = 2 ms.35 Meanwhile, lifetime data
recently communicated by Omary and Gabba¨ı show sP values
on the order of 10−1–100 ms for several 1 : 1 adducts of electron-
rich aromatic hydrocarbons (naphthalene, biphenyl, pyrene)
with the macrocyclic Lewis acid [l-(o-C6F4)3Hg3].14 Therefore,
it is reasonable to conclude that secondary p interactions of
aromatic luminophores with trinuclear complexes of 5d10 heavy
metal ions lead to an unusually strong external heavy-atom
effect that is comparable to the internal heavy-atom effect in
organic compounds. The long-range ordering of the acid–base
stacks, in which each organic triplet emitter is surrounded by
six heavy metal atoms, is likely the major contributing factor to
the unusually strong external heavy-atom effect seen here. This
effect, however, is smaller than the internal effect in which metals
are involved in coordinate covalent bonds with the organic
moiety, wherein sP values of 101–102 ls are common for ligand-
centered emissions in 5d10 complexes41 and even in lighter 3d10
and 4d10 complexes.42 The phosphorescence lifetimes are even
shorter for metal-centered emissions, wherein sP values of a
few ls and even 102 ns are quite common, e.g. for Au(I) complexes
that show gold-centered emissions.43
Fig. 3 Emission spectra of single crystals of the 1·C10F8 stacked adduct
and solid C10F8. No luminescence was detected at room temperature
from solid C10F8.
In order to gain insight into the photophysical processes that
lead to the enhanced phosphorescence in 1·C10F8, absorption,
diffuse-reflectance, and luminescence excitation spectra were
obtained. Fig. 5 shows the diffuse-reflectance spectrum of the
solid adduct 1·C10F8. Because the adduct stacks in a 1 : 1 manner,
the diffuse-reflectance data are compared with the absorption
spectra for dilute solutions of 1 and of C10F8, which represent
monomers of these species. Fig. 5 shows the 1·C10F8 adduct
exhibits not only absorptions characteristic of its two monomer
components, but also new red-shifted features (designated by
arrows) that extend the absorption edge of the adduct to
approach the visible region. Luminescence excitation spectra
suggest that these new absorptions represent the major excitation
route that leads to the yellow luminescence of 1·C10F8. Fig. 6
shows that distinct luminescence excitation peaks appeared near
335 and 360 nm for 1·C10F8, which correspond to the red-shifted
diffuse-reflectance features. The fact that these new features
become much more clearly discernible in the luminescence
excitation spectrum than they were in the diffuse reflectance
spectrum indicates the central role played by their corresponding
transitions in the excitation route of 1·C10F8. For comparison,
Fig. 6 also shows the luminescence excitation spectrum of C10F8
Fig. 4 Phosphorescence decay curve for the yellow emission band of
single crystals of the 1·C10F8. The emission is monitored at 550 nm
generated with 337.1 nm pulsed N2 laser excitation.
effect.29,31 The interactions between 1 and naphthalene are
secondary p interactions, as evidenced by the relatively long crys-
tallographic distances between the planes of the two components
˚
(∼3.5 A between the centroids; vide supra). The energies of such
interactions have been estimated to be in the range of only 1–2
kcal mol−1 in similar systems.32,33 Hence, one can consider the
heavy-atom effect seen here for the 1·C10F8 adduct to be more
comparable to the external heavy-atom effect known for organic
compounds when a heavy atom is present in the luminophore
environment (e.g., in the solvent, host matrix, or “through space”
interactions within the molecule), as opposed to the internal
heavy-atom effect where a heavy atom is involved in a direct
covalent bonding to the luminophore.34–38 The external heavy-
atom effect in organic compounds usually leads to a modest de-
crease in phosphorescence lifetimes while this decrease is much
more significant in the case of the internal heavy-atom effect.
For example, comparing (in a diethyl ether–isopentane–ethanol
5 : 5 : 2 glass) naphthalene (sP = 2.4 s) with 1-bromonaphthalene
and with norbornyl bromide fused to naphthalene (with the
Br over the naphthalene) reduces the lifetime to 0.014 s and
0.77 s, respectively, due to internal and external heavy-atom
effects.34,35 The reduction in phosphorescence lifetime seen for
Fig. 5 Solid-state diffuse-reflectance spectrum of the 1·C10F8 stacked
adduct in comparison to its free molecular components, represented by
dilute solutions of 1 and octafluoronaphthalene in acetonitrile.
2 6 0 0
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Fig. 6 Overlay of various electronic spectra of 1·C10F8 and free C10F8
Fig. 7 Proposed energy level diagram showing the interaction between
the excited states of 1 and C10F8 to form the charge transfer (CT) states
in the 1·C10F8 adduct.
for purposes of comparison.
alone, which exhibits luminescence excitation peaks at shorter
wavelengths and correspond to the monomer absorption peaks
in the absorption spectrum of dilute solutions of C10F8.
depopulated as a result of the charge transfer process. Thus,
the lowest-energy emitting state in the adduct remains as the T1
state with little perturbation of its original energy in the organic
component alone. The spectral data suggest that the S1 state
of C10F8 is not involved in the charge transfer process because
vibronic features corresponding to this state remain essentially
unperturbed in the electronic spectrum of the binary adduct (see
the dashed lines in Fig. 5). Hence, we illustrate in Fig. 7 that the
3CT and 1CT states arise from the molecular orbital interaction
of the T2 and S2 states of C10F8 with suitable frontier orbitals in
1 while both the S1 and T1 states of C10F8 remain non-bonding
in the 1·C10F8 adduct. The luminescence excitation spectrum
that monitors the yellow phosphorescence of 1·C10F8 (Fig. 6)
suggests that the charge transfer process represents the major
low-energy excitation route while excitation peaks due to direct
absorption to the triplet are absent (e.g., S0 → T1 absorptions for
naphthalenes lie in the range (20–25) × 103 cm−1).35 Finally, we
would like to address the different luminescence behavior seen
here for the 1·C10F8 adduct from that of the 1·C6F6 adduct we
reported earlier,20 which does not exhibit phosphorescence from
the T1 state of C6F6 on complexation with 1. An explanation
may lie in the fact that previous experimental data in the
organic literature suggest that hexafluorobenzene has a greater
ground to excited state separation than octafluoronaphthalene,
leading to an increased internal conversion rate, S1 → S0, for
hexafluorobenzene compared to intersystem crossing, S1 → T1,
which dominates in octafluoronaphthalene; for further details,
see refs. 30 and 46.
We assign the diffuse-reflectance/excitation peaks at ∼335
and 360 nm for 1·C10F8 to charge-transfer transitions in the
ground-state adduct. The trinuclear Au(I) complex 1 is strongly
nucleophilic,19 so it acts as a p-base and donates electron
density to the p-acid C10F8. The resulting charge-transfer adduct
exhibits characteristic absorptions that are red-shifted from
the absorptions of its individual components. Kisch et al.
suggested that donor-to-acceptor charge transfer bands should
appear in the diffuse-reflectance spectra of 1 : 1 donor–acceptor
inorganic–organic solid stacks.44 Also, previous work by us20
and by Balch and co-workers12b showed that charge-transfer
absorption bands in the visible and near-IR region arise in
neutral adducts that form upon interaction of trinuclear Au(I)
complexes with organic acceptors such as TCNQ and nitro-
substituted fluorenones. No luminescence data were reported
in these previous reports, but they support the charge-transfer
assignment of the lowest-energy diffuse-reflectance/excitation
peaks of 1·C10F8. Although the mechanism of the external
heavy-atom effect of organic luminophores has been subject
to numerous interpretations45 and there seems to be lack of
consensus on its origin, it has been suggested that charge transfer
might play a role in the external heavy-atom effect. Evidence
to this effect has been gathered, typically by analysis of the
emission data for rigid glasses at cryogenic temperatures for
a number of related luminophores that contain one or more
heavy atoms (usually halogens), or by theoretical studies.45 But,
to our knowledge, distinct peaks due to the suggested charge-
transfer process in the ground-state adduct, like those shown in
Figs. 5 and 6, have not been reported for conventional organic
luminophores that exhibit phosphorescence due to an external
heavy-atom effect.
The conventional heavy-atom effect in organic molecules
usually invokes a phosphorescence route that entails simply the
enhancement of the S1–T1 intersystem crossing of the organic
compound following direct absorption from S0 to either S1 or
a higher singlet (e.g. S2) that then relaxes to S1 via internal
conversion.34,35 In contrast, the spectral data for the 1·C10F8
adduct here suggest a different excitation route for the phos-
phorescence, as depicted in Fig. 7. Absorption occurs directly
to the resulting charge-transfer states in the 1·C10F8 adduct. The
significantly lower intensity of the lowest-energy feature near
360 nm in the diffuse reflectance and excitation spectra of the
adduct is consistent with a triplet charge transfer state (3CT)
while the stronger higher-energy feature near 335 nm is assigned
to a singlet charge transfer state (1CT). Fortunately, these charge
transfer states lie higher in energy than the energy of the T1 state
of the organic component such that the latter state will not be
Concluding remarks
This work demonstrates the extension of the range of applica-
tions resulting from the electron-rich nature of cyclic trinuclear
Au(I) complexes to include the synthesis of bright room-
temperature phosphors upon selective interaction with lumines-
cent aromatic molecules that are electron poor. In addition to
the fundamental significance in acid–base chemistry, the results
of this work demonstrate a potential application in molecular
light-emitting diodes because of the desired features for adducts
like 1·C10F8 as emitting materials for such devices,47 including:
(1) bright emission in the solid state at room temperature, (2)
phosphorescent instead of fluorescent emission; the theoretical
upper limit for the internal electroluminescence efficiency is 0.25
and 1.00 for fluorescent and phosphorescent materials, respec-
tively, (3) the luminescence is sensitized by the intermolecular
interactions in the solid state, where such interactions normally
cause self-quenching in common luminophores, and (4) the 1 : 1
acid–base stacking pattern renders desirable charge transport
properties of electrons and holes. Nevertheless, the relatively
D a l t o n T r a n s . , 2 0 0 5 , 2 5 9 7 – 2 6 0 2
2 6 0 1

View Article Online
long lifetimes and the crystalline as opposed to amorphous
nature of the solid are considered disadvantages for adducts
like 1·C10F8 with respect to certain display applications.
19 A. Burini, J. P. Fackler, Jr., R. Galassi, T. A. Grant, M. A. Omary,
M. A. Rawashdeh-Omary, B. R. Pietroni and R. J. Staples, J. Am.
Chem. Soc., 2000, 122, 11264.
20 M. A. Rawashdeh-Omary, M. A. Omary, J. P. Fackler, Jr., R. Galassi,
B. R. Pietroni and A. Burini, J. Am. Chem. Soc., 2001, 123, 9689.
21 For reviews, see: (a) A. Burini, A. A. Mohamed and J. P. Fackler,
Jr., Comments Inorg. Chem., 2003, 24, 253; (b) M. A. Omary, A. A.
Mohamed, M. A. Rawashdeh-Omary and J. P. Fackler, Jr., Coord.
Chem. Rev., 2005, DOI: 10.1016/j.ccr.2004.12.018.
Acknowledgements
The financial support of this project has been provided by
the Robert A. Welch Foundation of Houston, TX, to J. P. F.
and M. A. O., a National Science Foundation CAREER award
to M. A. O. (CHE-0349313), and an Advanced Technology
Program grant from the Texas Higher Education Coordinating
Board to M. A. O. We also thank Professor Alfredo Burini of
the University of Camerino for initiating collaborative studies
with the trinuclear Au(I) complexes.
22 F. Bonati, A. Burini, B. R. Pietroni and B. Bovio, J. Organomet.
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Analytical X-ray Systems, Madison, WI, 1995.
24 SAINT V 4.035. Software for the CCD Detector System, Bruker
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2 6 0 2
D a l t o n T r a n s . , 2 0 0 5 , 2 5 9 7 – 2 6 0 2