Excited-State Turn-On of Aurophilicity and Tunability of Relativistic Effects in a Series of Digold Triazolates Synthesized via iClick

Article abstract of DOI:10.1021/jacs.0c01624

iClick reactions between Au(I) acetylides PPh₃Au-CCR, where R = nitrophenyl (PhNO₂), phenyl (Ph), thiophene (Th), bithiophene (biTh), and dimethyl aniline (PhNMe₂), and Au(I)-azide PPh₃AuN₃ provide di

Full text of DOI:10.1021/jacs.0c01624

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Article
Excited State Turn-On of Aurophilicity and Tunability of Relativistic
Effects in a Series of Digold Triazolates Synthesized via iClick
Charles J. Zeman, Yu-Hsuan Shen, Jessica Heller, Khalil A. Abboud, Kirk S. Schanze, and Adam S. Veige
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c01624 • Publication Date (Web): 08 Apr 2020
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Excited State Turn-On of Aurophilicity and Tunability of Relativistic
Effects in a Series of Digold Triazolates Synthesized via iClick
Charles J. Zeman IV,a,b Yu-Hsuan Shen,a Jessica K. Heller,a Khalil A. Abboud,a Kirk S. Schanze,*b
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Adam S. Veige,*a
aUniversity of Florida, Department of Chemistry, Center for Catalysis, P.O. Box 117200, Gainesville, FL, 32611.
bUniversity of Texas at San Antonio, Department of Chemistry, One UTSA Circle, San Antonio, TX 78249.
ABSTRACT: iClick reactions between Au(I) acetylides PPh3Au-C≡CR where R = nitrophenyl (PhNO2), phenyl (Ph), thiophene
(Th), bithiophene (biTh), and dimethyl aniline (PhNMe2) with Au(I)-azide PPh3AuN3 provide digold complexes of the general
formula R-1,5-bis-triphenylphosphinegold(I) 1,2,3-triazolate (Au2-R). Within the digold triazolate complexes the Au(I) atoms
are held in close proximity but beyond the distance typically observed for aurophilic bonding. Though no bond exists in the
ground state, time-dependent density functional theory interrogation of the complexes reveals excited states with significant
aurophilic bonding. The series of complexes allows for tuning of the excited state “turn-on” of aurophilicity, where ligand to
metal charge transfer (LMCT) induces the aurophilic bonding. Complexes containing ligand localized excited states however,
do not exhibit aurophilicity in the excited state. As a control experiment, a monogold complex was synthesized. The computed
excited state of the monogold species exhibited LMCT to the gold ion as in the dinuclear cases, but without a partnering gold
ion only a distinct N-Au-P bending occurs revealing a potential mechanism for the excited state turn-on of aurophilic bonding.
Analysis of the steady-state electronic spectra indicate that LMCT states are achievable for compounds with sufficiently strong
electron donating ligands, and in digold complexes this is associated with enhanced fluorescence, suggestive of an aurophilic
interaction.
of the electron correlation induced binding interaction be-
tween Au(I) atoms is due to relativistic augmentation.10
Supporting this contention, Hartree-Fock methods, which
do not include Coulombic correlation contributions to the
total energy, fail to model aurophilicity.15–17
INTRODUCTION
Aurophilicity is the term given for the peculiarly strong
bond (7 – 12 kcal/mol binding energy) between the metal
centers of Au(I) complexes.1–3 Smaller than the sum of the
two van der Waals radii (3.8 Å), inter- and intramolecular
aurophilic bonds exhibit Au(I)-Au(I) equilibrium distances
between 2.5 – 3.5 Å.3 The observed bond length depends
greatly on steric limitations, with large ligands obstructing
close contacts such that formal definitions of aurophilicity
set the cutoff distance as 3.5 Å, but any distance between 3.5
and 3.8 Å is considered to be ambiguous.2–6
Comprised of two closed shell atoms with 5d10 electron
configurations, the peculiar aspect of the aurophilic bond is
that the atoms are not expected to experience attractive
forces through ordinary effects. Intuitively, the +1 formal
charge of gold cations is expected to give rise to significant
Coulombic repulsion within short distances from one an-
other. These contrarian physical properties of Au(I) beg the
question of why there should be any attractive forces pre-
sent at all. Evidence for Au(I)-Au(I) bonds began to accumu-
late in the 1970s, though the term “aurophilicity” was not
coined until 1988 when relativistic effects of electrons in
gold atoms were implicated.7–9 More specifically, similar to
Van der Waals theory,10 electron correlation energy is the
primary contributor to the attractive force between the two
Au(I) metal atoms.2 Known to be at a local maximum on the
periodic table for gold atoms,5,11–14 it is the relativistic ex-
pansion of d-orbitals that induces the interaction leading to
aurophilicity, referred to as “super van der Waals” or “rela-
tivistic bonding.”6 Theoretical treatments approximate 28%
Figure 1. (a) Classes of aurophilic bonding. (b) This work, ex-
cited state "turn-on" of aurophilic bonding.
Complexes featuring Au(I)-Au(I) bonds come in three
forms: 1) intermolecular unsupported, 2) intramolecular
semi-supported, and 3) intramolecular fully-supported
bonds (Fig. 1a). Often provided as evidence for the existence
of aurophilicity, compounds containing Au(I)-Au(I) bonds
exhibit intense luminescence in the solid state.18–21 How-
ever, the Au(I)-Au(I) bond often breaks upon dissolving the
complexes in solution and only a few unsupported soluble
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complexes featuring aurophilic bonds are known.20,22–25 Ma-
bonds in the ground state. At a distance of 3.9 Å, the Au(I)-
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terials applications of aurophilicity are rare, and despite
decades of work to understand the nature of the interaction,
only recently a gold cluster featuring aurophilic bonds was
incorporated into an organic light-emitting diode (OLED);
furthermore, only three systems26–29 have been shown to
provide white light. More importantly, solution phase pro-
cessing of opto-electronic materials is obviously advanta-
geous. However, unsupported aurophilic bonds are gener-
ally only stable in the solid state. One possible solution is to
design complexes that do not have aurophilic bonds in the
ground state but rather upon photoexcitation induce the in-
teraction in the excited state (Fig. 1b).
Au(I) distance is well beyond the accepted cutoff distance
for aurophilicity. However, during the course of investigat-
ing the photophysical properties and computational analy-
sis of the resulting excited states of the related flourenyl de-
rivative Au2-FO,28 we found evidence for an excited state
“turn on” of aurophilic bonding.
As identified through time-dependent density functional
theory (TDDFT), a geometry optimization of the excited
state S1 of Au2-FO revealed the interatomic Au(I)-Au(I) dis-
tance contracts to 3.5 Å after photoexcitation (Fig. 3). As
noted above, this distance is less than the sum of the Van der
Waals radii of the two atoms (3.8 Å). By analysis of the
charge difference density (CDD) of the excited state S1, the
fluorenyl moiety enables strong ligand to metal charge
transfer (LMCT) resulting in an increase in electron density
at the Au(I) ions in the excited state. This creates a shorter
separation distance of the potential energy minimum of the
two-coordinate Au(I) atoms compared to their ground state
positions. Presumably, the increase in the effective size of
the Van der Waals radii from LMCT enables the Au(I)-Au(I)
bonding between the two metal centers where such an in-
teraction would otherwise be unfeasible in the ground state.
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It is well understood that Au(I)-Au(I) bonds that are pre-
sent in the ground state will contract upon photoexcita-
tion.30,31 Some examples even exhibit excited state au-
rophilic interactions where the ground state distance be-
tween gold atoms was long, yet still within the accepted
range for aurophilic interactions.32 For example, Tobon,
Saillard, Liu, Boucekkine et al. demonstrated both theoreti-
cally and experimentally that a fully-supported digold com-
plex can undergo contraction from 3.32 to 2.61 Å in the ex-
cited state.33 Similar studies of photoinduced structural
change have even been extended to diPt(II) complexes.34,35
Inorganic click or “iClick” involves adapting typically all-
organic click reactions to include metal ions. For example,
combining Ph3PAuI-N3 (1) with Ph3PAuI-CCPh (2) results in
the formation of the digold triazolate complex 4-phenyl-1,5-
bis-triphenylphosphinegold(I) 1,2,3-triazolate (Au2-Ph; Fig.
2). The synthesis of digold complexes is generic and most
any alkyne substituent is tolerable. Synthesized for control
experiments to delineate the effects of the proximal Au(I)
ions, treating the Au(I)-azide complex 1 with 2-ethynylthi-
ophene provides the monogold complex 4,5-Au1-Th in 91.1 %
yield.
Figure 3. Structure of Au2-FO with illustration of excited state
turn-on of an aurophilic interaction where Au atoms separated
by 3.9 Å contract to 3.5 Å upon photo excitation.
In this work, we report a series of digold complexes out-
lined by Figure 4 that contain Au(I) ions well-separated and
beyond the limit of aurophilicity in the ground state, but
upon photoexcitation, the distance contracts to induce au-
rophilic bonding in the excited state (Fig. 3) as identified by
computational means and further characterized spectro-
scopically. These compounds were chosen for the range of
electron donating strengths presented by the aryl chromo-
phore. Importantly, the observation of excited state “turn on”
of a Au(I)-Au(I) interaction is dependent on these electron
donating properties of the ligand substituents, thus offering
an ability to fine-tune the interaction.
Figure 2. Top: Typical iClick synthesis of digold triazolate Au2-
Ph. Bottom: Synthesis of 4,5-Au1-Th.
It became immediately apparent by crystallography, com-
putational modeling, and photophysical characterizations
that compounds such as Au2-Ph do not have Au(I)-Au(I)
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3.5642(6), 3.5239(6), 3.4940(3) Å, for Au2-PhNMe2, Au2-
1
PhNO2, Au2-biTh, Au2-Th, and Au2-Ph.
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Figure 4. Synthesized compounds by iClick with nitrophenyl
(PhNO2), phenyl (Ph), thiophene (Th), bithiophene, (biTh) and
dimethyl aniline (PhNMe2) ligands, where the 3,4-ethylenedi-
oxythiophene (EDOT) ligand was only studied computation-
ally.
RESULTS
Preliminary Remarks. Using similar computational
methods for diAu(I) complexes of the same model structure
in Au2-FO,28 but exchanging the fluorenyl substituent for
any number of moieties with varying degrees of electron
donating or withdrawing character will probe the electronic
requirements to achieve excited sate turn-on of aurophilic
bonding. Figure 4 depicts the structures chosen for this
study. Chosen to span a range of relevant ionization poten-
tials (IPs) relative to the electron affinity of Au(I) ions, the
substituents should be sufficient at probing the effects of
donor strength on excited state turn-on of aurophilicity. Ad-
ditionally, being analogous to complex Au2-FO allows for
the synthesis of this series of similar compounds using the
same iClick reaction.36 Reported previously in the literature
are complexes Au2-Ph and Au2-PhNO2, whereas Au2-EDOT
was only investigated computationally. Synthetic details
and characterization data for new complexes Au2-Th, Au2-
biTh, and Au2-PhNMe2 are detailed in the supporting infor-
mation. All compounds were stored under inert atmos-
pheres and isolated from direct exposure to light. Typically,
all products exhibited air sensitivity and were handled in an
inert atmosphere, the most sensitive being Au2-PhNMe2
and the most stable being Au2-PhNO2.
Figure 5. Crystal structures taken from X-ray diffraction results
of single crystals.
Computational Modeling. Hartree-Fock (HF), second-
order Moeller-Plesset perturbation theory (MP2),39 Becke’s
three-parameter hybrid functional (B3LYP),40–42 and the
long-range corrected variant CAM-B3LYP43 levels of theory
were employed to optimize the ground state (S0) geome-
tries for compounds with triphenyl phosphine ligands re-
placed with trimethyl phosphine ligands to reduce compu-
tational costs (for validation, see supporting information).
Of these methods, HF is the only level of theory that does
not account for electron correlation energy. In turn, MP2 in-
cludes this contribution to the total energy as an adjustment
Au2-Ph, and Au2-PhNO2 are crystalline compounds and
single crystal X-ray diffraction data is available from the lit-
erature.37,38 For the complexes Au2-PhNMe2, Au2-Th and
Au2-biTh, X-ray structural data was acquired to assess the
influence, if any, the substituents have on the intramolecu-
lar Au(I)-Au(I) bond distance. Figure 5 presents the molec-
ular structures for the model complexes derived from the X-
ray structures. Each complex adopts the same general mo-
lecular structure in the solid state, where the substituent
and the triazolate are rings are nearly coplanar. The Au(I)
ions occupy positions 1 and 5 on the triazolate rings. The
identity of the substituents does influence the intramolecu-
lar Au(I) distances, but all of them are beyond the accepta-
ble cut-off for aurophilic bonding or display inconclusive
Au(I)-Au(I) interaction distances at 3.7359(2) 3.7298(2),
Figure 6. HOMO energy levels (-IP) of the aryl R-groups accord-
ing to different methods.
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Figure 7. Results of computational modeling of digold triazolates of varying -R groups according to Hartree-Fock (left) and CAM-
B3LYP (right). The structures show the optimized ground state geometry (S0), the charge difference density which moves from
purple to pink MOs upon photoexcitation (CDD), and the subsequent relaxed excited state geometry (S1) sequentially (red arrows).
From top to bottom, these structures correspond to Au2-PhNO2, Au2-Ph, Au2-Th, Au2-EDOT, Au2-biTh, and Au2-PhNMe2.
to the Hamiltonian while maintaining the exact solution of
the electron exchange energy from pure HF. B3LYP also ac-
counts for both the exchange and correlation potential,
through an iterative optimization. Finally, CAM-B3LYP im-
proves on B3LYP by more accurately describing charge sep-
aration at greater distances, which is vital for modeling ex-
cited states expected to display charge-transfer character.
MP2 and CAM-B3LYP are expected to most accurately
model the electronic structures for this series of compounds.
The results of the ground state geometry optimizations, in-
cluding the interatomic distance between gold(I) metal cen-
ters for each compound are shown in the supporting infor-
mation (SI). Generally, all methods predict the gold(I) atoms
reside at a distance from one another that is larger than the
sum of their Van der Waals radii, with MP2 predicting the
shortest distance of 3.726 Å for Au2-PhNO2, and HF predict-
ing the longest distance of 3.987 Å for Au2-Ph. From these
results alone, no immediately apparent trend is evident
across the series of aryl donor moieties.
To better evaluate the relationship between donor char-
acter and subsequently determined properties, the IP of
each R-group was assumed to be equivalent to the negative
of the HOMO energy level, as postulated by Koopman’s the-
orem. Figure 6 depicts these HOMO energies. For the pur-
pose of these calculations, the triazolate was replaced with
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a hydrogen atom to isolate the aryl R-group donors. Re-
EDOT
4.87
4.32
4.60
3.20
3.08
2.88
2.87
2.76
2.69
3.83
3.89
3.78
3.81
3.44
3.60
0.0273
0.4962
0.0253
0.4983
0.0001
0.0001
0.0001
0.0000
0.0002
0.5437
0.0016
0.0001
0.0041
0.5276
0.0006
Y
N
Y
N
Y
Y
Y
Y
Y
N
Y
Y
Y
N
Y
-0.051
+0.023
-0.162
-0.045
-0.163
-0.307
-0.322
-0.235
-0.312
+0.038
-0.734
-0.838
-0.800
+0.058
1
2
3
4
5
6
7
8
ported IPs represent those from fully optimized geometries.
The results indicate the chosen compounds display a wide
range of reducing electron donor strength, with nitrophenyl
(PhNO2) being the hardest to oxidize (lowest energy HOMO)
and dimethyl aniline (PhNMe2) being the easiest (highest
energy HOMO).
biTh
PhNMe2
TD-B3LYP PhNO2
Ph
For the sake of consistency, excited state analyses employ
time dependent (TD) methods throughout. Moreover, no
excited state methods exist for MP2,39 CIS is difficult to
make meaningful comparisons with TDDFT by way of em-
ploying an alternative method for characterizing excited
states,44 and analytical gradients are unavailable for
CIS(D)45 making geometry optimizations of excited states
identified by this method require a prohibitive number of
energy calculations. Only a single excitation was calculated
with singlet multiplicity, as the lowest lying excited state is
the only one of interest and subsequent geometry optimiza-
tions ultimately lead to excitation energies that are relaxed
to a minimum. As such, optimizations of higher ordered ex-
cited states identified by TDDFT become redefined as S1
upon completion of the calculation. For all compounds, S1
was comprised of a wide active space with several 1-elec-
tron transitions being involved in the configuration interac-
tion (CI). In these CIs, the HOMO-LUMO transition domi-
nates. Figure 7 depicts charge difference densities (CDDs)
derived from a coefficient-weighted summation of contrib-
uting 1-electron molecular orbital (MO) transitions where
charge moves from purple to pink MOs. In all cases, the
HOMO (or electron holes in the case of the CDDs) are π MOs
that are predominately localized on the aryl-triazolate lig-
and. In contrast, the LUMO is localized on the metal centers
and phosphine ligands for all compounds other than Au2-
PhNO2 and Au2-biTh. From natural bond orbital (NBO)
analysis, it was found that the LUMOs of Au2-Ph, Au2-Th,
Au2-EDOT, and Au2-PhNMe2 are comprised of approxi-
mately 40% gold atomic orbitals of predominately p-char-
acter with minimal (< 3%) contribution from phosphorous
atoms (Fig. S41). These results indicate S1 is a LMCT state
for aryl R-groups with sufficiently low IPs and high LUMO
energy. As such, Au2-PhNO2 and Au2-biTh display only a
ligand-localized excitation with minimal contributions from
the gold(I) atoms. One last noteworthy result is that TDDFT
methods predict a greater delocalization of the metal-cen-
tered virtual orbitals compared to TDHF.
Th
EDOT
biTh
PhNMe2
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TD-CAM
PhNO2
Ph
Th
EDOT
biTh
PhNMe2
-0.684
c
aEnergy of the vertical excitation, oscillator strength, charge
transfer character as identified by CDDs, and dthe change in the
Au(I)-Au(I) distance upon relaxation of the excited state.
b
Presented in Figure 7 and Table 1 are the ways in which
the Au(I)-Au(I) interaction changes in the excited state,
while all of the Au(I)-Au(I) distances at the stationary point
of the relaxed geometries are given in the SI. Generally, the
results indicate excited states identified through TDHF ex-
perience minimal structural change upon relaxation. In con-
trast, a contraction in the Au(I)-Au(I) distance for vertical
excitations that display charge-transfer character to the
metal centers are observed for TDDFT optimized excited
states. These contractions all reach values that are well be-
low the sum of the Van der Waals radii for the two Au(I) at-
oms. A bending of the angle between N-Au-P atoms from ap-
proximately 180° to 120° accompanies the Au(I)-Au(I) con-
tractions in all the examples. Changes in bond angles be-
tween Au and the triazolate ring were minimal. These ef-
fects were most extreme for CAM-B3LYP, which predicted
bond formation between Au atoms within 3.2 Å of one an-
other for Au2-Ph, Au2-Th, Au2-EDOT, and Au2-PhNMe2. Ta-
ble 1 summarizes the computational results including
greater detail regarding the properties of each individual
vertical excitation. That is, CT transitions have lower oscil-
Table 1. TDDFT Results for Au2-R Complexes.
CT?
Method
-R
ΔE (eV)a
f b
ΔRd (Å)
(Y/N)c
TD-HF
PhNO2
Ph
4.56
4.66
4.45
4.84
4.17
4.57
4.70
4.69
4.54
0.4812
0.0441
0.1493
0.0534
0.4956
0.0399
0.4695
0.0306
0.1320
N
Y
Y
Y
N
Y
N
Y
Y
-0.021
-0.004
-0.004
-0.094
+0.027
-0.173
-0.028
-0.075
-0.046
Th
EDOT
biTh
PhNMe2
PhNO2
Ph
CIS
Figure 8. Excited state analysis of (a) theoretical 1,4-Au1-Th
and (b) the as synthesized 4,5-Au1-Th isomers of the
monogold triazolate with a thiophene ligand using CAM-
B3LYP.
Th
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lator strengths than localized excitations, and excitation en-
Page 6 of 13
1
ergy becomes lower with decreasing IP of the aryl donor.
For completeness, calculations at the CAM-B3LYP level of
theory were repeated for triplet states, the results of which
are shown in Figure S42. It was found that the lowest energy
triplet excited states for all compounds were significantly
more delocalized compared to singlet excited states, with
ligand-centered π-π* character. These findings are con-
sistent with previous work and what is known about triplet
excited states in organometallic complexes.28,36,46,47 Without
the distinctive LMCT character of the singlet excited states
(Fig. 7), it was not unexpected that no contraction in the
Au(I)-Au(I) interatomic distance was observed for any com-
pound when relaxing the excited triplet state geometry.
As a control, and to distinguish the spectroscopic features
arising from the excited state turn on of Au(I)-Au(I) bonding,
the monogold complex 4,5-Au1-Th was synthesized (see SI)
and subjected to the same (CAM-B3LYP) computational
analysis as the digold compounds (Fig. 8). In addition to the
synthesized 4,5-Au1-Th isomer, the 1,4-Au1-Th isomer was
treated computationally. These results show a similar
charge-transfer to the Au(I) center, and subsequent optimi-
zation of the excited state reveals a significant bending dis-
tortion of the N-Au-P bond angle. Naturally, without a sec-
ond Au(I) atom, formation of an aurophilic bond is impossi-
ble, however this conformational change provides insight
into the Au(I)-Au(I) bond forming mechanism. Through
LMCT into a nonbonding orbital, electron density is added
to the Au(I) metal center and forces its observed bent struc-
ture.
Photophysics. Figure 9a depicts the steady-state absorp-
tion spectra of the synthesized compounds in THF. Gener-
ally, the transition energies were relatively large, with the
molecules absorbing primarily in the UV region. A notable
exception was for Au2-PhNO2 and Au2-biTh, which display
distinctly lower energy bands centered at 395 and 375 nm,
respectively. Extra caution was given to background correc-
tions during acquisition of these absorption spectra, so the
sharp rise in absorptivity at short wavelengths is real (i.e.
attributed to molecular absorption and not the medium).
The high energy absorption band (sub 350 nm) for these
compounds displays a red-shifting trend when going from
aryl donors of high IP to low IP. Namely, the long wave-
length absorption onset increases in the sequence of Au2-
Ph (340 nm), Au2-Th (365 nm), and Au2-PhNMe2 (380 nm),
which suggests that this feature is due to the LMCT absorp-
tion (vide infra). These LMCT states are further identified by
comparison with the calculated spectra (Fig. 9b), which
shows the results of 50 vertical excitations using CAM-
B3LYP that are broadened with gaussian functions (FWHM
= 50 nm). Compounds which display LMCT character in Fig-
ure 7 are in good agreement with experimental results, all
showing long-wavelength absorption onsets at roughly 375
nm. This also serves as validation of the computational
method, with CAM-B3LYP performing much better than
other excited state methods. The only outlier here was Au2-
PhNO2, which gave a 0.73 eV error compared to experi-
mental results. However, further validation of the chosen
density functional (CAM-B3LYP) is seen from the agree-
ment in spectral shape between calculated and experi-
mental results for all compounds. A more detailed compar-
ison can be found in the SI, which also includes spectra cal-
culated at the B3LYP level of theory.
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Figure 9. Normalized UV-Visible absorption spectra of digold
triazolates. (a) Experimentally measured spectra of synthetic
compounds collected in degassed THF. (b) Theoretical absorp-
tion spectra taken from the results of TDDFT calculations at the
CAM-B3LYP level of theory comprised of a total of 50 excita-
tions. Theoretical spectra are artificially broadened with gauss-
ian functions (FWHM = 50 nm).
Figure 10 depicts emission spectra collected by exciting at
285 nm under an inert atmosphere and after exposure to air
in all cases. Emission not quenched in air is from S1 (fluores-
cence) and quenched emission originates from T1 (phos-
phorescence). As such, Au2-PhNO2 and Au2-biTh are either
strictly fluorescent or weakly phosphorescent, respectively
(Figures 10a and 10d), with a fluorescence profile at much
lower energy (λmax = 475-500 nm) compared to other com-
pounds that display fluorescence. The spectral shape and
energetics of Au2-PhNO2 is independent of excitation wave-
length, differing only in relative emission intensity accord-
ing to the varying absorptivity at different wavelengths. At
higher electron donating strength, Au2-Th displays only
phosphorescence, with 100% of emission quenched upon
exposure to air (Figure 11c). Other compounds of interme-
diate CT-character show dual emission, with varying rela-
tive contribution of phosphorescence to the total photolu-
minescence. With the exception of Au2-PhNO2 and Au2-
biTh, the respective fluorescence and phosphorescence
(when present) of each compound red shifts across the se-
ries of high IP aryl donors to low IP donors, showing typical
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the mono and digold compounds is that the vibronic struc-
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ture is better resolved for Au1-Th compared to Au2-Th, giv-
ing an energy spacing of 590 cm-1, in the regime of bending
and twisting motions. As evidence for excited state Au(I)-
Au(I) bonding, inspection of the discrepancies between the
degassed and air saturated conditions as shown in the SI re-
vealed that there is still measurable fluorescence for Au2-
Th compared to Au1-Th, however difficult to detect due to
the fluorescence quantum yield being in competition with
efficient intersystem crossing to the triplet state.
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Figure 11. Normalized absorption (—) and emission (—, —,
and —) spectra of monogold 4,5-Au1-Th. Spectra were col-
lected in THF at room temperature and Me-THF at 77 K. The
air-saturated spectrum was scaled to reflect relative emission
intensity compared to degassed conditions.
Figure 10. Normalized photoluminescence intensity in THF for
(a) Au2-PhNO2, (b) Au2-Ph, (c) Au2-Th, (d) Au2-biTh, and (e)
Au2-PhNMe2 where black lines indicate degassed conditions
and red lines indicate air saturated conditions. (f) Emission at
77 K in Me-THF for Au2-PhNO2 (—), Au2-Ph (—), Au2-Th (—),
and Au2-PhNMe2 (—).
DISCUSSION
Au2-PhNO2 and Au2-biTh are distinct from the other
compounds studied in several notable ways. TDHF and
TDDFT results all indicate the S1 for Au2-PhNO2 and Au2-
biTh is primarily a localized excitation on the aryl triazolate
ligand (Fig. 7). This is clearly evident from the CDD of this
state, which actually shows a minor shift in electron density
from the metal centers to the nitrophenyl and bithiophene
moieties in the case of CAM-B3LYP. The electron withdraw-
ing strength of the nitro group lowers the LUMO energy suf-
ficiently to make the aryl ligand act as an acceptor rather
than an electron donor. This property makes Au2-PhNO2 an
excellent control, where any excited state characteristics
are not expected to originate from the effects of the Au(I)
atoms. The geometry optimizations offer more evidence
that Au2-PhNO2 and Au2-biTh do not exhibit excited state
turn-on of aurophilic bonding. In both the ground state and
excited state, the Au(I)-Au(I) distances remain beyond the
limits of aurophilicity. Moreover, the emission spectra for
Au2-PhNO2 exhibits only fluorescence (Fig. 10). Exclusion
of the Au(I) centers in the excited state wavefunction pre-
cludes the ability of the heavy atom effect to facilitate inter-
system crossing. Without the aid of spin-orbit coupling,
Au2-PhNO2 and Au2-biTh exclusively or predominately flu-
oresce, respectively, from a ligand-centered excited state.
The results indicate the lower energy absorption band for
Au2-PhNO2 and Au2-biTh is a localized excitation (LE) and
the higher energy absorption band for the other complexes
is a LMCT excitation to the Au(I) metal centers (Fig. 9). Re-
gardless of which state is excited, Au2-PhNO2 and Au2-biTh
always emit from the LE, due to Kasha’s rule.
LMCT emissive character. For stable compounds with rea-
sonable phosphorescence, some vibronic structure is evi-
dent in this contribution to the total emission.
To further examine the emission spectra and attempt to
elucidate evidence of Au(I)-Au(I) bond formation as pre-
dicted by the computational results, emission spectra were
collected at 77 K in Me-THF (Figure 11f). All fluorescence of
LMCT states quench at low temperature, meaning that the
acquired spectra for Ph, PhNMe2, and Th ligands are phos-
phorescence, while Au2-PhNO2 shows the same fluores-
cence spectrum as shown in Figure 10a. It is obvious that
the triplet energies decrease as the IP of the donor ligand
decreases. At liquid nitrogen temperatures, a reduction in
spectral broadening is expected to result in narrower line-
shape in the component vibrational bands of the emission
spectra. Interestingly, this was only observed for Au2-Ph,
which shows a spacing of 1500 cm-1 between the two most
prominent peaks at 410 nm and 437 nm, corresponding to
C=C stretching modes in aromatic compounds.
Subjected to the same spectroscopic analysis as the digold
series, the results of steady-state spectroscopy on the
monogold complex Au1-Th shown in Figure 8b are depicted
in Figure 11. Not surprising, considering emission of Au2-
Th is primarily phosphorescence, and previous work iden-
tified that the triplet state is characteristic of the ligand it-
self, the spectra for monogold 4,5-Au1-Th are very similar
to those obtained for Au2-Th.46,47 Some differences between
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A more complicated interplay between the Au(I) atoms the digold compounds to form the new bond. CT character
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and the excited state characteristics were expected of the
remaining compounds. From the computational results, S1
for all the other compounds display obvious CT-character
with charge density shifting towards the metal centers (Fig.
7). For a given computational method, the CDDs of this
LMCT state appear fundamentally the same. However, ob-
served as a red-shift in the electronic spectra across the se-
ries of synthesized compounds (Fig. 9), the excitation en-
ergy decreases for smaller donor IPs due to the raising of
the HOMO energy level where the electron-hole for these
excited states is located. This effect is further exemplified in
the calculated excitation energies (Table 1) decreasing with
decreasing IP, with the exception of Au2-PhNO2 and Au2-
biTh, which do not fit this trend due to the localization of
the excited state on the aryl component.
favoring one metal center over the other is likely the reason
the angle distortion only occurs for the N-bound Au(I) ion.
CONCLUSION
A series of iClick synthesized digold compounds that ex-
hibit a range of aryl donor-motif chromophores with differ-
ent IPs were studied computationally and spectroscopically.
At ~3.5-3.7 Å from each other and enforced by a semi-sup-
ported architecture, the sum of the Van der Waals radii for
each complex in the ground-state is beyond the accepted
value for an aurophilic bond. Vertical excitation calculations
reveal compounds with aryl ligands of sufficiently low IP ex-
hibit charge transfer character to the Au(I) metal centers.
Geometry optimizations of the excited states that include
Coulombic correlation exhibit bond contraction and ex-
cited-state activation of aurophilicity. Providing evidence
for an excited state aurophilic bond, an increase in triplet
quantum yield correlates with CT to Au(I). Case in point is
the example of Au2-PhNO2 and Au2-biTh that do not exhibit
phosphorescence and their computed S1 states are ligand
localized and do not exhibit Au(I)-Au(I) contraction. In ad-
dition, unable to partner with an adjacent gold ion, the
monogold complexes exhibit CT resulting in a bending in
the N-Au-P atoms, thus suggesting a possible mechanism for
the turn-on of aurophilcity in this series of complexes. As
such, it is now possible to choose donor chromophores such
that excited-state aurophilicity is either on or off, and sim-
ple choice of donor IP can tune the extent of intersystem
crossing and thus the magnitude of aurophilicity in the ex-
cited state. Since aurophilic bonds are often unstable in so-
lution, this new level of control may prove useful in light in-
duced self-assembly of supramolecular structures, device
fabrication, bio sensing, molecular switches, and general
processability.49
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Of particular interest is how the CDDs display a bonding
type MO between the Au(I) atoms in the excited state, re-
gardless of computational method. At a minimum, this indi-
cates that the two Au(I) atoms have shared electron density
after photoexcitation. Evidence of the metal contribution to
the excited state is seen in the emission spectra (Fig. 10),
where fluorescence is sharp and intense, as is typical of au-
rophilic emission.18–21 Similar to how Au2-PhNO2 and Au2-
biTh did not display appreciable phosphorescence because
a lack of augmented spin-orbit coupling, the added contri-
bution from the metal centers of the remaining compounds
gives rise to phosphorescence. Moreover, as ligand donor
strength is increased, the apparent contribution of the phos-
phorescence increases. These data demonstrate empirically
that the “turn-on” of aurophilicity in the excited state is
achievable by varying the HOMO energy level of a neighbor-
ing chromophore without lowering its associated LUMO en-
ergy below that of the Au(I) electron acceptor.
An equally important result from this work is the struc-
tural change upon relaxing this excited state through differ-
ent computational methods. Because HF implements the
mean-field approximation to combat the problem of corre-
lated electrons, dispersive forces are absent from the ener-
getics. Even for ground states, HF cannot model aurophilic
bonding, considering that dispersion is a fundamental com-
ponent of aurophilicity theory. In contrast, methods that in-
clude Coulombic correlation display contractions in the
Au(I)-Au(I) distance that are well within the cutoff for ac-
cepted aurophilic character. For methods that better model
CT excited states (CAM-B3LYP), the aurophilic bonding is
strengthened and bolsters the hypothesis that charge trans-
fer to the Au(I) metal centers augment Van der Waals to fa-
cilitate aurophilic bonding.
EXPERIMENTAL
Unless specified otherwise, all manipulations were
performed under an inert atmosphere using standard
Schlenk or glove-box techniques. Pentane, methylene chlo-
ride (DCM), diethyl ether, and tetrahydrofuran (THF) were
degassed by sparging with high purity argon, and were
dried using a GlassContour drying column. Methanol was
dried over anhydrous copper(II)sulfate, distilled and stored
over 4 Å molecular sieves; chloroform-d (Cambridge Iso-
topes) was dried over calcium hydride, distilled, and stored
over 4 Å molecular sieves. 1H, 13C{1H}, and 31P{1H} NMR
spectra were acquired on a Varian Mercury 300 MHz, a
Bruker 400 MHz, an Inova 500 MHz, or a Varian Agilent 600
MHz spectrometer. 2D NMR spectra were obtained on a
Bruker 400 MHz, an Inova 500 MHz or a Varian Agilent 600
MHz spectrometer. Chemical shifts are reported in δ (ppm).
For 1H and 13C NMR spectra, the residual solvent peaks were
used as an internal reference standard, while 31P{1H} spec-
tra were referenced to an 85% phosphoric acid external
standard (0 ppm). Data are reported as follows: chemical
shift, multiplicity (s = singlet, d = doublet, t = triplet, q =
quartet, dd = doublet of doublets, m = multiplet, b = broad)
and integration. ESI-MS analysis were performed by Manasi
Kamat, Subhradeep Bhar, and Atiye Ahmadi at Mass Spec-
trometry Research and Education Center, the Chemistry De-
partment. Elemental analyses were performed at the
CENTC Elemental Analysis facility at University of Roches-
ter, NY. The following materials were purchased and used
The bond angle decrease between the N-Au-P atoms is an-
other structural change to consider. Typically, aurophilic
bonding between two linear Au(I) complexes occurs with
the linear bonding axes of the individual metal centers be-
ing orthogonal with respect to one another.2,48 The struc-
ture of the compounds studied here precludes such an in-
teraction without excessive distortion. Unable to approach
orthogonally the complex distorts through the N-Au-P angle
to accommodate. This appears to be an inherent property of
the Au(I) metal center in the LMCT state, since the single
Au(I) ion complex Au1-Th also exhibits the distortion. Ulti-
mately, changing from linear to a trigonal planar geometry
allows for interaction with the other Au(I) metal center in
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as received: chlorotriphenylphosphinegold(I) (PPh3AuCl) 8.3 Hz, 2H, C5-H), 7.18-7.21 (m, 12H, C10-H and C14-H), 7.33-
1
2
3
4
5
6
7
8
(Acros), 2-ethynylthiophene (Accela), 4-ethynyl-N,N-dime-
thylaniline (Sigma-Aldrich), 2,2-bithiophene (Oakwood
Chemical), sodium azide (NaN3) (Sigma-Aldrich). The fol-
lowing were prepared by literature methods: PPh3AuN3,50
PPh3AuC≡CPh,51 PPh3AuC≡CC6H4NO2,52 Br(C4H2S)(C4H3S),53
PPh3AuC≡C(C4H2S)(C4H3S),54 (Au2-Ph),37 (Au2-PhNO2).38
7.35 (m, 6H, C11-H and C15-H), 7.43-7.47 (m, 12H, C9-H and
C13-H), 8.24 (d, 3JHH = 8.2 Hz, 2H, C4-H). 1C{1H} NMR (indirect
detection through 1H-13C gHMBC and 1H-13C gHSQC (126
MHz, CDCl3)): δ 41.0 (C7), 112.8 (C5), 126.1 (C3), 127.3 (C4),
129.0 (d, 3JCP = 11 Hz, C10 and C14), 131.4 (C11 and C15), 134.2
2
(d, J CP = 14 Hz, C9 and C13), 136.5 (C8 and C12), 149.2 (C6),
151.7 (C2). 31P{1H} NMR (121 MHz, CDCl3): δ 44.61 (s, P1),
31.17 (s, P2). ESI-MS: m/z calculated for C46H40Au2N4P2
[M+H]+ 1105.2138, found 1105.2104.
General procedure for the synthesis of tri-
phenylphoshinegold(I)
acetylides
(PPh3AuC≡CTh,
PPh3AuC≡CPhNMe2). H-C≡C-R (R = Th, PhNMe2) (0.5
mmol), PPh3Au-Cl (0.5 mmol), and K2CO3 (1.5 mmol) were
added into a solution mixture of 4 mL THF and 1 mL MeOH.
The solution was stirred in the dark for 24 h. Water was
added and the compound was extracted with chloroform,
then dried over anhydrous MgSO4. Crystals were grown
through pentane diffusion into a methylene chloride solu-
tion of gold acetylide at -25 oC.
9
Au2-biTh. 95.3% Yield (0.089 mmol, 102 mg). 1H-NMR
(600MHz, CDCl3), δ (ppm): 6.91 (dd, 3JHH = 3.5 Hz, 4JHH = 1.0
Hz, 1H, C8-H), 6.95 (dd, 3JHH = 5.0 Hz, 3JHH = 3.6 Hz, 1H, C9-H),
7.09 (d, 3JHH = 3.7 Hz, 1H, C5-H), 7.12 (d, 3JHH = 4.8 Hz, 1H, C10-
H), 7.24-7.38 (m, 12H, C13-H and C17-H), 7.40-7.49 (m, 6H,
C14-H and C18-H), 7.48-7.60 (m, 12H, C12-H and C16-H), 7.61
(d, 3JHH = 3.7 Hz, 1H, C4-H).13C{1H} NMR (indirect detection
through 1H-13C gHMBC and 1H-13C gHSQC) (600 MHz, CDCl3),
δ (ppm): 121.78 (C4), 122.42 (C8), 122.94 (C10), 124.25 (C5),
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General Procedure for the synthesis of digold-triazolate
(Au2-Th, Au2-PhNMe2, Au2-biTh). PPh3Au-C≡C-R (R = Th,
PhNMe2, biTh) (0.05 mmol), and PPh3Au-N3 (0.05 mmol)
were dissolved into 2 mL of CDCl3. The solution was added
to a J. Young tube and the product formation was monitored
via 1H and 31P NMR spectroscopy. The reaction was stopped
after 1 h and the solvent was removed in vacuo. Crystals
were grown through pentane diffusion into a methylene
chloride solution of the triazolate at -25 oC.
1
127.41 (C9), 128.48 (d, JCP = 12 Hz, C11 and C15), 129.12 (d,
3JCP = 11 Hz, C13 and C17), 131.56 (m, C14 and C18), 132.07 (C6),
134.25 (d, 2JCP = 14 Hz, C12 and C16), 138.80 (C7), 145.56 (C2).
31P{1H} NMR (242.9 MHz, CDCl3), δ (ppm): 44.58 (s, P1),
31.18 (s, P2). ESI-MS: m/z calculated for C46H35Au2N3P2S2
[M+H]+ 1150.1157, found 1150.1122.
4,5-Au1-Th. H-C≡C-Th (0.05 mmol) and PPh3Au-N3 (0.05
mmol) were added into 2 mL of DCM. The solution was
stirred in the dark for 5 h then solvents was removed in
vacuo. Crystals were grown through pentane diffusion into
a methylene chloride solution of the triazolate at -25 oC. 91.1%
Yield (0.046 mmol, 27.77 mg). 1H NMR (400 MHz, CDCl3): δ
7.00 (dd, 3JHH = 3.5, 5.0 Hz, 1H, C5-H), 7.06 (dd, 3JHH = 5.0 Hz,
4JHH = 1.2 Hz, 1H, C4-H), 7.74 (dd, 3JHH = 3.6 Hz, 4JHH = 1.2 Hz,
1H, C6-H), 7.49-7.63 (m, 15H, C8-H, C9-H and C10-H). 13C{1H}
1
PPh3AuC≡CTh. 74.4 % Yield (0.37 mmol, 209.6 mg). H
NMR (500 MHz, CDCl3): δ 6.89 (dd, 3JHH = 3.5, 5.1 Hz, 1H, C5-
H), 7.08 (dd, 3J HH = 5.2 Hz, 4J HH = 1.2 Hz, 1H, C4-H), 7.15 (dd,
3
3J HH = 3.7 Hz, J HH = 1.3 Hz, 1H, C6-H), 7.43-7.56 (m, 15H,
1
aromatic). 13C{1H} NMR (indirect detection through H-13C
1
gHMBC and H-13C gHSQC (126 MHz, CDCl3)): δ 125.1 (C4),
125.2 (C3), 126.5 (C5), 129.1 (d, 4J CP = 11 Hz, C10), 129.6 (d,
1J CP = 56 Hz, C7), 131.1 (C6), 131.5 (d, 3J CP = 3 Hz, C9), 134.3
(d, 2JCP = 14 Hz, C8). 31P{1H} NMR (121 MHz, CDCl3): δ 42.52.
ESI-MS: m/z calculated for C24H18AuPS [M+Na]+ 589.0430,
1
1
NMR (indirect detection through H-13C gHMBC and H-13C
gHSQC (101 MHz, CDCl3)): δ 122.1 (C4), 122.6 (C6), 127.4
3
1
(C5), 129.3 (d, J CP = 11 Hz, C9), 129.7 (d, J CP = 55 Hz, C7),
found 589.0408. Anal. Calcd. for C24H18AuPS: C, 50.89; H,
3.20. Found: C, 50.60; H, 3.11; N, -0.23.
4
2
131.7 (d, J CP = 2 Hz, C10), 134.3 (d, J CP = 13 Hz, C8), 137.8
(C8), 148.1 (C9). 31P{1H} NMR (161 MHz, CDCl3): δ 43.50. ESI-
MS: m/z calculated for C24H19AuN3PS [M+H]+ 610.0781,
found 610.0762.
PPh3AuC≡CPhNMe2. 62.3 % Yield (0.31 mmol, 187.9 mg).
1H NMR (500 MHz, CDCl3): δ 2.91 (s, 6H, C7-H), 6.57 (d, 3J HH
= 8.4 Hz, 2H, C5-H), 7.35-7.55 (m, 17H, C4-H and aromatic).
13C{1H} NMR (indirect detection through 1H-13C gHMBC and
1H-13C gHSQC (126 MHz, CDCl3)): δ 40.3 (C7), 111.8 (C5),
112.1 (C3), 129.1 (d, 4JCP = 11 Hz, C11), 130.0 (d, 1JCP = 55 Hz,
C8), 131.4 (d, 3J CP = 2 Hz, C10), 133.4 (C4), 134.3 (d, 2J CP = 14
Hz, C9), 149.2 (C6). 31P{1H} NMR (121 MHz, CDCl3): δ 42.52.
ESI-MS: m/z calculated for C28H25AuNP [M+H]+ 604.1468,
found 604.1438.
Au2-Th. 93.8% Yield (0.047 mmol, 50.1 mg). 1H NMR (500
MHz, CDCl3): δ 6.85-6.90 (m, 2H, C5-H and C6-H), 7.12-7.21
(m, 12H, C9-H and C13-H), 7.28-7.30 (m, 6H, C10-H and C14-H),
7.38-7.43 (m, 12H, C8-H and C12-H), 7.60-7.62 (m, 1H, C4-H).
13C{1H} NMR (indirect detection through 1H-13C gHMBC and
1H-13C gHSQC (126 MHz, CDCl3)): δ 120.7 (C4), 121.1 (C5),
Computational Methods. All calculations were done in
Gaussian 16 (Revision B.01) on a home-built 64-core local
machine.45 For all methods, a 6-31G(d) basis set was used
for C, H, N, and O; a 6-31+G(d) basis set was used for P; and
the SDD basis set was used for Au as it is employed in the
software package.55 This corresponds to the Stuttgart-
Dressden pseudopotential, ECP60MWB, which uses an in-
termediate treatment of relativistic effects for the effective
core potential.56 This way relativistic effects are derived
from the basis set, and not from the more computationally
expensive Dirac equation. For all compounds, triphenyl
phosphine ligands were reduced to trimethyl phosphines to
reduce computational costs. Validation of this simplification
can be found in the SI. A total of 50 vertical excitations were
used to generate the calculated absorption spectra, whereas
excited state geometry optimizations used only a single ex-
citation. In all cases, symmetry constraints were turned off,
an ultrafine integration grid was used, and calculations
were done in the gas phase. NBO analysis was done using
the Multiwfn software package.57
127.1 (C6), 129.1 (d, 3JCP = 11 Hz, C9 and C13), 131.5 (d, 4JCP
=
12 Hz, C10 and C14), 134.3 (d, 2J CP = 14 Hz, C8 and C12), 140.2
(C3). 31P{1H} NMR (121 MHz, CDCl3): δ 44.59 (s, P1), 31.50
(s, P2). ESI-MS: m/z calculated for C42H33Au2N3P2S [M+H]+
1068.1280, found 1068.1252.
Au2-PhNMe2. 92.5 % Yield (0.046 mmol, 51.1 mg). 1H
3
NMR (500 MHz, CDCl3): δ 2.89 (s, 6H, C7-H), 6.69 (d, J HH
=
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Monograph 16, Edited by R. J. H. Clark. Elsevier,
Amsterdam, Oxford, New York, 1978, 274 p. $49.75.” Synth.
React. Inorg. Met. Chem. 1978, 8, 503–504.
Spectroscopy. Samples for spectroscopic analysis were
prepared in an inert atmosphere glovebox and kept isolated
from light. A 1 cm path length quartz cell was used for these
measurements with THF that was dried and purified by an
MBraun MB-SPS-800 solvent purification system. Steady-
state UV-Vis absorption spectra were recorded on Shi-
madzu UV-1800 dual-beam spectrophotometer and emis-
sion spectra were collected with an Edinburgh FLS1000
photoluminescence spectrometer where an optical density
of 0.6 was used at the excitation wavelength. Low tempera-
ture measurements were made with the aid of liquid N2
poured into a dewar with glass windows.
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(8) Scherbaum, F.; Grohmann, A.; Huber, B.; Krüger, C.;
Schmidbaur, H. “Aurophilicity” as a Consequence of
Relativistic
Effects:
The
Hexakis
(Triphenyl
phosphaneaurio)Methane Dication [(Ph3PAu)6C]2+. Angew.
Chemie Int. Ed. 1988, 27, 1544–1546.
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Bonding Relationships: The Conformation-Determining
Attractive Interaction between Gold(I) Centers. Angew.
Chemie Int. Ed. 1988, 27, 417–419.
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ASSOCIATED CONTENT
Synthetic details, NMR spectra, photophysical characterization
data, computational details. This material is available free of
charge via the Internet at http://pubs.acs.org.
(10) Runeberg, N.; Schütz, M.; Werner, H.-J. The Aurophilic
Attraction as Interpreted by Local Correlation Methods. J.
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AUTHOR INFORMATION
Corresponding Author(s)
Chem. Rev. 1988, 88, 563–594.
(12) Pyykkö, P. Relativistic Effects on Periodic Trends BT
-
* veige@chem.ufl.edu
* kirk.schanze@utsa.edu
The Effects of Relativity in Atoms, Molecules, and the
Solid State; Wilson, S., Grant, I. P., Gyorffy, B. L., Eds.;
Springer US: Boston, MA, 1991; pp 1–13.
Author Contributions
All authors contributed to the writing of this manuscript. All
authors have given approval to the final version of the manu-
script.
(13) Pyykkö, P. Theoretical Chemistry of Gold. III. Chem. Soc.
Rev. 2008, 37, 1967–1997.
(14) Schwerdtfeger, P. Relativistic Effects in Properties of Gold.
Funding Sources
Heteroat. Chem. 2002, 13, 578–584.
Research supported by the U.S. Department of Energy, Office of
Basic Energy Sciences, Division of Materials Sciences and Engi-
neering under Award DE-SC0020008. The UF Mass Spectrom-
etry Research and Education Center (NIH S10 OD021758-01A1)
are thanked for acquisition of mass spectrometry data. K.A.A
acknowledges the NSF (CHE-0821346) for the purchase of X-
ray equipment.
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Bonding Relationships in a d10-d10 System. J. Am. Chem.
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Interactions. Bonding Relationships in d10-d10 Systems.
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