Gold(I) pyrenyls: Excited-state consequences of carbon-gold bond formation

Article abstract of DOI:10.1021/om700346v

Two 1-pyrenyl derivatives of gold(I) have been prepared by base-promoted transmetalation of 1-pyreneboronic acid. Triplet-state luminescence observed from the new complexes at 77 K is assigned to a pyrene π-π* state.

Full text of DOI:10.1021/om700346v

Organometallics 2007, 26, 3279-3282
3279
Gold(I) Pyrenyls: Excited-State Consequences of Carbon-Gold
Bond Formation
David V. Partyka,† Arthur J. Esswein,‡ Matthias Zeller,§ Allen D. Hunter,§ and
Thomas G. Gray*,†
Departments of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106,
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Youngstown State
UniVersity, 1 UniVersity Plaza, Youngstown, Ohio 44555
ReceiVed April 9, 2007
Scheme 1
Summary: Two 1-pyrenyl deriVatiVes of gold(I) haVe been
prepared by base-promoted transmetalation of 1-pyreneboronic
acid. Triplet-state luminescence obserVed from the new com-
plexes at 77 K is assigned to a pyrene π-π* state.
The design of photoactive compounds having both predefined
optical absorption and triplet-state luminescence is an imperative
in organic materials chemistry. Applications are varied and
include light-emitting diode fabrication, sensor technology, in
vivo photochemistry, and photodynamic therapy.1-6 A staple
of phosphorescence emitter design is the spin-orbit coupling
brought about by heavy-atom substituents.7 A recent report8
describes a general protocol for aromatic carbon-gold bond
formation from arylboronic acid precursors. The reaction
tolerates polar and reducible functionalities that recur in organic
fluorophores. Steric encumbrance, heterocyclic substrates, and
ring fusion are all accommodated. The carbon-gold bond is
minimally polar and nonchromophoric in the visible region.
Here, gold is intended as a relativistic spectator.9-13 Spin-orbit
coupling associated with the heavy-atom nucleus intermingles
singlet and triplet excited states. Optical access to the triplet
manifold of organic chromophores can thereby result. Bench-
mark properties associated with triplet excited states include
microsecond-scale or longer excited-state lifetimes and triplet-
state photosensitization. Presented here are syntheses and optical
characterization of gold-substituted 1-pyrenyls prepared by the
new method. Photophysical consequences of auration are
directly interrogated.
Commercially available 1-pyreneboronic acid was reacted
with Cy3PAuBr14 (Cy ) cyclohexyl) in isopropyl alcohol in
the presence of Cs2CO3 to afford Cy3PAu(1-pyrenyl) (1) as an
air-stable, yellow-brown solid in 80% isolated yield after
crystallization (eq 1 of Scheme 1).15 An analogous reaction with
the N-heterocyclic carbene complex IPrAuBr16 afforded the
corresponding 1-pyrenyl derivative 2 in 75% isolated yield (eq
2 of Scheme 1). Full experimental details appear in the
* To whom correspondence should be addressed. E-mail: tgray@case.edu.
† Case Western Reserve University.
‡ Massachusetts Institute of Technology.
§ Youngstown State University.
(1) Prasad, P. N. Introduction to Biophotonics; Wiley-Interscience: New
York, 2003; pp 433-463.
(14) Schneider, D.; Schier, A.; Schmidbaur, H. Dalton Trans. 2004,
1995-2005.
(2) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Chem. ReV. 2007,
107, 1233-1271.
(15) Synthesis of 1: isopropyl alcohol (5 mL) was added to a flask
charged with Cs2CO3 (113 mg, 0.35 mmol) and 1-pyreneboronic acid (84
mg, 0.34 mmol). To this suspension was added [(PCy3)AuBr] (95 mg, 0.17
mmol), and the mixture was stirred at 50 °C for 24 h. After it was cooled,
the mixture was stripped of solvent in vacuo and extracted into benzene,
and the extract was filtered through Celite and taken to dryness by rotary
evaporation. The solid was triturated with pentane (to remove traces of
isopropyl alcohol), the residual of which was removed by rotary evaporation.
The solid was again extracted into benzene, and the extract was filtered
through Celite and concentrated to saturation. Vapor diffusion of pentane
caused separation of yellow-brown crystals. The supernatant was decanted,
and the recrystallization was repeated. The light yellow crystals were
(3) Khalil, G. E.; Chang, A.; Gouterman, M.; Callis, J. B.; Dalton, L.
R.; Turro, N. J.; Jokusch, S. ReV. Sci. Instrum. 2005, 76, 054101.
(4) Brin˜as, R. P.; Troxler, T.; Hochstrasser, R. M.; Vinogradnov, S. A.
J. Am. Chem. Soc. 2005, 127, 11851-11862.
(5) Gao, R.; Ho, D. G.; Hernandez, B.; Selke, M.; Murphy, D.; Djurovich,
P. I.; Thompson, M. E. J. Am. Chem. Soc. 2002, 124, 14828-14829.
(6) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher, W. M.;
O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10619-10631.
(7) Lower, S. K.; El-Sayed, M. A. Chem. ReV. 1966, 66, 199-241.
(8) Partyka, D. V.; Zeller, M.; Hunter, A. D.; Gray, T. G. Angew. Chem.,
Int. Ed. 2006, 45, 8188-8191.
(9) Ferna´ndez, E. J.; Laguna, A.; Olmos, M. E. AdV. Organomet. Chem.
2005, 52, 77-142.
(10) Schmidbaur, H. In Gmelin Handbuch der Anorganischen Chemie,
8th ed.; Slawisch, A., Ed.; Springer-Verlag: Berlin, 1980.
(11) Schmidbaur, H.; Grohmann, A.; Olmos, M. E. In Gold: Progress
in Chemistry, Biochemistry and Technology; Schmidbaur, H., Ed.; Wiley:
Chichester, U.K., 1999.
1
collected and dried. Yield: 92 mg (80%). H NMR (100 MHz; C6D6): δ
9.32 (d, 1H, 1-pyrenyl), 8.78 (q, 1H, 1-pyrenyl), 8.25 (d, 1H, 1-pyrenyl),
8.16 (d, 1H, 1-pyrenyl), 7.97-8.01 (m, 3H, 1-pyrenyl), 7.77-7.86 (m, 2H,
1-pyrenyl), 0.97-1.98 (m, 33H, C6H11). 31P NMR (162 MHz; C6D6): δ
58.0 ppm. UV-vis (THF, 10-6 M): λ, nm (ꢀ, M-1 cm-1) 281 (48 900),
310 (sh, 8490), 324 (18 100), 338 (38 800), 354 (53 100), 378 (5200). Anal.
Calcd for C34H42AuP: C, 60.17; H, 6.24. Found: C, 60.14; H, 6.42.
(16) de Fre´mont, P.; Singh, R.; Stevens, E. D.; Petersen, J. L.; Nolan, S.
P. Organometallics 2007, 26, 1376-1385.
(12) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412-4456.
(13) Pyykko¨, P. Inorg. Chim. Acta 2005, 358, 4113-4130.
10.1021/om700346v CCC: $37.00 © 2007 American Chemical Society
Publication on Web 06/08/2007

3280 Organometallics, Vol. 26, No. 14, 2007
Communications
Figure 2. Emission spectra (77 K, red; 298 K, black; 355 nm
excitation) of (Cy3P)Au(1-pyrenyl) (1; 5 × 10-6 M) in degassed
2-MeTHF. Inset: emission of 1 at 5 × 10-8 M under otherwise
identical conditions. No luminescence is observed past 550 nm at
this concentration. Note that y axes of the two spectra have different
scales. An asterisk indicates the excitation pulse second harmonic.
Figure 1. Crystal structure of 2 (50% probability, 100 K). Selected
bond lengths (Å) and angle (deg): Au-C1, 2.000(6); Au-C2,
2.005(7); C1-Au-C2, 172.7(2).
Supporting Information. The new compounds have been char-
acterized by elemental analysis, optical absorption and emission,
multinuclear NMR, X-ray diffraction crystallography,17 and
combustion analysis.
At increased concentration (5 × 10-6 M), the same lumines-
cence occurs between 400 and 500 nm. However, structured
emission appears at 77 K, beginning near 600 nm and extending
to 800 nm. A vibronic progression is clearly resolved between
Diffraction-quality crystals of 1 and 2 were grown by vapor
diffusion of pentane into concentrated benzene solutions at room
temperature. A thermal ellipsoid projection of 2 appears in
Figure 1. The crystal structure of 1 is provided as Supporting
Information, along with full crystallographic data for both
compounds. Metric parameters in the new structures are
unexceptional. In both, gold adopts a nearly linear two-
coordinate geometry. In neither structure are aurophilic inter-
actions evident: the closest approach of neighboring gold atoms
in 2 is 7.01 Å; that of 1 is 8.16 Å. Neither structure exhibits
pyrene π-stacking.
600 and 700 nm, with an average peak spacing of 404 cm-1
.
Structured luminescence at 77 K extends to at least 800 nm,
albeit with lesser resolution. This structured luminescence is
quenched at room temperature. The 77 K emission lifetime of
the structured emission collected at 603 nm is 1.09 ( 0.01 ms.
Luminescence behavior of the N-heterocyclic carbene variant
2 is closely analogous. Here also, concentration-dependent
structured emission occurs at 77 K with a 1.08 ( 0.01 ms
lifetime. Emission spectra of 2 appear as Supporting Informa-
tion. Under identical conditions, pyrene is nonemissive at
wavelengths exceeding 600 nm.
Compounds 1 and 2 absorb light of wavelengths below 400
nm. Their absorption spectra are similar to that of pyrene and
are included as Supporting Information. Room-temperature
excitation at 355 nm in 2-methyltetrahydrofuran (2-MeTHF)
elicits weak luminescence between 400 and 500 nm (Figure
2). Cooling to 77 K leads to concentration-dependent emission.
At 5 × 10-8 M concentration (Figure 2, inset), the emission
from 400 to 500 nm is the more intense of the two observed
luminescence profiles. Structured emission is evident at low
Static and time-resolved density-functional theory calculations
have been performed18,19 on Me3PAu(1-pyrenyl) (1′), a truncated
analogue of 1. The geometry was optimized in the gas phase
with the TZVP basis set of Godbout and co-workers.20
A
harmonic frequency calculation confirmed the resulting structure
to be an energy minimum. Calculated (observed for 1) metrics
are as follows: Au-C ) 2.071 Å (2.073(8) Å), Au-P ) 2.351
Å (2.3115(19) Å), and C-Au-P ) 179.13° (174.0(2)°).
Implicit THF solvation was included in single-point calculations
temperatures, with a mean peak-to-peak separation of 971 cm-1
.
No higher wavelength emission is apparent at this concentration.
(18) Full details appear in the Supporting Information.
(19) Calculations employed the Gaussian03 suite of programs: Frisch,
M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant,
J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.;
Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada,
M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
revision D.01; Gaussian, Inc.: Wallingford, CT, 2004.
(17) (a) Crystallographic data for 1: colorless rods, crystal dimensions
0.40 × 0.13 × 0.12 mm3, space group I4h, a ) 24.9180(15) Å, b ) 24.9180-
(15) Å, c ) 9.2140(11) Å, V ) 5721.0(8) Å3, Z ) 8, Fcalcd ) 1.576 Mg/m3,
µ(Mo KR) ) 5.220 mm-1, data measured on a Bruker AXS SMART APEX
CCD-based diffractometer (Mo KR, λ ) 0.710 73 Å) at 100(2) K, structure
solved by direct methods, 29 261 reflections collected, 7109 unique
reflections (Rint ) 0.0472), 7109/127/375 data/restraints/parameters, final
R indices (I > 2σ(I)) R1 ) 0.0468 and wR2 ) 0.1204, R indices (all data)
R1 ) 0.0500 and wR2 ) 0.1240, largest difference peak and hole +6.089
and -1.243 e Å-3. (b) Crystallographic data for 2: colorless block, crystal
dimensions 0.39 × 0.32 × 0.20 mm3, space group P21/c, a ) 12.2122(13)
Å, b ) 12.1108(13) Å, c ) 23.060(3) Å, â ) 90.024(2)°, V ) 3410.6(6)
Å3, Z ) 4, Fcalcd ) 1.532 Mg/m3, µ(Mo KR) ) 4.347 mm-1, data measured
on a Bruker AXS SMART APEX CCD-based diffractometer (Mo KR, λ
) 0.710 73 Å) at 100(2) K, structure solved by direct methods, 28 480
reflections collected, 8437 unique reflections (Rint ) 0.0398), 8437/0/423
data/restraints/parameters, final R indices (I > 2σ(I)) R1 ) 0.0522 and wR2
) 0.1273, R indices (all data) R1 ) 0.0595 and wR2 ) 0.1310, largest
(20) Godbout, N.; Salahub, D.; Andzelm, J.; Wimmer, E. Can. J. Chem.
1992, 70, 560-571.
difference peak and hole +7.679 and -3.385 e Å-3
.

Communications
Organometallics, Vol. 26, No. 14, 2007 3281
Figure 3. (left) Kohn-Sham orbital energy-level diagram of (Me3P)Au(1-pyrenyl) (1′) (mpwpw91/Stuttgart ECP and basis on Au). Implicit
THF solvation is included through a polarizable continuum model. Percentage contributions of (phosphine)gold(I) cation and pyrenyl anion
fragments are indicated. (right) Plots of selected orbitals (contour level 0.03 au).
Scheme 2
through the polarizable continuum model of Tomasi and co-
workers.21,22 Figure 3 depicts a partial Kohn-Sham orbital
energy level diagram; plots of selected orbitals are given on
the right-hand side of the figure. The highest occupied Kohn-
Sham orbital and the lowest unoccupied Kohn-Sham orbital
(HOMO and LUMO, respectively) are pyrene π-orbitals, as are
the HOMO-1 and LUMO+1 (not shown). The HOMO-2
primarily accounts for the carbon-gold σ-bond. This orbital
has 51.9% gold, 9.0% phosphorus, and 32.0% pyrenyl character,
as apportioned by Mulliken analysis.23 Energetically this
σ-orbital lies 1.01 eV below the HOMO of 1′. Time-dependent
density functional calculations of 1′ (in the singlet geometry)
indicate that the lowest four triplet states have pyrene π-π*
character; all transform as A′ in Cs symmetry. The two lowest
lying singlet excited states are also π-π* in nature. The
calculations suggest that the intense absorption (ꢀ ) 52 000 M-1
cm-1) at 336 nm is attributable to an optically allowed transition
that is a combination of LUMO r HOMO and LUMO+1 r
HOMO-1 one-electron excitations.
2) leads promptly to a singlet excited state that decays to an
excimer. The excimer lies at higher energy than the triplet, to
which it decays. The excimer effectively pumps the triplet.
Higher temperatures suppress excimer formation. Alternatively,
nonradiative decay processes at higher temperature may subvert
triplet-state emission. This hypothesis qualitatively reconciles
the observed temperature- and concentration-dependent emission
of aurated hydrocarbons.8
Excited-state consequences of metalation with gold have been
evaluated for two 1-pyrenyl complexes. The absorption profile
of pyrene is substantially retained. Triplet-state photophysical
properties emerge at low temperature upon replacement of a
single terminal hydrogen with (phosphine)- or (carbene)gold-
(I) fragments. A structured luminescence of millisecond duration
occurs in a 2-MeTHF glass at 77 K; no comparable emission is
observed for pyrene under identical conditions. Density func-
tional calculations indicate an emitting triplet state of pyrenyl
π-π* parentage. Ongoing efforts seek to enhance intersystem
crossing by attaching multiple gold(I) fragments to the pyrene
core and to alter the luminescence temperature dependence by
varying the ancillary phosphine or N-heterocyclic carbene
ligand.
The structured luminescence encountered at low temperature
indicates triplet-state parentage, as does its millisecond-length
lifetime. The concentration dependence of the structured emis-
sion indicates oligomer formation. However, aurophilic24-27 and
excimer28,29 emissions are typically structureless. Scheme 2
depicts a plausible excited-state hypothesis. Excitation of 1 (or
(21) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117-
129.
(22) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. ReV. 2005, 105, 2999-
3093.
(23) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833-1840.
(24) Schmidbaur, H. Gold Bull. 2000, 33, 3-10.
(25) Balch, A. L. Gold Bull. 2004, 37, 45-50.
(26) Abdou, H.; Mohamed, A. A.; Fackler, J. P., Jr. Inorg. Chem. 2007,
46, 141-146.
(27) Elbjeirami, O.; Yockel, S.; Campana, C. F.; Wilson, A. K.; Omary,
M. A. Organometallics 2007, 26, 2250-2260.
(28) Gijzeman, O. L. J.; Lanelaar, J.; van Voorst, J. D. W. Chem. Phys.
Lett. 1970, 5, 269-272.
(29) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausalito, CA, 1991.
Acknowledgment. We thank Case Western Reserve Uni-
versity and the donors of the Petroleum Research Fund,
administered by the American Chemical Society (Grant 42312-
G3 to T.G.G.), for support. The diffractometer at YSU was
funded by NSF Grant 0087210, by the Ohio Board of Regents

3282 Organometallics, Vol. 26, No. 14, 2007
Communications
emission spectrum of 1 at 5 × 10-6 M and 5 × 10-8 M in
2-MeTHF, crystallographic data, and optimized Cartesian coordi-
nates of 1′ and CIF files giving crystal data for 1 and 2. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Grant CAP-491, and by Youngstown State University. We thank
J. Updegraff (CWRU) for experimental assistance. T.G.G.
thanks Professor D. G. Nocera (Massachusetts Institute of
Technology) for access to instrumentation and for stimulating
discussions and J. B. Updegraff (CWRU) for assistance.
Supporting Information Available: Text, figures, and tables
giving experimental and computational protocols, details of the
OM700346V