Influence of a gold(I)-acetylide subunit on the photophysics of Re(phen)(CO)3Cl

Article abstract of DOI:10.1021/ic048376r

The synthesis and photophysical properties of two new Re(l) complexes are reported: fac-Re(phenC≡CH)(CO)₃Cl (where phenC≡CH is 5-ethynyl-1,10-phenanthroline) and its Au(l)-acetylide analogue (fac-Re(phenC≡CAuPPh₃)(CO)₃-Cl). Also reported are the photophysical measurements obtained for the benchmark fac-Re(phen)(CO) ₃Cl chromophore, as well as the phenC≡CAuPPh₃ and phenC≡CH ligands. The unstable nature of the precursor gold-containing ligand illustrates the advantage of using the chemistry on the complex approach, which facilitated preparation of the Re-Au binuclear complex. Where possible, all compounds were studied by static and transient absorption (TA), as well as steady-state and time-resolved photoluminescence (TRPL), at room temperature (RT) and 77 K, as well as nanosecond time-resolved infrared (TRIR) spectroscopy. The spectroscopic information provided by these techniques enabled a thorough evaluation of excited-state decay in most cases. In fac-Re(phenC≡CH)-(CO)₃Cl, the RT excited-state decay is most consistent with a metal-to-ligand charge transfer (MLCT) assignment, whereas at 77 K, the lowest excited state is dominated by the triplet intraligand ( ³IL) state, localized within the diimine ligand. The lowest excited state in fac-Re(phenC≡CAuPPh₃)(CO)₃Cl seems to result from an admixture of Re-based MLCT and ³IL states resident on the phenC≡CAuPPh₃ moiety. TA and TRIR methods indicate that these excited states are thermally equilibrated at room temperature. At 77 K, the MLCT energy of fac-Re(phen-C≡CAuPPh₃)(CO)₃Cl is increased as a result of the glassy medium and the resulting excited state can be considered to be ligand-localized.

Full text of DOI:10.1021/ic048376r

Inorg. Chem. 2005, 44, 3412−3421
Influence of a Gold(I)
Re(Phen)(CO)3Cl
−Acetylide Subunit on the Photophysics of
Irina E. Pomestchenko, Dmitry E. Polyansky, and Felix N. Castellano*
Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403
Received November 17, 2004
The synthesis and photophysical properties of two new Re(I) complexes are reported: fac-Re(phenC
(where phenCtCH is 5-ethynyl-1,10-phenanthroline) and its Au(I)-acetylide analogue (fac-Re(phenCt
t
CH)(CO)3Cl
CAuPPh3)(CO)3-
Cl). Also reported are the photophysical measurements obtained for the benchmark fac-Re(phen)(CO)3Cl chromophore,
as well as the phenC CAuPPh3 and phenC CH ligands. The unstable nature of the precursor gold-containing
ligand illustrates the advantage of using the “chemistry on the complex” approach, which facilitated preparation of
the Re Au binuclear complex. Where possible, all compounds were studied by static and transient absorption
t
t
−
(TA), as well as steady-state and time-resolved photoluminescence (TRPL), at room temperature (RT) and 77 K,
as well as nanosecond time-resolved infrared (TRIR) spectroscopy. The spectroscopic information provided by
these techniques enabled a thorough evaluation of excited-state decay in most cases. In fac-Re(phenCtCH)-
(CO)3Cl, the RT excited-state decay is most consistent with a metal-to-ligand charge transfer (MLCT) assignment,
whereas at 77 K, the lowest excited state is dominated by the triplet intraligand (3IL) state, localized within the
diimine ligand. The lowest excited state in fac-Re(phenC
t
CAuPPh3)(CO)3Cl seems to result from an admixture of
CAuPPh3 moiety. TA and TRIR methods indicate that
Re-based MLCT and 3IL states resident on the phenC
t
these excited states are thermally equilibrated at room temperature. At 77 K, the MLCT energy of fac-Re(phen-
CAuPPh3)(CO)3Cl is increased as a result of the glassy medium and the resulting excited state can be considered
to be ligand-localized.
Ct
Introduction
new class of π-conjugated Re(I) chromophores where various
arylethynylenes are appended to the 5,5′-ends of 2,2-
bipyridine.25-28 These metal-organic oligomers possess pho-
tophysical properties that are not as predictable as their
Since the first report by Wrighton and Morse in 1974,1
the photophysics and photochemistry of complexes of the
general formula Re(L-L)(CO)3Cl (where L-L is a diimine
ligand) have been widely developed. Particularly noteworthy
are the contributions from Meyer’s group, which outlined
the systematic structure-photophysical property relationships
as a function of either nonchromophoric (replacement of Cl)
or chromophoric (diimine) ligand substitutions.2,3 Of course,
many investigators have used Re(I) complexes in a variety
of fundamental and applied research projects, because their
photophysical properties are largely predictable prior to
synthesis.2-24 Recently, Schanze’s group has developed a
(4) Luong, J. C.; Nadjo, L.; Wrighton, M. S. J. Am. Chem. Soc. 1978,
100, 5790-5795.
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2897.
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1979, 101, 7415-7417.
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1067-1069.
(8) Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K. S.; Anthon,
D.; Neveux, P. E., Jr.; Meyer, T. J. Inorg. Chem. 1987, 26, 1116-
1126.
(9) Mabrouk, P. A.; Wrighton, M. S. Inorg. Chem. 1986, 25, 526-531.
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2415.
* To whom correspondence should be addressed. Tel.: (419) 372-7513.
Fax: (419) 372-9809. E-mail: castell@bgnet.bgsu.edu.
(1) Wrighton, M. S.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998-
1003.
(2) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952-957.
(3) Worl, L. A.; Duesing, R.; Chen, P.; Della Ciana, L.; Meyer, T. J. J.
Chem. Soc., Dalton Trans. 1991, 849-858.
(11) Juris, A.; Campagna, S.; Bidd, I.; Lehn, J.-M.; Ziessel, R. Inorg. Chem.
1988, 27, 4007-4011.
(12) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.;
DeGraff, B. A. Inorg. Chem. 1990, 29, 4335-4340.
(13) Perkins, T. A.; Hauser, B. T.; Eyler, J. R.; Schanze, K. S. J. Phys.
Chem. 1990, 94, 8745-8748.
(14) Shaw, J. R.; Schmehl, R. H. J. Am. Chem. Soc. 1991, 113, 389-394.
3412 Inorganic Chemistry, Vol. 44, No. 10, 2005
10.1021/ic048376r CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/12/2005

Influence of Au(I)-Acetylide Subunit on Re(Phen)(CO)3Cl
structurally simpler predecessors, yielding room-temperature
(RT) excited-state decay that can best be described as a
composite of metal-to-ligand charge transfer (3MLCT) and
Au-CtC-fragment serves an electron-donating substituent,
while, at the same time, the gold(I)-acetylide linkage yields
a near-visible absorbing chromophore with low-lying triplet
states. Therefore, the opportunity presents itself to see if the
Au-acetylide states are able to sensitize the formation of,
as well as interact with, the lowest Re-based MLCT excited
state. Because the photophysical properties of the MLCT
excited states of Re(I) complexes are well-documented,
evaluating the properties of 1 and appropriate reference
chromophores (2-5) with a battery of spectroscopic tech-
niques will yield a detailed understanding of excited-state
decay in Re(I) complexes appended with Au-acetylide
conjugated substituents.
3
oligomer-based π-π* intraligand (3IL) character. Clearly,
the addition of the conjugated organic ethynylene units
dramatically affects the radiative decay processes in Re(I)
complexes. Over the past several years, Yam’s group in Hong
Kong has studied the photophysical properties of a variety
of Re(I)-acetylides of the general formula Re(L-L)(CO)3-
(CtCR) (where R represents various organic-based sub-
stituents).29-32 The σ-donating ability of the acetylides
renders the Re(I) center more electron-rich and, therefore,
systematic variation of the MLCT energies is observed as a
function of R. These structures have been further elaborated
into various carbon-rich polynuclear luminescent materials.32
Upon consideration of the aforementioned experimental
results, we rationalized that the placement of metal-contain-
ing conjugated substituents within the diimine ligand frame-
work should promote interesting electronic effects in Re(I)
complexes. As typically observed in Re(I) compounds that
contain a single chromophoric diimine unit, we expect any
electronic effects to be communicated through the resultant
photophysical and electrochemical properties. In addition,
we were interested in the question of whether the placement
of a metal-acetylide substituent possessing low-energy
triplet states within the diimine ligand structure of a simple
Re(I) complex would markedly influence excited-state decay.
In the current offering, we selected a gold(I)-acetylide
system to serve as a substituent in the 5-position of 1,10-
phenanthroline, which is appended to a Re(CO)3Cl skeleton.
The newly synthesized Ph3P-Au-CtC-phen-containing
Re(I) complex (1) possesses several distinguishing features
that facilitated its selection for the current study. The Ph3P-
The current report provides a comprehensive photophysical
study on the newly synthesized compounds Re(phen-
CtCAuPPh3)(CO)3Cl (1) and Re(phenCtCH)(CO)3Cl (2)
(where phenCtCH is 5-ethynyl-1,10-phenanthroline) (5), in
addition to the benchmark chromophore Re(phen)(CO)3Cl
(3). The phenCtCAuPPh3 ligand (4) was also independently
synthesized to facilitate spectroscopic comparisons. The
complexes were studied by static and transient absorption
(TA), as well as steady-state and time-resolved photolumi-
nescence (TRPL), at room temperature and 77 K, as well as
nanosecond time-resolved infrared (TRIR) spectroscopy
(1-4 only). The spectroscopic information provided by these
techniques enabled a thorough characterization of excited-
state decay in compounds 1 and 2. In 2, the RT excited state
decay is most consistent with a MLCT assignment, whereas
at 77 K, the lowest excited state is clearly 3IL-based, localized
within the diimine ligand. The photophysical behavior of 1
is somewhat more complex as the excited states at RT are
best characterized as an admixture of Re-based MLCT and
3
3IL character whereas at 77 K, the IL state dominates
excited-state decay.
(15) Leasure, R. M.; Sacksteder, L.; Nesselrodt, D.; Reitz, G. A.; Demas,
J. N.; DeGraff, B. A. Inorg. Chem. 1991, 30, 3722-3728.
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1992, 114, 1897-1898.
(17) Kotch, T. G.; Lees, A. J.; Fuerniss, S. J.; Papathomas, K. I. Chem.
Mater. 1992, 4, 675-683.
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Soc. 1993, 115, 8230-8232.
(19) Wang, Y.; Schanze, K. S. Inorg. Chem. 1994, 33, 1354-1362.
(20) Striplin, D. R.; Crosby, G. A. Chem. Phys. Lett. 1994, 221, 426-
430.
(21) Paolucci, F.; Marcaccio, M.; Paradisi, C.; Roffia, S.; Bignozzi, C. A.;
Amatore, C. J. Phys. Chem. B 1998, 102, 4759-4769.
(22) Schanze, K. S.; Lucia, L. A.; Cooper, M.; Walters, K. A.; Ji, H.-F.;
Sabina, O. J. Phys. Chem. A 1998, 102, 5577-5584.
(23) Yam, V. W.-W.; Lau, V. C.-Y.; Wu, L.-X. J. Chem. Soc., Dalton
Trans. 1998, 1461-1468.
(24) Guo, X.-Q.; Castellano, F. N.; Li, L.; Lakowicz, J. R. Anal. Chem.
1998, 70, 632-637.
(25) Ley, K. D.; Schanze, K. S. Coord. Chem. ReV. 1998, 171, 287-307.
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Chem. Commun. 1999, 1749-1750.
Experimental Section
General. All reagents not specifically listed below were obtained
from commercial sources and used as received. Anhydrous dichlo-
romethane was purchased from Aldrich Chemical Co. and distilled
over CaH2. Diisopropylamine and acetone were dried and distilled
according to standard procedures, prior to use. Potassium carbonate
(Aldrich Chemical Co.) was dried in an oven (140 °C) for at least
12 h before use. Copper(I) iodide, 1,10-phenanthroline (phen), and
Re(CO)5Cl were purchased from Aldrich Chemical Co. and used
as received. Au(PPh3)Cl was purchased from Alfa Aesar and used
as received. 5-Bromo-1,10-phenanthroline was available from a
(27) Walters, K. A.; Dattelbaum, D. M.; Ley, K. D.; Schoonover, J. R.;
Meyer, T. J.; Schanze, K. S. Chem. Commun. 2001, 1834-1835.
(28) Walters, K. A.; Ley, K. D.; Cavalaheiro, C. S. P.; Miller, S. E.;
Gosztola, D.; Wasielewski, M. R.; Bussandri, A. P.; van Willigen,
H.; Schanze, K. S. J. Am. Chem. Soc. 2001, 123, 8329-8342.
(29) Yam, V. W.-W.; Lau, V. C.-Y.; Cheung, K.-K. Organometallics 1995,
14, 2749-2753.
(30) Yam, V. W.-W.; Lau, V. C.-Y.; Cheung, K.-K. Organometallics 1996,
15, 1740-1744.
(31) Yam, V. W.-W.; Chong, S. H.-F.; Cheung, K.-K. Chem. Commun.
1998, 2121-2122.
(32) Yam, V. W.-W. Chem. Commun. 2001, 789-796.
Inorganic Chemistry, Vol. 44, No. 10, 2005 3413

Pomestchenko et al.
previous study.33 Water was deionized by means of Barnstead
E-Pure system. Silica gel used in chromatographic separations was
obtained from EM Science (Silica Gel 60, 230-400 mesh). All
synthetic manipulations were performed under an inert and dry
argon atmosphere using standard techniques. Elemental analyses
were performed by Atlantic Microlab (Norcross, GA) or QTI
dissolved in isooctane and the reaction mixture was brought to
reflux for 3 h. When the mixture was heated, a yellow precipitate
started to appear. The solution was cooled to room temperature
and the precipitate was collected by filtration and washed with cold
isooctane. The solid product was purified by column chromatog-
raphy (SiO2, 5 vol % of acetone in dichloromethane as an eluent)
1
1
(Whitehouse, NJ). H NMR spectra were recorded on a Varian
(0.489 g, 87% yield). Rf ) 0.44 (5 vol % acetone in CH2Cl2); H
model Gemini 300 (300 MHz) spectrometer. All chemical shifts
are referenced to residual solvent signals previously referenced to
tetramethylsilane (TMS) and splitting patterns are designated as s
(singlet), d (doublet), t (triplet), q (quartet), m (multiplet), and br
(broad). Static IR spectra of solid samples were measured with a
ThermoNicolet model IR 200 spectrometer. Electrospray ionization
(ESI) mass spectra were measured at the University of Toledo, using
an Esquire-LC spectrometer. EI mass spectra were measured in-
house using a Shimadzu model QP5050A spectrometer.
NMR (300 MHz, CDCl3): 9.44 (dd, J1 ) 5.1 Hz, J2 ) 1.2 Hz,
1H); 9.40 (dd, J1 ) 5.1 Hz, J2 ) 1.5 Hz, 1H); 9.0 (dd, J1 ) 8.4
Hz, J2 ) 1.2 Hz, 1H); 8.51 (dd, J1 ) 8.3 Hz, J2 ) 1.5 Hz, 1H);
8.25 (s, 1H); 7.96 (dd, J1 ) 8.4 Hz, J2 ) 5.1 Hz, 1H); 7.89 (dd, J1
) 8.4 Hz, J2 ) 5.1 Hz, 1H); 3.70 (s, 1H). Anal. Calcd. for C17H8-
ClN2O3Re: C, 40.04; N, 5.49; H, 1.58. Found: C, 40.23; N, 5.32;
H, 1.66. EI-MS (70 eV) m/z 510. FTIR (ATR): 2020, 1927, and
1888 cm-1 (νCtO ).
Re(phenCtCAuPPh3)(CO)3Cl (1). Compound 2 (0.1 g, 0.196
mmol) was dissolved in dry CH2Cl2 (75 mL) in a sealable reaction
vessel and the solution was degassed with argon for 15 min. Au-
(PPh3)Cl (0.097 g, 0.196 mmol) and freshly distilled (iPr)2NH (1
mL) were added to the reaction mixture under an argon atmosphere.
The reaction vessel was sealed with a Teflon screw cap that had
been fitted with an O-ring and was stirred at 40 °C for 1 week.
The reaction mixture was filtered, and the solvent was removed.
The remaining orange-yellow residue was dissolved in CH2Cl2
and precipitated by the addition of isooctane. The solid was
collected on a frit, washed with isooctane, and purified by column
chromatography on silica (5 vol % of acetone in dichloromethane
as an eluent). Rf ) 0.53 (5 vol % acetone in CH2Cl2). Experimental
yield ) 85% (0.160 g). 1H NMR (300 MHz; CDCl3): 9.37 (dd, J1
) 5.1 Hz, J2 ) 1.5 Hz, 1H); 9.25-9.30 (m, 2H); 8.41 (dd, J1 )
8.4 Hz, J2) 1.2 Hz, 1H); 8.11 (s, 1H); 7.88 (dd, J1 ) 8.4 Hz, J2 )
5.1 Hz, 1H); 7.79 (dd, J1 ) 8.1 Hz, J2 ) 5.1 Hz, 1H); 7.48-7.63
(m, 15H). Anal. Calcd. for C35H22AuClN2O3PRe‚C3H6O: C, 44.47;
N, 2.73; H, 2.75. Found: C, 44.77; N, 3.02; H, 2.93. ESI-MS: m/z
966 [M-H]+. FTIR (ATR): 2015, 1902, and 1880 cm-1 (νCtO).
phenCtCAuPPh3 (4). 5-Ethynyl-1,10-phenanthroline (0.1 g,
0.489 mmol) was dissolved in dry CH2Cl2 (100 mL) in a sealable
reaction vessel and the solution was degassed with argon for 15
min. Au(PPh3)Cl (0.242 g, 0.489 mmol) and freshly distilled
(iPr)2NH (2 mL) were added to the reaction mixture under an argon
atmosphere. The reaction vessel was sealed with a Teflon screw
cap that had been fitted with an O-ring and was stirred at 40 °C
for 6 days. Volatiles were removed under reduced pressure, yielding
a pale yellow solid. Crude product was purified by preparative thin-
layer chromatography (TLC) on silica (1 vol % of MeOH and 2
vol % of Et3N in dichloromethane as an eluent). Rf ) 0.12-0.30
(broad band). MALDI-MS: Calculated for C32H22AuN2P 662.47;
Spectroscopic Measurements. UV-Vis absorption spectra were
measured with a Hewlett-Packard model 8453 diode array spec-
trophotometer, which was accurate to (2 nm. Uncorrected (for
spectral response) steady-state photoluminescence spectra at RT
and 77 K were obtained with a single photon counting spectrof-
luorimeter (Edinburgh Analytical Instruments, model FL/FS 900).
Radiative quantum yields (Φr) of each metal complex were
measured, relative to Re(bpy)(CO)3Cl, for which Φr ) 0.005 in
deaerated CH2Cl2.2 Emission intensity decays were measured and
analyzed as previously described.33,34 TA spectra and decay kinetics
were obtained using the unfocused third harmonic provided by a
Continuum Surelite I Nd:YAG laser (355 nm, 5-7 ns fwhm), using
the apparatus in the Ohio Laboratory for Kinetic Spectrometry,
which has already been described.33,34 Typical excitation energies
of 4-7 mJ/pulse were maintained in all experiments. All samples
were thoroughly degassed prior to measurements with high-purity
argon and kept under an argon atmosphere throughout each
experiment conducted at ambient temperature (22 °C). TRIR
measurements were performed with a step-scan instrument based
on a modified Bruker model IFS55 FTIR spectrometer (step size
) 20 nm, resolution ) (8 cm-1), using the third harmonic (355
nm, 1 mJ/pulse) of a Spectra Physics GCR series YAG laser as
the pump source, operating at 10 Hz.35 The argon-degassed solution
samples were flowed at a rate of 10 mL/min between CaF2 windows
(4 mm thick) equipped with a 1 mm spacer. All other experimental
details have been described in a previous report published by our
colleagues at BGSU.35 Cyclic voltammetry was performed in a one-
compartment cell, using a three-electrode arrangement including a
platinum disk working electrode, a platinum wire auxiliary elec-
trode, and a Ag/AgCl reference electrode. All electrochemistry was
performed on a BAS Epsilon system, using a scan rate of 200 mV/s
in 0.1 M TBAH/CH3CN.
1
found 663.21. H NMR (300 MHz; CDCl3): 9.18 (dd, J1 ) 4.5
Hz, J2 ) 1.8 Hz, 1H); 9.12 (dd, J1 ) 4.5 Hz, J2 ) 1.8 Hz, 1H);
9.07 (dd, J1 ) 8.1 Hz, J2 ) 1.8 Hz, 1H); 8.16 (dd, J1 ) 8.4 Hz, J2
) 1.8 Hz, 1H); 8.02 (s, 1H); 7.67-7.70 (m, 2H), 7.497-7.631 (m,
15H).
Preparations. 5-Ethynyl-1,10-phenanthroline (5) was synthe-
sized from 5-bromo-1,10-phenanthroline, according to a literature
1
procedure, and yielded satisfactory mass and H NMR spectra.36
Re(phen)(CO)3Cl (3) was prepared as described in the literature.1-3,37
Re(phenCtCH)(CO)3Cl (2). 5-Ethynyl-1,10-phenanthroline
(0.232 g, 1.13 mmol) and Re(CO)5Cl (0.399 g, 1.10 mmol) were
Results and Discussion
Structures. Compounds 2 and 3 were prepared by
displacement of CO from Re(CO)5Cl, using the appropriate
phenanthroline ligand in refluxing isooctane.37 The synthesis
of 1 features a “chemistry on the complex” approach, where
the acetylene-terminated diimine ligand chelated to the
Re(I) center couples to Ph3PAuCl under mild reaction
conditions.38-41 Complexes 1-3 were purified by column
chromatography on silica gel, and final isolated yields were
(33) Tyson, D. S.; Henbest, K. B.; Bialecki, J.; Castellano, F. N. J. Phys.
Chem. A 2001, 105, 8154-8161.
(34) Tyson, D. S.; Castellano, F. N. J. Phys. Chem. A 1999, 103, 10955-
10960.
(35) Fedorov, A. V.; Daniolv, E. O.; Merzlikine, A. G.; Rodgers, M. A.
J.; Neckers, D. C. J. Phys. Chem. A 2003, 107, 3208-3214.
(36) Ziessel, R.; Suffert, J.; Youinou, M.-T. J. Org. Chem. 1996, 61, 6535-
6546.
(37) Lin, R.; Fu, Y.; Brock, C. P.; Guarr, T. F. Inorg. Chem. 1992, 31,
4346-4353.
3414 Inorganic Chemistry, Vol. 44, No. 10, 2005

Influence of Au(I)-Acetylide Subunit on Re(Phen)(CO)3Cl
Table 1. 1H NMR Chemical Shifts of the Phenanthroline Ring Protons
in Compounds 1, 2, 4, and 5
Chemical Shifta (ppm)
compound
H(2)
H(9)
H(4)
H(7)
H(6)
H(8)
H(3)
5
4
2
1
9.23
9.17
9.44
9.37
9.21
9.12
9.40
9.29
8.76
9.06b
9.00
9.27b
8.22
8.16
8.51
8.41
8.09
8.02
8.25
8.11
7.73
7.69
7.96
7.88
7.66
c
7.89
7.79
a The protons are numbered according to their corresponding positions
on the phenanthroline ring. b Downfield shift of H(4) is due to higher
through-space deshielding by acetylide. c The signal from H(3) is overlapped
with aromatic protons of the -Au(PPh3) group.
Table 2. Cyclic Voltammetry Data in in 0.1 M TBAH/CH3CN
compound
Eox (V)
Ered (V)
∆Ea (V)
1
1.45
1.45
1.41
-1.28
-1.19
-1.36
-1.13
2.73
2.64
2.77
2
3b
5c
a ∆E ) Eox - Ered b Data taken from ref 21. c Measured in CH2Cl2.
.
quite significant (g85%). All compounds have been structur-
1
ally characterized with H NMR, mass spectrometry, el-
emental analysis, and FTIR, where appropriate. The three-
band IR pattern observed in the CO stretching region is
consistent with the facial arrangement of the three COs in
the coordination sphere, as typically observed in related
1
Re(I) organometallic structures.1-3 In the H NMR spectra,
all the proton signals residing within the phenanthroline
ligand are shifted upfield in 1, relative to the acetylene
precursor 2 (Table 1). This upfield shift is consistent with
back-donation from the gold(I) center to the σ-ethynyl bond
in the substituted phenanthroline ligand. A similar upfield
shifting effect has been observed in the phenanthroline
protons in Ru(II)-systems-containing Au-acetylide molec-
ular wires.42 Elemental analysis and mass spectrometry data
are also consistent with the proposed structures.
Figure 1. (a) Room-temperature (RT) absorption spectra of 1 (solid line)
and 2 (dotted line) in CH3CN. (b) RT absorption spectra of 4 (solid line)
and 5 (dotted line) in an ethanol/methanol (EtOH/MeOH) mixture (4:1).
result from rereduction of Re(II) by the solvent or an
impurity. Partially reversible phen-based reductions are
observed in compounds 1 and 2, with the first wave centered
at -1.28 and -1.18 V, respectively. Electron back-donation
from the gold(I) unit is responsible for the negative shift of
the reduction potential of the phenanthroline ligand in 1
compared to 2. Additional independent evidence for the
electron-donating nature of the gold(I) unit was obtained by
comparing 1H NMR chemical shifts of compounds 1 and 2,
as well as 4 and 5 (see Table 1). No electrochemical
measurements could be performed on compound 4, because
of its decomposition under cyclic voltammetry experimental
conditions. The electrochemical properties of 1 and 2 are
consistent with those previously measured in 3, where the
first oxidation is metal-based and the first reduction takes
The newly synthesized complexes 1 and 2, as well as the
benchmark chromophore 3, are air-stable solids, in addition
to being thermally and photochemically stable in solution.
Compound 4 is photochemically unstable in solution;
therefore, multiple precautions were taken during all mea-
surements and fresh samples were used for all experiments.
The UV-Vis spectra of 4 were measured before and after
each photochemical exposure, to ensure data quality.
Electrochemistry. The redox behavior of the new com-
plexes measured in CH3CN by cyclic voltammetry are
consistent with data typically obtained in Re(L-L)(CO)3Cl
complexes (see Table 2).3,8,11,21 The first oxidation in 1 and
2 is rhenium-based, occurring at peak potentials of 1.45 and
1.46 V, respectively, versus Ag/AgCl. These oxidations are
not reversible on the electrochemical time scale and likely
place on the diimine ligand (Eox ) 1.41 V and E1/2,red
)
-1.36 V).21 The decrease in reduction potential of phen-
anthroline ligand in 2, relative to 3, is probably due to the
higher conjugation of the aromatic system.
(38) Pabst, G. R.; Pfuller, O. C.; Sauer, J. Tetrahedron 1999, 55, 8045-
8064.
(39) Aspley, C. J.; Williams, J. A. G. New. J. Chem. 2001, 25, 1136-
1147.
(40) Goze, C.; Kozlov, D. V.; Castellano, F. N.; Suffert, J.; Ziessel, R.
Tetrahedron Lett. 2003, 44, 8713-8716.
Absorption and Photoluminescence Properties. The
ground-state absorption spectra of complexes 1 and 2 in CH3-
CN are presented in Figure 1a, whereas the electronic spectra
of the “free” ethynylated ligands 4 and 5 are provided in
(41) Goze, C.; Kozlov, D. V.; Tyson, D. S.; Ziessel, R.; Castellano, F. N.
New. J. Chem. 2003, 27, 1679-1683.
(42) Shiotsuka, M.; Yamamoto, Y.; Okuno, S.; Kitou, M.; Nozaki, K.;
Onaka, S. Chem. Commun. 2002, 590-591.
Inorganic Chemistry, Vol. 44, No. 10, 2005 3415

Pomestchenko et al.
Figure 1b. The broad lowest-energy absorption feature in 1
and 2 are assigned to a dπ (Re) f π* (phen-R) (where R is
CtC-AuPPh3 and CtCH, respectively) MLCT transition
with more-intense features at higher energy being of diimine-
ligand π-π* origin. The gold(I)-acetylide chromophore is
responsible for the relatively strong near-visible π-π*
absorption band centered at 347 nm in 1.42-44 This assign-
ment is consistent with observations made in Ru(II) com-
plexes that contain gold(I)-acetylides linked in the 3- and
3,8-positions of 1,10-phenanthroline.42 We note that the
lowest-energy transition in 4 is somewhat blue-shifted,
relative to the position of the corresponding band in the
binuclear complex 1. Upon coordination to the Re(I) center,
the energy of this transition is reduced, relative to the free
ligand 4. This observation is consistent with the electro-
chemical data presented previously but represents only a
minor effect, because the Re-based MLCT transitions are
approximately the same energy in both 1 and 2 (see Figure
1a).
Complex 1 displays an emission band centered at 598 nm
in deaerated 2-methyltetrahydrofuran (MTHF) at RT, which
is completely invariant to the excitation wavelength over the
range of 300-480 nm (see Figure 2a). The corrected
excitation spectrum of 1 measured over a range of 300-
500 nm can be largely superimposed with its absorption
spectrum, suggesting that the gold(I)-acetylide chromophore
harvests near-visible light and efficiently converts it into a
broad luminescence in the visible range. The emission
3
spectrum in 1 is largely characteristic of MLCT-based
luminescence typically observed in Re(I) diimine complexes,
with the exception of a minor high-energy shoulder between
525 and 550 nm. The position of this shoulder coincides with
the most significant feature observed in the phosphorescence
of the free ligand 4 when excited at 337 nm (see Figure 2b).
Because the ligand-localized phosphorescence spectrum in
4 is dependent on excitation wavelength (see below), it is
difficult to interpret whether the high-energy emission
shoulder in 1 results from a ligand-localized contribution or
not. This point will be discussed further during the presenta-
tions of the time-resolved absorption and IR data.
Figure 2. (a) RT emission spectra of (a) 1-3 in MTHF using 400-nm
excitation. (b) RT emission spectra of 4 in CH2Cl2 using 360-nm (solid
line) and 337-nm (dashed line) excitation.
structureless nature of its emission band at RT is consistent
with a “pure” d-π* MLCT assignment.
The terminal acetylene-containing ligand 5 does not
display any phosphorescence at RT, whereas the gold-
containing diimine ligand 4 is phosphorescent and displays
excitation-wavelength-dependent phosphorescence emission
spectra (see Figure 2b), which are almost quantitatively
quenched by dissolved oxygen (only those features between
500 and 700 nm). Although 4 is photochemically unstable,
the absorption spectra acquired before and after the static
luminescence spectra indicated no significant sample de-
composition. The following discussion, in regard to free
ligand 4, assumes that the photoluminescence (PL) emanates
from only one species; however, we cannot exclude contri-
butions from minor impurities or decomposition products.
Although the PL data of 4 provide some insight into the
photophysical behavior of dinuclear 1, we believe that the
TA and TRIR data discussed later in this work provides
more-definitive experimental evidence for its proposed
“mixed” excited-state behavior. The excitation wavelength-
dependent phosphorescence in 4 is illustrated in Figure 2b,
where excitation at 337 nm in deaerated CH2Cl2 generates a
spectrum that consists of fluorescence and phosphorescence
bands with essentially equal intensity; the phosphorescence
maxima are positioned at 529 and 571 nm. Please note that
It is well-established that 3 possesses a lowest-energy
d-π* MLCT excited-state configuration, even at cryogenic
temperatures as low as 4 K.20 Similarly, photoluminescence
centered at 615 nm was observed for the model complex 2
upon excitation into the low-energy charge-transfer band (see
Figure 2a). The blue shift in the emission observed in 1,
relative to 2, is consistent with a higher reduction potential
of 1, compared to 2, and the electron-donating effect of the
1
gold(I)-acetylide unit, as demonstrated by the H NMR
chemical shifts (see Table 1). The electron-rich Ph3PAu
substituent is clearly responsible for this effect by rendering
the phen-ligand harder to reduce in 1. We note that there is
no evidence of a high-energy shoulder in 2 and the broad,
(43) Irwin, M. J.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1997, 16,
3541-3547.
(44) Che, C.-M.; Chao, H.-Y.; Miskowski, V. M.; Li, Y.; Cheung, K.-K.
J. Am. Chem. Soc. 2001, 123, 4985-4991.
3416 Inorganic Chemistry, Vol. 44, No. 10, 2005

Influence of Au(I)-Acetylide Subunit on Re(Phen)(CO)3Cl
Table 3. Measured and Calculated Photophysical Parameters in MTHF
λ
em,max (nm)
τ (ns)
τ (µs)
λabs,max
(nm)
b
b
compound
at RT
at 77 K
at RT
at 77 K
Φa
kr (s-1
)
knr (s-1
)
1
2
347, 400
400
598
615
527, 570, 620
504, 543, 585
960, 930 (TA)c
163, 156 (TA)c
456
0.018
1.9 × 104
1.02 × 106
30 (0.53),d 410 0.008
(0.47)d
4.9 × 104
6.08 × 106
3
386
328
606
535
227, 214 (TA)c
7.6
0.013 5.77 × 104 4.35 × 106
4e
(379, 403, 413),f
(529, 571),g
(518, 528, 564, 577),i
(532, 580)j
41 µs (TA),c 192 µs (TA)c
450g
0.005
(538, 581)h
5e
296, 308
(365, 375, 386,
(363, 376, 381, 398),f
(495, 505, 516, 537,
581)k
(7 ms, 80 ms)k
408),f no phos.
a Photoluminescence quantum yield, measured relative to Re(bpy)(CO)3Cl, ΦDCM ) 0.005;2 values are (10%. b Radiative (kr) and nonradiative (knr) rate
constants calculated by kr ) Φ/τ and knr ) (1 - Φ)/τ. c Excited-state lifetime determined by transient absorption spectroscopy, using 355-nm excitation.
d Relative amplitudes of each intensity decay component, determined from a biexponential fit. e RT data measured in CH2Cl2 and 77 K data obtained in
EtOH/MeOH (4:1). f Fluorescence peaks. g Phosphorescence peak, 337-nm excitation. h Phosphorescence peaks, 360-nm excitation. i Phosphorescence peaks,
330-nm excitation. j Phosphorescence peaks, 365-nm excitation. k Phosphorescenc peaks.
dichloromethane was used as a consequence to solubility
limitations in MTHF. On the other hand, 360-nm excitation
generates emission spectra with significantly more intense
singlet fluorescence at the same relative band positions.
However, the phosphorescence bands are red-shifted (∼10
nm), relative to that measured using 337-nm excitation,
perhaps suggesting the presence of another energetically
proximate triplet state. We believe the fluorescence bands
are derived mostly from a phen-like singlet state that is not
heavily influenced by the spin-orbit coupling provided by
the Au unit. The two distinct RT phosphorescene spectra
observed in 4 possess the same vibronic spacing within
experimental error (∼1380 cm-1), indicating a similar
excited-state structure. Because the CtC stretching fre-
quency occurs at higher energy (∼2100 cm-1), the two
phosphorescence spectra are more consistent with triplet
excited states intimately coupled to the CdC and/or CdN
ring stretching vibrations of the phenanthroline structure.
Therefore, the Au-acetylide unit serves to facilitate spin-
orbit coupling but does not seem to be the source of the RT
phosphorescence. These data imply that there are multiple,
close-lying intraligand triplet states based within the phenan-
throline structure in 4. Assuming that the integrity of 4 was
not compromised in the aforementioned static experiments,
it is reasonable to assume that binuclear complex 1 has at
least two closely lying triplet ligand manifolds with which
the Re-based MLCT excited state can potentially interact.
The RT single-exponential PL lifetimes for 1-3 measured
in deaerated MTHF are 960, 163, and 227 ns, respectively
(Table 3). These lifetimes were completely invariant to
monitoring wavelength. The longer lifetime observed in 1
reflects a larger energy gap between ground and excited
states, decreasing the nonradiative rate constant by a factor
of ∼6 in 1, relative to 2. However, there is likely a
contribution associated with electron delocalization into the
Au-acetylide fragment in 1 that enhances the excited-state
lifetime in the charge-transfer excited state.45,46 Alternatively,
the participation of a ligand-localized triplet state in the
excited-state manifold may be responsible for the lifetime
extension in 1, relative to 2 and 3. No phosphorescence
transients could be measured in optically dilute (OD < 0.2)
degassed solutions of 4 in CH2Cl2. All measured and derived
photophysical parameters are collected in Table 3 for
compounds 1-4. The photoluminescence data obtained for
2 and 3 at room temperature are most consistent with a
lowest-energy MLCT excited state, whereas an assignment
for 1 cannot be made by sole reliance on luminescence data.
The emission properties of 1-3 were studied in MTHF
low-temperature glasses at 77 K, whereas 4 was measured
in a 4:1 ethanol/methanol (EtOH/MeOH) mixture, as a
function of excitation wavelength (Figure 3). The emission
manifold in 3 is broad and structureless at 77 K (see Figure
3a), which is consistent with radiative decay from a MLCT
manifold whose vibronic components are broadened as a
result of large solvent reorganizational energies and greater
distortions in the low-frequency modes coupling the ground
and excited states.3 The large reorganization energy in 3 is
apparent from the large energetic difference between the RT
and 77 K spectra (2190 cm-1), also called the thermally
induced Stokes shift (∆Es), which can be roughly ap-
proximated as 2λs, where λs is the outer-sphere reorganization
energy for the MLCT excited state.47,48 The emission lifetime
of 3 at 77 K is 7.6 µs, thus benchmarking the lifetime
expected for a 3MLCT excited state in a Re(I)-phen complex
at this temperature. The static 77 K spectra of compounds 1
and 2 contain well-defined structured vibronic components
(Figure 3), indicating that the emission is not of MLCT
origin. (Static emission spectra of 1, 2, 4, and 5 are given in
the Supporting Information.) Although the low-temperature
spectra of 1 and 2 are shifted in energy, the major vibronic
spacing of the two highest-energy bands are identical (∼1450
cm-1), implying that these emissions are both diimine-ligand-
based. The 77 K emission maximum of 1, relative to that of
2, is 23 nm red-shifted compared to a 17-nm blue shift
measured in the RT spectra. We believe these observations
reflect the distinct natures of the emitting states as a function
(45) Boyde, S.; Strouse, G. F.; Jones, W. E., Jr.; Meyer, T. J. J. Am. Chem.
Soc. 1990, 112, 7395-7396.
(46) Strouse, G. F.; Schoonover, J. R.; Duesing, R.; Boyde, S.; Jones, W.
E., Jr.; Meyer, T. J. Inorg. Chem. 1995, 34, 473-487.
(47) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg.
Chem. 2001, 40, 4053-4062.
(48) Chen, P.; Meyer, T. J. Chem. ReV. 1998, 98, 1439-1477.
Inorganic Chemistry, Vol. 44, No. 10, 2005 3417

Pomestchenko et al.
each respective phenanthroline ligand. This phenomenon is
not unprecedented and, because it is well-established that
the 3MLCT level in Re(I) complexes increases as temperature
3
decreases, whereas the IL states are largely unaffected by
3
environment,1,12,15,18 it is not surprising that the IL and
3MLCT states invert at low temperature in 1 and 2. This
state inversion leads to emission from the lowest excited
state, which can be largely attributed to the 3IL state localized
on the diimine ligand structure in 1 and 2.
Transient Absorption. Nanosecond laser flash photolysis
experiments were performed on compounds 1-3 in MTHF
solutions and 4 in CH2Cl2 (due to solubility restrictions) at
RT using 355-nm excitation (see Figure 4). In complexes
1-3, the absorption transients decay with the same single
exponential kinetics observed in the respective luminescence
intensity decays, within a few percent (see Table 3). These
data illustrate that each luminescent species is mostly likely
of the same origin as that which produces the absorption
transients. The data obtained for 3 represent the absorption
spectral features associated with the MLCT excited state in
MTHF, which display TAs throughout the entire spectrum,
except at wavelengths of >520 nm, where the PL dominates
the signal(see Figure 4a). The ground-state bleaching of the
MLCT band appears as a depression between 375 nm and
450 nm, whereas broad features are observed in the range
of 450-520 nm while a large transient is obtained near 300
nm. This difference spectrum is in complete agreement with
that reported for the same complex in CH3CN, where the
absorptions near 305 and ∼480 nm were assigned to reduced
phen in the MLCT excited state.10 The absorption spectrum
of phen‚- possesses strong features in the 520-650 nm
region, as well in the ultraviolet (UV) region.49,50 Although
we cannot evaluate the long wavelength segment and the
MLCT bleach interferes with the 375-450 nm range, the
absorptions at <375 nm and in the range of 450-520 nm
are assigned to the reduced phen ligand in 3. Similar excited-
state spectroscopic features are observed in reference chro-
mophore 2, except the MLCT bleach is largely obscured by
the absorption transients assigned to the reduced phenCt
CH species (see Figure 4b).
Compound 1, on the other hand, displays a noticeably
different spectrum than 2 and 3. There is a detectable ground-
state bleaching near 350 nm in the region where the gold-
(I)-acetylide chromophore is the predominate absorber and
there are transients observed at >370 nm with peaks at ∼450
nm and <330 nm with a maximum at ∼320 nm (see Figure
4c). When the formation of the charge-transfer excited state
occurs, several spectroscopic features should be observed.
One is the ground-state bleaching of the MLCT transition,
which, in the case of 1, is completely obscured by other
transient absorptions, precluding its use as a diagnostic tool.
Another is the formation of the diimine ligand radical anion
that generally possesses strong, highly allowed absorption
features in the UV and visible regions. These would be
associated with the Ph3PAuCtCphen ligand in 1, and should
Figure 3. Emission spectra (at 77 K) of (a) 1-3 measured in MTHF
glasses using 400-nm excitation and (b) 4 measured in an EtOH/ MeOH
(4:1) glass using 330- and 365-nm excitation; the spectrum of 1 (in MTHF)
is superimposed in each panel, for comparative purposes.
of temperature, i.e., diimine-centered π-π* at 77 K and
d-π* charge transfer at RT.
Although measured in a different matrix (due to solubility
limitations), the 77 K phosphorescence of 4 displays excita-
tion-wavelength-dependent emission profiles (see Figure 3b).
Sandwiched between these two distinct profiles is the 77 K
spectrum of 1 measured in MTHF. Although the differences
are hard to interpret, the energetic proximity and structure
of these three spectra are approximately similar, suggesting
at least that the emissions are emanating from states
composed of similar electronic structures. The recovered
lifetime of 1 at 77 K is 456 µs, whereas the decay in 2 is
biexponential with two long-lifetime components: 30 and
410 µs. We note that the complex nature of the emission
intensity decay has been observed in other Re(I)-phen
complexes.12 However, the measured lifetimes are well
beyond that expected for Re(I)-diimine-based MLCT excited
states at 77 K. The excited-state lifetime of 4 measured at
77 K (λex ) 337 nm) in a 4:1 EtOH/MeOH glass is 450 µs,
which is in quantitative agreement with that observed for 1
at 77 K. Taken together, the 77 K results are consistent with
an intraligand-based π-π* phosphorescence emanating from
(49) Kato, T.; Shida, T. J. Am. Chem. Soc. 1979, 101, 6869-6876.
(50) Turro, C.; Chung, Y. C.; Leventis, N.; Kuchenmeister, M. E.; Wagner,
P. J.; Leroi, G. E. Inorg. Chem. 1996, 35, 5104-5106.
3418 Inorganic Chemistry, Vol. 44, No. 10, 2005

Influence of Au(I)-Acetylide Subunit on Re(Phen)(CO)3Cl
Figure 4. Excited-state absorption difference spectra of (a) 3 in MTHF, (b) 2 in MTHF, (c) 1 in MTHF, and (d) 4 in CH2Cl2, generated by 355-nm
excitation at 4, 7, 4, and 2 mJ/pulse, respectively. All delay times are specified in each graph.
be distinct from phen (3) or phenCtCH (2), which may be
the case. Compound 1 happens to possess low-energy π-π*
transitions centered near 350 nm, because of the Au-
acetylide linkage in strong electronic contact with the
phenanthroline ligand.42 It stands to reason that, when the
formation of the Re-based MLCT excited state occurs, the
spectral features associated with the Ph3PAuCtCphen ligand
would bleach and simultaneously yield strong diimine ligand
radical anion transients. If the ligand radical anion transients
are responsible for the large absorption features in the visible,
then the TA data are consistent with a lowest MLCT excited
state in 1. However, it is somewhat surprising that the
transient features, as well as the relative intensities, are so
markedly different when comparing compound 1 to com-
pounds 2 and 3, suggesting that the excited-state picture in
1 is more complex. Figure 4d displays the excited-state
absorption difference spectrum of the free ligand 4 in CH2-
Cl2, following 355-nm excitation. As a result of sample
decomposition, the solution was changed every 50 nm during
the excited-state difference spectrum measurements, which
were collected in 10-nm segments. Because transient lumi-
nescence measurements performed on 4 at RT were unsuc-
cessful, the TA kinetics provides an independent measure-
ment of the excited-state lifetime. At all wavelengths, the
kinetics was best modeled by a sum of two single exponen-
tials, with a short component of ∼41 µs and a longer
component of ∼192 µs. Given the unstable nature of free
ligand 4, we performed these experiments under the most
controlled conditions possible and believe that the data are
representative of a pure sample. The long-lived transients
are tentatively assigned to the lowest 3IL state of 4, because
no other low-lying long-lived species are expected to be
present. It stands to reason that the absorption transients
produced as a result of 355-nm excitation correspond to the
lowest-energy 3IL state, which seems to decay to the ground
state in a biexponential fashion, although we cannot com-
pletely exclude components arising from decomposition
products generated during the experiment. A comparison of
Figure 4c and 4d indicates that the difference spectra cannot
be completely superimposed, which may suggest that the
nature of the excited states of the two complexes is somewhat
different. However, there are also similarities in the transient
spectra of 1 and 4, which may indicate that there is some
3
degree of IL character in the excited state of 1. Up to this
point, the combined photophysical properties of 1 are most
consistent with an excited state of largely Re-based MLCT
Inorganic Chemistry, Vol. 44, No. 10, 2005 3419

Pomestchenko et al.
character yet apparently influenced by the 3IL state(s) resident
on the phenCtCAuPPh3 ligand. The nature of the proposed
interaction is uncertain but may have its origin in excited-
state equilibrium or configuration mixing between the MLCT
3
and IL states.
Time-Resolved Infrared Spectroscopy. The three strongly
IR-absorbing CtO groups present in the coordination shell
of molecules 1-3 provide a diagnostic probe that is valuable
in elucidating their excited-state behavior.27,51-53 Compound
3 has been well-studied in this regard, and the formation of
the MLCT excited state produces ground-state bleaching
signals, as well as characteristic ν(CtO) shifts to higher
frequency in all three bands.51 This shifting to higher
frequency is a direct result of the charge transfer: i.e.,
oxidation of the Re(I) metal center to Re(II) is accompanied
by the transient shortening of all three CtO bonds.
Therefore, in cases where TA spectra are ambiguous and
somewhat speculative, TRIR spectra can substantiate or even
diagnose the composition of the lowest excited state. This
notion has been convincingly argued by several investigators
over the past decade, and TRIR has been used to identify
the lowest excited-state configuration in several instances.27,52,53
Figure 5 presents the FTIR and TRIR spectra of compounds
1 and 2 measured in the CtO stretching region in CHCl3.
The TRIR spectrum of 4 was also measured but yielded no
observable transients in the window of the experiment. The
ν(CtO) frequencies measured in the FTIR and TRIR
experiments are indicated throughout Figure 5. In both cases,
the FTIR solution spectra are effectively the same within
experimental error ((8 cm-1) and are very similar to that
observed for Re(phen)(CO)3Cl in CH3CN.51 The FTIR
spectra measured 150 ns after 355-nm excitation display
distinguishing features in 1 and 2. In 2, all three original
ν(CtO) bands are bleached and their positions clearly shift
to higher energy following excitation, which is diagnostic
of the MLCT excited state (see Figure 5a). The difference
spectrum measured in 1 provides the most compelling
evidence for a “mixed” excited-state composition (see Figure
5b). It is clear that the origin vibrations are all bleached
following the 355-nm laser pulse, and each of these bands
seem to have a corresponding transient IR absorption at
higher energy, which is consistent with the formation of the
Re-based MLCT excited state. However, there is also a major
absorption feature at 2014 cm-1, whose band position is too
high in energy to result from the blue shift of the 1924 cm-1
absorption. We believe the intense positive transient at 2014
cm-1 actually results from a red shift of the 2026 cm-1 band,
Figure 5. Fourier transform infrared (FTIR) and time-resolved infrared
(TRIR) spectra of (a) 2 and (b) 1 in CHCl3; the TRIR spectra are measured
150 ns following 355-nm excitation at 1 mJ/pulse.
likely thermally equilibrated.33,34 This picture is consistent
with observations made in the TA and PL experiments
presented earlier. Importantly, the diagnosis of multiple,
coexisting excited states in Re(I) complexes has been
previously demonstrated using TRIR spectroscopy,53 and we
believe that, in the present study, transient IR yields the
most definitive evidence for the presence of multiple ex-
cited states. This may seem somewhat surprising, given the
fact that these observations result from indirectly probing
CtO vibrations, which respond to the oxidation changes at
the metal center. In any case, the TRIR technique can be
considered most valuable in understanding excited-state
equilibrium processes in molecules bearing the appropriate
IR-active moieties.
3
which is somewhat indicative of the presence of a IL
state.52,53 At the present time, we are not certain why only a
single vibration seems to display a red shift; however, note
that TRIR spectra measured as a function of time demonstrate
that all the transients decay to the ground state on the same
time scale. In essence, all of the vibrations are coupled to
the same dynamic excited-state processes, which indicate that
3
both MLCT and IL states coexist in 1 at RT and are most
(51) George, M. W.; Turner, J. J. Coord. Chem. ReV. 1998, 177, 201-
217.
(52) Schoonover, J. R.; Strouse, G. F. Chem. ReV. 1998, 98, 1335-1355.
(53) Schoonover, J. R.; Strouse, G. F.; Dyer, R. B.; Bates, W. D.; Chen,
P.; Meyer, T. J. Inorg. Chem. 1996, 35, 273-274.
3420 Inorganic Chemistry, Vol. 44, No. 10, 2005

Influence of Au(I)-Acetylide Subunit on Re(Phen)(CO)3Cl
Conclusion
excited state in 1 can best be described as being composed
of MLCT and IL states in thermal equilibrium.
3
The present study revealed that the excited-state composi-
tion of 1 can be best described as an admixture of metal-
to-ligand charge transfer (MLCT) and triplet intraligand (3IL)
states most likely in thermal equilibrium. One piece of
evidence for this observation comes from the fact that the
room-temperature excited-state lifetime of 1 is modestly
enhanced, relative to two structural model systems, possess-
ing a lowest d-π* MLCT excited state. Transient absorption
data independently obtained for 1, and its corresponding free
diimine ligand 4, demonstrated similarities in their difference
spectra, implying that some 3IL character was present in the
lowest excited state. Time-resolved infrared (TRIR) spec-
troscopy data were most definitive in demonstrating that the
Acknowledgment. This project was financially supported
by the National Science Foundation (CAREER Award CHE-
0134782) and the ACS-PRF (36156-G6). All TRIR and TA
measurements were performed in the Ohio Laboratory for
Kinetic Spectrometry at Bowling Green State University. We
thank Charles R. Luman for his assistance with the electro-
chemical measurements and Dr. Radek Pohl for the synthesis
of phen-CtC-TMS.
Supporting Information Available: Static emission spectra of
compounds 1, 2, 4, and 5 at 77 K (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
IC048376R
Inorganic Chemistry, Vol. 44, No. 10, 2005 3421