Synthesis, electrochemistry, and photophysical properties of a series of luminescent pyrene-thiophene dyads and the corresponding Co2(CO) 6 complexes

Article abstract of DOI:10.1021/ic801226p

A series of pyrene based dyad systems together with their dicobalt hexacarbonyl complexes (1b-6b) were synthesized. The pyrene-thiophene dyads are luminescent in room temperature solution with luminescence lifetimes on the nanosecond time scale. At room temperature the dyad emission is quenched by coordination to a Co₂(CO)₆ moiety via an acetylene bridge. However, at 77 K this emission is not fully quenched following complexation. Electrochemical studies suggest that an intraligand state is responsible for the emission. Photochemical studies in the presence of PPh₃ indicate that CO loss occurs following broadband irradiation with λ_exc > 400 nm, resulting in the formation of both -pentacarbonyl and -tetracarbonyl photoproducts.

Full text of DOI:10.1021/ic801226p

Inorg. Chem. 2008, 47, 10980-10990
Synthesis, Electrochemistry, and Photophysical Properties of a Series
of Luminescent Pyrene-Thiophene Dyads and the Corresponding
Co2(CO)6 Complexes
Anthony Coleman and Mary T. Pryce*
School of Chemical Sciences, SRC for Solar Energy ConVersion, Dublin City UniVersity,
Dublin 9, Ireland
Received July 2, 2008
A series of pyrene based dyad systems together with their dicobalt hexacarbonyl complexes (1b-6b) were
synthesized. The pyrene-thiophene dyads are luminescent in room temperature solution with luminescence lifetimes
on the nanosecond time scale. At room temperature the dyad emission is quenched by coordination to a Co2(CO)6
moiety via an acetylene bridge. However, at 77 K this emission is not fully quenched following complexation.
Electrochemical studies suggest that an intraligand state is responsible for the emission. Photochemical studies in
the presence of PPh3 indicate that CO loss occurs following broadband irradiation with λexc > 400 nm, resulting in
the formation of both -pentacarbonyl and -tetracarbonyl photoproducts.
Introduction
serve as models for linear molecular wires,11 where π-con-
jugation extends through the length of the organic ligand.2,7,12
The synthesis of aryl acetylene compounds using a
Sonogashira-Hagihara coupling reaction is an excellent route
to conjugated π-systems such as molecular wires and
molecular electronic devices13 and also facilitates incorpora-
tion of heteroaromatic rings such as thiophenes, pyridines,
and large highly conjugated aromatic systems. Coupled
thiophene units are known to be good electronic conduc-
tors,14 as they enhance π-conjugation and thus π-delocal-
ization,13 and promote electronic coupling between terminal
units.15 Moreover, thiophene subunits can be functionalized
to improve their solubility in water. They also offer the
possibility of tethering to a surface via the heteroatom lone
pair of electrons, and undergo electropolymerization.16 In
recent years there has been a growing interest in the use of
dicobalt hexacarbonyl derivatives for the development of
metallodendrimers,17 as precursors in the catalytic Pauson-
In recent years organometallic systems containing transi-
tion metals bound to highly π-conjugated systems have been
the subject of extensive investigations.1 These systems often
contain pyridyl, thienyl,2 phenyl,2,3 and ferrocenyl1,4 units
or large polycyclic aromatic hydrocarbons (PAHs) such as
pyrene,5 perylene,6 phenanthrene, or polypyridine ligands7
such as 2.2′-bipyridine8 or 1,10-phenanthroline9 linked by
alkene, alkyne, or azine type10 bridging units. Such systems
* To whom correspondence should be addressed. E-mail: mary.pryce@
dcu.ie. Phone: +353 1 700 8005.
(1) Caballero, A.; Ta´rraga, A.; Velasco, M. D.; Molina, P. Dalton Trans.
2006, 1390–1398.
(2) Sun, S.-S.; Lees, A. J. Coord. Chem. ReV. 2002, 230, 171–192.
(3) Benniston, A. C.; Chapman, G. M.; Harriman, A.; Sams, C. A. Inorg.
Chim. Acta 2006, 359, 753–758.
(4) (a) Sun, S.-S.; Tran, D. T.; Odongo, O. S.; Lees, A. J. Inorg. Chem.
2002, 41, 132–135.
(5) (a) Tyson, D. S.; Henbest, K. B.; Bialecki, J.; Castellano, F. N. J.
Phys. Chem. A 2001, 105, 8154–8161. (b) Goze, C.; Kozlov, D. V.;
Castellano, F. N.; Suffert, J.; Ziessel, R. Tetrahedron Lett. 2003, 44,
8713–8716.
(6) Harriman, A.; Izzet, G.; Ziessel, R. J. Am. Chem. Soc. 2006, 128,
10868–10875.
(7) Leroy-Lhez, S.; Fages, F. C. R. Chimie 2005, 8, 1204–1212.
(8) (a) Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel, R. Chem.sEur. J.
1999, 5 (11), 3366–381. (b) Harriman, A.; Hissler, M.; Ziessel, R.
Phys. Chem. Chem. Phys. 1999, 1, 4203–4211.
(9) Schmittel, M.; Kalsani, V.; Ja¨ckel, F.; Rabe, J. P.; Bats, J. W.; Fenske,
D. Eur. J. Org. Chem. 2006, 3079–3086.
(11) Paul, F.; Lapinte, C. Coord. Chem. ReV.; 1998, 178–180.
(12) Butler, J. M.; George, M. W.; Schoonover, J. R.; Dattelbaum, D. N.;
Meyer, T. J. Coord. Chem. ReV.; 2007, 251, 492–514.
(13) Orita, A.; Ye, F.; Babu, G.; Ikemoto, T.; Otera, J. Can. J. Chem. 2005,
83, 716–727.
(14) Benniston, A. C.; Harriman, A.; Lawrie, D. J.; Mayeux, A.; Rafferty,
K.; Russell, O. D. Dalton Trans. 2003, 4762–4769.
(15) Gao, L.-B.; Kan, J.; Fan, Y.; Zhang, L.-Y.; Liu, S.-H.; Chen, Z.-N.
Inorg. Chem. 2007, 46, 5651–5664.
(16) Pickup, P. G. J. Mater. Chem.; 1999, 9, 1641–1653.
(17) Shiotsuka, M.; Inui, Y.; Sekioka, Y.; Yamamoto, Y.; Onaka, S. J.
Organomet. Chem. 2007, 692, 2441–2447.
(10) Martinez, R.; Ratera, I.; Ta´rraga, A.; Molina, P.; Veciana, J. Chem.
Commun. 2006, 3809–3811.
10980 Inorganic Chemistry, Vol. 47, No. 23, 2008
10.1021/ic801226p CCC: $40.75  2008 American Chemical Society
Published on Web 10/29/2008

Luminescent Pyrene-Thiophene Dyads
Khand reaction18 and as redox active sensors in biological
probes.19 Much recent work has involved the generation of
luminescent metal-containing alkynyl compounds.20 Materi-
als containing a conjugated chromophore such as pyrene or
phenanthrene, and transition metals such as Ru or Pt are
highly luminescent.21 In most cases the excited-state behavior
Chart 1
3
3
of these compounds involves MLCT states, as the π-π*
charge transfer contributions are much higher in energy. This
results in the frequently observed MLCT based luminescence.
It has, however, been observed that by strategically placing
3
the acetylide unit, the energy of the IL (intraligand) state
can be altered.21,22 Castellano and co-workers have reported
3
that the MLCT excited state can be used as an internal
3
sensitizer for the formation of a IL excited state on an
adjacent pyrenylacetylide ligand which results in long-lived
phosphorescence.23,24 Potential applications for such systems
include luminescent probes,21 labels for biomaterials,25,26
DNA probes,27 devices for artificial photosynthesis,28,29
molecular switches and logic gates,30 optoelectronic devices31
such as organic field-effect transistors (OFETS) and light-
emitting diodes,31 or smart polymers.32
In this contribution, we describe the synthesis, character-
ization, electrochemistry, photophysical and photochemical
properties of a series of pyrenylacetylene compounds bearing
a thiophene unit and the corresponding Co2(CO)6 complexes
(Chart 1). Our interest in these systems stems from the
observation that such systems containing polycyclic aromatic
units such as pyrene are highly fluorescent and further
coupling to an extended π-conjugated system replaces the
typical pyrene based emission with CT (charge transfer)
based emission. These highly fluorescent systems are very
sensitive to changes in the properties of the π-conjugated
ligands used, with small changes resulting in notable shifts
in the absorption and emission spectra, as well as metal
coordination which may result in complete fluorescence
quenching. As such these type of systems show great
potential in the area of molecular sensors.14 Fairlamb and
co-workers33 have also shown the viability of metal carbonyl
systems as CO-releasing molecules for therapeutic use in
biological systems. In addition such complexes may be
employed in the synthesis of cyclopentenones via the
Pauson-Khand reaction (Reaction 1),34 where the Co2(CO)6
complexed ligand acts both as the ligand source for cyclo-
pentenone formation and also as the cobalt catalyst. The
cyclopentenone species formed may be further used in the
area of natural product chemistry in the formation of
prostaglandins35 and a number of other pharmaceutically
active compounds.36
Experimental Section
(18) (a) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E. J. Chem.
Soc. D 1971, 36. (b) Gibson, S. E.; Stevenazzi, A. Angew. Chem.,
Int. Ed. 2003, 42, 1800–1810.
(19) Esho, N.; Davies, B.; Lee, J.; Dembinski, R. Chem. Commun. 2002,
332–333.
(20) Lam, S. C.-F.; Yam, V. W.-W.; Wong, K. M.-C.; Cheng, E. C.-C.;
Zhu, N. Organometallics 2005, 24, 4298–4305. (a) Bruce, M. I.; Smith,
M. E.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 2001,
637-639, 484–499.
(21) Danilov, E. O.; Pomestchenko, I. E.; Kinayyigit, S.; Gentili, P. L.;
Hissler, M.; Ziessel, R.; Castellano, F. N. J. Phys. Chem. A 2005,
109, 2465–2471.
Materials. All manipulations were carried out under an atmo-
sphere of argon or nitrogen using standard Schlenk techniques. All
solvents were supplied by the Aldrich Chemical Co. Dichlo-
romethane, chloroform, diethyl ether, pentane and cyclohexane were
dried over MgSO4 prior to use. Ethanol and methanol were distilled
from magnesium turnings before use. Tetrahydrofuran was distilled
from sodium/benzophenone ketyl solution and diisopropylamine
was distilled from potassium hydroxide prior to use. All solvents
used in emission and lifetime experiments were of spectroscopic
grade and were used without further purification. Silica Gel (Merck)
was used as received. All mobile phases for column chromatography
(22) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg.
Chem. 2001, 40, 4053–4062.
(23) Goze, C.; Kozlov, D. V.; Tyson, D. S.; Ziessel, R.; Castellano, F. N.
New. J. Chem. 2003, 27, 1679–1683.
(24) Pomestchenko, I. E.; Castellano, F. N. J. Phys. Chem. A 2004, 108,
3485–3492.
(33) (a) Fairlamb, I. J. S.; Lynam, J. M.; Moulton, B. E.; Taylor, I. E.;
Duhme-Klair, A. K.; Sawle, P.; Motterlini, R. Dalton Trans. 2007,
3603–3605. (b) Fairlamb, I. J. S.; Duhme-Klair, A. K.; Lynam, J. M.;
Moulton, B. E.; O’Brien, C. T.; Sawle, P.; Hammad, J.; Motterlini,
R. Bioorg. Med. Chem. Lett., 16 2006, 995–998.
(34) (a) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E. J. Chem.
Soc., Perkin Trans. 1 1973, 975-977. (b) Khand, I. U.; Knox, G. R.;
Pauson, P. L.; Watts, W. E.; Foreman, M. I. J. Chem. Soc. Perkin
Trans. 1 1973, 977–981. (c) Gibson, S. E.; Stevenazzi, A. Angew.
Chem., Int. Ed.; 2003, 42, 1800–1810.
(25) Khatyr, A.; Ziessel, R. Org. Lett. 2001, 3 (12), 1857–1860.
(26) Evangelista, R. A.; Pollak, A.; Allore, B.; Templeton, E. F.; Morton,
R. C.; Diamandis, E. P. Clin. Biochem. 1988, 21, 173–178.
(27) Okamoto, A.; Ochi, Y.; Saito, I. Chem. Commun. 2005, 1128–1130.
(28) Gust, D.; Moore, T. A. Science 1989, 244, 35–41.
(29) Chakraborty, S.; Wadas, T. J.; Hester, H.; Schmehl, R.; Eisenberg, R.
Inorg. Chem. 2005, 44, 6865–6878.
(30) Van Dijk, E. H.; Myles, D. J. T.; van der Veen, M. H.; Hummelen,
J. C. Org. Lett. 2006, 8 (11), 2333–2336.
(31) Grosshenny, V.; Romero, F. M.; Ziessel, R. J. Org. Chem. 1997, 62,
1491–1500.
(35) (a) Daalman, L.; Newton, R. F.; Pauson, P. L.; Taylor, R. G.;
Wadsworth, A. J. Chem. Res.(s) 1984, 344. (b) Jeffer, H. J.; Pauson,
P. L. J. Chem. Res. (M) 1983, 2201.
(32) Roquet, S.; Cravino, A.; Leriche, P.; Ale´veˆque, O.; Fre`re, P.; Roncalli,
J. J. Am. Chem. Soc. 2006, 128, 3459–3466.
(36) Rautenstrauch, V.; Megard, P.; Conesa, J.; Kuster, W. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 1413.
Inorganic Chemistry, Vol. 47, No. 23, 2008 10981

Coleman and Pryce
were dried over MgSO4 before use. 5-bromo-2-thiophene carbox-
aldehyde was purified by distillation using a Bu¨chi Ku¨gelrohr
apparatus. The starting materials 1-trimethylsilylethynylpyrene and
1-ethynylpyrene were synthesized according to literature methods.8
All other chemicals were obtained commercially and were used
without further purification. All solutions were deoxygenated by
purging with pure argon or nitrogen for ∼10 min. Column
chromatography was carried out using either neutral silica gel pH
6.5-7.5 or neutral aluminum oxide.
General Methods. IR spectra were recorded on a Perkin-Elmer
2000 FTIR spectrophotometer (2 cm-1 resolution) in a 0.1 mm
sodium chloride liquid solution cell using spectroscopic grade
pentane, cyclohexane, or dichloromethane. 1H NMR and 13C NMR
were recorded on a Bruker AC 400 spectrophotometer in CDCl3
or acetone-d6 and were calibrated relative to the residual proton
peaks of the solvent. All UV-vis spectra were measured in
spectroscopic grade solvents on a Hewlett-Packard 8452A-photo-
diode-array spectrometer using a 1 cm quartz cell. Elemental
analysis of C, H, and N were carried out by the Chemical services
Unit, University College Dublin using an Exeter Analytical CE-
440 elemental analyzer. Melting points were measured on a
Gallenkamp melting point apparatus. Electrospray ionization (ESI)
mass spectra were obtained using a Bruker-Esquire LC_00050
electrospray ionization at positive polarity with cap-exit voltage of
77.2 V. Spectra were recorded in the scan range 100-1000 m/z
with an acquisition time of 300-3000 µs and a potential of 30-70
V. Recorded spectra were the summation of 10 scans. Because of
poor solubility and ionization problems, mass spectrometry mea-
surements could not be obtained for the dicobalt complexes using
the available ESI instrumentation.
Steady-State Photolysis. Samples of the dicobalt hexacarbonyl
complexes were dissolved in pentane solution in the presence of
an excess of the trapping ligand triphenylphosphine, such that the
optical density of the νCO bands were approximately 1 A.U. prior
to irradiation. A small sample of this solution was placed in an IR
liquid cell with a 1 mm Teflon spacer and NaCl optical windows.
The solution was irradiated with broadband irradiation at 60 s
intervals using a 300 W xenon-arc lamp, and spectral changes were
monitored by IR spectroscopy.
Syntheses. We report the synthesis of a new series of luminescent
pyrene based dyad systems containing thienyl terminal units linked
by an ethynyl bridge. This synthesis was achieved by reaction of
the previously synthesized 1-ethynylpyrene with the relevant
brominated thienyl ligand via a palladium-catalyzed Sonogashira
cross-coupling reaction. The dicyanovinyl derivatives were syn-
thesized by a Knoevenagel condensation of the relevant aldehyde
in the presence of malonitrile and piperidine. The dicobalt hexac-
arbonyl complexes of these compounds were synthesized by
reaction of the ethynylated ligand with dicobalt octacarbonyl under
mild conditions. These compounds were characterized by FT-IR,
UV-vis, 1H and 13C spectroscopy, mass spectrometry, and elemen-
tal analysis.
by column chromatography on silica gel with a suitable mobile
phase. Solvent was removed by rotary evaporation yielding the
desired compound.
2-Pyrenylacetylene-thiophene (3a). To diisopropylamine (25
mL) was added 1-ethynylpyrene (0.200 g, 0.88 mmol), PdCl2(PPh3)2
(0.037 g, 0.053 mmol, 6%), CuI (0.010 g, 0.053 mmol, 6%), PPh3
(0.028 g, 0.106 mmol, 12%) and 2-bromothiophene (0.085 mL,
0.88 mmol). This solution was then allowed to stir overnight under
inert conditions and subsequently heated to reflux for 5 h to yield
a bright yellow solution. Solvent was removed under reduced
pressure. The yellow residue was purified by column chromatog-
raphy on silica gel with a 4:1 petroleum ether/CH2Cl2 mobile phase.
The desired product eluted off the column as the first intense yellow
band. Solvent was removed by rotary evaporation yielding a brightly
colored yellow solid. Yield: 0.240 g, 0.78 mmol, 88%; IR (CH2Cl2):
(νC≡C) 2199, (νCdC) 1549 cm-1; UV-vis (CH2Cl2): 236, 286, 304,
370, 396 nm; 1H NMR (400 MHz, CDCl3): 8.61 ppm (d, 1H), 8.15
3
4
ppm (m, 8H), 7.45 ppm (q, 1H, J ) 2.4 Hz, J ) 1.2 Hz), 7.37
ppm (q, 1H, 3J ) 4.0 Hz, 4J ) 1.2 Hz), 7.08 ppm (q, 1H, 3J ) 3.6
Hz, J ) 1.6 Hz); 13C NMR (100 MHz, CDCl3): δ 131.0, 128.4,
4
127.4, 127.2, 126.4, 126.3, 126.2, 125.3, 124.7, 124.6, 124.4, 123.5,
116.4, 91.4, 87.2, 40.3, 28.7, 28.0, 21.6, 19.4, 13.3, 10.4; M.P. )
115-117 °C; Mass Spec. ESI m/z 308.1; Anal. Calcd for C22H12S,
C 85.68%, H 3.92%, Found C 85.18%, H 3.96%.
2-Bromo, 5-pyrenylacetylene-thiophene (4a). To 20 mL of
freshly distilled diisopropylamine was added 1-ethynylpyrene (0.400
g, 1.77 mmol), 2,5-dibromothiophene (0.199 mL, 1.77 mmol),
PdCl2(PPh3)2 (0.0742 g, 0.106 mmol), CuI (0.0201 g, 0.106 mmol),
and triphenylphosphine (0.055 g, 0.212 mmol). This dark brown
colored solution was then heated to reflux temperature overnight
under inert conditions to yield an orange colored solution. Solvent
was removed under reduced pressure, and the product purified by
column chromatography on silica gel with a 80:20 petroleum ether/
dichloromethane mobile phase. The desired product eluted off the
column as the first bright yellow band, and solvent was removed
by rotary evaporation. Yield: 0.210 g, 0.54 mmol, 30.6%; IR
(CH2Cl2): 2194 cm-1; UV-vis (CH2Cl2): 238, 286, 310, 378, 400,
1
436 nm; H NMR (400 MHz, CDCl3): 8.5 ppm (d,1H), 8.0 ppm
2
2
(m,8H), 7.0 ppm (d, 1H, J ) 3.8 Hz), 7.2 ppm (d, 1H, J ) 3.8
Hz); 13C NMR (100 MHz, d6-acetone): 134.15, 132.74, 132.23,
132.05, 130.39, 130.06, 129.87, 129.59, 128.16, 127.63, 127.48,
127.34, 126.99, 125.76, 117.41, 101.01, 92.96, 87.50; M.P.:
133-135 °C; Anal. Calcd for C22H11SBr C 68.23%, H 2.86%;
Found C 68.07%, H 2.88%.
(5-(1-Pyrenylacetylene)-2-thiophene)carboxaldehyde (5a). To
20 mL of freshly distilled, argon purged diisopropylamine was
added 1-ethynylpyrene (0.226 g, 1.00 mmol), PdCl2(PPh3)2 (0.0420
g, 0.06 mmol, 6%), CuI (0.0114 g, 0.06 mmol, 6%), and
triphenylphosphine (0.0314 g, 0.12 mmol, 12%) and freshly distilled
5-bromo-2-thiophenecarboxaldehyde (0.118 mL, 1.00 mmol). This
solution was then stirred overnight in the dark under inert conditions
to yield a bright yellow precipitate in solution. The solvent was
removed under reduced pressure to yield a bright yellow solid,
which was purified by column chromatography on silica gel and a
30:70 pentane/CH2Cl2 mobile phase. The desired product was
isolated as a bright yellow-orange solid. Yield: 0.230 g, 0.68 mmol,
68%; IR (CHCl3) 1672, 2197 cm-1; UV-vis (CH2Cl2): 246, 286,
326, 352(sh.), 390(sh.), 404, 420, 372 nm;1H NMR (400 MHz,
CDCl3): 9.86 ppm (s, 1H,), 8.50 ppm (d, 1H), 8.10 ppm (m, 8H),
General Procedure for Preparation of Pyrene-Thiophene
Dyads via a Sonogashira Cross-Coupling Reaction. To freshly
distilled and argon purged diisopropylamine ((i-Pr2)NH) was added
the appropriate substituted ethynylpyrene ligand, followed by
catalytic quantities of PdCl2(PPh3)2, copper(I) iodide, and triph-
enylphosphine. This solution was then purged with argon for a
further 5 min, and the brominated ligand added. The solution was
stirred overnight and subsequently heated to reflux temperature
under inert conditions. In certain cases stirring alone at room
temperature was sufficient for reaction to occur. Solvent was then
removed under reduced pressure. The resulting residue was purified
7.69 ppm (d, 1H, 2J ) 3.9 Hz), 7.42 ppm (d, 1H, 2J ) 3.9 Hz); 13
C
NMR (100 MHz, CDCl3) δ 133.13, 132.51, 132.10, 132.06, 131.18,
130.96, 129.67, 128.93, 128.85, 127.22, 126.48, 126.09, 126.04,
125.09, 124.61, 124.41, 124.17; M.P 120-122 °C; Mass Spec.:
10982 Inorganic Chemistry, Vol. 47, No. 23, 2008

Luminescent Pyrene-Thiophene Dyads
ESI m/z 336.1; Anal. Calcd for C23H12OS: C 82.17%; H 3.59%;
Found C 82.03%, H 3.60%.
136.83, 136.26, 134.43, 134.45, 133.39, 134.43, 13.20, 130.48,
129.92, 129.64, 129.11, 128.17, 127.65, 127.02, 125.78, 89.03,
84.96; M.P: 125-127 °C; Anal. Calcd for C24H12S C 86.72%, H
3.64%; Found C 86.48%, H 3.66%.
(5-(1-Pyrenylacetylene)-2-thiophene)ethylene Malonitrile (6a).
This dicyanovinyl derivative was synthesized via a Knoevenagel
condensation reaction of the previously prepared carboxaldehyde
(5a). To argon purged ethanol (40 mL) was added 5-(1-ethy-
nylpyrene)-2-thiophene carboxaldehyde (0.230 g, 0.68 mmol),
malonitrile (0.044 g, 0.042 mL, 0.68 mmol), and 3-4 drops of
piperidine. This yellow solution was then heated to reflux temper-
atures for 1 h under inert conditions to yield a dark red solution.
Solvent was removed under reduced pressure yielding a dark red-
maroon solid. This residue was purified by column chromatography
on silica gel and eluted with CH2Cl2. The red-orange band was
collected and the solvent removed by rotary evaporation to yield
the desired red product. Yield: 0.21 g, 0.55 mmol, 79%; IR
(CH2Cl2): (νCdC) 1574, (νC≡N) 2227, (νC≡C) 2187 cm-1; UV-vis
General Procedure 2 for the Preparation of Dicobalt Hexa-
carbonyl Complexes. To a flame dried reaction flask was added
pentane (10 mL), which was purged with argon for 10 min. To
this solution were then added molar equivalents of the relevant
substituted ethynylpyrene ligand and Co2(CO)8. This solution was
stirred overnight at room temperature under inert conditions to yield
a dark green-brown solution. The crude product was then subse-
quently purified by column chromatography on silica gel and eluted
with pentane. Solvent was removed by rotary evaporation to yield
a green residue.
1-Trimethylsilylethynylpyrene Dicobalt Hexacarbonyl (1b).
To an argon purged pentane solution (10 mL) was added 1-trim-
ethylsilylethynylpyrene (0.150 g, 0.50 mmol) and Co2(CO)8 (0.170
g, 0.50 mmol). This solution was then stirred overnight at room
temperature under inert conditions to yield a dark green solution.
The crude product was then purified by column chromatography
on silica gel and eluted with pentane to yield a dark green solution.
Solvent was removed to yield a green residue, which was then
recrystallized from hot hexane to yield the pure product. Yield:
0.20 g, 0.34 mmol, 68% (based on PyrCCTMS). IR (pentane): 2086,
2051, 2024 cm-1; UV-vis (pentane): 200, 238, 272, 284, 330, 344,
364, 384 nm; 1H NMR (400 MHz, CDCl3): 8.60 ppm (d, 1H), 8.10
ppm (m, 8H), 0.5 ppm (s, 9H); 13C NMR (100 MHz, CDCl3): δ
134.42, 133.91, 131.26, 130.83, 130.54, 130.40, 130.18, 129.95,
129.39, 129.16, 129.10, 126.32, 126.10, 125.88, 125.71, 125.25,
124.18, 124.05, 123.88, 123.75, 123.30, Anal. Calcd for
C27H18O6SiCo2: C 55.49%, H 3.10%; Found C 55.41%, H 3.22%.
1-Ethynylpyrene Dicobalt Hexacarbonyl (2b). To a flame dried
reaction flask was added 1-ethynylpyrene (0.100 g, 0.44 mmol),
Co2(CO)8 (0.150 g, 0.44 mmol), and 15 mL of pentane. 5 mL of
CH2Cl2 were also added to improve the solubility of the ligand.
This solution was then stirred overnight in darkness and under inert
conditions. This resulted in the formation of a dark green-brown
colored solution. Solvent was removed and the dark brown residue
was purified by column chromatography on silica gel with a 20:80
CH2Cl2/petroleum ether mobile phase. The desired product eluted
off the column as the first dark brown band. Yield: 0.160 g, 0.31
mmol, 70% (based on PyrCCH); IR (pentane): 2092, 2057, 2031
1
(CH2Cl2): 478, 396, 356, 284, 274, 240 nm; H NMR (400 MHz,
CDCl3): 8.5 ppm (d, 1H), 8.2 ppm (m, 8H), 7.8 ppm (s, 1H), 7.7
2
2
ppm (d, 1H, J ) 4.0 Hz), 7.45 ppm (d, 1H, J ) 4.0 Hz); 13C
NMR (100 MHz, CDCl3) δ 148.7, 137.5, 134.6, 131.5, 131.2,
130.86, 130.13, 129.91, 128.78, 128.18, 126.18, 125.55, 125.28,
125.22, 123.96, 123.65, 112.88, 99.62; M.P.: 120-122 °C; Mass
Spec.: ESI m/z 384.1; Anal. Calcd for C26H12N2S: C 81.22%; H
3.14%; N 7.28%; Found C 79.37%, H 3.33%, N 7.12%.
2-Pyrenylacetylene-5-trimethylsilylacetylene-thiophene (7a). To
freshly distilled and argon purged diisopropylamine (20 mL) was
added 2-ethynylpyrene-5-bromo-thiophene (0.237 g, 0.612 mmol),
PdCl2(PPh3)2 (0.514 g, 0.073 g, 6%), CuI (0.0139 g, 0.073 g, 6%),
and PPh3 (0.0385 g, 0.147 mmol, 12%). This solution was purged
for a further 5 min with argon. To this solution was added
trimethylsilylacetylene (0.259 mL, 0612 mmol). The solution was
then heated to reflux temperatures and stirred overnight under inert
conditions yielding a luminescent green-purple solution. Solvent
was removed under reduced pressure to yield a dark green-brown
solid. The crude product was purified by column chromatography
on silica gel with a 9:1 pentane/CH2Cl2 mobile phase. The desired
product eluted off the column as the first green colored luminescent
band. Solvent was removed by rotary evaporation to yield a dark
green-brown solid. Yield: 0.102 g, 0.25 mmol, 42%; IR (CH2Cl2):
(νc≡c) 2144, 2065 cm-1, (νc)c) 1644 cm-1; UV-vis (CH2Cl2): 240,
1
274, 286, 310, 324, 388, 414 nm; H NMR (400 MHz, CDCl3):
2
8.55 ppm (d, 1H), 8.15 ppm (m, 8H), 7.27 ppm (d, 1H, J ) 4
2
Hz), 7.16 ppm (d, 1H, J ) 4 Hz), 0.3 ppm (s, 9H). 13C-NMR
1
cm-1; UV-vis (pentane): 202, 234, 268, 386, 578 nm; H NMR
(100 MHz, CDCl3): δ 185.93, 185.60, 130.66, 130.27, 130.09,
129.74, 128.36, 127.12, 126.80, 126.28, 125.17, 124.61, 124.40,
124.38, 70.60, 69.14; M.P.124-125 °C.
(400 MHz, CDCl3): 8.6 ppm (s, 1H), 8.2 ppm (m, 8H), 7.0 ppm (s,
1H), 2.2 ppm (s, 1H); 13C NMR (100 MHz, CDCl3): 131.60, 131.37,
131.25, 131.12, 130.88, 128.05, 128.01, 127.50, 126.38, 125.79,
125.62, 124.83, 124.60, 124.18; Anal. Calcd for C24H10O6Co2: C
56.28%, H 1.97%; Found C 56.18%, H 1.98%.
2-Pyrenylacetylene-5-acetylene-thiophene (8a). To freshly
distilled and argon purged methanol (10 mL) was added 2-ethy-
nylpyrene-5-trimethylsilylacetylene-thiophene (0.1022 g, 0.25 mmol).
This was left to stir and once that the latter had dissolved, K2CO3
(0.0052 g, 0.0375 mmol, 15%) was added. This solution was then
allowed stir at room temperature for 4-5 h. The solvent was
removed under reduced pressure, and the resulting residue dissolved
in 50 mL of CH2Cl2. This solution was then washed with 4 × 25
mL aliquots of a 5% w/v aq. NaHCO3 solution. The organic
fractions were combined and dried over magnesium sulfate. The
combined fractions were filtered, and the solvent removed by rotary
evaporation to yield a cream colored solid with near quantitative
yields. Yield: 0.082 g, 0.25 mmol, 98%; IR (pentane): (νc≡c) 2038,
2159 cm-1; UV-vis (pentane): 284, 302, 316, 374, 382, and 404
2-Pyrenylacetylene-thiophene Dicobalt Hexacarbonyl (3b). To
argon purged hexane (20 mL) was added 2-pyrenylacetylene (0.104
g, 0.336 mmol) and Co2(CO)8 (0.116 g, 0.336 mmol) and a few
drops of CH2Cl2 to improve solubility of the ligand. This solution
was then stirred overnight under inert conditions and in darkness.
Overnight stirring resulted in the formation of a dark green-brown
colored solution. Solvent was removed by rotary evaporation to
yield a dark green-brown residue. This was purified by column
chromatography on silica gel with a hexane mobile phase. The
desired product eluted off the column as an intense dark green-
brown band, which was collected and solvent removed to yield a
dark green-brown colored solid. Yield: 0.070 g, 0.11 mmol, 33%
(based on compound 3a); IR (CH2Cl2): 2089, 2056, 2028, 1602
cm-1; UV-vis (CH2Cl2): 236, 286, 304, 352, 370, 396, 509 nm;
1H NMR (400 MHz, CDCl3): 8.61 ppm (d, 1 H), 8.13 ppm (m,
1
nm; H NMR (400 MHz, CDCl3): 8.6 ppm (d, 1H), 8.2 ppm (m,
8H), 7.3 ppm (d, 1H, 2J ) 3.8 Hz), 7.2 ppm (d, 1H, 2J ) 3.8 Hz),
3.4 ppm (s, 1H); 13C NMR (100 MHz, d6-acetone): 141.87, 141.76,
Inorganic Chemistry, Vol. 47, No. 23, 2008 10983

Coleman and Pryce
2
8H), 7.44 ppm (d, 1H, J ) 2.4 Hz), 7.37 ppm (d, 1H,2J ) 4.8
157.1, 152.3, 151.6, 142.0, 140.8, 131.4, 130.8, 129.2, 128.4, 127.7,
127.0, 126.9, 126.7, 125.8, 119.9, 111.6, 109.6, 108.9, 104.6, 100.5,
100.0, 95.4, 55.1; Anal. Calcd for C32H12N2O6SCo2: C 57.33%;
H1.80%; N 4.17%; Found C 57.03%, H 1.85%, N 4.18%.
Photophysical Measurements. Emission spectra (accuracy (
5 nm) were recorded at 298 K using a LS50B luminescence
spectrophotometer, equipped with a red sensitive Hamamatsu R928
PMT detector, interfaced with an Elonex PC466 employing Perkin-
Elmer FL WinLab custom built software. Luminescence lifetime
measurements were obtained using an Edinburgh Analytical Instru-
ments (EAI) Time-Correlated Single-Photon Counting apparatus
(TCSPC) as described by Browne et al.37 Emission lifetimes were
calculated using a single exponential fitting function, involving a
Levenberg-Marquardt algorithm with iterative reconvolution (Ed-
inburgh Instruments F900 software) and are (10%. The ꢀ2 and
residual plots were used to judge the quality of the fits.
Electrochemistry Measurements. Cyclic voltammetry experi-
ments were carried out using a CH instruments Model 600a
electrochemical workstation at a scan rate of 0.1 V s-1. Electro-
chemical studies were conducted using a three-electrode system
with a 0.1 M solution of TBAPF6 in anhydrous acetonitrile as the
supporting electrolyte. The working electrode was either a 3 mm
diameter Teflon shrouded glassy carbon or platinum electrode,
which was polished before each use. The counter electrode was a
platinum wire, and the reference electrode was a nonaqueous Ag/
Ag+ electrode. Deoxygenation of the solutions was achieved by
bubbling through with argon for approximately 10 min, and a
blanket of argon was maintained over the solution during all
experiments. The filling solution for the nonaqueous Ag/Ag+
reference electrode was 0.1 M TBAPF6 and 0.1 mM AgNO3 in
anhydrous acetonitrile. All potentials are quoted with respect to
the potentials of the ferrocene/ferrocenium couple (E0′ ) 0.31 V
vs Ag/Ag+).
Hz), 7.09 ppm (t, 1H,3J ) 4.4 Hz, 3J ) 3.6); 13C NMR (100 MHz,
CDCl3): δ 131.22, 129.39, 128.42, 128.38, 127.82, 127.40, 127.23,
127.00, 126.98, 126.64, 126.52, 126.44, 126.32, 126.26, 126.21,
124.67, 124.63, 124.67, 124.63, 124.60, 124.43, 123.43; Mass Spec.
ESI m/z 308 [M+ - Co2(CO)6], 230.2 [Co2(CO)4], 202.2
[Co2(CO)3]; Anal. Calcd for C28H12O6SCo2: C 56.59%, H 2.04%;
Found C 55.08%, H 2.15%.
2-Bromo,5-pyrenylacetylene-thiophene Dicobalt Hexacar-
bonyl (4b). To argon purged hexane (20 mL) was added 2-bromo,5-
ethynylpyrene-thiophene (0.100 g, 0.258 mmol) and Co2(CO)8
(0.088 g, 0.258 mmol). This solution was then stirred overnight
under inert conditions to yield a dark green-brown colored solution.
Solvent was removed under reduced pressure to yield a dark green-
brown residue, which was purified by column chromatography on
silica gel with petroleum ether (40:60°). The desired product eluted
off the column as a dark green band. Solvent was removed to give
a dark green solid. Yield: 0.098 g, 0.14 mmol, 54% (based on
compound 4a); IR (CH2Cl2): 2090, 2057, 2030 cm-1; UV-vis
1
(Hexane): 204, 236, 284, 308, 368, 396, 578 nm; H NMR (400
MHz, CDCl3): 8.43 ppm(d, 1H, J ) 8 Hz), 8.24 ppm (m, 8H),
7.18 ppm (d, 1H, J ) 4.0 Hz), 7.15 ppm (d, 1H, J ) 4.0 Hz); 13
C
NMR (100 MHz, CDCl3): 29.71, 30.96, 120.38, 121.74, 123.73,
124.10, 124.56, 125.30, 125.77, 126.33, 127.23, 128.11, 128.43,
128.59, 129.44, 130.24, 130.99, 131.20, 131.50, 132.27; Anal. Calcd
for C28H11O6SBrCo2: C 49.95%, H 1.65%; Found C 50.21%, H
1.47%.
(5-(1-Pyrenylacetylene)-2-thiophene)carboxaldehyde Dico-
balt Hexacarbonyl (5b). To a flame dried round-bottom flask was
added [5-(1-ethynylpyrene)-2-thienyl]carboxaldyde (0.100 g, 0.297
mmol) and Co2(CO)8 (0.111 g, 0.326 mmol). To this was added
pentane (20 mL) and 4-5 drops CH2Cl2 to improve the solubility
of the aldehyde. This solution was then stirred overnight in darkness
and under inert conditions. This resulted in the formation of a dark
green solution. Solvent was removed by rotary evaporation to yield
a dark green solid. This was purified by column chromatography
on silica gel with a 70:30 CH2Cl2/pentane mobile phase and the
solvent removed to give a dark green crystalline solid. Yield: 0.152
g, 0.24 mmol, 83% (based on 5a); IR (CH2Cl2): 2093, 2060, 2033,
1664 cm-1; UV-vis (CH2Cl2): 236, 282, 366, 390, and 602 nm;
1H NMR (400 MHz, CDCl3): 9.9 ppm (s, 1H), 8.3 ppm (d, 1H),
Results and Discussion
The structures of the pyrene-thiophene dyads and the
corresponding complexes synthesized in this study are
presented in Scheme 1. The ligands were prepared using a
Sonogashira cross-coupling reaction with 1-ethynylpyrene
in each case providing the pyrenyl group. 1-Ethynylpyrene
(2a) was initially prepared from 1-bromopyrene via a
trimethylsilyl protected derivative.8 The dicobalt hexacar-
bonyl (Co2(CO)6) substituted complexes were prepared by
stirring the appropriate ligand and Co2(CO)8 in pentane at
room temperature overnight under inert conditions. In the
case of compounds 5a and 6a dichloromethane was added
to improve the solubility of the ligand. The Sonogashira cross
coupling reactions were catalyzed by PdCl2(PPh3) (6 mol
%), copper (I) iodide (6 mol %), and triphenylphosphine (6
mol %) in the presence of i-Pr2NH to remove the nascent
acid formed (Scheme 2).
2
8.1 ppm (8H, m), 7.7 ppm (d, 1H, J ) 3.9 Hz), 7.2 ppm (s, 1H,
2J ) 3.9 Hz); 13C NMR (100 MHz, CDCl3): 182.80, 182.55, 182.49,
175.35, 143.49, 137.31, 133.46, 132.89, 131.69, 130.73, 128.30,
126.54, 125.99, 125.87, 125.62, 119.95, 118.94, 108.67; Anal. Calcd
for C29H12O7SCo2: C 55.97%, H 1.94%; Found C 55.65%, H 1.20%.
(5-(1-Pyrenylacetylene)-2-thiophene)ethyleneMalonitrileDi-
cobalt Hexacarbonyl (6b). To 20 mL of pentane was added 5-(1-
ethynylpyrene)-2-thienyl]methylene malonitrile (0.100 g, 0.26
mmol), dicobalt octacarbonyl (0.097 g, 0.0286 mmol, 10% excess),
and a few drops of CH2Cl2 to improve solubility. This solution
was then stirred overnight at room temperature under inert
conditions to yield a dark green colored solution. Solvent was
removed and the resulting green-brown residue purified by column
chromatography on silica gel with a 40:60 CH2Cl2/pentane mobile
phase. The second dark green band due to the desired product was
collected and the solvent removed to give a dark green solid. Yield:
0.107 g, 0.16 mmol, 62% (based on 6a); IR (CH2Cl2): (νCO) 2093,
2062, 2035, (νCN) 2226, (νCdC) 1569 cm-1; UV-vis (CH2Cl2): 236,
UV-vis Absorption. Electronic absorption data for the
compounds and complexes in this study are summarized in
Table 1. The absorption spectra for compounds 5a, 5b, and
6a in CH2Cl2 are presented in Figure 1. The pyrenylacetylene
chromophores in each case are responsible for the high
energy π f π* transitions, observed in the range 320-400
1
276, 286, 350, 368, 386, 590 nm; H NMR (400 MHz, CDCl3):
8.30 ppm (d, 1H), 8.20 ppm (m, 8H), 7.75 ppm (s, 1H), 7.75 ppm
(d, 1H, J ) 4.0 Hz), 7.30 ppm (d, 1H, J ) 4.0 Hz); 13C NMR
2
2
(37) Browne, W. R.; O’Connor, C. R.; Villani, C.; Vos, J. G. Inorg. Chem.
2001, 40, 5461–5464.
(100 MHz, d6-acetone) δ 172.3, 171.7, 166.6, 162.1, 158.4, 157.6,
10984 Inorganic Chemistry, Vol. 47, No. 23, 2008

Luminescent Pyrene-Thiophene Dyads
Scheme 1. Overview of the compounds synthesised in this study: (i) pyrene ligand, (i-Pr2)NH, PdCl2(PPh3)2, CuI, PPh3 (6, 6, and 12 mol %); (ii)
n-Pentane, Co2(CO)8; (iii) MeOH, K2CO3, CH2Cl2, 5% w/v NaHCO3 (aq.); (iv)EtOH, malonitrile, piperidine, 60 °C, 1 h
nm.38 The broad low energy band evident in compound 6a
at 478 nm is attributed to an intramolecular charge transfer
(ICT) transition from the pyrenylacetylene-thienyl moiety
to the electron accepting dicyanovinyl unit.32 Coordination
to Co2(CO)6 (compounds 1b-6b) results in broader, less
well-defined spectra with additional weak d-d bands between
580-600 nm, which extend to approximately 700 nm.17,39,40
In compound 6b, the presence of the Co2(CO)6 unit reduces
the λmax of the ICT transition to approximately 380 nm, and
this band overlaps the weak ligand based π f π* transition.
For both the thiophene and bromothiophene systems (3a and
4a, respectively), no noticeable shift in the λmax value of
approximately 400 nm assigned to π f π* transitions is
observed following complexation with Co2(CO)6 (3b and 4b).
Luminescence Studies. Room temperature emission stud-
ies of the ligands and complexes in this study were carried
out in CH2Cl2 as were luminescence lifetime measurements
using the SPC technique. For all organic ligands in this series,
irradiation of each of the different absorption bands resulted
in the same emission band. Solutions were prepared such
that they were isoabsorbtive at a selected wavelength, having
an optical density in solution of approximately 0.3 A.U. No
shift in the band position was observed, for the cobalt
complexes, when compared to the pyrene ligands, which
suggests that following complexation the origin of the
emission band is ligand based. As is evident in Figure 2
visible excitation led to structured luminescence for all
pyrenyl-acetylide compounds in this study.
Compounds 3a-8a are strongly luminescent at room
temperature (298 K) in dichloromethane solution. In com-
pounds 3a and 4a the emission maxima are close to the
lowest-energy absorption band (see Table 1), being separated
by ∼500 cm-1. However, in contrast the Stokes shifts
observed for compounds 5a and 6a are significantly larger
at ∼3800 cm-1 and ∼4600 cm-1, respectively. This may be
as a result of the destabilizing effect of the more electro-
negative aldehyde and dicyanovinyl terminal units.41 Com-
pound 3a exhibits two emission bands centered at 404 and
425 nm. The emission bands of the bromothiophene analogue
(4a) are red-shifted to 410 and 430 nm, respectively. As
shown in Figure 2, both 5a and 6a exhibit a single broad,
intense emission band at 503 and 612 nm, respectively. This
red-shift of ∼3550 cm-1 in the emission maximum, observed
(38) Kozlov, D. V.; Tyson, D. S.; Goze, C.; Ziessel, R.; Castellano, F. N.
Inorg. Chem. 2004, 43 (19), 6083–6092.
(39) Diederich, F.; Rubin, Y.; Chapman, O. L.; Goroff, N. S. HelV. Chim.
Acta 1994, 77, 1441.
(40) Low, P. J.; Rousseau, R.; Lam, P.; Udachin, K. A.; Enright, G. D.;
Tse, J. S.; Wayner, D. D. M.; Carty, A. J. Organometallics 1999, 18,
3885–3897.
(41) Raposo, M. M. M.; Sousa, A. M. R. C.; Kirdch, G.; Cardoso, P.;
Belsley, M.; de Matos Gomes, E.; Fonseca, A. M. C. Org. Lett., 2006,
8 (17), 3681–3684.
Inorganic Chemistry, Vol. 47, No. 23, 2008 10985

Coleman and Pryce
Scheme 2. Structures of Triphenylphosphine Derivatized
Photoproducts Formed, Following Steady State Photolysis with λexc.
520 nm and λexc. > 400 nm
>
Figure 1. Overlay of the electronic absorption spectra of 5a (black line),
5b (blue line), and 6a (red line) in CH2Cl2.
Table 1. Absorption Properties and Extinction Coefficients (ε) for the
Compounds in This Study
compound
λmax (nm), ε (× 107 M-1 cm-1
)
1a
272 (2.50), 284 (1.60), 330 (1.86), 344 (2.99), 364 (1.30),
384 (2.03)a
1b
2a
272 (2.94)b 284 (2.77)b, 3
Figure 2. Room temperature emission spectra of compounds 5a (solid line)
(7.65 × 10-9 mol L-1) and 6a (dashed line) (1.03 × 10-8 mol L-1) in
CH2Cl2 solution.
observed for thiophene-pyrene systems.13,14 One possibility
for the increase in lifetimes observed in this study is the
increase in electron delocalization within the molecule, on
replacing H (3a), by electron accepting groups such Br (4a),
CHO (5a), and CH(CN)2 (6a).
30 (1.34)b, 344(1.64)b, 364 (1.90)b, 388 (2.41)b, 580 (1.02)a
272 (2.30), 284 (1.04), 314 (7.11)a, 328 (1.84), 342 (2.11),
360 (1.31), 384 (3.75)a
2b
3a
3b
236 (4.93), 268 (3.16), 386 (3.06), 580 (2.35)a
286 (3.13), 304 (2.70), 370 (3.24), 396 (2.15)
286 (4.64)b, 304 (4.01)b,
352 (4.29)b, 370(3.92)b, 396 (2.60)b, 580
286 (3.39), 310 (2.97), 378 (4.06), 400 (3.06)
286 (3.44)b, 310 (2.42)b,
4a
4b
354 (2.49)b, 378 (3.33)b, 402 (3.31)b,
580 (7.33)a
As is evident in Figure 3, following coordination with
Co2(CO)6, (complexes 3b, 4b, 5b, and 6b), a large reduction
in emission intensity (up to 95%) is observed. However, no
shift in the position of the emission band is obvious, thereby
suggesting, that in complexes 3b-6b, the observed emission
is ligand based without significant MLCT contribution. The
emission lifetimes obtained for the dicobalt complexes (3b,
4b, 5b, 6b) are similar to those discussed above for the
uncomplexed pyrenyl ligands. The excitation spectra of
compounds 3a-8a have the same shape as the corresponding
absorption spectra in the range 280-400 nm, further sug-
gesting that the emissive excited state is centered on the
ligand. The excitation spectra of 6a (Figure 4) also has the
same shape as the corresponding electronic absorption
spectrum for this compound in the range 280-550 nm,
5a
286 (3.04), 326 (3.11), 352 (2.33), 372 (3.46), 390 (3.92),
402 (3.88), 420 (3.59)
5b
6a
280 (3.82), 360 (2.23), 388 (2.42), 580 (1.69)a
274 (1.88), 284 (2.07), 294 (1.26), 356 (3.37), 396 (1.93),
478 (2.92)
6b
276 (4.95), 282 (5.04), 350 (3.33), 368 (3.81), 386 (4.35),
590 (3.06)a
7a
8a
274, 286, 310, 324, 388, 414
274 (8.07)a, 286 (1.13), 306 (8.07)a,
318 (1.20), 379 (2.03), 388 (1.56), 410 (1.54)
a ε ) 1 × 106 M-1 cm-1
.
b ε ) 1 × 108 M-1 cm-1
.
on altering the terminal end, is attributed to the improved
electron-accepting ability of the dicyanovinyl unit. The
emission lifetimes were measured as 0.16, 0.64, 0.88, and
1.16 ns, for compounds 3a, 4a, 5a, and 6a, respectively.
These values are similar to emission lifetimes previously
10986 Inorganic Chemistry, Vol. 47, No. 23, 2008

Luminescent Pyrene-Thiophene Dyads
Table 2. Luminescence Properties (298 and 77 K) of the Ligands and
the Complexesc
298 Ka
λem (nm)
387, 406
391, 409
385, 405
395, 425, 540
404, 425
400, 424
410, 430
410, 430
503
77 Kb
λem (nm)
compound
τ (ns)
τ (ns)
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
2.1, 2.1
2.3, 2.4
17.0
17.9
0.16
0.14
0.64
0.63
0.88
0.23
1.16
0.99
1.1, 0.2
384, 404, 420
384, 405
382,393,403, 420
377, 420, 482, 530
404, 425
70.9
42.2,
91.7
3.35
1.77
0.88
0.45
0.75
55.0
0.52
1.73
0.96
1.49
400, 424
410, 430, 445
404, 430, 443
444, 468
503
612
612
420, 440
445
520
516
415, 440, 455
a Room temperature (298 K) luminescence spectra and excited-state
lifetimes recorded in spectrophotometric grade CH2Cl2. b Spectrum recorded
in hexane. c Low-temperature (77 K) luminescence spectra and excited-
state lifetimes recorded in 4:1 EtOH:MeOH.
Figure 3. Room temperature (298 K) emission spectra of compounds 5a
and 5b in CH2Cl2. The emission spectra of the uncomplexed aldehyde (5a)
is represented by (black solid line) while that of the Co2(CO)6 complexed
analogue (5b) is similarly indicated by (red solid line). Also shown are the
analogous 77 K emission spectra for both 5a (green dashed line) and 5b
(blue dashed line) in 4:1 EtOH:MeOH.
Emission quenching has previously been reported for a
few examples of metal carbonyl complexed fluorescent
organic compounds. These include [Co2(CO)6(µ-η2-RCH2-
CCH)], [Fe2(CO)6(µ-StBu)(µ-η2-RCH2C)CH2)], [Ru3(CO)9(µ-
CO)(µ3-η2-RCH2CCH)], and [Os3(CO)9(µ-CO)(µ3-η2-RCH2-
CCH)], where R are the highly fluorescent acridone or
5-(dimethylamino)naphthalene-1-sulfonyl (dansyl) lumino-
phores.42 The fluorescence quantum yields for the complexes
are 1 to 2 orders of magnitude less than either the acridone
or dansyl fluorophores. Ruthenium(II) polypyridyl complexes
anchored with [Co2(CO)4(dppm)] systems have also dem-
onstrated emission quenching upon coordination of
[Co2(CO)4(dppm)].17 Other metal carbonyl complexed pyre-
nyl ligands include tricarbonylrhenium complexes of N,N′-
bis[(pyrenyl)pyrazolyl]alkanes where the emission is also
quenched.43 This is in contrast to the luminescent N,N′-
diimine derivatives of tricarbonylrhenium.44 In this study we
have attributed quenching to energy transfer to the cobalt
carbonyl center, which is in agreement with the systems
previously reported.17,42,43 Support for such a mechanism
comes from quenching experiments using pyrene and (µ2-
C2(C6H5)2)Co2(CO)6 as model compounds, which were used
to construct a Stern-Volmer plot, generating a bimolecular
quenching constant of 2.15 × 1010 L mol-1s-1(see Supporting
Information for details).
Figure 4. Overlay of excitation spectrum (solid line) and electronic
absorption spectrum (dashed line) of the dicyanovinyl derivative, 6a, in
CH2Cl2 solution at 298 K.
thereby further confirming that the emissive excited state
originates from the ligand.
Electrochemistry. Cyclic voltammetry studies were car-
ried out on compounds 1a-8a in a 0.1 M TBAPF6/CH3CN
electrolyte solution (Table 3). All compounds exhibited an
oxidation in the range 1.0-1.1 V versus Ag/Ag+. This is
assigned to irreversible oxidation of the pyrene unit based
on previous reports in the literature,14,17 where irreversible
oxidation of the pyrene unit in a pyrene-thiophene dyad was
observed at 1.25 V versus Ag/AgCl. It is suggested that this
process is irreversible because of the generation of a dimer
cationic species by association of the oxidized pyrene with
Low temperature (77 K) emission studies were also
undertaken in 4:1 EtOH/MeOH glasses (Table 2). At 77 K
no change in the emission band position of compounds 3a
and 4a is observed while the emission band of compound
5a is blue-shifted by 2650 cm-1 to 444 nm (Figure 3). The
emission band of compound 6a is blue-shifted by ∼2890
cm-1 at 77 K. In all four cases, as observed in the room
temperature studies, complexation does not result in the
generation of new emission bands. This again suggests a
ligand based lowest lying excited state for the compounds
in this series. This is in agreement with a number of
studies15-17 in which the emissive excited state of pyreny-
lacetylene bearing compounds are assigned as being pre-
dominantly IL charge transfer in nature.
(42) Wong, W.-Y.; Choi, K.-H.; Lin, Z. Eur. J. Inorg. Chem. 2002, 2112–
2120.
(43) Reger, D. L.; Gardinier, J. R.; Pellechia, P. J.; Smith, M. D.; Brown,
K. J. Inorg. Chem. 2003, 42, 7635–7643.
(44) Lo, K. K.-W.; Tsang, K. H.-K.; Zhu, N. Organometallics 2006, 25,
3220–3227.
Inorganic Chemistry, Vol. 47, No. 23, 2008 10987

Coleman and Pryce
Table 3. Electrochemical Properties at Room Temperature
b
compound
Ep,A (V) [Irr]a
Ep,C (V) [Irr]
-1.95, -1.75, -1.60
-1.95, -1.76, -1.56, -1.28, -1.16
-0.39, -1.35
E1/2 (V) [∆Ep (mV)]c
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
0.41, 0.75, 0.97
0.16, 0.39, 0.51, 0.64, 0.86, 0.97
0.649, 1.09
0.51, 0.64, 0.95, 1.07
0.02, 0.90
0.56, 0.92
0.95, 1.07
0.67, 0.95, 1.02
1.02
0.56, 0.69
0.48, 1.03
-1.27 (71)
-2.11, -1.56, -1.81,
-1.91, -1.71,
-1.84, -1.22,
-1.07, -1.19, -1.88
-1.21 (81)
-1.20 (86)
-2.08 (49)
-1.88 (98), -1.67 (56)
0.99 (45), -1.86 (44), -1.66 (59)
-1.48, -1.18, -1.09
-1.64, -1.36, -1.10 (42)
-1.37
0.92, 0.71, 0.56, 0.36
1.00
-1.14 (74)
-1.06, -1.26, -1.83
a Indicates an electrochemically irreversible process. b Indicates a fully reversible electrochemical process. c E1/2 is the separation between oxidation and
reduction peak in mV (V vs Ag/Ag+, scan rate 0.1 V s-1 in a 0.1 M TBAPF6/CH3CN solution).
Figure 5. Cyclic voltamogramm of compound 5a in a TBAPF6/CH3CN
electrolyte solution.
Figure 6. IR difference spectra following broadband irradiation of 5b in
pentane at λexc > 400 nm. Negative bands indicate bleaching of the parent
Table 4. M-CO Band Positions in Pentane for the Parent Dicobalt
bands while positive bands indicate formation of the pentacarbonyl (*) and
Hexacarbonyl Complexes and the Photoproducts Observed in This Study
tetracarbonyl species (#). Spectra obtained at 60 s intervals over total
complexa
νCO (cm-1 ( 2 cm-1
)
irradiation time of 10 min.
1b
1c
1d
2b
2c
2d
3b
3c
3d
4b
4c
5b
5c
5d
6b
6c
6d
2086, 2051, 2024
2059, 2012. 2001
1984, 1968, 1941
2092, 2057, 2030
2064, 2017, 2005
1979, 1971, 1947
2090, 2057, 2031
2064, 2021, 2007
1973, 1924
2091, 2058, 2033
2064, 2022, 1974
2093, 2060, 2033
2067, 2022, 2013
2001, 1975
onto the working and counter electrode surfaces, which
perturbs further electrochemical measurements. This explains
loss of the characteristic redox processes following oxidation
at high potentials. Furthermore, this may suggest that
electropolymerization occurs at high positive potentials and
requires further study as the electropolymerization processes
in thiophene systems is known.45 Following complexation
with Co2(CO)6 an irreversible oxidation process is observed
in each case in the range 0.55-0.70 V versus Ag/Ag+. This
is assigned to oxidation of the cobalt metal center and is in
agreement with previous literature observations for similar
systems where E1/2(Co1+/0) was observed in the range
0.70-0.90 V.17,19,46
2093, 2062, 2035
2067, 2025, 2014, 2001
1964, 1948, 1977
a Parent complexes 1b-6b are indicated in bold. Complexes labeled c
and d refer to the corresponding pentacarbonyl and tetracarbonyl photo-
products, respectively.
The cyclic voltammogram of compound 1a also exhibits
a quasi-reversible oxidation wave at 0.41 V and a quasi-
reversible reduction wave at -1.60 V versus Ag/Ag+,
possessing an energy band gap (∆Ep) of 120 mV,
a second pyrene subunit.14 Oxidation at higher potentials
(+1.4 V) leads to gradual loss of all characteristic oxidation
and reduction potentials. This phenomenon has been previ-
ously observed38 and has been assigned to decomposition
of the complex at the electrode surface. We have however
observed that in each case oxidation at high positive
potentials results in “polymerization” or deposition of a film
(45) Areephong, J.; Kudernac, T.; de Jong, J. J. D.; Carroll, G. T.;
Pantorotta, D.; Hjelm, J.; Browne, W. R.; Feringa, B. L.; J. Am. Chem.
Soc., 2008 (in print).
(46) Bahadur, M.; Allen, C. W.; Geiger, W. E.; Bridges, A. Can. J. Chem.
2002, 80, 1393–1397.
10988 Inorganic Chemistry, Vol. 47, No. 23, 2008

Luminescent Pyrene-Thiophene Dyads
while ipa/ipc ) 1. This suggests that this process is a quasi-
reversible one-electron process and is assigned to the
reduction of the pyrene unit. However, in the case of the
associated dicobalt hexacarbonyl complex (1b), an irrevers-
ible oxidation assigned to oxidation of the cobalt metal center
is also observed at 0.511 V versus Ag/Ag+. Complexation
also shifts the quasi-reversible redox processes to lower
potentials by approximately 20 mV. This indicates that
coordination of the cobalt hexacarbonyl moiety reduces the
reversibility of the redox system perhaps as a result of
increased charge transfer from the pyrene unit to the metal
center. The presence of the irreversible pyrene based
oxidation is especially obvious in compound 2a, where this
oxidation is observed at 1.09 V versus Ag/Ag+. Following
complexation a broad irreversible oxidation assigned to
oxidation of the cobalt metal centers is observed at 0.64 V
versus Ag/Ag+. A quasi-reversible reduction at E1/2 ) -1.27
V versus Ag/Ag+ present may be due to the reduction of
the 1-ethynylpyrene unit, not previously observed for the free
ligand.
The cyclic voltammogram of 3a exhibits an irreversible
oxidation at 0.90 V because of oxidation of the pyrene
moiety. A quasi-reversible reductive process also observed
at E1/2 ) -1.80 V is assigned to the one-electron reduction
of the pyrene moiety on the basis of similarities with a
literature value of -1.59 V versus Ag/AgCl observed for
the reduction of pyrene in an analagous system.14 For the
associated complex 3b a cobalt based irreversible oxidation
is also observed at 0.56 V versus Ag/Ag+. The cyclic
voltammogram of the bromothiophene analogue (4a) also
exhibits an irreversible reduction process at -1.88 V assigned
to the one-electron reduction of the pyrene unit, while a
quasi-reversible reductive feature at -2.08 V is assigned to
the one electron reduction of the thiophene unit. This low
reduction potential for the thiophene unit may be attributed
to the electron withdrawing effect of the appended Br.
Following coordination of Co2(CO)6 (4b) an irreversible
cobalt centered oxidation is observed at 0.67 V. Although
the one-electron reduction of the pyrene unit remains at
approximately -1.8 V, going from -1.84 V for 4a to -1.88
V for the associated Co2(CO)6 complex, the band at -2.08
V previously assigned to the quasi-reversible reduction of
the thiophene unit is shifted to lower reduction potentials
on complexation. This lowering of the thiophene reduction
potential to -1.19 V is due to increased electron donation
following complexation and results in a thiophene based
reduction potential, which is more in line to those observed
in the literature, where the irreversible reduction of the
thiophene subunit in a similar dyad system was observed at
-1.40 V versus Ag/AgCl.47
with Co2(CO)6 an irreversible oxidation is observed at 0.67
V versus Ag/Ag+. Coordination of the cobalt metal center
also appears to lower the reduction potential for the thiophene
unit by ∼500 mV to -1.18 V versus Ag/Ag+. Furthermore,
the thiophene based reduction now appears to be irreversible.
As observed for 5a, the complexed analogue 5b also displays
a quasi-reversible reduction because of reduction of the
pyrene unit at a half-wave potential (E1/2) of -1.86 V versus
Ag/Ag+. The cyclic voltammogram of ethynylpyreneth-
iophene carboxaldeyde (5a) is shown in Figure 5, however,
the proximity of the reduction potentials to one another
makes definite assignment difficult.
Compounds 6a and 6b exhibit an apparent irreversible
reduction of the thiophene unit at -1.16 V and an irreversible
reduction of the dicyanovinyl moiety at -1.36 V versus Ag/
Ag+. These reduction potentials are in good agreement with
those observed in the range -1.37 to -1.23 V versus Ag/
AgCl by Roncali and co-workers for dicyanovinyl units in
triphenylamine-thienylenevinylene systems.32
The level of π-conjugation and thus delocalization of
electron density throughout the pyrenylacetylene ligand was
increased by the addition of a further ethynyl unit in the
5-position of the substituted thiophene ring resulting in the
generation of compound 7a. An apparent irreversible oxida-
tion of the pyrene unit was observed at positive potentials
of 1.00 V versus Ag/Ag+ with no further features evident in
this region. In contrast however a number of features are
evident at reductive potentials. The primary reductive process
involves the apparent quasi-reversible reduction of the
thiophene unit at -1.26 V followed by a secondary reductive
process at -1.83 V versus Ag/Ag+ involving reduction of
the pyrene unit.8 Furthermore, a very weak reductive feature
observed at -1.53 V versus Ag/Ag+ between the thiophene
and pyrene based reductions is assigned to the reduction of
the ethynyl terminal unit.8 However, because of its low
intensity and proximity to two very strong reductive poten-
tials, the assignment of this weak reductive feature must
remain tentative.
Steady-State Photolysis. As presented in Table 4, ex-
tended broadband irradiation of complex 1b with λexc. > 520
nm for 600 s in the presence of PPh3 resulted in bleaching
of the parent absorption bands together with the generation
of product bands at 2059, 2012, and 2001 cm-1, assigned to
the pentacarbonyl species (Reaction 2). Subsequent irradia-
tion with λexc > 400 nm for 15 min resulted in further
bleaching of the parent bands and an increase in absorbance
in the bands assigned to the pentacarbonyl species 1c.48,49
Bands were also observed under these conditions at 1984,
1968, and 1941 cm-1 and are assigned to the formation of
the tetracarbonyl species (1d, Reaction 3). Similar results
were obtained for complexes 2b, 3b, and 4b. The assign-
ments made in this study are based on comparisons with
νCO bands for similar systems, such as (µ2-C2H2)Co2-
(CO)5(PPh3) which has stretching vibrations at 2069, 2017,
The aldehyde derivative, 5a, also exhibits a fully reversible
one-electron reduction peak with a half-wave potential
(E1/2) of -1.67 V versus Ag/Ag+, assigned to the reduction
of the thiophene group.47 A quasi reversible reductive process
also observed at E1/2 ) -1.88 V is assigned to the one-
electron reduction of the pyrene moiety.8 On complexation
(48) Gordon, C. M.; Kiszka, M.; Dunkin, I. R.; Kerr, W. J.; Scott, J. S.;
Gebicki, J. J. Organomet. Chem. 1998, 554, 147–154.
(49) Chia, I. S.; Cullen, W. R.; Franklin, M.; Manning, A. R. Inorg. Chem.
1975, 14, 2521–2526.
(47) Benniston, A. C.; Harriman, A.; Lawrie, D. J.; Rostron, S. A.
Tetrahedron Lett. 2004, 45, 2503–2506.
Inorganic Chemistry, Vol. 47, No. 23, 2008 10989

Coleman and Pryce
2009, 1998, 1975 cm-1 and, the tetracarbonyl analogue, (µ2-
C2H2)Co2(CO)4(PPh3)2 which displays νCO bands at 2028,
1982, 1965, and 1948 cm-1.50
sponding Co2(CO)6 complexes have been synthesized. The
photophysical, photochemical, and electrochemical properties
of these novel systems have been studied. All of the
synthesized compounds absorb into the visible region, the
dicyanovinyl compound 6a absorbs further toward the red
region because of an intramolecular charge transfer transition,
while the presence of low energy MLCT transitions in the
dicobalt complexes result in a considerable red shift in the
low energy absorptions. Luminescence studies and cyclic
voltammetry indicate that in all cases the lowest lying excited
state is intraligand charge transfer in nature. It is suggested32
that this charge transfer involves the formation of a
pyrene+-thiophene- excited state. This is further supported
by electrochemical studies in acetonitrile solution. In these
experiments irreversible one-electron oxidation of the pyrene
unit was observed. Complexation with the Co2(CO)6 moiety
has been shown to significantly quench the observed ligand
based emission. This may be as a result of energy transfer
from the luminescent pyrene moiety to the cobalt metal
center. At low temperatures (77 K) this energy transfer
process is not observed, resulting in less efficient quenching
of the ligand based emission for the cobalt carbonyl
complexes. Electrochemical studies also indicate that the
coordinated Co2(CO)6 units are irreversibly oxidized at high
oxidation potentials. Irradiation of the dicobalt carbonyl
complexes in the presence of PPh3 yields both -pentacarbonyl
and -tetracarbonyl species. Low energy broadband irradiation
(λexc > 520 nm) resulted in the production of the pentacar-
bonyl species with apparent low yield, while higher energy
irradiation with λexc > 400 nm, resulted in formation of the
tetracarbonyl species.
Broadband photolysis of complex 5b with λexc > 520 nm
resulted in very little change in the IR spectrum. However
subsequent higher energy irradiation with λexc. > 400 nm,
resulted in the predominant formation of the pentacarbonyl
species 5c (Figure 6), with νCO bands at 2067, 2022, and
2013 cm-1, with additional νCO bands at 2001 and 1975 cm-1
which suggest tetracarbonyl formation. Under similar condi-
tions, broadband irradiation of complex 6b at λexc > 400
nm resulted in bleaching of the parent absorption bands and
generation of νCO bands at 2067, 2025, 2014, 2001, and 1977
cm-1 assigned to the pentacarbonyl species, 6c. No evidence
was obtained for formation of the tetracarbonyl photoproduct,
6d.
Acknowledgment. The authors wish to thank Dublin City
University and Roscommon Co. Council for financial sup-
port.
Conclusion
Supporting Information Available: Figures containg UV-vis
spectra and cyclic voltammograms of the compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
In summary, a series of novel pyrenylacetylene compounds
bearing various substituted thiophene units and their corre-
(50) Draper, S. M.; Long, C.; Myers, B. M. J. Organomet. Chem. 1999,
588, 195–199.
IC801226P
10990 Inorganic Chemistry, Vol. 47, No. 23, 2008