Photophysics of platinum-acetylide substituted hexa-peri-hexabenzocoronenes

Article abstract of DOI:10.1021/ic051788v

This manuscript reports the synthesis and photophysical investigation of two hexa-peri-hexabenzocoronenes (HBCs) that are functionalized with platinum(II) acetylide units of the type trans-(Ar-C≡C-) ₂Pt(PBu₃)₂. In one complex, the platinum is directly linked to the HBC chromophore by an ethynyl spacer, whereas in the second, the platinum is separated from the HBC via a 1,4-phenylene ethynylene spacer. The Pt-acetylide units introduce strong spin-orbit coupling into the HBC chromophore, giving rise to high yields of the triplet excited state along with moderately intense phosphorescence at ambient temperature. On the basis of emission spectroscopy, the triplet state of the HBC chromophore is located at 2.14 eV and the S-T splitting is 0.6 eV. The triplet-triplet absorption and radical cation absorption spectra of the Pt-HBCs are determined by laser flash photolysis. Aggregation of the Pt-HBCs in a poor solvent such as hexane leads to quenching of the triplet state, but spectroscopy provides no evidence for the formation of a triplet excimer, even under conditions where the molecules are strongly aggregated.

Full text of DOI:10.1021/ic051788v

Inorg. Chem. 2006, 45, 2509−2519
Photophysics of Platinum
−Acetylide Substituted
Hexa-peri-hexabenzocoronenes
Kye-Young Kim, Shengxia Liu, Muhammet E. Ko1se, and Kirk S. Schanze*
Department of Chemistry, UniVersity of Florida, P.O. Box 117200,
GainesVille, Florida 32611-7200
Received October 16, 2005
This manuscript reports the synthesis and photophysical investigation of two hexa-peri-hexabenzocoronenes (HBCs)
that are functionalized with platinum(II) acetylide units of the type trans-(Ar )2Pt(PBu3)2. In one complex, the
platinum is directly linked to the HBC chromophore by an ethynyl spacer, whereas in the second, the platinum is
separated from the HBC via a 1,4-phenylene ethynylene spacer. The Pt acetylide units introduce strong spin
−CtC−
−
−
orbit coupling into the HBC chromophore, giving rise to high yields of the triplet excited state along with moderately
intense phosphorescence at ambient temperature. On the basis of emission spectroscopy, the triplet state of the
HBC chromophore is located at 2.14 eV and the S
−T splitting is 0.6 eV. The triplet
−triplet absorption and radical
cation absorption spectra of the Pt HBCs are determined by laser flash photolysis. Aggregation of the Pt
−
−HBCs
in a poor solvent such as hexane leads to quenching of the triplet state, but spectroscopy provides no evidence
for the formation of a triplet excimer, even under conditions where the molecules are strongly aggregated.
Introduction
HBCs that are substituted on the periphery with solubi-
lizing alkyl groups aggregate into discotic liquid crystalline
phases due to their disk shape and large-area planar π
systems.7-9 A number of investigations have been carried
out to characterize the properties of all-organic HBC-based
materials.10,11 These studies have demonstrated that in the
solid state the materials display very high charge carrier
mobility, an effect which is believed to be due to strong π-π
interactions between adjacent molecules in columnar stacks.12
Several investigations have also demonstrated that the
absorption and fluorescence properties of the HBC chro-
mophore are influenced by aggregation, and the effects are
attributed to interchromophore interactions, e.g., π-stacking
and singlet excimer formation in the excited state.9
Large polycyclic aromatic hydrocarbons have been the
focus of considerable research interest during the past several
years. When substituted with alkyl or alkyloxy groups, the
structures are soluble in typical organic solvents, and due to
the planar, disk shape of the aromatic core the molecules
have a strong tendency to aggregate.1,2 Of particular recent
interest is the family of substituted hexa-peri-hexabenzo-
coronenes (HBCs), which have been studied intensively by
Mu¨llen and co-workers.3,4 Although there are early reports
concerning the properties of the unsubstituted parent HBC,5,6
work in this field has been fueled by the development of
synthetic approaches that allow relatively large-scale syn-
thesis of a variety of functionalized HBC derivatives.3,7
Introduction of a transition metal that is strongly coupled
to the HBC can provide a means to tune the optical and
electronic properties of the chromophore without sacrificing
its propensity to self-assemble into aggregates and liquid
* To whom correspondence should be addressed. Tel: 352-392-9133.
Fax: 352-392-2395. E-mail: kschanze@chem.ufl.edu.
(1) Gallivan, J. P.; Schuster, G. B. J. Org. Chem. 1995, 60, 2423-2429.
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118, 3767-3768.
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P.; Mu¨llen, K. Chem.-Eur. J. 2000, 6, 4327-4342.
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10.1021/ic051788v CCC: $33.50
Published on Web 02/11/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 6, 2006 2509

Kim et al.
Chart 1
provides clear evidence that aggregation of the Pt-HBCs
suppresses the triplet yield and lifetime. By contrast,
spectroscopic studies show that the electronic structure of
the triplet state in the Pt-HBCs is not strongly affected by
aggregation, suggesting that triplet state remains localized
on a single HBC chromophore despite the existence of
intermolecular π-π interactions in the aggregates.
Experimental Section
crystalline phases. Although a number of investigations have
explored the properties of all-organic HBCs, to date only
two metal-organic HBC complexes have been reported.13,14
This previous investigation examined the optical and elec-
trochemical properties of an N-heterocyclic HBC derivative
coordinated to -Ru(bpy)22+ or Pd(η3‚C3H5)+ units.13,14 The
Ru-HBC complex displayed a near-infrared photolumines-
cence arising from a Ru f HBC metal-to-ligand charge-
transfer excited state.
The present manuscript reports the synthesis and photo-
physical investigation of two HBCs that are functionalized
with platinum(II) acetylide units of the type trans-(Ar-Ct
C-)2Pt(PBu3)2 (structures 1 and 2, Chart 1). The Pt-
acetylide units are of interest because they introduce strong
spin-orbit coupling into the HBC chromophore, giving rise
to high yields of the triplet excited state along with
moderately efficient phosphorescence at ambient temper-
ature.15-20 The Pt-substituted HBCs are soluble in typical
organic solvents, and like the all-organic HBCs, they exhibit
a strong tendency to aggregate, even in dilute solution.
Because the platinum center enhances intersystem crossing
and phosphorescence, it is possible to probe the effect of
aggregation of the HBC chromophore on the properties of
the triplet excited state in this system. The work presented
herein is unique because most previous studies concerning
the effects of aggregation on the optical properties of
π-conjugated materials have focused on the singlet state.21-26
This is in part because the singlet state is more easily probed
spectroscopically compared to the triplet. The present work
General Methods. All starting materials were used without
further purification unless otherwise noted. Anhydrous iron(III)
chloride (Acros, 98%) was used as received. All reactions were
carried out under argon atmosphere with freshly distilled solvents.
4-Dodecylphenylacetylene and compounds 5, 8, and 9 were
synthesized according to literature procedures.3,18 Column chro-
matography was performed on silica gel (Supelco). H, 13C, and
1
31P NMR spectra were recorded on Varian Mercury 300 or Gemini
300 MHz spectrometers; chemical shifts (δ) are reported in ppm
and referenced to TMS or protiated solvent signals. 31P NMR
chemical shifts were referenced to 80% H3PO4. Mass spectra were
recorded on Finnigan MAT95Q Hybrid Sector (EI, HRMS) or
Bruker Reflex II (MALDI-TOF) mass spectrometers. 1H NMR
spectra of 1, 2, 3, 6, and 7 and 31P NMR spectra of 1 and 2 are
provided in the Supporting Information.
4-Iodo-4′-dodecyldiphenylacetylene (4). A degassed solution
of 4-dodecyl-phenylacetylene3 (1.50 g, 5.55 mmol) in THF (10 mL)
was added dropwise via a cannula to a degassed solution of
p-diiodobenzene (5.49 g, 16.6 mmol), PdCl2(PPh3)2 (12 mg, 17
µmol), CuI (3 mg), and diisopropylamine (10 mL) in THF (20 mL).
The mixture was stirred at room temperature for 5 h. The mixture
was passed through a pad of Celite, and the solvent was removed
in vacuo. The resulting residue was purified by column chroma-
tography on silica gel. Elution with hexane gave 4 as a white solid
(1.3 g, 50%): 1H NMR (300 MHz, CDCl3) δ 7.68 (d, J ) 8.5 Hz,
2H), 7.43 (d, J ) 8.5 Hz, 2H), 7.24 (d, J ) 8.5 Hz, 2H), 7.16 (d,
J ) 8.5 Hz, 2H), 2.61 (t, J ) 7.5 Hz, 2H), 1.60 (t, J ) 6.9 Hz,
2H), 1.31-1.25 (m, 18H), 0.88 (t, J ) 6.7 Hz, 3H). 13C NMR (76
MHz, CDCl3) δ 143.75, 137.42, 133.01, 131.46, 128.49, 122.99,
119.92, 93.81, 91.02, 87.78, 35.93, 31.93, 31.25, 29.68, 29.65,
29.59, 29.49, 29.37, 29.25, 22.71, 14.16. EI/HRMS calcd: 472.1627;
found: 472.1615.
4-Iodo-4′,4′′,4′′′,4′′′′,4′′′′′-pentadodecylhexaphenylbenzene (6).
Compound 4 (134 mg, 0.284 mmol), 5 (300 mg, 0.284 mmol), and
diphenyl ether (2 mL) were placed in a Teflon-top sealed pressure
tube, and the reaction mixture was heated at reflux for 7 h in a
sand bath. During the reflux period, the temperature of the reaction
vessel was carefully maintained at 260 °C. After cooling, the
reaction mixture was added dropwise to ethanol (100 mL) with
vigorous stirring. After this solution was kept in the refrigerator
overnight, the solvent was decanted to give a yellow-brown oil.
The oil was purified by column chromatography on silica gel with
hexane/CH2Cl2 (10:1) to afford 6 as a waxy solid (277 mg, 65%).
1H NMR (300 MHz, CDCl3) δ 7.14 (d, J ) 8.4 Hz, 2H), 6.65-
6.61 (m, 20H), 6.54 (d, J ) 8.4 Hz, 2H), 2.38-2.30 (m, 10H),
1.45-1.10 (m, 100H), 0.88 (t, J ) 6.8 Hz, 15H). EI/HRMS calcd:
1502.0782; found: 1502.0784.
(13) Draper, S. M.; Gregg, D. J.; Madathil, R. J. Am. Chem. Soc. 2002,
124, 3486-3487.
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M.; Vos, J. G.; Passaniti, P. J. Am. Chem. Soc. 2004, 126, 8694-
8701.
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M. R. A.; Khan, M. S.; Raithby, P. R. J. Chem. Phys. 2000, 113,
7627-7634.
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Khan, M. S.; Raithby, P. R.; Ko¨hler, A.; Friend, R. H. J. Am. Chem.
Soc. 2001, 123, 9412-9417.
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G. J. Phys. Chem. A 2002, 106, 10108-10115.
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K. S. J. Am. Chem. Soc. 2002, 124, 12412-12413.
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Phys. Chem. B 2004, 108, 4969-4978.
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A. B. Chem. Phys. 1998, 227, 75-82.
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11873.
2-Iodo-5,8,11,14,17-pentadodecylhexa-peri-hexabenzocoro-
nene (7). Compound 6 (250 mg, 0.168 mmol) was dissolved in
CH2Cl2 (120 mL), and a stream of argon was bubbled through the
solution. Iron(III) chloride (0.54 g, 3.3 mmol) was dissolved in
nitromethane (7 mL) and added dropwise to the solution of 6. The
mixture was stirred at room temperature for 2 h, and the resulting
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2510 Inorganic Chemistry, Vol. 45, No. 6, 2006

Photophysics of Platinum-Acetylide Substituted HBCs
green solution was added dropwise to MeOH (150 mL) with stirring.
The resulting yellow precipitate was collected by suction filtration
using a sintered glass filter and washed with MeOH followed by
hexane. The residue was redissolved in small amount of CH2Cl2,
and the above workup procedure was repeated twice to afford 7 as
an off-yellow powder. (175 mg, 70%). 1H NMR (300 MHz, CDCl3)
δ 8.10 (br s, 2H), 8.04 (br s, 2H), 7.97 (br s, 2H), 7.74 (br s, 2H),
7.65 (br s, 2H), 7.50 (br s, 2H), 3.02 (br t, 2H), 2.83 (br t, 4H),
2.62 (br t, 4H), 2.08 (br, 2H), 1.93 (br, 4H), 1.80 (br, 4H), 1.65-
1.2 (m, 90H), 0.90 (t, J ) 7.6 Hz, 15H). MS (LDL-TOF) calcd:
1490.1; found: 1490.8.
Hz, 18H), 0.89 (m, 15H). 31P NMR (121 MHz, CDCl3) δ 4.59 (JPt-P
) 2346 Hz). MS (LDL-TOF): calcd: 2188.2; found: 2188.5.
Photophysical Measurements. All photophysical studies were
carried out with solutions contained in 1 × 1 cm2 quartz cells and
degassed by using argon bubbling unless otherwise noted. For
emission measurements, sample concentrations were adjusted to
produce optically dilute solutions (Amax < 0.1, c ≈ 5 × 10-7 M).
Transient absorption measurements were carried out with solutions
having A ) 0.8-1.0 at 355 nm. UV-visible absorption spectra
were recorded on a Varian Cary 100 dual-beam spectrophotometer.
Corrected steady-state emission measurements were performed on
a SPEX Fluorolog 3 fluorescence spectrometer. Emission quantum
yields were measured by relative actinometry with 9,10-dicyanoan-
thracene in argon-degassed EtOH (Φem ) 0.89) as an actinometer.27
Fluorescence decays were obtained by time-correlated single photon
counting on an instrument that was constructed in-house. Excitation
was effected by using a violet diode laser (405 nm, IBH instruments,
Edinburgh, Scotland, pulse width 800 ps). The time-resolved
emission was collected using a red-sensitive, photon-counting PMT
(Hamamatsu R928), and the light was filtered using 10 nm band-
pass interference filters. Lifetimes were determined from the
observed decays with the DAS6 deconvolution software (IBH
instruments, Edinburgh, Scotland). The goodness of the fits was
evaluated by ø2 values <1.3 and by inspection of the residual plots.
Phosphorescence decays were measured on an apparatus consisting
of a Continuum Surelite II Nd:YAG laser as a source (third
harmonic, λ ) 355 nm, 10 ns fwhm) with time-resolved detection
provided by an intensified CCD detector (Princeton Instruments,
PI-MAX iCCD) coupled to an Acton SpectraPro 150 spectrograph.
Transient absorption spectra were obtained on an instrument that
has been previously described that uses the third harmonic of a
Nd:YAG laser (Spectra Physics GCR-14, 355 nm, 10 ns fwhm, 10
mJ pulse-1, 20 mJ cm-2 irradiance) as excitation source.28
2-Triisopropylsilylethynyl-5,8,11,14,17-pentadodecylhexa-
peri-hexabenzocoronene (3). Compound 7 (170 mg, 0.114 mmol),
triisopropylsilylacetylene (31 mg, 0.17 mmol), Pd(PPh3)4 (6.6 mg,
5.7 µmol), and CuI (2 mg) in piperidine/THF (1 mL/2 mL) were
stirred at 40 °C for 4 h. The solvent was evaporated, the residue
was redissolved in a minimal amount of CH2Cl2, and the solution
was added dropwise to MeOH. The resulting precipitate was
collected by suction filtration on a fritted glass filter and purified
by column chromatography on silica gel with hexane/CH2Cl2 2:1
to afford 3 as a yellow solid (137 mg, 78%). 1H NMR (300 MHz,
CDCl3) δ 8.29 (br s, 2H), 7.98 (br s, 2H), 7.88 (br s, 6H), 7.85 (br
s, 2H), 3.00 (br t, J ) 7.8 Hz, 4H), 2.90 (br t, J ) 6.3 Hz, 6H),
2.00 (br m, 10H), 1.67-1.25 (m, 108H), 0.87-0.83 (m, 15H). MS
(LDL-TOF) calcd: 1544.6; found: 1544.4.
Complex 1. A stirred solution of 3 (100 mg, 48 µmol), 8 (38
mg, 52 µmol), Pd(PPh3)4 (5.6 mg, 4.8 µmol), and CuI (2 mg) in
piperidine/THF (2 mL, 1:1 v/v) was degassed with argon for 20
min. Tetrabutylammonium fluoride (1 M in THF, 100 µL) was
added to the reaction mixture via a microsyringe. The mixture was
stirred at 40 °C. After 5 h, the solvent was removed under a stream
of argon. The residue was redissolved in a small amount of CH2-
Cl2, and the resulting solution was added dropwise to MeOH (100
mL) with stirring. The precipitate was collected by suction filtration
and purified by column chromatography on silica gel (petroleum
ether/CH2Cl2, 2:1-1:3) to give 1 (68 mg, 68%). A second
purification by preparative thin-layer chromatography on silica gel
(hexane/CH2Cl2 3:2) yielded 1 as a yellow solid (45 mg, 45%). 1H
NMR (300 MHz, CD2Cl2) δ 8.99 (br s, 2H), 8.75 (br s, 2H), 8.49
(br s, 2H), 8.23 (br s, 2H), 7.90 (br s, 2H), 7.76 (br s, 2H), 7.37 (d,
J ) 7.2 Hz, 2H), 7.28 (t, J ) 7.2 Hz, 2H), 7.18 (t, J ) 7.3 Hz,
1H), 3.24 (t, J ) 7.4 Hz, 4H), 2.88 (t, J ) 7.2 Hz, 4H), 2.68 (t, J
) 7.2 Hz, 2H), 2.44 (m, 12H), 2.11 (m, 4H), 1.92-1.20 (m, 110
H), 1.14 (t, J ) 7.2 Hz, 18H), 0.87-0.83 (m, 15H). 31P NMR (121
MHz, CD2Cl2) δ 4.77 (JPt-P ) 2354 Hz). MS (LDL-TOF): calcd:
2088.0; found: 2088.0.
Electrochemical Measurements. Electrochemical studies em-
ployed cyclic voltammetry using a BAS CV-50W voltammetric
analyzer. The three-electrode system was equipped with a glassy
carbon disk (1.6 mm diameter) working electrode, a silver wire
reference electrode, and a Pt wire counter electrode. The electro-
chemical potentials were calibrated relative to the saturated calomel
electrode (SCE) by using ferrocene as an internal standard (Fc/
Fc+ at +0.40 V vs SCE). Tetra-n-butylammonium hexafluorophos-
phate (0.1 M) in CH2Cl2 was used as supporting electrolyte.
Density Functional Calculations. Quantum chemical calcula-
tions were carried out by using the Gaussian 03 program.29 For the
parent structure, HBC, the B3LYP hybrid functional was used along
with the 6-31g* basis set. The molecule has D6h symmetry, and
Complex 2. A solution of 7 (80 mg, 54 µmol), 9 (49 mg, 59
µmol), Pd(PPh3)4 (6.2 mg, 5.4 µmol), and CuI (2 mg) in piperidine/
THF (2 mL, 1:1 v/v) was stirred and degassed for 20 min with
argon. The reaction mixture was then stirred while heating to 40
°C for 5 h. After cooling, the solvent was evaporated under a stream
of argon. The residue was redissolved in a small volume of CH2-
Cl2, and the resulting solution was added dropwise into MeOH (100
mL) with stirring. The precipitate was collected by suction filtration
and purified by column chromatography on silica gel (petroleum
ether/CH2Cl2, 3:1-1:3) to give 2 as a yellow solid (89 mg, 75%).
A second column chromatography of 2 on silica gel (hexane/CH2-
Cl2, 3:2) yielded pure 2 as a yellow solid (63 mg, 53%). 1H NMR
(300 MHz, CD2Cl2) δ 8.30 (br s, 2H), 8.16 (br s, 2H), 8.04 (br s,
4H), 8.01 (br s, 2H), 7.99 (br s, 2H), 7.78 (d, J ) 8.0 Hz, 2H),
7.50 (d, J ) 8.0 Hz, 2H), 7.31 (d, J ) 6.9 Hz, 2H), 7.25 (t, J ) 7.4
Hz, 2H), 7.15 (t, J ) 7.2 Hz, 1H), 2.91 (br m, 10H), 2.25 (m,
12H), 1.98 (br m, 10H), 1.91-1.23 (m, 104 H), 1.07 (t, J ) 7.2
(27) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Marcel Dekker: New York, 1993.
(28) Wang, Y. S.; Schanze, K. S. Chem. Phys. 1993, 176, 305-319.
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
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Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004.
Inorganic Chemistry, Vol. 45, No. 6, 2006 2511

Kim et al.
Scheme 1 a
used by Mu¨llen and co-workers to prepare the corresponding
monobrominated HBC derivative.3 Thus, the Diels-Alder
reaction of alkyl-substituted tetracyclone 5 with 4-iodo-4′-
dodecyldiphenylacetylene (4) in diphenyl ether affords
hexaphenylbenzene derivative 6 in 30-70% yield, and
subsequent oxidative dehydrogenation of 6 with iron(III)
chloride provides 7 in good yield. Although Diels-Alder
reactions of tetracyclones with diarylacetylenes to afford
polyphenyl-substituted aromatic hydrocarbons have been
reported many times,3,30-32 in our hands, it was observed
that the yield of 6 was highly variable. Careful study of this
reaction showed that the yield varied strongly with temper-
ature. It was found that the yield was optimum when the
reaction was carried out with the vessel maintained at 260
°C, a temperature at which the diphenyl ether solvent is
gently refluxing. If the reaction is run at slightly lower
temperatures (e.g., 200-240 °C), the Diels-Alder reaction
is slow, and 5 undergoes a competitive thermal decomposi-
tion reaction which reduces the yield of the desired product.
To accomplish the construction of Pt-acetylide HBCs, the
iodo-substituted HBC 7 was functionalized with TIPS-
acetylene to give 3. In-situ deprotection of the TIPS group
followed by Hagihara coupling with platinum(II) chloride
complex 8 afforded Pt-HBC 1. Sonogashira coupling of
iodo-HBC 7 with 9 afforded 2.
a (a) Ph2O, reflux, 65%; (b) FeCl3, CH3NO2/CH2Cl2, 70%; (c) triiso-
propylsilylacetylene, Pd(PPh3)4, CuI, piperidine, 40 °C, 78%; (d) CuI,
piperidine, nBu4NF, 40 °C, 45%; (e) Pd(PPh3)4, CuI, piperidine, 40 °C,
53%.
In contrast to other reported HBC derivatives, 1 and 2 are
soluble in most organic solvents, likely due to the symmetry
breaking effect of the Pt-substitution on the HBC, along with
the effect of the tributylphosphine ligands on the Pt center.
The good solubility characteristics of the compounds facili-
tated their purification by column chromatography and
allowed photophysical measurements to be carried out in a
variety of solvents. The structures of 1-3 were characterized
the calculations were carried out within this symmetry. Structure 3
was optimized under C2V symmetry restriction by using the same
functional and basis set used for HBC. Calculations on Pt-HBC 1
were carried out by using the B3LYP hybrid; however, for this
system, the SDD basis set was used to employ Stuttgart/Dresden
relativistic effective core potential for explicit treatment of platinum
electrons, and the geometry was optimized within Cs symmetry.
The alkyl chains in 1 and 3 were truncated to either hydrogen or
methyl groups to save computational time in calculations. All time-
dependent density functional theory (TD-DFT) calculations were
performed with the same basis sets and methods used for geometry
optimizations for the corresponding structure.
1
by H and 31P NMR spectroscopy and MALDI-TOF mass
spectrometry (the NMR spectra are included as Supporting
1
Information). The H NMR spectra of 1 and 2 in CD2Cl2
feature well-resolved, yet slightly broadened resonances in
the aromatic region compared to the aromatic resonance
signals from 6, indicating that the complexes aggregate as
previously reported for other HBC derivatives.9
Results and Discussion
More information concerning aggregation of 2 comes from
a study of the effect of concentration on its 1H NMR
spectrum (see Figure S-8 in the Supporting Information). In
particular, for a 0.01 M solution of complex 2 in CD2Cl2,
the resonance signals for the aromatic HBC protons appear
at δ ≈ 7.8-8.2 ppm. As the solution is diluted, the aromatic
HBC protons shift downfield. In a 3 × 10-5 M solution, the
five sets of HBC aromatic protons in 2 appear in the range
from 8.4 to 8.9 ppm. The R- and â-methylene groups on the
HBC-tethered alkyl chains also shift downfield with decreas-
ing concentration. For example, the R-CH2 signals shift from
2.65 to 3.15 ppm as the sample is diluted from 0.01 to 3 ×
10-5 M. The solvent dependence of the NMR spectrum of 2
Structure and Synthesis. Chart 1 shows the structures
of the three HBCs prepared in this study. Each of the HBCs
is functionalized with five linear C12 chains to facilitate
solubility. Complexes 1 and 2 contain the HBC unit coupled
to a trans-Pt(PBu3)2(CtC-Ar)2 chromophore. In complex
1, the HBC chromophore is coupled to the Pt center via a
single ethynyl linker, whereas in complex 2, the HBC
chromophore is separated from the Pt center by a 1,4-
phenylene ethynylene “spacer”. The effect of the phenylene
ethynylene spacer on the optical and aggregation properties
of the complexes will be evident below. HBC 3 is a synthetic
precursor for 1 and 2 and was selected as a model to provide
information on the photophysics of the ethynyl-substituted
HBC chromophore.
(30) Sadhukhan, S. K.; Viala, C.; Gourdon, A. Synthesis (Stuttgart) 2003,
1521-1525.
(31) Harvey, J. A.; Ogliaruso, M. A. J. Chem. Eng. Data 1977, 22, 110-
113.
(32) Samanta, S. R.; Mukherjee, A. K.; Bhattacharya, A. J. Indian J. Chem.,
Sect. B: Org. Chem. Incl. Med. Chem. 1985, 24, 960-961.
The overall synthetic route used to obtain the HBC
derivatives is illustrated in Scheme 1. The key intermediate
is the iodo-substituted HBC 7, which is a new compound.
This material was synthesized by a method analogous to that
2512 Inorganic Chemistry, Vol. 45, No. 6, 2006

Photophysics of Platinum-Acetylide Substituted HBCs
Figure 1. (a) Absorption and (b) photoluminescence spectra of 1 (red), 2 (black), and 3 (green) in THF solution. Photoluminescence spectra were obtained
in argon-deoxygenated solution, c ) 5 × 10-7 M.
is consistent with observations on a series of symmetrical
hexa-alkyl-chain substituted-HBCs, in which it was shown
that the signals for the aromatic HBC protons shift downfield
with decreasing concentration.9 The upfield shift at higher
concentration is attributed to aggregation of the HBC in
solution.
cm-1). This band is believed to arise due to the presence of
the Pt-acetylide unit, which breaks the symmetry of the
HBC chromophore and extends the π conjugation along the
Pt-acetylide axis. This assignment is also supported by the
calculations that are presented below. A subtle, yet note-
worthy, point is that the absorption bands in the spectrum
of 2 are less well resolved compared to those of 1. This
feature suggests that 2 has a larger self-association constant
than that of 1, and therefore is aggregated to some extent
even in THF solution with c ) 0.5 µM.
The photoluminescence spectra of 1-3 in argon degassed
solutions are shown in Figure 1b, and emission lifetimes and
quantum yields (τem and φem, respectively) are listed in Table
1. TIPS-HBC 3 exhibits a moderately intense fluorescence
that appears as a broad band with a complex vibronic
structure. The fluorescence is similar to that of symmetric
hexa-alkyl-substituted HBCs, which indicates that the ethynyl
substituent has little effect on the fluorescence.9,33,34,37 There
are several significant features with respect to the fluores-
cence of 3. First, the intensity at the emission’s origin is
low (λ ≈ 452 nm), and there is an apparent Stokes shift
between the fluorescence and absorption onsets. This strongly
suggests that the transition from the lowest singlet excited
state to the ground state is forbidden. This notion is supported
by the observation that the fluorescence lifetime is relatively
long (τ ≈ 45 ns). This finding is consistent with the premise
suggested above that the lowest singlet excited state in the
HBC chromophore is symmetry forbidden.34-36 In addition,
the complexity of the overall fluorescence spectrum strongly
suggests that there are several singlet excited states that are
in close energetic proximity which contribute to the fluo-
rescence, each having its own vibronic progression.
Absorption and Photoluminescence Spectroscopy: the
Singlet and Triplet Excited States in the HBC Chro-
mophore. The absorption and photoluminescence spectra of
1-3 were examined in a variety of solvents in very dilute
solutions (c ≈ 5 × 10-7 M), and in this study, it was found
that aggregation is minimized in THF (vide infra). Thus,
studies carried out in this solvent provide the best available
information concerning the photophysics of the Pt-HBCs
in a nonaggregated state. Figure 1a illustrates the absorption
spectra of 1-3 in THF solution. First, the absorption of
TIPS-HBC 3 is similar to that of other all-organic
HBCs;5,6,33,34 the spectrum features a weak onset at ca. 425
nm, with band maxima at 393, 363, and 346 nm. Interest-
ingly, the 363 nm band is the most intense, whereas the 393
nm band and the low-energy tail are considerably less
intense. In view of the fact that, in analogy to benzene, the
HBC chromophore has D6h symmetry, it is likely that the
unusual shape of the absorption spectrum derives from the
fact that the lowest-energy tail arises from one or more
symmetry-forbidden transitions, whereas the intense 363 nm
absorption is due to a symmetry-allowed transition.35,36 This
hypothesis is supported by density functional calculations
presented below.
The absorption spectra of Pt-HBCs 1 and 2 are very
similar to that of 3 with a few key differences. First, the
dominant HBC-based bands seen in 3 appear in the spectra
of 1 and 2 with slight red-shifts. More importantly, both of
the Pt-HBCs exhibit an additional, moderately intense
absorption band with λmax ≈ 416 nm (ꢀ ≈ 3-4 × 104 M-1
The striking feature in the photoluminescence of Pt-HBCs
1 and 2 is the additional, narrow-bandwidth emission with
λmax ≈ 578 nm. This band is clearly due to phosphorescence
from the HBC chromophore,38 and the phosphorescence
assignment is supported by the emission lifetime which is
greater than 20 µs for both complexes (Table 1). The
phosphorescence is observed in the Pt-HBCs because spin-
(33) Wang, Z. H.; Tomovic, E.; Kastler, M.; Pretsch, R.; Negri, F.;
Enkelmann, V.; Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 7794-7795.
(34) Biasutti, M. A.; Rommens, J.; Vaes, A.; De Feyter, S.; De Schryver,
F. D.; Herwig, P.; Mu¨llen, K. Bull. Soc. Chim. Belg. 1997, 106, 659-
664.
(35) Klevens, H. B.; Platt, J. R. J. Chem. Phys. 1949, 17, 470-481.
(36) Platt, J. R. J. Chem. Phys. 1949, 17, 484-495.
(37) Wu, J. S.; Watson, M. D.; Tchebotareva, N.; Wang, Z. H.; Mu¨llen,
K. J. Org. Chem. 2004, 69, 8194-8204.
Inorganic Chemistry, Vol. 45, No. 6, 2006 2513

Kim et al.
Table 1. Photophysical Data for 1-3a
PL (λmax/nm)
τF/nsd
τp/nse
τp(TA)/µsf
b,c
Φem
F
P
THF
0.13
CH2Cl2
THF
THF
THF
CH2Cl2
hexane
1
472
578
ΦF ) 0.0096
ΦP ) 0.12
0.17
ΦF ) 0.11
ΦP ) 0.058
0.12
0.028
0.079
32 (64%)
3.0 (36%)
47 (95%)
9.8 (5%)
215
33
130
40
22
20
12
8
2
3
472
470
579
-
0.097
45 (86%)
8.6 (14%)
-
-
-
-
a All measurements were conducted on argon-degassed THF solutions at 298 K unless otherwise noted. b Photoluminescence quantum yields are relative
to 9,10-dicyanoanthracene in ethanol solution, Φem ) 0.89. c Measurements were taken on solutions with matched optical density at 377 nm, c ≈ 2 × 10-7
M. d Fluorescence lifetimes. Decays were biexponential; numbers in parentheses indicated relative amplitude of component. e Phosphorescence decay lifetimes.
f Transient absorption decay lifetimes.
orbit coupling from the metal center increases the rates of
S1 f T1 intersystem crossing and T1 f So radiative decay.
Interestingly, in complex 1, where the Pt complex is
separated from the HBC chromophore by an ethynyl group,
very little fluorescence is observed (φf ≈ 0.01), suggesting
that S1 f T1 intersystem crossing is very rapid. By contrast,
in 2 where the Pt complex is separated from the HBC
chromophore by a phenyl-ethynyl spacer, the fluorescence
is considerably stronger (φf ≈ 0.11), signaling that in this
system S1 f T1 intersystem crossing is less competitive with
radiative decay of the singlet state. Despite these differences,
the phosphorescence energy and band shape are almost
identical in 1 and 2, which indicates that the lowest triplet
is localized on the HBC chromophore. In both complexes,
the phosphorescence is dominated by the 0-0 band, with a
weak, but discernible vibronic progression observed with
0-1, 0-2, and 0-3 bands at 1327, 1602, and 2126 cm-1,
respectively. The first two bands are likely C-C stretching
modes of the HBC aromatic ring system, whereas the 0-3
band is likely due to the CtC stretch. The fact that the
phosphorescence 0-0 band is considerably more intense
compared to the vibronic overtones indicates that the
geometry of the triplet state is very similar to that of the
ground state, consistent with the rigid structure of the HBC
ring system. The emission of 2 was also examined at 80 K
in a rigid 2-methyltetrahydrofuran (2-MTHF) solvent glass
(see Figure S-2 in the Supporting Information). Although
the emission energy and band shape at 80 K are very similar
to those observed at ambient temperature, one subtle effect
was noticed. In particular, the 0-0 phosphorescence band
has two shoulders that are shifted to slightly longer wave-
length. Similar splitting in the phosphorescence has been
observed in other platinum-acetylide systems at low tem-
perature, and in those systems, it has been attributed to the
existence of different conformers that cannot equilibrate in
the frozen matrix.39 A similar effect may be operating in
complex 2 because in the frozen 2-MTHF glass rotation
around the CtC bonds is not possible.
The observation of HBC-based phosphorescence from 1
and 2 allows us to assign the triplet energy of the HBC
chromophore as ca. 2.14 eV (49.4 kcal mol-1). Taking the
lowest singlet excited-state energy from the onset of the
fluorescence for 2 (λ ≈ 454 nm, 2.73 eV) allows derivation
of the singlet-triplet splitting for the HBC chromophore
(∆EST ) Es - ET ≈ 0.6 eV). This value is remarkably small
by comparison with values for other condensed polycyclic
aromatic hydrocarbons, which range from 1.5 eV for
naphthalene to 0.9 eV for chrysene.27,40 The small ∆EST for
the HBC chromophore underscores the fact that the singlet
excited state is strongly delocalized within the 14-ring
system. This is because a significant component of the
singlet-triplet splitting arises from electron-electron repul-
sion in the singlet state, which is decreased by spatial
delocalization of the excited-state wave function.41
Density Functional Calculations. To provide insight
regarding the electronic basis for the lowest excited states
of the HBC chromophore, density functional theory (DFT)
calculations were carried out on the parent molecule HBC,
as well as on derivatives 1 and 3. The parent HBC
chromophore has very high symmetry, and comparison of
the results on the parent with those of 3 provide insight
concerning the effect of the ethynyl substituent on the
electronic structure of the HBC chromophore. The calcula-
tions on Pt-HBC complex 1 reveal how the metal interacts
with the HBC chromophore. The geometry of each structure
was optimized by using DFT, and then time-dependent DFT
(TD-DFT) was used to compute the optical transition
energies. The results of the calculations for the three
compounds are listed in Table 2. Frontier orbital density plots
for 1 are shown in Figure 2, whereas those for HBC and 3
are provided as Supporting Information.
Calculations for the parent HBC were carried out in D6h
symmetry. Not surprisingly, the results show that there is a
direct parallel between the symmetry and structure of the
frontier orbitals for the HBC chromophore and benzene,
which also has D6h symmetry. In particular, the HOMO and
LUMO orbitals of HBC are doubly degenerate (E1g and E2u,
respectively, see Supporting Information for orbital plots).
Transitions between these levels give rise to three types of
(38) While this is the first unambiguous report of phosphorescence from
an HBC, in a hydrocarbon review published in 1962, E. Clar noted
that “A weak solution (of hexa-peri-benzocoronene) in trichloroben-
zene cooled in liquid nitrogen shows a strong orange phosphorescence
(band at 5750 Å) of long life after irradiation with a mercury lamp”,
ref 6.
(40) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cum-
mings: Menlo Park, CA, 1978.
(39) Haskins-Glusac, K., Ph.D. Dissertation, University of Florida, Gaines-
ville, 2003, http://www.chem.ufl.edu/∼kschanze/dissertations.htm.
(41) McGlynn, S. P.; Smith, F. J.; Cilento, G. Photochem. Photobiol. 1964,
3, 269-294.
2514 Inorganic Chemistry, Vol. 45, No. 6, 2006

Photophysics of Platinum-Acetylide Substituted HBCs
Table 2. Results of TD-DFT Calculations on HBC, 1, and 3
HBC
3
1
a
b
a
b
a
b
state
1
1B2u
428 nm
f ) 0.0000
H - 1 f L, 50%
H f L + 1, 50%
1B2
433 nm
f ) 0.0001
H - 1 f L, 51%
H f L + 1, 49%
1A′′
441 nm
f ) 0.0048
H - 2 f L + 1 5%
H - 1 f L, 39%
H f L + 1, 53%
H f L + 2, 3%
2
3
4
1B1u
409 nm
f ) 0.0000
1E2g
381 nm
f ) 0.0000
1E2g
377 nm
f ) 0.0000
H - 1 f L + 1, 50%
H f L, 50%
1A1
415 nm
f ) 0.0167
1B2
381 nm
f ) 0.0028
1A1
376 nm
f ) 0.0119
H - 1 f L + 1, 41%
H f L, 59%
1A′
423 nm
f ) 0.1513
1A′′
404 nm
f ) 0.0025
1A′
404 nm
f ) 0.1238
H - 2 f L, 2%
H - 1 f L + 1, 25%
H f L, 73%
H - 4 f L, 7%
H f L + 1, 7%
H f L + 2, 86%
H - 4 f L + 1, 11%
H - 2 f L, 2%
H - 1 f L + 1, 23%
H - 1 f L + 2, 57%
H f L, 7%
H - 5 f L + 1, 6%
H - 2 f L + 1, 24%
H - 1 f L, 34%
H f L + 1, 36%
H - 5 f L, 3%
H - 4 f L + 1, 4%
H - 2 f L, 23%
H - 1 f L + 1, 27%
H - 1 f L + 2, 27%
H f L, 16%
H - 2 f L, 20%
H f L + 2, 80%
H - 2 f L, 23%
H f L + 2, 77%
H - 2 f L + 1, 20%
H - 1 f L + 2, 80%
H - 2 f L + 1, 20%
H - 1 f L + 2, 80%
5
6
1E1u
377 nm
f ) 0.6473
H - 1 f L, 50%
H f L + 1, 50%
1A1
361 nm
f ) 0.8441
H - 1 f L + 1, 58%
H - 1 f L + 2, 4%
H f L, 38%
1A′′
394 nm
f ) 0.1949
1E1u
374 nm
f ) 0.6562
H - 1 f L + 1, 50%
H f L, 50%
1B2
361 nm
f ) 0.6724
H - 1 f L, 47%
H f L + 1, 49%
H f L + 2, 4%
1A′
383 nm
f ) 0.5409
triplet
n/a
3A1
3A′
589 nm
595 nm
606 nm
a State assignment, wavelength of optical transition and oscillator strength (f). b Molecular orbital basis for transitions.
1
1
1
configurations (E1g X E2u ) B2u + B1u + E1u), in direct
analogy to the lowest transitions in benzene. Of these three
excited-state configurations, transitions from the ground state
(1A1g) to 1B2u and 1B1u are dipole forbidden in D6h symmetry,
mophore. The intense transition observed at 363 nm in 3
likely corresponds to 1A1 f 1A1 and 1A1 f 1B2 (transitions
5 and 6, Table 2); these transitions correlate with the A1g
f 1E1u transitions which are dipole allowed in the D6h local
symmetry of the HBC chromophore.
1
1
whereas the transition to the E1u state is dipole allowed.42
Time-dependent DFT was carried out on HBC to calculate
the energies and oscillator strengths of the lowest optical
transitions, and the results are summarized in the first column
of Table 2. The TD-DFT calculations predict the dipole
forbidden 1A1g f 1B2u and 1A1g f 1B1u transitions (1 and 2,
Table 2) to occur at 428 and 409 nm, respectively, while
the dipole-allowed transitions occur at 377 and 374 nm
(transitions 5 and 6, Table 2).
Overall, the calculations provide insight into the absorption
and fluorescence properties of 3 and structurally related
HBCs. In particular, it is evident that the chromophore
features two energetically low-lying excited states that are
dipole-forbidden. This explains the relatively large apparent
Stokes-shift, and the comparatively long fluorescence lifetime
(45 ns for 3). It is possible that the complex structure of the
fluorescence spectrum observed for 3 and other HBCs arises
because at ambient temperature both the 1B2u and 1B1u states
are populated and undergo radiative decay on a comparable
time scale.
Turning to the calculation on Pt-HBC 1, it is first
necessary to consider the energetically preferred orientation
of the square planar platinum unit (as defined by the PtP2C2
moiety) with respect to the HBC plane. Calculations were
carried out to explore the ground-state energy as a function
of the dihedral angle (θ), which is defined as the relative
angle between the HBC plane and the plane defined by the
PtP2C2 unit. The calculations indicate that the “twisted”
conformation with θ ) 90° is preferred by ca. 0.6 kcal mol-1
relative to the conformation where the two units are coplanar
(θ ) 0°). This finding is consistent with recently published
theoretical studies of Pt-acetylides which show that the π
system of the arylacetylide ligands interacts most strongly
with in-plane (dxy) orbital on Pt.43,44 The HOMO and LUMO
Calculations on 3 were carried out in C2V symmetry.
Despite the lower symmetry, the spatial distribution of the
frontier orbitals of 3 correspond quite closely with the
HOMO (E1g) and LUMO (E2u) levels of HBC. The HOMO
and LUMO orbitals (both of B1 symmetry) feature contribu-
tions from the ethynyl substituent. The results of the TD-
DFT calculations are listed in Table 2 to allow comparison
between the corresponding transitions in 3 and HBC. Note
that there is very good agreement between the calculated
transitions and the observed absorption spectrum of 3. In
particular, the low-energy tail on the absorption spectrum
of 3 has its onset at ca. 425 nm, and the first clearly resolved,
weak band has λmax ) 393 nm. These bands likely correspond
1
1
1
1
to A1 f B2 and A f A1 (transitions 1 and 2, Table 2);
although these transitions are allowed in C2V symmetry, they
correlate with the 1B2u and 1B1u transitions, which are dipole
forbidden in the D6h local symmetry of the HBC chro-
(42) Cotton, F. A. Chemical Applications of Group Theory; John Wiley
(43) Emmert, L. A.; Choi, W.; Marshall, J. A.; Yang, J.; Meyer, L. A.;
and Sons: New York, 1963.
Brozik, J. A. J. Phys. Chem. A 2003, 107, 11340-11346.
Inorganic Chemistry, Vol. 45, No. 6, 2006 2515

Kim et al.
Figure 3. (a) Transient absorption spectra obtained for 1 (red) and 2 (blue)
in degassed THF solution 1 µs delay following 355 nm excitation. (b)
Transient absorption spectra obtained for 1 in the presence of MV2+ in
degassed THF/CH3CN (2:1) solution following 355 nm excitation. The black
spectrum was obtained with 1 µs delay, and for succeeding spectra (in order
of decreasing intensity) were obtained with 16 µs delay increments.
energy transition is anticipated to have a small degree of
charge-transfer character, and it is expected to be polarized
along the long axis of the molecule.
Figure 2. Frontier orbital plots for optimized ground state geometry of
Pt-HBC complex 1 computed by using DFT as described in the text.
The geometry and energy of the lowest triplet excited state
of HBC, 1, and 3 were also computed with DFT. The results
of the computations were very similar and indicate that the
triplet excitation is localized in the HBC chromophore. This
effect is clearly seen by comparing the computed bond
lengths for the ground and triplet states of 1 (see Supporting
Information). In particular, the bond lengths in the HBC
chromophore in triplet 1 differ significantly from those for
the ground state geometry; however, the bond lengths for
the Pt-acetylide unit are virtually the same in the ground
and first triplet state. The pattern of bond length differences
between the lowest singlet and triplet states is very similar
in HBC and 3. Consistent with the notion of an HBC-
localized triplet state, the calculated S0-T1 0-0 energy
difference (energy gap between optimized geometry of the
lowest triplet and singlet states) is very similar in the three
compounds (last row, Table 2). Interestingly, the computed
energy difference is remarkably close to the experimentally
observed phosphorescence energy.
Transient Absorption Spectroscopy: HBC Triplet and
Radical Cation Absorption. To provide additional informa-
tion regarding the spectroscopy of the HBC triplet state and
its reactivity in photoinduced electron transfer, nanosecond
to microsecond time-resolved absorption experiments were
carried out with Pt-HBCs 1 and 2 in THF solution (c ≈ 20
µM). Figure 3a illustrates the time-resolved difference
absorption spectra of 1 and 2 obtained at a series of delay
times following 355 nm pulsed excitation. Both complexes
levels computed for the low-energy “twisted” conformation
are shown in Figure 2. The interesting aspect here is that
the HOMO is delocalized considerably into the Pt-acetylide
substituent. In particular, there is a small contribution to the
orbital from the in-plane (dxy) orbital on Pt, and the HOMO
is delocalized significantly into the phenylacetylene ligand
that is trans to the ethynyl-HBC unit. By contrast, the
LUMO is more strongly localized on the HBC core. The
results of the TD-DFT calculation on 1 are shown in Table
2. Several features are significant with respect to the results.
First, the calculated transitions are shifted to lower energy
relative to the position of the corresponding transitions in
HBC. This shift arises because the energy of the HOMO in
1 is raised due to interaction with the Pt-acetylide unit. The
second point is that there is a significant increase in the
oscillator strength of the low-energy transitions; in particular,
transition 2, which is dominated by the HOMO f LUMO
configuration has f ) 0.15. The interesting point is that this
transition corresponds quite closely to the additional band
observed in the absorption spectra of 1 and 2 at λ ) 416
and 418 nm, respectively. Given that the HOMO is delo-
calized significantly into the Pt-acetylide substituent and
the LUMO is largely localized on the HBC core, this low-
(44) Cooper, T. M.; Blaudeau, J. P.; Hall, B. C.; Rogers, J. E.; McLean,
D. G.; Liu, Y. L.; Toscano, J. P. Chem. Phys. Lett. 2004, 400, 239-
244.
2516 Inorganic Chemistry, Vol. 45, No. 6, 2006

Photophysics of Platinum-Acetylide Substituted HBCs
exhibit bleaching (negative ∆A) in the region corresponding
to the ground-state absorption (λ < 420 nm) combined with
broad excited-state absorption bands extending throughout
the visible into the near-IR region. These transients are
attributed to the triplet excited state of the HBC chromophore.
The transient absorption decay lifetimes for 1 and 2 in THF
solution were 130 and 40 µs, consistent with the triplet
assignment. Although the spectra of two complexes exhibit
some subtle differences, they are generally very similar in
band shape, which underscores the hypothesis that the triplet
excitation is largely localized in the HBC chromophore.
Electrochemical studies were carried out on 1-3 (CH2-
Cl2 solution) in order to provide information concerning the
potentials for oxidation and reduction of the HBC moiety.
Unfortunately, well-resolved cyclic voltammetric waves were
not observed for any of the materials, a result we attribute
to aggregation and/or coating of the materials on the electrode
surface. Nevertheless, a broad, quasi-reversible oxidation
wave was observed for 1 at +0.8 V vs SCE. On the basis of
this approximate potential combined with the triplet energy
(2.14 eV), we estimate that the excited-state oxidation
potential of the Pt-HBCs is E1/2(3Pt-HBC*/Pt-HBC+•) ≈
-1.3 V vs SCE.45 This indicates that the Pt-HBC triplet
excited state is a strong reductant and photoinduced electron
transfer to electron acceptors with reduction potentials more
positive than -1.3 V will be thermodynamically favorable.
In accord with this prediction, Stern-Volmer quenching
studies using methyl viologen (MV2+), which has a reduction
potential of -0.40 vs SCE,46 demonstrate that MV2+
quenches the triplet excited state of Pt-HBC 1 at the
diffusion-controlled rate.
Figure 4. Absorption and photoluminescence spectra of (a) 1 and (b) 2
in THF (red), CH2Cl2 (blue), and hexane (green), c ≈ 5 µM. Emission
spectra were obtained on degassed solutions with matched absorbance at
the excitation wavelength, and therefore, the intensities reflect the relative
emission quantum yields.
Effects of Aggregation on the Triplet State. As noted
in the Introduction, one of the principal aims of this
investigation was to study the effect of aggregation on the
triplet state of the HBC chromophore. To pursue this line of
investigation, absorption, luminescence, and transient absorp-
tion spectra were obtained on the Pt-HBCs in a variety of
solvents. A clear trend emerges from comparison of the data
obtained in three solvents: THF, CH2Cl2, and hexane. With
respect to solvation of the HBC chromophore, this series
ranges from good (THF) to poor (hexane), and therefore,
the degree of chromophore aggregation varies in the order
hexane > CH2Cl2 > THF. In addition, as noted above, 2
has a stronger self-association constant, and consequently,
the effects of aggregation are more pronounced for this
complex in CH2Cl2 and hexane. Figure 4 compares the
emission and absorption spectra of the two complexes in the
three solvents (c ) 5 × 10-6 M). Several trends are clear
from the data. First, the absorption of the two complexes
becomes slightly broader as the solvent is varied from THF
to CH2Cl2 to hexane. This effect is most pronounced for
complex 2, where it is quite evident that the low-energy
bands are very poorly defined in CH2Cl2 and hexane. The
same effect is evident in the high-energy bands for complex
1. We attribute the effects seen in hexane and to a lesser
extent CH2Cl2 to the formation of aggregates, which are
presumably formed by π-stacking of the HBC chromophores.
Similar solvent effects on the absorption spectra of alkyl-
substituted HBCs have been reported by Mu¨llen and co-
Transient absorption spectroscopy of a mixture of 1 (c )
4 µM) and MV2+ (c ) 10 mM) in THF/CH3CN (2:1 v/v)
provides clear evidence that quenching involves photoin-
duced electron transfer to afford Pt-HBC radical cation (1+•)
and the reduced viologen (MV+•),
31* + MV2+ f 1+• + MV+•
(1)
As shown in Figure 3b, the transient absorption difference
spectrum of the solution 1 and MV2+ is dominated by strong
bleaching of the HBC ground-state absorption at ≈370 nm,
combined with a strong absorption band at 570 nm, with a
shoulder at 535 nm, along with another set of transitions in
the 450-500 nm region. All of these bands are believed to
arise from the Pt-HBC radical cation, 1+•. Although MV+•
exhibits absorption bands at 395 and 605 nm (ꢀ ≈ 3 × 104
and 1 × 104 M-1 cm-1, respectively),47 it is believed that
these spectral features are obscured by the absorption of 1+•,
which is considerably more intense. Given that 1+• and MV+•
are produced in equimolar amounts in the transient absorption
experiment and that the 1+• absorption bands are much
stronger compared to those of MV+• allows us to estimate
that the 570 nm absorption band of 1+• has ꢀ g 5 × 104
M-1 cm-1.
(45) E1/2(3Pt-HBC*/Pt-HBC+•) ) E1/2(Pt-HBC/Pt-HBC+•) - Etriplet
.
(46) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; Wiley: New York, 2000.
(47) Kosower, E. M.; Cotter, J. C. J. Am. Chem. Soc. 1964, 86, 5524-
5527.
Inorganic Chemistry, Vol. 45, No. 6, 2006 2517

Kim et al.
workers under conditions where the chromophore is aggre-
gated.9,48
More insight into the effects of aggregation on the
photophysics comes from the photoluminescence data, which
are also shown in Figure 4. First, it is quite evident that the
intensity (quantum efficiency) of the emission (fluorescence
and phosphorescence) decreases substantially along the series
THF > CH2Cl2 > hexane. In particular, for 1, the total
emission yield decreases from 0.13 in THF to 0.028 in CH2-
Cl2. The quenching is even stronger in hexane, where the
emission quantum yield is ,0.01. A similar trend is seen
for 2, but in this case because the relative fluorescence yield
is higher, it is also possible to discern that the vibronic
structure that is present on the fluorescence band in THF is
blurred in CH2Cl2 and hexane and the overall fluorescence
appears more as a broad, red-shifted “excimer-like” band.
While the phosphorescence of both complexes is strongly
quenched by aggregation in CH2Cl2 and hexane, close
inspection of the phosphorescence band shape of 1 reveals
that it is almost the same in CH2Cl2 and THF, suggesting
that the electronic structure of the triplet state is not strongly
affected by aggregation. (Similar aggregation-induced quench-
ing of the phosphorescence from 1 is observed in THF/
hexane solvent mixtures as the proportion of hexane is
increased, see Supporting Information for spectral data.)
Figure 5. Transient absorption spectra obtained for (a) 1 and (b) 2 in
degassed THF, CH2Cl2, and hexane solutions following 355 nm excitation.
Spectra were obtained on degassed solutions with matched absorbance at
355 nm at the same laser power, and therefore, the relative intensities reflect
the triplet yield.
The effect of aggregation on the triplet state of 1 and 2
was also probed by comparing the transient absorption
spectra and triplet decay lifetimes of the two complexes in
THF, CH2Cl2, and hexane. The spectra of the two complexes
immediately following laser excitation are shown in Figure
5, and the transient absorption decay lifetimes are listed in
Table 1. For each complex, the triplet decay lifetimes
decrease in the sequence THF > CH2Cl2 > hexane; note
that the triplet decays nearly 10-fold more rapidly in hexane
compared to THF. These results clearly indicate that ag-
gregation of the Pt-HBCs leads to quenching of the triplet
state.
concomitant with aggregation likely arises because the singlet
excimer state is produced rapidly, and intersystem crossing
in the excimer is less efficient than in the monomer singlet
state. The lack of evidence for a strong aggregation effect
on the phosphorescence or triplet-triplet absorption spectra
suggests that a triplet excimer is not formed in the Pt-HBC
aggregates (i.e., the triplet is localized on a single HBC
chromophore unit, even in the aggregate). This finding is
consistent with earlier work on triplet states in aromatic
hydrocarbons which suggests that triplet excimers are not
stabilized relative to the monomeric triplet states.49-51
While the available evidence indicates that the electronic
structure of the HBC triplet in 1 and 2 is not strongly affected
by aggregation, there is clear evidence that the lifetime of
the triplet is reduced by aggregation. The origin of this effect
is not clear; however, it likely arises due to the introduction
of new low-frequency modes that increase the Franck-
Condon factors for nonradiative decay of the triplet state.
While triplet-triplet annihilation in the aggregates was
considered as a possible pathway for the accelerated non-
radiative decay, a study of the triplet lifetime as a function
of laser power did not provide any evidence for the
involvement of this pathway.
While there is a distinct effect of aggregation on the triplet
lifetime, the effect of aggregation on the transient absorption
difference spectra is less pronounced. For both complexes,
the initial amplitude of the transient absorption is lower in
CH2Cl2 and hexane compare to THF, which qualitatively
indicates that the triplet yield is suppressed when the Pt-
HBCs are aggregated. In addition, the position of the ground-
state bleaching bands shifts slightly with solvent, consistent
with the solvent effect on the absorption spectra. For complex
2, the maximum of the visible triplet-triplet absorption band
is slightly red-shifted in hexane compared to THF. By
contrast, the visible region triplet-triplet absorption of
complex 1 is almost unaffected by solvent.
Summary and Conclusions
Taken together, the spectral data suggest that aggregation
has little effect on the electronic structure of the triplet state.
Rather aggregation decreases the triplet yield and also
accelerates triplet decay. The decrease in the triplet yield
This report describes the synthesis and photophysical
characterization of two HBCs that are functionalized with
(49) Lim, E. C. Acc. Chem. Res. 1987, 20, 8-17.
(50) Yanagidate, M.; Takayama, K.; Takeuchi, M.; Nishimura, J.; Shizuka,
H. J. Phys. Chem. 1993, 97, 8881-8888.
(51) Yamaji, M.; Tsukada, H.; Nishimura, J.; Shizuka, H.; Tobita, S. Chem.
Phys. Lett. 2002, 357, 137-142.
(48) Wu, J. S.; Fechtenkotter, A.; Gauss, J.; Watson, M. D.; Kastler, M.;
Fechtenkotter, C.; Wagner, M.; Mu¨llen, K. J. Am. Chem. Soc. 2004,
126, 11311-11321.
2518 Inorganic Chemistry, Vol. 45, No. 6, 2006

Photophysics of Platinum-Acetylide Substituted HBCs
organometallic Pt-acetylide units. The ground-state absorp-
tion spectra of these novel Pt-HBCs are dominated by the
bands characteristic of the HBC chromophore, with the
exception of an additional, moderately intense band that
appears at λ ≈ 417 nm that is assigned to an allowed
transition polarized along the axis of the π-conjugated Pt-
acetylide unit. Photoluminescence and transient absorption
spectroscopy reveal that the primary effect of the Pt-
acetylide unit on the photophysics of the Pt-HBCs is to
enhance the yield of the triplet excited state and promote its
radiative decay (phosphorescence). Thus, both of the Pt-
HBCs exhibit moderately efficient phosphorescence at room
temperature at λmax ≈ 578 nm. The strong similarity of the
phosphorescence of the two Pt-HBCs, combined with the
results of triplet state DFT calculations, leads to the conclu-
sion that the triplet is localized on the HBC chromophore.
The phosphorescence affords the triplet energy of the HBC
chromophore, ET ≈ 2.14 eV, and allows calculation of the
S-T splitting, ∆EST ≈ 0.6 eV.
strongly and favorably influenced by coupling to a transition
metal unit. In the Pt-acetylide systems that are the subject
of this investigation, the primary effect of the transition metal
is to significantly enhance the probability of S1-T1 inter-
system crossing and the rate of T1-S0 radiative decay. These
properties could prove useful in optoelectronic device
applications of metal-containing HBC systems. For example,
it is now well-established that the use of phosphorescent
chromophores can significantly enhance the quantum ef-
ficiency of light emission from organic light emitting
diodes.52,53 In addition, the efficiency of photoinduced charge
separation in organic photovoltaic devices might be enhanced
by the use of triplet excited-state chromophores as the active
materials. These and other applications of Pt-acetylide-
substituted HBCs are the subject of ongoing investigations.
Acknowledgment. We gratefully acknowledge the Na-
tional Science Foundation (Grant No. CHE-0515066) and
the Air Force Office of Scientific Research (Grant No.
F49620-03-1-0127) for support of this work.
By analogy to other alkyl-chain-substituted HBCs, the Pt-
HBCs have a strong tendency to aggregate, even in dilute
solutions. Aggregation of the Pt-HBCs in dilute CH2Cl2 and
hexane solutions is clearly evident from broadening of the
ground-state absorption spectra and the appearance of broad,
red-shifted excimer-like fluorescence. Interestingly, aggrega-
tion of the Pt-HBCs only leads to quenching of the triplet
state, as evidenced by a substantial decrease in the phos-
phorescence intensity and the triplet lifetime. The results do
not provide evidence for the existence of a triplet excimer,
even when the ground state complexes are strongly ag-
gregated in solution.
Supporting Information Available: Emission spectra of 1 for
THF/hexane solvent mixtures; emission spectrum of 2 at 80 K; 1H
NMR spectra of 1, 2, 3, 6, and 7; 31P NMR spectra of 1 and 2; and
frontier orbital density plots for HBC and 3 (11 figures). This
material is available free of charge via the Internet at http://
pubs.acs.org.
IC051788V
(52) Kwong, R. C.; Sibley, S.; Dubovoy, T.; Baldo, M.; Forrest, S. R.;
Thompson, M. E. Chem. Mater. 1999, 11, 3709-3713.
(53) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151-154.
This investigation provides an excellent example of how
the optical properties of the HBC chromophore can be
Inorganic Chemistry, Vol. 45, No. 6, 2006 2519