Delocalization in platinum-alkynyl systems: A metal-bridged organic mixed-valence compound

Article abstract of DOI:10.1021/ja045869m

A complex featuring two triarylamine redox centers bridged by Pt, trans-bis(triethylphosphine)-bis{4-[bis(4-methoxyphenyl)amino]phenylethynyl} platinum(II), has been synthesized as a model system for π-conjugated Pt-containing polymers. Analysis of the intervalence charge-transfer band displayed by its mixed-valence monocation affords a quantitative assessment of electronic delocalization through the Pt bridge; this is found to be only slightly smaller than that determined for a benzene-bridged analogue. These results are supported by density functional theory calculations, which show that the active orbitals involved in the electron-transfer process in both cases have similar delocalization through the bridging unit. Copyright

Full text of DOI:10.1021/ja045869m

Published on Web 09/03/2004
Delocalization in Platinum-Alkynyl Systems: A Metal-Bridged Organic
Mixed-Valence Compound
Simon C. Jones,† Veaceslav Coropceanu,† Stephen Barlow,† Tiffany Kinnibrugh,‡
Tatiana Timofeeva,‡ Jean-Luc Bre´das,† and Seth R. Marder*,†
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, and Department of
Natural Sciences, New Mexico Highlands UniVersity, Las Vegas, New Mexico 87701
Received July 9, 2004; E-mail: seth.marder@chemistry.gatech.edu
Organometallic “rigid-rod” polymers with π-conjugated metal-
organic units are currently receiving much attention because of their
potential use as advanced materials for electronic, optical, and liquid
crystal applications.1 Notably, Pt-alkynyl polymers and oligomers
have been employed in organic photocells2 and have been used as
model systems to characterize the triplet manifold of conjugated
organic polymers3 relevant to OLED devices.4
The performance of electronic devices is influenced to a large
extent by the efficiency of intrachain electron and hole migration,
or delocalization. However, the question concerning the degree to
which Pt disrupts or enhances the π-conjugation between organic
Figure 1. Molecular structure of 1 at 120 K with thermal ellipsoids at
units and, hence, alters the electronic structure has still not fully
been addressed. Some qualitative insight, in particular concerning
the delocalization of the singlet and triplet excitons in Pt-alkynyl
species, has been acquired from photophysical studies.5 More
recently, the π-conjugation has been probed by means of single-
molecule charge-transport measurements; it has been found, for
instance, that the molecular conductivity of a model Pt-bridged
phenylacetylide molecule is significantly higher than that of a
π-conjugated oligo(phenyleneethynylene) “molecular wire”,6 al-
though it is considerably lower than that of a 9,10-anthracene-
bridged analogue.7
50% probability and all H atoms omitted for clarity. Primed atoms are
generated by Ci symmetry. Selected distances (Å) and angles (deg): Pt1-
P1 2.2949(12), Pt1-C1 2.008(5), C1-C2 1.171(7), C2-C3 1.476(7), N1-
C6 1.399(6), N1-C9 1.442(6), N1-C16 1.432(6), N1-N1′ 17.780(7), C1-
Pt1-P1 93.07(12), C1-Pt1-P1′ 86.93(12), C4-C3-C3′-C8′ 3.0(7)
In this communication, we present a quantitatiVe assessment of
the effect Pt has on electronic delocalization. We describe a system,
1 (Figure 1), in which two triarylamine redox centers are bridged
by Pt; analysis of the intervalence charge-transfer (IVCT) band of
its mixed-valence (MV) derivative, [1]+, allows us to quantify
directly the electronic coupling through Pt for the first time and to
compare this to that in an all-organic structural analogue previously
reported by Lambert,8 [2]+ (see Figure 2). Moreover, while
inorganic MV systems with both inorganic and organic bridges have
been much studied and pure organic MV systems are gaining
increasing attention,9 [1]+ represents an unsual class of MV system
whereby two organic redox centers are coupled through a metal
atom.10
The Pt(II)-bridged triarylamine species, 1, was synthesized by
coupling of trans-dichlorobis(triethylphosphine)platinum(II) with
(4,4′-dimethoxy,4′′-ethynyl)triphenylamine. A single-crystal X-ray
diffraction study confirms the trans arrangement of ligands across
a square-planar Pt center (Figure 1). The electronic absorption
spectrum of 1 in CH2Cl2 features a strong transition at 373 nm
(log ꢀ ) 4.82), along with higher energy absorption at 307 nm
(log ꢀ ) 4.60). The luminescence spectrum shows a weak band at
405 nm assigned to S1 f S0 relaxation, along with a much more
intense band at 494 nm assigned to the T1 f S0 transition. Very
similar behavior is observed for Pt-alkynyl polymers,3 and hence
1 may be considered a good model for these systems in terms of
Figure 2. UV-vis-NIR spectra for [1]+ and [2]+ in CH2Cl2.
electronic structure and photophysics. Calculations of the excited
state of 1 performed at the time-dependent density functional theory
(TD-DFT) level reveal that both S1 and T1 states originate from
orbitals with mixed triarylamine and alkyne character and are, thus,
delocalized over the entire system. The energy of 3.4 eV (364 nm)
derived for S1 compares well with the experimental absorption data.
TD-DFT calculations also indicate that several more excited states
located just above S1 also contribute to the first absorption band,
with energies 3.6-3.7 eV.
Electrochemistry of 1 reveals two overlapping reversible oxida-
tions due to the triarylamine groups, with separation ∆E1/2 ) 65
mV resolvable by differential pulse voltammetry, indicating the
comproportionation constant for the MV monocation, KC, to be ca.
† Georgia Institute of Technology.
‡ New Mexico Highlands University.
9
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J. AM. CHEM. SOC. 2004, 126, 11782-11783
10.1021/ja045869m CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
In summary, we have shown that the electronic coupling through
an alkyne-Pt-alkyne bridge is only somewhat smaller than through
an alkyne-benzene-alkyne bridge. Thus, we may conclude that
electronic delocalization in Pt-alkynyl π-conjugated polymers is
not substantially decreased when compared to poly(phenyleneethy-
nylenes), and hence, the phosphorescent properties imparted to the
system by Pt3,4 do not seem to be outweighed significantly by a
deleterious reduction in conjugation along the polymer chain.
Acknowledgment. This material is based upon work supported
in part by the STC Program of the National Science Foundation
under Agreement Number DMR-0120967. We thank the ONR and
the NSF (CHE-0342321) for support, Dr. M. Yu. Antipin and Dr.
V. N. Khrustalev for low temperature X-ray data collection, and
Christina Bauer for the electronic spectra of 1.
Supporting Information Available: Synthetic, computational, and
crystallographic details (CIF); electronic spectra, voltammetry, and EPR
data for 1. This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 3. Localized orbitals of [1]+ and [2]+ involved in the ET process.
Table 1. IVCT Band Data for [1]+ and [2]+
1
1
cation
ν¯max
ꢀmax/M-1 cm-
∆ν¯1/
R/Å
V/cm-
2
References
[1]+
[2]+
6540
7730
3395
5257
4200
4280
17.78
350
440
(1) (a) Cooper, T. M. In Encyclopedia of Nanoscience and Nanotechnology;
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19.308
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13 at 298 K.11 Reaction of 1 with 0.1 equiv of tris(4-bromophenyl)-
aminium hexafluoroantimonate oxidant in CH2Cl2 affords a green
solution containing the monocation; the dication is formed by
similar reaction of 1 with 2 equiv of oxidant in CH2Cl2. The MV
monocation displays a band (6540 cm-1) in the near-IR region of
the spectrum attributable to IVCT, along with peaks at 10 000 and
15 000 cm-1 assigned to Pt f N+ charge transfer and a localized
triarylaminium excitation, respectively. The IVCT band is absent
in the electronic spectrum of homovalent [1]2+, while the other two
transitions are observed at similar energies to [1]+ but with greater
intensity; the neutral molecule does not display any features in this
region of the spectrum. EPR spectroscopy of [1]+ confirms the first
oxidation to be N-based (see Supporting Information). Optical
spectra of [1]+ and the benzene-bridged MV monocation [2]+
measured under identical conditions are shown in Figure 2.12 The
IVCT band for [1]+ is of low energy and low intensity; the width
at half-height is ca. 4200 cm-1, which is broader than the bandwidth
of 3140 cm-1 predicted by the Hush model,13 suggesting that [1]+
is a Robin-Day Class II system.14 In the framework of the Hush
model, the electronic coupling between nitrogen redox centers, V,
may be derived from the following equation:13
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and references therein.
(10) Interligand electronic coupling in MV semiquinonate-catecholate com-
plexes of Co10a and Cr10b has been determined from analysis of their IVCT
bands. (a) Adams, D. M.; Hendrickson, D. N. J. Am. Chem. Soc. 1996,
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V ) 2.06 × 10-2 (ꢀmax‚νjmax‚∆νj )1/2/R
1/2
(11) ∆E1/2 is similar to that for the benzene-bridged analogue, 2, (60 mV);
however, the electrochemical KC is often a poor quantitative measure of
electronic coupling. See: Barriere, F.; Camire, N.; Geiger, W. E.; Mueller-
Westerhoff, U. T.; Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262-7263.
where R is the donor-acceptor charge-transfer distance in Å; for
[1]+ this is taken to be the intramolecular N-N crystallographic
distance. The derived values of V along with the details of the IVCT
bands for [1]+ and [2]+ are listed in Table 1.
(12) [M+] was corrected for disproportionation using the physically reasonable
solution to (0.25 - (1/Kc)) [M+]2 - (0.5C) [M+] + (0.25X) (2C - X) )
0, where [M+] ) concentration of monocation at equilibrium, C ) total
concentration of all M-species in the solution, and X ) concentration of
oxidizing agent in the solution.
Our results reveal that the electronic coupling in the Pt-bridged
system is only slightly decreased when compared to that found for
the benzene-bridged triarylamine system [2]+. The electronic
couplings obtained by using Koopmans’ theorem15 (KT) are nearly
twice those derived from IVCT bands but reproduce the same trend.
The similarity between the electron-transfer processes in [1]+ and
[2]+ is additionally supported by the nature of the active orbitals
involved (see Figure 3);16 both sets of orbitals show very similar
delocalization of the triarylamine electron density onto the central
bridge.
(13) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391-444.
(14) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247-
422.
(15) (a) KT estimates of V obtained from half the energy difference between
HOMO and HOMO-1 in the neutral molecule; 560 and 882 cm-1 for
[1]+ and [2]+, respectively. (b) Coropceanu, V.; Malagoli, M.; Andre´, J.
M.; Bre´das, J.-L. J. Am. Chem. Soc. 2002, 124, 10519-10530.
(16) The localized orbitals have been obtained as a linear combination of
HOMO and HOMO-1.
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