Photophysics of monodisperse platinum-acetylide oligomers: Delocalization in the singlet and triplet excited states

Article abstract of DOI:10.1021/ja027639i

A series of monodisperse Pt-acetylide polymers that contain the [-C_≡C-(p-C₆H₄)-C_≡C-(t-Pt(PBu₃)₂)-]_n repeat unit has been prepared for n = 1, 2, 3, 4, 5, and 7. The photophysical properties of the series provide information concerning the relationship between the oligomer length and delocalization in the singlet and triplet excited states of the π-conjugated electron system. The results imply that the singlet excited state is delocalized over approximately 6 repeat units; however, the triplet state is considerably more localized. The triplet energy is almost invariant with oligomer length, but the phosphorescence spectra and triplet nonradiative decay rates indicate that the electron-vibrational coupling in the triplet state decreases with increasing oligomer length. Copyright

Full text of DOI:10.1021/ja027639i

Published on Web 09/26/2002
Photophysics of Monodisperse Platinum-Acetylide Oligomers: Delocalization
in the Singlet and Triplet Excited States
Yao Liu, Shujun Jiang, Ksenija Glusac, David H. Powell, Danielle F. Anderson, and Kirk S. Schanze*
Department of Chemistry, UniVersity of Florida, P.O. Box 117200, GainesVille, Florida 32611-7200
Received July 10, 2002
Platinum-acetylide-based π-conjugated polymers and oligomers
have attracted interest because their photophysics are dominated
by long-lived and phosphorescent 3π,π* excited states.1,2 This
characteristic leads to materials that may be used to fabricate high-
efficiency organic electroluminescent devices,3 and for applications
in laser protection.4
Considerable insight concerning excited-state structure and
delocalization in organic π-conjugated systems has been acquired
through the study of monodisperse oligomers.5 Comparatively less
information is available concerning the effect of delocalization on
3π,π* states in conjugated systems, because these states are
spectroscopically “silent” due to the forbidden character of the S0
T T1 transitions.
Herein we report the preparation and photophysical characteriza-
Figure 1. (a) UV-visible absorption spectra of PAOs in THF. In order of
increasing intensity: Pt-1, Pt-2, Pt-3, Pt-4, Pt-5, Pt-7. (b) Photolumines-
cence spectra in deoxygenated THF. F is the fluorescence: in order of
increasing intensity Pt-2, Pt-3, Pt-4, Pt-5, Pt-7. P is phosphorescence: in
order of decreasing intensity in 0-1 band Pt-2, Pt-3, Pt-4, Pt-5, Pt-7. Note
that the fluorescence intensity scale is magnified 100× as compared to
phosphorescence.
tion of the series of linear Pt-acetylide oligomers (PAOs) shown
in Scheme 1. PAOs with n > 2 luminesce from the 1π,π* and 3π,π*
manifolds of the Pt-acetylide π-conjugated system, which allows
us to probe in detail the effect of oligomer length on the spec-
troscopy, energetics, and dynamics of the long-lived excited states.
PAOs are synthesized by an iterative-convergent approach that
relies on the use of Pt-acetylide building blocks in which terminal
acetylenes are protected using the trimethylsilyl protecting group.5
Complete details of the synthesis and spectral characterization of
the PAOs are available as Supporting Information.
Scheme 1
Absorption spectra of the PAOs (Figure 1a) exhibit three well-
resolved bands. The spectra are dominated by an intense band (I)
arising from the long-axis polarized π,π* transition. The maximum
of band I red-shifts with increasing PAO length (Table 1), but the
difference in λmax between Pt-5 and Pt-7 is small, indicating that
the effective conjugation length as probed by the Franck-Condon
absorption event is ∼6 repeat units. The width of band I also
increases with PAO length, possibly signaling the existence of
conformers that differ with respect to the relative orientations of
the chromophoric units (i.e., the phenylene rings and the plane
defined by the PtP2R2 units) along the chain. Band broadening may
occur because the number of different conformers increases with
PAO length. Two other absorption bands (II and III) are resolved
at shorter wavelengths. The intensity of these bands increases with
PAO length, but their absorption maxima do not vary systematically.
This observation is consistent with the notion that the high-energy
bands arise from short-axis polarized transitions localized on the
phenylene rings. Importantly, the spectra of the longer PAOs are
very similar to the absorption of the analogous Pt-acetylide
polymer,1a,b which indicates that the oligomers are effective models
for the excited-state properties of the corresponding polymer.
Figure 1b illustrates the photoluminescence spectra of the series
of PAOs Pt-2-Pt-7 obtained in degassed THF solutions. (At room
temperature, Pt-1 is a nonemissive compound.) Each of the PAOs
Table 1. Photophysical Data for PAOsa
emission/nm
c
UV−vis λmax
/nm (log ꢀ)
τP
/µs
k c
knr
r
F
P
λmax
λmax
φPb/%
/103 s-1 /104 s-1
Pt-1 324 (4.44)
Pt-2 355 (4.85)
Pt-3 363 (5.10)
Pt-4 367 (5.22)
Pt-5 368 (5.30)
374.0 515.9
385.5 517.3
389.0 517.8
390.6 518.2
5.2
5.8
6.7
6.6
7.1
17.7
18.4
18.6
19.3
24.2
2.9
3.2
3.6
3.4
2.9
5.4
5.1
5.0
4.8
3.8
Pt-7 371.5 (5.46) 391.0 518.2
a All measurements carried out in vacuum-degassed THF solutions. b fac-
Ir(2-phenylpyridine)3 used as the actinometer (φ ) 0.4). c kr ) φP/τP; knr
)
(1 - φP)/τP . Decay rate computed under the assumption that the intersystem
crossing efficiency is unity.
exhibits a very weak fluorescence band that is Stokes-shifted very
little from the low-energy absorption (band I), indicating that the
emission emanates from the lowest singlet state (S1). In addition,
the PAOs feature a moderately intense phosphorescence band with
a maximum at ∼515 nm. Table 1 lists the λmax values for the
fluorescence and phosphorescence bands, along with the phospho-
rescence quantum yield (φP), the phosphorescence lifetime (τP), and
radiative and nonradiative decay rates (kr and knr, respectively) for
the triplet excited state.
* To whom correspondence should be addressed. E-mail: kschanze@
chem.ufl.edu.
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J. AM. CHEM. SOC. 2002, 124, 12412-12413
10.1021/ja027639i CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Despite the evidence which suggests that the triplet state is
strongly localized in the PAOs, several lines of evidence indicate
that the properties of the triplet exciton are influenced by PAO
P
length. First, the ratio I01P/I00 decreases systematically with
increasing PAO length (where IP is phosphorescence intensity, see
Figure 1b). This clearly indicates that electron-vibrational coupling
in T1 decreases with increasing PAO length, which signals that even
though the triplet energy is not affected by delocalization, the triplet
wave function is sensitive to oligomer length.
More quantitative information regarding the effect of PAO length
on the electron-vibrational coupling in the triplet state is provided
by the dependence of the nonradiative decay rate (knr) on PAO
length. Interestingly, as shown in Table 1, knr decreases by more
than 20% across the series Pt-2 f Pt-7. Application of nonradiative
decay theory under the assumption that an average high-frequency
mode with hω ) 1300 cm-1 is coupled to T1 decay indicates that
the observed 20% decrease in knr can be accounted for by a 5%
decrease in the Huang-Rhys constant across the series.7 (The
Huang-Rhys constant is a quantitative measure of the degree of
Figure 2. Plots of optical transition energies in eV versus the inverse
squared number of repeat units that represents relative molecular length
for each oligomer. (b) absorption; (9) fluorescence; (2) phosphorescence.
Note that the y-scale for the phosphorescence is expanded by a factor of 5.
The fluorescence intensity of all of the PAOs is low (φF < 10-3).
This implies that intersystem crossing is extremely rapid (e.g., k
. 109 s-1), and we conclude that the triplet yield is approximately
unity. The maximum of the fluorescence band red-shifts from n )
2 to 5, but it levels off between n ) 5 and 7. This trend is consistent
with the trend in the absorption spectra and implies that the effective
conjugation length in the relaxed singlet state (S1) is ∼6 repeat
units. Additional insight concerning the effect of delocalization on
S1 is provided by the fluorescence band shape. Specifically, the
electron-vibrational coupling (which decreases with increasing
electron-vibrational coupling.6,7
)
In conclusion, a new series of PAOs has been synthesized by an
iterative-convergent approach. Absorption and photoluminescence
spectroscopy of the oligomers provide considerable insight into the
extent of delocalization in the singlet and triplet excited states in
Pt-acetylide materials.
Acknowledgment. We gratefully acknowledge the National
Science Foundation (NSF CHE-9901862 and CHE-0211252) for
support of this work.
excited-state delocalization)6,7 in S1 is proportional to the ratio of
F
Supporting Information Available: Descriptions of the synthesis
and spectroscopic characterization of PAOs, and the photophysical
methods, figures showing expansion of fluorescence spectra, the
phosphorescence decays, and calculated fits (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
the 0-1 and 0-0 vibronic bands in the fluorescence spectrum (I01
/
I00F).6,7 Inspection of the fluorescence spectra in Figure 1b shows
that I01F/I00 decreases with increasing PAO length (see the
F
Supporting Information for an expansion of the fluorescence
spectra). Interestingly, the ratio drops sharply from 0.45 to 0.2
between n ) 4 and 5, but it is then approximately the same for n
) 5 and 7. This observation is consistent with the trends in the
absorption and fluorescence maxima, which imply that the S1 state
is delocalized over ∼6 repeat units.
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In contrast to the absorption and fluorescence, the phosphores-
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that the energy of the triplet state (T1) is influenced little by PAO
length, and it also implies that the triplet is localized on 1 or 2
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phos
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