Phenylene vinylene platinum(II) acetylides with prodigious two-photon absorption

Article abstract of DOI:10.1021/ja309393c

The linear and nonlinear optical properties of a series of linear and cross-conjugated platinum(II) acetylide complexes that contain extended p-(phenylene vinylene) chromophores are reported. The complexes exhibit very high femtosecond two-photon absorption (2PA) cross section values (σ₂ up to 10 000 GM), as measured by nonlinear transmission (NLT) and two-photon excited fluorescence (2PEF) methods. The large 2PA cross sections span a broad range of wavelengths, 570-810 nm, and they overlap with the triplet excited state absorption. Spectral coincidence of high cross section 2PA and triplet absorption is a key feature giving rise to efficient dual-mode optical power limiting (OPL).

Full text of DOI:10.1021/ja309393c

Communication
pubs.acs.org/JACS
Phenylene Vinylene Platinum(II) Acetylides with Prodigious Two-
Photon Absorption
Galyna G. Dubinina,*,† Randi S. Price,† Khalil A. Abboud,† Geoffrey Wicks,‡ Pawel Wnuk,§
Yuriy Stepanenko,§ Mikhail Drobizhev,‡ Aleksander Rebane,*,‡,∥ and Kirk S. Schanze*,†
†Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, United States
‡Department of Physics, Montana State University, Bozeman, Montana 59717, United States
§Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, 01-224, Poland
∥National Institute of Chemical Physics and Biophysics, Tallinn, 12618, Estonia
S
* Supporting Information
atom-assisted high intersystem crossing efficiency along with a
nearly perfect match between the broad 2PA spectrum and the
T−T absorption wavelength.
ABSTRACT: The linear and nonlinear optical properties
of a series of linear and cross-conjugated platinum(II)
acetylide complexes that contain extended p-(phenylene
vinylene) chromophores are reported. The complexes
exhibit very high femtosecond two-photon absorption
(2PA) cross section values (σ2 up to 10 000 GM), as
measured by nonlinear transmission (NLT) and two-
photon excited fluorescence (2PEF) methods. The large
2PA cross sections span a broad range of wavelengths,
570−810 nm, and they overlap with the triplet excited
state absorption. Spectral coincidence of high cross section
2PA and triplet absorption is a key feature giving rise to
efficient dual-mode optical power limiting (OPL).
In order to probe the nonlinear optical response of the new
organometallic chromophores, we experimentally evaluate the
femtosecond 2PA cross sections across the broad wavelength
range 570−1070 nm by using two alternative methods, where
the first relies on two-photon excited fluorescence (2PEF) and
the second directly detects the intensity-dependent nonlinear
transmission (NLT). The ability to compare 2PA cross sections
measured by two different approaches is critical in this study. In
particular, the accuracy of the 2PEF method suffers from the
fact that the fluorescence signal is weak due to the efficient
intersystem crossing that is characteristic of platinum(II)
acetylide chromophores.1a In order to overcome this short-
coming, in the present study we developed and utilized a novel
NLT experimental arrangement that features automated data
collection and accurate measurement of small changes in the
intensity-dependent transmission (ΔT < 0.1%) as a function of
the input laser wavelength (see Supporting Information (SI)).
The series of p-phenylene vinylene platinum(II) acetylides
TPV0, TPV1, TPV2, TPV1-Ph, crossTPV1, crossTPV3
(Figure 1) were prepared by CuI catalyzed reaction of cis-
Pt(PBu3)2Cl2 or trans-(chloro)(phenylacetylide)-bis-(tri-n-
butylphosphine)platinum(II) and the corresponding terminal
acetylenes in diethylamine with good yields (58−96%).4 The
terminal acetylene precursor compounds were synthesized by
multistep procedures that relied on the Wittig reaction as a key
step for chromophore construction (see SI for details).
Complex TPV2-T2 was synthesized by a sequential one-pot
reaction using 2 equiv of the corresponding terminal alkyne, cis-
Pt(PBu3)Cl2, and 1 equiv of 5,5′-diethynyl-2,2′-bithiophene
(67%). Full synthetic details and characterization data for all of
the compounds are included as SI, including the X-ray crystal
structure of crossTPV1.
rganometallic complexes comprising a platinum(II)
Oacetylide core linked with different π-conjugated
chromophores are promising materials for applications
requiring a strong nonlinear optical response.1 In dual-mode
optical power limiting (OPL),2 a chromophore first undergoes
ultrafast two-photon absorption (2PA) to produce the singlet
state, followed by rapid intersystem crossing promoted by the
heavy atom effect to afford the triplet state, which can
subsequently absorb via the T1−Tn transition. Even though a
number of Pt-complexes have been demonstrated to act as
efficient triplet absorbers, their maximum intrinsic 2PA cross
sections are relatively low,1e thus impairing the practical
usefulness of materials available to date. This problem is
exacerbated by the fact that the currently attainable maximum
peak 2PA values, σ2 ∼102−103 GM, typically occur only over a
relatively narrow range of wavelengths.1d
π-Conjugated chromophores featuring triphenylamine moi-
eties at the termini of p-(phenylene vinylene) repeat units are
known to exhibit very high 2PA cross sections in the visible-
and near-IR range of wavelengths,3 especially if several such
chromophores are linked together in a dendritic structure.3b
However, lacking incorporated heavy atoms, the triplet−triplet
(T−T) absorption in such systems is marginal. Here we
combine oligomeric p-(phenylene vinylene) π-conjugated
chromophores (OPV) with platinum(II) centers to achieve
more than 1 order of magnitude enhancement of peak 2PA, up
to σ2 ∼103−104 GM, while retaining the benefits of the heavy
The one-photon photophysical properties of the set of
chromophores are summarized in Table 1, and additional data,
including full absorption and emission spectra, are provided in
the SI.
Received: September 21, 2012
Published: November 7, 2012
© 2012 American Chemical Society
19346
dx.doi.org/10.1021/ja309393c | J. Am. Chem. Soc. 2012, 134, 19346−19349

Journal of the American Chemical Society
Communication
the OPV ligand increases, due to less mixing of Pt-centered
orbitals in the frontier orbitals. Second, TPV2-T2 and the cross
conjugated chromophores exhibit comparatively low Φfl and τfl
values, suggesting that ISC is efficient in these systems.
In order to provide more insight into the triplet states of the
chromophores, ns−μs transient absorption spectroscopy was
carried out. Figure 2 shows representative transient absorption
Figure 1. Structures and acronyms for p-(phenylene vinylene)
platinum(II) acetylide chromophores.
Table 1. One-Photon Photophysical Properties in THF
Solution
Figure 2. Transient absorption difference spectra of selected Pt-
complexes, following nanosecond-pulsed 355 nm excitation, ca. 10
mJ/pulse in argon-purged THF.
a
λabs
τT‑T
,
a
b
a
c
name
TPV0
(log ε)
λfl
Φfl
τfl, ns λT‑T
μs
Φ1O2
395
459
502
525
502
522
0.10
0.30
0.52
0.46
0.28
0.01
0.04
0.18
0.78
1.15
1.44
0.66
<0.1
<0.1
585
0.3
0.13
spectra, and the full set of spectra is in the SI. Each of the
chromophores exhibits strong, broad T−T absorption through-
out the visible and near-IR regions, with bleaching correspond-
ing to the ground state absorption. The T−T bands are
noticeably narrower and more intense for the cross-conjugated
complexes, signaling that triplet chromophore is influenced by
the Pt centers.
The triplet lifetimes are listed in Table 1 (τT‑T), and it can be
seen that τT‑T increases substantially with OPV length. This
trend parallels that seen in the fluorescence lifetimes, and it is
consistent with decreased Pt-induced spin−orbit coupling with
increased length of the OPV.8 In addition, the triplet lifetimes
of TPV2-T2 and the cross-conjugated chromophores are
comparatively low, suggesting significant spin−orbit coupling in
these systems. Finally, the last column of Table 1 lists the
singlet oxygen yields observed using the chromophores as
sensitizers. These values in general reflect lower limits for the
ISC yields (ΦISC), and the data indicate in general that the
yields are high and they are largest for TPV1, TPV2-T2, and
the cross-conjugated chromophores. In most cases Φfl + Φ1O2
≈ 1, which supports the notion that ΦISC ≈ Φ1O2 and further
that the dominant paths for relaxation of the singlet state are via
fluorescence and intersystem crossing.
The 2PA absorption spectra and absolute cross section values
were measured with the wavelength tunable output of an
amplified Ti-sapphire source (80−120 fs) by using the NLT
and 2PEF methods (pulse repetition rates 100 Hz and 1 kHz,
correspondingly) as described in the SI. Figure 3 shows plots of
2PA cross section vs wavelength over the 570−1070 nm range
(lower horizontal scale) for all seven Pt-acetylide chromo-
phores, along with three representative free ligands in THF
solution.
In general, data measured using the NLT method (blue filled
symbols) correlate very well with those measured by the 2PEF
method (red open symbols). The 2PEF-based values of TPV0-
L and TPV1-L correspond well with those reported earlier.3 A
(5.13)
TPV1
415
(5.32)
678
687
671
683
662
719
1.19
67.9
0.59
0.79
0.85
0.43
0.73
0.51
0.35
0.79
0.83
0.98
TPV2
424
(5.47)
TPV1-Ph
TPV2-T2
crossTPV1
crossTPV3
415
(4.97)
423
(5.39)
d
d
355
(5.05)
476,
507
415
(5.03)
501,
537
b
a
Wavelengths are in nm. Measured at rt using 9,10-diphenylan-
thracene in cyclohexane as a standard (Φfl = 0.75).5 Estimated error in
c
values is 15%. Measured in CDCl3 using terthiophene as a standard
(Φ1O2 = 0.73).6 Estimated error in values is 15%. Lifetime is less
d
than the instrument time resolution which is ∼100 ps.
For the TPVn series, the peak molar absorptivity values
increase with the length of the π-conjugated unit, while the
absorption and fluorescence maxima red-shift in the same
sequence. The cross-conjugated chromophores exhibit very
broad absorption for λ < 525 nm, with two band maxima. For
cross-TPV1 the absorption maximum is at 355 nm, and as the
length of the OPV unit increases in crossTPV3, the long
wavelength band (λ ≈ 415 nm) becomes more pronounced.
These trends suggest that the shorter wavelength absorption
arises from the Pt-CC-phenylene-CC-Pt unit,7 while the longer
wavelength transition arises from the OPV chromophore axis.
Inspection of the fluorescence quantum yields and lifetimes (Φfl
and τfl) in Table 1 reveals several interesting trends. First, for
the TPVn series Φfl and τfl increase with the length of the OPV
chromophore. This suggests that a competing nonradiative
decay process (likely intersystem crossing, ISC, vide infra)
becomes less efficient as the OPV length increases. This effect
is consistent with decreased spin−orbit coupling as the size of
19347
dx.doi.org/10.1021/ja309393c | J. Am. Chem. Soc. 2012, 134, 19346−19349

Journal of the American Chemical Society
Communication
Figure 3. 2PA spectra of Pt-complexes and their chromophore ligands in THF solution measured by NLT method (blue filled symbols) and by
fluorescence excitation method (red open symbols). Linear absorption spectrum (black line) calibrated in transition wavelength (upper horizontal
scale) is shown for comparison.
certain discrepancy in the NLT peak values in TPV0-L and
TPV0 may be caused by partial bleaching of high concentration
samples used in the NLT experiment. The linear absorption
spectrum of each chromophore is shown in Figure 3 for
comparison (black line).
Several features are significant with respect to the 2PA
spectra of the chromophores. Specifically, each system exhibits
pronounced 2PA with σ2 greater than 500 GM over a broad
spectral window, in some cases ranging from 600 to 900 nm. In
addition, the peak 2PA cross sections for the complexes that
contain the longest OPV segment exhibit σ2 > 10 000 GM.
Finally, the cross-conjugated chromophores exhibit particularly
broad and intense 2PA spectra, with σ2 > 1000 GM between
600 and 800 nm. In each case, there is an outstanding overlap
between the 2PA absorptions and the T-T absorptions (Figure
2), which is optimal when considering application of the
chromophores in dual-mode OPL. Preliminary z-scan experi-
ments using nanosecond pulses support the idea that these
19348
dx.doi.org/10.1021/ja309393c | J. Am. Chem. Soc. 2012, 134, 19346−19349

Journal of the American Chemical Society
Communication
McLean, D. G.; Fleitz, P. A.; Cooper, T. M.; Drobizhev, M.; Makarov,
N. S.; Rebane, A.; Kim, K.-Y.; Farley, R.; Schanze, K. S. Inorg. Chem.
2007, 46, 6483. (e) Vestberg, R.; Westlund, R.; Eriksson, A.; Lopes,
C.; Carlsson, M.; Eliasson, B.; Glimsdal, E.; Lindgren, M.; Malmstrom,
E. Macromolecules 2006, 39, 2238.
(2) (a) McKay, T. J.; Staromlynska, J.; Wilson, P.; Davy, J. J. Appl.
Phys. 1999, 85, 1337. (b) Sutherland, R. L.; Brant, M. C.; Heinrichs, J.;
Rogers, J. E.; Slagle, J. E.; McLean, D. G.; Fleitz, P. A. J. Opt. Soc. Am. B
2005, 22, 1939.
(3) (a) Albota, M.; Beljonne, D.; Bredas, J.-L.; Ehrlich, J. E.; Fu, J.-Y.;
Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.;
McCord-Maughon, D.; Perry, J. W.; Rockel, H.; Rumi, M.;
Subramaniam, G.; Webb, W. W.; Wu, X.-L.; Xu, C. Science 1998,
281, 1653. (b) McIlroy, S. P.; Clo, E.; Nikolajsen, L.; Frederiksen, P.
K.; Nielsen, C. B.; Mikkelsen, K. V.; Gothelf, K. V.; Ogilby, P. R. J. Org.
Chem. 2005, 70, 1134. (c) Drobizhev, M.; Rebane, A.; Suo, Z.;
Spangler, C. W. J. Luminesc. 2005, 111, 291. (d) He, G. S.; Tan, L.-S.;
Zheng, Q.; Prasad, P. N. Chem. Rev. 2008, 108, 1245.
(4) Sonogashira, K.; Fujikura, Y.; Yatake, T.; Toyoshima, N.;
Takahashi, S.; Hagihara, N. J. Organomet. Chem. 1978, 145, 101.
(5) Mardelli, M.; Olmsted, J. J. Photochem. 1977, 7, 277.
(6) Scaiano, J. C.; Redmond, R. W.; Mehta, B.; Arnason, J. T.
Photochem. Photobiol. 1990, 52, 655.
chromophores are very effective pulse limiters (see SI, Figure
S61b)
The 2PA spectra of the complexes show a distinct peak at
transition wavelengths shorter than the S0−S1 transition in the
one-photon absorption (1PA) spectra, which can be explained
by alternative parity selection rules for 1PA and 2PA
transitions.3c By analogy to the 1PA spectra, the 2PA bands
red-shift with increasing π-conjugation length. Note that the
peak 2PA values increase approximately 2-fold going from
single ligand chromophores TPV0-L and TPV1-L to the
corresponding Pt-complexes TPV0 and TPV1, and more than
2-fold going from TPV2-L to TPV2. The observation that in
TPV-T2 and crossTPV3 the NLT shows larger 2PA cross
sections at the shorter wavelengths than 2PEF may be due to
the excited state absorption occurring from excited singlet state
during the duration of the laser pulse, thus further augmenting
the strong nonlinear-optical response.
In conclusion, a series of p-(phenylene vinylene) substituted
platinum acetylide chromophores have been synthesized and
investigated under one- and two-photon excitation. Nano-
second transient absorption shows that all of the chromophores
feature strong, broad T−T absorption in the visible and near-
infrared. The 2PA spectra for the chromophores were measured
by using the traditional 2PEF approach, and with a newly
developed NLT approach, and two approaches afford
consistent spectral profiles and absolute maximum σ2 values.
The chromophores feature considerable σ2 in a spectral region
that overlaps nicely with the region of large T−T absorption.
The results suggest that the OPV-based platinum acetylide
systems are promising candidates for practical application as the
active chromophores in broad temporal and frequency
responsive optical power limiting materials. Investigations in
progress seek to characterize the efficacy of these chromo-
phores to exhibit a broad-band nonlinear absorption response
in solution and in solid matrices.
(7) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus,
M.; Khan, M. S.; Raithby, P. R.; Kohler, A.; Friend, R. H. J. Am. Chem.
Soc. 2001, 123, 9412.
(8) Rogers, J. E.; Hall, B. C.; Hafnagle, D. C.; Slagle, J. E.; Ault, A. P.;
McLean, D. G.; Fleitz, P. A.; Cooper, T. M. J. Phys. Chem. A 2005, 122,
214708.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray structure of crossTPV1, experimental details, character-
ization and photophysical data. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
kschanze@chem.ufl.edu; dubinina@chem.ufl.edu; rebane@
physics.montana.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Air Force Office of
Scientific Research (Grant Nos. FA-9550-06-1-1084 and
FA9550-09-1-0219). K.A.A. wishes to acknowledge the Na-
tional Science Foundation and the University of Florida for
funding of the purchase of the X-ray equipment.
REFERENCES
■
(1) (a) Liao, C.; Shelton, A. H.; Kim, K.-Y.; Schanze, K. S. ACS Appl.
Mater. Interfaces 2011, 3, 3225. (b) Haley, J. E.; Krein, D. M.;
Monahan, J. L.; Burke, A. R.; McLean, D. J.; Slagle, J. E.; Fratini, A.;
Cooper, T. M. J. Phys. Chem. A 2011, 115, 265. (c) Liu, R.; Li, Y.; Li,
Y.; Zhu, H.; Sun, W. J. Phys. Chem. A 2010, 114, 12639. (d) Rogers, J.
E.; Slagle, J. E.; Krein, D. M.; Burke, A. R.; Hall, B. C.; Fratini, A.;
19349
dx.doi.org/10.1021/ja309393c | J. Am. Chem. Soc. 2012, 134, 19346−19349