Stereochemical Effects on Platinum Acetylide Two-Photon Chromophores

Article abstract of DOI:10.1021/acs.jpca.9b07823

A series of cis-platinum(II) acetylide complexes containing two-photon-absorbing chromophores have been synthesized and characterized to explore the effects of stereochemistry on the nonlinear absorption properties. The molecules feature 4-(phenylethynyl)phenylethynylene (PE2), diphenylaminofluorene (DPAF), and benzothiazolylfluorene (BTF) ligands. The photophysical properties were investigated under one- and two-photon conditions and compared to the known trans analogues via UV-visible absorption, photoluminescence, femtosecond and nanosecond transient absorption (TA), nanosecond z-scan, and femtosecond two-photon absorption (2PA). The bent cis complexes exhibit blue shifts in the absorption, emission, femtosecond, and nanosecond TA spectra along with lower molar extinction coefficients and lower phosphorescence yields relative to the trans complexes suggesting less efficient Pt-induced spin-orbit coupling and intersystem crossing in the cis configuration. The cis chromophores are noncentrosymmetric and therefore show dipolar behavior with a pronounced 2PA in the 0-0 transition of the S₀ → S₁ band, while the trans complexes show quadrupolar behavior with a forbidden 0-0 transition. In the S₀ → S_n region, both cis and trans complexes show intense two-photon-absorption bands (up to 3700 GM by the peak cross section for cis-BTF) which contain a significant contribution from the excited state absorption (S₁ → S_n). All six complexes exhibit comparable nonlinear absorption response with a significant contribution from triplet-triplet absorption that slightly favors trans complexes but is more strongly dependent upon the structure of the π-conjugated chromophore.

Full text of DOI:10.1021/acs.jpca.9b07823

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Stereochemical Effects on Platinum Acetylide Two-Photon Chromophores
Abigail Shelton, Silvano R. Valandro, Randi S. Price, Galyna G. Dubinina, Khalil A.
Abboud, Geoffrey Wicks, Aleksander Rebane, Muhammad Younus, and Kirk S. Schanze
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b07823 • Publication Date (Web): 07 Oct 2019
Downloaded from pubs.acs.org on October 7, 2019
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Stereochemical Effects on Platinum Acetylide
Two-Photon Chromophores
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§
†
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§
Abigail H. Shelton, Silvano R. Valandro, Randi S. Price, Galyna G. Dubinina, Khalil A. Abboud,
§
‡,$
‡,#,
†,
†,
Geoffrey Wicks, Aleksander Rebane, * Muhammad Younus, * Kirk S. Schanze *
§Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, FL 32611
†Department of Chemistry, University of Texas at San Antonio, San Antonio, TX 78249
$ILX Lightwave, Bozeman, MT 59717
‡Physics Department, Montana State University, Bozeman, Montana, 59715
#
National Institute for Chemical Physics and Biophysics, Tallinn, 12618, Estonia
*Authors to whom correspondence should be addressed.
e-mail: kirk.schanze@utsa.edu, muhammad.younus@utsa.edu, rebane@physics.montana.edu
Abstract
.
A series of cis-platinum(II) acetylide complexes containing two-photon absorbing
chromophores have been synthesized and characterized to explore the effects of stereochemistry on the
non-linear absorption properties. The molecules feature 4-(phenylethynyl)phenylethynylene (PE2),
diphenylaminofluorene (DPAF), and benzothiazolylfluorene (BTF) ligands. The photophysical
properties were investigated under one- and two-photon conditions and compared to the known trans
analogs via UV-visible absorption, photoluminescence, femtosecond and nanosecond transient
absorption, nanosecond z-scan, and femtosecond two-photon absorption. The bent cis complexes exhibit
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blue-shifts in the absorption, emission, femtosecond and nanosecond TA spectra along with lower molar
extinction coefficients and longer fluorescence lifetimes relative to the trans complexes which indicates
less efficient Pt-induced spin-orbit coupling and intersystem crossing in the cis configuration. The cis
chromophores are non-centrosymmetric and therefore show dipolar behavior with a pronounced 2PA
absorption in 0-0 transition of the S →S band while the trans complexes show quadrupolar behavior
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with a forbidden 0-0 transition. In the S →S region, both cis and trans complexes show intense two-
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photon absorption bands (up to 3700 GM by the peak cross section for cis-BTF) which contains a
significant contribution from the excited state absorption (S →S ). All six complexes exhibit comparable
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non-linear absorption response with a significant contribution from triplet-triplet absorption that slightly
favors trans complexes but is more strongly dependent upon the structure of the -conjugated
chromophore.
Introduction
Platinum acetylide chromophores often display strong non-linear absorption (NLA) over a wide
spectral region and linear transmission through most of the visible region. This provides researchers with
the opportunity to investigate the spin-forbidden S →T state absorption, intersystem crossing (ISC) to
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-3
the triplet manifold, and triplet-triplet excited state absorption (T →T ). The introduction of different
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n

-conjugated ligands of varying lengths in platinum acetylide complexes have been shown to elicit
advantageous effects on the one- and two-photon photophysical properties.4-7 The platinum metal center
induces efficient ISC to a long lived triplet excited state; therefore, these platinum acetylides are well
suited for a dual-mode NLA process from a nanosecond laser pulse that combines 2PA from the singlet
8
-10
ground state (S →S ) with excited state absorption (ESA) from the triplet state (T →T ).
0
n
1
n
The majority of the photophysical investigations reported to date have focused on linear platinum
acetylide complexes in the trans configuration at the metal center.1,4,8,11-17 This focus is partially due to
the thermodynamic instability of the cis isomer with respect to the trans isomer in solution, specifically
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when the auxiliary ligands are mono-dentate phosphines such as PBu and PEt . However, platinum
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acetylides can be constrained into the cis configuration by the incorporation of chelating auxiliary groups
such as bidentate phosphines (e.g. 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-
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bis(diphenylphosphino)propane (dppp)),19-20 diimine ligands such as 2,2’-bipyridine,19,21 or more recently
bidentate bis(imidaozolin-2-ylidenes)).22
Stable cis-platinum acetylide complexes have been studied for their intraligand and charge transfer
properties and for use in white23-24 and red organic light emitting diodes, neutral molecular squares,26-29
coordination spheres,30-31 and as molecular tweezers,32-33 applications which are aided by the locked
geometry and the near 90° angles formed between the ligand arms. Most investigations within this
research area have focused on 1,4-diethynylbenzene or 4,4-diethynylbiphenyl ancillary ligands, and
variations, within monomeric or multi-platinum species. Despite the growing amount of literature on cis-
platinum complexes, very few studies have reported the photophysical response (photoluminescence,
transient absorption, or nonlinear absorption), and most are not designed to exhibit 2-photon absorption
(2PA) and excited state absorption (ESA).34-35
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Herein we report the synthesis of three novel cis platinum acetylide complexes and make comparisons
between their one- and two-photon photophysical properties relative to the previously reported trans
analogs. The diphenylaminofluorene (DPAF) and benzothiazolefluorene (BTF) chromophores utilized
are both large cross section 2PA chromophores; these donor and acceptor chromophores were
incorporated onto platinum metal cores by acetylide linkages in both cis and trans geometries. The trans-
PE2 (bis-((4-(phenylethynyl)phenyl)ethynyl)bis-(tributylphosphine) platinum(II)) has become a common
benchmark for platinum acetylide chemistry which shows moderate NLA.11,15,36-37 As such, the cis and
trans geometries of PE2 were also investigated. This series of Pt complexes, with structures shown in
Figure 1, has allowed for the elucidation of stereochemical effects on their photophysical properties. The
photophysical properties were evaluated by ground state absorption, steady state emission spectroscopy,
triplet-triplet transient absorption, and nonlinear absorption.
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PBu3
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R
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DPAF
BTF
Figure 1. Chemical structures of the two-photon absorbing platinum acetylides in the cis and trans
geometries
Experimental Section
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Materials and Synthesis. PE2, DPAF, and BTF chromophores, trans-PE2, trans-DPAF, trans-
BTF,8 1,5-cyclooctadiene platinum(II) dichloride,38 and cis-diphenylphosphineethaneplatinum(II)
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dichloride were synthesized according to literature procedures. The synthesis route for obtaining the cis
complexes is shown in Figure 2 and further described in the supporting information. The first intermediate
was synthesized by reaction of K PtCl with 1,5-cyclooctadiene in ethanol, according to the method
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described by Baker.38 The obtained COD product was then reacted with 1,2-
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bis(diphenylphosphino)ethane (dppe) to synthesize the Pt(dppe)Cl precursor. As has previously been
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examined with similar dppe-bound platinum complexes, the [Pt(dppe)Cl ] precursor was utilized to
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generate the target cis complexes by reaction with the corresponding chromophores.19,40 The dppe
chelating auxiliary group is incorporated into the cis complexes to increase the thermodynamic stability
of the generated complexes in addition to locking the geometry into the cis configuration.
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General Techniques. H, C, and P NMR spectra were recorded on a Varian Mercury 300
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spectrometer in deuterated chloroform; chemical shifts () are reported in ppm and referenced to
tetramethylsilane or protonated solvent signals. Elemental analyses were performed by the University of
Florida Spectroscopic Services.
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X-Ray intensity data was collected at 100 K on a Bruker SMART diffractometer using MoKα
radiation (λ = 0.71073 Å) and an APEXII CCD area detector. Raw data frames were read by program
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SAINT and integrated using 3D profiling algorithms. The resulting data was reduced to produce hkl
reflections, their intensities and estimated standard deviations. The data was corrected for Lorentz and
polarization effects and numerical absorption corrections were applied based on indexed and measured
faces. The molecular structure was solved and refined in SHELXTL6.1, using full-matrix least-squares
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refinement. The non-H atoms were refined with anisotropic thermal parameters and all of the H atoms
were calculated in idealized positions and refined riding on their parent atoms.
Unless noted, one-photon photophysical studies were carried out with samples contained in 1 x 1 cm
quartz or glass spectroscopic cuvettes. Ground state absorption spectra were measured on a Varian Cary
1
00 dual-beam spectrophotometer. Corrected steady-state emission measurements were performed on a
Photon Technology International (PTI) photon counting fluorescence spectrometer; sample
concentrations were adjusted to produce optically dilute solutions, O.D.max < 0.20. Samples were argon
purged for 20 minutes for phosphorescent measurements. Low-temperature measurements were
conducted in distilled HPLC grade 2-methyltetrahydrofuran and placed in a standard NMR tube. The
tube was then inserted into a liquid nitrogen-filled silvered finger dewar and placed into the PTI
spectrometer sample holder. Quantum yields were calculated against Ru(bpy) Cl in air-saturated
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deionized water ( = 0.0379).43
Fluorescence lifetimes for the PE2 complexes were obtained by PicoQuant FluoTime 100 Compact
Fluorescence Lifetime Spectrophotometer which has a minimum time resolution of 100 ps. A UV-pulsed
diode laser provided the excitation at 375 nm (P < 10 mW). The laser was pulsed using an external BK
Precision 4011A 5 MHz function generator. Decays were obtained using the biexponential fitting
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parameters within the PicoQuant PicoHarp software. Fluorescence lifetimes for the DPAF and BTF
complexes were obtained using a PicoQuant FluoTime 300 spectrometer equipped with a frequency
doubled, Coherent Chameleon Ti:sapphire femtosecond laser as the source. The temporal resolution of
the FT-300 system is ~ 5 ps.
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Nanosecond triplet-triplet transient absorption measurements were conducted using the third
harmonic of a Continuum Surelite II-10 Nd:YAG laser (λ = 355 nm), with an excitation pulse energy of
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mJ; this instrumentation has been described previously.44 Sample concentrations were adjusted to an
optical density of 0.7 at the excitation wavelength. The sample solutions were placed in a continuously
circulating 1 cm pathlength flow cell holding a volume of 10 mL. The sample solutions were prepared in
THF and deoxygenated by bubbling with argon.
Triplet excited state lifetimes were calculated from the transient absorption spectra with software
developed in-house using samples prepared identically to those used for nanosecond triplet-triplet
transient absorption measurements. The software and instrumentation have been described previously,
though the system has been modified to incorporate a Surelite Continuum I Nd:YAG laser for excitation
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at 355 nm. The lifetimes were such that the arc lamp pulser was not necessary.
Nanosecond NLA measurements were performed via an open-aperture z-scan apparatus developed
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in-house. The excitation wavelength was generated by a Continuum Surelite OPO Plus pumped with
the third harmonic (355 nm) of a Continuum Surelite II-10 Nd:YAG laser. The laser beam was split with
a 50:50 beam splitter to two OPH PE10-SH V2 pyroelectric detectors, which measured the transmitted
pulse energy as a function of the input pulse energy using an Ophir Laserstar dual-channel optical laser
energy meter. The beam was focused with a 25.4 mm diameter, 50.8 mm focal length concave lens. A
ThorLabs motorized translation stage (Z825B and TDC001) allowed mm movement along the z-axis.
To acquire femtosecond transient absorption a double-beam subpicosecond pump-probe technique
was used. Pump and probe pulses were generated by an amplified Ti-sapphire laser (Astrella, Coherent,
Inc.) displaying at 800 nm with pulse width 100 fs and 1 kHz repetition rate coupled with an optical
paramagnetic amplifier (Coherent) was used for pulse pump generation. The probe pulse was a continuum
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generated by the fundamental focused onto a sapphire plate. Transient absorption spectra were obtained
using a Helios Fire transient absorption spectrometer (Ultrafast Systems). The sample solutions were
excited at 380 nm and constantly stirred to avoid photodegradation. Transient absorption data were
analyzed using global analysis and singular value decomposition (SVD) by Surface Xplorer PRO program
from Ultrafast System.
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The femtosecond 2PA measurements were performed in benzene solutions by the non-linear
transmission method and 2PA cross section values were calculated as described in a previous article and
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using Rhodamine B in H O and PPV in DCM as a reference standard. Briefly, the optical parametric
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amplifier (OPA, wavelength 550 – 810 nm SHS and 800 -1140 nm SHI, pulse duration 80 – 100 fs)
output beam at a 100 Hz pulse repetition rate was directed through the sample prepared at concentrations
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c ~10 M in a standard spectroscopic cuvette. Two identical 10 mm diameter silicon photodetectors
(Thorlabs, DET100A) were used to measure the laser pulse reflected from two glass beam pick-off plates
positioned, accordingly, before and after the sample. The analog signals from the photodetectors were
digitized using a DAQ board (National Instruments, Model PCI-6011). The dependence of the sample
transmission (defined as ratio of the signal from the second detector dived by the signal from the first
detector) on the incident intensity was evaluated by varying the energy of the incident pulse in the range,
Pin= 1.0 – 100 J using a neutral density filter attenuator wheel (Thorlabs) mounted on a stepper motor.
The estimated accuracy of the measured transmission value is about 0.1%. A laser energy meter (Nova
II, Ophir) was used to measure the pulse energy at the input to the sample. The beam diameter at the
sample was 0.4 – 2.0 mm and varied depending on the OPA wavelength. A digital CCD camera (Stingray,
Allied) was used to measure the spatial beam intensity profile at the sample. The DAQ board, the
attenuator wheel, the energy meter and the CCD camera were controlled with a PC/LabView routine.
Second harmonic generation autocorrelator (Clark MXR) was used to measure the temporal
autocorrelation width of the laser pulses.
Results and Discussion
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Structures and Synthesis.
The new series of cis-[Pt(dppe)(L )] complexes (L = 4-
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(
phenylethynyl)phenylethynylene (PE2), diphenylaminofluorene (DPAF), and benzothiazolylfluorene
(BTF)) were prepared by the reaction of cis-[Pt(dppe)Cl ] with appropriate acetylene ligand in presence
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of CuI catalyst (Figure 2). The complexes were characterized by spectroscopic and elemental analysis.
The molecular structure of cis-PE2 and cis-BTF were determined by single crystal X-ray crystallography
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(
see SI section for the X-ray analysis data).
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In all complexes, the H NMR spectrum gave expected peaks around  2.32 – 2.46 for the
characteristic methylenic protons. The complexes with DAPF and BTF chromophores displayed signals
as quartets at  ~ 1.08 – 2.06 for the diastereotopic CH protons and  ~ 0.30 as triplets for the CH protons
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in the fluorene moiety. All complexes displayed complex multiplets and doublets at  ~ 6.97-8.02 for the
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1
aromatic protons. The P NMR spectrum of all complexes gave singlets at  ~ 42 with satellites ( J
(Pt-P)
=
2278 – 2280 Hz). These spectroscopic data are consistent with the previously published cis-Pt(dppe)
acetylide complexes. 39,48
P
P
PPh2
PPh2
Ph2P Pt
COD, H O, EtOH
2
CuI, iPr NH
CH2Cl2
2
o
5
0
K PtCl4
Pt Cl
Cl
Ph P Pt Cl
R
2
2
7
0 - 73 %
H
R
CHCl , 96%
3
Cl
R
Figure 2. Synthetic scheme for cis platinum acetylides
X-ray Crystallography. Suitable single crystals of cis-PE2 and cis-BTF were grown via vapor
diffusion of diethyl ether into THF and diethyl ether into DCM, respectively, for x-ray crystallography.
The molecular structures of cis-PE2 and cis-BTF are shown in Figure 3 and compared to the structures of
trans-PE2 28,49 and trans-BTF and select interatomic bond distances and angles are shown in Table 1.
The hydrogen atoms have been removed from both structures for clarity. The geometry around the
platinum metal center in cis-PE2 is approximately square planar, with a sum of the angles around the
8
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metal center being 359.97°. The cis-PE2 P-Pt-C1 and P-Pt-C1’angles of 90.1(2)° and 175.3(3)°
1
2
3
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8
9
1
1
1
1
1
1
1
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1
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2
2
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6
2
7
respectively, are similar to other known cis platinum acetylides. The smallest angle around the platinum
metal is that subtended by the dppe ligand, as anticipated due to the expected bite angle of the dppe ligand
onto a platinum center.27,50 The platinum acetylide units deviate more strongly from the expected linear
angle of 180° within the cis complex; the cis complex has a Pt-C1-C2 angle of 172.4(6)° while the trans
analogue exhibits an angle of 176.7(5)°. However, the angle observed in cis-PE2 is consistent with other
0
1
2
3
4
5
6
7
8
9
0
1
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9
0
1
2
3
4
5
6
7
8
9
0
27
similar cis platinum acetylides such as 174.3(5)° in Pt(CCH) (dppe), 168.0(3)° in Pt(CCC H -p-
2
6
4
2
0
20
CH ) (dppe), and 171.9(4)° in Pt(CCC H -p-C H -p-CCH) (dppe). The -CC- bond proximal to
3
2
6
4
6
4
2
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0
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2
3
4
5
6
7
8
9
0
Figure 3. Molecular structures for cis-PE2 (top) and cis-BTF (bottom) with atomic numbers. The
hydrogen atoms are omitted for clarity.
the platinum metal center in cis-PE2 is longer than the distal -CC- bond, 1.195(9) Å versus 1.184(8) Å.
This bond is longer than those observed in Cl-CC-Cl systems of 1.183 Å, but notably shorter than the
-
CC- bond length observed in trans-PE2 of 1.229(8) Å.49 Davy and co-workers suggest that the
lengthening of the acetylide bond observed in trans-PE2 is the result of d-* backbonding in the
49
phenylethynyl chromophore, as is especially evident for the -CC- adjacent to the metal center. The
shorter distal -CC- bond in trans-PE2 (1.191(9) Å), in addition to the average dihedral angle of 14.9°
between the two phenyl groups, indicates that the conjugation does not extend throughout the PE2
chromophore. The observed bond lengths in cis-PE2 suggest that d-* backbonding could occur in the
chromophore, particularly near the platinum center, but to a smaller extent than observed in trans-PE2.
Interestingly, the average dihedral angle in cis-PE2 of 11.0° is smaller than the 14.9° angle observed in
trans-PE2, which would promote conjugation throughout the chromophore in the cis geometry.
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Similar to cis-PE2, the geometry around the platinum center of cis-BTF is approximately square planar
1
2
and the angle subtended by the dppe ligand is 86.33(2)°, the smallest of the four angles around the metal
center. The P1-Pt-C1 and P1-Pt-C27 angles of 90.82(7) and 177.53(7)°, respectively, are consistent with
previously reported cis-platinum acetylides.27 The Pt-C1 and Pt-C27 bond lengths of 2.013(3) and
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
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2
2
2
2
2
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6
2
.010(3) Å within the cis-BTF complex are slightly longer than the comparative Pt-C1 bonds of the trans-
0
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9
0
BTF of 2.000(3) Å. Similarly, the Pt-P1 and Pt-P2 bond lengths of 2.2649(7) and 2.2709(7) Å within the
cis-BTF complex are slightly shorter than the comparative Pt-C1 bonds of the trans-BTF of 2.2941(7) Å.
Common within platinum acetylides is the observation that the Pt-P bond lengths are 0.2-0.4 Å longer
than the Pt-C bond lengths, regardless of the geometry around the platinum center.19-20,27,49,51 Taken
together, the observed cis-BTF and trans-BTF platinum bond lengths further indicate that the dppe ligand
and geometry around the platinum center affect the length deviation more so than the choice of
chromophore.
The crystal structure of cis-BTF displayed a small degree of disorder within one benzothiazole portion
of one chromophore. The effect of this disorder is most apparent when examining the bond angles of the
chromophores. The Pt-C27-C28 bond angle of 178.1(2)° is more linear than the Pt-C1-C2 angle of
1
70.8(2)°. However, both angles are within the expected region for cis platinum acetylides.20,27 The Pt-
CC bond angle of 176.6(3)° observed in trans-BTF also shows slight deviation from linearity. The
acetylide bond lengths are nearly identical in both complexes: 1.203(3) for C1-C2 and 1.201(4) for C27-
C28 within cis-BTF, and 1.208(4) for trans-BTF, suggesting similar conjugation across the acetylide
portions of the complexes.
Table 1. Select bond distances (Å) and angles (degrees) observed in cis-PE2 and cis-BTF.
Distance
Pt-P1
Pt-P2
P1-C29
Pt-C1
C1-C2
C2-C3
cis-PE2
2.271(2)
---
1.846(5)
2.011(8)
1.195(9)
1.435(7)
cis-BTF
2.2649(7) P1-Pt-P’
2.2709(7) C1-Pt-C1’
Angle
cis-PE2
85.77(11)
94.0(5)
90.1(2)
175.8(3)
172.4(6)
---
cis-BTF
86.33(2)
91.38(10)
90.82(7)
91.42(7)
170.8(2)
178.1(2)
---
P1-Pt-C1
P’-Pt-C1’
Pt-C1-C2
Pt-C27-C28
2.013(3)
1.203(3)
1.439(4)
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C6-C9
1.434(7)
1.184(8)
1.449(8)
---
---
---
---
C1-C2-C3
C27-C28-C29
C2-C3-C4
C6-C9-C10
C9-C10-C11
176.4(6)
---
121.2(5)
177.2(6)
175.8(8)
170.0(3)
176.9(3)
---
1
2
3
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9
1
1
1
1
1
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1
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1
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5
5
5
5
5
5
5
5
6
C9-C10
C10-C11
C6-C15
C32-C41
C12-C16
C38-C42
1.471(4)
1.461(3)
1.471(4)
1.468(4)
---
---
---
---
---
0
1
2
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9
0
Absorption and Emission Spectroscopy. The photophysical characterization (photoluminescence
and two-photon absorption and nonlinear absorption) of trans-PE2, trans-DPAF and trans-BTF has
previously been reported.1,4,8,11 As such, the trans complexes were examined here to provide benchmarks
for comparison to the novel cis complexes to elucidate the impact of the stereochemical effects on one
and two-photon photophysical properties.
platinum acetylide series in THF solution are shown in Figure 4a-c and the wavelength maxima and
molar absorption coefficients are listed in Table 2. The S →S absorption maxima for the complexes in
Ground state absorption spectra for the cis and trans
0
1
the cis geometry are blue-shifted relative to the absorption maxima observed for the corresponding trans
complexes. In addition to the observed blue-shift, the cis platinum acetylides exhibit a more pronounced
vibrational structure relative to the trans complexes. This observation has also been noted in similar 4-
1
9
ethynylstilbene cis and trans platinum acetylide complexes. The pronounced structure implies that
electron-vibrational coupling is stronger in the cis complexes. The blue shift suggests a weaker interaction
between the chromophores in the cis complexes, suggesting that the singlet excited state is more localized
on a single ligand. In contrast, the red-shift in trans platinum acetylides, and absence of vibronic structure,
suggesting weaker electron-vibrational coupling, which implies a larger degree of delocalization in the S1
excited state due to the proximity and mixing of the Pt dπ and acetylide π* orbitals.
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1
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7
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9
a
b
c
trans-PE2
cis-PE2
d
6
3
1
1
1
1
1
1
1
1
1
1
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9
0
0
15
e
trans-DPAF
cis-DPAF
12
9
6
3
0
1
1
5
2
9
6
3
0
f
trans-BTF
cis-BTF
300
350
400
400
500
600
700
Wavelength (nm)
Figure 4. Ground state absorption (on the left panel, in THF) and emission spectra (on the right panel,
in deoxygenated THF at rt) of cis complexes
The trans-complexes exhibit larger extinction coefficients than do the cis complexes for both the
DPAF and BTF series. Additionally, the DPAF complexes both exhibit similar, but larger, absorptivity
than do the BTF congeners. In comparison to the DPAF and BTF complexes, the cis- and trans-PE2
complexes display similar, but lower molar absorptivity values. The red shifts of the absorption maxima
and increases in extinction coefficients in the trans complexes relative to the cis complexes indicate
increased -conjugation that may extend through the platinum acetylide to a greater extent.
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Emission spectra of the cis and trans platinum acetylide series are shown in Figure 4d-f. The
fluorescence and phosphorescence of the complexes can be distinguished by comparing the emission in
air-saturated and deoxygenated solutions – the phosphorescence is largely quenched in the presence of
oxygen. As such, the bands at shorter wavelength (400 – 500 nm) in Figure 4d-f are the fluorescence,
whereas the longer wavelength bands (500 – 700 nm) arise from phosphorescence. All six complexes
except cis-BTF feature relatively weak fluorescence with low quantum yields (Table 2). The most
1
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6
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9
1
1
1
1
1
1
1
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2
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9
0
efficient fluorescence was observed in cis-BTF ( ~ 0.04) and the weakest emission was observed in
F
PE2 complexes ( ~ 0.0005). The generally low fluorescence yields precluded us from reliably
F
measuring the 2PA spectra by femtosecond two-photon excitation (2PEF) and therefore the non-linear
transmission (NLT) technique was used (see below).47 The fluorescence lifetimes (τ ) for the complexes
F
were found to be very short, consistent with the low emission yields, suggesting that a rapid non-radiative
process (ISC) dominates decay of the singlet state.
Table 2. One-photon photophysical properties of the cis and trans platinum acetylide series in THF.
Absmaxa
nm)
353
330 (345)
382
ε
Flmaxb
(nm)
391
Fc
τF, (ps) d
Phmaxe
(nm)
527
Pc
Complex
-1
-1
(
(M cm )
83,381
trans-PE2
cis-PE2
0.0005
0.0003
0.0073
0.0052
0.0087
0.041
< 100 f
< 100 f
9.6 g
0.011
0.0011
0.33
84,015
159,892
131,628
154,574
128,024
425
401
407
437
415
522
534
536
566
563
trans-DPAF
cis-DPAF
trans-BTF
cis-BTF
376
4.8 g
0.0353
0.11
402
11 g
375 (396)
36 g
0.042
a
c
Ground state absorption maxima. b Fluorescence maximum, obtained by excitation at ground state absorption maximum.
2+
d
Quantum yields relative to Ru(bpy)
3
in air-saturated DI water,  = 0.0379. Fluorescence lifetimes measured at
e
maximum of fluorescence emission. Phosphorescence maximum, obtained by excitation at ground state absorption
f
g
maxima. Measured with FT-100 instrument, lifetime is shorter than instrument response ~ 100 ps. Measured with FT-300
instrument, minimum response is ~5 ps. Lifetime listed is dominant decay component of two- or three-component decay
function. See table S-1 for full listing of decay parameters.
The fluorescence maxima of the cis complexes (PE2 and DPAF) are red-shifted relative to the trans
complexes (Table 2) showing that the cis complexes’ emissive states are slightly lower in energy than
their trans analogues. However, no significant red shift is observed for cis/trans BTF. The
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phosphorescence spectra of each cis and trans pair are almost identical, exhibiting only a small red shift
1
2
3
4
5
6
7
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
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5
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5
5
5
5
5
5
5
6
3
from the cis to the trans congeners. This finding suggests that the luminescent ,* state is the same
within each set, which implies that the ligand-based chromophore, not the coordination geometry, is
3
influencing the transition. The phosphorescence emission appears to originate from a ,* state which
is efficiently populated via spin-orbit coupling between the excited chromophore and the heavy metal
platinum atom. Moreover, it is likely that the triplet exciton is localized on a single π-conjugated ligand
and not delocalized across the metal center. This observation is consistent with similar -conjugated
platinum acetylide systems that also show a localized triplet excited state.8,52-56 The photoluminescence
spectra of the cis complexes also show no evidence for MLCT emission involving the ancillary ligand,
which would appear as broad, structureless emission bands.19 Note that the trans complexes show
generally higher phosphorescence quantum yields compared to their cis cogeners (Table 2).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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9
0
1
2
3
4
5
6
7
8
9
0
The phosphorescence lifetimes and triplet decay rates are summarized in Table 3. Assuming that the
triplet excited state is populated only through excitation of the ground state to form the singlet excited
state, followed by ISC, then the rate of radiative decay of the triplet, k , can be determined by equation 1,
p
k =  / (
τ )
ISC * p
(1)
p
p
where  and  are the quantum yields of phosphorescence and intersystem crossing, respectively, and
p
ISC
τ , the phosphorescence decay lifetime, is defined by τ = 1 / (k + k ), where k is the rate of all
P
p
p
nr
nr
nonradiative processes from the triplet excited state. Assuming that ISC ~ 1,[While it is clear that this is
an assumption, given the short singlet lifetimes and low fluorescence yields the assumption likely only
introduces at most a 10% error in the estimated kp and knr values.] we compute the k and k values in
p
nr
Table 3. The most notable aspect of these values is the difference in the radiative decay rate (k ) for the
p
corresponding cis and trans isomers. In particular, the radiative rates of the trans isomers are 3- to 5-
times higher in each pair. This difference is most likely due to a slightly more effective spin-orbit coupling
in the trans configuration compared to the cis which consequently leads to enhanced radiative decay for
the former.
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 3. Triplet excited state properties of cis and trans platinum acetylide series in THF.
τT / s a
τp / s b
2
-1 c
4
-1 d
Complex
λmax / nm
k / 10 s
k / 10 s
nr
p
trans-PE2
577
15.4
48.9
2.2
0.31
54
2.0
2.8
1.6
2.7
cis-PE2
556
612
597
17.8
8.9
35.9
61.3
36.7
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
trans-DPAF
cis-DPAF
3.3
9.6
trans-BTF
cis-BTF
507, 660
13.3
75.6
80.5
14
1.3
633
6.5
5.2
1.2
a
Triplet lifetime from transient absorption decay. Triplet lifetimes by transient absorption are shorter due to triplet-triplet
b
c
d
annihilation. Phosphorescence lifetimes. Radiative decay rate. Nonradiative decay rate. Rates calculated by using the
corresponding phosphorescence lifetime (τ ).
p
Transient Absorption Spectroscopy. Nanosecond transient absorption spectra of the six platinum
acetylides (Figure 5) were measured in deoxygenated THF to provide additional information about the
triplet excited states. Pulsed photoexcitation of the platinum acetylide complexes with the third harmonic
output (355 nm) of a Q-switched Nd:YAG laser affords transient absorption due to the triplet excited state
that are strong and broad throughout most of the visible region. An additional observation in the triplet-
triplet (TT) spectra is a blue-shift of the cis-complexes relative to their trans-analogs. This is in contrast
with the previously investigated stilbene-containing platinum acetylides, in which virtually identical
transient absorption spectra were observed for trans-Pt(PBu ) (4-ES) , cis-Pt(dppe)(4-ES) , and cis-Pt(t-
3
2
2
2
Bu-bpy)(4-ES) (where 4-ethynylstilbene is abbreviated 4-ES and 4,4’-di-tert-butyl-2,2’-bipryidine is
2
1
9
abbreviated t-Bu-bpy). However, the stilbene complexes also exhibited short-lived triplet excited state
lifetimes (~100 ns) and triplet-triplet (TT) absorption that extended from 400 to 600 nm and bore strong
resemblance to that of trans-stilbene. In contrast, the cis and trans complexes reported herein are unable
to undergo such nonradiative decay as a result of twisting; as such, the triplet excited state lifetimes of the
examined complexes are much longer than the stilbene derivatives. Sun et al. reported the
photophysical characterization of a cis-diimine platinum acetylide that contains two BTF
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chromophores.34-35 Similar to the observed cis and trans complexes of this study, the cis-diimine
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1
1
1
1
1
1
1
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1
2
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2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
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3
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4
4
4
4
4
4
4
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5
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5
5
6
platinum complex also exhibited a TT absorption spectrum that was blue-shifted approximately
50 nm from the TT absorption maximum of the trans-BTF analog and a triplet lifetime of 10.8 s.
34-35
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
0.4
0.2
0.0
a
trans-PE2
cis-PE2
-
-
-
0.2
0.4
b
0.2
0.0
0.2
trans-DPAF
cis-DPAF
0.4
c
0.2
0.0
0.2
trans-BTF
cis-BTF
400
500
600
700
800
Wavelength (nm)
Figure 5. T -T absorption spectra in deoxygenated THF after nanosecond-pulsed 355 nm excitation
1
n
of cis- and trans- Pt acetylides with PE2 (a), DPAF (b), and BTF (c) chromophores.
The superimposable nature of the phosphorescence bands within each cis and trans pair suggests that
the emissive triplet excited state is the same within each set. This trend has been observed in similar
platinum acetylide systems and is proposed to be the result of the triplet emissive state being localized on
a single chromophore.8,52-56 However, the observed shifts in the TT absorption between the cis and trans
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pairs suggest that the T state is more delocalized in the trans isomers, or that there is a difference in
n
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conformation for the two isomers in the triplet state that is influencing the TT absorption.
0
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9
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Figure 6. Femtosecond transient absorption spectra and decays of cis-DPAF (a) and trans-DPAF (b)
in THF after a 380 nm excitation pulse. Kinetics traces for cis-DPAF (c) and trans-DPAF (d) at 530 nm
(black) and 600 nm (red). The lifetimes shown in the insets were obtained by global analysis of the time
resolved transient absorption spectral data.
Femtosecond transient absorption spectra were measured for the DPAF and BTF complexes (Figures
6
and S-1) to examine the effect of stereochemistry on the early time excited state dynamics. The spectra
of both complexes exhibit complex changes in bandshape and amplitude occurring over the timescale of
00 fs – 7 ns. As a starting point, it is quite evident that the spectra for both complexes at the longest
1
delay time is the same as the spectra observed on the ns-s timescale (Fig. 5b) and is attributed to the
triplet state. However, at earlier times, there are substantial changes that are difficult to assign to specific
processes. First, for both complexes, at 500 fs after excitation there is a strong and relatively narrow band
with peak ~ 525 nm which decays within 20 ps. Global analysis of the transient absorption dynamics
reveals that for both complexes this relaxation has a lifetime of ~ 4 ps, with a slightly faster decay in cis
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The Journal of Physical Chemistry
compared to trans-DPAF. This lifetime of this process corresponds approximately to the fluorescence
lifetime (Table 2), which suggests that the relaxation process corresponds to ISC, suggesting that kISC in
1
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5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
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6
1
1 -1
the DPAF complexes is ~10 s . After the initial rapid spectral dynamics attributed to ISC, the changes
in the spectra are more subtle, being characterized by a blue-shift and decrease in amplitude of the
remaining excited state absorption band centered at ~600 nm. These slower dynamics take place on the
0
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0 – 500 ps timescale and are likely due to structural relaxation in the triplet excited state. Similar
dynamics are observed for the cis and trans-BTF complexes (Figure S-1); however, in this complex the
large amplitude dynamics associated with ISC occur in the 10 - 30 ps timescale. Note that this is consistent
with the observation that the fluorescence lifetimes of the BTF complexes are longer for these complexes.
Non-Linear Absorption Spectroscopy. Femtosecond two-photon absorption measurements
were performed for all six complexes with the goal of determining their two-photon absorption
spectra and cross sections ( ). In this study the non-linear transmittance (NLT) method was
2
47
used, as it is more accurate than the 2-photon excited fluorescence (2PEF) method due to the
0
relatively weak fluorescence for the complexes. However, at the shorter wavelength (S →Sn
57
region) it was recently reported that NLT measurement gave the peak 2PA cross section five
times as large as those obtained from the 2PEF measurements as a result of overlapping with
the singlet excited state absorption which occurs on the same ultrafast time scale.
Chromophores that are centrosymmetric (quadrupolar) typically exhibit 2PA at higher energy
(
shorter wavelength) compared to the 1 photon absorption. This is because the “dipolar” term
in the 2PA cross-section is zero for centrosymmetric chromophores, and therefore the S →S
0
1
transition is forbidden for 2PA (see ref. 58 for a cogent discussion of the theory). As a
consequence, the intensity in 2PA for centrosymmetric chromophores is in S →S and higher
0
2
transitions. By contrast, in chromophores that lack centrosymmetry (dipolar) the 2PA is
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Page 20 of 36
dominated by the dipolar term,58 and the 2PA spectrum is degenerate with the 1 photon
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absorption. In this case both the 2PA and 1 photon S →S transitions are allowed. Previous
0
1
studies have shown that trans-platinum complexes in 2PA spectra show typical quadrupolar
behavior59-60 with a forbidden 0-0 transition, more pronounced vibronic components at higher
energy than the S →S band and allowed transitions to higher-energy states (S →S ) at shorter
0
1
2
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0
n
wavelengths ( < 650 nm). By contrast, we anticipated in this work that the corresponding cis-
platinum complexes would exhibit typical dipolar behavior, with the 2PA spectra occurring at
the same energy as the 1 photon absorption spectra.
As expected, trans-PE2 and trans-DPAF do not exhibit significant two-photon absorption in
the region corresponding to the 0-0 transition but instead show peaks that are not completely
resolved at ~ 640-650 nm which overlap with the strong singlet excited state absorption at ~
5
80 - 600 nm with  as large as 1000 GM (Figure 7). However, due to the stronger electron
2
withdrawing effect of the benzothiazole ring, trans-BTF shows dipolar behavior in the two-
photon spectrum with significant absorption in the 0-0 transition (up to ~ 1000 GM) and ~2000
GM at shorter wavelengths with contribution from the singlet ESA. For the cis-complexes, the
lack of symmetry (Ci) results in a permanent dipole moment which increases with the
introduction of electron withdrawing or electron donating functionality. This typical dipolar
behavior of the ligand rings decreases according to BTF > DPAF >> PE2. As a result, pronounced
peaks in the 0-0 transition, which coincide with the one-photon absorption peaks observed for
cis-BTF and cis-DPAF complexes with lower intensities for cis-PE2 while the corresponding trans
isomers do not exhibit any significant peaks in this region. Large  in the 0-0 and S →S region
2
0
n
were obtained for cis-BTF (1640 GM at 755 nm and 3680 GM at 575 nm), lower values for cis-
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DPAF (270 GM at 755 nm and 1020 GM at 580 nm) and less for cis-PE2 (70 GM at 715 nm and
40 GM at 570 nm).
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1
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1PA Wavelength (nm)
1
PA Wavelength (nm)
300
350
400
2
000
2.0
1.5
1.0
0.5
0.0
3
00
350
400
2000
2.0
1.5
1.0
0.5
0.0
trans-PE2
cis-PE2
1500
1500
1000
1000
0
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0
5
00
500
0
0
600
700
800
600
700
800
2PA Wavelength (nm)
2PA Wavelength (nm)
1PA Wavelength (nm)
350 400
1
PA Wavelength (nm)
300
2
000
500
2.0
3
00
350 400
2000
2.0
1.5
1.0
0.5
0.0
trans-DPAF
1
1.5
1.0
0.5
0.0
cis-DPAF
1
500
000
1000
1
5
00
500
0
0
600
700
800
2PA Wavelength (nm)
6
00
700
800
2
PA Wavelength (nm)
1PA Wavelength (nm)
1PA Wavelength (nm)
300
350
400
300
350
400
4
000
000
000
2.0
1.5
1.0
0.5
0.0
4000
2.0
1.5
1.0
0.5
0.0
cis-BTF
3
2
1
000
000
000
0
trans-BTF
3
2
1000
0
600
700
800
600
700
800
2PA Wavelength (nm)
2PA Wavelength (nm)
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Figure 7. 2PA spectra and 2PA cross sections (left vertical axis) of cis (on the left column) and
1
2
3
trans (on the right column) complexes in THF; Lower horizontal axes: 2PA laser wavelength;
Upper horizontal: 1PA transition wavelength. 1PA normalized absorption spectra in THF (solid
line) are shown in comparison.
4
5
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9
1
1
1
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Nonlinear absorption (NLA) of the cis and trans complexes were conducted via an open-aperture
nanosecond z-scan apparatus in 1 mM solutions in THF using the 5 ns/600 nm pulses from an OPO
rd
pumped by the 3 harmonic output of Q-switched Nd:YAG laser, Figure 8. (Cis-DPAF was not readily
soluble in THF at the concentrations needed for the ns z-scan experiments; as such, the measurements of
this sample were conducted in a 1:24 DCM/THF solution mixture.) The 600 nm excitation wavelength
was selected due to the lack of appreciable ground state absorption at this wavelength for all complexes.
Therefore, the excitation mechanism is presumed to occur via 2PA. The NLA response of the platinum
acetylide complexes were compared to T2, a previously studied diplatinum acetylide complex with a
dithiophene core and end-capped with DPAF chromophores that exhibits strong NLA response at 600 nm
excitation (structure in Figure S-2).61 Figure 8a shows representative ns z-scan results for the seven
complexes. Figure 8b shows the average percent absorptance (absorptance = 1 - transmittance) at the
laser focus, z = 0 mm, for the seven complexes as the average of multiple z-scan trials. Trans-DPAF
exhibits the strongest overall NLA response, with an average absorptance of 0.29, whereas cis-DPAF
exhibits weaker absorptance of 0.11. The cis- and trans-BTF pair exhibit similar responses of 0.20 and
0
.21, respectively. Both PE2 complexes exhibit NLA response, but markedly less than the other
complexes. These results suggest that chromophore selection (e.g, DPAF vs. BTF) is more critical to the
NLA response strength than is the geometry (e.g., cis vs. trans) at the platinum center. However, it is
clear trans-DPAF exhibits noticeably stronger attenuation than its cis counterpart.
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1
0
.00
.95
0.30
A
B
0.25
0.90
0.20
0.15
0.10
0.05
0.00
trans-PE2
cis-PE2
trans-DPAF
cis-DPAF
trans-BTF
cis-BTF
T2
0
0
0
0
.85
.80
.75
.70
0
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-
10
-5
0
5
10
Z Position (mm)
Figure 8. (a) NLA nanossecond open-aperture z-scan response (1 mM in THF; for cis-DPAF in 1:24
DCM/THF mixture); samples were excited with ns pulses at 600 nm and 750 J input energy. (b) The
average nonlinear absorptance (1-transmittance) relative to DPAF-based di-platinum acetylide, T2.
General Discussion. As noted in the introduction, many previous studies have explored the
photophysics and non-linear absorption of platinum acetylide complexes.2-4,8,47,60,62-63 It is reasonably
well-established that complexes that feature acetylide ligands with -donor or -acceptor structures
display pronounced NLA for nanosecond laser pulses via a mechanism that combines 2PA from the
4
,8,47
singlet ground state (S →S ) with excited state absorption (ESA) due to the long lived triplet state.
0
n
The objective of the current investigation was to explore whether the stereochemistry at the Pt(II) center
elicits any effects on the photophysics of the complexes, and on their 2-photon absorption and long pulse
NLA response. Taken together, the results presented herein reveal that the stereochemistry has relatively
minor effects on the complexes’ photophysics. This is not be surprising, given that the triplet excited
states, which dominate the photophysics, are mainly localized on a single acetylide ligand.14,52,56
Nevertheless, careful consideration of the data presented here suggests that there are subtle, yet potentially
significant, differences in the photophysics of the trans- and cis- complexes. First, as noted above (Table
2
), the phosphorescence yields for the trans isomers are moderately enhanced relative to the
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corresponding cis forms. This effect is due to enhanced radiative decay rates (Table 3), which points to
the possibility that the spin-orbit coupling induced by the Pt(II) metal center is enhanced in the trans
isomers. Some insight into the origin of enhanced comes from the comparison of the acetylide -CC-
bond lengths in corresponding cis- and trans- isomers. The crystal structure comparison of cis- and trans-
PE2 shows longer platinum -CC- bond lengths in the trans complexes, suggesting a higher degree of
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
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3
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0
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d-* backbonding. However, this trend does not extend to the BTF complexes, so a definitive
correlation between the length of the acetylide CC bond, backbonding, and spin-orbital coupling remains
elusive.
Second, the femtosecond two-photon absorption spectra revealed that trans complexes show a typical
quadrupolar behavior with no significant peaks in 0-0 transition of S →S band whereas the cis
0
1
complexes in the lack of C symmetry exhibit more likely dipolar behavior with significant 2PA peak
i
cross-section in 0-0 transition region (up to 1670 GM by the peak cross section for cis-BTF). At the shorter
wavelengths (< 650 nm) both cis and trans isomers show large two-photon absorption (up to 3700 GM
for cis-BTF) to the higher energy state (S →S ) with presumably significant contribution of ESA from
0
n
the singlet manifold (S →S ).
1
n
The triplet excited states were further probed via triplet-triplet transient absorption. The six complexes
exhibit relatively strong and broad transient absorption across most of the visible region. The transient
absorption maxima appear to be dependent on chromophore selection, whereas the delta absorption
strength observed is more dependent on the stereochemistry of the platinum center.
The nanosecond NLA responses of the complexes were measured via a open-aperture z-scan using
6
00 nm/5 ns pulses benchmarked relative to the previously studied complex T2. The only clear correlation
to emerge from the z-scan is that the DPAF and BTF complexes give rise to much greater attenuation
compared to the PE2 complexes. This is due to the fact that the former have significantly greater  in
2
the 600 nm region. However, disappointingly there is not a clear correlation with respect to complex
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stereochemistry in the long pulse attenuation results. While trans-DPAF is clearly much more effective
compared to cis-DPAF, a analogous difference is not seen for the cis/trans-BTF pair.
1
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1
1
1
1
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6
Summary and Conclusions
An investigation was carried out to compare the photophysics and non-linear absorption behavior of
a series of Pt-acetylide complexes that consist of cis- and trans- isomer pairs. The results find that, in
general, the photophysical properties are not strongly influenced by stereochemistry. This may not be
surprising given that the long-lived excited states are mainly localized on a single -conjugated ligand.
However, there are substantial differences in the 2-photon absorption spectra that are related to the effect
of complex stereochemistry on the local symmetry of the -conjugated chromophores. The net effect is
that the trans isomers, which are centrosymmetric, display weak 2PA in the spectral region degenerate
with the 1PA absorption. By contrast, the cis isomers, which lack centrosymmetry, feature relatively
strong 2PA in the degenerate spectral region.
0
1
2
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Despite the lack of significant differences in photophysics, overall this family of Pt-acetylide
chromophores exhibits pronounced non-linear absorption in the red and near-infrared spectral regions
arising from the combination of instantaneous 2PA combined with excited state absorption due to the
long-lived triplet excited state. The properties of these complexes are favorable for application of the
chromophores in high performance frequency and temporally agile non-linear absorption materials.
Supporting Information. Complete synthesis procedures, analytical data, NMR data, and X-ray
crystallographic files of cis-PE2 and cis-BTF. This material is available free of charge via the Internet at
http://pubs.acs.org.
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Acknowledgement. This research was supported by the Air Force Office of Scientific Research (Grant
Nos. FA-9550-06-1-1084 and FA9550-09-1-0219). Support from the Welch Foundation (Award No.
AX-0045-20110629).
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References
1. McKay, T. J.; Bolger, J. A.; Staromlynska, J.; Davy, J. R. Linear and Nonlinear Optical Properties
of Platinum-Ethynyl. J. Chem. Phys. 1998, 108, 5537-5541.
2.
3.
4.
Silverman, E. E.; Cardolaccia, T.; Zhao, X.; Kim, K.-Y.; Haskins-Glusac, K.; Schanze, K. S. The
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