Photophysics and non-linear absorption of Au(i) and Pt(ii) acetylide complexes of a thienyl-carbazole chromophore

Article abstract of DOI:10.1039/c4dt02123a

In order to understand the photophysics and non-linear optical properties of carbazole containing π-conjugated oligomers of the type ET-Cbz-TE (E = ethynylene, T = 2,5-thienylene, Cbz = 3,6-carbazole), a detailed investigation was carried out on a series of oligomers that feature Au(i) or Pt(ii) acetylide end groups , as well as a Pt(ii)-acetylide linked polymer (CBZ-Au-1 and CBZ-Pt-1, CBZ-Poly-Pt). These organometallic chromophores were characterized by UV-visible absorption and variable temperature photoluminescence spectroscopy, nanosecond transient absorption spectroscopy, open aperture nanosecond z-scan and two photon absorption (2PA) spectroscopy. The Au(i) and Pt(ii) oligomers and polymer exhibit weak fluorescence in fluid solution at room temperature. Efficient phosphorescence is observed from the Pt(ii) systems below 150 K in a solvent glass; however, the Au(i) oligomer exhibits only weak phosphorescence at 77 K. Taken together, the emission results indicate that the intersystem crossing efficiency for the Pt(ii) chromophores is greater than for the Au(i) oligomer. Nonetheless, nanosecond transient absorption indicates that direct excitation affords moderately long-lived triplet states for all of the chromophores. Open aperture z-scan measurement shows effective optical attenuation can be achieved by using these materials. The 2PA cross section in the degenerate S₀→S₁ transition region was in the range 10-100 GM, and increased monotonically toward shorter wavelengths, reaching 800-1000 GM at 550 nm.

Full text of DOI:10.1039/c4dt02123a

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Photophysics and non-linear absorption of
Au(I) and Pt(II) acetylide complexes of a
thienyl-carbazole chromophore†
Cite this: Dalton Trans., 2014, 43,
17721
a
b
b,c
a
Subhadip Goswami, Geoffrey Wicks, Aleksander Rebane* and Kirk S. Schanze*
In order to understand the photophysics and non-linear optical properties of carbazole containing π-con-
jugated oligomers of the type ET-Cbz-TE (E = ethynylene, T = 2,5-thienylene, Cbz = 3,6-carbazole), a
detailed investigation was carried out on a series of oligomers that feature Au(I) or Pt(II) acetylide “end
groups”, as well as a Pt(II)-acetylide linked polymer (CBZ-Au-1 and CBZ-Pt-1, CBZ-Poly-Pt). These organo-
metallic chromophores were characterized by UV-visible absorption and variable temperature photo-
luminescence spectroscopy, nanosecond transient absorption spectroscopy, open aperture nanosecond
z-scan and two photon absorption (2PA) spectroscopy. The Au(I) and Pt(II) oligomers and polymer exhibit
weak fluorescence in fluid solution at room temperature. Efficient phosphorescence is observed from the
Pt(II) systems below 150 K in a solvent glass; however, the Au(I) oligomer exhibits only weak phosphor-
escence at 77 K. Taken together, the emission results indicate that the intersystem crossing efficiency for
the Pt(II) chromophores is greater than for the Au(I) oligomer. Nonetheless, nanosecond transient absorp-
tion indicates that direct excitation affords moderately long-lived triplet states for all of the chromophores.
Open aperture z-scan measurement shows effective optical attenuation can be achieved by using these
Received 12th July 2014,
Accepted 31st August 2014
DOI: 10.1039/c4dt02123a
0 1
materials. The 2PA cross section in the degenerate S →S transition region was in the range 10–100 GM,
www.rsc.org/dalton
and increased monotonically toward shorter wavelengths, reaching 800–1000 GM at 550 nm.
particular with respect to applications in frequency and tem-
poral agile non-linear absorption. Among various organo-
Introduction
1
2
Since the discovery of two-photon absorption (2PA) by metallic complexes, platinum acetylide containing π-conjugated
1
Göppert-Mayer in the 1930s and the first experimental obser- chromophores are attractive for NLA materials because of their
2
13–18
vation of the effect in 1961, there has been significant “dual mode” non-linear absorption mechanism.
This dual
research in this field, with investigations seeking to under- mode comprises of ultrafast 2PA to produce the singlet excited
stand the factors that control the 2PA cross-section (σ ) and to state followed by intersystem crossing (ISC) to a long-lifetime
2
harness the effect in diverse applications. Two-photon absorp- triplet state T promoted by the heavy atom effect, which can
1
1 n
tion has a number of applications ranging from ultrafast further absorb photons due to a strongly allowed T –T
3
–6
18,19
optical power limiting via non-linear absorption (NLA),
absorption.
7
8
20–23
24–26
photodynamic therapy, microscopy, and three-dimensional
Porphyrin
and phthalocyanine
metal complexes
9
–11
data storage.
Although a variety of organic dyes have proven have been widely investigated with respect to 2PA and long
to be efficient 2PA chromophores, heavy-metal containing pulse NLA via the dual mode mechanism; however, metal com-
organometallic chromophores are especially interesting, in plexes linked to π-conjugated segments that feature the carba-
2
7
zole moiety have not been well explored. Even though the
singlet excited state photophysics for carbazole containing
oligomers and polymers are well established, the triplet excited
state is relatively unexplored. Introduction of a heavy metal
such as platinum or gold into the conjugated backbone of carb-
azole containing oligomers provides insight into the triplet
excited state due to the “heavy atom effect” that is elusive in
a
Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, FL
2611, USA. E-mail: kschanze@chem.ufl.edu; Tel: +1-352-392-9133
3
b
Physics Department, Montana State University, Bozeman, Montana 59717, USA.
E-mail: rebane@physics.montana.edu; Tel: +1-406-994-7831
c
National Institute of Chemical Physics and Biophysics, Tallinn, EE 12618, Estonia
†
Electronic supplementary information (ESI) available: Detailed description of
the synthesis and characterization of CBZ-Au-1, CBZ-Pt-1 and CBZ-Poly-Pt.
Experimental set up for non-linear transmission (NLT) and transmission vs. inci-
dent laser pulse energy plot. See DOI: 10.1039/c4dt02123a
2
8–31
the strictly organic system.
Most carbazole derivatives find
application in OLEDs or bulk heterojunction solar cells as π
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 17721–17728 | 17721

View Article Online
Paper
Dalton Transactions
Chart 1
electron donor materials; however, there are few reports on
organometallic carbazole complexes that exhibit non-linear
absorption. Zhou et al. reported a carbazole platinum acety-
Experimental section
Synthesis
2
7
lide polymer and a model platinum complex substituting 3,6 The details of the synthetic procedures along with the syn-
position of the aromatic ring, as an efficient optical power lim- thetic scheme and all the characterizations are provided in the
3
2
iting material at 532 nm. Tao et al. reported a naphthalene ESI.†
platinum acetylide substituted carbazole as a two photon
Instrumentation and methods
All the NMR spectra were recorded using a Varian VXR-300
absorbing and two photon induced luminescence molecule
3
3
2
with a 2PA cross section (σ ) of 14 GM (λex = 740 nm).
1
31
In order to gain fundamental insight into the photophysics FT-NMR, operating at 300 MHz for H NMR, 121 MHz for
and non-linear optical properties of carbazole containing NMR and at 75.4 MHz for C NMR. Mass spectrometry for the
P
1
3
π-conjugated oligomers of the general structure ET-Cbz-TE (T = newly synthesized compounds were recorded on a Bruker
2
,5-thienylene, Cbz = 3,6-carbazole and E = ethynyl), here we APEX II 4.7 T Fourier Transform Ion Cyclotron Resonance
report a detailed investigation of a series of complexes that mass spectrometer (Bruker Daltonics, Billerica, MA). Gel per-
feature Au(I) or Pt(II) acetylide “end groups”, as well as a Pt(II)- meation chromatography (GPC) analysis of the polymer was
acetylide linked polymer (CBZ-Au-1 and CBZ-Pt-1, CBZ-Poly-Pt performed on a system containing a Shimadzu SPD-20A photo-
−
1
in Chart 1). We were particularly interested in developing NLA diode array (PDA) detector, using THF as eluent at 1 ml min
chromophores that have high transparency throughout the flow rate and the system was calibrated with respect to linear
visible region, and as such the 3,6-substitution pattern on the polystyrene standards in THF.
Cbz was used which gives rise to a relatively large HOMO–
Photophysical measurements were conducted with dry
3
4
LUMO gap and consequently good optical transparency HPLC grade THF as solvent in 1 × 1 cm quartz cuvettes unless
through most of the visible region. The photophysical pro- otherwise noted. UV-visible absorption spectra were obtained
perties of the series of complexes and polymer are fully ana- on a Varian Cary 100 dual beam spectrophotometer. Room
lyzed, including one and two-photon absorption spectra, temperature and low temperature emission spectra were
photoluminescence spectra, triplet–triplet absorption spectra recorded on a Photon Technology International (PTI) fluori-
and singlet oxygen sensitization studies. The results clearly meter and collected 90° relative to the excitation beam. The
suggest that this set of chromophores has the potential to be optical density of the sample solutions was kept at ≤0.1 at exci-
used in optical power limiting materials, and the results shed tation wavelength. Refractive index corrections were applied
light on how the yield of the triplet state depends on the for sample and standard solutions for emission quantum yield
nature of the heavy metal center. Previous studies have exam- measurements. Freshly distilled HPLC grade 2-methyl tetra-
ined in some detail the photophysics of π-conjugated oligo- hydrofuran was used for the low temperature experiments. The
mers linked to heavy metals, and this work builds on that base sample was placed in 1 cm diameter borosilicate glass tube
3
5–37
of understanding.
and deoxygenated by 4–5 freeze–pump–thaw cycles under
17722 | Dalton Trans., 2014, 43, 17721–17728
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
vacuum (10− Torr) prior to use. Temperatures to 77 K were spectroscopic cuvette. Two identical 10 mm diameter silicon
achieved by using liquid-nitrogen-cooled Oxford instrument photodetectors (Thorlabs, DET100A) were used to measure the
DN 1704 optical cryostat attached to an Omega CYC3200 auto- laser pulse reflected from two glass beam pick-off plates posi-
tuning temperature controller. The reported concentration for tioned accordingly, before and after the sample. The analog
polymer refers to the concentration of polymer Repeat Unit signals from the photodetectors were digitized using a DAQ
5
(PRU). Solutions for spectroscopic studies were prepared by board (National Instruments, Model PCI-6110). The depen-
dilution of stock solutions. Fluorescence lifetimes were col- dence of the sample transmission (defined as ratio of the
lected via a FluoTime 100 Fluorescence Lifetime Spectrometer signal from the second detector divided by the signal from the
(PicoQuant, USA).
first detector) on the incident intensity was evaluated by
Nanosecond timescale transient absorption spectroscopy varying the energy of the incident pulse in the range Pin
≈
was conducted on a home-built apparatus using the third har- 0.3–30 μJ using a neutral density filter attenuator wheel (Thor-
monic of a Continuum Surelite series Nd:YAG laser (λ = labs) mounted on a stepper motor (Fig. S13 in the ESI† shows
3
55 nm, 10 ns FWHM, 7 mJ per pulse). Probe light was pro- typical measured dependence of the transmission on the inci-
duced by a xenon flash lamp and the transient absorption dent pulse energy at different wavelengths). The estimated
signal was detected with a gated-intensified CCD mounted on accuracy of the measured transmission is about 0.1%. A laser
a 0.18 m spectrograph (Princeton PiMax/Acton Pro 180). power meter (Nova II, Ophir) was used to measure the average
Samples for transient absorption were contained in a 1 cm laser power at the input to the sample. The beam diameter at
path length flow cell with a total volume of 10 ml and continu- the sample was 0.8–1.5 mm and varied depending on the OPA.
ously circulated at the pump-probe region during the exper- For calibration of the 2PA cross section a known reference PPV
1
8
iment. The optical density of the solutions was kept ≈0.7 at in THF was measured under identical conditions.
3
55 nm and samples were degassed with argon for at least
4
5 minutes before the measurement. Singlet oxygen quantum
yields were measured using a Photon Technology International
Quantamaster near-IR spectrophotometer containing an
InGaAs photodiode detector, an optical chopper and a lock-in
amplifier. The optical density was kept below 0.12 for all the
samples at excitation wavelength 350 nm.
Results and discussion
Ground state absorption spectra
The absorption spectra of the TMS-protected oligomer
The open aperture nanosecond time domain z-scan CBZ-TMS, metal complexes CBZ-Au-1, CBZ-Pt-1 and polymer
measurements were performed on a system which employs the CBZ-Poly-Pt were measured in THF solution and are shown in
third harmonic output of a Continuum Surelite II Nd:YAG Fig. 1a, and the molar extinction coefficients (ε) and absorp-
laser equipped with a Continuum Surelite OPO PLUS for exci- tion band maxima (λmax) are listed in Table 1. In general, the
tation at 600 and 628 nm. All of the chromophores were metal complexes, polymer and the free oligomer show strongly
excited with a 600 nm laser beam through 50/50 beam splitter. allowed absorption in the 300–450 nm region. However, there
The beam was focused using 50.8 mm focus length, 38.1 mm are interesting and significant differences in the absorption
diameter plano-convex lens. Sample solutions (C = 1.0 mM) properties across the series. First, the absorption of the free
were taken in a 1 mm path length cuvette and moved along oligomer CBZ-TMS is the most blue shifted of the series, with
the focused beam using a manual one-directional translational the principal π,π* transition lying in the near-UV with an onset
stage placed exactly behind the focusing lens. The energy of ∼410 nm and λmax = 328 nm. Second, the absorption of the
the laser beams were obtained by using Ophir pyroelectric gold(I) congener, CBZ-Au-1, is quite similar to that of
heads and an Ophir Laserstar power/energy monitor and col- CBZ-TMS, but the onset and λmax are slightly red-shifted. This
lected using StarCom32 software.
fact can be attributed to the weak interaction between 5d orbi-
42
The laser system for the 2PA measurements comprised a tals of the Au(I) centers with the ligand π system. Interest-
Ti:Sapphire femtosecond oscillator (Lighthouse Inc.), femto- ingly, the absorption of the Pt(II) complex and polymer are
second regenerative amplifier (Legend F-HE, Coherent Inc.), significantly red-shifted and show enhanced molar absorptiv-
and an optical parametric amplifier, OPA (TOPAS-C, Light Con- ity compared to the free oligomer and Au(I) complex. The
version). The second harmonic of the signal output of the OPA absorption onsets are red-shifted by ∼25 nm, and the
was continuously tunable from 540 to 810 nm with the enhanced oscillator strength for the Pt(II) systems is clearly
maximum pulse energy in the range 10–30 μJ, and second har- due to enhanced π-conjugation due to mixing of the ligand
4
3
monic of the idler was tunable from 800 to 1070 nm with the π-levels with the dπ levels of the Pt(II) centers, (The enhanced
maximum pulse energy 10–30 μJ. The average pulse duration molar absorptivity of CBZ-Pt-1 compared to CBZ-Poly-Pt is in
was 80–120 fs (FWHM). A detailed description of the laser part artifact arising because the polymer repeat unit used in
3
8
system is provided elsewhere. The NLT method has been the calculation of the absorptivity contains only a single Pt(II)
1
8,39–41
described previously.
atus is provided in Fig. S12†). Briefly, the OPA output beam at Taken together, these results suggest that there is substantial
00 Hz pulse repetition rate was directed through the sample metal–ligand electronic interaction in the Pt(II) systems;
(The schematic of the NLT appar- acetylide unit, whereas oligomer CBZ-Pt-1 contains two).
1
−
3
prepared at concentration, C ∼ 10 M, in a standard 10 mm however, the interaction is at best weak in the Au(I) complex.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 17721–17728 | 17723

View Article Online
Paper
Dalton Transactions
Fig. 1 (a) Absorption and (b) room temperature emission spectra of CBZ-TMS, CBZ-Au-1, CBZ-Pt-1 and CBZ-Poly-Pt in THF solution. The emission
spectra were obtained with excitation wavelength at 350 nm and they are intensity normalized at λmax. Note that the molar absorptivity for
CBZ-Poly-Pt is based on the repeat unit concentration, where the repeat unit contains a single Pt(II) center.
Table 1 Photophysical properties in THF solution
ε/M− cm−1
1
λ
F
/nm
Φ
F
a
λPhos/nm
b
τ
F
/ns
c
Φ
d
τTA/µs
Compound
λmax/nm
Δ
CBZ-TMS
CBZ-Au-1
CBZ-Pt-1
328
341
362
386
46 000
44 000
105 000
58 000
407
406
418
428
0.43
0.04
0.003
0.01
—
—
—
0.36
0.63
0.42
0.34
591
605
610
<0.2
<0.2
<0.2
0.21 ± 0.02
0.15 ± 0.02
0.16 ± 0.02
CBZ-Poly-Pt
a
Fluorescence quantum yield was measured with respect to quinine sulfate in 0.1 M sulfuric acid (Φ
Phosphorescence was measured in distilled 2-MeTHF solvent. Lifetime was shorter than lower limit of the picoQuant instrument, 200 ps. All
F
= 0.54), and the estimated error is ±5%.
b
c
d
samples are excited at 375 nm. The emission was detected for CBZ-Au-1 at 400 nm and for CBZ-Pt-1 and CBZ-Poly-Pt at 425 nm. Singlet oxygen
quantum yield was determined in C using terthiophene (Φ = 0.73) as actinometer.
D
6 6
Δ
Steady-state photoluminescence spectroscopy
CBZ-Au-1, which has a noticeably larger fluorescence yield.
This fact can be attributed to stronger spin–orbit coupling
4
3
Room temperature photoluminescence spectra of the oligomer effect for platinum(II) than gold(I). Interestingly, gold(I) has a
CBZ-TMS, metal complexes CBZ-Au-1, CBZ-Pt-1 and the slightly larger spin–orbit coupling constant than platinum(II).
45
polymer CBZ-Poly-Pt in deoxygenated THF are shown in We believe that the larger spin–orbit coupling in the platinum(II)
Fig. 1b. At ambient temperature, and with excitation at λ = systems arises because there is greater dπ-metal/π*-ligand
3
50 nm, vibronically structured emission bands were observed orbital mixing than in the gold(I) system.
that show comparatively low Stokes shift from the absorption.
The lifetimes of this emission from the metal complexes (τ
In order to understand the effect of the different heavy
atoms in promoting phosphorescence in the oligomers, vari-
f
,
Table 1) are very short (<200 ps), and on the basis of the small able temperature photoluminescence spectroscopy was per-
Stokes shift and short lifetimes the emission is attributed to formed on the series in deoxygenated 2-methyltetrahydrofuran
1
28
fluorescence from the π,π* state. The fluorescence spectra (2-MeTHF) solvent (glass). As shown in Fig. 2, for the Pt(II)
show similar trends across the series as noted for the UV- oligomer and polymer, below 130 K a structured emission
visible absorption spectra. In particular, the emission maxima band is clearly seen in the region from 590–800 nm that is
1
are red-shifted from free oligomer CBZ-TMS in the sequence assigned to phosphorescence from the T state. Note that at
CBZ-Au-1 < CBZ-Pt-1 < CBZ-Poly-Pt. Two well defined vibronic 80 K, the emission is dominated by the phosphorescence, and
peaks were observed in the fluorescence spectrum for all of the given that the room temperature fluorescence yields are ∼0.01,
systems, which is a signature of 3,6-thienyl substituted carba- we estimate that the 80 K phosphorescence yields for CBZ-Pt-1
4
4
zole system. The fluorescence quantum yields (Φ , Table 1) and CBZ-Poly-Pt are in the range 0.05–0.10.
f
for the metal complexes and polymer are more than an order
In contrast to the emission of the Pt(II) systems, even at
of magnitude less than for the free oligomer CBZ-TMS, which 80 K the gold(I) complex CBZ-Au-1 exhibits vibronically struc-
can be attributed to efficient S →T intersystem crossing in the tured fluorescence and only a weak phosphorescence emission
1
1
former. Close inspection of the fluorescence quantum yield (Fig. 3). The weak phosphorescence from CBZ-Au-1 can be
data suggests both the platinum complex CBZ-Pt-1 and the attributed to less efficient spin–orbit coupling for gold(I) in
4
6
polymer CBZ-Poly-Pt exhibit the most efficient intersystem comparison to platinum(II). In all of the phosphorescence
crossing to the triplet manifold, compared to gold(I) complex spectra, there are two distinct vibronic progressions observed,
17724 | Dalton Trans., 2014, 43, 17721–17728
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Fig. 2 Low temperature emission spectra of (a) platinum(II) oligomer CBZ-Pt-1 (temperatures are 80, 90, 100, 120, 135 and 150 K) and (b) platinum(II)
polymer CBZ-Poly-Pt in 2-MeTHF (temperatures are 90, 110, 130, 150 and 180 K). The excitation wavelength was 350 nm. The frequencies of the
vibronic progressions discussed in the text were determined by the differences shown in the figures.
Fig. 4 Nanosecond transient absorption spectra of the ligand
CBZ-TMS, metal complexes CBZ-Au-1, CBZ-Pt-1 and the polymer
CBZ-Poly-Pt in deoxygenated THF solution for 45 minutes. All samples
were excited at 355 nm with 300 ns of camera delay increment. All the
samples have matched absorbance of ∼0.7 at the excitation wavelength.
Fig. 3 Low temperature emission spectrum (77 K) of (a) gold(I) complex
CBZ-Au-1 and (b) magnified phosphorescence region. Note that the
0
–0 band in the fluorescence is clipped due to the scaling used to show
the weak phosphorescence. The frequencies of the vibronic pro-
gressions discussed in the text were determined by the differences
shown in b.
THF solutions of the oligomers and polymer. Fig. 4 compares
the transient absorption spectra of oligomer CBZ-TMS with
CBZ-Au-1, CBZ-Pt-1 and CBZ-Poly-Pt, immediately following
the laser pulse in THF. The transients are quenched by oxygen,
and on this basis they are assigned to the absorption of the
lowest triplet excited state. The lifetimes of the transient
absorption of the triplet state are listed in Table 1 (τTA). The
triplet lifetimes for the oligomers are similar, ranging from
assigned as Δν
1
and Δν
2
. The coupling frequency differs
slightly from sample to sample. The Δν1 is 1430 cm− for
1
CBZ-Pt-1 and CBZ-Au-1, whereas for CBZ-Poly-Pt Δν
1
≈
460 cm− . For CBZ-Pt-1, CBZ-Au-1 and CBZ-Poly-Pt Δν
1
1
5
2
≈ 610,
75 and 660 cm , respectively. The low temperature fluo-
−
1
rescence spectra of CBZ-Pt-1 and CBZ-Au-1 exhibit a pro-
gression with nearly the same frequency (Δν
the high frequency progression in the phosphorescence. Inter-
estingly, the fluorescence of the polymer exhibits two pro-
gressions, one at a similar frequency (Δν ≈ 1430 cm ) and
≈ 1430 cm− ) as
1
1
0.35–0.65 μs, and they vary in the sequence CBZ-Au-1 > CBZ-Pt-
−
1
1 > CBZ-Poly-Pt. The difference absorption spectra generally
consist of ground state bleaching of the S –S absorption and
1
−
1
0
1
another a higher frequency (Δν ≈ 2090 cm ). The latter may
be due to coupling of the singlet state with the CuC modes.
1
broad T –T absorption in the visible region. Interestingly
1
n
CBZ-TMS also exhibits triplet absorption, possibly due to the
presence of the thiophene units, which are known to enhance
ISC in π-conjugated electronic systems.
Transient absorption spectroscopy
47
To further explore the properties of the triplet state, ns–μs tran-
1 n
The λmax for T –T absorption is shifted about 20 nm for
sient absorption spectroscopy was carried out on deoxygenated the gold complex CBZ-Au-1 and ∼164 nm for the platinum
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 17721–17728 | 17725

View Article Online
Paper
Dalton Transactions
complex CBZ-Pt-1 relative to the ligand CBZ-TMS. By compar-
ing the transient absorption signal of the metallated oligomers
with the polymer, it is found that the intensity of the signal for
the polymer is lower. As the experiment is carried out from
samples solutions of matched absorbance, with the same laser
power, and the same excitation wavelength, this difference
suggests either that the polymer CBZ-Poly-Pt has a lower triplet
yield than the metal complexes, or that there is a triplet
quenching pathway that operates in the polymer that is not
available in the oligomers. One possibility is that triplet–triplet
annihilation may take place in the polymer.
As direct excitation of the metal complexes and the polymer
produces the triplet state, we examined their ability to sensitize
1
the generation of singlet oxygen ( O ) in deuterated benzene
2
solution. These experiments were performed by monitoring
1
phosphorescence from
2
O at 1270 nm. All of the materials
sensitize the production of singlet oxygen, with observed
quantum yields ranging from 0.15–0.21 (Φ , Table 1), consist-
Δ
ent with moderately efficient intersystem crossing. However,
the singlet oxygen yields are somewhat less than might be
expected given that the fluorescence quantum yields of the
metallated oligomers are so much less than free oligomer.
This may be due to the relatively short triplet lifetimes
which result in incomplete triplet energy transfer to oxygen
whose concentration is comparatively low in the air saturated
solutions.
Two photon absorption spectroscopy
Two photon absorption spectra and cross sections of CBZ-Au-
1
, CBZ-Pt-1 and CBZ-Poly-Pt were measured in THF by the
non-linear transmission (NLT) method and are presented in
Fig. 5. The comparatively low fluorescence quantum yields for
the metal containing oligomers and polymer precluded the
use of the two photon excited fluorescence (2PEF) method. Fig. 5 2-Photon absorption spectra of the metal complexes and the
polymer are measured with femtosecond pulses via the non-linear
transmittance method in THF solution (red filled circles for CBZ-Au-1,
blue filled squares for CBZ-Pt-1 and magenta filled diamonds for
CBZ-Poly-Pt); corresponding wavelengths are in bottom horizontal
The normalized ground state absorption spectra are shown in
the plots for comparison. In all three compounds the 2PA
cross section values are relatively low in the S →S transition
0 1
region, σ2PA ∼ 10–100 GM, with only CBZ-Au-1 showing slightly scale. One photon absorption spectra (shown by black line) are
higher values. The cross section intensity increases rapidly measured in THF solution and corresponding wavelengths are in upper
horizontal scale.
toward shorter wavelengths, reaching values of σ ∼ 800–1000
2
GM at 550 nm. Because the maxima of the 2PA spectra are
blue-shifted relative to the ground state absorption maxima,
we may conclude that these systems display mainly centro- consequently excitation at this wavelength is only likely to
4
8
symmetric character.
Comparison of the nanosecond occur via 2PA. Fig. 6 compares the representative z-scan results
transient absorption and 2PA spectra shows that there is a for CBZ-Au-1, CBZ-Pt-1 and CBZ-Poly-Pt. From the z-scan
significant overlap between them, especially in the mid-visible result, it is evident that the non-linear response of these
region between 500 and 650 nm. This overlap suggests that the materials varied in the series CBZ-Poly-Pt ∼ CBZ-Pt-1 >
complexes will exhibit pronounced long-timescale non-linear CBZ-Au-1, with the Pt-oligomer and polymer sample exhibiting
absorption via the dual mode mechanism that combines 2PA approximately 10% reduction in transmittance at the peak of
and excited state absorption by the triplet state.
the z-scan. Recent work from our laboratories has explored the
In order to probe the ability of the oligomers and polymers nanosecond z-scan response of a series of Pt-acetylide com-
4
9
to display non-linear absorption via the dual mode mechan- plexes with high 2PA cross section conjugated ligands. The
ism, open-aperture z-scan experiments were performed by z-scan response for the present series of carbazole oligomers are
using 1 mM THF sample solutions with 600 nm, 5 ns pulses. somewhat less compared to that of the most active Pt-acetylide
Note that 600 nm is more than 100 nm to the red of the onset oligomers previously investigated. The attenuated response is
2
of the allowed singlet ground state absorption transition, and likely due to the fact that the σ values for the carbazole based
17726 | Dalton Trans., 2014, 43, 17721–17728
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
that seen in previous studies of Pt-acetylide chromophores, the
excellent visible transparency of the ET-Cbz-TE family makes
this system promising for non-linear optical applications
where transparency for λ < 500 nm is a requirement.
Acknowledgements
This work is supported by Air Force Office of Scientific
Research (grant AFOSR FA-9550-06-1-1084) and National
Science Foundation (grant no. CHE-115164). The work at
Montana State University is supported by Air Force Office of
Scientific Research (grant no. FA9550-09-1-0219) and Estonian
Science Foundation (grant ETF no. 4977).
Fig. 6 Open aperture nanosecond z-scan
1 mM THF solution of
CBZ-Au-1, CBZ-Pt-1 and CBZ-Poly-Pt at 600 nm excitation wavelength,
ns pulse width.
5
oligomers are lower compared to the previously investigated References
systems. Nonetheless, the response observed in the present
1
2
M. Göppert-Mayer, Ann. Phys., 1931, 401, 273–294.
W. Kaiser and C. G. B. Garrett, Phys. Rev. Lett., 1961, 7, 229–
31.
R. Westlund, E. Malmström, C. Lopes, J. Öhgren,
T. Rodgers, Y. Saito, S. Kawata, E. Glimsdal and
M. Lindgren, Adv. Funct. Mater., 2008, 18, 1939–1948.
C. Tang, Q. Zheng, H. Zhu, L. Wang, S.-C. Chen, E. Ma and
X. Chen, J. Mater. Chem. C, 2013, 1, 1771–1780.
series of complexes is respectable and given their relatively
goodopticaltransparency,especiallyforCBZ-Au-1,thesechromo-
phores may be useful in non-linear optical applications.
2
3
4
Summary and conclusions
This study highlights the photophysical and non-linear absorp-
tion properties of a novel series of organometallic chromo-
phores that are based on the carbazole containing ET-Cbz-TE
π-conjugated oligomer. The results provide insight regarding
the relative effects of the Au(I) and Pt(II) centers on S –T inter-
5 C. W. Spangler, J. Mater. Chem., 1999, 9, 2013–2020.
6 K. D. Belfield, M. V. Bondar, F. E. Hernandez and
O. V. Przhonska, J. Phys. Chem. C, 2008, 112, 5618–5622.
7 S. Kim, T. Y. Ohulchanskyy, H. E. Pudavar, R. K. Pandey
and P. N. Prasad, J. Am. Chem. Soc., 2007, 129, 2669–2675.
8 W. Denk, J. Strickler and W. Webb, Science, 1990, 248,
73–76.
1
1
1
system crossing, radiative decay of T (phosphorescence), and
non-linear absorption. Au(I) appears to have relatively little
effect on the ground state absorption of the ET-Cbz-TE
chromophore, whereas the Pt(II) units induce a significant red-
9 D. A. Parthenopoulos and P. M. Rentzepis, Science, 1989,
245, 843–845.
shift. This signals that there is greater mixing of the metal 10 H. E. Pudavar, M. P. Joshi, P. N. Prasad and
orbitals with the π electron system in the ET-Cbz-TE chromo- B. A. Reinhardt, Appl. Phys. Lett., 1999, 74, 1338–1340.
phore for the Pt(II) systems. This results in greater metal 11 D. Day, M. Gu and A. Smallridge, Opt. Lett., 1999, 24, 948–
induced spin–orbit coupling in the Pt(II) systems, and the 950.
effect is manifested in a reduced fluorescence yield and higher 12 N. J. Long, Angew. Chem., Int. Ed. Engl., 1995, 34, 21–38.
phosphorescence yield for CBZ-Poly-Pt ∼ CBZ-Pt-1. All of the 13 J. E. Rogers, J. E. Slagle, D. M. Krein, A. R. Burke,
systems investigated exhibit moderately intense mid-visible
transient absorption from a triplet state with a lifetime in the
sub-microsecond timescale, consistent with relatively efficient
intersystem crossing. Femtosecond non-linear transmittance
B. C. Hall, A. Fratini, D. G. McLean, P. A. Fleitz,
T. M. Cooper, M. Drobizhev, N. S. Makarov, A. Rebane,
K.-Y. Kim, R. Farley and K. S. Schanze, Inorg. Chem., 2007,
46, 6483–6494.
experiments were utilized to construct the 2-photon absorption 14 J. E. Haley, D. M. Krein, J. L. Monahan, A. R. Burke,
spectra of the chromophores. In each case, the 2PA absorption
is blue shifted relative to the transition that is degenerate with
D. G. McLean, J. E. Slagle, A. Fratini and T. M. Cooper,
J. Phys. Chem. A, 2010, 115, 265–273.
the primary ground state absorption band. This result is con- 15 K.-Y. Kim, A. H. Shelton, M. Drobizhev, N. Makarov,
sistent with the centrosymmetric structure of the metallated
ET-Cbz-TE chromophore structure. The peak 2PA for
A. Rebane and K. S. Schanze, J. Phys. Chem. A, 2010, 114,
7003–7013.
this series occurs in the region 550–600 nm with σ2 values 16 K. A. Nguyen, P. N. Day and R. Pachter, J. Phys. Chem. A,
800 GM. 2009, 113, 13943–13952.
While the overall non-linear response of this series of 17 Z.-D. Yang, J.-K. Feng and A.-M. Ren, Inorg. Chem., 2008,
>
organometallic oligomers to nanosecond pulses is lower than
47, 10841–10850.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 17721–17728 | 17727

View Article Online
Paper
Dalton Transactions
1
8 G. G. Dubinina, R. S. Price, K. A. Abboud, G. Wicks,
P. Wnuk, Y. Stepanenko, M. Drobizhev, A. Rebane and
M. Salvalaggio, A. Savoini and S. Spera, Electrochim. Acta,
2011, 56, 6638–6653.
K. S. Schanze, J. Am. Chem. Soc., 2012, 134, 19346– 35 W. Lu, W.-M. Kwok, C. Ma, C. T.-L. Chan, M.-X. Zhu and
9349. C.-M. Che, J. Am. Chem. Soc., 2011, 133, 14120–14135.
9 C. Liao, A. H. Shelton, K.-Y. Kim and K. S. Schanze, ACS 36 C. Ma, C. T.-L. Chan, W.-M. Kwok and C.-M. Che, Chem.
Appl. Mater. Interfaces, 2011, 3, 3225–3238. Sci., 2012, 3, 1883–1892.
0 K. McEwan, K. Lewis, G. Y. Yang, L. L. Chng, Y. W. Lee, 37 G. S. M. Tong, P. K. Chow and C.-M. Che, Angew. Chem.,
1
1
2
W. P. Lau and K. S. Lai, Adv. Funct. Mater., 2003, 13, 863–
67.
1 T. C. Wen and C. Y. Tsai, Chem. Phys. Lett., 1999, 311, 173–
78.
2 J. L. Humphrey and D. Kuciauskas, J. Am. Chem. Soc., 2006,
28, 3902–3903.
3 T. E. O. Screen, J. R. G. Thorne, R. G. Denning,
Int. Ed., 2010, 49, 9206–9209.
38 N. S. Makarov, M. Drobizhev and A. Rebane, Opt. Express,
2008, 16, 4029–4047.
39 A. Rebane, M. Drobizhev, N. S. Makarov, G. Wicks,
P. Wnuk, Y. Stepanenko, J. E. Haley, D. M. Krein, J. L. Fore,
A. R. Burke, J. Slagle, D. G. McLean and T. M. Cooper, J.
Phys. Chem. A, 2014, 118, 3749–3759.
8
2
2
2
1
1
D. G. Bucknall and H. L. Anderson, J. Mater. Chem., 2003, 40 M. Sheik-Bahae, A. A. Said, T.-H. Wei, D. J. Hagan and
1
3, 2796–2808.
E. W. Van Stryland, IEEE J. Quantum Electron., 1990, 26,
760–769.
2
2
2
2
2
2
4 J. W. Perry, D. Alvarez, I. Choong, K. Mansour, S. R. Marder
and K. J. Perry, Opt. Lett., 1994, 19, 625–627.
41 D. A. Oulianov, I. V. Tomov, P. M. Dvornikov and
P. M. Rentzepis, Opt. Commun., 2001, 191, 235–243.
42 H.-Y. Chao, W. Lu, Y. Li, M. C. W. Chan, C.-M. Che,
K.-K. Cheung and N. Zhu, J. Am. Chem. Soc., 2002, 124,
14696–14706.
5 D. Swain, R. Singh, V. K. Singh, N. V. Krishna, L. Giribabu
and S. V. Rao, J. Mater. Chem. C, 2014, 2, 1711–1722.
6 B. S. Wang, J. Wang and J.-Y. Chen, J. Mater. Chem. B, 2014,
2, 1594–1602.
7 R. Liu, H. Chen, J. Chang, Y. Li, H. Zhu and W. Sun, Dalton 43 W.-Y. Wong, L. Liu, S.-Y. Poon, K.-H. Choi, K.-W. Cheah
Trans., 2013, 42, 160–171. and J.-X. Shi, Macromolecules, 2004, 37, 4496–4504.
8 S. M. Aly, C.-L. Ho, W.-Y. Wong, D. Fortin and P. D. Harvey, 44 S.-i. Kato, S. Shimizu, H. Taguchi, A. Kobayashi, S. Tobita
Macromolecules, 2009, 42, 6902–6916. and Y. Nakamura, J. Org. Chem., 2012, 77, 3222–3232.
9 C.-L. Ho, C.-H. Chui, W.-Y. Wong, S. M. Aly, D. Fortin, 45 M. Montalti, A. Credi, L. Prodi and M. T. Gandolfi, in Hand-
P. D. Harvey, B. Yao, Z. Xie and L. Wang, Macromol. Chem.
Phys., 2009, 210, 1786–1798.
0 J. B. Seneclauze, P. Retailleau and R. Ziessel, New J. Chem.,
book of Photochemistry, 3rd edn, 2005, ch. 13.
46 G.-J. Zhou and W.-Y. Wong, Chem. Soc. Rev., 2011, 40,
2541–2566.
3
3
3
3
3
2
007, 31, 1412–1416.
47 A. Parthasarathy, S. Goswami, T. S. Corbitt, E. Ji,
D. Dascier, D. G. Whitten and K. S. Schanze, ACS Appl.
Mater. Interfaces, 2013, 5, 4516–4520.
1 N. Zhang, A. Hayer, M. K. Al-Suti, R. A. Al-Belushi,
M. S. Khan and A. Köhler, J. Chem. Phys., 2006, 124, 244701.
2 G.-J. Zhou, W.-Y. Wong, D. Cui and C. Ye, Chem. Mater., 48 F. Terenziani, A. Painelli, C. Katan, M. Charlot and
2
3 C.-H. Tao, H. Yang, N. Zhu, V. W.-W. Yam and S.-J. Xu,
Organometallics, 2008, 27, 5453–5458.
4 A. Tacca, R. Po, M. Caldararo, S. Chiaberge, L. Gila,
L. Longo, P. R. Mussini, A. Pellegrino, N. Perin,
005, 17, 5209–5217.
M. Blanchard-Desce, J. Am. Chem. Soc., 2006, 128, 15742–
15755.
49 G. G. Dubinina, R. S. Price, K. A. Abboud, G. Wicks,
P. Wnuk, Y. Stepanenko, M. Drobizhev, A. Rebane and
K. S. Schanze, J. Am. Chem. Soc., 2012, 134, 19346–19349.
17728 | Dalton Trans., 2014, 43, 17721–17728
This journal is © The Royal Society of Chemistry 2014