Two-photon spectroscopy of tungsten(0) arylisocyanides using nanosecond-pulsed excitation

Article abstract of DOI:10.1039/c7dt02632c

The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)₆) were determined in the 800-1000 nm region using two-photon luminescence (TPL) spectroscopy. The complexes have high TPA cross sections, in the range 1000-2000 GM at 811.8 nm. In comparison, the cross section at 811.8 nm for tris-(2,2′-bipyridine)ruthenium(ii), [Ru(bpy)₃]²⁺, is 7 GM. All measurements were performed using a nanosecond-pulsed laser system.

Full text of DOI:10.1039/c7dt02632c

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Two-photon spectroscopy of tungsten(0)
arylisocyanides using nanosecond-pulsed
Cite this: DOI: 10.1039/c7dt02632c
excitation†
Kana Takematsu,a,b Sara A. M. Wehlin,a,c Wesley Sattler,a,d Jay R. Winkler *a and
Harry B. Gray*a
The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)6) were
determined in the 800–1000 nm region using two-photon luminescence (TPL) spectroscopy. The com-
plexes have high TPA cross sections, in the range 1000–2000 GM at 811.8 nm. In comparison, the cross
section at 811.8 nm for tris-(2,2’-bipyridine)ruthenium(II), [Ru(bpy)3]2+, is 7 GM. All measurements were
performed using a nanosecond-pulsed laser system.
Received 19th July 2017,
Accepted 11th September 2017
DOI: 10.1039/c7dt02632c
rsc.li/dalton
The strong one-photon absorption cross sections and relatively
long lifetimes (∼μs) of metal–ligand charge transfer (MLCT)
Introduction
The many potential applications of two-photon absorption states suggest that metal-acceptor(ligand) complexes represent
(TPA) (e.g., microscopy, photodynamic therapy, microfabrica- attractive targets for TPA applications that could potentially
tion, and optical power limiting) have stimulated efforts to dis- combine the desired redox chemistry from one-photon chem-
cover new compounds with large TPA cross sections (δ).1–6 istry with the enhanced spatial resolution and increased pene-
Notably, owing in part to the enhancement of TPA cross sec- tration depth from two-photon chemistry.
tions in molecules with π-conjugated chains, organic macro-
With the exception of metalloporphyrins,1,19 most TPA
molecular and dendrimer chromophores have received much studies of inorganic complexes or templates have employed
attention.2,7–9 In addition, over the past twenty years, there ruthenium as the central metal.10–18 A handful of other metal
have been many studies exploring coordination complexes that chromophores have been explored, including ones containing
combine the chemical stability and tunability of metal centers lanthanide ions, Re, Ir, Fe, Pt, and Zn.11,13,20–28 Further devel-
with a diverse library of ligands and linking polymers that opment of transition metal TPA complexes will be required if
could potentially promote TPA.10–23 Early TPA studies of in- they are to compete with organic chromophores. With this
organic chromophores found disappointing cross sections objective in mind, we chose to work on homoleptic tungsten
compared to those of well-designed organic chromophores (δ > arylisocyanide complexes (W(CNAr)6) that exhibit intense one-
1000 GM): the TPA cross section of the popular tris-(2,2′-bipyri- photon absorption bands in the visible region and relatively
dine)Ru(II) ion ([Ru(bpy)3]2+) was reported to be just 4.3 GM at long excited-state lifetimes.29–32 The tungsten complex,
880 nm.10 Imaginative designs of transition metal complexes W(CNdipp)6 (CNdipp = 2,6-diisopropylphenyl-isocyanide), is a
have led to striking improvements in TPA cross sections: oligo strong photoreductant (E°(W+/*W0) = −2.8 V vs. Cp2Fe+/0 in
(p-phenylenevinylene) groups end-functionalized by Ru- THF), making it of interest as a photoredox sensitizer.29
alkynyl groups have δ > 12 500 GM (at 725 nm), although the Derivatization of CNdipp, shown in Fig. 1, resulted in tungsten
chromophore is localized on the phenylenevinylene moiety.18 complexes with longer lifetimes (τ = 75 ns − 1.56 μs in THF)
and relatively high luminescence quantum yields (Φ
=
0.01–0.25 in THF).32 The extended conjugation of the nearly
coplanar π-systems in crystal structures suggests that these
molecules are promising TPA candidates.
aBeckman Institute, California Institute of Technology, Pasadena CA 91125, USA.
E-mail: winklerj@caltech.edu, hbgray@caltech.edu
bDepartment of Chemistry, Bowdoin College, Brunswick, ME 04011, USA
cDepartment of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill,
NC 27599, USA
Two main methods have been used to obtain TPA cross sec-
tions. In z-scan,33 the transmitted light intensity is measured
as the sample is moved between focal points of the laser
source and detector. In two-photon luminescence (TPL), once
called two-photon-excitation fluorescence (TPEF),34 emission
spectra are collected as the sample is excited by a tightly
dThe Dow Chemical Company, Formulation Science, Core R&D, 400 Arcola Road,
Collegeville, PA 19426, USA
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7dt02632c
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dippPhOMe as shown in Fig. 1) and corresponding tungsten
complexes, W(CNAr)6, were synthesized and purified according
to previously published protocols.29,32 Briefly, all ligands were
synthesized from a single intermediate, N-formyl-4-bromo-2,6-
diisopropylaniline, by Suzuki coupling of an arylboronic acid
followed by dehydration with OPCl3. WCl4(THF)2 was then
reduced with Na(Hg) in the presence of the ligands to produce
the respective tungsten complexes. Solutions of the tungsten
complexes were prepared in a nitrogen-filled glovebox using
2-methyltetrahydrofuran (2MTHF) that was freshly distilled
from Na/benzophenone ketyl for experiments. Immediate UV-
vis spectra (Cary 50) were collected in 1 mm cuvettes upon
removal from the glovebox. Solutions for TPA experiments
2
were placed in custom-made long-neck
1 cm cuvettes
and flame-sealed on high-vacuum line upon removal
a
from the glovebox. Sample concentrations of W(CNdipp)6,
Fig. 1 Top: Tungsten(0) (oligo)arylisocyanide W(CNAr)6. Bottom: (Oligo)
W(CNdippPh)6, W(CNdippPhPh)6, and W(CNdippPhOMe
)
2
arylisocyanides CNdipp, CNdippPh, CNdippPhPh, and CNdippPhOMe
.
2
6
ranged from 5–25 µM. All samples were stored in the dark.
[Ru(bpy)3]Cl2 and our emission standard, fluorescein, were
obtained from Sigma Aldrich and used without further purifi-
cation. Sample solutions ([Ru(bpy)32+] = 5 mM; [fluorescein] =
78 μM) for TPL measurements were prepared in MilliQ water
and filtered with Millex GV filters (0.22 µm). The fluorescein
solution was adjusted to pH 11 using NaOH (1 M). The solu-
tions were placed in modified Schlenk cuvettes (1 cm) and
were degassed and backfilled with argon. All samples were
stored in the dark before measurement.
focused beam from a pulsed laser. In relative TPL measure-
ments, the TPL cross sections are determined by comparison
to standard or reference TPL dyes.34–36 The TPA cross sections
are extrapolated from TPL measurements, assuming similar
one-photon and two-photon photoluminescence quantum
efficiencies (Φ): δ = Φ−1σTPL
.
Both methods typically utilize ultrafast Ti:sapphire lasers for
temporal and spatial control of the light source. Despite the low-
ering cost and ready commercial availability of femtosecond
lasers, they remain a hurdle for many researchers interested in
pursuing TPA studies. Even with access, the additional cost and
difficulty of realigning wavelength-dependent optics tend to
limit TPA studies to single wavelength measurements.
Comparing the two methods, TPL is less demanding in terms of
laser pulse-width;37 indeed, the first evidence of TPL was
obtained in 1961 via detection of Eu2+ luminescence produced
by excitation with a nanosecond pulsed ruby laser.38 In 2000,
Rumi et al. showed that TPA cross sections obtained via TPL
using nanosecond and picosecond lasers were approximately the
same for organic chromophores (bis-dialkylamino- and diaryl-
amino-substituted diphenylpolyenes and bis(styryl) benzenes).37
Although nanosecond lasers were used in early TPA studies, they
have largely been replaced by femtosecond excitation sources.
Nevertheless, TPL measurements with nanosecond pulsed lasers
can provide reliable cross sections and absorption spectra.
Laser apparatus
Spectral experiments were performed in the Beckman Institute
Laser Resource Center (BILRC). The third harmonic (355 nm)
from a Q-switched Nd:YAG laser (Spectra-Physics Quanta Ray
PRO, 10 Hz, 8 ns pulse) pumped a tunable optical parametric
oscillator (OPO, Spectra-Physics Quanta Ray MOPO-700). A
Pellin-Broca prism mounted on a rotational stage separated
the visible and near infrared (NIR) components. Two broad-
band mirrors (Newport) directed the preferred component to
the sample holder (1 cm cuvette holder with a magnetic
stirrer). The visible OPO output was used for initial beam
alignment; two 1.5 cm apertures were introduced to the beam
path to fix the radius and aid in the alignment of the NIR
beam. A NIR detector card (Thorlabs VRC4) was then used to
adjust the Pellin-Broca prism such that the NIR output
(820 nm) was centered on the first broadband mirror. A
700 nm long pass filter was introduced to eliminate residual
interference from the visible laser light, and spherical lens
(100 or 25 cm focal length) was added to concentrate the
beam.
We report here our application of a pulsed Nd:YAG laser
optical parametric oscillator combination to obtain an
extended TPA spectrum for [Ru(bpy)3]2+, and TPA cross sec-
tions
for
tungsten(0)
arylisocyanides
W(CNdipp)6,
W(CNdippPh)6, W(CNdippPhPh)6, and W(CNdippPhOMe )6.
2
Excitation wavelength tuning only required OPO adjustment
and rotation of the Pellin-Broca prism stage. A power-meter
was placed before the sample to optimize the NIR alignment
using the rotation stage. A small fraction of the excitation
beam was picked off and recorded with a calibrated Si photo-
Experimental
Sample preparation
The arylisocyanide ligands (CNAr, Ar = aryl groups including diode detector to provide precise readouts at low excitation
dipp and para-modified dipp groups dippPh, dippPhPh, and powers.
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Steady-state emission spectra were collected at a 90° angle entrance slit of a double monochromator (Instruments SA
from the excitation path via a 2-in collection lens and an DH-10) and detected by a PMT (Hamamatsu R928). Data were
optical fiber attached to a spectrometer (Melles-Grigot 13 recorded with and without laser excitation and processed in
FOS200). The integration times for each relative emission spec- Labview. Details of the time-resolved luminescence instrumen-
trum varied by sample: [Ru(bpy)3]Cl2 (8000 ms), W(CNdipp)6 tation and electronics have been published.32
(20 000–30 000
ms),
W(CNdippPh)6
(1000–1500
ms),
W(CNdippPhPh
)
6
(5000–12 000 ms), and W(CNdippPhOMe
)
2
6
(600–1500 ms). At each excitation wavelength, four spectra
were collected, averaged, and corrected for the dark noise and
spectrometer response.
A typical TPL experiment consisted of spectral collection over
800–1050 nm of the sample of interest. The excitation wave-
lengths were scanned in random order to minimize systematic
bias in the laser power. All emission data were collected and
analyzed in Matlab (The MathWorks, Inc.) to determine the TPA
cross section (δ) according to eqn (1),34 where α is a scaling
factor independent of the chemical identity of the sample and
Results and discussion
Validation of the laser apparatus was performed using fluor-
escein, a TPL standard. Scaled TPA spectra of fluorescein from
multiple data sets were in reasonable agreement with reference
spectra collected using femtosecond-pulsed lasers in the
800–900 nm region (Fig. 2).35,36,39 The variations among the
three published TPA spectra of fluorescein illustrate the chal-
lenges associated with these measurements. Slight discrepan-
cies in our data at longer wavelengths could arise from
instability of the laser output. To cover the broad spectral
profile, previous studies required the interchange of multiple
optical elements, which may have introduced some error in
the spectral data. The reproducibility of the spectral contour,
the broad spectral window, and ease of alignment of Nd:YAG/
OPO apparatus were all well-suited for our experiments.
hFðtÞi
δ ¼ α
ð1Þ
2
λΦCnhPðtÞi
includes the effects of the beam profile, sample excitation, and
collection efficiency, λ is the wavelength of the excitation light,
Φ = photoluminescence quantum efficiency, C = concen-
tration, n = index of refraction of the solvent, (P(t)) = time-aver-
aged power of the laser, and (F(t)) = time-averaged emission
intensity. To generate absolute TPA spectra, reference (fluor-
escein) and sample luminescence spectra were collected with
811 nm excitation using a fixed geometry (ESI†). There is some
variability in reported values of δ811 for fluorescein;35,36,39 we
chose the smallest value (δ811 = 21 GM (ref. 39)) to determine
the scaling factor α. Owing to the sensitivity of these measure-
ments to several experimental parameters, we estimate that
the uncertainties in the TPA cross sections are as large as 30%.
The TPA spectrum of [Ru(bpy)3]Cl2 in deionized water was
determined from relative TPL measurements, and scaled to
Power-dependent studies of fluorescein, [Ru(bpy)3]2+, and
W(CNAr)6 complexes at λexc = 847.6 nm all showed quadratic
dependence on the excitation power (ESI†). Emission spectra
and time-resolved emission decays at 610 nm collected under
one and two-photon excitation showed no major discrepancies
(ESI†). These results are consistent with the earlier work on
[Ru(bpy)3]2+ by Castellano et al.10 For the W(CNAr)6 complexes,
the decays in 2MTHF were slightly longer than those in THF:
W(CNdippPh)6: τ = 1.55 μs vs. 1.32 μs, W(CNdippPhPh)6: τ =
610 ns vs. 350 ns, W(CNdippPhOMe )6: τ = 1.47 μs vs. 1.20 μs.
32
2
The lifetime ratios were used to adjust the quantum yields for
TPA spectral analysis. The long W(CNAr)6 MLCT lifetimes
under two-photon excitation likely will be exploited in future
applications.
the previously reported cross section (δ880
=
4.3).10
nm
The quantum yields for the tungsten complexes in THF
are: W(CNdipp)6 (Φ = 0.01), W(CNdippPh)6 (Φ = 0.21),
W(CNdippPhPh
)
6
(Φ = 0.07), and W(CNdippPhOMe
)
6
(Φ =
2
0.21).32 Quantum yields for the tungsten complexes in 2MTHF
were determined from the ratio of the lifetimes (τ) of the
excited state in THF and 2MTHF assuming that the radiative
rate constants (kr) are the same in the two solvents (eqn (2)).
Smoothed two-photon absorption cross sections were gener-
ated using a Savitzky–Golar algorithm implemented in Matlab
using a polynomial order of 3 and a frame length of 11. A table
of experimental details is given in ESI.†
Φ
krτ
τ
WðCNArÞ6:2MTHF
WðCNArÞ6:2MTHF
WðCNArÞ6:2MTHF
¼
¼
τ
WðCNArÞ6:THF
ð2Þ
Φ
krτ
WðCNArÞ6:THF
WðCNArÞ6:THF
Power-dependent luminescence measurements were made
to verify TPA. Laser power was attenuated by either addition of
neutral density filters or by minor tuning of the Nd:YAG har-
monic-generating crystal. For time-resolved measurements,
Fig. 2 TPA spectra of fluorescein. The reproducibility of the data and
agreement with the reference data are shown in the figure. Each set of
symbols corresponds to a different data collection date, while the solid,
dashed, and dot-dashed lines correspond to the spectra in ref. 39, 35,
sample luminescence was directed by optical fiber to the and 36, respectively.
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The TPA spectrum of [Ru(bpy)3]Cl2 is shown in Fig. 3. The
spectral region corresponds to the excitation of the MLCT
band. Comparison with the single-photon absorption spec-
trum showed a blue-shift in the TPA spectrum. This shift has
been observed in previous TPA experiments.34
What about TPA cross sections for other Ru(II) complexes?
Values for complexes with N-methyl/aryl-2,2′:4,4″:4′,4″-quater-
pyridinium,13 fluorene-substituted 1,10-phenanthroline,12 and
a
combination of phenanthroline and dipyridophenzine
ligands11 are somewhat greater than those for [Ru(bpy)3]2+: δ =
62–180 GM at 720 nm, δ = 90/15/15 GM at 750/850/930 nm,
and δ = 150 GM at 710 nm, respectively. If the TPA cross sec-
tions are corrected for molecular weight (δ/M or GM mol
g−1),40 [Ru(bpy)3]2+ could potentially be a better TPA chromo-
phore on a per mass basis at selected wavelengths.
Interestingly, the TPA cross sections of Ru(II) complexes can
be tuned by the degree of π-conjugation in the ligands, but the
enhancement appears largely to arise from intraligand charge
transfer (ILCT) rather than MLCT interactions. After installing
a triple bond between ligating phenanthroline and fluorene
groups, Girardot et al. observed an ILCT band at 400 nm,
leading to increased TPA cross sections in the 700–900 nm
region.12 In addition, examining dimeric versions of
[Ru(phen)2dppz]2+, Samoc et al. observed significant enhance-
ment in the ILCT excitation region (560 nm) but not at 710 nm
(MLCT).14 In-depth comparisons of experimental TPA results
with electronic structure calculations for [Ru(bpy)3]2+ would
shed light on the ILCT and MLCT states and could lead to the
design and construction of metal complexes with greatly
enhanced TPA cross sections.
The TPA spectra of W(CNdippPh)6, W(CNdippPhPh)6, and
Fig. 4 From top to bottom: TPA spectra of W(CNdippPh)6,
W(CNdippPhOMe
)
6
are all blue-shifted from one-photon
2
W(CNdippPhPh)6, and W(CNdippPhOMe )6. For each panel, the markers
2
absorptions (Fig. 4). The TPA spectrum for W(CNdipp)6 could
not be collected, owing to the relatively low quantum yield
(Φ ∼ 0.01). The TPA cross sections for the remaining complexes
are exceptionally high (Table 1), in the range of 1000–2000 GM
at 811.8 nm. Comparisons to one-photon absorption spectra
suggest that the cross sections could increase further at
correspond to different experimental sets. The solid lines are smoothed
curves produced using
a Savitzky–Golay filter (order 3, frame 11)
through the averages of each data set. The dashed lines are the one-
○
photon absorption spectra. For the middle panel, the data set ( ) was
collected with slightly unstable laser power at the edge of the scans; a
□
second data set ( ) was collected when the laser power was more
stable. The data sets were scaled and the long wavelength baselines
were matched; no other corrections were made to the data.
Table 1 Comparison of one-photon and two-photon absorption cross
sections for tungsten arylisocyanides in 2MTHF
ε × 10−4 a
Complex
λOP a (nm)
(M−1 cm−1
)
λTP (nm)
δ (GM)
W(CNdippPh)6
W(CNdippPhPh
406, 497
406, 508
406, 498
4.3, 13
4.0, 16
4.5, 13
811.8
811.8
811.8
2000
1700
1000
)
6
W(CNdippPhOMe
)
6
2
a Wavelengths corresponding to the two-photon absorption and
maximum MLCT bands; extinction coefficients are in ref. 29.
Fig. 3 TPA spectra of [Ru(bpy)3]2+. The open markers correspond to
different experimental sets. The solid line is a smoothed curve produced
using a Savitzky–Golay filter (order 3, frame 11) through the average of
the data sets. The dashed line is the one-photon absorption spectrum
shorter wavelengths. These values, which are larger than those
reported for many organic chromophores, are comparable to
some of the highest TPA cross sections of metal com-
for [Ru(bpy)3]2+
.
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plexes.17,18 If MWs of the species were included (or as
suggested by others, numbers of electrons),41,42 the weighted
TPA cross sections could be even higher.
Conflicts of interest
There are no conflicts to declare.
The large TPA cross sections of tungsten(0) complexes can
be understood in terms of elements that optimize TPA in dyes:
(1) centrosymmetric geometry; (2) extended π-conjugation in
the ligands; (3) strong one-photon absorption near the TPA
band.1 Previously published W(CNAr)6 solid-state structures
showed that the compounds are all nearly centrosymmetric.32
The trans-arylisocyanide ligands are oriented in a coplanar
manner, constrained by bulky ortho-isopropyl groups. These
ligands therefore form a fully conjugated π-system. TDDFT cal-
culations indicate that the MLCT excited states in these mole-
cules involve a transfer of charge from the metal centers onto
the ligands.43 These MLCT excited states are threefold degener-
ate in Oh or Th symmetry; this degeneracy contributes to the
large extinction coefficients and two-photon cross sections.
Comparison of the TPA spectra showed that neither the
addition of phenyl or methoxy groups to CNdippPh signifi-
cantly altered the cross sections. As the maximum one-photon
Acknowledgements
Research was supported by CCI Solar Fuels (NSF
CHE-1305124) and the Arnold and Mabel Beckman
Foundation.
Notes and references
1 M. Pawlicki, H. A. Collins, R. G. Denning and
H. L. Anderson, Angew. Chem., Int. Ed., 2009, 48, 3244–
3266.
2 F. Terenziani, C. Katan, E. Badaeva, S. Tretiak and
M. Blanchard-Desce, Adv. Mater., 2008, 20, 4641–4678.
3 G. S. He, L.-S. Tan, Q. Zheng and P. N. Prasad, Chem. Rev.,
2008, 108, 1245–1330.
4 L. W. Tutt and T. F. Boggess, Prog. Quantum Electron., 1993,
17, 299–338.
5 W. Denk, J. H. Strickler and W. W. Webb, Science, 1990,
248, 73.
6 F. Helmchen and W. Denk, Nat. Methods, 2005, 2, 932–940.
7 M. Albota, D. Beljonne, J.-L. Brédas, J. E. Ehrlich, J.-Y. Fu,
A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder,
D. McCord-Maughon, J. W. Perry, H. Röckel, M. Rumi,
G. Subramaniam, W. W. Webb, X.-L. Wu and C. Xu, Science,
1998, 281, 1653.
8 H. Rath, V. Prabhuraja, T. K. Chandrashekar, A. Nag,
D. Goswami and B. S. Joshi, Org. Lett., 2006, 8, 2325–2328.
9 M. Drobizhev, A. Karotki, A. Rebane and C. W. Spangler,
Opt. Lett., 2001, 26, 1081–1083.
absorption of W(CNdippPhPh
) (Table 1) is red-shifted com-
6
pared to the other complexes, it is possible that at other wave-
lengths the complex may have slightly higher TPA cross sec-
tions (λ < 811.8 nm). The addition of a phenyl group to
CNdipp most likely increased the TPA cross sections, however,
due to the poor quantum yield of W(CNdipp)6, the enhance-
ment could not be directly measured using TPL.
The UV/vis spectra of the W(CNAr)6 complexes exhibited
strong MLCT absorptions, with extinction coefficients an order
of magnitude larger (Table 1) than that for the MLCT band in
[Ru(bpy)3]2+ (1.45 × 104 M−1 cm−1 at 452 nm).44 Sattler et al.
suggested that the intensity of the bands was due to significant
CT character in both the ground and excited states, W 5dπ –
CN π* and W 6p – CN π*, respectively.32 Upon excitation, the
molecule slightly distorts, increasing its net dipole moment.
As relaxation of the excited state is solvent-dependent, the
dependence of the TPA cross section on solvent polarity 10 F. N. Castellano, H. Malak, I. Gryczynski and
should be examined further to see whether there is compe- J. R. Lakowicz, Inorg. Chem., 1997, 36, 5548–5551.
tition between the distortion of the excited state to increase 11 C. Girardot, G. Lemercier, J. C. Mulatier, J. Chauvin,
the net dipole change and conservation of the constrained
molecular symmetry to preserve π-conjugation in the ligands.
P. L. Baldeck and C. Andraud, Dalton Trans., 2007, 3421–
3426, DOI: 10.1039/B706715A.
12 C. Girardot, B. Cao, J.-C. Mulatier, P. L. Baldeck,
J. Chauvin, D. Riehl, J. A. Delaire, C. Andraud and
G. Lemercier, ChemPhysChem, 2008, 9, 1531–1535.
13 B. J. Coe, M. Samoc, A. Samoc, L. Zhu, Y. Yi and Z. Shuai,
J. Phys. Chem. A, 2007, 111, 472–478.
Concluding remarks
The TPA cross sections corresponding to MLCT
excitation regions in W(CNdippPh)6, W(CNdippPhPh)6, and 14 P. Hanczyc, B. Norden and M. Samoc, Dalton Trans., 2012,
W(CNdippPhOMe
)
6
determined from 800–1000 nm are excep-
41, 3123–3125.
2
tionally high, ranging from 1000–2000 GM at 811.8 nm. Work 15 E. Baggaley, M. R. Gill, N. H. Green, D. Turton,
on ruthenium-based TPA shows that enhancement could be
achieved in coupled chromophores.14,15 Humphrey et al. also
have established that TPA cross sections are enhanced in Ru-
I. V. Sazanovich, S. W. Botchway, C. Smythe, J. W. Haycock,
J. A. Weinstein and J. A. Thomas, Angew. Chem., Int. Ed.,
2014, 53, 3367–3371.
alkynyl dendrimers and in Ru-capped ligand chains.17,18 16 S. K. Hurst, M. P. Cifuentes, J. P. L. Morrall, N. T. Lucas,
Based on these findings, we suggest that one way forward
would be to develop W-centered dendrimers or W-based tem-
plates for ligand addition.
I. R. Whittall, M. G. Humphrey, I. Asselberghs, A. Persoons,
M. Samoc, B. Luther-Davies and A. C. Willis,
Organometallics, 2001, 20, 4664–4675.
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17 K. A. Green, P. V. Simpson, T. C. Corkery, M. P. Cifuentes, 29 W. Sattler, M. E. Ener, J. D. Blakemore, A. A. Rachford,
M. Samoc and M. G. Humphrey, Macromol. Rapid
Commun., 2012, 33, 573–578.
18 B. Gao, L. M. Mazur, M. Morshedi, A. Barlow, H. Wang,
P. J. LaBeaume, J. W. Thackeray, J. F. Cameron,
J. R. Winkler and H. B. Gray, J. Am. Chem. Soc., 2013, 135,
10614–10617.
C. Quintana, C. Zhang, M. Samoc, M. P. Cifuentes and 30 L. A. Büldt and O. S. Wenger, Angew. Chem., Int. Ed., 2017,
M. G. Humphrey, Chem. Commun., 2016, 52, 8301–8304. 56, 5676–5682.
19 J. L. Humphrey and D. Kuciauskas, J. Am. Chem. Soc., 2006, 31 K. R. Mann, M. Cimolino, G. L. Geoffroy, G. S. Hammond,
128, 3902–3903.
20 A. D’Aléo, G. Pompidor, B. Elena, J. Vicat, P. L. Baldeck,
A. A. Orio, G. Albertin and H. B. Gray, Inorg. Chim. Acta,
1976, 16, 97–101.
L. Toupet, R. Kahn, C. Andraud and O. Maury, 32 W. Sattler, L. M. Henling, J. R. Winkler and H. B. Gray,
ChemPhysChem, 2007, 8, 2125–2132. J. Am. Chem. Soc., 2015, 137, 1198–1205.
21 J. R. Lakowicz, F. N. Castellano, I. Gryczynski, Z. Gryczynski 33 M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan and
and J. D. Dattelbaum, J. Photochem. Photobiol., A, 1999, 122,
95–101.
E. W. V. Stryland, IEEE J. Quantum Electron., 1990, 26, 760–769.
34 C. Xu and W. W. Webb, J. Opt. Soc. Am. B, 1996, 13, 481–491.
22 S. Botchway, J. R. Dilworth and M. Salichou, Dalton Trans., 35 M. A. Albota, C. Xu and W. W. Webb, Appl. Opt., 1998, 37,
2010, 39, 5219–5220. 7352–7356.
23 R. M. Edkins, S. L. Bettington, A. E. Goeta and A. Beeby, 36 N. S. Makarov, M. Drobizhev and A. Rebane, Opt. Express,
Dalton Trans., 2011, 40, 12765–12770. 2008, 16, 4029–4047.
24 C.-K. Koo, K.-L. Wong, C. W.-Y. Man, Y.-W. Lam, L. K.-Y. So, 37 M. Rumi, J. E. Ehrlich, A. A. Heikal, J. W. Perry, S. Barlow,
H.-L. Tam, S.-W. Tsao, K.-W. Cheah, K.-C. Lau, Y.-Y. Yang,
J.-C. Chen and M. H.-W. Lam, Inorg. Chem., 2009, 48, 872–
878.
Z. Hu, D. McCord-Maughon, T. C. Parker, H. Röckel,
S. Thayumanavan, S. R. Marder, D. Beljonne and
J.-L. Brédas, J. Am. Chem. Soc., 2000, 122, 9500–9510.
25 C.-K. Koo, K.-L. Wong, C. W.-Y. Man, H.-L. Tam, S.-W. Tsao, 38 W. Kaiser and C. G. B. Garrett, Phys. Rev. Lett., 1961, 7,
K.-W. Cheah and M. H.-W. Lam, Inorg. Chem., 2009, 48,
7501–7503.
26 J. E. Rogers, J. E. Slagle, D. M. Krein, A. R. Burke,
229–231.
39 S. de Reguardati, J. Pahapill, A. Mikhailov, Y. Stepanenko
and A. Rebane, Opt. Express, 2016, 24, 9053–9066.
B. C. Hall, A. Fratini, D. G. McLean, P. A. Fleitz, 40 T. Schwich, M. P. Cifuentes, P. A. Gugger, M. Samoc and
T. M. Cooper, M. Drobizhev, N. S. Makarov, A. Rebane, M. G. Humphrey, Adv. Mater., 2011, 23, 1433–1435.
K.-Y. Kim, R. Farley and K. S. Schanze, Inorg. Chem., 2007, 41 M. G. Kuzyk, J. Chem. Phys., 2003, 119, 8327–8334.
46, 6483–6494.
42 J. P. Moreno and M. G. Kuzyk, J. Chem. Phys., 2005, 123,
27 K.-Y. Kim, A. H. Shelton, M. Drobizhev, N. Makarov,
194101.
A. Rebane and K. S. Schanze, J. Phys. Chem. A, 2010, 114, 43 H. Kvapilová, W. Sattler, A. Sattler, I. V. Sazanovich,
7003–7013.
I. P. Clark, M. Towrie, H. B. Gray, S. Záliš and A. Vlček,
Inorg. Chem., 2015, 54, 8518–8528.
44 K. Kalyanasundaram, Coord. Chem. Rev., 1982, 46, 159–244.
28 M. G. Vivas, L. De Boni, T. M. Cooper and C. R. Mendonca,
J. Phys. Chem. A, 2014, 118, 5608–5613.
Dalton Trans.
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