Syntheses, Electrochemical, Linear Optical, and Cubic Nonlinear Optical Properties of Ruthenium-Alkynyl-Functionalized Oligo(phenylenevinylene) Stars

Article abstract of DOI:10.1002/cplu.201500220

The syntheses of trans-[Ru(C≡CC₆H₄-4-CHO)(C≡CC₆H₄-4-R)(dppe)₂] (R=H (9a), NO₂ (9b), CHO (9c), C≡CC₆H₃-3,5-Et₂ (9d), (E)-CHCHC₆H₄-4-tBu (9e); dppe=1,2-bis(diphenylphosphino)ethane), trans-[Ru(C≡CC₆H₄-4-R)Cl(dppe)₂] (R=C≡CC₆H₃-3,5-Et₂ (11a), (E)-CHCHC₆H₄-4-tBu (11b), (E)-CHCHC₆H₄-4-NO₂ (11c)), 1,2,4,5-{trans-[(dppe)₂(RC₆H₄C≡C)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₄C₆H₂ (R=H (14a), C≡CC₆H₃-3,5-Et₂ (14b), (E)-CHCHC₆H₄-4-tBu (14c)), 1-I-3,5-{trans-[(L₂)₂(R)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₂C₆H₃ (L₂=1,1-bis(diphenylphosphino)methane (dppm)), R=Cl (15a); L₂=dppe, R=C≡CPh (15b), R=C≡CC₆H₄-4-NO₂ (15c)), 1-Me₃SiC≡C-3,5-{trans-[(L₂)₂(R)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₂C₆H₃ (L₂=dppm, R=Cl (16a); L₂=dppe, R=C≡CPh (16b)), 1-HC≡C-3,5-{trans-[(dppe)₂(R)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₂C₆H₃ (R=Cl (17a), R=C≡CPh (17b)), and 1,3,5-{trans-[(dppe)₂(3,5-R₂-C₆H₃C≡C)Ru{C≡CC₆H₄-4-(E)-CHCH}]}₃C₆H₃ (R=(E)-CHCHC₆H₄-4-C≡C-trans-[Ru(C≡CPh)(dppe)₂] (18)) are reported together with those of the precursor alkynes 1-RC≡C-3,5-Et₂C₆H₃ (R=SiMe₃ (2), H (3), C₆H₄-4-C≡CSiMe₃ (5), C₆H₄-4-C≡CH (6)). The identities of 9c, 9d, 9e, 11a, and trans-[Ru{C≡CC₆H₄-4-(E)-CHCHC₆H₄-4-tBu}₂(dppe)₂] (12 and 12′) were confirmed by single-crystal X-ray diffraction studies. The electrochemical properties of 9a-e, 11a-b, 14a-c, 15a-c, 16b, 17a, 17b, and 18 were assessed by cyclic voltammetry; the studies reveal that potentials for the fully/quasi-reversible metal-centered oxidation processes decrease upon introduction of solubilizing alkyl substituents and increase upon increasing acceptor substituent strength; other structural variations have little impact. UV/Vis-NIR spectroscopic studies on these complexes reveal lowest-energy metal-ligand charge transfer (MLCT) bands that redshift upon increasing the acceptor substituent strength, blueshift on alkyl incorporation, and gain in intensity on progression from linear to star complexes. Low-temperature UV/Vis-NIR spectroelectrochemical studies of 14a-c show the appearance of an intense low-energy band at 7400-7900cm^-1 that is redshifted upon π-system lengthening and alkyl substituent incorporation. The cubic nonlinear optical properties of 9d, 9e, 14a-c, 15a-c, 16b, 17a,b, and 18 were assayed by femtosecond Z-scan studies at benchmark wavelengths (750 and 800nm) in the near-IR region, with nonlinearity increasing upon nitro incorporation; the values for the E-ene-linked dendrimers in these studies are much larger than yne-linked analogues. Compounds 9d, 9e, 14a-c, and 18 were further examined by broad-spectral-range femtosecond Z-scan studies; the cruciform complexes have appreciable multiphoton absorption cross-sections, with maximal values close to two and three times the wavelength of the linear optical absorption maxima. Super stars: (4-Formylphenylethynyl)ruthenium complexes (see figure) are shown to undergo "chemistry-on-complex" Horner-Wadsworth-Emmons coupling to afford a range of tri- and tetraruthenium-functionalized star molecules and a nonaruthenium dendrimer. The products are nonlinear optical (NLO)-active, with linear optical properties that are redox-switchable.

Full text of DOI:10.1002/cplu.201500220

DOI: 10.1002/cplu.201500220
Full Papers
Syntheses, Electrochemical, Linear Optical, and Cubic
Nonlinear Optical Properties of Ruthenium–Alkynyl-
Functionalized Oligo(phenylenevinylene) Stars**
Zhiwei Chen,[a] Christopher J. Jeffery,[b] Mahbod Morshedi,[b] Graeme J. Moxey,[b]
Adam Barlow,[b] Xinwei Yang,[b] Bandar A. Babgi,[b, c] Gulliver T. Dalton,[b] Michael D. Randles,[b]
Matthew K. Smith,[b] Chi Zhang,[a] Marek Samoc,[d, e] Marie P. Cifuentes,[a, b] and
Mark G. Humphrey*[a, b]
The syntheses of trans-[Ru(CꢀCC6H4-4-CHO)(CꢀCC6H4-4-
R)(dppe)2] (R=H (9a), NO2 (9b), CHO (9c), CꢀCC6H3-3,5-Et2
(9d), (E)-CH=CHC6H4-4-tBu (9e); dppe=1,2-bis(diphenylphos-
versible metal-centered oxidation processes decrease upon in-
troduction of solubilizing alkyl substituents and increase upon
increasing acceptor substituent strength; other structural varia-
tions have little impact. UV/Vis-NIR spectroscopic studies on
phino)ethane),
trans-[Ru(CꢀCC6H4-4-R)Cl(dppe)2]
(R=Cꢀ
CC6H3-3,5-Et2 (11 a), (E)-CH=CHC6H4-4-tBu (11 b), (E)-CH= these complexes reveal lowest-energy metal–ligand charge
CHC6H4-4-NO2 (11 c)), 1,2,4,5-{trans-[(dppe)2(RC6H4CꢀC)Ru{Cꢀ
CC6H4-4-(E)-CH=CH}]}4C6H2 (R=H (14a), CꢀCC6H3-3,5-Et2 (14b),
(E)-CH=CHC6H4-4-tBu (14c)), 1-I-3,5-{trans-[(L2)2(R)Ru{CꢀCC6H4-
4-(E)-CH=CH}]}2C6H3 (L2 =1,1-bis(diphenylphosphino)methane
(dppm)), R=Cl (15a); L2 =dppe, R=CꢀCPh (15b), R=Cꢀ
transfer (MLCT) bands that redshift upon increasing the accept-
or substituent strength, blueshift on alkyl incorporation, and
gain in intensity on progression from linear to star complexes.
Low-temperature UV/Vis-NIR spectroelectrochemical studies of
14a–c show the appearance of an intense low-energy band at
7400–7900 cmÀ1 that is redshifted upon p-system lengthening
and alkyl substituent incorporation. The cubic nonlinear optical
properties of 9d, 9e, 14a–c, 15a–c, 16b, 17a,b, and 18 were
assayed by femtosecond Z-scan studies at benchmark wave-
lengths (750 and 800 nm) in the near-IR region, with nonlinear-
ity increasing upon nitro incorporation; the values for the E-
ene-linked dendrimers in these studies are much larger than
yne-linked analogues. Compounds 9d, 9e, 14a–c, and 18
were further examined by broad-spectral-range femtosecond
Z-scan studies; the cruciform complexes have appreciable mul-
tiphoton absorption cross-sections, with maximal values close
to two and three times the wavelength of the linear optical ab-
sorption maxima.
CC6H4-4-NO2
(15c)),
1-Me3SiCꢀC-3,5-{trans-[(L2)2(R)Ru{Cꢀ
CC6H4-4-(E)-CH=CH}]}2C6H3 (L2 =dppm, R=Cl (16a); L2 =dppe,
R=CꢀCPh (16b)), 1-HCꢀC-3,5-{trans-[(dppe)2(R)Ru{CꢀCC6H4-
4-(E)-CH=CH}]}2C6H3 (R=Cl (17a), R=CꢀCPh (17b)), and 1,3,5-
{trans-[(dppe)2(3,5-R2-C6H3CꢀC)Ru{CꢀCC6H4-4-(E)-CH=
CH}]}3C6H3
(R=(E)-CH=CHC6H4-4-CꢀC-trans-[Ru(Cꢀ
CPh)(dppe)2] (18)) are reported together with those of the pre-
cursor alkynes 1-RCꢀC-3,5-Et2C6H3 (R=SiMe3 (2), H (3), C6H4-4-
CꢀCSiMe3 (5), C6H4-4-CꢀCH (6)). The identities of 9c, 9d, 9e,
11 a, and trans-[Ru{CꢀCC6H4-4-(E)-CH=CHC6H4-4-tBu}2(dppe)2]
(12 and 12’) were confirmed by single-crystal X-ray diffraction
studies. The electrochemical properties of 9a–e, 11 a–b, 14a–c,
15a–c, 16b, 17a, 17b, and 18 were assessed by cyclic voltam-
metry; the studies reveal that potentials for the fully/quasi-re-
Introduction
The nonlinear optical (NLO) properties of molecular materials
have attracted significant interest.[1] Whereas the majority of
studies have focused on purely organic molecules, recent com-
parisons of NLO parameters have shown that organometallics
[a] Z. Chen, Prof. C. Zhang, Prof. M. P. Cifuentes, Prof. M. G. Humphrey
School of Chemical and Material Engineering, Jiangnan University
Wuxi 214122, Jiangsu Province (P. R. China)
[e] Prof. M. Samoc
Present address: Advanced Materials Engineering and Modeling Group
Faculty of Chemistry, Wroclaw University of Technology
Wroclaw 50-370 (Poland)
E-mail: mark.humphrey@jiangnan.edu.cn
[b] C. J. Jeffery, Dr. M. Morshedi, Dr. G. J. Moxey, Dr. A. Barlow, X. Yang,
Dr. B. A. Babgi, Dr. G. T. Dalton, Dr. M. D. Randles, Dr. M. K. Smith,
Prof. M. P. Cifuentes, Prof. M. G. Humphrey
[**] Organometallic Complexes for Nonlinear Optics. Part 54. Part 53: H. J.
Zhao, P. V. Simpson, A. Barlow, G. J. Moxey, M. Morshedi, N. Roy, R. Philip, C.
Zhang, M. P. Cifuentes and M. G. Humphrey, Chem. Eur. J. 2009, DOI:
10.1002/chem.201500951.
Research School of Chemistry
Australian National University, Canberra, ACT 2601 (Australia)
E-mail: mark.humphrey@anu.edu.au
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201500220. It contains complete details of
the syntheses and characterization of compounds 1–5, 7, and 8, and
complexes 9a–e, 10, 11 a–c, 13a, 14a–c, 15a–c, 16a, 16b, 17a, 17b,
and 18; crystallographic studies of 9c–e, 11 a, 12’, and 12’’; electrochem-
ical studies of all new complexes; spectroelectrochemical studies of
14a–14c; single-wavelength nonlinear optical studies of 15a–c, 16b,
17a, 17b, and 18; and broad-spectral-range nonlinear optical studies of
9d, 9e, 14a–c, and 18.
[c] Dr. B. A. Babgi
Permanent address: Department of Chemistry, Faculty of Science
King Abdulaziz University, P.O. Box 80203, Jeddah 21589 (Saudi Arabia)
[d] Prof. M. Samoc
Research School of Physics and Engineering, Australian National University
Canberra, ACT 2601 (Australia)
Part of a Special Issue to celebrate Singapore’s Golden Jubilee. To view
the complete issue, visit: http://dx.doi.org/10.1002/cplu.v80.8.
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can outperform organics, even when the data are scaled for
molecular weights, molecular volumes, numbers of electrons
strongly participating in polarization, or even costs of produc-
tion.[2] The results of these performance comparisons, coupled
with the additional functionality inherent in metal complexes
(e.g., a diversity of geometries, performance tuning by co-
ligand modification, and the possibility of switching by reversi-
ble metal-centered oxidation) have helped drive the continu-
ing interest in organometallics as potential NLO materials,[3]
and to maintain them as a genuine alternative to organics.
One structure–NLO property relationship that was devel-
oped in early studies of purely organic molecules was the ob-
servation of an increase in NLO merit on replacing an yne link-
age in a p-bridging unit with an E-ene linkage, an outcome as-
cribed to orbital energy mismatch of p orbitals of sp-hybrid-
ized acetylenic carbon atoms with the p orbitals of sp2-hybrid-
ized phenyl carbon atoms;[4] this effect presumably acts in
combination with the increased propensity for oligo(phenyle-
neethynylene) (OPE) units to rotate out of p-system coplanarity
when compared to oligo(phenylenevinylene) (OPV) analogues,
thereby reducing efficient p delocalization. Metal alkynyl com-
plexes were contemporaneously shown to function as efficient
NLO materials,[5] and they continue to provoke interest, but de-
spite the favorable characteristics of E-ene-linked species, the
vast majority of studies of the NLO properties of metal–alkynyl
complexes have considered those with an OPE-based rather
than OPV-based p bridge. We report herein studies directed at
addressing this deficiency, including the syntheses of metal–al-
kynyl complexes with OPV-based bridges by a methodology
that employs an unusual “chemistry-on-complex” approach at
a pendant formyl group, comprehensive evaluation of the elec-
trochemical and spectroscopic properties of the new com-
plexes, structural and spectroelectrochemical studies of select-
ed examples, and an evaluation of their cubic NLO parameters;
some of these results have
plexes that bear two bidentate diphosphines were found to
provide a favorable balance of reactivity and NLO efficiency,[9]
so the studies summarized below concentrated on this combi-
nation.
Several ruthenium complexes that bear phenylenevinylene-
containing alkynyl ligands have been reported previously,[9b,10]
but all were prepared by treatment of a (chloro)ruthenium pre-
cursor with a preformed E-ene-bearing alkyne. To enhance flex-
ibility in the synthesis of such complexes, in the chemistry de-
scribed below we explored the possibility of using preformed
(4-formylphenylethynyl)ruthenium complexes in Horner–Wads-
worth–Emmons coupling with phosphonate esters. One further
point guided the synthetic studies. Alkynyl ligands with multi-
ple phenyleneethynylene units have been shown to afford
complexes with significant NLO properties,[9c,10d,11] but these
extended p systems are often accompanied by reduced solu-
bility of the complexes in common organic solvents. With this
in mind, the syntheses targeted bis(alkynyl)ruthenium com-
plexes that contain solubilizing alkyl groups on the terminal
aryl ring.
Synthesis of acetylenes
Two new alkynes that bear solubilizing alkyl groups were tar-
geted for eventual attachment at the periphery of the rutheni-
um-containing OPE stars. Thus, 1,3-diethyl-5-{(4-ethynylpheny-
l)ethynyl}benzene (5) was prepared by means of successive So-
nogashira coupling[12]/deprotection reactions in four steps
(Scheme 1). 2,6-Diethyl-4-iodobenzene (1) was readily obtained
by means of a standard diazotization reaction of 2,6-diethyl-4-
iodoaniline[13] with sodium nitrite and fluoroboric acid that af-
forded 1-iodo-3,5-diethylphenyldiazonium fluoroborate, and
a subsequent in situ treatment with sodium methoxide in
methanol. Sonogashira coupling of 1 with trimethylsilylacetyle-
been published in a preliminary
form.[6]
Results and Discussion
Previous studies with metal–al-
kynyl complexes have estab-
lished several outcomes that in-
fluenced the research described
herein. NLO activity has been
shown to increase upon pro-
ceeding from 14-electron com-
plexes to 18-electron com-
plexes,[7] and from 3d metals to
4d and 5d metals,[8] thereby fo-
cusing subsequent research ef-
forts on 18-valence electron
ruthenium and osmium alkynyl
complexes. Although they are
less efficient in an absolute
sense than their osmium conge-
ners, ruthenium–alkynyl com-
Scheme 1. Syntheses of acetylenes 5 and 8.
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ne using [PdCl2(PPh3)2]/CuI as catalysts afforded 3,5-diethyl-1-
(trimethylsilylethynyl)benzene (2) in good yield, desilylation of
which under basic conditions gave 1,3-diethyl-5-ethynylben-
zene (3). A second Sonogashira coupling that used 1-iodo-4-
(trimethylsilylethynyl)benzene[14] afforded the p-extended de-
rivative 4-(3,5-Et2C6H3CꢀC)C6H4CꢀCSiMe3 (4), which was then
treated with KOH to give 1-(4-HCꢀCC6H4CꢀC)-3,5-Et2C6H3 (5)
in excellent yield. The new compounds were obtained as oils
and characterized using IR, proton and carbon NMR spectros-
copy, and EI and high-resolution mass spectrometry. A down-
field shift in the NMR spectroscopic signal for the proton para
to the iodo/ethynyl is seen on proceeding from 1 and 2 (d=
6.99 ppm) to 3 and 4 (d=7.06 ppm), with a similar downfield
shift in the acetylenic proton being seen upon proceeding
from 3 (d=3.01 ppm) to 5 (d=3.16 ppm). The stilbenylacety-
lene 8 was obtained from a trans-halogenation of (E)-1-(4-tert-
butylphenyl)-2-(4-bromophenyl)ethene[15] with butyl lithium/
iodine that gave the iodo derivative (6) (Scheme 1). Compound
6 was then coupled to trimethylsilylacetylene under Sonoga-
shira conditions to give (E)-1-(4-trimethylsilylethynylphenyl)-2-
(4-tert-butylphenyl)ethene (7), with deprotection using potassi-
um hydroxide giving (E)-1-(4-tert-butylphenyl)-2-(4-ethynylphe-
monitored by NMR spectroscopy, with the phosphorus reso-
nance shifting from around d=49 ppm for the mono(alkynyl)
complexes to d=54.1 (9a) and 53.7 ppm (9b) upon formation
of the bis(alkynyl) derivatives, and the proton spectrum show-
ing an upfield shift of the aldehyde proton from d=9.99 to
9.81 ppm on formation of 9a, and a corresponding resonance
appearing at d=9.86 ppm for complex 9b. The order of intro-
duction of the alkynyl ligands is crucial, with the formation of
9b by treatment of 4-nitrophenylacetylene[12] with trans-[Ru(4-
CꢀCC6H4CHO)Cl(dppe)2] under similar conditions being at-
tempted without success. The bis(4-formylarylalkynyl) complex
9c was prepared from cis-[RuCl2(dppe)2][17] by using an excess
amount of acetylene in dichloromethane heated under reflux
conditions, the product being readily identified by the NMR
spectroscopic signals at d=53.6 ppm for the equivalent phos-
phorus centers and at d=9.87 ppm for the aldehyde protons.
Ruthenium–alkynyl complex formation can frequently be fa-
cilitated by the use of the preformed five-coordinate
[RuCl(dppe)2]+ cation.[18] In the present studies, the five-coordi-
nate complex cation as its hexafluorophosphate salt [RuCl(dp-
pe)2]PF6 (10) was conveniently prepared in over 95% yield di-
rectly from cis-[RuCl2(dppe)2][17] by using NaPF6 in dichlorome-
thane. Reaction of 10 with 5 followed by treatment with NEt3
1
nyl)ethene (8). Compounds 6–8 were characterized by IR, H
and 13C NMR spectroscopy, and EI and high-resolution ESI mass
spectrometry.
afforded
trans-[Ru(CꢀCC6H4-4-CꢀCC6H3-3,5-Et2)Cl(dppe)2]
(11 a) in 56% yield (Scheme 3). Formation of the unsymmetrical
bis(alkynyl) complex trans-[Ru(CꢀCC6H4-4-CHO)(CꢀCC6H4-4-
CꢀCC6H3-3,5-Et2)(dppe)2] (9d) was achieved in 46% yield after
prolonged stirring of 4-ethynylbenzaldehyde[19] with 11 a. The
transformation from the five-coordinate cationic complex to
the mono- and bis-alkynyl complexes was readily confirmed
through the 31P NMR spectroscopic resonances for the dppe li-
gands, with the two triplets observed for 10 (d=55.9,
83.7 ppm) being replaced by characteristic singlets at d=49.2
(11 a) and 53.3 ppm (9d). A similar sequence of reactions using
[RuCl(dppe)2]PF6 (10) with 8 afforded trans-[Ru{CꢀCC6H4-4-(E)-
CH=CHC6H4tBu-4}Cl(dppe)2] (11 b), together with trace
amounts of the symmetric bis(alkynyl) trans-[Ru{CꢀCC6H4-4-
(E)-CH=CHC6H4tBu-4}2(dppe)2] (12), and then trans-[Ru(Cꢀ
Synthesis of {4-(formylphenyl)alkynyl}ruthenium complexes
The obvious complexes from which to attempt on-complex
Horner–Wadsworth–Emmons coupling—namely, trans-[Ru(Cꢀ
CC6H4-4-CHO)Cl(L2)2]
(L2 =1,2-bis(diphenylphosphino)ethane
(dppe); 1,1-bis(diphenylphosphino)methane (dppm))—have
been reported previously[9c] and were therefore employed in
the present studies, but we also targeted several new rutheni-
um–alkynyl complexes in which the trans-chloro ligand was re-
placed with alkynyl ligands to enhance stability. The prepara-
tion of the 4-(formylphenyl)alkynyl complexes 9a and 9b em-
ployed established procedures for the syntheses of rutheni-
um–alkynyl complexes, with the treatment of trans-[Ru(4-Cꢀ
CC6H4R)Cl(dppe)2] (R=CHO,[9c] NO2[16]) with the required termi-
nal acetylene and ammonium or sodium hexafluorophosphate
under basic conditions affording ruthenium bis(alkynyl) com-
plexes in acceptable yields (Scheme 2). The reactions can be
CC6H4-4-CHO){CꢀCC6H4-4-(E)-CH=CHC6H4tBu-4}(dppe)2]
(9e)
following the reaction of 11 b with 4-HCꢀCC6H4CHO. All com-
1
plexes were characterized by IR, H and 13C NMR spectroscopy,
and EI and high-resolution ESI mass spectrometry. As expected,
the remote alkyl ligand variations have little effect on the
1H NMR spectroscopic aldehyde
proton resonance (d=9.81–
9.89 ppm for 9a–e).
Extension of the classical ste-
reospecific Horner–Wadsworth–
Emmons C=C bond formation
reaction to metal-containing (4-
formylphenyl)ethynyl complexes
was then explored as a new
route to metal-ethynyl-function-
alized stilbenes. Our initial at-
tempt examined the synthesis
Scheme 2. Syntheses of (4-formylphenyl)ethynylruthenium complexes 9a–9c. [Ru]=trans-[Ru(k2-dppe)2]=trans-
[Ru(k2-PPh2CH2CH2PPh2)2].
of the previously reported com-
plex trans-[Ru{CꢀCC6H4-4-(E)-
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CH=CHC6H4-4-NO2}Cl(dppm)2][9b]
(Scheme 4). trans-[Ru(CꢀCC6H4-
4-CHO)Cl(dppm)2][9c] was treated
with sodium hydride and 4-
O2NC6H4CH2P(O)(OEt)2 in tetra-
hydrofuran (THF) for five hours,
the latter prepared by means of
an Arbuzov reaction of 1-(bro-
momethyl)-4-nitrobenzene with
triethyl phosphite to give the
stereospecifically coupled (4-{4-
nitrophenyl-(E)-ethenyl}phenyl)-
ethynyl product (11 c) in a great-
er overall yield than the pub-
lished procedure, which in-
volves coupling of the pre-
formed (E)-1-(4-nitrophenyl)-2-
(4-ethynylphenyl)ethene to the
ruthenium center. A similar cou-
pling of the tri-substituted ben-
Scheme 3. Syntheses of (4-formylphenylethynyl)ruthenium complexes 9d and 9e. [Ru]=trans-[Ru(k2-
dppe)2]=trans-[Ru(k2-PPh2CH2CH2PPh2)2].
zylic
phosphonate
1,3,5-
[20]
C6H3{CH2PO(OEt)2}3
and an
excess amount of trans-[Ru(Cꢀ
CC6H4-4-CHO)Cl(dppe)2] over
40 hours afforded 13a in ac-
ceptable yield, thereby confirm-
ing the applicability of this
chemistry-on-complex method
for the multisite incorporation
of E-ene linkages into rutheni-
um–alkynyl complexes.
Having demonstrated this
methodology on known com-
plexes, we then extended it to
the formation of new E-ene-link-
Scheme 4. Syntheses of 11 c and 13a by Horner–Wadsworth–Emmons reactions. [Ru]=trans-[Ru(k2-dppe)2].
age-containing
ruthenium–al-
kynyl cruciform complexes (Scheme 5). Treatment of 1,2,4,5-
[20]
C6H2{CH2P(O)(OEt)2}4 with the (4-formylphenylethynyl)ruthe-
nium complexes 9a, 9d, and 9e and sodium hydride in 1,2-di-
methoxyethane for 24–48 hours afforded the tetra-ruthenium
star complexes 14a–c in good yields. Formation of the bis(al-
kynyl) complex environments was confirmed by monitoring
the 31P NMR spectra (appearance of resonances at about d=
53 ppm).
The methodology was then employed in the construction of
a ruthenium-alkynyl-containing stilbenyl dendrimer. The target
necessitated the construction of a 1A-3,5-B2-functionalized
arene to serve as the branching point in the dendrimer arms.
In the present studies, introduction of a branching point re-
quired the use of a 3,5-substituted iodobenzene, for which
treatment with trimethylsilylacetylene under Sonogashira con-
ditions followed by removal of the silyl protecting group af-
forded the required 3,5-substituted “wedge” complexes
(Scheme 6). Attempts to form the dppm-ligated ruthenium
wedge complex 15a by treatment of the bis(phosphonate
ester) 1-I-C6H3-3,5-{CH2PO(OEt)2}2 with trans-[Ru(CꢀCC6H4-4-
CHO)Cl(dppm)2] resulted in low yields of the desired product,
Scheme 5. Syntheses of ene-linkage-containing ruthenium–alkynyl cruciform
complexes 14a–c. [Ru]=trans-[Ru(k2-dppe)2].
and attempts to optimize the reaction by changing the base
(sodium ethoxide or potassium tert-butoxide), reaction temper-
ature, and reaction times led to decomposition of the (4-for-
mylphenyl)alkynyl ruthenium complex. In contrast, synthesis of
the dppe-ligated ruthenium wedge complex 15b from 9a was
achieved in 59% yield by using a threefold excess amount of
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NMR spectroscopy and mass
spectrometry.
The
n(CꢀC)
bands in the IR spectra show
a shift to lower energy on alky-
nylation at the iodo substituent
(15a, 15b at 2060 and
2072 cmÀ1, respectively), with
the latter remaining invariant
between the trimethylsilyl- and
terminal-alkyne complexes (16b
at 2056 cmÀ1 and 17b at
2057 cmÀ1). All show a single
phosphorus NMR spectroscopic
signal, thus confirming persis-
tence of the trans disposition of
the diphosphine ligands (d=
53.9–54.4 ppm for the dppe
complexes, and d=À5.9 ppm
for the dppm examples). A mo-
lecular ion is observed for 16b
at 2422 mass units in both the
HR ESI and FAB mass spectra,
displaying the expected isotope
pattern. Fragmentation occurs
by the loss of the trimethylsilyl
Scheme 6. Syntheses of ene-linkage-containing “wedge” complexes 17a and 17b. [Ru]=trans-[Ru{k2-
PPh2(CH2)nPh2}2] [n=1 (dppm), 2 (dppe)].
potassium tert-butoxide in THF heated under reflux conditions,
although the same conditions afforded only a small amount of
the nitro-substituted analogue 15c from 9b. In each case, the
1H NMR spectrum shows the disappearance of the aldehyde
peak owing to the (4-formylphenyl)ethynyl ligand of the start-
ing complexes at d=9.8–10.0 ppm. In 15c, the protons ortho
to the nitro substituent resonate downfield of the usual
phenyl region at d=7.95 ppm. The disappearance of the reso-
nances owing to the phosphonate ester at d=25.9 ppm in the
31P NMR spectra is consistent with the formation of the wedge
complexes. The IR spectra of these wedge compounds show
characteristic alkynyl n(CꢀC) bands at 2059, 2072, and
2046 cmÀ1 for 15a–c, respectively. The alkynyl n(CꢀC) stretch-
ing frequency remains unchanged in 15a relative to that of
the starting complex; in contrast, the loss of the electron-with-
drawing aldehyde group in the formation of 15b sees the al-
kynyl n(CꢀC) band shift to 2072 from 2046 cmÀ1 in 9a. In
15c, the (4-nitrophenyl)ethynyl ligand affords a band at
2046 cmÀ1, as is found with 9b. The ESI mass spectrum of 15a
reveals exchange of chloro ligand with solvent (MeCN), a peak
being seen at 2270 mass units ([MÀCl+MeCN]+), as well as
a peak that corresponds to the loss of chloride at 2230 mass
units ([MÀCl]+). The FAB mass spectra of 15b and 15c display
molecular ions at 2453 and 2543 mass units, respectively, with
15b fragmenting by loss of the phenylethynyl ligand and iodo
group, and 15c through loss of the nitro substituent.
group or the phenylethynyl ligand. No molecular ion is ob-
served in the ESI mass spectrum of 17a, with a fragment
owing to the loss of both chloro ligands and the ethynyl
group being seen at 2077 mass units. The molecular ion is ob-
served in the FAB mass spectrum of 17b (2350 mass units)
with the characteristic loss of a phenylethynyl ligand.
The ethynyl-substituted “wedge” complex 17b was then
coupled to the previously reported triruthenium complex
13a[10a,b] to form the nonaruthenium dendrimer 18 (Scheme 7).
The reaction was monitored by 31P NMR spectroscopy, with the
disappearance of the resonance at d=50.1 ppm owing to the
mono(alkynyl) ruthenium environments at 13a and replace-
ment with a signal at d=54.2 ppm owing to the resulting bi-
s(alkynyl) ruthenium centers of 18.
Structural studies
The compositions of compounds 9c, 9d, 9e, 11 a, 12’, and 12’’
were confirmed by single-crystal X-ray diffraction studies.[21]
The molecular structures are displayed in Figures S1 (9c),
1 (9d), 2 (9e), S2 (11 a), S3 (12’), S4, and S5 (12’’) in the Sup-
porting Information, and selected bond length and angle data
are collected in the relevant figure captions. The bond lengths
and angles about the key C/Cl-Ru-C1-C2-C3 units in structures
9c, 9d, 9e, and 11 a are not unusual (see, for example,
Refs. [11], [22]); they fall within the ranges of the literature
precedents for arylalkynylbis(diphosphine)ruthenium com-
plexes. For example, the RuÀC1 vectors for the mono(alkynyl)-
(chloro)bis(diphosphine)ruthenium complexes (2.037(8) (11 a))
and the bis(alkynyl)bis(diphosphine)ruthenium complexes
(2.074(4) (9c), 2.070(10), 2.063(9) (9d), 2.065(7), 2.076(6) (9e))
are not unexpected. In contrast, RuÀC1 parameters for 12’
Sonogashira coupling of iodo complexes 15a and 15b with
an excess amount of trimethylsilylacetylene afforded ethynyl
derivatives 16a and 16b, respectively, with subsequent depro-
tection with potassium carbonate affording moderate yields of
the desired alkyne-terminated wedge complexes 17a and 17b
(Scheme 6). All wedge complexes were characterized by IR and
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Compound 11 a, which contains
solubilizing ethyl groups on the
terminal phenyl ring, shows
a fully reversible oxidation pro-
cess at 0.51 V, which is some-
what lower than that required
to oxidize its non-alkylated ana-
logue 11 d (0.55 V),[23] and
which is presumably due to the
slight electron-releasing effect
of the alkyl groups. Replace-
ment of the yne linkage furthest
from the metal center with an
E-ene-linked tert-butyl-substitut-
ed phenyl group, in proceeding
from 11 d to 11 b (0.56 V), has
little effect (Figures 1 and 2) on
the potential for the oxidation
process. The similar electronic
environment of the metal atom
in these related complexes has
Scheme 7. Synthesis of the dendritic complex 18. [Ru]=trans-[Ru(k2-dppe)2]=trans-[Ru(k2-PPh2CH2CH2PPh2)2].
Figure 2. Molecular structure of trans-[Ru(CꢀCC6H4-4-CHO){CꢀCC6H4-4-(E)-
CH=CHC6H4-4-tBu}(dppe)2] (9e), with thermal ellipsoids set at the 30% prob-
ability level. Hydrogen atoms and lattice dichloromethane molecules have
been omitted for clarity. Selected bond lengths [Š] and angles [8]: Ru1ÀC1
2.065(7), Ru1ÀC21 2.076(6), Ru1ÀP1 2.3667(15), Ru1ÀP2 2.3596(15), Ru1ÀP3
2.3532(15), Ru1ÀP4 2.3659(15); C1-Ru1-C21 179.7(3), C1-Ru1-P1 98.28(17),
C1-Ru1-P2 91.81(17), C1-Ru1-P3 87.83(17), C1-Ru1-P4 81.30(17), C21-Ru1-P1
81.44(17), C21-Ru1-P2 88.33(17), C21-Ru1-P3 92.03(16), C21-Ru1-P4 98.98(17),
P1-Ru1-P2 82.56(5), P1-Ru1-P3 97.66(5), P1-Ru1-P4 179.58(6), P2-Ru1-P3
179.60(7), P2-Ru1-P4 97.42(5), P3-Ru1-P4 82.36(5).
Figure 1. Molecular structure of trans-[Ru(CꢀCC6H4-4-CHO)(CꢀCC6H4-4-
CꢀCC6H3-3,5-Et2)(dppe)2] (9d) with thermal ellipsoids set at the 30% proba-
bility level. Hydrogen atoms have been omitted for clarity. Selected bond
lengths [Š] and angles [8]: Ru1ÀC1 2.070(10), Ru1ÀC21 2.063(9), Ru1ÀP1
2.373(2), Ru1ÀP2 2.353(2), Ru1ÀP3 2.372(2), Ru1ÀP4 2.357(2); C1-Ru1-C21
178.9(4), C1-Ru1-P1 80.5(3), C1-Ru1-P2 89.5(2), C1-Ru1-P3 98.7(3), C1-Ru1-P4
90.7(2), C21-Ru1-P1 98.5(3), C21-Ru1-P2 90.8(2), C21-Ru1-P3 82.3(3), C21-Ru1-
P4 89.0(2), P1-Ru1-P2 81.63(8), P1-Ru1-P3 179.21(9), P1-Ru1-P4 98.50(8), P2-
Ru1-P3 98.45(8), P2-Ru1-P4 179.80(10), P3-Ru1-P4 81.43(8).
(2.108(5)) and 12’’ (2.149(4)) are both very long, which is possi-
bly an artefact of the substitutional disorder in these specific
structural studies.
been noted previously, with structural changes occurring re-
motely from the metal center having only minor effects on the
potentials for the oxidation processes.[10d] Replacement of the
chloro ligand by a 4-formylphenylethynyl group to form the
unsymmetrical bis(alkynyl) complexes trans-[Ru(CꢀCC6H4-4-
CHO)(CꢀCC6H4-4-R)(dppe)2] (9) results in an increase in poten-
tials for the metal-centered oxidation processes. For the (4-for-
mylphenylethynyl)(phenylethynyl) examples 9a–9c, E1/2 in-
creases with increasing electron-withdrawing character of the
phenylethynyl substituent, with R=H (9a, 0.61 V)<R=CHO
(9c, 0.72 V)<R=NO2 (9b, 0.75 V), as expected. Extension of
the p bridge of 9a (by means of a second phenylethynyl unit,
R=CꢀCC6H3-3,5-Et2, or a stilbenyl group R=(E)-CH=CHC6H4-4-
Cyclic voltammetry studies
Cyclic voltammetry (CV) data for the complexes prepared in
these studies are collected in Table 1. All complexes show an
oxidation process between 0.48 and 0.76 V that is assigned to
the RuII/III oxidation. Ratios for the forward and reverse current
peaks range from 0.8 to 1.0, and peak separations are equiva-
lent to (or in some cases up to 0.03 V more than) DE for the
ferrocene/ferrocenium internal standard, thus indicating that
these processes are fully or quasi-reversible. The potentials for
the oxidation processes vary in a fairly systematic fashion.
tBu) results in little change in E1/2
.
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chemical environment of the metal atom in each
case. The single oxidation process observed in the
sweep range (0 to 1.6 V) is due to the symmetry of
the molecules and the lack of interaction between
the metal centers. The dendritic complex 18 also
shows a single oxidation process (0.52 V), which is in
agreement with the pseudo-equivalent metal envi-
ronments seen in the 31P NMR spectroscopic data.
Complexes that contain the nitro group also show
an irreversible nitro-centered reduction process at
À1.14 (9b), À0.98 (11 c), and À1.33 V (15c).
Table 1. Cyclic voltammetric and linear optical data.[a]
[c]
Complex,[b]
R
E1/2 [V] [DE, (ipc/ipa)]
lmax [nm] (e [104 mÀ1 cmÀ1])
trans-[Ru](CꢀCC6H4-4-CHO)(CꢀCC6H4-4-R)
9a, H
0.61 [0.07, 0.9]
418 (2.3), 313 (1.9)
463 (2.3), 411 (sh, 2.0)
426 (4.0)
414 (5.2)
420 (6.1)
9b, NO2
9c, CHO
0.75 [0.07, 0.8]
0.72 [0.07, 0.8]
0.63 [0.07, 1]
0.58 [0.07, 1]
9d, CꢀCC6H3-3,5-Et2
9e, (E)-CH=CHC6H4-4-tBu
trans-[Ru](CꢀCC6H4-4-R)Cl
11 a, CꢀCC6H3-3,5-Et2
0.51 [0.09, 1]
0.56 [0.07, 1]
0.56 [1][9c]
380 (2.9)
400 (4.1)
489 (2.6)[d][9c]
11 b, (E)-CH=CHC6H4-4-tBu
11 c, (E)-CH=CHC6H4-4-NO2
[e]
Linear optical studies
[10b]
1,3,5-{trans-(R)[Ru](CꢀCC6H4-(E)-4-CH=CH)}3C6H3
13a, Cl
0.51 [0.9]
0.49 [1]
426 (8.7)[d]
The UV/Vis-NIR spectra for all complexes (Table 1)
show characteristic bands at lowest energy (absorp-
tion maxima in the range 380–490 nm (26300–
20400 cmÀ1)), assigned from DFT calculations on re-
lated ruthenium–alkynyl complexes as being due to
metal-to-ligand charge transfer (MLCT), specifically
Ru dyz!C2R in nature.[24] Nitro-substituted com-
plexes are transparent at frequencies below
22000 cmÀ1, whereas all other complexes are trans-
parent below 26000 cmÀ1. Alkyl introduction at the
distal aryl ring of trans-[Ru(CꢀCC6H4-4-Cꢀ
13b, CꢀCPh
421 (13.0)[d]
1,2,4,5-{trans-(RC6H4CꢀC)[Ru]{CꢀCC6H4-4-(E)-CH=CH}}4C6H2
14a, H
0.54 [0.11, 1]
0.55 [0.07, 1]
0.51 [0.09, 1]
421 (11.5), 325 (11.5)
395 (20.0), 297 (14.6)
441 (21.6), 315 (13.9)
14b, CꢀCC6H3-3,5-Et2
14c, (E)-CH=CHC6H4-4-tBu
1-I-3,5-{trans-(R)[Ru](CꢀCC6H4-4-(E)-CH=CH)}2C6H3
15a, Cl[e]
0.62 [0.10, 1]
0.53 [0.09, 1]
0.63 [0.06, 1]
406
15b, CꢀCPh
425 (6.6), 320 (6.7).
481 (3.2), 417 (3.7), 312 (3.9)
15c, CꢀCC6H4-4-NO2
1-Me3SiCꢀC-3,5-{trans-(R)[Ru](CꢀCC6H4-4-(E)-CH=CH)}2C6H3
16a, Cl[e]
CPh)Cl(dppe)2] (lmax 25800 cmÀ1 [22]
to give 11 a
)
411 (6.8)
421 (7.2), 319 (7.5)
16b, CꢀCPh
0.52 [0.16, 0.9]
(26300 cmÀ1, +500 cmÀ1) results in a slight hypso-
chromic shift in the MLCT band, whereas replace-
ment of the -yne linkage with an E-ene group and
introduction of a tert-butyl group to give 11 b
(25000 cmÀ1, À800 cmÀ1) produces a bathochromic
shift. The linear optical spectrum of the nitro-substi-
tuted E-ene-linked complex 11 c (20500 cmÀ1) con-
tains the lowest-energy transition, as expected.[9c]
Substitution of the chloro ligand with a phenyle-
thynyl unit upon proceeding from trans-[Ru(Cꢀ
CC6H4-4-CHO)Cl(dppe)2] (lmax =24200 cmÀ1)[9c] to
trans-[Ru(CꢀCC6H4-4-CHO)(CꢀCC6H4-4-R)(dppe)2]
1-HCꢀC-3,5-{trans-(R)[Ru](CꢀCC6H4-4-(E)-CH=CH)}2C6H3
17a, Cl[e]
17b, CꢀCPh
0.59 [0.11, 0.8]
0.56 [0.15, 0.8]
402, 317
417, 320
1,3,5-{trans-(3,5-R2C6H3CꢀC)[Ru]{CꢀCC6H4-4-(E)-CH=CH}]}3C6H3
18, {(E)-CH=CHC6H4-4-
CꢀC}[Ru](CꢀCPh)
0.52 [0.14, 0.8]
417, 322
[a] Measured in CH2Cl2. [b] [Ru]=trans-[Ru(dppe)2]. [c] Pt disc working, Pt wire auxili-
ary, and Ag/AgCl reference electrodes; ferrocene/ferrocenium redox couple at 0.56 V
(ipc/ipa 0.9 À1, DEp =0.07–0.09 V). [d] Measured in THF. [e] [Ru]=trans-[Ru(dppm)2].
Potentials of the oxidation processes for the cruciform com-
(9a–c) results in a decrease in the energy of the MLCT, with
plexes 1,2,4,5-(trans-[(dppe)2(RC6H4CꢀC)Ru{CꢀCC6H4-4-(E)-CH= the electron-withdrawing nature of the phenylethynyl substitu-
CH}])4C6H2 (14) are all found in the narrow range 0.51–0.55 V,
with the diethyl-substituted yne-linked complex (R=CꢀCC6H3-
3,5-Et2, 14b, 0.55 V) again requiring a higher potential than the
tert-butyl-substituted ene-linked example (R=(E)-CH=CHC6H4-
4-tBu, 14c, 0.51 V). The “wedge” complexes 15–17 are oxidized
at comparable potentials, with only a slight variation observed
on moving from the 1-iodo-substituted core 15b (0.53 V)
through the 1-trimethylsilylethynyl- (16b, 0.52 V) and ethynyl-
(17b, 0.56 V) analogues, and a slight decrease in the reversibili-
ty of the process at each step. Substitution with the electron-
withdrawing nitro group (15c, 0.63 V) affords a shift to higher
potentials, as expected. The chloro-terminated dppm-contain-
ing wedge complexes 15a (0.62 V) and 17a (0.59 V) require
a slightly higher potential for metal-centered oxidation than
their phenylethynyl-containing analogues 15b and 17b.
ent determining the size of the shift (R=H, 9a; 23900,
À300 cmÀ1)<CHO (9c; 23500, À700 cmÀ1)<R=NO2 (9b;
21600, À2600 cmÀ1). Similarly, a redshift is observed on
moving from (chloro)ruthenium–alkynyl complexes 11 a and
11 b to the corresponding (4-formylphenylethynyl) complexes
9d (24200, À2100 cmÀ1) and 9e (23800, À1200 cmÀ1).
Compared to the monometallic complexes (9 and 11), the
cruciform complexes 1,2,4,5-(trans-[(dppe)2(RC6H4CꢀC)Ru{Cꢀ
CC6H4-4-(E)-CH=CH}])4C6H2 (14) show a significant increase in
the extinction coefficient of the MLCT band, an effect noted
previously with trisubstituted benzene-cored analogues such
as the star complex 1,3,5-{trans-[(dppe)2(PhCꢀC)Ru(CꢀCC6H4-
4-CꢀC)]}3C6H3 (19b, e=11.6”104 mÀ1 cmÀ1).[25] In the present
studies, the increase in absorbance at lmax for 14b (R=Cꢀ
CC6H3-3,5-Et2, e=20.0”104 mÀ1 cmÀ1
) and 14c (R=(E)-CH=
The general invariance in the potentials for the metal-local-
ized oxidation process in these complexes reflects the similar
CHC6H4-4-tBu, 21.6”104 mÀ1 cmÀ1) is approximately twofold
greater than that of 14a (R=H, 11.5”104 mÀ1 cmÀ1), which con-
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tains the shorter terminal phenylethynyl ligand, and twofold
greater than the trimetallic complex 19b. Transition energies
are lower for the cruciform star complexes 14b (25300 cmÀ1
)
and 14c (22700 cmÀ1) than for the related mononuclear chloro
complexes 11 a (26300 cmÀ1) and 11 b (25000 cmÀ1) and,
again, the ene-linked complex 14b shows the lowest-energy
MLCT band. Relative to the trimetallic star complexes, that is,
the yne-linked 19b (24300 cmÀ1) and the E-ene-linked 13b
(23800 cmÀ1),[10b] the cruciform star complexes show a higher-
energy MLCT band.
Spectroelectrochemical studies
In situ oxidation of metal-containing complexes by using an
optically transparent thin-layer electrochemical (OTTLE) cell
offers a convenient method for accessing spectroscopic data
of the oxidized metal complexes and has been employed
widely with ruthenium–alkynyl complexes.[10d,23–24,26] Applica-
tion of potentials slightly higher than indicated by the cyclic
voltammetric data to dichloromethane solutions of the star
complexes 14a–c resulted in reversible conversion to the tetra-
cationic (formally RuIII ) complexes [14a–c]4+; isosbestic points
4
are observed in the transformations in each case (Figure 3, Fig-
ure S6 in the Supporting Information), and comparative UV/
Vis-NIR data have been collected in Table 2. All complexes
show the emergence of low-energy NIR absorption bands as-
signed to alkynyl-to-RuIII charge transfer,[24b] with the band for
14b (7600 cmÀ1), which contains the extended p-delocalizable
ligand, occurring at a slightly lower energy than that of 14a
(7900 cmÀ1); the E-ene-linked complex 14c shows the lowest-
energy band (7300 cmÀ1). These bands are significantly red-
shifted relative to those of the bis(alkynyl) complex trications
trans-[Ru(CꢀCPh)(CꢀCC6H4-4-CꢀCPh)(dppe)2]+
Figure 3. UV/Vis-NIR spectral progressions during oxidation of cruciform
complex 1,2,4,5-[trans-(dppe)2(RC6H4CꢀC)Ru{CꢀCC6H4-4-(E)-CH=CH}]4C6H2
(R=H, 14a) (top), and overlay of the spectra for [14a–c]4+ (R=CꢀCC6H3-
3,5-Et2 (14b), R=(E)-CH=CHC6H4-4-tBu (14c)) (bottom) in CH2Cl2.
(8400 cmÀ1)[24a] and 1,3,5-{trans-[(dppe)2(PhCꢀC)Ru(CꢀCC6H4-
4-CꢀC)}3C6H3]3+ (8400 cmÀ1).[24b] This large bathochromic
effect is likely due to the E-ene-linked cruciform nature of com-
plexes 14, rather than the increase in the number of metal
centers, because the tri-substituted nonaruthenium nonacation
[1,3,5-{trans-[3,5-{trans-[(dppe)2(PhCꢀC)Ru(CꢀCC6H4-4-Cꢀ
plexes.[24a] The absorption intensities for the lowest-energy
bands for the cruciform star complexes [14a–c]4+ are uniform-
ly higher than those observed for the related trimetallic star
complexes (e of about 12 versus 8 (”104 mÀ1 cmÀ1)), with the E-
ene-linked complex 14c showing the highest e value.
C)])2C6H3(CꢀC)(dppe)2Ru(CꢀCC6H4-4-CꢀC)]}3C6H3]9+
shows
the corresponding low-energy band at 8400 cmÀ1 (measured
in THF, eꢁ8”104 mÀ1 cmÀ1).[24b] Vibronic progressions evident
in the spectra of [14a–c]4+ at approximately 9000 cmÀ1 can be
attributed to n(CꢀC), as noted previously in similar com-
Cubic hyperpolarizability studies
The third-order optical nonlinearities of the cruciform, wedge,
and dendritic molecules 14a–c, 15a–c, 16a, 16b, 17a, 17b,
Table 2. UV/Vis-NIR data for cruciform complexes 1,2,4,5-{trans-[(dppe)2(RC6H4CꢀC)Ru{CꢀCC6H4-4-(E)-CH=CH}])4C6H2 in the resting (14a–c) and tetra-oxi-
dized ([14a–c]4+) forms.[a]
Complex, R
l [cmÀ1] (e)[b]
l [cmÀ1] (e)[b]
14a, H
22200 (11.5), 30700 (11.2)
[14a]4+
[14b]4+
7900 (11.6), 16600 (2.6, sh), 20900 (4.2), 26000 (7.9), 38300 (19.7)
14b, CꢀCC6H3-3,5-Et2
24900 (20.0), 32100 (13.4, sh),
37000 (15.8, sh)
7600 (11.9), 16300 (2.3, sh), 20800 (7.3), 21900 (8.1), 25500 (11.1),
32100 (14.9, sh), 38000 (20.3)
14c, (E)-CH=CHC6H4-4-tBu
[a] CH2Cl2. [b] 104 mÀ1 cmÀ1
31600 (14.4), 23700 (21.7)
[14c]4+
7400 (18.1), 9800 (3.3, sh), 16300 (2.9, sh), 20000 (9.2), 28300 (16.3),
30000 (14.6, sh), 38000 (19.9)
.
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and 18 were assessed by the Z-scan technique[27] at a bench-
mark wavelength of 750 nm, which was chosen because it cor-
responds to a region of optical transparency for all of these
complexes (reducing the effects of resonance enhancement),
and also because of its technological importance (it corre-
sponds to a wavelength region of enhanced transparency of
biological materials such as tissue); an NLO performance com-
parison in this wavelength region is therefore of interest. Meas-
urements were carried out in dichloromethane except for
those of 15b and 16b, which were measured in THF for solu-
bility reasons; the resultant data are collected in Table 3. At
this wavelength, all new complexes exhibit negative nonlinear
refraction and positive nonlinear absorption, which is consis-
tent with the presence of two-photon resonance effects. The
real components of the cubic nonlinearities in particular are
characterized in most cases by large error margins, thus ren-
dering development of structure–property relationships prob-
lematic. Nevertheless, some comparisons might usefully be
made. If we focus on the wedge complexes, replacing the bi-
dentate diphosphine co-ligand dppm by dppe at the rutheni-
um centers (proceeding from 15a to 15b or 17a to 17b) re-
sults in no change in the nonlinearity. The addition of the elec-
tron-withdrawing nitro substituent to the arylalkynyl ligand
(proceeding from 15b to 15c) results in a twofold increase in
the nonlinearity. While being mindful of variations that result
from a difference in measurement wavelength (750 versus
800 nm, illustrated by data for 9d–e and 14a–c at both wave-
lengths (Table S2 in the Supporting Information)), the nonli-
nearity jgj varies little between the two classes of wedge com-
plex (with yne- or E-ene-containing p bridges). As expected for
Table 3. Linear optical and cubic nonlinear optical data for selected complexes.[a,b]
Complex, R
lmax [nm]
greal [10À36 esu]
gimag [10À36 esu]
jgj [10À36 esu]
s2 [10À50 cm4 s]
1,3,5-(trans-(R)[Ru]{CꢀCC6H4-(E)-4-CH=CH})3C6H3 measured at 800 nm
13a[c], Cl[10b]
426
421
À4600Æ2000
À11 200Æ3000
4200Æ800
6200Æ2000
1000Æ200
2100Æ500
13b[c], CꢀCPh[10b]
8600Æ2000
14000Æ4000
1,3,5-(trans-(R)[Ru]{CꢀCC6H4-4-CꢀC})3C6H3 measured at 800 nm
19a[c], Cl[28]
414
411
À330Æ100
À600Æ200
2200Æ500
2900Æ500
2200Æ600
3000Æ600
530Æ120
700Æ120
19b[c], CꢀCPh[28]
1,2,4,5-(trans-(RC6H4CꢀC)[Ru]{CꢀCC6H4-4-(E)-CH=CH})4C6H2
14a, H
440
393
415
À38200Æ7790
À36000Æ10600
À36000Æ10600
7370Æ3100
14200Æ5800
14200Æ5800
38900Æ8380
38700Æ12000
38700Æ12000
2090Æ880
4020Æ1640
4020Æ1640
14b, CꢀCC6H3Et2-3,5
14c, (E)-CH=CHC6H4tBu-4
1-I-3,5-(trans-(R)[Ru]{CꢀCC6H4-4-(E)-CH=CH})2C6H3
15a, Cl[d]
406
425
481
À1800Æ490
À2200Æ260[e]
À4200Æ1500
300Æ70
220Æ60[e]
1500Æ420
1800Æ500
2200Æ260[d]
4500Æ1600
–
–
–
15b[c], CꢀCPh
15c, CꢀCC6H4NO2-4
1-Me3SiCꢀC-3,5-(trans-(R)[Ru]{CꢀCC6H4-4-(E)-CH=CH})2C6H3
16a, Cl[d]
411
421
À510Æ500
4700Æ1500
410Æ60
4700Æ2000
2100Æ150
1100Æ360
16b[c], CꢀCPh
À2100Æ140
-
1-HCꢀC-3,5-(trans-(R)[Ru]{CꢀCC6H4-4-(E)-CH=CH})2C6H
17a, Cl[d]
402
417
À1900Æ230
À1900Æ260
140Æ20
300Æ40
1900Æ230
1900Æ260
–
–
17b, CꢀCPh
1,3,5-(trans-(3,5-R2C6H3CꢀC)[Ru]{CꢀCC6H4-4-(E)-CH=CH})3C6H3
18, (E)-CH=CHC6H4-4-
CꢀC)[Ru](CꢀCPh)
417
À6700Æ770
1300Æ130
6800Æ780
–
1,3,5-{trans-(3,5-R2C6H3CꢀC)[Ru](CꢀCC6H4-4-CꢀC)}3C6H3[27] measured at 800 nm
20[c], (CꢀC)[Ru](CꢀCPh)
402
À5050Æ500
20100Æ2000
20700Æ2000
4800Æ500
[a] Measurements are referenced to the nonlinear refractive index of silica n2 =3”10À16 cm2 WÀ1. [b] Complexes measured in CH2Cl2 as solutions and at
750 nm with [Ru]=trans-Ru(dppe)2 unless otherwise specified. [c] Measured in THF as solutions. [d] [Ru]=trans-Ru(dppm)2.
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a relatively subtle change at these large molecules, replacing
the iodo substituent with either CꢀCSiMe3 or CꢀCH does not
significantly affect the nonlinearity (15b, 16b, 17b).
If we focus on the dendrimer complexes, proceeding from
the zero-generation yne-linked dendrimers (19a, 19b) to the
comparable E-ene-linked dendrimers (13a, 13b) results in
a marked increase in the nonlinear response. Although increas-
ing the dendrimer generation in moving from the zero- (19b)
to first-generation yne-linked dendrimer (20) results in an ap-
proximately tenfold increase in the nonlinearity, this effect is
not seen with E-ene-linked dendrimers (proceeding from 13b
to 18); in fact, the nonlinearity decreases by a factor of two.
This might be caused by an increasing loss of coplanarity in
moving to 18, or by the wavelength dependence of the re-
sponse (although the near coincidence of the linear optical ab-
sorption maxima suggests that this might not be a critical
factor). Finally, the observed nonlinearities of the cruciform
complexes at this benchmark wavelength are the largest from
the present study. The overall jgj values are dominated by
very large greal contributions: Whereas the energies of transi-
tions for complexes across the present study are similar, it is
noteworthy that the extinction coefficients for the lowest-
energy one-photon absorption bands for 14a–c are much
larger than those for the other complexes, and this might sug-
gest significantly enhanced resonance effects.
The impressive NLO coefficients at the aforementioned
benchmark wavelengths prompted a more detailed study of
selected examples over the relatively broad spectral range
500–1600 nm, with a particular focus on the cruciform star
complexes. Representative s2 and g spectral dependences
(those for 14a) are shown in Figure 4, with the remainder
being shown in Figures S7 (9d), S8 (9e), S9 (14b), S10 (14c),
and S11 (18) in the Supporting Information, whereas maximal
data are collected in Table S3 of the Supporting Information.
All complexes studied are multiphoton absorbers and show
positive imaginary components of third-order nonlinearity in
the range 500–1500 nm. These complexes exhibit large appar-
ent two-photon absorption (2PA) cross-sections at short wave-
lengths (500/520 nm), which is probably a combination of 2PA
and excited-state absorption/reverse saturable absorption
(ESA/RSA) owing to the measurements being undertaken at
the low-energy end of the linear absorption; the reported 2PA
values at such wavelengths should therefore be considered
with caution. The monometallic compounds (9d and 9e) have
a common 2PA peak at 640 nm, with the ethenyl-linked 9e
having a larger cross-section. The 2PA maxima are at lower en-
ergies for 9e than 9d in the range 740–900 nm, which is con-
sistent with the lower-energy MLCT in the linear absorption
spectrum (to which these 2PA maxima are correlated).
Figure 4. Top: Plot of s2 (blue) for 14a overlaid on the UV-visible spectrum
(black), and including plots of the UV-visible spectrum at twice (red) and
three times (green) the wavelength. Bottom: The real (red) and imaginary
(black) parts of the third-order hyperpolarizability of 14a.
the lowest-energy linear absorption band, and probably arises
from three-photon absorption. This low-energy nonlinear ab-
sorption band is at a shorter wavelength than that observed
for the 1,3,5-substituted analogue 18 (1300 nm), which is pos-
sibly a consequence of changing the core geometry of these
star complexes. Attempts to ‘tune’ the NLO properties with al-
teration of the capping groups appeared to give negligible
changes in the values of the NLO parameters (while being
mindful of the associated errors), although the longer-periph-
eral p-system-containing 14b and 14c have stronger nonlinear
absorptive properties at the longer wavelength maxima than
the phenyl-capped complex 14a does. This behavior might in-
dicate that the NLO properties are overwhelmingly influenced
by the core geometry rather than the peripheral substitution.
The monometallic compounds, 9d and 9e, are “building
blocks” for the formation of 14b and 14c, respectively. Addi-
tion of the monomer 9d/9e to the core to afford 14b/14c ap-
preciably changes the 2PA spectrum; 2PA maxima are shifted
to lower energy by around 20–40 nm, and the maximal values
of the cross-section at these wavelengths increase seven- to
tenfold. A strong response at around 1150 nm also appears,
which correlates closely with three times the wavelength of
Conclusion
The studies described herein have demonstrated useful out-
comes from both the synthetic and materials perspectives. The
“chemistry-on-complex” stereospecific formation of E-config-
ured stilbene groups by Horner–Wadsworth–Emmons coupling
of metal–ethynyl-functionalized benzaldehydes employed in
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undergo a broad range of reactions and consequent function-
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transformation of metal–ethynyl-derivatized benzaldehydes re-
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Access to the range of ruthenium–alkynyl-functionalized oli-
go(phenylenevinylene) stars, which the present study has af-
forded, has permitted an internal comparison of their proper-
ties across the complexes described here as well as an external
comparison to those of the extant ruthenium–alkynyl-function-
alized oligo(phenyleneethynylene) analogues, thereby allowing
several structure–property outcomes to be highlighted. Alkyl
substituents were employed in the present study to ensure
sufficient solubility, but their electron-releasing nature is re-
flected in increased ease of oxidation and a blueshift in optical
absorption maximum. Incorporation of nitro substituents in-
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the maximal value of the cubic NLO properties. Progression
from 1,3,5-trisubstituted arene core to the 1,2,4,5-tetrasubsti-
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parency.
Acknowledgements
We thank the Australian Research Council (ARC), the National
Natural Science Foundation of China (51432006), and the Chi-
nese Government Ministry of Education and State Administration
of Foreign Experts Affairs (111 Project: B13025) for financial sup-
port. M.D.R. was the recipient of an Australian Postgraduate
Award, X.Y. is the recipient of a China Scholarship Council ANU
Postgraduate Scholarship, M.P.C. thanks the ARC for Australian
Research Fellowships, and M.G.H. thanks the ARC for Australian
Professorial Fellowships. M.S. acknowledges the NCN grant DEC-
2013/10A/ST4/00114.
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Keywords: electrochemistry
chemistry · ruthenium · transition metals
·
nonlinear optics
·
redox
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