Unexpectedly high second-order nonlinear optical properties of simple Ru and Pt alkynyl complexes as an analytical springboard for NLO-active polymer films

Article abstract of DOI:10.1039/c4cc02432j

The unexpectedly high quadratic hyperpolarizability values of simple Ru and Pt alkynyl complexes have been measured by the EFISH technique, and this prompted us to investigate their potential as molecular building blocks for composite films with second ha

Full text of DOI:10.1039/c4cc02432j

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Unexpectedly high second-order nonlinear optical
properties of simple Ru and Pt alkynyl complexes
as an analytical springboard for NLO-active
polymer films†
Cite this: Chem. Commun., 2014,
50, 7986
Received 2nd April 2014,
Accepted 3rd June 2014
Alessia Colombo,ab Filippo Nisic,ab Claudia Dragonetti,*abc Daniele Marinotto,abc
Ivan Pietro Oliveri,a Stefania Righetto,ab Maria Grazia Lobellod and
Filippo De Angelis*d
DOI: 10.1039/c4cc02432j
www.rsc.org/chemcomm
The unexpectedly high quadratic hyperpolarizability values of simple
Ru and Pt alkynyl complexes have been measured by the EFISH
technique, and this prompted us to investigate their potential as
molecular building blocks for composite films with second harmonic
generation properties.
Organotransition metal complexes with second-order nonlinear
optical (NLO) properties are important as molecular building block
materials for the exciting field of molecular photonics, in particular
for optical communications, optical data processing and storage and
electro-optical devices.1 In particular, metal s-acetylides represent a
widely investigated class of second-order NLO chromophores, mainly
developed by Humphrey et al.,2 where the metal acts as the donor
group of a donor–acceptor system connected by a p-linker. Pt(II)
alkynyl complexes have been studied by W.-Y. Wong et al., as
excellent candidates both for optical power limiting3 and for solar
cell applications.4 The almost linear M–CRC–R structure allows a
Fig. 1 Simple Ru and Pt alkynyl complexes investigated.
good coupling between the d orbitals of the metal and the p* system (complex 1a, see Fig. 1) as a potential donor material to combine with
of the s-acetylide bridge affording a significant NLO response electron-withdrawing fullerides in bulk heterojunction solar cells.10
controlled by low-energy MLCT excitations. Of particular interest Interestingly, the presence of the phenylalkynyl ancillary ligand
are ruthenium s-alkynyl complexes, due to their facile high-yielding instead of a chloro ligand should increase the donor properties
syntheses,5 enhanced NLO coefficients,6 easy construction of multi- of the ruthenium center11 in this particular D–A structure where
metallic dendrimers,7 and reversible redox properties which afford the 2,1,3-benzothiadiazole moiety acts as an acceptor. From the
the possibility of NLO switching.8 Ruthenium s-acetylides have been perspective of materials with multifunctional properties, we asked
studied through hyper-Rayleigh scattering (HRS) measurements, but, ourselves if these complexes could have interesting second-order
to our knowledge, only in one case through electric field second NLO properties. In fact, to our knowledge the second-order NLO
harmonic (EFISH) generation measurements.9
response of these kinds of diruthenium s-acetylides have never
Recently, some of us reported a new dinuclear Ru(II) complex in been investigated. Humphrey studied a diruthenium complex
which two Ru atoms are separated by a 2,1,3-benzothiadiazole bridge {trans-[Ru(CRCPh)(dppe)2]}2(m-4,40-CRCC6H4CRCCRCC6-
H4CRC) but only for its excellent TPA (two photon absorption)
a
response and not for the second-order nonlinear optical properties
because it was centrosymmetric.12
In this communication we report the unexpectedly high EFISH
values of complex 1a, and of two new complexes: complex 1b
(analogous to complex 1a, but with hexyl chains on the thiophenes)
and complex 2 (a platinum alkynyl compound analogous to
`
Dipartimento di Chimica dell’Universita degli Studi di Milano, Italy.
E-mail: claudia.dragonetti@unimi.it
b
`
UdR INSTM and Centro di Eccellenza CIMAINA dell’Universita degli Studi di
Milano, Italy
c ISTM-CNR, via Golgi 19, 20133 Milano, Italy
d Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), ISTM-CNR,
Via Elce di Sotto 8, 06213 Perugia, Italy. E-mail: filippo@thch.unipg.it
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc02432j complex 1b). These complexes have been characterized by UV-vis
7986 | Chem. Commun., 2014, 50, 7986--7989
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spectroscopy, computed dipole moments, and DFT calculations
(see the ESI,† for the synthesis details and the chemical and
spectroscopical characterization of the three complexes). Complex
1a was used as molecular building blocks for composite films with
second harmonic generation properties.
The use of the EFISH technique13 represents in many cases a
valuable alternative to HRS14 which suffers from the limitation
of possible overestimation of the value of the quadratic hyper-
polarizability due to multiphoton fluorescence. It can provide
direct information on the intrinsic molecular NLO properties
through eqn (1):
gEFISH = (mbEFISH/5kT) + g(ꢀ2o; o, o, 0)
(1)
where mbEFISH/5kT is the dipolar orientational contribution to
the molecular nonlinearity, and g(ꢀ2o; o, o, 0), the third order
polarizability at frequency o of the incident light, is a purely
Fig. 2 Schematic representation of the energy levels of complexes 1a, 1b
and 2. Isodensity surface plots (isodensity contour: 0.035) of selected
molecular orbitals are also shown.
electronic cubic contribution to gEFISH
.
We found that our complexes are characterized by good to
excellent values of mbEFISH (ꢀ520/ꢀ1370 ꢁ 10ꢀ48 esu, see Table 1),
working in CH2Cl2 at a concentration of 10ꢀ3 M with a non resonant
incident wavelength of 1.907 mm, obtained by Raman-shifting under
high H2 pressure at the fundamental wavelength of 1.064 mm
produced by using a Q-switched, mode-locked Nd3+:YAG laser. To
evaluate the quadratic hyperpolarizability of the various complexes it
is necessary to have the dipole moment values. We report in Table 1
the total calculated dipole moments (m) in vacuo.
The investigated systems show a bent M-core-M moiety,
which is the origin of their non-zero dipole moments. The
dipole moments are directed from the two symmetric ligands
(negative pole) to the metal (positive pole) (see Fig. 2). A larger
dipole moment is associated with the presence of hexyl substituents
(compare 1a and 1b) and with the use of Pt instead of Ru
(compare 1b and 2).
As evidenced in Table 1, complexes 1a and 1b present a
red-shift of the UV-vis absorption maxima with respect to
complex 2, leading to much higher quadratic hyperpolarizability
with respect to complex 2 (factor of ca. 3.5).
The large bEFISH values observed in the present work are
remarkable. In fact, ruthenium s-alkynyl complexes with good
dipolar second-order nonlinear optical properties are expected to
be classical push–pull systems with high dipole moments. For
example, L. Rigamonti et al.9 reported the synthesis and NLO
characterization of a series of variously substituted donor-bridge-
acceptor complexes of the type trans-[Ru(4,4,400-CRCC6H4X2C6H4-
Y2C6H4NO2)Cl(dppe)2] with high m (13.34–14.68 ꢁ 10ꢀ18 esu) and
fair bEFISH values (55–89 ꢁ 10ꢀ30 esu), as determined by the EFISH
technique.
Therefore, the very high bEFISH values (up to ꢀ566 ꢁ 10ꢀ30
esu) observed for our Ru complexes are surprising for such non-
classical donor-p delocalized acceptor structures characterized
by relatively low dipole moments (Table 1). It is worth pointing
out that these bEFISH values are not overestimated by resonance,
because we worked with an incident wavelength of 1.907 mm
(see UV-Vis characterization in the ESI†).
In Fig. 2 the isodensity plots of the main molecular orbitals
are shown (see the ESI† for orbital energies). The highest
occupied molecular orbitals (HOMOs) of all the investigated
complexes are Ru and Pt orbitals combined with the p orbitals of
thiophene and benzothiadiazole groups. The lowest unoccupied
molecular orbitals (LUMOs) are completely localized on the
benzothiadiazole moieties without any contribution from the
metal. The HOMO–LUMO gaps for 1a, 1b and 2 are 1.44, 1.43
and 1.60 eV, respectively, which nicely parallel the trend in the
measured UV-vis absorption maxima showing complex 2 to
exhibit a 0.2–0.3 eV blue shift of the lowest transition compared
to 1a and 1b.
Since the electronic mechanisms which yield the NLO response
in molecular systems are well-defined, it is now crucial to face their
molecular engineering in order to obtain organized molecular
materials showing a temporal, stable and high bulk second-order
NLO response.16 Applications of ruthenium s-acetylide complexes to
produce second-order bulk NLO materials or structured films are
very limited.1c In spite of their large molecular quadratic hyper-
polarizabilities, they form crystalline materials which exhibit
modest bulk second harmonic generation (SHG) efficiency,2a
due to the reluctance of acetylide complexes to crystallize in non-
centrosymmetric structures. However, a film of a ruthenium
oligothienylacetylide NLO chromophore, incorporated into a
polymethylmethacrylate (PMMA) matrix, revealed an acoustically
induced SHG signal with a good w(2) value (0.80 pm Vꢀ1).17
Table 1 Electronic spectra and mbEFISH, m, and bEFISH values of the
investigated complexes
mbEFISH
l (nm)
(ꢁ10ꢀ48 esu)a,b
m
bEFISH
Sample [e (Mꢀ1 cmꢀ1)]a (ref. 15)
(ꢁ10ꢀ18 esu)c (ꢁ10ꢀ30 esu)
1a
1b
2
393 [35 011],
633 [35 340]
398 [39 664],
656 [36 018]
378 [34 462],
561 [26 254]
ꢀ900
ꢀ1370
ꢀ520
1.6
2.8
3.4
ꢀ566
ꢀ498
ꢀ151
a
b
In anhydrous CH2Cl2. The error in EFISH measurements is 10%.
c
Computed dipole moments in vacuo, also see ESI.
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Fig. 3 In situ corona-wire poling dynamic of a PMMA film containing
complex 1a.
The unexpectedly high quadratic hyperpolarizability of complex 1a
in solution prompted us to investigate its potential as molecular
building blocks for composite films with second harmonic generation
properties.18 Therefore we dispersed and oriented complex 1a in a
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33
containing the simple complex 1a is very high and rather stable, the
signal remaining even after two months. This particularly high
stability, from the perspective of applications, could be attributed
to the relatively large size of the NLO chromophore which would
hinder its mobility. In conclusion, our work puts in evidence that
simple quite symmetrical complexes with a relatively low dipole
moment could have remarkably high bEFISH values, and can be
the building blocks of active polymeric films characterized by an
excellent second-order NLO response. Our ruthenium s-acetylide
complexes will be tested in the near future as third order nonlinear
optical chromophores, being surely promising candidates also for
two-photon absorption (TPA) applications.
This work was financially supported by MIUR (FIRB 2003:
RBNE033KMA and FIRB 2004: RBPR05JH2P) and CNR.
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7988 | Chem. Commun., 2014, 50, 7986--7989
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