Switching the cubic nonlinear optical properties of an electro-, halo-, and photochromic ruthenium alkynyl complex across six states

Article abstract of DOI:10.1002/anie.200903027

More than on and off: The title complex (see picture) comprises addressable modules that respond independently to protic (alkynyl ligand?vinylidene ligand), electrochemical (metal-centered redox: Ru^II?Ru ^III), and photochemical (dith

Full text of DOI:10.1002/anie.200903027

Angewandte
Chemie
DOI: 10.1002/anie.200903027
Molecular Switches
Switching the Cubic Nonlinear Optical Properties of an Electro-, Halo-,
and Photochromic Ruthenium Alkynyl Complex Across Six States**
Katy A. Green, Marie P. Cifuentes, T. Christopher Corkery, Marek Samoc,* and
Mark G. Humphrey*
The impending breakdown in Mooreꢀs law has prompted the
search for molecule-based information-processing compo-
nents such as molecular switches and logic gates.[1] Bistable
molecules that afford states possessing distinct linear optical
properties (absorbance, fluorescence) and that can be inter-
converted by chemical, redox, magnetic, or photonic stimuli
have attracted considerable interest for Boolean logic oper-
ations.[2] In principle, molecular computing could exploit
ternary or higher-order digit representations, which would
permit smaller device components. “Switchable” molecular
properties are also of intense interest for sensor applications.
Despite this interest, molecules that can exist in more than
two stable and independently addressable states, which could
be employed for complex and higher-order logic functions,
have been explored significantly less than two-state mole-
cules. One potentially very important procedure to exploit
molecular switches is to utilize their cubic nonlinear optical
(NLO) properties,[3] and in particular their nonlinear absorp-
tion.[4] However, this avenue is also poorly explored and is to
date restricted to switching between three states at most,[5,6]
although such switching offers 1) the prospect of broadening
the available spectral range (and in particular utilizing
telecommunications-relevant wavelengths) and 2) the possi-
bility of enhanced spatial control (compared to linear optical
switching) because of the higher-order dependence on the
intensity of the incident light. Herein we show that a specific
binuclear metal alkynyl complex[7] incorporating a function-
alized 5,5’-dithienylperfluorocyclopentene (DTE) bridge[8]
can afford six stable and switchable states that possess distinct
cubic NLO properties. The complex is comprised of inde-
pendently addressable modules that respond orthogonally to
protic (alkynyl ligandQvinylidene ligand), electrochemical
(metal-centered redox: RuIIQRuIII), and photochemical
(DTE ring-openingQring-closing) stimuli. The six states are
interconverted along seven pathways, all of which result in
distinct changes to cubic nonlinearity for specific regions of
the spectrum. Our results demonstrate that complexes of this
type have the potential to be used, among other things, in the
construction of multi-input logic gates responding to diverse
stimuli across a broad spectral range.
The synthesis of the dinuclear ruthenium alkynyl complex
oa(II) is depicted in Figure S1 in the Supporting Information.
The DTE unit is obtained in the “open” form as its 5,5’-
diethynyl derivative; the open DTE is thermally stable, and
this open form persists through the preparative steps that
ultimately afford oa(II). Complete synthetic and character-
ization details are given in the Supporting Information.
Complex oa(II) can be reversibly protonated to the di(viny-
lidene) complex ov(II), reversibly oxidized to the RuIII
complex dication oa(III), and photoisomerized with UV
light to the closed alkynyl complex ca(II). Complex ca(II) can
be reversibly protonated to cv(II) and reversibly oxidized to
ca(III) and undergoes photoreversion to oa(II) on irradiation
with red light. The vinylidene forms and the oxidized forms
undergo reversible photoisomerization under analogous irra-
diation conditions. Note that oxidation of ruthenium vinyli-
dene complexes with these coligands is an irreversible
process, as assessed by cyclic voltammetry,[9] so there is a
maximum of six switchable states for this complex.[10] The
interconversions of the six stable states of the complex are
depicted in Scheme 1, and key spectral data are tabulated in
the Supporting Information (Table S1). Cyclic voltammo-
grams and UV/Vis/NIR spectral progressions for the redox
processes, the latter acquired with an optically transparent
thin-layer spectroelectrochemical (OTTLE) cell and demon-
strating stable isosbestic points, are given in the Supporting
Information, together with NMR spectra demonstrating
spectroscopically complete 1) photoisomerization between
the open and closed forms and 2) protonation/deprotonation
to afford the vinylidene and alkynyl complex forms, and IR
spectra demonstrating spectroscopically complete oxidation
from the RuII to the RuIII form.
[*] K. A. Green, Dr. M. P. Cifuentes, T. C. Corkery, Prof. M. G. Humphrey
Research School of Chemistry, Australian National University
Canberra, ACT 0200 (Australia)
Fax: (+61)2-6125-0760
E-mail: mark.humphrey@anu.edu.au
Prof. M. Samoc
Institute of Physical and Theoretical Chemistry
Wroclaw University of Technology, 50-370 Wroclaw (Poland)
Fax: (+48)7-1320-3364
E-mail: marek.samoc@pwr.wroc.pl
[**] Organometallic Complexes for Nonlinear Optics. Part 46. This work
was supported by the Australian Research Council and the
Foundation for Polish Science. M.G.H. thanks ARC for an Australian
Professorial Fellowship. K.A.G. is an Australian Postgraduate
Awardee. M.S. is a laureate of the FPS Welcome programme.
Part 45: R. L. Roberts, T. Schwich, T. C. Corkery, M. P. Cifuentes,
K. A. Green, J. D. Farmer, P. J. Low, T. B. Marder, M. Samoc, M. G.
Humphrey, Adv. Mater. 2009, 21, 2318.
Several organometallic alkyne-functionalized DTE com-
plexes have been reported, and the effect of oxidation state on
photochromic behavior has been probed, although there was
no assessment of the effect of protonation/deprotonation on
5
5
ꢀ À
À ꢀ
photochromism.
[CpFe(h -C5H4C C DTE C C-h -
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903027.
C5H4)FeCp][11] and [(dppe)Cp Fe(C C DTE C C)FeCp -
*
*
ꢀ À
À ꢀ
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Scheme 1. Interconversions of oa(II), ov(II), oa(III), ca(II), cv(II), and ca(III). o=open DTE, c=closed DTE, a=alkynyl, v=vinylidene,
II/III=(formal) metal-centered oxidation states.
(dppe)][12] (Cp = h5-C5H5, Cp* = h5-C5Me5, dppe = 1,2-bis(di-
phenylphosphino)ethane) undergo ring opening of the closed
form on oxidation, with the latter undergoing decomposition
on ring closing of the oxidized open form, whereas oxidation
complex forms are inactive (oa(II)) and a two-photon
absorber (ca(II)) over the range 1100–1600 nm.
The orthogonal switching that is possible with the alkynyl
complex forms affords the possibility of two-input logic gates
corresponding to 1) pH and hn input or 2) V and hn input,
which can be interrogated through their nonlinear absorption.
For example, Table 1 illustrates this for (1) with a NOR gate,
while Table 2 illustrates this for (2) with an INH gate.[14]
Our results demonstrate an “orthogonal” extension of the
concept of photochromism into the nonlinear optics domain,
similar to that which we have previously demonstrated with
ꢀ À
À ꢀ
of the open form of [trans-Cl(dppe)2Ru(C C DTE C C)-
trans-RuCl(dppe)2] results in ring closing.[13] In stark contrast,
the present complex cleanly photoisomerizes in both the
resting state and the oxidized forms, with all seven inter-
conversions affording bistable molecular switches.
The cubic NLO properties of the six states were assessed
by femtosecond Z scan over a broad spectral range. Note that
a complete spectral dependence study of each state typically
requires many hours, thus demonstrating the stability of the
oxidized species oa(III) and ca(III). The marked differences
in absorptive nonlinearities for the six states of the complex
afford the possibility of NLO switching. This situation is
illustrated in Figure 1 (which depicts the two-photon absorp-
tion cross-sections), with (a) and (b) corresponding to
oxidation/reduction and (c) corresponding to photoisomeri-
zation. Note that there are broad ranges of the nonlinear
spectra in which the behavior of the different forms of the
complex is dramatically different. In the range 600–1000 nm,
the open forms of the complex comprise a two-photon
absorber (oa(II)), a saturable absorber (oa(III)), and an
inactive nonlinear absorber (ov(II)), corresponding to pos-
itive, negative, and near-zero values of the imaginary compo-
nent of the cubic nonlinearity. The two vinylidene complex
forms are inactive (ov(II)) and a two-photon absorber (cv(II))
over the spectral range 750–1600 nm, while the two alkynyl
Table 1: NOR gate from vinylidene (v)/alkynyl (a) and open (o)/closed
(c) switching of two-photon absorption at 900 nm.
Complex
v=0, a=1
o=0, c=1
s2 [GM]
Output
ov(II)
oa(II)
cv(II)
ca(II)
0
1
0
1
0
0
1
1
0
200
400
500
1
0
0
0
Table 2: INH gate from RuII/RuIII and open (o)/closed (c) switching of
two-photon absorption at 1550 nm.
Complex
RuII =0, RuIII =1
o=0, c=1
s2 [GM]
Output
oa(II)
oa(III)
ca(II)
ca(III)
0
1
0
1
0
0
1
1
0
0
50
0
0
0
1
0
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Angewandte
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Experimental Section
The identity and purity of oa(II), ca(II), ov(II), and cv(II) were
confirmed by 1H, 31P, and 13C NMR and UV/Vis spectroscopy, ESI
mass spectrometry, and elemental analyses, while the oxidized species
oa(III) and ca(III) were characterized by UV/Vis and IR spectros-
copy. Full details of the syntheses and characterization of all new
compounds are given in the Supporting Information. The protona-
tion/deprotonation sequences to interconvert oa(II) and ov(II) as well
as ca(II) and cv(II) were carried out ex situ by sequential use of HBF4
and NEt3. The oxidation/reduction sequences to interconvert oa(II)
and oa(III) as well as ca(II) and ca(III) were carried out in an OTTLE
cell while monitoring changes in the optical nonlinearity, as described
in the Supporting Information. The photoisomerization sequences to
interconvert the pairs ov(II) and cv(II), oa(II) and ca(II), and oa(III)
and ca(III) were carried out in situ in the spectroscopy cell employed
for the NLO studies, as described in the Supporting Information.
Received: June 5, 2009
Revised: July 16, 2009
Published online: September 16, 2009
Keywords: alkynyl complexes · molecular devices ·
.
nonlinear absorption · nonlinear optics · organometallics
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oa(III) (circles), b) ov(II) (squares) and cv(II) (circles), and (c) ca(II)
(diamonds) and ca(III) (squares). For ease of comparison, the oa(III)
and ca(III) values are plotted as 1/10 of their true value. The effective
two-photon cross-sections are presented in GM (Gꢀppert–Mayer)
units, where 1 GM=10À50 cm4 s.
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by laser techniques, for example, for sensing or for the
modification of the propagation characteristics of laser beams
in optical devices. These results also highlight the utility of
organometallics in such applications. While organics currently
dominate the field of molecular nonlinear optics, the facile
redox switching at metal centers in organometallics adds a
new dimension to this field.
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