Two-Photon Absorption Properties in “Push-Pull” Ruthenium Nitrosyl Complexes with various Fluorenylterpyridine-Based Ligands

Article abstract of DOI:10.1002/ejic.202100109

Using the compound [Ru^II(FT)(bipy)(NO)](PF₆)₃ (FT is the electron-rich 4’-(2-fluorenyl)-2,2’:6’,2’’-terpyridine ligand and bipy is 2–2’bipyridine) as a reference, two new compounds are presented in which carbon-carbon double and triple bonds are inserted between the fluorenyl substituent and the terpyridine to provide an extended conjugation path. The electronic properties of the three complexes are compared experimentally by UV-visible spectroscopy and computationally by means of the density functional theory. All of them exhibit a capability for NO release under irradiation on their low-energy transition located in the 400–500 nm range, with a quantum yield around 0.01. Their two-photon absorption (TPA) cross sections are investigated by the Z-scan technique at λ=800 nm. While the reference compound exhibits a cross-section equal to 108 GM, the introduction of double and triple bonds leads to increased cross-sections equal to 131 GM and 150 GM, respectively. These values are discussed in reference to the two-level model in use for “push-pull” dipolar TPA chromophores.

Full text of DOI:10.1002/ejic.202100109

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Two-Photon Absorption Properties in “Push-Pull”
Ruthenium Nitrosyl Complexes with various
Fluorenylterpyridine-Based Ligands
Valerii Bukhanko,[a] Andrés Felipe León-Rojas,[b] Pascal G. Lacroix,*[a] Marine Tassé,[a]
Gabriel Ramos-Ortiz,[c] Rodrigo M. Barba-Barba,[c] Norberto Farfán,[b] Rosa Santillan,[d] and
Isabelle Malfant*[a]
Using the compound [RuII(FT)(bipy)(NO)](PF6)3 (FT is the elec-
tron-rich 4’-(2-fluorenyl)-2,2’:6’,2’’-terpyridine ligand and bipy is
NO release under irradiation on their low-energy transition
located in the 400–500 nm range, with a quantum yield around
0.01. Their two-photon absorption (TPA) cross sections are
investigated by the Z-scan technique at λ=800 nm. While the
reference compound exhibits a cross-section equal to 108 GM,
the introduction of double and triple bonds leads to increased
cross-sections equal to 131 GM and 150 GM, respectively. These
values are discussed in reference to the two-level model in use
for “push-pull” dipolar TPA chromophores.
2–2’bipyridine) as
a reference, two new compounds are
presented in which carbon-carbon double and triple bonds are
inserted between the fluorenyl substituent and the terpyridine
to provide an extended conjugation path. The electronic
properties of the three complexes are compared experimentally
by UV-visible spectroscopy and computationally by means of
the density functional theory. All of them exhibit a capability for
*
Introduction
release NO under irradiation in the λ=300–500 nm domain,
taking advantage of the non-invasive and highly controllable
characteristics of light.[9–11] Nevertheless, and in order to be fully
applicable, the irradiation should be achieved in the λ=600—
1300 nm “therapeutic window” of relative transparency of the
biological media.[12]
Since it was identified as the endothelium-derived relaxing
*
factor by Ignarro and Palmer, in 1987,[1,2] nitric oxide (NO ) has
been attracting a growing interest in relation to its increasingly
recognized biological functions and possible therapeutic
*
*
applications.[3,4] However, the action of NO in biological media
To design alternative NO donors, compatible with the
may strongly depend on its concentration. For instance, at
micromolar concentration, it acts as a potential anticancer
agent, promoting apoptosis and inhibiting angiogenesis,
whereas lower concentrations promote cells growth with
therapeutic window, we have previously reported on a Ru(NO)
complex built up with fluorenylterpyridine and a bipyridine
ligand (13+, in Scheme 1).[13] Introducing a fluorene substituent
on the terpyridine was motivated by its well documented
capability to enhance the two-photon absorption (TPA) proper-
ties of molecules.[14] In the last decade, the TPA technique which
involves two photons at double wavelength of the one-photon
excitations (e.g. λ=800 nm instead of 400 nm) has emerged as
the most promising technic in the photo-dynamic therapy
(PDT) treatment of cancer, microsurgery, local drug delivery and
high resolution cell imaging.[15,16] These enhanced capabilities
are related to its low damage effects, high selectivity and
deeper penetration into biological tissues. Along this line, the
*
application for tissue healing.[5–7] In this context, exogenous NO
donors have been widely investigated,[8] but they must be able
*
to deliver NO locally and quantitatively to avoid undesirable
effect on untargeted cells. Among various potential candidates,
ruthenium nitrosyl complexes have emerged as an appealing
class of biological materials in relation to their low toxicity,
good stability, and moreover to their capability to exclusively
*
[a] Dr. V. Bukhanko, Dr. P. G. Lacroix, M. Tassé, Prof. I. Malfant
CNRS, Laboratoire de Chimie de Coordination (LCC),
205 route de Narbonne, 31077 Toulouse, France
E-mail: pascal.lacroix@lcc-toulouse.fr
isabelle.malfant@lcc-toulouse.fr
[b] A. F. León-Rojas, Dr. N. Farfán
Facultad de Química, Departamento de Química Orgánica, Universidad
Nacional Autónoma de México, 04510 CDMX., México
[c] Dr. G. Ramos-Ortiz, Dr. R. M. Barba-Barba
Centro de Investigaciones en Óptica,
A.P. 1–948, 37000 León, México
[d] Dr. R. Santillan
factor of merit of NO donors is an uncaging parameter arising
*
as the product of the release efficiency ϕNO (number of NO
*
generated per NO donors promoted in the excited state) by
*
the TPA capability of the NO donors, known as the TPA cross-
section (σTPA). Therefore, any attempt to enhance the capability
of Ru(NO) complexes will imply increasing either ϕNO or σTPA. In
13+, ϕNO lies around 0.01, a value that appears satisfactory.
Indeed, increasing ϕNO significantly complicates the handling of
the species, since a short exposition to light initiates the
decomposition of the complex with the outcome of potential
Departamento de Química,
Centro de Investigación y de Estudios Avanzados del IPN,
07000, A.P. 14–740, Ciudad de México, México
*
undesirable NO effect on healthy tissues. On the other hand,
the TPA cross section of 13+ is equal to 108.0 � 18.0 GM at
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202100109
800 nm, far below values determined for large size poly-
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Results and Discussion
Synthesis and characterization
The synthetic routes towards substituted terpyridine ligands A
and B used in the synthesis of complexes 23+ and 33+
,
respectively, are described in Scheme 2. Ligand A was obtained
through a 6 steps linear type synthesis. In a first step, the
fluorene moiety was modified by insertion of two hexyl chains,
to substantially improve the solubility of the final complex. This
approach was described in previous papers: high solubility of
complexes (around 10À 2 M) was crucial for measuring TPA
response of the complexes.[13] The incorporation of a double
bond into the structure was addressed through a Heck-type
coupling of 2a with methylacrylate, similarly to the procedure
described in the literature for analogous compounds.[19] The
resulting ester 2b was further converted to the corresponding
aldehyde 2d through reduction of 2b and subsequent
oxidation of the acrylic alcohol 2c. Two succeeding steps,
Claisen condensation with 2-acetylpyridine and Michael reac-
tion of the obtained chalcone 2e with Kröhnke salt in the
presence of ammonium acetate resulted in the formation of the
final terpyridine (A) core.
Ligand B was synthesized by a more convenient 7 steps
convergent approach depicted in Scheme 2. Thus, a triple bond
was introduced into the structure through Sonogashira cou-
pling of 2a with TMS-protected acetylene, and subsequent
deprotection of the acetylenic position resulted in formation of
3b. The synthesis of the terpyridine core (3e) was achieved
separately, following the procedure initially used by Constable
and Ward and well-described in the literature.[20,21] The final step
of the terpyridine synthesis consists on introduction of triflate
group, which further enables Sonogashira coupling between
compounds 3b and 3e, thus producing the desired ligand B
with 42% yield. Applying a similar convergent approach to the
synthesis of ligand A, with the only difference being introduc-
tion of a double bond instead of triple bond, was not successful
due to the low yield of the final Heck-coupling step (1–2%).
Therefore, an alternative linear way for the synthesis of ligand A
was preferred.
The final compounds [2](PF6)3 and [3](PF6)3 were obtained
by an approach previously used for the synthesis of bipyridine-
terpyridine ruthenium-nitrosyl complexes.[13] The first step of
complexes synthesis implies the reaction of ligands A or B with
RuCl3, which results in the formation of a paramagnetic
complex [Ru(L)Cl3] (Scheme 3). In a second step, [Ru(L)Cl3]
interacts with 2,2’-bipyridine in the presence of triethylamine to
form the diamagnetic species [Ru(L)(bipy)Cl]Cl. Use of triethyl-
amine facilitates the reduction of RuIII to RuII. A subsequent
ligand substitution provides a convenient approach to ruthe-
nium-nitro complex [Ru(L)(bipy)NO2]Cl, which is then converted
to ruthenium-nitrosyl species [Ru(L)(bipy)NO](PF6)3 upon treat-
ment with concentrated HCl.
Scheme 1. Ruthenium nitrosyl complexes under investigation
ruthenium-based dendrimers specially designed to target
record cross-sections (higher than 10000 GM).[17] These observa-
tions encouraged us to direct our research effort towards an
increase of σTPA by chemical modifications achieved on the 13+
complex.
The present contribution is devoted to the investigation of
alternative ruthenium nitrosyl complexes with enhanced TPA
capability, within the dipolar approach previously used in 13+
.
Owing to the comprehensive analysis on the origin of TPA
effects in molecules, which relates the properties to long range
charge transfer transitions,[15,16,18] the selected strategy aims at
the insertion of a À CH=CHÀ double and a À C�CÀ triple bonds
between the fluorenyl unit and the terpyridine, thus leading to
complexes 23+ and 33+, respectively (Scheme 1). After describ-
ing the synthesis and characterization of the new species, their
spectroscopic and TPA properties are presented and compared
with those of the parent 13+ species. The differences are
analyzed computationally within the framework of the time-
dependent density functional theory (TD-DFT) to evaluate the
capacities of extended π-delocalization to enhance the charge
transfer effects and hence the TPA response of these ruthenium
nitrosyl derivatives.
The formation of complexes [Ru(L)(bipy)NO2]Cl and [Ru-
1
(L)(bipy)NO](PF6)3 can be easily monitored by H NMR spectro-
scopy. The proton in position 6 of bipyridine (Scheme 3) is
spatially closed to the monodentate ligand present in the
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Scheme 2. Synthetic route towards Ligand A (top) and Ligand B (bottom).
Scheme 3. Synthetic approach for the preparation of [2](PF6)3 and [3](PF6)3, built from ligands A and B, respectively. R stands for the fluorenyl-based
substituents present in A and B. Hydrogen 6-H (in Blue) is used to monitor the Cl!NO2!NO substitution, by 1H-NMR.
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complex. Its chemical shift in the 1H-NMR spectra strongly
depends on the nature of this ligand (Cl, NO2, NO), as is
illustrated in Figure 1, for the series of complexes leading to
[3](PF6)3.
Table 1. Comparison between the experimental (UV-visible, acetonitrile)
and the computed (TD-DFT) data for the low-lying transition in the three
ruthenium complexes under investigations, with absorption maxima (λmax
in nm), extinction coefficients (ɛ in molÀ 1.L.cmÀ 1), and oscillator strengths
(f).
In the case of ruthenium complexes having a chlorido
ligand in the coordination sphere, the chemical shift of the
proton H(1) is 10.23 ppm (in CD3OD), whereas for its nitro-
analogue the peak of the proton H(1) is at 9.90 ppm (in CD3OD).
In the final ruthenium-nitrosyl complex [3](PF6)3 the peak of the
corresponding proton is located at 9.31 ppm (in CD3CN). The
structures of both complexes [2](PF6)3 and [3](PF6)3 were
confirmed by NMR, applying two-dimensional NMR-technics
HMBC, HMQC and COSY.
The NO-bond stretching vibration frequencies are
1942 cmÀ 1 and 1946 cmÀ 1 for complexes [2](PF6)3 and [3](PF6)3,
respectively. These values are very close to those previously
reported by our group for similar complexes, containing five
pyridine rings in coordination spheres of ruthenium atom.[13,22]
These values indicate that the Ru-NÀ O moiety adopts a highly
linear geometry, which is in a good agreement with previous
observations.[22]
Compounds
UV-visible
λmax
TD-DFT
λmax
ɛ
f
[1](PF6)3
[2](PF6)3
[3](PF6)3
455
494
466
16 700
25 100
20 900
430
484
457
0.314
0.802
0.802
to [2](PF6)3 and [3](PF6)3, respectively. The intensities of these
transitions (extinction coefficients ɛ) are slightly enhanced in
[2](PF6)3 and [3](PF6)3 versus that of the reference [1](PF6)3 (see
Table 1). It is generally expected that both effects (excitations at
longer wavelengths and enhanced intensities) lead to chromo-
phores of better TPA capabilities.[23,24]
The TD-DFT computed spectra of the three complexes 13+
,
23+ and 33+ are shown in Figure 3. The main difference versus
experiment is observed at the intermediate band 2 (325–
425 nm range), which appears well resolved by DFT, and spread
into several overlapping components in the UV-visible spectra.
Nevertheless, the low-energy bands of absorption are well
defined, and related to a single high intensity transition. The
data are gathered in Table 1 for these transitions, to be
compared to the experimental data. The agreement between
computation and experiment appears satisfactory. In particular,
the maximum energy difference is limited to 0.16 eV in the
worst case ([1](PF6)3), which is excellent for chromophores
having long range charge transfer capacities.[25] Moreover, the
trends observed in red shifts and changes in transition
intensities are qualitatively reproduced by DFT. Altogether,
these features allow us to use the computation to provide a
precise analysis of the origin of the transitions at the orbital
level (Table 2).
Spectroscopic Properties
The experimental UV-visible spectra of the three Ru(NO)
complexes are shown in Figure 2. Contrary to their related
substituted-terpyridine ligands, which possess bands with
absorption maxima below 350 nm (Supplementary Materials),
the complexes exhibit additional transitions at lower energy: an
intense low energy band with absorption maxima around λ=
450–500 nm (band 1), and a poorly resolved group of tran-
sitions in the λ=325–425 nm range (band 2). Intense tran-
sitions, reminiscent of those of the ligands are present below
350 nm (band 3), but will not be analyzed here. Interestingly, a
clear red shift of 39 nm and 11 nm is observed at absorption
maxima for the low-energy transitions on going from [1](PF6)3
Figure 1. Change in 1H-NMR-spectra upon substitution of monodentate ligand in the synthesis of complex [3](PF6)3. Spectra of [Ru(B)(bipy)NO2]Cl and
[Ru(B)(bipy)Cl]Cl were measured in CD3OD, spectrum of [3](PF6)3 in CD3CN.
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Figure 2. Experimental UV-vis spectra in MeCN for [1](PF6)3 (top), [2](PF6)3 (middle), and [3](PF6)3 (bottom).
*
The examination of Table 2 indicates that the dominant
“push-pull” charge transfer effect is observed in band 1 in any
case. Furthermore, the HOMO-LUMO levels provide most of the
contribution to the effect. These orbitals are shown in Figure 4.
While the LUMO levels are nearly identical with most of the
electron density located on the Ru(NO) fragments, the HOMO
levels involve the fluorenyl moieties with a contribution of the
π-conjugated bridge in 23+ and 33+. This conjugation extension
results in a slight enhancement of the energy of the HOMO
level and finally in the red shift observed in the UV-visible
spectra. The similarities encountered in the spectroscopic
NO release from [2](PF6)3 and [3](PF6)3
The growing interest devoted to Ru(NO) complexes is largely
due to their ability to release nitric oxide under irradiation,
according to the following equation (1):
(1)
Indeed, we have always observed the release of NO as a
*
neutral NO , thus leading to a subsequent RuII!RuIII conversion
*
properties suggest complexes with rather similar NO release
capabilities.
during the release process.[11,13,22,26–28] In the class of complexes
built up from substituted terpyridine and bipyridine ligand, we
have reported on various examples of such behavior with
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Figure 3. Computed TD-DFT spectra for 13+ (top), 23+ (middle), and 33+ (bottom).
quantum yield (ϕNO) ranging from 0.004 to 0.03 depending on
the donor substituent and the wavelength of
irradiation.[11,13,26–28]
Owing to the presence of the lowest energy band (band 1) in
the UV-vis spectrum having absorption maxima located at
494 nm and 466 nm for [2](PF6)3 and [3](PF6)3, respectively, the
irradiations were performed at 436 nm using a set of available
The resulting release of nitric oxide is followed by the
formation of a solvent bound ruthenium(III) photoproduct,
according to equation (1). This was clearly evidenced by X-ray
crystallography in the case of a good stability of the RuIII photo
product in previous reports.[26,29] Nevertheless, it should be
pointed out that equation (1) does not preclude any further
chemical evolution of [(L)RuIII(solvent)], with the outcome of a
possible photoproduct different than the expected species.
*
filters, in order to determine the NO release efficiencies.
The changes in the electronic absorption spectrum of
[2](PF6)3 and [3](PF6)3 exposed to 436 nm light in acetonitrile are
shown in Figure 5. The presence of isosbestic points at 256,
328, 354, 394 nm and 298, 310, 384, 460, 492 nm for [2](PF6)3
and [3](PF6)3 respectively indicates a clean conversion of the
RuII(NO+) complexes to related photolysed species. No back-
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Table 2. Details of the dominant transitions involved in band 1 and 2 for 13+, 23+, and 33+: absorption maxima (λ
composition of the interaction configuration expansion, and charge transfer character.
in nm), oscillator strengths (f),
Character
max
Compounds
13+
Transitions
λ max
f
Main composition[a]
Band 1
1!2
430
0.314
78.4% χ167!168
fluorene!RuNO
Band 2
1!7
362
346
0.370
0.301
31.3% χ164!169 +18.8% χ162!170 +14.2% χ159!169
46.1% χ167!170 +20.8% χ164!169
terpy!RuNO
1!9
fluorene!terpyRuNO
23+
Band 1
1!2
484
401
457
0.802
0.855
0.802
77.3% χ174!175
60.3% χ174!177
63.6% χ173!174
fluorene!RuNO
fluorene!terpy
Band 2
1!5
33+
Band 1
1!2
Band 2
1!5
393
381
0.144
0.699
67.3% χ165!175
55.2% χ173!176 +10.1% χ165!175
bipyRu!RuNO
1!7
fluorene!terpyRuNO
[a] Orbital 167(168) is the HOMO(LUMO) for 13+, 174(175) is the HOMO(LUMO) for 22+, and 173(174) is the HOMO(LUMO) for 33+
.
Figure 4. HOMO-LUMO frontier orbitals in 13+, 23+, 33+ with their relative
energies.
reaction is observed when the light is turned off. In the
photolysed species, new bands located at 288, 382, 484 nm and
288, 362, 480 nm arise for [2](PF6)3 and [3](PF6)3 respectively.
Due to a significant absorption of the photoproduct at 436 nm,
the release takes 6 hours for [2](PF6)3 and 4 hours for [3](PF6)3 to
reach completeness.
*
The quantum yield of NO release (ϕNO) observed for
[2](PF6)3 and [3](PF6)3 is equal to 0.008 and 0.009 under light
irradiation at 436 nm respectively.
*
Moreover, the photo-generation of NO can be demon-
Figure 5. Evolution in the absorption spectra of [2](PF6)3 (top) and [3](PF6)3
(bottom) in acetonitrile under irradiation at λ=436 nm. Blue line: before
irradiation; red line: after completeness of the photo-chemical process.
strated experimentally, by EPR spectroscopy under standard
excitation, since spin trapping combined with EPR spectroscopy
is considered as one of the best methods for the direct
*
detection of NO radicals.[30] Namely, we used Iron(II)-N-methyl-
*
D-glucamine dithiocarbamate [FeII(MGD)2] to trap NO due to its
high probability of adduct formation and to the high stability of
its spin adduct. The appearance of the characteristic triplet
2.040. These values are fully consistent with the literature report
for [FeII(MGD)2-NO] adduct.[31] The weak signal observed in the
*
*
signal of NO is shown in Figure 6, with a hyperfine splitting
control spectrum is due to a trace of NO and related to the fact
that the manipulation is never strictly conducted in the dark.
constant value of aN =1.21 × 10À 3 cmÀ 1 and a g-factor equal to
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TPA properties
At the molecular level, the quantification of the simultaneous
absorption of two-photons is expressed by the TPA cross-
section (σTPA), expressed in Göppert-Mayer unit (1 GM=
10À 50 cm4 s photonsÀ 1 moleculesÀ 1). Due to an absence of
fluorescence in the present ruthenium-nitrosyl complexes, the
TPA measurements were performed by the Z-scan technique,
which requires a good solubility of the investigated molecules
at relatively high concentrations (around 10À 2 mol.LÀ 1) to obtain
a good signal to noise ratio. This requirement explains the
design of ligands containing two hexyl chains introduced to
enhance the solubility of the final complexes. We have
previously reported the TPA properties of [1](PF6)3,[13] therefore
the present report focusses on the TPA properties of [2](PF6)3,
and [3](PF6)3, which are then discussed with reference to those
of [1](PF6)3.
The experimental data were recorded using an incident
laser beam at 800 nm. Although it does not correspond strictly
to the expected maximum of TPA of the ruthenium complexes
(988 nm and 932 nm corresponding to 2λmax for [2](PF6)3 and
[3](PF6)3, respectively), it is important to keep in mind that the
800 nm wavelength is a standard laser line produced by
femtosecond laser oscillators or laser amplifiers as in the case of
our experimental set-up; in addition, it is nearly in the middle
range of the therapeutic window (700–950 nm). Therefore, and
for practical reasons, it was the one selected here. Figure 7
presents typical normalized transmission (T(z)) (see Experimental
Section), with the fitting carried out using the standard formal-
ism in Z-scan analysis.[33] The nonlinear absorption was also
measured at different energies in the range 40—100 nJ per
pulse. As it is expected, the transmission T(z=0) in each sample
decreased as the energy of pulses was increased. It is observed
that the Z-scan traces exhibit good symmetry around the z=0
position. Also, in all cases a good fitting corresponding to TPA
was preserved independently of the energy and no artifacts on
the Z-scan traces were detected, i.e., thermo-optical effects
which usually introduce strong distortion and asymmetries on
the signal at high energies. Notice that asymmetric traces can
be also a sign of significant generation of photolysed species
(irreversible photo-reactions) during the acquisition of Z-scan
data. Thus, although photolysed species (after NO release from
the excited states generated by TPA) were present in our
experiments, their density was not significant and did not affect
the measurement of σTPA for the ruthenium complexes. Other
effects such as multistep two-photon excitation (absorption of
excited states) that sometimes appears as an artifact of
instantaneous TPA was precluded with the use of very short
pulses (80 fs). The average of TPA parameters extracted from
the Z-scan analyses (nonlinear absorption coefficient β, and
molecular TPA cross section σTPA) are gathered in Table 3 for the
two materials. These average values result from several plots of
Z-scans for each sample.
*
Figure 6. Triplet EPR signals from NO trapped by [Fe(MGD)2] for [2](PF6)3
(top) and [3](PF6)3 (bottom) upon one photon excitation at room temper-
ature and at λ>400 nm (Hg lamp) (bottom). Dashed lines correspond to
control signal before irradiation.
*
The observation of NO confirms the validity of the process
depicted in equation (1) with the outcome of a fast reduction of
the concomitant [RuIII(solvent)] species into [RuII(solvent)], in
such a way that no transient RuIII complex is detectable in the
reaction medium in the UV-vis spectra (Figure 5). Indeed, the
presence of RuIII species leads to the appearance of a broad and
weak band at 600 nm.[26] The reasons for a fast reduction of the
RuIII species was discussed elsewhere[22] and were related to the
presence of 5 pyridine rings in the coordination sphere of the
ruthenium center, thus leading to a significant increase of the
RuIII/RuII redox potential, the RuII being strongly stabilized by d-
π* back donation to the aromatic rings.
*
Under irradiation at 436 nm, the quantum yield of NO
release is found equal to 0.008, and 0.009 for [2](PF6)3, and
[3](PF6)3, respectively. These values lie in the same range than
the value of 0.011 obtained for [1](PF6)3, recorded as a reference
in the same conditions (Supplementary Materials). As a final
remark, it is worth pointing out that the NO release process is
assumed to arise after photon absorption (equation 1), there-
fore is regarded as a property related to the evolution of the
excited state, without reference to the process (one-photon or
two-photon absorption) employed for getting this excited state.
In other word, we assume that NO release induced by one or
two-photon absorption lead to the same quantum yield.
Nevertheless, we have previously observed that fluorenyl-
terpyridine based Ru(NO) chromophores can release NO after
both one and two-photon excitations.[32]
Owing to the present data, the issue of the relative effect of
double and triple bonds on the TPA properties is naturally
addressed in these ruthenium complexes. It is usually assumed
that ethynylene-linked (À C�CÀ ) systems are less conjugated
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Figure 7. Transmission in Z-scan experiments for [2](PF6)3 and [3](PF6)3 (top) and decrease in transmission with the increase of pulse energy (bottom). Symbols:
experimental data points. Lines: fitting to experimental data.
Table 3. Nonlinear absorption coefficients (β in 10À 11 cm/W), and TPA
cross-sections (σTPA in GM), for [1](PF6)3, [2](PF6)3, and [3](PF6)3, at the
incident laser wavelength of 800 nm.
where ω is the laser frequency, n the refractive index, c the
velocity of light in vacuum, and L the local field factor. Within
the framework of the perturbation theory, γ can be expressed
by an extensive sum-over-state expression, which involves
contribution of all hmjmjni ¼ mmn transitions between states m
and n, through the following equation (3):[38]
Compound
β
σTPA
Ref.
[1](PF6)3
[2](PF6)3
[3](PF6)3
2.63 � 0.43
3.17 � 0.45
3.63 � 0.71
108 � 18
131 � 19
150 � 29
[ref 13]
this work
this work
than vinylene-linked (À CH=CHÀ ) systems, because of energy
mismatches between π–π and π*–π* levels within the C(sp)À C-
(sp2) connection along the π-delocalized skeletons. This leads to
larger nonlinear optical responses observed in vinylene systems
in most cases.[34]
(3)
In this expression, P is a perturbation operator, 0, m, n, p are
the labels of the ground and excited states, i, j, k, l are molecular
axis, �hwm0 is the energy of state m, and Γm0 is the band width of
the 0!m transition.
However, this difference is not pronounced for TPA proper-
ties. Indeed, a literature survey indicates reports of larger cross-
[36]
sections either in the À CH=CHÀ ,[35] or in the À C�CÀ classes
In the case of pseudo 1-dimensional “push-pull” systems in
which the electronic spectra exhibit an intense transition
between the ground state (g) to a low-lying excited state (e)
with strong charge-transfer character, it was proposed to
simplify drastically equation (3) to the dominant contribution of
this current charge transfer (g!e) transition. Within this so-
called “two-level model”, σTPA is expressed as follows
equation (4):[24]
of chromophores.
Providing a simple picture for the origin and magnitude of
the TPA response of the three 1–3 ruthenium complexes can be
tentatively approached within the framework of a simplified
two-level model. At the most fundamental level, σTPA is related
to the imaginary part of the second hyperpolarizability (Imγ ) as
follows equation (2):[37]
(2)
(4)
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in which Ege is the energy of the g!e transition, f its oscillator
Ne.[40–42] Saturation effects can take place in the case of systems
of much bigger size than the present 23+ and 33+ species.
Therefore, one can conclude that the modest enhancement
observed in the σTPA value after introduction of double and
triple CÀ C bond is not due to the loss of conjugation frequently
encountered in large size chromophores. Instead, one may infer
that the two-level expression of σTPA (equation 4) cannot fully
account for providing a simple expression of the origin of the
TPA properties along this series, in part due to some energy
mismatches between the real absorption maxima and the
experimental requirement to have to work at incident wave-
length of the Ti-sapphire laser (800 nm).
After the observation of a modest σTPA enhancement
obtained within the present “push-pull” approach, the issue of a
more promising strategy is naturally addressed. It has recently
been reported that huge σTPA enhancement could be accessible
in complexes of higher dimensionality than those of the first
generation of “push-pull” chromophores.[43] Investigating multi-
polar chromophores will certainly require more sophisticated
synthetic approaches and computational analysis, to optimize
the best candidates. Nevertheless, we are now focusing our
research efforts in these directions.
strength, mgg and mee the dipole moment in the ground and
excited states, respectively, and Γ the linewidth broadening
parameter which is often taken at the constant value of 0.1 eV.
Equation 4 is valid at the maximum absorption, when the laser
energy (h�w) is half of the transition energy. Nevertheless, we
have tentatively applied this model on the basis of the
electronic
parameters
available
in
the
Gaussian09
computation,[39] which indicate that the first (1!2) transition is
the only one having high intensity (f) low energy (E), strong
charge transfer (μee–μgg) character for providing an intense TPA
response. Experimentally the corresponding UV-visible band 1
accounting for this transition is observed in the λ=425–525 nm
range, therefore not strictly at half wavelength of the 800 nm
output of the Ti-sapphire laser. On the other hand, and apart
from the fact that two-photon electronic spectra may be
significantly different than one-photon spectra, the 800 nm
wavelength was selected for practical reasons as Z-scan experi-
ments with short optical pulses require the use of Ti-sapphire
lasers whose emission is precisely around 800 nm. Further, this
wavelength is within the window of biomedical interest for
medical therapy based on light. Of course, extending the laser
emission to other wavelength would be possible but it cannot
be envisioned practically, for a question of high cost when laser
sources of short pulses are used. Under these assumptions, the Conclusion
computed two-level evolution of σTPA on going from 13+ to 23+
and 33+ are estimated in Table 4 and compared with the
experimental data.
With the aim of increasing the TPA properties of a Ru(NO)
complex capable of releasing NO under irradiation in the
*
The observation that the two-level approach fails to account
for the effect of double and triple bond insertion in [2](PF6)3,
and [3](PF6)3 immediately strikes in Table 4. Indeed, the
experimental σTPA enhancement appears far reduced compared
to the computed prediction. The issue of a possible saturation
observed experimentally in TPA material of long length has
been discussed in the literature.[15] As the π-system becomes
longer, it reaches a length known as the “conjugation length”,
beyond which the loss of planarity leads to electron density
confinement along the conjugated chains. However, this
possibility cannot be envisioned here, because 23+ and 33+
appear more planar than the reference 13+. Additionally, there
is no simple expression to relate the dependence of the
therapeutic window, a strategy based on inserting double and
triple carbon-carbon bond was explored within a family of
“push-pull” species in which the nitrosyl ligand acts as the
electron acceptor and the fluorenyl moieties act as the electron
donor. This leads to two new Ru(NO) complexes perfectly
characterized and capable of releasing NO with a quantum yield
of 0.01. The extension of the π-delocalized framework, leads to
an enhancement of the TPA cross-section from 108 to 150 GM,
which is lower than the value predicted using the crude but
commonly used “two-level” prediction. Although there is no
clear evidence for a TPA saturation in these species, the present
*
investigation suggests that the most efficient NO donors in the
family of Ru(NO) complexes would certainly benefit from
geometries of greater complexity than that of the first
generation of 1-dimensional (“push-pull”) systems. In this
regard, an enhancement of conjugation between structural
fragments of such complexes would result in enhanced TPA
parameters of equation 4 (f, μee-μgg, Ege) and hence that of σTPA
,
on the size of a molecule. A more fruitful approach is to use the
concept of normalized cross-section (σTPA/Ne) which refers to
the cross-section per π-electron.
*
Several investigations have been reported in the literature
response and consequently of their NO release capabilities via
with the aim of finding the optimized size to maximize σTPA
/
the uncaging cross section (σTPA ×ϕNO).
Table 4. Experimental cross-sections (σTPA in GM) compared with the computed data: Energies (Ege in eV), oscillator strength (f), change in dipole moment
during transition (μee–μgg in D).
Experimental data
Computed data (equation 4)
compound
[1](PF6)3
[2](PF6)3
[3](PF6)3
σTPA
108
131
150
Relative[a] σTPA
1
1.21
1.39
Ege
f
μee–μgg
31.39
36.12
37.05
relative1 σTPA
1
3.81
3.79
2.886
2.563
2.712
0.314
0.802
0.802
[a] [1](PF6)3 being used as the reference.
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doi.org/10.1002/ejic.202100109
116.80, 55.23, 51.81, 40.46, 31.60, 29.79, 23.86, 22.69, 14.12. HRMS
Experimental Section
(ESI): calculated for C29H39O2 (M+H)+: 419.2944, found: 419.2946.
Materials and Equipment
Synthesis of 2c and 2d. The solution of DIBALÀ H (30 mL, 1.5 M,
45 mmol) was added dropwise into the solution of 2b (6.28 g,
15 mmol) in anhydrous toluene (150 mL) under inert atmosphere at
Methylacrylate,
triethylamine,
diisobutylaluminum
hydride
(DIBALÀ H) solution in toluene, Pd(PPh3)2Cl2, Pd(CH3COO)2, CuI were
purchased from Sigma-Aldrich; LiCl, NH4PF6, 2-acetylpyridine and 2-
bromofluorene from Alfa Aesar; 2,2’-bipyridine, ethyl pyridine-2-
carboxylate, ethynyltrimethylsilane, trifluoromethanesulfonic anhy-
dride, Pd(PPh3)4, Pd(o-tol)3 from TCI; NaNO2 from Fluka, RuCl3 · xH2O
(Ru: 40–49%) from STREM. Chemicals and solvents were analytical
°
À 78 C. The reaction mixture was stirred vigorously until the
complete disappearance of the starting material (TLC control). Then
MeOH (3.6 mL) was added slowly to the reaction mixture until
complete evolution of H2 followed by quick addition of AcOEt
(300 mL). The mixture was allowed to reach room temperature and
stirring vigorously for 1 hour followed by filtration through celite;
the filtrate was evaporated under reduced pressure. The resulting
mixture was dissolved in CHCl3 (225 mL) and MnO2 (9.78 g,
112.5 mmol) was added. The heterogeneous mixture was stirred at
room temperature until complete disappearance of starting materi-
al (TLC control, for 2–3 days). After completion of the reaction, the
mixture was filtered through celite and the filtrate was concen-
trated under reduced pressure. The residues were purified by
column chromatography (eluent: n-hexane/EtOAc, 99:1) on silica
gel to yield 3.43 g (8.2 mmol, 70%) of 2d as a yellow oil. 1H NMR of
compound 2d (300 MHz, CDCl3), δ 9.74 (d, J=7.7 Hz, 1H), 7.77–7.70
(m, 2H), 7.58 (d, J=15.9 Hz, 1H), 7.56 (dd, J=7.9, 1.6 Hz, 1H), 7.53
(d, J=1.6 Hz, 1H), 7.39–7.34 (m, 3H), 6.79 (dd, J=15.9, 7.7 Hz, 1H),
2.03–1.94 (m, 4H), 1.15–0.97 (m, 12H), 0.75 (t, J=6.9 Hz, 6H), 0.67–
0.55 (m, 4H); NMR 13C (75 MHz, CDCl3) δ 193.88, 153.74, 151.72,
151.62, 144.83, 140.04, 132.88, 128.34, 128.18, 127.84, 127.17,
123.15, 122.82, 120.51, 120.34, 55.30, 40.42, 31.59, 29.76, 23.85,
22.68, 14.11. HRMS (ESI): calculated for C28H37O (M+H)+: 389.2839,
found: 389.2840.
[13]
grade and used without further purification. [1](PF6)3 and 3c, 3d,
3e[21] were prepared as previously described.
Elemental analyses were performed at LCC with a Perkin–Elmer
2400 series II Instrument. The H NMR and 13C NMR spectra were
1
recorded at 298 K with a Bruker Avance 400 spectrometer, using
CDCl3, CD3OD or CD3CN as an internal reference. The IR spectra
were recorded with a Perkin–Elmer (FTIR/FIR) 100 Spectrometer.
The ESI mass spectra were performed on a UPLC Xevo G2 Q TOF
(Waters) spectrometer and Bruker Daltonics–micrOTOFÀ Q III spec-
trometer. UV-Vis spectra were obtained on a Perkin Elmer Lambda
35 UV-Vis spectrometer. Electron paramagnetic resonance experi-
ments (EPR) were performed on a Bruker ESP 500E spectrometer.
The atom labeling for the complete assignment of [2](PF6)3, and [3]
(PF6)3 by NMR spectroscopy is provided in Supplementary Materials.
Synthesis
Synthesis of 2a. 2-Bromofluorene (11.65 g, 0.048 mol) and KІ (0.79 g,
4.76 mmol) were dissolved in DMSO (96 mL) upon slight heating.
After complete dissolution the mixture was degassed and placed
under inert atmosphere. KOH (9.16 g, 0.164 mol) was then pestled
and added carefully to the solution under Ar flow. The mixture was
stirred for 20 min followed by slow, dropwise addition of n-
bromohexane (17 mL, 0.121 mol) in 5 portions under vigorous
stirring. The interval between portions was 10 min. After the
addition was completed, the mixture was left stirring at room
temperature for 3 hours followed by water addition and extraction
with EtOAc. The organic layer was washed with brine and dried
over Na2SO4, and the solvent was evaporated under reduced
pressure. The product was purified by column chromatography
(eluent: pentane, 100%) with a quantitative yield of 19.63 g of 2a
as a colorless oil. 1H NMR (300 MHz, CDCl3) δ 7.70–7.64 (m, 1H), 7.56
(d, J=7.5, 1H), 7.46 (s, 1H), 7.45 (dd, J=8.0, 1.9 Hz, 1H), 7.34–7.31
(m, 3H), 2.02–1.85 (m, 4H), 1.19–0.97 (m,12H), 0.78 (t, J=7.0 Hz, 6H),
0.69–0.51 (m, 4H).
Synthesis of 2e. A solution of NaOH (1.35 mL, 1 M, 1.35 mmol) in
water was added into the solution of 2d (0.35 g, 0.90 mmol) and 2-
acetylpyridine (0.10 mL, 0.90 mmol) in EtOH under vigorous stirring.
The reaction mixture was stirred at room temperature until
complete conversion of starting material (TLC control). The
resulting precipitate was filtered off, rinsed with water and cold
EtOH and dried under vacuum to give 2e (0.20 g, 0.52 mmol) in
45% yield. After purification of the filtrate by column chromatog-
raphy (elution system: hexane–AcOEt, 9:1) additional 0.06 g were
obtained (13%). The total yield was 58%, yellow powder; mp. 104–
1
°
105 C; H NMR (400 MHz, THF-d8) δ 8.69 (ddd, J=4.7, 1.7, 0.9 Hz,
1H), 8.11 (ddd, J=7.7, 1.3, 0.9 Hz, 1H), 7.93 (d, J=15.4 Hz, 1H), 7.90
(ddd, J=7.7, 7.5, 1.7 Hz, 1H), 7.77–7.65 (m, 3H), 7.65 (d, J=1.5 Hz,
1H), 7.55 (dd, J=7.9, 1.5 Hz, 1H), 7.50 (ddd, J=7.5, 4.7, 1.3 Hz, 1H),
7.41–7.33 (m, 1H), 7.35–7.23 (m, 3H), 7.19 (d, J=15.4 Hz, 1H), 2.14–
1.97 (m, 4H), 1.17–0.96 (m, 12H), 0.75 (t, J=7.0 Hz, 6H), 0.70–0.55
(m, 4H); 13C NMR (100 MHz, THF-d8) δ 189.02, 155.49, 152.11, 151.99,
149.76, 144.95, 143.52, 143.21, 141.70, 137.78, 136.70, 128.34,
128.06, 127.80, 127.78, 127.54, 125.09, 123.68, 123.07, 122.29,
120.85, 120.84, 55.93, 41.28, 32.49, 30.69, 24.70, 23.48, 14.37. HRMS
(ESI): calculated for C35H42NO (M+H)+: 492.3261, found: 491.3261.
Synthesis of 2b. Methylacrylate (1.35 mL, 15 mmol) and anhydrous
DMF (10 mL) were added to the mixture of 2a (4.13 g, 10 mmol),
Pd(OAc)2 (67 mg, 0.30 mmol), P(o-tol)3 (228 mg, 0.75 mmol) and
Et3N (1.4 mL, 10 mmol) under inert atmosphere. The resulting
°
suspension was heated at 90 C under vigorous stirring over 14 h.
Synthesis of ligand A. 1-(2-Oxo-2-(pyridin-2-yl)ethyl)pyridin-1-ium
iodide (0.32 g, 0.99 mmol) and ammonium acetate (1.04 g,
13.5 mmol) were added to the suspension of 2e (0.44 g, 0.9 mmol)
in EtOH (5 mL) and the mixture was refluxed until complete
conversion of starting ketone 2e (TLC control). After the reaction
finished the mixture was cooled down and the solvent was
evaporated under reduced pressure. The residue was diluted with
water and extracted with EtOAc. The organic extracts were dried
over Na2SO4 followed by filtration and solvent evaporation under
reduced pressure. The residue was purified two times by column
chromatography on alumina, using CH2Cl2 in the first column;
hexane /acetone, 9:1 for the second column to yield 0.19 g
After the completion of the reaction, the suspension was cooled
down to room temperature followed by adding water (100 mL).
The mixture was extracted with EtOAc; the organic extracts were
washed with brine, dried over Na2SO4 and filtrated. The solvent was
evaporated under reduced pressure and the residues were purified
by column chromatography (eluent: n-hexane/acetone, 100:2) on
1
silica gel to yield 3.43 g (8.20 mmol, 82%) of 2b as a yellow oil. H
NMR (400 MHz, CDCl3) δ 7.79 (d, J=16.0 Hz, 1H), 7.74–7.66 (m, 2H),
7.51 (dd, J=7.9, 1.6 Hz, 1H), 7.49 (d, J=1.6 Hz, 1H), 7.36–7.32 (m,
3H), 6.50 (d, J=16.0 Hz, 1H), 3.83 (s, 3H), 2.01–1.93 (m, 4H), 1.14–
1.00 (m, 12H), 0.76 (t, J=7.1 Hz, 6H), 0.65–0.56 (m, 4H); 13C NMR
(100 MHz, CDCl3), δ 167.80, 151.50, 151.48, 145.68, 143.79, 140.31,
133.27, 127.96, 127.58, 127.07, 123.10, 122.47, 120.30, 120.15,
1
(0.32 mmol, 36%) of the ligand A as a yellow oil. H NMR (400 MHz,
CD3CN) δ 8.72 (ddd, J=4.8, 1.8, 0.9 Hz, 2H), 8.66 (ddd, J=7.8, 1.2,
0.9 Hz, 2H), 8.64 (s, 2H), 7.95 (ddd, J=7.8, 7.6, 1.8 Hz, 2H), 7.79–7.75
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(m, 3H), 7.68 (d, J=16.4 Hz, 1H), 7.65 (dd, J=7.8, 1.5 Hz, 1H), 7.48
water, chilled EtOH and diethyl ether. The complex was dried under
vacuum to yield brown-black solids.
(d, J=16.4 Hz, 1H), 7.43 (ddd, J=7.6, 4.8, 1.2 Hz, 2H), 7.43–7.40 (m,
1H), 7.37–7.31 (m, 2H), 2.12–1.98 (m, 4H), 1.14–0.94 (m, 12H), 0.73 (t,
J=7.0 Hz, 6H), 0.62–0.45 (m, 4H); NMR 13C (100 MHz, CD3CN) δ
156.93, 156.78, 152.32, 152.07, 150.23, 148.02, 142.88, 141.59,
138.09, 136.58, 134.57, 128.44, 127.98, 127.7, 126.89, 125.13, 124.00,
122.45, 121.91, 120.88, 120.83, 118.65, 56.02, 40.82, 32.18, 30.21,
24.64, 23.16, 14.20. HRMS (ESI): calculated for C42H46N3 (M+H)+:
592.3686, found: 592.3689.
Ru(A)Cl3: yield 90%. Ru(B)Cl3: yield 58%.
General procedure for synthesis of [Ru(L)(bipy)Cl]Cl: Complex Ru(L)Cl3
(1 equiv), 2,2’-bipyridine (1 equiv), LiCl (6 equiv) were added to the
mixture of 75% EtOH/25% H2O mixture (92 mL for 1 mmol of Ru(L)
Cl3), followed by Et3N (1.6 equiv) addition. The mixture was refluxed
for 3 h; the flask was covered with aluminum foil to protect the
complex from light. After completion of the reaction, an undesired
precipitate was filtered off from the hot solution. The precipitate
was washed by ethanol and the filtrate was concentrated under
reduced pressure to 1/6 of the initial volume and kept in the fridge
Synthesis of 3a and 3b. The mixture of 2a (3.14 g, 7.60 mmol),
activated CuI (72 mg, 0.38 mmol) and PdCl2(PPh3)2 (133 mg,
0.189 mmol) was placed under inert atmosphere followed by
addition of anhydrous THF (10 mL). The reaction mixture was
heated to reflux followed by dropwise addition of DIPA (6 mL,
43 mmol) and ethynyltrimethylsilane (2.7 mL, 19 mmol), the con-
denser being intensively chilled (the boiling point of
°
at 5 C over 2 h. The obtained precipitate was filtered off and
thoroughly rinsed with aqueous HCl solution (3 M), a small amount
of chilled EtOH and diethyl ether. The product was dried under the
vacuum to yield purple solids.
°
ethynyltrimethylsilane is 57 C). After 12 h, the solution was cooled
1
down, and EtOAc (15 mL) was added followed by washing with
saturated NH4Cl solution. The product was purified by column
chromatography (eluent: hexane, 100%) over silica gel to yield
1.84 g (4.26 mmol, 56%) of 3a, which was then suspended in the
mixture of methanol (10 mL) and diethyl ether (10 mL) followed by
potassium carbonate addition (2.11 g, 15 mmol). The mixture was
stirred for 90 min and filtered, the filtrate was evaporated under
reduced pressure. The residue was dissolved in hexane and filtered
through silica gel which was thoroughly rinsed with hexane. The
filtrate was evaporated under reduced pressure to yield 1.24 g
(3.46 mmol, 81%) of 3b as a colorless oil. 1H NMR of compound 3b
(400 MHz, CDCl3) δ 7.73–7.70 (m, 1H), 7.67 (dd, J=7.0, 1.3 Hz, 1H),
7.51 (dd, J=7.0, 1.4 Hz, 1H), 7.50 (s, 1H), 7.40–7.31 (m, 3H), 3.14 (s,
1H), 2.01–1.95 (m, 4H), 1.22–0.98 (m, 12H), 0.79 (t, J=7.1 Hz, 6H),
0.68–0.56 (m, 4H); 13C NMR (100 MHz, CDCl3) δ 151.2, 150.9, 142.1,
140.4, 131.2, 127.8, 127.0, 126.7, 123.0, 120.3, 120.2, 119.7, 84.9,
77.0, 55.2, 40.5, 31.6, 29.8, 23.8, 22.7, 14.1.
[Ru(A)(bipy)Cl]Cl: yield 69%. H NMR (400 MHz, CD3OD) δ 10.29 (d,
J=5.1 Hz, 1H), 8.89 (s, 2H), 8.78 (d, J=8.0 Hz, 1H), 8.56 (d, J=7.8 Hz,
2H), 8.49 (d, J=8.0 Hz, 1H), 8.31 (t, J=7.8 Hz, 1H), 8.10 (d, J=
16.3 Hz, 1H), 8.05–8.00 (m, 1H), 7.88–7.65 (m, 9H), 7.59 (d, J=
16.3 Hz, 1H), 7.42 (ddd, J=8.3, 6.7, 3.5 Hz, 4H), 7.33–7.26 (m, 2H),
7.05 (t, J=6.5 Hz, 1H), 2.23–2.07 (m, 4H), 1.17–1.00 (m, 12H), 0.78 (t,
J=7.0 Hz, 6H), 0.72–0.59 (m, 4H).
1
[Ru(B)(bipy)Cl]Cl: yield 83%. H NMR (400 MHz, CD3OD) δ 10.23 (d,
J=5 Hz, 1H), 8.82 (s, 2H), 8.79 (d, J=8.2 Hz, 1H), 8.57 (d, J=8.0 Hz,
2H), 8.50 (d, J=7.9 Hz, 1H), 8.34 (td, J=7.9, 1.5 Hz, 1H), 8.02 (ddd,
J=7.5, 5.6, 1.2 Hz, 1H), 7.91 (td, J=7.9, 1.5 Hz, 2H), 7.87 (d, J=
7.8 Hz, 1H), 7.85–7.83 (m, 1H), 7.78–7.67 (m, 5H), 7.47–7.38 (m, 4H),
7.35 (ddd, J=7.6, 5.5, 1.3 Hz, 2H), 7.06 (ddd, J=7.3, 5.8, 1.3 Hz, 1H),
2.14–2.07 (m, 4H), 1.19–0.99 (m, 12H), 0.79 (t, J=7.0 Hz, 6H), 0.66–
0.56 (m, 4H).
General procedure for synthesis of [Ru(L)(bipy)NO2]Cl: Complex [Ru-
(L)(bipy)Cl]Cl (1 equiv) and NaNO2 (10 equiv) were dissolved in 75%
EtOH/25% H2O mixture (124 mL for 1 mmol of [Ru(L)(bipy)Cl]Cl). The
mixture was refluxed over 3 h; the flask was covered with aluminum
foil to protect the complex from light. After completion of the
reaction, the flask was cooled down and the volume of solvent was
Synthesis of ligand B. The mixture of 3e (311 mg, 0.87 mmol), 3b
(330 mg, 0.87 mmol), [Pd(PPh3)4] (50 mg, 0.043 mmol) was degassed
and placed under inert atmosphere. Anhydrous toluene (60 mL)
and DIPA (8.2 mL) were added to the reaction mixture and the
°
resulting mixture was heated at 80 C over 36 h. After completion
°
reduced to 1/6 of the initial volume. The residue was kept at 5 C,
of the reaction (progress controlled by TLC), the reaction mixture
was cooled down and evaporated under reduced pressure. The
residue was dissolved in CH2Cl2, rinsed with water, brine and dried
over Na2SO4. The solvent was evaporated at reduced pressure. The
product was purified by two successive column chromatography
on silica gel: 1) elution with pentane/CH2Cl2, 9:1; 2) elution with
pentane/EtOAc, 10:1 to yield 213 mg (42%) of ligand B, as a
for 2 h. The obtained precipitate was filtrated off and rinsed with
cold water, a small amount of cold EtOH and diethyl ether. The
complex was dried under the vacuum to yield dark red solids.
1
[Ru(A)(bipy)NO2]Cl: yield 66%. H NMR (400 MHz, CD3OD) δ 9.91 (d,
J=4.8 Hz, 1H), 8.78 (d, J=8.1 Hz, 1H), 8.74 (s, 2H), 8.55 (d, J=8.1 Hz,
1H), 8.46 (d, J=8.0 Hz, 2H), 8.32 (td, J=8.0, 1.5 Hz, 1H), 8.04–7.99
(m, 1H), 7.96 (d, J=16.3 Hz, 1H), 7.92–7.80 (m, 6H), 7.77–7.73 (m,
2H), 7.73–7.68 (m, 1H), 7.52 (d, J=16.3 Hz, 1H), 7.47–7.43 (m, 2H),
7.42–7.36 (m, 2H), 7.36–7.31 (m, 2H), 7.19–7.13 (m, 1H), 2.21–2.07
(m, 4H), 1.16–1.00 (m, 12H), 0.76 (t, J=7.0 Hz, 6H), 0.70–0.58 (m,
4H).
1
yellowish oil. H NMR (400 MHz, CDCl3) δ 8.74 (dd, J=4.7, 1.7 Hz,
2H), 8.64 (m, 4H), 7.92–7.82 (td, J=7.7, 1.7 Hz, 2H), 7.75–7.68 (m,
2H), 7.58 (s, 1H), 7.57 (d, J=8.5 Hz, 1H), 7.40–7.30 (m, 5H), 2.08–1.92
(m, 4H), 1.20–0.99 (m, 12H), 0.80 (t, J=7.0 Hz, 6H), 0.72–0.55 (m,
4H). 13C NMR (100 MHz, CDCl3) δ 155.8, 155.6, 151.3, 151.0, 149.3,
142.3, 140.3, 137.0, 133.8, 131.0, 127.8, 127.0, 126.6, 124.1, 123.0,
122.9, 121.3, 120.6, 120.3, 119.8, 95.3, 87.7, 55.3, 40.5, 31.6, 29.8,
23.9, 22.7, 14.1. HRMS (APCI): calculated for C42H43N3 (M)+: 589.3451,
found: 589.3438.
1
[Ru(B)(bipy)NO2]Cl: yield 46%. H NMR (400 MHz, CD3OD) δ 9.90 (d,
J=5.3 Hz, 1H), 8.78 (d, J=8.2 Hz, 1H), 8.76 (s, 2H), 8.55 (d, J=8.1 Hz,
1H), 8.50 (d, J=8.0 Hz, 2H), 8.34 (td, J=7.9, 1.4 Hz, 1H), 8.02 (ddd,
J=7.5, 5.9, 1.1 Hz, 1H), 7.93 (td, J=7.9, 1.4 Hz, 2H), 7.90–7. 81 (m,
3H), 7.79 (s, 1H), 7.75 (d, J=8.6 Hz, 2H), 7.67 (dd, J=7.8, 1.3 Hz, 1H),
7.52–7.32 (m, 6H), 7.18 (ddd, J=7.6, 5.7,1.3 Hz, 1H), 2.19–2.00 (m,
4H), 1.21–1.01 (m, 12H), 0.78 (t, J=6.9 Hz, 6H), 0.70–0.54 (m, 4H).
General procedure for the synthesis of Ru(L)Cl3: Ligands A or B
(1 mmol) and RuCl3 ·xH2O (Ru: 40–49%; 251 mg for 1 mmol of
ligand, 1–1.22 eq.) were suspended in ethanol (65 mL for 1 mmol of
ligand). The mixture was refluxed over 3 h; the flask was covered
with aluminum foil to protect the complex from light. After 3 h the
flask was cooled down and the volume of solvent was reduced to
1/3 of the initial volume upon evaporation under reduced pressure.
General procedure for synthesis of [Ru(L)(bipy)NO](PF6)3: Complex
[Ru(L)(bipy)NO2]Cl (1 equiv) was dissolved in EtOH (108 mL for
1 mmol of [Ru(L)(bipy)NO2]Cl), and the solution was mixed with the
solution of HCl (12 M, 108 mL for 1 mmol of [Ru(L)(bipy)NO2]Cl,
1300 equiv) in EtOH (216 mL for 1 mmol of [Ru(L)(bipy)NO2]Cl). The
°
The residue was left in the fridge at 5 C for 2 h. The obtained
precipitate was filtered off and was thoroughly rinsed with cold
°
mixture was heated to 60 C and allowed to stand under stirring for
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1 h; the flask was covered by aluminum foil to protect the complex
allows reaching apparent stable absorption conditions. The UV-vis
spectra were recorded under irradiation realized with a Muller
reactor device equipped with a cooling water filter and a mercury
arc lamp equipped with appropriate interference filter to isolate the
desired irradiation wavelength (λmax =436 nm). The temperature
from light. After completion of the reaction, the flask was cooled
down and the volume of solvent was reduced to 1/5 of the initial
volume followed by addition of saturated solution of NH4PF6
°
(15 equiv) in water. The mixture was kept at 5 C for 2 h. The
°
obtained precipitate was filtrated off, rinsed with a small amount of
was maintained at 25 C during the whole experiment.
cold EtOH and diethyl ether. The complex was dried under vacuum.
Quantum yield measurements: Light intensities were determined
before each photolysis experiments by chemical actinometry
procedure. The actinometers used were potassium ferrioxalate and
measured at λirr =436 nm before each experiment with [2](PF6)3
and [3](PF6)3 (I0 =7.0.10À 7 mol.LÀ 1.sÀ 1 and 8.7.10À 7 mol.LÀ 1.sÀ 1 respec-
tively). The quantum yield (ϕΑ) for [2](PF6)3 and [3](PF6)3 was
determined by the program Sa3.3 written by D. Lavabre and V.
Pimienta.[50] It allows the resolution of the differential equation (5):
1
[2](PF6)3: yield 63%. H NMR (400 MHz, CD3CN) δ 9.30 (d, J=5.7 Hz,
1H), 8.92 (s, 2H), 8.81 (d, J=7.8 Hz, 1H), 8.74 (d, J=7.4 Hz, 2H), 8.69
(td, J=8.1, 1.4 Hz, 1H), 8.61 (d, J=7.7 Hz, 1H), 8.49 (td, J=7.9,
1.4 Hz, 2H), 8.32 (d, J=16.3 Hz, 1H), 8.33–8.27 (m, 1H), 8.27–8.23 (m,
1H), 8.01 (dd, J=5.6, 1.1 Hz, 2H), 7.95 (d, J=8.1 Hz,1H), 7.94 (s, 1H),
7.90–7.83 (m, 2H), 7.74 (d, J=16.2 Hz, 1H), 7.71 (ddd, J=7.5, 5.4,
1.1 Hz, 2H), 7.53–7.39 (m, 4H), 7.32 (dd, J=6.0, 0.9 Hz, 1H), 2.21–
2.09 (m, 4H), 1.15–0.97 (m, 12H), 0.77 (t, J=6.9 Hz, 6H), 0.67–0.52
(m, 4H). Elemental analysis calcd (%) for C52H53N6ORuP3F18: C 47.53,
H 4.07, N 6.40; found: C 47.26, H 4.09, N 6.74. IR (neat film): 2929
cmÀ 1, 1942 cmÀ 1 ν(NO), 1592 cmÀ 1, 1481 cmÀ 1, 1046 cmÀ 1, 836 cmÀ 1
(5)
where IAa is the intensity of the light absorbed by the precursor;
AbslA, the absorbance before irradiation; AbsTlot, the total absorb-
ance; I0, the incident intensity measured at 436 nm; and F, the
photokinetic factor given by equation (6):
(PF6), 557 cmÀ 1
.
1
[3](PF6)3: yield 66%. H NMR (300 MHz, CD3CN) δ 9.31 (d, J=5.8 Hz,
1H), 8.96 (s, 2H), 8.82 (d, J=8.1 Hz, 1H), 8.75–8.66 (m, 3H), 8.63 (d,
J=8.1 Hz, 1H), 8.48 (t, J=8.2 Hz, 2H), 8.31 (t, J=8.0, 1H), 8.28–8.21
(m, 1H), 8.02 (d, J=5.2 Hz, 2H), 7.97 (d, J=7.8 Hz, 1H), 7.92–7.79 (m,
3H), 7.78–7.68 (m, 2H), 7.56–7.39 (m, 4H), 7.34 (d, J=5.9 Hz, 1H),
2.27–2.05 (m, 4H), 1.21–0.95 (m, 12H), 0.77 (t, J=6.8 Hz, 6H), 0.66–
0.46 (m, 4H). Elemental analysis calcd (%) for C52H51N6ORuP3F18 : C
47.60, H 3.92, N 6.41; found: C47.61, H 3.66, N 6.42. IR (neat film):
2927 cmÀ 1, 2201 cmÀ 1 ν(C�C), 1945 cmÀ 1 ν(NO), 1600 cmÀ 1, 1480
(6)
The equation was fitted with the experimental data AbslTot ¼ fðtÞ
and 2 parameters ϕΑ and ɛΒ (ɛΒ is the molar extinction coefficient
measured at the end of the reaction) at two wavelengths (λirr =
436 nm, λobs =370 nm). λobs was chosen because it corresponds to a
large difference between molar extinction coefficient at the initial
and final time of the photochemical reaction. Simulation and
optimization procedures were performed by using numerical
integration and a non-linear minimization algorithm for the fitting
of the model to the experimental data.[50,51]
cmÀ 1, 835 cmÀ 1 (PF6), 557 cmÀ 1
.
Computational studies
The ruthenium complexes [2]+, and [3]3+ were fully optimized in
gas phase using the Gaussian-09 program package[44] within the
framework of the Density Functional Theory (DFT). The double-ζ
basis set 6–31G* was used for all atoms except the heavy
ruthenium atom, for which the LANL2DZ basis set was applied to
account for relativistic effects.[45] To be consistent with our previous
For [2](PF6)3:
report on [1]3+ [13]
we have selected the hybrid functional B3PW91
,
for the optimization. B3PW91 has been shown to outperform other
hybrid functionals (e.g. B3LYP) and pure functionals (e.g. PW91) in
numerous cases of ruthenium complexes, especially when back
bonding ligands (like NO) are present.[46,47] The vibrational analyses
were performed at the same level to verify that the stationary
points correspond to minima on the potential energy surfaces. The
UV-visible electronic spectra were then computed at the CAM-
B3LYP/6-31G* level, for consistency with our previous investigation
For [3](PF6)3:
Z-scan measurements
of 13+ [13]
This long-range corrected hybrid functional is also
.
The Z-scan technique[13] was used to measure the nonlinear
absorption coefficient of the samples at 800 nm using short laser
pulses of 80 fs at 1 KHz of repetition rate. Molecules under study
were dissolved in acetonitrile at the concentration of 1×
10À 2 mol.LÀ 1. Z-scan traces for each solution were measured at
different energies (40, 62 and 100 nJ). All samples were measured
at least five times for each energy. Before measuring the samples,
the nonlinear transmission of the laser dye rhodamine 6G (R6G)
was tested with the open aperture approach to calibrate the
apparatus. In this case, the value of σTPA of R6G used to calibrate
the system was the one widely accepted in the literature.[52] Then
the nonlinear absorption coefficient β of each sample was obtained
after fitting the normalized transmission T(z) to Z-scan formalism.
reported as being particularly well suited for studying molecules
with very delocalized excited states.[48] Solvent effects were
included by using the polarizable continuum model (PCM)
implemented in Gaussian09 for acetonitrile (ɛ=35.688). Molecular
orbitals were plotted with GABEDIT 2.4.8.[49]
Photochemical studies
Photochemistry: Kinetic studies on the photolysis reactions were
carried out with a diode array Hewlett Packard 8454 A spectropho-
tometer. Solutions of 2 mL of [2](PF6)3 (4.43 10À 5 mol.LÀ 1) and
[3](PF6)3 (4.48 10À 5 mol.LÀ 1) for irradiation at 436 nm in non-
deoxygenated acetonitrile were used. The optical fiber was fixed
laterally from the cuvette. Absorption spectra were taken after each
minute, in fast scan mode, during a period of irradiation at 436 nm
of 6 hours and 4 hours for [2](PF6)3 and [3](PF6)3 respectively, which
The TPA cross-section (σTPA
expression equation (7):
)
is obtained from the following
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doi.org/10.1002/ejic.202100109
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(7)
Were N is the molecular density and ω is the optical frequency.
Supporting Information (see footnote on the first page of this
article): UV-vis of the ligands; DFT atomic coordinates for 13+, 23+
,
and 33+; comparison of coordination spheres, computed (DFT) and
experimental (X-ray for [1](PF6)3); NMR spectra. Photorelease from
[1](PF6)3; Infra-red spectra for [2](PF6)3) and [3](PF6)3).
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: Ruthenium nitrosyl · Photochemistry · Two-photon
absorption · Ligands design
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Manuscript received: February 8, 2021
Revised manuscript received: March 15, 2021
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