Chemically regulating Rh(i)-Bodipy photoredox switches

Article abstract of DOI:10.1039/c4cc01345j

The ability of Rh(i) centers to undergo photoinduced electron transfer from discrete metal orbitals to Bodipy fluorophores is mediated through reversible coordination chemistry. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01345j

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Chemically regulating Rh(I)-Bodipy photoredox
switches†
Cite this: Chem. Commun., 2014,
50, 6850
A. M. Lifschitz,a R. M. Young,ab J. Mendez-Arroyo,a V. V. Roznyatovskiy,ab
C. M. McGuirk,a M. R. Wasielewskiab and C. A. Mirkin*a
Received 20th February 2014,
Accepted 2nd May 2014
DOI: 10.1039/c4cc01345j
www.rsc.org/chemcomm
The ability of Rh(I) centers to undergo photoinduced electron transfer coordination complexes incorporating Bodipy moieties can be used
from discrete metal orbitals to Bodipy fluorophores is mediated to switch Bodipy’s fluorescence on and off by modulating the ability
through reversible coordination chemistry.
of the metal center to undergo photoinduced electron transfer (PeT)
to the fluorophore. The inclusion of Bodipy as the electron acceptor
Rh(III) coordination complexes have been extensively utilized as provides us with a straightforward spectroscopic handle for the PeT
strong photo-oxidants in broad areas of chemistry, such as photo- process.7 Thus, we are able to achieve coordination chemistry-based
redox catalysis and materials for photovoltaic cells.1 While the control of PeT from a discrete dz2 Rh(I) orbital to Bodipy by either
generation of photo-oxidized Rh species can also have important controlling the oxidation potential of the metal center, yielding a
implications in organometallic and catalytic transformations,2 the switch in which PeT is disabled via ligand chelation (Fig. 1A); or by
role of monometallic Rh(I) complexes as photo-reductants remains affecting the Bodipy’s redox potential, resulting in a system in which
largely unexplored.3 Typically, monometallic Rh(I) complexes are PeT is triggered by ligand chelation (Fig. 1B). As a whole, the Bodipy-
unable to directly undergo the electronic transitions in solution that WLA platform enables the kinetically fast formation of photo-oxidized
efficiently transfer charge from the metal in ways that can be Rh species that decay slowly in time, thus offering both an easily
exploited for redox chemistry.4 However, the ability to modulate accessible platform for highly relevant photoredox transformations
the oxidation of Rh(I) by an ancillary photoactive species via simple and a novel means to control photoredox switches.
coordination transformations could be exploited for the generation
of novel molecular switches for photoredox processes with important on its redox potential, we first synthesized a pair of complexes in
applications in renewable energy and catalytic processes. which the charge of the first coordination sphere is varied. Thus,
To modulate the ability of Rh(I) to undergo photo-oxidation based
Rh(I) complexes are a powerful platform in the Weak-Link semiopen Rh(I) complex 1 incorporating Bodipy-functionalized
Approach (WLA), a methodology for the synthesis of supramolecular hemilabile ligand S3 (see Fig. S1, ESI†) was assembled via
constructs that can be toggled between active states through modula-
tion of the coordination chemistry of hemilabile ligands at d8 metal
nodes.5 Upon coordination to small analytes, Rh(I) serves as a
structural regulator to guide distances and orientations between
functional moieties embedded in the supramolecular structure,
resulting in molecular switches capable of sensing, signal amplifica-
tion, and control of catalytic activity.6 By exploiting high-lying, metal-
centered orbitals to engage in photoredox interactions within the
supramolecular environment, a further degree of control over photo-
active WLA structures could be achieved. Herein we show that Rh(I)
a Department of Chemistry and International Institute for Nanotechnology,
Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
E-mail: chadnano@northwestern.edu
b Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern
University, Evanston, Illinois 60208, USA
† Electronic supplementary information (ESI) available: Optical, time resolved and
Fig. 1 Modulation of PeT through coordination-based control of redox
potentials.
NMR spectra, and computational, spectroelectrochemical and electrochemical
data. See DOI: 10.1039/c4cc01345j
6850 | Chem. Commun., 2014, 50, 6850--6852
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Table 1 Spectral and electrochemical data
lmax abs
(nm)
lmax em
(nm)
QY
fluo. em
Bodipy
Ea (V)
Rh
Ea (V)
Complex
1
2
4
5
544
547
465/528
463
571
569
520
519
0.089
0.48
0.047
0.34
0.65
0.71
0.70
0.42
0.039
—
0.456
0.747
standard WLA procedures6 and reversibly converted to closed
complex 2 upon abstraction of a coordinated chloride. This trans-
formation effectively changes the charge of the Rh(I) first coordina-
tion sphere from neutral to positive. The Bodipy is appended to the
hemilabile coordinating moiety through a perfluorinated phenyl
bridge that does not allow for ground-state interactions with the
Rh(I). This particular ligand design precludes significant changes in
the inductive contributions to the Bodipy frontier orbitals upon
coordination changes.8 As a result, the introduction and abstraction
of chloride can be used to change the oxidation potential of the
metal center while retaining the redox properties of Bodipy (Table 1).
Ground-state computational and electrochemical analyses are
useful for assessing potential excited state interactions in Bodipy
systems, since the fluorophore displays relatively small Stokes shifts
and thus we can expect the energetics of both its ground- and
excited-states to be similar. In our system, DFT calculations of
complexes 1 and 2 show that the coordination of chloride results
in the destabilization of the metal dz2 orbital energy level to the
degree that it lies in between the Bodipy-centered frontier orbitals
(see Computational section, ESI†). Conversely, displacement of
chloride by the Bodipy-ligand thioether depletes the electron density
of Rh(I) and lowers the energy level of the dz2 orbital so that it now
lies below the Bodipy-centered HOMO. The computational results
are clearly projected onto the electrochemical behavior of complexes
1 and 2. As expected, the Bodipy-centered redox processes are largely
unaffected by the coordination chemistry of Rh(I) (Table 1). On the
other hand, an irreversible Rh-centered oxidation process is observed
at 39 mV for neutral complex 1 at a lower potential than the Bodipy
oxidation. This anodic wave disappears upon abstraction of chloride
in 2, supporting computational modelling of the effects of coordina-
tion on the energy level of the Rh(I)-centered dz2 orbital. Comparison
with Bodipy ligand S3 as well as with Rh(I) complexes S1 and S2,
which mimic the coordination environment of 1 and 2, respectively,
but do not incorporate a Bodipy moiety, supports the assignment of
the anodic wave at 39 mV to Rh(I) oxidation (see ESI†).
Fig. 2 Transient absorption spectra of 1 in CH2Cl2 (lex = 505 nm). Inset:
zoom-in of transient absorption from the Bodipy anion.
species can be found in its TA spectrum. Interestingly, the weak
Rh–S interaction in the perfluorinated Bodipy ligand precludes
significant stabilization of the meso-centered LUMO, so that PeT to
the perfluorinated phenyl spacer does not occur significantly.
Indeed, computational modelling of 2 shows that the LUMO is
spread throughout the Bodipy and the meso substituent, suggesting
that the energy of the two p systems is very similar.
The on–off PeT switch comprised by 1 and 2 provides the basis
for the design of a reverse off–on switch. That is, if one is to enable
PeT from Rh(I) in a WLA coordination complex in which a Bodipy
ligand is chelated to the metal centre and disable PeT when the
ligand is partially displaced, then neutral coordination spheres
should be avoided. In doing so, it can be ensured that partial
displacement of the of the Bodipy ligand results in a Rh(I) centre
that cannot be oxidized at lower potentials than the Bodipy moiety
itself. Furthermore, since the coordination changes effected by the
displacement of a neutral Bodipy ligand by another neutral coordi-
nating species are not expected to have a major effect on the
oxidation potential of Rh(I), then coordination changes should target
the oxidation potential of the Bodipy to control PeT. We can achieve
this by designing a novel Bodipy ligand 3 in which a phosphino-
alkylamine coordinating moiety is directly attached to the Bodipy
core, thereby allowing for control of the Bodipy ground state
energetics through coordination chemistry (Fig. 3, inset).
Amino-functionalization of the Bodipy 8-position has recently
attracted much attention.9 The effects of amino coordination on
sterically hindered 8-amino Bodipys, however, has not yet been
explored. We found that incorporation of ligand 3 into closed
complex 4 results in a red-shift of the Bodipy absorption, suggesting
that the fluorophore’s HOMO–LUMO energy levels can be modulated
through metal cation coordination (Fig. 3). Formation of semiopen
complex 5 via reaction with acetonitrile reconstitutes the original
absorption features of ligand 3. Since semiopen complex 5 is
The absorption peaks corresponding to the Bodipy-centered
p–p* and n–p* transitions are very similar in complexes 1 and 2,
supporting the lack of ground state interaction between Bodipy and
Rh(I) (see Fig. S5, ESI†). On the other hand, the transient absorption
(TA) spectrum of 1 shows that absorption is followed by fast
formation of a Bodipy radical anion species band at 600 nm
(Fig. 2), accompanied by a reduction in fluorescence quantum yield
(QY) to 8.9% (compared to 91% of the free ligand).8a A similar radical
Bodipy anion band is also observed upon controlled-potential,
bulk electrolysis of the Bodipy ligand at À1.6 V, thus providing
unequivocal spectral evidence of the PeT process (see Fig. S15, ESI†).
Closed complex 2, on the other hand, displays a QY that is 5.3 times
higher (48%), and no bands corresponding to reduced Bodipy
Fig. 3 Absorption spectra of ligand 3 and its Rh(I) complexes.
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generated through displacement with a neutral coordinating species, process capable of competing with other slower electron and energy
quenching through PeT from the Rh(I) is not possible as in the case transfer mechanisms. Thus, the use of Rh(I) as a photo-reductant can
of 1; the fluorescence QY thus increases from 4.7% in 4 to 34% in 5 allow for control of photocatalytic and photoredox systems by
(Table 1). These spectral effects are also accompanied by a large enabling fast energy funnelling pathways upon simple coordination
decrease in the absorption cross-section in 4, resulting in a fluores- changes. This approach stands in stark contrast with previous
cence intensity switching ratio between the two complexes of 27.4. methodologies for the control of active WLA systems, since in this
Interestingly, the sharp feature on the absorption spectrum of 4 at case the rational tuning of redox potentials rather than the use of
528 nm leads to very low fluorescence emission and it does not defined, and often unpredictable, supramolecular architectures is
appear in the fluorescence excitation spectrum (see ESI†), suggesting necessary to assure switchability. Furthermore, the interaction
that the peak in question corresponds to a charge transfer transition between Rh(I) and excited Bodipy moieties can affect the energetics
and that the Bodipy p–p* band is centered at 465 nm.
of redox and organometallic transformations based on the
The absorption and fluorescence behavior of complexes 4 and 5 metal, thus posing new venues to expand the already prominent
can be explained through computational modelling of their ground- capabilities of Rh(I) in the field of catalysis. The use of Bodipy is an
state, orbital energy levels. In these studies, closed complex 4 excellent complement for Rh(I) photoredox systems since the fluoro-
displays a Rh(I) dz2 HOMO flanked by the Bodipy frontier orbitals, phore’s redox and photophysical properties can also be rationally
suggesting that the low fluorescence QY may partly result from PeT addressed, as we have demonstrated in the development and
from Rh(I) to the fluorophore. Furthermore, while the Bodipy- successful use of novel 8-amino Bodipy ligand 3. Overall, through
centered HOMO displays a characteristic nodal plane on the meso the exploitation of Rh(I) as a photoreductant unit, we have intro-
position, the Bodipy-centered LUMO shows significant contribution duced a new paradigm for the development of molecular switches,
from the Rh–N coordination bond, thus providing a potential sensors and photo-mediated catalytic processes.
explanation for the charge transfer band in the absorption spectrum
This material is based upon work supported by the following
of 4. On the other hand, semiopen complex 5 displays HOMO and awards, National Science Foundation CHE-1149314, U.S. Army
LUMO orbitals fully centered on the Bodipy moiety with several W911NF-11-1-0229 (C.A.M.), and Chemical Sciences, Geosciences,
metal-centered occupied orbitals laying at lower energy levels.
and Biosciences Division, Office of Basic Energy Sciences,
The computational modelling of 4 and 5 is validated by the DOE under grant no. DE-FG02-99ER14999 (M.R.W.). J.M.A.
complexes’ spectroelectrochemical behaviour. The Rh-centered oxida- acknowledges a fellowship from Consejo Nacional de Ciencia
tion events occur at potentials lower than Bodipy-centered processes y Tecnologia (CONACYT).
in closed complex 4, while the opposite is observed in semiopen
complex 5 (Table 1). Model complexes S4 and S5 that mimic the
coordination environment of 4 and 5, respectively, but that do not
incorporate Bodipy moieties were also synthesized and characterized
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