12 research outputs found
Charge-Transfer Dynamics in Nanorod Photocatalysts with Bimetallic Metal Tips
CdSe@CdS
dot-in-rod nanostructures tipped with AuPt bimetallic
nanoparticles as cocatalyst show increased photon-to-hydrogen conversion
efficiency compared to their analogues with pure Au or Pt tips. The
underlying charge-separation and recombination processes are investigated
by time-resolved transient absorption spectroscopy, to unravel whether
the observed enhancement of photocatalytic activity is due to charge-separation/recombination
properties of the system or to higher reactivity for proton reduction
at the surface of the metal nanoparticle. We find that in the catalytically
active Pt- and AuPt-functionalized structures charge separation occurs
with similar time constants (Pt 3.5, 35, and 49 ps; AuPt 2.6, 31,
and 66 ps), and the charge-separated state shows a lifetime of âź20
Îźs in both cases. Hence, these processes should not be regarded
as a source of the increased catalytic efficiency in the AuPt-functionalized
nanorods. The results indicate that the proton reduction at the metal
nanoparticle surface itself determines the overall efficiency
Photophysical Dynamics of a Ruthenium Polypyridine Dye Controlled by Solvent pH
The photophysics of the novel ruthenium dye [Ru(tmBiBzIm)(dppz)(tbbpy)]<sup>2+</sup> (tmBiBzIm = 5,5â˛,6,6â˛-tetramethyl-2,2â˛-bibenzimidazole, dppz = dipyrido[3,2-<i>a</i>:2â˛,3,3â˛-<i>c</i>]phenazine, tbbpy = 4,4â˛-di-<i>tert</i>-butyl-2,2â˛-bipyridine) is investigated, which might be suitable as a model compound for intracellular DNA and pH sensors. The combination of three different bidentate ligands allows for controlling the photophysics by two distinct mechanisms: (i) protonation and deprotonation of the tmBiBzIm and (ii) hydrogen bonding to the phenazine nitrogens of the dppz ligand. As will be reported, deprotonation of the tmBiBzIm ligand causes a bathochromic shift of the metal-to-ligand charge-transfer transition, although the tmBiBzIm ligand itself does not directly contribute to the light absorption. Furthermore, tmBiBzIm deprotonation shortens the overall excited-state lifetime of the complex significantly. Although the protonation stage of the tmBiBzIm directly impacts the excited-state properties of the dye, the overall photoinduced dynamics is dominated by the dppz ligand. Consequently, addition of water to the solvent affects the excited-state relaxation pathway as known from, for example, [Ru(phen)<sub>2</sub>dppz]<sup>2+</sup> (phen = 1,10-phenanthroline) complexes
Structural Control of Photoinduced Dynamics in 4<i>H</i>âImidazole-Ruthenium Dyes
The photoinduced dynamics of a series of terpyridine
4<i>H</i>-imidazole-ruthenium complexes, which constitute
a new family of
panchromatic dyes, is investigated. The dynamics involves two excited
states localized within the 4<i>H</i>-imidazole sphere.
Upon MLCT excitation, an excited state is populated, which is localized
on the central part of the 4<i>H</i>-imidazole ligand caused
by its nonplanar conformation. The population of the second excited
state is connected to a planarization of the 4<i>H</i>-imidazole
ligand as revealed by viscosity-dependent measurements, and the excess
electronic charge on the ligand is shifted to the terminal rings.
The impact on the photoinduced dynamics of the electronic character
of the substituent at the terminal rings and the protonation state
of the 4<i>H</i>-imidazole ligand is studied. Significant
changes in the lifetime of the excitation and alterations of the decay
mechanism are observed depending on the interplay of the electronic
character of the substituent and ligand protonation. In a NMe<sub>2</sub> substituted complex, the character of the substituent is
changed from electron donating to electron withdrawing upon stepwise
protonation, resulting in pH switchable decay mechanism
Increased Charge Separation Rates with Increasing DonorâAcceptor Distance in Molecular Triads: The Effect of Solvent Polarity
Distance-dependent
electron transfer in donorâspacerâacceptor
systems is accepted to occur via two distinct mechanisms, that is,
by coherent superexchange or incoherent hopping. In general, the rate
of electron transfer (<i>k</i><sub>ET</sub>) decreases with
increasing donorâacceptor distances, irrespective of the actual
mechanism being responsible for the process. However, recently Wenger
and his group showed that in the frame of the superexchange mechanism
electron-transfer rates can pass a maximum when increasing the transfer
distance. This manuscript presents an investigation of the forward
electron transfer in a series of donor (<i>N</i>-methylphenothiazine)âphotocenter
(RuÂ(II) bisÂ(terpyridine) complex)âacceptor (<i>N</i>-methylfulleropyrrolidine) triads that reveals the control of the
electron-transfer rates by solvent variation to an extent that in
acetonitrile an increasing electron-transfer rate is observed with
increasing donorâacceptor distance, while in dichloromethane
an increase in the separation causes the electron transfer rate to
drop. This behavior is qualitatively rationalized based on a recently
introduced model. Nonetheless, the quantitative mismatch between the
results presented here and the theory indicates that nonexponential
distance-dependent couplings will have to be considered in extending
the theory
A Novel Ru(II) Polypyridine Black Dye Investigated by Resonance Raman Spectroscopy and TDDFT Calculations
The optical properties of a new (bipyridine)<sub>2</sub>RuÂ(4<i>H</i>-imidazole) complex presenting a remarkable
broad absorption in the visible range are investigated. The strong
overlap of the absorption with the solar radiation spectrum renders
the studied complex promising as a black absorber and hence as a starting
structure for applications in the field of dye-sensitized solar cells.
The correlations between structural and electronic features for the
unprotonated and protonated forms are studied by means of UVâvis
absorption and resonance Raman (RR) spectroscopy modeled with the
help of time-dependent density functional theory (TDDFT) calculations.
The absorption spectra show two bands in the visible region, which
TDDFT assigns to a metal-to-ligand charge-transfer (MLCT) state and
to a superposition of three excited states with MLCT and intraligand
charge-transfer character, respectively. Additionally, the analysis
of the molecular orbitals and RR spectra in resonance with the first
MLCT band shows that the effects of protonation favor a charge-transfer
photoexcitation to the 4<i>H</i>-imidazole ligand. The RR
spectra simulated for several excitation wavelengths covering the
visible region are in excellent agreement with experimental data.
In particular, it is noteworthy that the calculations are able to
reproduce the wavelength dependence of the RR spectra provided that
corrected excitation energies are employed. Interference effects between
the electronic states contributing to the RR scattering are small
for the investigated complex
Photoredox-active Dyads Based on a Ru(II) Photosensitizer Equipped with Electron Donor or Acceptor Polymer Chains: A Spectroscopic Study of Light-Induced Processes toward Efficient Charge Separation
A photosensitizerâmultielectron-acceptor
dyad (PâA<sub><i>n</i></sub>) was synthesized via
controlled nitroxide-mediated
polymerization of styrenic naphthalene diimide (NDI) and subsequent
functionalization with a [RuÂ(dqp)<sub>2</sub>]<sup>2+</sup> photosensitizer
(dqp is 2,6-diÂ(quinolin-8-yl)Âpyridine) at the chain terminus. The
optical and electrochemical analysis showed the preserved properties
of the individual subunits, corroborated by the analysis of the related
multielectron donor assembly (D<sub><i>n</i></sub>âP)
based on triarylamine (TARA). A detailed photophysical study of both
dyads is presented to elucidate the primary light-induced energy-
and electron-transfer events. While the D<sub><i>n</i></sub>âP dyad displays the unchanged <sup>3</sup>MLCT-based (MLCT
is metal-to-ligand charge transfer) emission of the pristine photosensitizer,
the PâA<sub><i>n</i></sub> system revealed efficient
emission quenching and the occurrence of the NDI radical anion signature.
The time-resolved emission data revealed a nonmonoexponential decay
attributed to the conformational freedom by the flexible linkage,
while the transient absorption data confirmed the rapid formation
of the reduced acceptor
Excited State Dynamics of a Photobiologically Active Ru(II) Dyad Are Altered in Biologically Relevant Environments
In
this study femtosecond and nanosecond time-resolved transient
absorption spectroscopy was used to investigate the influence of ionic
strength and complexity on the excited state dynamics of a RuÂ(II)-based
metalâorganic dyad. The bis-heteroleptic complex [RuÂ(bpy)<sub>2</sub>(ippy)]<sup>2+</sup> (<b>1</b>), where bpy = 2,2â˛-bipyridine
and ippy = 2-(1-pyrenyl-1<i>H-</i>imidazoÂ[4,5-<i>f</i>]Â[1,10]Âphenanthroline, is a potent photosensitizer for in vitro photodynamic
therapy (PDT) and photodynamic inactivation (PDI) of microorganisms
owing to a long-lived triplet excited state derived from a metal-to-ligand
charge-transfer (<sup>3</sup>MLCT) state that is equilibrium with
an intraligand (<sup>3</sup>IL) state. The prolonged lifetime provides
ample opportunity for bimolecular quenching of this state by oxygen;
thus singlet oxygen (<sup>1</sup>O<sub>2</sub>) sensitization is very
efficient. In simple aqueous solution, fast cooling within the <sup>3</sup>MLCT manifold is followed by energy transfer to an <sup>3</sup>IL state, which is facilitated by rotation of a pyrenyl unit about
the imidazoâpyrenyl (ip) coannular bond. For solutions of <b>1</b> in high ionic strength simulated biological fluid (SBF),
a more physiologically relevant solvent that contains a complex mixture
of ions at pH 7.4, femtosecond studies revealed an additional excited
state, possibly based on an ionâligand interaction. This new
state appearing in high ionic strength SBF was not observable in water,
simple buffers, or low ionic strength SBF. These photoinduced dynamics
were also affected by the presence of biomolecules such as DNA in
simple buffer, whereby relaxation on the picosecond time scale was
accelerated from 39 to 18 ps with DNA intercalation by <b>1</b>. The increased rate of coplanarization of the pyrene and the imidazole
units was attributed to DNA-induced conformational restriction of
the pyrenyl unit relative to the ip bond. Quantitative changes to
excited state decay rates of <b>1</b> in solutions of high ionic
strength were also observed when probed on the microsecond time scale.
Notably, the thermalized excited state decay pathways were altered
substantially with DNA intercalation, with access to some states being
completely blocked. Experimentally, this manifested in the absence
of the slowest microsecond decay channel, which is normally observed
for <b>1</b> in solution. The quantitative and qualitative observations
from this study highlight the importance of employing biologically
relevant solvents and potential biomolecule targets when the excited
state dynamics and photophysical properties (under cell-free conditions)
responsible for the potent photobiological effects are assessed in
the context of photodynamic therapy and photodynamic inactivation
New Ruthenium Bis(terpyridine) Methanofullerene and Pyrrolidinofullerene Complexes: Synthesis and Electrochemical and Photophysical Properties
A series of terpyridine (tpy) methanofullerene
and pyrrolidinofullerene dyads linked via <i>p</i>-phenylene
or <i>p</i>-phenyleneethynylenephenylene (PEP) units is
presented. The coordination to rutheniumÂ(II) yields donorâbridgeâacceptor
assemblies with different lengths. Cyclic voltammetry and UVâvis
and luminescence spectroscopy are applied to study the electronic
interactions between the active moieties. It is shown that, upon light
excitation of the rutheniumÂ(II)-based <sup>1</sup>MLCT transition,
the formed <sup>3</sup>MLCT state is readily quenched in the presence
of C<sub>60</sub>. The photoinduced dynamics have been studied by
transient absorption spectroscopy, which reveals fast depopulation
of the <sup>3</sup>MLCT (73â406 ps). As a consequence, energy
transfer occurs, populating a long-lived triplet state, which could
be assigned to the <sup>3</sup>C<sub>60</sub>* state
Efficient Energy Transfer and Metal Coupling in Cyanide-Bridged Heterodinuclear Complexes Based on (Bipyridine)(terpyridine)ruthenium(II) and (Phenylpyridine)iridium(III) Complexes
We report a series
of cyanide-bridged, heterodinuclear iridiumÂ(III)ârutheniumÂ(II)
complexes with the generalized formula [IrÂ((R<sub>2</sub>)<sub>2</sub>-ppy)<sub>2</sub>(CN)Â(Îź-CN)ÂRuÂ(bpy)Â(tpy-R<sub>1</sub>)]ÂPF<sub>6</sub> (ppy = 2-phenylpyridine, bpy = 2,2â˛-bipyridine, and
tpy = 2,2â˛:6â˛,2âł-terpyridine). The structural,
spectroscopic, and electrochemical properties were analyzed in the
context of variation of the electron-withdrawing (e.g., âF,
âBr, âCHO) and -donating (e.g., âMe) and extended
Ď-conjugated groups at several positions. In total, ten dinuclear
complexes and the appropriate model complexes have been prepared.
The iridiumÂ(III)-based emission is almost fully quenched in these
complexes, and only the rutheniumÂ(II)-based emission is observed,
which indicates an efficient energy transfer toward the Ru center.
Upon oxidation of the Ru center, the fluorinated complexes <b>2</b> exhibit a broad intervalence charge-transfer transition in the near-infrared
region. The complexes are assigned to a weakly coupled class II system
according to the RobinâDay classification. The electronic structure
was evaluated by density functional theory (DFT) and time-dependent
DFT calculations to corroborate the experimental data
Photophysics of Ru(II) Dyads Derived from Pyrenyl-Substitued Imidazo[4,5â<i>f</i>][1,10]phenanthroline Ligands
The
photophysics of a series of RuÂ(II) dyads based on the 2-(1-pyrenyl)-1<i>H</i>-imidazoÂ[4,5-<i>f</i>]Â[1,10]-phenanthroline ligand
was investigated. The ability of these metal complexes to intercalate
DNA and induce cell death upon photoactivation makes them attractive
photosensitizers for a range of photobiological applications, including
photodynamic therapy. In the present study, time-resolved transient
absorption and emission spectroscopy were used to interrogate the
photoinduced processes that follow metal-to-ligand charge transfer
excitation of the complexes in solution. It was found that energy
transfer to pyrene-localized intraligand triplet states, facilitated
by torsional motion of the pyrene moiety relative to the imidazoÂ[4,5-<i>f</i>]Â[1,10]Âphenanthroline ligand, was an important relaxation
pathway governing the photophysical dynamics in this class of compounds.
Biphasic decay kinetics were assigned to spontaneous (pre-equilibrium)
and delayed emission, arising from an equilibrium established between <sup>3</sup>MLCT and <sup>3</sup>IL states. TDDFT calculations supported
these interpretations