25 research outputs found
Ultrafast Circular Dichroism Study of the Ring Opening of 7-Dehydrocholesterol
UV femtosecond time-resolved circular dichroism (TRCD)
spectroscopy has been used to study the ultrafast changes of chirality
in a small molecular biological paradigm sample, 7-dehydrocholesterol
(7-DHC). Upon UV-photoexcitation, 7-DHC undergoes a ring opening to
produce previtamin D<sub>3</sub>, and two of the chiral centers of
7-DHC are removed, which impacts the overall chirality of the molecule.
Here, measurements of this chirality change connected to the ring
opening of 7-DHC with a time resolution of 280 fs in
the UV are reported. With this method, a previously described discrepancy
concerning the photophysics of 7-DHC was clarified. With our setup,
the relaxation time of the chirality change was measured to be 1â2
ps, which corresponds to the shortest time constant in the transient
absorption (TA) measurements, allowing
us to assign that time constant to the ring opening
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
Fate of Photoexcited Molecular Antennae - Intermolecular Energy Transfer versus Photodegradation Assessed by Quantum Dynamics
The
present computational study aims to unravel the competitive
photoinduced intermolecular energy transfer and electron transfer
phenomena in a light-harvesting antenna with potential applications
in dye-sensitized solar cells and photocatalysis. A series of three
thiazole dyes with hierarchically overlapping emission and absorption
spectra, embedded in a methacrylate-based polymer backbone, is employed
to absorb light over the entire visible region. Intermolecular energy
transfer in such antenna proceeds via energy transfer from dye-to-dye
and eventually to a photosensitizer. Initially, the ground and excited
state properties of the three pushâpull-chromophores (e.g.,
with respect to their absorption and emission spectra as well as their
equilibrium structures) are thoroughly evaluated using state-of-the-art
multiconfigurational methods and computationally less demanding DFT
and TDDFT simulations. Subsequently, the potential energy landscape
for the three dyads, formed by the Ď-stacked dyes as occurring
in the polymer environment, is investigated along linear-interpolated
internal coordinates to elucidate the photoinduced dynamics associated
with intermolecular energy and electron transfer processes. While
energy transfer among the dyes is highly desired in such antenna,
electron transfer, or rather a light-induced redox chemistry, leading
to the degradation of the chromophores, is disadvantageous. We performed
quantum dynamical wavepacket calculations to investigate the excited
state dynamics following initial light-excitation. Our calculations
reveal for the two dyads with adjusted optical properties exclusively
efficient intermolecular energy transfer within 200 fs, while in the
case of the third dyad intermolecular electron transfer dynamics can
be observed. Thus, this computational study reveals that statistical
copolymerization of the individual dyes is disadvantageous with respect
to the energy transfer efficiency as well as regarding the photostability
of such antenna
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
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
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 Properties of Heteroleptic Cu(I) 4<i>H</i>âImidazolate Complexes
The excited state
properties of three heteroleptic copperÂ(I) xantphos
4<i>H</i>-imidazolate complexes are investigated by means
of femtosecond and nanosecond time-resolved transient absorption spectroscopy
in dichloromethane solution. The subpicosecond spectral changes observed
after excitation into the MLCT absorption band are interpreted as
intersystem crossing from the singlet to the triplet manifold. This
interpretation is corroborated by DFT and TD-DFT results, indicating
a comparable molecular geometry in the ground state (and hence the
nonrelaxed singlet state) and the excited triplet state. Population
of the triplet state is followed by planarization of the <i>N</i>-aryl rings of the 4<i>H</i>-imidazolate ligand on a 10
ps time scale. The planarization strongly depends on the substitution
pattern of the <i>N</i>-aryls and correlates with the reduced
moment of inertia for the planarization motion. The triplet state
subsequently decays to the ground state in about 100 ns. These results
demonstrate that the excited state processes of copperÂ(I) complexes
depend on the specific ligand(s) and their substitution pattern. Thus,
the work presented points to a possibility to design copperÂ(I) complexes
with specific photophysical properties
Self-healing Functional Polymers: Optical Property Recovery of Conjugated Polymer Films by Uncatalyzed Imine Metathesis
The
implementation of a self-healing functionality into materials
has become a prevalent approach for materials which require long-term
reliability. As of today, the restoration of mechanical properties
has dominated the research on self-healing materials, whereas research
on healing of other functionalities (e.g., conductivity or optical
properties) is still in its infancy. Here, the first conjugated polymer,
which can restore its optical properties after photodamage is reported.
The proposed self-healing mechanism relies on a thermally triggered
imine metathesis between the conjugated polymer and additional macromolecular
healing agents with no catalyst needed
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