14 research outputs found
Hydrogen-Bonding Effects on the Formation and Lifetimes of Charge-Separated States in Molecular Triads
Photoinduced electron transfer in two molecular triads
comprised
of a triarylamine donor, a d<sup>6</sup> metal diimine photosensitizer,
and a 9,10-anthraquinone acceptor was investigated with particular
focus on the influence of hydrogen-bonding solvents on the electron
transfer kinetics. Photoexcitation of the rutheniumÂ(II) and osmiumÂ(II)
sensitizers of these triads leads to charge-separated states containing
an oxidized triarylamine unit and a reduced anthraquinone moiety.
The kinetics for formation of these charge-separated states were explored
by using femtosecond transient absorption spectroscopy. Strong hydrogen
bond donors such as hexafluoroisopropanol or trifluoroethanol cause
a thermodynamic and kinetic stabilization of these charge-separated
states that is attributed to hydrogen bonding between alcoholic solvent
and reduced anthraquinone. In the ruthenium triad this effect leads
to a lengthening of the lifetime of the charge-separated state from
âź750 ns in dichloromethane to âź3000 ns in hexafluoroisopropanol
while in the osmium triad the respective lifetime increases from âź50
to âź2000 ns between the same two solvents. In both triads
the lifetime of the charge-separated state correlates with the hydrogen
bond donor strength of the solvent but not with the solvent dielectric
constant. These findings are relevant in the greater context of solar
energy conversion in which one is interested in storing light energy
in charge-separated states that are as long-lived as possible. Furthermore
they are relevant for understanding proton-coupled electron transfer
(PCET) reactivity of electronically excited states at a fundamental
level because changes in hydrogen-bonding strength accompanying changes
in redox states may be regarded as an attenuated form of PCET
Photoinduced Electron Transfer in CdSe/ZnS Quantum DotâFullerene Hybrids
Photoinduced electron transfer (ET)
in CdSe/ZnS coreâshell
quantum dot (QD) â fullerene (COOHâC<sub>60</sub>) hybrids
was studied by the means of time-resolved emission and absorption
spectroscopy techniques. A series of four QDs with emission in the
range 540â630 nm was employed to investigate the dependence
of the electron transfer rate on the QD size. Emission of the QDs
is quenched upon hybrid formation, and the quenching mechanism is
identified as photoinduced electron transfer from the QD to the fullerene
moiety due to the fullerene anion signature observed in transient
absorption. In order to obtain quantitative information on the ET
reaction, several kinetic data analysis techniques were used, including
a conventional multiexponential fitting and a maximum entropy method
for emission decay analysis, as well as a distributed decay model
based on the Poisson distribution of fullerenes in the hybrids. The
latter gradually simplifies the interpretation of the transient absorption
spectra and indicates that the spectra of QD cations are essentially
similar to those of neutral QDs, differing only by a minor decrease
in the intensity and broadening. Furthermore, only a minor decrease
in the ET rate with the increasing QD size was observed, the time
constants being in the range 100â200 ps for all studied QDs.
The charge recombination is extended to 10 ns or longer for all hybrids
Quantitative Analysis of Intramolecular Exciplex and Electron Transfer in a Double-Linked Zinc PorphyrinâFullerene Dyad
Photoinduced charge transfer in a double-linked zinc
porphyrinâfullerene
dyad is studied. When the dyad is excited at the absorption band of
the charge-transfer complex (780 nm), an intramolecular exciplex is
formed, followed by the complete charge separated (CCS) state. By
analyzing the results obtained from time-resolved transient absorption
and emission decay measurements in a range of solvents with different
polarities, we derived a dependence between the observable lifetimes
and internal parameters controlling the reaction rate constants based
on the semiquantum Marcus electron-transfer theory. The critical value
of the solvent polarity was found to be Îľ<sub>r</sub> â
6.5: in solvents with higher dielectric constants, the energy of the
CCS state is lower than that of the exciplex and the relaxation takes
place via the CCS state predominantly, whereas in solvents with lower
polarities the energy of the CCS state is higher and the exciplex
relaxes directly to the ground state. In solvents with moderate polarities
the exciplex and the CCS state are in equilibrium and cannot be separated
spectroscopically. The degree of the charge shift in the exciplex
relative to that in the CCS state was estimated to be 0.55 Âą
0.02. The electronic coupling matrix elements for the charge recombination
process and for the direct relaxation of the exciplex to the ground
state were found to be 0.012 Âą 0.001 and 0.245 Âą 0.022 eV,
respectively
Monoisomeric Phthalocyanines and PhthalocyanineâFullerene Dyads with Polar Side Chains: Synthesis, Modeling, and Photovoltage
Synthesis, characterization, molecular
modeling, and photovoltage
responses of four phthalocyanineâfullerene (Pc-C<sub>60</sub>) dyads with two polar (âOH) side chains are described. The
synthesized dyads have polar tails either on the Pc (electron donor
D) side or on the fullerene (electron acceptor A) side of the dyad,
providing a possibility to produce oriented donorâacceptor
(DâA) monolayers with revised electron transfer direction when
deposited on an aqueous subphase or on a solid surface. In the dyads,
phthalocyanine and fullerene have different mutual orientations: in
the trans-dyads they have face-to-face orientation, while in the cis-dyads
the orientation is face-to-edge. Molecular modeling was used to examine
and confirm the spatial arrangements of the synthesized dyads. The
Pc-C<sub>60</sub> dyads were deposited successfully onto solid substrates
as highly oriented monolayers using the LangmuirâSchaĚfer
method. Formation of a vertically oriented monolayer and the following
electron transfer from the photoexcited phthalocyanine to fullerene
was demonstrated for all dyad monolayers by the time-resolved Maxwell
displacement charge method. The electron transfer direction was reversed
for the dyads with reversed polarity, demonstrating the ability to
control charge transfer direction in the film
Excited State Intramolecular Proton Transfer in Electron-Rich and Electron-Poor Derivatives of 10-Hydroxybenzo[<i>h</i>]quinoline
Eight previously inaccessible derivatives of 10-hydroxybenzoÂ[<i>h</i>]Âquinoline were prepared via a straightforward strategy
comprising formation of the benzoÂ[<i>h</i>]Âquinoline skeleton
followed by CâH acetoxylation at position 10. The occurrence
of excited state intramolecular proton transfer (ESIPT) was detected
in all cases since emission was observed only from the excited keto-tautomer.
Studies on derivatives bearing both electron-donating and electron-withdrawing
groups adjacent to the pyridine ring allowed us to identify some design
patterns giving rise to NIR emission and large Stokes shifts. For
a derivative of 10-hydroxybenzoÂ[<i>c</i>]Âacridine, emission
at 745 nm was observed, one of the lowest energy fluorescence ever
reported for ESIPT system. On the basis of time-resolved measurements,
proton transfer was found to be extremely fast with time constants
in the range (0.08â0.45 ps)
Large Stokes Shift Fluorescent Dyes Based on a Highly Substituted Terephthalic Acid Core
The synthesis of dyes based on a highly substituted terephthalic acid core is described, starting from readily available 2,5-dihydroxy-terephthalic acid diethyl ester. The dyes are highly colored, soluble in organic solvents and reasonably fluorescent in solution and in the solid state. The maxima for absorption and emission are around 402 and 502 nm, respectively. The fluorophores are readily cyclized to generate compounds which comprise the basic 6,13-dihydroxy-chromeno[2,3-<i>b</i>]xanthene-7,14-dione unit. These new derivatives are nonfluorescent
Large Stokes Shift Fluorescent Dyes Based on a Highly Substituted Terephthalic Acid Core
The synthesis of dyes based on a highly substituted terephthalic acid core is described, starting from readily available 2,5-dihydroxy-terephthalic acid diethyl ester. The dyes are highly colored, soluble in organic solvents and reasonably fluorescent in solution and in the solid state. The maxima for absorption and emission are around 402 and 502 nm, respectively. The fluorophores are readily cyclized to generate compounds which comprise the basic 6,13-dihydroxy-chromeno[2,3-<i>b</i>]xanthene-7,14-dione unit. These new derivatives are nonfluorescent
Synthesis and Photophysical Properties of Two DiazaporphyrinâPorphyrin Hetero Dimers in Polar and Nonpolar Solutions
Two diazaporphyrin (DAP)-porphyrin
hetero dimers, in βâ<i>meso</i> and βâβ
configurations, were prepared
to study their photoinduced intramolecular electron transfer properties.
The two <i>meso</i> nitrogen atoms in the porphyrin ring
of DAP change its redox potential, making DAP more easily reduced,
compared to its porphyrin counterpart. A charge-transfer from porphyrin
to DAP in both hetero dimers was verified by versatile optical spectroscopic
methods. The steady-state fluorescence spectra indicated an efficient
intramolecular exciplex formation for both dimers. For the βâ<i>meso</i> dimer, ultrafast time-resolved spectroscopic methods
revealed the subpicosecond formation of two types of primary short-living
(1â18 ps) intramolecular exciplexes, which relaxed in toluene
to form a long-living final exciplex (1.4 ns) followed by a longer-living
charge transfer complex (>5 ns). However, in benzonitrile, the
lifetime
of the final exciplex was longer (660 ps) as was that of the charge
transfer complex (180 ps). The βâβ analogue formed
similar short-living exciplexes in both solvents, but the final exciplex
and the charge transfer state had significantly shorter lifetimes.
The electrochemical redox potential measurements and density functional
theory calculations supported the proposed mechanism
Syntheses and Excitation Transfer Studies of Near-Orthogonal Free-Base PorphyrinâRuthenium Phthalocyanine Dyads and Pentad
A new series of molecular dyads and pentad featuring
free-base
porphyrin and ruthenium phthalocyanine have been synthesized and characterized.
The synthetic strategy involved reacting free-base porphyrin functionalized
with one or four entities of phenylimidazole at the meso position
of the porphyrin ring with ruthenium carbonyl phthalocyanine followed
by chromatographic separation and purification of the products. Excitation
transfer in these donorâacceptor polyads (dyad and pentad)
is investigated in nonpolar toluene and polar benzonitrile solvents
using both steady-state and time-resolved emission techniques. Electrochemical
and computational studies suggested that the photoinduced electron
transfer is a thermodynamically unfavorable process in nonpolar media
but may take place in a polar environment. Selective excitation of
the donor, free-base porphyrin entity, resulted in efficient excitation
transfer to the acceptor, ruthenium phthalocyanine, and the position
of imidazole linkage on the free-base porphyrin could be used to tune
the rates of excitation transfer. The singlet excited Ru phthalocyanine
thus formed instantly relaxed to the triplet state via intersystem
crossing prior to returning to the ground state. Kinetics of energy
transfer (<i>k</i><sub>ENT</sub>) was monitored by performing
transient absorption and emission measurements using pumpâprobe
and up-conversion techniques in toluene, respectively, and modeled
using a FoĚrster-type energy transfer mechanism. Such studies
revealed the experimental <i>k</i><sub>ENT</sub> values
on the order of 10<sup>10</sup>â10<sup>11</sup> s<sup>â1</sup>, which readily agreed with the theoretically estimated values. Interestingly,
in polar benzonitrile solvent, additional charge transfer interactions
in the case of dyads but not in the case of pentad, presumably due
to the geometry/orientation consideration, were observed
Sequential Photoinduced Energy and Electron Transfer Directed Improved Performance of the Supramolecular Solar Cell of a Zinc PorphyrinâZinc Phthalocyanine Conjugate Modified TiO<sub>2</sub> Surface
Improved performance of a photosynthetic antennaâreaction
center mimicking supramolecular solar cell is demonstrated. Toward
this, porphyrinâphthalocyanine conjugates connected by amide
linkers, as wide-band capturing solar energy harvesting materials,
have been newly synthesized and characterized. Efficient singletâsinglet
energy transfer from the zinc or free-base porphyrin to phthalocyanine
is evidenced from steady-state emission and transient absorption studies
in nonpolar and polar solvents. Further, the dyad was immobilized
via axial coordination of zinc porphyrin of the dyad on semiconducting
TiO<sub>2</sub> surface modified with axial coordinating ligand functionality,
phenylimidazole. Photoelectrochemical studies revealed improved performance
of this cell compared to either zinc porphyrin or zinc phthalocyanine
only modified electrodes under similar experimental conditions. Transient
absorption studies performed on the dyad immobilized on glass/TiO<sub>2</sub> surface suggested that upon excitation of the axially coordinated
zinc porphyrin of the dyad singletâsinglet energy transfer
to zinc phthalocyanine occurs within 0.2 ps instead of a competing
charge injection reaction from the singlet excited zinc porphyrin
to TiO<sub>2</sub>. Further, sequential photoinduced electron transfer
from the newly formed singlet excited zinc phthalocyanine to zinc
porphyrin producing ZnP<sup>â˘â</sup>âZnPc<sup>â˘+</sup> with a 2 ps time constant and followed by electron
injection from the ZnP<sup>â˘â</sup> to TiO<sub>2</sub> within 30 ps has been proposed as a mechanism of photocurrent generation
in the biomimetic supramolecular photocell