7 research outputs found
Supramolecular DonorâAcceptor Assembly Derived from TetracarbazoleâZinc Phthalocyanine Coordinated to Fullerene: Design, Synthesis, Photochemical, and Photoelectrochemical Studies
A functional photosynthetic antennaâreaction center
mimicking
donorâacceptor polyad has been newly designed, synthesized,
and characterized. The polyad was comprised of four entities of carbazole
covalently linked to the macrocycle periphery of a zinc phthalocyanine
(ZnPc). Efficient singlet excitation transfer from the carbazole to
zinc phthalocyanine has been witnessed from the emission studies.
Axial coordination of phenylimidazole-functionalized fulleropyrrolidine
to ZnPc served as an electron acceptor in the polyad. Optical absorption
and emission along with computational studies revealed stable complex
formation wherein the evaluated binding constant <i>K</i> was 7.7 Âą 0.2 Ă 10<sup>5</sup> M<sup>â1</sup>,
an order of magnitude higher than that observed earlier for similar
complexes due to the electronic effect induced by the carbazole entities.
From the free-energy calculations, photoinduced electron transfer
from the <sup>1</sup>ZnPc* to fullerene within the polyad was established
to be an exothermic process. Kinetics of charge separation, <i>k</i><sub>CS</sub>, monitored by time-resolved emission was
found to be 2.8 Ă 10<sup>9</sup> s<sup>â1</sup>, indicating
a relatively fast charge-separation process. The electron-transfer
products were characterized by nanosecond transient absorption spectroscopic
technique; the presence of ZnPc<sup>+â˘</sup> radical cation
at 890 nm and fulleropyrrolidine anion radical at 1000 nm was clear
from this study. The kinetics of charge recombination, <i>k</i><sub>CR</sub>, evaluated from the decay of either of the radical
ions, was found to be 6.25 Âą 0.2 Ă 10<sup>7</sup> s<sup>â1</sup>, revealing the persistence of the radical ion-pair
species to some extent. Further, photoelectrochemical studies, performed
by constructing photocells by electrophoretic deposition of the studied
polyad on nanocrystalline SnO<sub>2</sub> modified surface, revealed
a higher value of incident photon-to-current conversion efficiency
covering the wide visibleânear IR spectral region and good
onâoff switchability. Better charge injection from the excited
polyad to the conduction band of the semiconductor was evident from
the electrochemical impedance spectral measurements of electron recombination
resistance calculations
Phenothiazine-Sensitized Organic Solar Cells: Effect of Dye Anchor Group Positioning on the Cell Performance
Effect of positioning of the cyanoacrylic acid anchoring
group
on ring periphery of phenothiazine dye on the performance of dye-sensitized
solar cells (DSSCs) is reported. Two types of dyes, one having substitution
on the C-3 aromatic ring (Type 1) and another through the N-terminal
(Type 2), have been synthesized for this purpose. Absorption and fluorescence
studies have been performed to visualize the effect of substitution
pattern on the spectral coverage and electrochemical studies to monitor
the tuning of redox levels. B3LYP/6-31G* studies are performed to
visualize the frontier orbital location and their significance in
charge injection when surface modified on semiconducting TiO<sub>2</sub>. New DSSCs have been built on nanocrystalline TiO<sub>2</sub> according
to traditional two-electrode GraĚtzel solar cell setup with
a reference cell based on N719 dye for comparison. The lifetime of
the adsorbed phenothiazine dye is found to be quenched significantly
upon immobilizing on TiO<sub>2</sub> suggesting charge injection from
excited dye to semiconducting TiO<sub>2</sub>. The performances of
the cells are found to be prominent for solar cells made out of Type
1 dyes compared to Type 2 dyes. This trend has been rationalized on
the basis of spectral, electrochemical, computational, and electrochemical
impedance spectroscopy results
Control over Photoinduced Energy and Electron Transfer in Supramolecular Polyads of Covalently linked azaBODIPY-Bisporphyrin âMolecular Clipâ Hosting Fullerene
A âmolecular clipâ featuring a near-IR
emitting fluorophore,
BF<sub>2</sub>-chelated tetraarylazadipyrromethane (aza-BODIPY) covalently
linked to two porphyrins (MP, M = 2H or Zn) has been newly synthesized
to host a three-dimensional electron acceptor fullerene via a âtwo-pointâ
metalâligand axial coordination. Efficient singletâsinglet
excitation transfer from <sup>1</sup>ZnP* to aza-BODIPY was witnessed
in the dyad and triad in nonpolar and less polar solvents, such as
toluene and <i>o</i>-dichlorobenzene, however, in polar
solvents, additional electron transfer occurred along with energy
transfer. A supramolecular tetrad was formed by assembling bis-pyridine
functionalized fullerene via a âtwo-pointâ metalâligand
axial coordination, and the resulted complex was characterized by
optical absorption and emission, computational, and electrochemical
methods. Electron transfer from photoexcited zinc porphyrin to C<sub>60</sub> is witnessed in the supramolecular tetrad from the femtosecond
transient absorption spectral studies. Further, the supramolecular
polyads (triad or tetrad) were utilized to build photoelectrochemical
cells to check their ability to convert light into electricity by
fabricating FTO/SnO<sub>2</sub>/polyad electrodes. The presence of
azaBODIPY and fullerene entities of the tetrad improved the overall
light energy conversion efficiency. An incident photon-to-current
conversion efficiency of up to 17% has been achieved for the tetrad
modified electrode
Role of Surfactants and Salt in Aqueous Two-Phase Separation of Carbon Nanotubes toward Simple Chirality Isolation
Aqueous two-phase extraction has recently been demonstrated as a new method to separate single-wall carbon nanotubes (SWCNTs). In this work, we determined that the mechanism of separation is driven by the hydrophobicity of the surfactant, or combination of surfactants, at the SWCNT surface. This knowledge allowed us to develop a simple approach for obtaining highly enriched single-chirality suspensions in only 1 or 2 steps. These results were obtained by strategically combining multiple surfactants with different diameter-dependent binding affinities for SWCNTs and salts that readjust the surfactant structure within the mixed micelle surrounding the SWCNTs. The procedure is successfully applied to SWCNTs from different sources (CoMoCAT and HiPco) with various diameter distributions (from 0.53 to 1.2 nm). Each separation step is characterized by optical absorption, resonant Raman, and photoluminescence excitation spectroscopies. By determining the SWCNT sorting mechanism, we were able to develop a new set of parameters that separated another chirality
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
Developing Monolithic Nanoporous Gold with Hierarchical Bicontinuity Using Colloidal Bijels
We
report a universal platform for the synthesis of monolithic
porous gold materials with hierarchical bicontinuous morphology and
combined macro- and mesoporosity using a synergistic combination of
nanocasting and chemical dealloying. This robust and accessible approach
offers a new design paradigm for the parallel optimization of active
surface area and mass transport in porous metal electrodes
Pyrazinacenes: aza analogues of acenes
A series of edge-sharing condensed oligopyrazine analogues of acenes, the pyrazinacenes, were
synthesized and characterized. X-ray crystallographic determinations revealed intermolecular interactions
that affect the propensity of the molecules to undergo Ď-Ď stacking. Increasing heteroatom
substitution of the acene framework induces shorter intermolecular Ď-Ď stacking distances (shorter
than for graphite) probably due to lower van derWaals radius of nitrogen atoms. Hydrogen bonding
is also a determining factor in the case of compounds containing reduced pyrazine rings. Combined
electrochemical, electronic absorption, and computational investigations indicate the substantial
electron deficiency of the compounds composed of fused pyrazine rings. The pyrazinacenes are
expected to be good candidates as materials for organic thin film transistors