5 research outputs found
Preferential Through-Space Charge Separation and Charge Recombination in VâType Configured PorphyrinâazaBODIPYâFullerene Supramolecular Triads
Electron transfer
is one of the most fundamental and prevalent
processes occurring in chemistry, physics, and biology. In donorâacceptor
systems with one of the partnersâ a photosensitizer, upon photoexcitation,
transfer of an electron between the photoexcited and ground-state
molecules occurs. Factors affecting the geometry, energetics, and
dynamics of this process have been one of the intensively studied
scientific topics, often by building model donorâacceptor conjugates
or by utilizing natural systems. A wealth of information, applicable
to almost all areas of modern science and technology, has been generated
from these studies. In the present study, we demonstrate preferential
through-space charge separation and charge recombination in supramolecular
triads composed of porphyrin (free-base, zinc, or magnesium at the
central cavity) as excited-state electron donor, BF<sub>2</sub>-chealted
azadipyrromethene (azaBODIPY), and fullerene (C<sub>60</sub>) as electron
acceptors. Because of spatial close proximity of the terminal porphyrin
and fullerene entities of the triads as a consequence of the V-type
configuration, photoinduced charge separation from the singlet excited
porphyrin involves fullerene instead of energetically more favorable
covalently linked azaBODIPY entity. Interestingly, charge recombination
also follows this path of through-space instead of an electron migration
from the fullerene anion radical to the covalently linked azaBODIPY
entity. The present study highlights the importance of geometric disposition
of donor and acceptor entities in governing not only the forward photoinduced
electron transfer but also the dark reverse electron transfer in multimodular
donorâacceptor conjugates, applicable toward light-energy-harvesting
and building optoelectronic devices
Paddle-Wheel BODIPYâHexaoxatriphenylene Conjugates: Participation of Redox-Active Hexaoxatriphenylene in Excited-State Charge Separation to Yield High-Energy Charge-Separated States
Hexaoxatriphenylene,
a scaffold linker often utilized in building
covalent organic frameworks, is shown to be electroactive and a useful
entity to build light energy-harvesting donorâacceptor systems.
To demonstrate this, new donorâacceptor conjugates have been
synthesized by employing BODIPY as a sensitizer. Excited-state electron
transfer leading to high-energy charge-separated states, useful to
drive energy-demanding photocatalytic reactions, from the electron-rich
hexaoxatriphenylene to <sup>1</sup>BODIPY*, in the synthesized tri-BODIPYâhexaoxatriphenylene
âpaddle-wheelâ conjugates, has been successfully demonstrated
using femtosecond transient absorption spectroscopy. The measured
rate of charge separation was in the range of âŒ3â10
Ă 10<sup>11</sup> s<sup>â1</sup>, revealing ultrafast
charge separation
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 GraÌtzel 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
ÏâExtended Pyrazinepyrene-Fused Zinc Phthalocyanines: Synthesis and Excited-State Charge Separation Involving Coordinated C<sub>60</sub>
A series of pyrazinepyrene-fused zinc phthalocyanines
(ZnPc-Pyrn) have been newly synthesized
by reacting
quinoxaline and the corresponding diamino-functionalized phthalocyanines
as a new class of Ï-extended phthalocyanine systems. Bathochromically
shifted absorption as a function of the number of pyrazinepyrene entities
due to extended Ï-conjugation and quenched fluorescence due
to the presence of fused pyrazinepyrene were witnessed. The electronic
structures of these phthalocyanines were probed by systematic computational
and electrochemical studies, while the excited-state properties were
examined by pumpâprobe spectroscopies operating at the femto-
and nanosecond time scales. Similar to the excited singlet lifetimes,
the excited triplet states also revealed diminished lifetimes with
an increased number of pyrazinepyrene entities. Further, the coordinatively
unsaturated zinc in these molecules was coordinated with phenyl imidazole-functionalized
fullerene, ImC60, to form a new series of donorâacceptor
conjugates. Upon full characterization of these conjugates, the occurrence
of excited-state charge separation was established by transient pumpâprobe
spectroscopy, covering wide temporal and spatial regions. The lifetime
of the final charge-separated states was âŒ2 ns and decreased
with an increase in the number of fused pyrazinepyrene units
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 Ï-Ï stacking. Increasing heteroatom
substitution of the acene framework induces shorter intermolecular Ï-Ï 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