3 research outputs found
Nonparallel Stacks of Donor and Acceptor Chromophores Evade Geminate Charge Recombination
We report a nonparallel stacked arrangement of donor–acceptor
(D–A) pairs for prolonging the lifetime of photoinduced charge-separated
states. Hydrogen–hydrogen steric repulsion in naphthalimide-naphthalene
(NIN) dyad destabilizes the planar geometry between the constituent
units in solution/ground state. Sterically imposed nonplanar geometry
of the dyad allows the access of nonparallel arrangement of the donor
and acceptor stacks having triclinic space group in the crystalline
state. Antiparallel trajectory of excitons in nonparallel D–A
stacks can result in lower probability of geminate charge recombination,
upon photoexcitation, thereby resulting in a long-lived charge-separated
state. Upon photoexcitation of the NIN dyad, electron transfer from
naphthalene to the singlet excited state of naphthalimide moiety results
in radical ion pair intermediates that survive >10,000-fold longer
in the aggregated state (τ<sub>cr</sub><sup>a</sup> > 1.2 ns) as compared to that of monomeric
dyad (τ<sub>cr</sub><sup>m</sup> < 110 fs), monitored using femtosecond transient absorption spectroscopy
S···π, π–π, and C–H···π Contacts Regulate Solid State Fluorescence in Regioisomeric Bisthiazolylpyrenes
The formation of well-defined packing
motifs and control of π–π
stacking by rationalizing the effects of molecular substitution in
π-conjugated organic materials represent a significant challenge
in supramolecular design. To explore the influence of the thiazolyl
group in the solid state packing and photophysical properties of pyrene,
a new class of three regioisomeric bisthiazolylpyrenes (2-2″TP)
were designed and synthesized. Single crystal X-ray structure and
quantum theory of atoms in molecules analysis revealed the presence
of S···π interactions in 2′TP. Hirshfeld
analysis showed the γ and β packing motif in 2TP and 2″TP,
respectively, due to extensive π–π interactions,
observed S···π interactions led to herringbone
packing in 2′TP. The diverse packing arrangement of 2-2″TP
led to a notable difference in the fluorescence behavior in the crystalline
state. 2′TP possessing S···π interaction
showed remarkable enhanced emission in the crystalline state compared
to the amorphous state, in contrast to 2TP and 2″TP. Our results
provide new insight for the rational design and regulation of intermolecular
interactions of conjugated heterocyclic aromatics to achieve a diverse
degree of orbital overlap between neighboring chromophore units and
thereby favorable optoelectronic properties
<i>ansa</i>-Ferrocene-Incorporated Calixpyrroles and Calixphyrins: Syntheses and Spectral/Structural Characterization
The syntheses and spectral/structural characterization
of <i>ansa</i>-ferrocene-incorporated <i>normal</i> calixphyrins
and <i>core-modified</i> calixpyrroles and calixphyrins
are reported. Acid-promoted dehydrative condensation of 1,1′-bis(dimethylpyrrolylmethyl)ferrocene
and 2,5-bis(dimethylhydroxymethyl)thiophene/furan yielded <i>ansa</i>-ferrocene-based <i>core-modified</i> calixpyrroles,
while acid-catalyzed dehydrative condensation of 1,1′-bis(diphenylpyrrolylmethyl)ferrocene
with the aryl aldehydes and 2,5-bis(phenylhydroxymethyl)thiophene
followed by DDQ oxidation resulted in the formation of <i>ansa</i>-ferrocene-appended <i>normal</i> and <i>core-modified</i> calixphyrins, respectively. The newly synthesized macrocycles were
characterized by FAB-MS, NMR, and UV–vis spectral analyses
and finally confirmed by single-crystal X-ray structural analysis.
All these studies clearly revealed the introduction of ferrocene in
the main framework of the corresponding macrocycles in an <i>ansa</i>-type way. The <i>core-modified</i> calixpyrroles
adopt a 1,3-alternate conformation, while the corresponding calixphyrins
maintained partial planarity along the tripyrrin plane due to the
presence of <i>meso</i> sp<sup>2</sup> carbon and generated
curved staircase conformation. In addition to the intramolecular hydrogen-bonding
interactions, calixphyrins generate self-assembled dimers, one- and
two-dimensional supramolecular assemblies
through intermolecular hydrogen bonding in the solid state