6 research outputs found
Synthesis and Properties of a Covalently Linked Angular Perylene Imide Dimer
Utilizing the unexplored chemistry of a monocarbon analog to perylene bisimide, a covalently linked angular perylene dimer was synthesized. On the basis of measured optical properties and molecular modeling, the spectral changes relative to a monomeric reference perylene can be explained by an angle-dependent oblique exciton coupling model. With a roughly trigonal interchromophore arrangement, the dimer building block is promising for larger, cyclic assemblies to mimic naturally occurring light harvesting complexes
Epitaxially Intergrown Conformational Polymorphs and a Mixed Water/Methanol Solvate of 5′-Deoxy-5′-iodoguanosine
5′-Deoxy-5′-iodoguanosine
(<b>I</b>) crystals
deposited from mixtures of water and methanol grow as nonsolvated
hybrids of conformational polymorphs (<b>Ia</b>, <b><b>Ib</b></b>) and as a mixed solvate (<b>Ic</b>). Some
solvent-free crystals are purely <b>Ia</b>, while others have
varying amounts of <b>Ib</b> epitaxially intergrown with <b>Ia</b>. In <b>Ia</b> and <b>Ib</b> the conformations
differ primarily by torsion about the C4′–C5′
bond (guanosine numbering scheme), which dramatically affects the
iodine atom position. Powder diffraction and reconstructed reciprocal-lattice-slice
images had small peaks incompatible with <b>Ia</b>. Some solvent-free
crystals required lattices for both <b>Ia</b> and <b>Ib</b> to index all observable reflections. Unit-cell dimensions for <b>Ia</b> and <b>Ib</b> suggest the potential for epitaxial
intergrowth. Hydrogen-bond networks in <b>Ia</b> and <b>Ib</b> are essentially identical and result in double layers of molecules
in the <i>ab</i> plane, with layers of iodine at the layer
surfaces. The iodine layers of <b>Ia</b> and <b>Ib</b> are incompatible: in <b>Ia</b> adjacent iodine atom layers
interdigitate slightly, whereas in <b>Ib</b> they do not. Theoretical
calculations support the conclusion that at room temperature <b>Ia</b> is the thermodynamically more stable polymorph and that <b>Ib</b> represents a kinetic product
Epitaxially Intergrown Conformational Polymorphs and a Mixed Water/Methanol Solvate of 5′-Deoxy-5′-iodoguanosine
5′-Deoxy-5′-iodoguanosine
(<b>I</b>) crystals
deposited from mixtures of water and methanol grow as nonsolvated
hybrids of conformational polymorphs (<b>Ia</b>, <b><b>Ib</b></b>) and as a mixed solvate (<b>Ic</b>). Some
solvent-free crystals are purely <b>Ia</b>, while others have
varying amounts of <b>Ib</b> epitaxially intergrown with <b>Ia</b>. In <b>Ia</b> and <b>Ib</b> the conformations
differ primarily by torsion about the C4′–C5′
bond (guanosine numbering scheme), which dramatically affects the
iodine atom position. Powder diffraction and reconstructed reciprocal-lattice-slice
images had small peaks incompatible with <b>Ia</b>. Some solvent-free
crystals required lattices for both <b>Ia</b> and <b>Ib</b> to index all observable reflections. Unit-cell dimensions for <b>Ia</b> and <b>Ib</b> suggest the potential for epitaxial
intergrowth. Hydrogen-bond networks in <b>Ia</b> and <b>Ib</b> are essentially identical and result in double layers of molecules
in the <i>ab</i> plane, with layers of iodine at the layer
surfaces. The iodine layers of <b>Ia</b> and <b>Ib</b> are incompatible: in <b>Ia</b> adjacent iodine atom layers
interdigitate slightly, whereas in <b>Ib</b> they do not. Theoretical
calculations support the conclusion that at room temperature <b>Ia</b> is the thermodynamically more stable polymorph and that <b>Ib</b> represents a kinetic product
Theory-Driven Insight into the Crystal Packing of Trialkylsilylethynyl Pentacenes
The
functionalization of oligoacenes and similar π-conjugated
chromophores with trialkylsilylÂethynyl groups has proven to
be a versatile means to enhance solubility and solution processability
and engineer solid-state packing arrangements to produce organic semiconductors
that demonstrate outstanding charge-carrier transport characteristics.
While a general, empirical-based geometric model has been developed
and implemented to direct the solid-state packing arrangements of
these molecular materials, there exist numerous examples where the
model falters. Here, we employ electronic structure methods to probe
the noncovalent, intermolecular interactions of two closely related
systems that result in two very different crystal packing configurations:
triisopropylÂsilylethynyl (TIPS) pentacene and its triethylsilylÂethynyl
(TES) analog. The quantum-chemical evaluation details how the slightly
larger electron density contained within the volume of the TIPS moiety
with respect to TES is in part responsible for the solid-state packing
variations. We also make use of periodic density functional theory
(DFT) methods to develop in silico polymorphs of these systems and
explore the electronic characteristics of varied packing arrangements.
The results suggest that TES pentacene, if processed correctly, could
be developed into a material with improved charge-carrier transport
characteristics when compared to its native form. Overall, the theory-driven
insight developed in this work lays an important foundation to build
a more robust crystal engineering paradigm for these technologically
relevant organic semiconductors
Synthesis and Electrical Properties of Derivatives of 1,4-bis(trialkylsilylethynyl)benzo[2,3‑<i>b</i>:5,6‑<i>b</i>′]diindolizines
A new
class of nitrogen-containing arene organic semiconductors
incorporating fused indolizine units is described. This system, though
having a zigzag shape, mimics the electronic properties of its linear
analogue pentacene as a result of nitrogen lone pair incorporation
into the π-electron system. Solubilizing trialkylsilylethynyl
groups were employed to target crystal packing motifs appropriate
for field-effect transistor devices. The triethylsilylethynyl derivative
yielded hole mobilities of 0.1 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> and on/off current ratios of 10<sup>5</sup>
Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission
Singlet fission is an excitation
multiplication process in molecular
systems that can circumvent energy losses and significantly boost
solar cell efficiencies; however, the nature of a critical intermediate
that enables singlet fission and details of its evolution into multiple
product excitations remain obscure. We resolve the initial sequence
of events comprising the fission of a singlet exciton in solids of
pentacene derivatives using femtosecond transient absorption spectroscopy.
We propose a three-step model of singlet fission that includes two
triplet-pair intermediates and show how transient spectroscopy can
distinguish initially interacting triplet pairs from those that are
spatially separated and noninteracting. We find that the interconversion
of these two triplet-pair intermediates is limited by the rate of
triplet transfer. These results clearly highlight the classical kinetic
model of singlet fission and expose subtle details that promise to
aid in resolving problems associated with triplet extraction