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 trialkylsilyl­ethynyl 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: triisopropyl­silylethynyl (TIPS) pentacene and its triethylsilyl­ethynyl (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 cm² V^–1 s^–1 and on/off current ratios of 10⁵

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