Synthesis and Study of Electro-Active Organic Molecules for Optoelectronic Applications

Electro-active organic materials have received considerable attention in the emerging area of molecular electronics and nanotechnology not only because of the attractive optical and electronic properties but also the advantages of organic materials such as low cost, easy processing, and great opportunities for structural modification. These materials are now being considered as active components in electronic and optoelectronic devices such as light emitting diodes for display applications, thin film transistors for low-cost and ultra-dense logic and memory circuits, photodiodes for optical information processing, and photovoltaic cells for solar energy harvesting. Therefore the electro-active organic materials have become a focus of intense research. This thesis concentrates the synthesis and study of optoelectronic properties of several classes of novel electro-active organic materials. A series of electro-active tetraarylbenzo[1,2-b:4,5-b¡¦]difuran (BDF) and model diarylbenzofuran derivatives have been synthesized and their structures were established by X-ray crystallography. The single charge stabilization by benzodifuran and coplanar fÑ-aryl groups lying on the longitudinal suggests that the linear arrays of BDFs may allow the construction of molecular wires suitable for long-range electron transport. The synthesis of hexa alkyloxy substituted hexa-peri-hexabenzocoronene (HBC), a larger and eletron rich chromophore, via oxidative cyclodehydrogenation of hexakis(4-alkyloxyphenyl)-benzene produced a quantitative yield of an indenofluorene derivative rather than the expected HBC. The mechanistic considerations for the formation of the indenofluorene derivative led us to devise an alternative synthesis of elusive alkoxy-substituted HBC. Furthermore a series of alkyloxy substituted HBCs were prepared and their electronic properties were studied. A series of aryloxy-substituted tetraphenylethylenes (TPEs), tetraphenylethylene based dendrimers and a series of phenyl ethers were prepared and the effect of the diarylether linkage on their electronic and optical properties was studied. Although the diarylether linkage in TPEs did not affect the properties significantly, these linkages in poly-p-phenyl ethers seem to mediate the delocalization of the cationic charges. A series of cycloannulated aromatic donors were prepared for the preparation of stable cation radical salts. The availability of wide range of donors with varied redox potentials (0.82 V- 1.85 V) and spectroscopic features make these cation radicals to be valuable oxidants for variety of organic, inorganic and organometallic donors

Synthesis, Electronic Properties, and X-ray Structural Characterization of Tetrarylbenzo[1,2-\u3cem\u3eb\u3c/em\u3e:4,5-\u3cem\u3eb\u3c/em\u3e′]difuran Cation Radicals

Electroactive tetraarylbenzo[1,2-b:4,5-b′]difuran (BDF) and model diarylbenzofuran derivatives are synthesized and their structures are established by X-ray crystallography. Isolation and X-ray crystallographic characterization of the robust cation-radical salts of BDF derivatives confirm that a single charge in the BDFs is stabilized largely by the benzodifuran and coplanar α-aryl groups lying on the longitudinal axis. These findings suggest that the linear arrays of BDFs may allow the construction of molecular wires suitable for long-range electron transport

Unraveling the Coulombic Forces in Electronically Decoupled Bichromophoric Systems during Two Successive Electron Transfers

Coulombic forces are vital in modulating the electron transfer dynamics in both synthetic and biological polychromophoric assemblies, yet quantitative studies of the impact of such forces are rare, as it is difficult to disentangle electrostatic forces from simple electronic coupling. To address this problem, the impact of Coulombic interactions in the successive removal of two electrons from a model set of spirobifluorenes, where the interchromophoric electronic coupling is nonexistent, is quantitatively assessed. By systematically varying the separation of the bifluorene moieties using model compounds, ion pairing, and solvation, these interactions, with energies up to about 0.4 V, are absent at distances greater than about 9 Å. These findings can be (quantitatively) applied for the design of polychromophoric assemblies, whereby the redox properties of donors and/or acceptors can be tuned by judicious positioning of the charged groups to control the electron‐transfer dynamics

Charge Delocalization in Self-Assembled Mixed-Valence Aromatic Cation Radicals

The spontaneous assembly of aromatic cation radicals (D+•) with their neutral counterpart (D) affords dimer cation radicals (D2+•). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography

Through-Space or Through-Bond? The Important Role of Cofaciality in Orbital Reordering and Its Implications for Hole (De)stabilization in Polychromophoric Assemblies

Developing predictive tools for the elucidation of redox and optical properties of polychromophoric assemblies is crucial for the rational design of efficient charge-transfer materials. Here, such tools are introduced to explain the curious observation that a pair of bichromophoric electron donors based on ethanoanthracene (<b>5</b>) and dihydroanthracene (<b>DHA</b> or <b>11</b>), having similar interchromophoric separation between the carbons in the region of orbital overlap, show dramatically different (>300 mV) stabilization of their cation radicals. Analysis of molecular orbital diagrams reveals the important interplay between through-space and through-bond electronic couplings, which results in <i>HOMO/HOMO–1 swapping in <b>11</b></i> (HOMO = highest occupied molecular orbital). Unlike the antisymmetric singly occupied molecular orbital (SOMO) that stabilizes a hole by charge resonance in <b>5</b>^•+ (0.57 V), the symmetric SOMO in <b>11</b>^•+ (0.88 V) does not afford hole stabilization by charge resonance; rather, the hole localizes onto a single benzenoid unit. This important finding is expected to aid in the design of polychromophoric assemblies with chromophores of graded redox potentials for optimization of long-range charge transfer