The textures and the growth kinetics of the hexagons were analyzed using a polarizing microscope equipped with a Mettler hot stage and a Leica image analyzing system. AFM investigations were performed with a Digital Instruments setup and fluorescence microscopy investigations were performed on the discotic host materials containing fluorescent dyes as guest molecules.Poly((3-substituted)thiophenes) (PTs) represent a class of polymer that are environmentally stable, highly processable, and exhibit high electrical conductivity. These materials are promising candidates for field-effect transistors, optical and electronic sensors, light-emitting diodes (LEDs), and nonlinear optical materials, to name a few. [1] However, the polymerization of the non-symmetrical 3substituted thiophene leads to a mixture of PT structures containing three regiochemical linkages between repeat units (2,2¢, 2,5¢, 5,5¢). The 2,2¢ and 5,5¢ couplings are considered defects in the polymer structure since they diminish conjugation and prevent ideal solid-state packing, thus impairing these materials' electronic and photonic properties. However, the synthesis of the pure 2,5¢ (or head-to-tail (HT) coupled) regioregular PTs was first accomplished by our group by employing Kumada cross-coupling methods to regiospecifically polymerize 2-bromo-3-alkyl-5-magnesiobromothiophene. [2] Other methods have employed organozinc [3] reagents with nickel(II) catalysts. We have also shown that regioregular PTs can be made using Stille crosscoupling procedures, [4,5] while others have employed Suzuki cross-coupling. [6] Despite these new advances, the above synthetic procedures have some drawbacks. The method developed in our laboratory [2] requires highly purified starting materials, most important of which is the monomer, 2-bromo-3-alkylthiophene. In addition, this method requires cryogenic temperatures and long polymerization times ranging from 12 to 24 h or longer. The Rieke method starts with the easy-to-purify 2,5-dibromo-3-alkylthiophene (since the compound is the highest boiling fraction in the crude mixture in its preparation), however, it requires the non-trivial preparation of Rieke zinc via alkali metal reduction of zinc halides, employs cryogenic temperatures, and necessitates long reaction times. Both Suzuki and Stille coupling methods also suffer from many of the above drawbacks. In addition, there have been no reports of using the above methods for the large-scale synthesis of HT-PTs. This communication will describe a new method for the preparation of HT-poly(3-alkylthiophenes) (HT-PATs) that is simple, efficient, fast, and economical.[**] We are grateful to the NSF (CHE-9509959) for financial support. We also thank Lubrizol for providing a research fellowship for RSL and Eric van Inwegen for help with syntheses.See page 187 for ordering details.

An investigation of the new synthetic method to synthesize regioregular, head-to-tail coupled poly(3-alkylthiophenes) using magnesium−halogen exchange (Grignard metathesis) called the GRIM method is described. Treatment of 2,5-dibromo-3-alkylthiophenes with a variety of alkyl and vinyl Grignard reagents resulted in two metalated, regiochemical isomers, namely, 2-bromo-3-alkyl-5-bromomagnesiothiophene and 2-bromomagnesio-3-alkyl-5-bromothiophene in an 85:15 ratio. This ratio appears to be independent of reaction time, temperature, and Grignard reagent employed. Introduction of a catalytic amount of Ni(dppp)Cl2 to this isomeric mixture afforded poly(3-alkylthiophene) that contained greater than 95% HT−HT couplings (typically 98% HT couplings were seen). The high degree of regioregularity found in the polymer can be explained by a combination of kinetic and thermodynamic effects arising from steric and electronic effects found in the catalytic reaction. A series of reaction investigations led to a general explanation of the origin of regioregularity in polythiophene polymerization reactions. These reactions included kinetic studies and competition experiments.

Regioregular, head-to-tail coupled, poly(3-alkylthiophenes), synthesized by three different methods, were subjected to MALDI-TOF MS analysis. Polymer samples of both high and low polydispersities were examined. Polymer samples of narrow polydispersity were obtained by fractionation of the polymer by Soxhlet extraction with various solvents. Comparisons between the molecular weights calculated by MALDI and GPC of all fractionated polymer samples showed that GPC calculated molecular weights are a factor of 1.2-2.3 times higher than MALDI. The polydispersities calculated by MALDI were identical or slightly lower than those calculated by GPC. Polymer end-group compositions were also analyzed. We found that more than one type of end-group structure could be detected, and these structures are dependent on the synthetic method employed. Chemical modification of the end-group structure was also performed and monitored by MALDI with successful results. We observed that smaller polymer chains were subject to end-group fragmentation when the MALDI laser power was high. Larger polymer chains, however, showed no such fragmentation.

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Monolayers of two classes of Zn porphyrins have been prepared and examined on Si(100). These molecules, designated as ZnPBzX- and ZnPCH2X-, contain either a benzyl (-Bz-) or a methylene (-CH2-) unit terminated with a Group VI atom (X = O, S, Se) appended to a meso-position of the porphyrin, with the nonlinking meso-substituents consisting of either mesityl (-Bz- class) or p-tolyl and phenyl (-CH2- class) units. The two series of ZnPBzX- and ZnPCH2X- monolayers on Si(100) were examined using a variety of techniques, including X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The studies reveal the following characteristics of the ZnPBzX- and ZnPCH2X- monolayers. (1) Surface binding can be readily achieved to Si(100) with both relatively short (-Bz-) and very short (-CH2-) tethers regardless of the nature of the Group VI anchoring atom (O, S, Se). (2) The longer -Bz- tether affords monolayers with the porphyrin ring in a somewhat more upright orientation with respect to the surface than the shorter -CH2- tether. The more upright adsorption geometry of the porphyrins bearing the former type of linker leads to a higher packing density and more homogeneous redox thermodynamics. (3) The kinetics of electron transfer does not depend on the type of Group VI atom used for anchoring to the Si(100) surface. On the other hand, the type of linker does affect the electron-transfer rates, with the monolayers bearing the -CH2- linker exhibiting systematically faster rates than those bearing the -Bz- linker. Collectively, the studies reported herein provide a detailed picture of how the anchor atom and the linker type influence the structural and electron-transfer characteristics of these general classes of monolayers.