Michael G Walter

University of North Carolina at Charlotte | UNC Charlotte · Department of Chemistry

The synthesis, electrochemical, and photophysical characterization of N,N'-dialkylated and N,N'-dibenzylated dipyridinium thiazolo[5,4-d]thiazole derivatives are reported. The thiazolothiazole (TTz2+) viologens exhibit strong blue fluorescence with high quantum yields between 0.8 - 0.96. The dioctyl, dimethyl, and dibenzyl derivatives also show dis...

The effects of molecular arrangement on the singlet exciton diffusion properties of free-base carboalkoxyphenylporphyrins containing a variety of alkyl substituents were investigated in solution-cast thin films. Exciton diffusivity was calculated using relative quenching efficiencies (Q) obtained by measuring the singlet photoluminescent decay life...

The singlet exciton diffusion properties of solution-cast thin films prepared from mixed substituent monoiodophenyl or monobromophenyl derivatives of 5,10,15,20-tetrakis(4-carbomethoxyphenyl) porphyrin (TCM4PP) were investigated for use in solar energy conversion applications. The photoluminescent singlet decay lifetime PL(t) of pristine porphyrin...

Water splitting cells with direct semiconductor/liquid contacts are attractive because they avoid significant fabrication and systems costs involved with the use of separate electrolyzers wired to p-n junction solar cells. Another attractive advantage of photoelectrochemical water splitting directly at the semiconductor surface is the ease with whi...

The IUPAC Subcommittee on Polymer Education has been pursuing the development of a compact syllabus covering the essential topics required for a tertiary education in polymer science, with numerical and short answer exercises addressing each topic. The primary goal of the document is to provide a framework for a complete course made freely availabl...

Water-soluble dipyridinium thiazolo[5,4-d]thiazole (TTz) compounds are incorporated into inexpensive poly(vinyl alcohol) (PVA)/borax films and exhibit fast (<1 s), high-contrast photochromism, photofluorochromism, and oxygen sensing. Under illumination, the films change from clear/yellow TTz²⁺ to purple TTz•+ and then blue TTz⁰. The contrast and sp...

The complex dielectric function of photochromic thiazolo[5,4-d]thiazole embedded in polymer is reported in the spectral range from 0.8 to 2.5 eV. Strong absorption bands in the measured spectral range are observed depending upon its redox states.

There is a growing demand for new fluorescent small molecule dyes for solid state applications in the photonics and optoelectronics industry. Thiazolo[5,4-d]thiazole (TTz) is an organic heterocycle moiety which has previously shown remarkable properties as a conjugated polymer and in solution-based studies. For TTz-based small molecules to be incor...

Vertically oriented Si microwires (Si MWs) of high aspect ratio were grown on degenerately doped n-Si (111) substrate in an atmospheric pressure chemical vapor deposition (APCVD) system by a vapor–liquid-solid (VLS) mechanism. A lithography-free de-wetting technique was used on as-deposited Cu thin film to form the catalyst seed layer for the Si MW...

In this paper, the complex dielectric function of 2,5-bis(N,N-dibutyl-4-aminophenyl) thiazolo[5,4-d]thiazole is reported. Thin films of this material were obtained by spin coating on a silicon substrate. The samples were investigated using spectroscopic ellipsometry in the spectral range from 354 nm to 1907 nm at multiple angles of incidence. The e...

Solvatofluorochromic molecules provide strikingly high fluorescent outputs to monitor a wide range of biological, environmental, or materials‐related sensing processes. Here, thiazolo[5,4‐d]thiazole (TTz) fluorophores equipped with simple alkylamino and nitrophenyl substituents for solid‐state, high‐performance chemo‐responsive sensing applications...

Hexacoordinate silicon pincer complexes using 2,6-bis(benzimidazol-2-yl)pyridine (bzimpy) ligands have been developed as a multifunctional, molecular electronic materials platform. We report the synthesis, characterization, and device application of a variety of Si(pincer)2 complexes that exhibit tunable optoelectronic properties and excellent ther...

The study of excited-state energy diffusion has had an important impact in the development and optimization of organic electronics. For instance, optimizing excited-state energy migration in the photoactive layer in an organic solar cell device has been shown to yield efficient solar energy conversion. Despite the crucial role that energy migration...

A hexacoordinate organosilane with a SiC4N2 skeleton has been synthesized from the reaction of a tridentate, dianionic, CNC-diphenylpyridine (DPP) ligand with SiCl4. The complex has a distorted octahedral geometry with short S-N bond lengths averaging 1.9111(14) Å. The Si(DPP)2 complex is resistant to hydrolysis and thermally robust. Thermally evap...

These recommendations are specifically for polymers and polymer systems showing a significant response to an electromagnetic field or one of its components (electric field or magnetic field), i.e., for electromagnetic-field-responsive polymer materials. The structures, processes, phenomena and quantities relating to this interdisciplinary field of...

Solution-processable conductive polymers are advantageous materials for making inexpensive, electrical junctions to crystalline semiconductors. We have investigated methods to improve the device performance of hybrid solar cells made from n-type silicon and a conductive polymer glue based on a blend of poly(3,4-ethylenedioxythiophene):poly(styrenes...

There have been an increasing number of materials developed that show multifunctional chromogenic properties (such as electrochromism, electrofluorochromism, or photochromism), but to date, few materials have shown all three properties. Materials that are electrochemically and optically active are attractive for a diverse set of applications that i...

Organic electronics has developed rapidly over the past 40 years. In 1977, a seminal discovery was reported that showed that a polymer known as polyacetylene could conduct electricity as well as metals could. This was a groundbreaking discovery that led to a Nobel Prize in Chemistry in 2000. The polymers that are used in organic electronics have no...

Fundamental properties of conjugated copolymers are sensitively linked to their constitution and features of their repeat unit design that govern assembly and organization across multiple length scales, ultimately impacting device performance. Herein we report the efficient synthesis and characterization, and optical, electrochemical and transport...

These recommendations are specifically for polymers and polymer systems showing a significant response to an electromagnetic field or one of its components (electric field or magnetic field), i.e., for electromagnetic-field-responsive polymer materials. The structures, processes, phenomena and quantities relating to this interdisciplinary field of...

A family of asymmetric thiazolo[5,4-d]thiazole (TTz) fluorescent dye sensors have been developed and their photophysical sensing properties are reported. The π-conjugated, TTz-bridged compounds are synthesized via a single-step, double condensation/oxidation of dithiooxamide and two different aromatic aldehydes: one with strong electron donating ch...

ABSTRACT: Photoinduced electron and energy transfer through preorganized chromophore, donor, and acceptor arrays are key to light-harvesting capabilities of photosynthetic plants and bacteria. Mimicking the design principles of natural photosystems, we constructed a new luminescent pillared paddle wheel metal− organic framework (MOF), Zn2(NDC)2(DPT...

This paper provides a list of the most important terms from all areas of polymer science including polymer chemistry, polymer physics, polymer technology and polymer properties. These have been assembled into a representative list of terms that serves as an IUPAC recommended list of keywords for polymer science.

A neutral hexacoordinate silicon complex containing two 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) ligands has been synthesized and explored as a potential electron transport layer and electroluminescent layer in organic electronic devices. The air and water stable complex is fluorescent in solution with a λmax = 510 nm and a QY = 57%. Thin films...

An organic conductive glue based on a blend of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and D-sorbitol was examined for laminating conductors to crystalline silicon. The PEDOT:PSS glue (s-PEDOT:PSS) functions as a high work function processable conductor and exhibits ohmic contact on p-type silicon and a rectifying contac...

To improve polymer education for 9–12 and undergraduate students, a plastic electronics laboratory kit using polymer semiconductors has been developed. The three-module kit and curriculum use polymer semiconductors to provide hands-on inquiry activities with overlapping themes of electrical conductivity, light emission, and light-harvesting solar e...

Creating new building blocks for donor–acceptor conjugated systems is an important task for continued development of materials for organic electronics. Purines were introduced into small-molecule π-conjugated systems via Stille cross-coupling using stannylated derivatives of benzodithiophene, thiophene, or dithienylbenzothiadiazole to generate a se...

Sensitizers with high triplet quantum yields are useful for generating photovoltaics, photocatalysts and photodynamic therapy agents with increased efficiency. In this study, the heavy atom effect was used to optimize the triplet and singlet oxygen quantum yields of 5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrin (1-TCM4PP). The triplet quantum...

The development of a new science outreach program is described that uses a polymer semiconductor kit geared towards high school science classrooms. The kit is focused around educating students about polymeric semiconductors and contains materials and curriculum for three educational modules: a conductive polyaniline nanofiber module, a light-emitti...

5,10,15,20-Tetrakis(4-aminophenyl)porphyrin (TAPP) undergoes oxidative polymerization to form electronically conductive, nanofibrous structures in which the porphyrin units are linked by phenazine bridges. Polymerizations by chemical oxidation, electrochemical oxidation, and interfacial oxidative polymerization are described. Poly-TAPP (pTAPP) film...

The versatility of porphyrins as optoelectronic or catalytic units make them attractive elements for inclusion in functional materials. Polymers that include porphyrins have been created with a wide variety of structures and applied to a wide range of applications. This review covers recent developments in the synthesis, characterization, and appli...

Over the last 5 years, researchers at the California Institute of Technology and several partner institutions have been executing a set of public outreach programs unified under the name “The Solar Army.” These programs involve students of all ages, from those in primary school to adults, in the enterprise of cutting-edge solar energy research. Her...

The electronic and photovoltaic properties of junctions between the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and Si(111) surfaces have been investigated for a range of doping types, doping levels, and surface functionalization of the Si. PEDOT–poly(styrenesulfonate) (PSS) formed ohmic, low resistance contacts to H-terminated and...

The electronic and photovoltaic properties of junctions between the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and Si(111) surfaces have been investigated for a range of doping types, doping levels, and surface functionalization of the Si. PEDOT–poly(styrenesulfonate) (PSS) formed ohmic, low resistance contacts to H-terminated and...

The electrical properties of individual p- and n-type silicon microwires have been investigated using a direct contact formation technique, in which an Ohmic contact between the tungsten probe and microwire was produced by applying mechanical force to the probe/microwire junction. This method alleviated the need for lithography or a high-temperatur...

The optical absorption, ionic conductivity, electronic conductivity, and gas separation properties have been evaluated for flexible composite films of ionically conductive polymers that contain partially embedded arrays of ordered, crystalline, p-type Si microwires. The cation exchange ionomerNafion, and a recently developed anion exchange ionomer,...

The electrical (DC) behavior of single silicon microwires has been determined by the use of tungsten probes to make ohmic contact to the silicon microwires. The basic electrical properties of the microwires, such as their DC resistivity and the doping distribution along the length of the microwires, were investigated using this approach. The techni...

Arrays of B-doped p-Si microwires, diffusion-doped with P to form a radial n(+) emitter and subsequently coated with a 1.5-nm-thick discontinuous film of evaporated Pt, were used as photocathodes for H(2) evolution from water. These electrodes yielded thermodynamically based energy-conversion efficiencies >5% under 1 sun solar simulation, despite a...

The effects of introducing an n^+-doped emitter layer have been evaluated for both planar Si photoelectrodes and for radial junction Si microwire-array photoelectrodes. In contact with the pH-independent, one-electron, outer-sphere, methyl viologen redox system (denoted MV^(2+/+)), both planar and wire array p-Si photoelectrodes yielded open-circui...

Electrochemical oxidation of tetrakis-5,10,15,20-(4-aminophenyl)porphyrin (TAPP) on fluorine-doped tin oxide electrodes leads to a conductive polymeric film (poly-TAPP) with a nanostructured fibrous morphology. The nanofiber structure and growth rate are enhanced by the addition of pyridine (5 -15%) to the dichloromethane electrochemical solution....

This review summarizes recent advances in the use of porphyrins, phthalocyanines, and related compounds as components of solar cells, including organic molecular solar cells, polymer cells, and dye-sensitized solar cells. The recent report of a porphyrin dye that achieves 11% power conversion efficiency in a dye-sensitized solar cell indicates that...

Pyridine derivatives and dichloromethane (DCM) are commonly used together in a variety of different applications. However, DCM slowly reacts with pyridine and a variety of other representative pyridine derivatives to form methylenebispyridinium dichloride compounds under ambient conditions. The proposed mechanism (two consecutive S(N)2 reactions) w...

Porphyrins substituted at meso positions with aminophenyl groups undergo oxidative electropolymerization in a process analogous to the formation of polyaniline. Porphyrins that successfully generate polymer films on the electrode include those tetrasubstituted with four p-aminophenyl groups, trisubstituted, or trans-disubstituted, but not cis-disub...

Conductive nanostructured films of poly-tetrakis-5,10,15,20-(4-aminophenyl)porphyrin (TAPP) can be grown electrochemically or through interfacial oxidative polymerization. The poly-TAPP nanomorphology is sensitive to the electrochemical solvent, potentiometric method, and the aminophenyl porphyrin monomer utilized. To elucidate the molecular struct...

New mixed-substituent amino/carboxyphenylporphyrins for a dye-sensitized TiO2 solar cell were prepared using several synthetic routes. The reaction of 4-carbomethoxy- and 4-acetamidobenzaldehydes with pyrrole in propionic acid under aerobic conditions afforded mixtures of mixed amide/ester substituted tetraphenylporphyrins which were separated usin...

The molecules 2,5-Di-pyridin-2(and 4)-yl-thiazolo[5,4-d]thiazole show very interesting fluorescent properties under long wavelength UV radiation. In the neutral form solutions exhibit weak blue fluorescent emission. However addition of even a very small amount of strong acid will cause the fluorescence to shift to longer wavelength and strongly inc...