Harold Freeman

Research

Prof. Harold S. Freeman Conducts Research on Organic Dyes and Pigments

Our 31 years of research in this area has involved 3 principal types of studies: 1) the design of dyes having high photostability on hydrophobic fibers; 2) the environmental chemistry of synthetic dyes and pigments; and 3) the analytical chemistry of synthetic dyes. Most recently, we have conducted research pertaining to the design of dyes for use in Photodynamic Therapy (PDT) and Dye Sensitized Solar Cells (DSSCs).

1. Dyes having high photostability on hydrophobic fibersThe need for photostable (lightfast) dyes for hydrophobic fibers, especially polyester, stems from their use in automobile interiors, where stability to prolonged and repeated exposures to sunlight in combination with heat and humidity is paramount. Our research in this area began in 1985, with funding from a consortium of companies in the supply chain leading to fabric and carpet for automobile interiors and, subsequently, from NSF to characterize the contribution of heat and humidity to the dye fading process (cf. Fig 1). At the heart of this work was to determine the mechanisms of fading associated with prototype synthetic dyes used in this area of technology and to use the resulting information to design dyes immune to the observed mechanisms. This research led to a family of lightfast disperse dyes containing a built-in photostabilizer (e.g. 1-2). In this regard, it was found that the nature and location of the built-in stabilizer played a key role in the viability of the dyes obtained.

   

   Fig. 1. Dyed PET films before (left) and after (right) sunlight exposure.

   

   In closely related studies, we are investigating the influence of the host fiber on the photodegradation of adsorbed dyes and preliminary results indicate that the photostability of dyes on polyester is adversely influenced by the host fiber unless a photostabilizer is present. This also suggests that stabilization of the excited polymer molecules is critical to the life of the adsorbed dye.

2. Environmental chemistry of synthetic dyes and pigmentsSynthetic dyes, especially azo dyes, have come under increased scrutiny by regulatory agencies because certain precursors used in their manufacture were shown to pose a risk to human health and the environment. This led to the need to understand the relationships between azo dye structures and their genotoxic and ecotoxic potential. Our work in this area started in 1984 with funding from IBM, whose corporate interest was to generate results that could be used as a basis for knowing which dyes to avoid in commercial products, such as ink-jet printers, but our group saw an opportunity to use the same results to design safe alternatives to important dyes being eliminated from commerce because of their potential to undergo metabolism to produce carcinogenic arylamines such as benzidine (cf. 3, Fig. 2). This idea enabled the design of non-genotoxic aromatic amine precursors (e.g. 4-6) for azo dye and pigment synthesis and the use of the derivative colorants for textile and non-textile coloration.
   

   Fig. 2. Metabolic cleavage of an azo dye to the corresponding arylamines.

   

   Funding from the National Textile Center and Clariant Chemicals has permitted the design of metal complexed dyes for polyamide and protein fibers that would be devoid of the toxicity concerns associated with certain effluents from dye manufacture and textile dyeing operations. The main goal was to protect ecosystems associated with either drinking water or aquatic life (plants and animals). While the presence of chromium (Cr) and cobalt (Co) ions in textile dyes was deemed critical to producing photostable dyes for carpet fibers, the presence of free Cr and Co ions in ecosystems is undesirable. Our research in this area has produced environmental friendly iron (Fe) complexed azo and formazan dyes that maintain the desirable technical properties of the Cr- and Co-based prototypes. It was also found that the formation of low-spin rather than high-spin complexes is the key to overcoming the typically dull colors associated with Fe complexes (cf. Fig. 3).

   

   Fig. 3. Optimized geometries for low-spin (left) vs. high-spin (right) Fe complexed formazan dyes.

3. The analytical chemistry of synthetic dyesOur research in this area has been dedicated to the development of methods for purifying synthetic dyes for structure confirmation and toxicological studies. A variety of chromatographic, crystallographic, and spectrometric methods were developed and published for use by the color chemistry community (cf. Fig. 4). These methods also proved useful for characterizing complex mixtures and for correlating dye structure with choice of techniques to employ.
   
   

   Fig. 4. Mass spectrometry and x-ray results from synthetic dye analyses.

4. Dyes for use in photodynamic therapy (PDT)An opportunity to merge our long-standing interest in synthetic dyes with an initial career interest in pharmaceuticals was the inspiration for current research pertaining to the design and synthesis of dye sensitizers for photodynamic therapy (PDT). PDT is a type of photochemotherapy for various types of cancers and requires the presence of light, a sensitizer, and molecular oxygen for treatments. During PDT, a sensitizer is administered intravenously, intraperitoneally, or topically, and selectively localizes in a tumor due to physiological differences between the tumor and healthy tissue. Localization into cancer cells and achieving a maximum tumor-to-normal cell concentration ratio can take 3 to 96 hours, depending on the photosensitizer and tumor. Following localization, fluorescence from the sensitizer can be used to diagnose and detect the tumor. Irradiation at a wavelength specific to the photosensitizer produces singlet oxygen, which reacts with and destroys the tumor. Suitable sensitizers are mainly porphyrinoid compounds, phthalocyanines, and related structures. Our research focuses on the design of novel dyes having high singlet oxygen generating efficiency and high selectivity for tumors. Porphyrin and formazan systems containing appendages that facilitate tumor uptake are currently under investigation (e.g. 7-8).
   
5. Dyes for use in dye sensitized solar cells (DSSCs)Over the past 10 years, many metal-free organic dye sensitizers have been developed for application in DSSCs, with the typically structure based on a push-pull system (i.e. donor-spacer-acceptor system). Specifically, double bond based spacers have been frequently employed to give red-shifted absorption spectra aimed at high solar energy capture. However, widely employed C=C double bonds as spacers can limit DSSC efficiency, resulting in adverse effects such as self-quenching of generated electrons arising from cis-trans isomerization or dye aggregation. While the photovoltaic performance of a furan-based sensitizer has often been investigated with regard to recombination kinetics, fewer studies involving other heterocyclic ring systems as a spacer are reported. Hence, our contributions in this arena have involved 1) the synthesis of novel metal-free sensitizers by systematic variation of the spacer unit (cf. Fig. 5) and 2) determination of intrinsic factors affecting photovoltaic performance through use of the resultant dyes in DSSCs.Based on current-potential curves, current density increased in the order DP-TZ < DP-P < DP-F < DP-B < DP-T The highest plateau of quantum efficiency from IPCE spectra was obtained from DP-B, indicating a relationship between decreased (or increased) aggregation behavior of dyes on a TiO₂ surface and higher (or lower) electron transfer yield. The dependence of open-circuit voltage on electron lifetime was evident, resulting in enhanced V_oc in the order of devices based on DP-TZ< DP-F < DP-T < DP-B < DP-P. The lower polarizabilities of the molecules contributed to increased lifetimes but this benefit was weakened when efficient surface blocking was reduced, resulting in an unexpected drop in open-circuit voltage for DP-F. The DSSC device based N-methyl pyrrole (DP-P) showed good kinetic properties, resulting in high V_oc and making the pyrrole unit a logical spacer for further molecular design studies towards high efficiency DSSCs. Given the similar maximum quantum efficiency (from IPCE data) of the device based on DP-P compared to DP-T, an enhanced absorption spectral shift would give high current densities needed for large solar energy capture. It should be added that molecular engineering of sensitizers having an N-methyl pyrrole unit should be careful to reduce the dihedral angle between spacers and neighboring group. Such experiments should be augmented by molecular modeling studies designed to examine the behavior of dye sensitizers on TiO₂ in the presence of additives.
   

   Fig. 5. Target molecular structures based on different spacers in a D-π-A system.