Significance and Goals

The interest in organic dyes dates back to prehistoric times. The discovery that natural colorants found in plants and animals could be isolated and used for dyeing textiles had a remarkable influence on the development of modern society. Colorful fabrics soon became coveted objects of luxury, and expensively dyed garments were used for display of wealth and power. Toga picta, the most splendid variant of the ancient Roman male costume, was dyed solid purple and originally allowed to be worn only by triumphing generals, until it was adopted by emperors in the imperial period. Toga picta, the precursor of many regal outfits, was colored with Tyrian purple, an outrageously expensive dye, extracted by ancient Phoenicians from sea snails of the Murcidae family.

In a way, it was organic synthesis that put an end to dye production as a luxury business. In 1856, with the serendipitous discovery of mauveine by the adolescent William Perkin, dyestuff industry began its explosive growth, which continues to this day. Thanks to Perkin and his successors, prices dropped, and the once luxurious dyed textiles are now commonplace and easily affordable. Perkin’s discovery gave an indispensable research impulse to chemists: it spurred an interest in fundamental properties of organic dyes, and in synthetic methods to produce these compounds. It was soon realized that organic dyes can offer far more than just the color (Figure 1). These materials are now known to emit light when stimulated by radiation or electricity, conduct or produce electric current, and promote chemical reactions when irradiated. Consequently, all modern applications of organic colorants are based upon controlled interactions of dye molecules with radiation or electricity.

Figure 1. Origins of functional dye chemistry and its modern importance.

Molecules of all practically useful organic dyes contain highly mobile electrons, associated with so-called π-conjugated bonds. π-Conjugation is classically represented as a combination of single and multiple bonds, and is related to the old chemical concept of unsaturation. The π-conjugated part of the molecule, responsible for the color of the dye and other properties, is known as the chromophore. The energy of interacting photons can be lowered by increasing the number of π bonds, which is the method chemists use to create dyes absorbing visible light or near-infrared radiation. Design of new chromophores suitable for particular applications is however more complicated, because the optical and electronic characteristics of molecules can be predicted with limited accuracy. More importantly, for each new dye, a chemical synthesis has to be proposed and executed. This can become a challenging task, especially for original new structures, whose chemistry is not well understood.

Many of the most recent developments in dye chemistry, especially those related to semiconductor applications, have been inspired by the discovery of graphene, which has a perfectly ordered π-conjugated structure. However, because of their infinite size, graphene sheets have no energy gap and are not semiconducting. The energy gap can be opened by constraining the size of graphene in one or two dimensions. In particular, many large aromatic hydrocarbons, whose molecules can be considered graphene fragments (nanographenes, graphene quantum dots), are useful as organic semiconductors. This observation spurred great interest in large sheet-like molecules, with structures resembling graphene. By combining features of classical dyes (e.g., peryleneimides, phthalocyanines, etc.) with those of graphene, it is possible to create new families of chromophores with unprecedented electronic and optical properties.

The present proposal, built upon the expertise of the Stępień group, remains focused on fundamental aspects of modern chromophore science. With the support of the FNP-TEAM program, we will be able to develop new classes of π-conjugated molecules, making extensive use of our modular donor−acceptor approach. Through these investigations, we will expand the synthetic toolbox of heteroaromatic chemistry, and elaborate new hybrid systems based on oligopyrrole and rylene motifs. However, through the essential partnership with the groups of Kim, Galstyan, and Łapkowski, our research will be directed toward several areas with an increasing practical significance (Figure 2).

Figure 2. Potential applications of the proposed research. Envisaged future research directions are presented in gray boxes.

State-of-the-art photophysical investigations performed by the Kim group will provide insight into several aspects of exciton dynamics in the newly developed hybrid oligopyrroles. In particular, we hope that excited-state symmetry breaking in pyrrole dyads will help us design bright fluorophores for biomedical applications as well as efficient dyes for two-photon absorption (TPA). TPA-active compounds are widely explored as polymerization photoinitiators for 3D fabrication, fluorescent imaging dyes, and photosensitizers for photodynamic therapy. Practically useful compounds require high TPA brightness levels combined with good photostability, and should be easily functionalized for particular applications. The usability of our chromophores as TPA dyes cannot be predicted at this stage and has not been proposed as a direct objective of this work. However, because some of the proposed targets fulfill the initial structural criteria for a TPA dye and because excited-state symmetry breaking is usually associated with high TPA levels, this line of investigation is envisaged as a potential future outcome of the project.

Electrochemically generated polymers, developed in collaboration with the Łapkowski group, can be expected to exhibit marked electrochromism, as previously reported in various ladder-like and donor-acceptor polymers, two features that can be combined in the present designs. Further potential applications of these materials include electroluminescent devices and electrogenerated chemiluminescence, an effect used practically in clinical and industrial analysis.

Our collaboration with the Galstyan laboratory will let us identify useful designs of antimicrobial PDT photosensitizers based on donor−acceptor oligopyrroles. The potential clinical market for such developments includes treatment of various bacterial pathogens and infections, e.g., chronic sinusitis, periodontitis, streptococcal wounds, acne, etc. The combined expertise of Stępień and Galstyan groups can be extended to explore other biochemical applications of functional chromophores and fluorophores, such as the design of membrane-intercalating conjugated oligoelectrolytes, useful as cell stains or efficient antimicrobial agents.