Organic semiconductor materials are of interest for optoelectronic applications due to their low cost, solution processability, and tunable properties. Recently, organic pigments derived from natural products have attracted attention, exhibiting extraordinary environmental stability combined with high (photo)conductivity, in spite of their molecular structures not having a fully conjugated core. A subset of such pigments, fungi-derived pigments, represents a naturally sourced, sustainable class of materials that are completely unexplored as organic semiconductors. We explored optical and electronic properties of several fungi-derived pigments, an example of which is a blue-green pigment xylindein, which is secreted by the wood-staining fungi Chlorociboria (C.) aeruginosa and C. aeruginascens. Xylindein exhibits an extraordinary long-term thermal and photostability in solution and in films, a hole mobility of >0.2 cm2/(Vs) in amorphous films, and a photoresponse throughout the ultraviolet and visible wavelength range. In order to understand these properties, we carried out a detailed study of exciton and charge carrier dynamics in xylindein and selected other pigments (such as red and yellow pigments derived from Scytalidium cuboideum and Scytalidium ganodermophthorum, respectively), which will be presented.
A particularly important aspect of the pigment photophysics is related to their ability to form both intermolecular and intramolecular hydrogen bonds. The relative contribution of these interactions to the optical properties depends on the environment, which was established using measurements of optical absorption and time-resolved photoluminescence (PL) of pigments in various solvents and pH buffers, depending on the pigment concentration. In thin films, optical properties are determined by an interplay of p-p stacking and hydrogen bonding resulting in an aggregate formation. The aggregate properties were quantified using temperature and polarization dependent optical absorption and PL spectra and Spano’s theoretical analysis of molecular aggregate spectral characteristics. The optical properties were then related to the molecular packing and film morphology, obtained from XRD and SEM.
Pristine xylindein when deposited from solution forms porous amorphous films; towards improving film morphology, blends of xylindein with several polymers were explored. The (opto)electronic properties of xylindein and xylindein dispersed in polymer matrices were systematically studied to determine electron and hole mobilities, depending on the blend content and film structure and morphology. Measurements of photocurrents in pristine films and in donor-acceptor blends in which a polymer serves as a donor and xylindein as acceptor were also carried out. (Opto)electronic properties of solution-grown crystals formed by a red pigment derived from Scytalidium cuboideum will also be presented.