The thesis is devoted to the investigation of photoelectronic properties of structured biopolymer of melanin of natural and synthetic origin and composites based on them using optical spectroscopy methods, under excitation by light in visible and UV ranges.
Research of the molecular structure of melanin solutions and composites based on it have shown that melain is characterized by a nanocluster model, which implements a collective model of excitation through Frenkel excitons (EF) and CT excitons (CTE), which correlates with previous studies.
It was shown that the spectra and dynamics of PL decay of the studied synthetic and natural melanins at room temperature are similar in many respects.
The temperature dependence of PL melanin was studied. It is shown that both pre-dimeric and dimeric structures characteristic of stack packing of chromophores are possible in melanin solutions. Pre-dimeric and dimeric structures can generate monomeric, excimeric (exciplexic) and dimeric radiation in the PL spectra. Some bands of the PL spectra of melanin behave like excimer radiation depending on the temperature. As the temperature decreases due to structural difficulties in the convergence of the excimer molecules, the excimer PL associated with the predimeric states (PL bands at 600 and 680 nm) disappears, and with the dimeric ones it remains and has a very short lifetime.
As a result of spectral and kinetic studies of PL of natural and synthetic melanin, as well as melanin composites with surfactants SDBS and astrafloxin, it was concluded that the possibility of formation of intermolecular complexes with charge transfer (CT), nanoaggregation and formation of CT states, researched exciton lifetimes, energy structure of melanin. In particular, it was found that interacting with the pre-dimeric states of melanin with the above components, intermolecular CT complexes are formed. As a result of this interaction, the decay of excimer radiation and the increasing in the radiation intensity of free excitons in melanin nanoclusters are observed. That it is shown that melanin belongs to the class of CT crystals.
Based on the presented experimental data on the absorption and TRES spectra for melanin solutions at different temperatures (296 and 4.2 K), a model of the energy structure of the ground and excited states responsible for the optical properties of melanin was proposed.
The energy diagram shows that in the PL spectra of melanin during optical excitation we can distinguish radiation of EF (390-500 nm) for one-dimensional polymer chromophores of melanin with different degrees of conjugation, which form a molecular disk and CTE delocalized within two or more molecular disks.
Due to the delay time of 50–180 ps in the kinetic curves, it is shown that both EF and CTE can be barrier-free autolocalized.
Based on experimental data on absorption spectra and fluorescence spectroscopy with picosecond time resolution for aqueous solutions of melanin and melanin composites with electron acceptor TNF, the formation of a stable intermolecular complexes with CT in the ground and excited states have been concluded, its photoelectronic properties are investigated.
The study of TRES spectra of melanin with TNF showed that in the PL spectra can be distinguished fast and slow components of emission. The instantaneous spectra of the fast component consist of 3 bands, which have similar PL lifetimes, but different nature of excitation, the energy structure and exciton processes responsible for it are established.
The PL properties of melanin composite with PCBM electron acceptor were studied. It is shown the possibility of sensitization of PL molecules of PCBM by melanin due to physical and chemical absorption of PCBM molecules on their surface with the formation of CT complexes between melanin and PCBM molecules, the emergence of donor-acceptor interaction, the nature of these transitions is established.
