Lessons from chlorophylls : modifications of porphyrinoids towards optimized solar energy conversion

Practical applications of photosynthesis-inspired processes depend on a thorough understanding of the structures and physiochemical features of pigment molecules such as chlorophylls and bacteriochlorophylls. Consequently, the major structural features of these pigments have been systematically examined as to how they influence the S_{1} state energy, lifetimes, quantum yields, and pigment photostability. In particular, the effects of the macrocyclic π-electron system, central metal ion (CMI), peripheral substituents, and pigment aggregation, on these critical parameters are discussed. The results obtained confirm that the π-electron system of the chromophore has the greatest influence on the light energy conversion capacity of porphyrinoids. Its modifications lead to changes in molecular symmetry, which determine the energy levels of frontier orbitals and hence affect the S_{1} state properties. In the case of bacteriochlorophylls aggregation can also strongly decrease the S_{1} energy. The CMI may be considered as another influential structural feature which only moderately influences the ground-state properties of bacteriochlorophylls but strongly affects the singlet excited-state. An introduction of CMIs heavier than Mg^{2+} significantly improves pigments' photostabilities, however, at the expense of S_{1} state lifetime. Modifications of the peripheral substituents may also influence the S1 energy, and pigments’ redox potentials, which in turn influence their photostability

Influence of the C-17-3 chain length of chlorophylls on their aggregation properties

Chlorofile w kompleksach fotosyntetycznych występują wyłącznie w postaci agregatów zasocjowanych z białkiem. Głównym elementem strukturalnym wpływającym na ich właściwości agregacyjne jest 20-węglowy fityl przyłączony w pozycji C17-3.Celem projektu było określenie roli łańcucha węglowodorowego podstawionego w pozycji C17-3 na właściwości agregacyjne chlorofili. Wykonano eksperymenty agregacyjne z zastosowaniem barwników zestryfikowanych podstawnikami o różnej długości łańcucha alifatycznego. Pozwoliło to na określenie wpływu fitylu na proces formowania się agregatów w środowisku micelarnym. Przeprowadzono syntezę estrów chlorofilu a (kompleks magnezowy) oraz feofityny a (barwnik pozbawiony jonu centralnego) o zróżnicowanej długości podstawnika alifatycznego w pozycji C17-3 stosując metodę karbodiimidową. Użyto w niej aktywatorów DCC/OXYMA/DMAP oraz szeregu pierwszorzędowych alkoholi. Uzyskaną serię estrów oczyszczono metodami chromatografii cieczowej.Wykonano serię doświadczeń, w których agregowano barwniki wzorcowe z długim i krótkim łańcuchem węglowym w pozycji C17-3. Agregację prowadzono w roztworach wodnych niejonowych surfaktantów (LDAO, Triton X-100 i β-OG). Przebieg agregacji w układzie micelarnym analizowano technikami spektroskopii absorpcyjnej, emisyjnej, czasowo-rozdzielczej oraz dichroizmu kołowego. Umożliwiło to wybór układu modelowego do przeprowadzenia dalszych eksperymentów agregacyjnych na otrzymanych pochodnych. Agregację estrów prowadzono w wodnym roztworze β-OG. Postęp procesu monitorowano przy użyciu tych samych metod pomiarowych.Chlorophylls are principal photoactive constituents of the photosynthetic apparatus. They form highly ordered aggregates associated with proteins within all photosynthetic units. A characteristic structural feature, the most relevant for their molecular organization processes, is the 17-3-esterifying C20-phytyl chain. This hydrophobic group is essential both for specific interactions in aggregates and the pigments self-assembly.The purpose of this project was to determine a role played by the phytyl side chain in the formation of chlorophyll aggregates. This work reports a synthesis of a series of chlorophyll derivatives where the phytyl moiety was replaced with a set of aliphatic primary alcohols. The syntheses aimed at further investigations on the physiochemical properties of chlorophyll derivatives in micellar systems.Esters of chlorophyll a (Mg2+ complex) and pheophytin a (complex without central ion) were obtained via the DCC/OXYMA/DMAP mediated coupling with number of aliphatic alcohols. All products were purified by liquid chromatography methods. Aggregation of model pigments was carried out in aqueous solution of several nonionic surfactants (LDAO, Triton X-100, β-OG). These experiments were performed in order to choose a model system for the aggregation. A set of spectroscopic measurements including absorption, emission, circular dichroism and lifetime of excited singlet states was carried out in order to characterize the aggregates. The aqueous solution of β-OG was applied as a medium for the aggregation of the model esters

Effect of Solvent Polarizability on the Keto/Enol Equilibrium of Selected Bioactive Molecules from the 1,3,4-Thiadiazole Group with a 2,4-Hydroxyphenyl Function

Three novel 1,3,4-tiadiazole-derived compounds with biological-activity, i.e., 4-(5-(methylamino)-1,3,4-thiadiazol-2-yl)­benzene-1,3-diol (MDFT), 4-(5-(phenylamino)-1,3,4-thiadiazol-2-yl)­benzene-1,3-diol (PhATB), and 4-(5-(4-chlorophenylamino)-1,3,4-thiadiazol-2-yl)­benzene-1,3-diol (4-CIPhATB) were characterized with the use of several spectroscopic methods. Detailed UV–vis studies revealed keto/enol tautomerism of the examined compounds. The absorption spectra recorded in nonpolar solvents exhibited bands that were characteristic of keto tautomers, while in polar solvents the enol form is predominant. A number of spectra revealed the presence of both tautomeric forms in the solution. The keto/enol equilibria observed were both solvent- and temperature-dependent. The keto/enol equilibrium was also observed using FTIR spectroscopy. A detailed analysis of the spectroscopic data leads to a conclusion that the solvent-induced tautomerism of the selected compounds from the 1,3,4-thiadiazole group does not depend on the electric dipole moment of the solvent but more likely on its average electric polarizability. Additionally, a clear effect of the substituent present in the molecule on the tautomeric equilibrium in the selected 1,3,4-thiadiazole analogues was noted