The formation of J-aggregates of 21-thia-5,10,15,20-tetra-(4-sulfonatophenyl)-porphyrin in acidified water solutions is herein revealed for the first time and their spectral-luminescent properties are measured. It is shown that the spectral-luminescent properties of J-aggregates are due to the excitonic interactions, and the number of coherent interacting monomeric porphyrin molecules in the aggregate is evaluated. It is found
that J-aggregates of heteroporphyrin have fluorescence, and the fluorescence quantum yield Фfl is found to be as low as 1.8 . 10–4. The photolability of J-aggregates is found, namely, upon J-aggregate photoexcitation to the absorption band at 503 nm they collapse to the monomeric doubly protonated molecules. The process of photomonomerisation is reversible: upon keeping the solution in the dark, J-aggregates form again.
The temperature dependence of absorption and fl uorescence spectra of 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin in weakly acidic aqueous solutions was studied in the range 288-333 K. The fraction of molecules in the free-base form in the ground (S 0 ) and lower excited singlet states (S 1 ) was found to increase as the temperature increased because the fraction of molecules in the doubly protonated form decreased. The deprotonation was caused by a shift in the acid-base equilibrium in the macrocycle core due to decreases in pK a (S 0 ) and pK a (S 1 ) with increasing temperature. The difference pK a (S 1 ) -pK a (S 0 ) < 0 for solution temperatures >293 K with pK a (S 1 ) -pK a (S 0 ) > 0 for T < 293 K. The activation energies of deprotonation in the S 0 -and S 1 -states for T > 293 K were E a = 5.0 and 3.4 kJ/mol; for T < 293 K, they increased to 20.3 and 56.2 kJ/mol. These differences were explained by different specifi c solvation of the tetrapyrrole macrocycle in the ground S 0 -and lower excited S 1 -states because of a change in the ratio of two forms of water. So-called form A with disordered H-bonds dominated at higher temperatures. The fractions of forms A and B, which possessed a strongly ordered H-bond system, were comparable as the temperature decreased. Stabilization of the porphyrin free base prevailed at high temperatures; of the doubly protonated form, at low temperatures.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.