RESUMO
The singlet excited state of 4-nitroquinoline N-oxide (1NQNO*) has been characterized by different spectroscopic techniques, combining transient absorption with steady state and time-resolved emission spectroscopy. The energy of 1NQNO* has been established as 255â¯kJ/mol from the fluorescence spectrum, whereas its lifetime has been found to be 10â¯ps in the femto-laser flash photolysis (LFP) experiments, where a characteristic S1Sn absorption band with maximum centered at 425â¯nm is observed. In a first stage, the triplet excited state of NQNO (3NQNO*) has also been characterized by emission spectroscopy in solid matrix, at low temperature. Thus, from the steady-state phosphorescence spectrum the triplet energy has been estimated as 183â¯kJ/mol, whereas the setup with time resolution has allowed us to determine the phosphorescence lifetime as 3â¯ms. Formation of 3NQNO* by intersystem crossing in solution at room temperature, has been monitored by femto-LFP, which shows the appearance of a band with maximum at 560â¯nm (T1-Tn). It increases with the decreasing intensity of its precursor 425â¯nm (S1Sn) band, giving rise to an isosbestic point at 500â¯nm. The characterization of 3NQNO* has been completed by nano-LFP, using xanthone as photosensitizer and oxygen as well as ß-carotene as quenchers. In addition, quenching of 3NQNO* by electron donors (DABCO) is also observed in aprotic solvents, leading to the radical anion of NQNO (-NQNO). If there is a proton source in the medium (Et3N as electron donor or MeCN:H2O/4:1 as solvent system) protonation of the radical anion results in formation of the neutral radical of NQNO (NQNOH). In general, all processes are slower in protic solvents because of the solvation sphere. Overall, this information provides a deeper insight into the formation and behavior of excited states and radical ionic species derived from the title molecule NQNO.
RESUMO
The triazine derivative Tinosorb S has been shown to accept atomic oxygen generated by N-oxide photodeoxygenation. This is a unique example of a solar filter and mechanism-directed antioxidant. The concept has been proved by a combination of steady-state and laser-flash photolysis.