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Transient absorption spectroscopic studies on 4-nitroquinoline N-oxide: From femtoseconds to microseconds timescale

AuthorsDuran-Giner, N.; Carlotti, B.; Clementi, C.; Elisei, F.; Encinas, S.; Miranda, M. A.
Issue Date5-Jun-2019
CitationSpectrochimica Acta - A - Molecular and Biomolecular Spectroscopy 216: 265-272 (2019)
AbstractThe singlet excited state of 4-nitroquinoline N-oxide ( NQNO*) has been characterized by different spectroscopic techniques, combining transient absorption with steady state and time-resolved emission spectroscopy. The energy of NQNO* has been established as 255 kJ/mol from the fluorescence spectrum, whereas its lifetime has been found to be 10 ps in the femto-laser flash photolysis (LFP) experiments, where a characteristic S –S absorption band with maximum centered at 425 nm is observed. In a first stage, the triplet excited state of NQNO ( NQNO*) 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 183 kJ/mol, whereas the setup with time resolution has allowed us to determine the phosphorescence lifetime as 3 ms. Formation of NQNO* by intersystem crossing in solution at room temperature, has been monitored by femto-LFP, which shows the appearance of a band with maximum at 560 nm (T -T ). It increases with the decreasing intensity of its precursor 425 nm (S –S ) band, giving rise to an isosbestic point at 500 nm. The characterization of NQNO* has been completed by nano-LFP, using xanthone as photosensitizer and oxygen as well as β-carotene as quenchers. In addition, quenching of NQNO* by electron donors (DABCO) is also observed in aprotic solvents, leading to the radical anion of NQNO ([rad] NQNO). If there is a proton source in the medium (Et N as electron donor or MeCN:H O/4:1 as solvent system) protonation of the radical anion results in formation of the neutral radical of NQNO ([rad]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.
Publisher version (URL)http://dx.doi.org/10.1016/j.saa.2019.02.105
Identifiersdoi: 10.1016/j.saa.2019.02.105
issn: 1386-1425
Appears in Collections:(ITQ) Artículos
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