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

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 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 S₁S_n absorption band with maximum centered at 425 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 183 kJ/mol, whereas the setup with time resolution has allowed us to determine the phosphorescence lifetime as 3 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 560 nm (T₁-T_n). It increases with the decreasing intensity of its precursor 425 nm (S₁S_n) band, giving rise to an isosbestic point at 500 nm. The characterization of ³NQNO* has been completed by nano-LFP, using xanthone as photosensitizer and oxygen as well as β-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.