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

Abstract

The 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.