Excited State Processes of 2-Butylamino-6-methyl-4-nitropyridine N-oxide in Nonpolar Solvents. A Transient Absorption Spectroscopy Study

Earlier steady-state fluorescence studies showed that 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M) can undergo fast excited-state intramolecular proton transfer (ESIPT). In a nonpolar solvent such as n-octane, both normal and tautomeric fluorescence was observed. Strikingly, the relative ratio of those two emission bands and the fluorescence quantum yield of the normal emission were found to depend on the excitation wavelength in violation of the Kasha−Vavilov rule. In this work, the system was investigated further by means of transient absorption spectroscopy, followed by global and target analysis. Upon excitation at 420 nm, a normal excited singlet state S-1(N) is reached, which decays in about 12 ps via fluorescence and ESIPT (minor pathways) and to a long-lived "dark" state (major pathway) that is most probably the triplet T-1(N). Upon 330 nm excitation, however, a more complex pattern emerges and additional decay channels are opened. A set of four excited-state species is required to model the data, including a hot state S-1(N)* that decays in about 3 ps to the tautomer, to the long-lived "dark" state and to the relaxed S-1(N) state. A kinetic scheme is presented that can explain the observed transient absorption results as well as the earlier fluorescence data.