Excited State Spectra And Kinetics Of Protonated Adenosine And P-dimethylamino Acetophenones

Herein steady-state absorption,steady-state fluorescence spectra,density functional theory and time-dependent density functional theory,as well as femtosecond time-resolved fluorescence and transient absorption spectroscopy have been used to investigate excited states of protonated adenosine,p-dimethylaminoacetophenone and p-dimethylaminobenzaldehyde molecules in solvents of various properties.The results are as followings:(1)Steady-state and time-resolved fluorescence spectra of protonated adenosine were obtained.The excited state of protonated adenosine was observed exhibiting bi-exponential decay process with time constants determined to be ~0.9 ps and ~2933ps,respectively.The absorption and fluorescence energies and the corresponding molecular orbital features of protonated 9-methyladenine at neutral and the various protonated forms were calculated using the density functional theory and the time-dependent density functional theory.The experimentally observed spectra and dynamics was proposed arising mainly due to the N1 protoned species,though contribution due to the N7 protonated species is also possible.(2)DMAAP and DMABA are similar in structure,and exhibiting in the same solvent very similar deactivation pathways.The decay of photo-excited DMABA is faster than that of DMAAP,likely due to the hydrogen group in DMABA rendering less steric hindrance effect than the methyl group in DMAAP.The deactivation pathway of both DMAAP and DMABA is affected strongly by the solvent properties.Especially in the protic solvents,the formation of solute-solvent intermolecular hydrogen bonding can open up new non-radiative deactivation channels.This was found responsible for the faster decay of their charge transfer state in methanol,and accounting for also the extremely rapid decay of their locally excited state in water.