Chemistry, photophysics, and ultrafast kinetics of two structurally related Schiff bases containing the naphthalene or quinoline ring.

The two structurally related Schiff bases, 2-hydroxynaphthylidene-(8-aminoquinoline) (HNAQ) and 2-hydroxynaphthylidene-1(')-naphthylamine (HNAN), were studied by means of steady-state and time resolved optical spectroscopies as well as time-dependent density functional theory (TDDFT) calculations. The first one, HNAQ, is stable as a keto tautomer in the ground state and in the excited state in solutions, therefore it was used as a model of a keto tautomer of HNAN which exists mainly in its enol form in the ground state at room temperature. Excited state intramolecular proton transfer in the HNAN molecule leads to a very weak (quantum yield of the order of 10(-4)) strongly Stokes-shifted fluorescence. The characteristic time of the proton transfer (about 30 fs) was estimated from femtosecond transient absorption data supported by global analysis and deconvolution techniques. Approximately 35% of excited molecules create a photochromic form whose lifetime was beyond the time window of the experiment (2 ns). The remaining ones reach the relaxed S(1) state (of a lifetime of approximately 4 ps), whose emission is present in the decay associated difference spectra. Some evidence for the back proton transfer from the ground state of the keto form with the characteristic time of approximately 13 ps was also found. The energies and orbital characteristics of main electronic transitions in both molecules calculated by TDDFT method are also discussed.

DFT studies of the structure and vibrational spectra of 8-hydroxyquinoline N-oxide.

The geometry, frequency and intensity of the vibrational bands of 8-hydroxyquinoline N-oxide (8-HQNO) and its deuterated derivative (8-DQNO) were obtained by the density functional theory (DFT) with the BLYP and B3LYP functionals and 6-31G(d,p) basis set. The optimized bond lengths and bond angles are in good agreement with the X-ray data. The IR and INS spectra of 8-HQNO and 8-DQNO computed at the DFT level reproduce the vibrational wavenumbers and intensities with an accuracy, which allows reliable vibrational assignments.

The 15N and 13C solid state NMR study of intramolecular hydrogen bond in some Schiff's bases.

A series of 11 Schiff's bases derived from substituted salicylaldehyde and aliphatic amines has been studied in the solid state by 15N and 13C cross-polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR). 15N CPMAS is especially useful for investigation of the tautomerism in the compounds considered, owing to the large difference in the nitrogen chemical shifts of OH and NH tautomers. In the solid state, three of the compounds examined were shown by 15N NMR to exist as OH tautomeric forms, and the remaining eight as the corresponding NH forms. This was confirmed by 13C CPMAS. The results reported were compared with those obtained in CDCI3 solutions.