Effects of solvent and acid concentration on the absorption and fluorescence spectra of α,α-diaminonaphthalenes

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Abstract
Absorption and fluorescence spectra of 1,4-diaminonaphthalene (1,4-DAN), 1,5-diaminonaphthalene (1,5-DAN) and 1,8-diaminonapthalene (1,8-DAN) have been recorded in 11 different solvents and at various acid concentrations (H0/pH/H–, –10 to 16). These studies have indicated that the two amino groups are coplanar to the napthalene ring in 1,5-DAN, are twisted with respect to each other but coplanar to the ring in 1,8-DAN and are non-planar with respect to the naphthalene ring in 1,4-DAN. Based on molar absorption coefficient data the long wavelength band is found to be a mixture of the 1Ag→1La and 1Ag→1Lb transitions in non-polar solvents, whereas in aqueous solvents it is due to the 1Ag→1Lb transition. The change in dipole moment on excitation is zero for 1,5-DAN and greatest for 1,4-DAN. Prototropic equilibrium is not established between mono-cation and neutral species in the S1 state. This is because the radiative decay rate is greater than the protonation/deprotonation rates. Proton-induced fluorescence quenching is not observed for neutral species, whereas it is observed for the mono-cations before these are protonated to form di-cations. k′ q for 1,5-DAN and 1,8-DAN are 2.6 × 108 and 1.0 × 109 dm3 mol–1 s–1, respectively. Lifetime data have shown that the proton-induced fluorescence quenching is dynamic in nature. —NH2 and —NH+ 3 are stronger acids in the S1 state than in S0 state.

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