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

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 ¹A_g→¹L_a and ¹A_g→¹L_b transitions in non-polar solvents, whereas in aqueous solvents it is due to the ¹A_g→¹L_b 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.