Solvatochromism of 1-naphthol-4-sulfonate photoacid and its encapsulation in cyclodextrin derivatives

Abstract

Photophysical properties of the ground and excited states of 1-naphthol-4-sulfonate (1NO4S), photoacid, are investigated in different solvents and cyclodextrin derivatives of different size and structure. Fluorescence decay traces of 1NO4S are well fitted to double exponential decay function in all solvents. The short-lived traces with higher amplitude were assigned to emission of the neutral form (ROH*), while the long-lived traces with low amplitude were assigned to emission of the anionic form (RO−*). The photophysical properties of 1NO4S were correlated with a number of solvent parameters such as the solvent polarity parameters ETN and the reaction field factor Δf. Three linear solvation energy relationships developed by Kamlet-Taft, Catalán, and Laurence et al. were employed to assess the effect of non-specific and specific interactions on the photophysical properties of 1NO4S. All three solvation energy relationships showed that the non-specific interactions mainly control the photophysical properties of 1NO4S while the contribution of the specific interactions are weak. Molecular encapsulation of 1NO4S by α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), γ-cyclodextrin (γ-CD), methyl-β-cyclodextrin (Mβ-CD), 2-hydroxypropyl-β-cyclodextrin (HPβ-CD) in aqueous solution has been studied by steady state and time resolved fluorescence techniques. Steady state fluorescence measurements show 1:1 inclusion of 1NO4S with different cyclodextrin derivatives with an association constant of 7.9 ± 4.0, 59.6 ± 10.0, 0.4 ± 0.5, 169.8 ± 25.0, and 359.0 ± 20.0 M−1 for 1NO4S complexes with α-CD, β-CD, γ-CD, Mβ-CD or HPβ-CD, respectively following the fluorescence intensity changes of the anionic form (RO−*) at 435.0 nm. Association constants obtained following the fluorescence enhancement of the neutral form (ROH*) were the same as determined following fluorescence intensity decrease of RO−* at 372 nm. Data derived from time resolved measurements gave very close value for the association constants as determined from steady state fluorescence measurements.