Experimental and theoretical studies of the photophysics of 7-amino-3-phenyl-2H-benzo[b] [1,4]oxazin-2-one in homogeneous solvents and b-cyclodextrin aqueous solutions

The photophysical behavior of 7-amino-3-phenyl-2H-benzo[b][1,4]oxazin-2-one was studied in organic solvents and in aqueous solutions of beta-cyclodextrin using steady-state fluorescence and computational chemistry methods. In homogeneous media, fluorescence spectra show a noteworthy solvatochromic effect leading to large Stokes shifts. Linear solvation energy relationship and Lippert-Mataga equation analysis of the Stokes shifts indicate an increase of the dipolar moment in the singlet excited state and the participation of a partial charge-transfer state in the deactivation process. Incorporation of 7-amino-3-phenyl-2H-benzo[b][1,4]oxazin-2-one into the beta-cyclodextrin inner cavity was monitored by observing the increase of fluorescence as a function of the cyclodextrin concentration. Analysis of fluorescence data in terms of Job plots and the Benesi-Hildebrand equation are indicate the formation of a 1:1 complex. The binding constant obtained from Benesi-Hildebrand plots was 597 M-1 at 298 K. Also, the values of thermodynamics parameters determined from the dependence of the binding constant on the temperature show that inclusion is an enthalpy-driven process. Docking studies suggest that the complex stability is due to favorable van der Waals interactions within the cavity and a hydrogen bond interaction between the amino substituent and hydroxyl groups located in the narrow rim of the cavity. The same conclusion was achieved employing the Molecular Mechanics Poisson-Boltzmann Surface Area methodology to determine the energy contributions to the total free energy for the inclusion process.