Photophysical properties of 2,5-diphenyl-thiazolo[5,4-d]thiazole

2,5-Diphenyl-thiazolo[5,4-d]thiazole (DTTz) was prepared and its photophysical properties were determined by optical absorption and steady-state spectroscopy, both in solution and in the solid state. Solvents with different properties (polarity, dielectric constant and refractive index) were employed and the solvatochromic effect and Stokes's shift were analyzed by Lippert's equation. The fluorescence emission was a mirror image of the excitation (absorption) spectrum, indicating that no geometric distortion takes place upon excitation. Larger Stokes's shifts with significant broadening of the fluorescence emission were obtained in the presence of strong concentrated acidic solutions. There was also a simultaneous decrease of the fluorescence quantum yield. Quantum mechanical calculation shows that this molecule is planar in the electronic ground state and its dipole moment is null in this state. Quantum mechanics successfully simulates the spectral profile. The lower electronic transition is characterized as a pi-pi*. Charge densities determined by quantum mechanical calculation demonstrate that protonation occurs on the nitrogen atom of the heterocyclic ring. Although no excimer emission was detected in concentrated solutions and in the solid state, an exciton splitting of the absorption or excitation spectra was observed. This was explained on the basis of the excitonic spectral splitting by the pi -stage molecular configuration of the molecules in the solid state with parallel dipole moments. (C) 2001 Elsevier Science B.V. All rights reserved.