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

2,5-Diphenyl-thiazolo[5,4-d]thiazole (DTTz) was prepared and its photophysi cal properties were determined by optical absorption and steady-state spect roscopy, both in solution and in the solid state. Solvents with different p roperties (polarity, dielectric constant and refractive index) were employe d 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 fluorescen ce quantum yield. Quantum mechanical calculation shows that this molecule i s planar in the electronic ground state and its dipole moment is null in th is state. Quantum mechanics successfully simulates the spectral profile. Th e lower electronic transition is characterized as a pi-pi*. Charge densitie s 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. Thi s was explained on the basis of the excitonic spectral splitting by the pi -stage molecular configuration of the molecules in the solid state with par allel dipole moments. (C) 2001 Elsevier Science B.V. All rights reserved.