Materiais moleculares funcionais contendo n-heterociclos - capes

2854 R. Cristiano et al. / Tetrahedron 63 (2007) 2851–2858Figure 3. Normalized absorption and fluorescence spectra in chloroformsolution for compounds 8 and 10.Compounds 11–16 also present fluorescence in the solidstate. Optical absorption and emission of these materials inthe solid state were measured in thin films, obtained byspin coating from chloroform solution onto a quartz plate.The solid state optical absorption and emission spectra ofcompounds 11–16 are shown in Figure 5. The lowest energyabsorption bands relating to the 2,3-dicyanopyrazine ring,observed in solution, are also seen in the solid state. Thesolid state optical absorption bands are, however, red shiftedfor most of the compounds, as in 11, 13, and 14, showing anincrease of 22, 8, and 6 nm, respectively, compared to thecorresponding bands in solution. This may be due to a slightincrease in planarity in the solid state leading to an increasein the conjugation. 13 On the other hand, 12 exhibits a blueshift of 6 nm in the lowest energy absorption band in thesolid state, and 15 and 16 do not show changes in the spectra.The optical band gaps (E g ) of all compounds were determinedby their corresponding absorption in thin films, usingthe same method applied in the literature 14 (Table 1). Theoptical band gap varies from 2.92 eV in 10 to 3.32 eV in15. TheE g wavelengths were obtained from the derivativeof the UV curves, giving the midpoint between the thresholdenergy at which the lowest energy absorption band startsincreasing and that at which it starts decreasing (Supplementarydata).Thin films of 11, 13, and 14 exhibit blue fluorescence(Fig. 5b). Of these compounds only 13, containing a biphenylFigure 4. Optical absorption (a) and fluorescence (b) spectra of 11–16 inchloroform solution.Figure 5. Optical absorption (a) and fluorescence (b) spectra of 11–16 insolid state.