Issue in Honor of Prof. Nikolai Zefirov ARKIVOC 2005 (iv) 142-150
Fluorescence spectra of dimer 5 and trimer 6 were obtained in dilute solutions of degassed chloroform and are shown in Figure 3. When irradiated at 326 nm, both oligomers exhibit exactly the same blue emission bands at 359 and 381 nm, and with the increasing oligomeric chain length, there is no apparent red shift in emission wavelength. Furthermore, upon varying the excitation wavelength from 250 to 347 nm, both dimer 5 and trimer 6 gave the same emission maxima indicating that the emission is insensitive to the excitation wavelength. These results corroborate the observations from the electronic absorption analysis for 5 and 6, and confirms that the electronic properties of the oligomers are dominated by the longest linearly conjugated segment (shown in bold for trimer 6, Figure 4) and increased electronic communication is not obtained by increasing the oligomeric length.
i-Pr3Si Sii-Pr3 Figure 4. Schematic illustration of the longest linearly conjugated segment of trimer 6, shown in bold.
In conclusion, we have described the synthesis of the first members of a new class of crossconjugated organic oligomers. These compounds can be readily formed in a divergent manner using palladium catalyzed cross-coupling as a key synthetic step. Electronic absorption and emission spectroscopy suggests that electronic communication along the conjugated framework is severely limited due to cross conjugation, which results in similar absorption and emission wavelengths for oligomers regardless of length.
Experimental Section
General Procedures. Reagents and solvents were purchased reagent grade and used without further purification. Evaporation and concentration in vacuo was done at H2O-aspirator pressure. All reactions were performed in standard glassware. A positive pressure of argon or nitrogen was essential to the success of all Pd-catalyzed reactions. Degassing of solvents was accomplished by vigorously bubbling argon or nitrogen through the solution for at least 30 min. Column chromatography was done with Silica gel 60 (230-400 mesh) from Rose Scientific Ltd. using the solvent system described and thin-layer chromatography (TLC) used plastic sheets covered with silica gel 60 F254 from Macherey-Nagel and visualization by UV light or KMnO4 stain. UV/Vis spectroscopic measurements were done at room temperature using reagent grade CHCl3 at
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