S Sankararaman

The synthesis of a variety of 1,3-diarylpropynones that contain not only substituted phenyl groups but also fluorophoric 1-pyrenyl, 3-carbazolyl, and 1-naphthyl groups was achieved in good to excellent yields by using a carbonylative alkynylation reaction in the presence of cis-(Tz)Pd(Cl)2(PPh3) (Tz = 1,2,3-triazol-5-ylidene) as a precatalyst under 1.0 atm of CO. Products resulting from competing Sonogashira coupling reactions, namely 1,2-diarylacetylenes, were not observed. The synthesized ynones exhibit fluorescence not only in solution but also in the solid state. The catalytically active palladium species was recovered and reused for up to three cycles by adsorbing the cis-(Tz)Pd(Cl)2(PPh3) precatalyst onto silica gel and carrying out the reaction under heterogeneous conditions.

The synthesis of several 1,3,6,8-tetraethynylpyrene derivatives is reported. The effect of extended acetylenic conjugation on their absorption and fluorescence-emission properties is studied. Significant bathochromic shifts of both the absorption and fluorescence emission bands are observed. These derivatives emit fluorescence in the visible (400-550 nm) region. © Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Several 2-(phenylethynyl)triphenylene derivatives bearing electron donor and acceptor substituents on the phenyl rings have been synthesized. The absorption and fluorescence emission properties of these molecules have been studied in solvents of different polarity. For a given derivative, solvent polarity had minimal effect on the absorption maxima. However, for a given solvent the absorption maxima red shifted with increasing conjugation of the substituent. The fluorescence emission of these derivatives was very sensitive to solvent polarity. In the presence of strongly electron withdrawing (-CN) and strongly electron donating (-NMe2) substituents large Stokes shifts (up to 130 nm, 7828 cm-1) were observed in DMSO. In the presence of carbonyl substituents (-COMe and-COPh), the largest Stokes shift (140 nm, 8163 cm-1) was observed in ethanol. Linear correlation was observed for the Stokes shifts in a Lippert-Mataga plot. Linear correlation of Stokes shift was also observed with ET(30) scale for protic and aprotic solvents but with different slopes. These results indicate that the fluorescence emission arises from excited state intramolecular charge transfer in these molecules where the triphenylene chromophore acts either as a donor or as an acceptor depending upon the nature of the substituent on the phenyl ring. HOMO-LUMO energy gaps have been estimated from the electrochemical and spectral data for these derivatives. The HOMO and LUMO surfaces were obtained from DFT calculations. © 2010.