The influence of phenylethynyl linkers on the photo-physical properties ofmetal-free porphyrins

Series of 5,10,15,20-tetraarylporphyrins 1 and 5,10, 15,20-tetrakis[4-(aryl ethynyl)phenyl]porphyrins 2 were prepared via condensation of pyrrole with the appropriate benzaldehyde or 4-(arylethynyl)benzaldehyde derivative (3). Condensation of meso-phenyldipyrromethane with mixtures of benzaldehyde an d 4-(trimethylsilylethynyl)benzaldehyde gave a separable mixture of mono- ( 6), bis- (both cis-7 and trans-8) and tris[4-(trimethylsilylethynyl)phenyl] porphyrin (9). Following removal of the trimethylsilyl groups of 6-9, the 4-ethynylphenyl groups of 11-14 were coupled to 1-iodo-3,5-di(trifluorometh yl)benzene with Pd(OAc)(2) to give 15-18 bearing one, two (both cis- and tr ans-) and three 4-[bis-3,5-(trifluoromethyl)phenylethynyl]phenyl groups res pectively. Coupling of 11 and 1-iodo-4-nitrobenzene with Pd(OAc)(2) gave po rphyrin 19 with one 4-(4-nitrophenylethynyl)phenyl group. Porphyrin 24 with a p-quinone linked to the porphyrin core via a phenylethynyl group was pre pared via similar chemistry. The absorbance spectra, emission maxima, excit ed-state fluorescence lifetimes, quantum yields of fluorescence, rates of f luorescence and rates of non-radiative decay were measured for each of the porphyrins. Absorbance spectra and emission maxima were nearly identical fo r all the porphyrins of this study, which suggests that the aryl groups and 4-(arylethynyl)phenyl groups are not strongly coupled to the porphyrin cor e in these metal-free compounds. Fluorescence quantum yields and rates of r adiative decay were larger for porphyrins bearing 4-(arylethynyl)phenyl gro ups, while excited-state fluorescence lifetimes were somewhat shorter. Thes e effects were additive for each additional 4-(arylethynyl)phenyl group. Co pyright (C) 2000 John Wiley & Sons, Ltd.