Photophysical investigation of neutral and diprotonated free-base bis(arylethynyl)porphyrins.

The photophysical properties for a series of free-base arylethynyl porphyrins and the corresponding trans-disubstituted tetraphenylporphyrin (H(2)TPP) derivatives lacking arylethynyl functionalities have been studied via electronic absorption and emission spectroscopy in both neutral and diacid forms. Enhanced substituent effects on porphyrin absorption spectra are observed in the arylethynyl porphyrins relative to the H(2)TPP derivatives, owing to the presence of the ethynyl spacer that allows for a coplanar geometry between the porphyrin macrocycle and the appended phenyl substituents. Upon protonation, both series of porphyrins exhibit substantially red shifted absorption and emission spectra and enhanced oscillator strengths, with the magnitude of the spectral shifts being more substantial in the presence of the ethynyl functionalities. Spectral features of the arylethynyl porphyrin bearing p-dimethylamino substituents closely resemble those previously classified as "hyperporphyrin spectra" and are indicative of excited-state charge-transfer character. Protonation of both series of porphyrins results in reduced fluorescence lifetimes and enhanced nonradiative decay rates, and the impact of protonation on these parameters is attenuated in the presence of the arylethynyl functionalities. Our results coupled with previous structural data showing that arylethynyl porphyrins exhibit less structural distortion upon diacid formation relative to H(2)TPP further substantiate the proposal that significant alteration of porphyrin photophysical properties upon diacid formation can be attributed to nonplanar structural distortions induced by protonation.