Spectroscopic and computational study of β-ethynylphenylene substituted zinc and free-base porphyrins

A series of tetraphenylporphyrins appended at the β-pyrrolic position with an ethynylphenylene- or ethynylpyridine-substituent have been subjected to spectroscopic and density functional theory (DFT) analyses. The mean absolute deviation between corresponding experimental and DFT-derived vibrational spectra is up to 10.2 cm−1, suggesting that the DFT B3LYP/6-31G(d) method provides an accurate model of the β-substituted porphyrin systems. The configuration interactions that give rise to prominent electronic absorptions have been calculated using time-dependant DFT (TD-DFT) and have been rationalized with reference to the energy and topology of DFT calculated molecular orbitals. As the electron withdrawing capacity of the β-substituent increases the LUMO orbital gains appreciable amplitude over the substituent moiety and is stabilised. This represents a departure from the assumptions underpinning the Gouterman four-orbital model, resulting in atypical electronic absorption spectra. This phenomenon is also manifested in the enhancement patterns of the resonance Raman spectra insofar as B-band excitation engenders an enhancement of substituent based modes. These observations demonstrate that the β-substituent exerts an appreciable electronic influence on the porphyrin π-electron system and provides a means of introducing charge-transfer character to prominent electronic transitions.