Danilo Pannacci

Excited State Reactivity of Aza-Aromatics

The photochemical and photophysical behaviour of trans 9-phenanthryl, n-pyridylethylenes (n = 2, 3, 4) has been studied in acetonitrile-water in the neutral and cationic forms. The basicity constants have been determined as well as the fluorescence quantum yields and lifetimes and the cis-trans photoisomerization quantum yields for the free bases and protonated cations. The photoreaction yields have been also measured for the trans 1-naphthyl, n-pyridylethylenes previously described. A drastic reduction in photoreactivity when the heteroatom is in the meta position (n = 3) has been observed. The results obtained give a general picture of the competition among the various deactivating processes in these compounds and of its mechanistic implications.

P‐Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical, Photophysical and Theoretical Study

Two novel helical naphthopyrans have been synthesised. The helical scaffold has the interesting effect of increasing the thermal stability of the transoid-trans (TT) open isomer formed upon UV irradiation of the closed form (CF), which transforms these naphthopyrans from thermal to photochemical photochromes. The photochromic performance is excellent in both polar and apolar solvents and the conversion percentage from the CF to the TT form can be as high as 92.8 %. We propose a new method to determine the quantum yields of the photochemical processes that lead to transoid-cis (TC) and TT isomers, and their molar absorption coefficients. The thermal stability of the TT and TC isomers has been studied in different solvents. The quantum yields of fluorescence before and after irradiation, along with the decay lifetimes, have also been measured. TD-DFT calculations have been performed to determine the relative thermodynamic stability of the species involved in the photochromic mechanism and to rationalise their spectral properties.