While rotary molecular switches based on neutral and cationic organic π-systems have been reported, structurally homologous anionic switches providing complementary properties have not been prepared so far. Here we report the design and preparation of a molecular switch mimicking the anionic p-HBDI chromophore of the green fluorescent protein. The investigation of the mechanism and dynamics of the E/Z switching function is carried out both computationally and experimentally. The data consistently support axial rotary motion occurring on a sub-picosecond time scale. Transient spectroscopy and trajectory simulations show that the nonadiabatic decay process occurs in the vicinity of a conical intersection (CInt) between a charge transfer state and a covalent/diradical state. Comparison of our anionic p-HBDI-like switch with the previously reported cationic N-alkyl indanylidene pyrrolinium switch mimicking visual pigments reveals that these similar systems translocate, upon vertical excitation, a similar net charge in the same axial direction.

The photophysical and photochemical properties of some aza-analogues of 1,4-distyrylbenzene, bearing a nitrogen heteroatom in the side arene rings, have been investigated. The kinetic competition between the radiative and reactive relaxation channels of the lowest excited singlet state has been compared with that of the parent hydrocarbon. The replacement of the side phenyl rings with pyridyl or quinolyl rings was found to markedly affect the photobehavior of the three stereoisomers (EE, ZE, and ZZ). Adiabatic formation of the excited 1 EE* by irradiation of the other isomers was evidenced by fluorescence measurements. The effect of the presence of the heteroatom in the central ring, while maintaining the linear conjugation, was investigated for EE-2,5-distyrylpyridine, and it was found to be less important than that of N in the side rings. Information about the conformational equilibria of the compounds bearing side quinolyl groups or the pyridyl central ring, due to internal rotation around the quasi single bonds with the ethenic carbons, was obtained by selective photoexcitation.

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.

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