The fluorescence spectra of CyP-OBu have been obtained upon excitation at the maximum absorption band and are also recorded in Figure . Whatever the solvent, a broad fluorescence spectrum is observed, the emission maximum is red-shifted when the solvent polarity increases. This indicates that the dipolar moment of the excited-state is significantly larger than for the ground state, which is also consistent with the Lippert-Mataga plot (Figure ). Whatever the solvent, the fluorescence quantum yields (Φ) are low and do not exceed 4%, suggesting a predominance of photo-switching events over luminescence. The similarity of the optical properties between CyP and CyP-OBu also suggests the conservation of the good photoisomerization characteristics shown by the parent photo-switch, as also justified by the fact that the peripheral butyl group is decoupled from the chromophoric center. Due to solubility issues the photoisomerization of CyP-OBu was only investigated in organic solvents (CHCl 3 , MeCN, DMSO and EtOH). The isomerization is reversible and fully photoinduced, no thermal switching in the dark has been observed within one year. The direct isomerization consists in the conversion of the as-synthetized E into the Z-isomer upon irradiation at 375 nm, while the back-switch represents the reverse reaction promoted by irradiation at 300 nm. These processes can be followed by monitoring both the time-evolution of the UV absorption spectrum (Figures ), and the chemical shifts of the styryl and methoxy groups on the 1 H NMR spectra (Figures ).