Open form PSS (irr = 365 nm) The initial colourless solution of 2o turns dark blue upon irradiation at 365 nm with the emergence of two characteristic absorption bands in the visible, i.e.: a large band centered at 582 nm and a shoulder at 383 nm, respectively attributed to the S0-S1 and S0-S2 transitions on the closed DTE unit (Figure ). These features attest the formation of the closed isomer 2c. One can notice that the energies and shapes of the broad bands in the visible are identical for 1c and 2c. This is explained by the formation of a sp3 carbon at the central position upon ring-closure reaction, which prevents any electronic communication with the closed DTE core.