Preventing the formation of the long-lived colored transoid-trans photoisomer in photochromic benzopyrans.

A new photochromic fused benzopyran presenting a bridge between the pyran double bond and the benzenic ring was prepared. While the UV irradiation of usual benzopyrans leads to the formation of two colored photoisomers with very different thermal stabilities, studies by laser flash photolysis showed that the presence of this particular bridge prevents the formation of the undesirable long-lived colored TT isomer and therefore after laser irradiation the colored solution fades following a fast monoexponential decay.

Photochromic and photophysical properties of new benzo- and naphtho[1,3]oxazine switches.

A new set of photochromic hindered benzo- and naphtho[1,3]oxazines with fast switching speed between the closed and open form were synthesised. Laser excitation of these uncoloured molecules (with ∼10 ns FWHM pulses) leads to the cleavage of the C-O bond and opening of the oxazine ring in less than 20 ns, generating a thermally unstable coloured zwitterionic species that reverts in few nanoseconds to the initial oxazine closed form. For these derivatives a detailed spectroscopic and photophysical study was undertaken involving measurements of absorption and phosphorescence spectra, phosphorescence quantum yields (ϕ(Ph)), phosphorescence lifetimes (τ(Ph)), triplet energies (E(T)), singlet oxygen (ϕ(Δ)) and ring opening photoreaction (ϕ) quantum yields, reisomerization kinetics and the zwitterionic form lifetime. A remarkable solvent effect on the lifetime of the photogenerated species was observed.

Picosecond structural relaxation of abietic acid based amine end capped para-phenylenevinylene trimers in solution.

The synthesis and photophysical properties of six new abietic acid based amine end-capped p-phenylenevinylene trimers (AECPV3) in their lowest excited singlet states are presented. The AECPV3 compounds show a large red-shift of both the absorption (25-30 nm) and emission (37-42 nm) maxima with respect to those of the corresponding trimers. Picosecond time-resolved fluorescence data reveal the presence of a fast conformational relaxation process (40-62 ps) of the initially excited compounds, leading to more planar conformers. The conformational relaxation time is proportional to the volume of both the side chain and the amine groups.