Abnormal enhancement of the photoisomerization process in a trans-nitroalkoxystilbene dimer sequestered in...

We report on the synthesis and the photophysical properties of a trans-nitroalkoxystilbene dimer (DPNS). The fluorescence quantum yield (Φ(f)), the Stokes shift, and the quantum yield for the trans-to-cis photoisomerization (Φ(t→c)) are strongly dependent on the nature of the solvent. Upon increasing solvent polarity, Φ(f) increases together with the decrease of Φ(t→c). This solvent-induced reverse behavior mainly stems from the progressive stabilization of a highly polar twisted internal charge transfer state (TICT) at excited singlet level which opens a competing channel to photoisomerization. In the presence of hydroxylic substrates (i.e., alcohols or water), fluorescence of DPNS is strongly quenched due to a hydrogen bonding interaction at excited state. The efficiency of the process is clearly correlated to the H-bond donor ability of the quencher. In aqueous solution, the major formation of a 2:1 host-guest complex with β-cyclodextrins (β-CD) prevents the quenching by H(2)O and leads to a 50-fold increase of the fluorescence signal together with a strong band blue-shift with respect to that of the free chromophore. This latter effect was rationalized in terms of a severe reduction of the solvent-induced stabilization of the TICT state. As a consequence, the trans-to-cis photoisomerization reaction is reactivated and leads to a paradoxical 14-fold increase of Φ(t→c) even though DPNS is sequestered in β-CD cavities.