Abnormal Enhancement of the Photoisomerization Process in a trans-Nitroalkoxystilbene Dimer Sequestered in beta-Cyclodextrin Cavities

en

116

42

10328

10337

2012-10-25

2012-10-25

1089-5639

Journal of Physical Chemistry A

We report on the synthesis and the photophysical properties of a trans-nitroalkoxystilbene dimer (DPNS). The fluorescence quantum yield (Phi(f)), the Stokes shift, and the quantum yield for the trans-to-cis photoisomerization (Phi(t -> c)) are
Show moreWe report on the synthesis and the photophysical properties of a trans-nitroalkoxystilbene dimer (DPNS). The fluorescence quantum yield (Phi(f)), the Stokes shift, and the quantum yield for the trans-to-cis photoisomerization (Phi(t -> c)) are strongly dependent on the nature of the solvent. Upon increasing solvent polarity, Phi(f) increases together with the decrease of Phi(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 beta-cyclodextrins (beta-CD) prevents the quenching by H2O 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 Phi(t -> c) even though DPNS is sequestered in beta-CD cavities.
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American Chemical Society

J. Phys. Chem. A

journalArticle

ACL

Chimie/Chimie organique

alpha-cyclodextrin
aqueous-solution
dynamics
fluorescence
gamma-cyclodextrin
inclusion complexes
isomerization
p-dimethylaminobenzonitrile
quantum yields

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crp

International