Photoinduced Formation of Bithiophene Radical Cation via a Hole-Transfer Process from CdS Nanocrystals

The exciton dynamics in semiconductor nanocrystals can be strongly affected by coupling the nanocrystals to org. ligands. A deeper understanding of the interactions in semiconductororg. hybrid systems is important for the design of functional devices. In the present work, the interactions between CdS quantum dots and bithiophene mols. have been investigated. In particular, the photophys. behavior of CdS nanocrystals has been investigated in n-heptane in the presence of increasing bithiophene concn. by use of steady-state and time-resolved measurements. Bithiophene is a well-known electron donor (or hole acceptor), and it has a good affinity with CdS surface for the presence of sulfur atoms. The nanocrystal luminescence was efficiently quenched upon addn. of increasing concn. of the thiophene deriv., and modifications in the emission decay profiles of CdS were obsd.; the anal. of luminescence data suggests that quenching is mainly due to static interaction able to modify the dynamics of the exciton states of the hybrid nanomaterials. The transient absorption measurements enable to detect the bithiophene radical cation upon CdS excitation, thus revealing the occurrence of an efficient hole transfer process from the nanocrystals to the org. ligand, for which a quantum efficiency of 36% has been measured. The dependence of transient signal on bithiophene concn. and the formation of tetrathiophene intermediates indicate that CdS exciton states are able to photosensitize the polymn. of bithiophene after the hole transfer processes. The data indicate that in the investigated system the decay of charged species is not detd. by back-reactions.