Photoinduced Formation of Bithiophene Radical Cation via a Hole- Transfer Process from CdS Nanocrystals Alessandro Iagatti, † Rebecca Flamini, Morena Nocchetti, and Loredana Latterini* Dipartimento di Chimica and Centro Eccellenza Materiali Innovativi Nanostrutturati (CEMIN), Universita ̀ di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy * S Supporting Information ABSTRACT: The exciton dynamics in semiconductor nanocrystals can be strongly affected by coupling the nanocrystals to organic ligands. A deeper understanding of the interactions in semiconductor-organic hybrid systems is important for the design of functional devices. In the present work, the interactions between CdS quantum dots and bithiophene molecules have been investigated. In particular, the photophysical behavior of CdS nanocrystals has been investigated in n-heptane in the presence of increasing bithiophene concentration 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 addition of increasing concentration of the thiophene derivative, and modifications in the emission decay profiles of CdS were observed; the analysis 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 organic ligand, for which a quantum efficiency of 36% has been measured. The dependence of transient signal on bithiophene concentration and the formation of tetrathiophene intermediates indicate that CdS exciton states are able to photosensitize the polymerization of bithiophene after the hole transfer processes. The data indicate that in the investigated system the decay of charged species is not determined by back-reactions. 1. INTRODUCTION The design of hybrid nanomaterials based on colloidal semiconductor nanocrystals (or quantum dots, QDs) and organic moieties is an attractive research field in nano- technology. 1-4 This research area gives the possibility to obtain new nanocomposite materials with defined optoelec- tronic properties and photophysical behavior suitable for the advancement of nanodevices which can be used as emitting tools, 5 sensing apparatus, 6-8 or a photoelectron medium. 2,9,10 In order to achieve hybrid nanomaterials with defined behavior and functions, the interactions between semiconductor nano- crystals and organic species have to be deeply understood. In many studies, the interactions between QDs and organic molecules have been monitored through the modifications of the nanocrystal luminescent properties. In some cases the QD emission behavior can be determined by the degree of crystallinity or the nanocrystal surface properties, 11-13 which affect the monitoring signal and make difficult to reach a definitive assignment of the forces driving the interactions. Insights into the nature of the interactions can be achieved by a careful analysis of the time-resolved measurements, 14-18 and the occurrence of energy and/or charge transfer process has been suggested. In a recent study, Bhattacharyya et al. observed a remarkable modification of the luminescent decay of CdTe- QDs in the presence of oligothiophene nanomaterials for the occurrence of hole transfer processes from the valence band of CdTe to the HOMO orbital of the thiophene system which compete with energy transfer processes. 16 Xu et al. investigate the luminescence behavior of CdSe-ZnS-polymer in bulk and at single particle level and demonstrate that the hole transfer rate, in the CdSe-ZnS-polymer system, depends on the distance between donor and acceptor but also in the electronic coupling between the donor and acceptor components. 17 The investigation of intermediate species, formed upon interaction of QDs exciting with organic or bio-organic molecules, and their assignment to chemical structures is extremely useful to establish the nature of the interactions. 18-21 Huang et al. were able to reveal the nature CdSe- phenothiazine interactions by use of transient absorption measurements and observe that the dynamics of exciton bleaching was kinetically coupled to the formation of the radical cation of the organic moiety. 21 Recently, Tseng et al. suggested the occurrence of a multistep electron transfer interactions initiated by a hole transfer process from CdS nanorod to a Ru- Received: June 19, 2013 Revised: October 19, 2013 Published: October 21, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 23996 dx.doi.org/10.1021/jp406072n | J. Phys. Chem. C 2013, 117, 23996-24002