DOI: 10.1002/cphc.200800776 Host-Assisted Guest Self-Assembly: Enhancement of the Dimerization of Pyronines Y and B by g-Cyclodextrin Jorge Bordello, BelØn Reija, Wajih Al-Soufi,* and Mercedes Novo* [a] 1. Introduction Knowledge of the structural factors controlling non-covalent host–guest interactions is fundamental for the understanding of molecular recognition phenomena in biological systems, and it is also of great importance for the design of functional supramolecular systems with a wide range of applications. Structure and stability of supramolecular host–guest assem- blies depend sensitively on the properties of host and guest molecules. Analysis of the dynamics of supramolecular associa- tion by fluorescence correlation spectroscopy (FCS) has shown the important interplay between geometrical requirements and specific interactions between host and guest. [1, 2] The over- all stability of the assembly depends on the balance between the geometric and orientational requirements limiting the as- sociation rate constant and the strength of specific interactions controlling the dissociation process. Subtle changes in the geo- metry or electronic structure of host or guest can have a big influence on this balance and may lead to big changes in the stability or even on the type of the supramolecular species formed. Cyclodextrin inclusion complexes are simple and useful models for the study of this kind of intermolecular host–guest interactions. Cyclodextrins (CDs) are toroidally shaped polysac- charides with a highly hydrophobic central cavity that allows them to form inclusion complexes with many organic sub- strates. Earlier we studied the host–guest interactions between the pyronines Y (PY) and B (PB) and b-cyclodextrin (bCD). [1, 3] It was found that these two xanthene dyes (Figure 1) form inclusion complexes of stoichiometry 1:1 with bCD. The values of the corresponding stability constants show that the complex PB:bCD is about five times more stable than the complex PY:bCD. Specific interactions between the xanthene moiety and the electron-rich oxygens of the cyclodextrin cavity were suggested to be responsible for the stability of the complexes, as well as for their different photophysical properties with re- spect to free pyronines in aqueous solution. Dynamic studies performed by FCS yielded the individual rate constants of the association and dissociation processes. [1] The association rate constants of the two pyronines with bCD are similar, but much lower than that of a diffusion-controlled collision process, which is attributed to geometrical and orientational require- [a] J. Bordello, Dr. B. Reija, Prof. Dr. W. Al-Soufi, Prof. Dr. M. Novo Grupo de Fotofísica e Fotoquímica Molecular Departamento de Química Física, Facultade de Ciencias Universidade de Santiago de Compostela 27002 Lugo (Spain) Fax: (+ 34) 982-285-872 E-mail : wajih.al-soufi@usc.es m.novo@usc.es Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200800776. The role of small variations in the structural properties of host and guest molecules on the stoichiometry and strength of sup- ramolecular associations is analyzed. Earlier we found that a change in substituents from pyronine B to pyronine Y has a dramatic effect on both the stability and the dynamics of the association of these guests with b-cyclodextrin as host. Now we study the association between these two pyronines and a cyclodextrin with a bigger cavity (g-cyclodextrin) using UV/Vis absorption and fluorescence spectroscopy. The absorption spectra of the pyronines show complex variations with cyclo- dextrin concentration indicating that pyronine dimerization is strongly enhanced inside the cavity of the cyclodextrin. A full model is proposed and the equilibrium constants of the in- volved processes and the absorption and emission spectra of the different species are estimated. The equilibrium constants of the formation of complexed dimers are much higher than those for free dimerization or for the inclusion of a single guest. The g-cyclodextrin host acts like a belt to assist the guest self-assembly. The differences in the stability of pyronine B and pyronine Y dimers are explained on the basis of their structure and geometry. Figure 1. Structures and cavity sizes of bCD and gCD and structures of pyro- nines Y and B. ChemPhysChem 0000, 00, 1 – 10  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ