Effective Stabilization of cis-Isomer of Styryl Dye inside the Cucurbit[7]uril Cavity

Stabilization of unstable molecules by encapsulation in the different supramolecular hosts (e.g. cyclodextrines, calixarenes, cyclophanes, carcerands, etc) capable of formation of the rigid three-dimensional nanocages in the solution has attracted great attention over the past few decades. Cucurbiturils (CB [n], n=5-10) represent a new class of organic macrocyclic receptors comprising n glycoluril units connected by 2n methylene bridges. CB [n] possess an unique symmetrical structure having nonpolar interior cavity and high negative charge density on the carbonyl portals that allow to form the stable inclusive complexes with cationic organic molecules. Among the CB [n] homologues cucurbit[7]uril (CB [7]) merits a particular attention because of its relatively good water-solubility, ability to include aromatic molecules, catalyze their chemical and photochemical reactivity and modulate excited state processes in molecular level. In particular, a very promising idea of the controlled photoisomerization may be realized using complexation between photoactive molecules and CB [7]. However, despite the high practical potential of CB [7], there are only few examples of the controlled photoisomerization managed by interaction of photoactive species with CB [7]. To extend the aforementioned idea, we decided to check whether the interaction with CB [7] could initiate the light-induced E-Z-isomerization of photochemically inactive styryl dye. In this study we have systematically examined the reversible E-Z-photoisomerization of N-methyl-4-styrylpyridinium dye (SP) derivatized with ditia-dioxa-monoaza-15-crown-5 residue encapsulated into CB [7] cavity in aqueous solution. It is known that formation of TICT states (TICT - Twisted Intramolecular Charge Transfer) is a predominant excited state relaxation pathway for highly polar styryl (pyridinium) dyes bearing amino type substituent, so that no E-Z-photoisomerization can be observed for these species upon irradiation. After the studied dye was confined into the nonpolar and narrow cavity of CB [7], the rigid environment restricted the rotations of SP in the excited state, thereby preventing the formation of TICT states and favouring E-Z-photoisomerization. In darkness the reverse thermal Z-E-isomerization was observed. It should be noted that the cis-isomer of SP in complex with CB[7] remained stable for along time, whereas in the absence of CB [7] cis-SP is not stable. The remarkable stability of cis-SP encapsulated in the CB [7] cavity is attributed to the strong host-guest interactions including ion-dipole interaction between the positively charged guest and the carbonyl portals of the host along with hydrophobic interactions between the aromatic part of the dye and the nonpolar cavity of CB [7]. Moreover the fine correspondence of the size of guest molecule and five volume of the inner cavity of CB [7] has a substantial impact on the stabilization of the cis-isomer. The present work not only demonstrates the supramolecular control of molecular reactivity but also provides an insight into such important area of science and technology as design of photofunctional materials.