Strong Binding of Hydrocarbons to Cucurbituril Probed by Fluorescent Dye Displacement: A Supramolecular Gas‐Sensing Ensemble

Abstract

The inclusion of small hydrocarbons into molecular container compounds in solution has received considerable attention. [1][2][3][4][5][6][7][8][9] It allows for a puristic understanding of the solvophobic driving force for the formation of discrete host-guest complexes [10] and has additional potential for gas storage, uptake, and separation, thus complementing solid-state applications of porous materials [11,12] or surface-immobilized macrocycles. [13] Studies on the precipitation of complexes between the smallest alkanes with a-cyclodextrin date back to the 1950s. [14] Subsequently, synthetic hosts such as cryptophanes, [1] self-assembling capsules, [2][3][4][5] and hemicarcerands [6,7] have been investigated for their potential to entrap small hydrocarbons. Herein, we describe a highly sensitive fluorescence-based method for the quantification of volatile hydrocarbons binding with cucurbituril. We observe exceptionally strong, highly selective, and reversible binding in aqueous solution.Cucurbit[n]urils (CBn) are water-soluble, highly symmetric pumpkin-shaped synthetic macrocycles, [15,16] and their unique supramolecular chemistry is presently unfolding. [17,18] They are well-established to bind organic ammonium ions through a combination of hydrophobic interactions inside the nonpolar inner cavity and ion-dipole interactions with the carbonyl portals. While a CB5 derivative has been reported to bind very small guests such as methane and acetylene, [12, 19] the homologue which holds most promise for hydrocarbon binding is CB6, [20] the original cucurbituril, which possesses an intermediary size to allow inclusion of guests with up to seven heavy atoms into its inner cavity. [18,21] Unfortunately, CB6 has an intrinsically low water solubility of about 30 mm, which complicates the determination of actual binding constants. [21,22] In particular, it prevents 1 H NMR titrations, which have been routinely employed in all previous studies on solution-phase gas binding by molecular containers. [1][2][3][4][5][6][7][8][9] The binding of xenon with CB6 has been studied by 129 Xe NMR spectroscopy in the presence of 0.2 m Na 2 SO 4 , [23] where the solubility is increased (but where also the binding strength suffers). Alternatively, an alkylated CB6 derivative with higher water solubility has been employed, which was also investigated by isothermal titration calorimetry, to afford a binding constant of 3400 m À1 with xenon. [24] The binding constants, which we report herein for several simple hydrocarbons, are much larger than those observed for xenon, and, in fact, the largest ones reported for neutral guests with CB6. [18] In several cases, they exceed those previously observed for any molecular container. [1][2][3][4][5][6][7][8][9] In the quest for a convenient method to monitor volatile hydrocarbon binding to CB6, we selected an indicator displacement strategy [25] based on our recently developed anchor dye approach. [26] In detail, compound 1 possesses a putrescine anchor for strong binding with CB6, [27] and a ...

Switching Properties of a Spiropyran–Cucurbit[7]uril Supramolecular Assembly: Usefulness of the Anchor Approach

“…We have devised a much improved supramolecular strategy for the rate-accelerated formation of the merocyanine form of an anchor-substituted photo-and acidochromic spiropyran in the presence of the macrocyclic cucurbit [7]uril host. The commonly observed hydrolytic instability of the merocyanine form in aqueous solution is totally suppressed in the presence of the host, showing impressive long-term persistence (no sign of any decay during 2.5 days).…”

“…Indeed, the photochromic properties of spiropyrans are of elevated interest for the photocontrol of biological functions, [34][35][36][37][38][39][40] bioimaging, [41][42][43][44][45] optical signal processing and photoswitching in general, [46][47][48][49][50][51][52][53][54][55] as well as chemosensing. [56][57][58][59][60][61] In order to overcome the handicap of a limited photochromic performance of spiropyrans upon direct encapsulation in cucurbiturils we devised herein a novel strategy based on the use of anchor-substituted spiropyrans as guests of cucurbit [7]uril (CB7). This approach led to the observation of an impressive long-term protection of the merocyanine against hydrolytic degradation.…”

“…Cucurbiturils (CBs) are a recently widely explored class of water-soluble macrocyclic host compounds [1] with applications in chemosensing assemblies, [2][3][4][5][6][7] label-free enzyme assays, [8] nanovalves, [9] molecular self-sorting, [10] dye encapsulation and stabilization, [11][12][13] supramolecular catalysis, [14,15] drug delivery, [5,[16][17][18][19][20][21] molecular logic and switches, [22][23][24][25] etc. CBs are composed of a variable number of n glycouril units (n = 5,6,7,8,10) which are linked by methylene bridges.…”

“…CBs are composed of a variable number of n glycouril units (n = 5,6,7,8,10) which are linked by methylene bridges. [26] Their structure is characterized by two carbonyl-laced portals and an inner cavity with low polarity and low polarizability.…”

“…The host-guest complexation of the anchor by the cucurbit [7]uril macrocycle, evidenced by absorption titration, NMR spectroscopy, and electrospray ionization mass spectrometry, produced significant improvements of the switching properties of the photochrome: a) a ca. 70 times faster appearance of the merocyanine form, b) a practically unlimited hydrolytic stability of the merocyanine (two and a half days without any measureable decay), and c) a fast, clean, and fatigueresistant photoinduced ring-closure back to the spiro form.…”