Host−Guest Chemistry and Light Driven Molecular Lock of Ru(bpy)₃−Viologen with Cucurbit[7−8]urils

Abstract

Host-guest chemistry and photoinduced electron-transfer processes have been studied in the systems containing Ru(bpy)3 complex covalently linked to viologen as a guest molecule and cucurbit[n]urils (n = 7, 8) as host molecules in aqueous solution. The Ru(bpy)3-viologen complex, [Ru(2,2'-bipyridine)2(4-(4-(1'-methyl-4,4'-bipyridinediium-1-yl)butyl)-4'-methyl-2,2'-bipyridine)]Cl4 (denoted as Ru2+-MV2+, 1) was shown to form stable 1:1 inclusion complexes with cucurbit[7]uril (CB[7]) and cucurbit[8]uril (CB[8]). The binding modes are slightly different with CB[7] and CB[8]. CB[7] preferentially binds to part of the viologen residue in 1 together with the butyl chain, whereas CB[8] preferentially encloses the whole viologen residue. Photoinduced intramolecular electron transfer from the excited-state of the Ru moiety to MV2(+) which is inserted into the cavity of the CBs occurred. Long-lived charge-separated states Ru3(+)-MV(+*) were generated with the lifetimes of 280 ns with CB[7] and 2060 ns with CB[8]. This shows that CBs can slow down the charge recombination within supramolecular systems, and the difference in lifetimes seems to be due to the difference in binding modes. In the presence of a sacrificial electron donor triethanolamine, light-driven formation of a dimer of MV(+*) inside the CB[8] cavity was observed. This "locked" molecular dimer can be "unlocked" by molecular oxygen to give back the original form of the molecular dyad 1 with the MV2(+) moiety inserted in the cavity of CB[8]. The processes could be repeated several times and showed nice reversibility.

Study of the Counter Anions in the Host‐Guest Chemistry of Cucurbit[8]uril and 1‐Ethyl‐1′‐benzyl‐4,4′‐bipyridinium

“…However, the peak at 226 nm showed less reducing effect in comparison with that of 260 nm, demonstrating that the host-guest including process had less effect on the absorption of I − ; the same effect can be followed in control experiment on UV-Vis titration of 10  μ M KI with CB[8] (see Supplementary Figure S6), suggesting that some weak interaction existed between the counter anion I − and CB[8], which is in good agreement with the above mentioned results. According to the literature method [20], the binding constant of BEV-Cl 2 in the presence of CB[8] is 6.29 × 10 5  M −1 by sigmoidal fitting, while those for BEV-Br 2 , BEV-I 2 , BEV-(PF 6 ) 2 , and BEV-(ClO 4 ) 2 are 2.37 × 10 5  M −1 , 3.37 × 10 4  M −1 , 1.92 × 10 5  M −1 and 5.03 × 10 5  M −1 , respectively (see Supplementary Figure S7). The lower binding constant of BEV-I 2 can be ascribed to the afore mentioned weak interaction between I − and CB[8], and the more twisted conformation of BEV inside CB[8]; the real reason is under investigation.…”

“…In particularly, CB[8] can form a 2 : 1 including complex with the radical to form a dimer (MV +• ) 2 /CB[8]; the dimerization constant of MV +• in the presence of CB[8] is estimated to be 2 × 10 7  M −1 , which is about 10 5 times larger than that of MV +• alone in aqueous media. The characteristic UV-Vis absorption peaks for the radical dimer are around 370 nm, 540 nm, and 900 nm, while those for the free radical are around 400 nm and 600 nm; all these can be employed to distinguish them from each other [2–14, 18–20]. …”

Supramolecular Complexation between Porphyrin‐Viologen Dyad and Cucurbit[7]uril

“…The photoinduced electron transfer processes for ruthenium complexes covalently linked to viologen as a guest and cucurbit[n]urils (n = 7, 8) as host have been studied. [10] However, the complexation between porphyrin-viologen dyad with cucurbit[n]urils (n = 5-10) has not been vastly studied in the literature . 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Results and Discussion Synthesis of Porphyrin-Viologen Dyad: Porphyrin-viologen dyad 3 used in this study was synthesized by a series of reactions as shown in Scheme 1.…”

Selective Positioning of CB[8] on Two Linked Viologens and Electrochemically Driven Movement of the Host Molecule

“…[1][2][3][4][5][6][7][8] A common guest is methylviologen (N,N-dimethyl-4,4Ј-bipyridinium, MV 2+ ), [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] which is an efficient electron acceptor used for the construction of molecular devices. [13,[24][25][26][27] Recently, several groups have reported the movement of a CB [n] host to different positions of a guest using either cyclodextrins or proton-and electron-transfer reactions. [20,[28][29][30][31][32][33] It has previously been shown that dimethylmethylviologen (N,N-dimethyl-3,3Ј-dimethyl-4,4Ј-bipyridinium, DMV 2+ , Figure 1) is more difficult to reduce than MV 2+ .…”

“…Molecular oxygen quenched the dimer, and the CB [8] observed after photoinduced electron transfer. [26][27] Herein, we report the interactions between the host molecule CB [8] and guests DMV 2+ and MV 2+ and 1 and 2, in which DMV 2+ and MV 2+ are connected by a 3-or 6-carbon chain, respectively (see Figure 1).…”