tided as ca. 99% unreacted 'H NMR (vide supra). The components of the aqueous phase were identified by 'H NMR as paraquat (S 4.35 (s, 3 H), 8.37 (d, J = 1,2 H), 8.90 (d, J = 1,2 H)) and 2-(ethylamino)-2-methylpropanol hydrochloride (spectrum identical to that described above). Integrals of the NMR signals for paraquat and the amino alcohol indicated a 2.5% conversion of DEM-3 dimer to the amino alcohol.B. Buffered Methanol Medium. The reaction was performed in pH 7, Tris-buffered methanol. This time the reaction mixture turned dark blue. Again, the residue from solvent evaporation was extracted into 1 mL of D20 and 1 mL of CDC13. The components of the organic phase were identified by 'H NMR as DEM-3 dimer and 2-(ethylamino)-2methylpropanol (6 1.06 (s, 6 H), 1.07 (t, J = 7.2, 3 H), 2.44 (q, J = 1.2, 2 H), 3.33 (s, 2 H)). The components of the aqueous phase were identified as paraquat and a small amount of 2-(ethylamino)-2-methylpropanol hydrochloride, also from the 'H NMR spectrum. Integrals of the NMR signals indicated a 41% conversion of DEM-3 dimer to amino alcohol.Attempted Reduction of Daunomycin with DEM-3 Dimer. The reaction vessel was a 9 mm X 20 cm Pyrex tube equipped with a 2.5-cm side arm. The side arm was charged with 2.65 X 10"* mol of DEM-3 dimer dissolved in methylene chloride, and the methylene chloride was evaporated with a stream of nitrogen. The main tube was charged with 2 mL of 2 X 10~3 M 1:1 Tris/Tris-HCl buffered methanol containing 2.66 X 10"6 mol of daunomycin. The methanol solution was freeze-thaw-degassed, and the tube was sealed with a torch. After mixing the reagents, the solution was heated at 36 °C for 18 h. C-18 reverse-phase HPLC analysis as described earlier35 showed no formation of 7-deoxydaunomycinone.

The concept of machine can be extended to the molecular level by designing and synthesizing (supra)molecular species capable of performing mechanical movements. The energy needed to make a machine work can be supplied as chemical energy, electrical energy, or light. When a chemical "fuel" is used, waste products are formed, whereas this is not the case when suitable photochemical or electrochemical energy inputs are employed. A number of elementary functions performed by molecular-level machines are illustrated, and more complex ones are foreseen.

A molecular-level abacus-like system driven by light inputs has been designed in the form of a [2]rotaxane, comprising the pi-electron-donating macrocyclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shaped component that contains 1) a Ru(II) polypyridine complex as one of its stoppers in the form of a photoactive unit, 2) a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4'-bipyridinium unit and a 3,3'-dimethyl-4,4'-bipyridinium unit as pi-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodology to make 1) the so-called "west-side" comprised of the Ru(II) polypyridine complex linked by a bismethylene spacer to the p-terphenyl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called "east-side" comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismethylene spacer to an incipient 3,3'-dimethyl-4,4'-bipyridinium unit. Next, 3) the "west-side" and "east-side" were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, the co-conformational behavior of the molecular shuttle. The stable translational isomer is the one in which the BPP34C10 component encircles the 4,4'-bipyridinium unit, in keeping with the fact that this station is a better pi-electron acceptor than the other station. This observation raises the question- can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employed-one was based fully on processes that involved only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecu...

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Two novel [2]rotaxanes, comprised of a dibenzo[24]crown-8 (DB24C8) macroring bound mechanically to a chemical “dumbbell” possessing two different recognition sitesviz., secondary dialkylammonium (NH2 +) and 4,4‘-bipyridinium (Bpym2+) unitshave been synthesized by using the supramolecular assistance to synthesis provided by, inter alia, hydrogen bonding interactions. One of these rotaxanes bears a fluorescent and redox-active anthracene (Anth) stopper unit. NMR spectroscopy and X-ray crystallography have demonstrated that the DB24C8 macroring exhibits complete selectivity for the NH2 + recognition sites, i.e., that the [2]rotaxanes exist as only one of two possible translational isomers. Deprotonation of the rotaxanes' NH2 + centers effects a quantitative displacement of the DB24C8 macroring to the Bpym2+ recognition site, an outcome that can be reversed by acid treatment. The switching processes have been investigated by 1H NMR spectroscopy and, for the Anth-bearing rotaxane, by electrochemical and photophysical measurements. Furthermore, it is possible to drive the DB24C8 macroring from the dumbbell's Bpym2+ unit, in the deprotonated form of the Anth-bearing rotaxane, by destroying the stabilizing DB24C8−Bpym2+ charge-transfer interactions via electrochemical reduction. The photochemical and photophysical properties of this rotaxane (in both its protonated and deprotonated states) have also been investigated.

Abstract— Excited state emission and absorption decay measurements have been made on the cage‐type cryptate complexes [M bpy.bpy.bpy]n+, where Mn+= Na+, La3+, Eu3+, Gd3+ or Tb3+ and [bpy.bpy.bpy] is a tris‐bipyridine macrobicyclic cryptand. Excitation has been performed in the high intensity 1π‐π* cryptand band with maximum at about 300 nm. Experiments have been carried out in H2O or D2O solutions and at 300 and 77 K to evaluate the rate constants of radiative and nonradiative decay processes. For Mn+= Na+, La3+ and Gd3+ the lowest excited state of the cryptate is a 3ππ* level of the cryptand which decays in the microsecond time scale at room temperature in H2O solution and in the second‐millisecond time scale at 77 K in MeOH‐EtOH. For Mn+= Eu3+, the lowest excited state is the luminescent 5D0 Eu3+ level which in H2O solution is populated with 10% efficiency and decays to the ground state with rate constants 2.9 × 103 s_1 at room temperature and 1.2 × 103 s−′ at 77 K. The relatively low efficiency of 5D0 population upon 1ππ* excitation is attributed to the presence of a ligand‐to‐metal charge transfer level through which 1ππ* decays directly to the ground state. For Mn+= Tb3+ the lowest excited state is the luminescent 5D4 Tb3+ level. The process of 5D4 population upon 1ππ* excitation is ˜100% efficient, but at room temperature it is followed by a high‐efficiency, activated back energy transfer from the 5D4 Tb3+ level to the 3ππ* ligand level because of the relatively small energy gap between the two levels (1200 cm_1) and the intrinsically long lifetime of 5D4. At 77 K back energy transfer cannot take place and the 5D4 Tb3* level deactivates to the ground state with rate constant 5.9 × 102 s‐′ (H2O solution). The relevance of these results toward the optimization of Eu3+ and Tb3+ cryptates as luminescent probes is discussed.

Photochemical control of a selfassembled supramolccular I : 1 pseudorotaxane (formed between a tetracationic cyclophane, namely the tetrachloride salt of cyclobis(paraquat-p-phenylene) , and 1 .S-bis[2-(2-(2-hydroxy)ethoxy)ethoxy]naphthalene) has been achieved in aqueous solution. The photochemical one-electron reduction of the cyclophane to the radical trication weakens the noncovalent bonding interactions between the cyclophane and the naphthalene guest-n-n interactions between the n-electron-rich and nelectron-poor aromatic systems, and hydrogen-bonding interactions between the acidic r-bipyridinium hydrogen atoms of the cyclophane and the polyether oxygen atoms of the naphthalene derivative-sufficiently to allow thc gucst to dcthread from the cavity; the process can be monitored by the appearance of naphthalene fluorescence. The radical tricationic cyclophane can be oxidized back to the tetracation in the dark by allowing oxygen gas into the system. This reversible process is marked by the disappearance of naphthalene fluorescence as the molecule is recomplexed by the tetracationic cy-

In aqueous solution (2 < pH < 8) the thermodynamically stable form of the 4‘-methoxyflavylium ion (AH + ) is its hydrated derivative trans-4‘-methoxychalcone, C t . The C t compound shows a broad absorption band with λmax = 350 nm. In acid medium, irradiation of C t with near-UV light causes strong spectral changes with five isosbestic points and appearance of a very intense band in the visible region with maximum at 435 nm, corresponding to the AH + form. It has been shown that irradiation of C t causes a trans → cis photoisomerization reaction (Φ = 0.04 at λexc = 365 nm), which is followed by 100% conversion of the cis-chalcone form (C c ) to the AH + ion. The AH + ion is photochemically inactive and thermally inert in acid medium (half-life of the back conversion at 25 °C in the dark is 815 days at pH 1.0 and 20 h at pH 4.3, respectively). At high temperature (>50 °C) and/or pH ≥3, however, AH + can be quantitatively converted back to C t (half-life of 15 min at pH 4.0 and 60 °C). Owing to this unique behavior, this represents a novel molecular system in which the color can be controlled by light and changes in temperature and/or pH. The ability to photochemically convert the stable and colorless C t form to the kinetically inert and colored AH + form, and the possibility to reconvert AH + to C t at high temperature or by a pH jump make the system well-suited as the basis for an optical memory device with multiple storage and nondestructive readout capacity through a write−lock−read−unlock−erase cycle.