A Simple Molecular Machine Operated by Photoinduced Proton Transfer Serena Silvi, † Arturo Arduini, ‡ Andrea Pochini, ‡ Andrea Secchi, ‡ Massimiliano Tomasulo, § Franc ¸ isco M. Raymo, § Massimo Baroncini, † and Alberto Credi* ,† Dipartimento di Chimica “G. Ciamician”, UniVersita ` di Bologna, Via Selmi 2, 40126 Bologna, Italy, Dipartimento di Chimica Organica e Industriale, UniVersita ` di Parma, Via Usberti 17/a, 43100 Parma, Italy, and Center for Supramolecular Science, Department of Chemistry, UniVersity of Miami, 1301 Memorial DriVe, Coral Gables, Florida 33146-0431 Received July 19, 2007; E-mail: alberto.credi@unibo.it Molecular machiness(supra)molecular systems in which large- amplitude motions of some components can be controlled by appropriate stimuliscan be operated by means of chemical, electrochemical, or photochemical processes. 1 However, the use of light stimulation 2 has several advantages. For example, photons can be used to supply energy to the system and to gather information about its state without physically touching it. Light excitation can be carried out by a variety of sources (including the sun), with the possibility of a fine resolution in space and time. Many artificial molecular machines reported so far are powered by exoergonic chemical reactions, most typically acid-base reac- tions. 1,3 A modular construction of light-driven molecular machines, usually pursued by integrating photochemical functions with mechanically switchable systems, 4 is in general more difficult to carry out. It would therefore be useful to identify viable strategies for using light to operate “stand alone” chemically driven molecular machines. 5 Here we show that the acid-base controlled threading- dethreading of a pseudorotaxane in solution can be operated by photoinduced intermolecular proton transfer 6 with a molecular switch. Pseudorotaxanes whose molecular components can be threaded and dethreaded in response to external signals may be regarded as very simple prototypes of chemical machinery 7,8 and are important for the development of less trivial unimolecular machines based on rotaxanes, 9 catenanes, 10 and related interlocked compounds. 11 The calix[6]arene wheel C (Scheme 1) forms fairly stable pseudorotaxane complexes with 4,4′-bipyridinium compounds in apolar solvents. 12 Therefore, we envisaged that compound AH 2+ , obtained by protonation of the pyridine nitrogen of the 4,4′- pyridylpyridinium axle-like A + (as PF 6 - salt; Scheme 1), could thread into the cavity of C as well. In fact, spectrophotometric titrations and voltammetric experiments show that a very stable [K ) (6 ( 2) × 10 6 M -1 ] pseudorotaxane complex is formed between C and AH 2+ in CH 2 Cl 2 (see Supporting Information). For the present discussion, it is important to note that the pseudorotaxane [C‚AH] 2+ exhibits a broad and weak charge-transfer (CT) absorp- tion band in the visible region (λ max ) 478 nm, ǫ ) 500 M -1 cm -1 ), where none of the isolated molecular components exhibit absorption features. Deprotonation of AH 2+ with a base (e.g., tributylamine) in CH 2 Cl 2 leads to dethreading of the pseudorotaxane. In order to trigger the self-assembly and disassembly of this pseudorotaxane by light, a suitable species whose acid-base properties can be photocontrolled should be identified. Specifically, such a compound must occur in two forms, interconvertible into one another by light irradiation, exhibiting smaller and larger acid strength than that of AH 2+ , respectively. A species that fulfils these requirements is the spiropyran photochrome SP (Scheme 1). 13,14 In the presence of an acid, the colorless SP is converted into the yellow protonated merocyanine form MEH + . 15 Upon irradiation with visible light, MEH + releases a proton, isomerizing back to SP. 16 The coupled operation expected for the pseudorotaxane and spiropyran switches is represented in Scheme 1. The absorption spectrum of a 1:1 mixture of the protonated axle AH 2+ and the calixarene wheel C shows the typical 12 CT absorption band of the [C‚AH] 2+ complex (Figure 1, black curve). In our conditions (1 × 10 -4 M), more than 95% of the molecular components are associated together in the pseudorotaxane super- structure. The CT band of [C‚AH] 2+ does not change soon after the addition of 1 equiv of SP (blue curve). By keeping the solution in the dark, the absorption band typical 16 of MEH + at 417 nm gradually appears. Although the MEH + band partially overlaps with the much weaker CT band of the complex, it can be noticed that the absorbance at λ > 520 nm decreases concomitantly with the formation of MEH + (Figure 1, red curve). This change cannot be ascribed to the SP-MEH + transformation and has to be assigned to the dethreading of the [C‚AH] 2+ pseudorotaxane. When the equilibration is completed (after 7 days), it can be estimated on the basis of the MEH + absorption band at 417 nm that the SP: MEH + ratio is about 60:40; however, the decrease of the CT absorption would correspond to disassembling of 15% of the [C‚ AH] 2+ species, instead of the 40% expected from the amount of MEH + formed. Most likely, the decrease in the CT band is partially offset by the absorbance increase originating from the formation † Universita ´ di Bologna. ‡ Universita ´ di Parma. § University of Miami. Scheme 1. Control of Threading-Dethreading Processes in Pseudorotaxane [C‚AH] 2+ by Means of Light-Induced Proton Exchange with a Spiropyran-Merocyanine Photochromic System Published on Web 10/13/2007 13378 9 J. AM. CHEM. SOC. 2007, 129, 13378-13379 10.1021/ja0753851 CCC: $37.00 © 2007 American Chemical Society