Molecular Interactions, Proton Exchange, and Photoinduced Processes Prompted by an Inclusion Process and a [2]Pseudorotaxane Formation Amal Kumar Mandal, † Moorthy Suresh, † Manoj K. Kesharwani, † Monalisa Gangopadhyay, §,∥,# Manoj Agrawal, † Vinod P. Boricha, † Bishwajit Ganguly,* ,‡,† and Amitava Das* ,§,∥,# † CSIR-Central Salt and Marine Chemicals Research Institute, Bhavnagar, Gujarat 364002, India # Organic Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Rd., Pune, Maharashtra 411008, India ‡ Academy of Scientific and Innovative Research, CSIR-Central Salt and Marine Chemicals Research Institute, Bhavnagar, Gujarat 364002, India § Academy of Scientific and Innovative Research, CSIR-National Chemical Laboratory, Pune, Maharashtra 411008, India * S Supporting Information ABSTRACT: Appropriate design of the host and guest components allows formation of a novel [2]pseudorotaxane complex with an interrupted photoinduced electron transfer (PET)-coupled fluorescence resonance energy transfer (FRET) response. This is the first example of an inclusion complex with NO 6 -based azacrown ether as the host unit (H). Different guest molecules (G1, G2, G3, and G4) with varying stopper size are used for the studies. Unlike G1, G2, and G3, G4 with a relatively bulkier stopper fails to form a [2]pseudorotaxane complex. Isothermal titration microcalorimetry measurements reveal a systematic increase in the association constant for H·G1, H·G2, and H·G3 with a change in the stopper size. Thermodynamic data suggest that the formation of H·G1/ H·G2/H·G3 is exclusively driven by a large positive entropic gain (TΔS = 19.69/26.80/21.81 kJ·mol −1 ), while the enthalpy change is slightly negative for H·G1/H·G3 (−2.61/−1.97 kJ·mol −1 ) and slightly positive for H·G2 (ΔH = 5.98 kJ·mol −1 ). For these three inclusion complexes, an interrupted PET-coupled FRET response is observed with varying efficiency, which is attributed to the subtle differences in acidity of the NH 2 + unit of the guest molecules and thus the proton exchange ability between the host and respective guest. This is substantiated by the results of the computational studies. ■ INTRODUCTION It has long been known that crown ethers are capable of forming hydrogen-bonded adducts with organic ammonium ions. 1 Researchers have exploited this binding motif and have shown that a range of wire-type secondary ammonium ions (R 2 NH 2 + ) could be used to thread through the cavities of appropriately sized crown ether derivatives to afford interwoven complexes. This act of supramolecular recognition has very recently led to the development of new classes of molecular level devices, such as molecular switches, 2 motors, 3 rotors, 4 shuttles, 5 muscles, 6 extension cables, 7 etc. In this regard, the most studied crown ether is dibenzo-24-crown-8 (DB24C8) or its derivatives. 1e,8 Balzani et al. have also demonstrated the fluorescence resonance energy transfer (FRET)-based plug in socket function at a molecular level using inclusion complexes of a crown ether, derived from binapthol. 1c Such an idea can be further extended to achieve a more complex system, where one can have additional control on the input signal and the consequential modified output response. To demonstrate such a process, we have utilized a supramolecular assembly where the threading phenomena result in an interrupted photo- induced electron transfer (PET) and a fluorescence on response, which in turn initiates a FRET process. Here the choice of the azacrown moiety, used as a guest, is separated from the photoactive donor unit by a −CH 2 − spacer, which allows the unshared pair of electrons of the tertiary N crown to participate effectively in the PET process and thereby to participate in the switch-on/of f-type luminescence response. 9 Though this scheme has been successfully utilized for designing sensors for metal ion recognition, 9 to date no literature report states that such a concept has been utilized for the generation of a threaded inclusion complex with an interrupted response. The appropriate choice of the donor (pyrene) and acceptor (anthracene) units, which possess the fitting spectral properties for a probable FRET process, is also crucial in the present study. To the best of our knowledge, there is no example of a relatively simple supramolecular assembly available in the literature that is capable of exhibiting a FRET process, which is Received: April 12, 2013 Published: August 16, 2013 Article pubs.acs.org/joc © 2013 American Chemical Society 9004 dx.doi.org/10.1021/jo400752d | J. Org. Chem. 2013, 78, 9004−9012