Constructing highly sensitive sensors

Primary investigator: Kent Nielsen.

Our main interest is in molecular recognition: how to generate molecules that bind to others strongly and selectively with non-covalent interactions. In the event of molecular recognition (Figure 1) a host interacts with a guest to produce a detectable change which can be read by an observer or by an instrument.

Our unique knowledge about the redox-active tetrathiafulvalene unit[1] (TTF) has made it possible to prepare the firsts anion sensor based on TTF by annulating one TTF unit directly onto the upper rim of the calix[4]pyrrole skeleton (Figure 2), thus allowing the detection of anions by electrochemical means.[2] This new calix[4]pyrrole exhibit exceptionally high binding affinities and an approximately stoichiometric cyclic voltammetry response for anions.

Further, We have studied the dynamic structure of the tetraTTF-calix[4]pyrrole 1 (Figure 3) in the absence and presence of chloride anions, and shown (Figure 3a) that flat electron deficient aromatic guests (e.g., trinitrobenzene) are able to bind within the TTF-pyrrole "clefts" in the 1,3-alternate conformation[3] and (Figure 3b) that spherical electron deficient aromatic guests (e.g., fullerenes) are able to bind within the TTF-pyrrole “cage” in the chloride bound cone-conformation (1.Cl-).[4] Both of these interactions results in a remarkable easy to visualize yellow-to-green color change produced on binding.

Figure 3. Complexation between tetraTTF-calix[4]pyrrole 1 and a) trinitrobenzene in the 1,3-alternate conformation and b) fullerene in the chloride bound cone conformation (1.Cl-).

[1] J. O. Jeppesen, K. Takimiya, F. Jensen, T. Brimert, K. Nielsen, N. Thorup, J. Becher, J. Org. Chem. 2000, 65, 5794–5805.
[2] (a) K. A. Nielsen, J. O. Jeppesen, E. Levillain, J. Becher, Angew. Chem. Int. Ed. 2003, 42, 187–191. (b) K. A. Nielsen, W.-S. Cho, J. Lyskawa, E. Levillain, V. M. Lynch, J. L. Sessler, J. O. Jeppesen, J. Am. Chem. Soc. 2006, 128, 2444–2451.
[3] K. A. Nielsen, W.-S. Cho, J. O. Jeppesen, V. M. Lynch, J. Becher, J. L. Sessler, J. Am. Chem. Soc. 2004, 126, 16296–16297.
[4] K. A. Nielsen, W.-S. Cho, G. Sarova, B. M. Petersen, A. D. Bond, J. Becher, F. Jensen, D. M. Guldi, J. L. Sessler, J. O. Jeppesen, Angew. Chem. Int. Ed. 2006, 45, 6848–6853.

Molecular muscles

Primary investigators: Jan O. Jeppesen and Stinne W. Hansen.

The electron rich tetrathiafulvalene (TTF) unit has been used widely in combination with the electron poor cyclophane cyclobis(paraquat-p-phenylene) (CBPQT4+) in the construction of molecular machines such as rotaxanes and catenanes.

The aim of this project is to expand the concept of molecular switches, in which the ring component moves back and forth between two stations (for instance TTF and dioxynaphthalene), to systems where motion only proceeds in one direction, i.e. unidirectional motion.

Toward this end, a thread compound containing a TTF- and a monopyrrolo-TTF- (MPTTF) unit both capable of complexing CBPQT4+ has been designed and synthesized. The idea behind unidirectional linear motion in this system is explained below.

(a) The complexation between CBPQT4+ and the TTF unit happens quickly (<sec.) while (b), (c) the movement of the CBPQT4+ to the MPTTF unit is slow (~3h) because of the small barrier on the TTF unit. (d) Oxidation of the TTF and MPTTF units will force the CBPQT4+ to pass over the large barrier on the MPTTF unit. Subsequent reduction reestablishes the starting point (a). The barrier for passage of CBPQT4+ over the TTF2+ unit must be larger than the passage of CBPQT4+ over the barrier on the left hand side of the MPTTF unit.

In this sequence of events the ring component CBPQT4+ engages the thread compound from one end, slide along it unidirectionally, and finally falls off the other end. Subsequent reduction reestablishes the initial state of the system. Therefore, the system will be capable of multiple cycles of unidirectional linear motion.

Previous work, on which this work relies

  1. Nygaard S.; Laursen, B. W.; Flood, A. H.; Jeppesen, J. O.; Stoddart, J. F. Chem. Commun. 2006, 144-146.
  2. Jeppesen, J. O.; Vignon, S. A.; Stoddart, J.F. Chem. Eur. J. 2003, 9, 4611-4625