Fast and reversible interconversion between two stable states of a molecule is a topic of fundamental and practical importance.  Current efforts in many research groups focus on chemical systems that display predictable structural changes upon irradiation of light, changes in oxidation states, or interactions with externally added chemical species.  Undoubtedly, such bistability has immediate technological implications in molecular-level devices for energy conversion and chemical sensing.

Under the financial support of ACS-PRF, we have developed a new class of shape-adaptive molecules displaying predictable switching motions that directly impact their electrochemical and photophysical properties.  Our initial goal was establishing synthetic routes to a general structural platform that can amplify local structural distortions to large-scale conformational changes.  Building on the early success in the synthesis of such constructs, our efforts have recently been directed toward designing and implementing viable signal transduction mechanisms to convert mechanical signals to readable optoelectronic output in molecular devices and materials.  Summarized in the following paragraphs are the advent of this chemistry, our accomplishments, and the proposed evolution of the shape-adaptive chemical architectures for applications in assembly, sensing, switching, transport, and actuation.

We have developed biconcave receptor molecules in which opening and closing motions are mechanically coupled through correlated twisting of symmetrically disposed bulky aromatics.  Drawing inspirations from naturally occurring C₃-symmetric transmembrane protein complexes, we have designed and prepared a series of aromatic-rich biconcave structures 1 (Figure 1a).  Here, large van der Waals contacts direct correlated tilting motions of m-terphenyl subunits to open and close the molecular cavity.  Self-assembly of these bulky tris(N-salicylideneamine) derivatives afforded non-porous materials that can release entrapped volatile guest molecules through gating motions in the solid state.  This chemistry has significant implications for storage and transport with molecular materials, which we will continue to explore.

As a logical extension of the chemistry described above, we have subsequently designed and synthesized a series of dynamic two-dimensional (2-D) conjugated systems 2 (Figure 1b).  Noting that signal amplification in one-dimensional (1-D) molecular wire sensors often exploit binding-induced local "defects" in their extended electronic structures, we decided to implement conceptually parallel processes in 2-D structural settings in order to take advantage of the additional dimensionality of the space sampled by excitons.  Unlike 1-D systems, however, conjugation and deconjugation of 2-D systems require simultaneous rotation of multiple nonproximate bonds in order to maximize the effects.  As a practical solution to this challenge, we have exploited a delicate interplay between hydrogen-bonding interactions and van der Waals contacts to guide correlated tilting motions of multiple p-extended aryl groups extended from a common [p,p]/[n,p]-conjugated molecular core.  Structural folding and unfolding motions triggered by interactions with solvent molecules, changes in temperature, and chemical input signals (such as addition and removal of fluoride ion) elicited reversible turn-on and turn-off of the fluorescence response.  This chemistry was elaborated further to furnish "spring-loaded" turn-on fluorescent probes.  We are currently exploring the scope of covalently triggered conformational switching as a viable mechanism for chemical sensing.

The cooperative nature of the conformational switching depicted in Figure 1 was probed further by a dynamic multichromophore array.  In order to exploit an efficient fluorescence resonance energy transfer (FRET) between the tris(N-salicylideneaniline)-based energy donor (= D) at the core and BODIPY-based energy acceptors (= A) at the periphery, an "inverted antenna" molecule 3 (Figure 2) was designed.  Multiple pairwise D–A interactions within this construct collectively resulted in a highly unusual signal amplification behavior, along with a binary-function-like sigmoidal response curve characteristic of positive homotropic allosteric interactions.  In our on-going efforts to develop viable mechanisms to enhance sensory signal responses, we are currently exploring the effect of orientational factors (k) on the efficiency of Forster-type energy transfer.

The ACS-PRF grant was used effectively at the early stage of the PI's independent career to attract and retain two postdoctoral associates who played crucial roles in developing the chemistry described above and helped train graduate and undergraduate students.  The PI has been invited to two NSF-organized workshops to present the research.  Postdoctoral associates and graduate students who worked on these projects also attended National ACS Meetings in Atlanta (March, 2006) and in Boston (August, 2007) to give poster and oral presentations.  Findings originating from this ACS PRF-supported research not only resulted in 8 publications over a period of two years but also formed the basis of a NSF CAREER Award proposal (CHE 0547251) and a DOD/ARO proposal (W911NF-07-1-0533) that the PI was awarded recently.