Piezoluminescence at the Air−Water Interface through Dynamic Molecular Recognition Driven by Lateral Pressure Application

Abstract

The steroid cyclophanes with a cyclic core consisting of a 1,6,20,25-tetraaza[6.1.6.1]paracyclophane connected to four steroid moieties (cholic acid or cholanic acid) through a flexible l-lysine spacer were spread on water as Langmuir monolayers. The pi-A isotherm of the cholic-type steroid cyclophane includes a transition to the condensed phase with a limiting area of approximately 2 nm(2). This value is close to the cross-sectional area of the steroid cyclophane with a standing-up conformation of the cholic acid moieties, strongly suggesting that the cavity converts from a two-dimensional cavity to a three-dimensional cavity upon compressing the monolayer. Surface-reflective fluorescence spectroscopy of the monolayer using an aqueous fluorescent probe (6-(p-toluidino)naphthalene-2-sulfonate (TNS)) showed an abrupt increase in the TNS fluorescence intensity at a molecular area of 2 nm(2). Efficient binding of the guest probe would occur upon the completion of the three-dimensional cavity. Repeated compression and expansion induces periodic changes in the fluorescence intensity. This indicates a piezoluminescence effect through the catch and release of the TNS guest upon dynamic cavity formation. Analyses of the binding behavior of TNS to the steroid cyclophane resulted in binding constants in the range of approximately (5-9) x 10(4) M(-1) which are similar to that observed in lipid bilayer media (K = 5.1 x 10(4) M(-1)). The fluorescence intensity within the condensed phase was significantly increased with increasing pressure, suggesting that suppression of the molecular motion of the bound TNS may retard the nonemission process. Similar monolayer experiments were carried out with the monolayer of the cholanic-type steroid cyclophane that cannot form an open conformation on water. Both the phase transition in the pi-A isotherm and the change in the fluorescence intensity were negligible, confirming that the dynamic characteristic of the cavity is indispensable for the efficient pressure-induced binding of the guest and the consequent luminescence.

Synthesis and Mechanofluorochromism of 4-[Bis(4-methylphenyl)amino]acetophenone

“…Among them, materials that exhibit reversible change in fluorescent color induced by mechanical stress such as grinding, namely "mechanofluorochromism", have recently been received attention. Several examples of both organometallic and pure organic materials have been reported to exhibit mechanofluorochromism [1][2][3][4][5][6][7][8][9][10][11][12][13] and these phenomena were generally believed to be due to change in intermolecular interactions caused by altering the molecular arrangements in the crystalline state by mechanical stress.…”

Challenges and breakthroughs in recent research on self-assembly

“…Recent examples from the authors' group are briefly summarized below. The first example, shown in figure 78, involves monolayers of a steroid cyclophane, which contains a 1,6,20,25-tetraaza[6.1.6.1]-paracyclophane cyclic core connected to four steroid moieties (cholic acid) through a flexible L-lysine spacer [691][692][693][694]. The steroid cyclophane was reported to exhibit large binding constants for aqueous naphthalene guests such as 6-(p-toluidino)naphthalene-2-sulfonate (TNS) in a 1 : 1 stoichiometry, while the corresponding cyclophane lacking the steroidal residues only weakly binds this guest.…”

Functional Nanomaterials Prepared by Nanoarchitectonics-Based Supramolecular Assembly

“…In this example, molecular machines containing a central ring (1,6,20,25-tetraaza[6.1.6.1]-paracyclophan Fig. 3.14 Capture and release of a guest molecule with molecular machine driven by macroscopic mechanical motions cyclic core) and four rigid cholic acid walls are operated by mechanical macroscopic motions at the air-water interface [46,47]. The molecular machine can catch and release guest molecules based on hand-motion-like microscopic motions.…”