Squaraine rotaxanes with boat conformation macrocycles

Abstract

Mechanical encapsulation of fluorescent, deep-red bis(anilino)squaraine dyes inside Leigh-type tetralactam macrocycles produces interlocked squaraine rotaxanes. The surrounding macrocycles are flexible and undergo rapid exchange of chair and boat conformations in solution. A series of X-ray crystal structures show how the rotaxane co-conformational exchange process involves simultaneous lateral oscillation of the macrocycle about the center of the encapsulated squaraine thread. Rotaxane macrocycles with 1,4-phenylene sidewalls and 2,6-pyridine dicarboxamide bridging units are more likely to adopt boat conformations in the solid state than analogous squaraine rotaxane systems with isophthalamide-containing macrocycles. A truncated squaraine dye, with a secondary amine attached directly to the central C(4)O(2) core, is less electrophilic than the extended bis(anilino)squaraine analogue, but it is still susceptible to chemical and photochemical bleaching. Its stability is greatly enhanced when it is encapsulated as an interlocked squaraine rotaxane. An X-ray crystal structure of this truncated squaraine rotaxane shows the macrocycle in a boat conformation, and NMR studies indicate that the boat is maintained in solution. Encapsulation as a rotaxane increases the dye's brightness by a factor of 6. The encapsulation process appears to constrain the dye and reduce deformation of the chromophore from planarity. This study shows how mechanical encapsulation as a rotaxane can be used as a rational design parameter to fine-tune the chemical and photochemical properties of squaraine dyes.

FIGURE 1

Squaraine dyes and squaraine rotaxanes.

FIGURE 2

X-ray crystal structures of bis(anilino)squaraine rotaxanes 2a–e.

FIGURE 3

Atom labels for squaraine rotaxanes.

FIGURE 4

Partial ¹H NMR spectra at 22 °C in CDCl₃ showing the macrocycle methylene signals for squaraine rotaxanes; (a) 2d with J = 5.8 Hz in a 300 MHz spectrum; (b) 5 with J₁ = 8.2 Hz, J₂ = 14.4 Hz (left) and J₁ = 1.5 Hz, J₂ = 14.4 Hz (right) in a 600 MHz spectrum.

FIGURE 5

Co-conformational exchange in squaraine rotaxanes 2a–e induces linear oscillation of the macrocycle about the center of the squaraine thread. For clarity, not all possible conformational exchange pathways are shown.

FIGURE 6

X-ray crystal structure of truncated squaraine rotaxane 5.

FIGURE 7

Absorption spectra of 4 (– –, black) and 5 (– –, red) and fluorescence emission spectra of 4 (—, black) and 5 (—, red) in CHCl₃ (5 μM).

FIGURE 8

Change in absorption of 4 (◦) and 5 (☓) in CHCl₃ (10 μM) upon exposure to an unfiltered xenon arc lamp (150 W) at 25 °C.

FIGURE 9

Change in absorption upon addition of 2-mercaptoethanol (50 mM) to 4 (●), 5 (◦), and 1a (☓) in CHCl₃ (10 μM) at 25 °C.

SCHEME 1

Synthesis of Truncated Rotaxane 5