Rational Design of Fluorophores for In Vivo Applications - Progress in Molecular Biology and Translational Science - page 73

squaraine rotaxanes, see Ref. 112 ). Rotaxanes are the supramolecular struc-
tures in which a rode-like molecule is threaded through the macrocyclic
component, and the bulky groups located on both ends of the axle prevent
de-threading. 112 Squaraine rotaxanes have been assembled by the “clipping
method,” that is, macrocyclic amide was synthesized from diamine and
isophthalic chloride in situ in the presence of squaraines containing bulky
terminal substituents ( Fig. 3.3C ). 109,113-115 The macrocycle formed in situ
“clips” the axle (squaraine) and the process is facilitated by the hydrogen
bonding between amide N-H in the intermediate acyclic precursor of
macrocycle and oxygen atoms from the oxabutadiene core of squaraine
(template effect). In an alternative synthetic strategy (capping method),
macrocyclic lactam is synthesized first and forms a reversible host-guest
complex with squaraine (pseudo-rotaxane).

In subsequent chemical

reactions, bulky groups (stoppers) are installed at both ends of the

threaded squaraine and lock rotaxane irreversibly. 116
The resulting squaraine rotaxanes possess essentially the same optical prop-
erties as free squaraines (though they sometimes exhibit lower fluorescence
quantum yields than the corresponding free squaraines), but they are substan-
tially more inert toward nucleophiles in solution and less prone to aggregation
in polar solvents. 109 For example, both near-IR absorption and emission of
the free squaraine

are reduced two times within 5 min in the presence
of cysteine (due to the nucleophilic attack of thiol), whereas the corresponding
rotaxanes react substantially more slowly. 109 Similarly, whereas SQI exhibits a
very broad absorption in a dimethylsulfoxide/water mixture (due to the ag-
gregation), the corresponding rotaxanes show only minor broadening under
similar conditions. 109 Both effects, namely, reduced reactivity toward nucle-
ophiles and diminished tendencies for aggregation in polar media, are attrib-
uted to steric protection of the macrocycle on the squaraine core. 109 The
macrocyclic teralactam surrounds the squaraine, makes it less accessible by nu-
cleophiles, and prevents it from chromophore-chromophore interaction,
leading to aggregation.
Squaraine rotaxanes have been converted into water-soluble derivatives
by attaching multiple sulfonic, carboxylic, guanidine, and quarternary
ammonium groups as terminal “capping” groups. 117 In a similar fashion,
multivalent bioconjugatable derivatives have been prepared. 115 Squaraine
rotaxanes have been utilized subsequently for intracellular 118

SQI

and in vivo

imaging. 117,119
An increase in the application of squaraine can be expected in the future.
There is a wealth of squaraine derivatives described in the literature with