molecules. Given this unique optical property, we have
performed10 calculations on an electrochemically controllable
RGB (red/green/blue) dye, which is based on a [2]catenane
constitution consisting of an interlocked CBPQT4+ ring and
a crown ether, having three π-electron-rich stations. The color
of this three-station2 catenane changes as a function of the
CBPQT4+ ring location (Scheme 1) which, in principle, can
be controlled by the oxidation of each station in turn.
as well as a new [2]rotaxane, 1‚4PF6, using DFBZ-TEG
as a precursor to its dumbbell. Both the [2]pseudorotaxane
and the [2]rotaxane which have their optical (absorption
spectrum) and electrochemical properties measured, and
thermodynamic parameters deduced, display intense blue
colors.
The synthetic routes employed in the synthesis of 3,3′-
difluorobenzidine13 (DFBZ), DFBZ-TEG, and 1‚4PF6 are
summarized in Scheme 2.
Scheme 1. Color Change Scheme of the RGB Dye
Scheme 2. Synthesis of the Blue-colored [2]Rotaxane
Although for the best part of two decades we have been
identifying and examining a range of guest molecules5a,8,9,11
that can form inclusion complexes with the CBPQT4+ ring,
the blue color that is suitable for the design of the RGB dye
has eluded us.12 DFT calculations10 predict that bis-N-
oligoethyleneglycol-substituted 3,3′-difluorobenzidine should
give the required blue color when it forms a complex with
the CBPQT4+ ring. Here, we describe the synthesis of a new
guest compound, bis-N-tetraethyleneglycol-substituted 3,3′-
difluorobenzidine (DFBZ-TEG), for the host, CBPQT4+,
a
CTAB: Cethyltrimethylammonium bromide.
DFBZ was prepared, starting from 4-bromo-2-fluoro-
aniline (2), in 33% yield using a modification of a procedure
already reported in the literature:13b the reaction also afforded
3-monofluorobenzidine (15%) and benzidine (BZ) itself
(10%). DFBZ was obtained as a pure compound, free from
these byproducts, after column chromatography (SiO2: MeOH/
(2 M) NH4Cl (aq)/MeNO2, 5:4:1).
DFBZ-TEG was prepared by alkylation (NEt3/PhMe) of
the two amino groups in DFBZ with the monotosylate 3 of
tetraethyleneglycol.11,14 Because the reactivity of these amino
groups is lower than in BZspresumably on account of both
the steric and electronic influences of the fluorine substituents
in the 3,3′-positionssthe yield of DFBZ-TEG was only
30%. When CBPQT‚4PF6sa white powderswas added to
a colorless solution of DFBZ-TEG in MeCN, the color of
the solution turned blue immediately, indicating the formation
of the [2]pseudorotaxane. The [2]rotaxane 1‚4PF6 was
isolated in 18% yield as a result of capping11,15 the terminal
hydroxyl groups of this [2]pseudorotaxane with TIPS triflate
in DMF laced with 2,6-lutidine.
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(12) The color of the benzidine complex with CBPQT‚4PF6 is blue.
However, the color of the complex with the bis-N-ethyleneglycol-substituted
benzidine derivative is greenish blue.
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Org. Lett., Vol. 9, No. 8, 2007