Intrinsically thermochromic fluorans

Article abstract of DOI:10.1039/c1cc15854f

A Suzuki coupling strategy has been employed to access a series of novel fluorans substituted with a phenolic moiety. These fluorans display good thermochromism in methyl stearate and obviate the need for traditional complex formulations containing acidic colour developers.

Full text of DOI:10.1039/c1cc15854f

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COMMUNICATION
Intrinsically thermochromic fluoransw
Farid Azizian,a Amanda J. Field,b B. Mark Heron*b and Colin Kilnerb
Received 21st September 2011, Accepted 9th November 2011
DOI: 10.1039/c1cc15854f
A Suzuki coupling strategy has been employed to access a series
of novel fluorans substituted with a phenolic moiety. These
fluorans display good thermochromism in methyl stearate and
obviate the need for traditional complex formulations containing
acidic colour developers.
health issues.11 Of interest to this study are the recent reports
concerning the palladium-catalysed arylation of a 9-trifloxy-
xanthene with a triaryl boroxin as the key step in the synthesis
of a series of rosamines (9-arylxanthenes)12 and the related
arylation13 and borylation14 of fluorescein monotriflate in the
design of new sensor systems.
Three bromo-substituted fluorans 1–3 were employed as
substrates in order to investigate the coupling process and any
subsequent intrinsic thermochromic behaviour. The isomeric
diethylamino substituted bromobenzo[a]fluorans 1 and 2 were
obtained in moderate yield from the acid-catalysed condensation
between the ketoacid 4a and either 6-bromo-2-naphthol or
7-bromo-2-naphthol,15 respectively. The methoxy substituted
bromobenzo[a]fluoran 3 resulted from a similar sequence
from 6-bromo-2-naphthol and a mixture of the ketoacids
4b : 4c (B2 : 1) derived from the Friedel–Crafts benzoylation of
1,3-dimethoxybenzene with AlCl3 (Scheme 2). The new fluorans
The last few years have seen a resurgence in interest in organic
dye systems with a particular emphasis directed towards dyes
designed for solar cells,1 sensors and probes for a variety of
analytes,2 optical microscopy and nanoscopy3 and photochromic
systems4 amongst other technological applications.5 Dyes derived
from the xanthene system6 feature prominently amongst
these examples and additionally in electrochromic displays.7
Thermochromic formulations containing spiro[isobenzofuran-
1(3H),90(9H)xanthen]-3-ones, more commonly referred to as
fluorans, have attracted significant commercial interest for
image recording.8,9 In order for fluorans to exhibit a thermo-
chromic response, a formulation incorporating the fluoran
(colour former) and a Brønsted acid (colour developer) in an
inert relatively low melting matrix e.g. methyl stearate, is required.9
The reversible colour generation results from temperature
dependent association and proton transfer between the acidic
developer and the fluoran; at low temperature the fluoran is in
close association with the acidic developer and intense colour
is observed as a consequence of cationic dye generation
through lactone ring-opening. However, upon warming the
wax above its melting temperature, the association between
these partners is diminished and a colourless solution results
(Scheme 1).9,10
1
1–3 displayed the expected H and 13C NMR signals.16
The synthesis of biaryls employing the Suzuki coupling
reaction is a versatile and routinely employed reaction in
organic synthesis.17 Heating a toluene ethanol solution of
1 with 3-hydroxybenzene boronic acid in the presence of 3 mol%
Pd(PPh3)4 and K2CO3 gave, after aqueous work-up and
column chromatography, two new fluorans 90-diethylamino-
30-(3-hydroxyphenyl)benzo[a]fluoran 5a (69%)18 and the debro-
minated derivative 6a (15%) (Scheme 3); the latter structure was
verified by independent synthesis. The moderate yield of 5a
was somewhat disappointing in view of the relatively high
conversion indicated by TLC analysis of the reaction mixture.
It is likely that the moderate isolated yield of pure 5a is in part
attributed to a combination of the poor resolution between
the two new products and the chromatographic purification
process which results in some ring-opening of the fluorans on
the chromatography silica.
We now communicate our preliminary observations on the
value of transition metal-coupling chemistry for the derivatisation
of readily accessible bromo-substituted fluorans, and through such
a coupling process, incorporate a phenolic unit which obviates
the requirement for complex formulation preparation involving
the use of bisphenol A; the latter, in polymer formulations,
has recently been associated with undesirable environmental
The structure of 5a was unequivocally established by X-ray
crystallography (Fig. 1).19 The pale maroon crystal was obtained
by the slow solvent diffusion technique from MeOH/CH2Cl2 and
%
crystallised in the triclinic space group P1. The asymmetric unit
a St Mary Cray Research, Sun Chemical Ltd, St Mary Cray,
Orpington, Kent BR5 3PP, UK
b School of Chemistry, The University of Leeds, Leeds, LS2 9JT, UK.
E-mail: b.m.heron@leeds.ac.uk; Fax: +44 (0)113 3436565;
Tel: +44 (0)113 3432925
w Electronic supplementary information (ESI) available: Selected
spectroscopic data for new compounds and crystallographic data for
compound 5a are provided. CCDC 845168. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c1cc15854f
Scheme 1 Typical thermochromic equilibrium for a fluoran.
c
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Table
between donor (D) and acceptor (A) groups for chains of 5a
1
Bond lengths and angles for intermolecular H-bonds
D–Hꢁ ꢁ ꢁA
D–H
Hꢁ ꢁ ꢁA
1.95
1.92
Dꢁ ꢁ ꢁA
2.7823(15)
2.7507(14)
D–Hꢁ ꢁ ꢁA
170.4
172.0
O65A–H65Aꢁ ꢁ ꢁO20Ba
0.84
0.84
O65B–H65Bꢁ ꢁ ꢁO20Ab
Symmetry codes:a ꢀx + 2, ꢀy + 1, ꢀz + 1; ꢀx + 1, ꢀy + 2, ꢀz + 2.
b
Scheme 2 Synthesis of bromofluorans 1–3.
was noted and clean separation by chromatography could not
be accomplished thus accounting for only moderate yields of
the pure isolated products. The debromination of aromatic
bromides in Pd-catalysed reactions is a relatively common
problem.21
The evaluation of a typical fluoran colour former relies
upon examination of a formulation of the fluoran in methyl
stearate containing bisphenol A and monitoring the colour of
the mixture above and below the melting point of methyl
stearate.9 In the present work bisphenol A was not required
because of the intrinsic developer unit. Thus with the exception
of 5d, which was too insoluble in hot methyl stearate, the
remaining substituted fluorans 5a–c were dissolved in hot
(160 1C) methyl stearate and filtered through glass wool. The
foregoing colourless solutions were then cooled to ambient
temperature during which time 5a and 5c developed an intense
maroon colour as a consequence of ring-opening of the fluoran
unit presumably through intermolecular association between
the phenolic moieties and the spirolactone units.10 The colour
change of 5a in methyl stearate as a thin film sandwiched
between two microscope slides is clearly indicated (Fig. 3) and
also in a formulated ink screen printed label (Fig. 4).22
The difference in reflectance of 5a in methyl stearate at low
and high temperature is shown by the reflectance spectra
(Fig. 5). Furthermore, cycling the temperature of the printed
label above and below the range B27–34 1C resulted in
switching between the colourless and coloured states and
further confirmed the reversibility of the thermochromic
process. These observations are comparable to the performance
of current formulated thermochromic inks.23 It is noteworthy
that the solution of 5b, which does not contain a phenolic
residue, remained colourless throughout the temperature
cycling experiments, but upon addition of bisphenol A displayed
a typical thermochromic response with the reversible development
of a maroon shade (Fig. 6) with comparable spectroscopic
features to those noted for 5a.
Scheme 3 Synthesis of hydroxyphenyl substituted fluorans 5.
Fig. 1 X-ray crystal structure of 5a with thermal ellipsoids scaled at
the 50% probability level.
contained two molecules, labelled A and B, that displayed
slightly different conformations. The bond lengths and angles
of 5a were comparable to those of related simple fluorans.20
Interestingly, the unit cell indicates intermolecular H–bonding
between the phenolic moiety and the lactone CQO (Fig. 2),
and is perhaps suggestive of an extremely low concentration of
ring-opened fluoran molecules and whilst these could not be
detected they may account for the pale maroon colour of the
crystal. The intermolecular H-bonding of 5a generates infinite
linear chains with alternating A and B molecules (Fig. 2).
The chains run parallel to [ꢀ1, 1, 1] i.e. parallel to the body-
diagonal of the unit cell that intercepts points 1, 0, 0 and 0, 1, 1.
In each chain, hydrogen bonds are alternating intermolecular
O65A–H65Aꢁ ꢁ ꢁO20B and O65B–H65Bꢁ ꢁ ꢁO20A bonds with the
following geometry (A, 1) (Table 1).
Repeating the coupling protocol with 1 and benzene boronic
acid gave 5b (36%). The alternate bromobenzo[a]fluorans 2 and 3
gave 5c and 5d, respectively in 58% yield upon a similar coupling
reaction with 3-hydroxybenzene boronic acid (Scheme 3). The
formation of by-products 6a (9% from 2) and 6b (14% from 3)
The union of a phenolic moiety to benzofluoran by a Suzuki
coupling methodology provides a very simple and moderately
efficient approach to the design of new intrinsically thermo-
chromic fluorans from readily available bromofluorans. Prototype
formulations incorporating these new hydroxyphenyl substituted
benzofluorans demonstrate reversible and intense colour
development without the requirement for acidic colour
developers such as bisphenol A. These preliminary results
Fig. 2 Stick view of compound 5a showing the hydrogen-bonded
chains of alternating A and B molecules.
Fig. 3 Compound 5a in methyl stearate (lower cold, upper warm).
Chem. Commun., 2012, 48, 750–752 751
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18 Experimental methods for the coupling reactions leading to 5a–d
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19 Crystal data for 5a crystallised from MeOH/CH2Cl2 by solvent
vapour diffusion: C34H27NO4,M = 513.57, triclinic, space group
Fig. 6 Reflectance spectra of 5b in methyl stearate containing
bisphenol A at 20 1C and 50 1C.
should open up access to new more environmentally friendly
thermochromic image recording systems. Our studies concerning
the extension of this strategy involving more varied substituted
fluorans to enable a full colour gamut to be accessed are ongoing.
Notes and references
%
P 1, a = 13.1397(13) A, a = 72.535(5)1, b = 14.8298(14) A, b =
69.311(5)1, c = 16.2973(15) A, g = 68.257(5)1, volume = 2706.7(4) A3,
rc = 1.26 Mg mꢀ3, T = 150 K, Z = 4, reflections measured 81 602,
independent reflections 17 792 R(int) = 0.0381, R1 = 0.0719 (I 4
2s(I)), wR2 (all data) = 0.1344. Full data are provided in the
supplementary information (see also CCDC 845168).
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c
752 Chem. Commun., 2012, 48, 750–752
This journal is The Royal Society of Chemistry 2012