Two stage colour modulation of triarylmethine dyes derived from a photochromic naphthopyran

Article abstract of DOI:10.1002/ejoc.200800178

The synthesis of the unsymmetrical triarylmethine dyes 4 derived from a lithiated naphthopyran and diaryl ketones is reported. In toluene solution 4 display a typical reversible photochromic response becoming coloured upon irradiation with UV light and fa

Full text of DOI:10.1002/ejoc.200800178

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200800178
Two Stage Colour Modulation of Triarylmethine Dyes Derived from a
Photochromic Naphthopyran
Christopher D. Gabbutt,[a] B. Mark Heron,*[a] Suresh B. Kolla,[a] and Matthew Mcgivern[a]
Dedicated to Professor John D. Hepworth on the occasion of his 70th birthday
Keywords: Photochromism / Naphthopyran / Triarylmethine dye / Lithiation
The synthesis of the unsymmetrical triarylmethine dyes 4
derived from a lithiated naphthopyran and diaryl ketones is
reported. In toluene solution 4 display a typical reversible
photochromic response becoming coloured upon irradiation
with UV light and fading upon cessation of irradiation. At
low pH an intensely coloured dye cation is generated and
UV irradiation of this solution results in a further colour
change as a consequence of photochromic cycling of the
naphthopyran unit.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
naphthoxazine][11] has been investigated. However, the
union of a naphthopyran to a traditional dye chromophore
has, to the best of our knowledge, not been studied. In this
report we describe our initial investigations on the synthesis
and spectroscopic properties of some 3H-naphtho[2,1-b]-
pyrans that contain a triarylmethanol unit.
Introduction
The phenomenon of photochromism is well documen-
ted.[1] One class of organic photochromes that has attracted
considerable attention over the last decade are the
naphthopyrans, e.g. 3,3-diaryl-3H-naphtho[2,1-b]pyran (1),
which exhibit photochromism through a reversible electro-
cyclic ring-openingϪring-closing sequence (Scheme 1).[2]
Results and Discussion
A variety of triarylmethine dyes have been conveniently
accessed by the addition of an aryllithium or -magnesium
reagent, derived from an aryl iodide or bromide, to a benzo-
phenone.[12] Furthermore, a lithiated heterocycle, obtained
by a facile heteroatom-directed lithiation sequence, has
been reacted with a benzophenone to obtain heteroaryl di-
arylmethine dyes.[13] Adopting this latter simple methodol-
ogy requires access to a naphthopyran containing a suitable
heterocyclic moiety. The 3,3-diaryl-3H-naphtho[2,1-b]py-
rans can be obtained with a wide variety of substituents in
the geminal aryl units through the acid-catalysed reaction
between 2-naphthol and a 1,1-diarylprop-2-yn-1-ol.[2,14]
This procedure is also appropriate for the synthesis of 3-
aryl-3-heteroaryl-substituted naphthopyrans.[15,16] Thus
heating 2-naphthol with 1-(4-methoxyphenyl)-1-(2-thienyl)-
prop-2-yn-1-ol (2), derived from the addition of lithium tri-
methylsilylacteylide to 2-(4-methoxybenzoyl)thiophene with
subsequent base-promoted unmasking of the terminal
acetylene function, gave the naphthopyran 3 in 65% yield
after recrystallisation from a mixture of EtOAc and hexane
(Scheme 2). The 1H NMR spectrum of 3 displayed the typi-
cal doublet for 2-H at ca. δ = 6.2 ppm with J = 9.9 Hz and
a low field doublet at δ = 7.9 for 10-H.[15]
Scheme 1. Photochromic response of a 3H-naphtho[2,1-b]pyran.
Current interest in naphthopyrans has focussed on the
mechanism of colour generation,[3] spectroscopic character-
isation of the coloured species[4] and control over the hue
and persistence of the photo-generated colour.[5] The influ-
ence of the matrix,[6] metal-ion concentration,[7] solvent
polarity[8] and pH[9] on the photochromism of variously
substituted naphthopyrans has also been reported. Ad-
ditionally, linking of two photochromic naphthopyrans to-
gether[10] or linking a naphthopyran with a spiro[indoline-
[a] Department of Colour Science, School of Chemistry, The Uni-
versity of Leeds,
Leeds, LS2 9JT, UK
Fax: +44-(0)113-3432947
E-mail: b.m.heron@leeds.ac.uk
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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C. D. Gabbutt, B. M. Heron, S. B. Kolla, M. Mcgivern
SHORT COMMUNICATION
of these compounds in an acidic medium under UV irradia-
tion where combined behaviour may be expected. Scheme 4
depicts the structural and colour changes that the molecules
are predicted to undergo upon UV irradiation in neutral
(toluene) and acidic solutions.
Scheme 2. Preparation of 3H-naphtho[2,1-b]pyran 3.
The heteroatom-directed lithiation of 3 with n-butyl-
lithium in THF under N2 at –78 °C for 1 h followed by
quenching the mixture with 4-(dimethylamino)benzophe-
none resulted in a clean reaction mixture after aqueous
work-up that contained a new product (TLC) together with
unreacted 3 and the benzophenone (Scheme 3). Column
chromatography (50% EtOAc in hexane) gave the triaryl-
methanol 4a in 50% yield.[17] The 1H NMR spectrum of 4a
displayed the expected doublet for 2-H at δ = 6.24 ppm
(J = 9.9 Hz) and a singlet at δ = 2.79 attributed to the hy-
droxy proton. The thiophene ring protons appeared as an
AB system with a doublet at δ = 6.50 and at δ = 6.70 with
J = 3.7 Hz, the magnitude of this coupling constant is typi-
cal for J3,4 of a 2,5-disubstituted thiophene ring.[18] Repeat-
ing the low temperature metallation protocol provided ac-
cess to 4b from 4-(diethylamino)benzophenone in 31%
yield[19] (Scheme 3). Interestingly, compounds 4a,b were
formed as a mixture of diastereoisomers indicated by the
presence of an additional doublet at ca. δ = 6.70 ppm. Re-
Scheme 4. Structural reorganisations of 3H-naphtho[2,1-b]pyrans 4
under UV irradiation and at low pH.
The photochromic response of toluene solutions of 3 and
the series of diarylmethanol-substituted compounds 4 were
recorded. Irradiation with UV light (365 nm, 8 Watt) to a
steady state effected the ring-opening of the original colour-
less naphthopyran unit in each case to generate the orange-
red coloured species which upon cessation of irradiation
readily reverted to their original colourless state (Figure 1).
From these data (Table 1) it is evident that the incorpora-
tion of the diarylmethanol unit induces an approximately
12 nm bathochromic shift in λmax of these compounds with
4 absorbing at ca. 486 nm and 3 at 474 nm. Similar revers-
ible photochromic behaviour was noted for acetone solu-
tions of 3 and 4.
1
cording the H NMR spectrum of 4b in [D6]benzene en-
abled additional signals for the diastereoisomers to be ob-
served.
Scheme 3. Preparation of 3H-naphtho[2,1-b]pyrans 4.
It is noteworthy that initial attempts to obtain an accu-
rate mass determination of 4 by electron impact HRMS
were inconclusive. However, more extensive investigation
using softer ionising techniques (MALDI and electrospray)
revealed the expected molecular ions. The low intensity of
the ion in electrospray ionisation confirms the labile nature
of 4 arising from the resonance stabilisation of the cation
by the dialkylaminophenyl group.
The photophysical properties of 3 and 4 merit some com-
ment. With the exception of precursor 3 the naphthopyrans
are expected to exhibit both photochromism, under irradia-
tion with UV light, due to reversible pyran ring-opening
and also display absorption properties typical of a triaryl-
methine unit when dissolved in an acidic medium (low pH).
Figure 1. Visible spectra of the photochromic dyes 3 and 4 in PhMe
Perhaps of greatest interest are the spectroscopic properties recorded prior to and immediately after UV irradiation.
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Eur. J. Org. Chem. 2008, 2031–2034

Two Stage Colour Modulation of Triarylmethine Dyes
Table 1. Spectroscopic data for naphthopyrans 3 and 4 for solu-
tions at room temperature.
AcOH (no UV)
AcOH (with UV) PhMe (with UV)
λmax εmax
λmax
λmax
[nm] [mol–1 dm3 cm–1] [nm]
[nm]
3
–
–
500
564 (br)
566 (br)
474
486
485
4a 536
41100
42900
4b 538
The colour properties of the cationic dyes derived by pro-
tonation and facile elimination of water from 4 were next
studied. In accord with traditional λmax measurements of
triarylmethine dyes[12] solutions of 3 and 4a, b in 98%
aqueous acetic acid were examined (Table 1). The spectra
of the photochromic dyes 3 and 4 are presented in
Figure 2. The dyes 4 afford an intensely coloured cation
with λmax ca. 537 nm and extinction coefficients of ca.
41000 mol–1 dm3 cm–1 whereas 3 remains essentially colour-
less.
Figure 3. Visible spectra of the dyes 3 and 4 in 98% aq. AcOH at
room temperature immediately after UV irradiation.
species may be considered to be a resonance hybrid of 5
(Scheme 5) in which the bridging thiophene unit plays a
unique role because it is integral to both the cationic and
dienone chromophores (Scheme 5).
Figure 2. Visible spectra of photochromic dyes 3 and 4 in 98% aq.
AcOH at room temperature.
Scheme 5. Resonance stabilisation of cations derived from 4.
Irradiation of an acetic acid solution of the naphthopy-
To explore the reversibility of the combined UV irradia-
ran 3 resulted in a bathochromic shift in λmax of 26 nm rela- tion and pH switching process a cold (ca. +10 °C) acetone
tive to the value obtained for the toluene solution (Table 1 solution of 4b (Figure 4, far left image, prior to irradiation)
and Figure 3). This positive solvatochromism has been pre- was irradiated which resulted in the reversible development
viously noted for simple naphthopyrans and confirms a of an orange solution (Figure 4, left centre image). Once
quinone like rather than a zwitterionic structure for the the orange colour had faded, two drops of MeSO3H were
ring-opened species.[8]
added to the solution and the gradual development of a
Significant changes in the visible spectra of 4a and b in
AcOH were observed upon irradiation to a steady state
with the initially developed violet shades (λmax ca. 537 nm)
reversibly affording dull purple – brown solutions with a
complex line shape suggestive of three overlapping bands
in the visible region (Figure 3 and Table 1). The maximum
absorbance of this complex band appeared at ca. 565 nm
with a peak width at half absorbance intensity of ca.
250 nm. We propose that these complex absorption spectra
arise from the additive mixing of some of the remaining
ring-closed naphthopyran cationic species together with a
new coloured species that results when the naphthopyran
Figure 4. Images of dye 4b in acetone. Far left: solution prior to
irradiation; left centre: solution immediately after UV irradiation;
right centre: acidified solution; far right: UV irradiated acidified
unit of the cationic dye undergoes ring-opening. The new solution.
Eur. J. Org. Chem. 2008, 2031–2034
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C. D. Gabbutt, B. M. Heron, S. B. Kolla, M. Mcgivern
SHORT COMMUNICATION
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violet colour was noted (Figure 4, right centre image). This
solution was then irradiated and the reversible evolution of
a dull purple-brown shade was noted (Figure 4, far right
image). Once this shade had faded the acidic solution was
neutralised with aq. NaOH solution which resulted in the
immediate formation of a virtually colourless solution, due
to hydroxide ion interception of the cation. The complete
irradiation, protonation, irradiation sequence was con-
ducted for a second cycle, however when attempting to run
the sequence for a third cycle the solution developed a slight
haze, presumably due to salt formation upon acid neutral-
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Conclusions
[6]
[7]
The naphthopyrans 4a and 4b containing pendant tri-
arylmethanol units have been obtained by heteroatom-di-
rected metallation of a naphthopyran bearing a 2-thienyl
substituent and subsequent reaction with a benzophenone.
These new naphthopyrans display typical reversible photo-
chromism in toluene and also readily afford intense col-
oured solutions in acetic acid as a consequence of triaryl-
methine cation generation. Irradiation of these acidic solu-
tions enables an additional coloured species to be generated
as a consequence of interplay between the ring-opened
naphthopyran unit and the cationic centre. The reversibility
of the system has been demonstrated in acetone solution.
Our studies on the modulation of the colour of traditional
dyes through cycling of a photochromic unit are ongoing.
[8]
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[11]
[12]
Experimental Section
Visible spectra were recorded using an Analytik Jena Specord S100
diode array spectrophotometer for solutions of the dyes in either
spectroscopic grade toluene or acetic acid (98% aqueous) in the
concentration range 1.1–2.8ϫ10–5 moldm–3 in 1 cm quartz cells.
[13]
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Supporting Information (see footnote on the first page of this arti-
cle): A form of full experimental details for compounds 4a, b is
available.
Acknowledgments
We thank James Robinson Ltd. (Huddersfield) for financial sup-
port and the Engineering and Physical Sciences Research Council
(EPSRC) for the provision of a high-resolution mass spectrometry
service at the University of Wales, Swansea.
[15]
[16]
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[17]
[18]
See Supporting Information for compound 4a.
S. Gronowitz, A.-B. Hörnfeldt, in The Chemistry of Heterocy-
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See Supporting Information for compound 4b.
Received: February 14, 2008
[19]
Published Online: March 10, 2008
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Eur. J. Org. Chem. 2008, 2031–2034