Trivinylogs of Crystal Violet: ? synthesis and absorption properties of new near-IR dyes

Article abstract of DOI:10.1039/b006693l

Based on a Heck reaction protocol, trivinylogs of Crystal Violet have been prepared and show strong absorptions in the near-infrared region. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b006693l

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Trivinylogs of Crystal Violet:† synthesis and absorption properties
of new near-IR dyes
Saumitra Sengupta* and Subir Kumar Sadhukhan
1
Department of Chemistry, Jadavpur University, Calcutta 700 032, India
Received (in Cambridge, UK) 16th August 2000, Accepted 27th September 2000
First published as an Advance Article on the web 30th November 2000
Based on a Heck reaction protocol, trivinylogs of Crystal Violet have been prepared and show strong absorptions in
the near-infrared region.
Near-infrared (NIR) absorbing dyes (λmax > 700 nm) are of
much current interest because of their potential applications
in optical imaging systems, thermal writing displays, infrared
photography, as filter elements for NIR emitting lasers, etc.1
These dyes are usually prepared by end-capping of conjugated
organic chromophores with strong donor and acceptor sub-
stituents. A wide variety of NIR dyes are known today, some of
which e.g. the phthalocyanines, polymethines and squarylium
dyes have found important uses in laser-based applications.1,2
However, most of these dyes have large and complex archi-
tectures (long polyene chains with charged heterocyclic end-
groups) which require multi-step synthetic procedures resulting
in poor overall yields. Moreover, due to their large molecular
weights, these dyes are often found to be insoluble in common
organic solvents causing difficulties in their surface applications
Fig. 1 Absorption spectra of 4 (0.013 mM) in CH2Cl2. TFA Con-
centration: (A) 1.5 mM; (B) 3.0 mM; (C) 6.1 mM; (D) 27.4 mM;
by spin-coating techniques. In view of these, there is a sustained
interest in new NIR dyes that are easy to prepare, have
improved solubility properties and yet show strong absorptions
(log ε > 4)‡ at longer wavelengths.
Crystal Violet is a typical triphenylmethane dye which
strongly absorbs in the visible region (λmax = 588 nm). We
reasoned that trivinylogs of Crystal Violet would cause large
bathochromic shifts (vinylene red shifts amount to ca. 100 nm)3
and hence lead to absorptions well within the NIR range.
That this simple modification can indeed produce new NIR
dyes with strong absorptions at longer wavelengths is now
described in this paper.
Our synthesis started with the cheap commercial dye New
Fuchsin (1) which via threefold diazotization (NaNO2, aq.
H2SO4) and Sandmeyer reaction (KI, H2O) was converted to
the tris(iodoaryl)methanol 2 in 40% overall yield (Scheme 1).4
Threefold Heck reaction of 2 with p-dimethylaminostyrene,
under Jeffery’s conditions,5 then produced the dye base 3
(50%). The latter, upon protonation with TFA,6 gave rise to the
desired dye 4 which was studied spectrophotometrically as a
solution in CH2Cl2. The dye could also be generated in CHCl3
solutions but no attempt has been made towards its isolation
in the solid state. The electronic spectrum of 4 showed a strong
absorption in the near-infrared region (λmax = 1003 nm, log
ε = 4.76). Such a large bathochromic shift is no doubt caused
by the extended π-conjugation in 4 which greatly lowered
the energy of its charge-transfer transitions vis-à-vis Crystal
Violet. The trigonal symmetry in 4 also plays a significant role
towards this end. Thus, it ensures equal and extensive charge
delocalization among all three arms leading to a large con-
(E) 71.0 mM.
tribution of the resonance stabilized quinonoid forms to the
ground state. Interestingly, the absorption spectra of 4 showed
a pH dependent behavior (Fig. 1). Upon increasing the acid
concentration, the NIR band of 4 gradually decreased and a
new lower wavelength absorption band at ca. 530 nm gained in
intensity. In the process, the former also underwent a small blue
shift while the latter was shifted to longer wavelengths. In a
strongly acidic solution (curve E, Fig. 1), the NIR absorption
of 4 virtually vanished and the lower wavelength band at 615
nm (log ε = 5.08) became the sole maximum. We attribute this
to the formation of the N-protonated dye 4Hϩ whose reduced
charge-transfer capabilities lead to absorptions at lower wave-
lengths. The solution concentration of 4Hϩ gradually increased
with increasing acid strength and ultimately, in a strongly
acidic medium, it became the predominant species. Similar
acid-induced blue shifts have also been reported for some
hemicyanine dyes.7
For comparison, we have also prepared the methoxy analog
of 4 i.e. 6. Thus, threefold Heck reaction of 2 with p-meth-
oxystyrene produced the dye base 5 (55%) which upon proton-
ation (TFA, CH2Cl2), as before, gave rise to the trimethoxy dye
6 (Scheme 1). The latter showed a λmax value at 733 nm
(log ε = 4.93) which is greatly blue-shifted from that of 4. This
was, nevertheless, quite expected since the OMe group is a
weaker auxochrome than NMe2. However, the fact that 6,
despite having a weak auxochrome like OMe, is indeed capable
of a NIR absorption, is a significant result and attests to the
efficacy of the design elements (strong cationic acceptor group,
trigonal symmetry, extended π-conjugation) leading to its
preparation.
† The IUPAC name for Crystal Violet is N-(4-{bis[4-(dimethylamino)-
phenyl]methylene}cyclohexa-2,5-dien-1-ylidene)-N-methylmethan-
aminium chloride.
In summary, we have shown that new strongly absorbing
NIR dyes can be easily prepared by extending the π-con-
jugation of the existing octupolar visible dyes. Further studies
‡ The units of ε are dm3 molϪ1 cmϪ1 throughout.
4332
J. Chem. Soc., Perkin Trans. 1, 2000, 4332–4334
This journal is © The Royal Society of Chemistry 2000
DOI: 10.1039/b006693l

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Scheme 1 Reagents and conditions: i NaNO2, H2SO4, 0 ЊC then KI, rt; ii Pd(OAc)2, Bu4NBr, KOAc, 80 ЊC; iii CF3CO2H, CH2Cl2, rt.
on these dyes, especially on their aggregation phenomena,8 are
currently in progress.
Tris(4-iodo-3-methylphenyl)methanol 2
To a solution of New Fuchsin§ (1, 1.0 g, 2.73 mmol) in conc.
H2SO4 (1.70 cm3) and water (5 cm3), was added a solution
of NaNO2 (0.62 g, 3.0 mmol) in water (2 cm3) at 0 ЊC. After
15 min, the diazotized solution was treated dropwise with a
solution of KI (7.0 g, 42.1 mmol) in water (5 cm3). The mixture
was stirred at room temperature for 5 h and then at 80 ЊC for
30 min. It was then cooled, filtered and the residue thoroughly
Experimental
All melting points are uncorrected. IR spectra were taken on a
Perkin-Elmer R-297 spectrometer. NMR spectra were recorded
in CDCl3–TMS on a Bruker Avance 300 (300 MHz) instrument
and coupling constants, J, are given in Hz. Electronic spectra
were taken on a Shimadzu UV-160A spectrophotometer. Light
petroleum refers to the 60–80 ЊC fraction.
§ The IUPAC name for New Fuchsin is 4-[bis(4-amino-3-methylphenyl)-
methylene]-2-methylcyclohexa-2,5-dien-1-ylidenaminium chloride.
J. Chem. Soc., Perkin Trans. 1, 2000, 4332–4334
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washed with water. The residue was purified by column chroma-
tography over silica gel (5% EtOAc in light petroleum) to give 2
(0.74 g, 40%); mp 98–99 ЊC (MeOH); νmax (CHCl3)/cmϪ1 3650,
3560, 1450, 1365, 1185, 1000; δH (CDCl3) 2.40 (9 H, s), 6.72
(3 H, dd, J 8.4, 2.1), 7.18 (3 H, d, J 2.1), 7.74 (3 H, d, J 8.4);
δC (CDCl3) 28.2, 81.0, 100.3, 126.9, 128.9, 138.5, 141.2, 146.2;
found C, 39.12, H, 2.56; C22H19I3O requires C, 38.86, H, 2.79%.
TFA solution (0.5 cm3 for 4, 9 cm3 for 6) and the final volumes
adjusted to 10 cm3 with CH2Cl2. For the pH dependent study of
4, the solutions A–E (Fig. 1) were prepared by adding varying
amounts of the TFA solution (0.5, 1, 2, 9 cm3 and 9 cm3 ϩ 50
mg TFA) to a 1 cm3 aliquot of the stock solution of 3 and the
final volumes were adjusted to 10 cm3 with CH2Cl2.
Acknowledgements
A general procedure for threefold Heck reaction of 2:
preparation of the dye bases 3 and 5
The authors are grateful to Professor S. P. Moulik and S. K.
Hait for absorption measurements and to S. K. Ghorai for
some experimental assistance. Financial support for this work
was provided by DST (SP/S1/G-14/97) and CSIR (research
fellowship to S. K. S.).
Pd(OAc)2 (0.003 g, 0.014 mmol) was added to a solution of 2
(0.10 g, 0.15 mmol), 4-substituted styrene (0.90 mmol), Bu4NBr
(0.14 g, 0.44 mmol) and KOAc (0.11 g, 1.10 mmol) in DMF
(3 cm3) and the mixture heated at 80 ЊC for 30 h. It was then
diluted with water (5 cm3) and extracted with CH2Cl2. The
organic layer was washed with water, dried and evaporated
under reduced pressure. The residue was purified by preparative
thin-layer chromatography over silica gel (10% EtOAc in light
petroleum) to give 3 or 5.
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Sample preparation for absorption measurements
Stock solutions of the dye base 3 or 5 (0.135 mM) and TFA
(30.50 mM) were prepared in CH2Cl2. The dye solutions (4 or 6)
were prepared by mixing 1 cm3 of the dye base solution with the
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J. Chem. Soc., Perkin Trans. 1, 2000, 4332–4334