70
A. Abdullah et al. / Dyes and Pigments 121 (2015) 57e72
To conclude, the proposed organic mechanism concurs with the
4.2. Photometric processes
postulated physical kinetics argument, with the intermediate
structures being depicted in Fig. 9; and the overall mechanism
Solutions of compounds 1e7 (2.0 ꢁ 10ꢂ4 M) in dry methanol
(magnesium Grignard) were prepared under dark conditions.
Photometric measurements were made using thermostatted bo-
rosilicate glass cuvettes (1 cm).
consisting of
a multistep step first-order sequential process
(effectively ignoring the fast non-rate determining equilibrium
steps), graphically shown in Fig. 10.
To our knowledge these relatively stable dinitro-substituted
intermediates and extremely slow thermal isomerisation process
have never been experimentally observed in such a manner as re-
ported herein.
To observe decolourisation, the merocyanine isomers of the
mononitro-substituted compounds 6, 7 were generated by photo-
irradiating solutions (1.0 ꢁ 10ꢂ4 M) with a focussed wavelength
filtered (1 M CoSO4: 1 M Cu2SO4 solution) ultra-violet radiation
Lastly, at room temperature e an important factor when
considering the use of substituted-spirobenzopyrans as light
induced ionic switches or ion chelating systems e these dinitro-
substituted compounds behave differently, to a significant and
measurable degree, to the structurally identical but mono-nitro-
substituted spirobenzopyrans 6 and 7. These di-nitro-group sub-
stitution effects may also therefore prove practically useful in other
(photo)thermochromic systems when incorporated into the skel-
eton of a larger molecule - e.g. lariat-ethers and crown systems etc.
In these structures ion-chelation would thermodynamically bias
the equilibrium towards the zwitterionic structure, thereby
potentially allowing greater (photo)thermochromic (equilibrium)
control in these systems. The latter effect offers the potential of
being used in biological metal-ion sensing probes, particularly as
the presence of two-nitro substituents greatly enhances the known
aqueous solubility of these spirobenzopyran-based systems e
necessary for their operation in biological media.
In summary, this study has clearly highlighted that the thermal
formation of both the gem-methyl and 30-cyclohexyl- dinitro-
substituted merocyanines exhibit a previously unobserved, un-
usual and interesting sigmoidal absorbance phenomenon: This
most probably arises from the formation of two relatively long-
lived rate determining intermediates e the TCC merocyanine
conformer and the oxygen-protonated pyran ring of its associated
spirobenzopyran structure.
(high pressure HgeXe source, 200 W,
wards the spirocyclic forms was followed by monitoring (intervals
30 s) the visible absorption band (lmax z 550 nm).
l
¼ 365 nm). Reversion to-
4.3. Synthesis
4.3.1. Syntheses of the spirobenzopyrans
6,8-Dinitro-10,30,30-trimethylspiro-[2H-1-benzopyran-2,20-
indoline] 1. 2-Hydroxy-3,5-dinitrobenzaldehyde (0.20 g,
0.94 mmol) and 1,3,3-trimethyl-2-methylene indolenine (0.16 g,
0.92 mmol) were dissolved in ethanol (10 mL), and the resulting
solution heated under reflux for 24 h. After this period the solvent
reduced in volume by rotary evaporation to approximately 2 mL.
Cooling of the remaining solution yielded a crystalline solid which
was recrystallised from ethanol to yield the title compound as a
bottle green crystalline solid (0.22 g, 63%). mp > 220 ꢀC (from
ethanol) (lit., [21], 280e283 ꢀC). (Spectrum A) dH (CDCl3) (crude);
spectrum showing a mixture. Olefinic proton 5.66e5.68 (1H, d,
J ¼ 10), 1.16 (3H, s, CH3), (singlet suggesting trans-form) also 1.35,
1.33 (6H, d, gem-CH3) (suggesting cis-form). (Spectrum B) dH Sam-
ple recrystallised from ethanol producing a sharper spectrum (ar-
omatics less complicated). No olefinic proton as in (A). 1.16 (3H, s,
CH3) (singlet suggesting trans-form) also 1.33, 1.35 (6H, d, gem-CH3)
(suggesting cis-form). (Spectrum C) Spectrum re-acquired after
irradiating (B) with visible light: Spectrum similar to (B). Spectrum
(D). UV irradiation of (C); spectrum showing mixture. Olefinic
proton 5.66e5.67 (1H, d, J ¼ 10) 1.16 (3H, s, CH3), (singlet suggesting
trans-form). Doublet at 1.33 (6H, d, gem-CH3) suggesting cis-form.
n(max) (CDCl3)/cmꢂ1 3020 (sat CeH), 1600 (C]C), 1220 (CeC), 1200
(CeN), 1450 (NO2), 954 (CeO spiro), 771 (ArH, 4 adj H's). m/z 368
(Mþþ1, 31%), 367 (Mþ, base peak, 100), 366 (Mþꢂ1, 14.3). (Found: C
61.19, H 4.58, N 11.22. C19H17N3O5. 0.4H2O requires C 60.93, H 4.76,
N 11.22).
4. Experimental section
4.1. Instrumentation
1H NMR assignments were carried out with a JEOL FX2000
spectrometer using deuteriochloroform, dimethyl sulfoxide-d6 or
1,1,2,2-tetrachloroethene-d2 (TCE) as the solvent with either tet-
ramethylsilane (TMS) or (TCE) used as the internal reference.
Multiplicities are reported as (s) singlet, (d) doublet, (t) triplet, (q)
quartet and (m) multiplet. Assignments of hydroxyl and ammo-
nium protons were verified by deuterium exchange. Mass spectra
were recorded with a VG 7070H mass spectrometer interfaced with
a Finnegan Incos data system. Accurate mass measurements were
carried out at the EPSRC mass spectrometry service at the Univer-
sity of Wales, Swansea. UV spectroscopy was carried out using
PerkineElmer Lambda 5 and Lambda 9 spectrometers; both in-
struments are double beamed with thermostatically controlled cell
blocks. The Lambda 9 is additionally fitted with as RS 232 port,
which allows remote control by PC. All UV measurements were
taken at 25 ꢀC using 3-cm3 quartz cells with a 1-cm path length and
are referenced against air. IR spectra were recorded with a Per-
kineElmer 983 spectrometer. Melting points were determined in
open capillary tubes with an Electrothermal melting point appa-
ratus and are uncorrected. Elemental analyses were carried out in-
house. Thin-layer chromatography was performed over glass plates
coated with Merck silica gel 60 F254; flash chromatography was
10-Methyl-6,8-Dinitro-30-spirocyclohexylspiro-[2H-1-
benzopyran-2,20-indoline]
2.
1,2-Dimethyl-3-spirocyclohexyl
indolium triflate (1.02 g, 2.81 mmol) was dissolved in a 40% so-
dium hydroxide solution (10 mL). The resulting mixture was
vigorously stirred for 5 min after which time diethyether (15 mL)
was added. The diethylether layer was subsequently separated,
dried (anhydrous sodium sulphate), filtered and evaporated under
reduced pressure to produce
a yellow-orange oil (0.44 g,
2.06 mmol). The oil was isolated, dissolved in ethanol (3 mL), and
added to 2-hydroxy-3,5-dinitrobenzaldehyde (0.44 g, 2.07 mmol)
in ethanol (20 mL). The resulting solution was subsequently heated
under reflux for 24 h. Removal of the solvent under reduced pres-
sure to approximately 2 mL, and cooling, yielded a reddish coloured
crystalline solid. Recrystallisation from the minimum quantity of
ethanol yielded the title compound as a dark red coloured precip-
itate (0.45 g, 58%). mp > 209e211 ꢀC. dH (CDCl3) v. insoluble
resulting in a broad spectrum. dH (TCE); 8.40, 8.42 (1H, d, ArH J ¼ 8),
8.22, 8.24 (1H, d, ArH J ¼ 8), 7.65, 7.67 (1H, d, ArH, J ¼ 8), 7.39e7.41
(1H, t, ArH), 6.88, 6.90 (1H, d, CH]CH, J ¼ 10), 6.85, 6.87 (1H, t,
ArH), 6.74, 6.76 (1H, d, ArH J ¼ 8), 6.20, 6.22 (1H, d, CH]CH, J ¼ 10),
2.78 (3H, s, NeCH3), 1.22e1.95 (10H, m, CH2 x 5). n(max) (CDCl3)/
cmꢂ1 3020, 2930 (sat CeH), 1602 (C]C), 1430, 1330 (NO2), 1376
performed using Merck 7734 silica gel (20e63 mm).
Chemical intermediates were purchased from commercial
sources unless otherwise stated.