Steric and substituent control on the photoreversibility of some novel N-alkyl-3,3′-disubstituted-6-nitro-indolospirobenzopyrans: Evaluation using UV spectroscopic studies

Article abstract of DOI:10.1016/j.dyepig.2010.10.006

A series of novel 6-nitro substituted indolospirobenzopyrans are reported. The effects of substitution on their photoreversibility are investigated by: synthesising derivatives that possess sterically hindering (with regard to the spiropyran-opening ? closing) groups contained within the spirocyclic skeleton. Specifically, spirobenzopyrans possessing combinations of various N-alkyl, and/or, simultaneously, 3,3′-geminal methyl and/or cyclohexyl groups are synthesised. Further, these systems are evaluated in three solvents; methanol, acetonitrile and dichloroethane. The thermal decolourisation rates of the gem-dimethyl and 3,3′-cyclohexyl compounds are additionally evaluated at the temperature of 50 °C. The above systems, and changes in physical parameters are extensively evaluated, and the subsequent biasing of the equilibria established using UV spectroscopy. Rates of colourisation, decolourisation and the equilibrium constants are obtained. The overall photochromic 'tuneability' of these systems is subsequently established.

Full text of DOI:10.1016/j.dyepig.2010.10.006

Dyes and Pigments 90 (2011) 146e162
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Steric and substituent control on the photoreversibility of some
novel N-alkyl-3,30-disubstituted-6-nitro-indolospirobenzopyrans:
Evaluation using UV spectroscopic studies
,
b
b
Craig J. Roxburgh a , Peter G. Sammes , Ayse Abdullah
*
a Department of Chemistry, The Christopher Ingold Laboratories, University College, The University of London, London WC1H OAJ, UK
b Department of Chemistry, Surrey University, Guildford, GU2 7XH, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel 6-nitro substituted indolospirobenzopyrans are reported. The effects of substitution on
their photoreversibility are investigated by: synthesising derivatives that possess sterically hindering (with
regard to the spiropyran-opening 4 closing) groups contained within the spirocyclic skeleton. Specifically,
spirobenzopyrans possessing combinations of various N-alkyl, and/or, simultaneously, 3,30-geminal methyl
and/or cyclohexyl groups are synthesised. Further, these systems are evaluated in three solvents; methanol,
acetonitrile and dichloroethane. The thermal decolourisation rates of the gem-dimethyl and 3,30-cyclohexyl
compounds are additionally evaluated at the temperature of 50 ꢀC. The above systems, and changes in
physical parameters are extensively evaluated, and the subsequent biasing of the equilibria established
using UV spectroscopy. Rates of colourisation, decolourisation and the equilibrium constants are obtained.
The overall photochromic ’tuneability’ of these systems is subsequently established.
Received 8 June 2010
Received in revised form
12 October 2010
Accepted 15 October 2010
Available online 23 October 2010
Keywords:
6-Nitro-spirobenzopyrans
Photoreversible
Colourisation (k2) and decolourisation (k1)
rates
Ó 2010 Elsevier Ltd. All rights reserved.
Equilibrium constants (K)
Substituent control
UV spectroscopy
1. Introduction
photoreversible chelation of specific metal-ions has been reported
in other systems [4e7]. Some elegant work utilising the Gd3þ ion
There is ongoing interest in the development of reversible metal-
chelating agents in which chelation can be switched on-and off-by
exposure to light of different wavelengths [1]. Several research
groups have made notable contributions in this area [2]. A popular
substrate for such studies is the 6-nitrospiro[1-benzopyran-2,20-
indole] system (1a) and its analogues since these have well-docu-
mented photochemical properties [3]. Photoirradiation with UV
has been reported by Sakata et al. [8]. Finally, it is important to
emphasise the substantial contribution that the solvent [9] and
substituents [10e12] exert on the ring-openingeclosing reaction
kinetics in spirobenzopyrans.
We have previously reported that the open 4 closed dynamic
equilibrium, an inherent property of spirobenzopyrans, under both
dark conditions and photolysis could be greatly influenced by the
use of appropriately placed electronically-influencing substituents.
This was specifically the case when substituted in the 5-position of
the indole-ring [13e15]; and also alkyl with alkyl-substituents
placed in selected parts of the spirobenzopyran skeleton, including
the alkenic-bond of the pyran ring [16]. Additionally, appropriately
substituted spirobenzopyrans have been used to selectively trans-
port ions across solvent interfaces, and that selectively placed
skeletal substituents can be used to exert control over the process
[17]. We now report the synthesis of a series of novel indolo-
benzospiropyrans containing combinations of N-alkyl, 3,30-dialkyl
and 6-nitro-substituents; and subsequently studies of the dynamic
ring-opening 4 closing rates of reaction within these systems by
UV spectroscopy.
light at w
ocyanine) form (1b), which can be converted back to the ring-closed
form either by photoirradiation with visible light (w
l
¼ 380 nm leads to the ring-opened zwitterionic (mer-
l
¼ 550 nm) or
thermally Fig. 1. These photocolouration-thermodecolouration, or
photocolouration-photodecolourisation cycles may be repeated
between 5e10,000 times-depending on the specific type of system
and have thus formed the basis of robust light-induced ionic
switches. Further use of spirobenzopyrans, in particular, the
* Corresponding author. Present address: The Institute of Naval Medicine,
Alverstoke, Gosport, Hants PO12 2DL, UK.
E-mail address: INM-EMS-OccHygSCRL@mob.uk (C.J. Roxburgh).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.10.006

C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
147
UV light
NO2
visible light
or heat
N
O
NO2
N
+
-
O
1a (closed-form)
1b (open-form)
Fig. 1. Typical UVeVIS Absorption Spectra of the closed-(1a), and open-(1b)
Indolospirobenzopyrans.
2. Results and discussion
2.1. Syntheses of indolobenzospiropyrans 2e5
The syntheses of the N-alkylindolospiropyrans 2e5 were under-
taken using standard synthetic methods, or slight modifications
thereof. Typically, the appropriately substituted alkyl-and aryl-hy-
drazines and ketones were condensed together to yield the corre-
sponding hydrazones (cf. 7) which were then cyclised to the indoles
(cf. 10) using the Fischer indole synthesis, and variations thereof. N-
alkylation of the indoles was affected using various appropriate
alkylating agents, from which the indolenines (cf. 8) were obtained,
after treatment with aqueous sodium hydroxide. The final indolo-
spirobenzopyrans were obtained in good to excellent yields-ranging
from 55 to 89%-by a condensation reaction between the appropriate
indolenines and salicaldehydes in refluxing ethanol: typically, the
reactions had gone to completion within a 24 h time period.
Scheme 1. N0-Hexyl-6-nitro-30-spirocyclohexylspiro-[2H-1-benzopyran-2,20-indoline]
5 (typical synthetic procedure used for compounds 2-5).
spirocyclic carbon-oxygen bond, the open-merocyanine form is
complex, existing as a potential mixture of eight different cisoid-and
transoid-stereoisomers (Fig. 3). Therefore, the overall observed
ring-closure rate is comprised of the sum of the individual isomer’s
ring-closure rates. However, at the final transition stage of ring-
closure, there can only be the one cisoid-stereoisomer which closes
to form the spirocyclic structure. Clearly, some stereoisomers
are more thermodynamically stable than others, and at any given
temperature there will be a significant predominance of one or more
A ’general’ synthesis is exemplified below, in Scheme 1, for the
new N-hexyl-3,30-cyclohexyl-substituted spirocyclic system 5.
2.2. Kinetics and thermodynamic treatment of the reversible
interconversion of the trans-merocyanine form. Variable
temperature studies using UV spectroscopy
-
N
+
O
N
O
fig c - dipolar form
The kinetics of the spirobenzopyran ring-opening 4 closing
reaction have been described in several publications, [18e20] however
there are no specific reports detailing the effects on the kinetics of this
process where the spirobenzopran skeleton is substituted with various
N-hexyl and/or electronically biasing functional groupseas depicted
above (note: propyl- and phenyl- substituents in the 1-, 2- and 5-
positions of spirobenzopyrans have been reported [21]). We have
therefore studied, and report herein, the effects that these types of
substitutions have on the benzospiropyran ring-opening 4 closing
reactions, and their corresponding reaction rates (Fig. 2).
fig d - quinolic form
-
N+
O
fig e
Fig. 2. Electronic distribution between the mesomeric forms of the transoid-photo-
merocyanine. (Conjugation between the two trans- merocyanine forms Fig 2c and Fig
2d allows rotation about the single bonds as depicted in Fig 2e).
Whilst the initial ring-opening reaction of the spirobenzopyrans
is a relatively simple unimolecular reaction involving cleavage of the

148
C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
-
O
-
N
N
R
R
R
R
O
+
+
+
`
`
R
f
h
j
g
TTT
TTC
R
N
R
-
O
N
R
+
-
O
`
`
CCC
CCT
i
`
+
R
N
-
R
N
R
R
O
-
+
O
`
k
CTC
TTC
`
`
R
R
+
R
+
N
R
N
-
O
-
O
TCT
TCC
m
l
R= H
R`= n-alkyl
C = Cis-, T = Trans-
Fig. 3. The eight theoretically possible zwitterionic tautomeric isomers f-m of the open-form.
of themein fact there is a predominant mixture of trans-isomers.
Overall this is a verycomplex and multifactorial scenario the detailed
discussion and analysis being outside the scope of this paper.
It is evident from the UV solution spectrum of spirobenzopyrans
that the absorption peak present in the visible region (centered
not a formal rate constant]. The reverse process of decolourisation
may be treated similarly; the initial rate of the absorption vs. time
graph being proportional to the initial rate of decolourisation (kinit).
Further, measurement of the gradient of the plot of ’ln[AꢁAt] vs.
time (t) enables the decolourisation rate to be establishedethis is
defined here as the apparent rate constant (k1) (whereby A is the
initial absorbance, and At the absorbance at a time (t)). Similarly,
the reverse is true, for the colourisation reaction, but in this case the
plot would be ln[AtꢁA] vs. timeealso giving an apparent rate
constant, defined here as k2. Thus, as k2/k1 represents the equilib-
rium constant (K), the overall equilibrium position of these systems
(depicted below) can be calculated (Table 5).
The half-lifes (t1/2) for the reactions are obtained in the standard
manner from the absorbance vs. time graphs (i.e. the time between
any two points ’in time’ over which the absorbance value has either
halved, or doubled).
Lastly, by varying the temperature and subjecting the UV
absorption data to the methods described above allows an indication
of the equilibrium position, and thus the thermal stability at this
particular temperature, to be obtained. The reactions of the spi-
robenzopyrans were additionally studied in three typical solvents
(dichloroethane (DCE), acetonitrile and methanol) which provided
further important information and how the medium could
around
l
¼ 550 nm), during the formation, or disappearance, of the
trans-merocyanine form, increases or decreases respectively as the
ring-opening 4 closing process proceeds. This absorption peak
has therefore been used as the reference wavelength with which
to monitor the progress of these colourisation 4 decolourisation
reactions. Subsequently, a plot of absorbance at this particular
wavelengthefor the open-formevs. time (t-in minutes) has been
plotted for each of the various spirobenzopyran reactions studied.
Since the concentration of the open-form (effectively formed via
an intra-unimolecular reaction) can be taken as a first-order plot,
a linear relationship should, theoretically, subsequently be obtained.
For the colourisation reaction: calculation of the initial gradient, of
the absorbance [A] vs. t graph allows the initial (init) rate of change
in absorbance (which is directly proportional to the merocyanine
form concentration change) allows calculation and comparisons of
relative initial colourisation rates to be performed [The authors use
the term kinit but stress that this is a relatively proportional measure
of the initial rate of colourisation or decolourisation change and is

C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
149
potentially be utilised to influence their solution equilibria. This is
particularly important when considering the design and practical
operational efficiency of solution-based ion-chelating systems.
Overall, the above enables one to understand and quantify
the effects that various alkyl-group substitutions exert on the
spirobenzopyran ring-opening 4 closing equilibria, associated
solvent, and temperature effects. Subsequently, this enables one to
predict the effects that these substitutions will exert on the pho-
tochromicity of spirobenzopyrans, which in turn permits applica-
tion in the design of clean, or ’tuneable’, on 4 off photo-activated
ionic switches, and selective ion-chelating systems.
rates of reaction for compounds 2 and 3 from the graphs above are:
k1 ¼ 4.2 ꢂ 10ꢁ1 minꢁ1 and k1 ¼ 7.1 ꢂ 10ꢁ1 minꢁ1 respectively; and
the half-lifes (t1/2) ¼ 1 min and (t1/2) ¼ 2 min respectively.
Thermal decolourisation rate for compound 2 and compound 3 in acetonitrile at
2 ꢂ 10ꢁ4 M and at 21 ꢀC.
Compound 3
Compound 2
t (min)
Open-form
Ln A
t (min)
Open-form
Ln A
absorbance
at 550 nm
absorbance
at 550 nm
Dark
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0.81
0.547
0.386
0.277
0.204
0.155
0.122
0.1
ꢁ0.21072
ꢁ0.60331
ꢁ0.95192
ꢁ1.28374
ꢁ1.58964
ꢁ1.86433
ꢁ2.10373
ꢁ2.30259
ꢁ2.4651
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
0.746
0.618
0.508
0.405
0.325
0.263
0.216
0.180
0.152
0.131
0.115
0.103
0.094
0.087
<0.02
ꢁ0.29303
ꢁ0.48127
ꢁ0.67727
ꢁ0.90387
ꢁ1.12393
ꢁ1.33560
ꢁ1.53248
ꢁ1.71480
ꢁ1.88387
ꢁ2.03256
ꢁ2.16282
ꢁ2.27303
ꢁ2.36446
ꢁ2.44185
2.3. Thermal decolourisation of compounds 2 and 3
The thermal decolouration reaction of compounds 2 and 3, at
concentrations of 2 ꢂ 10ꢁ4 M, were studied in the three solvents
detailed above, and at various temperatures (below), after irradi-
ating with UV light for 1 min. Post irradiation, absorbance, due to
the merocyanine structure, was continuously measured against
time and the appropriate graphs plotted (Graphs 1e6).
The thermal decolourisation rates (k1) for the compounds 2 and
3 at 2 ꢂ 10ꢁ4 M in acetonitrile, at a temperature of 21 ꢀC, were
calculated from gradients of the log graph i.e. graph 2, and the half-
lifes (t1/2) calculated from Graph 1. The calculated colourisation
0.085
<0.02
<0.02
The above experiments were repeated at 21 ꢀC in DCE and the
resulting graphs reproduced below.
Lastly the above experiments were repeated at 21 ꢀC in
methanol and the resulting plots reproduced below.
The kinetic data is reproduced in Table 1 to facilitate immediate
comparison of the data obtained, and the ensuing discussion.
The variation of thermal decolourisation rates (k1) in the three
solvents were compared and contrasted by plotting graphs of k1
against: 1), acetonitrile; 2), DCE and 3), methanol (below) for both
compounds 2 and 3 respectively. The Burdick and Jackson reference
polarity (dielectric) index for organic solvents quotes values for
acetonitrile, DCE and methanol of: 5.8, 3.5 and 5.1 respectively [22].
Graphs 1e2. Thermal decolourisation (Graph 1), and log absorbance (Graph 2) of
compounds 2 and 3 in acetonitrile at a concentration of 2 ꢂ 10ꢁ4 M at 21 ꢀC against
time.
Graphs 3e4. Thermal decolourisation (Graph 3), and log absorbance (Graph 4) of
compounds 2 and 3 in DCE at a concentration of 2 ꢂ 10ꢁ4 M at 21 ꢀC against time.

150
C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
Compound 2 decolourisation rate vs. solvent
1.4
1.2
1
0.8
0.6
0.4
0.2
0
DCE
Acetonitrile
Solvent
MeOH
Graph 7. Rates of thermal decolourisation (k1) for compound 2 in DCE, acetonitrile and
methanol solvents.
In summary, at a concentration of 2 ꢂ 10ꢁ4 M, and at 21 ꢀC, some
clear trends in the rates of thermal decolourisations (k1) for
compounds 2 and 3 are realized. This is clearly evident when
examining the above decolourisation rate (k1) vs. solvent plots.
For compound 2 the rate of thermal decolourisation varies
considerably with solvent. The fastest rate of decolourisation occurs in
the least polar solvent (DCE: polarity ¼ 3.5) and is almost 3 times
faster than in acetonitrile (polarity ¼ 5.8). This result is perhaps not
surprising since the rate of decolourisation is partly dependent on the
degree of stability exerted on the zwitterionic form by the solvent.
Acetonitrile being considerably more polar than DCE ’solution-stabi-
lizes’ the open-form considerably more than DCE by restricting and
consequently slowing cyclisation. Significantly reduced interactions
are expected in a relatively ’inert’ solvent, such as DCE, consequently
relatively uninhibiting cyclisation. When observing the thermal
re-cyclisation in methanol a significant reduction in rate is noted.
Whilst the reference polarity reported for methanol is 5.1, which is
similar to that of acetonitrile, the relative rate of thermal decolour-
isation is just 0.02 sꢁ1-21 times lowerethe largest relative difference
between all three solvents. This observation is perhaps not surprising
since methanol is additionally able to stabilize the open form through
hydrogen bonding, around both the phenoxide and iminium ions.
Consideration of compound 3 reveals a similar relative trend in
thermal decolourisation rates to those observed for 2, although as
expected, with a difference in absolute values. These differences in
absolute values, obviously arising from the differing functional
group substitutions, are discussed below.
Graphs 5e6. Thermal decolourisation (Graph 5), and log absorbance (Graph 6) of
compounds 2 and 3 in methanol at a concentration of 2 ꢂ 10ꢁ4 M at 21 ꢀC against time.
Whilst the polarity of the solvents would be expected to play
a significant part in influencing the equilibrium position of the spi-
robenzopyran ring-opening 4 closing reaction, due to stabilization
of the zwitterionic form, this is not the only important physical factor.
The ability of the solvent to hydrogen bond, particularly to the
merocyanine phenoxide ion, would possibly be at least as important,
and probably more so. Additionally, the ability of the iminium ion to
form a partial ion-pair bond (i.e. ¼ Nþ 4 X
d
ꢁ(X ¼ an electronegative
atom) or ¼ Nþ 4 lone pair) results in this moiety being capable of
bonding to solvent atoms possessing a dꢁ þ dipole-element, and/or
ed
a lone pair such as a nitrogen atom (cf. acetonitrile), or an oxygen
atom (cf. methanol). Therefore, overall, it is likely that a combination
of these major factors, and probably other parameters, will play
a significant role in influencing and dictating the spirobenzopyr-
an 4 merocyanine equilibrium constants-which are in addition to
the substituent effects (Graph 7).
Comparing and contrasting compounds 2 and 3: the rates of
thermal decolourisation (k1) for compound 3 is greater than that of 2
in CH3CN and DCE, but to significantly differing degrees. The largest
difference (a factor of w1.7 times (0.71 sꢁ1/0.4 minꢁ1)-and a relative
difference of w0.3 minꢁ1) is recorded in acetonitrile with the
difference in DCE being negligible (1.25 minꢁ1 vs. 1.24 minꢁ1). This
would indicate, in the absence of solvent effectseor at least
the relatively minimal ones that would be expected to occur in
DCEethat the thermal decolourisation rates of these two
compounds are almost identical, with 3 possessing a modestly
greater rate (1.25 minꢁ1 vs. 1.24 minꢁ1). Therefore the indications
are that in this solvent, and at the given temperature, the ring-
closing reaction is similarly biased for both gem-dimethyl and 3,30-
cyclohexyl-substituted spirobenzopyrans. Additionally, as the fastest
rate of decolourisation for the merocyanines is in the least polar
solvent (DCE); this would indicate less polar solvation and hence less
stabilization of the merocyanine Zwitterion is occurring (a more
polar solvent would have greater ability to solvate the Zwitterion-
stabilizing it-and consequently reducing its rate of decolourisation).
It is of note that the initial decolourisation rates (kinit) are almost
identical the overall decolourisation rate (k1), and consequently the
reverse reaction rate (colourisation) (k2) is near to zero (within
decimal place rounding). This indicates a very slow decolourisation
Table 1
Summary of kinetic data for compounds 2 and 3 in acetonitrile, DCE and methanol at
21 ꢀC at concentrations of 1 ꢂ 10ꢁ4 M.
Kinetic Data for compounds 2 and 3 in acetonitrile, DCE and methanol at 21 ꢀ
C
Structure
Solvent
k1 (minꢁ1
)
Half-life (t1/2)-min
CH3CN
DCE
MeOH
7.1 ꢂ 10ꢁ1
1
0.5
1.25
1.0 ꢂ 10ꢁ2
509.7 (Calc’d)
N
O
CH3
3
NO2
CH3CN
DCE
MeOH
4.2 ꢂ 10ꢁ1
1.24
2
0.65
49.51 (Calc’d)
H3C
N
CH3
2 ꢂ 10ꢁ2
O
CH3
2
NO2

C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
151
reaction with almost insignificant colourisation reaction occurring:
calculated half-lifes of w50 min for compound 2 and 510 min for
compound 3 would support this (Graph 8).
Compound 2 decolourisation rate vs. solvent
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Thermal decolourisation rate for compound 3 and compound 2 in DCE at 2 ꢂ 10ꢁ4
M
and at 21 ꢀC.
Compound 3
Compound 2
Dark
Open-form
Ln A
Dark
Open-form
Ln A
absorbance
at 550 nm
absorbance
at 550 nm
DCE
Acetonitrile
MeOH
t (min)
t (min)
Solvent
0
0.5
1
1.5
2
2.5
3
0.608
0.311
0.174
0.127
0.095
ꢁ0.49758
ꢁ1.16796
ꢁ1.7487
0
0.5
1
1.5
2
2.5
3
0.47
0.283
0.136
ꢁ0.75502
ꢁ1.26231
ꢁ1.9951
Graph 9. Initial (init) rates of thermal decolourisation (kinit) for compound 2 in DCE,
acetonitrile and methanol solvents.
ꢁ2.06357
ꢁ2.35388
hydrogen bond. Lastly, the fact that the rates of thermal decolour-
isation for compounds 2 and 3 are of a similar magnitude is perhaps
surprising. It is possible that the effects discussed above produce
a thermal energy leveling effect between the compounds at this
temperature, largely negating steric/substituent effects, which
explains why their rates are near parity.
However, in acetonitrile 3 undergoes thermal decolourisation
(k1 ¼ 0.71 minꢁ1) at w1.7 times the rate of 2 (k1 ¼ 0.42 minꢁ1
)
indicating a relatively less thermally stable merocyanine form. Thus,
in this case, with merocyanine possessing a bulky 3,30-cyclohexyl-
grouping, solvent effects appear to be operating. Any polar solvent
interactions are expected to be maximally centered around the
phenoxide and iminium ions. For both compounds 2 and 3 solvent
interactions around the phenoxide ions are expected to be similar (as
they are sterically similarly available), however differences would be
expected to arise around the iminium ions since 3 possesses a nearby
sterically bulky 3,30-cyclohexyl group, as opposed to 2 possessing
a gem-dimethyl group. Differences in solvation around the iminium
ion, and consequently the merocyanine form, are therefore expected
and hence probably a major cause of the differing thermal decol-
ourisation rates.
It can be seen from the graphs of initial decolourisation rates (kinit
)
that these, to a degree, parallel those for the apparent decolourisation
rates obtained from the log graphs. Some slight variations are noted,
as might be expected. The initial decolourisation rates for compound
2 in DCE and acetonitrile are considerably less (almost 70% and 60%
respectively) than the overall decolourisation rates (k1), and do not
undergo a similar magnitude drop in changing from DCE to acetoni-
trile solvents; the graph depicts an uplift rather than the observed
dropdown. However, interestingly, both rates compare favorably
when contrasted in methanol solvent. This indicates the significant
influence on the ring-opening reaction exerted by the solvent. For
compound 3 a similar trend is noted but the initial decolourisation
rates drop by margins of 50% and 25% respectively. Similar to
compound 2, an uplift in initial decolourisation rate for compound 3 is
also noted in acetonitrile, but in this case it is w30% as opposed to
almost 70%. Rates in methanol remained similarly low and mirrored
the pattern observed for compound 2. In summary, as might be
expected, substituent effects substantially influence the initial
decolourisation rates, except where solvent effects dominate, as is the
case of methanol-through polarity and hydrogen bonding (Graph 9).
As extremely low thermal decolourisation reaction rates were
observed in methanol at 21 ꢀC, postulated to be due to significant
solvent interactions, it was decided to investigate whether this could
be biased, or at least alleviated by conducting the same experiments
at a higher temperature (Graph 10). This increased temperature
should reduce the transitional energy barrier to decolourisation.
Additionally, reduction of these solvent interactions would poten-
tially allow one to observe and study the important alkyl-substituent
The largest changes in decolourisation rates (k1) occur in
methanol where they significantly reduce i.e. in the case of
compound 3 k1 is 125 times slower than in DCE (and 71 times
slower in acetonitrile); and in the case compound 2 62 times slower
(and 21 times slower in acetonitrile). The lower polarity of meth-
anol (5.1) than acetonitrile (5.8) is contrary to a concomitantly
lower decolourisation rate and cannot alone explain this significant
reduction. In this case it is expected that methanol, which possesses
significant hydrogen bonding capability, hydrogen bonds to the
phenoxide ion greatly inhibiting ring-closure. Additionally, the
oxygen lone-pairs of methanol can coordinate towards the iminium
ion, which offers further stabilization, limiting the ability to re-
cyclise (decolourise). The above explains why the decolourisation
rates are significantly lower in methanol-likely to be so in other
polar solventseand increasingly so in ones that can additionally
Compound 3 decolourisation rate vs. solvent
Compound 3 decolourisation rate vs. solvent
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.4
1.2
1
0.8
0.6
0.4
0.2
0
DCE
Acetonitrile
Solvent
MeOH
DCE
Acetonitrile
Solvent
MeOH
Graph 8. Rates of thermal decolourisation (k1) for compound 3 in DCE, acetonitrile and
Graph 10. Initial rates of thermal decolourisation (kinit) for compound 3 in DCE,
methanol solvents.
acetonitrile and methanol solvents.

152
C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
(continued )
Compound 3
Compound 2
Dark
Open-form
absorbance
at 550 nm
Ln A
Dark
Open-form
absorbance
at 550 nm
Ln A
t (min)
t (min)
12.5
13
13.5
14
14.5
15
15.5
16
16.5
17
17.5
18
18.5
19
19.5
20
0.47
ꢁ0.75502
ꢁ0.77003
ꢁ0.78307
ꢁ0.79629
ꢁ0.80968
ꢁ0.82554
ꢁ0.83933
ꢁ0.85332
ꢁ0.86750
ꢁ0.87948
ꢁ0.89404
ꢁ0.90634
ꢁ0.92130
ꢁ0.93649
ꢁ0.94933
ꢁ0.96233
ꢁ0.97551
ꢁ0.98886
ꢁ1.00239
ꢁ1.01611
12.5
13
13.5
14
14.5
15
15.5
16
16.5
17
17.5
18
18.5
19
19.5
20
0.657
0.645
0.638
0.632
0.626
0.620
0.614
0.607
0.602
0.596
0.590
0.584
0.579
0.573
0.567
0.562
0.557
0.551
0.547
0.541
ꢁ0.42007
ꢁ0.4385
0.463
0.457
0.451
0.445
0.438
0.432
0.426
0.42
0.415
0.409
0.404
0.398
0.392
0.387
0.382
0.377
0.372
0.367
0.362
ꢁ0.44942
ꢁ0.45887
ꢁ0.46840
ꢁ0.47804
ꢁ0.48776
ꢁ0.49923
ꢁ0.50750
ꢁ0.51751
ꢁ0.52763
ꢁ0.53785
ꢁ0.54645
ꢁ0.55687
ꢁ0.56740
ꢁ0.57625
ꢁ0.58519
ꢁ0.59602
ꢁ0.60331
ꢁ0.61434
20.5
21
21.5
22
20.5
21
21.5
22
Thermal decolourisation rate for compound
3
and compound 2 in MeOH at
2 ꢂ 10ꢁ4 M, and at 50 ꢀC.
Graphs 11e12. Thermal decolourisation (Graph 11), and log absorbance (Graph 12) of
compounds 2 and 3 in methanol at a concentration of 2 ꢂ 10ꢁ4 M at 50 ꢀC against time.
Compound 3
Compound 2
Dark
Open-form
absorbance
at 550 nm
Ln A
Dark
Open-form
absorbance
at 550 nm
Ln A
effects that are known to operate in the spirobenzopyran ring-
opening 4 closing process. To this end we conducted the above
experiments in methanol at a temperature of 50 ꢀC calculating the
thermal decolourisation rates utilizing Graphs 11 and 12. The results
are subsequently summarized and compared to those obtained at
21 ꢀC (Table 2).
t (min)
t (min)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
0.582
0.51
ꢁ0.54128
ꢁ0.67334
ꢁ0.81419
ꢁ0.95972
ꢁ1.09961
ꢁ1.23443
ꢁ1.36258
ꢁ1.48281
ꢁ1.58964
ꢁ1.6874
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
0.949
0.885
0.846
0.817
0.788
0.763
0.737
0.712
0.687
0.662
0.638
0.615
0.591
0.568
0.546
0.524
0.503
0.483
0.463
0.444
0.426
0.408
0.391
0.375
0.359
0.345
0.331
0.318
0.306
0.294
0.283
0.274
0.264
0.255
0.247
0.240
0.233
0.228
ꢁ0.05235
ꢁ0.12217
ꢁ0.16723
ꢁ0.20212
ꢁ0.23826
ꢁ0.27050
ꢁ0.30517
ꢁ0.33968
ꢁ0.37542
ꢁ0.41249
ꢁ0.44942
ꢁ0.48613
ꢁ0.52594
ꢁ0.56563
ꢁ0.60514
ꢁ0.64626
ꢁ0.68717
ꢁ0.72774
ꢁ0.77003
ꢁ0.81193
ꢁ0.85332
ꢁ0.89649
ꢁ0.93905
ꢁ0.98083
ꢁ1.02443
ꢁ1.06421
ꢁ1.10564
ꢁ1.14570
ꢁ1.18417
ꢁ1.22418
ꢁ1.26231
ꢁ1.29463
ꢁ1.33181
ꢁ1.36649
ꢁ1.39837
ꢁ1.42712
ꢁ1.45672
ꢁ1.47841
0.443
0.383
0.333
0.291
0.256
0.227
0.204
0.185
0.169
0.156
0.145
0.138
0.131
0.127
0.124
0.121
0.119
0.117
0.115
0.113
0.112
0.111
0.110
0.110
0.110
0.109
0.109
0.108
0.108
0.108
0.108
0.108
0.108
0.108
0.108
0.108
Thermal decolourisation rate for compound
3 and compound 2 in MeOH at
2 ꢂ 10ꢁ4 M, and at 21 ꢀC.
Compound 3
Compound 2
ꢁ1.77786
ꢁ1.8579
Dark
Open-form
Ln A
Dark
Open-form
Ln A
absorbance
at 550 nm
absorbance
at 550 nm
t (min)
t (min)
ꢁ1.93102
ꢁ1.9805
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
0.665
0.651
0.646
0.637
0.629
0.621
0.612
0.604
0.596
0.587
0.579
0.571
0.563
0.556
0.548
0.54
0.533
0.525
0.518
0.511
0.504
0.497
0.49
ꢁ0.40797
ꢁ0.42925
ꢁ0.43696
ꢁ0.45099
ꢁ0.46362
ꢁ0.47642
ꢁ0.49102
ꢁ0.50418
ꢁ0.51751
ꢁ0.53273
ꢁ0.54645
ꢁ0.56037
ꢁ0.57448
ꢁ0.58699
ꢁ0.60148
ꢁ0.61619
ꢁ0.62923
ꢁ0.64436
ꢁ0.65778
ꢁ0.67139
ꢁ0.68518
ꢁ0.69917
ꢁ0.71335
ꢁ0.72447
ꢁ0.74024
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
0.85
ꢁ0.16252
ꢁ0.18754
ꢁ0.20089
ꢁ0.21072
ꢁ0.21816
ꢁ0.22941
ꢁ0.23953
ꢁ0.24974
ꢁ0.26007
ꢁ0.27050
ꢁ0.27971
ꢁ0.29035
ꢁ0.29975
ꢁ0.31061
ꢁ0.32021
ꢁ0.32989
ꢁ0.34108
ꢁ0.34956
ꢁ0.36241
ꢁ0.36962
ꢁ0.37980
ꢁ0.39008
ꢁ0.39899
ꢁ0.40947
ꢁ0.41703
ꢁ2.03256
ꢁ2.06357
ꢁ2.08747
ꢁ2.11196
ꢁ2.12863
ꢁ2.14558
ꢁ2.16282
ꢁ2.18037
ꢁ2.18926
ꢁ2.19823
ꢁ2.20727
ꢁ2.20727
ꢁ2.20727
ꢁ2.21641
ꢁ2.21641
ꢁ2.22562
ꢁ2.22562
ꢁ2.22562
ꢁ2.22562
ꢁ2.22562
ꢁ2.22562
ꢁ2.22562
ꢁ2.22562
ꢁ2.22562
0.829
0.818
0.810
0.804
0.795
0.787
0.779
0.771
0.763
0.756
0.748
0.741
0.733
0.726
0.719
0.711
0.705
0.696
0.691
0.684
0.677
0.671
0.664
0.659
7.5
8
8.5
9
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
13
13.5
14
14.5
15
15.5
16
16.5
17
17.5
18
18.5
9.5
10
10.5
11
11.5
12
12.5
13
13.5
14
14.5
15
15.5
16
16.5
17
17.5
18
18.5
0.483
0.477

C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
153
decolourisation rates (k1) between the gem-dimethyl 2 (0.1 minꢁ1
)
(continued)
and 3,30-cyclohexyl
3
(0.42 minꢁ1
)
compounds (4.2 times
Compound 3
Compound 2
(0.42 minꢁ1/0.1 minꢁ1)) becomes apparent, highlighting the
expected substituent effects that were not observed at the lower
temperature. Further, the increased rate of decolourisation for the
3,30-cyclohexyl substituted compound vindicates the fact that steri-
cally bulky groups destabilize the open-zwitterionic form promoting
ring-cyclisation. It is also apparent from Graph 12 that compound 2
possesses a slight ’hyperbolic’ element. As discussed earlier, and
particularly at this higher temperature, the merocycanine form can
exist in a number of conformations. This results in the existence of
several simultaneous ring-closure reactions-with their concomitant
individual rate constants (fast and slow fading-rates)epotentially
resulting in an apparent non-linear rate. In addition, there exists the
possibility that one or more of the merocyanine rotamers possesses
a longer half-life, also potentially contributing to the explanation of
the ’hyperbolic-shaped’ graph.
Further, these rate constant results are supported and enforced
by the corresponding half-lifes, which for compounds 2 and 3 are
8.75 (k1 ¼ 1.0 ꢂ 10ꢁ1 minꢁ1) and 2.5 (k1 ¼ 4.2 ꢂ 10ꢁ1 minꢁ1) min,
respectively.
Overall the above findings will find useful practical implications
in the design of ’tunable’ photoreversible spirobenzopyran systems.
Additionally, by studying the same decolourisation reactions
under identical physical conditions, but at two different tempera-
tures, the activation energies (Ea’s) of spirobenzopyran formation
can be calculated using Eq (1):
Dark
Open-form
absorbance
at 550 nm
Ln A
Dark
Open-form
absorbance
at 550 nm
Ln A
t (min)
t (min)
19
19.5
20
0.108
0.108
0.108
ꢁ2.22562
ꢁ2.22562
ꢁ2.22562
19
19.5
20
20.5
21
21.5
22
22.5
23
23.5
24
24.5
25
25.5
26
26.5
27
27.5
28
28.5
29
29.5
30
0.217
0.212
0.208
0.204
0.203
0.198
0.195
0.193
0.192
0.190
0.188
0.187
0.186
0.185
0.184
0.183
0.182
0.182
0.182
0.181
0.181
0.181
0.180
0.180
0.181
0.181
ꢁ1.52786
ꢁ1.55117
ꢁ1.57022
ꢁ1.58964
ꢁ1.59455
ꢁ1.61949
ꢁ1.63476
ꢁ1.64507
ꢁ1.65026
ꢁ1.66073
ꢁ1.67131
ꢁ1.67665
ꢁ0.68201
ꢁ1.68740
ꢁ1.69282
ꢁ1.69827
ꢁ1.70375
ꢁ1.70375
ꢁ1.70375
ꢁ1.70926
ꢁ1.70926
ꢁ1.79026
ꢁ1.71480
ꢁ1.71480
ꢁ1.70926
ꢁ1.70926
30.5
31
31.5
Lnðk2=k1Þ ¼ ꢁ
D
Ea=R½ð1=T2Þ ꢁ ð1=T1Þꢃ
(1)
R (gas constant) ¼ 8.314 J molꢁ1 Kꢁ1; T ¼ 21 ꢀC (294 K) and 50 ꢀC
(323 K); k2 and k1 are the decolourisation rate constants (sꢁ1) at the
different temperatures (T) in Kelvin (K).
It is clear that raising the temperature produces three significant
effects: 1) the initial rates of decolourisation (kinit) for compounds 3
and 2 are significantly highere14 times (0.14 minꢁ1 vs. 0.01 minꢁ1
)
For compounds 2 and 3 the positive activation energies are 44
and 102 kJ molꢁ1 respectively. The relatively lower activation
energy for 2, as compared to 3 (a ratio of 2.3 times) indicates 2 is
relatively more stable than 3. This is probably due to lower steric
strain operating in the closed-form of the gem-dimethyl compound
as compared to the 3,30-cyclohexyl analogue. Additionally, this
supports, and closely parallels the observed relative increased rates
of decolourisation values for the two compounds (described later)
i.e. value of 0.19 sꢁ1 for the 3,30-cyclohexyl compound 3 as opposed
to 0.02 sꢁ1 for the gem-dimethyl-substituted compound 2ea ratio
of 9.5 times for the more highly substituted compound. Again, this
is probably due to the greater relief in steric strain experienced
by the more highly substituted compound-in undergoing ring-
opening.
and 6 times (0.12 minꢁ1 vs. 0.02 minꢁ1) respectively, at 21 ꢀC; 2)
significantly higher thermal rate constants (k1) for compounds 3
(0.42 minꢁ1) and 2 (0.1 minꢁ1) are noted atꢁt1his temperature, as
opposed to that at 21 ꢀC (0.01 and 0.02 min respectively). This
calculates to 42 times (0.42 minꢁ1/0.01 minꢁ1) and 5 times
(0.1 minꢁ1/0.02 minꢁ1) respectively; and 3) a significant difference in
Table 2
Summary of kinetic data compounds for 2 and 3 in methanol at 50 ꢀC.
Kinetic Data for Compounds 2 and 3 in methanol at 50 ꢀC and 25 ꢀC
Structure
50 ꢀC
25 ꢀC
Lastly, the Gibbs free energy (G in kJ) for compounds 2e5 at
21 ꢀC can be deduced from their calculated equilibrium constants
(K) (Note: although the equilibrium constants are calculated later in
this paper (Table 5) it is sensible to calculate and reproduce the
Gibb’s free energy values at this point) using Eq (2):
k1 (minꢁ1) Half-life k1 (minꢁ1) Half-life
(t1/2)-min
(t1/2)-min
N
509.7 (Calc’d from
4.2 ꢂ 10ꢁ1 2.5
1.0 ꢂ 10ꢁ2
O
lnK ¼ ꢁ½
D
G=RTꢃ
(2)
Ln2/1.36 ꢂ 10ꢁ3
)
CH3
R (gas constant) ¼ 8.314 J molꢁ1 Kꢁ1; T ¼ 21ꢀC (294 K) and 50 ꢀC
3
NO2
(323 K).
The calculated values for G (kJ) for compounds 2e5 are: ꢁ0.662;
ꢁ3.036; 2.872 and 0.840 respectively. Examination of these values
enables the following relative ease of colourisation reaction order
to be established:
H3C
N
CH3
49.51 (Calc’d from
O
1.0 ꢂ 10ꢁ1 8.75
2.0 ꢂ 10ꢁ2
Ln2/5.50 ꢂ 10ꢁ4
)
CH3
3 > 2 > 5 > 4
2
NO2
This order exactly parallels the calculated trend (described later)
for the colourisation reaction equilibrium constants.

154
C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
Table 3
Using these Gibb’s free energy values we are also able to
calculate the entropy of formation (S, J Kꢁ1) for compounds 2 and 3
Summary of thermal colourisation kinetic data for compounds 2, 3, 4
and 5 in methanol at 21 ꢀC in methanol.
using the Eq (2):
Structure
H3C
k2 (minꢁ1
)
D
G ¼
D
H ꢁ T
D
S
(3)
CH3
Values for the entropies of formation (J Kꢁ1) for 2 and 3 at 21 ꢀC
are: 2060 and 4713 respectively. Therefore, formation of compound
3 is more than twice as entropically favorable as that of compound
2, which is in line with the free energy of formation values.
N
2.0 ꢂ 10ꢁ2
O
CH3
2
NO2
2.4. N-Hexyl-3,30-substituted-spirobenzopyrans
If a truly on 4 off UV-activated switchable photochromic system
is required then, in these structures, there exists a need for the
development of systems with larger energy barriers between the
various photoisomers. Therefore, photochromic systems possessing
additional sterically bulky and hindering groups were examined in
order to obtain an understanding of what effect these groups have
on the ease of the ring-closing 4 opening reaction. We therefore
decided to turn our attention towards the investigation of a further
two novel spirobenzopyrans 4 and 5, which in addition to containing
3,30-gem-dimethyl and 3,30-cyclohexyl-groups, possessed the steri-
cally bulky N-hexyl moiety. [It is noteworthy at this point to refer-
ence the important contribution made by Gabbutt and coworkers
who synthesised a number of alternative N-alkyl-containing indo-
lenine [23] intermediates, which were subsequently used to produce
a variety of photochromic spiroindolinonaphthoxazines]. These
were prepared in order to specifically investigate what effect the N-
hexyl-group exerts on the rates of colourisation (k2), decolourisation
(k1); and hence the resulting equilibrium constants (K). These
new compounds were subsequently compared and contrasted
against their structurally equivalent congenersemaintaining the
same 3,30-gem-dimethyl 2 or 3,30-cyclohexyl-3 substitution pattern-
but possessing an N-methyl group. In order to undertake these
comparisons a second set of thermal colourisation and decolour-
isation data sets, containing a similar number of data points, were
experimentally obtained: These new data sets allowed a more
practically convenient comparison. [Note: it was not practicable
to record as many measurements in these experiments due to time
constraints. This produced some difference in the values of k1 as
compared to those previously obtained when utilizing a far greater
number of data points. However, this offered a better relative
comparison]. The plots of absorption against time, for both the
colourisation and decolourisation reactions, and subsequently the
corresponding log rates against time, for compounds 2 and 3, are
thus revisited utilising these data sets. Similar data sets were also
experimentally obtained for the N-hexyl-substituted compounds,
which were subsequently subjected to the same graphical plots as
those of the N-methyl-substituted compounds. The ensuing results
for all four compounds, specifically; the rates of colourisation (k2),
decolourisation (k1), and calculated equilibrium constants (K), were
compared and contrasted. Some significant trends are observed
which are subsequently graphically represented via- scatter plots.
Firstly, the thermal colourisation reactions of compounds 2, 3, 4
and 5 in methanol at 21 ꢀC, and a concentration of 2 ꢂ 10ꢁ4 M were
considered. The data for the thermal colourisation reactions are
summarized below in Table 3.
N
O
1.9 ꢂ 10ꢁ1
CH3
3
NO2
H3C
N
CH3
O
5.0 ꢂ 10ꢁ2
C6H13
4
NO2
N
2.6 ꢂ 10ꢁ2
O
C6H13
5
NO2
Fig. 2c;-all linking bonds being trans(t)ꢁ16 i.e. (t,t,t) (Note: In
practice, the real structure will have relative partial, and opposite
charges (dþ and
d
ꢁ) localised on the indolium-nitrogen and
phenolic-oxygen atoms, respectively). Electronic conjugation
between the dipolar zwitterionic (Fig. 2c) and polar polyenic, or
quinonic (Fig. 2d) forms permits the alternating formation of the
three single bonds (n)-sandwiched between the indolinium and
phenoxide-rings (Fig. 2e). Concomitant rotation about these three
single bonds can potentially result in the formation of 2n theoretical
stereoisomers. Whilst some of these isomers are clearly more
thermodynamically stable than others, the isomer formed after
initial cleavage of the spirocyclic carbon-oxygen bond, or immedi-
ately prior to cyclisation, must, skeletally, be that depicted in Fig. 3
(l)ewhich has support [24]. Previous reports also enforce the
theoretical presence of eight potential isomersefour transoid- and
four cisoid-forms; with the relatively high energy cis-forms not
significantly proportionally contributing to the overall to the
equilibrium isomeric mixture [25]. Therefore, the isomers of rela-
tively higher importance in the photochromic process are those
possessing a transoid-structure.
Considering the observation that the thermal colourisation rate
plots-for all four compounds-appear to deviate from linearity
would seem to indicate the existence of a more complex ring-
opening scenario, as compared against the log rate decolourisation
plots for some of the same compounds discussed earlier. Thus, in
the case of colourisation it is postulated that one or more of the
intermediate mesomeric isomers-depicted in figs f-m below-are
implicated to a relatively greater extent. The existence of one or
Examination of the thermal log rate colourisation graphs indi-
cate some deviation from linearity (overall the correlation coeffi-
cients display values in the range of 0.94 to w0.99). This indicates
that a more complex (not a simple unimolecular spirocyclic ring-
opening) scenario is operating. The thermal colourisation process
involves initial cleavage of the spirocyclic carbon-oxygen bond
’eventually’ generating the trans-merocyanine structure depicted in

C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
155
more intermediate mesomeric isomers would probably indicate
they possess a relatively increased thermodynamic stability, and
hence possess a longer transient lifetime. This postulation can be
reinforced since there have been previous literature reports of
non-linear plots [26]. In these instances their non-linearity was
ascribed to the existence of short and long-lived transient inter-
mediates which were attributed to ’fast and slow fading-rates’.
Further, these non-linear plots have subsequently been resolved
into slow and fast-fading first-order rates. The above discussion
perhaps explains why, in these particular cases, a slight deviation
from a typical linear rate plot is observed.
We subsequently considered the reverse process of thermal
decolourisation for compounds 2, 3, 4 and 5 under identical
physical parameters. The results are summarized below in Graphs
13 and 14 and Table 4 below Graph 15 and 16.
Following the log concentration plots, and determination of the
colourisation (k2) and decolourisation (k1) rates, enabled calcula-
tion of the equilibrium constants K (¼k2/k1) for compounds 2, 3, 4
and 5efor the overall colourisation reaction.
2.5. Thermal decolourisaton rates (k1)
The rates of thermal decolourisation (ring-cyclisation rate-k1)
for compounds 2, 3, 4 and 5 in methanol at 21 ꢀC are 1.6 ꢂ 10ꢁ2
,
5.5 ꢂ 10ꢁ2, 8.4 ꢂ 10ꢁ2 and 6.9 ꢂ 10ꢁ2 minꢁ1 respectively; which are
all of the same order of magnitude. These produce the increasing
thermal decolourisation rate trend depicted below:
4 > 5 > 3 > 2
Two observations become immediately noticeable: 1); the
N-hexyl groups promote ring-cyclisation over their equivalent
N-methyl substituted compounds as both 4 and 5 possess higher
decolourisation rates; and 2) the general increase in steric bulk,
through alkyl-substituent does not lead to a general increase in
decolourisation rate e.g. the more highly substituted 3,30-cyclo-
hexyl-N-hexyl-substituted compound 5 possesses a slower rate
than its equivalent gem-dimethyl- congener 4. However, to the
contrary, this is true for the N-methyl-substituted compounds 2
and 3 which would indicate a more complex scenario than simply
the position and degree of alkyl-substitution.
The maximal decolourisation rate is noted for the 3,30-gem-
dimethyl-N-hexyl derivative 4, being approximately 22% higher
than its 3,30-cyclohexyl-N-hexyl-substituted congener 5. In this
case it is probable that the steric bulk associated with the 3,30-
cyclohexyl-group relatively promotes ring-opening to the mer-
ocyanine over that of gem-dimethyl-substituted compound 4.
Thus it is thermodynamically more favorable to remain in the open-
form, hence the lower rate of ring-closure (decolourisation). It is,
alternatively, and perhaps additionally possible that the mer-
ocyanine of compound 4, being relatively more planar than 5 is able
to exist or form an initial micelle-type structureeoriginating
around the n-hexyl grouping. This partly prevents solvation stabi-
lization by the methanol, thus increasing its rate of intramolecular
cyclisation. If not quite a micelle-type structure then the more
planar merocyanine, possessing long N-hexyl-chains, could lie in
a slightly staggered and stacked layer-like structure. The zwitter-
ionic moieties could additionally lie in a planar orientation, but
with the iminium and phenoxide ionic charges of each zwitterion
lying in such way that each molecules alternate counter-ions are
matched and paired. Additionally, there would be Van der Waal’s
bonding between the layers of hexyl-chains. Overall this structure
could, similarly, relatively promote intramolecular ring-cyclisation
(decolourisation), by, again, inhibiting the ability of the methanol
molecules to solvate the associated zwitterionic charges. In the case
Graphs 13e16. Thermal colourisations (Graph 13 and Graph 14) and log absorbance
(Graphs 15 and 16) of compounds 2, 3, 4 and 5 in methanol at a concentration of
2 ꢂ 10ꢁ4 M at 21 ꢀC against time (note: the four compounds had to be represented on
two different graphs due to the scale).

156
C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
Table 4
Table 5
Summary of thermal decolourisation kinetic data for compounds for
2, 3, 4 and 5 at 21 ꢀC in methanol.
Determination of the equilibrium constants (K) for compounds 2, 3, 4 and 5 in
methanol at 21 ꢀC for the overall colourisation reaction.
Structure
H3C
k1 (minꢁ1
)
Structure
Colouration
k2 (minꢁ1
(previous tables) (previous tables)
Decolouration
K (Cal’d from
(k2/k1))
)
k1 (minꢁ1
)
CH3
H3C
CH3
N
1.6 ꢂ 10ꢁ2
O
CH3
N
NO2
2.1 ꢂ 10ꢁ2
1.6 ꢂ 10ꢁ2
1.31
O
2
CH3
2
NO2
N
O
5.5 ꢂ 10ꢁ2
CH3
N
O
1.9 ꢂ 10ꢁ1
5.5 ꢂ 10ꢁ2
3.45
3
NO2
CH3
3
NO2
N
7.0 ꢂ 10ꢁ2
O
C6H13
N
5.0 ꢂ 10ꢁ2
7.0 ꢂ 10ꢁ2
0.71
5
O
NO2
C6H13
5
NO2
H3C
CH3
H3C
CH3
N
8.4 ꢂ 10ꢁ2
O
C6H13
N
2.6 ꢂ 10ꢁ2
8.4 ꢂ 10ꢁ2
0.31
O
4
NO2
C6H13
4
NO2
of its 3,30-cyclohexyl-substitued congener it is probable that the
extra steric bulk associated with the cyclohexyl group (which
would be expected to sit in the relatively, spatially expansive, open
chair- conformation) would prevent, or greatly inhibit these types
of effects. The resulting structure would thus be more open and
therefore exposed to stabilizing solvent interactions (polar and
hydrogen bonding), which would reduce its relative rate of decol-
ourisation (Graph 17e20).
pyramidal inversion relieving steric inhibition of phenoxide attack
at the base of the iminium-ion’s carbon atom (Graph 21).
2.6. Thermal colourisation rates k2
The rates of thermal colourisation (ring-opening) k2 for
compounds 2, 3, 4 and 5 in methanol at 21 ꢀC are 2.1 ꢂ 10ꢁ2
,
1.9 ꢂ 10ꢁ1, 2.6 ꢂ 10ꢁ2 and 5.0 ꢂ 10ꢁ2 minꢁ1 respectively; which, as
opposed to the above decolourisation rates, exhibit a difference in
the order of magnitude for one structure (3). These compounds
produce the increasing thermal colourisation rate trend depicted
below:
The merocyanine of the N-methyl-3,30-cyclohexyl-substitued
compound 3 possesses a zwitterionic structure which is more
stable in methanol than its N-hexyl equivalent 5 (rates of
decolourisation: 5.5 ꢂ 10ꢁ2 (3) vs. 7.0 ꢂ 10ꢁ2 minꢁ1 (5)). This is
probably due to the simple fact that it is more polar and
hydrogen bond solvated, the latter exerting the greatest effect on
decolourisation rates. Again, as discussed above, the larger
degree of steric bulk associated with 5, relative to compound 3,
limits the degree of solvent interactions. Also, the molecule
might be expected to form a ’lipophilic shell’ which would
encourage intramolecular cyclisation to a relatively larger degree
than that of 3.
The slowest decolourisation rate, and by the largest factor
(3.4 times), is observed for the least alkyl-substituted compound 2.
This is probably due to the fact that its zwitterionic structure is the
most relatively solvated, and hence solution (methanol) stable, of
all the four compounds considered herein. It is also likely that
the N-alkyl group, being either a methyl or an n-hexyl, makes
less relative difference to the ring-closure rate. This is because the
N-alkyl-during the ring-closing transition state-can undergo
3 > 5 > 4 > 2
Some clear observations are noted. The 3,30-cyclohexyl-
substituted compounds 3 and 5 undergo a faster ring-opening
than their gem-dimethyl substituted structural equivalents 4 and
2-approximately 9 times faster in the case of 3, and 2.4 times in the
case of 5. In this case it is likely that the additional steric bulk of the
cyclohexyl-substitution promotes ring-opening (colourisation) with
the resulting relief in steric strain acting as the intramolecular driving
force. Compound 5, in comparison to compound 3, additionally
possesses an N-hexyl group. This N-hexyl substitution results in
a colourisation rate approximately 4 times slower than compound 5
(0.19 minꢁ1 (3) vs. 0.05 minꢁ1 (5)). The slower rate of colourisation,
due to N-alkyl substitution, may arise from the increased steric bulk
of the open-form which reduces solvation (methanol) stabilization
relative to the less alkyl-substituted structure (3).

C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
157
R1
R1
R2
R2
Decolourisation rate vs. compounds 2, 3, 4 & 5
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
Thermal
decolouration (k )
R3
1
N
N
R5
+
O
-
5 R
O
2a-5a
R3
R4
2-5
4 R
2
3
4
5
R1 =R2 =CH3 , R3=NO2, R4=H, R5=CH3
R1 /R2 =cyclohexyl, R3=NO2, R4=H, R5=CH3
R1=R2=CH3, R3=NO2, R4=H, R5=N-C6H13
R1/R2=cyclohexyl, R3=NO2, R4=H, R5=N-C6H13
2
3
5
4
Compounds
Graph 21. Rate of thermal decolourisations (k1) plotted against their corresponding
compounds; 2, 3, 4 and 5 in methanol at 21 ꢀC.
The ring-opening steric effects exerted by the 3,30-cyclohexyl
grouping clearly outweigh that of the reduced solvent stabilization,
due to the N-hexyl group, as 5 possesses a decolourisation rate of
approximately twice that of 4eboth structures possessing N-hexyl
groups. On a reduced solvation, due to the alkyl- substitution
argument, 5 would be expected undergo colourisation slower than
4, however the reverse is true. This verifies the greater effect on
ring-opening is steric, and exerted by the 3,30-cyclohexyl group.
This is also vindicated when comparing structures 3 and 2, differing
only in structure by the virtue of a 3,30-cyclohexyl moiety vs. a gem-
dimethyl substitution. Here the solvation effect differences affected
by the N-hexyl groups are negated and emphasise the magnitude of
the steric effects induced on substitution of the 3,30-cyclohexyl-
group. This produces a colourisation rate 9 times faster than the
gem-dimethyl substituted structure 2. Structure 4 possesses the
third fastest colourisation rate (2.6 ꢂ 10ꢁ2 minꢁ1), and is 1.2 times
faster than 2ethe least alkyl-substituted compound. The difference
is relatively small when compared to the other observed differ-
ences and is probably, in the main, due to an N-hexyl solvation
stability effect. This is because the N-hexyl group substitution is
unlikely to sterically greatly affect the course of the ring-opening
reaction, particularly as pyramidal inversion through the nitrogen
atom can partially relieve experienced steric strain.
In summary the ring-opening process is complicated and
multifactorial, however the gross observed trends can be suitably
explained (Graph 22).
2.7. Equilibrium constants (K)
The equilibrium constants (Kedefined here as k2/k1) for
compounds 2, 3, 4 and 5 in methanol at 21 ꢀC are: 1.3, 3.45, 0.31 and
0.71 respectively. This enables establishment of the order depicted
below:
3 > 2 > 5 > 4
Examination of the structures (see overleaf) and relating them
to their equilibrium constants enables one to acknowledge some
clear trends, and to draw some firm and practically useful
conclusions.
Both N-methyl substituted compounds 3 and 2 possess similarly
relatively high equilibrium constants (3.46 and 1.31 respectively).
In contrast, both N-hexyl substituted compounds 5 and 4 possess
similar, but significantly relatively lower equilibrium constants
(0.71 and 0.31 respectively) (Graph 23).
Graphs 17e20. Thermal decolourisations (Graphs 17 and 18) and log absorbance
(Graph 19 and 20) of compounds 2, 3, 4 and 5 in methanol at a concentration of
2 ꢂ 10ꢁ4 M at 21 ꢀC against time.

158
C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
3. Summary and conclusions
Colourisation rate vs. compounds 2, 3, 4 & 5
0.2
0.15
0.1
The thermal decolourisation rate of the 3,30-cyclohexyl-
substituted spirocyclic-compound 3 is greater than that of the
simple gem-dimethyl-substituted spirocyclic-compound 2. This
trend is consistent in dichloroethane and acetonitrile solvents, but
not methanol where the rates are similarly lowethis is probably due
to a combination of solvation, and particularly hydrogen bonding,
which creates a ’rate-levelling effect’. In general, the more polar the
solvent, the slower the rate of merocyanine decolourisation. An
increase in the temperature (methanol at 50 ꢀC) was accompanied
by a faster rate (k1) of decolourisation (ring-cyclisation)ealmost an
0.05
0
2
4
5
3
Compounds
Graph 22. Rate of thermal colourisations (k2) plotted against their corresponding
compounds; 2, 3, 4 and 5 in methanol at 21 ꢀC.
order of magnitude-for both compounds 3 (1.4 ꢂ10ꢁ1 vs.1.0 ꢂ 10ꢁ2
)
and 2 (1.2 ꢂ 10ꢁ1 vs. 2.0 ꢂ 10ꢁ2).
The trends within the N-methyl and N-hexyl-substituted series
follow a pattern in that in both cases the equilibrium constants for
the 3,30-cyclohexyl-substituted compounds are correspondingly
higher than their gem-dimethyl congeners i.e. 3.35 (3) vs. 1.31 (2)
(a ratio of 2.6) and 0.7 (5) vs. 0.31 (4) (a ratio of 2.6) respectively. The
ratios exhibit remarkable similarity, indicating a ’constant effect’ on
substitution of a cyclohexyl group for a gem-dimethyl. This is
probably due to the significant relief in steric strain, present in the
sterically crowded cyclohexyl-substituted closed-forms, obtained
on ring-opening (in a methanolic solution at 21 ꢀC).
The fact that there is such a relatively large difference between
the N-hexyl and N-methyl-substituted compounds indicates that
there is, potentially, at least one particularly important additional
factor operating. For example, the equilibrium constant of the
N-methyl-3,30-cyclohexyl substituted compound 3 (K ¼ 3.45) is
almost 5 times that of the N-hexyl-3,30-cyclohexyl substituted
compound 5 (K ¼ 0.71). This could possibly be due to the fact that
the more highly alkyl-substituted N-hexyl derivative exhibits
reduced solvation stabilization of the zwitterion, due to the
increased steric bulk associated with this group. It is also possible
that the closed-form of 5 is generally relatively more lipophilically
stable-through the operation of Van der Waal’s bonding (being
more highly alkyl-substituted)-as compared to the closed-form of
the corresponding N-methyl congeners. Similar observations are
also noted for the compounds 2 (K ¼ 1.31) and 4 (K ¼ 0.31), the ratio
in this case being 4.2 times, remarkably similar, and enforcing the
trend noted for compounds 3 and 5.
The thermal decolourisation of both N-hexyl-substituted
compounds 4 and 5 occurs faster than the N-methyl substituted
compounds 2 and 3 indicating relative instability of the mer-
ocyanine forms; or increased relative thermal stability of the
corresponding spirocyclic structures, in methanol (lower solvation
stability due to greatly increased alkyl-substitution).
The rates of thermal colourisation were significantly dominated
by substitution of a 3,30-cyclohexyl group which, through the
release of steric strain, drove the ring-opening reaction. In this case
the N-hexyl-substitutions produced relatively decreased effects.
Explanations have been postulated for these observed trends.
In several instances the experimentally determined log rate vs.
time plots exhibited notable deviations from the expected linearity
of unimolecular, or first-order, kinetics. This indicates the occur-
rence of fast and slow fading-rates strongly suggesting the presence
of several intermediate merocyanine isomers. Previous reports
have confirmed these observations and resolved the kinetics into
fast and slow fading first-order reaction rates.
This has, in the case of several of the spirobenzopyrans
discussed herein, been offered as an explanation for the slight
deviation from non-linear plots (overall correlation coefficients
exhibited a range of 0.94 to w 0.99).
Examination of the equilibrium constants (K) reveal a clear
trend which enables one to predict how substituent effects may
bias and influence the overall equilibria in these systems.
The trend observed in the Gibbs free energy values enforces and
parallels that of the equilibrium constants.
Equilibrium constants (K) vs. compounds 2, 3, 4 & 5
4
3.5
3
2.5
2
1.5
1
0.5
0
4
5
2
3
Compounds
>
>
>
N
N
N
N
O
O
O
O
NO2
NO2
NO2
NO2
Graph 23. Equilibrium constants (K) for the spirobenzopyran ring-opening 4 closing reaction plotted against their corresponding compounds; 2, 3, 4 and 5 in methanol at 21 ꢀC.

C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
159
To conclude, the above clearly demonstrates the thermodynamic
control that appropriate functional group substitutions can exert on
the spirobenzopyran ring-opening 4 closing reaction, and their
magnitude. Additionally, significant and predictable solvent effects are
observed, which in turn can be practically used to bias the equilibrium
position within these systems. Further, changes in temperature
also allow one to bias the concentrations of spirobenzopyran and/
or zwitterionic forms. Overall, the above physical parameters, and
variations of, allows one to significantly influence and control the
interconversion of these systems - obviously important in the
design of clean on 4 off ionic spirobenzopyran-based switches-when
required. Ultimately, establishment of how these physical parameters
can qualitatively and quantitatively influence the equilibrium position
within these systems is extremely important. Further, this method-
ology can potentially be transferred, incorporated and applied into the
design of on 4 off ionic switches, and e.g. ion-chelating systems,
including those that additionally possess lariat-type ether appendages
etc, ea very useful ability.
4.1.1. Thermal decolourisation
The thermal decolourisation of the compounds at 1 ꢂ 10ꢁ4 M in
acetonitrile, DCE and methanol was observed principally after solu-
tions had been irradiated with the UV lamp for 1 min. Immediately
after photoirradiation the progressively decreasing merocyanine
concentration/absorbance at w
l
¼ 550 nm was continuously moni-
tored, at 21 ꢀC (also at 50 ꢀC for methanol), against time.
Freshly prepared 1 ꢂ 10ꢁ4 M solutions of the compounds were
prepared in the dark (allowing pre-photoirradiation equilibration)
in the following solvents: (a) acetonitrile (b) DCE and (c) MeOH.
Specifically, the position of the equilibria were measured using
UV spectroscopy adopting the following protocol:
(a) The solutions were left to pre-equilibrate for 1 h in the dark
prior to photoirradiation.
(b) The solutions were then exposed to UV light using a focused
200 W high pressure mercuryexenon light source, for 1 min-
promoting formation of the merocyanine form.
(c) Continuous measurement of the reducing merocyanine
concentration/absorption at (
l
¼ 550 nm) was undertaken.
4. Experimental section
4.1. Photoirradiation studies
4.1.2. Thermal colourisation
The photochromic properties of these compounds were evalu-
ated by preparing solutions (2 ꢂ 10ꢁ5 mol dmꢁ3 unless otherwise
stated), in the ’dark’, in freshly dried and redistilled solvents
(generally acetonitrile, dichloroethane (DCE) or methanol).
Perchlorate salts were dried over P2O5 for several days before use.
Solutions were placed in a stoppered 10 mm pathlength cuvette at
room temperature (20e25 ꢀC) and allowed to equilibrate for 1 h
before measurement of their UV/Vis absorption curves (’dark’
curves). The solutions were then irradiated for 1 min with UV light
The thermal colourisation process of the compounds at
1 ꢂ10ꢁ4 M in methanol were observed principally after the solutions
had been irradiated with visible light for 3 min (promoting forma-
tion of the closed-from). Immediately after photoirradiation the
progressively increasing merocyanine concentration/absorbance at
w
l
¼ 550 nm was continuously monitored, at 21 ꢀC against time.
Freshly prepared 1 ꢂ 10ꢁ4 M solutions of the compounds were
prepared in the dark (allowing pre-photoirradiation equilibration).
Specifically, the position of the equilibria were measured using
UV spectroscopy adopting the following protocol:
of
l
¼ 365 nm generated from a steady power source. The UV light
source was a 200 W mercury/xenon lamp, focussed in a LOT-Oriel
air-cooled lamp housing, with solution filters1 to eliminate light of
(a) The solutions were left to pre-equilibrate for 1 h in the dark
prior to photoirradiation.
400>l > 320 nm (this allows photoirradiation with a lmax centered
(b) The solutions were exposed to broad band visible light for
3 minepromoting formation of the closed-form.
(c) Continuous measurement of the increasing merocyanine
at 365 nm, the absorption wavelength of the spirobenzopyrans, and
additionally avoids photoirradiation of the formed merocyanine,
which has a lmax centered at 550 nm). The UV absorption spectra
(UV curve) were measured; this was followed by exposure of the
cuvette to a visible light source (3 min: 100 W tungsten spotlight)
and the UV absorption spectrum remeasured Fig. 1.
concentration/absorption at (
l
¼ 550 nm) was undertaken.
Thermal decolourisation and colourisation UV studies on 2, 3, 4
and 5.
Variable solvent and temperature Studies using UV Spectroscopy.
Thermal decolourisation rate for compound 3 and compound 2, compound 4 and compound 5, in MeOH at 2 ꢂ 10ꢁ4 M and at 21 ꢀC.
t (min)
Compound 2
Compound 3
Compound 4
Compound 5
Open-form absorbance
at 550 nm
Ln A
Open-form absorbance
at 550 nm
Ln A
Open-form absorbance
at 550 nm
Ln A
Open-form (A)
Ln A
0
2
4
6
0.648
0.622
0.603
0.583
0.57
0.556
0.542
0.53
ꢁ0.434
ꢁ0.475
ꢁ0.505
ꢁ0.540
ꢁ0.563
ꢁ0.600
ꢁ0.612
ꢁ0.639
ꢁ0.670
ꢁ0.689
0.429
0.388
0.370
0.354
0.339
0.327
0.314
0.303
0.291
0.280
ꢁ0.901
ꢁ0.947
ꢁ0.994
ꢁ1.038
ꢁ1.080
ꢁ1.120
ꢁ1.160
ꢁ1.190
ꢁ1.211
ꢁ1.273
0.502
0.472
0.459
0.446
0.433
0.423
0.416
0.409
0.404
0.397
ꢁ0.68916
ꢁ0.75078
ꢁ0.77871
ꢁ0.80744
ꢁ0.83702
ꢁ0.86038
ꢁ0.87707
ꢁ0.89404
ꢁ0.90634
ꢁ0.92382
0.403
0.369
0.346
0.327
0.310
0.290
0.278
0.263
0.253
0.244
ꢁ0.90882
ꢁ0.99696
ꢁ1.06132
ꢁ1.11780
ꢁ1.17118
ꢁ1.23787
ꢁ1.28013
ꢁ1.33560
ꢁ1.37437
ꢁ1.41059
8
10
12
14
16
18
0.517
0.502
1
A 1ꢀM cobalt and copper sulphate solution (1:1) contained within a 2ꢀmm
walled PyrexÒ glass cuvette was prepared: This dual combination of a solution and
a
glass filter possesses an irradiation window between 320-400ꢀnm, with
unwanted wavelengths outside this range effectively filtered out.

160
C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
Thermal colourisation rate for compound 3 and compound 2, compound 5 and compound 4, in MeOH at 2 ꢂ 10ꢁ4 M and at 21 ꢀC.
t (min)
Compound 2
Compound 3
Compound 4
Compound 5
Open-form absorbance
at 550 nm
Ln A
Open-form absorbance
at 550 nm
Ln A
Open-form absorbance
at 550 nm
Ln A
Open-form absorbance
at 550 nm
Ln A
0
2
4
6
0.046
0.049
0.054
0.056
0.057
0.059
0.063
0.065
0.066
0.067
ꢁ3.07911
ꢁ3.01593
ꢁ2.91877
ꢁ2.8824
0.02
0.025
0.03
0.035
0.039
0.043
0.046
0.047
0.048
0.048
ꢁ3.67
0.044
0.055
0.065
0.074
0.081
0.09
0.099
0.104
0.11
ꢁ3.005
0.057
0.06
0.064
0.07
0.074
0.078
0.081
0.084
0.087
0.089
ꢁ2.8647
ꢁ3.6
ꢁ2.90042
ꢁ2.73337
ꢁ2.60369
ꢁ2.51331
ꢁ2.40795
ꢁ2.31264
ꢁ2.26336
ꢁ2.20727
ꢁ2.16282
ꢁ2.81341
ꢁ2.74887
ꢁ2.65926
ꢁ2.60369
ꢁ2.55105
ꢁ2.51331
ꢁ2.47694
ꢁ2.44185
ꢁ2.41912
ꢁ3.50656
ꢁ3.35241
ꢁ3.24419
ꢁ3.14656
ꢁ3.07911
ꢁ3.05761
ꢁ3.03655
ꢁ3.03655
8
ꢁ2.8647
10
12
14
16
18
ꢁ2.83022
ꢁ2.76462
ꢁ2.73337
ꢁ2.7181
ꢁ2.70306
0.115
The thermal decolourisation and colourisation processes of the
four series were studied at 1 ꢂ10ꢁ4 M in methanol and measured at
21 ꢀC, versus time.
10-Methyl-6-nitro-30spirocyclohexylspiro-[2H-1-benzopyran-2,20-
indoline] 3. 1,2-Dimethyl-3-spirocyclohexyl indolium triflate (1.02 g,
2.81 mmol) was dissolved in a 40% sodium hydroxide solution (10 mL)
and the resulting solution stirred for 5 min. Diethylether (15 mL) was
added to the stirred solution and after 5 min separated from the
reaction mixture, dried (anhydrous sodium sulphate) and evaporated
under reduced pressure. The yellow/orange oil that resulted was
isolated (0.4 g, 1.87 mmol), dissolved in ethanol (3 mL), added to 2-
hydroxy-5-nitrobenzaldehyde (0.31 g, 1.87 mmol) in ethanol (10 mL),
and the resulting solution heated under reflux for 24 h. The ethanol
was reduced in volume to approximately one-quarter, under reduced
pressure, and left overnight at 0 ꢀC whereupon a slightly brown solid
formed. The solid was filtered off and recrystallised from ethanol to
yield the title compound as a tan coloured precipitate (0.37 g, 55%). mp
122e124 ꢀC. dH (CDCl3) 8.02 (1H, d, ArH J ¼ 2), 7.99 (1H, s, ArH),
7.44, 7.42 (1H, d, ArH), 7.26 (1H, s, ArH), 7.24, 7.20 (1H, t, ArH), 6.9 (1H,
s, ArH), 6.84e6.88 (1H, t, ArH), 6.74e6.76 (1H, d ArH J ¼ 8), 6.58e6.55
(1H, d, ArH), 5.88e5.90 (1H, d, ArH J ¼ 10), 2.7 (3H, s, NeCH3),
1.97e1.31 (10H, m, cyclohexyl 5 ꢂ CH2). n(max) (CDCl3)/cmꢁ1 2936,
3020 (sat CeH), 1608 (C]C), 1516 (C]N), 1338, 1450 (NO2), 1216
(CeN), 1088 (CeC), 954 (CeO spiro), 775 (ArH, 4 adj H’s). M/e 363
(Mþþ1, 23.6), 362 (Mþ, 65.0), 361 (Mþ ꢁ 1, 5.1), 83 (base peak 100%).
(Found: C 72.71, H 6.05, N 7.48. C22H22N2O3. requires C 72.91, H 6.12, N
7.73).
4.2. Chemical syntheses
1H NMR assignments were carried out with a JEOL FX2000
spectrometer using deuteriochloroform, [D6]dimethyl sulfoxide or
1,1,2,2-tetrachloroethene (TCE) as the solvent with tetramethylsi-
lane (TMS) as the internal reference. Multiplicities are reported as
(s) singlet, (d) doublet, (t) triplet, (q) quartet and (m) multiplet.
Assignments of hydroxyl and ammonium 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 University of Wales, Swansea. UV
spectroscopy was carried out using PerkineElmer Lambda 5 and
Lambda 9 spectrometers; both instruments 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 ml quartz
cells with a 1 cm path length and are referenced against air. IR
spectra were recorded with a PerkineElmer 983 spectrometer.
Melting points were determined in open capillary tubes with an
Electrothermal melting point apparatus and are uncorrected.
Elemental analyses were carried out in-house. Thin-layer chroma-
tography was performed over glass plates coated with Merck silica
gel 60 F254; flash chromatography was performed using Merck
10-Hexyl-6-nitro-30,30-dimethylspiro-[2H-1-benzopyran-2,20-
indoline] 4. 3,3-Dimethyl-1-hexyl-2-methyl indolium iodide
(0.91 g, 2.45 mmol) was dissolved in a stirred 40% sodium
hydroxide solution (30 mL). Following this, diethylether (100 mL)
was added and the mixture vigorously stirred for 15 min. After
this period the diethylether layer was separated, dried (anhydrous
sodium sulphate), filtered and evaporated under reduced pressure
to yield an yellow/orange oil (0.38 g, 1.56 mmol): this oil was
isolated, added to 2-hydroxy-5-nitrobenzaldehyde (0.26 g,
1.56 mmol) in ethanol (15 mL), and the resulting solution heated
under reflux overnight. Partial (approximately 75%) evaporation
of the solvent under reduced pressure, followed by overnight
refrigeration, produced a solid which was broken up and recrys-
tallised from chloroform:hexane (1:1) to yield the title compound
as a dusty pink solid (390 mg, 64%). mp 104e106 ꢀC. dH (CDCl3)
8.02 (1H, d (obs), ArH J ¼ 7), 7.99 (1H, d, ArH J ¼ 7), 7.16e7.20 (1H,
t, ArH J ¼ 7), 7.06, 7.08 (1H, d, ArH J ¼ 8), 6.88 (1H, d, CH ] CH
J ¼ 10), 6.85e6.90 (1H, t, ArH J ¼ 7), 6.73e6.75 (1H, d, ArH J ¼ 8),
6.56, 6.58 (1H, d, ArH J ¼ 8), 5.85, 5.87 (1H, d, CH ] CH (cis) J ¼ 10),
3.11e3.25 (10H, m, 5 ꢂ -CH2), 1.18e1.28 (6H, 2ꢂ s, gem eC(CH3)2),
0.83e0.87 (3H, t, hexyl-CH3). n(max) (CDCl3)/cmꢁ1 2856, 2932 (sat
CeH), 1610 (C]C), 1472 (CeO), 1336 (NO2), 1274 (CeN), 1090
(CeC), 954 (CeO spiro), 732 (ArH, 4 adjacent H’s). M/e 392 (Mþ,
7.6), 172 (base peak, 100%). C24H28N2O3 Acc. (EI) requires:
392.2100 Found: 392.2100.
7734 silica gel (20e63 mm).
Chemical intermediates were purchased from the Aldrich
Chemical Company unless otherwise stated.
4.2.1. Syntheses of spirobenzopyrans
6-Nitro-10,30,30-trimethylspiro-[2H-1-benzopyran-2,20-indoline
2. 1,3,3-Trimethyl-2-methylene indoline (2.50 g, 14.4 mmol) was
added to a solution of 2-hydroxy-5-nitrobenzaldehyde (2.42 g,
14.4 mmol), in ethanol (75 mL), and heated under reflux for 8 h. After
this period the solution was reduced to approximately one-quarter
of its volume, and cooled to 0 ꢀC, whereupon a yellow crystalline
solid formed. The solid was filtered off and recrystallised from
ethanol to yield the title compound as golden yellow crystals (4.13 g,
89%). mp 178e179 ꢀC. Lit 177e178 ꢀC [27]. dH (CDCl3) 7.92 (1H, d, ArH
J ¼ 2), 7.89 (1H, s, ArH), 7.13 (1H, t, ArH J ¼ 8,2), 6.99 (1H, d, ArH J ¼ 7),
6.83 (1H, d, CH ] CH J ¼ 10), 6.78 (1H, t, ArH J ¼ 8, 2), 6.70 (1H, d, ArH
J ¼ 8), 6.4 (1H, d, ArH J ¼ 7), 5.80 (1H, d, CH ] CH J ¼ 10), 2.70 (3H, s,
NeCH3),1.30 (3H, s, ArCCH3),1.2 (3H, s, ArCCH03). n(max) (CDCl3)/cmꢁ1
3080, 2980 (sat CeH), 1660, 1615 (C]C), 1575, 1500 (NO2), 1340
(CeO),1200 (CeN),1100 (CeC), 954 (CeO spiro). M/e 322 (Mþ, 82.6),
292 (9.2), 261 (14.5), 217 (8.6), 159 (base peak 100%).

C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
161
10-Hexyl-6-nitro-30-spirocyclohexylspiro-[2H-1-benzopyran-2,20-
indoline] 5. 1-Hexyl-2-methyl-3-spirocyclohexyl indolium iodide
(0.79 g, 1.92 mmol) was dissolved in a 40% sodium hydroxide solution
(30 mL) by vigorously stirring. Following this, diethylether (100 mL)
was added and the solution vigorously stirred for 15 min: After this
period the diethylether layer was separated from the reaction mixture,
dried (anhydrous sodium sulphate) and evaporated under reduced
pressure to yield an orange oil (0.36 g,1.27 mmol). The oil was isolated,
added to 2-hydroxy-5-nitrobenzaldehyde (0.21 g, 1.26 mmol) in
ethanol (15 mL) and the resulting solution refluxed overnight. After
this period the solution was allowed to cool, the ethanol reduced in
volume to approximately 5 mL and the resulting mixture left in
a refrigerator overnight. The solid that formed was filtered off and
recrystallised from chloroform: hexane (1:1) to yield the title
compound as a dusty pink solid (300 mg, 55%). mp 122e124 ꢀC. dH
(CDCl3) 7.99, 8.01 (1H, d (obscured), J ¼ 9, ArH, J ¼ 9), 7.99 (1H, s, ArH),
7.41, 7.43 (1H, d, Ar J ¼ 7), 7.21 (1H, t, ArH), 6.86, 6.89 (1H, d, CH ] CH,
J ¼ 10), 6.80e6.89 (1H, t, ArH), 6.71e6.71 (1H, d, ArH, J ¼ 9), 6.58, 6.58
(1H, d, ArH, J ¼ 7), 5.87, 5.90 (1H, d, CH ] CH, J ¼ 10ecis-form),
3.05e3.22 (10H, m, 5 ꢂ CH2), 1.26e1.97 (10H, m, cyclohexyl 5 ꢂ CH2),
0.84e0.86 (3H, t, CH3). n(max) (CDCl3)/cmꢁ1 2934, 2856 (sat CeH),1612
(C]C), 1472 (CeO), 1336 (NO2), 1276 (CeN), CeC (1090), 954 (CeO
spiro), 732 (ArH, 4 adj H’s). M/e 433 (Mþþ1, 37.7), 432 (base peak,
100%) 431 (Mþ ꢁ 1, 8.8). (Found: C 74.70, H 7.48, N 6.38. C27H32N2O3
requires C 74.79, H 7.48, N 6.47).
4.2.3. Syntheses of indolenines
1-Methyl-3-spirocyclohexyl-2-methylene indolenine. 1,2-
Dimethyl-3-spirocyclohexyl indolium triflate (5.19) (1.02 g,
2.81 mmol) was dissolved in a 40% sodium hydroxide solution
(10 mL) and stirred for 10 min. After this period diethylether
(10 mL) was added and the solution stirred for a further 5 min.
The diethylether layer was separated from the reaction mixture,
dried (anhydrous sodium sulphate), filtered, and removed under
reduced pressure to yield the title compound as a yellow/orange
oil (0.45 g, 75%). dH (CDCl3) 7.44 (1H, d, ArH J ¼ 7), 7.14 (1H, t,
ArH), 6.74 (1H, t, ArH), 6.56 (1H, d, ArH J ¼ 7), 3.88 (2H, dd, C]
CH2 J ¼ 10, 10), 3.02 (3H, s, NeCH3), 1.83 (10H, m, cyclohexyl
5 ꢂ CH2). n(max) (CDCl3)/cmꢁ1 2934 (sat CeH), 1644 (C]N), 1604
(C]C), 1908 (CeC), 908 (CeC), 775 (ArH, 4 adj H’s). M/e 215
(Mþþ1, 18.5), 214 (Mþ, 46.9), 213 (Mþ ꢁ 1, 80.0), 158 (base peak,
100%), 306 (Mþ ꢁ 1, 15.8). C15H19N Acc. (EI) requires: 213.1517
Found: 213.1517.
4.2.4. Synthesis of 3H-indoles
3-Cyclohexyl-2-methyl-3H-indole. Cyclohexylmethyl ketone
phenylhydrazone (4.65 g, 21.53 mmol) was added to zinc chloride
(1.00 g, 7.35 mmol) in glacial acetic acid (100 mL), and the resulting
solution heated on a steam bath, under nitrogen, for 3 h. The resulting
solution was cooled to room temperature, filtered, and the remaining
glacial acetic acid removed under reduced pressure to yield the title
compound as an orange oil (3.25 g, 76%). dH (CDCl3) 7.51e7.53 (1H, d,
ArH J ¼ 7), 7.31 (1H, d, ArH J ¼ 7), 7.15, 6.99 (2H, m, ArH), 2.7 (3H, s,
N]CeCH3), 1.25e2.31 (10H, m, cyclohexyl 5 ꢂ CH2). n(max) (CDCl3)/
cmꢁ1 2932 (sat CeH), 1688 (C]N), 1598 (C]C, Ar), 1216 (CeN), 1026
(CeC). M/e 200 (Mþþ1, 27.6),199 (Mþ, base peak,100%),198 (Mþ ꢁ 1,
4.2). C14H17N Acc. (EI) requires: 199.1361 Found: 199.1361.
4.2.2. Syntheses of N-alkyl indoles
1-Hexyl-2-methyl-3-spirocyclohexyl indolium iodide. 3-
Cyclohexyl-2-methyl-3H-indole (1.01 g, 5.09 mmol) and hexyl
iodide (1.08 g, 5.09 mmol) were heated under reflux for 24 h. After
this period the remaining solution was cooled, yielding the title
compound as a deep black/red waxy solid (1.41 g, 68%). mp
(formed a gum). dH (CDCl3) 7.94, 7.96 (1H, d, ArH J ¼ 7), 7.68, 7.70
(1H, d, ArH J ¼ 7), 7.60 (1H, t. ArH), 7.57 (1H, t, ArH), 4.70 (2H, t,
NþeCH2CH2), 3.16 (3H, s, eNþ ¼ CeCH3), 1.23e2.13 (18H, m,
9 ꢂ CH2), 0.91 (3H, t, hexyl, CH2CH3). n(max) (CDCl3)/cmꢁ1 2930,
3018 (sat CeH), 1600 (C]C), 1126 (C]N), 1216 (CeN), 1042 (CeC),
736 (ArH, 4 adj H’s). M/e 285 (Mþ þ 2-HI, 7.5), 284 (Mþ þ 1-HI,
3,30-Dimethyl-2-methyl-3H-indole.
A mixture of isopropyl
methyl ketone phenylhydrazone (1.69 g, 9.60 mmol) and hydro-
chloric acid (30 mL) was heated under reflux for 1 h prior to stirring
at room temperature for 3 h. After this period the resulting mixture
was filtered and the hydrochloric acid removed under reduced
pressure to yield a red oil. Column chromatography of the oil over
silica using ethyl acetate as the eluent yielded the title compound
as a reddish oil (1.19 g, 78%). dH (CDCl3) 7.0e7.6 (4H, m, ArH), 2.3
(3H, s, CH3), 1.3 (6H, s, ArCH3). n(max) (CDCl3)/cmꢁ1 3018 (sat CeH),
1450 (C]N), 1533 (C]C), 1210 (CeC), 1190 (CeN), 771 (ArH, 4 adj
H’s). M/e 159 (Mþ, 11.9), 146 (base peak, 100%).
44.8), 283 (Mþ ꢁ HI,100% base peak), 282 (Mþ-1-HI, 20.8). C20H29
N
Acc. (EI) requires: 283.2300 Found: 283.2300.
3,3-Dimethyl-1-hexyl-2-methyl indolium iodide. 3,3-Dimethyl-
2methyl-3H-indole (1.00 g, 6.28 mmol) and hexyl iodide (1.39 g,
6.55 mmol) were heated under reflux for 24 h. The solid that
formed was broken up and washed with diethylether to yield the
title compound as a deep red powder (1.21 g, 51%). mp (formed
a ’gum’). dH (CDCl3) 8.52e8.55 (4H, m, ArH), 4.66e4.51 (10H, t,
5 ꢂ CH2), 3.53 (3H, s, Nþ ¼ CeCH3), 1.18 (6H, 2 ꢂ s, gem eC(CH3)2),
0.63 (3H, t, hexyl CH2CH3). n(max) (CDCl3)/cmꢁ1 2940, 3018 (sat
C-H), 1606 (C]C), 1640 (C]N), 1216 (CeN), 1032 (CeC). M/e 243
(Mþ ꢁ HI, 50.1), 242 (Mþ ꢁ HI, 5.5), 144 (100% base peak).
1,2-Dimethyl-3-spirocyclohexyl indolium triflate. 2-Methyl-3-
spirocyclohexyl-3H-indole (3.22 g, 16.18 mmol) was added to
methyltrifluoromethane sulphonate (2.66 g, 16.22 mmol) in
a mixture of hexane (20 mL) and diethylether (30 mL). Instantly,
a canary yellow precipitate formed which was filtered off and
washed with cold diethylether to yield the title compound as pale
yellow crystals (4.59 g, 78%). mp 126e128 ꢀC. dH (CDCl3) 7.91, 7.93
(1H, d, ArH), 7.66e7.68 (1H, d, ArH), 7.62 (1H, t, Ar J ¼ 8), 7.56 (1H, t,
ArH J ¼ 8), 4.12 (3H, s, NþeCH3), 2.88 (3H, s, Nþ ¼ CeCH3), 1.54, 2.08
(10H, m, cyclohexyl 5 ꢂ CH2). n(max) (CDCl3)/cmꢁ1 3020 (sat CeH),
1590 (C]C), 1325 (CeC), 1210 (CeN), ArH (760), 758 (CeF). M/e 233
(Mþ ꢁ (SO3CF3), 6.7), 323 (Mþ ꢁ SO3CF3-1, 41.7), 218 (Mþ-SO3CF3-
15, 17.7), 217 (base peak, 100%) 203 (Mþ-SO3CF3-2 ꢂ 15, 4.2).
(Found: C 52.50, H 5.36, N 3.88. C16H20NO3F3S requires C 52.88, H
5.55, N 3.86).
4.2.5. Syntheses of hydrazones
Isopropyl methyl ketone phenylhydrazone. Phenyl hydrazine
(1.65 g, 15.28 mmol) was added to 2-methyl-3-pentanone (2.48 g,
24.80 mmol) in ethanol (10 mL) and the resulting solution heated
under reflux for 5 h. After this period the ethanol and excess
2-methyl-3-pentanone were removed under reduced pressure
(rotary evaporation) to yield title compound as mobile, slightly red
oil (3.20 g, 68%). dH (CDCl3) 6.8e7.3 (5H, m, ArH), 4.75 (1H, bs, NH),
2.55 (1H, hept, (CH3)2)CeH), 1.91 (3H, s, CH3), 1.14 (6H, d,
(CH3)2CeH). n(max) (CDCl3)/cmꢁ1 3300e3500 (¼NeH, w, secondary
amine), 3100 (CeH), 1650 (C]C), 1530 (C]N).
Cyclohexyl ketone phenylhydrazone [28]. Phenylhydrazine
(3.00 g, 27.77 mmol) was added to cyclohexylmethyl ketone (3.50 g,
27.77 mmol) in ethanol (100 mL) and the resulting solution heated
under reflux for 0.5 h. Removal of the ethanol under reduced
pressure yielded the title compound as a deep orange mobile oil
(4.78 g, 79%). dH (CDCl3) 11.0 (1H, s, NH, exchangeable D2O), 7.25
(2H, t, ArH), 7.03, 7.05 (2H, dd, ArH), 6.79, 6.83 (1H, t, ArH J ¼ 8),1.90
(1H, m, (CH2)2CHC), 1.82 (3H, s, N]CeCH3), 1.17e1.78 (10H, m,
cyclohexyl 5 ꢂ CH2). n(max) (CDCl3)/cmꢁ1 3650 (NeH), 3020 (CeH),
1616 (C]C), 1520 (CeN), 1020 (CeC), 760 (ArH). M/e 216 (Mþ, 6.6),
215 (Mþ ꢁ 1, 8.1), 18 (base peak, 100%).

162
C.J. Roxburgh et al. / Dyes and Pigments 90 (2011) 146e162
(f) Fedorova OA, Strokach YP, Gromov SP, Koshkin AV, Valova TM, Alfimov MV,
4.2.6. Syntheses of salicaldehydes
et al. New J Chem 2002;26:1137.
2-Hydroxy-5-nitrobenzaldehyde.
Hexamethylenetetramine
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(1.40 g, 10 mmol) was added in portions, over 15 min, to a stirred
mixture of 4-nitrophenol (1.39 g, 10 mmol) in 85% polyphosphoric
acid (8 mL). The resulting solution was heated to 100 ꢀC and
maintained at this temperature for 2 h. Cold water (40 mL) was
added to the stirred solution whilst maintaining the temperature at
near to 15 ꢀC by cooling over an ice bath. The solid that formed was
filtered off and washed with a little cold water (2 ꢂ 25 mL). Drying
yielded title compound as a cream coloured solid (0.97 g, 58%). mp
127e129 ꢀC. Lit 128 ꢀC [29]. dH (DMSO) 10.35 (1H, s, CHO), 8.42 (1H,
br s, ArH), 8.35e8.33 (1H, br d, ArH), 7.17e7.20 (1H, d, ArH). n(max)
(CDCl3)/cmꢁ1 3682 (OH), 3020 (CeH), 1722 (C]O), 1594 (C]C),
1320 (CeC), 1216 (NO2). M/e 169 (Mþþ 2, 3.0), 168 (Mþþ1, 23.8),
167 (Mþ, base peak 100%).
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314e466.
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interpretation of the kinetic data is gratefully acknowledged.
The authors thank the Science and Engineering Research
Council (SERC) for a grant supporting this research.
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