NMR structural and kinetic assignment of fluoro-3H-naphthopyran photomerocyanines

Article abstract of DOI:10.1562/0031-8655(2001)074<0694:NSAKAO>2.0.CO;2

The kinetic and structural behavior of a photochromic compound, 3-(2-fluorophenyl)-3-phenyl-3H-naphtho[2,1-b]pyran (F-Py), was investigated using ¹H and ¹⁹F nuclear magnetic resonance (NMR) spectroscopy. Upon irradiation, the four th

Full text of DOI:10.1562/0031-8655(2001)074<0694:NSAKAO>2.0.CO;2

NMR Structural and Kinetic Assignment of Fluoro-3 -naphthopyran
H
Photomerocyanines
Author(s): S. Delbaere, J-C. Micheau, Y. Teral, C. Bochu, M. Campredon, and G. Vermeersch
Source: Photochemistry and Photobiology, 74(5):694-699.
Published By: American Society for Photobiology
DOI: http://dx.doi.org/10.1562/0031-8655(2001)074<0694:NSAKAO>2.0.CO;2
URL: http://www.bioone.org/doi/full/10.1562/0031-8655%282001%29074%3C0694%3ANSAKAO
%3E2.0.CO%3B2
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Photochemistry and Photobiology, 2001, 74(5): 694–699
NMR Structural and Kinetic Assignment of Fluoro-3H-naphthopyran
Photomerocyanines¶
S. Delbaere*1, J.-C. Micheau2, Y. Teral3, C. Bochu1, M. Campredon3 and G. Vermeersch1†
1Laboratoire de Physique et d’Application R.M.N., Universite´ de Lille 2, Lille, France;
2Laboratoire IMRCP, Universite´ Paul Sabatier, Toulouse, France and
3Laboratoire de Photochimie Organique Applique´e, Universite´ de la Me´diterrane´e, Marseille, France
Received 17 April 2001; accepted 2 August 2001
ABSTRACT
substituted 3H-naphtho-[2,1-b]-pyrans, such as norbornyli-
dene (5), spiro-adamantane (6,7) and cyclopropyl (8,9) or
substituted phenyl (4,10–13) instead of alkyl groups, were
synthesized in order to improve the photochromic response.
Today, colors ranging from yellow to blue can be obtained
in the pyran series via substituent effects (14). This family
of compounds has been used in recent years for the manu-
facture of plastic photochromic lenses owing to its good pho-
tochromic properties, associated with high fatigue resistance
(15,16).
The photochromic activity of these compounds is caused
by the reversible light-induced cleavage of the bond between
the heterocyclic oxygen atom and the quaternary carbon: the
closed colorless forms undergo ring-opening to become the
corresponding open forms, the photomerocyanines, which
are responsible for coloration. Though a better understanding
of pyran properties has been obtained in recent years, the
number and nature of the transient species that are formed
must be clearly established in order to improve performance
further in commercial applications.
In the course of our studies on fluorochromenes at low
temperatures (17), we have observed that 3-(2-fluorophenyl)-
3-phenyl-3H-naphtho[2,1-b]pyran (F-Py) exhibited a partic-
ularly low thermal back rate constant in solution. A similar
property has been observed in plastic substrates for a number
of ortho-substituted diaryl naphthopyrans (12). To study this
phenomenon further, we have chosen an uncommon ap-
proach involving 19F nuclear magnetic resonance (NMR)
spectroscopy for our investigations.
The kinetic and structural behavior of a photochromic
compound,
3-(2-fluorophenyl)-3-phenyl-3H-na-
phtho[2,1-b]pyran (F-Py), was investigated using 1H and
19F nuclear magnetic resonance (NMR) spectroscopy.
Upon irradiation, the four theoretically predicted pho-
tomerocyanines appear along with a fifth form X, whose
final structure has not been elucidated. This last form
and two of the photomerocyanines are thermally labile,
whereas the other two do not show any signs of decay.
The system has been analyzed by NMR spectroscopy.
This led to the structural assignment of each photomer-
ocyanine. The kinetics of the thermal bleaching were
monitored by directly and separately measuring the
concentrations of each species at regular time intervals
using 19F NMR spectroscopy. We therefore propose a
plausible reaction mechanism. On the basis of this
mechanism, the mathematical treatment and the study
of the effects of temperature led to the determination of
the kinetic and thermodynamic parameters (rate coef-
ficients, enthalpy and entropy of activation) of this pho-
tochromic system. The leading role of the labile inter-
mediate X on the formation of trans–transoid–cis (TTC)
and cis–transoid–cis (CTC) photomerocyanines is point-
ed out.
INTRODUCTION
Benzo and naphthopyran derivatives are important classes of
photochromic materials (1,2). They were originally studied
for their potential to offer complementary colors (yellow to
orange) for the well-known (blue) indolinospironaphthoxa-
zines (3). Since the pioneering work of Becker and Michl
(4), who first established these properties in a rigid matrix
at 77 K, many studies have been carried out demonstrating
photochromic behavior at room temperature. Several novel
In a previous paper, we reported the results of 1H, 13C and
19F NMR studies on the structure of the photomerocyanines
after irradiation of 3,3-bis-(4-fluorophenyl)-3H-naphtho[2,1-
b]-pyran (18). Both the colored species, the transoid–cis
(TC) and the transoid–trans (TT) isomers of photomerocy-
anine, were produced and studied. Kinetic analyses were
also performed by UV and NMR spectroscopies. At the
same time, Malatesta and coworkers (19) have described a
spectrokinetic investigation of the photochromic reaction of
¶Posted on the website on 10 August 2001.
†Dedicated to Ralph Becker on the occasion of his 75th birthday.
*To whom correspondence should be addressed at: Laboratoire de
Physique et d’Application R.M.N., UPRESA CNRS 8009, Uni-
versite´ de Lille 2, Faculte´ des Sciences Pharmaceutiques et Biol-
ogiques, B.P. 83, F-59006 Lille Cedex, France. Fax: 33-3-2095-
9009; e-mail: stephanie.delbaere@phare.univ-lille2.fr
Abbreviations: CD3CN, acetonitrile-d3; CTC, cis–transoid–cis; CTT,
cis–transoid–trans; F-Py, 3-(2-fluorophenyl)-3-phenyl-3H-na-
phtho[2,1-b]pyran; HOESY, heteronuclear Overhauser effect
spectroscopy; NMR, nuclear magnetic resonance; TC, transoid–
cis; THF, tetrahydrofuran; TT, transoid–trans; TTC, trans–tran-
soid–cis; TTT, trans–transoid–trans.
᭧
2001 American Society for Photobiology 0031-8655/01
$5.00ϩ0.00
694

Photochemistry and Photobiology, 2001, 74(5) 695
Figure 1. 1H NMR spectrum of F-Py before (A) and after (B) UV
irradiation in CD3CN at 228 K.
Scheme 1. Photochromic equilibrium between closed and open
forms.
hydrofuran (THF) at
Ϫ10ЊC, a solution of the 2-fluorobenzophenone
(10 mmol) in THF was added dropwise. On completion of the ad-
dition, the mixture was allowed to warm to room temperature and
then poured into ice, and the two phases separated. The organic
phase was washed with saturated aqueous ammonium chloride so-
lution, dried (Na2SO4), filtered and concentrated under vacuum. 1-
(2-Fluorophenyl)-1-phenyl-2-propyn-1-ol was obtained as an oil in
76% yield. A 4 mmol amount of 1-(2-fluorophenyl)-1-phenyl-2-pro-
the nonfluorated equivalent molecule by spectrophotometric
methods using monochromatic irradiation in the UV and vis-
ible ranges. By these two methods, the kinetic parameters of
the thermal ring-closure were determined.
In the investigation described here, we set out to generate
photoproducts by irradiation of F-Py. In this case, the situ-
ation is complicated, as the photoreaction can yield four
transoid photomerocyanines that exhibit different thermal
reactivities. Indeed, the monosubstitution by a fluorine atom
on an ortho position of a phenyl substituent leads to asym-
metry of the pyran carbon (C3). Our aim was to confirm the
formation of the four transoid isomers of photomerocyanine
(Scheme 1) and to observe their behavior through NMR
spectroscopy.
pyn-1-ol was dissolved in methylene chloride at 25
catalytic amount of p-toluenesulfonic acid was added. After 10 min,
6 mmol of -naphthol dissolved in methylene chloride was added
ЊC and then a
␤
dropwise. After 2 h, the solvent was evaporated, and the organic
residue was purified by flash chromatography (pentane–diethylether)
and recrystallized from pentane (yield 43%, m.p. 133ЊC).
Samples (10Ϫ2 M in acetonitrile-d3 [CD3CN]) in the NMR tube (5
mm) were irradiated in a home-built apparatus. The emission spec-
trum of a 1000 W Xe–Hg high-pressure short-arc lamp (Oriel), fil-
tered by a band-pass glass filter (Schott 011FG09: 259 Ͻ ␭ Ͻ 388
nm with
␭
ϭ 330 nm and T ϭ 79%), was focused on the end of
max
a silica light-pipe (length 6 cm, diameter 8 mm), leading the light
to the spinning sample tube, which was inserted in a quartz dewar.
The temperature of the sample was controlled with a variable tem-
perature unit (B-VT1000, Bruker, over a range of 123 to 423 K).
After 15 min of irradiation, the NMR sample tube was rapidly trans-
ferred to the probe (QNP 1H, 13C, 19F, 31P) of a thermoregulated
Bruker AC300P NMR spectrometer. Irradiation and NMR experi-
ments were performed at 228, 232, 236, 239.5, 253 and 273 K. The
2D heteronuclear Overhauser effect spectroscopy (HOESY) (22–24)
experiment was recorded on a Bruker DPX-300 spectrometer
equipped with the QNP probe.
The D2O exchange experiment was performed at 228 K. After
irradiation, 1H and 19F NMR spectra were recorded to verify the
formation of X. Then a drop of D2O was added cautiously, taking
care not to stir the sample. The drop crossed the red solution and
froze at the bottom of the tube.
Typically, UV-visible spectroscopy (7,19,20) has been
used to study kinetics and has allowed the determination of
the intrinsic properties that are responsible for the photo-
chromic process. Nevertheless, this method gives only val-
ues of absorbance, with the disadvantage of having overlap-
ping spectra and unknown absorption coefficients. In this
paper, high-resolution 19F NMR spectroscopy has been used.
The 19F nucleus is 100% abundant, with spin ϭ ½ and a
large magnetic moment. This is a very sensitive method for
detecting minor fluorine-containing compounds. In particu-
lar, it is possible to use 19F NMR spectroscopy as a simple
way of determining the number of different fluorine-contain-
ing components present in a mixture simply by counting the
number of fluorine peaks in the spectrum. This is easier for
1
19F NMR than for H NMR as usually there are fewer fluo-
RESULTS AND DISCUSSION
rine atom environments present. The range of 19F NMR
1
The H NMR spectrum of closed F-Py in acetonitrile-d3 and
1
chemical shifts is wider than that of H NMR, and each
1
the complete assignments of H are presented in Fig. 1A
1
chemically distinct 19F nucleus in a H-decoupled spectrum
(25). After 15 min of UV irradiation performed at 228 K,
the solution had turned red-orange, and new 1H NMR signals
were observed as shown in the spectrum on Fig 1B. After
the irradiation was stopped, 1H NMR (Fig. 1B) and 19F NMR
(Fig. 2) spectra were recorded at regular time intervals,
maintaining the same spectrometer temperature for 18 h.
These observations are consistent with the generation of var-
ious photoproducts exhibiting different thermal stabilities.
One of them, characterized by the 19F NMR signal at
usually gives rise to only one single resonance. Provided that
the 19F NMR spectrum was acquired under conditions of full
T1 relaxation, it is possible to quantify the relative amounts
of the different components in the mixture by integrating the
minor fluorine peaks in the spectrum.
MATERIALS AND METHODS
F-Py was synthesized in two steps according to standard procedures
(2,21). To a solution of sodium acetylide in freshly distilled tetra-

696 S. Delbaere et al.
Figure 4. Examples of concentration evolutions after irradiation at
228 and 273 K (experimental points and fitted curves).
sults and NMR information, the nature of the photoproducts
was deduced.
The C3–O cleavage produced at first a short-lived species,
probably a zwitterionic form that is quenched by the water
present in the solvent (acetonitrile), leading to an ‘interme-
diate’ structure X. Its short lifetime and the overlapping part
Figure 2. 19F NMR spectra after UV irradiation, T
ϭ 228 K; time
ϭ
0 (A), 3020 (B), 6795 (C), 14 345 (D) and 67 195 s (E). *: deg-
radation product.
1
of the H spectrum between 7.2 and 7.9 ppm (which hides
Ϫ
112.43 ppm, presented no change and was identified as the
its resonances) did not allow 2D experiments to characterize
it. Padwa et al. observed previously an unstable photoprod-
uct resulting from the 1,4 addition of methanol on an o-qui-
noneallide intermediate (29). This structure is not consistent
with the X form because no aliphatic signals were detected
well-known degradation product (26) by comparison with
the spectrum of standard 2-fluorobenzophenone. The other
19F signals were assigned to the four isomers (photomero-
cyanine structures PM1 at
ppm, PM3 at
last signal, at
structure X. The H and 19F signals of PM3 and PM4 fully
disappeared after the NMR sample had been at room tem-
perature for several days. That allowed these to be differ-
entiated from the degradation product.
Ϫ
112.47 ppm, PM2 at
112.62 ppm and PM4 at 112.55 ppm). The
113.05 ppm, was attributed to an unknown
Ϫ112.66
1
Ϫ
Ϫ
in the H NMR spectra. Nevertheless, a singlet signal is de-
Ϫ
tected at 7.9 ppm. This singlet is exchanged when D2O is
added, although the 19F signal (␦ ϭ Ϫ113.05 ppm) is still
present. This exchange is characteristic of a labile proton,
which can therefore be assigned to the hydroxy function of
a naphthol.
1
The X isomer rapidly disappeared in the dark, whereas
the concentrations of PM1 and PM2 increased simultaneous-
ly. Then these two isomers reconverted significantly to F-
Py, by which time X had totally disappeared. The two other
isomers (PM3 and PM4) did not change throughout the
course of the experiment.
Previous work using ab initio calculations (27), Zerner
intermediate neglect of differential overlap (13), spectropho-
tometric methods (19) and NMR studies (3,18,27–28)
showed that the primarily formed metastable photomerocy-
anines preferentially exist as trans–transoid–cis (TTC) and
cis–transoid–cis (CTC) forms rather than as trans–transoid–
trans (TTT) and cis–transoid–trans (CTT). Using these re-
As expected, regarding the two isomers PM1 and PM2, the
doublet signals at 6.40 and 6.41 ppm on the H NMR spec-
trum were straightforwardly assigned to H-5 protons in TC
structures (18), as were the signals at 8.24 and 8.66 ppm,
which were assigned to H-2 protons deshielded by the CϭO
1
bond. The signal at 8.24 ppm presents a doublet–doublet
multiplicity, resulting from the H-1 coupling constants (3JH–
ϭ
11.5 Hz) and with F (5JH–F
ϭ 1.1 Hz). To determine the
H
exact structure of each isomer, a HOESY correlation was
performed (Fig. 3). This 2D NMR experiment yields infor-
mation on the spatial relationship between spins in the het-
eronuclear case (1H and 19F). The fluorine atom at
Ϫ112.66
ppm (PM2) presents a dipolar correlation with the proton
signal at 8.24 ppm (H-2), which can only be identified in
the TTC isomer. On the HOESY map, no correlation was
found between the fluorine atom at
Ϫ112.47 ppm and its
proton H-2 at 8.66 ppm. It was deduced that PM1 corre-
sponds to the CTC isomer.
Regarding the structure of the low-concentration isomers
(PM3 and PM4), the 1H doublet signals at 6.29 and 6.31 ppm
were assigned to H-5 protons in TT structures (18). Com-
parison of the 19F chemical shifts and environments with
those of isomers TTC and CTC led us to assign PM3 and
PM4 to the photomerocyanines CTT and TTT, respectively.
For the structures of the photoproducts being determined,
1
the integration of some H and all the 19F signals in each
NMR spectrum was performed.
Knowing the initial concentration of F-Py, we directly ob-
tained the exact concentration of each photoproduct during
the thermal relaxation (Fig. 4). On the basis of this kinetic
Figure 3. 19F–1H NMR HOESY of F-Py after irradiation in CD3CN
at 228 K.

Photochemistry and Photobiology, 2001, 74(5) 697
Scheme 2. Thermal bleaching mechanism.
behavior, we assumed a simpler mechanism such as the one
involving only thermal processes of the photoproducts that
have a significant evolution. As the concentrations of the
slow isomers TTT and CTT were extremely stable, they
were not inserted in this assumed model (Scheme 2).
This thermal bleaching mechanism was deduced from
chemical and 19F NMR data. It includes the four species
CTC, TTC, X and F-Py and four pseudoelementary pro-
cesses, k1, k2, k3 and k4. The kinetic equations were deduced
from this very simple mechanism:
d[F-Py]
ϭ
k2·[CTC]
dt
ϩ
k4·[TTC]
(1)
(2)
d[CTC]
dt
ϭ
ϭ
k1·[X]
Ϫ
Ϫ
k2·[CTC]
k4·[TTC]
d[TTC]
dt
k3·[X]
(3)
Figure 5. A: Eyring-type plot of the photobleaching of X, CTC and
TTC. B: plot of ⌬ ⌬
H# vs S# for the four processes.
d[X]
dt
ϭ Ϫ(k1
ϩ
k3)·[X]
(4)
(5)
[F-Py]
ϩ [CTC]ϩ [TTC]ϩ [X] ϭ A
This process was applied for all the experiments per-
formed at the various temperatures. The k⌬ and
2
Ϫ
3
␹ minimi-
where the concentrations are expressed in mol dm and the
thermal bleaching rates k1, k2, k3 and k4 in s , and A rep-
resents the constant sum [F-Py]0 [CTT] [TTT].
Ϫ
1
zation values are listed in Table 1 and the results are illus-
trated by the fitted curves in Fig. 4. The good agreement
between the experimental and calculated curves shows the
validity of our mechanistic assumptions.
To interpret the temperature effects, ln(k⌬/T) was plotted
vs 1/T (Fig. 5A) according to Eyring’s equation,
Ϫ
Ϫ
To resolve these equations, and in particular to extract the
four pseudoelementary rate constants (k1, k2, k3 and k4), we
used homemade software. The values of concentrations vs
time are injected into a model written from the differential
equations. The validity of these equations is demonstrated
by simulation and numerical fitting according to the semi-
implicit Runge–Kutta method (30), using physically realistic
initial values for all the kinetic rates (k1, k2, k3 and k4). These
parameters were fitted automatically using an iterative al-
k⌬
T
kB
h
⌬
S#
⌬
H#
ln
ϭ
ln
ϩ
Ϫ
R
R·T
where kB is Boltzmann’s constant and h is Planck’s constant,
and the values of activation enthalpy
H# and activation en-
tropy
S# can be deduced (Fig. 5B). The enthalpy values
⌬
gorithm of the Powell type (31), designed to minimize the
⌬
2
residual quadratic error
␹
(32) between the calculated and
agree with those generally calculated for photochromic com-
pounds (33). The ring-closure reactions of CTC and TTC
experimental concentrations.
2
Table 1. Kinetic rates (sϪ1) of bleaching in CD3CN and minimal error
␹
T (K)
228
232
236
239.5
253
273
5
4
6
5
9
4
4
6
5
9
4
4
6
5
9
4
4
5
5
9
k1 (X
k3 (X
→
→
CTC)
TTC)
9.37
1.40
ϫ
ϫ
ϫ
ϫ
ϫ
10Ϫ
10Ϫ
10Ϫ
10Ϫ
10Ϫ
1.49
2.64
4.86
2.11
4.88
ϫ
ϫ
ϫ
ϫ
ϫ
10Ϫ
10Ϫ
10Ϫ
10Ϫ
10Ϫ
2.28
4.11
7.53
3.73
2.51
ϫ
ϫ
ϫ
ϫ
ϫ
10Ϫ
10Ϫ
10Ϫ
10Ϫ
10Ϫ
2.83
ϫ
ϫ
ϫ
ϫ
ϫ
10Ϫ
X not detected
X not detected
7.10
1.14
5.73
6.67
10Ϫ
10Ϫ
10Ϫ
10Ϫ
5
4
8
4
k2 (CTC
k4 (TTC
→
→
F-Py) 2.02
F-Py) 1.21
1.65
8.29
2.96
1.33
ϫ
ϫ
ϫ
10Ϫ
10Ϫ
10Ϫ
3.56
2.32
6.47
ϫ
ϫ
ϫ
10Ϫ
10Ϫ
10Ϫ
3
2
10
␹

698 S. Delbaere et al.
Acknowledgements The 300 MHz NMR facilities were funded by
the Re´gion Nord-Pas de Calais (France), the Ministe`re de
l’Education Nationale, de l’Enseignement Supe´rieur et de la Re-
cherche (MENESR) and the Fonds Europe´ens de De´veloppement
Re´gional (FEDER).
give medium activation values although the extreme values
correspond to the process from X. Regarding the values of
the entropy changes, they are all negative, indicating a loss
of degrees of freedom between the open forms and the tran-
sition states.
(X CTC), we can assume the formation of TTC with
⌬ → TTC) being much greater than ⌬
S#(X S#
APPENDIX
→
greater ease than CTC.
In the particular case of our simple mechanism (see Scheme 2), the
differential equations are algebraically tractable.
Indeed, the bleaching of X follows a monoexponential function,
where X0 is the initial concentration and k1 ϩ k3 is the rate parameter
for the bleaching.
The intermediate form X was assumed to be twisted and
stabilized by a hydrogen bridge in a closed-like configura-
tion, but with the C–O bond open. The formation of TTC
and CTC from X requires twisting around some bonds, i.e.
around C1–C2 and C2–C3 in this naphtho-protonated form.
During this process, the presence of the proton allows the
formation of hydrogen bonding between O–H and F (the
dipole moment for fluorobenzene is 1.6 D) (34). If this hy-
drogen bond brings the two atoms (O and F) into close prox-
imity during the rotations, it could catalyze the formation of
TTC. On the other hand, if the O–H· · ·F hydrogen bonding
is not in place, the oxygen and fluorine atoms are inclined
to move away from each other by electrostatic repulsive
forces. The rotation around the C2–C3 bond is executed in
the opposite direction and could lead to the formation of
X
ϭ
X0· exp[
Ϫ
(k1
ϩ
k3)·t]
(6)
For CTC and TTC, analytical solution shows that all the time-
dependent profiles are biexponential curves with apparent rate con-
stants a1, a2, b1 and b2 (Eqs. 7 and 8).
[CTC]
[TTC]
ϭ
ϭ
A1·exp(
Ϫ
a1·t)
ϩ
ϩ
A2·exp(
Ϫ
a2·t)
(7)
(8)
B1·exp(
Ϫ
b1·t)
B2·exp(
Ϫ
b2·t)
The apparent rate constants a1, a2, b1 and b2 together with the am-
plitudes A1, A2, B1 and B2 can be deduced using biexponential fitting
of each kinetics using Eqs. 7 and 8. Then, the pseudo-elementary
rate constants k1 to k4 can be calculated from the S, P, SЈ and PЈ
values and the relative amplitudes A1, A2, B1 and B2.
In this paper this cumbersome manual extraction has been re-
placed by computer calculation using homemade software.
CTC. The lower value of the activation entropy for X
→
CTC could be explained by the absence of hydrogen bond
promotion.
Regarding the two remaining isomers (CTT and TTT),
their thermal stability can be interpreted by the requirement
For the CTC isomer, a2,1
k3, P k1 · k2 k2 · k3 and a2
For the TTC isomer, b2,1 (SЈ Ϯ ͙
k4, PЈ ϭ k1 · k4 k3 · k4 and b2
ϭ
(S Ϯ ͙S2
a1.
Ϫ
4P)/2 with S
ϭ
k1
ϩ
k2
ϩ
ϭ
ϩ
Ͼ
ϭ
S
Ͼ
Ј2 Ϫ 4P
b1.
Ј)/2 with SЈ ϭ k1
ϩ
k3
ϩ
ϩ
for three rotations (C3–C2
two for the ring-closure process.
ϩ
C2–C1
ϩ
C1–C1a) instead of
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CONCLUSIONS
1
Five species are produced upon irradiation of F-Py. The H
NMR and 19F NMR spectroscopies revealed a naphtho-pro-
tonated form (X) and led to the structural determination of
the four theoretically expected photomerocyanines, which
show different thermal stabilities. Thus we proposed the
most plausible thermal reaction mechanism, based on quan-
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the system was analyzed during its dynamic evolution.
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possible to propose some reasonable hypotheses about the
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is activated, whereas the thermal reactivity of the isomers
CTT and TTT in the back reaction to F-Py is negligible.
This work indicates NMR spectroscopy to be a promising
tool for studying photochromic compounds: the number and
the structure of each photomerocyanine can be unambigu-
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erably improved by following the changes occurring in the
photoproducts, and their interdependence. In addition, the
concentrations of the different open forms can be measured
directly from NMR spectra. This gives a significant advan-
tage over the methods used previously. The idea of combin-
ing UV–visible with NMR could bring a better understand-
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