Unexpected halogen substituent effects on the complex thermal relaxation of naphthopyrans after UV irradiation

Article abstract of DOI:10.1021/jo050620o

The kinetics of the thermal fading of four halogenated naphthopyrans (NP) have been analyzed using NMR spectroscopy in CD₃CN. Two photomerocyanines (TT and TC) were detected after UV irradiation. The main relaxation process TC → NP was coupled

Full text of DOI:10.1021/jo050620o

SCHEME 1. Complete Model of All of the Possible
Isomerization Equilibria between NP, TT, and TC
(X ) F, Cl, Br, and I)
Unexpected Halogen Substituent Effects on
the Complex Thermal Relaxation of
Naphthopyrans after UV Irradiation
Stephanie Delbaere,* Jean-Claude Micheau,†
Michel Frigoli, and Gaston Vermeersch
CNRS UMR 8009, Laboratoire de Physique, Faculte´ de
Pharmacie, Universite´ de Lille 2, Lille, France
sdelbaer@pharma.univ-lille2.fr
Received March 30, 2005
naphtho[1,2-b]pyrans (NP), substituted at the 6-position
with a F, Cl, Br, or I atom, were investigated after UV
irradiation in order to appreciate the effect of the nature
of the halogen on the thermal relaxation. This investiga-
tion is of particular interest because the effect of the
nature of the halogen substituents was soon noted for
its ability to influence hydrogen bond formation,8 pho-
tochemical reactivity,9 and enzymatic dehalogenation
reaction.10 All of the naphthopyran molecules were
labeled by two fluorine atoms at the 4 and 4′′ positions
in order to be detectable by 19F NMR spectroscopy. For
each compound, the two expected photomerocyanine
transoid isomers TC (transoid-cis) and TT (transoid-
trans) were characterized by variable-temperature (248
< T < 288 K) 1H, 13C, and 19F NMR spectroscopy in
acetonitrile solution (Scheme 1), and a kinetic analysis
of the complex thermal relaxation was carried out. The
activation and thermodynamic parameters of the various
thermal isomerization processes were determined and
rationalized in regard to the electronegativity and po-
larizability of the halogenated substituent and their
ability to participate in a nonbonded intermolecular
interaction11,12 with the electronegative nitrogen of CD3-
CN.
The kinetics of the thermal fading of four halogenated
naphthopyrans (NP) have been analyzed using NMR spec-
troscopy in CD3CN. Two photomerocyanines (TT and TC)
were detected after UV irradiation. The main relaxation
process TC f NP was coupled with TT/TC isomerization.
The activation parameters of the various processes were
rationalized by considering electronegativity and polariz-
ability of the halogen substituents and their selective
solvation by the electronegative nitrogen of acetonitrile.
Organic photochromism finds applications in data
storage, optical filters, displays, sensor protection,
waveguides, and ophthalmic plastic lenses.1-2 Among the
most useful and widely investigated photochromic mol-
ecules are the 2H-naphtho[1,2-b]pyrans, which lead,
under UV irradiation, to a thermoreversible ring opening.
This reaction gives rise to several photoisomers named
photomerocyanines. It has been reported that within a
given series, the photochromic properties strongly depend
on the nature and position of the substituents3-5 and
solvent environment.6-7 Four di(4-fluorophenyl)-2H-
The time evolution of the concentrations of the three
isomers NP, TC, and TT (Figure 1) were deduced from
the 1H and 19F 1D NMR spectra recorded at regular time
intervals during thermal relaxation. Examination of the
whole set of kinetics indicates that, whatever the tem-
* Corresponding author. Tel: 333 2096 4023. Fax: 333 2095 9009.
† Laboratoire IMRCP, UMR CNRS 5623, Universite´ Paul Sabatier,
F-31062 Toulouse, France.
(1) Becker, R. S.; Michl, J. J. Am. Chem. Soc. 1966, 88, 5931-5933.
(2) Photochromism: Molecules and Systems; Du¨rr, H., Bouas-
Laurent, H., Eds; Elsevier: Amsterdam, 1990. Organic Photochromic
and Thermodynamic Compounds; Crano, J. C., Guglielmetti, Eds.;
Plenum Press: New York, 1999.
(3) Jockusch, S.; Turro, N.; Blackburn, F. J. Phys. Chem. A 2002,
106, 9236-9241.
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Giusti, G. J. Photochem. Photobiol. A 2003, 159, 7-16.
(5) Gabbutt, C.; Heron, B.; Thomas, D.; Light, M.; Hursthouse, M.
Tetrahedron Lett. 2004, 45, 6151-6154.
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10.1021/jo050620o CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/27/2005
5302
J. Org. Chem. 2005, 70, 5302-5304

FIGURE 1. Evolution of the concentrations of the three
isomers of the NP-Cl compound during thermal relaxation at
273 K of an acetonitrile-d3 solution after UV irradiation.
(Symbols are experimental data; continuous lines correspond
to the numerical fitting of the three curves simultaneously.)
perature and the nature of the halogen atom, NP always
increases and TC always decreases. On the other hand,
TT isomer follows a different evolution exhibiting a more
or less intense overshoot or remaining quasiunchanged
during the monitoring time (see the Supporting Informa-
tion). Such a biexponential-like kinetic behavior is in
accordance with a three-species system undergoing the
six a priori possible connected isomerization processes.
To obtain the nature, number, and rate constants of each
individual isomerization path, a numerical kinetic analy-
sis of the plots was carried out using this complete model.
Data treatment of each naphthopyran at each temper-
ature demonstrated that, depending on the nature of the
halogen substituent, only a limited number of isomer-
ization processes occurred simultaneously. All of the
extracted rate constants were then reported on an Eyring
plot to check their regular evolution vs temperature
variations (Figure 2 and Supporting Information).
FIGURE 2. Eyring plots of the TC f NP relaxation process
in the four compounds. Each data point corresponds to an
independent determination of the kTCfNP rate constant by the
numerical modeling of the TT, TC, and NP kinetics.
TABLE 1. Activation Parameters ∆Hq (kJ mol-1) and
∆Sq (J mol-1 K-1) of the Various Pseudoelementary
Processes Occurring during the Thermal Relaxation of
Halogenated NP Moleculesa
TT f TC
∆Hq ∆Sq
84 ( 5 -45 ( 4
Cl 85 ( 4 -10 ( 1
Br 91 ( 4 3 ( 0.5 101 ( 4
n.d. n.d.
TC f TT
∆Hq ∆Sq
n.d.
80 ( 3 -13 ( 0.5 53 ( 4 -120 ( 10
53 ( 2 73 ( 4 -43 ( 2
61(3 -88 ( 4
TC f NP
∆Hq ∆Sq
76 ( 4 -42 ( 2
F
I
a n.d.: process too slow to be determined accurately or not
detected.
Table 1 shows that the investigated naphthopyrans do
not exhibit all of the expected pseudoelementary pro-
cesses. For instance, for the iodo compound, thermal
relaxation is limited to TC f NP irreversible isomeriza-
tion. For the fluoro compound, there are two consecutive
irreversible isomerizations: TT f TC and NP. Finally,
for the chloro and bromo compounds, the situation is
slightly more complicated because in addition to these
two consecutive reactions there is a reverse TT r TC
process; hence, there are TT a TC f NP. The corre-
sponding enthalpy ∆Hq and entropy ∆Sq of activation of
the various reaction paths have been extracted from the
Eyring plots.
For the TT f TC reaction, the observed ∆Hq and ∆Sq
values seem to follow some stereoelectronic substituent
parameter: the more electronegative and less bulky
substituent (F) giving rise to lower values while the less
electronegative and bulkier bromo-substituent, higher
values. Although the reverse TT r TC process was only
detected in chloro- and bromophotomerocyanines, the
same trend is observed (i.e., higher activation parameters
for the heavier and less electronegative bromo substitu-
ent). For the iodo compound, there is an observed lack of
TT a TC reactivity because the corresponding processes
are too slow to be detected in our low-temperature range.
For the fluoro compound, the TT f TC reaction is still
detectable but slow (∆Sq
) -45 J mol-1 K-1) while
TTfTC
the reverse TT r TC is also not detectable.
Examination of the TT a TC equilibrium13 in chloro-
and bromophotomerocyanines shows that in both cases,
∆G° ) (∆HqTTfTC - ∆Hq TCfTT) -T(∆SqTTfTC - ∆Sq
)
TCfTT
is positive (≈ 4-5 kJ mol-1 at 300 K for chloro- vs
bromophotomerocyanines, respectively). TT is more stable
than its isomer TC. However, this equilibrium is continu-
ously displaced in the opposite direction by the irrevers-
ible TC f NP ring closure.
Examination of the relative values of the activation
parameters of the TC f NP ring closure shows an
unexpected order, i.e., Cl < I < (Br ≈ F). Light and heavy
halogen atoms are mixed. No correlation can be estab-
lished between the extracted activation parameters and
any substituent constant related to the stereoelectronic
nature of the halogen substituent. This lack of correlation
is the sign that at least two competing effects are
(13) Allen, F. H.; Davies, J. E.; Johnson, O. J.; Kennard, O.; Macrae,
C. F.; Mitchell, E. M.; Mitchell, G. F.; Smith, J. M.; Watson, D. J. Chem.
Inf. Comput. Sci. 1991, 31, 187-204.
J. Org. Chem, Vol. 70, No. 13, 2005 5303

occurring simultaneously. To find a rational explanation
of this unexpected halogen substituent effect on the TC
f NP thermal relaxation, we have to consider the
involvement of some solute-solvent interactions. Indeed,
it is well-known14 that halogens Cl, Br, and I, but not F,
are able to form contacts at short interatomic distances
with electronegative atoms. As a consequence, I, Br, and
Cl compounds but not F compounds can be solvated in
acetonitrile solution and the solvation power follows the
polarizability scale (I > Br > Cl . F). Moreover, to be
significant, such a solvation effect must be associated to
some structural changes during the course of the reac-
tion. This is the case during the TC f NP ring closure
reaction, where there is a marked change in the electronic
distribution between the open TC and closed NP isomers.
Hence, there would be a solvation change between TC
and NP. Because the entropies of activation are negative,
a degree of freedom is lost when going from the TC-
photomerocyanine to a more ordered preclosed activated
complex. It is then expected that the closed NP would be
tighter (i.e., more solvated) than the open TC and that
solvation would decrease the activation parameters.
From these considerations, it appears that electrone-
gativity and polarizability act simultaneously as two
opposite factors to determine the relative reactivity of the
halogenated naphthopyrans TC f NP ring closure. Going
from F to I, intramolecular substituent effect gives rise
to an increase of the activation parameters with bulki-
ness while intermolecular solvation gives rise to a
decrease with polarizability. Hence, the two parameters
are operating simultaneously leading to apparently
scrambled activation parameter values.15
Because structural changes are minor during TT a TC
isomerization, there are only few solvation effects, and
the main halogen substituent effects are related to a
unique atomic parameter; for instance, activation pa-
rameters increase with the size of the halogen.
Acknowledgment. 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’Enseigne-
ment Supe´rieur et de la Recherche (MENESR), and the
Fonds Europe´ens de De´veloppement Re´gional (FEDER).
Part of this collaborative work was performed within
the framework of the “Groupe de Recherche: Photo-
chromes Organiques, Mole´cules, Me´canismes, Mode`les”,
GDR CNRS no. 2466. We thank Dr. M. Campredon
(Lab. Photochimie Organique Applique´e, University
Me´diterrane´e, Marseille, France) for the gift of NP-F,
NP-Cl, and NP-Br.
Supporting Information Available: Irradiation tech-
niques, NMR details and interpretation of NMR spectra, and
kinetic analysis. This material is available free of charge via
the Internet at http://pubs.acs.org.
JO050620O
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5304 J. Org. Chem., Vol. 70, No. 13, 2005