934
S. DELBAERE ET AL.
EXPERIMENTAL
irradiation period have been neglected. This assumption
is fully justified if F is strictly constant when the
irradiation wavelength is close to an isobestic point or if
the absorbance of the photochemical reacting solution is
sufficiently low. As the irradiation light was not strictly
monochromatic, Fij and ei values must be considered as
wavelength-averaged.
The calculated evolution in concentrations of different
species was obtained by numerical integration of the set
of differential equations written from the Scheme 5, using
homemade curve fitting software.17
From the general Scheme 5, all the values are taken into
account to start a fitting procedure. Starting values for the
kij or the hij were chosen around 10ꢂ3, and were then
refined by trial and error until a good fit was obtained.
Finally, the parameters were fitted automatically using an
iterative algorithm of the Powell type, designed to
minimize the residual error (RE ¼ SnSm [YcalꢂYobs)]2/
nm), between the experimental and the calculated curves
(n is the number of experimental data points and m the
number of kinetic curves). Parameters which were
estimated either too small or not significant were removed
one by one until a systematic increase of the residual error
was observed.
Degassing and irradiation
Experiments were carried out in degassed toluene-d8 at
2–3 mmol dmꢂ3 concentration. The degassing of solution
to remove oxygen was made by the technique of freeze-
pump-thaw cycles (five cycles: 2.25 ꢁ 10ꢂ6 Torr) directly
in the J. Young valve NMR sample tubes (Wilmad
507-JY-7). Photoirradiation was carried out directly into
the NMR tube in a home-built apparatus with a 1000 W
high-pressure Hg–Xe lamp equipped with filters. Mono-
chromatic UV light was obtained by passing the light
through a first filter (Schott 11FG09: 259 < l <388 nm
with lmax ¼ 330 nm, T ¼ 79%), then through an inter-
ferential one (l ¼ 313 nm and T ¼ 7%). Monochromatic
visible light was obtained by passing the light through a
first filter (Schott SCFIKG1503: 295 < l < 800 nm with
T ¼ 50% at l ¼ 330 and 700 nm), then through an
interferential one (l ¼ 434 nm and T ¼ 16%). NMR
spectra were recorded on a Bruker 500 spectrometer
(1H, 500 MHz) equipped with TXI probe, using standard
sequences. Data sets were processed using Bruker
Topspin 1.3 software.
We found that the fitting procedure was very selective
among the various possible processes taken under
consideration. For instance, the simulated kinetic curves
were significantly distant from their corresponding
experimental data if some important pathway was omitted
or if the presence of a forbidden process was artificially
imposed.
UV–Visible spectroscopy
UV–Visible spectra were performed in toluene solution
(5.02 ꢄ 0.21 ꢁ 10ꢂ5 M) of spectrometric grade (Aldrich)
at 208C. The analysis cell (optical path length 1 cm) was
placed in a thermostated copper block with magnetic
stirring inside the sample chamber of a Varian Cary 50
spectrometer. An Oriel 200 W high-pressure Xe–Hg lamp
was used for irradiation equipped with interferential filter
(l ¼ 313 nm and T ¼ 17%). The photostationary state was
attained after 40 min of irradiation, then the sample was
kept in the dark for thermal evolution (Fig. 4).
Acknowledgements
´
The 500 MHz NMR facilities were funded by the Region
Nord-Pas de Calais (France), the Ministere de la Jeunesse,
`
de l’Education Nationale et de la Recherche (MJENR)
´
´
´
and the Fonds Europeens de Developpement Regional
(FEDER). Part of this collaborative work was performed
within the framework of the ‘Groupe de Recherche:
Kinetic data analysis
A general scheme involving the seven species (OD-CN,
OD-TTC, OD-CTC, OD-TTT, CD-CN, CD-TTC and
CD-CTC) has been reduced to 20 processes. Each process
transforming the species i into the species j is
characterised by a rate vij. (Scheme 5).
´
´
Photochromes Organiques, Molecules, Mecanismes,
`
Modeles’, GDR CNRS n8 2466. S.D. and J-C. M.
acknowledge ‘Phenics’ (Photoswitchable Organic Mol-
ecular Systems & Devices) GDRI CNRS n8 93 for
financial support and G.V. is indebted to the French
Embassy in Japan, for the financial support, during
Japan–French Seminar in Japan. M.F. acknowledges sup-
port by the EPSRC (UK).
These rate can be expressed either thermally
227
(vij ¼ k [Xi]) or photochemically (vij ¼ lhij [Xi] ¼
ij
l
227
Fij ei l I0 F [Xi]) or both (vij ¼ (227kij þ hij) [Xi]). kij is
the thermal rate constant of bleaching at 227 K of
compound ‘Xi’ into compound ‘Xj’. lhij corresponds to an
apparent first order rate parameter at l ¼ 313 nm or
434 nm, Fij is the quantum yield of the photochemical
transformation, ei is the molar absorption coefficient of
compound Xi at the irradiation wavelength, l is the optical
path, I0 is the incident monochromatic photon flux and F
the photokinetic factor. Its variations during the UV
REFERENCES
1. In Photochromism, Brown GH (ed.). Wiley: New York, 1971.
2. In Photochromism Molecules and Systems, Durr H, Bouas-Laurent
H (eds). Elsevier: Amsterdam, 1990.
3. In Organic Photochromic and Thermochromic compounds, Crano
JC, Guglielmetti R (eds). Plenum Press: New-York, 1999.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 929–935
DOI: 10.1002/poc
Delbaere
