F. Wilkinson et al., PICOSECOND TIME-RESOLVED SPECTROSCOPY OF THE PHOTOCOLOURATION REACTION OF PHOTOCHROMIC NAPHTHOXAZINE-SPIRO-INDOLINES, Journal of the Chemical Society. Faraday transactions, 92(8), 1996, pp. 1331-1336
Citations number
19
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
The photochemical formation of the open merocyanine forms of several n
aphthoxazine-spiro-indolines in different solvents have been studied u
sing both picosecond transient absorption (PTA) and picosecond time-re
solved resonance Raman (PTR(3)) methods. The PTA studies have establis
hed the presence of several metastable species in the photochemical fo
rmation of the coloured merocyanine form of these photochromic compoun
ds. The primary photochemical step occurs on the sub-ps timescale and
is followed by the formation of a cisoid intermediate over the next 6-
30 ps. This cisoid species then isomerises to the equilibrated distrib
ution of transoid isomers of the merocyanine form with a lifetime that
is dependent upon both solvent viscosity and polarity as well as the
nature of the substituents on the naphthalene part of the molecule. Ho
wever, the rate of this cis --> trans isomerisation is unaffected on c
hanging the N-alkyl group on the indoline part of the molecule from a
methyl to an isobutyl group. The PTR(3) studies have demonstrated that
, in butan-1-ol, there are at least three different transient species
with characteristic vibrational spectra which evolve with different li
fetimes to give the final isomeric distribution over the first few ns
of the reaction. In cyclohexane it is likely that the formation of a s
ingle species is being probed, which is fully developed after the firs
t 200 ps of the reaction. This single species has a spectrum which is
the same as the equilibrated steady-state resonance Raman merocyanine
spectrum. It is likely that the evolution in the PTR(3) spectra obtain
ed here, in butan-1-ol, results from an equilibration of initially for
med transoid merocyanine isomers to give a more stable distribution in
this polar hydrogen-bonded solvent. Such an equilibration appears to
be unnecessary in a non-polar solvent such as cyclohexane and it is su
ggested that this is because the transoid isomer, initially formed, is
already in its most stable form.