Sub-picosecond transient absorption spectroscopy of substituted photochromic spironaphthoxazine compounds

Article abstract of DOI:10.1016/j.dyepig.2010.05.009

The photochromic reaction dynamics of spiroindolinenaphthoxazine and its 6′CN and 5′CHO substituted compounds is investigated in different solvents by femtosecond transient absorption spectroscopy. In addition to the formation of the merocyanine coloured form (ring-opened trans form, OF), another short-lived intermediate species is produced upon photoexcitation, which is not a precursor to the OF product but which is formed in parallel to it via a competing relaxation process. This species is ascribable to either a relaxed s-cis ring-opened isomer on the ground state potential energy surface or to a metastable minimum of the excited S₁ state potential energy surface of the ring-closed form. The observed kinetics suggest that the production of OF (photocoloraton reaction) is controlled by the efficiency of the competing process rather than by an s-cis - trans isomerisation energy barrier.

Full text of DOI:10.1016/j.dyepig.2010.05.009

Dyes and Pigments 89 (2011) 305e312
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Sub-picosecond transient absorption spectroscopy of substituted photochromic
spironaphthoxazine compounds
Guy Buntinx , Olivier Poizat , Sarah Foleya , Michel Sliwa , Stéphane Aloïse ,
a
a
,
,
1
a
a
*
b
b
Vladimir Lokshin , André Samat
Laboratoire de Spectrochimie Infrarouge et Raman (UMR 8516 du CNRS), Centre d’études et de recherches Lasers et Applications (FR 2416 du CNRS),
Université de Sciences et Technologies de Lille, Bat C5, 59655 Villeneuve d’Ascq Cedex, France
a
b
Centre Interdisciplinaire de Nanoscience de Marseille (UPR 3118), Universités d’Aix-Marseille, Campus de Luminy, Case 901, 13288 Marseille Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
0
0
Article history:
Received 22 December 2009
Received in revised form
The photochromic reaction dynamics of spiroindolinenaphthoxazine and its 6 CN and 5 CHO substituted
compounds is investigated in different solvents by femtosecond transient absorption spectroscopy. In
addition to the formation of the merocyanine coloured form (ring-opened trans form, OF), another short-
lived intermediate species is produced upon photoexcitation, which is not a precursor to the OF product
but which is formed in parallel to it via a competing relaxation process. This species is ascribable to either
a relaxed s-cis ring-opened isomer on the ground state potential energy surface or to a metastable
22 April 2010
Accepted 15 May 2010
Available online 24 May 2010
1
minimum of the excited S state potential energy surface of the ring-closed form. The observed kinetics
Keywords:
Photochromism
Spirooxazine
Spironaphthoxazine
Femtochemistry
Time-resolved absorption
Transient spectroscopy
suggest that the production of OF (photocoloraton reaction) is controlled by the efficiency of the
competing process rather than by an s-cis e trans isomerisation energy barrier.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
and leads to a mixture of coloured, ring-opened merocyanine
isomers (open forms, OF) with extended
p conjugation, which
1
Photochromic spirooxazine derivatives are the object of
considerable efforts of synthesis [1e5] motivated by their wide-
spread use in the commercial manufacture of ophthalmic lenses for
adjustable sunlight-protection glasses or in other applications
requiring the transparency of an object to vary according to
surrounding light intensity. They are also promising compounds for
a wide range of potential applications in photonics such as memo-
ries and switches [6]. Spironaphthoxazine molecules combine
indoline and naphthoxazine moieties linked in a perpendicular and
absorb in the 500e700 nm range. H NMR [10] and resonance
Raman [11] measurements, in agreement with theoretical predic-
tions [12], have shown that only the two most stable OF isomers,
TTC and CTC, are formed. The merocyanine isomers undergo
thermal back reaction in the seconds to minutes time domain at
room temperature [13].
A major purpose of recent experimental [2e5] and theoretical
[14] work on spironaphthoxazines in view of the applications is the
design of new structures with well-controlled and tunable optical
properties. In this regard, Metelitsa et al. showed that modifying
the spironaphthoxazine (SNO) parent molecule by substitution so
as to form push-pull type compounds leads to a notable red-shift of
the merocyanine absorption but simultaneously reduces the
3
electronically uncoupled configuration via a common sp (spiro)
carbon (ring-closed form, CF). Accordingly, their absorption spec-
trum is roughly the superposition of the UV absorption features of
both moieties. The photochromic transformation (Scheme 1)
induced by UV irradiation proceeds via the ultrafast cleavage of the
spiro carbon-oxygen single bond followed by isomerisation [7e9]
quantum yield of photocoloration,
Fcol (CF/OF reaction yield) [3].
To understand the parameters that control the efficiency of the
photochromic process, it is essential to be able to describe in detail,
at the molecular level, the photoinduced reaction pathway. The
ring-opening reaction mechanism and dynamics have been studied
by ultrafast time-resolved spectroscopy for SNO in solution [15e21]
and in the microcrystalline powder phase [22e26]. This reaction
has also been investigated theoretically by using quantum chemical
*
Corresponding author. Fax: þ33 3 20 33 63 54.
E-mail address: olivier.poizat@univ-lille1.fr (O. Poizat).
1
Present address: Université de Franche-Comté, Laboratoire de Chimie Physique
et Rayonnement (UMR CEA E4), 16 route de Gray, 25030, Besançon, France.
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.05.009

306
G. Buntinx et al. / Dyes and Pigments 89 (2011) 305e312
0
(Fcol) on going from SNO to 6 CNeSNO. To better characterize the
influence of substitution on the complex photophysics of this class
of molecules, we have thus undertaken an extensive investigation
by transient absorption spectroscopy of the photoinduced reac-
tivity of a series of substituted derivatives of SNO.
We present here a comparative analysis of the results obtained
0
in various solvents for the parent SNO molecule and its 6 CN and
0
5
CHO substituted compounds, for which the experimental F
col
values have been determined [2]. Whereas the efficiency of the
photochromic transformation is rather good for SNO (
F
col ¼ 0.41 in
methylcyclohexane, 0.32 in ethanol) but notably smaller for
0
0
6
5
CNeSNO (
F
col ¼ 0.17 in toluene, 0.11 in methanol), the behavior of
CHOeSNO is intermediate and can be compared to that of SNO in
0
nonpolar solvents (
in polar solvents (
F
col ¼ 0.35 in toluene) and to that of 6 CNeSNO
F
col ¼ 0.13 in methanol) [2]. These three repre-
Scheme 1. Representation of the photochromic transformation between the ring-
closed form (left) and main ring-open forms (right) of SNO.
sentative compounds are thus interesting since they may be able to
provide precious information on the influence of substituents and
of the solvent on the reaction mechanism.
calculations [12,27,28]. However, the detailed ring-opening mech-
anism remains unclear. In particular the nature of the primary
transient species precursor of the merocyanine form is still ques-
tioned. The CeO bond cleavage is generally assumed to occur in the
2. Experimental
Spironaphthoxazines have been synthesized by reaction of 2-
methylene-1,3,3-trimethylindoline (Fischer’s base) with the corre-
sponding nitroso (or amino) naphthols. Detailed synthesis has
1
excited S state in the sub-picosecond to picosecond time scale. The
growth of the strong red absorption band of the ring-opened
merocyanine ground state product takes place in a few picoseconds
and is accompanied by a spectral evolution (red-shift, band
narrowing, and notable increase in intensity) which has been
explained by the cumulative effects of solvation, vibrational cool-
ing, and cisetrans conformational rearrangement [17e21,28].
Absorption components observed in the 400e550 nm region and
decaying in the sub-picosecond time scale were tentatively attrib-
0
already been reported for the parent SNO molecule [2] and the 6 CN
0
[1] and 5 CHO [33] substituted compounds. Spectrophotometric
grade acetonitrile, ethanol, toluene, and n-hexane solvents were
obtained from Aldrich. Most of the measurements were performed
ꢀ
3
on 10 M solutions circulating in a 2 mm thick flow cell (equipped
with 200 m thick CaF windows). In addition, for some detailed
measurements of the early time dynamics (0e1.8 ps), the flowing
m
2
ꢀ
2
uted to a species belonging to the excited S
1
state surface with
jet sampling technique was adopted (200
mm thick jet, 10
M
contributions from the closed form [17,19] and also a cisoid ring-
opened form (so-called X primary photoproduct) [19], preceding
the formation of the ground state merocyanine form. On the other
hand, combined ab initio and semi-empirical quantum mechanical
calculations recently performed by Maurel et al. for SNO were
consistent with the picture of an ultrafast ring-opening process on
solutions). The apparatus used for obtaining the femtosecond
transient absorption measurements has been described previously
[34]. In the present investigation, 377 and 252 nm pump pulses
(100 fs, 1 kHz) were obtained by frequency doubling and tripling,
respectively, the output of the amplified Ti-Sapphire laser tuned at
756 nm in 300
to 10e20 J per pulse (1.0e2.0 mJ/cm ). The broadband white light
continuum probe pulses were generated by focussing 756 nm, 1
pulses in a CaF plate. In order to avoid rotational diffusion effects
mm thick BBO crystals. The pump power was limited
2
the excited S
1
state surface followed by a quasi-instantaneous
m
barrierless deactivation through a conical intersection to a ground
state s-cis opened intermediate considered as the first photo-
product along the ring-opening pathway [28]. It was concluded
that the only significant barrier to the photoinduced formation of
the final merocyanine form is that of cisetrans isomerization and,
consequently, the time-evolution of the UVevis absorption spectra
is probably essentially characterizing this ground state isomeriza-
tion process.
mJ
2
on the absorption spectra, the pump polarization was set at the
ꢁ
magic angle (54.7 ) relative to the probe. Spectra were recorded
over the 400e740 nm and 770e975 nm spectral ranges and were
corrected from group-velocity dispersion (GVD) effects. The time
resolution was better than 300 fs with the flow cell and 180 fs with
the flowing jet configuration.
Although the nature and position of the substituents greatly
influence the macroscopic properties of spirooxazines (colour-
ability for instance), thus reflecting a strong influence of substitu-
tion upon the photophysical and photochemical characteristics
3. Results and discussion
3.1. Spectral evolution
[3,29e31], very few substituted derivatives of spirooxazines have
been concerned by such ultrafast time-resolved spectroscopic
investigations. There is thus almost no report in the literature on
the influence of substitution on the reaction pathway at the
microscopic level. Recently, we investigated the photophysics of the
Pump-probe transient absorption spectra have been recorded
for n-hexane, acetonitrile, and ethanol solutions of SNO,
0
0
5 CHOeSNO, and 6 CNeSNO in the 400e740 nm range and in the
0e2 ns time domain following pump excitations at 377 and 252 nm.
0
0
0
6
-cyano substituted spironaphthoxazine compound (6 CNeSNO)
6 CNeSNO has also been studied in toluene solution. Typical tran-
ꢀ
3
in acetonirile solution by femtosecond transient absorption spec-
troscopy [32]. Besides the growth of the typical merocyanine
absorption spectrum, a strong band covering the 400e550 nm
region was tentatively assigned to a new, unidentified metastable
species formed concurrently to the merocyanine product, which
suggested the existence of an additional excited-state deactivation
process concurrent to the photochromic reaction and which could
explain the 50% reduction of the photocoloration quantum yield
sient absorption spectra of SNO (10 M) excited at 377 nm and
recorded in the 400e740 nm spectral range are shown in Figs.1 and
2 for solutions in n-hexane and acetonitrile, respectively. The
0 1
377 nm excitation corresponds to the low energy edge of the S /S
transition [35]. Similar spectra obtained in n-hexane and acetoni-
0
trile are shown in Figs. 3 and 4 for 5 CHOeSNO and in Figs. 5 and 6
0
for 6 CNeSNO. For all compounds, the spectra obtained in ethanol
(not shown) are somewhat comparable to those in acetonitrile and

G. Buntinx et al. / Dyes and Pigments 89 (2011) 305e312
307
0
ꢀ3
Fig. 3. Transient absorption spectra of 5 CHOeSNO (10 M) in n-hexane at different
pump-probe time delays in the 0e500 ps range after excitation at 377 nm. The ꢀ1 ps
trace corresponds to the spectral baseline in the absence of pump excitation.
0
differ mainly by their kinetic parameters. The spectra of 6 CNeSNO
in toluene resemble those recorded for this compound in n-hexane.
As a general rule, in all solvents, these transient spectra are char-
acterized by two main absorption components exhibiting opposite
time evolutions (lmax given in Table 1). On one hand, a “red” band
peaking at 550e650 nm is rising with time and corresponds
unambiguously to the coloured merocyanine (open form, OF) by
analogy with steady-state spectra previously reported for this
species [3]. As can be seen in Table 1, its peak position decreases in
energy upon substitution of the SNO skeleton and also on going
from nonpolar to polar solvents, in agreement with steady-state
measurements [3]. On the other hand, a “blue” band lying in the
Fig. 1. Transient absorption spectra of SNO (10 3 M) in n-hexane after excitation at
77 nm. For clarity, the spectral evolution is separated into 3 time domains 0e0.64 ps
top), 0.64e1.88 ps (middle), and 1.8e50 ps (bottom). Spectra below and above 1.8 ps
ꢀ
3
(
400e550 nm region is formed within the pump excitation pulse
width and decays with time. The relative initial intensity of this
band, as compared to the final intensity of the OF band, varies
strongly from one compound to the other and upon the changing of
are recorded with the flowing jet and the flow cell systems, respectively. The pump-
probe time delays are given. The ꢀ1 ps trace corresponds to the spectral baseline in the
absence of pump excitation.
0
the solvent: whereas it is high in the cases of 6 CNeSNO in all
Fig. 2. Transient absorption spectra of SNO (10 3 M) in acetonitrile after excitation at
ꢀ
0
ꢀ3
M) in acetonitrile after
377 nm. For clarity, the spectral evolution is separated into 3 time domains 0e0.8 ps
Fig. 4. Transient absorption spectra of 5 CHOeSNO (10
(
top), 0.8e5 ps (middle), and 5e300 ps (bottom). Spectra below and above 0.8 ps are
excitation at 377 nm. For clarity, the spectral evolution is separated in 3 time domains
0e5 ps (top), 5e20 ps (middle), and 30e500 ps (bottom). The pump-probe time delays
are given. The ꢀ1 ps trace corresponds to the spectral baseline in the absence of pump
excitation.
recorded with the flowing jet and the flow cell systems, respectively. The pump-probe
time delays are given. The ꢀ1 ps trace corresponds to the spectral baseline in the
absence of pump excitation.

308
G. Buntinx et al. / Dyes and Pigments 89 (2011) 305e312
Some spectral modifications accompany the decay of the blue
band and the growth of the OF band. These modifications are
solvent dependent and include band shifts and changes in band
shape. For example, the OF band maximum of SNO in acetonitrile
(Fig. 2) undergoes a 610e575 nm blue shift from 2 to 3 ps, then
a 575e595 nm from 3 to 100 ps. In parallel, the bandwidth
decreases from 20 to 100 ps whereas a shoulder appears at 565 nm.
These changes can have several origins, as already suggested
[17e21,28,32]. Firstly, is the probable presence, for any species
involved in the reaction pathway (excited states and ground state
intermediates, final OF form), of molecular relaxation processes
such as cooling or solvation. A second cause of band shape variation
can be the inhomogeneous character of the population of mole-
cules due to the possible existence, at each step of the reaction, of
a distribution of conformers evolving with specific kinetic charac-
teristics. This is particularly expected for the merocyanine form for
which 8 ground state OF isomers arising from rotation about the
three central bonds can exist, at least transiently, and which evolve
toward the most stable ones, TTC and CTC [10e12]. The effect of
ground state cisetrans conformational rearrangement on the OF
absorption band shape and intensity has been well predicted in the
case of SNO from theoretical calculations [28].
0
ꢀ3
Fig. 5. Transient absorption spectra of 6 CNeSNO (10 M) in n-hexane at different
pump-probe time delays in the 0e500 ps range after excitation at 377 nm. The ꢀ1 ps
trace corresponds to the spectral baseline in the absence of pump excitation.
0
solvents (Figs. 5 and 6) and of 5 CHOeSNO in acetonitrile (Fig. 4)
and ethanol, it is much weaker in the cases of 5 CHOeSNO in n-
0
hexane (Fig. 3) and of SNO (Figs. 1 and 2). For all compounds, the
final intensity of the OF band relative to the initial intensity of
the blue band appears systematically increased upon changing
the pump excitation from 377 to 252 nm, although the spectral
evolution remains unaffected.
In addition to the two absorption components mentioned
above, a decay of absorption is observed for all compounds above
00 nm, suggesting the presence in this region of the edge of a third
near-IR” band. Spectra recorded in the 770e975 nm region for
CNeSNO in n-hexane, ethanol, and acetonitrile confirmed the
presence of a broad absorption band peaking around 880 nm
lmax in Table 1), characterized by similar decay kinetics
discussion below) as the blue band but about three times less
intense. The near-IR and blue bands are thus likely to characterize
the same transient species.
In the case of SNO, the observed spectral evolution is consis-
tent with the most recently reported results [17e20]. For pump-
probe delays longer than 1 ps, the evolution resembles that
7
0
0
observed for the 6 CN and 5 CHO substituted compounds: a broad
and structureless blue band (w1 ps spectrum in Figs. 1 and 2)
declines and a red band typical of the OF species develops. In
addition, a sub-picosecond spectral evolution, not observed for the
substituted compounds, is noticed in the case of SNO. A very
short-lived band (0.3e0.4 ps spectra in Figs. 1 and 2), lying at the
same position as the blue band but showing a sharper structure,
disappears within 1 ps.
“
0
6
(
see
(
3.2. Kinetic analysis
The kinetic evolution is complex and differs noticeably for the
three compounds. It is also strongly dependent on the solvent.
As discussed above, this complexity can be accounted for by the
cumulative effect of molecular relaxation processes arising in the
picosecond time domain and solvent-dependent distribution of
conformers. Examples of plots extracted from the spectra and
monitored at the maxima of the blue and red bands are shown
0
0
for 5 CHOeSNO and 6 CNeSNO in Figs. 7 and 8, respectively.
Approximate fits with 2 or 3 exponential components were
generally satisfying and the corresponding time constants are
0
listed in Table 1. Curiously, for 5 CHOeSNO in ethanol and aceto-
nitrile, the blue band absorption decay is well fitted by a single-
exponential kinetics. On the contrary, in the case of SNO,
a supplementary very short decay component (about 0.2 ps) is
necessary to characterize the transition from the initially produced
blue absorption band to the less structured one. Although some
isosbestic points seem more or less apparent during the decay of
the blue and near-IR bands and the rise of the OF band, the blue
band decay and OF band rising kinetics are never really matching.
In some cases, they are even completely different. This is particu-
0
larly evident for 6 CNeSNO: in n-hexane a main time constant of
about 1 ps characterizes both the OF absorption growth and blue
band decay (Fig. 8, part A), leading to the observation of a quasi-
isosbestic point at 610 nm (Fig. 5) and suggesting that the blue-
absorbing intermediate is the precursor of the OF species; in
contrast, in acetonitrile, the blue band decay is much slower than
the OF band growth (Fig. 8, part B), which excludes unambiguously
the hypothesis that the blue-absorbing intermediate is the
precursor of the OF. The isosbestic point noticed in n-hexane is
0
ꢀ3
Fig. 6. Transient absorption spectra of 6 CNeSNO (10 M) in acetonitrile after exci-
tation at 377 nm. For clarity, the spectral evolution is separated in 3 time domains
e2 ps (top), 2e15 ps (middle), and 15e300 ps (bottom). The pump-probe time delays
are given. The ꢀ1 ps trace corresponds to the spectral baseline in the absence of pump
0
excitation.

G. Buntinx et al. / Dyes and Pigments 89 (2011) 305e312
309
Table 1
0
0
Wavelength of maximum absorption (nm) and main time constants (ps) measured by pump-probe absorption for the blue and OF absorption bands of 6 CNeSNO, 5 CHOeSNO,
and SNO, and for the near-IR band of 6 CNeSNO (pump excitation at 377 nm).
0
Compounds
Band type
n-hexane
Toluene
Ethanol
Acetonitrile
l
s
(%)
l
s
(%)
l
s
(%)
l
s
(%)
0
6
5
CNeSNO
blue band
NIR band
OF band
526
1.0 (72)
529
1.9 (70)
94 (30)
527
1.7 (33)
39 (67)
1.7 (40)
38 (60)
0.8 (60)
6.5 (40)
28 (100)
458
6.8 (15)
30 (85)
8.2 (10)
34 (90)
0.9 (70)
8.5 (30)
78 (100)
2
6 (28)
1.5 (70)
4 (30)
1.2 (68)
3 (32)
0.5 (60)
.5 (40)
3.3 (63)
4 (37)
0.17
.65 (67)
.9 (33)
a
878
623
505
608
485
875
642
501
630
488
880
637
494
623
489
2
635
1.5 (69)
15 (31)
1
0
CHOeSNO
blue band
OF band
7
1.8 (20)
7.5 (80)
0.25
0.45 (50)
4.1 (27)
2.4 (66)
12.5 (34)
0.25
0.5 (24)
2.2 (42)
62 (34)
0.96 (73)
11.6 (27)
1
SNO
blue band
0
9
43 (23)
OF band
550
0.75 (75)
0.5 (25)
604
1.1 (35)
8.5 (45)
595
1
40 (20)
a
Not measured.
0
thus fortuitous. In the case of 5 CHOeSNO, the blue band decay
3.3. Discussion of the photoinduced processes and
photocoloration quantum yield
kinetics is much shorter than the OF band rise kinetics in n-hexane
(Fig. 7, part A) but much longer in acetonitrile (Fig. 7, part B). The
fact that, in acetonitrile, the OF band reaches its maximum inten-
sity well before the full decay of the blue band is clearly visible in
the 15e300 ps spectral evolution observed for 6 CNeSNO (bottom
Previous interpretations of the femtosecond transient absorp-
tion spectra of SNO proposed that the first transient observed
around 490 nm corresponds to the excited S state that deactivates
1
0
of Fig. 6) and in the 50e500 ps spectral evolution recorded for
in less than 1 ps by ring-opening following the spiro carbon-oxygen
bond cleavage to yield a primary photoproduct of cisoid structure,
the so-called X intermediate, also absorbing in the blue region
[17e19]. The cisoid X intermediate is believed to belong to either
0
5
CHOeSNO (bottom of Fig. 4). In Fig. 7, part B, the fast rise that
precedes the main decay kinetics of the blue band (494 nm trace) is
due to the presence of a weak contribution of the OF absorption
band overlapping the blue band. Similarly, we ascribe the slight
decay that follows the fast rise of the OF band (623 nm trace) to the
presence of a weak contribution of the blue band’s foot overlapping
the OF signal. As a matter of fact, a three-exponential kinetic model
involving the two OF band growth components and the blue band
decay component leads to an excellent fit of the experimental
kinetic plot at all wavelengths.
1
the ground state surface [17,18] or to the S state surface [19]. It is
assumed to be a precursor of the final, solvent dependent distri-
bution of transoid OF isomers. The evolution between the X species
and the stable OF mixture at equilibrium arises in several tens of
picoseconds with multiexponential kinetics.
From a theoretical point of view, Maurel et al. recently reported
quantum mechanical results on the photocoloration reaction in the
0
Fig. 8. Kinetic traces for 6 CNeSNO in (A) n-hexane at 526 nm (B) and 623 nm (C)
0
Fig. 7. Kinetic traces for 5 CHOeSNO in (A) n-hexane at 505 nm (B) and 608 nm (C)
and (B) acetonitrile at 458 nm (B) and 637 nm (C). The solid lines are the best fits to
the data (time constants in Table 1). The insets show enlargements of the early time
domain kinetics.
and (B) acetonitrile at 494 nm (B) and 623 nm (C). The solid lines are the best fits to
the data (time constants in Table 1).

310
G. Buntinx et al. / Dyes and Pigments 89 (2011) 305e312
Table 2
Ratio (ROF/blue) of the final intensity of the OF absorption band (band surface
measured at 500 ps) and of the initial intensity of the blue absorption band (band
0
0
surface measured at 1 ps) for 6 CNeSNO, 5 CHOeSNO, and SNO in n-hexane,
toluene, ethanol, and acetonitrile, and quantum yield of photocoloration (
nonpolar and polar solvents.
Fcol) in
ROF/blue
Hex
F
a
col
Tol
EtOH
Aceto
Nonpolar
Polar
0.11e
0
0
c
6
5
CNeSNO
CHOeSNO
0.42
2.0
2.5
0.43
0.32
1.0
2.0
0.35
0.38
1.9
0.17
0.35
b
c
e
0.13
b
0.41d
0.32f
SNO
a
From Ref. [2].
b
c
d
e
f
Not measured.
In toluene.
In methylcyclohexane.
In methanol.
In ethanol.
Scheme 2. Representation of the ground and first excited potential energy surfaces for
the ring-opening reaction of SNO based on the reaction scheme established from
theoretical calculations (Fig. 9 in ref. [28]).
decay of the blue band, which points out the important result that
the blue-absorbing intermediate, previously named X, cannot be
the precursor of the OF product. The analogy of the blue band decay
kinetics and OF band rising kinetics observed for some SNO
derivatives is thus probably fortuitous. The second point is the
surprising constancy of the OF band growth kinetics. For almost all
compounds and all solvents, the appearance of this band can be
acceptably fitted by a two-exponential kinetic model with a main
component of time constant around 1 ps and a lower one of time
constant of about 10 ps (see Table 1). Such constancy in the OF
formation kinetics contrasts with the strongly substituent- and
solvent-dependent efficiency of OF production measured by the
SNO family [28] that were consistent with the picture of an ultrafast
ring-opening process on the excited S state surface followed by
1
a near barrierless deactivation to the ground state surface through
a conical intersection (CI), yielding both an s-cis opened interme-
diate and the initial closed form with a distribution ratio of about
1
:1 (Scheme 2). This predicted s-cis opened form could correspond
to the X intermediate observed experimentally. Then, the s-cis form
is expected to evolve via cisetrans isomerization toward the trans
forms (transition state TSt
closure process (transition state TSt
erned by the relative heights of the respective energy barriers.
Indeed a good correlation was found between experimental
2
) in competition with the back ring-
) with the partition ratio gov-
coloration quantum yields (see
Fcol values in Table 2). For instance,
1
in nonpolar solvents, the col value doubles on going from
F
0
0
6 CNeSNO to 5 CHOeSNO or SNO but the time constants of OF
appearance in n-hexane are not significantly different in the 3
0
quantum yields of photocoloration (
substituted SNO derivatives and the calculated cisetrans isomeri-
zation barriers E(TSt ), suggesting that this barrier is an important
F
col) reported for a series of
compounds. On the other hand, for 6 CNeSNO as well as for
0
5 CHOeSNO,
Fcol is notably enhanced on going from nonpolar to
2
polar solvents but the OF appearance time constants do not
decrease and are even slightly higher in n-hexane than in ethanol
or acetonitrile. The nearly constant value of the rate constants of
formation of OF is not consistent with the previous statements that
the substitution and solvent effects on the efficiency of this reac-
parameter in controlling the photocoloration reaction [28].
On the other hand Metelitsa et al. found, for a series of 17
substituted SNO compounds, that the quantum yield of photo-
coloration is systematically higher in toluene than in methanol [3].
This effect has been interpreted, with reference to the assumption
that an intermediate X is involved in the reaction mechanism, by
supposing a better stabilization by solvation of X and thus a struc-
ture closer to the initial ring-closed reactant in the polar solvent. In
this configuration, a weak energy barrier between X and the closed
form would favor the ring-closure back reaction. On the contrary, in
toluene, the less solvated X species would have a structure closer to
the final OF photoproduct, lowering the energy barrier toward the
OF form and thus favoring its formation. This interpretation is
consistent with the conclusion of Maurel et al. stating from
tion, measured by
Fcol, results from variations of the associated
barrier height [3,28]. Therefore, it must be assumed that a parallel
photoinduced process is competing with the formation of the OF
species and arises with an efficiency strongly dependent on the
substituent and solvent, thus controlling the
The third important piece of information arising from the time-
resolved data concerns the influence of the nature of the substit-
Fcol value.
uent and the solvent on the relative intensity of the blue and OF
OF/blue
absorption bands. The ratio R
of the final intensity of the OF
absorption band, as measured by the band surface in the 500 ps
spectrum, to the initial intensity of the blue absorption band, as
measured by the band surface at 1 ps, for 6 CNeSNO, 5 CHOeSNO,
2
quantum chemical results that the E(TSt ) reaction barrier between
a cisoid intermediate and the stable trans OF species is a crucial
parameter governing the photocoloration reaction [28].
0
0
and SNO in the different solvents is given in Table 2, together with
Confronting the transient absorption results obtained for the
CNeSNO, 5 CHOeSNO, and SNO compounds with the corre-
the
solvents [2]. There is an apparent correlation between the R
and col variations. On one hand, in a given solvent, both param-
Fcol values reported for these compounds in polar and nonpolar
0
0
OF/blue
6
sponding quantum yield of photocoloration is expected to provide
very complementary information on the influence of the nature of
the substituent and the solvent on the reaction mechanism. A key
issue for this task is certainly the assignment of the transient
absorption blue band that evidently plays a determinant role in the
photophysics of these compounds. In this regard, three major
conclusions can be stated from the analysis of the spectral and
F
0
0
eters increase on going from 6 CNeSNO to 5 CHOeSNO and SNO.
On the other hand, for a given spironaphthoxazine compound, both
parameters increase on going from polar to nonpolar solvents.
0
The strongest effect is observed for 5 CHOeSNO, for which both
the R
OF/blue
and Fcol factors are somewhat similar to those found for
SNO in nonpolar solvents but rather comparable to those of
0
0
kinetic evolution observed after excitation for 6 CNeSNO,
6 CNeSNO in polar solvents. This correlation suggests that an
0
5
CHOeSNO, and SNO. The first point concerns the dissimilar
kinetics of the appearance of the OF species (red band) and the
increase of the blue-absorbing species yield parallels a decrease of
the OF yield, i.e. both species are formed via competing

G. Buntinx et al. / Dyes and Pigments 89 (2011) 305e312
311
pathways. This result corroborates the above conclusion that the
blue-absorbing species is not the precursor of OF and reveals that it
is, instead, the product of the photoinduced process that competes
with the formation of OF and controls its efficiency.
With regards to these results, the exact nature of this blue-
absorbing species cannot be identified with certainty but two
hypotheses can be proposed on the basis of the schematic repre-
sentation of the ring-opening reaction path (Scheme 2) derived
from that proposed by Maurel et al. for SNO from quantum
chemical predictions [28]. A first possibility is an assignment to the
ground state s-cis form, as proposed by some authors for X
processes other than that driving the system to the CI [28].
However, it is not excluded that, in such complex molecules,
relaxation of the excited FranckeCondon geometry toward the S /S
1 0
CI is competing with relaxation along other valleys of the excited
state potential energy surface. In this case, it must be assumed that
1 0 1
another S /S CI allows fast deexcitation of the metastable S state
minimum to account for the fast decay observed for the blue
absorption band. This interpretation corresponds to that already
0
proposed in a previous report on 6 CNeSNO excited at 377 nm [32],
where a competing excited state deactivation route was assumed to
involve a blue-absorbing chemical intermediate called Y. In this
previous work it was suggested that both Y and a short-lived
intermediate species X, precursor of the OF product, were contrib-
uting to the transient blue absorption band. However this point
remains to establish. In fact, in the hypothesis that a blue-absorbing
excited state metastable minimum is populated in parallel to the
ring-opening deactivation route, it is not clear whether or not
another blue-absorbing species X precedes the formation of OF.
[17,18,28], but with the further restriction that the s-cis form is not
the precursor of OF. To take this constraint into account, it can be
envisaged that ultrafast (sub-picosecond) excited state relaxation
through the S /S conical intersection (CI) leads to an unsolvated
1 0
and vibrationally hot ring-opened cisoid species, which can either
relax by cooling/solvation toward a vibrationally relaxed s-cis form
at equilibrium geometry or evolve via cisetrans isomerisation to
2
the trans OF species (passage above the TSt barrier). The observed
growth and band shape evolution of the OF band can be due to
structural relaxation and thermal equilibration of the distribution
of OF conformers after the barrier crossing. Moreover, it must be
assumed that the barrier between the relaxed s-cis form and the
4
. Conclusion
We have presented an analysis of the photochromic reaction
0
0
dynamics of spiroindolinenaphthoxazine and its 6 CN and 5 CHO
substituted compounds in different solvents by femtosecond
transient absorption spectroscopy. In addition to the formation of
the merocyanine coloured form (ring-opened trans form, OF), we
have undeniably demonstrated the presence of another metastable
species, which can be attributed to either a relaxed ground state
s-cis ring-opened isomer or to a metastable minimum of the
trans form (transition state TSt
2
) is too large to be surpassed so that
barrier but vibrationally
(
i) hot s-cis molecules can cross the TSt
2
relaxed s-cis molecules cannot and (ii) the only possible deactiva-
tion of vibrationally relaxed s-cis is to cross the barrier of refor-
mation of the closed form (transition state TSt ). In support to this
1
assumption, the theoretical calculation performed by Maurel et al.
predict systematically, for various substituted SNO derivatives, that
1
excited S state potential energy surface of the ring-closed form,
the TSt
barrier [28]. A low TSt
2
barrier is one order of magnitude higher than the TSt
barrier implies that fast ring-closure reac-
1
the formation of which is competing with that of merocyanine. The
relative yields of these two concurrent processes appear strongly
dependent on the nature of the substituent and on the solvent and
their variation parallels the changes in quantum yield of photo-
coloration observed for these compounds [2]. On the basis of the
observed kinetics, it is concluded that, at least in the case of the
molecules investigated in this work, the production of OF (photo-
coloraton reaction) is controlled by the efficiency of the competing
process rather than by an s-cis e trans isomerisation energy barrier.
This result asserting the existence of a branching into two parallel
and competing excited-state deactivation processes is certainly
a key aspect of the photoreactivity of spirooxazines.
1
tion must yield back the CF molecule, which is consistent with the
fast decay observed for the blue-absorbing species. In this
hypothesis, the OF growth and relaxed s-cis form decay kinetics are
effectively unrelated, as found experimentally. As discussed above,
a near-IR band maximizing in the 800e900 nm region is associated
with the blue band and is thus also attributed to the s-cis species.
Both spectral features are consistent with the theoretical vertical
excitation wavelengths predicted, in the case of SNO, for the s-cis
species with the CS-INDO-CIPSI method [28]. The fact that the
formation of the OF product and s-cis isomer are competing
processes is easy to understand but the substitution and solvent
effects on the partitioning ratio is more difficult to rationalize. A
1 0
strong dependence of the position of the S /S CI on the substituent
Acknowledgments
nature and solvent is probably determinant in these effects. In any
case, the observation that the formation of OF is favored upon
switching the pump excitation wavelength from 377 nm to 252 nm
can be understood as the effect of a much larger excess of vibra-
The authors thank the Groupement de Recherche GDRI 93
0
“Phenics” from CNRS and the Centre d Etudes et de Recherches
Lasers et Applications (CERLA) for their help in the development of
this work. CERLA is supported by the Ministère chargé de la
Recherche, Région Nord/Pas de Calais, and the Fonds Européen de
Développement Economique des Régions.
tional energy given to the system. After fast S
through the CI, this excess energy contained in the hot s-cis
molecules is logically expected to promote the TSt barrier crossing
1 0
/S relaxation
2
to the detriment of vibrational cooling, thus enhancing the
formation of the trans OF form and reducing the production of the
blue-absorbing relaxed s-cis form. To summarize, this mechanism
assumes that both the final trans OF and relaxed s-cis form (blue-
absorbing species) arise simultaneously and concurrently from
a vibrationally hot cisoid population produced from ultrafast
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