Photochromism of nitrospiropyrans: Effects of structure, solvent and temperature

Article abstract of DOI:10.1039/b007708i

The thermal and photochemical ring opening and ring closure of the spiropyran/merocyanine couple of three nitro-substituted spiropyrans (6-NO₂: 6, 7 and 8-NO₂: 8) were studied and the results were compared with those of other 6-NO₂ spirobenzopyranindolines (BIPS) (1-5). The photocolouration, which occurs in the triplet manifold throughout, and the photochemical conversion and thermal relaxation of the two observable photomerocyanines into the closed spiropyran (Sp) form (decolouration) were quantified as a function of solvent polarity and temperature. The relaxation time (τ_t-Sp) at 25°C ranges from 2 s for 5 in methylcyclohexane to 10⁴ s for 7 in ethanol. This large variation in τ_t-Sp is due to changes in both the activation energy (E_a = 75-105 kJ mol^-1, increasing with polarity) and the pre-exponential factor (A = 10¹²-10¹⁵ s^-1). The quantum yield of colouration with 308 nm pulses is substantial in solvents of low polarity (φ_col = 0.3-0.8) and decreases (<0.2) with increasing polarity. The observed merocyanine triplet state is rather short-lived (<10 μs) and the precursor of the trans- and a cis-merocyanine. The same intermediates, the triplet state and the cis-isomer, were observed when the trans-merocyanine was excited at 530 nm. The thermal cis → trans isomerization takes place in the μs-ms range and has an activation energy of E_c→t = 30-40 kJ mol^-1. The effects of substituents and medium properties are described and the mechanism of photochromism is discussed.

Full text of DOI:10.1039/b007708i

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Photochromism of nitrospiropyrans: e†ects of structure, solvent and
temperature
Helmut Gorner
Max-Planck-Institut fur Strahlenchemie, D-45413 Mulheim an der Ruhr, Germany
Received 22nd September 2000, Accepted 30th November 2000
First published as an Advance Article on the web 9th January 2001
The thermal and photochemical ring opening and ring closure of the spiropyran/merocyanine couple of three
nitro-substituted spiropyrans (6-NO : 6, 7 and 8-NO : 8) were studied and the results were compared with
2
2
those of other 6-NO spirobenzopyranindolines (BIPS) (1È5). The photocolouration, which occurs in the
2
triplet manifold throughout, and the photochemical conversion and thermal relaxation of the two observable
photomerocyanines into the closed spiropyran (Sp) form (decolouration) were quantiÐed as a function of
solvent polarity and temperature. The relaxation time (q ) at 25 ¡C ranges from 2 s for 5 in
tvSp
methylcyclohexane to 104 s for 7 in ethanol. This large variation in q is due to changes in both the
tvSp
activation energy (E \ 75È105 kJ mol~1, increasing with polarity) and the pre-exponential factor
a
(A \ 1012È1015 s~1). The quantum yield of colouration with 308 nm pulses is substantial in solvents of low
polarity (U \ 0.3È0.8) and decreases (\0.2) with increasing polarity. The observed merocyanine triplet state
col
is rather short-lived (\10 ls) and the precursor of the trans- and a cis-merocyanine. The same intermediates,
the triplet state and the cis-isomer, were observed when the trans-merocyanine was excited at 530 nm. The
thermal cis ] trans isomerization takes place in the lsÈms range and has an activation energy of E \ 30È40
c?t
kJ mol~1. The e†ects of substituents and medium properties are described and the mechanism of
photochromism is discussed.
Other properties of 6-NO BIPS are the involvement of a
merocyanine-type triplet state prior to formation of the trans-
1
Introduction
2
The photoprocesses and subsequent thermal rearrangement of
isomer and of a cis-isomer as second intermediate in photo-
colouration.18 The pathway of photochemical ring opening
has been the subject of discussion over the years.1h7 For 5,7-
spiro[2H-1-benzopyran-2,2@-indolines] (BIPS),1h5 especially of
6-nitro-substituted derivatives (6-NO BIPS),1h25 have been
2
intensively investigated. For 6-NO BIPS in polar solvents at
2
dichloro-6-NO BIPS in acetonitrile at 24 ¡C Bach and Calvert
2
ambient temperature an equilibrium between the ring-closed
have concluded that 80% and 20% of the photocolouration
form (Sp) and the merocyanine (open form) is established; the
most stable isomer is indicated as trans-merocyanine. Four
merocyanine conformers with the central transoid segment
can be expected for BIPS after thermal or photochemical
cleavage of the CÈO bond. The eight coloured merocyanine
conformers have a zwitterionic or quinoid character.
occurs in the excited singlet and triplet states, respectively.25 A
singlet pathway for 6-NO BIPS has also been proposed by
2
other authors.11 The intermediacy of a cis-photomerocyanine,
which thermally converts into the trans-merocyanine in the ls
or ms range at room temperature,21 may have severe conse-
quences on the photodegradation and photooxidation.26 On
the other hand, for 1 in acetone no triplet state or cis-isomer
could be registered upon fs excitation and observation at 620
nm.24
This paper aims to provide a deeper insight into the photo-
chemical properties of NO BIPS and the relaxation processes.
2
For this purpose, two 6-NO BIPS (6, 7) and an 8-NO BIPS
2
2
Upon UV irradiation of 6-NO BIPS the colouration is
(8) were studied by time-resolved spectroscopy for the Ðrst
time. The results were compared with those of parent 6-
2
enhanced6 and this photochemical ring opening occurs in the
sub-ns range.7 The quantum yield (U ) is moderate or large,
NO BIPS (1) and four derivatives (2È5). The collected
col
2
but its dependence on the solvent properties has not been
material constitutes a Ðrm basis for mechanistic conclusions
completely understood.1h4 When the light is removed the
absorbance in the red spectral range decreases and reaches its
original stationary value. This deÐnes the thermal relaxation
and gives the possibility of evaluating the speciÐc role of sub-
stituents and solvent properties in the photochromism. The
photochemical ring opening of 6-NO BIPS in the excited
2
time (q ) which depends on the structure, solvent and tem-
singlet state can now be excluded.
tvSp
perature.8h17
2
Experimental
The
3@,3@-dimethyl-6-nitro-R -spiro[2H-1-benzopyran-2,2@-
8
indoline] derivatives 1È5, are the same as used pre-
viously.18h23 Compound 7 was synthesized according to
methods described elsewhere27 and 628,29 and 8 are from
416
Phys. Chem. Chem. Phys., 2001, 3, 416È423
This journal is ( The Owner Societies 2001
DOI: 10.1039/b007708i

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Aldrich and were puriÐed by recrystallization. The solvents
(Merck) were of the purest spectroscopic quality available, e.g.
dimethylformamide (DMF), acetone and acetonitrile: Uvasol;
methylcyclohexane (MCH) was puriÐed by distillation.
Time-resolved experiments were typically performed by
excitation with the 308 nm output of an excimer laser
(Lambda Physics, EMG 200). In some cases the second har-
formation was obtained from the values of *A
at the
TT
maximum, j \ 430È460 nm. Singlet molecular oxygen for-
TT
mation was measured using a standard method and the cor-
rection for the radiative rate constant in each solvent.23 The
experimental error of U is typically ^15%.
*
3
Results and discussion
monic from a Nd-laser (JK lasers), j \ 530 nm was used.
exc
The laser photolysis set-up was essentially the same as used
3.1 Spiro-merocyanine ground state equilibrium
previously.18h23 Absorbances of 0.5È2 (in 1 cm cuvettes) were
typically used for j \ 308 nm, corresponding to Sp concen-
The absorption spectrum of NO BIPS in polar solvents at
exc
2
trations of 0.1È0.4 mM. The molar absorption coefficient of
room temperature has two major bands with maxima in the
the trans-merocyanine (e ) shows some variation with structure
UV (j ) and the visible (j ). An example is shown for 7 in
t
Sp
t
and solvent; values of e \ 3.5 ] 104 and 5.2 ] 104 M~1 cm~1
acetonitrile (curve 1 in Fig. 1a), having maxima at j \ 336
t
Sp
nm and j \ 545 nm. The ground state properties of 1, 6, 7
t
were used for non-polar and polar solvents, respectively.1 The
absorption spectra were recorded on spectrophotometers
(Perkin-Elmer, 540 and a Hewlett Packard diode array, 8453).
and 8 in polar solvents are rather similar. The ratio of absorb-
ances at j and j , A /A , in these cases increases on going
from MCH or toluene to acetone and further increases on
going to acetonitrile or ethanol (Table 1). Only 5 seems to be
an exception since equilibrium (1) favours the Sp side even in
polar solvents, i.e. no remaining colour could be achieved. In
solvents of low polarity the A /A ratio is generally very
t
Sp
t
Sp
In several cases, e.g. where q
is long enough, the spectral
tvSp
and kinetic measurements were also possible under contin-
uous irradiation at room temperature, using either a Hg 150
W lamp (e.g. 366 nm) or the analyzing light of the laser set-up
(directly or after passing through an interference Ðlter). The
kinetics of decolouration (and colouration when appropriate)
in the longer time range ([1 s) were measured after irradia-
tion (at [450 and 308 nm, respectively) using the diode array
with appropriate interference Ðlters (if necessary). Plots of log
A@/A@max \ log(A [ A)/(A [ A ) and log(A [ A )/(A [ A )
t
Sp
small, as is shown for 7 (Fig. 2a) and 6 (Fig. 3a) in toluene.
The analysis of the spectroscopic changes depends signiÐ-
e
I
e
e
I
e
were obtained from the absorption decrease and increase,
respectively, (at j ) vs. time; A refers to the initial (light-
t
I
induced) value and A to the Ðnal (end) value after relaxation.
e
The same q
value was obtained from the increase of
tvSp
absorption after irradiation at j [ 500 nm. The experimental
errors in q
are typically ^7% (10 signal average) and
tvSp
^15% for values above and below 1 s, respectively and those
of E and log A are ^6%. The rate constant k was deter-
a
1
mined from the other data in Table 1 using the equilibrium
condition: k \ k A /A (e /e ). Note that this is only a
1
~1 t Sp Sp t
rough estimation because the trans-merocyanine has also a
maximum in the UV. The corrected 1/k value for 6 or 7 in
~1
polar solvents should be smaller by up to 30%. The increase
in absorbance at j , e.g. 1 ls after the pulse (*A ) was found
t
col
to be linear with the laser intensity (\1 MW cm2) for opti-
cally matched conditions (e.g. A \ 1.0 ^ 0.05). The slope
308
(Prel) was taken as a measure of colouration and the absolute
col
Fig. 1 Absorption spectra of 7 in acetonitrile at 25 ¡C. (a) Steady-
state (1) prior to and (2) after irradiation at 308 nm and (3) after irra-
quantum yield of photocolouration (U ) was obtained from
col
the values of Prel/e using 3 in toluene as reference. The U
diation at [450 nm; inset: relaxation kinetics, A@/A@max for j \ 308
col t
col
irr
value is taken as 0.72,21,23 being the average of two literature
values: 0.593 and 0.85.12 The relative quantum yield of triplet
nm (L) and [450 nm (…). (b) Transient spectra under argon at 30 ns
(L) and 1 ls (…) after the 308 nm pulse; insets: signals as indicated.
Table 1 Thermal equilibrium: absorption maximum, ratio of absorbances and inverse rate constantsa
Compound
Solvent
j /nm
A /A
(1/k )b/s
(1/k )c/s ] 103
t
t
Sp
~1
1
1d
Toluene
DMF
Acetonitrile
Ethanol
Toluene
DMF
Acetonitrile
Ethanol
Toluene
DMF
Acetonitrile
Ethanol
DMF
605
560
558
537
603
550
545
538
600
550
545
536
590
580
555
O0.002
0.03
0.05
0.08
O0.002
1.2
1.2
0.6
O0002
0.15
0.3
25
330
400
1000
40
3300
5000
10 000
25
2000
2500
3300
50
30
500
P50
70
50
80
6
7
8
P90
18
27
110
P60
90
50
27
3
2
10
0.8
0.1
0.1
0.3
Acetonitrile
Ethanol
a In air- or argon-saturated solution at 25 ¡C; A \ 1.5È2.5. b Obtained from q , following irradiation at 308 nm. c Calculated from A /A
308 tvSp
and k ; values of e \ 3.5 ] 104 and 5.2 ] 104 M~1 cm~1 were used in non-polar and polar solvents, respectively. d Values for 1 were partly
~1
taken from ref. 20 and 21.
t
Sp
t
Phys. Chem. Chem. Phys., 2001, 3, 416È423
417

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Table 2 Inverse relaxation timea
Compound
MCH
Toluene DMF
Acetonitrile
Ethanol
1b
2
3
4
5
6
7
8
50
70
70
40
60
30
3
8
10
20
25
0.3
0.5
20
2.5
10
10
3
1
3
6
3
500
30
300
25
40
30
8
0.2
0.4
30
0.1
0.3
2
40
60
a Values of 1/q
(s~1 ] 10~3), referring to trans ] Sp conversion,
tvSp
were obtained at 25 ¡C in air- (or argon-) saturated solution; A
1È3. b Values for 1È5 from ref. 21 were partly modiÐed.
\
308
330 and 400 s may be compared with literature values of 8,
300 and 330 s, respectively.8,9
In cases where the A /A ratio is large enough, a photo-
t
Sp
Fig. 2 Absorption spectra of 7 in air-saturated toluene. (a) Steady-
state relaxation (intervals of 10 s) after irradiation at 308 nm. (b) Tran-
sient spectra at 30 ns (L) and 1 ls (…) after the 308 nm pulse; insets:
signals as indicated.
chemical back-conversion into the Sp form can be achieved by
irradiation of the merocyanine at P450 nm. Such a photo-
decolouration has already been reported for 1.2,3,6 Examples
are shown for 7 in acetonitrile (Fig. 1a), for 6 in acetonitrile
(Fig. 4a) and for 8 in ethanol (Fig. 5a). The relaxation time is
the same as that after enhanced photocolouration by UV
light.
cantly on the molar absorption coefficients at the two
maxima. As typical values e \ 0.8 ] 10 M~1 cm~1 at j
The results described above refer to room temperature, but
Sp
Sp
and e \ 3.5 ] 104 and 5.2 ] 104 M~1 cm~1 for the trans-
1/q
depends strongly on the temperature. In each solvent
changes by several orders of magnitude over a wide
t
tvSp
merocyanine are generally accepted for 6-NO BIPS in non-
q
2
tvSp
polar and polar solvents, respectively.6,12,14
temperature range, e.g. 0 to 80 ¡C or up to 110 ¡C. Plots of log
1/q
vs. 1/T are shown in Fig. 6 for 3 in several solvents.
tvSp
3.2 E†ects of structure and solvent on the dynamics of
thermal decolouration
From the linear Arrhenius dependences in all cases, the values
of the activation energy (E ) and the pre-exponential factor are
a
derived (Table 3). For 1 in DMF and acetonitrile E is slightly
a
After removal of the UV light, the enhanced colouration
slowly reverts back to the original. The absorbance at j (A ),
measured directly after removal of the light, decreases and
smaller than the literature values of 96 and 105 kJ mol~1,
respectively.8 The pre-exponential factor varies from 1012 to
t
I
1015 s~1 and the E values from 75 to 105 kJ mol~1 with a
a
approaches the steady-state A (end) value. The decay of A@/
tendency to be higher in polar solvents. Pre-exponential
e
A@max is related to one component, which follows a Ðrst-order
factors of 1015 s~1, albeit rather high, have been reported for
law and is not sensitive towards oxygen throughout. The
the thermal cis ] trans isomerization of carbocyanine dyes.30
inverse relaxation times (1/q ) at 25 ¡C are listed in Table 2.
For 1 in three polymers E \ 78È99 kJ mol~1.15 For 6,8-
tvSp
a
Examples are shown for 7 in acetonitrile (inset in Fig. 1a), 6 in
(NO ) BIPS in aqueous solution at pH 7.4, E \ 72 kJ mol~1,
2 2
a
acetonitrile (inset in Fig. 4a) and 8 in ethanol (inset in Fig. 5a).
*HE \ 70 kJ mol~1, *GE \ 106 kJ mol~1 and *SE\ 119 J
mol~1 K~1.13
For 1 in toluene, DMF and acetonitrile at 25 ¡C q \ 25,
tvSp
Fig. 4 Absorption spectra of 6 in acetonitrile. (a) Steady-state (1)
Fig. 3 Transient absorption spectra of 6 in toluene at 25 ¡C. (a) Air-
saturated at \1 ls (|) and 1 ms (>) after the 308 nm pulse and (b) at
50 ns (L) and 5 ls (…). Insets: signals as indicated from upper to
lower in argon-, air- and oxygen-saturated toluene.
prior to and (2) after irradiation at 308 nm and (3) after irradiation at
[450 nm; inset: relaxation kinetics, A@/A@max for j \ 308 nm (L)
irr
and [450 nm (…). (b) Transient spectra under argon at 0.1 ls (L)
and 50 ls (…) after the 308 nm pulse; insets: signals as indicated.
418
Phys. Chem. Chem. Phys., 2001, 3, 416È423

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are mainly due to the activation energy (Fig. 6). On the other
hand, for most NO BIPS the pre-exponential factors also
2
change signiÐcantly (Table 3). Larger q
values in ethanol
tvSp
are found for all cases examined, except for 5. The trend of an
increasing q
on increasing the polarity is most pronounced
tvSp
for 6 and 7, where it increases ca. 100-fold in acetonitrile with
respect to toluene.
The equilibrium constant K \ k /k
~1
A /A and the ratio of the absorption coefficients (Table 1)
is estimated using
1
1
t
Sp
and the rate constant k for the forward reaction is obtained
1
from k ] k \ 1/q
(Bk ) and K . For 1 and 3 in ace-
1
~1
tvSp
~1
1
tonitrile the respective values are 1/k \ 400 and 100 s and
~1
1/k \ 5 ] 104 and 4 ] 103 s. Both values vary with structure
l
by ca. two orders of magnitude and tend to increase with
increasing polarity. For six 6-NO BIPS in acetonitrile at
2
45 ¡C the 1/k values were reported to be rather similar, (2È
1
7) ] 103 s, whereas 1/k
changes by more than 3 orders of
~1
magnitude.14 For 1È8 at 25 ¡C the 1/k values change signiÐ-
1
Fig. 5 Spectra of 8 in air-saturated ethanol at 25 ¡C. (a) Steady-state
(1) prior to and (2) after irradiation at 308 nm and (3) after irradiation
cantly, from (2È3) ] 103 s for 4 or 8 in DMF to 1 ] 105 s for 6
in ethanol, but the variation in 1/k is larger. The increase in
~1
at [450 nm; inset: relaxation kinetics, A@/A@max for j \ 308 nm (L)
the 1/k
value with increasing polarity is most pronounced
irr
and [450 nm (…). (b) Transient di†erence at 0.1 ls (L) and 20 ls
~1
for 7, where it increases 250-fold on going from ethanol to
toluene. A zwitterionic transition state rather than one with
non-ionic character has already been concluded for 1.8,9
(…); insets: signals as indicated.
The relaxation time at room temperature depends signiÐ-
cantly on the structure and solvent polarity.6,9 For eight 6-
NO BIPS k was found to be related to the ionic charge of
3.3 Dynamics and efficiency of photocolouration
UV excitation of the ring-closed form of a 6-NO BIPS gener-
ates two (or three) detectable species which are distinguished
by spectroscopy and kinetics. The transient which is formed
within the laser pulse width is short-lived, \10 ls, decays by
Ðrst-order kinetics and has a broad absorption spectrum. Its
2
~1
2
the substituent moiety.15 For 1 q
increases with solvent
at room temperature
tvSp
polarity.6,9 For 3 the di†erences in q
tvSp
two maxima, denoted as j , are listed in Table 4. This short-
TT
lived transient is quenched by oxygen and has been assigned
to a triplet state of the photomerocyanine in the cases of 1È
5.18h23 The triplet lifetime (q ) is in the 5È10 ls range at 25 ¡C
T
in the absence of oxygen, but is shorter for 7 (Table 4).
The other major species, which is long-lived or can be
quasi-permanent on the timescale of seconds, is observed sub-
sequent to the triplet intermediate and assigned to the photo-
merocyanine.18,23 In fact, it is preferentially the most stable
trans-merocyanine with a major maximum in the visible at j
t
and a minor one around 400 nm, but in several cases it also
includes a contribution from the cis-isomer. Examples of the
initial changes in the sub-ls range are shown for 6 in toluene
(Fig. 2b) and acetonitrile (Fig. 1b); j B 600 nm and j are at
TT
t
600 and 550 nm, respectively. Time-resolved absorption
spectra are shown for 6 in toluene (Fig. 3b) and acetonitrile
(Fig. 4b).
For NO BIPS the relative quantum yield of triplet forma-
2
tion at j (*A ) is almost proportional to the *A value,
TT
TT
col
denoting the relative yield of the trans-merocyanine form.
Both yields decrease with polarity and are smallest in alcohols
(Table 4). The quantum yields of photocolouration are listed
in Table 5. For 1È5 it has been shown that U is substantial
col
in solvents of low polarity and decreases with increasing
Fig. 6 Plots of log 1/q
vs. 1/T for 3 in MCH (L), toluene (K),
tvSp
DMF (@), acetonitrile (|) and ethanol ()); j \ 308 nm.
polarity.19,21h23 This trend and the smallest values in acetoni-
exc
Table 3 Activation energy and pre-exponential factor of thermal relaxationa
Compound
MCH
Toluene
DMF
Acetonitrile
Ethanol
1
2
3
4
5
6
7
8
75(11.7)b
85(13.4)
70(11.0)
80(12.6)
85(13.3)
85(13.2)
90(13.2)
95(14.5)
95(14.4)
80(13.2)
85(13.3)
100(13.8)
100(14.0)
100(15.6)
80(13.8)
95(14.7)
95(14.5)
80(12.5)
90(14.1)
100(13.6)
95(13.1)
100(15.8)
95(13.5)
100(14.8)
95(14.1)
90(11.1)
90(14.5)
105(14.2)
100(13.8)
105(15.5)
75(12.7)
90(14.0)
80(13.3)
90(14.0)
90(14.2)
85(13.5)
a E values (in kJ mol~1) were measured in air- or argon-saturated solution; A \ 1È3. b Values in parentheses: log A (s~1).
308
a
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419

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Table 4 TÈT Absorption maxima, triplet lifetime,a quantum yield of singlet oxygen formation and relative yields of triplet formation and
colouration
Compound
Solvent
j
/nm
TT
q /ls
*A
b
U c
*A b
T
TT
*
col
1d
3d
6
Toluene
Acetonitrile
Toluene
Acetonitrile
Toluene
Acetonitrile
Toluene
Acetonee
Acetonitrile
Ethanol
440, 580
440, 590
430, 560
430, 560
460, 590
460, 590
430, 600
430, 600
430, 600
430, 600
450, 580
6
8
7
1.0
0.1
0.9
0.2
0.5
0.1
0.4
0.2
0.75
0.10
0.72
0.19
0.45
0.08
0.38
1.0
0.1
0.8
0.2
0.6
0.1
0.4
0.3
8
10
10
0.4
0.2
0.2
0.08
5
7
0.15
0.1
0.3
0.09
0.08
0.28
0.09
0.12
0.3
8
Toluene
a In argon-saturated solution; A \ 2. b Using 1 in toluene as reference. c In oxygen-saturated solution. d Values for 1 and 3 are taken from ref.
308
21 and 23. e j \ 354 nm, with respect to 1 in toluene.
exc
trile or ethanol were also found for 6È8. The suggested e†ect
triplet yield; it is relatively large for 1È523 as well as for 6È8 in
solvents of low polarity and smaller in polar solvents (Table
4).
of solvent polarity on U is the presence of n,p and p,p triplet
col
states; ring opening occurs via the 3Sp(n,p)* state, which is
lower lying in solvents of low polarity and above the 3Sp(p,p)*
state in polar solvents.19 This model also explains the larger
In some cases a small but signiÐcant spectral change in the
sub-ms time range was monitored (Table 6), an example is
shown for 6 in toluene (Fig. 3a). This is due to a contribution
of a cis-photomerocyanine which thermally converts into the
trans-merocyanine. The lifetimes of the cis-isomers of 1È5 at
room temperature are in the ls or ms range.21 Triplet decay
leads to formation of both the long-lived (quasistable) trans-
isomer and the shorter-lived cis-isomer, the absorption
maximum of the latter is slightly red-shifted. The same behav-
iour for 1È8 indicates that a triplet mechanism also accounts
for 8 and probably other BIPS where the nitro group is sub-
stituted at the 8 position.
The pathway via triplet states for 1È8 (Scheme 1) is initiated
by population of the excited singlet state (1Sp*), reaction (2),
followed by intersystem crossing at the Sp geometry, reaction
(3). Further steps are: ring opening in the triplet manifold,
reaction (4), establishment of the triplet equilibrium, reaction
(5), intersystem crossing at the perpendicular conÐguration
and internal rotation in the ground state to the cis- and trans-
isomers, reaction (6). Eventually, thermal isomerization, reac-
tion (7) yields the trans-isomer.
quantum yield of colouration of 5,7-dichloro-6-NO BIPS in
2
acetonitrile upon triplet sensitized excitation.25
3.4 Mechanism of photocolouration
For 6-NO BIPS the activation energy of thermal colouration
2
is E \ 80È110 kJ mol~1.1,2 In particular, the values for 1 in
Sp
benzene, chloroform and ethanol are 70, 97 and 103 kJ
mol~1.6 There is general agreement that cleavage of the CÈO
bond in the benzopyran portion leads initially to the CCC
(cis-cis-cis) structure (which, however, could not be directly
observed) and then to one or several isomers with partial
trans-geometry.1h10,31,32
The time-resolved photocolouration of 6-NO BIPS occurs
2
via the trans-triplet state (3trans*) which is in equilibrium with
the perpendicular triplet conÐguration (3perp*).18h23 For 6-8
in non-polar solvents a substantial phosphorescence signal of
singlet molecular oxygen appeared. The quantum yield of
singlet oxygen formation (U ) follows the same trend as the
*
The observed triplet refers to the 3perp* ¢ 3trans equi-
librium throughout since that of the Sp form is too short-lived
Table 5 Quantum yield U of colourationa
col
Compound MCH Toluene Acetoneb Acetonitrile Ethanol
1c
2
3
4
5
6
7
8
0.83
0.60
0.85
0.83
0.18
0.72
0.24
0.09
0.34
0.12
0.29
0.12
0.04
0.2
0.12
0.12
0.07
0.07
0.03
0.17
0.21
0.83
0.5
0.63
0.6
0.4
0.02
0.09
0.05
0.4
0.4
a In air- or argon-saturated solution at 25 ¡C; A \ 2. Obtained
308
from P ; see Experimental. b j \ 354 nm. c Values for 1È5 are
col
exc
taken from ref. 21 and 23.
Scheme 1
Table 6 Lifetime of the cis-isomer and activation energya
Compound
(1/k )b/ls
c?t
Toluene
E
/kJ mol~1
c?t
DMF
Acetonitrile
DMF
Acetonitrile
1c
3
300
[500
380(400)
200
20(20)d
60(55)
5
16(16)
70(60)
4
30
17(12)
40(40)
40(40)
32
39
40(40)
35
6
7
8
300(320)
30
32
32
a In air- (or argon-) saturated solution; j \ 530 nm after pre-irradiation at \400 nm where necessary, A \ 0.2È1. b At 25 ¡C. c Values for 1
exc
530
and 3 from ref. 21 were partly modiÐed. d Values in parentheses refer to j \ 308 nm.
exc
420
Phys. Chem. Chem. Phys., 2001, 3, 416È423

View Article Online
in Ñuid media and the amount of cis-isomer is not sensitive to
oxygen.18h23 Owing to the intermediacy of the observed
triplet having a lifetime of 0.2È1 ls for 7 and 5È10 ls for the
other NO BIPS (Table 4), colour formation is delayed in the
2
absence and accelerated in the presence of oxygen. This triplet
quenching by oxygen is illustrated in Fig. 3b (insets) for
oxygen-, air- and argon-saturated toluene. It is interesting to
note that for 1 in acetone upon fs excitation and observation
at 620 nm no triplet state or cis-isomer could be registered;24
instead, bleaching recovery within 0.7 ps is complete and the
lifetime is 84 ps.
3.5 Photoprocesses from the trans-merocyanine
For those cases, where equilibrium (1) is not completely
shifted to the Sp form, selective excitation of Sp is not possible
since the trans-merocyanine has also a band in the UV, see
Fig. 4a. Therefore, the photochemistry of the trans-isomer is of
great importance. For the trans-merocyanine of 1È5 in polar
solvents at room temperature a weak emission signal centred
at 640È670 nm (U \ 0.002È0.02) has been attributed to Ñuo-
Fig. 8 Spectra of 6 in argon-saturated toluene at 25 ¡C (after pre-
irradiation at \400 nm). (a) Transient di†erence at 0.1 ls (L), 5 ls
(K), 0.3 ms (|) and 1 ms (…) after the 530 nm pulse; insets: signals as
indicated and (b) steady-state absorption spectra of trans-(full) and
cis-(dotted) isomers.
f
rescence of the
S
state (1trans* ] trans).21 In methyl-
1
tetrahydrofuran at [196 ¡C the Ñuorescence becomes much
larger (U is up to 0.7) since the competing twisting process is
f
retarded by the high viscosity.22
Detection of the triplet state of the trans-merocyanine and a
cis-isomer as major secondary species has been reported for
1È5 in solution at room temperature.21 The viscosity-induced
retardation of twisting at lower temperatures also enhances
the triplet yield at the expense of the yield of the cis-isomer.22
Both species were observed upon excitation at 530 nm of the
trans-merocyanine forms of 6È8 at room temperature. Exam-
ples are shown for 7 in ethanol (Fig. 5b), for 6 in acetonitrile
(Fig. 7a) and toluene (Fig. 8a). The transient absorption has
maxima at 420È460 nm and at 600È660 nm and strong blea-
The decay rate constant of the cis-isomer of 6-NO BIPS
2
increases with temperature. The lifetime (1/k ) at 25 ¡C is in
c?t
the lsÈms range. A plot of log k
depends linearly on 1/T
c?t
and the activation energy (E ) is up to 40 kJ mol~1.21 Com-
c?t
parable results were found for 6È8 (Fig. 9) and the 1/k
and
c?t
values are given in Table 6.The Ðndings are described by
E
c?t
a mixed singletÈtriplet mechanism (Scheme 2).
For 6-NO BIPS at room temperature the triplet route,
2
reactions (9) ] (5) ] (6), is minor and the singlet route, reac-
ching around j and below 400 nm. The kinetics of bleaching
tion (10) is major, but reduced in viscous media.22 Reactions
(5)È(7) can also be observed upon UV excitation of the Sp
form, because the spectral variations are consistent with
thermal conversion of the cis-component, present in the
mixture, in the trans-form. This is shown for 6 in toluene (Fig.
3a). The absorption spectrum of the cis-isomer can be esti-
mated from the di†erence spectrum with respect to that of the
trans-isomer by using the isosbestic points. This is illustrated
t
recovery and absorption decay are Ðrst-order and practically
identical (insets in Fig. 7a and 8a). The initial decay com-
ponent is due to triplet decay, reaction (6), and the second
component is due to decay of the cis-isomer, reaction (7). In
air-saturated solution the 3trans* state is rather short-lived
(\1 ls) and the cis-isomer can subsequently be detected (Fig.
7a). In the absence of oxygen the cis-isomer appears partly via
triplet decay after a few ls due to q \ 3È10 ls (Fig. 8a). Note
T
that the triplet lifetime upon population of the 1trans* state
with j \ 530 nm is practically the same as upon population
exc
of the 1Sp* state with j \ 308 nm.
exc
Fig. 7 Spectra of 6 in air-saturated acetonitrile at 25 ¡C. (a) Tran-
sient di†erence at 30 ns (L), 1 ls (|) and 10 ls (…) after the 530 nm
pulse; insets: signals as indicated and (b) steady-state absorption
spectra of trans-(full) and cis-(dotted) isomers.
Fig. 9 Plots of log 1/q
vs. 1/T for 6 (full symbols) and 8 (open
c?t
symbols) in toluene (squares), DMF (diamonds) and acetonitrile
(triangles), j \ 530 nm; in toluene: j \ 308 nm (circles).
exc
exc
Phys. Chem. Chem. Phys., 2001, 3, 416È423
421

View Article Online
Overhauser e†ect study that the TTC isomer is the most
stable structure.
Under discussion is the degree of zwitterionic and quinoid
character in BIPS.5,37 In favour of the zwitterionic character
of the merocyanine isomers of several NO BIPS is that q
at
2
tvSp
room temperature strongly increases with solvent polarity.9
Although this e†ect is observed for all NO BIPS (Table 2), the
2
maximum change varies from 10-fold (5, 8) to 100-fold (1È3) or
even 300-fold (6, 7). At least in the latter two cases a zwitter-
ionic character is demanded. A strong decline in the potential
energy surface of the relevant S and T states is expected on
1
1
going from the Sp form via the CCC to the TTC isomer or
another more stable merocyanine.To account for the result
that U is well below unity, even for NO BIPS, (Table 3),
col
2
there should be a deactivation pathway left of the maximum
of the E barrier in Scheme 2.21h23
t?Sp
4
Conclusion
Scheme 2
The photochromism and thermal decolouration of NO BIPS
2
6È8 and 1È5 are quite similar. The 1/q
values of 6È8 at
for 6 in acetonitrile (Fig. 7b) and toluene (Fig. 8b). A rather
similar spectrum with a red shift of ca. 10 nm for the cis-
isomer is reminiscent of the behavior of trans-merocyanines
which are not derived from spiropyrans.19
tvSp
room temperature are large in solvents of low polarity and
markedly smaller in polar solvents, such as acetonitrile or
ethanol, where the activation parameters are larger. The
and the 1/k value with increasing polarity is
most pronounced for 7 (6-NO -8-CO CH BIPS), where it
increases ca. 100- and 250-fold, respectively. Only one triplet
state, in particular that of the photomerocyanine(s) and one
cis-isomer were observed upon excitation of either the spiro
compound or its trans-merocyanine. The decrease in the U
values with polarity is ascribed to ring opening via the n,p
triplet state and an increasing energy gap with respect to the
3(p,p)* state (neither could be detected spectroscopically) due
to the usual shift of the 3(n,p)* state. The e†ects of solvent and
temperature on the decay rate constant of the observed cis-
isomer were analyzed. While the photomerocyanines are
exclusively formed via triplet states, a mixed singletÈtriplet
mechanism is suggested for deactivation of the excited trans-
merocyanine.
increase in q
tvSp
~1
2
2
3
3.6 E†ects of structure and solvent on the deactivation
processes
In non-polar media certain 6-NO BIPS undergo dimer-
2
ization.33 For 3 in cyclohexane at 8 ¡C a dimer of the Sp and
col
photomerocyanine forms has been reported, its equilibrium
constant was found to be 3.2 ] 104.16 In fact, NO BIPS in
2
non-polar solution show drastic changes at lower tem-
peratures (not shown). However, using small concentrations,
as in this work, no drastic di†erence of the various e†ects
could be revealed in MCH at and above room temperature
with respect to more polar solvents, see for example Fig. 6.
Nevertheless, a rather peculiar behavior was found in toluene,
where the largest lifetime of the cis-merocyanines of 1È8 and
the smallest activation energies were obtained (Fig. 9). Inter-
estingly, ring-opened spiropyrans with alkyl chains are able to
form J- or H-aggregates in a bilayer matrix.34
Acknowledgements
The author thanks Professor Kurt Scha†ner for his support,
Professor Alexander K. Chibisov from the Center of Photo-
chemistry in Moscow for providing compound 7, Dr. J.
Leitich for helpful discussions and Mr. Leslie J. Currell for
technical assistance.
It should be emphasized that the triplet mechanism upon
excitation of the Sp form and a mixed singletÈtriplet mecha-
nism upon excitation of the trans-merocyanine (Scheme 2) are
suggested for NO BIPS; the triplet data for 1, 3, 6È8 (Table 4)
2
are rather similar. This holds also for 8, the only 8-NO BIPS
2
examined in this work, where oxygen a†ects the growth
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423