Photochromism of spirobenzopyranindolines and spironaphthopyranindolines

Article abstract of DOI:10.1039/b007713p

The photocolouration of five spiro[2H-1-benzopyran-2,2′-indoline] compounds (1-5) and two spironaphthopyranindolines (6, 7) to give the ring-opened merocyanine(s) and the subsequent thermal relaxation back to the closed form was studied. A trans-merocyanine but no cis-photoisomer were detected for 1-7 using λ_exc = 308 nm and a singlet pathway accounts for photocolouration. In contrast to nitrospiropyrans the yield of colouration of spiropyrans is essentially independent of solvent polarity. The relaxation times at 25°C range from 0.01 s for 6 in methylcyclohexane to 17 s for 5 in ethanol, which is due to differences in both activation energies (65-80 kJ mol^-1) and the pre-exponential factor (10¹²-10¹⁴ s^-1). The photomerocyanine yield is substantially increased upon naphthalene- or acetone-sensitized excitation, where otherwise the same spectral changes, relaxation time and activation parameters were found. Upon excitation of the trans-merocyanine (λ^exc = 530 nm), a cis-merocyanine with lifetime in the sub ms range could be detected at lower temperatures. Deactivation of the excited trans-merocyanine occurs mainly in the excited singlet state. For 4 and 5, due to an internal heavy-atom effect of bromine substituents, and for 1, a triplet state as a precursor of the cis- and trans-merocyanines could be observed. The effects of structure and medium properties on the photoprocesses are described and the mechanisms of photochromism and thermal decolouration are discussed.

Full text of DOI:10.1039/b007713p

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Photochromism of spirobenzopyranindolines and
spironaphthopyranindolines
Alexander K. Chibisov¤ and 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 12th January 2001
The photocolouration of Ðve spiro[2H-1-benzopyran-2,2@-indoline] compounds (1È5) and two
spironaphthopyranindolines (6, 7) to give the ring-opened merocyanine(s) and the subsequent thermal
relaxation back to the closed form was studied. A trans-merocyanine but no cis-photoisomer were detected for
1È7 using j \ 308 nm and a singlet pathway accounts for photocolouration. In contrast to nitrospiropyrans
exc
the yield of colouration of spiropyrans is essentially independent of solvent polarity. The relaxation times at
25 ¡C range from 0.01 s for 6 in methylcyclohexane to 17 s for 5 in ethanol, which is due to di†erences in both
activation energies (65È80 kJ mol~1) and the pre-exponential factor (1012È1014 s~1). The photomerocyanine
yield is substantially increased upon naphthalene- or acetone-sensitized excitation, where otherwise the same
spectral changes, relaxation time and activation parameters were found. Upon excitation of the
trans-merocyanine (j \ 530 nm), a cis-merocyanine with lifetime in the sub ms range could be detected at
exc
lower temperatures. Deactivation of the excited trans-merocyanine occurs mainly in the excited singlet state.
For 4 and 5, due to an internal heavy-atom e†ect of bromine substituents, and for 1, a triplet state as a
precursor of the cis- and trans-merocyanines could be observed. The e†ects of structure and medium properties
on the photoprocesses are described and the mechanisms of photochromism and thermal decolouration are
discussed.
thopyrans at room temperature, one could assume that the
photochromism is di†erent. The present results show that this
is, however, not the case. Apart from di†erent pathways for
1
Introduction
The photochromism and the thermal relaxation proper-
ties of 6-nitro-spiro[2H-1-benzopyran-2,2@-indolines] (6-
photochemical ring opening for BIPS and NO BIPS, the
2
NO BIPS),1h13 as well as of spirooxazines (SO)14h23 have
thermal decolouration and photochromism follow a unique
2
been studied over the years. The photocolouration of all these
pattern with minor speciÐcity for each group of spiro com-
pounds.
spiro compounds occurs in the sub-ns range.8,15,19,24,25 The
thermal relaxation time (q ) depends on structure, solvent
tvSp
and temperature.1 In spite of these extensive investigations
and in contrast to 6-NO BIPS,7h13 little is still known for
2
BIPS.3,24h27 A likely reason is that BIPS compounds in solu-
tion at room temperature have no permanent colour, since
q
is generally too short. It is therefore not possible to
tvSp
measure the quantum yield (U ) by steady-state methods.
col
This is not the case for the SO, where a major longer-lived
and a minor shorter-lived relaxation time at various tem-
peratures indicate two distinctly di†erent photomerocyanines
and separate pathways for the thermal reaction back to the
closed form (Sp).23 The photochemical properties and relax-
ation kinetics of several groups of spiro compounds are
believed to be rather di†erent, but they have not yet been clas-
siÐed and may have major similarities. For example, what are
the relationships between a spironaphthopyran17,28,29 and a
spironaphthoxazine? It is also an open question as to why the
colour of SO is much stronger than for BIPS not carrying a
nitro group.
In this paper Ðve BIPS (1È5) and two spironaphthopyrans
(6, 7) were studied by photochemical techniques. A compari-
son of these results with those of a spironaphthoxazine (8) and
a spirophenanthroxazine (9, phenanthrospiroindolineoxazine)
provides the possibility of examining the speciÐc role of struc-
ture and substituents on the photoprocesses and relaxation
steps. Owing to the lack of colour of BIPS and spironaph-
2
The
Experimental
1@,3@,3@-trimethyl-spiro[2H-1-benzopyran-2,2@-indoline]
derivatives 130 and 2, 310 were as used previously and com-
pounds 4È7 were from Aldrich and puriÐed by rec-
rystallization. Naphthalene was recrystallized. The solvents
(Merck) were of the purest spectroscopic quality available, e.g.
¤ On leave from the Center of Photochemistry, Russian Academy of
Sciences, 117421 Moscow, Russia.
424
Phys. Chem. Chem. Phys., 2001, 3, 424È431
This journal is ( The Owner Societies 2001
DOI: 10.1039/b007713p

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dimethylformamide (DMF), acetone and acetonitrile: Uvasol;
methylcyclohexane (MCH) was puriÐed by distillation. For
excitation of the Sp form and the trans-merocyanine, j
\
exc
308 nm (excimer laser, EMG 200) and j \ 530 nm (second
exc
harmonic from a Nd-laser) were used, respectively. The
absorption spectra were recorded on a diode array detector
(HP, 8453). The absorbance increase at the absorption
maximum of the trans-merocyanine, j , e.g. 1 ls after the
t
pulse (*A ) for optically matched conditions (e.g., A \ 2.0
col
308
^ 0.05), is linear with the laser intensity and the slope was
taken as a measure of colouration: Prel \ (e ] U ). The
col
t
col
experimental errors in q , E and log A are typically ^7%.
tvSp
a
The other methods are as described elsewhere.13
3
Results and discussion
3.1 Photocolouration
Fig. 2 Absorption spectra of 2 in argon-saturated ethanol using
naphthalene as sensitizer at 25 ¡C (a) in 1 s intervals (upper to lower)
after the pulse and (b) of the transient at 50 ns (L) and 10 ls (…);
For BIPS1h6 and SO14,19h21 the ground state equilibrium (1)
between the ring-closed Sp form and the most stable opened
merocyanine is established (X: N for SO and CH for the
spiroindolines).
j
\ 308 nm; insets: signals at 410 nm (left) and 600 nm (right).
exc
(1)
The absorption spectrum of a given BIPS at low temperatures
(\0 ¡C) and of 7 in polar solvents at ambient temperature has
two major bands with maxima in the UV (j ) and the visible
Sp
(j ). In contrast, virtually no visible absorption could be
t
observed on a timescale of a few seconds and longer upon
excitation at 308 nm of 1È6 even in polar solvents at 25 ¡C.
However, on a shorter timescale a transient with maximum at
j \ 550È600 nm could be observed in all cases examined.
t
Examples for 1È3 and 5È7 are shown in Fig. 1È6, respectively.
The transient is attributed to a trans-merocyanine or a
mixture of two distinct isomers. The quantum yields of
colouration of 1È9 are compiled in Table 1. For 6 in toluene
U
\ 0.2 is in agreement with the literature value of 0.17.16,28
Fig. 3 Transient absorption spectra of 3 (a) in argon-saturated
ethanol at 25 ¡C using naphthalene as sensitizer at 1 ls (L), 10 ls (|)
and 0.3 ms (>) after the pulse; insets: signals at 430 nm (left) and 590
nm (right), and (b) in air-saturated toluene at 100 ¡C, at 1 ls (L), 0.3
col
At higher temperatures the same spectra as at 25 ¡C were
recorded, as illustrated for 3 in toluene [(Fig. 3(b)]. Moreover,
no signiÐcant change in U with temperature (typically 25È
col
ms (>) and 0.8 ms ()); j \ 308 nm; insets: signals at 590 nm.
exc
Fig. 1 Transient di†erence/absorption spectra of
1 in argon-
saturated acetonitrile (a) at 25 ¡C, 1 ls (L) and 10 ms (|) after the 308
nm pulse, inset: signal at 570 nm, and (b) after pre-irradiation with
white light at [50 ¡C, \1 ls (L) and 100 ls (…) after the 530 nm
pulse; solid line: trans ground state; insets: signals at 440, 540 and
610 nm (left to right).
Fig. 4 Absorption spectra of 5 at 25 ¡C (a) in air-saturated ethanol in
1 s intervals (upper to lower) after the pulse and (b) of the transient in
argon-saturated toluene at 0.1 ls (L) and 50 ls (…); j \ 308 nm;
exc
insets: signals at 590 nm.
Phys. Chem. Chem. Phys., 2001, 3, 424È431
425

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Table 1 Quantum yield U of colourationa
col
Compound
MCH
0.18
Toluene DMF
Acetonitrile
Ethanol
1
0.16
0.06
0.08
0.10
0.07
0.22
0.25
0.3
0.12
0.12
0.06
0.07
0.06
0.08
0.25
0.3
0.18
0.07
2
3
0.08
4
0.08
0.06
0.25
0.26
0.06
0.07
0.25
0.38
5
6
0.2
7
8b
9b
0.4
0.5
0.3
0.4
0.4
0.3
a In air- or argon-saturated solution at 25 ¡C; A \ 2. Obtained
308
from P and e values using e \ 3.0 ] 104, 4.0 ] 104 and 6.0 ] 104,
col
t
t
M~1 cm~1 for 1È7, 8 and 9, respectively; see Experimental. b Values
for 8, 9 are taken from ref. 10 and 23.
This is also the case for 4, where no heavy-atom e†ect of the
bromo group is involved in ring opening. A minor heavy-
atom e†ect (\5%) was found for 5 with two bromine atoms
(not shown). Compounds 1È4 and 6È9, i.e. the other spiro
compounds not containing a nitro group, do not show a
triplet state prior to formation of the photomerocyanine at all.
Fig. 5 Transient absorption spectra of 6 (a) in air-saturated MCH at
25 ¡C, \1 ls (L), 30 ms (>), and 100 ms ()) after the 308 nm pulse;
insets: signals at 520 nm, (c) acetone-sensitized at 25 ¡C in argon-
3.2 Sensitized photocolouration
saturated acetonitrile at 50 ns (L) and 10 ls (…), j \ 308 nm;
exc
inset: signal at 560 nm, and (b) in argon-saturated ethanol after pre-
Upon sensitized excitation the photomerocyanine yield is sub-
stantially increased. Examples with acetone in argon-saturated
acetonitrile are shown for 6 [(Fig. 5(c)]. The triplet state of
acetone, although spectroscopically not directly observable
due to its maximum below 330 nm, transfers the energy
towards that of the spiro compound, which then converts into
the merocyanine. The latter “grow-upÏ reÑects the energy
irradiation with white light at [5 ¡C, 1 ls (K) and 0.3 ms (=), j
\
exc
530 nm; insets: signals at 560 nm (left) and 590 nm (right).
100 ¡C for MCH, toluene, DMF) was registered throughout.
For NO BIPS U is substantial in solvents of low polarity
2
col
and decreases with increasing polarity.10,13 This trend was
not found for spiro compounds without a nitro group (Table
1).
transfer kinetics. The rate constant of energy transfer (k ) can
et
be better measured using naphthalene, as shown for 2 [(Fig.
2(b)], 3 [(Fig. 3(a)] and 7 [Fig. 6(a)] in ethanol. The decay of
the triplet state absorption of naphthalene in argon-saturated
ethanol at 415 nm becomes faster on addition of BIPS. For 2
Singlet molecular oxygen formation in substantial yield,
corresponding to that of intersystem crossing and U , has
col
been measured for the Sp form of NO BIPS at room tem-
2
and 7 the k value is ca. 5 ] 109 M~1 s~1.
perature,12,13 whereas for 1È3, 6 and 7 no signal under other-
et
wise the same conditions could be monitored (not shown).
3.3 Thermal ring closure
For 7 the ratio of absorbances at j and j , A /A , increases
t
Sp
t
Sp
on going from MCH or toluene to acetonitrile and further to
ethanol (Table 2). For 1È9 as well as for NO BIPS in solvents
2
of low polarity the A /A ratio is generally very small. The
t
Sp
absorbance at j (A ), measured directly after removal of the
UV light, decreases and approaches the steady-state A (end)
value which is close to zero in most cases. An example is
t
I
e
shown for 3 in toluene at 100 ¡C [(Fig. 3(b)]. The decay of
(A [ A )/(A [ A ) follows a Ðrst-order law and is insensitive
e
I
e
towards oxygen throughout. The inverse relaxation times
(1/q ) at 25 ¡C are compiled in Table 3. The results are more
tvSp
or less in agreement with literature values, for 4 and 5 in
DMF 1/q \ 0.12 and 0.032 s~1, respectively.9 For the
tvSp
spironaphthopyran 6 1/q \ 30 s~1 in DMF and acetonitri-
tvSp
le and two components with 1/q \ 3.6 and 70 s~1 have
tvSp
been reported in toluene.17 A second minor component as
reported for 8, 9 and other SO17,23 could not be registered in
the other cases.
The rate constant for the forward reaction (k \ k
1 ~1
and the equi-
] A /A ] e /e ) is obtained from k B 1/q
t
Sp
Sp t
~1
tvSp
librium constant K \ k /k
is estimated using A /A and
1
1
~1
t
Sp
the ratio of the absorption coefficients (Table 2). A satisfactory
analysis of the spectroscopic changes depends signiÐcantly on
the molar absorption coefficients at the two maxima. Typical
Fig. 6 Transient absorption spectra of 7 in argon-saturated ethanol
(a) naphthalene-sensitized at 25 ¡C, 100 ns (L) and 10 ls (…) after the
values for SO are e \ (0.5È1.0) ] 104 M~1 cm~1.20 For 6 in
Sp
toluene e \ 6.0 ] 104 M~1 cm~1 has been reported.27 It
308 nm pulse and (b) at [50 ¡C, 1 ls (|) and 1 ms (>), j \ 530
t
exc
appears that e of SO depends markedly on the solvent; the e
nm; insets: signals at 560 nm (left) and 590 nm (right), left and right
t
t
lines: trans and cis isomers, respectively.
values (]104 M~1 cm~1) are 5.1 for 9 in MCH and 8.7 in
426
Phys. Chem. Chem. Phys., 2001, 3, 424È431

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Table 2 Absorption maximum, ratio of absorbances and inverse rate constantsa
Compound
Solvent
j /nm
A /A
e /e
(1/k )b/s
(1/k )c/s ] 103
t
t
Sp
t
Sp
~1
1
1
2
3
4
Acetonitrile
Acetonitrile
Acetonitrile
Toluene
Acetonitrile
Ethanol
Toluene
Ethanol
Toluene
Acetonitrile
Ethanol
570
590
590
575
575
585
555
560
555
560
555
595
585
590
\0.01
\0.01
\0.01
O0.002
0.002
0.002
0.002
0.01
0.002
0.05
0.2
0.002
0.03
0.13
6
6
6
5
6
6
7
8
7
8
8
8
8
1.7
1
0.3
1
7
17
0.02
0.07
0.04
0.07
0.17
0.7
7
1
0.6
0.2
2.5
20
5
6
50
0.07
0.04
0.1
0.01
0.01
3
7
8d
9d
Acetonitrile
Toluene
Acetonitrile
2
0.03
10
0.4
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
t
Sp
and k . d Values for 8, 9 are taken from ref. 10 and 23.
~1
ethanol and 3.8, 3.1 and 5.6 for 8 in MCH, toluene and
The relaxation time depends strongly on the temperature.
ethanol, respectively.21 This e /e ratio from the above liter-
Plots of log 1/q
vs. 1/T are shown in Fig. 7 for 2 and 6 in
t Sp
tvSp
ature was taken into account. For 1È5, 8 and 9 in acetonitrile
several solvents in the range 20È70 ¡C (or even up to 110 ¡C).
at 25 ¡C k is of the order of 1 s~1 and 20È50 times larger for
From the linear dependences in all cases, the activation ener-
~1
6 and 7. The 1/k
value increases with polarity, but with
gies (E ) and the pre-exponential factors are derived (Table 4).
~1
a
respect to NO BIPS13 this e†ect is relatively small, e.g., 7-fold
For 1È7 the pre-exponential factor varies from 1012 to 1014
2
for 4. The 1/k value is somewhat related to 1/k
and
s~1. Values for E are 60È85 kJ mol~1 with no signiÐcant
1
~1
a
roughly 103 times larger.
e†ect of the medium, especially no tendency to increase in
Table 3 Inverse relaxation time 1/q (s~1) obtained from the trans photoisomera
tvSp
Compound
MCH
Toluene
DMF
Acetonitrile
Ethanol
1
10
10
15
5
3
3
10
1
0.5
1
3
0.1
0.08
30
12
0.6
1
3
0.15
0.1
30
16
1.5
2.4(60)
0.4
2
0.2[0.2]
0.4[0.4]
0.3[0.4]c
0.06
16
6[5]
3
4
5
0.3
60
25
0.7(20)
0.17(120)
6
90
80
0.7(18)d
0.08(160)
7
8b
9b
0.7
4(40)
a Obtained at 25 ¡C in air- or argon-saturated solution; A \ 1È3. b Values for 8, 9 are taken from ref. 10 and 23. c Values in square brackets
308
refer to naphthalene as sensitizer under argon. d Values in parentheses refer to the minor component, see text.
Table 4 Activation energy and pre-exponential factor of thermal relaxationa
Compound
MCH
Toluene
DMF
Acetonitrile
Ethanol
E /kJ mol~1
a
1
65
70
70
70
70
70
65
80
60
70
70
62/69
70/56
70
70
75
80
70
75
75
80
60
2
70
75
80
70[75]
75[80]
85[85]d
3
4
5
6
65
65
65/78c
60/63
75
70
75[75]
75[80]
85
7
8b
9b
log A/s~1
1
2
3
4
5
6
7
12.3
13.1
13.3
12.8
12.6
12.6
12.3
13.5
10
13.9
13.7
11.9
12.1
13.5
13.2
11
14.5
14.3
12.1
13.5
13.5
11.6[12.3]
12.6[13]
13.2[13]d
13.2
13.2
14.5
13.4
14.3[14]
13.8[14]
a In air- or argon-saturated solution; A \ 1È3. b Values for 8, 9 are taken from ref. 10 and 23. c The second value refers to the minor
308
component. d Values in square brackets refer to naphthalene as sensitizer under argon.
Phys. Chem. Chem. Phys., 2001, 3, 424È431
427

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Scheme 1
isomer with trans geometry. The potential energy surface of
the ground state along the reaction coordinate should have
one major maximum with the activation barrier E
least one minor barrier.
and at
t?Sp
Whether or not the potential energy surface at the position
of the CCC isomer has a minimum remains open as yet. An
open question is also the degree of zwitterionic and quinoid
character in BIPS and SO.5,31,32 A fact that argues in favour
of the zwitterionic character of the merocyanine isomers of
several NO BIPS at room temperature is that the A /A ratio
2
t
Sp
and q
tvSp
of the relationship between q
tvSp
BIPS is now possible (Table 3), but complicated by the small
increase with the solvent polarity.10,13 Examination
and the solvent polarity for
Fig. 7 Semi-logarithmic plots of 1/q
vs. 1/T for 2 (closed symbols)
U
values (Table 1). The di†erences in q
at room tem-
tvSp
and 6 (open symbols) in MCH (circles), toluene (squares), DMF
col
tvSp
perature are the consequence of changes in E rather than the
(triangles), and ethanol (diamonds); j \ 308 nm.
a
pre-exponential factor (Tables 4 and 5). NO BIPS, in contrast
2
exc
to BIPS, are coloured in polar solvents; this is essentially a
consequence of the smaller k values (0.1È10~4 s~1)13 com-
polar solvents. For NO BIPS the di†erences in q
at room
v1
pared with (1È100 s~1) for BIPS. Concerning the thermal ring-
2
tvSp
temperature are mainly due to variations in the activation
energy.1,13 On the other hand, for spiro compounds not
carrying a nitro group the pre-exponential factor rather than
the activation energy changes signiÐcantly. This is illustrated
by the almost parallel lines in Fig. 7 for 2 and 6.
opening, the rate constants of BIPS (0.1È10~3 s~1) are
comparable to, or even larger than, those of NO BIPS (10~2È
2
10~4 s~1).
The photocolouration of NO BIPS (Scheme 1) occurs via
2
the trans triplet state (3trans*) which is in equilibrium with the
Upon acetone- or naphthalene-sensitization, the same relax-
ation time and activation parameters (within experimental
error) were found as with direct excitation (Tables 3 and 4).
This indicates that the properties of the potential energy
surface of the ground state are the same after photochemical
ring opening either in the triplet or singlet states.
perpendicular triplet conÐguration (3perp*).9h13 For
NO BIPS the quantum yield of formation of the 3trans* state
2
is large in solvents of low polarity and decreases with increas-
ing polarity.10,11 Triplet decay leads to formation of both the
cis and trans isomers as short-lived and long-lived species,
respectively. The absorption maximum of the cis isomer is
slightly red-shifted with respect to the trans form. For BIPS,
3.4 Mechanism of photochemical ring opening of BIPS
The two principal pathways of photochemical ring opening
via the triplet state for NO BIPS and bypassing this route for
2
1-9 are illustrated in Schemes 1 and 2, respectively. Indoline
spiropyrans and their photomerocyanines are separated by an
energy barrier. Cleavage of the CÈO bond in the benzopyran
portion leads via the CCC (cis-cis-cis) geometry to a trans
isomer or a distribution of isomers.1h13,31,32 Cleavage of the
CÈO bond of the spiro compounds under examination should
lead to the CCC isomer and subsequently to the most stable
Scheme 2
Table 5 Bleaching and absorption maxima, lifetimes of the triplet and cis isomer and activation energya
Compound
Solvent
j /nm
j
/nm
q b/ls
q
c/ms
E
/kJ mol~1
bl
abs
T
c?t
c?t
1
4
5
DMF
Ethanol
DMF
Ethanol
DMF
Acetonitrile
Ethanol
DMF
Ethanol
DMF
Acetonitrile
Ethanol
380, 550
430, 610
440, 600
430, 620
450, 630
440, 620
440, 620
450, 620
8
40
3
8
8
2
3
\370, 540
\360, 550
\370, 560
\360, 560
\370, 560
\380, 550
5
23
25
28
6
7
2
0.5
0.7
2.5
0.2
25
25
24
24
23
\400, 560
560
560
\400, 560
460, 590
600
480, 590
480, 590
a In argon-saturated solution at [50 ¡C. b Quenchable by oxygen. c Independent of oxygen.
428
Phys. Chem. Chem. Phys., 2001, 3, 424È431

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spironaphthopyrans and SO, however, virtually no spectral
changes (no shift around j ) in the ls or ms time range were
t
monitored, see Fig. 1(a), 3(a) and 4(b), which could be ascribed
to
a
cis-photomerocyanine. Therefore, in contrast to
NO BIPS where the triplet equilibrium and
a
cis-
2
photomerocyanine are involved, a route via the perpendicular
singlet conÐguration (1perp*), reaction (8), is suggested
(Scheme 2). The alternative of a pathway via a very short-lived
triplet state can be rejected on the basis of the results for
NO BIPS.10h13 For 4 and 5 the internal heavy-atom e†ect of
2
the bromine substituents cannot successfully compete with
ring opening in the S state.
1
The energy transfer from the triplet state of acetone or
Fig. 8 Transient di†erence spectra of 4 in argon-saturated ethanol at
[50 ¡C (after pre-irradiation with white light) at 0.1 ls (L), 8 ls (|)
and 30 ls (…) after the 530 nm pulse; insets: signals at 450 nm (left)
and 550 nm (right); solid line: trans ground state.
naphthalene (3D) to the spiro compounds not containing a
nitro group, mimics the triplet route, reactions (9) and (4)È(6)
in Scheme 1. No spectroscopic and kinetic evidence could be
detected that reaction (9) leads to another trans-merocyanine
than reactions (2), (7) and (8) for direct excitation (Tables 3
and 4).
1(b)] and 4 (Fig. 8). For 5 in ethanol at room temperature the
colouration was alternatively enhanced by naphthalene (Fig.
9). The oxygen-sensitive short-lived and oxygen-independent
longer-lived transients are assigned, by analogy to 6-
The potential energy surfaces of the relevant S and T
1
1
states, leading from the Sp form via the CCC to the TTC
isomer or essentially a trans isomer, are illustrated in Scheme
3. A strong decrease of the potential energy of both states is
expected on going from the Sp form to the most stable
merocyanine. The potential energy surfaces of S and S have
NO BIPS, to the triplet state and the observable cis isomer,
2
respectively. For 2 and 3 neither a triplet state nor a cis
isomer could be detected, in contrast to 4 or 5 in DMF where
0
1
both appear (Table 5). The lifetime of the cis isomer (q
)
been calculated for BIPS.33 To account for the results that
c?t
depends strongly on temperature, whereas the triplet lifetime
is rather independent. The absorption spectrum of the cis
isomer can be estimated from the di†erence spectrum with
respect to that of the trans isomer by using the isosbestic
U
is well below unity (Table 1), there should be a deactiva-
col
tion pathway left of the maximum of the E
barrier, i.e. effi-
tvSp
cient internal conversion at the Sp geometry competes with
ring opening via the singlet mechanism. The low efficiency of
the overall photoinduced colour formation is quantiÐed by
the smaller U values of BIPS with respect to NO BIPS
points. From Arrhenius dependences of 1/q
vs. 1/T (Fig. 10)
activation energies of E \ 23È25 kJ mol~1 were obtained
c?t
c?t
col
2
(Table 5). The pre-exponential factor is 2 ] 108 and 0.5 ] 108
(Table 1) and the much shorter q
values (Table 3).
t?Sp
s~1 for 4 and 5 in DMF, respectively.
It appears surprising that for 1 [Fig. 1(b)] and 4 (Fig. 8),
where the photochemical ring opening occurs via singlet states
selectively, the photochemical ring closure occurs via a mixed
singletÈtriplet mechanism (Scheme 3). A possible reason could
be an activation barrier in the S -state, thereby enhancing at
1
lower temperatures Ñuorescence and intersystem crossing at
the expense of formation of the cis isomer. This hypothesis has
to be rejected since triplet population is rather efficient for 4
already at room temperature. Deactivation of the excitated
trans-merocyanine of 1È5 does not lead to a cis isomer and
therefore population of the 1perp* state is unlikely.
Scheme 3
3.6 Spirooxazines
3.5 Deactivation processes from the excited
trans-merocyanine derived from BIPS
The thermodynamic activation parameters of SO are well
known.19,20,23 For 8 and 9 in ethanol *SE \ 6.7 and [23 J
mol~1 K~1, respectively, *HE \ 63È79 kJ mol~1 and
*GE \ 70È77 kJ mol~1.20 The kinetics of ring opening are
very fast and the efficiency of photocolouration is rather
high.14h22 Replacing the naphth- by the phenanthroxazine
Excitation of the trans-merocyanine of 6-NO BIPS leads to a
Ñuorescence signal, the triplet state of the trans-merocyanine
and the intermediacy of the cis isomer.10,11,13 The Ðndings are
described by the mixed singletÈtriplet mechanism for deactiva-
tion of the 1trans* state (Scheme 3). For 6-NO BIPS the
triplet route, reactions (10), (11), (5) and (6), is minor at room
2
2
temperature and the singlet route (10) and (12) is major, but
reduced in viscous media.11 In addition, a permanent blea-
ching becomes signiÐcant in polar solvents, especially in alco-
hols. We ascribe this to the photochemical back-conversion
into the Sp form, reaction (13). An alternative could be photo-
addition of the solvent to a merocyanine chain. However, this
can only be a side route since the e†ect can be repeated, i.e.
essentially Sp is reformed.
For 1È7 at 25 ¡C, however, colouration as
a
quasi-
permanent process is difficult to reveal since the q
values at
tvSp
25 ¡C are rather short (Table 3). Therefore, excitation of the
trans-merocyanine of 1È7 (after pre-irradiation by UV or
white light) was carried out at lower temperatures, where the
Fig. 9 Transient di†erence spectra of 5 in argon-saturated ethanol at
25 ¡C, naphthalene-sensitized (after pre-irradiation in white light) at
0.1 ls (L) and 20 ls (…); insets: signals at 430, 540 and 630 nm (left
to right); solid line: trans ground state.
q
values are sufficiently long. Examples of the transient dif-
tvSp
ference spectra are shown in ethanol at [50 ¡C for 1 [Fig.
Phys. Chem. Chem. Phys., 2001, 3, 424È431
429

View Article Online
6(b). Deactivation of the excited trans-merocyanine of 6 or 7,
in contrast to BIPS, probably involves only the 1perp* state.
4
Conclusion
The photochromism of BIPS 1È5, two spironaphthopyrans (6,
7) and spirooxazines (8,9) is dominated by ring opening via the
singlet state. For the spiro compounds examined, which do
not contain a nitro group, virtually no triplet state and singlet
molecular oxygen could be detected at room temperature.
Only at lower temperatures do the triplet state of 1, 4 or 5
(e.g., at [50 ¡C) and a cis isomer of 4È7 become spectro-
scopically and kinetically observable. The major changes in
the thermal and photochemical behaviour with respect to
NO BIPS result from di†erences in the equilibrium constant
2
K
and the pathways for ring opening, respectively. Other-
1
wise, the properties of the four di†erent types of spiro com-
pound
(BIPS,
NO BIPS,
spironaphthopyrans
and
2
spirooxazines) are rather similar. The q
values, and thus the
tvSp
activation parameters, vary gradually for 1È7, depending more
on the structural characteristics than solvent polarity. The
quantum yield U of colouration of 1È5 at ambient tem-
Fig. 10 Semi-logarithmic plots of 1/q
vs. 1/T (after pre-irradiation
col
c?t
perature is presented for the Ðrst time. For 1È9 U does not
with white light) for 4 in DMF (K), 5 in DMF ()), 6 in ethanol (|)
col
exhibit a substantial dependence either on the type of spiro
and 7 in ethanol (L); j \ 530 nm.
exc
compound or on the solvent polarity. This latter e†ect is in
striking contrast to the behaviour of NO BIPS.
2
has no inÑuence on the overall pattern and only minor e†ects
on the thermal and photochemical parameters. A small e†ect
Acknowledgements
of solvent polarity on A /A and q
for 8 at room tem-
t
Sp
tvSp
We thank Professor Kurt Scha†ner for his support, Professor
Mannschreck for providing compound 1 and Mr. Leslie J.
Currell for technical assistance. A. K. C. is grateful to the
Deutsche Forschungsgemeinschaft and the Russian Fund of
Basic Research (No. 000332300) for Ðnancial support.
perature and larger e†ects for 9 could be due to the quinoid
and zwitterionic character of the merocyanine isomers, respec-
tively.23
SO compounds also show a second minor relaxation com-
ponent (q@ ). The values of q@ \ 5È50 ms at 25 ¡C may be
tvSp
tvSp
compared with q \ 0.5È5 s. The activation energy (E@ ) is
tvSp
comparable to the E
t?Sp
t?Sp
value of the major relaxation com-
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431