Opening a spiropyran ring by way of an exciplex intermediate

Article abstract of DOI:10.1021/jo062124l

(Chemical Equation Presented) A molecular dyad has been synthesized in which the main chromophore is a 1,4-diethynylated benzene residue terminated with pyrene moieties, this latter unit acting as a single chromophore. A spiropyran group has been condense

Full text of DOI:10.1021/jo062124l

Opening a Spiropyran Ring by Way of an Exciplex Intermediate
Andrew C. Benniston,* Anthony Harriman, Sarah L. Howell, Peiyi Li,
and Donocadh P. Lydon
Molecular Photonics Laboratory, School of Natural Sciences, Bedson Building, Newcastle UniVersity,
Newcastle Upon Tyne, NE1 7RU, United Kingdom
a.c.benniston@ncl.ac.uk
ReceiVed October 13, 2006
A molecular dyad has been synthesized in which the main chromophore is a 1,4-diethynylated benzene
residue terminated with pyrene moieties, this latter unit acting as a single chromophore. A spiropyran
group has been condensed to the central phenylene ring so as to position a weak electron donor close to
the pyrene unit. Illumination of the pyrene-based chromophore leads to formation of a fluorescent exciplex
in polar solvents but pyrene-like fluorescence is observed in nonpolar solvents. The exciplex has a lifetime
of a few nanoseconds and undergoes intersystem crossing to the pyrene-like triplet state with low efficiency.
Attaching a 4-nitrobenzene group to the open end of the spiropyran unit creates a new route for decay
of the exciplex whereby the triplet state of the spiropyran is formed. Nonradiative decay of this latter
species results in ring opening to form the corresponding merocyanine species. Rate constants for the
various steps have been obtained from time-resolved fluorescence spectroscopy carried out over a modest
temperature range. Under visible light illumination, the merocyanine form reverts to the original spiropyran
geometry so that the cycle is closed. Energy transfer from the pyrene chromophore to the merocyanine
unit leads to an increased rate of ring closure and serves to push the steady-state composition in favor of
the spiropyran form.
Introduction
display photochromism, the so-called spiropyrans4 have received
intensive study over many decades. Indeed, a wide variety of
important applications have been found for photochromic
spiropyrans. Such uses range from transient data storage,5 to
optical filters for eye protection,6 to self-developing photogra-
phy.7 Of late, considerable interest has been given to the
The term photochromism refers to the reversible transforma-
tion of a compound between two forms having disparate
absorption spectral profiles, with at least one of the formation
reactions being driven by light absorption.1 Although a great
number of molecules,2 including naturally occurring substances,3
(3) For a comprehensive review see: Hampp, N. Chem. ReV. 2000, 100,
1755 and references cited therein.
(4) For a comprehensive review see: Minkin, V. I. Chem. ReV. 2004,
104, 2751 and references cited therein.
* Address correspondence to this author. Phone: (44) 191-222-5706. Fax:
(44) 191-222-8660.
(1) Brown G. H., Ed. Techniques of Chemistry; Vol. III, Photochromism;
Wiley-Interscience: New York, 1971.
(2) For a comprehensive review see: Irie, M. Chem. ReV. 2000, 100,
1683 and references cited therein.
(5) Feringa, B.; Jager, W. F.; de Lange, B. Tetrahedron 1993, 37, 8267.
(6) Sanchez, C.; Lebeau, B.; Chaput, F.; Biolot, J.-P. AdV. Mater. 2003,
15, 1970.
(7) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. ReV. 2000, 100, 1741.
10.1021/jo062124l CCC: $37.00 © 2007 American Chemical Society
Published on Web 01/11/2007
888
J. Org. Chem. 2007, 72, 888-897

Photochromic Dyes
development of optical data storage systems with novel read-
write features.8 Other applications involve photochemical
switching,9 ion transfer through membranes,10 and surface
modulation and 3-D imaging.11 Paramount to all such applica-
tions is the light-induced conversion of the colorless spiropyran
(SP) to the colored merocyanine (MC) form.12 This transforma-
tion leads to a large increase in dipole moment, a feature that
allows the optical properties to be probed by both polarity and
electrical field effects, and a marked change in geometry.13
Conversion of the ring-opened MC to ring-closed SP is
accomplished by illumination with visible light, although there
is often a slow dark reaction that restores the SP form.14
The mechanisms and rates of the two photochemical steps
have been subjected to detailed examination by theoretical15 and
spectroscopic study.16 Evidence has accumulated to show that
photoconversion of the SP to MC forms can take place via both
singlet17 and triplet18 excited states but the reverse process seems
to occur exclusively by way of the singlet manifold. The triplet
mechanism allows sensitization of the ring-opening step and,
in turn, this leads to a marked improvement in the optical
properties of the light-absorbing unit.19 Alternatively, the rate
of ring opening can be influenced by changes in solvent
polarity.20 Here, we have sought to develop a different route
for the SP to MC conversion that allows for a wide choice of
chromophore and retains the polarity dependence. This new
approach to ring opening makes use of the amino donor site
inherent to the SP moiety to form an exciplex intermediate with
a covalently attached polycyclic aryl hydrocarbon.21 Provided
the energy levels are carefully balanced, intersystem crossing
within the exciplex provides a means by which to populate the
triplet excited state localized on the SP unit. Once formed, this
latter species acts as the precursor for the MC form.
FIGURE 1. Molecular formulas for the spiropyrans and pyrene-based
reference compound studied here.
As a test of this ring-opening strategy we have synthesized
a T-shaped, pyrene-based SP derivative whereby the ancillary
chromophore is provided by a highly delocalized polycyclic
hydrocarbon. The two molecular fragments are connected by
way of a rigid framework that brings the pyrene subunit into
close proximity with the amino donor that is an integral part of
the SP function. The resultant system, P-SP (Figure 1), has been
characterized in full detail. To activate the ring-opening step,
the SP unit has been functionalized with a nitro group on the
terminal phenylene ring, forming P-SPN (Figure 1). Our studies
have shown that illumination into the pyrene-based chromophore
leads to ring opening and transient formation of the MC species.
This latter geometric form, which is fluorescent and highly
colored, has a lifetime of ca. 20 s in methyltetrahydrofuran at
20 °C but returns to the closed form via both thermal and
photodriven processes. We now examine the mechanisms of
the ring-opening and ring-closing events in the context of
developing a next-generation Write-Read-Erase optical system.
It is also interesting to consider this system in terms of its dual
fluorescence properties, bearing in mind that the fluorescence
spectral profile is sensitive to the composition of the equilibrium
mixture and that intramolecular energy transfer can take place
within the open form.
(8) Evans, R. A.; Such, G. K. Aust. J. Chem. 2005, 58, 825.
(9) (a) Krysanov, S. A.; Alfimov, M. V. Chem. Phys. Lett. 1982, 91, 77.
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Results and Discussion
Synthesis. The general procedures used in the preparation
of spiropyran-based molecular systems are very well docu-
mented, and represent a convergent synthetic approach.22 The
formation of the linking sp3 spiro carbon center generally
involves condensation of a methyl indolinium salt and an
appropriately functionalized aromatic aldehyde.23 To this end,
we devised a synthetic strategy utilizing this well-established
procedure. However, care has to be taken regarding which unit
is functionalized. That is to say, we chose to graft the
(22) Cho, Y. J.; Rho, K. Y.; Kim, S. H.; Keum, S. R.; Yoon, C. M.
Dyes Pigm. 2000, 44, 19.
(23) (a) Voloshin, N. A.; Chernyshev, A. V.; Metalista, A. V.; Rashita,
I. M.; Voloshina, E. N.; Minkin, V. I. Russ. Chem. Bull. Int. Ed. 2005, 54,
705. (b) Minkovska, S.; Fedieva, M.; Jeliazkova, B.; Deligeogiev, T.
Polyhedron 2004, 23, 3147. (c) Yagi, S.; Maeda, K.; Nakazumi, H. J. Mater.
Chem. 1999, 9, 2991. (d) Hirano, M.; Miyahita, A.; Shitara, H.; Nohira, H.
Chem. Lett. 1991, 1873. (e) Nakano, S.; Miyashita, A.; Nohira, H. Chem.
Lett. 1993, 13. (f) Minkin, V. I.; Metalista, A. V.; Dorogan, I. V.; Lukyanov,
B. S.; Besugliy, S. O.; Micheau, J.-C. J. Phys. Chem. A 2005, 109, 9605.
(21) (a) Song, L.; Jares-Erijman, E. A.; Jovin, T. M. J. Photochem.
Photobiol. A 2002, 150, 177. (b) Ahmed, S. A.; Tanaka, M.; Ando, H.;
Tawa, K.; Kimura, K. Tetrahedron 2004, 60, 6029.
J. Org. Chem, Vol. 72, No. 3, 2007 889

Benniston et al.
SCHEME 1. General Methods Used in the Preparation of the T-Shaped Dyads
photoactive unit onto the benzene ring of the methyl indolinium
function, rather than at the nitrogen atomsthe latter being the
normal site for attachment of functionalizing groups.24 The main
gain from our approach is that it opens up the possibility for
making a wide range of spiropyran dyads. It should be noted
that this approach minimizes the number of model compounds
needed for photophysical characterization. The main reference
compound required, 1,4-bis(pyrenylethynyl)benzene (P), is
easily prepared from 1,4-bis(trimethylsilylethynyl)benzene by
deprotection with tert-butylammonium fluoride and palladium-
catalyzed cross-coupling with 2-bromopyrene.
(triisopropylsilyl)acetylene with a Negishi-type27 palladium
catalyzed cross-coupling reaction in refluxing toluene gave
2,3,3-trimethyl-4,7-bis[triisopropylsilyl)-ethynyl]-3H-indole 3,
in excellent yield. It should be noted that toluene as solvent is
essential since use of lower boiling point solvents (e.g., THF)
gave rise to the monoethynylated product. Methylation at the
1-position of the indole ring with iodomethane afforded the
tetramethyl indolinium iodide salt 4 in good yield. The precur-
sor-protected spiropyrans (6a,b) are formed by condensation
of 4 with the appropriate salicylaldehydes (5a,b) in ethanol in
the presence of triethylamine. Introduction of the required pyrene
units was achieved by in situ fluoride triisopropylsilyl depro-
tection of 6a,b and palladium catalyzed cross-coupling with
2-bromopyrene. The dyads were readily soluble in standard
organic solvents.
Photophysical Properties of the P-SP Dyad. The electronic
absorption spectrum of the pyrene-based reference compound
(P) shows a series of well-resolved bands with maxima between
320 and 450 nm. The spectral shape and position are quite
insensitive to changes in solvent polarity and the onset of
absorption is located at ca. 425 nm. There are marked similarities
between absorption spectra recorded for P and the corresponding
dyad P-SP in cyclohexane solution. The most notable differ-
ences are a modest red shift and a general loss of spectral
resolution for the dyad. The less intense absorption bands
associated with the SP moiety located at ca. 350 nm are obscured
by pyrene-based electronic transitions. Hence, it is not possible
to selectively illuminate into the SP unit for any of these dyads
but, in contrast, the pyrene-based excited singlet state can be
accessed with complete selectivity. Fluorescence is observed
for both compounds in deoxygenated cyclohexane. In each case,
Illustrated in Scheme 1 is a breakdown of the procedures used
to prepare the molecular dyads: note well, these systems are
termed dyads in that the pyrene-based chromophore and the
spiropyran-based switch are the two active components. The
starting point is the preparation of 4,7-dibromo-2,3,3-trimethyl-
3H-indole 2, since it was expected that the bromo groups could
be substituted later in the synthetic procedure with acetylene
groups under the usual cross-coupling conditions. Following the
standard Fisher indole synthesis25 reaction of 1 equiv of 2,5-
dibromophenylhydrazine, 1, with 2 equiv of 3-methyl-2-
butanone in dry acetic acid under reflux afforded, after workup,
compound 2 in 46% yield. The cross-coupling of 2 with
(triisopropylsilyl)acetylene under conventional Sonogashira26
coupling conditions was problematic, and a great number of
side products were isolated. However, the reaction of 2 with
(24) (a) Song, L.; Jares-Erijman, E. A.; Jovin, T. M. J. Photochem.
Photobiol. A 2002, 150, 177. (b) Li, X.; Li, J.; Wang, Y.; Matsura, T.;
Meng, J. J. Photochem. Photobiol. A 2004, 161, 201. (c) Cho, M. J.; Kim,
W.; Jun, W. G.; Lee, S. K.; Jin, J.-I.; Choi, D. H. Thin Solid Films 2006,
500, 52.
(25) Nakazaki, M.; Yamamoto, K. J. Org. Chem. 1976, 41, 1877.
(26) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
16, 4467.
(27) King, A. O.; Okukado, N.; Negishi, E. J. Chem. Soc., Chem.
Commun. 1977, 683.
890 J. Org. Chem., Vol. 72, No. 3, 2007

Photochromic Dyes
TABLE 1. Photophysical Properties of the Various Pyrene-Based
Compounds Measured in Deoxygenated Solution at Room
Temperature
property
solvent
P
P-SP
P-SPN
0.66
0.040
0.004
1.3
0.19 (84%)
4.7 (16%)
0.07 (95%)
2.8 (5%)
80
a
ΦF
cyclohexane
MeTHF
0.67
0.62
0.65
2.2
0.62
0.52
0.39
1.7
MeCNe
τF/nsb
cyclohexane
MeTHF
2.1
2.2
MeCN
2.4
3.4
τT/µsc
cyclohexane
MeTHF
90
80
85
80
80
MeCN
cyclohexane
MeTHF
80
70
65
700
1150
1900
SS/cm-1 d
400
420
480
730
1280
2140
MeCN
FIGURE 3. Transient differential absorption spectra recorded after
laser excitation (λ ) 355 nm) of P-SP in deoxygenated MeTHF. The
insert shows a decay profile recorded at 490 nm.
a Fluorescence quantum yield, (8%. b Fluorescence lifetime, (5%.
c Triplet lifetime, (10%. d Stokes shift, (80 cm-1 e Acetonitrile.
.
Laser flash photolysis studies carried out with P-SP in
deoxygenated solution at room temperature show a weak signal
that could be attributed to transient formation of the π,π* excited
triplet state (Figure 3). Transient bleaching is observed in the
region where the pyrene chromophore absorbs while weak
absorption is seen over the range from 440 to 760 nm. Addition
of 10% v/v iodoethane enhances the signal by a factor of about
4-fold but decreases the lifetime from 80 to 20 µs in acetonitrile.
Molecular oxygen quenches the signal. All indications point to
the transient species being the π,π* triplet state localized on
the pyrene unit,28 as confirmed by comparison to spectra
recorded for P under the same conditions (see the Supporting
Information). The quantum yield (ΦT) for formation of the triplet
state is ca. 0.10 in cyclohexane but increases to around 0.20 in
both MeTHF and acetonitrile. The triplet lifetime remains
comparable in all solvents at low laser intensity but there are
indications for triplet-triplet annihilation at higher laser power.
Our understanding of the P-SP excited-state manifold can
now be summarized as follows: Excitation in cyclohexane gives
rise to a first-excited singlet state that is pyrene-like and is
primarily of π,π* character. This species is highly fluorescent
but possesses increased charge-transfer character relative to
pyrene, probably due to interaction with a high-lying charge-
transfer state.29 Increasing the polarity of the solvent leads to
stabilization of this charge-transfer state such that it now lies at
lower energy than the corresponding pyrene-based π,π* state.
The charge-transfer state, which is destabilized below the glass
transition temperature due to the restriction in solvent reorienta-
tion,30 can be considered to be a further example of the well-
known exciplex family formed between pyrene and amine
donors.31 There is no indication in any of the spectral records
to suggest that the SP ring is opened following illumination.
Indeed, absorption bands due to the SP function are obscured
FIGURE 2. Normalized fluorescence spectra recorded for (a) P-SP
and (b) P-SPN in cyclohexane (black), MeTHF (dark gray), and
acetonitrile (gray) at room temperature following excitation into the
pyrene-based chromophore.
the corrected excitation spectrum matches well with the absorp-
tion spectrum but the Stokes’ shift increases from 400 cm-1
for P to 730 cm-1 for P-SP. Again, fluorescence from the dyad
is somewhat less well resolved than that from P. The fluores-
cence quantum yields (ΦF) and lifetimes (τF), however, are
comparable (Table 1).
For P-SP, the fluorescence maximum shifts progressively
toward longer wavelength with increasing polarity of the
solvent: appearing at 461, 473, and 493 nm, respectively, in
cyclohexane, MeTHF, and acetonitrile (Figure 2). Indeed, the
Stokes’ shift (SS) increases markedly with increasing solvent
polarity (Table 1). There is a progressive decrease in ΦF but a
slight increase in τF (Table 1) and further loss of vibrational
fine structure (Figure 2) with increasing solvent polarity. In each
solvent, fluorescence decay occurs by way of a monoexponential
process. The fluorescence spectral properties are independent
of concentration over the range of interest and there are no
indications for intermolecular interactions. The fluorescence
spectral profile and intensity recorded for P-SP in MeTHF
remain independent of temperature on progressively cooling the
solution from 298 to 150 K. Below 150 K the spectral profile
evolves steadily toward that found in cyclohexane; at 80 K, the
0,0 transition is at 465 nm (see the Supporting Information).
Phosphorescence was not detectable at 80 K, even when 10%
v/v iodoethane was added.
(28) Watkins, A. R. J. Phys. Chem. 1976, 80, 713.
(29) Harriman, A.; Hissler, M.; Ziessel, R. Phys. Chem. Chem. Phys.
1999, 1, 4203.
(30) Gaines, G. L.; O’Neil, M. P.; Svec, W. A.; Niemczyk, M. P.;
Wasielewski, M. R. J. Am. Chem. Soc. 1991, 113, 719.
(31) (a) Helsen, N.; Viaene, L.; Van der Auweraer, M.; De Schryver,
F. C. J. Phys. Chem. 1994, 98, 1532. (b) Hartley, R. J.; Faulkner, L. R.
J. Am. Chem. Soc. 1985, 107, 3436. (c) Van Haver, Ph.; Helsen, N.;
Depaemelaere, S.; Van der Auweraer, M.; De Schryver, F. C. J. Am. Chem.
Soc. 1991, 113, 6849. (d) The term “exciplex”, originating from “excited
state complex”, applies equally well to intermolecular and intramolecular
species. Distinction between an exciplex and an excited charge-transfer state
is less obvious.
J. Org. Chem, Vol. 72, No. 3, 2007 891

Benniston et al.
TABLE 2. Results from the Time-Resolved Fluorescence Studies
Carried out with P-SPN in Deoxygenated MeTHF as a Function of
Temperature
SCHEME 2. Outline for the Ring-Opening Processes That
Follow Illumination into the Pyrene-Based Chromophore
Present in P-SPN
a
b
c
temp/K
t1/ps
t2/ns
A1/%
kF
kB
kD
∆Ed
323
318
313
308
303
298
293
288
283
278
273
268
263
258
254
110
130
160
190
120
180
150
170
190
160
170
200
220
180
230
0.83
0.89
1.00
1.05
1.04
1.10
1.10
1.49
1.88
1.50
1.60
1.65
1.75
1.80
1.87
87
88
89
89
92
91
92
93
94
96
95
96
97
97
98
6.1
5.0
3.7
2.9
5.8
3.2
4.3
3.6
3.1
4.1
3.7
2.9
2.5
3.5
2.3
11.4
9.1
7.2
6.3
6.9
5.5
5.7
4.9
4.1
2.9
3.6
2.6
1.8
2.1
1.2
10.8
10.0
8.6
7.9
8.6
7.8
8.0
5.4
4.0
6.0
5.3
5.3
5.1
5.0
4.9
4.5
4.5
4.3
3.9
5.4
4.4
4.9
4.8
4.7
6.1
5.3
5.4
5.7
6.0
6.3
a (8%, units 109 s-1
. . .
b (8%, units 108 s-1 c (12%, units 108 s-1
d (20%, units kJ mol-1
.
by π,π* transitions localized on the pyrene-like chromophore
and it is clear that photons collected by this latter chromophore
are not used to open the SP ring in either polar or nonpolar
solvent.
profiles. Furthermore, time-gated fluorescence spectra32 were
identical for the short- and long-lived components. The simplest
explanation for this latter finding is that, in a polar solvent, the
exciplex lies in thermal equilibrium with a lower lying excited
state that itself undergoes fast nonradiative decay to a dark
state.33
Photophysical Properties of P-SPN. The electronic absorp-
tion spectrum of P-SPN remains remarkably similar to that of
P-SP in each of the three solvents investigated. Likewise, the
fluorescence spectra of the two compounds are closely com-
parable, except for a slight blue shift noted for P-SPN (Figure
2). This leads to a small reduction in the size of the Stokes’
shift (Table 1). It is notable that both ΦF and τF for P-SPN
remain similar to those of P-SP in deoxygenated cyclohexane
(Table 1), where ring opening does not take place, and where
the fluorescence is pyrene-like. Under such conditions, the
corrected fluorescence excitation spectrum is in good agreement
with the absorption spectrum while fluorescence decay profiles
are strictly monoexponential at all monitoring wavelengths. The
close similarity between the photophysical properties of the two
compounds can be used to argue that the same species is
responsible for the observed fluorescence in each case. Fluo-
rescence decay leads to partial formation of the π,π* triplet state
localized on the pyrene-like chromophore, ΦT being around
0.10, but no other transient species are observed in the spectral
records.
t
τ1
t
τ2
IF(t) ) A1 exp -
+ (1 - A1) exp -
(1)
(2)
(
)
(
)
k
K ) F ) exp -
∆E
(
)
kB
RT
The time-resolved fluorescence (IF) results can be analyzed
in terms of eq 1, where τ1 and τ2 are the two lifetimes and A1
refers to the fractional contribution of the shorter lived species.
In turn, the derived parameters can be used to calculate33 rate
constants for conversion of the exciplex into the new state (kF),
for the corresponding reverse process (kB) whereby the exciplex
is repopulated from the new state, and for nonradiative decay
of the new state (kD). Furthermore, the energy gap (∆E) between
the exciplex and the new state can be derived by virtue of eq 2,
where K refers to the equilibrium constant for the reversible
interchange between the exciplex and the new state (Scheme
2). The parameters derived for MeTHF over a modest temper-
ature range are collected in Table 2. Similar behavior was found
in acetonitrile, albeit over a smaller temperature range, where
it was found that kF, in particular, is much higher than in
MeTHF.
However, in polar solvents ΦF is much lower for P-SPN than
for P-SP (Table 1). In MeTHF, ΦF is reduced a factor of 13-
fold whereas acetonitrile leads to a 100-fold decrease in ΦF
relative to P-SP. In both solvents, the fluorescence decay profiles
were dual-exponential. In MeTHF, for example, the decay
profiles were dominated by a short-lived component of ca. 0.2
ns but there was clear evidence for a minor contribution from
a longer lived species (Table 1). Again, corrected fluorescence
excitation spectra were in good agreement with the correspond-
ing absorption spectrum and the fluorescence decay profiles
were independent of both monitoring and excitation wave-
lengths. It was clear, however, that the contribution of the longer
lived component increased progressively with increasing tem-
perature while both derived lifetimes increased with decreasing
temperature (Table 2). Repeated purification of the sample and
solvent had no effect on the time-resolved fluorescence decay
From this analysis it appears that there is a slight tendency
for both kF and kD to decrease on lowering the temperature
whereas kB is somewhat more sensitive to changes in temper-
ature. Because kF and kB show disparate temperature effects, it
follows that the corresponding equilibrium constant increases
with decreasing temperature and this, in turn, means that the
(32) Lamb, D. C.; Schenk, A.; Rocker, C.; Scalfi-Happ, C.; Nienhaus,
G. U. Biophys. J. 2000, 79, 1129.
(33) (a) Wasielewski, M. R.; Johnson, D. G.; Svec, W. A.; Kersey,
K. M.; Minsek, D. W. J. Am. Chem. Soc. 1988, 110, 7219. (b) Heitele, H.;
Finckh, P.; Weeren, S.; Pollinger, F.; Michel-Beyerle, M. E. J. Phys. Chem.
1989, 93, 5173. (c) Harriman, A.; Heitz, V.; Ebersole, M.; van Willigen,
H. J. Phys. Chem. 1994, 98, 4982.
892 J. Org. Chem., Vol. 72, No. 3, 2007

Photochromic Dyes
FIGURE 5. Transient differential absorption spectra recorded for
P-SPN in deoxygenated (a) MeTHF and (b) acetonitrile at delay times
10, 25, 45, 60, 100, and 160 µs following laser excitation at 355 nm.
FIGURE 4. Effect of decreasing temperature on the fluorescence
spectrum of P-SPN in MeTHF recorded at 10-deg intervals over the
range (a) 80-120 and (b) 120-300 K. Note the slit widths were
decreased from (b) 5 to (a) 2 nm to offset the effects of increased
intensity.
cis isomer while the higher energy fluorescence can be attributed
to the corresponding trans isomer. At ambient temperature, the
cis isomer converts rapidly into the trans form16a,35c but this
structural change becomes progressively more difficult as the
temperature is lowered. Literature work indicates that the trans
isomer reconverts, both thermally and photolytically, to the
original SP geometry.14,36 It is notable that the energy gaps
between the exciplex and the MC forms, taken simply as the
respective emission maxima, far exceed the ∆E values derived
from the time-resolved fluorescence spectra. This means that
the reservoir state, which in any case is a dark species, is not a
ring-opened MC.
A more likely assignment for the reservoir state is the lowest
energy triplet localized on the SP moiety. It is known that ring
opening of SP can be effected by triplet sensitization19 and we
have shown that a triplet state is involved in the formation of
the MC form. The corresponding SP singlet excited state lies
at an energy above that of the pyrene-based singlet state and
therefore well above the energy level associated with the
exciplex. The energy of the SP triplet state (SP*T) should be
close to that of the exciplex such that kF can be assigned to
intersystem crossing from the exciplex to SP*T while kB refers
to the reverse step (Scheme 2). Consequently, kD must refer to
the sum of all other processes leading to deactivation of SP*T
(kD ) kNR + kPT + kMC).
Laser flash photolysis studies carried out with 4-ns excitation
pulses delivered at 355 nm, where the pyrene-like chromophore
is the only absorbing species, show that the pyrene-like triplet
(P*T) excited state is present (Figure 5). This species has a
characteristic differential absorption spectrum, already identified
from studies made with P-SP, and seen clearly from its
absorption at long wavelengths. This species decays with a
lifetime of 80 µs in deoxygenated MeTHF at room temperature.
Other species are also evident in the transient records. One such
contributor appears as a permanent product on the available time
scale (<10 ms) (Figure 5) and can be identified from its
characteristic absorption spectrum as being the ring-opened MC
form.9c,16a,18a It is known that this latter species reverts to the
SP form with a lifetime of ca. 20 s under these conditions. A
second contributor decays with a lifetime of ca. 5 µs (Figure
6). The differential absorption spectrum of this latter species
energy gap (∆E) between the exciplex and the reservoir state
increases slightly as the temperature is lowered (Table 2). Since
the fluorescence maximum noted for the exciplex is insensitive
to changes in temperature, at least over the relevant range, it
seems likely that the energy of the reservoir state must decrease
progressively with decreasing temperature. The net effect is
small and it is clear the exciplex and reservoir states are close
in energy.
The steady-state fluorescence spectral profiles also exhibit
interesting temperature effects. On rapid cooling of the MeTHF
solution to 80 K the fluorescence spectrum changes from that
typical of the exciplex at room temperature to the pyrene-like
emission in the glassy matrix.34 The fluorescence quantum yield
and lifetime measured at 80 K are 0.51 and 2.6 ns, respectively.
No other fluorescent species could be seen under these condi-
tions. This behavior is to be contrasted with that observed on
slowly cooling the solution and recording spectra at regular
temperature intervals (Figure 4). Here, the fluorescence intensity
increases progressively until around the freezing point of
MeTHF (-136 °C) where the spectral profile begins to sharpen.
Below the glass transition temperature (ca. 120 K) the fluores-
cence spectrum is reminiscent of the pyrene-like π,π* emission
seen for P-SP under these conditions. An additional feature of
these emission spectra is the development of new fluorescence
bands at long wavelength (Figure 3). At 250 K, there are two
such bands with peaks at 678 and 640 nm but the emission
spectrum evolves in favor of the higher energy band as the
temperature is lowered. At temperatures below ca. 180 K, the
fluorescence maximum shifts progressively from 640 nm to
around 600 nm although the 678 nm band is still evident
(Figure 4).
These new fluorescence features arise from photolysis into
the pyrene-like band, as can be demonstrated by steady-state
illumination at fixed temperature. Corrected excitation spectra
indicate that both bands are due to ring-opened MC forms. Based
on earlier work9c,16a,18a,35 with the parent SP module, the species
responsible for fluorescence around 678 nm is most likely the
(34) Fujii, T.; Ishii, A.; Takusagawa, N.; Yamashita, H.; Anpo, M.
J. Photochem. Photobiol. 1995, 86, 219.
(35) (a) Chibisov, A. K.; Go¨rner, H. J. Phys. Chem. 1999, 103, 5211.
(b) Go¨rner, H. Chem. Phys. 1997, 222, 315. (c) Chibisov, A. K.; Go¨rner,
H. J. Phys. Chem. 1997, 101, 4305.
(36) (a) Yagi, S.; Maeda, K.; Nakazumi, H. J. Mater. Chem. 1999, 9,
2991. (b) Swansburg, S.; Buncel, E.; Lemieux, R. P. J. Am. Chem. Soc.
2000, 122, 6594.
J. Org. Chem, Vol. 72, No. 3, 2007 893

Benniston et al.
PERP*T, with the later triplet being responsible for ring opening.
For P-SP, ring opening is too slow to compete with decay of
PERP*T and, since formation of P*T is far from quantitative,
there must be an escape route to reform the ground state via
nonradiative decay. For P-SPN, however, PERP*T partitions
between P*T and ring opening.38 It is clear that these two triplets
are not in thermal equilibrium and population of P*T does not
facilitate ring opening. Since the triplet state of the ring-opened
form can be detected at early times in the flash photolysis
records following excitation of the pyrene-like chromophore,
it follows that there must be a direct link between PERP*T and
MC*T.
Solvent polarity plays an important role in formation of the
MC form.20 In nonpolar solvents, the mechanism is switched-
off since exciplex formation is inefficient and the photophysics
are localized on the pyrene-like unit. In moderately polar
solvents, such as MeTHF, ring opening is able to compete with
formation of P*T and the MC form is formed in modest yield.
On moving to more polar solvents, such as acetonitrile, there
is a significant increase in kF but a reduction in the yield of the
ring-opened MC form. This finding suggests that partitioning
favors formation of P*T in polar solvents, probably because the
energy of the MC-based triplet states will increase in polar
solvents.39 (Note well, the energy level for MC*T remains
unknown since this species does not show phosphorescence after
rapid cooling to 77 K.) The transient absorption spectral records
show the presence of the latter species (MC*T) at the end of
the laser pulse. Although the open form accumulates during
repetitive laser flashing, it is still possible to detect MC*T in a
flow cell where each laser shot is directed to a fresh aliquot of
solution. This means that the MC*T must arise, at least in part,
directly from the PERP*T. Finally, to allow for formation of
the cis isomer, it is necessary to consider that the SP*T and/or
PERP*T undergo(es) nonradiative decay to give a mixture of
open and closed forms in their respective ground state geom-
etries.
FIGURE 6. Transient differential absorption spectra recorded for
P-SPN in deoxygenated MeTHF at delay times 0.1, 0.2, 0.5, 1, 2, 5,
and 10 µs following laser excitation at 355 nm. The insert shows a
decay profile recorded at 500 nm.
can be resolved by removal of the P*T spectrum, normalizing
at 700 nm, and is consistent with that expected for the triplet
state of the MC form.37 At room temperature, we expect the
trans isomer to be the dominant, if not the only, MC form
present so the derived spectrum can be assigned to the triplet
state of this species.
Confirmation of this assignment was obtained from a two-
color experiment in which the sample was first subjected to
preirradiation at 355 nm to build up the MC form, before laser
photolysis at 560 nm (see the Supporting Information). Under
such conditions, the MC form is the only species able to absorb
at 560 nm and the resultant transient absorption spectrum, which
decays with a lifetime of 7 µs in deoxygenated MeTHF at 200
K, can be assigned to the corresponding triplet state. Both
spectrum and lifetime are consistent with the assignment made
on the basis of the 355-nm excitation studies.
It appears, therefore, that the SP*T formed from the exciplex
decays to form a mixture of P*T and the ring-opened MC form
(Scheme 2). Presumably, the latter species is formed by way of
the so-called perpendicular triplet state (PERP*T),16a,18a,35b which
itself decays to form a mixture of cis and trans MC isomers.
As already mentioned,14,36 the cis isomer rapidly converts to
the more stable trans isomer at room temperature but can be
seen by fluorescence spectroscopy at low temperature. As such,
kD can be partitioned into the three individual rate constants
leading to formation of P*T (kPT), MC*T (kMC), and the ground
state PERP (kNR).
Mechanism for Ring Opening. Relying heavily on earlier
work9d,16a-d carried out with simpler SP derivatives, the ring
opening process for P-SPN can be summarized in terms of
Scheme 2. The unusual feature of this scheme concerns the key
role played by the exciplex. Also, the onset of delayed
fluorescence due to thermal repopulation of the exciplex from
the dark reservoir state, now identified as the SP*T, allows
additional energy levels and rate constants to be quantified. A
key feature is that the SP*T lies at slightly lower energy than
the exciplex such that it can be populated by intersystem
crossing. The energy level of this triplet state seems to be slightly
dependent on temperature, which might reflect a modest
structural change. Once formed, the SP*T converts rapidly to
Ring Closing To Reform the Spiropyran. It has been shown
that the ring-opened MC form of P-SPN reverts back to the
corresponding SP form on standing in the dark. The lifetime of
the MC form is ca. 20 s in deoxygenated MeTHF at 20 °C and
the activation energy for ring closure is ca. 60 kJ mol-1 22
. These
values seem to be in line with results collected for the parent
SP unit in various solvents.40 Thermal reformation of the SP
form is very much slower (τ ≈ 36 min) in sucrose octaacetate
glass, no doubt due to the greatly increased viscosity although
ring opening is still highly efficient and light-induced ring
closure occurs readily. The ring-opened forms, both cis and trans
isomers, fluoresce at ambient temperature and also form
relatively long-lived excited triplet states. On illumination into
the pyrene-like absorption bands, the ring-opened form ac-
cumulates in solution but quickly reaches a steady-state value
(38) The role of the nitro group in promoting ring opening is unclear.
Ring opening does not take place when the nitro group is replaced with
either a hydrogen atom or a cyano group. Replacing pyrene with perylene
lowers the excitation energy of the exciplex to below that of the SP*T and
again there is no ring opening. The nitro group might facilitate intersystem
crossing, because of its efficient spin orbit coupling properties, or weaken
the bond around the spiro center, as shown by X-ray crystallography. Further
research is required to fully explore this effect. Note well, the perylene
derivative of P-SPN was particularly insoluble and limited analytical data
are collected in the Supporting Information.
(39) Harriman, A. J. Photochem. Photobiol. A 1992, 65, 79.
(40) (a) Sueishi, Y.; Ohcho, M.; Nishimura, N. Bull. Chem. Soc. Jpn.
1985, 58, 2608. (b) Pfeifer-Fukumura, U. J. Photochem. Photobiol. A 1997,
111, 145.
(37) Go¨rner, H. Phys. Chem. Chem. Phys. 2001, 3, 416.
894 J. Org. Chem., Vol. 72, No. 3, 2007

Photochromic Dyes
The ring-opened form should be more polar than the
corresponding SP form42 and the extent of resonance polarity
might be affected by the nature of the surrounding solvent.43
Increased polarity, or the increased contribution of zwitterionic
resonance forms, will lead to slower rates of ring closure. It is
also likely that such polar species will minimize the role of the
tertiary amine as an electron donor for the pyrene moiety. As
such, a further effect of ring opening might be to switch-off
exciplex emission and restore pyrene-like fluorescence. This
situation is very difficult to verify experimentally because of
the low concentration of MC forms present in the equilibrium
mixture.
Concluding Remarks
Ring opening has been achieved via an exciplex state that
makes direct use of the tertiary N atom as the primary electron
donor. The mechanism works only in moderately polar solvents;
the exciplex is not formed in nonpolar environments while there
is a reduced yield of MC in strongly polar solvents. Although
the choice of chromophore is restricted to UV-absorbing
materials it is far wider than that available to conventional triplet
sensitization19 where the singlet-triplet energy gap has to be
considered. Because SP-based triplet excited states are involved,
ring opening is affected by the presence of dissolved oxygen.
Temperature is not a limiting factor and the various forward
steps show small activation energies. This leads to a high overall
rate of ring opening in fluid solution. A limitation of the present
system involves the onset of intramolecular energy transfer from
the pyrene fluorophore to the ring-opened MC form. In viscous
media, where ring closure is relatively slow, this latter process
provides an unusual means for fluorescence modulation.44 Thus,
in the absence of preirradiation with an intense UV pulse, only
exciplex fluorescence is observed. After preirradiation, fluo-
rescence is observed in the far red region for both UV and visible
illumination. It is also important to note the dual fluorescence
found for the exciplex in certain solvents. This allows the energy
gap between exciplex and reservoir states to be measured
accurately.
FIGURE 7. Fluorescence (red) and corrected excitation (black) spectra
recorded for the MC form in deoxygenated MeTHF at 200 K and the
absorption spectrum recorded for the pyrene-like chromophore.
SCHEME 3. Proposed Outline for the Ring Closure Step,
Allowing for Both Light-Activated and Thermal Processes
and for Sensitization by the Pyrene-like Excited Singlet State
This latter situation can be considered in terms of a Write-
Read-Erase protocol.45 A UV pulse is used to write the signal.
The far red fluorescence provides the means by which to read
the signal. The intensity, peak wavelength, decay rate, dipole
moment, and absorption spectral change provide critical infor-
mation about the system under interrogation. An alternative way
to read the signal could involve monitoring the MC*T in
deoxygenated media. Additional benefits might accrue from the
use of Stark spectroscopy,46 electric field effects,47 or fluores-
cence correlation spectroscopy.48 An important advantage of this
system, perhaps unique in the field, is that a second UV pulse,
or a visible light pulse, can be used for fast erasure of the signal.
The entire cycle could be operable within times as short as 10
that is comprehensively on the side of the SP form. Prolonged
irradiation causes gradual decomposition of the material but does
not improve the steady-state composition. Illumination into the
MC absorption bands causes fast conversion to the original SP
geometry such that ring closure must be light activated (Scheme
3). Since the triplet lifetime is not unusually short for a MC
dye, it can be argued that photoisomerization occurs from the
singlet excited state of the ring-opened species.
In deoxygenated MeTHF at 200 K, where thermal ring closure
is relatively slow, the corrected fluorescence excitation spectrum
recorded for emission from the trans MC isomer clearly shows
that photons absorbed by the pyrene-like chromophore are
transferred to the MC form (Figure 7). This requires intramo-
lecular singlet energy transfer and is most likely to involve the
Fo¨rster dipole-dipole mechanism.41 Such a process is favored
by spectral overlap between pyrene-like emission and absorption
by the MC form and by their close proximity. Energy transfer
of this type decreases the amount of the ring-opened form in
the steady-state composition. It should be noted that, because
of the respective absorption spectral profiles, Fo¨rster energy
transfer from exciplex to the ring-closed SP form is extremely
unlikely.
(42) Bletz, M.; Pfeifer-Fukumura, U.; Kolb, U.; Baumann, W. J. Phys.
Chem. A 2002, 106, 2232.
(43) Benniston, A. C.; Harriman, A.; McAvoy, C. J. Chem. Soc., Faraday
Trans. 1998, 94, 519.
(44) Raymo, F. M.; Tomasulo, M. J. Phys. Chem. A 2005, 109, 7343.
(45) Lehn J.-M. Supramolecular Chemistry: Concepts and PerspectiVes;
VCH: Weinheim, Germany, 1995.
(46) Oh, D. H.; Sano, M.; Boxer, S. G. J. Am. Chem. Soc. 1991, 113,
6880.
(47) Andreeasson, J.; Terazono, Y.; Albinsson, B.; Moore, T. A.; Moore,
A. L.; Gust, D. Angew. Chem., Int. Ed. 2005, 44, 7591.
(48) Widengren, J.; Seidel, C. A. M. Phys. Chem. Chem. Phys. 2000, 2,
3435.
(41) Forster, T. Discuss. Faraday Soc. 1959, 27, 7.
J. Org. Chem, Vol. 72, No. 3, 2007 895

Benniston et al.
ns. Ongoing research with P-SPN dispersed in solid media49
are aimed at expanding on this aspect of the work and taking
advantage of the high fatigue resistance inherent to polymer-
bound material.
The reaction mixture was heated at reflux for 24 h. The solvent
was removed under vacuum and the residue was extracted with
ethyl acetate (50 mL), before being washed with water (2 × 30
mL) and brine (1 × 30 mL). The organic layer was separated and
dried over MgSO4 and the solvent was removed. Column chroma-
tography on silica gel, using EtOAc/petroleum ether (4:100) as
eluant, gave the product as a pale cream oily solid. Yield 2.54 g,
Experimental Section
1
68%. H NMR (CDCl3, 300 MHz) δ 7.32 (d, 1H, J ) 7.8 Hz),
General experimental methods are reported in the Supporting
Information. Preparations described below were carried out under
a N2 atmosphere unless stated otherwise.
7.17 (d, 1H, J ) 8.1 Hz), 2.29 (s, 3H), 1.45 (s, 6H), 1.16 (m, 42H).
13C{1H} NMR (CDCl3, 75 MHz) δ 189.5, 155.7, 146.4, 131.8,
129.4, 117.9, 116.1, 104.2, 104.1, 97.4, 96.9, 55.2, 20.3, 18.9, 15.6,
11.7. EI-MS m/z (%) 519 (32) [M]+, 476 (100) [M - CH(CH3)2]+.
FT-IR (cm-1) 2940, 2888, 2862, 2144, 1583, 1479. Elemental
analysis calcd (found) for C33H53NSi2(H2O)0.5: C 74.93 (75.23),
H 10.29 (10.23), N 2.65 (2.56). Mp 82 °C. [Note well, the oily
nature of the solid made accurate elemental analysis difficult even
after constant drying.]
Preparation of 2,5-Dibromophenylhydrazine (1). To concen-
trated HCl (110 mL) at 0 °C in a conical flask was added 2,5-
dibromoaniline (10 g, 39.7 mmol). The mixture was stirred for 30
min, followed by the dropwise addition at 0 °C of NaNO2 (3.01 g,
43.67 mmol) in water (20 mL). The initial light brown suspension
turned yellow and the reaction mixture, maintained at a temperature
of 0 °C, was stirred for a further 1 h. Urea (0.15 g, 2.49 mmol)
was added and, after stirring for 10 min, SnCl2 (18 g, 79.8 mmol)
in concentrated HCl (30 mL) that had been cooled to -30 °C was
added slowly, causing the reaction mixture to turn a cream/beige
color. The reaction was stirred for 3 h and finally left in a
refrigerator overnight. The resultant precipitate was filtered and
added to an ice bath and the pH was adjusted to above 10. The
crude product was extracted into ethyl acetate, which was washed
with dilute NaOH (50 mL). The separated organic fraction was
removed under vacuum to give a tan solid that was washed with
petroleum ether to afford the product. Yield 7.00 g, 66%. 1H NMR
(CDCl3, 300 MHz) δ 7.28 (d, 1H, J ) 2.1 Hz), 7.23 (d, 1H, J )
8.4 Hz), 6.76 (dd, 1H, J ) 8.3 Hz, J′ ) 2.1 Hz), 5.72 (s, br, 1H),
3.62 (s, br, 2H). 13C{1H} NMR (CDCl3, 75 MHz) δ 148.5, 133.2,
122.4, 122.1, 115.4, 105.9. FT-IR (cm-1) 3341, 3239, 3177, 3115,
3080, 1614, 1575, 1558, 1486, 1449, 1398, 1296, 1284, 1269, 1144,
1101, 1079, 1015, 940, 880, 836, 784, 734. EI-MS m/z 266 (M+).
Elemental analysis calcd (found) for C6H6Br2N2: C 27.10 (27.07),
H 2.27 (2.25), N 10.53 (10.26). Mp 90 °C.
Preparation of 1,2,3,3-Tetramethyl-4,7-bis[(triisopropylsilyl)-
ethynyl)]-1H-indolinium Iodide (4). Methyl iodide (4.25 g, 29.91
mmol) was added to 2,3,3-trimethyl-4,7-bis[(triisopropylsilyl)-
ethynyl)]-3H-indole (2.55 g, 4.88 mmol) in acetonitrile (40 mL),
and the mixture was heated overnight under reflux. The solvent
was removed under vacuum and the residue was purified by
washing with diethyl ether (15 mL), before being filtered to afford
the product as a yellow solid. Yield 2.26 g, 70%. 1H NMR (CDCl3,
300 MHz) δ 7.49 (d, 1H, J ) 9.0 Hz), 7.44 (d, 1H, J ) 9.0 Hz),
4.54 (s, 3H), 3.17 (s, 3H), 1.82 (s, 6H), 1.11 (m, 42H). 13C{1H}
NMR (CDCl3, 75 MHz) δ 198.3, 142.6, 140.7, 135.7, 133.7, 120.0,
112.5, 105.4, 101.3, 100.45, 52.0, 39.55, 20.6, 18.7, 11.5. MALDI-
MS m/z (%) 534.5 (100) [M - I]+. FT-IR (cm-1) 2941, 2889, 2862,
2151, 1486. Elemental analysis calcd (found) for C34H56INSi2: C
61.70 (61.57), H 8.53 (8.51), N 2.12 (2.03). Mp >178 °C dec.
General Method for the Preparation of Bis(triisopropylsi-
lylethynyl)spiropyrans (6a,b). 6a: 1,2,3,3-Tetramethyl-4,7-bis-
[(triisopropylsilyl)ethynyl)]-1H-indolinium iodide (0.100 g, 1.51 ×
10-4 mol) was dissolved in ethanol (88 mL) containing salicylal-
dehyde 5a (0.11 g, 9 × 10-4 mol) and triethylamine (1 mL). The
reaction mixture was refluxed for 8 h then cooled and the solvent
was removed under vacuum. The residue was dissolved in ethyl
acetate (40 mL) and washed with aqueous Na2CO3 (3 × 40 mL)
and brine (1 × 30 mL). The organic fraction was collected and
dried over MgSO4 and the solvent was removed under vacuum.
The crude material was purified by column chromatography on
silica gel, using petroleum ether/diethyl ether (20:1) as eluant to
afford the required compound as an oily solid. Yield 0.050 g, 52%.
1H NMR (CDCl3, 300 MHz) δ 7.19 (d, 1H, J ) 8.1 Hz), 7.14 (dt,
2H, J ) 7.8 Hz, J′ ) 1.3 Hz), 7.05 (dd, 1H, J ) 7.5 Hz, J′ ) 1.5
Hz), 6.86 (m, 2H), 6.78 (d, 1H, J ) 8.1 Hz), 5.63 (d, 1H,3J ) 10.2
Hz), 3.31 (s, 3H), 1.60 (s, 3H), 1.29 (s, 3H), 1.15 (m, 42H). 13C-
{1H} NMR (CDCl3, 75 MHz) δ 154.8, 148.4, 138.4, 133.9, 130.1,
129.9, 126.9, 124.4, 120.3, 119.2, 118.9, 118.7, 115.2, 106.2, 105.3,
97.1, 95.5, 52.2, 31.2, 23.3, 20.1, 19.1, 11.9. EI-MS m/z (%) 637
(100) [M]+, 594 (13), [M - CH(CH3)2]+. Accurate mass calcd
(found) for C41H59NOSi2 637.4135 (637.4134). FT-IR (cm-1) 2941,
2889, 2863, 2147, 2136, 1557, 1486. Elemental analysis calcd
(found) for C41H59NOSi2(H2O)0.5: C 76.10 (76.32), H 9.35 (9.26),
N 2.16 (1.81). Mp 67 °C.
Preparation of 4,7-Dibromo-2,3,3-trimethyl-3H-indole (2).
2,5-Dibromophenylhydrazine (3.12 g, 11.73 mmol) and 3-methyl-
2-butanone (2.02 g, 23.46 mmol) were added to acetic acid (50
mL) and the mixture was heated under reflux temperature for 24
h. The solvent was removed and the reaction mixture was extracted
with ethyl acetate (50 mL). The combined organics were washed
with aqueous Na2CO3 solution (2 × 30 mL) and brine (1 × 30
mL). The organic layer was separated and dried over MgSO4 and
the solvent was removed to afford the crude product. Column
chromatography on silica gel with ethyl acetate/petroleum ether
(3:1) as eluant afforded the pure product as a pale yellow solid.
1
Yield 1.70 g, 46%. H NMR (CDCl3, 300 MHz) δ 7.32 (d, 1H,
J ) 8.4 Hz), 7.16 (d, 1H, J ) 8.4 Hz), 2.31 (s, 3H) 1.45 (s, 6H).
13C{1H} NMR (CDCl3, 75 MHz) δ 190.7, 154.1, 144.5, 132.7,
130.5, 116.6, 113.5, 58.5, 19.7, 15.7. EI-MS m/z (%) 317 (80) [M+],
302 (56) [M - CH3]+. FT-IR (cm-1) 2979, 2967, 2928, 2866, 1597,
1579. Elemental analysis calcd (found) for C11H11Br2N: C 41.67
(41.75), H 3.50 (3.53), N 4.42 (4.32). Mp 132 °C.
Preparation of 2,3,3-Trimethyl-4,7-bis[(triisopropylsilyl)ethy-
nyl)]-2,3,3-trimethyl-3H-indole (3). 4,7-Dibromo-2,3,3-trimethyl-
3H-indole (2.20 g, 7.17 mmol) was added to a Schlenk flask charged
with a suspension of zinc(II) chloride (1.59 g, 11.68 mmol) and
triethylamine (20 mL) in toluene (20 mL). The reaction mixture
was degassed thoroughly and [Pd(PPh3)4] (0.828 g, 0.72 mmol)
and (triisopropylsilyl)acetylene (3.14 g, 17.21 mmol) were added.
6b: Yield 0.54 g, 80%. 1H NMR (CDCl3, 300 MHz) δ 8.04 (dd,
1H, J ) 9.0 Hz, J′ ) 2.7 Hz), 7.99 (d, 1H, J ) 2.7 Hz), 7.17 (d,
1H, J ) 8.1 Hz), 6.92 (d, 1H, J ) 10.2 Hz), 6.87 (d, 1H, J ) 8.1
Hz), 6.81 (d, 1H, J ) 8.7 Hz), 5.79 (d, 1H, J ) 10.5 Hz), 3.26 (s,
3H), 1.54 (s, 3H), 1.26 (s, 3H), 1.10 (m, 42H). 13C{1H} NMR
(CDCl3, 75 MHz) δ 159.9, 147.7, 141.6, 137.5, 134.1, 128.7, 126.1,
125.0, 122.9, 121.5, 119.0, 118.6, 115.7, 106.7, 105.6, 104.8, 104.0,
97.7, 96.3, 52.7, 31.2, 30.8, 23.3, 21.9, 18.9, 11.8, 11.7. EI-MS
(49) Preliminary Write-Read-Erase experiments were carried out on
P-SPN dispersed in sucrose octaacetate glass. Thus, pulsed illumination of
a P-SPN impregnated glass with short wavelength light (λmax ) 400 nm)
caused the development of the usual colored product. The so-formed MC
species could be detected by absorption spectroscopy. Subsequent illumina-
tion with a long wavelength pulse (λmax ) 600 nm) bleached the colored
spot and restored the original spectrum from the pyrene moiety. Repeated
cycling was possible by exposure of the glass to the two different
wavelengths (see the Supporting Information).
m/z (%) 682 (100) [M]+, 652 (14) [M - 2 CH3]+. FT-IR (cm-1
)
2941, 2890, 2863, 2144, 1578, 1522, 1464. Elemental analysis calcd
(found) for C41H58N2O3Si2: C 72.09 (71.80), H 8.56 (8.54), N 4.10
(3.83). Mp 97 °C.
896 J. Org. Chem., Vol. 72, No. 3, 2007

Photochromic Dyes
General Method for the Preparation of Dyads. To a refluxing
mixture of the spiropyran (6a,b) (0.400 g, 0.613 mmol), [Pd(PPh3)4]
(0.075 g, 0.065 mmol), and 2-bromopyrene (0.34 g, 1.21 mmol) in
THF (20 mL) were added over 10 h triethylamine (10 mL) and
tetrabutylammonium fluoride (1.8 mmol) in THF (10 mL). After
14 h, the solvent was removed and the residue dissolved in CH2-
Cl2 (100 mL), which was washed with water (1 × 50 mL),
separated, and dried over MgSO4. Removal of the solvent afforded
the crude product, which was purified by column chromatography
on silica gel eluting with CH2Cl2/petroleum ether (1:2). P-SP: Yield
0.21 g, 46%. 1H NMR (CDCl3, 300 MHz) δ 8.64 (2d, 2H, J ) 9.0
Hz), 8.09 (m, 15H), 7.57 (d, 1H, J ) 8.1 Hz), 7.24 (d, 2H, J ) 8.1
Hz), 7.17 (m, 1H), 7.09 (m, 1H), 6.94 (d, 1H, J ) 10.2 Hz), 6.88
(m, 2H), 5.87 (d, 1H, J ) 10.5 Hz), 3.50 (s, 3H), 1.78 (s, 3H),
1.48 (s, 3H). 13C{1H} NMR (CDCl3, 75 MHz) δ 155.7, 149.5,
139.3, 134.1, 133.0, 132.9, 132.48, 132.44, 132.30, 132.26, 132.18,
131.1, 131.0, 130.4, 130.04, 129.5, 129.3, 129.2. 129.03, 128.3,
127.26, 127.22, 126.66, 126.61, 126.55, 125.75, 125.55, 124.9,
121.3, 119.91, 119.86, 119.61, 119.13, 116.1, 105.6, 104.6, 95.52,
95.27, 94.6, 94.1, 68.94, 54.3, 53.4, 26.7, 24.7, 21.5. MALDI-MS
m/z 725 [M]+. Elemental analysis calcd (found) for C55H35NO: C
91.01 (90.74), H 5.04 (5.02), N 1.93 (2.16). Mp 226 °C.
P-SPN was purified by three consecutive column chromatogra-
phy separations on silica gel with first petroleum ether/diethyl ether
(1:1) as eluant, then petroleum ether/THF (2:1), and finally
petroleum ether/CH2Cl2 (1:2). Yield 0.29 g, 45%. 1H NMR (CDCl3,
300 MHz) δ 8.66 (d, 1H, J ) 9.3 Hz), 8.62 (d, 1H, J ) 9.3 Hz),
8.13 (m, 18 H), 7.59 (d, 1H, J ) 8.1 Hz), 7.27 (d, 1H, J ) 8.4),
7.01 (d, 1H, J ) 10.2 Hz), 6.90 (d, 1H, J ) 8.7 Hz), 5.92 (d, 1H,
J ) 10.2 Hz), 3.52 (s, 3H), 1.78 (s, 3H), 1.49 (s, 3H). 13C{1H}
NMR (CDCl3, 75 MHz) δ 160.1, 148.2, 141.8, 137.7, 133.6, 132.38,
132.23, 131.9, 131.8, 131.7, 131.6, 129.7, 129.4, 129.2, 128.9,
128.8, 128.5, 127.6, 126.7, 126.6, 126.4, 126.1, 125.9, 125.8, 125.1,
124.9, 123.1, 121.5, 119,3, 118.8, 118.6, 118.1, 115.9, 107.1, 104.4,
94.9, 94.1, 93.8, 93.3, 53.6, 53.2, 31.7, 27.3, 24.0, 20.6. MALDI-
MS m/z (%) 770 (14) [M]+, 621 (100) [M - (CH)OC6H3NO2]+,
[M - CH3 - (CH)OC6H3NO2]+. Elemental analysis calcd (found)
for C55H34N2O3: C 85.69 (85.54), H 4.45 (4.57), N 3.63 (3.73).
Mp >180 °C dec.
Emission lifetimes were measured at room temperature with a
spectrophotometer and at variable temperatures with a time-
correlated, single photon counting spectrometer. For all time-
resolved fluorescence measurements, the instrumental response
function was deconvoluted from the experimental decay record
before calculating the fluorescence lifetime. All emission studies
were made with optically dilute solutions having an absorbance of
ca. 0.08 at the excitation wavelength. A sample of P-SPN dispersed
in a glass was made by grinding together ca. 1 mg of the sample
and recrystallized sucrose octaacetate in a mortar. The solid was
transferred to a glass cuvette and melted at ca. 110 °C. Slow cooling
of the cuvette afforded a clear glass.
Transient differential absorption spectra were recorded with a
standard flash photolysis setup. The sample was deoxygenated by
a constant stream of N2 and irradiated with pulses (fwhm ) 4 ns)
delivered with a frequency-tripled, Nd:YAG laser. The monitoring
beam was a pulsed Xe arc lamp, filtered as appropriate, and directed
to a high radiance monochromator. Spectra were compiled point-
by-point with 3 individual spectra being averaged at each wave-
length. Kinetic measurements were made at a single wavelength
by averaging 50 individual traces. Control experiments were carried
out to estimate the efficiency of intersystem crossing in P-SP. Thus,
sufficient iodoethane was added to reduce ΦF by a fixed amount
in deoxygenated cyclohexane solution. Comparing the differential
absorption spectral signals observed before and after addition of
iodoethane, and allowing for the noted effect on the fluorescence
yield, we conclude that the quantum yield for formation of the triplet
state is about 0.1 in cyclohexane. Similar measurements were made
in other solvents.
To make spectroscopy measurements on the ring-opened form,
the N2-purged solution was preirradiated with light of λ ) 355 nm.
For fluorescence measurements, the sample was cooled in a
thermostated cell prior to preirradiation for 30 s with monochromatic
light from a Xe arc lamp, delivered through an optical fiber. During
illumination, the spectrometer slits were blocked by mechanical
shutters so as not to saturate the detector. Fluorescence and
excitation spectra were recorded on fast scan rates and averaged.
Such measurements were made at low temperature where the MC
form is relatively stable. The radiative lifetime for the MC form,
believed to be the trans isomer, was estimated from the Strickler-
Berg expression52 by using measured absorption and fluorescence
spectra and with the molar absorption spectrum at the peak
Preparation of 1,4-Bis(pyrenylethynyl)benzene (P). To 1,4-
bis(trimethylsilylethynyl)benzene (0.200 g, 7.39 × 10-4 mol) in
THF (20 mL) were added triethylamine (10 mL), [Pd(PPh3)4] (0.085
g, 0.0736 mmol), and 2-bromopyrene (0.42 g, 11.49 mmol). The
reaction mixture was brought to reflux and a solution of tetrabu-
tylammonium fluoride (0.57 g, 2.20 mmol) in THF (10.2 mL) was
added over 8 h. After an additional 10 h of reflux, the solvent was
removed and the residue dissolved in chloroform (100 mL), which
was washed with water (50 mL), separated, and dried over MgSO4.
Removal of the solvent afforded a crude product that was purified
by column chromatography on silica gel with petroleum ether/
diethyl ether (2:1) as eluant, followed by a second column with
petroleum ether/THF (1:1) to afford the required product. 1H NMR
(CDCl3, 300 MHz) δ 8.63 (d, 2H, J ) 9.0 Hz), 8.12 (m, 16H),
7.73 (s, 4H).
maximum in MeTHF being taken as 52 000 M-1 cm-1 32
. For the
corresponding transient absorption spectral studies the laser excita-
tion wavelength was 560 nm, as obtained with a frequency-tripled,
Nd:YAG laser fitted with an OPO. The sample was housed in an
optical cryostat set to a particular temperature and preirradiated as
above. The concentration of the MC form present in the sample
was measured by transmittance spectroscopy with a laser diode/
photocell system. Transient differential absorption spectra were
obtained in a single shot with a pulsed Xe lamp and cooled CCD
setup. Kinetic measurements were made at fixed wavelength with
a high radiance monochromator and PMT.
Spectroscopic grade solvents were obtained from commercial
sources and redistilled from appropriate drying agents. All solutions
were purged with a stream of dried nitrogen gas. All measurements
were made at 20 °C, unless stated otherwise, using N2-purged
spectroscopic grade solvent. Quantum yields were measured relative
to Coumarin 153 in cyclohexane50 and were corrected for changes
in refractive index as necessary.51 Variable temperature emission
spectroscopy was carried out with a cryostat and a commercial
fluorimeter, allowing 15 min equilibration at each temperature.
Acknowledgment. We thank EPSRC (EP/C007727), the
Leverhulme Trust (F/00125/J), and the University of Newcastle
for financial support. Dorota Rewinska is thanked for making
some of the steady-state measurements.
Supporting Information Available: 1H NMR spectra for all
new compounds; figures showing the temperature dependence for
P-SP fluorescence, a triplet absorption spectrum for P, absorption
spectra for P-SPN in different solvents, and a triplet absorption
spectrum for the MC form. This material is available free of charge
via the Internet at http://pubs.acs.org.
(50) Horng, M. L.; Gardechi, J. A.; Papazyan, A.; Maroncelli, M. J. Phys.
Chem. 1995, 99, 17311.
(51) Walter, J. B.; Garboczi, D. N.; Fan, Q. R.; Zhou, X.; Walker,
B. D.; Eisen, H. N. J. Lumin. 1998, 79, 211.
(52) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814.
JO062124L
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