Micro-environmental fine-tuning of electronic and kinetic properties of photochromic dyes

Article abstract of DOI:10.1039/c0jm00585a

Despite the recognition of the importance of Spiropyran (SP)-Merocyanine (MC) photochromic materials in molecular switching, the majority of studies have been reported in solution with relatively few in the solid-state as required in modern displays and other devices. In contrast, the present work bridges solution, condensed phase and solid-phase media and is to our knowledge the first micro-environmental study on electronic and kinetic effects for the SP-MC system. Expanding on our previous work we have investigated the photochromic properties of an SP molecule (1b) attached to a 100 μm commercially available polystyrene micro-particle (Wang Resin). The thermal MC → SP reversion kinetics (5a,b → 4a,b and 2a,b → 1a,b) which reflect the critical lifetime of the open MC form were measured. Reduction in k_obs of thermal reversion, MC → SP, up to two orders of magnitude, and a blue-shift up to 58 nm was observed with the potential for tailoring or fine tuning. The use of polymeric micro-particles in controlling both electronic and kinetic properties of photochromic molecular switches has been highlighted. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0jm00585a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Micro-environmental fine-tuning of electronic and kinetic properties of
photochromic dyes
a
b
a
c
Jamie Whelan, Dalia Abdallah, James Wojtyk and Erwin Buncel*
Received 2nd March 2010, Accepted 14th April 2010
First published as an Advance Article on the web 8th June 2010
DOI: 10.1039/c0jm00585a
Despite the recognition of the importance of Spiropyran (SP)–Merocyanine (MC) photochromic
materials in molecular switching, the majority of studies have been reported in solution with relatively
few in the solid-state as required in modern displays and other devices. In contrast, the present work
bridges solution, condensed phase and solid-phase media and is to our knowledge the first micro-
environmental study on electronic and kinetic effects for the SP–MC system. Expanding on our
previous work we have investigated the photochromic properties of an SP molecule (1b) attached to
a 100 mm commercially available polystyrene micro-particle (Wang Resin). The thermal MC / SP
reversion kinetics (5a,b / 4a,b and 2a,b / 1a,b) which reflect the critical lifetime of the open MC form
were measured. Reduction in kobs of thermal reversion, MC / SP, up to two orders of magnitude, and
a blue-shift up to 58 nm was observed with the potential for tailoring or fine tuning. The use of
polymeric micro-particles in controlling both electronic and kinetic properties of photochromic
molecular switches has been highlighted.
Introduction
a first-order reaction. This SP-MC interconversion is accom-
panied by a change in colour (from colourless SP to coloured
MC), a change in polarity (non-polar SP to polar MC) as well
as the aforementioned change in shape (orthogonal SP to
planar MC).
1
2
The fields of micro- and nanoelectronics have received much
activity in the past decade, the recent focus being on materials
whose properties can be easily manipulated by application of
heat, electron injection, or light. Many of these studies have
utilised compounds known as molecular switches, the basic
premise being that application of an external stimulus causes
a reversible structural/chemical change to occur. Examples of
Various efforts have been made to exploit the photochromic
nature of spiropyrans towards development of information
12
storage systems, MRI contrast agents, metal ion sensors,
13
14
15
16
fluorescent probes and photoswitchable magnets. An impor-
tant factor in implementation of these and other SP-based
applications is the thermal reversion reaction, MC / SP. Many
attempts have been made to suppress reversion, including
3
molecular switches under investigation include azobenzenes,
4
diarylethenes, fulgides, spirooxazines, spiropyrans and a new
5
6
7
8
family based on [1,3]oxazine rings.
Spiropyrans are a class of compounds symbolic of molecular
9
switches due to their thermochromic (heat), electrochromic
17
18
chemical modification, phenoxide protonation, metal ion
19
20
chelation, as well as choice of solvent. Degradation of the
SP–MC photochromic switch, or ‘‘fatigue’’ effects, upon repeti-
10
11
(
electron injection)
and photochromic (light exposure)
properties. The spiropyran (SP) molecule is composed of two
3
moieties, linked via a sp spiro-carbon (Scheme 1), imposing
tive switching cycles remains an issue for their long term use, with
21
both O -dependent and O -independent processes observed.
2 2
orthogonality on the molecule. Excitation of SP upon irradia-
tion in the UV (ca. 350 nm) results in heterolytic cleavage of
the spiro-carbon–oxygen bond, with subsequent cis-trans iso-
merisation producing an extended p-orbital isomeric system,
the merocyanine (MC). Accordingly, this new planar MC
absorbs in the visible spectral region (ca. 550 nm) and in polar
solvents displays a degree of charge separation which is sche-
matically represented as a zwitterionic resonance structure;
non-polar solvents inhibit this charge separation, resulting in
greater quinoidal character (Scheme 1). Thermal energy or
visible light irradiation reverses the ring-opening process in
Formation of the triplet-excited state following UV irradiation
of the SP and prior to ring opening is known to result in the
22
formation of singlet oxygen, and an improvement in cycling
efficiency has been observed on addition of singlet oxygen
23
scavengers.
24
SPs have been well studied in solution, and of late research
has begun to focus on the physical and chemical properties of SPs
in the solid-state. Work undertaken includes metal electrodes
25
usage, nanoparticles, polymer nanoparticles, as well as
26
27
28
several publications on attachment to polymer brushes. Work
carried out by Li and co-workers has examined the photo-
chromic properties of SP molecules attached to the periphery of
29
a
Department of Chemistry and Chemical Engineering, Royal Military
College of Canada, Kingston, ON, Canada K7K 7B4
nanocrystals with the aim of modulating crystal fluorescence. It
was found that the open MC isomeric form of the dye quenched
the nanocrystal fluorescence via a FRET mechanism, while the
SP isomer had no such effect. Following this, the authors
incorporated SP molecules in hydrophobic cavities of polymer
b
Reserves Development Division, Abu Dhabi Company for Onshore Oil
Operations, P.O. Box 270, Abu Dhabi, United Arab Emirates
c
Department of Chemistry, Queen’s University, Kingston, ON, Canada
K7L 3N6. E-mail: buncele@chem.queensu.ca
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Scheme 1
nanoparticles and observed a fluorescence behaviour of the
Gaining insight into the fundamentals of solid-state kinetic
behaviour is central towards integration of the SP-MC molecular
switch into novel micro- or nanotechnologies.
30
encapsulated MC.
Recently we have shown how a simple photochromic micro-
particle could be synthesised from readily available starting
materials using the cross-linked polystyrene Wang Resin (WR)
as solid support and following the kinetics of thermal MC / SP
Results
31
1. Synthesis and characterisation of photochromic
microparticles
reversion in condensed medium. For better understanding of
the potential uses of the SP system in microelectronic devices,
blade-coated thin film samples of the molecular dye and the two
photochromic microparticles were prepared and the kinetics of
thermal reversion followed. Thus herein we build on our
previous work by varying the photochromic microparticle size,
whilst also developing solid film studies. Synthesis and charac-
terisation of the photochromic microparticle will be detailed first,
followed by spectral and kinetic studies of the micro-particles in
condensed phase (non-polar viscous medium). Thereafter, these
measurements will be repeated on particles in the solid state by
incorporation into thin films, and comparison made of all
systems with free SP in solution.
32
From a family of insoluble polymer beads, the cross-linked
33
polystyrene Wang Resin (WR)
(3) containing tethered
hydroxymethyl reactive sites was chosen as the solid support
34
(
Scheme 2). Coupling with the carboxylated SP (1b) formed the
photochromic microparticle WR-SP (4), which upon UV irra-
diation converts into WR-MC (5). Structural characterisation
was carried out using FT-IR spectroscopy showing proof of
1
attachment, and H-MAS to determine percentage of resin
reactive sites containing SP molecules following a previously
35
reported procedure.
Two particle sizes were chosen, 50 and 100 mm, designated as
4a (WR-SP 50 mm) and 4b (WR-SP 100 mm), producing coloured
particles labelled 5a (WR-MC 50 mm) and 5b (WR-MC 100 mm)
following UV irradiation. Previously 1b was chosen having
a suitable linker group for attachment to the resin. Additionally,
6-nitro-BIPS (1a) is used herein for comparative purposes.
The use of photochromic particles in fine-tuning the electronic
properties of molecular switches is discussed employing the SP-
MC as a model system, including consideration of particle size.
Our work shows the potential for incorporation of the SP-MC
photochromic molecular switch into solid-state technologies.
Scheme 2
5
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2
. Kinetics of thermal reversion in condensed medium
36
Though 3 is known to swell in various solvents, the polymer
particles eventually sediment to the bottom of a vessel. To
combat this, the solution can be ‘‘hardened’’ with a suitable
thickener. Since the SP-MC system is sensitive to changes in
solvent polarity, an appropriate system was required so as not
to distort the kinetic experiments. Given that 3 is composed of
cross-linked polystyrene, the solvent and thickener chosen
which most matched this was toluene and polystyrene.
37
Reichardt’s dye was used to monitor the polarity changes
between toluene and the polystyrene thickened toluene. No
change in polarity was observed up to a medium viscosity of
1
1 centi-Stokes (cS), the condensed medium employed in this
study.
Kinetics of thermal reversion, MC / SP, were followed
Fig. 2 Solid state first order rate plot of thermal reversion 5a / 4a in ca.
50 mm polystyrene matrix thin film (kobs ¼ 7.25 ꢂ 10ꢁ4
ꢁ1
7
s
).
spectrophotometrically in a quartz cuvette with both 4a and,
separately, 4b, suspended in condensed medium. The thermal
reversion process WR-MC / WR-SP follows first order
kinetics, with results for both photochromic microparticles 4a
and 4b, as well as for unbound photochromes (1a and 1b) in
condensed medium and in solution given in Table 1.
Table 2 Observed rate constants and lmax for the kinetics of thermal
reversion MC / SP, for 2a, 2b, 5a and 5b in the solid state
ꢁ1
Kinetic process
k
obs, s (MC lmax
)
4
2a / 1a
2b / 1b
5a / 4a
5b / 4b
9.72 ꢂ 10ꢁ (596 nm)
3
.
Solid state kinetics
ꢁ4
10.5 ꢂ 10 (596 nm)
ꢁ
4
7.25 ꢂ 10 (588 nm)
Homogeneous films of the photochromic systems under study
were prepared from a blade-coated toluene/polystyrene solution
ꢁ4
5.69 ꢂ 10 (588 nm)
(
1)/suspension (4a and 4b), yielding a film thickness ca. 120 mm
and ca. 750 mm, respectively. After irradiation UV-Vis spectra
were recorded and kinetics of MC / SP thermal reversion
followed (Fig. 1 and 2). All thermal reversion reactions fol-
lowed first-order kinetics and solid-state results are given in
Table 2.
Table 1 Observed rate constants and MC lmax for thermal reversion 2a
/
1a and 2b / 1b in solution and condensed medium, and 5a / 4a and
5b / 4b in condensed medium
ꢁ
1
k
obs, s (MC lmax
)
Discussion
Kinetic process
Solution
Condensed medium
1
.
Medium polarity effects
2a / 1a
2b / 1b
5a / 4a
5b / 4b
12.2 ꢂ 10ꢁ (600 nm)
2
13.1 ꢂ 10 (600 nm)
ꢁ2
6.94 ꢂ 10ꢁ (604 nm)
2
7.43 ꢂ 10 (604 nm)
ꢁ2
Medium polarity is of paramount consequence for the SP–MC
photochromic system. Previously our group and others
have observed colouring and kinetic effects induced by solvent
ꢁ
4
32.2 ꢂ 10 (588 nm)
38.7 ꢂ 10ꢁ (588 nm)
4
(
solvatochromic and solvatokinetic behaviour) on a number of
20,37
SP-MC systems.
It was found that an increase in solvent
polarity, as measured by Reichardt’s dye, resulted in a hyp-
sochromic shift in lmax of the coloured MC form, as well as
a decrease in kobs for thermal MC / SP reversion. Scheme 1
shows the typical schematic representation of the SP-MC
interconversion, namely that structurally and electronically the
coloured MC isomeric form is situated somewhere between two
resonance forms, quinoidal (non-polar media) and zwitterionic
(
polar media). Depending on the predominant resonance
structure, a thermally-induced back reaction would reduce to
ꢁ1
C–C single bond rotation (ꢀ5 kcal mol ) for quinoidal MC
with electrocyclic ring closing, vs. trans / cis C]C isomer-
ꢁ1
isation (ꢀ65 kcal mol ) for zwitterionic MC, followed by C–O
bond recombination. Viewing the above as an increase in
solvent polarity resulting in an increase in bond order of the
central C–C bond, non-polar solvents favour greater quinoidal
character, while polar solvents favour a more charge-separated,
or zwitterionic (hybrid) resonance form as illustrated.
Fig. 1 Solid state first order rate plot of 2a / 1a thermal reversion in ca.
20 mm polystyrene-matrix thin film (kobs ¼ 9.72 ꢂ 10ꢁ
4
s ).
ꢁ1
1
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compared the band maximum of their MC in water (525 nm) to
that of their MC-nanoparticles in water (588 nm) and concluded
that the MC is situated in a hydrophobic environment. In the
present case, however, the solvent polarity is not being altered, the
condensed medium of toluene/polystyrene having the same
polarity as cross-linked polystyrene resin. Thus another factor
must be causing this decrease in lmax, which is likely to be a change
in the micro-environment of the open form, on going from a free-
moving fluid solution, to a more confined polymer micro-partic-
ulate environment. Interestingly, in our case and that of the
MC-polymeric nanoparticle of Li and co-workers, the MC isomer
becomes encased within a hydrophobic environment; the authors
conclude that the red shift of their MC attached to the periphery of
a nanocrystal is due to the non-polar micro-environment on the
particle surface. Lowering of lmax indicates a restricted orienta-
tion of the MC enforced by the resin, thereby limiting the degree to
which MC can achieve itslowest energy state, i.e. planar form. The
issue of planarity is important here as it is the extended p-conju-
gated network formed upon ring opening which gives rise to the
coloured MC form, not present in orthogonal SP. A reduction in
planarity hampers MC attaining its lowest energy state
Solvatokinetic studies showed that kobs for the thermal
reversion was only moderately influenced by solvents of low
polarity, e.g., toluene, diethyl ether and 1,4-dioxane. The mech-
anism in non-polar solvents for ring-closure of MC to SP is thus
considered as an electrocyclic process via the transition state as
20
shown.
In the present study we set out to determine the effect of
changes in the micro-environment on the electronic and kinetic
properties of the SP-MC system, building on the results of our
group’s previous works. Changing the solvent medium within the
20
non-polar range results in little change in both lmax and kobs
,
unlike in polar solvents. This is deemed to be a key requirement
from which to judge the micro-environmental effects on the
energies and kinetics of thermal reversion. It is expected that the
quinoidal resonance character will predominate in toluene,
chosen as base solvent.
(
i.e. reduced planar structure), evidenced by a decrease in lmax
.
Kinetic results (Table 1) in condensed medium show that kobs
for MC / SP thermal reversion decreases by two orders of
Two parameters are expected to exert influence on the UV-Vis
absorption and kinetic properties of this molecular switch
comprising solution, condensed and solid-state systems:
ꢁ2 ꢁ1
magnitude, comparing 2a (13.1 ꢂ 10 s ) and 2b (7.43 ꢂ
ꢁ2 ꢁ1
ꢁ4 ꢁ1
ꢁ4 ꢁ1
1
0
s ) to 5a (32.2 ꢂ 10 s ) and 5b (38.7 ꢂ 10 s ); results
for 5b mirror those for 5a. The decrease in rate of reversion can
be rationalised as resulting from constraints imposed by the
polymer resin. For thermal back reaction, a volume demanding
process, in condensed medium of the resin-bound MC vs. free
MC, the obstacles put in place by the polymer strands of the resin
could hinder the MC’s ability to rotate about the central C–C
bond, i.e. the polymer microparticles’ entangled internal strands
obstruct efficient rotation of the dye molecule, thus prolonging
the life-time of the MC isomer.
(
i) polarity, and (ii) state of micro-environment, i.e. solution,
condensed, solid. In this study, polarity as measured by Reich-
ardt’s dye is held constant; however, interaction between MC
and the micro-environment marks important change as one
traverses over solution, 2a and 2b (toluene); to condensed 2a and
2
b (toluene-polystyrene); condensed particulate, 5a and 5b
(
toluene-polystyrene); to solid-state 2a and 2b (polystyrene
matrix); and finally solid-state particulate, 5a and 5b (polystyrene
matrix). Concurrently, UV-Vis examination of photochromes 2a
and 2b in toluene shows lmax 600 and 604 nm, and thermal
Condensed phase photostability studies carried out on 5b
closely resemble 5a, with the cycling ability of the photochrome
5b and 5a relative to 2a and 2b increased by over 200% and 300%
respectively. Li and co-workers found that binding of the dye to
ꢁ
2
ꢁ1
reversion rates for MC / SP, k ¼ 12.2 ꢂ 10
obs
s
(2a) and
ꢁ2
ꢁ1
6
.94 ꢂ 10
s
(2b), respectively. Using this as our baseline, all
other systems will be compared on the basis of changing
parameter (ii) above.
28
a nanocrystal had no effect on photodegradative reactions;
however, the authors noted that incorporation of the dye into
a polymer particle minimized bimolecular degradative reactions
29
proceeding via the triplet-excited dye molecules. The corre-
spondence of our results with their system suggests similar
reasoning here.
2
. Condensed Phase Study of MC / SP Reversion
Variation in medium viscosity, from 1 to 11 cS, for 2a and 2b in
non-polar range was found to result in no change in lmax and
minimal change in kobs relative to baseline toluene. This implies
that rotation about the central C–C single bond is unimpeded in
viscous media for the free, unbound, MC.
Progression in the micro-environment from solution to
condensed medium, followed by incorporation into a polymer
microparticle, warranted further insight regarding future appli-
cations of this system; thus a solid-state investigation was initi-
ated. As with condensed medium analysis, solid-state kinetics of
thermal reversion of MCs 2a, 2b, 5a and 5b will be compared to
that of solution (2a and 2b) and condensed systems (2a, 2b, 5a
and 5b).
Concomitant to attachment of carboxylated SP (1b) to WR (3)
forming 4a and 4b, lmax of the open form in condensed medium
was found to shift from 600 (2a)/604 (2b) nm to 588 nm (both 5a
and 5b). Based on solvatochromic studies outlined earlier, this
shift would normally coincide with an increase in solvent polarity,
in accordance with increasing stabilisation of the developing
zwitterionic resonance form. The results bear a striking similarity
to the 588 nm absorption lmax found by Li and co-workers in both
3
.
Solid-state Studies of MC / SP Reversion
(a) Micro-environmental spectroscopic observations. The UV-
24
their MC-nanocrystal (THF) and polymeric MC-nanoparticle
Vis spectra of the prepared films of 1a and 2a (Fig. 3), and 1b and
2b (Fig. 4) following irradiation are pertinent. Fig. 3 shows lmax
29
beads (polystyrene-based in water). Li and co-workers have
5
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Fig. 3 Solid state UV/Vis spectra of 1a (SP) and 2a (MC) in ca. 120 mm
polystyrene matrix thin film.
Fig. 5 Solid state UV-Vis spectra of 4a (WR-SP 50) and 5a (WR-MC 50)
in ca. 750 mm polystyrene-matrix thin film.
As with the spectral characterisation of 2a in the solid state,
exhibiting a shoulder at 575 nm (Fig. 4), a similar and yet more
pronounced shoulder at 575 nm is seen for 5a and 5b (Fig. 5). It is
noted this shoulder peak appears at the same wavelength
(
574 nm) in two different systems, solid state 2a and solid state 5a
and 5b. The UV-Vis spectrum of 2b would suggest a medium
20
polarity close to that of ethanol. It is also worth noting that
there are two distinct peaks, 546 and 446 nm (the peak at 446 nm
was seen to decay within the same time period as that of 546 nm).
The micro-environmental effect on lmax of the MC could
potentially serve as a fine tuning feature of electronic properties
of photochromic molecules. As can be seen with the free MC 2a
in solid state compared to condensed medium, a small blue-shift
in lmax is observed (600 to 596 nm), while attachment to the
polystyrene resin produces a further blue-shift (600 to 588 nm).
Of note is the observation that there is no variation in MC lmax
for 5a or 5b, implying a lack of MC sensitivity towards macro-
environmental, compared to micro-environmental, effects. From
an optoelectronic device perspective this means freedom of
choice in terms of particle size, though the lower limits of this
effect still need to be established.
Fig. 4 Solid state UV/Vis spectra of 1b (SP) and 2b (MC) in ca. 120 mm
polystyrene matrix thin film.
of 2a as 596 nm, shoulder at 575 nm, which represents a slight
hypsochromic shift (4 nm) in lmax on progression from solution
(
600 nm) to solid state (596 nm). With no change in medium
polarity, this strongly points to micro-environment effected
restriction of MC planarity inducing higher energy excitation for
backbone p-electrons. It is noted that the reduction in planarity
of condensed 2a going to solid state 2a is less than for condensed
It would be of interest to study varying the degree of cross-
linking in 3 and its effect on spectral properties. If resin cross-
linking restricts achievement of planarity, then this could be used
for further fine tuning of electronic properties. As noted by Li
1
a to condensed 5a (4 vs. 12 nm). Ring-opening in the solid state
for 2a causes permanent deformation in the solid polystyrene
matrix, allowing for formation of planar MC. In comparison, for
29
5
a the polymeric strands of Wang resin are cross-linked, thereby
and co-workers, it is also possible that adjusting the length of
the tethering chain from the SP to the resin (currently 3 carbons)
could enable the indoline moiety more/less manoeuvring capa-
bility, thereby minimising/maximising the lmax shift. Clearly,
further defining studies would help elucidate the relationship
between the tethering chain length and the degree of crowding on
the electronic properties of the dye.
restricting the degree to which SP can fully ring open, thus
forming MC with a reduced planar structure and therefore
a lower MC lmax
.
The UV-Vis solid-state spectra of 4a and 5a (Fig. 5) are
identical to 4b and 5b with lmax (5a, 5b) 588 nm, as found for the
condensed phase lmax of both particles. Given the change in
micro-environment from condensed to solid state, it is not
unreasonable to expect a blue-shift in lmax on going from solu-
tion to solid state as observed for 2a and 2b. The polystyrene
strands from the film matrix do not affect the degree to which
MC can achieve planarity, in contrast to the resin itself. Thus
planarity is determined by 3 alone, and is unaffected by the
greater environment.
(b) Micro-environmental kinetic observations. Solid state MC
/ SP thermal reversion followed at 596 nm (2a) and 548 nm (2b)
exhibits first order kinetics with kobs ¼ 9.72 ꢂ 10ꢁ (Fig. 4) and
4
10.5 ꢂ 10ꢁ
4
s
ꢁ1
for 2a and 2b, respectively, and represents
a marked decrease in kobs MC / SP from solution/condensed
to solid state for 2a (12.2 ꢂ 10ꢁ to 9.72 ꢂ 10ꢁ4 s ) and 2b
2
ꢁ1
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ꢁ
2
ꢁ4 ꢁ1
(
6.94 ꢂ 10 to 10.5 ꢂ 10 s ). It also represents an increase in
All glassware used in this study was cleaned in the following
way: washed with acetone and then soaked in an acid bath (2 M
HCl) overnight. The glassware was then washed thoroughly with
deionised water, then acetone, before being dried overnight in
the lifetime of the coloured MC isomer from that of the resin-
bound micro-particle suspended in condensed medium, 32.2 ꢂ
ꢁ4
ꢁ1
ꢁ4 ꢁ1
1
0
s
(5a in condensed medium) to 10.5 ꢂ 10
s
(2b in solid
ꢃ
state). Once again the progression towards a more confined/
restrictive micro-environment yields a further decrease in MC /
SP thermal reversion. Thus, the MC micro-environment prog-
resses from one in which MC is free to undergo reversion
a vacuum oven at 60 C.
Synthesis of 4
ꢁ
1
(
solution/condensed medium), to one in which it is slightly
In a 100 ml conical flask, 3 (1.006 g, 50 or 100 mm, 1.0 mmol g )
was allowed to become swollen in dried tetrahydrofuran (THF)
(25 ml) for 30 min. To this suspension was added the SP (0.380 g,
1 mmol), dicyclohexylcarbodiimide (DCC) (0.226 g, 1.1 mmol)
and dimethylaminopyridine (DMAP) (0.035 g, 0.29 mmol). The
mixture was mechanically shaken at room temperature for 6 h,
under nitrogen. The resin was then vacuum-filtered and washed
consecutively with THF, acetone, EtOH, water and finally
acetone. The resin beads were then dried in a vacuum oven
crowded (condensed medium immobilised within a swollen
polystyrene resin), to a more crowded system (solid state) with
the film matrix polystyrene strands expected to act as obstruc-
tions for ring closing, thereby decreasing kobs. It could be
reasonably expected, therefore, that solid state kinetics of 5a and
5
b would reveal further decreases in kobs for thermal reversion,
due to additional crowding imposed by the resin, coupled with
the film matrix, as is indeed observed. Once again it should be
noted that the effect of particle size is insignificant, in this case in
terms of affecting thermal reversion kinetics. Coupled with the
lack of influence of particle size on MC electronic properties, this
is a strong positive towards using microparticle-encased photo-
chromic molecular switches for optoelectronic devices.
ꢃ
overnight, at 50 C, to give 4a (ꢀ1.130 g). Synthesis of 4b
followed the same procedure (Scheme 3).
T
Solvent Polarity, E (30)
Table 3 records a summary of the kinetic behaviour from
solution, to condensed, to solid state media of 2a, 2b, 5a and 5b.
It can clearly be seen that at constant polarity, modification of
the micro-environment surrounding the coloured MC form plays
a dominant role in determination of both electronic as well as
kinetic properties of this photochromic system.
Solvent polarity measurements were carried out on all viscosity
ranges for the non-polar medium using Reichardt’s dye as indi-
cator [ET (30)]. Briefly, a stock solution of Reichardt’s dye in
DMSO was prepared (ꢀ10ꢁ M). A sample of viscous solvent
(2.0 ml) was added to a quartz cuvette and a blank spectrum was
acquired. Next, 10 mL of dye solution was added to the cuvette
via syringe and placed in the UV-Vis spectrophotometer. A
2
T
UV-Vis spectrum was recorded and from lmax the E (30) was
calculated.
Experimental
UV-Vis spectra and kinetics were recorded using a Varian Cary 3
double-beam spectrophotometer equipped with a thermostatted
Non-polar viscous medium
ꢃ
1
cell compartment set to 25 C. H-MAS spectra were recorded on
a Bruker Avance-600 MHz spectrometer, using a Magic Angle
Spinning probe. Viscosity measurements were carried out on
A stock solution of non-polar medium was prepared by weighing
42 g of polystyrene (M
¼ 70, 000) into a conical flask and dis-
w
solved in toluene (150 ml). Solutions of varying viscosity were
prepared by changing the amount of stock solution added. For
example, 5 ml of stock solution was added to a 25 ml volumetric
flask and this was made up to the mark with spectophotometric
ꢃ
a Lauda viscometer, at 25 C, using an appropriate capillary
tube. Results are quoted in centi-Stokes (cS). Solvent polarity
was determined using Reichardt’s dye. TGA and DSC
measurements were carried out using a TA Instruments TGA
ꢃ ꢃ
ꢃ ꢃ
ꢁ1
2
050 (400 C @ 10 C min ) and a TA Instruments DSC 2010
ꢁ1
(
200 C @ 10 C min ), respectively.
Table 3 Observed rate constants and lmax for the kinetics of thermal
reversion 2a/ 1a, 2b / 1b, 5a / 4a and 5b / 4b in solution,
condensed medium and the solid state
ꢁ
1
k
obs, s , (MC lmax)
Kinetic
process
Condensed
medium
Solid
state
Solution
ꢁ
2
2
ꢁ2
ꢁ4
2a / 1a
2b / 1b
5a / 4a
5b / 4b
12.2 ꢂ 10
,
,
13.1 ꢂ 10
,
9.72 ꢂ 10
,
(
600 nm)
(600 nm)
7.43 ꢂ 10
(596 nm)
10.5 ꢂ 10
ꢁ
ꢁ2
ꢁ4
6.94 ꢂ 10
,
,
(
604 nm)
(604 nm)
ꢁ
(546 nm)
ꢁ4
4
32.2 ꢂ 10
,
7.25 ꢂ 10
,
(
588 nm)
4
(588 nm)
5.69 ꢂ 10
ꢁ
ꢁ4
38.7 ꢂ 10
,
,
(
588 nm)
(588 nm)
Scheme 3 Synthesis of 4
5
732 | J. Mater. Chem., 2010, 20, 5727–5735
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grade toluene to give a viscosity of 1.08 cS. Subsequent samples
were prepared in identical manner.
Kinetic analysis of thin film samples
A sample of the SP or WR-SP thin film was cut (ca. 1 ꢂ 5 cm)
and this was placed across the multi-cell chamber of the HP
UV-Vis spectrophotometer. A spectrum was recorded as
Kinetics of Condensed Phase WR-MC / WR-SP
‘‘Blank’’ and then the photochromic film was placed in the
Spectroline CX Series Ultravioloet Fluorescence Analysis
Cabinet equipped with a Spectroline XX-15N UV lamp (dual
Thermal ring-closing WR-MC / WR-SP was followed by the
creation of a stable suspension of the photochromic beads in
a suitable viscous solution. The suspension was formed by add-
ing WR-SP (ꢀ20 mg) to a quartz UV-Vis cuvette which was
manually shaken to distribute the beads within the viscous
media. Stability of the suspension was verified by monitoring the
lmax of the WR-SP in the non-polar viscous media to ensure no
sedimentation occurred during the time frame under study
1
5W, 365 nm) and irradiated for 90s, generating the coloured
MC isomer as evidenced by the change in colour of the photo-
chromic film. The coloured thin film was then placed back in
the spectrophotometer and the kinetics of thermal reversion
MC / SP followed at the respective MC lmax for 10 h. Spectra
were recorded every 30 s. Throughout this entire procedure,
lights were kept off and the UV-Vis spectrometer was covered
with aluminium foil to prevent any stray light causing unwanted
photochemical reversion of the MC / SP. Observed rate
constants, kobs, for thermal reversion MC / SP were calculated
(
ꢀ30 min). Once a solvent viscosity was found that generated
a stable suspension, a fresh sample was prepared with 2.0 ml of
viscous solvent and ꢀ10 mg of uncoloured WR-SP particles
added to a quartz cuvette, placed in the spectrometer and
a ‘‘Blank’’ spectrum recorded. The sample cuvette was then
placed in a Spectroline CX Series Ultravioloet Fluorescence
Analysis Cabinet equipped with a Spectroline XX-15N UV lamp
N t
fromlinear plots of log(A – A ) versus time.
Conclusions
(
dual 15W, 365 nm) and irradiated for 90 s, generating the col-
oured WR-MC isomer as evidenced by the blue colour change of
the micro-particle. The cuvette was immediately placed into the
spectrophotometer and the kinetics of thermal reversion WR-
MC / WR-SP was followed at 588 nm. Spectra were recorded
every 30 s. Throughout this entire procedure, lights were kept off
and the UV-Vis spectrometer was covered with aluminium foil to
prevent any stray light causing unwanted photochemical rever-
sion of WR-MC / WR-SP.
In keeping with the growing need for solid-state studies of
photochromic dyes as required in modern displays and other
devices, the present work has bridged solution, condensed phase
and solid-phase media, identifying steric, kinetic, electronic and
other micro-environmental factors. To facilitate incorporation of
the SP-MC molecular switch into novel micro-/nano-optoelec-
tronic devices, we have developed a means towards fine tuning of
key elements; electronic and kinetic properties. Herein we have
described the synthesis of photochromic polymer microparticles
(
4a and 4b) and studied the electronic and kinetic properties of
Thin film
thermal reversion in solution, condensed and solid state media
with comparisons made with the unbound dye in the same media
Thin film samples were prepared by a blade-coating procedure
that used blades from Paul N. Gardner Company, Inc. Film
thickness was achieved using blades with pre-set heights,
measured in mils (1 mil ¼ 1/1000 th of an inch, ¼ 25.4 mm).
A Mylar solid support was chosen for the fabrication of the thin
films.
(
1a and 1b).
UV-Vis analysis of the coloured MC (5a or 5b) within the
microparticle points to a more polar environment than that in
toluene, compared with the unbound dye, 604 (2a)/600 nm (2b)
to 588 nm (5a and 5b). However, Reichardt’s dye and the cross-
linked resin structure indicate unchanged polarity; rather it is
modification of the micro-environment in terms of spatial
confinement of the photochrome which is affected. The resulting
blue-shift in lmax is effected by restrictions imposed by the cross-
linked polystyrene resin micro-environment surrounding the
photochrome, precluding formation of the highly planar MC
form. Kinetics of thermal reversion were carried out for the
unbound (2a and 2b) and resin-bound (5a and 5b) MCs
in solution, condensed and solid state media. A decrease in
MC / SP kobs was observed upon changing the micro-envi-
ronment from solution/condensed to solid state for both
unbound and resin-bound dyes. Changes in kobs progressing
from solution/condensed to solid state were larger for unbound
than for resin-bound dyes, indicative of the importance played by
the micro-environment in determining kinetic properties of
molecular switches.
Preparation of thin film of 1a and 1b
Thin films of SP were prepared by blade coating a mixture of SP
(
50 mg) in viscous toluene (2.5 ml) onto Mylar. The thickness of
the thin film samples was set at 5 mils (127 mm or 0.127 mm).
Films were left to air-dry overnight.
Preparation of thin film of WR-SP
Thin films of WR-SP were prepared by blade coating a mixture
of WR-SP (200 mg) in viscous toluene (2.5 ml) onto Mylar. The
thickness of the thin film samples was set at 30 mils (762 mm or
0
.762 mm). The film was then air dried for one hour before
a second coating of toluene (150 ml)/polystyrene (42 g,
We have shown how modification of micro-environment for
a photochromic molecular switch can determine the fine-tuning
of both electronic and kinetic properties. This highlights the need
for consideration of support systems for dye-incorporating
M
w
¼ 70,000) solution with a 40 mils (1.016 mm) blade was
applied to smooth the film and refractive index match the resin
and the film. Films were then left to air-dry overnight.
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J. Mater. Chem., 2010, 20, 5727–5735 | 5733

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micro-/nano-optoelectronic devices, while at the same time
opening a window of opportunity to exploit further the electronic
and kinetic properties without resorting to extreme measures. We
have also shown how consideration of particle size does not play
a role in affecting either of these properties (electronic/kinetic), as
such making microparticle encased molecular switches ideal
candidates for incorporation into optoelectronic devices. Further
studies are proposed to deduce tethering chain length and resin
cross-linking effects on resin-confined dye molecules.
SPIE-Proceedings (Organic Photonic Materials and Devices VII),
2
005, 5724, 13–20.
7
M. Tomasulo, S. L. Kaanumal, S. Sortino and F. M. Raymo, J. Org.
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Modification of the electronic properties for any dye molecule
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new dye into varying support systems will have an impact. Here
we advocate a coupling of chemical modification (coarse tuning)
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molecules, thereby enabling achievement of tailored optoelec-
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Acknowledgements
The authors thank the Natural Sciences and Engineering
Research Council of Canada (EB) for funding this research.
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