Optimizing the photochromic performance of naphthopyrans in a rigid host matrix using poly(dimethylsiloxane) conjugation

Article abstract of DOI:10.1039/b904345d

A series of methoxy substituted 2,2-diaryl-2H-naphthopyran photochromic dyes were assembled incorporating hydroxy functionality to allow their subsequent attachment to flexible poly(dimethylsiloxane) oligomers. The photochromic performance of the generated PDMS-naphthopyran conjugates was studied in a thermoset host matrix and compared to non-conjugated, electronically equivalent control dyes. Both coloration and decoloration speeds were found to be greatly improved with critical T_1/2 decoloration times reduced by 42-80%. The extent of solution-like performance provided by PDMS conjugation in the rigid host was examined with reference to the fade performance of control dyes in solution, and found to range from 20 to 90%. These measures are believed to be influenced by the electronic nature and steric interactions of the photochromic dyes.

Full text of DOI:10.1039/b904345d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Optimizing the photochromic performance of naphthopyrans in a rigid host
matrix using poly(dimethylsiloxane) conjugation†
c
abc
Francesca Ercole,abc Nino Malic,ab Thomas P. Davis and Richard A. Evans
*
Received 4th March 2009, Accepted 8th May 2009
First published as an Advance Article on the web 19th June 2009
DOI: 10.1039/b904345d
A series of methoxy substituted 2,2-diaryl-2H-naphthopyran photochromic dyes were assembled
incorporating hydroxy functionality to allow their subsequent attachment to flexible
poly(dimethylsiloxane) oligomers. The photochromic performance of the generated PDMS–
naphthopyran conjugates was studied in a thermoset host matrix and compared to non-conjugated,
electronically equivalent control dyes. Both coloration and decoloration speeds were found to be
greatly improved with critical T1/2 decoloration times reduced by 42–80%. The extent of solution-like
performance provided by PDMS conjugation in the rigid host was examined with reference to the fade
performance of control dyes in solution, and found to range from 20 to 90%. These measures are
believed to be influenced by the electronic nature and steric interactions of the photochromic dyes.
donating substituents (e.g. methoxy or dialkyl amino)18 and
Introduction
concurrent steric effects of substituents have all been found to
influence photochromic characteristics such as de/coloration
rates and photogenerated color.12
Naphthopyrans are an important class of photochromic dyes
which display a reversible color change when exposed to UV light
and are well known for their commercial application in the
ophthalmic lens industry.1–5 Potential application of photo-
chromics in other areas such as electronic devices, optical
memories and photo-switches continues to grow and motivate
interesting research.6–11
The photochemical as well as the thermal behavior of the
photochromic molecules is also profoundly influenced by the
media in which they are incorporated into. Within rigid, optically
clear matrices, such as ophthalmic lens material, viscosity and
free volume of the host environment become major factors
governing the speed of the photochromic reaction.19–21 Photo-
chromic switching involves substantial mechanical movement,
the transition requiring a large intramolecular rotation during
coloration and decoloration. Therefore, in solid media the
molecules are restricted in their movement rendering the speed of
photochromic switching significantly slower. This is frequently
referred to as the matrix effect.19 We have been motivated to
develop strategies that enable fast switching of photochromic
dyes to occur even within the rigid material of a lens.22–24
The methodology developed in our laboratory which has
provided considerable control and improvement to photo-
chromic behavior in a host matrix is the use of polymer conju-
gation.22,25–27 Covalent attachment of oligomers directly to the
dye acts to alter the local environment in the vicinity of the
photochromic molecule. When incorporated within a host
matrix, entanglement and partitioning of polymer tails provide
encapsulation and insulation from the rigid bulk environment,
The photochromic reaction proceeds by UV irradiation of the
colorless closed form resulting in the electrocyclic ring opening of
the pyran moiety via cleavage of the C(sp3)–O bond (Scheme 1).12
This produces a distribution of open geometrical isomers (mer-
ocyanines) which are intensely colored due to their extended
conjugation and quasi-planar conformations. In the absence of
UV irradiation, the colored isomers gradually thermally or
photochemically revert back to the closed naphthopyran form,
undergoing a significant intramolecular rotation during the
conversion.
The optical lens market continues to be the largest consumer
of photochromic dyes in which substituted 2,2-diaryl-2H-naph-
tho[1,2-b]pyrans, with the base structure shown in Scheme 1, are
highly represented.1–3,13 Chemical developments aimed at opti-
mizing performance characteristics such as intensity of colora-
tion, fade (reversibility) rate, fatigue resistance and
photogenerated color have been extensive. For example,
5-alkoxycarbonyl substitution,14,15 heterocyclic fused moieties
(e.g. an indeno group),16,17 judiciously positioned electron
presenting the dye and its aggregates with
a localized
aCSIRO Molecular & Health Technologies, Bag 10, Clayton, VIC 3169,
Australia. E-mail: Richard.Evans@csiro.au; Fran.Ercole@csiro.au; Nino.
Malic@csiro.au
bThe Cooperative Research Centre for Polymers, 8 Redwood Drive,
Notting Hill, VIC 3168, Australia
cCentre for Advanced Macromolecular Design, School of Chemical
Sciences and Engineering, The University of New South Wales, Sydney,
NSW 2052, Australia
Scheme 1 Generic photochromic reaction for substituted 2,2-diaryl-2H-
naphtho[1,2-b]pyrans.
† Electronic supplementary information (ESI) available: Synthesis details
and thermal decoloration pathway scheme. See DOI: 10.1039/b904345d.
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environment of controlled viscosity. Attachment of PDMS
(poly(dimethylsiloxane)) oligomers manifests a strong lubri-
cating effect by introducing a local environment of low viscosity
and higher mobility, effectively improving switching ability.
The purpose of the work presented in this article was to exploit
PDMS conjugation to investigate its capacity to influence the
spectrometer using a cone voltage of 50 V with the source
maintained at 80 ꢀC. Methanol was used as solvent system with
a flow rate of 0.04 mL minꢁ1
.
1
Molecular weights of PDMS were obtained from H NMR
spectra from integration values of CH2 resonances along the
backbone with respect to those of Si(CH3)2. Final molecular
weights of PDMS–naphthopyran conjugates were additionally
confirmed from integration of characteristic naphthopyran end
groups.
photochromic behavior of
a variety of known methoxy
substituted 2,2-diaryl-2H-naphtho[1,2-b]pyrans within a rigid
host matrix. An OH group on the naphthopyran was used as the
tethering moiety for subsequent conjugation via condensation
chemistry. Practical strategies for synthesizing and accessing
appropriate starting materials and conjugates were also pre-
sented. Photochromic spectrokinetic properties of PDMS-
conjugated dyes within a rigid host matrix were compared to
those of electronically equivalent control dyes which lacked
PDMS tailing. The extent of the solution-like performance
provided in the host matrix by PDMS conjugation was investi-
gated by comparing their fade performance with that of their
control dyes in solution.
Photochromic analyses were performed on lenses composed of
PDMS–naphthopyran conjugates and their corresponding
controls dissolved individually in a standard industrial lens
formulation comprising 1 : 4 weight ratio of poly(ethylene
glycol)(400) dimethacrylate (PEGDMA) and 2,20-bis((4-meth-
acryloxyethoxy)phenyl)propane (EBPDMA) with 0.4% azobi-
s(isobutyronitrile) (AIBN) (Fig. 1). The formulation was then
cured at 80 ꢀC for 16 h in a standard mold to give optically clear
test samples of equivalent thickness (ꢂ2.4 mm). The doping
concentrations were chosen in order to maintain optical densities
in a meaningful detector range for photochromic kinetic tests
(refer to those entries in Table 2).
Experimental
Under continuous irradiation, the photochromic responses of
the lenses were analyzed on a light table (schematic representa-
tion available in literature)24 comprising a Cary 50 spectropho-
tometer to measure absorbance values and a 160 W Oriel xenon
lamp as an incident light source. A series of two filters (Edmund
Optics WG320 and Edmund Optics band-pass filter U-340) were
used to restrict the output of the lamp to a narrow band (350–400
nm). The samples were maintained at 20 ꢀC and monitored at the
maximum absorbance of the colored form for a period of 1000 s.
Then the thermal decoloration was monitored in the dark for
a further 4800 s (maximum). Control samples were also analyzed
in solution using toluene as the solvent in a quartz cell with 1 cm
path length. Concentrations ranged from 10ꢁ4 to 10ꢁ5 M, refer to
entries in Table 2.
Materials
All chemicals (reagents and solvents) were of high purity and
used as received unless otherwise stated. Reagents for synthesis
were obtained from Aldrich at the highest purity available and
used without further purification. Carbinol (hydroxyl-termi-
nated) poly(dimethylsiloxane) was purchased from Gelest Inc.
All chromatography was performed using silica gel (Kieselgel
Merck 60, 0.040–0.063 mm) and TLC was performed on Merck
Silica 60F254 plates.
Methyl 1,4-dihydroxy-2-naphthoate (2a) and methyl
4-hydroxy-6-methoxy-2-naphthoate (6a) were prepared using
literature procedures.28,29 The synthesis of 4-hydroxy-1-phenyl-2-
naphthoic acid (7) was performed using a patent procedure.30
5-Hydroxy-7H-benzo[c]fluoren-7-one (9) was synthesized
using the procedure of Aki et al.31 5-Hydroxy-7H-benzo[c]-
fluoren-7-ol (10) was synthesized using the procedure of Zeyni-
zadeh and Behyar.32 4-Hydroxy-40-methoxybenzophenone (11)
was synthesized using a procedure adapted from the literature.33
4-(3-Hydroxypropoxy)-phenyl-40-methoxybenzophenone (12)
was synthesized as reported in patent literature.34 Propynols 13,
14 and 15 were prepared using lithium trimethylsilylacetylide, as
in the literature.35 Complete characterization and additional
details of synthetic procedures are outlined in ESI†.
Preparation of starting materials
Methyl 4-hydroxy-1-methoxy-2-naphthoate (2b). 1,4-Dihy-
droxy-2-naphthoic acid (1) (2.00 g, 9.80 mmol) and K2CO3
(5.40 g, 39.18 mmol) were dissolved in a mixture of waterꢀ(50 mL)
and i-PrOH (10 mL). The solution was cooled to ꢁ15 C in an
EtOH–dry ice bath and propionyl chloride (1.30 mL, 1.36 g, 14.7
mmol) was added dropwise with vigorous stirring over a period
of 5 min. The mixture was left to stir at ꢁ15 ꢀC for an additional
15 min and then quenched by the dropwise addition of aqueous 6
M HCl to pH ꢂ5 (caution – foaming). The solid was then
collected by filtration, washed with water and dried under
General experimental measurements
1H (400 MHz/200 MHz) and 13C (100 MHz/50 MHz) NMR
spectra were obtained with a Bruker AV400 or a Bruker AC200
spectrometer at 25 ꢀC. Spectra were recorded for samples dis-
solved in deuterated solvent and chemical shifts are reported in
parts per million from external tetramethylsilane and J values are
given in Hz. Positive ion EI mass spectra were run on a Ther-
moQuest MAT95XL mass spectrometer using ionization energy
of 70 eV. Accurate mass measurements were obtained with a high
resolution of 5000–10 000 using PerFluoroKerosene (PFK) as
the reference sample. Positive and negative ion electrospray mass
spectra (ESI-MS) were acquired with a VG Platform mass
Fig. 1 Structures of monomers of thermally curable host matrix
formulation.
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vacuum overnight at 40 ꢀC to give 1-hydroxy-4-(propionyloxy)-
2-naphthoic acid (2.20 g, 75% pure by 1H NMR). The solid was
dissolved in dry acetone (180 mL) and then treated with finely
ground anhydrous K2CO3 (7.20 g, 52.1 mmol). The mixture was
re-extracted back into diethyl ether (3 ꢃ 100 mL). The organic
layers were combined and washed with saturated brine, dried
with MgSO4 and the solvent removed under reduced pressure to
give a thick amber oil (3a, 2.9 g, 75%) in sufficient purity for
subsequent steps. Friedel–Crafts acylation of the crude half ester
mixture (3a), using procedures outlined in the literature,29,36,37,39
followed by saponification yielded 4-hydroxy-6-methoxy-2-
naphthoic acid (5) as a pale pink powder (35% overall yield
ꢀ
heated to reflux for 30 min and then cooled to 0 C with an ice
bath. Iodomethane (4.5 mL, 10.3 g, 72.3 mmol) was then added
dropwise over 5 min via syringe and the resulting mixture was left
to stir at room temperature for 16 h. Removal of the solvent and
excess reagents under vacuum gave a brown paste that was
re-dissolved in diethyl ether and filtered through a plug of silica
gel to remove baseline material. The solvent was then removed
under vacuum and the brown gum was collected to afford methyl
1-methoxy-4-(propionyloxy)-2-naphthoate as a crude product.
Finely ground K2CO3 (1.64 g, 11.9 mmol) was added to
a suspension of the crude material (2.74 g) in methanol (15 mL)
and the resulting suspension was stirred at room temperature for
3 h. The mixture was then diluted with water (100 mL), acidified
by the addition of aqueous 2 M HCl and extracted into diethyl
ether (3 ꢃ 75 mL). The combined organic extracts were washed
with 10% NaHCO3 (3 ꢃ 75 mL) and water (1 ꢃ 100 mL). The
product was then extracted from the organic layer into an
aqueous basic layer using 0.1 M NaOH (5 ꢃ 50 mL). The
combined basic aqueous layers were then acidified with aqueous
2 M HCl to pH ꢂ5 and extracted with diethyl ether (3 ꢃ 75 mL).
The combined organic layers were then washed with saturated
brine and dried with MgSO4. Solvent removal yielded the pure
final product, methyl 4-hydroxy-1-methoxy-2-naphthoate (2b),
1
for the three steps). H NMR (400 MHz, d6-acetone) d: 3.95 (s,
3H, ArOCH3), 7.22 (dd, J 8.7, 1.5 Hz, 1H, ArH), 7.49 (br d, J
1.5 Hz, 1H, ArH), 7.58 (d, J 1.5 Hz, 1H, ArH), 7.92 (d, J 8.7 Hz,
1H, ArH), 8.12 (s, 1H, ArH), 9.21 (s, 1H, ArOH) ppm. 13C NMR
(100 MHz, d6-acetone) d: 55.1, 100.7, 108.0, 119.8, 122.6, 126.1,
128.8, 129.5, 131.0, 152.4, 159.4, 167.3 ppm. ESI-MS m/z
217.2 [M ꢁ H]ꢁ.
3-Hydroxypropyl 4-hydroxy-6-methoxy-2-naphthoate (6b). The
title compound was synthesized using an alkylation procedure
adapted from that of Hattori et al.28 4-Hydroxy-6-methoxy-2-
naphthoic acid (5) (4.82 g, 22.1 mmol) and anhydrous sodium
bicarbonate (1.86 g, 22.1 mmol) were added to DMF (20 mL)
ꢀ
and stirred at 100 C for 30 min under argon. The solution was
cooled with an ice bath and sodium iodide (6.63 g, 44.2 mmol)
was added, followed by the dropwise addition of 3-chlor-
opropanol (4.20 g, 44.2 mmol, 3.80 mL). The mixture was stirred
for an additional 3 h at 100 ꢀC and then poured into dilute
aqueous HCl and extracted into diethyl ether (3 ꢃ 75 mL).
The combined organic extracts were washed with 10% NaHCO3
(3 ꢃ 75 mL) and water (1 ꢃ 100 mL). The product was extracted
from the organic layer with 0.1 M NaOH (5 ꢃ 50 mL). The
combined basic aqueous layers were then acidified with aqueous
2 M HCl, extracted with diethyl ether (3 ꢃ 75 mL), washed with
saturated brine and dried (MgSO4). Solvent removal under
vacuum yielded the pure final product, 3-hydroxypropyl 4-
hydroxy-6-methoxy-2-naphthoate (6b), as an off-white solid
1
as a tan solid (1.70 g, 75%). H NMR (400 MHz, d6-acetone) d:
3.90 (s, 3H, ArOCH3), 3.96 (s, 3H, COOCH3), 7.22 (s, 1H, ArH),
7.59–7.64 (m, 2H, ArH), 8.17–8.21 (m, 1H, ArH), 8.22–8.27 (m,
1H, ArH), 9.07 (s, 1H, ArOH) ppm. 13C NMR (100 MHz, d6-
acetone) d: 52.4, 63.5, 108.3, 120.6, 123.5, 124.2, 127.9, 128.2,
129.0, 130.3, 149.9, 151.5, 167.1 ppm. Mass spec (EI): m/z 232.1
([M]+, 100%), 217.1 (68), 201.1 (22), 170.0 (20), 161.1 (27), 142.1
(17), 131.0 (18), 115.1 (24), 102 (24), 83.9 (89), 69.1 (54). Mass
spec (HR, EI): m/z 232.0729 (C13H12O4 requires 232.0736).
1
(3.95 g, 65%). H NMR (400 MHz, d6-acetone) d: 1.98 (p, J 6.2
Hz, 2H, CH2CH2CH2), 3.68 (br s, OH), 3.75 (t, J 6.2 Hz, 2H,
CH2OH), 3.95 (s, 3H, ArOCH3), 4.42 (t, J 6.2 Hz, 2H,
CH2CH2CH2) 7.22 (dd, J 8.7, 2.5 Hz, 1H, ArH), 7.47 (br d, J 1.5
Hz, 1H, ArH), 7.57 (d, J 1.5 Hz, 1H, ArH), 7.92 (d, J 8.7 Hz, 1H,
ArH), 8.09 (s, 1H, ArH), 9.19 (br s, 1H, ArOH) ppm. 13C NMR
(100 MHz, d6-acetone) d: 32.0, 54.8, 58.1, 61.7, 100.4, 107.4,
119.6, 122.0, 125.8, 128.5, 129.2, 130.7, 152.2, 159.1, 166.2 ppm.
Mass spec (EI): m/z 276.1 ([M]+, 100%), 218.0 (69), 201.0 (58),
175.0 (41), 145.1 (13), 130.0 (12), 115.1 (10), 102 (22). Mass spec
(HR, EI): m/z 276.0988 (C15H16O5 requires 276.0988).
4-Hydroxy-6-methoxy-2-naphthoic acid (5). The Stobbe
condensation of 4-methoxybenzaldehyde with diethyl succinate
was affected either by t-BuOK (2.2 equivalents, toluene, room
temperature, 16 h)36 or by sodium ethylate (Na in dry ethanol
(NaOEt), 2 equivalents, reflux 3 h).37 Both methods resulted in
a considerable proportion (ꢂ35%) of by-product, 2,3-bis(4-
methoxybenzylidene)succinic acid (3b), isolated as a pale yellow
crystalline material, as well as the desired half ester product, 3a.29
1
The H NMR for 3b showed a mixture of geometrical isomers
1
(both E/E and Z/Z, inconsistent proportions): H NMR (400
MHz, (CD3)2SO) d: 3.73 and 3.79 (2 ꢃ s, 6H, ArOCH3), 6.89 (d,
J 8.8 Hz, 2H, ArH), 7.01 (d, J 8.8 Hz, 2H, ArH), 7.38 (d, J 8.4 Hz,
2H, ArH), 7.50 (d, J 8.4 Hz, 2H, ArH), 7.68 and 7.72 (2 ꢃ s, 2H,
olefinic H) ppm. A second alternative literature procedure38 was
also used to prepare the half ester 3a which resulted in no by-
product: to a mixture of 4-methoxybenzaldehyde (2.00 g, 14.7
mmol) and diethyl succinate (2.56 g, 14.7 mmol) was added
powdered t-BuOK (2.17 g, 17.7 mmol). The mixture was ground
with a mortar and pestle for 10 min and then left to sit at room
temperature for 3 h. The thick paste was then dissolved in water
and the resulting basic solution washed with diethyl ether (3 ꢃ
100 mL). The basic solution was neutralized with 2 M HCl and
2-Hydroxyethyl 4-hydroxy-1-phenyl-2-naphthoate (8). The
synthesis of the title compound was also performed using the
alkylation procedure of Hattori et al.28 4-Hydroxy-1-phenyl-2-
naphthoic acid (7) (2.00 g, 7.57 mmol) and NaHCO3 (0.636 g,
7.57 mmol) were added to DMF ꢀ(20 mL) and stirred at 60 ꢀC for
2 h under nitrogen, then at 100 C for 15 min. Freshly distilled
and acid-free 2-bromoethanol (1.14 g, 9.08 mmol, 0.64 mL) was
thenꢀadded and the mixture stirred for an additional 1.25 h at
100 C. The mixture was poured into dilute aqueous HCl and
extracted with diethyl ether. The ether layer was washed several
times with water, then with saturated brine and dried with
MgSO4. The solvent was removed under vacuum to give the pure
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product as an off-white solid (2.16 g, 93%). 1H NMR (200 MHz,
d6-acetone) d: 3.43 (t, J 5.2 Hz, 2H, CH2OH), 4.00 (t, J 5.2 Hz,
2H, OCOCH2CH2), 7.26–7.31 (m, 2H, ArH), 7.35 (s, 1H, ArH),
7.40–7.62 (m, 6H, ArH), 8.35 (dd, J 8.3, 1.1 Hz, 1H, ArH) ppm.
13C NMR (50 MHz, d6-acetone) d: 60.3, 67.0, 108.1, 123.0, 127.1,
127.2, 127.8, 127.9, 128.0, 128.7, 129.9, 131.1, 133.1, 134.6, 140.4,
153.3, 168.7 ppm. ESI-MS m/z 307.4 [M ꢁ H]ꢁ.
Preparation of PDMS-conjugated naphthopyrans
COOH-terminated poly(dimethylsiloxane), n ¼ 12.6. Hydroxy
end-terminated poly(dimethylsiloxane) (25 g, ca. 0.0221 mol) and
succinic anhydride (2.65 g, 0.0265 mol) were added to dry
CH2Cl2 (ca. 40 mL) under nitrogen. Triethylamine (3.35 g, 4.6
mL, 0.033 mol) was then added in one portion, the mixture
stirred at room temperature for 30 min followed by heating at 35
ꢀC for 1 h. Polyethylene glycol methyl ether (3.86 g, 0.011 mol)
was added and the mixture stirred for an additional 30 min at 35
ꢀC. Hexane (ca. 40 mL) was then added, the mixture washed with
several portions of 1 M HCl and then dried with MgSO4. The
solvent was removed under vacuum giving the pure product as
Typical synthesis of hydroxyl naphthopyran
The synthesis of methyl 6-hydroxy-2,2-bis(4-methoxyphenyl)-
2H-naphtho[1,2-b]pyran-5-carboxylate (16)13 from methyl
1,4-dihydroxy-2-naphthoate
(2a)
and
1,1-bis(4-methox-
yphenyl)prop-2-yn-1-ol (14) has been described by us elsewhere24
using a procedure adapted from the literature.40 The product was
isolated by crystallization from diethyl ether as a bright yellow
1
solid (0.97 g, 45%). H NMR (400 MHz, d6-acetone) d 3.73 (s,
6H, 2 ꢃ ArOCH3), 4.05 (s, 3H, COOCH3), 6.32 (d, J 10.0 Hz,
1H, pyran-CH), 6.86 (d, J 8.8 Hz, 4H, Ar-H), 7.45 (d, J 8.8 Hz,
4H, ArH), 7.48 (d, J 10.0 Hz, 1H, pyran-CH), 7.59 (m, J 8.4 and
1.5 Hz, 1H, Ar-H), 7.75 (m, J 8.4 and 1.5 Hz, 1H, ArH), 8.31 (d, J
8.4 Hz, 1H, ArH), 8.39 (d, J 8.4 Hz, 1H, ArH), 12.18 (s, 1H,
ArOH) ppm. 13C NMR (100 MHz, d6-acetone) d: 53.8, 56.2, 82.6,
104.0, 115.0, 115.2, 123.5, 125.0, 125.6, 126.7, 128.4, 129.5, 129.9,
130.3, 131.5, 138.7, 142.7, 157.8, 160.7, 173.7 ppm. Mass spec
(EI): m/z 468.2 ([M]+, 92%), 436.2 (100), 408.2 (38), 391.2 (18),
377.1 (13), 330.1 (39), 218.1 (16). Mass spec (HR, EI): m/z
468.1564 (C29H24O6 requires 468.1573).
1
a colorless oil (26.34 g, 97%). H NMR (400 MHz, CDCl3) d:
0.04–0.08 (m, SiCH3), 0.50–0.55 (m, 4H, CH2-4,5), 0.88 (t, J 7.0
Hz, 3H, CH3-1), 1.26–1.36 (m, 4H, CH2-2,3), 1.57–1.65 (m, 2H,
CH2-6), 2.68 (br s, 4H, CH2-10,11), 3.42 (t, J 7.3 Hz, 2H, CH2-7),
3.63 (t, J 4.8 Hz, 2H, CH2-8), 4.25 (t, J 4.8 Hz, 2H, CH2-9) ppm.
Refer to graphic above for corresponding numbering system
used for CH2 NMR assignments.
Succinoyl chloride terminated poly(dimethylsiloxane). Carbox-
ylic acid terminated poly(dimethylsiloxane) (1.0 g, ca. 0.811
mmol) was dissolved in dry dichloromethane (10 mL) under
nitrogen and 1 small drop of DMF was added. To the mixture
was added oxalyl chloride (0.41 g, 0.28 mL, 3.24 mmol) in one
portion. The mixture was stirred at ambient temperature for no
more than 30 min whilst maintaining a slow nitrogen flow above
the reaction by means of a syringe needle through a rubber
septum. The solvent and excess reagents were removed under
vacuum with residual traces of oxalyl chloride removed with the
aid of 1,2-dichloroethane. The acid chloride product was used
immediately. Analysis by 1H NMR in d-chloroform showed
Typical synthesis of naphthopyran control
Methyl 2,2-bis(4-methoxyphenyl)-6-(propionyloxy)-2H-naph-
tho[1,2-b]pyran-5-carboxylate (16a). To a solution of methyl
6-hydroxy-2,2-bis(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran-
5-carboxylate (16) (0.20 g, 0.427 mmol) in dry dichloromethane
(10 mL) was added triethylamine (0.178 mL, 0.13 g, 1.28 mmol),
under nitrogen. Propionyl chloride (0.04 g, 0.427 mmol, 0.037
mL) was added dropwise via a syringe and the mixture stirred at
ambient temperature until TLC analysis showed the reaction to
be complete (ca. ꢂ15 min). The solvent and excess reagents were
evaporated under vacuum and the residue re-dissolved in diethyl
ether (20 mL) and washed successively with 0.1 M HCl, water,
0.1 M NaHCO3, water and brine. The organic layer was dried
with MgSO4 and the solvent removed under vacuum. The crude
glassy residue was then purified by column chromatography
(silica gel, diethyl ether–hexane, 2 : 1), giving the pure product as
a pale pink solid (0.180 g, 79%). 1H NMR (400 MHz, d6-acetone)
d: 1.26 (t, J 7.5 Hz, 3H, propionyl-CH3), 2.76 (q, J 7.5 Hz, 2H,
propionyl-CH2), 3.76 (s, 6H, 2 ꢃ ArOCH3), 3.93 (s, 3H,
COOCH3), 6.41 (d, J 10.0 Hz, 1H, pyran-CH), 6.89 (d, J 8.9 Hz,
4H, ArH), 6.96 (d, J 10.0 Hz, 1H, pyran-CH), 7.45 (d, J 8.9 Hz,
4H, ArH), 7.59 (m, 1H, ArH), 7.67 (m, 1H, ArH), 7.85 (d, J 8.4
Hz, 1H, ArH), 8.41 (d, J 8.4 Hz, 1H, ArH) ppm. 13C NMR
(50 MHz, d6-acetone) d: 9.4, 27.7, 52.8, 55.5, 83.5, 114.3, 114.4,
120.8, 121.6, 123.0, 123.5, 127.1, 128.3, 128.6, 128.8, 128.9, 130.2,
137.7, 140.3, 146.7, 160.1, 166.3, 173.1 ppm. Mass spec (EI): m/z
524.2 ([M]+, 36%), 568.2 (54), 436.2 (100), 407.1 (28), 391.1 (11),
372.1 (39). Mass spec (HR, EI): m/z 524.1835 (C32H28O7 requires
524.1819).
1
quantitative conversion. H NMR (400 MHz, CDCl3) d: 0.04–
0.08 (m, av. 80H, SiCH3), 0.50–0.55 (m, 4H, CH2-4,5), 0.88 (t, J
7.0 Hz, 3H, CH3-1), 1.26–1.36 (m, 4H, CH2-2,3), 1.57–1.65 (m,
2H, CH2-6), 2.72 (t, J 6.6 Hz, 2H, CH2-10), 3.22 (t, J 6.6 Hz, 2H,
CH2-11), 3.42 (t, J 7.0 Hz, 2H, CH2-7), 3.63 (t, J 4.8 Hz, 2H,
CH2-8), 4.26 (t, J 4.8 Hz, 2H, CH2-9) ppm. Refer to graphic
above (where n ¼ 12.6) for corresponding numbering system
used for CH2 NMR assignments.
Typical synthesis of PDMS–naphthopyran conjugates
Methyl
2,2-bis(4-methoxyphenyl)-6-(butyl(PDMS)propy-
loxyethoxy-succinoyloxy)-2H-naphtho[1,2-b]pyran-5-carboxylate
(16b). Methyl 6-hydroxy-2,2-bis(4-methoxyphenyl)-2H-naph-
tho[1,2-b]pyran-5-carboxylate (16) (0.25 g, 0.534 mmol) was
dissolved in dry dichloromethane (10 mL) followed by the
addition of triethylamine (119 mg, 0.16 mL, 1.17 mmol), under
an atmosphere of argon. Succinoyl chloride end-functionalised
PDMS, synthesized as described above, was then added dropwise
(0.485 mmol) and the mixture was left to stir at room
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temperature for 1 h. The solvent was then removed under
vacuum, the residue re-dissolved in diethyl ether–hexane (1 : 1)
and the mixture filtered through a plug of silica gel. The solvent
was removed and the remaining oily residue was purified by
column chromatography (silica gel, CH2Cl2–petroleum ether, 4 :
1) to give the pure PDMS conjugate as a viscous red oil (759 mg,
80%). 1H NMR (400 MHz, d6-acetone) d: 0.07–0.13 (m, SiCH3),
0.57 (m, 4H, CH2-4,5), 0.89 (br t, J 7.0 Hz, 3H, CH3-1), 1.35 (m,
4H, CH2-2,3), 1.59 (m, 2H, CH2-6), 2.78 (m, 2H, CH2-10 overlap
with H2O and HDO signals), 3.04 (t, J 6.5 Hz, 2H, CH2-11), 3.41
(t, J 6.8 Hz, 2H, CH2-7), 3.62 (t, J 5.0 Hz, 2H, CH2-8), 3.75 (s,
6H, 2 ꢃ ArOCH3), 3.95 (s, 3H, COOCH3), 4.23 (t, J 5.0 Hz,
2H, CH2-9), 6.40 (d, J 10 Hz, 1H, pyran-CH), 6.88 (d, J 8.8 Hz,
4H, ArH), 6.96 (d, J 10 Hz, 1H, pyran-CH), 7.44 (d, J 8.8 Hz, 4H,
ArH), 7.59 (t, J 7.5 Hz, 1H, ArH), 7.66 (t, J 7.5 Hz, 1H, ArH),
7.94 (d, J 8.5 Hz, 1H, ArH), 8.40 (d, J 8.5 Hz, 1H, ArH) ppm.
Refer to Fig. 4 for corresponding numbering system used for
CH2 NMR assignments.
catalyst (e.g. p-toluene sulfonic acid or refluxing in acidic
alumina) or with the addition of a dehydrating agent (trimethyl
orthoformate) to react with the generated water.40,41
It has been shown previously that the substitution patterns can
dramatically influence photogenerated color and switching speed
by effecting the stability of the generated ring-opened forms, as
investigated by Gabbutt et al.18,29 Alkoxycarbonyl substitution at
position 5 assists fade rate; 9-methoxy substitution results in
a faster fade rate compared to 6-methoxy substitution due to
large disparities in the stability of their open forms; bis vs. mono
p-methoxy substitution on the diaryl portion of the naph-
thopyran increases the fade rate and bathochromically shifts the
absorption of the colored form.18,42,43 An extra 6-phenyl moiety
substituted on the already bulky naphthol ring also poses further
steric considerations. Finally, an indeno moiety joining positions
5 and 6 is another significant structural modification of
the 2,2-diaryl-2H-naphtho[1,2-b]pyrans.16,17,30 Along with
p-methoxy substitution on the diaryl rings, such naphthopyrans
produce open forms containing an extended p-electron system
that are significantly shifted to higher wavelengths and capable
of displaying faster bleaching kinetics.
Results and discussion
The naphthopyran dyes exploited in this study were chosen to
display a broad range of switching speeds both in solution and in
a host matrix by variation of these known substitution patterns.
The preliminary requirement of this work was therefore the
synthesis of such naphthopyrans incorporating a free hydroxyl
group for subsequent attachment of PDMS oligomers. Overall,
the conditions for final assembly of the dyes were optimized in
order to achieve synthesis yields comparable to those found in
the literature for equivalent non-hydroxyl functionalized dyes
(30–70%).
Naphthopyrans
Substituted 2,2-diaryl-2H-naphtho[1,2-b]pyrans (NP) are
synthesized by reacting a 1-naphthol (N) with a diaryl prop-2-yn-
1-ol (P) in the presence of an acidic catalyst (Scheme 2). The
formation of the naphthopyran is confirmed by the appearance
of characteristic resonances in the 1H NMR spectrum between 6
and 8 ppm with coupling constants of J z 10 Hz due to pyran
ring protons H-3 and H-4. The mechanisms involved in forma-
tion are well known, involving the Claisen rearrangement of the
generated naphthyl propargyl ether.35 The reaction can be
carried out in a solvent (e.g. toluene) or in solid state (grinding
reagents in silica gel), with varying degrees of acid strength of
Hydroxyl functionalized naphthopyrans 16–21, listed in Table
1, were assembled from relevant starting materials and then
converted to their corresponding ester derivatives which
are displayed in Fig. 3. The naphthopyrans (NP) each contain
5-alkoxycarbonyl substitution, shown in the base structure in
Scheme 2. Hydroxyl indeno-fused naphthopyran, 22, shown in
Fig. 2, was also synthesized accordingly and converted to its
esterified derivatives also displayed in Fig. 3.
Starting materials
Naphthols. Methyl 1,4-dihydroxy-2-naphthoate, 2a (Scheme
3), was prepared from commercially available 1,4-dihydroxy-2-
naphthoic acid, 1.28 Carboxyl substitution present in 2a precludes
the formation of possible di-naphthopyran species,12 affording
naphthopyrans substituted with a free 6-hydroxy group. Such
by-products can be produced in significant proportion from
dihydroxy naphthalenes due to the presence of two reactive
centers.
The 6-hydroxy moiety present in, for example, naphthopyrans
16 and 17 can then be methylated as a way to access a 6-methoxy
substitution. However, this direct conversion also affords
a major ring-opened and non-photochromic by-product, as
reported by Gabbut et al.29 We therefore explored an alternative
strategy via the assembly of the appropriate starting material,
methyl 4-hydroxy-1-methoxy-2-naphthoate (2b), using the series
of transformations as shown in Scheme 3. Reaction of 1, in cold
and basic water–isopropanol solution, with one and a half
Scheme 2 Synthesis of naphthopyran from substituted starting mate-
rials.
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Table 1 Substituted hydroxyl naphthopyrans (NP) synthesized from corresponding reactants, naphthol (N) and propynol (P)
Reactants
N
Naphthopyran substituentsa
Naphthopyrans
NP
P
R1
R2
R3
R4
16
17
18
19
20
21
2a
2a
2b
6b
6b
8
14
15
13
14
15
14
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
H
–OCH2CH2CH2OH
OCH3
H
OCH3
CH3
CH3
CH3
6-OH
6-OH
6-OCH3
9-OCH3
9-OCH3
6-Ph
–CH2CH2CH2OH
–CH2CH2CH2OH
–CH2CH2OH
a
Substituents R1 to R4 refer to substituted naphthopyrans (NP) in Scheme 2.
Condensation of a large excess of a diol, such as ethylene
glycol, with the generated substituted naphthoic acids (5 and 7)
was a logical approach to introduce hydroxyl functionality to
naphthol derivatives.45 Esterification with methanol using
Fischer esterification conditions (high excesses of methanol and
catalytic quantities of acid) was straightforward, quantitatively
generating the methyl ester derivative, 6a (Scheme 5). However,
numerous attempts to condense ethylene glycol to 5 in an
equivalent manner proved futile, with numerous etherified and
inseparable by-products formed during the reaction. Lower
temperatures only resulted in low conversion to products. A
much more successful strategy was found to be the nucleophilic
displacement of a halogen by the selectively deprotonated
naphthoic acid. With one equivalent of NaHCO3 and dry DMF,
two equivalents of 3-chloropropanol and sodium iodide, 6b
could be synthesized in 65% yield and with exceptional purity
(Scheme 5). The use of 2-bromoethanol proved even more
successful giving yields in excess of 90% for 8 (Scheme 6).
Friedel–Crafts cyclization31 of 7, followed by the reduction of
the carbonyl group32 of the resultant fluorenone derivative, 9,
gave the hydroxyl substituted benzofluorenol 10, as shown in
Scheme 6.
Fig. 2 Substituted hydroxyl indeno-fused naphthopyran 22 synthesized
from propynol 14 and naphthol 10.
equivalents of propionyl chloride selectively acylated the phenol,
affording 1-hydroxy-4-(propionyloxy)-2-naphthoic acid in 75%
crude yield (with 25% of starting material 1 remaining present).
The crude mixture was alkylated with an excess of methyl iodide
and K2CO3 in dry acetone to give methyl 1-methoxy-4-(propio-
nyloxy)-2-naphthoate in the same proportions, followed by the
removal of the propionyl group using an alcoholic solution of
K2CO3. Importantly, 2b could then be isolated from the crude
organic mixture by selective extraction using 0.1 M NaOH, with
an overall isolatable yield of 70% from 1.
4-Hydroxy-6-methoxy-2-naphthoic acid 5 was required for the
synthesis of 6a and 6b. As shown in Scheme 4, the strategy used
to assemble 5 firstly involved the Stobbe condensation of 4-
methoxybenzaldehyde with diethyl succinate (using NaOEt–t-
BuOK)29,36,37,44 to produce the required half ester 3a. The crude
yield of this reaction was consistently 70% and comparable to
literature values. A pale yellow precipitate, however, formed on
standing from the amber viscous oil (ꢂ13% by mass) and was
found to be an isomeric mixture (both E/E and Z/Z isomers) of
3b, forming as a result of a second molecule of benzaldehyde
condensing with the generated half ester, 3a. The use of anhy-
drous ethanol gave a cleaner product29 with no by-product
Propynols. Diaryl prop-2-yn-1-ols can be conveniently
prepared by the addition of lithium trimethylsilylacetylide to
a benzophenone following the removal of the trimethylsilyl
group.35,46 In this work 13 and 14 were easily accessed from
commercially available benzophenones. Hydroxyl substitution
can also be introduced to the naphthopyran via the diaryl prop-2-
yn-1-ol,33,34 as exemplified with naphthopyran 18. Williamson
etherification of 11 with 3-chloropropanol33 generated 12 which
was then converted to diaryl propynol 13 (Scheme 7). However,
attempts to generate a propynol directly from the phenolic
benzophenone 11 were unsuccessful. Meyer–Schuster rear-
rangement of the propynol46,47 resulted in its degradation and the
material formed an intractable gum on standing. The generation
of propynol 13 from 12 was therefore preferable for our inves-
tigations.
1
evident by H NMR, but we found yields to be disappointing
(35%). Removal of the by-product could be easily carried out via
filtration, being insoluble in most solvents, however, the use of
solvent-free conditions38 provided us with the half ester in higher
yields and exclusive of 3b. Friedel–Crafts acylation of the
resulting crude half ester, 3a, gave the cyclized product 4, fol-
lowed by base hydrolysis to afford 5.
Conjugated naphthopyrans
The condensation of naphthopyrans 16–22 to their correspond-
ing ester derivatives was carried out using standard acid chloride
chemistry, as shown in Scheme 8.
1-Phenyl substituted naphthoic acid, 7, was also synthesized
from benzophenone using Stobbe condensation chemistry
though no analogous by-product was ever formed during its
synthesis.
Carboxylic acid terminated PDMS was prepared from
commercially available OH terminated PDMS by acylation with
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Fig. 3 Molecular glossary of substituted naphthopyrans tested.
1
succinic anhydride. The oligomeric product was shown by H
NMR to have an average molecular weight of approx. 1200 g
molꢁ1 where n ¼ ꢂ13. Esterification reactions generated both
PDMS conjugates (16b–22b) and propionate control naph-
thopyrans (16a–22a) in high yields, with products easily purified
by chromatography. The final substituted naphthopyrans (both
propionate controls and PDMS conjugates) and control dyes
18c, 19c and 20c are displayed in a glossary as Fig. 3.
Each PDMS conjugate produced displayed a high level of
purity; every methylene proton was accounted for, correct integral
ratios and expected peak shifts associated with ester bond
formation were exhibited (refer to ESI† for all 1H NMR
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Scheme 3 (i) MeI, DMF, NaHCO3 (1 equiv.), 100 ꢀC; (ii) K2CO3 (4
equiv.), H2O–i-PrOH (5 : 1), propionyl chloride (1.5 equiv.), ꢁ15 ꢀC,
neutralization; (iii) K2CO3 (4 equiv.), dry acetone, MeI (8 equiv.); (iv)
K2CO3 (1.5 equiv.), MeOH.
Scheme 6 (i) Stobbe condensation with dimethyl succinate, Friedel–
Crafts acylation, base hydrolysis; (ii) 2-bromoethanol (2 equiv.), dry
DMF, NaHCO3 (1 equiv.), 100 ꢀC; (iii) methanesulfonic acid, 60 ꢀC; (iv)
NaBH4, wet THF, 50 ꢀC.
Scheme 7 (i) 3-Chloropropanol (2 equiv.), NaI (2 equiv.), DMF, K2CO3
(2 equiv.), 100 ꢀC; (ii) lithium trimethylsilylacetylide, anhyd. THF, 0 ꢀ
C
Scheme 4 (i) Diethyl succinate, Na ethylate (2 equiv.), reflux; (ii)
anhydrous NaOAc (1.1 equiv.), acetic anhydride; (iii) 5% NaOH in
EtOH–H2O (4 : 1).
to rt; (iii) KOH, MeOH, then AcOH.
Scheme 8 Condensation chemistry for synthesizing propionate naph-
thopyran controls (16a–22a) and PDMS–naphthopyran conjugates (16b–
22b); (i) succinic anhydride, TEA, DCM, rt; (ii) (COCl)2, DCM, DMF (1
drop), rt; (iii) DCM, TEA, rt. See experimental in ESI† for n values
(normally n ¼ ꢂ13).
Scheme 5 (i) MeOH, H2SO4 (catalytic); (ii) 3-chloropropanol (2 equiv.),
NaI (2 equiv.), dry DMF, NaHCO3 (1 equiv.), 100 ꢀC.
assignments and final n values for PDMS-conjugated tail (nor-
1
sample, as a result of undesirable phase separation of PDMS–dye
conjugates, was not observed throughout our sample preparation.
This indicated an appropriate level of miscibility of the PDMS
with the lens host with the concentration levels used (Table 2).
mally n ¼ ꢂ13)). An example H NMR spectrum for PDMS
conjugate 19b with final peak assignments is shown below (Fig. 4).
The final photochromic test samples were prepared by dis-
solving the photochromic conjugates and controls individually in
a lens monomer formulation. Previous 2D NMR investigations
have confirmed intramolecular and localized associations
between PDMS oligomer chains and covalently bound spiroox-
azine photochromic dyes in solution.27 Within the environment of
a rigid matrix stronger interactions are envisaged due to the
reduced miscibility and chemical compatibility of the PDMS with
the host matrix. The partitioning of oligomer tails around bound
dye moieties is believed to provide an overall insulation and
encapsulation effect. Visibly obvious hazing in the cured lens
Photochromic properties
When irradiated with UV light an initially colorless photo-
chromic test sample becomes highly colored. Upon cessation of
the incident UV light, the thermal back reaction occurs as the
naphthopyran open form undergoes ring closure and samples
decolorize spontaneously in the dark. Spectrokinetic properties
are studied by monitoring absorption density with time at the
lmax of the colored form. Firstly the sample is continuously
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Fig. 4 1H NMR spectrum (d6-acetone) of PDMS conjugate 19b with peak assignments. *HDO, H2O (2.84 ppm) and acetone (2.05 ppm).
Table 2 Photokinetic analysis of the decoloration of control and PDMS-conjugated naphthopyrans in host lens matrix (PEGMA–EBPDMA 1 : 4 mass
composition) vs. control naphthopyrans in toluenea
T1/2/sc
Host (conc.)d
lmaxa/nm
b
A0
A1
k1/minꢁ1
A2
k2/minꢁ1
Ath
16a
16b
16a
17a
17b
17a
18c
18a
18b
18c
19c
19a
19b
19c
20c
20a
20b
20c
21a
21b
21a
22a
22b
22a
1.22
1.35
1.85
0.91
1.10
1.42
1.24
1.15
1.23
0.84
1.45
1.12
1.24
0.49
1.98
1.79
1.09
0.87
0.62
0.83
1.19
0.53
0.73
1.01
0.69
0.80
0.82
0.64
0.72
0.69
0.60
0.64
0.70
0.70
0.68
0.71
0.88
0.98
0.70
0.73
0.85
0.89
0.51
0.62
0.94
0.43
0.71
0.82
1.209
1.837
3.691
0.638
1.034
1.036
0.401
0.463
0.567
0.724
3.197
4.038
8.852
12.622
2.186
2.878
4.928
5.479
0.853
1.228
5.247
0.390
0.776
1.052
0.13
0.06
0.20
0.10
0.04
0.24
0.13
0.10
0.06
0.18
0.12
0.11
0.03
0.08
0.08
0.08
0.03
0.13
0.20
0.15
0.13
0.31
0.16
0.15
0.077
0.096
0.039
0.072
0.103
0.005
0.072
0.077
0.110
0.025
0.072
0.077
0.040
0.782
0.072
0.075
0.024
0.041
0.064
0.082
0.662
0.047
0.083
0.138
0.16
0.14
0.04
0.24
0.23
0
0.26
0.24
0.24
0.05
0.17
0.16
0.14
0
0.19
0.18
0.16
0.01
0.24
0.20
0.01
0.21
0.10
0.001
55
31
19
120
63
52
212
167
123
74
20
15
6
4
28
26
11
10
129
60
11
355
76
45
Lens (6 ꢃ 10ꢁ7
Lens (6 ꢃ 10ꢁ7
)
)
510
510
499
500
500
485
511
511
511
501
519
519
519
508
507
507
507
497
515
515
507
560
560
553
Toluene (9.5 ꢃ 10ꢁ3
)
)
Lens (1.5 ꢃ 10ꢁ7
)
)
Lens (1.5 ꢃ 10ꢁ7
Toluene (5.0 ꢃ 10ꢁ5
Lens (1.5 ꢃ 10ꢁ7
Lens (1.5 ꢃ 10ꢁ7
Lens (1.5 ꢃ 10ꢁ7
)
)
)
Toluene (3.0 ꢃ 10ꢁ5
)
)
Lens (1.2 ꢃ 10ꢁ6
Lens (1.2 ꢃ 10ꢁ6
Lens (1.2 ꢃ 10ꢁ6
)
)
)
Toluene (2.0 ꢃ 10ꢁ4
Lens (1.2 ꢃ 10ꢁ6
Lens (1.2 ꢃ 10ꢁ6
Lens (1.2 ꢃ 10ꢁ6
)
)
)
Toluene (6.4 ꢃ 10ꢁ5
)
)
)
Lens (1.5 ꢃ 10ꢁ7
)
)
Lens (1.5 ꢃ 10ꢁ7
Toluene (6.5 ꢃ 10ꢁ5
Lens (1.5 ꢃ 10ꢁ7
Lens (1.5 ꢃ 10ꢁ7
)
)
Toluene (4.7 ꢃ 10ꢁ5
a
Samples initially irradiated at 350–400 nm for 1000 s, then thermal decoloration monitored at lmax of the colored form (determined by wavelength scan
of colored formd) at 20 ꢀC in the dark for 4800 s. b Measured absorbance intensity at onset of thermal decoloration period. c Time taken for the initial
absorbance value A0 to decay to half. d Concentration in mole (naphthopyran) per gram matrix formulation (for lens); M (in toluene).
irradiated for 1000 s and then the decoloration kinetics of the
naphthopyrans in solution and in the host matrix is investigated
in the dark at 20 ꢀC upon cessation of UV irradiation. The
following empirical biexponential equation48,49 was used to
analyze the thermal ring closure kinetics within the host matrix:
where A(t) is the optical density at lmax of the open form; A1
and A2 are the contributions to the initial optical density A0; k1
and k2 are exponential decay rate constants of fast and slow
components, respectively, and Ath is the residual coloration
(offset).
The other standard photochromic parameters presented were
the lmax of the photocolored form and colorability, A0, which is
AðtÞ ¼ A1eꢁk t þ A2eꢁk t þ Ath
(1)
1
2
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the absorbance level achieved after 1000 s of continuous irradi-
ation. This equation has been used frequently to represent and
compare the decoloration behavior of both spirooxazines and
naphthopyrans within solid media33,48,50,51 and has consistently
fitted our decoloration curves with correlation coefficients (R)
greater than 0.99.23,24,52,53 An evaluation of T1/2 value, which is
the time taken for the sample to fade to half of the initial
absorbance value, is also insightful for comparing overall
kinetics.
As depicted in Scheme 1 (and specifically in ESI†), exposure of
the naphthopyran closed form (CF) to continuous UV irradia-
tion results in a distribution of colored merocyanine isomers.
Spectroscopy studies have underlined the formation of two main
classes of transoid open isomers: a short-lived and major
component, namely trans–cis (TC) geometrical isomers and
a longer-lived and minor trans–trans (TT) population. The latter
reverts to the closed form through a two-step process (TT / TC
/ CF), with the TC isomers being the intermediates. The
thermal decoloration behavior in solution can be attributed to
these two main classes of open isomers decaying with different
first-order rate constants, k1 and k2.49,54–56 Kinetics within a solid
substrate can be complicated by a more disperse environment in
terms of distributions of free volume and variations in chemical
composition within the matrix environment. Therefore, sepa-
rated constants k1 and k2, in the equation above, along with their
allocated contributions to initial optical density, are overall
empirical values representing only fast and slow components. It is
noteworthy, however, that PDMS conjugates tested in this study
displayed a diminished contribution term, A2, which is associated
with the slower kinetic component k2, compared to their control
samples. Their kinetics more closely approached the simplified
decays displayed by naphthopyrans in solution. We believe that
this trend is indicative of a more homogenous and less disperse
local environment created for naphthopyran molecules within
the matrix as a result of PDMS tailing.
Fig. 5 Normalized absorbance vs. time for the coloration and decolor-
ation of indeno-fused naphthopyran control dye 22a and corresponding
PDMS conjugate 22b in a rigid polymeric host matrix PEGDMA–
EBPDMA (1 : 4).
Table 2 summarizes the data obtained for kinetic fitting for all
samples tested. For visual convenience, data for the PDMS
conjugates are listed between that of the controls in solution
(which are fastest and at the bottom of each set) and that of the
controls in the host matrix (which are slowest and at the top of
each set).
Fig. 6 Normalized absorbance vs. time for the coloration and decolor-
ation of 9-methoxy substituted naphthopyran control dyes 19a and 19c
compared to corresponding PDMS conjugate 19b in a rigid polymeric
host matrix PEGDMA–EBPDMA (1 : 4).
On the whole, all naphthopyran dyes tested displayed accel-
erated switching speeds in the host matrix (both coloration and
decoloration) when conjugated to PDMS compared to their
controls. This was evidenced by reduced T1/2 values and
increasing rate constants throughout (see Table 2); T1/2 values
were 1.3–4.7 times lower (reduced by 42–80%) and fast rate
constants, k1, were 1.2–2.8 times greater for PDMS conjugates
compared to their non-conjugated controls. This is particularly
apparent in Fig. 5 which shows the overlaid coloration and
decoloration curves for indeno-fused naphthopyrans 22a and
22b.
improvement to the switching kinetics displayed by PDMS-
conjugated 19b compared to the slower controls 19a and 19c,
the latter displaying only minor differences with respect to one
another.
All naphthopyran controls showed slower kinetics in the
matrix compared to their solution behavior. This is expected due
to the restricted rotational mobility of the photochromic mole-
cules in the rigid host. The major fast decay constant, k1, for
6-phenyl substituted naphthopyran control 21a, dropped to
a sixth of the value in the matrix compared to solution, evidenced
also by significantly different T1/2 values (129 s for matrix
compared to 11 s for solution). Indeno-fused 22a also showed
considerably slower switching speed in the matrix (T1/2 355 s)
compared to its behavior in solution (T1/2 45 s).
A comparison of the kinetics of naphthopyran controls 18c,
19c and 20c with respect to their alkyl extended and electroni-
cally equivalent versions (18a, 19a and 20a) showed some
additional improvement to rates with an additional spacer (1.1–
1.3 times higher T1/2 values for controls containing extra
spacer). However, this effect was insignificant compared to that
displayed by PDMS conjugation. Fig. 6 shows the overall
The extent to which PDMS conjugation offers solution-like
behavior was estimated by directly comparing the T1/2 values of
the PDMS conjugate in the matrix to those of the control in
solution (set as the lowest limit). Overall this measure ranged
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J. Mater. Chem., 2009, 19, 5612–5623 | 5621

View Article Online
as well as the overall rigidity imposed by the local environment.
We therefore do not expect the impact on kinetics offered by
PDMS conjugation to be the same for each dye.
In previous investigations, the attachment of low Tg radically
polymerized tails, such as poly(n-butyl acrylate) to naph-
thopyran dyes of base structures 16a and 17a, displayed not only
improved de/coloration speeds but also enhanced optical densi-
ties (colorabilities) compared to their unconjugated controls.24,57
These concurrent fast and dark effects were not evidenced
throughout our entire investigations. Whilst a larger proportion
of PDMS–naphthopyrans tested did display higher colorabilties
some showed the contrary, such as the 9-methoxy substituted
PDMS conjugates (19b and 20b). It is well known that the
photocoloration period involves both thermal and photochem-
ical pathways inter-converting between the isomers.49,54,55
A diminished colorability is likely to be the result of competitive
thermal reverse processes which can affect the amount of
colorless form able to convert during irradiation.58 Nonetheless,
all coloration curves comprehensively displayed an overall
positive effect from PDMS conjugation with samples achieving
a photostationary state very quickly (mostly within 10 min)
compared to the controls which all showed reduced coloration
speeds.
Fig. 7 Normalized absorbance vs. time for the coloration and decolor-
ation of naphthopyran control dye 17a and corresponding PDMS
conjugate 17b in host matrix PEGDMA–EBPDMA (1 : 4), 1.50 ꢃ 10ꢁ7
mol gꢁ1, compared to the behavior of 17a in toluene (5.0 ꢃ 10ꢁ5 M).
from 20% for 21b to 90% for 20b with conjugates 17b, 19b, 20b
and 22b being particularly impressive. Several factors are likely
to be involved; the 6-phenyl substituted naphthopyran 21a is
more likely to experience steric hindrance to bond rotation
within the matrix due to an additional aromatic moiety whereas
naphthopyran dyes 19c and 20c have the benefit of a 9-methoxy
moiety which acts to positively influence the energetics of the
system for ring closure.29 Interestingly the indeno-fused naph-
thopyran 22b, also containing an extra bulky heterocyclic
moiety, showed superior performance by more closely
approaching its solution-like behavior in the matrix, in contrast
to 21b. Fig. 7 also displays superior performance for the
coloration and decoloration behavior of PDMS dye 17b in the
host matrix compared to the control 17a and its ability to
restore solution-like kinetics. Inspection of the overlaid curves
also indicates a weak yet residual coloration in the matrix which
could only be removed in a reasonable period of time on
exposure to visible light. In solution, however, this eventually
faded completely in the dark. This was also evident in all other
samples tested and is likely to be due to stable isomer pop-
ulations whose conversion back to their closed forms is partic-
ularly unfavorable within the matrix, even in the presence of
a lubricating tail.
It is noteworthy that all naphthopyran samples also exhibited
a bathochromic shift (10–15 nm) in the wavelength of their open
forms within the lens matrix with respect to that in toluene. This
can be accounted for by the partly polar nature of the lens matrix
incorporating substantial PEG units and its interaction with
their more polar transition states. Such effects are likely to have
little consequence on the overall speed, with all samples dis-
playing significantly slowed kinetics compared to those in
toluene as a result of the rigidity of the matrix.
Conclusions
In conclusion, we have shown that the attachment of PDMS to
various methoxy substituted naphthopyrans gives markedly
superior photochromic performance in coloration and decolor-
ation speeds and, for a majority of the samples tested, greater
colorabilties within a rigid host matrix. This is made possible by
providing a localized and favorable environment for switching of
attached dyes and their aggregates, all acting to overcome the
restrictions imposed on mobility by a rigid matrix.
Efficient routes that were used to access the relevant starting
materials necessary to generate the hydroxyl functionalized
naphthopyrans and subsequent conjugates were also presented
as part of the investigation.
Attachment of PDMS does not aim to modulate photo-
chromic speed by manipulating electronic characteristics of the
naphthopyran; it simply aims to restore the dye’s switching
potential when incorporated into a rigid media. Therefore, the
methodology cannot make an inherently slow dye become fast
and certainly not faster than its solution speed at a given
temperature. For example, PDMS-conjugated dye 18b, con-
taining a 6-methoxy moiety, could never achieve the decoloration
speeds in the matrix that are displayed by 9-methoxy substituted
control dyes 19c and 20c, despite the presence of a lubricating
tail—its electronic nature renders the dye inherently slow.
Therefore, the overall speed of the PDMS–naphthopyran dye
conjugate within the host is determined by many interplaying
factors: the thermodynamics of the photochromic transition,
influenced by electronic substitution, simultaneous steric affects,
Acknowledgements
FE thanks the CRC for Polymers for funding the research in
association with CAMD, School of Chem. Eng. and Ind. Chem.
and CSIRO Molecular and Health Technologies for providing
laboratory space. FE also acknowledges the assistance of Dr
Jonathan Campbell (University of New South Wales) for the set-
up and maintenance of photochromic testing equipment. TPD
acknowledges the receipt of a Federation fellowship from the
ARC.
5622 | J. Mater. Chem., 2009, 19, 5612–5623
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27 R. A. Evans, T. L. Hanley, M. A. Skidmore, T. P. Davis, G. K. Such,
L. H. Yee, G. E. Ball and D. A. Lewis, Nat. Mater., 2005, 4(3), 249–
254.
28 T. Hattori, H. Nobuyuki, S. Oi, H. Abe and S. Miyano, Tetrahedron:
Asymmetry, 1995, 6(5), 1043–1046.
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