Synthesis and photochromic properties of methacryloxy 6- nitrospirobenzopyrans

Article abstract of DOI:10.1016/j.dyepig.2013.03.010

The synthesis and spectroscopic characterization of several methacryloxy-modified spiropyran dyes is reported. The dyes were found to have similar photochromic isomerization rates relative to a commercially available reference compound in solution. The methacryloxy-modified dyes were also successfully incorporated into polymer matrices, where they retained their ability to photoisomerize and complex with cobalt (II) and zinc (II) ions.

Full text of DOI:10.1016/j.dyepig.2013.03.010

Dyes and Pigments 98 (2013) 437e441
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photochromic properties of methacryloxy
6-nitrospirobenzopyrans
*
Kelly A. Sennett, Brian K. Lindner, Navdeep Kaur, Shannon M. Fetner, Shannon E. Stitzel
Department of Chemistry, Fisher College of Science and Mathematics, Towson University, 8000 York Road, Towson, MD 21252, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and spectroscopic characterization of several methacryloxy-modified spiropyran dyes is
reported. The dyes were found to have similar photochromic isomerization rates relative to
commercially available reference compound in solution. The methacryloxy-modified dyes were also
successfully incorporated into polymer matrices, where they retained their ability to photoisomerize and
complex with cobalt (II) and zinc (II) ions.
Received 31 January 2013
Received in revised form
11 March 2013
Accepted 12 March 2013
Available online 26 March 2013
a
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Spiropyrans
Photochromism
Synthesis
Polymers
Metal complexation
UV spectroscopy
1. Introduction
group has made polymerization possible in acrylate polymers [12]
and polymer brushes [13,14]. This report presents the preparation
A spiroindolinobenzopyran 1 is a photo-switchable organic
molecule which can undergo cleavage at the CeO bond of the
neutral spirocyclic form (SP) upon exposure to UV light to generate
the merocyanine form (MC), a highly colored zwitterion. The MC
form can revert back slowly to the closed SP form through thermal
decay, or it can be forced closed upon exposure to visible light [1]
(Fig. 1). These types of photochromic compounds have been
investigated for various applications including optoelectronic de-
vices such as lenses and filters and molecular switches [2,3], and
photo-controlled polymeric materials [4,5]. Spiropyrans have also
been studied for their ability to complex with cations via the
phenolate oxygen atom in the MC form [6], and have potential for
development into chemical sensing materials [7,8].
Spiropyrans have previously been immobilized onto surfaces
and incorporated into polymer matrices through modification with
different functional groups at the nitrogen of the indole ring.
Modification with a thiol group has been used to generate self-
assembled monolayers of dye molecules on surfaces [9] and nano-
particles [10]. Addition of a hydroxyl or carboxy-propyl group has
facilitated covalent attachment to glass [11] and plastic surfaces [8].
The addition of a short alkyl chain terminating in a methacryloxy
and characterization of spiroindolinobenzopyran dyes with a
methacrylate moiety connected by alkyl chains of three different
lengths to enable their incorporation within polymer matrices.
While work with the shorter two chains has been reported for
different photo-controlled polymer materials applications [4,5,13],
these studies have used polymers with little to no cross-linking
within the polymer matrix. The results of this preliminary study
present a comparison of the photochromic properties of these
methacryloxy monomers in solution, as well as in cross-linked, rigid
polymer matrices. This is also the first reported synthesis of the
longest chain methacryloxy monomer.
2. Methods and materials
2.1. Equipment
Unless otherwise stated, reagents were used as supplied. Flash
column chromatography was performed using silica gel 60 (32e
63
mm; MP Biomedicals). All of the compound yields are unopti-
mized, and all were homogeneous by TLC (silica gel 60). Infrared
spectra were recorded on a Nicolet iS10 infrared spectrometer
equipped with a Smart iTR ATR attachment (neat sample). NMR
spectra were recorded on a JEOL 400 MHz instrument in either
chloroform-d or acetonitrile-d3 with tetramethylsilane (TMS) as the
* Corresponding author. Tel.: þ1 410 704 2948.
E-mail addresses: sstitzel@towson.edu, sestitzel@gmail.com (S.E. Stitzel).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.03.010

438
K.A. Sennett et al. / Dyes and Pigments 98 (2013) 437e441
(CDCl3) 1.16 (s, 3H, Me), 1.26 (s, 3H, Me), 1.34 (m, 4H), 1.53e1.66 (m,
4H), 1.91 (s, 3H, Me), 3.10e3.14 (m, 2H, J ¼ 3 Hz, NeCH2), 4.09 (t, 2H,
J ¼ 6.9 Hz, OeCH2), 5.53 (s, 1H, CHH]C), 5.82 (d, 1H, J ¼ 10.5 Hz, 3-
H), 6.06 (s, 1H, CHH]C), 6.54 (d, 1H, J ¼ 7.8 Hz, Ar-H), 6.72 (d, 1H,
J ¼ 8.7, Ar-H), 6.85 (t,1H, J ¼ 6.9 Hz, Ar-H), 6.87 (d, 1H, J ¼ 10.1 Hz, 4-
H), 7.06 (d, 1H, J ¼ 8, 1 Hz, Ar-H), 7.17 (t,1H, J ¼ 8, 1 Hz, Ar-H), 7.98 (s,
1H, Ar-H), 8.01 (d, 1H, J ¼ 3 Hz, Ar-H); dC (CD3CN) 17.54 (Me), 19.02
(Me), 25.43 (Me), 25.49 (CH2), 26.48 (CH2), 28.25 (CH2), 28.49
(CH2), 43.15 (CH2), 52.42 (q), 64.43 (CH2), 106.71 (CH), 107.04 (q),
115.32 (CH), 119.04 (q), 119.20 (CH), 121.76 (CH), 122.05 (CH), 122.79
(CH), 124.67 (C]CH2), 125.64 (CH), 127.76 (CH), 128.07 (CH), 136.14
(q), 136.82 (q, C]CH2), 140.94 (q), 147.22 (q), 159.46 (q), 167.06 (q,
Fig. 1. Photochemical conversion of 10,30,30-trimethyl-6-nitrospiro[2H-1-benzopyran-
2,20-indoline] between the spirocyclic and merocyanine forms.
C]O). Found [M
þ
H]þ
¼
477.2377, C28H32N2O5 requires
internal reference. Accurate mass FAB mass spectra were recorded
at Johns Hopkins University using a VG70SE double focusing
magnetic sector mass spectrometer (VG Analytical, Manchester, UK,
now Micromass/Waters) interfaced with an MSS data system
(MasCom, Bremen, Germany). 10-[2-hydroxyethyl]-30,30-dimethyl-
6-nitrospiro[2H-1-benzopyran-2,20-indoline] 2a, 10-[6-hydroxyh
exyl]-30,30-dimethyl-6-nitrospiro[2H-6-nitro-spiro[2H-1-benzopy-
ran-2,20-indoline] 2b, and 10-[10-hydroxydecyl]-30,30-dimethyl-6-
nitrospiro[2H-1-benzopyran-2,20-indoline] 2c were all prepared
according to the procedure of Inouye et al. [15].
[M þ H]þ ¼ 477.2389.
2.2.3. 10-[10-methacryloxydecyl]-30,30-dimethyl-6-nitrospiro[2H-1-
benzopyran-2,20-indoline] 3c
From 2c and methacryloyl chloride as a reddish-orange solid
after elution from silica with 20% EtOAc in hexane (0.600 g, 30.4%);
mp 58e59 ꢀC; nmax 2926, 2855, 1717, 1610, 1577, 1509, 1479, 1333,
1274, 1160, 1122, 1088, 950, 807, 747 cmꢁ1
, dH (CDCl3) 1.11 (s, 3H,
Me), 1.18 (s-broad, 10H), 1.21 (s, 3H, Me), 1.60 (m, 6H), 1.87 (s, 3H,
Me), 3.02e3.12 (m, 2H, J ¼ 8 Hz, NeCH2), 4.06 (t, 2H, J ¼ 6.9 Hz, Oe
CH2), 5.47 (s, 1H, CHH]C), 5.78 (d, 1H, J ¼ 10.1 Hz, 3-H), 6.02 (s, 1H,
CHH]C), 6.49 (d, 1H, J ¼ 7.8 Hz, Ar-H), 6.66 (d, 1H, J ¼ 8.2 Hz, Ar-H),
6.79 (t, 1H, J ¼ 7.6 Hz, Ar-H), 6.82 (d, 1H, J ¼ 10.5 Hz, 4-H), 7.00 (d,
1H, J ¼ 6.4 Hz, Ar-H), 7.11 (d, 1H, J ¼ 6.4 Hz, Ar-H), 7.93 (s, 1H, Ar-H),
7.95 (d,1H, J ¼ 3 Hz, Ar-H); dC (CD3CN) 18.42 (Me),19.90 (Me), 26.00
(CH2), 26.09 (Me), 27.37 (CH2), 28.64 (CH2), 29.01 (CH2), 29.27
(CH2), 29.45 (CH2), 29.51 (CH2), 29.56 (CH2), 43.81 (CH2), 52.70 (q),
64.87 (CH2), 106.71 (CH), 106.82 (q), 115.61 (CH), 118.56 (q), 119.30
(CH), 121.72 (CH), 122.14 (CH), 122.76 (CH), 125.24 (C]CH2), 125.92
(CH), 127.79 (CH), 128.12 (CH), 135.98 (q), 136.60 (q, C]CH2), 140.89
(q), 147.22 (q), 159.79 (q), 167.63 (q, C]O). Found
[M þ H]þ ¼ 533.3007, C32H40N2O5 requires [M þ H]þ ¼ 533.3016.
2.2. General preparation method of methylacryloxy 6-
nitrospirobenzopyran monomers
Typical procedures [16] employed for the preparation of meth-
ylacryloxy monomers were as follows. A stirred solution of the N-
hydroxyalkyl 6-nitrospirobenzopyran (1 equiv.) and triethylamine
(3 equiv.) in dry acetone (20 mL) was cooled to 0 ꢀC under a N2
atmosphere and protected from exposure to visible light. Meth-
acryloyl chloride (3 equiv.) in dry acetone (5 mL) was then added
dropwise over 30 min. After stirring at room temperature for 10e
12 h, the reaction mixture was filtered by gravity filtration and then
the solvent was removed under reduced pressure. The crude
product was then purified by flash column chromatography (80:20
hexane/ethyl acetate) to leave the product as a yellow to reddish-
orange solid. The identities of all products were confirmed by 1H
and 13C NMR, and carbon assignments were confirmed by DEPT-
135 experiments (q ¼ quaternary).
2.3. General method for the preparation of dye-containing
polymers
Polymers containing 3a, 3b, or 3c spiropyran dye were synthe-
sized according to the following method. Methyl methacrylate
(MMA) and ethylene glycol dimethacrylate (EDGMA), a cross-
linking monomer, were both passed through a hydroquinone
monomethyl ether removing column (SigmaeAldrich) to eliminate
any polymerization inhibitor prior to use. To a 25 mL round-bottom
flask were added, 3a, 3b or 3c (2 mL of 0.02 M solution in acetone),
2.2.1. 10-[2-methacryloxyethyl]-30,30-dimethyl-6-nitrospiro[2H-1-
benzopyran-2,20-indoline] 3a
From 2a and methacryloyl chloride as a yellow solid after
elution from silica with 20% EtOAc in hexane (0.998 g, 41.8%); mp
98e101 ꢀC (lit. mp 84 ꢀC [17]); nmax 2965, 1718, 1611, 1577, 1511,
1480, 1334, 1273, 1152, 1122, 1089, 952, 808, 750 cmꢁ1
,
dH (CDCl3)
acetone (2 mL), MMA (425 mL; 0.4 g), EDGMA (190 mL; 0.2 g) and
1.15 (s, 3H, Me), 1.27 (s, 3H, Me), 1.90 (s, 3H, Me), 3.38e3.58 (m, 2H,
J ¼ 6.4 Hz, NeCH2), 4.29 (t, 2H, J ¼ 6.4 Hz, OeCH2), 5.55 (s, 1H,
CHH]C), 5.85 (d, 1H, J ¼ 10.1 Hz, 3-H), 6.06 (s,1H, CHH]C), 6.68 (d,
1H, J ¼ 7.8 Hz, Ar-H), 6.73 (d, 1H, J ¼ 8.7 Hz, Ar-H), 6.88 (t, 1H,
J ¼ 4 Hz, Ar-H), 6.90 (d, 1H, J ¼ 10.5 Hz, 4-H), 7.07 (d, 1H, J ¼ 7.3 Hz,
Ar-H), 7.19 (t, 1H, J ¼ 7.3 Hz, Ar-H), 7.99 (s, 1H, Ar-H), 8.01 (d, 1H,
J ¼ 3 Hz, Ar-H); dC (CD3CN) 17.58 (Me),18.98 (Me), 25.23 (Me), 42.24
(CH2), 52.28 (q), 62.65 (CH2), 106.83 (CH), 115.39 (CH), 119.00 (q),
119.74 (CH), 121.82 (CH), 121.96 (CH), 122.84 (CH), 125.33 (C]CH2),
125.71 (CH), 127.77 (CH), 128.23(CH), 135.97(q), 136.43 (q, C]CH2),
141.23 (q), 146.96 (q), 159.38 (q), 166.90 (q, C]O).
2,20-azobisisobutylnitrile (40 mg; 0.01 wt% relative to MMA). The
flask was sealed with a septum, and purged with nitrogen for
15 min. After purging, the flask was placed in a thermostated sand
bath at 60 ꢀC, and the reaction was allowed to proceed for 8e12 h.
The resulting insoluble bulk polymer was manually ground by
mortor and pestle, followed by Soxhlet extraction in acetone for 8e
12 h. The polymer was then air dried and sieved to a particle size of
63 mm or smaller.
2.4. UVevisible measurements
All spectroscopic analyses were performed with an Ocean Optics
USB 4000 spectrometer and an LS-1 tungsten halogen lamp with a
400 nm longpass filter. UV and visible light exposures were per-
formed using a UV-UVP lamp (4 Watt, 365 nm) and a Fisher Sci-
entific Ultra-bright LED Illuminator (18 mW, 400e750 nm)
respectively. All spectroscopic analyses were carried out with
constant magnetic stirring at room temperature. Monomer solution
2.2.2. 10-[6-methacryloxyhexyl]-30,30,-dimethyl-6-nitrospiro[2H-1-
benzopyran-2,20-indoline] 3b
From 2b and methacryloyl chloride as an orange solid after
elution from silica with 20% EtOAc in hexane (1.048 g, 49.5%); mp
64e67 ꢀC (lit. mp 63e64 ꢀC [12]); nmax 2929, 2858, 1709, 1610, 1576,
1509, 1479, 1333, 1275, 1166, 1122, 1089, 951, 807, 748 cmꢁ1
, dH

K.A. Sennett et al. / Dyes and Pigments 98 (2013) 437e441
439
spectra were measured using a quartz cuvette containing a 1 mL
1
aliquot of a 1 ꢂ 10ꢁ3 M monomer solution in acetone. Polymer
spectra were obtained by suspending a 5 mg quantity of polymer in
2 mL of acetone in a 1 cm quartz cuvette. Absorbance spectra of the
MC forms of dyes were recorded after exposure to UV light for
2
1.5
1
3a
3b
3c
1 min. The metal complex spectra were obtained by adding a 20 mL
aliquot of a 0.02 M CoCl2 or ZnCl2 solution in acetone to the cuvette
after 1 min of UV light exposure, and then recording the spectrum.
Experiments to measure the rates of photochemical isomerization
from the SP to MC form (forward reaction) were conducted under
constant UV light, with absorbance measurements recorded at the
wavelength of maximum absorbance (lmax) every 10 s for a total of
10 min. To determine the rate of isomerization from the MC to SP
form (reverse reaction), the cuvette was first exposed to UV light
(solution ¼ 45 s; polymer ¼ 8 min) to convert the dye to the MC
form. The reverse reaction was then monitored during constant
visible light exposure, with absorbance measurements recorded at
lmax every 10 s for total of 15 min.
0.5
0
400
500
600
700
Wavelength (nm)
Fig. 2. UVevisible spectra of the MC form of reference compound 1 and methacryloxy
spiropyrans 3aec in solution.
to the monomer solution, a new peak was observed near 490 nm
due to the formation of a complex between the phenolate oxygen in
the MC form of the dye and the metal ion. A small bathochromic
shift was measured in the absorption maxima of the dye/metal
complexes for all three methacryloxy dyes relative to the methyl-
substituted analog 1, with the largest shifts (>10 nm) occurring
for 3a (Table 1). The measured absorption maxima for the zinc
complexes were also slightly lower than those for the cobalt
complexes with the same dye, with the 3b dye showing the largest
difference (Fig. 3).
The forward and reverse rates of photochemical isomerization
(Fig. 1) were determined for the reference compound 1 and the
three methacryloxy spiropyrans 3aec. The rate constants for the
colorization and bleaching processes of the dyes were determined
from the following equation:
3. Discussion
Methylacryloxy 6-nitrospirobenzopyran monomers (Scheme 1)
were obtained in moderate yield by esterification of the previously
reported alcohols (2aec). Purification of 3a was possible by crys-
tallization from ethanol or column chromatography, but the longer
alkyl chain length products (3b, 3c) could only be purified by col-
umn chromatography alone. Products 3aec were readily confirmed
by 1H and 13C NMR spectroscopy. The 1H NMR spectra displayed
characteristic doublets at 5.8 ppm (3-H) and 6.9 ppm (4-H) with a
coupling constant of 10.5 Hz indicating the cis arrangement of the
pyran ring protons. The addition of the methacryloxy group was
verified by the singlets at 5.5 ppm and 6.1 ppm for the vinyl pro-
tons. Also the alkyl chain attached to the indoline ring was
confirmed by a multiplet at 3.0e3.5 ppm for the diastereotopic Ne
CH2 protons, and a triplet at 4.1e4.3 ppm for the OeCH2 protons.
The 13C NMR spectra confirmed the successful addition of the
methacrylate group to the hydroxyl group on the alkyl chain of 2ae
c by the appearance of the carbonyl carbon at 167 ppm. While the
synthesis of 3a has been previously reported [16], the melting point
reported here is higher than previously stated in the literature [17].
The synthesis of 3b was also previously reported [5,12] but with
some differences in the method, and the 13C NMR data presented
here has not been published. There was no literature found
reporting the synthesis of 3c.
ꢁln½ðAt ꢁ AeÞ=ðAo ꢁ AeÞꢃ ¼ kt
(1)
where At and A0 are the absorbance values at time t and time zero
respectively, and Ae is the absorbance value at lmax after reaching
the photostationary state [2]. As seen in Table 2, the rate constants
for the methacryloxy-modified dyes are similar to the methyl
analog 1 for both the forward and reverse reactions, indicating that
the presence of the methacryloxy moiety does not hinder isomer-
ization of the dye in solution.
The main purpose of the synthetic modification of the spi-
ropyran dyes was to enable covalent attachment of the monomers
within polymer matrices. Rigid, cross-linked polymers containing
each of the three methacryloxy-modified dyes were synthesized
and characterized by UVeVisible spectroscopy. A control polymer
containing reference compound 1 was also synthesized. As com-
pound 1 does not have a methacrylate functional group however,
the dye was completely removed upon washing via Soxhlet
extraction, and therefore no reference polymer data could be ob-
tained. All three of the methacryloxy-modified dyes remained in
The UVeVisible spectroscopic characterization of the meth-
acryloxy spiropyran monomers 3aec began with a comparison of
the lmax values of the photoinduced MC form of each dye, as well as
their absorbance features in the presence of cobalt (II) and zinc (II).
It was found that the lmax was not significantly altered by substi-
tution of the methyl 1 for the methacryloxy-terminated alkyl
chains (Fig. 2), with all lmax values occurring within 7 nm of each
other at ca 570 nm. Upon addition of cobalt (II) or zinc (II) chloride
Scheme 1. Formation of methylacryloxy 6-nitrospirobenzopyran monomers.

440
K.A. Sennett et al. / Dyes and Pigments 98 (2013) 437e441
Table 1
3c MC
0.6
Wavelength of maximum absorbance (nm) for methacryloxy dyes in solution and
polymer matrices.
0.5
0.4
0.3
0.2
0.1
0
3c MC + Co(II)
3c MC + Zn(II)
1
3a
3b
3c
Solution: MC
568
486
485
e
575
500
496
578
510
510
570
494
489
572
505
500
570
493
490
573
509
501
Solution: MC þ Co(II)
Solution: MC þ Zn(II)
Polymer: MC
Polymer: MC þ Co(II)
e
Polymer: MC þ Zn(II)
e
400
500
600
700
1 MC
1.8
1.5
1.2
0.9
0.6
0.3
Wavelength (nm)
1 MC + Co(II)
1 MC + Zn(II)
3b MC
3b MC + Co(II)
3b MC + Zn(II)
Fig. 4. UVeVis spectra of the MC form of 3c with and without cobalt (II) and zinc (II) in
polymers.
Table 3
Forward and reverse rate constants (sꢁ1) for the photochromic isomerization in a
polymer matrix. Constants reported as mean ꢄ 95% confidence interval (n ¼ 3).
3a
3b
3c
Forward
Reverse
1.6 ꢄ 0.4 ꢂ 10ꢁ2
1.5 ꢄ 0.1 ꢂ 10ꢁ2
1.7 ꢄ 0.1 ꢂ 10ꢁ2
0
6.8 ꢄ 0.4 ꢂ 10ꢁ3
6.6 ꢄ 0.8 ꢂ 10ꢁ3
6 ꢄ 1 ꢂ 10ꢁ3
400
500
600
700
Wavelength (nm)
metal ions. While only subtle differences in the measured absor-
bance maxima between the cobalt and zinc complexes were
observed, small spectral variations could be utilized in sensing ap-
plications, and so further investigation into the metal complex for-
mation behavior of the photochromic polymers is ongoing.
Fig. 3. UVevisible spectra of the MC form of 3b and reference compound 1 with and
without cobalt (II) and zinc (II) in solution.
the polymer after washing, verifying that they were covalently
attached to the polymer backbone, rather than simply being trap-
ped within the polymer matrix.
Spiropyran dyes are known to be solvatochromic [18], and so
small shifts in the lmax values were expected between the dye
monomers in solution and their corresponding polymers. When in
the polymer, the photoinduced MC form of the dyes were found to
have similar lmax values, with only slight bathochromic shifts seen
as compared to the monomers in solution (Table 1). The MC form of
each dye was also able to complex with both cobalt and zinc ions
when introduced into the polymer, with the complex peaks
exhibiting greater than 10 nm bathochromic shifts relative to the
corresponding metal complex peaks observed for the monomers in
solution (Table 1). The measured absorption maxima for the zinc
complexes were again slightly lower than those for the cobalt
complexes with the same dye, but with the 3c dye showing the
largest difference (Fig. 4).
The rates of photochemical isomerization between the SP and MC
forms in the polymers (Table 3) were comparable to values in so-
lution, indicating that there is minimal steric hindrance from the
rigid polymer matrix. Even the shortest alkyl chain seems to insulate
the spiropyran from the influence of the polymer backbone. In
addition, the three methacryloxy dyes were found to have the same
forward and reverse rates relative to each other within a given
polymer matrix, indicating that the difference in alkyl chain length
does not have a large impact on the photochromic properties of the
dyes in this type of cross-linked polymer matrix. Based on the re-
sults, it appears that the all three of the methacryloxy-modified dyes
were successfully incorporated into polymers, while retaining their
abilities to undergo photochromic isomerization and complex with
4. Conclusions
In this study, the synthesis of a new methacryloxy-modified
spiropyran dye and the characterization of three methacryloxy-
modified dyes are presented. To our knowledge, this is the first
reported synthesis of 3c, and the first report of 3b synthesized by
this method. Comparisons of the monomers 3aec in solution show
that they are all able to complex with both cobalt (II) and zinc (II) in
solution, and they have similar rates of photoisomerization as
compared to a methyl analog 1 and to each other. More impor-
tantly, all three dyes were successfully incorporated into rigid,
cross-linked polymer matrices, where they retained their ability to
photoisomerize and complex with metal ions. Future studies will
investigate the application of these types of spiropyran-containing
polymers to metal ion sensing.
Acknowledgment
The authors would like to thank the NSF (MRI 0923051) for
support. The authors also thank the Faculty Development and
Research Committee and the Jess and Mildred Fisher College of
Science and Mathematics at Towson University for financial sup-
port. Dr. Timothy Brunker is also thanked for his helpful discussions
regarding syntheses.
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Table 2
Forward and reverse rate constants (sꢁ1) for the photochromic isomerization in
solution. Constants reported as mean ꢄ 95% confidence interval (n ¼ 3).
1
3a
3b
3c
Forward 6.8 ꢄ 0.2 ꢂ 10ꢁ3 1.3 ꢄ 0.4 ꢂ 10ꢁ2 7.0 ꢄ 0.1 ꢂ 10ꢁ3 6.4 ꢄ 0.6 ꢂ 10ꢁ3
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