X-ray, kinetics and DFT studies of photochromic substituted benzothiazolinic spiropyrans

Article abstract of DOI:10.1016/j.molstruc.2010.01.012

Photochromic molecules have the potential to find utility in a wide variety of applications including photoswitchable binding and optical memory. This work explores the relationship between photochromism and structural parameters such as particular bond lengths for this class of compounds for which very few crystal structures have been published. Photochemical kinetics, Density Functional Theory (DFT) and X-ray crystallography were used to study the benzothiazolinic spiropyran 3-methyl-6-nitro-3′-methylspiro-[2H-l-benzopyran-2,2′-benzothiazoline]. A second benzothiazolinic spiropyran 3-methyl-8-methoxy-6-nitro-3′-methylspiro-[2H-l-benzopyran-2,2′-benzothiazoline] was synthesized and subjected to photochemical and computational studies. Selected structural and photochemical data for these, related benzothiazolinic spirooxazines and spiropyrans, and related thiazolidinic spiropyrans are compared. Both benzothiazolinic spiropyrans exhibit photochromic properties that are influenced by substituents, solvent, and temperature. The crystallographic C_spiro-O bond distance of 3-methyl-6-nitro-3′-methylspiro-[2H-l-benzopyran-2,2′-benzothiazoline] that has been shown to correlate with photochromic properties is 1.458 ?. The crystallographic C_spiro-O bond distance matches that of the structure generated by DFT calculations exactly. The effect of substituents on calculated bond lengths and photochemical parameters was determined. For this class of compounds, both X-ray geometry and DFT optimized geometry may be used to predict photochromism, but not degree of photocolorability.

Full text of DOI:10.1016/j.molstruc.2010.01.012

Journal of Molecular Structure 968 (2010) 13–18
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
X-ray, kinetics and DFT studies of photochromic substituted benzothiazolinic
spiropyrans
1
*
Satish Kumar , Kriya Velasco, Alison McCurdy
Department of Chemistry and Biochemistry, California State University, Los Angeles, 5151 State University Dr., Los Angeles, CA 90032, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Photochromic molecules have the potential to find utility in a wide variety of applications including
photoswitchable binding and optical memory. This work explores the relationship between photochro-
mism and structural parameters such as particular bond lengths for this class of compounds for which
very few crystal structures have been published. Photochemical kinetics, Density Functional Theory
Received 29 September 2009
Received in revised form 21 December 2009
Accepted 5 January 2010
Available online 14 January 2010
(
3
DFT) and X-ray crystallography were used to study the benzothiazolinic spiropyran 3-methyl-6-nitro-
0
0
-methylspiro-[2H-l-benzopyran-2,2 -benzothiazoline].
A
second benzothiazolinic spiropyran 3-
Keywords:
Photochromism
X-ray
0
0
methyl-8-methoxy-6-nitro-3 -methylspiro-[2H-l-benzopyran-2,2 -benzothiazoline] was synthesized
and subjected to photochemical and computational studies. Selected structural and photochemical data
for these, related benzothiazolinic spirooxazines and spiropyrans, and related thiazolidinic spiropyrans
are compared. Both benzothiazolinic spiropyrans exhibit photochromic properties that are influenced
by substituents, solvent, and temperature. The crystallographic Cspiro–O bond distance of 3-methyl-6-
Benzothiazolinic spiropyran
0
0
nitro-3 -methylspiro-[2H-l-benzopyran-2,2 -benzothiazoline] that has been shown to correlate with pho-
tochromic properties is 1.458 Å. The crystallographic Cspiro–O bond distance matches that of the structure
generated by DFT calculations exactly. The effect of substituents on calculated bond lengths and photo-
chemical parameters was determined. For this class of compounds, both X-ray geometry and DFT opti-
mized geometry may be used to predict photochromism, but not degree of photocolorability.
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Photochromic spiropyrans have attracted considerable attention
lengths are found to be shorter than those found in 5-membered
heterocycles of 1.47–1.48 Å. For spirooxazines, better photocolor-
ability correlates with longer Cspiro–O bond lengths. These correla-
tions are thought to reflect the degree of interaction between the
lone pair on one heteroatom bound to the spiro carbon with the r*
antibonding orbital of the polar Cspiro–O bond that breaks during
recently owing to their diverse applications, such as in optical mem-
ory devices, light filters, drug delivery systems, and biological
calcium signal mimics [1–6]. These spiropyrans isomerize reversibly
between two structures (closed, colorless and open, colored) in
response to light as shown in Fig. 1 for 3-methyl-6-nitro-3 -methyl-
spiro-[2H-l-benzopyran-2,2 -benzothiazoline] (1). In many cases,
thermal (dark) processes compete with the photochemical conver-
sion of the less stable to the more stable form. Photochromismof spi-
ropyran compounds has been reviewed extensively by Bertelson
photochemical ring opening [12].
0
Our interest in this subject derives from the fact that suitably
functionalized spiropyran compounds have potential for applica-
tion in mimicking calcium oscillations in biological systems
[14,15]. While spirobisbenzopyran compounds do not have appro-
priate photophysical properties at room temperature, benzothiaz-
olinic spiropyran compounds offer a significant improvement. In
order to understand the photochromic properties of benzothiazoli-
nic spiropyrans, we have studied some photochemical properties
and determined the crystal structure of compound 1. The related
compound 2 was also synthesized and characterized. The synthesis
of 1 and 2 were first reported by Guglielmetti et al. [16,17].
However, no crystal structure was reported. In fact, no crystal
structure for a closed form of a benzothiazolinic spiropyran has
been reported in the literature. The structure of the open merocy-
anine form of a benzothiazolinic spiropyran was reported by Miler-
Srenger and Guglielmetti [18]. In this work, the crystal structure of
0
[
7,8]. The Cspiro–O bond length and substituents have been corre-
lated with photochromic properties of spiropyrans and spirooxa-
zines [9–13]. Specifically, for spiropyrans and spirooxazines, such
as those shown in Fig. 2, the Cspiro–O bond lengths are elongated
by 0.03–0.06 Å from the normal Csp3–O bond length in 6-membered
heterocycles of 1.41–1.43 Å. The corresponding Cspiro–N
1
bond
*
Corresponding author. Tel.: +1 323 343 2362; fax: +1 323 343 6490.
E-mail address: amccurd@calstatela.edu (A. McCurdy).
Present address: Department of Chemistry, The University of Memphis, Memphis,
1
TN 38152, United States.
0
022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.01.012

1
4
S. Kumar et al. / Journal of Molecular Structure 968 (2010) 13–18
5
0 mL round bottom flask. The reaction flask was evacuated and
2
filled with N (gas). Ethanol (15 mL) and piperidine (0.1 mL) were
added to the reaction flask. The reaction mixture was refluxed
for 1 h. The solvent was evaporated and impure product was puri-
fied using flash column chromatography (10% Ethyl acetate in hex-
1
ane) to give 1 (0.12 g, 60%) as a pure product. H NMR (CDCl
3
,
4
7
3
1
1
3
00 MHz, d): 8.05 (dd, J = 8.88 Hz, 1H), 7.98 (d, J = 2.24 Hz, 1H),
.20–7.14 (m, 2H), 6.91–6.84 (m, 2H), 6.84–6.64 (m, 2H), 3.04 (s,
H), 2.10 (s, 3H); C NMR (CDCl , 400 MHz, d): 155.8, 144.6,
3
42.4, 131.6, 125.9, 125.6, 125.0, 122.1, 121.7, 121.6, 121.0,
20.5, 117.1, 114.8, 108.0, 30.5, 19.62. HRMS (m/z): Calcd,:
26.0725; found: 326.0725. Crystals suitable for single-crystal X-
Fig. 1. Photochromism of the benzothiazolinic spiropyran 1; the colorless closed
form 1 is interconverted with the colored open form.
1
3
ray determinations were grown by keeping a solution of 1 in THF
for several weeks in the dark.
1
Compound 2 (51% yield). H NMR (CDCl
3
, 400 MHz, d): 7.68 (d,
J = 3.2 Hz, 1H), 7.63 (d, J = 3.6 Hz, 1H), 7.154 (m, 2H), 6.86 (t,
J = 10 Hz, 1H), 6.66 (d, J = 10 Hz, 1H), 6.59 (d, J = 1.60 Hz, 1H),
Fig. 2. Representative spiropyran compounds with correlations between photo-
chromism and Cspiro–O bond length. Left: spiroindolinopyran; Right: spirooxazine.
1
3
3
4
1
3
.81 (s, 3H), 3.07 (s, 3H), 2.07 (d, J = 2.0 Hz, 3H); C NMR (CDCl ,
00 MHz, d): 148.7, 145.4, 144.7, 142.0, 131.7, 125.8, 122.2,
1
(Fig. 5) is compared with the equilibrium geometry obtained
using density functional calculations shown in Fig. S4 (B3LYP/6-
1G*) as well as certain structural reported and calculated features
21.5, 121.0, 119.8, 114.9, 114.3, 107.8, 107.6, 56.5, 30.4, 19.4.
ꢁ5
Photochemistry: A 30 mL solution of 1 (3.68 ꢀ 10 M) or 2
ꢁ5
3
(
6.17 ꢀ 10 M) was prepared in the appropriate solvent in a 3-neck
of related photochromic and nonphotochromic spiropyran scaf-
folds shown below (Fig. 3).
flask containing a stirbar. Theflaskwascooled to ꢁ30 °C in a thermo-
statted cryobath. Light from a 200 W Mercury Xenon lamp, filtered
by water and an Oriel #59800, 330 nm bandpass filter, was directed
via a 3 foot fiber optic bundle into the top of a 3-neck flask while the
solution was stirred. UV–Vis observations were made using a Hellma
2
. Experimental and computational
1
0 mm immersion probe connected to a Varian Cary 50 spectropho-
The compounds 2, 7 and 8 were synthesized according to the pro-
tometer. Rates are reported as averages of three runs, except for 1 in
isopropanol and acetonitrile, in which the reaction was performed
once and the rate was averaged from five different wavelengths.
Computation: The program Spartan ‘04 was used. AM1 was used
to generate the initial geometry for 1. The compound 1 was then
cedure reported in Guglielmetti et al. [19]. The procedure was
adapted to synthesize compound 1 as described below and in
Scheme 1.
Compound 1 (60% yield). A mixture of 8 (0.28 g, 0.80 mmol) and
of 5-nitrosalicylaldehyde (0.14 g, 0.83 mmol) were added to a
Fig. 3. Structures of related spiropyrans (1, 2, 3, 4) and spirooxazines (5, 6).
Scheme 1.

S. Kumar et al. / Journal of Molecular Structure 968 (2010) 13–18
15
Table 1
First order rate constants of thermal bleaching at ꢁ30°.
Solvent
2-Propanol
CH
3
CN
THF
Toluene
ꢁ
1
ꢁ3
ꢁ3
ꢁ3
ꢁ2
k
k
T
min (1)
5.28 ± 0.29 ꢀ 10
6.00 ± 0.81 ꢀ 10
7.40 ± 0.54 ꢀ 10
1.11 ± 0.19 ꢀ 10
Insoluble photoproduct
4.14 ± 1.33 ꢀ 10
ꢁ
1
ꢁ2
T
min (2)
Negligible
Negligible
subjected to geometry optimization (B3LYP/6-31G*), and the
resulting geometry was used to determine single point energy.
Cartesian coordinates and energies are listed in Table S6. For com-
pound 2, AM1 was used to generate conformers. These conformers
were then subjected to geometry optimization (B3LYP/6-31G*),
and the resulting geometries were used to determine single point
energies. Cartesian coordinates and energies are listed in Tables
S7–S9.
3.2. Photochemical studies
At several temperatures, solutions of 1 and 2 in a variety of sol-
vents were irradiated with UV light, and subsequently were allowed
toreturntotheiroriginalcolorlessforminthedark. Compound1 was
found to exhibit weak photochromism at room temperature only in
polar solvents (isopropanol, but not in toluene or THF). Photochro-
mismin1wasobservedinseveralsolvents(THF, acetonitrile, isopro-
panol) tested at lower temperatures to slow thermal closure rates
(Table 1). Closure rates had been reported earlier only for photoirra-
diation in toluene at 25 °C. After irradiation at low temperature, the
solution is colored, as shown in Fig. 4, and then thermally bleaches to
a colorless solution over time. Thermal fading rates are slower in
more polar solvents, due to solvent stabilization of the polar zwitter-
ionic open forms. The addition of an electron-donating group (2) has
the effect of increasing the rate of thermal closure, as the nucleophi-
licity of the phenolic oxygen is enhanced, or as the stability of the
zwitterionic form is decreased.
2.1. Crystal data
C
M
17
H
14
N
2
O
3
S
Monoclinic, P2
Z = 4
1
/n
r
326.36
MoK radiation
a
T = 100 (2) K
V = 1522.24(11) Å3
a = 9.1899 (4) Å
b = 11.8526 (5) Å
c = 13.9833 (6) Å
b = 91.9440 (10)°
3
0.29 ꢀ 0.28 ꢀ 0.28 mm
ꢁ1
l
= 0.229 mm
3
Density = 1.424 Mg/m
3
.3. Crystallography and intermolecular contacts
2
2
.2. Data collection
1
The target compound 1 crystallized in the P2 /n space group
Bruker SMART 1000
Absorption correction: none
25987 measured reflections
6105 independent reflections
I > 2 (I), 4660 reflections
int = 0.0777
from THF as yellow crystals (Fig. 5). One interesting feature of
the structure is the presence of several short intermolecular dis-
tances (Table 2 and Fig. 6). For example, edge to face interactions
are observed between aromatic systems, such as the intermolecu-
T
T
max = 0.9386
min = 0.9365
r
R
lar short contact observed between H11 and C
centroid (H11-centroid distance is 3.166 Å) [25]. In addition, there
(H17A-cen-
4
, displaced from the
.3. Refinement
is a C–Hꢂꢂꢂpi interaction between the methyl H17A and C
4
troid distance is 3.307 Å). The crystal lattice is further stabilized by
the presence of short intermolecular contacts between carbon
2
2
R1 (w = 1/
r
(Fo )) = 0.0443
S = 1.689
max = 0.621 e Å
ꢁ3
D
q
2
3 10
atoms. For example, the C –C distance is 3.391 Å and the C10–
C
the crystal structure of a related compound 3-ethyl-8-methoxy-
6
wR = 0.0872
ꢁ
3
11 distance is 3.343 Å. Similar short contacts were reported in
Parameters 264
Restraint 0
D
q
min = ꢁ0.356 e Å
H atom unrestrained
0
0
-nitro-2H-1-benzopyran-2-spiro-2 -(3 -methylthiazolidine)
(4)
by Miler-Srenger and Guglielmetti [26]. However, the crystal struc-
tures of two benzothiazolinic spirooxazines (3-methyl-2,3-dihy-
dro-l,3-benzothiazolespiro-2,2 -(3-methyl-2H-phenanthr[9,10-b]
Data collection: Bruker SMART v5.630; cell refinement: Bruker SAINT
v6.45A; data reduction: Bruker SAINT v6.45A; program(s) used to
solve structure: SHELXS97 [20]; program(s) used to refine structure:
SHELXL97 [21]; molecular graphics: Bruker XP v5.1; Ortep3 [22];
Mercury [23]; software used to prepare material for publication:
Bruker XCIF v6.12.
0
[
2
1,4]oxazine) (5), 3-methyl-2,3-dihydro-1,3-benzothiazolespiro-
,2 -(3-methyl-2H-naphth[2,I-b][1,4]-oxazine) (6) reported earlier
0
Crystallographic data have been deposited at the CCDC, 12 Un-
ion Road, Cambridge CB2 1EZ, UK and copies can be obtained on re-
quest, free of charge, by quoting the publication citation and the
deposition number 632419.
3
. Results and discussion
3.1. Synthesis and characterization of 1 and 2
As detailed in the experimental section, the known targets 1
and 2 were prepared from a quaternized ethylbenzothiazole. Addi-
tion of piperidine generated the enamine base, which was con-
densed with the appropriately substituted salicylaldehyde to
make the spiropyran. The reported 1H spectrum (for 2) and
C
spectra (for 1 and 2) are consistent with those determined in this
work [24].
13
Fig. 4. Absorption spectra of an irradiated solution of spiropyran
1 in THF
ꢁ5
(
3.68 ꢀ 10 M) at ꢁ30 °C as it thermally (without light) reverts from open to
closed form.

1
6
S. Kumar et al. / Journal of Molecular Structure 968 (2010) 13–18
Table 2
Short intermolecular contacts.
Atom1
Atom2
Length
Length-VdW
C
C
H
3
4
C
H
C
C
10
3.391
2.798
2.859
3.343
ꢁ0.009
ꢁ0.102
ꢁ0.041
ꢁ0.057
11
17A
4
C
10
11
Fig. 6. Crystal packing diagram (Mercury [23]) of 1 showing short contacts.
0
0
Fig. 5. Ortep view [22] of 3-methyl-6-nitro-3 -methylspiro-[2H-l-benzopyran-2,2 -
sued extensively for benzothiazolinic spiropyrans by several inves-
tigators such as Vandewyer et al. [31] and Kister et al. [32]. The
presenceofanelectron-withdrawinggroupinposition6andanelec-
tron-donating group in position 8 of the benzopyran ring system is
often required for good photochromic properties. Compound 1 has
one nitro substituent at position 6, and it is weakly photochromic
at room temperature. By comparison, a second benzothiazolinic spi-
ropyran (2) with both nitro and methoxy substituents was synthe-
sized and characterized. This compound did not form suitable
crystals to obtain a crystal structure, however, photochemical and
computational studies were pursued. Compound 2 was photochro-
mic, and using the measure of colorability, or the steady state absor-
bance in the visible region upon irradiation, it was found to be more
efficient at ring opening than 1 in THF at ꢁ30 °C. This substituent ef-
fect in a different solvent and at a different temperature is also doc-
umented by other investigators (Table 4). Compound 3 was reported
to be not photochromic [16]. The slight shortening of the Cspiro–O
bond length with the added methoxy group (1 versus 2) is consistent
with that found for the same substituent changes in photochromic
spiroindolinopyrans [9]. For spiropyrans 1–4, as the Cspiro–O bond
increases in length, the Cspiro–N bond decreases, which is consistent
with what is observed crystallographically for indolinospiropyrans.
benzothiazoline] with displacement ellipsoids at the 50% probability level.
by Sun et al. [27] show no short intermolecular contacts, possibly
due to the absence of electron donating or withdrawing substitu-
ents. Compound 2 did not crystallize well.
3.4. Computational studies
Examination of various bond distances and bond angles reveals
that in the solid state, the molecule 1 adopts a geometry very close
to that obtained using DFT calculations (Tables 3, S1, S11, and S12).
This suggests that the weak C–Hꢂꢂꢂpi interactions do not affect the
geometry significantly for 1. For example, the bond distance Cspiro
–
0
1 8 1
O (C –O ) is 1.458 ÅA in both structures. This bond distance is of
particular interest, due to its potential for use in prediction of pho-
tochromic properties. For the related spiropyrans 4, 5, and 6, the
calculated bond lengths are slightly less accurate, but are within
.01 Å. The calculated Cspiro–N bond lengths are within 0.01 Å of
experimental values for 1 and 4, but do not accurately reproduce
crystallographic distances for spirooxazines 5 and 6.
0
3
.5. Correlating structural features with photochromism and
thermochromism
3
.5.1. Cspiro–N
A crystallographic Cspiro–N
benzothiazolinic spiropyran 1, which is slightly longer than the re-
1
bond length
As for other classes of spiropyrans and spirooxazines, efforts to
correlate substituent effects with photochromism have been pur-
1
bond length of 1.434 is observed for
Table 3
Calculated bond lengths and crystallographic bond lengths.
E
mol)
rel (kcal/
C
(Å)
spiro–O bond length (calc)
C
(Å)
spiro–N bond length (calc)
C
(Å)
spiro–O bond length (crystal)
Cspiro–N bond length
(crystal), Å
a
1
2
2
2
3
3
4
5
6
–
0
1.458
1.454
1.458
1.462
1.437
1.440
1.462
1.436
1.436
–
1.437
1.437
1.436
1.440
1.443
1.442
1.436
1.438
1.438
–
1.458
–
–
–
–
–
1.469 [26]
1.422 [27]
1.430 [27]
1.41–1.43
1.434
–
–
–
–
–
1.421 [26]
1.402 [27]
1.409 [27]
1.47–1.48
-a
-b
-c
-a
-b
1.689
2.595
0
0.652
–
–
–
–
Saturated heterocycles
12]
[
a
Relative to that of the lowest energy conformer.

S. Kumar et al. / Journal of Molecular Structure 968 (2010) 13–18
17
Table 4
Bond lengths and photochemical properties for 1 and related spiropyran compounds.
C
spiro–O (Å)
C
spiro–N (Å)
Colorability = (A
0
/cL); (solvent, T)
k
T
4
ꢁ1
ꢁ1
1
2
3
4
5
6
1.458
1.434
1.437
1.443
1.421
1.402
1.409
4916 (THF, ꢁ30°) 700 (toluene, 25°) [28]
8995 (THF, ꢁ30°) 3700 (toluene, 25°) [28]
1.11 ꢀ 10ꢁ
s
s
(THF, ꢁ30°) 1.8 s (toluene, 25°) [26]
1.454a
a
a
1.85 ꢀ 10
ꢁ4 ꢁ1
(THF, ꢁ30°) 6.5 s (toluene, 25°) [28]
ꢁ1
1.437a
1.469 [26]
1.422 [27]
1.430 [27]
0
b
–
ꢁ
1
2300 (toluene, 25°) [29]
76 s (toluene, 25°) [30]
b
0
0
–
–
b
a
Calculated using DFT for the most stable isomer (Table 3).
Not photochromic.
b
ported value 1.421 Å for the related thiazolidinic spiropyran 4 and
also longer than those found for the benzothiazolinic spirooxazines
wavelength-dependent quantum yields for photochemical ring
opening.
5
1
and 6 (Table 3). Calculated bond lengths for Cspiro–N in benzo-
thiazolinic spiropyrans 2 and 3 are close to that observed for 1. Ta-
4
. Conclusions
ken together, bond lengths in all six compounds are significantly
3
3
shorter than C(sp )–N(sp ) bond lengths found in five-membered
heterocycles. A shorter Cspiro–N bond is an indication of donation
of lone pair electrons from N to Cspiro, which would stabilize the
cation that develops on Cspiro when the compound is in the open
form. There does not appear to be a correlation between Cspiro
bond length and either photochromism or the extent of photo-
colorability for the series of compounds 1–6 (Table 4). It is possible,
however, that the uncertainty of the calculated Cspiro–N bond
lengths in the spirooxazines masks any trend that may exist.
Few crystallographic studies have been published for this sub-
1
class of benzothiazolinic spiropyrans, and no crystal structure of
the closed form of any benzothiazolinic spiropyran has been re-
ported. Therefore, correlations between experimental molecular
geometry and photochromism or the degree of photocolorability
for this subclass of spiropyrans are not yet well substantiated. In
this work, we confirm that for benzothiazolinic or thiazolidinic spi-
ropyrans and spirooxazines, the crystallographic bond distances
such as Cspiro–O can be used as an indicator of photochromism,
but not for the degree of photocolorability. Moreover, for com-
pounds such as these, the crystal structure and the computational
results are similar for Cspiro–O bonds. Therefore, computation may
also serve as a useful tool to predict photochromism from ground
state structures for benzothiazolinic or thiazolidinic spiropyrans
and spirooxazines.
1
–
N
1
1
The crystallographic bond angles around the spiro carbon atom
of 1 are found to be between 106.15° and 114.01°. This distortion
from the tetrahedral angle and the short Cspiro–N
indicate that the spiro carbon possesses a degree of sp character.
The sum of the bond angles at N was found to be 348.21°, which
1
bond length
2
1
3
lies between the sum for sp hybridized bond angles (327°) and the
2
sum for sp hybridized bond angles (360°). The sum of the bond
angles, as well as the shorter Cspiro–N distance described above,
2
indicates that N
1
has some sp character to stabilize a developing
Acknowledgments
positive charge on Cspiro, suggesting a possibility of thermochro-
mism. This possibility was also reported by Sun et al. [27] for their
nonphotochromic benzothiazolinic spirooxazines 5 and 6. Thermo-
chromism was not observed for 1 at room temperature, or up to
We thank Dr. Michael W. Day and Lawrence M. Henling (Cal-
tech) for performing single-crystal X-ray crystallographic studies.
Thanks to Josh Cohen for assistance in the preparation of this man-
uscript. This work was supported by a grant from the National
Institutes of Health (NIGMS MBRS S06 GM08101).
5
0 °C in isopropanol.
3.5.2. Cspiro–O bond length
As discussed in the introduction, a longer Cspiro–O bond is asso-
Appendix A. Supplementary data
ciated with photochromism in spiropyrans and spirooxazines. In
addition, for spirooxazines, this bond length correlates with the de-
gree of photocolorability. The extent to which either photochro-
mism or the degree of photocolorability of thiazolidinic and
benzothiazolinic spiropyrans and benzothiazolinic spirooxazines
may also be predicted by Cspiro–O bond length was explored using
data collected and reported for compounds 1–6.
Taking together both the crystallographic and computational
distances in Table 4, it appears that compounds with Cspiro–O bond
distances greater than or equal to 1.454 Å (4, 1, 2) are photochro-
mic, while compounds with Cspiro–O bond distances less than or
equal to 1.437 Å (3, 6, 5) are not photochromic. Within the uncer-
tainty of calculated bond Cspiro–O bond distances of up to 0.01 Å
CCDC 632419 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via Exter-
nal link http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033). Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.molstruc.2010.01.012.
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