Synthesis and photochromic properties of spiro[naphthopyran-7′H- benzocyclohepta-5′,8′-dienes]

Article abstract of DOI:10.1016/j.dyepig.2012.03.031

A series of spironaphthopyrans has been synthesised by the acid-catalysed condensation between either a substituted 1-naphthol or 2-naphthol and the propargylic alcohol derived from the addition of lithium trimethylsilylacetylide to a 7H-benzocycloheptan-7-one with subsequent in situ silyl group removal. The photochromism of the novel spironaphthopyrans was characterised by a bathochromically shifted λ_max coupled with a relatively quick initial fade of the photogenerated species relative to the comparable diphenyl substituted model compounds. After initial fading a relatively intense residual colour persisted for the spironaphthopyrans.

Full text of DOI:10.1016/j.dyepig.2012.03.031

Dyes and Pigments 95 (2012) 62e68
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Short communication
Synthesis and photochromic properties of spiro[naphthopyran-70H-
benzocyclohepta-50,80-dienes]
*
Daniel L. Crossley, Christopher D. Gabbutt, B. Mark Heron , Paul Kay, Martin Mogstad
Department of Colour Science, School of Chemistry, The University of Leeds, Leeds LS2 9JT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of spironaphthopyrans has been synthesised by the acid-catalysed condensation between either
a substituted 1-naphthol or 2-naphthol and the propargylic alcohol derived from the addition of lithium
trimethylsilylacetylide to a 7H-benzocycloheptan-7-one with subsequent in situ silyl group removal. The
photochromism of the novel spironaphthopyrans was characterised by a bathochromically shifted lmax
coupled with a relatively quick initial fade of the photogenerated species relative to the comparable
diphenyl substituted model compounds. After initial fading a relatively intense residual colour persisted
for the spironaphthopyrans.
Received 6 March 2012
Received in revised form
26 March 2012
Accepted 28 March 2012
Available online 4 April 2012
Keywords:
Synthesis
Ó 2012 Elsevier Ltd. All rights reserved.
Photochromism
Spiropyran
Naphthopyran
Benzocycloheptanone
Propynol
1. Introduction
spironaphthopyrans derived from the condensation between
a naphthol and a 7-ethynyl-7H-benzocyclohepta-5.8-dien-7-ol; the
Labile spirocyclic systems have featured prominently in stimuli
responsive colorant chemistry over the years (Fig. 1) [1]. The spi-
roindolinobenzopyrans 1 came to the fore in the mid 70’s and were
noted for their photo- and thermochromic properties and continue
to be of interest today as a model responsive system for exploring
a range of optical phenomena [2]. Spiroindolizines 2 [3], spi-
roindolinonaphthoxazines 3 [4] and spiroindolinonaphthopyrans 4
[5] have all been investigated for their photochromic properties
with the most commercially successful series being the spi-
roindolinonaphthoxazines 3, which have generated appreciable
commercial interest in the ophthalmic photochromic lens industry
[6]. The fluorescence of the rhodamines e.g. 5 is well-known and
has been harnessed in a variety of sensor and labelling applications
[7]. The closely related fluoran (spiro[isobenzofuran-1(3H),90-(9H)
xanthen]-3-one) system is a key unit in commercial, reversible
thermochromic inks and which is still attracting interest today with
linked colour former e developer systems such as 6 [8]. The
photochromism of a series of spirofluorene-7 [9] and spirothiox-
anthene e naphthopyrans 8 has been discussed [10]. Given the
interest in such a diversity of spirocyclic compounds we elected to
examine the synthesis and photochromism of a new series of
results of this preliminary study are described in this short paper.
2. Experimental
2.1. Equipment
Unless otherwise stated, reagents were used as supplied. NMR
spectra were recorded on a Bruker Avance 400 MHz instrument (1H
NMR 400 MHz, 13C NMR 100 MHz) for sample solutions in CDCl3
with either tetramethylsilane as an internal reference or referenced
to the residual protonated solvent signal. FT-IR spectra were recor-
ded on a Perkin Elmer Spectrum One spectrophotometer system
equipped with a golden gate ATR attachment (neat sample).
UVevisible spectra were recorded for freshly prepared spectroscopic
grade toluene solutions of the samples, previously unexposed to
daylight, with a 4 min activation with UV irradiation (UV exposure
time determined to give a maximum absorption value), absorption
decay data collected over either 15 or 30 min at 18 s intervals,10 mm
pathlength quartz fluorescence cuvette, PTFE capped, ca. 1 ꢀ 10ꢁ4
e
10ꢁ6 mol dmꢁ3 using a Cary 50 Probe spectrophotometer equipped
with a single cell Peltier temperature controlled (20 ꢂC) stirred cell
attachment with activating irradiation provided by an Oriel 150 W
xenon arc lamp source (Newport 66906), xenon ozone free arc lamp
(Newport 6255), distilled water liquid filter (Newport 6177),
* Corresponding author. Tel.: þ44 (0)1482 667422.
E-mail address: b_mark_heron@hotmail.co.uk (B. Mark Heron).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.03.031

D.L. Crossley et al. / Dyes and Pigments 95 (2012) 62e68
63
Fig. 1. Some examples of spirocyclic colorants.
multiple filter holder (Newport 62020), UG11 filter (Newport FSO-
UG11), fibre optic coupler (Newport 77799) and liquid light guide
(Newport 77557). Flash column chromatography was performed on
chromatography grade silica gel (Fluorochem, 40e63 micron particle
size distribution). All the compound yields are unoptimised. All new
compounds were homogeneous by TLC (Merck TLC Aluminium
sheets, silica gel 60 F254) using a range of eluent systems of differing
polarity. Mass spectra were recorded at the National EPSRC Mass
Spectrometry Service Centre, Swansea using a Thermo Scientific LTQ
Orbitrap XL Fourier transform mass spectrometer. The naphthols
employed were either commercially available (Alfa-Aesar, Aldrich)
or were prepared according to published protocols. 1,1-
Diphenylprop-2-yn-1-ol was obtained by the well-established
protocol of the addition of lithium trimethylsilylacetylide to benzo-
phenone with subsequent in situ base promoted desilylation [11];
physical data for this propynol was in accord with that previously
reported [12]. Ethyl 4-hydroxy-1-phenylnaphthalene-2-carboxylate
[13] and methyl 4-hydroxy-1-methylnaphthalene-2-carboxylate
[14] were prepared according to literature procedures.
of trimethylsilylacetylene (2 mol equiv.) in anhydrous THF (200 mL)
under nitrogen. On completion of the addition the solution was
stirred at ꢁ15 ꢂC for 30 min 6,8-Diphenyl-7H-benzocycloheptan-7-
one (10.67 g, 34.6 mmol) was then added in a single portion and
the reaction mixture was stirred whilst warming to room tempera-
ture until TLC examination showed no further ketone remained (ca.
3 h). The reaction mixture was then cooled to ꢁ15 ꢂC and a meth-
anolic KOH [(2 mol equiv.) in methanol (100 mL)] was added. The
reaction mixture was stirred at ꢁ15 ꢂC until TLC confirmed that no
further change in composition occurred (ca. 30 min). The reaction
mixture was then poured into water (500 mL), extracted with ethyl
acetate (3 ꢀ 50 mL), washed with water (2 ꢀ 100 mL) and dried
(anhyd. MgSO4). The solvent was removed under reduced pressure
and the residue was crystallised from hexane to yield the title
compound [16] as an off-white powder (9.11 g, 79%), mp 128e132 ꢂC,
nmax 3500, 3264, 3022, 1615, 1598, 1490, 1443, 1241, 1032, 753,
696 cmꢁ1
, dH 2.13 (1H, s, acetylene-H), 2.17 (1H, s, OH), 6.81 (2H, s, 5-
H, 9-H), 7.28e7.50 (14H, overlapping m, AreH). Found
[M þ Na]þ ¼ 357.1253, C25H18O requires [M þ Na]þ ¼ 357.1250.
2.2. Preparation of 6,8-diphenyl-7H-benzocycloheptan-7-one 9
2.4. General method for the preparation of naphthopyrans
A solution of potassium hydroxide (1.06 g, 19 mmol) in ethanol
(30 mL) was added dropwise over 5 min to a warm stirred solution
of o-phthalaldehyde (10 g, 74.6 mmol) and 1,3-diphenylacetone
(15.7 g, 74.6 mmol) in ethanol (100 mL). A colour change from
pale gold to red and finally to orange was then observed upon the
addition. The reaction mixture was then heated under reflux for
1.5 h. After cooling to 5 ꢂC the resulting pale yellow precipitate was
collected by vacuum filtration and washed with cold ethanol. The
resulting solid was then recrystallised from ethyl acetate and
hexane, to afford the title compound as pale yellow microcrystals
(15.1 g, 65%), mp ¼ 117e119 ꢂC (lit. mp ¼ 119 ꢂC [15]).
A stirred solution of the naphthol (1 equiv.) and the prop-2-yn-
1-ol (1 equiv.), trimethyl orthoformate (2 equiv.) with a catalytic
amount of pyridinium para-toluenesulfonate (0.05 equiv.) in 1,2-
dichloroethane (40 mL) was heated under reflux until TLC exami-
nation confirmed no further conversion had occurred (ca. 8 h). The
cooled mixture was diluted with CH2Cl2 (30 mL), washed with
water (100 mL), dried (anhyd. MgSO4) and the solvent removed
under reduced pressure. The residue was then either crystallised
from ethyl acetate and hexane or eluted from silica to afford the
naphthopyran. The following compounds were obtained in this
manner:
2.3. Preparation of 6,8-diphenyl-7-ethynyl-7H-benzocyclohepta-
5.8-dien-7-ol 10
2.4.1. 60,80-Diphenyl-spiro[3H-naphtho[2,1-b]pyran-3,70-70H-
benzocyclohepta-50,80-diene] 12a
From 2-naphthol 11a and propynol 10 as a pale pink solid after
elution from silica with 20% EtOAc in hexane (0.51 g, 37%), mp
202e204 ꢂC, nmax 3050, 1634, 1587, 1488, 1441, 1243, 1171, 1093,
n-Butyllithium (1.6 M in hexanes) (2 mol equiv.) was added
portionwise over 5 min, via syringe to a cold (ꢁ15 ꢂC) stirred solution

64
D.L. Crossley et al. / Dyes and Pigments 95 (2012) 62e68
880, 812, 754, 697 cmꢁ1
,
dH 5.60 (1H, d, J ¼ 10.0 Hz, 2-H), 6.14 (1H, d,
J ¼ 8.8 Hz, 6-H), 7.70 (1H, d, J ¼ 8.2 Hz, 7-H), 7.94 (1H, d, J ¼ 8.2 Hz,
J ¼ 9.0 Hz, 5-H), 6.765 (2H, s, 50-H, 90-H), 6.77 (1H, d, J ¼ 10.0 Hz, 1-
H), 7.19 (6H, m, AreH), 7.27 (5H, m, AreH), 7.32 (1H, d, J ¼ 9.0 Hz, 6-
H), 7.38 (2H, m, AreH), 7.41 (1H, m, AreH), 7.51 (2H, m, AreH), 7.62
(1H, d, J ¼ 8.0 Hz, 7-H), 7.81 (1H, d, J ¼ 8.5 Hz, 10-H). Found
[M þ H]þ ¼ 461.1903, C35H24O requires [M þ H]þ ¼ 461.1900.
10-H). Found [M
þ
H]þ
¼
335.1434, C25H18
O
requires
[M þ H]þ ¼ 335.1430.
2.4.7. 3,3-Diphenyl-8-methoxy-3H-naphtho[2,1-b]pyran 16b
From 6-methoxy-2-naphthol 11b and propynol 15 as pale beige
microcrystals after recrystallisation from EtOAc and hexane (0.71 g,
40.6%), mp 179e181 ꢂC (lit. mp 173e175 ꢂC [18]), nmax 2937, 1620,
1590, 1471, 1446, 1363, 1328, 1246, 1195, 1160, 1095, 1055, 1009, 818,
2.4.2. 60,80-Diphenyl-8-methoxy-spiro[3H-naphtho[2,1-b]pyran-
3,70-70H-benzocyclohepta-50,80-diene] 12b
From 6-methoxy-2-naphthol 11b and propynol 10 as a pale pink
solid after elution from silica with 10% EtOAc in hexane (0.26 g,
24%), mp 162e164 ꢂC, nmax 3048, 1617, 1592, 1489, 1373, 1242, 1204,
764, 731, 696, 676 cmꢁ1
, dH 3.86 (3H, s, 8-OMe), 6.26 (1H, d,
J ¼ 9.9 Hz, 2-H), 7.02 (1H, d, J ¼ 2.4 Hz, 7-H), 7.12 (1H, dd, J ¼ 8.8,
2.4 Hz, 9-H), 7.19 (1H, d, J ¼ 8.7 Hz, 5-H), 7.22e7.33 (7H, m, AreH, 1-
H), 7.47 (4H, m, AreH), 7.53 (1H, d, J ¼ 8.7 Hz, 6-H), 7.84 (1H, d,
J ¼ 8.8 Hz, 10-H). Found [M þ H]þ ¼ 365.1539, C26H20O2 requires
[M þ H]þ ¼ 365.1536.
1192, 1166, 1099, 1028, 852, 748, 696, 678 cmꢁ1
, dH 3.85 (3H, s, 8-
OMe), 5.60 (1H, d, J ¼ 9.9 Hz, 2-H), 6.12 (1H, J ¼ 9.0 Hz, 5-H), 6.73
(1H, d, J ¼ 9.9 Hz, 1-H), 6.77 (2H, s, 50-H, 90-H), 6.95 (1H, d,
J ¼ 2.5 Hz, 7-H), 7.09 (1H, dd, J ¼ 9.0, 2.5 Hz, 9-H), 7.20 (6H, m,
AreH), 7.28 (5H, m, AreH), 7.37 (2H, m, AreH), 7.50 (2H, m, AreH),
7.71 (1H, d, J ¼ 9.0 Hz, 10-H). Found [M þ H]þ ¼ 491.2009, C36H26O2
requires [M þ H]þ ¼ 491.2006.
2.4.8. 3,3-Diphenyl-9-methoxy-3H-naphtho[2,1-b]pyran 16c
[18,19]
From 7-methoxy-2-naphthol 11c and propynol 15 as off-white
microneedles after recrystallisation from EtOAc and hexane
(0.45 g, 25.6%), mp 176e179 ꢂC, nmax 3058, 1634, 1618, 1592, 1509,
2.4.3. 60,80-Diphenyl-9-methoxy-spiro[3H-naphtho[2,1-b]pyran-
3,70-70H-benzocyclohepta-50,80-diene] 12c
1444, 1244, 1220, 1019, 968, 914, 828, 754, 698, 670 cmꢁ1
, dH 3.91
From 7-methoxy-2-naphthol 11c and propynol 10 as a pale pink
solid after recrystallisation from EtOAc and hexane (0.36 g, 30%),
mp 198e201 ꢂC, nmax 3061, 3938, 1621, 1591, 1469, 1447, 1364, 1241,
(3H, s, 9-OMe), 6.24 (1H, d, J ¼ 9.9 Hz, 2-H), 6.96 (1H, dd, J ¼ 8.7,
2.1 Hz, 8-H), 7.05 (1H, d, J ¼ 8.7 Hz, 5-H), 7.21e7.33 (8H, m, AreH, 1-
H), 7.48 (4H, m, AreH), 7.57 (2H, app. t, AreH). Found
[M]þ ¼ 364.1460, C26H20O2 requires [M]þ ¼ 364.1458.
1160, 1095, 1056, 1009, 819, 763, 732, 695 cmꢁ1
, dH 3.86 (3H, 9-
OMe), 5.70 (1H, d, J ¼ 10.0 Hz, 2-H), 6.01 (1H, d, J ¼ 8.8 Hz, 5-H),
6.71 (1H, d, J ¼ 10.0 Hz, 1-H), 6.76 (2H, s, 50-H, 90-H), 6.94 (1H, dd,
J ¼ 8.8, 2.2 Hz, 8-H), 7.05 (1H, d, J ¼ 2.2 Hz, 10-H), 7.19e7.28 (11H, m,
AreH), 7.36 (2H, m, AreH), 7.51 (3H, m, AreH, 7-H). Found
[M þ H]þ ¼ 491.2012, C36H26O2 requires [M þ H]þ ¼ 491.2006.
2.4.9. Ethyl 2,2,6-triphenyl-2H-naphtho[1,2-b]pyran-5-carboxylate
17a
From ethyl 1-hydroxy-4-phenylnaphthalene-3-carboxylate 13a
and propynol 15 as off-white microcrystals after elution from silica
with 20% EtOAc in hexane (0.58 g, 24.0%), mp 174e177 ꢂC, nmax
2921, 1718, 1446, 1366, 1228, 1165, 1114, 1041, 938, 762, 698, 672,
2.4.4. 5-Ethoxycarbonyl-6,60,80-triphenyl-spiro[2H-naphtho[1,2-b]
pyran-2,70-70H-benzocyclohepta-50,80-diene] 14a
From ethyl 1-hydroxy-4-phenylnaphthalene-3-carboxylate 13a
and propynol 10 as off-white microcrystals after elution from silica
with 30% EtOAc in hexane (0.52 g, 36%), mp 112e114 ꢂC, nmax 2952,
1713, 1606, 1505, 1461, 1442, 1432, 1372, 1295, 1225, 1165, 1108,
606 cmꢁ1
,
dH 0.91 (3H, t, J ¼ 7.2 Hz, CH2CH3), 4.02 (2H, q, J ¼ 7.2 Hz,
CH2CH3), 6.25 (1H, d, J ¼ 9.9 Hz, 3-H), 6.85 (1H, d, J ¼ 9.9 Hz, 4-H),
7.28e7.63 (18H, m, AreH), 8.44 (1H, dd, J ¼ 8.7, 1.5 Hz, 10-H). Found
[M þ H]þ ¼ 483.1947, C34H26O3 requires [M þ H]þ ¼ 483.1955.
Spectroscopic data comparable to that reported previously [20].
1037, 1025, 998, 897, 749, 677 cmꢁ1
,
dH 0.88 (3H, t, J ¼ 7.2 Hz,
CH2CH3), 3.94 (2H, q, J ¼ 7.2 Hz, CH2CH3), 5.55 (1H, d, J ¼ 10.3 Hz, 3-
H), 6.35 (1H, d, J ¼ 10.2 Hz, 4-H), 6.50 (1H, d, J ¼ 8.4 Hz, 10-H), 6.91
(3H, s, 50-H, 90-H and m, AreH), 7.13e7.24 (7H, m, AreH), 7.28e7.41
(10H, m, AreH), 7.42 (2H, m, AreH), 7.52 (2H, m, AreH). Found
[M þ H]þ ¼ 609.2437, C44H32O3 requires [M þ H]þ ¼ 609.2424.
2.4.10. Methyl 2,2-diphenyl-6-methyl-2H-naphtho[1,2-b]pyran-5-
carboxylate 17b [21]
From methyl 1-hydroxy-4-methylnaphthalene-3-carboxylate
13b and propynol 15 as a pale yellow powder after recrystallisa-
tion from EtOAc and hexane (1.36 g, 58.6%), mp 175e178 ꢂC, nmax
2952,1725, 1489,1445,1432,1365,1288,1251, 1220,1114,1043, 985,
2.4.5. 60,80-Diphenyl-5-methoxycarbonyl-6-methyl-spiro[2H-
naphtho[1,2-b]pyran-2,70-70H-benzocyclohepta-50,80-diene] 14b
From methyl 1-hydroxy-4-methylnaphthalene-3-carboxylate
13b and propynol 10 as a pale orange microcrystals after elution
from silica with 25% EtOAc in hexane (0.39 g, 31%), mp 176e178 ꢂC,
nmax 3017, 1715, 1432, 1362, 1292, 1250, 1223, 1204, 1115, 1050, 890,
956, 761, 696, 658 cmꢁ1
, dH 2.56 (3H, s, Me), 4.01 (3H, s, OMe), 6.22
(1H, d, J ¼ 9.9 Hz, 3-H), 6.70 (1H, d, J ¼ 9.9 Hz, 4-H), 7.26 (6H, m,
AreH), 7.60 (6H, m, AreH), 7.94 (1H, m, 7-H), 8.43 (1H, dd, J ¼ 8.8,
1.6 Hz, 10-H). Found [M þ NH4]þ ¼ 424.1907, C28H22O3 requires
[M þ NH4]þ ¼ 424.1907.
765, 750, 707, 689, 642 cmꢁ1
,
dH 2.52 (3H, s, 6-Me), 3.92 (3H, s,
3. Discussion
OMe), 5.53 (1H, d, J ¼ 10.0 Hz, 3-H), 6.22 (1H, d, J ¼ 10.0 Hz, 4-H),
6.45 (1H, d, J ¼ 8.4 Hz, 10-H), 6.92 (2H, s, 50-H, 90-H), 6.96 (1H, m,
AreH), 7.20e7.29 (8H, m, AreH), 7.35 (1H, m, AreH), 7.45 (2H, m,
AreH), 7.57 (2H, m, AreH), 7.82 (1H, d, J ¼ 8.4 Hz, 7-H). Found
[M þ H]þ ¼ 533.2123, C38H28O3 requires [M þ H]þ ¼ 533.2111.
The base-catalysed condensation between o-phthalaldehyde
and 1,3-diphenylacetone proceeded smoothly to afford 6,8-
diphenyl-7H-benzocycloheptan-7-one 9 as pale yellow crystals in
65% yield after recrystallisation from ethyl acetate/hexane. Addi-
tion of lithium trimethylsilylacetylide to 9 with in situ hydroxide-
mediated removal of the trimethylsilyl group gave, after aqueous
work-up, the propynol 10 in 79% yield (Scheme 1). The formation of
10 was readily confirmed by 1H NMR spectroscopy which displayed
2.4.6. 3,3-Diphenyl-3H-naphtho[2,1-b]pyran 16a
From 2-naphthol 11a and propynol 15 as colourless microcrys-
tals after elution from silica with 10% EtOAc in hexane (3.1 g 67%),
mp 158e160 ꢂC (lit. mp 158e159 ꢂC [17]), nmax 3062, 1635, 1588,
1489, 1445, 1241, 1215, 1093, 1083, 1008, 945, 803, 751, 744, 696,
a singlet at
d 2.13 and at d 2.17 (D2O exchangeable) assigned to the
acetylene and hydroxyl protons, respectively. The condensation of
a naphthol with a propynol to afford a propargylic ether with
subsequent rearrangement and isomerisation is the classical
678 cmꢁ1
,
dH 6.26 (1H, d, J ¼ 9.9 Hz, 2-H), 7.19 (1H, d, J ¼ 8.8 Hz, 5-
H), 7.23e7.33 (8H, AreH, 1-H), 7.43e7.50 (5H, m, AreH), 7.65 (1H, d,

D.L. Crossley et al. / Dyes and Pigments 95 (2012) 62e68
65
approach to naphthopyrans [11,19,22]. Many variations in condi-
Ph
Ph
OH
HO
Ph
(i)
tions have been reported to offer enhanced yields in the foregoing
condensation and include the use of various acidic catalysts
including acidic alumina [23], substituted acetic acids [24], sulfonic
acids [18] and pyridinium p-toluenesulfonate (PPTS) [20]; the
application of solvent free conditions has also been advocated [25].
In the present work PPTS in conjunction with trimethyl ortho-
formate as a dehydrating agent was employed.
R1
+
O
Ph
R1
11a R1 = H
15
11b R1 = 6-MeO
11c R1 = 7-MeO
16a R1 = H
16b R1 = 8-MeO
16c R1 = 9-MeO
Thus heating 2-naphthol 11a and the propynol 10 under reflux
for 6 h in 1,2-DCE containing PPTS and (MeO)3CH gave, after
aqueous work up and elution of the crude product from chro-
matography silica, the pure spironaphthopyran 12a in a moderate
37% yield. The remaining naphthopyrans were obtained by
a similar protocol in 24e67% yield (Scheme 2). The 1H NMR
Ph
O
Ph
OH
HO
Ph
(i)
+
CO2R2
Ph
R1
15
CO2R2
R1
13a R1 = Ph, R2 = Et
13b R1 = R2 = Me
17a R1 = Ph, R2 = Et
17b R1 = R2 = Me
spectrum of 12a displayed a singlet at
d 6.77 for the equivalent
benzocycloheptadiene alkene protons (50-H and 90-H) together
with some interesting chemical shifts for the pyran ring protons
Reagents and conditions: (i) PPTS, (MeO)3CH, 1,2-DCE, reflux
with 2-H appearing as a doublet at
d 5.6 and 1-H as a doublet at
Scheme 2. Formation of diphenyl substituted naphthopyrans.
d
6.7 with a mutual coupling constant of 10 Hz. Whilst the
magnitude of J1,2 is typical at ca. 10 Hz [11,26] the signal for 2-H
and for 1-H have been shifted upfield by ca. 0.7 ppm and
0.6 ppm, respectively from the chemical shift of the comparable
protons in the simple diphenyl substituted naphthopyran 16a.
Furthermore, 5-H in 12a, which appears as a doublet with
Preliminary energy minimized modelling (Molecular Mechanics
3, Chem 3D Ultra, version 12) of 12a (Fig. 2) revealed that 5-H is
located within the shielding zone of one of the pendant phenyl
rings resulting in an upfield shift. Furthermore, due to the puckered
nature of the seven-membered ring it appears that 2-H, and to
a lesser extent 1-H, experience a shielding effect from the benzene
ring of the benzotropone unit and thus experience a significant
upfield shift. Attempts to obtain suitable crystals for X-ray analysis
are ongoing.
J ¼ 9.0 Hz and resonates at
d 6.14 is also shifted upfield relative to
that in 16a (ca. 7.2) (Table 1). It is clear that such upfield chemical
d
shifts must arise through the unique interaction between the C-3
spiro linked diphenylbenzocycloheptane residue and these
protons. A similar upfield shift was noted for the pyran ring
protons of the spironaphtho[1,2-b]pyrans 14 with 3-H resonating
at ca. d 5.5 and 4-H at ca. d 6.3 relative to 3-H (d 6.2) and 4-H (d 6.8)
in the model 2,2-diphenyl-2H-naphtho[1,2-b]pyrans 17.
The photochromic response, wavelength of maximum absorp-
tion of the photoinduced dye (lmax) and the time taken for the
OH
1'
R1
9'
3'
Ph
1
11a
11b
11c
R = H
7'
1
1
R = 6-MeO
R = 7-MeO
3
5
5'
1
O
Ph
R1
8
(ii)
1
12a
12b
R = H
Ph
O
Ph
1
Ph
HO
R = 8-MeO
(i)
12c R1 = 9-MeO
3'
Ph
9
10
1'
9'
5'
(ii)
7'
2
Ph
Ph
O
OH
1
4
9
CO2R2
CO2R2
7
R1
R1
1
2
1
2
14a
R = Ph, R = Et
13a
R = Ph, R = Et
14b 1 = Me, R2 = Me
1
2
R
13b
R = Me, R = Me
Reagents and conditions: (i) lithium trimethylsilylacetylide, THF, 0 oC - rt then KOH, MeOH, 0 oC - rt;
(ii) PPTS, (MeO)3CH, 1,2-DCE, reflux
Scheme 1. Formation of spironaphthopyrans.

66
D.L. Crossley et al. / Dyes and Pigments 95 (2012) 62e68
Table 1
Selected 1H NMR data (CDCl3) for spironaphtho[2,1-b]pyrans 12 and model 3,3-
diphenyl-3H-naphtho[2,1-b]pyrans 16.
d2-H
d1-H
J1,2 (Hz)
d5-H
12a
16a
12b
16b
12c
16c
5.60
6.26
5.60
6.26
5.70
6.24
6.77
10.0
9.9
9.9
9.9
10.0
9.9
6.14
7.19
6.12
7.19
6.01
7.05
7.22e7.33a
6.73
7.22e7.33a
6.71
7.21e7.33a
a
1-H is obscured by other aromatic signals.
Fig. 3. Absorption spectra of spironaphthopyran 12a and reference compound 16a.
Fig. 2. Molecular Mechanics 3 (MM3) representation of spironaphthopyran 12a.
photoinduced colour to fade to half of its original intensity under
steady state irradiation immediately after cessation of the irradia-
tion (half-life, t ), of colourless freshly prepared toluene solutions
½
Fig. 4. Absorption spectra of spironaphthopyran 14a and reference compound 17a.
of the spironaphthopyrans 12 and 14 were compared with those of
the simple diphenyl substituted analogues 16 and 17 (Table 2).
From these data it is evident that the replacement of the diphenyl
unit with the spirocycle induces a bathochromic shift of w33 nm in
lmax of the naphtho[2,1-b]pyran isomers 12a e c with an associated
decrease in the half-life of the photogenerated colour (Fig. 3).
Whilst as expected, the 2H-naphtho[1,2-b]pyrans 14 and 17 affor-
ded longer wavelength absorbing photomerocyanines relative to
relatively quickly but never fades completely under ambient labo-
ratory lighting to that of the initial colourless, freshly prepared
solutions. This latter feature is easily demonstrated by examining
the selected fading spectra (Fig. 5) for 14a; these spectra show
intense colour generation [spectrum 14a (1) recorded immediately
after cessation of activating radiation] with gradual decrease of the
their 3H-naphtho[1,2-b]pyran analogues [11];
a significantly
smaller bathochromic shift of w12 nm was noted upon replace-
ment of the diphenyl moiety with the spirocycle in the naphtho
[1,2-b]pyran isomers 14a,b with a decrease in the persistence of the
photoinduced colour (Fig. 4). A significant and limiting feature of all
of these new spirocyclic compounds is their residual solution
colour, i.e. after cessation of irradiation the intense colour fades
Table 2
Photochromic response of toluene solutions of naphthopyrans.
lmax (nm)
t (s)
½
12a
16a
12b
16b
12c
16c
14a
17a
14b
17b
464
428
497
468
457
424
496
482
482
472
24
40
81
136
22
46
107
202
62
104
Fig. 5. Selected absorption spectra of 14a recorded after cessation of irradiation.

D.L. Crossley et al. / Dyes and Pigments 95 (2012) 62e68
67
Ph
Ph
Ph
Ph
Ph
+
Ph
Ph
O
Ph
O
O
O
12a
18
trans,cis
19
trans,trans
20
Scheme 3. Photomerocyanines derived from the ring-opening of the spironaphthopyran 12a.
chemical technology. 5th ed., 6. Hoboken N.J.: Wiley & Sons; 2004. p. 614e31;
[c] Minkin VI . Photo-, thermo-, solvato-, and electro-chromic spiroheter-
ocyclic compounds. Chem Rev 2004;104:2751e76.
photogenerated colour [spectrum 14a (2) through to spectrum 14a
(9)] until the final spectrum [14a (10) recorded 30 min after
cessation of activating radiation] which retains an appreciable
absorption at 496 nm (Fig. 5).
[2] Aldoshin SM. Spiropyrans: structural features and photochemical properties.
Russ Chem Rev 1990;59:1144e78;
[a] Lukyanov BS, Lukyanova MB. Spiropyrans: synthesis, properties and appli-
cations. Chem Heterocycl Compd 2005;41:281e311;
The lifetime of the isomeric photomerocyanines derived from
the irradiation of a naphthopyran has been discussed with photo-
bleaching of the persistent isomer to the parent naphthopyran
reported [27]. Additionally, the structures of photomerocyanines
have been examined by NMR spectroscopy which revealed that the
most persistent isomer was that possessing a trans,trans geometry
[26e28] which is equivalent to 19 in this study, which arises from
photochemical isomerisation of 18 (trans,cis) (Scheme 3). A further
complication in this work may arise from a contribution from the
zwitterionic species 20 which may be favoured since the benzo-
tropylium species is stabilised through aromaticity. Further inves-
tigation of the structure and spectroscopic features including
photobleaching and degradation of this series of compounds is
ongoing.
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4. Conclusions
A series of novel spironaphthopyrans, spiro[naphthopyran-70H-
benzocyclohepta-50,80-dienes], derived from the reaction between
either a 1- or 2-naphthol and 6,8-diphenyl-7-ethynyl-7H-benzo-
cyclohepta-5.8-dien-7-ol are reported. The photochromic response
of these new spironaphthopyrans is characterised by a bath-
ochromically shifted lmax accompanied by more rapid fading than
their corresponding diphenyl substituted analogues. Unfortunately,
complete thermal fading to the pre-irradiated colourless state is not
observed with moderately intensely coloured solutions remaining;
[b] Mitronova GY, Belov VN, Bossi ML, Wurm CA, Meyer L, Medda R, et al. New
fluorinated rhodamines for optical microscopy and nanoscopy. Chem Eur J
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[8] Azizian F, Field AJ, Heron BM, Kilner C. Intrinsically thermochromic fluorans.
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[10] Coelho CJ, Carvalho LM, Abrantes S, Manuel OM, Oliveira-Campos AMF,
Samat A, et al. Tetrahedron 2002;58:9505e11;
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a
feature which limits the application of these interesting
compounds.
[b] Gabbutt CD, Heron BM, Kolla SB, Kilner C, Coles SJ, Horton PN, et al. Ring
contraction during the 6
p-electrocyclisation of naphthopyran valence tauto-
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Acknowledgement
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J Chem Soc; 1943:10e3.
[15] Breeze Bentley J, Everard KB, Marsden RJB, Sutton LE. Electric dipole moment
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The authors thank The Worshipful Company of Clothworkers’ of
the City of London for a millennium grant for the purchase of
a Bruker Avance NMR spectrophotometer and for an annual grant
for the purchase of a Cary 50 Probe spectrophotometer and asso-
ciated UVevisible irradiation sources. The EPSRC are thanked for
access to the National Mass Spectrometry Service (Swansea).
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