The intramolecular capture of thermally generated merocyanine dyes derived from naphthopyrans: Photochromism of 5-(diarylhydroxymethyl)-2H-naphtho[1,2-b] pyrans

Article abstract of DOI:10.1016/j.dyepig.2011.07.004

A series of 2H-naphtho[1,2-b]pyrans bearing a diarylmethanol unit at C-5 have been synthesised by the addition of an excess of an aryllithium reagent to alkyl 2H-naphtho[1,2-b]pyran-5-carboxylate precursors. These naphthopyrans show photochromism when exp

Full text of DOI:10.1016/j.dyepig.2011.07.004

Dyes and Pigments 92 (2012) 995e1004
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The intramolecular capture of thermally generated merocyanine dyes derived
from naphthopyrans: Photochromism of 5-(diarylhydroxymethyl)-2H-naphtho
[1,2-b]pyrans
*
Christopher D. Gabbutt, Lucy V. Gibbons, B. Mark Heron , Suresh B. Kolla
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 2H-naphtho[1,2-b]pyrans bearing a diarylmethanol unit at C-5 have been synthesised by the
addition of an excess of an aryllithium reagent to alkyl 2H-naphtho[1,2-b]pyran-5-carboxylate precur-
sors. These naphthopyrans show photochromism when exposed to ultraviolet irradiation and also
generate intense colours at low pH through triarylmethine cation generation. However, photochromism
of the triarylmethine cation derived from the naphthopyran unit could not be detected. An irreversible
cascade process initiated by the thermally-induced ring-opening of the diarylmethanol substituted
2H-naphtho[1,2-b]pyrans in the presence of an acid catalyst afforded novel benzopentalenonaph-
thalenone dyes.
Received 22 June 2011
Received in revised form
7 July 2011
Accepted 11 July 2011
Available online 22 July 2011
Keywords:
Synthesis
Photochromism
Ó 2011 Elsevier Ltd. All rights reserved.
Naphthopyran
Benzo[5,6]pentaleno[1,2-b]naphthalene
Triarylmethine cationic dye
Absorption
1. Introduction
2. Experimental
There has been a resurgence of interest in the design, synthesis
and properties of dye systems over the last decade or so. Of
particular note is the interest in probe and sensor systems [1], dyes/
sensitizers for dye sensitised solar cells [2] and organic conducting
and emitting systems [3]. A healthy interest has also been main-
tained in established functional dye systems, such as those used in
photodynamic therapy [4], bio-labelling [5] and photochromism
[6,7]. The control of various phenomena through the cycling
sequence of a photochromic unit [8] and the properties of hybrid
photochromic systems [9] have recently been reviewed.
Our interest has focussed upon cationic dyes whose colour can
be modulated by the variation of pH and also through the switching
of a photochromic naphthopyran unit [10]. In this study the cationic
centre was generated at the terminus of a thienyl group on the
3H-naphtho[2,1-b]pyran 1 and through variation of pH and irra-
diation switching between one colourless and three coloured states
was accomplished. We now report our full observations [11] on the
2H-naphtho[1,2-b]pyran system, wherein the cationic centre is
generated at C-5 adjacent to the naphthalene unit in close prox-
imity to the photomerocyanine moiety Scheme 1.
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 tetramethylsilane as an internal reference. FT-IR spectra were
recorded on a Perkin Elmer Spectrum One spectrophotometer
system equipped with a golden gate ATR attachment (neat sample).
UVevisible spectra were recorded for spectroscopic grade CH2Cl2
solutions of the samples (4 min activation with UV irradiation,
10 mm pathlength quartz fluorescence cuvette, PTFE capped, ca.
1 ꢀ10ꢁ4e10ꢁ6 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 Watt xenon arc lamp source (Newport 66906), xenon
ozone free arc lamp (Newport 6255), distilled water liquid filter
(Newport 6177), multiple filter holder (Newport 62020), UG11 filter
(Newport FSO-UG11), fibre optic coupler (Newport 77799) and
liquid light guide (Newport 77557). All compounds with the
exception of 4c 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. Ethyl 9-methoxy-2,
* Corresponding author. Tel.: þ44 (0) 113 3432925; fax: þ44 (0) 113 3436565.
E-mail address: b.m.heron@leeds.ac.uk (B. M. Heron).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.07.004

996
C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 995e1004
Scheme 1. Colour modulation of naphthopyran 1 through changes in pH and irradiation.
2-bis(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran-5-carboxylate
2a was obtained according to the literature procedure in 72% yield
and was identical in all respects to authentic material [12]; lmax after
irradiation ¼ 514, 432 nm (CH2Cl2), dH (CDCl3) 1.42 (3H, t, J ¼ 7.1 Hz,
CH2CH3), 3.76 (6H, s, OMe), 3.94 (3H, s, 9-OMe), 4.38 (2H,
q, J ¼ 7.1 Hz, CH2CH3), 6.15 (1H, d, J ¼ 10.0 Hz, 3-H), 6.81 (4H,
m, AreH), 7.14 (1H, dd, J ¼ 8.9, 2.6 Hz, 8-H), 7.41 (4H, m, AreH), 7.56
(1H, d, J ¼ 2.6 Hz, 10-H), 7.67 (1H, d, J ¼ 8.9 Hz, 7-H), 7.69 (1H,
d, J ¼ 10.0 Hz, 4-H), 8.02 (1H, s, 6-H); dH (CD3CO2D) 1.26 (3H,
t, J ¼ 7.1 Hz, CH2CH3), 3.59 (6H, s, OMe), 3.80 (3H, s, 9-OMe), 4.24 (2H,
q, J ¼ 7.1 Hz, CH2CH3), 6.13 (1H, d, J ¼ 10.0 Hz, 3-H), 6.71 (4H, m,
AreH), 7.01 (1H, dd, J ¼ 8.9, 2.6 Hz, 8-H), 7.31 (4H, m, AreH), 7.51 (1H,
d, J ¼ 2.6 Hz, 10-H), 7.52 (1H, d, J ¼ 10.0 Hz, 4-H), 7.59 (1H, d,
J ¼ 8.9 Hz, 7-H), 7.90 (1H, s, 6-H).
dC 14.68, 40.74, 55.81, 61.02, 63.76, 82.83, 100.91, 112.08, 115.85,
119.73, 121.51, 122.61, 124.47, 128.33, 128.52, 129.39, 130.70, 133.44,
148.17, 149.99, 159.58, 167.56. Found [M þ H]þ ¼ 523.2591,
C33H34N2O4 requires [M þ H]þ ¼ 523.2587.
2.3. Preparation of ethyl 2-(4-dimethylaminophenyl)-9-methoxy-2-
(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran-5-carboxylate 2c
A suspension of ethyl 4-hydroxy-6-methoxynaphthalene-2-
carboxylate 3.0 g (12.2 mmol), 1-(4-dimethylaminophenyl)-1-
(4-methoxyphenyl)prop-2-yn-1-ol 3.42 g (12.2 mmol) and acidic
alumina (6 g) in toluene (120 mL) were heated under reflux until TLC
examination of the mixture indicated that no propynol remained.
The cooled solution was filtered and the spent acidic alumina
catalyst was washed with toluene (2 ꢀ 30 mL). Removal of the
combined toluene washings and reaction solvent gave a purple gum
that was eluted from silica with 5% ethyl acetate in toluene to afford
the title compound as a pale blue microcrystalline solid (4.72 g) 76%,
mp ¼ 140e142 ꢂC, nmax 1696.0, 1607.4, 1508.4, 1287.1, 1209.6,1190.7,
2.2. Preparation of ethyl 2,2-bis(4-dimethylaminophenyl)-9-
methoxy-2H-naphtho[1,2-b]pyran-5-carboxylate 2b
A suspension of ethyl 4-hydroxy-6-methoxynaphthalene-2-
carboxylate 3.0 g (12.2 mmol), 1,1-bis(4-dimethylaminophenyl)
1172.7, 1032.0, 995.4, 812.9 cmꢁ1
, lmax after irradiation 570, 476 (sh)
nm (CH2Cl2), dH 1.42 (3H, t, J ¼ 7.1 Hz, CH2CH3), 2.91 (6H, s, NMe2),
3.76 (3H, s, OMe), 3.94 (3H, s, OMe), 4.80 (2H, q, J ¼ 7.1 Hz, CH2CH3),
6.15 (1H, d, J ¼ 10.1 Hz, 3-H), 6.63 (2H, m, AreH), 6.82 (2H, m, AreH),
7.10 (1H, dd, J ¼ 8.9, 2.5 Hz, 8-H), 7.33 (2H, m, AreH), 7.42 (2H, m,
AreH), 7.57 (1H, d, J ¼ 2.5 Hz,10-H), 7.64 (1H, d, J ¼ 10.1 Hz, 4-H), 7.67
(1H, d, J ¼ 8.9 Hz, 7-H), 8.00 (1H, s, 6-H); dC 14.59, 40.60, 55.39, 55.72,
60.97, 82.52, 100.78, 111.98, 113.52, 115.76, 119.70, 121.81, 122.48,
124.58, 128.24, 128.39, 128.48, 128.94, 130.67, 132.90, 137.78, 147.90,
149.99,158.97,159.57,167.40. Found [M þ H]þ ¼ 510.2264C32H31NO5
requires [M þ H]þ ¼ 510.2275.
prop-2-yn-1-ol 3.60
g (12.2 mmol) and Sasol Pural MG-30
(magnesium aluminium hydroxy carbonate, 30% Mg content on
silica) (10 g) in toluene (120 mL) were heated under reflux until TLC
examination of the mixture indicated that no propynol remained.
The cooled solution was filtered and the spent MG-30 catalyst was
washed with toluene (2 ꢀ 30 mL). Removal of the combined
toluene washings and reaction solvent gave a dark brown gum that
was eluted from silica with 2% ethyl acetate in toluene to afford the
title compound 2b as colourless microcrystals (3.85 g), 60.5%,
mp ¼ 185e186 ꢂC, nmax 1694.1, 1607.0, 1513.1, 1435.8, 1350.4, 1287.8,
1194.6, 1167.2, 997.0, 809.9 cmꢁ1
,
lmax after irradiation ¼ 614,
472 nm (CH2Cl2), dH 1.41 (3H, t, J ¼ 7.1 Hz, CH2CH3), 2.90 (12H, s,
NMe2), 3.93 (3H, s, OMe), 4.37 (2H, q, J ¼ 7.1 Hz, CH2CH3), 6.15 (1H,
d, J ¼ 10.1 Hz, 3-H), 6.34 (4H, m, AreH), 7.09 (1H, dd, J ¼ 8.9, 2.5 Hz,
8-H), 7.34 (4H, m, AreH), 7.58 (1H, d, J ¼ 2.5 Hz, 10-H), 7.62 (1H, d,
J ¼ 10.1 Hz, 4-H), 7.66 (1H, d, J ¼ 8.9 Hz, 7-H), 7.98 (1H, s, 6-H),
2.4. General method for the preparation of the 5-[1,1-bis
(4-methoxyphenyl)-1-hydroxymethyl] substituted naphthopyrans
n-Butyllithium 10.1 mL (16.1 mmol,1.6 M in hexanes) was added
via syringe to a cold (w ꢁ70 ꢂC) stirred solution of 4-bromoanisole

C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 995e1004
997
3.01 g (16.1 mmol) in anhydrous THF (60 mL) under nitrogen. After
2 h the naphthopyran (4 mmol) was added in a single portion and
the solution was allowed to stir and warm to rt overnight. The
reaction mixture was quenched with water (30 mL) and the layers
separated. The aqueous phase was extracted with EtOAc
(2 ꢀ 30 mL) and the combined organic layers were washed with
water (50 mL), dried (anhyd. Na2SO4) and evaporated to afford the
crude product as an off-white powder which was purified by
column chromatography. The following compounds were obtained
in this manner.
(3H, s, OMe), 3.82 (6H, s, OMe), 3.92 (3H, s, OMe), 5.78 (1H, d,
J ¼ 10.0 Hz, 3-H), 6.62 (2H, m, AreH), 6.67 (1H, s, 6-H), 6.82 (7H, m,
AreH, 4-H), 7.01 (1H, dd, J ¼ 8.9, 2.4 Hz, 8-H), 7.13 (4H, m, AreH), 7.27
(2H, m, AreH),7.34 (2H, m, AreH), 7.36 (1H, d, J ¼ 8.9 Hz, 7-H), 7.57
(1H, d, J ¼ 2.4 Hz, 10-H), dC 40.35, 40.47, 55.25, 55.30, 55.37, 55.52,
81.77, 82.11, 91.84, 99.79, 100.64, 111.74, 113.16, 113.27, 113.57, 113.91,
114.37, 116.22, 118.74, 119.07, 121.46, 122.85, 123.47, 125.08, 126.01,
126.99, 128.01, 128.09, 128.33, 128.72, 128.99, 129.24, 129.44, 129.73,
130.47, 131.07, 131.36, 135.95, 138.03, 138.12, 139.35, 143.14, 144.67,
148.53, 149.72, 150.60, 157.30, 158.03, 158.51, 158.69, 158.90, 159.46.
Found [M þ H]þ ¼ 680.3002C44H41NO6 requires [M þ H]þ
¼
2.4.1. 9-Methoxy-5-[1,1-bis(4-methoxyphenyl)-1-hydroxymethyl]-
2,2-bis(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran 3a
680.3007.
9-Methoxy-5-[1,1-bis(4-methoxyphenyl)-1-hydroxymethyl]-2,2-
bis(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran 3a as colourless
microcrystals (2.13 g) 79% after elution from silica with 40% EtOAc in
hexane and recrystallisation from acetone and methanol,
mp ¼ 181e182 ꢂC, nmax 3446.8, 1607.1, 1504.9, 1297.7, 1246.7, 1171.9,
2.5. General method for the intramolecular capture of the
merocyanine form of a naphthopyran
A solution of the 5-[1,1-bis(4-methoxyphenyl)-1-hydroxymethyl]-
2H-naphtho[1,2-b]pyran (0.75 mmol) in toluene (30 mL) containing
a catalytic amount of 4-TsOH 20 mg (0.1 mmol) was heated until TLC
examination of the reaction mixture indicated that no naphthopyran
remained (see individual examples for temperature and time). The
cooled solution was diluted with EtOAc (20 mL) and the mixture
washed withwater(2 ꢀ 30mL). Removal of the dried (Na2SO4)solvent
gave a deep purple solid that was eluted from silica. The following
compounds were obtained in this fashion.
1032.3, 944.4, 824.9, 585.9 cmꢁ1
,
lmax after irradiation ¼ 514, 429 nm
(CH2Cl2) then upon addition of acid lmax ¼ 498 nm,
3
¼ 7.87 ꢀ 104
max
molꢁ1 dm3 cmꢁ1
(CH2Cl2 þ 2
,
lmax ¼ 648 nm,
3
¼ 1.26 ꢀ 104 molꢁ1 dm3 cmꢁ1
max
mL MeSO3H), dH 2.96 (1H, s, OH), 3.76 (6H, s, OMe), 3.82
(6H, s, OMe), 3.92 (3H, s, OMe), 5.78 (1H, d, J ¼ 10.1 Hz, 3-H), 6.68 (1H,
s, 6-H), 6.82 (8H, m, AreH), 6.89 (1H, d, J ¼ 10.1 Hz, 4-H), 7.03 (1H, dd,
J ¼ 9.2, 2.4 Hz,8-H), 7.14 (4H, m, AreH), 7.32(4H, m, AreH), 7.39 (1H, d,
J ¼ 9.2 Hz, 7-H), 7.56 (1H, d, J ¼ 2.4 Hz, 10-H), dC 55.47, 55.51, 55.76,
81.80, 82.36, 100.86, 113.44, 113.55, 116.52, 119.02, 121.86, 124.10,
126.20, 126.84, 128.28, 128.57, 129.24, 130.04, 137.53, 138.29, 139.52,
148.58,158.35,158.80,159.07. Found [M þ Na]þ ¼ 689.2503C43H38O7
requires [M þ Na]þ ¼ 689.2510.
2.5.1. 3,7-Dimethoxy-11,11,12-tris(4-methoxyphenyl)-11H-benzo
[5,6]pentaleno[1,2-b]naphthalene-5-one 4a
3,7-Dimethoxy-11,11,12-tris(4-methoxyphenyl)-11H-benzo[5,6]
pentaleno[1,2-b]naphthalene-5-one 4a from heating 3a at 60 ꢂC for
20 h, as deep purple microcrystals 0.37 g (76%) after elution from
silica with 5% ethyl acetate in toluene and recrystallisation from
acetone and methanol, mp ¼ 252e255 ꢂC, nmax 3002.7, 2949.2,
2831.4, 1583.4, 1505.3, 1491.4, 1244.1, 1222.9, 1175.1, 1030.5, 824.8,
2.4.2. 2,2-Bis(4-dimethylaminophenyl)-9-methoxy-5-[1,1-bis
(4-methoxyphenyl)-1-hydroxymethyl]-2H-naphtho[1,2-b]pyran 3b
2,2-Bis(4-dimethylaminophenyl)-9-methoxy-5-[1,1-bis(4-
methoxyphenyl)-1-hydroxymethyl]-2H-naphtho[1,2-b]pyran 3b as
pale green microcrystals (1.64 g), 59% after elution from silica with
10% ethyl acetate in toluene and recrystallisation from CH2Cl2 and
hexane, mp ¼ 136e138 ꢂC, nmax 3489.9, 1604.5, 1506.4, 1348.7,
787.2, 551.9 cmꢁ1
dm3 cmꢁ1
lmax ¼ 376 nm,
,
lmax ¼ 586 nm,
3
¼ 1.06 ꢀ 104 molꢁ1
max
,
3
¼ 1.59 ꢀ 104 molꢁ1 dm3 cmꢁ1
max
(CH2Cl2), dH 3.75 (6H, s, OMe), 3.78 (3H, s, OMe), 3.92 (3H, s, OMe),
3.95 (3H, s, OMe), 6.42 (1H, s, 10-H), 6.71 (6H, m, AreH), 6.75 (1H,
dd, J ¼ 8.2, 2.6 Hz, 2-H), 6.95 (3H, m, AreH), 7.11 (1H, d, J ¼ 8.4,
2.8 Hz, 8-H), 7.19 (5H, m, AreH), 7.80 (1H, d, J ¼ 2.8 Hz, 6-H), 8.51
(1H, d, J ¼ 2.6 Hz, 4-H), dC 55.66, 56.13, 56.35, 58.66, 110.15, 113.72,
113.94, 115.54, 116.33, 120.66, 122.28, 122.80, 126.58, 130.17, 130.21,
130.56, 131.93, 131.99, 132.16, 133.21, 136.52, 140.85, 144.10, 150.53,
156.95, 158.65, 159.50, 159.91, 160.34, 160.46, 184.16. Found
[M þ H]þ ¼ 647.2419C43H34O6 requires [M þ H]þ ¼ 647.2428.
1245.0, 1169.4, 1030.9, 996.5, 816.2, 547.2 cmꢁ1
, lmax after
irradiation ¼ 615, 495 nm (CH2Cl2) then upon addition of acid lmax
¼
476 nm,
3
¼ 5.11 ꢀ 104 molꢁ1 dm3 cmꢁ1
,
lmax ¼ 568 nm,
max
3
¼ 1.72 ꢀ 104 molꢁ1 dm3 cmꢁ1 (CH2Cl2 þ 2
mL MeSO3H), dH 2.96
max
(6H, s, NMe2), 2.99 (6H, s, NMe2), 3.77 (3H, s, OMe), 3.78 (3H, s, OMe),
3.92 (3H, s, OMe), 5.69 (1H, s, OH), 5.87 (1H, d, J ¼ 10.1 Hz, 3-H), 6.12
(1H, d, J ¼ 10.1 Hz, 4-H), 6.63 (2H, m, AreH), 6.74 (2H, m, AreH), 6.81
(4H, m, AreH), 7.09 (1H, dd, J ¼ 8.9, 2.5 Hz, 8-H), 7.10 (1H, s, 6-H), 7.23
(2H, m, AreH), 7.29 (2H, m, AreH), 7.37 (4H, m, AreH), 7.44 (1H, d,
J ¼ 2.5 Hz, 10-H), 7.63 (1H, d, J ¼ 8.9 Hz, 7-H), dC 40.44, 55.23, 55.26,
55.37, 77.53, 91.80, 99.87,111.86,112.08,113.14,113.26,114.26,119.00,
121.36, 123.02, 125.09, 128.81, 129.15, 129.23, 129.41, 130.46, 131.17,
136.21, 138.27, 143.15, 144.76, 149.02, 157.25, 158.71, 158.87. Found
[M þ H]þ ¼ 693.3320C45H44N2O5 requires [M þ H]þ ¼ 693.3323.
2.5.2. 3-Dimethylamino-12-(4-dimethylaminophenyl)-7-methoxy-
11,11-bis(4-methoxyphenyl)-11H-benzo[5,6]pentaleno[1,2-b]
naphthalene-5-one 4b
3-Dimethylamino-12-(4-dimethylaminophenyl)-7-methoxy-
11,11-bis(4-methoxyphenyl)-11H-benzo[5,6]pentaleno[1,2-b]
naphthalene-5-one 4b, from heating 3b at 100 ꢂC for 48 h, as deep
purple microcrystals 0.29 g (58%) after elution from silica with 10%
ethyl acetate, 10% hexane in toluene and recrystallisation from
acetone and methanol, mp ¼ 266e269 ꢂC, nmax 2901.7, 2833.8,
1600.2, 1586.6, 1504.9, 1489.4, 1243.8, 1430.8, 1284.1, 1243.8, 1178.1,
2.4.3. 2-(4-Dimethylaminophenyl)-9-methoxy-2-(4-methoxyphenyl)-
5-[1,1-bis(4-methoxyphenyl)-1-hydroxymethyl]-2H-naphtho[1,2-b]
pyran 3c
2-(4-Dimethylaminophenyl)-9-methoxy-2-(4-methoxyphenyl)-
5-[1,1-bis(4-methoxyphenyl)-1-hydroxymethyl]-2H-naphtho[1,2-b]
pyran 3c as colourless microcrystals (1.82 g) 67% after elution from
silica with 25% EtOAc in hexane and recrystallisation from Et2O and
hexane, m.p. 189e190 ꢂC, nmax 3517.5, 1607.2, 1506.1, 1243.8, 1173.1,
1033.1, 832.9, 817.1, 801.2 cmꢁ1
molꢁ1 dm3 cmꢁ1
lmax ¼ 370 nm,
,
lmax ¼ 545 nm,
3
¼ 1.61 ꢀ 104
,
3
¼ 1.04 ꢀ 104 mmaoxlꢁ1 dm3 cmꢁ1
max
(CH2Cl2), dH 2.95 (6H, s, NMe2), 3.12 (6H, s, NMe2), 3.75 (6H, s, OMe),
3.92 (3H, s, OMe), 6.41 (1H, s, 10-H), 6.45 (1H, dd, J ¼ 8.5, 2.5 Hz, 2-
H), 6.49 (2H, m, AreH), 6.70 (4H, m, AreH), 7.01 (2H, m, AreH), 7.05
(1H, d, J ¼ 8.5 Hz, 1-H), 7.08 (1H, dd, J ¼ 8.5, 2.5 Hz, 8-H), 7.21 (1H, d,
J ¼ 8.5 Hz, 9-H), 7.25 (4H, m, AreH), 7.82 (1H, d, J ¼ 2.5 Hz, 6-H),
8.53 (1H, d, J ¼ 2.5 Hz, 4-H), dC 40.24, 41.01, 55.21, 55.69, 58.44,
109.32, 111.42, 111.61, 113.17, 115.28, 118.64, 121.59, 121.99, 123.19,
1019.7, 823.8 cmꢁ1
,
lmax ¼ after 30 s of irradiation 547, 462(sh) nm
(CH2Cl2) then upon addition of acid lmax ¼ 478 nm,
3
¼ 6.30 ꢀ 104
max
molꢁ1 dm3 cmꢁ1 lmax ¼ 582 nm,
3
¼ 1.90 ꢀ 104 molꢁ1 dm3 cmꢁ1
max
(CH2Cl2 þ 2
mL MeSO3H), dH 2.90, (1H, s, OH), 2.92 (6H, s, NMe2), 3.76

998
C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 995e1004
OH
Ar2
O
Ar2
MeO
Ar1
1
Ar1
3
O
CO2Et
UV
rt
acidic Al2O3
PhMe, reflux
10
MeO
MeO
4
+
8
CO2Et
CO2Et
OH
Ar2
6
colourless
Ar1 = Ar2 = 4-MeOC6H4
coloured
,
2a' 2b' 2c'
Ar1
,
2a
2b
2c
Ar1 = Ar2 = 4-Me2NC6H4
Ar1 = 4-Me2NC6H4, Ar2 = 4-MeOC6H4
Scheme 2. Preparation and Photochromic Response of naphthopyrans 2.
129.54, 129.79, 129.88, 130.11, 131.92, 132.38, 132.97, 136.52, 138.16,
143.33, 147.46, 150.23, 150.81, 157.49, 158.04, 158.69, 162.82, 183.11.
158.93, 159.90, 183.53. Found [M þ H]þ ¼ 660.2740C44H37NO5
requires [M þ H]þ ¼ 660.2744.
Found [M þ H]þ ¼ 673.3054C45H40N2O4 requires [M þ H]þ
¼
673.3061.
3. Discussion
2.5.3. 3-Dimethylamino-7-methoxy-11,11,12-tris(4-
methoxyphenyl)-11H-benzo[5,6]pentaleno[1,2-b]naphthalene-5-
one 4c
Alkyl 2,2-diaryl-2H-naphtho[1,2-b]pyran-5-carboxylates 2aec
were readily prepared in good yield using the established acid-
catalysed route from a 1,1-diarylprop-2-yn-1-ol and an alkyl
4-hydroxynaphthalene-2-carboxylate (Scheme 2) [12,13]. The 1H
Elution from silica with 5% EtOAc in PhMe of the crude reaction
mixture from 5-[1,1-bis(4-methoxyphenyl)-1-hydroxymethyl]-2H-
naphtho[1,2-b]pyran 3c after heating at 60 ꢂC for 30 h, gave two
fractions: Fraction 1, 3-dimethylamino-7-methoxy-11,11,12-tris
(4-methoxyphenyl)-11H-benzo[5,6]pentaleno[1,2-b]naphthalene-
5-one 4c as a deep purple powder 150 mg (30%)1 after recrystalli-
sation from acetone and methanol, nmax 2933.9, 2832.5, 1599.5,
1573.6, 1504.5, 1488.5, 1433.1, 1286.8, 1242.7, 1172.9, 1087.1, 1027.2,
NMR spectra of 2aec were characterised by a doublet at ca.
with 10 Hz coupling which is assigned to 3-H and which is coupled
to 4-H (ca. 7.6). A low field singlet was also noted at ca. 8.0 (6-H)
d 6.1
d
d
due to the peri disposition of this proton and the adjacent electron
withdrawing ester function.
The photochromic response of these naphthopyrans 2 in CH2Cl2
solution upon UV irradiation (4 min) to a steady state was charac-
terised by the generation of two overlapping absorption bands
which were typical for the generation of photomerocyanines 20. The
long wavelength absorption maxima followed the expected trend in
accord with terminal aryl ring donor strength [7] with that of 2b
appearing at 614 nm (Ar1 ¼ Ar2 ¼ 4-Me2NC6H4), 2c at 570 nm
(Ar1 ¼4-Me2NC6H4, Ar2 ¼ 4-MeOC6H4) and 2a at 514 nm associated
with the least electron donating aryl group (Ar1 ¼ Ar2 ¼ 4-MeOC6H4)
(Fig. 1). The persistence of the photomerocyanine was also in
agreement with the increasing electron donating character of the
geminal aryl unit with the photomerocyanine derived from 2b
fading most rapidly and that derived from 2a relatively slowly; 2c
exhibited an intermediate lifetime.
805.3, 789.2, 643.9, 628.8, 523.2 cmꢁ1
,
green solution,
lmax ¼ 426 nm,
lmax ¼ 390 nm,
3
¼ 1.2 ꢀ 104 molꢁ1 dm3 cmꢁ1
,
max
3
¼ 9.3 ꢀ 103 molꢁ1 dm3 cmꢁ1 and lmax ¼ 571 nm,
3
max
¼ 7.8 ꢀ
max
103 molꢁ1 dm3 cmꢁ1 (CH2Cl2), dH 3.14 (6H, s, NMe2), 3.78 (6H, s,
OMe), 3.81 (3H, s, OMe), 3.95 (3H, s, OMe), 6.44 (1H, s, 10-H), 6.48
(1H, dd, J ¼ 8.3, 2.3, Hz, 2-H), 6.72 (5H, m, AreH), 6.95 (1H, d,
J ¼ 8.3 Hz,1-H), 6.99 (2H, m, AreH), 7.12 (1H, dd, J ¼ 8.5, 2.7 Hz, 8-H)
7.24 (6H, m, AreH), 7.84 (1H, d, J ¼ 2.7 Hz, 6-H), 8.53 (1H, d, J ¼ 2.3 Hz,
4-H), dC 41.41, 55.62, 56.10, 58.62, 109.91, 112.23, 113.61, 113.80,
115.81, 119.70, 122.10, 123.18, 126.94, 130.05, 130.53, 131.03, 132.23,
133.27, 136.76, 138.79, 142.51, 149.02, 151.22, 157.38, 158.52, 159.25,
159.80, 183.78; and Fraction 2, 12-(4-dimethylaminophenyl)-3,7-
dimethoxy-11,11,bis(4-methoxyphenyl)-11H-benzo[5,6]pentaleno
[1,2-b]naphthalene-5-one 4d as deep purple microcrystals 100 mg
(20%) after recrystallisation from acetone and methanol,
mp ¼ 278e280 ꢂC, nmax 3003.6, 2931.9, 2833.5, 1582.8, 1554.9,
1505.0, 1490.5, 1427.1, 1243.6,1175.7, 1025.7, 815.8, 524.3 cmꢁ1, blue
The addition of an excess of 4-methoxyphenyllithium to 2aec at
low temperature gave the diarylmethanols 3aec in good yield
(59e79%) (Scheme 3). The 1H NMR spectra of these compounds
displayed a readily identifiable doublet at
d
5.8 (J ¼ 10 Hz) for the
solution, lmax ¼ 369 nm,
3
¼ 1.0 ꢀ 104 molꢁ1 dm3 cmꢁ1 and
max
lmax ¼ 642 nm (broad with shoulder at 499 nm),
3
¼ 1.2 ꢀ 104
max
molꢁ1 dm3 cmꢁ1 (CH2Cl2), dH 2.95 (6H, s, NMe2), 3.75 (6H, s, OMe),
3.92 (3H, s, OMe), 3.95 (3H, s, OMe), 6.41 (1H, s, 10-H), 6.50 (2H, m,
AreH), 6.71 (4H, m, AreH), 6.76 (1H, dd, J ¼ 8.2, 2.6 Hz, 2-H), 6.99
(2H, m, AreH), 7.09 (1H, dd, J ¼ 8.4, 2.8, Hz, 8-H), 7.10 (1H, d,
J ¼ 8.2 Hz,1-H), 7.20 (1H, d, J ¼ 8.4 Hz, 9-H), 7.24 (4H, m, AreH), 7.80
(1H, d, J ¼ 2.8 Hz, 6-H), 8.53 (1H, d, J ¼ 2.6 Hz, 4-H), dC 40.25, 55.24,
55.71, 55.93, 58.46, 109.55, 111.51, 113.27, 114.92, 115.68, 119.60,
121.77, 122.76, 126.11, 129.69, 129.86, 130.09, 130.67, 131.83, 132.13,
132.90, 136.29, 141.60, 143.49, 148.86, 150.20, 150.29, 157.03, 158.16,
1
It was not possible to isolate analytically pure material by column chroma-
tography due to the poor resolution between 4c and 4d in a variety of solvent
systems. The sample of 4c isolated contains w3% of the isomer 4d by 1H NMR
spectroscopy. All spectroscopic analyses were undertaken on the mixture.
Fig. 1. Absorption spectra for irradiated naphthopyrans 2aec.

C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 995e1004
999
Ar2
O
Ar1
Ar2
O
Ar1
pyran ring proton, 3-H [7]. 6-H now resonates much further upfield
in the range 6.6e7.1 relative to that of 6-H (w 8.0) in 2 as
d
d
H+
a consequence of conversion of the ester function into a diary-
lmethanol unit. The presence of the hydroxyl unit was confirmed by
IR spectroscopy which displayed a broad stretching band at ca.
3480 cmꢁ1. The successful addition of 4-methoxyphenyllithium to
2aec was further confirmed by the absence of an ester CO signal in
MeO
MeO
Ar3
OH
Ar3
Ar3
Ar3
3a''
3a
the 13C NMR spectra of 3aec which appeared at w
d 167 in the
UV
UV
precursors.
The electrocyclic ring-opening of the diarylmethanol
substituted naphthopyran 3a to afford merocyanine 3a0 (Scheme 4)
in CH2Cl2 solution (ca. 1 ꢀ10ꢁ5 mol dmꢁ3) through irradiation with
ultraviolet light (275e375 nm) for 4 min resulted in the generation
of a red solution with lmax 514 nm which faded gradually to col-
ourless upon cessation of irradiation (Fig. 2). Treatment of the
Ar2
Ar2
O
Ar1
O
Ar1
MeO
MeO
H+
Ar3
OH
Ar3
Ar3
colourless solution of 3a in CH2Cl2 with MeSO3H (98%, 2 mL injected
Ar3
3a'''
3a'
directly into 10 mm pathlength cuvette) resulted in the develop-
ment of an intense violet colour derived from the cationic dye 3a00
which displayed two new bands in the absorption spectrum that
Scheme 4. Proposed relationship between merocyanine and cationic species derived
from 3a.
could only be measured after dilution to ca. 1 ꢀ 10ꢁ6 mol dmꢁ3
.
These
new
bands
had
lmax
¼
498
nm,
¼ 7.87 ꢀ 104 molꢁ1 dm3 cmꢁ1 and lmax ¼ 648 nm,
(Scheme 4). The 2:1 ratio of 3a:3a00 was also reflected in the NMR
signals assigned to the MeO groups in the d 3.5e4.1 region. In CDCl3
solution 3a exhibits three signals accounting for two sets of
equivalent MeO groups and the 9-MeO group. However, in
CD3CO2D solution there are 6 signals, presumably the three original
signals (shifted slightly due to the change in solvent) and three
further signals associated with the MeO groups in 3a00. The 1H NMR
3
max
3
¼ 1.26 ꢀ 104 molꢁ1 dm3 cmꢁ1 and may be considered typical
max
for unsymmetrically substituted triarylmethine cationic dyes
[10,14]. In marked contrast to the behaviour of 1, UV irradiation of
this acidified solution of 3a resulted in no appreciable change in the
absorption spectrum (Fig. 2).
In comparison with the thienyl substituted naphthopyran 1
where different shades could be reversibly generated by either
irradiation or treatment with acid or a combination of both stimuli
[10] the present compound 3a is insensitive to irradiation after
protonation. This behaviour suggests that the absorption spectrum
results from either a combination of pyran ring-opening and cation
generation upon protonation to afford 3a000 or that once the cation
3a00 is formed its resonance stabilisation with disruption of the
spectrum of 3a in CDCl3 to which had been added 1 mL of MeSO3H
was complex and further examination of the 1H NMR spectra of this
compound in a variety of media both with and without added acid
is ongoing. Finally, the UVevisible absorption spectrum of an irra-
diated CH2Cl2 solution of 3a was recorded after the addition of
MeSO3H (98%, 2
mL). This spectrum was essentially identical to
those obtained for the addition of acid and the addition of acid
followed by irradiation further suggesting that species 3a000 is not
present under these conditions.
naphthalene
p-electron density, inhibits the electrocyclic ring-
opening of the pyran unit which would also lead to 3a000 (Scheme 4).
In order to establish which of the foregoing phenomena may
operate, the 1H NMR spectra of 2a and 3a in CD3CO2D solution were
examined. The 1H NMR spectrum of a colourless solution of 2a in
CD3CO2D was remarkably similar to the spectrum recorded in
CDCl3 with only minor solvent induced shifts noted. No low field
doublets associated with a merocyanine unit and exhibiting trans-
coupling [15] which could be attributed to the acid-induced ring-
opening of the pyran ring were apparent. The 1H NMR spectrum of
3a in CD3CO2D (deep purple solution) was more complex and
indicated a mixture of two pyran containing species in an
approximate 2:1 ratio after standing for 10 h at room temperature
Ultra-violet irradiation of a solution (ca. 1 ꢀ 10ꢁ5 mol dmꢁ3) of
3b led to the development of photomerocyanine 3b0 with a weak
blue colour, lmax 615 nm, which faded very rapidly, a feature that
was not wholly unexpected since it has been established that
a combination of strong electron donating groups at 2-C of
a naphtho[1,2-b]pyran combined with a bulky substituent at the 5-
position results in only transient photocolouration [7]. Subsequent
treatment of this solution with 2 mL of MeSO3H similarly resulted in
the generation of two new absorption bands with lmax ¼ 476 nm,
3
¼
¼
5.11
ꢀ
104 molꢁ1 dm3 cmꢁ1
,
lmax
¼
568 nm,
max
3
1.72 ꢀ 104 molꢁ1 dm3 cmꢁ1 (after dilution to ca.
max
1 ꢀ 10ꢁ6 mol dmꢁ3); further examination under UV irradiation
resulted in no discernable photochromism (Fig. 3), a feature
comparable with observations noted for 3a.
with signals at
d
5.68 (J ¼ 9.9 Hz) and
d
5.71 (J ¼ 10.1 Hz), each of
which is indicative of 3-H in a pyran ring [7,15]. It is likely that these
signals are due to 3-H in 3a and 3-H in the cationic species 3a00
Ar2
O
Ar1
Ar2
O
Ar1
1
3
10
4 eq. Ar3Li, THF
MeO
4
MeO
-78 oC to rt then aq NH4Cl
OH
Ar3
8
CO2Et
6
Ar3
Ar1 = Ar2 = 4-MeOC6H4
Ar1 = Ar2 = 4-Me2NC6H4
Ar1 = Ar2 = Ar3 = 4-MeOC6H4
2a
2b
2c
3a
3b
3c
Ar1 = Ar2 = 4-Me2NC6H4, Ar3 = 4-MeOC6H4
Ar1 = 4-Me2NC6H4, Ar2 = Ar3 = 4-MeOC6H4
Ar1 = 4-Me2NC6H4, Ar2 = 4-MeOC6H4
Scheme 3. Preparation of Substituted Naphthopyrans 3.

1000
C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 995e1004
stock solution with a simple TLC inspection lamp (Spectroline, 8
Watt, 365 nm) resulted in the instantaneous development of a red-
violet shade attributed to the formation of photomerocyanine 3c0. A
solution of 3c was next examined in the spectrophotometer under
continuous irradiation using the 150 Watt xenon lamp irradiation
system in which the irradiation beam is delivered perpendicularly
to the recording beam. Spectra of 3c were collected at regular
intervals (10 s) over 4 min. From a selection of the spectra (Fig. 4) it
is evident that 3c0 is generated rapidly with a maximum absorption
at 547 nm noted after ca. 30 s of irradiation. After this time
continued irradiation presumably results in the degradation of the
photomerocyanine with an attendant decrease in the 547 nm
absorption and the shoulder at 462 nm ultimately resulting in
a near colourless solution. The cause of this unexpected facile
photodegradation of 3c is presently under investigation. However,
our preliminary observations upon repeating the irradiation
sequence on a more concentrated solution of 3c in CDCl3 resulted in
the formation of a complex (1H NMR) reaction product.
Fig. 2. Absorption spectra of 3a before and after treatment with acid.
For comparative purposes a solution of 3c was also examined
upon treatment with acid in an identical manner to that reported for
3a, b. Protonation of 3c afforded the predictable intense absorption
bands with lmax ¼ 478 nm,
3
¼ 6.30 ꢀ 104 molꢁ1 dm3 cmꢁ1
,
max
lmax ¼ 582 nm,
3
¼ 1.90 ꢀ 104 molꢁ1 dm3 cmꢁ1 (after dilution to
max
ca. 1 ꢀ 10ꢁ6 mol dmꢁ3) (Fig. 5). Once again subsequent irradiation
(8 Watt) of this acidified solution resulted in no discernible colour
change.
We have recently communicated that on heating a toluene
solutions of 3a and b containing a catalytic amount of 4-TsOH, the
initial colour gradually intensified and 3a and b had been irreversibly
converted into the intensely violet benzopentalenonaphthalenones
4a and b respectively in good yield (Scheme 5); the pentacyclic
structure 4a was confirmed by X-ray crystallography [11].
It was of interest to explore the behaviour of the unsymmetri-
cally substituted naphthopyran 3c towards a similar acid-catalysed
thermal rearrangement since the opportunity exists for the
formation of a mixture of isomers. Thus heating 3c with 4-TsOH in
PhMe resulted in a complex reaction product from which two new
isomeric benzopentalenonaphthalenones 4c and 4d were isolated
by column chromatography. The structural characterisation of
these two new isomers was accomplished by comparison of key
signals in their 1H NMR spectra with those of 4a and b. In
Fig. 3. Absorption spectra of 3b before and after treatment with acid.
Finally, examination of the photochromic properties of a solu-
tion of 3c (CH2Cl2, ca. 1 ꢀ 10ꢁ5 mol dmꢁ3) in a similar manner to
that of 3a and b resulted in a quite unusual observation. Irradiation
of 3c for the standard 4 min period resulted in no apparent
photochromic response. Re-examination of a fresh sample of the
compound 4a 2-H, ortho to a methoxy group resonates at d 6.75 and
Fig. 4. Absorption spectra of 3c during UV irradiation.

C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 995e1004
1001
Fig. 5. Absorption spectra of 3c before and after treatment with acid.
R1
Ar2
O
Ar1
2
4
O
1
cat. 4-TsOH, PhMe, heat
MeO
6
MeO
12
Ar4
OH
Ar3
Ar3
Ar3
Ar3
9
11
Ar1 = Ar2 = Ar3 = 4-MeOC6H4
Ar3 = Ar4 = 4-MeOC6H4, R1 = OMe
3a
3b
3c
4a
4b
4c
4d
Ar1 = Ar2 = 4-Me2NC6H4, Ar3 = 4-MeOC6H4
Ar1 = 4-Me2NC6H4, Ar2 = Ar3 = 4-MeOC6H4
Ar3 = 4-MeOC6H4, Ar4 = 4-Me2NC6H4, R1 = NMe2
Ar3 = Ar4 = 4-MeOC6H4, R1 = NMe2
Ar3 = 4-MeOC6H4, Ar4 = 4-Me2NC6H4, R1 = OMe
Scheme 5. Acid-catalysed thermal rearrangement of naphthopyrans 3.
Fig. 6. 1H NMR chemical shifts of selected protons in 4aed.

1002
C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 995e1004
Me2N
OMe
NMe2
NMe2
O
O
MeO
H+, heat
O
MeO
MeO
Ar
OH
Ar
Ar
Ar
OMe
Ar
Ar
OMe
3c
-H+
Me2N
Me2N
NMe2
O
OH
H
O
MeO
H
H
MeO
MeO
Ar
Ar
Ar
Ar
Ar
Ar
OMe
OMe
5
OMe
6
Me2N
H
Me2N
O
O
H
MeO
MeO
Ar
Ar
Ar
Ar
4c
Scheme 6. Proposed mechanism for the formation of 4c.
OMe
OMe
in 4b 2-H, ortho to an NMe2 group resonates further upfield at
6.45. Thus 4c in which 2-H resonates at 6.48 was assigned to the
3-dimethylamino isomer and 4d in which 2-H resonates at 6.76
was assigned to the 3-methoxy compound (Fig. 6). The insensitivity
of the chemical shift of 4-H, which resonated at
d
8.5, to the elec-
d
d
tronic nature of the adjacent OMe/NMe2 group was noticeable;
presumably the chemical shift of 4-H is dominated by the proximal
CO group at C-5. Isomers 4c and d each displayed a low field signal
d
Fig. 7. Uv-visible absorption spectrum of 4a (ca. 1 ꢀ 10ꢁ4 mol dmꢁ3) in CH2Cl2.

C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 995e1004
1003
Fig. 8. Uv-visible absorption spectrum of 4b (ca. 1 ꢀ 10ꢁ4 mol dmꢁ3) in CH2Cl2.
Fig. 9. Uv-visible absorption spectrum of 4c (ca. 1 ꢀ 10ꢁ4 mol dmꢁ3) in CH2Cl2.
at w
d
183.6 assigned to the CO group which was confirmed by
4c (green) lmax ¼ 390 nm,
3
¼ 1.2 ꢀ 104 molꢁ1 dm3 cmꢁ1
,
infrared spectroscopy which revealed a CO stretching band at ca.
lmax ¼ 426 nm,
3
¼ 9.3 ꢀ103mmaxolꢁ1 dm3 cmꢁ1 and lmax ¼ 571 nm,
max
1590 cmꢁ1 indicative of the extended conjugation.
3
¼ 7.8 ꢀ 103 molꢁ1 dm3 cmꢁ1 (Fig. 9) and 4d (blue)
max
The formation of the isomers 4c and d may be rationalised by
initial intramolecular trapping of the bis(4-methoxyphenyl) stabi-
lised cation to afford a mixture of isomeric dienones 5. At this
juncture the formation of either 4c or 4d is determined since only
the aryl group which is in a cis-relationship with the enone moiety
can undergo intramolecular cyclisation in the presence of 4-TsOH to
afford the naphthol 6. Enoleketo tautomerism of 6 followed by
lmax ¼ 369 nm,
3
¼ 1.0 ꢀ 104 molꢁ1 dm3 cmꢁ1 and lmax ¼ 642 nm
max
aerial oxidation generations the stabilising extended conjugated
system of the product (Scheme 6).
p
The planar, highly conjugated pentalenones 4a and 4b exhibited
two significant absorption bands in CH2Cl2 solution with 4a
(turquoise) lmax ¼ 586 nm,
¼ 1.1 ꢀ 104 molꢁ1 dm3 cmꢁ1 and
lmax ¼ 376 nm,
3
¼ 1.6 ꢀm3 1a0x4 molꢁ1 dm3 cmꢁ1 (Fig. 7) and 4b
max
(violet) lmax ¼ 545 nm,
3
¼ 1.6 ꢀ 104 molꢁ1 dm3 cmꢁ1 and
max
lmax ¼ 370 nm,
3
¼ 1.0 ꢀ 104 molꢁ1 dm3 cmꢁ1 (Fig. 8).
max
However, more complex solution spectra (CH2Cl2), displaying
three broad bands were observed for the pentelenones 4c and
d derived from the unsymmetrically substituted precursor 3c, with
Fig. 10. Uv-visible absorption spectrum of 4d (ca. 1 ꢀ 10ꢁ5 mol dmꢁ3) in CH2Cl2.

1004
C.D. Gabbutt et al. / Dyes and Pigments 92 (2012) 995e1004
¼ 1.2 ꢀ 104 molꢁ1 dm3 cmꢁ1
dye-sensitized solar cells. Eur J Org Chem; 2009:2903e34;
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(broad with shoulder at 499 nm),
(Fig. 10).
3
max
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Readily accessible alkyl 2H-naphtho[1,2-b]pyran-5-carboxylates
2 afford diarylmethanol substituted naphthopyrans 3 upon reac-
tion with excess organolithium reagents. These naphthopyrans
exhibit photochromism through the reversible electrocyclic ring-
opening of the pyran unit. The addition of methanesulfonic acid
to solutions of 3 leads to the formation of intensely coloured
cationic dyes 300; 1H NMR studies of which indicate that the pyran
ring remains intact. However these cationic dye solutions no longer
exhibit photochromism.
The thermally generated merocyanine derived from the ring-
opening of the naphthopyran unit of 3c has been trapped with
a proximal cationic centre to afford a mixture of isomeric benzo-
pentalenonaphthalenones 4c, d which have been distinguished
using key 1H NMR chemical shifts. Further examination of the
syntheses of these unusual polycyclic dye systems is ongoing.
(c) Wu W, Liu Y, Zhu D.
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field-effect transistors: design, synthesis and application. Chem Soc Rev 2010;
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[6] In: Dürr H, Bouas-Laurent H, editors. Photochromism molecules and systems.
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[10] Gabbutt CD, Heron BM, Kolla SB, Mcgivern M. Two stage colour modulation of
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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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