The synthesis and properties of naphthopyran- boradiazaindacene conjugates

Article abstract of DOI:10.1016/j.dyepig.2012.01.002

Three new 3,3-diaryl-3H-naphtho[2,1-b]pyran-boradiazaindacene conjugates have been synthesised. The naphthopyran-boradiazaindacene conjugates exhibit a weaker photochromic response relative to the simple naphthopyrans with the photomerocyanines fading rel

Full text of DOI:10.1016/j.dyepig.2012.01.002

Dyes and Pigments 94 (2012) 175e182
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis and properties of naphthopyrane boradiazaindacene conjugates
*
Christopher D. Gabbutt, B. Mark Heron , Colin Kilner, 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:
Three new 3,3-diaryl-3H-naphtho[2,1-b]pyraneboradiazaindacene conjugates have been synthesised.
The naphthopyraneboradiazaindacene conjugates exhibit a weaker photochromic response relative to
the simple naphthopyrans with the photomerocyanines fading relatively quickly. Photochromic
switching of the naphthopyran unit results in a decrease in the fluorescence intensity for only the most
persistent photomerocyanine. A crystal structure shows that the units are essentially orthogonally
disposed and that the boradiazaindacene core is extensively delocalised.
Received 30 November 2011
Received in revised form
4 January 2012
Accepted 5 January 2012
Available online 10 January 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Photochromism
Fluorescence
Synthesis
3H-Naphtho[2,1-b]pyran
BODIPY
Crystal structure
1. Introduction
characterisation and photochromic response of such BODIPY-
naphthopyran conjugates (Scheme 1).
Fluorescent dyes containing the 4,4-difluoro-4-bora-3a,4a-
diaza-s-indacene core 1, commonly referred to as BODIPY dyes,
have been widely studied [1]. Their impressive fluorescence prop-
erties have resulted in a broad range of structural variants designed
to tailor absorption/emission wavelength [2], improve two-photon
absorption [3], enhance solid state emission [4], sensitivity to
analytes [5] and singlet oxygen generation [6], participate in energy
transfer systems [7], behave as pH probes [8] and as fluorescence
probes for catalyst evaluation in high-throughput screening [9] and
serve as sensitizers in dye sensitized solar cell assemblies [10].
There has recently been considerable activity in the fluorescence
switching of BODIPY dyes through the use of hydrogen bonded [11]
and covalently bonded photochromic systems including diary-
lethenes 2 [12], spiropyrans 3 [13] and oxazines 4 [14]. Our interests
are focussed on photochromic naphthopyrans [15] and we have
previously reported the colour modulation of a triarylmethine dye
through the photochromic cycling of a naphthopyran [16]. We
became curious as to whether a naphthopyran unit could be
coupled to a BODIPY dye and subsequently be used to modulate
the photophysical properties of such a coupled system. We now
report our preliminary results concerning the synthesis, structural
2. Experimental
2.1. Equipment
Unless otherwise stated, reagents were used as supplied by the
major chemical catalogue companies. NMR spectra were recorded
on
a
Bruker Avance 400 MHz spectrophotometer (1H NMR
400 MHz, 13C NMR 100 MHz) for sample solutions in CDCl3 with
tetramethylsilane as an internal reference. Melting points were
either determined in capillary tubes or obtained by differential
scanning calorimetry using a TA Instrument DSC 2010. The crystal
structure determination was carried out at 150 K on a Bruker-
Nonius Apex X8 diffractometer equipped with an Apex II CCD
detector and using graphite monochromated Mo-Ka radiation from
a FR591 rotating anode generator. The structure was solved by
direct methods and refined using SHELXL-97. FT-IR spectra were
recorded on a Perkin Elmer Spectrum One spectrophotometer
system equipped with a golden gate ATR attachment (neat sample).
Fluorescence spectra were recorded in 10 mm path length quartz
cuvettes in spectroscopic grade toluene using a Perkin Elmer LS55
fluorescence spectrometer and relative fluorescence quantum
yields (Ff ) were determined using an aqueous acidic, degassed
solution of quinine bisulfate (Ff ¼ 0.55 in 1 N H2SO4) [17].
UVevisible spectra were recorded for spectroscopic grade CH2Cl2
* 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 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.01.002

176
C.D. Gabbutt et al. / Dyes and Pigments 94 (2012) 175e182
F
F
F
F
F
F
8
1
7
N
N
S
S
B
3
5
F
F
1
N
B
N
B
F
F
N
N
F
F
2
F
B
N
NMe2
N
N
O2N
O
F
O
O
O
O
O
N
N
F
N
F
NO2
B
4
3
Scheme 1. The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene nucleus 1 and photochromic BODIPY dye systems 2e4.
solutions of the samples (10 mm path length quartz cuvette, PTFE
capped, concentration in the range 1.3e2.0 ꢀ 10ꢁ5 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). Spectra (350e750 nm) were recorded at 6 s intervals over
a 600 s period. All compounds were homogeneous by TLC [Merck
TLC aluminium sheets either silica gel 60 F254 (cat. No 105554)]
using a range of eluent systems of differing polarity and flash
column chromatography was performed on chromatography silica
gel (Fluorochem, 40e63 micron particle size distribution). Mass
spectra were recorded at the National EPSRC Mass Spectrometry
Service Centre, Swansea.
8.5 Hz, 4-H), 8.43 (1H, d, J 0.6 Hz, 1-H), 10.04 (1H, s, CHO), 10.35 (1H,
bs, OH).
2.3. Preparation of 6-[Bis-(3,5-dimethyl-1H-pyrrol-2-yl)methyl]-2-
naphthol 7
6-Hydroxy-2-naphthaldehyde 5 (1.5 g, 8.7 mmol) and 2,4-
dimethylpyrrole (1.65 g, 17.4 mmol) were dissolved in CH2Cl2
(400 mL) under a nitrogen atmosphere. Trifluoroacetic acid
(0.1 mL 0.87 mmol) was added and the solution was stirred at
room temperature (rt) for 3 h. The CH2Cl2 was removed under
vacuum and the crude brown gum was purified by flash chro-
matography (20:79:1 EtOAc:hexane:triethylamine) to afford the
title compound 7 as a pale brown solid (1.68 g) 56%, dH 1.84 (6H, s,
(5-CH3)2), 2.14 (6H, s, (3-CH3)2), 5.55 (1H, s, methine), 5.73 (2H, d,
J 2.6 Hz, pyrrole 4-H), 7.05 (1H, dd, J 8.8, 2.5 Hz, AreH), 7.10 (1H,
d, J 2.2 Hz, AreH), 7.25 (2H, m, AreH), 7.46 (2H, bs, pyrrole NH),
7.60 (1H, d, J 8.5 Hz, 4-H), 7.65 (1H, d, J 8.8 Hz, 8-H).This sample
was used directly for the preparation of 4,4-difluoro-1,3,5,7-
tetramethyl-8-(6-hydroxynaphth-2-yl)-4-bora-3a,4a-diaza-s-
indacene 6b.
2.2. Preparation of 6-hydroxy-2-naphthaldehyde 5
n-BuLi (29.4 mL, 47.1 mmol, 1.6 M solution in hexanes) was
added dropwise to a cold (ꢁ78 ꢂC) stirred solution of 6-bromo-2-
naphthol (5 g, 22.4 mmol) in anhydrous THF (80 mL), after
15 min anhydrous 1-formylpiperidine (7.5 mL, 67.2 mmol) was
added and the mixture was stirred for 15 min at ꢁ78 ꢂC and
allowed to gradually warm to room temperature. Water (150 mL)
was added and the mixture acidified (wpH 5) with conc. HCl. The
aqueous layer was extracted with ethyl acetate (3 ꢀ 50 mL) and the
combined organic layers were dried over anhyd. Na2SO4 and the
solvent removed under vacuum to afford the title compound 5
(3.13 g) 81% yield, mp 177e178 ꢂC (lit. mp 180 ꢂC [18]), dH
(d6-DMSO) 7.21 (2H, m, AreH), 7.79 (2H, m, AreH), 8.02 (1H, d, J
2.4. Preparation of 4,4-difluoro-1,3,5,7-tetramethyl-8-(6-
hydroxynaphth-2-yl)-4-bora-3a,4a-diaza-s-indacene 6b
Dichlorodicyanoquinone (DDQ) (1.82 g, 8.0 mmol) was added to
a stirred solution of 6-[bis-(3,5-dimethyl-1H-pyrrol-2-yl)-methyl]-
2-naphthol 7 (2.78 g, 8.1 mmol) in CH2Cl2 (150 mL). After 15 min at
rt triethylamine (30 mL, 280 mmol) and BF3.OEt2 (30 mL,
237 mmol) were added and the mixture was stirred at RT for 1 h.
The reaction mixture was washed with water (600 mL) and
aqueous NaOH (200 mL, 2 M aq.). The aqueous solution was back

C.D. Gabbutt et al. / Dyes and Pigments 94 (2012) 175e182
177
extracted with CH2Cl2 (2 ꢀ 50 mL), all of the CH2Cl2 extracts were
Other compounds prepared via this method:
combined, dried over anhyd. Na2SO4, filtered and evaporated. The
crude compound was purified by silica gel chromatography (10%
EtOAc in hexane) to afford the title compound 6b (0.91 g) 29% yield
as a bright orange solid, mp 263.8 ꢂC, nmax 3433.2, 2917.1, 1629.5,
1606.9, 1538.3, 1504.1, 1463.7, 1394.1, 1307.7, 1262.2, 1195.1, 1155.2,
2.5.2. 4,4-Difluoro-1,3,5,7-tetramethyl-8-[30-(2-fluorophenyl)-30-
(4-pyrrolidinophenyl)-30H-naphtho[2,1-b]pyran-80-yl]-4-bora-
3a,4a-diaza-s-indacene 9b
Compound 9b from 6b and 1-(2-fluorophenyl)-1-(4-pyrro-
lidinophenyl)prop-2-yn-1-ol 8b after elution from silica with 30%
EtOAc in hexane as orange microcrystals (71.8 mg) 28%, mp
255.4 ꢂC, lmax 488 nm, ε 80400 Mꢁ1 cmꢁ1 (CH2Cl2), lmax post-
928.4, 903.6, 800.8, 746.4, 608.0, 567.4, 470.6 cmꢁ1
, dH 1.31 (6H, s,
(CH3)2), 2.57 (6H, s, (CH3)2), 5.55 (1H, bs, OH), 5.97 (2H, s, pyrrole 4-
H), 7.16 (1H, dd, J 8.5, 2.5 Hz, 3-H), 7.22 (1H, d, J 2.4 Hz, 1-H), 7.31
(1H, dd, J 8.5, 2.5 Hz, 7-H), 7.69 (1H, bs, 5-H), 7.77 (1H, d, J 8.5 Hz,
4-H), 7.82 (1H, d, J 8.5 Hz, 8-H), dC 14.62, 93.34, 109.56, 118.68,
121.20, 126.25, 127.16, 127.47, 128.75, 129.97, 130.12, 131.75, 134.42,
141.76, 143.17, 154.24, 155.40, 204.37.Found M þ Hþ 391.1791.
C23H21ON2BF2 requires M þ Hþ 391.1788.
irradiation 580 nm with t ¼ 8 s (CH2Cl2), labs 507 nm, lem
½
514 nm, Ff 0.44 (PhMe), nmax 2962.9, 1609.6, 1540.2, 1510.5, 1470.6,
1363.5, 1305.2, 1188.3, 1153.7, 1078.4, 1051.2, 974.9, 809.6, 752.2,
476.4 cmꢁ1
, dH 1.31 (3H, s, CH3), 1.32 (3H, s, CH3), 1.96 (4H, m,
N(CH2)2), 2.56 (6H, s, (CH3)2), 3.26 (4H, m, N(CH2)2), 5.96 (1H, s,
pyrrole-H), 5.97 (1H, s, pyrrole-H), 6.47 (1H, dd, J ¼ 10.0, 4.2 Hz, 20-
H) 6.50 (2H, m, AreH), 7.03 (1H, m, AreH), 7.15 (1H, m, AreH), 7.28
(6H, m, AreH, 50-H, 90-H), 7.63 (1H, d, J ¼ 1.5 Hz, 70-H), 7.66 (1H, d,
J ¼ 8.8 Hz, 60-H), 7.71 (1H, m, AreH), 8.07 (1H, d, J ¼ 8.5 Hz, 100-H),
dC 14.61, 14.81, 25.49, 47.47, 81.48, 110.95, 113.41, 116.49, 118.62,
119.10, 121.15, 122.50, 123.73, 126.15, 126.70, 127.84, 127.99, 128.03,
128.08,129.22,129.36,129.66,129.78,129.83,129.94,131.74,132.04,
141.73, 143.24, 147.48, 151.11, 155.35. Found M þ Hþ 668.3058.
C42H37ON3BF3 requires M þ Hþ 668.3055.
2.5. General method for the preparation of 4,4-difluoro-1,3,5,7-
tetramethyl-8-(30,30-bis-(diaryl)-30H-naphtho[2,1-b]pyran-80-yl)-4-
bora-3a,4a-diaza-s-indacenes 9
4-TsOH (0.05 g) was added in a single portion to a warm (ca.
50 ꢂC) stirred solution of the 4,4-difluoro-1,3,5,7-tetramethyl-8-(6-
hydroxynaphth-2-yl)-4-bora-3a,4a-diaza-s-indacene 6b (0.15 g,
0.38 mmol) and the 1,1-diarylprop-2-yn-1-ol (0.38 mmol) in
toluene (25 mL). The resulting suspension was heated under reflux
until no propynol remained by TLC (ca. 1.5 h). The solution was
cooled to RT and washed with water (3 ꢀ 20 mL). The combined
aqueous layers were extracted with toluene (2 ꢀ 20 mL). The
toluene extracts were combined with the original toluene layer and
dried over Na2SO4. Evaporation of the toluene afforded the crude
product that was purified by elution from silica. The following
compounds were obtained by this protocol:
2.5.3. 4,4-Difluoro-1,3,5,7-tetramethyl-8-[30-(2-bromophenyl)-30-
(4-pyrrolidinophenyl)-30H-naphtho[2,1-b]pyran-80-yl]-4-bora-
3a,4a-diaza-s-indacene 9c
Compound 9c from 6b and 1-(2-bromophenyl)-1-(4-pyrro-
lidinophenyl)prop-2-yn-1-ol 8c after elution from silica with 35%
EtOAc in hexane as orange microcrystals (120.4 mg) 43%, mp
227.5 ꢂC, lmax 488 nm, ε 82100 Mꢁ1 cmꢁ1 (CH2Cl2), lmax post-
irradiation 582 nm with t ¼ 112 s (CH2Cl2), labs 507 nm, lem
½
514 nm, Ff 0.39 (PhMe), nmax 2959.8, 1607.8, 1539.9, 1507.9, 1462.1,
2.5.1. 4,4-Difluoro-1,3,5,7-tetramethyl-8-(30,30-bis-(4-
methoxyphenyl)-30H-naphtho[2,1-b]pyran-80-yl)-4-bora-3a,4a-
diaza-s-indacene 9a
1364.3, 1306.1, 1187.6, 1153.6, 972.6, 810.0, 795.8, 752.1, 578.3,
476.2 cmꢁ1
,dH 1.313 (3H, s, CH3), 1.317 (3H, s, CH3), 1.98 (4H, m,
N(CH2)2), 2.56 (6H, s, (CH3)2), 3.30 (4H, m, N(CH2)2), 5.97 (2H, s,
pyrrole-H), 6.52 (2H, m, AreH), 6.66 (1H, d, J ¼ 10.0 Hz, 20-H), 7.14
(1H, m, AreH), 7.30 (6H, m, AreH, 50-H, 90-H), 7.59 (1H, dd, J ¼ 7.6,
1.2 Hz, AreH), 7.63 (1H, d, J ¼ 1.6 Hz, 70-H), 7.65 (1H, d, J ¼ 8.8 Hz, 60-
H), 7.79 (1H, dd, J ¼ 8.0, 1.6 Hz, AreH), 8.07 (1H, d, J ¼ 8.7 Hz, 100-H),
dC 14.61, 14.82, 25.49, 47.46, 83.77, 110.96, 113.60, 119.10, 119.24,
121.15, 121.29, 122.54, 125.82, 126.17, 126.90, 127.86, 128.71, 129.05,
129.18, 129.40, 129.47, 129.67, 129.78, 129.90, 131.73, 135.23, 141.75,
143.01, 143.23, 147.36, 151.25, 155.35. Found M þ Hþ 727.2285.
C42H37ON3BBrF2 requires M þ Hþ 727.2290.
Compound 9a from 6b and 1,1-bis(4-methoxyphenyl)prop-2-
yn-1ol 8a after elution from silica with 30% EtOAc in hexane as
orange microcrystals (51.6 mg) 21% yield, mp 290.8 ꢂC, lmax 488 nm,
ε 58400 Mꢁ1 cmꢁ1 (CH2Cl2), labs 507 nm, lem 513 nm, Ff 0.43
(PhMe), nmax 2960.8, 2923.3, 1602.8, 1544.8, 1508.3, 1305.6, 1248.2,
1184.8, 1154.7, 1016.3, 972.4, 795.9, 764.1, 709.8, 581.9, 475.2 cmꢁ1
,
dH 1.31 (6H, s, (CH3)2), 2.56 (6H, s, (CH3)2), 3.79 (6H, s, (OMe)2), 5.96
(2H, s, pyrrole-H), 6.25 (1H, d, J ¼ 10 Hz, 20-H), 6.87 (4H, m, AreH),
7.22 (1H, d, J ¼ 8.8 Hz, 50-H), 7.29 (1H, d, J ¼ 10.0 Hz, 10-H), 7.33 (1H,
dd, J ¼ 8.5, 1.5 Hz, 90-Hp), 7.38 (4H, m, AreH), 7.63 (1H, d, J ¼ 1.5 Hz,
70-H), 7.65 (1H, d, J ¼ 8.8 Hz, 60-H), 8.07 (1H, d, J ¼ 8.5 Hz, 100-H), dC
14.61, 14.81, 55.26, 82.48, 113.44, 113.59, 118.46, 119.18, 121.17,
122.46, 126.23, 127.88, 128.14, 128.33, 129.19, 129.66, 129.83, 129.94,
137.26, 141.68, 143.21, 151.30, 155.38, 158.96. Found Mþ 639.2737.
C40H35O3N2BF2 requires Mþ 639.2740.
3. Discussion
Photochromic naphthopyrans are most conveniently accessed
from the acid-catalysed condensation between a naphthol and
R2
OH
OH
R1
OH
(i)
N
(ii)
N
B
F
H
F
R2
OHC
N
HN
5
R1
7
6a R1 = Me, R2 = H
6b R1 = R2 = Me
Reagents: (i) 2,4-dimethylpyrrole, cat. TFA, CH2Cl2, rt; (ii) DDQ, CH2Cl2, rt then Et3N, BF3.OEt2, rt
Scheme 2. Synthesis of 8-(6-hydroxynaphthalen-2-yl)-1,3,5,7-tetramethyl-BODIPY dye 6b.

178
C.D. Gabbutt et al. / Dyes and Pigments 94 (2012) 175e182
Scheme 3. Synthesis of naphthopyran substituted BODIPY dyes 9aec.
Fig. 1. X-ray crystal structure of 9a (50% probability thermal ellipsoids).
a 1,1-diarylprop-2-yn-1-ol [19,20]. The BODIPY unit is invariably
obtained from the reaction between an excess of a substituted
pyrrole and an aromatic aldehyde in the presence of a catalytic
amount of trifluoroacetic acid, oxidation of the intermediate
dipyrromethane unit affording a typically unstable dipyrromethene
which is directly transformed into the BODIPY dye upon treatment
with a tertiary amine and BF3$OEt2 [1].
at d 5.73 and a singlet at d 5.55 assigned to the methine proton.
Oxidation of 7 was achieved with DDQ and the resulting crude
product was reacted directly with an excess of triethylamine and
BF3.OEt2 in dichloromethane to afford 6b in 29% yield. The structure
of 6b was confirmed by the presence of key 1H NMR signals for the
Considering the two key functional group requirements (a
naphthol and an aromatic aldehyde) a convenient starting material
for our initial exploration was 6-hydroxy-2-naphthaldehyde 5, in
which the hydroxyl group is conjugated to the aldehyde function.
6-Hydroxy-2-naphthaldehyde was readily synthesised in 81% yield
by the low temperature dilithiation of 6-bromo-2-naphthol with n-
butyllithium and quenching the resulting dianion with anhydrous
N-formylpiperidine. Examination of the literature revealed that an
8-(6-hydroxynaphthalen-2-yl)-3,5-dimethyl-BODIPY dye 6a had
been previously reported and its pH sensing properties explored
[8]. Encouraged by this fact, but also aware of the reactive nature of
unsubstituted pyrrole ring carbon atoms we elected to prepare the
tetramethyl substituted analogue 6b commencing from 2,4-
dimethylpyrrole (Scheme 2).
Condensation of
5 with 2,4-dimethylpyrrole catalysed by
trifluoroacetic acid gave 6-[bis-(3,5-dimethyl-1H-pyrrol-2-yl)-
methyl]naphthalen-2ol 7 in 56% yield. The 1H NMR spectrum of 7
Fig. 2. Absorption spectra of 9aec recorded immediately after cessation of UV
displayed signals for the pyrrole NH at
d
7.46, pyrrole ring protons
irradiation.

C.D. Gabbutt et al. / Dyes and Pigments 94 (2012) 175e182
179
Scheme 4. Model photochromic compounds 10aec.
pyrrole ring protons at
d
5.97, the pyrrole methyl group signals at
5.08. The
It should be noted that lmax for the simple 3,3-bis(4-
methoxyphenyl) substituted naphthopyran 10a appears at
475 nm (PhMe solution) and is relatively short-lived [half-life
d
1.26 and 2.57 and a broadened hydroxyl signal at d
d
mass spectrum of 6b displayed the expected molecular ion
[M þ H]þ of m/z 391.1791 with a base peak at m/z 371.1739 assigned
to [M-F]þ.
Heating 6b with 1,1-bis(4-methoxyphenyl)prop-2-yn-1ol 8a
which was available from our other studies [21] in PhMe containing
a catalytic amount of 4-TsOH gave 4,4-difluoro-1,3,5,7-tetramethyl-
8-(30,30-bis-(4-methoxyphenyl)-30H-naphtho[2,1-b]pyran-80-yl)-4-
bora-3a,4a-diaza-s-indacene 9a as orange-red crystals in 21% yield
(Scheme 3). The structure of this product was clearly indicated by
1H NMR spectroscopy which showed the typical 2-H pyran ring
(t ) ¼ 3 s] [24]. It may be that the photochromic response of this
½
new BODIPY substituted naphthopyran 9a results in a weak band at
ca. 475 nm which fades rapidly and is thus masked by the intense
absorption associated with the BODIPY unit (Scheme 4).
Photochromic response e structure relationships are well-
documented for the 3H-naphtho[2,1-b]naphthopyran isomers
[19,20,25e27]. We thus decided to simultaneously bathochromi-
cally shift lmax and decrease the rate of fade of the subsequent
BODIPY substituted naphthopyran. These features were conve-
niently accomplished using propynol 8b wherein the pyrrolidine
unit is responsible for a bathochromic shift in lmax and the fluorine
atom leads to a decrease in the rate of fade through steric inter-
actions which hinder the rate of ring-closure of the photo-
merocyanine [28]. The new naphthopyran 9b was obtained in 28%
yield by the previously described method for 9a and displayed
doublet at
pyrrole ring protons at
d
6.25 (J ¼ 10 Hz) [17] and a signal for the equivalent
d
5.96 and completely established by an X-
ray crystal structure (Fig.1) [22]. The bond lengths and angles of the
BODIPY unit of 9a compare favourably with those of previously
reported boraindacene fluorophores [23] and indicate a highly
delocalised cyanine dye like system with little distinction between
single and double bonds. Steric interactions between the proximal
methyl groups (C-1 and C-7) and the naphthalene moiety result in
a near orthogonal arrangement of the boraindacene and naph-
thopyran units with an angle between planes of 85ꢂ.
a characteristic dd (J ¼ 10.0, 4.2 Hz) for 2-H at
d
6.47 in its 1H NMR
spectrum through coupling with 1-H and the F atom of the o-flu-
orophenyl unit [28,29]. The absorption spectrum of 9b displayed
a weak photochromic response with lmax of 580 nm and a t of ca. 8
½
The absorption spectrum of 9a in CH2Cl2 solution showed no
evidence of photochromism upon UV irradiation, with the sample
exhibiting the expected intense absorption of the BODIPY unit at
488 nm (Fig. 2).
s (Figs. 2 and 3). The comparable data for the naphthopyran 10b are
lmax ¼ 554 nm and t ¼ 40 s [28]. The difference in lmax suggests
½
that the BODIPY unit at C-8 behaves as an electron releasing
Fig. 3. Absorption spectra of 9b illustrating fade of photomerocyanine 9b0 over time
after cessation of UV irradiation to a steady state.
Fig. 4. Absorption spectra of 9c illustrating fade of photomerocyanine over time.

180
C.D. Gabbutt et al. / Dyes and Pigments 94 (2012) 175e182
Scheme 5. Photochromic response of BODIPY substituted naphthopyrans 9aec.
substituent comparable in its effect to a C-8 methoxy group [24,25].
The shorter half-life associated with photomerocyanine 9b0 infers
that this species is less stable than that derived from 10b.
In a third attempt to further enhance the photochromic
response propynol 8c was employed. The bromine atom in 8c
significantly hinders the ring-closure of the photomerocyanine
such that the photomerocyanine 10c0 derived from 10c has lmax
absorption of photons generating an excited singlet state from
which ring-opening of the pyran ring ensues [30]. Fluorescence
emission is also derived from a singlet excited state and it may be
that the most efficient (or rapid) energy dissipation process occurs
via fluorescence rather than the required electrocyclic bond reor-
ganisation leading to the photomerocyanine and hence photo-
chromism. Detailed fast spectroscopic studies are required to
establish this feature but are outside the scope of this preliminary
synthetic chemistry investigation.
554 nm and t of 1024 s [28]. Heating 8c with 6b gave 9c in 43%
½
yield. The 1H NMR spectrum of 9c displayed a doublet at
d 6.66
(J ¼ 10.0 Hz) which confirmed the formation of the pyran unit. The
The fluorescence spectra of the three new BODIPY substituted
naphthopyrans 9aec were next examined. All three compounds
9aec displayed good fluorescence with narrow Stokes’ shifts
typical of the BODIPY fluorophore [1]. Excitation and emission
spectra were recorded for each compound before (unirradiated)
and immediately post-UV irradiation; the normalised absorption
and emission spectra are presented in Figs. 5e7. Unlike the signif-
icant changes in fluorescence emission intensity that were noted
for the photochromic cycling of the dithienyletheneeBODIPY
system 2 [12], no such dramatic differences were noted for the
series 9aec. For 9a, which displayed no evidence of a photochromic
response under the applied irradiation conditions, there was only
persistence of the photomerocyanine 9c0 [lmax ¼ 582 nm t ¼ 112 s
½
(Figs. 2 and 4)] is ca. fourteen fold greater than that observed for
9b0, derived from 9b upon UV irradiation, which confirms the
success of our structural modification. However, the half-life of 9c0
is again significantly lower than that of 10c0, indicating the relative
instability of the BODIPY substituted photomerocyanine 9c0. It
should be noted that during the photochromic measurements of
9aec the intensity of the sharp absorption band at 488 nm char-
acteristic of the BODIPY unit remained essentially unchanged,
a feature that is consistent with observations made by Neckers
et al., concerning the influence of photochromic cycling of the P-
type photochromic dithienyletheneeBODIPY system
2 [12]
(Scheme 5).
The reason for the extremely weak photochromic response of
9aec relative to 10aec may stem from the fact that it has been
established that the initial event in the photochromic process is
Fig. 5. Excitation and emission fluorescence spectra of 9a in PhMe.
Fig. 6. Excitation and emission fluorescence spectra of 9b in PhMe.

C.D. Gabbutt et al. / Dyes and Pigments 94 (2012) 175e182
181
reorganisation leading to photochromism. Photoinduced ring-
opening of the naphthopyran unit resulted in a decrease in the
fluorescence intensity which was most significant for the naph-
thopyran with the most persistent photomerocyanine.
Acknowledgement
The authors thank Yorkshire Forward and James Robinson Ltd.,
[Vivimed Laboratories (Europe)] for a studentship (SBK). The
Worshipful Company of Clothworkers’ of the City of London are
thanked for a millennium grant for the purchase of a Bruker Avance
NMR instrument and for an annual grant for the purchase of a Cary
50 Probe spectrophotometer and associated irradiation sources.
The EPSRC are thanked for access to the National Mass Spectrom-
etry Service (Swansea).
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1.541(3), N46eC42 1.403(2), C42eC41 1.395(3), C41eC52 1.395(3),
C52eN48 1.402(2), N48eB47 1.542(3), F47eB47 1.388(2), F47eB47eF48
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[22] Crystallographic data for compound 9a (CCDC855708): formula
C40H35BF2N2O3; formula weight
¼
640.51;
T
¼
150K;
l
¼
0.71073
Å
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[Mo-Ka]; crystal system ¼ monoclinic; space group
¼
P21/c; unit cell
b
Z
¼
dimensions, a ¼ 9.6248(5) Å,
19.7794(11) Å,
(calculated)
1.339 Mg mꢁ3
a
¼ 90ꢂ; b ¼ 16.7178(8) Å,
¼ 93.465(2)ꢂ;
c
¼
g
¼
90ꢂ; volume
¼
3176.8(3) Å3;
¼
4; density
[30] (a) Moine B, Buntix G, Poizat O, Rehault J, Moustrou C, Samat A. Transient
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Picosecond and nanosecond photo-dynamics of a naphthopyran merocyanine.
Phys Chem Chem Phys 2002;4:180e4.
¼
;
absorption coefficient
0.092 mmꢁ1
;
F(000) ¼ 1344; crystal ¼ red prism; crystal size ¼ 0.18 x 0.18 x 0.18 mm;
q
range for data collection
ranges ꢃ11, ꢁ19
ꢁ11
collected 32968; independent reflections
absorption correction ¼ none; refinement method ¼ full matrix least-
squares on F2; data/restraints/parameters
6442/0/439; goodness-of-
¼
1.6
20, ꢁ24
ꢃ
q
ꢃ
ꢃ
26.52ꢂ; index
24; reflections
¼
ꢃ
h
ꢃ
k
ꢃ
l
ꢃ
¼
¼
6442 [R(int)
¼
0.0293];
¼