Synthesis and optical properties of naphthopyran dyes conjugated with fluorescent stilbazolium moieties

Article abstract of DOI:10.1039/c3nj41130c

The synthesis of new 3H-naphtho[2,1-b]pyran dyes substituted at position 8 of the naphthalene ring by either a stilbazolium or an 4-ethenyl-quinolinium group and their photochemical properties are described. In non-polar media the naphthopyran-stilbazoliu

Full text of DOI:10.1039/c3nj41130c

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Synthesis and optical properties of naphthopyran dyes
conjugated with fluorescent stilbazolium moieties†
Cite this: DOI: 10.1039/c3nj41130c
Ingolf Kahle,a Oliver Trober,b Hannes Richterb and Stefan Spange*a
¨
The synthesis of new 3H-naphtho[2,1-b]pyran dyes substituted at position 8 of the naphthalene ring by
either a stilbazolium or an 4-ethenyl-quinolinium group and their photochemical properties are
described. In non-polar media the naphthopyran–stilbazolium conjugates do not show any
photochromic response while the formation of the ring opened species is promoted in strong polar
media. Additional investigations of the new dyes within the pores of mesoporous MCM 41 particles
showed a significant enhancement of the photochromism compared with that in solution. The
generation of the ring-opened species upon irradiation with UV-light is accompanied by a decrease of
the fluorescence intensity, which could be recovered by illumination with visible light.
Received (in Montpellier, France)
12th December 2012,
Accepted 20th February 2013
DOI: 10.1039/c3nj41130c
www.rsc.org/njc
different working groups. Some attempts were made for
bonding a highly fluorescent boradiazaindacene core covalently
Introduction
In recent years, photochromic materials have attracted much
attention in various fields of science due to their great potential
for applications such as ophthalmic lenses,1 optical storage,2
optical switching,3–5 nonlinear devices,6 and especially for
decorative purposes, e.g. textiles and cosmetics.7,8 Among various
photochromic systems including spiropyrans, spirooxazines,
fulgides, diarylethenes and azo compounds, naphthopyrans are
of current interest because of the large spectral range of their
light induced open coloured forms. The photochromism of
naphthopyrans was first reported in the year 1965 and involves a
ring-opening reaction via photochemical cleavage of the C(sp3)–O
bond.9 This process is associated with the generation of coloured
ring-opened isomers, which can be either zwitterionic or more
likely quinoidal depending on the substituent R (Scheme 1).
While the transoid-cis isomer is cyclised thermally over a
period of time, the transoid-trans species is more stable and its
ring-closing takes place upon irradiation with visible light.10,11
Besides these interesting properties in colour switching, recent
investigations were made on fluorescence modulation and
fluorescence switching.12 Therefore, a combination of photo-
chromic and fluorescent dyes is the focus of current studies of
or through hydrogen bonding to diarylethenes, spiropyrans,
oxazines and recently even naphthopyrans.13–17 There has also
been a report on a photochromic naphthopyran bearing a
fluorescent naphthalimide chromophore.18
In this work, we report our results on the synthesis, character-
ization and optical properties of some new 3H-naphtho[2,1-b]pyran
dyes in conjugation with stilbazolium moieties. Especially
styrylpyridinium and styrylquinolinium merocyanines have
been known not only for their fluorescence or solvatochromic
properties and structure–colour relationships, but also for their
a Department of Polymer Chemistry, Chemnitz University of Technology, Strasse der
Nationen 62, Chemnitz, 09111, Germany. E-mail: stefan.spange@chemie.tu-chemnitz.de;
Fax: +49 (0)371 531 21239; Tel: +49 (0)371 531 21230
b Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Hermsdorf Branch of
the Institute, Michael-Faraday-Straße 1, Hermsdorf, 07629, Germany
† Electronic supplementary information (ESI) available: NMR spectra of synthe-
sized compounds, UV/Vis spectroscopic investigation of host–guest systems,
fluorescence spectra of host–guest systems. See DOI: 10.1039/c3nj41130c
Scheme 1 Structure of naphthopyran dyes and their photo-induced stereo-
isomers of the coloured ring-opened form.
c
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potential applicability in non-linear optics, such as optical (1H, d, J = 4.9 Hz); 7.28 (1H, d, J = 4.9 Hz); 7.37 (4H, d, J = 8.8 Hz);
sensors, and in physiology and biochemistry areas.19–23
7.80 (1H, d, J = 9.0 Hz); 7.92 (1H, dd, J = 8.8 Hz and 1.4 Hz); 8.03
In several cases, it is necessary to incorporate photochromic (1H, d, J = 8.8 Hz); 8.19 (1H, d, J = 1.4 Hz); 10.07 (1H, s). 13C NMR
compounds into a rigid host material for practical applications.4,24–26 (CDCl3): 55.4; 82.9; 113.6; 114.4; 118.7; 119.7; 122.5; 123.8; 128.4;
Therefore, the polarity of the surrounding medium is of great 128.9; 131.6; 132.2; 133.2; 135.2; 136.9; 153.5; 159.2; 192.1.
importance for the photochromic response of such materials. Found: m/z 423.1603 calculated for C28H22O4 + H: m/z 423.1591.
Accordingly, the investigation of the naphthopyran-conjugates was
General method for the synthesis of 1-methyl-(4-(30,30-diaryl-[3H]-
naphtho[2,1-b]pyran-80-yl)vinyl)pyridinium or quinolinium iodides
performed once in solution as well as within surface-modified and
non-functionalized MCM 41 particles in order to make comparative
statements about the influence of the surrounding medium with 1.05 mmol of the required formyl-substituted naphthopyran
regard to polarity and rigidity.
(compound 1 or 2) and 1.15 mmol of 1,4-dimethylpyridinium
or, respectively, quinolinium iodide were dissolved in 15 mL of
methanol. After the addition of one drop of piperidine as a
catalyst, the mixture was stirred under reflux for 3 h. Subsequently,
the mixture was cooled to room temperature and the precipitate
Experimental
Materials
Unless otherwise stated, reagents were used as supplied by the was filtered, washed with methanol and diethyl ether and dried
major chemical catalogue companies. All used solvents were under ambient conditions.
redistilled over appropriate drying agents prior to use.
1-Methyl-(4-(30,30-diphenyl-[3H]-naphtho[2,1-b]pyran-80-yl)-vinyl)-
pyridinium iodide (3). Yellow solid (67% yield), Mp. 226 1C, 1H NMR
(DMSO d6): 4.25 (3H, s); 6.67 (1H, d, J = 10.0 Hz); 7.24–7.41 (7H, m);
Instrumentation
1H NMR (250 MHz) and 13C NMR (69.9 MHz) spectra were 7.50 (4H, dd, J = 8.2 Hz and 1.3 Hz); 7.54 (1H, d, J = 9.9 Hz); 7.60 (1H,
recorded on a Bruker Avance 250 NMR spectrometer. The d, J = 16.9 Hz); 7.89 (1H, d, J = 9.0 Hz); 7.94 (1H, dd, J = 9.0 Hz and
residue signals of the solvents were used as internal standards. 1.4 Hz); 8.11 (1H, s); 8.12 (1H, d, J = 16.1 Hz); 8.23 (2H, d, J = 6.7 Hz);
UV/Vis absorption measurements in solution were performed 8.23 (1H, d, J = 9.0 Hz); 8.85 (2H, d, J = 6.7 Hz). 13C NMR (DMSO d6):
using a MCS 400 diode array UV/Vis spectrometer from Carl 46.9; 82.0; 114.3; 119.1; 119.2; 122.8; 122.9; 123.4; 124.7; 126.3;
Zeiss, connected via glass-fibre optics. UV/Vis investigation of 127.6; 128.2; 128.8; 130.0; 130.4; 130.8; 140.5; 144.5; 145.0; 151.3;
reflectance was carried out on a Zeiss multi channel spectrometer 152.5. Found: m/z 452.2010 calculated for C33H26NO+: m/z 452.2009;
system MCS 601/CLD using a deuterium lamp (215–620 nm) and a
CLX 75 W/Sch xenon lamp (290–900 nm) as light sources. Fluores-
cence spectra were recorded on a FluoroMaxs-4 from Horiba.
e .
(MeOH) = 23300 L molꢀ1 cmꢀ1
1-Methyl-(4-(30,30-bis-(4-methoxyphenyl)-[3H]-naphtho[2,1-b]-
pyran-80-yl)vinyl)pyridinium iodide (4). Yellow solid (61% yield),
1
Mp. 230 1C, H NMR (DMSO d6): 3.71 (6H, s); 4.25 (3H, s); 6.54
General method for the synthesis of 3,3-diaryl-8-formyl-
[3H]-naphtho[2,1-b]pyrans
(1H, d, J = 10.0 Hz); 6.91 (4H, d, J = 8.8 Hz); 7.32 (1H, d, J =
9.0 Hz); 7.36 (4H, d, J = 8.8 Hz); 7.49 (1H, d, J = 10.1 Hz); 7.60 (1H,
The synthesis of the formyl-substituted naphthopyrans was d, J = 15.3 Hz); 7.86 (1H, d, J = 9.0 Hz); 7.94 (1H, dd, J = 9.0 Hz
performed in analogy to published procedures.27 Therefore, a and 1.1 Hz); 8.10 (1H, d, J = 1.1 Hz); 8.12 (1H, d, J = 16.1 Hz); 8.20
solution of 10 mmol of 6-hydroxy-2-naphthaldehyde, 10 mmol of (1H, d, J = 9.0 Hz); 8.22 (2H, d, J = 6.7 Hz); 8.85 (2H, d, J = 6.7 Hz).
1,1-diphenylpropyn-l-ol (for compound 1) or 1,1-bis-(4-methoxy- 13C NMR (DMSO d6): 46.9; 55.1; 81.8; 113.5; 114.2; 118.7; 119.1;
phenyl)-propyn-1-ol (for compound 2) and a catalytic amount of 122.8; 123.4; 124.6; 127.7; 128.7; 129.2; 130.0; 130.7; 136.7; 140.5;
p-toluenesulfonic acid, in 40 mL of methylene chloride was stirred 145.0; 151.4; 152.5; 158.5. Found: m/z 512.2210 calculated for
at ambient temperature overnight. The solution was washed C35H30NO3 : m/z 512.2220; e(MeOH) = 20 800 L molꢀ1 cmꢀ1
with a solution of NaHCO3 (5% in water) and then with water.
.
+
1-Methyl-(4-(30,30-diphenyl-[3H]-naphtho[2,1-b]pyran-80-yl)-vinyl)-
The organic layer was dried over MgSO4, filtered and evaporated. quinolinium iodide (5). Red solid (72% yield), Mp. 278 1C, 1H NMR
The crude product was recrystallized from ethyl acetate–hexane (DMSO d6): 4.54 (3H, s); 6.68 (1H, d, J = 10.0 Hz); 7.25–7.3 (8H, m);
(1/20 v/v).
7.52 (4H, d, J = 8.3 Hz); 7.58 (1H, d, J = 9.9 Hz); 7.91 (1H, d, J =
3,3-Diphenyl-8-formyl-[3H]-naphtho[2,1-b]pyran (1). White 8.9 Hz); 8.06 (1H, t, J = 7.6 Hz); 8.21–8.30 (4H, m); 8.42 (1H, d, J =
solid (71% yield), Mp. 134 1C, 1H NMR (CDCl3): 6.32 (1H, d, 15.1 Hz); 8.43 (1H, d, J = 9.0 Hz); 8.53 (1H, d, J = 6.5 Hz); 9.11 (1H, d,
J = 10.0 Hz); 7.26–7.39 (8H, m); 7.49 (4H, dd, J = 8.0 Hz and 1.6 Hz); J = 8.5 Hz); 9.35 (1H, d, J = 6.5 Hz). 13C NMR (DMSO d6): 44.6; 82.1;
7.80 (1H, d, J = 9.1 Hz); 7.91 (1H, dd, J = 8.6 Hz and 1.5 Hz); 8.02 114.3; 116.0; 119.0; 119.2; 119.3; 122.5; 125.5; 126.2; 126.3; 126.5;
(1H, d, J = 8.6 Hz); 8.17 (1H, d, J = 1.5 Hz); 10.07 (1H, s). 127.6; 128.3; 128.8; 129.1; 130.1; 130.9; 131.1; 131.2; 134.9; 138.6;
13C NMR (CDCI3): 83.2; 114.4; 119.0; 119.6; 122.5; 123.8; 127.1; 142.8; 144.6; 147.9; 151.4; 152.5. Found: m/z 502.2154 calculated for
127.9; 128.3; 128.4; 128.5; 131.7; 132.3; 133.2; 135.2; 144.5; C37H28NO+: m/z 452.2165; e(MeOH) = 28 000 L molꢀ1 cmꢀ1
.
153.4; 192.1. Found: m/z 363.1368 calculated for C26H18O2
H: m/z 363.1380.
+
1-Methyl-(4-(30,30-bis-(4-methoxyphenyl)-[3H]-naphtho[2,1-b]-
pyran-80-yl)vinyl)quinolinium iodide (6). Red solid (82% yield),
1
3,3-Bis-(4-methoxyphenyl)-8-formyl-[3H]-naphtho[2,1-b]pyran Mp. 237 1C, H NMR (DMSO d6): 3.72 (6H, s); 4.54 (3H, s); 6.55
1
(2). Pale yellow solid (67% yield), Mp. 176 1C, H NMR (CDCl3): (1H, d, J = 10.0 Hz); 6.91 (4H, d, J = 8.9 Hz); 7.33 (1H, d, J =
3.78 (6H, s); 6.26 (1H, d, J = 10.0 Hz); 6.86 (4H, d, J = 8.8 Hz); 7.24 8.9 Hz); 7.37 (4H, d, J = 8.9 Hz); 7.53 (1H, d, J = 10.1 Hz); 7.88
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(1H, d, J = 8.9 Hz); 8.06 (1H, t, J = 8.1 Hz); 8.20–8.30 (5H, m); 8.42
Considering these two reaction types, 6-hydroxy-2-naphth-
(1H, d, J = 15.6 Hz); 8.44 (1H, d, J = 9.0 Hz); 8.52 (1H, d, J = aldehyde was the initial starting material which was converted
6.8 Hz); 9.11 (1H, d, J = 8.4 Hz); 9.35 (1H, d, J = 6.5 Hz). 13C NMR to the respective formyl-substituted naphthopyran dye in the
(DMSO d6): 44.7; 55.1; 81.9; 113.5; 114.2; 116.1; 118.8; 119.1; first step, followed by the condensation with either 1,4-dimethyl-
119.3; 122.6; 125.5; 126.3; 126.6; 127.7; 128.8; 129.2; 130.2; 130.8; pyridinium iodide or 1,4-dimethylquinolinium iodide in the
131.1; 133.3; 135.0; 136.7; 138.7; 142.9; 148.0; 151.5; 152.6; 158.5. second step (Scheme 2).
+
Found: m/z 562.2375 calculated for C39H32NO3 : m/z 562.2377;
The molecular structure of the products was clearly evidenced
by electrospray ionization time-of-flight mass spectrometry and
NMR spectroscopic measurements (see ESI†). The 1H-NMR
spectra of all naphthopyran compounds showed the typical
e(MeOH) = 24000 L molꢀ1 cmꢀ1
.
Synthesis of MCM 41 particles
The MCM-41 was synthesized according to the literature.28 2.4 g 2-H pyran ring doublet in the region between d 6.24 ppm and
of n-cetyltrimethylammonium bromide was dissolved in 120 g 6.68 ppm ( J = 10 Hz). All observed signals for the vinylic hydrogens
of deionised water and stirred until the solution was homo- of the conjugated vinylpyridinium or vinylquinolinium units
geneous and clear. Then 8 mL of ammonium hydroxide was showed typical coupling constants for the trans-conformation
added and stirred for 5 min after which 10 mL of tetraethoxy- ( J = 15–16 Hz).
silane (TEOS) was added to the solution. The mixture was
Optical properties
stirred overnight at room temperature, and the products were
collected by filtration, dried and calcined in air. Calcination
was performed at 823 K for 5 h. The gel molar composition was
1 M TEOS : 1.64 M NH4OH : 0.15 M CTAB : 126 M H2O.
The optical properties of the new naphthopyran-conjugates 3–6
have been investigated in several organic liquids to study the
influence of solvent polarity of the surrounding medium. For
this reason, UV/Vis spectra have been measured once during
UV-irradiation (at 350 nm) and also without illumination with
UV-light to observe the photochromic behaviour. In addition,
fluorescence measurements were also made for each solution.
The results are summarized in Table 1. Generally, the naphtho-
pyran-conjugates did not show any solvatochromism, but a
significant hypsochromic shift was observed for the emission
maxima in acidic media. This means that Brønsted acidic sites
strongly interact with the dye molecules in their excited states.
But only the methoxy-substituted naphthopyran-conjugates 4
and 6 showed a ring opening process in stronger acidic media,
e.g. formic acid, as could be seen in Fig. 1, while no ring-
cleavage was observed in weaker acids, like acetic acid.
Surface modification of MCM 41 particles
1,1,1,3,3,3-Hexamethyldisilazane (HMDS) was used for the
silylation of the mesoporous materials. For this purpose 1 g
of the calcined sample was heated at 120 1C for 2 h under
vacuum to remove water adsorbed in the pores of the material.
15 mL of toluene was added to the powder under a nitrogen
atmosphere. 4 mmol of HMDS was then added and the mixture
was gently refluxed for 4 h. The powder was filtered off, washed
with abs. ethanol, and dried at room temperature under
vacuum (5 mbar) for 8 h. The amount of trimethylsilyl groups
on MCM materials was 2.14 mmol gꢀ1 which was calculated
from C-content measured by elemental analysis.
The observed ring-cleavage of dyes 4 and 6 in formic acid
was solely forced by the interaction with Brønsted acidic sites.
Therefore, several UV/Vis absorption bands appeared in the
visible region. The simple ring-opened form is assigned to
the absorption band at longer wavelengths of about 630 nm.
Impregnation of MCM 41 particles with photochromic dyes
The MCM 41 particles were dried at 120 1C under vacuum for
3 h. Afterwards the host material was impregnated with the
synthesized photochromic dyes from solution by the incipient
wetness technique.
5 wt% naphthopyran with respect to the amount of used
MCM 41 was dissolved in a small amount of dichloromethane
and added to the mesoporous powder under vacuum. This
mixture was shaken for 3 h. Capillary forces draw the solution
into the pores. The loaded materials were filtered and dried
under ambient conditions.
Results and discussion
Synthesis
Photochromic naphthopyrans are readily accessible by the acid-
catalysed condensation of naphthol and 1,1-diarylprop-2-yn-1-
ol. Styrylpyridinium or styrylquinolinium merocyanines are
commonly obtained by Knoevenagel condensation: alkylated
4-methylpyridinium or 4-methylquinolinium salts are condensed
with an aromatic aldehyde in methanol or absolute ethanol with
piperidine as the catalyst.
Scheme 2 Synthesis of naphthopyrans substituted in the 8-position with
a vinylpyridinium or vinylquinolinium unit. Reaction conditions: (i) CH2Cl2, cat.
p-toluenesulfonic acid, RT, 12 h; (ii) CH3OH, cat. piperidine, 3 h reflux.
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Table 1 UV/Vis absorption maxima/emission maxima (values in nm) of conjugates 3–6 in several organic solvents (for recording the emission spectra, the samples
were excited at 450 nm)
Solvent
3
4
5
6
THF
Acetone
DMF
Formamide
DMSO
417/614
418/615
416/613
416/620
419/622
419/621
463/666
453/660
454/670
453/662
453/669
458/666
456/670
458/646
457/627
469/668
455/665
456/672
456/669
456/677
462/672
459/669
416/612
419/619
417; 650a/615
420/616
417; 650a/618
424/618
MeOH
Tetramethyl-urea
Acetic acid
Formic acid
417; 650a/613
417/607
420; 650a/619
420/610
465/658
417/585
400; 478b; 629/562; 667b
430; 480b; 630/613; 648b
a
b
Increasing UV/Vis absorption band upon UV-irradiation. UV/Vis absorption bands or emission of the protonated species.
Scheme 3 Assumed interaction of stilbazolium-conjugated naphthopyrans
with Brønsted acidic sites and the resulting equilibrium.
did not show ring-cleavage in formic acid, we conclude that
electron donating substituents at the C-3 aryl rings seem to
promote the formation of the open species by stabilizing the
carbocation. The isomerization and the resulting equilibrium is
depicted in Scheme 3.
Besides these processes in acidic solution, the stilbazolium-
conjugates
3 and 4 showed photochromism in strong
polar, non acidic solvents, like tetramethylurea and dimethyl
sulfoxide (DMSO), as exemplified in Fig. 2. Concerning these
observations, a strong interaction between the dyes and strong
polar media seems to be necessary for a photochromic
response. By comparing the spectral evolution of the two
compounds 3 and 4 in DMSO upon UV-irradiation, one can
see that the methoxy substituted naphthopyran-conjugate 4
shows a much more pronounced photochromism. This observa-
Fig. 1 Time-dependent spectral evolution of dyes 4 and 6 in formic acid,
showing an equilibrium between protonated and unprotonated ring-opened
species with isosbestic points at 494 nm (4) and 498 nm (6).
This strong bathochromic shift is explained by the increased tion supports the hypothesis that electron donating groups at the
conjugated p-electron system. This species undergoes an acid– C-3 geminal aryl units promote the ring-opening reaction. But in
base equilibrium with the protonated species showing a UV/Vis contrast, the quinolinium dyes did not show any photochromic
absoption maximum at 480 nm. This strong hypsochromic response in polar solution upon illumination with UV-light,
shift results from the two electron-withdrawing substituents whether electron donating groups are present or not. For this
at the naphthalene ring, namely the stilbazolium unit and the reason, the quinolinium group, as a strong electron withdrawing
carbo cation. The UV/Vis absorption bands of the initial closed substituent, seems to impair the photochromic process in solution.
species were also observed at shorter wavelengths (400 nm for The pyridinium unit, however, as a weaker electron-withdrawing
dye 4 and about 430 nm for dye 6). As the two other dyes 3 and 5 group, allows photochromism in such polar environments.
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Fig. 3 Spectral evolution of dye 4 in the pores of surface-modified MCM 41
particles (above) and non-functionalized MCM 41 particles (below) upon irradia-
tion with UV-light (at 350 nm).
Fig. 2 Spectral evolution of dyes 3 and 4 in DMSO (c B 3 ꢁ 10ꢀ5 mol Lꢀ1) upon
irradiation with UV-light.
Therefore, additional investigations were performed within of all samples are presented in the ESI†). This result is a further
the pores of MCM 41 particles, which are known for confirmation for the requirement of a highly polar environ-
their strong polar surface. It was shown previously that ment to promote a photochromic reaction. But mainly it was
the incorporation of photochromic molecules in micro- and shown that a photochromic process is significantly enhanced
mesoporous matrices leads to advanced optical materials with within a solid matrix compared with the behaviour in solution.
the advantage of being able to tune the optical properties by A very fast response was indicated by an increasing UV/Vis
surface modification.25,26
But the open coloured forms of photochromic naphthopyrans with UV-light.
absorption band at 626 nm just immediately after irradiation
or spiropyrans were often stabilized in the pores of porous silica
Since stilbazolium dyes are known to be highly fluorescent,
matrices, leading to disturbance of photochromism. Also acidic the influence of the photoinduced ring-cleavage on their
sites at the surface of such aluminosilicate compounds may lead fluorescence properties was also the focus of this work.22 The
to ring-opening and simultaneous protonation reactions.26–30
fluorescence of the naphthopyran-conjugates shows a kind
For comparison purposes, optical measurements were performed of ‘‘on–off switch’’ mechanism caused by the photoinduced
within the pores of non-functionalized and also surface-modified conversion between the closed and open forms. After
MCM 41 particles, which were treated with 1,1,1,3,3,3-hexamethyl- UV-irradiation of the photochromic naphthopyran materials
disilazane (HMDS) to produce trimethylsilyl groups at the surface, for several seconds, the emission band diminishes quickly.
lowering the acidity and polarity of the material. In this way, the This may also be a result of a possible cis–trans-isomerization,
influence of surface-polarity on the spectroscopic behaviour was for which stilbazolium dyes are generally known for.31,32 But
studied within a rigid host material.
this effect is rather negligible, as could be seen in Fig. 3.
Here the fluorescence quenching of the stilbazolium moieties in
The UV/Vis absorption spectra of dyes 3–6 within the
surface-modified MCM 41 particles showed no or only very the open form of the naphthopyran-conjugates may result from an
weak photochromic response upon irradiation with UV-light, intramolecular energy transfer mechanism. The emission of the
comparable with that in solution. But a significant enhance- respective stilbazolium or vinylquinolinium iodides overlaps
ment of the photochromism was observed for all fluorophore- significantly with the UV/Vis absorption bands of the photoinduced
conjugates within the pores of non-functionalized MCM 41 ring-opened merocyanines, thus allowing energy transfer leading
particles, as exemplified in Fig. 3 for compound 4 (the spectra to fluorescence quenching. This effect is reversible as shown by
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1
Table 2 Half-lives t of fading and recovery of the fluorescence intensity, UV/Vis
2
absorption maxima of the closed and opened forms of dyes 3–6 in MCM 41
particles and their respective emission wavelengths
Abs. lmax
Abs. lmax
t1/2 (Fade) t1/2 (Recover) [nm] closed [nm] open Emission
Dyes [s]
[s]
9.56
3.27
18.65
2.21
form
form
lmax [nm]
3
4
5
6
28.78
439
430
475
470
600
626
617
630
580
570
625
625
6.90
20.15
2.60
was recovered and a faster response was shown by the methoxy-
substituted dyes again, as it is exemplefied for compounds 3 and
4 in Fig. 5 (all curves are shown in the ESI†).
The results of the optical investigation of naphthopyran–
fluorophore conjugates 3–6 within the pores of MCM 41
particles are summerized in Table 2.
Fig. 4 Observed emission maxima of compounds 3–6 upon excitation with UV-
light at 350 nm over a period of time.
additional fluorescence measurements. For this purpose, the
emission maxima of the samples were observed over a period of
time during simultaneous irradiation with UV-light (excitation at
350 nm) and subsequent irradiation with visible light (excitation
at 500 nm). The normalized bleaching curves, which were
obtained during the irradiation at 350 nm, are shown in Fig. 4.
It can be seen that the emission reaches a constant value, which
is due to the resultant equilibrium between open and closed
species. The stilbazolium-conjugated dyes 3 and 4 showed the
strongest decrease of about 56% and 55% of their initial
fluorescence intensity. The quinolinium-conjugated dyes 5 and
6 showed a less marked decrease in fluorescence (upto 70% and
67% of their initial fluorescence intensity) due to the hindered
ring-opening process which was discussed before. Additionally,
we observed that the fluorophore-conjugates 4 and 6 with
substituted methoxy groups showed a much faster response
compared with the non-substituted dyes 3 and 5. This result
further supports the hypothesis that electron donating substituents
at the C-3 aryl rings seem to promote the formation of the ring-
opened species.
Conclusions
Some new photochromic naphthopyrans bearing either a stil-
bazolium or a vinylquinolinium moiety have been synthesized
and investigated. The optical properties were studied in
solution and within modified and non-modified MCM 41
particles to study the influence of the environmental polarity.
The presence of the conjugated stilbazolium or rather vinyl-
quinolinium units located at the 8-position of the naphthalene
ring results in a minor photoactivity in less polar media
compared with common chromene dyes. In contrast to this,
the light-induced ring-opening reaction is promoted in strong
polar media. Acidic media like formic acid facilitated the ring-
opening reaction of the methoxy-substituted naphthopyran-
conjugates and the ring-opened species underwent protona-
tion, while no reaction was observed for dyes 3 and 5 without
substituted aryl rings. Further investigations showed that only
naphthopyran dyes in conjugation with stilbazolium moieties
show a photochromic response in strong polar solvents, like
DMSO. Only a very weak photochromism was observed within
the surface-modified MCM 41 particles as well. In contrast,
photochromism was significantly increased within the pores of
non-modified MCM 41 particles for all compounds. The
obtained results further showed that quinolinium units disturb
the photochromism, while electron donating substituents at the
C-3 aryl rings accelerate the photochromic process. Additionally,
the photoinduced ring-opening of the naphthopyrans leads to a
decrease of the fluorescence intensity, possibly caused by an
energy transfer between closed and open species. This effect is
reversible upon illumination with visible light. This leads to a
kind of ‘‘on–off’’-fluorescence switch.
Subsequently, the samples were excited at 500 nm and the
emission maxima was observed again. The fluorescence intensity
Acknowledgements
This work was financially supported by the Deutsche
Forschungsgemeinschaft (SP 392/25-2, RI 930/2-2). We thank
Dr Roy Buschbeck and Brigitte Kempe for mass spectrometric
investigation.
Fig. 5 Evolution of the emission intensity of compounds 3 (solid curves) and 4
(dotted curves) upon excitation at 350 nm and 500 nm.
c
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