Photochromic 1-benzofurylfulgides with modulated fluorescence

Article abstract of DOI:10.24820/ark.5550190.p009.806

Spectral luminescent and photochromic properties of 5-alkoxy-2-methyl-1-benzofuran-3-yl fulgides with increasing alkyl chain length were investigated. All compounds exist in the form of ring-opened Z-isomers (λmax 344-346 nm). Under UV-irradiation they rearrange into thermally stable colored ring-closed isomers (λmax - 500 nm) exhibiting fluorescence emission (λmax 603-608 nm). Fluorescence in ring-closed form C is modulated by light, irradiation with visible light (546 nm) results in formation of the initial opened form O and complete quenching of the emission. The quantum efficiency of photodecoloration is by 3.47-4.25 times higher than that of the photocoloration. 5-Alkoxy-2-methyl-1-benzofuran-3-yl fulgides demonstrate high fatigue resistance and are characterized by excellent thermal stability of the photoinduced cyclic form.

Full text of DOI:10.24820/ark.5550190.p009.806

General Papers
ARKIVOC 2016 (vi) 1-10
Photochromic 1-benzofurylfulgides with modulated fluorescence
Nadezhda I. Makarova,a Anatoly V. Chernyshev,a Vladimir P. Rybalkin,b
Sofiya Yu. Zmeeva,a Lidiya L. Popova,a Karina S. Tikhomirova,a Anatoly V. Metelitsa,a
Alexander D. Dubonosov,*b Vladimir A. Bren,a and Vladimir I. Minkina,b
a Institute of Physical and Organic Chemistry, Southern Federal University, 194/2, Stachka Av.,
344 090 Rostov on Don, Russian Federation
b Southern Scientific Center of Russian Academy of Sciences,41,Chekhov Pr.,
344 006 Rostov on Don, Russian Federation
E-mail: aled@ipoc.sfedu.ru
DOI: https://doi.org/10.24820/ark.5550190.p009.806
Abstract
Spectral luminescent and photochromic properties of 5-alkoxy-2-methyl-1-benzofuran-3-yl
fulgides with increasing alkyl chain length were investigated. All compounds exist in the form of
ring-opened Z-isomers (λmax 344-346 nm). Under UV-irradiation they rearrange into thermally
stable colored ring-closed isomers (λmax ~ 500 nm) exhibiting fluorescence emission (λmax 603-
608 nm). Fluorescence in ring-closed form C is modulated by light, irradiation with visible light
(546 nm) results in formation of the initial opened form O and complete quenching of the
emission. The quantum efficiency of photodecoloration is by 3.47-4.25 times higher than that of
the photocoloration. 5-Alkoxy-2-methyl-1-benzofuran-3-yl fulgides demonstrate high fatigue
resistance and are characterized by excellent thermal stability of the photoinduced cyclic form.
Keywords: Fulgides, 1-benzofuran, photochromism, fluorescence
Introduction
Photochromic compounds have already found an important place in the design of novel
polyfunctional materials for molecular electronics, molecular logic devices, molecular switches
and motors, rewritable optical memory, photodynamic chemo- and biosensors.1-7 The light-
induced rearrangement between two metastable isomeric forms of photochromic compounds
results in significant changes in various physical properties of the system, such as electronic
absorption spectra, fluorescence, dipole moment and nonlinear optic parameters. The most
thoroughly studied classes of organic photochromic compounds are represented by spiropyrans
and spirooxazines,8,9 azobenzenes,10 dihetarylethenes,11 fulgides and fulgimides.12,13 Fulgides,
the compounds with the structural skeleton of bismethylenesuccinic anhydride and an aromatic
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or heterocyclic substituent linked to a methylene group, are susceptible to the light-induced
hexatriene-cyclohexadiene rearrangement of their ground state colourless ring-opened forms to
give coloured ring-closed isomers possessing high fatigue resistance, thermal and photostability
capable to display fluorescent, chemosensor and complexing properties.14-16 These isomeric
forms do not interconvert in dark and have markedly different physical chemical characteristics,
which makes fulgides to be promising candidates for the use in high capacity three-dimensional
multilayer optical memory devices.17-21 The most studied groups of the heterocyclic fulgides are
currently represented by furyl-, thienyl-, oxazolyl-, pyrryl-, and indolylfulgides; reports on the
synthesis and properties of fulgides with benzofuryl and naphthofuryl groups are relatively
scarce.22-24
Therefore, the main goal of this work was the investigation into the spectral luminescent and
photochromic properties (kinetics of photodecoloration and photocoloration processes, quantum
efficiency of photorearrangements, quantum yields of fluorescence) of 5-alkoxy-2-methyl-1-
benzofuran-3-yl fulgides 1 bearing long alkyl groups allowing application of these photochromic
compounds in Langmuir-Blodgett films, which are widely used in the developing of
multifunctional materials possessing advanced properties.25 A preliminary information on the
synthesis of compounds 1 was reported earlier.26
Results and Discussion
According to 1H NMR data26 fulgides 1a-c have the structure of ring-opened Z-isomers, whereas
previously synthesized 5-methoxy-2-methyl-1-benzofuran-3-yl (1a)23 and 5-methoxy-2-
methylnaphtho[1,2-b]furan-3-yl (2) fulgides22 existed in the form of E-isomers (Scheme 1).
Scheme 1. Structures of 1a-c and 2.
The three-proton singlet signals of isopropylidene methyl groups are found, indeed, in the
region of 2.04-2.46 ppm, while for the E-structures the characteristic signal appears at 1.12
ppm.23 The E-configuration of 2 was unequivocally confirmed by the X-ray structural
determination.22 Spectral and photochromic properties of 1a-c were studied in toluene solutions
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at ambient temperature. Electronic absorption spectra of fulgides 1a-c are characterized by long-
wave absorption bands with the maxima in the region 344-346 nm and the molar extinction
coefficients 9150-9300 L mol–1 cm–1 (Table 1).
Table 1. Spectral absorption and spectral fluorescent characteristics of isomeric forms of 1a-c
and 222 in toluene at T = 293 К
Compound Isomeric Absorption
Fluorescence
excitation,
λexc (nm)
Fluorescence,
λmax (nm),
(quantum yield)
form
λmax
max
(nm)
(L mol–1 cm-1)
1a
1b
1с
2
Z
344
500
9300
9200
9150
10100
С
370 (S0S2)
500 (S0S1)
603 (0.003)
608 (0.004)
608 (0.005)
648 (0.002)
Z
346
500
С
370 (S0S2)
500 (S0S1)
Z
346
500
С
370 (S0S2)
500 (S0S1)
E
C
344
546
340 (S0S2)
542 (S0S1)
The intensity and the position of the absorption maxima of ring-opened Z-isomers of the
fulgides 1a-c do not practically depend on the length of the alkyl chain. The observed
bathochromic shift of a long-wavelength band absorption maximum in the sequence of
compounds R = Me (1a), (CH2)5Me (1b) and (CH2)15Me (1c) changes by not more than 2 nm.
Ring-opened Z-isomers 1a-c do not exhibit fluorescent properties in toluene at ambient
temperature. Irradiation of toluene solutions of colorless ring-opened isomers 1a-c with light of
365 nm wavelength leads to their rose red photocoloration due to the appearance of a new
absorption band with a maximum at 500 nm, the intensity of which increases upon irradiation
while the intensity of the initial long wavelength absorption band decreases (Fig. 1). These
spectral changes are indicative of the occurrence of the electrocyclic rearrangement of hexatriene
ring-opened isomer O into 1,3-cyclohexadiene ring-closed form C that involves a prior step of
Z/E-isomerization (Table 1, Scheme 2). Ring-closed form C of 5-methoxy-2-methylnaphtho[1,2-
b]furan-3-yl fulgide 2 absorbs in a longer wavelength region (λmax = 546 nm, Δλ = 46 nm)22 due
to the additional stabilization of aromatic heterocyclic system caused by benzoannelation.
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Scheme 2. Photoisomerization of fulgides 1a-c.
Figure 1. Absorption spectra of fulgide 1c Z-O in toluene (4·10-5 mol L–1, T = 293 K)
corresponding to the initial state (1), to the photostationary state established after irradiation for
12 min with light of 365 nm wavelength (2) and to the Z/E-O mixture obtained upon subsequent
irradiation for 13 min with light of 546 nm wavelength (3).
The prolonged UV-irradiation does not lead to the full conversion of 1a-c into colored ring-
closed form C due to the establishment of the photostationary state including all isomeric forms,
Z-O, E-O and C, of the photochromic fulgides (Fig. 1, curve 2). Its formation is caused by the
substantial overlap of the absorption bands related to the S0→S1 transition of the initial ring-
opened form O and the S0→S2 transition of the photoinduced isomer C (Table 1).12-15 The
subsequent irradiation of colored solutions of 1a-c with visible light (λirr = 546 nm) leads to their
photodecoloration due to the reverse ring-opening photoreaction C → E/Z-O (Fig.1, curve 3).12,15
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Therefore, no complete restoration of the initial (observed before irradiation) absorption
spectra occurs. The recurrence of the initial spectral characteristics can, however, be achieved
under a series of repeated cycles of photocoloration-photodecoloration. No spectral changes for
the solutions of C forms of 1a-c were found at room temperature after seven days in dark
conditions, which indicates the absence of backward thermal processes and high thermal stability
of the ring-closed isomers of the studied fulgides.
Ring-closed C forms of fulgides 1a-c exhibit fluorescence of the toluene solutions (Table 1,
Fig. 2). Their fluorescence bands maxima are located in the region at 603-608 nm, while the
maxima of the fluorescence band of C isomer of 5-methoxy-2-methylnaphtho[1,2-b]furan-3-yl
fulgide 2 was found in the longer wavelength region (λmax = 648 nm).22 Elongation of the alkyl
chain in 1b and 1c leads to a rather small (Δλ = 5 nm) bathochromic shift of the fluorescence
bands as compared with the methoxy substituted fulgide 1a.
Figure 2. Spectra of fluorescence (1) (exc = 500 nm) and fluorescence excitation (2) (obs = 600
nm) of photoisomer C in toluene obtained after irradiation of 1c for 12 min with light of 365 nm
wavelength (4·10-5 mol L–1, T = 293 K).
Quantum yields of fluorescence of fulgides 1a-c tend to increase with increasing the length of
the 5-alkoxy chain: from 0.003 (a, R = Me) to 0.004 (b, R = (CH2)5Me) and up to 0.005 (c, R =
(CH2)15Me). For comparison, we determined the quantum yield of fluorescence of 2 and found
that it is only 0.002 (Table 1). The fluorescence excitation spectra are in good agreement with the
long-wavelength absorption of C isomers, which confirms the conclusion that only ring-closed
isomers of fulgides 1a-c are responsible for the observed emission properties. In addition, these
spectra demonstrate that in the spectral region from 300 to 400 nm there are two additional bands
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with the maxima at 320 and 370 nm corresponding to the S0→S3 and S0→S2 transitions of the
ring-closed isomers of 1a-c (Table 1, Fig. 2). After irradiation of colored solutions of 1a-c with
visible light (λirr = 546 nm) the intensity of fluorescence decreases to zero. Consequently,
fulgides 1a-c are capable of efficient modulation of the emission by UV/visible light (at least for
10 cycles) while for previously known 1-benzofuryl fulgides little modulation of the
fluorescence was achieved23 and their resistance to photodegradation was insufficient (two
switching cycles).
Fulgides 1a-c manifest high resistance to photodegradation. As shown in Fig. 3 by an
example of 1c, the optical density in the maximum of absorption band of C form (in the
photostationary state) remained practically unchanged within 10 photocoloration-
photodecoloration cycles. Accordingly, no changes in fluorescence intensity of C form (in the
photostationary state) were detected.
Figure 3. Optical density dynamics of fulgide 1c in toluene at λobs = 500 nm (the absorbance
band maximum of the ring-closed C isomer) under photocoloration (λ1 = 365 nm, τ = 12 min)-
photodecoloration (λ2 = 546 nm, τ = 13 min) cycling; n is the number of cycles.
As a convenient integral parameter for the evaluation of efficiency of the photocoloration and
photodecoloration processes, we used colorability, which is defined as the product of quantum
yield of photocoloration (photodecoloration) and molar absorption coefficient of the
photoinduced form:27 ФZ(E)∙max (for the overall process Z→E→C, including two sequential
C
photoinduced reactions – Z/E-photoisomerization and subsequent E→C photocyclization) and
ФCE∙maxC (for the photodecoloration reaction C→E).
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Table 2. The values of colorabilities of fulgides 1a-c in toluene at T = 293 K: (photocoloration -
C
a
ФZ(E)C∙maxC and photodecoloration - ФCE∙max
)
C
C
Compound
ФZ(E)C∙max
ФCE∙max
M-1 cm-1
ФZ(E)C/ФCE
1a
1b
1c
337
273
277
1170
1160
1074
0.29
0.24
0.26
a
ФZ(E)C and ФCE – quantum yields of photocoloration (cyclization) and photodecoloration
C
(opening of the ring) reactions, correspondingly; max – the value of a molar extinction
coefficient at the absorption band maximum of the photoinduced ring-closed form C.
The determined values of the efficiencies of the photocoloration reactions Z→E→C of
fulgides 1a-c show a slight dependence on the substituents (277-337 M-1 cm-1). The efficiencies
of the photodecoloration reactions C→E are at 3.47-4.25 times higher than the corresponding
parameters of the photocoloration Table 2. The ratio of the quantum yields of the
photocoloration to the quantum yields of the photodecoloration (ФZ(E)C/ФCE) obtained for 1a-c on
the basis of these data are 0.24-0.29 that indicates significantly higher efficiency of the
photoinduced reaction of the cycle opening (C→E) (Table 2).
Conclusions
Thus, 5-alkoxy-2-methyl-1-benzofuran-3-yl fulgides 1b,c, possesing long alkyl substituents,
represent novel photochromic compounds with modulated fluorescence. UV-irradiation of their
ring-opened isomers O results in the rearrangement into thermally stable pink-colored ring-
closed forms C exhibiting fluorescence properties in contrast to the initial O structures. The
irradiation of ring-closed isomeric forms with visible light leads to their re-opening and complete
fluorescence quenching.
Experimental Section
General. 3-[1-(5-Methoxy- (1a), 3-[1-(5-hexyloxy- (1b) and 3-[1-(5-hexadecyloxy-2-methyl-1-
benzofuran-3-yl)ethylidene]-4-(1-methylethylidene)dihydro-2,5-furandione
(1c)
were
synthesized earlier following the principles of the Stobbe condensation.26 Their purity (98%)
was confirmed by thin layer chromatography and by 1H NMR spectroscopy.
The electronic absorption spectra were recorded on a Varian Cary 100 spectrophotometer, the
emission spectra - on a Varian Cary Eclipse spectrofluorimeter. Toluene of the spectroscopic
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grade (Aldrich) was used to prepare solutions. The fluorescence quantum yields were determined
by the Parker-Rice method28 using methylene blue in water (φ = 0.02, λirr = 540 nm) as a
standard luminophore.29 The solutions were irradiated in a quartz cell (l = 1 cm) with a DRSh-
250 mercury lamp using a set of interferential light filters to allocate mercury spectral lines (365
and 546 nm). The kinetic photocoloration curves of fulgide solutions were recorded directly
during irradiation on a Cary 50 spectrophotometer equipped with the temperature-controlled cell.
A xenon lamp with a monochromator for allocation of narrow spectral lines (Newport) was used
as a radiation source. The intensity of light source was determined using a Newport 2935 power
meter for optical radiation. The irradiation light intensity at 365 and 546 nm was determined as
3.83∙1015 and 5.68∙1015 photon∙s-1, correspondingly. Colorability parameters ФZ(E)C∙max and
C
C
ФCE∙макс were calculated on the basis of kinetic photocoloration and photodecoloration curves
using the procedure.27
Acknowledgements
This research was supported by The Ministry of Education and Science of Russian Federation in
the framework of the State Assignment for Research (Project No. 1895).
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