Photochromic 1-benzofurylfulgides with modulated fluorescence

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 (λ max 344-346 nm). Under UV-irradiation they rearrange into thermally stable colored ring-closed isomers (λ max ~ 500 nm) exhibiting fluorescence emission (λ max 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.


Introduction
2][3][4][5][6][7] The lightinduced 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][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-1benzofuran-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. 25A preliminary information on the synthesis of compounds 1 was reported earlier. 26

Scheme 1. Structures of 1a-c and 2.
The three-proton singlet signals of isopropylidene methyl groups are found, indeed, in the region of 2.04-2.46ppm, while for the E-structures the characteristic signal appears at 1.12 ppm. 23The E-configuration of 2 was unequivocally confirmed by the X-ray structural determination. 22Spectral and photochromic properties of 1a-c were studied in toluene solutions at ambient temperature.Electronic absorption spectra of fulgides 1a-c are characterized by longwave 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).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[1,2b]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.3][14][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). 12,15herefore, 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). 22Elongation 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.Quantum 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 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 achieved 23 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 photocolorationphotodecoloration cycles.Accordingly, no changes in fluorescence intensity of C form (in the photostationary state) were detected.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 C (for the overall process Z→E→C, including two sequential photoinduced reactions -Z/E-photoisomerization and subsequent E→C photocyclization) and ФCE•max C (for the photodecoloration reaction C→E).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 3.47-4.25times 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 ringclosed forms C exhibiting fluorescence properties in contrast to the initial O structures.Their purity (98%) was confirmed by thin layer chromatography and by 1 H NMR spectroscopy.

Experimental Section
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 grade (Aldrich) was used to prepare solutions.The fluorescence quantum yields were determined by the Parker-Rice method 28 using methylene blue in water (φ = 0.02, λirr = 540 nm) as a standard luminophore. 29The 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•10 15 and 5.68•10 15 photon•s -1 , correspondingly.Colorability parameters ФZ(E)C•max C and ФCE•макс C were calculated on the basis of kinetic photocoloration and photodecoloration curves using the procedure. 27

Figure 1 .
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).

Table 1 .
22ectral absorption and spectral fluorescent characteristics of isomeric forms of 1a-c and 222in toluene at T = 293 К

Table 2 .
The values of colorabilities of fulgides 1a-c in toluene at T = 293 K: (photocoloration -ФZ(E)C•max C and photodecoloration -ФCE•max C ) a ФZ(E)C and ФCEquantum yields of photocoloration (cyclization) and photodecoloration (opening of the ring) reactions, correspondingly; max Cthe value of a molar extinction coefficient at the absorption band maximum of the photoinduced ring-closed form C. a