Synthesis and photochromism of naphtho[2,1-b ]furyl fulgides

A series of novel 2-and 5-naphtho[2,1-b ]furyl fulgides with different combinations of substituents (H, Me, Et) in the furan ring and methylidene bridge were synthesized and their spectral, luminescent and photochromic properties were studied. The most sterically overcrowded ring-opened fulgides 4 Z O and 14 Z O under UV irradiation rearrange into the colored fluorescent ring-closed isomers C . The reverse dark reaction C → E O is not observed at room temperature over 48 h. The exposure of C forms to visible light results in the backward isomerization into the initial non-fluorescent form O . Thus, fulgides 4 and 14 represent photochromic compounds with modulated fluorescence which are sufficiently fatigue-resistant with respect to photodegradation, surviving at least ten cycles of photocoloration-photobleaching without notable decrease in the optical density at the absorption maximum of the cyclic form C .


Introduction
2][3][4][5][6][7][8][9] Heterocyclic fulgides are widely studied in this regard due to their high thermal stability and the fatigue resistance of their photocolored cyclic forms.Photoirradiation of these compounds results in the electrocyclic hexatriene-cyclohexadiene rearrangement of ring-opened isomers O into colored ring-closed isomers C. The isomeric forms O and C possessing markedly different physical chemical characteristics do not interconvert when irradiation ceases.1][12] Previously, we synthesized and studied spectral-luminescent and photochromic properties of 1-benzofuryl and naphtho [1,2-b]furyl fulgides. 13,14Taking into account that spectral and fatigue resistance characteristics of this class of photochromic compounds are considerably dependent on the nature of their heterocyclic moieties, 11,15 we have synthesized novel naphtho [2,1-b]furyl fulgides and studied their properties in comparison with their [1,2-b] analogues.

Results and Discussion
Fulgides 4Z and 5E were synthesized starting from 1-methylnaphtho[2,1-b]furan 1 according to Scheme 1. Acetylation and formylation of 1 led to 2-acetyl-1-methylnaphtho[2,1-b]furan 2 and 1-methylnaphtho-[2,1-b]furan-2-carbaldehyde 3, respectively.The trienes 4Z and 5E were prepared by Stobbe condensation of 2 and 3, correspondingly, with diethyl isopropylidenesuccinate in THF followed by hydrolysis and cyclization.The intermediate mono ethyl esters 4M, 5M and the corresponding fulgenic acids 4F, 5F were purified and used in the next step of the reaction without isolation.The 5Z-isomer was obtained from 5E according to a specially developed technique based on differential solubilities of Z and E forms in benzene coupled with the use of irradiation (see Experimental Section).
2-Naphtho[2,1-b]furyl 11 and 5-naphtho[2,1-b]furyl fulgides 13, 14 were synthesized as shown in Scheme 2. 2-Acetylnaphtho[2,1-b]furan 6 under the Stobbe condensation conditions gave rise to fulgide 11E and to side product 12, which is rather unusual for this reaction.Here, one of the methyl groups of the isopropylidene fragment was involved in the condensation.The 11Z-Isomer was obtained from 11E according to the technique based on different solubility of Z and E forms in acetonitrile and irradiation of the E form as in the 5E→5Z conversion (see Experimental Section).
The Vilsmeier-Haack formylation of 2-ethylnaphtho[2,1-b]furan 7 led to the 5-formyl product 8 as a single product, while the Friedel-Crafts acetylation of 7 generated a mixture of 1-acetyl-9 and 5-acetyl-2ethylnaphtho[2,1-b]furan 10 as the dominant product.It should be mentioned that the last result is in disaccord with the data 16 that reported the exclusive formation of the product of 2-acylation.The Stobbe condensation of 8 and 10 with diethyl isopropylidenesuccinate in THF followed by hydrolysis and cyclization gave rise to fulgides 13E and 14Z, respectively.Intermediate mono ethyl esters 11M, 13M, 14M and the corresponding fulgenic acids 11F, 13F, 14F were purified and involved in the next step of the reaction without isolation.The structures of the ring-opened Z-and E-isomeric forms O of fulgides 4, 5, 11, 13 and 14 were confirmed by 1 H and 13 C NMR and IR spectrometry.The IR spectra of these compounds exhibit characteristic spectral bands of two exocyclic furandione carbonyl groups at 1744-1823 cm -1 .The 1 H NMR spectra of 5E, 11E and 13E contain signals of methyl group protons at 1.33-1.95and 2.40-2.80ppm, unambiguously indicative of their Zconfiguration. 17Analogues signals of 4Z, 5Z, 11Z and 14Z are observed at 2.01-2.73ppm.
The electronic absorption spectra of fulgides 4, 5, 11 and 13 in toluene are characterized by long wavelength bands with maxima in the range of 403-431 nm (Table 1).Compound 14 absorbs in a somewhat shorter-wave region of the spectrum.Table 1  Irradiation of toluene solutions of fulgides 5E, 11E and 13E with light of 365 nm leads to the spectral changes characteristic of E/Z-isomerization processes 18,19 not followed by the formation of the ring-closed forms C, known to absorb in the visible region 20 (Scheme 3, Fig. 1).In all cases, a slight bathochromic shift of 5-8 nm accompanied by a moderate hyperchromic effect was detected.The reversible transformations are initiated by exposure to light of 436 nm.Similarly, irradiation of toluene solutions of 5Z and 11Z with UV-light resulted in only Z/E-isomerization without formation of the ring-closed forms C.However, irradiation of the most sterically overcrowded fulgides 4Z and 14Z in toluene with light of 365 nm led to a rose red photocoloration of the solution due to the appearance of a new absorption band with a maximum at 480 nm, the intensity of which increased upon irradiation while the intensity of the initial long wavelength absorption band decreased (Fig. 2, Table 2).These spectral changes are indicative of the occurrence of the electrocyclic rearrangement of hexatriene ringopened isomer O into 1,3-cyclohexadiene ring-closed form C that involves a prior Z/E-isomerization step of O (Scheme 4) and results in the formation of the photostationary state. 1,11,19The establishment of the photostationary state 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. 11,15 It should be mentioned that the ring-closed form C of naphtho [   No changes in the absorption maxima of photoinduced isomers 4 C, 14 C were found at room temperature after three days in dark conditions, which indicates the absence of backward thermal processes and a high thermal stability of the ring-closed isomers of these fulgides.However, subsequent irradiation of the colored solutions of 4 C, 14 C with light of 436 nm leads to their photodecoloration due to the reverse ring-opening photoreaction C → E/Z-O. 11,15ing-closed forms of fulgides 4 C, 14 C display fluorescence in solution in toluene (Table 1).The fluorescence excitation spectra are in good agreement with the long-wavelength absorption of C isomers, which confirms that only ring-closed isomers C are responsible for the observed emission properties.After photodecoloration of 1a-c the intensity of fluorescence decreases to zero.Fulgides 4 and 14 are capable of modulation of the emission by UV/visible light at least for 10 cycles without change of absorption and fluorescence intensities.

Conclusions
In conclusion, novel 2-and 5-naphtho[2,1-b]furyl fulgides with different combinations of substituents (H, Me, Et) in the furan ring and methylidene bridge were synthesized.Irradiation of fulgides 5E, 11E, 13E and 5Z, 11Z with UV-light resulted in E/Z (Z/E)-isomerization processes without formation of the ring-closed forms C. On the contrary, the most sterically overcrowded fulgides 4 and 14 behave as photochromic compounds with modulated fluorescence and possess high resistance to photocoloration-photobleaching cycling.UVirradiation of their ring-opened isomers O induces the rearrangement into thermally stable rose red ringclosed forms C exhibiting fluorescence properties in contrast to the initial O structures.The irradiation of ringclosed isomeric forms with visible light leads to their re-opening and complete fluorescence quenching.

Experimental Section
General.The IR spectra were recorded on a Varian Excalibur 3100 FT-IR instrument using the attenuated total internal reflection technique (ZnSe crystal).The 1 H NMR spectra in CDCl3 and DMSO-d6 were recorded on a Varian Unity 300 spectrometer (300 MHz) and on integrated analytical LC-SPE-NMR-MS system Bruker Avance-600 (600 MHz for 1 H, 120 MHz for 13 C), the signals were referred with respect to the signals of residual protons of deutero-solvents (7.24 and 2.50 ppm), δ values were measured with precision 0.01 ppm.Mass spectra were recorded on a Shimadzu GCeMSQP2010SE instrument with direct sample entry into the ion source (EI, 70 eV).The electronic absorption spectra were recorded on a Varian Cary 100 spectrophotometer.

Table 1 .
Electronic absorption spectra of the isomeric forms of fulgides 4

Table 2 .
Absorption and fluorescent spectra of fulgides 4 and 14 in toluene