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European Journal of Natural History
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NOVEL PHOTOCHROMIC SPIROPYRANS DERIVED FROM 6-HYDROXY-4-METHYL-5-FORMYLCOUMARIN

Dubonosov A.D., Nikolaeva O.G., Tsukanov A.V.
Novel indoline, isobenzofuran and naphtopyran spiropyrans (SPPs) containing in 2H-chromene part of the molecules fused coumarin moiety were synthesized. According to 1H NMR, IR and UV/Vis spectral data these compounds exist in cyclic spiroform SP. Under irradiation of their solutions in isopentane-isopropanole (4:1) mixture at Т < 250 К the formation of merocyanine (MC) isomer was observed. The back reaction MC ® SP is thermally reversible.

hotochromic spiropyrans (SPPs) are widely used for data recording, as photocontrolled organic molecular switches, in devices of molecular electronics and as chemosensors for metal cations [1-4]. Some representatives of coumarin-containing SPPs were synthesized earlier [5-9]. In order to study influence of annelation manner of coumarin moiety to 2H-chromene part of the molecule on photochromic and spectral properties of SPPs novel indoline, isobenzofuran and naphtopyran spiropyrans were synthesized.

6-Hydroxy-4-methyl-5-formylcoumarin 1 as aldehyde component was used for the synthesis of above mentioned SPPs. Perchlorates of N-substituted 3H-indolium, isobenzofuranium and benzo[f]chromenium were employed to form hetaryl part of the molecules. Spiropyrans 2 were obtained by condensation of corresponding 3H-indolium perchlorates with aldehyde 1 in isopropanol in the presence of triethylamine
(scheme 1).

Scheme 1

Spiropyrans 3,4 were synthesized in two steps (scheme 2). The first one represented condensation of isobenzofuranium and benzo[f]chromenium perchlorates with aldehyde 1 in acetic acid. The second step consisted in treatment of intermediate product by dry ammonia in benzene
solution.

Scheme 2

In upfield region of 1H NMR spectra of spiropyrans 2-4 are present two signals of magnetic unequivalent gem-methyl groups. Signals of diastereotopic methylen group protons of N-benzyl substituent in SPP 2a are observed as two doublets at 4,20-4,30 ppm. Signal of Н-2 proton in double bond of pyran cycle of compounds 2-4 are registered as doublet at
5,32-5,85 ppm. Doublet of Н-1 proton is closed by aromatic proton signals at 7,00-8,50 ppm. However for molecules 2b,c,d and 4 this doublet is seen at 7,40-7,60 ppm. These data confirm that SPPs 2-4 exist in cyclic SP form.

Singlet signal of Н-9 proton in coumarin moiety is registered in the region 6,15-6,19 ppm. The location of Н-5 and Н-6 protons is very specific: doublet at 6,56-6,90 ppm. The second doublet of these protons is closed by aromatic proton signals at 7,00-8,50 ppm, however for 2d it is seen at 7,32-7,36 ppm.

 

Table 1 1H NMR spectra of spiropyrans Н NMR spectra of spiropyrans 2-4 in СDCl3

Comp,

Chemical shift, ppm (J/Гц)

Hetarene fragment

Pyran fragment

С1Н = С2Н (1Н, d)*

2а**

1,10 s, 1,25 s (6Н, 2 gem-CH3);

4,00 d, 4,50 d (2Н, NCH2)

5,40-5,55

2b

0,86 t (3Н, СН3); 1,19 s, 1,55 s (6H, 2 gem-CH3);
1,28 s (2Н, СН2); 3,08 m (2Н, NСН2)

5,83-5,85; 7,56-7,60

2c

0,86 t (3Н, СН3); 1,18-1,28 m (12H, 2CH3, 3СН2);
1,55 s (2Н, СН2);3,05-3,26 m (2Н, N-СН2)

5,80-5,84; 7,56-7,60

2d

0,86 t (3Н, СН3); 1,18-1,28 m (16H, gem- 2CH3, 5СН2);
1,56 s (2Н, СН2); 3,05-3,26 m (2Н, NСН2)

5,80-5,84; 7,56-7,60

3**

1,30 s, 1,45 s (3Н, 2 gem-2CH3)

5,65-5,75

4**

1,82 s (3Н, CH3)

5,60-5,80; 7,40-7,50

Notes:

* Signal of the second proton is in the region of aromatic proton signals;

** In С6D6.

 

Electronic absorption spectra of SPPs 2-4 in isopentane-isopropanol mixture (4:1) show long-wavelength bands with maxima centered around 364-402 nm with strongly marked vibration structure which are indicative for cyclic forms SP [2, 10, 11]. The irradiation of compounds 2-4 SP in above mentioned solution (λirr 365 nm, Т < 250 К) leads to formation of merocyanine isomers 2-4 MC. In dark conditions these isomers thermally convert into the initial spiroforms
(table 2).

Scheme 3

　

Table 2 Spectral characteristics of isomers SP and MC for 2-4 in isopentane-isopropanol mixture (4:1) at 203 K

Comp.

Spiroform SP, lmax, nm

Photoinduced form MC, lmax, nm

2a

382

609

2b

380

606

2c

384

609

2d

383

606

3

364

550

4

402

602

According to these data a new type of photochromic coumarin-containing SPPs was synthesized.

Acknowledgments

This work was supported by Russian Foundation for Basic Research (grants №09-03-00052), Ministry of Education and Science of Russian Federation (grant N RNP 2.2.1.1/2348), and President`s of RF grant (NSh-3233.2010.3).

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