6-Nitro-substituted BIPS

Further, Takahashi et al. [57] reported no spectral change in an air atmosphere between 200 nsec and 2 msec for 6-nitro-BIPS in methanol and acetonitrile, a result that is consistent with Lenoble and Becker s findings [56]. Kaliskey and co-workers [58] collected data for a nitro-substituted BIPS in acetonitrile with many similarities to that of Lenoble and Becker and further noted that the buildup of the 560 nm absorption is concentration dependent, which implies a bimolecular reaction such as aggregation leads to the absorption changes in this spectral region. 

Nitro-substitution especially at the 6-position of BIPS opens up a triplet pathway for photo-isomerization. This pathway runs in parallel to the singlet manifold. This increases the yield and, in turn, may lead to photo-aggregation that is observed for these compounds. Photochemical ring closure to the spiropy-ran form is more efficient for these 6-nitro-substituted compounds. The photochemistry of 6-nitro-BIPS merocyanine is similar to that of unsubstituted BIPS(s) however, the 6,8-dinitro compound efficiently cyclizes upon excitation to form the spiropyran closed form via a singlet manifold. 

The third approach is useful when the spiropyran itself (or, more correctly, its intermediates) is easily available, the necessary substituted intermediates are not, and the spiropyran tolerates the chemical transformations involved. Halogenation and nitration of BIPS and spiro(dinaphthopyrans) can be carried out by standard methods bromo substituents replaced by cyano or lithium, nitro substituents reduced to amino, etc. For example, the aldehyde group in 6-formylBIPS can be reduced, oxidized, caused to react with organometallics, condensed with active methylene groups, converted to cyano, and derivatized to give a wide variety of 6-substituted BIPS. These reactions cannot be usefully carried out directly on 5-formylsalicylal-... 

In attempts to prepare 3-acyl-substituted BIPS (113), various salicylaldehydes were condensed with l,3,3-trimethyl-2-formylmethyleneindoline (Fischer s aldehyde). In alcohol, dimethylformamide, tetrahydrofuran, Y-mcthylpyrrolidonc. and hexamethylphosphoramide as the sole solvents, decarbonylation occurred and the only products were the BIPS compounds. However, in dimethyl sulfoxide, 5-nitro-and 3-bromo-5-nitrosalicylaldehydes gave 11 and 5.6% of the desired 3-formyl-BIPS, respectively. The structures were assigned from their IR and NMR spectra. 

The nitration of 5-chloromethylsalicylaldehyde in the absence of water gave the 3-nitro derivative in 43% yield, which was very easily converted to the hydroxymethyl and acetoxymethyl compounds. These gave the corresponding photochromic 8-nitro-6-substituted BIPS, which had Xmax 590 and 600 nm, respectively. Compared with 8-nitroBIPS itself, the thermal fade rates of the compounds having the additional 6-substituents were slightly faster in toluene and slightly slower in dioxane.109... 

Spiropyrans, especially the nitro-substituted ones, are in general endowed with good colorability. However, one of the major deficiencies of spiropyrans for many potential applications is their short life cycle because of the extensive thermal and photochemical decomposition that they undergo by repeated and/or continuous light exposure. In 1968 Gautron28 was the first to investigate the photodegradation of 6-nitro-BIPS (6) and four other derivatives. 

These results may be compared with those of recent SERS experiments performed by Schneider et al.,54 who postulated that SERS spectra ofnitro-BIPS derivatives in methanolic solutions originated from neutral open merocyanine species. The presence of open forms in solutions of 7, in the absence of any UV light, probably arises from solvatochromism, which is an efficient process in the opening of nitro-substituted spiro compounds in polar solvents.55 Moreover, as discussed above, the detection of photomerocyanines, even at trace levels, is favored with respect to that of the nonresonant closed form of the photochromes by a SERRS effect excited at 5l4.5nm. It should be noted that in the case of cyanine dyes, SERRS spectra, in Ag colloids, from 10 I7M solutions have been recently reported.56... 

Another class of merocyanines consists of ring-opened spiropyrans, such as spiro[2H-l-benzopyran-2,2 -indolines] (BIPS), especially of 6-nitro-substituted derivatives (NOjBIPS). The photochromism of spiro... 

The salicylaldehydes commonly used have nitro, alkoxy, or halogen substituents, and need no further comment. Some unusual ones have cationic substituents 3-, 4- and 5-methylthio- and dimethylaminosalicylaldehydes have been prepared and methylated to the corresponding dimethylsulfonium and trimethylammonium salicylaldehydes. 1012 The salicylaldehyde substituted in the 4-position with a 3,5-diphenyl-2-pyrazolin-l-yl group gives a series of BIPS and spiro(dipyrans) having highly fluorescent spiro forms and nonfluorescent open forms, which is the reverse of the usual situation. 13... 

A BIPS having hydrogen rather than alkyl on the l -nitrogen atom could possibly be formed from the reaction between a 2,3,3-trialkylindolenine and a salicylaldehyde. However, the reactions between 4-nitro-2,3,3,7-tetramethylindole-nine and five different substituted salicylaldehydes (3-methoxy-5-nitro 5-nitro 3-bromo-5-nitro 3-nitro-5-methoxy and 3-nitro) gave only the 2-hydroxy(substitu-ted)styryl dyes, which were not solvatochromic or photochromic. The yields were 3-7% based on 5-nitro-2-methylphenylhydrazine, the precursor of the indolenine used. The NMR, UV, and mass spectrometry (MS) evidence excluded spiro or open zwitterionic structures for these products.81... 

Upon treatment with acid (or on contact with an acidic surface), many spiropyrans give the salt of the open form or the open form itself, depending upon the relative base strengths of the spiro and open forms. Thus, treatment of several BIPS (7-diethylamino, 6-nitro, and 5 -nitro) with trifluoroacetic acid in the nonprotic solvents acetonitrile and chloroform gave the protonated merocyanine form, which upon neutralization with base gave the open colored form.180 This sequence of operations causes coloration by a non-thermal, non-photochemical route the adsorption coloration was utilized in the early applications of spiropyrans in carbonless (pressure-sensitive) copy papers. In this application, dialkylamino-substituted spirodi(benzopyrans) were preferred paper containing BIPS compounds turned pink on storage. 

Following their first experiments devoted to 6-nitro-BIPS,17 Takahashi and co-workers conducted a series of nanosecond TR3 studies24-28 on a limited number of photochromic compounds, namely r,3, 3-trimethylspiro[2ff-l-benzo-pyran-2,2 -indoline] (BIPS), 6-nitro-BIPS, and spirobibenzopyran (SBP) (Scheme 4). This work was undertaken to elucidate the electronic distribution of the metastable photomerocyanines (ort/jo-quinoidal or zwitterionic) and the number of stereoisomers involved in the photochromic process. Therefore, the effects of temperature, solvent, laser excitation wavelengths, and isotopic substitutions (I3C and, 5N) on the resonance Raman spectra were intensively studied to clarify the photochromic mechanism in the compounds investigated. 

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