Nitro-BIPS

Detection by SERS of Degradation Products from UV-Irradiated Solutions of 8-Methoxy-6-Nitro-BIPS 

Hfcure ll. SFRS spc iw of S-nicdimy-f-nilic-BlPS 1 SchL- K in Ag cotloiils. Laser iMocililian 514.5 am. 20m1V. 

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 

In these experiments, SERS spectra were obtained by taking smaU aliquots at different time intervals from a continuously UV-irradiated solution. The aliquots 

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In the presence of acid, unsubstituted BIPS assumes a thermally stable protonated colored form, and shows reversed photochromism, in which the phenolate form changes to a phenol.20 22 In this case, the absorption band remarkably shifts to the short wavelength. For example, in the presence of acid the absorption band of 6-nitro-BIPS occurs at 405 nm in acetone. 

Thermal ring opening to form the merocyanine form is less dependent upon the type of substituent keeping a AG of near to 101 kJmol for BIPS, 6-nitro-BIPS and 6,8-dinitro-BIPS. [37] The ring opening reaction s rate-determining step is probably a rotation cis to trans about the central p-methine bond and this is also consistent with the polar CCC or CCT transition state which have a rather double p-methine bond. 

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. 

Sato et al. [42] measured resonance Raman spectra of 6-nitro-BIPS solid aggregates. They assigned the aggregates that they collected from cyclohexane as being the same as Takahashi groups mixture of isomers in acetonitrile [57] (not cyclohexane) because their spectra were similar. Sato et al. concluded that the aggregates were pure merocyanines with no ring-closed form present. 

The thermal reverse reaction s value of AG increased from 77 to 92 to 111 kJ mol respectively for DIPS, 6-nitro-BIPS, and 6,8-dinitro-BIPS probably due to stabilization of the merocyanine form by delocalization of the negative charge on the C9 oxygen [37]. 

A cooperative model for isomerization was proposed to explain the effect of intense laser irradiation. In this model, the formation of excited or transient states in close proximity can transiently provide enough free volume for isomerization to occur. This work with NOSH has been complemented by studies using 6-nitro-BIPS and NIPS [109,110]. This cooperative model is shown in Figure 12. 

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 absorption maxima in toluene and ethanol, the thermal fade rate constants in toluene and dioxane, and the kinetic parameters AH and AS for a series of F-phenyl- and T-carbethoxymethyl-6-nitro BIPS having also methoxy, bromo, or nitro substituents in the 8-position have been recorded. The effects of the substituents on the Xmax and the rate constants are discussed. The preparation of 1-carbethoxy-methylFischer s base is given in detail.112... 

The role of the X isomer in the photocoloration of 6-nitro- l, 3, 3 -trimcthyl-spiro 2H-1 -bcnzopyran-2,2 -indoline (6-nitro-BIPS, 6) has been discussed by several authors. 

In a picosecond transient absorption spectroscopy study of 6-nitro-BIPS derivatives, Krysanov and Alfimov8 identified a transient absorbing at 440 nm and proposed its formation within 8 ps from the excited singlet state SPNo2 of the... 

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. 

Equation (2.4.2.14), which considers only photodegradation, describes then only approximately the behavior of irradiated spiropyran solutions. The observed deviation is, in the author s opinion, derived from an oxidation process not included in Scheme 14 as the data reported in Figure 4 for 8-methoxy-6-nitro-BIPS (11) seem to indicate. 

Photochemical fatigue of 6-nitro-BIPS (6) depended on the presence in solution of triplet sensitizers or quenchers. The triplet energy Er for 6 is 64.3 kcal mol-1. When benzophenone, a triplet-state sensitizer having a triplet energy Er— 69.2 kcal mol -was added to solutions of 6, a considerable acceleration of the fatigue was observed (Figure 14). On the other hand, 1,1 -binaphthyl ( >=54.3 kcal mol-1) acted as a triplet-state quencher, and its presence in solution decreased the colorability (Figure 15). 

Of the three compounds studied, 4, 8, and 11, 8-methoxy-6-nitro-BIPS (11) had the worst performance although it exhibited a very good colorability. The color-ability of spironaphthopyrans was low, and their fatigue resistance slightly better. Spirooxazine 8 has the best photostability and an acceptable colorability (Table 24). 

We have shown38 that 8-methoxy-6-nitro-BIPS (11) is the only photochrome able to sensitize the formation of (h with a quantum yield A = 0.15 that is about one-fourth that of a commonly used singlet-oxygen sensitizer, i.e. methylene blue, for which 0A = 0.57.39 This benzopyran behaves as a good quencher of l02 as well. Unfortunately, in the paper by Guglielmetti and co-workers an analysis of the... 

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. 

H. Takahashi, H. Murakawa, Y. Sakaino, T. Ohzeki, J. Abe, and O. Yamada, Time-resolved resonance Raman studies of the photochromic reaction of 6-nitro-l, 3, 3 -tnmethylspiro[27/-1 -benzopyran-2,2 -indoline] (6-nitro BIPS), J. Photochem. Photobiol. 45, 233-241 (1988). 

H. Sato, A. Matsuzaki, S. Nishio, M. Yoshimi, K. Yamamoto, and H. Tomioka, A Raman Spectroscopic study of merocyanine aggregates of a N-15 isomer of 6-nitro BIPS. A direct evidence for the association through the NOj bridges, Chem. Lett. 8, 715-716 (1996). 

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