Spiropyrans Raman

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

The various techniques of Raman spectroscopy have recently been applied to the spiropyrans, and this is discussed more extensively in Chapter 8 in Volume 2. BIPS and its 6-nitro-, 6-nitro-8-methoxy-, and 6-nitro-5,7-methoxy derivatives were studied by surface-enhanced (SERS) and resonance-enhanced Raman spectroscopy in neutral, acidic, and basic solutions. The complex results are too numerous to summarize here, but two unusual ones are that even in acid solutions the tested compounds gave the SERS spectra of the unprotonated open forms, and that an aged solid sample of 6-nitroBIPS gave results very different from a fresh sample. This was attributed to the existence of two enantiomers of the spiro form, which are slowly interconvertible in the solid state and which have greatly different photocoloration quantum yields.167... 

Y. Onai, K. Kasatani, M. Kobayashi, H. Shinohara, and H. Sato, Long-lived colored merocyanine conformers in the aggregates formed on UV irradiation of spiropyran. A Raman spectroscopic study, Chem. Lett., 1990, 1809-1812. 

S. Schneider, H. Grau, J. Ringer, and M. Melzig, Surface-enhanced resonance Raman studies of spiropyrans (BIPS and derivatives), Mol. Cryst. Liq. Cryst., 246, 267-274 (1994). 

T. Yuzawa and H. Takahashi, Time-resolved resonance Raman and absorption spectroscopies of reaction intermediates in the photochromism of spiropyrans, Mol. Cyst. Liq. Cryst., 246, 279-282 (1994). 

Chapter 8 (New Trends in Raman Studies of Organic Photochromes). This chapter presents an overview of past and recent vibrational data on spiropyrans and spirooxazines, obtained by various Raman approaches. 

As briefly outlined above, recent improvements in Raman techniques have opened up new possibilities for the study of the structure and dynamics of species involved in the photochromic reaction. Therefore, the purpose of this chapter is not to cover all aspects of vibrational spectroscopy, particularly Raman spectroscopy, but merely to focus on the new research trends in work on spiropyran and spirooxazine compounds. 

In view of the difficulties encountered in direct interpretation of the vibrational spectra of spiropyrans, Davin-Pretelli et al.w proposed a more rational approach. They investigated first the IR and Raman spectra of the two constituent moieties of the photochrome under study. Thus, from a preliminary IR and Raman study of benzothiazolines and 2/f-chromenes, which served as models, they were able to give a satisfactory assignment of the spectra of a series of benzothiazolinic spiropyrans.9 Moreover, on the basis of the vibrational data for 2//-chromenes, the set of bands at ca. 1250 cm-1 was unambiguously assigned to vibrations involving... 

In order to demonstrate the usefulness of the method, SERS spectra of indolino-spironaphthopyran (SPP, 4) and indolino-spironaphthoxazine (SPOX, 3) (see Scheme 7) were recorded at low concentration (10 7-10-8 A/) and their spectral characteristics analysed.48 Figure 8 shows the SERS spectrum of 4 in Ag colloids (spectrum a) along with the corresponding near-infrared Fourier transform (NIR-FT) Raman spectrum of the neat compound (spectrum b). Strong differences between the SERS and spontaneous Raman spectra are apparent. As stated above, these differences arise mainly from the particular orientation of the adsorbed spiropyran with to the metal surface in SERS experiments. 

M. Guiliano, E. Davin-Pretelli, G. Mille, J. Chouteau, and R. Guglielmetti, Infrared and Raman spectroscopic studies of benzothiazolinic spiropyrans and merocyanines, Helv. Chim, Acta 61, 1072-1085 (1978). 

J. Aubard, C. M Bossa, J. P. Bertigny, R. Dubest, G. Levi, E. Boschet, and R. Guglielmetti, Surface enhanced Raman spectroscopy of photochromic spirooxazines and related-spiropyrans, Mol. Cryst. Liq. Cryst. 246, 275-278 (1994). 

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