Spirobenzopyrans photochromism

Comprehensive and important reviews on photochromism of spiropyrans have been published by Bertelson1 and Guglielmetti2 In the present chapter, general synthetic methods and physical properties of spiropyrans with special reference to leuco dyes will be described. The chapter is divided into the spirobenzopyran, spironaphthooxazine, and spirothiopyran and related compounds. 

Solid films of spiropyrans are important in optical data storage. Thin films of spirobenzopyran (1.0 (tm) have been prepared by vacuum deposition, and its reversible photochromism has been confirmed.39 The J-aggre-... 

A class of crown spirobenzopyrans 10 exhibit thermally irreversible photochromism only in the presence of alkali-metal cations (Scheme 3). The favored complexation between 10 and lithium cation was thus revealed by the effect of lithium salt on the electronic absorption spectra of 10. Lithium complexation was also corroborated on the basis of FAB mass experiments. Before addition of Lil to 10, ion peaks for MH, [M + Na] and [M + K] were detected after the addition the signals decreased, while a peak for [M + Li]+ appeared (Figure 10) . 

As mentioned in sections 1.2.2.2 and 1.2.3.2, the photochromic reactions of spirobenzopyran and spironaphthoxazines show a marked solvent dependency and this is also the case with benzo and naphthopyrans. Consequently, spectral data collected from the literature is only comparable within any one study or where the same solvent has been used. This accounts for any discrepancies between one set of results and any other one listed in this and related sections of this chapter. The data normally quoted when discussing the properties of photochromic materials relate to the absorption maximum (2. ) of the coloured state, the change in optical density (absorbance) on exposure to the xenon light source (AOD) and the fade rate which is the time in seconds for the AOD to return to half of its equilibrium value. 

Salicylaldehyde is the starting material for the preparation (Scheme 12) of most spirobenzopyrans, which are important photochromic compounds. The reader interested in their synthesis and properties is referred to the recent excellent book by Bertelson.5... 

The chromene ring of spirobenzopyrans such as (168) and (169) is opened by treatment with mineral add and closed again by alkali — a reaction of potential importance in the thermochromic or photochromic application of such compounds (69T5995,74JOU1516). 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

A recent review discusses the synthesis, spectral properties, and photochemical behavior of novel spirobenzopyrans, Spiro-1,4-oxazines, and 2//-chromenes. It emphasizes the structural features needed for infrared-absorbing photoisomers, luminescent stable forms, improved fatigue resistance, and spiropyrans exhibiting photochromism in vacuum-deposited films.147... 

V. Minkin, Structural variation and responses in photochromic properties of spirocyclic molecular systems related to spirobenzopyrans, Mol. Cryst. Liq. Cryst., 246, 9-16 (1994). 

K. Horie, K. Hirao, N. Kenmochi, and I. Mita, Photochromic reactions of spirobenzopyran, azo-benzene and fulgide at 4 K in polymer films, Makromol. Chem. Rapid Commun., 9, 267-273 (1988). 

The photochromism of spirobenzopyrans is a well-documented phenomenon that arises from the photoinduced reversible isomerization between spiropyran and merocyanine forms . In spirobenzopyrans carrying a crown ether moiety (e.g., Ill), this interconversion process is affected by metal ion complexation. A strong interaction of the crown ether unit with a metal ion caused the thermal isomerization of the spirobenzopyran residue to the corresponding merocyanine form with simultaneous suppression of the UV-induced isomerization process (negative photochromism) (Scheme 3). Conversely, a weak metal ion interaction induced a positive photochromism <2001JOC1533, 2002EJ0655>. 

It was only recently that these phenomena were detected photochemically using photochromic molecules whose chromophores undergo a reversible photoisomerization reaction. The photochemical reactions involved in these processes are mainly the trans-ds isomerization of aromatic azo compounds and stilbene derivatives as well as the ring opening/dosure reaction of spirobenzopyran derivatives . ... 

For polyester VIII characterized by the presence of a bisphotochnomic chromophore in the main diain, a still more pronounced temperature dependeme should be observed. Indeed, an apparent activation energy around Tg of 120 kcal/mol emphasizes the strong influence of bulky spirobenzopyran groups in the main chain on the photochromic behavior. 

A similar but much more detailed interpretation was recently given by Eisen-bach who showed the general validity of the WLE-equation in the interpretation of the photochromism of azobenzenes and spirobenzopyrans linked to amorphous bulk polymers. Eisenbach used directly the WLF-equation ... 

Noteworthy is also the discussion of Krysrewski and Nadolski on the photochromic properties of spirobenzopyran copolymers and copolyesters. They interpreted the photo-chromic feding on the basis of the formation of defects within the matrix resulting from the excess of energy on irradiation as mentioned above. 

Smets and coworkers worked with stretched spirobenzopyran rubber networks obtained by copolymerization of ethyl aaylate with variaWe amounts of a bis-photochrome dimethacrylate as cross-linking agent, namely l,l -(a,a -p-xylyl)-bis-[3, 3 -dimethyl-8-methacryloyloxymethyl-6-nitro-spiro(2H-l-benzopyran-2,2 -indoline)]. The chemical structure of these DIPS-rubbers is given in Fig. 11. 

Luminescence properties of and phenomena in polymer systems continues to be widely researched in connection with mechanisms of polymer degradation and stabilization, molecular dynamics, solubility, blend miscibility, and solar energy harnessing. A number of interesting reviews have appeared. Molecular dynamics of polymers in solution and in the solid state have been covered, as has excimer formation,photoresponsive polymers,behaviour of polymer gels, and photochromic phenomena. Photoisomerization of enzymes and model compounds has also been discussed in depth, with particular emphasis on proteins and synthetic polymers containing azo-compounds or spirobenzopyrans. ... 

Besides azobenzene, spirobenzopyran, and salicylidene derivatives, other photochromic dyes have been used in LBK films. For example, anthocyanine dyes 5 (see Figure 6.5) have been used to obtain monolayers that change the area at constant pressure upon irradiation. Diphenyldiacetylerie chromophores 6 (Figure 6.5) have shown photoinduced anisotropy upon polar-... 

P. M. Rentzepis et al. first demonstrated a bit-oriented 3D optical memory using a photochromic spirobenzopyran shown in Figure 16.3. Isomer A has an absorption band shorter than 450 nm upon irradiation with ultraviolet (UV) light, it converts to isomer B, which has an absorption band around 600 nm, as shown in Figure 16.4. Isomer B gives fluorescence around 700 nm upon photoexcitation with 500-700 nm light. 

Polymer films mixed with low-molecular-weight photochromic compounds, such as nylon film- 3-carotene [28], nylon film-a-cyanostilbene [28], and polystyrene — spirobenzopyran [29] were shown to undergo photostimulated reversible size changes. 

The research was initiated by Kato et al. [51] in 1976, who used an acetyl cellulose film containing photochromic spirobenzopyran and phosphatidyl diloride. Figure 23 shows a schematic diagram of the apparatus used for the measurement of the membrane potential. The concentration ratio, y = C1/C2, of the electrolytes in compartments I and II is a parameter to vary the potential in the dark. In the dark before photoirradiation, the membrane exhibited a steady state potential difference Aq> of —28mv. The membrane potential shifted to — lOmv when the membrane was irradiated with ultraviolet light, and it reverted to the initial value upon visible irradiation. The change in the membrane potential was thus reversible. 

In solution, the thermal reversion of methacrylate polymers substituted with spirobenzopyrans was often accompanied by a spectral shift of the photomerocyanines and a deviation from first-order kinetics. These observations were ascribable to intermolecular interaction between the merocyanine form and the ester residue of methyl methacrylate. This was supported by the emergence of photoviscosity effects in solutions of these photochromic polymers. 

Crowned spirobenzopyrans as ion-responsive photochromic materials 03YGK322. 

---