Spirobenzopyrans merocyanine form

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 ... 

More recently, a spirobenzopyran dimer bridged by a diaza-18-crown-6 moiety through the 8-position (67) was developed by Kimura and co-workers.119 121 Crowned bis(spirobenzopyran) 67 shows a similar coloration selectivity to that of 63. Complexation of multivalent metals, especially Ca2+ and La3, by 67 enhanced the isomerization of the spirobenzopyran moiety to the corresponding merocyanine form due to an effective intramolecular interaction between a crown-complexed cation and the two phenolate anions in the cation complexes of the merocyanine form. 

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>. 

Chemically modified crowned spirobenzopyran 112, containing a pyrenyl fluorophore attached at the nitrogen atom, can function as a fluorescence emission switch <2004T6029>. This sensor displayed a quenching of the PET fluorescence emission of the fluorophore in the absence of metal ions (the merocyanine form was not produced). When, however, the spiro form of 112 was converted into the merocyanine form by metal ion complexation of the crown ether portion of the molecule, a fluorescence resonance energy transfer (FRET) from the pyrene to the merocyanine moiety took place, producing fluorescence emission. 

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. 

The transference number for Li+ in the 13a-containing lilm was decreased significantly when the spirobenzopyran unit isomerized to its merocyanine form. The results was explained by the enhanced Li+ binding in the merocyanine form. 

The crowned spiropyran in the electrically neutral form can bind an alkali metal ion with the crown ether moiety. At the same time, the spirobenzopyran portion isomerizes to the corresponding merocyanine form photo-chemically. The zwitterionic merocyanine form of crowned spirobenzopyran moiety brings about a significant change in the metal-ion binding ability. This prompted the authors to apply the compound to photo-responsive ion-conductive materials. They observed that the ion-conductivity was increased by ultraviolet light and decreased by visible light [259]. 

Two types of cteviation from the first-orda kinetics are noted for photo-aialthamal isomerization reactions in polymer films. The first is the normal type, in which the reaction rate is the same as or smaller than that in solutions at the initial stage and then progressively becomes smaller. Typical examples are thermal decoloration of the photocolored merocyanine form of spirobenzopyran molecularly dispersed in or chemically bound to a polymer matrix > and photoisomerization of the transazobenzene residue incorporated in polymer main chains The first interpretation for the decoloration of the merocyanine form assumed the existence of different isomers, each of which fades independently following first-order kinetics On the other hand, Kryszewski et al. proposed the kinetic matrix effect, which means that the distribution of free volume may lead to the deviation from first-order kinetics. His idea was based on the finding that deviations from first-order kinetics can be observol even in simple molecules such as azobenzene which has only one trans or cis isomeric form. The effect of free volume distribution on reactivity was further demonstrated by studies of annealed polymer films The distribution function of free volume as well as the critical free volume v were estimated for the merocyanine form of spiropyran in poly(methyl methacrylate) derivatives of azobenzene in polystyrene and azobenzene in polycarbonate The deviation from first-order kinetics was also observed in cyclizing imidization of model poly(amic acid) in a polyamide matrix... 

In general, chemical reactions and random physical processes taking place in solid media such as amorphous polymers follow non-exponential kinetics even if there is good reason to expect that the elen ntaiy step be a first-order process. The nonexponential decoloration rate of the merocyanine form of spirobenzopyran was explained by Kryszewski et al. in terms of the distribution of free volumes in the... 

Kryszewski et al. first proposed the idea of a kinetic matrix effect which related the non-exponential decoloration rate of the merocyanine form of spirobenzopyran (MC (a) and (b)) in PMMA to the distribution of free volume in the matrix, though it had been suggested that the deviation from first-order kinetics in the photo-... 

Horie, K., Hirao, K., Mita, I., Takubo, Y., Okamoto, T., Washio, W., Tagawa, S., and Tabata, Y, Red fluorescence from the merocyanine form of spirobenzopyran, Chem. Phys. Lett., 119, 499, 1985. 

Poly(methacrylic acid) with spirobenzopyran pendant groups is soluble in polar solvents, such as methanol or water, when the content of spirobenzopyran is less than 20 mol%. A methanol solution of the polymer has a weak red color under thermal equilibrium conditions, which indicates the presence of merocyanine form in equilibrium with spiropyran form. Visible irradiation completely bleached the absorption (merocyanine - spiropyran) after the light was removed, the absorption gradually reappeared in the dark (spiropyran merocyanine). Ultraviolet irradiation caused enhancement of the absorption near 530 nm (merocyanine merocyanine) the intense absorption again returned to the thermal equilibrium intensity after removal of the light (merocyanine - spiropyran). Together with these isomerizations. 

The hydrophobic nature of the merocyanine is considered weak compared with that of the spirobenzopyran form because of its zwitterionic structure. The increase in hydrophobic nature on isomerization of the merocyanine to the spiropyran form causes the polymer to contract, resulting in a change in the conformation. The dissociation of the carboxylic acid residue in the side chain is influenced by this conformational change of the main chain, giving... 

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