Negative photochromism

Polyelectrolyte glasses can be cast as optically clear films from a solution of poly(dimethylalkylamines) and BIPS-6-sulfonate as the counterion. The films and solutions of the polymer exhibit negative photochromism, the colored form (A,max = 408-430 nm) bleaching significantly by irradiation or heating.156... 

S. Nakano, A. Miyashita, and H. Nohira, Metastable solution structures of spirobenzoselenazoli-nobenzopyrans and their negative photochromic properties, Chem. Lett., 1993, 13-16. 

Like azobenzenes (III.A), polypeptides carrying spiropyrans have been studied to control the conformations of polypeptides. In polar solvents such as hexafluoropropanol, the colored merocyanine form is thermally more stable than the spiro form. Therefore it shows reverse (or negative) photochromism irradiation by visible light on the colored species bleaches it, while the spiro form formed... 

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>. 

Negative photochromic polymers were obtained by introducing a spiroben-zoselenazolinobenzopyran (SSeP) moiety onto PMMA backbones. Monomers (7)... 

J. Sunamoto, K. Iwamoto, M. Akutagawa, M. Nagase, and H. Kondo, Rate control by restricting mobility of substrate in specific reaction field. Negative photochromism of water-soluble spiropyran in AOT reversed micelles, J.Am. Chem. Soc. 104, 4904-4907 (1982). 

Among the diarylethene family, the dimethyidihydropyrene (DHP)/cyclophanediene (CPD) system is interesting because it is a rare example of negative photochrome, where the thermally stable DHP closed form is colored. Upon irradiation with visible light, DHP is converted to the colorless, open CPD form, and the reverse reaction occurs either photochemically with UV light or thermally [116, 117]. 

In addition to the photoswitching on of the luminescence with photochromic ligands that show negative photochromism, the switching off of the luminescence through the development and incorporation of ligands with positive photochromism, such as spiropyran, spirooxazine, and diarylethene, into different luminescent transition-metal complexes could also be achieved. 

Abe prepared a photochromic compound (236) bearing l,l -binaphthyl and two diphenylimidazole units, which exhibited unusual negative photochromism, in which the stable colored species (236) converted into the metastable colorless species (237) via short-lived radical (Scheme 51) Spiropyrans and related photochromic molecules were prepared and characterized/ ... 

In the original paper these values are 100 times greater, but this should be an error in their table. See values for the solvents. negative photochromism... 

Hydroxyl ILs were compared with non-hydroxyl analogs. Scheme 4.7.[45] The Et(30) parameter is lower (51.6-52.4) Kcal.moH for non-hydroxyl than for hydroxyl ILs (55.5-62.0) Kcal moH. Positive photochromism takes place only for the lower Et(30) values (< 56.2), while negative photochromism occurs for higher Et(30) values (>60.1) ILs based on [HOEMIM]+ and the anions [PFe]-, [NTf2]- and [CIO4]-. This is in accordance with a stabilization of the merocyanine form in polar solvents. No linear correlation was obtained for the back thermal reaction with the Et(30) value. 

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