Positive photochromism

Positive imaging techniques, 19 201 Positive ion spectroscopy, 24 107 Positive photochromism, 6 588 Positive photoresists, 20 280-281 Positive photosensitive polyimides,... 

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>. 

Crystalline-state photochromism usually proceeds with considerably lower interconversion ratios of less than 15% because the light penetration into the bulk crystal is prohibited by the absorption of the photo-generated isomer (inner-filter effects) [3,4]. The fully reversible crystalline-state photochromism of 1 can be partly attributed to its photochromic property. The rhodium dithionite complex 1 belongs to a unique class of photochromic compounds, which exhibits a unimolecular type T inverse photochromism [13]. The type T inverse photochromism means that the back reaction occurs thermally and the A.max of the absorption spectrum of 1 is longer than that of 2. If the back reaction occurs photochemically and the XmaK of the initial absorption spectrum is shorter than that of the photo-generated isomer, it is called type P positive photochromism and is known as a common photochromic system. 

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. 

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. 

Instead of metal chelation, an intramolecular hydrogen bonding between the oxygen atom of phenolate and a hydrogen atom of a carboxylic acid in the 8-position leads to stabilization of the colored form, such as compound 12.20,21 This spiropyran exhibits reversed photochromism, which means that thermally stable species change from the spiro form to the colored form, and thus the colorless form produced by photoirradiation soon converts to thermally stable colored form. 

The colored form of spironaphthopyran 32 absorbs at A,max of ca. 450 m,70 and the closed spiro form is colorless, which has no absorption band above 400nm. Bulky substituent group is especially important for photochromic sunglass. Introduction of the spiroadamantane or spirobi-cyclo[3.3.1]heptane into the 2-position of naphthopyran increases the resistance to photo-fatigue reaction, since endocyclic double bond induced by 1,7-hydrogen shift in the colored form cannot be formed in 2-adamantyl or 2-bicycloheptanyl group. 

Spirooxazine is an aza analogue of spiropyran in which the carbon atom at 3-position is replaced by a nitrogen atom. Historically, the photo-chromic phenomenon of spiroindolinooxazine derivatives was found after discovery of photochromic spiroindolinobenzopyran.72... 

Another approach to shift absorption bands for the colored form is the extension of Jt-conjugation outside the spiro skeleton. Procedures of molecular designs for such photochromic compounds are shown in Scheme 20. (1) Position for extension of additional -conjugation in spirothiopyran... 

Color Change. The development of color change within a photopolymer structure has been utilized in a number of commercial products. Specifically, the light induced change of a colorless leuco dye into a colored image forms the basis of several monotone positional pre-press proofing products. Other color change schemes (e.g. photochromic chemistry) have been used in a variety of products. 

The mixed liposomal solutions were prepared by the ethanol-injection method(13) in order to obtain completely transparent solutions. It is interesting to note that miscibility of the photochromic amphiphiles with DPPC depend on the position of bulky azobenzene. If azobenzene is incorporated close to the end of long alkyl chain, a stable mixed bilayer state cannot be formed. On the other hand, when the azobenzene moiety is located near the head group or at the center of the hydrocarbon tail, the azobenzene amphiphiles are successfully incorporated into the bilayer membrane. No individual micelle formation nor phase separation in the bilayer was observed at 25 °C by absorption spectroscopy. However, the microstructure of the mixed liposomes depends on the type of azobenzene amphiphiles. 

This selection scheme was used to evolve the orthogonal E. coli tRNA u -TyrRS pair in yeast. A synthetase library (10 in size) was similarly constructed by randomizing five active-site residues in E. coli TyrRS corresponding to the five residues randomized in the Af/TyrRS. Mutant synthetases were identified after several rounds of positive and negative selection that incorporate a number of unnatural amino acids into proteins, albeit with rather low protein yields (about 0.05 mgl A similar approach has been used to evolve orthogonal E. coli leucyl tRNAcuA LeuRS pairs that selectively incorporate photochromic and fluorescent amino acids into proteins in yeast. ... 

Products 11-15 are convenient starting compounds for the synthesis of photochromes with fused heterocycles, as well as of polymers. However, the reactivity of the bromine atoms in 11 is limited by steric factors. Thus, lithium (and then carboxy) derivatives of 13 are formed in high yields, whereas attempts to introduce formyl groups at positions 4 and 4 with DMF failed. 

In a series of studies, substituents were introduced or varied at the 2-positions of the thiophene rings of di(benzothienyl)ethenes. The reactions of 3-bromo-2(n-aIkyl)-l-benzothiophene with BuLi and octafluorocyclo-pentene gave bis(2-n-alkyl-l -benzothiophen-3-yl)perfluorocyclo-pentenes 60 containing alkyl substituents with a different chain length at position 2 of the benzothiophene ring. The authors stated that the Et-, Pr-, and Bu-containing derivatives exhibit photochromic properties even in the crystalline state (06JPP(A)162). 

As mentioned in Scheme 3, in spite of the presence of the benzothiazole rings at positions 2 and 2 of the thienyl moieties in compoimd 5, the latter exhibits photochromic properties. Compound 61 was synthesized under similar conditions, and was shown also to have photochromic properties (OlRCBllO) (Scheme 17). 

More recently, dithienylethenes 64 containing the (4-pyridyl)ethyl and (4-pyridyl)ethynyl groups at position 2 of the thiophene ring (080L2051) have been synthesized in a similar way. Photochrome 65 was prepared from 2-hydroxymethylbenzo[b]thiophene according to Scheme 19 (05IZV2697). 

Because it is much easier to obtain derivatives containing lithium at position 3 of the thiophene ring compared to the replacement of halogen in the benzene ring, it became possible to synthesize photochrome 66, in which the chlorine atom preintroduced into the benzene ring remains intact (07MC125) (Scheme 20). 

The synthesis of photochromic 6n conjugate systems having a bis(2,3 -benzothienyl) unit 75 was described (04CC1010) (Scheme 23). These systems have unusual structures, in which the fluorine atom, the methyl group, or the trifluoromethyl group rather than the heterocycle is located in a vicinal position adjacent to the benzothienyl substituent in the per-fluorocyclopentene moiety. 

The reaction of 2,5-disubstituted 3-furyl bromides with butyl-lithium and octafluorocyclopentene in dry tetrahydrofuran (THF) at —78°C gave bis(furyl)ethenes 82 (06JMC4690). Photochrome 83 (05JOC10323, 06EJ03105) (29-46% yield) and a series of its derivatives 84 (55-65%) containing different substituents at position 6 of the ben-zofuran ring were synthesized from 3-bromo-2-methyl-1-benzofuran (08JPP(A)146). 

The influences on the absorption spectra and the other photochromic properties of compounds with substituents in the 3//-naphtho[2,l-h]pyran ring and on the 3,3 -aryl groups have been stndied in detail. Electron-donating gronps in one or both of the 3-phenyl gronps, especially in the p-position, show a marked bathochromic shift in the absorption maxima of the coloured state, whilst electron-withdrawing groups have the opposite effect (Table 1.4). Substitutions in the a-position have little effect on the absorption maxima but have a very marked effect on the rate of return back to the colourless state, presumably due to stabilisation of the open chain form (Table 1.4). 

Valence tautomerism is responsible for the photochromic transformation exhibited by the xanthenone (143). This phenomenon is a type of dynamic isomerism in which the primary change is a shift in the position of the valence bonds as illustrated in Scheme 16 (68JOC3469). 

Among chromenes, only the spiropyrans and their heterocyclic derivatives have found a wide practical application as photochromic substances.5 6,7-Chromenediols have been proposed as analytical reagents for the spectrophotometric assay of rare earth cations.279 Chromenes with structure and activity similar to those of hashish constituents have been prepared.280 A number of chromenes, mostly with aryl substituents in positions 2,3, and 4 have been patented as biologically active substances.126,280 290... 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

The melting points of photochromes are a function of (a) the initial position of equilibrium between the two forms, (b) the thermal active tion of the back reaction, and (c) the possible thermal activation to the normal photoinduced equilibrium. These complications obviously make melting point data difficult to interpret. 

A0 is expressed by A0 = Sqf coiMCFIo (at low concentration), where k depends on the experimental conditions, Sqf is the molar absorptivity of the open form at Amax, co is the photocoloration quantum yield, and [CF]0 is the initial concentration of the closed form. All the photochromic parameters are very sensitive to the substitution on the naphthopyran ring. There is an important bathochromic shift of the absorption wavelength of the open forms (up to 88 nm) when a bithienyl entity is fixed at position 3. Moreover, the closed-form absorption spectra show minor but significant variations with a maximum shift of 20 nm. 

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