Effect on Photochromism

Because photochromic reactions in polymer matrices deviate in general from first-order kinetics of both photochemical and thermal isomerization below the Tg (vide infra), polymers with Tgs lower than room temperature are of interest in order to perform photochromic reactions smoothly even in the solid state. In this respect, polysiloxanes17 and polyphosphazenes18 with flexible main chains are attractive. 

In his excellent review,2 Krongauz has summarized the effects of polymer matrices on spiropyran photochromism as reported in papers appearing before 

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 conclusions drawn from studies of the thermal reversion of low-molar-mass photomerocyanines derived from 6-nitrospiropyrans in polymer solids can be summarized as follows according to Krongauz s review12  

The decoloration deviates from first-order kinetics below the glass transition temperature (Tg) although the reaction obeys first-order kinetics above T 

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Table 3.4. Substitution of Methoxy on the Naphtho Portion of 3,3-Diphenyl-3//-naphtho[2,1-6]pyran Effect on Photochromic Properties of Compounds Imbibed into Poly-merizates of Diethyleneglycol bis(allyl carbonate)...

Table 4.5. Substituent Effect on Photochromism of Phenyl Fulgides 289 21...

H. Durr, A. Thome, H. Kilburg, S. Bossmann, E. Blasius, K. Janzen, and C. Kranz, Supramolecular effects on photochromism—Properties of crown ether-modified dihydroindolizines, J. Phys. Org. Chem. 5, 689-698 (1992). 

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). 

The complex formation and photochromic behaviour of the azacrown-containing chromene 15b and its crown-free analogue 15a were studied in detail [38,39], The addition of Ca(C104)2 to a solution of 15b led to the shift of the band at 400 nm, indicating the binding of Ca2+ by the macrocyclic unit of 15b. In contrast, the presence of Ca(C104)2 in a solution of 15a even at a high concentration of 0.1 mol dm-3 had no effect on the absorption spectrum of 15a. 

S.-R Heum, M.-S. Hur, P. M. Kazmaier, and E. Buncel, Thermo- and photochromic dyes Indolino-benzospiropyrans. Part 1. UV-VIS spectroscopic studies of 1, .3,.3-spiro(2 //-1 -benzopyran-2,2 -indolines) and the open-chain merocyanine forms solvatochromism and medium effects on spiro ring formation, Can. J. Chem., 69, 1940-1947 (1991). 

C. Salemi, G. Giusti, and R. Guglielmetti, DABCO effect on the photodegradation of photochromic compounds in spiro[indoline-pyran] and spiro[indoline-oxazine] series, J. Photochem. Photobiol. A Chem., 86, 247-252 (1995). 

V. A. Krongauz, Environmental effects on organic photochromic systems, in Photochromism Molecules and Systems (H. Durr and H. Bouas-Laurent, eds.), pp. 793-821, Elsevier, Amsterdam (1990). 

Hovey et al.3 reported an enhancement in photochromic response by the addition of a methoxy group in the 9 -position or a bromine in the 8 -position, as shown in Table 2.1. Although dramatically improving the photochromic response, these substituents had little effect on the position of the visible absorption band. 

S. Schneider, F. Baumann, U. Kluter, and M. Melzig, Photochromism of spirooxazines, II. CARS-investigation of solvent effects on the isomeric distribution, Ber. Bunzenges Phys. Chem., 91, 1225-1228 (1987). 

Substituent Effects on the Quantum Yield of Photochromic Reactions of... 

Heller and Langan38 reported that the quantum yield for photocoloration (c) of fulgide 35 to the 7,7a-DHBF (36) in toluene was 0.20, and the E->c value appeared to be wavelength independent over the range 313-366 nm. Temperature (10-40°C) had little effect on the quantum yield for photocoloration. Furthermore, the cycles of photochromism of 35-36-35 did not affect the (E - c) value. These results showed that fulgide 35 is well suited for chemical actinometry in the near-UV and visible spectral region. 

Steric Effects on the Photochromic Behavior of the Alkylidene Group of Furyl Fulgides... 

Substituent effects on the quantum yield of photoreactions of indolyl fulgide 58 have also been reported by Uchida et al.,15 as shown in Table 4.14. The molecular structure and photochromic reactions are shown in Scheme 18. 

Photochromic reactions of the heterocyclic fulgides are also in accordance with the Woodward-Hoffmann selection rules. The molecular structures have dramatic effects on the quantum yields of photoinduced ring-closure and ring-opening reactions of fulgides. The photochemical reactions of furyl fulgides are shown in Scheme 35 and the quantum yields are summarized in Table 4.29.44 45 104... 

A. Tomoda, A. Kaneko, H. Tsuboi, and R. Matsushima, Effect of two election-donating substituents on photochromism of aryl fulgide, 2-(2,6-dimethyl-3,5-substituted benzylidene)-3-isopropylidene succinic anhydride, Bull. Chem. Soc. Jpn., 65, 2352-2358 (1992). 

K. Uchida, S. Nakamura, and M. Irie, Thermally irreversible photochromic systems. Substituent effect on the absorption wavelength of 1 l,12-dicyano-5a,5b-dihydro-5a,5b-dimethylbenzo [1,2-6 6,5-6 ]bis[l]benzothiophene, Bull. Chem. Soc. Jpn. 65, 430-435 (1992). 

This section relates to the reversible color change of viologens via a photoreduction mechanism principally in the solid state. The effect of the kind of viologens (bipyridinium rings and counteranions) and polymers on photochromism is discussed. The viologens and their abbreviation under discussion in this section are given in Scheme 3. 

Considering that the photochromes IX and X only differ in their N-substituent, the comparison of these data shows that the nature of the photochrome must also be taken into consideration, and that it would be hazardous to draw general conclusions about the influence of photochrome incorporation. In the case of azobenzene side groups (see Table 6) binding or not of the photochrome on the polymer chain does not exert a significant effect on the decoloration kinetics. 

In most cases, the fractional firee volume amounts to 0.025 — 0.030. a value which is considered as a normal one for amorphous polymers. For the photochromic polyesters inadminissibly high fg values were however found (corrected 0.13 to 0.197 instead of 0.27 to 0.45 as indicated erroneously). On the basis of the calculation of Sevens is 0.048 which is much more reasonable when taking into account the bulkiness of the bisphotochrome and the disturbing effects on its direct vicinity. 

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