Photoinduced yellowing

Changes in physicochemical properties of the drug substance (e.g., color, crystal modification) may take place upon irradiation. Efforts should be made to observe such changes during the in vitro assay. The sample absorption spectrum should be recorded before and after irradiation surface color of solid samples should be evaluated by appropriate methods and the identity of the sample crystal modification should be confirmed when the drug is irradiated in the solid state. The humidity in the test chamber can influence the photochemical stability of certain solid samples, as demonstrated for mefloquine. The photoinduced yellowing of uncoated mefloquine tablets is accelerated by an increase in humidity. These tablets are mainly used in tropical countries and the real in-use conditions will include high relative humidity. In such cases, the influence of the humidity on the photostability must be taken into account (Tpnnesen et al., 1997). 

Raithby and coworkers have observed the reversible 100% conversion of the red [(dppe)Ni( 7 -N02)Cl] complex to its photoinduced yellow linkage isomer [(dppe)Ni( 7 -ONO)Cl] (Equation 2) (135). Raman spectral data revealed a band at 1333 cm in the parent nitro compound and a 1404 cm band in the nitrito isomer. The nitrito isomer was further identified by photocrystallography and was found to be stable between 100 and 160 K and observed to reconvert to the ground state at 300 K. 

Photoinduced methyl transfer. The yellow mixture of 4-phenylpyridinium cation (with relatively low reduction potential) and tetramethylborate anion in tetrahydrofuran persists for 24 h without any reaction (in the dark). However, the deliberate irradiation of the charge-transfer band (at /exc = 370 nm)... 

The DHAs undergo an interesting photoinduced rearrangement to the corresponding VHFs, and this is accompanied by a change of color from, in the case of the phenyl derivative 18a (Scheme 2), yellow to dark red.[3a,19] In this case, it was... 

To effectively prevent photoinduced discoloration of modern paper, one must consider the environment in which it may be stored, exhibited, or used, and the causes of its yellowing. In view of the photochemistry involved, some form of photostabilization of modern paper is essential if adequate protection against the detrimental effects of solar irradiation is to be achieved. 

For actinometric measurements, a 1cm cuvette containing a =5mM solution of the furyl fulgide (Aberchrome 540) in toluene is used, which completely absorbs UV radiation between 313 and 366 nm. The color change from yellow to red due to the photoinduced (conrotatory) ring closure is monitored at 494 nm, and the resulting linear increase in absorbance (AA ) with irradiation time (Af) is evaluated using Eq. (18) to calculate the intensity (l ) of the incident UV radiation, i.e.. 

Reactions of 1,4-Diketones. - The hula-twist mechanism is suggested as the only possible mechanism for the ,Z-isomerism of tru s-1,2-dibenzoylethene in the crystalline phase. The irradiation in sunlight of the yellow bi-indenylidenedione (152) brings about a colour change to red. The red compound has been shown to be the biradical (153). The red colour can be dispersed thermally when the radical (153) reverts to the parent (152). The photoinduced electron transfer within (154) has been studied. ... 

Zueolotto V, Barbosa Neto NM, Rodrigues JJ Jr, Constantino CJL, Zilio SC, Mendon9a CR, Aroca RF, Oliveira ON Jr. 2004a. Photoinduced phenomena in layer by layer films of poly(allylamine hydrochloride) and brilliant yellow azodye. J Nanosci Nanotech 4 855 860. 

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