Absorption spectra, photoinduced changes

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). 

FIGURE 11 (a) Photoinduced absorption spectrum of PPV A7 is the change in transmission and T is the transmission through the sample, (b) Energy scheme of optical transition corresponding to the peaks in the spectrum in (a) 1101. 

A molecule that undergoes a reversible photoinduced change between two states with different optical properties such as color or absoiption spectrum is considered to be photochromic. The chromogenic effect, photochromism, can be illustrated by the photochemical transformation of molecule A to B upon absorption of light ... 

Photoexcitation spectroscopy is a powerful method for studying conducting pol5mers charges are injected into the 71-electron system without chemically changing the polymer and without introducing additional disorder. Since the background absorptions are completely suppressed in the photoinduced absorption spectrum. 

Sub-picosecond photoinduced absorption studies were employed to demonstrate the speed of the photoinduced electron transfer. Upon addition of C(M to P30T, the P1A spectrum, decay kinetics, and intensity dependence all change dramatically 36J. Already at 1 ps after photoexcitation by a 100 fs pump pulse at... 

By adding PCBM to the polymer matrix, the excited state evolution scenario changes dramatically. Figure 1.18b shows a sequence of A T/T spectra for MDMO-PPV/PCBM composites excited by a sub-10-fs pulse. At early time delays (see the 15 fs and 33 fs data) the spectrum closely resembles that of pure MDMO-PPV, confirming the predominant excitation of this molecule. The SE band from MDMO-PPV rapidly gives way to a photoinduced absorption (PA) feature, the formation of which is completed within... 

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