Light initial absorption

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

Compound 24(00) underwent photochromic reaction by alternate irradiation with UV and visible light. Upon irradiation of the ethyl acetate solution of 24(00) with 313-nm light, an absorption at 560 nm appeared, as shown in Fig. 9.10. This absorption grew and shifted and the system reached the photostation-ary state after 120 min. The color of the solution changed from pale blue to red-purple and then to blue-purple. Such a red spectral shift suggests the formation of 24(CC). The isosbestic point was maintained at an initial stage of irradiation,... 

The light-initiated reaction has a high quantum yield. This means that many molecules of the product are formed for every photon of light absorbed. Our mechanism must explain how hundreds of individual reactions of methane with chlorine result from the absorption of a single photon by a single molecule of chlorine. 

It seems reasonable to assume that the initial absorption of a photon of light leads to an electronically excited singlet azide (335) which, a priori, may react in several ways. It could decompose in a concerted or stepwise process to give product alternatively, it might undergo intersystem crossing (ISC) to the triplet species (336) (equation 152). This could tlien decompose to product, again in a concerted or a step-... 

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. 

Reactive species formed by the CDOM absorption of UVR can then undergo indirect, or secondary, photochemical reactions. In fact, the many possible reactions caused by photosensitized transient intermediates probably account for most of the photodegradation of CDOM that we observe. The many different photoprocesses involved in CDOM photochemistry make for a very complex pathway of reactions beginning with the initial absorption of light energy and ending with the final products of these multiple reactions. 

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