Following a brief historical review of solid-state photochemistry current research is discussed under four parallel headings: first, analysis of the topochemical postulate according to which the course of the solid-state reaction and the stereochemistry of the photodimer (if any) can be predicted from the configuration and nearest-neighbour geometry of closest monomer molecules in the crystal lattice; secondly, the study of the lOCUS of the reaction, that is the dependence of the course of the reaction or (dimerization, cis -trans isomerization) on crystal texture (dislocation, grain and phase boundaries); thirdly, a better understanding of the packing principles of organic molecules in their crystal structure to enable us to construct photoreactive (or lightstable) crystal structures of any given monomer as well as mixed crystals of potentially codimerizable monomers for synthetic purposes or energy-transfer studies.Several approaches to this problem of 'crystal engineering' based on generalizations of packing modes of primary amides and of dichiorophenyl derivatives are outlined, and their application to a systematic photochemistry of the solid state discussed.

No abstract

No abstract

In recipients of allogeneic bone marrow, asymptomatic CMV infection of the lung is a major risk factor for subsequent CMV interstitial pneumonia. Prophylactic ganciclovir is effective in preventing the development of CMV interstitial pneumonia in patients with asymptomatic infection.

Some crystalline salicylideneanilines (anils) undergo reversible colour change when irradiated with ultraviolet light (photochromy) . This phenomenon is largely topochemically dominated since : (i) no correlation exists between photosensitivity and the chemical nature of the ring-substituents, and (ii) polymorphic forms of a given anil differ markedly in light-sensitivity.Absorption spectra measured in solid films of several (monomorphic and dimorphic) anils in temperature ranges from -180" to near the melting points suggest the following classification : a-type ; photochromic, pale yellow a t all temperatures, not thermochromic : P-type ; not photochromic, colour varies with temperature, thermochromic.Further experimental data include the crystallographic constants of 15 anils in their various polymorphic forms, the fluorescence spectra of thermochromic and photochromic crystals, and the thermal fading rates of several photo-coloured materials, including O-deuterated anils.HAVING discussed in previous communications of this series the geometry of irreversible reactions in the solid state (dimerisation, isomerisation) we proceed to analyse in this and subsequent Papers the mechanism of reversible reactions. As example of the latter type of system we have chosen the photochemistry of anils of o-hydroxybenzaldehydes, partly because of the large amount of information available on the polymorphism and photo-behaviour of these compounds (see early summaries 1*2), and partly because of the ease with which they can be prepared. Preliminary accounts of our work on this system have been pre~ented.~" Photochromy.-Salicylideneanilines (I) (referred to as '' anils " in this Paper; and for convenience of discussion locants in the aniline ring are unprimed, those in the salicylidene ring are primed, when the term " a n i l " is used) are pale yellow to dark orange when freshly prepared. Some of the pale yellow anils are sensitive to ultraviolet light (eg., R' * N a 3' OH3655 A) in which their colour deepens to red or brown; certain anils exist in more than one crystal modification,not all of which are necessarily photochromic. Previous work6" has shown, and we have confirmed, that the colour change in photochromic crystals is not accompanied by any observable changes in the X-ray diffraction pattern and infrared (i.r.) spectrum of the crystal. Many previous attempts have been made to correlate photochemical activity with other properties of the members of this family of compounds; no definite correlation has as yet been established.The photo-colour can be "eradicated" by heat or irradiation with visible light (e.g., 4358 A). Fading rates are characteristic for each compound and, wherever polymorphism occurs, for each (photochromic) crystal modification. Stobbe had found that certain systems (fulgides, hydrazones, and stilbene derivatives) were photochromic 5' 2 R2* Part V, preceding paper.

The crystal structures of substituted cinnamic acids can be divided into three groups (a,P,y) according to their cell dimensions. These structure groups coincide with the three types of photochemical behaviour reported in Part I1 : dimerisation to (1) a-truxillic or (2) @-truxinic acid, and (3) lightst ability.X-Ray analyses of members of the a-and @-classes indicate that in these photo-active structures the conformation of the dimers can be correlated with the packing arrangement of nearest-neighbours in the monomer lattice : dimerisation takes place a t >C=c( separations of 3-6-4-1 and preserves in the (molecular) symmetry of the cyclobutane derivative the (crystallographic) symmetry element relating nearest-neighbour monomers. The separation between y = C < of 4-7-51 in the light-stable y-structures is interpreted as too long for dimerisation to be possible.

No abstract

No abstract