**1. Introduction**

Intensive research over the past few decades has been devoted to the synthesis of crystalline cavity-containing, framework, and coordination polymer species of a porous nature suited to the storage, immobilisation, sensing, or reaction of a wide variety of substrates of environmental and economic importance [1–15]. (The references cited here are a somewhat eclectic selection intended to illustrate the range of chemistry involved, rather than to be comprehensive, which is far from the case.) This has resulted not only in real advances towards practical objectives in gas storage [1,5,16,17] but also in unanticipated developments such as that of the "crystalline sponge" method [18,19] of determining the molecular structures of molecules otherwise difficult to crystallise. While metal ions clearly have a fundamental role in determining the structure of these materials, of equal importance is that they endow the solids with functionality specific to the given metal ion. One such function is that of photoactivity, a property which may have various manifestations [2,8], but which in the case of uranium(VI) as uranyl ion, UO2<sup>2</sup>+, derivatives, is anticipated to be that of photo-oxidation catalysis, long known in their solution chemistry [20–22].

Although photocatalysis by metal-organic framework (MOF) systems in particular could be described in 2017 as a "largely unexplored field" [23], it has rapidly become a popular area of study [24]. Investigations of hetereogeneous photocatalysis by uranyl-containing solids [25–35], however, have remained largely limited to those of oxidative destruction of environmental pollutants or to basic mechanistic work, although water splitting has been frequently cited as a possible application. Selectivity of these reactions has not been a major focus and in some early instances [34] would be expected to have been determined by the nature of the preformed support upon which uranyl centres were immobilised. Given that the use of a radioactive material would pose problems in any large-scale application for environmental remediation or water splitting, an alternative, more appealing prospect is that of selective photochemical synthesis within cavities of a porous uranyl complex crystal, a prospect which parallels what has already been realised for synthesis in general with other metal

ion derivatives [2,36–38] and which is rendered worthy of wider investigation by the observation of selective incorporation of various materials into the cavities of some known uranyl ion coordination polymers [39–45].

In general, polycarboxylates, often in the company of aza-aromatic species, are the most important class of ligands giving rise to coordination polymers, metal-organic frameworks, and closed metallo-clusters [1–14,46]. This is particularly true of uranyl ion containing systems [30,47–51], and it is for this reason that the present report is focussed upon uranyl polycarboxylates, though this is not to say that less-investigated species such as, for example, those based on polyphosphonates [52–55] are not of equal potential interest. We do note, however, that while uranyl ion photocatalysed oxidation of carboxylic acids is a long known reaction [20,21], it is slow and there is little evidence that the synthesis of uranyl carboxylates [49] is significantly influenced by it, so that the extraordinary variety of known carboxylate systems is open to exploration. With the particular objective of defining possibly more efficient pathways to photoactive closed uranyl-polycarboxylate oligomers, expected to be the most stringent form of receptor, we present an analysis of both positive and negative aspects of the crystal structures and composition of currently known system.
