Élisabeth Anstett

What Is a Mass Grave? Toward an Anthropology of Human Remains Treatment in Contemporary Contexts of Mass Violence

. No continent has been spared these mass crimes, which have marked every decade of the twentieth century. Europe in particular has seen the Armenian genocide, the Holocaust, and, more recently, the genocide perpetrated in Bosnia. Africa has not only seen the genocide committed against the Herero and the Rwandan genocide, but also several million civilian deaths in the Great Lakes region and the Horn of Africa, in particular. Asia has seen the Cambodian genocide and the massacres that have repeatedly taken place in Indonesia, Sri Lanka, East Timor, and many other regions, against a background of religious, cultural, and/or political conflict. In the Americas, the twentieth century saw the emergence of such new social phenomena as the globalization of systematic, planned killing (in particular, through the implementation of Operation Condor in South America) and a degree of spatial and sociological contamination by extreme violence (in Guatemala, Peru, Colombia, and Mexico, for example) so total as to make it difficult to find words to describe this endlessly shifting cycle of atrocities carried out -sometimes simultaneously -for political, economic, or purely sordid motives.

The Mass Production of Mass Graves However, these tens of millions of victims of mass violence have not disappeared; their corpses have not simply evaporated. Their bodies have in fact always undergone specific and complex forms of treatment: in some cases quite intentionally left to lie in spaces earmarked for pollution or conquest; in others exhibited as trophies; and in yet others meticulously destroyed or hidden (Anstett and Dreyfus 2012). Yet it should be emphasized that the remains of the victims of mass violence have, in the vast majority of cases (including the Holocaust), been deposited directly into the ground by their killers or by accomplices of the latter. These deposition sites are innumerable, and across the planet today there exist tens of thousands of such mass graves. It has been possible to map these sites fairly systematically in certain countries, such as Poland and Spain, and less completely elsewhere, such as in Rwanda, where work on a preliminary inventory of mass graves began in the summer of 1994. Work has yet to begin on such studies in other contexts, such as the mass graves of the Soviet camps [START_REF] Anstett | Cadavres impensables, cadavres impensés: Approches méthodologiques du traitement des corps dans les violences de masse et les genocides [Cadavers, unthinkable and inconceived : methodological approaches to dead body treatment in mass violence and genocide[END_REF]. All of these examples attest to the scale of the practice of mass burial. The form of these depositions varies from one case to another. Their size ranges from trenches containing a few or dozens of bodies (in the context of the Spanish Civil War, for example) to gigantic pits containing hundreds or even thousands of bodies, as seen in Rwanda. The technological complexity involved also varies enormously [START_REF] Fowler | A Mere Technical Exercise? Challenges and Technological Solutions to the Identification of Individuals in Mass Grave Scenarios in the Modern Context[END_REF][START_REF] Thompson | Deconstructing the Ideal of Standardization in Forensic Anthropology[END_REF]. It can range from simply using the characteristics of the terrain to maximum effect, as seen in Cambodia, where corpses were left in the furrows of rice paddies, and the use of karst crevices, or Foibe, by Yugoslav troops in northern Italy, for example [START_REF] Zamparutti | Foibe litterature: Documentation or victimhood narrative?[END_REF], to the digging of pits several meters deep using earth-moving machinery and the creation of secondary or even tertiary graves, as took place in Bosnia [START_REF] Jugo | Disassembling the Pieces, Reassembling the Social: The Forensic and Political Lives of Secondary Mass Graves in Bosnia and Herzegovina[END_REF]. Whether in their immediate aftermath or at a later stage, these crimes and their associated charnel pits have given rise to a massive search process aimed at locating the victims, undertaken in some cases by survivors and in others by local or international organizations [START_REF] Rosenblatt | Digging for the Disappeared: Forensic Science after Atrocity[END_REF]. For one of the side effects of the mass violence of the twentieth century has been to generate mass exhumations, thus ushering in a new phenomenon of large-scale disinterments, which Holocaust historian Robert Jan van Pelt has qualified as a true "forensic turn" in human history [START_REF] Anstett | Why Exhume, Why Identify?[END_REF].

A New Social Phenomenon: Mass Exhumations

For the moment, any history of this entirely new process, which has seen social groups disinterring bodies on a massive scale, can necessarily be sketchy at best. As far as its methodological premises are concerned, there can be no doubt that the extensive practice of informal exhumations and large-scale pillaging alike carried out on Amerindian tombs at the end of the nineteenth century in the United States [START_REF] Platt | Bitter Legacies: A War of Extermination, Grave Looting, and Culture Wars in the American West[END_REF], and the collection at the turn of the twentieth century by German, French, and British anthropologists in Africa of human remains resulting from their colonial wars [START_REF] Harrison | Skulls and Scientific Collecting in the Victorian Military: Keeping the Enemy Dead in British Frontier Warfare[END_REF], directly contributed to the birth of physical anthropology as an academic discipline, constituting the foundations of a system of knowledge that would later be put to use in order to identify soldiers who had fallen in battle [START_REF] Wagner | A curious trade: The recovery and repatriation of Vietnam MIAs[END_REF]. These new ways of appropriating human bodies would at the same time legitimize not only the constitution of human osteological collections on a vast scale but also the very principle of mass disinterment. The geography of this process of large-scale exhumations is no less rich and complex than its history. Indeed, the rapid globalization of the practices involved in opening mass graves in order to locate and identify bodies appears to be linked as much to the spread of a new system of technical knowledge that brings together funerary archaeology, physical anthropology, and forensic science (Signoli 2008) as it is to the ability of societies to face up to their past and to commit themselves, for various reasons, to disinterring their dead (Thomas 2010), whether in Argentina, Poland, Guatemala, Bosnia, Spain, Rwanda, or elsewhere. Nevertheless, exhumation as a social phenomenon remains relatively little studied if we take into account its dramatic contemporary extension. The issues raised by, and the very real consequences of, exhuming bodies "en masse" have yet to be thoroughly examined, despite the fact that this practice has already radically modified the sociotechnical space of funerary practices and, indeed, the very notion of what these are.

The Question of Burial

Although the main objective of these exhumations remains the location and identification of the victims, the survivors also tend to have considerable expectations as far as understanding the violence in question is concerned (Anstett and Dreyfus 2015b). The examination of the bodies and the study of these charnels can provide crucial information regarding the context of the process of killing and of the perimortem treatment of the corpses. In this respect, this information also enables us to take an entirely fresh approach to the notion of burial itself. The term "charnel pit" will be used here to render the original, etymologically related French term charnier, which in French is used virtually interchangeably with the term fosse commune (literally "common pit"), which is in turn usually translated as "mass grave." While, in an archaeological context, "charnel pit" most often refers to a secondary burial site in which skeletal remains are placed following the decomposition of soft tissues, it is also used in relation to primary burial places linked to modern episodes of mass violence. It also presents the advantage of lacking the funerary connotations of the term "mass grave," a distinction that will be of some importance in the analysis presented in this chapter. According to the now classic definition provided by the Dictionnaire de la préhistoire de Leroi-Gourhan (Leclerc and Tarrête 1988), a burial (une sépulture) is indeed the "place where the remains of one or more dead individuals have been deposited, and where enough indications remain to allow the archeologist to perceive in this deposition the intention of carrying out a funerary act." The notion of burial thus closely associates intentional deposition with a funerary intention: "what makes a burial is the intentional nature of the deposition, and the desire to carry out a funerary act" (Leclerc 1990: 14), to the extent that Duday even refers to an "intentional character of the deposition which is obviously necessary if it is to be described as sepulchral in character" (Duday et al. 1990: 30;emphasis added). Yet this definition implicitly conflates the authors of the deposition with those of the possible funerary acts when in fact this association is far from being a given. The criminology of extreme violence in fact shows us that the relations existing between the killers and the corpses of their victims belong to a complex set of registers, which combine not only psychological and potentially psychiatric elements but, above all, sociological, cultural, ideological, and political factors [START_REF] Shute | Moral Discourse and Action in Relation to the Corpse: Integrative Concepts for a Criminology of Mass Violence[END_REF][START_REF] Ranaletti | When Death Is Not the End: Toward a Typology of the Treatment of Corpses of 'Disappeared Detainees' in Argentina from 1975 to 1983[END_REF]. Positing a "necessary" association between the funerary act on the one hand, and the voluntary deposition of the corpse on the other, thus proves extremely problematic in the context of mass crimes, as an examination of the charnels of the twentieth century shows that funerary acts may be organized on sites where the voluntary nature of the deposition of bodies is debatable and, conversely, that technically highly elaborate depositions may be carried out without any funerary intention whatsoever. Archaeologists Jesse and Skinner (2005) have shown perfectly in their pioneering work how complex and difficult it may be to develop a typology of mass graves and mass grave-related sites from a simple technological perspective.

While the problems raised by the use of the word "burial" in contexts of mass violence are manifest in the very definition of the term, other limits of the notion are revealed by the comparative analysis of various charnel pits, as I shall shortly argue. An important question may thus be asked: in such cases, might the funerary act not in fact be much more important than the voluntary nature of the deposition? In this respect, situations of mass violence raise key questions regarding the nature and identity of mass graves. Do they cease to be simply charnel pits when they become the object of devotional practices? This in fact often occurs when the areas in which massacres have taken place see the killers living alongside their victims permanently, when the locations of charnels are known to neighbors and survivors, and when sites of interment become spaces (publicly declared or otherwise) for commemorative rituals, thus becoming instituted as proper burial spaces even though they had not initially been conceived of as such by the killers, as is the case in the killing fields of Cambodia [START_REF] Jarvis | Powerful Remains: The Continuing Presence of Victims of the Khmer Rouge Regime in Today's Cambodia[END_REF]. The study of the fate of corpses in situations of extreme violence thus allows us to ask questions that go to the very heart of what constitutes a burial.

Framing the Analysis of the Treatment of Human Remains : the Advantages of a Comparative Approach Addressing the very issues described here, the interdisciplinary research program entitled "Corpses of Mass Violence and Genocide," which ran from 2012 to 2016 thanks to funding from the European Research Council, enabled a group of researchers to carry out a study and comparative analysis of the treatment of victims' remains in contexts of mass violence and genocides. In this project, the treatment of corpses was placed at the heart of research carried out jointly between anthropologists, legal specialists, and historians. The general objective of the program was to arrive at a better understanding of the processes of production and reabsorption of extreme violence, the study of which had hitherto generally ignored the precise functions assigned to the bodies of victims. The different axes of analysis corresponded to the three stages constituted successively by the treatment of these remains by the perpretrators (Anstett and Dreyfus 2014), by the search for bodies (Anstett and Dreyfus 2015a), and finally by their treatment within society (Dreyfus and Anstett 2016). These stages were separated not so much for chronological reasons as for the fact that they each seemed to relate to the types of logic and issues involved in the practices of the destruction, identification, and reclaiming of bodies, respectively, each of which can be considered as a distinct area of study. The research produced in the context of this program was based on a number of case studies carried out in Europe (Poland, Spain, Bosnia, Belarus, Armenia), as well as in Africa (Rwanda, South Africa, Namibia, Zimbabwe), Latin America (Argentina, Uruguay, Mexico, Columbia, Peru, Guatemala), and Asia (Cambodia, Malaysia, Indonesia), thereby allowing a wide range of situations involving the extermination of civilian populations to be examined. This program has not only helped to confirm the status of an entirely new field of research devoted to the study of the social uses of corpses [START_REF] Anstett | Des cadavres en masse: Sociétés et sciences sociales face à l'impensé" [Cadavers en masse : societies and social sciences facing what remains inconceived[END_REF]), but has above all enabled a certain number of differences and similarities between the different fates that befall the corpses of victims of mass crimes to be revealed, thus opening up the way for a structural study of the production of violence. The study of the often surprising details of the stage of the treatment of corpses by the killers, which involves some form of manipulation of the body along with, in the majority of cases, its interment, produces some particularly revealing outputs. This study has allowed us to establish two sets of oppositions.

Visibility regimes

A first major distinction may thus be drawn between two visibility regimes that are directly revealed by the treatment of corpses: on the one hand stands a logic of the invisibilization of human remains, to which belong all those practices that involve hiding bodies from the eyes of perpetrators, from future victims, and/or from survivors; and, on the other, stands a converse logic of display which characterizes all practices that involve exhibiting bodies -directly or indirectly, for example through the use of images -and the instrumentation of the visual impact of the dead body. Examples of practices of display are easy to find since they relate to the larger question of the use of trophies, the long history and universal distribution of which is attested by historians and anthropologists. As the British scholar Simon [START_REF] Harrison | Dark Trophies: Hunting and the Enemy Body in Modern War[END_REF] has brilliantly argued, the use of human trophies was revived in the context of colonization through the collection of skeletal remains, a practice given scientific legitimacy by racialist physical anthropology. This is the case, for instance, with the Herero skeletons that German universities have to this day been unable to fully return to Namibia [START_REF] Shigwedha | The Return of Herero and Nama Bones from Germany: The Victims' Struggle for Recognition and Recurring Genocide Memories in Namibia[END_REF]. The practices surrounding the display of dead bodies are also highly varied, involving widely differing degrees of sophistication, from simply leaving bodies on the site of killing to staging elaborate spectacles using corpses. Examples of the latter include the corpses that were left to decay on gibbets beside roads during the Armenian genocide [START_REF] Kevorkian | Earth, Fire, Water: Or How to Make the Armenian Corpses Disappear[END_REF], the practice seen in Guatemala and South Africa of dismembering the bodies of victims and leaving the body parts by the roadside [START_REF] Rousseau | Death and Dismemberment: The Body and Counter-Revolutionary Warfare in Apartheid South Africa[END_REF], and the distribution of booklets containing photographs of corpses, produced for propaganda purposes in northern Italy [START_REF] Dato | Chained Corpses: Warfare, Politics and Religion after the Habsburg Empire in the Julian March, 1930s-70s[END_REF]. Examples of the invisibilization of corpses in contexts of mass violence are similarly numerous. The most elaborate example -which provides further evidence of the organized character of the enterprise of destruction -is without a shadow of a doubt the creation of hidden extermination sites and the associated use of crematory ovens during the crimes of the Nazis (Van Pelt 2014). Another example involving a particularly elaborate logistical operation is Operation Carrot (Lopez-Mazz 2015), in which the Uruguayan army exhumed the corpses of murdered political opponents, which had initially been hastily buried, in order to carry out a meticulous process of destruction, before dumping any remains over the sea. These processes of concealment are not always so sophisticated, and may manifest themselves through purely pragmatic measures, such as throwing corpses into naturally formed holes or waterways, as some examples of pits in Bosnia and Rwanda have shown. In general, however, the simplest and most widespread practice of invisibilization still involves the burying of bodies.

Property regimes

It is also possible to draw a second major distinction between two opposing regimes of property: on the one hand, a logic of appropriation by the killers (involving a principle of objectification, in which corpses become the property of the perpetrators) may be perceived, while on the other is a logic of abandonment or rejection, which stems from a desire to expel the victims from a territory and a social or symbolic order, leading the killers to get rid of corpses using a variety of practices, ranging from simple disposal to careful destruction. Numerous examples of corpses being appropriated by perpetrators exist, from the "disappeared" of the Latin American dictatorships [START_REF] Crossland | Buried Lives: Forensic Archeology and the Disappeared in Argentina[END_REF] to the prisoners of the gulags, whose remains were never returned to their families. In such cases, corpses are confiscated not only in order to deprive loved ones of the ability to mourn but also as part of an attempt to erase the victims' very existence. Generally speaking, these logics of appropriation manifest themselves through the implementation of practices of concealment, which may involve recourse to interment. In this sense, mass graves become places for the storage of bodies, which are thus seen as "belongings," either watched over or kept carefully hidden in order to remain accessible to the killers alone. Conversely, there are innumerable examples of disposal, from the Cambodian killing fields, where corpses were left to rot in the open air (Guillou 2015), to the Rwandan genocide, where the bodies of Tutsis were thrown into latrines and rivers, or left lying in the banana fields and forests. In other cases, perpetrators have disposed of bodies by burying them summarily, the charnel pit serving the purpose of removing the undesirable and inconvenient presence of human remains from the public space; by incinerating them, as was the case in the Holocaust and also for the victims of the Stalinist Great Purge in Moscow and the Argentine dictatorship in Buenos Aires, in particular; or, finally, by dumping them in the sea, as was the case with the "death flights," again in Argentina.

Analytical Matrix and Research Question

Although property and visibility regimes both arise from distinct factors that vary in relative importance according to their context, the two regimes are nevertheless always linked to one another. Their intimate connection enables us to draw up an analytical matrix that is applicable to all examples of charnel pits studied to date

The comprehensive analysis of each mass grave, nevertheless, reminds us that it remains essential to deal with the specific facts of violence in a global manner, taking into account both the general logic of the production of death en masse in a given sociohistorical context (i.e., the diachronic character of the process of legitimizing extreme violence) but also a local situation and a modus operandi that are specific to each massacre (i.e., the synchronic character of the perpetration of this violence). In this respect, the function attributed by the perpetrators to the site where corpses are deposited remains highly variable, even when the pits in question are identical from a purely technological point of view. This function may also on occasions include elements of a mortuary or funerary nature, when the status of human beings is accorded to the victims or, for instance, when elements of a religious grammar are pressed into service by the perpetrators' ideology. It may equally contain none of these when the killers see themselves as simply manipulating bodies, which they perceive as detritus. The use of these sites of deposition by survivors and the local population is also highly variable, ranging from avoidance to sacralization. It is, unfortunately, not possible to go into the details of these practices in the space available here. However, while the matrix in Table 13.1 facilitates a better understanding of the symbolic and social functions assigned to these sites by the people who created them, it leaves open the question of the exact nature of these sites. For what, in the end, is a mass grave? And to what extent can we consider this peculiar place as a "grave" or even a burial place? I would argue that several elements need to be taken into consideration here.

Mass Graves and Burial: The question of intentional deposition

Here too, it may be useful to return to the definition of the term "charnel pit." "Charnel" derives from the same Latin root carnis (meat/flesh) as the French charnier, which is in turn defined thus: A place where flesh is piled up, the charnel pit [charnier] is simultaneously a delimited funerary space, having a social function and being identifiable as such, and a place where death is not accompanied by a tomb, attesting to catastrophe … Furthermore, it is through reference to the original sense of the term, which designated a place where salted meat was kept that, by analogy, the charnel pit still carries its pound of flesh. (Signol 2008b: 71) In contexts of extreme violence, the charnel pit as the very first expression of a mass grave is thus positioned as the product of an intentional deposition that signals a highly specific stage in the perpetration of the atrocities in question, namely, the bringing together of dead bodies. The victims were either assembled when still alive and murdered on the site of their future burial, as in the case of some pits from the Holocaust in Belarus [START_REF] Straede | The Dead Bodies of Bobruisk, Belarus 1941-45[END_REF], open-air charnel pits in Cambodia (Guillou 2015), and pits linked to the Stalinist Great Purge that have been exhumed at the Butovo complex in Moscow, for example [START_REF] Rousselet | L'Europe et ses représentations du passé: Les tourments de la mémoire[END_REF], or transported there following their murder, as in the case of the mass graves of the Holocaust in Poland (Sturdy Colls 2015), the secondary and tertiary graves in Bosnia [START_REF] Jugo | Disassembling the Pieces, Reassembling the Social: The Forensic and Political Lives of Secondary Mass Graves in Bosnia and Herzegovina[END_REF], and virtually all the mass graves identified in Rwanda. The intentional nature of the deposition is in this sense linked to the intentionality of the crime (an essential element which all judicial instances recognize as a qualification of the latter), of which it is both an indication and a piece of evidence. For, even if the postmortem treatment of bodies was not initially specified as part of the pre-established plan for killing, it is always subject to choices made on the ground by the killers according to considerations that induce variation in the regimes of the visibility and appropriation of victims' bodies, as was explained earlier. Even in the technically highly complex cases of comingled remains (Adams and Byrd 2008), the work of archaeologists and forensic anthropologists allows light to be shed on these choices. The chief indicator regarding the intentionality of the deposition of corpses is thus the location of the sites chosen for their interment. The choice of a site is made according to several criteria: accessibility, degree of frequentation (whether already abandoned and rarely visited or, in contrast, of particular importance for the victim group), and the pre-existing symbolic significance of the site, following a logic of pollution/contamination or, conversely, of conquest/cleansing. Similarly, the site may subsequently be abandoned by the perpetrators or placed under their surveillance. Nevertheless, in the case of situations of mass violence at any rate, the intentionality of the deposition of remains does not systematically imply the existence of a "funerary" act -far from it. It would, for example, be extremely problematic to argue that any funerary intention was present in the case of Holocaust cremations, given that here the perpetrators specifically denied their victims the status of human beings [START_REF] Mcconnell | Lands of Unkultur: Mass Violence, Corpses, and the Nazi Imagination of the East[END_REF]. Similarly, one would be hard pressed to prove the funerary intention of those who threw the bodies of Tutsis down latrines, thereby explicitly associating them with excrement, during the Rwandan genocide [START_REF] Korman | The Tutsi Body in the 1994 Genocide: Ideology, Physical Destruction, and Memory[END_REF]. In contexts of mass violence, then, particularly close attention needs to be paid to the distinction that can be drawn between mortuary space and funerary space.

The question of ideology However, qualifying these practices of interment as mortuary in nature also raises questions insofar as acts of desecration carried out upon victims' remains are both numerous and varied. Such profanatory intentions not only are visible in the practices of postmortem dismemberment or mutilation observed in a great number of contexts (Armenia, Rwanda, Guatemala, South Africa), but are also apparent in the discourse of the perpetrators. The stage of the treatment of victims' corpses by their killers in situations of mass violence bears the imprint not only of a specific historical and sociocultural context (including a religious grammar shared by the majority of individuals, along with usual mortuary customs and funerary practices) but also of a political and/or ideological project, all of which underpin, in a sometimes surprising but always highly specific manner, the production of mass death. For this reason, taking the specifically ideological dimension of mass killing into account is, we would argue, absolutely essential. Doing so involves documenting the discourses of leaders and ideologues in just as much depth as the practices of the killers. As shown by the studies carried out on dictatorship-era Argentina [START_REF] Feitlowitz | A Lexicon of Terror: Argentina and the Legacies of Torture[END_REF], on Rwanda during the genocide [START_REF] Korman | The Tutsi Body in the 1994 Genocide: Ideology, Physical Destruction, and Memory[END_REF], on Belarus under Nazi occupation [START_REF] Mcconnell | Lands of Unkultur: Mass Violence, Corpses, and the Nazi Imagination of the East[END_REF], and on Guatemala (Nyberg Sørensen 2014), the semantic and lexical registers mobilized in hate speech are invariably interpreted at face value and translated in an utterly literal manner into the actual modes of treatment applied to victims' bodies. The killing stage of extreme violence thus relates directly to representations of the enemy, the racialist, sexist, xenophobic, or segregationist dimensions of which are translated into dehumanizing metaphors, through which those to be eliminated become subhumans, animals, or base matter. According to the perspective of the killers, then, what point would there be in creating a "mortuary" space for rats, vermin, pigs, or any other living thing that is loathed or held in contempt, let alone for filth or refuse? In order to understand their precise nature and specific characteristics more fully, then, would it not be more accurate to consider the peculiar spaces that are charnel pits in relation to rubbish dumps or landfills, rather than mortuary or funerary spaces? And should less attention perhaps be given to the intentional character of the deposition in itself, in order to concentrate on the careful study of the exact motivations behind this deposition? Practices of dehumanisation Indeed, work carried out by archaeologists and forensic anthropologists involved in the excavation of mass graves linked to mass violence has shown that these pits are often sited near rubbish dumps. This is the case for a number of sites from the Spanish Civil War [START_REF] Ferrándiz | Exhuming the Defeated: Civil War Mass Graves in 21st-Century Spain[END_REF]. It is also the case for the mass graves in the Blagovchina forest which served the Nazi extermination camp of Maly Trostenets, and which are now partially covered by the vast municipal rubbish dump of the city of Minsk (in the 1940s it was still some distance away). Similarly, Crematoria II and III of Auschwitz-Birkenau were intentionally sited in a remote part of the concentration camp, which contained sewage ponds and waste treatment areas. In Rwanda, there are also innumerable examples of latrines being used by the killers to dispose of the bodies of Tutsis during the genocide. Without reading more than is necessary into the fundamental semantic ambiguity of the term "pit," it does seem to be the case that in contexts of extreme violence there is a strong symbolic link between the treatment of the dead and the treatment of refuse. In many situations of mass violence, victims' corpses have indeed been likened to rubbish, which suggests that the notion of refuse has a paradigmatic role, as archaeologists of the contemporary past have been pointing out for some time [START_REF] Rathje | Integrated Archaeology: A Garbage Paradigm[END_REF]. Recent developments in the archaeology of mass violence (González-Ruibal and Moshenska 2015;[START_REF] Rosenblatt | Digging for the Disappeared: Forensic Science after Atrocity[END_REF][START_REF] Colls | Holocaust Archaeologies: Approaches and Future Directions[END_REF], along with the findings of the archaeology of waste (Rathje and Murphy 2001), thus suggest that funerary practices and practices surrounding the treatment of refuse need to be studied simultaneously, within a comparative critical framework, in order to shed light in particular on the logics of inclusion and exclusion applied respectively to the "good" and the "bad" dead. To conclude, it would seem that fundamental epistemological and methodological issues are involved in the integration into the analysis of mass graves of the findings of what Alfredo Gonzalez-Ruibal (2008) has termed an "archaeology of the super-modern," which is prepared to confront the unthinkable question of the intentional large-scale destruction of humans by other humans. Aside from the fact that such an archaeology quite rightly poses afresh essential questions regarding the relation of researchers to the human remains that they disinter, given the highly politically and culturally sensitive contexts of the digs in question (Crossland and Joyce 2015), it also forces us to reconsider the ontology of the corpse itself [START_REF] Domanska | Toward the Archaeontology of the Dead Body[END_REF], the threshold object that we have long known has been thought of in different ways in different eras and places but which, as the examples provided by genocides and situations of mass violence have shown, may also be the subject of radically discordant representations within a single society.


Oleh Stetsiuk

Patricia Bolle

Marie Cordier

Julien Boixel

Rémi Dessapt

Structure-property relationships in normal and mixed dithienylethenes -polyoxometalates supramolecular assemblies with fast solid-state photochromic properties

Five new highly photochromic hybrid organic-inorganic materials were successfully prepared by supramolecular assembly of normal (1 + ) and mixed (2 + ) cationic dithienylethenes (DTEs) and polyoxometalates (POMs) units. Single-crystal X-ray diffraction studies reveal that (1)3

) is an isotype of (1)4[Mo8O26]4DMF (1-Mo8) i.e., the first POM-DTE ionic assembly, recently reported. The solid-state photochromic properties of these materials at room temperature are highly tunable with the nature of the DTEs and POMs used, which has been interpreted taking into account steric and electronic factors. Especially, while 1-PMo12 and 2-PMo12 exhibit negligible photoresponses which have not been considered herein, thorough investigations of the photocoloration and fading kinetics in ambient conditions have highlighted the precise impact of both organic and inorganic components on the photoswitching abilities of 1-Mo8, 2-Mo8, 1-PW12 and 2-PW12. Upon UV-light irradiation, the absorption of the normal and mixed DTEs in their closed-ring form exhibits a hypsochromic shift which is more pronounced in hybrid systems integrating -[Mo8O26] 4-rather than -[PW12O40] 3-, due to the higher negative charge density of the octamolybdate unit. The photocyclization rates of DTEs are also systematically increased when combined with the -[Mo8O26] 4-unit, while in contrast, their back cycloreversion rates are not affected whatever the nature of the POMs used. In addition, within isostructural series, hybrids integrating mixed DTEs exhibit faster fading processes than their normal DTE counterparts when exposed to visible light. Finally, among this new series, 2-Mo8 shows the best photochromic performances with a fast photocoloration rate associated with a high coloration contrast, a fast and complete fading process and a high cyclability.

Introduction

Polyoxometalates are a unique class of redox and optically active metal-oxide anionic clusters which have been widely studied in many domains including catalysis, 1, 2 biology, 3,4 magnetism, 5 optics, 6,7 and materials sciences. 8,9 Among all investigated POMs materials, hybrid organic-inorganic polyoxometalates (hybrid-POMs) are designed to combine the intrinsic properties of the distinct molecular subunits into optimized devices for synergistic effects and new applications. 10 In the field of photochromic materials, the hybrid-POM systems have been at first built by ionic assembling of POMs and organoammonium cations (OACs). 11,10,12 Their photochromic behavior in the solid state, which is based on the photoreduction of the POM units assisted by H-atom transfer from OACs, 11,12 has proven its efficiency in many fields with applications such as ultraviolet sensors, 13 sustainable materials, 14,15 catalysis, 2 and memory devices, 16,17 taking advantage of the contactless and energy tunable properties of light. Another class of photochromic supramolecular hybrid-POMs has been later designed where OACs are replaced by organosulfonium cations (OSCs). In these systems, the mechanism of photocoloration does not require direct Hbonding interactions between the POMs and the organic cations. 18 Instant color change upon photoirradiation along the photochromic cycle is generally a sought-after feature in these domains, and more particularly for optical data storage application that requires rapid "ON -OFF" states. While fast photocoloration rates have been raised in various optimized hybrid-POM systems, large improvements are still needed to accelerate and enhance the fading process. For this purpose, changing the common OAC (or OSC) cations by another organic part with intrinsic photochromic properties, is a promising alternative strategy. Following this concept, several photoactive hybrid materials incorporating spiropyrans or spirooxazines photochromic units covalently 7,[19][20][21] or ionically 22,23 assembled with POMs have been already reported. This coupling allows improving the thermal stability of the organic switches, and finely directing their photoisomerization in the solid state. To go further in designing efficient solid-state photoswitches with faster photoresponses and improved cyclabilities, a Univ Rennes, CNRS, ISCR -UMR6226, F-35000, Rennes, France b Université de Nantes, CNRS, Institut des Matériaux Jean Rouxel, IMN, F-44000 Nantes, France E-mails: remi.dessapt@cnrs-imn.fr and julien.boixel@univ-rennes1.fr Electronic Supplementary Information (ESI) available: Detailed syntheses, 1 H, 19 F and 13 Please do not adjust margins Please do not adjust margins dithienylethenes (DTE)-based systems are particularly attractive given their well-adapted photochromic features of DTEs gathering rapid and reversible photoswitching reactions together with thermally stable open and closed-ring isomers in solution and in the solid state, as well as remarkable photofatigue resistance. 24 Moreover, their two forms exhibit distinct electronic and optical characteristics highly tunable by rational design, which has motivated their use for the photoswitching of numerous properties, such as luminescence, 21,25 conductivity, 26,27 nonlinear optics, 28,29 magnetism, 30 and catalytic ability. 31 Considering the potential of coupling DTE photochromes with POMs, we recently designed (1)4[Mo8O26]4DMF (1-Mo8) which is the first ionic assembly of an asymmetrical methylpyridinium DTE cation (1 + ) (Scheme 1) and a -[Mo8O26] 4-unit. 32 The POM unit modifies the solid-state photoresponse of 1 + via both steric and electronic effects, resulting in a considerable blue-shift of almost 50 nm in its closed form absorption band, while maintaining the efficiency of its almost complete and fast photocyclization process, as well as its good cyclability. In order to deeper understand the synergic processes between POMs and DTEs during photoirradiation, and to improve further the fading rate and the photofatigue resistance of such supramolecular assemblies, we investigate another type of DTE called "mixed" formed with one 3-methyl-thiophene and one 2-methyl-thiophene, while "normal" DTE backbones bear two 3-methyl-thiophene. Although mixed DTEs have been much less studied than their normal analogues, it has been shown that their quantum yields of photo-cycloreversion reaction are significantly higher than the ones of normal DTEs, leading to faster and complete reopening reaction in solution. 33 We thus envision to take advantage of this specific feature, and transfer the photoswitching efficiency of mixed DTEs from the solution to the solid state, through supramolecular assembling with POMs. We report herein the straightforward preparation and photophysical investigations of five new POM-DTE ionic assemblies. We sequentially changed the organic (DTE) and inorganic (POM) components in order to performed a step-bystep photophysical investigation, and to establish a detailed comparison on their optical properties. Based on our previous study, we firstly modified the nature of the DTE and successfully synthetized the new (2)4[Mo8O26]4DMF (2-Mo8) assembly, an isotype of 1-Mo8, where 2 + is a new asymmetrical methylpyridinium-containing mixed DTE cation (Scheme 1). Secondly, four new DTE-POM systems have been prepared, namely (1)

3[PM12O40]•5ACN (M = W, Mo) (1-PW12 and 1-PMo12) and (2)3[PM12O40]•5ACN (M = W, Mo) (2-PW12 and 2-PMo12),
where the DTE is either a normal type (1 + ) or a mixed type (2 + ), associated with the well-known Keggin-type POM. Singlecrystal X-ray diffraction studies revealed that all these assemblies are isostructural. The thorough investigation of their solid-state photochromic properties at room temperature gives fine understanding on synergistic effects between DTE (normal vs mixed) and POM (molybdenate vs tungstenate) components.

Results and discussion

Synthesis and characterization of [2]OTf and the five DTE-POM assemblies Synthesis [1]OTf has been prepared following our reported procedure. The triflate salts of the second targeted cationic DTE [2]OTf have been obtained by the treatment of the corresponding new neutral pyridyl DTE 2, with an excess of methyl triflate in dry CH2Cl2 solution (Scheme 1). 34 2 was obtained through an adapted procedure, by replacing the 3-bromo-2,5dimethylthiophene precursor by the 2-bromo-3,5dimethylethiophene (Fig. S1, ESI †). 35,36 The purity of 2 and [2]OTf has been confirmed by means of 1 H, 13 C, 19 F NMR spectroscopies (Fig. S2 andS3, ESI †), mass spectrometry (Fig. S2 and S4, ESI †), FT-IR spectroscopy (Fig. S5, ESI †), and elemental analysis (ESI †). Single-crystals of the DTE-POM assemblies have been obtained by mixing in dry acetonitrile (ACN) or in DMF a stoichiometric amount of a cationic DTE ( [1]OTf or [2]OTf) with the tetrabutylammonium salts of the considered POM anions (NBu4)4[-Mo8O26] or (NBu4)3[-PM12O40] (M = W or Mo) (Scheme 1), following the procedure recently reported by our group. 32 The presence of positively charged sp 2 pyridyl-N-atom in [1]OTf and [2]OTf favored the electrostatic assemblies with the POMs in comparison with the starting tetrabutylammonium cation, due to the shielded sp 3 positive charge by the alkylchains. The composition and purity of the obtained hybrid materials have been further confirmed by FT-IR spectroscopy (Fig. S6-S8, ESI †) and elemental analysis (ESI †). The IR spectrum of 2-Mo8 shows the Mo=O and Mo-O-Mo vibration modes of the -[Mo8O26] 4-anion in the range 1000-400 cm -1 , associated with the vibrational signature of 2 + and DMF molecules in the range of 1650-1000 cm -1 (Fig. S6, ESI †). The IR spectra of the tungstenate (1-PW12, 2-PW12) and molybdenate (1-PMo12, 2-PMo12) series show similar features with four characteristic bands at 1078, 979, 895, 810 cm -1 and 1060, 956, 878 and 800 cm -1 , respectively, which correspond to ν(P-O), ν(M-Oterminal), ν(M-Oa-M octahedral corner-sharing) and ν(M-Ob-M octahedral edge-sharing) vibrational modes (Fig. S7 and S8, ESI †). 32,37,38 The absence of splitting of the ν(M-Oa-M) band well characterizes the -isomer of the Keggin-type anions.

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Crystal structures

Both 2 and [2]OTf crystallize in the monoclinic system P21/c space group (Table S1, ESI †), with one independent DTE in its open form (respectively labeled as 2o and 2o + ) in the unit cell. The thienyl groups of both molecules adopt a photoactive antiparallel conformation (Fig. 1a and S9, ESI †). The distances between the two reactive carbon atoms are 3.592(2) and 3.643(4) Å for 2 and [2]OTf, respectively (Tables S2 andS3, ESI †), slightly shorter than that observed for [1]OTf (3.687(33) Å), 32 which indicates that they are close enough to undergo photocyclization reaction. The unit cells of 2 and [2]OTf (Fig. 1b and Fig. S9 and S10, ESI †) are additionally stabilized with numerous intermolecular H-bonds between the fluorine atoms of the bridge and the hydrogen atoms of both the methyl groups and the pyridinium ring, as detailed in Table 1 and Table S4, ESI †.

d(F•••H) [Å] d(A•••D) [Å] ∠ (AHD) [°]

F(5A)•••H(20A) = 2.3788(55) C(20)•••F(5A) = 3.3162(71) 160
F(7)•••H(21C) = 2.6377(35) C(21)•••F(7) = 3.5913(55) 164
2-Mo8 crystallizes in the triclinic system (Table S1, ESI †). Its unit cell shows a -[Mo8O26] 4-anion, two crystallographically independent 2 + cations in their open form and two crystallized DMF molecules as independent fragments (Fig. 2a). The 2 + molecules are in their photoactive antiparallel conformation, and the distances between the two reactive carbon atoms (3.550(15) and 3.607(12) Å) are slightly shorter than that observed for [2]OTf (3.643(4) Å). Noticeably, the crystal packing of 2-Mo8 is similar to that previously observed for 1-Mo8, 32 as a consequence of very comparable structures for the 1 + and 2 + molecules (Scheme 1). The octamolybdate block is surrounded by four 2 + cations whom the aromatic pyridinium rings are involved in the formation of additional π•••π interactions with the oxygen POM facets, the shortest distances being of about 2.95 Å (Fig. 2b The four 1-PM12 and 2-PM12 (M = Mo, W) compounds are all isostructural and crystallize in the triclinic system (Table S1, ESI †).

Their unit cells show one entire -[PM12O40] 3-anion, three cationic DTE molecules and non-coordinated ACN molecules (Fig. 3 and Fig. S12, ESI †). Attempts to obtain single-crystals of 1-PW12 suitable for X-ray diffraction analysis were unsuccessful, and its structure is not reported herein. The number of ACN molecules in 2-PW12, 2-PMo12 and 1-PMo12 could not be precisely determined directly from X-ray diffraction data. The residual electron densities were squeezed with Platon, and the total number of electrons per unit cell is in a good agreement with the TGA and elemental analyses, therefore resulted in five crystallized ACN molecules. The structure of 2-PW12 has been chosen herein to described the crystal packing in this series, and detailed structures of 1-PMo12 and 2-PMo12 are presented in Fig. S12-S16, ESI † for comparison. Please do not adjust margins

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The three DTE units are in their open forms and exhibit suitable photoactive metrics with antiparallel conformations and typical CC distances between the two reactive carbon atoms (Fig. 3 and Table 2). In the unit cell, the bulky -[PW12O40] 3-anions are surrounded by the pyridinium rings of the DTEs (Fig. 4 and Fig. S12-S17, ESI †). Numerous π•••π interactions between the oxygen-POM facets and the cationic heterocycles are observed, with oxygen-to-centroid distances ranging from 3.08 to 3.27 Å (Table 2). These distances are comparable with those observed in 2-Mo8 (3.34 Å) and in 1-Mo8 (3.38 Å).

Thermogravimetric analysis

The thermal behaviors of 1-PMo12, 1-PW12, 2-PMo12 and 2-PW12 have been examined by thermogravimetric analysis, performed under N2/O2 atmosphere. All the recorded thermograms exhibit similar trends, and that of 2-PMo12 is described as an example (

Photochromic properties

Photochromism of 2 and [2]OTf in solution

The photochromic properties in CH2Cl2 solution of both 2 and [2]OTf have been monitored by UV-visible absorption and 1 H NMR spectroscopies. 350 and 525 nm light irradiations are used to trigger the photocyclization and the back cycloreversion reactions, respectively. The electronic absorption spectra of 2 and [2]OTf are presented in Fig. 5, and the related data are summarized in Table 3. Monitoring the ring-closure reaction by 1 H NMR spectroscopy gives access to the percentage of conversion at the PSS. Fig. 6 shows the 1 H NMR spectra of

Solid-state photochromic properties

The solid-state photochromic properties of [2]OTf, 2-Mo8, 2-PW12 and 1-PW12 have been investigated at room temperature by diffuse reflectance spectroscopy of microcrystalline powders, and compared with those of the previously described [1]OTf and 1-Mo8. 32 In this present work, 1-PMo12 and 2-PMo12 have been synthesized to obtain detailed crystal structures of the Keggin-type POM based assemblies, but their photochromic behaviors have not been investigated, because the absorption properties of the -[PMo12O40] 3-unit interfere with those of the DTEs (see below).

Photocyclization of [2]

OTf upon UV irradiation at 365 nm gives rise to a drastic color change, from off white to blue, owing to the gradual growth of a new broad absorption band centered at 610 nm (Fig. 7a and Fig. S19, ESI †) of similar origin than recorded in solution (Fig. 5).

The full width half maximum of the absorption profile in the visible is smaller for

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As shown in Fig. 7, 2-Mo8, 2-PW12 and 1-PW12 exhibit strong solidstate photochromic properties upon UV-light irradiation (ex = 365 nm). The color of 2-Mo8 (Fig. 7b) and 2-PW12 (Fig. 7c) shifts from offwhite to purplish-blue, which slightly differs from that observed for [2]OTf (Fig. 7a). The photocoloration process is faster and much more pronounced for 2-Mo8 than for 2-PW12. Upon similar UV exposure, 1-PW12 gradually shifts from off-white to blue (Fig. 7d) with a high photocoloration contrast, similarly as already observed for 1-Mo8 and [1]OTf. 32 These color-change effects deal with the growth of broad absorption bands relative to the closed-ring isomer of the DTE units, whom the most intense is located at max = 580 nm, 595 nm and 646 nm for 2-Mo8, 2-PW12 and 1-PW12, respectively (Fig. 7 A large blue shift of 52 nm was also observed for 1-Mo8, 32 in well agreement with the electron-withdrawing nature of the POM units which decreases the delocalization of the -conjugation throughout the closed-ring isomer of the DTEs, shifting its absorption at a lower wavelength. Our present work clearly evidences that the amplitude of this hypsochromic shift varies with the nature of the POM used, and is smaller for hybrid assemblies built upon the -[PW12O40] 3-unit than for those integrating the -[Mo8O26] 4-one. This could be explained considering that the octamolybdate anion has a smaller size and a higher negative charge, in comparison to those of -[PM12O40] 3-, leading to a higher negative charge density 39 which increases its electron-withdrawing character and has a stronger impact on the conjugation throughout the DTEs in their closed form.

Upon visible-light irradiation (ex = 590 nm) at room temperature, the fading processes of [2]OTf, 2-Mo8, and 2-PW12 rapidly occur (Fig. 8), and their pronounced coloration totally disappear in only 30 seconds, revealing highly fast reversible systems. Upon visible-light irradiation at ex = 630 nm, the 1-PW12 is slower, and the bleaching is complete after ten minutes. OTf, 595 nm for 2-PW12, 580 nm for 2-Mo8, and 646 nm for 1-PW12, as a function of the UV-light irradiation time (Fig. 9). The Abs λmax (t) vs t plots have been satisfactory fitted thought a biexponential rate law. The color fading rates have been measured under visible light at 590 nm for [2]OTf, 2-Mo8 and 2-PW12, and at 630 nm for 1-PW12, by monitoring the temporal decay of Abs λmax (t) of samples once irradiated under UV excitation. Decays were fitted using a mono or a biexponential rate law. The coloration and fading kinetic parameters of all the DTE-POM assemblies together with those of [1]OTf and [2]OTf are presented in Table 4. First, the photocoloration and fading rates of the compounds have been compared by considering their coloration (t c 1/2) and fading (t f 1/2) half-life times. Upon UV-light irradiation, [2]OTf exhibits a coloration rate (t c 1/2 = 0.50 min, Fig. 9a) about four-fold slower than that of [1]OTf (t c 1/2 = 0.12 min). However, the opposite trend is observed if considered the fading process under visible-light irradiation. The fading rate of Moreover, although the cycloreversion of the closed-ring form in [1]OTf is not complete, 31 [2]OTf exhibits a quantitative bleaching, and noticeably, its absorption loss reaches 98% after only 30s. This is in direct accordance with previous studies on normal 3-methylthiophene and mixed 2-,3-methyl-thiophene DTEs, which revealed higher photochromic quantum yields of ring-closure reaction for normal DTEs but higher ring-opening reaction for mixed ones. 42 Although 1-Mo8 and 2-Mo8 are isostructural, the nature of the DTE strongly dictates the photoresponses of the hybrid assemblies which follow the same trend than that observed for [1]OTf and [2]OTf. The photocoloration rate of 2-Mo8 (t c 1/2 = 0.46 min, Fig. 9c) is 4-fold slower than that of 1-Mo8 (t c 1/2 = 0.12 min). However, its fading rate is considerably 70-fold faster (Fig. 9d), with t f 1/2 = 0.05 min and 3.50 min for 2-Mo8 and 1-Mo8, respectively, and its absorption loss reaches 95% after only 2 min. The isostructural 1-PW12 and 2-PW12 compounds follow a similar trend. The coloration rate of 2-PW12 (t c 1/2 = 1.37 min, Fig. 9e) is just over 2-fold slower than that of 1-PW12 (t c 1/2 = 0.60 min, Fig. 9g). In addition, 2-PW12 exhibits a completed bleaching (the absorption loss reaches 97% after 2 min), and its fading rate (t f 1/2 = 0.08 min, Fig. 9f) is 40-fold faster than that of its 1 + analogue (t f 1/2 = 3.87 min, Fig. 9h). These results clearly indicate that 2-Mo8 and 2-PW12 are efficient reversible photochromic systems which exhibit improved fading rates compared to 1-Mo8 and 1-PW12. a The Abs max (t) vs t plots were fitted as Abs max (t) = (A1+A2) -A1exp(-k c 1t) -A2exp(-k c 2t) for the coloration process and as Abs max (t) = (A0-A1-A2) + A1exp(-k f 1t) + A2exp(-k f 2t) for the fading ones. b Coloration (k c ) and fading (k f ) rate constants. c For the coloration process, the half-life time t c 1/2 is defined as the UV irradiation time required for Abs max (t) to reach half of its maximum value. For the fading ones, t f 1/2 is defined as the irradiation time required for Abs max (t) to reach the A0-(A1+A2)/2 value. d Regression coefficient for the Abs max (t) vs t plot. e ex = 590 nm for [2]OTf and 2-PW12; ex = 630 nm for 1-PW12.

For a given DTE, the UV-induced coloration processes of the hybrid assemblies are significantly impacted by the nature of the POM unit. Indeed, the coloration rates of 2-Mo8 (t c 1/2 = 0.46 min) is as fast as that of [2]OTf (t c 1/2 = 0.50 min), and a similar trend was observed for 1-Mo8 (t c 1/2 = 0.12 min) and [1]OTf (t c 1/2 = 0.12 min). Thus, the photocyclization processes of 1 + and 2 + are not disturbed by their assembling with the -[Mo8O26] 4-unit. In contrast, the coloration rates of 2-PW12 (t c 1/2 = 1.37 min) and 1-PW12 (t c 1/2 = 0.60 min) are slower than those of [1]OTf and [2]OTf, revealing that the coupling of the DTEs with the -[PW12O40] 3-unit slightly affects the efficiency of the photocyclization processes. This is in direct line with the energy of the LMCT transitions of the POM units which decreases in the trend - S22, ESI †). The low-energy LMCT transitions of the -[PW12O40] 3- polyanion strongly overlap the absorption bands of the DTEs in the range 300-350 nm, and thus, the POM dramatically competes to absorb the excitation UV energy needed to activate their ring-closure process. This competition is even more marked in hybrid assemblies built upon the -[PMo12O40] 3-entity, which explains why their poor photochromic properties have not been considered in our present work. In marked contrast, the fading rate of 2-Mo8 upon visible light irradiation (t f 1/2 = 0.05 min) is well comparable with that of 2-PW12 (t f 1/2 = 0.08 min), and a similar trend is observed for 1-Mo8 (t f 1/2 = 3.50 min) and 1-PW12 (t f 1/2 = 3.87 min). This evidences that the back cycloreversion process of the DTEs is not significantly impacted by the POMs, essentially because both the -[Mo8O26] 4-and -[PW12O40] 3-units have large transparency windows in the range 400-800 nm.

[Mo8O26] 4-> -[PW12O40] 3-> -[PMo12O40] 3-(Fig.
Our complete kinetics study finds that within this new series, 2-Mo8 is the most efficient photochromic hybrid POM-DTE system, which combines fast coloration rate, high coloration contrast, and very fast and complete fading process. Thus, its robustness for repeatable ''recording-erasing'' processes at room temperature was investigated by monitoring the evolution of the absorption at 580 nm of a sample alternatively irradiated for 20 secs with UV light (365 nm), and then for 1 min with visible light (590 nm) (Fig. 10). 2-Mo8 exhibits a high cyclability on sixty coloration/fading cycles (n), with only a very limited photofatigue. The empirical linear evolution of the Abs 580 (n) vs. n plot allows predicting that the color-change effect should disappear at the end of about 1500 cycles. Mo8) is strictly similar to that of 1-Mo8. Combined X-ray crystallographic and spectroscopic analyses have enabled to highlight the precise impact of both DTEs and POMs on the photochromic properties of the assemblies, as well as synergistic effects between the organic and inorganic components. Noticeably, the presence of the POMs significantly modifies the photoresponses of 1 + and 2 + in the solid-state. Upon UV-light irradiation, the absorption of their closed-ring isomer undergoes a blue shift which increases with the electron-withdrawing character of the POMs, and is more pronounced for -[Mo8O26] 4-than for -[PW12O40] 3-. The photocyclization process of the DTEs also varies with the nature of the POM used. The beneficial association of 1 + and 2 + with -[Mo8O26] 4-which has a high energy LMCT transitions prevents a strong competition between both organic and inorganic components to absorb the excitation UV energy, that causes a decrease of the photocoloration rates, as observed in POM-DTE assemblies built upon the -[PM12O40] 3-(M = W, Mo) unit. In addition, upon visiblelight excitation, 2-Mo8 and 2-PW12 exhibit complete bleaching with cycloreversion rates faster than those of their 1 + counterparts. In particular, 2-Mo8 exhibits remarkable photochromic properties at room temperature, which include a marked photocoloration contrast, fast photoswitching processes, and a high cyclability. 2-Mo8 stands as the most efficient system for fast and complete fading process among hybrid photochrome-based POM assemblies. Thus, this strategy of assembling allows transferring the photoswitching efficiency of mixed DTEs from the solution to the solid state, and it opens the way towards the design of novel robust photoresponsive materials with improved fading processes and high light-driven "recording-erasing" potential.

  C, ESI-HRMS, FT-IR, UV-vis spectra, X-ray crystallographic data, TGA measurements. CCDC 2057188-2057193. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/x0xx00000x ARTICLE Journal Name 2 | J. Name., 2012, 00, 1-3

Fig. 1

 1 Fig. 1 Crystal structure of [2]OTf. (a) Structure of 2o + with atom labelling. (b) Stacking of the 2o + and triflate molecules with intermolecular H-bonds (green dotted lines).

  and Fig. S11, ESI †). One of the two DMF molecules involves π•••π interactions with the pyridinium ring of one 2 + unit, with a short O•••N distance of 3.062(9) Å, while the second DMF molecule forms no significant interaction.

Fig. 2

 2 Fig. 2 (a) Crystal structure of 2-Mo8. The red dotted lines illustrate π•••π interactions between DMF molecules and the pyridinium ring of 2 + . (b) Crystal packing of 2-Mo8 showing the π•••π interactions (red dotted lines) between oxygen POM facets and the pyridinium ring of 2 + (blue octahedra: MoO6, red sphere: oxygen, yellow sphere: sulfur, blue sphere: nitrogen, green sphere: fluorine, grey sphere: carbon). H atoms were omitted for clarity.

Fig. 3 4 |

 34 Fig.3Crystal structure of 2-PW12. The -[PW12O40] 3-anion is represented in polyhedral mode (blue octahedra: WO6, orange tetrahedron: PO4, red sphere: oxygen, yellow sphere: sulfur, blue sphere: nitrogen, green sphere: fluorine, grey sphere: carbon). H atoms and acetonitrile molecules were omitted for clarity.

Table 2

 2 Comparison of the CC distances between the reactive carbon atoms of DTEs, and π•••π interactions (oxygen-to-centroid) distances around the -[PM12O40] 3-anions in 1-PMo12, 2-PMo12, and 2 -PW12. CC distances [Å] π•••π interactions distances [Å] 1-PMo12 3.561(12); 3.566(19); 3.70(9) 3.08; 3.15; 3.22 2-PMo12 3.627(18); 3.63(2); 3.88(3) 3.11; 3.17; 3.20 2-PW12 3.64(3); 3.65(2); 3.88(3) 3.13; 3.22; 3.27

Fig. 4

 4 Fig. 4 (a) Unit cell of 2-PW12 along the a-axis. (b) Crystal packing of 2-PW12 showing the π•••π interactions (red dotted lines) between oxygen POM facets and pyridinium rings (blue octahedra: WO6, orange tetrahedron: PO4, red sphere: oxygen, yellow sphere: sulfur, blue sphere: nitrogen, green sphere: fluorine, grey sphere: carbon). H atoms were omitted for clarity.

  Fig. S18, ESI †). The first weight loss (5.26 %) appears from 20-185 o C, that corresponds to five non-coordinated ACN molecules (theoretical weight loss of 5.94 %). A second broad weight loss (42.30 %) occurs at ca. 240-650 o C, which corresponds to the gradual decomposition of the three cationic DTEs (theoretical weight loss of 41.22 %).

Fig. 5

 5 Fig. 5 Absorption spectra in CH2Cl2 at 298 K (c = 2.10 -5 M) of (a) 2 and (b) [2]OTf, before (black line) and after (red line) irradiation at 350 nm.The electronic absorption spectrum of 2o (Fig.5a) shows two maxima in the UV region at 265 and 304 nm which are assigned to 1 (π-π*) transitions. Upon UV-light irradiation (350 nm), the initial colorless solution of 2o turns pink with the emergence of a large absorption band in the visible region centered at 514 nm, and attributed to 1 (π-π*) transition of the closed-ring isomer (2c).40, 41 Irradiation at 525 nm of 2c, at the photostationary state (PSS), quantitatively yields back the open form 2o, revealing the full reversibility of the system. The open-ring form [2o]OTf exhibits absorption maxima at 280 and 380 nm assigned to 1 (π-π*) and charge transfer (toward the pyridinium moiety) transitions, respectively. Similarly, UV-light irradiation of a solution of [2o]OTf produces the closed-ring isomer [2c]OTf evidenced by the appearance of the lower-lying 1 (π-π*) transition in the visible (centered at 607 nm), and resulting in a drastic change in color, from pale yellow to dark blue (Fig. 5b). The presence of the cationic pyridinium moiety in [2]OTf induces a large bathochromic shift of the absorption bands relative to 2 ([2]OTf/[2] ≈ 90 nm) both in the open and closed-ring forms, due

[ 2 ]

 2 OTf before and after irradiation at 350 nm. The characteristic upfield shifts of the thienyl protons are observed, from 8.11 to 7.52 ppm and from 6.66 to 5.72 ppm. The central methyl groups are differently affected by the ring-closure reaction, i.e., the central 2-methyl is upfield shifted (from 2.45 to 2.10 ppm) while the central 3-methyl being downfield shifted (from 1.78 to 1.87 ppm). The PSS of [2]OTf have been determined to be 67 % of closed isomer, by the relative integration of the above signals between the open and the closed forms. No thermal cycloreversion has been observed in the dark for a month. The expected moderate conversion at the PSS (0.67) of [2]OTf is due to its 2-methyl-thiophene that increases the quantum yield of the ring-opening reaction, in comparison with the normal analogue bis-3-methyl-thiophene (PSS > 0.95).

Fig. 6 1 H

 1 Fig. 6 1 H NMR spectra in CD3OD of [2]OTf before (bottom) and after (top) irradiation at 350 nm.

[ 2 ] 2 ]

 22 Photocyclization of[2]OTf upon UV irradiation at 365 nm gives rise to a drastic color change, from off white to blue, owing to the gradual growth of a new broad absorption band centered at 610 nm (Fig.7aand Fig.S19, ESI †) of similar origin than recorded in solution (Fig.5). The full width half maximum of the absorption profile in the visible is smaller for [2]OTf than for [1]OTf, on account of reduced delocalization. Qualitatively, the color-change effect is slower for [2]OTf than for [1]OTf (60 min for [2]OTf and 20 min for [1]OTf, to reach the PSS), which is attributed to the faster cycloreversion process of the mixed 2 + DTE (see below), similarly as observed in solution. Before UV irradiation, the solid-state absorption spectra of 2-Mo8, 2-PW12 and 1-PW12 (Fig. S20, ESI †) exhibit intense and broader absorption bands from 250 to 450 nm, attributed to both the 1 (π-π*) transitions of the DTE units (1o + or 2o + ) and the ligand-to-metal charge-transfer (O→M LMCT) transitions of the POM unit. While the absorption thresholds of 2-Mo8 and 1-Mo8 are well comparable with those of [2]OTf and [1]OTf, those of 1-PW12 and 2-PW12 are systematically red-shifted in the range 400-450 nm. The -[PW12O40] 3-unit has a LMCT absorption band (max = 322 nm) much lower in energy than that of -[Mo8O26] 4-(max = 300 nm).Consequently, the shift of the absorption thresholds of 1-PW12 and 2-PW12 could be consistent with the existence of an additional absorption band, assignable to an intermolecular charge-transfer transition. This latter results in direct excitation of an electron from the HOMO of the electron-rich DTE donors into the LUMO of the electron-poor -[PW12O40] 3-acceptor, in close proximity through many - stacking interactions, which has been already observed in other charge-transfer assemblies based on Keggin-type POM units.23

Fig. 7 6 |

 76 Fig. 7 Temporal evolution of the photogenerated absorption for (a) [2]OTf, (b) 2-Mo8, (c) 2-PW12 and (d) 1-PW12, during UV-light irradiation (365 nm, time range of 0-60 min for [2]OTf, 2-Mo8 and 2-PW12, and of 0-30 min for 1-PW12). Insets: photographs of the associated color-change effects.

  et Fig. S21, ESI †). All these absorption bands are blue shifted with respect to those of [1]OTf and [2]OTf (DTE/DTE-POM = 30, 24 and 15 nm for 2-Mo8, 1-PW12 and 2-PW12, respectively).

Fig. 8

 8 Fig. 8 Photographs of the color-change effects of [2]OTf, 2-Mo8, 2-PW12 and 1-PW12 at different times (in min and sec) during the coloration (upon UV-light irradiation) and fading (upon visible-light irradiation) processes. The solid-state photocoloration and fading kinetics of [2]OTf, 2-Mo8, 2-PW12 and 1-PW12 have been investigated under air and at room temperature, and compared with those of [1]OTf and 1-Mo8. The coloration kinetics have been quantified by monitoring the

Fig. 9

 9 Fig. 9 Temporal Evolution of Abs λmax (t) during the photocoloration and visible-light driven fading processes for [2]OTf (a and b), 2-Mo8 (c and d), 2-PW12, (e and f) and 1-PW12 (g and h). Black lines show the fits of the Abs λmax (t) vs. t plots according to the rate laws Abs max (t) = (A1+A2) -A1exp(-k c 1t) -A2exp(-k c 2t) for the coloration process under UV light (365 nm), and Abs max (t) = (A0-A1-A2) + A1exp(-k f 1t) + A2exp(-k f 2t) for the fading one under visible light.

[ 2 ]

 2 OTf (t f 1/2 = 0.01 min, Fig.9b) is adjust margins Please do not adjust margins considerably increased compared to that of [1]OTf (t f 1/2 = 5.60 min).

Fig. 10

 10 Fig.10 Evolution of the absorbance monitored at 580 nm for 2-Mo8 during successive coloration/bleach cycles (n). The solid red line corresponds to the empirical linear fit of Abs 580 (n) vs. n according to the relationships Abs 580 (n) = -2.680. 10 -4 n + 0.402.

ConclusionFive new photochromic supramolecular assemblies which combine a normal ( 1 +

 1 ) or a mixed (2 + ) cationic dithienylethene (DTE) with three different POM units have been successfully prepared. This study completes well our recent work devoted to (1)4[Mo8O26]•4DMF (1-Mo8), the very first member of this DTE-POM series. All these hybrid materials show a high degree of organization, conducive to the establishment of structure-properties relationships. The (1)3[PM12O40]•5ACN (M = W, Mo) (1-PW12 and 1-PMo12) and (2)3[PM12O40]•5ACN (M = W, Mo) (2-PW12 and 2-PMo12) compounds are isostructural, while the crystal packing of (2)4[Mo8O26]•4DMF (2-

Table 1

 1 Distances and angles values of the intermolecular H-bonds in [

2]OTf.

Table 3

 3 Absorption data in solution of compounds 2 and [2]OTf.

		abs a /nm (/10 -4 M -1 cm -1 )	abs a,b /nm
		Open (o) forms	Closed (c) forms (PSS)
	2	265 (1.80), 304 (2.30), 345sh (0.75)	267, 297sh, 348sh, 518
	[2]OTf	281 (0.87), 331 (2.33)	278, 320sh, 385, 552, 607

a Measured in CH2Cl2 solution at 298 K. b After irradiation at 350 nm.

Table 4

 4 Photochromic kinetic parameters at room temperature of the DTE-POM

assemblies, [1]OTf and [2]OTf.

Visible-light driven fading process e Compound A0 a A1 a A2 a k f 1 (s -1 ) b k f 2 (s -1 ) b t f 1/2 (min) c R 2 d

	ARTICLE							Journal Name
			Photocoloration process
	Compound	A1 a	A2 a	k c 1 (s -1 ) b	k c 2 (s -1 ) b	t c 1/2 (min) c	R 2 d
	[1]OTf 43	0.408	1.720	0.001	0.138		0.12	0.9995
	1-Mo8 43	0.391	1.790	0.015	0.123		0.12	0.9998
	1-PW12	1.004	0.841	0.005	0.076		0.60	0.9957
	[2]OTf	0.73	0.27	0.035	0.002		0.50	0.9995
	2-Mo8	0.773	0.252	0.037	0.003		0.46	0.99966
	2-PW12	0.377	0.297	0.020	0.001		1.37	0.9958
	[1]OTf 43	2.128	1.720	-	0.002	-	5.60	0.9878
	1-Mo8 43	2.181	2.118	-	0.003	-	3.50	0.9822
	1-PW12	1.714	1.537	-	0.003	-	3.87	0.9974
	[2]OTf	1.01	0.899	0.108	0.533	0.066	0.01	0.9999
	2-Mo8	0.959	0.896	0.033	0.215	0.003	0.05	0.9999
	2-PW12	0.654	0.238	0.400	0.382	0.068	0.08	0.9996

8 | J. Name., 2012, 00, 1-3 Please do not adjust margins Please do not adjust margins

Scheme 1 Synthetic routes for the targeted POM/DTE assemblies from normal (1 + ) and mixed (2 + ) DTE cations. i) (NBu4)4[-Mo8O26] or (NBu4)3[-PM12O40] (M = W, Mo), CH3CN or DMF.Accepted manuscript / Final version

J. Name., 2013, 00, 1-3 | 3

Acknowledgements

This work was supported by the CNRS, the Université de Nantes, the Université de Rennes 1, the Université Bretagne Loire, the Ministère de l'Enseignement Supérieur et de la Recherche, and the LUMOMAT project supported by the Région des Pays de la Loire. Thierry Roisnel is gratefully acknowledged for help with X-ray analysis of 2 and [2]OTf.

Conflicts of interest

There are no conflicts to declare.