Xavier Guillot

Bertrand Hervieu

Jean Viard

L ' Archipel Paysan

Bernard Kayser

Urbanization Brian J L Berry

Counterurbanization

Teaching design in rural contexts

Since the late 1980s a significant "spatial and demographic rebalancing" has occurred between what was "yesterday's" city and countryside 1 . In France, at the scale of national territory, what was previously called the "rural exodus", has given way to a process called "rural renaissance", as identified by Bernard Kayser twenty years ago 2 . In fact, nowadays, French countryside registers a positive demographic curve.

This "rural renaissance" is complex to understand, because countryside has become enbedded with numerous conflicting development issues linked with the extension of urbanisation at the scale of the territory 3 . One of the main issues is the ambiguous spatial status of the hybrid rural/urban space which has developed around every French town and city, generally called "espace peri-urbain", and which is the consequence of a process that emerged decades ago, and was identified and coined in the late 1970s by British urban planners as a process of counterubanization 4 . Another conflicting issue linked with this process is the changing status of villages, in particular those which are located at a convenient distance from cities, making is possible for commuters to live in rural areas while working in cities.

This multi layer change in the pattern of urbanization is even more complex to tackle, as this issue must be addressed in the context of another major planning challenge : developing sustainable human settlement to ensure the good of the future generations. In this context, "new alliances" must be found at many levels of developmentspatial, social, cultural and economic -to accommodate the evolution of these conflictual urbanization dynamics between urban and rural areas. New conceptual frameworks must be formulated to imagine and plan tomorrow's contemporary rurality : this "metropolised elsewhere" as Martin Vanier calls it 5 .

Clearly, rural areas are a major design challenge for architects and planners, due to the multitude of issues regarding the sustainable development of our society. In addition to the complexity of these economic, social and cultural issues, is the fact that rural areas are at the heart of a major challenge directly linked with sustainable development : the evolution of our energy production system. The rural area is the site of the evolution towards the production of 1

renewable energy in different forms: solar, geothermal, biogas, wind, water, etc. This variety of energy sources also consitutes a cultural and spatial issue, as it can -and must -promote local and regional diversity in the design of our living space at the territorial scale.

However, despite the importance of these issues, in France, the question of rural areas is largely ignored in architectural education. In the last two decades, Schools of architecture have failed to renew their cultural background and design tools to tackle issues of this so called "rural renaissance" .

Identifying a turning point in our design approach

Introducing this issue in architectural education today will start from the following observation: during the last thirty years, architectural design has been subject to a form of "urbanocentrism" in the approach of issues regarding human settlement in general. I mean here that issues related with the future of cities have dominated the planning and design agenda. Furthermore, when architectural design in rural areas is addressed, it is often through design tools determined by "sub-concept" from urban design and urbanism based on density, which has proved to be largely inefficient, as we can observe nowadays through the unsightly settlements that have developed in rural areas around cities and towns in France 6 . It is on the basis of this observation that we have formulated the hypothesis that a turning point in our approach must be foreseen with the corollary, the re-evaluation of a set of assumptions underlying our design curriculum in schools of architecture.

In relation to this hypothesis, an immediate clarification is needed : our purpose is not to promote a project design focused exclusively on rural areas while ignoring what happens in urban areas. Such an approach would be contrary to our beliefs. Our approach assumes that rural areas "work" in a dialectical and complementary relationship with urban areas. This approach has been borrowed from the Italian territorialist school. Following this idea, actions should be directed towards building a new "alliance" between the rural and urban areas 7 .Through this reference, I wish to emphasise that ongoing discussions about rural areas can not isolate themselves from urban space issues and the dialectic that occurs between these two spatial entities at a cultural and economic level. Understanding the nature of this dialectic, its mode of operation and the different forms of territorial occupation, is the general framework of our reflection in this domain.

Engaging a debate on the long term spatial organization of our society

Regarding current management practices, one of the major criticisms made by the Italian territorialists is the gradual disappearance of the integrated nature of the whole city and 6 Martin Chénot, « pour une alternative au modèle de la ville compacte», in Guillot, X., Espace rural & projet architectural. Volume 2.Vers un nouveau pacte ville-campagne. Publications de l'Université de Saint-Etienne, 2011, pp. 12-18-7 Xavier Guillot, « Du projet spatial en milieu rural. Bilan d'étape d'une réflexion en cours », in Guillot, X., Espace rural & projet architectural. Volume 2.Vers un nouveau pacte ville-campagne. Publications de l'Université de Saint-Etienne, 2011, pp. 18-29. territory. For Daniela Poli : "The project of modern society is not built, it is 'superimposed' on the territory, which is considered a ground without geological and historical depth, and perceived as a 'simple support' of various functions which are more and more dissociated from local know-how in the field of maintenance, "care" given to the land by those who live there on a daily basis" 8 . This process tends to transform local contexts in generic spaces without specific characters. Understanding today's transformation of rural areas requires a critical appraisal of twentieth century planning doctrines, and to question their failure in terms of planning sustainable settlements in relation to a specific geographic area and its resources. Indeed, such an appraisal would lead to questioning the ability of our contemporary society to transform a territory inherited from past generations and to give meaning and legitimacy in relation to our new era. Hence the need to link our thinking to the current debate on sustainable development in schools of architecture. -How can issues specific to rural areas find their place in this debate on sustainability while giving these areas a particular meaning? -Conversely, how can the debate on sustainability help us to consolidate our reflection on rural areas and participate in the construction of a new field of knowledge?

These are the key questions that guide our thinking in the conduct of our teaching, and which are shared with other members of the academic network "espace rural et projet spatial", established in 2008. Based on these observations and analyses, and through seminars and symposiums organised during the past five years 9 , three working hypotheses, three areas of reflection have been identified to guide us to a new conceptualization of architectural and spatial design in rural areas.

-Establishing new economic and cultural alliances

The first area of reflection raises the question of the changing spatial and economic status of rural land and the role that the project can play in its evolution. It starts from the premise that beyond the agricultural status inherited from past generations, rural areas are now shaped by different activities resulting from economic and social changes of our time. Nowadays, they are "multifunctional spaces"10 . Such territorial multifunctionality requires the development of new social, economic and cultural alliances to formulate action and the transformation of space. For example, the requirement of such alliances arises when integrating tourism which can have paradoxical effects on the evolution of rural areas. The concept of an agricultural park as 8 Daniéla Poli, "La dimension locale dans le projet du parc agricole de la Tosacane centrale", in Xavier Guillot, Espace rural & projet spatial. Volume 3. Du terrain à la recherche : objets et stratégies, Publications de l'Université de Saint-Etienne, 2012, pp. 42-60. 9 In october 2009 at Aydat in Auvergne, in octobre 2010, at Jaujac in Ardèche, in octobre 2011à Saint-Etienne, in 2012at Nancy, in 2013, in Clermont-Ferrand. All this symposiums are co organised by school architecture and other partner of our netwoks, in particular Regional natural parc, National School of agronomy (ENGREF Agro paristech) and other regional and governemental bodies such as CAUE (conseil d'architecture, d'urbanisme et d'environnnement). For more details, see the web site « espace rural et projet spatial » implemented in Italy, is a remarkable case study of mixed economic response to this demand 11 . Other forms of development within the logic of economic diversity should be explored.

-Promote territorial resources

The second area of reflection deals with the multifunctionality of rural areas regarding use of territorial resources. In the context of rural areas, the concept of territorial resources is intended broadly and refers not only to the so-called natural resources. This concept suggests the idea that every territory has a potential that should be highlighted by the project 12 . Through this concept, the goal is to explore the medium and long-term economic sustainability of our settlements in formulating scenarios for developing new human space organizations. This approach raises the question of the ability of our society to organize the permanent settlement of new populations in a rural setting inherited from past generations, and to transform this setting while giving shape to new ecosystems 13 . From the perspective of spatial transformation of our human settlements, this line of thought introduced a major issue on the "relocation" of the building process. By using the expression of "relocation", we refer to the ability of our societies to rethink the "anchor" in an area and produce architectural and landscape differences that contribute to the construction of local specificities.

-Building the common good

The third area of reflection deals more directly with architectural design practice and takes into account the issues just mentioned, and the need to work with other disciplines: geography, agronomy and landscape design in particular. In formulating new practices we encounter a key concept: building for the common good 14 . This approach aims in particular to identify areas of common interest. By dismissing the cultural and professional determinism that freezes our reading of the rural areas, it calls for another perspective and other tools to deal with their transformation 15 . The design of such spaces introduces in particular the idea of engaging a dialogue in which the architect would not only be in a position of expert designer, separate from, or above others, but also a mediator and an incentive to invent or reclaim, with labors, methods of design and construction. This is to emphasize that the project throughout the area or building is primarily a social action. Simultaneously addressing these three types of issues in terms of design leads us to consider and to reconsider -the potential and the shortcomings of the design tools and concepts inherited from architecture as a discipline. Concretely, it leads us to open our design practice to other disciplines, mainly to landscape architecture, but also, to a certain extent, to geography and agronomy. In other words, at the core of our approach regarding the future of rural areas, is a multidisciplinary design approach that gives landscape architecture a major role.

Introducing landscape architecture as a key design component of a multidisciplinary design approach

The world of theory of landscape architecture is wide. It is therefore necessary to clarify our perspective of this discipline and the concepts we will rely on in teaching design in a school of architecture. Regarding these expectations, one author in particular, and his work, occupies a singular place in our approach to landscape design : John Brinkerhoff Jackson 16 . Why this author? Why is the thought embodied in his work a key reference in our teaching? How does the landscape -and the view it offers on the organization of human space -allow us to meet the expectations outlined above, concerning the renewal of our conceptual approach on architectural design in a rural context ? To answer these questions, we revisit Discovering the vernacular landscape 17 , a book which best expresses his thoughts. We also refer to the analysis in Le goût du monde 18 by Jean-Marc Besse, which explains the different interpretations of the notion of landscape , both in theory and practice.

In his approach to landscape, Jackson provides a seminal terminology reminder from its Anglo-Saxon equivalent, which demonstrates the conceptual scope of this discipline. The term landscape is composed of two words, land and scape, which individually and by their combination gives a unique sense of the term. For Jackson, the word 'land' is used by farmers to refer to an area that is both closed and part of a larger, open area. The word 'scape', meanwhile, refers to a family of terms which describe "the collective aspects of the environment" sheaf, shape, ship. In this sense, it refers to the overall concepts, collectables, system19 . This double connection with geography and anthropology which characterizes the Jacksonian landscape approach, is crucial to identifying the knowledge and concepts that we associate with landscape architecture in our approach of design. It is part of its central premise: that the landscape is primarily a "social space"20 . Thus, "reading the landscape, explains Jean-Marc Besse is to identify ways of organizing space" to understand how such a "community draws a border dividing the land among families, builds roads and places for public meetings, and reserves land for communal use» 21 etc.. Introducing this concept generally implies an interest in the diversity of spatial forms, "the structural elements and dynamics, the morphologies, the flows and upheavals, the discontinuities of space for circulation, because all these features are used to characterize a landscape", adds the author 22 . This approach to landscape clearly relies on geographical knowledge in its definition. Simultaneously, it gives a secondary role to other approaches, in particular those focusing on an aesthetic ideal 23 .

In our studio, our choice is to consider geography -especially the methodological point of view that this discipline develops regarding human spaceas the main knowledge resource to address the analysis of rural space and the question of its transformation, including on the territorial level. In this domain, the contribution of geography seems particularly relevant with respect to a key educational goal : to address transformation in relation to the issue of sustainability. Indeed, as explained by Jean-Marc Besse: "The landscape has a substantial and intrinsic thickness: it is a complex set of articulated objects, or at least a domain of material reality, larger and deeper than the representations that accompany it. The landscape is as wind, rain, water, heat, climate, rocks, living world around humans: in short, a whole environment, in which of course the changes are affected more or less directly by an action, emotion and human thought, but which ultimately we must recognize, exists and develops without the human being, existed before humans, and will out-survive humans in one way or another» 24 .

However, the fact that we give prime importance to the physical dimension of the landscape, does not mean that we finally adopt a deterministic approach to human organization, depending on the spatial and material configuration of the territory. On this point, we recall the genealogy of thought between JB Jackson and French human geography 25 : for scholars belonging to this school of thought, landscape analysis must begin with the question of habitat as being the main factor structuring landscapes. A connection with Maurice Le Lannou is at this level revealing. "Inhabiting for the human being", writes Maurice Le Lannou, "is more than just shelter and a place to stay. It means "to inhabit a part of the planet and learn through work, enough to satisfy their needs (primary and redundant), depending on opportunities and constraints within the nature and culture . And more fundamentally, it is to organize collectively, a space of existence." 26 We should emphasize here a remarkable concept coined 21 Ibidem 22 Ibidem 23 According to J.M. Besse, Jacksonian landscape theory was forged in a constant dialogue with human geography and the social sciences of his time (...) It is based on two major statements: the landscape is an organized space, i.e. composed and designed by men on the earth's surface: the landscape is a collective work of societies that transforms the substrate nature. "In this perspective, the landscape value of a place is not considered only aesthetic (even if it is) said Besse, but rather is considered in relation to the sum of experiments, habits, practices developed by a human group in place Voir Besse, ibidem.

by Maurice Le Lannou in his Rêveries d'un géographe 27 : that of " inhabitant man" ( l'homme habitant ). For Le Lannou, '' the dwelling man is primarily a human group that occupies a part of the earth's surface and organizes a specific way. However, not all is geography in the human community. As indicated by the author, what geography focuses on, and what works with our approach to landscape architecture, are the visible and tangible manifestations of this occupation and the human appropriation of the world, which differ from one place to the other" 28 .

Implication in architectural education : "l'Atelier rural" and the design studio "Prospective territoriale et soutenabilité architecturale" at the National Advanced School of Architecture in Saint-Etienne

At the National Advanced School of Architecture in Saint-Etienne, the above issues and their implications in architectural education are developed in the "Atelier rural" : a collaborative experience which is conducted with Regional Nature Reserves, at the bachelor and master levels. L'Atelier Rural gives a prominent place to work in the field, "atelier hors les murs" ,organized in partnership with territorial proceedings related to the project site -here the Parc Naturel Régional des Monts d'Ardèche. This partnership allows students direct contact with the reality of issues in rural areas, through the different actors.

Another feature of the "Atelier rural" is the gathering of both students of bachelor and master levels during the first six weeks of the semester. Its goal is to jointly achieve an analysis of the project territory they have in common and to develop a shared knowledge of this territory, called the "Landscape Atlas" . The supervision of this Atlas is specifically supervised by Pierre Albert Perillat and Evelyne Chalaye who are also in charge of the bachelor design studio « Objet d'architecture -Matériau de paysage » and of the master design studio « Naturel -Artificiel -Matériel ». Based on this Atlas, students will identify project sites on which they intervene with different approaches in terms of program: agriculture, tourism, dwelling. The articulation of the different programs can assert a point of view on the future of rural areas in the medium and long term.

The objectives of the design studio Prospective territoriale & Soutenabilité architecturale which I conducted with the collaboration of three colleagues with different expertise, François Chomienne (landscape designer), Marc Lemarié (architect, specialist in building conservation) and Marcel Ruchon (architect and specialist in ecological approach to construction) 29 are part of this educational and research perspective we have just developed. The specificity of the objectives of this studio can be identified by the two terms that make up its name. They refer to two scales of the living space and two approaches to action: planning and foresight on one hand, and the architectural project, on the other hand. This double pronged thought and action François Chomienne is a landscape designer, Marc Lemarié is an architect, and "architecte en chef des batiments de France" and a specialist in building conservation) and Marcel Ruchon is an architect and specialist in ecological approach in construction establishes the link between the design of buildings and the territory. "The environmental issue" plays a central role in this multiscale project approach.

In which way is landscape architecture as defined earlier and its link with geography, particularly useful in our practice of teaching design in the context of this studio ? The answer to this question is threefold. It has led us to identify three main directions to approach the design task, representing three types of interaction between humans and territory in its design process. These three types of interaction establish the link with the notion of "inhabitant-man" as previously mentioned, and the interpretation we have made here.

Indeed, prospective approach in the context of territory addresses the large scale of human space and long term time-frame of the construction of landscape. Its purpose is to "explore possible futures to identify tomorrow's challenges and inform today's decisions" 30 . The prospective work informs the project. It suggests a direction: where one can go, instead of the project that explains how to get there. Regarding our goal, the prospective allows the scenarios to anticipate contemporary configurations of the rural over the medium-and longterm base.

Regarding our main question, the prospective approach will anticipate, based on hypothetical scenarios and interaction between people and territory, leading to specific modes of organization of places. Three types of scenarios are explored. Each refers to a particular research domain on the future of the human space as previously identified 31

-Building an economically sustainable territory

The first type of scenario addresses the economic and social future of the project site. In the case of rural areas, the identification of new local economic life usually implies a renewal of the local economic fabric. Moreover, it requires the definition of new economic agents which are not necessarily in line with the economic history of territory. Our view is to rethink the economic status of the territory and its spatial and human potential, taking into account the demographic rebalancing across the national territory as described above, and resulting societal and cultural changes. In this area, the renewal of farming combined with other activities, is subject to special attention. In addition, we seek any form of hybrid economic activity encouraging the establishment of non-seasonal activities, providing the inhabitants with jobs throughout the year (crafts, industrial or commercial, educational and cultural activities, tertiary type professions, etc ...). Like the oldest human settlements in rural areas -but in other ways -the issue of dwelling is combined with other functions to counteract the syndrome of "dormitory town".

-Developing local channels for the construction of local buildings

The second type of alliance focuses on building construction. The objective is to explore the potential of the territory in terms of production of new materials and constructive channels, based on agriculture (straw, hemp, etc.), forestry (wood) or soil (stone, earth). The introduction of new constructive channels requires new know-how (alongside those to be revived) and the creation of new jobs through new businesses, enabling us to establish a link with the previous scenarios. The development of these scenarios will set up a new construction economy for new buildings, as well as for the rehabilitation of existing buildings, that will in turn induce local changes in building practices. Besides using locally produced materials, these new practices are based on the renewal of manual labor: a principle that also reveals what Marcel Ruchon calls "metabolic energy" : the energy produced by the strength of our bodies which accompanies a decrease in energy devoted to the industrialization and mechanization of construction processes.

-Promoting energy self-sufficient habitat.

The third type of scenario addresses at a comprehensive level the question of energy in our way of living and settlement patterns. Primarily, it is rethinking the energy dependence (heating and electrical manufacturing) to achieve a form of domestic self-sufficiency. It also extends our thinking to the large technical system in place that feeds our everyday life in all activities, and in turn to imagine, at the local level, the establishment of a network of new sources of energy production related to the potential of the area, allowing inhabitants to be, in the medium term, disconnected from the national network. In this regard, innovative solutions must be devised based on "energy mix" taking advantage of the four basic elements of the territory -earth, wind, sun, water. By eliminating our dependence on national energy systems, it is also to eliminate our dependence on fossil fuels and nuclear energy sources. At this last level, as with the previous one, the landscape changes brought about by the introduction of new constructive economic and energy sectors are outlined. Through the development of these scenarios, the goal is to prefigure the establishment of a new economy of living which induces, in the long term, a new form of spatial organization at the scale of the territory.

Reflecting a plurality of possible relationships between man and territory

At a methodological level, the simultaneous consideration of these three issues allows a multiscale approach to design, integrating both the scale of architecture design referring to the short term development of the implementation of the building, and the scale of the territory referring to its long term formation, illustrated by the multiple parameters, physical economic and human, represented by the three scenarios which participate "together" in the transformation of our living environment.

The purpose of the studio is not to formulate "one" architectural and territorial project. The purpose of the studio is not about designing an ideal type of sustainable living space at the architectural and territorial levels conforming to the idea of landscape as aesthetic ideal. The purpose of the studio is to formulate several hypotheses of spatial organization that reflect the information provided by the scenarios. And therefore to reflect a plurality of possible relationships between the territory and the settlement process of a community of people.

Accordingly, one can understand the relevance of the contribution of human geography and the concept of "inhabiting human". No scenario is written in advance. The territory, as inherited with its physical and human characteristics, should not be over-emphasized to the detriment of the imagination of man, who is able to -and has to -transform and adapt it to the present economic, technical, social and cultural context 32 . 32 In this transformation process, we find one of the key issues of the thinking of the Italian territorialist School in their use of the notion of heritage

16

  John Brinkerhoff Jackson, A la découverte du paysage vernaculaire, (titre original : Discovering the Vernacular Landscape) Actes sud / Ecole nationale supérieure du paysage, 2003 ; By the same author, another book which we refer to in our analysis is A Sense of Place, a Sense of Time, Yale University Press, New Haven etLondre, 1994.

  17

  Jean-Marc Besse et Gilles A. Tiberghien, « L'expérience du Paysage », dans John B. Jackson, A la découverte du paysage vernaculaire, (titre original : Discovering the Vernacular Landscape) Actes sud / Ecole nationale supérieure du paysage, 2003, pp. 9-34 18 Jean-Marc Besse, Le Goût du monde. Exercice de paysage, Actes sud / Ecole nationale supérieure du paysage, 2009

27

  Maurice Le Lannou, Le déménagement du territoire. Rêverie d'un géographe, Paris, Editions du Seuil, 1967 28 Quote of Le Lannou taken from Besse & Tiberghien, "L'expérience du paysage", ibid., P. 22 29

  Pourquoi des architectes dans les campagnes », dans Xavier Guillot (dir.), Espace rural et Projet spatial, vol. I, Réflexions introductives/stratégiespédagogiques, op.cit., p. 226-231.

	11	Daniéla Poli, "La dimension locale dans le projet du parc agricole de la Tosacane centrale", in Xavier
	Guillot, Espace rural & projet spatial. Volume 3. Du terrain à la recherche : objets et stratégies, Publications de
	l'Université de Saint-Etienne, 2012, pp. 42-60. 12 Hervé Gumuchian et Bernard Pecqueur, La ressource territoriale, Economica, 2007. 13 Alberto Magnaghi. Le projet local, Editions Mardaga, Liège, 2003. 14 See André Micoud, «

15

Alexis Pernet, « Les enjeux contemporains de l'accompagnement du projet spatial en milieu rural : nouveaux objets, nouveaux terrains de recherche », dans Xavier Guillot, Espace rural & projet spatial. Volume 3. Du terrain à la recherche : objets et stratégies, Publications de l'Université de

Saint-Etienne, 2012, p. 142-175.

  Penser et construire ensemble l'espace rural de demain. Acquis et recherches en cours », dans Xavier Guillot (dir), Espace rural & projet spatial. Volume3. Du terrain à la recherche : objets et stratégies, Publications de l'Université deSaint-Etienne, 2011, pp. 14-27

	30	Pierre Chapuy, quoted in « L'urbaniste est il un prospectiviste qui s'ignore ? » in Urbanisme n° 386, sept-
	oct 2012 , pp . 29-37.

31

Xavier Guillot«

Jean Renard, Les Mutations des campagnes. Paysages et structures agraires dans le monde,Paris, Armand Colin, 2005.

On this point see chapter « Le mot paysage » inJackson, Ibidem, and in Besse, ibidem, p. 34

 Ibidem,

VoirBesse, ibidem, p 42. 25 When he founded the magazine Landscape in 1952, Jackson placed himself under the patronage of Peter Deffontaines and Maurice Le Lannou, and translated La Géographie humaine(Flammarion, Paris, 1949). 26 Ibidem


Thomas Delouche

Muriel Hissler

Pierre-Antoine Bouit
email: pierre-antoine.bouit@univ-rennes1.fr

Polycyclic aromatic hydrocarbons containing heavy Group 14 elements: from synthetic challenges to optoelectronic devices

Keywords:

Polycyclic aromatic hydrocarbons (PAHs) are important organic building blocks for optoelectronic applications. Among the various molecular engineering strategies (-extension, edge structure modification, exocyclic functionalization), this review will focus on the insertion of heavy group 14 elements (Si, Ge and Sn) into the polyaromatic backbone. The main synthetic approaches toward these compounds will be discussed. Their optical and redox properties will be described with a particular attention of the specific effect of the heteroatom. Finally, there insertion into optoelectronic devices will be highlighted.

I Introduction 1

2 PAHs containing 5-membered heteroc ycles 4

3 PAHs containing 6-membered heterocycles 10

4 PAHs containing 7-membered heterocycles 16 5 Conclusion 18 Declaration of Competing Interest 18

I. Introduction

Polycyclic aromatic hydrocarbons, usually denominated through their abbreviation PAHs, are molecules constituted of sp 2 hybridized C atoms organized in ortho-and peri-fused aromatic rings. 1 When larger than one nanometer, they are often called nanographenes. [START_REF] Narita | New advances in nanographene chemistry[END_REF] These -conjugated systems proved during the last 20 years their efficiency as semi-conductors in various applications (Organic Field-Effect Transistors (OFETs), Organic Solar Cells (OSCs), organic batteries …). Different synthetic strategies have been developed to modify their molecular structure and thus their electronic properties and device performances. The first is to increase the size of the -systems from the prototypic hexaperibenzocoronene 1 (Fig. 1) to Graphene Nanoribbons (GNR). [START_REF] Narita | Synthesis of structurally well-defined and liquid-phaseprocessable graphene nanoribbons[END_REF] A second strategy is to modify the C-sp2 skeleton by introducing various ring-sizes (5, 6 or 7-membered rings such in 2, Fig. 1). [START_REF] Kawasumi | A grossly warped nanographene and the consequences of multiple odd-membered-ring defects[END_REF] In addition to the modification of -delocalization and self-assembly, this strategy allows preparing chiral PAHs (the so called chiral nanographenes). [START_REF] Qiu | Negatively Curved Nanographene with Heptagonal and [5]Helicene Units[END_REF] Finally, another strategy consists of tuning the edge configuration. [START_REF] Konishi | Synthesis and Characterization of Teranthene: A Singlet Biradical Polycyclic Aromatic Hydrocarbon Having Kekulé Structures[END_REF] While introducing fjord configuration also allows preparing chiral PAHs, the introduction of zigzag edges configurations allows stabilizing open shell diradicaloid such 3 (Fig. 1). All these strategies rely on the modification of the C-based skeleton. However, an alternative approach is to selectively replace C-atoms by heteroatoms. 7 For example, during the last decade, this approach was successfully applied to triels (B [START_REF] Zhou | Planarized Triarylboranes: Stabilization by Structural Constraint and Their Plane-to-Bowl Conversion[END_REF] ), pnictogens (N, [START_REF] Draper | Heterosuperbenzenes: A New Family of Nitrogen-Functionalized, Graphitic Molecules[END_REF] and P 10 ) or chalcogens (O 11 , S [START_REF] Feng | Triangle-Shaped Polycyclic Aromatic Hydrocarbons[END_REF] ) (compounds 4-7, Fig. 1) and even recently to heavier main group elements. [START_REF] Hupf | A Modular Approach to Phosphorescent π-Extended Heteroacenes[END_REF] Figure 1: Examples of planar(1), contorted (2), open-shell (3) and heteroatoms containing PAHs (4-7)

Despite the "vertical analogy" with carbon, there are surprisingly few examples of PAHs containing heavy Group 14 elements, which is somehow surprising considering the important role played by -conjugated siloles (Fig. 2) in optoelectronic applications such OSCs or Organic-Light-Emitting Diodes (OLEDs). 14 Hence, siloles display a general property in Group 14 metalloles: they possess a low lying lowest unoccupied molecular orbital (LUMO) due to hyperconjugation between the *-orbitals of the exocyclic ER bonds (E = Si, Ge, Sn) and a *-orbital of the dienic system. [START_REF] Hissler | Linear organic π-conjugated systems featuring the heavy Group 14 and 15 elements[END_REF] As a consequence, such metalloles display red-shifted absorptions and lowest reduction potential than the corresponding cyclopentadiene. [START_REF] Yamaguchi | Silole-containing σ-and π-conjugated compounds[END_REF] In addition, hexaphenyl-siloles became one of the flagship compounds of the Aggregation-Induced Emission (AIE) literature and their intense light emission in the condensed phase was used in various applications (from biology to material science). [START_REF] Hong | Aggregation-induced emission[END_REF] Following these observations, various -conjugated oligomers and polymers, were prepared mainly containing siloles, but also germoles and stannoles (8-9, Fig. 2). [START_REF] Parke | Marriage of heavy main group elements with πconjugated materials for optoelectronic applications[END_REF][START_REF] Ramirez Y Medina | Tuning the Optoelectronic Properties of Stannoles by the Judicious Choice of the Organic Substituents[END_REF]20,[START_REF] Amb | Dithienogermole As a Fused Electron Donor in Bulk Heterojunction Solar Cells[END_REF] However, many group-14 containing -systems are only used as precursors for the corresponding B-containing compounds, (for example the Bacenes). [START_REF] Al | Benzo-and Naphthoborepins: Blue-Emitting Boron Analogues of Higher Acenes[END_REF] On the other hand, the field of Pb-containing -systems is almost unexplored, mainly because of the weakness of Pb-C bonds. [START_REF] Saito | Synthesis, Structures, and Properties of Plumboles, Phosphorus, Sulfur Silicon[END_REF][START_REF] Kuwabara | Stannoles, and Plumboles[END_REF] In addition, organosilicon derivatives offer the possibility to prepare stable compounds displaying unusual electronic configurations (either high- [START_REF] Yamaguchi | Photophysical Properties Changes Caused by Hypercoordination of Organosilicon Compounds: From Trianthrylfluorosilane to Trianthryldifluorosilicate[END_REF] or low-coordination number [START_REF] Su | Isolable Dibenzo[a,e]disilapentalene with a Dichotomic Reactivity toward CO2[END_REF] or single bonding [START_REF] Nukazawa | An Isolable Tetrasilicon Analogue of a Planar Bicyclo[1.1.0butane with π-Type Single-Bonding Character[END_REF] ). All these peculiar properties make them valuable building blocks for -conjugated system engineering.

In this article, we will focus on PAHs incorporating Si, Ge or Sn atoms, their synthesis, the impact of the heteroatoms on the electronic properties, and their insertion in optoelectronic devices. As the definition of PAHs varies depending on the articles, we decided to limit this review to compounds featuring heterocycles that are orthoand peri-fused and featuring at least 14 C-sp 2 atoms in the -conjugated framework. This thus excludes ortho-fused PAHs such linear hetero-acenes and -helicenes, which were previously reviewed. 28 Bridged triarylsilanes 29 and heterofluorenes [START_REF] Parke | Marriage of heavy main group elements with πconjugated materials for optoelectronic applications[END_REF] will not be covered by this review either. All along this review, a particular focus will be devoted to the description of the synthetic approaches, as obtaining such derivatives often represents a synthetic challenge. This is classical with PAHs, but here PAHs chemistry has to be combined with the specificity of organo-main group elements, that usually display high reactivity due to large bond lengths and poor orbital overlap.

II. PAHs containing 5-membered heterocycles

As mentioned in the introduction, the most widely studied class of tetrel containing systems are by far the metalloles. Such heterocycle are usually prepared trough lithiation-substitution, it is thus normal that this methodology has been widely used to introduce these subunits into PAHs. However, as we will see interesting alternative approaches have also been proposed recently.

In 2009, Kawashima and coworkers developed a sila-Friedel-Crafts reaction toward dibenzosilole 11 (Scheme 1a) that they further applied to the synthesis of trisilasumanene 13. [START_REF] Furukawa | Development of a Sila-Friedel-Crafts Reaction and Its Application to the Synthesis of Dibenzosilole Derivatives[END_REF] While sumanenes and trithiasumanenes display a concave surface, 13 is planar, as evidenced by its crystallographic structure (Fig. 4). This planarization is explained by the longer distances of the C-Si bonds compared to C-C or C-S bonds. The insertion of the three siloles in the structure of 13 is accompanied by the appearance in the UV-vis absorption of a weak transition at low energy (> 365 nm) compared to its triphenylene analogue. As expected due to its rigid polyaromatic framework, 13 is fluorescent (λem = 427 nm) in diluted dichloromethane solution (c = 2.10 -5 M). Interestingly, it is also luminescent in solidstate (λem = 447 nm). 32

Scheme 1: Sila-Friedel-Crafts approach to trisilasumanene 13

At the same period, Saito and coworkers developed a stepwise lithiation of the bay region of triphenylenes that offer an alternative strategy toward trisilasumanenes (Scheme 2). [START_REF] Tanikawa | Synthesis, structures and optical properties of trisilasumanene and its related compounds[END_REF][START_REF] Saito | The Chemistry of Heterasumanenes[END_REF] The authors could isolate the intermediates and thus study the effect of successive insertion of siloles on the optical properties. Despite the expected   - * conjugation in the LUMO, typical of metalloles, an almost neglectable redshift has been observed ( max abs(14-18) = 8 nm). However, this redshift is larger in emission, as a consequence of a more important reorganization in the excited state. Such observation is not intuitive as 14 and 18 have both fully planar and rigid framework. Interestingly, the blue-shift between 13 and 18 (em= 47 nm) points out the major role played by the electron rich exocyclic alkoxy groups of 13. The same synthetic approach allowed the authors to prepare the trigermasumanene 19. [START_REF] Tanikawa | Synthesis, Structures, and Optical Properties of Heterasumanenes Containing Group 14 Elements and Their Related Compounds[END_REF] As previously observed in metallole oligomers, 18 and 19 display similar absorption/emission properties.

The authors also applied this strategy to form triphenylenodistannole 20 (Fig. 4) in low yield (9%). Tristannasumanene could not be observed, presumably because of competition between lithiation and cleavage of the Sn-C bonds. The optical properties are similar to the similar derivatives with Si (16) (291 nm < abs < 292 nm, DCM, c  10 -5 M). [START_REF] Tanikawa | Synthesis, Structures, and Optical Properties of Heterasumanenes Containing Group 14 Elements and Their Related Compounds[END_REF] Scheme 2: Direct lithiation approach toward heterasumanenes 17-20

Finally, using their lithiation method, Saito's group could also prepare in very low yield heterasumanenes 22-24 featuring different heteroatoms starting from triphenylenothiophene 21 (Scheme 3). [START_REF] Saito | Synthesis and structures of heterasumanenes having different heteroatom functionalities[END_REF] At that time, this was the first synthetic approach toward heterasumanenes featuring two or three different heteroatoms. Nevertheless, no optical properties are reported in the article.

Scheme 3: Synthesis of heterasumanenes 23-24 with 2 or 3 different heteroatoms

More recently, Xu and coworkers developed a Rh-catalysed approach to trisilasumanene and trigermasumanene through a threefold cyclodehydrogenation of Si/Ge-H and C-H bonds (Scheme 4). [START_REF] Zhou | Synthesis of Silicon and Germanium-Containing Heterosumanenes via Rhodium-Catalyzed Cyclodehydrogenation of Silicon/Germanium-Hydrogen and Carbon-Hydrogen Bonds[END_REF] One advantage of this strategy is to avoid the multiple lithiation steps described previously which led to poor yields. Similarly to its Si analogue 27, 28 adopts a planar structure (Fig. 4). They also confirmed Saito's observation on the weak impact of changing the tetrel in heterasumanene, as 27-28 display similar absorption/emission properties (Fig. 3). The electronic properties were also stdudied by density functional theory (DFT) calculations. Such computational studies are of prime importance in metalloles in order to evaluate the involvement of E in the FMOs. Here, authors show that while the highest occupied molecular orbital (HOMOs) of sumanene (E = CH2), 27 and 28 are similar, their LUMOs are delocalized over the entire hetero-polyaromatic skeleton, indicating the σ*-π* conjugations of the metaloles. Calculations also predict a lower gap for 27 compared to 28, which is not reflected in the experimental spectroscopic data. Recently, Xu group also demonstrated that iodine-doped sumanene can be used as intermediates for the preparation of heterasumanenes such 23. 38 Finally, Saito and coworkers recently took advantage of the specific ability of heavy group 14 elements to form spiro derivatives. [START_REF] Furakawa | Synthesis and properties of spiro-type heterasumanenes containing group 14 elements as bridging atoms[END_REF] The synthesis of 30-32 was achieved with low yield using their direct-lithiation strategy (Scheme 5). In the key step, they used ECl4 (E = Si, Ge, Sn) as tetrel source for the formation of the spiro derivatives.

For comparison, they also prepared in good yields the corresponding non-spiro derivatives 33-35. The two planar PAHs backbones of 30-32 were almost perpendicular (dihedral angles between 85° and 89°) as evidenced by X-ray crystallography and DFT calculations (Fig. 5).

Scheme 5: Synthesis of (spiro)-heterasumanenes 30-35

UV-vis absorption spectra (DCM, c = 10 -5 M) of the spiro-type 30-32 showed slightly longer absorption maxima (with max 400-403 nm) compared to the non-spiro compounds 33-35 (390-397 nm). The FMOs give insight into this phenomenon as the LUMOs are stabilized through *-* conjugation thus leading to extended conjugation between the PAHs in the spiro forms (Fig. 5). The non-spiro 33-35 display only one reversible oxidation in cyclic voltammetry (CV) at +0.40 V vs Fc + /Fc. The spiro 30-32 show two reversible oxidation waves, with the first potential being in the same range than the non-spiro analogue (+0.40 V vs Fc + /Fc). This can be explained by looking at the FMOs as HOMO / HOMO-1 do not show orbital contributions from the heteroatom (Fig. 5). In all cases, the potential seems independent of the heteroatom. Additional DFT calculations suggest the presence of spiro-conjugation in the LUMO between the orthogonal PAHs bridged by group 14 atoms (Fig. 5). Iyoda and coworkers described the synthesis of cyclic tetrathiophenes planarized by silyl groups 38 (Scheme 6). [START_REF] Ohmae | Cyclic Tetrathiophenes Planarized by Silicon and Sulfur Bridges Bearing Antiaromatic Cyclooctatetraene Core: Syntheses, Structures, and Properties[END_REF] This was achieved in moderate yield through the Cumediated coupling of dithienosiloles 37. Contrary to its S-analog, the presence of the Si bridges does not fully planarize the central cyclooctatetraene (COT) unit, due to longer Si-C bonds compared to the S-C bond. Hence, the structure displays some distortion (Fig. 6). However, the COT ring display a high degree of antiaromaticity according to Nucleus Independent Chemical Shift (NICS(0) = +12.7). The UV-vis absorption spectrum of 38 (abs = 483 nm) is considerably red-shifted compared to its non sililated analogue which is colourless (abs = 335 nm), as a result of a better planarization. In addition to the oxidation wave at +0.45 V vs Fc + /Fc, which is present in the non sililated analogue, 38 displays two reversible reduction waves (Ered 1 = -2.09 V vs Fc + /Fc). Likewise, the radical cation 38 •+ , radical anion 38 •-and dianion 38 2- could be characterized by electron paramagnetic resonance and UV-Vis absorption.

In diluted DCM solution, the longest absorption maxima of 38 •+ (at) and 38 •-were observed respectively in the near infrared (1178 nm) and the visible (766 nm) respectively.

Scheme 6: Synthesis of disilatetrathia [8]circulene 38 and tetrasilatetrathia [8]circulene 41 [8]circulenes are currently attracting lot of attention in the field of novel aromatics. 41 During their work on hetero [8]circulenes, [START_REF] Miyake | Hetero[8]circulenes: synthetic progress and intrinsic properties[END_REF] Miyake and coworkers reported the preparation of the first tetrasilatetrathia [8]circulene 41. [START_REF] Serizawa | Synthesis of Tetrasilatetrathia[8]circulenes by a Fourfold Intramolecular Dehydrogenative Silylation of C-H Bonds[END_REF] This silane fragment was introduced by mean of Pd-catalysed silylation of iodo-precursor 39 (Scheme 6). The [8]circulene 41 was then constructed through fourfold Rh-catalysed dehydrogenative silylation in excellent yield. As evidenced by its X-ray structure (Fig. 6), 41 is almost planar with the central eight-membered ring being a regular octagon. 41 is more asily oxidized (Eox = +0.74 V vs Fc + /Fc) than its non silylated analogue (Eox = +1.31 V vs Fc+/Fc) due to better conjugation in the planar 41. Similarly, a red shift is observed in UV-Vis absorption is observed for 41 with a longest absorption wavelength at 341 nm in DCM. However, such absorption are blue shifted compared to fully carbonated [8]circulene despite their -curvature. 41 Of particular interest, an efficient intersystem crossing takes place at low temperature with the appearance of phosphorescence at low energy (fluo = 397 nm; phos = 548 nm). The same team later reported the Gecontaining analogue of 41 using a similar synthetic approach. [START_REF] Akahori | Synthesis and Photodynamics of Tetragermatetrathia[8]circulene[END_REF] In this case, the HOMO-LUMO gap appeared larger than the Si-congener, but the optical properties are not strongly affected, even if the authors could observe differences in the excited state dynamics.

S

Si formation on tetrachloro-perylene diimide. The planarity of the perylene diimide core is not affected by the presence of the Si-rings. Of particular interest, -stacking is observed in the structure of both compounds (average distance: 3.43 -3.44 Å). As expected for perylene diimide derivatives 43-44 display intense absorption in the visible region (abs = 540-550 nm) accompanied by fluorescence with excellent quantum yields (> 88%) and very small Stokes shift ( 410 cm -1 ). The typical PDI-like two reversible reductions are observed without significant modulation compared to the non-silated analog (Ered -0.90 V vs Fc+/Fc). To take advantage of the good electron affinity and favorable solid-state arrangement, crystalline sample of 43 and 44 were inserted in OFET showing efficient electron transport with mobility up to 0.30 cm 2 .V -1 .s -1 with on/off ratios of 1.3 10 5 . Such excellent electron mobilities nicely illustrate that Si-containing PAHs can act as efficient n-type semi-conductors in optoelectronic devices.

Scheme 7: Synthesis of silaperylenes diimides 43-44

This section describes the synthesis of PAHs fused with group 14 metalloles. While the early compounds were prepared according to lithiation-substitution, the development of catalytic metalloles synthesis (M = Pd, Rh) allowed diversifying the structures to obtain heteroatom-containing sumanenes, perylenes or [8]circulenes. All compounds feature the classical metalloles - interactions or spiro conjugation which affect their optical and redox properties. However, the effect of changing the tetrel (Si to Ge to Sn) has only a limited impact on these properties, which is rather surprising given, for example, the tremendous impact of P-environment on phosphole-based PAHs. 10a

II. PAHs containing 6-membered heterocycles

Contrary to metalloles, there are limited example of -conjugated 6-membered rings featuring heavy group 14 elements in the literature, mainly due to synthetic reasons. The presence of PAHs featuring those elements are thus even more limited. However, as we will see in this part, the development of catalytic methods recently allowed this field to rise quickly.

In 2000, Tamao and coworkers described the synthesis of silaphenalene 46 from pentacoordinate fluorodisilanes 45 through an ammonium sila-ylide intermediate (Scheme 8). [START_REF] Saeki | Thermolysis of bis(pentacoordinate) silicon compound bearing two 1-fluorodisilanyl units at 4-and 8-positions on 1,5bis(dimethylamino)naphthalene ring: construction of disilapyrene skeleton and cyclophaneshaped siloxane dimer[END_REF] The authors postulate that the reaction proceeds through a complex mechanism involving silylene trapping with two acetylenes together with an amino migration to the Si atom. Later, the same team adapted this strategy for the preparation of disilapyrene 49 (Scheme 8). [START_REF] Saeki | Thermolysis of bis(pentacoordinate) silicon compound bearing two 1-fluorodisilanyl units at 4-and 8-positions on 1,5bis(dimethylamino)naphthalene ring: construction of disilapyrene skeleton and cyclophaneshaped siloxane dimer[END_REF] Interestingly, the authors could prepare the cage-like cyclophane dimer 50 by hydrolysis of intermediate 48 followed by dehydro-condensation. Both monomer and dimer display a planar polyaromatic framework. The photophysical properties of both "monomeric" 49 and dimer 50 were characterized. While 49 display a pyrene like UV-vis absorption at abs = 394 nm, [START_REF] Crawford | Experimental and Theoretical Studies of the Photophysical Properties of 2-and 2,7-Functionalized Pyrene Derivatives[END_REF] dimer 50 features an additional red-shifted transition of low intensity (abs = 412 nm). Dimer emission (abs = 488 nm) is also red-shifted with a 10 fold increase in quantum yield compared to the monomer ((50)= 8%) and display the typical broad shape associated with pyrene excimer.

Scheme 8: Synthesis of silaphenalene 46, disilaphenalene monomer 49 and dimer 50.

In 2015, Fukuzawa and coworkers developed a Ru-catalysed approach toward silaphenalenes (Scheme 9). [START_REF] Tokoro | Synthesis of Silaphenalenes by Ruthenium-Catalyzed Annulation between 1-Naphthylsilanes and Internal Alkynes through C-H Bond Cleavage[END_REF] This reaction consists of an annulation of 1naphthylsilanes with alkynes catalysed by RuH2(CO)(P(p-FC6H4)3)3, proceeding through the cleavage of a C-H bond. It is tolerant to various naphthyl platform, silanes (52) and alkyne 53 (electron rich or electron poor). Such strategy is efficient and tolerant to many functional groups. Furthermore, it remains efficient with commercially available RuH2(CO)(PPh3)3 as catalyst. One important factor is that such heterocycle synthesis does not involve n Buli in the last step contrary to many metalloles synthesis and is thus compatible with the introduction of many chemical functions. This methodology was used by Oyama's team to prepare with good yields -extended version of these silaphenalenes (54-56, Scheme 9) using the commercial catalyst. 50 X-ray crystallography confirmed that these compounds are fully planar.

The UV-vis absorption and emission of the compounds were recorded and compared to their silane precursors. As expected, the annulation lead to a red-shift both in absorption and emission. All compounds are fluorescent both in diluted solution and in solid-state with quantum yields between 10% and 40%.

Scheme 9: Synthesis of sila-and disilaphenalenes 52-59

Finally, we also used this strategy to prepare a novel family of PAHs featuring two heteroatoms in the scaffold with various -extension and Si-substituents (57-59, Scheme 9). [START_REF] Delouche | Si-containing Polycyclic Aromatic Hydrocarbons: Synthesis and Opto-electronic Properties[END_REF] All those systems remain planar according to X-ray crystallography and DFT. Contrary to what was observed on simpler silaphenalenes such 53, the double annulation strategy did not allow introducing functional groups on the alkyne. Importantly, the absorption of disilapyrenes 57-58 (395 nm <abs < 404 nm) is strongly redshifted from pyrene (abs = 338 nm). Accordingly, even if the delocalized -* transition has limited Si contributions according to TD-DFT, the electronic distribution strongly differs from pyrene [START_REF] Crawford | Experimental and Theoretical Studies of the Photophysical Properties of 2-and 2,7-Functionalized Pyrene Derivatives[END_REF] and the redshift is reproduced (Fig. 7). In our comprehensive structure-property relationships study, we observed that the size of the -system has a moderate effect on the absorption (abs(57a-59)= 9 nm). The luminescence in solution (DCM, c = 10 -5 M) can however be finely tuned taking in consideration both the size of the -system and the exocyclic substituents as the emission (abs(57b-59)= 66 nm. The redox properties are also tunable as the use of more -extended 59 (Eox= +0.83 V vs Fc+/Fc) or richer exocyclic group at the Si (Me /57a(Eox= +0.88 V vs Fc + /Fc) vs Ph / 57b (Eox= +0.95 V vs Fc+/Fc allows reducing the oxidation potential. Taking advantage of the solid-state luminescence of these derivatives, associated with suitable redox properties and thermal stability (Td10 = 411°C), we prepared multilayered fluorescent OLEDs using 57b as dopant in two commercial host matrices, either through vacuum deposition or solution process. The devices display modest performances (maximal External Quantum Efficiency (EQE) = 1.5 %). [START_REF]A Brief History of OLEDs-Emitter Development and Industry Milestones[END_REF] However, such performances may be ameliorated upon device engineering or by screening other host matrices, which paves the way toward the preparation of electroluminescent devices with enhanced properties. . 53 The reaction is tolerant to various substituents on the silane or on the alkyne involved in the annulation (electron rich/ electron poor). The optical properties were briefly examined and moderate blue fluorescence both in solution and in solid-state was reported (10%<<20%). Investigation of an asymmetric version with a chiral phosphine as ligand illustrates that this reaction allows preparing compounds with stereogenic Si with promising enantioselectivity (ee = 80% for 62, Scheme 10).

Scheme 10: Rh-catalysed access to benzonaphthosilines 61-62

Shintani's team also developed a Pd-catalysed method to prepare benzophenanthrosilines 64 in good to excellent yields (> 72%, Scheme 11). [START_REF] Tsuda | Palladium-Catalyzed Synthesis of Benzophenanthrosilines by C-H/C-H Coupling through 1,4-Palladium Migration/Alkene Stereoisomerization[END_REF] The proposed mechanism involves a C-H/C-H coupling through Pd migration and alkene stereoisomerization. The scope of the reaction is rather broad as it is compatible with different silanes (64a-d), different -platforms (65) and with the introduction of electron-rich/electron poor substituents (66). It also allows the preparation of PAHs featuring two Si-cycles (67). The X-ray structure reveals that the -platform is slightly distorted from planarity. Examination of the UV-vis/emission spectra illustrates that the optical properties of these benzophenanthrosilines can be effectively tuned in the visible range by modifying the substituents. Without surprise, insertion of electrondonating D group at the 10-position (66a) or electron-accepting A group at 3-position (66b) or both (66c) led to red-shifted absorption and emission.

Si Si

Scheme 11: Synthesis of benzophenanthrosilines 64-67

In 2015, Wagner's group prepared boron-containing PAH 6 (Fig. 1 and Scheme 12). [START_REF] Zhou | Planarized Triarylboranes: Stabilization by Structural Constraint and Their Plane-to-Bowl Conversion[END_REF][START_REF] Hertz | Steric Shielding vs Structural Constraint in a Boron-Containing Polycyclic Aromatic Hydrocarbon[END_REF] A key compound in this procedure was the disilabisanthene 70 which was prepared through Peterson olefination followed by photocyclization. Contrary to 6 which is nearly planar, 70 displays a negative -curvature (saddle-shape, see Fig. 8) as illustrated by its X-ray structure. Again, the average C-Si distance (dC-Si = 1.85 Å, while the average dC-B = 1.55 Å) imposes some steric congestion. 70 absorbs and emit in the blue with an excellent quantum yield (em = 408 nm,  = 70%). It also displays reversible oxidation (while the bora-analogues also display reversible reductions, as expected for tricoordinated boron-based -systems).

Scheme 12: Synthesis of sila-and disilabisanthene 70 and 72?

In their study of multiple heteroatom doped bisanthenes, the same team prepared compound 72, featuring Si and O atoms in the scaffold (Scheme 12). [START_REF] Hertz | En Route to Stimuli-Responsive Boron-, Nitrogen-, and Sulfur-Doped Polycyclic Aromatic Hydrocarbons[END_REF] This time, the -platform is nearly planar (Fig. 8). Indeed, the steric congestion is released by the presence of short C-O bonds (average distances: d = 1.37 Å) in the polyaromatic skeleton. The fluorescence in solution is red-shifted compared to 70 due to the presence of electron rich O atom. Mitake et al described a sequence of hexadehydro-Diels-Alder followed by tetradehydro-Diels-Alder reactions of silicon-tethered tetraynes 73 to give PAHs featuring a 5-membered Si-cycle and a 6-membered Si-cycle 74 in moderate to excellent yield (81% for 74a, Scheme 13). [START_REF] Mitake | Consecutive HDDA and TDDA reactions of silicon-tethered tetraynes for the synthesis of dibenzosilole-fused polycyclic compounds and their unique reactivity[END_REF] The reaction scope is rather large as electron-rich and electron poor aryls could be used. The structure of 74a reveals an helical -framework imposed by the [5]helicene fragment (Fig. 9). Interestingly, a [2 + 2 + 2 + 2] cycloaddition by-product 75a could be characterized and an optimized yield of 49% was obtained. This was the first example of a thermal [2 + 2 + 2 + 2] cycloaddition of alkynes for the preparation of -conjugated eight-membered rings. The X-ray structure reveals a complex saddle-shaped -framework (Fig. 9). All helicenoïd derivatives 74a-c are luminescent in solution and in solid state. Substitution does not affect their absorption/emission properties.

III. PAHs containing 7-membered heterocycles

The development fully-carbonated -systems featuring 7-membered rings is currently in fast development. [START_REF] Chaolumen | Construction of Heptagon-Containing Molecular Nanocarbons[END_REF] In contrast, the development of heteroatom containing 7-membered unsaturated heterocycles was limited due to fast rearomatization into aryl derivative. 59 However, the presence of substituents on the heteropine recently allowed stabilizing them and could lead to their insertion into functional materials. [START_REF] Wang | Recent Advances in Luminescent Annulated Borepins, Silepins, and Phosphepins[END_REF] Regarding polyaromatic silepines, Shintani's research group recently reported that tuning the Pd catalyst (using of bisphosphine ligands possessing a biaryl scaffold) led to the preparation of benzofluorenosilepine 76 instead of benzophenanthrosilines 64 (Scheme 14). [START_REF] Tsuda | Palladium-Catalyzed Synthesis of Dibenzosilepin Derivatives via 1,n-Palladium Migration Coupled with anti-Carbopalladation of Alkyne[END_REF] Under optimal conditions (with BINAP (2,2′-Bis(diphenylphosphino)-1,1′-binaphthalene) as ligand), 76a could be obtained in 67% yields. The proposed mechanism involves a 1,5-palladium migration followed by anti-carbopalladation of alkyne. As observed with related heteropines, the 7membered cycle of 76a displays boat shape structure. Various silanes, aromatic platforms (77-78) or functional groups (for example electron rich or electron poor such 79-80) could be introduced. Importantly, preliminary tests with enantiopure (R)-BINAP leads to promising enantioselectivity (60% ee). Compared to 64 (abs = 336 nm/(em = 371 nm), 76a displays a red-shifted absorption (abs = 376 nm) and emission (em = 523 nm) in DCM (c = 5. 10 -5 M) (Fig. 10). This red-shift was attributed to a lower LUMO level, as evidenced by cyclic voltammetry (Ered(76a) = -2.05 V vs Fc + /Fc, reversible wave, which is more positive by 0.20 V compared to the irreversible Si Si b) a) reduction of 64) and corroborated by DFT computations (Fig. X). In addition, while the HOMOs have similar shapes and energy, the electronic distribution strongly differs between 64 and 76a. Furthermore, the modification of the -platform allows fine-tuning the optical/redox properties of the silepines as observed in Fig. 10 with the red-shift between 76a and 76b due to -extension. During our research on naphthyl-fused heteropines, [START_REF] Delouche | Naphthyl-fused phosphepines: Luminescent contorted polycyclic Pheterocycles[END_REF] we prepared a dimer of naphthyl-fused germepine 82 through a Yamamoto coupling as the fusing step. [START_REF] Delouche | Mixing Polyaromatic Scaffolds and Main Group Elements: Synthesis, Coordination and Optical Properties of Naphthyl-Fused Heteropines[END_REF] This compound displays absorption / emission in the blue range. Compared to its B and P-analogues, 82 displays a blue shifted absorption and higher Stokes shift. Such behaviour comes from the longer Ge-C bonds that make the polyaromatic scaffold less planar and rigid.

Scheme 15: Synthesis of naphthyl-fused germepine 82.

While the field of PAHs featuring 7-membered rings is clearly rising, there are only limited example of such derivatives with Si or Ge. However, the prepared compounds display interesting luminescent properties which should trigger the development of novel synthetic approaches to fully explore the potential of these novel -systems.

IV. Conclusions

This review aims to shed light on the synthetic strategies to access PAHs featuring heavy group 14 elements (Si, Ge and Sn) and their optical and redox properties. As presented, a great diversity of structure has already been prepared with 5, 6 or 7membered heterocycles and organotetrels including Si, Ge and Sn. While the first PAHs were prepared using to lithiation-substitution, innovative synthetic approaches have been developed including Sila-Friedel-Crafts, metal catalysed approaches (Rh, Pd) or hexadehydro-Diels-Alder. This allowed to greatly diversify this family of derivatives. Even if changing the nature of the group heavy 14 element does not strongly impact its optical/redox properties, depending on the structure, the optical (absorption/emission) properties of the compounds could be tuned all over the visible spectral region, and their redox potential could be finely tuned. Some particular properties of the tetrels such as longer bond lengths than their C-counterparts or their ability to form spiro-derivatives could be used to tune both structures and properties. Finally, various example of insertion into opto-electronic devices such as OFETs or OLEDs nicely illustrate that these derivatives will soon play a role in the field optoelectronic devices. However, following the fast development of both PAHs and main-group chemistry, many challenges remain to be addressed by combining these two domains such as (i) the preparation of larger nanoribbons based on these structures [START_REF] Marangoni | Heterostructures through Divergent Edge Reconstruction in Nitrogen-Doped Segmented Graphene Nanoribbons[END_REF] (ii) the incorporation of group-14 elements with unconventional valence (such as hyperconjugation or Si=C bonds) into PAHs, [START_REF] Yamaguchi | Photophysical Properties Changes Caused by Hypercoordination of Organosilicon Compounds: From Trianthrylfluorosilane to Trianthryldifluorosilicate[END_REF][START_REF] Su | Isolable Dibenzo[a,e]disilapentalene with a Dichotomic Reactivity toward CO2[END_REF] (iii) the preparation of roomtemperature phosphorescent compounds taking advantage of the presence of heavy main group elements [START_REF] Parke | Aggregation induced phosphorescence in the main group[END_REF] (iv) the study and optimization of the chiroptical properties of chiral compounds (the so-called chiral nanographenes or PAH featuring stereogenic tetrels), (v) stabilization of mono-and polyradical structures 66 (vi) the preparation of optimized opto-electronic devices.

II. Conflicts of interests

No conflict of interests.

Figure 2 :

 2 Figure 2: a) Partial MO description of group 14 metalloles (E = Si, Ge, Sn); b)example of -conjugated siloles, germoles and stannoles[START_REF] Pyykkö | Molecular Single-Bond Covalent Radii for Elements 1-118[END_REF]

Scheme 4 :Figure 3 :

 43 Scheme 4: Rh-catalysed approach to heterasumanenes 27-28

Figure 4 :

 4 Figure 4: X-ray structures of trithiasumanene (a), trisilasumanene 13 (b) (butyloxy groups omitted for clarity) and trigermasumanene 28 (c).

Figure 5 :

 5 Figure 5: DFT calculated (B3LYP/6-31G(d)) frontier molecular orbitals of 30. Reprinted with permission from ref 39.

Figure 6 :

 6 Figure 6: X-ray structures of 38 (a) and 41 (b)

Figure 7 :

 7 Figure 7: Electron density difference computed for 57a (Blue / red regions respectively the decrease / increase of electron density upon absorption). Reprinted with permission from ref 51.

Figure 8 :

 8 Figure 8: X-ray structures of 6 (a), 70 (b) and 72 (c)

Scheme 13 :Figure 9 :

 139 Scheme 13: Dehydro-Diels-Alder approach to 74a-c and 75a

Scheme 14 :

 14 Scheme 14: Synthetic access to benzofluorenosilepines 76-80

Acknowledgements 18

References 19 Acknowledgements

This work is supported by the Ministère de la Recherche et de l'EnseignementSupérieur, the CNRS, the Région Bretagne, the French National Research Agency (ANR Heterographene ANR-16-CE05-0003-01).

IV.

III.