Marc-Olivier Killijian
email: mkilliji@laas.fr

Matthieu Roy

Gaétan Séverac

Christophe Zanon

Data Backup for Mobile Nodes : a Cooperative Middleware and Experimentation Platform

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Problem Statement

Finding the proper abstractions to design middleware for the provision of dependable distributed applications on mobile devices is still a big challenge [START_REF] Roy | Geo-registers: An abstraction for spatial-based distributed computing[END_REF]. The number of mobile communicating devices one can meet in every-day life is dramatically increasing: mobile phones, PDAs, handheld GPS, laptops and notebooks, portable music and video players. Those devices benefit from an amazing number of sensors and communication interfaces. The interconnection of these systems does not only result in a huge distributed system. New technical and scientific challenges emerge due to the mobility of users and of their devices, or due to the massive scale of uncontrolled devices that constantly connect and disconnect, fail, etc. To handle those systems' dynamics, cooperation-based approaches à la peer-to-peer seem attractive. An important question is thus to know if and how can we design a sound middleware that offers useful building blocks for this type of system. Another crucial question is to study how we can correctly evaluate those highly mobile and dynamic systems. This paper answers these two questions: we present a middleware architecture dedicated to the provision of cooperative data backup on mobile nodes and a platform for its experimental evaluation. This architecture is exemplified by implementing a Distributed Black-Box (DBB) application which provides a virtual device, whose semantics is similar to avionics blackboxes, that tracks cars' history in a way that can be replayed in the event of a car accident. This application ensures information is securely stored using replication mechanisms, by means of exchanging positions between cars. Our implementation is based on three original services: a Proximity Map, a Trust and Cooperation Oracle, and a Cooperative Data Backup.

This DBB application is a good illustration of the use of the various middleware services and applications that users can benefit thanks to mobile communicating devices, such as in the automobile context with carto-car communication. As a "classical" black-box, its aim is to record critical data, such as: engine / vehicle speed, brake status, throttle position, and even the state of the driver's seat belt switch. As a "smart" blackbox, it can also be used for extending the recorded information with contextual information concerning the neighboring vehicles, possibly the various vehicles involved in an accident. Indeed, information stored by the application leverages vehicle-based parameters and communication-induced information.

The proposed architecture is based on four main middleware building blocks, namely a Networking service, a Proximity Map, a Trust and Cooperation Oracle, and a Cooperative Data Backup service. This architecture and the DBB application will be described in Section 2. This distributed architecture being targeted to mobile nodes (e.g. automobiles), it has been implemented and evaluated on top of a mobile robot platform described in section 3. Finally, we give some further trails of research in Section 4. The overall architecture used in this work is depicted on Fig. 1. The mobile platform, the hardware and other experimental settings will be described later in Section 3. This architecture is a partial implementation of the Hidenets architecture and has been detailed in the projects deliverables, see e.g. [START_REF]Hidenets. revised reference model[END_REF] or [START_REF]Hidenets resilient architecture (final version)[END_REF]. Apart from standard hardware-related services (networking, localization...), we propose three new building blocks, targeted for mobile systems, that are described in the following subsections. The rationale of these building blocks is as follows:

• Proximity map. Before being able to backup data, a mobile node has first to discover its neighbors and the resources and services they offer. The proximity map represents the local knowledge a node has about its vicinity.

• Trust and cooperation oracle. In order to interact with a priori unknown neighbors for critical services (e.g., collaborative backup), a node has to evaluate the level of trust it can assign to each of its neighbors. The purpose of the trust and cooperation oracle is to evaluate this level of trust and to incite nodes to cooperate with one another.

• Cooperative data backup. The provision of a cooperative backup service at the middleware level is the major contribution of the architecture described in this paper. This service acts as a peer-to-peer storage resource sharing service for backup and restoration of critical data.

Communication and network layer

Since Java provides no specific support for ad hoc networking, we implemented a specific package for handling multiple WiFi interfaces. This package supports both UDP broadcasting and TCP unicasting. It handles indexing, choping and unchoping of arbitrary size messages and deals with typed messages. As we are only interested in local interactions within an entity's neighborhood, our network layer implements one-hop interactions only, and does not address the problem of routing in an ad-hoc network.

Localization and Proximity Map

For many applications for mobile nodes, and especially for cooperation-based applications, a node needs to interact with its neighbors. Furthermore, the quality of service that may be provided by a given component can vary according to the vicinity, e.g. the quantity of neighbors, their density, etc. It is then necessary to formalize this view of the vicinity into a more abstract representation. To that means, we propose the Proximity Map building block, that provides an abstraction of the physically attainable network of entities. The aim of this building block is to provide applications with information aggregated from localization and networking layers.

Indeed, the goal of the proximity map is to gather physical information about nodes in the vicinity. When using its proximity map, a given node has a view of the nodes in its vicinity (defined as being the nodes which are reachable within H hops), their location information, and the freshness of the pieces of information.

This problem has similarities with neighbor discovery protocols for ad-hoc routing algorithms, that can be divided into pro active schemes and reactive schemes. In a reactive scheme, information about routing is constructed on demand, i.e., as soon as a message has to be sent to a previously unknown destination. In a pro active scheme, the entity periodically sends messages on the network to look for new neighbors, and to check the availability and reachability of already discovered requested by the application/caller. Since we are only interested in local interactions, and due to the fact that the set of entities is large and unknown to participants, we designed the proximity map as a pro active service.

Intuitively each node periodically beacons its proximity map to its 1-hop neighbors, and collects similar information from its direct neighbors. When merging these pieces of information, it is able to update its proximity map with new nodes that appeared as neighbors of neighbors, nodes which have moved, nodes whose connectivity changed, etc. The preliminary ideas about the proximity map can be found in [START_REF] Killijian | Towards group communication for mobile participants[END_REF].

To implement the proximity map, we use locationstamped beacons. Each node keeps a map of its knowledge of the location and connectivity of other nodes, which is represented as a graph.This graph is regularly updated when the node receives a beacon and is also regularly sent to the node's neighbors in its beacons.

Trust and Cooperation Oracle

The trust and cooperation oracle (TCO) is our second basic building block for cooperative services. A cooperative service emerges from the cooperation of entities that are generally unknown to one another. Therefore, these entities have no a priori trust relationship and may thus be reluctant to cooperate. In cooperative systems without cooperation incentives, entities tend to behave in a rational way in order to maximize their own benefit from the system. The goal of the trust and cooperation oracle is therefore to evaluate locally the level of trust of neighboring entities and to manage cooperation incentives [START_REF] Courtès | Security rationale for a cooperative backup service for mobile devices[END_REF].

Synergy is the desired positive effect of cooperation, i.e., that the accrued benefits are greater than the sum of the benefits that could be achieved without cooperation. However synergy can only be achieved if nodes do cooperate rather than pursuing some individual short-term strategy, i.e. being rational. Therefore, cooperative systems need to have cooperation incentives and rationality disincentives. There are several approaches to this, some are based on microeconomy and some others are based on trust. Typically, for micro-economic approaches, a node has to spend "money" for using a service and earns "money" for servicing other nodes. Regarding trust, a common approach is to use the notion of reputation, a level representing the level of trust that may be placed on a node, which can be computed locally by a single node, or collectively and transitively by a set of nodes. Another approach based on the notion of trust relies on the use of trusted hardware, e.g. a smart-card. Whatever the most appropriate approach in a given context, the TCO leverages this information by providing a single interface with simple semantics. Given a node identifier n, it returns the probability that this node n cooperates correctly for the next interaction: float trustLevel(NodeID n)

In an automotive context, we consider that there are a limited number of different middleware providers. We can also state that it is at least unusual and potentially dangerous for vehicle owners to modify the software their vehicle is running, and that software updates are relatively rare. As a result, there are only a few different legacy middleware versions. We can thus consider that the middleware is certified, i.e., a trusted authority within the infrastructure domain can generate and distribute certificates. These certificates can be verified in the ad-hoc domain by a trusted hardware, in our platform a smart-card.

Cooperative Data Backup service

The cooperative backup service aims to improve the dependability of data stored by participating nodes by providing them with mechanisms to tolerate hardware or software faults, including permanent faults such as loss, theft, or physical damage. To tolerate permanent faults, the service must provide mechanisms to store the users' data on alternate storage nodes using the available communication means. The problem of cooperative backup of critical data can be divided in three steps: i) discovering storage resources in the vicinity (this step is performed using the proximity map service), ii) negotiating a contract with the neighboring nodes for the use of their resources (this step uses the trust and cooperation oracle), and iii) handling a set of data chunks to backup and assigning these chunks to the negotiated resources according to a data encoding scheme and with respect to desired properties of dependability, privacy, availability and confidentiality. The service is also in charge of the recovery phase, i.e., the data restoration algorithm.

The Cooperative Data Backup service provision is designed using the following principles.

• A client of the service provides a data stream to be backed up to the backup operation with a unique identifier for the stream.

• The stream passes through a series of chopping and indexing operations in order to produce a set of small (meta-) data chunks to be backed up (more details can be found in [START_REF] Courtes | Storage tradeoffs in a collaborative backup service for mobile devices[END_REF]).

• A backup thread runs periodically. It processes the block buffer, queries the Proximity Map service and the Trust and Cooperation Oracle in order to produce a potential contributors list. Then it places data blocks on contributors according to given placement and replication strategies, as described in [START_REF] Courtes | Storage tradeoffs in a collaborative backup service for mobile devices[END_REF] and [START_REF] Courtes | Dependability evaluation of cooperative backup strategies for mobile devices[END_REF]. When the client wants to restore data, it can either submit the unique identifier of the stream to the asyn-chronous restore operation and then poll it periodically, or it can directly call the synchronous restore operation that will return when the data has been successfully restored. To that means, a periodic thread handles the restoration waiting queue: it looks for given IDs, unpacks the received blocks and potentially adds new identifiers to the waiting queue according to the decoding operation on received data chunks (i.e. data or meta-data).

The Distributed Black-Box application

Using the above described services, we implemented a Distributed Black-Box application. This application backs up a stream of data for every car that consists of a periodic sampling of a car's proximity map. In our implementation, the cooperative backup service replicates these streams among neighboring cars, or to an infrastructure when connectivity permits it. The stream of any participant (be it crashed or not) can then be restored either from neighboring devices (cars in ad-hoc mode), or from the infrastructure.

The ARUM experimental platform

To the best of our knowledge, little research has been done on the evaluation of resilience in ubiquitous systems. Most of the literature in this domain concerns evaluation of users experience and human-computer interfaces. However, some work is also looking at defining appropriate metrics for the evaluation of distributed applications running on ubiquitous systems [START_REF] Basu | Metrics for performance evaluation of distributed application execution in ubiquitous computing environments[END_REF], [START_REF] Castro | Evaluating distibuted query processing systems for ubiquitous computing[END_REF]. [START_REF] Burnett | A hybrid evaluation approach for ubiquitous computing environments[END_REF] is looking at a general approach to evaluate ubiquitous systems. In the paper, the authors argue that quantitative measurements should be complemented with qualitative evaluation. The argument is that there is a number of problems for which evaluation cannot be easily quantified. Thus an evaluation should be conducted using an hybrid quantitative/qualitative strategy.

It is clear that the area of resilient computing has proposed a number of contributions concerning the evaluation of distributed systems and this paper will not survey this domain. Analytical evaluation is probably the most popular technique, such as within Assert [START_REF] Arlat | Dependability needs and preliminary solutions concerning evaluation, testing and wrapping[END_REF] in the avionics application domain. More recently, experimental evaluation started to gain attention. The approach taken is often based on dependability benchmarking, for example DBench [START_REF] Kanoun | Dbench -dependability benchmarking[END_REF] addresses dependability benchmarking of operating systems.

In the ubiquitous and mobile computing area, evaluation of resilient mechanisms remains an open problem. In most cases, the proposed algorithms are evalu-ated and validated using network simulators [START_REF] Das | Simulation-based performance evaluation of routing protocols for mobile ad hoc networks[END_REF], [START_REF] Hamida | On the complexity of an accurate and precise performance evaluation of wireless networks using simulations[END_REF]. Since simulators use a model of physical components, such as wireless network cards and location systems, this raises concerns on the coverage of the assumptions that underlie the simulation [START_REF] Cavin | On the accuracy of manet simulators[END_REF]. Little work concerning the evaluation of algorithms in a realistic environment is available. Scale ability. This calls for the development of a realistic platform, at a laboratory scale, to evaluate and validate fault-tolerance algorithms (e.g., group membership and replication protocols, storage mechanisms, etc.) targeting systems comprising a large number of communicating mobile devices equipped with various sensors and actuators. The goal is to have an experimentation platform allowing for reproducible experiments (including mobility aspects) that will complement validation through simulation. As we will see, an important issue within this platform is related to changes of scale so as to emulate as many various systems as possible.

We are developing an experimental evaluation platform composed of both fixed and mobile devices [START_REF] Killijian | Experimental evaluation of resilience for ubiquitous mobile systems[END_REF]. Technically speaking, each mobile device is composed of some programmable mobile hardware able to carry the device itself, a lightweight processing unit equipped with one or several wireless network interfaces and a positioning device. The fixed counterpart of the platform contains the corresponding fixed infrastructure: an indoor positioning system, wireless communication support, as well as some fixed servers. Our platform is set up in a room of approximately 100m 2 where mobile devices can move around. By changing scale, we can emulate systems of different sizes. Hardware modeling of this type of system requires a reduction or increase of scale to be able to conduct experiments within the laboratory. To obtain a realistic environment, all services must be modified according to the same scale factor.

For example, if we consider a VANET experiment like the DBB, a typical GPS in a moving car is accurate to within 5 -20m. So, for our 100m 2 indoor environment to be a scaled down representation of (say) a 250000m 2 outdoor environment (a scale reduction factor of 50), the indoor positioning accuracy needs to be 10 -40cm. Technological aspects. To reach such a level of accuracy for indoor positioning, we used a dedicated motion capture technology that tracks objects based on real-time analysis of images captured by infra-red cameras. The Cortex system is able to localize objects at the millimeter scale, which is more than enough for the VANET setting.

Another important question is how to make the devices actually mobile. Obviously, when conducting experiments, a human operator cannot be behind each device, so mobility has to be automated. This is why we considered the use of simple small robot platforms in order to carry around the platform devices. The task of these robots is to "implement" the mobility of the nodes. The carried devices communicate with the robot through a serial port. This way they can control the mobility, i.e. the trajectory, the stops and continuations, the fault-injection, etc. The last and most important design issue for the platform concerns wireless communications. Indeed, the communication range of the participants (mobile nodes and infrastructure access-points) has to be scaled according to the experiment being conducted. For example, with a VANET experiment, a typical automobile has a wireless communication range of a few hundred meters, say 200m. With a scale reduction factor fixed at 50, the mobile devices communication range has to be limited to 4m. However, to cope with other experiments and other scale reduction factors, this communication range should ideally be variable.

satisfying solution consists in using, for this purpose, signal attenuators placed between the WiFi network interfaces and their antennas. An attenuator is an electronic device that reduces the amplitude or power of a signal without appreciably distorting its waveform. Attenuators are passive devices made from resistors. The degree of attenuation may be fixed, continuously adjustable, or incrementally adjustable. In our case, the attenuators are used to reduce the signal received by the network interface. The necessary capacity of the attenuators depends on many parameters such as the power of the WiFi interfaces and the efficiency of the antennas, but also on the speed of the robot movements, the room environment, etc. We tried to characterize the relationship between all these parameters but had to fall back to an empirical method to choose the appropriate attenuators. For example, for the Distributed Black-Box case we used 26DB attenuators. Application scenario. As can be seen on Fig 1, the middleware described in this article is running on top of Apple OS X.5.6 and Java 1.5. The hardware (Macbook with additional WiFi interface and some localization hardware) is carried by a Lynxmotion 4WD rover. The resulting platform can be seen on Fig. 2. We currently own four fully equipped robots. We were thus able to emulate the Distributed Black-Box in a setting with three cooperating cars and a police coming after an accident has taken place. During the first part of the scenario, the three cars backup the Black-Box data for each other, then one of the cars looses control and leaves the circuit track to crash in a wall. After the accident has been reported, including the ID of the crashed car and the approximated time of the accident, the police enters the scene and requests restoration of the black box data for a given period of time that surrounds the accident. Once the data is successfully restored, the police is then able to replay the film of the accident, and to identify the other involved cars if there is any. A movie of this scenario is available at http://theresumeexperience.blogspot.com/ and will be shown at DSN'2009. The software described in this paper should be released under a GPL license soon, a link will be posted on the same blog.

Conclusion and Further Work

We presented a middleware architecture for dependable ubiquitous mobile applications that provides new building blocks based on cooperative approaches. We believe that those building blocks provide useful abstractions in the context of future emerging applications in the future mobiquitous society. In this work, we were concerned with the provision of a critical data backup service, a trust and cooperation oracle, a proximity map. These services help building reliable mobile applications as we shown on a distributed black box example for automobiles.

We also described a new platform for the experimental evaluation of this type of mobile application. Indeed, the evaluation of mobile systems is often based on the use of network simulators, which are often not satisfying, especially when dependability is an important issue. This platform is able to emulate mobile systems of variable size.

We believe that the usual mobility models used for the evaluation of mobile systems are not satisfactory. A mobility model dictates how the nodes, once distributed in the space, move. A mobility model involves the nodes' location, velocity and acceleration over time. The topology and movement of the nodes are key factors in the performance of the system under study. Because the mobility of the nodes directly impacts the performance of the protocol, if the mobility model does not reflect realistically the environment under study, the result may not reflect the performance of the system in the reality. The majority of the existing mobility models for ad hoc networks do not provide realistic movement scenarios [START_REF] Musolesi | Mobility Models for Systems Evaluation. A Survey[END_REF]. We are currently working on the use of real mobility traces from various sources in order to build more realistic mobility models to use in our analytical and experimental evaluation.

We are also improving the software that controls the mobility aspects of the platform. At the moment the robots follow a tape track on the ground. The next version will enable the setup of virtual tracks and allow for the differentiation of the various robots: i.e. each robot will be able to follow its own trajectory. Once upgraded, the evaluation platform described in this paper will be able to run experiments according to the realistic mobility models mentioned above, since the platform will be able to use any "mobility pattern" as an input for performing reproducible experiments.

Figure 1 . Overall Architecture 2 .

 12 Figure 1. Overall Architecture

Figure 2 .

 2 Figure 2. The ARUM platform

This work was partially supported by the French National Center for Scientific Research (CNRS), the European Hidenets project (EU-IST-FP6-26979), and the European ReSIST network of excellence (EU-IST-FP6-26764).


Marta Rosati

Angela Acocella

Andrea Pizzi

Giorgio Turtù

Giulia Neri

Nicola Demitri

Nonappa Nonappa

Giuseppina Raffaini

Bertrand Donnio

Francesco Zerbetto

Giuseppina Nonappa

Bertrand Raffaini

Francesco Donnio

Francesca Baldelli Zerbetto

Bombelli

Gabriella Cavallo

Pierangelo Metrangolo

Janus-Type Dendrimers Based on Highly Branched Fluorinated Chains with Tunable Self-Assembly and 19 F Nuclear Magnetic Resonance Properties

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

Dendrimers are a class of highly structured three-dimensional macromolecules characterized by well-defined periodically branched structure, high monodispersity, and tunable size and shape. [START_REF] Astruc | Dendrimers Designed for Functions: From Physical, Photophysical, and Supramolecular Properties to Applications in Sensing, Catalysis, Molecular Electronics, Photonics, and Nanomedicine[END_REF][START_REF] Sowinska | Advances in the Chemistry of Dendrimers[END_REF] Particularly interesting are Janus-type dendrimers (JDs), i.e., synthetic amphiphilic dendritic macromolecules bearing two domains of different polarity, which can also have different sizes. [START_REF] Percec | Self-Assembly of Janus Dendrimers into Uniform Dendrimersomes and Other Complex Architectures[END_REF][START_REF] Caminade | Dendrimers: Syntheses and Properties[END_REF] These macromolecules self-assemble in water, giving rise to a wide variety of ordered morphologies such as micelles, tubular vesicles, cubosomes, disks, helical ribbons, and dendrimersomes. [START_REF] Wilner | Dendrimersomes Exhibit Lamellar-to-Sponge Phase Transitions[END_REF][START_REF] Zhang | Amphiphilic Janus Dendrimers Self-Assemble Into Uniform Dendrimersomes With Predictable Size[END_REF][START_REF] Sherman | Mimicking Complex Biological Membranes and Their Programmable Glycan Ligands with Dendrimersomes and Glycodendrimersomes[END_REF] A tailored balance between hydrophilic and hydrophobic portions allows the control of their self-assembly mode, which ultimately is responsible for the emergence of specific functions. [START_REF] Sun | From Structure to Function via Complex Supramolecular Dendrimer Systems[END_REF][START_REF] Tomalia | Dendritic Effects: Dependency of Dendritic Nano-Periodic Property Patterns on Critical Nanoscale Design Parameters (CNDPs)[END_REF][START_REF] Park | Functional Supramolecular Assemblies Derived from Dendritic Building Blocks[END_REF][START_REF] Frechet | Dendrimers and Other Dendritic Macromolecules: From[END_REF] A powerful tool for tuning the selfassembly behavior of JDs and increasing their colloidal stability, also in biological media, is the introduction of fluoroalkyl groups in their molecular structure. [START_REF] Tomalia | Supramolecular Chemistry: Fluorine Makes a Difference[END_REF][START_REF] Percec | Transformation of a Spherical Supramolecular Dendrimer into a Pyramidal Columnar Supramolecular Dendrimer Mediated by the Fluorophobic Effect[END_REF][START_REF] Berger | Organic Fluorine Compounds: A Great Opportunity for Enhanced Materials Properties[END_REF][START_REF] Gladysz | Physical, and Chemical Properties of Fluorous Compounds[END_REF] Formation of ordered arrangements is expected as a consequence of combined hydrophobicity and lipophobicity of fluoroalkyl moieties that induce segregation in both organic and aqueous environments (fluorophobic effect). [START_REF] Gladysz | Physical, and Chemical Properties of Fluorous Compounds[END_REF][START_REF] Barthel-Rosa | Chemistry in Fluorous Media: A User's Guide to Practical Considerations in the Application of Fluorous Catalysts and Reagents[END_REF] This phase separation has, for example, promoted generation of compartmentalized supramolecular structures with larger interfaces than their nonfluorinated analogues; [START_REF] Krafft | Fluorocarbons and Fluorinated Amphiphiles in Drug Delivery and Biomedical Research[END_REF][START_REF] Krafft | Highly Fluorinated Compounds Induce Phase Separation in, and Nanostructuration of Liquid Media. Possible Impact on, and Use in Chemical Reactivity Control[END_REF] that is, vesicles are preferred over micelles and tubules over vesicles. Moreover, fluorination can also induce pronounced conformational changes, [START_REF] Shiyanovskaya | Self-Organization of Supramolecular Helical Dendrimers into Complex Electronic Materials[END_REF] as is the case of semi-fluorinated dendrons, [START_REF] Percec | Self-Assembly of Semifluorinated Minidendrons Attached to Electron-Acceptor Groups into Pyramidal Columns[END_REF][START_REF] Rosen | Single-Electron Transfer and Single-Electron Transfer Degenerative Chain Transfer Living Radical Polymerization[END_REF][START_REF] Wilson | Disassembly via an Environmentally Friendly and Efficient Fluorous Phase Constructed with Dendritic Architectures[END_REF][START_REF] Wu | Complex Columnar Hexagonal Polymorphism in Supramolecular Assemblies of a Semifluorinated Electron-Accepting Naphthalene Bisimide[END_REF][START_REF] Ho | Screening Libraries of Semifluorinated Arylene Bisimides to Discover and Predict Thermodynamically Controlled Helical Crystallization[END_REF] which can selfassemble in the bulk state into supramolecular columns exhibiting collective liquid crystalline behavior. [START_REF] Percec | Transformation of a Spherical Supramolecular Dendrimer into a Pyramidal Columnar Supramolecular Dendrimer Mediated by the Fluorophobic Effect[END_REF][START_REF] Hernández-Ainsa | Fluorinated Liquid Crystalline Dendrimers[END_REF][START_REF] Percec | Self-Assembly of Semifluorinated Dendrons Attached to Electron-Donor Groups Mediates Their π-Stacking via a Helical Pyramidal Column[END_REF][START_REF] Dukeson | Application of Isomorphous Replacement in the Structure Determination of a Cubic Liquid Crystal Phase and Location of Counterions[END_REF] JDs bearing linear fluorinated chains at their periphery have been extensively studied by Percec et al., [START_REF] Wilner | Dendrimersomes Exhibit Lamellar-to-Sponge Phase Transitions[END_REF][START_REF] Sherman | Mimicking Complex Biological Membranes and Their Programmable Glycan Ligands with Dendrimersomes and Glycodendrimersomes[END_REF][START_REF] Xiao | Self-Sorting and Coassembly of Fluorinated, Hydrogenated, and Hybrid Janus Dendrimers into Dendrimersomes[END_REF][START_REF] Xiao | Janus Dendrimersomes Coassembled from Fluorinated, Hydrogenated, and Hybrid Janus Dendrimers as Models for Cell Fusion and Fission[END_REF][START_REF] Percec | Self-Assembly of Semifluorinated Janus-Dendritic Benzamides into Bilayered Pyramidal Columns*[END_REF][START_REF] Percec | Self-Organizable Vesicular Columns Assembled from Polymers Dendronized with Semifluorinated Janus Dendrimers Act as Reverse Thermal Actuators[END_REF][START_REF] Torre | Encapsulation of Hydrophobic Components in Dendrimersomes and Decoration of Their Surface with Proteins and Nucleic Acids[END_REF] who reported the formation of unilamellar and multilamellar dendrimersomes with bilayered structures arising from the interdigitation of the fluorinated chains. [START_REF] Sherman | Mimicking Complex Biological Membranes and Their Programmable Glycan Ligands with Dendrimersomes and Glycodendrimersomes[END_REF][START_REF] Xiao | Janus Dendrimersomes Coassembled from Fluorinated, Hydrogenated, and Hybrid Janus Dendrimers as Models for Cell Fusion and Fission[END_REF] Their bilayer thickness, comparable to that of biological membranes, together with their imaging capability via 19 F magnetic resonance imaging (MRI), renders them promising candidates for the development of theranostic systems and personalized drug therapies. [START_REF] Lv | Fluoropolymers in Biomedical Applications: State-of-the-Art and Future Perspectives[END_REF][START_REF] Jiang | Novel Fluorinated Polymers Containing Short Perfluorobutyl Side Chains and Their Super Wetting Performance on Diverse Substrates[END_REF][START_REF] Zhang | The Fluorination Effect of Fluoroamphiphiles in Cytosolic Protein Delivery[END_REF][START_REF] Kostina | Membrane-Mimetic Dendrimersomes Engulf Living Bacteria via Endocytosis[END_REF][START_REF] Celentano | Design of Fluorinated Hyperbranched Polyether Copolymers for 19F MRI Nanotheranostics[END_REF] Indeed, permeability across fluorinated membranes is lower than their non-fluorinated analogues since the former are less dynamic than the latter, and exchange of components among micelles or vesicles is considerably slower. [START_REF] Krafft | Fluorocarbons and Fluorinated Amphiphiles in Drug Delivery and Biomedical Research[END_REF][START_REF] Chowdhury | Dendronized Fluorosurfactant for Highly Stable Water-in-Fluorinated Oil Emulsions with Minimal Inter-Droplet Transfer of Small Molecules[END_REF] In this regard, JDs containing perfluoropolyether chains were also used as nanocarriers for encapsulating polar guest molecules and generate stable water-in-fluorinated oil emulsions with possible application as polar nanoreactors in non-polar reaction media such as fluorous phases or supercritical CO2. [START_REF] Chowdhury | Dendronized Fluorosurfactant for Highly Stable Water-in-Fluorinated Oil Emulsions with Minimal Inter-Droplet Transfer of Small Molecules[END_REF][START_REF] Garcia-Bernabé | Syntheses and Phase-Transfer Properties of Dendritic Nanocarriers That Contain Perfluorinated Shell Structures[END_REF][START_REF] Wagner | Perfluoroalkyl-Functionalized Hyperbranched Polyglycerol as Pore Forming Agents and Supramolecular Hosts in Polymer Microspheres[END_REF] However, to fully exploit the unique features offered by fluorinated JDs, there is a pressing need to design more sustainable fluorinated synthons. [START_REF] Krafft | Per-and Polyfluorinated Substances (PFASs): Environmental Challenges[END_REF][START_REF] Bowman | Fluorotechnology Is Critical to Modern Life: The FluoroCouncil Counterpoint to the Madrid Statement[END_REF][START_REF] Dichiarante | Natural Surfactants towards a More Sustainable Fluorine Chemistry[END_REF] In fact, perfluoroalkanes (PFAs) with chain lengths ≥6 carbon atoms are to be avoided for their bioaccumulative potential. On the other hand, PFAs with shorter chains do not lead to satisfactory results in terms of self-assembling performance. [START_REF] Schuster | Effects of Chemical Structure on the Dynamic and Static Surface Tensions of Short-Chain, Multi-Arm Nonionic Fluorosurfactants[END_REF] We have recently demonstrated that a multibranched super-fluorinated thiol bearing perfluoro-tert-butoxyl groups is a valuable substitute for long-chain PFA thiols in the construction of omniphobic self-assembled monolayers [START_REF] Dichiarante | A Short-Chain Multibranched Perfluoroalkyl Thiol for More Sustainable Hydrophobic Coatings[END_REF] and stabilization of gold nanoclusters. [START_REF] Dichiarante | Superfluorinated and NIR-Luminescent Gold Nanoclusters[END_REF] The unique design of the above-mentioned molecule allows multiplicity of equivalent fluorine atoms (27 F) for 19 F-MRI purposes, along with a stable and dense packing, due to the intrinsic tendency to crystallize of these branched fluorinated chains. [START_REF] Dichiarante | Superfluorinated and NIR-Luminescent Gold Nanoclusters[END_REF][START_REF] Tedeschi | Waterproof-Breathable Films from Multi-Branched Fluorinated Cellulose Esters[END_REF] Moreover, the presence of four ether bonds in the core may hasten molecule degradation in the environment because of possible cleavage of these bonds in physiological conditions, thus overcoming PFA bioaccumulation issues. (45) A similar fluorinated branched structure was also applied by Yu et al. in the synthesis of nonassociative and water-soluble fluorinated amphiphiles. [START_REF] Taraban | Monitoring Dendrimer Conformational Transition Using 19F and 1H2O NMR[END_REF][START_REF] Liu | Monitoring Fluorinated Dendrimer-Based Self-Assembled Drug-Delivery Systems with 19F Magnetic Resonance[END_REF][START_REF] Taraban | Conformational Transition of a Non-Associative Fluorinated Amphiphile in Aqueous Solution[END_REF][START_REF] Jiang | Symmetry-Guided Design and Fluorous Synthesis of a Stable and Rapidly Excreted Imaging Tracer For19F MRI[END_REF] Conversely, under the hypothesis that short-chain and multibranched fluorinated synthons may effectively drive JDs self-assembly in a manner similar to their linear counterparts, we targeted polyester dendrons of different generations bearing multiple perfluoro-tert-butoxyl groups. Interestingly, such macromolecules showed a tunable self-assembly behavior in aqueous media as a function of a subtle balance between fluorinated and hydrophilic domains (Figure 1). Coarse-grain simulations in combination with dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM) experiments confirmed the formation of supramolecular structures of different morphologies and allowed rationalization of the molecular factors governing their self-assembly. In particular, the highest generation JD exhibited an interesting morphological transition from multilamellar dendrimersomes to tubules driven by a delicate balance of intra-and intermolecular forces that compete at the interface. Noticeably, the morphology of the resulting assemblies strongly impacts the intensity of their 19 F NMR signal, which is switched off on passing from dendrimersomes to tubules, as a likely consequence of the reduction of CF3 local mobility. Interactions with cosolvents of different nature (i.e., fluorinated) have been demonstrated to promote conformational changes, which are ultimately reflected in the supramolecular organization. Overall, we propose here a new family of fluorinated JDs (FJDs), which show a versatile and tunable self-assembly behavior, paving the way for the development of functional supramolecular materials promising as 19 F-MRI probes for biological applications.

Results and Discussion

Design and Synthesis of Fluorinated Janus Dendrimers (FJDs)

The fluorinated JDs of generations from 1 to 3 (FJD1-3) were synthesized by using a click chemistry approach to connect hydrophilic and hydrophobic moieties. In particular, we exploited the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction, which requires mild conditions and guarantees the orthogonal linkage of two different molecular building blocks through the formation of a 1,4-disubstituted 1,2,3triazole ring with good yields. [START_REF] Medina | Dendrimers as Carriers for Delivery of Chemotherapeutic Agents[END_REF] The azide derivative (F27-N3) (Scheme S1) was designed on the basis of the previously reported multibranched fluorinated thiol. [START_REF] Dichiarante | A Short-Chain Multibranched Perfluoroalkyl Thiol for More Sustainable Hydrophobic Coatings[END_REF] The synthesis occurred through a multistep reaction, starting from pentaerythritol as reported in the Supporting Information (Scheme S1). Its structure was confirmed by ATR-FTIR, 1 H, 13 C, and 19 F NMR measurements, and elemental analysis (Figures S1-S3).

FJDs1-3 were synthesized following either a convergent or a divergent pathway (Schemes S2 and S3) according to the literature. (4) Both methodologies were highly efficient with yields higher than 70% in all cases. FJDs1-3 were fully characterized by 1 H, 13 C, and 19 F NMR, ATR-FTIR, and HRESI-MS spectroscopy. Formation of the sought amphiphilic dendrimers is evident from the presence in 1 H NMR spectra of the peak at about 8 ppm related to the proton of the triazole ring (Figure S4). Moreover, the presence of one single peak at around -71.5 ppm in the 19 F NMR spectra (Figure S5) confirms the presence of only one fluorinated compound and its purity. The successful formation of "clicked" dendrimers is also confirmed by ATR-FTIR spectra (Figures S6-S8) showing the disappearance of the bands at 2100 and 3300 cm - 1 associated respectively with the N═N═N stretching of the azide F27-N3 and the C-H stretching of the alkyne-terminated dendrimeric precursors. Furthermore, the presence of C═O stretching vibrations at 1730 cm -1 and O-H stretching vibrations between 3600 and 3100 cm -1 , typical of polyester dendrons, together with C-F stretching and bending vibrations respectively at 1300 and 650 cm -1 , associated with the fluorinated moiety, confirm the linking between hydrophobic and hydrophilic portions.

Noncovalent Interactions Involved in the Formation of the Bulk Structure

FJD1 was crystallized by slow evaporation of an acetonitrile/toluene (1:10) solution. Single crystals suitable for X-ray diffraction analysis were measured at 100 K by means of synchrotron radiation. FJD1 crystallizes in the triclinic P-1 space group, with the unit cell containing two dendrimeric molecules related by a crystallographic inversion center (Figure 2a). Specifically, two facing FJD1 molecules share the π-surface of their triazole rings, the latter being stacked in antiparallel fashion. In addition to π•••π stacking, the structure grows along the crystallographic b-axis thanks to a network of hydrogen bonds involving hydroxyl groups and triazole rings of neighboring molecules (Figure 2b). The incompatibility between fluorocarbon and hydrocarbon groups forces their segregation, producing a lamellar structure with a regular alternation of fluorinated and non-fluorinated regions (Figure 2c). Of note, the fluorocarbon layers are tightly packed, with an entangled network of weak F•••F interactions that confers further stability to this supramolecular architecture. Overall, the high fluorine content of FJD1, combined with the presence of polar groups, determines the typical self-assembly that is expected in these amphiphilic materials.

Thermal Behavior and Liquid Crystalline Properties

It is known for dendritic molecules that properties observed in bulk are often related to their self-assembling behavior in solution. [START_REF] Tomalia | Dendritic Effects: Dependency of Dendritic Nano-Periodic Property Patterns on Critical Nanoscale Design Parameters (CNDPs)[END_REF][START_REF] Peterca | Predicting the Size and Properties of Dendrimersomes from the Lamellar Structure of Their Amphiphilic Janus Dendrimers[END_REF] Therefore, small-and wide-angle Xray scattering (SWAXS), polarized optical microscopy (POM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) were performed to study the molecular organization of FJDs1-3 in bulk. TGA performed in a nitrogen atmosphere showed the good thermal stability of all FJDs, as reported in Figure S9. For FJD1 and FJD2, the onset decomposition temperature is at 206 and 248 °C, respectively, where a 5% weight loss occurs (T5%), and then degradation extends gradually up to 300 °C. FJD3 instead shows a two-step degradation at 170 °C (T5%) and 330 °C, suggesting that for the highest generation dendrimer thermal degradation starts from the outermost layer and then extends to inner layers. [START_REF] Fedeli | Self-Assembling Amphiphilic Janus Dendrimers: Mesomorphic Properties and Aggregation in Water[END_REF] DSC and SWAXS analyses confirmed the crystallinity of FJD1 in the pristine state with a sharp melting peak at 91 °C, while on cooling from isotropic liquid, it froze into a glassy state (Figure S10). Loss of crystallinity was expected on increasing the dendritic generation according to the literature. [START_REF] Núñez | Crystal Structure, Melting Behaviour and Equilibrium Melting Point of Star Polyesters with Crystallisable Poly(ε-Caprolactone) Arms[END_REF] In fact, FJD2 is only partly crystalline in the pristine state. It melts at 72 °C to a quite fluid and non-birefringent mesophase with a low-energy (0.29 kJ/mol) and reversible transition around 104 °C to an isotropic liquid phase (Figure S11). The presence of only one single peak at low angles in SWAXS pattern precluded phase identification. However, the total absence of birefringence at POM suggested the formation of a cubic mesophase, [START_REF] Zeng | Liquid Crystals A Self-Assembled Bicontinuous Cubic Phase with a Single-Diamond Network[END_REF][START_REF] Poppe | A Skeletal Double Gyroid Formed by Single Coaxial Bundles of Catechol Based Bolapolyphiles[END_REF] which is frozen in a glassy solid state on cooling to room temperature, as suggested from SWAXS (Figure S11a). This liquid crystalline behavior can be explained considering the high tendency of fluorinated moieties to microsegregate. [START_REF] Hernández-Ainsa | Fluorinated Liquid Crystalline Dendrimers[END_REF] Likely, the hydroxyl terminating segments tend to form compact double layers to compensate for the space requirement discrepancy with fluorinated segments, thus resulting ultimately in the fusion of strata into cubic network. Interestingly, SWAXS spectra show one halo at ca. 16° (i.e., 5.5 Å) that could be associated with molten CF3 groups, while the second more extended halo of lesser intensity (ca. 10°) probably is related to average molecular distances (Figure S11a). FJD3 exhibits coexistence of several phases in the pristine state. DSC and POM revealed a broad transition at 86 °C, where a liquid suspension is formed, suggesting that only a partial melting has occurred, followed by a second broad transition at 100-110 °C, probably related to conformational rearrangements (Figure S12a). This is also confirmed by POM and SWAXS pattern observed at 90 °C (Figure S12b,c) and hints at coexistence of different polymorphs with a different thermal behavior. [START_REF] Corvis | New Melting Data of the Two Polymorphs of Prednisolone[END_REF][START_REF] Zhao | Melt Temperature and Initial Polymorphs Dependencies of Polymorphs Selection during Subsequent Crystallization in Propylene-Ethylene Random Copolymer[END_REF]) A similar behavior is reported in the literature for polyester dendritic amphiphiles of the same generation, [START_REF] Tuuttila | Synthesis and Thermal Behavior of Janus Dendrimers, Part 1[END_REF] thus suggesting that increased mobility of the polyester chains can induce different molecular conformations, which are eventually responsible for the formation of different polymorphs.

Tuning the Self-Assembly of FJDs in Water

All synthesized FJDs can easily be dispersed in water by injection of their diluted solutions in ethanol (typically 10 mg/mL). This method has been reported to be extremely efficient for the formation of monodisperse dendrimersomes starting from amphiphilic JDs. (3) The resulting assemblies were analyzed over time by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), and 19 F NMR.

Cryo-TEM images of aged FJD1 and FJD2 dispersions showed the coexistence of two families of nanoparticles (Figure 3a, Figure S13 and Figure 3e, Figure S14, respectively). A statistical analysis of the analyzed images indicated that for FJD1 about 57% of the nanoparticles have an average diameter of 14 nm, while the remaining 43% show an average diameter of 105 nm (Figures 3b,c). In the case of FJD2, about 87% of the nanoparticles show an average diameter of 14 nm and only the 13% show a diameter of 52 nm (Figures 3f,g). These results were also coherent with DLS experiments as no significant changes were observed during sample aging in terms of size and polydispersity (Figure S15). 19 F NMR spectra of such formulations show a single resonance peak at around -72 ppm, and no significant changes were observed over time, thus confirming the colloidal stability of FJD1 and FJD2 (Figures 3d and3h, respectively). A peculiar self-assembly behavior was instead observed for FJD3, which shows substantial changes in its aggregation mode upon aging. As schematically shown in Figure 4a, cryo-TEM images of the freshly prepared formulation revealed the presence of well-defined dendrimersomes of about 100 nm of diameter, composed of up to six layers, with an interlayer distance of 5 nm and a total layer width of 20 nm. Such dendrimersomes evolve over time forming structures of increasing complexity. Within 24 h at room temperature, vesicles start to interact and fuse with each other forming sheet-like structures, which gradually wrap around themselves giving rise to helical nanostructures, which eventually after 48 h evolve in tubule-like structures, as shown in Figure 4b. Simultaneously, a strong quenching of the 19 F NMR signal at -71.6 ppm was clearly observed at 48 h aging (Figure 4c). Because no precipitation occurred, quenching of the 19 F NMR signal would suggest, according to the literature, (61) a reduced mobility of CF3 groups, probably related to the change in the morphology of the self-assembled structures. The change in the aggregate morphology was also confirmed by DLS, where both polydispersity and hydrodynamic radius increased significantly at 48 h aging (Figure 4d). Interestingly, changing ethanol with trifluoroethanol (TFE), as cosolvent, the formation of tubular aggregates is prevented, and the presence of only spherical particles was confirmed by TEM. DLS and 19 F NMR did not change over time (Figure S16), confirming the stability of such dispersions. This suggests that FJD3 selfassembly is driven by a complicated pattern of forces that compete intra-and intermolecularly, in which the cosolvent plays a critical role, defining the final morphology of the supramolecular structures.

Considering the molecular structure of the three FJDs, we can assume that the interactions involved in their self-assembly are mainly fluorine-fluorine interactions, segregating fluorinated moieties from water, hydrogen bonding between polar headgroups and also with solvent, and stacking interactions among triazole rings. It is thus reasonable to expect the formation of assemblies in which fluorine moieties are located in the inner strata, isolated from water, while hydroxyl groups are exposed on the external surface in contact with the aqueous medium. This is supported by the single crystal structure of FJD1 that shows formation of fluorinated double layers as a consequence of the microsegregation of fluorinated segments, as also evident in FJD3 multilayer dendrimersomes. Although the observed different self-assembly behavior could also be rationalized considering that while all FJDs contain the same fluorinated moiety, bis-MPA dendron increases in generation on going from FJD1 to FJD3. Probably, the morphology of the final supramolecular aggregate is regulated by a subtle balance between fluorinated and hydrophilic moieties and changing the hydrophilic/hydrophobic balance promotes morphological changes from spherical nanoparticles to tubules. As suggested by cryo-TEM analyses and also supported by the literature for similar compounds, (62)FJD3 multilamellar dendrimersomes observed in water/ethanol solutions might be metastable kinetic aggregates evolving to more thermodynamically stable tubular assemblies. This transition from aggregates with a higher curvature, i.e., dendrimersomes, to morphologies characterized by a lower curvature, i.e., tubules, produces an increased rigidity of the fluorinated chains, working as shut-off switch of the 19 F NMR signal. [START_REF] Preslar | F Magnetic Resonance Imaging Signals from Peptide Amphiphile Nanostructures Are Strongly Affected by Their Shape[END_REF][START_REF] Ramanathan | Amphiphile Nanoarchitectonics: From Basic Physical Chemistry to Advanced Applications[END_REF] This hypothesis is also supported by the high tendency of 27F group-containing molecules to crystallize, as reported elsewhere, [START_REF] Dichiarante | A Short-Chain Multibranched Perfluoroalkyl Thiol for More Sustainable Hydrophobic Coatings[END_REF][START_REF] Tedeschi | Waterproof-Breathable Films from Multi-Branched Fluorinated Cellulose Esters[END_REF][START_REF] Tirotta | A Superfluorinated Molecular Probe for Highly Sensitive in Vivo 19 F -MRI[END_REF][START_REF] Huynh | Small BODIPY Probes for Combined Dual 19 F MRI and Fluorescence Imaging[END_REF][START_REF] Tirotta | F Magnetic Resonance Imaging (MRI): From Design of Materials to Clinical Applications[END_REF] and is strengthened by the experimental evidence that changing ethanol with trifluoroethanol (TFE) as cosolvent prevents the formation of tubules, and only spherical nanoparticles are observed by TEM, while DLS and 19 F NMR do not show any changes over time (Figure S16). We speculate that TFE might completely change the interaction pattern at the interface interacting with the fluorinated moieties within the bilayer and rendering them more mobile (i.e., 19 F NMR signal on).

In Silico Modeling of FJD3 Nanostructures

A minimalistic model of coarse-grained interaction forces able to reproduce the experiments could shed light on the complex FJD3 self-assembling process. Initially, we performed all-atom molecular dynamics simulations (AA-MD) of a single FJD3 molecule and dimers dissolved in either water/ethanol or water/TFE solutions at different cosolvent concentrations. Figures 5a,b show two representative snapshots of the AA-MD of the monomer. The AA-MD results of both monomer and dimer simulations were analyzed by means of three structural parameters: (i) the central dihedral angle, ϑ (Figure 5c), (ii) the radius of gyration, Rg (Figure 5d), and (iii) the radial distribution function (RDF) of the main chemical groups (Figure S17). The central dihedral angle of the single molecule displays a certain amount of mobility, which reaches a limiting value in about 50 ns. In all the conditions, ϑ reaches an intermediate value between trans and cis conformations (0° ≤ ϑ ≤ 90° for cis structures, 90° ≤ ϑ ≤ 180° for trans structures). Interestingly, ϑ and Rg values are on average higher in the presence of ethanol as cosolvent, indicating an unfolded transoid conformation for FJD3 in water/ethanol solutions. In the presence of 5% of TFE, instead, the dendrimer collapses in a cisoid conformation, probably due to specific F•••F interactions with the cosolvent (Figures 5c,d). In fact, RDF analysis (Figures S17a-d) shows that TFE molecules are mainly located around the fluorinated end of FJD3, showing a plethora of short contacts between carbonyl oxygens of FJD3 and hydroxyl group of TFE. AA-MD dynamics of the dimers did not show significant differences from those on the single molecules (details reported in Figures S18a-h andS19). Importantly, the unfolding of the dendrimer in the presence of ethanol could explain the lower mobility of the fluorinated portions, in agreement with 19 F NMR switch-off.

We have also run coarse-grained (CG) molecular dynamics simulations to gain insights into the FJD3 self-assembly process, with particular interest in its morphological transitions. Our minimal CG model has three different types of beads, as reported in Figure S20: (i) three CF beads with hydrophobic character representing the fluorinated end, (ii) two OA beads with hydrophilic character representing the polyester moiety, and (iii) three OC beads without a specific character, reproducing the linking group between the hydrophilic and hydrophobic ends. The associated parameters used in a dissipative particle dynamics (DPD) model are given in Table S3. Initially, CG simulations were set to reproduce FJD3 self-assembly in aqueous solutions. To rationalize the solvatophilic effect on self-assembled morphology, the interaction parameter (aOA,S) between dendrimer hydrophilic beads (OA) and solvent (S) has been varied from 20 (higher hydrophilic interaction between dendrimer and solvent) to 35 (lower hydrophilic interaction between dendrimer and solvent). Cubic simulation boxes of 40rc × 40rc × 40rc and 80rc × 80rc × 80rc with periodic boundary conditions were applied, and the bead density was set to 3. Following Groot and Rabone, (67) the physical length scale rc corresponds to 10 Å, resulting in boxes of 40 and 80 nm 3 , respectively, the latter representing a reasonable size to accommodate the 60 nm diameter multilamellar dendrimersomes observed by means of cryo-TEM images. In this box, to reproduce the experimental concentration, a total of 1536000 beads were used (see the Supporting Information for details). As shown in Figure 6, referring to the largest boxes, at the highest hydrophilicity (aOA,S = 20), we observed the formation of unilamellar hollow dendrimersomes and disk-like aggregates, while amphiphilic oligomers started to associate forming larger and multilamellar dendrimersomes upon reduction of the hydrophilicity. Finally, at aOA,S = 35, the inner cavity closes with formation of dendrimersomes characterized by the presence of up to four layers (Figures 6e,f). These results demonstrate that the hydrophilic parameter governs the formation of dendrimersomes, in agreement with what observed in similar systems by Arai et al. [START_REF] Arai | Self-Assembly of Janus Oligomers into Onion-like Vesicles with Layer-by-Layer Water Discharging Capability: A Minimalist Model[END_REF] The results of the smaller box are similar; the major difference is the number of layers and the size of the dendrimersomes (Figures S21a-f). A time evolution study of the transformation of the aggregates into dendrimersomes obtained with the largest box for aOA,S = 30 was also performed by calculating the mean aggregation number (Nagg) (Figure S22). Figure S22 shows snapshots taken at different times of the dendrimersomes formation process, starting from an initial random distribution. The mean aggregation number abruptly increases with time (vertical lines of Figure S22). At short simulation time (200 ns) (Figure S23b), FJD3 molecules start aggregating into disk-like structures, which fold into dendrimersomes with a mechanism that resembles the movement of a hand palm forming a fist. After 300 ns, single dendrimersomes start coalescing into larger spherical particles, gradually embedding nearby smaller structures (represented by Figures S23c-f). At longer simulation times (>1400 ns), spherical particles form dendrimersomes merging into large multiporous structures, which ultimately form multilayered systems (Figures S23g-j) with a mechanism already described by Chang et al. [START_REF] Chang | Structural Characteristics and Fusion Pathways of Onionlike Multilayered Polymersome Formed by Amphiphilic Comb-like Graft Copolymers[END_REF] Additionally, we performed a new set of simulations that explicitly included the cosolvents (EtOH or TFE) to unravel their role in the time-dependent morphological changes experimentally observed for FJD3. [START_REF] Ivnitski | The Strong Influence of Structure Polymorphism on the Conductivity of Peptide Fibrils[END_REF][START_REF] Gillissen | Triple Helix Formation in Amphiphilic Discotics: Demystifying Solvent Effects in Supramolecular Self-Assembly[END_REF][START_REF] Po | Supramolecular Self-Assembly of Amphiphilic Anionic Platinum(II) Complexes: A Correlation between Spectroscopic and Morphological Properties[END_REF] The cosolvents were represented as two beads, where one is hydrophobic and the other hydrophilic. The cosolvent beads were labeled EO and EC for ethanol and TO and TC for TFE; coarse-grained interaction parameters are reported in Table S3. We performed three new CG simulations: two at a high concentration of cosolvent and an additional one at a lower EtOH concentration. Cubic simulation boxes of 40rc × 40rc × 40rc dimensions with periodic boundary conditions were applied with bead density set to 3 with a total number of 192000 beads (as described in detail in the Supporting Information). Figure 7 reports the final morphologies of the aggregates obtained for the three simulations, which are in good agreement with cryo-TEM results. In particular, at the lowest ethanol concentration, unilamellar dendrimersomes were observed, characterized by a large central hollow core, with a diameter of about 22 nm and a double-layer thickness of about 6 nm. As ethanol concentration increases, elongated tubular aggregates are formed, with a central cavity of about 12 nm. In particular, it is evident that ethanol molecules fully cover the external and internal walls of the tubule, forming inner and outer coatings. Contrarily, the presence of TFE as cosolvent promotes the formation of dendrimersomes, characterized by a very large cavity of about 32 nm. A deeper inspection of the dendrimersome double layer showed the presence of interdigitated TFE molecules in the hydrophobic stratum, contrary to what observed in ethanol (Figure S24).

Conclusions

Here we reported the synthesis and structural characterization of a new family of fluorinated Janus-type dendrimers, FJDs, bearing a short-chain multibranched fluorinated moiety with 27 magnetically equivalent fluorine atoms linked to bis-MPA polyester dendrons of different generations. All synthesized FJDs are easily dispersed in aqueous solutions by injection of their diluted solution in ethanol, forming selfassembled nanostructures with a morphology dependent on a subtle balance between hydrophilic and hydrophobic portions in the molecule.

For the lowest generation dendrimers (FJD1 and FJD2), spherical nanoparticles identified by cryo-TEM and characterized by a high colloidal stability (DLS) are observed. Associated 19 F NMR spectra to these nanostructures show a single peak with a good signal-to-noise ratio making them traceable by 19 F MRI. On the contrary, the highest generation derivative, FJD3, self-assembles into multilamellar dendrimersomes displaying a morphological transition into tubular structures after 48 h of aging. Simultaneously, the switching-off of the 19 F NMR signal is observed, indicating lower mobility of the fluorine atoms as a consequence of a higher rigidity of the fluorinated portions. Interestingly, substituting ethanol with trifluoroethanol as cosolvent, the transition to tubules is prevented and 19 F NMR spectra do not change over time.

Computational studies allowed rationalization of the molecular factors governing the self-assembly of the highest generation dendrimer showing that depending on the nature of the cosolvent, FJD3 assumes different conformations. In fact, with TFE as cosolvent, FJD3 mostly visits bent structures with the fluorinated moiety and hydrophilic groups arranged in a sort of cisoid conformation, probably driven by specific F•••F interactions with the cosolvent. On the contrary, with ethanol as cosolvent, FJD3 adopts an unfolded transoid conformation. These conformational changes are at the basis of the complex self-assembly behavior seen for FJD3 in aqueous media. In particular, in the presence of ethanol as cosolvent, the fluorinated moieties of FJD3, confined in the inner strata of the nanostructures, are more strongly interdigitated by F•••F noncovalent interactions, while polyester chains are involved in hydrogen bonds with the cosolvent, driving the formation of longitudinal stable aggregates. In the presence of a fluorinated cosolvent(TFE), instead, F•••F interactions between dendrimer and TFE provide a higher solvation of FJD3, stabilizing a cisoid conformation and preventing the formation of tubular structures. A stronger interdigitation among FJD3 fluorinated moieties in the tubular structures also justifies a lower mobility of the CF3 groups with a significant reduction of their 19 F NMR signal.

In conclusion, short and branched fluorinated chains are here demonstrated as novel moieties for the design of JDs with tunable self-assembly behavior and functional properties. This novel design paves the way toward a plethora of applications from transport of gases to 19 F MRI tracking. The ability of similar fluorinated JDs to deliver genetic material is currently under patenting (73) and will be reported elsewhere.

Figure 1 .

 1 Figure 1. Fluorinated Janus-type dendrimers. The copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction allowed the orthogonal linkage of the fluorinated azideterminating derivative (F27-N3) and alkyne terminating polyester dendrimers of generations I, II, and III (DG1, DG2, and DG3). The three fluorinated Janus dendrimers (FJDs) obtained showed different magnetic properties at 19 F NMR depending on their self-assembly in aqueous media.

Figure 2 .

 2 Figure 2. Single-crystal X-ray structure of FJD1. (a) Dimers formed by antiparallel stacking. (b) Network of hydrogen bonds stabilizing the hydrocarbon regions formed by the packing of FJD1 molecules. (c) Fluorine-fluorine interactions next to π•••π stacking generate a layered structure. Color code: C, dark gray or light blue; H, light gray; F, yellowish-green; O, red; N, violet. Short contacts are depicted as black dotted lines.

Figure 3 .

 3 Figure 3. Self-assembly in water media of FJD1 (a-d) and FJD2 (e-h). (a) Cryo-TEM images show the formation of large spherical nanoparticles (light blue arrow) of around 100 nm of diameter and small nanoparticles, e.g., micelles (orange arrow) of 14 nm of diameter. (b) Size distribution profile of smaller nanoparticles from cryo-TEM images. (c) Size distribution profile of larger nanoparticles from cryo-TEM images. (d) 19 F NMR spectra over time (solvent: solution + D2O 10% v/v). (e) Cryo-TEM images show the formation of large spherical nanoparticles (light blue arrow) of around 50 nm of diameter and small micelles (orange arrow) of 14 nm of diameter. (f) Size distribution profile of larger nanoparticles from cryo-TEM images. (g) Size distribution profile of smaller from cryo-TEM images. (h) 19 F NMR spectra over time (solvent: solution + D2O 10% v/v).

Figure 4 .

 4 Figure 4. FJD3 self-assembly in water media. (a) Cartoon scheme of the aggregates transition.(b) Cryo-TEM images showed the presence of multilamellar dendrimersomes that after 24 h fuse to form sheet-like structures, which finally form tubules after 48 h from sample preparation. Dendrimersomes are composed of an interlayer distance of 5 nm, and a total layer width of 20 nm (in red). (c)19 F NMR spectra over time: after 48 h of aging there is the switching off of the signal (solvent: solution + D2O 10% v/v). (d) DLS results: changes in the hydrodynamic radius (RH, blue) and polydispersity (PDI, black) over time.

Figure 5 .

 5 Figure 5. AA-MD simulations of a single FJD3 molecule. Snapshots of (a) transoid and (b) cisoid conformations; fluorine atoms are green, carbon atoms gray, oxygen atoms red, hydrogen atoms white, and nitrogen atoms blue. The dihedral ϑ angle is also shown; the atom numbering is introduced to identify the dihedral angle. Solvent not shown to assist the eye. Summary of results in four solvent conditions; water (black line), 5% of ethanol in water (red line), 25% of ethanol in water (blue line), 5% of TFE in water (green line). (c) Plot of central dihedral angle of FJD3 vs time. (d) Plot of Rg vs time.

Figure 6 .

 6 Figure 6. Snapshots of morphologies (upper row) and corresponding cross sections of the inner core (bottom row) of aggregates obtained with different aOA,S parameters: (a, d) aOA,S = 20; (b, e) aOA,S = 30; (c, f) aOA,S = 35. Simulation box of dimensions 80rc × 80rc × 80rc. Color code: hydrophilic groups in red and hydrophobic groups in green. Water and OC beads are not represented here for the sake of clarity.

Figure 7 .

 7 Figure 7. Snapshots of morphologies obtained in different conditions (upper row) and corresponding cross sections (bottom row) showing the hollow cores of the aggregates: (a, d) lowest ethanol concentration; (b, e) highest ethanol concentration; (c, f) TFE as a cosolvent. Color code: hydrophilic groups in red and hydrophobic groups in green. OC water and cosolvent beads are not represented to assist the eye.

Acknowledgements

Nonappa acknowledges the provision of facilities at the Nanomicroscopy Centre at Aalto University (OtaNano) and Photonics Research and Innovation (PREIN) flagship. B.D. thanks the CNRS and University of Strasbourg.

Funding F.B.B., G.C., and P.M. are thankful to the NEWMED project, ID 1175999 (funded by Regione Lombardia, POR FESR 2014-2020), and to the project NiFTy funded by MIUR (PRIN2017, no. 2017MYBTXC).

Supporting Information

Additional experimental and computational details, analytical graphs, references, Tables S1-S4, and Figures S1-S23. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.macromol.2c00129. Deposition Number 2009095 contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformation zentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Author Information

Abbreviations

AA-MD, all-atom molecular dynamics; ATR-FTIR, attenuated total reflectance Fouriertransform infrared spectroscopy; bisMPA, 2,2-bis(hydroxymethyl)propionic acid; CG, coarse grained; cryo-TEM, cryogenic transmission electron microscopy; CuAAC, copper-catalyzed azide-alkyne cycloaddition; DLS, dynamic light scattering, DPD, dissipative particle dynamics; DSC, differential scanning calorimetry; EtOH, ethanol; FJD1, fluorinated Janus dendrimer of generation 1; FJD2, fluorinated Janus dendrimer of generation 2; FJD3, fluorinated Janus dendrimer of generation 3; HRESI-MS, highresolution electrospray ionization mass spectrometry; JDs, Janus dendrimers; MRI, magnetic resonance imaging; NMR, nuclear magnetic resonance spectroscopy; PFCs, perfluorocarbons; POM, polarized optical microscopy; RDF, radial distribution function; SWAXS, small-and wide-angle X-ray scattering; TEM, transmission electron microscopy, TFE, 2,2,2-trifluoroethanol; TGA, thermal gravimetric analysis.