Andrey Getmanskiy

Semen Sechenev a , Igor Ryadchikov

Alexander Gusev

Nikita Mikhalkov

Dmitry Kazakov

Alexey Simulin

Dmitry Sokolov
email: dmitry.sokolov@univ-lorraine.fr

Real-time system architecture design practices

Keywords: real-time systems, hardware, software

In this paper we give an overview of both hardware and software architectures of real time systems used in devices for various purposes -from lab bench contraptions to cars. The goal of the paper is to reveal separate classes of such architectures as well as to define preconditions for choosing a particular architecture.

Introduction

Due to the intensive growth of applications of mobile robotic platforms in recent years, the development of onboard real-time control systems is coming to the fore. Increasingly, open-source robotics (OSR) platforms are the foundation for development, providing unified blueprints, schematics, software, and infrastructure to implement the final ideas of third-party developers. Like in the case of a non-anthropomorphic robot we are currently developing in the Laboratory of Robotics and Mechatronics of the Kuban State University (refer to Fig. 1). Thus, the task of selecting the architecture of the on-board control system from a large variety of ready-made alternatives comes to the fore, in accordance with the specific operating conditions of the mobile platform. The development of criteria and the construction of a methodology for selecting the architecture of the on-board real-time systems, thus, are an urgent research task.

In this paper we propose an overview of the hardware and software solutions for a selection of representative samples that cover very different areas varying from laboratory test benchs to electric cars, ATVs performing "aggressive maneuvers", humanoid robots, four-legged walking robots and manipulators. The idea is to reveal main architecture classes, summarize the identified strengths and weaknesses of the most popular solutions. This allows us to identify the main issues affecting the choice of a particular architecture.

So, let us start the overview of a selection of representative examples.

Architecture desing practices

Lab test benches

We start this overview with a one-dimensional prototype of a balancing cube bot (Cubly) [START_REF] Gajamohan | The cubli: A cube that can jump up and balance[END_REF] . The Fig. 2a shows schematically the main elements of the system and their interaction. In this document we provide such schemes for each of the architectures to clearly show their distinctive features. The degree of schemes' details depends on the completeness of the system's description in public sources and also on the expediency of mentioning some elements, especially for complex systems.

The system consists of two controllers connected by a CAN interface, the main controller and the motor controller. IMU sensors, encoder and brake motor servo are connected to the main controller. IMU sensors are connected to the master controller via SPI interface. The main controller is the STM3210E debugging board for STM32F103E microcontrollers with Cortex M3 core. The advantages of this choice are the possibility of rapid prototyping and availability of community support.

FreeRTOS real-time system is used as an operating system. As advantages of FreRTOS the authors note the functionality that allows to perform multitasking and small kernel size -only 4KB. There is a wide choice of real time operating systems: the article 6 contains a rather complete list of them. FreeRTOS is described there as one of the most common free open source systems, along with eCos. The list of officially supported freeRTOS microcontrollers is quite impressive [START_REF]freeRTOS supported MCUs[END_REF] . It is similar to openRTOS (for commercial use) and safeRTOS (for industrial and medical use).

In this case the network of controllers (CAN) is used for communication between the main controller and the motor drivers. CAN is a popular decision both in research, and in the industry. Its advantages are high resistance to interference, simplicity, low cost and ample documentation. However, the disadvantages of this bus are speed (a small amount of data can be transmitted in one package, because a lot of work for service data), limited working distance (30m for 1Mb/s), lack of predictability and limited reliability. There are corresponding higher-level protocols: CANopen, DeviceNet, FTT-CAN and TTCAN [START_REF] Wilamowski | Industrial communication systems[END_REF] . This is a quite typical architecture, let us consider another speciment of the family, namely the triple inverted pendulum on a cart [START_REF] Glück | Swing-up control of a triple pendulum on a cart with experimental validation[END_REF] . There is one important point: while the final version of this system is controlled via a dedicated controller (dSPACE DS1103 10 control module), all initial experiments were performed in Matlab/Simulink environment. Some values were pre-calculated, interpolated and stored in look-up tables. Using dSPACE DS1103 is convenient because all I/O configuration can be done in Simulink and then compiled, loaded and started automatically. This can greatly speed up the prototyping of the system as well as reduce the experiment time. The architecture diagram is shown in Fig. 2b.

Electric cars

Distinctive feature of hardware and software architecture of cars is grouping of electronic systems into electronic control units (ECU). ECU can control one or more electrical systems of the car. Modern cars have a large number of ECU, which requires finding ways to reduce the number of wires connecting these systems. The most common solution is CAN Network [START_REF] Miranda Bermejo | Design and Implementation of a Control System for Testing an Experimental Electrical Vehicle[END_REF][START_REF] Nolte | Automotive communications-past, current and future[END_REF][START_REF] Fredriksson | Can for critical embedded automotive networks[END_REF][START_REF] Leen | Expanding automotive electronic systems[END_REF] . It is a reliable way of data transmission even in case of electromagnetic interference, with a transmission speed of up to 1 Mb/s, which may be sufficient for a certain range of tasks. CAN Nwtwork is used for communication with all devices of the system described in the article 2 , except for GPS. Due to the requirements of some devices it was necessary to divide the described system into several subnets: some devices require different network settings, some should be isolated, anb in some cases it is necessary to filter messages. Network gateways (such as ADFweb and DSPACE MicroAutoBox) are used to communicate between these networks.

The dSPACE MicroAutoBox II is used as the main microcontroller that controls all ECU in the vehicle 2 . Car PC (Intel P8700, 1 GB RAM and 500GB HDD) is connected to dSPACE MicroAutoBox II via Ethernet and used for logging, experimenting and development via dSPACE ControlDesk software. In its turn, dSPACE ControlDesk is used in conjunction with Simulink for rapid prototyping. The Fig. 3 shows a simplified scheme of the architecture.

Humanoid robots

Two-legged walking robots also often use the CAN bus [START_REF] Kaneko | Humanoid robot hrp-3[END_REF][START_REF] Metta | The icub humanoid robot: an open platform for research in embodied cognition[END_REF][START_REF] Lim | Robot system of drc-hubo+ and control strategy of team kaist in darpa robotics challenge finals[END_REF] . Another popular solution is EtherCAT [START_REF] Tsagarakis | Walk-man: A high-performance humanoid platform for realistic environments[END_REF][START_REF] Stasse | A new humanoid research platform targeted for industrial applications[END_REF] , which solves the CAN bus problem, namely the inability to provide high poll frequency of low level controllers [START_REF] Stasse | A new humanoid research platform targeted for industrial applications[END_REF] . As stated in the article [START_REF] Stasse | An overview of humanoid robots technologies[END_REF] , PCI, PCI104 or PCIe are often used to connect motherboards, network cards and analog-to-digital converters. The advantage of tboth CAN and EtherCAT is reliability, and they are both used to connect motors and encoders. However, the CAN bus has a limited bandwidth (1 Mbit/sec) and limited number of devices connected to the bus. For this reason, several buses are often used in parallel in humanoid robots [START_REF] Metta | The icub humanoid robot: an open platform for research in embodied cognition[END_REF][START_REF] Kaneko | Humanoid robot hrp-3[END_REF] . Some robots 19 also use SERCOS III, which is similar to EtherCAT principles and has a similar performance [START_REF] Wilamowski | Industrial communication systems[END_REF] .

Let's consider the hardware and software architecture of the Walk-man robot [START_REF] Tsagarakis | Walk-man: A high-performance humanoid platform for realistic environments[END_REF] . COM Express computing module based on Pentium i7 quad core processor was used to control the robot's movements. It runs the Ethercat Master device manager. It is responsible for the operation of Ethercat slaves, synchronizing them by exchanging information about their relative state in real time. YARP [START_REF] Metta | Yarp: yet another robot platform[END_REF] was chosen as the middle layer software for remote access to the robot motors, meeting the requirements of high speed and low latency.

In the original version, the robot architecture YarpServer and RosCore could communicate directly with each other. But in the real world, their direct communication can result in frequent communication losses. Centralized YARP/ROS servers may not recover from this. Therefore, the controlling PC and the robot were separated and each running its own RosCore and YarpServer, as shown in Fig. 4.

EtherCAT is a rather popular solution, especially for complex systems with large numbers of sensors. For example, there are "robot skin" projects [START_REF] Baglini | Design of an embedded networking infrastructure for whole-body tactile sensing in humanoid robots[END_REF] , four-legged [START_REF] Semini | Design overview of the hydraulic quadruped robots[END_REF][START_REF] Guo | Real-time quadruped robot control system based on xenomai[END_REF] and six-legged [START_REF] Gui | Stability analysis for a hexapod robot walking on slopes[END_REF][START_REF] Deng | Object carrying of hexapod robots with integrated mechanism of leg and arm[END_REF][START_REF] Zhao | Obstacle avoidance and motion planning scheme for a hexapod robot octopus-iii[END_REF] robots, medical rehabilitation robots and exoskeletons [START_REF] Allouche | Design and control of a sit-to-stand assistive device via ethercat fieldbus[END_REF][START_REF] Grosu | Real-time physical layer architecture for corbys gait rehabilitation robot[END_REF][START_REF] Rebelo | Bilateral robot teleoperation: A wearable arm exoskeleton featuring an intuitive user interface[END_REF] .

A typical example of a humanoid robot is Talos Pyréne, developed by PAL-Robotics [START_REF] Stasse | A new humanoid research platform targeted for industrial applications[END_REF] . It uses the architecture is built around ROS and EtherCAT. The robot is equipped with two 8-core computers, one for the control and the other is responsible for vision and high-level computations. All PAL-Robotics robots, including Talos Pyréne, are deeply integrated with the Robotic Operating System (ROS). They active use ros control, thus quickly switching between simulations and tests on a real robot. The authors, however, note that although ros control is easier to use than openRTM, it is currently inferior to openRTM [START_REF] Ando | Rt-middleware: distributed component middleware for rt (robot technology)[END_REF] .

Aggressively maneuvering quadrocopters

The architecture of ATVs that perform "aggressive maneuvers" often includes the Vicon motion capture system. The frequency of system operation and the number of sensors used can vary. A typical example is provided in [START_REF] Bangura | Real-time model predictive control for quadrotors[END_REF] , where the Vicon system with the frequency of 200Hz is chosen. Vicon connects via Ethernet to a ground station where ROS is installed. Then the data is sent via UDP bridge over MAVLink (micro air vehicle communication) protocol to Pixhawk px4 autopilot from which I2C commands to engine drivers are sent. The architecture diagram is shown in the Fig. 5. There are also variants using ROS-Matlab Bridge 32 .

Manipulators

Many KUKA industrial manipulators are controlled with Kuka KR C4 [START_REF] Kuka | KUKA kr c4 extended documentation[END_REF] . The central element of the controller is the Cabinet Control Unit (CCU), the main board that is the interface for all components of the controller. You can see the wiring diagram of the devices in the Fig. 7. The user interface is located on the KUKA Control PC (KPC). The KUKA Servo Pack (KSP) is a motor controller. KUKA Power Pack (KPP) -robot power system with its own controller. Resolver Digital Converter (RDC) is responsible for collecting data on motor position and temperature. The Safety Interface BOARD (SIB) is responsible for the option to safely operate the robot. In addition, an Ethernet card, Controller System Panel (CSP), Dual NIC and SmartPAD are connected to the CCU, which is the operator's console. These elements are connected to the CCU via the following five interfaces: 1) KUKA Extension Bus (KEB), 2) KUKA System Bus (KSB), 3) KUKA Service Interface (KSI), 4) KUKA (KCB), 5) KUKA (KOI). Via the KEB interface, the controller can be connected to industrial Internet networks such as PROFIBUS and DeviceNet. Support for PROFINET, INTERBUS, EtherCAT, Ethernet/IP and VARANBUS is also claimed [START_REF] Kuka | KUKA kr c4 booklet[END_REF] . For comparison, the Universal Robots E-series collaborative control robots use a control box that supports only 3 industrial Internet standards: ModbusTCP, ProfiNet and EthernetIP [START_REF]Universal Robots[END_REF] . And all these standards are characterized by soft real time, non-deterministic, without guarantees about the transfer time [START_REF] Wilamowski | Industrial communication systems[END_REF] .

Four-legged walking robots

An architecture to control a complex and inherently unstable system such as StarlETH in an unknown environment is described in the article [START_REF] Hutter | Starleth: A compliant quadrupedal robot for fast, efficient, and versatile locomotion[END_REF] . One of the main requirements for such an architecture is reliability. The basis of the architecture chosen by the authors is a centralized computer responsible for all high-level management. Schematic representation of the architecture is shown in the Fig. 6. The authors use real time environment for simulation and management SL [START_REF] Schaal | The sl simulation and real-time control software package[END_REF] , which simulates the dynamics of a large number of objects, visualizes the robot and the environment, as well as communicates between different services via shared memory. It also includes tools for logging and post-processing.

Motor tracking system is responsible for synchronization and coordination of separate executive motors and consists of low-level regulators (position, speed, torque, etc.) and signal filters. The motor tracking system uses a shared memory to access the simulation and a CAN interface to access the actual robot. This makes it possible to experiment with the robot and run the simulation with exactly the same controller.

The authors note that the factor limiting the speed of the system is the connection between the centralized computer and the motor controllers via CAN interface. Despite parallel channels (one channel per leg), the 1 Mb/s bandwidth limits the total polling frequency to 400 Hz.

How to choose an architecture

The considered architectures of real-time systems can be classified by the area -aggressive flight copters, lab test benches such as inverted pendulums, four-legged walking robots and so on. The are popular architecture solutions for each use case, however, there are no clear boundaries between these architectures. Prior to choosing the architecture for a specific task we need to answer a number of questions, and only these answers would allow us to make a good choice. To give an example, the answer to the question whether it is planned to use a large number of different sensors gives a clear tip if you need to think about using ROS. Here we list all the questions that we found relevant:

1. Are there highly specialized hardware and software solutions for the task you are solving? A positive answer to this question can narrow down the search area -specialized solutions can use the same technologies as general purpose solutions, but will take into account special requirements. For example, most specialized flight controllers are lightweight and small in size, while specialized controllers for industrial manipulators support a large set of standards such as industrial internet for more flexible integration into existing production lines. Many ready-made software solutions for robotic tasks become available using ROS or Matlab. 2. Do you plan to connect a large number of different sensors? When you connect a large number of sensors, the network load grows. In this case you can use EtherCAT or SERCOS III. However, if you connect a large number of sensors from different manufacturers, you should pay attention to ROS, which already has interfaces for many different sensors in its open database. 3. How complex is the system: how many drives and how many non-motorized degrees of freedom? If the system is complex, you can divide it into separate logical units. This will help to avoid some problems and increase reliability of the system. For example, centralized ROS and YARP servers may not recover after losing communication with each other 3 . To increase speed, it may be worth parallelize channels into separate groups of motors [START_REF] Hutter | Starleth: A compliant quadrupedal robot for fast, efficient, and versatile locomotion[END_REF] . For example, it is possible to partially compensate for the lack of CAN transmission speed. But you should be careful, as even in spite of paralleling CAN, the bus may become a bottleneck of the system and limit the speed in general. 4. Do you need quick prototyping possibilites? If you need to experiment a lot, you should consider using Matlab/Simulink and systems that support it, such as dSPACE DS1104, MicroAutoBox and external PCs for development and debugging. If you plan to test different algorithms for the same hardware architecture, using Matlab/Simulink can significantly speed up their testing cycle. For example, there is HDL coder -a Matlab package that allows you to translate Matlab/Simulink code and models into C++ code and run it on microcontrollers from different manufacturers. It also allows SIL, PIL and HIL testing to be performed quickly. 5. How critical is the reliability of the system? If one of the main requirements of the architecture is reliability (e.g. the brake system of a car or a complex dynamic system), you should pay attention to the CAN Network, which has established itself as a network with high reliability [START_REF] Wilamowski | Industrial communication systems[END_REF] . In general, of course, the reliability of the system depends on many factors -all nodes of the system and the connection between them must be stable. As a rule of thumb, industrial solutions offered by large companies are more reliable than free and open source solutions.

A software solution can be to program the behavior of the device in the event of a system failure. It is also worth making sure that the system can recover from a failure -as, for example, in [START_REF] Tsagarakis | Walk-man: A high-performance humanoid platform for realistic environments[END_REF] the authors have divided RosCore and YarpServer into different devices so that after the loss of communication these servers could recover. 6. What computing power is required for the system operation? If a large computing power is required, and for some reason it cannot be placed inside the device under development, it can be transferred to an external calculation module. Also, calculations can be divided logically between different nodes of system (for example, preprocessing of the data received from sensors on the controller near to sensors). If large computing power is only needed in the experiment phase, it is reasonable to use a separate external computer for the experiments. 7. What are the maximum bandwidth requirements? It is necessary to calculate the worst case of the channel load (for example, in the project 2 the author performs such calculations) and to make sure that the chose network type will allow to transmit this amount of data at the speed suitable for you. If not, you should either choose another network (the bandwidth of different networks are given in [START_REF] Wilamowski | Industrial communication systems[END_REF] ) or parallelize the data transmission to separate nodes as done in [START_REF] Hutter | Starleth: A compliant quadrupedal robot for fast, efficient, and versatile locomotion[END_REF] . It is also important to pay attention not only to the bandwidth, but also to the delivery time. It may be crucial to use deterministic systems guaranteeing the time of delivery such as EtherCAT, EPL, Profinet IRT and SERCOS III. 8. What is the planned carrying capacity of the system? The question of limited payload is crucial for selfcontained/autonomous devices, whereas in other cases some system elements can be externalized. If the system's payload is small and the computing capacity is large, you can take some of the computations to an external computer. Then the controller on the device itself may be smaller, with less power consumption. 9. Do the scenarios consider the interaction of the device with another device, do you need access to some shared information? In this case one should think about using cloud services or consider a choice of data transmission channel between the devices which will not become a bottleneck of the real-time system being developed. 10. What is the maximum planned speed of movement of the device? Quite often an action or task must be performed under a hard deadline, otherwise the result will have no meaning [START_REF] Wilamowski | Industrial communication systems[END_REF] . One of the factors that determine this deadline is the velocity of the (components of the) device. The system must have time to react to the changes in the environment. Otherwise, for example, an ATV flying at a high speed will not have time to react to an obstacle on its way and may crash. 11. Will device operate in a known or unknown environment? In an unknown environment, much more sensor data needs to be processed. This should be taken into account in the calculations as a time spent on data processing. Correspondingly, the network load will also increase. 12. Are the hardware dimensions critical? If so (e.g., if it is a satellite to be placed into orbit), then smaller microcontrollers can be used. For example, the use of System on a Chip allows us to greatly reduce the system dimensions. 13. Is this device susceptible to interference?. CAN is a popular solution if noise immunity is critical. However, other industrial standards (EtherCAT, SERCOS III) also have good immunity. 14. What is the distance between the interacting modules?. This must be taken into account, as some networks have serious limitations in this respect. For example, CAN, while having undeniable reliability advantages over short networks, has considerable speed losses for longer (typ. 30 meters) networks.

Choosing the architecture for a non-anthropomorphic biped

Here we give an example of architecture choice for the non-anthropomorphic robot we are currently developing in the Laboratory of Robotics and Mechatronics of the Kuban State University (refer to Fig. 1). The biped has an auxiliary dynamic stabilization system which consists of two reaction wheels inside the robot's body and we are currently studying the improvement of the stabilization system on the basis of a scissored pair control moment gyroscope [START_REF] Aranovskiy | Experimental comparison of velocity observers: A scissored pair control moment gyroscope case study[END_REF] .

Let us see how answering the above questions narrows down the search area for the architecture choice.

1. There are there highly specialized solutions, mainly software. It is possible to use open drivers for sensors and ready algorithms for, say, motion planning, machine vision, SLAM and so on. We can choose ROS, as it supports a large number of different sensors, it uses code from many open sources, can automatically update it, apply patches and so on [START_REF] Quigley | an open-source robot operating system[END_REF] .

2. We do plan to use many sensors, but we are not talking about hundreds and thousands, so using EtherCAT or SERCOS III is not critical. However, completely different sensors -lidars, cameras, pressure sensors, IMUs and others -will be used. For this case, again, a popular solution is to use ROS with its Peer-to-Peer topology and support for a large number of sensors. 3. Our system is rather complex: it has 2 legs with 4 servomotors on each [START_REF] Ryadchikov | Design and control of self-stabilizing angular robotics anywalker[END_REF] . Inside the robot body there is a stabilization system with 3 motors. Part of the sensors are responsible for the stabilization of the robot -Hall sensors in the motors of the stabilization system, pressure sensors on the feet of the robot, gyroscope. Leg servo motors can also be included in the group of robot devices that affect stabilization. Sensors responsible for machine vision, SLAM can be separated into a different system. These two groups can be separated and connected to different computing modules, as done in [START_REF] Tsagarakis | Walk-man: A high-performance humanoid platform for realistic environments[END_REF] . Also, it makes sense to create parallel networks to servomotors increase the available bandwidth. 4. The possibility of rapid prototyping is very important, as a number of studies are planned during the development process. This will require flexibility, the ability to quickly remake individual system blocks and to test in a new configuration. One of the popular means of rapid prototyping is Matlab. 5. The reliability is of course important but not crucial: while it is planned to commercialize the final product as an educational kit, it would not be very dangerous for the user. 6. Raspberry Pi 3B (ARM Cortex-A53 x64, 1.2 GHz) should suffice for the stabilization task; as for the total computing power for the final robot it is hard to tell at this stage, probably we well need something comparable to Walk man 3 . 7. CAN bus with paralallel channels should suffice for the controller-legs communication, Ethernet interface suffices for the high-level control tasks. 8. The carrying capacity of the robot is only 3.8 kg, therefore, the control module need to be externalized. 9. No interaction with another robot/device is planned at this stage. 10. The speed limit is 5 km/h. 11. The robot will move in a known environment at first, but unknown environment is planned. We need to plan for an external acquisition system. 12. The hardware dimensions are very important, although it is less critical than for satellites. 13. The robot may operate in a very noisy environment, so fault tolerant interfaces are a must. 14. The maximum distance between system's modules is 5 meters, thus CAN bus is a reasonable choice.

While main hardware choices for the robot itself were made in [START_REF] Ryadchikov | Design and control of self-stabilizing angular robotics anywalker[END_REF] (carrying capacity, movement speed etc), the above answers orient our choice for control architecture to something very similar to Walk-man depicted in Fig. 4, the main difference is that we do not need EtherCAT, as CAN bus suffices for our needs.

Conclusion

The examples considered in this article show some of the most popular real-time hardware and software architectures. Many of the listed solutions similar characteristics (SERCOS III and EtherCAT), some can be (in some cases) mutually interchangeable, and many can be used in different combinations with each other. We believe that by answering the questions in section 3, you can define the basic requirements for the hardware and software architecture, choose one of the described solutions, or use these examples as a starting point for further searches.

Fig. 1 .Fig. 2 .

 12 Fig. 1.We are developing a biped that uses uses reaction wheels (shown in red) as an auxiliary stabilization system, see[START_REF] Efimov | Adaptive stabilization by delay with biased measurements[END_REF]

Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .Fig. 7 .

 34567 Fig. 3. Hardware and software architecture of the car from 2 .


Jianhong Chang

# Hui Li

# Xinyu Guan

Cuimei Li

Guangtao Yu

Valentin Valtchev

Yushan Yan

Shilun Qiu

Qianrong Fang
email: qrfang@jlu.edu.cn

Iodine adsorption in tetrathiafulvalene-based covalent organic frameworks

To safeguard the development of nuclear energy, practical techniques for capture and storage of radioiodine are of critical importance but remains a significant challenge. Here we report the synergistic effect of physical and chemical adsorption of iodine in tetrathiafulvalene-based covalent organic frameworks (COFs), which can markedly improve both iodine adsorption capacity and adsorption kinetics due to their strong interaction. These functionalized architectures are designed to have high specific surface areas (up to 2359 m 2 g -1 ) for efficient physisorption of iodine, and abundant tetrathiafulvalene functional groups for strong chemisorption of iodine. We demonstrate that these frameworks achieve excellent iodine adsorption capacity (~ 8.15 g g -1 ) and adsorption kinetics (~ 0.69 g g -1 h -1 ), which are much higher than other materials reported so far, including silver-doped adsorbents, inorganic porous materials, metal-organic frameworks, porous organic frameworks, and other COFs. Furthermore, a combined theoretical and experimental study, including DFT calculations, electron paramagnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, reveals the strong chemical interaction between iodine and framework of material. Our study thus opens an avenue to construct functional COFs for a critical environment-related application.

Nuclear power is one of the most important energy sources due to its high power density and low carbon emission. [START_REF] Kintisch | Congress tells DOE to take fresh look at recycling spent reactor fuel[END_REF][START_REF] Vienna | Nuclear waste vitrification in the United States: recent developments and future options[END_REF] However, an urgent safety issue related to nuclear power is the administration of waste produced from nuclear fission. [START_REF] Ewing | Nuclear waste management in the United States-starting over[END_REF][START_REF] Soelberg | Radioactive iodine and krypton control for nuclear fuel reprocessing facilities[END_REF] Radioiodine ( 129 I and 131 I) is a typical pollutant that accompanies nuclear fission, and attracts a particular attention since it can disperse rapidly in air and causes a radiological environment up to ten million years (15.7 million year for half-life of 129 I). [START_REF] Ojovan | An introduction to nuclear waste immobilization[END_REF] For example, radioactive iodine leaked out from Chernobyl nuclear accident has been incorporated into the human metabolic system and resulted in human diseases like thyroid cancer. To safeguard the future development of atomic energy, efficient strategies for adsorption and removal of radioiodine are of necessary but remain elusive.

The traditional technology to remove radioactive iodine is based on a chemical transformation, that is, using silver-doped adsorbents to produce harmless AgI; however, these materials show a low efficiency. For instance, their theoretical and practical adsorption capacity is 1.18 g g -1 and 0.10-0.31 g g -1 (per gram Ag), respectively, which is far to face the practical needs. [START_REF] Riley | Materials and processes for the effective capture and immobilization of radioiodine: A review[END_REF] In recent years, porous materials including inorganic porous materials, [START_REF] Hertzsch | Supramolecular-wire confinement of I 2 molecules in channels of the organic zeolite tris(o-phenylenedioxy)cyclotriphosphazene[END_REF][START_REF] Mohanambe | Insertion of iodine in a functionalized inorganic layered solid[END_REF] metal-organic frameworks (MOFs), [START_REF] Long | The pervasive chemistry of metal-organic frameworks[END_REF][START_REF] Zhou | Introduction to metal-organic frameworks[END_REF] and porous organic frameworks (POFs), [START_REF] Hasell | Molecular doping of porous organic cages[END_REF][START_REF] Pei | Ultrahigh iodine adsorption in porous organic frameworks[END_REF] have been shown to be a suitable choice for the removal of radioiodine. In particular, covalent organic frameworks (COFs), [START_REF] Côté | Porous, crystalline, covalent organic frameworks[END_REF][START_REF] Colson | Rationally synthesized two-dimensional polymers[END_REF][START_REF] Ding | Covalent organic frameworks (COFs): from design to applications[END_REF][START_REF] Geng | Covalent organic frameworks: design, synthesis, and functions[END_REF][START_REF] Guan | Design and applications of three dimensional covalent organic frameworks[END_REF] as a unique class of crystalline organic polymers with ordered porous channels, have been studied extensively for the uptake of iodine. These COF materials with specific pore environments and tunable chemistry can be easily functionalized to obtain active sites for more efficient iodine capture by electrostatic forces, Lewis acid-base interactions and coordinative sites. Nevertheless, most studies are still limited to exploitation of physical interaction, i.e., iodine uptake capacity in porous materials is driven by pore volume and uptake kinetics is dominated by pore connectivity and size. For example, although a state-of-the-art COF, TPB-DMTP COF, has achieved remarkable iodine adsorption capacity (6.26 g g -1 ), its adsorption kinetics is quite slow (0.13 g g -1 h -1 ). [START_REF] Wang | Exceptional iodine capture in 2D covalent organic frameworks[END_REF] In principle, the combination of high physisorption and strong chemical interaction in a material will be beneficial to improving both iodine adsorption capacity and adsorption kinetics. However, this avenue is still not properly explored.

Herein, we report the synergistic effect of physical and chemical iodine adsorption in tetrathiafulvalene (TTF)-based COFs, JUC-560 (JUC = Jilin University China) and JUC-561, leading to ultrahigh iodine adsorption capacity and ultrafast adsorption kinetics. These architectures are designed to have large specific surface areas (1815 m 2 g -1 for JUC-560 and 2359 m 2 g -1 for JUC-561) for high iodine uptakes through the physical process, and plentiful TTF functional groups for powerful iodine chemisorption. As a result, JUC-561 exhibits an unprecedented iodine uptake of 8.15 g g -1 and adsorption kinetics of 0.69 g g -1 h -1 at 333 K under ambient pressure. These values are much higher than other materials reported so far, including silver-doped adsorbents, inorganic porous materials, MOFs, POFs, and COFs. The strong interaction between the COF framework and iodine has been probed by DFT calculations on model molecules, TTF•I 2 , TTF + •I 2 , TTF + •(I -) 2 , TTF + •(I 3 -), and TTF + •(I 5 -), and confirmed by experimental studies including X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy, and Raman spectroscopy.

Designed synthesis and structural analysis

To achieve high iodine adsorption, we introduced a TTF-based derivative, tetrathiafulvalenetetrabenzaldehyde (TTF-TBA, Fig. 1b), as a planar 4-connected building unit. TTF or TTF-based derivatives are known to form the radical cations with electron acceptors (e.g., I 2 ). [START_REF] Li | Three-dimensional tetrathiafulvalene-based covalent organic frameworks for tunable electrical conductivity[END_REF] Therefore, by the incorporation of TTF-based units, the resulting COF materials will promote iodine adsorption based on this strong interaction. We further chose two typical monomers, 2',5'-dimethyl-[1,1':4',1''-terphenyl]-4,4''-diamine (DTDA, Fig. 1a) as a linear building unit and 2,4,6-tris(4-aminophenyl)amine (TAPA, Fig. 1c) as a 3-connected building unit. Thus, we developed two novel structures, two-dimensional (2D) JUC-560 with sql topology 20 and three-dimensional (3D) JUC-561 with ffc topology (Fig. 1d-i), [START_REF] Lan | Materials genomics methods for high-throughput construction of COFs and targeted synthesis[END_REF] constructed from the condensation of TTF-TBA and linear DTDA or non-coplanar 3-connected TAPA (Supplementary Fig. 1), respectively.

The targeted COFs were synthesized by suspending TFP-TTF and DTDA or TAPT in a mixture of mesitylene/1,4-dioxane or o-dicholobenzene/n-butanol in the presence of acetic acid, followed by heating at 120 °C for 3 days. JUC-560 shows powder X-ray diffraction (PXRD) peaks at 2.69°, 3.48°, 5.38°, 5.60°, and 8.07° for 2θ, which are assigned to the (110), ( 200), ( 220), (310), and (330) facets, respectively. This structure adopts a 2D sql net with an eclipsed AA-stacking mode that yields a PXRD pattern similar to the experimentally observed one (Fig. 2a and Supplementary Figs. 101112). On the other hand, JUC-561 exhibits PXRD peaks at 1.99°, 3.21°, 4.01°, 5.20°, 6.06°, 6.89°, 7.77°, 8.44°, and 22.00° for 2θ, which correspond to the (020), (001), (040), (041), (060), (061), (042), (110), and (313) facets, respectively. JUC-561 assumes a 3D ffc net that exhibits a PXRD profile that consistent with the experimental one (Fig. 2b and Supplementary Figs. 13-16). PXRD patterns based on Pawley refinements 22 also confirm the assignments of PXRD peaks, as evidenced by their negligible differences (residuals Rp = 1.31%, Rwp = 2.53% for JUC-560, and residuals Rp = 3.07%, Rwp = 4.51% for JUC-561). Based on these results, both TTF-based COFs have mesoporous frameworks with a diameter of about 2.62 nm for JUC-560 and about 2.55 nm for JUC-561 (Fig. 1f and1g).

Thermal and chemical stability

Both COFs were observed to be chemically stable in different organic solvents, including acetone, ethanol, cyclohexanone, hexane, tetrahydrofuran, N,N-dimethylformamide and dimethylsulfoxide. After being treated in these solvents for 24 h, JUC-560 and JUC-561 maintained their crystallinity (Supplementary Figs. 17 and18). Furthermore, both COFs retained their structures in boiling water (100 °C) and even in strong acid (3 M HCl, 25 °C) and strong base (3 M NaOH, 25 °C) for 24 h (Supplementary Figs. 19 and20). According to the thermogravimetric analysis (TGA), these COFs did not decompose until 400 °C under nitrogen (Supplementary Figs. 21 and22). Therefore, these TTFbased COFs are chemically and thermally stable, crystalline, porous materials.

Analysis of nitrogen gas adsorption

The porosities of both COFs were determined by N 2 adsorption and desorption isotherms at 77 K (Fig. 2c and2d). For each COF, a sharp uptake below P/P 0 = 0.05 and a step, indicating the presence of mesopores, was observed. The inclination of isotherms between P/P 0 = 0.9-1.0 and slight desorption hysteresis is related to the presence of textural mesopores from the agglomeration of COF crystals. [START_REF] Fang | 3D porous crystalline polyimide covalent organic frameworks for drug delivery[END_REF] The Brunauer-Emmett-Teller (BET) specific surface areas were calculated to be 1815 m 2 g -1 for JUC-560 and 2359 m 2 g -1 for JUC-561, respectively (Supplementary Figs. 23-26). The pore size distribution obtained by the nonlocal density functional theory (NLDFT) showed monomodal mesopores, 2.50 nm for JUC-560 and 2.46 nm for JUC-561 (Fig. 2e and2f), which are in good agreement with those of the proposed model (2.62 nm for JUC-560 and 2.55 nm for JUC-561). Furthermore, the pore volume was determined to be 1.11 cm 3 g -1 for JUC-560 and 1.92 cm 3 g -1 for JUC-561, matching well with the simulated ones (1.16 cm 3 g -1 for JUC-560 and 2.04 cm 3 g -1 for JUC-561).

Analysis of iodine adsorption

We conducted iodine adsorption experiment by exposing COFs to iodine vapor at 60 °C under ambient pressure. JUC-560 exhibited a quick iodine uptake, showing a nearly linear increment in 7 h and then reaching adsorption saturation within 14 h (Fig. 3a). Similarly, JUC-561 showed a rapid adsorption in 8 h and achieved saturation in 18 h (Fig. 3b). JUC-560 obtained iodine uptakes as high as 5.17 g g -1 .

Remarkably, JUC-561 showed much higher iodine uptakes of 8.15 g g -1 due to its 3D framework with higher specific surface areas and interconnected channels. On the basis of the pore volume together with the iodine density (4.93 g cm -3 ), their theoretical capacities (4.93 g cm -3 × pore volume) upon the full occupation of pores by iodine are 5.47 g g -1 for JUC-560 and 9.47 g g -1 for JUC-561 (Supplementary Table 1). The channels of both COFs are almost entirely occupied by iodine (95% for JUC-560 and 86%

for JUC-561 based on their theoretical maxima), which is consistent with the high weight lost (Supplementary Figs. 27 and28) for iodine-loading samples.

We summarized the iodine uptakes of typical adsorbents in Fig. 4 and Supplementary Table 2. As can be seen, the adsorption capacity of JUC-561 (8.15 g g -1 ) is far superior to those reported previously, such as 29-fold higher than that of the typical silver-doped zeolite mordenite (Ag-MOR, 0.28 g g -1 ), [START_REF] Chapman | Radioactive iodine capture in silver-containing mordenites through nanoscale silver iodide formation[END_REF] two orders of magnitude higher than those of nonporous materials, and much better to those of representative porous materials, including zeolitic imidazolate framework-8 (ZIF-8, 1.20 g g -1 ), [START_REF] Sava | Capture of volatile iodine, a gaseous fission product, by zeolitic imidazolate framework-8[END_REF] porous aromatic framework 24 (PAF-24, 2.76 g g -1 ), [START_REF] Yan | Highly efficient enrichment of volatile iodine by charged porous aromatic frameworks with three sorption sites[END_REF] azo-bridged porphyrin-phthalocyanine network (AzoPPN, 2.90 g g -1 ), [START_REF] Li | Porous azo-bridged porphyrin-phthalocyanine network with high iodine capture capability[END_REF] covalent organic polymers (COP 1 0 , 3.80 g g -1 ), [START_REF] Das | Multifunctional redox-tuned viologen-based covalent organic polymers[END_REF] and a state-of-the-art COF (TPB-DMTP COF, 6.26 g g -1 ). [START_REF] Wang | Exceptional iodine capture in 2D covalent organic frameworks[END_REF] Notably, although the specific surface area of JUC-561 (2358 m 2 g -1 ) is lower than that of some of reported porous materials, such as COF-320 (2400 m 2 g -1 ), NiP-conjugated microporous polymer (NiP-CMP, 2630 m 2 g -1 ), and porous carbon (Uassis-PC800, 3053 m 2 g -1 ), its iodine adsorption is higher (COF-320, 4.00 g g -1 ; 29 NiP-CMP, 2.02 g g -1 ; 30 and Uassis-PC800, 2.25 g g -1 ) 31 due to the synergistic effect of physical and chemical adsorption of iodine in JUC-561.

The adsorption kinetics of iodine in various materials were also summarized in Supplementary Table S3. In fact, the available relevant reports are limited due to low adsorption kinetics in most of previously reported materials. Remarkably, the adsorption kinetics of TTF-based COFs (0.48 g g -1 h -1 for JUC-560 and 0.69 g g -1 h -1 for JUC-561) is superior to those adsorbents reported, such as nitrogen-rich triptycenebased porous polymer (NTP, 0.11 g g -1 h -1 ), [START_REF] Ma | Nitrogen-rich triptycene-based porous polymer for gas storage and iodine enrichment[END_REF] TTA-TTB COF (0.15 g g -1 h -1 ), [START_REF] Wang | Exceptional iodine capture in 2D covalent organic frameworks[END_REF] NiP-CMP (0.17 g g -1 h - 1 ), [START_REF]Highly efficient and reversible iodine capture using a metalloporphyrin-based conjugated microporous polymer[END_REF] AzoPPN (0.18 g g -1 h -1 ), [START_REF] Li | Porous azo-bridged porphyrin-phthalocyanine network with high iodine capture capability[END_REF] and even more than 34 times higher than that (0.02 g g -1 h -1 ) of Ag-loaded zeolites applied in real environment. [START_REF] Chapman | Radioactive iodine capture in silver-containing mordenites through nanoscale silver iodide formation[END_REF] These results confirm that the strong physical and chemical interaction between TTF units and iodine is favorable to enhance iodine adsorption ability.

The iodine-adsorbed COFs can preserve iodine upon exposure to air under ambient conditions (25 °C and 1 bar), and do not display iodine escape from the frameworks (Fig. 3c and3d). Adsorbed iodine could be removed by immersing these samples in ethanol solution at room temperature. Iodine release was monitored by optical images and UV/Vis spectroscopy, leading to a linear increase based on the absorbance over time (Supplementary Figs. 29-36). After ethanol rinse, these COFs were recyclable and retained similar iodine uptakes (Fig. 3e and3f). FT-IR spectroscopy verified the chemical integrity of these COFs after cycled application (Supplementary Figs. 37 and38). These regenerated COFs maintained their crystallinity and porosity, as evidenced by their unchanged PXRD patterns and retained specific surface areas (Supplementary Figs. 39404142). These excellent cycle performances suggest that these materials are robust against oxidative iodine over long-term exposure.

Experimental study of the interaction between iodine and framework

Electron paramagnetic resonance (EPR) studies of the solid samples at 110 K confirmed the generation of TTF radical cations after iodine adsorption (Fig. 5a and5b). Clearly, the pristine samples showed a very weak EPR signal (black curve) while there is an approximately two orders of magnitude increase in paramagnetic intensity for the iodine-doped samples (red curve). After I 2 doping, sharp peaks were observed at g = 2.0182 for JUC-560 and 2.0189 for JUC-561 respectively, clearly demonstrating the presence of TTF radical cations from oxidation by iodine. [START_REF] Berridge | Incorporation of fused tetrathiafulvalenes (TTFs) into polythiophene architectures: varying the electroactive dominance of the TTF species in hybrid systems[END_REF] Furthermore, we tested the electrochemical behavior of JUC-560 and JUC-561 by cyclic voltammetry (CV). In Supplementary Figs.

43 and 44, two clear redox processes at 0.72 and 1.15 V vs Ag/AgCl for JUC-560 and 0.69 and 1.08 V vs Ag/AgCl for JUC-561 were observed in the reversible CV profiles, revealing that TTF units retain their redox activities in the frameworks and can be oxidized by iodine which has a reduction potential at 0.78 V. [START_REF] Qi | Electrochemical behaviour of gold in iodide solutions[END_REF] To investigate the existing state of iodine captured in TTF-based COFs, we conducted X-ray photoelectron spectroscopy (XPS). In Fig. 5c and5d, two conspicuous peaks located at 629.8 eV and 618.3 eV for JUC-560 as well as 630.0 eV and 618.6 eV for JUC-561, belonging to I 3d 3/2 and I 3d 5/2 orbital of iodine molecules respectively, were observed, which indicates that the captured iodine partially exists as a molecule. Furthermore, another two peaks at 630.9 eV and 619.6 eV for JUC-560 as well as 631.4 eV and 619.8 eV for JUC-561 were found, which are attributed to the formation of polyiodine anions (such as I 3 -and I 5 -). [START_REF] Hsu | Highly conducting iodine derivatives of polyacetylene: Raman, XPS and X-ray diffraction studies[END_REF] In addition, the species of captured iodine were detected by Raman spectroscopy. As shown in Fig. 5e and5f, the Raman spectra of TTF-based COFs showed no distinct peaks before iodine adsorption (black curve). However, after iodine capture, the low-frequency spectral region was dominated by three peaks at 107, 136 and 174 cm -1 for JUC-560 and 108, 135 and 181 cm -1 for JUC-561 (red curve). The peaks can be assigned to the symmetric and asymmetric stretching vibrations of I 3 -as well as the stretching vibrations of I 5 -, respectively. [START_REF] Harijan | Radioactive iodine capture and storage from water using magnetite nanoparticles encapsulated in polypyrrole[END_REF] Thus the results show that the adsorbed iodine species in TTF-based COFs exist as both iodine molecule and polyiodine anions, and the process is the combination of physisorption and chemisorption.

Theoretical study of the interaction between iodine and framework

To gain insight into how the COF frameworks bind iodine, we performed DFT calculations with the M06-2X-D3 function based on Gaussian 09 package (Revision D.01). [START_REF] Frisch | Revision D.01[END_REF] Full geometry optimizations of model compounds, TTF•I 2 , TTF •+ •I 2 , TTF •+ •(I -), TTF •+ •(I 3 -), and TTF •+ •(I 5 -), offered related systems with the I 2 molecule, I -, I 3 -or I 5 -anion situating on the center of TTF or TTF •+ unit (Supplementary Fig. 45).

No symmetry constraints were imposed during the optimizations. The standard def2-SVP basis set was used for geometry optimizations, and the vibration analysis and def2-TZVP basis set was applied for calculating single point energy. The binding energy of I 2 molecule to the TTF or TTF •+ unit calculated at the M06-2X-D3 level was low (-8.3 kcal mol -1 and -7.1 kcal mol -1 respectively). However, the association of I -anion with the TTF •+ unit showed a large binding energy as high as -74.6 kcal mol -1 , which is the result of the electrostatic interaction between the anions and the positively charged viologen. [START_REF] Das | Multifunctional redox-tuned viologen-based covalent organic polymers[END_REF] Similarly, the I 3 -or I 5 -anion also displayed a relatively high binding energy of -70.6 kcal mol -1 or -65.3 kcal mol -1 to TTF •+ unit, respectively. Thus, these results suggest that TTF molecule reducing to its diradical form leads to the change of I 2 to I -, I 3 -or I 5 -anion, which at last is tightly bound to the framework of COFs through electrostatic interactions.

Outlook

By developing TTF-based COF materials, we have successfully obtained high iodine adsorption capacity and quick iodine adsorption kinetics. These results point out that the integration of virtues of physical and chemical adsorption is of paramount importance in the designed synthesis of porous materials for the very challenging capture of radioiodine. The pre-designed COF frameworks exhibiting synergy between physisorption and chemisorption showed a dual impact on the iodine adsorption: substantially improved total iodine capacity, in particular the 3D COF (JUC-561); and accelerated transport rate due to their strong interaction between functionalized units and iodine. Consequently, the TTF-based COFs achieved the highest iodine uptake (~ 8.15 g g -1 ) and adsorption kinetics (~ 0.69 g g -1 h -1 ), which are far superior to other materials reported so far, such as silver-doped adsorbents, inorganic porous materials, MOFs, POFs, and other COFs. The set of experimental results thus demonstrate the potential of functionalized COF materials as a platform for radioiodine capture and storage. Some prospective directions for future work can be exploited to obtain efficient iodine adsorption.

COF materials feature open pores and excellent specific surface areas, and thus hold great promises in iodine adsorption. Unlike 2D structures with characteristic eclipsed AA stacking, 3D COFs have attracted widespread attentions owing to their interconnected channels and impressive specific surface areas.

Therefore, the development of 3D COFs with different architectures are interesting for iodine capture due to their unique pore shape and pore environment. In addition, COFs are well-defined in which their organic building units are precisely integrated into extended structures, and thus they can be easily functionalized to obtain strong chemisorption of iodine. A number of functionalized groups as electron donors, such as carbazole and ionic liquid, can be decorated into the frameworks by bottom-up or postsynthetic approaches. Most importantly, by the integration of higher specific surface areas and abundant functional units, we predict that the iodine adsorption in COF materials could be further improved. These functionalized porous frameworks would be applicable in environmental protection for the capture of radioiodine and other hazardous substances.

References:

spectroscopy. Solid-state [START_REF] Côté | Porous, crystalline, covalent organic frameworks[END_REF] C NMR spectroscopy was recorded on an AVIII 500 MHz solid-state NMR spectrometer. FT-IR spectroscopy (KBr) was obtained using a SHIMADZU IRAffinity-1 Fourier transform infrared spectrophotometer. Thermogravimetric analysis (TGA) was recorded on a SHIMADZU DTG-60 thermal analyzer under N 2 . The operational range of the instrument was from 30 °C to 600 °C at a heating rate of 10 °C min -1 with N 2 flow rate of 30 mL min -1 . PXRD data were collected on a PANalytical B.V. Empyrean powder diffractometer using a Cu Kα source (λ = 1.5418 Å) over the range of 2θ = 2.0-40.0° with a step size of 0.02° and 2 s per step. The sorption isotherm for N 2 was measured by using a Quantachrome Autosorb-IQ analyzer with ultra-high-purity gas (99.999% purity). Before gas adsorption measurements, the as-synthesized COFs (~50.0 mg) were immersed in DMF for 12 h (3  5.0 ml) and then acetone for another 36 h (3  5.0 ml). The acetone was then extracted under vacuum at 85 °C to afford the samples for sorption analysis. To estimate pore size distributions for TTF-COFs, nonlocal density functional theory (NLDFT) was applied to analyze the N 2 isotherm on the basis of the model of N 2 @77K on carbon with slit pores and the method of non-negative regularization.

For scanning electron microscopy (SEM) images, JEOL JSM-6700 scanning electron microscope was applied. The transmission electron microscopy (TEM) images were obtained on JEM-2100 transmission electron microscopy. The Electrochemistry experiments were conducted on a CHI660C Electrochemical Workstation (Shanghai Chenhua Electrochemical Instrument). Electron paramagnetic resonance (EPR) spectra were measured on JES-FA200. Raman scattering spectra were measured on INVIA, and the Raman band of the silicon wafer at 520.7 cm -1 was used to calibrate the spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 XPS system. Synthesis of JUC-560. DTDA (0.05 mmol, 17.3 mg) and TFP-TTF (0.025 mmol, 18.6 mg) were weighted into a Pyrex tube (volume: ca. 20.0 ml with a body length of 18.0 cm and neck length of 9.0 cm). Then, the mixture of mesitylene (0.7 mL), 1,4-dioxane (0.3 mL) and 0.1 ml of aqueous acetic acid (6.0 mol/L) was added. The Pyrex tube was flash-frozen in a liquid nitrogen bath and evacuated to an internal pressure of ca. 19.0 mbar and flame-sealed, reducing the total length by ca. 10.0 cm. Upon warming to room temperature, the tube was placed in an oven at 120 °C for three days. As a result, a pale brown powder was isolated by centrifugation and washed with acetone (3  5.0 ml) and the yield is about Synthesis of JUC-561. TAPA (0.04 mmol, 13.4 mg) and TFP-TTF (0.03 mmol, 21.7 mg) were weighted into a Pyrex tube (volume: ca. 20.0 ml with a body length of 18.0 cm and neck length of 9.0 cm). Then, the mixture of o-dicholobenzene (0.2 mL), n-butanol (0.8 mL), and aqueous acetic acid (9.0 mol/L, 0.1 mL) was added. The Pyrex tube was flash-frozen in a liquid nitrogen bath and evacuated to an internal pressure of ca. 19.0 mbar and flame-sealed, reducing the total length by ca. 10.0 cm. Upon warming to room temperature, the tube was placed in an oven at 120 °C for five days. As a result, a pale brown powder was isolated by centrifugation and washed with acetone (3  5.0 ml) and the yield is about 81%. General procedure for iodine vapor sorption. An open small vial (2.0 mL) containing the COF sample (20.0 mg) was placed in a large vial (10.0 mL) containing iodine (1.0 g). The large vial was sealed and kept in an oven at 60 °C. After a certain period, the large vial was cooled to room temperature. The small vial containing the COF sample was weighted and placed back into the iodine-containing large vial. The large vial was sealed and put back in the oven at 60 °C to continue the adsorption till the mass of the small vial containing the COF sample did not change.

General procedure for recycling the COF samples. The iodine-captured COF sample was added to ethanol (20.0 mL) in a vial at 25 °C, and ethanol was refreshed every 2 h until no color of the solution was observed. The COF sample was collected by filtration, washed with ethanol, dried under vacuum at 120 °C overnight and reused for the next cycle.

General procedure for iodine escape investigation. An open vial (2.0 mL) containing the iodine-loaded COF sample was placed in a large empty vial (10.0 mL). The large vial was sealed and kept in room temperature. After a certain period, the small vial containing the COF sample was weighted and placed back into the large empty vial. The large vial was sealed to continue the desorption. The investigation was carried out for a period of 8 days.

Cyclic voltammetry (CV) measurements. After grinding each as-synthesized COF (6.0 mg) and carbon black (4.0 mg) in agate mortar and pestle for 10 mins, the grinded powder was subjected to a mixture of water (0.25 mL), ethanol (0.25 mL) and Nafion soluion (50.0 µL) and sonicated for 2 h. Then the electrode was prepared by drop-casting such mixture onto the surface of carbon fiber which was dried before use. The CV measurements were performed using an electrochemical analyzer CHI 760i E and a standard three-electrode setup with a carbon working electrode, platinum wire auxiliary electrode and Ag/AgCl as the reference electrode. The scan rate was 20 mV•s -1 with potential range 0 V to 1.5 V in supporting electrolyte (0.1 M NBu 4 PF 6 in CH 2 Cl 2 ).

Figures and captions:

Fig. 1

 1 Fig. 1 Schematic Representation of the Strategy for Preparing TTF-based COFs. (a-c) Molecular

Fig. 2

 2 Fig. 2 PXRD profiles and porosity. (a and b) PXRD patterns of JUC-560 (a) and JUC-561 (b). (c and

Fig. 3

 3 Fig. 3 Study of iodine capture. (a and b) Uptake of iodine of JUC-560 (a) and JUC-561 (b) as a function

Fig. 4

 4 Fig. 4 Comparison of iodine adsorption capacity in different adsorbents. A plot of iodine adsorption

Fig. 5

 5 Fig. 5 Spectroscopy study. (a and b) EPR spectroscopy of JUC-560 (a) and JUC-561 (b) before (black

  83%. Anal. Cald for C 37 H 26 N 2 S 2 : C: 78.97; H: 4.66; N: 4.98; S: 11.39. Found: C: 79.12; H: 4.61; N: 5.06; S: 11.21. Solid-state 13 C NMR (500 MHz)  (ppm): 159.4, 150.8, 141.1, 131.9, 121.2, 20.2. FT-IR: (KBr), ν/cm -1 : 3024.24, 2866.98, 2733.86, 2361.54, 1774.92, 1603.03, 1555.49, 1483.80, 1307.52, 1102.72, 1012.02, 840.13.

Acknowledgements

We thank National Natural Science Foundation of China (21571079, 21621001, 21390394, 21571076, 21673094, and 21571078), 111 project (BP0719036 and B17020), the program for JLU Science and

Technology Innovative Research Team for funding, and the Computing Center of Jilin Province for supercomputer time. We thank Prof. L. Chen for help at SEM images. We thank Prof. J. Jiang for help at EPR spectroscopy. We thank Prof. C. Hou for help at XPS spectroscopy. We thank Prof. S. Xu for help at Raman spectroscopy.

Author contributions

J.C. and H.L. performed the synthesis and characterization of COF samples and measurements of nitrogen and iodine adsorption. X.G., C.L. and G.Y. performed the DFT calculations. Q.F. led the overall design and direction of the project. Q.F., V.V., Y.Y., and S.Q. prepared the manuscript with help from all authors.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information is available for this paper.

Correspondence and requests for materials should be addressed to Q.F.

Methods

Materials. All starting materials and solvents, unless otherwise noted, were obtained from J&K scientific LTD. The reagents and purity of the solvents were of 95% and used without further purification. TFP-TTF, DTDA, and TAPA were synthesized using a modified literature method. All products were isolated and handled under nitrogen using either glovebox or Schlenk line techniques.

Instruments. A Bruker AV-400 NMR spectrometer was applied to record the liquid 1 H NMR