Aggregology: Exploration and innovation at aggregate level

An aggregate is not a simple mixture of its molecular constituents. Understanding the operations and interplays of various forces and processes involved in an aggregate system is of great academic value and has far-reaching practical implications. We are launching Aggregate with the aim of building a new platform for aggregology study and in the hope to open up new avenues for explorations at higher levels of structural hierarchy and system complexity. I am pleased to announce the inauguration of Aggregate, a new journal devoted to aggregate science or aggregology and co-published by South China University of Technology (SCUT), AIE Institute, and Wiley. To understand the nature, scientists have viewed the world from all different angles. Various research frameworks have been built according to the level of inquiry: for example, in the area of materials research, macro- and microsciences are the platforms for studying bulk substances and molecular species, respectively. A philosophical linkage between the macro- and microsciences is the reductionism conjecture, assuming that every macrosystem (e.g., a substance), no matter how complex it is, is reducible to simple microelements (e.g., molecules). The enthusiastic efforts of scientists over the past centuries along this line of epistemology have led to the booming development and firm establishment of molecular science. In 1873, James C. Maxwell delivered a lecture before the British Association at Bradford, articulating that "A molecule is the smallest possible portion of a particular substance."[1] Merriam-Webster, an authoritative English dictionary, defines a molecule as "the smallest particle of a substance that retains all the properties of the substance."[2] This reductionism doctrine has placed molecules in the central position of scientific research. To learn how a single molecule behaves, experiments are usually conducted in extremely dilute solutions in order to circumvent the interference from molecular interactions. Many scientific laws, rules, theorems, equations, formulas, etc., have been derived and developed from experimental data in dilute solutions; for example, the Beer's law governing light transmission through, or photon absorption by, isolated molecular species. The Beer's law, however, becomes invalid in a concentrated solution when molecules are aggregated. In some extreme cases, molecules and aggregates show completely different behaviors or properties. Many chromophores, for example, are luminescent in dilute solutions as single molecules but become less emissive or even totally nonluminescent when the molecules are aggregated. This process is widely known as aggregation-caused quenching (ACQ). On the other hand, some luminogens exhibit aggregation-induced emission (AIE), which is diametrically opposed to the ACQ effect. The AIEgens are nonluminescent as free molecules but become emissive when aggregated. The ACQ and AIE effects demonstrate that a molecular quality can disappear (1→0) and a new materials property can emerge (0→1) at the aggregate level, respectively. These examples are in sharp contrast to the general belief that molecular behaviors dictate materials properties (1→1). The properties of an aggregate are not necessarily a simple, linear addition of those of its molecular constituents or elementary particles. In as early as the classical period, Greek philosopher Aristotle already advocated his emergentism or constructionism idea that "The whole is greater than the sum of its parts." In a Regents' Lecture given in 1967 at the University of California in La Jolla, Philip W. Anderson voiced his holism opinion that "The behavior of large and complex aggregates of elementary particles is not to be understood in terms of a simple extrapolation of the properties of a few particles."[3] When many molecular species are admixed or assembled into an aggregate, its properties will be affected in a complex fashion by different factors, such as quantity (number of components), geometry (size, shape and dimension), morphology (amorphous or crystalline), and interaction (attraction or repulsion). Decipherment of such a complex system calls for the development of aggregology, a new scientific framework for aggregate research.[4, 5] Although molecular science deals with elementary entities of isolated molecules free from molecular interactions, aggregate science or aggregology concerns complex systems where various forces, effects, channels, and processes (e.g., antogony, synergy, emergence, divergence, and multeity) are intricately convoluted. An aggregate is often a two- or multibody system, where kinetics, thermodynamics, amorphization, crystallization, flexibilization, rigidification, etc., may operate antagonistically or synergistically. A symbiotic aggregate may be generated in a guest–host, donor–acceptor, or sensitizer–emitter system when the constituents work in perfect synergy. Entirely new properties or functionalities, for example, clusteroluminescence and circularly polarized luminescence, may emerge from the clusterization of nonconjugated molecules or polymers and the helical assembly of achiral building blocks. An aggregate may produce multifaceted outputs and simultaneously perform multiple tasks, for example, polymorphism-associated multicolor luminescence and multimodality biosensing, imaging, and therapy. Understanding the operations and interplays of antagonism, synergism, emergentism, multiplicity, etc., in an aggregate system is of great fundamental importance and has far-reaching practical implications.[5, 6] The aggregology study will generate new models, hypotheses, diagrams, and theories and create new knowledge to boost our comprehension of natural processes and to solve the issues and problems unsolvable by the traditional reductionism approaches. The establishment of new fundamental principles and working mechanisms will enable rational design of novel aggregate systems and judicious development of advanced materials. With these considerations in mind, we are launching here this new journal of Aggregate, to offer a new stage or podium for disseminating new research discoveries and breakthroughs, discussing matters, challenges, and opportunities in the area, and exchanging views, opinions, and ideas for advancing aggregate study. Thanks to the great support from the international research community, we have quickly assembled a strong editorial team for Aggregate. I am thrilled to introduce Dr. Anjun Qin (Guangzhou) as Deputy Editor and Drs. Yuning Hong (Melbourne), Gen-ichi Konishi (Tokyo), Paul R. McGonigal (Durham), Kazuo Tanaka (Kyoto), Shuang-Quan Zang (Zhengzhou), and Yu Shrike Zhang (Boston) as associate editors of our journal. Dr. Qin is an expert in synthetic polymer chemistry and advanced functional materials. Dr. Hong's research is focused on chemical biology of protein folding or aggregation and biomedical applications of AIEgens. Dr. Konishi studies photochemistry and macromolecular chemistry. Dr. McGonigal works on supramolecular chemistry and functional organic materials. Dr. Tanaka is designing heteroatom-containing functional materials and organic–inorganic hybrids. Dr. Zang is an inorganic chemist with expertise in metal clusters and functional metal–organic frameworks. Dr. Zhang is a biomedical pioneer in the research area of engineered living systems. Aggregate will run under the auspices of SCUT, a top-ranking research university in China, AIE Institute, a newly built institute for advanced study in Huangpu, and Wiley, a prestigious multinational scientific publisher. We sincerely thank the financial support from SCUT, AIE Institute and Huangpu District, and the professional guidance and assistance from Wiley, especially Drs. José Oliveira and Xin Su, without which, the journal would have not been launched in such a short period of time. Aggregate strives to swiftly publish research articles with the highest quality. To reach a wide spectrum of audience, the papers published in Aggregate will be openly accessible. Given the great importance of aggregate science, we are confident that Aggregate will grow quickly and develop healthily. There are plentiful rooms and abundant opportunities in the mesoregion and the aggregate mesoscience will help fill the gaps between the macro- and microworlds.[5, 6] It is envisioned that aggregology study will lead to a paradigm shift in research epistemology and methodology and open up new avenues for explorations and innovations at higher levels of structural hierarchy and system complexity. Professor, South China University of Technology, and The Hong Kong University of Science & Technology Director, AIE Institute Editor-in-Chief, Aggregate