Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.

Excitons in nanoscale systems

The current state of understanding of molecular resonance energy transfer (RET) and recent developments in the field are reviewed. The development of more general theoretical approaches has uncovered some new principles underlying RET processes. This review brings many of these important new concepts together into a generalization of Forster's original theory. The conclusions of studies investigating the various approximations in Forster theory are summarized. Areas of present and future activity are discussed. The review covers Forster theory for donor-acceptor pairs and electronic coupling for singlet-singlet, triplet-triplet, and superexchange-mediated energy transfer. This includes the transition density picture of Coulombic coupling as well as electronic coupling between molecular aggregates (excitons). Spectral overlaps and ensemble energy transfer rates in disordered aggregates, the role of dielectric properties of the medium, weak versus strong coupling, and new models for energy transfer in complex molecular assemblies are also described.

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Long-Range Resonance Energy Transfer in Molecular Systems

Triplet excitons in organic molecules underscore a variety of processes and technologies as a result of their long lifetime and spin multiplicity Organic phosphorescence, which originates from triplet excitons, has potential for the development of a new generation of organic optoelectronic materials and biomedical agents However, organic phosphorescence is typically only observed at cryogenic temperatures and under inert conditions in solution, which severely restricts its practical applications In the past few years, room-temperature-phosphorescent systems have been obtained based on organic aggregates Rapid advances in molecular-structure design and aggregation-behaviour modulation have enabled substantial progress, but the mechanistic picture is still not fully understood because of the high sensitivity and complexity of triplet-exciton behaviour This Review analyses key photophysical processes related to triplet excitons, including intersystem crossing, radiative and non-radiative decay, and quenching processes, to illustrate the intrinsic structure–property relationships and draw clear and integrated design principles The resulting strategies for the development of efficient and persistent room-temperature-phosphorescent systems are discussed, and newly emerged applications based on these materials are highlighted Advances in molecular-structure design and modulation of the aggregation behaviour have enabled much progress in the observation of room-temperature phosphorescence from organic aggregates This Review analyses key photophysical processes related to triplet excitons, illustrating the intrinsic structure–property relationships and identifying strategies to design efficient and persistent room-temperature-phosphorescent systems

Room-temperature phosphorescence from organic aggregates

H. Wang, L. Meng, Prof. Y. Wang, Prof. P. Wang Key Laboratory of Photochemical Conversion and Optoelectronic Materials Technical Institute of Physics and Chemistry Chinese Academy of Sciences Beijing 100190 , China E-mail: wangy@mail.ipc.ac.cn; wangpf@mail.ipc.ac.cn H. Wang, L. Meng University of Chinese Academy of Sciences Beijing 100049 , China Dr. Q. Peng, Prof. Y. Yi Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190 , China L. Xie College of Life Science and Technology Beijing University of Chemical Technology Beijing , China

Novel Thermally Activated Delayed Fluorescence Materials–Thioxanthone Derivatives and Their Applications for Highly Efficient OLEDs

Polysilsesquioxanes are regarded as organic–inorganic hybrid materials at a molecular level. We can readily tune the properties of the materials for realizing the desired ones by changing the contents or types of organic components. In addition, the hierarchical structures have potential for generating further characteristics distinctly different from those of the constituents. From this viewpoint, polyhedral oligomeric silsesquioxane (POSS) is attractive as a cornerstone in highly ordered materials. In this review, we survey recent studies on modified POSS-based functional materials and particularly explain the design concepts for applying the significant characteristics of POSS for the material properties. In the initial sections, we introduce the examples concerning intrinsic properties of POSS such as thermal stability, mechanical property, and structural features. In the latter sections, we describe the application of POSS-based materials for bio-relating functional materials. We describe the validity of POSS for molecular designing of the advanced imaging tools.

Advanced functional materials based on polyhedral oligomeric silsesquioxane (POSS)

This paper deals with triplet—triplet annihilation in pure and mixed organic crystals. In crystals containing a small concentration of impurity traps, triplet excitation migration may proceed from trap to trap on a time scale which is short compared with the long triplet state lifetime but which is long compared with the normal fluorescence lifetime. Nearest‐neighbor and long‐range mutual annihilation of two triplets may then take place giving rise to delayed fluorescence. The rates of long‐range triplet excitation migration and annihilation show a concentration dependence, a temperature dependence, and a solvent dependence. Providing the triplet—triplet annihilation rate is not too fast, the intensity of the delayed fluorescence can be shown to depend upon the square of the intensity of the exciting light. This expectation is borne out by experiments, briefly reported here, on delayed fluorescence in dilute isotopic mixed crystals. In crystals containing high concentrations of such impurity traps, or in pure crystals, the annihilation rate becomes extremely rapid and this mechanism effectively quenches phosphorescence in many, but not all, classes of pure organic crystals. The kinetics of the over‐all process are discussed in both the limits of fast and slow annihilation rates. A theoretical investigation of the origin of the annihilation matrix element is carried out, and it is shown that exchange interactions play the largest role in determining annihilation rates. Past work [H. Sponer, Y. Kanda, and L. A. Blackwell, J. Chem. Phys. 29, 721 (1958); N. W. Blake and D. S. McClure, ibid., p. 722] on delayed fluorescence of assumed pure naphthalene crystals containing very small amounts of β‐methyl naphthalene (traps) can be understood within the framework of this paper. The need in organic crystals for the major delayed fluorescence mechanism to be based upon ionization and electron trapping seems now to be considerably lessened.

Triplet—Triplet Annihilation and Delayed Fluorescence in Molecular Aggregates

Errata: Triplet—Triplet Annihilation and Delayed Fluorescence in Molecular Aggregates; and Comments Concerning Ruby‐Laser‐Induced Fluorescence in Anthracene Crystals

H. Sternlicht

Papers

Triplet—Triplet Annihilation and Delayed Fluorescence in Molecular Aggregates

Errata: Triplet—Triplet Annihilation and Delayed Fluorescence in Molecular Aggregates; and Comments Concerning Ruby‐Laser‐Induced Fluorescence in Anthracene Crystals