Artificial Plasmonic Molecules and Their Interaction with Real Molecules Gilad Haran* ,† and Lev Chuntonov* ,‡ † Chemical and Biological Physics Department, Weizmann Institute of Science, Rehovot 760001, Israel ‡ Schulich Faculty of Chemistry, TechnionIsrael Institute of Technology, Haifa 3200008, Israel ABSTRACT: Plasmonic molecules are small assemblies of nanosized metal particles. Interactions between the particles modify their optical properties and make them attractive for multiple applications in spectroscopy and sensing. In this review, we focus on basic properties rather than on applications. Plasmonic molecules can be created using either nanofabrication methods or self-assembly techniques in solution. The interaction of plasmonic molecules with light leads to excitations that are classified using the concept of normal modes. The simplest plasmonic molecule is a dimer of particles, and its lowest energy excitation takes the form of a symmetric dipolar mode. More complex excitations take place when a larger number of particles is involved. The gaps between particles in a plasmonic molecule form hotspots in which the electromagnetic field is concentrated. Introducing molecules into these hotspots is the basis of a vast spectrum of enhanced spectroscopies, from surface-enhanced Raman scattering to surface-enhanced fluorescence and others. We show in this review how these spectroscopic methods can be used to characterize the fields around plasmonic molecules. Furthermore, the strong fields can be used to drive new phenomena, from plasmon-induced chemical reactions to strong coupling of quantum emitters with the plasmonic fields. We systematically discuss these phenomena, introducing in each case the theoretical basis as well as recent experimental realizations. CONTENTS 1. Introduction A 2. Plasmonic Molecules C 2.1. The Plasmonic Molecule Paradigm C 2.2. Coupling between Plasmonic Excitations in Nanoparticles C 2.3. Normal Modes and the Plasmon Hybrid- ization Theory G 2.4. Trimers and Larger Plasmonic Molecules I 2.5. Interference Effects in Plasmonic Spectral Lineshapes N 2.6. Quantum Effects in Plasmonic Molecules Q 2.7. Chirality Effects in Plasmonic Molecules Q 3. Interaction between Plasmonic Molecules and Their Neighbors T 3.1. Surface-Enhanced Raman Spectroscopy As a Probe of Plasmonic Fields T 3.2. Surface-Enhanced Raman Spectroscopy As a Probe of Chemical Reactions X 3.3. Surface-Enhanced Fluorescence, Infrared Absorption, and Nonlinear Optical Signals X 3.4. Strong Coupling of Plasmonic Molecules and Quantum Emitters AA 4. Conclusions and Outlook AC Appendix: Some Introductory Comments on the Methodology Used for Studying Plasmonic Mole- cules AC A.1. Theoretical Methods: Mie Theory and the Static Approximation AC A.2. Theoretical Methods: Plasmon Hybridization AD A.3. Theoretical Methods: Symmetry Analysis AE A.4. Experimental Methods: Optical Spectrosco- py of Individual Plasmonic Molecules AE A.5. Experimental Methods: Spectroscopy within the Electron Microscope AF Associated Content AF Special Issue Paper AF Author Information AF Corresponding Authors AF ORCID AF Notes AF Biographies AF Acknowledgments AF References AG 1. INTRODUCTION The constant demand for device miniaturization as well as for improving the resolution and lowering the detection limit in spectroscopic measurements initiated an extensive research in the field of plasmonics. 1−7 Conduction electrons on a metal surface, driven by the electromagnetic field of light, enhance the Received: October 27, 2017 Review pubs.acs.org/CR Cite This: Chem. Rev. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.chemrev.7b00647 Chem. Rev. XXXX, XXX, XXX−XXX