Chiral Spectroscopy of Nanostructures

Conspectus Chirality is ubiquitous in the universe andin living creaturesover detectable length scales from the subatomic to the galactic,as exemplified in the two extremes by subatomic particles (neutrinos)and spiral galaxies. Between them are living creatures that displaymultiple levels of chirality emerging from hierarchically assembledasymmetric building blocks. Not too far from the bottom of this pyramidare the foundational building blocks with chiral atomic centers on sp (3) carbon atoms exemplifiedby l-amino acids and d-sugars that are self-assembledinto higher-order structures with increasing dimensions forming highlycomplex, amazingly functional, and energy-efficient living systems.The organization and materials employed in their construction inspiredscientists to replicate complex living systems via the self-assemblyof chiral components. Multiple studies pointed to unexpected and uniqueelectromagnetic properties of chiral structures with nanoscale andmicroscale dimensions, including giant circular dichroism and collectivecircularly polarized scattering that their constituent units did notpossess. To address the wide variety of chiral geometries observedin continuousmaterials, singular particles, and their complex systems, multipleanalytic techniques are needed. Simultaneously, their spectroscopicproperties create a pathway to multiple applications. For example,mirror-asymmetric vibrations at chiral centers formed by sp (3) carbon atoms lead to optical activityforthe infrared (IR) wavelength regions. At the same time, understandingthe optical activity in, for example, the IR region enables biomedicalapplications because multiple modalities of biomedical imaging andvibrational optical activity (VOA) of biomolecules are known for IRrange. In turn, VOA can be realized in both absorption and emissionmodalities due to large magnetic transition moments, as vibrationalcircular dichroism (VCD) or Raman optical activity (ROA) spectroscopy.In addition to the VOA, in the range of longer wavelengths, latticevibrational mode or phononic behavior occurs in chiral crystals andnanoassemblies, which can be readily detected by terahertz circulardichroism (TCD) spectroscopy. Meanwhile, chiral self-assembly caninduce circularly polarized light emission (CPLE) regardless of theexistence of chirality in coassembled fluorophores. The CPLE fromself-assembled chiral materials is particularly interesting becausethe CPLE can originate from both circularly polarized luminescenceand circularly polarized scattering (CPS). Furthermore, because self-assemblednanostructures often exhibit stronger optical activity than theirbuilding blocks owing to dimension and resonance effects, the opticalactivity of single assembled nanostructures can be investigated byusing microscopic technology combined with chiral optics. Here, wedescribe the state of the art for spectroscopic methods for the comprehensiveanalysis of chiral nanomaterials at various photon wavelengths, addressedwith special attention given to new tools emerging both for materialswith self-organized hierarchical chirality and single-particle spectroscopy.