DOI: 10.1002/chem.200902286 Two-Photon Absorption Circular Dichroism: A New Twist in Nonlinear Spectroscopy Carlos Toro, [a] Leonardo De Boni, [a] Na Lin, [b] Fabrizio Santoro, [c] Antonio Rizzo, [c] and Florencio E. Hernandez* [a, d] Introduction Among several phenomena, homochirality seems to rule the activity of nearly all natural products in nature, from pro- teins, nucleic acids, lipids, sugars, and amino acids to hor- mones, antibiotics, and vitamins. [1] Because the specificity of ligand–receptor interactions plays a dominant role in life processes, a better understanding of the chiroptical proper- ties of optically active molecules has been fundamental for all studies in biological activity, essentially those associated with the origin of life itself. [2] Molecules that are not superimposable on their mirror image, such as pure enantiomers, present differences in linear absorption coefficients and refractive indices for left and right circularly polarized light. The former effect is the so-called circular dichroism (CD), whereas the latter is known as optical rotation (OR). [3] CD and OR are a mani- festation of the interaction between the radiation fields and the induced electric-dipole, magnetic-dipole and electric- quadrupole moments. Because the latter vanishes in isotrop- ic media due to rotational averaging, the former two in co- operation determine the intensity of a transition by the action of photons in solution. Because of the OR, chiral molecules rotate the plane of polarization of light, a well recognized effect known as optical activity. During the last two decades, CD and OR have facilitated remarkable progress in the understanding of principles un- derlying chirality and the development of new applications. CD has strongly contributed to the study of the physical- chemical and conformational properties of chiral molecules, proteins, and polypeptides, in solutions and at interfaces. [4] Nonetheless, the progress and current status of this method presents an important limitation: it is based on the linear absorption (one-photon absorption, 1PA) of optically active compounds, which is typically observed in the far and near UV region of the spectrum. In addition, when working in solution, the linear absorption of common organic solvents in the same spectral region usually overwhelms the CD signal from the sample of interest. Despite the success of techniques such as vibrational circular dichroism (VCD) [5] and Raman optical activity (ROA), [6] which have yielded major advances in the determination of the absolute config- uration of small molecules as well as in the elucidation of secondary structure in proteins, we believe that the analysis of interesting biological systems such as natural amino acid structures and the study of innovative optically active drugs soluble in organic solvents demands alternative approaches Keywords: chirality · circular di- chroism · two-photon absorption Abstract: Herein we report on the full experimental measurement of the two- photon absorption circular dichroism spectra of (S)-()-1,1’-bi(2-naphthol) and (R)-(+)-1,1’-bi(2-naphthol), their analysis, and theoretical support. The finding of new nonlinear optical fingerprints in chiral molecules offers new opportunities in the recognition and understanding of optically active systems in regions where CD could present strong limitations. [a] C. Toro, Dr. L. De Boni, Dr. F. E. Hernandez Department of Chemistry, University of Central Florida P. O. Box 162366, Orlando, Fl 32816-2366 (USA) E-mail: florenzi@mail.ucf.edu [b] Dr. N. Lin Department of Theoretical Chemistry, Royal Institute of Technology AlbaNova, 106 91 Stockolm (Sweden) [c] Dr. F. Santoro, Dr. A. Rizzo Istituto per i Processi Chimico-Fisici del Consiglio Nazionale delle Ricerche (IPCF-CNR) Area della Ricerca, G. Via Moruzzi 1, 56124 Pisa (Italy) [d] Dr. F. E. Hernandez The College of Optics and Photonics, CREOL University of Central Florida P. O. Box 162366, Orlando, Fl 32816-2366 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200902286.  2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 3504 – 3509 3504