Two-Photon Polarization Dependent Spectroscopy in Chirality: A Novel Experimental-Theoretical Approach to Study Optically Active Systems

Abstract

Many phenomena, including life itself and its biochemical foundations are fundamentally rooted in chirality. Combinatorial methodologies for catalyst discovery and optimization remain an invaluable tool for gaining access to enantiomerically pure compounds in the development of pharmaceuticals, agrochemicals, and flavors. Some exotic metamaterials exhibiting negative refractive index at optical frequencies are based on chiral structures. Chiroptical activity is commonly quantified in terms of circular dichroism (CD) and optical rotatory dispersion (ORD). However, the linear nature of these effects limits their application in the far and near-UV region in highly absorbing and scattering biological systems. In order to surmount this barrier, in recent years we made important advancements on a novel non linear, low-scatter, long-wavelength CD approach called two-photon absorption circular dichroism (TPACD). Herein we present a descriptive analysis of the optics principles behind the experimental measurement of TPACD, i.e., the double L-scan technique, and its significance using pulsed lasers. We also make an instructive examination and discuss the reliability of our theoretical-computational approach, which uses modern analytical response theory, within a Time-Dependent Density Functional Theory (TD-DFT) approach. In order to illustrate the potential of this novel spectroscopic tool, we first present the experimental and theoretical results obtained in C2-symmetric, axially chiral R-(+)-1,1'-bi(2-naphthol), R-BINOL, a molecule studied at the beginning of our investigation in this field. Next, we reveal some preliminary results obtained for (R)-3,3′-diphenyl-2,2′-bi-1-naphthol, R-VANOL, and (R)-2,2′-diphenyl-3,3′-(4-biphenanthrol), R-VAPOL. This family of optically active compounds has been proven to be a suitable model for the structure-property relationship study of TPACD, because its members are highly conjugated yet photo-stable, and easily derivatized at the 5- and 6-positions. With the publication of these outcomes we hope to motivate more members of the scientist community to engage in state-of-the-art TPACD spectroscopy.

A Combined Experimental–Computational Investigation to Uncover the Puzzling (Chiro‐)optical Response of Pyridocyclophanes: One‐ and Two‐Photon Spectra

“…Purification by flash chromatography on silica gel (hexane/AcOEt 70:30) afforded (P,P,P,P)-D-OH (6 mg, 62 %) as aw hite solid. [a] 20 D =+1090 (c = 1.12 E À3 gmL À1 ,C HCl 3 ). 1…”

Damped response theory description of two-photon absorption

“…In Sec. V, we summarize the results of calculations on R-(+)-1,1 -bi(2-naphtol), or BINOL, a system which was recently the subject of a mixed experimental/theoretical study [70][71][72][73] of its two-photon absorption and circular dichroism 74,75 spectra. Section VI contains our conclusions and outlook.…”

Two-photon absorption and two-photon circular dichroism of hexahelicene derivatives: a study of the effect of the nature of intramolecular charge transfer

“…19 The latter is the nonlinear analogue of ECD, which is defined, in the degenerate case, as  are the two-photon absorption (TPA) cross-sections for LCPL and RPCL, respectively, measured at a specific wavelength  . [20][21][22] TPA is particularly advantageous over OPA in many applications because the typical excitation wavelength for this nonlinear absorption process is approximately two folds longer (degenerate case) than that necessary for OPA. Therefore, the linear absorption in the twophoton excitation region is negligible and scattering can be minimized.…”