Computational Challenges in Simulating and Analyzing Experimental Linear and Nonlinear Circular Dichroism Spectra. R-(+)-1,1′-Bis(2-naphthol) as a Prototype Case

Pentacene-Bridge Interactions in an Axially Chiral Binaphthyl Pentacene Dimer

Molecular chirality can be exploited as a sensitive reporter of the nature of intra- and interchromophore interactions in π-conjugated systems. In this report, we designed an intramolecular singlet fission (iSF)-based pentacene dimer with an axially chiral binaphthyl bridge (2,2'-(2,2'-dimethoxy-[1,1'-binaphthalene]-3,3'-diyl) n-octyl-di-isopropyl silylethynyl dipentacene, BNBP) to utilize its chiroptical response as a marker of iSF chromophore-bridge-chromophore (SFC-β-SFC) interactions. The axial chirality of the bridge enforces significant one-handed excitonic coupling of the pentacene monomer units; as such, BNBP exhibits significant chiroptical response in the ground and excited states. We analyzed the chiroptical response of BNBP using the exciton coupling method and quadratic response density functional theory calculations to reveal that higher energy singlet transitions in BNBP involve significant delocalization of the electronic density on the bridging binaphthyl group. Our results highlight the promising application of chiroptical techniques to investigate the nature of SFC-β-SFC interactions that impact singlet fission dynamics.

Excited-State Absorption of Uracil in the Gas Phase: Mapping the Main Decay Paths by Different Electronic Structure Methods

We present a computational study of the one-photon and excited-state absorption (ESA) from the two lowest energy excited states of uracil in the gas phase: an nπ* dark state (1n) and the lowest energy bright ππ* state (1π). The predictions of six different linear response electronic structure methods, namely, TD-CAM-B3LYP, EOM-CCSD, EOM-CC3, ADC(2), ADC(2)-x, and ADC(3) are critically compared. In general, the spectral shapes predicted by TD-CAM-B3LYP, EOM-CCSD, EOM-CC3, and ADC(3) are fairly similar, though the quality of TD-CAM-B3LYP slightly deteriorates in the high-energy region. By computing the spectra at some key structures on different potential energy surfaces (PES), that is, the Franck-Condon point, the 1n minimum, and structures representative of different regions of the 1π PES, we obtain important insights into the shift of the ESA spectra, following the motion of the wavepacket on the excited-state PES. Though 1π has larger ESA than 1n, some spectral regions are dominated by these latter signals. Aside from its methodological interest, we thus obtain interesting indications to interpret transient absorption spectra to disentangle the photoactivated dynamics of nucleobases.

Unveiling Excited-State Chirality of Binaphthols by Femtosecond Circular Dichroism and Quantum Chemical Calculations

Time-resolved circular dichroism (TR-CD) is a powerful tool for probing conformational dynamics of biomolecules over large time scales that are crucial for establishing their structure-function relationship. However, such experiments, notably in the femtosecond regime, remain challenging due to their extremely weak signals, prone to polarization artifacts. By using binol and two bridged derivatives (PL1 and PL2) as chiral prototypes, we present here the first comprehensive study of this type in the middle UV, combining femtosecond TR-CD and quantum mechanical calculations (TD-DFT). We show that excitation of the three compounds induces large variations of their transient CD signals, in sharp contrast to those of their achiral transient absorption. We demonstrate that these variations arise from both the alteration of the electronic distribution and the dihedral angle in the excited state. These results highlight the great sensitivity of TR-CD detection to signals hardly accessible to achiral transient absorption.

Ab initio study of the enantio-selective magnetic-field-induced second harmonic generation in chiral molecules

We present a systematic ab initio study of enantio-selective magnetic-field-induced second harmonic generation (MFISHG) on a set of chiral systems ((l)-alanine, (l)-arginine and (l)-cysteine; 3,4-dehydro-(l)-proline; (S)-α-phellandrene; (R,S)- and (S,S)-cystine disulphide; N-(4-nitrophenyl)-(S)-prolinol, N-(4-(2-nitrovinyl)-phenyl)-(S)-prolinol, N-(4-tricyanovinyl-phenyl)-(S)-prolinol, (R)-BINOL, (S)-BINAM and 6-(M)-helicene). The needed electronic frequency dependent cubic response calculations are performed within a density functional theory (DFT) approach. A study of the dependence of the property on the choice of electron correlation, on one-electron basis set extension and on the choice of magnetic gauge origin is carried out on a prototype system (twisted oxygen peroxide). The magnetic gauge dependence analysis is extended also to the molecules of the set. An attempt to analyze the structure-property relationships is also made, based on the results obtained for biphenyl (in a frozen twisted conformation), for prolinol and for some of their derivatives. The strength of the effect is discussed, in order to establish its measurability with a proposed experimental setup.

A complex-polarization-propagator protocol for magneto-chiral axial dichroism and birefringence dispersion

A schematic representation of magneto-chiral effects.

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

Effect of the strength and nature of the extension of the π-electronic delocalization on TPA and TPCD of helicenes derivatives.

Theoretical study of two-photon circular dichroism on molecular structures simulating aromatic amino acid residues in proteins with secondary structures

Calculation and comparative analysis of the theoretical two-photon circular dichroism (TPCD) spectra of l-His, l-Phe, and l-Tyr simulating residues in proteins with secondary structures (α-helix, β-strand and random coil), down to the far-UV region (FUV).

Ab initio study of the circular intensity difference in electric-field-induced second harmonic generation of chiral natural amino acids

We present a systematic computational study of circular intensity difference (CID) in electric-field-induced second harmonic generation (EFISHG) of some representative chiral natural amino acids (Alanine, Arginine, Aspartic Acid, Cysteine and Tryptophan), taking into account the electric-dipole, electric-quadrupole and magnetic-dipole interactions. The calculations are performed by applying cubic response theory at both Hartree-Fock and Density Functional theory levels, the latter with the popular Becke-three parameters, Lee, Yang and Parr (B3LYP) functional. Special focus is given to the basis set, electron correlation and origin dependence of the properties. The full set of molecular parameters defined by Lam and Thirunamachandran in their reference theoretical paper published in 1982 [J. Chem. Phys., 1982, 77, 3810] is obtained and discussed. This permits the prediction of the CID observable for different possible experimental setups.

The Effect of the π-Electron Delocalization Curvature on the Two-Photon Circular Dichroism of Molecules with Axial Chirality

Herein we report on the theoretical-experimental study of the effect of curvature of the π-electron delocalization on the two-photon circular dichroism (TPCD) of a family of optically active biaryl derivatives (S-BINOL, S-VANOL, and S-VAPOL). The comparative analysis of the influence of the different transition moments to their corresponding TPCD rotatory strength reveals an enhanced contribution of the magnetic transition dipole moment on VAPOL. This effect is hereby attributed to the additional twist in the π-electron delocalization on this compound. TPCD measurements were done using the double L-scan technique in the picosecond regime. Theoretical calculations were completed using modern analytical response theory, within a time-dependent density functional theory (TD-DFT) approach, at both, B3LYP and CAM-B3LYP levels, with the aug-cc-pVDZ basis set for S-BINOL and S-VANOL, and 6-31G* for S-VAPOL. Solvent effects were included by means of the polarizable continuum model (PCM) in CH2Cl2.

Two-photon polarization dependent spectroscopy in chirality: a novel experimental-theoretical approach to study optically active systems

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 C(2)-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.