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

Abstract

A combined experimental-computational analysis of the one- and two-photon absorption (OPA, TPA) and the electronic circular dichroism (ECD) of complex chiral shape-persistent (2,6)-pyrido[142 ]allenoacetylenic cyclophane, (P,P,P,P)-MC[142 ] enantiomers, sheds light on the origin of their peculiar spectral signatures. The study on MC[142 ], which is represented in solution by three possible conformers, indicates that two of them (chair and twist) are the prevalent conformers at room temperature; the population ratio depending on the solvent. The complex shape of the red-edge of the UV and ECD spectra is qualitatively reproduced by using vibronic calculations and assigned to progressions on the ethynyl stretchings observable only in the chair and twist conformers. The picture arising from the analysis of the linear spectra is confirmed by two-photon spectroscopies, showing that they constitute valuable tools for the exploration of high-energy electronic transitions in pyridoallenoacetylenic cyclophanes. Discrepancies among computed and experimental spectra point out that an accurate description of such complex systems with a large delocalized π framework and a dense manifold of electronic excited state remains a challenge for theoretical models.

Going beyond the vertical approximation with time‐dependent density functional theory

“…While up to now we focused on one‐photon absorption (OPA) and emission, the model PESs described in the previous sections can be used, in combination with TD‐DFT data, to simulate many other achiral and chiral, linear and nonlinear spectroscopies. This includes Electronic Circular Dichroism (ECD), Magnetic Circular Dichroism (MCD), Circularly Polarized Luminescence (CPL), Near Edge X‐ray Absorption Fine Structure (NEXAFS), Two‐Photon Absorption (TPA), and Two‐Photon Circular Dichroism (TPCD). Going in further detail, for nonlinear processes some additional approximations are required in order to write the spectra in a form equivalent to Eqs or .…”

Predicting the Helical Sense of Poly(phenylacetylene)s from their Electron Circular Dichroism Spectra

“…For a better match of the CD traces and in order to correct the TD‐DFT overestimation of the excitation energies, theoretical wavelengths from the calculated ECD and UV spectra were rescaled using the experimental CD/UV spectra as reference (Figure c,d) . Additionally, the wavelength and intensity in the stationary point of the first Cotton effect were also adjusted (see the Supporting Information).…”

“…The 3‐21G basis set and the rCAM‐B3LYP functional were selected to perform the calculations after carrying out a basis set and a DFT functional selection study on PPAs with nine monomers. In the former investigation we used the 3‐21G, 6‐31G, 6‐31G* and 6‐31G** bases, and in the latter the B3LYP, the coulomb attenuated method B3LYP (CAM‐B3LYP), the revised version of CAM‐B3LYP (rCAM‐B3LYP), and Grimme's reparametrization of the B97‐1 functional (B97D) density functionals. When comparing the 3‐21G spectra with those obtained with the larger basis sets, we obtained insignificant differences; and among the functionals, those performing best were CAM‐B3LYP and rCAM‐B3LYP.…”

How and How Much Molecular Conformation Affects Electronic Circular Dichroism: The Case of 1,1-Diarylcarbinols

“…The dependence of ECD on conformational factors has been discussed in several instances, especially in combination with ECD quantum-chemical calculations [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. In fact, it is easy to demonstrate “in silico” that different conformations of the same molecule exhibit different chiroptical responses.…”