From linear to cyclic oligoparaphenylenes: electronic and molecular changes traced in the vibrational Raman spectra and reformulation of the bond length alternation pattern

Abstract

Cyclic paraphenylenes, [n]CPPs, and linear paraphenylenes, [n]LPPs, formed by n benzenes, are investigated by Raman spectroscopy for n = 5 to 12 and density functional theory (DFT) for n = 4 to 20. The information on the experimental Raman frequencies and intensities, combined with DFT computations and reported X-ray diffraction structures, provides a consistent interpretation of the Raman spectra and allows establishing relevant structure-property trends. Structural and electronic effects such as benzene ring bending, inter-ring torsions, π-conjugation (aromaticity) and orbital energy gaps as a function of the linear elongation in [n]LPPs versus the macrocyclic curvature in [n]CPPs and of the molecular size (i.e., polymer limit) are systematically analyzed on the basis of the vibrational Raman properties. Changes in the BLA as an indicator of the degree of quinonoid character are analyzed and linked to the Effective Conjugation Coordinate (ECC) model. The BLA patterns involved in twisted and non-twisted conformations and in different species (bipolarons, quinonoid tautomers, and ECC active modes) are compared and their differences are discussed. This paper offers a unified interpretation of structural and electronic aspects in relation to the evolution from linear 1D π-systems to cyclic 2D structures.

Strain‐Promoted Reactivity of Alkyne‐Containing Cycloparaphenylenes

“…[39] To more accurately assess the bond strength of the bent alkyne in macrocycles 1-3,F ourier transform (FT)-Raman spectra were recorded of the neat solids (Figure 2b,T able 1). In agreement with previous reports of CPPs [40] and bent polyynes [41] the Raman bands are deactivated with increasing bending of the carbon backbone.M ore importantly,t he CCs tretching band successively shifts to smaller wavenumbers from 2232 cm À1 for DPA to 2182 cm À1 for the smallest macrocycle 1,which is lower than the value reported for cyclooctyne (ṽ CC 2206 cm À1 ). In agreement with previous reports of CPPs [40] and bent polyynes [41] the Raman bands are deactivated with increasing bending of the carbon backbone.M ore importantly,t he CCs tretching band successively shifts to smaller wavenumbers from 2232 cm À1 for DPA to 2182 cm À1 for the smallest macrocycle 1,which is lower than the value reported for cyclooctyne (ṽ CC 2206 cm À1 ).…”

High‐Pressure Chemistry and the Mechanochemical Polymerization of [5]‐Cyclo‐p‐phenylene

“…In the middle region =1150–1350 cm −1 there is a clear cut splitting of the two groups of bands. The intensity ratio of these two bands is closely related to the bond length alternation and the torsion . An initial increase of the frequency as a function of the pressure is well reproduced by the computations.…”

“…The BLA values are computed for the r , r 1 , and r 2 values as defined in Figure a, and they clearly reflect the aromatic versus quinonoid nature of the nanohoops, which, in turn, depends on a combination of factors, such as the bending angles (shown in Figures S13 and S14 in the Supporting Information) and thus, also depends on the local π conjugation. Given the highly strained nature of the [5]CPP molecules even at ambient pressure, the molecules display cyclic conjugation …”

“…Next, we turn to the highest frequency region that provides most of the information on the changes of the BLA . There are two main contributions (G and G ), represented as red and blue profiles in Figure , at =1555 and 1572 cm −1 , respectively.…”

Expanding the Chemical Space of Biocompatible Fluorophores: Nanohoops in Cells

“…[24][25][26][27] First, the bending of the π-system increases delocalization around the hoop due to induction of a small amount of quinoidal character in these strained systems. [28] Second, the hoop architecture forces neighboring aromatic units to have smaller dihedral angles than in an acyclic oligomeric system due to conformational constraints of the macrocyclic geometry, again leading to increased conjugation. [29] These two factors together results in a sizedependent fluorescence emission (lem) where the HOMOàLUMO gap narrows as nanohoop diameter decreases.…”