Exploring the Relation Between Intramolecular Conjugation and Band Dispersion in One-Dimensional Polymers

Abstract

Making use of the inherent surface anisotropy of different high index surface planes vicinal to the low index Au(111) orientation, one-dimensional polymers have been synthesized following established procedures from two different precursor molecules. The successful polymerization of both 4,4″-dibromo-p-terphenyl and 5,5′-dibromo-salophenato-Co(II) precursors into poly(p-phenylene) and poly-[salophenato-Co(II)], respectively, has been confirmed by scanning tunneling microscopy and low energy electron diffraction. Angle-resolved photoemission spectroscopy data reveal a highly dispersive band in the case of poly(p-phenylene) while no significant dispersion is resolved for poly[salophenato-Co(II)]. On the basis of density functional theory calculations, we explain this observation as a result of a high conjugation along the aromatic phenyl groups in poly(p-phenylene) that is absent in the case of poly[salophenato-Co(II)], where intramolecular conjugation is interrupted in the salophenato-Co(II) unit. Furthermore, we make use of multicenter and delocalization indexes to characterize the electron mobility (corresponding to a high band dispersion) along different paths associated with individual molecular orbitals.

How do the Hückel and Baird Rules Fade away in Annulenes?

“…The calculation of the electronic aromaticity indices (AV1245, AV min , BOA and FLU) uses a QTAIM atomic partition [105] performed by the AIMAll software [106]. The AOM resulting from the QTAIM partition and the molecular geometries are the input for the in-house ESI-3D program [27,31,107], which provides AV1245 [55], AV min [56], BLA, BOA, DIs [29,30], FLU [27], HOMA [33] (included in the Supplementary Material), I ring [42], I ring and MCI [43] values. The numerical accuracy of the QTAIM calculations has been assessed using two criteria: (i) The integration of the Laplacian of the electron density (∇ 2 ρ(r)) within an atomic basin must be close to zero; (ii) the number of electrons in a molecule must be equal to the sum of all the electron populations of the molecule, and also equal to the sum of all the localization indices and half of the delocalization indices in the molecule.…”

“…Both I ring and MCI are among the less fallible aromaticity indices available in the literature [2,36,45] and, therefore, they have been used in a plethora of cases involving a difficult assessment of aromaticity [46][47][48][49][50][51][52][53][54]. However, these indices present some drawbacks that prevent their use in large rings [55] and, therefore, we have recently designed [55] and tested [40,56] a new electronic aromaticity index, AV1245, based on MCI but free of the shortcomings of this index. AV1245 is defined as the average value of the four-atom MCI index between relative positions 1-2 and 4-5 constructed from each five-atom fragment (five consecutive atoms) along the perimeter of the ring [55].…”

“…The latter gives an average picture of the electron delocalization among the atoms in the ring. However, for the purpose of measuring aromaticity, it has been found more adequate to use the minimum absolute MCI value evaluated along the perimeter, AV min [56]. AV min identifies the weakest link, i.e., the fragment with the lowest electron delocalization which is usually responsible for the loss of aromaticity in the ring.…”

How Aromatic Are Molecular Nanorings? The Case of a Six-Porphyrin Nanoring

“…In order to solve the drawbacks of I ring and MCI, one of us defined the AV1245 index 13 as the average value of the fouratom MCI index between relative positions 1-2 and 4-5 constructed from every five consecutive atoms along the perimeter of the ring. Lately, it has been seen that the average value could hinder some important values and that the minimal value of the index, AV min , 14,34 could be more useful. The AV min value corresponds to the least delocalized fragment of the ring.…”

Aromaticity as a Guiding Concept for Spectroscopic Features and Nonlinear Optical Properties of Porphyrinoids

“…Nevertheless, these macrocycles sustain multiple conjugation pathways and, in principle, Hückel´s rule should not be applied to polycyclic π-systems. By this reason, we have investigated the conjugated pathways in porphyrinoid compounds using several local aromaticity indicators [ 48 ], including the recently introduced AV1245 [ 49 ] and AV min [ 50 ] electronic indices. All indices agree on the general features of these compounds such as the fulfilment of Hückel´s rule, but only AV min is capable of recognizing the annulene pathway as the most aromatic one in all studied porphyrinoids.…”