Through versus Cross Electron Delocalization in Polytriacetylene Oligomers: A Computational Analysis

Abstract

Herein, we report a systematic theoretical investigation of the molecular and electronic properties of unsubstituted polytriacetylene (PTA) and iso-polytriacetylene (iso-PTA) oligomers, which are characterized by through and cross pi-conjugation pathways, respectively. The goal of the study is to compare through versus cross conjugation on the basis of the computed molecular geometries of the neutral, anionic, and cationic species, the electron affinities, ionization potentials, excitation energies, and nonlinear optical properties for oligomers up to the nonamer. Differences in the effective conjugation length are directly related to electron delocalization in cross- and through-conjugated pathways. As in the through-conjugated oligomers, that is, the PTAs, the frontier orbitals of the iso-PTA oligomers are delocalized along the entire carbon backbone, suggesting that pi-delocalization can extend through cross-linked carbon atoms. However, in contrast to the PTA oligomers, the bond lengths remain strictly constant and the reduction of the energy gap beyond the trimer is completely due to the correlation contribution. On the other hand, in the anions and cations, the bond lengths do change significantly with increasing chain length. Therefore, oxidation or reduction of the iso-PTA oligomer appears to switch on delocalization through cross-linked carbon atoms. Obviously, the effective conjugation length is specific and depends on the observable considered.

Donor/Acceptor Effects on the Linear and Nonlinear Optical Properties of Geminal Diethynylethenes (g‐DEEs)

“…The strongly electronwithdrawing nitrophenyl group, in conjugation with the enediyne backbone, exerts little influence on the observed bond lengths. The alkyne bond lengths of C(1)ÀC (2) and C(7)ÀC (8) are comparable to the other derivatives of this study ( Table 1), as well as to other cross-conjugated enediynes [10] [17] [20 -23]. The alkylidene bond angle C(2)ÀC(3)ÀC(7) of 117.80 (14)8 is, however, one of the largest observed to date for g-DEE structures.…”

“…In general, the BLA index for the crossconjugated enynes is defined by Eqn. 1, where the C D ÀC T , C Ar ÀC T , C¼C, and CC refer to the average bond distances for each type of single or multiple bond, respectively [8]. The dR values for each type of g-DEE (Table 6) obtained from DFT calculations show that dR decreases monotonically from 18a to 18e and suggest that the degree of delocalization in the enediyne backbone tends to increase as more D/A chromophores are appended.…”

On the Structure of Cross‐Conjugated 2,3‐Diphenylbutadiene

“…It is known that bond length alternation phenomena are not particularly pronounced in cross-conjugated compounds. [22] Short intramolecular contacts between the atoms or pair of atoms C11/H11 and C18/H18, C21/H21 and C28/H28, C12 and H24, and C22 and H14 are observed. This suggests that the region where the phenyl groups are linked to the butadiene moiety is crowded, and this may well be responsible for the long C12-C13 and C22-C23 bonds.…”

Oligomeric and Polymeric Systems with a Cross-conjugated π-Framework

“…The electronic delocalization of iso-PTAs in comparison to linearly-conjugated PTAs has also been studied theoretically by Lüthi and coworkers. 75,76 Similar in structure to 79-81, symmetrical dimeric isoPTAs with a range of terminal and pendent functionality have been reported to date, and their structures are summarized in Scheme 22. In nearly all cases, these molecules are readily available via an oxidative homocoupling reaction connecting the two halves via the central bond.…”