Helical and Nonhelical Structures of Vinylene- and Azomethine-Linked Heterocyclic Oligomers: A Computational Study of Conformation-Dependent Optoelectronic Properties

Abstract

A systematic computational investigation has been carried out at the density functional level of theory to characterize various conformational isomers of furan, pyrrole, and thiophene based oligomers and consequently study their stabilities and electronic properties, especially in the cases of long oligomers. In these oligomers, adjacent heterocyclic rings are connected by either vinylene or azomethine linkages. B3LYP and B3LYP-D3 functionals are used to observe the effect of dispersion energy. Our results show that a combination of the B3LYP-D3 functional and the 6-31G(d,p) basis set is suitable for ground-state studies of these systems. For long vinylene-linked oligomers, folding isomers are comparatively more stable than their respective linearly conjugated isomers, due to intramolecular noncovalent interactions. In the case of azomethine-linked oligomers, geometries and stabilities of conformers depend on the type of heterocyclic ring in the repeating unit. For vinylene-linked heterocyclic oligomers, first optically allowed electronic transitions of linearly conjugated oligomers have the largest oscillator strengths, and these absorption bands are dominated by HOMO to LUMO transitions. In the case of a few linear azomethine-linked oligomers, two major electronic transitions, S 0 → S 1 and S 0 →S 2 , are noticed. However, transitions from S 0 to higher electronic states are the most prominent transitions in cases of foldamers, except azomethine-linked thiophene foldamers. Major absorption bands of these helical oligomers are dominated by transitions from HOMO−N to LUMO+N orbitals. All the helical conformers are found to be circular dichroism active.

On the variation of the belt and chiral screw and spring conformations of substituted regioregular HT undecathiophenes

“…In all conformations, they exhibited excellent pelectron delocalization and dense valence and conduction bands. 21 Moreover, it was demonstrated that doping with both electron donor or acceptor species reduced the band gaps and pushed behaviour of the systems towards those typical for metals. 21 Oligothiophenes, oligofurans and oligopyrroles with vinylene or azomethine ring linkers have been recently studied at the B3LYP/6-31G** level combined with and without Grimme's D3 correction.…”

“…21 Moreover, it was demonstrated that doping with both electron donor or acceptor species reduced the band gaps and pushed behaviour of the systems towards those typical for metals. 21 Oligothiophenes, oligofurans and oligopyrroles with vinylene or azomethine ring linkers have been recently studied at the B3LYP/6-31G** level combined with and without Grimme's D3 correction. 22 Strikingly, the relative stability of the conformers and difference in electronic transitions depended on the linker, but for the azomethine linker also on the heterocycle unit.…”

“…In all conformations, they exhibited excellent π-electron delocalization and dense valence and conduction bands. 21 Moreover, it was demonstrated that doping with both electron donor or acceptor species reduced the band gaps and pushed behaviour of the systems towards those typical for metals. 21 …”

“… 21 Moreover, it was demonstrated that doping with both electron donor or acceptor species reduced the band gaps and pushed behaviour of the systems towards those typical for metals. 21 …”

“…One of the reviewers of this letter rightly pointed out that to characterize helices, the helix pitch, helix radius and number of units per turn as well as bond length alternation (BLA) are worth to be given. 21 Finding the helix parameters require fitting the helix coordinates by a least square routine. 38 The pitch, radius, and number of thiophene rings per turn for the studied helical structures (Table 7, ESI † ) were found here using the HELFIX program 38 operating on coordinates of 11 S-atoms.…”