Charge Delocalization over Stacked π-Electron Systems

Abstract

Cation radicals of oligomers and polymers of dibenzofulvene having a -stacked conformation showed broad absorption bands based on charge resonance in near IR region, which red-shifted with increasing degree of polymerization, indicating that charge delocalization takes place through multiple, stacked fluorene moieties.Polymers and oligomers that may transport injected charges are gathering attention as potential molecular wire.1 In this regard, various main-chain conjugating polymers have been designed. Apart from such long-range through-bond conjugating molecules, charge transport through stacked -electronic systems has been proposed as represented by the discussion on conduction behavior of DNAs. 2 We recently reported the synthesis and properties of poly(dibenzofulvene) (poly(DBF)) having a stable -stacked structure with the main-chain C-C bondings being all-trans and the side-chain fluorene chromophores being tightly stacked on top of each other.3,4 Based on the findings that this polymer shows a high hole mobility in a bulk film form 5 and a reduced oxidation potential in electrochemical analysis, 4 it has been proposed that the stacked -electron systems of poly(DBF) may be an effective molecular wire. However, no direct information has been obtained on whether poly(DBF) delocalizes and transports charges. Here, we report the chemical oxidation of DBF oligomers having two to eight monomeric units isolated by size-exclusion chromatography (SEC) and a polymeric mixture (n ! 9, M n 1890) and the observation of charge resonance (CR) 6 in multiple, stacked fluorene groups by near infra-red (NIR) absorption 7 spectroscopy. CR is directly connected to charge delocalization. Although there are several clear examples of CR band detection involving two aromatic molecules, [8][9][10][11][12][13][14] no systematic study has been reported on CR in regularly stacked, multiple aromtic groups in a molecule.The oligomers and the polymeric mixture having different degrees of polymerization (n in Scheme 1, left) were obtained by SEC separation from the anionic polymerization products of DBF prepared using MeLi as an initiator and EtI as a terminating reagent in tetrahydrofuran at À78 C. 4 The oligomers and the polymer were dissolved in dry CH 2 Cl 2 , and SbCl 5 in CH 2 Cl 2 was added to the solution. 15 The mixed solution showed a blue-green color, indicating the generation of radical cation. Figure 1 indicates the NIR absorption spectra of the oxidized oligo-and poly(DBF) samples and that of oxidized fluorene as a model of monomeric unit. In the spectrum of oxidized fluorene, a clear band was observed at 635 nm due to cation radical. 9 The spectra of the oxidized -stacked DBF oligomers and polymer indicated broad absorption bands due to CR in the near IR region in addition to the peak at 660-680 nm due to cation radical. Using the polymer sample (n ! 9, mixture), it was confirmed that the peak position of the near-IR band was not significantly affected by the sample concentration ([DBF unit] = 1:5 Â 10 À4 M, 5:9 Â 10 À5 M;...

Synthesis, structure and function of π-stacked polymers

“…93 Although oxidized fluorene as a monomeric unit model showed only absorption bands based on radial cation species around 650 nm, p-stacked poly(DBF) with various DP showed a broad hole resonance band in the NIR region. Further, the peak top energy decreased with an increase in DP (Figure 23, bottom).…”

[2.2]Paracyclophane-Layered Polymers End-Capped with Fluorescence Quenchers

“…The introduction of a π−π stacking structure in the conjugated polymer backbone would allow effective hole, electron, and energy transfer in a polymer chain and in an aggregate of polymers. Considering this possibility, Nakano and co-workers have synthesized poly(dibenzofulvene)s with a π-stacked conformation that exhibited a higher hole drift mobility than the through-bond conjugated poly(phenylenevinylene)…”

“…In this Note, we report the synthesis and properties of [2.2]paracyclophane-layered polymers in which fluorescence quenchers exist at the ends of the polymer chain; these are responsible for photoexcited energy transfer from the layered [2.2]paracyclophane to the terminal units. These polymers give rise to a new class of molecular wires comprising π-stacked aromatic rings. , …”

Synthesis, Structure, and Properties of Aromatic Ring-Layered Polymers Containing Ferrocene as a Terminal Unit

“…A π-π stacking structure and orientation of aromatic rings play an important role in enabling effective charge or energy transfer. Although the syntheses of new conjugated polymers have been intensively pursued in recent years [1][2][3], few reports have described conjugated polymers consisting of layered aromatic rings via the through-space interaction of the π-π stacking [4][5][6][7][8][9][10][11][12][13][14][15][16]. Conjugated polymers and molecular wires such as poly(phenylene-ethynylene)s (PPEs) and oligothiophenes synthesized thus far consist of sp or sp 2 carbon frameworks [17][18][19][20][21][22][23][24][25][26].…”