Effective conjugation in conjugated polymers with strongly twisted backbones: a case study on fluorinated MEHPPV

Abstract

Conjugated polymers with strongly twisted backbones, such as MEHPPV with fluorinated vinylene units (F-MEHPPV), demand a redefinition of the all-important 'effective conjugation length' ECL, which we extract here by a facile graphical method. In MEHPPV (being essentially planar), the ECL coincides with the 'maximum conducive chainlength'MCC and extends over about n  9 repetition units (RU). In F-MEHPPV, the MCC is similarly long with n  8, while ECL localizes on just one RU. The strong twist in F-MEHPPV persists in the excited state, broadening the emission spectrum and quenching the fluorescence by reduced radiative and enhanced non-radiative rates.The strong twist in F-MEHPPV persists in the excited state, broadening the emission spectrum, reducing the radiative rate, and enhancing the non-radiative deactivation, so that the fluorescence quantum yield of 30% in MEHPPV drops to 4% in F-MEHPPV. This example strikingly demonstrates how already small geometrical distortions (substitution of H by F) can lead to strong sterical effects, generating dramatic changes in the optical absorption and emission process.

Twist‐Elasticity‐Controlled Crystal Emission in Highly Luminescent Polymorphs of Cyano‐Substituted Distyrylbenzene (βDCS)

“…Understanding the solid‐state fluorescence spectra of crystal polymorphs is a nontrivial issue due to different contributions which impact spectral positions and bandshapes. One important contribution arises from intramolecular factors, that is, (i) the geometry of the compound, in particular dihedral twists, which hypsochromically (blue) shift the electronic transition by up to several hundred meV; these twists may additionally broaden the spectrum, inducing a bathochromic (red) shift of the emission maximum; this intramolecular‐induced spectral broadening is clearly seen in the present case for PM1 exhibiting strongly twisted geometries. Intermolecular effects include (ii) excitonic coupling, (iii) (anisotropic) polarizability effects as well as (iv) intermolecular vibronic coupling; the latter increases with the amount of π–π overlap and leads to a general broadening and bathochromic shift of the emission .…”

Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes

“…The resulting increase in persistence length was recently highlighted as a new design aspect to optimize optical absorption and thus solar cell performance (13). However, a high persistence length alone is not sufficient; only a concomitant backbone planarization enhances absorption due to an increase in transition dipole moment (14)(15)(16). However, more bulky side groups are usually associated with a larger torsion of the conjugated backbone, i.e., with a large distribution of interring dihedral angles (17)(18)(19), which localizes electronic excitations and gives rise to less favorable material properties.…”

Tetrakis{[(p‐dodecacarboranyl)methyl]stilbenyl}ethylene: A Luminescent Tetraphenylethylene (TPE) Core System

“…The fluorescence spectra of both compounds have a peak at λ em = 563 nm (2.21 eV); they are equally as unstructured as in the absorption spectra, that is, they are very different to that of stilbene, where only the absorption is unstructured (vide supra), but the emission is found to be structured; the unstructured emission of the TDSE compounds also arises from the twisted TPE unit in the S 1 state (calculated to θ = 30°), due to the steric demands of the molecule; similar effects were, for example, observed for substituted stilbene‐type materials …”