Electronic Structure Trumps Planarity: Unexpected Narrow Exciton Delocalization in PNDIT2 Revealed by Time‐Resolved Electron Paramagnetic Resonance (EPR) Spectroscopy

Abstract

exception. Furthermore, this polymer has a remarkable electron mobility of up to µ e = 0.85 cm 2 Vs −1 , [8] which is correlated with a high degree of molecular order on the micro-and nanoscale. [9,10] The degree of (de)localization of excitons in organic semiconductors is of outstanding importance, as it directly relates to efficiency. Depending on the desired application, either localized or delocalized excitons are sought-after. In a simple picture, delocalization is directly connected to planarity and conjugation, as the orbital overlap is maximized for parallel orientation of adjacent p z orbitals. [11] However, deviations in both directions are known, such as strong conjugation despite large torsional angles [12] as well as exciton delocalization confined to a rather small part of the available π system. [13] Eventually, (de)localization highly depends on structure and ordering primarily on the mole cular scale, as organic semiconductors show normally rather restricted large-scale ordering as compared to their inorganic and often highly crystalline counterparts. However, planarity within a molecular backbone resulting in high molecular order does not necessarily lead to higher carrier mobility. [14] Therefore a detailed understanding of the electronic structure of polymers and their building blocks is essential to develop efficient materials for organic electronics.To gain insight into the electronic structure of polymers, starting with their building blocks has proven to be valuable. [15] To the best of our knowledge, no systematic spectroscopic study of PNDIT2 starting from its building blocks has been performed yet. Here, we investigate four different building blocks of PNDIT2 and the polymer itself with two different chain lengths (see Figure 1 for chemical structures), using timeresolved electron paramagnetic resonance (TREPR) spectroscopy of their triplet excitons in combination with steady-state absorption spectroscopy and quantum-chemical calculations density-functional theory (DFT).Electron paramagnetic resonance (EPR) spectroscopy is intrinsically sensitive to the local environment of the electron spin, rendering it particularly well-suited to probe both electronic structure and local ordering on a molecular level. The key is its unrivalled sensitivity and selectivity for paramagnetic states not only allowing for unambiguous assignment of the spin multiplicity, but outperforming optical spectroscopy by far in terms of resolution. While conventional EPR spectroscopy can characterize structural features in the range Exciton delocalization in organic semiconductors, due to its direct relation to device efficiency, is of outstanding importance. Time-resolved electron paramagnetic resonance spectroscopy of light-induced triplet excitons gives access to the delocalization length in a unique way, connecting it to both, electronic structure and overall conformational flexibility. Systematically investigating building blocks of increasing length and comparing the results with the polymer deepens the unders...

Tuning the Porosity and Photocatalytic Performance of Triazine‐Based Graphdiyne Polymers through Polymorphism

“…This indicates a higher concentration of induced polaron‐like centers in TzG Cu than in TzG Pd/Cu —presumably caused by denser, crystalline packing. There is no qualitative difference between the spin delocalization along the π‐aromatic backbone of both polymorphs, as observed previously for compounds with π‐aromatic domains of varying sizes . Hence, the observed optical differences between the two polymorphs are purely a morphological effect.…”

“…There is no qualitative differenceb etween the spin delocalization along the p-aromatic backbone of both polymorphs, as observed previously for compounds with p-aromatic domains of varying sizes. [24] Hence, the observed optical differences between the two polymorphs are purely amorphological effect.…”

Structure–Function Relationship of Organic Semiconductors: Detailed Insights From Time-Resolved EPR Spectroscopy

“…Fully axial triplet spectra are rare, as they require a particular high symmetry of the excited state. Naphthalenediimide is one notable example of a triplet spectrum that can be ascribed to a fully axial excited state (Meyer et al, 2018b). Sometimes, an increase in rhombicity when proceeding from small building blocks to the polymer can be ascribed to the curvature of the polymer backbone (Matt et al, 2018b).…”

“…Depending on the desired application, either localized (OLEDs) or delocalized (OPVs) excitons are sought-after. Due to the direct relationship of the dipolar coupling of the two electron spins in a triplet exciton, TREPR spectroscopy gives a handle on exciton delocalization, revealing striking differences for different polymers (Matt et al, 2018b; Meyer et al, 2018b).…”

“…Applying this approach to PNDIT2 (Meyer et al, 2018b) known for its remarkable charge-carrier mobility quite surprisingly reveals the triplet exciton in this polymer to be strongly confined to no more than two repeat units (cf. Figure 8).…”