Direct S₀→T Excitation of a Conjugated Polymer Repeat Unit: Unusual Spin-Forbidden Transitions Probed by Time-Resolved Electron Paramagnetic Resonance Spectroscopy

Abstract

A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. (Time-resolved) electron paramagnetic resonance (EPR) is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior resolution compared to optical methods. We present here evidence for a direct S→T optical excitation of distinct triplet states in the repeat unit of a conjugated polymer used in organic photovoltaics. These states differ in their electronic structure from those populated via intersystem crossing from excited singlet states. This is an additional and so far unconsidered route to triplet states with potentially high impact on efficiency of organic electronic devices.

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

“…[20,21] Optical excitation at the respective wavelengths in the visible range was carried out using an optical parametric oscillator (OPO) pumped by an Nd:YAG laser. [20,21] Optical excitation at the respective wavelengths in the visible range was carried out using an optical parametric oscillator (OPO) pumped by an Nd:YAG laser.…”

“…EPR Instrumentation: All TREPR experiments were performed at 80 K using a setup described previously. [20,21] Optical excitation at the respective wavelengths in the visible range was carried out using an optical parametric oscillator (OPO) pumped by an Nd:YAG laser. The repetition rate of the laser was set to 10 Hz and the final pulse energy (after the OPO) to 1 mJ.…”

“…[5,6] Using organic molecules as a replacement for conventional inorganic semiconductors comes with a number of advantages, such as mechanical flexibility, low cost, and the nearly infinite possibility of tailoring molecules for each special application. Whereas for conventional inorganic semiconductors, it is rather straightforward to create either p-type or n-type materials using well-established protocols, only very few n-type conjugated polymers are known to date, with poly{[N,N′bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDIT2), also known as P(NDI2OD-T2) or Polyera ActivInk N2200, [7] being a notable between 0.5 and 5 nm in systems that lack long-range order [16] only TREPR spectroscopy [17] with a time resolution of up to 10 ns allows for real-time observation, e.g., of short-lived radical-pair [18,19] and triplet states [20,21] generated by pulsed laser excitation. Furthermore, as EPR is exclusively sensitive to paramagnetic states, but polymers are normally diamagnetic, applying conventional EPR spectroscopy requires to insert spin probes into the material of interest.…”

“…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 between 0.5 and 5 nm in systems that lack long‐range order only TREPR spectroscopy with a time resolution of up to 10 ns allows for real‐time observation, e.g., of short‐lived radical‐pair and triplet states generated by pulsed laser excitation. Furthermore, as EPR is exclusively sensitive to paramagnetic states, but polymers are normally diamagnetic, applying conventional EPR spectroscopy requires to insert spin probes into the material of interest.…”

Bildung Stabiler All‐Silicium Varianten von 1,3‐Cyclobutandiyl im Gleichgewicht

“…Abbildung 3 zeigt das TR-EPR-Spektrum, das nach gepulster Laseranregung des Komplexes in gefrorener Lçsung bei 80 K aufgenommen wurde, zusammen mit der entsprechenden Spektralsimulation. [29,30] Es wird ein breites Signal beobachtet, das bei etwa 340 mT zentriert ist, ein Wert, der sich gut mit den bekannten Silicium-zentrierten Diradikalen vergleichen lässt. [31] Die Breite des Spektrums legt nahe, dass es von der dipolaren Kopplung zwischen zwei ungepaarten Elektronen eines Triplettzustands dominiert wird.…”