Conjugation-Induced Thermally Activated Delayed Fluorescence: Photophysics of a Carbazole-Benzophenone Monomer-to-Tetramer Molecular Series

Abstract

Materials exhibiting thermally activated delayed fluorescence (TADF) have been extensively explored in the last decade. These emitters have great potential of being used in organic light-emitting diodes because they allow for high quantum efficiencies by utilizing triplet states via reverse intersystem crossing. In small molecules, this is done by spatially separating the highest occupied molecular orbital from the lowest unoccupied molecular orbital, forming an intramolecular charge-transfer (iCT) state and leading to a small energy difference between lowest excited singlet and triplet states (ΔE ST ). However, in polymer emitters, this is harder to achieve, and typical strategies usually include adding known TADF units as sidechains onto a polymer backbone. In a previous work, we proposed an alternative way to achieve a TADF polymer by repeating a non-TADF unit, polymerizing it via electron-donating carbazole moieties. The extended conjugation on the backbone reduced the ΔE ST and allowed for an efficient TADF polymer. In this work, we present a more in-depth study of the shift from a non-TADF monomer to TADF oligomers. The monomer shows non-TADF emission, and we find the delayed emission to be of triplet−triplet annihilation origin. An iCT state is formed already in the dimer, leading to a much more efficient TADF emission. This is confirmed by an almost two-fold increase of photoluminescence quantum yield, a decrease in the delayed luminescence lifetime, and the respective spectral lineshapes of the molecules.

From main-chain conjugated polymer photosensitizer to hyperbranched polymer photosensitizer: expansion of the polymerization-enhanced photosensitization effect for photodynamic therapy

“…As a consequence, the sidechain conjugated polymers will show a smaller DE ST value and red-shifted absorption, which is beneficial for ROS production. [36][37][38] For the hyperbranched polymers, the most important feature is the three-dimensional dendritic structures, which can endow the polymers with a good spatial separation effect (Fig. 1), 39,40 consequently improving the contact between the polymers and oxygen (necessary for the photosensitization process) and enhancing the ROSgenerating efficiency.…”

Investigation of the Characteristic of Solution‐Processed Tetraphenyldibenzoperiflanthene (DBP) Film and Its Application on Organic Photovoltaic Cells

“…For the PL spectra, the spectrum shapes for the DBP film and the DBP solution are completely different; it may be on account of that the DBP molecular aggregation in the film causes conjugate expansion, which results in dimer luminescence. [ 21 ] The PL peak for the DBP film is very broad with an unclear boundary between the acromions, whereas all the PL spectra for the DBP solutions manifest two explicit and sharp peaks. The PL peak positions indicate a large difference, and all the PL peaks of the DBP films red shift 35–40 nm relative to that of the DBP solutions.…”

Modulating the optical properties and functions of organic molecules through polymerization

“…9D). 79 In all these oligomers, the LUMO distributions are localized on the benzophenone side chains, showing very similar LUMO energy levels of around À1.70 eV. However, the HOMOs are distributed in the conjugated backbone of the carbazole donor.…”