Thermally activated delayed fluorescence (TADF) compounds with a flexible donor–acceptor structure suffer from conformational disorder in solid-state, which deteriorates their emission properties as well as OLED performance.

Molecular rotors are ac lass of fluorophores that enable convenient imaging of viscosity inside microscopic sampless uch as lipid vesicleso rlive cells. Currently, rotor compounds containingaboron-dipyrromethene( BODIPY) group are among the most promisingv iscosity probes. In this work, it is reportedt hat by addingh eavy-electron-withdrawing ÀNO 2 groups, the viscosity-sensitiver ange of a BODIPYp robe is drasticallye xpandedf rom 5-1500cPt o 0.5-50 000 cP.T he improved range makes it, to our knowledge, the first hydrophobic molecular rotor applicable not only at moderate viscosities but also for viscosity measurements in highlyv iscous samples. Furthermore, the photo-physicalm echanism of the BODIPY molecular rotors under study has been determined by performing quantum chemical calculations and transienta bsorption experiments. This mechanism demonstrates how BODIPY molecular rotors work in general,w hy the ÀNO 2 group causes such an improvement, and why BODIPYm olecular rotors suffer from undesirable sensitivity to temperature. Overall,b esides reporting av iscosity probe with remarkable properties, the results obtainede xpandt he general understanding of molecular rotors and show away to use the knowledge of their molecular actionm echanism for augmenting their viscositysensing properties.[**] BODIPY = boron-dipyrromethene.Supporting Information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.

Thermally activated delayed fluorescence (TADF) materials, combining high fluorescence quantum efficiency and short delayed emission lifetime, are highly desirable for application in organic light-emitting diodes (OLEDs) with negligible external quantum efficiency (EQE) roll-off. Here, we present the pathway for shortening the TADF lifetime of highly emissive 4,6-bis[4-(10-phenoxazinyl)phenyl]pyrimidine derivatives. Tiny manipulation of the molecular structure with methyl groups was applied to tune the singlet–triplet energy-level scheme and the corresponding coupling strengths, enabling the boost of the reverse intersystem crossing (rISC) rate (from 0.7 to 6.5) × 10 6 s –1 and shorten the TADF lifetime down to only 800 ns in toluene solutions. An almost identical TADF lifetime of roughly 860 ns was attained also in solid films for the compound with the most rapid TADF decay in toluene despite the presence of inevitable conformational disorder. Concomitantly, the boost of fluorescence quantum efficiency to near unity was achieved in solid films due to the weakened nonradiative decay. Exceptional EQE peak values of 26.3–29.1% together with adjustable emission wavelength in the range of 502–536 nm were achieved in TADF OLEDs. Reduction of EQE roll-off was demonstrated by lowering the TADF lifetime.

TADF compounds with flexible molecular core show large conformational disorder and emission wavelength instability in solid films.