Triplet harvesting with 100% efficiency by way of thermally activated delayed fluorescence in charge transfer OLED emitters.

Organic light-emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up-conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (ΔEST ) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates.

Precise control of intramolecular charge-transport: the interplay of distance and conformational effects.

A new series of donor-bridge-acceptor (D-B-A) compounds consisting of π-conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron-donor and a fullerene (C60 ) electron-acceptor have been synthesized. In addition to varying the length of the bridge (i.e.

The β phase formation limit in two poly(9,9-di-n-octylfluorene) based copolymers.

Random copolymers of poly(9,9-di-n-octylfluorene) (PF8) incorporating 0, 8, 12, 15, and 20% dibenzothiophene (DBT), and copolymers with 2, 5, 8, 12, and 15% dibenzothiophene-S,S-dioxide (S-unit) were synthesised. Absorption and emission spectra of thin films indicate that the DBT system shows a linear decrease of toluene vapour induced β phase with increasing DBT content to a 20% cutoff, whilst in the S-unit copolymers the β phase is present up to 12% co-monomer content, and at 15% the characteristic absorption peak is absent or masked. These results demonstrate the limits, in thin films, at which the β phase can be formed in widely used PF8 copolymer systems for device applications and clearly show that it is practical to use copolymers having electron or hole transport units in the polyfluorene backbone and still be able to form efficient β phase emission sites.

Tuning the intramolecular charge transfer emission from deep blue to green in ambipolar systems based on dibenzothiophene S,S-dioxide by manipulation of conjugation and strength of the electron donor units.

The efficient synthesis and photophysical properties of a series of ambipolar donor-acceptor-donor systems is described where the acceptor is dibenzothiophene S,S-dioxide and the donor is fluorene, carbazole, or arylamine. The systems exhibit intramolecular charge transfer (ICT) states (of variable ICT character strengths) leading to fluorescence emission ranging from deep blue to green with moderate to high photoluminescence quantum yields. The emission properties can be effectively tuned by systematically changing the position of substitution on both donor and acceptor units (which affects the extent of conjugation) and the redox potentials of the donor units.

The interplay of conformation and photophysical properties in deep-blue fluorescent oligomers.

We report the synthesis, X-ray crystal structures and photophysics of new donor-acceptor oligomers of fluorene (F) and dibenzothiophene-S,S-dioxide (S) with constrained dihedral angles in the backbone. The materials display bright deep-blue fluorescence and evidence is presented for a planarised intramolecular charge-transfer (PICT) state in the F-S systems.