Realization of efficient deep-blue anthracene-based emitters with superior film-forming and charge transport properties is challenging. A series of non-symmetric 9,10-diphenylanthracenes (DPA) with phenyl and pentyl moieties at the 2nd position and alkyl groups at para positions of the 9,10-phenyls were synthesized and investigated. The non-symmetric substitution at the 2nd position enabled to improve film forming properties as compared to those of the unsubstituted DPA and resulted in glass transition temperatures of up to 92 °C. Small-sized and poorly conjugated substituents allowed to preserve emission in the deep blue range (<450 nm). Substitution at the 2nd position enabled to achieve high fluorescence quantum yields (up to 0.7 in solution, and up to 0.9 in the polymer host), although it caused an up to 10-fold increase in the intersystem crossing rate as compared to that of the unsubstituted DPA. Further optimization of the film forming properties achieved by varying the length of the alkyl groups attached at the 9,10-phenyls enabled to attain very high hole drift mobilities (∼5 × 10(-3)-1 × 10(-2) cm(2) V(-1) s(-1)) in the solution-processed amorphous films of the DPA compounds.

Triplet–triplet annihilation (TTA) is an attractive way to boost the efficiency of conventional fluorescent organic light-emitting diodes (OLEDs). TTA-active anthracene derivatives are often considered as state-of-the-art emitters due to the proper energy level alignment. In this work, TTA properties of a series of highly fluorescent nonsymmetrical anthracene compounds bearing 9-(4-arylphenyl) moiety and 10-(4-hexylphenyl) fragments were assessed. Two different methods to enhance the TTA efficiency are demonstrated. First, the intensity of TTA-based delayed fluorescence directly depended on the intersystem crossing (ISC) rate. This ISC rate can be significantly enhanced in more conjugated compounds due to the resonant alignment of S1 and T2 energy levels. While enhanced ISC rate slightly quenches the intensity of prompt fluorescence, the rise of the triplet population boosts the intensity of resultant delayed fluorescence. Second, the triplet annihilation rate can be significantly enhanced by optimization of triplet exciton diffusion regime in the films of anthracene derivatives. We show that the proper layer preparation technology has a crucial influence on uniformity and energetic disorder of the film. This enhances the nondispersive triplet diffusion and increases the resulting delayed fluorescence intensity.

Efficient triplet exciton hopping (diffusion) in amorphous solid films is essential for triplet−triplet annihilation (TTA) and TTA-mediated photon upconversion (UC) at low excitation power densities. However, enhanced triplet diffusion, particularly in high-emitter-content UC films, also facilitates their trapping and quenching at nonradiative decay sites, thus deteriorating UC efficiency. In this work, triplet exciton diffusion and quenching are studied in matrix-free solid UC films based on two novel bisfluoreneanthracene (BFA) emitters, i.e., one with methyl substitution (BFA-Me) and the other with a phenyl substitution (BFA-Ph), and a standard platinum octaethylporphyrin (PtOEP) sensitizer. By analyzing temperature-dependent TTA-UC dynamics and accounting for various singlet exciton-related processes, we are able to discern triplet exciton quenching occurring explicitly in the emitter and show that it is one of the dominating mechanisms impeding the UC performance of BFA/PtOEP films, particularly at elevated temperatures. Regardless of the lower density of quenchers present in the BFA-Ph film, twice as large triplet diffusivity estimated in this film (D = (2.13 ± 0.64) × 10 −9 cm 2 •s −1 ) at room temperature as compared to that in the BFA-Me film caused more rapid triplet quenching. This resulted in the shifting of the optimal UC performance of BFA-Ph to lower temperatures (T = 160 K) with respect to that of BFA-Me (T = 220 K). To obtain a high UC quantum yield, which for these materials can be estimated to reach >5% at room temperature and above, the excessive diffusion to the remaining quenching sites needs to be suppressed, e.g., by increasing the intermolecular distance through side groups.

Deliberate control of intermolecular interactions in fluorene- and benzofluorene-cored oligomers was attempted via introduction of different-length alkyl moieties to attain high emission amplification and low amplified spontaneous emission (ASE) threshold at high oligomer concentrations. Containing fluorenyl peripheral groups decorated with different-length alkyl moieties, the oligomers were found to express weak concentration quenching of emission, yet excellent carrier drift mobilities (close to 10−2 cm2/V/s) in the amorphous films. Owing to the larger radiative decay rates (&gt;1.0 × 109 s−1) and smaller concentration quenching, fluorene-cored oligomers exhibited down to one order of magnitude lower ASE thresholds at higher concentrations as compared to those of benzofluorene counterparts. The lowest threshold (300 W/cm2) obtained for the fluorene-cored oligomers at the concentration of 50 wt % in polymer matrix is among the lowest reported for solution-processed amorphous films in ambient conditions, what makes the oligomers promising for lasing application. Great potential in emission amplification was confirmed by high maximum net gain (77 cm−1) revealed for these compounds. Although the photostability of the oligomers was affected by photo-oxidation, it was found to be comparable to that of various organic lasing materials including some commercial laser dyes evaluated under similar excitation conditions.

Organic single crystals (SCs) expressing long-range periodicity and dense molecular packing are an attractive amplifying medium for the realization of electrically driven organic lasers. However, the amplified spontaneous emission (ASE) threshold (1-10 kW/cm) of SCs is still significantly higher compared to those of amorphous neat or doped films. The current study addresses this issue by investigating ASE properties of rigid bridging group-containing bifluorene SCs. Introduction of the rigid bridges in bifluorenes enables considerable reduction of nonradiative decay, which, along with enhanced fluorescence quantum yield (72-82%) and short excited state lifetime (1.5-2.5 ns), results in high radiative decay rates (∼0.5 × 10 s) of the SCs, making them highly attractive for lasing applications. The revealed ASE threshold of 400 W/cm in acetylene-bridged bifluorene SCs is found to be among the lowest ever reported for organic crystals. Ultrafast transient absorption spectroscopy enabled one to disclose pronounced differences in the excited state dynamics of the studied SCs, pointing out the essential role of radiative traps in achieving a record low ASE threshold. Although the origin of the trap states was not completely unveiled, the obtained results clearly evidence that the crystal doping approach can be successful in achieving extremely low ASE thresholds required for electrically pumped organic laser.