The low statistical probability factor of the rubrene emitter (f = 15.5 ± 3%) was determined to be the prime TTA-UC efficiency-limiting factor.

Enhanced TTA-UC performance of t-butyl-rubrene films as compared to that of unsubstituted-rubrene films is achieved due to suppressed singlet fission and non-radiative triplet quenching.

Light upconversion (UC) via triplet-triplet annihilation (TTA) by using noncoherent photoexcitation at subsolar irradiance power densities is extremely attractive, particularly for enhanced solar energy harvesting. Unfortunately, practical TTA-UC application is hampered by low UC efficiency of upconverting polymer glasses, which is commonly attributed to poor exciton diffusion of the triplet excitons across emitter molecules. The present study addresses this issue by systematically evaluating triplet exciton diffusion coefficients and diffusion lengths (LD) in a UC model system based on platinum-octaethylporphyrin-sensitized poly(methyl methacrylate)/diphenylanthracene (emitter) films as a function of emitter concentration (15-40 wt %). For this evaluation time-resolved photoluminescence bulk-quenching technique followed by Stern-Volmer-type quenching analysis of experimental data was employed. The key finding is that although increasing emitter concentration in the disordered PMMA/DPA/PtOEP films improves triplet exciton diffusion, and thus LD, this does not result in enhanced UC quantum yield. Conversely, improved LD accompanied by the accelerated decay of UC intensity on millisecond time scale degrades TTA-UC performance at high emitter loadings (>25 wt %) and suggests that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor.

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.

Solution-processable donor−acceptor molecules consisting of triphenylamine core and 1,8-naphthalimide arms were designed and synthesized by palladium-catalyzed Heck reaction. Dilute solutions of the synthesized compounds show strong absorption peaks in the visible wavelength range from 400 to 550 nm, which can be ascribed to the intramolecular charge transfer. Fluorescence quantum yields of dilute solutions of the synthesized materials range from 0.45 to 0.70, while those of the solid samples are in the range of 0.09−0.18. The synthesized molecules exhibit high thermal stability with the thermal degradation onset temperatures ranging from 431 to 448 °C. The compounds form glasses with glass-transition temperatures of 55−107 °C. DFT calculations show that HOMO and LUMO orbitals are almost entirely localized on the donor and acceptor moieties, respectively. Consequently, the frontier orbital energies for the three synthesized compounds are similar and practically do not depend on the number of 1,8-naphthalimide moieties. Ionization potentials of the solid samples (5.75− 5.80 eV) are comparable. The charge-transporting properties of the synthesized materials were studied using xerographic time-offlight method. Hole mobilities in the layers of the compounds having one and two 1,8-naphthalimide moieties exceed 10 −3 cm 2 •V −1 •s −1 at high electric fields at room temperature. The differences on the hole mobilities between the three synthesized compounds are discussed in the frame of Marcus theory by comparing the reorganization energy and electronic coupling parameters.

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.

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Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.