Regio- and conformational isomerization are fundamental in chemistry, with profound effects upon physical properties, however their role in excited state properties is less developed. Here two regioisomers of bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide, a donor–acceptor–donor (D–A–D) thermally-activated delayed fluorescence (TADF) emitter, are studied. 2,8-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide exhibits only one quasi-equatorial conformer on both donor sites, with charge-transfer (CT) emission close to the local triplet state leading to efficient TADF via spin-vibronic coupling. However, 3,7-bis(10H-phenothiazin-10-yl)dibenzo[b,d]thiophene-S,S-dioxide displays both a quasi-equatorial CT state and a higher-energy quasi-axial CT state. No TADF is observed in the quasi-axial CT emission. These two CT states link directly to the two folded conformers of phenothiazine. The presence of the low-lying local triplet state of the axial conformer also means that this quasi-axial CT is an effective loss pathway both photophysically and in devices. Importantly, donors or acceptors with more than one conformer have negative repercussions for TADF in organic light-emitting diodes.

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During the last few years, great interest has been given to research in organic solar cells. Apart from the low cost techniques needed for the production of the active layers, the plasticity of these types of materials offers an additional benefit compared to traditional silicon-based solar cells. However, the commercialization of organic photovoltaic technologies is limited by rapid degradation and low efficiency. Recently, the use of luminescent down-shifting (LDS) films has been explored as a tool for increasing the external quantum efficiency of organic-based devices. A previously observed superposition in the emission–absorption spectra of Kremer blue (Kb) and Coumarin 7 (C7) dyes suggests that an energy transfer process can take place among these molecules. Decay time photoluminescence measurements in Kb100−xC7x samples were carried out to confirm a Föster resonance energy transfer process. An optimization process following the design of experiments methodology was carried out to adjust the optical properties of Kb100−xC7x films for applications in the P3HT:PC61BM organic photovoltaic device. An increment of 18.6% in the short circuit current density (Jsc) of P3HT:PC61BM devices coated with a Kb92.5C77.5 LDS film was observed. A degradation test shows a rapid drop in the electric current density of devices without the LDS film, which we attribute to the so-called burn-in, widely reported in the literature. For devices coated with the Kb92.5C77.5 LDS film, no rapid drop in the electric current density was observed, suggesting that the use of the luminescent layer mitigates the photoinduced burn-in loss.

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