Narrow Results

While exciplex formation is considered to have a negative impact on the performance of organic light-emitting devices (OLEDs), it can be utilized as a simple means for manipulating the emission colors. Here, we explore various important factors influencing the exciplex formation in order to provide a simple way for tailoring the emission color of OLEDs involving exciplex emission. In addition to the well-known effects of the energy difference between electron affinity and ionization potential (I_P) of organic materials used, we found that electron mobility and I_P of electron-transporting layer (ETL) are also critical factors in controlling the intensity of exciplex emission via the influence on charge accumulation intensity at the organic/organic interface. Interestingly, devices with green, blue, and white electroluminescence colors can be attained by simply replacing different ETL materials in contact with the same blue fluorophore underneath. The underlying mechanism for the color change in OLEDs will be discussed.

Photophysical properties of meso-tetrakis(4-N-methylpyridiniumyl)porphyrin (TMpyP4) and its metallocomplexes M(II)TMpyP4 (M = Zn, Cu, Ni, Co) bound to natural DNA and synthetic poly-, oligo- and mononucleotides are considered with a primary emphasis placed upon intermolecular interaction of the photoexcited porphyrins with the nearest environment. Quenching of the fluorescent S₁ (but not triplet T₁) state due to guanine to porphyrin electron transfer is observed for TMpyP4 intercalated between GC base pairs of the double-strand helixes, whereas in the case of TMpyP4 complexed with guanosine monophosphate (GMP) both S₁ and T₁ states of the porphyrin are quenched. Furthermore, a dependence of the efficiency of TMpyP4 triplet state quenching by the dissolved molecular oxygen from air on the porphyrin localization enables one to readily distinguish porphyrin groove binding mode from intercalation. Excited states of the TMpyP4 complexes with transition metals, in spite of their very short lifetimes, also interact with nucleic acid components by means of an axial ligand binding/release to/from the metal. A possible structure of the five-coordinate excited complex (“exciplex”) formed in case of CuTMpyP4 groove binding to some single- and double-strand polynucleotides is discussed.

Although photoexcited state lifetimes of transition-metal porphyrin complexes are usually too short to exhibit any appreciable reactivity in intermolecular redox reactions with substrates, there are several cases to show enough lifetimes of the photoexcited states of transition-metal porphyrins and analogues to be involved in intermolecular photoredox reactions. This minireview focuses on such examples of photochemistry and photocatalysis of transition-metal porphyrins and analogues. For example, supramolecular complexes of transition-metal porphyrins with fullerene and its derivatives undergo fast charge separation to produce the charge-separated states upon photoexcitation, the lifetimes of which are longer than those of transition-metal porphyrins themselves. The photoexcited state of [Co${}^{\text{III}}$TPP]${}^{+}$ (TPP = tetraphenylporphyrin dianion) acts as a good electron acceptor to form an exciplex with hexamethylbenzene, leading to the photocatalytic oxygenation of hexamethylbenzene with dioxygen. In contrast to [Co${}^{\text{III}}$TPP]${}^{+}$, the photoexcited state of [Mn${}^{\text{III}}$(TBP₈Cz): TBP₈Cz = octakis(p-tert-butylphenyl)corrolazinato${}^{3-}$] acts as an electron donor to react with O₂ (electron acceptor), producing the superoxo-Mn(IV) complex, by which hexamethylbenzene is oxidized to yield pentamethylbenzyl alcohol. A manganese porphyrin complex that is covalently linked to graphene oxide (GO) sheets is shown to act as a photoredox catalyst for photocatalytic asymmetric epoxidation of trans-stilbene with O₂ and isobutyraldehyde in the presence of imidazole to show high enantioselectivity (ee 100%) under photoirradiation. The photoexcited states of photo-labile Mn-porphyrin chlorates or bromates undergo homolytic cleavage to produce active radical species which are responsible for photocatalysis in the photocatalytic oxidation with substrates.