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Time-resolved emission of [Ir(ppy)2dcbpy]Cl (1a, ppy = 2-phenylpyridine, dcbpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) dissolved in DMF revealed a biexponential decay of the excited state. Prior studies on cyclometalated iridium complexes indicated that such dual emission behavior can be attributed to an impurity, compounds that have two LUMO states that are close in energy, or the presence of ion-pairing. To probe each of these effects, a series of homoleptic, Ir(ppy)3 (2), and heteroleptic complexes, [Ir(ppy)2dcbpy]X [X = PF6−(1b), BPh4−(1c)] and [Ir(ppy)2bpy]X [bpy = 2,2′-bipyridine, X = Cl− (3a) PF6−(3b)], were synthesized and photophysically characterized. Among these complexes, only 1a exhibited dual emission with the emission lifetimes of 430 ± 5 ns (80%) and 125 ± 6 ns (20%) in DMF. These lifetimes were further confirmed with the nanosecond transient absorption kinetics, indicating that the dual emission likely did not originate from an impurity. When probing the emission and absorption kinetics of the salts 1b and 1c only one lifetime of 550 ± 10 ns was observed, indicating that the short lifetime of the Cl− salt (1a) comes from a putative ion-paired complex. Density-Functional Theory (DFT) calculations on 1 and 2 fragments in DMF helped uncover the reason behind the ion-pair formation, with carboxyl substituents in 1a causing a loss in electron density on the 3,3′-position of dcbpy. Additionally, the loss in electron density was experimentally validated with 1H NMR, which showed a noticeable downfield shift of the 3,3′-protons in relative to the shifts in 1b and 1c. The data supports that the dual emission observed in 1a is not from two separately emissive states, rather from the dissociated and ion-paired complexes.

In this review, we have attempted to briefly summarize the influence of an external electric field on an assembly of tautomeric molecules and to what experimentally observable effects this interaction can lead to. We have focused more extensively on the influence of an oriented external electric field (OEEF) on excited-state intramolecular proton transfer (ESIPT) from the studies available to date. The possibilities provided by OEEF for regulating several processes and studying physicochemical processes in tautomers have turned this direction into an attractive area of research due to its numerous applications.

Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorption partially blocked and screened by the photo-excited electron occupation of the conduction band and hole occupation of the valence band (absorption bleaching). Exciton absorption turns spectrally narrower upon pumping and sustains the Mott transition, indicating Mahan excitons. Simultaneously, intra-valence-band transitions occur at sub-picosecond time scales after holes scatter to the edge of the Brillouin zone. Our results pave new ways for the understanding of non-equilibrium charge-carrier dynamics in materials by reliably distinguishing between changes in absorption coefficient and refractive index, thereby separating competing processes. This information will help to overcome the limitations of materials for high-power optical devices that owe their properties from dynamics in the ultrafast regime.