RESUMO
Three-dimensional (3D) hybrid organic-inorganic lead halide perovskites (HOIPs) feature remarkable optoelectronic properties for solar energy conversion but suffer from long-standing issues of environmental stability and lead toxicity. Associated two-dimensional (2D) analogues are garnering increasing interest due to superior chemical stability, structural diversity, and broader property tunability. Toward lead-free 2D HOIPs, double perovskites (DPs) with mixed-valent dual metals are attractive. Translation of mixed-metal DPs to iodides, with their prospectively lower bandgaps, represents an important target for semiconducting halide perovskites, but has so far proven inaccessible using traditional spacer cations due to either intrinsic instability or formation of competing non-perovskite phases. Here, we demonstrate the first example of a 2D Ag-Bi iodide DP with a direct bandgap of 2.00(2) eV, templated by a layer of bifunctionalized oligothiophene cations, i.e., (bis-aminoethyl)bithiophene, through a collective influence of aromatic interactions, hydrogen bonding, bidentate tethering, and structural rigidity. Hybrid density functional theory calculations for the new material reveal a direct bandgap, consistent with the experimental value, and relatively flat band edges derived principally from Ag-d/I-p (valence band) and Bi-p/I-p (conduction band) states. This work opens up new avenues for exploring specifically designed organic cations to stabilize otherwise inaccessible 2D HOIPs with potential applications for optoelectronics.
RESUMO
The prevalence of ultrafast electron-transfer processes in light-harvesting materials has motivated a deeper understanding of coherent reaction mechanisms. Kinetic models based on the traditional (equilibrium) form of Fermi's Golden Rule are commonly employed to understand photoinduced electron-transfer dynamics. These models fail in two ways when the electron-transfer process is fast compared to solvation dynamics and vibrational dephasing. First, electron-transfer dynamics may be accelerated if the photoexcited wavepacket traverses the point of degeneracy between donor and acceptor states in the solvent coordinate. Second, traditional kinetic models fail to describe electron-transfer transitions that yield products which undergo coherent nuclear motions. We address the second point in this work. Transient absorption spectroscopy and a numerical model are used to investigate coherent back-electron-transfer mechanisms in a transition metal complex composed of titanium and catechol, [Ti(cat)3](2-). The transient absorption experiments reveal coherent wavepacket motions initiated by the back-electron-transfer process. Model calculations suggest that the vibrationally coherent product states may originate in either vibrational populations or coherences of the reactant. That is, vibrational coherence may be produced even if the reactant does not undergo coherent nuclear motions. The analysis raises a question of broader significance: can a vibrational population-to-coherence transition (i.e., a nonsecular transition) accelerate electron-transfer reactions even when the rate is slower than vibrational dephasing?
RESUMO
Analogues of 2D photon echo methods in which two population times are sampled have recently been used to expose heterogeneity in chemical kinetics. In this work, the two population times sampled for a transition metal complex are transformed into a 2D rate spectrum using the maximum entropy method. The 2D rate spectrum suggests heterogeneity in the vibrational cooling (VC) rate within the ensemble. In addition, a cross peak associated with VC and back electron transfer (BET) dynamics reveals correlation between the two processes. We hypothesize that an increase in the strength of solute-solvent interactions, which accelerates VC, drives the system toward the activationless regime of BET.