We discuss the nature of the optical excitations of Cs 2 AgBiBr 6 , the archetypal compound of lead-free double-perovskites. Such quaternary material shows an indirect electronic bandgap with a broad optical absorption spectrum above 2 eV. By means of ab-initio excited-state methods we show that the first absorption peak is due to a bound direct exciton (X point of the Brillouin Zone), while the photoluminescence spectrum is explained in terms of phonon-assisted radiative recombination of indirect-bound excitons with transferred momenta along the L-X and Γ-X directions. To address the role of metal and halide atoms on the electronic and optical properties of this materials class, we investigate two additional ternary double-perovskites, i.e. Cs 2 In 2 X 6 (X=F, Br). Based on the accurate determination of the absorption coefficients and minimum gaps we estimate the spectroscopic limited maximum efficiency of solar cells based on such compounds, providing relevant information for their application in photovoltaics. Bulk Hybrid Organic Inorganic Halide Perovskites (OIHPs) and derived systems have completely changed the scenario of modern, cheap, photovoltaics (PV), 1-7 as testified by the huge conversion efficiencies (PCEs) reached by the perovskite solar cells (PSCs) that nowadays pass 23%. 8 Despite the so high PCEs reached by PSCs based on the archetypal compound MAPbX 3 (MA=CH 3 NH + 3 , methylammonium; X=halide) as light harvester, there remain issues mainly related to device stability and toxicity. For the former, the dimensionality reduction (3D → 2D, via the partial/total replacement of MA with longer-chain, hydrophobic, aliphatic (BA=butylammonium) or aromatic (PEA=phenethylammonium), organic cations), at the price of reduced PCEs, seems one of the best direction to follow. 9-12

The absorption-spectral and kinetic behaviors of radical ions and neutral hydrogenated radicals of seven 3-styryl-quinoxalin-2(1 H)-one (3-SQ) derivatives, one without substituents in the styryl moiety, four others with electron-donating (R = -CH, -OCH, and -N(CH)) or electron-withdrawing (R = -OCF) substituents in the para position in their benzene ring, and remaining two with double methoxy substituents (-OCH), however, at different positions (meta/para and ortho/meta) have been studied by UV-vis spectrophotometric pulse radiolysis in neat acetonitrile saturated with argon (Ar) and oxygen (O) and in 2-propanol saturated with Ar, at room temperature. In acetonitrile solutions, the radical anions (4R-SQ) are characterized by two absorption maxima located at λ = 470-490 nm and λ = 510-540 nm, with the respective molar absorption coefficients ε = 8500-13 100 M cm and ε = 6100-10 300 M cm, depending on the substituent (R). All 4R-SQ decay in acetonitrile via first-order kinetics, with the rate constants in the range (1.2-1.5) × 10 s. In 2-propanol solutions, they decay predominantly through protonation by the solvent, forming neutral hydrogenated radicals (4R-SQH), which are characterized by weak absorption bands with λ = 480-490 nm. Being oxygen-insensitive, the radical cations (4R-SQ) are characterized by a strong absorption with λ = 450-630 nm, depending on the substituent (R). They are formed in a charge-transfer reaction between a radical cation derived from acetonitrile (ACN) and substituted 3-styryl-quinoxalin-2-one derivatives (4R-SQ) with a pseudo-first-order rate constant k = (2.7-4.7) × 10 s measured in solutions containing 0.1 mM 4R-3-SQ. The Hammett equation plot gave a very small negative slope (ρ = -0.08), indicating a very weak influence of the substituents in the benzene ring on the rate of charge-transfer reaction. The decay of 4R-SQ in Ar-saturated acetonitrile solutions occurs with a pseudo-first-order rate constant k = (1.6-6.2) × 10 s and, in principle, is not affected by the presence of O, suggesting charge-spin delocalization over the whole 3-SQ molecule. Most of the radiolytically generated transient spectra are reasonably well-reproduced by semiempirical PM3-ZINDO/S (for 4R-SQ) and density functional theory quantum mechanics calculations employing M06-2x hybrid functional together with the def2-TZVP basis set (for 4R-SQ).

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Local vibrational coupling models predict that intramolecular vibrational energy redistribution (IVR) is not completely statistical even at the dissociation limit of polyatomic molecules. Thus states protected from IVR and from rapid dissociation form regular progressions and can be assigned vibrational quantum numbers. We previously observed such regular progressions of states in vibrational spectra of the molecule SCCl2, but a discrepancy in the density of such states remained between theory and experiment. Here we show that the gap can be closed by observing and assigning additional vibrational transitions above the dissociation limit of SCCl2, and by carefully analyzing the theoretically expected density of protected states. The newly observed transitions originate from recently assigned and more highly excited vibrational levels in the B̃ electronic state, connecting to the X̃ ground state by different Franck-Condon factors. Based on our analysis of Franck-Condon activity, we conclude that theory and experiment agree within measurement uncertainty. Consistency between theory and experiment implies that even more protected states should be observed for larger molecules, leading to nonstatistical dissociation reactions.