Organohalide lead perovskites have revolutionized the scenario of emerging photovoltaic technologies. The prototype MAPbI3 perovskite (MA = CH3NH3(+)) has dominated the field, despite only harvesting photons above 750 nm (∼1.6 eV). Intensive research efforts are being devoted to find new perovskites with red-shifted absorption onset, along with good charge transport properties. Recently, a new perovskite based on the formamidinium cation ((NH2)2CH(+) = FA) has shown potentially superior properties in terms of band gap and charge transport compared to MAPbI3. The results have been interpreted in terms of the cation size, with the larger FA cation expectedly delivering reduced band-gaps in Pb-based perovskites. To provide a full understanding of the interplay among size, structure, and organic/inorganic interactions in determining the properties of APbI3 perovskites, in view of designing new materials and fully exploiting them for solar cells applications, we report a fully first-principles investigation on APbI3 perovskites with A = Cs(+), MA, and FA. Our results evidence that the tetragonal-to-quasi cubic structural evolution observed when moving from MA to FA is due to the interplay of size effects and enhanced hydrogen bonding between the FA cations and the inorganic matrix altering the covalent/ionic character of Pb-I bonds. Most notably, the observed cation-induced structural variability promotes markedly different electronic and optical properties in the MAPbI3 and FAPbI3 perovskites, mediated by the different spin-orbit coupling, leading to improved charge transport and red-shifted absorption in FAPbI3 and in general in pseudocubic structures. Our theoretical model constitutes the basis for the rationale design of new and more efficient organohalide perovskites for solar cells applications.

We computationally investigate organometal CH3NH3PbX3 and mixed halide CH3NH3PbI2X perovskites (X = Cl, Br, I), which are key materials for high efficiency solid-state solar cells. CH3NH3PbX3 perovskites exhibited the expected absorption blue shift along the I → Br → Cl series. The mixed halide systems surprisingly showed the CH3NH3PbI3 and the CH3NH3PbI2Cl (or CH3NH3PbI3–x Cl x ) perovskites to have similar absorption onset at ∼800 nm wavelength, whereas CH3NH3PbI2Br absorbs light below ∼700 nm. To provide insight into the structural and electronic properties of these materials, in light of their application as solar cell active layers, we perform periodic DFT calculations on the CH3NH3PbX3 and CH3NH3PbI2X perovskites. We find a good agreement between the calculated band structures and the experimental trend of optical band gaps. For the mixed halide perovskites our calculations show the existence of two different types of structures with different electronic properties, whose relative stability varies by varying the X group. For these systems, the calculated formation energies decrease in the order I > Br > Cl, in line with the observed miscibility of CH3NH3PbI3 and CH3NH3PbBr3 compounds, while suggesting a comparatively smaller chlorine incorporation into CH3NH3Pb(I1–x Cl x )3 compounds. We also show that Cl atoms preferentially occupy the apical positions in the PbI4X2 octahedra, while Br atoms may occupy both apical and equatorial positions, consistent with reported lattice parameters. The interplay of the organic and inorganic components of the perovskites, possibly mediated by hydrogen bonding between the ammonium groups and the halides, seems to be the key to the observed structural variability.

A facile and fast approach, based on microwave-enhanced Sonogashira coupling, has been employed to obtain in good yields both mono- and, for the first time, disubstituted push-pull Zn(II) porphyrinates bearing a variety of ethynylphenyl moieties at the β-pyrrolic position(s). Furthermore, a comparative experimental, electrochemical, and theoretical investigation has been carried out on these β-mono- or disubstituted Zn(II) porphyrinates and meso-disubstituted push-pull Zn(II) porphyrinates. We have obtained evidence that, although the HOMO-LUMO energy gap of the meso-substituted push-pull dyes is lower, so that charge transfer along the push-pull system therein is easier, the β-mono- or disubstituted push-pull porphyrinic dyes show comparable or better efficiencies when acting as sensitizers in DSSCs. This behavior is apparently not attributable to more intense B and Q bands, but rather to more facile charge injection. This is suggested by the DFT electron distribution in a model of a β-monosubstituted porphyrinic dye interacting with a TiO2 surface and by the positive effect of the β substitution on the incident photon-to-current conversion efficiency (IPCE) spectra, which show a significant intensity over a broad wavelength range (350-650 nm). In contrast, meso-substitution produces IPCE spectra with two less intense and well-separated peaks. The positive effect exerted by a cyanoacrylic acid group attached to the ethynylphenyl substituent has been analyzed by a photophysical and theoretical approach. This provided supporting evidence of a contribution from charge-transfer transitions to both the B and Q bands, thus producing, through conjugation, excited electrons close to the carboxylic anchoring group. Finally, the straightforward and effective synthetic procedures developed, as well as the efficiencies observed by photoelectrochemical measurements, make the described β-monosubstituted Zn(II) porphyrinates extremely promising sensitizers for use in DSSCs.

The absorption and emission properties of the two components of the yellow color extracted from weld (Reseda luteola L.), apigenin and luteolin, have been extensively investigated by means of DFT and TDDFT calculations. Our calculations reproduce the absorption spectra of both flavonoids in good agreement with the experimental data and allow us to assign the transitions giving rise to the main spectral features. For apigenin, we have also computed the electronic spectrum of the monodeprotonated species, providing a rationale for the red-shift of the experimental spectrum with increasing pH. The fluorescence emission of both apigenin and luteolin has then been investigated. Excited-state TDDFT geometry optimizations have highlighted an excited-state intramolecular proton transfer (ESIPT) from the 5-hydroxyl to the 4-carbonyl oxygen of the substituted benzopyrone moiety. By computing the potential energy curves at the ground and excited states as a function of an approximate proton transfer coordinate for apigenin, we have been able to trace an ESIPT pathway and thus explain the double emission observed experimentally.

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