The electronic properties of three popular high spin complexes [TM(acac)3, TM = Cr, Mn, and Fe] revisited: an experimental and theoretical study.

The occupied and unoccupied electronic structures of three high spin TM(acac)3 (TM = Cr, Mn, and Fe) complexes (I, II, and III, respectively) were studied by revisiting their literature vapour-phase He(i) and, when available, He(ii) photoemission (PE) spectra and by means of original near-edge X-ray absorption fine structure (NEXAFS) spectroscopic data recorded at the O K-edge (OK-edge) and TM L2,3-edges (TML2,3-edges). The assignments of the vapour-phase He(i)/He(ii) PE spectra were guided by the results of spin-unrestricted non-relativistic Slater transition state calculations, while the OK-edge and TML2,3-edge spectroscopic pieces of evidence were analysed by exploiting the results of spin-unrestricted scalar-relativistic time-dependent density functional theory (DFT) and DFT/ROCIS calculations, respectively. Although the actual symmetry (D3, in the absence of any Jahn-Teller distortion) of the title molecules allowed an extensive mixing between TM t2g-like and eg-like atomic orbitals, the use of the Nalewajski-Mrozek TM-O bond multiplicity index combined with a thorough analysis of the ground state (GS) outcomes allowed the assessment of the TM-O bond weakening associated with the progressive TM 3d-based eg-like orbital filling. The experimental information provided by OK-edge spectra was rather poor; nevertheless, the combined use of symmetry, orbitals and spectra allowed us (i) to rationalise minor differences characterizing spectral features along the series, (ii) to quantify the contribution provided by the ligand-to-metal-charge-transfer (LMCT) excitations to the different spectral features, and (iii) to recognize the t2g-/eg-like nature of the TM 3d-based orbitals involved in LMCT transitions. As far as the TML2,3-edge spectra and the DFT/ROCIS results were concerned, the lowest lying I,IIL3 spectral features included states having either the GS spin multiplicity (S(I) = 3/2, S(II) = 2) or, at higher excitation energies (EEs), states with ΔS = ±1. In contrast to that, only states with ΔS = 0, -1 significantly contributed to the IIIL3 spectral pattern. Along the whole series, the L3 higher EE side was systematically characterized by states involving TM2p → π4 MLCT excitations; as such, coupled-single excitations with ΔS = 0 were involved in I and II, while single MLCT TM2p → π4 transitions with ΔS = -1 were involved in III.

L2,3-edges absorption spectra of a 2D complex system: a theoretical modelling.

L2,3-edges absorption spectra of FePc (I) and FePc(η2-O2) (II) on Ag(110) have been modelled using the DFT/ROCIS method. Despite disregarding the presence of the substrate, the agreement between experiment and theory is remarkable. Moreover, theoretical results confirm the fraction of II (70%) present on the surface, thus allowing a thorough assignment of each experimental spectral feature. Ground state (GS) theoretical outcomes pertaining to I and II provide an intimate understanding of the electron transfer pathway ruling the I-based catalytic oxygen reduction reaction. DFT/ROCIS outcomes indicate that the lower excitation energy (EE) side of the I/IIL3 intensity distributions mainly includes states having the GS number of unpaired electrons (two in I and six in II), whereas states with higher/lower spin multiplicity contribute to the I/IIL3 higher EE side. The occurrence of states involving metal to ligand charge transfer transitions implying low lying empty π* ligand-based orbitals on the I/IIL3 higher EE sides have been confirmed.

Multipolar symmetric squaraines with large two-photon absorption cross-sections in the NIR region.

The two-photon absorption (TPA) properties of two extended symmetric squaraine dyes are thoroughly characterized from the experimental and quantum-chemical point of view. The two molecules are specially engineered to have a multipolar structure with increasing complexity, D-π-A-π-D and A'-π-D-π-A-π-D-π-A', respectively. The experimental TPA spectra, measured by means of the Z-scan technique in the femtoseconds regime, display considerably high values of TPA cross sections (σ(TPA)) for both molecules. In particular, the squaraine with the more extended structure shows the highest value of σ(TPA) ever reported for this class of molecules. CIS and TDDFT calculations of the one and two-photon absorption properties are carried out to clarify the origin of the observed TPA properties and fully characterize the electronic properties of these compounds. The calculations, in good agreement with the experimental data, suggest that the origin of this exceptionally high σ(TPA) can be ascribed to the presence of a peripheral A' group, that increases the density of excited states involved in the TPA process.

Dimers of polar chromophores in solution: role of excitonic interactions in one- and two-photon absorption properties.

The possibility to exploit a bottom-up approach to design and synthesize multichromophoric structures from a single molecular unit is strategic for the targeted synthesis of molecular compounds with well defined linear and nonlinear absorption properties. In this view, it is important to be able to predict the properties of multichromophoric units, based on the knowledge of the properties of the individual chromophores and their mutual arrangement. To this end, we present a combined experimental and theoretical study on 4-(para-di-n-butylaminostyryl)-pyridine, a push-pull molecule, and its dimer, 4,4'-bis(para-di-n-butylaminostyryl)-2,2'-bipyridine, formed by connecting the two pyridine groups into a bipyridine structure. One photon absorption and fluorescence spectra are measured in solvents of different polarity, and two-photon absorption spectra are recorded in dichloromethane. Experimental results are compared with results of TDDFT (Time-Dependent Density Functional Theory) and CIS (Configuration Interaction with Single excitation) methods implemented in the Gaussian03 program suite. An essential-state analysis of optical spectra is used to rationalize the observed behavior.