A local point of view of the Cu(100) → NiTPP charge transfer at the NiTPP/Cu(100) interface.

A precise understanding, at the molecular level, of the massive substrate → adsorbate charge transfer at the NiTPP/Cu(100) interface has been gained through the application of elementary symmetry arguments to the structural determination of the NiTPP adsorption site by photoelectron diffraction (PED) measurements and Amsterdam density functional calculations of the free D4h NiTPP electronic structure. In particular, the PED analysis precisely determines that, among the diverse NiTPP chemisorption sites herein considered (fourfold hollow, atop, and bridge), the fourfold hollow one is the most favorable, with the Ni atom located at 1.93 Å from the surface and at an internuclear distance of 2.66 Å from the nearest-neighbors of the substrate. The use of elementary symmetry considerations enabled us to provide a convincing modeling of the NiTPP-Cu(100) anchoring configuration and an atomistic view of the previously revealed interfacial charge transfer through the unambiguous identification of the adsorbate π* and σ* low-lying virtual orbitals, of the substrate surface atoms, and of the linear combinations of the Cu 4s atomic orbitals involved in the substrate → adsorbate charge transfer. In addition, the same considerations revealed that the experimentally reported Ni(II) → Ni(I) reduction at the interface corresponds to the fingerprint of the chemisorption site of the NiTPP on Cu(100).

On surface chemical reactions of free-base and titanyl porphyrins with r-TiO2(110): a unified picture.

In this Perspective we present a comprehensive study of the multiple reaction products of metal-free porphyrins (2H-Ps) in contact with the rutile TiO2(110) surface. In the absence of peripheral functionalization with specific linkers, the porphyrin adsorption is driven by the coordination of the two pyrrolic nitrogen atoms of the macrocycle to two consecutive oxygen atoms of the protruding Obr rows via hydrogen bonding. This chemical interaction favours the iminic nitrogen uptake of hydrogen from near surface layers at room temperature, thus yielding a stable acidic porphyrin (4H-P). In addition, a mild annealing (∼100 °C) triggers the incorporation of a Ti atom in the porphyrin macrocycle (self-metalation). We recently demonstrated that such a low temperature reaction is driven by a Lewis base iminic attack, which lowers the energy barriers for the outdiffusion of Ti interstitial atoms (Tiint) [Kremer et al., Appl. Surf. Sci., 2021, 564, 150403]. In the monolayer (ML) range, the porphyrin adsorption site, corresponding to a TiO-TPP configuration, is extremely stable and tetraphenyl-porphyrins (TPPs) may even undergo conformational distortion (flattening) by partial cyclo-dehydrogenation, while remaining anchored to the O rows up to 450 °C [Lovat et al., Nanoscale, 2017, 9, 11694]. Here we show that, upon self-metalation, isolated molecules at low coverage may jump atop the rows of five-fold coordinated Ti atoms (Ti5f). This configuration is associated with the formation of a new coordination complex, Ti-O-Ti5f, as determined by comparison with the deposition of pristine titanyl-porphyrin (TiO-TPP) molecules. The newly established Ti-O-Ti5f anchoring configuration is found to be stable also beyond the TPP flattening reaction. The anchoring of TiO-TPP to the Ti5f rows is, however, susceptible to the cross-talk between phenyls of adjacent molecules, which ultimately drives the TiO-TPP temperature evolution in the ML range along the same pathway followed by 2H-TPP.

Ferrous to Ferric Transition in Fe-Phthalocyanine Driven by NO2 Exposure.

Due to its unique magnetic properties offered by the open-shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d-states of FePc and the sp-band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature.

Multireference Ab Initio Investigation on Ground and Low-Lying Excited States: Systematic Evaluation of J-J Mixing in a Eu3+ Luminescent Complex.

A theoretical protocol combining density functional theory (DFT) and multireference (CAS) calculations is proposed for a Eu3+ complex. In the complex, electronic levels of the central Eu3+ ion are correctly calculated at the CASPT2 level of theory, and the effect of introducing different numbers of states in the configuration interaction matrices is highlighted as well as the shortcomings of DFT methods in the treatment of systems with high spin multiplicity and strong spin-orbit coupling effects. For the 5D0 state energy calculation, the inclusion of states with different multiplicity and the number of states considered for each multiplicity are crucial parameters, even if their relative weight is different. Indeed, the addition of triplet and singlets is important, while the number of states is relevant only for the quintets. The herein proposed protocol enables a rigorous, full ab initio treatment of Eu3+ complex, which can be easily extended to other Ln3+ ions.

Nature of the Ligand-Centered Triplet State in Gd3+ ß-Diketonate Complexes as Revealed by Time-Resolved EPR Spectroscopy and DFT Calculations.

A series of Gd3+ complexes (Gd1-Gd3) with the general formula GdL3(EtOH)2, where L is a ß-diketone ligand with polycyclic aromatic hydrocarbon substituents of increasing size (1-3), was studied by combining time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy and DFT calculations to rationalize the anomalous spectroscopic behavior of the bulkiest complex (Gd3) through the series. Its faint phosphorescence band is observed only at 80 K and it is strongly red-shifted (∼200 nm) from the intense fluorescence band. Moreover, the TR-EPR spectral analysis found that triplet levels of 3/Gd3 are effectively populated and have smaller |D| values than those of the other compounds. The combined use of zero-field splitting and spin density delocalization calculations, together with spin population analysis, allows us to explain both the large red shift and the low intensity of the phosphorescence band observed for Gd3. The large red shift is determined by the higher delocalization degree of the wavefunction, which implies a larger energy gap between the excited S1 and T1 states. The low intensity of the phosphorescence is due to the presence of C-H groups which favor non-radiative decay. These groups are present in all complexes; nevertheless, they have a relevant spin density only in Gd3. The spin population analysis on NaL models, in which Na+ is coordinated to a deprotonated ligand, mimicking the coordinative environment of the complex, confirms the outcomes on the free ligands.

cis-[(η5-C5H5)Fe(η1-CO)(µ-CO)]2, the poor relative between cis and trans tautomers. A theoretical study of the gas-phase Fe L3-edge and C and O K-edge XAS of trans-/cis-[(η5-C5H5)Fe(η1-CO)(µ-CO)]2.

The relative stability of trans-[(η5-C5H5)Fe(η1-CO)(µ-CO)]2 (trans-I) and cis-I tautomers in a vacuum and in solvents with different dielectric constants (ε) has been investigated by exploiting density functional theory (DFT). Theoretical results indicate that, in agreement with experimental evidence, trans-I is more stable than cis-I in a vacuum (∼1.5 kcal mol-1; ε = 1), while the opposite is true in media with ε > 7. Differently from solution, DFT outcomes pertaining to the vapor-phase cis-I ⇆ trans-I equilibrium at T = 368 K, the temperature at which the Fe L2,3-edges and the C and O K-edge X-ray absorption spectroscopy (XAS) data of I have been recorded, ultimately indicate the trans-I predominance (∼93%). Compositions, oscillator strengths (f) and excitation energy (EE) values of cis-I transitions substantially mirror those of trans-I; nevertheless, the weighted cis-If(EE) distributions negligibly contribute to the diverse simulated XA spectra of I.

On-surface synthesis of extended linear graphyne molecular wires by protecting the alkynyl group.

In this paper we report on the use of an Ullmann-like aryl halide homocoupling reaction to obtain long Graphyne Molecular Wires (GY MWs) organized in dense, ordered arrays. Instead of using highly reactive terminal alkynes, we resort to a precursor wherein the acetylenic functional group is internal, namely protected by two phenyl rings, each bearing a Br atom in the para position to allow for linear homocoupling. In addition, two further factors concur with the production of dense and highly ordered arrays of very long GY MWs, namely the geometric compatibility between the substrate and both the organometallic intermediates and the final polymeric products of the synthesis, coupled with the presence of surface-adsorbed bromine atoms separating the MWs, which minimize inter-wire cross-linking secondary reactions.

On-Surface Photochemistry of Pre-Ordered 1-Methyl-2-phenyl-acetylenes: C-H Bond Activation and Intermolecular Coupling on Highly Oriented Pyrolytic Graphite.

In this contribution we report on light-induced metal-free coupling of propynylbenzene molecular units on highly oriented pyrolytic graphite. The reaction occurs within the self-assembled monolayer and leads to the generation of covalently coupled 1,5-hexadiyne and para-terphenyl derivatives under topological control. Such photochemical uncatalysed pathway represents an original approach in the field of topological C-C coupling at the solid/liquid interface and provides new insight into the low temperature formation of aromatic compounds at the surface of carbonaceous supports.

Comparative Experimental and Theoretical Study of the Fe L2,3-Edges X-ray Absorption Spectroscopy in Three Highly Popular, Low-Spin Organoiron Complexes: [Fe(CO)5], [(η5-C5H5)Fe(CO)(µ-CO)]2, and [(η5-C5H5)2Fe].

The occupied and unoccupied electronic structures of three highly popular, closed shell organoiron complexes ([Fe(CO)5], [(η5-C5H5)Fe(CO)(µ-CO)]2, and [(η5-C5H5)2Fe]) have been theoretically investigated by taking advantage of density functional theory (DFT) calculations coupled to the isolobal analogy ( Elian et al. Inorg. Chem. 1976 , 15 , 1148 ). The adopted approach allowed us to look into the relative role played by the ligand → Fe donation and the Fe → ligand back-donation in title molecules, as well as to investigate how CO- (terminal or bridging) and [(η5-C5H5)]--based π* orbitals compete when these two ligands are simultaneously present as in [(η5-C5H5)Fe(CO)(µ-CO)]2. Insights into the nature and the strength of the bonding between Fe and the C donor atoms have been gained by exploiting the Nalewajski-Mrozek bond multiplicity index ( Nalewajski et al. Int. J. Quantum Chem. 1994 , 51 , 187 ), which have been found especially sensitive even to tiny bond distance variations. The bonding picture emerging from ground state DFT results proved fruitful to guide the assignment of original, high-resolution, gas-phase L2,3-edges X-ray absorption spectra of the title molecules, which have been modeled by the two-component relativistic time-dependent DFT including spin orbit coupling and correlation effects and taking advantage of the full use of symmetry. Assignments alternative to those reported in the literature for both [Fe(CO)5] and [(η5-C5H5)2Fe] are herein proposed. Despite the high popularity of the investigated molecules, the complementary use of symmetry, orbital, and spectroscopy allowed us to further look into the metal-ligand symmetry-restricted-covalency and the differential-orbital covalency, which characterize them.

Comparative Experimental and Theoretical Study of the C and O K-Edge X-ray Absorption Spectroscopy in Three Highly Popular, Low Spin Organoiron Complexes: [Fe(CO)5], [(η5-C5H5)Fe(CO)(µ-CO)]2, and [(η5-C5H5)2Fe].

The unoccupied electronic structures of three closed-shell, highly popular organoiron complexes ([Fe(CO)5], [(η5-C5H5)Fe(CO)(µ-CO)]2, and [(η5-C5H5)2Fe]; 0, I, and II, respectively) have been investigated both experimentally and theoretically by combining original gas-phase X-ray absorption spectroscopy (XAS) outcomes recorded at the C and O K-edge with results of scalar relativistic time-dependent density functional calculations carried out within the zeroth order regular approximation. Experimental evidence herein discussed complement the Fe L2,3-edges XAS ones we recently recorded, modeled, and assigned for the same complexes (Carlotto et al. Inorg. Chem. 2019, 58, 5844). The first-principle simulation of the C and O K-edge features allowed us to univocally identify the electronic states associated to the ligand-to-metal charge transfer (LMCT) transitions both in I and in II. At variance to that, LMCT transitions with sizable oscillator strengths do not play any role in determining neither the C nor the O K-edge spectral pattern of 0. The higher π-acceptor capability of the CO ligand, regardless of its terminal or bridging coordination, with respect to [(η5-C5H5)]- is herein ultimately confirmed.

Theoretical Investigation of the Electronic Properties of Three Vanadium Phthalocyaninato (Pc) Based Complexes: PcV, PcVO, and PcVI.

The electronic properties of three vanadium phthalocyaninato (Pc) based complexes (PcV, PcVO, and PcVI; I-III, respectively) were theoretically investigated and corresponding VL2,3-edge XAS spectra modeled. Ground state (GS) DFT outcomes indicated that II is more stable than III by 141 kcal/mol; moreover, the Ziegler transition state method allowed us to estimate the PcV-X bond dissociation energy and to quantify σ/π contributions to the V-X interaction. As such, the Nalewajski-Mrozek V-X and V-N bond multiplicity indexes (V-O/V-I = 2.48/1.22; V-N = 0.64, 0.51, and 0.58 in I-III, respectively) state that the V-X bond strength and nature affect the V-N interaction. The coordination of X to V in the I → II/I → III reactions implies the transfer of two/one electrons from I to X. In both cases, the oxidation involves only the V ion; moreover, V 3d based orbitals from which electrons are transferred were identified. Literature I/IIL2,3-edge XAS data were modeled by exploiting the DFT/ROCIS method. The same protocol was adopted to predict IIIL2,3-edge XAS spectra. Theoretical results indicated that, along the whole series, spectral features lying at the lowest excitation energies (EEs) are mostly generated by states having the same GS spin multiplicity and involve 2pV → SOMO (single occupied molecular orbital) single electronic excitations. XAS features at higher EEs include only states with the same GS spin multiplicity in I, while states with both ΔS = 0 and ΔS = +1 (S = total spin quantum number) are present in II and III with significant, in some cases prevailing, contributions from metal to ligand charge transfer (MLCT) excitations. Beyond the role played by MLCT transitions in determining XAS patterns, it is noteworthy that they involve only Pc-based empty orbitals with no participation of the X-based virtual levels.

Reaction between Indazole and Pd-Bound Isocyanides-A Theoretical Mechanistic Study.

The mechanism of the addition of indazole (Ind)-a bifunctional aromatic N,NH-nucleophile-to cyclohexyl isocyanide coordinated to the palladium(II) center in the model complex cis-[PdCl2(CNMe)(CNCy)] (1) to give the corresponding aminocarbene ligand was investigated in detail by theoretical (DFT) methods. The most plausible mechanism of this reaction is that of the associative type involving nucleophilic attack of Ind by its unprotonated N atom at the isocyanide carbon atom followed by the stepwise proton transfer from the nucleophile molecule to the isocyanide N atom via deprotonation/protonation steps. Two reaction channels based on two tautomeric forms of indazole were found. The channel leading to the experimentally isolated aminocarbene product is based on the less stable tautomeric form. Another channel based on the more stable tautomer of Ind is slightly kinetically more favorable but it is endergonic. Thus, the regioselectivity of this reaction is thermodynamically rather than kinetically driven. The bonding situation in key species was analyzed.

Metal-Free on-Surface Photochemical Homocoupling of Terminal Alkynes.

On-surface synthesis involving the homocoupling of aryl-alkynes affords the buildup of bisacetylene derivatives directly at surfaces, which in turn may be further used as ingredients for the production of novel functional materials. Generally, homocoupling of terminal alkynes takes place by thermal activation of molecular precursors on metal surfaces. However, the interaction of alkynes with surface metal atoms often induces unwanted reaction pathways when thermal energy is provided to the system. In this contribution we report about light-induced metal-free homocoupling of terminal alkynes on highly oriented pyrolitic graphite (HOPG). The reaction occurred with high efficiency and selectivity within a self-assembled monolayer (SAM) of aryl-alkynes and led to the generation of large domains of ordered butadiynyl derivatives. Such a photochemical uncatalyzed pathway represents an original approach in the field of topological C-C coupling at the solid/liquid interface.

Theoretical modeling of the L2,3-edge X-ray absorption spectra of Mn(acac)2 and Co(acac)2 complexes.

Mn(acac)2 (I) and Co(acac)2 (II) L2,3-edge absorption spectra have been modeled using the DFT/ROCIS method. In addition to the agreement between experiment and theory, the combined use of the B3LYP exchange-correlation functional and the def2-TZVP(-f) basis set provided useful information about the coordinative geometry around the M(ii) ions as well as about the nature and the strength of the Mn-O and Co-O interaction. The lower excitation energy (EE) side of both (I)(/)(II)L3 and (I)(/)(II)L2 intensity distributions mainly includes states having ground state spin multiplicity (S = 5/2 in I and S = 3/2 in II), whereas states with lower spin multiplicity (S = 3/2 in I and S = 1/2 in II) significantly contribute to the higher EE side of both (I)(/)(II)L3 and (I)(/)(II)L2. Hence, the occurrence of states involving metal to ligand charge transfer transitions in the presence of ligands with low lying empty π* orbitals on the L3 and L2 higher EE sides is herein confirmed.

Electronic structures of CuTPP and CuTPP(F) complexes. A combined experimental and theoretical study I.

Copper complexes of tetraphenylporphyrin (H2TPP) and tetrakis(pentafluorophenyl)porphyrin (H2TPP(F)) deposited as thin films on Au(111) have been studied experimentally and theoretically. Core level emissions from C 1s, N 1s, F 1s and Cu 2p as well as valence states of CuTPP and CuTPP(F) have been investigated using surface photoelectron spectroscopy. The interpretation of experimental results has been guided by theoretical calculations carried out on isolated species in the habit of the density functional theory. Reference to experimental and theoretical outcomes pertaining to H2TPP and H2TPP(F) allowed a confident and detailed assignment of the title molecules' X-ray and ultraviolet photoemission data. With specific reference to the latter, similar to copper phthalocyanine (CuPc), whose coordinative pocket mirrors the CuTPP/CuTPP(F) ones, the lowest ionization energy of the title compounds implies electron ejection from a ring orbital rather than from the Cu 3d-based singly occupied molecular orbital. Moreover, analogous to CuPc, the ionic contribution appears to play an important role in the Cu-N bonding. Nevertheless, differences in the number, symmetry, nature and relative position of CuTPP/CuTPP(F) occupied frontier orbitals compared to CuPc may be stated only by considering in great detail the Cu-ligand covalent interactions.

Electronic structure of CuTPP and CuTPP(F) complexes: a combined experimental and theoretical study II.

The unoccupied electronic structure of thick films of tetraphenylporphyrin and tetrakis(pentafluorophenyl)porphyrin Cu(ii) complexes (hereafter, CuTPP and CuTPP(F)) deposited on Au(111) has been studied by combining the outcomes of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy with those of spin-unrestricted time-dependent density functional (TD-DFT) calculations carried out either within the scalar relativistic zeroth order regular approximation (ZORA) framework (C, N and F K-edges) or by using the Tamm-Dancoff approximation coupled to ZORA and including spin-orbit effects (Cu L2,3-edges). Similarly to the modelling of NEXAFS outcomes pertaining to other Cu(ii) complexes, the agreement between theory and experiment is more than satisfactory, thus confirming the open-shell TD-DFT to be a useful tool to look into NEXAFS results pertinent to Cu(ii) compounds. The combined effect of metalation and phenyl (Ph) fluorine decoration is found to favour an extensive mixing between (Ph)σ* and pristine porphyrin macrocyle (pmc) (pmc)π* virtual levels. The lowest lying excitation in the C and N K-edge spectra of both CuTPP and CuTPP(F) is associated with a ligand-to-metal-charge-transfer transition, unambiguously revealed in the (CuTPP)N K-edge spectral pattern. Moreover, the comparison with literature data pertaining to the modelling of the (Cu(II))L2,3 features in the phthalocyanine-Cu(ii) (CuPc) complex provided further insights into how metal-to-ligand-charge-transfer transitions associated with excitations from 2p(Cu(II)) AOs to low-lying, ligand-based π* MOs may contribute to the Cu(ii) L2,3-edge intensity and thus weaken its believed relationship with the Cu(ii)-ligand symmetry-restricted covalency. Despite the coordinative pocket of CuTPP/CuTPP(F) mirroring CuPc, the ligand-field strength exerted by the phthalocyanine ligand on the Cu(ii) centre is experimentally found and theoretically confirmed to be slightly stronger than that experienced by Cu in CuTPP and CuTPP(F). On the whole, the obtained results complement those published in the near past by the same group on the occupied and empty states of the H2TPP and H2TPP(F) free ligands as well as on the occupied states of both CuTPP and CuTPP(F), thus providing the final piece to get a thorough description of electronic perturbations associated with the metalation and the Ph halogen decoration of H2TPP.

Molecules-oligomers-nanowires-graphene nanoribbons: a bottom-up stepwise on-surface covalent synthesis preserving long-range order.

We report on a stepwise on-surface polymerization reaction leading to oriented graphene nanoribbons on Au(111) as the final product. Starting from the precursor 4,4″-dibromo-p-terphenyl and using the Ullmann coupling reaction followed by dehydrogenation and C-C coupling, we have developed a fine-tuned, annealing-triggered on-surface polymerization that allows us to obtain an oriented nanomesh of graphene nanoribbons via two well-defined intermediate products, namely, p-phenylene oligomers with reduced length dispersion and ordered submicrometric molecular wires of poly(p-phenylene). A fine balance involving gold catalytic activity in the Ullmann coupling, appropriate on-surface molecular mobility, and favorable topochemical conditions provided by the used precursor leads to a high degree of long-range order that characterizes each step of the synthesis and is rarely observed for surface organic frameworks obtained via Ullmann coupling.

Hydrogen capture by porphyrins at the TiO2(110) surface.

Metal-free porphyrin molecules adsorb on the rutile TiO2(110) surface with their pyrrolic nitrogen atoms atop the O-bridge rows, whereas the iminic nitrogen atoms capture two additional hydrogen atoms. Hydrogenation occurs spontaneously at room temperature, irrespective of the distance of the polypyrrolic macrocycle from the surface, as varied by changing the porphyrin functionalization.

XAS of tetrakis(phenyl)- and tetrakis(pentafluorophenyl)-porphyrin: an experimental and theoretical study.

The unoccupied electronic structure of tetrakis(phenyl)- and tetrakis(pentafluorophenyl)-porphyrin thick films deposited on SiO2/Si(100) native oxide surfaces has been thoroughly studied by combining the outcomes of near-edge X-ray absorption fine structure spectroscopy at the C, N, and F K-edges with those of scalar relativistic zeroth order regular approximation time-dependent density functional theory calculations carried out on isolated molecules. Both experimental and theoretical results concur to stress the electronic inertness of pristine porphyrin macrocycle based 1s(C)→π* and 1s(N)→π* transitions whose excitation energies are substantially unaffected upon fluorination. The obtained results complement those published by the same group about the occupied states of both molecules, thus providing the missing tile to get a thorough description of the halide decoration effects on the electronic structure of the tetrakis(phenyl)-porphyrin.

Stereoselective photopolymerization of tetraphenylporphyrin derivatives on Ag(110) at the sub-monolayer level.

We explore a photochemical approach to achieve an ordered polymeric structure at the sub-monolayer level on a metal substrate. In particular, a tetraphenylporphyrin derivative carrying para-amino-phenyl functional groups is used to obtain extended and highly ordered molecular wires on Ag(110). Scanning tunneling microscopy and density functional theory calculations reveal that porphyrin building blocks are joined through azo bridges, mainly as cis isomers. The observed highly stereoselective growth is the result of adsorbate/surface interactions, as indicated by X-ray photoelectron spectroscopy. At variance with previous studies, we tailor the formation of long-range ordered structures by the separate control of the surface molecular diffusion through sample heating, and of the reaction initiation through light absorption. This previously unreported approach shows that the photo-induced covalent stabilization of self-assembled molecular monolayers to obtain highly ordered surface covalent organic frameworks is viable by a careful choice of the precursors and reaction conditions.