Density functional theory investigations of geometries and electronic spectra of lithium phthalocyanines

Tetrakis(1,2,5-thiadiazolo)porphyrazines. 9. Synthesis and spectral and theoretical studies of the lithium(i) complex and its unusual behaviour in aprotic solvents in the presence of acids

The template cyclotetramerization of 1,2,5-thiadiazolo-3,4-dicarbonitrile in the presence of lithium n-butoxide in n-butanol leads to the Li(i) complex of tetrakis(1,2,5-thiadiazolo)porphyrazine. Various possible structures of dilithium and monolithium complexes have been considered by DFT/B3LYP molecular modelling using the cc-pvtz basis set, and their theoretical IR and UV-VIS spectra have been calculated. The experimental ⁷Li NMR, IR and UV-VIS spectra measurements show that the complex contains two inequivalent lithium atoms - one is coordinated to the macrocyclic dianion to form the anionic lithate complex [TTDPaLi]^-, while the other forms the solvated countercation [Li(Solv)₄]⁺. The lithate complex is stable in protic solvents, such as methanol, and is soluble in water to give aggregated solutions. Its demetallation occurs in the presence of acids (CH₃COOH, CF₃COOH, H₂SO₄). In aprotic solvents (DMF, DMSO), the acid-catalyzed formation of the [TTDPa]^2- dianion is observed which is followed by the formation of the meso-protonated form {H[TTDPa]}^- at higher acid concentrations. Both processes can be reversed by the addition of a lithium salt excess or neutralization of the acid. The fluorescence quantum yield for the lithate complex [TTDPaLi]^- is much higher than that for the [TTDPa]^2- dianion (0.34 and 0.01 in DMSO), and this can be used for detecting low concentrations of acids and Li⁺ in aprotic solvents (10^-6-10^-5 M). The first reversible reduction of the macrocycle in the anionic lithate complex (-0.94 V vs. SCE in DMSO) is ∼0.5 V more difficult than that in the complexes with divalent metals [TTDPaM] (M = Mg^II, Zn^II, Cu^II).

Determination of the optical GAP in thin films of amorphous dilithium phthalocyanine using the Tauc and Cody models

Semiconducting thin films were grown on quartz substrates and crystalline silicon wafers, using dilithium phthalocyanine and the organic ligands 2,6-dihydroxyanthraquinone and 2,6-diaminoanthraquinone as the starting compounds. The films, thus obtained, were characterized by Fourier Transform infrared (FTIR), fast atomic bombardment (FAB+) mass and ultraviolet-visible (UV-Vis) spectroscopies. The surface morphology of these films was analyzed by means of atomic force microscopy (AFM) and scanning electron microscopy (SEM). It was found that the temperature-dependent electric current in all cases showed a semiconductor behavior with conductivities on the order of 10⁻⁶·S cm⁻¹, whereas the highest value corresponded to the thin film based upon the bidentate amine. The Tauc and Cody optical band gap values of thin films were calculated from the absorption coefficients and were found to be around 1.5 eV, with another strong band between 2.3 and 2.43 eV, arising from non-direct transitions. The curvature in the Tauc plot influencing the determination of the optical gap, the Tauc optical gap corresponding to the thicker film is smaller. The dependence of the Cody optical gap on the film thickness was negligible.

Theoretical and experimental investigation on the electronic properties of the shuttlecock shaped and the double-decker structured metal phthalocyanines, MPc and M(Pc)2 (M = Sn and Pb)

The molecular geometries, electronic structures, and excitation energies of tin and lead phthalocyanine compounds, SnPc, PbPc, Sn(Pc)(2), and Pb(Pc)(2), were investigated using the B3LYP method within a framework of density functional theory (DFT). The geometries of SnPc, PbPc, Sn(Pc)(2), and Pb(Pc)(2) were optimized under C(4v), C(4v), D(4d), and D(4d) molecular symmetries, respectively. The excitation energies of these molecules were computed by the time-dependent DFT (TD-DFT) method. The calculated results for the excited states of three compounds other than the unknown Pb(Pc)(2) corresponded well with the experimental results of electronic absorption spectroscopy. The non-planar C(4v) molecular structure of SnPc and PbPc influences especially on the orbital energy of the HOMO-1 through mixing of the s-type atomic orbital of the central metal atom to the π system of the Pc ring in an anti-bonding way; however, the HOMO and the LUMO have little effect of the deviation from the planar structure because they have no contribution from the atomic orbital of the central metal. This orbital mixing pushes up the orbital energy of the HOMO-1, and reduces the energy of the metal-to-ligand charge transfer band of SnPc and PbPc. The calculated results also reproduced well the excitation profile of Sn(Pc)(2), which was quite different from that of SnPc. The strong interactions between the π-type orbitals of two Pc moieties altered the electronic structure resulting in the characteristic excitation profile of Sn(Pc)(2). In addition, this caused a reduction of about 0.8 eV in the ionization potential as compared to usual MPcs including SnPc, which was consistent with the experimental results.

Theoretical and experimental study on the excited states of the X-, α- and β-forms of lithium phthalocyanine

The electronic structures and absorption spectra for three different types (X, α and β) of model dimers of lithium phthalocyanine (LiPc) were investigated by the density functional theory (DFT) and compared with a LiPc monomer. We quantitatively investigated the excited states of the three LiPc dimers using time-dependent DFT calculations. The differences and similarities of the observed absorption spectra in the solution and the polymorphic solids of LiPc were clearly interpreted by the calculated excited states of the monomer and dimers. The calculated results for the dimers presumed that the X-form showed a different electronic spectral pattern from the monomer and the other two forms, whereas the α- and β-forms presented similar electronic absorption profiles to each other and to the monomer. The calculated excited states also explained the differences in absorption profiles between LiPc and typical phthalocyanine compounds. These characteristic features of LiPc would be closely related to its molecular orbitals, especially those which originated from the singly occupied molecular orbital (SOMO) of the LiPc monomer. It was shown that the next highest occupied π-type orbital to the SOMO of the monomer reduced the energy of the low-lying excited states, which corresponded to the Q- and B-bands of the dimers.

Structures and Spectroscopic Properties of Bis(phthalocyaninato) Yttrium and Lanthanum Complexes: Theoretical Study Based on Density Functional Theory Calculations

Density functional theory (DFT) calculations were carried out to describe the molecular structures, molecular orbitals, atomic charges, UV-vis absorption spectra, IR, and Raman spectra of bis(phthalocyaninato) rare earth(III) complexes M(Pc)(2) (M = Y, La) as well as their reduced products [M(Pc)(2)](-) (M = Y, La). Good consistency was found between the calculated results and experimental data. Reduction of the neutral M(Pc)(2) to [M(Pc)(2)]- induces the reorganization of their orbitals and charge distribution and decreases the inter-ring interaction. With the increase of ionic size from Y to La, the inter-ring distance of both the neutral and reduced double-decker complexes M(Pc)(2) and [M(Pc)(2)](-) (M = Y, La) increases, the inter-ring interaction and splitting of the Q bands decrease, and corresponding bands in the IR and Raman spectra show a red shift. The orbital energy level and orbital nature of the frontier orbitals are also described and explained in terms of atomic character. The present work, representing the first systemic DFT study on the bis(phthalocyaninato) yttrium and lanthanum complexes sheds further light on clearly understanding structure and spectroscopic properties of bis(phthalocyaninato) rare earth complexes.