Synthesis, Spectroscopy, and Electrochemistry of Tetra-tert-butylated Tetraazaporphyrins, Phthalocyanines, Naphthalocyanines, and Anthracocyanines, together with Molecular Orbital Calculations

Magnetic Circular Dichroism of Porphyrins Containing M = Ca, Ni, and Zn. A Computational Study Based on Time-Dependent Density Functional Theory

A theoretical study is presented on the magnetic circular dichroism (MCD) exhibited by the porphyrin complexes MP (M = Mg,Ni,Zn), MTPP (M = Mg,Ni,Zn), and NiOEP, where P = porphyrin, TPP = tetraphenylporphyrin, and OEP = octaethylporphyrin. The study makes use of a newly implemented method for the calculation of A and B terms from the theory of MCD and is based on time-dependent density functional theory (TD-DFT). It is shown that the MCD spectrum is dominated by a single positive A term in the Q-band region in agreement with experiment where available. The band can be fully explained as the first transition in Gouterman's four-orbital model for the type of porphyrins studied here. For the Soret band, the experimental MCD spectrum appears as a single positive A term. This is also what is found computationally for NiP and NiTPP, where the second transition in Gouterman's four-orbital model give rise to a positive A term. However, for the remaining systems, the simulated MCD spectrum is actually due to two B terms that have the appearance of one positive pseudo A term. The two B terms appear because the second Gouterman state is coupled strongly to a second excited state (b(2u) --> 2e(g)) of nearly the same energy by the external magnetic field.

Near-infrared two-photon absorption in phthalocyanines: enhancement of lowest gerade-gerade transition by symmetrical electron-accepting substitution

This paper presents, to the best of our knowledge, the first study of two-photon absorption (2PA) spectra of a number of symmetrically substituted phthalocyanines in the excitation wavelength region from lambda(ex)=800 to 1600 nm. The selected molecules vary by position of substitution (alpha or beta), number of substituent groups (4, 8, or 16), and presence or absence of metal (Zn or Al) in the center. For all phthalocyanines we find a moderately strong (sigma(2) approximately 100-200 GM), pure electronic, gerade-gerade (g-g) 2PA transition, which shows up as a well-resolved relatively narrow peak in the energy region between Q and B bands (lambda(ex)=870-1100 nm). In metallophthalocyanines (MPcs) this lowest g-g transition is followed by the onset of other higher-frequency 2PA transitions. In some metal-free phthalocyanines (H(2)Pcs) we also reveal a second, broader 2PA transition at slightly higher frequency. In both MPcs and H(2)Pcs, we find a strong monotonic increase of integrated strength of the lowest g-g transition as a function of electron-accepting ability of peripheral substituents, expressed as their aggregated Hammett constant. By using few essential states models (three states for MPcs and four states for H(2)Pcs) we demonstrate the primary role of excited-state transition dipole moment in this effect.

Subphthalocyanine-Fused Dimers and Trimers: Synthetic, Electrochemical, and Theoretical Studies

Subphthalocyanine (SubPc)-fused dimers and trimers bearing fluorine, iodine, and thioether peripheral substituents were synthesized and characterized. Absorption spectroscopy and electrochemical studies revealed (i) that the substituents have a strong effect on the electronic properties of the macrocycles and (ii) that there is good communication between the subphthalocyaninic moieties within the oligomeric structures. Theoretical calculations at DFT/6-31G(d,p) computational level and electron density studies support the experimental findings. The frontier orbitals in the dimers and trimers were also shown to be significantly altered with respect to those of SubPcs as a consequence of the extension of the conjugation associated with symmetry breaking. Time-dependent density functional theory calculations reproduced the differences observed in the UV-vis spectra of the fused dimers and the monomeric SubPcs.

Tautomerization in naphthalenediimines: a keto-enamine Schiff base macrocycle

[reaction: see text] A new [3 + 3] Schiff base macrocycle incorporating naphthalene groups has been prepared. By examination of its properties, X-ray crystallography of model compounds, and calculations, it has been determined that the macrocycle exists predominantly as the keto-enamine tautomer. This unexpected tautomerization presents an unusual hexaketo interior in the macrocycle.

NMR spectra of free-base porphine, porphyrazine, phthalocyanine and naphthalocyanine as well as their metal complexes: density functional calculations

Nuclear magnetic shielding tensors of porphine have been calculated at density functional B3LYP and PBE level using the gauge independent atomic orbital (GIAO) method. The geometries used were optimized using the 6-31G(d) basis set and the NMR calculations were performed using 6-31G(d) and 6-311G(d,p) basis sets, respectively. The calculated NMR shielding tensors and chemical shifts of porphine are compared with previous calculations as well as experimental data and satisfying results are obtained. Further NMR calculations are extended to metal-free and metallo-porphyrazine, -phthalocyanine, and -naphthalocyanine for the first time and the results are compared with experimental data available. The chemical shifts of the atoms in these compounds are assigned according to the experimental data available.

Application of MCD spectroscopy and TD-DFT to a highly non-planar porphyrinoid ring system. New insights on red-shifted porphyrinoid spectral bands

The first magnetic circular dichroism (MCD) spectra are reported for tetraphenyltetraacenaphthoporphyrin (TPTANP). The impact on the electronic structure of steric interactions between the fused acenaphthalene rings and the meso-tetraphenyl substituents is explored based on an analysis of the optical spectra of the Zn(II) complex (ZnTPTANP) and the free base dication species ([H4TPTANP]2+). In the case of ZnTPTANP, significant folding of the porphyrinoid ligand induces a highly unusual MCD-sign reversal providing the first direct spectroscopic evidence of ligand nonplanarity. Density functional theory (DFT) geometry optimizations for a wide range of Zn(II) porphyrinoids based on the B3LYP functional and TD-DFT calculations of the associated UV-visible absorption spectra are reported, allowing a complete assessment of the MCD data. TPTANP complexes are found to fall into a class of cyclic polyenes, termed as soft MCD chromophores by Michl (J. Pure Appl. Chem. 1980, 52, 1549.), since the signs of the Faraday A1 terms observed in the MCD spectrum are highly sensitive to slight structural changes. The origin of an unusually large red shift of the main B (or Soret) band of MTPTANP (the most red shifted ever reported for fused-ring-expanded metal porphines) and of similar red shifts observed in the spectra of other peripherally crowded porphyrinoid complexes is also explored and explained on this basis.

Synthesis and Photophysical Properties of Annulated Dinuclear and Trinuclear Phthalocyanines

Metal-free mononuclear, dinuclear and trinuclear phthalocyanines were prepared by a mixed cyclotetramerisation of a 1,2,4,5-tetracyanobenzene derivative and 4,5-bis(2,6-dimethylphenoxy)phthalonitrile. For the first time, a pi-electron-conjugated trinuclear phthalocyanine was synthesised with phthalocyanine units connected by common annulated benzene rings. The Q band of the trinuclear compound in solution occurs at lambda = 944 nm whereas those of the dinuclear and mononuclear compounds are at lambda = 853/830 and 701/664 nm, respectively. Fluorescence quantum yields, fluorescence lifetimes and singlet-oxygen quantum yields of the compounds were determined.

Electron-Donating or -Withdrawing Nature of Substituents Revealed by the Electrochemistry of Metal-Free Phthalocyanines

The effect of substituents on the electrochemistry of metal-free phthalocyanines was examined for 17 phthalocyanine compounds. This work also provides new information about the electron-donating or -withdrawing nature of various substituents, namely, alkoxy, alkylthio, alkyl, alkynyl, phenyloxy, and phenylthio groups attached to the phthalocyanine system, from the viewpoint of electrochemistry. Most of the effects of peripheral and nonperipheral substitution and changes in the ring (pi-conjugated system) size on the electrochemistry of metal-free phthalocyanines can be reasonably explained by considering the energy levels of frontier molecular orbitals of the corresponding compounds, which were obtained by calculations using the semiempirical PM3 method.

Synthesis, DFT calculations, linear and nonlinear optical properties of binuclear phthalocyanine gallium chloride

The axially substituted binuclear GaCl/GaCl phthalocyanine 1 with an unsymmetrical pattern of substitution has been prepared and its nonlinear optical (NLO) properties determined. The resulting binuclear complex retains approximately the same transition energies of monomeric (RO)8PcGaCl as far as the linear optical spectrum is concerned, although 1 has a double concentration of central atoms per molecule and an enlarged conjugated ligand. The lack of significant spectral shifts in passing from mononuclear to binuclear complexes has been rationalized theoretically by means of density functional theory calculations. The purpose of the present study is to determine whether binuclearity affects the optical limiting behavior of 1 with respect to monomeric (RO)8PcGaCl in the NLO regime determined by nanosecond laser pulses.