Elena A. Makarova

Spectroscopy, electrochemistry, and molecular orbital calculations of metal-free tetraazaporphyrin, -chlorin, -bacteriochlorin, and -isobacteriochlorin.

Metal-free tetraazachlorin (TAC), -bacteriochlorin (TAB), and -isobacteriochlorin (TAiB) were characterized by electronic absorption, magnetic circular dichroism (MCD), fluorescence, and time-resolved ESR (TR-ESR) spectroscopy, and by cyclic voltammetry. The results are compared with those of metal-free tetraazaporphyrin (TAP). The potential difference DeltaE between the first oxidation and reduction couples decreases in the order TAP>TAiB>TAC>TAB. The splitting of both the Q and Soret bands decreases in the order TAB>TAC>TAP>TAiB. Corresponding to the split absorption bands, MCD spectra show a minus-to-plus pattern with increasing energy in both the Q and Soret regions, which suggests that the energy difference between the HOMO and second HOMO is larger than that between the LUMO and second LUMO. These spectroscopic properties and redox potentials were reproduced by molecular orbital calculations using the ZINDO/S Hamiltonian. The fluorescence quantum yields of the reduced species are much smaller than that of TAP. The zero-field splitting (ZFS) parameters D and E of the excited triplet states (T1) of these species decrease and increase, respectively, on going from TAP to TAC and further to TAB. The D and E values of TAiB are larger than those of the other species. The results are supported by the absence of interaction between the spin over reduced pyrrole moieties of the HOMO and over the LUMO, and by calculations of ZFS under a half-point-charge approximation.

New synthesis of tetraazachlorins and tetraazabacteriochlorins by Diels–Alder reaction of tetraazaporphine

The use of the Diels–Alder reaction of unsubstituted tetraazaporphine as dienophile with some dienes of the anthracene series for the synthesis of novel substituted tetraazachlorins, tetraazabacteriochlorins and tetraazaisobacteriochlorins is reported. The condensation of tetraazaporphine with anthracene, 9,10-dimethyl- or 2,6-di(tert-butyl)anthracene in boiling chlorobenzene gives novel dibenzobarreleno-substituted tetraazachlorins 3–5 in yields of up to 71%. When AlCl3 was used as catalyst in the reaction of tetraazaporphine with 2,6-di(tert-butyl) anthracene, the corresponding cis- and trans-dibenzobarreleno-substituted tetraazabacteriochlorins 6a and 6b were obtained. The reaction of tetraazaporphine with the more active diene 2,3-benzanthracene at 130°C gives mainly benzonaphtho[2,3-b]barreleno-substituted tetraazachlorin 7, while the same reaction conducted at 220°C led to a mixture of cis- and trans isomers of tetraazabacteriochlorins 8a and 8b and tetraazaisobacteriochlorins 9a and 9b. The products were characterized using elemental analysis data and mass, 1HNMR and visible spectra. The effect of hydrogenation of one or two double bonds of the tetraazaporphine macrocycle on the electronic absorption spectra is discussed.

Design and synthesis of tetraazachlorins, tetraazabacteriochlorins and tetraazaisobacteriochlorins

Three approaches to the synthesis of reduced derivatives of tetraazaporphine such as tetraazachlorin, tetraazabacteriochlorin, and tetraazaisobacteriochlorin are reviewed. The first synthesis of alkyl-substituted tetraazachlorins was achieved by the catalytic hydrogenation of magnesium complexes of tetraazaporphines. Two other synthesis approaches for reduced tetraazaporphines based on the mixed condensation of the precursors with different hydrogenation levels and β–β addition reactions have been developed in the last decade. The use of tetramethylsuccinonitrile as the saturated component in the mixed condensation with derivatives of aromatic and heteroaromatic dicarboxylic acids enable the synthesis of oxidation-resistant benzo-; 1,2- and 2,3-naphtho-; pyrazino-; 2,3- and 3,4-pyridino-fused tetraazachlorins, tetraazabacteriochlorins, and tetraazaisobacteriochlorins. Fullerene conjugates of reduced tetraazaporphine derivatives were obtained using 1,2-dicyanofullerene as the source of the hydrogenated sites. Tetraazaporphine can act as dienophile in Diels-Alder reaction with dienes of the anthracene and cyclopentadiene series and also as dipolarophile in 1,3-dipolar cycloadditions with azomethine ylides and nitrones. In addition, the effect of the reduction of peripheral double bonds in tetraazaporphine macrocycle alone, and in combination with annelation of benzene or heterocyclic rings on the absorption spectra is discussed.

Synthesis and spectroscopic and electrochemical studies of pyrazine- or pyridine-ring-fused tetraazachlorins, bacteriochlorins, and isobacteriochlorins.

Mixed condensation of tetramethylsuccinonitrile and either 2,3-dicyano-5,6-diethylpyrazine, 2,3-dicyanopyridine, or 3,4-pyridinedicarboximide in the presence of nickel chloride forms novel pyrazine-, 2,3-pyridine-, or 3,4-pyridine-ring-fused tetraazachlorin (TAC), tetraazabacteriochlorin (TABC), and tetraazaisobacteriochlorin (TAiBC) derivatives. All possible structural isomers were separated using repeated thin-layer chromatography and have been investigated by absorption and magnetic circular dichroism spectroscopy. Similarly to previously reported TAC analogues, the TAC and TABC derivatives show large splitting of the Q band, while a single, intense absorption band is observed for the TAiBC derivatives. Although the absorption spectra are practically identical in shape for the separated structural isomers of TACs and TABCs, the Q-band maxima of the TAiBCs depend significantly on their structures. The observed spectroscopic properties were interpreted on the basis of electrochemical data and the results of (time-dependent) DFT calculations.

An efficient synthesis of metal-free tetraazachlorins via indium complexes

An efficient synthetic route for the preparation of benzene or 1,2-naphthalene fused and phenyl substituted metal-free tetraazachlorins with yields up to 40% was developed using In(III) as a removable template. New substituted tribenzotetraazachlorins derivatives with tert-butyl and phenylsulfanyl groups in β and α position of fused benzene rings, correspondingly, were synthesized by novel approach and their spectral properties were investigated.

Alternative selection rules for one- and two-photon transitions in tribenzotetraazachlorin: Quasi-centrosymmetrical π-conjugation pathway of formally non-centrosymmetrical molecule

We compare the two-photon absorption (2PA) spectra of non-centrosymmetrical metal-free tribenzo-tetraazachlorin (H2TBTAC) and analogous symmetrical tetra-tert-butyl-phthalocyanine (H2TtBuPc). Surprisingly, despite formal lack of center of inversion, the 2PA spectrum of H2TBTAC displays a two-photon allowed transition at 935 nm, similar to gerade-gerade (g-g) transitions observed in H2TtBuPc and in other symmetrical phthalocyanines. This transition is even better resolved in the singlet-singlet excited-state absorption spectrum. We tentatively explain the survival of the g-g transition in H2TBTAC by assuming that the main π-electron conjugation pathway in the tetraaza-substituted tetrapyrrole macrocycle bypasses the outer parts of the two oppositely located isoindole rings and thus renders the optically responsive core of the chromophore quasi-centrosymmetrical. By using the independently measured ground- and excited-state absorption extinction coefficients, we also show that the two-photon absorptivity can be quantitatively explained by a simple three-level model with the lowest energy Q1 state serving as an intermediate level.

Phthalocyanines and related compounds: XLII. Synthesis of phenyl-substituted tetraazachlorins and tetraazabacteriochlorins

New oxidation-resistant hexaphenyl-substituted tetramethyltetraazachlorin, its nickel complex, and tetraphenyl-substituted octamethyltetraazabacteriochlorin and octamethyltetraazaisobacteriochlorin nickel complexes were synthesized for the first time by mixed condensation of diphenylm aleonitrile with tetramethylsuccinonitrile in the presence of lithium dimethylaminoethoxide or nickel chloride. The products were characterized by electronic absorption, 1H NMR, and mass spectra. Mixed condensation of diphenylsuccinonitrile with phthalonitrile in the presence of nickel chloride was found to give nickel complexes of phenylsubstituted benzo-fused tetraazaporphyrins rather than their hydrogenated derivatives.

Synthesis and properties of tetra- and octacationic meso-tetrakis(3-pyridyl)bacteriochlorin derivatives

The synthesis and properties of new water-soluble tetra- and octacationic derivatives of meso-tetrakis(3-pyridyl)bacteriochlorin are reported. Tetracationic salt was synthesized by quaternization of the pyridine groups of starting bacteriochlorin with 1,4-dibromobutane. Octacationic salts were obtained by treating of tetracationic salt with excess of pyridine or N,N-dimethylaminoethanol. Zinc complexes of water-soluble bacteriochlorin derivatives were prepared by direct metalation of corresponding non-metal compounds with zinc acetylacetonate, unlike to neutral bacteriochlorin which forms zinc complex only by transmetalation of the corresponding cadmium complex. The new bacteriochlorins have strong absorption in red and NIR spectral region. Free-base BCs possess good stability in aqueous solutions as compared to their zinc complexes and efficiently generate singlet oxygen.

Phthalocyanines and related compounds: XLVI. [4+2]-cycloaddition of tetraazaporphine to dienes of the cyclopentadiene series

The [4+2]-cycloaddition reaction of unsubstituted tetraazaporphine as the dienophile with a series of dienes of the cyclopentadiene series gave novel norbornene-condensed tetraazachlorins, tetraazabacteriochlorins, and tetraazaisobacteriochlorins. It was shown that, depending on the type of the diene, the reagent ratio, reaction time, and temperature mono-or bis-adducts are formed. The compounds obtained were characterized by the elemental analyses, electronic absorption spectra, 1H NMR spectra, and mass spectra.

Resonance enhancement of two-photon absorption of phthalocyanines for 3D optical storage in the presence of hot-band absorption

We study a new series of asymmetrical phthalocyanines with promising properties for two-photon 3D optical memory. The chromophores have a strong and narrow one-photon absorption (1PA) S0 -> S1 band at 730-760 nm, with a 200-400 cm-1 width. Tuning the laser frequency 500-1000 cm-1 below the S0 -> S1 band allows for very high values of two-photon absorption (2PA) cross-section &sgr;2 > 104 GM. At temperatures T < 110K the molecules can be switched between two tautomer forms by 2PA. We measure 2PA spectra as a function of frequency and sample temperature. We provide an analysis of changes in 2PA spectra and cross sections due to different substituent groups. As a result, we found that among the set of studied molecules, there is one which is the most promising for rewritable 3D optical memory.