Esmeralda Caballero

Scrutinizing the chemical nature and photophysics of an expanded hemiporphyrazine: the special case of [30]trithia-2,3,5,10,12,13,15,20,22,23,25,30-dodecaazahexaphyrin.

Thirty π-electron-expanded hemiporphyrazines 1a-c have been prepared by crossover condensation reaction of 2,5-diamino-1,3,4-thiadiazole and the corresponding phthalonitrile (3) or diiminoisoindoline (4) derivatives. The expanded azaporphyrin hexamers have been unequivocally characterized by means of spectroscopic, crystallographic, and electrochemical techniques. Weak intramolecular hydrogen bonding imposes a planar conformation to macrocycles. However, the overall electronic delocalization is low, and the nature of the resulting [30]heteroannulene is nonaromatic, as confirmed by NMR studies, XR diffraction analysis, and calculation of the NICS(0) value. Studies on a wide range of physicochemical features including ground, excited, reduced, and oxidized states provide evidence for the wide applicability of these 30 π-electron-expanded hemiporphyrazines in processes involving electron transfer. A key asset of our work is the systematic development of spectroscopic and kinetic markers for the formation and decay of all of the aforementioned species. Thirty π-electron-expanded hemiporphyrazines evolve as broadly absorbing light harvesters with excited state energies of around 2.3 eV that are susceptible to facile one-electron reduction and one-electron oxidation reactions.

Cyclopentadienylruthenium π Complexes of Subphthalocyanines: A “Drop‐Pin” Approach To Modifying the Electronic Features of Aromatic Macrocycles

Facing facts: Coordination of Cp*Ru (Cp*=C(5)Me(5)) to the concave and convex π surfaces of subphthalocyanines constitutes a new approach to the functionalization of subazaporphyrins. While the convex face shows higher reactivity, coordination to the concave side produces a stronger diatropic influence on the Cp* ligand and a greater perturbation of the macrocyclic π-electronic features.

Physicochemical Characterization of Subporphyrazines—Lower Subphthalocyanine Homologues

Physicochemical characterization of boron(III) subporphyrazines (SubPzs)--lower subphthalocyanine (SubPc) homologues--has been carried out for the first time. The SubPz macrocycle can act both as an oxidizing and a reducing entity, giving rise to stable radical anion or radical cation species, respectively. SubPzs are luminescent and exhibit fluorescence quantum yields that are in the range known for SubPcs. The peripheral substitution plays a dramatic role with respect to the luminescence properties. Moreover, as with SubPcs, deactivation of the singlet excited state of the SubPzs by intersystem crossing affords long-lived triplet excited states, which are amenable to being used as singlet-oxygen generators. Subporphyrazines are also promising electro- and photoactive materials for molecular photovoltaics.

TDDFT study of the UV-vis spectra of subporphyrazines and subphthalocyanines

The UV-vis spectra of a series of subporphyrazines, SubPz(A,R), and subphthalocyanines, SubPc(A,R) (A = F, Cl; R = H, F, CH3, C3H7, SCH3, SC2H5 and SPh), where A is the substituent attached to the central boron atom and R is the substituent attached to the periphery of the molecule have been analyzed through the use of TD–DFT calculations in vacuum and using chloroform as a solvent. The absorption spectra depend on both, the characteristics of the substituent attached to the periphery of the molecule and the extension of the π-system on going from SubPz to the SubPc analog. These latter effects lead to a red-shift of both the Q-band and the B-band, although the effect is larger for the former, mainly due to the increase of HOMO–LUMO energy gap on going from the SubPz to the SubPc analog. The effect of the substituents R is more intricate, because the profile of the absorption spectra changes depending on whether both substituents are on the same side (uu or dd) or on opposite sides (ud) of the molecular cone. Since the three conformers are rather close in energy, the observed spectra correspond, very likely, to the sum of the spectra of all of them.

Stereochemical issues related to the synthesis and reactivity of pyrazino[2′,1′-5,1]pyrrolo[2,3-b]indole-1,4-diones

Abstract The cyclization of all four diastereoisomers of cyclo -(Trp-Ala) to the corresponding 3,5a,6,10b,11,11a-hexahydro-2 H -pyrazino[2′,1′-5,1]pyrrolo[2,3- b ]indoles is studied from the stereochemical point of view. Epimerization of the tryptophan stereocenter during the cyclization and during acylation reactions of the N-2 atom of the tetracyclic derivatives is discussed.

Assembling a Phthalocyanine and Perylenediimide Donor–Acceptor Hybrid through a Platinum(II) Diacetylide Linker

In a novel electron-donor-acceptor conjugate, phthalocyanine (Pc) and perylenediimide (PDI) are connected through a trans-platinum(II) diacetylide linker to yield Pc-Pt-PDI 1. In the ground state, the presence of Pt(II) disrupts the electronic communication between the two electroactive components, as revealed by UV/Vis spectroscopy and electrochemical studies. The photophysical behavior of 1 is compared with that of the corresponding Pc-PDI electron-donor-acceptor conjugate 2 in terms of charge separation and charge recombination. The insertion of Pt(II) between Pc and PDI impacts the results in a longer-lived Pc(.) (+) /PDI(.) (-) radical ion-pair state. In addition, the intermediately formed Pc triplet excited state is formed with higher quantum yields in 1 than in 2.

STERIC AND STEREOCHEMICAL EFFECTS ON THE FREE-RADICAL BROMINATION OF TETRACYCLIC AND HEXACYCLIC FRAGMENTS OF THE MDR INHIBITOR N-ACETYLARDEEMIN

Abstract The bromination of several tetracyclic 3,5a,6,10b,11,11a-hexahydro-2H-pyrazino[2′,1′-5,1]pyrrolo[2,3-b]indole-1,4-diones under free radical conditions was studied. In contrast with literature data for related pyrrolo[2,3-b]indole derivatives, the reaction occurs normally at the 11a position rather than at the benzylic 10b position and was followed by elimination, leading to 11,11a unsaturated derivatives. Compounds with an increased steric hindrance at 11a afforded B-C ring aromatized derivatives. Hexacyclic derivatives of the, 7,9a,10,14b,15,15a-hexahydroindolo[3″,2″−4′,5′]pyrrolo[2′,1′−3,4]-pyrazino[2,1-b]quinazoline-5,8-dione system showed a behaviour similar to the ‘non-hindered’ tetracycles, leading to unsaturated analogues of the natural MDR inhibitor N-acetylardeemin. Unsaturated ardeemin analogues were also obtained by bromination of 2-(o-azidobenzoyl) derivatives of the tetracyclic systems followed by aza-Wittig cyclization.

On the fate of the tryptophan stereocenter during the synthesis of hexacyclic analogues of N-acetylardeemin

Abstract The reaction between 6-acetyl-3-alkyl-1-ethoxy-3,4,5a,6,11,11a-hexahydro-10b H -pyrazino-[2′,1′-5,1]pyrrolo[2,3- b ]indole-1,4-diones and anthranilic acid was studied from a stereochemical point of view. Various degrees of epimerization of the tryptophan and alanine stereocenters were observed in compounds with a cis relationship between their H-3 and H-11a hydrogen atoms, and stereochemistry was retained in the trans compounds. These observations are explained in terms of steric compression between the C 5 O, C 7 –alkyl and C 8 O groups in the hexacyclic reaction products. Acylation at N-2 with o -azidobenzoyl chloride followed by an intramolecular aza Wittig reaction afforded the target 10-acetyl-5,7,8,9a,10,14b,15,15a-octahydroindolo[3″,2″-4′,5′]pyrrolo[2′,1′-3,4]pyrazino[2,1- b ]quinazoline-5,8-diones with retention of all stereocenters.

Tuning the Electron Acceptor in Phthalocyanine-Based Electron Donor-Acceptor Conjugates.

Zinc phthalocyanines (ZnPc) have been attached to the peri-position of a perylenemonoimide (PMI) and a perylenemonoanhydride (PMA), affording electron donor-acceptor conjugates 1 and 2, respectively. In addition, a perylene-monoimide-monoanhydride (PMIMA) has been connected to a ZnPc through its imido position to yield the ZnPc-PMIMA conjugate 10. The three conjugates have been studied for photoinduced electron transfer. For ZnPc-PMIMA 10, electron transfer occurs upon both ZnPc and PMIMA excitation, giving rise to a long-lived (340 ps) charge-separated state. For ZnPc-PMI 1 and ZnPc-PMA 2, stabilization of the radical ion pair states by using polar media is necessary. In THF, photoexcitation of either ZnPc or PMI/PMA produces charge-separated states with lifetimes of 375 and 163 ps, respectively.

Introducing rigid π-conjugated peripheral substituents in phthalocyanines for DSSCs

A new carboxy-Zn(II)phthalocyanine bearing rigid 2,6-diarylphenyl peripheral substituents linked to the macrocycle through alkynyl spacers has been prepared by a convergent approach, which implies a triple Sonogashira coupling between the formyltriiodoZnPc and the corresponding terminal alkyne. Indeed, the introduction of this type of π-conjugated peripheral substituents accounts for a remarkable red shift of the phthalocyanine Q-band until ca. 700 nm. However, aggregation phenomena could not be supressed and may explain the moderate overall efficiencies achieved with these devices.