RESUMEN
A selection of dimeric tetraethynylethenes (TEEs) and perethynylated expanded radialenes, containing different donor/acceptor substitution patterns, have been prepared and fully characterized. The first X-ray crystal structure of an expanded [6]radialene, with twelve peripheral 3,5-di(tert-butyl)phenyl substituents, is presented. This macrocycle, the all-carbon core of which is isomeric with fullerene C60, adopts a non-planar, "chair-like" conformation. Also a TEE dimer, carrying N,N-dimethylaniline donor substituents, has been subjected to an X-ray crystallographic analysis. The electronic properties were studied by UV/Vis spectroscopy and electrochemistry, providing fundamental insight into mechanisms of pi-electron delocalization in the acyclic and macrocyclic chromophores. Donor or donor-acceptor-substituted dimeric TEE derivatives show very strong absorptions extending over the entire UV/Vis region; their longest wavelength absorption bands have high charge-transfer character. Macrocyclic cross-conjugation in the expanded radialenes becomes increasingly efficient with increasing donor-acceptor polarization. A dual, strongly solvent-polarity-dependent fluorescence was observed for a tetrakis(N,N-dimethylaniline)-substituted dimeric TEE; this interesting emission behavior is explained by the twisted intramolecular charge-transfer (TICT) state model. Donor-substituted expanded radialenes display huge resonance-enhanced third-order nonlinear optical coefficients.
RESUMEN
A new star-like pentaporphyrin, bearing nucleosidic linkers, has been synthesized, and the unexpected duality of the physico-chemical properties of its core is reported; beside the quenching of the fluorescence of the peripheral porphyrins by the central chromophore, this pentaporphyrin exhibits an unexpected shielding of the redox capabilities of its central core.
RESUMEN
Novel extended tetrathiafulvalenes (TTFs) with hexa-2,4-diyne-1,6-diylidene spacers between the two 1,3-dithiole rings and laterally appended alkynyl moieties for one- and two-dimensional scaffolding were synthesised and investigated for their electronic properties.
RESUMEN
Two series of monodisperse, terminally donor-donor [D-D, D=4-(dimethylamino)phenyl] and acceptor-acceptor [A-A, A=4-nitrophenyl] functionalized poly(triacetylene) (PTA) oligomers ranging from monomer to hexamer were synthesized by oxidative Hay oligomerization under end-capping conditions. The corresponding D-D and A-A end-substituted polymers with an average degree of polymerization (DP) of n approximately 18 and n approximately 12, respectively, were also prepared and served as reference points for the corresponding infinitely long polymers. These terminally functionalized PTA oligomers and polymers are yellow- to orange-colored compounds, displaying excellent solubility in aprotic solvents with melting points above 200 degrees C for the hexamers. For the 4-(dimethylamino)phenyl substituted compounds, a consistent first oxidation potential around +0.42 V versus Fc/Fc+ (ferrocene/ferricinium) was observed, whereas the 4-nitrophenyl functionalized systems underwent a reversible reductive two-electron transfer around -1.40V versus Fc/Fc+. The nature of the end-groups has a dramatic influence on the electronic absorption spectra. Saturation of the linear optical properties in the D-D series occurs at significantly shorter chain-length [effective conjugation length (ECL) of n approximately 4 monomer units] than in the A-A substituted or the previously reported Me3Si- and Et3Si-end-capped PTA oligomer series (ECL: n approximately 10 monomer units). Similar observations with respect to the ECL were made by measurement of the Raman-active v(C triple bound C) stretches. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) experiments showed that shorter oligomers of terminally D-D or A-A functionalized PTAs display higher second hyperpolarizabilities gamma than the corresponding R3Si-end-capped series (R=alkyl). Moreover, they disclose a distinct peak of the nonlinearity per monomer unit at intermediate backbone lengths. In THG experiments, the second hyperpolarizabilities for long D/A-functionalized PTA oligomers attained the same saturation values as observed for the corresponding R3Si-end-capped rods. The nonlinearities measured by DFWM of the D-D and A-A substituted PTAs were found to be larger than for the silylated ones, which can be explained by the closeness of the two-photon resonance.
RESUMEN
Corrphycene 3 (Cn) is a structural isomer of porphyrin 1 that was synthesized for the first time 5 years ago. This paper reports on the redox properties of free-base octaethylcorrphycene H2OECn and 16 metal complexes derived therefrom. In CH2Cl2 solution, the free base and the metallo(II) octaethylcorrphycenes, M(II)OECn, typically undergo four distinct one-electron redox steps involving the tetrapyrrolic macrocycle, of which two are reduction steps and two are oxidations. One exception to this general pattern is displayed by the Co(II)OECn complex. In this instance, the first one-electron reduction is metal-centered and produces Co(I)OECn. A comparison of the redox potentials of corrphycenes with those of porphyrins and porphycenes indicates that the first reduction potentials of the free base and of the metallo-octaethylcorrphycenes are between those of the porphycenes-the easiest to reduce molecules in this set of isomeric tetrapyrrolic systems-and those of the porphyrins. The oxidation potentials of corrphycenes and porphyrins are found to be quite similar. On the other hand, porphycenes are oxidized at less positive potentials. The redox gap deltaE1/2 = E1/2Ox1 - E1/2Red1 is equal to 2.15 +/- 0.08 V for the free base corrphycene and the various metallocorrphycenes that were subjected to study. This redox gap is not much different from that observed in porphyrins (deltaE1/2 = 2.25 +/- 0.1 V), whereas if differs significantly from that observed in porphycenes (deltaE1/2 = 1.85 +/- 0.15 V). The sequence of these deltaE1/2 values parallels the lowest energy absorption maxima observed in the UV-vis spectra of these three isomers.