Conceptual design of tetraazaporphyrin- and subtetraazaporphyrin-based functional nanocarbon materials: electronic structures, topologies, optical properties, and methane storage capacities

Accurate Prediction of the Excited States in the Fully Conjugated Porphyrin Tapes across the Full Spectral Range: A Story of the Interplay between π-π* and Intramolecular Charge-Transfer Transitions in Soft Chromophores

The ability of density functional theory (DFT) and time-dependent DFT (TDDFT) methods for the accurate prediction of the energies and oscillator strengths of the excited states in a series of fully conjugated meso-meso β-β β-β triple-linked porphyrin oligomers (porphyrin tapes 2-12) was probed in the gas phase and solution using several exchange-correlation functionals. It was demonstrated that the use of the hybrid B3LYP functional provides a good compromise for the accurate prediction of the localized π-π* and intramolecular charge-transfer transitions, thus allowing confident interpretation of the UV-vis-NIR spectra of porphyrin oligomers. The TDDFT-based sum-over-state (SOS) calculations for the porphyrin tape dimer 2 and trimer 3 as well as parent monomer 1 correctly predicted the signs and shapes of the magnetic circular dichroism (MCD) signals in the low-energy region of the spectra.

Detection of a weak ring current in a nonaromatic porphyrin nanoring using magnetic circular dichroism

We compare the absorption and magnetic circular dichroism (MCD) spectra of a series of porphyrin oligomers - dimer, tetramer, and hexamer - bound in a linear or cyclic fashion. The MCD signal is extremely weak for low energy transitions in the linear oligomers, but it is amplified when the cyclic porphyrin hexamer binds a template, restricting rotational freedom. The appearance of Faraday A terms in the MCD spectra demonstrates the presence of a magnetic moment, and thus, uncompensated electronic current. The value of the excited state magnetic moment estimated from the A term is very low compared with those of monomeric porphyrins, which confirms the nonaromatic character of the cyclic array and the lack of a global ring current in the ground state of the neutral nanoring. DFT calculations predict the absorption and MCD patterns reasonably well, but fail to reproduce the MCD sign inversion observed in substituted monomeric zinc porphyrins ("soft" chromophores). Interestingly, a correct sign pattern is predicted by INDO/S calculations. Analysis of the MCD spectra of the monomeric porphyrin unit allowed us to distinguish between two close-lying lowest energy transitions, which some previous assignments placed further apart. The present results prove the usefulness of MCD not only for deconvolution and assignment of electronic transitions, but also as a sensitive tool for detecting electronic ring currents.