Magnetic Circular Dichroism of meso-Phenyl-Substituted Pd-Octaethylporphyrins

Abstract

Absorption and magnetic circular dichroism (MCD) spectra have been measured and theoretically simulated for a series of palladium octaethylporphyrins substituted at the meso positions with phenyl groups ( n = 0–4). Analysis of the spectra included the perimeter model and time-dependent density functional theory (TDDFT) calculations. With the increasing number of phenyl substituents, the molecule is transformed from a positive hard (ΔHOMO > ΔLUMO) to a soft (ΔHOMO ≈ ΔLUMO) chromophore. This is manifested by a drastic decrease of the absorption intensity in the 0–0 region of the Q-band and by the strongly altered ratio of MCD intensities in the Q and Soret regions. Such behavior can be readily predicted using perimeter model, by analyzing frontier orbital shifts caused by various perturbations: alkyl and aryl substitution, insertion of a metal, and deviations from planarity. TDDFT calculations confirm the trends predicted by the perimeter model, but they fail in cases of less symmetrical derivatives to properly reproduce the MCD spectra in the Soret region. Our results confirm the power of the perimeter model in predicting absorption and MCD spectra of large organic molecules, porphyrins in particular. We also postulate, contrary to previous works, that the isolated porphyrin dianion is not a soft chromophore, but rather a strongly positive-hard one.

Photophysical Exploration of Two Isomers of Octaethyltrioxopyrrocorphin

“…According to the perimeter model that proved to be very useful for the interpretation of the MCD spectra of a wide variety of porphyrinoids, − the negative-to-positive sequences (in ascending energy) for the Q x and Q y transitions in 3-O 2,7,12 and 3-O 2,7,18 are indicative of a ΔHOMO > ΔLUMO relationship when using the classic Gouterman’s four-orbital description (whereby ΔHOMO is the energy difference between Gouterman’s a 1u and a 2u occupied orbitals and ΔLUMO is the energy difference between Gouterman’s e g( x ) and e g( y ) pair of unoccupied orbitals in the standard D 4h point group notation). , As will be shown below, DFT calculations correlate well with this interpretation.…”

Recent advances in triplet–triplet annihilation upconversion and singlet fission, towards solar energy applications

“…177 The earliest examples of near infrared to visible TTA-UC utilise metalloporphyrin complexes, 155 having a high triplet yield. [178][179][180][181] Indeed many groups continue using porphyrin complexes as sensitisers (1, 2), [182][183][184][185][186] although due to the breadth of literature concerning porphyrins we will not specifically cover them in this section. Instead, we will address here recent works related to other classes of organometallic and coordination complexes.…”

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

“…Time-dependent density functional theory (TDDFT) methodology offers a reasonable computational cost, a variety of specifically tuned exchange-correlation functionals, and good accuracy for the large π-extended systems. − The TDDFT approach was successfully applied for the accurate prediction of the vertical excitation energies in porphyrins, phthalocyanines, and their structural analogues. Two research groups have recently attempted to find the best TDDFT methodology for a large group of phthalocyanines and their analogues substituted with electron-donating groups. , These systems have UV–vis spectra that are composed of the combination of the macrocycle-centered π–π* transitions and interligand charge-transfer bands.…”