A Theoretical Study of Brominated Porphycenes: Electronic Spectra and Intersystem Spin−Orbit Coupling

Incorporating spin–orbit effects into surface hopping dynamics using the diagonal representation: a linear-response time-dependent density functional theory implementation with applications to 2-thiouracil

Evaluation of SOC values employs Casida's wave functions and the Breit–Pauli spin–orbit Hamiltonian with effective charge approximation.

Spectroscopy and Tautomerization Studies of Porphycenes

Tautomerization in porphycenes, constitutional isomers of porphyrins, is strongly entangled with spectral and photophysical parameters. The intramolecular double hydrogen transfer occurring in the ground and electronically excited states leads to uncommon spectroscopic characteristics, such as depolarized emission, viscosity-dependent radiationless depopulation, and vibrational-mode-specific tunneling splittings. This review starts with documentation of the electronic spectra of porphycenes: Absorption and magnetic circular dichroism are discussed, together with their analysis based on the perimeter model. Next, photophysical characteristics are presented, setting the stage for the final part, which discusses the developments in research on tautomerism. Porphycenes have been studied in different experimental regimes: molecules in condensed phases, isolated in supersonic jets and helium nanodroplets, and, recently also on the level of single molecules investigated by optical and scanning probe microscopies. Because of the rich and detailed information obtained from these diverse investigations, porphycenes emerge as very good models for studying the complex, multidimensional phenomena involved in the process of intramolecular double hydrogen transfer.

Can Expanded Bacteriochlorins Act as Photosensitizers in Photodynamic Therapy? Good News from Density Functional Theory Computations

The main photophysical properties of a series of expanded bacteriochlorins, recently synthetized, have been investigated by means of DFT and TD-DFT methods. Absorption spectra computed with different exchange-correlation functionals, B3LYP, M06 and ωB97XD, have been compared with the experimental ones. In good agreement, all the considered systems show a maximum absorption wavelength that falls in the therapeutic window (600-800 nm). The obtained singlet-triplet energy gaps are large enough to ensure the production of cytotoxic singlet molecular oxygen. The computed spin-orbit matrix elements suggest a good probability of intersystem spin-crossing between singlet and triplet excited states, since they result to be higher than those computed for 5,10,15,20-tetrakis-(m-hydroxyphenyl)chlorin (Foscan©) already used in the photodynamic therapy (PDT) protocol. Because of the investigated properties, these expanded bacteriochlorins can be proposed as PDT agents.

Effects induced by axial ligands binding to tetrapyrrole-based aromatic metallomacrocycles

The axial positions of planar metallomacrocycles are unoccupied. The positively charged metal is thus a potential binding site for electron-donating groups. The binding strength is affected by the central metal, the ligand, and the macrocycle. One ligand leads to the out-of-plane displacement of the central metal, whereas two ligands from two sides structurally neutralize each other. The axial ligand donates charge to the central metal and the macrocycle when the lone pair orients along the interaction axis. The frontier orbital levels are elevated because of the charge donated to the macrocycle. Even though the singlet-triplet gap and the absorption maximum do not change significantly upon binding, the redox chemistry is considerably affected by the shifts of orbital levels. The macrocyclic M-N bonds are weakened by the binding, but their natures remain almost unchanged. Calcium phthalocyanine is a special case, as the central calcium is too large to fit the cavity. Accordingly, multiple ligands facilely bind to the calcium from one side. The aluminum phthalocyanine halogen is another special case, as it has a halogen ligand coordinating to the aluminum through a nondative bond. This leads to some effects different from those caused by dative binding. When there is no considerable steric demand, the lone pair points along the interaction axis to facilitate the donation. When in a stacked dimer, the electron-rich group is part of a large molecule, and the orientation of the lone pair is approximately perpendicular to the interaction axis. This induces the charge loss of the central metal. Because metallomacrocycles are widespread in the biological, medical, and material sciences, the results from this study are expected to bring useful insights to these fields.