Recent advances in subphthalocyanines and related subporphyrinoids

Subporphyrinoids constitute a class of extremely versatile and attractive compounds. Herein, a comprehensive review of the most recent advances in the fundamentals and applications of these cone-shaped aromatic macrocycles is presented.

Light induced intramolecular energy and electron transfer events in carbazole–corrole and phenothiazine-corrole dyads

We report two corrole based donor–acceptor (D–A) dyads, Cbz-Cor and Ptz-Cor to understand the energy/electron transfer reactions. In these D–A systems, the donor, either carbazole (Cbz) or phenothiazine (Ptz), is covalently connected at the meso-phenyl position of 10-(phenyl)-5,15-bis-(pentafluorophenyl)corrole (Ph-Cor) by C–N linkage. Both the dyads were characterized by 1H NMR, MALDI-TOF MS, UV-vis, electrochemical, computational methods, study state fluorescence and TCSPC techniques. A comparison of absorption spectra with their reference monomeric compounds (Cbz-Ph, Ptz-Ph and Ph-Cor) revealed minimal ground-state interactions between chromophores in both dyads. Fluorescence studies suggested that singlet–singlet energy transfer from 1Cbz* to corrole is the major photochemical pathway in the Cbz-Cor dyad with a quenching efficiency of [Formula: see text]99%. Detailed analysis of the data suggests that Forster’s dipole–dipole mechanism does not adequately explain this energy transfer. However, at a 410 nm excitation, florescence quenching is detected in Ptz-Cor (49%) supporting a photo induced electron transfer (PET) process from the ground state of PTZ to the excited state of corrole macrocycle. The electron-transfer rates ([Formula: see text] of Ptz-Cor are found in the range [Formula: see text] to [Formula: see text] and are concluded to be solvent dependent.

Electron transfer reactions in sub-porphyrin-naphthyldiimide dyads

A series of donor-acceptor compounds based on a sub-porphyrin (SubP) as an electron donor and naphthyldiimide (NDI) as an acceptor has been designed, synthesized and investigated by time-resolved emission and transient absorption measurements. The donor and acceptor are separated by a single phenyl spacer substituted by methyl groups in order to systematically vary the electronic coupling. The electron transfer reactions in toluene are found to be quite fast; charge separation is quantitative and occurs within 5-10 ps and charge recombination occurs in 1-10 ns, depending on the substitution pattern. As expected, when steric bulk is introduced on the adjoining phenyl group, electron transfer rates slow down because of smaller electronic coupling. Quantum mechanical modelling of the potential energy for twisting the dihedral angles combined with a simplified model of the electronic coupling semi-quantitatively explains the observed variation of the electron transfer rates. Investigating the temperature variation of the charge separation in 2-methyltetrahydrofuran (2-MTHF) and analyzing using the Marcus model allow experimental estimation of the electronic coupling and reorganization energies. At low temperature, relatively strong phosphorescence is observed from the donor-acceptor compounds with onset at 660 nm signaling that charge recombination occurs, at least partially, through the sub-porphyrin localized triplet excited state. Finally, it is noted that charge separation in all SubP-NDI dyads is efficient even at cryogenic temperatures (85 K) in 2-MTHF glass.

Synthesis, characterization and photophysical properties of ferrocenyl and mixed sandwich cobaltocenyl ester linked meso-triaryl corrole dyads

We report here the design and synthesis of corrole-metallocene dyads consisting of a metallocene (either ferrocene (Dyad 1) or mixed sandwich [Formula: see text]-[C[Formula: see text]H[Formula: see text](COOH)]Co([Formula: see text]-C[Formula: see text]Ph[Formula: see text] (Dyad 2)) connected via an ester linkage at meso phenyl position. Both the dyads were characterized by [Formula: see text]H NMR, MALDI-TOF, UV-visible, fluorescence spectroscopies (steady-state, picosecond time-resolved), femtosecond transient absorption spectroscopy (fs-TA) and electrochemical methods. The absorption spectra of these dyads showed slight broadening and splitting of the Soret band that indicates a weak ground state interaction between the corrole macrocycle and metallocene part of the present donor–acceptor (D–A) system. However, in both the dyad systems, fluorescence emission of the corrole was quenched in polar solvents as compared to its parent compound 10-(4-hydroxyphenyl)-5,15-bis-(pentafluorophenyl ) corrole (Ph-Corr). The quenching was more pronounced in ferrocene derivatives than in cobaltocenyl derivatives. Transient absorption studies confirm the absence of photoinduced electron transfer from metallocene to correl for these dyad systems and the quenching of singlet state of corrole is found to enhance intersystem crossing due to heavy atom effect.