Singlet-singlet energy transfer in carbazole-porphyrin dyads and triads

Donor-π-Acceptor (D-π-A) type carbazole-porphyrin dyads and triads were synthesized in good yields through palladium catalyzed coupling reactions. The length of the spacer between the D (carbazole) and A (porphyrin) was varied from “phenylethyne” to “ethyne linkage”. The effect of the linker length on the spectral properties of these D-π-A dyads and triads was studied. The N-butyl-3-ethynyl-carbazole was coupled with different porphyrins bearing either one or two meso-p-iodophenyl group/meso-bromo groups using palladium catalyst. All the dyads and triads were characterized by MALDI-MS, NMR, UV–vis absorption, fluorescence and cyclic voltammetry techniques. The S<inf>1</inf> emission dynamics were analyzed by time-resolved spectroscopy (TCSPC); an efficient energy transfer up to 84% was observed from donor groups to the porphyrin core. Electrochemical studies revealed that, compounds having freebase porphyrin components are difficult to reduce as indicated by the cathodic shift in their reduction potentials. On the contrary, the compounds having zinc porphyrin components are easy to reduce as reflected by the anodic shift in their reduction potentials. The lowest HOMO-LUMO gap was found for the dyad which has short linker between D and A.