Enhancement of Tetraphenylporphyrin Electrochemiluminescence by Means of Symmetry Breaking

Lowering the geometric symmetry that has been shown to break the degeneracy of the four Gouterman molecular orbitals offers a means of tuning the electronic structures and properties of porphyrins. Such designs can lead to significant technological applications, especially through the enhancement of the luminescence and red-region absorption band properties. By investigating two simplest synthetic porphyrins of 5,10,15,20-tetraphenyl-21H,23H-porphine zinc (ZnTPP) and 5,10,15,20-tetraphenyl-21H,23H-porphine (H2TPP) as representatives, the connections between their geometric structures and electrochemiluminescence (ECL) were revealed. The photophysical spectroscopic characterization and density functional theory calculations along with electrochemical analyses identified the fact that reducing the symmetry from D-4h in ZnTPP to D-2h for H2TPP removes the accidental degeneracy of the highest occupied molecular orbital (HOMO)/HOMO - 1 resulting in significantly augmented photoluminescence, decreased electronic and electrochemical HOMO-lowest unoccupied molecular orbital (LUMO) gaps, and enhanced ECL. Especially, the nonemissive decay processes were found to be minimized, the radical cations and anions generated electrochemically stabilized, and the energy required to produce electrochemiluminescence (ECL) decreased. ECL efficiencies of 11 and 90% were determined for H2TPP in the annihilation pathway and the tri-n-propylamine (TPrA) coreactant route, which were 3 and 1.6 times higher than those of ZnTPP systems, respectively. For the two porphyrins, monomeric excited states were elucidated by means of spooling ECL spectroscopy. The combination of ECL data and detailed analysis of the contributing electronic structures provides direction to guide future designs of synthetic porphyrin architectures for technological applications.