Comparative (magneto)optical, redox, and photophysical properties of phosphorous octacarbazole- and tetra-tert-butyltetrabenzotriazacorroles

Abstract

The spectroscopy and electronic structure of the novel phosphorous(V) octacarbazole tetrabenzotriazacorrole (TBC(Cz)8P=O) compound were evaluated using a large array of experimental and theoretical methods, and compared to the reference terta-tert-butyl tetrabenzotriaazacorrole (TBC(tBu)4P=O). The first coordination sphere in TBC(Cz)8P(XY) and TBC(tBu)4P(XY) in a solid state and solution was probed by MALDI-TOF mass spectrometry, 31P NMR spectroscopy, XPS, UV–Vis, MCD, and elemental analysis. Based on the 31P NMR, mass spectrometry, UV–Vis, and MCD data, it was demonstrated that the initial pentacoordinated TBC(tBu)4P=O complex can be easily transformed to the hexa-coordinated species in the presence of water and methanol following TBC(tBu)4P=O ⇌ TBC(tBu)4P(OH)2 ⇌ TBC(tBu)4P(OH)(OMe) ⇌ TBC(tBu)4P(OMe)2 equilibrium. In wet solvents, the UV–Vis and MCD spectroscopies revealed that the two structures possess similar absorbance profiles; however, the TBC(Cz)8P(OH)2 compound had a more intense and red-shifted Q-band than the TBC(tBu)4P(OH)2 compound. The first oxidation under spectroelectrochemical conditions was found to be completely reversible for the TBC(Cz)8P(OH)2 compound; conversely, these experiments were completely irreversible for the TBC(tBu)4P(OH)2 compound. DFT and TDDFT calculations were performed using three different exchange-correlation functionals. The DFT results revealed several (16 in total) carbazole-centered MOs at energies in between the classical Gouterman's a1u and a2u frontier orbitals while the four Gouterman's frontier orbitals were found to be energetically sequential for the TBC(tBu)4P(OH)2 compound. The TDDFT calculations provided a very reasonable agreement between theory and experiment and demonstrated the presence of the inter-ligand (carbazole-to-TBC) charge-transfer transitions in TBC(Cz)8P(OH)2 in ∼500 nm region. Steady-state fluorescence spectroscopy data are indicative of the mid-range fluorescence quantum yields in both compounds. Transient absorption spectroscopy was utilized to probe triplet state formation and lifetimes (τT) which were found to be 112 μs for TBC(Cz)8P(OH)2 and 91 μs for TBC(tBu)4P(OH)2.