Electronic structure of tetra(4-aminophenyl)porphyrin studied by photoemission, UV-Vis spectroscopy and density functional theory

The valence and conduction bands of a thin film of tetra(4-aminophenyl)porphyrin (TAPP) are investigated by direct and inverse photoemission as well as by comparison to density functional theory (DFT) calculations. By projecting the electronic eigenfunctions onto the molecular framework it was possible to interpret the origin of each spectroscopic feature. Although the majority of the photoemission spectrum is attributed to the unsubstituted tetraphenylporphyrin (TPP) parent molecule, several features are clearly due to the amino substitution. Substitution also has important consequences for the energy positions of the frontier orbitals and therefore on the low-energy electronic excitations. The measured electronic transport energy gap (Eg = 1.85 eV) between the highest occupied molecular orbital (HOMO) and lowest unoccupied (LUMO) in TAPP is found to be significantly reduced with respect to TPP. Moreover, an increased energy separation between the two highest occupied states (HOMO and HOMO−1) is found both experimentally and by DFT calculations. Such evidence is attributed to an increased HOMO orbital destabilization due to an enhanced electron-donor character of the phenyl substituents upon amino functionalization. Finally, the above findings together with further time-dependent DFT calculations are used to interpret the effect of the amino groups on the UV-Vis absorption spectrum, namely an overall red-shift of the spectrum and remarkable intensity changes within the Q band.