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

摘要

The valence and conduction bands of a thin film of tetra(4-aminophenyl)porphyrin (TAPP) are investi-gated 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 possibleto interpret the origin of each spectroscopic feature. Although the majority of the photoemission spec-trum is attributed to the unsubstituted tetraphenylporphyrin (TPP) parent molecule, several featuresare clearly due to the amino substitution. Substitution also has important consequences for the energypositions of the frontier orbitals and therefore on the low-energy electronic excitations. The measuredelectronic 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. More-over, an increased energy separation between the two highest occupied states (HOMO and HOMO−1) isfound both experimentally and by DFT calculations. Such evidence is attributed to an increased HOMOorbital destabilization due to an enhanced electron-donor character of the phenyl substituents uponamino functionalization. Finally, the above findings together with further time-dependent DFT calcula-tions are used to interpret the effect of the amino groups on the UV–Vis absorption spectrum, namely anoverall red-shift of the spectrum and remarkable intensity changes within the Q band.© 2017 Elsevier B.V.