Energy Alignment, Molecular Packing, and Electronic Pathways: Zinc(II) Tetraphenylporphyrin Derivatives Adsorbed on TiO₂(110) and ZnO(11–20) Surfaces

Abstract

The relation between energy alignment, adsorption geometry, and electron transfer between a chromophore and an oxide surface has been explored for a series of Zn(II) tetraphenylporphyrin derivatives adsorbed on TiO2(110) and ZnO(112̅0) surfaces. The electronic occupied and unoccupied structure has been obtained using UV-photoemission and inverse photoemission spectroscopies. From these results, a full picture of the energetics at the chromophore–oxide interface was established. The alignment of the molecular levels relevant for optical transition was found independent of the functionalization of the meso-phenyl groups. However, to explain the observation of different optical properties and electron transfer efficiencies of these different dyes, the adsorption geometry of two of these dyes was determined using scanning tunnel microscopy and near edge absorption fine structure spectroscopy. Functionalization of the meso-phenyls with COOH groups in the meta-position results in the ZnP macrocycle adsorbed parallel to the surface. Functionalization of the meso-phenyl groups with COOH groups in the para position results in a bounding geometry where the ZnP macrocycle makes an angle of ∼50° from the surface normal. This geometry, which allows face-to-face stacking of the porphyrin rings, opens a new electronic channel for exciton delocalization that competes with direct electron injection into the substrate conduction band.

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

“…Although not rigorously appropriate for TPP and TAPP, the molecular symmetry of the MO and excited states refers, for simplicity, to D 2h free-base porphine. Measuring both the filled and empty states of a system gives access to its electronic transport energy gap (E g ) [48][49][50]. This is given by the energy separation between the leading edges at the HOMO and LUMO intersecting the zero-emission baseline (see Fig.…”

Zinc(II) Tetraphenylporphyrin on Ag(100) and Ag(111): Multilayer Desorption and Dehydrogenation

“…A variety of techniques were used such as scanning tunneling microscopy (STM) to gain insight into adsorption geometries, near edge X-ray absorption fine structure (NEXAFS) to elucidate intramolecular features such as the relative orientation of the phenyl groups with respect to the macrocycle, 25,26,32,[40][41][42][43][44][45] Xray, 17,21,33,34,[37][38][39][41][42][43][44][45][46][47] ultraviolet, 17,28,34,40,46,47 and inverse 17,28,40 photoemission spectroscopy (XPS, UPS, and IPS) to probe directly the electronic structure and chemistry, and temperature programmed desorption (TPD) to track molecular desorption and hydrogen evolution with temperature. 27,33,48 On metal surfaces, due a strong porphyrin core iminic nitrogenssubstrate interaction, the mesophenyls are typically rotated and can even become coplanar with the porphyrin core.…”

Influence of Molecular Aggregation on Electron Transfer at the Perylene Diimide/Indium-Tin Oxide Interface

“…15 Song et al 4 showed that diarylamine chlorosilane (Ar 2 N-(CH 2 ) n -SiCl 3 , Ar = 3,4-difluorophenyl) redox-active modifiers can serve as a hole-selective interfacial layer on an ITO electrode in bulk-heterojunction OPVs; the PCE of these devices was slightly higher than that of devices in which PEDOT:PSS was utilized as the interfacial layer. 4 These and other papers [15][16][17][18][19][20][21] have demonstrated that the molecular design of a redox-active modifier, such as the type, position and length of its bridging moiety and the presence of solubilizing groups or bulky substituents, may significantly affect the charge-collection kinetics and efficiency at modified TCO surfaces, because these structural parameters dictate properties such as molecular orientation, tunneling distance, degree of aggregation, and/or interface dipole magnitude and orientation.…”