On surface chemical reactions of free-base and titanyl porphyrins with r-TiO2(110): a unified picture

In this Perspective we present a comprehensive study of the multiple reaction products of metal-free porphyrins (2H-Ps) in contact with the rutile TiO₂(110) surface. In the absence of peripheral functionalization with specific linkers, the porphyrin adsorption is driven by the coordination of the two pyrrolic nitrogen atoms of the macrocycle to two consecutive oxygen atoms of the protruding O_br rows via hydrogen bonding. This chemical interaction favours the iminic nitrogen uptake of hydrogen from near surface layers at room temperature, thus yielding a stable acidic porphyrin (4H-P). In addition, a mild annealing (∼100 °C) triggers the incorporation of a Ti atom in the porphyrin macrocycle (self-metalation). We recently demonstrated that such a low temperature reaction is driven by a Lewis base iminic attack, which lowers the energy barriers for the outdiffusion of Ti interstitial atoms (Ti_int) [Kremer et al., Appl. Surf. Sci., 2021, 564, 150403]. In the monolayer (ML) range, the porphyrin adsorption site, corresponding to a TiO-TPP configuration, is extremely stable and tetraphenyl-porphyrins (TPPs) may even undergo conformational distortion (flattening) by partial cyclo-dehydrogenation, while remaining anchored to the O rows up to 450 °C [Lovat et al., Nanoscale, 2017, 9, 11694]. Here we show that, upon self-metalation, isolated molecules at low coverage may jump atop the rows of five-fold coordinated Ti atoms (Ti_5f). This configuration is associated with the formation of a new coordination complex, Ti-O-Ti_5f, as determined by comparison with the deposition of pristine titanyl-porphyrin (TiO-TPP) molecules. The newly established Ti-O-Ti_5f anchoring configuration is found to be stable also beyond the TPP flattening reaction. The anchoring of TiO-TPP to the Ti_5f rows is, however, susceptible to the cross-talk between phenyls of adjacent molecules, which ultimately drives the TiO-TPP temperature evolution in the ML range along the same pathway followed by 2H-TPP.

Morphology dependent interaction between Co(II)-tetraphenylporphyrin and the MgO(100) surface

Porphyrins are key elements in organic–inorganic hybrid systems for a wide range of applications. Understanding their interaction with the substrate gives a handle on structural and electronic device properties. Here we investigate a single transition-metal porphyrin, namely Co(ii)-tetraphenylporphyrin (CoTPP), on the MgO(100) surface and the effect of multilayer film formation within hybrid density-functional theory and many-body perturbation theory. We focus on the relevant adsorption sites, simulate their photoemission spectra as a key fingerprint and compare with experiments on MgO(100) films on Ag(100). While we find only weak interaction between the cobalt centre and terrace sites on the MgO(100) surface, a strong interaction manifests itself with the low-coordinated sites. This leads to distinct features in both the valence and core-level regions of the electronic structure, as observed in the ultraviolet and X-ray photoemission spectra, corroborated by simulated spectra and calculated cobalt core-level shifts. Our work thus demonstrates the relevance of morphology-related low-coordinated sites and their properties in the adsorption of CoTPP on the MgO(100) surface.

The adsorption of Co-tetraphenylporphyrin at relevant low-coordinated sites on MgO(100) shows distinct features from terrace-site and multilayer films in the near-valence and corelevel regions of the electronic structure.

The interface between chloroaluminum phthalocyanine and titanium dioxide: the influence of surface defects and substrate termination

Interface properties of chloroaluminum(iii) phthalocyanine (AlClPc) on two different rutile titanium dioxide (TiO2) single crystal surfaces ((100) and (001)) have been studied using X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). It is shown that the strength of the interaction clearly depends on the substrate termination and preparation. Generally, the (001) surface is more reactive compared to the (100) surface. The most important interaction channel involves the nitrogen atoms of the phthalocyanine macrocycle. An exposure to oxygen during the annealing steps of the preparation procedure allows diminishing the extent of interaction of nitrogen with titanium dioxide. The work function of AlClPc/TiO2 is rather independent of the substrate, indicating a pinning regime at all interfaces, where the HOMO of the molecule is aligned at the maximum of the defect states of the substrate.

Hybrid density functional study on the electronic structures and properties of P3HT-PbS and P3HT-CdS hybrid interface for photovoltaic applications

The efficiency of charge transport mainly depends on the interfacial energy level alignment between the conjugated polymer and the inorganic substrate. It provides an accurate understanding, predicting as well as controlling the optimal power conversion efficiency of various type of hybrid photovoltaic systems. In this article, we use hybrid functional (HSE06) to study the electronic structures and properties at the interface of poly(3-hexylthiophene)(P3HT)/CdS and P3HT/PbS for solar cell applications. We found that the dangling bonds at the inorganic surface introduce in-gap states and greatly reduce the device performance. We used pseudo-hydrogen atoms as the passivation agent to remove the dangling bonds and eliminate the in-gap states to construct the energy alignment at the hybrid interface. The calculated interfacial density of states reveal a better performance in P3HT/CdS, compared to P3HT/PbS. P3HT/CdS possesses a LUMO_P3HT /CBM_CdS and HOMO_P3HT /VBM_CdS energy offset large enough for sufficient exciton separation across the interface and prevents charge recombination. In contrast, the reason for low power conversion efficiency in P3HT/PbS lies on its HOMO_P3HT /VBM_PbS offset which is too small to break the exciton binding energy for charge separation. Moreover, we reported the dependency of the energy level alignment and open circuit voltage on the interfacial molecular orientations. Our DFT calculation can be used to predict candidate materials for the development of efficiency optoelectronic devices. © 2018 Wiley Periodicals, Inc.