Nonplanar Phthalocyanine Research Articles

Submolecular Imaging of Parallel Offset π–π Stacking in Nonplanar Phthalocyanine Bilayers

In aromatic systems, π–π interaction plays a central role in determining the stacking geometry and binding strength of molecules and thus a detailed microscopic understanding is highly desirable. Herein, by using scanning tunneling microscopy with submolecular resolution complemented with first-principles calculations based on density functional theory, we report the atomic-scale imaging of π–π interaction in nonplanar phthalocyanine (Pc) bilayers on different substrates, including graphite and Au(111) with weak interaction and Cu(111) with strong binding. We reveal that nonplanar Pc of the second layer on all substrates exhibits an in-plane rotation angle of 15° with a parallel offset of 1.19 A, which minimizes π–π repulsion. Interestingly, on Cu(111), it is found that the inequivalent charge distribution along with the alternating orientation of Pc molecules in the first layer creates a preferable anchoring site for Pc of the second layer, leading to the assembly of the √2 × √2R45° superstructure, consi...

Current mapping of lead phthalocyanine thin films in the presence of gaseous dopants.

The structural organization and its effect on conducting pathways in lead phthalocyanine (PbPc) thin films, a nonplanar phthalocyanine, deposited on Si and highly oriented pyrolytic graphite (HOPG) substrates in the presence of iodine and ammonia vapors are presented. Two-dimensional grazing incidence X-ray diffraction studies reveal that the crystalline ordering in pristine PbPc films on Si and HOPG substrates undergoes a drastic molecular rearrangement and surface reconstruction upon iodine doping. The structural rearrangement leads to morphological changes and higher surface roughness in iodine doped PbPc (I-PbPc) films. The obvious enhancement in the current values of I-PbPc is attributed to the introduction of holes as charge carriers. Nanoscale current mapping reveals the presence of percolation pathways in I-PbPc films, on both Si and HOPG substrates, being responsible for the observed high conductance in contrast to the isolated conducting domains in the pristine PbPc system. The broad distribution of current values across various conducting domains on Si is attributed to a mixture of crystalline phases and disordered fractions of I-PbPc, while the narrow distribution of current values observed in the case of HOPG arises from the majorly disordered PbPc molecules. These films also show enhanced sensitivity towards ammonia that is almost four times higher in magnitude than for pristine PbPc films. The current maps show that the adsorption of ammonia molecules disrupts the iodine percolation pathways, thereby imposing a detrimental effect on the conductivity of the PbPc films.

The Structure of VOPc on Cu(111): Does V=O Point Up, or Down, or Both?

The local structure of the nonplanar phthalocyanine, vanadyl phthalocyanine (VOPc), adsorbed on Cu(111) at a coverage of approximately one-half of a saturated molecular layer, has been investigated by a combination of normal-incidence X-ray standing waves (NIXSW), scanned-energy mode photoelectron diffraction (PhD), and density-functional theory (DFT), complemented by scanning tunnelling microscopy (STM). Qualitative assessment of the NIXSW data clearly shows that both “up” and “down” orientations of the molecule (with V=O pointing out of, and into, the surface) must coexist on the surface. O 1s PhD proves to be inconclusive regarding the molecular orientation. DFT calculations, using two different dispersion correction schemes, show good quantitative agreement with the NIXSW structural results for equal co-occupation of the two different molecular orientations and clearly favor the many body dispersion (MBD) method to deal with long-range dispersion forces. The calculated relative adsorption energies of the differently oriented molecules at the lowest coverage show a strong preference for the “up” orientation, but at higher local coverages, this energetic difference decreases, and mixed orientation phases are almost energetically equivalent to pure “up”-oriented phases. DFT-based Tersoff–Hamann simulations of STM topographs for the two orientations cast some light on the extent to which such images provide a reliable guide to molecular orientation.

Unraveling giant Cu(110) surface restructuring induced by a non-planar phthalocyanine

The surface stability of coinage metals is paramount when they are used as electrode materials for functional electronic devices incorporating organic semiconductors. In this work, it is shown that the adsorption of non-planar vanadyl phthalocyanine molecules on Cu(110) drastically restructured the metal surface at room temperature, which was further enhanced upon moderate annealing. Scanning tunneling microscopy imaging demonstrated that the surface was restructured at step edges into sawtooth features that gradually replaced the (110) terraces. The edges of the modified steps were preferentially composed of chiral (1×6) kink sites decorated with vanadyl phthalocyanine molecules adsorbed in a tilted configuration with the oxygen atom pointing downwards. These results can have a strong impact on the optimization of the performance of organic devices integrated with phthalocyanine molecules.

Multifunctional Bilayer Template for Near-Infrared-Sensitive Organic Solar Cells.

For organic solar cells (OSCs) based on nonplanar phthalocyanines, it has previously been reported that a thin film composed of triclinic crystals with face-on (or flat-lying)-oriented molecules, typically obtained with a CuI template layer, is desired for optical absorption in the near-infrared (NIR) spectral region. However, this work demonstrates that for a PbPc-C60 donor-acceptor pair, less face-on orientation with a broader orientation distribution obtained with a new template layer consisting of a ZnPc/CuI bilayer is more desirable in terms of solar cell efficiency than the face-on orientation. A NIR-sensitive PbPc-C60 OSC employing this bilayer-templated PbPc film is found to increase the internal quantum efficiency (IQE) by 36% on average in the NIR spectral region compared to a device using a CuI-templated PbPc film. Analyses of the change in IQE using the exciton diffusion model and the entropy- and disorder-driven charge-separation model suggest that the improved IQE is attributed to the facilitated dissociation of charge-transfer excitons as well as the reduction in exciton quenching near the indium tin oxide surface.

Adsorption Behavior of Nonplanar Phthalocyanines: Competition of Different Adsorption Conformations.

Using density functional theory augmented with state-of-the-art van der Waals corrections, we studied the geometric and electronic properties of nonplanar chlorogallium-phthalocyanine GaClPc molecules adsorbed on Cu(111). Comparing these results with published experimental data for adsorption heights, we found indications for breaking of the metal–halogen bond when the molecule is heated during or after the deposition process. Interestingly, the work-function change induced by this dissociated geometry is the same as that computed for an intact adsorbate layer in the “Cl-down” configuration, with both agreeing well with the experimental photoemission data. This is unexpected, as the chemical natures of the adsorbates and the adsorption distances are markedly different in the two cases. The observation is explained as a consequence of Fermi-level pinning due to fractional charge transfer at the interface. Our results show that rationalizing the adsorption configurations on the basis of electronic interface properties alone can be ambiguous and that additional insight from dispersion-corrected DFT simulations is desirable.

Film formation of non-planar phthalocyanines on copper(i) iodide

Structural templating and high substrate temperature growth are combined to study the differences in structure and morphology of two non-planar phthalocyanines using X-ray diffraction, atomic force microscopy and low energy ion scattering.

Understanding domain symmetry in vanadium oxide phthalocyanine monolayers on Au (111)

Understanding the growth of organic semiconductors on solid surfaces is of key importance for the field of organic electronics. Non planar phthalocyanines have shown great promise in organic photovoltaic (OPV) applications, but little of the fundamental surface characterization to understand their structure and properties has been performed. Acquiring a deeper understanding of the molecule/substrate interaction in small molecule systems is a vital step in controlling structure/property relationships. Here we characterize the vanadium oxide phthalocyanine (VOPc)/Au (111) surface using a combination of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM), obtaining complex diffraction patterns which can be understood using two dimensional fast Fourier transform (2D-FFT) analysis of STM images. These measurements reveal coexistence of three symmetrically equivalent in-plane orientations with respect to the substrate, each of which is imaged simultaneously within a single area. Combining scanning probe and diffraction measurements allows symmetrically related domains to be visualized and structurally analyzed, providing fundamental information useful for the structural engineering of non-planar phthalocyanine interfaces.

Orientational Ordering of Nonplanar Phthalocyanines on Cu(111): Strength and Orientation of the Electric Dipole Moment

In order to investigate the orientational ordering of molecular dipoles and the associated electronic properties, we studied the adsorption of chlorogallium phthalocyanine molecules (GaClPc, Pc=C32N8H16(-2) on Cu(111) by using the x-ray standing wave technique, photoelectron spectroscopy, and quantum mechanical calculations. We find that for submonolayer coverages on Cu(111) the majority of GaClPc molecules adsorb in a Cl-down configuration by forming a covalent bond to the substrate. For bilayer coverages the x-ray standing wave data indicate a coexistence of the Cl-down and Cl-up configurations on the substrate. The structural details established for both cases and supplementary calculations of the adsorbate system allow us to analyze the observed change of the work function.

Can phthalocyanines and their substituted α-para-(methoxy)phenyl derivatives act as photosensitizers in photodynamic therapy? A TD-DFT study

A time-dependent density functional theory study (TD-DFT) is presented regarding the substituent effects on the Q-bands of two classes of non-planar phthalocyanines: the alpha-octaphenyl and p-alpha-octamethoxyphenyl substituted compounds, in their free-base and zinc complex forms. Singlet vertical excitation energies, computed at the PBE0/SVP//BP86/SVP level of theory also including bulk solvent effects (COSMO model), resulted within 0.1 eV of experiment. The experimental red-shift for the Q-band, going from the phenylated to the methoxyphenylated case, was well-reproduced theoretically and in the latter case it was found to depend mainly on the nature of the substituents and partly on structure distortion effects. The energetic gap between the singlet ground and first triplet excited state was calculated in solvent to be 1.28 eV for the free-base phthalocyanine (H2Pc) and 1.45 eV for the unsubstituted zinc complex (ZnPc) and lower than 0.98 eV for all the other compounds, which is the energetic lower limit for a molecule to act as photosensitiser in photodynamic therapy according to a Type II reaction mechanism. As a consequence, since this property-requirement for drugs used in photodynamic therapy is not fulfilled by the investigated near-infrared photosensitizers, they cannot be proposed as candidates for their use in this medical treatment.

Skeletal modification of a non-planar phthalocyanine analogue under very mild conditions

A sterically congested tetraphenyl-substituted dibenzo-tetraaza-isobacteriochlorinato nickel derivative unexpectedly reacts with methanol at room temperature to release internal molecular strains, leading to the formation of a ligand having a skeleton bearing two cis-methoxy substituents.

Self-assembled monolayer of tin-phthalocyanine on InSb(0 0 1)-(4 × 2)/c(8 × 2)

We have herein studied the adsorption of tin-phthalocyanine on the indium rich 4 × 2/c(8 × 2) reconstruction of the InSb(0 0 1) surface, using scanning tunneling microscopy, low-energy electron diffraction and high-resolution electron energy loss spectroscopy. The tin-phthalocyanine molecules form a stable monolayer with a characteristic long-range one-dimensional order, imposed by the twofold symmetry of the substrate surface reconstruction. High-resolution electron energy loss studies of both the vibrational and the electronic eigenmodes of the molecules show that phthalocyanines lie flat on the surface. In that, they behave the same way as lead phthalocyanines, which leads us to suggest that a common adsorption configuration may be adopted by non-planar phthalocyanines on the InSb(0 0 1)-4 × 2/c(8 × 2) surface.

Non-planar phthalocyanines with Q-bands beyond 800 nm

Free base and zinc 1,4,8,11,15,18,22,25-octa( p-methoxyphenyl) phthalocyanine have been synthesized. The Q-bands are located beyond 800 nm due to combined effects of red-shifts caused by ligand deformation and the electron-donating properties of the methoxy substituents. The phthalocyanines reported in this study could potentially replace naphthalocyanines as absorbers in the 800 nm region within practical applications.

Transparent Organic Thin‐Film Transistor with a Laterally Grown Non‐Planar Phthalocyanine Channel

An improved‐performance transparent organic thin‐film transistor (OTFT) is described, which has a vanadyl‐phthalocyanine (VOPc) film as an active p‐channel, lattice‐matched (Sc0.7Y0.3)2O3 film as a high‐k gate dielectric, and atomically flat indium tin oxide (ITO) film as the bottom contact (see Figure). These features—lattice matching and atomic flatness—are expected to lead to further improvements in transparent OTFTs.

Extremely non-planar phthalocyanines with saddle or helical conformation: synthesis and structural characterizations.

Extremely non-planar phthalocyanines with saddle or helical conformation: synthesis and structural characterizations.