Free-base Phthalocyanine Research Articles

Exploring the Role of Central Metals in Bulky Phthalocyanines for Dye-Sensitized Solar Cells.

Two series of metallo-(Zn(II), Mg(II), and Ru(II)) and free-base phthalocyanines (Pcs) with a carboxyl anchoring group and well-established bulky peripheral substituents (either tert-butyl or bulky 2,6-diisopropylphenoxy) were synthesized and tested as sensitizers in dye-sensitized solar cells (DSSCs). The trend of photovoltaic efficiencies (PCEs) for free-base and metallo Pcs followed the order Zn(II)Pc>Mg(II)Pc≫H2Pc ≈ Ru(II)Pc regardless of the peripheral substitution. Higher efficiencies (4.95 versus 3.63 for the Zn(II) derivatives) were achieved with Pcs bearing the bulkier 2,6-diisopropylphenoxy group, indicating a lower aggregation and more suitable HOMO-LUMO levels. Furthermore, these derivatives showed a morelevant influence of the metal on the PCE values (from the highest 4.95 for the Zn(II)Pc to the lowest 0.23 for the Ru(II)Pc. In both series, the best PCEs observed with the Zn(II) derivatives were mainly due to their highest Jsc values. The lowest efficiencies found for the free-bases and Ru(II) derivatives were attributed to a mismatch between their LUMO levels and the conduction band of the TiO2,and lower light-harvesting capabilities, respectively. In conclusion, Zn(II) derivatives are still the best Pc candidates to use as sensitizers in molecular photovoltaics.

Solvothermal Synthesis of Rare Earth Bisphthalocyanines.

Rare earth bisphthalocyanines (MPc2) are of particular interest because of their behavior as single-molecular magnets, which makes them suitable for applications in molecular spintronics, high-density data storage and quantum computation. Nevertheless, MPc2 are not commercially available, and the synthesis routes are mainly focused on obtaining substituted phthalocyanines. Two preparation routes depend on the precursor: synthesis from phthalonitrile (PN) and the metalation of free or dilithium phthalocyanine (H2Pc and Li2Pc). In both options, byproducts such as free-base phthalocyanine and in the first route additional PN oligomers are generated, which influence the MPc2 yield. There are three preparation methods for these routes: heating, microwave radiation and reflux. In this research, solvothermal synthesis was applied as a new approach to prepare yttrium, lanthanum, gadolinium and terbium unsubstituted bisphthalocyanines using Li2Pc and the rare earth(III) acetylacetonates. Purification by sublimation gave high product yields compared to those reported, namely 68% for YPc2, 43% for LaPc2, 63% for GdPc2 and 62% for TbPc2, without any detectable presence of H2Pc. Characterization by infrared, Raman, ultraviolet-visible and X-ray photoelectron spectroscopy as well as elemental analysis revealed the main featuresof the four bisphthalocyanines, indicating the success of the synthesis of the complexes.

Ab Initio Modeling of Mixed-Dimensional Heterostructures: A Path Forward.

Understanding the electronic structure of mixed-dimensional heterostructures is essential for maximizing their application potential. However, accurately modeling such interfaces is challenging due to the complex interplay between the subsystems. We employ a computational framework integrating first-principles methods, including GW, density functional theory (DFT), and the polarizable continuum model, to elucidate the electronic structure of mixed-dimensional heterojunctions formed by free-base phthalocyanines and monolayer molybdenum disulfide. We assess the impact of dielectric screening across various scenarios, from isolated molecules to organic films on a substrate-supported monolayer. Our findings show that while polarization effects cause significant renormalization of molecular energy levels, band energies and alignments in the most relevant setup can be accurately predicted through DFT simulations of the individual subsystems. Additionally, we analyze orbital hybridization, revealing potential pathways for interfacial charge transfer. This study offers new insights into hybrid inorganic/organic interfaces and provides a practical computational protocol suitable for scaled-up studies.

Analysis on high-resolution spectrum of the S1-S0 transition of free-base phthalocyanine.

A high-resolution absorption spectrum of the S1-S0 transition of free-base phthalocyanine was observed and analyzed with improved reliability. The spectrum, with a partially resolved rotational structure, was obtained by using the buffer-gas cooling technique and a single-mode tunable laser. Our new analysis reveals that the S1←S0000 band belongs to the a-type transition, where the electronic transition moment aligns parallel to the NH-HN direction, allowing the assignment of the S1 state to 1B3u. These results agree with a prior study using supersonic expansion and are well supported by theoretical calculations. Interestingly, the rotational constant B in the S1 state, which is often smaller than that in the ground state for typical molecules, was found to be slightly larger than that in the S01Ag state. This suggests a change in the character of π bonds with the electronic excitation.

Submolecular-scale control of phototautomerization.

Optically activated reactions initiate biological processes such as photosynthesis or vision, but can also control polymerization, catalysis or energy conversion. Methods relying on the manipulation of light at macroscopic and mesoscopic scales are used to control on-surface photochemistry, but do not offer atomic-scale control. Here we take advantage of the confinement of the electromagnetic field at the apex of a scanning tunnelling microscope tip to drive the phototautomerization of a free-base phthalocyanine with submolecular precision. We can control the reaction rate and the relative tautomer population through a change in the laser excitation wavelength or through the tip position. Atomically resolved tip-enhanced photoluminescence spectroscopy and hyperspectral mapping unravel an excited-state mediated process, which is quantitatively supported by a comprehensive theoretical model combining ab initio calculations with a parametric open-quantum-system approach. Our experimental strategy may allow insights in other photochemical reactions and proof useful to control complex on-surface reactions.

Electroluminescence from a phthalocyanine monolayer encapsulated in a van der Waals tunnel diode

Monolayers of free base phthalocyanine (H2Pc) are grown on monolayer and few-layer exfoliated flakes of hexagonal boron nitride (hBN) which are subsequently integrated into a van der Waals tunnel diode. This heterostructure consists of two thin hBN flakes between which the H2Pc monolayer is sandwiched and also incorporates upper and lower few-layer graphene contacts. When a voltage is applied between the contacts, a tunnel current flows and the embedded molecules can be excited resulting in the emission of photons with wavelengths which are close to the peaks observed in photoluminescence. We also observe electroluminescence at voltages where the energy gained by a tunnelling electron is lower than the energy of the emitted photon implying a multi-electron excitation pathway which we attribute to the formation of an intermediate triplet state. Our results provide insights into the differences in excitation and relaxation of molecules in supramolecular monolayers and bulk crystals and we discuss how the alignment of the energy levels of the molecules and contact layers determine the emission process.

Preferred catalysis distinctly determined by metals doped with nitrogen in three-dimensionally ordered porous carbon materials.

Three-dimensionally ordered nanoporous structures were generated in carbon materials doped with metals and nitrogen as catalytically active sites for electrochemical reactions. Free-base and metal phthalocyanines with a strategically designed molecular structure were used as carbon sources to obtain an ordered porous structure via homogeneous self-assembly with Fe3O4 nanoparticles as the pore template and the prevention of melting away during carbonization. The doping of Fe and nitrogen was achieved by a reaction between the free-base phthalocyanine and Fe3O4 through carbonization at 550 °C, while Co and Ni were doped using the corresponding metal phthalocyanines. The preference of these three types of ordered porous carbon materials for catalytic reactions was distinctly determined by the doped metals. Fe-N-doped carbon showed the highest activity for O2 reduction. Additional heat treatment at 800 °C enhanced this activity. CO2 reduction and H2 evolution were preferred by the Ni- and Co-N-doped carbon materials, respectively. A change in the template particle size was capable of controlling the pore size to enhance mass transfer and improve performance. The technique presented in this study enabled systematic metal doping and pore size control in the ordered porous structures of carbonaceous catalysts.

Experimental and theoretical study of resonant core-hole spectroscopies of gas-phase free-base phthalocyanine.

We studied N 1s-1 inner-shell processes of the free base Phthalocyanine molecule, H2Pc, in the gas-phase. This complex organic molecule contains three different nitrogen sites defined by their covalent bonds. We identify the contribution of each site in ionized, core-shell excited or relaxed electronic states by the use of different theoretical methods. In particular, we present resonant Auger spectra along with a tentative new theoretical approach based on multiconfiguration self-consistent field calculations to simulate them. These calculations may pave the road towards resonant Auger spectroscopy in complex molecules.

High-resolution spectroscopy of buffer-gas-cooled phthalocyanine

For over five decades, studies in the field of chemical physics and physical chemistry have primarily aimed to understand the quantum properties of molecules. However, high-resolution rovibronic spectroscopy has been limited to relatively small and simple systems because translationally and rotationally cold samples have not been prepared in sufficiently large quantities for large and complex systems. In this study, we present high-resolution rovibronic spectroscopy results for large gas-phase molecules, namely, free-base phthalocya-nine (FBPc). The findings suggest that buffer-gas cooling may be effective for large molecules introduced via laser ablation. High-resolution electronic spectroscopy, combined with other experimental and theoretical studies, will be useful in understanding the quantum properties of molecules. These findings also serve as a guide for quantum chemical calculations of large molecules.

(Digital Presentation) Monitoring and Controlling Tautomerization in Phthalocyanines, Porphyrines and Porphycenes By Optical Single-Molecule Imaging and Spectroscopy

Tautomerization is an intramolecular chemical reaction defined as structural interconversion of a molecule between two isomers separated by a low energy barrier. Tautomerization in free base phthalocyanines, porphyrins or porphycenes has fascinated and attracted chemists and physicists alike over the last decades for its fundamental importance in various chemical and biological systems and many applications in technology [1,2]. Particularly interesting is the reaction rate which can range over orders of magnitude depending on the temperature and the local environment. In solution, this reaction in porphycenes occurs usually in several picoseconds or even faster, as determined via anisotropy studies by ultrafast transient absorption spectroscopy [3] and in porphyrins at cryogenic temperatures it can last for hours or days as determined form the live time of narrow spectral holes photochemically burned in the inhomogeneously broadened absorption band with a narrow band laser [4]. However, a spectacular and unexpected observation has been made by optical single molecule imaging and spectroscopy [5]. Examining a large number of single molecules embedded at low concentration in various polymer matrices at room temperature has revealed unquestionably that the rate in these molecules can vary dramatically over time such that tautomerization can be frozen for seconds and longer under ambient conditions[6]. The interconversion of the inner protons of phthalocyanine changes the orientation of the transition dipole moment by 90o degrees with respect to the molecular structure. Hence, if these molecules are embedded with a fixed orientation in a transparent solid polymer film at high dilution and tautomerization is frozen, the orientation of their transition dipole moments can be determined by polarization spectroscopy.Here we report on our recent experimental and theoretical studies of monitoring tautomerization by fluorescence imaging of single molecules of phthalocyanines, porphyrins, or porphycenes excited in the tightly focused azimuthally and radially polarized laser beam with a sample scanning confocal optical microscope equipped with single photon counting electronics and a fluorescence spectrometer. This technique has been used to determine the transition dipole moment orientation of single molecules, the polarizability of plasmonic nanoparticles or the dimensionality of single quantum dots. Different diffraction limited fluorescence intensity patterns are observed for molecules that undergo tautomerization and for those that do not. The latter corresponds to a molecule with a fixed transition dipole moment direction, whereas the tautomerizing molecule can be treated as the superposition of two emitters with different transition dipole moment orientations.Previous studies have shown that tautomerization in porphyrins or phthalocyanines can occur via ground state tunneling and is frozen at low temperatures, particularly at cryogenic temperatures. The structural heterogeneity in a polymer matrix leads to a distribution of asymmetrically distorted ground state double well potential and hence can lead to long residence times in the deeper energy minimum. However, even at helium temperatures tautomerization can be induced by electronic excitation to the first excited state, making photochemical spectral hole-burning possible. Recently we could demonstrate in a single-molecule study that the tautomerization rate of phthalocyanine tetrasulfonate embedded in a thin PVA-film at room temperature could be controlled in a tunable λ/2 Fabry Perot resonator by weak coupling between the electronically excited state and the vacuum electromagnetic field of a resonant cavity mode.

Free base phthalocyanine coating as a superior corrosion inhibitor for copper surfaces: A combined experimental and theoretical study

In the quest of novel, eco-benign copper corrosion inhibitors, free base and a set of six metal Phthalocyanine (Pc) coatings (where metal is Zn, Cu, Fe, Co, Ni, Mn) are systematically evaluated for their corrosion inhibition efficiency. The study demonstrates that the free base Phthalocyanine coatings are superior corrosion inhibitors for Cu surface in HCl with a corrosion inhibition efficiency of 88% as compared to its metal counterparts. A detailed and indepth study involving concerted electrochemical polarisation, impedance measurements at various concentrations of HCl assert the superior corrosion inhibition efficiency of free base Pc coatings as compared to its metal counterparts. Underlying factors/interface chemistry contributing to it is brought out using first principle based studies. First principles based computational studies establish that superior corrosion inhibition ability of free base phthalocyanine molecule is attributed to its innate binding ability with copper surface and its better inter-molecular packing. Thus, the theoretical adsorption trends of various studied phthalocyanine molecules while clearly following the experimental corrosion inhibition efficiency trends undermine the importance of contribution of chelating factors for the formation of a stable and efficient corrosion inhibition coating on a surface and that the adhesion/adsorption strength of a molecule correlates directly to its corrosion inhibition efficiency. The experimental observations are further substantiated through FE-SEM, EDAX and XPS studies. The corrosion inhibition efficiency of free base phthalocyanine is followed by its zinc based counterpart at 85%.

Interaction of free-base and 3d metal(II) phthalocyanines with open-shell endohedral fullerenes species N@C60 and P@C60

We studied theoretically the interactions of phthalocyanines (Pcs) of general formula MPc (M = 2H, Mn, Fe, Co, Ni, Cu, Zn) with endohedral fullerenes (EFs) N@C60 and P@C60, at the PBE-D/DNP theoretical level. All MPcs can form stable noncovalent complexes with both N@C60 and P@C60. The binding energies vary between −17.7 and −31.1 kcal/mol. Complexation pattern depends on the central atom of MPc, but not on the endohedral species (N or P): for H2Pc, two central H atoms approach the carbon atoms of C60 of adjacent 5:6:6 junctions; for MnPc, manganese atom coordinates to two carbon atoms of 6,6 C-C bond; for other MPcs, the central metal coordinates to 5:6:6 junction of fullerene. In MPc+N@C60 HOMO is localized exclusively on phthalocyanine, with LUMO found mostly on fullerene. In MPc+P@C60 complexes, HOMO has essentially the same localization and shape, but LUMO distribution is variable. After complexation with MPcs, no tangible changes are observed in the N and P spin state. MnPc, CoPc and CuPc having open-shell metal atoms behave differently: while the complexation has almost no effect on Cu spin, and causes only a moderate decrease in Co spin, Mn atom transfers roughly two-thirds of its unpaired electrons to fullerene.

Orbital-resolved visualization of single-molecule photocurrent channels.

Given its central role in utilizing light energy, photoinduced electron transfer (PET) from an excited molecule has been widely studied1-6. However, even though microscopic photocurrent measurement methods7-11 have made it possible to correlate the efficiency of the process with local features, spatial resolution has been insufficient to resolve it at the molecular level. Recent work has, however, shown that single molecules can be efficiently excited and probed when combining a scanning tunnelling microscope (STM) with localized plasmon fields driven by a tunable laser12,13. Here we use that approach to directly visualize with atomic-scale resolution the photocurrent channels through the molecular orbitals of a single free-base phthalocyanine (FBPc) molecule, by detecting electrons from its first excited state tunnelling through the STM tip. We find that the direction and the spatial distribution of the photocurrent depend sensitively on the bias voltage, and detect counter-flowing photocurrent channels even at a voltage where the averaged photocurrent is near zero. Moreover, we see evidence of competition between PET and photoluminescence12, and find that we can control whether the excited molecule primarily relaxes through PET or photoluminescence by positioning the STM tip with three-dimensional, atomic precision. These observations suggest that specific photocurrent channels can be promoted or suppressed by tuning the coupling to excited-state molecular orbitals, and thus provide new perspectives for improving energy-conversion efficiencies by atomic-scale electronic and geometric engineering of molecular interfaces.

Quantifying Force and Energy in Single-Molecule Metalation.

An atomic force microscope is used to determine the attractive interaction at the verge of adding a Ag atom from the probe to a single free-base phthalocyanine molecule adsorbed on Ag(111). The experimentally extracted energy for the spontaneous atom transfer can be compared to the energy profile determined by density functional theory using the nudged-elastic-band method at a defined probe-sample distance.

Internal Stark effect of single-molecule fluorescence

The optical properties of chromophores can be efficiently tuned by electrostatic fields generated in their close environment, a phenomenon that plays a central role for the optimization of complex functions within living organisms where it is known as internal Stark effect (ISE). Here, we realised an ISE experiment at the lowest possible scale, by monitoring the Stark shift generated by charges confined within a single chromophore on its emission energy. To this end, a scanning tunneling microscope (STM) functioning at cryogenic temperatures is used to sequentially remove the two central protons of a free-base phthalocyanine chromophore deposited on a NaCl-covered Ag(111) surface. STM-induced fluorescence measurements reveal spectral shifts that are associated to the electrostatic field generated by the internal charges remaining in the chromophores upon deprotonation.

Vibrationally Resolved Absorption and Luminescence Spectra of Mass-Selected Free-Base and Zinc Phthalocyanine Radical Cations Isolated in Solid Ne.

We report the first vibrationally well-resolved absorption and laser-induced fluorescence spectra of the radical cations of free-base phthalocyanine (H2Pc+) and zinc phthalocyanine (ZnPc+) isolated in 5 K neon matrices and compare them to the spectral properties of the corresponding neutrals. The samples were generated by low-energy deposition of the mass-selected ions. The spectra are also discussed in terms of time-dependent density functional theory calculations and compared with recently reported scanning tunneling microscopy-induced single-molecule luminescence of the same species adsorbed on NaCl-covered Au(111) or Ag(111) single crystal supports.

Investigation of Electric and Thermoelectric Properties of Phthalocyanine monomer/dimer Molecular Junctions

In this work, we carried out a theoretical calculation to present the electrical and thermoelectrical properties of five monomers/dimers structures based on Phthalocyanine molecule sandwiched between gold electrodes. The calculation was preformed based on density functional theory (DFT) implemented by SIESTA. The results reveal that the transmission calculations T(E) of all five monomers/dimers show no spin dependent with highest value for monomer structure around Fermi energy. The phthalocyanine monomer structures show higher conductivity calculation around the Fermi energy in comparison to conductivity values of phthalocyanine dimer structures. Further, thermoelectric properties such as Seebeck coefficient (S), thermal conductance (k) and figure of merit (ZT) are also presented. It is found that the highest positive Seebeck coefficient (S) value belongs to zinc phthalocyanine monomer, while the highest negative (S) value belongs to free base phthalocyanine dimer structure over a wide range of Fermi energies. Moreover, zinc phthalocyanine monomer structure shows highest thermal conductance (k), while phthalocyanine dimer structures exhibit lowest thermal conductance (k) in the vicinity of DFT predicted Fermi energy. As a result, high room-temperature figure of merit (ZT ≈1.7) is reported for free base phthalocyanine dimer, which might be due to the low thermal conductivity and high Seebeck coefficient and electrical conductance values, which makes it a preferred candidate for potential thermoelectric applications.

Tuning the Electrical and Thermoelectric Properties of Phthalocyanine and Metallo-Phthalocyanine Molecular Junction

In this work, a comparative study is presented that analyses the electrical and thermoelectrical properties of free base-Phthalocyanine (Bare-Pc) and Metallo-Phthalocyanine (MPc) molecule sandwiched between gold electrodes. The study investigates the transmission and conductance characteristics of Bare-Pc without central metal ion and with different transition metal ions at the centre of (Pc) such as (Mn, Co, Ni, and Zn) in two different molecular configurations, cis and trans. The results reveal that the conductance changed by varying the transition metal-centre in the order of Co > Ni > Zn in trans, and closely similar pattern Co > Ni > Mn in cis with higher values in trans configuration. This reflects the correlation between the spin-dependent transport properties with the transition metal-centre type and the molecular structure. Further, thermoelectrical properties such as thermal conductance (k), Seebeck coefficient (S) and figure of merit (ZT) are investigated. It was found that (k), (S) and (ZT) have higher values in case of trans configuration for Co-Pc, Ni-Pc and Zn-Pc structures. Thus, by varying the transition metal-centre type and the molecular configuration of (Pc) molecule, transport properties can be tuned to produce a good value of the transmission and electrical conductance, thermal conductance, Seebeck coefficient, and figure of merit of the (Pc) molecular junction for potential high conductance and efficient thermoelectric applications.

Water soluble near infrared dyes based on PEGylated-Tetrapyrrolic macrocycles

The design and synthesis of new near infrared (NIR) biocompatible dyes have been of great interest over the last years. Herein, the synthesis of two new water soluble NIR dyes is reported: a PEGylated (PEG = polyethyleneglycol) fluorinated bacteriochlorin [5,10,15,20-tetrakis(4-PEG500-2,3,5,6-fluorophenyl)bacteriochlorin], prepared in a straightforward synthetic strategy involving a solventless reduction; and an improved strategy to prepare a PEGylated tetra-substituted free-base phthalocyanine [2(3),9(10),16(17),23(24)-tetrakis(4-PEG200)phthalocyanine], obtained in 52% overall yield, from the PEGylated phthalonitrile. Photophysical characterization and photostability studies were performed, where both dyes showed favorable photostability and the PEGylated fluorinated bacteriochlorin presented good singlet oxygen quantum yield, showing potential to use as a photosensitizer for PDT (photodynamic therapy). Additionally, the free-base PEGylated phthalocyanine displayed reasonably high Stokes shift, suggesting high potential to use in optical imaging.

Complexation of free-base and 3d transition metal(II) phthalocyanines with endohedral fullerenes H@C60, H2@C60 and He@C60: The effect of encapsulated species

We studied theoretically the bonding of unsubstituted free-base phthalocyanine H2Pc and its 3d transition metal complexes MPc (MMn, Fe, Co, Ni, Cu, Zn) with endohedral non-metalofullerenes H@C60, H2@C60 and He@C60, by using PBE functional with Grimme's dispersion correction and DNP basis set. The calculated formation energies, geometries and electronic parameters (energies and distribution of HOMO and LUMO orbitals, electrostatic potential and spin density plots) were analyzed and compared to those of the dyads formed between the same series of unsubstituted Pcs and empty-cage C60. Particular differences were found in MnPc coordination pattern and the resulting frontier orbital distribution.