Free-base Phthalocyanine Research Articles

Structure of phthalocyanine overlayers on the [formula omitted] surfaces of InSb and InAs

The structure of free-base phthalocyanine (H 2Pc) and copper phthalocyanine (CuPc) overlayers deposited on the indium-terminated InSb(1 0 0) and InAs(1 0 0) (4×2)/c(8×2) reconstructed surfaces have been studied by low-energy electron diffraction and van der Waals (vdW) intermolecular interaction energy calculations. The diffraction pattern for H 2Pc grown on InSb(1 0 0) is a superposition of a (3×3) and (√10×√10) R±18.4° structure, whilst no ordered structure is formed when H 2Pc is grown on InAs(1 0 0). In contrast, CuPc films exhibit a (3×3) structure on InSb(1 0 0) and a (√10×√10) R±18.4° structure on InAs(1 0 0). These differences can be rationalised by the vdW intermolecular interaction energy calculations of the quadratic unit cells of H 2Pc and CuPc. The results show that the lattice structure during the initial stages of thin film growth is influenced strongly by the intermolecular interaction energy of their unit cells.

Stark effect on holes in the absorption spectra of phthalocyanine and the aluminum-phthalocyanine complex in polymer matrices

The effect of an external electric field on narrow spectral holes in S1←S0 absorption bands of molecules of free-base phthalocyanine and its complex with Al in polyvinyl butyral films at 4.2 K was studied. It was found that the average values of the change in the electric dipole moment due to phototransitions in the free-base phthalocyanine and its complex amount to 0.135 and 0.145 D, respectively. Both these values considerably exceed the relevant values for phthalocyanine complexes with Zn and Mg in the same matrix. Possible causes of the observed effects are discussed. Most probably, they are related to the differences in the structure of the central part of phthalocyanine-type molecules.

Non-photochemical versus photochemical hole burning in hyperquenched glassy water and cubic ice

The Shu–Small mechanism (J. Opt. Soc. Am. B 9 (1992) 724) for non-photochemical hole burning (NPHB) of electronic transitions of chromophores in amorphous hosts at low temperatures is based on a hierarchy of configurational tunneling events that begin in the outer shell and terminate in the inner shell that surrounds the chromophore. Connectivity between the intrinsic and extrinsic two level systems that stem from structural disorder and the excess free volume of glasses are key features of this mechanism. This mechanism has recently been modified to take into account multilevel extrinsic systems (J. Chem. Phys. 114 (2001) 9105). An important prediction of the Shu–Small mechanism is that NPHB should cease upon formation of a crystalline phase produced, for example, by warming of a vitrified glass. In this paper data are presented that are consistent with that prediction. The systems studied are aluminum phthalocyanine tetrasulphonate (APT) and free base phthalocyanine tetrasulphonate (PcT) in hyperquenched glassy films of water (HGW) and cubic ice ( I c). The hole-burning mechanisms for APT and PcT are non-photochemical and photochemical, respectively. Surprisingly, the hole-growth kinetics for PcT are as highly dispersive as are the kinetics for APT. This means that the kinetics for photoinduced proton tautomerization in PcT are strongly influenced by its nanoenvironment of water molecules, some of which are involved in H-bonding to the –NH groups of PcT.

NIR to UV absorption spectra and the optical constants of phthalocyanines in glassy medium

Optical absorption studies of phthalocyanines (Pc-s) in borate glass matrix have been reported for the first time. Measurements have been done corresponding to photon energies between 1.1 and 6.2 eV for free base, manganese, iron, nickel, molybdenum, cobalt and copper phthalocyanines. Several new discrete transitions are observed in the UV–vis region of the spectra in addition to a strong continuum component of absorption in the IR region. Values of some of the important optical constants viz. absorption coefficient ( α), molar extinction coefficient ( ε), absorption cross-section ( σ a), band width (Δ λ), electric dipole strength ( q 2) and oscillator strength ( f) for the relevant electronic transitions are also presented. All the data reported for Pc-s in the new matrix have been compared with those corresponding to solution, vapor and thin film media.

Model for the α→β1 phase transition in phthalocyanine thin films

The α→β1 phase transition induced by annealing free-base phthalocyanine thin films grown by organic molecular-beam deposition has been studied by powder x-ray diffraction, electronic absorption spectroscopy, and Nomarski interference microscopy. The final β1 phase consists of two domains having a different orientation relative to the substrate surface. Starting with the α phase, by careful control of the annealing temperature, a series of intermediate states are observed consisting of coexisting α and β1 phases. At the beginning of the phase transition, only one domain is observed with the β1 crystal phase aligned with the (001) plane parallel to the substrate. As the transition proceeds, however, the amount of the other domain having the (201) plane parallel to the substrate increases rapidly. A mechanism is proposed for the transition that can account for all the experimental results.

Investigation of phthalocyanine crystals exposed to NO 2 ambient gas

Crystals of free base phthalocyanine (H 2pc) and its unsubstituted metal derivatives Mpc (where M=Ni, Cu, Zn, Pb, and pc=C 32H 16N 8 2− =phthalocyanine ligand) were examined after being exposed to NO 2 ambient gas. Scanning electron microscopy (SEM) supported by X-ray microprobe analysis (EDAX), UV–Vis and FTIR spectroscopy as well as X-ray powder diffractometry (XRD) were applied to study the crystals chemistry and surface morphology. The NO 2 molecules were found to penetrate the crystal unit which led to the breakdown of the phthalocyanine macrocycle. The surface of the doped crystals has been scaled and covered by a layer composed predominantly of phthalimide. Despite vacuum desorption, it has been proved some free NO 2 to be still adsorbed by the studied material.

Comparative studies on the structures, infrared spectrum, and thermodynamic properties of phthalocyanine using ab initio Hartree–Fock and density functional theory methods

AbstractThe molecular structure, vibrational spectrum, standard thermodynamic functions, and enthalpy of formation of free base phthalocyanine (Pc) have been studied using the density functional theory B3LYP procedure, as well as the ab initio Hartree–Fock method. Various basis sets 3‐21G, 6‐31G*, and LANL2DZ have been employed. The results obtained at various levels are discussed and compared with each other and with the available experimental data. It is shown that calculations performed at the Hartree–Fock level cannot produce a reliable geometry and related properties such as the dipole moment of Pc and similar porphyrin‐based systems. Electron correlation must be included in the calculations. The basis set has comparatively less effect on the calculated results. The results derived at the B3LYP level using the smaller 3‐21G and LANL2DZ basis sets are very close to those produced using the medium 6‐31G* basis set. The geometry of Pc obtained at the B3LYP level has D2h symmetry and the diameter of the central macrocycle is about 4 Å. The enthalpy of formation of Pc in the gas phase has been predicted to be 1518.50 kJ/mol at the B3LYP/6‐311G(2d,2p)//B3LYP/6‐31G* level via an isodesmic reaction. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Third-order optical nonlinearities and optical limiting of octa-octyloxy phthalocyanine free base in toluene

The third-order nonlinear optical properties of the octa-octyloxy phthalocyanine free base were studied by Z-scan using 35-ps laser pulses at 532 nm in toluene. The nonlinear refractive index or the refractive cross section, and the excited singlet state absorption cross section were determined. The optical limiting performances at both nanosecond and picosecond time scales have been investigated. The results suggest that the octa-octyloxy phthalocyanine free base is promising for optical limiting materials.

Unoccupied electronic states in ordered PTCDA and H 2Pc thin films grown on InAs and InSb: a resonance HREELS study

High resolution electron energy loss spectroscopy has been used to investigate the unoccupied electronic states of ordered thin films of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and free-base phthalocyanine (H 2Pc) grown on InAs and InSb substrates. Measurement of the vibrational excitation functions over a wide range of electron beam energies shows that both molecules exhibit strong resonant behaviour due to temporary negative ion formation. For PTCDA/InAs(1 1 1)A, there is no mode selectivity and all the vibrational modes of the molecule are resonantly enhanced at 2.2 eV. By contrast, two resonances are observed for H 2Pc/InSb(1 0 0) at 0.8 and 7.0 eV respectively, with the higher energy resonance demonstrating mode selectivity. Identification of the resonant states is assisted by comparison to previous near edge X-ray absorption fine structure measurements for PTCDA and Pc films.

Phthalocyanine-Sensitized Nanostructured TiO2 Electrodes Prepared by a Novel Anchoring Method

A novel method for anchoring phthalocyanines substituted with ester groups onto nanostructured TiO2 films is described. Such phthalocyanines did not adsorb on nanostructured TiO2 film by the ordinary methods. In our new method, the TiO2 film is pretreated with (CH3)3COLi to change the surface hydroxyl groups (−OH) into oxygen anions (−O-), thus making the surface more reactive toward the ester functionalities of the dye. The dye can then be anchored onto the semiconductor surface through the produced carboxylate group(s). The amount of anchored dye on the semiconductor shows a dependence on both the time of base treatment and the time of dye treatment. Electrodes treated with the free base phthalocyanine and zinc phthalocyanine were characterized by absorption spectroscopy, photocurrent action spectroscopy, and photocurrent−photovoltage measurements. The homogeneous blue-green color and the absorption bands in the far-red region are indicative of an attachment of the dye on TiO2 film. A monochromatic incident photo-to-current conversion efficiency of 4.3% was achieved at 690 nm for a cell where the base-treated electrode was treated with ZnPcBu.

A tightly coupled linear array of perylene, bis(porphyrin), and phthalocyanine units that functions as a photoinduced energy-transfer cascade

We have prepared a linear array of chromophores consisting of a perylene input unit, a bis(free base porphyrin) transmission unit, and a free base phthalocyanine output unit for studies in artificial photosynthesis and molecular photonics. The synthesis involved four stages: (1) a rational synthesis of trans-AB2C-porphyrin building blocks each bearing one meso-unsubstituted position, (2) oxidative, meso,meso coupling of the zinc porphyrin monomers to afford a bis(zinc porphyrin) bearing one phthalonitrile group and one iodophenyl group, (3) preparation of a bis(porphyrin)-phthalocyanine array via a mixed cyclization involving the bis(free base porphyrin) and 4-tert-butylphthalonitrile, and (4) Pd-mediated coupling of an ethynylperylene to afford a perylene-bis(porphyrin)-phthalocyanine linear array. The perylene-bis(porphyrin)-phthalocyanine array absorbs strongly across the visible spectrum. Excitation at 490 nm, where the perylene absorbs preferentially, results in fluorescence almost exclusively from the phthalocyanine (phi(f) = 0.78). The excited phthalocyanine forms with time constants of 2 ps (90%) and 13 ps (10%). The observed time constants resemble those of corresponding phenylethyne-linked dyads, including a perylene-porphyrin (< or = 0.5 ps) and a porphyrin-phthalocyanine (1.1 ps (70%) and 8 ps (30%)). The perylene-bis(porphyrin)-phthalocyanine architecture exhibits efficient light-harvesting properties and rapid funneling of energy in a cascade from perylene to bis(porphyrin) to phthalocyanine.

Polymorphism in Phthalocyanine Thin Films: Mechanism of the α → β Transition

Two different polymorphic forms of free base phthalocyanine films have been grown on glass substrates by ultrahigh vacuum organic molecular beam deposition. Postgrowth annealing of films grown at room temperature leads to transformation from the α to the β1 phase. The effects of annealing lead to a number of transition states whose morphological, structural, and spectroscopic properties can be identified using atomic force and optical interference microscopy, X-ray diffraction, and Raman and electronic absorption spectroscopy. Detailed morphological studies indicate that the transition occurs via a discrete number of nucleations and is preceded by an elongation of the α crystallites. Quantitative analysis of the crystallites and domain size shows that the individual β1 crystals grow but are confined to domains of similar orientation. The film thickness plays a critical role and three regimes have been identified. The α → β1 transformation is only complete for films thicker than ∼940 Å, and thick films lead to a higher degree of orientation and larger domains.

Physical and optical properties of phthalocyanine doped inorganic glasses

Physical and optical properties of various free base and metallic phthalocyanine (Pc) doped glass matrix are reported for the first time. Absorption spectral measurements of H2Pc, MnPc, NiPc, CoPc, CuPc, MoOPc, ZnPc and FePc doped borate glass matrix have been made in the 200–1100 nm region and the spectra obtained are analyzed in the 2.1–6.2 eV region to obtain the optical band gap (Eg) and the width of the band tail (Et). Other important optical and physical parameters viz. refractive index (n), molar extinction coefficient (∈), density (ρ), glass transition temperature (Tg), molecular concentration (N), polaron radius (rp), intermolecular separation (R), molar refractivity (Rm) are also reported.

Correlation between linear optical constants and Raman enhancement in phthalocyanine thin solid films with incorporated silver clusters

Surface-enhanced Raman spectroscopic (SERS) experiments on thin films of free base phthalocyanine (H2Pc) with embedded silver clusters are reported. The thin films were produced by thermal evaporation of H2Pc together with different amounts of silver (8, 15 and 26 vol.%). From transmission electron microscopy it turned out that the average silver cluster diameter changes from approximately 4 to 7 nm with increasing silver concentration. The plasmon wavenumber of the silver clusters was obtained from spectrophotometric measurements and was close to 22 000 cm−1 (approximately 450 nm), nearly independent of the silver filling factor. Raman spectra of H2Pc were recorded at two different exciting laser wavelengths (441.6 and 568.2 nm), being either close to or well separated from the plasmon resonance. The intensity of the Raman scattering increased with increasing silver filling factor in the films, while the lineshape of the Raman spectra was not seriously influenced by the cluster incorporation. A systematic correlation between the intensity of the plasmon absorption line and the Raman enhancement could be established and is discussed in terms of an electromagnetic enhancement mechanism. Copyright © 1999 John Wiley & Sons, Ltd.

Laser-Desorption Supersonic Jet Spectroscopy of Phthalocyanines

The laser-induced fluorescence excitation spectra of zinc phthalocyanine, chloroaluminum phthalocyanine, magnesium phthalocyanine, and free-base phthalocyanine have been investigated under jet-cooled conditions, by using pulsed infrared laser desorption as the means of vaporization. Assignment of the observed vibronic transitions reveals that many of them gain intensity from coupling of theS1(Q) state with theS2(B) andQ′ states. Low-frequency out-of-plane modes of the macrocycle are observed in the region below 100 cm−1; the vibronic structure in this region of the spectrum is sensitive to deviation of the molecule from a planar geometry. TheS1–S0excitation energy shows a weak dependence on the hole size of the macrocycle.

Effect of Annealing on the Properties of thin Films of Free Base Phthalocyanine and Perylene-3,4,9,10-Tetracarboxylic Dianhydride Deposited by Organic Molecular Beam Deposition

ABSTRACTOrganic molecular beam deposition (OMBD) has been used to deposit thin films of free base phthalocyanine (H2Pc) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). The structural, optical and morphological properties of the films have been measured as a function of annealing temperature using x-ray diffraction (XRD), electronic absorption spectroscopy and atomic force microscopy (AFM). A phase transition, (α-β), is observed for H2Pc films which is accompanied by changes in the bulk film properties and surface morphology. In the case of PTCDA increasing temperature does not affect the structural and optical properties of the film, but different surface morphologies are observed.

Calculation of the structure and absorption spectra of phthalocyanines in the gas-phase and in solution

Ab initio calculations have been carried out on both gas-phase and solvated phthalocyanines in order to determine equilibrium structures, vibrational spectra, and electronic spectra. Density functional theory (DFT) optimized geometries have the expected symmetries of D 2h for free-base phthalocyanine, D 4h for copper phthalocyanine, and C 4v for tin phthalocyanine, whereas Hartree–Fock optimized geometries, which also include lead phthalocyanine, are slightly displaced from the expected symmetries. Both sets of optimized geometries agree reasonably well with the available experimental structures. Vibrational spectra, calculated by the Hartree–Fock method, are in partial agreement with measured spectra, whereas visible absorption spectra calculated using molecular geometries obtained at three different levels of theory (DFT, HF, and PM3) showed good agreement with the measured vapor-phase spectra in the Q-band, and using the DFT optimized geometry resulted in very good agreement with measurements in the B-band. The self-consistent reaction field method for including bulk solvent effects in HF calculations had a small, nearly negligible, effect on the molecular structure of SnPc, whereas the DFT/ cosmo solvation model yielded the predictable result of increasing the Sn–N bond length by pulling the Sn atom further out of the molecular plane.

Structural relaxations in amorphous water

In order to study the structural relaxations of vapor condensed amorphous water (ASW) by means of spectral hole-burning water matrices were doped with free-base phthalocyanine as a molecular probe. Structural relaxations are found which change the energy landscape of the amorphous solid upon annealing to temperatures far below the glass transition.

Non-logarithmic spectral diffusion dynamics evidence for interactions between tunneling centers

We investigated the time evolution of spectral holes burnt in a sample of polymethylmethacrylate doped with free-base phthalocyanine at temperatures of 0.5 and 1 K between 10 and 10 6 s. For the tunneling dynamics in thermal equilibrium conditions two different time regimes can be distinguished: At times exceeding hours the evolution is strongly non-logarithmic (algebraic), which we interpret in terms of TLS-TLS interactions; for shorter times, however, the deviation from a logarithmic hole broadening is less pronounced and can be explained by strong TLS-phonon interaction.

Excited States of Free Base Phthalocyanine Studied by the SAC-CI Method

The SAC (symmetry adapted cluster)/SAC-CI method was used to calculate the ground and excited states of free base phthalocyanine (FBPc). This is the first accurate ab initio study of the excited states of FBPc. The calculated electronic spectrum agrees reasonably well with experimental results with regard to both energy and intensity. The relationships among the molecular structure, excitation energy, and spectral intensity are discussed, and the present results are compared with those for free base porphine (FBP) and free base tetrazaporphine (FBTAP) studied previously. Two important effects of skeletal changes are clarified; meso-tetraaza substitution and tetrabenzo substitution cause a large splitting between the HOMO and next HOMO levels and lead to a breakdown of the quasi-degeneracy of the two main configurations of the Q band, resulting in strong visible absorption due to the incomplete cancellation of the individual contributions to the transition dipole moment. This explains why Pc's are so useful as pigments. Further, a new assignment of the B (Soret) band is proposed. The broad experimental B band is composed of at least four states. The main peak of the B band is due to transitions from the orbital lower than the so-called “four orbitals”, and the transitions arising from the “four orbitals” constitute the shoulder in the lower energy side of the band.