Purificación Vázquez

Phthalocyanines and related compounds:organic targets for nonlinear optical applications

Phthalocyanines (Pcs) and related compounds with their extended two-dimensional pi;-electronic delocalization are important targets to study nonlinear optical responses. The tailorability of these macrocycles allows the fine-tuning of the chemical structure and nonlinear optical response. In this article, the design and main properties of phthalocyanines for second- and third-order nonlinear optical (NLO) and optical limiting applications are discussed, both at the microscopic and macroscopic level. The points of view of synthetic organic chemists and physicists are accorded, the main aim of the review being to highlight the key problems of the field and place them within the general context of NLO materials.

Long-lived photoinduced charge separation for solar cell applications in phthalocyanine-fulleropyrrolidine dyad thin films{

The photophysical properties of a new dyad molecule composed of a covalently linked Zn-phthalocyanine (antenna/donor) and a C60 derivative (acceptor) have been investigated. We report experimental evidence of long-lived charge separation in the solid state with a lifetime several orders of magnitude higher than in solution. Such a long lifetime, unusual for phthalocyanine–fullerene dyads, is the basis for possible photovoltaic applications. A first demonstration of a working solar cell using phthalocyanine–fullerene dyads as the active material is presented. Though the power conversion efficiency under simulated solar illumination of 80 mW cm−2 is found to be moderate (0.02%), it is an encouraging result for application of C60 dyad molecules to photovoltaics.

Structure-function relationships in unsymmetrical zinc phthalocyanines for dye-sensitized solar cells.

A series of unsymmetrical zinc phthalocyanines bearing an anchoring carboxylic function linked to the phthalocyanine ring through different spacers were designed for dye-sensitised solar cells (DSSC). The modification of the spacer group allows not only a variable distance between the dye and the nanocrystalline TiO(2), but also a distinct orientation of the phthalocyanine on the semiconductor surface. The photovoltaic data show that the nature of the spacer group plays a significant role in the electron injection from the photo-excited dye into the nanocrystalline TiO(2) semiconductor, the recombination rates and the efficiency of the cells. The incident monochromatic photon-to-current conversion efficiency (IPCE) for phthalocyanines bearing an insulating spacer is as low as 9%, whereas for those with a conducting spacer an outstanding IPCE 80% was obtained.

Towards Tunable Graphene/Phthalocyanine–PPV Hybrid Systems†

The sheer explosion of interest in graphene has undoubtedly shown that it is the rising star in the emerging field of nanotechnology. Its extraordinary properties render it an outstanding material for electronics, material sciences, and photoconversion systems. As a zero-gap semiconductor for example, a flat monolayer of graphene is almost transparent and exhibits the lowest known electrical resistivity for any material at room temperature. The remarkably high electron mobility of graphene gives rise to its implementation in transparent conducting electrodes as a viable alternative to indium tin oxide (ITO). 4] Recent results demonstrate, however, that doping is a necessity to harvest the full potential of graphene. 6] Therefore, the aim herein is the tuning/altering of the features of photochemically transparent graphene by integrating a versatile electron donor system in solution. High-quality graphene flakes have been formed by means of solution processing, which involves exfoliating and dispersing them directly from graphite. 8] Such mild strategies stand in strong contrast to high throughput exfoliation of graphite with the assistance of strong oxidants. Moreover, reduction of graphene oxide to graphene is by no means quantitative and results in irreversible coagulation and permanent lattice defects. 13] To date, samples that exhibit high charge mobilities are large-area graphene samples obtained by micromechanical cleavage of pyrrolitic graphite. Other notable breakthroughs in this area rely on sheets grown onto solid substrates. The investigation of novel electron donor–acceptor hybrids involving low-dimensional allotropes of carbon is far more challenging than the exploitation of carbon nanotubes in the same context. The reason is primarily the lack of photospectroscopic signatures/markers, which makes the study of graphene more challenging. In fact, we have selected a spectator molecule to circumvent this impediment and to assist in identifying and visualizing electron donor–acceptor interactions. The unique absorption with high extinction coefficients in the red and near infrared regions, fluorescence, and the strong electron-donating character of zinc phthalocyanines (ZnPc) make a ZnPc-based PPV oligomer (1) PPV = poly(p-phenylene vinylene) the molecule of choice (Supporting Information, Scheme S1). Apart from supporting several ZnPc units, the oligomeric backbone is expected to be a great asset with regard to graphite exfoliation and stabilization of novel nanographene (NG) hybrids bearing ZnPc oligomer 1 that are thus formed (Figure 1).

A Panchromatic Supramolecular Fullerene‐Based Donor–Acceptor Assembly Derived from a Peripherally Substituted Bodipy–Zinc Phthalocyanine Dyad

A panchromatic 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene-zinc phthalocyanine conjugate (Bodipy-ZnPc) 1 was synthesized starting from phthalocyanine aldehyde 4, via dipyrromethane 3 and dipyrromethene 2. Conjugate 1 represents the first example in which a Bodipy unit is tethered to the peripheral position of a phthalocyanine core. Electrochemical and optical measurements provided evidence for strong electronic interactions between the Bodipy and ZnPc constituents in the ground state of 1. When conjugate 1 is subjected to photoexcitation in the spectral region corresponding to the Bodipy absorption, the strong fluorescence characteristic of the latter subunit is effectively quenched (i.e., > or = 97%). Excitation spectral analysis confirmed that the photoexcited Bodipy and the tethered ZnPc subunits interact and that intraconjugate singlet energy transfer occurs with an efficiency of ca. 25%. Treatment of conjugate 1 with N-pyridylfulleropyrrolidine (8), an electron-acceptor system containing a nitrogen ligand, gives rise to the novel electron donor-acceptor hybrid 1<-->8 through ligation to the ZnPc center. Irradiation of the resulting supramolecular ensemble within the visible range leads to a charge-separated Bodipy-ZnPc(*+)-C(60)(*-) radical-ion-pair state, through a sequence of excited-state and charge transfers, characterized by a remarkably long lifetime of 39.9 ns in toluene.

SYNTHESIS OF ALKYNYL-LINKED PHTHALOCYANINE DYADS: PUSH - PULL HOMO- AND HETERODIMETALLIC BISPHTHALOCYANINATO COMPLEXES

Metallophthalocyanine dimers linked by butadiynyl and ethynyl bridges 3a - c and 5 a -c , respectively, have been synthesized by metal-mediated coupling methodologies. The key to the synthesis of these chromophores was the ready availability of appropriately functionalized unsymmetrical phthalocyanines 2a - c that bear a terminal ethynyl group. Following the same methodology, push - pull homo- and heterodimetallic ethynyl-bridged bisphthalocyaninato complexes 10 a - c, that contain electron-donor and electron-acceptor substituents in each of the two phthalocyanine subunits, have been also prepared.

Increasing the efficiency of zinc-phthalocyanine based solar cells through modification of the anchoring ligand

Several zinc-based phthalocyanines have been synthesized and used in Dye-Sensitized Solar Cells (DSSC). The results have been compared with the standard TT1 phthalocyanine, which shows good light-to-energy conversion efficiencies in comparison with other IR sensitizers used in DSSC. We show herein that the anchoring moiety is critical for both achieving high injection yields and slow back electron transfer dynamics that affect the overall device efficiency. Moreover, based on these results, we have synthesized a new phthalocyanine with a superior performance, when compared to the TT1 dye, with a subtle change on the anchoring moiety, thus leading to a higher photocurrent response.

Alkynyl substituted phthalocyanine derivatives as targets for optical limiting

One of the key issues in the development of efficient optical limiters is the search for appropriate materials with improved nonlinear absorption. Phthalocyanine materials are among the most promising nonlinear absorbers described in the literature. Ethynyl and butadiynyl-bridged bisphthalocyanines and related ethynyl-containing mononuclear compounds have been synthesized and experimentally studied using the z-scan technique with nanosecond pulses at 532 nm. We intend to ascertain the effect of the electronic interaction between macrocycles in the optical limiting performance of phthalocyanines.

Effect of anchoring groups in zinc phthalocyanine on the dye-sensitized solar cell performance and stability

We have designed and developed an unsymmetrical zinc phthalocyanine (TT9) sensitizer that consists of three tert-butyl and two carboxylic acid groups that act as “push” and “pull”, respectively. The two carboxylic acid groups graft the sensitizer onto the semiconductor surface resulting in enhanced stability under heat and light compared to the similar unsymmetrical zinc phthalocyanine (TT1) sensitizer that consists of three tert-butyl and only one carboxylic acid groups. The solar cells containing the TT9 and TT1 sensitizers with non-volatile electrolyte were subjected to light soaking conditions at 60 °C. Under these conditions, the short circuit current of the TT1 sensitized solar cell after 1000 h decreases to half of its initial value where as the TT9 sensitized solar cell remained the same demonstrating the influence of number of anchoring groups on the stability of zinc phthalocyanine sensitized solar cells.

Synthesis and Photoinduced Electron‐Transfer Properties of Phthalocyanine–[60]Fullerene Conjugates

A series of three novel ZnPc-C60 conjugates (Pc=phthalocyanine) 1 a-c bearing different spacers (single, double, and triple bond) between the two electroactive moieties was synthesized and compared to that of ZnPc-C60 conjugate 2, in which the two electroactive moieties are linked directly. The synthetic strategy- towards the preparation of 1 a-c- involved palladium-catalyzed cross-coupling reactions over a monoiodophthalocyanine precursor 4 to introduce the corresponding spacer, and subsequent dipolar cycloaddition reaction to C60. Detailed photophysical investigations of 1 a-c and 2 prompted an intramolecular electron transfer that evolves from the photoexcited ZnPc to the electron-accepting C60. In particular, with the help of femtosecond laser photolysis charge separation was indeed confirmed as the major deactivation channel. Complementary time-dependent density functional calculations supported the spectral assignment, namely, the spectral identity of the ZnPc(*+) radical cation and the C60 (*-) radical anion as seen in the differential absorption spectra. The lifetimes of the correspondingly formed radical ion-pair states depend markedly on the solvent polarity: they increase as polarity decreases. Similarly, although to a lesser extent, the nature of the linker impacts the lifetime of the radical ion-pair states. In general, the lifetimes of these states tend to be shortest in the system that lacks any spacer at all (2), whereas the longest lifetimes were found in the system that carries the triple-bond spacer (1 a).