Gloria Zanotti

Bridged Phthalocyanine Systems for Sensitization of Nanocrystalline TiO 2 Films

Phthalocyanines based-dyes represent attractive alternatives to the expensive and polluting pyridyl based Ru complexes because of their photochemical and thermal stability, they do show in fact intense absorption in the UV/blue (Soret band) and the red/near IR (Q band) spectral regions and appear very promising as sensitizer dyes for DSSC. In this contribution we review the state of the art and the recent progress in the application of these materials as dyes for DSSC and present three new dyes which are bridged derivatives of Iron phthalocyanine. Synthesis, optical properties, electrochemical characterization and device performances are discussed with regard to the different substitution degree of the macrocycle.

Metal–organic green dye: chemical and physical insight into a modified Zn-benzoporphyrin for dye-sensitized solar cells

A new green unsymmetrical zinc triphenyl-tetrabenzoporphyrin compound, i.e. 5,10,15-(triphenyl),20-[ethynyl-(4-carboxy)phenyl]tetrabenzoporphyrinate Zn(II), has been synthesized as the first of a new class of dyes suitable for dye-sensitized solar cells. The molecule shows a hybrid porphyrin–phthalocyanine structure that allows a finer chemical and physical properties tuning with respect to phthalocyanines, by means of substitutions at the meso-positions, and a UV-vis spectrum that shows both an intense Soret band at 456 nm and a detectable Q band at 655 nm. The photophysical and redox properties of the molecule have been studied and show that HOMO and LUMO energy values are properly positioned for an effective charge transfer. DFT-based ab initio calculations have confirmed the energetic position of frontier orbitals and highlight the intricate structure of the visible spectrum and the charge transfer behavior of the molecule seen as a part of a complex device. Finally, dye-sensitized solar cells have been realized and IPCE and photovoltaic parameters have been measured, showing preliminary efficiency values of about 2%.

Iodine doping of substituted μ-carbido iron diphthalocyanines

Hemi and amphi-substituted μ-carbido iron diphthalocyanine having general formula (t-bu)4PcFe-C-FePc and (t-bu)4PcFe-C-FePc(t-bu)4 have been oxidized with I2. The oxidized compounds have been characterized and their physicochemical properties have been studied; the results obtained lead to assign them the formula [(t-Bu)4PcFe-C-FePc] (I5)0.66 and [[(t-Bu)4PcFe]2C]] (I5)0.66 respectively. The conduction properties, related to the role played by the pushing groups inserted in the peripheral macrocycles, are discussed. Moreover, an effective purification method for improving the yield and purity of iron tetra-tert-butyl phthalocyanine is described, the difficulty being able to obtain this precursor with a high purity grade.

Spectroscopic and Morphological Studies of Metal-Organic and Metal-Free Dyes onto Titania Films for Dye-Sensitized Solar Cells

We have investigated the spectroscopic behavior of three different sensitizers adsorbed onto titania thin films in order to gain information both on the electron transfer process from dye to titania and on the anchorage of the chromophore onto the semiconductor. We have examined by UV-Vis and fluorescence spectroscopy the widely used ruthenium complex cis-di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N719), the more recently developed organic molecular 3-(5-(4-(diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D5), and a push-pull zinc phthalocyanine sensitizer (ZnPc). Three type of titania films with different morphology, characterized by SEM and FT-IR measurement, were considered: a mesoporous transparent film deposited by spin-coating (TiMS), a semiopaque film deposited by doctor-blade from mesoporous titania (TiMS_DB) and a semiopaque film deposited by doctor-blade form commercial P25 titania (P25_DB). The use of TiMS is responsible for the adsorption of a higher amount of dye since the mesoporous structure allows increasing the interfacial area between dye and titania. Moreover, the fluorescence emission peak is weaker when the sensitizers are adsorbed onto TiMS. These findings suggest that mesostructured films could be considered the most promising substrates to realize photoanodes with a fast electron transfer process.

Unexpected Rotamerism at the Origin of a Chessboard Supramolecular Assembly of Ruthenium Phthalocyanine

We have investigated the formation and the properties of ultrathin films of ruthenium phthalocyanine (RuPc)2 vacuum deposited on graphite by scanning tunneling microscopy and synchrotron photoemission spectroscopy measurements, interpreted in close conjunction with ab initio simulations. Thanks to its unique dimeric structure connected by a direct Ru-Ru bond, (RuPc)2 can be found in two stable rotameric forms separated by a low-energy barrier. Such isomerism leads to a peculiar organization of the molecules in flat, horizontal layers on the graphite surface, characterized by a chessboard-like alternation of the two rotamers. Moreover, the molecules are vertically connected to form π-stacked columnar pillars of akin rotamers, compatible with the high conductivity measured in (RuPc)2 powders. Such features yield an unprecedented supramolecular assembly of phthalocyanine films, which could open interesting perspectives toward the realization of new architectures of organic electronic devices.

Modifications of an unsymmetrical phthalocyanine: Towards stable blue dyes for dye-sensitized solar cells

A new copper phthalocyanine, namely 9(10),16(17),23(24)-tri-tert-butyl-2-[acetynyl-(4-carboxy)phenyl]phthalocyaninatocopper and its related free base have been synthesized as potential stable blue dyes for dye sensitized solar cells. The molecule structure consists on an unsymmetrically-substituted macrocycle bearing three tert-butyl groups and one phenylethynyl moiety as peripheral substituents and it is analogue to that of a previously published zinc derivative. Chemical and optical characterizations, as well as theoretical calculations of the frontier orbitals of both molecules are discussed. To evaluate the effect of the central metal on the photovoltaic behavior of this dye, the novel molecules have been both tested and confronted with the zinc derivative. This last one has shown efficiency values significantly higher than those previously published with a 2.10% maximum efficiency, while the other two dyes yielded 1.66% and 1.38% maximum efficiency respectively.

Reply to the ‘Comment on “Metal–organic green dye: chemical and physical insight into a modified Zn-benzoporphyrin for dye-sensitized solar cells”’ by R. Steer, RSC Advances, 2018, DOI: 10.1039/c8ra00213d

The authors reply to the comment by R. P. Steer discussing the reasons for their incorrect assignment of the luminescence decay of the novel compound 5,10,15-(triphenyl),20-[ethynyl-(4-carboxy)phenyl]tetrabenzoporphyrinate Zn(II) (PETBP). Further DFT and TDDFT calculations have been performed on the compound to investigate the possibility of a direct S2–S0 decay instead of a S2–S1 conversion with a subsequent emission to the ground state. In addition, the presence of traces of very luminescent contaminants of the ring-opened type has been considered on the grounds of calculated absorption and fluorescence spectra. The results of these investigations confirm that the S2–S0 emission reported in the commented paper is not attributable to the target molecule but rather to a neglected luminescent impurity.

Understanding the influence of disorder on the exciton dynamics and energy transfer in Zn-phthalocyanine H-aggregates

The photophysics of 9(19),16(17),23(24)-tri-tert-butyl-2-[ethynyl-(4-carboxymethyl)phenyl]phthalocyaninatozinc(ii) and its H-aggregates is studied in different solvents by means of ultrafast non-linear optical spectroscopy and computational modeling. In non-coordinating solvents, both stationary and time-resolved spectroscopies highlight the formation of extended molecular aggregates, whose dimension and spectral properties depends on the concentration. In all the explored experimental conditions, time-resolved transient absorption experiments show multi exponential decay of the signals. Additional insights into the excited state relaxation mechanisms of the system is obtained with 2D electronic spectroscopy, which is employed to compare the deactivation channels in the absence or presence of aggregates. In ethanol and diethylether, where only monomers are present, an ultrafast relaxation process among the two non-degenerate Q-states of the molecule is evidenced by the appearance of a cross peak in the 2D-maps. In chloroform or CCl4, where disordered H-aggregates are formed, an energy transfer channel among aggregates with different composition and size is observed, leading to the non-radiative decay towards the lower energy dark state of the aggregates. Efficient coupling between less and more aggregated species is highlighted in two-dimensional electronic spectra by the appearance of a cross peak. The kinetics and intensity of the latter depend on the concentration of the solution. Finally, the linear spectroscopic properties of the aggregate are reproduced using a simplified structural model of an extended aggregate, based on Frenkel Hamiltonian Calculations and on an estimate of the electronic couplings between each dimer composing the aggregate computed at DFT level.