Photonics and Reactivity

Photodynamic therapy of cancer

Starting in 1996 new photosensitizers were developed for the photodynamic therapy of cancer. This work lead to two patents of the University of Coimbra, which have been licensed to a start-up pharmaceutical company: Luzitin SA. The company is now leading the project and clinical trials are scheduled to start in 2013. The scientific grounds for the development of such photosensitizers, their properties and their achievements are regularly published in the scientific literature.

Transdermal drug delivery with photoacoustic waves

Building on the expertise of the laboratory since 1990, applications of photoacoustic waves to various processes lead to the finding of their effects on biological membranes. Subject to photoacoustic waves generated by carefully designed piezophotonic materials, the permeability of the skin or of cell membranes was found to increase temporarily without compromising skin recovery of cell viability. The technology was patented by the University of Coimbra and licensed to a start-up technological company: LaserLeap Technologies. The company is pursuing applications of the technology to transdermal drug delivery, instrumental cosmetics and gene therapy.

Optical biopsy dyes

The new frontiers of cancer detection and treatment demand the ability to visualize microstructures that target the diseased tissue. By imaging where, when, and in what extent a marker chromophore appears, it becomes possible to refine basic preclinical research and improve cancer therapy. Optical imaging techniques have the advantage of high sensitivity, high time resolution, strong contrast and good spatial resolution. The optimization of cancer detection using optical imaging requires new contrast agents with strong absorption in the therapeutic window (700 to 850 nm) where tissues are more transparent. This project focuses on the development of contrast agents for fluorescence imaging and for photoacoustic tomography.

Solar energy conversion

Approximately 4.3 × 1020 J of photon energy hit the surface of our planet each hour, which is enough to cover our yearly energy needs. Photochemical conversion of solar photons is one of the most promising and sought after solutions to the current global energy problem. Dye-sensitized solar cells (DSSC) and organic bulk heterojunction solar cells (BHJ) are two examples of systems that allow the conversion of visible sunlight into electricity by inorganic or organic semiconductor materials, which are inexpensive and easy to process on a large scale. The lab is actively involved in the development and characterization of solar cells using strongly near-IR absorbing dyes, new techniques to measure their efficiency, and new forms of photo-piezo-pyroelectric conversion.

Intersecting/Interacting-State Model

The driving force of the work in the lab is the ability to understand and predict how molecular structure influences chemical reactivity and the properties of molecules in general. Rather than relying on time-consuming calculations, the lab specialized in semi-empirical models capable of providing reliable estimates of rate constants for fundamental chemical reactions in a wide variety of media. The Intersecting/Interacting-State Model has been extensively applied to atom transfer, proton transfer, electron and energy transfer, proton-coupled electron transfer and SN2 reactions. It provides the fundamental guidance to the more applied projects carried out in the lab.

Peptide and protein folding

One of the major challenges in the field of Biophysical Chemistry is the study of protein folding mechanisms, i.e., how an unstructured polypeptide chain can rapidly adopt a unique, densely packed, three dimensional structure. Erroneous folding is the molecular basis for a wide range of human disorders, including Alzheimer's and Parkinson's disease. Our aim is to make the early stages of folding experimentally accessible. We use ultrafast pH-jump technique to induce peptide and protein unfolding. The conformational changes during unfolding are monitored by time-resolved photoacoustics calorimetry, enabling the determination of kinetic constants, enthalpy and volume changes accompanying the unfolding process. We focus our study on model peptides with well defined secondary structure and small proteins.

Molecular Cryospectroscopy and Biospectroscopy

Conformational flexibility in organic molecules

The general subject of conformational mobility is of great interest to many important scientific domains, such as molecular recognition and nanotechnology. We considerably improved the knowledge on the phenomenon ofconformational cooling and discovered the inverse conformational cooling effect. In the case of matrix isolation experiments, these effects result from relaxation of higher energy conformers to other forms of lower energy during deposition of the matrix or upon annealing of the already deposited matrix. We demonstrated that the conformational cooling capabilities of noble gases increase from argon to xenon. Extensive conformational cooling in highly conformationally flexible molecules led to observation and allow for characterization of a single conformer in cryogenic matrices.

Intermolecular interactions in biologically relevant molecules

The subject of intermolecular interactions has been explored intensively. Annealing of the matrices allows for controlling molecular diffusion and production of small aggregates. We have shown that aggregation can be conformation selective even in small molecules like simple amino acids. The study of intermolecular interactions for compounds in their neat solid state, including hydrogen bonding, has been addressed. We were the first to observe the non-ionic forms of simple aminoacids in their neat solid phase. We also developed a very general empirical relationship between spectroscopic parameters and H-bond energies and bond lengths that can be used to shed light on the H-bond properties in sugars, amino acids and small peptides, nucleic acid bases, among other biologically relevant systems.

Conformation modulated photochemistry

An important active field of research is the study of the processes of energy relaxation in molecules through conformer ground state excitation with narrowband selective pumping in the near-infrared. Once one can selectively produce the desired reactant conformer and understand the mechanisms of vibrational energy redistribution and relaxation that might compete with the photochemically-induced processes, selective UV-excitation can be performed on that conformer and its photochemistry probed spectroscopically. These studies provide solid fundamental bases for understanding hot vibrational chemistry and for the development of new methods with important practical applications in the fields of phototherapy, synthetic photochemistry and molecular biology.

Environmental and astrophysical related chemistry

For several years we have been investigating the structure, spectroscopy and photochemical reactivity of molecular species of environmental and astrophysical interest, in laboratorial experimental conditions that mimic their real conditions in the interstellar space or frozen planetary bodies. These studies also intend to contribute to the understanding of the atmospheric chemistry, in particular the phenomenon of ozone depletion by CFCs, HCFCs and related compounds.

Biodegradable polymers

Another important field of research consists in the study of the structure and spectroscopic properties of biodegradable polymers and their precursors. The approach that has been followed in these studies considers the initial structural characterization of polymer precursors by matrix isolation spectroscopy and theoretical methods, followed by the investigation of the main intermolecular interactions in solid phase using also chemical imaging methods.

Heterocyclic chemistry

We are very much involved in the spectroscopic and photochemical study of different families of heterocycles, like tetrazoles, oxazoles, isoxazoles, phenols, saccharins, azirines and diazirines, pyrones, thiopyrones and coumarins.

Spectroscopy of ionic liquids

A new research line has been established, in the field of spectroscopy of ionic liquids. It was recently demonstrated that many ionic liquids can be distilled at low pressure without decomposition, opening good perspectives for the detailed study of the ionic liquid constituents by matrix isolation spectroscopy. On the other hand, ionic liquids may also present interesting properties for acting as matrices themselves for the vibrational study of highly ionic species, like, for example, amino acid zwitterions, either at low or high temperatures, by IR or Raman spectroscopy. These different novel approaches are currently being developed.

Spectroscopy of biological systems

The general subject of spectroscopy in biologically relevant systems is of particular interest. Develop more efficient vitrifying procedures involving new cryoprotectants which present lower toxicity than those actually available is one of the objectives. A related project deals with food technology. The project grounds on the concept of using effluents from milk industries (milk serum) to obtain novel probiotics. The final objective is to obtain a suitable product that can be easily delivered and used for animal feeding. Our contribution is centered in the physicochemical characterization of the starting materials, of the intermediates and of the final products.

Photochemical behavior of new azides: Gateway to nitrenes

Azides release easily molecular nitrogen and give rise to highly reactive nitrenes. This project studies the complex physics and chemistry underlying the reactivity of azides and nitrenes, and identifies their reactive intermediates and products. In the photochemical studies, the azides are trapped in cryogenic matrices. In situ irradiation is performed using a tunable laser, from near IR (1800 nm) till UV (220 nm). In the thermal reactivity studies, the azides are subjected to pulsed vacuum pyrolysis, in a specially designed pyrolyzer that achieves temperatures up to 1000 K. The intermediates and products of either photochemical or thermal reactions are trapped in cryogenic matrices (at 10-15 K) and characterized spectroscopically.

Near-infrared induced generation of less stable conformers

The matrix isolation method is employed to study conformational structures of small and medium-sized molecules. The compounds are initially trapped from the gas phase into low-temperature cryogenic inert gas environments (e.g., argon, xenon, nitrogen). The new higher-energy conformers not populated in the gas phase are then generated directly in matrices, by selective excitation of their vibrational overtones with near infrared narrow-band light. Upon near-infrared generation, the conformational structures of matrix-isolated molecules, including new forms, are characterized by spectroscopic techniques.


Conjugated organic oligomers and polymers

The complete electronic spectral and photophysical characterization of several conjugated organic oligomers (e.g. benzothidiazole, pyrrol, thiophene, etc., derivatives), polymers (including PT, PF and PPV) and hyperbranched polymer is a benchmark of our laboratory. This includes the determination of all the relevant quantum yields, lifetimes and rate constants associated to the deactivation of the excited states. This is made as a function of solvent, temperature and thin films.

Molecules of Art

Molecules associated to the colors produced by the ancients such as indigo, mauveine, alizarine, Maya Blue, etc., and relevant aspects associated to its photochemistry and photophysics are also part of ongoing projects in this lab. This includes the study of the stability mechanisms associated to these molecules, the synthesis of new Mayan-based hybrids and the chemistry behind mauveine history.

Fluorescent Chemosensors

Synthesis and photophysical characterization of chemical sensors able to differentiate metal anions and cations involving fluorescence quenching/enhancement or formation of excimers and exciplexes are being investigated in solution with sensors based on pyrene, naphthalene and coumarins.

Polymers and oligomers containing pyrene

The dynamics of water soluble poly(acrylic acid) polymers labeled with pyrene and several oligomers with different chain sizes linking two pyrene units are being investigated in solution media aiming to understand, amongst others, the effect of the position and degree of substitution on the photophysical characteristics of these polymers and oligomers. This includes the solving of complex kinetic schemes with 2, 3 and 4 excited species; e.g. two monomers and two excimers.

Vibronic Effect in Photochemistry

We are currently investigating the competition between vibrational relaxation and fluorescence in photchromic molecules such as benzochromene derivatives. This is known as the “vibronic effect” and establishes that there is (at low temperatures) competition between vibrational relaxation and fluorescence. Determination of fluorescence and photochemical quantum yields as a function of the excitation wavelength within a vibronic series in an electronic transition.

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