PDT

Medical Applications of Photochemistry

Group participants
Introduction
Phototherapy and Photochemotherapy
Photodynamic Therapy
Quantum Mechanics
Our research
Publications
References

Group Participants

Manuela Merchán
Luis Serrano-Andrés
Juan José Serrano-Pérez
Introduction:

Where do we come from? What is our future? What is the matter of our dreams? Such questions may well be asked by the first hominid in whose eyes glittered Human´s brilliant. Nowadays, the role of science in everyday is extremely important. We can assure that Physics and Chemistry are the basis of our knowledge. To tell the truth, Quantum Mechanics and Relativity allow us to explain every natural phenomena. From the XVI century physicians have noticed that Physics and Chemistry are very important in Medicine. Actually, all natural phenomena could be explained by physical and chemical theories, embracing the processes which take place in the organism like breathing or blood circulation, and modern Medicine resorts to this knowledge in order to understand how changes are made in the body and how they would be provoked or avoided. It was Paracelsus who introduced chemical therapy by means of his book Labyrinthus medicorum errantium (1553). Since then, the significance of the role of Physics and Chemistry in life sciences has been recognized by several scientists who pointed out that modern Medicine should be relied on experimental sciences, and ancient medical theories based on teachings learnt by heart should have been avoided. Of course, old-fashioned pharmacists and physicians showed their unwillingness to these progresses towards such new conception of their chores and knowledge they had not been trained for, but this classical way to heal gave way to modern Medicine in the end. Actually, modern Medicine has little to do with the classical one practised by Hippocrates or Galen. Nowadays, it is stressed that a proper alteration of DNA would entail beneficial effects. This is the origin of the medical area which represents better the connection between Physics, Chemistry and Medicine: Phototherapy. It consists of the employment of electromagnetic radiation coupled with a drug (the photosensitizer). The absorption of electromagnetic radiation by this chromophore triggers a chain of photochemical reactions which are the key of Phototherapy. The benefits of sunlight are known from antiquity. Indeed, Herodotus and Hippocrates pleaded for the use of sunlight to treat several diseases.Niels Ryberg Finsen is known as the father of Phototherapy since he was the first who studied it in the light of scientific method, and he was recognized with the Nobel Prize on Medicine and Physiology in 1903. From this moment a plethora of diseases has been successfully treated by means of Phototherapy, such as psoriasis, vitiligo, jaundice, rickets or some sorts of cancer. Not only does therapeutic photomedicine pursue the suppression of ongoing deterioration processes, but also tries to prevent, modulate or abrogate the pathogenic mechanisms causing the problem. PUVA therapy, in which furocoumarins are used as photosensitizers, is one of the most promising strategies against a plethora of diseases. Understanding the photochemical mechanism of that sort of photosensitization is crucial in order to put forward better drugs. In that regard, it is important to study previously the photophysics of the compounds and the most efficient way the first excited triplet state (the protagonist of the photosensitizing action) is populated in.
Therefore certain types of electromagnetic radiation can be used to treat some skin disorders and other diseases. The radiation is usually used in combination with a photosensitizer. A photosensitizer is a drug that is harmless in the dark, but it absorbs electromagnetic radiation in order to produce some biological effects.
Phototherapy is the use of ultraviolet, visible or near infrared light in the treatment of disease. In cases where no photosensitiser is administered the light is absorbed by chromophores present in the tissue, e.g., phototherapy of neonatal hyperbilirubinemia; photogeneration of vitamin D ₃.
Photochemotherapy is the use of ultraviolet, visible or near infrared light together with an administered photosensitizer (the photochemotherapeutic agent) in the treatment of disease. Light is absorbed by the photosensitizer, e.g. PUVA therapy of psoriasis and other skin diseases.
Photodynamic Therapy (PDT) is the treatment of disease by the use of visible or near infrared light together with an administered photosensitizer and in the presence of molecular oxygen at ambient levels. Light is absorbed by the photosensitizer, which then serves to activate oxygen in some way. The activated oxygen (principally singlet oxygen) then causes damage to the living systems, e.g. PDT of cancer; photodynamic destruction of microbes.
Phototherapy and Photochemotherapy:

In recent years, naturally occuring plant furocoumarins (psoralens) have been actively investigated with respect to their photosensitizing ability to induce skin erythema. Psoralens have also been found to posess carcinogenic and mutagenic properties when applied in conjunction with near ultraviolet light exposure. Psoralen plus near UV light treatment (PUVA therapy) has also been shown to induce various other adverse biological changes. These changes include pigmentation on human and guinea pig skin following erythema, killing and mutagenesis of bacteria, inactivation of DNA viruses, inhibition of tumor transmitting capacity of varios tumor cells, and disorders in the development of fertilized sea urchin eggs.
Psoralen-containing plants have been used for hundred of years in the treatment of skin disorders such as vitiligo and psoralen. Thus in India, extracts of Psoralea corylifolia were given orally, followed by exposure to sunlight, in order to treat vitiligo. The active component of these plants, psoralens, was not isolated until 1834, when Kalbruner isolated 5-methoxypsoralen from bergamot oil. Nearly 100 years later, it was showns by Phyladelphy that sunlight played a vital role in the interaction between psoralen and biological molecules. By the 1930s it had been shown that various plants of the Rutaceae and Umbelliferae families contained linear furocoumarins and angular furocoumarins, and the structures and syntheses of these compounds were investigated. The compounds had interesting physiological properties: they included compounds which were highly toxic to fish, but not to mammals; and they caused phototoxicity if ingested. As early as 1953, Lerner, Denton and Fitzpatrick had reported promising studies on the treatment of vitiligo with 8-methoxy psoralen. In 1972, the crystal and molecular structure of 8-methoxypsoralen (8-MOP) was elucidated by Stemple and Watson. In 1974, Parrish and his colleagues revealed that oral administration of 8-MOP, followed by exposure to ultraviolet light in the 320-400 nm range, could be used to treat psoriasis, and this observation started another field of phototherapy: the PUVA therapy. Nowadays 8-MOP and 5-MOP are used as an effective drug in the photochemotherapy of psoriasis, and khellin are used against vitiligo.

More recently, researchers have found useful applications for psoralens as molecular probes in studies dealing with chromatin structure, secondary structure in viral DNA and Drosophila ribosomal RNA, satellite DNA structure, bacterial and viral DNA repair mechanism, viral DNA-RNA hybrid structure and virus RNA duplexes.
The molecular basis for photobiological (skin photosensitization) and photochemotherapeutic effects of psoralens involve the stable covalent photoconjugation of furocoumarins with DNA of the target cells giving rise to both the monofunctional (single strand) adducts and interstrand cross-link adducts between the pyrimidine bases belonging to opposite strands of DNA. The photochemotherapeutic effectiveness of psoralen plus UVA is believe to be due: (a) the inhibition of DNA synthesis and subsequent decrease in macromolecular synthesis, (b) the killing of abnormal proliferative cells and the inhibition of recruitment of new cells in the G₀ and G₂ phases, (c)the inhibition of blood vessel cells (angiogenic effect), and (d) the inhibition of leukocytes and subsequent immunosuppression effect on the immune system.
The major chemical reaction involving the photoconjugation of psoralen and pyrimidine bases can be classified as a Type-I reaction that requires the transfer of hydrogen atoms or electrons but no direct involvement of molecular O₂.

The photoreactive process is thought to involve three steps. The first step takes place in the dark: the psoralen insert themselves between adjacent thymine base pairs in the DNA duplex. In the second step, the psoralen compound absorbs a photon and forms a monoadduct. The psoralen compounds can form two types of monoadducts, the furan monoadduct and the pyrone monoadduct, by reacting with the C5-C6 double bond of the thymine via the C4´-C5´ double bond in the furan ring or the C3-C4 double bond in the pyrone ring. In the third step, the monoadduct also absorbs one photon, inducing the other photoreactive double bond of the monoadduct to react with a thymine on the opposite strand of DNA. Therefore, in the third step a diadduct that crosslinks the DNA is formed. The formation of the diadduct severely damages the DNA, requiring a long time for its restoration. The diadduct causes adverse effects and it is carcinogen and mugaten. Several studies have suggested that, whereas the furan monoadduct forms a diadduct, the pyrone monoadduct does not.

Animation of mechanism

Interactive DNA

Psoralens also undergo Type-II reactions involving the formation of ¹O₂ and O₂^·- (or HO₂^·-) through a sensitized mechanism (photodynamic action) in which the photoexcited psoralen (in its triplet state) reacts with molecular oxygen to form reactive ¹O₂ or O₂^·-. These reactive forms of oxygen cause cell damage that eventually contributes to skin photosensitization, mutation, error-prone repair and skin carcinogenesis.

The reason that singlet oxygen is generated in the cells is because of simple photophysics. Provided that the psoralen possesses an absorption maximum at a wavelength corresponding with that of the incident laser light, shining light on a highly colored psoralen causes excitation to the singlet excited state. The singlet excited posoralen can decay back to the ground state with release of energy in the form of fluorescence - enabling identification of tumor tissue or skin disease. If the singlet state lifetime is suitable (and this is true for many psoralens) it is possible for the singlet to be converted into the triplet excited state which is able to transfer energy to another triplet. One of the very few molecules with a triplet ground state is dioxygen, which is found in most cells. Energy transfer therefore takes place to afford highly toxic singlet oxygen from ground state dioxygen, provided the energy of the triplet excited state psoralen is higher than that of the product singlet oxygen.
Therefore we can distinguish between two pathways of photosensitization: an oxygen-dependent pathway (lipid peroxidation, formation of cross- links in membrane proteins, cross-linking of enzyme subunits of oligomeric proteins) and an oxygen-independent pathway (convalent binding between furocoumarins and unsaturated fatty acids and lecithins in lipid membranes, that they form cross-links with DNA, and that they inactivate enzymes and ribosomes, dark binding of furocoumarins to lymphocites and skin cells). We present here a quantum chemical study of the electronic excited states and the photophysics of psoralen molecule, the molecule basis of furocoumarins family, in order to interpret its electronic spectra and understand its photochemotherapeutic effect against some skins diseases such as psoriasis. In the future we are going to study the mechanism of the photochemical reactions involved too. Our study aim to contribute to the understanding of the direct and dynamic mechanisms of antitumoral phototherapeutic action and phototherapeutic processes in a general way, and to help to design new pharmacons with efective photoinduced properties, trying to optimize aspects such as the proper absorption or emission energies, energy transfer to reactive oxygen, or the formation of adducts without unwanted secondary efects.
On balance, a good photosensitizer should be amphiphilic (a lot of psoralens are poor water-soluble and it is a disadvantage in injected administration of the drug because of the the aggregation of the molecules, but their lipophilic behaviour is very important too since the intercalative properties psoralens towards DNA are due mainly to hydrophobic forces with the adjacement nu- cleic acid base pairs), it should form monoadducts and no diadducts with DNA to avoid several mutagenic side-efects, it should be effective in transferring the energy to molecular oxygen (high singlet oxygen quantum yield, i.e. high phosphorescence/fuorescence ratio), its triplet state must be very long-lived, its triplet de-excitation energy must lie above the triplet-singlet excitation energy of oxygen (22.1kcal · mol⁻¹), it should be harmless in the absence of light (no dark toxicity), it should be easily accesible (either by chemical synthesis or isolation from natural sources), and it should be removed quickly from the body (nowadays patients who receive this therapy become profoundly sensitive to outdoor light for a few weeks period). And, of course, the longer wavelength the photosensitizer absorbs, the deeper electromagnetic radiation penetrates in the body (this fact is very useful in cancer treatment).
Photodynamic Therapy:

A new form of treatment called "Photodynamic Therapy" has recently been approved in Canada, Japan, Holland, and the United States for patients with certain types of cancer involving the lung, bladder, and esophagus. Like PUVA therapy, photodynamic therapy (also known as "PDT") involves the administration of a drug followed by light exposure. In PDT, drugs known as porphyrins are administered intravenously into the body to sensitize diseased tissue to visible light (in the case of skin diseases, a photosesitizing cream is applied to the skin). The affected tissue is then exposed to laser light in order to activate the porphyrin drug and cause chemical reactions in the tissue. The drug in its excited state excites molecular oxygen to singlet oxygen, that is very toxic and it kills sick cells because of the fact that it reacts with a lot of membrane components. As the drug is always accumulated at sick tissue, singlet oxygen damages sick cells but not healthy cells. The net result is selective tumor destruction.

Nowadays PDT is used to treat cancers of the skin or those that are on, or near, the lining of internal organs. It can be used to treat cancers in the skin, the head and neck area, the lining of the mouth, the lining of the lung, the lining of the gullet (esophagus), the lining of the stomach and the lining of the bladder.
Several different drugs have been developed for PDT, but porfimer sodium (Photofrin®) is the only drug that has received official approval for routine use in patients, and only then for the treatment of certain cancers. Pilot research studies in patients shown that porfimer sodium and light can also be useful for psoriasis, but there is a significant side effect-patients who receive porfimer sodium become profoundly sensitive to outdoor light for a six to eight weeks period. If patients must go outdoors, they need to wear protective clothing, including sunglasses. The cytotoxic and antitumor actions of PHOTOFRIN are light and oxygen dependent.

Researchers have investigated the use of laser light and a new drug known as "BPD verteporfin" at UBC for PDT of skin cancer and psoriasis. BPD does not appear to cause prolonged photosensitivity as does porfimer sodium and thus is of potential practical use for psoriasis. To date, BPD and red laser light appear to possess significant anti-psoriasis activity.

The laser light used in PDT can be directed through a fiber-optic (a very thin glass strand). The fiber-optic is placed close to the cancer to deliver the proper amount of light. The fiber-optic can be directed through a bronchoscope into the lungs for the treatment of lung cancer or through an endoscope into the esophagus for the treatment of esophageal cancer.
An advantage of PDT is that it causes minimal damage to healthy tissue. However, because the laser light currently in use cannot pass through more than about 3 centimeters of tissue (a little more than one and an eighth inch), PDT is mainly used to treat tumors on or just under the skin or on the lining of internal organs.
The abnormal blood vessels in the wet form of macular degeneration are traditionally treated with laser. The thermal energy of the laser destroys both the blood vessels as well as the surrounding tissue (retina). Unfortunately, the majority of patients with wet macular degeneration have abnormal blood vessels located beneath the center of vision. Thus, laser treatment will destroy the center of vision- the very thing we want to protect. Photodynamic therapy could allow selective destruction of abnormal blood vessels without damage to surrounding tissues. The PDT dye is selectively absorbed by the abnormally proliferating blood vessels. When exposed to low levels of light ("non-thermal") the dye is activated and the abnormal blood vessels are selectively destroyed. PDT therapy is currently being evaluated in clinical trials. Two drugs are being evaluated: verteporfin (Vysudine) from CIBA vision and tin ethyl etiopurpurin (SnET2) from Miravant. Initial results are promising and have shown at least temporary recovery or preservation of vision in some patients. FDA approval of the PDT trials await completion of these studies to assess the overall safety and efficacy of these new agents.
Quantum Mechanics:

Since macroscopic properties (in this case the photosensitizing ability) of a molecule are provided by its electronic structure, we have to resort to Quantum Mechanics in order to study the photosensitizing effectiveness of furocoumarins. Indeed, a theoretical model for any complex process is an approximate but well-defined mathematical procedure of simulation. When applied to Chemistry, the task is to use input information about the number and character of component particles (nuclei and electrons) to derive information and understanding of resultant molecular behaviour.
Quantum Mechanics is an extension of Classical Mechanics, which is only a rough approximation (but useful in our daily and macroscopic activities) of Nature. In the beginning of the 20th century physicians realized that Classical Mechanics was uncapable of explaining several phenomena related to microscopic world, and as a result Quantum Theory appeared. Until then physicists believed that Physics had reached its ceiling. For instance, the Nobel Laureate A.A. Michelson, when opening a new laboratory at the University of Chicago in 1894, stated “our future discoveries must be looked for in the sixth decimal place”. Nevertheless, only six years later, Max Planck succeeded in explaining the form of the black-body spectrum, considering that light energy is concentrated in packets (quanta) of energy E=h·ν.Furthermore, in 1905 Albert Einstein, starting with the idea that signals can not be transmitted faster than light, proved that natural laws must have the same aspect for all the observers who are moving freely. He also showed that the mass of a body increased with its energy content and so led to his famous equation E=m·c², , and showed as well that the definition of simultaneity, length and time interval all depend on relative uniform motion. Hence, what is important is the relative movement. Quantum Mechanics and Relativity have been the most important scientific revolutions in History and two of the most amazing mankind´s achievements in spite of being linked to words like difficult, unintelligible, complex and so on. However, the overwhelming majority of the advanced which we enjoy have their source in both theories, from mobile phone to computers, from lasers to transistors. Furthermore, these theories have a great impact in many philosophical topics.
One important topic of Quantum Mechanics is that matter and light have dual nature: wave and particle. Louis de Broglie made a breakthrough considering that electrons (matter) possess ondulatory properties λ=h/p. Therefore we have a code of translation from the description in terms of particles to the description in terms of waves (the relation p=h·k, linking the linear momentum with the wave number, is also fulfilled).
The technical development of Quantum Mechanics was spreaded over by Erwin Schrödinger and Werner Heisenberg in the mid-1920s. The wave and particle aspects of matter are reconciled by the Schrödinger equation, HΨ=EΨ. The Hamiltonian operator, H, is the operator related to the total energy of the system. The resolution of this equation, within an appropriate methodology, provide us the eigenfunctions of the Hamiltonian operator (Ψ, wavefunctions) and the eigenvalues (energies E).
The wave function of a particle is a complex function that, as its name implies, has wave-like properties: it has an amplitude and a phase and may show constructive or destructive interference. The square of the modulus of the wave function is everywhere positive, and when normalized is interpreted as being a probability density: the probability of finding the particle in a volume dV is the probability density times dV. The probability interpretation highlights one of the most important features of Quantum Mechanics: it is not always possible to predict with certainty the result of a measurement. Often a distribution of probabilities is the best that we can obtain. Another feature emerges when we solve the Schrödinger equation: physical quantities such as energy or momentum are restricted to certain discrete values instead of having a large continous range.
The electron density or probability density representations of electrons in atoms describe an electron as located within a specific region of space with a particular electron density at each point in space. This space and a description of its contained electron density is called an orbital. We say that electrons are located in orbitals owing to the Heisenberg´s Uncertainty Principle: we can not know the momentum and the position of a particle simultaneously. Thus we have to speak in probability terms. Orbitals are characterized for their quantum numbers, which report us about their energy, shape and space orientation. The molecular orbital is the most fundamental quantity in contemporary Quantum Chemistry. Almost all of our understanding of “what electrons are doing in molecules” is based on the molecular orbital concept. What is more, the overwhelming majority of the computational methods used today start by a calculation of the molecular orbitals of the system. The molecular orbital describes the “motion” of one electron in the electric field generated by the nuclei and some average distribution of the other electrons. It is in the simplest model occupied by zero, one or two electrons. In the case of two electrons occupying the same orbital, the Pauli principle demands that they have opposite spin. Such an approach leads to a total wave function for the system, which is an antisymmetrized product of molecular spin orbitals (a spin orbital is the product of a molecular orbital times a spin function).
The main challenge in Quantum Chemistry is that we can not solve exactly this equation except for one-electron systems (hydrogen atom, free particle, particle in a box…) owing to the electron repulsion term present in the Hamiltonian. This is the reason why a large number of numerical analysis techniques have been developed in the last decades. Quantum chemical methods look for approximate solutions of the Schrödinger equation, employing computational numerical methods based on the variational principle or on perturbation theory. This is the origin of methods like Hartree-Fock (SCF), DFT, CI, CC, CASSCF, CASPT2… If the 2n electrons in a closed-shell molecule are assigned to a set of n orthonormal molecular orbitals ψ_i/i= 1,2,...,n, the corresponding many-electron wave function can be written as ψ₀=(n!)^1/2 det [(ψ₁α) (ψ₁β) (ψ₂α)...], which is a Slater determinant. According to the variational principle, the molecular orbitals are varied to minimize the energy, calculated as the expectation value of the full Hamiltonian, [P&si;|H|Ψ].This procedure leads to a set of coupled differential equations: the Hartree-Fock theory. In addition, it was in 1951 that Roothaan published their equations, considering molecular orbitals that were restricted to be linear combinations of a set of prescribed three-dimensional one-electron functions χ_μ/μ=1,2,...,n. Thus: ψ_i=Σ^N_μ=1C_μiχ _μ. Variation of the total energy was then carried out respect to the coefficients C_μi. This lead to a set of algebraic equations which can be written in matrix form, FC=SCE, where F is the Fock operator acting over the coefficients matrix C ,S is the overlap matrix and E is the diagonal energy matrix. The set χ_μ is referred to as the basis set. The SCF method assumes that various electrons move independently affecting each other only through the requirement of antisymmetry of the wave function. On the other hand, two electrons, repelling each other by Coulomb repulsion, will be less likely to be found at the same point of space than at points separated from each other. In other words, there should properly be included in the calculation an effect of the correlation of the motion of the various electrons. We should go beyond Hartree-Fock method to include such correlation effects, for instance with CI, CASSCF, CASPT2… Since all possible determinant can be described by reference to the Hartree-Fock determinant Ψ₀, we can write the exact wave function for any state of the system as:
|Φ₀>=C₀|Ψ₀> + C_a^r|Ψ_a^r>+C_ab ^rs|Ψ_ab^rs>+...
including one, doubly,triply, and higher excited determinants, replacing occupied orbitals by virtual ones. At CASSCF level we optimize both the many-electron- function coefficients of the expansion and the basis functions included in each molecular orbital.
Essentially, we construct a multiconfigurational wavefunction that includes every configuration that it would be generated by a set of active orbitals and active electrons with the same spin and eigenvalues of spatial symmetry that a given state. This provides us the non- dynamic correlation due to states which are very close in energy. On the other hand, CASPT2 method includes the dynamic correlation due to short-range electronic interactions. This method can be seen as a conventional non-degenerate perturbation theory, that is, a single reference is considered, with the particularity that this zeroth-order wavefunction is multiconfigurational (CASSCF). The accuracy is about 0.1-0.2 eV.
To sum up, at the simplest (and less accurate) level, the wave function is represented by a single Slater determinant. At the most complex level, we represent the true wave function by a variationally determined superposition of all determinants in the N-particle Fock space. None of these models is applicable under all circumstances. As a result, we should look for the best method for our interests, depending on the size of our system and the properties we are analyzing, in order to find the right answer for the right reason.
Quantum chemistry and computer modeling nowadays has a major impact on the chemist's ways of thinking and working, as the role of both theoretical understanding and computational modeling is becoming increasingly important in chemical research. I think that this feeling is perfectly expressed in the first announcement of the European Summer School in Quantum Chemistry (ESQC), which has been arranged every odd year since 1989: Computational chemistry has today reached a status where available methods and programs systems are also used by scientists who are not specialists in the field. We also see increasing interest from the chemical and pharmaceutical industry to apply modern quantum chemical methods in their research. As a result there is a growing need to acquire the background knowledge necessary for a skilful use of these methods.
It is different to study a system in its ground state (the most favourable energetic situation) or in an excited state. The latter tends to be trickier than the former. The study of excited molecules has undergone an increasing interest in the last decades because of the large amount of processes owing to excitations: phototherapy, photosynthesis, the process of vision… Spectroscopic experiments demonstrate that energy can be absorbed or emitted by molecules and atoms in discrete amounts, corresponding to precise changes in energy of the molecule or atom concerned. Life on Earth depends, both directly and indirectly, on the influence that light has on chemistry. As matter, light is a form of energy that exhibits both wave-like and particle-like properties. Maxwell showed that light is a form of electromagnetic radiation, which consists of oscillating electric and magnetic fields which can be transmitted through space. The wave theory accounts for many intrinsic properties of light such as propagation, reflection, refraction, diffraction, interference and polarization. On the other hand, Einstein demonstrated that radiation also behaves as though it consists of a stream of particles called photons of energy E=h·ν. Different types of radiation possess different energies (X-rays, ultraviolet radiation, infra-red radiation and so on), giving rise to the electromagnetic spectrum. Energy levels in matter are quantized. Thus, molecules have only certain discrete energy states, and absorption of the relevant frequencies from incident radiation raises molecules from lower to higher levels. Absorption may be detected in three different areas of the electromagnetic spectrum. First, the electrons in molecules occupy molecular orbitals with precise energy levels. Transitions from lower, filled orbitals to upper (higher energy) empty orbitals usually involve absorption of radiation in the UV and visible parts of the spectrum. Much smaller quantities of energy are related to changes in the rotational energy of a molecule and in its vibrational energy (of course, both properties are also quantized). Therefore, the overall energy of a molecule is a sum of contributions from electronic, vibrational and rotational energy, that is, E = E_elec + E_vib + E_rot. In a sample of a stable substance at room temperature, all the molecules will be in the E₀ level (the ground electronic state). When the appropriate energy is incident it is possible to excite the molecule to the higher energy electronic state E₁ (which again has similarly quantized rotational and vibrational energy levels). To sum up, a direct result of the quantization of energy is that for each individual species only specific energies of radiation can be absorbed or emitted. The characteristic line and band spectra of chemical species are a consequence of this behaviour. We label the electronic states with a number that points out its relative energy (0 –ground state-, 1, 2, 3…) and a letter that indicates its spin multiplicity (S, singlet; T, triplet). The electronic absorption spectra of a wide variety of organic compounds show that only certain types of molecule exhibit absorption in the UV-VIS range of the spectrum. These must contain double or triple bonds (or, in some cases, lone-pairs of electrons), which are essentially responsible for the absorption; these fragments are called chromophores. When two or more chromophores are adjacent to each other (conjugation) the absorptions become more pronounced and occur at lower energy. This behaviour is related to the different types of molecular orbitals imply in such excitations. There are three types of molecular orbitals, and the electrons in these orbitals have somewhat different environments. First, there are the electrons in the σ orbitals orbitals which constitute the bonding framework of a molecule. Second, there are electrons in π orbitals, responsible for the double and triple bonds. Third, there are lone-pair electrons in non-bonding orbitals n (orbitals which basically belong to one atom in a molecule because of there are not atoms of the same symmetry in the neighbour atoms). When two electrons in atomic orbitals are brought together to form a bond (when a filled molecular orbital is produced) a higher energy anti-bonding orbital is also formed. When excitations take place, an electron from one of the filled orbitals (σ,π or n) becomes excited to a vacant anti-bonding orbital (σ´,π´) so that a new excited state is reached. In general, the order of decreasing energy for the absorption is as follows: σσ^* > σπ^* = πσ^* > ππ^* = nσ^* > nπ^*
The first ones are so energetic that they appear beyond the UV and VIS light. Only those of the last three types normally account for absorption in this region. The molecule in excited state is often prone to react in an easier way than in the ground state (photochemical reaction) as a result of the new electronic arrangement. The excess energy of an excited species can alter its reactivity, and is particularly significant in the case of electronic excitation because of the energies involved. First, the energies are of the same order of magnitude as bond energies, so that electronic excitation can have a considerable effect on the bonds in a species. Secondly, the energies correspond roughly with typical activation energies for several reactions, and the excitation energy can help a species to surmount an activation barrier. Otherwise, the molecule in its grounds state is uncapable of overcoming such barrier. Nevertheless, the excess energy possessed by an excited state is not the only factor that affects the reactivity of a molecule. The electronic rearrangement is the other important factor. For instance, the molecule in an excited state may exhibit more nucleophilic or electrophilic properties than in its ground state. The concept of potential energy surface (PES) comes from Born- Oppenheimer approximation, based on the separation of electronic and nuclear motion owing to the large difference in mass between these particles, and assumes that the electrons follow the nuclei instantaneously during the motion of the latter. Therefore we can define and electronic Hamiltonian and a nuclear Hamiltonian. Solving the electronic Schrödinger equation provides us a description of the movement of electrons, whereas solving the nuclear counterpart provides us a description of the rotation, vibration and translation of the molecule. When we solve the electronic Schrödinger equation we obtain the energy of a particular nuclear configuration. The value of this potential energy for every possible nuclear configuration is specifically the potential energy surface. Photophysical and photochemical processes take place by means of the relations between potential surfaces, such as conical intersections, singlet-triplet crossings and avoided crossings.
Our research:

We have undertaken a project to study, using quantum-chemical methods, the photophysics and photochemistry of furocoumarins and related compounds in order to rationalize the basic features responsible for their phototherapeutic properties. Is is known that furocoumarins undergo photocycloaddition reaction with thymine. As a result, important changes in DNA may well be produced.
Quantum-chemical methods are centered in the approximate resolution of Schrodinger equation (HΨ=EΨ) with computational techniques (numerical calculations).
The spectroscopic behaviour of this compounds (photophysic and photochemical properties) could be rationalized in terms of the relations between the potential energy surfaces of the electronic states implied in the process.
Using the CASSCF multiconfigurational wave functions as reference, the second-order perturbation theory through the CASPT2 method is employed to include the dynamic correlation energy in the calculation of the electronic excited states. The CASPT2 method calculates the first-order wave function and the energy to second-order.In order to prevent the possibility of intruder states in the calculated wave function, the level-shift (LS) technique is used. An atomic natural orbital (ANO-L) type basis set contracted to C,O[4s3p1d]/H[2s1p] are usually used throughout. The carbon and oxygen 1s core electrons are kept frozen at the second-order level.
Geometries are obtained by computing analytical gradients at the RASSCF level of calculation for the ground and the lowest singlet and triplet excited states.
CASSCF wave functions are generated as state-average (SA) CASSCF roots of a given symmetry. Based on preliminar RASSCF calculations, the CASSCF active space is selected. The CAS state interaction method (CASSI) is being used to compute transition properties. All calculations are being performed with the MOLCAS-6.0 quantum chemistry software and GAUSSIAN98.

Publications:

See the section named "Publications" in the main page of this website.

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