Diapositiva 1

1
Light is the electromagnetic radiation that has
frequencies in the visible, ultraviolet and
infrared region.
2
Wave model l c/n where l is the wavelength, c
is the velocity (3,00 x 108 ms-1) in the vacuum
and n is the frequency.
Quantum model a beam of radiation is regarded as
a stream of photons, particles with no mass but
with a specific energy. E hn where E is the
energy, h is Planck constant (6.63 x 10-34 Js)
and n is the frequency.
The energy of a photon with a wavelenght of 100
nm is 1,99 x10-18 J The energy of a photon with a
wavelenght of 1000 nm is 1,99 x10-19 J
3
One molecule interacts with one photon
If we consider a mole of photons, the energy
is 1,99 x10-18 x NA 1198 KJ for photons of
100 nm 1,99 x10-19 x NA 119,8 KJ for photons
of 1000 nm
These energy values are of the same order of
magnitude of those required to break chemical
bonds. So when a molecule absorbs a photon of
light it gains a not negligigle amount of
energy. The excited molecule A can be quite
different from A in the ground state and can be
considered as a new chemical species.
4
energy gap between ground state and excited state
has to match with the photon energy. Photons of
visible and near ultraviolet region have the
right energy
DE
Vibrational levels
5
Consider now the relation between electronic
configuration and electronic energy states
An organic molecule is described by

• product of monoelectronic wavefunctions
• Each consists of an orbital part and of a spin
  part
• -The orbital part defines the electronic
  configuration to which the electronic state
  belongs

Jablonski diagram of electronic states
Electronic configuration
6
Inorganic molecules of interest in photochemistry
are coordination compounds of transition metals.
Consider a schematic molecular orbital diagram
for an octahedral complex
in the ground state of a complex containing a
transition metal with a configuration dn, all the
bonding orbitals are filled while n electrons
occupy PM and sM.
Types of possible transitions MC metal centred
(d-d) LC ligand centred (P-P) LMCT ligand to
metal charge transfer MLCT metal to ligand
charge transfer
7
Intramolecular excited state decay
Lifetime of an excited state is the time needed
to reduce the excited state population by a
factor of 1/e ( 2.718)
t(S1) 1/(kic kfl kISC) t(T1) 1/(kph
kISC) t(S1) is about 10-9-10-7 s t(S1) is about
10-3-100 s
Quantum yields is the ratio between the number of
moles of species produced and the number of moles
of photons absorbed. It ranges from 0 to 1
Ffl kfl / (kic kfl kisc) Fph kph x kisc/
(kisc kph)(kic kfl kisc)
Absorption of light is allowed only between
states with the same multiplicity. It occurs in
10-15 s
8
Bimolecular processes
Bimolecular processes
in fluid solution, an excited molecule (with a
lifetime longer than 10-9 s) can deactivate not
only by the discussed intramolecular processes
but may have the chance of meeting another
molecule. In this case the deactivation of the
excited state follows a second order kinetic. The
two main bimolecular processes are
9
Energy transfer process
A is quenched by B. B is sensitized by A
Exchange mechanism (short range mechanism,
physical contact and overlap between donor and
acceptor is required)
A(T1).B(S0) A(S0).B(T1)
Simultaneous exchange of two electrons
10
Metal porphyrins as photosensitizers

• Emitting porphyrins
• Closed shell metals (such as Zn2)
• Long lived excited state
• When the metal belongs to the 2nd or 3rd
  transition series there is a strong spin-orbit
  coupling. Efficiency of ISC to T1 is unitary and
  lifetime of T1 is long enough (ms) for
  bimolecular processes

11
Metal porphyrins as photosensitizers oxygen
activation
T1 of Pd-porphyrin has a higher energy than that
required from promoting oxygen from ground to
excited state FT1 is near 1 ken is high because
the overall spin in maintained
12
Photodynamic therapy
It is a non surgical therapy for the treatment of
skin cancers. It involves the presence of three
key components
1O2 is toxic and causes the death of the cell
13
The photosensitizer
5-ALA is not a drug, it is a small molecule able
to penetrate skin and be easily adsorbed. It
prefers accumulation in tumoral cells. It is
converted to PPIX by the cellular metabolic
pathway that leads to heme synthesis.
5-aminolevulinic acid (5-ALA)
14
considering the visible spectrum of PPIX, light
is provided by lamps or lasers in the range
550-600 nm
1O2 produced in situ. It triggers chain reactions
leading to cellular death.
metabolic pathways lead to the accumulation of
PPIX in cells
15
Photodynamic therapy developments

• finding of other efficient photosensitizers
  (synthetic porphyrins and cationic colorants such
  as Blue Nile)
• a) shift the absorption at wavelength
  higher than 600 nm
• b) extend the therapy to other kinds of
  cancer
• set simple and cheap apparatus for irradiation
  with constant and homogenous emission, for the
  determination of the correct time of exposition
• red light with wavelength higher than 660
  nm is preferred for skin photodynamic therapy
  since it has a very good penetration through the
  skin.

16

• Non emitting porphyrins es Fe(III)porphyrins
• open shell metals
• short lived excited state
• No photosensitization reactions
• Intramolecular photoinduced electron transfer
  reactions

quantum yields of the photoreaction range from
10-4 to 10-1 and depend on the nature of iron
porphyrin and on the possibility for the radical
species to escape from metal before back electron
transfer occurs. Photogenerated Fe(II) porphyrins
can start the reductive activation of O2,
mimicking the catalytic sites of some natural
enzymes.
17
Catalytic cycle of cytochrome P450

• Reducing agent is NADPH
• The proteic environment allows the coexistence of
  NADPH with hypervalent iron-oxo species

18
Biomimetic photocatalysis

• Its ambition is to contribute to the
  realization of photocatalytic cycles which mimic
  cytochrome P450 oxygenase in its bielectronic
  reductive activation of O2. In order to pursue
  this objective the following basic conditions
  have to be satisfied
• in the reaction between Fe(II)porphyrin and
  oxygen, the formation of m-oxo dimers has to be
  avoided
• on the other hand Fe(II)O2 should undergo a
  further reduction.

• Fe(III)TDCPP has bulky chlorine atoms preventing
  the formation of photochemically inactive dimers
• Photogenerated radical contributes to the release
  of the second electron.

19
Biomimetic photocatalysis
Solvent non polar neat cyclohexane
L OH- RH2C6H12
Bielectronic reduction of O2 occurs with the
involvement of both metal and axial ligand
hn (300-400 nm)
20
Biomimetic photocatalysis
Solvent polar and protic
Protic environment favors heterolytic cleavage of
O-O bond leading to the formation of PFe(IV)O
like in P450
21
Biomimetic photocatalysis
Photochemical excitation of iron porphyrins
(Fe(III)TDCPP) is a suitable means of inducing
oxidation of hydroxy-guanidines. The reaction
presents important similarities with the
oxidation of endogenous N-hydroxy-L-arginine by
heme-containing metallo-enzyme nitric oxide
synthases (NOSs) and their model systems. The use
of Fe(III)(TpivPP)Cl provided the first
experimental evidence for the photoinduced
formation of an Fe(II)(TpivPP)(O2)(1-MeIm). A.
Maldotti, A.Molinari, P.Battioni, D. Mansuy et
al., Eur. J. Inorg. Chem., 2004, p. 3127