Most of the molecules presented here have strong colours, so one of their properties is that they absorb in a certain region of the visible light. This absorption also extends into the UV region, and, for some complexes, into the IR region.
In order to understand their behaviour, the compounds and complexes were studied by optical spectroscopy. Thus, the photochemical and photophysical properties of the different compounds were obtained through the interaction between the electromagnetic radiation, that is, light, and the molecules.
When molecular entities interact with electromagnetic radiation, three main processes can occur:
" absorption of energy resulting in a transition between two energy levels, the latter
being higher in energy, such that the molecule ends up in an excited state.
" stimulated emission or spontaneous emission of energy, that is, radiative
relaxation of the molecules back to the ground state.
In most of the cases, before any interaction with light, the molecules are in a stable state, namely the ground state, which is the one with the lowest possible total energy. In that case, the principal process when the interaction with light is switched on is the absorption of radiation. When atoms or molecules absorb light, incident photons allow the excitation from the ground state at an energy E0, to a higher energy level at Ei. In that process, the photon energy must be equal to h" = (Ei - E0).
After the interaction with light and the absorption of a photon, the molecule has received energy and takes on an electronic configuration of an excited state, which in an orbital picture means that one of its electrons has been promoted to a molecular orbital higher in energy. The excited state is usually photochemically or photophysically unstable. In order to come back to the ground state configuration, the molecule can dissipate the excess energy in different ways: [1-5]
i) the disappearance of the original molecule, by photochemical reactions
ii) by radiative relaxation leading to the emission of a photon usually at a lower energy than the one originally absorbed by the molecule (Stokes shift, Kasha's rule).
iii) by non-radiative relaxation, such as internal conversion and intersystem crossing followed by vibrational relaxation.
iv) if the molecule is in solution, it may interact with other species also present in solution, for example by luminescence quenching processes such as excitation energy transfer and light-induced electron transfer.
Figure 1.1: Scheme of different excited state deactivation processes. [6]
Spectroscopy involves the absorption of the electromagnetic radiation (from a lamp and/or a laser) by the molecules, as well as the one eventually emitted after excitation.
This includes different spectroscopic methods, such as absorption or emission spectroscopies. The different peaks in the obtained spectra represent transitions between different energy levels of the molecules. The strong light of Lasers, in particular pulsed Lasers, can be used to induce and probe the transformation of matter in real time on different timescales ranging from sub-picoseconds to hours and days.
The energy levels of a molecule represent the characteristic states of this molecule, which allow the identification of the transitions occurring in it. The energy is absorbed by quanta, in a discontinuous way and the energy of a photon is:
!
One of the most important aspects of a given absorption band is its integrated intensity, which is proportional to the oscillator strength containing information on the allowed or forbidden character of the transition. Transitions from ground to excited states having the same spin value are spin-allowed and if they have opposite parity they are also
electric dipole allowed, thus giving rise to intense absorption bands. Transition between states of the same parity, such as dd transitions in transition metal complexes, are electric dipole forbidden and give rise to only weak absorption bands. Transitions to excited states of different spin values are likewise forbidden and give very weak absorption bands. They acquire their intensity via spin-orbit coupling and may thus have a certain non-negligible intensity for complexes of heavier transition metal ions.
Transitions, which are both spin and parity forbidden, are very weak indeed and only observable in very special cases.
The photochemical and photophysical processes and most of all the states involved in those processes, can be illustrated in a so-called Jablonski diagram. The Jablonski diagram represents the different energy levels of a molecule. The total energy of the molecule can be described as the sum of the vibrational, rotational and electronic energies.
Figure 1.2: Schematic energy level diagram (Jablonski diagram)
According to the Jablonski diagram, two different relaxation pathways lead to luminescence (emission of light): it is called fluorescence when the excited and the ground state have the same spin and phosphorescence when their spins are different.
In the same way, non-radiative processes are called either internal conversion or intersystem crossing when they occur between states of the same or different spins, respectively.
Fluorescence and internal conversion are spin-allowed processes, versus phosphorescence and intersystem crossing which are spin-forbidden.
Every intramolecular decay process is characterised by a rate constant and the excited states can be characterized by lifetimes: [3, 4, 7, 8]
!
For luminescence processes, a quantum yield can be defined:
!
A molecule is a multielectron system, which can be described by molecular orbitals. An approximate wavefunction for a molecule is given by the antisymmetrised product function:
si is a spin eigenfunction. The orbital part of the wavefunction represents the electronic configuration. In a zero order description, the energy associated with an electronic configuration is given by the sum of the energies of the occupied MOs. [9] However, in order to obtain a more realistic description of the energy states of the molecules, two elements should be taken in account. The spin functions must be added to orbital functions for the description of the electronic configuration, and the interelectronic repulsion should be taken into account. [9]
For metal complexes, different MOs can be assigned according to their atomic orbital contributions. Two groups can be distinguished: the orbitals centred principally on the ligands (#L, !L, #n*
and !L*
) and the partially occupied orbitals centred predominantly on the metal (t2g*
Principal excited configurations of metal complexes can thus be classified as:
- metal centred (MC) transitions (!M*
$#M*
) - ligand centred (LC) transitions (!L$!L*
)
- ligand to metal charge transfer (LMCT) transitions (!L$!M*
, #M*
) - metal to ligand charge transfer (MLCT) transitions (!M$!L*
)