IMPORTANCIA DE LA GESTIÓN DEL TALENTO HUMANO

As discussed in the previous section, following absorption of light and concomitant population of excited states, a molecule can relax back to the ground state by emitting light (fluorescence or phosphorescence) or by radiationless deactivation such as internal conversion. However, over the past fifty years the dynamic quenching of excited states involving donor-acceptor systems based on combinations of multi-chromophoric arrays has been an area of great interest to photochemists.[20, 21] A bimolecular quenching processes involving a donor chromophore and an acceptor (quencher) molecule is classified into two general pathways: energy transfer and electron transfer.[22, 23]

Figure 3. Schematic description of electron motion in electron- and energy-transfer quenching mechanisms (D = donor and A = acceptor). The solid circles represent electrons while green lines represent HOMO and LUMO levels. a) electron transfer results in a radical ion pair. Energy transfer proceeds byb)an electron-exchange or c)dipole-dipole (Coulombic) mechanism. Blue squares indicate initial states following photoexcitation; yellow squares represent intermediate states and red squares represent final states after electron or energy transfer.

According to a simplified molecular orbital picture (Figure 3), energy and electron transfer between two chromophoric units can be formally described in term of electronic

motion between occupied and unoccupied molecular orbitals of a donor (D) and an acceptor (A) molecule.

Quenching by electron transfer strictly requires orbital overlap between the donor and the acceptor. It is a one electron process in which the electron jumps from an occupied orbital of one reactant to an unoccupied orbital of the other (Figure 3a).[22, 24–26] This process leads to the formation of a radical ion pair or a charge-transfer complex [D]·+_-[A]·- _{that is generally non-emissive. The energy of the radical ion pair state (E}

CS)

and free energy change (∆G_CS) associated with the formation of the charge-separated

state is generally determined by the Rehm–Weller equation,[27] which takes into account both the spectroscopic excited state energy of the donor (E0,0) and the thermodynamics

of the overall electrochemical redox characteristics associated with the electron transfer process. Importantly, electron transfer between two chromophores can only take place if the excited state energy of the donor, E0,0, is all available as free energy to promote the

excited state redox processes. Photoinduced electron transfer is particularly relevant in chapter 4 when discussing the optoelectronic interactions between supramolecular dynamic iridium-porphyrins and ruthenium-porphyrins diads and triads. Therefore, more insights about the nature and understanding of electron transfer processes will be highlighted in chapter 4.

Energy transfer between two chromophoric units can take place through three funda- mentally different mechanisms: 1) electron exchange, which is generally known as Dexter energy transfer; 2) dipole-dipole, which is known as Förster resonance energy transfer and 3) radiative mechanism.[28] Similar to electron transfer, the electron exchange mechanism requires orbital overlap between the donor and the acceptor, and involve two single inde- pendent electron transfers that result in the formation of the donor’s ground state (D) and acceptors’ excited state (A*) (red square in Figure 3b).[29] This process can be promoted as a single step involving two concomitant electron transfers, or in two steps characterised by an initial one-electron transfer promoting the formation of a radical ion pair (yellow squares in Figure 3b) and a subsequent electron transfer to generate the final excited state acceptor, A*, and ground state donor, D.[22]

Energy transfer by the dipole-dipole mechanism (Figure 3c), on the other hand, does not require orbital overlap between the donor and the acceptor molecules. Indeed, it operates by Coulombic resonance interactions (the transmitter-antenna mechanism)[30], in which the oscillating electrons of an excited-state donor are coupled with those of the acceptor and are quenched by an induced dipole interaction.[31, 32]

The Stern-Volmer relationship (eq. 2) is generally used to explore the kinetics of a photophysical intermolecular excited state deactivation process by bimolecular energy transfer via Dexter or Förster mechanisms.[33]

I0f

I_f = 1 +kqτ

0_{[Q] (2)}

In eq. 2 I0f is the emission intensity, or rate of luminescence, of the donor molecule

without a quencher, I_f is the emission intensity, or rate of luminescence, of the donor in

the presence of the quencher, kq is the bimolecular quenching rate constant, τ0 is the

lifetime of the emissive excited state of the donor in the absence of the quencher and [Q] is the concentration of the quencher. The product k_q·τ0 is generally known as the Stern-Volmer Constant,KSV. KSV can be easily determined from the slope of the plotI0f/If

- 1, which can be experimentally obtained by spectroscopic analysis, against the quencher concentration [Q]. However, eq. 2 is only valid with the assumption that the quenching process is diffusion-limited and purely collisional quenching takes place.[34]

Electron transfer and energy transfer by electron exchange require a close approach for effective orbital overlap. As a result, these mechanisms can only be promoted in bimolecular systems where the donor-acceptor distance is less than 10 Å.[22, 28] In contrast, Coulombic energy transfer does not involve orbital overlap and can be effective from collision distances of less than 10 Å, and up to separation distances as large as 100 Å.[30, 31, 35, 36] However, spin conservation is normally observed in both electron and energy transfer as the overall spin of the radical ion pair (electron transfer) or the spin of the acceptor’s excited state (energy transfer) match the spin of the donor’s excited state.

So far we have briefly discussed electron transfer processes and energy transfer mecha- nisms that operate via electron exchange (Dexter energy transfer) or dipole-dipole interac- tions (Förster resonance energy transfer). However, energy transfer can also be promoted via a mechanism that does not involve electronic interactions between the donor and the acceptor, which therefore behave as independent species. This mechanism is generally known as radiative energy transfer. Immediately one might ask, if there is no interaction between the donor and the acceptor chromophores, how can energy transfer occur? the answer is simple. Energy transfer can easily occur when the emission of light by the excited donor, D*, is subsequently absorbed by a ground state acceptor.[37] In this mechanism the acceptor does not influence the emission properties of the donor molecule, whose excited state characteristics remain unchanged by the presence of the acceptor. Furthermore, in contrast to the Dexter and Förster mechanisms, in radiative energy transfer the "energy delivery" mechanism does not involve a physical encounter between the chromophoric units. Instead, it only requires that the emission spectrum of the excited donor (D*) partially overlaps with the absorption spectrum of acceptor (A). Examples of this mechanism will be given in chapter 4 when discussing the iridium-porphyrin assemblies.