Verdad como Urphänomen (fenómeno originario)

Over the last two decades, as a result of improved analytical techniques, there have been an increasing number of studies on porphyrin photophysics. When a porphyrin undergoes light excitation, the excited state can be 'transferred' to a nearby acceptor through non-radiative mechanisms. Mg, Zn, Fe Au, Ru and free-base porphyrins have been included in systems that undergo energy and electron transfer, often with a quantum yield close to one.133,134 Examples where porphyrins play the donating role and the acceptors are either perylene derivatives or fullerene are common (Figure 4-

5).135-137,158

b)

Figure 4-5 (a) Porphyrin-perylene dyad by Tomizaki et a/. 136 and

(b) porphyrin-fullerene dyad by Schuster et al. 135

As previously discussed, the nature of the coordinated metal is critical for the photophysical properties of porphyrins. Mg(IIy37 and Zn(II) porphyrins, for example, are typically electron rich, therefore they have been shown to be very effective as energy/electron donors; on the contrary, Au(III)138 and Fe(III) porphyrins can use easily available lower oxidation states and are ideal as electron acceptors. When multielectron transfers are required, non-metallic porphyrins show the best chemical and physical properties (SnIl�SnIV, Sbm�Sb v). 139 By connecting electron-rich and electron-poor porphyrins, it has been possible to make dyads in which the excitation

energy of one porphyrin (donor) is efficiently transferred to the other one (acceptor). The transfer must be between isoenergetic states, therefore the emission spectrum of the donor and the absorption spectrum of the acceptor have to overlap; when this condition is fulfilled, excited state transfer can occur by one of two possible mechanisms. The first involves the fonnation of a resonant oscillating dipole in the acceptor produced by the electric field of the excited donor dipole. This pathway, known as the Forster or Coulomb mechanism, does not involve any orbital overlap,

being a pure electrostatic interaction. The second type of transfer is known as the Dexter or exchange mechanism; in this case, there is an exchange of electrons between donor and acceptor, which leads to an excited state localized on the acceptor. When the excited state transfer does not involve oxidation/reduction (e.g. energy transfer vs. electron transfer), the Dexter mechanism can be seen as a synchronized electron transfer between the two species (Figure 4-6).

electric field

�

bv hv e -

�

�

B B

F orster mechanism De..1er mechanism

Figure 4-6 Forster vs. Dexter energy transfer mechanisms

Many systems have already been created in which both electron and energy transfers have been observed; both Dexter and Forster processes have been obtained by choosing the nature of the connecting bridge between the porphyrin units and their relative distance/orientation. For example, Kadish et al.87 prepared a series of Zn/Zn and fb/fb dyads in which the porphyrins are very close but have little electronic interaction (Figure 4-7); when the two porphyrin were the same (e.g. TPP), partial fluorescence quenching was observed while, with different porphyrins, only fluorescence from TPP was noted. In both cases, energy transfer through the Forster mechanism was found to be the reason for the emission quenching.

M = Zn or 2H

Figure 4-7 Homometallic dyads by Kadish et al. 87

The Lindsey group examined a series of ethyne-linked porphyrin arrays and obtained fast and efficient energy transfer from photo excited Zn to free-base porphyrins; the Dexter mechanism in such systems prevails and is supported by the fact that the electron transfer rate and efficiency is dependent on the ability of the bridges to adopt conformations which are favourable for electronic communication (Figure 4-8a).16 Similar results were obtained by Osuka et al.78 in the study of polyene and polyyne linked ZnJfree base dyads (Figure 4-8b).

1oI, .Io\t-2H M, - lI\, Ma-2H M, • Zn, 1012. Fe(I�)CI � I �I 1 -0--0-1 I �I I �I 1 -0--0-1 1

-()-"""-Q-I

I-�,,-o--

I

I �o-I

In arrays containing Fe(III) porphyrins, electron transfer becomes the favoured relaxation pathway and many reports have been published in which such electron transfer occurs efficiently. For example, the homologues of molecules in F igure 4-8b containing a Fe(III) in place of a free base porphyrin showed practically the same 'transfer' efficiency and bridge dependence as for the Zn!fb homologue;78 in fact, in both cases, a Dexter (through bond) mechanism is involved. Similar work has been done by Helms et al. (Figure 4_9). 140 Here, the porphyrin units are connected through phenyl ring chains, which do not allow the electrons to flow as easily from one porphyrin to the other. It was found that the e fficiency of the electron transfer is dependent on the number of conjugation interruptions (the transfer rate drop was 6- fold per phenyl ring).

n = 0, 1 , 2

Figure 4-9 ZnlFe(III) dyads by Helms et a/. 140

It is worth making a comment about the nature of the bridges utilized in the last examples. The Osuka group78 compared polyene to polyyne bridges and realized that energy/electron transfer is more efficient through the double bonds than through alkyne linkers; in both cases, however, efficiency was higher than what was obtained by the Helms group (polyphenyl chains). It seems clear that the loss of coplanarity, along with the consequent interruption of the conjugation, is responsible for the drop in the electron transfer quantum yield.

As for energy transfer, conjugation is important only for long distance processes. Fujita et at. 141 showed that efficient electron transfer from Zn to Fe(III) porphyrins can be achieved by cofacial disposition, with the condition that the distance between the porphyrin units is not too long (Figure 4- 1 0).

CHI�H2�H2�H3

I

�HI�H2-N-C--CHr-

Figure 4-1 0 Zn/Fe(III) dyad by Fujita et al. 141 U

o

Interestingly, in the great majority of the reported work, the porphyrins are connected through their meso positions and it is likely that substitution at ,B-pyrro lic carbon would produce different photophysical properties. To the best of our knowledge, the only extensive investigation on the photophysics of ,B-pyrrolic connected porphyrin arrays is due to the Therien group. 159·161 A series of monomers, dimers and trimers featuring alkynyl substitution at ,B-pyrrolic position were prepared as intermediate for the preparation of face-to-face arrays (Figure 4-1 1 ). Interestingly, even though the alkynyl bridges should allow conjugation and high electronic communication between the units, the photophysical and EPR investigations on such systems showed that very little electronic communication is possible along the arrays; UV -visible spectra showed that exciton coupling was the only new feature deriving from the intramolecular interactions 159 and EPR showed that excited triplet states are completely localized on one porphyrin.l60 All these data are consistent with weak electronic interactions due to electrostatic perturbation between close but perpendicular porphyrin rings; therefore, the possibility of conjugation deriving from alkynyl linkage is not sufficient to ' force' the porphyrin into assuming a planar conformation, which would allow efficient electronic de localization over the array.

The synthetic route to porphyrin arrays described in the course o f this thesis allows the creation of structures with different porphyrins connected through their �pyrrolic positions; moreover, contrarily to the just described alkynyl linkages, the nature of the phenylene-vinylene bridges should constrain the array in the planar conformation, allowing long range energy/electron transfer via the Dexter mechanism through the conjugated system.