Emission from Charge-Separated States

Spontaneous emission from higher excited singlet (S2, S3, etc.) states is rare, since the system quickly finds the lowest excited singlet state by nonradiative transitions (internal conversion. Kasha s rule). Usually, emission spectra are formed either from the lowest excited singlet state of a molecule (fluorescence) or from the lowest triplet state (phosphorescence). In some other cases, low-lying CT states, singlets or triplets, are deexcited radiatively. 

The lowest CT state is reached in an electron transfer (ET) process on the lowest excited potential energy surface (PES). As in other cases of ET, the rate may be calculated if we know the free energy, reorganization energy, and electronic coupling. The final CT state is usually reached and stabilized within a nanosecond. The time for this process depends on the solvent If the solvent is nonpolar, polarization is electronic and rather fast Only structural reorgauization r ains to slow down the speed of the process. The calculation of the PIET rate can be done by ordinary methods (see below). 

In the case of a polar environment on the other hand, the Stoke shift is time-dependent due to solvent dynamics. The calculation of rate may be complicated. Usually, molecules have anissiou only from the locally excited state. Others have dual fluorescence from both the locally excited state and the CT state. 

The lower energy fluorescence band is clearly a CT band, since its position depends on the dielectric constant of the solvent. 

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On the other hand, since biradical excited states are in principle easy polarizable,39 105 they will have something in common with the charge-separated states of a broken dative bond. In order to find out the essentials about the excited states of the dative bonds, especially about the geometries corresponding to minima from which emission might take place, we will proceed in four steps. 

After the TICT minimum is reached, the transition moment between charge-separated state and the ground state represented by A-B and AB, respectively, is expected to be fairly small owing to almost no overlap between part A and B. Therefore, the fluorescence intensity will be small and significant contributions most probably stem from neighboring geometries (6 90°) for which the emission from admixed locally excited states can occur. The return from Sj or Tj minimum to S0, which can proceed in radiative or radiationless manner, usually does not lead to formation of cis-trans isomers as one would expect from the assumed energy surfaces. This is due most probably to rapid thermal cis-trans interconversion in the S() state. So far in this Section electronic properties of the free molecules have been addressed. 

The important determining factor for the relative energy of the charge-separated state A B to the hole-pair state AB is certainly the nature of the environment (polar solvent). Note that, although the emission from the TICT state has been observed in the gas phase, most of the observations have been made in polar solvents. We can again use the... 

Figure 10 Potential energy diagram for emission from normal planar (NP) and twisted intramolecular charge transfer (TICT) excited states. The charge-separated state is stabilized by twisting and by the polarity of the environment.

This exciplex has a large amount of charge transfer character, as shown by the solvent dependence of its fluorescence emission spectrum. The exciplex can then receive an electron from the secondary donor to form the final charge separated state ... 

It will be noted from Figure II that the final C -P-Qa-Qb state is not only long-lived and formed with a reasonable quantum yield, but also preserves a significant fraction of the excitation energy of the porphyrin as chemical potential. The porphyrin first excited singlet state lies at 1.9 eV, based on the absorption and emission spectra, and the final charge-separated state at about 1 eV. 

Besides returning to the ground state So from the S state by emission, the S, state may alternatively go to So by radiationless deactivation [step , butfrom S to So, followedby step ]. As the name implies, no radiation is emitted and the electronic excitation energy is converted into vibrational excitation of So, which is then transferred to the adjacent solvent. For certain excited molecules the most useful decay pathway is energy transfer [step ], which eventually leads to the so-called charge-separated state, and is of vital importance to photosynthesis. 

The most fascinating development in this field of CIDNP within the last years has been the observation, by Zysmilich and McDermott [146], of nuclear spin polarized (solid state) 15NNMR spectra from photosynthetic reaction centers in which the forward electron transfer from the primary charge-separated state to the accepting quinone was blocked. The all-emissive polarizations were proposed to be due to a radical pair mechanism, though many of the details are still not very clear. The reaction scheme is virtually identical to that of Chart VIII (Section V.A.2), the donor D being the special pair and the acceptor A the pheophytin. As in that example, the polarizations from the triplet exit channel are hidden in the triplet product 3D for the lifetime of the latter. This feature, in combination with the fact that nuclear spin relaxation in the molecular triplet localized on the special pair is relatively fast, serves to avoid the cancellation of CIDNP that would occur otherwise because the products from both exit channels are identical. 

In the most common scheme (Fig 12a), photoinduced electron transfer in a two-component supermolecule ("dyad") involves (i) excitation of a photosensitizer molecular component (P), (ii) electron transfer from the excited photosensitizer to an electron acceptor component (A)(a process often called "charge separation"), (iii) back electron transfer from the reduced acceptor to the oxidized photosensitizer (often designated as "charge recombination", not shown in the figure) [17,82]. The practical consequences of this sequence of processes may vary from system to system. Quenching of the excited photosensitizer is always observed (usually from emission intensity and lifetime measurements). The formation and disappearance of the charge separated state can in principle be monitored by fast spectroscopic techniques. The possibility of observation depends on both instrumental factors (sensitivity and time resolution) and on kinetic... 

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