Poly Excited State Lifetime

Lochmuller and coworkers used the formation of excimer species to answer a distance between site question related to the organization and distribution of molecules bound to the surface of silica xerogels such as those used for chromatography bound phases. Pyrene is a flat, poly aromatic molecule whose excited state is more pi-acidic than the ground state. An excited state of pyrene that can approach a ground state pyrene within 7A will form an excimer Pyr +Pyr (Pyr)2. Monomer pyrene emits at a wavelength shorter than the excimer and so isolated versus near-neighbor estimates can be made. In order to do this quantitatively, these researchers turned to measure lifetime because the monomer and excimer are known to have different lifetimes in solution. This is also a way to introduce the concept of excited state lifetime. 

Excited state resonance Raman spectra of CuTMPyP bound to DNA or poly[d(A-T)] have been recorded [167,168], These are assigned to an exciplex formed between the porphyrin and the A-T sites of the polynucleotide. The excited state lifetime is estimated to be ca. 20 ps. Weak emission from CuTMPyP" bound to DNA has been reported and has been assigned to originate in a tripdoublet or tripquartet level [169]. It is believed that the emissive complexes are intercalated, whereas groove-bound CuTMPyP does not emit because of solvent quenching of the excited state. 

In poly crystalline semiconductor samples, the excited-state lifetime of electron-hole pairs is so short that photocurrent collection is efficient only for carriers created within the space charge (depletion) region. Thin-film processes offer an inexpensive way to prepare large solar arrays, but the semiconductors formed by such processes are almost inevitably polycrystalline. It is not wise to use semiconductor films thicker than the depletion width in such devices because the additional thickness contributes only extra grain barrier boundaries for the majority of carriers to surmount on their way to the back contact. The additional thickness does not provide any additional photocurrent. 

The Ni and Pt complexes can also be incorporated into polymer films of quaternized poly(vinylpyridine) (PVP) and deposited onto the transparent electrode (84). Photocurrents are enhanced to microamps (pA), an increase that may be attributed to either the effect of immobilization of the complexes near the electrode surface or an increase of the excited-state lifetimes in the polymer matrix. However, the effective concentrations of the complexes in this study were much greater than for the acetonitrile solutions in their earlier work. The polymer films are not stable to continuous photolysis, and voltammograms of the films are quite sensitive to anions used in the supporting electrolyte. The system can be stabilized by using a polymer blend of PVP and a copolymer containing quaternary ammonium ion and including [Fe(CN)6]4- in the electrolyte solution (85). Upon irradiation of the visible MLCT bands of [M(mnt)2]2 (M = Ni, Pt), photocurrents are produced. The mechanism (Scheme 4) is believed to involve photooxidation of the metal bis(dithiolene) triplet state by the Sn02 electrode, followed by [Fe(CN)6]4 reduction of the monoanion, with completion of the ET cycle as ferricyanide, Fe(CN)6 3, diffuses to the other electrode and is reduced. 

In a later paper, the same authors [10] reported that naphthalene inhibited chain scission by a triplet energy transfer mechanism. Golemba and Guillet [11] determined the 0 of chain scission of poly(phenyl vinyl ketone) to be 0.25 of 313 nm. They determined that the excited state lifetime of the carbonyl group on the polymer was of the same order of magnitude as that of the analogous model compound. 

The efficiency of the immobilization is related to the glass transition temperature (Tg) and the free volume of the polymer. Fluctuations of the polymer environment surrounding the single molecules, both in time and in space, affect the local density and subsequently the excited-state lifetime. In fact, the mobility and exdted-lifetimes of single molecules in low Tg polymers, such as poly(methyl acrylate) (PMA Tg 281K), can act as a reporter for the dynamics of the polymer matrix itself (Section 2.18.6.2). 

The excited states of doubly-charged negative ions larger than monatomic are also of importance in providing for long lifetimes of these species. Studies of the energy stabilities of di-, tri-, and poly-atomic l ions involve molecular orbital treatments, however, rather than the approaches just discussed for atomic l species. A variety of semiempirical molecular calculation methods are possible. 

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