Poly excited states

Poly(aryl ether) branches of generation 1 to 3 have been appended to a pho-totautomerizable quinoHne core to investigate the effect of dendritic architecture on the excited state intramolecular proton transfer [45]. The changes observed in the absorption and emission spectra on increasing dendrimer generation indicate that the dendritic branches affect the planarity of the core and therefore the efficiency of the excited state intramolecular proton transfer and of the related fluorescence processes. 

Photoluminescence (PL) in the polysilanes is well documented,34b,34c and for the poly(diarylsilane)s occurs typically with a small Stokes shift and almost mirror image profile of the UV absorption.59 This is due to the similarity of the chromophore and fluorophore structures in the ground and excited states, respectively, which is a result of the fact that little structural change occurs on excitation of the electrons from the a to the a orbitals. As PL is the emissive counterpart to UV, the emissive counterpart to CD is circularly polarized pho-toluminescence (CPPL). Where the fluorophore is chiral, then the photoexcited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/R and /l, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality... 

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. 

Gamma radiolysis of simple carboxylic acids and N-acetyl amino acids results in loss of the carboxyl group with formation of carbon monoxide and carbon dioxide. In the carboxylic acids, the ratio of C0/C02 produced is approximately 0.1, while in the N-acetyl amino acids the ratio is much smaller. In the poly carboxylic acids and poly amino acids, radiolysis also results in the loss of the carboxyl group, but here the ratio of C0/C02 is greater than 0.1. Incorporation of aromatic groups in the poly amino acids provides some protection for the carboxyl group. The degradation of the poly acids is believed to involve radical and excited state pathways. 

For each of the poly carboxylic acids investigated, the sum of the yields of carbon monoxide and carbon dioxide is much larger than the yield of radical products. This observation differs from that noted for the model compounds, where the two were of similar magnitude. This suggests that excited state processes may play a more significant role in the degradation of the poly acids than they do in the small molecule, model compounds. 

In the poly carboxylic acids, carbon dioxide is the major product of radiolysis, but the carbon monoxide yields are greater than they are for the aliphatic carboxylic acids. However, the radical yields are not greater than expected on the basis of the model compounds, which suggests that excited states play an important role in the degradation of these poly acids. 

Figure 1. Adiabatic potential curves in the main chain scission of a model compound of poly(isobutylene) 2,2-, 4,4-tetramethylpentane (4). AE3l(=0.61eV), aET,(—0.35eV), and AEf (=2.05eV) are the activation energies of the main chain scission in the lowest singlet excited state (S,), the lowest triplet state (T,), and the ground state, respectively.

FPA measurements on poly(dA-dT) were also undertaken/146) but the excited-state decay function S(t) contained additional intermediate compo-... 

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. 

A further possibility is that the signals arise from hydrated electrons or base radical ions produced by monophotonic ionization of the polymers. However, the quantum yield for photoionization of adenosine is reported to be approximately the same as that of poly(A) and poly(dA) [25], It is unlikely that photoionization of the polymers can account for the signals seen here since there is no detectable signal contribution from the photoionization of single bases [4], The most compelling argument that our pump-probe experiments monitor excited-state absorption by singlet states is the fact that ps and ns decay components have been observed in previous time-resolved emission experiments on adenine multimers [23,26-28]. 

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