Excited-state processes, PDAs

The absence of energy shift between absorption and emission thresholds indicates minimal conformational relaxation in the excited state. There is an overall similarity to the PDAs, including the presence of a very strongly bound triplet exciton. The absence of polaron-related transitions in the PIA could be explained if the singlet were as strongly bound as in PDAs. All these properties are important in the electroluminescence processes discussed in Section V.C. 

We are therefore left with the possibility that triplet excitons are formed in this polydiacetylene and that the 1.24 eV signal results from a triplet-triplet transition, in agreement with previous findings in other PDA single-crystals. The observed laser intensity dependence may in fact indicate the presence of both monomolecular and bimo-lecular decays. This is consistent with excited-states spontaneous relaxation of the triplet exciton and bimolecular annihilation processes of triplet excitons as observed in many organic molecular crystals as well as in PDA-TS single crystal... 

On the basis of the overall results, an energy diagram for CT-allowing AN-s-BPDA-AN can be depicted as shown in Fig. 19. In the proposed mechanism, the intermolecular CT fluorescence emission occurs via two different pathways first, the photoinduced electron transfer from a local excited state at the biphenyldiimide unit to the spatially adjacent ground-state PDA residue, and second, the direct excitation at the CT absorption band. The first process is possible even at very low CTC concentration as in the PI film cured at a low temperature such as 200°C. Such a photoinduced electron transfer mechanism will be theoretically discussed again later. In fully aromatic s-BPDA-PDA, both the fluorescence from Si (tt, tt ) and phosphorescence from Ti (tt, tt ) are not observed practically. The results are probably attributed to the considerably fast CT process from Si (it, tt ). 

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