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1.3 PHYSICAL SCIENCES
A series of pi(п) conjugated oligomers containing 1 to 6 monomer units were studied by absorption and photoluminescence spectroscopies. The results are discussed and examined with regard to the variation of the optical properties with the increase of effective conjugation length. It was found that there was a linear relationship between the positioning of the absorption and photoluminescence maxima plotted against inverse conjugation length. The relationships are in good agreement with the simple particle in a box method, one of the earliest descriptions of the properties of one-dimensional organic molecules. Variations in the infinite chain limit band gap for a range of oligomeric systems are correlated with backbone structure and bond alternation parameters. In addition to the electronic transition energies, it was also observed that the Stokes shift exhibited a power dependence with inverse conjugation length, implying a correlation between the electron-vibrational coupling and chain length. This correlation is extended to relative luminescence yields and is examined via Raman spectrosocopy. There is a clear indication that the vibrational activity and thus by extension nonradiative decay processes are controllable through molecular structure. The implications in terms of the design of molecular materials for optical applications are discussed. The structure property relationships models are further used for the investigation of novel organic conjugated polymers. The polymers used are a sequential family of PPV derivatives containing a systematic alternation of the п conjugated back bone by the inclusion of acene units of differing number of repeat units. The substitution consisted of the insertion of higher order acene units into the polymer back-bone, maintaining the side chains in the same relative structural position. It was established that the absorption and emission are a trade-off between the electronic properties of the more conjugated naphthyl and anthryl units and their reduced contribution across the connecting vinyl bond. The results are discussed and examined with regard to the variation of the optical properties with increasing total electron affinity, a parameter which represents the total electron affinities of the backbone constituents (EAtotal). It was found that there was a linear relationship between the positioning of the absorption and the photoluminescence maxima plotted against EAtotal). The vibrational correlations observed in the simple oligomeric systems are extended to show that the vibrational characteristics of complex polymers can be examined using Raman spectroscopy. It is shown that in the polymer system the Raman spectrum and thus electron vibrational coupling is dominated by modes along the conjugated backbone. These modes are suppressed by the introduction of the larger, electron retaining and more rigid naphthyl and anthryl units. It is shown that the dominant effect is the reduced electronic coupling across the linking vinyl bond. However it is also established that the mismatching of the vibrational frequencies of the components of the conjugated back-bone contributes to the reduction of the electron vibrational coupling of the system. Finally the fluorescence yield was shown to be optimised by the reduction in the vibrational intensity. The vibrational study elucidates the benefit of reducing the pathways of non-radiative decay and illustrates how fluorescence yield can be optimised through the progressive restriction of the available vibrational modes. To further elucidate the change in the non-radiative rate as a function of the continual substitution, the Strickler-Berg equation was used to calculate the radiative rate and hence, using well-known photophysical equations, the non-radiative rate. The radiative rate was shown to be the dominate factor in the increase of the fluorescence yield and shown to be proportional to the integrated Raman intensity. TCSPC was employed as a further tool to investigate the transient process and their variation with structural changes.
O'Neill, Luke. (2008). Structure property relationships in organic conjugated systems. Technological University Dublin. doi:10.21427/D7RP5T