Bond relaxation

IV. Semiconducting Polymers Electronic Structure and Bond Relaxation in Excited States... 

In the spirit of the goal of this review, we focus on those aspects of the science of conjugated polymers that make them unique as NLO materials i.e. on the role of bond relaxation in the excited state (soliton and polaron formation) in the NLO response of conjugated polymers. As emphasized in Section IV, when photoexcited, bond relaxation in the excited state leads to the formation of electronic states within the energy gap of the semiconductor. These gap states change the optical properties of the polymer (photoinduced absorption). In this sense, semiconducting polymers are inherently nonlinear in their optical response. This process is shown schematically in Fig. VE-1. 

When semiconducting polymers are photoexcited, bond relaxation in the excited state leads to the formation of electronic states within the energy gap of the... 

These results seem Incompatible with the assumption of a "crankshaft-like motion". We believe that they can be understood if we asstime that the transition from the cis to the trans form does not take place In a single step but rather by a large number of oscillations around the bond angle by which the transition state Is approached. If we then Impede these oscillations by Incorporating the azobenzene group into a polymer chain, we reduce equally the rate at which the transition state Is approached and the rate at which a strained bond relaxes to its initial shape. 

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