Cholesteric lifetime

Fluorescence studies have shown that the radiative lifetime of 25f increases upon increasing DP, suggesting that the mobile excitons move through the supramolecular polymers and relax at their ends.139 Insertion of electron acceptors between the triphenylenes accounts for the formation of longer polymers and increases the order within the column. An X-ray diffraction ring with a diffraction spacing of 3.5 A indicates a short intermolecular distance, a feature not present for undoped samples.140 A chiral electron acceptor resulted in the formation of a cholesteric mesophase. 

Following a similar approach to that of pyrene exclmer formation, activation energies for pyrene- 4. exciplex formation can be obtained from expression of Eqn. 9 in an Arrhenius form and differentiation by 1/T. ki+k2 are obtained from data taken in eyelohexane (32), and A3 and E3 from the lifetime taken at f CA1 - 0. E3 can also be obtained from the slope of the phase dependent dynamic Stern-Volmer plots. As seen in Table 1 the data from each method are in good agreement. The small differences in activation parameters measured in the cholesteric and isotropic phases probably reflect changes in viscosity that accompany phase transitions. 

In accordance with (de MeUo et al. 2006) the biexponential PL decay of CdSe QDs is generally associated with the recombination of bright and dark excitons in QDs. The lifetimes Xi and X2 were calculated for the PL of CdSe QDs in smectic and cholesteric... 

First it was argued to be due to a chiral molecular configuration characteristic of the particular type of bent-shape molecules, such as twisted or propeller shape (conformational chirality). The concept of conformational chirality was supported by simulations by Earl et al. [61], and was demonstrated by the observation that doping calamitic cholesteric liquid crystal by achiral bent-core molecules can lead to a decrease of the helical pitch, indicating an enhanced rotatory power of the mixture [62]. Unfortunately there is no proof that the decrease of the pitch is not due to a decrease of the twist elastic constant caused by the addition of bent-core units. Although the conformational chirality is usually not questioned in the solid B4 phase [20], its role has been questioned by Walba et al. [20] by arguing that these chiral conformations have very short lifetime, therefore they average out in fluid smectic, such as SmCP or SmCo phases. 

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