Photophysical properties of discogenic triaryl pyrylium salts Excimer migration in columnar liquid crystals

The photophysical properties of a homologous series of 2, 4-6-triaryl pyrylium tetrafluoroborates substituted by six alkoxy chains (C_nH_2n+1O, n = 2, 3, 4, 5, 8, 12) are reported. in dilute solution, the electronic absorption and fluorescence spectra do not depend on the length of the alkoxy chains while both fluorescence lifetimes and fluorescence quantum yields increase when the chain length increases. Monomer and excimer fluorescence is observed with the pure compounds; the steady state and time resolved emission spectra and the decay kinetics which do not depend on the chain length suggest that the columnar structure exist with very short lateral chains. Excimer lifetimes determined in the liquid-crystalline phases follow an exponential law as a function of the reciprocal temperature: τ ∝ exp (E_M/RT). The activation energy for excimer migration is 0.046 eV, 0.061 eV and 0.087 eV respectively for n = 5, 8 and 12.