Structure and dynamics of the triplet state of oligothiophenes in isotropic and partially oriented matrices

The photoexcited triplet states of 4,4'-dipentoxy-2,2'-dithiophene (4-T2), 3,3'-dipentoxy-2,2'-dithiophene (3-T2), and 4,4'''-dipentoxy-2,2':5',2'':5'',2'''-tetrathiophene (4-T4) have been investigated by time-resolved electron paramagnetic resonance in glassy toluene and in a frozen oriented liquid crystal, which provides a partially ordered medium. The preferential orientation of the rod-like 4-T2 and 4-T4 is compared to that of the disk-like 3-T2. The use of an oriented matrix coupled to simple semiempirical calculations allowed us to determine the orientation of the principal axes of the fine interaction with respect to the molecular axes. The motional behavior of the molecules in the isotropic and anisotropic matrices has been studied by comparing the spectral profiles of the Echo detected EPR (Echo-EPR) spectra with those of the continuous wave time resolved EPR (TR-EPR). A model considering the modulation of the zero-field splitting (ZFS) by molecular libration accounts for the Echo-EPR lineshape, on the basis of motions around preferential axes depending on the embedding matrix. The differences in the ZFS parameters of the two isomers 4-T2 and 3-T2 are attributed to a mesomeric effect due to the substituents.