Ultrafast Exciton Dynamics in a Branched Molecule Investigated by Time-Resolved Fluorescence, Transient Absorption, and Three-Pulse Photon Echo Peak Shift Measurements

Abstract

The excited-state dynamics of a branched octupolar molecule (tris-4,4‘,4‘ ‘-(4-nitrophenilethynyl)triphenylamine (T-NPTPA)) with charge-transfer character is investigated by complementary measurements of time-resolved fluorescence, transient absorption, and three-pulse photon echo peak shift. The data are compared with those obtained for the linear molecule dimethylnitroaminotolane (DMNAT) representing a linear building block of the octupolar system. Time-resolved fluorescence as well as transient absorption experiments showed the presence of an intermediate C3 symmetry state for the trimer with a depolarization time faster than ∼50 fs. Three-pulse photon echo peak shift measurements for the trimer revealed a large initial fast component of the peak shift decay. The initial peak shift during the first 100 fs was found to be much higher for the trimer as compared to the linear building block. This is in accordance with the time-resolved anisotropy results suggesting a different character of the initial excitation of the trimer. The higher peak shift value for the trimer suggests a smaller total coupling of electronic transition frequency to the nuclear motions, which may be the result of the delocalized character of this initial excited state. A residual peak shift (∼2 fs) at long population times was also observed and was interpreted as an indication of the residual static inhomogeneity (inhomogeneous broadening) in the system.