Characterization of Ultrafast Intramolecular Charge Transfer Dynamics in Pyrenyl Derivatives: Systematic Change of the Number of Peripheral N,N-Dimethyaniline Substituents

We have comparatively investigated the charge transfer (CT) dynamics of a series of covalently linked N,N-dimethylaniline (DMA) and pyrenyl derivatives (N1, N2C, N2T, N3, and N4, where 1, 2, 3, and 4 represent the number of DMA groups, and C and T are abbreviations for cis and trans, respectively). Their CT characters have been estimated in the order of N1 > N2C congruent to N2T > N3 > N4 by an increase of Stokes shift with increasing the solvent polarity. The femtosecond time-resolved fluorescence spectra show very early excited-state dynamics, a transition from the initially populated locally excited (LE) state to the CT state. The spectral analysis reveals that CT stabilization processes of N2T and N4 are relatively constant in comparison with those of N1, N2C, and N3. These results indicate that the CT characters of N1, N2C, N2T, N3, and N4 molecular systems are strongly influenced by the number and position of donor (DMA) groups onto the acceptor (pyrene) moiety.