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

We have comparatively investigated the charge transfer (CT) dynamics of a series of covalently linked <i>N,N</i>-dimethylaniline (DMA) and pyrenyl derivatives (<b>N1</b>, <b>N2C</b>, <b>N2T</b>, <b>N3</b>, and <b>N4</b>, where <b>1, 2, 3,</b> and <b>4</b> represent the number of DMA groups, and C and T are abbreviations for <i>cis</i> and <i>trans</i>, respectively). Their CT characters have been estimated in the order of <b>N1</b> > <b>N2C</b> ≅ <b>N2T</b> > <b>N3</b> > <b>N4</b> 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 <b>N2T</b> and <b>N4</b> are relatively constant in comparison with those of <b>N1</b>, <b>N2C</b>, and <b>N3</b>. These results indicate that the CT characters of <b>N1</b>, <b>N2C</b>, <b>N2T</b>, <b>N3</b>, and <b>N4</b> molecular systems are strongly influenced by the number and position of donor (DMA) groups onto the acceptor (pyrene) moiety