Controlled nanodimensional supramolecular self-assembly of tetra-alkylated naphthalene diimide derivatives.

Construction of thermodynamically stable nanostructures on the nano- to millimeter scales through noncovalent bonding plays an important role in material science. The self-assembly of tetra-alkylamino core-substituted naphthalene diimides (cNDIs) with variable alkyl chains (C8H17, C12H25, and C16H33) added on to the core leads to the formation of a variety of controlled morphologies and well-defined nanostructures. Such structures include nanorods, vesicular, belts, twisted ribbons, and donutlike morphologies (formed in CHCl3/MeOH and CHCl3/hexane mixtures) generated through solvophobic control. UV/Vis absorption and fluorescence spectroscopy demonstrate molecular aggregation in solution. Furthermore, SEM was employed to visualize the supramolecular self-assembled nanostructures. The growth of these structures is mainly due to packing of hydrophobic alkyl chains and π-π stacking of the cNDI core. The present study paves the way to rational and controlled designs of nanostructures made of optically active dyes (naphthalene diimide); this may open a new avenue towards tuning nanodimensional morphology.

Excitation wavelength dependence of the charge separation pathways in tetraporphyrin-naphthalene diimide pentads.

The excited-state dynamics of two multichromophoric arrays composed of a naphthalene diimide centre and four zinc or free-base porphyrins substituted on the naphthalene core via aniline bridges has been investigated using a combination of stationary and ultrafast spectroscopy. These pentads act as efficient antennae as they absorb over the whole visible region, with a band around 700 nm, associated with a transition to the S1 state delocalised over the whole arrays, and bands at higher energy due to transitions centred on the porphyrins. In non-polar solvents, population of these porphyrin states is followed by sub-picosecond internal conversion to the S1 state. The existence of a charge-separated state located above the S1 state could enhance this process. The decay of the S1 state is dominated by non-radiative deactivation on the 100 ps timescale, most probably favoured by the small S1-S0 energy gap and the very high density of vibrational states of these very large chromophores. In polar solvents, the charge-separated state lies just below the S1 state. It can be populated within a few picoseconds by a thermally activated hole transfer from the S1 state as well as via sub-picosecond non-equilibrium electron transfer from vibrationally hot porphyrin excited states. Because of the small energy gap between the charge-separated state and the ground state, charge recombination is almost barrierless and occurs within a few picoseconds. Despite their very different driving forces, charge separation and recombination occur on similar timescales. This is explained by the electronic coupling that differs considerably for both processes.