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 spectroscopies. 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.

The excited-state dynamics of two triads consisting of a naphthalenediimide (cNDI) substituted at the core by two zinc (ZnP) or free-base tetraphenylporphyrins (FbP) was investigated by ultrafast fluorescence and transient absorption spectroscopy. The electronic absorption spectra of the triads are almost the composites of those of the constituents, pointing to a weak electronic coupling and to a localization of the excitation energy on one of the porphyrins. In cyclohexane, the excited-state dynamics of the triads are essentially the same as those of the individual porphyrins, with the exception of the Soret emission of the ZnP triad, whose lifetime exhibits a more than 10 fold shortening compared to ZnP. A similarly ultrafast fluorescence decay was measured in tetrahydrofuran and benzonitrile. In these two solvents, charge separation from the excited porphyrin to the cNDI was found to take place with ~1 ps and ~25 ps time constants in the ZnP and FbP triads, respectively. The build up of the charge-separated state population in the ZnP triad is independent on the excitation wavelength, indicating that charge separation takes place from the lowest singlet excited state. Charge recombination occurs with a time constant around 8 ps in both triads, i.e. is slower than charge separation in the ZnP triad but faster in the FbP triad. These differences are rationalized in terms of the driving forces for charge separation and recombination.

The self-assembly of core-substituted naphthalene diimides bearing triethylene glycol motifs leads to the formation of stable vesicles in DMSO and CHCl₃/MeOH (6 : 4, v/v) solvents. The vesicles were evaluated by means of UV/vis and fluorescence spectroscopy, transmission electron microscopy, atomic force microscopy and dynamic light scattering.

The photophysics and excited-state dynamics of two dyads consisting of either a free-base or a zinc-tetraphenylporphyrin linked through a rigid bridge to a core-substituted naphthalenediimide (NDI) have been investigated by femtosecond-resolved spectroscopy. The absorption and fluorescence spectra differ substantially from those of the individual units, pointing to a substantial coupling and to a delocalisation of the excitation over the whole molecule, as confirmed by quantum chemistry calculations. A strong dependence of their excited-state dynamics on the solvent polarity has been observed. In toluene, the fluorescence quantum yield of the dyads is of the order of a few percent and the main decay channel of the emitting state is proposed as intersystem-crossing to the triplet state. However, in a medium polarity solvent like dichloromethane, the emitting state undergoes charge separation from the porphyrin to the NDI unit within 1–3 ps, and the ensuing charge-separated state recombines in about 10–20 ps. This solvent dependence can be explained by the weak driving force for charge separation in polar solvents and the large electronic coupling between the porphyrin and NDI moieties, making charge separation a solvent-controlled adiabatic process.

The objective of this study was to evaluate the possibility of photoinduced stack/rod electron transfer in surface “zipper� architectures composed of stacks of blue (B) naphthalenediimides (NDIs) along strings of oligophenylethynyl (OPE) rods. The synthesis and characterization of anionic and cationic multichromophoric OPE-B systems are reported. Absorption spectra suggest that in OPE-B systems, planarity and thus absorption and conductivity of the OPE can possibly be modulated by intramolecular stacking of the surrounding NDIs, although interfering contributions from aggregation remain to be differentiated. Among surface architectures constructed with OPE-B and POP-B systems by zipper and layer-by-layer (LBL) assembly, photocurrents generated by OPE-B zippers exhibit the best critical thickness and fill factors. These findings confirm the existence and functional relevance of topologically matching zipper architectures. In OPE-B zippers, OPEs generate much more photocurrent than the blue NDIs. Ultrafast electron transfer from OPEs to NDIs accounts for these photocurrents, providing wavelength-controlled access to rod–stack charge separation, and thus to formal supramolecular n/p-heterojunctions (SHJs). NDI excitation is not followed by the complementary hole transfer to the OPE rod. Scaffolds with higher HOMOs will be needed to integrate blue NDIs into SHJ photosystems.

The photophysical properties of multichromophoric systems consisting of eight red or blue naphthalene diimides (NDIs) covalently attached to a p-octiphenyl scaffold, as well as a blue bichromophoric system with a biphenyl scaffold, have been investigated in detail using femtosecond time-resolved spectroscopy. The blue octachromophoric systems have been recently shown to self-assemble as supramolecular tetramers in lipid bilayer membranes and to enable generation of a transmembrane proton gradient upon photoexcitation (Bhosale, S.; Sisson, A. L.; Talukdar, P.; Fürstenberg, A.; Banerji, N.; Vauthey, E.; Bollot, G.; Mareda, J.; Röger, C.; Würthner, F.; Sakai, N.; Matile, S. Science2006, 313, 84). A strong reduction of the fluorescence quantum yield was observed when going from the single NDI units to the multichromophoric systems in methanol, the effect being even stronger in a vesicular lipid membrane. Fluorescence up-conversion measurements reveal ultrafast self-quenching in the multichromophoric systems, whereas the formation of the NDI radical anion, evidenced by transient absorption measurements, points to the occurrence of photoinduced charge separation. The location of the positive charge could not be established unambiguously from the transient absorption measurements, but energetic considerations indicate that charge separation should occur between two NDI units in the blue systems, whereas both an NDI unit and the p-octiphenyl scaffold could act as electron donor in the red system. The lifetime of the charge-separated state was found to increase from 22 to 45 ps by going from the bi- to the octachromophoric blue systems in methanol, while a 400 ps decay component was observed in the lipid membrane. This lifetime lengthening is explained in terms of charge migration that is most efficient when the octachromophoric systems are assembled as supramolecular tetramers in the lipid membrane. Furthermore, the average charge-separated state lifetime of the red system in methanol is even larger and amounts to 750 ps. This effect cannot be simply explained in terms of Marcus inverted regime as the driving force for charge recombination in the red system is only slightly larger than in the blue one. A better spatial separation of the charges in the red system stemming from the localization of the hole on the p-octiphenyl scaffold could additionally contribute to the slowing down of charge recombination.

Design, synthesis and evaluation of advanced rigid-rod π-stack photosystems with asymmetric scaffolds are reported. The influence of push–pull rods on self-organization, photoinduced charge separation and photosynthetic activity is investigated and turns out to be surprisingly small overall.

Rigid p-octiphenyl rods were used to create helical tetrameric π-stacks of blue, red-fluorescent naphthalene diimides that can span lipid bilayer membranes. In lipid vesicles containing quinone as electron acceptors and surrounded by ethylenediaminetetraacetic acid as hole acceptors, transmembrane proton gradients arose through quinone reduction upon excitation with visible light. Quantitative ultrafast and relatively long-lived charge separation was confirmed as the origin of photosynthetic activity by femtosecond fluorescence and transient absorption spectroscopy. Supramolecular self-organization was essential in that photoactivity was lost upon rod shortening (from p-octiphenyl to biphenyl) and chromophore expansion (from naphthalene diimide to perylene diimide). Ligand intercalation transformed the photoactive scaffolds into ion channels.