Designing and constructing multichromophoric, artificial light-harvesting antennas with controlled interchromophore distances, orientations, and defined donor-acceptor ratios to facilitate efficient unidirectional energy transfer is extremely challenging. Here, we demonstrate the assembly of a series of structurally well-defined artificial light-harvesting triads based on the principles of structural DNA nanotechnology. DNA nanotechnology offers addressable scaffolds for the organization of various functional molecules with nanometer scale spatial resolution. The triads are organized by a self-assembled seven-helix DNA bundle (7HB) into cyclic arrays of three distinct chromophores, reminiscent of natural photosynthetic systems. The scaffold accommodates a primary donor array (Py), secondary donor array (Cy3) and an acceptor (AF) with defined interchromophore distances. Steady-state fluorescence analyses of the triads revealed an efficient, stepwise funneling of the excitation energy from the primary donor array to the acceptor core through the intermediate donor. The efficiency of excitation energy transfer and the light-harvesting ability (antenna effect) of the triads was greatly affected by the relative ratio of the primary to the intermediate donors, as well as on the interchromophore distance. Time-resolved fluorescence analyses by time-correlated single-photon counting (TCSPC) and streak camera techniques further confirmed the cascading energy transfer processes on the picosecond time scale. Our results clearly show that DNA nanoscaffolds are promising templates for the design of artificial photonic antennas with structural characteristics that are ideal for the efficient harvesting and transport of energy.

The past few years have witnessed significant progress toward the design of supramolecular architectures of nano-to micrometer dimensions with intriguing properties. [1,2] Of particular interest to chemists is the self-assembly of rigid pconjugated molecules with controlled size and shape, as active components of organic electronic devices. [3, 4] In this context, self-assembly of oligo(p-phenyleneethynylene)s (OPEs) has received considerable attention.[5] Control of the morphology of self-assembled structures requires rational design of molecular components.[6] For example, spherical to tubular or cylindrical assemblies are obtained by structural variation of the self-assembling units, [7] whereas nonhelical to helical transformation can be achieved by attaching chiral handles [8][9][10][11] or by the "sergeant-and-soldiers" coassembly approach. [12][13][14] However, transitions of linear p-conjugated molecules from vesicles to helical tubules or vice versa are rare.[15] Herein we reveal a spectacular chirality-amplification effect and an unprecedented transition from vesicles to helical tubules during the sergeant-and-soldiers coassembly of chiral and achiral OPEs.We chose OPE1 and its chiral analogue OPE2 for our studies. These compounds were synthesized by palladiumcatalyzed Sonogashira-Hagihara cross-coupling reactions [16] and characterized by 1 H NMR, 13 C NMR, and FAB-MS techniques.[17] Compound OPE1 self-assembles in nonpolar hydrocarbon solvents to form nanoparticles, microspheres, giant superstructures, and blue-light-emitting organogels. [18] Surprisingly, OPE2 failed to form aggregates in nonpolar hydrocarbon solvents, as evident from absorption, emission, and CD spectral features. This behavior of OPE2 is different from that of an analogous chiral oligo(p-phenylenevinylene), OPV, for which formation of helical structures in decane leads to gelation. [19] These observations reveal the differences in the self-organization propensities of OPEs and OPVs, which reiterate that a subtle difference in p interaction is sufficient to induce a large difference in the morphological features.The UV/Vis absorption spectrum of OPE1 in decane (1 10 À5 m) at 258C exhibited a maximum at 384 nm with a shoulder at 420 nm. At this concentration, a decrease in the intensity of the p-p* transition band with first-order kinetics (k = 47.41 min À1 ) is observed over 6 h (Figure 1 a). Interestingly, addition of OPE2 (5-30 mol %) to this solution at a total concentration of 1 10 À5 m resulted in reversal of the absorption, which indicates probable destruction of the initial OPE1 aggregates to form coassembled or molecularly dissolved species. On the other hand, addition of OPE2 (5-30 mol %) to a fresh solution of OPE1 in decane (without aging) at a total concentration of 0.7 10 À5 m led to a decrease in the intensity of the aggregate band at 420 nm.[17] After heating to 80 8C and cooling to room temperature, this solution did not show any considerable change in the absorption spectrum, that is, the time-dependent process is ...

Let's twist again: Transcription of molecular chirality from common stereogenic centers to amplified supramolecular chirality during the coassembly of gel‐forming π‐conjugated molecules by inverted helicity results in the formation of longitudinally fused M and P helices. This “sergeants and soldiers” approach to coassembly under stereomutation has been confirmed by CD and AFM techniques (see picture).

Self-assembled nanotapes of a few tailor-made oligo(p-phenylenevinylene)s (OPVs) have been prepared and used as supramolecular donor scaffold to study the fluorescence resonance energy transfer (FRET) to a suitable acceptor. In nonpolar solvents, FRET occurs with nearly 63-81% efficiency, exclusively from the self-assembled OPVs to entrapped Rhodamine B, resulting in the quenching of the donor emission with concomitant formation of the acceptor emission at 625 nm. The efficiency of FRET is considerably influenced by the ability of the OPVs to form the self-assembled aggregates and hence could be controlled by structural variation of the molecules, and polarity of the solvent. Most importantly, FRET could be controlled by temperature as a result of the thermally reversible self-assembly process. The FRET efficiency was significantly enhanced (ca. 90%) in a xerogel film of the OPV1 which is dispersed with relatively less amount of the acceptor (33 mol %), when compared to that of the aggregates in dodecane gel. FRET is not efficient in polar solvents due to weak self-organization of the chromophores. These results indicate that energy transfer occurs exclusively from the self-assembled donor and not directly from the individual donor molecules. The present study illustrates that the self-assembly of chromophores facilitates temperature and solvent controlled FRET within pi-conjugated nanostructures.

Nucleic acids have been emerging as supramolecular structural scaffolds for the helical organization of chromophores in the creation of functional nanomaterials mainly because of the their unique structural features and synthetic accessibility. A large number of chromophores have been successfully incorporated into DNA or RNA as C-nucleosides, as base surrogates or as modified sugars using solid phase phosphoramidite chemistry. Moreover, multiple incorporations yield the helical organization of the chromophores inside or outside the DNA or RNA double helix depending upon the conjugation of the chromophores. Significant photophysical interactions are observed in the chromophore stacks resulting in unique optical properties that are significantly different from the monomer properties. In this feature article, multichromophore labelled nucleic acids are reviewed with special emphasis on the self-assembly induced modulation of the optical properties.