Perylene Diimide-Based Hj- and hJ-Aggregates: The Prospect of Exciton Band Shape Engineering in Organic Materials

Abstract

The exciton band dispersion in π-stacks of conjugated organic chromophores is a critical factor in determining the photophysical response and transport properties. In such stacks, the exciton band width and, in particular, the curvature at the band center, is determined by an interference between short-range coupling due to wave function overlap and long-range Coulomb coupling arising from transition dipole− dipole interactions. The interference can be completely destructive, yielding a dispersionless "flat" band resulting in an unusual situation where the aggregate displays monomer-like properties, despite having closely spaced chromophores. Coupled chromophores such as these are called "null aggregates" and the perfect balance of interactions that leads to them are referred to as "null points". Here, we study two perylene diimide (PDI) derivatives where positive long-range coupling induces H-aggregate behavior, whereas counteracting short-range coupling induces J-aggregate behavior. As such, both derivatives display so-called HJ-aggregate properties but are shown here to straddle a null point. In N-phenyl PDI π-stacks, the stronger Coulomb coupling tilts the scales in favor of overall H-like behavior resulting in Hj-aggregates, characterized by a weak 0−0 vibronic photoluminescence (PL) peak, which increases with temperature. By contrast, in tetraphenyl PDI π-stacks, the short-range coupling dominates, resulting in hJ-aggregates, as characterized by dominant 0−0 emission. Furthermore, in tetraphenyl PDI, the 0−0/0−1 PL ratio remains approximately twice the monomer value, independent of temperature, indicating strong Peierls-like dimerization. Identifying the null points in PDI derivatives provides reference geometries for band shape engineering through, for example, chemically induced or pressureinduced changes in molecular packing.

Safeguarding long-lived excitons from excimer traps in H-aggregated dye-assemblies

“…The E 0 → E 2 (0°) spectrum shows a strong H-type character with a considerably reduced A 0–0 / A 0–1 ratio of 0.81, while the E 0 → E 1 (90°) spectrum has a relatively weaker H-character with a higher A 0–0 / A 0–1 ratio of 1.15. Similar contrasting vibronic signatures have been reported in aggregates, where a combination of dominant long-range coulombic coupling and a relatively weaker short-range charge transfer interaction gives rise to an Hj-type coupling, 19 as well as in systems where exciton-coupled chromophores are related through a C 2 rotation. 20 From the energy difference between the lowest-energy vibronic peaks of the two transitions at 543 and 583 nm, respectively, we estimate a 2 J of ∼1265 cm –1 .…”

Ring fusion in tetrathienylethene cored perylene diimide tetramers affords acceptors with strong and broad absorption in the near-UV to visible region

“…[43][44][45] Since aggregation is inhibited by the non-planar structure of the molecules, the high I 0-1 (450 nm)/I 0-0 (500 nm) = 1.15 of FTTE-PDI4 in films thus suggests a degree of long-range coupling of the PDI chromophores possibly related to the rigid arrangement of the PDI units leading to H-aggregation of the PDI chromophores. 46 Notably, fused FTTE-PDI4 also shows higher absorptivity in the 300-450 region nm and a blueshifted absorption onset compared to TTE-PDI4, resulting in a higher band gap of 2.04 compared to 1.90 eV (calculated by the crossing point between absorption and emission, Fig. S6, ESI †), in agreement with the theoretical calculations.…”

Perylene Bisimide Cyclophanes: Structure–Property Relationships upon Variation of the Cavity Size

“…28b For a perfect H-aggregate, the 0-0 fluorescence band is suppressed since the transition from the lowest excited state is forbidden. 28b However, cyclophanes 3a-e all show an intense 0-0 emission band, which can be caused by a rotational displacement of the chromophores, 30 thermal activation of higher allowed excited states, 31 and/or by disorder, 32 presumably arising from the conformational flexibility of the bay substituents. For cyclophanes 3c-e also the fluorescence quantum yields remain high (88-94%), whilst the fluorescence quantum yields are significantly quenched for cyclophanes 3a and 3b (Table 1 and Figure 4b).…”