Chromophore aggregation strongly impacts the efficiency of organic photovoltaics (OPVs). Perylene-3,4:9,10-bis­(dicarboximide) (PDI)-based electron acceptors have been shown to be excellent alternatives to fullerenes in OPVs, provided their supramolecular assemblies do not form excimers. In order to study this phenomenon in a controlled fashion, we have prepared two PDI derivatives that were incorporated into an anodic aluminum oxide (AAO) membrane. In one system, the PDI molecule has an n-propyl silatrane attached to one of its imide nitrogens, while a 12-tricosanyl group is attached to the other imide nitrogen. The silatrane reacts with the AAO surface to covalently bind the PDI. The other PDI has 12-tricosanyl groups on both imide nitrogens, which intercalate with n-octadecylsilane chains covalently bound to an AAO membrane. Because aluminum oxide is a wide band gap semiconductor, photoexcitation of PDI does not result in charge injection into the AAO membrane; thus, the intrinsic electronic properties of the aggregated PDI molecules within the membrane can be studied. Both PDI derivatives form excimers upon photoexcitation with and without the solvent in the AAO membrane pores, which display increasing charge transfer character with increasing solvent polarity. Because the AAO membrane allows for any choice of solvent to be infiltrated into its pores, the PDI photophysics can be modulated over an arbitrary range of solvent polarities, irrespective of whether PDI is soluble in a particular solvent. The results presented here show how to tune the intermolecular interactions of PDI and related rylene dyes attached to walls of the AAO pores to understand the intermediate regime between solution and the solid state.

Excimers usually serve as low energy trap sites in supramolecular chromophore assemblies; however, if the trap is not too deep, excimers may diffuse throughout the structure, making it possible to deliver excitation energy to distant sites. To investigate this phenomenon, a supramolecular assembly of 9,10-bis(phenylethynyl)anthracene (BPEA) chromophores was prepared by covalently linking a BPEA molecule to the walls of nanoporous anodic aluminum oxide (AAO) membranes. The BPEA molecules self-associate and form excimers upon photoexcitation. Excimer formation in the BPEA assemblies on the AAO membranes is a multistep process and involves intermolecular structural reorganization between the chromophores. Describing the system using exciton theory reveals that the BPEA excimer is mobile, despite its frequent role as a lower energy trap state. The excimer diffusivity in the BPEA on the AAO membranes is higher than that of other reported excimers, approaching that of singlet excitons in efficient organic photovoltaic systems.

Femtosecond transient absorption microscopy of organic donor–acceptor single co-crystals shows that photo-initiated charge transfer exciton diffusion as well as charge recombination rates depend critically on crystal morphology.

Triplet−triplet annihilation-based molecular photon upconversion (TTA-UC) is a photophysical phenomenon that can yield high-energy emitting photons from low-energy incident light. TTA-UC is believed to fuse two triplet excitons into a singlet exciton through several consecutive energy-conversion processes. When organic aromatic dyes�i.e., sensitizers and annihilators� are used in TTA-UC, intermolecular distances, as well as relative orientations between the two chromophores, are important in an attempt to attain high upconversion efficiencies. Herein, we demonstrate a host−guest strategy�e.g., a cage-like molecular container incorporating two porphyrinic sensitizers and encapsulating two perylene emitters inside its cavity�to harness photon upconversion. Central to this design is tailoring the cavity size (9.6−10.4 Å) of the molecular container so that it can host two annihilators with a suitable [π•••π] distance (3.2−3.5 Å). The formation of a complex with a host:guest ratio of 1:2 between a porphyrinic molecular container and perylene was confirmed by NMR spectroscopy, mass spectrometry, and isothermal titration calorimetry (ITC) as well as by DFT calculations. We have obtained TTA-UC yielding blue emission at 470 nm when the complex is excited with low-energy photons. This proof-of-concept demonstrates that TTA-UC can take place in one supermolecule by bringing together the sensitizers and annihilators. Our investigations open up some new opportunities for addressing several issues associated with supramolecular photon upconversion, such as sample concentrations, molecular aggregation, and penetration depths, which have relevance to biological imaging applications.

Singlet fission (SF) is a spin-allowed process in which a photogenerated singlet exciton down-converts into two triplet excitons. Perylene-3,4-dicarboximide (PMI) has singlet and triplet state energies of 2.4 and 1.1 eV, respectively; thus making SF slightly exoergic and providing triplet excitons that have sufficient energy to raise the efficiency of single-junction solar cells by reducing thermalization losses from hot excitons formed when absorbed photons have energies higher than the semiconductor bandgap. However, PMI SF in the solid state has not been studied previously. Here, we show that 2,5diphenyl-N-(2-ethylhexyl)perylene-3,4-dicarboximide (dp-PMI) crystallizes into a slip-stacked intermolecular morphology favorable for SF. Transient absorption microscopy and spectroscopy show that dp-PMI SF occurs in ≤50 ps in both single crystals and polycrystalline thin films with a triplet yield of 150 ± 20%. Ultrafast SF in the solid state, the high triplet yield, and its photostability make dp-PMI an attractive candidate for SF-enhanced solar cells.