Reorganization of perylene bisimide J-aggregates: from delocalized collective to localized individual excitations

Regulating steric hindrances of perylenediimide to construct NIR photothermal J-aggregates with a large red-shift

Perylene diimide (PDI)-based photothermal agents (PTAs) possess excellent stability and high photothermal conversion efficiency. However, developing PDIs with strong near-infrared absorption under biological conditions remains a challenge. In this study, we introduce a novel approach to facilitate the formation of J-aggregate-based PTAs with significantly red-shifted absorption by modulating steric hindrances of PDIs. PDIA, featuring larger steric hindrances at the bay position and smaller steric hindrances at the imide position, self-assembles into J-aggregates which exhibit a remarkable red-shift of over 100 nm. After encapsulation by DPSE-PEG, PDIA nanoparticles (PDIA-NPs) demonstrated a uniform and stable size, while retaining their significant red-shift. In vitro experiments demonstrated the great potential of PDIA-NPs in photothermal therapies for tumors and thrombi under 808 nm laser irradiation. This research provides valuable insights into the design of stable J-aggregates based on PDIs suitable for biological applications, paving the way for the development of more effective PTAs.

Deciphering Spectroscopic and Structural Insights into the Photophysical Behavior of 2,2'-Dipyridylamine: An Efficient Environment Sensitive Fluorescence Probe

Excited state deactivation properties and the effects of solvent hydrogen bonding (HB) on the photophysical behavior of 2,2'-dypyridylamine (DPyA) were investigated by steady state and time-resolved fluorescence experiments, molecular docking, and density functional theory (DFT) calculations. In addition to the polarity effect, the contributions of solvent HB donation (HBD) acidity and HB acceptance (HBA) basicity to modulate the solvatochromic spectral properties were estimated from multiparametric linear regression analysis using Kamlet-Taft (KT) and Catalán formalisms. The importance of C-N bond torsion, leading to the trans → cis conversion, was manifested by substantial increase in DPyA fluorescence yield in the presence of cyclodextrin (CD) and glycerol. The unusually low fluorescence yield in aqueous medium was explained on the basis of synergistic effect of solvent hydrogen bonding combined with excited state conformational isomerization, which renders DPyA to be an excellent environment sensitive fluorescence reporter. The experimental results were verified with structural insights obtained from DFT calculations at B3LYP/6-311++G(d,p) level and construction of potential energy surface (PES) in the ground state as well as in the excited states.

Supramolecularly Engineered J-Aggregates Based on Perylene Bisimide Dyes

The discovery of the self-assembly of cyanine dyes into J-aggregates had a major impact on the development of dye chemistry due to the emergence of new useful properties in the aggregated state. The unique optical features of these J-aggregates are narrowed, bathochromically shifted absorption bands with almost resonant fluorescence with an increased radiative rate that results from the coherently coupled molecular transition dipoles arranged in a slip-stacked fashion. Because of their desirable properties, J-aggregates gained popularity in the field of functional materials and enabled the efficient photosensitization of silver halide grains in color photography. However, despite a good theoretical understanding of structure-property relationships by the molecular exciton model, further examples of J-aggregates remained scarce for a long time as supramolecular designs to guide the formation of dye aggregates into the required slip-stacked arrangement were lacking.Drawing inspiration from the bacteriochlorophyll c self-organization found in the chlorosomal light-harvesting antennas of green sulfur bacteria, we envisioned the use of nature's supramolecular blueprint to develop J-aggregates of perylene bisimides (PBIs). This class of materials is applied in high-performance color pigments and as n-type organic semiconductors in transistors and solar cells. Combining outstanding photochemical and thermal stability, high tinctorial strength and excellent fluorescence, PBIs are therefore an ideal model system for the preparation of J-aggregates with a wide range of potential applications.In this Account, we elucidate how a combination of steric constraints and hydrogen bonding receptor sites can guide the self-assembly of PBI dyes into slip-stacked packing motifs with J-type exciton coupling. We will discuss the supramolecular polymerization of multiple hydrogen-bonded PBI strands in organic and aqueous media and how minor structural modifications in monomeric PBI molecules can be used to obtain near-infrared absorbing J-aggregates, organogels, or thermoresponsive hydrogels. Pushing the boundaries of self-assembly into the bulk, engineering of the substituents' steric requirements by a dendron-wedge approach afforded adjustable numbers of helical strands of PBI J-aggregates in the columnar liquid-crystalline state and the preparation of lamellar phases. To fully explore their potential, we have studied PBI J-aggregates in collaborative work with spectroscopists, physicists, and theoreticians. In this way, exciton migration over distances of up to 180 nm was shown, and insights into the influence of static disorder on the transport of excitation energy in PBI J-aggregates were derived. Furthermore, the application of PBI J-aggregates as functional materials was demonstrated in photonic microcavities, thin-film transistors, and organic solar cells.

Enhanced Room-Temperature Photoluminescence Quantum Yield in Morphology Controlled J-Aggregates

Supramolecular assemblies from organic dyes forming J-aggregates are known to exhibit narrowband photoluminescence with full-width at half maximum of ≈9 nm (260 cm-1). Applications of these high color purity emitters, however, are hampered by the rather low photoluminescence quantum yields reported for cyanine J-aggregates, even when formed in solution. Here, it is demonstrated that cyanine J-aggregates can reach an order of magnitude higher photoluminescence quantum yield (increase from 5% to 60%) in blend solutions of water and alkylamines at room temperature. By means of time-resolved photoluminescence studies, an increase in the exciton lifetime as a result of the suppression of non-radiative processes is shown. Small-angle neutron scattering studies suggest a necessary condition for the formation of such highly emissive J-aggregates: the presence of a sharp water/amine interface for J-aggregate assembly and the coexistence of nanoscale-sized water and amine domains to restrict the J-aggregate size and solubilize monomers, respectively.

Femtosecond Transient Absorption Microscopy of Singlet Exciton Motion in Side-Chain Engineered Perylene-Diimide Thin Films

We present a statistical analysis of femtosecond transient absorption microscopy applied to four different organic semiconductor thin films based on perylene-diimide (PDI). By achieving a temporal resolution of 12 fs with simultaneous sub-10 nm spatial precision, we directly probe the underlying exciton transport characteristics within 3 ps after photoexcitation free of model assumptions. Our study reveals sub-picosecond coherent exciton transport (12-45 cm2 s-1) followed by a diffusive phase of exciton transport (3-17 cm2 s-1). A comparison between the different films suggests that the exciton transport in the studied materials is intricately linked to their nanoscale morphology, with PDI films that form large crystalline domains exhibiting the largest diffusion coefficients and transport lengths. Our study demonstrates the advantages of directly studying ultrafast transport properties at the nanometer length scale and highlights the need to examine nanoscale morphology when investigating exciton transport in organic as well as inorganic semiconductors.

Solvent Effects in Highly Efficient Light-Induced Molecular Aggregation

It has been reported that when irradiated with laser light non-resonant with the main absorption peaks, porphyrin molecules (4-[10,15,20-tris(4-sulfophenyl)-21,24-dihydroporphyrin-5-yl]benzenesulfonic acid, TPPS) in an aqueous solution become 10,000 to 100,000 times more efficient in light-induced molecular aggregation than expected from the ratio of gradient force potential to the thermal energy of molecules at room temperature. To determine the mechanism of this phenomenon, experiments on the light-induced aggregation of TPPS in alcohol solutions (methanol, ethanol, and butanol) were performed. In these alcohol solutions, the absorbance change was orders of magnitude smaller than in the aqueous solution. Furthermore, it was found that the absorbance change in the aqueous solution tended to be saturated with the increase of the irradiation intensity, but in the ethanol solution, the absorbance change increased linearly. These results can be qualitatively explained by the model in which intermolecular light-induced interactions between molecules within a close distance among randomly distributed molecules in the laser irradiation volume are highly relevant to the signal intensity. However, conventional dipole–dipole interactions, such as the Keesom interaction, are not quantitatively consistent with the results.

Improving the Quantum Yields of Perylene Diimide Aggregates by Increasing Molecular Hydrophobicity in Polar Media

Here we report the quantum yield of four aggregated perylene diimide (PDI) species that vary by the length of the branched side chains attached at the N,N' imide positions. The PDI molecules were dissolved in binary water:methanol solvents as a means to vary the solvent polarity and control the degree of aggregation in solution. By performing spectroscopy, kinetics, and light scattering experiments, the nature of the molecular interactions in the solutions was determined. The maximum quantum yield of the aggregated molecules increased from 0.04 for the shortest chain molecule (B2) to 0.20 for the largest chain molecule (B13). The higher quantum yield of B13 compared with B2 correlates well with an increase in the fluorescence lifetime. The monomer emission lifetime was 4.8 ns whereas a lifetime as high as 21.2 ns was measured for the B13 aggregate fluorescence. A shorter sub-nanosecond lifetime was also measured for suspended colloids in B5, B9, and B13. The enhanced quantum yield is attributed to an increase of disorder in the B13 aggregates. As the polarity of the solution increases, the hydrophobic effect further enhances the disorder, and, therefore, the quantum yields in these particles.

A solution-based single-molecule study of surface-bound PBIs: solvent-mediated environmental effects on molecular flexibility

We have investigated the fundamental photophysical properties of surface-bound perylene bisimide (PBI) molecules in solution at the single-molecule level.

Thrombin Ultrasensitive Detection Based on Chiral Supramolecular Assembly Signal-Amplified Strategy Induced by Thrombin-Binding Aptamer

Thrombin plays a critical role in hemostasis and hemolysis. It is of high importance to develop a system toward thrombin detection with high sensitivity and high selectivity for both research and clinical diagnosis applications. In this paper, we developed a thrombin detection assay by taking advantage of the novel signal amplified strategy based on the chiral supramolecular assembly in physiological K⁺ background. This assay could detect thrombin as low concentration as about 2 pM and provided a highly specific selectivity among several common interferences. Furthermore, the assay can discriminate thrombin from other nonspecific analogous proteins with high selectivity and can be used to detect thrombin in diluted real human serum samples, which suggested its great potential for rapid detection of thrombin in the clinic.

Nanotubular J-aggregates and quantum dots coupled for efficient resonance excitation energy transfer

Resonant coupling between distinct excitons in organic supramolecular assemblies and inorganic semiconductors is supposed to offer an approach to optoelectronic devices. Here, we report on colloidal nanohybrids consisting of self-assembled tubular J-aggregates decorated with semiconductor quantum dots (QDs) via electrostatic self-assembly. The role of QDs in the energy transfer process can be switched from a donor to an acceptor by tuning its size and thereby the excitonic transition energy while keeping the chemistry unaltered. QDs are located within a close distance (<4 nm) to the J-aggregate surface, without harming the tubular structures and optical properties of J-aggregates. The close proximity of J-aggregates and QDs allows the strong excitation energy transfer coupling, which is around 92% in the case of energy transfer from the QD donor to the J-aggregate acceptor and approximately 20% in the reverse case. This system provides a model of an organic-inorganic light-harvesting complex using methods of self-assembly in aqueous solution, and it highlights a route toward hierarchical synthesis of structurally well-defined supramolecular objects with advanced functionality.

Mechanistic insights into perovskite photoluminescence enhancement: light curing with oxygen can boost yield thousandfold

Propagation of the light-induced trap passivation reaction through a thick perovskite crystal.

Single Lévy states-disorder induced energy funnels in molecular aggregates

Using fluorescence super-resolution microscopy we studied simultaneous spectral, spatial localization, and blinking behavior of individual 1D J-aggregates. Excitons migrating 100 nm are funneled to a trap appearing as an additional red-shifted blinking fluorescence band. We propose that the trap is a Frenkel exciton state formed much below the main exciton band edge due to an environmentally induced heavy-tailed Lévy disorder. This points to disorder engineering as a new avenue in controlling light-harvesting in molecular ensembles.

Energy flow dynamics within cofacial and slip-stacked perylene-3,4-dicarboximide dimer models of π-aggregates

Robust perylene-3,4-dicarboximide (PMI) π-aggregates provide important light-harvesting and electron-hole pair generation advantages in organic photovoltaics and related applications, but relatively few studies have focused on the electronic interactions between PMI chromophores. In contrast, structure-function relationships based on π-π stacking in the related perylene-3,4:9,10-bis(dicarboximides) (PDIs) have been widely investigated. The performance of both PMI and PDI derivatives in organic devices may be limited by the formation of low-energy excimer trap states in morphologies where interchromophore coupling is strong. Here, five covalently bound PMI dimers with varying degrees of electronic interaction were studied to probe the relative chromophore orientations that lead to excimer energy trap states. Femtosecond near-infrared transient absorption spectroscopy was used to observe the growth of a low-energy transition at ~1450-1520 nm characteristic of the excimer state in these covalent dimers. The excimer-state absorption appears in ~1 ps, followed by conformational relaxation over 8-17 ps. The excimer state then decays in 6.9-12.8 ns, as measured by time-resolved fluorescence spectroscopy. The excimer lifetimes reach a maximum for a slip-stacked geometry in which the two PMI molecules are displaced along their long axes by one phenyl group (~4.3 Å). Additional displacement of the PMIs by a biphenyl spacer along the long axis prevents excimer formation. Symmetry-breaking charge transfer is not observed in any of the PMI dimers, and only a small triplet yield (<5%) is observed for the cofacial PMI dimers. These data provide structural insights for minimizing excimer trap states in organic devices based on PMI derivatives.