Structure-property relationship of perylene bisimide macrocycles probed by atomic force microscopy and single-molecule fluorescence spectroscopy

Monitoring tautomerization of single hypericin molecules in a tunable optical λ/2 microcavity

Hypericin tautomerization that involves the migration of the labile protons is believed to be the primary photophysical process relevant to its light-activated antiviral activity. Despite the difficulty in isolating individual tautomers, it can be directly observed in single-molecule experiments. We show that the tautomerization of single hypericin molecules in free space is observed as an abrupt flipping of the image pattern accompanied with fluorescence intensity fluctuations, which are not correlated with lifetime changes. Moreover, the study can be extended to a λ/2 Fabry-Pérot microcavity. The modification of the local photonic environment by a microcavity is well simulated with a theoretical model that shows good agreement with the experimental data. Inside a microcavity, the excited state lifetime and fluorescence intensity of single hypericin molecules are correlated, and a distinct jump of the lifetime and fluorescence intensity reveals the temporal behavior of the tautomerization with high sensitivity and high temporal resolution. The observed changes are also consistent with time-dependent density functional theory calculations. Our approach paves the way to monitor and even control reactions for a wider range of molecules at the single molecule level.

Highly Strained Nanoscale Bicyclophane Monolayers Entering the Third Dimension: A Combined Synthetic and Scanning Tunneling Microscopy Investigation

Tetrabromo aromatics can be synthesized by the Fischer-Zimmermann condensation of appropriate pyrylium salts with arylene dicarboxylic acid salts. Their cyclization by intramolecular Yamamoto coupling yields strained bicyclophanes with adjustable sizes and different intraannular bridges. All compounds adsorb at the solid/liquid interface on highly oriented pyrolytic graphite (HOPG) and are investigated by scanning tunneling microscopy (STM) with submolecular resolution. The observed two-dimensional (2D) supramolecular nanopatterns depend only on the sizes and alkoxy periphery of the cyclophanes and are independent of the specific structures of the intraannular bridges. Since the central arylene moieties of the smaller species are oriented perpendicular to the planes of the bicyclophanes, their substituents protrude from the surface by up to 1.6 nm after adsorption. Therefore, these molecules are attractive platforms for addressing the volume phase above the graphite surface.

Study of the exciton dynamics in perylene bisimide (PBI) aggregates with symmetrical quasiclassical dynamics based on the Meyer–Miller mapping Hamiltonian

The exciton dynamics in one-dimensional stacked PBI (Perylene Bisimide) aggregates was studied with SQC-MM dynamics (Symmetrical Quasiclassical Dynamics based on the Meyer–Miller mapping Hamiltonian).

Fluorescence Anisotropy Reloaded-Emerging Polarization Microscopy Methods for Assessing Chromophores' Organization and Excitation Energy Transfer in Single Molecules, Particles, Films, and Beyond

Fluorescence polarization is widely used to assess the orientation/rotation of molecules, and the excitation energy transfer between closely located chromophores. Emerging since the 1990s, single molecule fluorescence spectroscopy and imaging stimulate the application of light polarization for studying molecular organization and energy transfer beyond ensemble averaging. Here, traditional fluorescence polarization and linear dichroism methods used for bulk samples are compared with techniques specially developed for, or inspired by, single molecule fluorescence spectroscopy. Techniques for assessing energy transfer in anisotropic samples, where the traditional fluorescence anisotropy framework is not readily applicable, are discussed in depth. It is shown that the concept of a polarization portrait and the single funnel approximation can lay the foundation for alternative energy transfer metrics. Examples ranging from fundamental studies of photoactive materials (conjugated polymers, light-harvesting aggregates, and perovskite semiconductors) to Förster resonant energy transfer (FRET)-based biomedical imaging are presented. Furthermore, novel uses of light polarization for super-resolution optical imaging are mentioned as well as strategies for avoiding artifacts in polarization microscopy.

Review of fluorescent steroidal ligands for the estrogen receptor 1995-2018

The development of fluorescent ligands for the estrogen receptor (ER) continues to be of interest. Over the past 20 years, most efforts have focused on appending an expanding variety of fluorophores to the B-, C- and D-rings of the steroidal scaffold. This review highlights the synthesis and evaluation of derivatives substituted primarily at the 6-, 7α- and 17α-positions, culminating with our recent work on 11β-substituted estradiols, and proposes an approach to new fluorescent imaging agents that retain high ER affinity.

Macrocyclic poly(p-phenylenevinylene)s by ring expansion metathesis polymerisation and their characterisation by single-molecule spectroscopy

Ring expansion metathesis polymerisation (REMP) has proven to be a viable approach to prepare high purity macrocyclic phenylenevinylene polymers.

Ring expansion metathesis polymerisation (REMP) has proven to be a viable approach to prepare high purity cyclic polymers. Macrocyclic polymers with a fully conjugated defect free backbone are of particular interest as these polymers have no end groups that can act as charge traps. In this work soluble macrocyclic poly(p-phenylenevinylene)s (cPPVs) have been prepared directly via the REMP of substituted paracyclophanedienes. Single-molecule spectroscopy of the two topological forms of PPV i.e., linear (lPPV) and cyclic (cPPV) revealed that lPPV exists in an extended conformation whereas the cPPV adopts a restricted ring-like conformation. Despite such large differences in the chain conformation, the spectral properties of the two compounds are unexpectedly very similar, and are dominated by torsional deformations in relatively short conjugated segments.

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.

Organized Aggregation Makes Insoluble Perylene Diimide Efficient for the Reduction of Aryl Halides via Consecutive Visible Light-Induced Electron-Transfer Processes

The consecutive photo-induced electron-transfer (conPET) process found with perylene diimide (PDI) overcomes the limitation of visible-light photocatalysis and sheds light on effective solar energy conversion. By the incorporation of PDI into a metal-organic polymer Zn-PDI, a heterogeneous approach was achieved to tackle the poor solubility and strong tendency to aggregate of PDIs that restricted the exploitation of this outstanding homogeneous process. The interplay between metal-PDI coordination and π···π stacking of the organized PDI arrays in Zn-PDI facilitates the conPET process for the visible light-driven reduction of aryl halides by stabilizing the radical-anion intermediate and catalyst-substrate interacted moiety. These synergistic effects between the PDI arrays and Zn sites further render Zn-PDI photoactivity for fundamental oxidation of benzyl alcohols and amines. The tunable and modular nature of the two-dimensional metal-organic polymers makes the catalyst-embedding strategy promising for the development of ideal photocatalysts toward the better utilization of solar energy.

Spectroscopic and electrochemical behavior of newly synthesized high fluorescent symmetric 4'-nitrophenyl-3,4,9,10-perylenebisdiimide-azo hybrid dyes

The investigation has been made in the synthesis of azo hybrid rylene dyes. The hybridization of perylene bis-diimide with phenolic azo-dyes was carried out by the nucleophilic substitution (SNAr) reaction of tetrachloroperylene-3,4,9,10-bisdiimide 3 with phenolic azo-dyes 4a-g in basic medium. The hybrid dyes exhibited two absorption maxima λmax in the range 300-350, 426-438 nm in ethanol due to presence of azo linkage and highly conjugated framework of π bonds. Fluorescence spectra of these dyes in water showed sharp emission peaks with small bandwidths in the range 490-495 nm, and fluorescence quantum yield was 0.71-0.83 in comparison with standard reference fluorescein. The structures of perylene-azo dyes were elucidated by FTIR and NMR spectroscopy. Luminescence was determined by LS-100 meter which was found to be excellent in limits 0.208-0.239 cd/m(2). Cyclic voltammetric studies were made by Electrochemical Analyzer CH1830C which showed the oxidation chemical potential of these hybrid dyes.

Mesoscopic quantum emitters from deterministic aggregates of conjugated polymers

An appealing definition of the term "molecule" arises from consideration of the nature of fluorescence, with discrete molecular entities emitting a stream of single photons. We address the question of how large a molecular object may become by growing deterministic aggregates from single conjugated polymer chains. Even particles containing dozens of individual chains still behave as single quantum emitters due to efficient excitation energy transfer, whereas the brightness is raised due to the increased absorption cross-section of the suprastructure. Excitation energy can delocalize between individual polymer chromophores in these aggregates by both coherent and incoherent coupling, which are differentiated by their distinct spectroscopic fingerprints. Coherent coupling is identified by a 10-fold increase in excited-state lifetime and a corresponding spectral red shift. Exciton quenching due to incoherent FRET becomes more significant as aggregate size increases, resulting in single-aggregate emission characterized by strong blinking. This mesoscale approach allows us to identify intermolecular interactions which do not exist in isolated chains and are inaccessible in bulk films where they are present but masked by disorder.

Temporal Fluctuations in Excimer-Like Interactions between π-Conjugated Chromophores

Inter- or intramolecular coupling processes between chromophores such as excimer formation or H- and J-aggregation are crucial to describing the photophysics of closely packed films of conjugated polymers. Such coupling is highly distance dependent and should be sensitive to both fluctuations in the spacing between chromophores as well as the actual position on the chromophore where the exciton localizes. Single-molecule spectroscopy reveals these intrinsic fluctuations in well-defined bichromophoric model systems of cofacial oligomers. Signatures of interchromophoric interactions in the excited state--spectral red shifting and broadening and a slowing of photoluminescence decay--correlate with each other but scatter strongly between single molecules, implying an extraordinary distribution in coupling strengths. Furthermore, these excimer-like spectral fingerprints vary with time, revealing intrinsic dynamics in the coupling strength within one single dimer molecule, which constitutes the starting point for describing a molecular solid. Such spectral sensitivity to sub-Ångström molecular dynamics could prove complementary to conventional FRET-based molecular rulers.

Intramolecular Interactions of Highly π-Conjugated Perylenediimide Oligomers Probed by Single-Molecule Spectroscopy

Highly π-conjugated perylenediimide (PDI) oligomers are promising low band gap organic materials for various applications in optoelectronics. In this work, individual fluorescence dynamics of ethynylene- and butadiynylene-bridged dimeric and trimeric PDIs (PEP, PBP, and PEPEP) were monitored and analyzed by single-molecule fluorescence spectroscopy to gain information on the degree of extension of π-conjugation through the acetylene bridge in PDI multichromophores. The simultaneous measurements of fluorescence intensity, lifetime, and spectrum indicate a sequential decrease in π-conjugation upon photobleaching of PDI monomer units. Furthermore, Huang-Rhys (HR) factors, S, are obtained to evaluate the degree of electronic coupling in view of π-conjugation and overall rigidity between the PDI units in PDI oligomers at the single-molecule level. In addition, butadiynylene-bridged dimeric PDI (PBP) reveals conformational heterogeneity due to the long butadiynylene linker. These results suggest a new way to control the photophysical properties of the PDI multichromophoric system by expansion of π-conjugation and modification with different linker groups.

Reconstruction of the Molecular Structure of a Multichromophoric System Using Single-Molecule Defocused Wide-Field Imaging

Single-molecule defocused wide-field imaging (DWFI) has been demonstrated to be useful to determine molecular structure parameters, such as the orientations of transition dipole moments and the angular relationships between chromophores in multichromophoric molecular systems. For a series of acetylene-linked perylene bisimide (PBI) macrocycles with different ring size comprising three to six PBI dyes, we reconstructed the molecular structure of the multichromophoric system using DWFI method. Furthermore, we revealed that the structural heterogeneities and distortions depend on the ring size. Our findings illustrate the use of DWFI to gain deeper insight into the structure-property relationships of artificial light-harvesting molecular systems.

Microscopic evidence of "necklace and bead"-like morphology of polymer-surfactant complexes: a comparative study on poly(vinylpyrrolidone)-sodium dodecyl sulfate and poly(diallyldimethylammonium chloride)-sodium dodecyl sulfate systems

Here, we report the microscopic evidence of "necklace and bead"-like morphology, which has long been the most widely accepted model for polymer-surfactant complexes. The lack of microscopic evidence of the initial complexation between surfactant and polymer has resulted in many contradictory reports in the literature. In this paper, we visualized these initial complexes formed between negatively charged surfactant sodium dodecyl sulfate (SDS) with neutral poly(vinylpyrrolidone) (PVP) and cationic poly(diallyldimethylammonium chloride) (PDADMAC) polymer through photoluminescence (PL) microscopy and atomic force microscopy (AFM) using silicon quantum dot (Si QD) as an external PL marker. It is observed that, for the PVP-SDS system, SDS molecules bind at the hydrophobic sites on the random-coiled PVP chain through their hydrocarbon tails, while for the PDADMAC-SDS system, SDS head groups are associated with the positively charged nitrogen centers of the polymer, where the polymer chain wraps around the surfactant head groups.

Stepwise decrease of fluorescence versus sequential photobleaching in a single multichromophoric system

For individual molecules from the newly synthesized calix[4]arene tethered perylene bisimide (PBI) trimer, we studied the emitted fluorescence intensity as a function of time. Owing to the zigzag arrangement of PBI dyes in these trimers, the polarization state of the emission provides directly information about the emitting subunit within the trimer. Interestingly, we observed emission from all neutral subunits within a trimer rather than exclusively from the subunit with the lowest site energy. This can be understood in terms of thermally activated uphill energy transfer that repopulates the higher energetic chromophores. Together with the simultaneously recorded polarization-resolved emission spectra, this reveals that the emission from a multichromophoric system is governed by a complex interplay between the temporal variations of the photophysical parameters of the subunits, bidirectional hopping processes within the trimer, and unavoidable photobleaching. Moreover, it is demonstrated that the typically observed stepwise decrease of the signal from a multichromophoric system does not necessarily reflect sequential bleaching of the individual chromophores within the macromolecule.