Martin Vacha

Relating conformation and photophysics in single MEH-PPV chains.

We use a novel fluorescence polarization microscope in combination with molecular dynamics calculations to determine the conformation of individual isolated chains of the conjugated polymer MEH-PPV. We found a narrow distribution of defect cylinder conformations in a poor-solvent matrix and two types of defect coil conformations in a good-solvent matrix. The conformations were related to photophysical properties of MEH-PPV by measuring fluorescence intermittency on the same chains. We obtained direct evidence that the photophysics is determined by the chain conformations and that small changes in the polymer microscopic structure can qualitatively affect the photophysical properties.

Multimode Diffusion of Ring Polymer Molecules Revealed by a Single- Molecule Study**

Diffusion processes of synthetic polymer molecules are crucial in deciding their rheological properties, and subsequently in polymer processing and fabrication of plastics, films, and fibers. The topology of a polymer, whether linear, branched, or cyclic, can dramatically affect the motion in a dense entangled solution or in a melt. For linear and branched polymers, the reptation model has been accepted as a valid diffusion mechanism and verified experimentally by light scattering, NMR, and viscoelastic measurements. In the reptation model for linear and branched polymers, an entangled polymer diffuses in a dynamic tube confined by neighboring polymer chains. Single-molecule studies on naturally occurring macromolecules and also on synthetic polymers have demonstrated the reality of the tube and the diffusion scaling laws. The number and structure of end groups is a critical factor in the dynamics of the diffusion of linear or branched polymers. Ring polymers are, on the other hand, topologically unique by the absence of free chain ends. Therefore, their diffusion mechanism has attracted continuous attention, but is still an important challenge. Apart from cyclic DNA, a variety of synthetic ring polymers of sufficiently long chains and of guaranteed purity have recently become accessible. 21] As a result, unequivocal topology effects have now been disclosed by using custom-made ring polymers with specific segment structures and optional functional groups. Herein, we show, at the single-chain level, the direct and real-time observation of diffusion dynamics of synthetic ring and linear polymers incorporating a fluorophore. Singlemolecule spectroscopy is recognized as a powerful tool for monitoring of polymer dynamics, and is capable of revealing topology effects in their diffusion process. 32] We synthesized linear (1) and cyclic (2) poly(tetrahydrofuran)s (poly(THF)s) containing perylene diimide unit as a fluorophore by means of an electrostatic self-assembly and covalent fixation process 28] (Figure 1; see the Supporting

Large reverse saturable absorption under weak continuous incoherent light

In materials showing reverse saturable absorption (RSA), the optical absorbance increases as the power of the light incident on them increases. To date, RSA has only been observed when very intense light sources, such as short-pulse lasers, are used. Here, we show that hydroxyl steroidal matrices embedding properly designed aromatic molecules as acceptors and transition-metal complexes as donors exhibit high RSA on exposure to weak incoherent light at room temperature and in air. Accumulation by photosensitization of long-lived room-temperature triplet excitons in acceptors with a large triplet-triplet absorption coefficient allows a nonlinear increase in absorbance also under low-power irradiation conditions. As a consequence, continuous exposure to weak light significantly decreases the transmittance of thin films fabricated with these compounds. These optical limiting properties may be used to protect eyes and light sensors from exposure to intense radiation generated by incoherent sources and for other light-absorption applications that have not been realized with conventional RSA materials.

Absorption Linear Dichroism Measured Directly on a Single Light-Harvesting System: The Role of Disorder in Chlorosomes of Green Photosynthetic Bacteria

Chlorosomes are light-harvesting antennae of photosynthetic bacteria containing large numbers of self-aggregated bacteriochlorophyll (BChl) molecules. They have developed unique photophysical properties that enable them to absorb light and transfer the excitation energy with very high efficiency. However, the molecular-level organization, that produces the photophysical properties of BChl molecules in the aggregates, is still not fully understood. One of the reasons is heterogeneity in the chlorosome structure which gives rise to a hierarchy of structural and energy disorder. In this report, we for the first time directly measure absorption linear dichroism (LD) on individual, isolated chlorosomes. Together with fluorescence-detected three-dimensional LD, these experiments reveal a large amount of disorder on the single-chlorosome level in the form of distributions of LD observables in chlorosomes from wild-type bacterium Chlorobaculum tepidum . Fluorescence spectral parameters, such as peak wavelength and bandwidth, are measures of the aggregate excitonic properties. These parameters obtained on individual chlorosomes are uncorrelated with the observed LD distributions and indicate that the observed disorder is due to inner structural disorder along the chlorosome long axis. The excitonic disorder that is also present is not manifested in the LD distributions. Limiting values of the LD parameter distributions, which are relatively free of the effect of structural disorder, define a range of angles at which the excitonic dipole moment is oriented with respect to the surface of the two-dimensional aggregate of BChl molecules. Experiments on chlorosomes of a triple mutant of Chlorobaculum tepidum show that the mutant chlorosomes have significantly less inner structural disorder and higher symmetry, compatible with a model of well-ordered concentric cylinders. Different values of the transition dipole moment orientations are consistent with a different molecular level organization of BChls in the mutant and wild-type chlorosomes.

Inhomogeneous and single molecule line broadening of terrylene in a series of crystalline n-alkanes

We present a study of single molecule linewidth broadening of terrylene in dodecane, tetradecane, and hexadecane matrices. The Shpolskii bulk absorption spectra exhibit increase of the bandwidths and site complexity with increasing n-alkane chain length. Single molecule lines are broadened above the lifetime limit even at 1.6 K in all three solvents. The linewidth distributions suggest the existence of relaxing two-level systems coupled to the molecular transitions. Spectral diffusion was observed in the forms of spontaneous and photoinduced frequency jumps. Temperature dependencies of the single molecule linewidths show a wide range of power-law dependencies below 3 K and, in some cases, exponentially activated behavior above 3 K. Possible origins of the line broadening are discussed.

Single-molecule electroluminescence and photoluminescence of polyfluorene unveils the photophysics behind the green emission band

Optoelectronic properties of polyfluorene, a blue light-emitting organic semiconductor, are often degraded by the presence of green emission that originates mainly from oxidation of the polymer. Here, we use single-molecule electroluminescence (EL) and photoluminescence (PL) spectroscopy on polyfluorene chains confined in vertical cylinders of a phase-separated block copolymer to spectrally resolve the green band and investigate in detail the photophysical processes responsible for its appearance. In both EL and PL, a substantial fraction of polyfluorene chains shows spectrally stable green emission which is ascribed to a keto defect. In addition, in EL, we observe a new type of vibrationally resolved spectra distributed over a wide range of frequencies and showing strong spectral dynamics. Based on quantum chemical calculations, this type is proposed to originate from charge-assisted formation and stabilization of ground-state aggregates. The results are expected to have broad implications in the fields of photophysics and material design of polyfluorene materials.

Circularly Polarized Persistent Room-Temperature Phosphorescence from Metal-Free Chiral Aromatics in Air

Circularly polarized room-temperature phosphorescence (RTP) with persistent emission characteristics was observed from metal-free chiral binaphthyl structures. Enantiomers of the binaphthyl compounds doped into an amorphous hydroxylated steroid matrix produced blue fluorescence and yellow persistent RTP in air. The lifetime and quantum yield of the yellow persistent RTP were 0.67 s and 2.3%, respectively. The dissymmetry factors of circular dichroism (CD) in the first absorption band, circularly polarized fluorescence (CPF), and circularly polarized persistent RTP were |1.1 × 10(-3)|, |4.5 × 10(-4)|, and |2.3 × 10(-3)|, respectively. A comparison between the experimental data and calculations by time-dependent density functional theory for transient CD spectra confirmed that the binaphthyl conformations in the lowest singlet excited state (S1) and the lowest triplet state (T1) were different. The large difference in the dissymmetry factors for the CPF and the circularly polarized persistent RTP was likely caused by this conformational change between S1 and T1.

Single-Molecule Study on Polymer Diffusion in a Melt State: Effect of Chain Topology

We report a new methodology for studying diffusion of individual polymer chains in a melt state, with special emphasis on the effect of chain topology. A perylene diimide fluorophore was incorporated into the linear and cyclic poly(THF)s, and real-time diffusion behavior of individual chains in a melt of linear poly(THF) was measured by means of a single-molecule fluorescence imaging technique. The combination of mean squared displacement (MSD) and cumulative distribution function (CDF) analysis demonstrated the broad distribution of diffusion coefficient of both the linear and cyclic polymer chains in the melt state. This indicates the presence of spatiotemporal heterogeneity of the polymer diffusion which occurs at much larger time and length scales than those expected from the current polymer physics theory. We further demonstrated that the cyclic chains showed marginally slower diffusion in comparison with the linear counterparts, to suggest the effective suppression of the translocation through the threading-entanglement with the linear matrix chains. This coincides with the higher activation energy for the diffusion of the cyclic chains than of the linear chains. These results suggest that the single-molecule imaging technique provides a powerful tool to analyze complicated polymer dynamics and contributes to the molecular level understanding of the chain interaction.

Onset of exciton-polariton behavior observed in individual fibers of pseudoisocyanine J-aggregates by sub-micron scale reflectance spectroscopy

Abstract We report results on reflection microscopy and local spectroscopy of aggregates of pseudoisocyanine chloride prepared in thin film of polyvinylsulfate. The aggregates assemble into fiber-like structures of relatively high reflectivity of up to 20%. Locally, the fibers show a large variety of reflectance spectra from sharp peaks at the position of the absorption J-band to broad bands with further structures near their maxima. Polarization experiments confirm that transition dipoles responsible for the J-band are oriented along the fibers. A simple theoretical model taking into account polariton-like excitations and finite dimensions of the fibers is used to simulate the experimental results.

1-, 3-, 6-, and 8-Tetrasubstituted Asymmetric Pyrene Derivatives with Electron Donors and Acceptors: High Photostability and Regioisomer-Specific Photophysical Properties

The systematic synthesis of five 1-, 3-, 6-, and 8-tetrasubstituted asymmetric pyrenes with electron donor and acceptor moieties is presented, together with an examination of their photophysical properties. Pyrene derivative PA1, containing one formyl and three piperidyl groups, showed bright solvatochromic fluorescence from green (λem = 557 nm, ΦFL = 0.94 in hexane) to red (λem = 648 nm, ΦFL = 0.50 in methanol), suggesting potential applications for PA1 as an environmentally responsive probe. Although the synthesis of simple 1- and 3-disubstituted pyrene derivatives is considered difficult, PA13, with two formyl groups at the 1- and 3-positions and two piperidyl groups at the 6- and 8-positions, could be synthesized successfully. PA13 exhibited less pronounced solvatochromism, but displayed a narrow fluorescent band with high ΦFL in all solvents (ΦFL > 0.75). Moreover, its absorption band displayed an exceptional bathochromic shift compared to the other derivatives (e.g., λabs = 480 and 522 nm in ethanol for PA1 and PA13, respectively), suggesting that such modifications of pyrene may be quite important for the modulation of its energy gap. Additionally, all compounds exhibited exceptionally high photostability, which highlights the advantage of these new dyes and provides new insights on the design of photostable fluorophores.