Confocal microscopy is a powerful tool for single molecule investigation of fluorescent macromolecules. Besides the
commonly studied features in single molecule detection, the 3D orientation determination of the emission dipole enables
the analysis of different conformational states. These conformational states can be represented as state depending dipole
orientations intrinsic to the fluorescent molecule and/or in relation to the molecular frame. They might be subject to
intramolecular dynamics, which may lead to spectral diffusion, fluorescence intensity and/or lifetime fluctuations and
changes in the orientation of the emission dipole. We demonstrate a detection scheme that allows for simultaneous
determination of the full 3D emission dipole orientation, the fluorescence intensity, the fluorescence lifetime and the
emission spectra of single fluorescent molecules. We evaluate the feasibility of our approach using pyridyl
functionalized perylene bisimide (PBI) as a model system exhibiting conformational changes. Moreover, MC
simulations demonstrate the full potential of our detection scheme to distinguish between intensity fluctuations due to
conformational changes and changes in the out-of-plane orientation or changes in both of them.
We report on the functionalization of silicon oxide nanostructures using luminescent dye molecules and the
characterization of these systems by optical microscopy. The nanostructures are prepared by local anodic oxidation
(LAO) of a dodecyl-terminated silicon substrate using an atomic force microscope (AFM). The silicon oxide
nanostructures are negatively charged and the cationic dye rhodamine 6G could be successively bound to the structures
by electrostatic interactions. A quenching of luminescence due to the interaction of the excited states with the silicon was
found. The luminescence signal is attributed to monomeric Rh6G molecules with a slight blue shift of the emission due
to the changed chemical environment.
Positive and negative charges are stored locally in thin films of silicon oxide on silicon by applying a voltage between an
AFM cantilever tip and the silicon substrate. The stored charges are displayed by Kelvin probe force microscopy
(KPFM). The process of charge storing is investigated with respect to different dwell times and different voltages. The
amount of stored charges increases both with applied voltage and dwell time. A decay mechanism of the charges with
two different time regimes is discussed. A fast decay is attributed to a migration parallel to the surface, while the second
one is dominated by a transport perpendicular to the substrate surface.
We study the fluorescence intermittency of individual Dil-molecules on silicon dioxide surfaces with the focus on intermittency statistics on timescales above 15 milliseconds. On these time-scales intermittency statistics is no longer dominated by triplet blinking. We show that rather broad distributions of dark states must be present which give rise to power law distributions for on- and off-times. The off- time distribution depends on excitation intensity.
The directed surface passivation of semiconductor CdSe, 0r CdSe/ZnS quantum dots (QD) by meso-pyridyl substituted porphyrins (H2P) has been realized via a reversible non-covalent self-assembly interaction of H2P meso-pyridyl nitrogens with ions of the ZnS shell or Cd atoms of the CdSe core in various solvents at ambient temperature. The formation of "QD-porphyrin" nanoassemblies leads to a QD photoluminescence (PL) quenching (intensity decrease and PL decay shortening) accompanied by a H2P fluorescence enhancement. The analysis of experimental Foerster resonance energy transfer efficiencies EFRET (FRET) found via acceptor (H2P) sensibilization and donor (QD) PL quenching shows that EFRET values obtained from fluorescence enhancement are of the order of 6 - 8 % for most QD studied and are thus much smaller as compared to the PL quenching efficiency. With respect to QD PL quenching efficiencies, smaller values of EFRET might be due to different competing reasons: the presence of two independent quenching processes in the nanoassemblies, energy transfer QD -> H2P and photoinduced (electron/hole) charge transfer (CT) or time-dependent QD interface dynamics leading to a noticeable QD PL quenching. The analysis of spectroscopic and kinetic findings reveals that a limited number of "vacancies" accessible for porphyrin attachment is available on the QD surface. Simultaneous presence of porphyrin triads/pentads and QDs in a solution leads to the formation of higly organzed nanoassemblies.
Self-assembled nanoscale arrays of controllable geometry and composition (up to 8 tetrapyrroles) have been formed via non-covalent binding interactions of the meso-phenyl bridged Zn-octaethylporphyrin chemical dimers or trimers with di- /tetrapyridyl substituted porphyrin extra-ligands. In these complexes using steady-state and time-resolved (ps fluorescence and fs pump-probe) measurements pathways and efficiencies of the energy transfer photoinduced charge separation as well as exchange d-π effects have been studied in solutions of variable polarity at 77-293 K. The same principles of aggregation via the key-hole scheme "Zn-pyridyl" have been used also for the surface passivation of pyridylsubstituted tetrapyrroles on the coreshell semiconductor CdSe/ZnS quantum dots (QD) showing quantum confinement effects. Picosecond time-resolved and steady-state data reveal that CdSe/ZnS QD emission is multiexponential and the efficiency of its quenching by attached porphyrins (due to energy transfer and photoinduced charge separation) depends strongly on the number of anchoring groups their arrangement in the porphyrin molecule as well as on QD size and number of ZnS monolayers. The analysis of spectroscopic and kinetic findings reveals that on average only ~l/5 porphyrin molecules are assembled on the QD and a limited number of "vacancies" accessible for porphyrin attachment is available on the QD surface.
In this work we present the results showing how the extra-ligation, spacer properties and porphyrin macrocycle screening may influence on the conformational dynamics and photophysical properties of multiporphyrin arrays as well as on their interaction with molecular oxygen in solutions at 293 K. Steady-state and time-resolved studies indicate that for a sequence of porphyrin or chlorin chemical dimers Zn- cyclodimer yields (ZnOEP)2Ph yields (ZnOEP)2 yields (ZnOEChl)2 with relative lowering of excited S1- and T1-states, the extra-ligation by pyridine does not influence essentially on fluorescence parameters but leads to an increase of T1-states non-radiative decay. At 293 K the T1-state quenching by O2 for Zn-dimer- pyridine complexes depends on the nature and flexibility of the spacer between macrocycles and donor-acceptor interactions with pyridine. In triads and pentads the dimeric subunit plays the role of screen weakening O2 interaction with the second subunit. As a result, the T1-state quenching by O2 in triads and pentads is decreased by 50 divided by 70 percent with respect to that for the corresponding individual monomers.
Fluorescence detected magnetic resonance (FDMR) coherent phenomena on single triplet-state chromophore guest molecule in low-temperature organic host matrix are analyzed within the stochastic approach to describe triplet electron spin dephasing due to frequency fluctuations Ut induced by host-matrix proton spins dynamics. Exact equations for density matrix of a molecule averaged over histories of the fluctuations Ut are constructed using the model of N random telegraph process. The equations are applied to calculate FDMR responses of a molecule to cw/pulsed MW field and to describe a wide range of available experimental data on (1) the power-broadened FDMR line shapes, (2) the FDMR nutations, (3) the FDMR Hahn echo for pentacene+p- terphenyl pair supposing the fluctuations Ut to be slow. The failure of the standard Bloch equations for this system is demonstrated and the effects of microwaves-suppressed dephasing are discussed.
We present experiments on the optical properties of ultrathin (a few nanometers thick) films (copperphthalocyanine, amorphous silicon) with an incorporated metal cluster film (silver, indium). Due to the spatially close interface, the plasmon absorption may be displaced from its resonance frequency in the bulk, and its average position may be controlled by the average thickness of the ultrathin optical film. For example, we observe a shift of the plasmon resonance of silver clusters in amorphous silicon films (on quartz glass) from 440 nm to 740 nm, when the silicon thickness increases from `zero' up to 15 nm. The deposition experiments are accompanied by investigation of the film structure, particularly in order to estimate the silver cluster diameter, which is around 3 nm or less. Additionally, numerical simulations are in progress to optimize the island film preparation conditions.
Thin copperphthalocyanine layers have been deposited on quartz glass substrates and investigated by means of transmission and reflection spectroscopy. The film thickness ranged between 20 nm and the subnanometer region. The determination of the optical constants allowed the estimation of the oscillator strengths for the relevant molecular transitions. A thickness dependence of the Q-band absorption maximum position could be established for layers with a thickness below 5 nm. The contributions of several physical mechanisms to such lineshifts are discussed.
KEYWORDS: Molecules, Luminescence, Picosecond phenomena, Absorption, Atomic, molecular, and optical physics, Thermodynamics, Temperature metrology, Energy transfer, Performance modeling, Life sciences
Supramolecular ensembles stable at room temperature (complexation constant and activation energy range from 5 (DOT) 106 M-1 to 5 (DOT) 107 M-1 and from 0.5 to 1.0 eV correspondingly) containing up to five macrocyclic fragments have been constructed using two-fold ligation of Zn-porphyrin and Zn-chlorin chemical dimers by pyridyl substituted porphyrin or related molecules. Spectral, photophysical and thermodynamic properties of triadic and pentadic arrays have been studied in a temperature range from 140 to 360 K. Kinetic behavior of the complexes was investigated using a fluorescent picosecond laser setup ((Delta) t approximately equals 30 ps) with 2-D (wavelength-lifetime) registration. Observed spectral properties are explained in terms of extra-ligation (red shift of all electronic bands <EQ 550 cm-1)) and excitonic splitting ((Delta) E < 1900 cm-1). Nonradiative for- and backward excitation energy transfer (K > 1010 c-1), electron transfer and d-(pi) interactions are discussed as the main paths of electronic excitation deactivation in the complexes.
Electron spin resonance (ODMR) experiments via fluorescence excitation spectroscopy on single pentacence molecules in p-terphenyl crystals provide information on spectral jump and temperature dependent dephasing processes. It turns out that ODMR transitions in the excited triplet and optical transitions in the excited singlet state are decoupled from each other in a way that different mechanisms determine the respective spectral behavior.
Zn-Tetraphenylporphyrin-(SO-3)4 forms two types of complexes with a covalently linked viologenanthraquinone (VA) compound both in H2O and methanol. In both complexes fluorescence quenching occurs on a time scale < 10 ps and 0.5 ns, respectively, and its tentatively assigned to photo-induced electron transfer from porphyrin to VA.
Substituting copper porphyrin (CuP) with pyridine substituents results in the formation of dimers with zinc porphyrins (ZnP). Different forms of self-assembling shorten the fluorescence lifetime of ZnP from 2 ns to 247 and 80 ps. This can be explained by electron transfer from ZnP to CuP.
KEYWORDS: Zinc, Luminescence, Absorption, Fluorescence spectroscopy, Picosecond phenomena, Physics, Laser applications, Life sciences, Energy transfer, Chemical species
Fast energy transport between subunits in covalently linked porphyrin heterodi mers is observed by time resolved fluorescence experiments. Using different central atoms in the porphyr in rings optical absorption of the subunits can be clearly separated. This results in a unidirectional transport which is strongly dependent on the distance between the subunits.
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