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Single Molecule Spectroscopy of Mg Tetra-azaporphyrin in Solid Xenon
Abstract
We report initial results on the spectroscopy of single Mg-tetra-azaporphyrin molecules in solid xenon. Samples were prepared following the conventional matrix-isolation procedure. Single molecules, detected by fluorescence microscopy, gave remarkably stable signals. Their fluorescence excitation spectra revealed sharp zero-phonon lines, with most represented linewidths around 50 MHz. This is the first study of single molecules of a tetrapyrrolic compound.
Laser confocal microscopy and spatially-resolved fluorescence spectroscopy have been used for the study of the interaction of di(p-aminophenyl)etioporphyrin (DAPEP) and tetra(p-aminophenyl)porphyrin (TAPP) molecules with the surface of thin (7-15 μm) polypropylene films treated by 0.5 M KCl solution and activated by air non-equilibrium plasma at the normal atmospheric pressure. Confocal microscopy data (using laser scanning microscope LSM 510, Carl Zeiss) show that after treatment the polymer surface becomes inhomogeneous, and porphyrin moieties are randomly distributed on both film surfaces with a penetration depth of ~1 mm. On the basis of spatially resolved fluorescence measurements (using a home-built confocal microscope with a time resolution up to 100 ms/frame and high spatial resolution) it has been found that fluorescence spectra of individual spots correspond to monomeric species. It means that spatially closed few porphyrin molecules in the spot are not aggregated and do not interact significantly.
The properties of zero-phonon lines (ZPLs) for individual dibenzo-anthanthrene (DBATT) molecules in solid Xe have been studied. Systematic differences in the widths of ZPLs for molecules occupying different matrix sites were observed. For the majority of samples a monotonic increase of the average linewidth with temperature was found and satisfactorily described within a model, which combined (i) the line broadening caused by the interaction with two-level systems and (ii) dephasing due to the coupling with a local mode. For about 30% of studied samples, however, a surprisingly flat dependence of ZPL width on temperature was found.
Single light-harvesting complexes LH-2 from Rhodopseudomonas acidophila were immobilized on various charged surfaces under physiological conditions. Polarized light experiments showed that the
complexes were situated on the surface as nearly upright cylinders. Their fluorescence lifetimes and photobleaching properties
were obtained by using a confocal fluorescence microscope with picosecond time resolution. Initially all molecules fluoresced
with a lifetime of 1 ± 0.2 ns, similar to the bulk value. The photobleaching of one bacteriochlorophyll molecule from the
18-member assembly caused the fluorescence to switch off completely, because of trapping of the mobile excitations by energy
transfer. This process was linear in light intensity. On continued irradiation the fluorescence often reappeared, but all
molecules did not show the same behavior. Some LH-2 complexes displayed a variation of their quantum yields that was attributed
to photoinduced confinement of the excited states and thereby a diminution of the superradiance. Others showed much shorter
lifetimes caused by excitation energy traps that are only ≈3% efficient. On repeated excitation some molecules entered a noisy
state where the fluorescence switched on and off with a correlation time of ≈0.1 s. About 490 molecules were examined.
Recent experimental and theoretical studies concerning single-molecule spectroscopy in solids are discussed. Pure quantum effects--such as photon bunching, antibunching, and spectral jumps--and more classical phenomena--such as near-field excitation, saturation, ac/dc Stark shifts, spectral diffusion, two-photon excitation, and customary spectroscopic analysis--are considered. The emphasis of this review is on physical results and their interpretation. This is preceded by a general introduction, where fundamentals of single-molecule spectroscopy are explained.
In recent years, the development of single molecule detection (SMD) techniques has opened up a new era of life science. The dynamic properties of biomolecules and the unique operations of molecular machines, which were hidden in averaged ensemble measurements, have been unveiled. The SMD techniques have rapidly been expanding to include a wide range of life science. The experiments on molecular motors, DNA transcription, enzyme reactions, protein dynamics, and cell signaling are summarized in this short review.
The quantum yields and decay times of bromo-substituted perylenes are nearly identical with those of perylene. The lack of an expected internal heavy atom effect is due to the absence of a triplet energetically close to the fluorescing state.
The refractive indices of several frozen gas matrices at 12 K have been calculated from the measured fluorescence life-times of 9,10-diphenylanthracene, which is found to have a fluorescence quantum yield of unity even in a xenon matrix.
Picosecond transient spectroscopy was used to study excited state deactivation processes of porphycene embedded in solid argon and nitrogen matrices at low temperatures. Comparison of time profiles of the signals due to ground state bleaching and stimulated fluorescence reveals that the latter is delayed by about 100 ps with respect to the former when the molecule is excited into the S3 or S4 electronic states, corresponding to Soret bands. No delay between the two signals is observed for excitations into Q bands, corresponding to S1 and S2 transitions. This is evidence of slow vibrational relaxation from S3 and S4 into S1 and S2, due to the considerable S1–S3 energy gap and similarity of the potential energy profiles in different electronic states. On the contrary, for nonrigid dibenzo-derivatives of porphycene which have an `intruder' state located between the Q and Soret bands, the relaxation from excited electronic states is faster than the experimental time resolution of about 30 ps.
Matrix-isolated free-base porphine and its deuterated analogues show highly resolved intense fluorescence in neon, with evidence of strong b{sub 1g} vibronic activity, and highly resolved intense phosphorescence in xenon, dominated by totally symmetric vibrations. Triplet lifetimes were determined from T-T absorption decay, which revealed a new band at 310 nm. It was concluded that the heavy-atom enhancement of the T{sub 1} {yields} S{sub 0} radiative rate is dramatic. Both emissions exhibit site structure, and single-site spectra were obtained upon narrow-bandwidth excitation. The T{sub 1} {yields} S{sub 0} emission site pattern is identical with that of S{sub 1} {yields} S{sub 0} emission and S{sub 1} {l arrow} S{sub 0} absorption and totally different from that of S{sub 2} {l arrow} S{sub 0} absorption, suggesting that T{sub 1} and S{sub 1} are both Q{sub x} and have a common orbital origin, contrary to the currently accepted assignment. The implications of the results for the mechanism of the photoinduced double proton transfer in free-base porphine are considered. All the results are compatible with the proposal that the isomerization occurs in T{sub 1} in a stepwise fashion through the cis isomer. The advantages of interferometric recording of highly resolved visible and near-IR emission spectra are pointed out.
We report on a new, highly fluorescent aromatic compound for single molecule spectroscopy, dibenzanthanthrene, studied in an n‐hexadecane matrix. Narrow homogeneous lines are compatible with the measured lifetime. From the two components in the fluorescence correlation function, we determined the intersystem crossing rates towards and from the triplet states. The very weak triplet yields indicate that the molecule behaves as a nearly perfect optical two‐level system, with negligible buildup of triplet population. © 1996 American Institute of Physics.
The authors have investigated the effects of rare gas external heavy atoms on rate constants of spin-forbidden processes of (dimethylamino)benzonitrile isolated in cryogenic matrices by means of steady-state and time-resolved emission spectroscopy. We interpret the dramatic increase in the phosphorescence yield and the decreases in the fluorescence yield and lifetime and the phosphorescence lifetime in the heavy atom matrices of krypton and xenon in terms of a model in which the rate constant for phosphorescence increases 300-fold in xenon compared to argon, while the rate constants for intersystem crossing to the triplet state and nonradiative relaxation from the triplet state increase by factors of less than 5. Measurements in argon matrices doped with heavy atoms indicate that even one heavy atom neighbor has a significant effect on both single and triplet lifetime. We include higher resolution (up to 0.1 nm) emission spectra of (dimethylamino)benzonitrile detailing vibrational structure which has not previously been observed in the condensed phase. Comparison of the vibrational spacings with a matrix FTIR spectrum indicates that there is distortion of the aromatic ring in the Sâ state, while the shifts in the 0-0 transitions in acetonitrile matrix indicate that the Sâ state is somewhat more polar than either the Sâ or Tâ states.
Room-temperature solutions of two synthetic isobacteriochlorins are shown to have dual fluorescence with 0-0 bands near 635 and 585 nm. The full two-dimensional fluorescence intensity as a function of the excitation and emission wavelength is presented. It is concluded that the dual fluorescence results from two different tautomeric forms which involve a diagonal and an adjacent arrangement of the two free base hydrogens. The long-wavelength fluorescence has a lifetime of about 3 ns, and the short-wavelength emission has a lifetime of about 8 ns. Semiempirical calculations of the QCFF/PI and the CNDO/CI type were performed in order to discuss the absorption spectra of the different tautomers.
Individual antenna complexes from different photosynthetic units have been investigated by single molecule spectroscopy. In such energy transfer systems the polarization of the fluorescence emission gives valuable information about the nature of the emitting state, which is not readily available with other methods like fluorescence excitation or emission spectroscopy. The peripheral antenna light harvesting complex from purple bacterium Rhodopseudomonas Acidophila shows predominantly linear polarized fluorescence emission at low temperature, whereas at room temperature the fluorescence is randomly polarized. This is attributed to the fact, that at low temperature in the fluorescence emitting state the excitation energy is localized mainly on 4-5 chromophores. Analysis of the fluorescence emission of single peripheral antenna complexes of green plants indicate that for trimers of this species more than one Chlorophyll is responsible for the final fluorescence emission, which points towards a weak intermonomer coupling in the complex.
A fluorescence microscope for the detection of spatially resolved single molecules at low temperature has been developed. The main part consists of a microscope objective which is inserted in superfluid helium. Two versions of the optical system are described. Both setups have a high numerical aperture and thus high collection efficiency. A video camera with an image intensifier is used to detect the fluorescence images. Detection of single molecule fluorescence has been achieved. It was even possible to visualize individual fluorescing molecules on a video monitor in real time. Spectroscopic applications of this parallel detection scheme are discussed. © 1996 American Institute of Physics.
Fluorescence microscopy coupled with matrix isolation techniques have allowed the observation of dibenzanthanthrene molecules in rare gas matrices. Fluorescence excitation spectra of single molecules were measured in both Kr and Xe matrices. Spectral lines with Lorentzian shapes as well as irregular noisy profiles were observed.
The electronic structure and electronic spectra of azaporphyrin molecules are subjected to joint experimental and theoretical study. Spectral-luminescent investigations have been carried out for monoazaetioporphyrin II, trans-diazaetioporphyrin II, tetrazaporphin and their zinc complexes. The absorption, luminescence, fluorescence polarization and phosphorescence polarization spectra have been obtained, the parameters determining the energetics of primary photophysical processes have been measured as well. Calculations of azaporphin molecules have been performed using the all-valence-electron approximation by the CNDO/S method and in the π-electron approximation by the PPP method. Detailed juxtaposition of the experimental and computational data obtained has been made. A new interpretation of the azaporphyrin electronic spectra in the near-UV range is given.
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