Jan Jasny

Multifunctional spectrofluorimetric system

Abstract A new type of spectrofluorimetric research system is constructed. It is composed of a number of easily interchangeable optical instruments and is characterized by its wide aperture, 1 : 3, versatility and extendability. Some methods of testing the performance and of optimizing the construction are described. The system serves for recording emission and excitation spectra, their anisotropies (in-phase or our-of-phase, at several geometries), as well as the low-temperature absorption spectra. A pressurized sample compartment serves for the O 2 -quenching measurements in the kinetics of excited-state processes.

Fluorescence microscopy in superfluid helium: Single molecule imaging

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.

Single molecules observed by immersion mirror objective. The orientation of terrylene molecules via the direction of its transition dipole moment

Abstract Fluorescence from single molecules of terrylene in a Shpolskii matrix (n-hexadecane or n-dodecane) was detected with an immersion mirror objective. An analysis of fluorescence intensity distribution outside the image plane has been performed according to a method presented earlier (preceding Letter). Spatial orientations of a long molecular axis for several individual terrylene molecules have been found. One of the angles from the pair describing spatial orientation was found to match the value available from routine polarisation experiments.

Single molecules observed by immersion mirror objective. A novel method of finding the orientation of a radiating dipole

Abstract A novel method for determining the angular orientation of the transition dipole moment of single emitting molecules is presented. It is based on the analysis of light intensity distribution in the image space of the mirror objective.

Picosecond isomerisation kinetics of excited p-dimethylaminobenzonitriles studied by oxygen quenching of fluorescence

Abstract Fluorescence quenching with O 2 under high pressure was used for the study of reaction kinetics in the 10 picosecond region. Quenching of the dual fluorescence of 2-methyl-4-dimethylaminobenzonitrile (II) in acetonitrile was analysed. The process B * → A * occurs with a lifetime of 11 ps, while the solvent reorientation correlation time is only 1 ps, which supports the isomerisation mechanism which involves twisting the amino group. The lifetime of A * is 4.5 ns.

Excited state proton transfer in jet-cooled 2,5-di-(2-benzoxazolyl)phenol

Abstract The molecule 2,5-di-(2-benzoxazolyl)phenol (DBP) has been studied by laser spectroscopy in a supersonic free jet. In the jet the fluorescence with a Stokes shift of 7000 cm −1 arises exclusively from the proton-transferred species. The fluorescence excitation spectrum of DBP exhibits the broad fluorescence background underlying some weak, sharp features. The first prominent band has been observed at 27830 cm −1 , but from 250 cm −1 above the origin the spectrum becomes congested and broad. OH→OD substitution of DBP produces a blue shift of 103 cm −1 . For the deuterated sample (DBP-d 1 ) resolved line structure in the fluorescence excitation spectrum has been observed up to an excess energy of 400 cm −1 . To identify spectral features corresponding to deuterated DBP and other species double-resonance depletion spectroscopy has been applied. From analysis of the width of the vibronic bands of DBP-d 1 , we estimated an upper limit of the excited state deuteron transfer rate constant, k DT =1.7×10 12 s −1 .

Electrochemical generation of excited TICT states. Part I

Abstract The electrogenerated chemiluminescence (ECL) of intramolecular donor-acceptor compounds was studied in acetonitrile electrolyte solutions using the triple-step method. The compounds studied were the 9-anthryl derivatives of N,N-dialkylanilines. In the electrogenerated spectra, only the twisted intramolecular charge-transfer (TICT) band was observed. High emission efficiencies (φ ecl = 0.1–1.1%) as well as high yields of the excited states (φ ES = 0.3–11%) were found. The (φ ES values obtained have been interpreted in the terms of the Marcus theory.

Ground and excited state tautomerization in 9-acetoxy-2,7,12,17-tetra-n-propylporphycene

Abstract Substitution of 2,7,12,17-tetra-n-propylporphycene by an acetoxy group lifts the degeneracy in two trans tautomeric species which, by symmetry, are identical in the parent compound. Absorption spectra reveal the presence of both tautomers in comparable quantities in the ground electronic state, even at cryogenic temperatures. Analysis of the absorption pattern at low temperatures points to ground state tunneling of two hydrogens between two non-equivalent forms. On the contrary, the fluorescence occurs from only one tautomeric form, indicating the localization of the protons on two nitrogen atoms in the lowest excited singlet state, and a rapid tautomerization upon excitation of the higher energy species.

Nanosecond transient absorption spectrophotometer with dye laser probing and computer control

A computer controlled apparatus for laser flash spectroscopy has been constructed. Nitrogen laser (energy 1 mJ, pulse width 1 ns) is used both for excitation of the sample and for pumping a tunable dye laser which serves as a source of monitoring light. Two different scan modes are possible: (i) a scan with a fixed wavelength of the monitoring light: the temporal evolution of the transient absorption in the range from −2 up to 100 ns can be observed by changing the length of the variable self‐compensating optical delay line, or (ii) a scan with a fixed time delay: the corresponding transient absorption in the range 380–800 nm can be measured. For illustration, the transient absorption spectrum of benzophenone in methylcyclohexane, decay curves of the singlet state of benzanthracene in ethanol in the presence of a quencher, and a ‘‘negative absorption’’ of rhodamine B in nitriles are reported.

Detection and spectroscopy of single molecules in rare gas matrices: dibenzanthanthrene in krypton and xenon

Abstract 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.