Arlentin Laisaar

Spectral Hole Burning in Doped Organic Crystals and Polymer Glasses under Hydrostatic Pressure

We give an overview of results obtained in our isobaric studies of spectral holes in chlorin-doped glassy polystyrene, frozen n-octane and crystalline p-terphenyl under various fixed pressures up to 8.4 kbar at 4.2–18 K. Some results on low-pressure (<0.1 kbar) tuning of spectral holes at 1.5–4.2 K in benzophenone, biphenyl and frozen n-octane doped with chlorin and in frozen n-hexane doped with dimethyl-s-tetrazine are also described along with a theoretical work on the pressure effects in such non-isobaric studies. The most interesting effect, observed in our high-pressure isobaric experiments and explained theoretically, is the pressure-induced narrowing of spectral holes in glassy polystyrene, caused mainly by reduction in the number of soft localized modes in a glass under pressure.

Electron transfer and electronic energy relaxation under high hydrostatic pressure

The following question has been addressed in the present work. How external high (up to 8 kbar) hydrostatic pressure acts on photoinduced intramolecular electron transfer and on exciton relaxation processes? Unlike phenomena, as they are, have been studied in different systems: electron transfer in an artificial Zn-porphyrin-pyromellitimide (ZnP-PM) supramolecular electron donor-acceptor complex dissolved in toluene measured at room temperature; exciton relaxation in a natural photosynthetic antenna protein called FMO protein measured at low temperatures, between 4 and 100 K. Spectrally selective picosecond time-resolved emission technique has been used to detect pressure-induced changes in the systems. The following conclusions have been drawn from the electron transfer study: (i) External pressure may serve as a potential and sensitive tool not only to study, but also to control and tune elementary chemical reactions in solvents; (ii) Depending on the system parameters, pressure can both accelerate and inhibit electron transfer reactions; (iii) If competing pathways of the reaction are available, pressure can probably change the branching ratio between the pathways; (iv) The classical nonadiabatic electron transfer theory describes well the phenomena in the ZnP-PM complex, assuming that the driving force or/and reorganisation energy depend linearly on pressure; (v) A decrease in the ZnP-PM donor-acceptor distance under pressure exerts a minor effect on the electron transfer rate. The effect of pressure on the FMO protein exciton relaxation dynamics at low temperatures has been found marginal. This may probably be explained by a unique structure of the protein [D.E. Trondrud, M.F. Schmid, B.W. Matthews, J. Mol. Biol. 188 (1986) p. 443; Y.-F. Li, W. Zhou, E. Blankenship, J.P. Allen, J. Mol. Biol., submitted]. A barrel made of low compressibility beta-sheets may, like a diving bell, effectively screen internal bacteriochlorophyll a molecules from external influence of high pressure. The origin of the observed slow pico = and subnanosecond dynamics of the excitons at the exciton band bottom remains open. The phenomenon may be due to weak coupling of phonons to the exciton states or/and to low density of the relevant low-frequency ( approximately 50 cm(-1)) phonons. Exciton solvation in the surrounding protein and water-glycerol matrix may also contribute to this effect. Drastic changes of spectral, kinetic and dynamic properties have been observed due to protein denaturation, if the protein was compressed at room temperature and then cooled down, as compared to the samples, first cooled and then pressurised.

PRESSURE-INDUCED DYNAMICS IN SOLID N-ALKANES AS PROBED BY OPTICAL SPECTROSCOPY

The dependence of frequency, width, and area of spectral holes on pressure were measured at 1.6 K in the pressure range up to 2.5 MPa for dimethyl-s-tetrazine (DMST) doped n-hexane (Shpol’skii system), and as reference systems, for DMST-doped durene (“hard” molecular crystal) and ethanol:methanol glass. For the Shpol’skii system, in addition the inhomogenous fluorescence spectra were measured for normal and high (200 MPa) pressures. The main observations were the following: (i) spectral holes in the Shpol’skii system exhibit very large pressure-induced broadening (up to 65 GHz/MPa) depending essentially on the prehistory (freezing pressure) and exceeding the corresponding values for durene (by far) and glass; (ii) spectral holes in the Shpol’skii system exhibit strong, and to a large extent, reversible, area reduction with applied pressure; and (iii) the inhomogeneous fluorescence lines show quite a moderate (as compared to holes) pressure broadening of about several GHz/MPa. The results for the Shpol’ski...

Pressure effects on relaxation in a polymer glass: A persistent spectral hole burning study

The influence of hydrostatic pressure in the range of some kilobars on low-temperature (T < 20 K) relaxation in a polymer (polystyrene) glass after optical excitation of a probe chromophore in it is studied using two different kinds of spectral hole burning experiments—under isothermal-isobaric and in temperature cycling conditions. In the first case, the temperature dependence of the hole width reflects the dynamics of interaction of the electronic transition in a probe molecule with soft localized vibrational modes and with two-level systems, whereas, in the second case, the observed residual hole broadening after the temperature cycle arises from activated (overbarrier) transitions in almost symmetric double-well soft potentials. It is shown that both these processes are essentially suppressed by the applied hydrostatic pressure (the hole width in the first case and its increment in the second case are both reduced about twofold at 5 kbar). An extension of the soft potential model for glasses is proposed explaining in a coherent manner both effects. Its essential points are the presence in the potential of an extra term linear in pressure and the soft coordinate and an assumption about asymmetric distribution of the cubic anharmonicity parameter ξ in the potential.

Self-trapped excitons in LH2 bacteriochlorophyll–protein complexes under high pressure

The absorption and emission spectra of excitons in LH2 antenna complexes from the photosynthetic purple bacterium Rhodobacter sphaeroides have been studied under hydrostatic pressure. The measurements made between ambient pressure and 6 kbar over a broad temperature range reveal largely different rates of the pressure-induced shifts for the absorption and emission bands. Numerical calculations based on exciton polaron model provide evidence for the exciton self-trapping at ambient pressure as well as for the pressure stabilization of the self-trapped exciton states responsible for the emission, whereas the light absorbing states belong to nearly free excitons over the whole pressure and temperature ranges studied.

Optical study of terrylene molecules in crystalline biphenyl: effects of pressure and temperature on the luminescence spectra

The photoluminescence spectra of terrylene guest molecules in polycrystalline biphenyl host were measured at various temperatures between 4.7 and 295 K under ambient pressure (1 atm) as well as under various pressures up to 4.4 kbar at 4.7 K and up to 4.9 kbar at 295 K. With increasing pressure, all four spectral peaks studied shift to the red. By elevating the temperature from 4.7 to 295 K at 1 atm, the most intense peak at 17,220 cm−1 first shifts slightly to the red, but at ∼40 K begins to shift to the blue. Comparison of the temperature and pressure shifts for this peak reveals that its temperature shift is mainly determined by the thermal expansion of the host crystal rather than by the change in the electron–phonon coupling with temperature.

Pressure effects on the spectra of dye molecules in incommensurate and commensurate phases of biphenyl

Abstract Low-pressure tuning of spectral holes, burned in the spectra of chlorin molecules doped into polycrystalline biphenyl, was studied for the incommensurate phase III of biphenyl at T =2 K and P=0.1 to 2.5 MPa. A blue pressure shift of holes burned in the outermost red line of an inhomogeneous spectral triplet was found, in contrast to the red pressure shifts of the other two lines. Extrapolation of these shifts to higher pressures shows the convergence of that triplet at a pressure above 200 MPa. Such behaviour was confirmed by high-pressure measurements of two-dimensional (2D) excitation–emission spectra, from which the inhomogeneous distribution function (IDF) was extracted. At 5 K the low-pressure triplet shape of IDF converges to a high-pressure singlet at 170 MPa, close to the critical pressure for the incommensurate–commensurate transition. The results support a view that the optical spectra reflect interaction of the impurity molecule with the incommensurate modulation wave in biphenyl host matrix disappearing at transition to the commensurate phase.

Persistent spectral hole-burning in doped organic crystals and polymers at high hydrostatic pressures

Abstract We present a short review of results obtained from our studies of hole widths in chlorin-doped glassy polystyrene and polycrystalline n-octane and p-terphenyl under various pressures up to 8.4 kbar at 4.2–15 K.

High pressure narrowing of zero-phonon lines in polymer glasses at different temperatures

Abstract Pressure-induced narrowing of zero-phonon lines, stronger at higher temperatures, established for chlorin molecules in a glassy polystyrene by using the spectral hole burning technique, is explained theoretically by pressure dependence of pseudolocal vibrations and two-level systems.

An immersion cryostat for mounting a high-pressure optical cell surrounded by nonboiling liquid nitrogen

A special bubble-free cryostat for precise optical measurements of samples compressed in a high-pressure cell has been built and put into operation. Liquid nitrogen around the high-pressure cell is prevented from boiling thanks to a slight overpressure in the closed cryovessel; instead, cryoliquid is boiling at atmospheric pressure inside an evaporator.