Indrek Renge

Sharp‐line structure in the fluorescence and excitation spectra of greening etiolated leaves

Vibrationally resolved sharp‐line optical fluorescence and excitation spectra are observed for etiolated and greening barley leaves at T = 5 K. Conditions necessary for obtaining fine‐structured spectra in the case of chromophores in biological cells are discussed. The observed fine‐line spectra confirm earlier results that the inactive protochlorophyll F630 and initial chlorophyll(ide) forms are present as monomeric pigment molecules weakly bound to proteins, while the active holochrome F655 is an aggregated protochlorophyllide‐protein complex

Site-selection optical spectra of bacteriochlorophyll and bacteriopheophytin in frozen solutions

Abstract Site-selection spectra of title compounds in different frozen solvents at 5K have been obtained: fluorescence spectra on selective excitation in vibronic as well as in the O-O absorption region and excitation spectra with narrow-band fluorescence recording. Frequencies of vibrational modes active in fluorescence and excitation spectra of bacteriochlorophyll a (BChl) and its metal-free derivative have been determined from these fine-line spectra. Most favourable vibronic line-to-background intensity ratios have been found in non-polar aprotic glassy environments (triethylamine, di-iso-amylether). The intensity of zero-phonon lines in microcrystallic protic matrices was low, indicating strong electron-phonon coupling. The vibrational frequencies of the excited electronic state characteristic of axially (at Mg atom) mono- and disolvated BChl species have been identified. Narrow spectral holes of about 20% of the initial absorption could be burned with ≈10-4 quantum yield within the O-O band of BChl and bacteriopheophytin.

Spectral hole burning study of electron–phonon coupling in polymers

Persistent hole burning in the S1←S0 transition of tetra-tert-butyl-tetraazaporphine was used to investigate the electron–phonon coupling in a broad range of polymeric solids between 5 and 30–50 K. The maxima of pseudophonon sidebands (νph) are displaced by 5–20 cm−1 from the 0–0 hole. The relationships between the νph values and the velocities of transversal and longitudinal ultrasound waves as well as the Young’s modulus of polymers were established. At the same time νph compare well to inelastic neutron scattering maxima, the first boson peaks in the Raman scattering spectra, and the characteristic modes responsible for extra specific heat and heat conductivity plateau. Mutual correlations of the molecular structure, nanoscopic, and bulk properties in glassy and partially crystalline polymers are pointed out. The quasihomogeneous hole width (Γqh) at fixed temperature (T) increases when νph becomes smaller and the polarity of the host increases. Hole widths measured at 15 and 25 K also display a common ...

Spectral shift mechanisms of chlorophylls in liquids and proteins

Origins of non-excitonic spectral shifts of chlorophylls that can reach -1,000 cm(-1) in pigment-protein complexes are actively debated in literature. We investigate possible shift mechanisms, basing on absorption and fluorescence measurements in large number of liquids. Transition wavelength in solvent-free state was estimated (±2 nm) for chlorophyll a (Chl a, 647 nm), Chl b (624 nm), bacteriochlorophyll a (BChl a, 752 nm), and pheophytines. The dispersive-repulsive shift is a predominating mechanism. It depends on polarizability difference between the ground and the excited state Δα and the Lorenz-Lorentz function of refractive index of solvent (n). The approximate (± 2Å(3)) increase of polarizability Δα is close to 15Å(3) for S(1) bands of Chl a, BChl a, and BPheo a, slightly larger for Chl b (18Å(3)), and less for Pheo a (11Å(3)). The effect of solvent polarity, expressed in terms of static dielectric permittivity (ε) is relatively minor, but characteristic for different pigments and transitions. Remarkably, maximum influence of ε on S(1) band of BChl a is less (-20 ± 10 cm(-1)) than that for Chl a (-50 ± 10 cm(-1)), and not correlated with dipole moment changes on excitation Δμ (∼2D and 0.1 ± 0.1D, respectively). Hydrogen bonding in protic solvents produces red shifts in Chl a (-60 cm(-1)) and BChl a (-100 cm(-1)), but not in Chl b. Second axial ligand of BChl a has no influence on the S(1) band, whereas the S(2) transition suffers a -400 to -600 cm(-1) down shift. Aromatic character of solvent is responsible for a ∼-100 cm(-1) red shift of both Q transitions in BChl a. The S(1) bands in chlorophylls are relatively insensitive with respect to dielectric properties and specific solvation. Therefore, nontrivial mechanisms, yielding large site-energy shifts are expected in photosynthetic chlorophyll-proteins.

Nonlinear Polarization of Solvatochromic Betaine 30

Evidence is presented that solvatochromism of 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)-phenolate (Reichardts dye or Betaine 30) results not only from a large reduction of dipole moment and hydrogen bonding, but also from the modulation of mesomeric effect. The polarizability change between the ground and the excited state, estimated from the refractive index (n) dependence of absorption energy E(T)(30), increases from 20 to approximately 150 A(3) on going from apolar to highly polar media. The dependence of E(T)(30) on dielectric permittivity (epsilon) could be linearized using an empirical function of susceptibility difference (epsilon-n(2)), but not in terms of conventional expressions written as phi(epsilon)-phi(n(2)). As a result of solvent-induced change in ionicity and bond alternation, the dipole moments and polarizabilities of the chromophore cannot be treated as invariables. Hypsochromic shifts caused by solvent quadrupoles and bond dipoles were quantified.

A spectral hole-burning study of long-wavelength chlorophyll a forms in greening leaves at 5 K

Persistent holes have been burnt in the spectra of post‐etiolated maize leaves in the spectral interval 700–745 nm at 5 K. Narrow (< 1 cm−1) holes detectable both in fluorescence and excitation spectra correspond to the zero‐phonon purely electronic S1◀S0 transition of chlorophyll a rather than to the S1←S0 vibronic or S 1 ←S0 higher singlet transitions. The spectral composition of the inhomogeneously broadened absorption and fluorescence contours of long‐wavelength chlorophyll a forms in plants is discussed in terms of the contributions of phonon‐wings, vibronic satellites, purely electronic S1◀S0 transitions and its higher excitonic components.

Fluorescence-detected triplet kinetics study of the specifically solvated chlorophyll a and protochlorophyll in frozen solutions

Abstract The triplet lifetimes (τ T ) of the title compounds have been measured in a number of solvent hosts by using an indirect triplet state detection through fluorescence fading kinetics at 77 K. Three types of complexes with solvent molecules, i.e. with a mono-(L 1 -type) and bisligated (L 2 -type) central magnesium atom and the latter with hydrogen bridges (L 2 H-type), have average τ T values of 2.5, 1.4 and 0.8 ms and 10, 4.5 and 2.5 ms for chlorophyll a (Chl a ) and protochlorophyll (PChl), respectively. Hydrogen-bonded L 1 H-type complexes of chl a occurring in tert-alcohols and perfluoroaniline have the same τ T as L 1 -type solvates. The solvent deuteration isotopic effects show that in L 2 H-complexes (e.g. in methanol) triplet quenching arises partially due to electronic energy transfer to the vibrational over tones of OH groups attached via Mg atom to the central part of Chl a macrocycle. The dependence of fluorescence band-shapes and maxima on the excitation wavelength indicates the presence of several types of pigment-solvent complexes in most matrices. Minor amounts (∼ 1 %) of L 2 - and L 2 H-type solvates on the background of dominating monoligated species could be selectively excited with a 457.9 nm Ar + laser line.

Measurement of internal strain in polymers by means of optical hole burning

Abstract Spectral holes were investigated in polymers doped with octaethylporphine and other tetrapyrrolic chromophores. A single hole was burned at a fixed temperature ( T ) or pressure ( P ) and its width has been examined when T or P was subject to a change. The width of a hole created at higher T (30–45 K) shows surprisingly little variation upon cooling to 5 K. It even broadens slightly at the lowest T . The cooling produces negligible deviation from the Lorentzian hole shape. By contrast, the hole broadening under the He gas pressure leads to a Gaussian distortion. The reversible, non-dynamical broadening is discussed in terms of local stress affecting a small number of atoms adjacent to the dye molecule.

Influence of high pressure on optical impurity spectra.

Expressions are derived for inhomogeneous band shapes of impurity spectra in highly compressed glassy matrices. Intermolecular guest-host interactions are approximated to isotropic two-body Lennard-Jones 6-12 potentials having different parameters in the ground state and the excited state. Calculated shifts and widths are compared to published values for the absorption spectra of phenanthrene, anthracene [B. Y. Okamoto and H. G. Drickamer, J. Chem. Phys. 61, 2870 (1974)], and several polymethine dyes [G. A. Samara et al., J. Chem. Phys. 37, 1482 (1962)] embedded in polymer hosts and subject to pressures up to 140 kbars. The magnitudes of barochromic shifts of the band maxima and the inhomogeneous broadening suggest that the equilibrium coordinates of the excited state are typically less by 5+/-2%.

INFLUENCE OF INTERMOLECULAR INTERACTIONS ON THE OPTICAL SPECTRA OF AROMATIC IMPURITY MOLECULES IN SOLVENT GLASSES AT 8 K

Abstract The S1-S0 energy distribution functions (IDF) have been determined for pyrene, anthracene and 9-cyanoanthracene in frozen solvent glasses. The matrix shift of the maxima of IDF can be calculated from the common solvent shift equations provided the liquid-phase dielectric permittivity is used. The width of IDF is correlated with the matrix shift. The Debye-Waller factors have been estimated and the matrix dependence of linear electron-phonon coupling have been discussed.