Color-tuning is a critical survival mechanism for photosynthetic organisms. Calcium ions are believed to enhance both spectral tuning and thermostability in obligatory calcium-containing sulfur purple bacteria. This study examined the thermo- and piezo stability of the LH1-RC complexes from two calcium-containing sulfur purple bacteria notable for their extreme red-shifted spectra. The results generally show limited reversibility of both temperature and pressure effects related to the malleability of calcium-binding sites. While the pressure-induced decomposition product closely resembles the calcium-depleted form of the chromoproteins, the thermally induced products reveal monomeric B777 and dimeric B820 forms of bacteriochlorophyll a, similar to those seen in non-sulfur purple bacteria treated with detergent. The study further found nearly unison melting of the protein tertiary and secondary structures. Overall, our findings do not support a direct link between color adjustment and thermodynamic stability in light-harvesting chromoproteins.

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=0.1to2.5 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.

Broadening and shifts of spectral holes by changing the hydrostatic pressure at 2K were measured for dimethyl-s-tetrazine probe molecules in durene crystal, both pure and doped with hexachlorobenzene (HCB) at 2mol%. The HCB molecules act as elastic defects and cause increase in inhomogeneous broadening as well as in the pressure-induced broadening of spectral holes. However, the relative pressure broadening (scaled to the inhomogeneous linewidth) decreases with pressure. The HCB concentration also affects the color effect in pressure broadening, i.e. its dependence on the spectral position. The observations are qualitatively interpreted proceeding from the diaelastic model [J. Chem. Phys. 104 (1996) 5384].

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’skii system are shown to be inconsistent with existing theories. They are qualitatively explained by pressure-induced dynamics of vacancy defects in the frozen n-alkanes.

A statistical theory is developed in order to describe the pressure-induced shift and broadening of spectral holes in pressure-tuning experiments in crystals. The theory accounts for the defect-related diaelastic effect (induction of internal inhomogeneous strain fields by the applied hydrostatic pressure due to the host-defect compressibility and/or size mismatch). General results are specified and analyzed in the case of similar defects and for two different types of point defects. The former case yields no hole broadening, while the latter one does. A similar consideration applies to electric- and magnetic-field-induced effects on spectral holes in crystals as well.

Storage density as a function of a tolerable level of the cross-talking Fm is calculated for different packing geometries in spectral- hole-burning-based frequency-selective 2+1-dimensional (2 spatial+1 frequency dimension) optical memories. Spatial intercalation of adjacent isofrequency layers is demonstrated to increase the storage density, the highest density obtained for the rhombohedric 3D memory lattice.