I. Rückmann

Correlation of photophysical parameters with macrocycle distortion in porphyrins with graded degree of saddle distortion

Different porphyrin conformations are believed to play a role in controlling the cofactor properties in natural tetrapyrrole-protein complexes. In order to study the correlation between macrocycle nonplanarity and physicochemical properties in detail, a series of six porphyrins with graded degree of macrocycle distortion was investigated. These conformationally designed porphyrins are based on the successive introduction of beta-ethyl groups into the tetraphenylporphyrin parent macrocycle and the degree of nonplanarity is dependent on the number and localization of the beta-ethyl meso-phenyl interactions. The electronic properties of the complete series of porphyrins were investigated in solution. It was found that the singlet and triplet properties depend not only on the out-of-plane distortion parameter but also on the type of central metal. Moreover, it was found that macrocycle distortion affects the singlet state properties significantly stronger than the triplet properties. In addition, the efficiency of energy transfer to molecular oxygen was investigated. It was shown that the singlet oxygen quantum yield depends strongly on the triplet state lifetime of the porphyrins, resulting in differences between the decrease of intersystem crossing and singlet oxygen quantum yield. The observed gradual change of electronic parameters of base free tetraphenylporphyrins with increasing deformation of the macrocycle indicates the validity of using conformationally designed porphyrins to fine-tune photophysical properties.

Non-linear and transient absorption spectroscopy of magnesium(II)-tetrabenzoporphyrin in solution

The excited state properties of magnesium(II)-tetrabenzoporphyrin (Mg-TBP) were studied by using intensity dependent transmission (non-linear absorption), ps-transient absorption and time resolved luminescence spectroscopy. It was found that there is a strong excited absorption in the region around 500 nm. Because there is no or little ground state absorption in this region the dye is suitable as an optical limiter.

Growth and characterization of semiconductor microcrystallites in a glass matrix with respect to their nonlinear optical properties

Abstract We report on the optimization of diffusion controlled growth of CdSe-microcrystals embedded in glass, as proved by linear optical absorption spectra at different temperatures and small angle X-ray scattering. To study the relation between homogeneous and inhomogeneous spectral broadering ps-hole burning experiments at room temperature were performed on samples having different size distributions. The glass matrix is characterized alone by investigating borosilicate glass without and with Cd and/or Se ions before heat treatment.

Growth of CdSe microcrystallites in a borosilicate glass matrix. I. Preparation and X-ray analysis

CdSe microcrystals were grown at 580 and 600°C for 10–240 h from a K 2 O ZnO B 2 O 3 SiO 2 glass containing Cd 2+ and Se 2− ions. Size distribution of the microcrystals was qualitatively characterized by linear absorption measurements: a uniform size of CdSe microcrystals is indicated by a symmetric and well developed absorption band which becomes more narrow and intense with decreasing temperature of measurement. These results were confirmed by small angle X-ray scattering (SAXS) with Cu K α radiation of a SAXS device of Kratky-type. At 580°C, increasing heat treatment duration resulted in an increase of the amount of the precipitated CdSe microcrystals. A nearly complete precipitation of uniform microcrystals with radii of 1.6±0.2 nm was achieved after 80 h. Increasing treatment temperatures and/or periods caused a coarsening of the microcrystals. The size distribution became more asymmetric (Ostwald ripening).

Interface transfer processes and spectral hole burning in CdSe quantum confined clusters

Abstract The interface of very small CdSe microclusters embedded in borosilicate glass is investigated, analysing the spectral bleaching behaviour and its kinetics before and after a surface passivation due to hydrogenation. We observe a quenching of the charged carrier transfer from resonantly excited microparticles into the glass matrix via interface states.

Orientational relaxation of pheophorbide-a molecules in the ground and in the first excited state measured by transient dichroism spectroscopy

We report on transient dichroism spectroscopy carried out on pheophorbide-a in ethanol to measure the response time of the pump-beam induced anisotropy after selective excitation at high excitation levels. From the observed time behaviors of induced absorption, transient amplification, and of the bleaching signal, the orientational relaxation times of pheophorbide-a in the first excited singlet state and in the ground state were obtained separately to be 370 and 250 ps, respectively. It is found that pheophorbide-a behaves like a spherical rotor in the first excited state, while in the ground state randomization of molecules is dominated by the rotation around its quasi-symmetry axis. Because of rotation free measurement of ordinary transient absorption spectra fail in the case of ground state depletion, the observed orientational relaxation time of the ground state have to be taken into account to fit the bleaching kinetics obtained from transient absorption spectra very well to determine the intersystem crossing yield of pheophorbide-a to be 0.46.

Picosecond photorefractive diffraction in CdTe at 1 μm

Using a ps-transient grating technique the contribution of the ps-photorefractive effect to the first-order probe-beam diffraction signal has been studied in CdTe at 1 μm investigating diffraction kinetics at different sample orientations in the thin grating regime. A fast photorefractive grating formation time during the pump pulses and a characteristic decay time of 600 ps, shorter than the free-carrier lifetime, have been observed.

Influence of hydrogenation of defects at the microcrystallite(MC)-glass interface on the nonlinear optics of CdSe-MC's

Abstract Using frequency doubled 35 ps pulses of a YAG laser to excite resonantly CdSe-MCs (R = 1.6 nm) embedded in glass, the absorption saturation and its kinetics are investigated at room temperature. A passivation of the MC-glass interface is realized by a hydrogen treatment of one sample. In the hydrogenated sample a larger switching factor between the two transmission levels and a slower relaxation are observed supporting the assumption of carrier transfer into the glass matrix advanced by surface states.