Time-resolved Stokes shift measurements and steady-state absorption and fluorescence measurements of Coumarin 153 (C153) at different temperatures were used to explore the solvation dynamics of binary mixtures of alcohols and alkanes at various alcohol concentrations. These solvent mixtures show even at alcohol concentrations as low as 0.3% a strong Stokes shift of about 1200 cm -1 . Depending on alcohol concentration, this Stokes shift takes place on a time scale ranging from 300 ps up to several nanoseconds. These characteristic times are 2 orders of magnitude longer than the relaxation times typically found in alcohols, which is attributed to rotational reorientation of the solvent molecules. A monoexponential temporal behavior of the Stokes shift and a linear dependence of the time constants on the alkane viscosity are observed. These results and temperature dependent static fluorescence measurements strongly support a diffusion-controlled process of solvation in these mixtures.

Molecular dynamics simulations are applied to the preferential solvation of coumarin 153 (C153) by alcohol in an alcohol/alkane mixture, indicated by recent steady-state and time-resolved spectroscopic measurements. Simulations of weakly polar mixtures are done for the ground and the excited states of C153, using detailed models of the dye. Solvation of C153 by the alcohol is almost negligible in the ground state, with correspondingly little effect on the absorption spectrum of the dye, whereas preferential solvation of the excited state leads to a large solvation shift of the fluorescence spectrum, in agreement with experiment. The simulated solvation shell and its dynamics are described and related to the solvation shifts.

Principles of formation, electronic absorption and fluorescence spectra are reported for self-organized pentameric arrays of tetrapyrrolic macrocycles. In these arrays two molecules of Zn-porphyrin dimers, Zn(II)l,4-bis [5-(10,15,20+tri-p-hexylphenylporphyrinyl)]-benzene ((ZnHTPP) 2) are bound via one molecule of a tetrapyridyl-substituted free base of porphyrin or tetrahydroporphyrin. The process of self-assembly is based on the twofold coordination of the central Zn ions !n the dimer with the nitrogen atoms of the pyridyl rings in the free base which is strong enough to make the complexes stable at room temperature. The formation of the complexes can be followed by changes in the absorption bands of (ZnHTPP) 2 characteristic of an axial extra-ligation of Zn-porphyrins with pyridine or pyridyl-substituted compounds. The spectral behavior of the free bases in the pentads is determined by a non-planar distortion of their macrocycle caused by the two-point binding with the dimers. The fluorescence intensity of the Zn-porphyrin dimer decreases essentially upon complexation with the tetrapyridyl-substituted free bases. This quenching effect is assigned to a singlet-silaglet energy transfer from the complexed Zn-porphyrin dimers to the free base subunit in the pentad.

Using static and time-resolved measurements, dynamics of non-radiative relaxation processes have been studied in self-assembled porphyrin triads of various geometry, containing the main biomimetic components, Zn-porphyrin di mers, free-base extra-ligands (porphyrin, chlorin or tetrahydroporphyrin), and electron acceptors A (quinone or pyromellitimide). The strong quenching of the dimer fluorescence is due to energy and sequential electron transfer (ET) processes to the extra-ligand (~0.9-1.7 ps), which are faster than a slower ET (34-135 ps) from the dimer to covalently linked A in toluene at 293 K. The extra-ligand Si -state decay (sS = 940-2670 ps) is governed by competing processes: a bridge (dimer) mediated long-range (rDA = 18-24 A) superexchange ET to an acceptor, and photoinduced hole transfer from the excited extra-ligand to the dimer followed by possible superexchange ET steps to low-lying charge transfer states of the triads. The subsequent ET steps dimer ! monomer ! A taking place in the triads, mimic the sequence of primary ET reactions in photosynthetic reaction centers in vivo.

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