Jędrzej Łukasiewicz

Photocurrent generation in an electrochemical cell with substituted metalloporphyrins

Abstract Photocurrent generation in a semiconducting–metallic electrochemical cell filled with metallotetraphenyloporphyrins dissolved in polyvinylalcohol and metallotetranaphthyloporphyrins in nematic liquid crystal was investigated. The photopotential action spectra showed that the dye molecules are responsible for photoresponse. It has been shown that the intensity of photocurrent was dependent on the type of central metal complexed with the porphyrin ring of the dye molecule and also on the substituents attached to the methine bridge. The dependencies of the photopotential signal and of incident light–photosignal phase shift as a function of modulation light frequency indicate that the observed effect is due to more than one mechanism of electron transport. The results for metalloporphyrin dyes in fluid polyvinylalcohol and in nematic liquid crystal (D. Wrobel, J. Goc, R.M. Ion, J. Mol. Struct. 450 (1998) 239) were compared. The reasons of the different effectiveness of photoresponse in the electrochemical cell with different porphyrins complexed with various metals and substituted by different functional groups are discussed.

Molecular symmetry determines the mechanism of a very efficient ultrafast excitation-to-heat conversion in Ni-substituted chlorophylls.

In the Ni-substituted chlorophylls, an ultrafast (<60 fs) deactivation channel is created, which is not present in Ni-porphyrins. This observation prompted us to investigate in detail the mechanism of excitation-to-heat conversion in Ni-substituted chlorophylls, experimentally, using time-resolved laser-induced optoacoustic spectroscopy, and theoretically, using group theory approach. The Ni-substituted chlorophylls show exceptional photostability and the optoacoustic measurements confirm the prompt and very efficient (100%) excitation-into-heat conversion in these complexes. Considering their excellent spectral properties and the loss-free excitation-into-heat conversion they are likely to become a new class of versatile photocalorimetric references. The curious features of the Ni-substituted chlorophylls originate from the symmetry of a ligand field created in the central cavity. The central N-Ni(2+) bonds, formed via the donation of two electrons from each of the sp(2) orbitals of two central nitrogens to an empty [Formula: see text] hybrid centered on Ni(2+), have a considerable covalent character. The extreme rate of excited state relaxation is then not due to a ladder of the metal centered d-states, often invoked in metalloporphyrins, but seems to result from a peculiar topology of the potential energy surface (a saddle-shaped crossing) due to the covalent character of the N-Ni(2+) bonds. This is confirmed by a strong 0→0 character of electronic transitions in these complexes indicating a similarity of their equilibrium geometries in the ground (S(0)) and the excited states (both Q(X) and Q(Y)). The excitation energy is very efficiently converted into molecular vibrations and dissipated as heat, involving the central Ni(2+). These Ni-substituted pigments pose a fine exemplification of symmetry control over properties of excited states of transition metal complexes.

Spectral properties of stilbazolium merocyanines oriented in stretched polymer films and Langmuir-Blodgett monolayers

Abstract Three stilbazolium merocyanines: 1-(6′-hydroxyhexyl)-4-[(4-oxocyclohexa-2,5-dienylidene)ethylidene]-1,4-dihydropyridine; 1-(11′-hydroxyhexyl)-4-[(4-oxocyclohexa-2,5-dienylidene)ethylidene]-1,4-dihydropyridine; 1-(10′-carboxydecyl)-4-[4-oxocyclohexa-2,5-dienylidene)ethylidene]-1,4-dihydropyridine salt HCl located in Langmuir–Blodgett monolayers and deposited on quartz plate as well as embedded in isotropic or stretched polyvinyl alcohol films were investigated. Dyes occur in protonated and free-base forms. The concentration ratio of these forms depends on pH of subphase (for monolayers) or resin addition (in polymer film). The polarized absorption and fluorescence spectra were measured and coefficients of absorption and emission anisotropy were calculated. Dye molecules were in both matrices oriented, but degrees of orientation of various forms of dye were different. The anisotropies of absorption and emission are also different which strongly suggests the occurrence of dye forms with different yield of fluorescence and various orientations. The formation of mixed aggregates of protonated and free-base form is suggested. The orientation of stilbazolium merocyanines in anisotropic matrix is important in the application of these dyes as the sensors for measurements of the local electric field in biological membranes as well as for the formation polarizing absorption polymer film used in some types of two-colours liquid crystal displays.