N. V. Tkachenko

Near infra-red emission of charge-transfer complexes of porphyrin–fullerene films

Abstract Solid porphyrin–fullerene films were prepared by evaporating a mixture of porphyrin and fullerene in solution and by deposition of Langmuir–Blodgett films where the dyes were covalently linked to each other. Both types of the films exhibited an emission band shifted to the near infrared region, ca. 800 nm, where neither porphyrin nor fullerene emits alone nor mixed in solution. The emission is attributed to an emitting charge transfer (CT) state formed by porphyrin and fullerene moieties. It is characterized by a long lifetime, ca. 2 ns, and high quantum yield, 0.14%. Remarkably small total reorganization energies, less than 0.2 eV, are deduced from the CT emission spectra of the porphyrin–fullerene films.

Optical spectroscopy of perfluorothiophenyl, perfluorothionaphthyl, xanthate and dithiophosphinate radicals

Laser flash photolysis has been used to record the optical spectra of sulfur-containing radicals forming from photodissoci- ation of diphenyl disulfide, perfluorodiphenyl disulfide, perfluoro-2,2 X -dinaphthyl disulfide, diisopropyldixantogene and . bis diisobutylthio-phosphoryl-disulfane . The extinction coefficients of absorption bands were determined from the reaction of S-radicals with a stable nitroxyl radical. The rate constant of this reaction was for all radicals close to 10 9 M y1 s y1 and successfully competes with the reaction of recombination. The presence of a narrow and strong absorption band in the optical spectrum of a nitroxyl radical and the absence of absorption in the reaction products allows one to accurately determine the extinction coefficients of the absorption bands of S-radicals. q 2000 Published by Elsevier Science B.V.

PHOTOELECTRIC SIGNALS OF CHLOROPHYLL A LANGMUIR-BLODGETT FILMS

Abstract Photoelectric cells of ITO/insulator/chlorophyll a /insulator/InGa liquid metal, containing 1, 2, 4 or 10 LB monolayers of chlorophyll a as photoactive layers and 9–10 stearyl amine films as insulator layers have been studied by measuring the transient photovoltage and transient photocurrent responses in the micro- to millisecond and second time domains, respectively. The signals were created either by a laser pulse or by step-wise excitation. The cells exhibited light-induced charge transport inside the chlorophyll layers in the direction from the InGa to the ITO electrode. The charge movement is explained by an internal electric field due to the metal surface potentials, and was confirmed by measurements for when an external voltage was applied. A model based on a single-step light-induced charge shift is discussed, and the quantum yield for the light-induced charge generation has been estimated to be close to unity.

Irregular behavior of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin in Langmuir–Blodgett films

Abstract Multilayer Langmuir–Blodgett films containing 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (PFP) in octadecylamine and in fatty acid matrices were prepared. The films revealed relatively long fluorescence lifetimes with an irregular concentration dependence: the longer lifetime corresponded to the higher concentration of PFP. Fluorescence decay-associated spectra indicated that energy migration contributed to the relaxation of the singlet excited state at all concentrations studied. In comparison with solutions, the absorption spectra of the films revealed a distinct band narrowing and an increase in molar extinction of the Soret band. Presumably, PFP formed unusually uniform structures, a kind of highly ordered aggregates, in the plane of monolayers.

Photoinduced vectorial electron transfer in multilayered Langmuir-Blodgett films of porphyrin and phtalocyanine derivatives

A series of new phtalocyanines and phtalocyanine-fullerene dyads was synthesized. The dyads were transferred to solid substrates in the form of the Langmuir-Blodgett films. Poly(hexylthiophene) (PHT) and/or phtalocyanine (B6PH) were used as secondary electron donors in the multilayered structures, together with molecules of phtalocyanine-fullerene (B6PF) or porphyrin-fullerene (DHD6ee) dyads in a matrix of octadecylamine (ODA) molecules in a ratio of 1: 9. Directed photoinduced electron transfer in films was studied by means of the time-resolved Maxwell displacement charge method. It was established that the addition of a monolayer of molecules of the secondary B6PH donor to a monolayer of molecules of the DHD6ee/ODA (1: 9) dyad resulted in a thirty-fold increase in the sample’s photovoltaic response; it did not, however, change the recombination rate of the charges as compared to a single monolayer of the dyad molecules. In the case of bilayer samples consisting of B6PF/ODA (1: 9) and PHT/ODA (3: 2) monolayers, both quantities increased. The absorption of visible light over a wide spectral range was achieved via the use of three-component (PHT, B6PH and DHD6ee) multilayered structures. The relative sensitivities of the samples to excitation radiation were assessed and the efficiencies of their transformation of light energy to electric potential were compared. The largest sensitivity values were obtained for three-component samples prepared from PHT, B6PH, and DHD6ee monolayers in which the sensitivity value was 500 times larger than that for a separate monolayer of DHD6ee dyad molecules.

Interlayer photoinduced electron transfer in Langmuir-Blodgett films based on porphyrin and phthalocyanine derivatives

Vectorial photoinduced electron transfer was studied in Langmuir-Blodgett films. The monolayers of polythiophene and phthalocyanine were used as secondary electron donors in multilayers together with the monolayer of porphyrin-fullerene donor-acceptor dyad molecules mixed with octadecylamine (1:9). Simultaneous utilization of all the three compounds in the preparation of three-layer samples allowed us to widen the range of the visible light absorption by the structure. The relative sensitivity of the sample to the excitation laser light increased by about 350 times compared with that of a dyad monolayer.