The energy transfer in amorphous polyvinyl carbazole (PVCA) has been studied by fluorescence quenching experiments using perylene, trinitrofluorenone, and hexachloro‐p‐xylene as guest molecules. The PVCA shows excimer fluorescence. The results are discussed in terms of exciton diffusion and competition between excimer‐forming sites and guest molecules for the trapping of excitons. Single‐step transfer from the excimer state by dipole resonance is considered as an alternative mechanism of energy transfer; in all three cases, however, exciton migration is the more probable energy‐transfer mechanism. The concentration of excimer‐forming sites is estimated to be 10−3 mole/mole basic unit of the polymer. In the pure polymer the exciton covers about a thousand basic units during its lifetime.

Transfer of Electronic Excitation Energy in Polyvinyl Carbazole

The photogeneration efficiency of holes in films of highly purified poly (N‐vinylcarbazole) (PVCA) and of PVCA containing 0.1% by weight trichloroacetic acid (TCAA) has been studied as a function of applied field (5×104−2×106 V/cm) and excitation energy (3.4–5.2 eV), and the influence of extrinsic effects such as aging, ambient and photodecomposition upon the carrier generation efficiency, fluorescence, and absorption has been examined. It is observed that light and aging degrades the film. Possible photo‐oxidation processes are discussed. Oxygen is found to reduce the yields for carrier generation and fluorescence. The carrier generation in PVCA becomes less field dependent but more efficient with increasing excitation energy. The addition of TCAA to PVCA reduces the field dependence of the carrier generation, sensitizes it at low fields, but quenches it at high fields. A kinetic model is developed which assumes that the field dependence of the generation yield can be described in terms of the Onsager model for geminate recombination. The essential parameters from this analysis are φ0∼0.14 and ∼0.11 for the primary quantum yield for pure and acid doped PVCA, respectively. For PVCA the separation of the photoexcited electron‐hole pair increases from r0∼22.5 A for excitation into the first singlet state (1S1) to r0∼30 A for excitation into the third singlet state (1S3). For the acid doped film, we find r0∼30 A for 1S1 excitation. The kinetic model enables us to relate φ0 to molecular parameters, to account for the reduction of φ0 on doping with acids, and to predict the influence of extrinsic effects upon the spectral dependence of the carrier generation.

Photogeneration processes in poly (N‐vinylcarbazole)

Preparation of ultrathin polymer films based on two-dimensional molecular ordering

Sensitized fluorescence and fluorescence quenching has been studied in pyrene layers, doped with small amounts of coronene, anthraquinone and acenaphthylene. Coronene exhibits strong sensitized fluorescence at room temperature, but not at 77°K. This behaviour is due to decreased exciton mobility. The activation energy of the excition hopping process has been determined to be Ea = 0.055 eV. The lifetime of pyrene excimer fluorescence is decreased by quenchers in accordance with the steady-state measurements of relative quantum efficiencies. Three possible mechanisms of excimer state hopping in jpyrene are discussed

Exciton Diffusion in Pyrene-A Thermally Activated Hopping Process

The three new complexes TpPyLn(CH3CO2)2(H2O) (Ln = Eu (1), Gd(2), or Tb (3)) were prepared and characterized crystallographically. In the crystal lattices of these complexes, separate molecules are connected in infinite chains by π-stacking interactions. Complexes 1 and 3 display intense photoluminescence and triboluminescence (red and green respectively), while compound 3 exhibits electroluminescence commencing at 9 V in an ITO/PVK/3/Al device (ITO = indium-tin oxide, PVK = poly(N-vinylcarbazole)). A series of Eu/Tb-doped Gd compounds was prepared by cocrystallization from mixtures of 1 and 2 or 2 and 3, respectively. It was shown that π-stacking interactions are involved in increasing the efficiency of energy transfer from the gadolinium complex to emitting [TpPyEu]2+ or [TpPyTb]2+ centers, and this energy transfer occurs through hundreds of molecules, resembling the process of energy harvesting in chloroplast stacks.

Manifestation of π-π stacking interactions in luminescence properties and energy transfer in aromatically-derived Tb, Eu and Gd tris(pyrazolyl)borate complexes.

The electronic excitation energy transfer from the host material to guest molecules has been studied in the system perylene/N‐isopropyl carbazole by measuring the fluorescence intensity ratio guest/host over a great range of guest concentrations. Exciton diffusion and single‐step transfer are discussed as possible transfer mechanisms; it is shown that the former process prevails in this system. According to the hopping model, the number of steps during the lifetime of the exciton is calculated to be 4 × 104, and the average displacement from the starting point amounts to 1350 A.

W. Klöpffer

Papers

Transfer of Electronic Excitation Energy in Solid Solutions of Perylene in N‐Isopropylcarbazole