Highly ordered films of neat calix[4]arenes for second order nonlinear optics

Abstract

Communications ADVANCED MATERIALSfact, the devices could not be operated in air owing to significant degradation as a function of oxygen and/or water partial pressure. Whereas storage in air hardly changed the I-V characteristic (e.g., 40 % reduction of conductivity after 3 weeks for EC4T) and luminance or output power, operation in air led to a rapid degradation. For example, the output power decayed by a factor of five within 10 s for EC6T and within 20 s for EC7T; EC4T and ECST could not even be measured. Thus, we operated the LEDs in an inert N, atmosphere, which provided relatively stable conditions such that EC7T and EC6T showed little degradation (e.g., 25 Yo) after 15 min of operation at 2 mA. The reason for the rapid degradation in air (and also the slight degradation in a nitrogen atmosphere) is most likely oxidation of the Al electrode, which increases the barrier height and hence reduces the injection probability of electrons. The question remains why different oligothiophenes cause different degradation behavior. At present we can think of two possible answers: a) a different degree of dissociation or follow-up reactions of the oligothiophenes, perhaps at the inner electrode surface, in oxidizing atmosphere and in the presence of a current and/or electric field, and b) different diffusion velocities of 0, and/or H,O through the thin molecular film. The thermal molecular motion induced by the current may be responsible for rapid diffusion in operation while the different film morphologies (see above) may have caused the differences amongst the oligomers. More work is needed to give a clearer answer and to solve the problem of poor LED stability.Finally, we should briefly comment on the output power, which was measured by a calibrated laser power meter and is also displayed in Figure 2 for EC6T (dash-dotted curve, right hand scale). The light emission starts at voltages as low as 2.5 V (for a 160 nm thick EC6T film) and reaches a maximum around 8 V, as can be derived from the inset, which shows the power conversion (not quantum) efficiency curve. This curve varies relatively little (factor of two) as a function of voltage but has only a modest maximum of 4 x YO.(The value for EC7T is even smaller by a factor of four; the value for ECST is markedly higher but could not be measured with sufficient accuracy.) These values have not been corrected for geometric effects, for internal reflection and absorption, and for internal energy conversion processes. If estimated corrections are taken into account the values become considerably higher (by about a factor of 100) and the quantum efficiency may be 1 0~2 -1 0 -3 % . This is still very low and far from being satisfactory, in particular in view of applications. We have some evidence that the low efficiency has various causes, such as film morphology, electrode material with too high a work function, interface bonding and reactions, and the physical properties of the molecules. We believe that it can be improved by orders of magnitude if materials and preparat...

Nonlinear Optical Properties of Correlated Chromophores in Organic Mesoscopic Superstructures

“…Calixarenes are very attractive in the field of second-order nonlinear optics. They have been incorporated in neat films with large susceptibility and exceptional stability, [39] and are, therefore, interesting for application. The possibility of substituting the calyx with one, two, three, or four groups also promised them to be good candidates for a thorough vector analysis of the hyperpolarizability of the calixarene superstructure.…”

Monolithic integration of microelectronics and photonics using molecularly engineered materials

“…Increasing the size of the dopant (films of large concentrations of nitrocalix{4}arenes in a PMMA matrix and of 100% dopant were obtained) increases the temporal polar order stability. 17 To obtain the desired properties, the chromophores were covalently bonded to linear polymers as a side chain or in the main chain. However, larger number densities will lower the polymer's Tg by the plasticizing effect.…”

Enhanced Second-Order Optical Nonlinearity of Dye Molecules Adsorbed onto Laponite Particles

“…First, the requirements for materials to be used in device applications that utilize nonlinear interactions are very demanding. New approaches to materials research can yield materials with favorable properties compared to traditional nonlinear materials, as shown recently in several supramolecular systems (Cox et al 1990;Stucky and MacDougall 1990;Clays et al 1993;Kelderman et al 1993;Kauranen et al 1995;Lehn 1995). This is particularly true for 2nd-order nonlinear optics that require noncentrosymmetric materials (Prasad and Williams 1991).…”