We investigated the capability of molecular triads, consisting of two strong fluorophores that were covalently linked to a photochromic molecule, for optical gating. Therefore we monitored the fluorescence intensity of the fluorophores as a function of the isomeric state of the photoswitch. From the analysis of our data we develop a kinetic model that allows us to predict quantitatively the degree of the fluorescence modulation as a function of the mutual intensities of the lasers that are used to induce the fluorescence and the switching of the photochromic unit. We find that the achievable contrast for the modulation of the fluorescence depends mainly on the intensity ratio of the two light beams and appears to be very robust against absolute changes of these intensities. The latter result provides valuable information for the development of all-optical circuits which would require to handle different signal strengths for the input and output levels.
To obtain novel low‐bandgap materials with tailored hole‐transport properties and extended absorption, electron rich 3,4‐ethylenedioxythiophene is introduced as a comonomer in diketopyrrolo[3,4‐c]pyrrole copolymers with different aryl flanking units. The polymers are characterized by absorption and photoluminescence spectroscopy, dynamic scanning calorimetry, cyclic voltammetry, and X‐ray diffraction. The charge transport properties of these new materials are studied carefully using an organic field effect transistor geometry where the charge carriers are transported over a narrow channel at the semiconductor/dielectric interface. These results are compared to bulk charge carrier mobilities using space‐charge limited current (SCLC) measurements, in which the charge carrier is transported through the complete film thickness of several hundred nanometers. Finally, charge carrier mobilities are correlated with the electronic structure of the compounds. We find that in particular the thiophene‐flanked copolymer PDPP[T]₂‐EDOT is a very promising candidate for organic photovoltaics, showing an absorption response in the near infrared region with an optical bandgap of 1.15 eV and a very high bulk hole mobility of 2.9 × 10⁻⁴ cm² V⁻¹ s⁻¹ as measured by SCLC. This value is two orders of magnitudes higher than SCLC mobilities reported for other polydiketopyrrolopyrroles and is in the range of the well‐known hole transporting polymer poly(3‐hexylthiophene). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 639–648
We investigate in a combined theoretical and experimental study the influence that the specific arrangement, e.g., alternating or random, of donor and acceptor units has on the optical absorption of extended molecules. Because of its important role in low gap polymers we discuss in particular the energetic position of the first electronic transition. We theoretically determine the excitations in extended oligomers with thiophene as the donor and 2,1,3-benzothiadiazole as the acceptor component by using time-dependent density functional theory based on non-empirically tuned range separated hybrid functionals. Corresponding systems are synthesized and theoretical and experimental data are critically compared to each other. We conclude that the influence of the specific arrangement of donor and acceptor monomers on the optical gap is limited and that effects beyond the single molecule level effectively limit the size of the experimentally observed optical gap.
The synthesis and analysis of solution processable polymers for organic solar cells is crucial for innovative solar cell technologies such as printing processes. In the field of donor materials for photovoltaic applications, polymers based on tetraphenylamine (TPA) are well known hole conducting materials. Here, we synthesized two conjugated TPA containing copolymers via Suzuki polycondensation. We investigated the tuning of the energy levels of the TPA based polymers by two different concepts. Firstly, we introduced an acceptor unit in the side chain. The main-chain of this copolymer was built from TPA units. The resulting copolymer 2-(4-((4'-((4-(2-ethylhexyloxy)phenyl)(paratolyl) amino)biphenyl-4-yl)(para-tolyl)amino)benzylidene) malononitrile P1 showed a broader absorption up to 550 nm. Secondly, we used a donor-acceptor concept by synthesizing a copolymer with alternating electron donating TPA and electron withdrawing Thieno[3,4-b]thiophene ester units. Consequently, the absorption maximum in the copolymer octyl-6-(4-((4-(2-ethylhexyloxy)phenyl)(p-tolyl)amino)phenyl)-4-methylthieno[3,4-b]thiophene-2-carboxylate P2 was red shifted to 580 nm. All three polymers showed high thermal stability. By UV-vis and Cyclic voltammetry measurements the optical and electrochemical properties of the polymers were analyzed.
We report a novel preparation method for nanocrystalline TiO2 networks with controlled pore sizes using spherical polyelectrolyte brushes (SPB) as templates. The SPB consists of a solid polystyrene core from which anionic polyelectrolytes are densely grafted. The SPB templates are synthesized via conventional photoemulsion polymerization with efficient control of core size and brush length. Subsequently, the TiO2 precursor is hydrolyzed at room temperature within the anionic brush to obtain anatase nanocrystals of 12–20 nm size. These stable and form-persistent composite particles of SPB decorated with anatase nanocrystals are then assembled on a conductive substrate. The subsequent calcination of this composite layer leads to a robust nanocrystalline TiO2 network, in which the pores and the wall thickness are directly correlated to the polystyrene core size and the amount of TiO2 hydrolyzed within the brush respectively. In this study, we optimized different thin-film preparation methods and characterized the resulting nanocrystalline TiO2 networks using SEM and XRD. Moreover, the applicability of these nanocrystalline networks as electron transport layers are tested in solid-state dye-sensitized solar cells (SDSCs). The first test devices exhibited efficiencies up to 0.8%. The precise and individual control of parameters such as porosity, thickness and crystallinity makes this concept highly attractive for the realization of efficient solid-state hybrid devices.
The performance of organic light-emitting devices (OLEDs) is strongly influenced by the electronic properties of the employed materials. In order to determine the effect of these materials' parameters, several different hole-transporting 1,3,5-tris(4-diphenylaminophenyl)benzenes (TDAPBs) were synthesised. These TDAPBs contained different substituents, different numbers of substituents and different positions of theses substituents. For the evaluation of the electronic properties, cyclic voltammetry was employed in order to determine the HOMO values, and time-of-flight (TOF) measurements to obtain the hole mobilities. OLEDs were prepared consisting of the TDAPBs blended in a polymer matrix, and of Alq3 as electron-conducting and light-emitting layer. These devices were investigated regarding their current density/voltage characteristics, efficiencies, onset voltages for electroluminescence, and lifetimes. For hole-transporting blend systems an exponential relationship between the current density and the HOMO levels of the TDAPBs was found. However, even though the HOMO values cover a range from −5.09 to −5.35 eV, no effects on the performance of the OLEDs were detected for electroluminescent two-layer systems. In this case the initial voltage seems to be a determining parameter for the behaviour of the devices during operation. Copyright © 1999 John Wiley & Sons, Ltd.
