Photophysical and Protonation Time Resolved Studies of Donor-Acceptor Branched Systems With Pyridine Acceptors.

A comparative study of the photophysical properties of octupolar pyridyl-terminated triphenylamine molecule, with its quadrupolar and dipolar analogues, by means of ambient and low temperature steady state spectroscopy and femtosecond to nanosecond time-resolved fluorescence spectroscopy is reported. The push-pull molecules bear triphenylamine electron donating core, pyridine peripheral electron acceptors, and acetylene π-bridge. The samples were studied in solvents of varying polarity and also upon addition of small amounts of acetic acid to induce protonation of the pyridine group. All samples exhibit significant positive fluorescence solvatochromism as well as a relaxation of their excited state to a solvent relaxed intramolecular charge transfer state on the picosecond time scale. For the octupolar compound, excited state relaxation occurs simultaneously with excitation energy hopping among the branches. The hopping time is solvent polarity controlled since it becomes slower as the polarity increases. The experimental hopping times are compared to those predicted by Förster and Fermi formulations. The samples are capable of emitting broadband light covering almost the whole visible spectrum by careful control of protonation. Energy transfer from the neutral toward the protonated species on the 1 ps time scale is revealed.

Low Work Function Lacunary Polyoxometalates as Electron Transport Interlayers for Inverted Polymer Solar Cells of Improved Efficiency and Stability.

Effective interface engineering has been shown to play a vital role in facilitating efficient charge-carrier transport, thus boosting the performance of organic photovoltaic devices. Herein, we employ water-soluble lacunary polyoxometalates (POMs) as multifunctional interlayers between the titanium dioxide (TiO2) electron extraction/transport layer and the organic photoactive film to simultaneously enhance the efficiency, lifetime, and photostability of polymer solar cells (PSCs). A significant reduction in the work function (WF) of TiO2 upon POM utilization was observed, with the magnitude being controlled by the negative charge of the anion and the selection of the addenda atom (W or Mo). By inserting a POM interlayer with ∼10 nm thickness into the device structure, a significant improvement in the power conversion efficiency was obtained; the optimized POM-modified poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2- 33 ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl-C70 butyric acid methyl ester (PTB7:PC70BM)-based PSCs exhibited an efficiency of 8.07%, which represents a 21% efficiency enhancement compared to the reference TiO2 cell. Similar results were obtained in POM-modified devices based on poly(3-hexylthiophene) (P3HT) with electron acceptors of different energy levels, such as PC70BM or indene-C60 bisadduct (IC60BA), which enhanced their efficiency up to 4.34 and 6.21%, respectively, when using POM interlayers; this represents a 25-33% improvement as compared to the reference cells. Moreover, increased lifetime under ambient air and improved photostability under constant illumination were observed in POM-modified devices. Detailed analysis shows that the improvements in efficiency and stability synergistically stem from the reduced work function of TiO2 upon POM coverage, the improved nanomorphology of the photoactive blend, the reduced interfacial recombination losses, the superior electron transfer, and the more effective exciton dissociation at the photoactive layer/POM/TiO2 interfaces.

Formation of a highly-ordered rigid multichromophoric 3D supramolecular network by combining ionic and coordination-driven self-assembly.

We present here the self-assembly of a green-emitting metallosupramolecular rhomboid into a rigid, highly-ordered 3D multichromophoric network through the mediation of a tetra-anionic violet-blue molecular emitter. Control was obtained on the spatial topology, the electronic energy landscape and the fluorescence polarization of the interacting dipoles.

Energy transfer within self-assembled cyclic multichromophoric arrays based on orthogonally arranged donor-acceptor building blocks.

We herein present the coordination-driven supramolecular synthesis and photophysics of a [4+4] and a [2+2] assembly, built up by alternately collocated donor-acceptor chromophoric building blocks based, respectively, on the boron dipyrromethane (Bodipy) and perylene bisimide dye (PBI). In these multichromophoric scaffolds, the intensely absorbing/emitting dipoles of the Bodipy subunit are, by construction, cyclically arranged at the corners and aligned perpendicular to the plane formed by the closed polygonal chain comprising the PBI units. Steady-state and fs time-resolved spectroscopy reveal the presence of efficient energy transfer from the vertices (Bodipys) to the edges (PBIs) of the polygons. Fast excitation energy hopping - leading to a rapid excited state equilibrium among the low energy perylene-bisimide chromophores - is revealed by fluorescence anisotropy decays. The dynamics of electronic excitation energy hopping between the PBI subunits was approximated on the basis of a theoretical model within the framework of Förster energy transfer theory. All energy-transfer processes are quantitatively describable with Förster theory. The influence of structural deformations and orientational fluctuations of the dipoles in certain kinetic schemes is discussed.

Highly efficient and unidirectional energy transfer within a tightly self-assembled host-guest multichromophoric array.

The coordination-driven synthesis of a rhomboid cavitand composed of two different Bodipys and its inclusion complex with 1,3,6,8-tetrasulfopyrene via ionic self-assembly was established by X-ray crystallography. Highly efficient and unidirectional intra-host and guest-to-host energy transfer was demonstrated by femtosecond fluorescence spectroscopy.