Intramolecular excimer formation between 3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione chromophoric groups linked by a flexible alkyl spacer

Bichromophoric molecules containing two 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) moieties linked via aliphatic spacers of different length are synthesized. Optical absorption spectroscopy indicates that the molecules adopt an extended conformation in solution. Fluorescence spectroscopy shows that photons are emitted from the locally excited singlet state in an extended conformation. In sufficiently polar solvents, quenching of fluorescence occurs and fluorescence quantum yield (FF) and fluorescence lifetime (tF) measurements indicate formation of an intramolecular excimer as the quenching mechanism. The redox potentials of the molecules and the solvent polarity dependence of the quenching are consistent with significant charge-transfer character of the excimer state. Photoinduced absorption measurements show enhanced intersystem crossing to the triplet state in polar solvents. Results indicate that in donor–acceptor p-conjugated materials involving the DPP moiety, excimer-like interchain polaron pair excited states could play an important role in the photophysics because of their close proximity in energy to the lowest singlet excited

Photophysics of Self-Assembled Monolayers of a π-Conjugated Quinquethiophene Derivative

The photophysics of fully and partially covered self-assembled monolayers (SAMs) of a quinquethiophene (5T) derivative have been investigated. The monolayers behave as H-aggregates. The fluorescence of fully covered SAMs is weak and red-shifted, and the extinction is blue-shifted as compared to that of single molecules. The fluorescence of partially covered SAMs is dominated by that of single molecules on the surface. The extinction spectra are similar for fully and partially covered monolayers, which show that even the smallest islands are H-aggregates. The extinction spectra furthermore closely resemble those for 5T single crystals, which demonstrates that in oligothiophene crystals the intermolecular interactions within one layer molecules are stronger than the interlayer electronic coupling.

Synthesis and optical properties of Pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione (iDPP)-based molecules

We describe the synthesis and photophysical properties of a series of derivatives of pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione-3,6-diyl (iDPP) linked to two oligothiophenes of variable length (nT). The iso-DPP-oligothiophenes (iDPPnTs) differ from the common pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl-oligothiophene analogues (DPPnTs) by a different orientation of the two lactam rings in the bicyclic iDPP unit compared to DPP. In contrast to the highly fluorescent DPPnTs, the new isomeric iDPPnTs exhibit only very weak fluorescence. We demonstrate with the help of quantum-chemical calculations that this can be attributed to a different symmetry of the lowest excited state in iDPPnT (A in C2 symmetry) compared to DPPnTs (B) and the corresponding loss in oscillator strength of the lowest energy transition. Upon extending the oligothiophene moiety in the iDPPnTs molecules, the charge transfer character of the lowest A excited state becomes more pronounced. This tends to preclude high fluorescence quantum yields even in extended iDPPnTs systems

Influence of the Position of the Side Chain on Crystallization and Solar Cell Performance of DPP-Based Small Molecules

Three isomeric π-conjugated molecules basedon diketopyrrolopyrrole and bithiophene (DPP2T) substitutedwith hexyl side chains in different positions areinvestigated for use in solution-processed organic solar cells.Efficiencies greater than 3% are obtained when a mildannealing step is used. The position of the side chains onthe DDP2Ts has a major influence on the optical andelectronic properties of these molecules in thin semicrystallinefilms. By combining optical absorption and fluorescencespectroscopy, with microscopy (AFM and TEM) andscattering techniques (GIWAXS and electron diffraction), we find that the position of the side chains also affects themorphology and crystallization of these DPP2Ts when they are combined with a C70 fullerene derivative in a thin film. The studydemonstrates that changing the side chain position is an additional, yet complex, tool to influence behavior of conjugatedmolecules in organic solar cells.KEYWORDS: small molecules, solar cells, side chain engineering, morphology, crystallinit

Influence of the Position of the Side Chain on Crystallization and Solar Cell Performance of DPP-Based Small Molecules

Three isomeric π-conjugated molecules based on diketopyrrolopyrrole and bithiophene (DPP2T) substituted with hexyl side chains in different positions are investigated for use in solution-processed organic solar cells. Efficiencies greater than 3% are obtained when a mild annealing step is used. The position of the side chains on the DDP2Ts has a major influence on the optical and electronic properties of these molecules in thin semicrystalline films. By combining optical absorption and fluorescence spectroscopy, with microscopy (AFM and TEM) and scattering techniques (GIWAXS and electron diffraction), we find that the position of the side chains also affects the morphology and crystallization of these DPP2Ts when they are combined with a C₇₀ fullerene derivative in a thin film. The study demonstrates that changing the side chain position is an additional, yet complex, tool to influence behavior of conjugated molecules in organic solar cells

Phototuning Selectively Hole and Electron Transport in Optically Switchable Ambipolar Transistors

One of the grand challenges in organic electronics is to develop multicomponent materials wherein each component imparts a different and independently addressable property to the hybrid system. In this way, the combination of the pristine properties of each component is not only preserved but also combined with unprecedented properties emerging from the mutual interaction between the components. Here for the first time, that tri‐component materials comprised of an ambipolar diketopyrrolopyrrole‐based semiconducting polymer combined with two different photochromic diarylethene molecules possessing ad hoc energy levels can be used to develop organic field‐effect transistors, in which the transport of both, holes and electrons, can be photo‐modulated. A fully reversible light‐switching process is demonstrated, with a light‐controlled 100‐fold modulation of p‐type charge transport and a tenfold modulation of n‐type charge transport. These findings pave the way for photo‐tunable inverters and ultimately for completely re‐addressable high‐performance circuits comprising optical storage units and ambipolar field‐effect transistors