Optoelectronic properties of diketopyrrolopyrrole homopolymers compared to donor-acceptor copolymers.

Diketopyrrolopyrrole (DPP) is a component of a large number of materials used for optoelectronic applications. As it is exclusively used in combination with aromatic donors, the properties of its homopolymers are unknown. Because donor-acceptor character has been shown for other systems to reduce bandwidths, DPP homopolymers should have even larger conduction bands and better n-type conductivity than the thiophene-flanked systems, which have exceptional n-type conductivity and ambipolar character.

Spin density encodes intramolecular singlet exciton fission in pentacene dimers.

The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers.

Air-Stable n-channel Diketopyrrolopyrrole-Diketopyrrolopyrrole Oligomers for High Performance Ambipolar Organic Transistors.

n-channel organic semiconductors are prone to oxidation upon exposed to ambient conditions. Herein, we report design and synthesis of diketopyrrolopyrrole (DPP)-based oligomers for ambipolar organic thin-film transistors (OFETs) with excellent air and bias stability at ambient conditions. The cyclic voltammetry measurements reveal exceptional electrochemical stability during the redox cycle of oligomers.

Effect of Donor-Acceptor Substitution on Optoelectronic Properties of Conducting Organic Polymers.

The impact of donor-acceptor substitution on optical and electronic properties of conducting polymers was investigated with time-dependent density functional theory (TDDFT). A series of donor-acceptor systems with thiophene, 3,4-ethylenedioxythiophene, and pyrrole as donors and 3,4-difluorothiophene, diketopyrrolopyrrole, 2,1,3-benzothiadiazole, 4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b']dithiophene, and indeno[1-2b]-fluorene-6,12-dimalonitrile as acceptors as examples of donor-acceptor systems with increasing donor-acceptor character was studied. Spectral properties were analyzed in terms of differences in ionization potentials and electron affinities of donors and acceptors, charge separations between donors and acceptors in ground and excited states, and electron distribution in the acceptor units.

Effect of Chalcogens on Electronic and Photophysical Properties of Vinylene-Based Diketopyrrolopyrrole Copolymers.

Three vinylene linked diketopyrrolopyrrole based donor-acceptor (D-A) copolymers have been synthesized with phenyl, thienyl, and selenyl units as donors. Optical and electronic properties were investigated with UV-vis absorption spectroscopy, cyclic voltammetry, near edge X-ray absorption spectroscopy, organic field effect transistor (OFET) measurements, and density functional theory (DFT) calculations. Optical and electrochemical band gaps decrease in the order phenyl, thienyl, and selenyl.

Use of side-chain for rational design of n-type diketopyrrolopyrrole-based conjugated polymers: what did we find out?

The primary role of substituted side chains in organic semiconductors is to increase their solubility in common organic solvents. In the recent past, many literature reports have suggested that the side chains play a critical role in molecular packing and strongly impact the charge transport properties of conjugated polymers. In this work, we have investigated the influence of side-chains on the charge transport behavior of a novel class of diketopyrrolopyrrole (DPP) based alternating copolymers.

Quantitatively Correct UV-vis Spectrum of Ferrocene with TDB3LYP.

The ultraviolet-visible light (UV-vis) absorption spectrum of ferrocene is modeled with time-dependent density functional theory employing LSDA, BLYP, B3LYP, and CAM-B3LYP functionals in combination with 6-31G*, 6-31+G*, CC-PVTZ, and aug-CC-PVTZ basis sets. With the exception of LSDA, all functionals predict a reasonable Fe-CP distance of ∼1.67 Å.

Improved Prediction of Properties of π-Conjugated Oligomers with Range-Separated Hybrid Density Functionals.

Range-separated hybrid functionals along with global hybrids and pure density functionals have been employed to calculate geometries, ionization energies (IP)s, electron affinities (EA)s, and excitation energies of neutral and oxidized polyenes, thiophene, and furan oligomers. Long-range correction with 100% HF exchange solves the problem of density functional theory with incorrect chain length dependence of IPs and energy gaps. There is a possibility of overcorrection, if the short-range part of the functional with no or low HF exchange is too small.

Modeling photoelectron spectra of conjugated oligomers with time-dependent density functional theory.

With the aim of producing accurate band structures of conjugated systems by employing the states of cations, TDDFT calculations on conjugated oligomer radical cations of thiophene, furan, and pyrrole with one to eight rings were carried out. Benchmarking of density functional theory and ab initio methods on the thiophene monomer shows that the ΔSCF ionization potential (IP) is most accurate at the B3LYP/6-311G* level. Improvement of the basis set beyond 6-311G* leads to no further changes.

Effects of perfluorination on thiophene and pyrrole oligomers.

The effect of perfluorination on thiophene and pyrrole oligomers in neutral, cationic, and anionic states was investigated with density functional theory at the (TD)B3P86-30%/6-31G* level. For the title compounds fluorination leads to planarization. For pyrroles a band gap reduction of 0.

Tuned range-separated hybrids in density functional theory.

We review density functional theory (DFT) within the Kohn-Sham (KS) and the generalized KS (GKS) frameworks from a theoretical perspective for both time-independent and time-dependent problems. We focus on the use of range-separated hybrids within a GKS approach as a practical remedy for dealing with the deleterious long-range self-repulsion plaguing many approximate implementations of DFT. This technique enables DFT to be widely relevant in new realms such as charge transfer, radical cation dimers, and Rydberg excitations.

Koopmans' springs to life.

The meaning of orbital energies (OOEs) in Kohn-Sham (KS) density functional theory (DFT) is subject to a longstanding controversy. In local, semilocal, and hybrid density functionals (DFs) a Koopmans' approach, where OOEs approximate negative ionization potentials (IPs), is unreliable. We discuss a methodology based on the Baer-Neuhauser-Livshits range-separated hybrid DFs for which Koopmans' approach "springs to life.

Theoretical modeling of the doping process in polypyrrole by calculating UV/Vis absorption spectra of neutral and charged oligomers.

Changes in absorption spectra during doping of oligopyrroles were investigated with time-dependent density functional theory on optimized structures of neutral, singly, and doubly charged pyrrole oligomers with up to 24 rings. In the absence of counterions, defects are delocalized. Counterions induce localization.

Theoretical investigation of excited states of oligothiophene anions.

Electron-hole symmetry upon p- and n-doping of conducting organic polymers is rationalized with Hückel theory by the presence of symmetrically located intragap states. Since density functional theory (DFT) predicts very different geometries and energy level diagrams for conjugated pi-systems than semiempirical methods, it is an interesting question whether DFT confirms the existence of electron-hole symmetry predicted at the Hückel level. To answer this question, geometries of oligothiophene anions with 5-19 rings were optimized and their UV/vis spectra were calculated with time-dependent DFT.

Investigation of charge carriers in doped thiophene oligomers through theoretical modeling of their UV/Vis spectra.

The nature of the charge carriers in conducting organic polymers (COPs) is a long standing problem. Polythiophene is one of the prototypes of COPs and intensively studied. Because doping leads to changes in UV/vis spectra that are characteristic of the absorbing species, UV/vis spectra of charged thiophene oligomers with up to 25 rings were calculated with time-dependent density functional theory.

Theoretical Investigation of Excited States of Oligothiophenes and of Their Monocations.

Excitation energies of neutral thiophene oligomers with chain lengths of up to 25 rings and charged thiophene oligomers with chain lengths of up to 20 rings were calculated with time-dependent Hartree-Fock and time-dependent density functional theory (TDDFT). As recently for polyene cations, very good agreement is found between TDDFT and high-level ab initio calculations and with experimental results wherever data are available. For short thiophene oligomer cations, two sub-band transitions are predicted; for long chains, a third transition develops.

Theoretical Investigation of Excited States of Large Polyene Cations as Model Systems for Lightly Doped Polyacetylene.

Electronic excitations of polyene cations with chain lengths of up to 101 CH units were investigated as model systems for lightly doped polyacetylene (PA). Since high level ab initio calculations such as complete active space perturbation theory (CASPT2) are limited to systems with about 14 CH units, the performances of time-dependent Hartree-Fock (TDHF) and time-dependent density functional theory (TDDFT) were evaluated. It turned out that TDDFT excitations energies are much more accurate for polyene cations than for neutral polyenes.

Does the donor-acceptor concept work for designing synthetic metals? III. Theoretical investigation of copolymers between quinoid acceptors and aromatic donors.

Homopolymers of quinoxaline (QX), benzothiadiazole (BT), benzobisthiadiazole (BBT), thienopyrazine (TP), thienothiadiazole (TT), and thienopyrazinothiadiazole (TTP) and copolymers of these acceptors with thiophene (TH) and pyrrole (PY) were investigated with density functional theory. Theoretical band-gap predictions reproduce experimental data well. For all but six copolymers, band-gap reductions with respect to either homopolymer are obtained.

Pyramidalized Double Bonds Containing Endoperoxide Linkages: Photooxygenation of Dimethyl cis-3,8-Dihydroheptalene-3,8-dicarboxylate.

Diels-Alder cycloaddition utilizing singlet oxygen as the dienophile with dimethyl cis-3,8-dihydroheptalene-3,8-dicarboxylate (5) has been investigated, and monoaddition product 7 has been isolated. The addition of a second singlet oxygen to the cycloheptatriene unit in 7 gave syn-bis(norcaradiene) bis(endoperoxide) 4. (1)H NMR spectral studies and theoretical calculations indicate the increased pyramidalization in syn-4 compared with carbon analogue.

Theoretical Analysis of Substituent Effects on Building Blocks of Conducting Polymers: 3,4'-Substituted Bithiophenes.

Substituents are widely used to modify the properties of conducting polymers. To study substituent effects on energy levels and energy gaps systematically, CH(3)-, OH-, NH(2)-, CN-, and CCH-substituted bithiophenes were examined with density functional theory and NBO analysis. Total charges and pi-electron densities were analyzed separately to examine pi- and sigma-effects.