Organic binary charge-transfer compounds of 2,2' : 6',2'' : 6'',6-trioxotriphenylamine and a pyrene-annulated azaacene as donors

Three binary charge-transfer (CT) compounds resulting from the donor 2,2' : 6',2'' : 6'',6-trioxotriphenylamine (TOTA) and the acceptors F4TCNQ and F4BQ and of a pyrene-annulated azaacene (PAA) with the acceptor F4TCNQ are reported. The identity of these CT compounds are confirmed by single-crystal X-ray diffraction as well as by IR, UV-vis-NIR and EPR spectroscopy. X-ray diffraction analysis reveals a 1 : 1 stoichiometry for TOTA·F4TCNQ, a 2 : 1 donor : acceptor ratio in (TOTA)2·F4BQ, and a rare 4 : 1 stoichiometry in (PAA)4·F4TCNQ, respectively. Metrical parameters of the donor (D) and acceptor (A) constituents as well as IR spectra indicate full CT in TOTA·F4TCNQ, partial CT in (TOTA)2·F4BQ and only a very modest one in (PAA)4·F4TCNQ. Intricate packing motifs are present in the crystal lattice with encaged, π-stacked (F4TCNQ-)2 dimers in TOTA·F4TCNQ or mixed D/A stacks in the other two compounds. Their solid-state UV-vis-NIR spectra feature CT transitions. The CT compounds with F4TCNQ are electrical insulators, while (TOTA)2·F4BQ is weakly conducting.

Reversible Multielectron Release from Redox-Active Three-Dimensional Molecular Barrels with Ruthenium-Alkenyl Moieties

Three-dimensional molecular barrels Ru₆-4 and Ru₆-5 were synthesized in high yields from dinuclear ruthenium-vinyl clamps and tritopic triphenylamine-derived carboxylate linkers and characterized by multinuclear NMR spectroscopy including ¹H-¹H COSY and ¹H DOSY measurements, high-resolution electrospray ionization mass spectrometry, and X-ray crystallography. The metal frameworks of the cages adopt the shape of twisted trigonal prisms, and they crystallize as racemic mixtures of interdigitating Δ- and Λ-enantiomers with a tight columnar packing in Ru₆-4. Electrochemical studies and redox titrations revealed that the cages are able to release up to 11 electrons on the voltammetric timescale and that their cage structures persist up to the hexacation level. IR and UV-vis-near-infrared spectroelectrochemical studies confirm substituent-dependent intramolecular electronic communication within the π-conjugated 1,3-divinylphenylene backbone in the tricationic states, where all three divinylphenylene-bridged diruthenium clamps are present in mixed-valent radical cation states. The formation of 1:3 charge-transfer salts with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane as the electron acceptor is also demonstrated.

Conformation and Quantum-Interference-Enhanced Thermoelectric Properties of Diphenyl Diketopyrrolopyrrole Derivatives

Manipulating the connectivity of external electrodes to central rings of carbon-based molecules in single molecule junctions is an effective route to tune their thermoelectrical properties. Here we investigate the connectivity dependence of the thermoelectric properties of a series of thiophene-diketopyrrolopyrrole (DPP) derivative molecules using density functional theory and tight-binding modeling, combined with quantum transport theory. We find a significant dependence of electrical conductance on the connectivity of the two thiophene rings attached to the DPP core. Interestingly, for connectivities corresponding to constructive quantum interference (CQI), different isomers obtained by rotating the thiophene rings possess the same electrical conductance while those corresponding to destructive quantum interference (DQI) show huge conductance variations upon ring rotation. Furthermore, we find that DQI connectivity leads to enhanced Seebeck coefficients, which can reach 500-700 μV/K. After including the contribution to the thermal conductance from phonons, the full figure of merit (ZT) for the CQI molecules could reach 1.5 at room temperature and it would further increase to 2 when temperature elevates to 400 K. Finally, we demonstrate that doping with tetracyanoquinodimethane can change the sign of the Seebeck coefficients by forming a charge-transfer system with the DPP.

Ground-state charge-transfer interactions in donor:acceptor pairs of organic semiconductors - a spectroscopic study of two representative systems

We investigate blended donor:acceptor (D:A) thin films of the two donors diindenoperylene (DIP) and poly(3-hexylthiophene) (P3HT) mixed with the strong acceptor 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F₆TCNNQ) using Polarization-Modulation Infrared Reflection-Absorption Spectroscopy (PMIRRAS). For DIP:F₆TCNNQ thin films we first carry out a comprehensive study of the structure as a function of the D : A mixing ratio, which guides the analysis of the PMIRRAS spectra. In particular, from the red-shift of the nitrile (C[triple bond, length as m-dash]N) stretching of F₆TCNNQ in the different mixtures with DIP, we quantify the average ground-state charge-transfer (GS-CT) to be ρ_avg = (0.84 ± 0.04) e. The PMIRRAS data for P3HT:F₆TCNNQ blended films reveal nearly the same shift of the CT-affected C[triple bond, length as m-dash]N stretching peak for this system. This points towards a very similar CT strength for the two systems. We extend the analysis to the relative intensity of the C[triple bond, length as m-dash]N to the C[double bond, length as m-dash]C stretching modes of F₆TCNNQ in the mixtures with DIP and P3HT, respectively, and support it with DFT calculations for the isolated F₆TCNNQ. Such comparison allows to identify the vibrational signatures of the acceptor mono-anion in P3HT:F₆TCNNQ, thus indicating a much stronger, integer CT-type interactions for this system, in agreement with available optical spectroscopy data. Our findings stress the importance of a simultaneous analysis of C[triple bond, length as m-dash]N and C[double bond, length as m-dash]C stretching vibrations in F₆TCNNQ, or similar quinoid systems, for a reliable picture of the nature of GS-CT interactions.

Unusual Electronic Structure of the Donor-Acceptor Cocrystal Formed by Dithieno[3,2-a:2',3'-c]phenazine and 7,7,8,8-Tetracyanoquinodimethane

Mixed cocrystals derived from electron-rich donor (D) and electron-deficient acceptor (A) molecules showcase electronic, optical, and magnetic properties of interest for a wide range of applications. We explore the structural and electronic properties of a cocrystal synthesized from dithieno[3,2-a:2',3'-c]phenazine (DTPhz) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), which has a mixed-stack packing arrangement of the (π-electronic) face-to-face stacks in a 2:1 D:A stoichiometry. Density functional theory investigations reveal that the primary electronic characteristics of the cocrystal are not determined by electronic interactions along the face-to-face stacks, but rather they are characterized by stronger electronic interactions orthogonal to these stacks that follow the edge-to-edge donor-donor or acceptor-acceptor contacts. These distinctive electronic characteristics portend semiconducting properties that are unusual for semiconducting mixed cocrystals and suggest further potential to design organic semiconductors with orthogonal transport characteristics for different charge carriers.

Evidence for Anisotropic Electronic Coupling of Charge Transfer States in Weakly Interacting Organic Semiconductor Mixtures

We present a comprehensive investigation of the charge-transfer (CT) effect in weakly interacting organic semiconductor mixtures. The donor-acceptor pair diindenoperylene (DIP) and N,N'-bis(2-ethylhexyl)-1,7-dicyanoperylene-3,4/9,10-bis(dicarboxyimide) (PDIR-CN₂) has been chosen as a model system. A wide range of experimental methods was used in order to characterize the structural, optical, electronic, and device properties of the intermolecular interactions. By detailed analysis, we demonstrate that the partial CT in this weakly interacting mixture does not have a strong effect on the ground state and does not generate a hybrid orbital. We also find a strong CT transition in light absorption as well as in photo- and electroluminescence. By using different layer sequences and compositions, we are able to distinguish electronic coupling in-plane vs out-of-plane and, thus, characterize the anisotropy of the CT state. Finally, we discuss the impact of CT exciton generation on charge-carrier transport and on the efficiency of photovoltaic devices.

Optical-quality controllable wet-chemical doping of graphene through a uniform, transparent and low-roughness F4-TCNQ/MEK layer

We developed a uniform, transparent and low-roughness F4-TCNQ/MEK layer to controllably dope graphene with superior optical quality.