Tailoring Ultrafast Singlet Fission by the Chemical Modification of Phenazinothiadiazoles.

Quantum chemistry and time-resolved spectroscopy are applied to rationalize how singlet fission (SF) is affected by systematic chemical modifications introduced into phenazinothiadiazoles (PTD). Substitution of the terminal aromatic ring of TIPS-tetracene by a thiadiazole group leads to a considerable change in the relative energies of its S1 and T1 states. Thus, in contrast to TIPS-tetracene, SF becomes exothermic for various PTD derivatives, which show S1-2T1 energy differences as high as 0.15 eV. This enables SF in PTD as corroborated by femtosecond transient absorption spectroscopy and TD-DFT calculations. The latter report T-T spectra consistent with thin film UV-vis femtosecond transient absorption of PTDs at long delays. TD-DFT calculations also show that the S1-T1 energy gap can be rationally tuned by introducing N atoms into the aromatic scaffold and by the halogenation of one side ring of the PTD. In addition, the specific S1-to-1(T1T1) electronic coupling depends on the crystal morphology and the electronic properties simultaneously. Thus, both of them govern the strength and the interplay between direct and superexchange couplings, which in the most favorable cases accelerate SF to rate constants beyond (100 fs)-1. Remarkably, direct coupling was found to contribute considerably to the total effective coupling and even to dominate it for some PTDs investigated here. A quantum yield of 200% is obtained on the early picosecond time scale for all compounds studied here, which is reduced to 100% due to triplet-triplet annihilation after a few nanoseconds.

Pristine Poly( para-phenylene): Relating Semiconducting Behavior to Kinetics of Precursor Conversion.

We investigated unsubstituted poly( para-phenylene) (PPP), a long-desired prototype of a conjugated polymer semiconductor. PPP was accessed via thermal aromatization of a precursor polymer bearing kinked, solubility-inducing dimethoxycyclohexadienylene moieties. IR spectroscopy and Vis ellipsometry studies revealed that the rate of conversion of the precursor to PPP increases with temperature and decreases with film density, indicating a process with high activation volume. The obtained PPP films were analyzed in thin-film transistors to gain insights into the interplay between the degree of conversion and the resulting p-type semiconducting properties. The semiconducting behavior of PPP was further unambiguously proven through IR and transistor measurements of molybdenum trioxide p-doped films.

Thiadiazolo-Azaacenes.

This work reports the synthesis and characterization of bis- and tetrakis(thiadiazolo)-appended di- and tetraazaacenes, displaying up to seven catenated benzene/pyrazine rings. The targets are obtained by condensation of benzo-bis(thiadiazole)-4,5-dione with aromatic di- and tetraamines. The condensation products-up to a heptacene-like species-are stable but can be insoluble. Soluble derivatives are readily processible, but do not show enhanced electron affinities, as the two or four attached benzothiadiazole units are effectively resonance-separated from the acene body, maximizing the number of Clar-sextets.

Azaacenes Bearing Five-Membered Rings.

We report the synthesis of processible (dihydro)pyracyclene- and acenaphthylene-substituted azaacenes using condensation reactions in solution. The targets are characterized via cyclic voltammetry, X-ray crystallography, UV/Vis, fluorescence spectroscopy and DFT/NICS calculations. Formal hydrogenation of the annulated five-membered ring surprisingly alters emission in the solid-state as a consequence of modulation of aromaticity and HOMO-LUMO overlap. Five highly fluorescent, crystalline azaacenes were investigated as emitters in organic light-emitting diodes, and their performance with respect to luminance and efficiency was compared to that of structurally related azaacenes.

Bromination Improves the Electron Mobility of Tetraazapentacene.

A cyclocondensation of TIPS-ethynyl-substituted diaminoarenes with in situ obtained 4,5-dibromocyclohexa-3,5-diene-1,2-dione has led to the synthesis of tetrabromotetraazapentacene (BrTAP). BrTAP is easily reduced to its air-stable radical anion and electron mobilities >0.56 cm2 V-1 s-1 can be achieved in thin-film transistors.

Yamamoto Coupling for the Synthesis of Benzophenes and Acene-Based Cyclooctatetraenes.

Two TIPS-ethynylated dibromoacenes were used in a shotgun Yamamoto-reaction with 1,2-dibromobenzene or 4,5-dibromoveratrol. Four soluble benzononaphenes/benzoundecaphenes were isolated in satisfactory yields. Examples of tetracene- and pentacene-fused tetrabenzocyclooctatetraenes also isolated from this reaction are reported. All compounds were characterized by UV/Vis and fluorescence spectroscopy, cyclic voltammetry, X-ray crystallography as well as DFT and NICS calculations.

"Butterfly Wings" Stabilize Heptacene.

The synthesis of bisalkynylated derivatives of tetrabenzo[a,c,p,r]heptacene and tetrabenzo[a,c,l,n]pentacene via two-/fourfold Stille reactions involving a 9-stannafluorene and suitable tetrabromoacenes is reported. These triphenylene-"winged" heptacenes are surprisingly stable and maintain a significant portion of the electronic properties of heptacenes.

Phenylene Bridged Cyclic Azaacenes: Dimers and Trimers.

The synthesis and characterization of novel macrocyclic, phenylene-bridged azaacenes is reported. These species were obtained either by a conventional benzoin- diamine condensation, as shown for the case of the cyclotrimers, in which the azaacene units are separated by meta-connected phenylene bridges, or by a Buchwald-Hartwig-type Pd-catalyzed coupling, which employs 1,2,5,6-tetrabromodibenzocyclooctatetraene as the substrate and bis-TIPS-ethynylated diaminobenzene, -naphthalene or -anthracene as the coupling partner to give the double coupling products azaacene-annulated dibenzocyclooctatetraenes in moderate yields. The macrocycles show strong emission and light emitting diodes have been built with brightnesses exceeding 1600 cd m-2 . We evaluated the optical and electronic properties and the solid-state structures of the molecules and discuss their properties through comparison with their linear and tetrameric N-heteroacene counterparts.

A Golden Access to Acenopentalenes.

By employing gold catalysis, starting from dialkynylated acenes a series of novel un-symmetrical aceno-annulated dibenzopentalenes has been prepared. The achieved yields range from 62-68 %. The fused systems contain naphthalene, anthracene, tetracene, and pentacene units. All new compounds are soluble and stable under standard conditions. The optical properties of the systems are dominated by the dibenzopentalene core for the smaller representatives, while for the anthracene-pentacene-based aceno-benzopentalenes the optical properties of the acene group dominates. Preliminary morphology tests on thin films showed a reverse trend between crystal size and molecule size.

Tetraazapentacene constructs: controlling bulk-morphology through molecular dimensionality.

Multi-core TIPSTAP-constructs of different dimensionality were created via "geometrization" of the monomeric, highly crystalline parent using alkyne linkers. Morphological diversity is produced, while the material acceptor strength remains untouched. We establish structure-function relationships as the bulk morphology is predicted from the molecular geometry.

Dibenzohexacene: Stabilization Through Additional Clar Sextets.

Triphenylene units stabilize hexacenes as dibenzohexacenes. The hexacenes formed by a statistical Yamamoto coupling of dibromobenzene with dibromo-TIPS-pentacene or, alternatively, Stille coupling with dimethyldibenzostannole. UV/Vis spectroscopy and quantum chemical calculations show that despite the double benzannulation, the species have hexacene-type character, as evidenced by their redshifted absorption characteristics. The charge carrier mobility of dibenzohexacene 1 a was determined up to 0.2 cm2 /(Vs).

Alkyne-Substituted N-Heterophenes.

The synthesis and characterization of novel S-shaped N-heterophenes is reported. The targets contain a central anthracene connected to two heterophenanthrene units. N-Heterophenes are synthesized by reacting Boldt's 1,2,6,7-anthracenetetraone with different dialkynylated ortho-diaminobenzenes. The condensation yields range from 63 to 75 %; all of the N-heterophenes are stable and isolable.

Dicyano-Substituted Diazaacenes.

Herein, we report the synthesis, spectroscopic, and structural properties of novel 2,3-dicyano-substituted azaacenes containing three to six annelated rings as a framework. The targets were prepared by Pd-catalyzed coupling of diamino-bis(tri-isopropylsilylethynyl)benzene, -naphthalene, and -anthracene with either 4,5-dibromophthalonitrile or 6,7-dibromonaphthalene-2,3-dicarbonitrile into the respective N,N'-dihydrodiazaacenes. Oxidation with MnO2 or PbO2 furnishes the dicyano-substituted diazaacenes in reasonable to good yields. An exception is the diazahexacene, where the N,N'-dihydrodiazaacene is difficult to oxidize. Most of the targets could be crystallized and pack in isolated pairs, stacks, and brick-wall motifs. The n-type behavior of these molecules was illustrated in a proof-of-concept organic field-effect transistor, showing mobilities up to 7×10-3  cm2 V-1 s-1 .

Dibenzobarrelene-Based Azaacenes: Emitters in Organic Light-Emitting Diodes.

The synthesis of soluble dibenzobarrelene-substituted azaacenes either by condensation reactions in solution or, more effectively, in the solid state by using a ball mill is reported herein. We relate their optical properties in thin films to their packing in the solid state. The targets are characterized by cyclic voltammetry, X-ray crystallography and UV/Vis and fluorescence analysis. Three highly fluorescent, amorphous azaacenes were processed into organic light-emitting diodes, and their improved performance (luminance, efficiency and efficacy) was compared with that of structurally related iptycene-based azaacenes.

Bent N-Heteroarenes.

The syntheses and optical, electronic, and structural properties of a series of bent N-heteroarenes are described. The targets were obtained by condensation reactions of substituted aromatic o-diamines with 1,2-naphthoquinone and 1,2-anthraquinone in yields between 34 and 94%; naphthoquione-based products are generally formed in higher yields.