This review highlights recent developments concerning stabilization strategies of large (hetero-)acenes and discusses the resulting impact on the aromatic system.

Generating two long-living low-energy excitations after absorption of a single high-energy photon has stoked interest in singlet fission (SF) to enhance solar energy conversion in photovoltaics. To this end, survival of the triplet states is critical. This process is investigated in diethynylbenzene-linked tetraaza-triisopropylsilylethynylpentacene dimers, for which SF is energetically feasible and facilitated by the close distances between the azapentacenes. The ortho and meta connectivities are explored and compared with the tetraazapentacene molecule and the (1,3,5) trimer. Efficient SF (potential Φ T ≥ 160%) is demonstrated in all oligomers by quantitative kinetic analysis of broadband transient absorption and fluorescence signals. Together with dynamics of the starting singlet, the triplet pair, and the final free triplet state, our results show an intermediate component with spectral properties compatible with a biexcitonic state. Long-living triplets represent only a fraction of the high number of transient triplet pair intermediates, which undergo triplet−triplet annihilation as well as fusion between neighboring pentacenes. Therefore, our work provides new insight into the SF in covalent dimers and paves the way for the application of these materials for carrier multiplication.

We have synthesized a brominated N,N'-dihydrotetraazapentacene using a condensation route. Sonogashira reactions replace the Br-substituent by an alkynyl group, placed on the azaacene core. Sonogashira coupling of brominated dihydro tetraazapentacene 1H2 with alkynes and subsequent oxidation afford several functionalized TIPS-tetraazapentacene derivatives with energetically stabilized FMOs. These TIPS-TAPs are either crystalline or amorphous, depending upon their substitution pattern.

Singlet fission is the photoinduced conversion of a singlet exciton into two triplet states of half-energy. This multiplication mechanism has been successfully applied to improve the efficiency of single-junction solar cells in the visible spectral range. Here we show that singlet fission may also occur via a sequential mechanism, where the two triplet states are generated consecutively by exploiting oxygen as a catalyst. This sequential formation of carriers is demonstrated for two acene-like molecules in solution. First, energy transfer from the excited acene to triplet oxygen yields one triplet acene and singlet oxygen. In the second stage, singlet oxygen combines with a ground-state acene to complete singlet fission. This yields a second triplet molecule. The sequential mechanism accounts for approximately 40% of the triplet quantum yield in the studied molecules; this process occurs in dilute solutions and under atmospheric conditions, where the single-step SF mechanism is inactive.

Single crystal structural analysis of [FeII(tame)2]Cl2⋅MeOH (tame=1,1,1‐tris(aminomethyl)ethane) as a function of temperature reveals a smooth crossover between a high temperature high‐spin octahedral d 6 state and a low temperature low‐spin ground state without change of the symmetry of the crystal structure. The temperature at which the high and low spin states are present in equal proportions is T 1/2=140 K. Single crystal, variable‐temperature optical spectroscopy of [FeII(tame)2]Cl2⋅MeOH is consistent with this change in electronic ground state. These experimental results confirm the spin activity predicted for [FeII(tame)2]2+ during its de novo artificial evolution design as a spin‐crossover complex [Chem. Inf. Model. 2015, 55, 1844], offering the first experimental validation of a functional transition‐metal complex predicted by such in silico molecular design methods. Additional quantum chemical calculations offer, together with the crystal structure analysis, insight into the role of spin‐passive structural components. A thermodynamic analysis based on an Ising‐like mean field model (Slichter–Drickammer approximation) provides estimates of the enthalpy, entropy and cooperativity of the crossover between the high and low spin states.