The electrochemistry, photophysics, and electrochemically generated chemiluminescence (ECL) of a family of polysulfurated dendrimers with a pyrene core have been thoroughly investigated and ...complemented by theoretical calculations. The redox and luminescence properties of dendrimers are dependent on the generation number. From low to higher generation it is both easier to reduce and oxidize them and the emission efficiency increases along the family, with respect to the polysulfurated pyrene core. The analysis of such data evidences that the formation of the singlet excited state by cation–anion annihilation is an energy‐deficient process and, thus, the ECL has been justified through the triplet–triplet annihilation pathway. The study of the dynamics of the ECL emission was achieved both experimentally and theoretically by molecular mechanics and quantum chemical calculations. It has allowed rationalization of a possible mechanism and the experimental dependence of the transient ECL on the dendrimer generation. The theoretically calculated Marcus electron‐transfer rate constant compares very well with that obtained by the finite element simulation of the whole ECL mechanism. This highlights the role played by the thioether dendrons in modulating the redox and photophysical properties, responsible for the occurrence and dynamics of the electron transfer involved in the ECL. Thus, the combination of experimental and computational results allows understanding of the dendrimer size dependence of the ECL transient signal as a result of factors affecting the annihilation electron transfer.
Generation gap: Pyrene‐based dendrimers show stable and intense electrochemiluminescence (ECL) emission thanks to their excellent photophysical and electrochemical properties (see figure; kann=rate of electron transfer in the annihilation process). They are therefore an ideal benchmark to establish how and to what extent structural and electronic parameters affect and control the dynamics of the ECL properties.
The copper‐mediated Ullmann coupling of 1,7‐dibromoperylene bisimides afforded structurally perfect singly‐linked perylene bisimide (PBI) arrays, whilst the homo‐coupling of 1,12‐dibromoperylene ...bisimides gave doubly‐linked and triply‐linked diperylene bisimides. The interactions of three bay‐linked diperylene bisimides that differed in their linkage (singly, doubly, and triply) were investigated in their neutral and reduced forms (mono‐anion to tetra‐anion). UV/Vis absorption and fluorescence spectroscopy revealed different degrees of interaction, which was explained by exciton coupling and conjugation effects. The electrochemical properties and spectroelectrochemistry also showed quite‐different degrees of PBI interactions in the reduced mixed‐valence species, which was apparent by the observation of CT bands. The interpretation of the experimental findings was supported by spin‐restricted and ‐unrestricted DFT and time‐dependent TD‐DFT calculations with the long‐range‐corrected CAM‐B3LYP functional. Accordingly, the degree of interaction in both the neutral and reduced forms of the bay‐linked PBIs was qualitatively in the order doubly linked<singly linked≪triply linked, owing to the different degrees of twisting and flexibility between the two PBIs moieties. Only triply linked diPBI showed completely delocalized wavefunctions over the entire π‐system.
Charge of the linked brigade: The degree of interaction in the neutral and the reduced forms of bay‐linked perylene bisimides (PBIs; see figure) that differ in their linkage is due to the different degrees of twisting and flexibility between the PBI moieties.
The photophysical properties of a multicomponent 1rotaxane bearing a β‐cyclodextrin ring covalently connected to an axle comprising an azobenzene photoisomerisable moiety and a naphthalimide‐type ...fluorescent stopper are investigated by a combined experimental and computational study. The absorption and fluorescence spectra, and particularly the induced circular dichroism (ICD) signal, are determined. The latter shows a sign relation that cannot be rationalised in terms of the simple general rules commonly employed to analyse the ICD spectra of achiral guests encircled by chiral hosts. To assist the interpretation of experimental results, DFT and time‐dependent (TD) DFT calculations are performed to explore the availability of low‐energy conformations and to model their spectroscopic response. Molecular dynamics simulations performed in water show the interconversion of a number of conformers, the contribution of which to the ICD signal is in agreement with the observation.
The perfect host: Spectroscopic and computational techniques were used to investigate the absorption, luminescence and induced circular dichroism (ICD) spectra of a multicomponent 1rotaxane. DFT calculations revealed the availability of low‐energy conformations that contribute to the observed signal (see figure). The results were compared with those predicted on the basis of established ICD rules for cyclodextrin host–guest complexes.
The electronic structure of highly crystalline picene films with a standing-up orientation grown epitaxially on the Ag(110) surface was investigated. Upon exposure to oxgen gas, O2 molecules ...incorporate at the interstitial sites within the a–b plane of the film. Features related to the highest three occupied molecular orbitals shift toward a lower binding energy which results in the inactivation of traps and the reduction of the charge injection barrier by about 1 eV. It is suggested that the highest two picene orbitals are inverted due to the strong interactions between the singly occupied oxygen π orbital and the highest occupied orbital of picene.
Evidence of the biradicaloid and polyenic character of quinoidal oligothiophenes is reported by proving at the CASSCF//CASPT2 computational level the presence of a low-lying double exciton state ...responsible for the weak features observed in the NIR absorption region of the longest members of this class of molecules. The energy lowering of this state, accompanying the length increase in the oligomers, causes a displacement of the ground-state equilibrium geometry toward more biradicaloid structures because of the more efficient S0-S1 state mixing. Furthermore, it is shown that the doubly excited state is strongly coupled to the ground electronic state, and the coupling is mediated by a collective mode dominated by the out-of-phase stretching of adjacent CC bonds, recently shown to govern the Raman activity. All together, this evidence offers a unified view of the low-lying electronic states for quinoidal oligothiophenes and polyenes.
Tetracene tetracarboxylic diimides have been synthesized based on direct double ring extension of electron-deficient naphthalene diimides involving metallacyclopentadienes. Atomic structure and ...electronic transitions responsible for their NIR absorption spectra are investigated with quantum-chemical calculations. In light of their unique structure and admirable photophysical and electronic properties, this new molecular skeleton is promising candidate for n-type semiconductors.
Hybrid rylene arrays have been prepared via a combination of Stille coupling and C–H transformation. The ability to extend the π system along the equatorial axis of rylenes not only leads to ...broadened light absorption but also increases the electron affinity, which can facilitate electron injection and transport with ambient stability.
A new synthetic method toward ethylene-annulated di(perylene diimides) from easily available ethylene-annulated di(perylene esters), which is conducted by ICl-induced cyclization and Mizoroki–Heck ...coupling of ethynylene-linked di(perylene esters), is reported.
The effects of core twisting on charge transport properties of three tetrachloro-substituted perylene bisimide (PBI) and an octachloro PBI derivative are investigated in the framework of the ...nonadiabatic hopping mechanism. Charge transfer rate constants are computed within the Marcus–Levich–Jortner formalism including a single effective mode treated quantum mechanically and are injected in a kinetic Monte Carlo scheme to propagate the charge carrier in the crystal and to estimate charge mobilities at room temperature with and without the influence of an electric field. Thermally induced dynamical effects are investigated by means of an integrated computational approach including molecular dynamics simulations accompanied by quantum-chemical evaluation of electronic couplings, and the lattice vibrations responsible for fluctuations are identified. It is shown that dimer trapping, driven by the presence of both enantiomers in the crystal, is responsible for the limited mobilities of columnar core-twisted PBI derivatives. In addition, such an effect is not mitigated by thermally induced electronic coupling fluctuations since these are shown to be drastically reduced in core-twisted PBI compared to planar-core derivatives.
We report a highly efficient synthetic methodology toward tri-N-annulated hexarylenes from easily available N-annulated perylenes, which is favored by the oxidative ring fusion driven by ...DDQ/Sc(OTf)3. To assist the characterization of the new compounds, quantum-chemical calculations of structural and spectroscopic properties have been carried out for three oligomers of N-annulated rylenes. It is shown that tri-N-annulated hexarylene dyes display remarkably large dipole moments likely associated with the formation of H aggregates, as suggested by the marked concentration dependence of the measured UV−vis spectra. It is suggested that the combination of π−π stacking interactions and dipole−dipole interactions may favor the formation of highly ordered supramolecular structures, resulting in enhanced charge carrier mobilities.
