10 research outputs found
A push-pull unsymmetrical subphthalocyanine dimer
Unsymmetrical subphthalocyanine fused dimers have been prepared from appropriate ortho-dinitrile SubPc precursors. In particular, either electron-donating or electron-accepting substituents have been introduced on each SubPc constituent unit, resulting in unprecedented pushâpull Ï-extended curved aromatic macrocycles. From fluorescence experiments in solvents of different polarity we conclude a dual fluorescence, namely a delocalized singlet excited state (1.73 eV) and a polarized charge transfer state (<1.7 eV). Pump probe experiments corroborate the dual nature of the fluorescence. On one hand, the delocalized singlet excited state gives rise to a several nanosecond lasting intersystem crossing yielding the corresponding triplet excited state. On the other hand, the polarized charge transfer state deactivates within a few picosesonds. Visualization of the charge transfer state was accomplished by means of molecular modeling with a slight polarization of the HOMO towards the electron donor and of the LUMO towards the electron acceptor
Modulating the dynamics of Förster resonance energy transfer and singlet fission by variable molecular spacers
Optical gap and fundamental gap of oligoynes and carbyne
International audienceThe optoelectronic properties of various carbon allotropes and nanomaterials have been well established, while the purely sp-hybridized carbyne remains synthetically inaccessible. Its properties have therefore frequently been extrapolated from those of defined oligomers. Most analyses have, however, focused on the main optical transitions in UV-Vis spectroscopy, neglecting the frequently observed weaker optical bands at significantly lower energies. Here, we report a systematic photophysical analysis as well as computations on two homologous series of oligoynes that allow us to elucidate the nature of these weaker transitions and the intrinsic photophysical properties of oligoynes. Based on these results, we reassess the estimates for both the optical and fundamental gap of carbyne to below 1.6âeV, significantly lower than previously suggested by experimental studies of oligoynes
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Davydov splitting and singlet fission in excitonically coupled pentacene dimers
Singlet fission (SF) allows two charges to be generated from the absorption of a single photon and is, therefore, potentially transformative toward improving solar energy conversion. Key to the present study of SF is the design of pentacene dimers featuring a xanthene linker that strictly places two pentacene chromophores in a rigid arrangement and, in turn, enforces efficient, intramolecular Ï-overlap that mimics interactions typically found in condensed state (e.g., solids, films, etc.). Inter-chromophore communication ensures Davydov splitting, which plays an unprecedented role toward achieving SF in pentacene dimers. Transient absorption measurements document that intramolecular SF evolves upon excitation into the lower Davydov bands to form a correlated triplet pair at cryogenic temperature. At room temperature, the two spin-correlated triplets, one per pentacene moiety within the dimers, are electronically coupled to an excimer state. The presented results are transferable to a broad range of acene morphologies including aggregates, crystals, and films. © 2019 The Royal Society of Chemistry
Singlet fission in pentacene dimers
Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% ShockleyâQueisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule
Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers
We show unambiguous and compelling evidence by means of pumpâprobe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 ± 10% and (0.7 ± 0.1) Ă 1012 sâ1, respectively, in room temperature solutions
CCDC 1812258: Experimental Crystal Structure Determination
Related Article: RubĂ©n Casillas, Matthias Adam, Pedro B. Coto, Andreas R. Waterloo, Johannes Zirzlmeier, Seelam Rajagopala Reddy, Frank Hampel, Robert McDonald, Rik R. Tykwinski, Michael Thoss, Dirk M. Guldi|2018|Adv.Energy Mater.|8|1802221|doi:10.1002/aenm.201802221,An entry from the Cambridge Structural Database, the worldâs repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Pentacene Appended to a TEMPO Stable Free Radical: The Effect of Magnetic Exchange Coupling on Photoexcited Pentacene
Understanding the fundamental spin
dynamics of photoexcited pentacene
derivatives is important in order to maximize their potential for
optoelectronic applications. Herein, we report on the synthesis of
two pentacene derivatives that are functionalized with the [(2,2,6,6-tetraÂmethylÂpiperidin-1-yl)Âoxy]
(TEMPO) stable free radical. The presence of TEMPO does not quench
the pentacene singlet excited state, but does quench the photoexcited
triplet excited state as a function of TEMPO-to-pentacene distance.
Time-resolved electron paramagnetic resonance experiments confirm
that triplet quenching is accompanied by electron spin polarization
transfer from the pentacene excited state to the TEMPO doublet state
in the weak coupling regime