Femtosekunden Lasersystem für zeitaufgelöste Transientenabsorptionsspektroskopie
Final Report Abstract
The extension of the time window (8 ns) of the existing pump-probe spectrometer to 400 µs with the new spectrometer has been decisive in revolutionary experiments in the Guldi lab in the area of Molecular Singlet Fission, Short & Long Range Charge Transfer Events, and Nanocarbons in Electron Donor-Acceptor Hybrids. a) Highlight from work on Molecular Singlet Fission: When molecular dimers, crystalline films, or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission (SF) may produce two triplet excitons that can be used to generate two electron-hole pairs leading to a predicted ~50% enhancement in maximum solar cell performance. The SF mechanism is still not well understood. Our synthetic, spectroscopic, and computational efforts highlighted the importance of establishing the right balance between triplet excited state formation / singlet excited state deactivation and triplet excited state decay towards the optimization of SF. Nevertheless, the corresponding lifetimes of around 450 ps were still rather short and hampered an in-depth analysis of spin decoherence etc. To overcome the aforementioned obstacles, we linked two pentacene moieties using a rigid, nonconjugated 1,3-diethynyladamantyl spacer. Importantly, the 1,3-diethynyladamantyl spacer was expected to result in sufficient electronic coupling between the pentacenes to render SF facile, while at the same time keeping the coupling sufficiently weak to allow spin decoherence leading to two independent triplet states to compete successfully with triplettriplet annihilation leading back to the 1(S1S0) state. As a matter of fact, we have deconvoluted the SF process in a highly viscous butyronitrile matrix – at room temperature and at 105 K – and provided the first unambiguous evidence for all the key intermediates along the reaction coordinate of SF. At cryogenic temperatures, for example, we directly observe spin mixing of 1(T1T1) with 5(T1T1) followed by spin decoherence that converts the latter into an uncorrelated, not entangled pair of triplet excited states (T1 + T1). The combination of transient optical and EPR data provided a means of identifying and analyzing the SF mechanism over a very broad time range, which has the potential to be very useful in optimizing molecular designs for SF. b) Highlight from work on Short & Long Range Charge Transfer Events: A general disadvantage of p-phenylene (opP), p-phenyleneethylenes (opPE), and p-phenylenevinylenes (opPV) is the ability to rotate around the single bonds that connect the individual phenyl units. This causes a deviation from planarity along the oligomer chain. Moreover, in all these examples electronic coupling, that is, the effectiveness of the π-conjugation, controls the molecular wire behavior. In contrast, electronvibration coupling is usually neglected, primarily because electron transfer excites only low-energy torsional motions of C–C σ-bonds rather than vibrations of the C–C π-skeleton. Therefore, elastic tunneling/superexchange and hopping mechanisms, which occur much more slowly than inelastic tunneling, dominate charge transfer through conventional molecular wires. Interestingly, Marcus theory predicts potentially fast electron-transfer reactions – even in the Marcus inverted region – which arise from electronvibration (e-v) coupling throughout the bridge. Hitherto, the lack of suitable organic wires has hampered experimental verification. To rule out the effect of deviation from planarity along the oligomer chain, rigid and flat carbon-bridged oligop-phenylenevinylene (CopPV) wires were synthesized. Importantly, in polar solvents such as benzonitrile, the formation of ZnP•+–CopPVn–C60•– (n = 1–4) was found to dominate the overall photoreactivity. ZnP– CopPV3–C60 exhibited an 840-fold increase in electron transfer rate compared with ZnP–opPV3–C60 in the Marcus inverted region, where e–v coupling greatly affects the electron transfer. These new CopPVs feature rigidity as well as planarity, both of which are crucial criteria favoring strong e-v coupling and, in turn, enabling inelastic electron tunneling. c) Highlight from work on Nanocarbons in Electron Donor-Acceptor Hybrids: Low-dimensional carbon allotropes represent interesting nanoscale building blocks, since they feature physical and chemical properties, which are of tremendous interest for applications in the emerging field of nanotechnology. The most prominent examples for applications are found in the areas of organic solar cells, etc. To this date, exploring the full potential of SWCNT and graphene constitutes one of the biggest challenges in the field. A clear bottleneck is their individualization/exfoliation in combination with their stabilization in dispersion. At the focal point of our studies were mutual interactions between an anionic heptamethine cyanine and individualized single walled carbon nanotubes (SWCNT)/exfoliated nanographene (NG). Unprecedented was the distinct NIR absorption of the heptamethine cyanine, which assisted in visualizing the electronic interactions in the ground state with ease and precision. In statistical Raman assays, we concluded from downshifted 2D- and G-modes for SWCNT, as well as, upshifted 2D- and downshifted G-modes for NG that these electronic interactions resulted in stable n-doping of SWCNT/NG. Key to this shift of charge density was the electron donating character of the heptamethine cyanine. In the excited state, a complete, but metastable transfer of charges was accompanied by a several nanoseconds lived radical ion pair state. Note such a time domain has never been realized, for example, in NG to this date.
Publications
- “Controlling the Crystalline Three-Dimensional Order in Bulk Materials by Single-Wall Carbon Nanotubes”, Nat. Comm. 2014, 5, 3763
J. Lopez-Andarias, J.L. Lopez, C. Atienza, F.G. Brunetti, C. Romero-Nieto, D.M. Guldi, N. Martin
(See online at https://doi.org/10.1038/ncomms4763) - “Electron Transfer Through Rigid Organic Molecular Wires Enhanced by Electronic and Electron-Vibration Coupling”, Nat. Chem. 2014, 6, 899
J. Sukegawa, C. Schubert, X.Z. Zhu, H. Tsuji, D.M. Guldi, E. Nakamura
(See online at https://doi.org/10.1038/nchem.2026) - “Molecular wires-impact of π-conjugation and implementation of molecular bottlenecks”, Chem. Soc. Rev. 2015, 44, 988
C. Schubert, J.T. Margraf, T. Clark, D.M. Guldi
(See online at https://doi.org/10.1039/C4CS00262H) - “Pentacene appended to a TEMPO stable free radical: The effect of magnetic exchange coupling on photoexcited pentacene”, J. Am. Chem. Soc. 2015, 137, 857
E.T. Chernick, R. Casillas, J. Zirzlmeier, D.M. Gardner, M. Gruber, H. Kropp, K. Meyer, M.R. Wasielewski, D.M. Guldi, R.R. Tykwinski
(See online at https://doi.org/10.1021/ja510958k) - “Singlet fission in pentacene dimers”, Proc. Natl. Aca. Sci. 2015, 112, 5325
J. Zirzlmeier, D. Lehnherr, P.B. Coto, E.T. Chernick, R. Casillas, B.S. Basel, M. Thoss, R.R. Tykwinski, D.M. Guldi
(See online at https://doi.org/10.1073/pnas.1422436112) - “Efficient Energy-Conversion Materials for the Future: Understanding and Tailoring Charge-Transfer Processes in Carbon Nanostructures”, Chem. 2016, 1, 531
V. Strauss, A. Roth, M. Sekita, D.M. Guldi
(See online at https://doi.org/10.1016/j.chempr.2016.09.001) - “Intense Ground-State Charge-Transfer Interactions in Low-Bandgap, Panchromatic Phthalocyanine-Tetracyanobuta-1,3-diene Conjugates”, Angew. Chem. 2016, 55, 5560
M. Sekita, B. Ballesteros, F. Diederich, D.M. Guldi, G. Bottari, T. Torres
(See online at https://doi.org/10.1002/anie.201601258) - “Individualized low-dimensional carbon allotropes: Enabling ground state and excited state charge transfer by NIR absorbing heptamethine cyanine”, Chem. 2017, 3, 164
A. Roth, C. Schierl, A. Ferrer-Ruiz, M. Minameyer, L. Rodríguez-Pérez, C. Villegas, Mª Á. Herranz, N. Martín, D.M. Guldi
- “Unified model for singlet fission within a non-conjugated covalent pentacene dimer”, Nat. Comm. 2017, 8, Article number 15171
B.S. Basel, J. Zirzlmeier, C. Hetzer, B.T. Phelan, M.D. Krzyaniak, S.R. Reddy, P.B. Coto, N.E. Horwitz, R.M. Young, F.J. White, F. Hampel, T. Clark, M. Thoss, R.R. Tykwinski, M.R. Wasielewski, D.M. Guldi
(See online at https://doi.org/10.1038/ncomms15171)
