Optical Pumping of Poly(3-hexylthiophene) Singlet Excitons Induces Charge Carrier Generation

The dynamics of high-energy excitons of poly­(3-hexylthiophene) (P3HT) are shown to consist of torsional relaxation and exciton dissociation to form free carriers. In this work, we use pump–push–probe femtosecond transient absorption spectroscopy to study the highly excited states of P3HT in solution. P3HT excitons are generated using a pump pulse (400 nm) and allowed to relax to the lowest-lying excited state before re-excitation using a push pulse (900 or 1200 nm), producing high-energy excitons that decay back to the original excited state with both subpicosecond (0.16 ps) and picosecond (2.4 ps) time constants. These dynamics are consistent with P3HT torsional relaxation, with the 0.16 ps time constant assigned to ultrafast inertial torsional relaxation. Additionally, the signal exhibits an incomplete recovery, indicating dissociation of high-energy excitons to form charge carriers due to excitation by the push pulse. Our analysis indicates that charge carriers are formed with a yield of 11%

Optical Pumping of Poly(3-hexylthiophene) Singlet Excitons Induces Charge Carrier Generation

The dynamics of high-energy excitons of poly­(3-hexylthiophene) (P3HT) are shown to consist of torsional relaxation and exciton dissociation to form free carriers. In this work, we use pump–push–probe femtosecond transient absorption spectroscopy to study the highly excited states of P3HT in solution. P3HT excitons are generated using a pump pulse (400 nm) and allowed to relax to the lowest-lying excited state before re-excitation using a push pulse (900 or 1200 nm), producing high-energy excitons that decay back to the original excited state with both subpicosecond (0.16 ps) and picosecond (2.4 ps) time constants. These dynamics are consistent with P3HT torsional relaxation, with the 0.16 ps time constant assigned to ultrafast inertial torsional relaxation. Additionally, the signal exhibits an incomplete recovery, indicating dissociation of high-energy excitons to form charge carriers due to excitation by the push pulse. Our analysis indicates that charge carriers are formed with a yield of 11%

Artificial quantum photosynthetic materials

Photosynthesis has been shown to be a highly efficient process for energy transfer in plants and bacteria. It has been proposed that quantum mechanics plays a key role in this energy transfer process. There has been evidence that photosynthetic systems may exhibit quantum coherence. As artificial light-harvesting complexes have been proposed to mimic photosynthesis, it is prudent that artificial photosynthetic materials should also be tested for quantum coherence. To date, such studies have not been reported. In this work, we examine one such system, the BODIPY light harvesting complex (LHC), which has been shown to exhibit classical energy transfer via Förster resonance energy transfer. We compare the photon absorption of the LHC with the BODIPY chromophore by performing UV-visible, transient absorption, broadband pump-probe (BBPP) and two-dimensional electronic (2DES) spectroscopy. The 2DES and BBPP show evidence for quantum coherence, with oscillation frequencies of 100 cm-1 and 600 cm-1, which are attributable to vibronic, or exciton-phonon type coupling. Further computational analysis suggests strong couplings of the molecular orbitals of the LHC resulting from the stacking of neighbouring BODIPY chromophore units may contribute to undesirable hypochromic effects

Molecular-Level Details of Morphology-Dependent Exciton Migration in Poly(3-hexylthiophene) Nanostructures

The morphology dependence of exciton transport in the widely used conjugated polymer poly­(3-hexylthiophene) (P3HT) is elucidated by combining an accurate mesoscale coarse-grained molecular dynamics simulation model of P3HT structure with a Frenkel–Holstein exciton model. This model provides a more realistic representation than previously achieved of the molecular-level details of exciton transport on large length scales relevant to electronic applications. One hundred 300-monomer regioregular P3HT chains are simulated at room temperature for microseconds in two implicit solvents of differing solvent quality in which the polymer chains adopt contrasting morphologies: nanofiber-like aggregates or well-separated extended conformations. The model gives reasonable quantitative agreement with steady-state absorption and fluorescence and time-resolved fluorescence experiments, and provides valuable insight into the mechanism of exciton transport in conjugated polymers. In particular, exciton transfer in nanofiber aggregates is shown to occur mainly through interchain hops from chromophores on the aggregate surface toward the aggregate core, a behavior with important implications for organic electronic applications. Furthermore, the counterbalancing effects of greater orientational order and faster exciton transport in nanofiber aggregates than in extended chains are found to explain the puzzling observation of similar fluorescence anisotropy decay rates in nanofibers and free chains

Characterization of the ultrafast spectral diffusion and vibronic coherence of TIPS-pentacene using 2D electronic spectroscopy

TIPS-pentacene is a small-molecule organic semiconductor that is widely used in optoelectronic devices. It has been studied intensely owing to its ability to undergo singlet fission. In this study, we aim to develop further understanding of the coupling between the electronic and nuclear degrees of freedom of TIPS-pentacene (TIPS-Pn). We measured and analyzed the 2D electronic spectra of TIPS-Pn in solutions. Using center line slope (CLS) analysis, we characterized the frequency-fluctuation correlation function of the 0-0 vibronic transition. Strong oscillations in the CLS values were observed for up to 5 ps with a frequency of 264 cm-1, which are attributable to a large vibronic coupling with the TIPS-Pn ring-breathing vibrational mode. In addition, detailed analysis of the CLS values allowed us to retrieve two spectral diffusion lifetimes, which are attributed to the inertial and diffusive dynamics of solvent molecules. Amplitude beating analysis also uncovered couplings with another vibrational mode at 1173 cm-1. The experimental results can be described using the displaced harmonic oscillator model. By comparing the CLS values of the simulated data with the experimental CLS values, we estimated a Huang-Rhys factor of 0.1 for the ring-breathing vibrational mode. The results demonstrated how CLS analysis can be a useful method for characterizing the strength of vibronic coupling.Ministry of Education (MOE)Published versionH.-S.T. acknowledges support from the Singapore Ministry of Education Academic Research Fund (Grant No. Tier 1 RG2/19). J.M.d.l.P. was supported by an Australian Government Research Training Program (RTP) scholarship. T.W.K. and G.D.S. acknowledge funding from the Australian Research Council (Grant Nos. DP160103797, LE0989747, and LE130100158

Endothermic singlet fission is hindered by excimer formation

Singlet fission is a process whereby two triplet excitons can be produced from one photon, potentially increasing the efficiency of photovoltaic devices. Endothermic singlet fission is desired for a maximum energy-conversion efficiency, and such systems have been considered to form an excimer-like state with multiexcitonic character prior to the appearance of triplets. However, the role of the excimer as an intermediate has, until now, been unclear. Here we show, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as a prototypical example, that, rather than acting as an intermediate, the excimer serves to trap excited states to the detriment of singlet-fission yield. We clearly demonstrate that singlet fission and its conjugate process, triplet-triplet annihilation, occur at a longer intermolecular distance than an excimer intermediate would impute. These results establish that an endothermic singlet-fission material must be designed to avoid excimer formation, thus allowing singlet fission to reach its full potential in enhancing photovoltaic energy conversion.Cameron B. Dover, Joseph K. Gallaher, Laszlo Frazer, Patrick C. Tapping, Anthony J. Petty II, Maxwell J. Crossley, John E. Anthony, Tak W. Kee and Timothy W. Schmid