14:00 - 14:25InvitedID: 129
MANIPULATING MATTER WITH STRONG COUPLING: HARVESTING TRIPLET EXCITONS IN ORGANIC EXCITON MICROCAVITIES
1Department of Physics and Astronomy, University of Sheffield, Sheffield, United Kingdom; 2Department of Chemistry, University of Cambridge, Cambridge, United Kingdom; 3Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States of America
Exciton-polaritons are quasiparticles with mixed photon and exciton character that demonstrate rich quantum phenomena, novel optoelectronic devices and the potential to modify chemical properties of materials. Organic semiconductors are of current interest for their room-Temperature polariton formation. However, within organic optoelectronic devices, it is often the ‘dark’ spin-1 triplet excitons that dominate operation. These triplets have been largely ignored in treatments of polariton physics. Here we demonstrate polariton population from the triplet manifold via triplet-triplet annihilation, leading to polariton emission that is longer-lived (>μs) even than exciton emission in bare films. This enhancement arises from spin-2 triplet-pair states, formed by singlet fission or triplet-triplet annihilation, feeding the polariton. We propose this is possible due to state mixing, which – in the strong coupling regime – enables sharing of photonic character with states that are formally non-emissive. Our observations offer the enticing possibility of harvesting or manipulating states that are formally dark.
14:25 - 14:40ID: 222
INTRAMOLECULAR SINGLET FISSION IN EXTENDED SYSTEMS: OLIGOMERS, POLYMERS, AND FILMS
1City University of New York, New York, United States of America; 2Columbia University, New York, United States of America; 3Wuhan University of Technology, Wuhan, China; 4University of Southern Mississippi, Hattiesburg, United States of America; 5University of New South Wales, Sydney, Australia
Singlet fission (SF) is a multiple exciton generation process which coverts one singlet exciton into two triplet excitons in multichromophore organic systems. The recent discovery of intramolecular singlet fission compounds (iSF), in which SF occurs in isolated molecules without nearest neighbor interactions, has facilitated the development of structure-function relationships that can be used to optimize the generation and lifetime of triplet excitons. However, iSF materials have mostly taken the form of molecular dimers. While these compounds highlight the minimum requirements for achieving efficient SF, they offer limited degrees of freedom in terms of tunability in their structural and electronic properties. Furthermore, the transport of free triplets is constrained by the few numbers of chromophores. Here, I will discuss the evolution and dynamics of multiexciton states as we move beyond dimers to extended multichromophore systems such as oligomers and polymers. I will demonstrate how molecular engineering concepts can be used to independently optimize the rates of triplet pair formation, triplet pair dephasing, and triplet decay in both solution and thin films. Finally, I will discuss a general framework that can be used to quantify and evaluate the overall performance of SF materials.
14:40 - 14:55ID: 149
A MOLECULAR MOVIE OF ULTRAFAST SINGLET FISSION
1Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; 2Physical and Theoretical Chemistry Laboratory, Oxford University, Oxford, United Kingdom; 3Department of Chemistry, National University of Singapore, Singapore; 4Department of Physics, University of Warwick, Coventry, United Kingdom; 5Centre National de la Recherce Scientifique, Institute des Nanosciences de Paris, Sorbonne Uni-versite, Paris, France; 6Department of Physics and Astronomy, University of Sheffield, Sheffield, United Kingdom
The complex dynamics of ultrafast photoinduced reactions are governed by their evolution along vibronically coupled potential energy surfaces. While it is now often possible to identify such processes both theoretically and experimentally, a detailed depiction of the crucial nuclear degrees of freedom driving the reaction typically remains elusive and subject to interpretation.
The origin of this uncertainty can be attributed to a disparity between experimental and theoretical methods. Structurally-sensitive experimental techniques that can access the earliest photoreactive transformations are often only available for large and complex molecular systems, while accurate first-principles computational modelling for such non-Born-Oppenheimer dynamic processes is only affordable for much smaller model systems. Thus, even if experimentally-retrieved structural information is available, it can rarely be accurately projected onto the molecular origin of the crucial coupling and tuning modes involved in the processes, preventing identification of the operative reaction mechanisms and consequently the extraction of guiding design principles required for improved functional materials.
Here, combining ultrafast excited-state time-domain Raman spectroscopy and tree-tensor network state simulations, we construct the full 108-atom molecular movie of the ultrafast singlet fission reaction in a pentacene dimer, taking into account 252 vibrational modes strongly coupled to 5 electronic states. The simulations closely reproduce both the frequencies and the intensities of the experimentally retrieved vibrational coherence signatures (Fig. 1a-b). The remarkable structural agreement between theory and experiment for a system of this dimension enables us to construct a complete ‘molecular movie’ of the fission process and to determine the nature of the critical coupling and tuning modes driving this coherent ultrafast reaction (Fig. 1c-f). Our results provide quantitative insight into the molecular mechanism of singlet fission and allow us to visualise ultrafast quantum dynamics in an experimentally verifiable way, putting forth a general framework to identify ultrafast photochemical reaction mechanisms in complex systems.
14:55 - 15:10ID: 127
PHOTON EMISSION FROM NON-GEMINATE TRIPLET-PAIR STATES
1Department of Physics and Astronomy, The University of Sheffield, Sheffield, United Kingdom; 2Institute for Physical Chemistry, Universität Heidelberg, Heidelberg, Germany; 3School of Chemistry, University of Birmingham, Birmingham, United Kingdom; 4Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States of America
Singlet exciton fission, whereby one singlet exciton is converted into a pair of triplets, proceeds via an intermediate spin-entangled triplet-pair state 1(TT). This same state also mediates the reverse process of triplet-triplet annihilation. Whilst the understanding of the nature of the triplet-pair state has progressed significantly in recent years, there are still many aspects with no clear consensus. One of the outstanding questions is whether or not the triplet-pair state can emit photons. Three recent studies covering a range of singlet fission materials have observed a red-shifted, vibronically structured feature in transient photoluminescence spectra on timescales of tens of nanoseconds –. This has been assigned to emission from geminate triplet-pair states. Other studies, however, claim that defect emission  or excimer states  are responsible. One of the principal arguments used to counter the idea that the triplet-pair state can emit photons is that such emission has never been observed in the regime of non-geminate triplet-triplet annihilation . Here we show, using temperature-dependent, time-resolved photoluminescence measurements on thin films of 2,8‑difluoro‑5,11‑bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT), that vibronically structured emission from triplet-pair states formed by non-geminate triplet-triplet annihilation can in fact be clearly detected on timescales of hundreds of nanoseconds. These observations indicate that certain features in the photoluminescence spectra of some of the most well-studied singlet fission materials such as tetracene and pentacene may in fact arise from triplet-pair states and not from defects, excimers or self-trapped excitons as previously thought.
 H. L. Stern et. al. Nat. Chem., 9, 1205, (2017)
 C. K. Yong et. al. Nat. Commun., 8, 15953, (2017)
 S. Lukman et. al. J. Am. Chem. Soc., 139, 50, 18376 (2017)
 J. J. Burdett et. al. J. Chem. Phys., 133, 14, 144506, (2010)
 C. B. Dover et. al. Nat. Chem., 10, 305, (2018)
15:10 - 15:25ID: 162
QUANTITATIVE SINGLET FISSION AND TRIPLET EXTRACTION IN SOLUTION AT EFFECTIVE SOLAR FLUENCE
1University of Cambridge, Cambridge, United Kingdom; 2Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States of America; 3Mc Darmond Instute, Victoria University of Wellington, Wellington, New Zealand
Singlet fission is a carrier multiplication process in organic molecules, that offers a mechanism to break the Shockley–Queisser limit by performing photon multiplication in combination with traditional Silicon Photovoltaics. To take advantage of singlet fission, efficient extraction of the “dark” triplet excitons followed by radiative recombination in an “emitter” material is required. Here, we show in solution phase the transfer of molecular triplet excitons from TIPS-tetracene (tips-tc) to inorganic quantum dots of PbS can be as high as 95 ± 5 % efficient, using various optical techniques, including transient absorption, time-resolved photoluminescence and magnetic dependent photoluminescence. The effect of the organic ligands surrounding the quantum dots, the concentration of the triplet donor and acceptor and incident light fluence are investigated. The photon multiplication efficiency is shown to be limited by the singlet fission yield of tips-tc, which is found to be 130 ± 5 %; as well as bi-molecular decay of triplet excitons. This bi-molecular decay of triplets is shown to depend on the triplet acceptor concentration. In the high acceptor regime, we show that the photon multiplication efficiency is maintained up to fluences equivalent to effective solar fluence. Our results draw on the analogous field photon-up conversion to bring new physical insights to the mechanism of triplet transfer from organic to inorganic semiconductors and provide an important framework for future devices that harvest triplet excitons.
15:25 - 15:40ID: 157
OPTIMISING SINGLET FISSION DYNAMICS AND PHASE IN TIPS-TETRACENE FILMS AND BLENDS FOR DOWN-CONVERSION OF LIGHT
University of Cambridge, Cambridge, United Kingdom
Singlet fission in tetracene and its derivatives has been investigated extensively due to singlet and triplet energy levels that are favourable for down conversion of light for enhancing solar energy conversion efficiency in photovoltaics.
We investigate singlet fission dynamics and energy transfer dynamics in triisopropylsilyl-tetracene (TIPS-Tc) thin films and blends. In TIPS-Tc films, we study free triplet formation in amorphous TIPS-Tc films produced by thermal evaporation and spin coating. We observe a phase transition in TIPS-Tc films from amorphous to polycrystalline phase over time and study dynamics of singlet fission at various stages of this transition. We use transient absorption and emission spectroscopy to map exciton dynamics in the film over twelve orders of time (femto to microsecond). We find that TIPS-Tetracene films that exhibit a mixed morphology and partial transformation to the polycrystalline phase undergo singlet fission in tens of picoseconds (three orders of time faster than previously observed in TIPS-Tc films). Our insights allow us to produce TIPS-Tc films with optimal morphologies for efficient singlet fission and triplet energy transfer.
Further, in TIPS-Tc and Yb-complex blends formed via co-evaporation, we observe triplet energy transfer from TIPS-Tc to the Yb-complex and subsequent lanthanide emission. Luminescent harvesting of triplets (generated via singlet fission) from TIPS-Tc in these blends is a major step towards applications of singlet fission in solar cell technologies, providing an elegant and non-invasive pathway for increasing the energy efficiency of photovoltaics, without changes to the manufacturing process.
Apart from envisioning real-world applications(via proof of concept designs) of singlet fission and luminescent triplet harvesting, we gain significant insight into the underlying photophysics of these processes (also studying the role of entropy in these processes). Using magnetic field and temperature dependent transient spectroscopy, we map the thermodynamics and spin of exciton and excimer species in these materials through twelve orders of time.