Conference Agenda

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Session Overview
S-14: Energy conversion 2: Singlet fission, triplet fusion
Thursday, 11/Jul/2019:
16:10 - 18:00

Session Chair: Andrew Musser
Location: Main Hall
"Artis Centrum Hotel" (Address: Totoriu str. 23, Vilnius, LT-01120, Lithuania)

16:10 - 16:35
ID: 237
Oral presentation


Sumit Mazumdar, Souratosh Khan

University of Arizona, Tucson, United States of America

Singlet Fission is the photophysical process in which an optically allowed spin-singlet state dissociates into two spin-triplet excitons. The process is of strong interest in the area of organic photovoltaics as in principle it allows the doubling of the efficiency of organic solar cells, thereby beating the Shockley-Queisser limit of efficiency for solar cells. Until recently, it was believed that singlet fission occurs at ultrafast timescales of hundreds of femtoseconds or less, based on transient absorption studies that had detected rapid decay of the singlet exciton and what was thought to be the growth of the triplet exciton. A central assumption behind these studies was that the triplet-triplet bound biexciton, acknowledged to be the key intermediate in the singlet-fission process, was short-lived. Key questions in singlet fission are: (a) How do we know that the process is complete, i.e.,
transient absorption studies are not detecting the bound triplet-triplet instead of free triplets? (b) What is the binding energy of the triplet-triplet biexciton and how does it depend on chemical structure?
In the present talk I will present the results of our theoretical investigations of correlated-electron Hamiltonians on the triplet-triplet biexciton. Our work gives precise guidance to experimentalists on how to identify the triplet-triplet from free triplets from transient absorption in a variety of materials including crystalline acenes, acene dimers and donor-acceptor polymers. We show that the triplet-triplet not only absorbs at the same visible wavelengths where free triplets also absorb, but they exhibit additional absorptions in the infrared where free triplets do not absorb. We will also present what we believe to be precise estimates of the binding energy of the triplet-triplet, emphasizing in particular differences between long polyenes and acene-based materials.

16:35 - 16:50
ID: 185
Oral presentation


Ashish Sharma1, Elango Kumarasamy2, Amir Asadpoordarvish3, Dane R. McCamey3, Luis M. Campos2, Akshay Rao4, Murad J. Y. Tayebjee5, Girish Lakhwani1

1ARC Centre of Excellence in Exciton Science, School of Chemistry, The University of Sydney, Sydney, Australia; 2Department of Chemistry, Columbia University, New York, United States of America; 3School of Physics and ARC Centre of Excellence in Exciton Science, School of Physics, University of New South Wales, Sydney, Australia; 4Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; 5School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, Australia

Thermalization of high-energy photons is a dominant loss mechanism in conventional photovoltaics (PVs), which limits the maximum achievable power conversion efficiency to about 29% in single junction PVs, commonly known as the Shockley-Queisser limit. Singlet Fission (SF) is a spin-conserved exciton multiplication process, where a high-energy photo-excited singlet exciton splits into two low energy triplet excitons via a triplet pair (TT) intermediate. SF offers a promising pathway to overcome thermalization losses and surpass the Schokley-Queisser limit. While several pentacene dimers and acene analogues have been shown to demonstrate SF, a major challenge that remains is to achieve free triplets in high yield.

Here, we utilise a new pentacene dimer that incorporates a binaphthyl bridge (BNBP) to show near unity quantum efficiency separation of TT state into uncoupled triplets. To our knowledge, this is the first report demonstrating a complete conversion of the TT state into free triplets. We employed time resolved transient absorption (TA) and time-resolved electron spin resonance (tr-ESR) spectroscopy to study the nature and dynamics of the TT state generated through SF and found that triplet-triplet recombination is completely suppressed in BNBP. Consequently, the rate of decoupling of TT into free triplet states outcompetes TT decay. We utilize SF dynamics in BNBP to predict the coupling regimes that permit efficient TT separation and inform on design rules for future SF materials.

16:50 - 17:15
ID: 235
Oral presentation


Angelo Monguzzi

Department of Materials Science, University of Milano Bicocca, Milan, Italy

The conversion of low-energy photons into radiation of higher energy is useful for bioimaging, 3D displays, photovoltaics and other applications. In particular, upconversion of the infrared portion of the solar spectrum (which is typically not absorbed by the optically active materials used in solar cells and photocatalytic systems) allows additional photons to be harnessed and boosts the efficiency of these devices. For this reason, low power sensitized photon up-conversion based on triplet-triplet annihilation (sTTA-UC) has been recently recognized as a potential viable approach towards enhancing the efficiency of sunlight-powered devices through sub-bandgap photon harvesting.

The sTTA-UC permits the conversion of light into radiation of higher energy involving a sequence of photophysical processes between two dyes, namely a light harvester/sensitizer and an annihilator/emitter. High up­-con­version yields can be observed for low viscosity solutions of dyes, but in solid materials, which are better suited for integration in devices, the process is usually less efficient. Therefore, the capability to control triplet excitons in the solid state is crucial to obtain high performance in various organic devices. We will discuss the results obtained in several materials, such as dye-doped organic nanoparticles, hybrid nanocrystals and covalently-bonded supramolecular nanoparticles, highlighting the materials design guidelines to obtain efficient up-converters at the solid state that can match the strict requirements of solar and imaging technologies.

17:15 - 17:30
ID: 186
Oral presentation


Yoichi Sasaki1, Nobuhiro Yanai1,2, Nobuo Kimizuka1

1Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, Fukuoka, Japan; 2JST-PRESTO, Saitama, Japan

The efficient use of near-infrared (NIR) light has been much coveted in energy and biological applications. For example, although the NIR light from 700 nm to 900 nm shows good tissue transparency, the photon energy in this region is too weak for important biological applications such as photodynamic therapy and drug release. The blue light with a wavelength shorter than 500 nm is known to trigger drug release and in vivo photochemical reactions, and therefore it is imperative to produce such higher-energy visible light under NIR excitation.

Triplet-triplet annihilation based UC (TTA-UC) is especially promising since it converts lower-energy photons into higher-energy photons under weak excitation intensity like sunlight. Despite its importance, it has been difficult to obtain NIR (>700 nm) to blue (<500 nm) TTA-UC. This is because the energy loss associated with the intersystem crossing (ISC) of donors limits the use of acceptors with high T1 and S1 energy levels, and consequently the width of the anti-Stokes shift. Recent efforts have been accordingly devoted to realizing new mechanisms that circumvent the energy loss of ISC such as TADF molecules, QDs, perovskites, and heavy metal complexes. These sensitizers, however, tend to show short excited state lifetime (< 10 ns), and triplet energy transfer (TET) from the sensitizers to acceptor molecules is often inefficient.

In this work, we demonstrate an efficient NIR (>700 nm)-to-blue (<500 nm) TTA-UC by judicious developments of new Os(II) bis(terpyridine) complexes with singlet-to-triplet (S-T) direct transition. We achieved NIR-to-blue TTA-UC for the first time with a large anti-Stokes shift and a good UC efficiency.

17:30 - 17:45
ID: 126
Oral presentation


Marine Louis, Heidi Thomas, Max Gmelch, Anna Haft, Felix Fries, Sebastian Reineke

Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Institute for Applied Physics, Technische Universität Dresden, Dresden, Germany

Biluminescent materials are characterized by a dual-state emission, displaying simultaneous fluorescence and long-lived phosphorescence (>100 ms) at room temperature. While their use in various photonic applications has lately become an ever-growing research field, the development of efficient and competitive systems under aerated conditions remains a daunting challenge. Moreover, the few existing examples still suffer from the mandatory use of UV-light to trigger the luminescence response[1].

Here we report the design and synthesis of a large range of new pure-organic metal-free biluminescent emitters absorbing in the UV and visible range (up to 420 nm). Their spectroscopic properties (absorption, emission, lifetimes…) will be compared, analyzed and discussed. The most promising targets are used for the creation of original programmable luminescent tags (PLTs). A simple host/guest system, in which the emitters are embedded into a PMMA (Poly (methyl methacrylate) matrix, is used and top coated with an oxygen barrier layer. The latter enables a definite control of the oxygen concentration inside the films, which leads to the precise activation of the phosphorescence to encrypt information on their surfaces with a very high resolution (>700 dpi)[2],[3]. The temperature dependence of the oxygen barrier makes it possible to erase any encrypted information under heating, making the PLTs reusable at will.

Figure 1: A. Chemical structure of targets PhenDpa and PhenTpa. B. Photograph of the phosphorescence emission after: global activation (left) controlled activation (middle, right). Information can be erased and rewritten through control of the oxygen concentration within the thin films. λexc = 420 nm.

[1] Y. Su, A. Z. F. Phua, Y. Li, X. Zhou, D. Jana, G. Liu, W. Q. Lim, W. K. Ong, C. Yang, Y. Zhao, Sci. Adv. 2018, 4 : eaas9732.

[2] M. Gmelch, H. Thomas, F. Fries, S. Reineke, Sci. Adv. 2019, 5 : eaau7310.

[3] M. Louis, H. Thomas, M. Gmelch, A. Haft, F. Fries, S. Reineke, Adv. Mater. 2019, 1807887.

17:45 - 18:00
ID: 274
Oral presentation


Erki Enkvist, Asko Uri

University of Tartu, Tartu, Estonia

We have discovered organic tandem luminophores incorporating a covalently bound sulfur- or selenium-comprising heteroaromatic fragment and a green, orange or red fluorescent dye whose absorption spectrum overlaps with the phosphorescence emission spectrum of the phosphor. The tandem luminophores possess efficient Förster-type radiationless resonant energy transfer (FRET) from the excited triplet state of the low-QY donor phosphor (3D*) to the adjacent acceptor fluorophore (3D* + 1A → 1D + 1A*) that leads to emission of light from the fluorescent dye. Substantial (20- – 500-fold) enhancement of luminescence intensity compared to separated phosphors was achieved. We have used probes comprising tandem luminophores for analysis of kinases in biological samples, screening of inhibitors in biochemical assays, and for mapping and monitoring activity of kinases in living cells using time-gated luminescence microscopy.

Now we have established that in solid PVA matrix the same phosphors possess remarkable phosphorescence with lifetimes in 0.8-220 ms range. When acceptor dyes were covalently added to these compounds almost complete interchromophore ET from the phosphor, initially in excited triplet state, to an adjacent covalently linked acceptor fluorescent dye takes place. Variation of length of the linker enables tuning of luminescence lifetime in 100 μs to 5 ms range.

Excellent harvesting of energy of the triplet exited state of otherwise dim (possessing low quantum yield at room temperature) organic phosphors by adjacent fluorophores seems to be a general technology that may be applicable for construction of photoluminescence-based sensors for temperature and analytes (e.g., oxygen). The ET mechanism might also be applicable for development of efficient OLEDs that act through harvesting of energy of electrically generated triplet excitons via emitting light by adjacent fluorescent dyes. Heavier atoms than sulfur are not required to construct bright green, orange, red or near-infrared organic emitters possessing PL decay time in microsecond range.