Invited Speaker Australian & New Zealand Society of Magnetic Resonance Conference 2017

Spin Control and Dynamics in Pentacene Dimers (#27)

Dane McCamey 1 , M. J. Y. Tayebjee 2 , S.N. Sanders 3 , E Kumarasamy 3 , A Asadpoordarvish 1 , T.W. Schmidt 4 , J Beves 4 , M.Y. Sfeir 5
  1. School of Physics, University of New South Wales, Sydney, NSW, Australia
  2. School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney , NSW
  3. Department of Chemistry, Columbia University, New York, USA
  4. School of Chemistry, University of New South Wales, Sydney, NSW
  5. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York

Singlet exciton fission is a process in which an optically prepared singlet state splits into two triplet excitons with (anti-)correlated spins. If harnessed efficiently, this process can be exploited to enhance the photocurrent of solar cells, raising the limiting power conversion efficiency from 33.7% to 45.9% under 1 Sun [1], which has motivated interest in this field. Fission is usually studied via optical spectroscopic techniques, such as pump-probe transient absorption, which drive electronic transitions and give details about the rate and yield of fission. However experimental insight into the nature of the triplet-pair state generated upon fission is lacking, as such approaches do not have the resolution required to identify the small spin couplings which influence these processes. 

Transient electron paramagnetic resonance (EPR) spectroscopy involves continually measuring an EPR signal, usually following optical excitation of a spin system with transient properties. This is particularly useful in characterising singlet fission systems, as it allows us to drive transitions of the different spin species which contribute to the fission process. The spectral resolution this provides allows us to unambiguously determine the nature and spin dynamics of the triplet-pair states on the fission reaction coordinate.

By applying this approach to a range of novel molecules comprising pentacene dimers with engineered coupling, we show that singlet fission proceeds via a strongly coupled Quintet (S=2) state, before dissociating to two correlated but uncoupled Triplet (S=1) states [3]. The assignment of spin states is confirmed by the relative ratios of the coherent nutation frequency of the resolved spectral features.

Alongside the applications in solar energy generation, singlet fission provides access to an engineerable molecular platform in which to study and exploit fundamental physics of correlated spin systems. In this vein we will discuss coherence and spin lifetimes of the generated triplet states. Finally, we will discuss approaches to obtaining control of the coupling between the correlated spins which are prepared via fission. We will show that modification of the coupling between dimers has a strong impact on the fission process, but that fission proceeds in dimers with both homoconjugated and non-conjugated bridges [4]. We will also describe approaches to optical dynamical control of the coupling between the weakly-coupled pairs of triplets.

  1. [1] Tayebjee, M.J.Y., et al., J. Phys. Chem. Lett. 3, 2749-2754 (2012) [2] Sanders, S.N., et al., J. Am. Chem. Soc. 137, 8965-8972 (2015) [3] Tayebjee, M. J. T. et al, Nature Physics 13, 182 (2017) [4] Kumarasamy, E. et al., J. Am. Chem. Soc. 139, 12488 (2017)