Breakdown of the Perturbative Approach to Molecular Packing Dependence of Singlet Fission Rates in Pentacene Dimer Models: A Systematic Comparison with the Quantum Master Equation Approach
Abstract:We
have compared the molecular packing dependences of singlet fission
(SF) rates calculated by perturbative and quantum master equation
approaches using slip-stacked pentacene dimer models to clarify the
applicability of the perturbative treatment and the importance of
vibronic coupling effects on the SF dynamics. Several significant
differences between these two approaches are found in the peak positions
as well as in the relative peak amplitudes on the two-dimensional
map of the SF rate as a function of the … Show more
“…In our previous work on DP-Mes 48 , a combination of machine learning and entanglement renormalisation techniques was employed to capture the non-perturbative and non-Markovian physics arising from strong coupling to a large number of vibrational modes (see Methods and Supplementary Discussion, section 1). A recent theoretical study highlighted the breakdown of perturbative approaches in predicting singlet fission rates in pentacene dimer systems 62 , further emphasising that non-perturbative approaches such as the TTNS method 48 are necessary to describe fission dynamics. Our earlier DP-Mes simulation confirmed that singlet fission is mediated by super-exchange and is driven by a chain of cooperative vibronic processes involving modes of different timescales and symmetries.Fig.…”
The complex dynamics of ultrafast photoinduced reactions are governed by their evolution along vibronically coupled potential energy surfaces. It is now often possible to identify such processes, but a detailed depiction of the crucial nuclear degrees of freedom involved typically remains elusive. Here, combining excited-state time-domain Raman spectroscopy and tree-tensor network state simulations, we construct the full 108-atom molecular movie of ultrafast singlet fission in a pentacene dimer, explicitly treating 252 vibrational modes on 5 electronic states. We assign the tuning and coupling modes, quantifying their relative intensities and contributions, and demonstrate how these modes coherently synchronise to drive the reaction. Our combined experimental and theoretical approach reveals the atomic-scale singlet fission mechanism and can be generalized to other ultrafast photoinduced reactions in complex systems. This will enable mechanistic insight on a detailed structural level, with the ultimate aim to rationally design molecules to maximise the efficiency of photoinduced reactions.
“…In our previous work on DP-Mes 48 , a combination of machine learning and entanglement renormalisation techniques was employed to capture the non-perturbative and non-Markovian physics arising from strong coupling to a large number of vibrational modes (see Methods and Supplementary Discussion, section 1). A recent theoretical study highlighted the breakdown of perturbative approaches in predicting singlet fission rates in pentacene dimer systems 62 , further emphasising that non-perturbative approaches such as the TTNS method 48 are necessary to describe fission dynamics. Our earlier DP-Mes simulation confirmed that singlet fission is mediated by super-exchange and is driven by a chain of cooperative vibronic processes involving modes of different timescales and symmetries.Fig.…”
The complex dynamics of ultrafast photoinduced reactions are governed by their evolution along vibronically coupled potential energy surfaces. It is now often possible to identify such processes, but a detailed depiction of the crucial nuclear degrees of freedom involved typically remains elusive. Here, combining excited-state time-domain Raman spectroscopy and tree-tensor network state simulations, we construct the full 108-atom molecular movie of ultrafast singlet fission in a pentacene dimer, explicitly treating 252 vibrational modes on 5 electronic states. We assign the tuning and coupling modes, quantifying their relative intensities and contributions, and demonstrate how these modes coherently synchronise to drive the reaction. Our combined experimental and theoretical approach reveals the atomic-scale singlet fission mechanism and can be generalized to other ultrafast photoinduced reactions in complex systems. This will enable mechanistic insight on a detailed structural level, with the ultimate aim to rationally design molecules to maximise the efficiency of photoinduced reactions.
“…The slip-stacked geometry is commonly identified as most favourable for singlet fission, 12,23,24 but theoretical studies suggest that herringbone packing could also be suitable. 25 The exciton delocalisation on non-covalent dimers has been studied, [26][27][28][29][30] but the effect of molecular configurations on SF dynamics is not explicitly addressed.…”
mentioning
confidence: 99%
“…Calculations have predicted that the optimal geometry for SF is that which results in a substantial orbital overlap for both HOMO and LUMO, however this has been attributed to both herringbone and slip-stacked configurations. 12,30,[32][33][34][35] Therefore, a combined experimental and theoretical methodology that allows to isolate and distinguish the kinetics of herringbone and slip-stacked dimers in the SF process would be a valuable addition to the field.…”
mentioning
confidence: 99%
“…The slip-stacked geometry is commonly identified as most favorable for singlet fission, ,, but theoretical studies suggest that herringbone packing could also be suitable . Exciton delocalization on non-covalent dimers has been studied, − but the effect of the molecular configuration on the SF dynamics was not explicitly addressed. Structural considerations and the effect of dilution of acene molecules on the intertriplet interactions have been studied, but the measurements could not distinguish between herringbone and slip-stacked geometries. , Several theoretical studies have compared molecular systems with stacked or herringbone motifs in their crystal packing, addressing the effect of the relative orientation between neighboring chromophores on the spin and thermodynamics of SF and subsequent exciton migration, − but to our knowledge no such experimental study of solid-state crystalline systems has been reported.…”
Singlet fission -whereby one absorbed photon generates two coupled triplet excitons -is a key process for increasing the efficiency of optoelectronic devices by overcoming the Shockley-Queisser limit. A crucial parameter is the rate of dissociation of the coupled triplets, as this limits the number of free triplets subsequently available for harvesting and ultimately the overall efficiency of the device. Here, we present an analysis of the thermodynamic and kinetic parameters for this process in parallel and herringbone dimers measured by electron paramagnetic resonance spectroscopy in co-evaporated films of pentacene in p-terphenyl. The rate of dissociation is faster for parallel dimers than for their herringbone counterparts, as is the recombination to the ground state. DFT calculations, which provide the magnitude of the electronic coupling as well as the distribution of molecular orbitals for each geometry, suggest that lower triplet coupling in the parallel dimer is the driving force for faster dissociation. Conversely, localization of the molecular orbitals and stronger triplet-triplet interaction result in slower dissociation and recombination. The identification and understanding of how intermolecular geometry promote efficient triplet dissociation provides the basis for control of triplet coupling and thereby the optimization of one important parameter of device performance.
“…In a previous work on DP-Mes [51], a combination of machine learning and entanglement renormalisation techniques was employed to capture the non-perturbative and non-Markovian physics arising from strong coupling to a large number of vibrational modes. A recent theoretical study highlighted the breakdown of perturbative approaches in predicting singlet fission rates in pentacene dimer systems [259], further emphasising the fact that non-perturbative approaches such as the TTNS method [51] are necessary to describe fission dynamics. The DP-Mes simulations in [51] confirmed that singlet fission is mediated by superexchange and is driven by a chain of cooperative vibronic processes involving modes of different timescales and symmetries.…”
This thesis is the result of my own work and includes nothing which is the outcome of work done in collaboration except as declared in the Preface and specified in the text. It is not substantially the same as any that I have submitted, or, is being concurrently submitted for a degree or diploma or other qualification at the University of Cambridge or any other University or similar institution except as declared in the Preface and specified in the text.I further state that no substantial part of my thesis has already been submitted, or, is being concurrently submitted for any such degree, diploma or other qualification at the University of Cambridge or any other University or similar institution except as declared in the Preface and specified in the text. It does not exceed the prescribed word limit for the Physics and Chemistry Degree Committee.
Antonios M. AlvertisI thank my good friends Vasilis and Stratis for all the incredibly fun times in Cambridge and Athens, for hosting me in their house countless times, for sharing their food which was always tasty, for the music evenings, for having drinks at Darbar and so much more that I could never put on paper. I thank Iasonas and Georgia, that together with Stratis formed my St Edmund's crew of my last year, making me to always hang around college and looking forward to our dinners together and to our chats in the CR.Almost from my first day in Cambridge, and up to the last as a Ph.D. student, I stayed in Link House. It was amazing how many good friends from all over the world I made in this international community. I am thankful to the wardens, Gary and Anne, for making sure that the atmosphere was always so great, and for always being there when I needed support in any way possible. There is a lot I could say about my friends from the house, but a big thanks to Daniele and Jee-Yeon, Diego, Elena, Toshi and Chiara will have to suffice for now.Even lockdown was a nice time in Link House, for which special thanks go to Beatrice for all the fun things we did and for teaching me so much about movies.I would never have managed to achieve the things I did had it not been for my family and friends in Greece. I cannot imagine that one could wish for better and more supportive parents than my mother Cornelia and my father Yannis. Without them, and without my grandparents Antonis and Vasiliki, I would not be the person I am today. I owe eternal gratitude to our family friend Dr. Panagiotis Komninos, who so tragically passed away.It was largely thanks to our countless discussions on physics and what it's like to be an academic abroad that I was able to obtain a better understanding of the possible paths I could follow. It was also always such good fun to hear his stories about the Greek community in Munich. Finally, I owe the deepest thanks to my aunts Zeta and Mary for always being there in the difficult times, and my aunt Monica and cousin Marina for their unconditional love. Contents 1 Introduction I Background 2 Organic semiconductors and their properties 2.1 The chemis...
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