Let Digons be Bygones: The Fate of Excitons in Curved π-Systems

Many optoelectronic devices based on organic materials require rapid and long-range singlet exciton transport. Key factors controlling exciton transport include material structure, exciton–phonon coupling and electronic state symmetry. Here, we employ femtosecond transient absorption microscopy to study the influence of these parameters on exciton transport in one-dimensional conjugated polymers. We find that excitons with 2 1 A g – symmetry and a planar backbone exhibit a significantly higher diffusion coefficient (34 ± 10 cm 2 s –1 ) compared to excitons with 1 1 B u + symmetry (7 ± 6 cm 2 s –1 ) with a twisted backbone. We also find that exciton transport in the 2 1 A g – state occurs without exciton–exciton annihilation. Both 2 1 A g – and 1 1 B u + states are found to exhibit subdiffusive behavior. Ab initio GW -BSE calculations reveal that this is due to the comparable strengths of the exciton–phonon interaction and exciton coupling. Our results demonstrate the link between electronic state symmetry, backbone torsion and phonons in exciton transport in π-conjugated polymers.

Section: Resultsmentioning

confidence: 99%

Exciton Diffusion in Highly-Ordered One Dimensional Conjugated Polymers: Effects of Back-Bone Torsion, Electronic Symmetry, Phonons and Annihilation

Pandya

Alvertis

et al. 2021

“…An accurate description of nonequilibrium dynamics related to hot carries and excited states requires treatment of coupling between electronic and nuclear degrees of freedom and goes beyond the Born–Oppenheimer approximation. , Prevalent methods (e.g., mean-field Ehrenfest, surface hopping, , and Redfield , ) are typically used to simulate such dynamics relying on nonadiabatic couplings. Such modeling incorporating nonadiabatic effects provides mechanistical pictures of photoexcited dynamics and nonradiative relaxation for molecular systems. − A broad variety of previous studies addressed excited state properties and dynamics of various silicon systems, such as photochemistry of silane molecules using surface hopping methods, surface photovoltage of Si thin films by density matrix treatment, , multiple exciton generation in silicon–carbon nanotube quantum heterostructures through applying Boltzmann transport equation, charge carrier relaxation for Si quantum dots, , nanowires, and Si/Au nanointerfaces , adopting Redfield formalism, and nonradiative recombination in Si nanocrystals through the characterization of conical intersections. , …”

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confidence: 99%

Hot Carrier Dynamics at Ligated Silicon(111) Surfaces: A Computational Study

Han

Iduoku

Grant

et al. 2021

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We provide a case-study for thermal grafting of benzenediazonium bromide onto a hydrogenated Si(111) surface using ab initio molecular dynamics (AIMD) calculations. A sequence of reaction steps is identified in the AIMD trajectory, including the loss of N 2 from the diazonium salt, proton transfer from the surface to the bromide ion that eliminates HBr, and deposition of the phenyl group onto the surface. We next assess the influence of the phenyl groups on photophysics of hydrogen-terminated Si(111) slabs. The nonadiabatic couplings necessary for a description of the excited-state dynamics are calculated by combining ab initio electronic structures and reduced density matrix formalism with Redfield theory. The phenylterminated slab shows reduced nonradiative relaxation and recombination rates of hot charge carriers in comparison with the hydrogen-terminated slab. Altogether, our results provide atomistic insights revealing that (i) the diazonium salt thermally decomposes at the surface allowing the formation of covalently bonded phenyl group, and (ii) the coverage of phenyl groups on the surface slows down charge carrier cooling driven by electron−phonon interactions, which increases photoluminescence efficiency at the near-infrared spectral region.

“…19−21 For example, curved bichromophore molecular polygons (digons) act as unpolarized light absorbers and emitters as a result of bending strain and self-trapping of the emissive state on equivalent vertex units, respectively. 15,22 In ring-shaped cycloparaphenylenes (CPPs), the lowest-energy excited state is forbidden by symmetry. However, strong vibronic coupling during the internal conversion process in CPPs causes excited-state geometry distortions that result in exciton localization and a large transition dipole moment for the emissive state.…”

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confidence: 99%

“…For example, localization of the final fluorescent state following photoexcitation in spiro-linked conjugated polyfluorene stabilizes its light emission . In fact, exciton self-trapping or spatial confinement of the lowest-energy emissive state following photoexcitation has been observed in many conjugated organic systems, particularly those consisting of equivalent chromophores. − In these systems, exciton migration from an initial state delocalized over two or more equivalent chromophores to a single chromophore unit (or localized site within a single unit) seems to be a ubiquitous feature of the relaxation dynamics. Beyond that, additional hopping between equivalent localized sites on the lowest-energy surface may or may not occur depending on the specific electronic and vibronic couplings …”

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confidence: 99%

“…The π donors that induce energy transfer in the excited states are typically electron-rich heteroarenes or arenes with electron-donating groups. The discovery that curved oligophenylenes undergo energy transfer in the excited state provides a new option for designing π donor–acceptor molecular systems. − For example, curved bichromophore molecular polygons (digons) act as unpolarized light absorbers and emitters as a result of bending strain and self-trapping of the emissive state on equivalent vertex units, respectively. , In ring-shaped cycloparaphenylenes (CPPs), the lowest-energy excited state is forbidden by symmetry. However, strong vibronic coupling during the internal conversion process in CPPs causes excited-state geometry distortions that result in exciton localization and a large transition dipole moment for the emissive state .…”

mentioning

confidence: 99%

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Exciton Spatial Dynamics and Self-Trapping in Carbon Nanocages

Rodríguez-Hernández

Nelson

Oldani

et al. 2020

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Three-dimensional cage-shaped molecules formed from chainlike structures hold potential as unique optoelectronic materials and host compounds. Their optical, structural, and dynamical features are tunable by changes in shape and size. We perform a comparison of these properties for three sizes of strained conjugated [n.n.n]carbon nanocages composed of three paraphenylene chains (bridges) of length n = 4, 5, or 6. The exciton intramolecular redistribution occurring during nonradiative relaxation has been explored using nonadiabatic excited-state molecular dynamics. Our results provide atomistic insight into the conformational features associated with the observed red-and blue-shift trends in the absorption and fluorescence spectra, respectively, with increasing nanocage size. Their internal conversion processes involve intramolecular energy transfer that leads to exciton self-trapping on a few phenylene units at the center of a single bridge. The dependence of these dynamical features on the size of the nanocage can be used to tune their host−guest chemical properties and their use for organic electronics and catenane-like applications.