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The spatial extent of excitons in molecular systems underpins their photophysics and utility for optoelectronic applications. Phonons are reported to lead to both exciton localization and delocalization. However, a microscopic understanding of phonon-induced (de)localization is lacking, in particular how localized states form, the role of specific vibrations, and the relative importance of quantum and thermal nuclear fluctuations. Here we present a first-principles study of these phenomena in solid pentacene, a prototypical molecular crystal, capturing the formation of bound excitons, exciton-phonon coupling to all orders, and phonon anharmonicity, using density functional theory, the \emph{ab initio} $GW$-Bethe-Salpeter equation approach, finite difference, and path integral techniques. We find that for pentacene zero-point nuclear motion causes uniformly strong localization, with thermal motion providing additional localization only for Wannier-Mott-like excitons. Anharmonic effects drive temperature-dependent localization, and while such effects prevent the emergence of highly delocalized excitons, we explore the conditions under which these might be realized.

Triplet energy transfer between inorganic quantum dots (QDs) and organic materials plays a fundamental role in many optoelectronic applications based on these nanocomposites. Attaching organic molecules to the QD as transmitter ligands has been shown to facilitate transfer both to and from QDs. Here we show that the often disregarded thiol anchoring group can achieve quantitative triplet energy transfer yields in a PbS QD system with 6,11-bis[(triisopropylsilyl)ethynyl]tetracene-2-methylthiol (TET-SH) ligands. We demonstrate efficient triplet transfer in a singlet fission-based photon multiplication system with 5,12-bis[(triisopropylsilyl)ethynyl]tetracene generating triplets in solution that transfer to the PbS QDs via the thiol ligand TET-SH. Importantly, we demonstrate the increased thermal stability of the PbS/TET-SH system, compared to the traditional carboxylic acid counterpart, allowing for higher photoluminescence quantum yields.

Accurate predictions of electronic band gaps are key to the computational design of functional materials with tailored optical and thermoelectric properties. While an accurate description of the underlying electronic structure is indispensable, electron-phonon interactions also play a prominent role in determining the band gap. The harmonic vibrational approximation is used almost universally to describe electron-phonon coupling and provides a satisfactory description in most systems. Here, we demonstrate that this is not the case for molecular crystals due to the presence of strongly anharmonic motions. We demonstrate the breakdown of the harmonic approximation in acene molecular crystals, and show that converged band gaps can be obtained by rigorously accounting for vibrational anharmonicity using path-integral molecular dynamics. Finally, we characterize the most anharmonic vibrational modes and their contributions to the band-gap renormalization.

Recent synthesis of cyclo[18]carbon has spurred increasing interest in carbon rings. We focus on a comparative inspection of ground and excited states, as well as of hole transfer properties of cumulenic and polyynic cyclo[18]carbon via Density Functional Theory (DFT), time-dependent DFT (TD-DFT) and real-time time-dependent DFT (RT-TDDFT). Zero-point vibrations are also accounted for, using a Monte Carlo sampling technique and a less exact, yet mode-resolved, quadratic approximation. The inclusion of zero-point vibrations leads to a red-shift on the HOMO-LUMO gap and the first singlet and triplet excitation energies of both conformations, correcting the values of the 'static' configurations by 9% to 24%. Next, we oxidize the molecule, creating a hole at one carbon atom. Hole transfer along polyynic cyclo[18]carbon is decreased in magnitude compared to its cumulenic counterpart and lacks the symmetric features the latter displays. Contributions by each mode to energy changes and hole transfer between diametrically opposed atoms vary, with specific bond-stretching modes being dominant.

Thermally-activated delayed fluorescence (TADF) enables organic semiconductors with charge transfer (CT)-type excitons to convert dark triplet states into bright singlets via a reverse intersystem crossing (rISC) process. Here, we consider the role of the dielectric environment in a range of TADF materials with varying changes in dipole moment upon optical excitation. In a dipolar reference emitter, TXO-TPA, environmental reorganisation after excitation in both solution and doped films triggers the formation of the full CT product state. This lowers the singlet excitation energy by 0.3 eV and minimises the singlet-triplet energy gap ({\Delta}EST). Using impulsive Raman measurements, we observe the emergence of two (reactant-inactive) modes at 412 and 813 cm-1 as a vibrational fingerprint of the CT product. In contrast, the dielectric environment plays a smaller role in the electronic excitations of a less dipolar material, 4CzIPN. Quantum-chemical calculations corroborate the appearance of these new product modes in TXO-TPA and show that the dynamic environment fluctuations are large compared to {\Delta}EST. The analysis of the energy-time trajectories and the corresponding free energy functions reveals that the dielectric environment significantly reduces the activation energy for rISC, thus increasing the rISC rate by up to three orders of magnitude when compared to a vacuum environment., Comment: 67 pages, 7 main-text figures

Thermally activated delayed fluorescence enables organic semiconductors with charge transfer-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing. However, thus far, the contribution from the dielectric environment has received insufficient attention. Here we study the role of the dielectric environment in a range of thermally activated delayed fluorescence materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganization after excitation triggers the full charge transfer exciton formation, minimizing the singlet-triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the charge transfer product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy-time trajectories and their free-energy functions reveals that the dielectric environment substantially reduces the activation energy for reverse intersystem crossing in dipolar thermally activated delayed fluorescence emitters, increasing the reverse intersystem crossing rate by three orders of magnitude versus the isolated molecule.

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 21Ag– symmetry and a planar backbone exhibit a significantly higher diffusion coefficient (34 ± 10 cm2 s–1) compared to excitons with 11Bu+ symmetry (7 ± 6 cm2 s–1) with a twisted backbone. We also find that exciton transport in the 21Ag– state occurs without exciton–exciton annihilation. Both 21Ag– and 11Bu+ 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.

The results presented in this chapter have been published in Ref. [1] (https://doi.org/10.1063/1.5115239), where parts of the text and figures appear. Reprinted from.

The results presented in this chapter have been published in Ref. [1] (https://doi.org/10.1103/PhysRevB.102.081122).

For systems with a time-independent Hamiltonian, the Schrodinger equation dictates a simple time dependence that manifests itself as phase factors multiplying the solutions of the time-independent Schrodinger equation

Carbon atoms form the basis of organic molecules and polymers, which are the building blocks of organic semiconducting materials. There are millions of possible organic structures, which allow one to imagine countless ways of designing materials with the desired properties. The small atomic number of carbon, as well as hydrogen, nitrogen and oxygen, which are the other elements often participating in organic compounds, gives organic semiconductors some of their unique properties, compared to their inorganic counterparts such as silicon.

The results presented in this chapter have been published in Ref. [1] (https://doi.org/10.1063/5.0019009), where parts of the text and figures appear.

The experimental work presented in this chapter was not undertaken by the author of this thesis, but by a number of collaborators who are listed as co-authors in the aforementioned publication.

This chapter aims to introduce a number of concepts which are important for modelling the quantum dynamics of an exciton system interacting with an environment of molecular and/or crystal vibrations.

Organic semiconductors exhibit properties of individual molecules and extended crystals simultaneously. The strongly bound excitons they host are typically described in the molecular limit, but excitons can delocalize over many molecules, raising the question of how important the extended crystalline nature is. Using accurate Green's function based methods for the electronic structure and non-perturbative finite difference methods for exciton-vibration coupling, we describe exciton interactions with molecular and crystal degrees of freedom concurrently. We find that the degree of exciton delocalization controls these interactions, with thermally activated crystal phonons predominantly coupling to delocalized states, and molecular quantum fluctuations predominantly coupling to localized states. Based on this picture, we quantitatively predict and interpret the temperature and pressure dependence of excitonic peaks in the acene series of organic semiconductors, which we confirm experimentally, and we develop a simple experimental protocol for probing exciton delocalization. Overall, we provide a unified picture of exciton delocalization and vibrational effects in organic semiconductors, reconciling the complementary views of finite molecular clusters and periodic molecular solids.

The energies of molecular excited states arise as solutions to the electronic Schr\"{o}dinger equation and are often compared to experiment. At the same time, nuclear quantum motion is known to be important and to induce a red-shift of excited state energies. However, it is thus far unclear whether incorporating nuclear quantum motion in molecular excited state calculations leads to a systematic improvement of their predictive accuracy, making further investigation necessary. Here we present such an investigation by employing two first-principles methods for capturing the effect of quantum fluctuations on excited state energies, which we apply to the Thiel set of organic molecules. We show that accounting for zero-point motion leads to much improved agreement with experiment, compared to `static' calculations which only account for electronic effects, and the magnitude of the red-shift can become as large as 1.36 eV. Moreover, we show that the effect of nuclear quantum motion on excited state energies largely depends on the molecular size, with smaller molecules exhibiting larger red-shifts. Our methodology also makes it possible to analyze the contribution of individual vibrational normal modes to the red-shift of excited state energies, and in several molecules we identify a limited number of modes dominating this effect. Overall, our study provides a foundation for systematically quantifying the shift of excited state energies due to nuclear quantum motion, and for understanding this effect at a microscopic level.

International audience; Singlet fission in organic semiconductors causes a singlet exciton to decay into a pair of triplet excitons and holds potential for increasing the efficiency of photovoltaic devices. In this combined experimental and theoretical study, we reveal that a covalent dimer of the organic semiconductor tetracene undergoes activated singlet fission by qualitatively different mechanisms depending on the solvent environment. We show that intramolecular vibrations are an integral part of this mechanism, giving rise to mixing between charge transfer and triplet pair excitations. Either coherent or incoherent singlet fission can occur, depending on the transient solvent-induced energetic proximity between the states, giving rise to complex variation of the singlet fission mechanism and time scale in the different environments. Our results suggest a more general principle for controlling the efficiency of photochemical reactions by utilizing transient interactions to tune the energetics of reactant and product states and switch between incoherent and coherent dynamic

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., Ultrafast photo-induced processes in complex systems require theoretical models and their experimental validation which are still lacking. Here the authors investigate singlet fission in a pentacene dimer by a combined experimental and theoretical approach providing a real-time visualisation of the process.

International audience; Exciton-polaritons in organic materials are hybrid states that result from the strong interaction of photons and the bound excitons that these materials host. Organic polaritons hold great interest for optoelectronic applications; however, progress toward this end has been impeded by the lack of a first principles approach that quantifies light–matter interactions in these systems, which would allow the formulation of molecular design rules. Here, we present a theoretical framework that combines first principles calculations for excitons with classical electrodynamics in order to quantify light–matter interactions. We exemplify our approach by studying variants of the conjugated polymer polydiacetylene, and we show that a large polymer conjugation length is critical toward strong exciton–photon coupling, hence underlying the importance of pure structures without static disorder. By comparing to our experimental reflectivity measurements, we show that the coupling of excitons to vibrations, manifested by phonon side bands in the absorption, has a strong impact on the magnitude of light–matter coupling over a range of frequencies. Our approach opens the way toward a deeper understanding of polaritons in organic materials, and we highlight that a quantitatively accurate calculation of the exciton–photon interaction would require accounting for all sources of disorder self-consistently.