Role of Conical Intersections on the Efficiency of Fluorescent Organic Molecular Crystals

Organic molecular crystals are attractive materials for luminescent applications due to their promised tunability. However, the link between chemical structure and emissive behaviour is poorly understood due to the numerous interconnected factors which are at play in determining radiative and non-radiative behaviours at the solid state level. In particular, the decay through conical intersection dominates the nonadiabatic regions of the potential energy surface, and thus their accessibility is a telling indicator of the luminosity of the material. In this study, we investigate the radiative mechanism for five organic molecular crystals which display solid state emission, with a focus on the role of conical intersections in their photomechanisms. The objective is to situate the importance of the accessibility of conical intersections with regards to emissive behaviour, taking into account other nonradiative decay channels, namely vibrational decay, and exciton hopping. We begin by giving a brief overview of the structural patterns of the five systems within a larger pool of thirteen crystals for a richer comparison. We observe that due to the prevalence of sheet-like and herringbone packing in organic molecular crystals, the conformational diversity of crystal dimers is limited. Additionally, similarly spaced dimers have exciton coupling values of similar order within a 50 meV interval. Next, we focus on three exemplary cases, where we disentangle the role of nonradiative decay mechanisms and show how rotational minimum energy conical intersections in vacuum lead to puckered ones in crystal, increasing their instability upon crystallisation in typical packing motifs. In contrast, molecules with puckered conical intersections in vacuum tend to conserve this trait upon crystallisation, and therefore their quantum yield of fluorescence is determined predominantly by other nonradiative decay mechanisms.

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The role of conical intersections on the efficiency of fluorescent organic molecular crystals

10.26434/chemrxiv.12058953 ◽

2020 ◽

Author(s):

Miguel Rivera ◽

Ljiljana Stojanovic ◽

Rachel Crespo Otero

Keyword(s):

Solid State ◽

Minimum Energy ◽

Molecular Crystals ◽

State Level ◽

Conical Intersections ◽

Decay Channels ◽

Nonradiative Decay ◽

Exciton Coupling ◽

Organic Molecular Crystals

Organic molecular crystals are attractive materials for luminescent applications due to their promised tunability. However, the link between chemical structure and emissive behaviour is poorly understood due to the numerous interconnected factors which are at play in determining radiative and non-radiative behaviours at the solid state level. In particular, the decay through conical intersection dominates the nonadiabatic regions of the potential energy surface, and thus their accessibility is a telling indicator of the luminosity of the material. In this study, we investigate the radiative mechanism for five organic molecular crystals which display solid state emission, with a focus on the role of conical intersections in their photomechanisms. The objective is to situate the importance of the accessibility of conical intersections with regards to emissive behaviour, taking into account other nonradiative decay channels, namely vibrational decay, and exciton hopping. We begin by giving a brief overview of the structural patterns of the five systems within a larger pool of thirteen crystals for a richer comparison. We observe that due to the prevalence of sheet-like and herringbone packing in organic molecular crystals, the conformational diversity of crystal dimers is limited. Additionally, similarly spaced dimers have exciton coupling values of similar order within a 50 meV interval. Next, we focus on three exemplary cases, where we disentangle the role of nonradiative decay mechanisms and show how rotational minimum energy conical intersections in vacuum lead to puckered ones in crystal, increasing their instability upon crystallisation in typical packing motifs. In contrast, molecules with puckered conical intersections in vacuum tend to conserve this trait upon crystallisation, and therefore their quantum yield of fluorescence is determined predominantly by other nonradiative decay mechanisms.

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Competition between radiative and nonradiative excited state processes in photoluminescent organic molecular crystals

10.26434/chemrxiv.12058953.v1 ◽

2020 ◽

Author(s):

Miguel Rivera ◽

Ljiljana Stojanovic ◽

Rachel Crespo Otero

Keyword(s):

Excited State ◽

Molecular Crystals ◽

State Level ◽

High Energy ◽

Quantum Yields ◽

Conical Intersections ◽

Fluorescence Quantum Yields ◽

Exciton Coupling ◽

Organic Molecular Crystals

Organic molecular crystals are attractive materials for luminescent applications due to their promised tunability. However, the link between chemical structure and emissive behaviour is poorly understood due to the numerous interconnected factors which are at play in determining radiative and non-radiative behaviours at the solid state level. In this study, we investigate thirteen luminescent molecular crystals and apply newly implemented tools to study their geometric properties and constituent dimer excitonic coupling values. We then focus on the excited state decay pathways of five of the molecules. The competition between radiative and nonradiative processes was used to rationalise the different fluorescence quantum yields across systems. We found that due to the prevalence of sheet and herringbone packing in organic molecular crystals, the conformational diversity of crystal dimers is limited. Additionally, similarly spaced dimers have exciton coupling values of similar order within a 50 meV interval. Finally, we found that the accessibility of conical intersection geometries was a robust indicator of the role of nonradiative decay in the excited state mechanism of most molecules. The conical intersections all displayed a measure of rotation and puckering, where purely rotational conical intersections in vacuum lead to high energy puckered conical intersections in the crystal phase.

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Origin and role of gap states in organic semiconductor studied by UPS: as the nature of organic molecular crystals

Journal of Physics D Applied Physics ◽

10.1088/1361-6463/aa840f ◽

2017 ◽

Vol 50 (42) ◽

pp. 423002 ◽

Cited By ~ 46

Author(s):

Jin-Peng Yang ◽

Fabio Bussolotti ◽

Satoshi Kera ◽

Nobuo Ueno

Keyword(s):

Organic Semiconductor ◽

Molecular Crystals ◽

Organic Molecular Crystals ◽

Gap States

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The role of structural defects in the luminescence of organic molecular crystals

Surface and Defect Properties of Solids ◽

10.1039/9781847556950-00229 ◽

2007 ◽

pp. 229-249 ◽

Cited By ~ 2

Author(s):

J. O. Williams

Keyword(s):

Molecular Crystals ◽

Structural Defects ◽

Organic Molecular Crystals

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Adiabatic large polarons in anisotropic molecular crystals

Journal of Research in Physics ◽

10.2478/v10242-012-0002-2 ◽

2011 ◽

Vol 35 (1) ◽

pp. 15-27

Author(s):

Zoran Ivić ◽

Željko Pržulj

Keyword(s):

Energy Transfer ◽

Effective Mass ◽

Molecular Crystals ◽

Single Chain ◽

Proper Understanding ◽

One Dimensional ◽

Interchain Coupling ◽

Large Polaron ◽

The One

Adiabatic large polarons in anisotropic molecular crystals We study the large polaron whose motion is confined to a single chain in a system composed of the collection of parallel molecular chains embedded in threedimensional lattice. It is found that the interchain coupling has a significant impact on the large polaron characteristics. In particular, its radius is quite larger while its effective mass is considerably lighter than that estimated within the one-dimensional models. We believe that our findings should be taken into account for the proper understanding of the possible role of large polarons in the charge and energy transfer in quasi-one-dimensional substances.

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