Coherent exciton-vibrational dynamics and energy transfer in conjugated organics

Coherence, signifying concurrent electron-vibrational dynamics in complex natural and man-made systems, is currently a subject of intense study. Understanding this phenomenon is important when designing carrier transport in optoelectronic materials. Here, excited state dynamics simulations reveal a ubiquitous pattern in the evolution of photoexcitations for a broad range of molecular systems. Symmetries of the wavefunctions define a specific form of the non-adiabatic coupling that drives quantum transitions between excited states, leading to a collective asymmetric vibrational excitation coupled to the electronic system. This promotes periodic oscillatory evolution of the wavefunctions, preserving specific phase and amplitude relations across the ensemble of trajectories. The simple model proposed here explains the appearance of coherent exciton-vibrational dynamics due to non-adiabatic transitions, which is universal across multiple molecular systems. The observed relationships between electronic wavefunctions and the resulting functionalities allows us to understand, and potentially manipulate, excited state dynamics and energy transfer in molecular materials.Fil: Nelson, Tammie R.. Los Alamos National Laboratory; Estados UnidosFil: Ondarse Alvarez, Dianelys. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Oldani, Andres Nicolas. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rodríguez Hernández, Beatriz. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Alfonso Hernandez, Laura. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Galindo, Johan F.. Universidad Nacional de Colombia; ColombiaFil: Kleiman, Valeria D.. University of Florida; Estados UnidosFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados Unido

Photoinduced non-adiabatic energy transfer pathways in dendrimer building blocks

The efficiency of the intramolecular energy transfer in light harvesting dendrimers is determined by their well-defined architecture with high degree of order. After photoexcitation, through-space and through-bond energy transfer mechanisms can take place, involving vectorial exciton migration among different chromophores within dendrimer highly branched structures. Their inherent intramolecular energy gradient depends on how the multiple chromophoric units have been assembled, subject to their inter-connects, spatial distances, and orientations. Herein, we compare the photoinduced nonadiabatic molecular dynamics simulations performed on a set of different combinations of a chain of linked dendrimer building blocks composed of two-, three-, and four-ring linear polyphenylene chromophoric units. The calculations are performed with the recently developed ab initio multiple cloning-time dependent diabatic basis implementation of the Multiconfigurational Ehrenfest (MCE) approach. Despite differences in short time relaxation pathways and different initial exciton localization, at longer time scales, electronic relaxation rates and exciton final redistributions are very similar for all combinations. Unlike the systems composed of two building blocks, considered previously, for the larger 3 block systems here we observe that bifurcation of the wave function accounted by cloning is important. In all the systems considered in this work, at the time scale of few hundreds of femtoseconds, cloning enhances the electronic energy relaxation by ∼13% compared to that of the MCE method without cloning. Thus, accurate description of quantum effects is essential for understanding of the energy exchange in dendrimers both at short and long time scales

Ultrafast electronic energy relaxation in a conjugated dendrimer leading to inter-branch energy redistribution

Dendrimers are arrays of coupled chromophores, where the energy of each unit depends on its structure and conformation. The light harvesting and energy funneling properties are strongly dependent on their highly branched conjugated architecture. Herein, the photoexcitation and subsequent ultrafast electronic energy relaxation and redistribution of a first generation dendrimer (1) are analyzed combining theoretical and experimental studies. Dendrimer 1 consists of three linear phenylene-ethynylene (PE) units, or branches, attached in the meta position to a central group opening up the possibility of inter-branch energy transfer. Excited state dynamics are explored using both time-resolved spectroscopy and non-adiabatic excited state molecular dynamics simulations. Our results indicate a subpicosecond loss of anisotropy due to an initial excitation into several states with different spatial localizations, followed by exciton self-trapping on different units. This exciton hops between branches. The absence of an energy gradient leads to an ultrafast energy redistribution among isoenergetic chromophore units. At long times we observe similar probabilities for each branch to retain significant contributions of the transition density of the lowest electronic excited-state. The observed unpolarized emission is attributed to the contraction of the electronic wavefunction onto a single branch with frequent interbranch hops, and not to its delocalization over the whole dendrimer.Fil: Ondarse Alvarez, Dianelys. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología; ArgentinaFil: Kömürlü, S.. University of Florida; Estados UnidosFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Pierdominici Sottile, Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología; ArgentinaFil: Tretiak, S.. Los Alamos National Laboratory; Estados UnidosFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología; ArgentinaFil: Kleiman, V. D.. University of Florida; Estados Unido

Modification of Optical Properties and Excited-State Dynamics by Linearizing Cyclic Paraphenylene Chromophores

Cyclic and bent conjugated molecular systems have tunable optical, structural, and dynamical features that differentiate them from their linear counterparts. Examples of such systems are [<i>n</i>]­cycloparaphenylenes (CPPs), which consist of nanorings composed of <i>n</i> para-linked benzene units. Circular geometry and tunability of π-orbital overlaps and bending strains enrich them with unique physicochemical and electronic properties compared to those of the corresponding linear oligoparaphenylenes. Herein, we explore the changes of these properties on alkyl-tethered-<i>p</i>-heptaphenylenes by modifying the methylene tether lengths from 1 to 19 carbons, leading to a gradual linearization of the conjugated backbone conformation. For this purpose, the photoinduced internal conversion processes of different alkyl-tethered-<i>p</i>-heptaphenylenes are simulated using nonadiabatic excited-state molecular dynamics. We found that the greater the strain introduced on the conjugated system, the slower the electronic and vibrational energy relaxation process. All bent <i>p</i>-heptaphenylenes exhibit similar patterns of intramolecular energy redistribution that finally spatially localize the exciton on phenylene units in the middle of the conjugated chain. This behavior is opposite to the random exciton localization previously reported for [<i>n</i>]­CPPs. Moreover, the nonadiabatic S₂ → S₁ electronic transition activates specific collective asymmetric vibrational excitations that promote periodic oscillatory evolution of the excitonic wave function before an excessive energy dissipates into the bath degrees of freedom

Modification of Optical Properties and Excited-State Dynamics by Linearizing Cyclic Paraphenylene Chromophores

Cyclic and bent conjugated molecular systems have tunable optical, structural, and dynamical features that differentiate them from their linear counterparts. Examples of such systems are [<i>n</i>]­cycloparaphenylenes (CPPs), which consist of nanorings composed of <i>n</i> para-linked benzene units. Circular geometry and tunability of π-orbital overlaps and bending strains enrich them with unique physicochemical and electronic properties compared to those of the corresponding linear oligoparaphenylenes. Herein, we explore the changes of these properties on alkyl-tethered-<i>p</i>-heptaphenylenes by modifying the methylene tether lengths from 1 to 19 carbons, leading to a gradual linearization of the conjugated backbone conformation. For this purpose, the photoinduced internal conversion processes of different alkyl-tethered-<i>p</i>-heptaphenylenes are simulated using nonadiabatic excited-state molecular dynamics. We found that the greater the strain introduced on the conjugated system, the slower the electronic and vibrational energy relaxation process. All bent <i>p</i>-heptaphenylenes exhibit similar patterns of intramolecular energy redistribution that finally spatially localize the exciton on phenylene units in the middle of the conjugated chain. This behavior is opposite to the random exciton localization previously reported for [<i>n</i>]­CPPs. Moreover, the nonadiabatic S₂ → S₁ electronic transition activates specific collective asymmetric vibrational excitations that promote periodic oscillatory evolution of the excitonic wave function before an excessive energy dissipates into the bath degrees of freedom

Modification of Optical Properties and Excited-State Dynamics by Linearizing Cyclic Paraphenylene Chromophores

Cyclic and bent conjugated molecular systems have tunable optical, structural, and dynamical features that differentiate them from their linear counterparts. Examples of such systems are [<i>n</i>]­cycloparaphenylenes (CPPs), which consist of nanorings composed of <i>n</i> para-linked benzene units. Circular geometry and tunability of π-orbital overlaps and bending strains enrich them with unique physicochemical and electronic properties compared to those of the corresponding linear oligoparaphenylenes. Herein, we explore the changes of these properties on alkyl-tethered-<i>p</i>-heptaphenylenes by modifying the methylene tether lengths from 1 to 19 carbons, leading to a gradual linearization of the conjugated backbone conformation. For this purpose, the photoinduced internal conversion processes of different alkyl-tethered-<i>p</i>-heptaphenylenes are simulated using nonadiabatic excited-state molecular dynamics. We found that the greater the strain introduced on the conjugated system, the slower the electronic and vibrational energy relaxation process. All bent <i>p</i>-heptaphenylenes exhibit similar patterns of intramolecular energy redistribution that finally spatially localize the exciton on phenylene units in the middle of the conjugated chain. This behavior is opposite to the random exciton localization previously reported for [<i>n</i>]­CPPs. Moreover, the nonadiabatic S₂ → S₁ electronic transition activates specific collective asymmetric vibrational excitations that promote periodic oscillatory evolution of the excitonic wave function before an excessive energy dissipates into the bath degrees of freedom

Modification of Optical Properties and Excited-State Dynamics by Linearizing Cyclic Paraphenylene Chromophores

Cyclic and bent conjugated molecular systems have tunable optical, structural, and dynamical features that differentiate them from their linear counterparts. Examples of such systems are [<i>n</i>]­cycloparaphenylenes (CPPs), which consist of nanorings composed of <i>n</i> para-linked benzene units. Circular geometry and tunability of π-orbital overlaps and bending strains enrich them with unique physicochemical and electronic properties compared to those of the corresponding linear oligoparaphenylenes. Herein, we explore the changes of these properties on alkyl-tethered-<i>p</i>-heptaphenylenes by modifying the methylene tether lengths from 1 to 19 carbons, leading to a gradual linearization of the conjugated backbone conformation. For this purpose, the photoinduced internal conversion processes of different alkyl-tethered-<i>p</i>-heptaphenylenes are simulated using nonadiabatic excited-state molecular dynamics. We found that the greater the strain introduced on the conjugated system, the slower the electronic and vibrational energy relaxation process. All bent <i>p</i>-heptaphenylenes exhibit similar patterns of intramolecular energy redistribution that finally spatially localize the exciton on phenylene units in the middle of the conjugated chain. This behavior is opposite to the random exciton localization previously reported for [<i>n</i>]­CPPs. Moreover, the nonadiabatic S₂ → S₁ electronic transition activates specific collective asymmetric vibrational excitations that promote periodic oscillatory evolution of the excitonic wave function before an excessive energy dissipates into the bath degrees of freedom

Modification of Optical Properties and Excited-State Dynamics by Linearizing Cyclic Paraphenylene Chromophores

Cyclic and bent conjugated molecular systems have tunable optical, structural, and dynamical features that differentiate them from their linear counterparts. Examples of such systems are [<i>n</i>]­cycloparaphenylenes (CPPs), which consist of nanorings composed of <i>n</i> para-linked benzene units. Circular geometry and tunability of π-orbital overlaps and bending strains enrich them with unique physicochemical and electronic properties compared to those of the corresponding linear oligoparaphenylenes. Herein, we explore the changes of these properties on alkyl-tethered-<i>p</i>-heptaphenylenes by modifying the methylene tether lengths from 1 to 19 carbons, leading to a gradual linearization of the conjugated backbone conformation. For this purpose, the photoinduced internal conversion processes of different alkyl-tethered-<i>p</i>-heptaphenylenes are simulated using nonadiabatic excited-state molecular dynamics. We found that the greater the strain introduced on the conjugated system, the slower the electronic and vibrational energy relaxation process. All bent <i>p</i>-heptaphenylenes exhibit similar patterns of intramolecular energy redistribution that finally spatially localize the exciton on phenylene units in the middle of the conjugated chain. This behavior is opposite to the random exciton localization previously reported for [<i>n</i>]­CPPs. Moreover, the nonadiabatic S₂ → S₁ electronic transition activates specific collective asymmetric vibrational excitations that promote periodic oscillatory evolution of the excitonic wave function before an excessive energy dissipates into the bath degrees of freedom