Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron−phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, π-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes.

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.

Cycloparaphenylenes represent the smallest possible fragments of armchair carbon nanotubes. Due to their cyclic and curved conjugation, these nanohoops own unique photophysical properties. Herein, the internal conversion processes of cycloparaphenylenes of sizes 9 through 16 are simulated using Non-Adiabatic Excited States Molecular Dynamics. In order to analyze effects of increased conformational disorder, simulations are done at both low temperature (10 K) and room temperature (300 K). We found the photoexcitation and subsequent electronic energy relaxation and redistribution lead to different structural and electronic signatures such as planarization of the chain, electron-phonon couplings, wavefunction localization, and intra-ring migration of excitons. During excited state dynamics on a picosecond time-scale, an electronic excitation becomes partially localized on a portion of the ring (about 3-5 phenyl rings), which is not a mere static contraction of the wavefunction. In a process of non-radiative relaxation involving non-adiabatic transitions, the latter exhibits significant dynamical mobility by sampling uniformly the entire molecular structure. Such randomized migration involving all phenyl rings, occurs in a wave-like fashion coupled to vibrational degrees of freedom. These results can be connected to unpolarized emission observed in single-molecule fluorescence experiments. Observed intra-ring energy transfer is subdued for lower temperatures and adiabatic dynamics involving low-energy photoexcitation to the first excited state. Overall our analysis provides a detailed description of photo excited dynamics in molecular systems with circular geometry, outlines size-dependent trends and connotes specific spectroscopic signatures appearing in time-resolved experimental probes.

The non-adiabatic excited state molecular dynamics (NA-ESMD) approach is applied to investigate photoexcited dynamics and relaxation pathways in a spiro-linked conjugated polyfluorene at room (T = 300 K) and low (T = 10 K) temperatures. This dimeric aggregate consists of two perpendicularly oriented weakly interacting α-polyfluorene oligomers. The negligible coupling between the monomer chains results in an initial absorption band composed of equal contributions of the two lowest excited electronic states, each localized on one of the two chains. After photoexcitation, an efficient ultrafast localization of the entire electronic population to the lowest excited state is observed on the time scale of about 100 fs. Both internal conversion between excited electronic states and vibronic energy relaxation on a single electronic state contribute to this process. Thus, photoexcited dynamics of the polyfluorene dimer follows two distinct pathways with substantial temperature dependence on their efficiency. One relaxation channel involves resonance electronic energy transfer between the monomer chains, whereas the second pathway concerns the relaxation of the electronic energy on the same chain that has been initially excited due to electron-phonon coupling. Despite the slower vibrational relaxation, a more efficient ultrafast electronic relaxation is observed at low temperature. Our numerical simulations analyze the effects of molecular geometry distortion during the electronic energy redistribution and suggest spectroscopic signatures reflecting complex electron-vibrational dynamics.

Using the Non-Adiabatic Excited States Molecular Dynamics (NA-ESMD) approach, we investigate the ultrafast electronic relaxation in a recently synthesized small molecule donor, p-DTS(PTTh 2 ) 2 , which belongs to the dithienosilole-pyridylthiadiazole family of chromophores. In combination with the PC 70 BM acceptor, p-DTS(PTTh 2 ) 2 can be used to fabricate high efficiency bulk heterojunction organic solar cells. After photoexcitation to its broad high-energy peak in the 3-4 eV range, associated with multiple excited states, p-DTS(PTTh 2 ) 2 undergoes efficient ultrafast internal conversion to its lowest excited state. During this process, about 1-2 eV electronic energy transfers to the vibrational degrees of freedom leading to rapid heating of the molecule. Nevertheless, our simulations do not detect possible bond-breaking or decomposition of the system. This suggests minimal intra-molecular photodamage after photoexcitation to high-energy states in the 3-4 eV region. Calculated radiationless deactivation mainly consists of a sequential mechanism that involves electronic transitions between the current transient state and the corresponding state directly below in energy. Changes in the density of states along the relaxation process lead to pronounced variations and time-dependence of the accumulated populations of the different intermediate electronic excited states. Visualization of the electronic transition density during internal conversion reveals spatial intramolecular delocalization of electronic excitation from the thiophene moieties to the entire chromophore. Finally, our analysis of non-adiabatic coupling vectors suggests characteristic vibrational degrees of freedom coupled to the electronic system during various stages of non-radiative relaxation. Broader contextConsiderable world-wide effort in both industry and academia has been focused on developing organic photovoltaic (OPV) cell and module technologies, which can potentially meet multiple requirements for solar energy conversion. Here, power conversion efficiencies (PCEs) exceeding 10% have been achieved in the case of bulk-heterojuntion (BHJ) solar cell structures. These values of PCE are still below those corresponding to inorganic solar cells but steady improvements have been made. In recent years, small molecules have emerged as favorable novel organic compounds owing to their exceptional light-harvesting capacity and efficient charge transport. For example, solar cells fabricated using a recently synthesized molecular donor p-DTS(PTTh 2 ) 2 exhibited the highest record value of 6.7% for the PCE, this revealing the potential of small donor molecular materials. The performance of such OPV devices depends on a complex interplay of charge and energy transfer timescales competing with radiative and non-radiative relaxations, and various energy-loss mechanisms. Subsequently, design of new molecules involves optimization of many important parameters including detailed understanding of excited state dynamics relevant to charge transfer reactions in ...

Carbon nanobelts are cylindrical molecules composed of fully fused edge-sharing arene rings. Because of their aesthetically appealing structures, they acquire unusual optoelectronic properties that are potentially suitable for a range of applications in nanoelectronics and photonics. Nevertheless, the very limited success of their synthesis has led to their photophysical properties remaining largely unknown. Compared to that of carbon nanorings (arenes linked by single bonds), the strong structural rigidity of nanobelts prevents significant deformations away from the original high-symmetry conformation and, therefore, impacts their photophysical properties. Herein, we study the photoinduced dynamics of a successfully synthesized belt segment of (6,6)CNT (carbon nanotube). Modeling this process with nonadiabatic excited state molecular dynamics simulations uncovers the critical role played by the changes in excited state wave function localization on the different types of carbon atoms. This allows a detailed description of the excited state dynamics and spatial exciton evolution throughout the nanobelt scaffold. Our results provide detailed information about the excited state electronic properties and internal conversion rates that is potentially useful for designing nanobelts for nanoelectronic and photonic applications.

Photoexcitation of multichromophoric light harvesting molecules induces a number of intramolecular electronic energy relaxation and redistribution pathways that can ultimately lead to ultrafast exciton self-trapping on a single chromophore unit.

Computer simulations on the generation of bimetallic nanoparticles are presented in this work. Two different generation mechanisms are simulated: (a) cluster-cluster collision by means of atom dynamics simulations; and (b) nanoparticle growth from a previous seed through grand canonical Monte Carlo (gcMC) calculations. When two metal nanoparticles collide, different structures are found: core/shell, alloyed and three-shell (A-B-A). On the other hand, the growth mechanism at different chemical potentials by means of gcMC reveals the same results as atom dynamics collisions do.