Quantum Dynamics Simulations Reveal Vibronic Effects on the Optical Properties of [<i>n</i>]Cycloparaphenylenes

The size-dependent ultraviolet/visible photophysical property trends of [<i>n</i>]­cycloparaphenylenes ([<i>n</i>]­CPPs, <i>n</i> = 6, 8, and 10) are theoretically investigated using quantum dynamics simulations. For geometry optimizations on the ground- and excited-state Born–Oppenheimer potential energy surfaces (PESs), we employ density functional theory (DFT) and time-dependent DFT calculations. Harmonic normal-mode analyses are carried out for the electronic ground state at Franck–Condon geometries. A diabatic Hamiltonian, comprising four low-lying singlet excited electronic states and 26 vibrational degrees of freedom of CPP, is constructed within the linear vibronic coupling (VC) model to elucidate the absorption spectral features in the range of 300–500 nm. Quantum nuclear dynamics is simulated within the multiconfiguration time-dependent Hartree approach to calculate the vibronic structure of the excited electronic states. The symmetry-forbidden <i>S</i>₀ → <i>S</i>₁ transition appears in the longer wavelength region of the spectrum with weak intensity due to VC. It is found that the Jahn–Teller and pseudo-Jahn–Teller effects in the doubly degenerate <i>S</i>₂ and <i>S</i>₃ electronic states are essential in the quantitative interpretation of the experimental observation of a broad absorption peak around 340 nm. The vibronic mixing of the <i>S</i>₁ state with higher electronic states is responsible for the efficient photoluminescence from the <i>S</i>₁ state. The fluorescence properties are characterized on the basis of the stationary points of the excited-state PESs. The findings reveal that vibronic effects become important in determining the photophysical properties of CPPs with increased ring size

The Role of Through-Space Interactions in Modulating Constructive and Destructive Interference Effects in Benzene

Quantum interference effects, whether constructive or destructive, are key to predicting and understanding the electrical conductance of single molecules. Here, through theory and experiment, we investigate a family of benzene-like molecules that exhibit both constructive and destructive interference effects arising due to more than one contact between the molecule and each electrode. In particular, we demonstrate that the π-system of meta-coupled benzene can exhibit constructive interference and its para-coupled analog can exhibit destructive interference, and vice versa, depending on the specific through-space interactions. As a peculiarity, this allows a meta-coupled benzene molecule to exhibit higher conductance than a para-coupled benzene. Our results provide design principles for molecular electronic components with high sensitivity to through-space interactions and demonstrate that increasing the number of contacts between the molecule and electrodes can both increase and decrease the conductance

Synthesis, Characterization, and Computational Studies of Cycloparaphenylene Dimers

Two novel arene-bridged cycloparaphenylene dimers (<b>1</b> and <b>2</b>) were prepared using a functionalized precursor, bromo-substituted macrocycle <b>7</b>. The preferred conformations of these dimeric structures were evaluated computationally in the solid state, as well as in the gas and solution phases. In the solid state, the trans configuration of <b>1</b> is preferred by 34 kcal/mol due to the denser crystal packing structure that is achieved. In contrast, in the gas phase and in solution, the cis conformation is favored by 7 kcal/mol (dimer <b>1</b>) and 10 kcal/mol (dimer <b>2</b>), with a cis to trans activation barrier of 20 kcal/mol. The stabilization seen in the cis conformations is attributed to the increased van der Waals interactions between the two cycloparaphenylene rings. These calculations indicate that the cis conformation is accessible in solution, which is promising for future efforts toward the synthesis of short carbon nanotubes (CNTs) via cycloparaphenylene monomers. In addition, the optoelectronic properties of these dimeric cycloparaphenylenes were characterized both experimentally and computationally for the first time

Influence of Nanostructure on the Exciton Dynamics of Multichromophore Donor–Acceptor Block Copolymers

We explore the synthesis and photophysics of nanostructured block copolymers that mimic light-harvesting complexes. We find that the combination of a polar and electron-rich boron dipyrromethene (BODIPY) block with a nonpolar electron-poor perylene diimide (PDI) block yields a polymer that self-assembles into ordered “nanoworms”. Numerical simulations are used to determine optimal compositions to achieve robust self-assembly. Photoluminescence spectroscopy is used to probe the rich exciton dynamics in these systems. Using controls, such as homopolymers and random copolymers, we analyze the mechanisms of the photoluminescence from these polymers. This understanding allows us to probe in detail the photophysics of the block copolymers, including the effects of their self-assembly into nanostructures on their excited-state properties. Similar to natural systems, ordered nanostructures result in properties that are starkly different than the properties of free polymers in solution, such as enhanced rates of electronic energy transfer and elimination of excitonic emission from disordered PDI trap states

Fast Singlet Exciton Decay in Push–Pull Molecules Containing Oxidized Thiophenes

A common synthetic strategy used to design low-bandgap organic semiconductors employs the use of “push–pull” building blocks, where electron -rich and electron-deficient monomers are alternated along the π-conjugated backbone of a molecule or polymer. Incorporating strong “pull” units with high electron affinity is a means to further decrease the optical gap for infrared optoelectronics or to develop n-type semiconducting materials. Here we show that the use of thiophene-1,1-dioxide as a strong acceptor in “push–pull” oligomers affects the electronic structure and carrier dynamics in unexpected ways. Critically, the overall excited-state lifetime is reduced by several orders of magnitude relative to unoxidized analogs due to the introduction of low-energy optically dark states and low-energy triplet states that allow for fast internal conversion and intramolecular singlet fission. We found that the electronic structure and excited-state lifetime are strongly dependent on the number of sequential thiophene-1,1-dioxide units. These results suggest that both the static and dynamical optical properties are highly tunable via small changes in chemical structure that have drastic effects on the optoelectronic properties, which can impact the types of applications that involve these materials

Supporting information from A helical perylene diimide-based acceptor for non-fullerene organic solar cells: synthesis, morphology and exciton dynamics

Synthesis and Characterization detail

Breakdown of Interference Rules in Azulene, a Nonalternant Hydrocarbon

We have designed and synthesized five azulene derivatives containing gold-binding groups at different points of connectivity within the azulene core to probe the effects of quantum interference through single-molecule conductance measurements. We compare conducting paths through the 5-membered ring, 7-membered ring, and across the long axis of azulene. We find that changing the points of connectivity in the azulene impacts the optical properties (as determined from UV–vis absorption spectra) and the conductivity. Importantly, we show here that simple models cannot be used to predict quantum interference characteristics of nonalternant hydrocarbons. As an exemplary case, we show that azulene derivatives that are predicted to exhibit destructive interference based on widely accepted atom-counting models show a significant conductance at low biases. Although simple models to predict the low-bias conductance do not hold with all azulene derivatives, we demonstrate that the measured conductance trend for all molecules studied actually agrees with predictions based on the more complete GW calculations for model systems

Quantitative Intramolecular Singlet Fission in Bipentacenes

Singlet fission (SF) has the potential to significantly enhance the photocurrent in single-junction solar cells and thus raise the power conversion efficiency from the Shockley–Queisser limit of 33% to 44%. Until now, quantitative SF yield at room temperature has been observed only in crystalline solids or aggregates of oligoacenes. Here, we employ transient absorption spectroscopy, ultrafast photoluminescence spectroscopy, and triplet photosensitization to demonstrate intramolecular singlet fission (iSF) with triplet yields approaching 200% per absorbed photon in a series of bipentacenes. Crucially, in dilute solution of these systems, SF does not depend on intermolecular interactions. Instead, SF is an intrinsic property of the molecules, with both the fission rate and resulting triplet lifetime determined by the degree of electronic coupling between covalently linked pentacene molecules. We found that the triplet pair lifetime can be as short as 0.5 ns but can be extended up to 270 ns